Mac-1(low) early myeloid cells in the bone marrow-derived SP fraction migrate into injured skeletal muscle and participate in muscle regeneration

Muscle stem cells as immunomodulator during regeneration

The skeletal muscle is well known for its remarkable ability to regenerate after injuries. The regeneration is a complex and dynamic process that involves muscle stem cells (also called muscle satellite cells, MuSCs), fibro-adipogenic progenitors (FAPs), immune cells, and other muscle-resident cell populations. The MuSCs are the myogenic cell populaiton that contribute nuclei directly to the regenerated myofibers, while the other cell types collaboratively establish a microenvironment that facilitates myogenesis of MuSCs. The myogenic process includes activation, proliferation and differentiationof MuSCs, and subsequent fusion their descendent mononuclear myocytes into multinuclear myotubes. While the contributions of FAPs and immune cells to this microenvironment have been well studied, the influence of MuSCs on other cell types remains poorly understood. This review explores recent evidence supporting the potential role of MuSCs as immunomodulators during muscle regeneration, either through cytokine production or ligand-receptor interactions.

The Role of Supporting Cell Populations in Satellite Cell Mediated Muscle Repair

Skeletal muscle has a high capacity to repair and remodel in response to damage, largely through the action of resident muscle stem cells, termed satellite cells. Satellite cells are required for the proper repair of skeletal muscle through a process known as myogenesis. Recent investigations have observed relationships between satellite cells and other cell types and structures within the muscle microenvironment. These findings suggest that the crosstalk between inflammatory cells, fibrogenic cells, bone-marrow-derived cells, satellite cells, and the vasculature is essential for the restoration of muscle homeostasis. This review will discuss the influence of the cells and structures within the muscle microenvironment on satellite cell function and muscle repair.

Netrin-4 synthesized in satellite cell-derived myoblasts stimulates autonomous fusion

Satellite cells are indispensable for skeletal muscle regeneration and hypertrophy by forming nascent myofibers (myotubes). They synthesize multi-potent modulator netrins (secreted subtypes: netrin-1, -3, and -4), originally found as classical neural axon guidance molecules. While netrin-1 and -3 have key roles in myogenic differentiation, the physiological significance of netrin-4 is still unclear. This study examined whether netrin-4 regulates myofiber type commitment and myotube formation. Initially, the expression profiles indicated that satellite cells isolated from the extensor digitorum longus muscle (EDL muscle: fast-twitch myofiber-abundant) expressed slightly more netrin-4 than the soleus muscle (slow-type abundant) cells. As netrin-4 knockdown inhibited both slow- and fast-type myotube formation, netrin-4 may not directly regulate myofiber type commitment. However, netrin-4 knockdown in satellite cell-derived myoblasts reduced the myotube fusion index, while exogenous netrin-4 promoted myotube formation, even though netrin-4 expression level was maximum during the initiation stage of myogenic differentiation. Furthermore, netrin-4 knockdown also inhibited MyoD (a master transcriptional factor of myogenesis) and Myomixer (a myoblast fusogenic molecule) expression. These data suggest that satellite cells synthesize netrin-4 during myogenic differentiation initiation to promote their own fusion, stimulating the MyoD-Myomixer signaling axis.

Abundant Synthesis of Netrin-1 in Satellite Cell-Derived Myoblasts Isolated from EDL Rather Than Soleus Muscle Regulates Fast-Type Myotube Formation

Resident myogenic stem cells (satellite cells) are attracting attention for their novel roles in myofiber type regulation. In the myogenic differentiation phase, satellite cells from soleus muscle (slow fiber-abundant) synthesize and secrete higher levels of semaphorin 3A (Sema3A, a multifunctional modulator) than those derived from extensor digitorum longus (EDL; fast fiber-abundant), suggesting the role of Sema3A in forming slow-twitch myofibers. However, the regulatory mechanisms underlying fast-twitch myotube commitment remain unclear. Herein, we focused on netrin family members (netrin-1, -3, and -4) that compete with Sema3A in neurogenesis and osteogenesis. We examined whether netrins affect fast-twitch myotube generation by evaluating their expression in primary satellite cell cultures. Initially, netrins are upregulated during myogenic differentiation. Next, we compared the expression levels of netrins and their cell membrane receptors between soleus- and EDL-derived satellite cells; only netrin-1 showed higher expression in EDL-derived satellite cells than in soleus-derived satellite cells. We also performed netrin-1 knockdown experiments and additional experiments with recombinant netrin-1 in differentiated satellite cell-derived myoblasts. Netrin-1 knockdown in myoblasts substantially reduced fast-type myosin heavy chain (MyHC) expression; exogenous netrin-1 upregulated fast-type MyHC in satellite cells. Thus, netrin-1 synthesized in EDL-derived satellite cells may promote myofiber type commitment of fast muscles.

