Transcriptional profiling of skeletal muscle reveals factors that are necessary to maintain satellite cell integrity during ageing

Accelerating aging with dynamic biomaterials: Recapitulating aged tissue phenotypes in engineered platforms

Aging is characterized by progressive decline in tissue function and represents the greatest risk factor for many diseases. Nevertheless, many fundamental mechanisms driving human aging remain poorly understood. Aging studies using model organisms are often limited in their applicability to humans. Mechanistic studies of human aging rely on relatively simple cell culture models that fail to replicate mature tissue function, making them poor surrogates for aged tissues. These culture systems generally lack well-controlled cellular microenvironments that capture the changes in tissue mechanics and microstructure that occur during aging. Biomaterial platforms presenting dynamic, physiologically relevant mechanical, structural, and biochemical cues can capture the complex changes in the cellular microenvironment in a well-defined manner, accelerating the process of cellular aging in model laboratory systems. By enabling selective tuning of relevant microenvironmental parameters, these biomaterials systems may enable identification of new therapeutic approaches to slow or reverse the detrimental effects of aging.

Molecular hallmarks of long non-coding RNAs in aging and its significant effect on aging-associated diseases

Aging is linked to the deterioration of many physical and cognitive abilities and is the leading risk factor for Alzheimer's disease. The growing aging population is a significant healthcare problem globally that researchers must investigate to better understand the underlying aging processes. Advances in microarrays and sequencing techniques have resulted in deeper analyses of diverse essential genomes (e.g., mouse, human, and rat) and their corresponding cell types, their organ-specific transcriptomes, and the tissue involved in aging. Traditional gene controllers such as DNA- and RNA-binding proteins significantly influence such programs, causing the need to sort out long non-coding RNAs, a new class of powerful gene regulatory elements. However, their functional significance in the aging process and senescence has yet to be investigated and identified. Several recent researchers have associated the initiation and development of senescence and aging in mammals with several well-reported and novel long non-coding RNAs. In this review article, we identified and analyzed the evolving functions of long non-coding RNAs in cellular processes, including cellular senescence, aging, and age-related pathogenesis, which are the major hallmarks of long non-coding RNAs in aging.

Secreted Protein Acidic and Rich in Cysteine (SPARC)-Mediated Exercise Effects: Illustrative Molecular Pathways against Various Diseases

The strong benefits of exercise, in addition to the development of both the therapeutic applications of physical activity and molecular biology tools, means that it has become very important to explore the underlying molecular patterns linking exercise and its induced phenotypic changes. Within this context, secreted protein acidic and rich in cysteine (SPARC) has been characterized as an exercise-induced protein that would mediate and induce some important effects of exercise. Herein, we suggest some underlying pathways to explain such SPARC-induced exercise-like effects. Such mechanistic mapping would not only allow us to understand the molecular processes of exercise and SPARC effects but would also highlight the potential to develop novel molecular therapies. These therapies would be based on mimicking the exercise benefits via either introducing SPARC or pharmacologically targeting the SPARC-related pathways to produce exercise-like effects. This is of a particular importance for those who do not have the ability to perform the required physical activity due to disabilities or diseases. The main objective of this work is to highlight selected potential therapeutic applications deriving from SPARC properties that have been reported in various publications.

Ageing of skeletal muscle extracellular matrix and mitochondria: finding a potential link

Aim: To discuss the progress of extracellular matrix (ECM) characteristics, mitochondrial homeostasis, and their potential crosstalk in the pathogenesis of sarcopenia, a geriatric syndrome characterized by a generalized and progressive reduction in muscle mass, strength, and physical performance.Methods: This review focuses on the anatomy and physiology of skeletal muscle, alterations of ECM and mitochondria during ageing, and the role of the interplay between ECM and mitochondria in the pathogenesis of sarcopenia.Results: Emerging evidence points to a clear interplay between mitochondria and ECM in various tissues and organs. Under the ageing process, the ECM undergoes changes in composition and physical properties that may mediate mitochondrial changes via the systematic metabolism, ROS, SPARC pathway, and AMPK/PGC-1α signalling, which in turn exacerbate muscle degeneration. However, the precise effects of such crosstalk on the pathobiology of ageing, particularly in skeletal muscle, have not yet been fully understood.Conclusion: The changes in skeletal muscle ECM and mitochondria are partially responsible for the worsened muscle function during the ageing process. A deeper understanding of their alterations and interactions in sarcopenic patients can help prevent sarcopenia and improve its prognoses.

NRF2 Regulates Viability, Proliferation, Resistance to Oxidative Stress, and Differentiation of Murine Myoblasts and Muscle Satellite Cells

Increased oxidative stress can slow down the regeneration of skeletal muscle and affect the activity of muscle satellite cells (mSCs). Therefore, we evaluated the role of the NRF2 transcription factor (encoded by the Nfe2l2 gene), the main regulator of the antioxidant response, in muscle cell biology. We used (i) an immortalized murine myoblast cell line (C2C12) with stable overexpression of NRF2 and (ii) primary mSCs isolated from wild-type and Nfe2l2 (transcriptionally)-deficient mice (Nfe2l2^tKO). NRF2 promoted myoblast proliferation and viability under oxidative stress conditions and decreased the production of reactive oxygen species. Furthermore, NRF2 overexpression inhibited C2C12 cell differentiation by down-regulating the expression of myogenic regulatory factors (MRFs) and muscle-specific microRNAs. We also showed that NRF2 is indispensable for the viability of mSCs since the lack of its transcriptional activity caused high mortality of cells cultured in vitro under normoxic conditions. Concomitantly, Nfe2l2^tKO mSCs grown and differentiated under hypoxic conditions were viable and much more differentiated compared to cells isolated from wild-type mice. Taken together, NRF2 significantly influences the properties of myoblasts and muscle satellite cells. This effect might be modulated by the muscle microenvironment.

