Expression pattern of M-cadherin in normal, denervated, and regenerating mouse muscles

Current technology and industrialization status of cell-cultivated meat

Interest and investment in cultivated meat are increasing because of the realization that it can effectively supply sufficient food resources and reduce the use of livestock. Nevertheless, accurate information on the specific technologies used for cultivated meat production and the characteristics of cultivated meat is lacking. Authorization for the use of cultivated meat is already underway in the United States, Singapore, and Israel, and other major countries are also expected to approve cultivated meat as food once the details of the intricate process of producing cultivated meat, which encompasses stages such as cell proliferation, differentiation, maturation, and assembly, is thoroughly established. The development and standardization of mass production processes and safety evaluations must precede the industrialization and use of cultivated meat as food. However, the technology for the industrialization of cultivated meat is still in its nascent stage, and the mass production process has not yet been established. The mass production process of cultivated meat may not be easy to disclose because it is related to the interests of several companies or research teams. However, the overall research flow shows that equipment development for mass production and cell acquisition, proliferation, and differentiation, as well as for three-dimensional production supports and bioreactors have not yet been completed. Therefore, additional research on the mass production process and safety of cultivated meat is essential. The consumer's trust in the cultivated meat products and production technologies recently disclosed by some companies should also be analyzed and considered for guiding future developments in this industry. Furthermore, close monitoring by academia and the government will be necessary to identify fraud in the cultivated meat industry.

Flow Cytometer Analyses, Isolation, and Staining of Murine Muscle Satellite Cells

Fluorescence-activated cell sorting (FACS) is a powerful and requisite tool for the analysis and purification of adult stem cells. However, it is difficult to separate adult stem cells from solid organs than from immune-related tissues/organs. This is because of the presence of large amounts of debris, which increases noise in the FACS profiles. In particular, it is extremely difficult for unfamiliar researchers to identify muscle stem cell (also known as muscle satellite cell: MuSC) fraction because all myofibers, which are mainly composed of skeletal muscle tissues, become debris during cell preparation. This chapter describes our FACS protocol, which we have used for more than a decade, to identify and purify MuSCs.

Skeletal Muscle Stem Cells in Aging: Asymmetric/Symmetric Division Switching

In aged muscle, satellite cells’ symmetric and asymmetric divisions are impaired, and intrinsic and extrinsic complex mechanisms govern these processes. This review presents many updated aspects regarding muscle stem cells’ fate in normal and aging conditions. The balance between self-renewal and commitment divisions contributes to muscle regeneration, muscle homeostasis, aging, and disease. Stimulating muscle regeneration in aging could be a therapeutic target, but there is still a need to understand the many mechanisms that influence each other in satellite cells and their niche. We highlight here the general outlines regarding satellite cell divisions, the primary markers present in muscle stem cells, the aging aspects concerning signaling pathways involved in symmetric/asymmetric divisions, the regenerative capacity of satellite cells and their niche alteration in senescent muscle, genetics and epigenetics mechanisms implied in satellite cells aging and exercise effect on muscle regeneration in the elderly.

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

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

Preserved stem cell content and innervation profile of elderly human skeletal muscle with lifelong recreational exercise

Abstract

Muscle fibre denervation and declining numbers of muscle stem (satellite) cells are defining characteristics of ageing skeletal muscle. The aim of this study was to investigate the potential for lifelong recreational exercise to offset muscle fibre denervation and compromised satellite cell content and function, both at rest and under challenged conditions. Sixteen elderly lifelong recreational exercisers (LLEX) were studied alongside groups of age‐matched sedentary (SED) and young subjects. Lean body mass and maximal voluntary contraction were assessed, and a strength training bout was performed. From muscle biopsies, tissue and primary myogenic cell cultures were analysed by immunofluorescence and RT‐qPCR to assess myofibre denervation and satellite cell quantity and function. LLEX demonstrated superior muscle function under challenged conditions. When compared with SED, the muscle of LLEX was found to contain a greater content of satellite cells associated with type II myofibres specifically, along with higher mRNA levels of the beta and gamma acetylcholine receptors (AChR). No difference was observed between LLEX and SED for the proportion of denervated fibres or satellite cell function, as assessed in vitro by myogenic cell differentiation and fusion index assays. When compared with inactive counterparts, the skeletal muscle of lifelong exercisers is characterised by greater fatigue resistance under challenged conditions in vivo, together with a more youthful tissue satellite cell and AChR profile. Our data suggest a little recreational level exercise goes a long way in protecting against the emergence of classic phenotypic traits associated with the aged muscle.

Key points

The detrimental effects of ageing can be partially offset by lifelong self‐organized recreational exercise, as evidence by preserved type II myofibre‐associated satellite cells, a beneficial muscle innervation status and greater fatigue resistance under challenged conditions.

Satellite cell function (in vitro), muscle fibre size and muscle fibre denervation determined by immunofluorescence were not affected by recreational exercise.

Individuals that are recreationally active are far more abundant than master athletes, which sharply increases the translational perspective of the present study. Future studies should further investigate recreational activity in relation to muscle health, while also including female participants.

Cell adhesion an important determinant of myogenesis and satellite cell activity

Skeletal muscles represent a complex and highly organised tissue responsible for all voluntary body movements. Developed through an intricate and tightly controlled process known as myogenesis, muscles form early in development and are maintained throughout life. Due to the constant stresses that muscles are subjected to, skeletal muscles maintain a complex course of regeneration to both replace and repair damaged myofibers and to form new functional myofibers. This process, made possible by a pool of resident muscle stem cells, termed satellite cells, and controlled by an array of transcription factors, is additionally reliant on a diverse range of cell adhesion molecules and the numerous signaling cascades that they initiate. This article will review the literature surrounding adhesion molecules and their roles in skeletal muscle myogenesis and repair.

Pannexin 1 Regulates Skeletal Muscle Regeneration by Promoting Bleb-Based Myoblast Migration and Fusion Through a Novel Lipid Based Signaling Mechanism

Adult skeletal muscle has robust regenerative capabilities due to the presence of a resident stem cell population called satellite cells. Muscle injury leads to these normally quiescent cells becoming molecularly and metabolically activated and embarking on a program of proliferation, migration, differentiation, and fusion culminating in the repair of damaged tissue. These processes are highly coordinated by paracrine signaling events that drive cytoskeletal rearrangement and cell-cell communication. Pannexins are a family of transmembrane channel proteins that mediate paracrine signaling by ATP release. It is known that Pannexin1 (Panx1) is expressed in skeletal muscle, however, the role of Panx1 during skeletal muscle development and regeneration remains poorly understood. Here we show that Panx1 is expressed on the surface of myoblasts and its expression is rapidly increased upon induction of differentiation and that Panx1^-/- mice exhibit impaired muscle regeneration after injury. Panx1^-/- myoblasts activate the myogenic differentiation program normally, but display marked deficits in migration and fusion. Mechanistically, we show that Panx1 activates P2 class purinergic receptors, which in turn mediate a lipid signaling cascade in myoblasts. This signaling induces bleb-driven amoeboid movement that in turn supports myoblast migration and fusion. Finally, we show that Panx1 is involved in the regulation of cell-matrix interaction through the induction of ADAMTS (Disintegrin-like and Metalloprotease domain with Thrombospondin-type 5) proteins that help remodel the extracellular matrix. These studies reveal a novel role for lipid-based signaling pathways activated by Panx1 in the coordination of myoblast activities essential for skeletal muscle regeneration.

