Muscle stem cell renewal suppressed by Gas1 can be reversed by GDNF in mice

Skeletal Muscle Satellite Cells Co-Opt the Tenogenic Gene Scleraxis to Instruct Regeneration

Skeletal muscles connect bones and tendons for locomotion and posture. Understanding the regenerative processes of muscle, bone and tendon is of importance to basic research and clinical applications. Despite their interconnections, distinct transcription factors have been reported to orchestrate each tissue's developmental and regenerative processes. Here we show that Scx expression is not detectable in adult muscle stem cells (also known as satellite cells, SCs) during quiescence. Scx expression begins in activated SCs and continues throughout regenerative myogenesis after injury. By SC-specific Scx gene inactivation (ScxcKO), we show that Scx function is required for SC expansion/renewal and robust new myofiber formation after injury. We combined single-cell RNA-sequencing and CUT&RUN to identify direct Scx target genes during muscle regeneration. These target genes help explain the muscle regeneration defects of ScxcKO, and are not overlapping with Scx -target genes identified in tendon development. Together with a recent finding of a subpopulation of Scx -expressing connective tissue fibroblasts with myogenic potential during early embryogenesis, we propose that regenerative and developmental myogenesis co-opt the Scx gene via different mechanisms.

Hydrogel biomaterials that stiffen and soften on demand reveal that skeletal muscle stem cells harbor a mechanical memory

Muscle stem cells (MuSCs) are specialized cells that reside in adult skeletal muscle poised to repair muscle tissue. The ability of MuSCs to regenerate damaged tissues declines markedly with aging and in diseases such as Duchenne muscular dystrophy, but the underlying causes of MuSC dysfunction remain poorly understood. Both aging and disease result in dramatic increases in the stiffness of the muscle tissue microenvironment from fibrosis. MuSCs are known to lose their regenerative potential if cultured on stiff plastic substrates. We sought to determine whether MuSCs harbor a memory of their past microenvironment and if it can be overcome. We tested MuSCs in situ using dynamic hydrogel biomaterials that soften or stiffen on demand in response to light and found that freshly isolated MuSCs develop a persistent memory of substrate stiffness characterized by loss of proliferative progenitors within the first three days of culture on stiff substrates. MuSCs cultured on soft hydrogels had altered cytoskeletal organization and activity of Rho and Rac guanosine triphosphate hydrolase (GTPase) and Yes-associated protein mechanotransduction pathways compared to those on stiff hydrogels. Pharmacologic inhibition identified RhoA activation as responsible for the mechanical memory phenotype, and single-cell RNA sequencing revealed a molecular signature of the mechanical memory. These studies highlight that microenvironmental stiffness regulates MuSC fate and leads to MuSC dysfunction that is not readily reversed by changing stiffness. Our results suggest that stiffness can be circumvented by targeting downstream signaling pathways to overcome stem cell dysfunction in aged and disease states with aberrant fibrotic tissue mechanics.

Tumor necrosis factor α deficiency promotes myogenesis and muscle regeneration

Tumor necrosis factor α (TNFα) exhibits diverse biological functions; however, its regulatory roles in myogenesis are not fully understood. In the present study, we explored the function of TNFα in myoblast proliferation, differentiation, migration, and myotube fusion in primary myoblasts and C2C12 cells. To this end, we constructed TNFα muscle-conditional knockout ( TNFα-CKO) mice and compared them with flox mice to assess the effects of TNFα knockout on skeletal muscles. Results indicated that TNFα-CKO mice displayed phenotypes such as accelerated muscle development, enhanced regenerative capacity, and improved exercise endurance compared to flox mice, with no significant differences observed in major visceral organs or skeletal structure. Using label-free proteomic analysis, we found that TNFα-CKO altered the distribution of several muscle development-related proteins, such as Hira, Casz1, Casp7, Arhgap10, Gas1, Diaph1, Map3k20, Cfl2, and Igf2, in the nucleus and cytoplasm. Gene set enrichment analysis (GSEA) further revealed that TNFα deficiency resulted in positive enrichment in oxidative phosphorylation and MyoD targets and negative enrichment in JAK-STAT signaling. These findings suggest that TNFα-CKO positively regulates muscle growth and development, possibly via these newly identified targets and pathways.

