Characterization of mitochondrial bioenergetics in skeletal muscle disorders: an in vitro model of dystonia/spastic paraplegia and an in vivo model of delayed sarcopenia

Mitochondria are intracellular organelles present in almost all eukaryotic cells that are involved in several aspects of the cellular life, with a pivotal role in energy production and cell death. With the discovery of their link to various diseases, including cancer, aging and neurodegenerative diseases, the interest regarding mitochondrial studies has greatly increased in the scientific community. Dysfunctional mitochondria are indeed unable to provide the energy needed by tissues that require high metabolism, such as the heart, brain, and muscles, causing altered phenotypes that result in several pathological conditions. In this context, my PhD thesis focused on two different physio-pathological conditions involving alterations in the mitochondrial structure and function, with a great impact on cell metabolism, with the aim to clarify the underlying molecular mechanisms and thus contribute to identify new molecular targets for a pharmacological rescue of the phenotypes. In the first part (Project 1) we investigated a new movement-disorder syndrome caused by an autosomal-dominant variant in ATP Synthase Membrane Subunit C Locus 3 (ATP5MC3), using as experimental model primary skin-derived fibroblasts from a 13-year-old boy. The patient's cells showed an altered bioenergetic profile and a desensitization of the permeability transition pore (PTP), whose opening in the inner mitochondrial membrane can lead to cell death when long-lasting, while, when short-lived, regulates Ca2+ and ROS homeostasis. Remarkedly, the N106K substitution in the c3 subunit led to an accumulation of subunit c, possibly as a consequence of an altered stability/assembly of the ATP synthase complex. Consistently, electron microscopy analyses revealed that many mitochondria were damaged. Moreover, the patient-derived fibroblasts showed an abnormal accumulation of aberrant autophagy/mitophagy and lysosomal structures that were paralleled by increased levels of autophagy markers, thus suggesting an inability of these cells to maintain a proper turnover and clearance of the mutated subunit c by the lysosomal degradation pathways. In the second part (Project 2) we investigated the mitochondrial function in skeletal muscles of adult mice Knockout for the gene Pin1, used as a model with potential protection against sarcopenia. These mice showed an increase in maximal ADP-stimulated mitochondrial respiration due to myofibers switching towards a more oxidative phenotype, and also an increased sensitivity to ADP, made possible by a more efficient transport system of critical metabolites at the level of the mitochondrial inner membrane. This mitochondrial remodelling could counteract the loss of skeletal muscle and metabolism in aged individuals. A third Project is also presented regarding a study on the inflammatory response during tendon injury. The results suggested a new potential role of CD200 in the regulation of inflammation during tendon injury. In addition, genes (IRF, NFkb, TGRF2, CAAT, DOK2, RAS-GAP) that might be involved in the inflammatory response of TSPCs were highlighted.