Macrophage-released ADAMTS1 promotes muscle stem cell activation

Comparative Transcriptome Analysis of mRNA and miRNA during the Development of Longissimus Dorsi Muscle of Gannan Yak and Tianzhu White Yak

The Gannan yak, a superior livestock breed found on the Tibetan Plateau, exhibits significantly enhanced body size, weight, and growth performance in comparison to the Tianzhu white yak. MiRNAs play a pivotal role in regulating muscle growth by negatively modulating target genes. In this study, we found the average diameter, area, and length of myofibers in Gannan yaks were significantly higher than those of Tianzhu white yaks. Further, we focused on analyzing the longissimus dorsi muscle from both Gannan yaks and Tianzhu white yaks through transcriptome sequencing to identify differentially expressed (DE)miRNAs that influence skeletal muscle development. A total of 254 DE miRNAs were identified, of which 126 miRNAs were up-regulated and 128 miRNAs were down-regulated. GO and KEGG enrichment analysis showed that the target genes of these DE miRNAs were significantly enriched in signaling pathways associated with muscle growth and development. By constructing a DE miRNA- DE mRNA interaction network, we screened 18 key miRNAs, and notably, four of the candidates (novel-m0143-3p, novel-m0024-3p, novel-m0128-5p, and novel-m0026-3p) targeted six genes associated with muscle growth and development (DDIT4, ADAMTS1, CRY2, AKIRIN2, SIX1, and FOXO1). These findings may provide theoretical references for further studies on the role of miRNAs in muscle growth and development in Gannan yaks.

Enhancing high-quality fat survival: A novel strategy using cell-free fat extract

High-quality fat (HQF) improves the survival rate of fat and volumetric filling compared to traditional Coleman fat. However, this HQF strategy inevitably leads to a significant amount of unused fat being wasted. "CEFFE" (cell-free fat extract) is an acellular aqueous-phase liquid, rich in bioactive proteins. The remaining fat from preparing HQF can be further processed into CEFFE to promote the survival of HQF. HQF was obtained and the remaining fat was processed into CEFFE, then HQF was transplanted subcutaneously in nude mice. Animal studies showed that CEFFE significantly improved the survival rate of HQF. Histological analysis revealed that CEFFE improved the survival rate of HQF, by enhancing cell proliferation activity, reducing apoptosis, increasing angiogenesis, and improving the inflammatory state. Under simulated anaerobic conditions, CEFFE also improved the viability of HQF. In vitro, studies demonstrated that CEFFE enhanced the survival rate of HQF through multiple mechanisms. Transcriptomic analysis and qPCR showed that CEFFE increased the expression of angiogenesis-related genes in ADSCs while enhancing their proliferation-related gene expression and suppressing the expression of three differentiation-related genes. Moreover, functional experiments demonstrated that CEFFE-induced ADSCs exhibited stronger proliferation and adipogenic differentiation abilities. Tube formation and migration assays revealed that CEFFE promoted tube formation and migration of HUVECs, indicating its inherent pro-angiogenic properties. CEFFE facilitated the development of M0 to M2 macrophages, suggesting its role in improving the inflammatory state. This innovative clinical strategy optimizes HQF transplantation strategy, minimizing fat wastage and enhancing the efficiency of fat utilization.

Skeletal muscle: molecular structure, myogenesis, biological functions, and diseases

Skeletal muscle is an important motor organ with multinucleated myofibers as its smallest cellular units. Myofibers are formed after undergoing cell differentiation, cell-cell fusion, myonuclei migration, and myofibril crosslinking among other processes and undergo morphological and functional changes or lesions after being stimulated by internal or external factors. The above processes are collectively referred to as myogenesis. After myofibers mature, the function and behavior of skeletal muscle are closely related to the voluntary movement of the body. In this review, we systematically and comprehensively discuss the physiological and pathological processes associated with skeletal muscles from five perspectives: molecule basis, myogenesis, biological function, adaptive changes, and myopathy. In the molecular structure and myogenesis sections, we gave a brief overview, focusing on skeletal muscle-specific fusogens and nuclei-related behaviors including cell-cell fusion and myonuclei localization. Subsequently, we discussed the three biological functions of skeletal muscle (muscle contraction, thermogenesis, and myokines secretion) and its response to stimulation (atrophy, hypertrophy, and regeneration), and finally settled on myopathy. In general, the integration of these contents provides a holistic perspective, which helps to further elucidate the structure, characteristics, and functions of skeletal muscle.

Cellular interactions and microenvironment dynamics in skeletal muscle regeneration and disease

Skeletal muscle regeneration relies on the intricate interplay of various cell populations within the muscle niche-an environment crucial for regulating the behavior of muscle stem cells (MuSCs) and ensuring postnatal tissue maintenance and regeneration. This review delves into the dynamic interactions among key players of this process, including MuSCs, macrophages (MPs), fibro-adipogenic progenitors (FAPs), endothelial cells (ECs), and pericytes (PCs), each assuming pivotal roles in orchestrating homeostasis and regeneration. Dysfunctions in these interactions can lead not only to pathological conditions but also exacerbate muscular dystrophies. The exploration of cellular and molecular crosstalk among these populations in both physiological and dystrophic conditions provides insights into the multifaceted communication networks governing muscle regeneration. Furthermore, this review discusses emerging strategies to modulate the muscle-regenerating niche, presenting a comprehensive overview of current understanding and innovative approaches.

Inhibition of p53-MDM2 binding reduces senescent cell abundance and improves the adaptive responses of skeletal muscle from aged mice

Skeletal muscle adaptation to external stimuli, such as regeneration following injury and hypertrophy in response to resistance exercise, are blunted with advanced age. The accumulation of senescent cells, along with defects in myogenic progenitor cell (MPC) proliferation, have been strongly linked as contributing factors to age-associated impairment in muscle adaptation. p53 plays an integral role in all these processes, as upregulation of p53 causes apoptosis in senescent cells and prevents mitotic catastrophe in MPCs from old mice. The goal of this study was to determine if a novel pharmaceutical agent (BI01), which functions by upregulating p53 through inhibition of binding to MDM2, the primary p53 regulatory protein, improves muscle regeneration and hypertrophy in old mice. BI01 effectively reduced the number of senescent cells in vitro but had no effect on MPC survival or proliferation at a comparable dose. Following repeated oral gavage with 2 mg/kg of BI01 (OS) or vehicle (OV), old mice (24 months) underwent unilateral BaCl2 injury in the tibialis anterior (TA) muscle, with PBS injections serving as controls. After 7 days, satellite cell number was higher in the TA of OS compared to OV mice, as was the expression of genes involved in ATP production. By 35 days, old mice treated with BI01 displayed reduced senescent cell burden, enhanced regeneration (higher muscle mass and fiber cross-sectional area) and restoration of muscle function relative to OV mice. To examine the impact of 2 mg/kg BI01 on muscle hypertrophy, the plantaris muscle was subjected to 28 days of mechanical overload (MOV) in OS and OV mice. In response to MOV, OS mice had larger plantaris muscles and muscle fibers than OV mice, particularly type 2b + x fibers, associated with reduced senescent cells. Together our data show that BI01 is an effective senolytic agent that may also augment muscle metabolism to enhance muscle regeneration and hypertrophy in old mice.