Sustained expression of HeyL is critical for the proliferation of muscle stem cells in overloaded muscle

In overloaded and regenerating muscle, the generation of new myonuclei depends on muscle satellite cells (MuSCs). Because MuSC behaviors in these two environments have not been considered separately, MuSC behaviors in overloaded muscle remain unexamined. Here, we show that most MuSCs in overloaded muscle, unlike MuSCs in regenerating muscle, proliferate in the absence of MyoD expression. Mechanistically, MuSCs in overloaded muscle sustain the expression of Heyl, a Notch effector gene, to suppress MyoD expression, which allows effective MuSC proliferation on myofibers and beneath the basal lamina. Although Heyl-knockout mice show no impairment in an injury model, in a hypertrophy model, their muscles harbor fewer new MuSC-derived myonuclei due to increased MyoD expression and diminished proliferation, which ultimately causes blunted hypertrophy. Our results show that sustained HeyL expression is critical for MuSC proliferation specifically in overloaded muscle, and thus indicate that the MuSC-proliferation mechanism differs in overloaded and regenerating muscle.

Muscle regeneration is disrupted by cancer cachexia without loss of muscle stem cell potential

Cancer cachexia is a severe, debilitating condition characterized by progressive body wasting associated with remarkable loss of skeletal muscle weight. It has been reported that cancer cachexia disturbs the regenerative ability of skeletal muscle, but the cellular mechanisms are still unknown. Here, we investigated the skeletal muscle regenerative process in mouse colon-26 (C26) tumor cell-bearing mice as a C26 cancer cachexia model. Although the proliferation and differentiation abilities of muscle stem cells derived from the C26 tumor cell-bearing mice were sustained in vitro, the proliferation and differentiation were severely impaired in the cachexic mice. The numbers of both macrophages and mesenchymal progenitors, which are critical players in muscle regeneration, were reduced in the cancer cachexic mice, indicating that the skeletal muscle regeneration process was disrupted by cancer cachexia. Furthermore, the number of infiltrated neutrophils was also reduced in cancer cachexia mice 24 hours after muscle injury, and the expression of critical chemokines for muscle regeneration was reduced in cancer cachexia model mice compared to control mice. Collectively, although the ability to regeneration of MuSCs was retained, cancer cachexia disturbed skeletal muscle regenerative ability by inhibiting the orchestrated muscle regeneration processes.

Surgical and immune reconstitution murine models in bone marrow research: Potential for exploring mechanisms in sepsis, trauma and allergy

Bone marrow, the vital organ which maintains lifelong hemopoiesis, currently receives considerable attention, as a source of multiple cell types which may play important roles in repair at distant sites. This emerging function, distinct from, but closely related to, bone marrow roles in innate immunity and inflammation, has been characterized through a number of strategies. However, the use of surgical models in this endeavour has hitherto been limited. Surgical strategies allow the experimenter to predetermine the site, timing, severity and invasiveness of injury; to add or remove aggravating factors (such as infection and defects in immunity) in controlled ways; and to manipulate the context of repair, including reconstitution with selected immune cell subpopulations. This endows surgical models overall with great potential for exploring bone marrow responses to injury, inflammation and infection, and its roles in repair and regeneration. We review three different murine surgical models, which variously combine trauma with infection, antigenic stimulation, or immune reconstitution, thereby illuminating different aspects of the bone marrow response to systemic injury in sepsis, trauma and allergy. They are: (1) cecal ligation and puncture, a versatile model of polymicrobial sepsis; (2) egg white implant, an intriguing model of eosinophilia induced by a combination of trauma and sensitization to insoluble allergen; and (3) ectopic lung tissue transplantation, which allows us to dissect afferent and efferent mechanisms leading to accumulation of hemopoietic cells in the lungs. These models highlight the gain in analytical power provided by the association of surgical and immunological strategies.

Transmembrane proteoglycans syndecan-2, 4, receptor candidates for the impact of HGF and FGF2 on semaphorin 3A expression in early-differentiated myoblasts

Regenerative mechanisms that regulate intramuscular motor innervation are thought to reside in the spatiotemporal expression of axon-guidance molecules. Our previous studies proposed an unexplored role of resident myogenic stem cell (satellite cell)-derived myoblasts as a key presenter of a secreted neural chemorepellent semaphorin 3A (Sema3A); hepatocyte growth factor (HGF) and basic fibroblast growth factor (FGF2) triggered its expression exclusively at the early differentiation phase. In order to advance this concept, the present study described that transmembrane heparan/chondroitin sulfate proteoglycans syndecan-2, 4 may be the plausible receptor candidates for HGF and FGF2 to signal Sema3A expression. Results showed that mRNA expression of syndecan-2, 4 was abundant (two magnitudes higher than syndecan-1, 3) in early-differentiated myoblasts and their in vitro knockdown diminished the HGF/FGF2-induced expression of Sema3A down to a baseline level. Pretreatment with heparitinase and chondroitinase ABC decreased the HGF and FGF2 responses, respectively, in non-knockdown cultures, supporting a possible model that HGF and FGF2 may bind to heparan and chondroitin sulfate chains of syndecan-2, 4 to signal Sema3A expression. The findings, therefore, extend our understanding that HGF/FGF2-syndecan-2, 4 association may stimulate a burst of Sema3A secretion by myoblasts recruited to the site of muscle injury; this would ensure a coordinated delay in the attachment of motoneuron terminals onto fibers early in muscle regeneration, and thus synchronize the recovery of muscle fiber integrity and the early resolution of inflammation after injury with reinnervation toward functional recovery.

