Deaccelerated Myogenesis and Autophagy in Genetically Induced Pulmonary Emphysema

Marine-Derived Bioactive Compounds: A Promising Strategy for Ameliorating Skeletal Muscle Dysfunction in COPD

Chronic obstructive pulmonary disease (COPD) is frequently accompanied by skeletal muscle dysfunction, a critical and severe extrapulmonary complication. This dysfunction contributes to reduced exercise capacity, increased frequency of acute exacerbations, and elevated mortality, serving as an independent risk factor for poor prognosis in COPD patients. Owing to the unique physicochemical conditions of the marine environment, marine-derived bioactive compounds exhibit potent anti-inflammatory and antioxidant properties, demonstrating therapeutic potential for ameliorating COPD skeletal muscle dysfunction. This review summarizes marine-derived bioactive compounds with promising efficacy against skeletal muscle dysfunction in COPD, including polysaccharides, lipids, polyphenols, peptides, and carotenoids. The discussed compounds have shown bioactivities in promoting skeletal muscle health and suppressing muscle atrophy, thereby providing potential strategies for the prevention and treatment of COPD skeletal muscle dysfunction. These findings may expand the therapeutic strategies for managing COPD skeletal muscle dysfunction.

Autophagy in skeletal muscle dysfunction of chronic obstructive pulmonary disease: implications, mechanisms, and perspectives

Skeletal muscle dysfunction is a common extrapulmonary comorbidity of chronic obstructive pulmonary disease (COPD) and is associated with decreased quality-of-life and survival in patients. The autophagy lysosome pathway is one of the proteolytic systems that significantly affect skeletal muscle structure and function. Intriguingly, both promoting and inhibiting autophagy have been observed to improve COPD skeletal muscle dysfunction, yet the mechanism is unclear. This paper first reviewed the effects of macroautophagy and mitophagy on the structure and function of skeletal muscle in COPD, and then explored the mechanism of autophagy mediating the dysfunction of skeletal muscle in COPD. The results showed that macroautophagy- and mitophagy-related proteins were significantly increased in COPD skeletal muscle. Promoting macroautophagy in COPD improves myogenesis and replication capacity of muscle satellite cells, while inhibiting macroautophagy in COPD myotubes increases their diameters. Mitophagy helps to maintain mitochondrial homeostasis by removing impaired mitochondria in COPD. Autophagy is a promising target for improving COPD skeletal muscle dysfunction, and further research should be conducted to elucidate the specific mechanisms by which autophagy mediates COPD skeletal muscle dysfunction, with the aim of enhancing our understanding in this field.

ADAMTS4 Reduction Contributes to Extracellular Matrix Deposition and Impaired Myogenesis in the Skeletal Muscle of Cigarette Smoke-Exposed Mice

Background: The extracellular matrix (ECM) plays a critical role in the proper regeneration of skeletal muscle. ECM remodeling has been reported in the skeletal muscle of chronic obstructive pulmonary disease (COPD), while the mechanisms remain poorly understood. Methods: In this study, we examined the dynamic interplay between ECM components and ECM enzymes in COPD skeletal muscle and cigarette smoke (CS) extract-treated C2C12 cells. C2C12 cells were further used to evaluate the role of a disintegrin and metalloproteinase with thrombospondin motif 4 (ADAMTS4) in ECM remodeling and myogenesis. Results: Chronic CS exposure induced the development of COPD and comorbid sarcopenia in C57BL/6J mice. Muscle fibrosis was observed in the gastrocnemius muscle of CS-exposed mice, accompanied by an upregulation of protein expression but a downregulation of mRNA levels of fibronectin and versican. We found that the discrepancy of mRNA and protein expression was attributed to the aberrant secretion of some ECM enzymes belonging to matrix metalloproteinases and ADAMTS proteases, especially ADAMTS4. CS exposure reduced ADAMTS4 expression in gastrocnemius muscles and C2C12 cells, and Adamts4 knockdown induced fibronectin and versican accumulation and impeded myogenic process. Conclusions: Considering that recent studies have indicated an impaired skeletal muscle regeneration in COPD, we suggested that the restrained production of ADAMTS4 in response to CS could be involved in the damaged muscle regeneration through regulating skeletal muscle ECM in COPD. Targeting ECM enzymes may benefit the rehabilitation of COPD-related sarcopenia.

