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Brain Barriers and brain fluids research in 2020 and the fluids and barriers of the CNS thematic series on advances in in vitro modeling of the blood–brain barrier and neurovascular unit

Abstract

This editorial discusses advances in brain barrier and brain fluid research in 2020. Topics include: the cerebral endothelium and the neurovascular unit; the choroid plexus; the meninges; cerebrospinal fluid and the glymphatic system; disease states impacting the brain barriers and brain fluids; drug delivery to the brain. This editorial also highlights the recently completed Fluids Barriers CNS thematic series entitled, ‘Advances in in vitro modeling of the blood–brain barrier and neurovascular unit’. Such in vitro modeling is progressing rapidly.

Brain barriers and brain fluids research continues to be a vibrant field. For example, over 10,000 articles were published in 2020 (as accessed on Medline/Ovid) on either the blood–brain barrier (BBB), the brain endothelium, the choroid plexus (CP), cerebrospinal fluid (CSF), brain edema or hydrocephalus. That is too large a body of work to address in this review, but we aim to highlight some of the current themes of such research. As always, the choice of papers to highlight is idiosyncratic, reflecting in part the interests of the editors of Fluids and Barriers of the CNS. We apologize for the many important papers that are not cited.

In addition to reviewing the general literature, we also discuss the recently completed Fluids Barriers CNS thematic series entitled, ‘Advances in in vitro modeling of the blood–brain barrier and neurovascular unit’. In vitro models are advancing rapidly and providing greater insight into the mechanisms underpinning the BBB and neurovascular unit (NVU) in health and disease.

Elements of the blood–brain barriers and the brain fluid systems

Brain endothelium

Molecular mechanisms regulating BBB function, including tight junctions (TJs), are potential therapeutic targets for a variety of neurological conditions. Sladojevic et al. [1] found that Regulator of G-protein Signaling 5 (RGS5) regulates brain endothelial nitric oxide synthase and TJs in vitro. In vivo, an endothelial specific RGS5 mouse knockout had larger infarcts, worse neurological deficits and more brain edema after stroke, suggesting RGS5 might be a therapeutic target.

Brain endothelial intercellular junctions are complex dynamic structures. To further understand how to manipulate claudin-5, Roudnicky et al. [2] have used human pluripotent stem cell-derived endothelial cells to create a stable cell line expressing claudin5-green fluorescent protein (CLDN5-GFP). They then screened a chemical library and identified 62 compounds that activated CLDN5-GFP. One of which, RepSox, a TGFβ pathway inhibitor, was further examined and found to stabilize the vasculature and induce the expression of other TJ proteins and transporters. Kakogiannos et al. [3] described an interesting interaction between the TJ proteins, claudin-5 and junctional adhesion molecule A (JAM-A), whereby JAM-A can upregulate claudin-5 expression via the transcription factor C/EBP-alpha. Previously, it was found that JAM-A can also act as a leukocyte adhesion molecule at the BBB [4]. Together, these results suggest important roles for this relatively-understudied brain endothelial TJ protein.

The endothelial cytoskeleton is important for regulating multiple functions and Samus et al. [5] found that the actin-binding protein cortactin may be a therapeutic target in multiple sclerosis. In mouse experimental autoimmune encephalomyelitis (a model of multiple sclerosis), gene inactivation of cortactin reduced neuroinflammation and a lack of cortactin ameliorated leukocyte migration across the brain endothelium in vivo and in vitro. Mehra et al. [6] also reported that activation of N-Methyl-d-Aspartate Receptors (NMDARs) in brain endothelial cells upregulates immune cell infiltration into brain by phosphorylating myosin light chain and subsequent cell shrinkage. Interestingly, endothelial and neuronal NMDARs differ in structure, function and pharmacology.

Age has an enormous impact on the burden of cerebrovascular disease. However, disease mechanisms and treatments are often studied in young animals. Chen et al. [7] examined the effect of normal aging on capillary, arterial and venous brain endothelial cells using single-cell RNA sequencing in mice. They found the biggest changes with age occurred in the capillary endothelial cell transcriptome and that they could be ameliorated by exposure to plasma from young animals, indicating the importance of circulatory factors. Zhao et al. [8] employed a similar approach to examine the effects of aging on the transcriptome of capillary, arterial and venous brain endothelial cells in mice. Aging impacted inflammatory signaling in all segments and particularly impacted energy metabolism and barrier permeability in capillary endothelial cells. The effects of aging could be reversed with a glucagon-like peptide-1 receptor agonist. Yang et al. [9] have identified a switch from ligand-specific receptor-mediated transcytosis at the BBB to a non-specific caveolar transcytosis with aging in mice. That change may be linked to reduced pericyte coverage.

At the other end of the age spectrum, during brain blood vessel development, Chen et al. [10] examined the role of the gene encoding prion protein 2 (Prnd), which encodes the protein doppel. Prnd knockout mice had impaired blood vessel morphogenesis, sprouting defects and BBB dysfunction. Similarly, Cottarelli et al. [11] found that fibroblast growth factor binding protein 1 (Fgfbp1) is a novel Wnt/beta-catenin regulated gene and that endothelial cell-specific loss of Fgfbp1 results in transient hypervascularization and a delay in BBB maturation. They found Fgfbp1 concentrates Wnt ligands near endothelial junctions. Interestingly, Veys et al. [12] found that the major brain endothelial glucose transporter, Glut1, is crucial for CNS angiogenesis but not BBB barrier function. Major facilitator superfamily domain-containing 2a (Mfsd2a) is another protein important in BBB development. Wang et al. [13] found that Mfsd2a binds with another protein, Spinster homolog 2, to regulate sphingosine-1-phosphate release from brain endothelial cells which is important for BBB formation and maintenance.

Species differences are a potential confounder for the translation of preclinical data to the clinic. Song et al. [14] compared the transcriptomes of human and mouse brain microvascular endothelial cells and this should provide a valuable resource particularly since much research is mouse based. As noted throughout this review, major advances in our understanding of the brain endothelium and the NVU have utilized transcriptomics and particularly the multiple uses of RNA sequencing (RNA-Seq). Some guidelines on the appropriate approach for performing, analyzing and publishing such studies have recently been published [15].

Neurovascular unit

The importance of astrocytes in regulating BBB and other cerebrovascular functions has long been recognized (reviewed in [16]). For example, Wnt signaling is important for maintaining the BBB and Guerit et al. [17] recently demonstrated the importance of astrocytic Wnt release in that function in mice. Blocking such release increased BBB permeability, endothelial vesicle formation and brain edema.

Interestingly, Batiuk et al. [18] used single-cell RNA sequencing to identify multiple astrocyte subtypes that vary across and within mouse brain regions raising the possibility of another level of cerebrovascular regulation. Uchida et al. [19] recently described regional differences in transporter expression in brain capillaries from rats and humans. The extent that these changes are related to inherent differences at the endothelium or differences in signals from other cells of the NVU (e.g., astrocytes) merits investigation.

Pericytes are another important component of the NVU: Mae et al. [20] examined the effects of pericyte loss on the cerebral endothelium at the single cell level. They found that pericytes are important for a limited set of BBB functions but have a role in regulating endothelial arterio-venous zonation and angiogenesis. Further evidence on the importance of pericytes in BBB function comes from Gautam et al. [21] who used a pericyte-specific laminin knockout. They found that loss of pericyte laminin caused age-dependent BBB disruption including altered para- and transcellular pathways. Similarly, Sheikh et al. [22] found that neural-specific depletion of members of the non-specific lethal chromatin complex led to a TLR4-mediated inflammatory signaling cascade in neighboring pericytes that in turn led to marked cerebrovascular defects and cerebral hemorrhage. An interesting advance is the description of tunneling nanotubes linking retinal pericytes that have an important role in neurovascular coupling [23].

While the role of astrocytes and pericytes in regulating the BBB has been the subject of extensive investigation, macrophages/microglial cells have received less attention. Ronaldson and Davis [24] recently reviewed the role of microglial cells in BBB regulation. In addition, Delaney et al. [25] found that mutations in colony stimulating factor-1 receptor (Csf-1r) in a rare condition called adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP), were associated with cerebrovascular pathologies. Csf-1r is critical for macrophage/microglia function and they found that attenuating Csf-1r signaling resulted in remodeling of BBB TJs. Santisteban et al. [26] provided evidence of the importance of brain endothelial cell to perivascular macrophage crosstalk in BBB dysfunction that can occur in hypertension, with endothelial angiotensin II type-1 receptors playing a role in initiating dysfunction but perivascular macrophages being required for a full phenotype.

Similarly, little is known about brain endothelial cell oligodendrocyte interactions. The evidence on those bi-directional interactions and the role of Wnt/β-catenin signaling was recently reviewed [27]. While most focus has been on regulation of endothelial cell function by parenchymal cells, there are other examples of brain endothelial cell signals regulating parenchymal cells. For example, endothelial cells in gliomas promote glioma cell migration by secreting extracellular vesicles [28].

Neuronal activity also impacts brain endothelial cell function [29]. Pulido et al. [30] recently identified a core set of brain endothelial genes whose expression is regulated by neuronal activity. Prominent amongst those were efflux transporters. They also found that effects of neuronal activity on the expression of circadian clock genes in the brain endothelium was important in that regulation. Neuronal regulation of brain endothelial function also occurs in areas without a BBB. Thus, Jiang et al. [31] found that melanin-concentrating hormone-expressing neurons regulate the permeability of blood vessels of the median eminence via vascular endothelial growth factor signaling. Another type of BBB regulation can occur with parenchymal cell death. Nishibori et al. [32] review how the nuclear protein High Mobility Group Box-1 (HMGB-1), released after cell injury, acts to induce BBB disruption and neuroinflammation. HMGB-1 is a damage-associated molecular pattern (DAMP) important in brain injury. It and other DAMPs may be therapeutic targets.

As well as regulating brain endothelial function, other elements of the NVU are also directly impacted by disease. Thus, pericytes are prone to HIV-1 infection and Torices et al. [33] found that caveolin-1, occludin and Alix (an early acting endosomal factor) form a complex that regulates infection.

One generally neglected area of barriers research is the blood-nerve barrier. Ubogu [34] and Reinhold and Rittner [35] recently reviewed our current state of knowledge on this barrier. Stubbs [36] discusses the importance of blood-nerve-barrier dysfunction in peripheral neuropathies while Takeshita et al. [37] have developed a human blood-nerve barrier model.

Choroid plexus

The CP has a circadian rhythm. Thus, Yamaguchi et al. [38] examined the circadian rhythm of CP clock genes and the relationship between the CP rhythm and that in the suprachiasmatic nucleus and the pineal gland. Further, Liska et al. [39] found that the CP circadian rhythm can be reset by circulating glucocorticoids and Furtado et al. [40] found the circadian rhythmicity of the CP clock gene, Bmal1, was disrupted in a mouse Alzheimer’s disease (AD) model. These findings suggest that CP function is altered during the day-night cycle. This may impact fluid, solute and cell movement across this blood-CSF interface.

In addition to other brain barriers, the CP may be involved in neuroinflammation. Solar et al. [41] have recently reviewed the CP and the blood-CSF barrier in different diseases including those inducing neuroinflammation. Rodriguez-Lorenzo et al. [42] examined inflammatory changes in the CP in multiple sclerosis. While evidence of inflammation was found, they suggest that it plays only a minor role in immune cell infiltration in patients with chronic multiple sclerosis. In contrast, Mottahedin et al. [43] provide evidence of the importance of the CP in neutrophil entry after hypoxia–ischemia in neonatal rats and Saul et al. [44] recently described structural and functional alterations at the CP in amyotrophic lateral sclerosis (ALS). Rayasam et al. [45] also found that the CP was an important route for myeloid cell entry after neonatal stroke and that CX3CR1 and CCR2 signaling plays an important role in that infiltration. Nishihara et al. [46] compared the ability of CD4+ T helper subsets to cross the BBB and blood-CSF-barrier models in vitro and their results indicate different subsets use alternate routes for migration.

