Wnt5b Mediated Regulation of Cell Shape Changes During Tissue Folding at the Midbrain-Hindbrain Boundary

In the developing vertebrate brain a highly conserved structure forms in the neuroepithelial tissue known as the midbrain-hindbrain boundary (MHB). During MHB formation, cells at the point of deepest constriction shorten, narrow, basally constrict, and apically expand to form a sharp fold in the neural tube. Using the zebrafish model, we previously determined that two ubiquitously expressed isoforms of the cytoskeletal motor protein, non-muscle myosin II (NMII), differentially regulate cell shape with non-muscle myosin IIA regulating cell length and non-muscle myosin IIB regulating cell width. However, their upstream signaling pathways are unknown. wnt5b is a candidate mediator of MHB morphogenesis because of its known roles in early morphogenetic events and it is specifically expressed at the MHB during the onset of its formation. Using wnt5b knockdown, cell shape was analyzed using live confocal imaging of membrane GFP-injected embryos combined with a new morphometric technique that we developed using digital sectioning to reveal 3D cell shape. Results from these studies led to the discovery of a differential role for Wnt5b signaling in shaping MHB cell width and basal constriction. We further examined a potential connection between Wnt5b and NMII activity using immunohistochemistry and revealed that Wnt5b mediates Rho-associated kinase-1 (ROCK1), a known regulator of NMII activity, localization basally at the MHB during morphogenesis. Additionally, cell proliferation was not found to contribute to Wnt5b-dependent cell shape. Together, these findings suggest that Wnt5b mediates cell shape during embryonic brain morphogenesis and could possibly play a role in shaping other epithelial cells and tissues throughout development