Cellular and molecular events during early chick myogenesis

2017-03-01T02:41:51Z (GMT) by Sieiro Mosti, Daniel Alejandro
In chick embryos, skeletal muscle formation is initiated in selected epithelial somite progenitor cells, which, upon receiving external molecular cues, translocate into the nascent muscle where they differentiate into multinucleated myofibres. Here, two highly regulated mechanisms involving complex gene networks and choreographed cellular movements ensure the correct formation of the future musculature. The first involves a crucial myogenic cell-fate choice on whether to remain in a proliferative state or to begin differentiation. While this choice is known to involve both WNT and NOTCH pathways, the exact mechanisms and relationships between these pathways remains unknown. During the course of my doctoral studies, I uncovered at the heart of this response a signalling module encompassing NOTCH, GSK-3β,SNAI1 and WNT. This module transduces the activation of NOTCH into i) an inhibition of GSK-3β activity by non-transcriptional NOTCH signalling; ii) a SNAI1-induced epithelial to mesenchymal transition (EMT) leading to iii) the recruitment of membranal β-catenin to trigger WNT/β-catenin signalling and myogenesis independently of WNT ligand. While these results intimately associate the initiation of myogenesis to a change in cell adhesion, they may have wider implications, as they may reveal a general principle for coupling cell-fate changes to EMT in many developmental and pathological processes. The second process that is crucial for the correct formation of skeletal muscle, which I focused on extensively during my studies, is the fusion of myoblasts into multinucleated fibres. This event, which can occur up to tens of thousand of times within each muscle fibre, ensures not only the correct growth of muscle but also the proper functioning of repair mechanisms in adult muscles. During my PhD, I focused on investigating how early myoblasts initiate fusion and with which cellular partners. Here, I show that a complex fusion interaction takes place in the trunk, where primary myotome cells from the medial somite border rarely fuse to one another, but readily do so with other border cells. Conversely, resident muscle progenitors actively fuse with one another, but do so poorly with the primary myotome. I have also characterised fusion events in limb muscles, demonstrating key differences in the progression of this event. These studies revealed important physical interactions while suggesting complex molecular networks are in play. Based on this knowledge, by performing functional studies on potential novel fusion genes. I have gained further insight into the fusion process.