Muscle biology
Abstract
Sarcopenia burdens the elderly population through loss of muscle energy and mass, yet treatments to functionally rescue both parameters are missing. The glucocorticoid prednisone remodels muscle metabolism based on frequency of intake, but its mechanisms in sarcopenia are unknown. We found that once-weekly intermittent prednisone rescued muscle quality in aged 24-month-old mice to levels comparable to young 4-month-old mice. We discovered an age- and sex-independent glucocorticoid receptor transactivation program in muscle encompassing PGC1α and its co-factor Lipin1. Treatment coordinately improved mitochondrial abundance through isoform 1 and muscle mass through isoform 4 of the myocyte-specific PGC1α, which was required for the treatment-driven increase in carbon shuttling from glucose to amino acid biogenesis. We also probed the myocyte-specific Lipin1 as non-redundant factor coaxing PGC1α upregulation to the stimulation of both oxidative and anabolic effects. Our study unveils an aging-resistant druggable program in myocytes to coordinately rescue energy and mass in sarcopenia.
Authors
Ashok Daniel Prabakaran, Kevin McFarland, Karen Miz, Hima Bindu Durumutla, Kevin Piczer, Fadoua El Abdellaoui-Soussi, Hannah Latimer, Cole Werbrich, Hyun-Jy Chung, N. Scott Blair, Douglas P. Millay, Andrew J. Morris, Brendan Prideaux, Brian N. Finck, Mattia Quattrocelli
Abstract
Proline substitutions within the coiled-coil rod region of the β-myosin gene (MYH7) are the predominant mutations causing Laing distal myopathy (MPD1), an autosomal dominant disorder characterized by progressive weakness of distal/proximal muscles. We report that the MDP1 mutation R1500P, studied in what we believe to be the first mouse model for the disease, adversely affected myosin motor activity despite being in the structural rod domain that directs thick filament assembly. Contractility experiments carried out on isolated mutant muscles, myofibrils, and myofibers identified muscle fatigue and weakness phenotypes, an increased rate of actin-myosin detachment, and a conformational shift of the myosin heads toward the more reactive disordered relaxed (DRX) state, causing hypercontractility and greater ATP consumption. Similarly, molecular analysis of muscle biopsies from patients with MPD1 revealed a significant increase in sarcomeric DRX content, as observed in a subset of myosin motor domain mutations causing hypertrophic cardiomyopathy. Finally, oral administration of MYK-581, a small molecule that decreases the population of heads in the DRX configuration, significantly improved the limited running capacity of the R1500P-transgenic mice and corrected the increased DRX state of the myofibrils from patients. These studies provide evidence of the molecular pathogenesis of proline rod mutations and lay the groundwork for the therapeutic advancement of myosin modulators.
Authors
Massimo Buvoli, Genevieve C.K. Wilson, Ada Buvoli, Jack F. Gugel, Abbi Hau, Carsten G. Bönnemann, Carmen Paradas, David M. Ryba, Kathleen C. Woulfe, Lori A. Walker, Tommaso Buvoli, Julien Ochala, Leslie A. Leinwand
Abstract
Stromal interaction molecule 1 (STIM1) is a Ca2+ sensor located in the sarcoplasmic reticulum (SR) of skeletal muscle where it is best known for its role in store operated Ca2+ entry (SOCE). Genetic syndromes resulting from STIM1 mutations are recognized as a cause of muscle weakness and atrophy. Here, we focus on a gain of function mutation that occurs in humans and mice (STIM1+/D84G mice) where muscles exhibit constitutive SOCE. Unexpectedly, this constitutive SOCE did not affect global Ca2+ transients, SR Ca2+ content or excitation contraction coupling (ECC) and was therefore unlikely to underlie the reduced muscle mass and weakness observed in these mice. Instead, we demonstrate that the presence of D84G STIM1 in the nuclear envelope disrupts nuclear-cytosolic coupling causing severe derangement in nuclear architecture of STIM1+/D84G muscle, DNA damage and altered lamina A associated gene expression. Functionally, we found D84G STIM1 reduced the transfer of Ca2+ from the cytosol to the nucleus in myoblasts resulting in a reduction of [Ca2+]N. Taken together, we propose a novel role for STIM1 in the nuclear envelope that links Ca2+ signaling to nuclear stability in skeletal muscle.
Authors
Victoria Bryson, Chaojian Wang, Zirui Zhou, Kavisha Singh, Noah M. Volin, Eda Yildirim, Paul Rosenberg
Abstract
Myotonic dystrophy type 1 (DM1) involves misregulated alternative splicing for specific genes. We used exon or nucleotide deletion to mimic altered splicing of genes central to muscle excitation-contraction coupling in mice. Mice with forced skipping of exon 29 in the CaV1.1 calcium channel combined with loss of ClC-1 chloride channel function displayed markedly reduced lifespan, whereas other combinations of splicing mimics did not affect survival. The Ca2+/Cl– bi-channelopathy mice exhibited myotonia, weakness, and impairment of mobility and respiration. Chronic administration of the calcium channel blocker verapamil rescued survival and improved force generation, myotonia, and respiratory function. These results suggest that Ca2+/Cl– bi-channelopathy contributes to muscle impairment in DM1 and is potentially mitigated by common clinically available calcium channel blockers.
