Determination, diversification and multipotency of mammalian myogenic cells.

In amniotes, myogenic commitment appears to be dependent upon signaling from neural tube and dorsal ectoderm, that can be replaced by members of the Wnt family and by Sonic hedgehog. Once committed, myoblasts undergo different fates, in that they can differentiate immediately to form the myotome, or later to give rise to primary and secondary muscle fibers. With fiber maturation, satellite cells are first detected; these cells contribute to fiber growth and regeneration during post-natal life. We will describe recent data, mainly from our laboratory, that suggest a different origin for some of the cells that are incorporated into the muscle fibers during late development. We propose the possibility that these myogenic cells are derived from the vasculature, are multi-potent and become committed to myogenesis by local signaling, when ingressing a differentiating muscle tissue. The implications for fetal and perinatal development of the whole mesoderm will also be discussed.

Shear stress increases the release of interleukin-1 and interleukin-6 by aortic endothelial cells.

BACKGROUND: The aim of this study was to determine the correlation between shear stress and the release of interleukin-1 (IL-1) and interleukin-6 (IL-6) by endothelial cells (EC). METHODS: Bovine aortic EC were seeded in fibronectin-coated cylinders at 1.0 x 10(6) cells/tube and allowed to reach confluence and to adhere for 48 hours. The experimental groups were subjected to a laminar flow of 100 ml/min (6 dyne/cm2). The control group was subjected to similar incubation conditions without flow. The release of IL-1 and IL-6 by EC was measured by enzyme-linked immunosorbent assay. RESULTS: Shear stress increased significantly (p < 0.01) the release of IL-1 and IL-6 by EC. The release of these two cytokines had different kinetics. CONCLUSIONS: Increasing shear stress facilitates release of IL-1 and IL-6 by EC. Previous reports have shown that IL-1 and IL-6 promote vascular smooth-muscle cell proliferation. Thus abnormal flow conditions with increasing shear stress may predispose to smooth-muscle cell proliferation that characterizes early atherosclerotic plaque development by an interleukin-mediated mechanism.

The mitotic clock in skeletal muscle regeneration, disease and cell mediated gene therapy.

The regenerative capacity of skeletal muscle will depend on the number of available satellite cells and their proliferative capacity. We have measured both parameters in ageing, and have shown that although the proliferative capacity of satellite cells is decreasing during muscle growth, it then stabilizes in the adult, whereas the number of satellite cells decreases during ageing. We have also developed a model to evaluate the regenerative capacity of human satellite cells by implantation into regenerating muscles of immunodeficient mice. Using telomere measurements, we have shown that the proliferative capacity of satellite cells is dramatically decreased in muscle dystrophies, thus hampering the possibilities of autologous cell therapy. Immortalization by telomerase was unsuccessful, and we currently investigate the factors involved in cell cycle exits in human myoblasts. We have also observed that insulin-like growth factor-1 (IGF-1), a factor known to provoke hypertrophy, does not increase the proliferative potential of satellite cells, which suggests that hypertrophy is provoked by increasing the number of satellite cells engaged in differentiation, thus possibly decreasing the compartment of reserve cells. We conclude that autologous cell therapy can be applied to specific targets when there is a source of satellite cells which is not yet exhausted. This is the case of Oculo-Pharyngeal Muscular Dystrophy (OPMD), a late onset muscular dystrophy, and we participate to a clinical trial using autologous satellite cells isolated from muscles spared by the disease.

Myoblast differentiation during mammalian somitogenesis is dependent upon a community effect.

The differentiation potential of early mammalian myogenic cells was tested under clonal culture conditions. Cells were isolated from paraxial mesoderm and limb buds of transgenic mouse embryos at 9.5 days after conception and grown in culture at clonal density either on collagen-coated dishes or on various feeder cell layers. The transgene used contained a reporter gene encoding beta-galactosidase with a nuclear localization signal under the control of regulatory sequences from the gene for fast myosin light chain 3, so that beta-galactosidase staining indicated the presence of differentiated muscle cells. After 5 days in culture, the number and size of beta-galactosidase-positive (beta-gal+) clones were recorded. Cells isolated from somites I-V (the last five somites to have formed) or from unsegmented paraxial mesoderm did not give rise to any beta-gal+ clones. Cells isolated from somites VI-X or from the forelimb bud gave rise to beta-gal+ clones, but only on feeder cells. Cells from somites XI or older gave rise to beta-gal+ clones independently of the substrate. However, when cells isolated from unsegmented paraxial mesoderm or somites I-V were cultured with nontransgenic cells from the trunk (including neural tube and notochord), differentiation occurred on condition that the cells were in a three-dimensional aggregate, even though their specific position in the somite had been lost. By culturing explants ranging in size from 1 to < 100 cells in the presence of an inhibitor of cell division, we determined that a minimal number of 30-40 cells is required for mesodermal cells to differentiate.

