Thymic myoid cells as a source of cells for myoblast transfer

Gill morphology and morphometry of the facultative air-breathing armoured catfish, Corydoras paleatus, in relation on aquatic respiration

The Neotropical armoured catfish Corydoras paleatus is a facultative air-breathing teleost commonly exported as ornamental fish. In this species, air breathing enables it to survive and inhabit freshwater environments with low oxygen levels. Therefore, it is important to analyse the gills from a morphological aspect and its dimensions in relation to body mass with reference to aquatic respiration. For that, the gills were analysed using a stereoscopic microscope for morphometric studies, and structural and ultrastructural studies were carried out to compare the four branchial arches. Furthermore, two immunohistochemical techniques were used to locate and identify the presence of a Na⁺ /K⁺ pump. The characterization of the potential for cell proliferation of this organ was assessed using an anti-PCNA antibody. The results show that gills of C. paleatus present some characteristics related to its diet and lifestyle, such as the limited development of gill rakers and the abundance of taste buds. In addition, other special features associated with the environment and bimodal breathing were observed: scarce and absent mucous cells (MCs) in the gill filaments and branchial lamellae, respectively, and the localization of mitochondria-rich cells (MRCs) covering the basal third of the branchial lamellae, which reduces the gill respiratory area. A peculiar finding in the gill epithelium of this armoured catfish was the presence of mononuclear cells with sarcomeres similar to myoid cells, whose functional importance should be determined in future studies. Finally, in C. paleatus, the interlamellar space of gill filaments is an important site for cell turnover and ionoregulation; the latter function is also performed by the branchial lamellae.

The Inherent Human Aging Process and the Facilitating Role of Exercise

Arguably the best available depictions of the global physiological changes produced by age are the profiles of world record performance times in swimming, athletics, and cycling, depicting the trajectory of decline in maximal integrated physiological performance capability. The curves suggest that the aging process produces a synchronized, controlled decrease in physiological performance over the human lifespan. The shape of the performance profile by age is essentially independent of discipline, distance, or phenotype. Importantly, the specific times of performance are not the driving force in the production of the shape of the declining performance profile. We suggest that in these highly trained individuals the shape of the curve is generated by the aging process operating on a physiology optimized for any given age. We hypothesize that with adequate training this same profile and trajectory, but with lower performance times, would be generated by all individuals who engage in sufficient physical activity/exercise. Unlike performance, data obtained from examining individual physiological systems or tissues do not give information on the unceasing and changing global integrating functions of the aging process. However, these data do give valuable information about the integrity of physiological systems at a particular age and allow a direct comparison to be made between the effects of inactivity and physical activity/exercise. Being physically active has been shown to have global protective effects on physiological systems and thus facilitates the aging process by maintaining physiological integrity. There is emerging evidence which suggests that physiological regulation of aging may be multi-compartmentalized. We do not advocate exercise as a panacea, but all the evidence indicates that being physically active and exercising is far superior to any other alternative for achieving optimal aging.

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.

The satellite cell as a companion in skeletal muscle plasticity: currency, conveyance, clue, connector and colander

Satellite cells are companions to voluntary muscle fibres, and are named for their intimate positional or ;satellite' relationship, as if revolving around fibres, like a satellite moon around the earth. Studies on the nature of at least some satellite cells, including their capabilities for self-renewal and for giving rise to multiple lineages in a stem cell-like function, are exploring the molecular basis of phenotypes described by markers of specialized function and gene expression in normal development, neuromuscular disease and aging. In adult skeletal muscle, the self-renewing capacity of satellite cells contributes to muscle growth, adaptation and regeneration. Muscle remodeling, such as demonstrated by changes in myofibre cross-sectional area and length, nerve and tendon junctions, and fibre-type distribution, occur in the absence of injury and provide broad functional and structural diversity among skeletal muscles. Those contributions to plasticity involve the satellite cell in at least five distinct roles, here described using metaphors for behaviour or the investigator's perspective. Satellite cells are the 'currency' of muscle; have a 'conveyance' role in adaptation by domains of cytoplasm along a myofibre; serve researchers, through a marker role, as 'clues' to various activities of muscle; are 'connectors' that physically, and through signalling and cell-fibre communications, bridge myofibres to the intra- and extra-muscular environment; and are equipped as metabolic and genetic filters or 'colanders' that can rectify or modulate particular signals. While all these roles are still under exploration, each contributes to the plasticity of skeletal muscle and thence to the overall biology and function of an organism. The use of metaphor for describing these roles helps to clarify and scrutinize the definitions that form the basis of our understanding of satellite cell biology: the metaphors provide the construct for various approaches to detect or test the nature of satellite cell functions in skeletal muscle plasticity.

