Induction of muscle stem cell quiescence by the secreted niche factor Oncostatin M

Trained innate immunity modulates osteoblast and osteoclast differentiation

Macrophages are key regulators in bone repair and regeneration. Recent studies have shown that long-term epigenetic changes and metabolic shifts occur during specific immune training of macrophages that affect their functional state, resulting in heightened (trained) or reduced (tolerant) responses upon exposure to a second stimulus. This is known as innate immune memory. Here, we study the impact of macrophages' memory trait on osteoblast differentiation of human mesenchymal stromal cells (hMSCs) and osteoclast differentiation. An in vitro trained immunity protocol of monocyte-derived macrophages was employed using inactivated Candida albicans and Bacillus Calmette-Guérin (BCG) to induce a 'trained' state and Pam3CSK4 (PAM) and Lipopolysaccharides (LPS) to induce a 'tolerance' state. Macrophages were subsequently cocultured with hMSCs undergoing osteogenic differentiation during either resting (unstimulated) or inflammatory conditions (restimulated with LPS). Alkaline phosphatase activity, mineralization, and cytokine levels (TNF, IL-6, oncostatin M and SDF-1α) were measured. In addition, macrophages underwent osteoclast differentiation. Our findings show that trained and tolerized macrophages induced opposing results. Under resting conditions, BCG-trained macrophages enhanced ALP levels (threefold), while under inflammatory conditions this was found in the LPS-tolerized macrophages (fourfold). Coculture of hMSCs with trained macrophages showed mineralization while tolerized macrophages inhibited the process under both resting and inflammatory conditions. While osteoclast differentiation was not affected in trained-macrophages, this ability was significantly loss in tolerized ones. This study further confirms the intricate cross talk between immune cells and bone cells, highlighting the need to consider this interaction in the development of personalized approaches for bone regenerative medicine.

Oncostatin M signaling drives cancer-associated skeletal muscle wasting

Progressive weakness and muscle loss are associated with multiple chronic conditions, including muscular dystrophy and cancer. Cancer-associated cachexia, characterized by dramatic weight loss and fatigue, leads to reduced quality of life and poor survival. Inflammatory cytokines have been implicated in muscle atrophy; however, available anticytokine therapies failed to prevent muscle wasting in cancer patients. Here, we show that oncostatin M (OSM) is a potent inducer of muscle atrophy. OSM triggers cellular atrophy in primary myotubes using the JAK/STAT3 pathway. Identification of OSM targets by RNA sequencing reveals the induction of various muscle atrophy-related genes, including Atrogin1. OSM overexpression in mice causes muscle wasting, whereas muscle-specific deletion of the OSM receptor (OSMR) and the neutralization of circulating OSM preserves muscle mass and function in tumor-bearing mice. Our results indicate that activated OSM/OSMR signaling drives muscle atrophy, and the therapeutic targeting of this pathway may be useful in preventing muscle wasting.

Denervation alters the secretome of myofibers and thereby affects muscle stem cell lineage progression and functionality

Skeletal muscle function crucially depends on innervation while repair of skeletal muscle relies on resident muscle stem cells (MuSCs). However, it is poorly understood how innervation affects MuSC properties and thereby regeneration of skeletal muscle. Here, we report that loss of innervation causes precocious activation of MuSCs concomitant with the expression of markers of myogenic differentiation. This aberrant activation of MuSCs after loss of innervation is accompanied by profound alterations on the mRNA and protein level. Combination of muscle injury with loss of innervation results in impaired regeneration of skeletal muscle including shifts in myogenic populations concomitant with delayed maturation of regenerating myofibers. We further demonstrate that loss of innervation leads to alterations in myofibers and their secretome, which then affect MuSC behavior. In particular, we identify an increased secretion of Osteopontin and transforming growth factor beta 1 (Tgfb1) by myofibers isolated from mice which had undergone sciatic nerve transection. The altered secretome results in the upregulation of early activating transcription factors, such as Junb, and their target genes in MuSCs. However, the combination of different secreted factors from myofibers after loss of innervation is required to cause the alterations observed in MuSCs after loss of innervation. These data demonstrate that loss of innervation first affects myofibers causing alterations in their secretome which then affect MuSCs underscoring the importance of proper innervation for MuSC functionality and regeneration of skeletal muscle.

The extracellular matrix niche of muscle stem cells

Preserving the potency of stem cells in adult tissues is very demanding and relies on the concerted action of various cellular and non-cellular elements in a precise stoichiometry. This balanced microenvironment is found in specific anatomical "pockets" within the tissue, known as the stem cell niche. In this review, we explore the interplay between stem cells and their niches, with a primary focus on skeletal muscle stem cells and the extracellular matrix (ECM). Quiescent muscle stem cells, known as satellite cells are active producers of a diverse array of ECM molecules, encompassing major constituents like collagens, laminins, and integrins, some of which are explored in this review. The conventional perception of ECM as merely a structural scaffold is evolving. Collagens can directly interact as ligands with receptors on satellite cells, while other ECM proteins have the capacity to sequester growth factors and regulate their release, especially relevant during satellite cell turnover in homeostasis or activation upon injury. Additionally, we explore an evolutionary perspective on the ECM across a range of multicellular organisms and discuss a model wherein satellite cells are self-sustained by generating their own niche. Considering the prevalence of ECM proteins in the connective tissue of various organs it is not surprising that mutations in ECM genes have pathological implications, including in muscle, where they can lead to myopathies. However, the particular role of certain disease-related ECM proteins in stem cell maintenance highlights the potential contribution of stem cell deregulation to the progression of these disorders.

Oncostatin M: Dual Regulator of the Skeletal and Hematopoietic Systems

PURPOSE OF THE REVIEW

The bone and hematopoietic tissues coemerge during development and are functionally intertwined throughout mammalian life. Oncostatin M (OSM) is an inflammatory cytokine of the interleukin-6 family produced by osteoblasts, bone marrow macrophages, and neutrophils. OSM acts via two heterodimeric receptors comprising GP130 with either an OSM receptor (OSMR) or a leukemia inhibitory factor receptor (LIFR). OSMR is expressed on osteoblasts, mesenchymal, and endothelial cells and mice deficient for the Osm or Osmr genes have both bone and blood phenotypes illustrating the importance of OSM and OSMR in regulating these two intertwined tissues.

RECENT FINDINGS

OSM regulates bone mass through signaling via OSMR, adaptor protein SHC1, and transducer STAT3 to both stimulate osteoclast formation and promote osteoblast commitment; the effect on bone formation is also supported by action through LIFR. OSM produced by macrophages is an important inducer of neurogenic heterotopic ossifications in peri-articular muscles following spinal cord injury. OSM produced by neutrophils in the bone marrow induces hematopoietic stem and progenitor cell proliferation in an indirect manner via OSMR expressed by bone marrow stromal and endothelial cells that form hematopoietic stem cell niches. OSM acts as a brake to therapeutic hematopoietic stem cell mobilization in response to G-CSF and CXCR4 antagonist plerixafor. Excessive OSM production by macrophages in the bone marrow is a key contributor to poor hematopoietic stem cell mobilization (mobilopathy) in people with diabetes. OSM and OSMR may also play important roles in the progression of several cancers. It is increasingly clear that OSM plays unique roles in regulating the maintenance and regeneration of bone, hematopoietic stem and progenitor cells, inflammation, and skeletal muscles. Dysregulated OSM production can lead to bone pathologies, defective muscle repair and formation of heterotopic ossifications in injured muscles, suboptimal mobilization of hematopoietic stem cells, exacerbated inflammatory responses, and anti-tumoral immunity. Ongoing research will establish whether neutralizing antibodies or cytokine traps may be useful to correct pathologies associated with excessive OSM production.

