Mechanisms of brain ventricle development

The brain ventricles are a conserved system of fluid-filled cavities within the brain that form during the earliest stages of brain development. Abnormal brain ventricle development has been correlated with neurodevelopmental disorders including hydrocephalus and schizophrenia. The mechanisms which regulate formation of the brain ventricles and the embryonic cerebrospinal fluid are poorly understood. Using the zebrafish, I initiated a study of brain ventricle development to define the genes required for this process. The zebrafish neural tube expands into the forebrain, midbrain, and hindbrain ventricles rapidly, over a four-hour window during mid-somitogenesis. In order to determine the genetic mechanisms that affect brain ventricle development, I studied 17 mutants previously-identified as having embryonic brain morphology defects and identified 3 additional brain ventricle mutants in a retroviral-insertion shelf-screen. Characterization of these mutants highlighted several processes involved in brain ventricle development, including cell proliferation, neuroepithelial shape changes (requiring epithelial integrity, cytoskeletal dynamics, and extracellular matrix function), embryonic cerebrospinal fluid secretion, and neuronal development. In particular, I investigated the role of the Na+K+ATPase alpha subunit, Atp1a1, in brain ventricle formation, elucidating novel roles for its function during brain development. This study was facilitated by the snakehead mutant, which has a mutation in the atp1a1 gene and undergoes normal brain ventricle morphogenesis but lacks ventricle inflation. Analysis of the temporal and spatial requirements of atp1a1 revealed an early requirement during formation, but not maintenance, of the neuroepithelium. I also demonstrated a later neuroepithelial requirement for Atp1a1-driven ion pumping that leads to brain ventricle inflation, likely by forming an osmotic gradient that drives fluid flow into the ventricle space. (cont) Moreover, I have discovered that the forebrain ventricle is particularly sensitive to Na+K+ATPase function, and reducing or increasing Atp1a1 levels leads to a corresponding decrease or increase in ventricle size. Intriguingly, the Na+K+ATPase beta subunit atp1b3a, expressed in the forebrain and midbrain, is specifically required for their inflation, and thus may highlight a distinct regulatory mechanism for the forebrain and midbrain ventricles. In conclusion, my work has begun to define the complex mechanisms governing brain ventricle development, and I suggest that these mechanisms are conserved throughout the vertebrates.