Hemogenic endothelium generates mesoangioblasts that contribute to several mesodermal lineages in vivo

The embryonic endothelium is a known source of hematopoietic stem cells. Moreover, vessel-associated progenitors/stem cells with multilineage mesodermal differentiation potential, such as the ‘embryonic mesoangioblasts’, originate in vitro from the endothelium. Using a genetic lineage tracing approach, we show that early extra-embryonic endothelium generates, in a narrow time-window and prior to the hemogenic endothelium in the major embryonic arteries, hematopoietic cells that migrate to the embryo proper, and are subsequently found within the mesenchyme. A subpopulation of these cells, distinct from embryonic macrophages, co-expresses mesenchymal and hematopoietic markers. In addition, hemogenic endothelium-derived cells contribute to skeletal and smooth muscle, and to other mesodermal cells in vivo, and display features of embryonic mesoangioblasts in vitro. Therefore, we provide new insights on the distinctive characteristics of the extra-embryonic and embryonic hemogenic endothelium, and we identify the putative in vivo counterpart of embryonic mesoangioblasts, suggesting their identity and developmental ontogeny.

Satellite cells and the muscle stem cell niche

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

A murine model of volumetric muscle loss and a regenerative medicine approach for tissue replacement

Volumetric muscle loss (VML) resulting from traumatic accidents, tumor ablation, or degenerative disease is associated with limited treatment options and high morbidity. The lack of a reliable and reproducible animal model of VML has hindered the development of effective therapeutic strategies. The present study describes a critical-sized excisional defect within the mouse quadriceps muscle that results in an irrecoverable volumetric defect. This model of VML was used to evaluate the efficacy of a surgically placed inductive biologic scaffold material composed of porcine small intestinal submucosa-extracellular matrix (SIS-ECM). The targeted placement of an SIS-ECM scaffold within the defect was associated with constructive tissue remodeling including the formation of site-appropriate skeletal muscle tissue. The present study provides a reproducible animal model with which to study VML and shows the therapeutic potential of a bioscaffold-based regenerative medicine approach to VML.

Muscle side population cells from dystrophic or injured muscle adopt a fibro-adipogenic fate

Muscle side population (SP) cells are rare multipotent stem cells that can participate in myogenesis and muscle regeneration upon transplantation. While they have been primarily studied for the development of cell-based therapies for Duchenne muscular dystrophy, little is known regarding their non-muscle lineage choices or whether the dystrophic muscle environment affects their ability to repair muscle. Unfortunately, the study of muscle SP cells has been challenged by their low abundance and the absence of specific SP cell markers. To address these issues, we developed culture conditions for the propagation and spontaneous multi-lineage differentiation of muscle SP cells. Using this approach, we show that SP cells from wild type muscle robustly differentiate into satellite cells and form myotubes without requiring co-culture with myogenic cells. Furthermore, this myogenic activity is associated with SP cells negative for immune (CD45) and vascular (CD31) markers but positive for Pax7, Sca1, and the mesenchymal progenitor marker PDGFRα. Additionally, our studies revealed that SP cells isolated from dystrophic or cardiotoxin-injured muscle fail to undergo myogenesis. Instead, these SP cells rapidly expand giving rise to fibroblast and adipocyte progenitors (FAPs) and to their differentiated progeny, fibroblasts and adipocytes. Our findings indicate that muscle damage affects the lineage choices of muscle SP cells, promoting their differentiation along fibro-adipogenic lineages while inhibiting myogenesis. These results have implications for a possible role of muscle SP cells in fibrosis and fat deposition in muscular dystrophy. In addition, our studies provide a useful in vitro system to analyze SP cell biology in both normal and pathological conditions.

Hypoxia increases mouse satellite cell clone proliferation maintaining both in vitro and in vivo heterogeneity and myogenic potential

Satellite cells (SCs) are essential for postnatal muscle growth and regeneration, however, their expansion potential in vitro is limited. Recently, hypoxia has been used to enhance proliferative abilities in vitro of various primary cultures. Here, by isolating SCs from single mouse hindlimb skeletal myofibers, we were able to distinguish two subpopulations of clonally cultured SCs (Low Proliferative Clones - LPC - and High Proliferative Clones - HPC), which, as shown in rat skeletal muscle, were present at a fixed proportion. In addition, culturing LPC and HPC at a low level of oxygen we observed a two fold increased proliferation both for LPC and HPC. LPC showed higher myogenic regulatory factor (MRF) expression than HPC, particularly under the hypoxic condition. Notably, a different myogenic potential between LPC and HPC was retained in vivo: green fluorescent protein (GFP)+LPC transplantation in cardiotoxin-injured Tibialis Anterior led to a higher number of new GFP+muscle fibers per transplanted cell than GFP+HPC. Interestingly, the in vivo myogenic potential of a single cell from an LPC is similar if cultured both in normoxia and hypoxia. Therefore, starting from a single satellite cell, hypoxia allows a larger expansion of LPC than normal O₂ conditions, obtaining a consistent amount of cells for transplantation, but maintaining their myogenic regeneration potential.

