Physiological consequences of tropomyosin mutations associated with cardiac and skeletal myopathies.

Mutations have been identified in alpha-tropomyosin (Tm), a key regulatory protein in striated muscle cells, that are associated with a human cardiac myopathy, hypertrophic cardiomyopathy (HCM) and human skeletal myopathy, nemaline myopathy (NM). This dissertation describes experiments aimed at identifying the underlying mechanisms by which mutations in alpha-Tm cause inherited diseases of cardiac and skeletal muscle. The overall hypothesis was that the HCM and NM mutant alpha-Tm would act as dominant negatives but the HCM and NM mutation would differentially change the Ca2+ sensitivity of contraction within the differential ranges of Ca2+ that are important for regulation of contraction in cardiac and skeletal muscle, respectively. Thus, the different clinical presentation of HCM and NM results from the mutations differentially altering the ability of Tm to regulate muscle contraction. Gene transfer of normal and mutant alpha-Tm to isolated adult cardiac myocytes was used to study the primary effects of mutant alpha-Tm proteins on the structure and contractile function of fully differentiated striated muscle cells. Both HCM and NM mutant alpha-Tm proteins incorporated normally into the muscle sarcomere similar to normal Tm but exerted differential dominant-negative effects on the contractile function of the muscle cell. HCM mutant alpha-Tm. proteins produced a hypersensitivity of Ca2+ activated force production with a hierarchy that was related to the clinical severity of each mutation. Conversely, the NM mutant alpha-Tm produced a hyposensitivity of Ca2+-activated force production that may be related to muscle weakness observed in NM. Interestingly, these changes produced direct effects on the kinetics of contraction of intact muscle cells. HCM mutant alpha-Tm produced a marked slowing of muscle cell relaxation that suggests the mutation itself may be directly contributing to diastolic dysfunction in HCM patients. NM mutant alpha-Tm produced an acceleration of muscle cell relaxation, that may alter the force/frequency relationship in skeletal muscle. Taken together, these results suggest that direct effects of HCM mutations in alpha-Tm to cause myofilament sensitization to activating Ca2+ and slow muscle cell relaxation, and NM mutations in alpha-Tm cause myofilament desensitization to activating Ca2+ and speed muscle cell relaxation underlies the differential clinical presentation of cardiac and skeletal muscle myopathies associated with mutations in alpha-Tm.Ph.D.Animal PhysiologyBiological SciencesCellular biologyMolecular biologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132424/2/9963850.pd