Regenerative inflammation: When immune cells help to re-build tissues

Inflammation is an essential immune response critical for responding to infection, injury and maintenance of tissue homeostasis. Upon injury, regenerative inflammation promotes tissue repair by a timed and coordinated infiltration of diverse cell types and the secretion of growth factors, cytokines and lipids mediators. Remarkably, throughout evolution as well as mammalian development, this type of physiological inflammation is highly associated with immunosuppression. For instance, regenerative inflammation is the consequence of an in situ macrophage polarization resulting in a transition from pro-inflammatory to anti-inflammatory/pro-regenerative response. Immune cells are the first responders upon injury, infiltrating the damaged tissue and initiating a pro-inflammatory response depleting cell debris and necrotic cells. After phagocytosis, macrophages undergo multiple coordinated metabolic and transcriptional changes allowing the transition and dictating the initiation of the regenerative phase. Differences between a highly efficient, complete ad integrum tissue repair, such as, acute skeletal muscle injury, and insufficient regenerative inflammation, as the one developing in Duchenne Muscular Dystrophy (DMD), highlight the importance of a coordinated response orchestrated by immune cells. During regenerative inflammation, these cells interact with others and alter the niche, affecting the character of inflammation itself and, therefore, the progression of tissue repair. Comparing acute muscle injury and chronic inflammation in DMD, we review how the same cells and molecules in different numbers, concentration and timing contribute to very different outcomes. Thus, it is important to understand and identify the distinct functions and secreted molecules of macrophages, and potentially other immune cells, during tissue repair, and the contributors to the macrophage switch leveraging this knowledge in treating diseases.

Muscle stem cell dysfunction in rhabdomyosarcoma and muscular dystrophy

Muscle stem cells (MuSCs) are crucial to the repair and homeostasis of mature skeletal muscle. MuSC dysfunction and dysregulation of the myogenic program can contribute to the development of pathology ranging from cancers like rhabdomyosarcoma (RMS) or muscle degenerative diseases such as Duchenne muscular dystrophy (DMD). Both diseases exhibit dysregulation at nearly all steps of myogenesis. For instance, MuSC self-renewal processes are altered. In RMS, this leads to the creation of tumor propagating cells. In DMD, impaired asymmetric stem cell division creates a bias towards producing self-renewing stem cells instead of committing to differentiation. Hyperproliferation of these cells contribute to tumorigenesis in RMS and symmetric expansion of the self-renewing MuSC population in DMD. Both diseases also exhibit a repression of factors involved in terminal differentiation, halting RMS cells in the proliferative stage and thus driving tumor growth. Conversely, the MuSCs in DMD exhibit impaired differentiation and fuse prematurely, affecting myonuclei maturation and the integrity of the dystrophic muscle fiber. Finally, both disease states cause alterations to the MuSC niche. Various elements of the niche such as inflammatory and migratory signaling that impact MuSC behavior are dysregulated. Here we show how these seemingly distantly related diseases indeed have similarities in MuSC dysfunction, underlying the importance of considering MuSCs when studying the pathophysiology of muscle diseases.

Epigenetic integration of signaling from the regenerative environment

Skeletal muscle has an extraordinary capacity to regenerate itself after injury due to the presence of tissue-resident muscle stem cells. While these muscle stem cells are the primary contributor to the regenerated myofibers, the process occurs in a regenerative microenvironment where multiple different cell types act in a coordinated manner to clear the damaged myofibers and restore tissue homeostasis. In this regenerative environment, immune cells play a well-characterized role in initiating repair by establishing an inflammatory state that permits the removal of dead cells and necrotic muscle tissue at the injury site. More recently, it has come to be appreciated that the immune cells also play a crucial role in communicating with the stem cells within the regenerative environment to help coordinate the timing of repair events through the secretion of cytokines, chemokines, and growth factors. Evidence also suggests that stem cells can help modulate the extent of the inflammatory response by signaling to the immune cells, demonstrating a cross-talk between the different cells in the regenerative environment. Here, we review the current knowledge on the innate immune response to sterile muscle injury and provide insight into the epigenetic mechanisms used by the cells in the regenerative niche to integrate the cellular cross-talk required for efficient muscle repair.

Genome-wide association study of early liveweight traits in fat-tailed Akkaraman lambs

Small ruminants, especially sheep, are essential for sustainable agricultural production systems, future food/nutrition security, and poverty reduction in developing countries. Within developed countries, the ability of sheep to survive on low-quality forage intake could act as buffer against climate change. Besides sheep's importance in sustainable agricultural production, there has been less ongoing work in terms of sheep genetics in Near East, Middle East and in Africa. For lamb meat production, body weight and average daily gain (ADG) until weaning are critical economic traits that affects the profitability of the industry. The current study aims to identify single nucleotide polymorphisms (SNPs) that are significantly associated with pre-weaning growth traits in fat tail Akkaraman lambs using a genome-wide association study (GWAS). A total of 196 Akkaraman lambs were selected for analysis. After quality control, a total of 31,936 SNPs and 146 lambs were used for subsequent analyses. PLINK 1.9 beta software was used for the analyses. Based on Bonferroni-adjusted p-values, one SNP (rs427117280) on chromosome 2 (OAR2) had significant associations with weaning weight at day 90 and ADG from day 0 to day 90, which jointly explains a 0.8% and 0.9% of total genetic variation respectively. The Ovis aries natriuretic peptide C (NPPC) could be considered as a candidate gene for the defined significant associations. The results of the current study will help to increase understanding of the variation in weaning weight and ADG until weaning of Akkaraman lambs and help enhance selection for lambs with improved weaning weight and ADG. However, further investigations are required for the identification of causal variants within the identified genomic regions.

Skeletal muscle regeneration failure in ischemic-damaged limbs is associated with pro-inflammatory macrophages and premature differentiation of satellite cells

BACKGROUND

Chronic limb-threatening ischemia (CLTI), a severe manifestation of peripheral arterial disease (PAD), is associated with a 1-year limb amputation rate of approximately 15-20% and substantial mortality. A key feature of CLTI is the compromised regenerative ability of skeletal muscle; however, the mechanisms responsible for this impairment are not yet fully understood. In this study, we aim to delineate pathological changes at both the cellular and transcriptomic levels, as well as in cell-cell signaling pathways, associated with compromised muscle regeneration in limb ischemia in both human tissue samples and murine models of CLTI.

METHODS

We performed single-cell transcriptome analysis of ischemic and non-ischemic muscle from the same CLTI patients and from a murine model of CLTI. In both datasets, we analyzed gene expression changes in macrophage and muscle satellite cell (MuSC) populations as well as differential cell-cell signaling interactions and differentiation trajectories.

RESULTS

Single-cell transcriptomic profiling and immunofluorescence analysis of CLTI patient skeletal muscle demonstrated that ischemic-damaged tissue displays a pro-inflammatory macrophage signature. Comparable results were observed in a murine CLTI model. Moreover, integrated analyses of both human and murine datasets revealed premature differentiation of MuSCs to be a key feature of failed muscle regeneration in the ischemic limb. Furthermore, in silico inferences of intercellular communication and in vitro assays highlight the importance of macrophage-MuSC signaling in ischemia induced muscle injuries.

CONCLUSIONS

Collectively, our research provides the first single-cell transcriptome atlases of skeletal muscle from CLTI patients and a murine CLTI model, emphasizing the crucial role of macrophages and inflammation in regulating muscle regeneration in CLTI through interactions with MuSCs.

