Regeneration of skeletal and cardiac muscle in mammals: do nonprimate models resemble human pathology?

Regeneration of the digestive system in the crinoid Lamprometra palmata (Mariametridae, Comatulida)

The morphology and regeneration of the digestive system and tegmen after autotomy of the visceral mass in the crinoid Lamprometra palmata (Clark 1921) was studied. The gut has a five-lobed shape and is covered by a tegmen. The tegmen consists of epidermis and underlying connective tissue. The digestive tube can be divided into three parts: esophagus, intestine, and rectum. At 6 h post-autotomy, the calyx surface is covered by a layer of amoebocytes and juxtaligamental cells (JLCs). At 14-18 h, post-autotomy transdifferentiation of JLCs begins and give rise to the epidermis and cells of digestive system. On days 1-2 post-autotomy, JLCs undergo the mesenchymal-epithelial transition. Some JLCs turn into typical epidermal cells, while other JLCs form small closed epithelial structures that represent the gut anlage. On day 4 post-autotomy, the animals have a mouth opening and a small anal cone. On day 7 post-autotomy, the visceral mass and the digestive system become fully formed but are smaller than normal. A 24-h exposure of L. palmata individuals to a 10^-7 M colchicine solution did not slow down regeneration, and the timing of gut formation was similar to that in the control animals. We conclude that JLCs are the major cell source for gut and epidermis regeneration in L. palmata. The main mechanisms of morphogenesis are cell migration, mesenchymal-epithelial transition, and transdifferentiation.

Isolation and characterization of myogenic precursor cells from human cremaster muscle

Human myogenic precursor cells have been isolated and expanded from a number of skeletal muscles, but alternative donor biopsy sites must be sought after in diseases where muscle damage is widespread. Biopsy sites must be relatively accessible, and the biopsied muscle dispensable. Here, we aimed to histologically characterize the cremaster muscle with regard number of satellite cells and regenerative fibres, and to isolate and characterize human cremaster muscle-derived stem/precursor cells in adult male donors with the objective of characterizing this muscle as a novel source of myogenic precursor cells. Cremaster muscle biopsies (or adjacent non-muscle tissue for negative controls; N = 19) were taken from male patients undergoing routine surgery for urogenital pathology. Myosphere cultures were derived and tested for their in vitro and in vivo myogenic differentiation and muscle regeneration capacities. Cremaster-derived myogenic precursor cells were maintained by myosphere culture and efficiently differentiated to myotubes in adhesion culture. Upon transplantation to an immunocompromised mouse model of cardiotoxin-induced acute muscle damage, human cremaster-derived myogenic precursor cells survived to the transplants and contributed to muscle regeneration. These precursors are a good candidate for cell therapy approaches of skeletal muscle. Due to their location and developmental origin, we propose that they might be best suited for regeneration of the rhabdosphincter in patients undergoing stress urinary incontinence after radical prostatectomy.

Cell Therapy in Myology: Dynamics of Muscle Precursor Cell Death after Intramuscular Administration in Non-human Primates

Cell therapy could be useful for the treatment of myopathies. A problem observed in mice, with different results and interpretations, is a significant death among the transplanted cells. We analyzed this problem in non-human primates, the animal model more similar to humans. Autologous or allogeneic myoblasts (with or without a reporter gene) were proliferated in vitro, labeled with [¹⁴C]thymidine, and intramuscularly injected in macaques. Some monkeys were immunosuppressed for long-term follow-up. Cell-grafted regions were biopsied at different intervals and analyzed by radiolabel quantification and histology. Most radiolabel was lost during the first week after injection, regardless of whether the cells were allogeneic or autologous, the culture conditions, and the use or not of immunosuppression. There was no significant difference between 1 hr and 1 day post-transplantation, a significant decrease between days 1 and 3 (45% to 83%), a significant decrease between days 3 and 7 (80% to 92%), and no significant differences between 7 days and 3 weeks. Our results confirmed in non-human primates a progressive and significant death of the grafted myoblasts during the first week after administration, relatively similar to some observations in mice but with different kinetics.

