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      Myogenic Akt signaling upregulates the utrophin–glycoprotein complex and promotes sarcolemma stability in muscular dystrophy

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          Abstract

          Duchenne muscular dystrophy is caused by dystrophin mutations that lead to structural instability of the sarcolemma membrane, myofiber degeneration/regeneration and progressive muscle wasting. Here we show that myogenic Akt signaling in mouse models of dystrophy promotes increased expression of utrophin, which replaces the function of dystrophin thereby preventing sarcolemma damage and muscle wasting. In contrast to previous suggestions that increased Akt in dystrophy was a secondary consequence of pathology, our findings demonstrate a pivotal role for this signaling pathway such that modulation of Akt can significantly affect disease outcome by amplification of existing, physiological compensatory mechanisms.

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          Most cited references21

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          The circuitry of a master switch: Myod and the regulation of skeletal muscle gene transcription.

          The expression of Myod is sufficient to convert a fibroblast to a skeletal muscle cell, and, as such, is a model system in developmental biology for studying how a single initiating event can orchestrate a highly complex and predictable response. Recent findings indicate that Myod functions in an instructive chromatin context and directly regulates genes that are expressed throughout the myogenic program, achieving promoter-specific regulation of its own binding and activity through a feed-forward mechanism. These studies are beginning to merge our understanding of how lineage-specific information is encoded in chromatin with how master regulatory factors drive programs of cell differentiation.
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            Expression of full-length utrophin prevents muscular dystrophy in mdx mice.

            Duchenne muscular dystrophy (DMD) is a lethal, progressive muscle wasting disease caused by a loss of sarcolemmal bound dystrophin, which results in the death of the muscle fiber leading to the gradual depletion of skeletal muscle. The molecular structure of dystrophin is very similar to that of the related protein utrophin. Utrophin is found in all tissues and is confined to the neuromuscular and myotendinous junctions in mature muscle. Sarcolemmal localization of a truncated utrophin transgene in the dystrophin-deficient mdx mouse significantly improves the dystrophic muscle phenotype. Therefore, up-regulation of utrophin by drug therapy is a plausible therapeutic approach in the treatment of DMD. Here we demonstrate that expression of full-length utrophin in mdx mice prevents the development of muscular dystrophy. We assessed muscle morphology, fiber regeneration and mechanical properties (force development and resistance to stretch) of mdx and transgenic mdx skeletal and diaphragm muscle. The utrophin levels required in muscle are significantly less than the normal endogenous utrophin levels seen in lung and kidney, and we provide evidence that the pathology depends on the amount of utrophin expression. These results also have important implications for DMD therapies in which utrophin replacement is achieved by delivery using exogenous vectors.
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              Association of dystrophin and an integral membrane glycoprotein.

              Duchenne muscular dystrophy (DMD) is caused by a defective gene found on the X-chromosome. Dystrophin is encoded by the DMD gene and represents about 0.002% of total muscle protein. Immunochemical studies have shown that dystrophin is localized to the sarcolemma in normal muscle but is absent in muscle from DMD patients. Many features of the predicted primary structure of dystrophin are shared with membrane cytoskeletal proteins, but the precise function of dystrophin in muscle is unknown. Here we report the first isolation of dystrophin from digitonin-solubilized skeletal muscle membranes using wheat germ agglutinin (WGA)-Sepharose. We find that dystrophin is not a glycoprotein but binds to WGA-Sepharose because of its tight association with a WGA-binding glycoprotein. The association of dystrophin with this glycoprotein is disrupted by agents that dissociate cytoskeletal proteins from membranes. We conclude that dystrophin is linked to an integral membrane glycoprotein in the sarcolemma. Our results indicate that the function of dystrophin could be to link this glycoprotein to the underlying cytoskeleton and thus help either to preserve membrane stability or to keep the glycoprotein non-uniformly distributed in the sarcolemma.
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                Author and article information

                Journal
                Hum Mol Genet
                hmg
                hmg
                Human Molecular Genetics
                Oxford University Press
                0964-6906
                1460-2083
                15 January 2009
                4 November 2008
                4 November 2008
                : 18
                : 2
                : 318-327
                Affiliations
                [1 ]Department of Physiological Science
                [2 ]Molecular Biology Institute, University of California , Los Angeles, CA 90095, USA
                [3 ]Molecular Cardiology, Whitaker Cardiovascular Institute, Boston University School of Medicine , Boston, MA 02118, USA
                Author notes
                [†]

                These authors contributed equally to this work.

                [* ]To whom correspondence should be addressed at: Department of Physiological Science, University of California Los Angeles , 621 Charles E. Young Drive South, Life Sciences Building Room 5804, Los Angeles, CA 90025, USA. Tel: +1 3107942103; Fax: +1 3102063987; Email: rcrosbie@ 123456physci.ucla.edu
                Article
                ddn358
                10.1093/hmg/ddn358
                2638781
                18986978
                664b3deb-f400-4bc3-a8ce-0d173c2261a5
                © 2008 The Author(s)

                This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 8 September 2008
                : 25 October 2008
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                Genetics
                Genetics

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