Matrisome, innervation and oxidative metabolism affected in older compared with younger males with similar physical activity

BACKGROUND

Due to the interaction between skeletal muscle ageing and lifestyle factors, it is often challenging to attribute the decline in muscle mass and quality to either changes in lifestyle or to advancing age itself. Because many of the physiological factors affecting muscle mass and quality are modulated by physical activity and physical activity declines with age, the aim of this study is to better understand the effects of early ageing on muscle function by comparing a population of healthy older and young males with similar physical activity patterns.

METHODS

Eighteen older (69 ± 2.0 years) and 20 young (22 ± 2.0 years) males were recruited based on similar self-reported physical activity, which was verified using accelerometry measurements. Gene expression profiles of vastus lateralis biopsies obtained by RNA sequencing were compared, and key results were validated using quantitative polymerase chain reaction and western blot.

RESULTS

Total physical activity energy expenditure was similar between the young and old group (404 ± 215 vs. 411 ± 189 kcal/day, P = 0.11). Three thousand seven hundred ninety-seven differentially expressed coding genes (DEGs) were identified (adjusted P-value cut-off of <0.05), of which 1891 were higher and 1906 were lower expressed in the older muscle. The matrisome, innervation and inflammation were the main upregulated processes, and oxidative metabolism was the main downregulated process in old compared with young muscle. Lower protein levels of mitochondrial transcription factor A (TFAM, P = 0.030) and mitochondrial respiratory Complexes IV and II (P = 0.011 and P = 0.0009, respectively) were observed, whereas a trend was observed for Complex I (P = 0.062), in older compared with young muscle. Protein expression of Complexes I and IV was significantly correlated to mitochondrial capacity in the vastus lateralis as measured in vivo (P = 0.017, R2  = 0.42 and P = 0.030, R2  = 0.36). A trend for higher muscle-specific receptor kinase (MUSK) protein levels in the older group was observed (P = 0.08).

CONCLUSIONS

There are clear differences in the transcriptome signatures of the vastus lateralis muscle of healthy older and young males with similar physical activity levels, including significant differences at the protein level. By disentangling physical activity and ageing, we appoint early skeletal muscle ageing processes that occur despite similar physical activity. Improved understanding of these processes will be key to design targeted anti-ageing therapies.

Decreased Serum NCAM Levels Associated with Cognitive Impairment in Vascular Dementia

Objective

Neural cell adhesion molecule (NCAM), a glycoprotein widely distributed in the brain, has recently been shown to regulate neuroplasticity. However, the role of NCAM in vascular dementia (VaD) is still unclear. The purpose of this study is to determine whether NCAM is involved in the course of VaD.

Methods

Continuous recruitment of VaD patients and control population to join this study. Doctors or nurses are responsible for collecting their clinical characteristics including age, gender, formal education, heart rate, supine systolic blood pressure, supine diastolic blood pressure, fasting glucose, high-density lipoprotein, and low-density lipoprotein. Each participant received the Montreal Cognitive Assessment (MoCA) scale after being enrolled in the group. At the same time, their peripheral blood was collected, and their serum NCAM levels were measured by enzyme-linked immunosorbent assay (ELISA).

Results

98 VaD patients and 83 age- and sex-matched controls were enrolled. There was no significant statistical difference between the VaD group and the control group in terms of the comparison of clinical characteristics (p > 0.05). The MoCA score of VaD patients was significantly lower than that of the controls (27.9 ± 1.4 vs. 23.0 ± 2.1 points, p < 0.001). In addition, the circulating NCAM level of VaD patients was also significantly lower than that of controls (21.7 ± 3.8 vs. 17.6 ± 4.2 ng/mL, p < 0.001). The circulating NCAM level of VaD patients was significantly positively correlated with MoCA score (r = 0.285, p = 0.026). After adjusting for clinical characteristics, circulating NCAM levels are still an independent pathogenic factor of VaD (regression coefficient = 0.223, p = 0.034).

Conclusions

VaD patients have low circulating NCAM levels, which can be used as a potential predictor of VaD.

Exercise/Resistance Training and Muscle Stem Cells

Skeletal muscle has attracted attention as endocrine organ, because exercise-dependent cytokines called myokines/exerkines are released from skeletal muscle and are involved in systemic functions. While, local mechanical loading to skeletal muscle by exercise or resistance training alters myofiber type and size and myonuclear number. Skeletal muscle-resident stem cells, known as muscle satellite cells (MuSCs), are responsible for the increased number of myonuclei. Under steady conditions, MuSCs are maintained in a mitotically quiescent state but exit from that state and start to proliferate in response to high physical activity. Alterations in MuSC behavior occur when myofibers are damaged, but the lethal damage to myofibers does not seem to evoke mechanical loading-dependent MuSC activation and proliferation. Given that MuSCs proliferate without damage, it is unclear how the different behaviors of MuSCs are controlled by different physical activities. Recent studies demonstrated that myonuclear number reflects the size of myofibers; hence, it is crucial to know the properties of MuSCs and the mechanism of myonuclear accretion by MuSCs. In addition, the elucidation of mechanical load-dependent changes in muscle resident cells, including MuSCs, will be necessary for the discovery of new myokines/exerkines and understating skeletal muscle diseases.

M-Cadherin Is a PAX3 Target During Myotome Patterning

PAX3 belongs to the paired-homeobox family of transcription factors and plays a key role as an upstream regulator of muscle progenitor cells during embryonic development. Pax3-mutant embryos display impaired somite development, yet the consequences for myotome formation have not been characterized. The early myotome is formed by PAX3-expressing myogenic cells that delaminate from the dermomyotomal lips and migrate between the dermomyotome and sclerotome where they terminally differentiate. Here we show that in Pax3-mutant embryos, myotome formation is impaired, displays a defective basal lamina and the regionalization of the structural protein Desmin is lost. In addition, this phenotype is more severe in embryos combining Pax3-null and Pax3 dominant-negative alleles. We identify the adhesion molecule M-Cadherin as a PAX3 target gene, the expression of which is modulated in the myotome according to Pax3 gain- and loss-of-function alleles analyzed. Taken together, we identify M-Cadherin as a PAX3-target linked to the formation of the myotome.