Regeneration versus scarring in vertebrate appendages and heart

Injuries to complex human organs, such as the limbs and the heart, result in pathological conditions, for which we often lack adequate treatments. While modern regenerative approaches are based on the transplantation of stem cell-derived cells, natural regeneration in lower vertebrates, such as zebrafish and newts, relies predominantly on the intrinsic plasticity of mature tissues. This property involves local activation of the remaining material at the site of injury to promote cell division, cell migration and complete reproduction of the missing structure. It remains an unresolved question why adult mammals are not equally competent to reactivate morphogenetic programmes. Although organ regeneration depends strongly on the proliferative properties of cells in the injured tissue, it is apparent that various organismic factors, such as innervation, vascularization, hormones, metabolism and the immune system, can affect this process. Here, we focus on a correlation between the regenerative capacity and cellular specialization in the context of functional demands, as illustrated by appendages and heart in diverse vertebrates. Elucidation of the differences between homologous regenerative and non-regenerative tissues from various animal models is essential for understanding the applicability of lessons learned from the study of regenerative biology to clinical strategies for the treatment of injured human organs.

Cell therapy in muscular dystrophies: many promises in mice and dogs, few facts in patients

INTRODUCTION

Muscular dystrophies (MDs) are genetic diseases that produce progressive loss of skeletal muscle fibers. Cell therapy (CT) is an experimental approach to treat MD. The first clinical trials of CT in MD conducted in the 1990s were based on myoblast transplantation (MT). Since they did not yield the expected results, several researchers sought to discover other cells with more advantageous properties than myoblasts whereas others sought to improve MT.

AREAS COVERED

We explain the properties that are required for a cell to be used in CT of MD. We briefly review most of the cells that were proposed for this CT, and to what extent these properties were met not only in laboratory animals but also in clinical trials.

EXPERT OPINION

Although the repertoire of cells proposed for CT of MD has been expanded since the 1990s, only myoblasts have currently demonstrated unequivocally to significantly engraft in humans. Indeed, MT for MD involves significant technical challenges that need be solved. While it would be ideal to find cells involving less technical challenges for CT of MD, there is so far no clinical evidence that this is possible and therefore the work to improve MT should continue.

Skeletal muscle tissue engineering: strategies for volumetric constructs

Skeletal muscle tissue is characterized by high metabolic requirements, defined structure and high regenerative potential. As such, it constitutes an appealing platform for tissue engineering to address volumetric defects, as proven by recent works in this field. Several issues common to all engineered constructs constrain the variety of tissues that can be realized in vitro, principal among them the lack of a vascular system and the absence of reliable cell sources; as it is, the only successful tissue engineering constructs are not characterized by active function, present limited cellular survival at implantation and possess low metabolic requirements. Recently, functionally competent constructs have been engineered, with vascular structures supporting their metabolic requirements. In addition to the use of biochemical cues, physical means, mechanical stimulation and the application of electric tension have proven effective in stimulating the differentiation of cells and the maturation of the constructs; while the use of co-cultures provided fine control of cellular developments through paracrine activity. This review will provide a brief analysis of some of the most promising improvements in the field, with particular attention to the techniques that could prove easily transferable to other branches of tissue engineering.

Electroporation as a method to induce myofiber regeneration and increase the engraftment of myogenic cells in skeletal muscles of primates

Engraftment of intramuscularly transplanted myogenic cells in mice can be optimized after induction of massive myofiber damage that triggers myofiber regeneration and recruitment of grafted cells; this generally involves either myotoxin injection or cryodamage. There are no effective methods to produce a similar process in the muscles of large mammals such as primates. In this study, we tested the use of intramuscular electroporation for this purpose in 11 macaques. The test sites were 1 cm of skeletal muscle. Each site was treated with 3 penetrations of a 2-needle electrode with 1 cm spacing, applying 3 pulses of 400 V/cm, for a duration of 5 milliseconds and a delay of 200 milliseconds during each penetration. Transplantation of β-galactosidase-labeled myoblasts was done in electroporated and nonelectroporated sites. Electroporation induced massive myofiber necrosis that was followed by efficient muscle regeneration. Myoblast engraftment was substantially increased in electroporated compared with nonelectroporated sites. This suggests that electroporation may be a useful tool to study muscle regeneration in primates and other large mammals and as a method for increasing the engraftment of myoblasts and other myogenic cells in intramuscular transplantation.

