Biomaterial and stem cell-based strategies for skeletal muscle regeneration

Adult skeletal muscle can regenerate effectively after mild physical or chemical insult. Muscle trauma or disease can overwhelm this innate capacity for regeneration and result in heightened inflammation and fibrotic tissue deposition resulting in loss of structure and function. Recent studies have focused on biomaterial and stem cell-based therapies to promote skeletal muscle regeneration following injury and disease. Many stem cell populations besides satellite cells are implicated in muscle regeneration. These stem cells include but are not limited to mesenchymal stem cells, adipose-derived stem cells, hematopoietic stem cells, pericytes, fibroadipogenic progenitors, side population cells, and CD133⁺ stem cells. However, several challenges associated with their isolation, availability, delivery, survival, engraftment, and differentiation have been reported in recent studies. While acellular scaffolds offer a relatively safe and potentially off-the-shelf solution to cell-based therapies, they are often unable to stimulate host cell migration and activity to a level that would result in clinically meaningful regeneration of traumatized muscle. Combining stem cells and biomaterials may offer a viable therapeutic strategy that may overcome the limitations associated with these therapies when they are used in isolation. In this article, we review the stem cell populations that can stimulate muscle regeneration in vitro and in vivo. We also discuss the regenerative potential of combination therapies that utilize both stem cell and biomaterials for the treatment of skeletal muscle injury and disease. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1246-1262, 2019.

Skeletal Muscle Regenerative Potential of Human MuStem Cells following Transplantation into Injured Mice Muscle

After intra-arterial delivery in the dystrophic dog, allogeneic muscle-derived stem cells, termed MuStem cells, contribute to long-term stabilization of the clinical status and preservation of the muscle regenerative process. However, it remains unknown whether the human counterpart could be identified, considering recent demonstrations of divergent features between species for several somatic stem cells. Here, we report that MuStem cells reside in human skeletal muscle and display a long-term ability to proliferate, allowing generation of a clinically relevant amount of cells. Cultured human MuStem (hMuStem) cells do not express hematopoietic, endothelial, or myo-endothelial cell markers and reproducibly correspond to a population of early myogenic-committed progenitors with a perivascular/mesenchymal phenotypic signature, revealing a blood vessel wall origin. Importantly, they exhibit both myogenesis in vitro and skeletal muscle regeneration after intramuscular delivery into immunodeficient host mice. Together, our findings provide new insights supporting the notion that hMuStem cells could represent an interesting therapeutic candidate for dystrophic patients.

In vivo myomaker-mediated heterologous fusion and nuclear reprogramming

Knowledge regarding cellular fusion and nuclear reprogramming may aid in cell therapy strategies for skeletal muscle diseases. An issue with cell therapy approaches to restore dystrophin expression in muscular dystrophy is obtaining a sufficient quantity of cells that normally fuse with muscle. Here we conferred fusogenic activity without transdifferentiation to multiple non-muscle cell types and tested dystrophin restoration in mouse models of muscular dystrophy. We previously demonstrated that myomaker, a skeletal muscle-specific transmembrane protein necessary for myoblast fusion, is sufficient to fuse 10T 1/2 fibroblasts to myoblasts in vitro. Whether myomaker-mediated heterologous fusion is functional in vivo and whether the newly introduced nonmuscle nuclei undergoes nuclear reprogramming has not been investigated. We showed that mesenchymal stromal cells, cortical bone stem cells, and tail-tip fibroblasts fuse to skeletal muscle when they express myomaker. These cells restored dystrophin expression in a fraction of dystrophin-deficient myotubes after fusion in vitro. However, dystrophin restoration was not detected in vivo although nuclear reprogramming of the muscle-specific myosin light chain promoter did occur. Despite the lack of detectable dystrophin reprogramming by immunostaining, this study indicated that myomaker could be used in nonmuscle cells to induce fusion with muscle in vivo, thereby providing a platform to deliver therapeutic material.-Mitani, Y., Vagnozzi, R. J., Millay, D. P. In vivo myomaker-mediated heterologous fusion and nuclear reprogramming.

Identification and Characterization of the Dermal Panniculus Carnosus Muscle Stem Cells

The dermal Panniculus carnosus (PC) muscle is important for wound contraction in lower mammals and represents an interesting model of muscle regeneration due to its high cell turnover. The resident satellite cells (the bona fide muscle stem cells) remain poorly characterized. Here we analyzed PC satellite cells with regard to developmental origin and purported function. Lineage tracing shows that they originate in Myf5⁺, Pax3/Pax7⁺ cell populations. Skin and muscle wounding increased PC myofiber turnover, with the satellite cell progeny being involved in muscle regeneration but with no detectable contribution to the wound-bed myofibroblasts. Since hematopoietic stem cells fuse to PC myofibers in the absence of injury, we also studied the contribution of bone marrow-derived cells to the PC satellite cell compartment, demonstrating that cells of donor origin are capable of repopulating the PC muscle stem cell niche after irradiation and bone marrow transplantation but may not fully acquire the relevant myogenic commitment.

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PC satellite cells originate from Myf5⁺, Pax3/Pax7⁺ cell lineages

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Skin and muscle wounding increase PC myofiber turnover

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Donor bone marrow cells repopulate the PC satellite niche after BMT

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Dermis-derived myogenesis originates from the PC satellite cell population

Where do we stand in trial readiness for autosomal recessive limb girdle muscular dystrophies?

Autosomal recessive limb girdle muscular dystrophies (LGMD2) are a group of genetically heterogeneous diseases that are typically characterised by progressive weakness and wasting of the shoulder and pelvic girdle muscles. Many of the more than 20 different conditions show overlapping clinical features with other forms of muscular dystrophy, congenital, myofibrillar or even distal myopathies and also with acquired muscle diseases. Although individually extremely rare, all types of LGMD2 together form an important differential diagnostic group among neuromuscular diseases. Despite improved diagnostics and pathomechanistic insight, a curative therapy is currently lacking for any of these diseases. Medical care consists of the symptomatic treatment of complications, aiming to improve life expectancy and quality of life. Besides well characterised pre-clinical tools like animal models and cell culture assays, the determinants of successful drug development programmes for rare diseases include a good understanding of the phenotype and natural history of the disease, the existence of clinically relevant outcome measures, guidance on care standards, up to date patient registries, and, ideally, biomarkers that can help assess disease severity or drug response. Strong patient organisations driving research and successful partnerships between academia, advocacy, industry and regulatory authorities can also help accelerate the elaboration of clinical trials. All these determinants constitute aspects of translational research efforts and influence patient access to therapies. Here we review the current status of determinants of successful drug development programmes for LGMD2, and the challenges of translating promising therapeutic strategies into effective and accessible treatments for patients.

Mesoangioblast delivery of miniagrin ameliorates murine model of merosin-deficient congenital muscular dystrophy type 1A

Background

Merosin-deficient congenital muscular dystrophy type-1A (MDC1A) is characterized by progressive muscular dystrophy and dysmyelinating neuropathy caused by mutations of the α2 chain of laminin-211, the predominant laminin isoform of muscles and nerves. MDC1A has no available treatment so far, although preclinical studies showed amelioration of the disease by the overexpression of miniagrin (MAG). MAG reconnects orphan laminin-211 receptors to other laminin isoforms available in the extracellular matrix of MDC1A mice.