In vitro-generated human muscle reserve cells are heterogeneous for Pax7 with distinct molecular states and metabolic profiles

BACKGROUND

The capacity of skeletal muscles to regenerate relies on Pax7+ muscle stem cells (MuSC). While in vitro-amplified MuSC are activated and lose part of their regenerative capacity, in vitro-generated human muscle reserve cells (MuRC) are very similar to quiescent MuSC with properties required for their use in cell-based therapies.

METHODS

In the present study, we investigated the heterogeneity of human MuRC and characterized their molecular signature and metabolic profile.

RESULTS

We observed that Notch signaling is active and essential for the generation of quiescent human Pax7+ MuRC in vitro. We also revealed, by immunofluorescence and flow cytometry, two distinct subpopulations of MuRC distinguished by their relative Pax7 expression. After 48 h in differentiation medium (DM), the Pax7High subpopulation represented 35% of the total MuRC pool and this percentage increased to 61% after 96 h in DM. Transcriptomic analysis revealed that Pax7High MuRC were less primed for myogenic differentiation as compared to Pax7Low MuRC and displayed a metabolic shift from glycolysis toward fatty acid oxidation. The bioenergetic profile of human MuRC displayed a 1.5-fold decrease in glycolysis, basal respiration and ATP-linked respiration as compared to myoblasts. We also observed that AMPKα1 expression was significantly upregulated in human MuRC that correlated with an increased phosphorylation of acetyl-CoA carboxylase (ACC). Finally, we showed that fatty acid uptake was increased in MuRC as compared to myoblasts, whereas no changes were observed for glucose uptake.

CONCLUSIONS

Overall, these data reveal that the quiescent MuRC pool is heterogeneous for Pax7 with a Pax7High subpopulation being in a deeper quiescent state, less committed to differentiation and displaying a reduced metabolic activity. Altogether, our data suggest that human Pax7High MuRC may constitute an appropriate stem cell source for potential therapeutic applications in skeletal muscle diseases.

EMT/MET plasticity in cancer and Go-or-Grow decisions in quiescence: the two sides of the same coin?

Epithelial mesenchymal transition (EMT) and mesenchymal epithelial transition (MET) are genetic determinants of cellular plasticity. These programs operate in physiological (embryonic development, wound healing) and pathological (organ fibrosis, cancer) conditions. In cancer, EMT and MET interfere with various signalling pathways at different levels. This results in gross alterations in the gene expression programs, which affect most, if not all hallmarks of cancer, such as response to proliferative and death-inducing signals, tumorigenicity, and cell stemness. EMT in cancer cells involves large scale reorganisation of the cytoskeleton, loss of epithelial integrity, and gain of mesenchymal traits, such as mesenchymal type of cell migration. In this regard, EMT/MET plasticity is highly relevant to the Go-or-Grow concept, which postulates the dichotomous relationship between cell motility and proliferation. The Go-or-Grow decisions are critically important in the processes in which EMT/MET plasticity takes the central stage, mobilisation of stem cells during wound healing, cancer relapse, and metastasis. Here we outline the maintenance of quiescence in stem cell and metastatic niches, focusing on the implication of EMT/MET regulatory networks in Go-or-Grow switches. In particular, we discuss the analogy between cells residing in hybrid quasi-mesenchymal states and G_Alert, an intermediate phase allowing quiescent stem cells to enter the cell cycle rapidly.

OSMR deficiency aggravates pressure overload-induced cardiac hypertrophy by modulating macrophages and OSM/LIFR/STAT3 signalling

BACKGROUND

Oncostatin M (OSM) is a secreted cytokine of the interleukin (IL)-6 family that induces biological effects by activating functional receptor complexes of the common signal transducing component glycoprotein 130 (gp130) and OSM receptor β (OSMR) or leukaemia inhibitory factor receptor (LIFR), which are mainly involved in chronic inflammatory and cardiovascular diseases. The effect and underlying mechanism of OSM/OSMR/LIFR on the development of cardiac hypertrophy remains unclear.

METHODS AND RESULTS

OSMR-knockout (OSMR-KO) mice were subjected to aortic banding (AB) surgery to establish a model of pressure overload-induced cardiac hypertrophy. Echocardiographic, histological, biochemical and immunological analyses of the myocardium and the adoptive transfer of bone marrow-derived macrophages (BMDMs) were conducted for in vivo studies. BMDMs were isolated and stimulated with lipopolysaccharide (LPS) for the in vitro study. OSMR deficiency aggravated cardiac hypertrophy, fibrotic remodelling and cardiac dysfunction after AB surgery in mice. Mechanistically, the loss of OSMR activated OSM/LIFR/STAT3 signalling and promoted a proresolving macrophage phenotype that exacerbated inflammation and impaired cardiac repair during remodelling. In addition, adoptive transfer of OSMR-KO BMDMs to WT mice after AB surgery resulted in a consistent hypertrophic phenotype. Moreover, knockdown of LIFR in myocardial tissue with Ad-shLIFR ameliorated the effects of OSMR deletion on the phenotype and STAT3 activation.

CONCLUSIONS

OSMR deficiency aggravated pressure overload-induced cardiac hypertrophy by modulating macrophages and OSM/LIFR/STAT3 signalling, which provided evidence that OSMR might be an attractive target for treating pathological cardiac hypertrophy and heart failure.

Inactivating IL34 promotes regenerating muscle stem cell expansion and attenuates Duchenne muscular dystrophy in mouse models

Background: The balance between the differentiation and self-renewal of satellite cells (SCs) is essential for skeletal muscle homeostasis and regeneration. Our knowledge of this regulatory process is incomplete. Methods: Using global and conditional knockout mice as in vivo models and isolated satellite cells as in vitro system, we investigated the regulatory mechanisms of IL34 in the process of skeletal muscle regeneration in vivo and in vitro. Results: Myocytes and regenerating fibers are major source of IL34. Deletion of interleukin 34 (IL34) sustains expansion by sacrificing the differentiation of SCs and leads to significant muscle regeneration defects. We further found that inactivating IL34 in SCs leads to hyperactivation of NFKB1 signaling; NFKB1 translocates to the nucleus and binds to the promoter region of Igfbp5 to synergistically disturb protein kinase B (Akt) activity. Notably, augmented Igfbp5 function in SCs led to deficient differentiation and Akt activity. Furthermore, disrupting Akt activity both in vivo and in vitro mimicked the phenotype of IL34 knockout. Finally, deleting IL34 or interfering Akt in mdx mice ameliorates dystrophic muscles. Conclusion: We comprehensively characterized regenerating myofibers-expressed IL34 plays a pivotal role in controlling myonuclear domain. The results also indicate that impairing IL34 function by promoting SC maintenance can lead to improved muscular performance in mdx mice in which the stem cell pool is compromised.