The multifaceted role of macrophages in homeostatic and injured skeletal muscle

Skeletal muscle is essential for body physical activity, energy metabolism, and temperature maintenance. It has excellent capabilities to maintain homeostasis and to regenerate after injury, which indispensably relies on muscle stem cells, satellite cells (MuSCs). The quiescence, activation, and differentiation of MuSCs are tightly regulated in homeostatic and regenerating muscles. Among the important regulators are intramuscular macrophages, which are functionally heterogeneous with different subtypes present in a spatiotemporal manner to regulate the balance of different MuSC statuses. During chronic injury and aging, intramuscular macrophages often undergo aberrant activation, which in turn disrupts muscle homeostasis and regenerative repair. Growing evidence suggests that the aberrant activation is mainly triggered by altered muscle microenvironment. The trained immunity that affects myeloid progenitors during hematopoiesis may also contribute. Aged immune system may contribute, in part, to the aging-related sarcopenia and compromised skeletal muscle injury repair. As macrophages are actively involved in the progression of many muscle diseases, manipulating their functional activation has become a promising therapeutic approach, which requires comprehensive knowledge of the cellular and molecular mechanisms underlying the diverse activation. To this end, we discuss here the current knowledge of multifaceted role of macrophages in skeletal muscle homeostasis, injury, and repair.

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.

Intravenous Injection of Sodium Hyaluronate Diminishes Basal Inflammatory Gene Expression in Equine Skeletal Muscle

Following strenuous exercise, skeletal muscle experiences an acute inflammatory state that initiates the repair process. Systemic hyaluronic acid (HA) is injected to horses routinely as a joint anti-inflammatory. To gain insight into the effects of HA on skeletal muscle, adult Thoroughbred geldings (n = 6) were injected with a commercial HA product weekly for 3 weeks prior to performing a submaximal exercise test. Gluteal muscle (GM) biopsies were obtained before and 1 h after exercise for gene expression analysis and HA localization. The results from RNA sequencing demonstrate differences in gene expression between non-injected controls (CON; n = 6) and HA horses. Prior to exercise, HA horses contained fewer (p < 0.05) transcripts associated with leukocyte activity and cytokine production than CON. The performance of exercise resulted in the upregulation (p < 0.05) of several cytokine genes and their signaling intermediates, indicating that HA does not suppress the normal inflammatory response to exercise. The transcript abundance for marker genes of neutrophils (NCF2) and macrophages (CD163) was greater (p < 0.05) post-exercise and was unaffected by HA injection. The anti-inflammatory effects of HA on muscle are indirect as no differences (p > 0.05) in the relative amount of the macromolecule was observed between the CON and HA fiber extracellular matrix (ECM). However, exercise tended (p = 0.10) to cause an increase in ECM size suggestive of muscle damage and remodeling. The finding was supported by the increased (p < 0.05) expression of CTGF, TGFβ1, MMP9, TIMP4 and Col4A1. Collectively, the results validate HA as an anti-inflammatory aid that does not disrupt the normal post-exercise muscle repair process.

The Role of Macrophage Populations in Skeletal Muscle Insulin Sensitivity: Current Understanding and Implications

Insulin resistance is a crucial factor in the development of type 2 diabetes mellitus (T2DM) and other metabolic disorders. Skeletal muscle, the body's largest insulin-responsive tissue, plays a significant role in the pathogenesis of T2DM due to defects in insulin signaling. Recently, there has been growing evidence that macrophages, immune cells essential for tissue homeostasis and injury response, also contribute to the development of skeletal muscle insulin resistance. This review aims to summarize the current understanding of the role of macrophages in skeletal muscle insulin resistance. Firstly, it provides an overview of the different macrophage populations present in skeletal muscle and their specific functions in the development of insulin resistance. Secondly, it examines the underlying mechanisms by which macrophages promote or alleviate insulin resistance in skeletal muscle, including inflammation, oxidative stress, and altered metabolism. Lastly, the review discusses potential therapeutic strategies targeting macrophages to improve skeletal muscle insulin sensitivity and metabolic health.

Fibrillin-1 regulates endothelial sprouting during angiogenesis

Fibrillin-1 is an extracellular matrix protein that assembles into microfibrils which provide critical functions in large blood vessels and other tissues. Mutations in the fibrillin-1 gene are associated with cardiovascular, ocular, and skeletal abnormalities in Marfan syndrome. Here, we reveal that fibrillin-1 is critical for angiogenesis which is compromised by a typical Marfan mutation. In the mouse retina vascularization model, fibrillin-1 is present in the extracellular matrix at the angiogenic front where it colocalizes with microfibril-associated glycoprotein-1, MAGP1. In Fbn1C1041G/+ mice, a model of Marfan syndrome, MAGP1 deposition is reduced, endothelial sprouting is decreased, and tip cell identity is impaired. Cell culture experiments confirmed that fibrillin-1 deficiency alters vascular endothelial growth factor-A/Notch and Smad signaling which regulate the acquisition of endothelial tip cell/stalk cell phenotypes, and we showed that modulation of MAGP1 expression impacts these pathways. Supplying the growing vasculature of Fbn1C1041G/+ mice with a recombinant C-terminal fragment of fibrillin-1 corrects all defects. Mass spectrometry analyses showed that the fibrillin-1 fragment alters the expression of various proteins including ADAMTS1, a tip cell metalloprotease and matrix-modifying enzyme. Our data establish that fibrillin-1 is a dynamic signaling platform in the regulation of cell specification and matrix remodeling at the angiogenic front and that mutant fibrillin-1-induced defects can be rescued pharmacologically using a C-terminal fragment of the protein. These findings, identify fibrillin-1, MAGP1, and ADAMTS1 in the regulation of endothelial sprouting, and contribute to our understanding of how angiogenesis is regulated. This knowledge may have critical implications for people with Marfan syndrome.

Physiology and diseases of tissue-resident macrophages

Embryo-derived tissue-resident macrophages are the first representatives of the haematopoietic lineage to emerge in metazoans. In mammals, resident macrophages originate from early yolk sac progenitors and are specified into tissue-specific subsets during organogenesis-establishing stable spatial and functional relationships with specialized tissue cells-and persist in adults. Resident macrophages are an integral part of tissues together with specialized cells: for instance, microglia reside with neurons in brain, osteoclasts reside with osteoblasts in bone, and fat-associated macrophages reside with white adipocytes in adipose tissue. This ancillary cell type, which is developmentally and functionally distinct from haematopoietic stem cell and monocyte-derived macrophages, senses and integrates local and systemic information to provide specialized tissue cells with the growth factors, nutrient recycling and waste removal that are critical for tissue growth, homeostasis and repair. Resident macrophages contribute to organogenesis, promote tissue regeneration following damage and contribute to tissue metabolism and defence against infectious disease. A correlate is that genetic or environment-driven resident macrophage dysfunction is a cause of degenerative, metabolic and possibly inflammatory and tumoural diseases. In this Review, we aim to provide a conceptual outline of our current understanding of macrophage physiology and its importance in human diseases, which may inform and serve the design of future studies.

Macrophage Involvement in Aging-Associated Skeletal Muscle Regeneration

The skeletal muscle is a dynamic organ composed of contractile muscle fibers, connective tissues, blood vessels and nerve endings. Its main function is to provide motility to the body, but it is also deeply involved in systemic metabolism and thermoregulation. The skeletal muscle frequently encounters microinjury or trauma, which is primarily repaired by the coordinated actions of muscle stem cells (satellite cells, SCs), fibro-adipogenic progenitors (FAPs), and multiple immune cells, particularly macrophages. During aging, however, the capacity of skeletal muscle to repair and regenerate declines, likely contributing to sarcopenia, an age-related condition defined as loss of muscle mass and function. Recent studies have shown that resident macrophages in skeletal muscle are highly heterogeneous, and their phenotypes shift during aging, which may exacerbate skeletal muscle deterioration and inefficient regeneration. In this review, we highlight recent insight into the heterogeneity and functional roles of macrophages in skeletal muscle regeneration, particularly as it declines with aging.

