Here we analysed and characterized the mouse CP transcriptome under normal basal physiological conditions. Unravelling the transcriptome signature of a tissue is a valuable tool for better understanding its functions in health and in disease. In this particular case, as part of the barriers that separate the brain from the periphery, it has become clear that the CP is more than an obstacle for molecule and cell trafficking into and out of the brain; it is an active participant in brain homeostasis.
Not surprisingly, we confirmed that among the genes most highly expressed were those encoding for proteins that are secreted into the CSF, such as transthyretin and transferrin. These are carriers for the ligands thyroxine and iron respectively, which are relevant for proper brain function during development and throughout adulthood. While CP transthyretin in itself does not seem essential for thyroid hormone to reach and distribute within the brain parenchyma , the available data does not exclude a role for the CP in thyroid hormone homeostasis. In fact, the thyroid gland is the sole site recognized for thyroid hormone synthesis. However, when the thyroid gland is ablated, it takes several months for the tissues to be depleted of thyroid hormones [22, 23], which raises the possibility of extrathyroidal sites for thyroid hormone synthesis. The CP could be such a place, not only because it expresses the sodium-iodine symporter , which uptakes iodine against a concentration gradient, but also several peroxidases (glutathione peroxidase 4, 1, 3, 7, myeloperoxidase, thyroid peroxidase) as we now show in this study. Future investigation should address whether these peroxidases have the same ability as thyroid peroxidase in the thyroid to oxidise iodine and incorporate it into thyroglobulin (also present in the basal CP transcriptome) or other tyrosine containing protein.
Analysis of the basal transcriptome revealed genes involved in iron homeostasis such as transferrin; however, it is notable that participation in brain iron homeostasis seems to be particularly relevant in response to stimuli such as peripheral inflammation . These observations highlight the ability of the CP to respond/adapt to physiological and disease states.
Several genes were shown to be expressed in the CP for the first time. These include those encoding for the opioid receptors, for which we found mRNA for all members of the family, the opioid receptor sigma 1 being the most highly expressed. Other studies, using autoradiographic techniques with selective ligands, failed to detect the mu and delta opioid receptors in rat CP brain [25, 26], probably due to the lower sensitivity of the methodologies used. The present study also confirmed the presence of other neurotransmitter receptors previously described in the CP , such as the dopamine receptors. Again, this emphasizes an active role of the CP as a potential site for modulating response to neurotransmitters, which should be further investigated in the context of mood disorders.
We consider it of interest that the adult CP expresses genes encoding for several growth factors and axonal guidance molecules. While semaphorin 3f  and Slits 2 and 3 [28, 29] were previously described in the developing CP, we described here for the first time the expression of various netrins and ephrins. The present study also adds to previously described growth and trophic factors [30, 31], such as insulin-like growth factor II , glial cell line-derived neurotrophic factor , transforming growth factor alpha , fibroblast growth factor 2  and vascular endothelial growth factor . The expression of genes encoding for several members of the FGF family (those with higher expression value being FGF 18, 9 and 3), neuregulin, betacellulin and epiregulin should be highlighted. Betacellulin and its receptor ErbB-4 are highly expressed in neurons, implying important roles in neuronal cell functions; and inducing ErbB-4 activation promotes neurite outgrowth in PC12 cells . Numerous genetic modelling studies in mice demonstrate that neuregulin 1 signalling is important in the development of normal neuronal connectivity. Of particular interest to this discussion are findings linking deficits in neuregulin 1-ErbB4 signalling to perturbations of synaptic transmission, myelination, and the survival of particular sets of neurons and glia . It will be of interest to next investigate the participation of the CP in demyelinating diseases.
The CP expression of growth factors is also relevant in the context of neurogenesis. The subventricular zone is, together with the hippocampus, a site of adult neurogenesis. We showed here that the genes encoding for molecules that modulate adult neural stem cell proliferation and fate are transcribed in the CP. It is surprising that few studies so far have considered the CP as a relevant source of such mediators. The present study draws attention to the CP, as a modulator of the CSF composition, in the distribution of key molecules for neuronal differentiation.
Finally, the data we presented here confirms that the CP transcriptome displays a specific signature, different from all other tissues for which transcriptome data is available . While one could anticipate resemblance with other secretory epithelia, such as the lung, the closest similarity is with that of the PECAM-1+ BBB microvascular endothelial cells . This most likely reflects the fact that both have a barrier function, mediated by tight junctions; participate in the transport of molecules in and out of the brain, through several transporters and receptors; and in detoxifying processes, through several enzymes [31, 40]. Despite these similarities, there are striking differences in gene expression between these two barriers, such as is the case for carbonic anhydrase 12, which is involved in CSF production (13.0 versus 4.4).