Methods

Mesoangioblasts (MABs) are vessel-associated progenitors that can form the skeletal muscle and have been shown to restore defective protein levels and motor skills in animal models of muscular dystrophies. As gene therapy in humans still presents challenging technical issues and limitations, we engineered MABs to overexpress MAG to treat MDC1A mouse models, thus combining cell to gene therapy.

Results

MABs synthesize and secrete only negligible amount of laminin-211 either in vitro or in vivo. MABs engineered to deliver MAG and injected in muscles of MDC1A mice showed amelioration of muscle histology, increased expression of laminin receptors in muscle, and attenuated deterioration of motor performances. MABs did not enter the peripheral nerves, thus did not affect the associated peripheral neuropathy.

Conclusions

Our study demonstrates the potential efficacy of combining cell with gene therapy to treat MDC1A.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0055-5) contains supplementary material, which is available to authorized users.

Adult skeletal muscle stem cells

Skeletal muscles in vertebrates have a phenomenal regenerative capacity. A muscle that has been crushed can regenerate fully both structurally and functionally within a month. Remarkably, efficient regeneration continues to occur following repeated injuries. Thousands of muscle precursor cells are needed to accomplish regeneration following acute injury. The differentiated muscle cells, the multinucleated contractile myofibers, are terminally withdrawn from mitosis. The source of the regenerative precursors is the skeletal muscle stem cells-the mononucleated cells closely associated with myofibers, which are known as satellite cells. Satellite cells are mitotically quiescent or slow-cycling, committed to myogenesis, but undifferentiated. Disruption of the niche after muscle damage results in their exit from quiescence and progression towards commitment. They eventually arrest proliferation, differentiate, and fuse to damaged myofibers or make de novo myofibers. Satellite cells are one of the well-studied adult tissue-specific stem cells and have served as an excellent model for investigating adult stem cells. They have also emerged as an important standard in the field of ageing and stem cells. Several recent reviews have highlighted the importance of these cells as a model to understand stem cell biology. This chapter begins with the discovery of satellite cells as skeletal muscle stem cells and their developmental origin. We discuss transcription factors and signalling cues governing stem cell function of satellite cells and heterogeneity in the satellite cell pool. Apart from satellite cells, a number of other stem cells have been shown to make muscle and are being considered as candidate stem cells for amelioration of muscle degenerative diseases. We discuss these "offbeat" muscle stem cells and their status as adult skeletal muscle stem cells vis-a-vis satellite cells. The ageing context is highlighted in the concluding section.

Syngeneic Myoblast Transplantation Improves Muscle Function in a Murine Model of X-Linked Myotubular Myopathy

X-linked myotubular myopathy (XLMTM) is an isogenic muscle disease characterized by progressive wasting of skeletal muscle, weakness, and premature death of affected male offspring. Recently, the XLMTM gene knock-in mouse, Mtm1 p.R69C, was found to have a similar phenotype as the Mtm1 gene mutation in humans (e.g., central nucleation of small myofibers, attenuated muscle strength, and motor unit potentials). Using this rodent model, we investigated whether syngeneic cell therapy could mitigate muscle weakness. Donor skeletal muscle-derived myoblasts were isolated from C57BL6 wild-type (WT) and Mtm1 p.R69C (KI) mice for transplantation into the gastrocnemius muscle of recipient KI mice. Initial experiments demonstrated that donor skeletal muscle-derived myoblasts from WT and KI mice remained in the gastrocnemius muscle of the recipient KI mouse for up to 4 weeks posttransplantation. KI mice receiving syngeneic skeletal muscle-derived myoblasts displayed an increase in skeletal muscle mass, augmented force generation, and increased nerve-evoked skeletal muscle action potential amplitude. Taken together, these results support our hypothesis that syngeneic cell therapy may potentially be used to ameliorate muscle weakness and delay the progression of XLMTM, as application expands to other muscles.

Limb-girdle muscular dystrophies: where next after six decades from the first proposal (Review)

Limb-girdle muscular dystrophies (LGMD) are a heterogeneous group of disorders, which has led to certain investigators disputing its rationality. The mutual feature of LGMD is limb-girdle affection. Magnetic resonance imaging (MRI), perioral skin biopsies, blood-based assays, reverse-protein arrays, proteomic analyses, gene chips and next generation sequencing are the leading diagnostic techniques for LGMD and gene, cell and pharmaceutical treatments are the mainstay therapies for these genetic disorders. Recently, more highlights have been shed on disease biomarkers to follow up disease progression and to monitor therapeutic responsiveness in future trials. In this study, we review LGMD from a variety of aspects, paying specific attention to newly evolving research, with the purpose of bringing this information into the clinical setting to aid the development of novel therapeutic strategies for this hereditary disease. In conclusion, substantial progress in our ability to diagnose and treat LGMD has been made in recent decades, however enhancing our understanding of the detailed pathophysiology of LGMD may enhance our ability to improve disease outcome in subsequent years.

Fate choice of post-natal mesoderm progenitors: skeletal versus cardiac muscle plasticity

Regenerative medicine for skeletal and cardiac muscles still constitutes a fascinating and ambitious frontier. In this perspective, understanding the possibilities of intrinsic cell plasticity, present in post-natal muscles, is vital to define and improve novel therapeutic strategies for acute and chronic diseases. In addition, many somatic stem cells are now crossing the boundaries of basic/translational research to enter the first clinical trials. However, it is still an open question whether a lineage switch between skeletal and cardiac adult myogenesis is possible. Therefore, this review focuses on resident somatic stem cells of post-natal skeletal and cardiac muscles and their plastic potential toward the two lineages. Furthermore, examples of myogenic lineage switch in adult stem cells are also reported and discussed.

Adult stem cells and heart regeneration

Cardiovascular disease remains the single greatest cause of death in the Western world, claiming more lives in the USA than the next four leading causes combined. Among these diseases, the incidence of heart failure continues to rise at a staggering rate. Recent advances in medical and device therapies have dramatically improved both the survival and quality of life of many of these patients; however, limited strategies are available to address the central pathophysiology underlying the development of heart failure, namely, the loss of functional cardiomyocytes. Therefore, one recent strategy has been the development of cell-based therapies, aiming towards the replacement of injured or lost cardiomyocytes and thereby improved cardiac function. In this review, we will examine the cell types undergoing investigation as potential cell-based therapies and provide an overview of current clinical trials utilizing cell-based therapeutic approaches in patients with heart disease.