Transforming L1000 profiles to RNA-seq-like profiles with deep learning

The L1000 technology, a cost-effective high-throughput transcriptomics technology, has been applied to profile a collection of human cell lines for their gene expression response to > 30,000 chemical and genetic perturbations. In total, there are currently over 3 million available L1000 profiles. Such a dataset is invaluable for the discovery of drug and target candidates and for inferring mechanisms of action for small molecules. The L1000 assay only measures the mRNA expression of 978 landmark genes while 11,350 additional genes are computationally reliably inferred. The lack of full genome coverage limits knowledge discovery for half of the human protein coding genes, and the potential for integration with other transcriptomics profiling data. Here we present a Deep Learning two-step model that transforms L1000 profiles to RNA-seq-like profiles. The input to the model are the measured 978 landmark genes while the output is a vector of 23,614 RNA-seq-like gene expression profiles. The model first transforms the landmark genes into RNA-seq-like 978 gene profiles using a modified CycleGAN model applied to unpaired data. The transformed 978 RNA-seq-like landmark genes are then extrapolated into the full genome space with a fully connected neural network model. The two-step model achieves 0.914 Pearson’s correlation coefficients and 1.167 root mean square errors when tested on a published paired L1000/RNA-seq dataset produced by the LINCS and GTEx programs. The processed RNA-seq-like profiles are made available for download, signature search, and gene centric reverse search with unique case studies.

Genome-Wide Selective Analysis of Boer Goat to Investigate the Dynamic Heredity Evolution under Different Stages

Boer goats, as kemp in meat-type goats, are selected and bred from African indigenous goats under a long period of artificial selection. Their advantages in multiple economic traits, particularly their plump growth, have attracted worldwide attention. The current study displayed the genome-wide selection signature analyses of South African indigenous goat (AF), African Boer (BH), and Australian Boer (AS) to investigate the hereditary basis of artificial selection in different stages. Four methods (principal component analysis, nucleotide diversity, linkage disequilibrium decay, and neighbor-joining tree) implied the genomic diversity changes with different artificial selection intensities in Boer goats. In addition, the θπ, FST, and XP-CLR methods were used to search for the candidate signatures of positive selection in Boer goats. Consequently, 339 (BH vs. AF) and 295 (AS vs. BH) candidate genes were obtained from SNP data. Especially, 10 genes (e.g., BMPR1B, DNER, ITGAL, and KIT) under selection in both groups were identified. Functional annotation analysis revealed that these genes are potentially responsible for reproduction, metabolism, growth, and development. This study used genome-wide sequencing data to identify inheritance by artificial selection. The results of the current study are valuable for future molecular-assisted breeding and genetic improvement of goats.

Effect of resistance training on satellite cells in old mice - a transcriptome study : implications for sarcopenia

Aims

The decrease in the number of satellite cells (SCs), contributing to myofibre formation and reconstitution, and their proliferative capacity, leads to muscle loss, a condition known as sarcopenia. Resistance training can prevent muscle loss; however, the underlying mechanisms of resistance training effects on SCs are not well understood. We therefore conducted a comprehensive transcriptome analysis of SCs in a mouse model.

Methods

We compared the differentially expressed genes of SCs in young mice (eight weeks old), middle-aged (48-week-old) mice with resistance training intervention (MID+ T), and mice without exercise (MID) using next-generation sequencing and bioinformatics.

Results

After the bioinformatic analysis, the PI3K-Akt signalling pathway and the regulation of actin cytoskeleton in particular were highlighted among the top ten pathways with the most differentially expressed genes involved in the young/MID and MID+ T/MID groups. The expression of Gng5, Atf2, and Rtor in the PI3K-Akt signalling pathway was higher in the young and MID+ T groups compared with the MID group. Similarly, Limk1, Arhgef12, and Araf in the regulation of the actin cytoskeleton pathway had a similar bias. Moreover, the protein expression profiles of Atf2, Rptor, and Ccnd3 in each group were paralleled with the results of NGS.

Conclusion

Our results revealed that age-induced muscle loss might result from age-influenced genes that contribute to muscle development in SCs. After resistance training, age-impaired genes were reactivated, and age-induced genes were depressed. The change fold in these genes in the young/MID mice resembled those in the MID + T/MID group, suggesting that resistance training can rejuvenate the self-renewing ability of SCs by recovering age-influenced genes to prevent sarcopenia.

Cite this article: Bone Joint Res 2022;11(2):121–133.

Pim1 knockout alleviates sarcopenia in aging mice via reducing adipogenic differentiation of PDGFRα+ mesenchymal progenitors

BACKGROUND

Sarcopenia widely exists in elderly people and triggers numerous age-related events. The essential pathologic change lies in the increased intramuscular adipose tissue after aging with no exception to non-obese objects. Pim1 appears to be associated with adipogenic differentiation in recent studies, inspiring us to explore whether it regulates adipogenesis in aging muscles and affects sarcopenia.

METHODS

Wild-type and Pim1 knockout C57/BL6J mice were randomized into young and old groups. Histo-pathological and molecular biological methods were applied to assess the intramuscular adipose tissue content, the atrophy and regeneration, and the expressions of Pim1 and adipogenic transcription factors. PDGFRα+ mesenchymal progenitors were separated and their replicative aging model were established. Different time of adipogenic induction and different amounts of Pim1 inhibitor were applied, after which the adipogenic potency were evaluated. The expressions of Pim1 and adipogenic transcription factors were measured through western blotting.