Rapamycin improves satellite cells' autophagy and muscle regeneration during hypercapnia

Both CO2 retention, or hypercapnia, and skeletal muscle dysfunction predict higher mortality in critically ill patients. Mechanistically, muscle injury and reduced myogenesis contribute to critical illness myopathy, and while hypercapnia causes muscle wasting, no research has been conducted on hypercapnia-driven dysfunctional myogenesis in vivo. Autophagy flux regulates myogenesis by supporting skeletal muscle stem cell - satellite cell - activation, and previous data suggest that hypercapnia inhibits autophagy. We tested whether hypercapnia worsens satellite cell autophagy flux and myogenic potential and if autophagy induction reverses these deficits. Satellite cell transplantation and lineage-tracing experiments showed that hypercapnia undermined satellite cells' activation, replication, and myogenic capacity. Bulk and single-cell sequencing analyses indicated that hypercapnia disrupts autophagy, senescence, and other satellite cell programs. Autophagy activation was reduced in hypercapnic cultured myoblasts, and autophagy genetic knockdown phenocopied these changes in vitro. Rapamycin stimulation led to AMPK activation and downregulation of the mTOR pathway, which are both associated with accelerated autophagy flux and cell replication. Moreover, hypercapnic mice receiving rapamycin showed improved satellite cell autophagy flux, activation, replication rate, and posttransplantation myogenic capacity. In conclusion, we have shown that hypercapnia interferes with satellite cell activation, autophagy flux, and myogenesis, and systemic rapamycin administration improves these outcomes.

Multi-Omics Analysis Identified Drug Repurposing Targets for Chronic Obstructive Pulmonary Disease

Despite recent advances in chronic obstructive pulmonary disease (COPD) research, few studies have identified the potential therapeutic targets systematically by integrating multiple-omics datasets. This project aimed to develop a systems biology pipeline to identify biologically relevant genes and potential therapeutic targets that could be exploited to discover novel COPD treatments via drug repurposing or de novo drug discovery. A computational method was implemented by integrating multi-omics COPD data from unpaired human samples of more than half a million subjects. The outcomes from genome, transcriptome, proteome, and metabolome COPD studies were included, followed by an in silico interactome and drug-target information analysis. The potential candidate genes were ranked by a distance-based network computational model. Ninety-two genes were identified as COPD signature genes based on their overall proximity to signature genes on all omics levels. They are genes encoding proteins involved in extracellular matrix structural constituent, collagen binding, protease binding, actin-binding proteins, and other functions. Among them, 70 signature genes were determined to be druggable targets. The in silico validation identified that the knockout or over-expression of SPP1, APOA1, CTSD, TIMP1, RXFP1, and SMAD3 genes may drive the cell transcriptomics to a status similar to or contrasting with COPD. While some genes identified in our pipeline have been previously associated with COPD pathology, others represent possible new targets for COPD therapy development. In conclusion, we have identified promising therapeutic targets for COPD. This hypothesis-generating pipeline was supported by unbiased information from available omics datasets and took into consideration disease relevance and development feasibility.

