Phd in MORPHOGENESIS AND TISSUE ENGINEERING

Thesis title: Unveiling Histone Deacetylase 4 multiple functions in dystrophic skeletal muscle

Histone deacetylase 4 (HDAC4) is a stress-responsive epigenetic factor able to mediate multiple cellular responses in skeletal muscle, upon different pathological conditions. The cytoplasmic functions of HDAC4 in skeletal muscle are poorly characterized so far. HDAC4 expression is upregulated in skeletal muscles of mdx mice, a mouse model for studying Duchenne Muscular Dystrophy (DMD), suggesting a role in this disease. DMD is a genetic, progressive disorder, characterized by muscle degeneration and weakness, ultimately leading to the premature death of patients. Pan-HDAC inhibitors are presently in clinical trial for the treatment of DMD, preventing fibrosis and adipogenesis and promoting compensatory regeneration, despite presenting several important limitations. The identification of the specific functions of different HDAC members is a prerequisite for the development of more selective drugs for the treatment of DMD. With the aim to clarify HDAC4 functions in DMD, we generated dystrophic mice with a skeletal muscle-specific deletion of HDAC4 (mdx;HDAC4mKO mice). Deletion of HDAC4 in skeletal muscle worsens the pathological features of DMD, increasing muscle damage and compromising muscle regeneration and function. HDAC4 affected Fibro Adipogenic Progenitors potential and decreased their ability to support mdx satellite cells, in addition to compromise mdx satellite cell differentiation via paracrine signals. Moreover, HDAC4 orchestrates membrane repair mechanism in mdx muscles and satellite cells, affecting muscle necrosis, satellite cell survival and myogenic capacity. Importantly, ectopic expression of Trim72, a major player in the membrane repair mechanism, or cytoplasmic HDAC4 rescues mdx;HDAC4mKO mice phenotype in vitro and in vivo. We demonstrated that HDAC4 cytoplasmic functions are independent of its deacetylase activity and crucial for allowing a proper response to muscle damage and preserving muscle homeostasis in DMD context. These findings should be considered for future therapeutic approaches.