Eliazer S, Muncie JM, Christensen J, Sun X, D'Urso RS, Weaver VM, Brack AS. Wnt4 from the Niche Controls the Mechano-Properties and Quiescent State of Muscle Stem Cells. Cell Stem Cell 2019; 25: 654-665. e4. [PMID: 31495781 DOI: 10.1016/j.stem.2019.08.007]

Crosstalk among canonical Wnt and Hippo pathway members in skeletal muscle and at the neuromuscular junction

Skeletal muscles are essential for locomotion, posture, and metabolic regulation. To understand physiological processes, exercise adaptation, and muscle-related disorders, it is critical to understand the molecular pathways that underlie skeletal muscle function. The process of muscle contraction, orchestrated by a complex interplay of molecular events, is at the core of skeletal muscle function. Muscle contraction is initiated by an action potential and neuromuscular transmission requiring a neuromuscular junction. Within muscle fibers, calcium ions play a critical role in mediating the interaction between actin and myosin filaments that generate force. Regulation of calcium release from the sarcoplasmic reticulum plays a key role in excitation-contraction coupling. The development and growth of skeletal muscle are regulated by a network of molecular pathways collectively known as myogenesis. Myogenic regulators coordinate the differentiation of myoblasts into mature muscle fibers. Signaling pathways regulate muscle protein synthesis and hypertrophy in response to mechanical stimuli and nutrient availability. Several muscle-related diseases, including congenital myasthenic disorders, sarcopenia, muscular dystrophies, and metabolic myopathies, are underpinned by dysregulated molecular pathways in skeletal muscle. Therapeutic interventions aimed at preserving muscle mass and function, enhancing regeneration, and improving metabolic health hold promise by targeting specific molecular pathways. Other molecular signaling pathways in skeletal muscle include the canonical Wnt signaling pathway, a critical regulator of myogenesis, muscle regeneration, and metabolic function, and the Hippo signaling pathway. In recent years, more details have been uncovered about the role of these two pathways during myogenesis and in developing and adult skeletal muscle fibers, and at the neuromuscular junction. In fact, research in the last few years now suggests that these two signaling pathways are interconnected and that they jointly control physiological and pathophysiological processes in muscle fibers. In this review, we will summarize and discuss the data on these two pathways, focusing on their concerted action next to their contribution to skeletal muscle biology. However, an in-depth discussion of the non-canonical Wnt pathway, the fibro/adipogenic precursors, or the mechanosensory aspects of these pathways is not the focus of this review.

Tumors and their microenvironments: Learning from pediatric brain pathologies

Early clues to tumors and their microenvironments come from embryonic development. Here we review the literature and consider whether the embryonic brain and its pathologies can serve as a better model. Among embryonic organs, the brain is the most heterogenous and complex, with multiple lineages leading to wide spectrum of cell states and types. Its dysregulation promotes neurodevelopmental brain pathologies and pediatric tumors. Embryonic brain pathologies point to the crucial importance of spatial heterogeneity over time, akin to the tumor microenvironment. Tumors dedifferentiate through genetic mutations and epigenetic modulations; embryonic brains differentiate through epigenetic modulations. Our innovative review proposes learning developmental brain pathologies to target tumor evolution-and vice versa. We describe ways through which tumor pharmacology can learn from embryonic brains and their pathologies, and how learning tumor, and its microenvironment, can benefit targeting neurodevelopmental pathologies. Examples include pediatric low-grade versus high-grade brain tumors as in rhabdomyosarcomas and gliomas.

Misregulation of the Ubiquitin-Proteasome System and Autophagy in Muscular Dystrophies Associated with the Dystrophin-Glycoprotein Complex

The stability of the sarcolemma is severely impaired in a series of genetic neuromuscular diseases defined as muscular dystrophies. These are characterized by the centralization of skeletal muscle syncytial nuclei, the replacement of muscle fibers with fibrotic tissue, the release of inflammatory cytokines, and the disruption of muscle protein homeostasis, ultimately leading to necrosis and loss of muscle functionality. A specific subgroup of muscular dystrophies is associated with genetic defects in components of the dystrophin-glycoprotein complex (DGC), which plays a crucial role in linking the cytosol to the skeletal muscle basement membrane. In these cases, dystrophin-associated proteins fail to correctly localize to the sarcolemma, resulting in dystrophy characterized by an uncontrolled increase in protein degradation, which can ultimately lead to cell death. In this review, we explore the role of intracellular degradative pathways-primarily the ubiquitin-proteasome and autophagy-lysosome systems-in the progression of DGC-linked muscular dystrophies. The DGC acts as a hub for numerous signaling pathways that regulate various cellular functions, including protein homeostasis. We examine whether the loss of structural stability within the DGC affects key signaling pathways that modulate protein recycling, with a particular emphasis on autophagy.

Interleukin-11 expressed in the polyp-enriched fibroblast subset is a potential therapeutic target in Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS) is associated with early-onset gastrointestinal polyposis caused by hereditary inactivating pathogenic variants in the tumor suppressor gene STK11 (LKB1). Due to lack of prophylactic therapies, management of PJS polyps requires frequent surveillance. Interestingly, studies in mouse models have revealed that stromal cells drive the polyp formation, but detailed understanding of the cell types and interactions involved has been lacking. Using single-cell RNA sequencing of PJS mouse model polyps, we here identify a polyp-enriched crypt top fibroblast (pCTF) cluster characterized by a transcriptional signature also enriched in PJS patient polyps. The pCTF signature was also noted in primary fibroblasts in vitro following acute STK11 loss. Targeted deletion of Stk11 in crypt top fibroblasts using Foxl1-Cre led to upregulation of the pCTF signature genes and later to polyposis. pCTFs displayed similarity to inflammation-associated fibroblasts, and polyposis was exacerbated by inflammation. Cell-cell communication analysis identified interleukin 11 (IL-11) as a potential pCTF inducer, and consistent with this, IL-11 was required for fibroblast reprogramming toward pCTFs following STK11 loss. Importantly, a neutralizing IL-11 antibody efficiently reduced polyp formation in a PJS model indicating a key, targetable role for IL-11 in polyp development. Together the results characterize pCTFs as a PJS polyp-enriched fibroblast subset and identify IL-11 as a key mediator of fibroblast reprogramming and a potential therapeutic target in PJS. © 2025 The Pathological Society of Great Britain and Ireland.