Model systems for cardiovascular regenerative biology

There is an urgent clinical need to develop new therapeutic approaches to treat heart failure, but the biology of cardiovascular regeneration is complex. Model systems are required to advance our understanding of biological mechanisms of cardiac regeneration as well as to test therapeutic approaches to regenerate tissue and restore cardiac function following injury. An ideal model system should be inexpensive, easily manipulated, easily reproducible, physiologically representative of human disease, and ethically sound. In this review, we discuss computational, cell-based, tissue, and animal models that have been used to elucidate mechanisms of cardiovascular regenerative biology or to test proposed therapeutic methods to restore cardiac function following disease or injury.

Intramuscular transplantation of myogenic cells in primates: importance of needle size, cell number, and injection volume

The aim of this study was to quantitatively define the main measurable technical parameters for the intramuscular transplantation of myogenic cells in primates. Myoblasts transduced with the gene coding for β-galactosidase were injected into the skeletal muscles of 15 monkeys. The following parameters were studied: needle size, number of cells per injection, and volume of cell suspension per injection. Monkeys were immunosuppressed with tacrolimus. The cell-injected sites were biopsied 1 or 2 months later. Biopsies were examined histologically to assess the myoblast engraftment and the muscle structure. The conclusions were as follows: (1) Needles should be thin enough to avoid important tissue damage and allow muscle regeneration as satisfactory as possible. Among those tested, 27G should be the choice if the length is consistent with depth of injection. (2) At least 100,000 cells should be delivered per centimeter of needle trajectory. (3) The smallest volumes of cell suspension per injection should be used. In this study, 1 µl/cm of injection trajectory was sufficient. In principle, these parameters apply to muscles in which no damage occurred other than the injections.

Intramuscular cell transplantation as a potential treatment of myopathies: clinical and preclinical relevant data

INTRODUCTION

Myopathies produce deficits in skeletal muscle function and, in some cases, literally progressive loss of skeletal muscles. The transplantation of cells able to differentiate into myofibers is an experimental strategy for the potential treatment of some of these diseases.

AREAS COVERED

Among the two routes used to deliver cells to skeletal muscles, that is intramuscular and intravascular, this paper focuses on the intramuscular route due to our expertise and because it is the most used in animal experiments and the only tested so far in humans. Given the absence of recent reviews about clinical observations and the profusion based on mouse results, this review prioritizes observations made in humans and non-human primates. The review provides a vision of cell transplantation in myology centered on what can be learned from clinical trials and from preclinical studies in non-human primates and leading mouse studies.

EXPERT OPINION

Experiments on myogenic cell transplantation in mice are essential to quickly identify potential treatments, but studies showing the possibility to scale up the methods in large mammals are indispensable to determine their applicability in humans and to design clinical protocols.

Intramuscular transplantation of human postnatal myoblasts generates functional donor-derived satellite cells

Myogenic cell transplantation is an experimental approach for the treatment of myopathies. In this approach, transplanted cells need to fuse with pre-existing myofibers, form new myofibers, and generate new muscle precursor cells (MPCs). The last property was fully reported following myoblast transplantation in mice but remains poorly studied with human myoblasts. In this study, we provide evidence that the intramuscular transplantation of postnatal human myoblasts in immunodeficient mice generates donor-derived MPCs and specifically donor-derived satellite cells. In a first experiment, cells isolated from mouse muscles 1 month after the transplantation of human myoblasts proliferated in vitro as human myoblasts. These cells were retransplanted in mice and formed myofibers expressing human dystrophin. In a second experiment, we observed that inducing muscle regeneration 2 months following transplantation of human myoblasts led to myofiber regeneration by human-derived MPCs. In a third experiment, we detected by immunohistochemistry abundant human-derived satellite cells in mouse muscles 1 month after transplantation of postnatal human myoblasts. These human-derived satellite cells may correspond totally or partially to the human-derived MPCs evidenced in the first two experiments. Finally, we present evidence that donor-derived satellite cells may be produced in patients that received myoblast transplantation.

