|M.Sc Student||Odeh Maaly|
|Subject||Regulation of Skeletal Muscle Atrophy by p38 MAPK Signaling|
|Department||Department of Medicine||Supervisor||Professor Eyal Bengal|
|Full Thesis text|
Muscle atrophy is a major medical problem occurring due to several conditions such as denervation, inactivity, fasting and in various diseases like amyotrophic lateral sclerosis (ALS), AIDS and cancer. Loss of muscle tissue known as sarcopenia is a major health problem in older people. Atrophy is a catabolic process in which myofibrillar, soluble proteins and damaged organelles are removed by the ubiquitin-proteasome and autophagy degradation systems. The balanced activity of these systems is necessary to preserve fiber homeostasis and protect the muscle from undergoing cell death. The stress-induced p38 mitogen-activated protein kinases (p38 MAPKs) are a class of evolutionarily conserved serine/threonine kinases are involved in many pathologies including those promoting muscle atrophy. It has recently been shown that p38 MAPK induces the expression of muscle-specific E3 ligases functioning in the Ubiquitin-proteasome system. However, its role in the second major proteolytic pathway, autophagy and in energy metabolism has not been investigated. We hypothesize that induced p38 MAPK activity in denervated muscles is involved in atrophy by perturbing the fine balance between autophagy and Ubiquitin-mediated protein degradation systems and by the modification of energy metabolism. Our aims are to explore how p38 MAPK signaling affects skeletal muscle atrophy in a model of muscle denervation, to elucidate the mechanism by which p38 regulates the two major proteolytic systems involved in muscle atrophy; autophagy and Ubiquitin-proteasome and to analyze its effect on energy metabolism. For this purpose, we take advantage of a mouse strain designated p38αAF that harbors dominant-negative mutations of the activating phosphorylation sites of the p38MAPK gene that result in specifically attenuated p38MAPK signaling.
Our results indicate that p38αAF mice undergo less muscle atrophy relative to wild type mice following denervation of the leg muscles. We found several distinct metabolic differences between denervated muscles of control mice and p38αAF mice that could explain why muscles of the mutant mouse are relatively protected from atrophy. First, unlike in denervated muscles of the control mouse, markers of endoplasmic reticulum (ER) stress and unfolded protein response (UPR) are not induced in denervated muscles of p38αAF mouse. In addition, proteolysis by the ubiquitin-proteasome system appears low in denervated muscles of the mutant mouse relative to denervated muscles of the control mouse. In contrast, we find evidence to support the occurrence of autophagy in denervated muscles of p38αAF mice but not in denervated muscles of WT mice. Our results also indicate that atrophied muscles of p38αAF mice undergo slow-to-fast fiber type switching post denervation and utilize more anaerobic glycolysis while denervated muscles of WT mice undergo fast-to-slow fiber type switching post denervation and utilize more oxidative phosphorylation to produce energy.
Collectively, we conclude that attenuation of p38 activity diminishes denervation-mediated muscle atrophy. Blockage of p38 activity reduces proteasome-mediated protein degradation while inducing autophagy and a switch from wasteful oxidative phosphorylation to preservative anaerobic glycolysis. The involvement of p38 MAPK in catabolic pathways that regulate muscle mass place this kinase as a potential therapeutic target for the prevention and treatment of muscle atrophy in metabolic and neuromuscular diseases.