Project End Date January 2020
One unique feature of the vertebrate brain is that it is tubular. The lumen of the tube is filled with cerebrospinal fluid (CSF), and forms the brain ventricular system, a circulatory system within the brain. Ventricular abnormalities lead to devastating brain disorders such as anencephaly and hydrocephalus. The embryonic development and function of this system are poorly understood. We analyze embryonic brain ventricle development in the zebrafish, an excellent system, as imaging the brain at single cell resolution in living embryos is feasible, and as many brain ventricle mutants have been identified.
Brain ventricles form over a six-hour period, during mid-somitogenesis, and require normal junctions and ion pump activity. Thus, mutants in nagie oko, which encodes a MAGUK family protein, with no clear midline and disrupted epithelial junctional protein expression fail to inflate their ventricles. In the snakehead mutant brain, the ventricles also do not inflate due to a mutation in the NaK ATPase pump a subunit, Atp1a1, that leads to an absence of embryonic CSF (eCSF). Interestingly, Atp1a1 function is also required for normal junction formation, especially in conjunction with other pump subunits.
In addition to eCSF secretion, we have demonstrated that, in the zebrafish hindbrain, ventricular lumen expansion requires a “stretchy” epithelium. A mutation in the myosin phosphatase regulator, mypt1, results in a small hindbrain ventricle, due to inability of the surrounding neuroepithelium to stretch. 3D reconstruction of cell shape demonstrates that characteristic cell shapes within the hindbrain are also abnormal in mypt1 mutants. As wild type embryonic brain ventricles form, levels of phosphorylated myosin regulatory light chain (pMRLC) change dynamically; however, mutants show continuously high levels of pMRLC, with apical concentration of pMRLC and myosin II. Brain ventricle lumen expansion and cell shape are rescued by the inhibition of myosin II function, indicating that defects are a consequence of overactive myosin contraction. These results show that the epithelium must “relax”, via the activity of myosin phosphatase, in order to allow for normal expansion of the brain ventricular lumen. Epithelial relaxation may facilitate inflation of tubes in many organs.