A population of myogenic stem cells that survives skeletal muscle aging

Molecular and Structural Alterations of Skeletal Muscle Tissue Nuclei during Aging

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. The mechanisms underlying this phenomenon are certainly multifactorial and still remain to be fully elucidated. Changes in the cell nucleus structure and function have been considered among the possible contributing causes. This review offers an overview of the current knowledge on skeletal muscle nuclei in aging, focusing on the impairment of nuclear pathways potentially involved in age-related muscle decline. In skeletal muscle two types of cells are present: fiber cells, constituting the contractile muscle mass and containing hundreds of myonuclei, and the satellite cells, i.e., the myogenic mononuclear stem cells occurring at the periphery of the fibers and responsible for muscle growth and repair. Research conducted on different experimental models and with different methodological approaches demonstrated that both the myonuclei and satellite cell nuclei of aged skeletal muscles undergo several structural and molecular alterations, affecting chromatin organization, gene expression, and transcriptional and post-transcriptional activities. These alterations play a key role in the impairment of muscle fiber homeostasis and regeneration, thus contributing to the age-related decrease in skeletal muscle mass and function.

Potential Application of Muscle Precursor Cells from Male Specific-Pathogen-Free (SPF) Chicken Embryos in In Vitro Agriculture

This study examined the potential benefits of male specific-pathogen-free (SPF) White Leghorn embryos in cellular agriculture for sustainable and ethical poultry meat production-addressing traditional farming challenges, including disease outbreaks of Salmonella and Avian influenza. We isolated myogenic precursor cells (MPCs) from the thigh muscles (Musculus femoris) of 12.5-day-old embryos from 10 SPF White Leghorns that tested negative for Salmonella. We randomly selected MPCs from three males and three females, isolated them using a modified pre-plating (pp) method, and compared their in vitro development. After 1 h (pp1) and 2 h (pp2) of incubation, they were transferred to a new dish to remove fast-adhering cells and cultured (pp3). Isolated MPCs had a 69% positive reaction to Pax7. During proliferation, no differences were observed in PAX7, MYF5, or MYOD expression between the male and female MPCs. However, after five days of differentiation, the expression of late myogenic factors-MYOG and MYF6-significantly increased in all MPCs. Notably, MYOG expression was 1.9 times higher in female than in male MPCs. This impacted MYMK's expression pattern. Despite this, the myotube fusion index did not differ between the sexes. Muscle cells from male SPF-laying chicken embryos are promising for developing clean animal-cell-derived protein sources via resource recycling.

Loss of the matrix metalloproteinase-10 causes premature features of aging in satellite cells

Aged muscles accumulate satellite cells with a striking decline response to damage. Although intrinsic defects in satellite cells themselves are the major contributors to aging-associated stem cell dysfunction, increasing evidence suggests that changes in the muscle-stem cell local microenvironment also contribute to aging. Here, we demonstrate that loss of the matrix metalloproteinase-10 (MMP-10) in young mice alters the composition of the muscle extracellular matrix (ECM), and specifically disrupts the extracellular matrix of the satellite cell niche. This situation causes premature features of aging in the satellite cells, contributing to their functional decline and a predisposition to enter senescence under proliferative pressure. Similarly, reduction of MMP-10 levels in young satellite cells from wild type animals induces a senescence response, while addition of the protease delays this program. Significantly, the effect of MMP-10 on satellite cell aging can be extended to another context of muscle wasting, muscular dystrophy. Systemic treatment of mdx dystrophic mice with MMP-10 prevents the muscle deterioration phenotype and reduces cellular damage in the satellite cells, which are normally under replicative pressure. Most importantly, MMP-10 conserves its protective effect in the satellite cell-derived myoblasts isolated from a Duchenne muscular dystrophy patient by decreasing the accumulation of damaged DNA. Hence, MMP-10 provides a previously unrecognized therapeutic opportunity to delay satellite cell aging and overcome satellite cell dysfunction in dystrophic muscles.

Molecular regulation of satellite cells via intercellular signaling

Somatic stem cells are tissue-specific reserve cells tasked to sustain tissue homeostasis in adulthood and/or effect tissue regeneration after traumatic injury. The stem cells of skeletal muscle tissue are the satellite cells, which were originally described and named after their localization beneath the muscle fiber lamina and attached to the multi-nucleated muscle fibers. During adult homeostasis, satellite cells are maintained in quiescence, a state of reversible cell cycle arrest. Yet, upon injury, satellite cells are rapidly activated, becoming highly mitotically active to generate large numbers of myoblasts that differentiate and fuse to regenerate the injured muscle fibers. A subset self-renews to replenish the pool of muscle stem cells.Complex intrinsic gene regulatory networks maintain the quiescent state of satellite cells, or upon injury, direct their activation, proliferation, differentiation and self-renewal. Molecular cues from the satellite cells' environment provide the essential information as to when and where satellite cells are to stay quiescent or break quiescence and effect regenerative myogenesis. Predominantly, these cues are secreted, diffusible or membrane-bound ligands that bind to and activate their specific cognate receptors on the satellite cell to activate downstream signaling cascades and elicit context-specific cell behavior. This review aims to offer a concise overview of major intercellular signaling pathways regulating satellite cells during quiescence and in injury-induced skeletal muscle regeneration.

Ageing and rejuvenation of tissue stem cells and their niches

Most adult organs contain regenerative stem cells, often organized in specific niches. Stem cell function is critical for tissue homeostasis and repair upon injury, and it is dependent on interactions with the niche. During ageing, stem cells decline in their regenerative potential and ability to give rise to differentiated cells in the tissue, which is associated with a deterioration of tissue integrity and health. Ageing-associated changes in regenerative tissue regions include defects in maintenance of stem cell quiescence, differentiation ability and bias, clonal expansion and infiltration of immune cells in the niche. In this Review, we discuss cellular and molecular mechanisms underlying ageing in the regenerative regions of different tissues as well as potential rejuvenation strategies. We focus primarily on brain, muscle and blood tissues, but also provide examples from other tissues, such as skin and intestine. We describe the complex interactions between different cell types, non-cell-autonomous mechanisms between ageing niches and stem cells, and the influence of systemic factors. We also compare different interventions for the rejuvenation of old regenerative regions. Future outlooks in the field of stem cell ageing are discussed, including strategies to counter ageing and age-dependent disease.

Organotypic cultures as aging associated disease models

Aging remains a primary risk factor for a host of diseases, including leading causes of death. Aging and associated diseases are inherently multifactorial, with numerous contributing factors and phenotypes at the molecular, cellular, tissue, and organismal scales. Despite the complexity of aging phenomena, models currently used in aging research possess limitations. Frequently used in vivo models often have important physiological differences, age at different rates, or are genetically engineered to match late disease phenotypes rather than early causes. Conversely, routinely used in vitro models lack the complex tissue-scale and systemic cues that are disrupted in aging. To fill in gaps between in vivo and traditional in vitro models, researchers have increasingly been turning to organotypic models, which provide increased physiological relevance with the accessibility and control of in vitro context. While powerful tools, the development of these models is a field of its own, and many aging researchers may be unaware of recent progress in organotypic models, or hesitant to include these models in their own work. In this review, we describe recent progress in tissue engineering applied to organotypic models, highlighting examples explicitly linked to aging and associated disease, as well as examples of models that are relevant to aging. We specifically highlight progress made in skin, gut, and skeletal muscle, and describe how recently demonstrated models have been used for aging studies or similar phenotypes. Throughout, this review emphasizes the accessibility of these models and aims to provide a resource for researchers seeking to leverage these powerful tools.