Unloading stress disturbs muscle regeneration through perturbed recruitment and function of macrophages

Skeletal muscle is one of the most sensitive tissues to mechanical loading, and unloading inhibits the regeneration potential of skeletal muscle after injury. This study was designed to elucidate the specific effects of unloading stress on the function of immunocytes during muscle regeneration after injury. We examined immunocyte infiltration and muscle regeneration in cardiotoxin (CTX)-injected soleus muscles of tail-suspended (TS) mice. In CTX-injected TS mice, the cross-sectional area of regenerating myofibers was smaller than that of weight-bearing (WB) mice, indicating that unloading delays muscle regeneration following CTX-induced skeletal muscle damage. Delayed infiltration of macrophages into the injured skeletal muscle was observed in CTX-injected TS mice. Neutrophils and macrophages in CTX-injected TS muscle were presented over a longer period at the injury sites compared with those in CTX-injected WB muscle. Disturbance of activation and differentiation of satellite cells was also observed in CTX-injected TS mice. Further analysis showed that the macrophages in soleus muscles were mainly Ly-6C-positive proinflammatory macrophages, with high expression of tumor necrosis factor-α and interleukin-1β, indicating that unloading causes preferential accumulation and persistence of proinflammatory macrophages in the injured muscle. The phagocytic and myotube formation properties of macrophages from CTX-injected TS skeletal muscle were suppressed compared with those from CTX-injected WB skeletal muscle. We concluded that the disturbed muscle regeneration under unloading is due to impaired macrophage function, inhibition of satellite cell activation, and their cooperation.

Barx homeobox family in muscle development and regeneration

Homeobox transcription factors are key intrinsic regulators of myogenesis. In studies spanning several years, we have characterized the homeobox factor Barx2 as a novel marker for muscle progenitor cells and an important regulator of muscle growth and repair. In this review, we place the expression and function of Barx2 and its paralogue Barx1 in context with other muscle-expressed homeobox factors in both embryonic and adult myogenesis. We also describe the structure and regulation of Barx genes and possible gene/disease associations. The functional domains of Barx proteins, their molecular interactions, and cellular functions are presented with particular emphasis on control of genes and processes involved in myogenic differentiation. Finally, we describe the patterns of Barx gene expression in vivo and the phenotypes of various Barx gene perturbation models including null mice. We focus on the Barx2 null mouse model, which has demonstrated the critical roles of Barx2 in postnatal myogenesis including muscle maintenance during aging, and regeneration of acute and chronic muscle injury.

Skeletal Muscle-derived Hematopoietic Stem Cells: Muscular Dystrophy Therapy by Bone Marrow Transplantation

For postnatal growth and regeneration of skeletal muscle, satellite cells, a self-renewing pool of muscle stem cells, give rise to daughter myogenic precursor cells that contribute to the formation of new muscle fibers. In addition to this key myogenic cell class, adult skeletal muscle also contains hematopoietic stem cell and progenitor cell populations which can be purified as a side population (SP) fraction or as a hematopoietic marker CD45-positive cell population. These muscle-derived hematopoietic stem/progenitor cell populations are surprisingly capable of differentiation into hematopoietic cells both after transplantation into irradiated mice and during in vitro colony formation assay. Therefore, these muscle-derived hematopoietic stem/progenitor cells appear to have characteristics similar to classical hematopoietic stem/progenitor cells found in bone marrow. This review outlines recent findings regarding hematopoietic stem/progenitor cell populations residing in adult skeletal muscle and discusses their myogenic potential along with their role in the stem cell niche and related cell therapies for approaching treatment of Duchenne muscular dystrophy.

Abcg2 labels multiple cell types in skeletal muscle and participates in muscle regeneration

Abcg2-expressing cells proliferate after muscle injury and are required for effective regeneration of multiple muscle cell lineages.

Skeletal muscle contains progenitor cells (satellite cells) that maintain and repair muscle. It also contains muscle side population (SP) cells, which express Abcg2 and may participate in muscle regeneration or may represent a source of satellite cell replenishment. In Abcg2-null mice, the SP fraction is lost in skeletal muscle, although the significance of this loss was previously unknown. We show that cells expressing Abcg2 increased upon injury and that muscle regeneration was impaired in Abcg2-null mice, resulting in fewer centrally nucleated myofibers, reduced myofiber size, and fewer satellite cells. Additionally, using genetic lineage tracing, we demonstrate that the progeny of Abcg2-expressing cells contributed to multiple cell types within the muscle interstitium, primarily endothelial cells. After injury, Abcg2 progeny made a minor contribution to regenerated myofibers. Furthermore, Abcg2-labeled cells increased significantly upon injury and appeared to traffic to muscle from peripheral blood. Together, these data suggest an important role for Abcg2 in positively regulating skeletal muscle regeneration.

Surgical Therapy of End-Stage Heart Failure: Understanding Cell-Mediated Mechanisms Interacting with Myocardial Damage

Worldwide, cardiovascular disease results in an estimated 14.3 million deaths per year, giving rise to an increased demand for alternative and advanced treatment. Current approaches include medical management, cardiac transplantation, device therapy, and, most recently, stem cell therapy. Research into cell-based therapies has shown this option to be a promising alternative to the conventional methods. In contrast to early trials, modern approaches now attempt to isolate specific stem cells, as well as increase their numbers by means of amplifying in a culture environment. The method of delivery has also been improved to minimize the risk of micro-infarcts and embolization, which were often observed after the use of coronary catheterization. The latest approach entails direct, surgical, transepicardial injection of the stem cell mixture, as well as the use of tissue-engineered meshes consisting of embedded progenitor cells.

