Despite significant advances in diagnosis and treatment of heart diseases, cardiomyopathies remain the most prevalent cause of death in developed countries. Mutations in cardiac troponin T (cTnT) are responsible for ~7% of all familial hypertrophic cardiomyopathy (FHC) cases. Patients carrying these FHC related cTnT mutations show a high incidence of sudden death without the classical increase in the left ventricular heart wall seen in FHC patients with mutations in other proteins. This proposal focuses on the role of the proteasome in troponin related cardiomyopathies. The molecular mechanisms that regulate the cardiac proteasome and their role in cardiomyopathies are unknown. Transgenic mice expressing a mutant (I79N) cTnT that is associated with FHC exhibited changes in both post-translational modifications of proteasome subunits and the activity of the proteasome, but did not cause significant changes in the expression of the proteasome subunits investigated. Changes in phosphorylation levels of the proteasome were observed concomitant with decreases in all three 20S and 26S proteasome activities. It is critical to understand the importance of the proteasome system in cardiomyopathies to be able to properly target this key proteolytic complex for future cardiovascular benefit. The amount of a key phosphatase associated with the 20S proteasome is also decreased in 20S proteasomes isolated from I79N hearts. This is important since our results also suggest that the kinases and phosphatases associated with the proteasome complex inside the heart are important modulators of the proteasome activity, and that the cardiac proteasome is unlike proteasomes from other tissues. Based upon our results we hypothesize: 1) Significant increases in myofilament Ca2+-sensitivity (>0.1pCa units) contribute to cellular alterations in signaling that lead to proteasome dysfunction which results in increased ubiquitinated proteins and cardiac dysfunction, 2) Some FHC related mutations in troponin directly affect the ability of these proteins to be degraded by the proteasome and alter proteasome activity, and 3) Kinases and phosphatases function as associating proteins for proteasomal complexes in the Troponin-related cardiomyopathies;they play a critical role in modulating the proteasomal function as part of the proteasomal subproteome. To investigate these hypotheses we will investigate three specific aims: 1) To Delineate the Roles of the 20S and 26S Proteasomes in Troponin-related Cardiomyopathies, 2) To Characterize the Temporal Profile of the Proteasome complexes in Troponin-related Cardiomyopathies, and 3) To Characterize Cardiomyopathy Induced Phosphorylation Changes in 26S Proteasomes.
Familial hypertrophic cardiomyopathy (FHC) is the most common inherited heart disease with a prevalence of at least 0.2% and is the most common cause of sudden death in adolescents and young adults, especially in athletes. Dysfunction of the ubiquitin proteasome system (UPS) has been associated pressure overload induced hypertrophy, ischemia/reperfusion, and congestive heart failure. Investigating the role of the proteasome in FHC will give critical insights into proteasomal function in cardiomyopathies.
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