Physiologie, homéostasie calcique et électrophysiologie du muscle squelettique.

Responsable : Stéphane Sebille

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Myopathies, representing about 300 different diseases at 80% genetic, lead to a muscle mass / force loss. Early works in the laboratory on calcium in muscle cells started in 1990s and have been focused on excitation-contraction coupling installation and relationships between calcium increase and myoblast fusion mechanisms. More recently, a large part of our work was focused on calcium regulation in dystrophin‐deficient skeletal muscle cells. Our results support two hypotheses of an over‐activation of Ca2+-release in dystrophin-deficient cells, via inositol 1,4,5-trisphosphate receptors (IP3R) (16–18) as well as an increase of some TRPs channels activity (18–20). We will take advantage of these works for the present proposal to explore the following scientific issues.

Our first issue concerns the relationships between calcium homeostasis and myogenesis. The muscle-generating process is defined as « myogenesis » which is a multistep process controlled by a core network of transcription factors (Pax3, pax7, MRFs) and involving myoblast determination, cycle activation, proliferation, alignment, fusion in multinucleated myotubes and the formation of mature myofibres. Because myogenesis depends on both membrane potential and calcium signaling, we developed optogenetic approaches in muscle cells and demonstrated the potential of such technology to control membrane potential and consequent muscle cell activities as migration (21) or excitation-contraction coupling (22). Our aim in this field is to investigate relationships between calcium homeostasis and myogenesis with the use of recent tools. In building new knowledge on relationships between cell excitability, calcium signatures and cell fusion or migration, we will estimate the potential of optogenetics to modulate muscle differentiation processes, in collaboration with Véronique Blanquet’s team (Lab PEREINE, Limoges), in optical-stimulated skeletal muscle cells and tissues.

Given the essential role of calcium in muscle function, numerous pathologies are related to dysregulations of calcium homeostasis such as Duchenne muscular dystrophy (16,17). The generation of human induced pluripotent stem cells (iPSCs) from somatic cells opens new areas for precision medicine with personalized cell therapy. These cellular models are of great interest in the case of Duchenne muscular dystrophy (DMD), whose pathophysiology is still poorly understood and for which the existing treatments are not curative (23). Our aim in this field is to investigate relationships between calcium dysregulation and muscle disease first in applying new tools and methods, in combination with electrophysiology and calcium imaging. A particular interest will be focused on calcium dysregulation of muscle cells derived from DMD iPSCs, in collaboration with Christian Pinset’s team (I-Stem, Evry).

Funding: AFM grant

We also planned to explore the involvement of autophagy in muscle physiopathology. Autophagy is an important proteolytic system constantly active in skeletal muscle and its role in tissue homeostasis is complex: at high levels, it can contribute to muscle wasting; at low levels, it can cause weakness and muscle degeneration as in DMD (24). The interplay between calcium homeostasis and autophagy is not clear. Many studies suggest that imposition of cytosolic Ca²⁺ signals can trigger autophagy. However, it is also apparent that in some contexts Ca²⁺ signals have the ability to suppress autophagy (for review see (25)). Our aim in this field is to investigate the links between autophagy, calcium regulation and myogenesis for the physiological part and myopathies for the pathological part. Muscle primary cultures or cell lines such as C2C12 myoblasts and iPSCs will give opportunity to decipher the molecular mechanisms involved in these interplays as well as yeast will be used as a toolkit already developed in the lab (26,27) for the study of molecular interactions between calcium regulation and autophagy.

Muscle tissue can be remodeled in physiological conditions as exercice but also in pathological conditions as obesity (sarcopenic obesity) or hypertension. We already explored, in collaboration with the MOVE Laboratory (Poitiers) the effects of intermittent exercise on the cardiac remodeling of hypertensive rats and evaluated the impact of calcium channels on this remodeling (28).  We also explored the effect of an High Fat High Fructose diet on the physiological response of skeletal muscle and demonstrated that adaptations occurring  in  response  to  this  diet  result  in  a  general muscle  force  loss consistent with that observed in humans (29). Our aim in this field is to investigate relationships between calcium homeostasis and physiological or pathological remodeling in muscle cells. Autophagic fluxes will also be explored in remodeling muscles.