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

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

ADAMTS1 as potential prognostic biomarker promotes malignant invasion of glioma

BACKGROUND

Glioma is the most common intracranial malignancy in adults with a high degree of malignancy and poor prognosis, which is largely attributed to the existence of glioma stem cells (GSCs). Previous evidence indicated that the matrix metalloproteinase ADAMTS1 was implicated in the process of tumor invasion, but the involvement of ADAMTS1 in glioma malignant invasion remains poorly understood.

METHODS

The expression and prognosis values of ADAMTS1 were investigated in patients with glioma based on ONCOMINE and GEPIA databases. ADAMTS1 expression of different malignancy grade tissues was determined by immunohistochemistry. The effects of ADAMTS1 on cell proliferation and invasion were determined by clone formation assay and Transwell migration assay. The animal experiment was performed in an intracranial orthotopic xenograft model by knockout of ADAMTS1. Stemness properties and Notch1-SOX2 pathway were examined in stable ADAMTS1 knockdown GSCs.

RESULTS

The expression levels of ADAMTS1 were significantly higher in glioma tissues and significantly correlated with the grade of malignancy and prognosis of glioma. Elevated ADAMTS1 expression was associated with SOX2, N-cadherin and the resistance of chemoradiotherapy of glioma patients. ADAMTS1 knockout suppressed the intracranial orthotopic xenograft growth and prolonged the survival of xenograft mice in vivo. Mechanistically, we found a blockade of the migration and invasiveness of GSCs and the expression levels of Notch1 and SOX2 in absence of ADAMTS1.

CONCLUSION

As a biomarker for prediction of prognosis, ADAMTS1 may affect the invasive phenotype of GSCs by regulating Notch1-SOX2 signaling pathway, thereby promoting the invasive growth of glioma.

Meteorin-like is an injectable peptide that can enhance regeneration in aged muscle through immune-driven fibro/adipogenic progenitor signaling

Pathologies associated with sarcopenia include decline in muscular strength, lean mass and regenerative capacity. Despite the substantial impact on quality of life, no pharmacological therapeutics are available to counteract the age-associated decline in functional capacity and/or, resilience. Evidence suggests immune-secreted cytokines can improve muscle regeneration, a strategy which we leverage in this study by rescuing the age-related deficiency in Meteorin-like through several in vivo add-back models. Notably, the intramuscular, peptide injection of recombinant METRNL was sufficient to improve muscle regeneration in aging. Using ex vivo media exchange and in vivo TNF inhibition, we demonstrate a mechanism of METRNL action during regeneration, showing it counteracts a pro-fibrotic gene program by triggering TNFα-induced apoptosis of fibro/adipogenic progenitor cells. These findings demonstrate therapeutic applications for METRNL to improve aged muscle, and show Fibro/Adipogenic Progenitors are viable therapeutic targets to counteract age-related loss in muscle resilience.

Transcriptomic mapping of the metzincin landscape in human trophoblasts

The metzincin family of metalloproteases coordinates tissue developmental processes through regulation of growth factor availability, receptor signaling, and cell-cell/cell-matrix adhesion. While roles for select metzincins in controlling trophoblast functions in human placental development have been described, a comprehensive understanding of metzincin dynamics during trophoblast differentiation is lacking. To address this knowledge gap, single cell transcriptomic datasets derived from first trimester chorionic villi and decidua were used to decipher metzincin expression profiles and kinetics in diverse cell types within the utero-placental interface. Further, specific protease-substrate interactions within progenitor trophoblasts were examined to better define the progenitor niche. Within the uterine-placental compartment, 43 metzincin proteases were expressed across 15 cell-type clusters. Metzincin subgroups expressed in placental trophoblasts, placental mesenchymal cells, uterine stromal, and immune cells included multiple matrix metalloproteases (MMPs), a disintegrin and metalloproteases (ADAMs), a disintegrin and metalloproteases with thrombospondin repeats (ADAMTSs), pappalysins, and astacins. Within the trophoblast compartment, eight distinct trophoblasts states were identified: four cytotrophoblast (CTB), one syncytiotrophoblast precursor (SCTp), two column CTB (cCTB), and one extravillous trophoblast (EVT). Within these states 7 MMP, 8 ADAM, 4 ADAMTS, 2 pappalysin, and 3 astacin proteases were expressed. Cell trajectory modeling shows that expression of most (19/24) metzincins increase during EVT differentiation, though expression of select metalloproteases increase along the villous pathway. Eleven metzincins (ADAM10, -17, MMP14, -15, -19, -23B, ADAMTS1, -6, -19, TLL-1, -2) showed enrichment within CTB progenitors, and analysis of metzincin-substrate interactions identified ∼150 substrates and binding partners, including FBN2 as an ADAMTS6-specific substrate. Together, this work characterizes the metzincin landscape in human first trimester trophoblasts and establishes insight into the roles specific proteases perform within distinct trophoblast niches and across trophoblast differentiation. This resource serves as a guide for future investigations into the roles of metzincin proteases in human placental development.

Macrophages play a key role in tissue repair and regeneration

Tissue regeneration after body injury has always been a complex problem to resolve for mammals. In adult mammals, the repair process after tissue injury is often accompanied by continuous and extensive fibrosis, which leads to scars. This process has been shown to severely hinder regeneration. Macrophages, as widely distributed innate immune cells, not only play an important role in various pathological processes, but also participate in the repair process before tissue regeneration and coordinate the regeneration process after repair. This review will discuss the various forms and indispensability of macrophages involved in repair and regeneration, and how macrophages play a role in the repair and regeneration of different tissues.

The MyoD1 Promoted Muscle Differentiation and Generation by Activating CCND2 in Guanling Cattle

The purpose of this study was to analyze the transcriptome of MyoD1 gene knockout MDBK cells (bovine kidney cells) using high-throughput sequencing. For the first time, CRISPR/CAS9 technology was used to construct a MyoD1 knockout in MDBK cells and transcriptome sequence analysis was used to examine MyoD1-related target gene expression. Transcriptome sequencing indicated the presence of 723 differentially expressed genes (DEGs) by comparing wild type and MyoD1 knockout MDBK cells and included 178 upregulated and 72 downregulated genes. The DEGs are mainly enriched in Pl-3-kinase and AKT, p53 signaling pathways. Quantitative RT-PCR confirmed that PDE1B, ADAMTS1, DPT, and CCND2 were highly expressed in the leg muscle, longissimus dorsi, and shoulder of Guanling cattle, and CCND2 was inhibited after MyoD1 knockout, suggesting it may be a key downstream gene of MyoD1 and associated with muscle formation and differentiation in Guanling cattle. This provides experimental data for subsequent studies on the regulatory mechanisms of muscle differentiation in Guanling cattle.