Creatine Kinase-MM/Proto-oncogene Tyrosine-Protein Kinase Receptor as a Sensitive Indicator for Duchenne Muscular Dystrophy Carriers

Duchenne muscular dystrophy (DMD), a lethal X-linked recessive genetic disease, is characterized by progressive muscle wasting which will lead to premature death by cardiorespiratory complications in their late twenties. And 2.5-19% DMD carriers that also suffer from skeletal muscle damage or dilated cardiomyopathy when diagnosed as soon as possible is meaningful for prenatal diagnosis and advance warning for self-health. The current DMD carrier screening mainly relies on detecting serum creatine kinase activity, covering only 50-70% DMD carriers which will cause many false negatives and require the discovery of highly effective biomarker and simple detection procedure for DMD carriers. In this article, we have compiled a comprehensive summary of all documented biomarkers associated with DMD and categorized them based on their expression patterns. We specifically pinpointed novel DMD biomarkers, previously unreported in DMD carriers, and conducted further investigations to explore their potential. Compared to creatine kinase activity alone in DMD carriers, creatine kinase-MM can improve the specificity from 73 to 81%. And our investigation revealed another promising protein: proto-oncogene tyrosine-protein kinase receptor (RET). When combined with creatine kinase-MM (creatine kinase-MM/RET ratio), it significantly enhances the specificity (from 81 to 83%) and sensitivity (from 71.4 to 93%) of detecting DMD carriers in serum. Moreover, we successfully devised an efficient method for extracting RET from dried blood spots. This breakthrough allowed us to detect both creatine kinase-MM and RET using dried blood spots without compromising the detection rate.

Dynamic changes in butyrate levels regulate satellite cell homeostasis by preventing spontaneous activation during aging

The gut microbiota plays a pivotal role in systemic metabolic processes and in particular functions, such as developing and preserving the skeletal muscle system. However, the interplay between gut microbiota/metabolites and the regulation of satellite cell (SC) homeostasis, particularly during aging, remains elusive. We propose that gut microbiota and its metabolites modulate SC physiology and homeostasis throughout skeletal muscle development, regeneration, and aging process. Our investigation reveals that microbial dysbiosis manipulated by either antibiotic treatment or fecal microbiota transplantation from aged to adult mice, leads to the activation of SCs or a significant reduction in the total number. Furthermore, employing multi-omics (e.g., RNA-seq, 16S rRNA gene sequencing, and metabolomics) and bioinformatic analysis, we demonstrate that the reduced butyrate levels, alongside the gut microbial dysbiosis, could be the primary factor contributing to the reduction in the number of SCs and subsequent impairments during skeletal muscle aging. Meanwhile, butyrate supplementation can mitigate the antibiotics-induced SC activation irrespective of gut microbiota, potentially by inhibiting the proliferation and differentiation of SCs/myoblasts. The butyrate effect is likely facilitated through the monocarboxylate transporter 1 (Mct1), a lactate transporter enriched on membranes of SCs and myoblasts. As a result, butyrate could serve as an alternative strategy to enhance SC homeostasis and function during skeletal muscle aging. Our findings shed light on the potential application of microbial metabolites in maintaining SC homeostasis and preventing skeletal muscle aging.

Restoration of CPEB4 prevents muscle stem cell senescence during aging

Age-associated impairments in adult stem cell functions correlate with a decline in somatic tissue regeneration capacity. However, the mechanisms underlying the molecular regulation of adult stem cell aging remain elusive. Here, we provide a proteomic analysis of physiologically aged murine muscle stem cells (MuSCs), illustrating a pre-senescent proteomic signature. During aging, the mitochondrial proteome and activity are impaired in MuSCs. In addition, the inhibition of mitochondrial function results in cellular senescence. We identified an RNA-binding protein, CPEB4, downregulated in various aged tissues, which is required for MuSC functions. CPEB4 regulates the mitochondrial proteome and activity through mitochondrial translational control. MuSCs devoid of CPEB4 induced cellular senescence. Importantly, restoring CPEB4 expression rescued impaired mitochondrial metabolism, improved geriatric MuSC functions, and prevented cellular senescence in various human cell lines. Our findings provide the basis for the possibility that CPEB4 regulates mitochondrial metabolism to govern cellular senescence, with an implication of therapeutic intervention for age-related senescence.