Mitochondrial fatty acid oxidation regulates adult muscle stem cell function through modulating metabolic flux and protein acetylation

During homeostasis and regeneration, satellite cells, the resident stem cells of skeletal muscle, have distinct metabolic requirements for fate transitions between quiescence, proliferation and differentiation. However, the contribution of distinct energy sources to satellite cell metabolism and function remains largely unexplored. Here, we uncover a role of mitochondrial fatty acid oxidation (FAO) in satellite cell integrity and function. Single-cell RNA sequencing revealed progressive enrichment of mitochondrial FAO and downstream pathways during activation, proliferation and myogenic commitment of satellite cells. Deletion of Carnitine palmitoyltransferase 2 (Cpt2), the rate-limiting enzyme in FAO, hampered muscle stem cell expansion and differentiation upon acute muscle injury, markedly delaying regeneration. Cpt2 deficiency reduces acetyl-CoA levels in satellite cells, impeding the metabolic flux and acetylation of selective proteins including Pax7, the central transcriptional regulator of satellite cells. Notably, acetate supplementation restored cellular metabolic flux and partially rescued the regenerative defects of Cpt2-null satellite cells. These findings highlight an essential role of fatty acid oxidation in controlling satellite cell function and suggest an integration of lipid metabolism and protein acetylation in adult stem cells.

Autophagy in adult stem cell homeostasis, aging, and disease therapy

Autophagy is a crucial cellular process that facilitates the degradation of damaged organelles and protein aggregates, and the recycling of cellular components for the energy production and macromolecule synthesis. It plays an indispensable role in maintaining cellular homeostasis. Over recent decades, research has increasingly focused on the role of autophagy in regulating adult stem cells (SCs). Studies suggest that autophagy modulates various cellular processes and states of adult SCs, including quiescence, proliferation, self-renewal, and differentiation. The primary role of autophagy in these contexts is to sustain homeostasis, withstand stressors, and supply energy. Notably, the dysfunction of adult SCs during aging is correlated with a decline in autophagic activity, suggesting that autophagy is also involved in SC- and aging-associated disorders. Given the diverse cellular processes mediated by autophagy and the intricate mechanisms governing adult SCs, further research is essential to elucidate both universal and cell type-specific regulatory pathways of autophagy. This review discusses the role of autophagy in regulating adult SCs during quiescence, proliferation, self-renewal, and differentiation. Additionally, it summarizes the relationship between SC aging and autophagy, providing therapeutical insights into treating and ameliorating aging-associated diseases and cancers, and ultimately promoting longevity.

Aging: A struggle for beneficial to overcome negative factors made by muscle and bone

Musculoskeletal health is strongly influenced by regulatory interactions of bone and muscle. Recent discoveries have identified a number of key mechanisms through which soluble factors released during exercise by bone exert positive effects on muscle and by muscle on bone. Although exercise can delay the negative effects of aging, these beneficial effects are diminished with aging. The limited response of aged muscle and bone tissue to exercise are accompanied by a failure in bone and muscle communication. Here, we propose that exercise induced beneficial factors must battle changes in circulating endocrine and inflammatory factors that occur with aging. Furthermore, sedentary behavior results in the release of negative factors impacting the ability of bone and muscle to respond to physical activity especially with aging. In this review we report on exercise responsive factors and evidence of modification occurring with aging.

Multiple Wnt signaling pathways direct epithelial tubule interconnection in the regenerating zebrafish kidney

Epithelial tubule fusion is fundamental for kidney morphogenesis. Differentiating nephron tubules interconnect with collecting system epithelia to generate a lumenal pathway for fluid excretion. In the adult zebrafish kidney, nephrogenesis occurs as a regenerative response to injury and provides a model to explore cell signaling pathways required for tubule interconnection. We show that canonical Wnt signaling at the junction between two tubules induces a mesenchymal, invasive cell phenotype and is required, along with Src kinase and rac1, to generate basal cell protrusions. The Wnt ligands wnt9b and wnt4 are both required for new nephron formation after injury. Mutation in wnt4 or treatment with the canonical Wnt inhibitor IWR1 blocks formation of basal protrusions in forming nephrons. Mutation in the Wnt receptor frizzled9b reveals a fusion-associated non-canonical Wnt pathway that acts to 1) restrict canonical Wnt gene expression, 2) drive Rho kinase-dependent apical constriction of epithelial cells, and 3) position basal protrusions and generate orthogonal tubule lumenal connections. As a result, frizzled9b mutant nephrons fail to fully interconnect with target distal tubules. Our results indicate that canonical and non-canonical Wnt signaling interact in the same cells to orient and drive tubule interconnection in the regenerating zebrafish kidney.

Mechanisms, assessment, and exercise interventions for skeletal muscle dysfunction post-chemotherapy in breast cancer: from inflammation factors to clinical practice

Chemotherapy remains a central component of breast cancer treatment, significantly improving patient survival rates. However, its toxic side effects, along with cancer-related paraneoplastic syndromes, can lead to the loss of skeletal muscle mass and function, impairing physical abilities and increasing the risk of complications during treatment. Chemotherapeutic agents directly impact skeletal muscle cells by promoting protein degradation, inhibiting protein synthesis, and triggering systemic inflammation, all of which contribute to muscle atrophy. Additionally, these drugs can interfere with the proliferation and differentiation of stem cells, such as satellite cells, disrupting muscle regeneration and repair while inducing abnormal differentiation of intermuscular tissue, thereby worsening muscle wasting. These effects not only reduce the effectiveness of chemotherapy but also negatively affect patients' quality of life and disease prognosis. Recent studies have emphasized the role of exercise as an effective non-pharmacological strategy for preventing muscle loss and preserving muscle mass in cancer patients. This review examines the clinical manifestations of muscle dysfunction following breast cancer chemotherapy, the potential mechanisms underlying these changes, and the evidence supporting exercise as a therapeutic approach for improving muscle function.

Metabolic regulation in adult and aging skeletal muscle stem cells

Adult stem cells maintain homeostasis and enable regeneration of most tissues. Quiescence, proliferation, and differentiation of stem cells and their progenitors are tightly regulated processes governed by dynamic transcriptional, epigenetic, and metabolic programs. Previously thought to merely reflect a cell's energy state, metabolism is now recognized for its critical regulatory functions, controlling not only energy and biomass production but also the cell's transcriptome and epigenome. In this review, we explore how metabolic pathways, metabolites, and transcriptional and epigenetic regulators are functionally interlinked in adult and aging skeletal muscle stem cells.

Weighted gene co-expression network analysis reveals the hub genes and molecular mechanism of quiescence

The hematopoietic stem cell (HSC) continues their functional integrity and return to quiescence quickly even after inflammatory and other proliferative stress. The mechanism which is responsible for this highly regulatory process is not understood clearly. Previous results have shown that CD53 is noticeably upregulated in HSCs in response to a variety of stimuli. Gene expression profile using RNASeq data of HSCs from the bone marrow and spleen of CD53 knock out and their wild-type littermate had been deposited by Greenberg and co-authors, in GEO database, "GSE219050". They reported that knockout of CD53 promotes continued cell cycle. To identify key genes and specific processes are affected in absence of CD53, we applied weighted gene co-expression analysis. The results show that cyan module is correlated and dark red and light cyan are anti-correlated with CD53 loss. CDK1 is identified as more connected gene or hub gene in cyan module and it is upregulated in the absence of CD53. Likewise, hub genes from dark-red module are EP300, EGF, MCL1, LPL and IGF1R. The gene enrichment analysis depicts, two biological processes, MAPK cascade and Delta Notch signalling were suppressed. Similarly, the biological processes involved in light-cyan module are chromatin organisation and hub genes are Ehmt2, Ezh2, Kdm1a, Rbbp4, Esr1 and Mysm1. It uncovers the roles of CD53 in chromatin organisation, and MAPK cascade and Delta Notch signalling are the major contributors in quiescence mechanism. These findings might provide a new avenue in quiescence research.