Generation of a monoclonal antibody reactive to prefusion myocytes

We established a novel monoclonal antibody, Yaksa that is specific to a subpopulation of myogenic cells. The Yaksa antigen is not expressed on the surface of growing myoblasts but only on a subpopulation of myogenin-positive myocytes. When Yaksa antigen-positive mononucleated cells were freshly prepared from a murine myogenic cell by a cell sorter, they fused with each other and formed multinucleated myotubes shortly after replating while Yaksa antigen-negative cells scarcely generated myotubes. These results suggest that Yaksa could segregate fusion-competent, mononucleated cells from fusion–incompetent cells during muscle differentiation. The Yaksa antigen was also expressed in developing muscle and regenerating muscle in vivo and it was localized at sites of cell–cell contact between mono-nucleated muscle cells and between mono-nucleated muscle cells and myotubes. Thus, Yaksa that marks prefusion myocytes before myotube formation can be a useful tool to elucidate the cellular and molecular mechanisms of myogenic cell fusion.

Non-proteolytic functions of calpain-3 in sarcoplasmic reticulum in skeletal muscles

Mutations in CAPN3/Capn3, which codes for skeletal muscle-specific calpain-3/p94 protease, are responsible for limb-girdle muscular dystrophy type 2A. Using "knock-in" (referred to as Capn3(CS/CS)) mice, in which the endogenous calpain-3 is replaced with a mutant calpain-3:C129S, which is a proteolytically inactive but structurally intact calpain-3, we demonstrated in our previous studies that loss of calpain-3 protease activity causes muscular dystrophy [Ojima, K. et al. (2010) J. Clin. Invest. 120, 2672-2683]. However, compared to Capn3-null (Capn3(-/-)) mice, Capn3(CS/CS) mice showed less severe dystrophic symptoms. This suggests that calpain-3 also has a non-proteolytic function. This study aimed to elucidate the non-proteolytic functions of calpain-3 through comparison of Capn3(CS/CS) mice with Capn3(-/-) mice. We found that calpain-3 is a component of the sarcoplasmic reticulum (SR), and that calpain-3 interacts with, but does not proteolyze, typical SR components such as ryanodine receptor and calsequestrin. Furthermore, Capn3(CS/CS) mice showed that the nonenzymatic role of calpain-3 is required for proper Ca(2+) efflux from the SR to cytosol during muscle contraction. These results indicate that calpain-3 functions as a nonenzymatic element for the Ca(2+) efflux machinery in the SR, rather than as a protease. Thus, defects in the nonenzymatic function of calpain-3 must also be involved in the pathogenesis of limb-girdle muscular dystrophy type 2A.

Dynamic distribution of muscle-specific calpain in mice has a key role in physical-stress adaptation and is impaired in muscular dystrophy

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a genetic disease that is caused by mutations in the calpain 3 gene (CAPN3), which encodes the skeletal muscle-specific calpain, calpain 3 (also known as p94). However, the precise mechanism by which p94 functions in the pathogenesis of this disease remains unclear. Here, using p94 knockin mice (termed herein p94KI mice) in which endogenous p94 was replaced with a proteolytically inactive but structurally intact p94:C129S mutant protein, we have demonstrated that stretch-dependent p94 distribution in sarcomeres plays a crucial role in the pathogenesis of LGMD2A. The p94KI mice developed a progressive muscular dystrophy, which was exacerbated by exercise. The exercise-induced muscle degeneration in p94KI mice was associated with an inefficient redistribution of p94:C129S in stretched sarcomeres. Furthermore, the p94KI mice showed impaired adaptation to physical stress, which was accompanied by compromised upregulation of muscle ankyrin-repeat protein-2 and hsp upon exercise. These findings indicate that the stretch-induced dynamic redistribution of p94 is dependent on its protease activity and essential to protect muscle from degeneration, particularly under conditions of physical stress. Furthermore, our data provide direct evidence that loss of p94 protease activity can result in LGMD2A and molecular insight into how this could occur.

Bone marrow-derived hematopoietic cells undergo myogenic differentiation following a Pax-7 independent pathway

Several reports showed that hematopoietic stem cells (HSCs) participate in muscle regeneration, raising hope for their therapeutic potential for degenerative muscle diseases. However, proof that HSCs are able to reprogram their fate and enter a myogenic pathway, remains elusive. We demonstrate that murine bone marrow (BM)-derived hematopoietic cells, carrying reporter genes controlled by muscle-specific regulatory elements from the Myf5, myosin light chain (MLC3F), or MCK genes, are induced by myoblasts to activate muscle-specific genes. This potential resides in the more undifferentiated progenitors, expressing surface markers typical of HSCs. Comparative gene expression profiling of CD45(+)/Sca1(+) cells isolated from muscle or BM shows that hematopoietic cells participate to muscle regeneration, by undergoing a profound although incomplete myogenic reprogramming on interaction with the muscle microenviroment. These cells undergo specification and differentiation independently from Pax7 and MyoD, and lack Pax7-associated properties, such as self-renewal and proliferation, distinguishing from satellite cells. Our findings indicate that hematopoietic cells, on seeding in the muscle, become a distinct cell population endowed with myogenic potential.