Multiomics reveals glutathione metabolism as a driver of bimodality during stem cell aging

With age, skeletal muscle stem cells (MuSCs) activate out of quiescence more slowly and with increased death, leading to defective muscle repair. To explore the molecular underpinnings of these defects, we combined multiomics, single-cell measurements, and functional testing of MuSCs from young and old mice. The multiomics approach allowed us to assess which changes are causal, which are compensatory, and which are simply correlative. We identified glutathione (GSH) metabolism as perturbed in old MuSCs, with both causal and compensatory components. Contrary to young MuSCs, old MuSCs exhibit a population dichotomy composed of GSHhigh cells (comparable with young MuSCs) and GSHlow cells with impaired functionality. Mechanistically, we show that antagonism between NRF2 and NF-κB maintains this bimodality. Experimental manipulation of GSH levels altered the functional dichotomy of aged MuSCs. These findings identify a novel mechanism of stem cell aging and highlight glutathione metabolism as an accessible target for reversing MuSC aging.

GAS1RR, an immune-related enhancer RNA, is related to biochemical recurrence-free survival in prostate cancer

Prostate cancer (PCa) is one of the malignant tumors of urinary system with a high morbidity. Enhancer RNA is a subclass of long non-coding RNA transcribed from active enhancer regions, which plays a critical role in gene transcriptional regulation. However, the role of enhancer RNA (eRNA) in PCa remains extremely mysterious. This study is aimed at exploring key prognostic eRNAs in PCa. First, we downloaded gene expression data and clinical data of 33 cancer types from UCSC Xena platform. Second, we selected reported putative eRNA-target pairs and performed the Kaplan-Meier survival and correlation analysis to determine the crucial eRNAs most related to biochemical recurrence (BCR)-free survival. Third, we explored the clinical characteristics with the key eRNA GAS1 adjacent regulatory RNA (GAS1RR) and performed a computational difference algorithm and the Cox regression analysis. Next, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed to explore the underlying mechanisms. Finally, we used the pan-cancer data from The Cancer Genome Atlas (TCGA) and performed reverse transcription-quantitative polymerase chain reaction (RT-qPCR) of 18 pairs of specimens to prove the results we acquired. Among all 2695 putative eRNAs, 6 pairs of eRNA-target genes were prominently related to BCR-free survival. Growth arrest-specific protein 1 (GAS1) was a target gene of GAS1RR (r = 0.86, P < 0.001). Patients with low GAS1RR expression were likely to have unfavorable clinical characteristics. The result of computational Cox regression analysis demonstrated that GAS1RR may predict the prognosis of PCa independently. RT-qPCR results illuminated that GAS1RR and GAS1 were both downregulated in PCa tissues, and they show a strong positive correlation. GO and KEGG analyses revealed biological processes that GAS1RR was mainly associated with. Immune infiltration analysis indicated that GAS1RR expression is correlated with the infiltration level of six kinds of immune cells. Our results suggest that GAS1RR may be clinically useful in the prediction of PCa prognosis. Moreover, it may also be a prognostic predictor and theoretic target with great promise in PCa.

