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      The p38α/β MAPK functions as a molecular switch to activate the quiescent satellite cell

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          Abstract

          Somatic stem cells cycle slowly or remain quiescent until required for tissue repair and maintenance. Upon muscle injury, stem cells that lie between the muscle fiber and basal lamina (satellite cells) are activated, proliferate, and eventually differentiate to repair the damaged muscle. Satellite cells in healthy muscle are quiescent, do not express MyoD family transcription factors or cell cycle regulatory genes and are insulated from the surrounding environment. Here, we report that the p38α/β family of mitogen-activated protein kinases (MAPKs) reversibly regulates the quiescent state of the skeletal muscle satellite cell. Inhibition of p38α/β MAPKs (a) promotes exit from the cell cycle, (b) prevents differentiation, and (c) insulates the cell from most external stimuli allowing the satellite cell to maintain a quiescent state. Activation of satellite cells and p38α/β MAPKs occurs concomitantly, providing further support that these MAPKs function as a molecular switch for satellite cell activation.

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          Most cited references 37

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          Myogenic satellite cells: physiology to molecular biology.

          Adult skeletal muscle has a remarkable ability to regenerate following myotrauma. Because adult myofibers are terminally differentiated, the regeneration of skeletal muscle is largely dependent on a small population of resident cells termed satellite cells. Although this population of cells was identified 40 years ago, little is known regarding the molecular phenotype or regulation of the satellite cell. The use of cell culture techniques and transgenic animal models has improved our understanding of this unique cell population; however, the capacity and potential of these cells remain ill-defined. This review will highlight the origin and unique markers of the satellite cell population, the regulation by growth factors, and the response to physiological and pathological stimuli. We conclude by highlighting the potential therapeutic uses of satellite cells and identifying future research goals for the study of satellite cell biology.
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            p38 MAP kinases: key signalling molecules as therapeutic targets for inflammatory diseases.

            The p38 MAP kinases are a family of serine/threonine protein kinases that play important roles in cellular responses to external stress signals. Since their identification about 10 years ago, much has been learned of the activation and regulation of the p38 MAP kinase pathways. Inhibitors of two members of the p38 family have been shown to have anti-inflammatory effects in preclinical disease models, primarily through the inhibition of the expression of inflammatory mediators. Several promising compounds have also progressed to clinical trials. In this review, we provide an overview of the role of p38 MAP kinases in stress-activated pathways and the progress towards clinical development of p38 MAP kinase inhibitors in the treatment of inflammatory diseases.
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              Single-cell analysis of regulatory gene expression in quiescent and activated mouse skeletal muscle satellite cells.

              Repair and regeneration of adult skeletal muscle are mediated by satellite cells. In healthy muscle these rare mononucleate muscle precursor cells are mitotically quiescent. Upon muscle injury or degeneration, members of this self-renewing pool are activated to proliferate and then differentiate. Here we analyzed in single satellite cells the expression of a set of regulatory genes that are candidates for causal roles in satellite cell activation, maturation, and differentiation. Individual cells were identified as satellite cells and selected for analysis based on their physical association with single explanted myofibers or their position beneath the basal lamina in unperturbed muscle tissue. Using a multiplex single-cell RT-PCR assay we simultaneously monitored expression of all four MyoD family regulators of muscle determination and differentiation (MRFs) together with two candidate markers of satellite cell identity, c-met and m-cadherin. By making these measurements on large numbers of individual cells during the time course of satellite cell activation, we were able to define which expression states (possible combinations of the six genes) were represented and to specify how the representation of each state changed with time. Activated satellite cells began to express either MyoD or myf5 first among the MRFs; most cells then expressed both myf-5 and MyoD simultaneously; myogenin came on later in cells expressing both MyoD and myf5; and many cells ultimately expressed all four MRFs simultaneously. The results for fiber-associated satellite cells from either predominantly fast or slow muscles were indistinguishable from each other. The c-met receptor tyrosine kinase was also monitored because it is a candidate for mediating activation of quiescent satellite cells (Allen et al., 1995) and because it might also be a candidate molecular marker for satellite cells. A significant difficulty in studying mouse satellite cells has been the absence of molecular markers that could identify them in the quiescent state before expression of MRFs or desmin and distinguish them from fibroblasts. We show here that c-met receptor is present beneath the basal lamina on presumptive satellite cells in intact muscle and that c-met mRNA and protein are expressed by all myofiber-associated satellite cells from the time of explant through the course of activation, proliferation, and differentiation. c-met was not detected in muscle-derived fibroblasts or in other mononucleate cells from healthy muscle explants. When compared directly with m-cadherin, which has previously been suggested as a marker for quiescent satellite cells, m-cadherin mRNA was detected only in a small subset of satellite cells at early times after myofiber explant. However, at late times following activation (by 96 hr in this fiber culture system), c-met and m-cadherin were uniformly coexpressed. From the individual satellite cell expression types observed, a model of the satellite cell population at rest and during the time course of activation was generated.
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                Author and article information

                Journal
                J Cell Biol
                The Journal of Cell Biology
                The Rockefeller University Press
                0021-9525
                1540-8140
                11 April 2005
                : 169
                : 1
                : 105-116
                Affiliations
                [1 ]Bayer Corporation, Research Triangle Park, NC 27709
                [2 ]Dharmacon Research, Lafayette, CO 80026
                [3 ]Department of Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, CO 80309
                Author notes

                Correspondence to Bradley B. Olwin: Bradley.Olwin@ 123456colorado.edu

                Article
                200408066
                10.1083/jcb.200408066
                2171902
                15824134
                Copyright © 2005, The Rockefeller University Press
                Categories
                Research Articles
                Article

                Cell biology

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