Muscle stem cell adaptations to cellular and environmental stress

Exposome on skeletal muscle system: a mini-review

Exposomics is an ever-expanding field which captures the cumulative exposures to chemical, biological, physical, lifestyle, and social factors associated with biological responses. Since skeletal muscle is currently considered as the largest secretory organ and shows substantial plasticity over the life course, this reviews addresses the topic of exposome and skeletal muscle by reviewing the state-of-the-art evidence and the most intriguing perspectives. Muscle stem cells react to stressors via phosphorylated eukaryotic initiation factor 2α and tuberous sclerosis 1, and are sensible to hormetic factors via sirtuin 1. Microplastics can delay muscle regeneration via p38 mitogen-activated protein kinases and induce transdifferentiation to adipocytes via nuclear factor kappa B. Acrolein can inhibit myogenic differentiation and disrupt redox system. Heavy metals have been associated with reduced muscle strength in children. The deep study of pollutants and biological features can shed new light on neuromuscular pathophysiology. The analysis of a time-varying and dynamic exposome risk score from a panel of exposure and phenotypes of interest is promising. The systematization of hormetic factors and the role of the microbiota in modulating the effects of exposure on skeletal muscle responses are also promising. The comprehensive exposure assessment and its interactions with endogenous processes and the resulting biological effects deserve more efforts in the field of muscle health across the lifespan.

Mitochondrial stress response and myogenic differentiation

Regeneration and repair are prerequisites for maintaining effective function of skeletal muscle under high energy demands, and myogenic differentiation is one of the key steps in the regeneration and repair process. A striking feature of the process of myogenic differentiation is the alteration of mitochondria in number and function. Mitochondrial dysfunction can activate a number of transcriptional, translational and post-translational programmes and pathways to maintain cellular homeostasis under different types and degrees of stress, either through its own signaling or through constant signaling interactions with the nucleus and cytoplasm, a process known as the mitochondrial stress responses (MSRs). It is now believed that mitochondrial dysfunction is closely associated with a variety of muscle diseases caused by reduced levels of myogenic differentiation, suggesting the possibility that MSRs are involved in messaging during myogenic differentiation. Also, MSRs may be involved in myogenesis by promoting bioenergetic remodeling and assisting myoblast survival during myogenic differentiation. In this review, we will take MSRs as an entry point to explore its concrete regulatory mechanisms during myogenic differentiation, with a perspective to provide a theoretical basis for the treatment and repair of related muscle diseases.

Novel Bioresorbable Drug-Eluting Mesh Scaffold for Therapy of Muscle Injury

A novel bioresorbable drug-eluting polycaprolactone (PCL) mesh scaffold was developed, utilizing a solvent-cast additive manufacturing technique, to promote therapy of muscle injury. The degradation rate and mechanical properties strength of the PCL mesh were characterized after immersion in a buffer solution for different times. The in vitro release characteristics of vancomycin, ceftazidime, and lidocaine from the prepared mesh were evaluated using a high-performance liquid chromatography (HPLC) assay. In addition, the in vivo efficacy of PCL meshes for the repair of muscle injury was investigated on a rat model with histological examinations. It was found that the additively manufactured PCL meshes degraded by 13% after submission in buffered solution for four months. All PCL meshes with different pore sizes exhibited greater strength than rat muscle and survived through 10,000 cyclic loadings. Furthermore, the meshes could offer a sustained release of antibiotics and analgesics for more than 3 days in vitro. The results of this study suggest that drug-loaded PCL mesh exhibits superior ability to pure PCL mesh in terms of effectively promoting muscle repair in rat models. The histological assay also showed adequate biocompatibility of the resorbable meshes. The additively manufactured biodegradable drug-eluting meshes may be adopted in the future in humans for the therapy of muscle injuries.

