|M.Sc Student||Lavee Dagan|
|Subject||Complex Interactions of Morphogens and their|
Role in Myogenesis
|Department||Department of Biology||Supervisors||Professor Dale Frank|
|Mr. Reshef Ram|
|Full Thesis text|
During the transition from sea to land, vertebrates went through many changes in body plan, one of the most significant being the change in bone-muscle ratio. This change is reflected in the developing somites. In terrestrial vertebrates, the skeletal, or sclerotomal domain is the major component of the somite, while in aquatic vertebrates the somite consists almost entirely of muscle progenitor, or myotomal cells. Myogenesis also occurs much earlier in aquatic vertebrates; while in aquatic vertebrates muscle markers such as MyoD are present in the presomitic mesoderm and even in gastrulating embryos, in terrestrial vertebrates they are expressed only in the segmented somite. Despite the major difference at a phenotypic level of somite compartmentalization across species, the process seems to be largely conserved at a molecular level.
In terrestrial vertebrates, the relationships between the signaling factors involved in the process of myogenesis have been well defined. Wnt, Shh, and BMP inhibitors all interact to lead to muscle differentiation. In this work we analyze the role of the molecular cascades which are known to be essential for terrestrial myogenesis in Xenopus laevis, which represents an aquatic vertebrate. We show that ectopic Wnt can induce upregulation and lateral expansion of MyoD and BMP antagonists when activated at early gastrula. This inductive ability, however, is gradually lost over developmental time. This result suggests a role for Wnt in earlier mesodermal patterning which may lead to a larger accumulation of muscle progenitor cells in this species. In addition, inhibition of Shh in combination with Wnt overexpression shows a potentially conserved role of Shh in regulating the BMP antagonist Noggin. Surprisingly, Wnt overexpression also seems to cause positive regulation of the BMP pathway, as is shown in two ways: (1) Stabilization of the BMP downstream effector, phosphorylated smad1/5/8 and (2) Possible induction of an antagonist of Noggin and Chordin (both BMP antagonists). Upregulation of BMP antagonists is also shown to cause upregulation of muscle markers, which is consistent with the terrestrial vertebrate model. Based on the results, we postulate that in an aquatic common ancestor, BMP antagonists had a more promiscuous and dominant role in early patterning. Over evolutionary time, complex interactions between signaling factors, both inductive and inhibitory, evolved to properly regulate the process of myogenesis.