X-linked myopathy with excessive autophagy (XMEA) is a rare neuromuscular disorder characterized by muscle atrophy and weakness. Mutations in the gene VMA21 have been identified as the genetic cause of the disease in 2013. Since then, the number of XMEA patients with VMA21 mutations has increased, along with varying ages of onset and disease severity. Intriguingly, mutations in VMA21 have also been recently linked to a liver disease. VMA21 is a crucial chaperone protein that regulates the v-ATPase proton pump, which ensures the acidification of lysosomes and other organelles. Proper acidification is required for processes like autophagy, which are vital for muscle cell homeostasis. While autophagy defects have been described in cells from XMEA patients, the molecular mechanisms underlying muscle pathology in XMEA and the distinct clinical manifestations associated with VMA21 mutations remain largely unknown.
During my PhD, I explored the expression pattern and pathophysiological roles of VMA21. In a first study, I uncovered that VMA21 is expressed as two main isoforms in humans and mice: the previously known, ubiquitous VMA21-101 isoform and a yet unknown isoform, VMA21-120. I demonstrated that VMA21-120 is expressed specifically in skeletal muscle cells after their differentiation. Notably, both isoforms were absent in cells from two XMEA patients, opening new insights into the potent pathogenic role of VMA21-120 in XMEA-associated muscle alterations. In a second study, I characterized the consequences of VMA21 depletion in skeletal muscle using a mouse model newly generated in the lab. While 50% reduction in the expression of VMA21 did not perturb muscle homeostasis, the loss of the two VMA21 isoforms in skeletal muscle (Vma21mKO mice) resulted in early lethality and profound muscle degeneration. Importantly, VMA21 depletion caused a blockage in the autophagic flux at the degradation step, prior muscle degeneration. In parallel, RNA sequencing analysis revealed perturbations in several key pathways, such as endocytosis, as well as processes previously associated with VMA21- related disorders, including ER stress and lipid metabolism. Consistently, there was a dramatic extrusion of vesicles resembling autophagic vesicles and/or multivesicular bodies in Vma21mKO muscles before their degeneration. The combined disruption of lysosomal, autophagic and endocytic pathways may precipitate muscle fibres to death and trigger the extremely severe phenotype observed in Vma21mKO mice.
Overall, my research projects highlight the crucial physiological role of VMA21 in maintaining muscle cell homeostasis. My work confirmed that VMA21 deficiency leads to autophagy impairment in muscle fibres, and unveiled alterations in other processes, such as the endocytic pathway, expanding our understanding of XMEA pathogenesis. These discoveries open new avenues for future translational investigations on VMA21-related disorders and the development of therapeutic strategies.
