Thursday, April 3, 2025
4/3/2025
Junctophilin-1 in the heart - PROJECT SUMMARY / ABSTRACT Heart failure (HF) is a leading cause of morbidity and mortality worldwide. HF is associated with high risk of fatal ventricular arrhythmias and sudden cardiac death. The pathophysiological basis of systolic HF includes the impaired function of existing cardiomyocytes and their loss. While cardiac regeneration is a hot area of study to regenerate new cardiomyocytes, it is equally important to continue improving our understanding of the mechanisms underlying cardiomyocyte damage and dysfunction in HF. At a cellular level, defects in excitation- contraction (E-C) coupling or Ca2+ signaling is a hallmark of HF. Cardiomyocyte E-C coupling occurs in specialized ultrastructural domains, i.e., cardiac dyads or junctional membrane complexes (JMCs), which are established by physical couplings between the transverse tubules (T-tubules) and the terminal cisternae of sarcoplasmic reticulum (SR). The Junctophilin family proteins (JPs) are critical in establishing and organizing JMCs. There are four isoforms of JPs found in excitable tissues (JP1-4). It is generally accepted that JP2 is the only JP isoform found in cardiac muscle, responsible for the formation of JMCs. Our lab recently found that JP1, enriched in skeletal muscle, is also expressed in cardiac muscle, although at a much lower level in both human and mouse heart tissues. To dissect the function of JP1 in the heart, we generated cardiac specific JP1 knockout (JP1cKO) and 3xHA-tagged JP1 knockin (3xHA-JP1KI) mouse models. Our preliminary data demonstrate that JP1 co-localizes with JP2 and RyR2, the primary Ca2+ release channel, in the Z-disc. JP1cKO mice progressively develop systolic HF and die prematurely. Despite genetic ablation of JP1 in these mice, JP2 protein expression does not change indicating that JP2 is not sufficient to overcome the loss of JP1 in the heart. This led us to hypothesize that JP1 is required for normal cardiac function and is functionally distinct from JP2 in cardiomyocytes within the cardiac dyad. The goal of this proposal is to unravel the physiological function of JP1 in the heart and its potential significance in heart disease. With the necessary unique mouse models already available in our laboratory, we will test the hypothesis using a multidisciplinary approach in three specific aims: Aim 1, to investigate the functional role of JP1 in cardiac muscle by determining the structural and functional consequences of depleting JP1 from cardiomyocytes; Aim 2, to define the mechanism by which JP1 regulates cardiomyocyte structure and function by illustrating JP1-interacting molecular targets and signaling using candidate and unbiased proteomics-based approaches, co-immunoprecipitation, and high resolution imaging. Aim 3, to investigate the mechanism and significance of JP1 dysregulation during cardiac stress. Accomplishment of this project will have fundamental implications for furthering our understanding of the mechanisms underlying cardiac function and its regulation by characterizing a previously unappreciated protein, JP1, in the heart.
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