A transcriptionally repressed quiescence program is associated with paused RNAPII and is poised for cell cycle reentry

Adult stem cells persist in mammalian tissues by entering a state of reversible quiescence/ G0, associated with low transcription. Using cultured myoblasts and muscle stem cells, we report that in G0, global RNA content and synthesis are substantially repressed, correlating with decreased RNA Polymerase II (RNAPII) expression and activation. Integrating RNAPII occupancy and transcriptome profiling, we identify repressed networks and a role for promoter-proximal RNAPII pausing in G0. Strikingly, RNAPII shows enhanced pausing in G0 on repressed genes encoding regulators of RNA biogenesis (Nucleolin, Rps24, Ctdp1); release of pausing is associated with their increased expression in G1. Knockdown of these transcripts in proliferating cells leads to induction of G0 markers, confirming the importance of their repression in establishment of G0. A targeted screen of RNAPII regulators revealed that knockdown of Aff4 (positive regulator of elongation) unexpectedly enhances expression of G0-stalled genes and hastens S phase; NELF, a regulator of pausing appears to be dispensable. We propose that RNAPII pausing contributes to transcriptional control of a subset of G0-repressed genes to maintain quiescence and impacts the timing of the G0-G1 transition.

The State of the Art of Piezo1 Channels in Skeletal Muscle Regeneration

Piezo1 channels are highly mechanically-activated cation channels that can sense and transduce the mechanical stimuli into physiological signals in different tissues including skeletal muscle. In this focused review, we summarize the emerging evidence of Piezo1 channel-mediated effects in the physiology of skeletal muscle, with a particular focus on the role of Piezo1 in controlling myogenic precursor activity and skeletal muscle regeneration and vascularization. The disclosed effects reported by pharmacological activation of Piezo1 channels with the selective agonist Yoda1 indicate a potential impact of Piezo1 channel activity in skeletal muscle regeneration, which is disrupted in various muscular pathological states. All findings reported so far agree with the idea that Piezo1 channels represent a novel, powerful molecular target to develop new therapeutic strategies for preventing or ameliorating skeletal muscle disorders characterized by an impairment of tissue regenerative potential.

Estradiol deficiency reduces the satellite cell pool by impairing cell cycle progression

The size of the satellite cell pool is reduced in estradiol (E2)-deficient female mice and humans. Here, we use a combination of in vivo and in vitro approaches to identify mechanisms, whereby E2 deficiency impairs satellite cell maintenance. By measuring satellite cell numbers in mice at several early time points postovariectomy (Ovx), we determine that satellite cell numbers decline by 33% between 10 and 14 days post-Ovx in tibialis anterior and gastrocnemius muscles. At 14 days post-Ovx, we demonstrate that satellite cells have a reduced propensity to transition from G0/G1 to S and G2/M phases, compared with cells from ovary-intact mice, associated with changes in two key satellite cell cycle regulators, ccna2 and p16INK4a. Further, freshly isolated satellite cells treated with E2 in vitro have 62% greater cell proliferation and require less time to complete the first division. Using clonal and differentiation assays, we measured 69% larger satellite cell colonies and enhanced satellite cell-derived myoblast differentiation with E2 treatment compared with vehicle-treated cells. Together, these results identify a novel mechanism for preservation of the satellite cell pool by E2 via promotion of satellite cell cycling.

Cellular Aquaculture: Prospects and Challenges

Aquaculture plays an important role as one of the fastest-growing food-producing sectors in global food and nutritional security. Demand for animal protein in the form of fish has been increasing tremendously. Aquaculture faces many challenges to produce quality fish for the burgeoning world population. Cellular aquaculture can provide an alternative, climate-resilient food production system to produce quality fish. Potential applications of fish muscle cell lines in cellular aquaculture have raised the importance of developing and characterizing these cell lines. In vitro models, such as the mouse C2C12 cell line, have been extremely useful for expanding knowledge about molecular mechanisms of muscle growth and differentiation in mammals. Such studies are in an infancy stage in teleost due to the unavailability of equivalent permanent muscle cell lines, except a few fish muscle cell lines that have not yet been used for cellular aquaculture. The Prospect of cell-based aquaculture relies on the development of appropriate muscle cells, optimization of cell conditions, and mass production of cells in bioreactors. Hence, it is required to develop and characterize fish muscle cell lines along with their cryopreservation in cell line repositories and production of ideal mass cells in suitably designed bioreactors to overcome current cellular aquaculture challenges.

The regenerating skeletal muscle niche drives satellite cell return to quiescence

Skeletal muscle stem cells, or satellite cells (SCs), are essential to regenerate and maintain muscle. Quiescent SCs reside in an asymmetric niche between the basal lamina and myofiber membrane. To repair muscle, SCs activate, proliferate, and differentiate, fusing to repair myofibers or reacquiring quiescence to replenish the SC niche. Little is known about when SCs reacquire quiescence during regeneration or the cellular processes that direct SC fate decisions. We find that most SCs reacquire quiescence 5–10 days after muscle injury, following differentiation and fusion of most cells to regenerate myofibers. Single-cell sequencing of myogenic cells in regenerating muscle identifies SCs reacquiring quiescence and reveals that noncell autonomous signaling networks influence SC fate decisions during regeneration. SC transplantation experiments confirm that the regenerating environment influences SC fate. We define a window for SC repopulation of the niche, emphasizing the temporal contribution of the regenerative muscle environment on SC fate.

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Satellite cells primarily produce myonuclei in the first 4 days following injury

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Most satellite cells reacquire quiescence 5–7 days following injury

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Asymmetric division pathways are elevated in satellite cells reacquiring quiescence

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The regenerating muscle environment directs satellite cell fate decisions

Aging and “rejuvenation” of resident stem cells — a new way to active longevity?