The impact of diet on the CP has received little attention. However, Alimajstorovic et al. [47] recently reported that female rats fed a high fat diet have twice the CSF secretion rate of control diet rats and this may be a mechanism for idiopathic intracranial hypertension that occurs in obese individuals. Obata and Narita [48] reported that a high cholesterol diet or hereditary hyperlipidemia alters CP structure in rabbits.

Our understanding of the role of the CP in health and disease is aided by in vivo imaging measurements in humans. Evans et al. [49] recently described using magnetic resonance imaging (MRI) to measure the movement of water from arterial blood to CSF in human brain as well as mouse. MRI was also used by Zhao et al. [50] to examine CP hemodynamic parameters (flow, transit time) in humans. Eide et al. [51] used MRI to track the clearance of gadobutrol after intrathecal administration in healthy individuals and those with idiopathic normal pressure hydrocephalus. They found a delayed clearance of the tracer by the CP in the hydrocephalus patients. Positron emission tomography (PET) imaging is also being used to study the human CP. Thus, total AV1451 (tau) PET binding to the CP using Gaussian Mixed Model segmentation can distinguish between patients with AD from those with mild cognitive impairment [52].

Another method that may help to advance understanding of the CP is the development by Pellegrini et al. [53] of human induced pluripotent stem cell (iPSC)-derived CP organoids. They already demonstrated fluid secretion and identified multiple functions of different epithelial cell populations.

Meninges

The meninges have long been an understudied tissue. This is gradually changing because of the potential importance of meningeal lymphatics in CSF drainage and a route for immune surveillance of the brain (reviewed in [54]). Interestingly, Haugland et al. [55] recently found that implanting EEG electrodes in mice was enough to induce meningeal lymphangiogenesis and enhance the glymphatic pathway and Chen et al. [56] provided evidence that the meningeal lymphatics play a role in the clearance of red blood cells from CSF after subarachnoid hemorrhage (SAH). They found that ablating the meningeal lymphatics in mice greatly exacerbated SAH-induced brain injury. Being able to track meningeal lymphatic function in patients and how it is impacted by disease would be very useful and Ringstad and Eide [57] are currently using MRI to track the route for tracer movement from CSF to dural lymphatic vessels in humans.

Apart from the dural lymphatic vessels, Shibata-Germanos et al. [58] have identified a cell type they name Leptomeningeal Lymphatic Endothelial Cells (LLECs) within the leptomeninges. These cells have lymphatic and macrophage properties and while they do not form lumens, they phagocytose macromolecules including amyloid-β and suggesting they may have homeostatic and immune roles.

Interestingly, Uchida et al. [59] have also stressed the importance of transport at the blood-arachnoid barrier. They found that the total protein expression of several transporters at the arachnoid membrane, including p-glycoprotein and breast cancer resistance protein, was greater than at the CP. The importance of the arachnoid membrane in regulating CSF composition is understudied. Emerging evidence indicates the importance of the meninges as a niche for neural progenitor cells [60]. These cells may be a therapeutic target for treating neurological disorders.

Glymphatic system

The glymphatic system continues to stimulate much research and many controversies [61] and imaging techniques feature prominently in addressing these issues [62]. In particular, rapid advances in non-invasive MR techniques are promising, but have yet to definitively identify the glymphatic system in humans, and PET imaging is also being developed using radio-labeled tracers [63]. There is considerable interest in the effects of the sleep cycle on the glymphatic system with potential implications for toxin clearance in neurodegenerative diseases such as Alzheimer’s disease. For example, Hablitz et al. [64] have shown a circadian rhythm in the glymphatic system in mice with a peak glymphatic influx and clearance during the mid-rest phase. Loss of aquaporin-4 abolished the day-night differences. As noted elsewhere in this review, the CP and the BBB also show circadian rhythms. How these systems integrate and impact both toxin and drug clearance is an interesting and important area of future research.

A recent modeling study has emphasized the role of intracranial pressure in determining the relative contribution of the glymphatic system to CSF clearance [65]. Also, Goodman and Iliff [66] highlight the critical importance of maintaining physiological blood gases in glymphatic studies. They found that hypercapnia (as can occur with anesthesia) profoundly reduced both the brain uptake of tracers injected into the subarachnoid space and the appearance of tracers in deep cervical lymph nodes after injection into mouse brain parenchyma.

CSF analysis

CSF analysis is used in the diagnosis of some neurological diseases [67, 68] although there are concerns over reproducibility [69]. One recent advance is in the use of metagenomic next-generation sequencing (mNGS) of CSF RNA and DNA (reviewed in [70]). It was used to simultaneously screen for a wide range of infectious agents in an un-biased manner.

Many neurological conditions differentially affect women and men. Kamitaki et al. [71] examined the role of differences in the complement system in such sexual dimorphism. A genetic analysis indicated that complement component 4 (C4) genes C4A and C4B contribute to differential risk in systemic lupus erythematosus and Sjogren’s syndrome. They also found that CSF concentrations of both C4 and C3 (a downstream effector) were higher in men than women.

One area where CSF biomarkers are increasingly used is in attempting to distinguish types of neurodegenerative disorders. While p-tau181 is an established AD biomarker [67], Janelidze et al. [72] recently reported that p-tau217 may be a more useful biomarker. Similarly, Blennow et al. [73] proposed reductions in a tau fragment, tau368, in CSF as a novel marker of AD as it is sequestered into tangles. It should be noted that particular tau profiles change during AD progression [74]. For vascular dementia patients, Llorens et al. [75] recently found that elevated CSF lipocalin 2 levels can distinguish them from other types of neurodegenerative dementia.

CSF antibodies to glutamic acid decarboxylase (GAD) are found in several neurological conditions. Whether or not these GAD antibodies participate in pathology with GAD autoimmunity was recently reviewed by Graus et al. [76].

Identification of CSF leukocyte populations in disease states has also advanced. Schafflick et al. [77] used single cell transcriptomics to identify CSF leukocyte populations in multiple sclerosis and found that compartmentalized populations were driven by local T cell/B cell interactions. Gate et al. [78] found clonally expanded CD8+ T effector memory CD45RA+ cells in the CSF of AD patients. These clonally expanded cells had enhanced T cell receptor signaling and had specificity to two separate Epstein-Barr virus antigens.

He et al. [79] have developed an interesting approach for sampling brain lymph fluid from the afferent lymph vessels of deep cervical lymph nodes. A different analyte profile may result from this fluid compared to the CSF sampled from the usual sites (e.g., lumbar puncture).

Neurological condtions

SARS-CoV-2/ COVID-19

Neurological symptoms including long-term ones, are common in patients with COVID-19 [80, 81]. The underlying mechanisms for these effects are under intense investigation. There is evidence that the spike protein, S1, of SARS-CoV-2 can cross into mouse brain after IV or intranasal administration [82]. Furthermore, in a few cases of COVID-19 patients with neurological symptoms, SARS-CoV-2 was detected in CSF by PCR [83, 84]. In contrast to a direct parenchymal effect of the virus, it was suggested that antibodies against SARS-CoV-2 cross the blood–brain barriers and cause the neurological symptoms by an autoimmune-like response [85, 86].

Another possibility is that SARS-CoV-2 may directly affect cells of the NVU and lead to neurological dysfunction [87]. In vitro, Buzhdygan et al. [88] found that the spike protein of SARS-CoV-2 induced a loss of brain endothelial cell barrier integrity and triggered a pro-inflammatory response. However, it should be noted that questions have arisen over whether human brain endothelial cells normally express the angiotensin converting enzyme-2 (ACE2) necessary for infection [89]. An alternate barrier site may be the CP and Jacob et al. [90] and Pellegrini et al. [91] have both used human iPSC-derived organoids and found that the virus can infect the CP and disrupt choroid plexus function. Another alternate potential mechanism is that the systemic inflammatory response to SARS-CoV-2 causes secondary effects on the brain [92].

The effect of SARS-CoV-2 is a rapidly evolving area of research. Doubtless, much more will be discovered in 2021 including the impact on neurological function of different SARS-CoV-2 variants.

Spaceflight

Spaceflight and associated microgravity have emerged as new challenges with respect to the brain and CSF. It has been known for several years that long-duration spaceflight in particular, results in upward shifts in brain tissue, a narrowing of the CSF spaces at the vertex, raised intracranial pressure and increased ventricular volume [93]. The cardiovascular system adapts to weightlessness with an increased cardiac output and accumulation of venous blood in the head [94]. Spaceflight Neuro-ocular syndrome (SANS) is characterized by edema of the optic disc and flattening of the globe with optic nerve tortuosity leading to ophthalmic abnormalities [95]. Changes in CSF hydrostatic gradients and intracranial pressure may be responsible (reviewed in [96]).

Hydrocephalus

Hydrocephalus is a diverse disorder characterized by enlarged lateral ventricles (ventriculomegaly) with or without increased intracranial pressure. Underlying causes include failure of correct brain development, hypersecretion of CSF, obstructed CSF circulation, insufficient CSF absorption and atrophy of brain tissue.

Genetic causes for ventriculomegaly

The molecular mechanisms underlying brain ventricular development are still incompletely understood. Yang et al. [97] have identified a member of the neural CAM gene family, Camel, that regulates cell adhesion in zebrafish. Loss of Camel causes hydrocephalus, scoliosis of the spine and failure of the Reissner fiber to form in the ventricular system, whereas increasing Camel mRNA induced Reissner fiber misdirection. Prior results indicated that loss of the Reissner fiber (secreted by the subcommissural organ) can lead to hydrocephalus [98].

Correct cilia function is important for the brain and many examples of defective cilia are associated with hydrocephalus. For example, Robson et al. [99] examined patients with mutations in the multicilin gene, MCIDAS, using MRI. They found that all the patients had hydrocephalus, arachnoid cysts, and choroid plexus hyperplasia, possibly related to CSF overproduction. Another cilia protein, Cfap206 is regulated by FOXJ1 and Cfap206 mutant mice develop hydrocephalus, Beckers et al. [100]. Similarly, Zou et al. [101] found that loss of another motile cilia protein, RSPH9, caused hydrocephalus and ependymal cell loss in mice, along with some parenchymal effects. The loss of membrane type 1-matrix metalloproteinase in mice causes a hydrocephalus that is associated with reduced and disorganized motile cilia and altered brain development, (Jiang et al. [102]). Wu et al. [103] found that vacuolar protein sorting associated protein-35 (VPS35) promotes differentiation, survival and ciliogenesis in ependymal cells. It also prevents local microglial cell activation and knock out of VPS35 in ependymal progenitor cells resulted in hydrocephalus.

Clearly, genetic defects are a leading cause for brain to develop abnormally with resultant ventriculomegaly as emphasized by Jin et al. [104] who used whole-exosome sequencing in patients with sporadic congenital hydrocephalus. They found that de novo damaging mutations accounted for ~ 20% of sporadic congenital hydrocephalus cases that required neurosurgical treatment.

Hydrocephalus–other mechanisms

Reduced glycine decarboxylase function in mice and humans with non-ketotic hyperglycinemia is associated with hydrocephalus. Santos et al. [105] found this reflects a defect in folate metabolism and hydrocephalus in glycine decarboxylase deficient mice was prevented by supplementing the maternal diet with formate.