Ginsenoside Rb1 Prevents Oxidative Stress-Induced Apoptosis and Mitochondrial Dysfunction in Muscle Stem Cells via NF-κB Pathway

Sarcopenia, featured by the progressive loss of skeletal muscle function and mass, is associated with the impaired function of muscle stem cells (MuSCs) caused by increasing oxidative stress in senescent skeletal muscle tissue during aging. Intact function of MuSCs maintains the regenerative potential as well as the homeostasis of skeletal muscle tissues during aging. Ginsenoside Rb1, a natural compound from ginseng, exhibited the effects of antioxidation and against apoptosis. However, its effects of restoring MuSC function during aging and improving age-related sarcopenia remained unknown. In this study, we investigated the role of Rb1 in improving MuSC function and inhibiting apoptosis by reducing oxidative stress levels. We found that Rb1 inhibited the accumulation of reactive oxygen species (ROS) and protected the cells from oxidative stress to attenuate the H₂O₂-induced cytotoxicity. Rb1 also blocked oxidative stress-induced apoptosis by inhibiting the activation of caspase-3/9, which antagonized the decrease in mitochondrial content and the increase in mitochondrial abnormalities caused by oxidative stress via promoting the protein expression of genes involved in mitochondrial biogenesis. Mechanistically, it was proven that Rb1 exerted its antioxidant effects and avoided the apoptosis of myoblasts by targeting the core regulator of the nuclear factor-kappa B (NF-κB) signal pathway. Therefore, these findings suggest that Rb1 may have a beneficial role in the prevention and treatment of MuSC exhaustion-related diseases like sarcopenia.

Muscle satellite cells are impaired in type 2 diabetic mice by elevated extracellular adenosine

Muscle regeneration is known to be defective under diabetic conditions. However, the underlying mechanisms remain less clear. Adult quiescent muscle satellite cells (MuSCs) from leptin-receptor-deficient (i.e., db/db) diabetic mice are defective in early activation in vivo, but not in culture, suggesting the involvement of pathogenic niche factors. Elevated extracellular adenosine (eAdo) and AMP (eAMP) are detected under diabetic conditions. eAdo and eAMP potently inhibit cell cycle re-entry of quiescent MuSCs and injury-induced muscle regeneration. Mechanistically, eAdo and eAMP engage the equilibrative Ado transporters (ENTs)-Ado kinase (ADK)-AMPK signaling axis in MuSCs to inhibit the mTORC1-dependent cell growth checkpoint. eAdo and eAMP also inhibit early activation of quiescent fibroadipogenic progenitors and human MuSCs by the same mechanism. Treatment of db/db diabetic mice with an ADK inhibitor partially rescues the activation defects of MuSCs in vivo. Thus, both ADK and ENTs represent potential therapeutic targets for restoring the regenerative functions of tissue stem cells in patients with diabetes.

A Long Journey before Cycling: Regulation of Quiescence Exit in Adult Muscle Satellite Cells

Skeletal muscle harbors a pool of stem cells called muscle satellite cells (MuSCs) that are mainly responsible for its robust regenerative capacities. Adult satellite cells are mitotically quiescent in uninjured muscles under homeostasis, but they exit quiescence upon injury to re-enter the cell cycle to proliferate. While most of the expanded satellites cells differentiate and fuse to form new myofibers, some undergo self-renewal to replenish the stem cell pool. Specifically, quiescence exit describes the initial transition of MuSCs from quiescence to the first cell cycle, which takes much longer than the time required for subsequent cell cycles and involves drastic changes in cell size, epigenetic and transcriptomic profiles, and metabolic status. It is, therefore, an essential period indispensable for the success of muscle regeneration. Diverse mechanisms exist in MuSCs to regulate quiescence exit. In this review, we summarize key events that occur during quiescence exit in MuSCs and discuss the molecular regulation of this process with an emphasis on multiple levels of intrinsic regulatory mechanisms. A comprehensive understanding of how quiescence exit is regulated will facilitate satellite cell-based muscle regenerative therapies and advance their applications in various disease and aging conditions.