Forcing the code: tension modulates signaling to drive morphogenesis and malignancy

Development and disease are regulated by the interplay between genetics and the signaling pathways stimulated by morphogens, growth factors, and cytokines. Experimental data highlight the importance of mechanical force in regulating embryonic development, tissue morphogenesis, and malignancy. Force not only sculpts tissue movements to drive embryogenesis and morphogenesis but also modifies the context of biochemical signaling and gene expression to regulate cell and tissue fate. Not surprisingly, experiments have demonstrated that perturbations in cell tension drive malignancy and metastasis by altering biochemical signaling and gene expression through modifications in cytoskeletal tension, transmembrane receptor structure and function, and organelle phenotype that enhance cell growth and survival, alter metabolism, and foster cell migration and invasion. At the tissue level, tumor-associated forces disrupt cell-cell adhesions to perturb tissue organization, compromise vascular integrity to induce hypoxia, and interfere with antitumor immunity to foster metastasis and treatment resistance. Exciting new approaches now exist with which to clarify the relationship between mechanotransduction, biochemical signaling, and gene expression in development and disease. Indeed, gaining insight into these interactions is essential to unravel molecular mechanisms that regulate development and clarify the molecular basis of cancer.

Mll4 in skeletal muscle fibers maintains muscle stem cells

BACKGROUND

Muscle stem cells (MuSCs) undergo numerous state transitions throughout life, which are critical for supporting normal muscle growth and regeneration. Epigenetic modifications in skeletal muscle play a significant role in influencing the niche and cellular states of MuSCs. Mixed-lineage leukemia 4 (Mll4) is a histone methyltransferase critical for activating the transcription of various target genes and is highly expressed in skeletal muscle. This raises the question of whether Mll4 has a regulatory function in modulating the state transitions of MuSCs, warranting further investigation.

METHODS

To assess if myofiber-expressed Mll4, a histone methyltransferase, contributes to the maintenance of MuSCs, we crossed MCKCre/+ or HSAMerCreMer/+ mice to Mll4f/f mice to generate myofiber-specific Mll4-deleted mice. Investigations were conducted using 8-week-old and 4-week-old MCKCre/+;Mll4f/f mice, and adult HSAMerCreMer/+;Mll4f/f mice between the ages of 3 months and 6 months.

RESULTS

During postnatal myogenesis, Mll4 deleted muscles were observed with increased number of cycling MuSCs that proceeded to a differentiation state, leading to MuSC deprivation. This phenomenon occurred independently of gender. When Mll4 was ablated in adult muscles using the inducible method, adult MuSCs lost their quiescence and differentiated into myoblasts, also causing the depletion of MuSCs. Such roles of Mll4 in myofibers coincided with decreased expression levels of distinct Notch ligands: Jag1 and Dll1 in pubertal and Jag2 and Dll4 in adult muscles.

CONCLUSIONS

Our study suggests that Mll4 is crucial for maintaining MuSCs in both pubertal and adult muscles, which may be accomplished through the modulation of distinct Notch ligand expressions in myofibers. These findings offer new insights into the role of myofiber-expressed Mll4 as a master regulator of MuSCs, highlighting its significance not only in developmental myogenesis but also in adult muscle, irrespective of sex.

Comparative Analysis of Different Extracellular Matrices for the Maintenance of Bovine Satellite Cells

Cultured meat produced using satellite cells has emerged to address issues such as overpopulation, the ethical conundrums associated with the breeding environment, and the methane gas emissions associated with factory farming. To date, however, the challenges of maintaining satellite cells in vitro and reducing the costs of the culture media are still substantial. Gelatin, collagen, and fibronectin are commonly used extracellular matrices (ECMs) that facilitate signal integration with the cells and promote cell adhesion. In this study, we compared the proliferation, cell cycle, immunocytochemistry, and expression levels of Pax7, Pax3, Myf5, MyoD1, and MyoG genes in bovine satellite cells (BSCs) cultured on gelatin-, collagen- and fibronectin-coated dishes as part of short- and long-term cultures. We observed that BSCs cultured on gelatin-coated dishes showed higher levels of Pax7 expression than BSCs cultured on collagen- and fibronectin-coated dishes in both short- and long-term cultures, indicating that BSCs cultured on gelatin effectively maintained the satellite cell population in both the short- and long-term cultures. Our study highlights that gelatin is an effective ECM for the maintenance of BSCs and the production of cultured meat.

The L27 domain of MPP7 enhances TAZ-YY1 cooperation to renew muscle stem cells

Stem cells regenerate differentiated cells to maintain and repair tissues and organs. They also replenish themselves, i.e. self-renew, to support a lifetime of regenerative capacity. Here we study the renewal of skeletal muscle stem cell (MuSC) during regeneration. The transcriptional co-factors TAZ/YAP (via the TEAD transcription factors) regulate cell cycle and growth while the transcription factor YY1 regulates metabolic programs for MuSC activation. We show that MPP7 and AMOT join TAZ and YY1 to regulate a selected number of common genes that harbor TEAD and YY1 binding sites. Among these common genes, Carm1 can direct MuSC renewal. We demonstrate that the L27 domain of MPP7 enhances the interaction as well as the transcriptional activity of TAZ and YY1, while AMOT acts as an intermediate to bridge them together. Furthermore, MPP7, TAZ and YY1 co-occupy the promoters of Carm1 and other common downstream genes. Our results define a renewal program comprised of two progenitor transcriptional programs, in which selected key genes are regulated by protein-protein interactions, dependent on promoter context.

Mll4 in Skeletal Muscle Fiber Maintains Muscle Stem Cells by Regulating Notch Ligands

Background

Muscle stem cells (MuSCs) undergo numerous state transitions throughout life, which are critical for supporting normal muscle growth and regeneration. Therefore, it is crucial to investigate the regulatory mechanisms governing the transition of MuSC states across different postnatal developmental stages.

Methods

To assess if myofiber-expressed Mll4 contributes to the maintenance of MuSCs, we crossed MCK Cre/+ or HSA MerCreMer/+ mice to Mll4 f/f mice to generate myofiber-specific Mll4-deleted mice. Investigations were conducted using 8-week-old and 4-week-old MCK Cre/+ ;Mll4 f/f mice Investigations were conducted using 8-week-old and 4-week-old HSA Cre/+ ;Mll4 f/f mice were utilized.