Succinate dehydrogenase-complex II regulates skeletal muscle cellular respiration and contractility but not muscle mass in genetically induced pulmonary emphysema

Reduced skeletal muscle mass and oxidative capacity coexist in patients with pulmonary emphysema and are independently associated with higher mortality. If reduced cellular respiration contributes to muscle atrophy in that setting remains unknown. Using a mouse with genetically induced pulmonary emphysema that recapitulates muscle dysfunction, we found that reduced activity of succinate dehydrogenase (SDH) is a hallmark of its myopathic changes. We generated an inducible, muscle-specific SDH knockout mouse that demonstrates lower mitochondrial oxygen consumption, myofiber contractility, and exercise endurance. Respirometry analyses show that in vitro complex I respiration is unaffected by loss of SDH subunit C in muscle mitochondria, which is consistent with the pulmonary emphysema animal data. SDH knockout initially causes succinate accumulation associated with a down-regulated transcriptome but modest proteome effects. Muscle mass, myofiber type composition, and overall body mass constituents remain unaltered in the transgenic mice. Thus, while SDH regulates myofiber respiration in experimental pulmonary emphysema, it does not control muscle mass or other body constituents.

Chronic hypoxia impairs skeletal muscle repair via HIF-2α stabilization

BACKGROUND

Chronic hypoxia and skeletal muscle atrophy commonly coexist in patients with COPD and CHF, yet the underlying physio-pathological mechanisms remain elusive. Muscle regeneration, driven by muscle stem cells (MuSCs), holds therapeutic potential for mitigating muscle atrophy. This study endeavours to investigate the influence of chronic hypoxia on muscle regeneration, unravel key molecular mechanisms, and explore potential therapeutic interventions.

METHODS

Experimental mice were exposed to prolonged normobaric hypoxic air (15% pO2, 1 atm, 2 weeks) to establish a chronic hypoxia model. The impact of chronic hypoxia on body composition, muscle mass, muscle strength, and the expression levels of hypoxia-inducible factors HIF-1α and HIF-2α in MuSC was examined. The influence of chronic hypoxia on muscle regeneration, MuSC proliferation, and the recovery of muscle mass and strength following cardiotoxin-induced injury were assessed. The muscle regeneration capacities under chronic hypoxia were compared between wildtype mice, MuSC-specific HIF-2α knockout mice, and mice treated with HIF-2α inhibitor PT2385, and angiotensin converting enzyme (ACE) inhibitor lisinopril. Transcriptomic analysis was performed to identify hypoxia- and HIF-2α-dependent molecular mechanisms. Statistical significance was determined using analysis of variance (ANOVA) and Mann-Whitney U tests.

RESULTS

Chronic hypoxia led to limb muscle atrophy (EDL: 17.7%, P < 0.001; Soleus: 11.5% reduction in weight, P < 0.001) and weakness (10.0% reduction in peak-isometric torque, P < 0.001), along with impaired muscle regeneration characterized by diminished myofibre cross-sectional areas, increased fibrosis (P < 0.001), and incomplete strength recovery (92.3% of pre-injury levels, P < 0.05). HIF-2α stabilization in MuSC under chronic hypoxia hindered MuSC proliferation (26.1% reduction of MuSC at 10 dpi, P < 0.01). HIF-2α ablation in MuSC mitigated the adverse effects of chronic hypoxia on muscle regeneration and MuSC proliferation (30.9% increase in MuSC numbers at 10 dpi, P < 0.01), while HIF-1α ablation did not have the same effect. HIF-2α stabilization under chronic hypoxia led to elevated local ACE, a novel direct target of HIF-2α. Notably, pharmacological interventions with PT2385 or lisinopril enhanced muscle regeneration under chronic hypoxia (PT2385: 81.3% increase, P < 0.001; lisinopril: 34.6% increase in MuSC numbers at 10 dpi, P < 0.05), suggesting their therapeutic potential for alleviating chronic hypoxia-associated muscle atrophy.

CONCLUSIONS

Chronic hypoxia detrimentally affects skeletal muscle regeneration by stabilizing HIF-2α in MuSC and thereby diminishing MuSC proliferation. HIF-2α increases local ACE levels in skeletal muscle, contributing to hypoxia-induced regenerative deficits. Administration of HIF-2α or ACE inhibitors may prove beneficial to ameliorate chronic hypoxia-associated muscle atrophy and weakness by improving muscle regeneration under chronic hypoxia.

