Shroom3 and planar cell polarity couple apical constriction and convergent extension during neural tube morphogenesis

Neural tube closure is a critical developmental event that provides an extraordinary example of how coordinated cell behavior orchestrates the shaping of a complex tissue during vertebrate embryogenesis. Neural tube closure relies on actomyosin contractility to drive specific cellular behaviors such as apical constriction, tissue bending, and directional cell rearrangements. These complicated processes are facilitated by coordinating the activity of Rho-Kinase (Rock), to regulate cytoskeletal dynamics and actomyosin contractility, and the Planar Cell Polarity (PCP) pathway, which controls polarized cellular reorganization to mediate convergent extension of the tissue. Failure to properly coordinate these processes results in a class of highly prevalent, severely debilitating congenital malformations known as neural tube defects. The work described here investigates the role of Shroom3, a regulator of localized Rock activity, as a direct linker between planar cell polarity and actomyosin contractility to facilitate apical constriction, directional bending, cell topography, and subsequent neural tube morphogenesis. \ud In mice, simultaneous depletion of Shroom3 and the planar cell polarity components Vangl2 or Wnt5a results in convergent extension failure and an increased liability to neural tube defects. The Shroom3 and planar cell polarity pathways intersect at Dishevelled, as Dishevelled2 (Dvl2), Shroom3, and Rock co-distribute in cells. Dvl2 and Shroom3 form a physical complex mediated by discrete protein-protein domain interactions. Imaging of the neural plate of E8.5 embryos demonstrates that multiple components of the Shroom3 pathway are planar polarized along mediolateral cell junctions in vivo in a PCP-dependent manner. Finally, Shroom3 mutant embryos exhibit defects in planar cell arrangement during neural tube closure, suggesting a role for Shroom3 activity in convergent extension. These findings support a model in which the Shroom3 and planar cell polarity pathways interact to control convergent extension and polarized bending of the neural plate. The results of this study lend insight into the intricate set of mechanisms that regulate neural tube closure and provide a clear illustration of the complex genetic basis of neural tube defects.\u