This proposal is designed to study the mechanisms underlying initial brain ventricle inflation, using a genetic approach in zebrafish. The brain ventricles are a conserved system of cavities containing cerebrospinal fluid that are required for normal brain function. Abnormalities in brain ventricle structure can lead to hydrocephaly and are correlated with many mental health disorders including autism and schizophrenia. The molecular mechanisms underlying brain ventricle development are poorly understood. The zebrafish is an excellent model for this study, as the brain ventricles are visible throughout development, and many genetic, molecular, and embryological tools are available. Research in the Sive laboratory has demonstrated that brain ventricle formation is a multi-step process, involving at least two steps: formation and maintenance of epithelial integrity by junction-associated proteins, and initial inflation of the ventricles with fluid, which requires the snk (atp1a1a.1) gene encoding a Na K ATPase. This proposal will test the hypothesis that the Snk protein functions in initial brain ventricle inflation by acting locally. Tissue transplants and mosaic analysis will be used to ask whether limited wild-type Atp1a1a.1 function can rescue snk mutants. Additionally, this proposal will test the hypothesis that multiple genes synergize with Snk to direct initial brain ventricle inflation. Five mutants isolated from chemical screens and two from an insertion mutagenesis screen show a putative brain inflation phenotype. These will be tested for synergistic effects with the Snk protein. This analysis will exploit the zebrafish to determine the mechanisms underlying brain ventricle inflation and that are required to build normal brain structure, which may be perturbed in mental health disorders.