The emergence of the stem cell niche

Stem cell niches are composed of dynamic microenvironments that support stem cells over a lifetime. The emerging niche is distinct from the adult because its main role is to support the progenitors that build organ systems in development. Emerging niches mature through distinct stages to form the adult niche and enable proper stem cell support. As a model of emerging niches, this review highlights how differences in the skeletal muscle microenvironment influence emerging versus satellite cell (SC) niche formation in skeletal muscle, which is among the most regenerative tissue systems. We contrast how stem cell niches regulate intrinsic properties between progenitor and stem cells throughout development to adulthood. We describe new applications for generating emerging niches from human pluripotent stem cells (hPSCs) using developmental principles and highlight potential applications for regeneration and therapeutics.

Transcriptional reprogramming of skeletal muscle stem cells by the niche environment

Adult stem cells are indispensable for tissue regeneration, but their function declines with age. The niche environment in which the stem cells reside plays a critical role in their function. However, quantification of the niche effect on stem cell function is lacking. Using muscle stem cells (MuSC) as a model, we show that aging leads to a significant transcriptomic shift in their subpopulations accompanied by locus-specific gain and loss of chromatin accessibility and DNA methylation. By combining in vivo MuSC transplantation and computational methods, we show that the expression of approximately half of all age-altered genes in MuSCs from aged male mice can be restored by exposure to a young niche environment. While there is a correlation between gene reversibility and epigenetic alterations, restoration of gene expression occurs primarily at the level of transcription. The stem cell niche environment therefore represents an important therapeutic target to enhance tissue regeneration in aging.

Machine learning-based classification of dual fluorescence signals reveals muscle stem cell fate transitions in response to regenerative niche factors

The proper regulation of muscle stem cell (MuSC) fate by cues from the niche is essential for regeneration of skeletal muscle. How pro-regenerative niche factors control the dynamics of MuSC fate decisions remains unknown due to limitations of population-level endpoint assays. To address this knowledge gap, we developed a dual fluorescence imaging time lapse (Dual-FLIT) microscopy approach that leverages machine learning classification strategies to track single cell fate decisions with high temporal resolution. Using two fluorescent reporters that read out maintenance of stemness and myogenic commitment, we constructed detailed lineage trees for individual MuSCs and their progeny, classifying each division event as symmetric self-renewing, asymmetric, or symmetric committed. Our analysis reveals that treatment with the lipid metabolite, prostaglandin E2 (PGE2), accelerates the rate of MuSC proliferation over time, while biasing division events toward symmetric self-renewal. In contrast, the IL6 family member, Oncostatin M (OSM), decreases the proliferation rate after the first generation, while blocking myogenic commitment. These insights into the dynamics of MuSC regulation by niche cues were uniquely enabled by our Dual-FLIT approach. We anticipate that similar binary live cell readouts derived from Dual-FLIT will markedly expand our understanding of how niche factors control tissue regeneration in real time.

Mechanical compression creates a quiescent muscle stem cell niche

Tissue stem cell niches are regulated by their mechanical environment, notably the extracellular matrix (ECM). Skeletal muscles consist of bundled myofibers for force transmission. Within this macroscopic architecture, quiescent Pax7-expressing (Pax7⁺) muscle stem cells (MuSCs) are compressed between ECM basally and myofiber apically. Muscle injury causes MuSCs to lose apical compression from the myofiber and re-enter the cell cycle for regeneration. While ECM elasticities have been shown to affect MuSC's renewal, the significance of apical compression remains unknown. To investigate the role of apical compression, we simulate the MuSCs' in vivo mechanical environment by applying physical compression to MuSCs' apical surface. We demonstrate that compression drives activated MuSCs back to a quiescent stem cell state, regardless of basal elasticities and chemistries. By mathematical modeling and cell tension manipulation, we conclude that low overall tension combined with high axial tension generated by compression leads to MuSCs' stemness and quiescence. Unexpectedly, we discovered that apical compression results in up-regulation of Notch downstream genes, accompanied by the increased levels of nuclear Notch1&3 in a Delta ligand (Dll) and ADAM10/17 independent manner. Our results fill a knowledge gap on the role of apical compression for MuSC fate and have implications to stem cells in other tissues.

Quantification of Muscle Satellite Stem Cell Divisions by High-Content Analysis

High-content screening is commonly performed on 2D cultured cells, which is high throughput but has low biological relevance. In contrast, single myofiber culture assay preserves the satellite cell niche between the basal lamina and sarcolemma and consequently has high biological relevance but is low throughput. We describe here a high-content screening method that utilizes single myofiber culture that addresses the caveats of both techniques. Our method utilizes the transgenic reporter allele Myf5-Cre:R26R-eYFP to differentiate stem and committed cells within a dividing couplet that can be quantified by high-content throughput immunodetection and bioinformatic analysis.

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.

Exogenous Oncostatin M induces Cardiac Dysfunction, Musculoskeletal Atrophy, and Fibrosis

Musculoskeletal diseases such as muscular dystrophy, cachexia, osteoarthritis, and rheumatoid arthritis impair overall physical health and reduce survival. Patients suffer from pain, dysfunction, and dysmobility due to inflammation and fibrosis in bones, muscles, and joints, both locally and systemically. The Interleukin-6 (IL-6) family of cytokines, most notably IL-6, is implicated in musculoskeletal disorders and cachexia. Here we show elevated circulating levels of OSM in murine pancreatic cancer cachexia and evaluate the effects of the IL-6 family member, Oncostatin M (OSM), on muscle and bone using adeno-associated virus (AAV) mediated over-expression of murine OSM in wildtype and IL-6 deficient mice. Initial studies with high titer AAV-OSM injection yielded high circulating OSM and IL-6, thrombocytosis, inflammation, and 60% mortality without muscle loss within 4 days. Subsequently, to mimic OSM levels in cachexia, a lower titer of AAV-OSM was used in wildtype and Il6 null mice, observing effects out to 4 weeks and 12 weeks. AAV-OSM caused muscle atrophy and fibrosis in the gastrocnemius, tibialis anterior, and quadriceps of the injected limb, but these effects were not observed on the non-injected side. In contrast, OSM induced both local and distant trabecular bone loss as shown by reduced bone volume, trabecular number, and thickness, and increased trabecular separation. OSM caused cardiac dysfunction including reduced ejection fraction and reduced fractional shortening. RNA-sequencing of cardiac muscle revealed upregulation of genes related to inflammation and fibrosis. None of these effects were different in IL-6 knockout mice. Thus, OSM induces local muscle atrophy, systemic bone loss, tissue fibrosis, and cardiac dysfunction independently of IL-6, suggesting a role for OSM in musculoskeletal conditions with these characteristics, including cancer cachexia.

Muscle cell-derived cytokines in skeletal muscle regeneration

Regeneration of the mammalian adult skeletal muscle is a well-orchestrated process regulated by multiple proteins and signalling pathways. Cytokines constitute a major class of regulators of skeletal myogenesis. It is well established that infiltrating immune cells at the site of muscle injury secrete cytokines, which play critical roles in the myofibre repair and regeneration process. In the past 10-15 years, skeletal muscle itself has emerged as a prolific producer of cytokines. Much attention in the field has been focused on the endocrine effects of muscle-secreted cytokines (myokines) on metabolic regulation. However, ample evidence suggests that muscle-derived cytokines also regulate myogenic differentiation and muscle regeneration in an autocrine manner. In this review, we survey cytokines that meet two criteria: (a) evidence of expression by muscle cells; (b) evidence demonstrating a myogenic function. Dozens of cytokines representing several major classes make up this group, and together they regulate all steps of the myogenic process. How such a large array of cytokines coordinate their signalling to form a regulatory network is a fascinating, pressing question. Functional studies that can distinguish the source of the cytokines in vivo are also much needed in order to facilitate exploration of their full therapeutic potential.