A non-genetic cause of hydrocephalus is intraventricular hemorrhage (IVH). The lysis of red blood cells with the hemorrhage may trigger events contributing to the hydrocephalus. For example, peroxiredoxin 2, the 3rd most common protein in red blood cells, was shown to be a contributor to IVH-induced hydrocephalus in rats and a powerful inflammatory mediator (Tan et al. [106]).

Idiopathic normal pressure hydrocephalus is a condition where CSF circulation and /or absorption may be defective and different MRI techniques are giving improved insight into hydrocephalus and the glymphatic system. Eide et al. [107] used long-term MRI after intrathecal injection of a contrast agent, gadobutrol, to examine CSF tracer dynamics in patients with idiopathic normal pressure hydrocephalus and compared those changes to alterations in CSF system anatomy and neurodegeneration. This may prove a useful tool for examining mechanisms underlying hydrocephalus. Determining which normal pressure hydrocephalus patients will potentially benefit from shunt surgery and which tests are predictive of success continues to be a subject of major concern. For example, Wolfsegger et al. [108] used a quantitative gait analysis scale together with radiological and psychological assessments with CSF tap tests to refine the diagnosis.

Hydrocephalus treatment

Shunt failure continues to plague hydrocephalus treatment. Work from the Hydrocephalus Clinical Research Network identified factors that may predict fast and ultrafast shunt failure [109]. Age at time of surgery, hydrocephalus etiology and a history of prior failures were important predictors. In contrast, slit or enlarged ventricles were not. A large-scale study of failed shunts (shunt biobank) was developed with the aim of producing a prognostic algorithm [110].

There is a dire need for therapeutics that can alleviate the burden of hydrocephalus. Hochstetler et al. [111] recently reported that antagonists of transient receptor potential vanilloid 4 (TRPV4) channels can ameliorate hydrocephalus in a rat genetic model. Zhang et al. [112] developed a novel, potent SPAK kinase inhibitor that regulates brain cation-Cl cotransporters. Intracerebroventricular administration of the inhibitor reduced CSF hypersecretion in a model of post-hemorrhagic hydrocephalus. It also reduced brain edema and improved outcomes in a model of stroke. The development of treatment strategies may be assisted by in vitro modeling. Castaneyra-Ruiz et al. [113] presented a model for examining the effects of intraventricular hemorrhage on the developing ventricular zone and the associated stem cell niche.

Dementia

The effects of aging on the brain that lead to cognitive decline and dementia are complex, involving such agents as β-amyloid, tau and the susceptibility gene, APOE4. Expression of the latter increases the risk for AD and is associated with earlier onset. Importantly, Montagne et al. [114] recently found that BBB disruption contributes to APOE4-associated cognitive decline in patients independently of AD pathology and that higher levels of ApoE and tau, but not β-amyloid and APOE4 gene, were correlated with lower levels of claudin-5 and occludin in AD patient brains (Liu et al. [115]). This suggests there may be multiple NVU targets for reducing the cerebrovascular and cognitive effects of aging. Montagne et al. [114] also found that high CSF levels of a pericyte injury marker predicted cognitive decline in patients that carried the APOE4 gene, but not non-carriers. Furthermore, Blanchard et al. [116], using a human in vitro BBB model, found that APOE4 induces a cerebral amyloid angiopathy-like pathology in pericytes.

Johnson et al. [117] performed a large-scale proteomic analysis of brain and CSF in AD and identified changes in brain glucose metabolism and protein markers associated with an anti-inflammatory state that were also elevated in CSF.

There has been interest in examining changes in the retinal vasculature in AD patients to provide insight into cerebrovascular changes. Shi et al. [118] identified pericyte loss and vascular amyloidosis in AD retina post-mortem and these changes correlated with brain β-amyloid burden, cerebral amyloid angiopathy and clinical symptoms. Because β-amyloid and tau play a central role in AD pathogenesis there has been great interest in how these molecules are cleared from brain at the blood–brain barriers and via CSF and the glymphatic system and how such clearance is impacted by the disease itself. Harrison et al. [119] recently reported evidence of impaired glymphatic function and tau clearance in a mouse model of tauopathy, and that such clearance can be affected by an aquaporin-4 inhibitor.

Stroke and traumatic brain injury

In stroke, there is a question over whether brain endothelial injury is purely a consequence of parenchymal injury or whether it contributes to parenchymal injury. Evidence is accruing, using models with endothelial specific genetic deletions or overexpression, that supports the latter, indicating that brain endothelium is a target for reducing ischemic brain damage. For example, Ma et al. [120] found that endothelial-selective deletion of the microRNA cluster, miR-15a/16–1, reduced brain infarct, BBB dysfunction and neuroinflammation after stroke in mice. That same deletion increased the expression of claudin-5 and, interestingly, the authors also found that miR-15a/16–1 binds to the 3’ untranslated region of claudin-5. Similarly, Sun et al. [121] found that the endothelial-selective deletion of miR-15a/16–1 also promoted angiogenesis after stroke and improved long-term neurological outcomes in mice. In addition, endothelial specific overexpression of Kruppel-like factor 11 reduced infarct size, BBB disruption, edema and neuroinflammation in mice (Zhang et al. [122]). As noted above, Sladojevic et al. [1] found that mice with an endothelial-specific RGS5 knockout had larger infarcts, worse neurological deficits and more brain edema after stroke. Pericyte dysfunction also plays an important role in stroke pathology. Sun et al. [123] showed that transplantation of human pluripotent stem cell-derived pericyte-like cells improves functional outcomes after stroke in mice.

While severe and moderate traumatic brain injuries are known to cause BBB disruption, the impact of milder injuries is less clear, particularly with repetitive events. O’Keefe et al. [124] examined such injury using imaging and blood biomarkers of BBB injury in rugby players and mixed martial arts fighters. They found evidence of BBB dysfunction can occur in a subset of people after repetitive sub-concussive forces. Similarly, Veksler et al. [125] detected BBB dysfunction in American Football players using dynamic contrast-enhanced MRI. They also found BBB dysfunction in rodents exposed to mild repetitive closed-head injury. There is also evidence that acute but mild head trauma causes extravasation from the meningeal vessels into the subarachnoid space [126].

Brain edema

Brain edema is a major complication in a variety of neurological conditions including stroke, traumatic brain injury and brain tumors and novel treatment strategies and targets are badly needed. Targeting the subcellular distribution of aquaporin 4 (preventing cell surface expression) was found to reduce edema after spinal cord injury in rats (Kitchen et al. [127]). Mestre et al. [128] have proposed the provocative idea that CSF influx into brain parenchyma drives early edema after cerebral ischemia based on evidence that glymphatic influx from CSF to brain is doubled in the early stages of ischemia. Ischemic brain edema is associated with a net accumulation of brain ions and it has been also proposed that ion transport inhibitors may be a method of suppressing edema formation. It is interesting that the SPAK inhibitor developed by Zhang et al. [112] regulates brain cation-Cl cotransporters and reduces ischemic brain edema. It should be noted, however, that it also reduced infarct size which may itself impact edema formation.

Psychiatric disorders

There has been recent interest in the role of brain endothelial cell/claudin-5 dysfunction in psychiatric disorders, including depression and schizophrenia [129]. Dudek et al. [130] identified factors that may explain vulnerability or resilience of the BBB against the effects of chronic social stress (a model of depression) in mice, effects that also seem to occur in humans. These results may help to identify novel therapeutic strategies for depression. There may be molecular targets within the NVU as a whole as well as specifically the brain endothelium. Thus, Sugimoto et al. [131] reported that serotonin/5HT-1A signaling in the NVU enhances brain endothelial claudin-5 expression and may be linked to altered serotonin signaling found in multiple psychiatric disorders.

Lehmann et al. [132] examined transcriptional changes in the brain endothelial cells of mice exposed to chronic social stress. They found changes related to inflammation, oxidative stress, growth factor signaling and angiogenesis. Interestingly, cessation of the social stress led to a recruitment of leukocytes that may participate in vascular repair. Ouellette et al. [133] also found a vascular component to autism spectrum disorder. Using a mouse model of 16p11.2 deletion autism spectrum disorder syndrome, they found structural and functional cerebrovascular changes and that endothelial cell-specific 16p11.2 deletion recapitulated some of the behavioral changes found in 16p11.2 deletion syndrome.

Drug delivery

The delivery of therapeutics to the brain for treatment of neurological disorders continues to be a subject for much research. For example, enzyme replacement therapy is being used for the treatment of patients with lysosomal storage disorders. However, such proteins do not cross the NVU/BBB limiting their use in such disorders with CNS involvement and one approach is to use methods to enhance blood–brain transport. For example, Sun et al. [134] have used nanovesicles of saposin C and dioleoylphophatidylserine to transport β-glucosidase into brain in a mouse model of Gaucher disease and found a marked improvement in the neurological phenotype. Also, Hede et al. [135] used a gene therapy approach to examine whether it is possible to induce brain endothelial cells to produce the required protein. For NPC2, the protein that is mutated in Niemann Pick type C2, they have shown, at least in vitro, the feasibility of such an approach. Interestingly, Gorick et al. [136] showed that pulsed low-pressure focused ultrasound in conjunction with gas-filled microbubbles can be used to transfect the cerebral endothelium without causing BBB disruption.

A more established use of focused ultrasound with microbubbles is to increase the permeability of the NVU/BBB, a technique now in clinical trials. Thus, Rezai et al. [137] demonstrated that it can safely cause transient enhanced permeability in the hippocampus of patients with early AD. Furthermore, D’Haese et al. [138] used focused ultrasound-induced BBB/NVU disruption to induce a modest reduction in β-amyloid plaque burden in early AD patients.

There continues to be a major focus on targeting receptor-mediated transcytosis at the BBB/NVU for drug delivery. Thus, Kariolis et al. and Ullman et al. [139, 140] used a Fc fragment that targets the transferrin receptor, a receptor that is highly expressed in brain endothelial cells. This fragment has then been used to create antibody transport vehicle molecules for evaluation in mice and monkeys. The Ullmann et al. [140] study demonstrated delivery of iduronate 2-sulfatase to the brain in a mouse model of mucopolysaccharidosis type II, another lysosomal storage disorder, and improvement of brain-related pathology. Stocki et al. [141] also identified single domain antibodies with high affinity for the transferrin receptor, one of which crosses the blood–brain interface and is taken up by neurons. Intravenous administration of the antibody fused with neurotensin caused a reduction in body temperature (i.e., the construct induced a physiological response).

Georgieva et al. [142] used a human iPSC-based BBB model combined with a human single-chain variable fragment phage display to screen for potential targets for transcytosis. They identified a number of candidates, one of which showed markedly increased uptake into mouse brain. Gregory et al. [143] produced a synthetic protein nanoparticle based on polymerized human serum albumin chemically linked to a cell-penetrating peptide (iRGD). The nanoparticles were loaded with a siRNA against Signal Transducer and Activator of Transcription Factor 3 (STAT3) and given systemically along with ionized radiation to mice with glioblastoma. The treatment resulted in tumor regression and long-term survival in 87.5% of mice. Targeting the cerebral endothelium was also examined by Gonzalez-Carter et al. [144]. They used the low rate of endocytosis in brain endothelial cells to specifically target nanoparticles to the surface of those cells with minimal accumulation in other organs. The cerebral endothelium is capable of undergoing remodeling in disease states (e.g., upregulation of adhesion molecules participating in leukocyte infiltration). Marcos-Contreras et al. [145] used this feature to target the brain in neuroinflammation by creating vascular cell adhesion molecule-1 (VCAM-1) antibody/liposomes as drug carriers.

A novel approach to enhancing drug delivery to the brain was employed by Zhao et al. [146] who used the lymphatic vasculature. They found a subcutaneous injection in the neck close to a lymph node resulted in 44-fold higher drug uptake into brain compared to an intravenous injection.