Proximity Tagging Identifies the Glycan-Mediated Glycoprotein Interactors of Galectin-1 in Muscle Stem Cells

Myogenic differentiation, the irreversible developmental process where precursor myoblast muscle stem cells become contractile myotubes, is heavily regulated by glycosylation and glycan-protein interactions at the cell surface and the extracellular matrix. The glycan-binding protein galectin-1 has been found to be a potent activator of myogenic differentiation. While it is being explored as a potential therapeutic for muscle repair, a precise understanding of its glycoprotein interactors is lacking. These gaps are due in part to the difficulties of capturing glycan-protein interactions in live cells. Here, we demonstrate the use of a proximity tagging strategy coupled with quantitative mass-spectrometry-based proteomics to capture, enrich, and identify the glycan-mediated glycoprotein interactors of galectin-1 in cultured live mouse myoblasts. Our interactome dataset can serve as a resource to aid the determination of mechanisms through which galectin-1 promotes myogenic differentiation. Moreover, it can also facilitate the determination of the physiological glycoprotein counter-receptors of galectin-1. Indeed, we identify several known and novel glycan-mediated ligands of galectin-1 as well as validate that galectin-1 binds the native CD44 glycoprotein in a glycan-mediated manner.

The protein inputs of an ultra-predictive aging clock represent viable anti-aging drug targets

Machine learning models capable of predicting age given a set of inputs are referred to as aging clocks. We recently developed an aging clock that utilizes 491 plasma protein inputs, has an exceptional accuracy, and is capable of measuring biological age. Here, we demonstrate that this clock is extremely predictive (r = 0.95) when used to measure age in a novel plasma proteomic dataset derived from 370 human subjects aged 18-69 years. Over-representation analyses of the proteins that make up this clock in the Gene Ontology and Reactome databases predominantly implicated innate and adaptive immune system processes. Immunological drugs and various age-related diseases were enriched in the DrugBank and GLAD4U databases. By performing an extensive literature review, we find that at least 269 (54.8 %) of these inputs regulate lifespan and/or induce changes relevant to age-related disease when manipulated in an animal model. We also show that, in a large plasma proteomic dataset, the majority (57.2 %) of measurable clock proteins significantly change their expression level with human age. Different subsets of proteins were overlapped with distinct epigenetic, transcriptomic, and proteomic aging clocks. These findings indicate that the inputs of this age predictor likely represent a rich source of anti-aging drug targets.

Murine muscle stem cell response to perturbations of the neuromuscular junction are attenuated with aging

During aging and neuromuscular diseases, there is a progressive loss of skeletal muscle volume and function impacting mobility and quality of life. Muscle loss is often associated with denervation and a loss of resident muscle stem cells (satellite cells or MuSCs); however, the relationship between MuSCs and innervation has not been established. Herein, we administered severe neuromuscular trauma to a transgenic murine model that permits MuSC lineage tracing. We show that a subset of MuSCs specifically engraft in a position proximal to the neuromuscular junction (NMJ), the synapse between myofibers and motor neurons, in healthy young adult muscles. In aging and in a mouse model of neuromuscular degeneration (Cu/Zn superoxide dismutase knockout - Sod1-/-), this localized engraftment behavior was reduced. Genetic rescue of motor neurons in Sod1-/- mice reestablished integrity of the NMJ in a manner akin to young muscle and partially restored MuSC ability to engraft into positions proximal to the NMJ. Using single cell RNA-sequencing of MuSCs isolated from aged muscle, we demonstrate that a subset of MuSCs are molecularly distinguishable from MuSCs responding to myofiber injury and share similarity to synaptic myonuclei. Collectively, these data reveal unique features of MuSCs that respond to synaptic perturbations caused by aging and other stressors.

Extensive remodeling of the extracellular matrix during aging contributes to age-dependent impairments of muscle stem cell functionality

During aging, the regenerative capacity of skeletal muscle decreases due to intrinsic changes in muscle stem cells (MuSCs) and alterations in their niche. Here, we use quantitative mass spectrometry to characterize intrinsic changes in the MuSC proteome and remodeling of the MuSC niche during aging. We generate a network connecting age-affected ligands located in the niche and cell surface receptors on MuSCs. Thereby, we reveal signaling by integrins, Lrp1, Egfr, and Cd44 as the major cell communication axes perturbed through aging. We investigate the effect of Smoc2, a secreted protein that accumulates with aging, primarily originating from fibro-adipogenic progenitors. Increased levels of Smoc2 contribute to the aberrant Integrin beta-1 (Itgb1)/mitogen-activated protein kinase (MAPK) signaling observed during aging, thereby causing impaired MuSC functionality and muscle regeneration. By connecting changes in the proteome of MuSCs to alterations of their niche, our work will enable a better understanding of how MuSCs are affected during aging.