Oncostatin M: Risks and Benefits of a Novel Therapeutic Target for Atherosclerosis

BACKGROUND

Cardiovascular disease (CVD) is a leading cause of death worldwide. It is predicted that approximately 23.6 million people will die from CVDs annually by 2030. Therefore, there is a great need for an effective therapeutic approach to combat this disease. The European Cardiovascular Target Discovery (CarTarDis) consortium identified Oncostatin M (OSM) as a potential therapeutic target for atherosclerosis. The benefits of modulating OSM - an interleukin (IL)-6 family cytokine - have since been studied for multiple indications. However, as decades of high attrition rates have stressed, the success of a drug target is determined by the fine balance between benefits and the risk of adverse events. Safety issues should therefore not be overlooked.

OBJECTIVE

In this review, a risk/benefit analysis is performed on OSM inhibition in the context of atherosclerosis treatment. First, OSM signaling characteristics and its role in atherosclerosis are described. Next, an overview of in vitro, in vivo, and clinical findings relating to both the benefits and risks of modulating OSM in major organ systems is provided. Based on OSM's biological function and expression profile as well as drug intervention studies, safety concerns of inhibiting this target have been identified, assessed, and ranked for the target population.

CONCLUSION

While OSM may be of therapeutic value in atherosclerosis, drug development should also focus on de-risking the herein identified major safety concerns: tissue remodeling, angiogenesis, bleeding, anemia, and NMDA- and glutamate-induced neurotoxicity. Close monitoring and/or exclusion of patients with various comorbidities may be required for optimal therapeutic benefit.

Regulation of stem cell dynamics through volume exclusion

The maintenance and regeneration of adult tissues rely on the self-renewal of stem cells. Regeneration without over-proliferation requires precise regulation of the stem cell proliferation and differentiation rates. The nature of such regulatory mechanisms in different tissues, and how to incorporate them in models of stem cell population dynamics, is incompletely understood. The critical birth-death (CBD) process is widely used to model stem cell populations, capturing key phenomena, such as scaling laws in clone size distributions. However, the CBD process neglects regulatory mechanisms. Here, we propose the birth-death process with volume exclusion (vBD), a variation of the birth-death process that considers crowding effects, such as may arise due to limited space in a stem cell niche. While the deterministic rate equations predict a single non-trivial attracting steady state, the master equation predicts extinction and transient distributions of stem cell numbers with three possible behaviours: long-lived quasi-steady state (QSS), and short-lived bimodal or unimodal distributions. In all cases, we approximate solutions to the vBD master equation using a renormalized system-size expansion, QSS approximation and the Wentzel–Kramers–Brillouin method. Our study suggests that the size distribution of a stem cell population bears signatures that are useful to detect negative feedback mediated via volume exclusion.

Nutrition and stem cell integrity in aging

Adult stem cells (SCs) represent the regenerative capacity of organisms throughout their lifespan. The maintenance of robust SC populations capable of renewing organs and physiological systems is one hallmark of healthy aging. The local environment of SCs, referred to as the niche, includes the nutritional milieu, which is essential to maintain the quantity and quality of SCs available for renewal and regeneration. There is increased recognition that SCs have unique metabolism and conditional nutrient needs compared to fully differentiated cells. However, the contribution of SC nutrition to overall human nutritional requirements is an understudied and underappreciated area of investigation. Nutrient needs vary across the lifespan and are modified by many factors including individual health, disease, physiological states including pregnancy, age, sex, and during recovery from injury. Although current nutrition guidance is generally derived for apparently healthy populations and to prevent nutritional deficiency diseases, there are increased efforts to establish nutrient-based and food-based recommendations based on reducing chronic disease. Understanding the dynamics of SC nutritional needs throughout the life span, including the role of nutrition in extending biological age by blunting biological systems decay, is fundamental to establishing food and nutrient guidance for chronic disease reduction and health maintenance. This review summarizes a 3-day symposium of the Marabou Foundation (www.marabousymposium.org) held to examine the metabolic properties and unique nutritional needs of adult SCs and their role in healthy aging and age-related chronic disease.

Contribution of muscle satellite cells to sarcopenia

Sarcopenia, a disorder characterized by age-related muscle loss and reduced muscle strength, is associated with decreased individual independence and quality of life, as well as a high risk of death. Skeletal muscle houses a normally mitotically quiescent population of adult stem cells called muscle satellite cells (MuSCs) that are responsible for muscle maintenance, growth, repair, and regeneration throughout the life cycle. Patients with sarcopenia are often exhibit dysregulation of MuSCs homeostasis. In this review, we focus on the etiology, assessment, and treatment of sarcopenia. We also discuss phenotypic and regulatory mechanisms of MuSC quiescence, activation, and aging states, as well as the controversy between MuSC depletion and sarcopenia. Finally, we give a multi-dimensional treatment strategy for sarcopenia based on improving MuSC function.

Fluid shear stress-induced mechanotransduction in myoblasts: Does it depend on the glycocalyx?

Muscle stem cells (MuSCs) are involved in muscle maintenance and regeneration. Mechanically loaded MuSCs within their native niche undergo tensile and shear deformations, but how MuSCs sense mechanical stimuli and translate these into biochemical signals regulating function and fate is still poorly understood. We aimed to investigate whether the glycocalyx is involved in the MuSC mechanoresponse, and whether MuSC morphology affects mechanical loading-induced pressure, shear stress, and fluid velocity distribution. FSS-induced deformation of active proliferating MuSCs (myoblasts) with intact or degraded glycocalyx was assessed by live-cell imaging. Glycocalyx-degradation did not significantly affect nitric oxide production, but reduced FSS-induced myoblast deformation and modulated gene expression. Finite-element analysis revealed that the distribution of FSS-induced pressure, shear stress, and fluid velocity on myoblasts was non-uniform, and the magnitude depended on myoblast morphology and apex-height. In conclusion, our results suggest that the glycocalyx does not play a role in NO production in myoblasts but might impact mechanotransduction and gene expression, which needs further investigation. Future studies will unravel the underlying mechanism by which the glycocalyx affects FSS-induced myoblast deformation, which might be related to increased drag forces. Moreover, MuSCs with varying apex-height experience different levels of FSS-induced pressure, shear stress, and fluid velocity, suggesting differential responsiveness to fluid shear forces.