Results

During postnatal myogenesis, Mll4 deleted muscles were observed with increased number of cycling MuSCs that proceeded to a differentiation state, leading to MuSC deprivation. This phenomenon occurred independently of gender. When Mll4 was ablated in adult muscles using the inducible method, adult MuSCs lost their quiescence and differentiated into myoblasts, also causing the depletion of MuSCs. Such roles of Mll4 in myofibers coincided with decreased expression levels of distinct Notch ligands: Jag1 and Dll1 in pubertal and Jag2 and Dll4 in adult muscles.

Conclusions

Our study suggests that Mll4 is crucial for maintaining MuSCs in both pubertal and adult muscles, which may be accomplished through the modulation of distinct Notch ligand expressions in myofibers. These findings offer new insights into the role of myofiber-expressed Mll4 as a master regulator of MuSCs, highlighting its significance not only in developmental myogenesis but also in adult muscle, irrespective of sex.

Suppressing PDGFRβ Signaling Enhances Myocyte Fusion to Promote Skeletal Muscle Regeneration

Muscle cell fusion is critical for forming and maintaining multinucleated myotubes during skeletal muscle development and regeneration. However, the molecular mechanisms directing cell-cell fusion are not fully understood. Here, we identify platelet-derived growth factor receptor beta (PDGFRβ) signaling as a key modulator of myocyte fusion in adult muscle cells. Our findings demonstrate that genetic deletion of Pdgfrβ enhances muscle regeneration and increases myofiber size, whereas PDGFRβ activation impairs muscle repair. Inhibition of PDGFRβ activity promotes myonuclear accretion in both mouse and human myotubes, whereas PDGFRβ activation stalls myotube development by preventing cell spreading to limit fusion potential. Transcriptomics analysis show that PDGFRβ signaling cooperates with TGFβ signaling to direct myocyte size and fusion. Mechanistically, PDGFRβ signaling requires STAT1 activation, and blocking STAT1 phosphorylation enhances myofiber repair and size during regeneration. Collectively, PDGFRβ signaling acts as a regenerative checkpoint and represents a potential clinical target to rapidly boost skeletal muscle repair.

Overexpression of CPT1A disrupts the maintenance and regenerative function of muscle stem cells

The skeletal muscle satellite cells (SCs) mediate regeneration of myofibers upon injury. As they switch from maintenance (quiescence) to regeneration, their relative reliance on glucose and fatty acid metabolism alters. To explore the contribution of mitochondrial fatty acid oxidation (FAO) pathway to SCs and myogenesis, we examined the role of carnitine palmitoyltransferase 1A (CPT1A), the rate-limiting enzyme of FAO. CPT1A is highly expressed in quiescent SCs (QSCs) compared with activated and proliferating SCs, and its expression level decreases during myogenic differentiation. Myod1Cre-driven overexpression (OE) of Cpt1a in embryonic myoblasts (Cpt1aMTG) reduces muscle weight, grip strength, and contractile force without affecting treadmill endurance of adult mice. Adult Cpt1aMTG mice have reduced number of SC, impairing muscle regeneration and promoting lipid infiltration. Similarly, Pax7CreER-driven, tamoxifen-inducible Cpt1a-OE in QSCs of adult muscles (Cpt1aPTG) leads to depletion of SCs and compromises muscle regeneration. The reduced proliferation of Cpt1a-OE SCs is associated with elevated level of acyl-carnitine, and acyl-carnitine treatment impedes proliferation of wildtype SCs. These findings indicate that aberrant level of CPT1A elevates acyl-carnitine to impair the maintenance, proliferation and regenerative function of SCs.

Organoid culture promotes dedifferentiation of mouse myoblasts into stem cells capable of complete muscle regeneration

Experimental cell therapies for skeletal muscle conditions have shown little success, primarily because they use committed myogenic progenitors rather than true muscle stem cells, known as satellite cells. Here we present a method to generate in vitro-derived satellite cells (idSCs) from skeletal muscle tissue. When transplanted in small numbers into mouse muscle, mouse idSCs fuse into myofibers, repopulate the satellite cell niche, self-renew, support multiple rounds of muscle regeneration and improve force production on par with freshly isolated satellite cells in damaged skeletal muscle. We compared the epigenomic and transcriptional signatures between idSCs, myoblasts and satellite cells and used these signatures to identify core signaling pathways and genes that confer idSC functionality. Finally, from human muscle biopsies, we successfully generated satellite cell-like cells in vitro. After further development, idSCs may provide a scalable source of cells for the treatment of genetic muscle disorders, trauma-induced muscle damage and age-related muscle weakness.

Satellite cell-derived TRIM28 is pivotal for mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discover that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Interestingly, different from the role reported in a previous study based on C2C12 myoblasts, multiple lines of both in vitro and in vivo evidence reveal that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 are not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discover that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

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.

Both intrinsic and microenvironmental factors contribute to the regulation of stem cell quiescence

Precise regulation of stem cell quiescence is essential for tissue development and homeostasis. Therefore, its aberrant regulation is intimately correlated with various human diseases. However, the detailed mechanisms of stem cell quiescence and its specific role in the pathogenesis of various diseases remain to be determined. Recent studies have revealed that the intrinsic and microenvironmental factors are the potential candidates responsible for the orderly switch between the dormant and activated states of stem cells. In addition, defects in signaling pathways related to internal and external factors of stem cells might contribute to the initiation and development of diseases by altering the dormancy of stem cells. In this review, we focus on the mechanisms underlying stem cell quiescence, especially the involvement of intrinsic and microenvironmental factors. In addition, we discuss the relationship between the anomalies of stem cell quiescence and related diseases, hopefully providing therapeutic insights for developing novel treatments.

Mechanical state transitions in the regulation of tissue form and function

From embryonic development, postnatal growth and adult homeostasis to reparative and disease states, cells and tissues undergo constant changes in genome activity, cell fate, proliferation, movement, metabolism and growth. Importantly, these biological state transitions are coupled to changes in the mechanical and material properties of cells and tissues, termed mechanical state transitions. These mechanical states share features with physical states of matter, liquids and solids. Tissues can switch between mechanical states by changing behavioural dynamics or connectivity between cells. Conversely, these changes in tissue mechanical properties are known to control cell and tissue function, most importantly the ability of cells to move or tissues to deform. Thus, tissue mechanical state transitions are implicated in transmitting information across biological length and time scales, especially during processes of early development, wound healing and diseases such as cancer. This Review will focus on the biological basis of tissue-scale mechanical state transitions, how they emerge from molecular and cellular interactions, and their roles in organismal development, homeostasis, regeneration and disease.