New thematic series on advances in in vitro modeling of the BBB and NVU

In vitro models have been very important in understanding the BBB and the NVU. They can provide insights that cannot be obtained by in vivo experiments. However, conventional in vitro models do not fully replicate the BBB/NVU properties found in vivo leading to major efforts to improve the models. Fluids Barriers of the CNS published a thematic series aimed at detailing advances in in vitro modeling and providing new insights into brain microvascular endothelial cell (BMEC) and NVU function that have been obtained from in vitro models. Here are some highlights.

A very wide range of in vitro models are now being used to study the BBB/NVU. As reviewed by Bhalerao [147], the field has expanded from static BMEC mono- and co-cultures to include brain organoids, organ-on-a-chip models, spheroids and 3D microfluidic devices. Historically, BMEC primary cultures or BMEC-derived cell lines have been used, but one major advance in the past eight years has been in the use of human induced pluripotent stem cells (iPSCs) to produce BMECs and other cells of the NVU. Unlike conventional BMEC primary cultures and BMEC-derived cell lines, mono- and co-cultures of iPSC-derived BMECs have transendothelial electrical resistances (TEERs) and paracellular permeabilities close to that in vivo [148, 149]. In the thematic series, Workman and Svendsen [150] reviewed recent advances in using such iPSCs to model the BBB in conventional transwell experiments, as well as in 2D microfluidic chips and 3D microvessels.

It should be noted that there are controversies over the phenotype of iPSC-derived BMECs and whether they are ‘endothelial’ or ‘epithelial’. This controversy is addressed by Lippmann et al. in a commentary [151] and they strongly suggest the term ‘BMEC-like’ be used for these cells. As always, there is a need, where possible, to compare results from such models to in vivo measurements. It is important to benchmark in vitro models [152] and the article of Francisco et al. on the uses of RNAseq in studying the blood–brain barriers highlights the use of that technique for such benchmarking [15]. With regard to the phenotype of brain endothelial cells, the importance of the microenvironment in vivo for inducing differentiation by signaling pathways (e.g., Wnt/β-catenin and others pathways) and dedifferentiation under culture conditions was further characterized [153].

There are still major efforts to improve iPSC-derived BBB/NVU models as shown in the thematic series. One focus has been on the impact of extracellular matrix components. Aoki et al. and Motallebnejad & Azarin [154, 155] both showed the importance of laminin (an important component of the BMEC basement membrane) in enhancing barrier integrity. One major advantage of such human iPSC-derived models is that they can assess the impact of patient-specific genetic mutations on BBB/NVU functions. An example of this is the work of Katt et al. [156] on the effects of genetic mutations on the barrier functions in neurodegenerative disorders.

Cells in the NVU (e.g., astrocytes and pericytes) are also an important determinant of BMEC function. Two papers in this thematic series address the role of pericytes. Jamieson et al. [157] have examined the effect of human iPSC-derived pericytes in 2D and 3D BBB models in vitro and found effects that are model and stress dependent. Heymans et al. [158] have examined the effects of pericyte co-culture on BMEC gene expression, identifying signaling pathways that may underly changes in BMEC function.

Brain endothelial cell tight junctions, transporters and levels of endocytosis/transcytosis are important determinants of BBB function. This thematic series includes studies addressing all those areas in vitro. One use of in vitro models is to examine the mechanisms by which the BBB/NVU is impacted by disease. For example, two are related to stroke: Andjelkovic et al. [159], who review methods of modeling cerebrovascular disease in vitro and Gerhartl et al. [160] who examined the effects of astrocyte and pericyte co-culture on the BMEC response to in vitro ‘ischemia’. Neuroinflammation is an important component of many neurological diseases. As reviewed by Erickson et al. [161], the brain endothelium and the NVU play multiple roles in neuroinflammation and in vitro models have enhanced our understanding of those roles. Such models also provide a method for testing potential therapies to limit BBB/NVU dysfunction in disease. Thus, Ge et al. [162] showed the ability of human embryonic stem cell-derived mesenchymal stem cells to repair the BBB dysfunction induced by a major inflammatory mediator, tumor necrosis factor (TNF)-α, in vitro.

In summary, the thematic series on advances in in vitro modeling of the BBB and NVU highlights the strides that are being made in the area. In vitro models have helped and will continue to help us gain insights into normal BBB and NVU function as well as the impact of different pathologies.

Other studies on in vitro modeling of the BBB and NVU

Outside the thematic series, there were important studies on in vitro modeling. A plethora of new models have been developed (too many to review fully here). For example, Nishihara et al. [163] used what they term an extended endothelial culture method for human iPSCs which allows the study of immune cell interactions with BMEC-like cells and Linville et al. [164] have developed a novel 3-D model using human iPSCs to study human brain angiogenesis. Also, brain organoids were made from human embryonic stem cells that form blood vessel-like structures (Ham et al. [165]) and Ahn et al. [166] developed a microphysiological human BBB platform that allows 3D tracking of nanoparticles in the vascular and perivascular spaces.

Conclusions

We thank the readers, authors, reviewers and editorial board members of Fluids and Barriers of the CNS. Despite the impact of COVID-19, 2020 has produced a bumper crop of major advances in the fields of brain barriers and brain fluids research. Thank you all for your contributions.

Availability of data and materials

Not applicable.

References

  1. 1.

    Sladojevic N, Yu B, Liao JK. Regulator of G-protein signaling 5 Maintains brain endothelial cell function in focal cerebral ischemia. J Am Heart Assoc. 2020;9(18):e017533.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  2. 2.

    Roudnicky F, Zhang JD, Kim BK, Pandya NJ, Lan Y, Sach-Peltason L, Ragelle H, Strassburger P, Gruener S, Lazendic M, et al. Inducers of the endothelial cell barrier identified through chemogenomic screening in genome-edited hPSC-endothelial cells. Proc Natl Acad Sci USA. 2020;117(33):19854–65.

    CAS  PubMed  Article  Google Scholar 

  3. 3.

    Kakogiannos N, Ferrari L, Giampietro C, Scalise AA, Maderna C, Rava M, Taddei A, Lampugnani MG, Pisati F, Malinverno M, et al. JAM-A Acts via C/EBP-alpha to promote Claudin-5 expression and enhance endothelial barrier function. Circ Res. 2020;127(8):1056–73.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  4. 4.

    Sladojevic N, Stamatovic SM, Keep RF, Grailer JJ, Sarma JV, Ward PA, Andjelkovic AV. Inhibition of junctional adhesion molecule-A/LFA interaction attenuates leukocyte trafficking and inflammation in brain ischemia/reperfusion injury. Neurobiol Dis. 2014;67:57–70.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  5. 5.

    Samus M, Li YT, Sorokin L, Rottner K, Vestweber D. Actin-binding protein cortactin promotes pathogenesis of experimental autoimmune encephalomyelitis by supporting leukocyte infiltration into the central nervous system. J Neurosci. 2020;40(7):1389–404.

    PubMed  PubMed Central  Article  Google Scholar 

  6. 6.

    Mehra A, Guerit S, Macrez R, Gosselet F, Sevin E, Lebas H, Maubert E, De Vries HE, Bardou I, Vivien D, et al. Nonionotropic action of endothelial NMDA receptors on blood-brain barrier permeability via Rho/ROCK-mediated phosphorylation of myosin. J Neurosci. 2020;40(8):1778–87.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  7. 7.

    Chen MB, Yang AC, Yousef H, Lee D, Chen W, Schaum N, Lehallier B, Quake SR, Wyss-Coray T. Brain endothelial cells are exquisite sensors of age-related circulatory cues. Cell Rep. 2020;30(13):4418-4432.e4414.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  8. 8.

    Zhao L, Li Z, Vong JSL, Chen X, Lai HM, Yan LYC, Huang J, Sy SKH, Tian X, Huang Y, et al. Pharmacologically reversible zonation-dependent endothelial cell transcriptomic changes with neurodegenerative disease associations in the aged brain. Nat Commun. 2020;11(1):4413.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  9. 9.

    Yang AC, Stevens MY, Chen MB, Lee DP, Stahli D, Gate D, Contrepois K, Chen W, Iram T, Zhang L, et al. Physiological blood-brain transport is impaired with age by a shift in transcytosis. Nature. 2020;583(7816):425–30.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  10. 10.

    Chen Z, Morales JE, Avci N, Guerrero PA, Rao G, Seo JH, McCarty JH. The vascular endothelial cell-expressed prion protein doppel promotes angiogenesis and blood-brain barrier development. Development. 2020;147(18):23.

    Google Scholar 

  11. 11.

    Cottarelli A, Corada M, Beznoussenko GV, Mironov AA, Globisch MA, Biswas S, Huang H, Dimberg A, Magnusson PU, Agalliu D, et al. Fgfbp1 promotes blood-brain barrier development by regulating collagen IV deposition and maintaining Wnt/beta-catenin signaling. Development. 2020;147(16):24.

    Google Scholar 

  12. 12.

    Veys K, Fan Z, Ghobrial M, Bouche A, Garcia-Caballero M, Vriens K, Conchinha NV, Seuwen A, Schlegel F, Gorski T, et al. Role of the GLUT1 glucose transporter in postnatal CNS angiogenesis and blood-brain barrier integrity. Circ Res. 2020;127(4):466–82.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  13. 13.

    Wang Z, Zheng Y, Wang F, Zhong J, Zhao T, Xie Q, Zhu T, Ma F, Tang Q, Zhou B, et al. Mfsd2a and Spns2 are essential for sphingosine-1-phosphate transport in the formation and maintenance of the blood-brain barrier. Sci Adv. 2020;6(22):eaay8627.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  14. 14.

    Song HW, Foreman KL, Gastfriend BD, Kuo JS, Palecek SP, Shusta EV. Transcriptomic comparison of human and mouse brain microvessels. Sci Rep. 2020;10(1):12358.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  15. 15.

    Francisco DMF, Marchetti L, Rodriguez-Lorenzo S, Frias-Anaya E, Figueiredo RM, Bt RN, Winter P, Romero IA, de Vries HE, Engelhardt B, et al. Advancing brain barriers RNA sequencing: guidelines from experimental design to publication. Fluids Barriers CNS. 2020;17(1):51.

    PubMed  PubMed Central  Article  Google Scholar 

  16. 16.

    Cohen-Salmon M, Slaoui L, Mazare N, Gilbert A, Oudart M, Alvear-Perez R, Elorza-Vidal X, Chever O, Boulay AC. Astrocytes in the regulation of cerebrovascular functions. Glia. 2021;69(4):954–70.

    PubMed  Article  PubMed Central  Google Scholar 

  17. 17.

    Guerit S, Fidan E, Macas J, Czupalla CJ, Figueiredo R, Vijikumar A, Yalcin BH, Thom S, Winter P, Gerhardt H, et al. Astrocyte-derived Wnt growth factors are required for endothelial blood-brain barrier maintenance. Prog Neurobiol. 2020;199:101937.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  18. 18.

    Batiuk MY, Martirosyan A, Wahis J, de Vin F, Marneffe C, Kusserow C, Koeppen J, Viana JF, Oliveira JF, Voet T, et al. Identification of region-specific astrocyte subtypes at single cell resolution. Nat Commun. 2020;11(1):1220.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  19. 19.

    Uchida Y, Yagi Y, Takao M, Tano M, Umetsu M, Hirano S, Usui T, Tachikawa M, Terasaki T. Comparison of absolute protein abundances of transporters and receptors among blood-brain barriers at different cerebral regions and the blood-spinal cord barrier in humans and rats. Mol Pharm. 2020;17(6):2006–20.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  20. 20.