Molecular and cellular regulation of skeletal myogenesis

Since the seminal discovery of the cell-fate regulator Myod, studies in skeletal myogenesis have inspired the search for cell-fate regulators of similar potential in other tissues and organs. It was perplexing that a similar transcription factor for other tissues was not found; however, it was later discovered that combinations of molecular regulators can divert somatic cell fates to other cell types. With the new era of reprogramming to induce pluripotent cells, the myogenesis paradigm can now be viewed under a different light. Here, we provide a short historical perspective and focus on how the regulation of skeletal myogenesis occurs distinctly in different scenarios and anatomical locations. In addition, some interesting features of this tissue underscore the importance of reconsidering the simple-minded view that a single stem cell population emerges after gastrulation to assure tissuegenesis. Notably, a self-renewing long-term Pax7+ myogenic stem cell population emerges during development only after a first wave of terminal differentiation occurs to establish a tissue anlagen in the mouse. How the future stem cell population is selected in this unusual scenario will be discussed. Recently, a wealth of information has emerged from epigenetic and genome-wide studies in myogenic cells. Although key transcription factors such as Pax3, Pax7, and Myod regulate only a small subset of genes, in some cases their genomic distribution and binding are considerably more promiscuous. This apparent nonspecificity can be reconciled in part by the permissivity of the cell for myogenic commitment, and also by new roles for some of these regulators as pioneer transcription factors acting on chromatin state.

Microcarrier-based expansion of adult murine side population stem cells

The lack of reliable methods to efficiently isolate and propagate stem cell populations is a significant obstacle to the advancement of cell-based therapies for human diseases. One isolation technique is based on efflux of the fluorophore Hoechst 33342. Using fluorescence-activated cell sorting (FACS), a sub-population containing adult stem cells has been identified in a multitude of tissues in every mammalian species examined. These rare cells are referred to as the ‘side population’ or SP due to a distinctive FACS profile that results from weak staining by Hoechst dye. Although the SP contains multi-potent cells capable of differentiating toward hematopoietic and mesenchymal lineages; there is currently no method to efficiently expand them. Here, we describe a spinner-flask culture system containing C2C12 myoblasts attached to spherical microcarriers that act to support the growth of non-adherent, post-natal murine skeletal muscle and bone marrow SP cells. Using FACS and hemocytometry, we show expansion of unfractionated EGFP⁺ SP cells over 6 wks. A significant number of these cells retain characteristics of freshly-isolated, unfractionated SP cells with respect to protein expression and dye efflux capacity. Expansion of the SP will permit further study of these heterogeneous cells and determine their therapeutic potential for regenerative and reparative therapies.

Clinical experience with stem cells and other cell therapies in neurological diseases

To overcome the limited capacity of the CNS for regeneration, the theoretical alternative would be to use stem cells for more effective management of chronic degenerative and inflammatory neurological conditions, and also of acute neuronal damage from injuries or cerebrovascular diseases. Although the adult brain contains small numbers of stem cells in restricted areas, this intrinsic stem cell repertoire is small and does not measurably contribute to functional recovery. Embryonic cells carrying pluripotent and self-renewal properties represent the stem cell prototype, but there are additional somatic stem cells that may be harvested and expanded from various tissues during adult life. Stem cell transplantation is based on the assumption that such cells may have the potential to regenerate or support the survival of the existing, partially damaged cells. This review summarizes the state-of-the-art and the clinical worldwide experience with the use of various types of stem cells in neurological diseases.

Satellite cells are essential for skeletal muscle regeneration: the cell on the edge returns centre stage

Following their discovery in 1961, it was speculated that satellite cells were dormant myoblasts, held in reserve until required for skeletal muscle repair. Evidence for this accumulated over the years, until the link between satellite cells and the myoblasts that appear during muscle regeneration was finally established. Subsequently, it was demonstrated that, when grafted, satellite cells could also self-renew, conferring on them the coveted status of 'stem cell'. The emergence of other cell types with myogenic potential, however, questioned the precise role of satellite cells. Here, we review recent recombination-based studies that have furthered our understanding of satellite cell biology. The clear consensus is that skeletal muscle does not regenerate without satellite cells, confirming their pivotal and non-redundant role.

Fetal muscle contains different CD34+ cell subsets that distinctly differentiate into adipogenic, angiogenic and myogenic lineages

We have previously demonstrated that CD34(+) cells isolated from fetal mouse muscles are an interesting source of myogenic progenitors. In the present work, we pinpoint the tissue location of these CD34(+) cells using cell surface and phenotype markers. In order to identify the myogenic population, we next purified different CD34(+) subsets, determined their expression of relevant lineage-related genes, and analyzed their differentiation capacities in vitro and in vivo. The CD34(+) population comprised a CD31(+)/CD45(-) cell subset exhibiting endothelial characteristics and only capable of forming microvessels in vivo. The CD34(+)/CD31(-)/CD45(-)/Sca1(+) subpopulation, which is restricted to the muscle epimysium, displayed adipogenic differentiation both in vitro and in vivo. CD34(+)/CD31(-)/CD45(-)/Sca1(-) cells, localized in the muscle interstitium, transcribed myogenic genes, but did not display the characteristics of adult satellite cells. These cells were distinct from pericytes and fibroblasts. They were myogenic in vitro, and efficiently contributed to skeletal muscle regeneration in vivo, although their myogenic potential was lower than that of the unfractionated CD34(+) cell population. Our results indicate that angiogenic and adipogenic cells grafted with myogenic cells enhance their contribution to myogenic regeneration, highlighting the fundamental role of the microenvironment on the fate of transplanted cells.

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.

Human adipose tissue-derived stem cells exhibit proliferation potential and spontaneous rhythmic contraction after fusion with neonatal rat cardiomyocytes

Various types of stem cells have been shown to have beneficial effects on cardiac function. It is still debated whether fusion of injected stem cells with local resident cardiomyocytes is one of the mechanisms. To better understand the role of fusion in stem cell-based myocardial regeneration, the present study was designed to investigate the fate of human adipose tissue-derived stem cells (hASCs) fused with neonatal rat cardiomyocytes in vitro. hASCs labeled with the green fluorescent probe Vybrant DiO were cocultured with neonatal rat cardiomyocytes labeled with the red fluorescent probe Vybrant DiI and then treated with fusion-inducing hemagglutinating virus of Japan (HVJ). Cells that incorporated both red and green fluorescent signals were considered to be hASCs that had fused with rat cardiomyocytes. Fusion efficiency was 19.86 ± 4.84% at 5 d after treatment with HVJ. Most fused cells displayed cardiomyocyte-like morphology and exhibited spontaneous rhythmic contraction. Both immunofluorescence staining and lentiviral vector labeling showed that fused cells contained separate rat cardiomyocyte and hASC nuclei. Immunofluorescence staining assays demonstrated that human nuclei in fused cells still expressed the proliferation marker Ki67. In addition, hASCs fused with rat cardiomyocytes were positive for troponin I. Whole-cell voltage-clamp analysis demonstrated action potentials in beating fused cells. RT-PCR analysis using rat- or human-specific myosin heavy chain primers revealed that the myosin heavy-chain expression in fused cells was derived from rat cardiomyocytes. Real-time PCR identified expression of human troponin T in fused cells and the presence of rat cardiomyocytes induced a cardiomyogenic protein expression of troponin T in human ASCs. This study illustrates that hASCs exhibit both stem cell (proliferation) and cardiomyocyte properties (action potential and spontaneous rhythmic beating) after fusion with rat cardiomyocytes, supporting the theory that fusion, even if artificially induced in our study, could indeed be a mechanism for cardiomyocyte renewal in the heart.

