Regulation of cell polarity during zebrafish gastrulation

During the vertebrate gastrulation movements of convergence and extension (C&E) the entire embryo and most organ rudiments narrow along the mediolateral axis while extending their anterior–posterior dimension. In zebrafish C&E movements are driven by directed cell migration and mediolateral intercalation of polarized cells and require noncanonical Wnt signaling. Mutations in knypek gene encoding a member of the glypican family of heparan sulfate proteoglycans impair C&E movements due to abnormal cell polarity, as mutant cells fail to elongate and align mediolaterally. Knypek potentiates Silberblick (Wnt11) signaling mediating C&E via a noncanonical signal transduction cascade. Mosaic analyses demonstrate that knypek has both cell-autonomous and nonautonomous roles in cell polarity determination. Zebrafish Rho kinase ? (Rok2) is expressed in zebrafish gastrula and overexpression of a dominant-negative Rok2 (dnRok2) disrupts C&E, phenocopying noncanonical Wnt signaling mutants. Epistatic analyses place Rok2 downstream of Wnt 11 in regulation of C&E. Impaired dorsal convergence movements in Rok2-deficient embryos are associated with defective cell polarity and consequently inability of a cell to migrate persistently dorsally along straight path. Transplantation experiments reveal both cell-autonomous and cell nonautonomous roles for Rok2 function during cell elongation and orientation.