Senolytic treatment rescues blunted muscle hypertrophy in old mice

With aging, skeletal muscle plasticity is attenuated in response to exercise. Here, we report that senescent cells, identified using senescence-associated β-galactosidase (SA β-Gal) activity and p21 immunohistochemistry, are very infrequent in resting muscle, but emerge approximately 2 weeks after a bout of resistance exercise in humans. We hypothesized that these cells contribute to blunted hypertrophic potential in old age. Using synergist ablation-induced mechanical overload (MOV) of the plantaris muscle to model resistance training in adult (5-6-month) and old (23-24-month) male C57BL/6 J mice, we found increased senescent cells in both age groups during hypertrophy. Consistent with the human data, there were negligible senescent cells in plantaris muscle from adult and old sham controls, but old mice had significantly more senescent cells 7 and 14 days following MOV relative to young. Old mice had blunted whole-muscle hypertrophy when compared to adult mice, along with smaller muscle fibers, specifically glycolytic type 2x + 2b fibers. To ablate senescent cells using a hit-and-run approach, old mice were treated with vehicle or a senolytic cocktail consisting of 5 mg/kg dasatinib and 50 mg/kg quercetin (D + Q) on days 7 and 10 during 14 days of MOV; control mice underwent sham surgery with or without senolytic treatment. Old mice given D + Q had larger muscles and muscle fibers after 14 days of MOV, fewer senescent cells when compared to vehicle-treated old mice, and changes in the expression of genes (i.e., Igf1, Ddit4, Mmp14) that are associated with hypertrophic growth. Our data collectively show that senescent cells emerge in human and mouse skeletal muscle following a hypertrophic stimulus and that D + Q improves muscle growth in old mice.

The Many Roles of Macrophages in Skeletal Muscle Injury and Repair

Skeletal muscle is essential to physical activity and energy metabolism. Maintaining intact functions of skeletal muscle is crucial to health and wellbeing. Evolutionarily, skeletal muscle has developed a remarkable capacity to maintain homeostasis and to regenerate after injury, which indispensably relies on the resident muscle stem cells, satellite cells. Satellite cells are largely quiescent in the homeostatic steady state. They are activated in response to muscle injury. Activated satellite cells proliferate and differentiate into myoblasts. Myoblasts fuse to form myotubes which further grow and differentiate into mature myofibers. This process is tightly regulated by muscle microenvironment that consists of multiple cellular and molecular components, including macrophages. Present in both homeostatic and injured muscles, macrophages contain heterogeneous functional subtypes that play diverse roles in maintaining homeostasis and promoting injury repair. The spatial-temporal presence of different functional subtypes of macrophages and their interactions with myogenic cells are vital to the proper regeneration of skeletal muscle after injury. However, this well-coordinated process is often disrupted in a chronic muscle disease, such as muscular dystrophy, leading to asynchronous activation and differentiation of satellite cells and aberrant muscle regeneration. Understanding the precise cellular and molecular processes regulating interactions between macrophages and myogenic cells is critical to the development of therapeutic manipulation of macrophages to promote injury repair. Here, we review the current knowledge of the many roles played by macrophages in the regulation of myogenic cells in homeostatic, regenerating, and dystrophic skeletal muscles.

Bidirectional roles of skeletal muscle fibro-adipogenic progenitors in homeostasis and disease

Sarcopenia and myopathies cause progressive muscle weakness and degeneration, which are closely associated with fat infiltration and fibrosis in muscle. Recently, experimental research has shed light on fibro-adipogenic progenitors (FAPs), also known as muscle-resident mesenchymal progenitors with multiple differentiation potential for adipogenesis, fibrosis, osteogenesis and chondrogenesis. They are considered key regulators of muscle homeostasis and integrity. They play supportive roles in muscle development and repair by orchestrating the regulatory interplay between muscle stem cells (MuSCs) and immune cells. Interestingly, FAPs also contribute to intramuscular fat infiltration, fibrosis and other pathologies when the functional integrity of the network is compromised. In this review, we summarize recent insights into the roles of FAPs in maintenance of skeletal muscle homeostasis, and discuss the underlying mechanisms regulating FAPs behavior and fate, highlighting their roles in participating in efficient muscle repair and fat infiltrated muscle degeneration as well as during muscle atrophy. We suggest that controlling and predicting FAPs differentiation may become a promising strategy to improve muscle function and prevent irreparable muscle damage.

New Progress in Basic Research of Macrophages in the Pathogenesis and Treatment of Low Back Pain

Low back pain (LBP) is quite common in clinical practice, which can lead to long-term bed rest or even disability. It is a worldwide health problem remains to be solved. LBP can be induced or exacerbated by abnormal structure and function of spinal tissue such as intervertebral disc (IVD), dorsal root ganglion (DRG) and muscle; IVD degeneration (IVDD) is considered as the most important among all the pathogenic factors. Inflammation, immune response, mechanical load, and hypoxia etc., can induce LBP by affecting the spinal tissue, among which inflammation and immune response are the key link. Inflammation and immune response play a double-edged sword role in LBP. As the main phagocytic cells in the body, macrophages are closely related to body homeostasis and various diseases. Recent studies have shown that macrophages are the only inflammatory cells that can penetrate the closed nucleus pulposus, expressed in various structures of the IVD, and the number is positively correlated with the degree of IVDD. Moreover, macrophages play a phagocytosis role or regulate the metabolism of DRG and muscle tissues through neuro-immune mechanism, while the imbalance of macrophages polarization will lead to more inflammatory factors to chemotaxis and aggregation, forming an “inflammatory waterfall” effect similar to “positive feedback,” which greatly aggravates LBP. Regulation of macrophages migration and polarization, inhibition of inflammation and continuous activation of immune response by molecular biological technology can markedly improve the inflammatory microenvironment, and thus effectively prevent and treat LBP. Studies on macrophages and LBP were mainly focused in the last 3–5 years, attracting more and more scholars’ attention. This paper summarizes the new research progress of macrophages in the pathogenesis and treatment of LBP, aiming to provide an important clinical prevention and treatment strategy for LBP.

The Notch signaling network in muscle stem cells during development, homeostasis, and disease

Skeletal muscle stem cells have a central role in muscle growth and regeneration. They reside as quiescent cells in resting muscle and in response to damage they transiently amplify and fuse to produce new myofibers or self-renew to replenish the stem cell pool. A signaling pathway that is critical in the regulation of all these processes is Notch. Despite the major differences in the anatomical and cellular niches between the embryonic myotome, the adult sarcolemma/basement-membrane interphase, and the regenerating muscle, Notch signaling has evolved to support the context-specific requirements of the muscle cells. In this review, we discuss the diverse ways by which Notch signaling factors and other modifying partners are operating during the lifetime of muscle stem cells to establish an adaptive dynamic network.

Notch signaling pathway: architecture, disease, and therapeutics

The NOTCH gene was identified approximately 110 years ago. Classical studies have revealed that NOTCH signaling is an evolutionarily conserved pathway. NOTCH receptors undergo three cleavages and translocate into the nucleus to regulate the transcription of target genes. NOTCH signaling deeply participates in the development and homeostasis of multiple tissues and organs, the aberration of which results in cancerous and noncancerous diseases. However, recent studies indicate that the outcomes of NOTCH signaling are changeable and highly dependent on context. In terms of cancers, NOTCH signaling can both promote and inhibit tumor development in various types of cancer. The overall performance of NOTCH-targeted therapies in clinical trials has failed to meet expectations. Additionally, NOTCH mutation has been proposed as a predictive biomarker for immune checkpoint blockade therapy in many cancers. Collectively, the NOTCH pathway needs to be integrally assessed with new perspectives to inspire discoveries and applications. In this review, we focus on both classical and the latest findings related to NOTCH signaling to illustrate the history, architecture, regulatory mechanisms, contributions to physiological development, related diseases, and therapeutic applications of the NOTCH pathway. The contributions of NOTCH signaling to the tumor immune microenvironment and cancer immunotherapy are also highlighted. We hope this review will help not only beginners but also experts to systematically and thoroughly understand the NOTCH signaling pathway.