Sarcoglycanopathies: molecular pathogenesis and therapeutic prospects

Sarcoglycanopathies are a group of autosomal recessive muscle-wasting disorders caused by genetic defects in one of four cell membrane glycoproteins, alpha-, beta-, gamma- or delta-sarcoglycan. These four sarcoglycans form a subcomplex that is closely linked to the major dystrophin-associated protein complex, which is essential for membrane integrity during muscle contraction and provides a scaffold for important signalling molecules. Proper assembly, trafficking and targeting of the sarcoglycan complex is of vital importance, and mutations that severely perturb tetramer formation and localisation result in sarcoglycanopathy. Gene defects in one sarcoglycan cause the absence or reduced concentration of the other subunits. Most genetic defects generate mutated proteins that are degraded through the cell's quality control system; however, in many cases, conformational modifications do not affect the function of the protein, yet it is recognised as misfolded and prematurely degraded. Recent evidence shows that misfolded sarcoglycans could be rescued to the cell membrane by assisting their maturation along the ER secretory pathway. This review summarises the etiopathogenesis of sarcoglycanopathies and highlights the quality control machinery as a potential pharmacological target for therapy of these genetic disorders.

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.

Skeletal muscle as a paradigm for regenerative biology and medicine

Tissue development and regeneration share common features, since modules of regulatory pathways and transcription factors that are crucial for prenatal development are redeployed for tissue reconstruction after trauma. Regenerative medicine has therefore gained important insights through the study of developmental and regenerative biology. Moreover, diverse experimental models have been used to investigate the regeneration process in different tissues and organs. Paradoxically, little is known regarding the relative contribution of stem cells with respect to the supporting tissue during tissue regeneration. Particular attention will be given to mouse models using distinct injury paradigms to investigate the regenerative biology of skeletal muscle. An understanding of the response of stem and parenchymal cells is crucial for the development of clinical strategies to combat the normal decline in tissue performance during aging or its reconstitution after trauma and during disease. This review addresses these issues, focusing on muscle regeneration and how different factors, including genes, cells and the environment, impinge on this process.

Skeletal myogenesis by human primordial germ cell-derived progenitors

We have isolated and cultured human primordial germ cells (PGCs) from early embryos. The PGCs expressed embryonic germ (EG) cell-specific surface markers, including Oct4 and Nanos. We derived a cell population from these PGCs that we termed embryoid body-derived (EBD) cells. EBD cells can be extensively expanded in vitro for more than 50 passages and express lineage markers from all three primary germ layers. The myogenic potential of the EBD cells was examined both in vitro and in vivo.In vitro, the EBD cells can be induced to form multinucleated myotubes, which express late skeletal muscle-specific markers, including MHC and dystrophin, when exposed to human galectin-1. In vivo, the EBD cells gave rise to all the myogenic lineages, including the skeletal muscle stem cells known as satellite cells. Strikingly, these cells were able to partially restore degenerated muscles in the SCID/mdx mouse, an animal model of the Duchenne's muscular dystrophy. These results indicate the EBD cells may be a promising source of myogenic stem cells for cell-based therapies for muscle degenerative disorders.

Cell-based therapy for myocardial ischemia and infarction: pathophysiological mechanisms

Cell-based cardiac repair has emerged as an attractive approach to preventing or reversing heart failure resulting from myocyte dysfunction-e.g., due to infarction-and to enhancing the development of collaterals in patients with symptoms of myocardial ischemia. These two problems involve both overlapping and differing mechanisms, and these differences must be considered in cell-based therapies. In terms of myocardial dysfunction due to infarction, only committed cardiomyocytes have been shown to form new myocardium that is electrically coupled with the host heart. Despite this, multiple cell populations appear to improve function of the infarcted heart, including many that are clearly nonmyogenic. In terms of myocardial ischemia, although cell-based strategies improve ischemia in animal models, clinical trials to date have not shown robustly beneficial results. We review the evidence for potential mechanisms underlying the benefits of cell transplantation in the heart and discuss the clinical contexts in which they may be relevant.

Long-term survival of transplanted stem cells in immunocompetent mice with muscular dystrophy

Satellite cells refer to resident stem cells in muscle that are activated in response to damage or disease for the regeneration and repair of muscle fibers. The use of stem cell transplantation to treat muscular diseases has been limited by impaired donor cell survival attributed to rejection and an unavailable stem cell niche. We isolated a population of adult muscle mononuclear cells (AMMCs) from normal, strain-matched muscle and transplanted these cells into delta-sarcoglycan-null dystrophic mice. Distinct from other transplant studies, the recipient mice were immunocompetent with an intact endogenous satellite cell pool. We found that AMMCs were 35 times more efficient at restoring sarcoglycan compared with cultured myoblasts. Unlike cultured myoblasts, AMMC-derived muscle fibers expressed sarcoglycan protein throughout their entire length, consistent with enhanced migratory ability. We examined the capacity of single injections of AMMCs to provide long-term benefit for muscular dystrophy and found persistent regeneration after 6 months, consistent with augmentation of the endogenous stem cell pool. Interestingly, AMMCs were more effectively engrafted into aged dystrophic mice for the regeneration of large clusters of sarcoglycan-positive muscle fibers, which were protected from damage, suggesting that the stem cell niche in older muscle remains permissive.

Green fluorescent protein incorporation by mouse myoblasts may yield false evidence of myogenic differentiation of human haematopoietic stem cells

AIMS

Haematopoietic CD34+ stem cells are able to differentiate into skeletal muscle, a potentially invaluable tool for treating degenerative diseases such as muscular dystrophy. However, some studies argue that the differentiative potential of these cells might have been overestimated. In vitro studies provide a controlled environment in which to investigate this point.

METHODS

CD34+ stem cells from human peripheral blood, labelled with green fluorescent protein (GFP), were co-cultured with mouse myogenic C2C12 cells. The functional properties of mononucleated GFP+ cells were determined using electrophysiological techniques and were related to protein profiling determined by immunofluorescence staining and single-cell RT-PCR. Mouse mesoangioblasts co-cultured with human myotubes provided methodological controls.

RESULTS

After 2-4 days, mononucleated adherent GFP+ cells showed acetylcholine-evoked current responses, typical of myogenic cells, as if stem cells had integrated into the host environment. In contrast to this hypothesis, human nuclei could not be detected in adherent GFP+ cells by immunofluorescence. Moreover, single-cell RT-PCR showed that adherent GFP+ cells responsive to acetylcholine expressed mouse markers while loose unresponsive GFP+ cells were of human origin. The transcripts of the human alpha1 subunit of the acetylcholine muscle receptor were not amplified in co-cultures.