This review presents the current data on the methodology for assessing the biological and epigenetic age, describes the concept of the epigenetic clock, and characterizes the main types of resident stem cells and the specifics of their aging. It has been shown that age-related changes in organs and tissues, as well as age-related diseases, are largely due to the aging of resident stem cells. The latter represent an attractive target for cell rejuvenation, as they can be isolated, cultured ex vivo, modified, and re-introduced into the resident niches. Two main methodologies for the cellular rejuvenation are presented: genetic reprogramming with zeroing the age of a cell using transient expression of transcription factors, and various approaches to epigenetic rejuvenation. The close relationship between aging, regeneration, and oncogenesis, and between these factors and the functioning of resident stem cell niches requires further precision studies, which, we are sure, can result in the creation of an effective anti-aging strategy and prolongation of human active life.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

A neuromuscular perspective of sarcopenia pathogenesis: deciphering the signaling pathways involved

The escalation of life expectancy is accompanied by an increase in the prevalence of age-related conditions, such as sarcopenia. Sarcopenia, a muscle condition defined by low muscle strength, muscle quality or quantity, and physical performance, has a high prevalence among the elderly and is associated to increased mortality. The neuromuscular system has been emerging as a key contributor to sarcopenia pathogenesis. Indeed, the age-related degeneration of the neuromuscular junction (NMJ) function and structure may contribute to the loss of muscle strength and ultimately to the loss of muscle mass that characterize sarcopenia. The present mini-review discusses important signaling pathways involved in the function and maintenance of the NMJ, giving emphasis to the ones that might contribute to sarcopenia pathogenesis. Some conceivable biomarkers, such as C-terminal agrin fragment (CAF) and brain-derived neurotrophic factor (BDNF), and therapeutic targets, namely acetylcholine and calcitonin gene-related peptide (CGRP), can be retrieved, making way to future studies to validate their clinical use.

Fibroblast growth factor 6 regulates sizing of the muscle stem cell pool

Skeletal muscle stem cells, i.e., satellite cells (SCs), are the essential source of new myonuclei for skeletal muscle regeneration following injury or chronic degenerative myopathies. Both SC number and regenerative capacity diminish during aging. However, molecular regulators that govern sizing of the initial SC pool are unknown. We demonstrate that fibroblast growth factor 6 (FGF6) is critical for SC pool scaling. Mice lacking FGF6 have reduced SCs of early postnatal origin and impaired regeneration. By contrast, increasing FGF6 during the early postnatal period is sufficient for SC expansion. Together, these data support that FGF6 is necessary and sufficient to modulate SC numbers during a critical postnatal period to establish the quiescent adult muscle stem cell pool. Our work highlights postnatal development as a time window receptive for scaling a somatic stem cell population via growth factor signaling, which might be relevant for designing new biomedical strategies to enhance tissue regeneration.

Aging-associated stem/progenitor cell dysfunction in the salivary glands of mice

Although stem cell aging leads to a decline in tissue homeostasis and regenerative capacity, it remains unclear whether salivary gland stem cell function changes during this process. However, the salivary glands are gradually replaced by connective tissue during aging. Here, we show a decline in the stem cell ability of CD133-positive stem/progenitor cells in the salivary glands of aged mice. The CD133-positive cells were isolated from young, adult, and aged mice. The number of CD133-positive cells was significantly decreased in aged mice. They also showed a lower sphere formation capacity compared to young and adult mice. RNA sequencing revealed that CD133-positive cells in aged mice exhibited lower gene expression of several aging-related genes, including FoxO3a, than those in young and adult mice. Salivary gland cells infected with a recombinant lentivirus encoding the FoxO3a gene showed a reduction in oxidative stress induced by hydrogen peroxide compared with those infected with a control virus. Thus, FoxO3a may inhibit stem cell aging via oxidative stress.

Regulation of muscle hypertrophy: Involvement of the Akt-independent pathway and satellite cells in muscle hypertrophy

Skeletal muscles are composed of multinuclear cells called myofibers and have unique abilities, one of which is plasticity. In response to the mechanical load induced by physical activity, skeletal muscle exerts several local adaptations, including an increase in myofiber size and myonuclear number, known as muscle hypertrophy. Protein synthesis and muscle satellite cells (MuSCs) are mainly responsible for these adaptations. However, the upstream signaling pathways that promote protein synthesis remain controversial. Further, the necessity of MuSCs in muscle hypertrophy is also a highly debated issue. In this review, we summarized the insulin-like growth factor 1 (IGF-1)/Akt-independent activation of mammalian target of rapamycin (mTOR) signaling in muscle hypertrophy and the involvement of mTOR signaling in age-related loss of skeletal muscle function and mass and in sarcopenia. The roles and behaviors of MuSCs, characteristics of new myonuclei in muscle hypertrophy, and their relevance to sarcopenia have also been updated in this review.

It takes all kinds: heterogeneity among satellite cells and fibro-adipogenic progenitors during skeletal muscle regeneration

Vertebrate skeletal muscle is composed of multinucleate myofibers that are surrounded by muscle connective tissue. Following injury, muscle is able to robustly regenerate because of tissue-resident muscle stem cells, called satellite cells. In addition, efficient and complete regeneration depends on other cells resident in muscle - including fibro-adipogenic progenitors (FAPs). Increasing evidence from single-cell analyses and genetic and transplantation experiments suggests that satellite cells and FAPs are heterogeneous cell populations. Here, we review our current understanding of the heterogeneity of satellite cells, their myogenic derivatives and FAPs in terms of gene expression, anatomical location, age and timing during the regenerative process - each of which have potentially important functional consequences.

Epigenetic Clock and Circadian Rhythms in Stem Cell Aging and Rejuvenation

This review summarizes the current understanding of the interaction between circadian rhythms of gene expression and epigenetic clocks characterized by the specific profile of DNA methylation in CpG-islands which mirror the senescence of all somatic cells and stem cells in particular. Basic mechanisms of regulation for circadian genes CLOCK-BMAL1 as well as downstream clock-controlled genes (ССG) are also discussed here. It has been shown that circadian rhythms operate by the finely tuned regulation of transcription and rely on various epigenetic mechanisms including the activation of enhancers/suppressors, acetylation/deacetylation of histones and other proteins as well as DNA methylation. Overall, up to 20% of all genes expressed by the cell are subject to expression oscillations associated with circadian rhythms. Additionally included in the review is a brief list of genes involved in the regulation of circadian rhythms, along with genes important for cell aging, and oncogenesis. Eliminating some of them (for example, Sirt1) accelerates the aging process, while the overexpression of Sirt1, on the contrary, protects against age-related changes. Circadian regulators control a number of genes that activate the cell cycle (Wee1, c-Myc, p20, p21, and Cyclin D1) and regulate histone modification and DNA methylation. Approaches for determining the epigenetic age from methylation profiles across CpG islands in individual cells are described. DNA methylation, which characterizes the function of the epigenetic clock, appears to link together such key biological processes as regeneration and functioning of stem cells, aging and malignant transformation. Finally, the main features of adult stem cell aging in stem cell niches and current possibilities for modulating the epigenetic clock and stem cells rejuvenation as part of antiaging therapy are discussed.

Cell Types Used for Cultured Meat Production and the Importance of Myokines

The world’s population continues to increase, meaning we require more consistent protein supply to meet demand. Despite the availability of plant-based protein alternatives, animal meat remains a popular, high-quality protein source. Research studies have focused on cultured meat (meat grown in vitro) as a safe and more efficient alternative to traditional meat. Cultured meat is produced by in vitro myogenesis, which involves the processing of muscle satellite and mature muscle cells. Meat culture efficiency is largely determined by the culture conditions, such as the cell type and cell culture medium used and the biomolecular composition. Protein production can be enhanced by providing the optimum biochemical and physical conditions for skeletal muscle cell growth, while myoblasts play important roles in skeletal muscle formation and growth. This review describes the cell types used to produce cultured meat and the biological effects of various myokines and cytokines, such as interleukin-6, leukemia inhibitory factor, interleukin-4, interleukin-15, and interleukin-1β, on skeletal muscle and myogenesis and their potential roles in cultured meat production.