RESULTS

The aging mice demonstrated decreased forelimb grip strength (P = 0.0003), hanging impulse (P < 0.0001), exhaustive running time (P < 0.0001), tetanic force (P = 0.0298), lean mass (P = 0.0008), and percentage of gastrocnemius weight in body weight (P < 0.0001), which were improved by Pim1 knockout (P = 0.0015, P = 0.0222, P < 0.0001, P = 0.0444, P = 0.0004, and P = 0.0003, respectively). To elucidate the mechanisms, analyses showed that Pim1 knockout decreased the fat mass (P = 0.0005) and reduced the intramuscular adipose tissue content (P = 0.0008) by inhibiting the C/EBPδ pathway (P = 0.0067) in aging mice, resulting in increased cross-sectional area of all and fast muscle fibres (P = 0.0017 and 0.0024 respectively), decreased levels of MuRF 1 and atrogin 1 (P = 0.0001 and 0.0329 respectively), and decreased content of Pax7 at the basal state (P = 0.0055). In vitro, senescent PDGFRα+ mesenchymal progenitors showed significantly increased the intracellular adipose tissue content (OD510) compared with young cells after 6 days of adipogenic induction (P < 0.0001). The Pim1 expression was elevated during adipogenic differentiation, and Pim1 inhibition significantly reduced the OD510 in senescent cells (P = 0.0040) by inhibiting the C/EBPδ pathway (P = 0.0047).

CONCLUSIONS

Pim1 knockout exerted protective effects in sarcopenia by inhibiting the adipogenic differentiation of PDGFRα+ mesenchymal progenitors induced by C/EBPδ activation and thus reducing the intramuscular adipose tissue content in aging mice. These results provide a potential target for the treatment of sarcopenia.

Extracellular matrix: an important regulator of cell functions and skeletal muscle development

Extracellular matrix (ECM) is a kind of connective tissue in the cell microenvironment, which is of great significance to tissue development. ECM in muscle fiber niche consists of three layers: the epimysium, the perimysium, and the endomysium (basal lamina). These three layers of connective tissue structure can not only maintain the morphology of skeletal muscle, but also play an important role in the physiological functions of muscle cells, such as the transmission of mechanical force, the regeneration of muscle fiber, and the formation of neuromuscular junction. In this paper, detailed discussions are made for the structure and key components of ECM in skeletal muscle tissue, the role of ECM in skeletal muscle development, and the application of ECM in biomedical engineering. This review will provide the reader with a comprehensive overview of ECM, as well as a comprehensive understanding of the structure, physiological function, and application of ECM in skeletal muscle tissue.

Roles of the synaptic molecules Hevin and SPARC in mouse neuromuscular junction development and repair

Hevin and secreted protein acidic and rich in cysteine (SPARC) are highly homologous matricellular proteins that function in concert to guide the formation of brain synapses. Here, we investigated the role of these glycoproteins in neuromuscular junction (NMJ) maturation, stability, and repair following injury. Hevin and SPARC mRNA levels in developing (postnatal day 9), adult (postnatal days 90 and 120), and injured (fibular nerve crush) skeletal muscles were assessed with qPCR. Muscle fiber size was analyzed in developing (P9) mice lacking SPARC, Hevin, and both SPARC and Hevin. NMJ morphology was assessed in developing (P9), adult (P90) and injured (fibular nerve crush) mice lacking SPARC, Hevin, and both SPARC and Hevin skeletal muscle. Hevin and SPARC are expressed in skeletal muscles and are upregulated following nerve injury. Hevin^-/- mice exhibited delayed NMJ and muscle fiber development but displayed normal NMJ morphology in adulthood and accelerated NMJ reinnervation following nerve injury. Mice lacking SPARC displayed normal NMJ and muscle fiber development but exhibited smaller NMJs with fewer acetylcholine receptor islands in adulthood. Further, SPARC deletion did not result in overt changes in NMJ reformation following nerve injury. The combined deletion of Hevin and SPARC had little effect on NMJ phenotypes observed in single knockouts, however deletion of SPARC in combination with Hevin reversed deficiencies in muscle fiber maturation observed in Hevin^-/- muscle. These results identify SPARC and Hevin as extracellular matrix proteins with roles in NMJ development and repair.

Skewed macrophage polarization in aging skeletal muscle

Skeletal muscle aging is a major cause of disability and frailty in the elderly. The progressive impairment of skeletal muscle function with aging was recently linked to a disequilibrium between damage and repair. Macrophages participate in muscle tissue repair, first as pro-inflammatory M1 subtype and then as anti-inflammatory M2 subtype. However, information on the presence of macrophages in skeletal muscle is still sporadic and the effect of aging on macrophage phenotype remains unknown. In this study, we sought to characterize the polarization status of macrophages in skeletal muscle of persons across a wide range of ages. We found that most macrophages in human skeletal muscle are M2, and that this number increased with advancing age. On the contrary, M1 macrophages declined with aging, making the total number of macrophages invariant with older age. Notably, M2 macrophages colocalized with increasing intermuscular adipose tissue (IMAT) in aging skeletal muscle. Similarly, aged BALB/c mice showed increased IMAT and M2 macrophages in skeletal muscle, accompanied by slightly increased collagen protein production. Collectively, we report that polarization of macrophages to the major M2 subtype is associated with IMAT and propose that increased M2 in aged skeletal muscle may impact upon muscle metabolism associated with aging.