The cooperation of cis-elements during M-cadherin promoter activation

M-cadherin is a skeletal muscle-specific transmembrane protein mediating the cell-cell adhesion of myoblasts during myogenesis. It is expressed in the proliferating satellite cells and highly induced by myogenic regulatory factors (MRFs) during terminal myogenic differentiation. Several conserved cis-elements, including 5 E-boxes, 2 GC boxes, and 1 conserved downstream element (CDE) were identified in the M-cadherin proximal promoter. We found that E-box-3 and -4 close to the transcription initiation site (TIS) mediated most of its transactivation by MyoD, the strongest myogenic MRF. Including of any one of the other E-boxes restored the full activation by MyoD, suggesting an essential collaboration between E-boxes. Stronger activation of M-cadherin promoter than that of muscle creatine kinase (MCK) by MyoD was observed regardless of culture conditions and the presence of E47. Furthermore, MyoD/E47 heterodimer and MyoD ∼ E47 fusion protein achieved similar levels of activation in differentiation medium (DM), suggesting high affinity of MyoD/E47 to E-boxes 3/4 under DM. We also found that GC boxes and CDE positively affected MyoD mediated activation. The CDE element was predicted to be the target of the chromatin-modifying factor Meis1/Pbx1 heterodimer. Knockdown of Pbx1 significantly reduced the expression level of M-cadherin, but increased that of N-cadherin. Using ChIP assay, we further found significant reduction in MyoD recruitment to M-cadherin promoter when CDE was deleted. Taken together, these observations suggest that the chromatin-modifying function of Pbx1/Meis1 is critical to M-cadherin promoter activation before MyoD is recruited to E-boxes to trigger transcription.

The Satellite Cell at 60: The Foundation Years

The resident stem cell for skeletal muscle is the satellite cell. On the 50th anniversary of its discovery in 1961, we described the history of skeletal muscle research and the seminal findings made during the first 20 years in the life of the satellite cell (Scharner and Zammit 2011, doi: 10.1186/2044-5040-1-28). These studies established the satellite cell as the source of myoblasts for growth and regeneration of skeletal muscle. Now on the 60th anniversary, we highlight breakthroughs in the second phase of satellite cell research from 1980 to 2000. These include technical innovations such as isolation of primary satellite cells and viable muscle fibres complete with satellite cells in their niche, together with generation of many useful reagents including genetically modified organisms and antibodies still in use today. New methodologies were combined with description of endogenous satellite cells markers, notably Pax7. Discovery of the muscle regulatory factors Myf5, MyoD, myogenin, and MRF4 in the late 1980s revolutionized understanding of the control of both developmental and regerenative myogenesis. Emergence of genetic lineage markers facilitated identification of satellite cells in situ, and also empowered transplantation studies to examine satellite cell function. Finally, satellite cell heterogeneity and the supportive role of non-satellite cell types in muscle regeneration were described. These major advances in methodology and in understanding satellite cell biology provided further foundations for the dramatic escalation of work on muscle stem cells in the 21st century.

PEPITEM/Cadherin 15 Axis Inhibits T Lymphocyte Infiltration and Glomerulonephritis in a Mouse Model of Systemic Lupus Erythematosus

Control of lymphocyte infiltration in kidney is a potential therapeutic strategy for lupus nephritis, considering that control of lymphocyte migration by sphingosine 1 phosphate has been implicated in inflammation-related pathology. The peptide inhibitor of the transendothelial migration (PEPITEM)/cadherin (CDH) 15 axis was recently reported to promote sphingosine 1 phosphate secretion. In this study, we investigated whether CDH15 is expressed in the kidney of MRL/lpr mice and whether lymphocyte infiltration is suppressed by exogenously administered PEPITEM. Mice (18 wk old) were randomized into 4-wk treatment groups that received PEPITEM or PBS encapsulated in dipalmitoylphosphatidylcholine liposomes. Enlargement of the kidney, spleen, and axillary lymph nodes was suppressed by PEPITEM treatment, which also blocked infiltration of double-negative T lymphocytes into the kidney and glomerular IgG/C3 deposition, reduced proteinuria, and increased podocyte density. Immunohistochemical analysis revealed that the PEPITEM receptor CDH15 was expressed on vascular endothelial cells of glomeruli and kidney arterioles, skin, and peritoneum in lupus mice at 22 wk of age but not in 4-wk-old mice. These results suggest that PEPITEM inhibits lymphocyte migration and infiltration into the kidney, thereby preserving the kidney structure and reducing proteinuria. Thus, PEPITEM administration may be considered as a potential therapeutic tool for systemic lupus erythematosus.

Human and mouse skeletal muscle stem and progenitor cells in health and disease

The proper functioning of tissues and organs depends on their ability to self-renew and repair. Some of the tissues, like epithelia, renew almost constantly while in the others this process is induced by injury or diseases. The stem or progenitor cells responsible for tissue homeostasis have been identified in many organs. Some of them, such as hematopoietic or intestinal epithelium stem cells, are multipotent and can differentiate into various cell types. Others are unipotent. The skeletal muscle tissue does not self-renew spontaneously, however, it presents unique ability to regenerate in response to the injury or disease. Its repair almost exclusively relies on unipotent satellite cells. However, multiple lines of evidence document that some progenitor cells present in the muscle can be supportive for skeletal muscle regeneration. Here, we summarize the current knowledge on the complicated landscape of stem and progenitor cells that exist in skeletal muscle and support its regeneration. We compare the cells from two model organisms, i.e., mouse and human, documenting their similarities and differences and indicating methods to test their ability to undergo myogenic differentiation.

Stem cells in skeletal muscle growth and regeneration in amniotes and teleosts: Emerging themes

Skeletal muscle is a contractile, postmitotic tissue that retains the capacity to grow and regenerate throughout life in amniotes and teleost. Both muscle growth and regeneration are regulated by obligate tissue resident muscle stem cells. Given that considerable knowledge exists on the myogenic process, recent studies have focused on examining the molecular markers of muscle stem cells, and on the intrinsic and extrinsic signals regulating their function. From this, two themes emerge: firstly, muscle stem cells display remarkable heterogeneity not only with regards to their gene expression profile, but also with respect to their behavior and function; and secondly, the stem cell niche is a critical regulator of muscle stem cell function during growth and regeneration. Here, we will address the current understanding of these emerging themes with emphasis on the distinct processes used by amniotes and teleost, and discuss the challenges and opportunities in the muscle growth and regeneration fields. This article is characterized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Early Embryonic Development > Development to the Basic Body Plan Vertebrate Organogenesis > Musculoskeletal and Vascular.

A Promising Future for Stem-Cell-Based Therapies in Muscular Dystrophies-In Vitro and In Vivo Treatments to Boost Cellular Engraftment

Muscular dystrophies (MD) are a group of genetic diseases that lead to skeletal muscle wasting and may affect many organs (multisystem). Unfortunately, no curative therapies are available at present for MD patients, and current treatments mainly address the symptoms. Thus, stem-cell-based therapies may present hope for improvement of life quality and expectancy. Different stem cell types lead to skeletal muscle regeneration and they have potential to be used for cellular therapies, although with several limitations. In this review, we propose a combination of genetic, biochemical, and cell culture treatments to correct pathogenic genetic alterations and to increase proliferation, dispersion, fusion, and differentiation into new or hybrid myotubes. These boosted stem cells can also be injected into pretreate recipient muscles to improve engraftment. We believe that this combination of treatments targeting the limitations of stem-cell-based therapies may result in safer and more efficient therapies for MD patients. Matricryptins have also discussed.