Myoblast deactivation within engineered human skeletal muscle creates a transcriptionally heterogeneous population of quiescent satellite-like cells

Satellite cells (SCs), the adult Pax7-expressing stem cells of skeletal muscle, are essential for muscle repair. However, in vitro investigations of SC function are challenging due to isolation-induced SC activation, loss of native quiescent state, and differentiation to myoblasts. In the present study, we optimized methods to deactivate in vitro expanded human myoblasts within a 3D culture environment of engineered human skeletal muscle tissues ("myobundles"). Immunostaining and gene expression analyses revealed that a fraction of myoblasts within myobundles adopted a quiescent phenotype (3D-SCs) characterized by increased Pax7 expression, cell cycle exit, and activation of Notch signaling. Similar to native SCs, 3D-SC quiescence is regulated by Notch and Wnt signaling while loss of quiescence and reactivation of 3D-SCs can be induced by growth factors including bFGF. Myobundle injury with a bee toxin, melittin, induces robust myofiber fragmentation, functional decline, and 3D-SC proliferation. By applying single cell RNA-sequencing (scRNA-seq), we discover the existence of two 3D-SC subpopulations (quiescent and activated), identify deactivation-associated gene signature using trajectory inference between 2D myoblasts and 3D-SCs, and characterize the transcriptomic changes within reactivated 3D-SCs in response to melittin-induced injury. These results demonstrate the ability of an in vitro engineered 3D human skeletal muscle environment to support the formation of a quiescent and heterogeneous SC population recapitulating several aspects of the native SC phenotype, and provide a platform for future studies of human muscle regeneration and disease-associated SC dysfunction.

Integrative molecular roadmap for direct conversion of fibroblasts into myocytes and myogenic progenitor cells

Transient MyoD overexpression in concert with small molecule treatment reprograms mouse fibroblasts into induced myogenic progenitor cells (iMPCs). However, the molecular landscape and mechanisms orchestrating this cellular conversion remain unknown. Here, we undertook an integrative multiomics approach to delineate the process of iMPC reprogramming in comparison to myogenic transdifferentiation mediated solely by MyoD. Using transcriptomics, proteomics, and genome-wide chromatin accessibility assays, we unravel distinct molecular trajectories that govern the two processes. Notably, only iMPC reprogramming is characterized by gradual up-regulation of muscle stem cell markers, unique signaling pathways, and chromatin remodelers in conjunction with exclusive chromatin opening in core myogenic promoters. In addition, we determine that the Notch pathway is indispensable for iMPC formation and self-renewal and further use the Notch ligand Dll1 to homogeneously propagate iMPCs. Collectively, this study charts divergent molecular blueprints for myogenic transdifferentiation or reprogramming and underpins the heightened capacity of iMPCs for capturing myogenesis ex vivo.

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.

The mini-IDLE 3D biomimetic culture assay enables interrogation of mechanisms governing muscle stem cell quiescence and niche repopulation

Adult skeletal muscle harbours a population of muscle stem cells (MuSCs) that are required for repair after tissue injury. In youth, MuSCs return to a reversible state of cell-cycle arrest termed 'quiescence' after injury resolution. Conversely, some MuSCs in aged muscle remain semi-activated, causing a premature response to injuries that results in incomplete repair and eventual stem cell depletion. Regulating this balance between MuSC quiescence and activation may hold the key to restoring tissue homeostasis with age, but is incompletely understood. To fill this gap, we developed a simple and tractable in vitro method, to rapidly inactivate MuSCs freshly isolated from young murine skeletal muscle, and return them to a quiescent-like state for at least 1-week, which we name mini-IDLE (Inactivation and Dormancy LEveraged in vitro). This was achieved by introducing MuSCs into a 3D bioartificial niche comprised of a thin sheet of mouse myotubes, which we demonstrate provides the minimal cues necessary to induce quiescence. With different starting numbers of MuSCs, the assay revealed cellular heterogeneity and population-level adaptations that converged on a common niche repopulation density; behaviours previously observed only in vivo. Quiescence-associated hallmarks included a Pax7⁺CalcR⁺DDX6⁺MyoD^-c-FOS^- signature, quiescent-like morphologies, and polarized niche markers. Leveraging high-content bioimaging pipelines, we demonstrate a relationship between morphology and cell fate signatures for possible real-time morphology-based screening. When using MuSCs from aged muscle, they displayed aberrant proliferative activities and delayed inactivation kinetics, among other quiescence-associated defects that we show are partially rescued by wortmannin treatment. Thus, the assay offers an unprecedented opportunity to systematically investigate long-standing queries in areas such as regulation of pool size and functional heterogeneity within the MuSC population, and to uncover quiescence regulators in youth and age.

Innovation in culture systems to study muscle complexity

Endogenous skeletal muscle development, regeneration, and pathology are extremely complex processes, influenced by local and systemic factors. Unpinning how these mechanisms function is crucial for fundamental biology and to develop therapeutic interventions for genetic disorders, but also conditions like sarcopenia and volumetric muscle loss. Ex vivo skeletal muscle models range from two- and three-dimensional primary cultures of satellite stem cell-derived myoblasts grown alone or in co-culture, to single muscle myofibers, myobundles, and whole tissues. Together, these systems provide the opportunity to gain mechanistic insights of stem cell behavior, cell-cell interactions, and mature muscle function in simplified systems, without confounding variables. Here, we highlight recent advances (published in the last 5 years) using in vitro primary cells and ex vivo skeletal muscle models, and summarize the new insights, tools, datasets, and screening methods they have provided. Finally, we highlight the opportunity for exponential advance of skeletal muscle knowledge, with spatiotemporal resolution, that is offered by guiding the study of muscle biology and physiology with in silico modelling and implementing high-content cell biology systems and ex vivo physiology platforms.

Recent advances in tissue stem cells

Stem cells are undifferentiated cells capable of self-renewal and differentiation, giving rise to specialized functional cells. Stem cells are of pivotal importance for organ and tissue development, homeostasis, and injury and disease repair. Tissue-specific stem cells are a rare population residing in specific tissues and present powerful potential for regeneration when required. They are usually named based on the resident tissue, such as hematopoietic stem cells and germline stem cells. This review discusses the recent advances in stem cells of various tissues, including neural stem cells, muscle stem cells, liver progenitors, pancreatic islet stem/progenitor cells, intestinal stem cells, and prostate stem cells, and the future perspectives for tissue stem cell research.

Oncostatin M sensitizes keratinocytes to UVB-induced inflammation via GSDME-mediated pyroptosis

BACKGROUND

Oncostatin M (OSM), an interleukin-6 (IL-6) family proinflammatory cytokine, plays a critical role in inflammatory skin diseases, but its mechanism of action is not well understood.

OBJECTIVE

To demonstrate the mechanism of OSM induced pyropotosis in normal human epidermal keratinocytes (NHEKs) and immortalized human keratinocytes (HaCaT cells).

METHODS

NHEKs and HaCaT cells were treated with OSM. Knockout of OSM receptor (OSMR) with CRISPR/Cas9 system, knockdown of GSDME with small interfering RNA and primary keratinocytes from Osmr-/- and Gsdme-/- mice were used to study the effect of OSMR and GSDME. After treatment of OSM, NHEKs and HaCaT cells were irradiated with UVB. The mRNA was analyzed by quantitative real-time polymerase chain reaction (qRT-PCR) and RNA sequencing, protein level was detected by Western Blotting, Elisa and immunofluorescence. Cell death was examined by lactate dehydrogenase (LDH) releasing.