    Mae MA, He L, Nordling S, Vazquez-Liebanas E, Nahar K, Jung B, Li X, Tan BC, Foo JC, Cazenave Gassiot A, et al. Single-cell analysis of blood-brain barrier response to pericyte loss. Circ Res. 2021;128(4):e46–62.

    PubMed  Article  PubMed Central  Google Scholar 

  21. 21.

    Gautam J, Cao Y, Yao Y. Pericytic laminin maintains blood-brain barrier integrity in an age-dependent manner. Transl Stroke Res. 2020;11(2):228–42.

    PubMed  Article  PubMed Central  Google Scholar 

  22. 22.

    Sheikh BN, Guhathakurta S, Tsang TH, Schwabenland M, Renschler G, Herquel B, Bhardwaj V, Holz H, Stehle T, Bondareva O, et al. Neural metabolic imbalance induced by MOF dysfunction triggers pericyte activation and breakdown of vasculature. Nat Cell Biol. 2020;22(7):828–41.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  23. 23.

    Alarcon-Martinez L, Villafranca-Baughman D, Quintero H, Kacerovsky JB, Dotigny F, Murai KK, Prat A, Drapeau P, Di Polo A. Interpericyte tunnelling nanotubes regulate neurovascular coupling. Nature. 2020;585(7823):91–5.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  24. 24.

    Ronaldson PT, Davis TP. Regulation of blood-brain barrier integrity by microglia in health and disease: A therapeutic opportunity. J Cereb Blood Flow Metab. 2020;40((1_suppl)):6–24.

    Article  Google Scholar 

  25. 25.

    Delaney C, Farrell M, Doherty CP, Brennan K, O’Keeffe E, Greene C, Byrne K, Kelly E, Birmingham N, Hickey P, et al. Attenuated CSF-1R signalling drives cerebrovascular pathology. EMBO Mol Med. 2020;13:e12889.

    PubMed  PubMed Central  Google Scholar 

  26. 26.

    Santisteban MM, Ahn SJ, Lane D, Faraco G, Garcia-Bonilla L, Racchumi G, Poon C, Schaeffer S, Segarra SG, Korbelin J, et al. Endothelium-macrophage crosstalk mediates blood-brain barrier dysfunction in hypertension. Hypertension. 2020;76(3):795–807.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  27. 27.

    Manukjan N, Ahmed Z, Fulton D, Blankesteijn WM, Foulquier S. A Systematic review of WNT signaling in endothelial cell oligodendrocyte interactions: potential relevance to cerebral small vessel disease. Cells. 2020;9(6):25.

    Article  CAS  Google Scholar 

  28. 28.

    Tian Y, Wang Z, Wang Y, Yin B, Yuan J, Qiang B, Han W, Peng X. Glioma-derived endothelial cells promote glioma cells migration via extracellular vesicles-mediated transfer of MYO1C. Biochem Biophys Res Commun. 2020. S0006-291X(20)30283-7. https://doi.org/10.1016/j.bbrc.2020.02.017.

    Article  PubMed  PubMed Central  Google Scholar 

  29. 29.

    Kaplan L, Chow BW, Gu C. Neuronal regulation of the blood-brain barrier and neurovascular coupling. Nat Rev Neurosci. 2020;21(8):416–32.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  30. 30.

    Pulido RS, Munji RN, Chan TC, Quirk CR, Weiner GA, Weger BD, Rossi MJ, Elmsaouri S, Malfavon M, Deng A, et al. Neuronal activity regulates blood-brain barrier efflux transport through endothelial circadian genes. Neuron. 2020;108(5):937-952.e937.

    CAS  PubMed  Article  Google Scholar 

  31. 31.

    Jiang H, Gallet S, Klemm P, Scholl P, Folz-Donahue K, Altmüller J, Alber J, Heilinger C, Kukat C, Loyens A, et al. MCH Neurons Regulate Permeability of the Median Eminence Barrier. Neuron. 2020;107(2):306-319.e309.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  32. 32.

    Nishibori M, Wang D, Ousaka D, Wake H. High mobility group box-1 and blood-brain barrier disruption. Cells. 2020;9(12):10.

    Article  CAS  Google Scholar 

  33. 33.

    Torices S, Roberts SA, Park M, Malhotra A, Toborek M. Occludin, caveolin-1, and Alix form a multi-protein complex and regulate HIV-1 infection of brain pericytes. FASEB J. 2020;34(12):16319–32.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  34. 34.

    Ubogu EE. Biology of the human blood-nerve barrier in health and disease. Exp Neurol. 2020;328:113272.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  35. 35.

    Reinhold AK, Rittner HL. Characteristics of the nerve barrier and the blood dorsal root ganglion barrier in health and disease. Exp Neurol. 2020;327:113244.

    CAS  Article  Google Scholar 

  36. 36.

    Stubbs EB Jr. Targeting the blood-nerve barrier for the management of immune-mediated peripheral neuropathies. Exp Neurol. 2020;331:113385.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  37. 37.

    Takeshita Y, Sato R, Kanda T. Blood-Nerve Barrier (BNB) Pathology in Diabetic Peripheral Neuropathy and In Vitro Human BNB Model. Int J Mol Sci. 2020;22(1):62.

    PubMed Central  Article  CAS  Google Scholar 

  38. 38.

    Yamaguchi T, Hamada T, Matsuzaki T, Iijima N. Characterization of the circadian oscillator in the choroid plexus of rats. Biochem Biophys Res Commun. 2020;524(2):497–501.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  39. 39.

    Liska K, Sladek M, Cecmanova V, Sumova A. Glucocorticoids reset circadian clock in choroid plexus via period genes. J Endocrinol. 2021;248(2):155–66.

    PubMed  Article  Google Scholar 

  40. 40.

    Furtado A, Astaburuaga R, Costa A, Duarte AC, Goncalves I, Cipolla-Neto J, Lemos MC, Carro E, Relogio A, Santos CRA, et al. The rhythmicity of clock genes is disrupted in the choroid plexus of the APP/PS1 mouse model of alzheimer’s disease. J Alzheimers Dis. 2020;77(2):795–806.

    CAS  PubMed  Article  Google Scholar 

  41. 41.

    Solar P, Zamani A, Kubickova L, Dubovy P, Joukal M. Choroid plexus and the blood-cerebrospinal fluid barrier in disease. Fluids Barriers CNS. 2020;17(1):35.

    PubMed  PubMed Central  Article  Google Scholar 

  42. 42.

    Rodriguez-Lorenzo S, Konings J, van der Pol S, Kamermans A, Amor S, van Horssen J, Witte ME, Kooij G, de Vries HE. Inflammation of the choroid plexus in progressive multiple sclerosis: accumulation of granulocytes and T cells. Acta Neuropathol Commun. 2020;8(1):9.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  43. 43.

    Mottahedin A, Blondel S, Ek J, Leverin AL, Svedin P, Hagberg H, Mallard C, Ghersi-Egea JF, Strazielle N. N-acetylcysteine inhibits bacterial lipopeptide-mediated neutrophil transmigration through the choroid plexus in the developing brain. Acta Neuropathol Commun. 2020;8(1):4.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  44. 44.

    Saul J, Hutchins E, Reiman R, Saul M, Ostrow LW, Harris BT, Van Keuren-Jensen K, Bowser R, Bakkar N. Global alterations to the choroid plexus blood-CSF barrier in amyotrophic lateral sclerosis. Acta Neuropathol Commun. 2020;8(1):92.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  45. 45.

    Rayasam A, Faustino J, Lecuyer M, Vexler ZS. Neonatal stroke and TLR1/2 ligand recruit myeloid cells through the choroid plexus in a CX3CR1-CCR2- and context-specific manner. J Neurosci. 2020;40(19):3849–61.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  46. 46.

    Nishihara H, Soldati S, Mossu A, Rosito M, Rudolph H, Muller WA, Latorre D, Sallusto F, Sospedra M, Martin R, et al. Human CD4(+) T cell subsets differ in their abilities to cross endothelial and epithelial brain barriers in vitro. Fluids Barriers CNS. 2020;17(1):3.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  47. 47.

    Alimajstorovic Z, Pascual-Baixauli E, Hawkes CA, Sharrack B, Loughlin AJ, Romero IA, Preston JE. Cerebrospinal fluid dynamics modulation by diet and cytokines in rats. Fluids Barriers CNS. 2020;17(1):10.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  48. 48.

    Obata F, Narita K. Hypercholesterolemia negatively influences morphology and molecular markers of epithelial cells within the choroid plexus in rabbits. Fluids Barriers CNS. 2020;17(1):13.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  49. 49.

    Evans PG, Sokolska M, Alves A, Harrison IF, Ohene Y, Nahavandi P, Ismail O, Miranda E, Lythgoe MF, Thomas DL, et al. Non-invasive MRI of blood-cerebrospinal fluid barrier function. Nat Commun. 2020;11(1):2081.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  50. 50.

    Zhao L, Taso M, Dai W, Press DZ, Alsop DC. Non-invasive measurement of choroid plexus apparent blood flow with arterial spin labeling. Fluids Barriers CNS. 2020;17(1):58.

    PubMed  PubMed Central  Article  Google Scholar 

  51. 51.

    Eide PK, Valnes LM, Pripp AH, Mardal KA, Ringstad G. Delayed clearance of cerebrospinal fluid tracer from choroid plexus in idiopathic normal pressure hydrocephalus. J Cereb Blood Flow Metab. 2020;40(9):1849–58.

    CAS  PubMed  Article  Google Scholar 

  52. 52.

    Tadayon E, Moret B, Sprugnoli G, Monti L, Pascual-Leone A, Santarnecchi E. Alzheimer’s disease neuroimaging I: improving choroid plexus segmentation in the healthy and diseased brain: relevance for Tau-PET imaging in dementia. J Alzheimers Dis. 2020;74(4):1057–68.

    PubMed  Article  Google Scholar 

  53. 53.

    Pellegrini L, Bonfio C, Chadwick J, Begum F, Skehel M, Lancaster MA. Human CNS barrier-forming organoids with cerebrospinal fluid production. Science. 2020;369(6500):10.

    Article  CAS  Google Scholar 

  54. 54.

    Papadopoulos Z, Herz J, Kipnis J. Meningeal lymphatics: from anatomy to central nervous system immune surveillance. J Immunol. 2020;204(2):286–93.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  55. 55.

    Hauglund NL, Kusk P, Kornum BR, Nedergaard M. Meningeal lymphangiogenesis and enhanced glymphatic activity in mice with chronically implanted EEG electrodes. J Neurosci. 2020;40(11):2371–80.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  56. 56.

    Chen J, Wang L, Xu H, Xing L, Zhuang Z, Zheng Y, Li X, Wang C, Chen S, Guo Z, et al. Meningeal lymphatics clear erythrocytes that arise from subarachnoid hemorrhage. Nat Commun. 2020;11(1):3159.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  57. 57.

    Ringstad G, Eide PK. Cerebrospinal fluid tracer efflux to parasagittal dura in humans. Nat Commun. 2020;11(1):354.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  58. 58.

    Shibata-Germanos S, Goodman JR, Grieg A, Trivedi CA, Benson BC, Foti SC, Faro A, Castellan RFP, Correra RM, Barber M, et al. Structural and functional conservation of non-lumenized lymphatic endothelial cells in the mammalian leptomeninges. Acta Neuropathol. 2020;139(2):383–401.

    CAS  PubMed  Article  Google Scholar 

  59. 59.

    Uchida Y, Goto R, Takeuchi H, Luczak M, Usui T, Tachikawa M, Terasaki T. Abundant expression of OCT2, MATE1, OAT1, OAT3, PEPT2, BCRP, MDR1, and xCT transporters in blood-arachnoid barrier of pig and polarized localizations at CSF- and blood-facing plasma membranes. Drug Metab Dispos. 2020;48(2):135–45.

