In the adult the interface between the cerebrospinal fluid (CSF) and the brain is lined by the ependymal cells, which are joined by gap junctions. These intercellular connections do not provide a diffusional restrain between the two compartments. However during development this interface, initially consisting of neuroepithelial cells and later radial glial cells, is characterized by “strap” junctions, which limit the exchange of different sized molecules between CSF and the brain parenchyma. Here we provide a systematic study of permeability properties of this inner CSF-brain barrier during mouse development from embryonic day, E17 until adult. Results show that at fetal stages exchange across this barrier is restricted to the smallest molecules (286Da) and the diffusional restraint is progressively removed as the brain develops. By postnatal day P20, molecules the size of plasma proteins (70kDa) diffuse freely. Transcriptomic analysis of junctional proteins present in the CSF-brain interface showed expression of adherens junctional proteins, actins, cadherins and catenins changing in a development manner consistent with the observed changes in the permeability studies. Gap junction proteins were only identified in the adult as was claudin-11. Immunocytochemistry was used to localize at the cellular level some of the adherens junctional proteins of genes identified from transcriptomic analysis. N-cadherin, β– and α-catenin immunoreactivity was detected outlining the inner CSF-brain interface from E16; most of these markers were not present in the adult ependyma. Claudin-5 was present in the apical-most part of radial glial cells and in endothelial cells in embryos, but only in endothelial cells including plexus endothelial cells in adults. Claudin-11 was only immunopositive in the adult, consistent with results obtained from transcriptomic analysis. These results provide information about physiological, molecular and morphological-related permeability changes occur

Thyroid hormones (THs) are key players in regulating brain development. In humans, insufficient THs during gestation can lead to cretinism and mental retardation. Thus, transfer of appropriate quantities of THs from the blood into the brain at specific stages of development is critical. The choroid plexus forms the blood-cerebrospinal fluid (CSF) barrier. In reptiles, birds and mammals, the main protein synthesised by the choroid plexus is a TH distributor protein: transthyretin (TTR). TTR is secreted into the CSF and moves THs from blood into CSF. The structure of TTR has been highly conserved, implying strong selection pressure and an important function. The choroid plexus develops faster than other parts of the brain. In precocial animals rapid brain growth occurs prior to birth, whereas in altricial animals it is after birth. For both groups, maximal TTR expression in the choroid plexus is just prior to rapid brain growth, suggesting that choroid plexus-derived TTR moves THs from blood into CSF just prior to when THs are required for rapid brain growth. TTR null mice have no overt phenotype, bringing into question the role of TTR synthesised by the choroid plexus. This has been in part resolved by the discovery of TH transmembrane transporters, which have been identified in many tissues (including choroid plexus) and transport THs into and out of cells. In mammals, TTR binds T4 (pro-TH) with higher affinity than T3 (active TH). In all other vertebrates, TTR binds T3 with higher affinity than T4. As mammals are the exception, we should not base our thinking solely on mammalian data. THs enter the choroid plexus via TH transmembrane transporters and leave the choroid plexus to enter the CSF via either TH transmembrane transporters or via choroid plexus-derived TTR secreted into the CSF. The quantitative contribution of each route during development remains to be elucidated. This is part of a review series on ontogeny and phylogeny of brain barrier mechanisms.

A cell culture model for the blood-cerebrospinal fluid barrier in choroid plexus was developed. The relationship between synthesis and secretion of transthyretin across a layer of epithelial cells derived from rat choroid plexus and the transport of T4 was analyzed in a two-chamber system. Choroid plexus cells were dispersed and placed on a porous filter suspended in cell culture medium. A monolayer of polarized epithelial cells developed after 5 days in culture, separating fluid in the upper (apical) chamber from fluid in the lower (basal) chamber. Electrical resistance across the cell layer was 100 Ohm/cm2. Transthyretin was synthesized and secreted by these cells. Over 32 h, transthyretin accumulated in the fluid in the apical chamber to twice the concentration in the basal chamber. [125I]T4 added to the basal chamber permeated to the apical fluid and accumulated in the apical chamber to twice the concentration in the basal fluid. Upon inhibition of protein synthesis, T4 equilibrated to a similar concentration in the two chambers. Thus, the accumulation of T4 in the apical chamber required continuing protein synthesis. Competitive inhibition of T4 binding to transthyretin by EMD 21388 also prevented the accumulation of T4 to a higher concentration in the upper than in the lower chamber. These data suggest that T4 partitions through the choroid plexus and that transthyretin synthesis and secretion by the choroid plexus determines the concentration of T4 in the apical fluid. A model is proposed for the involvement of transthyretin secreted by the choroid plexus, in the in vivo distribution of T4 in the brain.