Postdevelopmental knockout of Orai1 improves muscle pathology in a mouse model of Duchenne muscular dystrophy

Loss of Calpain 3 dysregulates store-operated calcium entry and its exercise response in mice

Limb-Girdle Muscular Dystrophy R1/2A (LGMD R1/2A) is caused by mutations in the CAPN3 gene encoding Calpain 3, a skeletal-muscle specific, Ca2+-dependent protease. Localization of Calpain 3 within the triad suggests it contributes to Ca2+ homeostasis. Through live-cell Ca2+ measurements, muscle mechanics, immunofluorescence, and electron microscopy (EM) in Capn3 deficient (C3KO) and wild-type (WT) mice, we determined whether loss of Calpain 3 altered Store-Operated Calcium Entry (SOCE) activity. Direct Ca2+ influx measurements revealed loss of Capn3 elicits elevated resting SOCE and increased resting cytosolic Ca2+, supported by high incidence of calcium entry units (CEUs) observed by EM. C3KO and WT mice were subjected to a single bout of treadmill running to elicit SOCE. Within 1HR post-treadmill running, C3KO mice exhibited diminished force production in extensor digitorum longus muscles and a greater decay of Ca2+ transients in flexor digitorum brevis muscle fibers during repetitive stimulation. Striking evidence for impaired exercise-induced SOCE activation in C3KO mice included poor colocalization of key SOCE proteins, stromal-interacting molecule 1 (STIM1) and ORAI1, combined with disappearance of CEUs in C3KO muscles. These results demonstrate that Calpain 3 is a key regulator of SOCE in skeletal muscle and identify SOCE dysregulation as a contributing factor to LGMD R1/2A pathology.

Constitutive, Muscle-Specific Orai1 Knockout Results in the Incomplete Assembly of Ca2+ Entry Units and a Reduction in the Age-Dependent Formation of Tubular Aggregates

Store-operated Ca2+ entry (SOCE) is a ubiquitous cellular mechanism that cells use to activate extracellular Ca2+ entry when intracellular Ca2+ stores are depleted. In skeletal muscle, SOCE occurs within Ca2+ entry units (CEUs), intracellular junctions between stacks of SR membranes containing STIM1 and transverse tubules (TTs) containing ORAI1. Gain-of-function mutations in STIM1 and ORAI1 are linked to tubular aggregate (TA) myopathy, a disease characterized by the atypical accumulation of tubes of SR origin. Moreover, SOCE and TAs are increased in the muscles of aged male mice. Here, we assessed the longitudinal effects (from 4-6 months to 10-14 months of age) of constitutive, muscle-specific Orai1 knockout (cOrai1 KO) on skeletal muscle structure, function, and the assembly of TAs and CEUs. The results from these studies indicate that cOrai1 KO mice exhibit a shorter lifespan, reduced body weight, exercise intolerance, decreased muscle-specific force and rate of force production, and an increased number of structurally damaged mitochondria. In addition, electron microscopy analyses revealed (i) the absence of TAs with increasing age and (ii) an increased number of SR stacks without adjacent TTs (i.e., incomplete CEUs) in cOrai1 KO mice. The absence of TAs is consistent with TAs being formed as a result of excessive ORAI1-dependent Ca2+ entry.

Exploring the Gut Microbiota-Muscle Axis in Duchenne Muscular Dystrophy

The gut microbiota plays a pivotal role in maintaining the dynamic balance of intestinal epithelial and immune cells, crucial for overall organ homeostasis. Dysfunctions in these intricate relationships can lead to inflammation and contribute to the pathogenesis of various diseases. Recent findings uncovered the existence of a gut-muscle axis, revealing how alterations in the gut microbiota can disrupt regulatory mechanisms in muscular and adipose tissues, triggering immune-mediated inflammation. In the context of Duchenne muscular dystrophy (DMD), alterations in intestinal permeability stand as a potential origin of molecules that could trigger muscle degeneration via various pathways. Metabolites produced by gut bacteria, or fragments of bacteria themselves, may have the ability to migrate from the gut into the bloodstream and ultimately infiltrate distant muscle tissues, exacerbating localized pathologies. These insights highlight alternative pathological pathways in DMD beyond the musculoskeletal system, paving the way for nutraceutical supplementation as a potential adjuvant therapy. Understanding the complex interplay between the gut microbiota, immune system, and muscular health offers new perspectives for therapeutic interventions beyond conventional approaches to efficiently counteract the multifaceted nature of DMD.