    CAS  Article  Google Scholar 

  60. 60.

    Decimo I, Dolci S, Panuccio G, Riva M, Fumagalli G, Bifari F. Meninges: A widespread niche of neural progenitors for the brain. Neuroscientist. 2020. https://doi.org/10.1177/1073858420954826.

    Article  PubMed  PubMed Central  Google Scholar 

  61. 61.

    Mestre H, Mori Y, Nedergaard M. The brain’s glymphatic system: current controversies. Trends Neurosci. 2020;43(7):458–66.

    CAS  PubMed  Article  Google Scholar 

  62. 62.

    Naganawa S, Taoka T. The glymphatic system: a review of the challenges in visualizing its structure and function with MR imaging. Magn Reson Med Sci. 2020. https://doi.org/10.2463/mrmsrev.2020-0122.

    Article  PubMed  PubMed Central  Google Scholar 

  63. 63.

    Benveniste H, Lee H, Ozturk B, Chen X, Koundal S, Vaska P, Tannenbaum A, Volkow ND. Glymphatic cerebrospinal fluid and solute transport quantified by MRI and PET imaging. Neuroscience. 2020. S0306-4522(20)30730-2. https://doi.org/10.1016/j.neuroscience.2020.11.014.

    Article  PubMed  Google Scholar 

  64. 64.

    Hablitz LM, Pla V, Giannetto M, Vinitsky HS, Staeger FF, Metcalfe T, Nguyen R, Benrais A, Nedergaard M. Circadian control of brain glymphatic and lymphatic fluid flow. Nat Commun. 2020;11(1):4411.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  65. 65.

    Vinje V, Eklund A, Mardal KA, Rognes ME, Stoverud KH. Intracranial pressure elevation alters CSF clearance pathways. Fluids Barriers CNS. 2020;17(1):29.

    PubMed  PubMed Central  Article  Google Scholar 

  66. 66.

    Goodman JR, Iliff JJ. Vasomotor influences on glymphatic-lymphatic coupling and solute trafficking in the central nervous system. J Cereb Blood Flow Metab. 2020;40(8):1724–34.

    CAS  PubMed  Article  Google Scholar 

  67. 67.

    Ashton NJ, Hye A, Rajkumar AP, Leuzy A, Snowden S, Suarez-Calvet M, Karikari TK, Scholl M, La Joie R, Rabinovici GD, et al. An update on blood-based biomarkers for non-Alzheimer neurodegenerative disorders. Nat Rev Neurol. 2020;16(5):265–84.

    PubMed  Article  Google Scholar 

  68. 68.

    Simren J, Ashton NJ, Blennow K, Zetterberg H. An update on fluid biomarkers for neurodegenerative diseases: recent success and challenges ahead. Curr Opin Neurobiol. 2020;61:29–39.

    CAS  PubMed  Article  Google Scholar 

  69. 69.

    Mattsson-Carlgren N, Palmqvist S, Blennow K, Hansson O. Increasing the reproducibility of fluid biomarker studies in neurodegenerative studies. Nat Commun. 2020;11(1):6252.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  70. 70.

    Ramachandran PS, Wilson MR. Metagenomics for neurological infections-expanding our imagination. Nat Rev Neurol. 2020;16(10):547–56.

    PubMed  Article  PubMed Central  Google Scholar 

  71. 71.

    Kamitaki N, Sekar A, Handsaker RE, de Rivera H, Tooley K, Morris DL, Taylor KE, Whelan CW, Tombleson P, Loohuis LMO, et al. Complement genes contribute sex-biased vulnerability in diverse disorders. Nature. 2020;582(7813):577–81.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  72. 72.

    Janelidze S, Stomrud E, Smith R, Palmqvist S, Mattsson N, Airey DC, Proctor NK, Chai X, Shcherbinin S, Sims JR, et al. Cerebrospinal fluid p-tau217 performs better than p-tau181 as a biomarker of Alzheimer’s disease. Nat Commun. 2020;11(1):1683.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  73. 73.

    Blennow K, Chen C, Cicognola C, Wildsmith KR, Manser PT, Bohorquez SMS, Zhang Z, Xie B, Peng J, Hansson O, et al. Cerebrospinal fluid tau fragment correlates with tau PET: a candidate biomarker for tangle pathology. Brain. 2020;143(2):650–60.

    PubMed  Article  Google Scholar 

  74. 74.

    Barthelemy NR, Li Y, Joseph-Mathurin N, Gordon BA, Hassenstab J, Benzinger TLS, Buckles V, Fagan AM, Perrin RJ, Goate AM, et al. A soluble phosphorylated tau signature links tau, amyloid and the evolution of stages of dominantly inherited Alzheimer’s disease. Nat Med. 2020;26(3):398–407.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  75. 75.

    Llorens F, Hermann P, Villar-Pique A, Diaz-Lucena D, Nagga K, Hansson O, Santana I, Schmitz M, Schmidt C, Varges D, et al. Cerebrospinal fluid lipocalin 2 as a novel biomarker for the differential diagnosis of vascular dementia. Nat Commun. 2020;11(1):619.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  76. 76.

    Graus F, Saiz A, Dalmau J. GAD antibodies in neurological disorders - insights and challenges. Nat Rev Neurol. 2020;16(7):353–65.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  77. 77.

    Schafflick D, Xu CA, Hartlehnert M, Cole M, Schulte-Mecklenbeck A, Lautwein T, Wolbert J, Heming M, Meuth SG, Kuhlmann T, et al. Integrated single cell analysis of blood and cerebrospinal fluid leukocytes in multiple sclerosis. Nat Commun. 2020;11(1):247.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  78. 78.

    Gate D, Saligrama N, Leventhal O, Yang AC, Unger MS, Middeldorp J, Chen K, Lehallier B, Channappa D, De Los Santos MB, et al. Clonally expanded CD8 T cells patrol the cerebrospinal fluid in Alzheimer’s disease. Nature. 2020;577(7790):399–404.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  79. 79.

    He W, You J, Wan Q, Xiao K, Chen K, Lu Y, Li L, Tang Y, Deng Y, Yao Z, et al. The anatomy and metabolome of the lymphatic system in the brain in health and disease. Brain Pathol. 2020;30(2):392–404.

    PubMed  Article  PubMed Central  Google Scholar 

  80. 80.

    Paterson RW, Brown RL, Benjamin L, Nortley R, Wiethoff S, Bharucha T, Jayaseelan DL, Kumar G, Raftopoulos RE, Zambreanu L, et al. The emerging spectrum of COVID-19 neurology: clinical, radiological and laboratory findings. Brain. 2020;143(10):3104–20.

    PubMed  PubMed Central  Article  Google Scholar 

  81. 81.

    Romoli M, Jelcic I, Bernard-Valnet R, Garcia Azorin D, Mancinelli L, Akhvlediani T, Monaco S, Taba P, Sellner J. Infectious disease panel of the European Academy of N: A systematic review of neurological manifestations of SARS-CoV-2 infection: the devil is hidden in the details. Eur J Neurol. 2020;27(9):1712–26.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  82. 82.

    Rhea EM, Logsdon AF, Hansen KM, Williams LM, Reed MJ, Baumann KK, Holden SJ, Raber J, Banks WA, Erickson MA. The S1 protein of SARS-CoV-2 crosses the blood-brain barrier in mice. Nat Neurosci. 2021;24(3):368–78.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  83. 83.

    Destras G, Bal A, Escuret V, Morfin F, Lina B, Josset L. Group CO-DHS: Systematic SARS-CoV-2 screening in cerebrospinal fluid during the COVID-19 pandemic. Lancet Microbe. 2020;1(4):e149.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  84. 84.

    Domingues RB, Mendes-Correa MC, de Moura Leite FBV, Sabino EC, Salarini DZ, Claro I, Santos DW, de Jesus JG, Ferreira NE, Romano CM, et al. First case of SARS-COV-2 sequencing in cerebrospinal fluid of a patient with suspected demyelinating disease. J Neurol. 2020;267(11):3154–6.

    PubMed  Article  PubMed Central  Google Scholar 

  85. 85.

    Franke C, Ferse C, Kreye J, Reincke SM, Sanchez-Sendin E, Rocco A, Steinbrenner M, Angermair S, Treskatsch S, Zickler D, et al. High frequency of cerebrospinal fluid autoantibodies in COVID-19 patients with neurological symptoms. Brain Behav Immun. 2021;93:415–9.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  86. 86.

    Guilmot A, Maldonado Slootjes S, Sellimi A, Bronchain M, Hanseeuw B, Belkhir L, Yombi JC, De Greef J, Pothen L, Yildiz H, et al. Immune-mediated neurological syndromes in SARS-CoV-2-infected patients. J Neurol. 2021;268(3):751–7.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  87. 87.

    MacLean MA, Kamintsky L, Leck ED, Friedman A. The potential role of microvascular pathology in the neurological manifestations of coronavirus infection. Fluids Barriers CNS. 2020;17(1):55.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  88. 88.

    Buzhdygan TP, DeOre BJ, Baldwin-Leclair A, Bullock TA, McGary HM, Khan JA, Razmpour R, Hale JF, Galie PA, Potula R, et al. The SARS-CoV-2 spike protein alters barrier function in 2D static and 3D microfluidic in-vitro models of the human blood-brain barrier. Neurobiol Dis. 2020;146:105131.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  89. 89.

    Nascimento Conde J, Schutt WR, Gorbunova EE, Mackow ER. Recombinant ACE2 expression is required for SARS-CoV-2 to infect primary human endothelial cells and induce inflammatory and procoagulative responses. mBio. 2020;11(6):e03185-20.

    PubMed  PubMed Central  Article  Google Scholar 

  90. 90.

    Jacob F, Pather SR, Huang WK, Zhang F, Wong SZH, Zhou H, Cubitt B, Fan W, Chen CZ, Xu M, et al. Human pluripotent stem cell-derived neural cells and brain organoids reveal SARS-CoV-2 neurotropism predominates in choroid plexus epithelium. Cell Stem Cell. 2020;27(6):937-950.e939.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  91. 91.

    Pellegrini L, Albecka A, Mallery DL, Kellner MJ, Paul D, Carter AP, James LC, Lancaster MA. SARS-CoV-2 infects the brain choroid plexus and disrupts the blood-CSF barrier in human brain organoids. Cell Stem Cell. 2020;27(6):951-961.e955.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  92. 92.

    Huang X, Hussain B, Chang J. Peripheral inflammation and blood-brain barrier disruption: effects and mechanisms. CNS Neurosci Ther. 2021;27(1):36–47.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  93. 93.

    Roberts DR, Albrecht MH, Collins HR, Asemani D, Chatterjee AR, Spampinato MV, Zhu X, Chimowitz MI, Antonucci MU. Effects of spaceflight on astronaut brain structure as indicated on MRI. N Engl J Med. 2017;377(18):1746–53.

    PubMed  Article  PubMed Central  Google Scholar 

  94. 94.

    Norsk P. Adaptation of the cardiovascular system to weightlessness: Surprises, paradoxes and implications for deep space missions. Acta Physiol (Oxf). 2020;228(3):e13434.

    CAS  Article  Google Scholar 

  95. 95.

    Scott RA, Tarver WJ, Brunstetter TJ, Urquieta E. Optic nerve tortuosity on earth and in space. Aerosp Med Hum Perform. 2020;91(2):91–7.

    PubMed  Article  PubMed Central  Google Scholar 

  96. 96.

    Martin Paez Y, Mudie LI, Subramanian PS. Spaceflight associated neuro-ocular syndrome (SANS): A systematic review and future directions. Eye Brain. 2020;12:105–17.

