Our results indicate that 1,25(OH)2D3 enhances cerebral clearance of Aβ(1-40) from mouse brain across the BBB. Specifically, we have obtained in vivo evidence that 1,25(OH)2D3 treatment significantly increased [125I]hAβ(1-40) elimination from mouse brain across the BBB (Figurs 1, 2), and resulted in a significant reduction in the level of endogenous soluble Aβ(1-40) in mouse brain (Figure 3). The enhancement of [125I]hAβ(1-40) elimination from the brain might be explained in terms of an increase of [125I]hAβ(1-40) efflux transport activity across the BBB, or a decrease of [125I]hAβ(1-40) binding to brain parenchymal cells, or both. However, the kinetic data shown in Figure 1 indicate that 1,25(OH)2D3 treatment increased the efflux transport activity, but did not affect the binding.
The present study examined the [125I]hAβ(1-40) elimination from the brain for 60 min, and considerable degradation of injected [125I]hAβ(1-40) might occur during this period. However, we showed previously that the elimination of [125I]hAβ(1-40) from mouse brain was inhibited to 21.2% by unlabeled hAβ(1-40) and that intact [125I]hAβ(1-40) was detected in plasma after its microinjection into rat brain [6, 33]. Furthermore, the elimination of [125I]hAβ(1-40) from rat brain was not significantly inhibited by L-tyrosine . These results suggest that the present BEI findings mainly reflect the elimination of intact [125I]hAβ(1-40) and possibly also to some extent partially-degraded [125I]hAβ(1-40), but not [125I]L-tyrosine, from the brain. It is also possible that free [125I] was generated by the degradation of [125I]hAβ(1-40) in the brain. It was reported that rat brain capillary endothelial cells possess iodide efflux activity and that the activity was affected by intracellular ATP level . Therefore, we cannot rule out the possibility that the enhancement effect of 1,25(OH)2D3 shown in Figure 1 is partially due to facilitation of free [125I] elimination from the brain. Nevertheless, the reduction of endogenous Aβ(1-40) level in the brain by 1,25(OH)2D3 treatment, as shown in Figure 3, suggests that, at least, hAβ(1-40) elimination from the brain was enhanced by the treatment.
The biological effects of 1,25(OH)2D3 are mediated by a mixture of genomic and non-genomic actions. The genomic action of 1,25(OH)2D3 is mediated by binding of VDR-retinoid × receptor (RXR) complex at the vitamin D response elements on target genes. RT-PCR analysis confirmed the expression of VDR mRNA in mouse brain capillaries (Figure 4). We previously demonstrated the expression of RXRs in rat brain capillaries . These results indicate that brain capillary endothelial cells possess VDR-mediated genomic pathway(s), which may be involved in the enhancement of Aβ efflux transport activity.
As regards non-genomic actions, 1,25(OH)2D3 has been proposed to play roles in the generation of intracellular second messengers and various signal-transduction cascades [36, 37]. 1,25(OH)2D3 increased the intracellular level of cAMP in several cell lines as efficiently as did forskolin [30, 38], and the MEK-mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) signaling pathway is known to be activated through elevation of intracellular cAMP. The present BEI study demonstrated that forskolin enhanced [125I]hAβ(1-40) elimination from mouse brain (Figure 4) and the in vitro uptake study showed that forskolin increased [125I]hAβ(1-40) internalization into TM-BBB4 cells, and this action was inhibited by MEK inhibitor (Figure 6B). These results suggest that the MEK-MAPK-ERK signaling pathway is involved in enhancing the brain-to-blood efflux transport of hAβ(1-40) at the BBB. Therefore, 1,25(OH)2D3-mediated enhancement of brain-to-blood hAβ(1-40) efflux transport is likely to involve both genomic and nongenomic actions of 1,25(OH)2D3. The enhancing effect of forskolin on [125I]hAβ(1-40) elimination (16.4%, Figure 5) is smaller than that of 1,25(OH)2D3 (25.4%, Figure 1A). This slight difference may be explained by a difference in the activating activity of 1,25(OH)2D3 and forskolin at different doses, although it is also possible that forskolin activated only a part of the signaling pathways activated by 1,25(OH)2D3.