Authors
Lily A. Cisco, Matthew T. Sipple, Katherine M. Edwards, Charles A. Thornton, John D. Lueck
Abstract
Heterozygous (HET) truncating mutations in the TTN gene (TTNtv) encoding the giant titin protein are the most common genetic cause of dilated cardiomyopathy (DCM). However, the molecular mechanisms by which TTNtv mutations induce DCM are controversial. Here we investigated 127 clinically identified DCM human cardiac samples with next-generation sequencing (NGS), high-resolution gel electrophoresis, Western blot analysis and super-resolution microscopy in order to dissect the structural and functional consequences of TTNtv mutations. The occurrence of TTNtv was found to be 15% in the DCM cohort. Truncated titin proteins matching, by molecular weight, the gene-sequence predictions were detected in the majority of the TTNtv+ samples. Full length titin was reduced in TTNtv+ compared to TTNtv- samples. Proteomic analysis of washed myofibrils and Stimulated Emission Depletion (STED) super-resolution microscopy of myocardial sarcomeres labeled with sequence-specific anti-titin antibodies revealed that truncated titin is structurally integrated in the sarcomere. Sarcomere length-dependent anti-titin epitope position, shape and intensity analyses pointed at possible structural defects in the I/A junction and the M-band of TTNtv+ sarcomeres, which likely contribute, possibly via faulty mechanosensor function, to the development of manifest DCM.
Authors
Dalma Kellermayer, Hedvig Tordai, Balázs Kiss, György Török, Dániel M. Péter, Alex Ali Sayour, Miklós Pólos, István Hartyánszky, Bálint Szilveszter, Siegfried Labeit, Ambrus Gángó, Gábor Bedics, Csaba Bödör, Tamás Radovits, Bela Merkely, Miklós S.Z. Kellermayer
Abstract
X-linked myotubular myopathy (XLMTM) is a fatal congenital disorder caused by mutations in the MTM1 gene. Currently, there are no approved treatments, though AAV8-mediated gene transfer therapy has shown promise in animal models and preliminarily in patients. However, four patients with XLMTM treated with gene therapy have died from progressive liver failure, and hepatobiliary disease has now been recognized more broadly in association with XLMTM. In an attempt to understand whether loss of MTM1 itself is associated with liver pathology, we have characterized a novel liver phenotype in a zebrafish model of this disease. Specifically, we have found that loss-of-function mutations in mtm1 lead to severe liver abnormalities including impaired bile flux, structural abnormalities of the bile canaliculus, and improper endosomal-mediated trafficking of canalicular transporters. Using a reporter tagged Mtm1 zebrafish line, we have established localization of Mtm1 in the liver in association with Rab11 and canalicular transport proteins, and demonstrated that hepatocyte specific re-expression of Mtm1 can rescue the cholestatic phenotype. Lastly, we completed a targeted chemical screen, and found that Dynasore, a dynamin II inhibitor, is able to partially restore bile flow and transporter localization to the canalicular membrane. In summary, we demonstrate for the first time liver abnormalities that are directly caused by MTM1 mutation in a pre-clinical model, thus establishing the critical framework for better understanding and comprehensive treatment of the human disease.
Authors
Sophie Karolczak, Ashish R. Deshwar, Evangelina Aristegui, Binita M. Kamath, Michael W. Lawlor, Gaia Andreoletti, Jonathan R. Volpatti, Jillian L. Ellis, Chunyue Yin, James J. Dowling
Abstract
Mutations in genes encoding nuclear envelope proteins lead to diseases known as nuclear envelopathies, characterized by skeletal muscle and heart abnormalities, such as Emery-Dreifuss muscular dystrophy (EDMD). The tissue-specific role of the nuclear envelope in the etiology of these diseases has not been extensively explored. We previously showed that global deletion of the muscle-specific nuclear envelope protein NET39 in mice leads to neonatal lethality due to skeletal muscle dysfunction. To study the potential role of the Net39 gene in adulthood, we generated a muscle-specific conditional knockout (cKO) of Net39 in mice. cKO mice recapitulated key skeletal muscle features of EDMD, including muscle wasting, impaired muscle contractility, abnormal myonuclear morphology, and DNA damage. The loss of Net39 rendered myoblasts hypersensitive to mechanical stretch, resulting in stretch-induced DNA damage. Net39 was downregulated in a mouse model of congenital myopathy, and restoration of Net39 expression through AAV gene delivery extended life span and ameliorated muscle abnormalities. These findings establish NET39 as a direct contributor to the pathogenesis of EDMD that acts by protecting against mechanical stress and DNA damage.