The origin, molecular regulation and therapeutic potential of myogenic stem cell populations

Satellite cells, originating in the embryonic dermamyotome, reside beneath the myofibre of mature adult skeletal muscle and constitute the tissue-specific stem cell population. Recent advances following the identification of markers for these cells (including Pax7, Myf5, c-Met and CD34) (CD, cluster of differentiation; c-Met, mesenchymal epithelial transition factor) have led to a greater understanding of the role played by satellite cells in the regeneration of new skeletal muscle during growth and following injury. In response to muscle damage, satellite cells harbour the ability both to form myogenic precursors and to self-renew to repopulate the stem cell niche following myofibre damage. More recently, other stem cell populations including bone marrow stem cells, skeletal muscle side population cells and mesoangioblasts have also been shown to have myogenic potential in culture, and to be able to form skeletal muscle myofibres in vivo and engraft into the satellite cell niche. These cell types, along with satellite cells, have shown potential when used as a therapy for skeletal muscle wasting disorders where the intrinsic stem cell population is genetically unable to repair non-functioning muscle tissue. Accurate understanding of the mechanisms controlling satellite cell lineage progression and self-renewal as well as the recruitment of other stem cell types towards the myogenic lineage is crucial if we are to exploit the power of these cells in combating myopathic conditions. Here we highlight the origin, molecular regulation and therapeutic potential of all the major cell types capable of undergoing myogenic differentiation and discuss their potential therapeutic application.

Muscle CD31(-) CD45(-) side population cells promote muscle regeneration by stimulating proliferation and migration of myoblasts

CD31(-) CD45(-) side population (SP) cells are a minor SP subfraction that have mesenchymal stem cell-like properties in uninjured skeletal muscle but that can expand on muscle injury. To clarify the role of these SP cells in muscle regeneration, we injected green fluorescent protein (GFP)-positive myoblasts with or without CD31(-) CD45(-) SP cells into the tibialis anterior muscles of immunodeficient NOD/scid mice or dystrophin-deficient mdx mice. More GFP-positive fibers were formed after co-transplantation than after transplantation of GFP-positive myoblasts alone in both mdx and NOD/scid muscles. Moreover, grafted myoblasts were more widely distributed after co-transplantation than after transplantation of myoblasts alone. Immunohistochemistry with anti-phosphorylated histone H3 antibody revealed that CD31(-) CD45(-) SP cells stimulated cell division of co-grafted myoblasts. Genome-wide gene expression analyses showed that these SP cells specifically express a variety of extracellular matrix proteins, membrane proteins, and cytokines. We also found that they express high levels of matrix metalloproteinase-2 mRNA and gelatinase activity. Furthermore, matrix metalloproteinase-2 derived from CD31(-) CD45(-) SP cells promoted migration of myoblasts in vivo. Our results suggest that CD31(-) CD45(-) SP cells support muscle regeneration by promoting proliferation and migration of myoblasts. Future studies to further define the molecular and cellular mechanisms of muscle regeneration will aid in the development of cell therapies for muscular dystrophy.

Molecular regulation of myogenic progenitor populations

Skeletal muscle regeneration and adaptation to exercise require the actions of muscle satellite cells. Muscle satellite cells are thought to play an integral role in the process of exercise adaptation, but have also been shown to possess the capacity to fully regenerate muscle tissue following destructive muscle injury. We now know that molecular regulation of satellite cells involves the coordinated actions of a series of transcriptional networks that leads to myogenic commitment, cell-cycle entry, proliferation, and terminal differentiation. Additionally, Pax7 is a paired-box transcription factor that has been identified as playing a critical role in satellite cell regulation. It remains debatable, however, whether Pax7 is required for the specification of satellite cells and (or) whether it is playing a vital role in self-renewal and maintenance of the satellite cell population. In recent years, the emergence of atypical myogenic progenitor populations has added a new dimension to muscle repair, and significant interest has been focused on identifying populations such as bone-marrow-derived stem cells that have the ability to contribute to muscle. Interestingly, elucidating the molecular regulation of myogenic progenitor populations has involved animal models of muscle regeneration, with questionable relevance for human muscle adaptation to exercise. This paper highlights the current state of knowledge on the molecular regulation of satellite cells, explores the potential contribution of atypical myogenic progenitors, and discusses the information gathered from animal regeneration models in terms of its relevance to the process of exercise adaptation.

Molecular signature of quiescent satellite cells in adult skeletal muscle

Skeletal muscle satellite cells play key roles in postnatal muscle growth and regeneration. To study molecular regulation of satellite cells, we directly prepared satellite cells from 8- to 12-week-old C57BL/6 mice and performed genome-wide gene expression analysis. Compared with activated/cycling satellite cells, 507 genes were highly upregulated in quiescent satellite cells. These included negative regulators of cell cycle and myogenic inhibitors. Gene set enrichment analysis revealed that quiescent satellite cells preferentially express the genes involved in cell-cell adhesion, regulation of cell growth, formation of extracellular matrix, copper and iron homeostasis, and lipid transportation. Furthermore, reverse transcription-polymerase chain reaction on differentially expressed genes confirmed that calcitonin receptor (CTR) was exclusively expressed in dormant satellite cells but not in activated satellite cells. In addition, CTR mRNA is hardly detected in nonmyogenic cells. Therefore, we next examined the expression of CTR in vivo. CTR was specifically expressed on quiescent satellite cells, but the expression was not found on activated/proliferating satellite cells during muscle regeneration. CTR-positive cells reappeared at the rim of regenerating myofibers in later stages of muscle regeneration. Calcitonin stimulation delayed the activation of quiescent satellite cells. Our data provide roles of CTR in quiescent satellite cells and a solid scaffold to further dissect molecular regulation of satellite cells. Disclosure of potential conflicts of interest is found at the end of this article.

