Myosin II in early Xenopus morphogenesis

Abstract

This thesis focuses on demonstrating the relative roles of cytoskeletal myosins IA and IIB in early Xenopus morphogenesis. Cytoskeletal myosin II is a motor protein implicated at a cellular level in functions such as cell motility, adhesion and cortical integrity, and at a tissue level in diverse morphogenetic events, such as Drosophila germ band extension and dorsal closure. Vertebrates have different isoforms of myosin II, including myosins IIA and IIB, which have distinct subcellular localizations, as well as different biochemical properties, suggesting that they may have separate roles in vertebrate morphogenesis. I show that in Xenopus these isoforms also have distinct localizations in the embryo, with IIB expressed predominantly in the dorsal tissues, and IIA ubiquitously throughout the embryo, suggesting region-specific functions. With morpholino-oligonucleotide reduction or knock-down of myosins IIA and IIB, I show that myosin IIB is necessary for dorsal morphogenesis, particularly the convergence and extension (CE) of the dorsal mesodermal and neural tissues, which produce the main force that drives blastopore closure and dorsal body elongation in the normal Xenopus embryo. Myosin IIB is necessary for the organization of the contractile actin cortical cytoskeleton, polarized cell shapes and protrusive activity that drive both mesodermal and neural CE. In addition, myosin IIB is required for neurulation events other than CE, particularly for the apical constriction of neurepithelial cells, which drives neural plate bending during Ill neural tube closure. Myosin IIB is also necessary for antero-posterior elongation of the dorsal side of the embryo. In contrast, myosin IIA functions mostly in ventral morphogenetic events. During gastrulation, myosin IIA is involved in the process of convergent thickening that occurs in the ventral marginal zone of normal embryos and that aids in closure of the ventral blastoporal lip. Later, myosin IIA is involved in elongation of the ventral side of the embryos during body axis elongation.