Secreted protein acidic and rich in cysteine and bioenergetics: Extracellular matrix, adipocytes remodeling and skeletal muscle metabolism

The extracellular matrix (ECM) remodeling plays important roles in both adipocytes shape/expansion remodeling and the skeletal muscle (SM) metabolism. Secreted protein acidic and rich in cysteine (SPARC) is expressed in divers tissues including adipose tissue (AT) and SM where it impacts a variety of remodeling as well as metabolic functions. SPARC, also known as osteonectin or BM-40, is a glycoprotein associated with the ECM. Numerous researches attempted to elucidate the implications of SPARC in these two key metabolic tissues under different conditions. Whereas SPARC deficiency tends to shape the remodeling of the adipocytes and the fat distribution, this deficiency decreases SM metabolic properties. On the other hand, SPARC seems to be an enhancer of the metabolism and a mediator of the exercise-induced adaptation in the SM and as well as an adipogenesis inhibitor. Some findings about the SPARC effects on AT and SM seem "contradictory" in terms of tissue development and energy profile therefore highlighting the mechanistic role of SPARC in both is a priority. Yet, within this review, we expose selected researches and compare the results. We conclude with explanations to "reconcile" the different observations, hypothesize the feedback and regulatory character of SPARC and put its roles within the energetic and structural maps of both adipocytes and myocytes in homeostasis and in situations such as obesity or exercise. These properties explain the modifications and the remodeling seen in AT and SM undergoing adaptive changes (obesity, exercise, etc.) and represent a starting point for precise therapeutic targeting of SPARC-related pathways is conditions such as obesity, sarcopenia and diabetes.

Muscle injury-induced hypoxia alters the proliferation and differentiation potentials of muscle resident stromal cells

Background

Trauma-induced heterotopic ossification (HO) is a complication that develops under three conditions: the presence of an osteogenic progenitor cell, an inducing factor, and a permissive environment. We previously showed that a mouse multipotent Sca1⁺ CD31⁻ Lin⁻ muscle resident stromal cell (mrSC) population is involved in the development of HO in the presence of inducing factors, members of the bone morphogenetic protein family. Interestingly, BMP9 unlike BMP2 causes HO only if the muscle is damaged by injection of cardiotoxin. Because acute trauma often results in blood vessel breakdown, we hypothesized that a hypoxic state in damaged muscles may foster mrSCs activation and proliferation and trigger differentiation toward an osteogenic lineage, thus promoting the development of HO.

Methods

Three- to - six-month-old male C57Bl/6 mice were used to induce muscle damage by injection of cardiotoxin intramuscularly into the tibialis anterior and gastrocnemius muscles. mrSCs were isolated from damaged (hypoxic state) and contralateral healthy muscles and counted, and their osteoblastic differentiation with or without BMP2 and BMP9 was determined by alkaline phosphatase activity measurement. The proliferation and differentiation of mrSCs isolated from healthy muscles was also studied in normoxic incubator and hypoxic conditions. The effect of hypoxia on BMP synthesis and Smad pathway activation was determined by qPCR and/or Western blot analyses. Differences between normally distributed groups were compared using a Student’s paired t test or an unpaired t test.

Results

The hypoxic state of a severely damaged muscle increased the proliferation and osteogenic differentiation of mrSCs. mrSCs isolated from damaged muscles also displayed greater sensitivity to osteogenic signals, especially BMP9, than did mrSCs from a healthy muscle. In hypoxic conditions, mrSCs isolated from a control muscle were more proliferative and were more prone to osteogenic differentiation. Interestingly, Smad1/5/8 activation was detected in hypoxic conditions and was still present after 5 days, while Smad1/5/8 phosphorylation could not be detected after 3 h of normoxic incubator condition. BMP9 mRNA transcripts and protein levels were higher in mrSCs cultured in hypoxic conditions. Our results suggest that low-oxygen levels in damaged muscle influence mrSC behavior by facilitating their differentiation into osteoblasts. This effect may be mediated partly through the activation of the Smad pathway and the expression of osteoinductive growth factors such as BMP9 by mrSCs.

Conclusion

Hypoxia should be considered a key factor in the microenvironment of damaged muscle that triggers HO.

Electronic supplementary material

The online version of this article (10.1186/s13395-019-0202-5) contains supplementary material, which is available to authorized users.

Overexpression of PGC-1α in aging muscle enhances a subset of young-like molecular patterns

PGC-1α is a transcriptional co-activator known as the master regulator of mitochondrial biogenesis. Its control of metabolism has been suggested to exert critical influence in the aging process. We have aged mice overexpressing PGC-1α in skeletal muscle to determine whether the transcriptional changes reflected a pattern of expression observed in younger muscle. Analyses of muscle proteins showed that Pax7 and several autophagy markers were increased. In general, the steady-state levels of several muscle proteins resembled that of muscle from young mice. Age-related mtDNA deletion levels were not increased by the PGC-1α-associated increase in mitochondrial biogenesis. Accordingly, age-related changes in the neuromuscular junction were minimized by PGC-1α overexpression. RNA-Seq showed that several genes overexpressed in the aged PGC-1α transgenic are expressed at higher levels in young when compared to aged skeletal muscle. As expected, there was increased expression of genes associated with energy metabolism but also of pathways associated with muscle integrity and regeneration. We also found that PGC-1α overexpression had a mild but significant effect on longevity. Taken together, overexpression of PGC-1α in aged muscle led to molecular changes that resemble the patterns observed in skeletal muscle from younger mice.