MuSK-BMP signaling in adult muscle stem cells maintains quiescence and regulates myofiber size

A central question in adult stem cell biology is elucidating the signaling pathways regulating their dynamics and function in diverse physiological and age-related contexts. Muscle stem cells in adults (Satellite Cells; SCs) are generally quiescent but can activate and contribute to muscle repair and growth. Here we tested the role of the MuSK-BMP pathway in regulating adult SC quiescence by deletion of the BMP-binding MuSK Ig3 domain ('ΔIg3-MuSK'). At 3 months of age SC and myonuclei numbers and myofiber size were comparable to WT. However, at 5 months of age SC density was decreased while myofiber size, myonuclear number and grip strength were increased - indicating that SCs had activated and productively fused into the myofibers over this interval. Transcriptomic analysis showed that SCs from uninjured ΔIg3-MuSK mice exhibit signatures of activation. Regeneration experiments showed that ΔIg3-MuSK SCs maintain full stem cell function. Expression of ΔIg3-MuSK in adult SCs was sufficient to break quiescence and increase myofiber size. We conclude that the MuSK-BMP pathway regulates SC quiescence and myofiber size in a cell autonomous, age-dependent manner. Targeting MuSK-BMP signaling in muscle stem cells thus emerges a therapeutic strategy for promoting muscle growth and function in the settings of injury, disease, and aging.

Highlights

MuSK, in its role as a BMP co-receptor, regulates adult muscle stem cell quiescenceThe MuSK-BMP pathway acts cell autonomouslyIncreased muscle size and function with preservation of myonuclear density and stemness in mice with attenuated MuSK-BMP signaling.

RhoA-mediated G12-G13 signaling maintains muscle stem cell quiescence and prevents stem cell loss

Multiple processes control quiescence of muscle stem cells (MuSCs), which is instrumental to guarantee long-term replenishment of the stem cell pool. Here, we describe that the G-proteins G12-G13 integrate signals from different G-protein-coupled receptors (GPCRs) to control MuSC quiescence via activation of RhoA. Comprehensive screening of GPCR ligands identified two MuSC-niche-derived factors, endothelin-3 (ET-3) and neurotensin (NT), which activate G12-G13 signaling in MuSCs. Stimulation with ET-3 or NT prevented MuSC activation, whereas pharmacological inhibition of ET-3 or NT attenuated MuSC quiescence. Inactivation of Gna12-Gna13 or Rhoa but not of Gnaq-Gna11 completely abrogated MuSC quiescence, which depleted the MuSC pool and was associated with accelerated sarcopenia during aging. Expression of constitutively active RhoA prevented exit from quiescence in Gna12-Gna13 mutant MuSCs, inhibiting cell cycle entry and differentiation via Rock and formins without affecting Rac1-dependent MuSC projections, a hallmark of quiescent MuSCs. The study uncovers a critical role of G12-G13 and RhoA signaling for active regulation of MuSC quiescence.

We need to talk-how muscle stem cells communicate

Skeletal muscle is one of the tissues with the highest ability to regenerate, a finely controlled process which is critically depending on muscle stem cells. Muscle stem cell functionality depends on intrinsic signaling pathways and interaction with their immediate niche. Upon injury quiescent muscle stem cells get activated, proliferate and fuse to form new myofibers, a process involving the interaction of multiple cell types in regenerating skeletal muscle. Receptors in muscle stem cells receive the respective signals through direct cell-cell interaction, signaling via secreted factors or cell-matrix interactions thereby regulating responses of muscle stem cells to external stimuli. Here, we discuss how muscle stem cells interact with their immediate niche focusing on how this controls their quiescence, activation and self-renewal and how these processes are altered in age and disease.

Mitogen signaling strength and duration can control cell cycle decisions

Decades ago, mitogen-promoted signaling duration and strength were observed to be sensed by the cell and to be critical for its decisions: to proliferate or differentiate. Landmark publications established the importance of mitogen signaling not only in the G1 cell cycle phase but also through the S and the G2/M transition. Despite these early milestones, how mitogen signal duration and strength, short and strong or weaker and sustained, control cell fate has been largely unheeded. Here, we center on cardinal signaling-related questions, including (i) how fluctuating mitogenic signals are converted into cell proliferation-differentiation decisions and (ii) why extended duration of weak signaling is associated with differentiation, while bursts of strong and short induce proliferation but, if too strong and long, induce irreversible senescence. Our innovative broad outlook harnesses cell biology and protein conformational ensembles, helping us to define signaling strength, clarify cell cycle decisions, and thus cell fate.

Fibro-adipogenic progenitors in physiological adipogenesis and intermuscular adipose tissue remodeling

Excessive accumulation of intermuscular adipose tissue (IMAT) is a common pathological feature in various metabolic and health conditions and can cause muscle atrophy, reduced function, inflammation, insulin resistance, cardiovascular issues, and unhealthy aging. Although IMAT results from fat accumulation in muscle, the mechanisms underlying its onset, development, cellular components, and functions remain unclear. IMAT levels are influenced by several factors, such as changes in the tissue environment, muscle type and origin, extent and duration of trauma, and persistent activation of fibro-adipogenic progenitors (FAPs). FAPs are a diverse and transcriptionally heterogeneous population of stromal cells essential for tissue maintenance, neuromuscular stability, and tissue regeneration. However, in cases of chronic inflammation and pathological conditions, FAPs expand and differentiate into adipocytes, resulting in the development of abnormal and ectopic IMAT. This review discusses the role of FAPs in adipogenesis and how they remodel IMAT. It highlights evidence supporting FAPs and FAP-derived adipocytes as constituents of IMAT, emphasizing their significance in adipose tissue maintenance and development, as well as their involvement in metabolic disorders, chronic pathologies and diseases. We also investigated the intricate molecular pathways and cell interactions governing FAP behavior, adipogenesis, and IMAT accumulation in chronic diseases and muscle deconditioning. Finally, we hypothesize that impaired cellular metabolic flexibility in dysfunctional muscles impacts FAPs, leading to IMAT. A deeper understanding of the biology of IMAT accumulation and the mechanisms regulating FAP behavior and fate are essential for the development of new therapeutic strategies for several debilitating conditions.

Post-natal muscle growth and protein turnover: a narrative review of current understanding

A model explaining the dietary-protein-driven post-natal skeletal muscle growth and protein turnover in the rat is updated, and the mechanisms involved are described, in this narrative review. Dietary protein controls both bone length and muscle growth, which are interrelated through mechanotransduction mechanisms with muscle growth induced both from stretching subsequent to bone length growth and from internal work against gravity. This induces satellite cell activation, myogenesis and remodelling of the extracellular matrix, establishing a growth capacity for myofibre length and cross-sectional area. Protein deposition within this capacity is enabled by adequate dietary protein and other key nutrients. After briefly reviewing the experimental animal origins of the growth model, key concepts and processes important for growth are reviewed. These include the growth in number and size of the myonuclear domain, satellite cell activity during post-natal development and the autocrine/paracrine action of IGF-1. Regulatory and signalling pathways reviewed include developmental mechanotransduction, signalling through the insulin/IGF-1-PI3K-Akt and the Ras-MAPK pathways in the myofibre and during mechanotransduction of satellite cells. Likely pathways activated by maximal-intensity muscle contractions are highlighted and the regulation of the capacity for protein synthesis in terms of ribosome assembly and the translational regulation of 5-TOPmRNA classes by mTORC1 and LARP1 are discussed. Evidence for and potential mechanisms by which volume limitation of muscle growth can occur which would limit protein deposition within the myofibre are reviewed. An understanding of how muscle growth is achieved allows better nutritional management of its growth in health and disease.