RESULTS

Here we found that OSM induced pyropotosis in NHEKs and HaCaT cells, but knockout of OSMR abolished pyropotosis. RNA sequencing revealed an upregulation of several key genes involved in NLRP3 inflammasome activation following OSM treatment, among which NLRP3, GSDME, and IL-1β were confirmed by qRT-PCR and Western Blotting. Knockdown of GSDME alleviated OSM-induced pyropotosis. Pretreatment of OSM boosted UVB-induced pyroptosis and inflammation in NHEKs and HaCaT cells, and this priming function was lost in keratinocytes of Osmr-/- and Gsdme-/- mice. Similar results were obtained in a 3-dimensional culture of human epidermis.

CONCLUSION

OSM functions as a priming cytokine to enhance UVB-induced inflammation in keratinocytes, providing insight into the pathogenesis of inflammatory skin diseases.

Control of satellite cell function in muscle regeneration and its disruption in ageing

Skeletal muscle contains a designated population of adult stem cells, called satellite cells, which are generally quiescent. In homeostasis, satellite cells proliferate only sporadically and usually by asymmetric cell division to replace myofibres damaged by daily activity and maintain the stem cell pool. However, satellite cells can also be robustly activated upon tissue injury, after which they undergo symmetric divisions to generate new stem cells and numerous proliferating myoblasts that later differentiate to muscle cells (myocytes) to rebuild the muscle fibre, thereby supporting skeletal muscle regeneration. Recent discoveries show that satellite cells have a great degree of population heterogeneity, and that their cell fate choices during the regeneration process are dictated by both intrinsic and extrinsic mechanisms. Extrinsic cues come largely from communication with the numerous distinct stromal cell types in their niche, creating a dynamically interactive microenvironment. This Review discusses the role and regulation of satellite cells in skeletal muscle homeostasis and regeneration. In particular, we highlight the cell-intrinsic control of quiescence versus activation, the importance of satellite cell-niche communication, and deregulation of these mechanisms associated with ageing. The increasing understanding of how satellite cells are regulated will help to advance muscle regeneration and rejuvenation therapies.

Muscle stem cells and rotator cuff injury

The incidence of reinjury after treatment of rotator cuff tears (RCTs) remains very high despite the variety of nonoperative treatments and the high volume of surgical interventions performed. Muscle stem cells (MuSCs), also known as satellite cells, have risen to the forefront of rotator cuff tear research as a potential adjuvant therapy to aid unsatisfactory surgical outcomes. MuSCs are adult stem cells exhibiting the capacity to proliferate and self-renew, both symmetrically and asymmetrically. As part of this niche, they have been shown to adopt an activated phenotype in response to musculoskeletal injury and decrease their cellular populations during aging, implicating them as key players in both pathologic and normal physiological processes. While commonly connected to the regenerative phase of muscle healing, MuSCs also have the potential to differentiate into adverse morphologies. For instance, if MuSCs differentiate into adipocytes, the ensuing fatty infiltration serves as an obstacle to proper muscle healing and has been associated with the failure of surgical management of RCTs. With the potential to both harm and heal, we have identified MuSCs as a key player in RCT repair. To better understand this dichotomy, the following review will identify key studies regarding the morphology, function, and behavior of MuSCs with respect to RCTs and healing.

Biophysical matrix cues from the regenerating niche direct muscle stem cell fate in engineered microenvironments

Skeletal muscle stem cells (MuSCs) are essential for efficacious muscle repair, making MuSCs promising therapeutic targets for tissue engineering and regenerative medicine. MuSCs are presented with a diverse and temporally defined set of cues from their microenvironment during regeneration that direct stem cell expansion, differentiation, and return to quiescence. Understanding the complex interplay among these biophysical and biochemical cues is necessary to develop therapies targeting or employing MuSCs. To probe the role of mechanical cues presented by the extracellular matrix, we leverage chemically defined hydrogel substrates with controllable stiffness and adhesive ligand composition to characterize the MuSC response to matrix cues presented during early and late phases of regeneration. We demonstrate that relatively soft hydrogels recapitulating healthy muscle stiffness promote MuSC activation and expansion, while relatively stiff hydrogels impair MuSC proliferation and arrest myogenic progression. These effects are seen on soft and stiff hydrogels presenting laminin-111 and exacerbated on hydrogels presenting RGD adhesive peptides. Soluble factors present in the MuSC niche during different phases of regeneration, prostaglandin E2 and oncostatin M, synergize with matrix-presented cues to enhance stem cell expansion on soft substrates and block myogenic progression on stiff substrates. To determine if temporally varied matrix stiffness reminiscent of the regenerating microenvironment alters MuSC fate, we developed a photoresponsive hydrogel system with accelerated reaction kinetics that can be rapidly softened on demand. MuSCs cultured on these materials revealed that the cellular response to a stiff microenvironment is fixed within the first three days of culture, as subsequent softening back to a healthy stiffness did not rescue MuSC proliferation or myogenic progression. These results highlight the importance of temporally controlled biophysical and biochemical cues in regulating MuSC fate that can be harnessed to improve regenerative medicine approaches to restore skeletal muscle tissue.

Hematopoietic stem cells retain functional potential and molecular identity in hibernation cultures

Advances in the isolation and gene expression profiling of single hematopoietic stem cells (HSCs) have permitted in-depth resolution of their molecular program. However, long-term HSCs can only be isolated to near purity from adult mouse bone marrow, thereby precluding studies of their molecular program in different physiological states. Here, we describe a powerful 7-day HSC hibernation culture system that maintains HSCs as single cells in the absence of a physical niche. Single hibernating HSCs retain full functional potential compared with freshly isolated HSCs with respect to colony-forming capacity and transplantation into primary and secondary recipients. Comparison of hibernating HSC molecular profiles to their freshly isolated counterparts showed a striking degree of molecular similarity, further resolving the core molecular machinery of HSC self-renewal while also identifying key factors that are potentially dispensable for HSC function, including members of the AP1 complex (Jun, Fos, and Ncor2), Sult1a1 and Cish. Finally, we provide evidence that hibernating mouse HSCs can be transduced without compromising their self-renewal activity and demonstrate the applicability of hibernation cultures to human HSCs.

Cellular Mechanisms and Regulation of Quiescence

Quiescence is a state of reversible proliferative arrest in which cells are not actively dividing and yet retain the capacity to reenter the cell cycle upon receiving an appropriate stimulus. Quiescent cells are remarkably diverse-they reside in different locations throughout the body, serve distinct roles, and are activated by a variety of signals. Despite this diversity, all quiescent cells must be able to persist in a nondividing state without compromising their proliferative potential, which requires changes to core cellular programs. How drastically different cell types are able to implement extensive changes to their gene-expression programs, metabolism, and cellular structures to induce a common cellular state is a fascinating question in cell and developmental biology. In this review, we explore the diversity of quiescent cells and highlight the unifying characteristics that define the quiescent state.

Muscle Stem Cell Quiescence: Controlling Stemness by Staying Asleep

Muscle stem cells (MuSCs) are tissue-resident stem cells required for growth and repair of skeletal muscle, that are otherwise maintained in a cell-cycle-arrested state called quiescence. While quiescence was originally believed to be a state of cellular inactivity, increasing evidence suggests that quiescence is dynamically regulated and contributes to stemness, the long-term capacity to maintain regenerative functions. Here, we review the current understanding of MuSC quiescence and highlight recently discovered molecular markers, which differentiate depth of quiescence and influence self-renewal capacity. We also discuss how quiescent MuSCs integrate paracrine factors from their niche and dynamically regulate cell signaling, metabolism and proteostasis as they anticipate physiological needs, and how perturbing these cues during aging impairs muscle regeneration.