Loss of calpain 3 dysregulates store-operated calcium entry and its exercise response in mice

Limb-Girdle Muscular Dystrophy 2A (LGMD2A) is caused by mutations in the CAPN3 gene encoding Calpain 3, a skeletal-muscle specific, Ca2+-dependent protease. Localization of Calpain 3 within the triad suggests it contributes to Ca2+ homeostasis. Through live-cell Ca2+ measurements, muscle mechanics, immunofluorescence, and electron microscopy (EM) in Capn3 deficient (C3KO) and wildtype (WT) mice, we determined if loss of Calpain 3 altered Store-Operated Calcium Entry (SOCE) activity. Direct Ca2+ influx measurements revealed loss of Capn3 elicits elevated resting SOCE and increased resting cytosolic Ca2+, supported by high incidence of calcium entry units (CEUs) observed by EM. C3KO and WT mice were subjected to a single bout of treadmill running to elicit SOCE. Within 1HR post-treadmill running, C3KO mice exhibited diminished force production in extensor digitorum longus muscles and a greater decay of Ca2+ transients in flexor digitorum brevis muscle fibers during repetitive stimulation. Striking evidence for impaired exercise-induced SOCE activation in C3KO mice included poor colocalization of key SOCE proteins, stromal-interacting molecule 1 (STIM1) and ORAI1, combined with disappearance of CEUs in C3KO muscles. These results demonstrate that Calpain 3 is a key regulator of SOCE in skeletal muscle and identify SOCE dysregulation as a contributing factor to LGMD2A pathology.

Longitudinal assessment of skeletal muscle functional mechanics in the DE50-MD dog model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin (DMD) gene, is associated with fatal muscle degeneration and atrophy. Patients with DMD have progressive reductions in skeletal muscle strength and resistance to eccentric muscle stretch. Using the DE50-MD dog model of DMD, we assessed tibiotarsal joint (TTJ) flexor and extensor force dynamics, and the resistance of dystrophic muscle to eccentric stretch. Male DE50-MD and wild-type (WT) dogs were analysed every 3 months until 18 months of age. There was an age-associated decline in eccentric contraction resistance in DE50-MD TTJ flexors that discriminated, with high statistical power, WT from DE50-MD individuals. For isometric contraction, at the majority of timepoints, DE50-MD dogs had lower maximum absolute and relative TTJ flexor force, reduced TTJ muscle contraction times and prolonged relaxation compared to those in WT dogs. Cranial tibial muscles, the primary TTJ flexor, of 18-month-old DE50-MD dogs had significant numbers of regenerating fibres as expected, but also fewer type I fibres and more hybrid fibres than those in WT dogs. We conclude that these parameters, in particular, the eccentric contraction decrement, could be used as objective outcome measures for pre-clinical assessment in DE50-MD dogs.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

Rapid restitution of contractile dysfunction by synthetic copolymers in dystrophin-deficient single live skeletal muscle fibers