TM-BBB4 cells retain the expression of various molecules and functions of the in vivo BBB . However, the cells do not have polarity, and internalization into the cells may therefore reflect both uptake from the brain to the brain capillary endothelial cells and from the blood to the brain capillary endothelial cells. The effect of forskolin in enhancing the [125I]hAβ(1-40) internalization is likely to reflect enhanced hAβ(1-40) uptake from the brain to the cells, since in vivo forskolin treatment increased the elimination of [125I]hAβ(1-40) from the brain (Figure 5). A study of the effect of 1,25(OH)2D3 on the [125I]hAβ(1-40) internalization by TM-BBB4 failed to provide consistent results (data not shown). This may be due to the more complex activation pathways of 1,25(OH)2D3 than forskolin, as well as the non-polarized character of TM-BBB4 cells. Optimization of uptake conditions would be necessary for further studies. The molecules involved in hAβ(1-40) internalization in TM-BBB4 cells have not yet been identified. Our previous report using conditionally immortalized rat brain capillary endothelial cells (TR-BBB) showed that low-density lipoprotein receptor-related protein 1 indirectly contributes to [125I]hAβ(1-40) internalization and a P-glycoprotein substrate, verapamil, did not inhibit the uptake .
Soluble Aβ levels correlate with synaptic dysfunction in AD brain [41–43], and soluble Aβ dimers and oligomers initially generated from Aβ monomers are involved in the impairment of synaptic plasticity and memory [44, 45]. Our present findings indicate that 1,25(OH)2D3 modulates brain-to-blood efflux transport of hAβ(1-40) at the BBB, and we therefore consider that impairment of 1,25(OH)2D3 signaling leads to a decrease in the brain-to-blood efflux transport activity of hAβ(1-40) at the BBB, which in turn results in an increase of Aβ levels in the brain. Clinical data suggest that low serum 25(OH)D3 is associated with increased risk of cognitive impairment in elderly people [10–12], and higher serum 25(OH)D3 levels were found to be associated with better cognitive test performance in AD patients . Epidemiological studies support an association between VDR gene polymorphisms in the ligand-binding site and the development of late-onset AD [15, 16]. These relations between vitamin D status and AD development seem likely to arise, at least partially, from impairment of the brain-to-blood Aβ efflux transport activity at the BBB due to attenuation of basal 1,25(OH)2D3-mediated signaling in brain capillary endothelial cells.
Reduction of Aβ in the CNS is considered to be a primary therapeutic target for AD. Based on our findings here, 1,25(OH)2D3 appears to be a candidate agent for reduction of Aβ(1-40) level in the brain through enhancement of Aβ elimination across the BBB. The enhancing effect of 1,25(OH)2D3 was observed at 24 h after administration, but was not observed at 48 h or 72 h (Figure 2B). This is likely to be due to rapid clearance of 1,25(OH)2D3 from the circulation, since the plasma concentration of 1,25(OH)2D3 reaches a maximum within 2 h and returns to baseline 24 h after i.p. administration . The first experiment shown in Figure 1 was conducted at 24 h after the treatment, to allow sufficient time for functional expression, which involves following gene induction, protein synthesis and translocation. Several in vivo studies have examined the effect of vitamin D3 at 24 h after the last treatment [47–49].
To maintain the enhancing effect, repeated administration of 1,25(OH)2D3 would be necessary. However, an effective dose of 1,25(OH)2D3 might induce adverse effects such as hypercalcemia, because mice receiving over 0.1 μg of 1,25(OH)2D3 every other day for 2 weeks had hypercalcemia of over 12 mg/dL . Indeed, mice exhibited reduced body weight and abnormal behavior from day 4 during treatment with 1,25(OH)2D3 (1 μg/day, data not shown).
Several vitamin D analogues appear to induce less hypercalcemia than 1,25(OH)2D3, but might retain the Aβ-transport-enhancing activity of 1,25(OH)2D3. However, paricalcitol, a vitamin D analogue, did not significantly enhance [125I]hAβ(1-40) elimination from mouse brain (data not shown). Therefore, 1,25(OH)2D3 and its analogues are unlikely to be candidate disease-modifying agents for AD. Nevertheless, since serum 25(OH)D3 levels tend to decrease with aging , maintaining a normal serum level (32 to 70 ng/mL) could be helpful for prevention of AD. Furthermore, since complex genomic and nongenomic pathways appear to be involved in the action of 1,25(OH)2D3, separation of these effects by structural modification might be feasible. The present in vivo and in vitro results suggest that the increase in cAMP levels by forskolin might be a candidate pathway for disease-modification of AD, although the in vivo effects of forskolin are still poorly understood.