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Mitochondrial quality control is essential in highly structured cells such as neurons and muscles. In skeletal muscle the mitochondrial fission proteins are reduced in different physiopathological conditions including ageing sarcopenia, cancer cachexia and chemotherapy-induced muscle wasting. However, whether mitochondrial fission is essential for muscle homeostasis is still unclear. Here we show that muscle-specific loss of the pro-fission dynamin related protein (DRP) 1 induces muscle wasting and weakness. Constitutive Drp1 ablation in muscles reduces growth and causes animal death while inducible deletion results in atrophy and degeneration. Drp1 deficient mitochondria are morphologically bigger and functionally abnormal. The dysfunctional mitochondria signals to the nucleus to induce the ubiquitin-proteasome system and an Unfolded Protein Response while the change of mitochondrial volume results in an increase of mitochondrial Ca 2+ uptake and myofiber death. Our findings reveal that morphology of mitochondrial network is critical for several biological processes that control nuclear programs and Ca 2+ handling. Muscle loss is associated with altered expression of proteins involved in mitochondrial homeostasis, but whether this is causative remains unclear. Here, the authors show that genetic ablation of the pro-fission protein DRP1 leads to accumulation of abnormal mitochondria that induce muscle atrophy by altering Ca 2+ homeostasis and cellular stress responses.

In the last decades the term Store-operated Ca2+ entry (SOCE) has been used in the scientific literature to describe an ubiquitous cellular mechanism that allows recovery of calcium (Ca2+) from the extracellular space. SOCE is triggered by a reduction of Ca2+ content (i.e. depletion) in intracellular stores, i.e. endoplasmic or sarcoplasmic reticulum (ER and SR). In skeletal muscle the mechanism is primarily mediated by a physical interaction between stromal interaction molecule-1 (STIM1), a Ca2+ sensor located in the SR membrane, and ORAI1, a Ca2+-permeable channel of external membranes, located in transverse tubules (TTs), the invaginations of the plasma membrane (PM) deputed to propagation of action potentials. It is generally accepted that in skeletal muscle SOCE is important to limit muscle fatigue during repetitive stimulation. We recently discovered that exercise promotes the assembly of new intracellular junctions that contains colocalized STIM1 and ORAI1, and that the presence of these new junctions increases Ca2+ entry via ORAI1, while improving fatigue resistance during repetitive stimulation. Based on these findings we named these new junctions Ca2+ Entry Units (CEUs). CEUs are dynamic organelles that assemble during muscle activity and disassemble during recovery thanks to the plasticity of the SR (containing STIM1) and the elongation/retraction of TTs (bearing ORAI1). Interestingly, similar structures described as SR stacks were previously reported in different mouse models carrying mutations in proteins involved in Ca2+ handling (calsequestrin-null mice; triadin and junctin null mice, etc.) or associated to microtubules (MAP6 knockout mice). Mutations in Stim1 and Orai1 (and calsequestrin-1) genes have been associated to tubular aggregate myopathy (TAM), a muscular disease characterized by: (a) muscle pain, cramping, or weakness that begins in childhood and worsens over time, and (b) the presence of large accumulations of ordered SR tubes (tubular aggregates, TAs) that do not contain myofibrils, mitochondria, nor TTs. Interestingly, TAs are also present in fast twitch muscle fibers of ageing mice. Several important issues remain un-answered: (a) the molecular mechanisms and signals that trigger the remodeling of membranes and the functional activation of SOCE during exercise are unclear; and (b) how dysfunctional SOCE and/or mutations in Stim1, Orai1 and calsequestrin (Casq1) genes lead to the formation of tubular aggregates (TAs) in aging and disease deserve investigation.