Muscle stem cell niche dynamics during muscle homeostasis and regeneration

The process of skeletal muscle regeneration involves a coordinated interplay of specific cellular and molecular interactions within the injury site. This review provides an overview of the cellular and molecular components in regenerating skeletal muscle, focusing on how these cells or molecules in the niche regulate muscle stem cell functions. Dysfunctions of muscle stem cell-to-niche cell communications during aging and disease will also be discussed. A better understanding of how niche cells coordinate with muscle stem cells for muscle repair will greatly aid the development of therapeutic strategies for treating muscle-related disorders.

Ex vivo two-photon imaging of whole-mount skeletal muscles to visualize stem cell behavior

Quiescent skeletal muscle stem cells (MuSCs) are morphologically and functionally heterogeneous and exhibit different lengths of cellular extensions, which we call protrusions. Here, we present a protocol for ex vivo two-photon imaging of MuSCs in their native environment. We describe steps for muscle dissection, fixation, embedding, imaging, and analysis of datasets. This protocol allows the examination of MuSC morphology and protrusions at the single-cell level as well as stem cell numbers. For complete details on the use and execution of this protocol, please refer to Ma et al. (2022).1.

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.

Squishy matters - Corneal mechanobiology in health and disease

The cornea, as a dynamic and responsive tissue, constantly interacts with mechanical forces in order to maintain its structural integrity, barrier function, transparency and refractive power. Cells within the cornea sense and respond to various mechanical forces that fundamentally regulate their morphology and fate in development, homeostasis and pathophysiology. Corneal cells also dynamically regulate their extracellular matrix (ECM) with ensuing cell-ECM crosstalk as the matrix serves as a dynamic signaling reservoir providing biophysical and biochemical cues to corneal cells. Here we provide an overview of mechanotransduction signaling pathways then delve into the recent advances in corneal mechanobiology, focusing on the interplay between mechanical forces and responses of the corneal epithelial, stromal, and endothelial cells. We also identify species-specific differences in corneal biomechanics and mechanotransduction to facilitate identification of optimal animal models to study corneal wound healing, disease, and novel therapeutic interventions. Finally, we identify key knowledge gaps and therapeutic opportunities in corneal mechanobiology that are pressing for the research community to address especially pertinent within the domains of limbal stem cell deficiency, keratoconus and Fuchs' endothelial corneal dystrophy. By furthering our understanding corneal mechanobiology, we can contextualize discoveries regarding corneal diseases as well as innovative treatments for them.

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.

Advances and Challenges in Cell Biology for Cultured Meat

Cultured meat is an emerging biotechnology that aims to produce meat from animal cell culture, rather than from the raising and slaughtering of livestock, on environmental and animal welfare grounds. The detailed understanding and accurate manipulation of cell biology are critical to the design of cultured meat bioprocesses. Recent years have seen significant interest in this field, with numerous scientific and commercial breakthroughs. Nevertheless, these technologies remain at a nascent stage, and myriad challenges remain, spanning the entire bioprocess. From a cell biological perspective, these include the identification of suitable starting cell types, tuning of proliferation and differentiation conditions, and optimization of cell-biomaterial interactions to create nutritious, enticing foods. Here, we discuss the key advances and outstanding challenges in cultured meat, with a particular focus on cell biology, and argue that solving the remaining bottlenecks in a cost-effective, scalable fashion will require coordinated, concerted scientific efforts. Success will also require solutions to nonscientific challenges, including regulatory approval, consumer acceptance, and market feasibility. However, if these can be overcome, cultured meat technologies can revolutionize our approach to food.

The Third Annual Symposium of the Midwest Aging Consortium

The geroscience hypothesis suggests that addressing the fundamental mechanisms driving aging biology will prevent or mitigate the onset of multiple chronic diseases, for which the largest risk factor is advanced age. Research that investigates the root causes of aging is therefore of critical importance given the rising healthcare burden attributable to age-related diseases. The third annual Midwest Aging Consortium symposium was convened as a showcase of such research performed by investigators from institutions across the Midwestern United States. This report summarizes the work presented during a virtual conference across topics in aging biology, including immune function in the lung-particularly timely given the Corona Virus Immune Disease-2019 pandemic-along with the role of metabolism and nutrient-regulated pathways in cellular function with age, the influence of senescence on stem cell function and inflammation, and our evolving understanding of the mechanisms underlying observation of sex dimorphism in aging-related outcomes. The symposium focused on early-stage and emerging investigators, while including keynote presentations from leaders in the biology of aging field, highlighting the diversity and strength of aging research in the Midwest.

Native collagen VI delays early muscle stem cell differentiation

Adult muscle stem cells (MuSCs) are critical for muscle homeostasis and regeneration, and their behavior relies on a finely regulated niche made of specific extracellular matrix (ECM) components and soluble factors. Among ECM proteins, collagen VI (Col6) influences the mechanical properties of the niche and, in turn, MuSC self-renewal capabilities. Here, we investigated whether Col6 can exert a direct function as a biochemical signal for regulating the stemness and differentiation of murine MuSCs and myoblasts. Native Col6, but not its pepsin-resistant fragment, counteracts the early differentiation of myogenic cells by reducing the expression of differentiation marker genes and preserving stemness features, with inhibition of the canonical Wnt pathway. Our data indicate that extracellular Col6 acts as a soluble ligand in delaying early myogenic differentiation by regulating intracellular signals involved in adult myogenesis.

WNT Signaling in Stem Cells: A Look into the Non-Canonical Pathway

Tissue homeostasis is crucial for multicellular organisms, wherein the loss of cells is compensated by generating new cells with the capacity for proliferation and differentiation. At the origin of these populations are the stem cells, which have the potential to give rise to cells with both capabilities, and persevere for a long time through the self-renewal and quiescence. Since the discovery of stem cells, an enormous effort has been focused on learning about their functions and the molecular regulation behind them. Wnt signaling is widely recognized as essential for normal and cancer stem cell. Moreover, β-catenin-dependent Wnt pathway, referred to as canonical, has gained attention, while β-catenin-independent Wnt pathways, known as non-canonical, have remained conspicuously less explored. However, recent evidence about non-canonical Wnt pathways in stem cells begins to lay the foundations of a conceivably vast field, and on which we aim to explain this in the present review. In this regard, we addressed the different aspects in which non-canonical Wnt pathways impact the properties of stem cells, both under normal conditions and also under disease, specifically in cancer.

