Mechanisms That Determine the Internal Environment of the Developing Brain: A Transcriptomic, Functional and Ultrastructural Approach

Abstract

We provide comprehensive identification of embryonic (E15) and adult rat lateral ventricular choroid plexus transcriptome, with focus on junction-associated proteins, ionic influx transporters and channels. Additionally, these data are related to new structural and previously published permeability studies. Results reveal that most genes associated with intercellular junctions are expressed at similar levels at both ages. In total, 32 molecules known to be associated with brain barrier interfaces were identified. Nine claudins showed unaltered expression, while two claudins (6 and 8) were expressed at higher levels in the embryo. Expression levels for most cytoplasmic/regulatory adaptors (10 of 12) were similar at the two ages. A few junctional genes displayed lower expression in embryos, including 5 claudins, occludin and one junctional adhesion molecule. Three gap junction genes were enriched in the embryo. The functional effectiveness of these junctions was assessed using blood-delivered water-soluble tracers at both the light and electron microscopic level: embryo and adult junctions halted movement of both 286Da and 3kDa molecules into the cerebrospinal fluid (CSF). The molecular identities of many ion channel and transporter genes previously reported as important for CSF formation and secretion in the adult were demonstrated in the embryonic choroid plexus (and validated with immunohistochemistry of protein products), but with some major age-related differences in expression. In addition, a large number of previously unidentified ion channel and transporter genes were identified for the first time in plexus epithelium. These results, in addition to data obtained from electron microscopical and physiological permeability experiments in immature brains, indicate that exchange between blood and CSF is mainly transcellular, as well-formed tight junctions restrict movement of small water-soluble molecules from early in development. These data strongly indicate the brain develops within a well-protected internal environment and the exchange between the blood, brain and CSF is transcellular and not through incomplete barriers.

Molecular anatomy and functions of the choroidal blood-cerebrospinal fluid barrier in health and disease

“…4a). A functional correlate has been established by permeability studies at early embryonic stages [80]. Claudin-2 expression appears to follow the developmental increase in CSF secretion capacity.…”

“…Indeed, such an approach has already been successfully applied to define new stages of lung maturation in humans. Importantly, claudin proteins are expressed at the junctions linking the epithelial cells in the nascent choroid plexus in human, and evidence of the tightness of these junctions has been provided in embryonic rodents ( [74,80], and see infra). This suggests that the barrier originates as early as the choroid plexus develops.…”

A balanced view of choroid plexus structure and function: Focus on adult humans

“…Vigorous choroidal blood flow, together with the activities of key transport enzymes -Na + -K + -ATPase and carbonic anhydrase -drives the CPmanufactured CSF (containing Na + , Cl − , HCO 3 − ) into the four ventricles (Lorenzo and Snodgrass, 1972). Transcriptome analysis of Na-K-ATPase isoforms and carbonic anhydrase 2 in rat CP, both integral to CSF formation, reveals that these key transport enzymes appear (mRNA) in embryonic life (Liddelow et al, 2013); and thereafter their expression is augmented postnatally as CSF formation and pressure continuously rise to the adult level. It is impressive that CP can actively secrete CSF even when a substantial hydrostatic pressure is exerted on choroidal cells, either perpendicular to the epithelium as a monolayer in a transwell in vitro or when freely suspended in the ventricles under elevated intracranial pressure in situ (as in hydrocephalus).…”

Comment on “Multiple repressive mechanisms in the hippocampus during memory formation”

“…2D), following peripheral administration of an estrogen receptor antagonist. While we did not repeat these pharmacological studies, prior investigations have demonstrated direct actions of estrogens on ChP transcription of Ttr (11), its most highly expressed gene (3,4). …”

“…We re-examined the top 15 differentially-expressed genes (DEGs) reported by Cho et al . (1) at 4h following contextual fear conditioning, and observed very low levels of expression in the hippocampus [see Allen Brain Atlas (2)], but noted that 11 of the genes rank among the top expressed transcripts in the choroid plexus (ChP), and together belong to the characteristic transcriptomic signature of the ChP that we (3), and others (4) have characterized.…”