HDAC9 inhibition reduces skeletal muscle atrophy and enhances regeneration in mice with cigarette smoke-induced COPD

Cigarette smoke (CS) is the major risk factor for chronic obstructive pulmonary disease (COPD), and sarcopenia is one of the significant comorbidities of COPD. However, the pathogenesis of CS-related deficient skeletal muscle regeneration has yet to be clarified. The impact of CS on myoblast differentiation was examined, and then we determined which HDAC influenced the myogenic process and muscle atrophy in vitro and in vivo. Finally, we further investigated the potential mechanisms via RNA sequencing. Long-term CS exposure activated skeletal muscle primary satellite cells (SCs) while inhibiting differentiation, and defective myogenesis was also observed in C2C12 cells treated with CS extract (CSE). The level of HDAC9 changed in vitro and in vivo in CS exposure models as well as COPD patients, as detected by bioinformatics analysis. Our data showed that CSE impaired myogenic capacity and myotube formation in C2C12 cells via HDAC9. Moreover, inhibition of HDAC9 in mice exposed to CS prevented skeletal muscle dysfunction and promoted SC differentiation. The results of RNA-Seq analysis and verification indicated that HDAC9 knockout improved muscle differentiation in CS-exposed mice, probably by acting on the AKT/mTOR pathway and inhibiting the P53/P21 pathway. More importantly, the serum of HDAC9 KO mice exposed to CS alleviated the differentiation impairment of C2C12 cells caused by serum intervention in CS-exposed mice, and this effect was inhibited by LY294002 (an AKT/mTOR pathway inhibitor). These results suggest that HDAC9 plays an essential role in the defective regeneration induced by chronic exposure to CS.

MuSCs and IPCs: roles in skeletal muscle homeostasis, aging and injury

Skeletal muscle is a highly specialized tissue composed of myofibres that performs crucial functions in movement and metabolism. In response to external stimuli and injuries, a range of stem/progenitor cells, with muscle stem cells or satellite cells (MuSCs) being the predominant cell type, are rapidly activated to repair and regenerate skeletal muscle within weeks. Under normal conditions, MuSCs remain in a quiescent state, but become proliferative and differentiate into new myofibres in response to injury. In addition to MuSCs, some interstitial progenitor cells (IPCs) such as fibro-adipogenic progenitors (FAPs), pericytes, interstitial stem cells expressing PW1 and negative for Pax7 (PICs), muscle side population cells (SPCs), CD133-positive cells and Twist2-positive cells have been identified as playing direct or indirect roles in regenerating muscle tissue. Here, we highlight the heterogeneity, molecular markers, and functional properties of these interstitial progenitor cells, and explore the role of muscle stem/progenitor cells in skeletal muscle homeostasis, aging, and muscle-related diseases. This review provides critical insights for future stem cell therapies aimed at treating muscle-related diseases.

Autophagy in sarcopenia: Possible mechanisms and novel therapies

With global population aging, age-related diseases, especially sarcopenia, have attracted much attention in recent years. Characterized by low muscle strength, low muscle quantity or quality and low physical performance, sarcopenia is one of the major factors associated with an increased risk of falls and disability. Much effort has been made to understand the cellular biological and physiological mechanisms underlying sarcopenia. Autophagy is an important cellular self-protection mechanism that relies on lysosomes to degrade misfolded proteins and damaged organelles. Research designed to obtain new insight into human diseases from the autophagic aspect has been carried out and has made new progress, which encourages relevant studies on the relationship between autophagy and sarcopenia. Autophagy plays a protective role in sarcopenia by modulating the regenerative capability of satellite cells, relieving oxidative stress and suppressing the inflammatory response. This review aims to reveal the specific interaction between sarcopenia and autophagy and explore possible therapies in hopes of encouraging more specific research in need and unlocking novel promising therapies to ameliorate sarcopenia.

