Epithelial relaxation mediated by the myosin phosphatase regulator Mypt1 is required for brain ventricle lumen expansion and hindbrain morphogenesis

Abstract

SUMMARYWe demonstrate that in the zebrafish hindbrain, cell shape, rhombomere morphogenesis and, unexpectedly, brain ventricle lumen expansion depend on the contractile state of the neuroepithelium. The hindbrain neural tube opens in a specific sequence, with initial separation along the midline at rhombomere boundaries, subsequent openings within rhombomeres and eventual coalescence of openings into the hindbrain ventricle lumen. A mutation in the myosin phosphatase regulator mypt1 results in a small ventricle due to impaired stretching of the surrounding neuroepithelium. Although initial hindbrain opening remains normal, mypt1 mutant rhombomeres do not undergo normal morphological progression. Three-dimensional reconstruction demonstrates cell shapes within rhombomeres and at rhombomere boundaries are abnormal in mypt1 mutants. Wild-type cell shape requires that surrounding cells are also wild type, whereas mutant cell shape is autonomously regulated. Supporting the requirement for regulation of myosin function during hindbrain morphogenesis, wild-type embryos show dynamic levels of phosphorylated myosin regulatory light chain (pMRLC). By contrast, mutants show continuously high pMRLC levels, with concentration of pMRLC and myosin II at the apical side of the epithelium, and myosin II and actin concentration at rhombomere boundaries. Brain ventricle lumen expansion, rhombomere morphology and cell shape are rescued by inhibition of myosin II function, indicating that each defect is a consequence of overactive myosin. We suggest that the epithelium must 'relax', via activity of myosin phosphatase, to allow for normal hindbrain morphogenesis and expansion of the brain ventricular lumen. Epithelial relaxation might be a widespread strategy to facilitate tube inflation in many organs.

Identifying candidate genes for 2p15p16.1 microdeletion syndrome using clinical, genomic, and functional analysis

“…Although the forebrain and midbrain of the 1-dpf xpo1a and rel morphants seemed comparable to those of the controls, the hindbrain ventricle for both genes was abnormally developed and not properly expanded in approximately 50% to 70% of knockdowns compared with controls, as the ventricle walls were not stretched enough to allow brain cavities to form (Figure 8). This feature has previously been reported for suppression of other genes that have a role in brain and neural development (36), supporting the role of xpo1a and rel in neurodevelopment. The most dramatic brain abnormality was noted in bcl11aa and bcl11ab morphants, in which the brain lacked differentiation of structures or expansion and resembled an underdeveloped neural tube.…”

Mitotic cell rounding and epithelial thinning regulate lumen growth and shape

“…In the brain ventricle, actomyosin-dependent relaxation of the epithelium is required to expand the lumen 35 . During Drosophila tracheal development, forces from tip migratory cells stimulate cell intercalation to elongate the tube 6 .…”

Functional antagonism of voltage-gated K+ channel α-subunits in the developing brain ventricular system

“…kcng4b inhibits cell proliferation During neurogenesis in mammals and zebrafish, the nuclei of dividing neuroepithelial cells prior to mitosis undergo interkinetic nuclear migration (INM) towards the apical surface, where round cells appear (Merkle and Alvarez-Buylla, 2006;Gutzman and Sive, 2010;Leung et al, 2011). Such cells were often found close to the midbrain lumen in 24 hpf wild-type embryos ( Fig.…”