SEPN1-related myopathy (SEPN1-RM) is a muscle disorder due to mutations of the SEPN1 gene, which is characterized by muscle weakness and fatigue leading to scoliosis and life-threatening respiratory failure. Core lesions, focal areas of mitochondria depletion in skeletal muscle fibers, are the most common histopathological lesion. SEPN1-RM underlying mechanisms and the precise role of SEPN1 in muscle remained incompletely understood, hindering the development of biomarkers and therapies for this untreatable disease. To investigate the pathophysiological pathways in SEPN1-RM, we performed metabolic studies, calcium and ATP measurements, super-resolution and electron microscopy on in vivo and in vitro models of SEPN1 deficiency as well as muscle biopsies from SEPN1-RM patients. Mouse models of SEPN1 deficiency showed marked alterations in mitochondrial physiology and energy metabolism, suggesting that SEPN1 controls mitochondrial bioenergetics. Moreover, we found that SEPN1 was enriched at the mitochondria-associated membranes (MAM), and was needed for calcium transients between ER and mitochondria, as well as for the integrity of ER-mitochondria contacts. Consistently, loss of SEPN1 in patients was associated with alterations in body composition which correlated with the severity of muscle weakness, and with impaired ER-mitochondria contacts and low ATP levels. Our results indicate a role of SEPN1 as a novel MAM protein involved in mitochondrial bioenergetics. They also identify a systemic bioenergetic component in SEPN1-RM and establish mitochondria as a novel therapeutic target. This role of SEPN1 contributes to explain the fatigue and core lesions in skeletal muscle as well as the body composition abnormalities identified as part of the SEPN1-RM phenotype. Finally, these results point out to an unrecognized interplay between mitochondrial bioenergetics and ER homeostasis in skeletal muscle. They could therefore pave the way to the identification of biomarkers and therapeutic drugs for SEPN1-RM and for other disorders in which muscle ER-mitochondria cross-talk are impaired.

Store-operated Ca 2+ entry (SOCE), a ubiquitous mechanism that allows recovery of Ca 2+ ions from the extracellular space, has been proposed to limit fatigue during repetitive skeletal muscle activity. However, the subcellular location for SOCE in muscle fibers has not been unequivocally identified. Here we show that exercise drives a significant remodeling of the sarcotubular system to form previously unidentified junctions between the sarcoplasmic reticulum (SR) and transverse-tubules (TTs). We also demonstrate that these new SR-TT junctions contain the molecular machinery that mediate SOCE: stromal interaction molecule-1 (STIM1), which functions as the SR Ca 2+ sensor, and Orai1, the Ca 2+ -permeable channel in the TT. In addition, EDL muscles isolated from exercised mice exhibit an increased capability of maintaining contractile force during repetitive stimulation in the presence of 2.5 mM extracellular Ca 2+ , compared to muscles from control mice. This functional difference is significantly reduced by either replacement of extracellular Ca 2+ with Mg 2+ or the addition of SOCE inhibitors (BTP-2 and 2-APB). We propose that the new SR-TT junctions formed during exercise, and that contain STIM1 and Orai1, function as Ca 2+ Entry Units (CEUs) , structures that provide a pathway to rapidly recover Ca 2+ ions from the extracellular space during repetitive muscle activity.

Exercise promotes the formation of intracellular junctions in skeletal muscle between stacks of sarcoplasmic reticulum (SR) cisternae and extensions of transverse-tubules (TT) that increase co-localization of proteins required for store-operated Ca2+ entry (SOCE). Here, we report that SOCE, peak Ca2+ transient amplitude and muscle force production during repetitive stimulation are increased after exercise in parallel with the time course of TT association with SR-stacks. Unexpectedly, exercise also activated constitutive Ca2+ entry coincident with a modest decrease in total releasable Ca2+ store content. Importantly, this decrease in releasable Ca2+ store content observed after exercise was reversed by repetitive high-frequency stimulation, consistent with enhanced SOCE. The functional benefits of exercise on SOCE, constitutive Ca2+ entry and muscle force production were lost in mice with muscle-specific loss of Orai1 function. These results indicate that TT association with SR-stacks enhances Orai1-dependent SOCE to optimize Ca2+ dynamics and muscle contractile function during acute exercise.

Children are not fully able to associate severity and injury probability with cognitive, emotional, and social factors. This study focused on physical risk-taking by exploring the associations between risk propensity and physical balance during the developmental age. Specifically, sixteen girls aged 58 to 108 months (≈ 5 to 9 years), of whom 43.75% normal-weighted and all were active in rhythmic gymnastics, were subjected to a combination of physical tests (i.e., static and dynamic) and questionnaires to evaluate balance, sensation seeking, and risk propensity, respectively. Our results showed that the better the static balance, the higher the sensation seeking and risk propensity, while age had a negligible effect on these associations. Our study suggested that balance affects propensity towards risky behaviors in children.