Main Pathogenic Mechanisms and Recent Advances in COPD Peripheral Skeletal Muscle Wasting

Chronic obstructive pulmonary disease (COPD) is a worldwide prevalent respiratory disease mainly caused by tobacco smoke exposure. COPD is now considered as a systemic disease with several comorbidities. Among them, skeletal muscle dysfunction affects around 20% of COPD patients and is associated with higher morbidity and mortality. Although the histological alterations are well characterized, including myofiber atrophy, a decreased proportion of slow-twitch myofibers, and a decreased capillarization and oxidative phosphorylation capacity, the molecular basis for muscle atrophy is complex and remains partly unknown. Major difficulties lie in patient heterogeneity, accessing patients' samples, and complex multifactorial process including extrinsic mechanisms, such as tobacco smoke or disuse, and intrinsic mechanisms, such as oxidative stress, hypoxia, or systemic inflammation. Muscle wasting is also a highly dynamic process whose investigation is hampered by the differential protein regulation according to the stage of atrophy. In this review, we report and discuss recent data regarding the molecular alterations in COPD leading to impaired muscle mass, including inflammation, hypoxia and hypercapnia, mitochondrial dysfunction, diverse metabolic changes such as oxidative and nitrosative stress and genetic and epigenetic modifications, all leading to an impaired anabolic/catabolic balance in the myocyte. We recapitulate data concerning skeletal muscle dysfunction obtained in the different rodent models of COPD. Finally, we propose several pathways that should be investigated in COPD skeletal muscle dysfunction in the future.

Cellular interplay in skeletal muscle regeneration and wasting: insights from animal models

Skeletal muscle wasting, whether related to physiological ageing, muscle disuse or to an underlying chronic disease, is a key determinant to quality of life and mortality. However, cellular basis responsible for increased catabolism in myocytes often remains unclear. Although myocytes represent the vast majority of skeletal muscle cellular population, they are surrounded by numerous cells with various functions. Animal models, mostly rodents, can help to decipher the mechanisms behind this highly dynamic process, by allowing access to every muscle as well as time-course studies. Satellite cells (SCs) play a crucial role in muscle regeneration, within a niche also composed of fibroblasts and vascular and immune cells. Their proliferation and differentiation is altered in several models of muscle wasting such as cancer, chronic kidney disease or chronic obstructive pulmonary disease (COPD). Fibro-adipogenic progenitor cells are also responsible for functional muscle growth and repair and are associated in disease to muscle fibrosis such as in chronic kidney disease. Other cells have recently proven to have direct myogenic potential, such as pericytes. Outside their role in angiogenesis, endothelial cells and pericytes also participate to healthy muscle homoeostasis by promoting SC pool maintenance (so-called myogenesis-angiogenesis coupling). Their role in chronic diseases muscle wasting has been less studied. Immune cells are pivotal for muscle repair after injury: Macrophages undergo a transition from the M1 to the M2 state along with the transition between the inflammatory and resolutive phase of muscle repair. T regulatory lymphocytes promote and regulate this transition and are also able to activate SC proliferation and differentiation. Neural cells such as terminal Schwann cells, motor neurons and kranocytes are notably implicated in age-related sarcopenia. Last, newly identified cells in skeletal muscle, such as telocytes or interstitial tenocytes could play a role in tissular homoeostasis. We also put a special focus on cellular alterations occurring in COPD, a chronic and highly prevalent respiratory disease mainly linked to tobacco smoke exposure, where muscle wasting is strongly associated with increased mortality, and discuss the pros and cons of animal models versus human studies in this context. Finally, we discuss resident cells metabolism and present future promising leads for research, including the use of muscle organoids.