Multiplexed RNAscope and immunofluorescence on whole-mount skeletal myofibers and their associated stem cells

Skeletal muscle myofibers are large syncytial cells comprising hundreds of myonuclei, and in situ hybridization experiments have reported a range of transcript localization patterns within them. Although some transcripts are uniformly distributed throughout myofibers, proximity to specialized regions can affect the programming of myonuclei and functional compartmentalization of transcripts. Established techniques are limited by a lack of both sensitivity and spatial resolution, restricting the ability to identify different patterns of gene expression. In this study, we adapted RNAscope fluorescent in situ hybridization technology for use on whole-mount mouse primary myofibers, a preparation that isolates single myofibers with their associated muscle stem cells remaining in their niche. This method can be combined with immunofluorescence, enabling an unparalleled ability to visualize and quantify transcripts and proteins across the length and depth of skeletal myofibers and their associated stem cells. Using this approach, we demonstrate a range of potential uses, including the visualization of specialized transcriptional programming within myofibers, tracking activation-induced transcriptional changes, quantification of stem cell heterogeneity and evaluation of stem cell niche factor transcription patterns.

ZEB1 protects skeletal muscle from damage and is required for its regeneration

The mechanisms linking muscle injury and regeneration are not fully understood. Here we report an unexpected role for ZEB1 regulating inflammatory and repair responses in dystrophic and acutely injured muscles. ZEB1 is upregulated in the undamaged and regenerating myofibers of injured muscles. Compared to wild-type counterparts, Zeb1-deficient injured muscles exhibit enhanced damage that corresponds with a retarded p38-MAPK-dependent transition of their macrophages towards an anti-inflammatory phenotype. Zeb1-deficient injured muscles also display a delayed and poorer regeneration that is accounted by the retarded anti-inflammatory macrophage transition and their intrinsically deficient muscle satellite cells (MuSCs). Macrophages in Zeb1-deficient injured muscles show lower phosphorylation of p38 and its forced activation reverts the enhanced muscle damage and poorer regeneration. MuSCs require ZEB1 to maintain their quiescence, prevent their premature activation following injury, and drive efficient regeneration in dystrophic muscles. These data indicate that ZEB1 protects muscle from damage and is required for its regeneration.

Following muscle damage, an inflammatory response is associated to activation of satellite cells, which drive muscle repair. Here, the authors show that upregulation of Zeb1 in macrophages and muscle fibres regulates inflammation, and also show a role for Zeb1 in maintenance of satellite cell quiescence.

An Overview about the Biology of Skeletal Muscle Satellite Cells

The peculiar ability of skeletal muscle tissue to operate adaptive changes during post-natal de-velopment and adulthood has been associated with the existence of adult somatic stem cells. Satellite cells, occupying an exclusive niche within the adult muscle tissue, are considered bona fide stem cells with both stem-like properties and myogenic activities. Indeed, satellite cells retain the capability to both maintain the quiescence in uninjured muscles and to be promptly activated in response to growth or re-generative signals, re-engaging the cell cycle. Activated cells can undergo myogenic differentiation or self-renewal moving back to the quiescent state. Satellite cells behavior and their fate decision are finely controlled by mechanisms involving both cell-autonomous and external stimuli. Alterations in these regu-latory networks profoundly affect muscle homeostasis and the dynamic response to tissue damage, con-tributing to the decline of skeletal muscle that occurs under physio-pathologic conditions. Although the clear myogenic activity of satellite cells has been described and their pivotal role in muscle growth and regeneration has been reported, a comprehensive picture of inter-related mechanisms guiding muscle stem cell activity has still to be defined. Here, we reviewed the main regulatory networks determining satellite cell behavior. In particular, we focused on genetic and epigenetic mechanisms underlining satel-lite cell maintenance and commitment. Besides intrinsic regulations, we reported current evidences about the influence of environmental stimuli, derived from other cell populations within muscle tissue, on satel-lite cell biology.

EGFR-Aurka Signaling Rescues Polarity and Regeneration Defects in Dystrophin-Deficient Muscle Stem Cells by Increasing Asymmetric Divisions

Loss of dystrophin expression in Duchenne muscular dystrophy (DMD) causes progressive degeneration of skeletal muscle, which is exacerbated by reduced self-renewing asymmetric divisions of muscle satellite cells. This, in turn, affects the production of myogenic precursors and impairs regeneration and suggests that increasing such divisions may be beneficial. Here, through a small-molecule screen, we identified epidermal growth factor receptor (EGFR) and Aurora kinase A (Aurka) as regulators of asymmetric satellite cell divisions. Inhibiting EGFR causes a substantial shift from asymmetric to symmetric division modes, whereas EGF treatment increases asymmetric divisions. EGFR activation acts through Aurka to orient mitotic centrosomes, and inhibiting Aurka blocks EGF stimulation-induced asymmetric division. In vivo EGF treatment markedly activates asymmetric divisions of dystrophin-deficient satellite cells in mdx mice, increasing progenitor numbers, enhancing regeneration, and restoring muscle strength. Therefore, activating an EGFR-dependent polarity pathway promotes functional rescue of dystrophin-deficient satellite cells and enhances muscle force generation.

Skeletal Muscle Progenitor Cell Heterogeneity

Tissue-specific stem cells contribute to adult tissue maintenance, repair, and regeneration. In skeletal muscle, many different mononuclear cell types are capable of giving rise to differentiated muscle. Of these tissue stem-like cells, satellite cells (SCs) are the most studied muscle stem cell population and are widely considered the main cellular source driving muscle repair and regeneration in adult tissue. Within the satellite cell pool, many distinct subpopulations exist, each exhibiting differential abilities to exit quiescence, expand, differentiate, and self-renew. In this chapter, we discuss the different stem cell types that can give rise to skeletal muscle tissue and then focus on satellite cell heterogeneity during the process of myogenesis/muscle regeneration. Finally, we highlight emerging opportunities to better characterize muscle stem cell heterogeneity, which will ultimately deepen our appreciation of stem cells in muscle development, repair/regeneration, aging, and disease.

Persistent disruption of lateral junctional complexes and actin cytoskeleton in parotid salivary glands following radiation treatment

Xerostomia and hyposalivation are debilitating side effects for patients treated with ionizing radiation for head and neck cancer. Despite technological advances, collateral damage to the salivary glands remains a significant problem for patients and severely diminishes their quality of life. During the wound healing process, restoration of junctional contacts is necessary to maintain polarity, structural integrity, and orientation cues for secretion. However, little is known about whether these structural molecules are impacted following radiation damage and more importantly, during tissue restoration. We evaluated changes in adherens junctions and cytoskeletal regulators in an injury model where mice were irradiated with 5 Gy and a restoration model where mice injected postradiation with insulin-like growth factor 1 (IGF1) are capable of restoring salivary function. Using coimmunoprecipitation, there is a decrease in epithelial (E)-cadherin bound to β-catenin following damage that is restored to untreated levels with IGF1. Via its adaptor proteins, β-catenin links the cadherins to the cytoskeleton and part of this regulation is mediated through Rho-associated coiled-coil containing kinase (ROCK) signaling. In our radiation model, filamentous (F)-actin organization is fragmented, and there is an induction of ROCK activity. However, a ROCK inhibitor, Y-27632, prevents E-cadherin/β-catenin dissociation following radiation treatment. These findings illustrate that radiation induces a ROCK-dependent disruption of the cadherin-catenin complex and alters F-actin organization at stages of damage when hyposalivation is observed. Understanding the regulation of these components will be critical in the discovery of therapeutics that have the potential to restore function in polarized epithelium.

