MICRODOMAIN BASED CALCIUM INFLUX PATHWAYS THAT REGULATE PATHOLOGICAL CARDIAC HYPERTROPHY AND CONTRACTILITY

Abstract

Pathological cardiac stressors, including persistent hypertension or damage from ischemic heart disease, induce a chronic demand for enhanced contractile performance of the heart. The cytosolic calcium (Ca2+) transient that regulates myocyte contraction must be persistently increased in disease states in order to maintain cardiac output to sustain the metabolic requirements of the body. Associated with this enhanced intracellular Ca2+ ([Ca2+]i) state is pathological cardiac myocyte hypertrophy, which results in large part from the activation of Ca2+-dependent activation of calcineurin (Cn)-nuclear factor of activated T cells (NFAT) signaling. The puzzling feature of this hypertrophic signaling is that the cytosolic [Ca2+] that controls contractility appears to be separate from the [Ca2+] which activates Cn-NFAT signaling. The overarching theme of this dissertation is to explore the source and spatial constraints of pathological hypertrophic signaling Ca2+ and to investigate how it is possible that sensitive and finely tuned Ca2+-dependent signaling pathways are regulated in the background of massive Ca2+ fluctuations that oscillate between 100nM and upwards of 1-2μM during each cardiac contractile cycle. L-type Ca2+ channels (LTCCs) are a major source of Ca2+ entry in cardiac myocytes and are known to play an integral role in the initiation of myocyte excitation contraction-coupling (EC-coupling). We performed a number of experiments to show that a small population of LTCCs reside outside of EC-coupling domains within caveolin (Cav-3) signaling microdomains where they provide a local source of Ca2+ to activate Cn-NFAT signaling. We designed a Cav-targeted LTCC blocker that could eliminate Cn-NFAT activation but did not reduce myocyte contractility. The activity of Cav-targeted LTCCs could also be upregulated to enhance hypertrophic signaling without affecting contractility. Therefore, we believe that caveolae-localized LTCCs do not participate in EC-coupling, but instead act locally to control the coordinated activation of Cn-NFAT signaling that drives pathological remodeling. Transient Receptor Potential (TRP) channels are also thought to provide a source of Ca2+ for activation of hypertrophic signaling. The canonical family of TRP channels (TRPC) is expressed at low levels in normal adult cardiac tissue, but these channels are upregulated in disease conditions which implicates them as stress response molecules that could potentially provide a platform for hypertrophic Ca2+ signaling. We show evidence that TRPC channel abundance and function increases in cardiac stress conditions, such as myocardial infarction (MI), and that these channels are associated with hypertrophic responses, likely through a Ca2+ microdomain effect. While we found that TRPC channels housed in caveolae membrane microdomains provides a source of [Ca2+] for induction of cardiac hypertrophy, this effect also requires interplay with LTCCs. We also found that TRPC channels have negative effects on cardiac contractility, which we believe are due to local activation of Ca2+/calmodulin-dependent protein kinase (CaMKII) and subsequent modulation of ryanodine receptors (RyRs). Further, we found that inhibiting TRPC channels in a mouse model of MI led to increased basal myocyte contractility and reduced hypertrophy and cardiac structural and functional remodeling, as well as increased survival. Collectively, the data presented in this dissertation provides comprehensive evidence that Ca2+ regulation of Cn-NFAT signaling and resultant pathological hypertrophy can be coordinated by spatially localized and regulated Ca2+ channels. The compartmentalization of LTCCs and TRPC channels in caveolae membrane microdomains along with pathological hypertrophy signaling effectors makes for an attractive explanation for how Ca2+-dependent signaling pathways are regulated under conditions of continual Ca2+ transients that mediate cardiac contraction during each heart beat. Elucidation of additional Ca2+ signaling microdomains in adult cardiac myocytes will be important in more comprehensively resolving how myocytes differentiate between signaling versus contractile Ca2+.