The formation and function of the brain ventricular system

Abstract

The brain ventricular system is composed of a highly conserved set of cavities that contain cerebrospinal fluid (CSF), a protein-rich fluid essential for brain function. However, little is known about the function of embryonic CSF (eCSF), or the mechanisms of CSF production, retention, and circulation that regulate brain ventricle shape and size. Here we present data that begins to dissect the mechanisms governing CSF dynamics during zebrafish embryonic development. Our data indicate that the Na,K-ATPase regulates three aspects of brain ventricle development essential for normal function - neuroepithelial formation, permeability, and CSF production. Formation of a cohesive neuroepithelium requires both the alpha subunit (Atp1a1) and the regulatory subunit, Fyxd1, while only Atp1a1 modulates neuroepithelial permeability. Further, RhoA regulates both neuroepithelium formation and permeability, downstream of the Na,KATPase. Finally, we identified a RhoA-independent process, likely CSF production, which requires Atp1a1, but not Fxyd1. Therefore, formation of the vertebrate brain ventricles requires both production and retention of CSF. Although the embryonic brain ventricles contain large quantities of eCSF little is known about the function of the fluid or the mechanisms that drive fluid production. We developed a method to manually drain eCSF from zebrafish brain ventricles and show that eCSF is necessary for cell survival within the neuroepithelium. Further, increased retinol binding protein 4 (Rbp4), retinoic acid synthesis, and retinoic acid signaling via the PPAR? (peroxisome proliferatoractivated receptor gamma) receptors, prevents neuroepithelial cell death. Thus, we present a novel role for Rbp4 and retinoic acid synthesis and signaling during embryonic brain development. Finally, we also developed an assay to visualize CSF flow in the embryonic zebrafish. We found that the midbrain-hindbrain boundary acts as a barrier preventing CSF movement between the midbrain and hindbrain, while CSF moves freely between the midbrain and forebrain. Additionally, the heartbeat contributes to CSF movement increasing mixing between the hindbrain and forebrain/midbrain compartments. Furthermore, we determined that hydrocephalic phenotypes observed in zebrafish are due to abnormalities in CSF production, retention and flow. These data demonstrate the importance of CSF dynamics during development and further suggest that disruption of these processes can all result in hydrocephalus.