    PubMed  PubMed Central  Article  Google Scholar 

  97. 97.

    Yang S, Emelyanov A, You MS, Sin M, Korzh V. Camel regulates development of the brain ventricular system. Cell Tissue Res. 2021;383(2):835–52.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  98. 98.

    Vio K, Rodriguez S, Navarrete EH, Perez-Figares JM, Jimenez AJ, Rodriguez EM. Hydrocephalus induced by immunological blockage of the subcommissural organ-Reissner’s fiber (RF) complex by maternal transfer of anti-RF antibodies. Exp Brain Res. 2000;135(1):41–52.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  99. 99.

    Robson EA, Dixon L, Causon L, Dawes W, Benenati M, Fassad M, Hirst RA, Kenia P, Moya EF, Patel M, et al. Hydrocephalus and diffuse choroid plexus hyperplasia in primary ciliary dyskinesia-related MCIDAS mutation. Neurol Genetics. 2020;6(4):e482.

    Article  Google Scholar 

  100. 100.

    Beckers A, Adis C, Schuster-Gossler K, Tveriakhina L, Ott T, Fuhl F, Hegermann J, Boldt K, Serth K, Rachev E, et al. The FOXJ1 target Cfap206 is required for sperm motility, mucociliary clearance of the airways and brain development. Development. 2020;147(21):15.

    Google Scholar 

  101. 101.

    Zou W, Lv Y, Liu ZI, Xia P, Li H, Jiao J. Loss of Rsph9 causes neonatal hydrocephalus with abnormal development of motile cilia in mice. Sci Rep. 2020;10(1):12435.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  102. 102.

    Jiang Z, Zhou J, Qin X, Zheng H, Gao B, Liu X, Jin G, Zhou Z. MT1-MMP deficiency leads to defective ependymal cell maturation, impaired ciliogenesis, and hydrocephalus. Jci Insight. 2020;5(9):07.

    Article  Google Scholar 

  103. 103.

    Wu KY, Tang FL, Lee D, Zhao Y, Song H, Zhu XJ, Mei L, Xiong WC. Ependymal Vps35 promotes ependymal cell differentiation and survival, suppresses microglial activation, and prevents neonatal hydrocephalus. J Neurosci. 2020;40(19):3862–79.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  104. 104.

    Jin SC, Dong W, Kundishora AJ, Panchagnula S, Moreno-De-Luca A, Furey CG, Allocco AA, Walker RL, Nelson-Williams C, Smith H, et al. Exome sequencing implicates genetic disruption of prenatal neuro-gliogenesis in sporadic congenital hydrocephalus. Nat Med. 2020;26(11):1754–65.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  105. 105.

    Santos C, Pai YJ, Mahmood MR, Leung KY, Savery D, Waddington SN, Copp AJ, Greene N. Impaired folate 1-carbon metabolism causes formate-preventable hydrocephalus in glycine decarboxylase-deficient mice. J Clin Investig. 2020;130(3):1446–52.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  106. 106.

    Tan X, Chen J, Keep RF, Xi G, Hua Y. Prx2 (Peroxiredoxin 2) as a cause of hydrocephalus after intraventricular hemorrhage. Stroke. 2020;51(5):1578–86.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  107. 107.

    Eide PK, Pripp AH, Ringstad G. Magnetic resonance imaging biomarkers of cerebrospinal fluid tracer dynamics in idiopathic normal pressure hydrocephalus. Brain Commun. 2020;2(2):fcaa187.

    PubMed  PubMed Central  Article  Google Scholar 

  108. 108.

    Wolfsegger T, Hauser A, Wimmer S, Neuwirth K, Assar H, Topakian R. A comprehensive clinico-radiological, neuropsychological and biomechanical analysis approach to patients with idiopathic normal pressure hydrocephalus. Clin Neurol Neurosurg. 2021;201:106402.

    PubMed  Article  PubMed Central  Google Scholar 

  109. 109.

    Hauptman JS, Kestle J, Riva-Cambrin J, Kulkarni AV, Browd SR, Rozzelle CJ, Whitehead WE, Naftel RP, Pindrik J, Limbrick DD, et al. Predictors of fast and ultrafast shunt failure in pediatric hydrocephalus: a hydrocephalus clinical research network study. J Neurosurg Pediatr. 2020 Dec 18:1-10. https://doi.org/10.3171/2020.7.PEDS20111.

    Article  PubMed  PubMed Central  Google Scholar 

  110. 110.

    Gluski J, Zajciw P, Hariharan P, Morgan A, Morales DM, Jea A, Whitehead W, Marupudi N, Ham S, Sood S, et al. Characterization of a multicenter pediatric-hydrocephalus shunt biobank. Fluids Barriers CNS. 2020;17(1):45.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  111. 111.

    Hochstetler AE, Smith HM, Preston DC, Reed MM, Territo PR, Shim JW, Fulkerson D, Blazer-Yost BL. TRPV4 antagonists ameliorate ventriculomegaly in a rat model of hydrocephalus. Jci Insight. 2020;5(18):17.

    Article  Google Scholar 

  112. 112.

    Zhang J, Bhuiyan MIH, Zhang T, Karimy JK, Wu Z, Fiesler VM, Zhang J, Huang H, Hasan MN, Skrzypiec AE, et al. Modulation of brain cation-Cl<sup>-</sup> cotransport via the SPAK kinase inhibitor ZT-1a. Nat Commun. 2020;11(1):78.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  113. 113.

    Castaneyra-Ruiz L, McAllister JP 2nd, Morales DM, Brody SL, Isaacs AM, Limbrick DD Jr. Preterm intraventricular hemorrhage in vitro: modeling the cytopathology of the ventricular zone. Fluids Barriers CNS. 2020;17(1):46.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  114. 114.

    Montagne A, Nation DA, Sagare AP, Barisano G, Sweeney MD, Chakhoyan A, Pachicano M, Joe E, Nelson AR, D’Orazio LM, et al. APOE4 leads to blood-brain barrier dysfunction predicting cognitive decline. Nature. 2020;581(7806):71–6.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  115. 115.

    Liu CC, Yamazaki Y, Heckman MG, Martens YA, Jia L, Yamazaki A, Diehl NN, Zhao J, Zhao N, DeTure M, et al. Tau and apolipoprotein E modulate cerebrovascular tight junction integrity independent of cerebral amyloid angiopathy in Alzheimer’s disease. Alzheimers Dement. 2020;16(10):1372–83.

    PubMed  PubMed Central  Article  Google Scholar 

  116. 116.

    Blanchard JW, Bula M, Davila-Velderrain J, Akay LA, Zhu L, Frank A, Victor MB, Bonner JM, Mathys H, Lin YT, et al. Reconstruction of the human blood-brain barrier in vitro reveals a pathogenic mechanism of APOE4 in pericytes. Nat Med. 2020;26(6):952–63.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  117. 117.

    Johnson ECB, Dammer EB, Duong DM, Ping L, Zhou M, Yin L, Higginbotham LA, Guajardo A, White B, Troncoso JC, et al. Large-scale proteomic analysis of Alzheimer’s disease brain and cerebrospinal fluid reveals early changes in energy metabolism associated with microglia and astrocyte activation. Nat Med. 2020;26(5):769–80.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  118. 118.

    Shi H, Koronyo Y, Rentsendorj A, Regis GC, Sheyn J, Fuchs DT, Kramerov AA, Ljubimov AV, Dumitrascu OM, Rodriguez AR, et al. Identification of early pericyte loss and vascular amyloidosis in Alzheimer’s disease retina. Acta Neuropathol. 2020;139(5):813–36.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  119. 119.

    Harrison IF, Ismail O, Machhada A, Colgan N, Ohene Y, Nahavandi P, Ahmed Z, Fisher A, Meftah S, Murray TK, et al. Impaired glymphatic function and clearance of tau in an Alzheimer’s disease model. Brain. 2020;143(8):2576–93.

    PubMed  PubMed Central  Article  Google Scholar 

  120. 120.

    Ma F, Sun P, Zhang X, Hamblin MH, Yin KJ. Endothelium-targeted deletion of the miR-15a/16-1 cluster ameliorates blood-brain barrier dysfunction in ischemic stroke. Sci Signaling [Electronic Resource]. 2020;13(626):07.

    Google Scholar 

  121. 121.

    Sun P, Zhang K, Hassan SH, Zhang X, Tang X, Pu H, Stetler RA, Chen J, Yin KJ. Endothelium-targeted deletion of microRNA-15a/16-1 promotes poststroke angiogenesis and improves long-term neurological recovery. Circ Res. 2020;126(8):1040–57.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  122. 122.

    Zhang X, Tang X, Ma F, Fan Y, Sun P, Zhu T, Zhang J, Hamblin MH, Chen YE, Yin KJ. Endothelium-targeted overexpression of Kruppel-like factor 11 protects the blood-brain barrier function after ischemic brain injury. Brain Pathol. 2020;30(4):746–65.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  123. 123.

    Sun J, Huang Y, Gong J, Wang J, Fan Y, Cai J, Wang Y, Qiu Y, Wei Y, Xiong C, et al. Transplantation of hPSC-derived pericyte-like cells promotes functional recovery in ischemic stroke mice. Nat Commun. 2020;11(1):5196.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  124. 124.

    O’Keeffe E, Kelly E, Liu Y, Giordano C, Wallace E, Hynes M, Tiernan S, Meagher A, Greene C, Hughes S, et al. Dynamic blood-brain barrier regulation in mild traumatic brain injury. J Neurotrauma. 2020;37(2):347–56.

    PubMed  Article  PubMed Central  Google Scholar 

  125. 125.

    Veksler R, Vazana U, Serlin Y, Prager O, Ofer J, Shemen N, Fisher AM, Minaeva O, Hua N, Saar-Ashkenazy R, et al. Slow blood-to-brain transport underlies enduring barrier dysfunction in American football players. Brain. 2020;143(6):1826–42.

    PubMed  PubMed Central  Article  Google Scholar 

  126. 126.

    Turtzo LC, Jikaria N, Cota MR, Williford JP, Uche V, Davis T, MacLaren J, Moses AD, Parikh G, Castro MA, et al. Meningeal blood-brain barrier disruption in acute traumatic brain injury. Brain Commun. 2020;2(2):fcaa143.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  127. 127.

    Kitchen P, Salman MM, Halsey AM, Clarke-Bland C, MacDonald JA, Ishida H, Vogel HJ, Almutiri S, Logan A, Kreida S, et al. Targeting aquaporin-4 subcellular localization to treat central nervous system edema. Cell. 2020;181(4):784-799.e719.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  128. 128.

    Mestre H, Du T, Sweeney AM, Liu G, Samson AJ, Peng W, Mortensen KN, Staeger FF, Bork PAR, Bashford L, et al. Cerebrospinal fluid influx drives acute ischemic tissue swelling. Science. 2020;367(6483):13.

    Article  CAS  Google Scholar 

  129. 129.

    Greene C, Hanley N, Campbell M. Blood-brain barrier associated tight junction disruption is a hallmark feature of major psychiatric disorders. Transl Psychiatry Psychiatry. 2020;10(1):373.

    CAS  Article  Google Scholar 

  130. 130.

    Dudek KA, Dion-Albert L, Lebel M, LeClair K, Labrecque S, Tuck E, Ferrer Perez C, Golden SA, Tamminga C, Turecki G, et al. Molecular adaptations of the blood-brain barrier promote stress resilience vs. depression. Proc Natl Acad Sci USA. 2020;117(6):3326–36.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  131. 131.