Duchenne muscular dystrophy (DMD) is caused by the lack of dystrophin, a cytoskeletal protein essential for the preservation of the structural integrity of the muscle cell membrane. DMD patients develop severe skeletal muscle weakness, degeneration, and early death. We tested here amphiphilic synthetic membrane stabilizers in mdx skeletal muscle fibers (flexor digitorum brevis; FDB) to determine their effectiveness in restoring contractile function in dystrophin-deficient live skeletal muscle fibers. After isolating FDB fibers via enzymatic digestion and trituration from thirty-three adult male mice (9 C57BL10, 24 mdx), these were plated on a laminin-coated coverslip and treated with poloxamer 188 (P188; PEO75-PPO30-PEO75; 8400 g/mol), architecturally inverted triblock (PPO15-PEO200-PPO15, 10,700 g/mol), and diblock (PEO75-PPO16-C4, 4200 g/mol) copolymers. We assessed the twitch kinetics of sarcomere length (SL) and intracellular Ca2+ transient by Fura-2AM by field stimulation (25 V, 0.2 Hz, 25 °C). Twitch contraction peak SL shortening of mdx FDB fibers was markedly depressed to 30% of the dystrophin-replete control FDB fibers from C57BL10 (P < 0.001). Compared to vehicle-treated mdx FDB fibers, copolymer treatment robustly and rapidly restored the twitch peak SL shortening (all P < 0.05) by P188 (15 μM =  + 110%, 150 μM =  + 220%), diblock (15 μM =  + 50%, 150 μM =  + 50%), and inverted triblock copolymer (15 μM =  + 180%, 150 μM =  + 90%). Twitch peak Ca2+ transient from mdx FDB fibers was also depressed compared to C57BL10 FDB fibers (P < 0.001). P188 and inverted triblock copolymer treatment of mdx FDB fibers increased the twitch peak Ca2+ transient (P < 0.001). This study shows synthetic block copolymers with varied architectures can rapidly and highly effectively enhance contractile function in live dystrophin-deficient skeletal muscle fibers.

Searching for Mechanisms Underlying the Assembly of Calcium Entry Units: The Role of Temperature and pH

Store-operated Ca2+ entry (SOCE) is a mechanism that allows muscle fibers to recover external Ca2+, which first enters the cytoplasm andthen, via SERCA pump, also refills the depleted intracellular stores (i.e., the sarcoplasmic reticulum, SR). We recently discovered that SOCE is mediated by Calcium Entry Units (CEUs), intracellular junctions formed by: (i) SR stacks containing STIM1; and (ii) I-band extensions of the transverse tubule (TT) containing Orai1. The number and size of CEUs increase during prolonged muscle activity, though the mechanisms underlying exercise-dependent formation of new CEUs remain to be elucidated. Here, we first subjected isolated extensor digitorum longus (EDL) muscles from wild type mice to an exvivo exercise protocol and verified that functional CEUs can assemble alsoin the absence of blood supply and innervation. Then, we evaluated whetherparameters that are influenced by exercise, such as temperature and pH, may influence the assembly of CEUs. Results collected indicate that higher temperature (36 °C vs. 25 °C) and lower pH (7.2 vs. 7.4) increase the percentage of fibers containing SR stacks, the n. of SR stacks/area, and the elongation of TTs at the I band. Functionally, assembly of CEUs at higher temperature (36 °C) or at lower pH (7.2) correlates with increased fatigue resistance of EDL muscles in the presence of extracellular Ca2+. Taken together, these results indicate that CEUs can assemble in isolated EDL muscles and that temperature and pH are two of the possible regulators of CEU formation.

Ion Channels of the Sarcolemma and Intracellular Organelles in Duchenne Muscular Dystrophy: A Role in the Dysregulation of Ion Homeostasis and a Possible Target for Therapy

Duchenne muscular dystrophy (DMD) is caused by the absence of the dystrophin protein and a properly functioning dystrophin-associated protein complex (DAPC) in muscle cells. DAPC components act as molecular scaffolds coordinating the assembly of various signaling molecules including ion channels. DMD shows a significant change in the functioning of the ion channels of the sarcolemma and intracellular organelles and, above all, the sarcoplasmic reticulum and mitochondria regulating ion homeostasis, which is necessary for the correct excitation and relaxation of muscles. This review is devoted to the analysis of current data on changes in the structure, functioning, and regulation of the activity of ion channels in striated muscles in DMD and their contribution to the disruption of muscle function and the development of pathology. We note the prospects of therapy based on targeting the channels of the sarcolemma and organelles for the correction and alleviation of pathology, and the problems that arise in the interpretation of data obtained on model dystrophin-deficient objects.

Orai1 as a potential "fits-all approach" therapeutic target for the treatment of DMD

Obliterating skeletal muscle Orai1 expression improves muscle pathology in the mdx mouse model of DMD—could Orai1 emerge as a potential drug target?