Preservation of satellite cell number and regenerative potential with age reveals locomotory muscle bias

BACKGROUND

Although muscle regenerative capacity declines with age, the extent to which this is due to satellite cell-intrinsic changes vs. environmental changes has been controversial. The majority of aging studies have investigated hindlimb locomotory muscles, principally the tibialis anterior, in caged sedentary mice, where those muscles are abnormally under-exercised.

METHODS

We analyze satellite cell numbers in 8 muscle groups representing locomotory and non-locomotory muscles in young and 2-year-old mice and perform transplantation assays of low numbers of hind limb satellite cells from young and old mice.

RESULTS

We find that satellite cell density does not decline significantly by 2 years of age in most muscles, and one muscle, the masseter, shows a modest but statistically significant increase in satellite cell density with age. The tibialis anterior and extensor digitorum longus were clear exceptions, showing significant declines. We quantify self-renewal using a transplantation assay. Dose dilution revealed significant non-linearity in self-renewal above a very low threshold, suggestive of competition between satellite cells for space within the pool. Assaying within the linear range, i.e., transplanting fewer than 1000 cells, revealed no evidence of decline in cell-autonomous self-renewal or regenerative potential of 2-year-old murine satellite cells.

CONCLUSION

These data demonstrate the value of comparative muscle analysis as opposed to overreliance on locomotory muscles, which are not used physiologically in aging sedentary mice, and suggest that self-renewal impairment with age is precipitously acquired at the geriatric stage, rather than being gradual over time, as previously thought.

Changes in Myonuclear Number During Postnatal Growth -Implications for AAV Gene Therapy for Muscular Dystrophy

Adult skeletal muscle is a relatively stable tissue, as the multinucleated muscle fibres contain post-mitotic myonuclei. During early postnatal life, muscle growth occurs by the addition of skeletal muscle stem cells (satellite cells) or their progeny to growing muscle fibres. In Duchenne muscular dystrophy, which we shall use as an example of muscular dystrophies, the muscle fibres lack dystrophin and undergo necrosis. Satellite-cell mediated regeneration occurs, to repair and replace the necrotic muscle fibres, but as the regenerated muscle fibres still lack dystrophin, they undergo further cycles of degeneration and regeneration.AAV gene therapy is a promising approach for treating Duchenne muscular dystrophy. But for a single dose of, for example, AAV coding for dystrophin, to be effective, the treated myonuclei must persist, produce sufficient dystrophin and a sufficient number of nuclei must be targeted. This latter point is crucial as AAV vector remains episomal and does not replicate in dividing cells. Here, we describe and compare the growth of skeletal muscle in rodents and in humans and discuss the evidence that myofibre necrosis and regeneration leads to the loss of viral genomes within skeletal muscle. In addition, muscle growth is expected to lead to the dilution of the transduced nuclei especially in case of very early intervention, but it is not clear if growth could result in insufficient dystrophin to prevent muscle fibre breakdown. This should be the focus of future studies.

Human muscle stem cells are refractory to aging

Age-related loss of muscle mass and strength is widely attributed to limitation in the capacity of muscle resident satellite cells to perform their myogenic function. This idea contains two notions that have not been comprehensively evaluated by experiment. First, it entails the idea that we damage and lose substantial amounts of muscle in the course of our normal daily activities. Second, it suggests that mechanisms of muscle repair are in some way exhausted, thus limiting muscle regeneration. A third potential option is that the aged environment becomes inimical to the conduct of muscle regeneration. In the present study, we used our established model of human muscle xenografting to test whether muscle samples taken from cadavers, of a range of ages, maintained their myogenic potential after being transplanted into immunodeficient mice. We find no measurable difference in regeneration across the range of ages investigated up to 78 years of age. Moreover, we report that satellite cells maintained their myogenic capacity even when muscles were grafted 11 days postmortem in our model. We conclude that the loss of muscle mass with increasing age is not attributable to any intrinsic loss of myogenicity and is most likely a reflection of progressive and detrimental changes in the muscle microenvironment such as to disfavor the myogenic function of these cells.

The Epic of In Vitro Meat Production-A Fiction into Reality

Due to a proportionally increasing population and food demands, the food industry has come up with wide innovations, opportunities, and possibilities to manufacture meat under in vitro conditions. The amalgamation of cell culture and tissue engineering has been the base idea for the development of the synthetic meat, and this has been proposed to be a pivotal study for a futuristic muscle development program in the medical field. With improved microbial and chemical advancements, in vitro meat matched the conventional meat and is proposed to be eco-friendly, healthy, nutrient rich, and ethical. Despite the success, there are several challenges associated with the utilization of materials in synthetic meat manufacture, which demands regulatory and safety assessment systems to manage the risks associated with the production of cultured meat. The role of 3D bioprinting meat analogues enables a better nutritional profile and sensorial values. The integration of nanosensors in the bioprocess of culture meat eased the quality assessment throughout the food supply chain and management. Multidisciplinary approaches such as mathematical modelling, computer fluid dynamics, and biophotonics coupled with tissue engineering will be promising aspects to envisage the future prospective of this technology and make it available to the public at economically feasible rates.

Skeletal Muscle Gene Expression Profile in Response to Caloric Restriction and Aging: A Role for SirT1

SirT1 plays a crucial role in the regulation of some of the caloric restriction (CR) responsive biological pathways. Aging suppresses SirT1 gene expression in skeletal muscle, suggesting that aging may affect the role of CR in muscle. To determine the role of SirT1 in the regulation of CR regulated pathways in skeletal muscle, we performed high-throughput RNA sequencing using total RNA isolated from the skeletal muscles of young and aged wild-type (WT), SirT1 knockout (SirT1-KO), and SirT1 overexpression (SirT1-OE) mice fed to 20 wk ad libitum (AL) or 40% CR diet. Our data show that aging repressed the global gene expression profile, which was restored by CR via upregulating transcriptional and translational process-related pathways. CR inhibits pathways linked to the extracellular matrix and cytoskeletal proteins regardless of aging. Mitochondrial function and muscle contraction-related pathways are upregulated in aged SirT1 KO mice following CR. SirT1 OE did not affect whole-body energy expenditure or augment skeletal muscle insulin sensitivity associated pathways, regardless of aging or diet. Overall, our RNA-seq data showed that SirT1 and CR have different functions and activation of SirT1 by its activator or exercise may enhance SirT1 activity that, along with CR, likely have a better functional role in aging muscle.