    Sugimoto K, Ichikawa-Tomikawa N, Nishiura K, Kunii Y, Sano Y, Shimizu F, Kakita A, Kanda T, Imura T, Chiba H. Serotonin/5-HT1A signaling in the neurovascular unit regulates endothelial CLDN5 expression. Int J Mol Sci. 2020;22(1):29.

    Article  CAS  Google Scholar 

  132. 132.

    Lehmann ML, Poffenberger CN, Elkahloun AG, Herkenham M. Analysis of cerebrovascular dysfunction caused by chronic social defeat in mice. Brain Behav Immun. 2020;88:735–47.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  133. 133.

    Ouellette J, Toussay X, Comin CH, Costa LDF, Ho M, Lacalle-Aurioles M, Freitas-Andrade M, Liu QY, Leclerc S, Pan Y, et al. Vascular contributions to 16p11.2 deletion autism syndrome modeled in mice. Nature Neurosci. 2020;23(9):1090–101.

    CAS  PubMed  Article  Google Scholar 

  134. 134.

    Sun Y, Liou B, Chu Z, Fannin V, Blackwood R, Peng Y, Grabowski GA, Davis HW, Qi X. Systemic enzyme delivery by blood-brain barrier-penetrating SapC-DOPS nanovesicles for treatment of neuronopathic Gaucher disease. EBioMedicine. 2020;55:102735.

    PubMed  PubMed Central  Article  Google Scholar 

  135. 135.

    Hede E, Christiansen CB, Heegaard CW, Moos T, Burkhart A. Gene therapy to the blood-brain barrier with resulting protein secretion as a strategy for treatment of Niemann Picks type C2 disease. J Neurochem. 2021;156(3):290–308.

    CAS  PubMed  Article  Google Scholar 

  136. 136.

    Gorick CM, Mathew AS, Garrison WJ, Thim EA, Fisher DG, Copeland CA, Song J, Klibanov AL, Miller GW, Price RJ. Sonoselective transfection of cerebral vasculature without blood-brain barrier disruption. Proc Natl Acad Sci USA. 2020;117(11):5644–54.

    CAS  PubMed  Article  Google Scholar 

  137. 137.

    Rezai AR, Ranjan M, D’Haese PF, Haut MW, Carpenter J, Najib U, Mehta RI, Chazen JL, Zibly Z, Yates JR, et al. Noninvasive hippocampal blood-brain barrier opening in Alzheimer’s disease with focused ultrasound. Proc Natl Acad Sci USA. 2020;117(17):9180–2.

    CAS  PubMed  Article  Google Scholar 

  138. 138.

    D’Haese PF, Ranjan M, Song A, Haut MW, Carpenter J, Dieb G, Najib U, Wang P, Mehta RI, Chazen JL, et al. Beta-amyloid plaque reduction in the hippocampus after focused ultrasound-induced blood-brain barrier opening in alzheimer’s disease. Front Human Neurosci. 2020;14:593672.

    CAS  Article  Google Scholar 

  139. 139.

    Kariolis MS, Wells RC, Getz JA, Kwan W, Mahon CS, Tong R, Kim DJ, Srivastava A, Bedard C, Henne KR, et al. Brain delivery of therapeutic proteins using an Fc fragment blood-brain barrier transport vehicle in mice and monkeys. Sci Transl Med. 2020;12(545):27.

    Article  CAS  Google Scholar 

  140. 140.

    Ullman JC, Arguello A, Getz JA, Bhalla A, Mahon CS, Wang J, Giese T, Bedard C, Kim DJ, Blumenfeld JR, et al. Brain delivery and activity of a lysosomal enzyme using a blood-brain barrier transport vehicle in mice. Sci Transl Med. 2020;12(545):27.

    Article  CAS  Google Scholar 

  141. 141.

    Stocki P, Szary J, Rasmussen CLM, Demydchuk M, Northall L, Logan DB, Gauhar A, Thei L, Moos T, Walsh FS, et al. Blood-brain barrier transport using a high affinity, brain-selective VNAR antibody targeting transferrin receptor 1. FASEB J. 2020;25:25.

    Google Scholar 

  142. 142.

    Georgieva JV, Goulatis LI, Stutz CC, Canfield SG, Song HW, Gastfriend BD, Shusta EV. Antibody screening using a human iPSC-based blood-brain barrier model identifies antibodies that accumulate in the CNS. FASEB J. 2020;34(9):12549–64.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  143. 143.

    Gregory JV, Kadiyala P, Doherty R, Cadena M, Habeel S, Ruoslahti E, Lowenstein PR, Castro MG, Lahann J. Systemic brain tumor delivery of synthetic protein nanoparticles for glioblastoma therapy. Nat Commun. 2020;11(1):5687.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  144. 144.

    Gonzalez-Carter D, Liu X, Tockary TA, Dirisala A, Toh K, Anraku Y, Kataoka K. Targeting nanoparticles to the brain by exploiting the blood-brain barrier impermeability to selectively label the brain endothelium. Proc Natl Acad Sci USA. 2020;117(32):19141–50.

    CAS  PubMed  Article  Google Scholar 

  145. 145.

    Marcos-Contreras OA, Greineder CF, Kiseleva RY, Parhiz H, Walsh LR, Zuluaga-Ramirez V, Myerson JW, Hood ED, Villa CH, Tombacz I, et al. Selective targeting of nanomedicine to inflamed cerebral vasculature to enhance the blood-brain barrier. Proc Natl Acad Sci USA. 2020;117(7):3405–14.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  146. 146.

    Zhao P, Le Z, Liu L, Chen Y. Therapeutic delivery to the brain via the lymphatic vasculature. Nano Lett. 2020;20(7):5415–20.

    CAS  PubMed  Article  Google Scholar 

  147. 147.

    Bhalerao A, Sivandzade F, Archie SR, Chowdhury EA, Noorani B, Cucullo L. In vitro modeling of the neurovascular unit: advances in the field. Fluids Barriers CNS. 2020;17(1):22.

    PubMed  PubMed Central  Article  Google Scholar 

  148. 148.

    Canfield SG, Stebbins MJ, Faubion MG, Gastfriend BD, Palecek SP, Shusta EV. An isogenic neurovascular unit model comprised of human induced pluripotent stem cell-derived brain microvascular endothelial cells, pericytes, astrocytes, and neurons. Fluids Barriers CNS. 2019;16(1):25.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  149. 149.

    Lippmann ES, Azarin SM, Kay JE, Nessler RA, Wilson HK, Al-Ahmad A, Palecek SP, Shusta EV. Derivation of blood-brain barrier endothelial cells from human pluripotent stem cells. Nat Biotechnol. 2012;30(8):783–91.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  150. 150.

    Workman MJ, Svendsen CN. Recent advances in human iPSC-derived models of the blood-brain barrier. Fluids Barriers CNS. 2020;17(1):30.

    PubMed  PubMed Central  Article  Google Scholar 

  151. 151.

    Lippmann ES, Azarin SM, Palecek SP, Shusta EV. Commentary on human pluripotent stem cell-based blood-brain barrier models. Fluids Barriers CNS. 2020;17(1):64.

    PubMed  PubMed Central  Article  Google Scholar 

  152. 152.

    DeStefano JG, Jamieson JJ, Linville RM, Searson PC. Benchmarking in vitro tissue-engineered blood-brain barrier models. Fluids Barriers CNS. 2018;15(1):32.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  153. 153.

    Sabbagh MF, Nathans J. A genome-wide view of the de-differentiation of central nervous system endothelial cells in culture. Elife. 2020;9:e51276.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  154. 154.

    Aoki H, Yamashita M, Hashita T, Iwao T, Matsunaga T. Laminin 221 fragment is suitable for the differentiation of human induced pluripotent stem cells into brain microvascular endothelial-like cells with robust barrier integrity. Fluids Barriers CNS. 2020;17(1):25.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  155. 155.

    Motallebnejad P, Azarin SM. Chemically defined human vascular laminins for biologically relevant culture of hiPSC-derived brain microvascular endothelial cells. Fluids Barriers CNS. 2020;17(1):54.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  156. 156.

    Katt ME, Mayo LN, Ellis SE, Mahairaki V, Rothstein JD, Cheng L, Searson PC. The role of mutations associated with familial neurodegenerative disorders on blood-brain barrier function in an iPSC model. Fluids Barriers CNS. 2019;16(1):20.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  157. 157.

    Jamieson JJ, Linville RM, Ding YY, Gerecht S, Searson PC. Role of iPSC-derived pericytes on barrier function of iPSC-derived brain microvascular endothelial cells in 2D and 3D. Fluids Barriers CNS. 2019;16(1):15.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  158. 158.

    Heymans M, Figueiredo R, Dehouck L, Francisco D, Sano Y, Shimizu F, Kanda T, Bruggmann R, Engelhardt B, Winter P, et al. Contribution of brain pericytes in blood-brain barrier formation and maintenance: a transcriptomic study of cocultured human endothelial cells derived from hematopoietic stem cells. Fluids Barriers CNS. 2020;17(1):48.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  159. 159.

    Andjelkovic AV, Stamatovic SM, Phillips CM, Martinez-Revollar G, Keep RF. Modeling blood-brain barrier pathology in cerebrovascular disease in vitro: current and future paradigms. Fluids Barriers CNS. 2020;17(1):44.

    PubMed  PubMed Central  Article  Google Scholar 

  160. 160.

    Gerhartl A, Pracser N, Vladetic A, Hendrikx S, Friedl HP, Neuhaus W. The pivotal role of micro-environmental cells in a human blood-brain barrier in vitro model of cerebral ischemia: functional and transcriptomic analysis. Fluids Barriers CNS. 2020;17(1):19.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  161. 161.

    Erickson MA, Wilson ML, Banks WA. In vitro modeling of blood-brain barrier and interface functions in neuroimmune communication. Fluids Barriers CNS. 2020;17(1):26.

    PubMed  PubMed Central  Article  Google Scholar 

  162. 162.

    Ge S, Jiang X, Paul D, Song L, Wang X, Pachter JS. Human ES-derived MSCs correct TNF-alpha-mediated alterations in a blood-brain barrier model. Fluids Barriers CNS. 2019;16(1):18.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  163. 163.

    Nishihara H, Gastfriend BD, Soldati S, Perriot S, Mathias A, Sano Y, Shimizu F, Gosselet F, Kanda T, Palecek SP, et al. Advancing human induced pluripotent stem cell-derived blood-brain barrier models for studying immune cell interactions. FASEB J. 2020;34(12):16693–715.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  164. 164.

    Linville RM, Arevalo D, Maressa JC, Zhao N, Searson PC. Three-dimensional induced pluripotent stem-cell models of human brain angiogenesis. Microvascu Res. 2020;132:104042.

    CAS  Article  Google Scholar 

  165. 165.

    Ham O, Jin YB, Kim J, Lee MO. Blood vessel formation in cerebral organoids formed from human embryonic stem cells. Biochem Biophys Res Commun. 2020;521(1):84–90.

    CAS  PubMed  Article  Google Scholar 

  166. 166.

    Ahn SI, Sei YJ, Park HJ, Kim J, Ryu Y, Choi JJ, Sung HJ, MacDonald TJ, Levey AI, Kim Y. Microengineered human blood-brain barrier platform for understanding nanoparticle transport mechanisms. Nat Commun. 2020;11(1):175.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

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RFK wrote the initial draft. HCJ and LRD modified that draft. All authors read and approved the final manuscript.

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Keep, R.F., Jones, H.C. & Drewes, L.R. Brain Barriers and brain fluids research in 2020 and the fluids and barriers of the CNS thematic series on advances in in vitro modeling of the blood–brain barrier and neurovascular unit. Fluids Barriers CNS 18, 24 (2021). https://doi.org/10.1186/s12987-021-00258-z

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