The role of satellite cell-derived TRIM28 in mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discovered that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Unexpectedly, multiple lines of both in vitro and in vivo evidence revealed that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 were not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discovered that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

The muscle stem cell niche at a glance

Skeletal muscle stem cells (MuSCs, also called satellite cells) are the source of the robust regenerative capability of this tissue. The hallmark property of MuSCs at homeostasis is quiescence, a reversible state of cell cycle arrest required for long-term preservation of the stem cell population. MuSCs reside between an individual myofiber and an enwrapping basal lamina, defining the immediate MuSC niche. Additional cell types outside the basal lamina, in the interstitial space, also contribute to niche function. Quiescence is actively maintained by multiple niche-derived signals, including adhesion molecules presented from the myofiber surface and basal lamina, as well as soluble signaling factors produced by myofibers and interstitial cell types. In this Cell Science at a Glance article and accompanying poster, we present the most recent information on how niche signals promote MuSC quiescence and provide perspectives for further research.

Overcoming muscle stem cell aging

Reduced muscle strength and mass is one of the hallmarks of physiological aging in humans and can result in severe impairment of the quality of life. In part this is caused by a functional loss of the highly specialized muscle stem cells (MuSCs), which in healthy conditions provide maintenance, growth, and regeneration. Recent progress in understanding of the stem cell niche and results from single cell technologies reveal exciting insights at unprecedented detail into MuSCs and muscle biology during aging. Here, we review this field and discuss the implications of current findings with a focus on cellular reprogramming approaches as a novel therapeutic avenue for age-related muscle decline.

Regenerating human skeletal muscle forms an emerging niche in vivo to support PAX7 cells

Skeletal muscle stem and progenitor cells including those derived from human pluripotent stem cells (hPSCs) offer an avenue towards personalized therapies and readily fuse to form human-mouse myofibres in vivo. However, skeletal muscle progenitor cells (SMPCs) inefficiently colonize chimeric stem cell niches and instead associate with human myofibres resembling foetal niches. We hypothesized competition with mouse satellite cells (SCs) prevented SMPC engraftment into the SC niche and thus generated an SC ablation mouse compatible with human engraftment. Single-nucleus RNA sequencing of SC-ablated mice identified the absence of a transient myofibre subtype during regeneration expressing Actc1. Similarly, ACTC1+ human myofibres supporting PAX7+ SMPCs increased in SC-ablated mice, and after re-injury we found SMPCs could now repopulate into chimeric niches. To demonstrate ACTC1+ myofibres are essential to supporting PAX7 SMPCs, we generated caspase-inducible ACTC1 depletion human pluripotent stem cells, and upon SMPC engraftment we found a 90% reduction in ACTC1+ myofibres and a 100-fold decrease in PAX7 cell numbers compared with non-induced controls. We used spatial RNA sequencing to identify key factors driving emerging human niche formation between ACTC1+ myofibres and PAX7+ SMPCs in vivo. This revealed that transient regenerating human myofibres are essential for emerging niche formation in vivo to support PAX7 SMPCs.

The L27 Domain of MPP7 enhances TAZ-YY1 Cooperation to Renew Muscle Stem Cells

Stem cells regenerate differentiated cells to maintain and repair tissues and organs. They also replenish themselves, i.e. self-renewal, for the regenerative process to last a lifetime. How stem cells renew is of critical biological and medical significance. Here we use the skeletal muscle stem cell (MuSC) to study this process. Using a combination of genetic, molecular, and biochemical approaches, we show that MPP7, AMOT, and TAZ/YAP form a complex that activates a common set of target genes. Among these targets, Carm1 can direct MuSC renewal. In the absence of MPP7, TAZ can support regenerative progenitors and activate Carm1 expression, but not to a level needed for self-renewal. Facilitated by the actin polymerization-responsive AMOT, TAZ recruits the L27 domain of MPP7 to up-regulate Carm1 to the level necessary to drive MuSC renewal. The promoter of Carm1, and those of other common downstream genes, also contain binding site(s) for YY1. We further demonstrate that the L27 domain of MPP7 enhances the interaction between TAZ and YY1 to activate Carm1. Our results define a renewal transcriptional program embedded within the progenitor program, by selectively up-regulating key gene(s) within the latter, through the combination of protein interactions and in a manner dependent on the promoter context.

RhoA Is a Crucial Regulator of Myoblast Fusion

Satellite cells (SCs) are adult muscle stem cells that are mobilized when muscle homeostasis is perturbed. Here we show that RhoA in SCs is indispensable to have correct muscle regeneration and hypertrophy. In particular, the absence of RhoA in SCs prevents a correct SC fusion both to other RhoA-deleted SCs (regeneration context) and to growing control myofibers (hypertrophy context). We demonstrated that RhoA is dispensable for SCs proliferation and differentiation; however, RhoA-deleted SCs have an inefficient movement even if their cytoskeleton assembly is not altered. Proliferative myoblast and differentiated myotubes without RhoA display a decreased expression of Chordin, suggesting a crosstalk between these genes for myoblast fusion regulation. These findings demonstrate the importance of RhoA in SC fusion regulation and its requirement to achieve an efficient skeletal muscle homeostasis restoration.

The L27 Domain of MPP7 enhances TAZ-YY1 Cooperation to Renew Muscle Stem Cells

Stem cells regenerate differentiated cells to maintain and repair tissues and organs. They also replenish themselves, i.e. self-renewal, for the regenerative process to last a lifetime. How stem cells renew is of critical biological and medical significance. Here we use the skeletal muscle stem cell (MuSC) to study this process. Using a combination of genetic, molecular, and biochemical approaches, we show that MPP7, AMOT, and TAZ/YAP form a complex that activates a common set of target genes. Among these targets, Carm1 can direct MuSC renewal. In the absence of MPP7, TAZ can support regenerative progenitors and activate Carm1 expression, but not to a level needed for self-renewal. Facilitated by the actin polymerization-responsive AMOT, TAZ recruits the L27 domain of MPP7 to up-regulate Carm1 to the level necessary to drive MuSC renewal. The promoter of Carm1, and those of other common downstream genes, also contain binding site(s) for YY1. We further demonstrate that the L27 domain of MPP7 enhances the interaction between TAZ and YY1 to activate Carm1. Our results define a renewal transcriptional program embedded within the progenitor program, by selectively up-regulating key gene(s) within the latter, through the combination of protein interactions and in a manner dependent on the promoter context.

