Steven S. Welc

Duchenne muscular dystrophy (DMD) is an X-linked disorder mainly affecting young boys characterized by repeated cycles of myofiber injury/regeneration and chronic muscle inflammation. Although glucocorticoids can slow disease progression, their potential long-term adverse effects highlight the need for new steroid-sparing therapies for DMD. Annexin A1 (ANXA1) is a glucocorticoid-induced pro-resolving protein that signals primarily through formyl peptide receptor 2 (FPR2). We investigated whether the ANXA1 mimetic peptide Ac2-26 could reduce inflammation, stimulate tissue regeneration, and improve skeletal muscle function in muscular dystrophy. 3-week-old dystrophin-deficient male mdx mice were randomized to receive daily intraperitoneal injections of vehicle control, deflazacort (1 mg/kg/day), or Ac2-26 (1 mg/kg/day) for a duration of 5-weeks. Body composition, skeletal muscle mass, contractile function, and tissue histopathology/immune-cell infiltration were assessed at 8-weeks of age. Direct effects on myogenesis were also examined in vitro using primary D2- mdx myoblasts. ANXA1 was expressed predominantly by myeloid cells rather than resident dystrophic muscle cells. Both deflazacort and Ac2-26 markedly reduced intramuscular inflammation and increased myofiber size. However, Ac2-26 produced broader anabolic and functional benefits, increasing body-weight gain, lean body mass, hindlimb muscle weights, and absolute contractile force. Ac2-26 treatment also directly promoted myogenesis of dystrophic myoblasts in vitro , while pharmacological FPR2 blockade impaired myotube development. Overall, Ac2-26 mimicked the anti-inflammatory effects of deflazacort while providing superior benefits for body weight and composition, skeletal muscle growth, myofiber regeneration, and absolute muscle strength. These findings support therapeutic targeting of the ANXA1–FPR2 axis as a potential safe and effective steroid-sparing immunomodulatory strategy for DMD.

Duchenne muscular dystrophy (DMD) is a fatal genetic disorder characterized by skeletal muscle degeneration and cardiomyopathy without a cure. This study examined the therapeutic potential of the sodium-glucose cotransporter 2 (SGLT2) inhibitor empagliflozin (EMPA) on cardiac function in the dystrophin-deficient mdx mouse model of DMD. Male mice were fed control chow or EMPA-containing chow (∼25 mg/kg/day), and cardiac function was evaluated longitudinally by four-dimensional ultrasound imaging. EMPA did not alter left ventricular mass or chamber volume but preserved ejection fraction (EF) for 12 weeks, maintained significantly higher EF through 24 weeks, and attenuated global impairment of systolic and diastolic myocardial deformation. These functional improvements were accompanied by reduced cardiomyocyte hypertrophy and decreased expression of cardiac stress genes. EMPA reduced mitochondrial DNA damage, increased mitochondrial DNA copy number, and induced transcriptional signatures consistent with enhanced fatty acid and ketone metabolism, contributing to increased myocardial ATP content. Systemically, EMPA improved body mass trajectory, preserved relative lean mass, enhanced skeletal muscle torque, and did not adversely affect renal function. Together, these findings demonstrate that EMPA improves cardiac performance and mitochondrial integrity while enhancing myocardial energy availability in mdx mice, supporting SGLT2 inhibitors as a promising therapeutic strategy for individuals with DMD.