Induction of bone marrow-derived cells myogenic identity by their interactions with the satellite cell niche

Background

Skeletal muscle regeneration is possible thanks to unipotent stem cells, which are satellite cells connected to the myofibers. Populations of stem cells other than muscle-specific satellite cells are considered as sources of cells able to support skeletal muscle reconstruction. Among these are bone marrow-derived mesenchymal stem cells (BM-MSCs), which are multipotent, self-renewing stem cells present in the bone marrow stroma. Available data documenting the ability of BM-MSCs to undergo myogenic differentiation are not definitive. In the current work, we aimed to check if the satellite cell niche could impact the ability of bone marrow-derived cells to follow a myogenic program.

Methods

We established a new in-vitro method for the coculture of bone marrow-derived cells (BMCs) that express CXCR4 (CXCR4⁺BMCs; the stromal-derived factor-1 (Sdf-1) receptor) with myofibers. Using various tests, we analyzed the myogenic identity of BMCs and their ability to fuse with myoblasts in vitro and in vivo.

Results

We showed that Sdf-1 treatment increased the number of CXCR4⁺BMCs able to bind the myofiber and occupy the satellite cell niche. Moreover, interaction with myofibers induced the expression of myogenic regulatory factors (MRFs) in CXCR4⁺BMCs. CXCR4⁺BMCs, pretreated by the coculture with myofibers and Sdf-1, participated in myotube formation in vitro and also myofiber reconstruction in vivo. We also showed that Sdf-1 overexpression in vivo (in injured and regenerating muscles) supported the participation of CXCR4⁺BMCs in new myofiber formation.

Conclusion

We showed that CXCR4⁺BMC interaction with myofibers (that is, within the satellite cell niche) induced CXCR4⁺BMC myogenic commitment. CXCR4⁺BMCs, pretreated using such a method of culture, were able to participate in skeletal muscle regeneration.

Botulinum Neurotoxin Application to the Severed Femoral Nerve Modulates Spinal Synaptic Responses to Axotomy and Enhances Motor Recovery in Rats

Botulinum neurotoxin A (BoNT) and brain-derived neurotrophic factor (BDNF) are known for their ability to influence synaptic inputs to neurons. Here, we tested if these drugs can modulate the deafferentation of motoneurons following nerve section/suture and, as a consequence, modify the outcome of peripheral nerve regeneration. We applied drug solutions to the proximal stump of the freshly cut femoral nerve of adult rats to achieve drug uptake and transport to the neuronal perikarya. The most marked effect of this application was a significant reduction of the axotomy-induced loss of perisomatic cholinergic terminals by BoNT at one week and two months post injury. The attenuation of the synaptic deficit was associated with enhanced motor recovery of the rats 2–20 weeks after injury. Although BDNF also reduced cholinergic terminal loss at 1 week, it had no effect on this parameter at two months and no effect on functional recovery. These findings strengthen the idea that persistent partial deafferentation of axotomized motoneurons may have a significant negative impact on functional outcome after nerve injury. Intraneural application of drugs may be a promising way to modify deafferentation and, thus, elucidate relationships between synaptic plasticity and restoration of function.

Niche Cadherins Control the Quiescence-to-Activation Transition in Muscle Stem Cells

Many adult stem cells display prolonged quiescence, promoted by cues from their niche. Upon tissue damage, a coordinated transition to the activated state is required because non-physiological breaks in quiescence often lead to stem cell depletion and impaired regeneration. Here, we identify cadherin-mediated adhesion and signaling between muscle stem cells (satellite cells [SCs]) and their myofiber niche as a mechanism that orchestrates the quiescence-to-activation transition. Conditional removal of N-cadherin and M-cadherin in mice leads to a break in SC quiescence, with long-term expansion of a regeneration-proficient SC pool. These SCs have an incomplete disruption of the myofiber-SC adhesive junction and maintain niche residence and cell polarity, yet show properties of SCs in a state of transition from quiescence toward full activation. Among these is nuclear localization of β-catenin, which is necessary for this phenotype. Injury-induced perturbation of niche adhesive junctions is therefore a likely first step in the quiescence-to-activation transition.

A CREB-MPP7-AMOT Regulatory Axis Controls Muscle Stem Cell Expansion and Self-Renewal Competence

Skeletal muscle regeneration requires resident muscle stem cells, termed satellite cells (SCs). SCs are largely quiescent during homeostasis yet become activated upon injury to supply myonuclei and self-renewed SCs. Molecular mechanisms underlying the competence of SCs to proliferate and self-renew in response to injury remain unclear. Here, we show that CREB activity establishes proliferative potential during SC quiescence. SCs with inhibited CREB activity remain quiescent and positioned in their niche, but upon injury, they cannot enter or maintain a proliferative state for expansion and self-renewal. We demonstrate mechanistically that Mpp7 is a CREB target and its functional mediator. MPP7 loss affects the level and sub-cellular localization of AMOT and YAP1 in quiescent SCs. Furthermore, MPP7 and AMOT are required for YAP1 nuclear accumulation, and the three are individually required for a proliferative state in myoblasts. We propose that the CREB-MPP7-AMOT-YAP1 axis establishes the competence of quiescent SCs to expand and self-renew, thereby preserving stem cell function.

PAX7 Targets, CD54, Integrin α9β1, and SDC2, Allow Isolation of Human ESC/iPSC-Derived Myogenic Progenitors

Pluripotent stem (PS)-cell-derived cell types hold promise for treating degenerative diseases. However, PS cell differentiation is intrinsically heterogeneous; therefore, clinical translation requires the development of practical methods for isolating progenitors from unwanted and potentially teratogenic cells. Muscle-regenerating progenitors can be derived through transient PAX7 expression. To better understand the biology, and to discover potential markers for these cells, here we investigate PAX7 genomic targets and transcriptional changes in human cells undergoing PAX7-mediated myogenic commitment. We identify CD54, integrin α9β1, and Syndecan2 (SDC2) as surface markers on PAX7-induced myogenic progenitors. We show that these markers allow for the isolation of myogenic progenitors using both fluorescent- and CGMP-compatible magnetic-based sorting technologies and that CD54+α9β1+SDC2+ cells contribute to long-term muscle regeneration in vivo. These findings represent a critical step toward enabling the translation of PS-cell-based therapies for muscle diseases.

Skeletal muscle cells actively shape (auto)immune responses

Histopathological analyses of muscle specimens from myositis patients indicate that skeletal muscle cells play an active role in the interaction with immune cells. Research over the last few decades has shown that skeletal muscle cells exhibit immunobiological properties that perfectly define them as non-professional antigen presenting cells. They are able to present antigens via major histocompatibility complex molecules, exhibit costimulatory molecules and secrete soluble molecules that actively shape the immune response in an either pro- or anti-inflammatory manner. Skeletal muscle cells regulate both innate and adaptive immune responses and are essentially involved in the pathophysiological processes of idiopathic inflammatory myopathies. Understanding the role of skeletal muscle cells might help to identify new therapeutic targets for these devastating diseases. This review summarizes the immunobiological features of skeletal muscle cells, especially in the context of idiopathic inflammatory myopathies, and discusses shortcomings and limitations in skeletal muscle related research providing potential perspectives to overcome them in the future.