Fibromodulin and regulation of the intricate balance between myoblast differentiation to myocytes or adipocyte-like cells

Interactions between myoblasts and the surrounding microenvironment led us to explore the role of fibromodulin (FMOD), an extracellular matrix protein, in the maintenance of myoblast stemness and function. Microarray analysis of FMOD_kd myoblasts and in silico studies were used to identify the top most differentially expressed genes in FMOD_kd, and helped establish that FMOD-based regulations of integral membrane protein 2a and clusterin are essential components of the myogenic program. Studies in knockout, obese, and diabetic mouse models helped characterize the operation of a novel FMOD-based regulatory circuit that controls myoblast switching from a myogenic to a lipid accumulation fate. FMOD regulation of myoblasts is an essential part of the myogenic program, and it offers opportunities for the development of therapeutics for the treatment of different muscle diseases.-Lee, E. J., Jan, A. T., Baig, M. H., Ahmad, K., Malik, A., Rabbani, G., Kim, T., Lee, I.-K., Lee, Y. H., Park, S.-Y., Choi, I. Fibromodulin and regulation of the intricate balance between myoblast differentiation to myocytes or adipocyte-like cells.

Intrinsic muscle clock is necessary for musculoskeletal health

KEY POINTS

The endogenous molecular clock in skeletal muscle is necessary for maintenance of phenotype and function. Loss of Bmal1 solely from adult skeletal muscle (iMSBmal1(-/-) ) results in reductions in specific tension, increased oxidative fibre type and increased muscle fibrosis with no change in feeding or activity. Disruption of the molecular clock in adult skeletal muscle is sufficient to induce changes in skeletal muscle similar to those seen in the Bmal1 knockout mouse (Bmal1(-/-) ), a model of advanced ageing. iMSBmal1(-/-) mice develop increased bone calcification and decreased joint collagen, which in combination with the functional changes in skeletal muscle results in altered gait. This study uncovers a fundamental role for the skeletal muscle clock in musculoskeletal homeostasis with potential implications for ageing.

ABSTRACT

Disruption of circadian rhythms in humans and rodents has implicated a fundamental role for circadian rhythms in ageing and the development of many chronic diseases including diabetes, cardiovascular disease, depression and cancer. The molecular clock mechanism underlies circadian rhythms and is defined by a transcription-translation feedback loop with Bmal1 encoding a core molecular clock transcription factor. Germline Bmal1 knockout (Bmal1 KO) mice have a shortened lifespan, show features of advanced ageing and exhibit significant weakness with decreased maximum specific tension at the whole muscle and single fibre levels. We tested the role of the molecular clock in adult skeletal muscle by generating mice that allow for the inducible skeletal muscle-specific deletion of Bmal1 (iMSBmal1). Here we show that disruption of the molecular clock, specifically in adult skeletal muscle, is associated with a muscle phenotype including reductions in specific tension, increased oxidative fibre type, and increased muscle fibrosis similar to that seen in the Bmal1 KO mouse. Remarkably, the phenotype observed in the iMSBmal1(-/-) mice was not limited to changes in muscle. Similar to the germline Bmal1 KO mice, we observed significant bone and cartilage changes throughout the body suggesting a role for the skeletal muscle molecular clock in both the skeletal muscle niche and the systemic milieu. This emerging area of circadian rhythms and the molecular clock in skeletal muscle holds the potential to provide significant insight into intrinsic mechanisms of the maintenance of muscle quality and function as well as identifying a novel crosstalk between skeletal muscle, cartilage and bone.

The central role of muscle stem cells in regenerative failure with aging

Skeletal muscle mass, function, and repair capacity all progressively decline with aging, restricting mobility, voluntary function, and quality of life. Skeletal muscle repair is facilitated by a population of dedicated muscle stem cells (MuSCs), also known as satellite cells, that reside in anatomically defined niches within muscle tissues. In adult tissues, MuSCs are retained in a quiescent state until they are primed to regenerate damaged muscle through cycles of self-renewal divisions. With aging, muscle tissue homeostasis is progressively disrupted and the ability of MuSCs to repair injured muscle markedly declines. Until recently, this decline has been largely attributed to extrinsic age-related alterations in the microenvironment to which MuSCs are exposed. However, as highlighted in this Perspective, recent reports show that MuSCs also progressively undergo cell-intrinsic alterations that profoundly affect stem cell regenerative function with aging. A more comprehensive understanding of the interplay of stem cell-intrinsic and extrinsic factors will set the stage for improving cell therapies capable of restoring tissue homeostasis and enhancing muscle repair in the aged.

Increased Stiffness in Aged Skeletal Muscle Impairs Muscle Progenitor Cell Proliferative Activity

BACKGROUND

Skeletal muscle aging is associated with a decreased regenerative potential due to the loss of function of endogenous stem cells or myogenic progenitor cells (MPCs). Aged skeletal muscle is characterized by the deposition of extracellular matrix (ECM), which in turn influences the biomechanical properties of myofibers by increasing their stiffness. Since the stiffness of the MPC microenvironment directly impacts MPC function, we hypothesized that the increase in muscle stiffness that occurs with aging impairs the behavior of MPCs, ultimately leading to a decrease in regenerative potential.

RESULTS

We showed that freshly isolated individual myofibers from aged mouse muscles contain fewer MPCs overall than myofibers from adult muscles, with fewer quiescent MPCs and more proliferative and differentiating MPCs. We observed alterations in cultured MPC behavior in aged animals, where the proliferation and differentiation of MPCs were lower and higher, respectively. These alterations were not linked to the intrinsic properties of aged myofibers, as shown by the similar values for the cumulative population-doubling values and fusion indexes. However, atomic force microscopy (AFM) indentation experiments revealed a nearly 4-fold increase in the stiffness of the MPC microenvironment. We further showed that the increase in stiffness is associated with alterations to muscle ECM, including the accumulation of collagen, which was correlated with higher hydroxyproline and advanced glycation end-product content. Lastly, we recapitulated the impaired MPC behavior observed in aging using a hydrogel substrate that mimics the stiffness of myofibers.