Myogenic stage, sarcomere length, and protease activity modulate localization of muscle-specific calpain

p94/calpain 3 is a Ca(2+)-binding intracellular protease predominantly expressed in skeletal muscles. p94 binds to the N2A and M-line regions of connectin/titin and localizes in the Z-bands. Genetic evidence showing that compromised p94 proteolytic activity leads to muscular dystrophy (limb-girdle muscular dystrophy type 2A) indicates the importance of p94 function in myofibrils. Here we show that a series of p94 splice variants is expressed immediately after muscle differentiation and differentially change localization during myofibrillogenesis. We found that the endogenous N-terminal (but not C-terminal) domain of p94 was not only localized in the Z-bands but also directly bound to sarcomeric alpha-actinin. These data suggest the incorporation of proteolytic N-terminal fragments of p94 into the Z-bands. In myofibrils localization of exogenously expressed p94 shifted from the M-line to N2A as the sarcomere lengthens beyond approximately 2.6 and 2.8 microm for wild-type and proteaseinactive p94, respectively. These data demonstrate for the first time that p94 proteolytic activity is involved in responses to muscle conditions, which may explain why p94 inactivation causes limb-girdle muscular dystrophy.

Differentiation of a hepatic phenotype after heterotropic transplantation of heart, kidney, brain, and skin tissues into liver in F344 rats

While organ-specific stem cells with roles in tissue injury repair have been documented, their pathogenic significance in diseases and the factors potentially responsible for their activation remain largely unclear. In the present study, heart, kidney, brain, and skin samples from F344 transgenic rats carrying the GFP gene were transplanted into normal F344 rat liver one day after an intraperitoneal injection (i.p.) of carbon tetrachloride (CCl(4)) to test their differentiation capacity. The transplantation was carried out by female donors to male recipients, and vice versa. One week after transplantation, GFP antigen-positive cells with phenotypic characteristics of hepatocytes were noted. After two weeks, their extent increased, and at 4 weeks, large areas of strongly GFP-stained cells developed. All recipient livers had GFP antigen-positive hepatocyte cells. PCR analysis coupled with laser capture micro-dissection (LCM) revealed those cells to contain GFP DNA. Thus, our results indicate that tissue stem cells have multipotential ability, differentiating into hepatocytes when transplanted into an injured liver.

Regenerative capacity of skeletal muscle

PURPOSE OF REVIEW

The aim of this review is to highlight advances in the field of skeletal muscle regeneration that have been made in the last year.

RECENT FINDINGS

Studies have increased our understanding of the activation of satellite cells within their niche on the muscle fibre, the contribution of satellite cell-derived muscle precursor cells to skeletal muscle regeneration and the reduction of satellite cell function in old muscle. Although other stem cells, either bone marrow derived or present within skeletal muscle or other tissues, do contribute to muscle regeneration, recent studies have highlighted that this is at best minimal compared with the ability of satellite cells to regenerate skeletal muscle. The effect of the host muscle environment has been shown to have a profound effect on skeletal muscle regeneration. Age and denervation have a detrimental effect and certain types of muscle injury a positive effect. Work continues on the effect of growth factors on muscle cell lines in vitro and muscle regeneration in vivo.

SUMMARY

Recent work has focused on the contribution of satellite-cell derived muscle precursor cells and other stem cells to skeletal muscle regeneration. The muscle environment has a profound effect on the regenerative capacity of resident and implanted cells. Muscle regeneration may be optimized by using the best stem cell population and by modifying the host muscle environment.

Functional heterogeneity of side population cells in skeletal muscle

Skeletal muscle regeneration has been exclusively attributed to myogenic precursors, satellite cells. A stem cell-rich fraction referred to as side population (SP) cells also resides in skeletal muscle, but its roles in muscle regeneration remain unclear. We found that muscle SP cells could be subdivided into three sub-fractions using CD31 and CD45 markers. The majority of SP cells in normal non-regenerating muscle expressed CD31 and had endothelial characteristics. However, CD31(-)CD45(-) SP cells, which are a minor subpopulation in normal muscle, actively proliferated upon muscle injury and expressed not only several regulatory genes for muscle regeneration but also some mesenchymal lineage markers. CD31(-)CD45(-) SP cells showed the greatest myogenic potential among three SP sub-fractions, but indeed revealed mesenchymal potentials in vitro. These SP cells preferentially differentiated into myofibers after intramuscular transplantation in vivo. Our results revealed the heterogeneity of muscle SP cells and suggest that CD31(-)CD45(-) SP cells participate in muscle regeneration.