Authors
Yichi Zhang, Andres Ramirez-Martinez, Kenian Chen, John R. McAnally, Chunyu Cai, Mateusz Z. Durbacz, Francesco Chemello, Zhaoning Wang, Lin Xu, Rhonda Bassel-Duby, Ning Liu, Eric N. Olson
Abstract
Understanding how skeletal muscle fiber proportions are regulated is vital to understanding muscle function. Oxidative and glycolytic skeletal muscle fibers differ in their contractile ability, mitochondrial activity, and metabolic properties. Fiber-type proportions vary in normal physiology and disease states, although the underlying mechanisms are unclear. In human skeletal muscle, we observed that markers of oxidative fibers and mitochondria correlated positively with expression levels of PPARGC1A and CDK4 and negatively with expression levels of CDKN2A, a locus significantly associated with type 2 diabetes. Mice expressing a constitutively active Cdk4 that cannot bind its inhibitor p16INK4a, a product of the CDKN2A locus, were protected from obesity and diabetes. Their muscles exhibited increased oxidative fibers, improved mitochondrial properties, and enhanced glucose uptake. In contrast, loss of Cdk4 or skeletal muscle–specific deletion of Cdk4’s target, E2F3, depleted oxidative myofibers, deteriorated mitochondrial function, and reduced exercise capacity, while increasing diabetes susceptibility. E2F3 activated the mitochondrial sensor PPARGC1A in a Cdk4-dependent manner. CDK4, E2F3, and PPARGC1A levels correlated positively with exercise and fitness and negatively with adiposity, insulin resistance, and lipid accumulation in human and rodent muscle. All together, these findings provide mechanistic insight into regulation of skeletal muscle fiber–specification that is of relevance to metabolic and muscular diseases.
Authors
Young Jae Bahn, Hariom Yadav, Paolo Piaggi, Brent S. Abel, Oksana Gavrilova, Danielle A. Springer, Ioannis Papazoglou, Patricia M. Zerfas, Monica C. Skarulis, Alexandra C. McPherron, Sushil G. Rane
Abstract
Genetic testing is essential for patients with a suspected hereditary myopathy. More than 50% of patients clinically diagnosed with a myopathy carry a variant of unknown significance in a myopathy gene, often leaving them without a genetic diagnosis. Limb-girdle muscular dystrophy (LGMD) type R4/2E is caused by mutations in β-sarcoglycan (SGCB). Together, β-, α-, γ-, and δ-sarcoglycan form a 4-protein transmembrane complex (SGC) that localizes to the sarcolemma. Biallelic loss-of-function mutations in any subunit can lead to LGMD. To provide functional evidence for the pathogenicity of missense variants, we performed deep mutational scanning of SGCB and assessed SGC cell surface localization for all 6,340 possible amino acid changes. Variant functional scores were bimodally distributed and perfectly predicted pathogenicity of known variants. Variants with less severe functional scores more often appeared in patients with slower disease progression, implying a relationship between variant function and disease severity. Amino acid positions intolerant to variation mapped to points of predicted SGC interactions, validated in silico structural models, and enabled accurate prediction of pathogenic variants in other SGC genes. These results will be useful for clinical interpretation of SGCB variants and improving diagnosis of LGMD; we hope they enable wider use of potentially life-saving gene therapy.
Authors
Chengcheng Li, Jackson Wilborn, Sara Pittman, Jil Daw, Jorge Alonso-Pérez, Jordi Díaz-Manera, Conrad C. Weihl, Gabe Haller
Abstract
Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by absence of the protein dystrophin, which acts as a structural link between the basal lamina and contractile machinery to stabilize muscle membranes from mechanical stress. In DMD, mechanical stress leads to exaggerated membrane injury and fiber breakdown, with fast fibers being the most susceptible to damage. A major contributor to this injury is muscle contraction, controlled by the motor protein myosin. However, the relationship between how muscle contraction and fast muscle fiber damage contribute to the pathophysiology of DMD has not been well characterized. We explored the role of fast skeletal muscle contraction in DMD with a novel, selective, orally active inhibitor of fast skeletal muscle myosin, EDG-5506. Surprisingly, even modest decreases of contraction (<15%) were sufficient to protect skeletal muscles in dystrophic mdx mice from stress injury. Longer-term treatment also decreased muscle fibrosis in key disease-implicated tissues. Importantly, therapeutic levels of myosin inhibition with EDG-5506 did not detrimentally affect strength or coordination. Finally, in dystrophic dogs, EDG-5506 reversibly reduced circulating muscle injury biomarkers and increased habitual activity. This unexpected biology may represent an important alternative treatment strategy for Duchenne and related myopathies.
Authors
Alan J. Russell, Mike DuVall, Benjamin Barthel, Ying Qian, Angela K. Peter, Breanne L. Newell-Stamper, Kevin Hunt, Sarah J. Lehman, Molly R. Madden, Stephen T. Schlachter, Benjamin D. Robertson, Ashleigh Van Deusen, Hector M. Rodriguez, Carlos D. Vera, Yu Su, Dennis R. Claflin, Susan V. Brooks, Peter P. Nghiem, Alexis Rutledge, Twlya I. Juehne, Jinsheng Yu, Elisabeth R. Barton, Yangyi E. Luo, Andreas Patsalos, Laszlo Nagy, H. Lee Sweeney, Leslie A. Leinwand, Kevin Koch
Copyright © 2025 American Society for Clinical Investigation
ISSN: 0021-9738 (print), 1558-8238 (online)