Deiodinases and stem cells: an intimate relationship

Thyroid hormone is a major determinant of tissue functions in vivo. The deiodinase family controls the tissue-specific activation or inactivation of intracellular thyroid hormones. Precise control of the T3-dependent transcriptional program is required by multiple cell systems, including the stem cell. In this context, the identification of a close connection between thyroid hormones and different signal pathways involved in the control of stem cell functions suggested that the deiodinases may play a role in the definition of stem cell biology and physiology. Stem cells have an unlimited self-renewal capacity and the potential to differentiate into different types of mature cells. Deciphering how all these events are achieved, how the T3 signal is controlled and integrated in stem cells and their niches, and how it can impact on them is essentially unknown and represents a challenge for coming years. In this review, I will explore the role played by the deiodinases in the modulation of the TH signal in stem cells of adult tissues, namely muscle and intestine, and how their actions control the delicate balance among self-renewal, proliferation and differentiation. Elucidation of the molecular mechanisms presiding thyroid hormone action in stem cells may reveal therapeutic potential, for example in the fields of regenerative diseases and cancer.

The Muscle Stem Cell Niche in Health and Disease

The regulation of stem cells that maintain and regenerate postnatal tissues depends on extrinsic signals originating from their microenvironment, commonly referred to as the stem cell niche. Complex higher-order regulatory interrelationships with the tissue and factors in the systemic circulation are integrated and propagated to the stem cells through the niche. The stem cell niche in skeletal muscle tissue is both a paradigm for a structurally and functionally relatively static niche that maintains stem cell quiescence during tissue homeostasis, and a highly dynamic regenerative niche that is subject to extensive structural remodeling and a flux of different support cell populations. Conditions ranging from aging to chronically degenerative skeletal muscle diseases affect the composition of the niche and thereby impair the regenerative potential of muscle stem cells. A holistic and integrative understanding of the extrinsic mechanisms regulating muscle stem cells in health and disease in a broad systemic context will be imperative for the identification of regulatory hubs in the niche interactome that can be targeted to maintain, restore, or enhance the regenerative capacity of muscle tissue. Here, we review the microenvironmental regulation of muscle stem cells, summarize how niche dysfunction can contribute to disease, and discuss emerging therapeutic implications.

Greater Oxidative Capacity in Primary Myotubes from Endurance-trained Women

PURPOSE

Exercise training promotes skeletal muscle mitochondrial biogenesis and an increase in maximal oxygen consumption. Primary myotubes retain some metabolic properties observed in vivo but it is unknown whether this includes exercise-induced mitochondrial adaptations. The goal of this study was to test if primary myotubes from exercise-trained women have higher mitochondrial content and maximal oxygen consumption compared with untrained women.

METHODS

Six trained and nine untrained white women participated in this study. Muscle biopsies from the vastus lateralis muscle of the right leg were obtained and primary muscle cells were isolated. Maximal respiration rates, mitochondrial mRNA and protein content, and succinate dehydrogenase activity were measured in skeletal muscle and primary myotubes from trained and untrained women.

RESULTS

Trained women, compared with untrained women, had higher maximal whole-body oxygen consumption (+18%, P = 0.03), in vivo maximal skeletal muscle oxidative capacity measured with near infrared spectroscopy (+48%, P < 0.01), and maximal oxygen consumption in permeabilized muscle fibers (+38%, P = 0.02), which coincided with higher protein levels of muscle mitochondrial enzymes. Primary myotubes from trained women had higher maximal oxygen consumption (+38%, P = 0.03), suggesting that some elements of exercise-induced metabolic programming persists ex vivo. Consistent with this idea, myotubes from trained women had higher mRNA levels of transcriptional regulators of mitochondrial biogenesis in addition to higher protein levels of mitochondrial enzymes.

CONCLUSIONS

These data suggest the existence of an "exercise metabolic program," where primary myotubes isolated from exercise-trained individuals exhibit greater mitochondrial content and oxidative capacity compared with untrained individuals. These myotubes may be a useful model to study molecular mechanisms relevant to exercise adaptations in human skeletal muscle.

Impairment of cold injury-induced muscle regeneration in mice receiving a combination of bone fracture and alendronate treatment

Alendronate, a nitrogen-containing bisphosphonate, is well established as a treatment for osteoporosis through regulation of osteoclast activity. Previously, the pharmacological effects of bisphosphonates on cells outside the bone environment have been considered irrelevant because bisphosphonates target bone. Here we show that administration of alendronate impairs muscle regeneration in mice after bone fracture. A series of injections of alendronate alone or bone fracture alone did not affect muscle regeneration induced by cold injury. In contrast, alendronate treatment plus bone fracture severely impaired the regeneration of muscle that closely contacts the bone fracture site after cold injury. After cold injury, M-cadherin-positive myogenic cells disappeared in the damaged muscle areas of mice receiving the combination of alendronate treatment and bone fracture. The present results suggest that the muscle regeneration capacity is impaired by bone fracture in mice receiving alendronate treatment. The present research on the pharmacological effects of alendronate on muscle regeneration will aid in understanding of the in vivo action of alendronate on skeletal muscles.

A tale of two niches: differential functions for VCAM-1 in satellite cells under basal and injured conditions

Cell-cell adhesion molecules play key roles in maintaining quiescence or promoting activation of various stem cells in their niche. Muscle stem cells called satellite cells (SC) are critical for skeletal muscle regeneration after injury, but little is known about the role of adhesion molecules in regulating the behavior of these stem cells. Vascular cell adhesion molecule-1 (VCAM-1) is a cell-cell adhesion protein expressed on quiescent and activated SC whose function is unknown in this context. We deleted Vcam1 from SC using an inducible Cre recombinase in young mice. In the injured niche, Vcam1^-/- SC underwent premature lineage progression to a more differentiated state as well as apoptosis leading to a transient delay in myofiber growth during regeneration. Apoptosis was also increased in Vcam1^-/- SC in vitro concomitant with decreased levels of phosphorylated Akt, a prosurvival signal activated by VCAM-1 signaling in other cell types. During muscle regeneration, we observed an influx of immune cells expressing α4 integrin, a component of the major, high-affinity VCAM-1 ligand, α4β1 integrin. Furthermore, α4 integrin mRNA and protein were induced in SC 2 days after injury. These results suggest that SC interact with other SC as well as immune cells through α4β1 integrin in the injured niche to promote expansion of SC. In the uninjured niche, multiple cell types also expressed α4 integrin. However, only basal fusion of Vcam1^-/- SC with myofibers was decreased, contributing to decreased myofiber growth. These studies define differential roles for VCAM-1 in SC depending on the state of their niche.

Genome Editing and Muscle Stem Cells as a Therapeutic Tool for Muscular Dystrophies

Purpose of Review

Muscular dystrophies are a group of severe degenerative disorders characterized by muscle fiber degeneration and death. Therapies designed to restore muscle homeostasis and to replace dying fibers are being experimented, but none of those in clinical trials are suitable to permanently address individual gene mutation. The purpose of this review is to discuss genome editing tools such as CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR-associated), which enable direct sequence alteration and could potentially be adopted to correct the genetic defect leading to muscle impairment.