Methods to Mimic In Vivo Muscle Cell Biology in Primary Human Myoblasts Using Quiescence as a Guidepost in Regenerative Medicine Research

Regenerative medicine research and testing of new therapeutics for muscle-related human diseases call for a deeper understanding of how human myoblasts gain and maintain quiescence in vitro versus in vivo. The more closely we can experimentally simulate the in vivo environment, the more relevance in vitro research on myoblasts will have. In this context, isolation of satellite cells from muscle tissue causes activation while myoblasts remain activated in culture, thus not simulating quiescence as in their in vivo niche. Cells synchronized for cell cycle present a good starting point for experimental intervention. In the past, myoblast quiescence has been induced using suspension culture (SuCu) and, recently, by knockout serum replacement (KOSR)-supplemented culture media. We assessed the proportion of cells in G0 and molecular regulators after combining the two quiescence-inducing approaches. Quiescence was induced in primary human myoblasts (PHMs) in vitro using KOSR-treatment for 10 days or suspension in viscous media for 2 days (SuCu), or suspension combined with KOSR-treatment for 2 days (blended method, SuCu-KOSR). Quiescence and synchronization were achieved with all three protocols (G0/G1 cell cycle arrest >90% cells). Fold-change of cell cycle controller p21 mRNA for KOSR and SuCu was 3.23 ± 0.30 and 2.86 ± 0.15, respectively. Since this was already a significant change (p < 0.05), no further change was gained with the blended method. But SuCu-KOSR significantly decreased Ki67 (p = 0.0019). Myogenic regulatory factors, Myf5 and MyoD gene expression in PHMs were much more suppressed (p = 0.0004 and p = 0.0034, respectively) in SuCu-KOSR, compared to SuCu alone. In conclusion, a homogenous pool of quiescent primary myoblasts synchronized in the G0 cell cycle phase was achieved with cells from three different donors regardless of the experimental protocol. Myogenic dedifferentiation at the level of Myogenic Regulatory Factors was greater when exposed to the blend of suspension and serum-free culture. We suggest that this blended new protocol can be considered in future biomedical research if differentiation is detected too early during myoblast expansion. This shall also inform new ways to bridge the in vitro and in vivo divides in regenerative medicine research.

Stem cell therapy for muscular dystrophies

Muscular dystrophies are a heterogeneous group of genetic diseases, characterized by progressive degeneration of skeletal and cardiac muscle. Despite the intense investigation of different therapeutic options, a definitive treatment has not been developed for this debilitating class of pathologies. Cell-based therapies in muscular dystrophies have been pursued experimentally for the last three decades. Several cell types with different characteristics and tissues of origin, including myogenic stem and progenitor cells, stromal cells, and pluripotent stem cells, have been investigated over the years and have recently entered in the clinical arena with mixed results. In this Review, we do a roundup of the past attempts and describe the updated status of cell-based therapies aimed at counteracting the skeletal and cardiac myopathy present in dystrophic patients. We present current challenges, summarize recent progress, and make recommendations for future research and clinical trials.

Perspectives on skeletal muscle stem cells

Skeletal muscle has remarkable regeneration capabilities, mainly due to its resident muscle stem cells (MuSCs). In this review, we introduce recently developed technologies and the mechanistic insights they provide to the understanding of MuSC biology, including the re-definition of quiescence and G_alert states. Additionally, we present recent studies that link MuSC function with cellular heterogeneity, highlighting the complex regulation of self-renewal in regeneration, muscle disorders and aging. Finally, we discuss MuSC metabolism and its role, as well as the multifaceted regulation of MuSCs by their niche. The presented conceptual advances in the MuSC field impact on our general understanding of stem cells and their therapeutic use in regenerative medicine.

CCAAT/enhancer-binding protein beta promotes muscle stem cell quiescence through regulation of quiescence-associated genes

Regeneration of skeletal muscle depends on resident muscle stem cells called satellite cells that in healthy, uninjured muscle remain quiescent (noncycling). After activation and expansion of satellite cells postinjury, satellite cell numbers return to uninjured levels and return to mitotic quiescence. Here, we show that the transcription factor CCAAT/enhancer-binding protein beta (C/EBPβ) is required to maintain quiescence of satellite cells in uninjured muscle. We show that C/EBPβ is expressed in quiescent satellite cells in vivo and upregulated in noncycling myoblasts in vitro. Loss of C/EBPβ in satellite cells promotes their premature exit from quiescence resulting in spontaneous activation and differentiation of the stem cell pool. Forced expression of C/EBPβ in myoblasts inhibits proliferation by upregulation of 28 quiescence-associated genes. Furthermore, we find that caveolin-1 is a direct transcriptional target of C/EBPβ and is required for cell cycle exit in muscle satellite cells expressing C/EBPβ. The induction of mitotic quiescence is considered necessary for the long-term maintenance of adult stem cell populations with dysregulation driving increased differentiation of progenitors and depletion of the stem cell pool. Our findings place C/EBPβ as an important transcriptional regulator of muscle satellite cell quiescence.

Stem cell quiescence and its clinical relevance

Quiescent state has been observed in stem cells (SCs), including in adult SCs and in cancer SCs (CSCs). Quiescent status of SCs contributes to SC self-renewal and conduces to averting SC death from harsh external stimuli. In this review, we provide an overview of intrinsic mechanisms and extrinsic factors that regulate adult SC quiescence. The intrinsic mechanisms discussed here include the cell cycle, mitogenic signaling, Notch signaling, epigenetic modification, and metabolism and transcriptional regulation, while the extrinsic factors summarized here include microenvironment cells, extracellular factors, and immune response and inflammation in microenvironment. Quiescent state of CSCs has been known to contribute immensely to therapeutic resistance in multiple cancers. The characteristics and the regulation mechanisms of quiescent CSCs are discussed in detail. Importantly, we also outline the recent advances and controversies in therapeutic strategies targeting CSC quiescence.

Tissue Stem Cells: Architects of Their Niches

Stem cells (SCs) maintain tissue homeostasis and repair wounds. Despite marked variation in tissue architecture and regenerative demands, SCs often follow similar paradigms in communicating with their microenvironmental "niche" to transition between quiescent and regenerative states. Here we use skin epithelium and skeletal muscle-among the most highly-stressed tissues in our body-to highlight similarities and differences in niche constituents and how SCs mediate natural tissue rejuvenation and perform regenerative acts prompted by injuries. We discuss how these communication networks break down during aging and how understanding tissue SCs has led to major advances in regenerative medicine.

pH-Gated Succinate Secretion Regulates Muscle Remodeling in Response to Exercise

In response to skeletal muscle contraction during exercise, paracrine factors coordinate tissue remodeling, which underlies this healthy adaptation. Here we describe a pH-sensing metabolite signal that initiates muscle remodeling upon exercise. In mice and humans, exercising skeletal muscle releases the mitochondrial metabolite succinate into the local interstitium and circulation. Selective secretion of succinate is facilitated by its transient protonation, which occurs upon muscle cell acidification. In the protonated monocarboxylic form, succinate is rendered a transport substrate for monocarboxylate transporter 1, which facilitates pH-gated release. Upon secretion, succinate signals via its cognate receptor SUCNR1 in non-myofibrillar cells in muscle tissue to control muscle-remodeling transcriptional programs. This succinate-SUCNR1 signaling is required for paracrine regulation of muscle innervation, muscle matrix remodeling, and muscle strength in response to exercise training. In sum, we define a bioenergetic sensor in muscle that utilizes intracellular pH and succinate to coordinate tissue adaptation to exercise.

gp130 Controls Cardiomyocyte Proliferation and Heart Regeneration

Background:

A key cause of the high mortality of cardiovascular diseases is the cardiomyocyte inability to renew after cardiac injury. As a promising strategy to supplement functional myocytes for cardiac repair, there is a pressing need to understand the cellular and molecular mechanisms of heart regeneration.

