Project

human muscle; SOCE; EGFR; myoblast differentiation; myogenic transcription factors; calcium; membrane potential; ionic channels; TRPC

Lead
Lay summary
The general goal of our lab is to study human skeletal muscle regeneration. Muscle regeneration is a paradigm of a differentiation process as it involves myogenic differentiation factors, is controlled by extracellular growth factors and mediators, and it works through the activation of multiple intracellular signaling pathways. Upon muscle injury, satellite cells, the myogenic stem cell of skeletal muscle, proliferate as myoblasts and, after a process of differentiation, fuse together to generate new muscle fibers. Our objective is to understand the interplay between variations of the resting membrane potential, the ionic channels, the intracellular calcium, and various enzymes or myogenic regulatory factors activated during the differentiation process of myoblasts. We showed that EGFR is down-regulated during the early steps of myoblast differentiation. The role of EGFR is to maintain proliferation and inhibit differentiation likely by preventing Kir2.1-induced hyperpolarization. We previously showed that membrane hyperpolarization is required for the proper establishment of the differentiation program. The purpose of the differentiation-linked hyperpolarization is to generate calcium influxes mainly via store-operated Orai1 channels. These channels are gated by the ER-resident protein STIM1, and in absence of STIM1, the differentiation is abolished. Recently, we found a new isoform of STIM1, STIM1L that is highly expressed in muscle cells and is crucial for rapid Ca2+ entry taking place during muscle contraction. Overall, the calcium influx, by activating the CaMK and calcineurin pathways activates the myogenic transcription factors myogenin and MEF2, two transcription factors crucial for myoblast differentiation.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

Myoblast proliferation and differentiation into myofibers is a crucial event occurring during both embryogenesis and post-natal muscle growth and regeneration. The aim of our laboratory is to decipher the cellular and molecular mechanisms that control the transition from proliferation to differentiation of human primary myoblasts. In our previous work, we showed that activation of Ca2+-dependent signaling molecules, calcineurin and CaMK, lead to the induction of myogenin and MEF2 transcription factors, both required for the initiation of myogenesis. More recently, we identified two early events that allow the initiation of myoblast differentiation: (i) activation of Store-Operated Ca2+ Entry (SOCE) due to STIM activation and opening of Orai channels, and (ii) down-regulation of Epidermal Growth Factor Receptor (EGFR). This down-regulation is required for myoblast differentiation as it induces Kir2.1 channel activation that is responsible for a membrane hyperpolarization, which in turn enhances Ca2+ entry. We propose that these two events result in the generation of two Ca2+ signals. The aim of the present proposal is to characterize these two Ca2+ signals and to elucidate their role in the activation of downstream pathways. Specifically we will study:1. The molecular mechanisms that induce the expression of myogenin and MEF2 transcription factors. We will focus on the role of Ca2+-dependent pathways (calcineurin and CaMK) in the induction of these factors.2. The role of STIM/Orai and other Ca2+ entry channels (TRPC, transient receptor potential canonical) in the activation of calcineurin and CaMK. We intend to study whether the different routes of Ca2+ entry are associated with specific downstream signaling events. In addition, the role of TRPC channels in later events (fusion) will be investigated.3. The regulation of EGFR and its implication in Kir2.1 activation. We shall study the processes leading to EGFR degradation in differentiating myoblasts, and the putative Ca2+-dependence of EGFR and Kir2.1 regulation.These three projects will improve our understanding of human myoblast differentiation. In addition, our work should bring new highlights regarding the role of Ca2+ signals and ionic channels on cellular differentiation.