Immortalized Canine Dystrophic Myoblast Cell Lines for Development of Peptide-Conjugated Splice-Switching Oligonucleotides

Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disease caused by frameshift or nonsense mutations in the DMD gene, resulting in the loss of dystrophin from muscle membranes. Exon skipping using splice-switching oligonucleotides (SSOs) restores the reading frame of DMD pre-mRNA by generating internally truncated but functional dystrophin protein. To potentiate effective tissue-specific targeting by functional SSOs, it is essential to perform accelerated and reliable in vitro screening-based assessment of novel oligonucleotides and drug delivery technologies, such as cell-penetrating peptides, before their in vivo pharmacokinetic and toxicity evaluation. We have established novel canine immortalized myoblast lines by transducing murine cyclin-dependent kinase-4 and human telomerase reverse transcriptase genes into myoblasts isolated from beagle-based wild-type or canine X-linked muscular dystrophy in Japan (CXMDJ) dogs. These myoblast lines exhibited improved myogenic differentiation and increased proliferation rates compared with passage-15 primary parental myoblasts, and their potential to differentiate into myotubes was maintained in later passages. Using these dystrophin-deficient immortalized myoblast lines, we demonstrate that a novel cell-penetrating peptide (Pip8b2)-conjugated SSO markedly improved multiexon skipping activity compared with the respective naked phosphorodiamidate morpholino oligomers. In vitro screening using immortalized canine cell lines will provide a basis for further pharmacological studies on drug delivery tools.

"Empowering" Cardiac Cells via Stem Cell Derived Mitochondrial Transplantation- Does Age Matter?

With cardiovascular diseases affecting millions of patients, new treatment strategies are urgently needed. The use of stem cell based approaches has been investigated during the last decades and promising effects have been achieved. However, the beneficial effect of stem cells has been found to being partly due to paracrine functions by alterations of their microenvironment and so an interesting field of research, the “stem- less” approaches has emerged over the last years using or altering the microenvironment, for example, via deletion of senescent cells, application of micro RNAs or by modifying the cellular energy metabolism via targeting mitochondria. Using autologous muscle-derived mitochondria for transplantations into the affected tissues has resulted in promising reports of improvements of cardiac functions in vitro and in vivo. However, since the targeted treatment group represents mainly elderly or otherwise sick patients, it is unclear whether and to what extent autologous mitochondria would exert their beneficial effects in these cases. Stem cells might represent better sources for mitochondria and could enhance the effect of mitochondrial transplantations. Therefore in this review we aim to provide an overview on aging effects of stem cells and mitochondria which might be important for mitochondrial transplantation and to give an overview on the current state in this field together with considerations worthwhile for further investigations.

The Satellite Cell at 60: The Foundation Years

The resident stem cell for skeletal muscle is the satellite cell. On the 50th anniversary of its discovery in 1961, we described the history of skeletal muscle research and the seminal findings made during the first 20 years in the life of the satellite cell (Scharner and Zammit 2011, doi: 10.1186/2044-5040-1-28). These studies established the satellite cell as the source of myoblasts for growth and regeneration of skeletal muscle. Now on the 60th anniversary, we highlight breakthroughs in the second phase of satellite cell research from 1980 to 2000. These include technical innovations such as isolation of primary satellite cells and viable muscle fibres complete with satellite cells in their niche, together with generation of many useful reagents including genetically modified organisms and antibodies still in use today. New methodologies were combined with description of endogenous satellite cells markers, notably Pax7. Discovery of the muscle regulatory factors Myf5, MyoD, myogenin, and MRF4 in the late 1980s revolutionized understanding of the control of both developmental and regerenative myogenesis. Emergence of genetic lineage markers facilitated identification of satellite cells in situ, and also empowered transplantation studies to examine satellite cell function. Finally, satellite cell heterogeneity and the supportive role of non-satellite cell types in muscle regeneration were described. These major advances in methodology and in understanding satellite cell biology provided further foundations for the dramatic escalation of work on muscle stem cells in the 21st century.

Restoring aged stem cell functionality: Current progress and future directions

Stem cell dysfunction is a hallmark of aging, associated with the decline of physical and cognitive abilities of humans and other mammals [Cell 2013;153:1194]. Therefore, it has become an active area of research within the aging and stem cell fields, and various techniques have been employed to mitigate the decline of stem cell function both in vitro and in vivo. While some techniques developed in model organisms are not directly translatable to humans, others show promise in becoming clinically relevant to delay or even mitigate negative phenotypes associated with aging. This review focuses on diet, treatment, and small molecule interventions that provide evidence of functional improvement in at least one type of aged adult stem cell.

Human Rotator Cuff Tears Have an Endogenous, Inducible Stem Cell Source Capable of Improving Muscle Quality and Function After Rotator Cuff Repair

BACKGROUND

The muscle quality of the rotator cuff (RC), measured by atrophy and fatty infiltration (FI), is a key determinant of outcomes in RC injury and repair. The ability to regenerate muscle after repair has been shown to be limited.

PURPOSE

To determine if there is a source of resident endogenous stem cells, fibroadipogenic progenitor cells (FAPs), within RC injury patients, and if these cells are capable of adipogenic, fibrogenic, and pro-myogenic differentiation.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 20 patients between the ages of 40 and 75 years with partial- or full-thickness RC tears of the supraspinatus and evidence of atrophy and FI Goutallier grade 1, 2, or 3 were selected from 2 surgeons at an orthopaedic center. During the surgical repair procedure, supraspinatus muscle biopsy specimens were obtained for analysis as were deltoid muscle biopsy specimens to serve as the control. FAPs and satellite cells were quantified using fluorescence-activated cell sorting. Muscle FI and fibrosis was quantified using Oil Red O and Masson trichrome staining. FAP differentiation and gene expression profiles were compared across tear sizes after culture in adipogenic, fibrogenic, and beta-3 agonist (amibegron) conditions. Analysis of variance was used for statistical comparisons between groups, with P < .05 as statistically significant.

RESULTS

Histologic analysis confirmed the presence of fat in biopsy specimens from patients with full-thickness tears. There were more FAPs in the full-thickness tear group compared with the partial-thickness tear group (9.43% ± 4.25% vs 3.84% ± 2.54%; P < .01). Full-thickness tears were divided by tear size, with patients with larger tears having significantly more FAPs than those with smaller tears. FAPs from muscles with full-thickness tendon tears had more adipogenic and fibrogenic potential than those with partial tears. Induction of a beige adipose tissue (BAT) phenotype in FAPs was possible, as demonstrated by increased expression of BAT markers and pro-myogenic genes including insulin-like growth factor 1 and follistatin.

CONCLUSION

Endogenous FAPs are present within the RC and likely are the source of FI. These FAPs were increased in muscles with in larger tears but are capable of adopting a pro-myogenic BAT phenotype that could be utilized to improve muscle quality and patient function after RC repair.

The eroding chromatin landscape of aging stem cells

Adult stem cells undergo both replicative and chronological aging in their niches, with catastrophic declines in regenerative potential with age. Due to repeated environmental insults during aging, the chromatin landscape of stem cells erodes, with changes in both DNA and histone modifications, accumulation of damage, and altered transcriptional response. A body of work has shown that altered chromatin is a driver of cell fate changes and a regulator of self-renewal in stem cells and therefore a prime target for juvenescence therapeutics. This review focuses on chromatin changes in stem cell aging and provides a composite view of both common and unique epigenetic themes apparent from the studies of multiple stem cell types.