Methods:

Seven genetic mouse lines were used: global OSM (oncostatin M) knockout, monocyte-/macrophage-specific OSM deletion, cardiomyocyte-specific lines, including OSM receptor deletion, gp130 (glycoprotein 130) deletion, gp130 activation, and Yap (yes-associated protein) ablation with gp130 activation mice. A series of molecular signaling experiments, including RNA sequencing, immunostaining, coimmunoprecipitation, and imaging flow cytometry, were conducted. Two models of cardiac injury, apical resection and myocardial infarction operation, were performed in neonatal, juvenile, and adult mice. Heart regeneration and cardiac function were evaluated by Masson staining and echocardiography, respectively. Gene recombinant adenovirus-associated virus was constructed and infected myocardial-infarcted mice as a gene therapy.

Results:

OSM was identified by RNA sequencing as a key upstream regulator of cardiomyocyte proliferation during neonatal heart regeneration in mice. Cardiomyocyte proliferation and heart regeneration were suspended in neonatal mice after cardiac injury when OSM was conditionally knockout in macrophages. The cardiomyocyte-specific deficiency of the OSM receptor heterodimers, OSM receptor and gp130, individually in cardiomyocytes reduced myocyte proliferation and neonatal heart regeneration. Conditional activation of gp130 in cardiomyocytes promoted cardiomyocyte proliferation and heart regeneration in juvenile and adult mice. Using RNA sequencing and functional screening, we found that Src mediated gp130-triggered cardiomyocyte proliferation by activating Yap (yes-associated protein) with Y357 phosphorylation independently of the Hippo pathway. Cardiomyocyte-specific deletion of Yap in Myh6-gp130 ACT mice blocked the effect of gp130 activation–induced heart regeneration in juvenile mice. Gene therapy with adenovirus-associated virus encoding constitutively activated gp130 promoted cardiomyocyte proliferation and heart regeneration in adult mice after myocardial infarction.

Conclusions:

Macrophage recruitment is essential for heart regeneration through the secretion of OSM, which promotes cardiomyocyte proliferation. As the coreceptor of OSM, gp130 activation is sufficient to promote cardiomyocyte proliferation by activating Yap through Src during heart regeneration. gp130 is a potential therapeutic target to improve heart regeneration after cardiac injury.

Secretome-Based Screening in Target Discovery

Secreted proteins and their cognate plasma membrane receptors regulate human physiology by transducing signals from the extracellular environment into cells resulting in different cellular phenotypes. Systematic use of secretome proteins in assays enables discovery of novel biology and signaling pathways. Several secretome-based phenotypic screening platforms have been described in the literature and shown to facilitate target identification in drug discovery. In this review, we summarize the current status of secretome-based screening. This includes annotation, production, quality control, and sample management of secretome libraries, as well as how secretome libraries have been applied to discover novel target biology using different disease-relevant cell-based assays. A workflow for secretome-based screening is shared based on the AstraZeneca experience. The secretome library offers several advantages compared with other libraries used for target discovery: (1) screening using a secretome library directly identifies the active protein and, in many cases, its cognate receptor, enabling a rapid understanding of the disease pathway and subsequent formation of target hypotheses for drug discovery; (2) the secretome library covers significant areas of biological signaling space, although the size of this library is small; (3) secretome proteins can be added directly to cells without additional manipulation. These factors make the secretome library ideal for testing in physiologically relevant cell types, and therefore it represents an attractive approach to phenotypic target discovery.

Five transcriptional factors reprogram fibroblast into myogenic lineage cells via paraxial mesoderm stage

It is hard to supply satellite cells as a cell source for therapy of muscle degenerative disease since the sampling of muscle tissue is very invasive to a patient with muscular disease. Direct conversion allows us to get specific cell types by transduction of defined transcriptional factors. To induce myogenic direct conversion, we transduced five transcriptional factors including Pax3, Sox2, Klf4, c-Myc, and Esrrb into mouse embryonic fibroblasts. We found that the transduction of the five transcriptional factors induced myogenic direct conversion of fibroblast. We revealed that the transduced cells with the five transcriptional factors were converted to myogenic lineage cells through a paraxial mesoderm-like stage. The expression level of myogenic-related genes of the transduced cells gradually increased as the passage increased. The induced myogenic lineage cells differentiated into muscle fibers in virto and in vivo. The current study revealed that the five transcription factors generated myogenic lineage cells from fibroblast via a paraxial mesoderm stage. The induced myogenic lineage cells may have a potential being applied as cell source for degenerative muscle disease.

Satellite cells in ageing: use it or lose it

Individuals that maintain healthy skeletal tissue tend to live healthier, happier lives as proper muscle function enables maintenance of independence and actuation of autonomy. The onset of skeletal muscle decline begins around the age of 30, and muscle atrophy is associated with a number of serious morbidities and mortalities. Satellite cells are responsible for regeneration of skeletal muscle and enter a reversible non-dividing state of quiescence under homeostatic conditions. In response to injury, satellite cells are able to activate and re-enter the cell cycle, creating new cells to repair and create nascent muscle fibres while preserving a small population that can return to quiescence for future regenerative demands. However, in aged muscle, satellite cells that experience prolonged quiescence will undergo programmed cellular senescence, an irreversible non-dividing state that handicaps the regenerative capabilities of muscle. This review examines how periodic activation and cycling of satellite cells through exercise can mitigate senescence acquisition and myogenic decline.

From gut to glutes: The critical role of niche signals in the maintenance and renewal of adult stem cells

Stem cell behavior is tightly regulated by spatiotemporal signaling from the niche, which is a four-dimensional microenvironment that can instruct stem cells to remain quiescent, self-renew, proliferate, or differentiate. In this review, we discuss recent advances in understanding the signaling cues provided by the stem cell niche in two contrasting adult tissues, the rapidly cycling intestinal epithelium and the slowly renewing skeletal muscle. Drawing comparisons between these two systems, we discuss the effects of niche-derived growth factors and signaling molecules, metabolic cues, the extracellular matrix and biomechanical cues, and immune signals on stem cells. We also discuss the influence of the niche in defining stem cell identity and function in both normal and pathophysiologic states.

Epigenetic metabolites license stem cell states

It has become clear during recent years that stem cells undergo metabolic remodeling during their activation process. While these metabolic switches take place in pluripotency as well as adult stem cell populations, the rules that govern the switch are not clear. In this review, we summarize some of the transitions in adult and pluripotent cell types and will propose that the key function in this process is the generation of epigenetic metabolites that govern critical epigenetic modifications, and therefore stem cell states.