Engineered clustered myoblast cell injection augments angiogenesis and muscle regeneration in peripheral artery disease

The low survival rate of administered cells due to ischemic and inflammatory environments limits the efficacy of the current regenerative cell therapy in peripheral artery disease (PAD). This study aimed to develop a new method to enhance the efficacy of cell therapy in PAD using cell sheet technology. Clustered cells (CCs) from myoblast cell sheets obtained from C57/BL6 mice were administered into ischemic mouse muscles 7 days after induction of ischemia (defined as day 0). Control groups were administered with single myoblast cells (SCs) or saline. Cell survival, blood perfusion of the limb, angiogenesis, muscle regeneration, and inflammation status were evaluated. The survival of administered cells was markedly improved in CCs compared with SCs at days 7 and 28. CCs showed significantly improved blood perfusion, augmented angiogenesis with increased density of CD31⁺/α-smooth muscle actin⁺ arterioles, and accelerated muscle regeneration, along with the upregulation of associated genes. Additionally, inflammation status was well regulated by CCs administration. CCs administration increased the number of macrophages and then induced polarization into an anti-inflammatory phenotype (CD11c^-/CD206⁺), along with the increased expression of genes associated with anti-inflammatory cytokines. Our findings suggest clinical potential of rescuing severely damaged limbs in PAD using CCs.

Home Away From Home: Bioengineering Advancements to Mimic the Developmental and Adult Stem Cell Niche

The inherent self-organizing capacity of pluripotent and adult stem cell populations has advanced our fundamental understanding of processes that drive human development, homeostasis, regeneration, and disease progression. Translating these principles into in vitro model systems has been achieved with the advent of organoid technology, driving innovation to harness patient-specific, cell-laden regenerative constructs that can be engineered to augment or replace diseased tissue. While developmental organization and regenerative adult stem cell niches are tightly regulated in vivo, in vitro analogs lack defined architecture and presentation of physicochemical cues, leading to the unhindered arrangement of mini-tissues that lack complete physiological mimicry. This review aims to highlight the recent integrative engineering approaches that elicit spatio-temporal control of the extracellular niche to direct the structural and functional maturation of pluripotent and adult stem cell derivatives. While the advances presented here leverage multi-pronged strategies ranging from synthetic biology to microfabrication technologies, the methods converge on recreating the biochemical and biophysical milieu of the native tissue to be modeled or regenerated.

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.

Therapeutic Application of Extracellular Vesicles-Capsulated Adeno-Associated Virus Vector via nSMase2/Smpd3, Satellite, and Immune Cells in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by loss-of-function mutations in the dystrophin gene on chromosome Xp21. Disruption of the dystrophin–glycoprotein complex (DGC) on the cell membrane causes cytosolic Ca²⁺ influx, resulting in protease activation, mitochondrial dysfunction, and progressive myofiber degeneration, leading to muscle wasting and fragility. In addition to the function of dystrophin in the structural integrity of myofibers, a novel function of asymmetric cell division in muscular stem cells (satellite cells) has been reported. Therefore, it has been suggested that myofiber instability may not be the only cause of dystrophic degeneration, but rather that the phenotype might be caused by multiple factors, including stem cell and myofiber functions. Furthermore, it has been focused functional regulation of satellite cells by intracellular communication of extracellular vesicles (EVs) in DMD pathology. Recently, a novel molecular mechanism of DMD pathogenesis—circulating RNA molecules—has been revealed through the study of target pathways modulated by the Neutral sphingomyelinase2/Neutral sphingomyelinase3 (nSMase2/Smpd3) protein. In addition, adeno-associated virus (AAV) has been clinically applied for DMD therapy owing to the safety and long-term expression of transduction genes. Furthermore, the EV-capsulated AAV vector (EV-AAV) has been shown to be a useful tool for the intervention of DMD, because of the high efficacy of the transgene and avoidance of neutralizing antibodies. Thus, we review application of AAV and EV-AAV vectors for DMD as novel therapeutic strategy.

Deletion of SA β-Gal+ cells using senolytics improves muscle regeneration in old mice

Systemic deletion of senescent cells leads to robust improvements in cognitive, cardiovascular, and whole-body metabolism, but their role in tissue reparative processes is incompletely understood. We hypothesized that senolytic drugs would enhance regeneration in aged skeletal muscle. Young (3 months) and old (20 months) male C57Bl/6J mice were administered the senolytics dasatinib (5 mg/kg) and quercetin (50 mg/kg) or vehicle bi-weekly for 4 months. Tibialis anterior (TA) was then injected with 1.2% BaCl2 or PBS 7- or 28 days prior to euthanization. Senescence-associated β-Galactosidase positive (SA β-Gal+) cell abundance was low in muscle from both young and old mice and increased similarly 7 days following injury in both age groups, with no effect of D+Q. Most SA β-Gal+ cells were also CD11b+ in young and old mice 7- and 14 days following injury, suggesting they are infiltrating immune cells. By 14 days, SA β-Gal+/CD11b+ cells from old mice expressed senescence genes, whereas those from young mice expressed higher levels of genes characteristic of anti-inflammatory macrophages. SA β-Gal+ cells remained elevated in old compared to young mice 28 days following injury, which were reduced by D+Q only in the old mice. In D+Q-treated old mice, muscle regenerated following injury to a greater extent compared to vehicle-treated old mice, having larger fiber cross-sectional area after 28 days. Conversely, D+Q blunted regeneration in young mice. In vitro experiments suggested D+Q directly improve myogenic progenitor cell proliferation. Enhanced physical function and improved muscle regeneration demonstrate that senolytics have beneficial effects only in old mice.

RhoA within myofibers controls satellite cell microenvironment to allow hypertrophic growth

Adult skeletal muscle is a plastic tissue that can adapt its size to workload. Here, we show that RhoA within myofibers is needed for overload-induced hypertrophy by controlling satellite cell (SC) fusion to the growing myofibers without affecting protein synthesis. At the molecular level, we demonstrate that RhoA controls in a cell autonomous manner Erk1/2 activation and the expressions of extracellular matrix (ECM) regulators such as Mmp9/Mmp13/Adam8 and macrophage chemo-attractants such as Ccl3/Cx3cl1. Their decreased expression in RhoA mutants is associated with ECM and fibrillar collagen disorganization and lower macrophage infiltration. Moreover, matrix metalloproteinases inhibition and macrophage depletion in controls phenocopied the altered growth of RhoA mutants while having no effect in mutants showing that their action is RhoA-dependent. These findings unravel the implication of RhoA within myofibers, in the building of a permissive microenvironment for muscle hypertrophic growth and for SC accretion through ECM remodeling and inflammatory cell recruitment.