CONCLUSION

Single-cell analysis of functional properties combined with other markers revealed that, under the co-culture conditions used, GFP was transferred from human CD34+ stem cells to C2C12 myoblasts by mechanisms unrelated to myogenic stem cell differentiation. Our results emphasize the need for careful controls using several markers when investigating the myogenic differentiation of circulating stem cells.

Cell-lineage regulated myogenesis for dystrophin replacement: a novel therapeutic approach for treatment of muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized in skeletal muscle by cycles of myofiber necrosis and regeneration leading to loss of muscle fibers and replacement with fibrotic connective and adipose tissue. The ongoing activation and recruitment of muscle satellite cells for myofiber regeneration results in loss of regenerative capacity in part due to proliferative senescence. We explored a method whereby new myoblasts could be generated in dystrophic muscles by transplantation of primary fibroblasts engineered to express a micro-dystrophin/enhanced green fluorescent protein (muDys/eGFP) fusion gene together with a tamoxifen-inducible form of the myogenic regulator MyoD [MyoD-ER(T)]. Fibroblasts isolated from mdx(4cv) mice, a mouse model for DMD, were efficiently transduced with lentiviral vectors expressing muDys/eGFP and MyoD-ER(T) and underwent myogenic conversion when exposed to tamoxifen. These cells could also be induced to differentiate into muDys/eGFP-expressing myocytes and myotubes. Transplantation of transduced mdx(4cv) fibroblasts into mdx(4cv) muscles enabled tamoxifen-dependent regeneration of myofibers that express muDys. This lineage control method therefore allows replenishment of myogenic stem cells using autologous fibroblasts carrying an exogenous dystrophin gene. This strategy carries several potential advantages over conventional myoblast transplantation methods including: (i) the relative simplicity of culturing fibroblasts compared with myoblasts, (ii) a readily available cell source and ease of expansion and (iii) the ability to induce MyoD gene expression in vivo via administration of a medication. Our study provides a proof of concept for a novel gene/stem cell therapy technique and opens another potential therapeutic approach for degenerative muscle disorders.

Genetic complementation of human muscle cells via directed stem cell fusion

Duchenne muscular dystrophy (DMD) is caused by mutations in the X chromosome-linked DMD gene, which encodes the sarcolemma-stabilizing protein-dystrophin. Initial attempts at DMD therapy deployed muscle progenitor cells from healthy donors. The utilization of these cells is, however, hampered by their immunogenicity, while those from DMD patients are scarce and display limited ex vivo replication. Nonmuscle cells with myogenic capacity may offer valuable alternatives especially if, to allow autologous transplantation, they are amenable to genetic intervention. As a paradigm for therapeutic gene transfer by heterotypic cell fusion we are investigating whether human mesenchymal stem cells (hMSCs) can serve as donors of recombinant DMD genes for recipient human muscle cells. Here, we show that forced MyoD expression in hMSCs greatly increases their tendency to participate in human myotube formation turning them into improved DNA delivery vehicles. Efficient loading of hMSCs with recombinant DMD was achieved through a new tropism-modified high-capacity adenoviral (hcAd) vector directing striated muscle-specific synthesis of full-length dystrophin. This study introduces the principle of genetic complementation of gene-defective cells via directed cell fusion and provides an initial framework to test whether transient MyoD synthesis in autologous, gene-corrected hMSCs increases their potential for treating DMD and, possibly, other muscular dystrophies.

Spontaneous myogenic differentiation of Flk-1-positive cells from adult pancreas and other nonmuscle tissues

At the embryonic or fetal stages, autonomously myogenic cells (AMCs), i.e., cells able to spontaneously differentiate into skeletal myotubes, have been identified from several different sites other than skeletal muscle, including the vascular compartment. However, in the adult animal, AMCs from skeletal muscle-devoid tissues have been described in only two cases. One is represented by thymic myoid cells, a restricted population of committed myogenic progenitors of unknown derivation present in the thymic medulla; the other is represented by a small subset of adipose tissue-associated cells, which we recently identified. In the present study we report, for the first time, the presence of spontaneously differentiating myogenic precursors in the pancreas and in other skeletal muscle-devoid organs such as spleen and stomach, as well as in the periaortic tissue of adult mice. Immunomagnetic selection procedures indicate that AMCs derive from Flk-1(+) progenitors. Individual clones of myogenic cells from nonmuscle organs are morphologically and functionally indistinguishable from skeletal muscle-derived primary myoblasts. Moreover, they can be induced to proliferate in vitro and are able to participate in muscle regeneration in vivo. Thus, we provide evidence that fully competent myogenic progenitors can be derived from the Flk-1(+) compartment of several adult tissues that are embryologically unrelated to skeletal muscle.

Deciphering the beta-adrenergic response in human embryonic stem cell-derived-cardiac myocytes: closer to clinical use?

Human embryonic stem cells (hESCs) are a pluripotent cell type considered to have high potential for the treatment of cardiovascular disease by cell replacement therapies. Several groups have shown that hESCs can be differentiated in vitro into cardiac myocytes, which may be used to facilitate tissue regeneration by injection directly into damaged myocardium. However, several hurdles still need to be overcome before these cells can be used in clinical trials. In particular, because transplanted hESC-cardiac myocytes should integrate fully within the damaged heart, these cells must be functionally compatible with the host myocardium. To assess this aspect of hESC-cardiac myocytes, Brito-Martins et al. (2008) in this issue of the BJP, describe the responses of hESC-cardiac myocytes to beta-adrenoceptor stimulation, compared to those of myocytes from adult human hearts. Data obtained using specific beta-adrenoceptor agonists showed good compatibility of hESC-cardiac myocytes with adult human myocardium in terms of beta-adrenoceptor response.

Consequences of disrupting the dystrophin-sarcoglycan complex in cardiac and skeletal myopathy

Mutations that disrupt the dystrophin glycoprotein complex lead to plasma membrane instability of cardiomyocytes and skeletal muscle myofibers. Instability of the plasma membrane leads to degeneration largely due to activation of a necrotic process in these disorders. In response to ongoing degeneration, skeletal muscle exhibits robust regeneration while in cardiac muscle regeneration is not obvious. The dystrophin complex is concentrated along the plasma membrane in costameric structures that correspond to the Z bands of sarcomeres, thus positioning the dystrophin complex to transmit force between the sarcomere and the plasma membrane to the extracellular matrix. Although it is apparent that this position is important for perpendicular force transmission, it is clear that the dystrophin complex also fulfills signaling roles. Nitric oxide synthase and stress-induced signaling cascades are activated to participate in protection but may also contribute to pathology.