Impaired muscle stem cell function and abnormal myogenesis in acquired myopathies

Skeletal muscle possesses a high plasticity and a remarkable regenerative capacity that relies mainly on muscle stem cells (MuSCs). Molecular and cellular components of the MuSC niche, such as immune cells, play key roles to coordinate MuSC function and to orchestrate muscle regeneration. An abnormal infiltration of immune cells and/or imbalance of pro- and anti-inflammatory cytokines could lead to MuSC dysfunctions that could have long lasting effects on muscle function. Different genetic variants were shown to cause muscular dystrophies that intrinsically compromise MuSC function and/or disturb their microenvironment leading to impaired muscle regeneration that contributes to disease progression. Alternatively, many acquired myopathies caused by comorbidities (e.g., cardiopulmonary or kidney diseases), chronic inflammation/infection, or side effects of different drugs can also perturb MuSC function and their microenvironment. The goal of this review is to comprehensively summarize the current knowledge on acquired myopathies and their impact on MuSC function. We further describe potential therapeutic strategies to restore MuSC regenerative capacity.

Mechanisms of spermidine-induced autophagy and geroprotection

Aging involves the systemic deterioration of all known cell types in most eukaryotes. Several recently discovered compounds that extend the healthspan and lifespan of model organisms decelerate pathways that govern the aging process. Among these geroprotectors, spermidine, a natural polyamine ubiquitously found in organisms from all kingdoms, prolongs the lifespan of fungi, nematodes, insects and rodents. In mice, it also postpones the manifestation of various age-associated disorders such as cardiovascular disease and neurodegeneration. The specific features of spermidine, including its presence in common food items, make it an interesting candidate for translational aging research. Here, we review novel insights into the geroprotective mode of action of spermidine at the molecular level, as we discuss strategies for elucidating its clinical potential.

RNA-Sequencing Reveals Upregulation and a Beneficial Role of Autophagy in Myoblast Differentiation and Fusion

Myoblast differentiation is a complex process whereby the mononuclear muscle precursor cells myoblasts express skeletal-muscle-specific genes and fuse with each other to form multinucleated myotubes. The objective of this study was to identify potentially novel mechanisms that mediate myoblast differentiation. We first compared transcriptomes in C2C12 myoblasts before and 6 days after induction of myogenic differentiation by RNA-seq. This analysis identified 11,046 differentially expressed genes, of which 5615 and 5431 genes were upregulated and downregulated, respectively, from before differentiation to differentiation. Functional enrichment analyses revealed that the upregulated genes were associated with skeletal muscle contraction, autophagy, and sarcomeres while the downregulated genes were associated with ribonucleoprotein complex biogenesis, mRNA processing, ribosomes, and other biological processes or cellular components. Western blot analyses showed an increased conversion of LC3-I to LC3-II protein during myoblast differentiation, further demonstrating the upregulation of autophagy during myoblast differentiation. Blocking the autophagic flux in C2C12 cells with chloroquine inhibited the expression of skeletal-muscle-specific genes and the formation of myotubes, confirming a positive role for autophagy in myoblast differentiation and fusion.

Impaired regenerative capacity contributes to skeletal muscle dysfunction in chronic obstructive pulmonary disease

Locomotor skeletal muscle dysfunction is a relevant comorbidity of chronic obstructive pulmonary disease (COPD) and is strongly associated with worse clinical outcomes including higher mortality. Over the last decades, a large body of literature helped characterize the process, defining the disruptive muscle phenotype caused by COPD that involves reduction in muscle mass, force-generation capacity, fatigue-tolerance, and regenerative potential following injury. A major limitation in the field has been the scarcity of well-calibrated animal models to conduct mechanistic research based on loss- and gain-of-function studies. This article provides an overall description of the process, the tools available to mechanistically investigate it, and the potential role of mitochondrially driven metabolic signals on the regulation muscle regeneration after injury in COPD. Finally, a description of future avenues to further expand on the area is proposed based on very recent evidence involving mitochondrial metabolic cues affecting myogenesis.