Roles of canonical Wnt signaling during skeletal muscle regeneration – WntReg

Skeletal muscles are one of the two sorts of striated muscles of the vertebrate body (the other being the cardiac muscle). They are fixed to the bones by the tendons and have the function to move the body in space under the control of the central nervous system. A skeletal muscle is constituted of individual components known as muscle fibres (myofibres), cylindrical multinucleated cells containing contracting myofibrils. Homeostasis of the adult skeletal muscle tissue relies on a pool of resident quiescent precursors located in a niche around the myofibers: the Satellite Cells (SCs). SCs are committed muscle stem cells responsible for the growth and the regenerative capacity of the muscle tissue. Upon activation following injury or repeated exercise, SCs leave quiescence to proliferate and then differentiate to form new myofibers, while a sub-population exit the cell cycle to self-renew and replenish the stem cell niche in the new tissue. In the course of this process, signals from the local milieu and the microenvironment instruct cycling SCs and control myogenic fate choice.
During aging, the degenerative loss of skeletal muscle mass and strength is associated with reduced proliferative capacity of SCs. In progressive myo-degenerative diseases such as Duchenne muscular dystrophy, regeneration cannot compensate for the loss of muscle tissue. The constant degeneration processes exhaust the SC pool and disease-related systemic factors diminish their myogenic potential.
The canonical Wnt cascade is a critical regulator of stem cells in many adult tissues and our previous report highlighted that numerous Wnt proteins are secreted during muscle regeneration. As currently understood, Wnt proteins bind to receptors of the Frizzled and LRP families on the cell surface of receiving cells. Through several cytoplasmic relay components, the signal is transduced to ß-catenin, which enters the nucleus and forms a complex with TCF to activate transcription of Wnt target genes. However, conflicting reports proposed roles for the canonical Wnt/ß-Catenin pathway during skeletal muscle regeneration. These previous data were mainly based on in vitro cell culture experiments, and/or on injection of recombinant Wnt proteins in vivo. Our goal is to comprehend the roles of Wnt/ß-Catenin signals during adult muscle tissue repair. We thus designed a research strategy based on the generation of mouse genetic models with SC-specific Wnt/ß-Catenin gain- and loss-of-function mutations. First, we will study the effects of conditional activation of the canonical Wnt pathway in vivo within the SC lineage. We will analyze the involvement of Wnt/ß-Catenin signaling during SC quiescence, activation and the control of the balance between proliferation and differentiation in vitro and in vivo. We then plan to screen for novel Wnt target genes by microarray analysis and ChIP-sequencing in cultured primary myogenic cells, and validate the impact of candidate genes on SC myogenesis. The last part of our project will focus on characterizing the molecular signals involved in SC self-renewal and niche occupancy in vivo and the interactions between Wnt canonical and polarity signalings.
This proposal is primarily basic in its setup. However, our data will help us better our understanding of adult muscle tissue repair and SC biology. We chose to focus our research on Wnt/ß-Catenin signaling, since we patented the use of in vivo injection of diffusible Wnt ligands and antagonists (Patent CA2010/000734). Our aim is to establish a landmark for the creation of methodologies to enhance muscle repair via pharmacological control of endogenous Wnt activity that will allow development of novel therapeutic strategies toward ameliorating the loss of muscle function in human pathologies.