Intracellular Sorting of Connexin 43 by the Ubiquitin Ligase Wwp1

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DESCRIPTION (provided by applicant): Cardiac arrhythmias associated with hypertrophic myopathy (HCM, the most common genetic myocardial disease) are the leading cause of sudden death in athletes and young people. Grossly, HCM is characterized by left ventricular hypertrophy (LVH) in the absence of identifiable clinical causes. Histologically, the myocardium of affected individuals displays myocyte hypertrophy and disarray, disorganization of intercalated discs with altered localization of Connexin 43 (Cx43), and, usually, increased amounts of interstitial fibrosis. Although there is a correlation among prognosis, the degree of LVH, amount of fibrosis, and causative mutation, this risk stratification methodology is limited in scope. In particular, the precise mechanisms underlying ventricular arrhythmias in patients with little-to-moderate LVH and are poorly defined, especially in pediatric HCM patients, although the regulation of Cx43 likely plays a critical role. We have recently created an inducible transgenic mouse model which develops a HCM-like phenotype when the ubiquitin ligase Wwp1 is globally overexpressed. Interestingly, these mice die very suddenly at 6-12 weeks of age (equivalent to adolescence or early adulthood in humans), and this phenotype is 100% penetrant. Similar to human HCM, transgenic animals display an increase in heart weight-to-body weight ratio, moderate LVH with an accompanying transcriptional activation of hypertrophic markers, myocyte disarray, a disruption of intercalated discs, dramatically decreased Cx43 protein levels, and an irregular heart rate. Importantly, we have been able to show that WWP1 is normally expressed in vesicles within human cardiomyocytes, WWP1 is upregulated and mislocalized in HCM, and Wwp1 can ubiquitinate Cx43. Based on these observations, we posit the following central hypothesis: increased Wwp1-mediated ubiquitination of Cx43 causes lysosomal degradation of Cx43, ensuing arrhythmia, and sudden death. In order to test this hypothesis, we aim to 1) define the role of Wwp1 overexpression in cardiomyocytes and its ability to promote the HCM phenotype; 2) determine the mechanism of Wwp1-mediated intracellular trafficking of Cx43; and 3) identify the Wwp1-mediated ubiquitination site(s) on Cx43 to determine the functional significance of ubiquitination of this/these site(s). Accomplishing the Specific Aims outlined here will elucidate the molecular mechanism underlying gap junction remodeling. This information can then be applied to HCM, a frequent and devastating disease known to display mislocalized Cx43 and increased WWP1 expression, in order to identify patient populations that might be at greater risk for fatal arrhythmias.