Recent Findings

Recent findings show that advances in gene therapy, when combined with traditional viral vector-based approaches, are bringing the field of regenerative medicine closer to precision-based medicine.

Summary

The use of such programmable nucleases is proving beneficial for the creation of more accurate in vitro and in vivo disease models. Several gene and cell-therapy studies have been performed on satellite cells, the primary skeletal muscle stem cells involved in muscle regeneration. However, these have mainly been based on artificial replacement or augmentation of the missing protein. Satellite cells are a particularly appealing target to address these innovative technologies for the treatment of muscular dystrophies.

Discovering novel oncogenic pathways and new therapies using zebrafish models of sarcoma

Sarcoma is a type of cancer affecting connective, supportive, or soft tissue of mesenchymal origin. Despite rare incidence in adults (<1%), over 15% of pediatric cancers are sarcoma. Sadly, both adults and children with relapsed or metastatic disease have devastatingly high rates of mortality. Current treatment options for sarcoma include surgery, radiation, and/or chemotherapy; however, significant limitations exist with respect to the efficacy of these strategies. Strong impetus has been placed on the development of novel therapies and preclinical models for uncovering mechanisms involved in the development, progression, and therapy resistance of sarcoma. Over the past 15 years, the zebrafish has emerged as a powerful genetic model of human cancer. High genetic conservation when combined with a unique susceptibility to develop sarcoma has made the zebrafish an effective tool for studying these diseases. Transgenic and gene-activation strategies have been employed to develop zebrafish models of rhabdomyosarcoma, malignant peripheral nerve sheath tumors, Ewing's sarcoma, chordoma, hemangiosarcoma, and liposarcoma. These models all display remarkable molecular and histopathological conservation with their human cancer counterparts and have offered excellent platforms for understanding disease progression in vivo. Short tumor latency and the amenability of zebrafish for ex vivo manipulation, live imaging studies, and tumor cell transplantation have allowed for efficient study of sarcoma initiation, growth, self-renewal, and maintenance. When coupled with facile chemical genetic approaches, zebrafish models of sarcoma have provided a strong translational tool to uncover novel drug pathways and new therapeutic strategies.

Primary Mouse Myoblast Purification using Magnetic Cell Separation

Primary myoblasts can be isolated from mouse muscle cell extracts and cultured in vitro. Muscle cells are usually dissociated manually by mincing with razor blades or scissors in a collagenase/dispase solution. Primary myoblasts are then gradually enriched by pre-plating on collagen-coated plates, based on the observation that mouse fibroblasts attach quickly to collagen-coated plates, and are less adherent. Here, we describe an automated muscle dissociation protocol. We also propose an alternative to pre-plating using magnetic bead separation of primary myoblasts, which improve myoblast purity by minimizing fibroblast contamination.

Prenatal myonuclei play a crucial role in skeletal muscle hypertrophy in rodents

Multinucleated muscle fibers are formed by the fusion of myogenic progenitor cells during embryonic and fetal myogenesis. However, the role of prenatally incorporated myonuclei in the skeletal muscle fibers of adult animals is poorly understood. We demonstrated, using muscle-specific reporter mice, that the prenatal myonuclei remained in the adult soleus muscle, although cardiotoxin injection caused the loss of prenatal myonuclei. Overloading by the tendon transection of synergists failed to induce compensatory hypertrophy in regenerated soleus muscle fibers of adult rats, whereas unloading by tail suspension normally induced the fiber atrophy. Loss of hypertrophying function correlated with the lowered histone acetylation at the transcription start site of Igf1r gene, which was one of the genes that did not respond to the overloading. These parameters were improved by the transplantation of cells harvested from the juvenile soleus muscles of neonatal rats in association with enhanced histone acetylation of Igf1r gene. These results indicated that the presence of prenatal myonuclei was closely related to the status of histone acetylation, which could regulate the responsiveness of muscle fibers to physiological stimuli.

Satellite cell activation induced by aerobic muscle adaptation in response to endurance exercise in humans and rodents

Although the requirement of satellite cells activation and expansion following injury, mechanical load or growth stimulus provoked by resistance exercise has been well established, their function in response to aerobic exercise adaptation remains unclear. A clear relationship between satellite cell expansion in fiber-type specific myosin heavy chain and aerobic performance has been related, independent of myonuclear accretion or muscle growth. However, the trigger for this activation process is not fully understood yet and it seems to be a multi-faceted and well-orchestrated process. Emerging in vitro studies suggest a role for metabolic pathways and oxygen availability for satellite cell activation, modulating the self-renewal potential and cell fate control. The goal of this review is to describe and discuss the current knowledge about the satellite cell activation and expansion in response to aerobic exercise adaptation in human and rodent models. Additionally, findings about the in vitro metabolic control, which seems be involved in the satellite cell activation and cell fate control, are presented and discussed.

Skeletal Muscle Satellite Cells, Mitochondria, and MicroRNAs: Their Involvement in the Pathogenesis of ALS

Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND), is a fatal motor neuron disorder. It results in progressive degeneration and death of upper and lower motor neurons, protein aggregation, severe muscle atrophy and respiratory insufficiency. Median survival with ALS is between 2 and 5 years from the onset of symptoms. ALS manifests as either familial ALS (FALS) (~10% of cases) or sporadic ALS (SALS), (~90% of cases). Mutations in the copper/zinc (CuZn) superoxide dismutase (SOD1) gene account for ~20% of FALS cases and the mutant SOD1 mouse model has been used extensively to help understand the ALS pathology. As the precise mechanisms causing ALS are not well understood there is presently no cure. Recent evidence suggests that motor neuron degradation may involve a cell non-autonomous phenomenon involving numerous cell types within various tissues. Skeletal muscle is now considered as an important tissue involved in the pathogenesis of ALS by activating a retrograde signaling cascade that degrades motor neurons. Skeletal muscle heath and function are regulated by numerous factors including satellite cells, mitochondria and microRNAs. Studies demonstrate that in ALS these factors show various levels of dysregulation within the skeletal muscle. This review provides an overview of their dysregulation in various ALS models as well as how they may contribute individually and/or synergistically to the ALS pathogenesis.

Increased hypertrophic response with increased mechanical load in skeletal muscles receiving identical activity patterns

It is often assumed that mechanical factors are important for effects of exercise on muscle, but during voluntary training and most experimental conditions the effects could solely be attributed to differences in electrical activity, and direct evidence for a mechanosensory pathway has been scarce. We here show that, in rat muscles stimulated in vivo under deep anesthesia with identical electrical activity patterns, isometric contractions induced twofold more hypertrophy than contractions with 50-60% of the isometric force. The number of myonuclei and the RNA levels of myogenin and myogenic regulatory factor 4 were increased with high load, suggesting that activation of satellite cells is mechano dependent. On the other hand, training induced a major shift in fiber type distribution from type 2b to 2x that was load independent, indicating that the electrical signaling rather than mechanosignaling controls fiber type. RAC-α serine/threonine-protein kinase (Akt) and ribosomal protein S6 kinase β-1 (S6K1) were not significantly differentially activated by load, suggesting that the differences in mechanical factors were not important for activating the Akt/mammalian target of rapamycin/S6K1 pathway. The transmembrane molecule syndecan-4 implied in overload hypertrophy in cardiac muscle was not load dependent, suggesting that mechanosignaling in skeletal muscle is different.