Translational research on Myology and Mobility Medicine: 2021 semi-virtual PDM3 from Thermae of Euganean Hills, May 26 - 29, 2021

On 19-21 November 2020, the meeting of the 30 years of the Padova Muscle Days was virtually held while the SARS-CoV-2 epidemic was hitting the world after a seemingly quiet summer. During the 2020-2021 winter, the epidemic is still active, despite the start of vaccinations. The organizers hope to hold the 2021 Padua Days on Myology and Mobility Medicine in a semi-virtual form (2021 S-V PDM3) from May 26 to May 29 at the Thermae of Euganean Hills, Padova, Italy. Here the program and the Collection of Abstracts are presented. Despite numerous world problems, the number of submitted/selected presentations (lectures and oral presentations) has increased, prompting the organizers to extend the program to four dense days.

Pro-myogenic small molecules revealed by a chemical screen on primary muscle stem cells

Satellite cells are the canonical muscle stem cells that regenerate damaged skeletal muscle. Loss of function of these cells has been linked to reduced muscle repair capacity and compromised muscle health in acute muscle injury and congenital neuromuscular diseases. To identify new pathways that can prevent loss of skeletal muscle function or enhance regenerative potential, we established an imaging-based screen capable of identifying small molecules that promote the expansion of freshly isolated satellite cells. We found several classes of receptor tyrosine kinase (RTK) inhibitors that increased freshly isolated satellite cell numbers in vitro. Further exploration of one of these compounds, the RTK inhibitor CEP-701 (also known as lestaurtinib), revealed potent activity on mouse satellite cells both in vitro and in vivo. This expansion potential was not seen upon exposure of proliferating committed myoblasts or non-myogenic fibroblasts to CEP-701. When delivered subcutaneously to acutely injured animals, CEP-701 increased both the total number of satellite cells and the rate of muscle repair, as revealed by an increased cross-sectional area of regenerating fibers. Moreover, freshly isolated satellite cells expanded ex vivo in the presence of CEP-701 displayed enhanced muscle engraftment potential upon in vivo transplantation. We provide compelling evidence that certain RTKs, and in particular RET, regulate satellite cell expansion during muscle regeneration. This study demonstrates the power of small molecule screens of even rare adult stem cell populations for identifying stem cell-targeting compounds with therapeutic potential.

Aging Induced p53/p21 in Genioglossus Muscle Stem Cells and Enhanced Upper Airway Injury

Aging of population brings related social problems, such as muscle attenuation and regeneration barriers with increased aging. Muscle repair and regeneration depend on muscle stem cells (MuSCs). Obstructive sleep apnea (OSA) rises in the aging population. OSA leads to hypoxia and upper airway muscle injury. However, little is known about the effect of increasing age and hypoxia to the upper airway muscle. The genioglossus (GG) is the major dilator muscle to keep the upper airway open. Here, we reported that muscle fiber and MuSC function declined with aging in GG. Increasing age also decreased the migration and proliferation of GG MuSCs. p53 and p21 were high expressions both in muscle tissue and in GG MuSCs. We further found that hypoxia inhibited GG MuSC proliferation and decreased myogenic differentiation. Then, hypoxia enhanced the inhibition effect of aging to proliferation and differentiation. Finally, we investigated that hypoxia and aging interact to form a vicious circle with upregulation of p53 and p21. This vicious hypoxia plus aging damage accelerated upper airway muscle injury. Aging and hypoxia are the major damage elements in OSA patients, and we propose that the damage mechanism of hypoxia and aging in GG MuSCs will help to improve upper airway muscle regeneration.