Muscle Myopathies

 

striated-muscle-group

Muscle contraction is essential for the movement of joints (in the case of skeletal muscle) and propulsion of blood through the vasculature (in the case of cardiac muscle), and normal breathing (contraction of the diaphragm). Muscle cells are composed of densely packed myofibrils, which are, in turn, composed of sarcomeres arranged in series. Sarcomeres are a strikingly regular lattice of interdigitating actin “thin” filaments and myosin “thick” filaments that slide past one another and generate the forces of muscle contraction. Thin and thick filament lengths and overlap determine the force of muscle contractions. Thin filament lengths vary in different muscles and during development and aging, and can become mis-specified in some congenital myopathies, such as in some nemaline myopathies (NM) with short thin filaments, and in mouse models of Duchenne muscular dystrophy resulting in abnormally long thin filaments. Tropomodulins (Tmod1 and Tmod4) cap thin filament pointed ends, CapZ caps barbed ends, and tropomyosin (TM), copolymerizes with actin along the entire thin filament length and is critical for regulating actin/myosin crossbridge formation. Our work has shown that Tmod1 is required for the precise regulation of thin filament length in sarcomeres.

Nemaline myopathy (NM) is a congenital skeletal muscle disease that results in muscle weakness and pathological actin filament aggregation into nemaline rods within muscle cells. NM is caused by mutations in proteins that encode thin filament and actin-regulatory proteins, including tropomyosins, troponins, nebulin and cofilin. Recently, human leiomodin-3 (Lmod3) mutations were found to cause nemaline myopathy with shorter thin filaments and impaired contractility, leading to severe muscle weakness and respiratory insufficiency. Using muscle biopsies from human NM patients or mouse models, our research investigates developmental or adaptive regulation of thin filament lengths, using confocal microscopy and super-resolution quantitative image analysis with our DDecon software to obtain precise length measurements. To understand the cellular mechanisms of NM pathogenesis, NM mutations in actin-regulatory proteins with effects on thin filament length or dynamics in humans (e.g., TM, nebulin, cofilin, and Lmod3) will be expressed in mouse muscle cells, where actin dynamics, myofibril assembly and homeostasis will be examined. Mutant proteins will also be tested for functionality in actin binding and pyrene-actin polymerization assays in vitro. A novel mouse model of human NM with a mutation in the tropomyosin3 gene has been developed and will be characterized structurally and functionally for preclinical studies. These proposed experiments will have broad relevance to the NM clinical community and suggest novel and unconventional therapeutic interventions for this debilitating disorder.

 

Relevant publications:

Gokhin DS, Fowler VM. Software-based measurement of thin filament lengths: an open-source GUI for Distributed Deconvolution analysis of fluorescence images. J Microsc. 2017 Jan; 265(1):11-20

Gokhin DS, Ochala J, Domenighetti AA, Fowler VM Tropomodulin 1 directly controls thin filament length in both wild-type and tropomodulin 4-deficient skeletal muscle. Development. 2015 Dec 15;142(24):.