CONCLUSIONS

These findings provide novel evidence that the low regenerative potential of aged skeletal muscle is independent of intrinsic MPC properties but is related to the increase in the stiffness of the MPC microenvironment.

Increased microenvironment stiffness in damaged myofibers promotes myogenic progenitor cell proliferation

BACKGROUND

The stiffness of the myogenic stem cell microenvironment markedly influences the ability to regenerate tissue. We studied the effect of damaged myofibers on myogenic progenitor cell (MPC) proliferation and determined whether the structural integrity of the microenvironment contributes to phenotypic changes.

METHODS

Individual myofibers were isolated and cultured for 6 days. During this period, the cytoskeleton of myofibers and transcription factors regulating MPC differentiation were characterized by immunostaining. Atomic Force Microscopy (AFM) was performed to measure stiffness of cultured myofibers. Healthy and damaged myofibers, and their associated MPCs, were studied in skeletal muscle from dystrophic and tenotomy mouse models. MPCs were cultured on stiffness-tunable substrates, and their phenotypes were assessed by immunostaining of myogenic transcription factors.

RESULTS

We showed that individual myofibers tend to shrink or collapse when cultured ex vivo starting from day 1 and that this is associated with a marked increase in the number of proliferative MPCs (Pax7(+)MyoD(+)). The myofibers collapsed due to a loss of viability as shown by Evans blue dye uptake and the disorganization of their cytoskeletons. Interestingly, collapsed myofibers in mdx skeletal muscles were similar to damaged myofibers in that they lose their viability, have a disorganized cytoskeleton (actin and α-actinin), and display local MPC (MyoD(+)) proliferation at their periphery. In a tenotomy model that causes loss of muscle tension, the cytoskeletal disorganization of myofibers also correlated with the activation/proliferation of MPCs. A deeper analysis of collapsed myofibers revealed that they produce trophic factors that influence MPC proliferation. In addition, collapsed myofibers expressed several genes related to the basal lamina. Immunostaining revealed the presence of fibronectin in the basal lamina and the cytoplasm of damaged myofibers. Lastly, using atomic force microscopy (AFM), we showed that collapsed myofibers exhibit greater stiffness than intact myofibers. Growing MPCs on a 2-kPa polyacrylamide-based substrate, exempt of additional microenvironmental cues, recapitulated proliferation and reduced spontaneous differentiation compared to growth on a 0.5-kPa substrate.

CONCLUSIONS

Our results support the notion that collapsed or damaged myofibers increase the structural stiffness of the satellite cell microenvironment, which in addition to other cues such as trophic factors and changes in extracellular matrix composition, promotes the proliferation and maintenance of MPCs, required for myofiber repair.

Wnt protein-mediated satellite cell conversion in adult and aged mice following voluntary wheel running

Muscle represents an abundant, accessible, and replenishable source of adult stem cells. Skeletal muscle-derived stem cells, called satellite cells, play essential roles in regeneration after muscle injury in adult skeletal muscle. Although the molecular mechanism of muscle regeneration process after an injury has been extensively investigated, the regulation of satellite cells under steady state during the adult stage, including the reaction to exercise stimuli, is relatively unknown. Here, we show that voluntary wheel running exercise, which is a low stress exercise, converts satellite cells to the activated state due to accelerated Wnt signaling. Our analysis showed that up-regulated canonical Wnt/β-catenin signaling directly modulated chromatin structures of both MyoD and Myf5 genes, resulting in increases in the mRNA expression of Myf5 and MyoD and the number of proliferative Pax7(+)Myf5(+) and Pax7(+) MyoD(+) cells in skeletal muscle. The effect of Wnt signaling on the activation of satellite cells, rather than Wnt-mediated fibrosis, was observed in both adult and aged mice. The association of β-catenin, T-cell factor, and lymphoid enhancer transcription factors of multiple T-cell factor/lymphoid enhancer factor regulatory elements, conserved in mouse, rat, and human species, with the promoters of both the Myf5 and MyoD genes drives the de novo myogenesis in satellite cells even in aged muscle. These results indicate that exercise-stimulated extracellular Wnts play a critical role in the regulation of satellite cells in adult and aged skeletal muscle.

Early gene expression changes in skeletal muscle from SOD1(G93A) amyotrophic lateral sclerosis animal model

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by loss of motor neurons. Familial ALS is strongly associated to dominant mutations in the gene for Cu/Zn superoxide dismutase (SOD1). Recent evidences point to skeletal muscle as a primary target in the ALS mouse model. Wnt/PI3 K signaling pathways and epithelial-mesenchymal transition (EMT) have important roles in maintenance and repair of skeletal muscle. Wnt/PI3 K pathways and EMT gene expression profile were investigated in gastrocnemius muscle from SOD1(G93A) mouse model and age-paired wild-type control in the presymptomatic ages of 40 and 80 days aiming the early neuromuscular abnormalities that precede motor neuron death in ALS. A customized cDNA microarray platform containing 326 genes of Wnt/PI3 K and EMT was used and results revealed eight up-regulated (Loxl2, Pik4ca, Fzd9, Cul1, Ctnnd1, Snf1lk, Prkx, Dner) and nine down-regulated (Pik3c2a, Ripk4, Id2, C1qdc1, Eif2ak2, Rac3, Cds1, Inppl1, Tbl1x) genes at 40 days, and also one up-regulated (Pik3ca) and five down-regulated (Cd44, Eef2 k, Fzd2, Crebbp, Piki3r1) genes at 80 days. Also, protein-protein interaction networks grown from the differentially expressed genes of 40 and 80 days old mice have identified Grb2 and Src genes in both presymptomatic ages, thus playing a potential central role in the disease mechanisms. mRNA and protein levels for Grb2 and Src were found to be increased in 80 days old ALS mice. Gene expression changes in the skeletal muscle of transgenic ALS mice at presymptomatic periods of disease gave further evidence of early neuromuscular abnormalities that precede motor neuron death. The results were discussed in terms of initial triggering for neuronal degeneration and muscle adaptation to keep function before the onset of symptoms.