Molecular and spatial landmarks of early mouse skin development

A wealth of specialized cell populations within the skin facilitates its hair-producing, protective, sensory, and thermoregulatory functions. How the vast cell-type diversity and tissue architecture develops is largely unexplored. Here, with single-cell transcriptomics, spatial cell-type assignment, and cell-lineage tracing, we deconstruct early embryonic mouse skin during the key transitions from seemingly uniform developmental precursor states to a multilayered, multilineage epithelium, and complex dermal identity. We identify the spatiotemporal emergence of hair-follicle-inducing, muscle-supportive, and fascia-forming fibroblasts. We also demonstrate the formation of the panniculus carnosus muscle (PCM), sprouting blood vessels without pericyte coverage, and the earliest residence of mast and dendritic immune cells in skin. Finally, we identify an unexpected epithelial heterogeneity within the early single-layered epidermis and a signaling-rich periderm layer. Overall, this cellular and molecular blueprint of early skin development-which can be explored at https://kasperlab.org/tools-establishes histological landmarks and highlights unprecedented dynamic interactions among skin cells.

Intravital microscopy of satellite cell dynamics and their interaction with myeloid cells during skeletal muscle regeneration

Skeletal muscle regeneration requires the highly coordinated cooperation of muscle satellite cells (MuSCs) with other cellular components. Upon injury, myeloid cells populate the wound site, concomitant with MuSC activation. However, detailed analysis of MuSC-myeloid cell interaction is hindered by the lack of suitable live animal imaging technology. Here, we developed a dual-laser multimodal nonlinear optical microscope platform to study the dynamics of MuSCs and their interaction with nonmyogenic cells during muscle regeneration. Using three-dimensional time-lapse imaging on live reporter mice and taking advantages of the autofluorescence of reduced nicotinamide adenine dinucleotide (NADH), we studied the spatiotemporal interaction between nonmyogenic cells and muscle stem/progenitor cells during MuSC activation and proliferation. We discovered that their cell-cell contact was transient in nature. Moreover, MuSCs could activate with notably reduced infiltration of neutrophils and macrophages, and their proliferation, although dependent on macrophages, did not require constant contact with them. These findings provide a fresh perspective on myeloid cells' role during muscle regeneration.

Migration and differentiation of muscle stem cells are coupled by RhoA signalling during regeneration

Skeletal muscle is highly regenerative and is mediated by a population of migratory adult muscle stem cells (muSCs). Effective muscle regeneration requires a spatio-temporally regulated response of the muSC population to generate sufficient muscle progenitor cells that then differentiate at the appropriate time. The relationship between muSC migration and cell fate is poorly understood and it is not clear how forces experienced by migrating cells affect cell behaviour. We have used zebrafish to understand the relationship between muSC cell adhesion, behaviour and fate in vivo. Imaging of pax7-expressing muSCs as they respond to focal injuries in trunk muscle reveals that they migrate by protrusive-based means. By carefully characterizing their behaviour in response to injury we find that they employ an adhesion-dependent mode of migration that is regulated by the RhoA kinase ROCK. Impaired ROCK activity results in reduced expression of cell cycle genes and increased differentiation in regenerating muscle. This correlates with changes to focal adhesion dynamics and migration, revealing that ROCK inhibition alters the interaction of muSCs to their local environment. We propose that muSC migration and differentiation are coupled processes that respond to changes in force from the environment mediated by RhoA signalling.

Odontoblasts release exosomes to regulate the odontoblastic differentiation of dental pulp stem cells

BACKGROUND

Dental pulp stem cells (DPSCs) play a crucial role in dentin-pulp complex regeneration. Further understanding of the mechanism by which DPSCs remain in a quiescent state could contribute to improvements in the dentin-pulp complex and dentinogenesis.

METHODS

TSC1 conditional knockout (DMP1-Cre+; TSC1f/f, hereafter CKO) mice were generated to increase the activity of mechanistic target of rapamycin complex 1 (mTORC1). H&E staining, immunofluorescence and micro-CT analysis were performed with these CKO mice and littermate controls. In vitro, exosomes were collected from the supernatants of MDPC23 cells with different levels of mTORC1 activity and then characterized by transmission electron microscopy and nanoparticle tracking analysis. DPSCs were cocultured with MDPC23 cells and MDPC23 cell-derived exosomes. Alizarin Red S staining, ALP staining, qRT‒PCR, western blotting analysis and micro-RNA sequencing were performed.

RESULTS

Our study showed that mTORC1 activation in odontoblasts resulted in thicker dentin and higher dentin volume/tooth volume of molars, and it increased the expression levels of the exosome markers CD63 and Alix. In vitro, when DPSCs were cocultured with MDPC23 cells, odontoblastic differentiation was inhibited. However, the inhibition of odontoblastic differentiation was reversed when DPSCs were cocultured with MDPC23 cells with mTORC1 overactivation. To further study the effects of mTORC1 on exosome release from odontoblasts, MDPC23 cells were treated with rapamycin or shRNA-TSC1 to inactivate or activate mTORC1, respectively. The results revealed that exosome release from odontoblasts was negatively correlated with mTORC1 activity. Moreover, exosomes derived from MDPC23 cells with active or inactive mTORC1 inhibited the odontoblastic differentiation of DPSCs at the same concentration. miRNA sequencing analysis of exosomes that were derived from shTSC1-transfected MDPC23 cells, rapamycin-treated MDPC23 cells or nontreated MDPC23 cells revealed that the majority of the miRNAs were similar among these groups. In addition, exosomes derived from odontoblasts inhibited the odontoblastic differentiation of DPSCs, and the inhibitory effect was positively correlated with exosome concentration.

CONCLUSION

mTORC1 regulates exosome release from odontoblasts to inhibit the odontoblastic differentiation of DPSCs, but it does not alter exosomal contents. These findings might provide a new understanding of dental pulp complex regeneration.

Regulation of the Wnt signaling pathway during myogenesis by the mammalian SWI/SNF ATPase BRG1

Skeletal muscle differentiation is a tightly regulated process, and the importance of the mammalian SWI/SNF (mSWI/SNF) chromatin remodeling family for regulation of genes involved in skeletal myogenesis is well-established. Our prior work showed that bromodomains of mSWI/SNF ATPases BRG1 and BRM contribute to myogenesis by facilitating the binding of mSWI/SNF enzymes to regulatory regions of myogenic and other target genes. Here, we report that pathway analyses of differentially expressed genes from that study identified an additional role for mSWI/SNF enzymes via the regulation of the Wnt signaling pathway. The Wnt pathway has been previously shown to be important for skeletal muscle development. To investigate the importance of mSWI/SNF enzymes for the regulation of the Wnt pathway, individual and dual knockdowns were performed for BRG1 and BRM followed by RNA-sequencing. The results show that BRG1, but not BRM, is a regulator of Wnt pathway components and downstream genes. Reactivation of Wnt pathway by stabilization of β-catenin could rescue the defect in myogenic gene expression and differentiation due to BRG1 knockdown or bromodomain inhibition using a specific small molecule inhibitor, PFI-3. These results demonstrate that BRG1 is required upstream of β-catenin function. Chromatin immunoprecipitation of BRG1, BRM and β-catenin at promoters of Wnt pathway component genes showed binding of BRG1 and β-catenin, which provides further mechanistic insight to the transcriptional regulation of these genes.