Possible functions of p94 in connectin-mediated signaling pathways in skeletal muscle cells

Calpains are intracellular Ca2+ -requiring 'modulator proteases', which modulate cellular functions by limited and specific proteolysis. p94/calpain3, a skeletal-muscle specific calpain, has been one of the representative calpain species which indicates physiological importance of calpain proteolytic system; a defect of proteolytic activity of p94 causes limb girdle muscular dystrophy type2A (LGMD2A, also called 'calpainopathy'). Immunohistochemical studies on myofibrils showed that p94 localizes at the Z- and N2-line regions of sarcomeres. It was also identified by the yeast two hybrid studies that p94 binds to the N2A and M-line regions of connectin. Furthermore, genetic studies indicate that p94 is indispensable for skeletal muscles, although its precise functions are still unclear. Interestingly, connectin provides sarcomere not only with elasticity but also with binding sites to various multi-functional proteins such as muscle ankyrin repeat proteins (MARPs), muscle RING finger proteins (MURFs), titin-capping protein (T-cap/telethonin), sarcomeric-alpha-actinin, p94 etc. Binding sites for these proteins are not randomly placed along connectin but rather accumulated in the Z-, N2-, and/or M-line regions, indicating the existence of 'signal complexes' unique to each regions. The concept of these complexes are strongly supported by the facts that mutations of connectin or its binding proteins in these regions severely perturb muscle functions, as in the case of LGMD2A caused by mutations in the p94 gene. Therefore, it is hypothesized that the 'signal complexes' in the Z-, N2-, and M-lines modulate muscle cell homeostasis by transducing signals of external stimulations/stresses to trigger appropriate response at various different cellular events such as protein modification and gene expressions. In this article, we performed detailed immunohistochemical analyses of p94 on isolated single myofibers. Together with recent findings about p94, it is suggested that sarcomeric localization of p94, especially its M-line localization, is affected by the combination of cellular contexts such as contractile status of myofibrils, fiber type compositions, sarcomeric maturation, and the composition of the 'signal complexes' in each region.

IGF-I increases bone marrow contribution to adult skeletal muscle and enhances the fusion of myelomonocytic precursors

Muscle damage has been shown to enhance the contribution of bone marrow-derived cells (BMDCs) to regenerating skeletal muscle. One responsible cell type involved in this process is a hematopoietic stem cell derivative, the myelomonocytic precursor (MMC). However, the molecular components responsible for this injury-related response remain largely unknown. In this paper, we show that delivery of insulin-like growth factor I (IGF-I) to adult skeletal muscle by three different methods-plasmid electroporation, injection of genetically engineered myoblasts, and recombinant protein injection-increases the integration of BMDCs up to fourfold. To investigate the underlying mechanism, we developed an in vitro fusion assay in which co-cultures of MMCs and myotubes were exposed to IGF-I. The number of fusion events was substantially augmented by IGF-I, independent of its effect on cell survival. These results provide novel evidence that a single factor, IGF-I, is sufficient to enhance the fusion of bone marrow derivatives with adult skeletal muscle.

Advancing cancer biotherapy with proteomics

Proteomics is becoming increasingly important for cancer biotherapy. The development of high-throughput platforms now allows the analysis of multiple proteins from small quantities of material. While these techniques are being used to discover new biomarkers, they are particularly important for assessing complex biologic processes such as immunotherapy for cancer. Recent advances in this field are reviewed, as well as the use of proteomics to assess the effectiveness and toxicities of high-dose IL-2 cancer therapy. Proteomics is becoming useful in assessing cancer biotherapies and in unraveling their mechanisms of action. High-throughput proteomic technologies have now advanced to a stage where they have the potential to become effective discovery tools for biomarkers/predictors of disease, disease recurrence, and response to therapy.

Participation of bone marrow-derived cells in fibrotic changes in denervated skeletal muscle

In denervated skeletal muscle, mononuclear interstitial cells accumulate in the perisynaptic regions before fibrotic change occurs. These cells are currently considered to be fibroblasts that originate from muscle tissue. However, when we denervated hind limbs of GFP-bone marrow chimeric mice by excising the sciatic nerve unilaterally, many bone marrow-derived cells (BM-DCs) infiltrated the interstitial spaces and accumulated in the perisynaptic regions, peaking 14 days after denervation. They accounted for nearly one-half of the increase in mononuclear interstitial cells. Although BM-DCs did not incorporate into satellite cells, immunohistochemical and FACS analyses revealed that BM-DCs were both CD45 and CD11b positive, indicating that they were of macrophage/monocyte lineage. BrdU staining showed inactive proliferation of BM-DCs. Reverse transcriptase-polymerase chain reaction of mononuclear cells isolated by FACS revealed that BM-DCs did not express type I collagen or tenascin-C; however, they did express transforming growth factor-beta1, suggesting that they regulate the fibrotic process. In contrast, muscle tissue-derived interstitial cells expressed type I collagen and tenascin-C, suggesting that these populations were the final effectors of fibrosis. These findings identify elementary targets that may regulate the migration, homing, differentiation, and function of BM-DCs, leading to amelioration of the excessive fibrosis of denervated skeletal muscle.

Muscle stem cells and regenerative myogenesis

A population of myogenic progenitors termed satellite cells undertakes postnatal development and repair of skeletal muscle. Studies have indicated that atypical myogenic precursors can also participate in muscle regeneration. The source of this regenerative capacity has been attributed to "adult stem cells" that represent poorly understood multipotent cell lineages, believed to reside in all adult tissue populations. Here we review the origin and location of muscle satellite cells and stem cells, as well as the mechanisms by which they may be specified. We discuss how the experimental models utilized raise important questions regarding the validity of extrapolating these findings.