Frequency of M-Cadherin-stained Satellite Cells Declines in Human Muscles During Aging

To answer the question of whether the satellite cell pool in human muscle is reduced during aging, we detected satellite cells in 30-μm-thick transverse sections under the confocal microscope by binding of M-cadherin antibody. The basal lamina was detected with laminin. Nuclei were stained with bisbenzimide or propidium iodide. Satellite cells were counted by applying the disector method and unbiased sampling design. To determine if there are age-related differences in muscle fiber types, morphometric characteristics of muscle fibers were examined on thin sections stained for myofibrillar ATPase. Autopsy samples of vastus lateralis muscle from six young (28.7 ± 2.3 years) and six old (70.8 ± 1.3 years) persons who had suffered sudden death were analyzed. Numbers of satellite cells per fiber length (Nsc/Lfib) and number of satellite cells per total number of nuclei (satellite cell nuclei + myonuclei) (Nsc/Nnucl) were significantly lower in the old group ( p<0.05). We demonstrate the importance of proper sampling and counting in estimation of sparsely distributed structures such as satellite cells. Our results support the hypothesis that the satellite cell fraction declines during aging.

Satellite Cells and Skeletal Muscle Regeneration

Skeletal muscles are essential for vital functions such as movement, postural support, breathing, and thermogenesis. Muscle tissue is largely composed of long, postmitotic multinucleated fibers. The life-long maintenance of muscle tissue is mediated by satellite cells, lying in close proximity to the muscle fibers. Muscle satellite cells are a heterogeneous population with a small subset of muscle stem cells, termed satellite stem cells. Under homeostatic conditions all satellite cells are poised for activation by stimuli such as physical trauma or growth signals. After activation, satellite stem cells undergo symmetric divisions to expand their number or asymmetric divisions to give rise to cohorts of committed satellite cells and thus progenitors. Myogenic progenitors proliferate, and eventually differentiate through fusion with each other or to damaged fibers to reconstitute fiber integrity and function. In the recent years, research has begun to unravel the intrinsic and extrinsic mechanisms controlling satellite cell behavior. Nonetheless, an understanding of the complex cellular and molecular interactions of satellite cells with their dynamic microenvironment remains a major challenge, especially in pathological conditions. The goal of this review is to comprehensively summarize the current knowledge on satellite cell characteristics, functions, and behavior in muscle regeneration and in pathological conditions.

Muscle Satellite Cells: Exploring the Basic Biology to Rule Them

Adult skeletal muscle is a postmitotic tissue with an enormous capacity to regenerate upon injury. This is accomplished by resident stem cells, named satellite cells, which were identified more than 50 years ago. Since their discovery, many researchers have been concentrating efforts to answer questions about their origin and role in muscle development, the way they contribute to muscle regeneration, and their potential to cell-based therapies. Satellite cells are maintained in a quiescent state and upon requirement are activated, proliferating, and fusing with other cells to form or repair myofibers. In addition, they are able to self-renew and replenish the stem pool. Every phase of satellite cell activity is highly regulated and orchestrated by many molecules and signaling pathways; the elucidation of players and mechanisms involved in satellite cell biology is of extreme importance, being the first step to expose the crucial points that could be modulated to extract the optimal response from these cells in therapeutic strategies. Here, we review the basic aspects about satellite cells biology and briefly discuss recent findings about therapeutic attempts, trying to raise questions about how basic biology could provide a solid scaffold to more successful use of these cells in clinics.

Evidence for caspase-dependent programmed cell death along with repair processes in affected skeletal muscle fibres in patients with mitochondrial disorders

Mitochondrial disorders are heterogeneous multisystemic disorders due to impaired oxidative phosphorylation causing defective mitochondrial energy production. Common histological hallmarks of mitochondrial disorders are RRFs (ragged red fibres), muscle fibres with abnormal focal accumulations of mitochondria. In contrast with the growing understanding of the genetic basis of mitochondrial disorders, the fate of phenotypically affected muscle fibres remains largely unknown. We investigated PCD (programmed cell death) in muscle of 17 patients with mitochondrial respiratory chain dysfunction. We documented that in affected muscle fibres, nuclear chromatin is condensed in lumpy irregular masses and cytochrome c is released into the cytosol to activate, along with Apaf-1 (apoptotic protease-activating factor 1), caspase 9 that, in turn, activates effector caspase 3, caspase 6, and caspase 7, suggesting the execution of the intrinsic apoptotic pathway. Whereas active caspase 3 underwent nuclear translocation, AIF (apoptosis-inducing factor) mainly stayed within mitochondria, into which an up-regulated Bax is relocated. The significant increase in caspase 2, caspase 3 and caspase 6 activity strongly suggest that the cell death programme is caspase-dependent and the activation of caspase 2 together with PUMA (p53 up-regulated modulator of apoptosis) up-regulation point to a role for oxidative stress in triggering the intrinsic pathway. Concurrently, in muscle of patients, the number of satellite cells was significantly increased and myonuclei were detected at different stages of myogenic differentiation, indicating that a reparative programme is ongoing in muscle of patients with mitochondrial disorders. Together, these data suggest that, in patients with mitochondrial disorders, affected muscle fibres are trapped in a mitochondria-regulated caspase-dependent PCD while repairing events take place.

Satellite cells in human skeletal muscle plasticity

Skeletal muscle satellite cells are considered to play a crucial role in muscle fiber maintenance, repair and remodeling. Our knowledge of the role of satellite cells in muscle fiber adaptation has traditionally relied on in vitro cell and in vivo animal models. Over the past decade, a genuine effort has been made to translate these results to humans under physiological conditions. Findings from in vivo human studies suggest that satellite cells play a key role in skeletal muscle fiber repair/remodeling in response to exercise. Mounting evidence indicates that aging has a profound impact on the regulation of satellite cells in human skeletal muscle. Yet, the precise role of satellite cells in the development of muscle fiber atrophy with age remains unresolved. This review seeks to integrate recent results from in vivo human studies on satellite cell function in muscle fiber repair/remodeling in the wider context of satellite cell biology whose literature is largely based on animal and cell models.

Impaired regeneration: A role for the muscle microenvironment in cancer cachexia

While changes in muscle protein synthesis and degradation have long been known to contribute to muscle wasting, a body of literature has arisen which suggests that regulation of the satellite cell and its ensuing regenerative program are impaired in atrophied muscle. Lessons learned from cancer cachexia suggest that this regulation is simply not a consequence, but a contributing factor to the wasting process. In addition to satellite cells, evidence from mouse models of cancer cachexia also suggests that non-satellite progenitor cells from the muscle microenvironment are also involved. This chapter in the series reviews the evidence of dysfunctional muscle repair in multiple wasting conditions. Potential mechanisms for this dysfunctional regeneration are discussed, particularly in the context of cancer cachexia.