Skeletal muscle fiber type composition affects muscle function, metabolism, and disease vulnerability. In addition, muscle fiber type analysis informs disease diagnosis and underlying pathophysiology. Multiple methodologies can be used to assess muscle fiber type; however, immunofluorescence (IF) for myosin heavy chain (MyHC) isoforms is the most widely used modern approach due to its relative ease, time‐effectiveness, single‐cell resolution, and capacity to preserve spatial positioning within the native tissue architecture. Here, we present a protocol for IF for MyHC labeling on formalin‐fixed paraffin‐embedded (FFPE) mouse and human muscle sections. We then describe a modified procedure for fiber type analysis of the intact mouse lower hindlimb, enabling high‐throughput muscle composition and morphological analysis across distinct muscles on a single tissue section. Traditionally, IF labeling for MyHC isoforms required fresh tissue flash‐frozen in liquid nitrogen–cooled isopentane, which, while effective, presents challenges for sample processing and preservation, long‐term storage, transport, and biosafety. Comparatively, embedding tissue in paraffin after formalin fixation streamlines clinical workflows, preserves morphology, improves long‐term sample stability, and simplifies sample storage and transport. Furthermore, FFPE effectively inactivates most infectious agents, which can be retained in frozen sections. Thus, FFPE samples are typically safe for standard laboratory handling and are not classified as biohazardous. This approach can be adapted for use with a range of downstream applications, including integration of fiber type analysis with emerging next‐generation techniques that favor FFPE samples. In sum, this method offers a robust alternative to traditional fresh‐frozen protocols and allows for simultaneous fiber type analysis across multiple muscle tissues. © 2025 The Author(s). Current Protocols published by Wiley Periodicals LLC. Basic Protocol : Multiplex Immunofluorescence for MyHC Labeling in FFPE Skeletal Muscle Tissues Alternate Protocol 1 : Immunofluorescence for MyHC 2x Labeling in FFPE Skeletal Muscle Tissues Alternate Protocol 2 : Multiplex Immunofluorescence for MyHC Labeling in FFPE Whole Hindlimb Sections

Muscle wasting and weakness are critical clinical problems that limit mobility and independence, reduce health span, and increase the risk of physical disability. The molecular basis for this has not been fully determined. Klotho expression is downregulated in conditions associated with muscle wasting, including aging, chronic kidney disease, and myopathy. The objective of this study was to investigate a mechanistic role for Klotho in regulating muscle wasting and weakness. Body weight, lean mass, muscle mass, and myofiber caliber were reduced in Klotho-deficient mice. In the tibialis anterior muscle of Klotho-null mice, type IIa myofibers were resistant to changes in size, and muscle composition differed with a higher concentration of type IIb fibers to the detriment of type IIx fibers. Glycolytic GPDH enzymatic activity also increased. Klotho-deficient mice showed impaired muscle contractility, with reduced twitch force, torque, and contraction–relaxation rates. RNA sequencing revealed upregulation of synaptic and fetal sarcomeric genes, prompting us to examine muscle innervation. Klotho deficiency led to neuromuscular junction remodeling, myofiber denervation, and functional motor unit loss. Loss of motor units correlated with absolute torque. Collectively, these findings reveal a novel mechanism through which systemic Klotho deficiency disrupts muscle synapses and motor unit connectivity, potentially contributing to muscle wasting and weakness.

Muscle wasting and weakness are important clinical problems that impact quality of life and health span by restricting mobility and independence, and by increasing the risk for physical disability. The molecular basis for this has not been fully determined. Klotho expression is downregulated in conditions associated with muscle wasting, including aging, chronic kidney disease, and myopathy. The objective of this study was to investigate a mechanistic role for Klotho in regulating muscle wasting and weakness. Body weight, lean mass, muscle mass, and myofiber caliber were reduced in Klotho-deficient mice. In the tibialis anterior muscle of Klotho null mice, type IIa myofibers were resistant to changes in size, and muscle composition differed with a higher concentration of type IIb fibers to the detriment of type IIx fibers. Glycolytic enzymatic activity also increased. The composition of the soleus muscle was unaffected and myofiber caliber was reduced comparably in type I, IIa, and IIx fibers. Muscle contractile function declined in Klotho-deficient mice, as evidenced by reduced absolute twitch and torque, and decreased rates of contraction and relaxation. RNA-sequencing analysis identified increased transcriptional expression of synaptic and fetal sarcomeric genes, which prompted us to test effects on muscle innervation. Klotho-deficiency induced morphological remodeling of the neuromuscular junction, myofiber denervation, and a functional loss of motor units. Loss of motor units correlated with absolute torque. Collectively, our findings have uncovered a novel mechanism through which Klotho-deficiency leads to alterations to the muscle synapse affecting motor unit connectivity that likely influences muscle wasting and weakness. Key points summary: Maintaining skeletal muscle mass and function is critical to preserve physical capacity and independence. Clinical observations implicate longevity factor ⍺Klotho as a key regulator of muscle mass and weakness. Low Klotho levels are reported to correlate with muscle weakness and frailty. Using Klotho null mice, our study shows that Klotho-deficiency promotes skeletal muscle weakness and impaired motor unit connectivity. RNA-sequencing analysis identified altered expression of sarcomeric and synaptic genes suggesting changes to the muscle synapse in Klotho-deficient mice. Histopathological analyses revealed Klotho-deficiency is associated with reduced myofiber caliber, altered muscle composition, and increased prevalence of NCAM+ denervated fibers. Imaging of the NMJ further showed morphological changes and reduced area of synaptic contact. Overall, our findings show that Klotho regulates the structure and function of the NMJ affecting motor unit connectivity which may have an important role in the pathogenesis of muscle wasting and weakness.