Sarcopenia: Current treatments and new regenerative therapeutic approaches

Sarcopenia is characterized by loss of muscle and reduction in muscle strength that contributes to higher mortality rate and increased incidence of fall and hospitalization in the elderly. Mitochondria dysfunction and age-associated inflammation in muscle are two of the main attributors to sarcopenia progression. Recent clinical trials on sarcopenia therapies such as physical exercise, nutraceutical, and pharmaceutical interventions have revealed that exercise is the only effective strategy shown to alleviate sarcopenia. Unlike nutraceutical and pharmaceutical interventions that showed controversial results in sarcopenia alleviation, exercise was found to restore mitochondria homeostasis and dampen inflammatory responses via a complex exchange of myokines and osteokines signalling between muscle and bone. However, as exercise have limited benefit to immobile patients, the use of stem cells and their secretome are being suggested to be novel therapeutics that can be catered to a larger patient population owing to their mitochondria restoration effects and immune modulatory abilities. As such, we reviewed the potential pros and cons associated with various stem cell types/secretome in sarcopenia treatment and the regulatory and production barriers that need to be overcome to translate such novel therapeutic agents into bedside application.

Translational potential: This review summarizes the causes underlying sarcopenia from the perspective of mitochondria dysfunction and age-associated inflammation, and the progress of clinical trials for the treatment of sarcopenia. We also propose therapeutic potential of stem cell therapy and bioactive secretome for sarcopenia.

Inflamma-miR-21 Negatively Regulates Myogenesis during Ageing

Ageing is associated with disrupted redox signalling and increased circulating inflammatory cytokines. Skeletal muscle homeostasis depends on the balance between muscle hypertrophy, atrophy and regeneration, however during ageing this balance is disrupted. The molecular pathways underlying the age-related decline in muscle regenerative potential remain elusive. microRNAs are conserved robust gene expression regulators in all tissues including skeletal muscle. Here, we studied satellite cells from adult and old mice to demonstrate that inhibition of miR-21 in satellite cells from old mice improves myogenesis. We determined that increased levels of proinflammatory cytokines, TNFα and IL6, as well as H₂O₂, increased miR-21 expression in primary myoblasts, which in turn resulted in their decreased viability and myogenic potential. Inhibition of miR-21 function rescued the decreased size of myotubes following TNFα or IL6 treatment. Moreover, we demonstrated that miR-21 could inhibit myogenesis in vitro via regulating IL6R, PTEN and FOXO3 signalling. In summary, upregulation of miR-21 in satellite cells and muscle during ageing may occur in response to elevated levels of TNFα and IL6, within satellite cells or myofibrillar environment contributing to skeletal muscle ageing and potentially a disease-related decline in potential for muscle regeneration.

MicroRNAs in Skeletal Muscle Aging: Current Issues and Perspectives

Skeletal muscle is one of the major organs responsible for body movements and metabolism making up approximately 40% of the total body mass. During aging, skeletal muscle exhibits a degenerative age-associated decline in mass and function termed sarcopenia. This age-associated dysfunction of skeletal muscle is a major criterion of morbidity, mortality, and overall declines of quality of life in the elderly people. Therefore, researchers have focused on identifying modulators of muscle aging process including messenger RNAs, proteins, and recently small noncoding RNAs such as microRNAs (miRNAs). In particular, miRNAs have been demonstrated to play a critical role in skeletal muscle development and homeostasis. Recent studies revealed that miRNAs were also involved in muscle aging processes and the rejuvenation of aged muscle by regulating important molecules and pathways of aging including insulin-like growth factors, nicotine-adenine dinucleotide (+)-dependent protein deacetylase sirtuin-1, telomerase reverse transcriptase, and transforming growth factor-β signaling pathway. Over the years, miRNAs have emerged as promising candidates for biomarkers of sarcopenia and targets for interventions to slow muscle aging. Here, we comprehensively review the current knowledge on the role of miRNAs in skeletal muscle aging and highlight their potential as biomarkers or therapeutic targets for skeletal muscle health.

Current perspectives on the cellular and molecular features of epigenetic ageing

IMPACT STATEMENT

The field of epigenetic ageing is relatively new, and the speed of its expansion presents a challenge in keeping abreast with new discoveries and their implications. Several reviews have already addressed the great number of pathologies, health conditions, life-style, and external stressors that are associated with changes to the rate of epigenetic ageing. While these associations highlight and affirm the ability of epigenetic clock to capture biologically meaningful changes associated with age, they do not inform us about the underlying mechanisms. In this very early period since the development of the clock, there have been rather limited experimental research that are aimed at uncovering the mechanism. Hence, the perspective that we proffer is derived from available but nevertheless limited lines of evidence that together provide a seemingly coherent narrative that can be tested. This, we believe would be helpful towards uncovering the workings of the epigenetic clock.

Stem Cell Aging in Skeletal Muscle Regeneration and Disease

Skeletal muscle comprises 30-40% of the weight of a healthy human body and is required for voluntary movements in humans. Mature skeletal muscle is formed by multinuclear cells, which are called myofibers. Formation of myofibers depends on the proliferation, differentiation, and fusion of muscle progenitor cells during development and after injury. Muscle progenitor cells are derived from muscle satellite (stem) cells (MuSCs), which reside on the surface of the myofiber but beneath the basement membrane. MuSCs play a central role in postnatal maintenance, growth, repair, and regeneration of skeletal muscle. In sedentary adult muscle, MuSCs are mitotically quiescent, but are promptly activated in response to muscle injury. Physiological and chronological aging induces MuSC aging, leading to an impaired regenerative capability. Importantly, in pathological situations, repetitive muscle injury induces early impairment of MuSCs due to stem cell aging and leads to early impairment of regeneration ability. In this review, we discuss (1) the role of MuSCs in muscle regeneration, (2) stem cell aging under physiological and pathological conditions, and (3) prospects related to clinical applications of controlling MuSCs.

Skeletal muscle as an experimental model of choice to study tissue aging and rejuvenation

Skeletal muscle is among the most age-sensitive tissues in mammal organisms. Significant changes in its resident stem cells (i.e., satellite cells, SCs), differentiated cells (i.e., myofibers), and extracellular matrix cause a decline in tissue homeostasis, function, and regenerative capacity. Based on the conservation of aging across tissues and taking advantage of the relatively well-characterization of the myofibers and associated SCs, skeletal muscle emerged as an experimental system to study the decline in function and maintenance of old tissues and to explore rejuvenation strategies. In this review, we summarize the approaches for understanding the aging process and for assaying the success of rejuvenation that use skeletal muscle as the experimental system of choice. We further discuss (and exemplify with studies of skeletal muscle) how conflicting results might be due to variations in the techniques of stem cell isolation, differences in the assays of functional rejuvenation, or deciding on the numbers of replicates and experimental cohorts.