Activation of different myogenic pathways: myf-5 is induced by the neural tube and MyoD by the dorsal ectoderm in mouse paraxial mesoderm.

Newly formed somites or unsegmented paraxial mesoderm (UPM) have been cultured either in isolation or with adjacent structures to investigate the influence of these tissues on myogenic differentiation in mammals. The extent of differentiation was easily and accurately quantified by counting the number of beta-galactosidase-positive cells, since mesodermal tissues had been isolated from transgenic mice that carry the n-lacZ gene under the transcriptional control of a myosin light chain promoter, restricting expression to striated muscle. The results obtained showed that axial structures are necessary to promote differentiation of paraxial mesoderm, in agreement with previous observations. However, it also appeared that the influence of axial structures could be replaced by dorsolateral tissues, adjacent to the paraxial mesoderm. To elucidate which of these tissues exerts this positive effect, we cultured the paraxial mesoderm with a variety of adjacent structures, either adherent to the mesoderm or recombined in vitro. The results of these experiments indicated that the dorsal ectoderm exerts a positive influence on myogenesis but only if left in physical proximity to it. In contrast, lateral mesoderm delays the positive effect of the ectoderm (and has no effect on its own) suggesting that this tissue produces an inhibitory signal. To investigate whether axial structures and dorsal ectoderm induce myogenesis through common or separate pathways, we dissected the medial half of the unsegmented paraxial mesoderm and cultured it with the adjacent neural tube. We also cultured the lateral half of the unsegmented paraxial mesoderm with adjacent ectoderm. The induction of the myogenic regulatory factors myf-5 and MyoD was monitored by double staining of cultured cells with antibodies against MyoD and beta-galactosidase since the tissues were isolated from mouse embryos that carry n-lacZ targeted to the myf-5 gene, so that myf-5 expressing cells could be easily identified by either histochemical or immunocytochemical staining for beta-galactosidase. After 1 day in culture myogenic cells from the medial half expressed myf-5 but not MyoD, while myogenic cells from the lateral half expressed MyoD but not myf-5. By the next day in vitro, however, most myogenic cells expressed both gene products. These data suggest that the neural tube activates myogenesis in the medial half of paraxial mesoderm through a myf-5-dependent pathway, while the dorsal ectoderm activates myogenesis through a MyoD-dependent pathway. The possible developmental significance of these observations is discussed and a model of myogenic determination in mammals is proposed.

Transplantation of normal and DMD myoblasts expressing the telomerase gene in SCID mice.

The limited proliferative capacity of dystrophic human myoblasts severely limits their ability to be genetically modified and used for myoblast transplantation. The forced expression of the catalytic subunit of telomerase can prevent telomere erosion and can immortalize different cell types. We thus tested the ability of telomerase to immortalize myoblasts and analyzed the effect of telomerase expression on the success of myoblast transplantation. Telomerase expression did not significantly extend the human myoblast life span. The telomerase expressing myoblasts were nonetheless competent to participate in myofiber formation after infection with the retroviral vector. Although the new fibers obtained are less numerous than after the transplantation of normal myoblasts, these results demonstrate that the forced expression of telomerase does not block the ability of normal or dystrophic myoblasts to differentiate in vivo. It will be now necessary to determine the factors that prevent telomerase from extending the life span of human myoblasts before the potential of this intervention can be fully examined.

Regenerative capacity of human satellite cells: the mitotic clock in cell transplantation.

In this communication, we will review the problems caused by cell-mediated gene therapy, taking skeletal muscle as a physiological model. In particular we have utilised vectors transferring telomerase under the control of retroviral promoters into human satellite cells. The set of results presented here has several implications regarding gene therapy trials. Nevertheless, more experiments will be required to fully validate this cellular model and to use telomerase to safely extend the lifespan of putative gene therapy vectors.