We propose to investigate the mechanisms of force transmission in skeletal muscles. In particular, we will investigate the contribution of desmin and dystrophin, intracellular components of the membrane cytoskeleton, the membrane receptor alpha-7-integrin, and the extracellular molecular merosin to force transmission in diaphragm muscle. Desmin deficiency leads to desminopathy, a rare disease. Deficiencies of dystrophin, merosin, or alpha-7-integrin lead to various form of muscular dystrophy, which are more common diseases. Lack of any of these proteins causes skeletal muscle degeneration, chronic inspiratory muscle weakness, and ultimately respiratory insufficiency that leads to respiratory failure and eventually death. The diaphragm, unlike most other skeletal muscles, is loaded biaxially in vivo. That is the diaphragm experiences loads along muscle fibers and transverse to fibers during contractile activity. This application is an initial first step towards understanding the mechanical behavior of diaphragm muscle at the cellular level. Our central hypothesis is that force transmission in the diaphragm is modulated by transverse fiber loading and mediated by the linkage of specific intra- and extracellular members of the transmembrane protein network. This hypothesis will be tested by studying spontaneous and engineered mutant mouse strains; using strains missing key elements of the transmembrane protein network, we will test the response of the biaxial mechanical properties of the diaphragm and hindlimb muscles to the absence of these proteins. The long term goals of this research program are to understand muscle force transmission in skeletal muscles at the protein level and build a detailed model of mechanical coupling in normal skeletal muscles that explains the mechanism(s) by which force is transmitted from cytoskeleton to extracellular matrix.
The specific aims of this project are to determine passive mechanical properties of the mouse diaphragm and their influence on contractile function and to evaluate the role of intracellular, transmembrane, and extracellular elements on the biaxial transmission of force in the diaphragm. Using a electron microscopy and biaxial loading technique applied to whole diaphragm and limb skeletal muscles in vitro, we will test the following hypotheses at both tissue and sarcomere levels: (1) transverse stress mediates force transmission in the normal diaphragm at both tissue and at sarcomere levels, and both passive and contractile properties of the diaphragm are altered by the presence of transverse stress; (2) intracellular members of the transmembrane protein network, desmin and dystrophin, are essential in integrating transverse and longitudinal mechanical properties of the diaphragm, and the strength of the mechanical linkage between myofibrils and the plasma membrane is determined primarily by these proteins; and (3) the mechanical coupling between myofibrils and extracellular matrix is crucial to force transmission along and transverse to the fibers in normal skeletal muscles, and force transmission is compromised by loss of either alpha-7-integrin or merosin.
These aims address the mechanism(s) by which force transmission is mediated by specific cytoskeletal and extracellular proteins in skeletal muscles.
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