Insulin resistance and type 2 diabetes impair cellular regeneration in multiple tissues including skeletal muscle. The molecular basis for this impairment is largely unknown. Glucose uptake via glucose transporter GLUT4 is impaired in insulin resistance. In healthy muscle, acute injury stimulates glucose uptake. Whether decreased glucose uptake via GLUT4 impairs muscle regeneration is presently unknown. The goal of this study was to determine whether GLUT4 regulates muscle glucose uptake and/or regeneration following acute injury. Tibialis anterior and extensor digitorum longus muscles from wild-type, control, or muscle-specific GLUT4 knockout (mG4KO) mice were injected with the myotoxin barium chloride to induce muscle injury. After 3, 5, 7, 10, 14, or 21 days (in wild-type mice), or after 7 or 14 days (in control & mG4KO) mice, muscles were isolated to examine [3H]-2-deoxyglucose uptake, GLUT4 levels, extracellular fluid space, fibrosis, myofiber cross-sectional area, and myofiber centralized nuclei. In wild-type mice, muscle glucose uptake was increased 3, 5, 7, and 10 days post-injury. There was a rapid decrease in GLUT4 protein levels that were restored to baseline at 5–7 days post-injury, followed by a super-compensation at 10–21 days. In mG4KO mice, there were no differences in muscle glucose uptake, extracellular fluid space, muscle fibrosis, myofiber cross-sectional areas, or percentage of centrally nucleated myofibers at 7 days post-injury. In contrast, at 14 days injured muscles from mG4KO mice exhibited decreased glucose uptake, muscle weight, myofiber cross sectional areas, and centrally nucleated myofibers, with no change in extracellular fluid space or fibrosis. Collectively, these findings demonstrate that glucose uptake via GLUT4 regulates skeletal myofiber regeneration following acute injury.

Cardiomyopathy is the leading cause of death in Duchenne muscular dystrophy (DMD); however, in the mdx mouse model of DMD, the cardiac phenotype differs from that seen in DMD-associated cardiomyopathy. Although some have used pharmacologic stress to stimulate injury and enhance cardiac pathology in the mdx model, many methods lead to high mortality with variable cardiac outcomes, and do not recapitulate the structural and functional cardiac changes seen in human disease. Here, we describe a simple and effective method to enhance the cardiac phenotype model in mdx mice using advanced 2D and 4D high-frequency ultrasound to monitor cardiac dysfunction progression in vivo. mdx and wild-type mice received daily low-dose (2 mg/kg/day) isoproterenol injections for 10 days. Histopathological assessment showed that isoproterenol treatment increased myocyte injury, elevated serum cardiac troponin I levels and enhanced fibrosis in mdx mice. Ultrasound revealed reduced ventricular function, decreased wall thickness, increased volumes and diminished cardiac reserve in mdx compared to wild-type mice. Our findings highlight the utility of challenging mdx mice with low-dose isoproterenol as a valuable model for exploring therapies targeting DMD-associated cardiac pathologies.