Paxillin genes and actomyosin contractility regulate myotome morphogenesis in zebrafish.

Paxillin (Pxn) is a key adapter protein and signaling regulator at sites of cell-extracellular matrix (ECM) adhesion. Here, we investigated the role of Pxn during vertebrate development using the zebrafish embryo as a model system. We have characterized two Pxn genes, pxna and pxnb, in zebrafish that are maternally supplied and expressed in multiple tissues. Gene editing and antisense gene knockdown approaches were used to uncover Pxn functions during zebrafish development. While mutation of either pxna or pxnb alone did not cause gross embryonic phenotypes, double mutants lacking maternally supplied pxna or pxnb displayed defects in cardiovascular, axial, and skeletal muscle development. Transient knockdown of Pxn proteins resulted in similar defects. Irregular myotome shape and ECM composition were observed, suggesting an "inside-out" signaling role for Paxillin genes in the development of myotendinous junctions. Inhibiting non-muscle Myosin-II during somitogenesis altered the subcellular localization of Pxn protein and phenocopied pxn gene loss-of-function. This indicates that Paxillin genes are effectors of actomyosin contractility-driven morphogenesis of trunk musculature in zebrafish. Together, these results reveal new functions for Pxn during muscle development and provide novel genetic models to elucidate Pxn functions.