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.

Muscle-derived stem cells: Implications for effective myoblast transfer therapy

Stem cells have been proposed as a wonder solution for tissue repair in many situations and have attracted much attention in the media for both their therapeutic potential and ethical implications. In addition to the excitement generated by embryonic stem cells, research has now identified a number of stem cells within adult tissues which pose much more realistic targets for therapeutic interventions. Myoblast transfer therapy (MTT) has long been viewed as a potential therapy for the debilitating muscle-wasting disorder Duchenne Muscular Dystrophy. This technique relies on the transplantation of committed muscle precursor cells directly into the muscle fibres but has had little success in clinical trials. The recent discovery of a population of cells within adult muscle with stem cell-like characteristics has interesting implications for the future of such putative cell transplantation therapies. This review focuses on the characterization and application of these potential muscle-derived stem cells (MDSC) to MTT.

Thymic myoid cells express high levels of muscle genes

To explore the possible contribution of thymic myoid cells in tolerance induction mechanisms, we quantified by real-time RT-PCR, the expression of 12 muscle genes (the five subunits of acetylcholine receptor, Musk, rapsyn, utrophin, ErbB2, ErbB3, troponin T, and MCK) in a human thymic myoid cell line (MITC), compared to thymic epithelial cells (TEC) and thymocytes. Although expression of all the genes analyzed was detected in TEC and thymocytes, the level of expression in these cells was much lower than in MITC, except for -AChR, utrophin and ErbB3 genes. Since myoid cells express high level of most muscle genes and are consistently found in the thymic medulla, they may contribute to the mechanisms involved in the induction and maintenance of immune tolerance.

Differentiation of skeletal muscle from pituitary folliculo-stellate cells and endocrine progenitor cells

We previously reported the ectopic differentiation of skeletal muscle cells in a pituitary gland transplanted beneath a kidney capsule. Morphological observation suggested that the skeletal muscle cells may have differentiated from folliculo-stellate (FS) cells in the anterior pituitary gland. However, at that time, we did not confirm this directly with an in vitro system. To obtain direct evidence, we used the Tpit/F1 cell line. The Tpit/F1 cell line was recently established from the pituitary gland of a temperature-sensitive T antigen transgenic mouse and has the characters of pituitary FS cells. Using Tpit/F1 cells, we have found that FS cells of the pituitary are able to differentiate into muscle cells in vitro. Additionally, we showed that the cells have some characteristics of pituitary FS cells and also express pituitary endocrine cell-specific transcription factor (pit-1) and prolactin genes, and can differentiate into striated muscle cells. The anterior pituitary gland is known to be of ectodermal origin, so the differentiation of its cells into striated muscle is completely unexpected. This is the first report of direct evidence of ectopic differentiation of skeletal muscle cells from pituitary cells.