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RhoA within myofibers controls SC fusion and muscle hypertrophic growth

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RhoA controls the expression of Mmps and of macrophage chemoattractants (Ccl3/Cx3cl1)

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RhoA controls ECM remodeling and macrophage recruitment upon hypertrophy

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Mmp inhibition and macrophage depletion phenocopy the blunted growth of RhoA mutant muscles

OUP accepted manuscript

Numerous components of the immune system, including inflammatory mediators, immune cells and cytokines, have a profound modulatory effect on the homeostatic regulation and regenerative activity of endogenous stem cells and progenitor cells. Thus, understanding how the immune system interacts with stem/progenitor cells could build the foundation to design novel and more effective regenerative therapies. Indeed, utilizing and controlling immune system components may be one of the most effective approaches to promote tissue regeneration. In this review, we first summarize the effects of various immune cell types on endogenous stem/progenitor cells, focusing on the tissue healing context. Then, we present interesting regenerative strategies that control or mimic the effect of immune components on stem/progenitor cells, in order to enhance the regenerative capacity of endogenous and transplanted stem cells. We highlight the potential clinical translation of such approaches for multiple tissues and organ systems, as these novel regenerative strategies could considerably improve or eventually substitute stem cell-based therapies. Overall, harnessing the power of the cross-talk between the immune system and stem/progenitor cells holds great potential for the development of novel and effective regenerative therapies.

Cell-cell contact and signaling in the muscle stem cell niche

Muscle stem cells (also called satellite cells or SCs) rely on their local niche for regulatory signals during homeostasis and regeneration. While a number of cell types communicate indirectly through secreted factors, here we focus on the significance of direct contact between SCs and their neighbors. During quiescence, SCs reside under a basal lamina and receive quiescence-promoting signals from their adjacent skeletal myofibers. Upon injury, the composition of the niche changes substantially, enabling the formation of new contacts that mediate proliferation, self-renewal, and differentiation. In this review, we summarize the latest work in understanding cell-cell contact within the satellite cell niche and highlight areas of open questions for future studies.

Benefits and pathologies associated with the inflammatory response

Adult skeletal muscle regenerates completely after a damage, thanks to the satellite cells, or muscle stem cells (MuSCs), that implement the adult myogenic program. This program is sustained by both robust intrinsic mechanisms and extrinsic cues coming from the close neighborhood of MuSCs during muscle regeneration. Among the various cell types present in the regenerating muscle, immune cells, and particularly macrophages, exert numerous functions and provide sequential transient niches to support the myogenic program. The adequate orchestration of the delivery of these cues ensures efficient muscle regeneration and full functional recovery. The situation is very different in muscular dystrophies where asynchronous and permanent microinjuries occur, triggering contradictory regenerating cues at the same time in a specific area, that lead to chronic inflammation and fibrogenesis. Here we review the beneficial effects that leukocytes, and particularly macrophages, exert on their neighboring cells during skeletal muscle regeneration after an acute injury. Then, the more complicated (and less beneficial) roles of leukocytes during muscular dystrophies are presented. Finally, we discuss how the inflammatory compartment may be a target to improve muscle regeneration in both acute muscle injury and muscle diseases.

Current Thoughts of Notch's Role in Myoblast Regulation and Muscle-Associated Disease

The evolutionarily conserved signaling pathway Notch is unequivocally essential for embryogenesis. Notch’s contribution to the muscle repair process in adult tissue is complex and obscure but necessary. Notch integrates with other signals in a functional antagonist manner to direct myoblast activity and ultimately complete muscle repair. There is profound recent evidence describing plausible mechanisms of Notch in muscle repair. However, the story is not definitive as evidence is slowly emerging that negates Notch’s importance in myoblast proliferation. The purpose of this review article is to examine the prominent evidence and associated mechanisms of Notch’s contribution to the myogenic repair phases. In addition, we discuss the emerging roles of Notch in diseases associated with muscle atrophy. Understanding the mechanisms of Notch’s orchestration is useful for developing therapeutic targets for disease.

Cellular and molecular modulation of rotator cuff muscle pathophysiology

Rotator cuff (RC) tendon tears are common shoulder injuries that result in irreversible and persistent degeneration of the associated muscles, which is characterized by severe inflammation, atrophy, fibrosis, and fatty infiltration. Although RC muscle degeneration strongly dictates the overall clinical outcomes, strategies to stimulate RC muscle regeneration have largely been overlooked to date. In this review, we highlight the current understanding of the cellular processes that coordinate muscle regeneration, and the roles of muscle resident cells, including immune cells, fibroadipogenic progenitors, and muscle satellite cells in the pathophysiologic regulation of RC muscles following injury. This review also provides perspectives for potential therapies to alleviate the hallmarks of RC muscle degeneration to address current limitations in postsurgical recovery.

Tissue-specific parameters for the design of ECM-mimetic biomaterials

The extracellular matrix (ECM) is a complex network of biomolecules that mechanically and biochemically directs cell behavior and is crucial for maintaining tissue function and health. The heterogeneous organization and composition of the ECM varies within and between tissue types, directing mechanics, aiding in cell-cell communication, and facilitating tissue assembly and reassembly during development, injury and disease. As technologies like 3D printing rapidly advance, researchers are better able to recapitulate in vivo tissue properties in vitro; however, tissue-specific variations in ECM composition and organization are not given enough consideration. This is in part due to a lack of information regarding how the ECM of many tissues varies in both homeostatic and diseased states. To address this gap, we describe the components and organization of the ECM, and provide examples for different tissues at various states of disease. While many aspects of ECM biology remain unknown, our goal is to highlight the complexity of various tissues and inspire engineers to incorporate unique components of the native ECM into in vitro platform design and fabrication. Ultimately, we anticipate that the use of biomaterials that incorporate key tissue-specific ECM will lead to in vitro models that better emulate human pathologies. STATEMENT OF SIGNIFICANCE: Biomaterial development primarily emphasizes the engineering of new materials and therapies at the expense of identifying key parameters of the tissue that is being emulated. This can be partially attributed to the difficulty in defining the 3D composition, organization, and mechanics of the ECM within different tissues and how these material properties vary as a function of homeostasis and disease. In this review, we highlight a range of tissues throughout the body and describe how ECM content, cell diversity, and mechanical properties change in diseased tissues and influence cellular behavior. Accurately mimicking the tissue of interest in vitro by using ECM specific to the appropriate state of homeostasis or pathology in vivo will yield results more translatable to humans.

Fusion and beyond: Satellite cell contributions to loading-induced skeletal muscle adaptation

Satellite cells support adult skeletal muscle fiber adaptations to loading in numerous ways. The fusion of satellite cells, driven by cell-autonomous and/or extrinsic factors, contributes new myonuclei to muscle fibers, associates with load-induced hypertrophy, and may support focal membrane damage repair and long-term myonuclear transcriptional output. Recent studies have also revealed that satellite cells communicate within their niche to mediate muscle remodeling in response to resistance exercise, regulating the activity of numerous cell types through various mechanisms such as secretory signaling and cell-cell contact. Muscular adaptation to resistance and endurance activity can be initiated and sustained for a period of time in the absence of satellite cells, but satellite cell participation is ultimately required to achieve full adaptive potential, be it growth, function, or proprioceptive coordination. While significant progress has been made in understanding the roles of satellite cells in adult muscle over the last few decades, many conclusions have been extrapolated from regeneration studies. This review highlights our current understanding of satellite cell behavior and contributions to adaptation outside of regeneration in adult muscle, as well as the roles of satellite cells beyond fusion and myonuclear accretion, which are gaining broader recognition.