Navigating translational control of gene expression in satellite cells

Satellite cells, named for their satellite position around the sarcolemma of the myofibre, are responsible for skeletal muscle regeneration. Satellite cells normally reside in a quiescent state, but rapidly activate the myogenic program and the cell cycle in response to injury. Translational control of gene expression has emerged as an important regulator of satellite cell activity. Quiescent satellite cells maintain low levels of protein synthesis and selectively translate specific mRNAs to conserve limited energy. Activated satellite cells rapidly restore global protein synthesis to meet the demands of proliferating myogenic progenitors that participate in muscle repair. We propose a model by which translational control enables rapid protein level changes in response to injury-induced environmental shifts, serving as both a brake mechanism during quiescence and an accelerator for injury response. In this Chapter, we navigate the processing, translation and metabolism of newly transcribed mRNAs. We review the modifications of mRNA that occur during mRNA processing in the nucleus of satellite cells, and illustrate how these modifications impact the translation and stability of mRNAs. In the cytoplasm, we review how pathways work in concert to regulate protein synthesis globally, while trans acting microRNAs and RNA binding proteins modify specific mRNA translation within a context of tightly regulated protein synthesis. While navigating translational control of gene expression in satellite cells, this chapter reveals that despite significant progress, the field remains nascent in the broader scope of translational control in cell biology. We propose that future investigations will benefit from incorporating emerging global analyses to study translational control of gene expression in rare satellite cells, and we pose unanswered questions that warrant future exploration.

Regulation of adult stem cell quiescence and its functions in the maintenance of tissue integrity

Adult stem cells are important for mammalian tissues, where they act as a cell reserve that supports normal tissue turnover and can mount a regenerative response following acute injuries. Quiescent stem cells are well established in certain tissues, such as skeletal muscle, brain, and bone marrow. The quiescent state is actively controlled and is essential for long-term maintenance of stem cell pools. In this Review, we discuss the importance of maintaining a functional pool of quiescent adult stem cells, including haematopoietic stem cells, skeletal muscle stem cells, neural stem cells, hair follicle stem cells, and mesenchymal stem cells such as fibro-adipogenic progenitors, to ensure tissue maintenance and repair. We discuss the molecular mechanisms that regulate the entry into, maintenance of, and exit from the quiescent state in mice. Recent studies revealed that quiescent stem cells have a discordance between RNA and protein levels, indicating the importance of post-transcriptional mechanisms, such as alternative polyadenylation, alternative splicing, and translation repression, in the control of stem cell quiescence. Understanding how these mechanisms guide stem cell function during homeostasis and regeneration has important implications for regenerative medicine.

Re-organization of nucleolar architecture in myogenic differentiation

Myogenesis, the process of muscle differentiation, requires an extensive remodeling of the cellular transcriptome and proteome. Whereas the transcriptional program underpinning myogenesis is well characterized, the required adaptation in protein synthesis is incompletely understood. Enhanced protein synthesis necessitates ribosome biogenesis at the nucleolus. Nucleolar size and activity are inextricably linked with altered gene expression. Here, we report changes in nucleolar morphology and function during myogenic differentiation. Immunofluorescence analysis revealed alterations in nucleolar morphology that were dependent on the cellular state - proliferative or quiescent myogenic progenitors (myoblasts or reserve cells) contained multiple small nucleoli with a characteristic spherical shape, whereas multinucleated myotubes typically contained one large, often irregularly shaped nucleolus. These morphological alterations are consistent with changes to nucleolar phase separation properties. Re-organization of the nucleolar structure was correlated with enhanced rRNA production and protein translation. Inhibition of mTOR signaling with rapamycin perturbed nucleolar re-organization. Conversely, hyperactivated mTOR enhanced alterations in nucleolar morphology. These findings support the idea that there is an mTOR dependent re-organization of nucleolar structure during myogenesis, enhancing our understanding of myogenesis and possibly facilitating new approaches to therapeutic interventions in muscle pathologies.

Ribonucleotide reductase M2B in the myofibers modulates stem cell fate in skeletal muscle

The balance among quiescence, differentiation, and self-renewal of skeletal muscle stem cells (MuSCs) is tightly regulated by their intrinsic and extrinsic properties from the niche. How the niche controls MuSC fate remains unclear. Ribonucleotide reductase M2B (Rrm2b) modulates MuSC quiescence/differentiation in muscle in response to injury. Rrm2b knockout in myofibers, but not in MuSCs, led to weakness of muscles, such as a loss of muscle mass and strength. After muscle injury, damaged myofibers were more efficiently repaired in the Rrm2b myofiber-specific knockout mice than the control mice, but these myofibers were thinner and showed weak functioning. Rrm2b-deleted myofibers released several myokines, which trigger MuSCs to differentiate but not re-enter the quiescent stage to replenish the stem cell pool. Overall, Rrm2b in the myofibers plays a critical role in modulating the MuSC fate by modifying the microenvironment, and it may lead to a possible strategy to treat muscle disorders.