Arrhythmogenic cardiomyopathy (AC) is a genetic based heart disease characterized by ventricular dysfunction, fibrofatty replacement of the myocardium and ventricular arrhythmias leading to sudden cardiac death. Recent human clinical data has revealed that only 40% of AC patients harbor mutations in components of the desmosome (i.e. desmoplakin (DSP), desmoglein, desmocollin, plakoglobin, plakophilin), an intercellular structure that establishes a mechanical link between adjacent cells. The remaining causative mutations are largely unknown, suggesting that identification and characterization of novel desmosomal interacting proteins is of utmost importance. To address this challenge, we set out to identify novel desmosome associated proteins by performing a yeast-2-hybrid screen with DSP as bait in an adult human heart cDNA library. In this screen, we identified synaptosomal associated protein 29 (SNAP29), a vesicular trafficking protein with roles in mediating membrane fusion, though its function in the heart has not been established. In humans, SNAP29 mutations are associated with the desmosomal disease, CEDNIK syndrome, which includes neurological manifestations as well as skin defects associated with desmosomal instability. Our preliminary data show that SNAP29 colocalizes with DSP at the distal ends of cells where desmosomes are located in adult mouse and human hearts, suggesting that this interaction occurs in the intact heart. In hearts of DSP cardiac specific knockout (DSP-cKO) mice which exhibit features of AC, SNAP29 is reduced in the junctional fraction indicating a functional interaction between SNAP29 and DSP in the heart. Conversely, using hearts from our recently generated SNAP29 global knockout (SNAP29-gKO) mice which exhibit preweaning lethality, increased heart weight to body weight ratios, and skin defects, there is a reduction in DSP in the junctional fraction providing striking evidence for a functional relationship between SNAP29, DSP, and the desmosome. The importance of SNAP29 in heart function is further supported by our experiments knocking down SNAP29 in zebrafish which result in cardiac defects and arrhythmias. We hypothesize that SNAP29 is a novel component of the cardiac desmosome and that loss of SNAP29 will lead to defects in cardiac desmosome ultrastructure and stability leading to impaired heart function and arrhythmias. To test this hypothesis, our aims are as follows: i) to characterize the molecular interactions between SNAP29 and DSP as well as understand the relevance of these interactions in AC using forced yeast-2-hybrid and cardiomyocyte overexpression of mutant sequences and ii) to characterize the role of SNAP29 in the developing and adult heart by assessing the cardiac phenotype of global SNAP29 null mice and cardiac-specific SNAP29 null mice by determining a) levels and localization of desmosomal and vesicular trafficking proteins using fluorescent immunostaining and western blot, b) desmosome ultrastructure using transmission electron microscopy, c) cardiac morphology and function using histological techniques, echocardiography, MRI, and surface electrocardiography.
The goal of this project is to understand the role of new molecular players in the early heart development and progression of a genetic form of heart disease; arrhythmogenic cardiomyopathy (AC). Using gene targeted animal and in vitro model systems; these studies will allow us to improve our understanding of the molecular mechanisms underlying the development and disease pathogenesis of AC by highlighting new molecular pathways and potential therapeutic targets important to these processes.
|Mezzano, Valeria; Pellman, Jason; Sheikh, Farah (2014) Cell junctions in the specialized conduction system of the heart. Cell Commun Adhes 21:149-59|