HDAC4 mediates the crosstalk between skeletal muscle and fibro adipogenic progenitors in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked, progressive, muscle-wasting disease caused by the lack of the dystrophin protein. The absence of dystrophin compromises muscle integrity leading to the substitution of skeletal muscle fibers with fibrotic and adipose tissues. This is due to the altered activity of a mesenchymal stem cell population, named fibro-adipogenic progenitors (FAPs). FAPs are the main cellular target of Givinostat, a pan-histone deacetylase inhibitor, recently approved by the FDA as a treatment for DMD patients. However, deletion of histone deacetylase 4 (HDAC4) in mdx muscles worsens DMD progression, demonstrating the importance of preserving the functions of this specific HDAC in dystrophic skeletal muscle. The main objective of my Ph.D. project is to characterize in detail multiple functions of HDAC4 in skeletal muscle in DMD, focusing on the crosstalk between muscle fibers and FAPs. We found that dystrophic mice lacking HDAC4 (mdx;KO) exhibit higher fibrotic and adipose tissue infiltration in the musculature than mdx littermates. Consistently, FAPs isolated from mdx;KO mice display a higher fibrotic and adipogenic potential. We hypothesized a paracrine function of HDAC4 which is mediated by EVs. Thus, we expressed the muscle-specific, cytoplasmic-restricted form of HDAC4 and noticed that this reduces the levels of fibrotic and adipose tissues in mdx;KO muscles, in addition to reducing muscle degeneration. All of the above, in addition to showing an effect which is independent of the deacetylase activity of HDAC4, confirms the importance of preserving its cytoplasmic functions in DMD. Proteomic and non-coding RNA transcriptomic analyses identified interesting candidates differentially secreted between mdx and mdx;KO myofibers, i.e. adiponectin and miR-5106, that may be crucial mediators of the paracrine functions of HDAC4 in DMD skeletal muscle.