Myogenic cell proliferation and generation of a reversible tumorigenic phenotype are triggered by preirradiation of the recipient site

Environmental influences have profound yet reversible effects on the behavior of resident cells. Earlier data have indicated that the amount of muscle formed from implanted myogenic cells is greatly augmented by prior irradiation (18 Gy) of the host mouse muscle. Here we confirm this phenomenon, showing that it varies between host mouse strains. However, it is unclear whether it is due to secretion of proliferative factors or reduction of antiproliferative agents. To investigate this further, we have exploited the observation that the immortal myogenic C2 C12 cell line forms tumors far more rapidly in irradiated than in nonirradiated host muscle. We show that the effect of preirradiation on tumor formation is persistent and dose dependent. However, C2 C12 cells are not irreversibly compelled to form undifferentiated tumor cells by the irradiated muscle environment and are still capable of forming large amounts of muscle when reimplanted into a nonirradiated muscle. In a clonal analysis of this effect, we discovered that C2 C12 cells have a bimodal propensity to form tumors; some clones form no tumors even after extensive periods in irradiated graft sites, whereas others rapidly form extensive tumors. This illustrates the subtle interplay between the phenotype of implanted cells and the factors in the muscle environment.

The role of stem cells in skeletal and cardiac muscle repair

In postnatal muscle, skeletal muscle precursors (myoblasts) can be derived from satellite cells (reserve cells located on the surface of mature myofibers) or from cells lying beyond the myofiber, e.g., interstitial connective tissue or bone marrow. Both of these classes of cells may have stem cell properties. In addition, the heretical idea that post-mitotic myonuclei lying within mature myofibers might be able to re-form myoblasts or stem cells is examined and related to recent observations for similar post-mitotic cardiomyocytes. In adult hearts (which previously were not considered capable of repair), the role of replicating endogenous cardiomyocytes and the recruitment of other (stem) cells into cardiomyocytes for new cardiac muscle formation has recently attracted much attention. The relative contribution of these various sources of precursor cells in postnatal muscles and the factors that may enhance stem cell participation in the formation of new skeletal and cardiac muscle in vivo are the focus of this review. We concluded that, although many endogenous cell types can be converted to skeletal muscle, the contribution of non-myogenic cells to the formation of new postnatal skeletal muscle in vivo appears to be negligible. Whether the recruitment of such cells to the myogenic lineage can be significantly enhanced by specific inducers and the appropriate microenvironment is a current topic of intense interest. However, dermal fibroblasts appear promising as a realistic alternative source of exogenous myoblasts for transplantation purposes. For heart muscle, experiments showing the participation of bone marrow-derived stem cells and endothelial cells in the repair of damaged cardiac muscle are encouraging.

Molecular pathophysiology and targeted therapeutics for muscular dystrophy

Experimental therapeutics of the muscular dystrophies has made impressive advances on several fronts. Adeno-associated virus has emerged as the clear 'vector of choice' for muscle gene delivery, with successful functional rescue of dystrophic muscle in rodent models. Correction of the dystrophin gene mutation in a dog model has been reported, and several reports of progress on myogenic stem cell characterization are resurrecting cell transplantation as a possible therapeutic approach. The downstream consequences of dystrophin deficiency are being defined quickly using microarray experiments, and drugs targeting specific biochemical pathways are being tested rapidly in animal models. Such targeted drug discoveries, which are discussed in this article, have begun to be implemented in human clinical trials.

Immunobiology and the future of myoblast transfer therapy

Myoblast transfer therapy (MTT) is a cell-mediated gene transfer method aimed at the restoration of normal dystrophin expression in Duchenne muscular dystrophy (DMD). Initial clinical MTT trials were conducted amid much controversy, as they were based on very few animal studies. Unfortunately, the trials were of little therapeutic benefit. As a result, there has been a renaissance of interest in experimental studies in animal models. In MTT, myoblasts are obtained by muscle biopsy from normal, i.e., dystrophin-positive, donors, expanded in culture, and injected directly into the muscles of dystrophic recipients. The major requirement for successful MTT is the survival of injected donor myoblasts in the host environment. However, a vast majority of donor cells fail to survive for more than 1 h after injection, and very few last beyond the first week. This review on the immunological aspects of MTT focuses in particular on the roles of specific components of the host immune response, the effects of tissue culture on donor cells, and strategies under development to circumvent the problem of donor myoblast death after injection in vivo.