Disrupted macrophage metabolic reprogramming in aged soleus muscle during early recovery following disuse atrophy

Aged skeletal muscle is characterized by poor muscle recovery following disuse coinciding with an impaired muscle pro-inflammatory macrophage response. Macrophage inflammatory status is regulated by its metabolic state, but little is understood of macrophage metabolism and its relation to macrophage inflammation in the context of muscle recovery and aging. Therefore, the purpose of this study was to thoroughly characterize macrophage metabolism and inflammation in aged muscle during early recovery following disuse atrophy using single cell transcriptomics and functional assays. Young (4-5 months) and old (20-22 months) male C57BL/6 mice underwent 14 days of hindlimb unloading followed by 4 days of ambulatory recovery. CD45+ cells were isolated from solei muscles and analyzed using 10x Genomics single cell RNA sequencing. We found that aged pro-inflammatory macrophage clusters were characterized with an impaired inflammatory and glycolytic transcriptome, and this dysregulation was accompanied by a suppression of HIF-1α and its immediate downstream target, Glut1. As a follow-up, bone marrow-derived macrophages were isolated from a separate cohort of young and old mice at 4-d recovery and were polarized to a pro-inflammatory phenotype and used for glycolysis stress test, phagocytosis activity assay, and targeted GC-MS metabolomics. Aged bone marrow-derived pro-inflammatory macrophages were characterized with impaired glycolysis and phagocytosis function, decreased succinate and an accumulation of glycolytic metabolic intermediates overall supporting reduced glycolytic flux and macrophage function. Our results indicate that the metabolic reprograming and function of aged skeletal muscle pro-inflammatory macrophages are dysfunctional during early recovery from disuse atrophy possibly attributing to attenuated regrowth.

An anti-ADAMTS1 treatment relieved muscle dysfunction and fibrosis in dystrophic mice

Duchenne Muscular Dystrophy (DMD) is caused by mutations in the dystrophin gene, accompanied by aberrant extracellular matrix synthesis and muscle damage. ADAMTS1 metalloproteinase was reported increased in dystrophin-deficient mdx mouse. The aim of this study was to explore the role of ADAMTS1 in muscle function, fibrosis and damage, and respiratory function of mdx mice. 102 DMD patients and their mothers were included in this study. Multiplex ligation dependent probe amplification (MLPA) assay and Next-generation sequencing (NGS) were adopted to do genetic diagnosis. Dystrophin-deficient mdx mice were treated with anti-ADAMTS1 antibody (anti-ADAMTS1) for three weeks. The results showed that ADAMTS1 was increased in gastrocnemius muscle of mdx mice and serum of DMD patients. Anti-ADAMTS1 treatment increased Versican transcription but suppressed versican protein expression. Besides, we found anti-ADAMTS1 improved muscle strength, diaphragm and extensor digitorum longus muscles functions in mdx mice. Meanwhile, muscle fibrosis and damage were attenuated in anti-ADAMTS1 treated dystrophic mice. In summary, anti-ADAMTS1 antibody relieved muscle dysfunction and fibrosis in dystrophic mice. It is suggested that ADAMTS1 is a potential target for developing new biological therapies for DMD.

Inflammation during post-injury skeletal muscle regeneration

The adult skeletal muscle fully regenerates after injury thanks to the properties of muscle stem cells that follow the adult myogenic program to replace damaged myofibers. Muscle regeneration also relies upon the coordinated actions of several other cell types, among which immune cells. Leukocytes infiltrate the damaged muscle soon after injury and support the regeneration process in a variety of ways, from the activation of muscle stem cells to the maturation of newly formed myofibers. Leukocytes also interact with other cell types such as fibroadipogenic precursors and endothelial cells. This review presents the interactions that leukocytes develop with the cells present in their vicinity and the impact they have on skeletal muscle regeneration.

In vivo partial reprogramming of myofibers promotes muscle regeneration by remodeling the stem cell niche

Short-term, systemic expression of the Yamanaka reprogramming factors (Oct-3/4, Sox2, Klf4 and c-Myc [OSKM]) has been shown to rejuvenate aging cells and promote tissue regeneration in vivo. However, the mechanisms by which OSKM promotes tissue regeneration are unknown. In this work, we focus on a specific tissue and demonstrate that local expression of OSKM, specifically in myofibers, induces the activation of muscle stem cells or satellite cells (SCs), which accelerates muscle regeneration in young mice. In contrast, expressing OSKM directly in SCs does not improve muscle regeneration. Mechanistically, expressing OSKM in myofibers regulates the expression of genes important for the SC microenvironment, including upregulation of p21, which in turn downregulates Wnt4. This is critical because Wnt4 is secreted by myofibers to maintain SC quiescence. Thus, short-term induction of the Yamanaka factors in myofibers may promote tissue regeneration by modifying the stem cell niche.

Short term systemic expression of the reprogramming factors Oct-3/4, Sox2, Klf4, c-Myc (OSKM) rejuvenates aging cells and promotes tissue regeneration. Here the authors show that myofiber-specific expression of OSKM accelerates muscle regeneration by reducing secretion of muscle stem cell quiescence promoting Wnt4.

Myeloid cell diversification during regenerative inflammation: Lessons from skeletal muscle

Understanding the mechanisms of tissue and organ regeneration in adult animals and humans is of great interest from a basic biology as well as a medical, therapeutical point of view. It is increasingly clear that the relatively limited ability to regenerate tissues and organs in mammals as oppose to lower vertebrates is the consequence of evolutionary trade-offs and changes during development and aging. Thus, the coordinated interaction of the immune system, particularly the innate part of it, and the injured, degenerated parenchymal tissues such as skeletal muscle, liver, lung, or kidney shape physiological and also pathological processes. In this review, we provide an overview of how morphologically and functionally complete (ad integrum) regeneration is achieved using skeletal muscle as a model. We will review recent advances about the differentiation, activation, and subtype specification of circulating monocyte to resolution or repair-type macrophages during the process we term regenerative inflammation, resulting in complete restoration of skeletal muscle in murine models of toxin-induced injury.

Macrophages and Stem Cells-Two to Tango for Tissue Repair?

Macrophages (MCs) are present in all tissues, not only supporting homeostasis, but also playing an important role in organogenesis, post-injury regeneration, and diseases. They are a heterogeneous cell population due to their origin, tissue specificity, and polarization in response to aggression factors, depending on environmental cues. Thus, as pro-inflammatory M1 phagocytic MCs, they contribute to tissue damage and even fibrosis, but the anti-inflammatory M2 phenotype participates in repairing processes and wound healing through a molecular interplay with most cells in adult stem cell niches. In this review, we emphasize MC phenotypic heterogeneity in health and disease, highlighting their systemic and systematic contribution to tissue homeostasis and repair. Unraveling the intervention of both resident and migrated MCs on the behavior of stem cells and the regulation of the stem cell niche is crucial for opening new perspectives for novel therapeutic strategies in different diseases.

Targeting intramuscular adipose tissue expansion to preserve contractile function in volumetric muscle loss: A potentially novel therapy?

In volumetric muscle loss (VML), the severity of trauma exceeds a muscle's regenerative capacity. VML causes permanent functional impairments for which there are no rehabilitative, pharmacological, or regenerative medicine interventions. Driving failed regeneration in VML is a hostile microenvironment characterized by heightened inflammation, fibrosis, and denervation, which may reduce the remaining muscle tissue's quality, and stimulate intramuscular adipose tissue (IMAT) expansion. IMAT is increased in various muscle disease states, and has known lipotoxic effects on regeneration and contractile function. The contribution of ectopic fat deposition to the hostile VML microenvironment at the injury site and in the remaining tissue warrants further investigation. Targeting IMAT may lead to novel therapeutic strategies for improving functional outcomes in VML.