Regulating fibrosis and muscle growth in the muscular dystrophies

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DESCRIPTION (provided by applicant): Muscular dystrophy is a genetic disease for which there is no cure. One of the most severe forms of muscular dystrophy is Duchenne Muscular Dystrophy (DMD). DMD and a subset of the limb girdle muscular dystrophies have in common disruption of the dystrophin protein complex. Disrupting the dystrophin complex lead to a fragile muscle membrane, loss of myofibers and replacement of the muscle with fibrosis or scarring. Multiple lines of evidence point to fibrosis is as a driver of muscular dystrophy pathology. We hypothesize that fibrosis provides a scaffold that promotes an unfavorable cytokine profile that further damages muscle. We further hypothesize that the primary components of the unfavorable cytokine profile are TGFp and the related TGFp family member myostatin. Together, TGFp and myostatin, lead to increased fibrosis, reduced muscle mass and regeneration, and aggravated membrane fragility. Therefore, we propose to determine the means by which TGFp and myostatin are normally sequestered by the matrix and held unavailable for receptor engagement and signaling and to determine how to promote inactivation of TGFp and myostatin in muscular dystrophy (Project 1). We will also demonstrate necessary proteolytic cleavage steps for release and processing of myostatin, and related molecules, and the degree to which soluble receptors can be effective in treating muscular dystrophy (Project 2). We will also sequentially assess the distinct intracellular signaling pathways that are triggered by TGFp and myostatin and test whether inhibiting these pathways improves muscle function and pathology in muscular dystrophy (Project 3). Three established investigators (McNally, Lee, and Molkentin) will lead these projects forming a distinctive team where their combined expertise will define the TGFp/myostatin pathway for therapeutic intent in muscular dystrophy. Three Cores will support the Projects; Core A will integrate the efforts at our three institutions to assure seamless collaboration and transfer of materials. Core B will provide histopathological assessment of muscular dystrophy after genetic manipulation and treatments, and Core C will perform functional analysis in vivo and provide support to Core B. PUBLIC HEALTH RELEVANCE: Muscular dystrophy is a devastating disorder that affects children and adults and for which there is no cure. We now understand that scarring plays an important role in furthering muscle weakness in the muscular dystrophies. Three premier investigators in the field of muscular dystrophy research will synergistically devise and implement new strategies to treat this disease. Our focus is on a pathway regulated by TGPp and myostatin and the goals of manipulating this pathway to reduce scarring, increase growth and promote muscle stability. PROJECT 1 Principal Investigator: Elizabeth M. McNally, M.D., Ph.D. Title: LTBPS as Regulators of TGF¿ and Myostatin Description (provided by applicant): Disruption of the dystrophin complex causes the muscle membrane to become fragile and highly susceptible to damage. Muscular dystrophy is defined by ongoing muscle degeneration combined with insufficient regeneration. Although muscle regeneration is ongoing in these disorders, it is ineffective and fibrofatty infiltration ultimately replaces muscle resulting in muscle loss and weakness. We recently used a mouse model of Limb Girdle Muscular Dystrophy (LGMD), the Sgcg null mouse lacking y-sarcoglycan, to ask whether genetic modifier genes can alter the outcome in muscular dystrophy. We measured outcome using two quantitative assessments of muscle pathology, Evans blue dye uptake (dye uptake) to measure membrane fragility and leakiness and collagen deposition to reflect scarring in the muscle. We found that both membrane fragility and scarring were both strongly modified by Ltbp4, the gene encoding the latent TGFB binding protein. Genetic and molecular data support that cleavage of LTBP4 releases TGFB, making it more available to the cells within muscle. Enhanced TGFp signaling promotes both membrane disruption and scarring in muscular dystrophy. Similarly, an increase in full length LTBP4 is protective of both membrane leakiness and fibrosis. Based on studies performed with the related LTBP family members, we hypothesize that LTBP4 also binds myostatin. We posit a model for muscular dystrophy pathogenesis where LTBP4 cleavage releases both myostatin and TGFB, thereby promoting muscular dystrophy through increased scarring and decreased muscle growth. In Aim 1, we will test this hypothesis by overexpressing the protective form of LTBP4 in myofibers and determining whether this reduces muscular dystrophy and downstream signaling. In Aim 2, we will determine whether LTBP4 binds myostatin and the degree to which LTBP4 mediates its effects through myostatin using myostatin null mice. In Aim 3, we will assess which proteases cleave LTBP4 since it is likely that both myostatin and LTBP4 are activated by the same proteases. The signaling aspects and protease experiments will be conducted in conjunction with Projects 2 and Project 3. Public Health Relevance: The McNally laboratory recently discovered a gene, called Ltbp4, that can modify the outcome of muscular dystrophy. We will now test that genetic discovery using techniques to increase LTBP4 and to determine whether it exerts its effects through myostatin as well as TGFp.