Cytoglobin modulates myogenic progenitor cell viability and muscle regeneration

Mammalian skeletal muscle can remodel, repair, and regenerate itself by mobilizing satellite cells, a resident population of myogenic progenitor cells. Muscle injury and subsequent activation of myogenic progenitor cells is associated with oxidative stress. Cytoglobin is a hemoprotein expressed in response to oxidative stress in a variety of tissues, including striated muscle. In this study, we demonstrate that cytoglobin is up-regulated in activated myogenic progenitor cells, where it localizes to the nucleus and contributes to cell viability. siRNA-mediated depletion of cytoglobin from C2C12 myoblasts increased levels of reactive oxygen species and apoptotic cell death both at baseline and in response to stress stimuli. Conversely, overexpression of cytoglobin reduced reactive oxygen species levels, caspase activity, and cell death. Mice in which cytoglobin was knocked out specifically in skeletal muscle were generated to examine the role of cytoglobin in vivo. Myogenic progenitor cells isolated from these mice were severely deficient in their ability to form myotubes as compared with myogenic progenitor cells from wild-type littermates. Consistent with this finding, the capacity for muscle regeneration was severely impaired in mice deficient for skeletal-muscle cytoglobin. Collectively, these data demonstrate that cytoglobin serves an important role in muscle repair and regeneration.

Integrated analysis of global mRNA and protein expression data in HEK293 cells overexpressing PRL-1

BACKGROUND

The protein tyrosine phosphatase PRL-1 represents a putative oncogene with wide-ranging cellular effects. Overexpression of PRL-1 can promote cell proliferation, survival, migration, invasion, and metastasis, but the underlying mechanisms by which it influences these processes remain poorly understood.

METHODOLOGY

To increase our comprehension of PRL-1 mediated signaling events, we employed transcriptional profiling (DNA microarray) and proteomics (mass spectrometry) to perform a thorough characterization of the global molecular changes in gene expression that occur in response to stable PRL-1 overexpression in a relevant model system (HEK293).

PRINCIPAL FINDINGS

Overexpression of PRL-1 led to several significant changes in the mRNA and protein expression profiles of HEK293 cells. The differentially expressed gene set was highly enriched in genes involved in cytoskeletal remodeling, integrin-mediated cell-matrix adhesion, and RNA recognition and splicing. In particular, members of the Rho signaling pathway and molecules that converge on this pathway were heavily influenced by PRL-1 overexpression, supporting observations from previous studies that link PRL-1 to the Rho GTPase signaling network. In addition, several genes not previously associated with PRL-1 were found to be significantly altered by its expression. Most notable among these were Filamin A, RhoGDIα, SPARC, hnRNPH2, and PRDX2.

CONCLUSIONS AND SIGNIFICANCE

This systems-level approach sheds new light on the molecular networks underlying PRL-1 action and presents several novel directions for future, hypothesis-based studies.

Loss of SPARC in mouse skeletal muscle causes myofiber atrophy

INTRODUCTION

The expression of secreted protein acidic and rich in cysteine (SPARC) in skeletal muscle decreases with age. Here, we examined the role of SPARC in skeletal muscle by reducing its expression.

METHODS

SPARC expression was suppressed by introducing short interfering RNA (siRNA) into mouse tibialis anterior muscle. Myofiber diameter, atrogin1, and muscle RING-finger protein 1 (MuRF1) expression, and tumor necrosis factor-α (TNFα) and transforming growth factor-β (TGFβ) signaling were then analyzed.

RESULTS

Reduced SPARC expression caused decreases in the diameter of myofibers, especially fast-type ones, accompanied by upregulation of atrogin1, but not MuRF1, at 10 days after siRNA transfection. The expression of TNFα and TGFβ and the phosphorylation status of p38 were not affected by SPARC knockdown, whereas Smad3 phosphorylation was increased at 2 days after siRNA transfection.

CONCLUSIONS

The loss of SPARC not only upregulates atrogin1 expression but also enhances TGFβ signaling, which may in turn cause muscle atrophy.

Heterochronic parabiosis for the study of the effects of aging on stem cells and their niches

Aging is unmistakable and undeniable in mammals. Interestingly, mice develop cataracts, muscle atrophy, osteoporosis, obesity, diabetes and cognitive deficits after just 2-3 postnatal years, while it takes seven or more decades for the same age-specific phenotypes to develop in humans. Thus, chronological age corresponds differently with biological age in metazoan species and although many theories exist, we do not understand what controls the rate of mammalian aging. One interesting idea is that species-specific rate of aging represents a ratio of tissue attrition to tissue regeneration. Furthermore, current findings suggest that the age-imposed biochemical changes in the niches of tissue stem cells inhibit performance of this regenerative pool, which leads to the decline of tissue maintenance and repair. If true, slowing down stem cell and niche aging, thereby promoting tissue regeneration, could slow down the process of tissue and organismal aging. In this regard, recent studies of heterochronic parabiosis provide important clues as to the mechanisms of stem cell aging and suggest novel strategies for enhancing tissue repair in the old. Here we review current literature on the relationship between the vigor of tissue stem cells and the process of aging, with an emphasis on the rejuvenation of old tissues by the extrinsic modifications of stem cell niches.