Delivering genes to the organ-localized immune system: long-term results of direct intramarrow transduction

We studied the distribution of transgene-expressing cells after direct gene transfer into the bone marrow (BM). Rats received direct injection into the femoral BM of SV(Nef-FLAG), a Tag-deleted recombinant SV40 carrying a marker gene (FLAG epitope). Controls received an unrelated rSV40 or saline. Blood cells (5%) and femoral marrow cells (25%) expressed FLAG throughout. FLAG expression was assessed in different organs at 1, 4 and 16 months. FLAG+ macrophages were seen throughout the body, and were prominent in the spleen. FLAG+ cells were common in pulmonary alveoli. The former included alveolar macrophages and type II pneumocytes. These cells were not detected at 1 month, occasional at 4 months and common at 16 months after intramarrow injection. Rare liver cells were positive for both FLAG and ferritin, indicating that some hepatocytes also expressed this BM-delivered transgene. Control animals were negative. Thus: (a) fixed tissue phagocytes may be accessible to gene delivery by intramarrow transduction of their progenitors; (b) transduced BM-resident cells or their derivatives may migrate to other organs (lungs) and may differentiate into epithelial cells; and (c) intramarrow injection of rSV40s does not detectably transduce parenchymal cells of other organs.

Which skeletal myoblasts and how to be transplanted for cardiac repair?

Clinical efficacy of skeletal myoblast (skMb) transplantation is controversial whether this treatment produces beneficial outcome in patients with dilated cardiomyopathy (DCM). Based on immunological tolerance between wild-type and DCM hamsters with the deletion of delta-sarcoglycan (SG) gene, skMb engraftment in TO-2 myocardium (3x10(5) cells in approximately 100mg heart) was verified by the donor-specific expression of delta-SG transgene constitutively produced throughout myogenesis. At 5 weeks after the transplantation, the cell rates expressing fast-myosin heavy chain (MHC) exceeded slow-MHC in delta-SG(+) cells. Fifteen weeks after (corresponding to approximately 12 years in humans), fast MHC(+) cells nullified, but the delta-SG(+) and slow MHC(+) cell number remained unaltered. These skMbs fused with host cardiomyocytes via connexin-43 and intercalated disc, modestly improving the hemodynamics without arrhythmia, when engrafted skMbs were sparsely disseminated in autopsied myocardium. These results provide us evidence that disseminating delivery of slow-MHC(+) myoblasts is promising for repairing DCM heart using histocompatible skeletal myoblasts in future.

New therapies for Duchenne muscular dystrophy: challenges, prospects and clinical trials

Muscular dystrophies primarily affect skeletal muscle. Mutations in a large number of genes, mainly encoding cytoskeletal proteins, cause different forms of dystrophy that compromise patient mobility and quality of life, and in the most severe cases lead to complete paralysis and premature death. Although muscular dystrophies still lack an effective therapy, several novel strategies are entering or are ready to enter clinical trials. Here we review the main experimental strategies, namely drug, gene and cell therapies, outlining their goals and limitations. We also provide an update of ongoing or planned clinical trials based on these strategies.

Reduced life span with heart and muscle dysfunction in Drosophila sarcoglycan mutants

In humans, genetically diverse forms of muscular dystrophy are associated with a disrupted sarcoglycan complex. The sarcoglycan complex resides at the muscle plasma membrane where it associates with dystrophin. There are six known sarcoglycan proteins in mammals whereas there are only three in Drosophila melanogaster. Using imprecise P element excision, we generated three different alleles at the Drosophila delta-sarcoglycan locus. Each of these deletions encompassed progressively larger regions of the delta-sarcoglycan gene. Line 840 contained a large deletion of the delta-sarcoglycan gene, and this line displayed progressive impairment in locomotive ability, reduced heart tube function and a shortened life span. In line 840, deletion of the Drosophila delta-sarcoglycan gene produced disrupted flight muscles with shortened sarcomeres and disorganized M lines. Unlike mammalian muscle where degeneration is coupled with ongoing regeneration, no evidence for regeneration was seen in this Drosophila sarcoglycan mutant. In contrast, line 28 was characterized with a much smaller deletion that affected only a portion of the cytoplasmic region of the delta-sarcoglycan protein and left intact the transmembrane and extracellular domains. Line 28 had a very mild phenotype with near normal life span, intact cardiac function and normal locomotive activity. Together, these data demonstrate the essential nature of the transmembrane and extracellular domains of Drosophila delta-sarcoglycan for normal muscle structure and function.

Myogenic reprogramming of retina-derived cells following their spontaneous fusion with myotubes

Satellite cells are recognized as the main source for myoblasts in postnatal muscle. The possible participation of other cell types in myofiber maintenance remains a subject of debate. Here, we investigated the potential of vascular preparations from mouse retina to undergo myogenesis when cultured alone or with differentiated primary myogenic cultures. The choice of retina, an organ richly supplied with capillary network and anatomically separated from skeletal muscles, ensures that the vasculature preparation is devoid of satellite cells. We demonstrate that retina-derived cells spontaneously fuse with preexisting myotubes and contribute additional myonuclei, some of which initiate expression of muscle-specific genes after fusion. Myogenic differentiation of retinal cells prior to their fusion with preexisting myotubes was not detected. Although originating from vasculature preparations, nuclei undergoing myogenic reprogramming were contributed by cells that were neither endothelial nor blood borne. Our results suggest smooth muscle/pericytes as the possible source, and that myogenic reprogramming depends on the muscle specific transcription factor MyoD. Our studies provide insights into a novel avenue for myofiber maintenance, relying on nuclei of non-myogenic origin that undergo fusion and subsequent myogenic conversion within host myofibers. This process may support ongoing myofiber maintenance throughout life.

Bone Marrow–Derived Cells Are Involved in the Pathogenesis of Cardiac Hypertrophy in Response to Pressure Overload

Background— Bone marrow (BM) cells possess broad differentiation potential and can form various cell lineages in response to pathophysiological cues. The present study investigated whether BM-derived cells contribute to the pathogenesis of cardiac hypertrophy, as well as the possible cellular mechanisms involved in such a role.

Methods and Results— Lethally irradiated wild-type mice were transplanted with BM cells from enhanced green fluorescent protein–transgenic mice. The chimeric mice were subjected to either prolonged hypoxia or transverse aortic constriction. BM-derived enhanced green fluorescent protein–expressing cardiomyocytes increased in number over time, emerging predominantly in the pressure-overloaded ventricular myocardium, although they constituted <0.01% of recipient cardiomyocytes. To determine whether BM-derived cardiomyocytes were derived from cell fusion or transdifferentiation at the single-cell level, lethally irradiated Cre mice were transplanted with BM cells from the double-conditional Cre reporter mouse line Z/EG. BM-derived cardiomyocytes were shown to arise from both cell fusion and transdifferentiation. Interestingly, BM-derived myofibroblasts expressing both vimentin and α-smooth muscle actin were concentrated in the perivascular fibrotic area. These cells initially expressed MAC-1/CD14 but lost expression of these markers during the chronic phase, which suggests that they were derived from monocytes. A similar phenomenon occurred in cultured human monocytes, most of which ultimately expressed vimentin and α-smooth muscle actin.

Conclusions— We found that BM-derived cells were involved in the pathogenesis of cardiac hypertrophy via the dual mechanisms of cell fusion and transdifferentiation. Moreover, the present results suggest that BM-derived monocytes accumulating in the perivascular space might play an important role in the formation of perivascular fibrosis via direct differentiation into myofibroblasts.