Background Skeletal muscle is a plastic tissue that can adapt to different stimuli. It is well established that Mammalian Target of Rapamycin Complex 1 (mTORC1) signalling is a key modulator in mediating increases in skeletal muscle mass and function. However, the role of mTORC1 signalling in adult skeletal muscle homeostasis is still not well defined. Methods Inducible, muscle‐specific Raptor and mTOR k.o. mice were generated. Muscles at 1 and 7 months after deletion were analysed to assess muscle histology and muscle force. Results We found no change in muscle size or contractile properties 1 month after deletion. Prolonging deletion of Raptor to 7 months, however, leads to a very marked phenotype characterized by weakness, muscle regeneration, mitochondrial dysfunction, and autophagy impairment. Unexpectedly, reduced mTOR signalling in muscle fibres is accompanied by the appearance of markers of fibre denervation, like the increased expression of the neural cell adhesion molecule (NCAM). Both muscle‐specific deletion of mTOR or Raptor, or the use of rapamycin, was sufficient to induce 3–8% of NCAM‐positive fibres (P < 0.01), muscle fibrillation, and neuromuscular junction (NMJ) fragmentation in 24% of examined fibres (P < 0.001). Mechanistically, reactivation of autophagy with the small peptide Tat‐beclin1 is sufficient to prevent mitochondrial dysfunction and the appearance of NCAM‐positive fibres in Raptor k.o. muscles. Conclusions Our study shows that mTOR signalling in skeletal muscle fibres is critical for maintaining proper fibre innervation, preserving the NMJ structure in both the muscle fibre and the motor neuron. In addition, considering the beneficial effects of exercise in most pathologies affecting the NMJ, our findings suggest that part of these beneficial effects of exercise are through the well‐established activation of mTORC1 in skeletal muscle during and after exercise.

Among the numerous changes that occur in skeletal muscle during aging, the reduced regeneration potential after an injury is largely due to the impaired ability of satellite cells to proliferate and differentiate. Herein, using the freeze-fracture electron microscopy technique, we analyzed both the incidence and size of dihydropyridine receptors (DHPRs) tetrads (4 particles) in cultured myotubes from a young subject (28 years) after 9 days of differentiation and from an old subject (71 years) after 9 and 12 days of differentiation. Compared to young myotubes, at 9 days of differentiation old myotubes exhibited: i) a lower incidence and a smaller size of DHPR clusters and ii) a lower number of complete tetrads. At 12 days of differentiation values of incidence, size and number of complete tetrads in old myotubes were instead comparable with those of young myotubes at 9 days of differentiation. Collectively, these results indicate that in aged myotubes the synthesis process of the proteins involved in the excitation-contraction coupling mechanism, such as the DHPR, is somehow slowed, supporting previous studies evidence of a decrease in the differentiation potential of myotubes from elderly individuals.

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Creatine (Cr) is essential for cellular energy homeostasis, particularly in muscle and brain tissues. Creatine Transporter Deficiency (CTD), an X-linked disorder caused by mutations in the SLC6A8 gene, disrupts Cr transport, leading to intellectual disability, speech delay, autism, epilepsy, and various non-neurological symptoms. In addition to neurological alterations, Creatine Transporter knockout (CrT −/y ) mice exhibit severe muscle atrophy and functional impairments. This study provides the first characterization of the skeletal muscle phenotype in CrT −/y mice, revealing profound ultrastructural abnormalities accompanied by reduced fiber cross-sectional area and muscle performance. Notably, mitochondria are involved, as evidenced by disrupted cristae, increased mitochondrial size, impaired Ca 2+ uptake, reduced membrane potential and ATP production. Mechanistically, the expression of atrophy-specific E3 ubiquitin ligases and suppression of the IGF1-Akt/PKB pathway, regulated by mitochondrial Ca 2+ levels, further support the atrophic phenotype. These findings highlight the profound impact of Cr deficiency on skeletal muscle, emphasizing the need for targeted therapeutic strategies to address both the neurological and peripheral manifestations of CTD. Understanding the underlying mechanisms, particularly mitochondrial dysfunction, could lead to novel interventions for this disorder.