Origin of cardiac fibroblasts and the role of periostin

Cardiac fibroblasts are the most populous nonmyocyte cell type within the mature heart and are required for extracellular matrix synthesis and deposition, generation of the cardiac skeleton, and to electrically insulate the atria from the ventricles. Significantly, cardiac fibroblasts have also been shown to play an important role in cardiomyocyte growth and expansion of the ventricular chambers during heart development. Although there are currently no cardiac fibroblast-restricted molecular markers, it is generally envisaged that the majority of the cardiac fibroblasts are derived from the proepicardium via epithelial-to-mesenchymal transformation. However, still relatively little is known about when and where the cardiac fibroblasts cells are generated, the lineage of each cell, and how cardiac fibroblasts move to reside in their final position throughout all four cardiac chambers. In this review, we summarize the present understanding regarding the function of Periostin, a useful marker of the noncardiomyocyte lineages, and its role during cardiac morphogenesis. Characterization of the cardiac fibroblast lineage and identification of the signals that maintain, expand and regulate their differentiation will be required to improve our understanding of cardiac function in both normal and pathophysiological states.

Paraoxical relationship between protein content and nucleolar activity in mammalian cardiomyocytes

It was recently demonstrated that polyploidization of the avian myocardium is associated with a reduction of cardiac aerobic capacity evaluated by the heart mass to body mass ratio (heart index). To investigate possible cellular correlates of polyploidization, the protein content and nucleolar activity per cell and per genome were examined by image cytometry in 21 mammalian species, differing in the degree of heart polyploidization and heart index. We found that average cardiomyocyte ploidy level correlates negatively with the animal heart index (r = –0.75, p < 10–4), i.e., the large heart of athletic mammals is polyploidized to a lesser degree than the relatively smaller heart of sedentary species, which confirms the picture observed in birds. The protein content per genome decreased with the elevation of cardiomyocyte ploidy level. This inverse correlation was especially pronounced with the removed effect of body mass (r = –0.79, p < 10–4). Surprisingly, these changes were accompanied by the increase of nucleolar activity per genome (r = 0.61, p < 10–3). In the two species, for which the microarray gene expression data were available (human and mouse), this increase was paralleled by the elevated expression of ribosomal protein genes (but there was no increase in the expression of tissue-specific genes). Thus, in the polyploid cardiomyocytes there is a misbalance between protein content per genome and ribosome biogenesis. The reduction of protein content (per genome) of polyploid cardio my ocytes should further curtail heart functionality (in addition to reduction of heart index), because it is known that cardio myocyte protein content consists of more than 90% contractile proteins. This finding makes doubtful a widespread notion that polyploidization is necessary for cell function. Because somatic polyploidization is associated with stressful conditions and impaired energetics, we suppose that additional genomes can serve for cell regeneration and as a defense against oxidative damage in the organs that work at the limit of their metabolic capacity.Key words: somatic polyploidy, heart, functional capacity, protein–DNA ratio.

Regenerative capacity of the myocardium: implications for treatment of heart failure

Research into myocardial regeneration has an exciting future, shown by the results of experimental and clinical work challenging the dogma that the heart is a postmitotic non-regenerating organ. Such studies have initiated a lively debate about the feasibility of novel treatment approaches leading to the recovery of damaged myocardial tissue. The possibility of reconstituting dead myocardium by endogenous cardiomyocyte replication, transplantation, or activation of stem cells--or even cloning of an artificial heart--is being advanced, and will be a major subject of future research. Although health expenditure for heart failure in the industrial world is high, we are still a long way from being able to treat the cause of reduced myocardial contractility. Despite the hopes of some people, conventional treatment for heart failure does not achieve myocardial regeneration. We present a virtual case report of a patient with acute myocardial infarction; we discuss treatment options, including strategies aimed at organ regeneration.

Spallanzani's mouse: a model of restoration and regeneration

The ability to regenerate is thought to be a lost phenotype in mammals, though there are certainly sporadic examples of mammalian regeneration. Our laboratory has identified a strain of mouse, the MRL mouse, which has a unique capacity to heal complex tissue in an epimorphic fashion, i.e., to restore a damaged limb or organ to its normal structure and function. Initial studies using through-and-through ear punches showed rapid full closure of the ear holes with cartilage growth, new hair follicles, and normal tissue architecture reminiscent of regeneration seen in amphibians as opposed to the scarring usually seen in mammals. Since the ear hole closure phenotype is a quantitative trait, this has been used to show-through extensive breeding and backcrossing--that the trait is heritable. Such analysis reveals that there is a complex genetic basis for this trait with multiple loci. One of the major phenotypes of the MRL mouse is a potent remodeling response with the absence or a reduced level of scarring. MRL healing is associated with the upregulation of the metalloproteinases MMP-2 and MMP-9 and the downregulation of their inhibitors TIMP-2 and TIMP-3, both present in inflammatory cells such as neutrophils and macrophages. This model has more recently been extended to the heart. In this case, a cryoinjury to the right ventricle leads to near complete scarless healing in the MRL mouse whereas scarring is seen in the control mouse. In the MRL heart, bromodeoxyuridine uptake by cardiomyocytes filling the wound site can be seen 60 days after injury. This does not occur in the control mouse. Function in the MRL heart, as measured by echocardiography, returns to normal.