Age-related decrease in muscle satellite cells is accompanied with diminished expression of early growth response 3 in mice

Skeletal muscle regeneration is mostly dependent on muscle satellite cells. Proper muscle regeneration requires enough number of satellite cells. Recent studies have suggested that the number of satellite cells in skeletal muscle declines as we age, leading to the impairment of muscle regeneration in older population. Our earlier study demonstrated that zinc finger transcription factor early growth response 3 (Egr3) plays an important role for maintaining the number of myoblasts, suggesting that age-related decrease in muscle satellite cell should be associated with the expression levels of Egr3. The aim of this study was to investigate whether aging would alter the Egr3 expression in satellite cells. A couple groups of male C57BL/6J mice were examined in this study: young (3 Mo) and old (17 Mo). Immunohistochemical staining showed that the satellite cell number decreased in normal and injured muscles of old mice. In fluorescence-activated cell sorting-isolated muscle satellite cells from normal and injured muscles, the mRNA expression of Egr3 was significantly decreased with age regardless of injury. In harmony with these results, Pax7 mRNA levels also decreased in the satellite cells from old mice. Alternatively, inhibition of Egr3 expression by shRNA decreased Pax7 protein expression in cultured myoblasts. These results suggest that Egr3 is associated with the age-related decline of muscle satellite cells in older population. Also, Egr3 might be implicated in the regulation of Pax7. Therefore, the loss of Egr3 expression may elucidate attenuated MSCs function and muscle regeneration in older age.

Aging: A cell source limiting factor in tissue engineering

Tissue engineering has yet to reach its ideal goal, i.e. creating profitable off-the-shelf tissues and organs, designing scaffolds and three-dimensional tissue architectures that can maintain the blood supply, proper biomaterial selection, and identifying the most efficient cell source for use in cell therapy and tissue engineering. These are still the major challenges in this field. Regarding the identification of the most appropriate cell source, aging as a factor that affects both somatic and stem cells and limits their function and applications is a preventable and, at least to some extents, a reversible phenomenon. Here, we reviewed different stem cell types, namely embryonic stem cells, adult stem cells, induced pluripotent stem cells, and genetically modified stem cells, as well as their sources, i.e. autologous, allogeneic, and xenogeneic sources. Afterward, we approached aging by discussing the functional decline of aged stem cells and different intrinsic and extrinsic factors that are involved in stem cell aging including replicative senescence and Hayflick limit, autophagy, epigenetic changes, miRNAs, mTOR and AMPK pathways, and the role of mitochondria in stem cell senescence. Finally, various interventions for rejuvenation and geroprotection of stem cells are discussed. These interventions can be applied in cell therapy and tissue engineering methods to conquer aging as a limiting factor, both in original cell source and in the in vitro proliferated cells.

Multiplexed RNAscope and immunofluorescence on whole-mount skeletal myofibers and their associated stem cells

Skeletal muscle myofibers are large syncytial cells comprising hundreds of myonuclei, and in situ hybridization experiments have reported a range of transcript localization patterns within them. Although some transcripts are uniformly distributed throughout myofibers, proximity to specialized regions can affect the programming of myonuclei and functional compartmentalization of transcripts. Established techniques are limited by a lack of both sensitivity and spatial resolution, restricting the ability to identify different patterns of gene expression. In this study, we adapted RNAscope fluorescent in situ hybridization technology for use on whole-mount mouse primary myofibers, a preparation that isolates single myofibers with their associated muscle stem cells remaining in their niche. This method can be combined with immunofluorescence, enabling an unparalleled ability to visualize and quantify transcripts and proteins across the length and depth of skeletal myofibers and their associated stem cells. Using this approach, we demonstrate a range of potential uses, including the visualization of specialized transcriptional programming within myofibers, tracking activation-induced transcriptional changes, quantification of stem cell heterogeneity and evaluation of stem cell niche factor transcription patterns.

Rejuvenating Strategies of Tissue-specific Stem Cells for Healthy Aging

Although aging is a physiological process, it has raised interest in the science of aging and rejuvenation because of the increasing burden on the rapidly aging global population. With advanced age, there is a decline in homeostatic maintenance and regenerative responsiveness to the injury of various tissues, thereby contributing to the incidence of age-related diseases. The primary cause of the functional declines that occur along with aging is considered to be the exhaustion of stem cell functions in their corresponding tissues. Age-related changes in the systemic environment, the niche, and stem cells contribute to this loss. Thus, the reversal of stem cell aging at the cellular level might lead to the rejuvenation of the animal at an organismic level and the prevention of aging, which would be critical for developing new therapies for age-related dysfunction and diseases. Here, we will explore the effects of aging on stem cells in different tissues. The focus of this discussion is on pro-youth interventions that target intrinsic stem cell properties, environmental niche component, systemic factors, and senescent cellular clearance, which are promising for developing strategies related to the reversal of aged stem cell function and optimizing tissue repair processes.

Micro(RNA)-managing muscle wasting

Progressive skeletal muscle wasting is a natural consequence of aging and is common in chronic and acute diseases. Loss of skeletal muscle mass and function (strength) often leads to frailty, decreased independence, and increased risk of hospitalization. Despite progress made in our understanding of the mechanisms underlying muscle wasting, there is still no treatment available, with exercise training and dietary supplementation improving, but not restoring, muscle mass and/or function. There has been slow progress in developing novel therapies for muscle wasting, either during aging or disease, partially due to the complex nature of processes underlying muscle loss. The mechanisms of muscle wasting are multifactorial, with a combination of factors underlying age- and disease-related functional muscle decline. These factors include well-characterized changes in muscle such as changes in protein turnover and more recently described mechanisms such as autophagy or satellite cell senescence. Advances in transcriptomics and other high-throughput approaches have highlighted significant deregulation of skeletal muscle gene and protein levels during aging and disease. These changes are regulated at different levels, including posttranscriptional gene expression regulation by microRNAs. microRNAs, potent regulators of gene expression, modulate many processes in muscle, and microRNA-based interventions have been recently suggested as a promising new therapeutic strategy against alterations in muscle homeostasis. Here, we review recent developments in understanding the aging-associated mechanisms of muscle wasting and explore potential microRNA-based therapeutic avenues.

Autophagy as a Therapeutic Target to Enhance Aged Muscle Regeneration

Skeletal muscle has remarkable regenerative capacity, relying on precise coordination between resident muscle stem cells (satellite cells) and the immune system. The age-related decline in skeletal muscle regenerative capacity contributes to the onset of sarcopenia, prolonged hospitalization, and loss of autonomy. Although several age-sensitive pathways have been identified, further investigation is needed to define targets of cellular dysfunction. Autophagy, a process of cellular catabolism, is emerging as a key regulator of muscle regeneration affecting stem cell, immune cell, and myofiber function. Muscle stem cell senescence is associated with a suppression of autophagy during key phases of the regenerative program. Macrophages, a key immune cell involved in muscle repair, also rely on autophagy to aid in tissue repair. This review will focus on the role of autophagy in various aspects of the regenerative program, including adult skeletal muscle stem cells, monocytes/macrophages, and corresponding age-associated dysfunction. Furthermore, we will highlight rejuvenation strategies that alter autophagy to improve muscle regenerative function.