Evaluation of Sca-1 and c-Kit As Selective Markers for Muscle Remodelling by Nonhemopoietic Bone Marrow Cells

Bone marrow (BM)-derived cells (BMCs) have demonstrated a myogenic tissue remodeling capacity. However, because the myoremodeling is limited to approximately 1%-3% of recipient muscle fibers in vivo, there is disagreement regarding the clinical relevance of BM for therapeutic application in myodegenerative conditions. This study sought to determine whether rare selectable cell surface markers (in particular, c-Kit) could be used to identify a BMC population with enhanced myoremodeling capacity. Dystrophic mdx muscle remodeling has been achieved using BMCs sorted by expression of stem cell antigen-1 (Sca-1). The inference that Sca-1 is also a selectable marker associated with myoremodeling capacity by muscle-derived cells prompted this study of relative myoremodeling contributions from BMCs (compared with muscle cells) on the basis of expression or absence of Sca-1. We show that myoremodeling activity does not differ in cells sorted solely on the basis of Sca-1 from either muscle or BM. In addition, further fractionation of BM to a more mesenchymal-like cell population with lineage markers and CD45 subsequently revealed a stronger selectability of myoremodeling capacity with c-Kit/Sca-1 (p < .005) than with Sca-1 alone. These results suggest that c-Kit may provide a useful selectable marker that facilitates selection of cells with an augmented myoremodeling capacity derived from BM and possibly from other nonmuscle tissues. In turn, this may provide a new methodology for rapid isolation of myoremodeling capacities from muscle and nonmuscle tissues. Disclosure of potential conflicts of interest is found at the end of this article.

Multipotential mesoangioblast stem cell therapy in the mdx/utrn-/- mouse model for Duchenne muscular dystrophy

Background: Duchenne muscular dystrophy is a progressive, lethal muscle-wasting disease for which there is no treatment. Materials & methods: We have isolated wild-type mesoangioblasts from aorta and tested their effectiveness in alleviating severe muscle disease in the dystrophin/utrophin knockout (mdx/utrn-/-) mouse model for Duchenne muscular dystrophy. Results: Mesoangioblast clones express Sca-1 and Flk-1 and differentiate into smooth and skeletal muscle, glial cells and adipocytes in vitro. Mesoangioblasts proliferate in vivo, incorporate into muscle fibers, form new fibers, and promote synthesis of dystrophin and utrophin. Muscle fibers that have incorporated mesoangioblasts, as well as surrounding fibers, are protected from damage, with approximately 50-fold less damage than fibers in muscle injected with saline. Some mesoangioblasts localize beneath the basal lamina and express c-met, whereas others differentiate into smooth muscle cells at the periphery of vessels and express α-smooth muscle actin. In mdx/utrn-/- muscle, some mesoangioblasts also form Schwann cells. Discussion & conclusion: Mesoangioblasts differentiate into multiple cell types damaged during the progression of severe muscle disease and protect fibers from damage. As such, they are good candidates for therapy of Duchenne muscular dystrophy and perhaps other neuromuscular diseases.

Early management of ST elevation myocardial infarction: a review of practice

The last two decades of the 20th century witnessed continuous evolution in the understanding of the pathophysiology of ST elevation myocardial infarction. In parallel, the management of these patients developed steadily throughout this time and into the early years of the 21st century. From humble beginnings involving oxygen therapy, bed rest and analgesia, the relative merits of different strategies to open 'infarct-related arteries' (IRAs) are now being debated: pharmacological reperfusion, mechanical reperfusion or a combination of both these modalities. The current understanding of the process of thrombotic occlusion of the coronary artery has led to the appreciation of the importance of not simply opening the IRA, but also maintaining its patency once opened. Considerable attention is now being afforded to the significant minority of patients who do not achieve early, complete myocardial reperfusion, despite restoration of adequate flow down the epicardial IRA. Those patients who fail to achieve myocardial reperfusion, either due to late presentation or failure of reperfusion therapy, and are left with permanent myocardial scarring can now be considered. This article critically appraises the recent and emerging evidence and clinical implications of the contemporary management of ST elevation myocardial infarction.

Ins and outs of therapy in limb girdle muscular dystrophies

Muscular dystrophies are hereditary degenerative muscle diseases that cause life-long disability in patients. They comprise the well-known Duchenne Muscular Dystrophy (DMD) but also the group of Limb Girdle Muscular Dystrophies (LGMD) which account for a third to a fourth of DMD cases. From the clinical point of view, LGMD are characterised by predominant effects on the proximal limb muscles. The LGMD group is still growing today and consists of 19 autosomal dominant and recessive forms (LGMD1A to LGMD1G and LGMD2A to LGMD2M). The proteins involved are very diverse and include sarcomeric, sarcolemmal and enzymatic proteins. With respect to this variability and in line with the intense search for a potent therapeutic approach for DMD, many different strategies have been tested in rodent models. These include replacing the lost function by gene transfer or stem cell transplantation, using a related protein for functional substitution, increasing muscle mass, or blocking the molecular pathological mechanisms by pharmacological means to alleviate the symptoms. The purpose of this review is to summarize current data arising from these preclinical studies and to examine the potential of the tested strategies to lead to clinical applications.

Alpha-linolenic acid-enriched diet prevents myocardial damage and expands longevity in cardiomyopathic hamsters

Randomized clinical trials have demonstrated that the increased intake of omega-3 polyunsaturated fatty acids significantly reduces the risk of ischemic cardiovascular disease, but no investigations have been performed in hereditary cardiomyopathies with diffusely damaged myocardium. In the present study, delta-sarcoglycan-null cardiomyopathic hamsters were fed from weaning to death with an alpha-linolenic acid (ALA)-enriched versus standard diet. Results demonstrated a great accumulation of ALA and eicosapentaenoic acid and an increased eicosapentaenoic/arachidonic acid ratio in cardiomyopathic hamster hearts, correlating with the preservation of myocardial structure and function. In fact, ALA administration preserved plasmalemma and mitochondrial membrane integrity, thus maintaining proper cell/extracellular matrix contacts and signaling, as well as a normal gene expression profile (myosin heavy chain isoforms, atrial natriuretic peptide, transforming growth factor-beta1) and a limited extension of fibrotic areas within ALA-fed cardiomyopathic hearts. Consequently, hemodynamic indexes were safeguarded, and more than 60% of ALA-fed animals were still alive (mean survival time, 293+/-141.8 days) when all those fed with standard diet were deceased (mean survival time, 175.9+/-56 days). Therefore, the clinically evident beneficial effects of omega-3 polyunsaturated fatty acids are mainly related to preservation of myocardium structure and function and the attenuation of myocardial fibrosis.

Adult bone marrow–derived stem cells for organ regeneration and repair

Stem cells have been recognized as a potential tool for the development of innovative therapeutic strategies. There are in general two types of stem cells, embryonic and adult stem cells. While embryonic stem cell therapy has been riddled with problems of allogeneic rejection and ethical concerns, adult stem cells have long been used in the treatment of hematological malignancies. With the recognition of additional, potentially therapeutic characteristics, bone marrow-derived stem cells have become a tool in regenerative medicine. The bone marrow is an ideal source of stem cells because it is easily accessible and harbors two types of stem cells. Hematopoietic stem cells give rise to all blood cell types and have been shown to exhibit plasticity, while multipotent marrow stromal cells are the source of osteocytes, chondrocytes, and fat cells and have been shown to support and generate a large number of different cell types. This review describes the general characteristics of these stem cell populations and their current and potential future applications in regenerative medicine.