Stem Cells for the Heart, Are We There Yet?

Although several repair mechanisms have been described in the human heart, all fall too short to prevent clinical heart disease in most acute or chronic pathological cardiac conditions. Moreover, despite many breakthroughs in cardiovascular medicine, the complications of a myocardial infarction such as chronic heart failure remains a serious worldwide problem. Bone marrow stem cells could provide for a promising strategy to restore myocardial infarctions and prevent postinfarct congestive heart failure, because there is growing body of evidence that bone marrow stem cells, such as mesenchymal stem cells, can generate new cardiomyocytes in animals and humans. In this review, we will discuss important issues on stem cell therapy for cardiac regeneration after myocardial infarction, which might be of paramount importance when considering future human trials.

The scarless heart

Over the past several years many mechanisms by which myocardial replacement could be achieved have been described. These include resident cardiac stem cells or circulating stem cells that can either differentiate into, or fuse to cardiomyocytes, or mature cells that can transdifferentiate into cardiomyocytes. However, the fact remains that after injury to the heart, the overriding response is scar formation with little myocardial replacement. One exception to this response is the MRL mouse, which heals with little scarring and shows nearly full myocardial replacement after injury. Results obtained with this model will be discussed.

Efficacy of myoblast transplantation in nonhuman primates following simple intramuscular cell injections: toward defining strategies applicable to humans

Nonhuman primates were used to define myoblast transplantation strategies applicable to humans. Nevertheless, previous experiments were based on the use of myotoxins concomitant with the myoblast injections. Since myotoxins must be avoided for clinical applications, we analyzed the efficacy of simple myoblast injections (i.e., myoblasts resuspended only in saline) into monkey muscles. We also evaluated different FK506 dosages (in combination or not with mycophenolate mofetil) for immunosuppression. Allogeneic myoblasts transduced with the beta-galactosidase (beta-Gal) gene were implanted in the muscles of 19 monkeys by injections placed 1 to 2 mm from each other. A biopsy was performed at the implanted sites 1 month later, and histologically studied for demonstration of beta-Gal+ myofibers, lymphocyte infiltration, and CD8+ cells. The presence of antibodies against the donor myoblasts and the blood levels of FK506 were analyzed. Our results show that: (1) If myoblast injections are sufficiently close to each other, high percentages of hybrid myofibers can be obtained following myoblast transplantation in primates (25 to 67% with an interinjection distance of 1 mm). (2) Efficient immunosuppression can be reached by increasing FK506 dosages, but also by combining this drug with mycophenolate mofetil, a combination that reduces toxic effects. The present results represent a step towards a better designing of myoblast transplantation strategies in humans.

Heart regeneration in adult MRL mice

The reaction of cardiac tissue to acute injury involves interacting cascades of cellular and molecular responses that encompass inflammation, hormonal signaling, extracellular matrix remodeling, and compensatory adaptation of myocytes. Myocardial regeneration is observed in amphibians, whereas scar formation characterizes cardiac ventricular wound healing in a variety of mammalian injury models. We have previously shown that the MRL mouse strain has an extraordinary capacity to heal surgical wounds, a complex trait that maps to at least seven genetic loci. Here, we extend these studies to cardiac wounds and demonstrate that a severe transmural, cryogenically induced infarction of the right ventricle heals extensively within 60 days, with the restoration of normal myocardium and function. Scarring is markedly reduced in MRL mice compared with C57BL/6 mice, consistent with both the reduced hydroxyproline levels seen after injury and an elevated cardiomyocyte mitotic index of 10-20% for the MRL compared with 1-3% for the C57BL/6. The myocardial response to injury observed in these mice resembles the regenerative process seen in amphibians.