In vivo GDF3 administration abrogates aging related muscle regeneration delay following acute sterile injury

Tissue regeneration is a highly coordinated process with sequential events including immune cell infiltration, clearance of damaged tissues, and immune‐supported regrowth of the tissue. Aging has a well‐documented negative impact on this process globally; however, whether changes in immune cells per se are contributing to the decline in the body’s ability to regenerate tissues with aging is not clearly understood. Here, we set out to characterize the dynamics of macrophage infiltration and their functional contribution to muscle regeneration by comparing young and aged animals upon acute sterile injury. Injured muscle of old mice showed markedly elevated number of macrophages, with a predominance for Ly6C^high pro‐inflammatory macrophages and a lower ratio of the Ly6C^low repair macrophages. Of interest, a recently identified repair macrophage‐derived cytokine, growth differentiation factor 3 (GDF3), was markedly downregulated in injured muscle of old relative to young mice. Supplementation of recombinant GDF3 in aged mice ameliorated the inefficient regenerative response. Together, these results uncover a deficiency in the quantity and quality of infiltrating macrophages during aging and suggest that in vivo administration of GDF3 could be an effective therapeutic approach.

Expansion capacity of human muscle progenitor cells differs by age, sex, and metabolic fuel preference

Activation of satellite cells and expansion of the muscle progenitor cell (MPC) population are essential to generate a sufficient number of cells to repair damaged skeletal muscle. Proliferating MPCs have high energetic and biosynthetic material requirements, and the ability to utilize oxidative phosphorylation (OXPHOS) and/or glycolysis may affect expansion capacity of MPCs. In the present study, we investigated the effect of donor age and sex on human (h)MPC expansion capacity and metabolic fuel preference. hMPCs from young and old male and female donors were grown for 408 h (17 days). Percent confluence, live nuclei count, and dead cell count were measured every 24 h. Metabolic phenotype was assessed by glucose uptake, expression of genes related to glycolysis and OXPHOS, and the Seahorse XF24 Phenotype Test Kit during the exponential phase of growth. hMPCs from old male donors had impaired expansion capacity secondary to heightened cell death early in expansion compared with hMPCs from young male donors, an effect not observed in female hMPCs. Age-related differences in metabolism were also sex dependent; markers of OXPHOS were altered in old (vs. young) male hMPCs, whereas markers of metabolism were largely unaffected by age in female hMPCs. For the first time, we identify sex-specific differences in cell death and OXPHOS that contribute to impaired expansion capacity of hMPC cell populations with age.

The AMPK/p27Kip1 Axis Regulates Autophagy/Apoptosis Decisions in Aged Skeletal Muscle Stem Cells

Skeletal muscle stem cell (MuSC) function declines with age and contributes to impaired muscle regeneration in older individuals. Acting through AMPK/p27Kip1, we have identified a pathway regulating the balance between autophagy, apoptosis, and senescence in aged MuSCs. While p27Kip1 is implicated in MuSC aging, its precise role and molecular mechanism have not been elucidated. Age-related MuSC dysfunction was associated with reduced autophagy, increased apoptosis, and hypophosphorylation of AMPK and its downstream target p27Kip1. AMPK activation or ectopic expression of a phosphomimetic p27Kip1 mutant was sufficient to suppress in vitro apoptosis, increase proliferation, and improve in vivo transplantation efficiency of aged MuSCs. Moreover, activation of the AMPK/p27Kip1 pathway reduced markers of cell senescence in aged cells, which was, in part, dependent on p27Kip1 phosphorylation. Thus, the AMPK/p27Kip1 pathway likely regulates the autophagy/apoptosis balance in aged MuSCs and may be a potential target for improving muscle regeneration in older individuals.

Synergistic effects of TGFβ2, WNT9a, and FGFR4 signals attenuate satellite cell differentiation during skeletal muscle development

Satellite cells play a key role in the aging, generation, and damage repair of skeletal muscle. The molecular mechanism of satellite cells in these processes remains largely unknown. This study systematically investigated for the first time the characteristics of mouse satellite cells at ten different ages. Results indicated that the number and differentiation capacity of satellite cells decreased with age during skeletal muscle development. Transcriptome analysis revealed that 2,907 genes were differentially expressed at six time points at postnatal stage. WGCNA and GO analysis indicated that 1,739 of the 2,907 DEGs were mainly involved in skeletal muscle development processes. Moreover, the results of WGCNA and protein interaction analysis demonstrated that Tgfβ2, Wnt9a, and Fgfr4 were the key genes responsible for the differentiation of satellite cells. Functional analysis showed that TGFβ2 and WNT9a inhibited, whereas FGFR4 promoted the differentiation of satellite cells. Furthermore, each two of them had a regulatory relationship at the protein level. In vivo study also confirmed that TGFβ2 could regulate the regeneration of skeletal muscle, as well as the expression of WNT9a and FGFR4. Therefore, we concluded that the synergistic effects of TGFβ2, WNT9a, and FGFR4 were responsible for attenuating of the differentiation of aging satellite cells during skeletal muscle development. This study provided new insights into the molecular mechanism of satellite cell development. The target genes and signaling pathways investigated in this study would be useful for improving the muscle growth of livestock or treating muscle diseases in clinical settings.

Transcript profile distinguishes variability in human myogenic progenitor cell expansion capacity

Primary human muscle progenitor cells (hMPCs) are commonly used to understand skeletal muscle biology, including the regenerative process. Variability from unknown origin in hMPC expansion capacity occurs independently of disease, age, or sex of the donor. We sought to determine the transcript profile that distinguishes hMPC cultures with greater expansion capacity and to identify biological underpinnings of these transcriptome profile differences. Sorted (CD56+/CD29+) hMPC cultures were clustered by unbiased, K-means cluster analysis into FAST and SLOW based on growth parameters (saturation density and population doubling time). FAST had greater expansion capacity indicated by significantly reduced population doubling time (-60%) and greater saturation density (+200%), nuclei area under the curve (AUC, +250%), and confluence AUC (+120%). Additionally, FAST had fewer % dead cells AUC (-44%, P < 0.05). RNA sequencing was conducted on RNA extracted during the expansion phase. Principal component analysis distinguished FAST and SLOW based on the transcript profiles. There were 2,205 differentially expressed genes (DEgenes) between FAST and SLOW (q value ≤ 0.05); 362 DEgenes met a more stringent cut-off (q value ≤ 0.001 and 2.0 fold-change). DEgene enrichment suggested FAST (vs. SLOW) had promotion of the cell cycle, reduced apoptosis and cellular senescence, and enhanced DNA replication. Novel (RABL6, IRGM1, and AREG) and known (FOXM1, CDKN1A, Rb) genes emerged as regulators of identified functional pathways. Collectively the data suggest that variation in hMPC expansion capacity occurs independently of age and sex and is driven, in part, by intrinsic mechanisms that support the cell cycle.