Widespread distribution and muscle differentiation of human fetal mesenchymal stem cells after intrauterine transplantation in dystrophic mdx mouse

Duchenne muscular dystrophy (DMD) is a common X-linked disease resulting from the absence of dystrophin in muscle. Affected boys suffer from incurable progressive muscle weakness, leading to premature death. Stem cell transplantation may be curative, but is hampered by the need for systemic delivery and immune rejection. To address these barriers to stem cell therapy in DMD, we investigated a fetal-to-fetal transplantation strategy. We investigated intramuscular, intravascular, and intraperitoneal delivery of human fetal mesenchymal stem cells (hfMSCs) into embryonic day (E) 14-16 MF1 mice to determine the most appropriate route for systemic delivery. Intramuscular injections resulted in local engraftment, whereas both intraperitoneal and intravascular delivery led to systemic spread. However, intravascular delivery led to unexpected demise of transplanted mice. Transplantation of hfMSCs into E14-16 mdx mice resulted in widespread long-term engraftment (19 weeks) in multiple organs, with a predilection for muscle compared with nonmuscle tissues (0.71% vs. 0.15%, p < .01), and evidence of myogenic differentiation of hfMSCs in skeletal and myocardial muscle. This is the first report of intrauterine transplantation of ontologically relevant hfMSCs into fully immunocompetent dystrophic fetal mice, with systemic spread across endothelial barriers leading to widespread long-term engraftment in multiple organ compartments. Although the low-level of chimerism achieved is not curative for DMD, this approach may be useful in other severe mesenchymal or enzyme deficiency syndromes, where low-level protein expression may ameliorate disease pathology.

Skeletal myosphere-derived progenitor cell transplantation promotes neovascularization in delta-sarcoglycan knockdown cardiomyopathy

Bone marrow cells have been shown to contribute to neovascularization in ischemic hearts, whereas their impaired maturation to restore the delta-sarcoglycan (delta-SG) expression responsible for focal myocardial degeneration limits their utility to treat the pathogenesis of cardiomyopathy. Here, we report the isolation of multipotent progenitor cells from adult skeletal muscle, based on their ability to generate floating-myospheres. Myosphere-derived progenitor cells (MDPCs) are distinguishable from myogenic C2C12 cells and differentiate into vascular smooth muscle cells and mesenchymal progeny. The mutation in the delta-SG has been shown to develop vascular spasm to affect sarcolemma structure causing cardiomyopathy. We originally generated delta-SD knockdown (KD) mice and transplanted MDPCs into the hearts. MDPCs enhanced neoangiogenesis and restored delta-SG expression in impaired vasculatures through trans-differentiation, leading to improvement of cardiac function associated with paracrine effectors secretion. We propose that MDPCs may be the promising progenitor cells in skeletal muscle to treat delta-sarcoglycan complex mutant cardiomyopathy.

Hybrid cardiomyocytes derived by cell fusion in heterotopic cardiac xenografts

Cardiomyocytes expressing host markers, such as the Y chromosome in sex-mismatched transplants, have been described in human allografts, suggesting that circulating cells can contribute to cardiac regeneration. It has not been established, however, whether host-derived cardiomyocytes result from transdifferentiation of stem cells or cell fusion. To address this issue, we used heterotopic heart xenografts and looked for markers of donor and recipient cells. Golden Syrian hamsters or transgenic mice expressing nuclear beta-galactosidase under the control of the cardiac troponin I promoter served as organ donors, while GFP+ transgenic rats were used as recipients. GFP+ cells, including abundant CD-45+ inflammatory cells and rare undifferentiated cells expressing early cardiac markers (GATA-4 or MEF2C), were found in xenografts harvested two weeks after surgery. In addition, rare GFP+ mature cardiomyocytes were found in 7 of 8 hamster xenografts and 6 of 6 mouse xenografts. The proportion of these cells was very low (0.0001% to 0.0344% in hamster xenografts) but similar to the one observed in control rat heart allografts. Without exception, all GFP+ cardiomyocytes also expressed donor markers, i.e., hamster membrane antigens or lacZ, so they must derive from cell fusion, not transdifferentiation.

The skeletal muscle satellite cell: the stem cell that came in from the cold

The muscle satellite cell was first described and actually named on the basis of its anatomic location under the basement membrane surrounding each myofiber. For many years following its discovery, electron microscopy provided the only definitive method of identification. More recently, several molecular markers have been described that can be used to detect satellite cells, making them more accessible for study at the light microscope level. Satellite cells supply myonuclei to growing myofibers before becoming mitotically quiescent in muscle as it matures. They are then activated from this quiescent state to fulfill their roles in routine maintenance, hypertrophy, and repair of adult muscle. Because muscle is able to efficiently regenerate after repeated bouts of damage, systems must be in place to maintain a viable satellite cell pool, and it was proposed over 30 years ago that self-renewal was the primary mechanism. Self-renewal entails either a stochastic event or an asymmetrical cell division, where one daughter cell is committed to differentiation whereas the second continues to proliferate or becomes quiescent. This classic model of satellite cell self-renewal and the importance of satellite cells in muscle maintenance and repair have been challenged during the past few years as bone marrow-derived cells and various intramuscular populations were shown to be able to contribute myonuclei and occupy the satellite cell niche. This is a fast-moving and dynamic field, however, and in this review we discuss the evidence that we think puts this enigmatic cell firmly back at the center of adult myogenesis.

A side order of stem cells: the SP phenotype

A defining property of murine hematopoietic stem cells (HSCs) is low fluorescence after staining with Hoechst 33342 and Rhodamine 123. These dyes have proven to be remarkably powerful tools in the purification and characterization of HSCs when used alone or in combination with antibodies directed against stem cell epitopes. Hoechst low cells are described as side population (SP) cells by virtue of their typical profiles in Hoechst red versus Hoechst blue bivariate fluorescent-activated cell sorting dot plots. Recently, excitement has been generated by the findings that putative stem cells from solid tissues may also possess this SP phenotype. SP cells have now been isolated from a wide variety of mammalian tissues based on this same dye efflux phenomenon, and in many cases this cell population has been shown to contain apparently multipotent stem cells. What is yet to be clearly addressed is whether cell fusion accounts for this perceived SP multipotency. Indeed, if low fluorescence after Hoechst staining is a phenotype shared by hematopoietic and organ-specific stem cells, do all resident tissue SP cells have bone marrow origins or might the SP phenotype be a property common to all stem cells? Subject to further analysis, the SP phenotype may prove invaluable for the initial isolation of resident tissue stem cells in the absence of definitive cell-surface markers and may have broad-ranging applications in stem cell biology, from the purification of novel stem cell populations to the development of autologous stem cell therapies.