Age-related resistance of skeletal muscle-derived progenitor cells to SPARC may explain a shift from myogenesis to adipogenesis

Aging causes phenotypic changes in skeletal muscle progenitor cells (SMPCs) that lead to the loss of myogenicity and adipogenesis. Secreted protein acidic and rich in cysteine (SPARC), which is secreted from SMPCs, stimulates myogenesis and inhibits adipogenesis. The present study aimed to examine whether changes in SPARC expression, its signaling pathway, or both are involved in age-related phenotypic changes in SMPCs. SPARC expression levels were comparable in SMPCs derived from young and old rats. However, when SPARC expression was reduced by a SPARC-specific siRNA, SMPCs from young rats showed reduced myogenesis and increased adipogenesis. In striking contrast, old rats showed little changes in these functions. Recombinant SPARC was effective in inhibiting adipogenesis and promoting myogenesis of SMPCs from young rats but had no effect on SMPCs from old rats when endogenous SPARC levels were reduced by the SPARC-siRNA. Further, the level of integrin α5, a subunit of the putative SPARC receptor, was decreased in SMPCs from old rats, and its inhibition in SMPCs from young rats by siRNA reduced adipogenesis in response to SPARC. These results suggest that, although SPARC plays a role in regulating SMPC function, SMPCs become refractory to the action of SPARC with age. Our data may explain an age-related shift from myogenesis to adipogenesis, associated with sarcopenia.

The effect of physiological stimuli on sarcopenia; impact of Notch and Wnt signaling on impaired aged skeletal muscle repair

The age-related loss of skeletal muscle mass and function that is associated with sarcopenia can result in ultimate consequences such as decreased quality of life. The causes of sarcopenia are multifactorial and include environmental and biological factors. The purpose of this review is to synthesize what the literature reveals in regards to the cellular regulation of sarcopenia, including impaired muscle regenerative capacity in the aged, and to discuss if physiological stimuli have the potential to slow the loss of myogenic potential that is associated with sarcopenia. In addition, this review article will discuss the effect of aging on Notch and Wnt signaling, and whether physiological stimuli have the ability to restore Notch and Wnt signaling resulting in rejuvenated aged muscle repair. The intention of this summary is to bring awareness to the benefits of consistent physiological stimulus (exercise) to combating sarcopenia as well as proclaiming the usefulness of contraction-induced injury models to studying the effects of local and systemic influences on aged myogenic capability.

Transcriptional changes of secreted Wnt antagonists in hindlimb skeletal muscle during the lifetime of the C57BL/6J mouse

The canonical Wnt pathway plays a critical role in myogenesis and age-related inefficient muscle regeneration. To gain insights into changes in Wnt signaling in muscle during the lifetime of a mouse, mRNA levels of secreted Wnt antagonists were investigated. Among 13 analyzed antagonists, seven genes were found to be down-regulated in skeletal muscles of adult and old mice. Epigenetic modifications at the promoter regions of these seven Wnt antagonists were then examined to understand how these correlate with this transcriptional repression. DNA methylation was stably maintained, while chromatin modifications changed to transcriptionally inactive states over the course of a lifetime. Similar patterns of changes in chromatin modifications were observed at the promoters of all of the studied genes. The observations indicated that an upstream factor might regulate the chromatin states and the transcriptional repression of Wnt antagonists. Several bioinformatic analyses revealed that a FOXD3 binding motif is present within promoter regions of the seven antagonists. Furthermore, age-dependent differential FOXD3 binding is observed at the motifs of the seven gene promoters. Our results suggest that FOXD3 as a potential epigenetic regulator may mediate the transcriptional repression of the seven antagonists, possibly through regulation of histone modifications.

BMP-9-induced muscle heterotopic ossification requires changes to the skeletal muscle microenvironment

Heterotopic ossification (HO) is defined as the formation of bone inside soft tissue. Symptoms include joint stiffness, swelling, and pain. Apart from the inherited form, the common traumatic form generally occurs at sites of injury in damaged muscles and is often associated with brain injury. We investigated bone morphogenetic protein 9 (BMP-9), which possesses a strong osteoinductive capacity, for its involvement in muscle HO physiopathology. We found that BMP-9 had an osteoinductive influence on mouse muscle resident stromal cells by increasing their alkaline phosphatase activity and bone-specific marker expression. Interestingly, BMP-9 induced HO only in damaged muscle, whereas BMP-2 promoted HO in skeletal muscle regardless of its state. The addition of the soluble form of the ALK1 protein (the BMP-9 receptor) significantly inhibited the osteoinductive potential of BMP-9 in cells and HO in damaged muscles. BMP-9 thus should be considered a candidate for involvement in HO physiopathology, with its activity depending on the skeletal muscle microenvironment.

A muscle resident cell population promotes fibrosis in hindlimb skeletal muscles of mdx mice through the Wnt canonical pathway

Previous work has pointed to a role for the Wnt canonical pathway in fibrosis formation in aged skeletal muscles. In the present study, we studied the dystrophic mdx mouse, which displays skeletal muscle fibrosis. Our results indicated that the muscle resident stromal cell (mrSC) population in the muscles of dystrophic mice is higher than in the muscles of age-matched wild-type mice. Wnt3a promoted the proliferation of and collagen expression by cultured mrSCs but arrested the growth of and collagen expression by cultured myoblasts. Injections of Wnt3A in the tibialis anterior muscles of adult wild-type mice significantly enhanced the mrSC population and collagen deposition compared with the contralateral muscles. Conversely, an injection of the Wnt antagonist Dickkof protein (DKK1) into the skeletal muscles of mdx mice significantly reduced collagen deposition. These results suggested that the Wnt canonical pathway expands the population of mrSCs and stimulates their production of collagen as observed during aging and in various myopathies.