A well-patterned and functioning heart is required for the growth and survival of embryos. While many genes critical for early cardiogenesis have been identified by previous genetic studies, genetic networks required for establishing and maintaining embryonic cardiac function remain to be explored. We have previously shown that calcium homeostasis has an important role in maintaining embryonic cardiac rhythmicity in zebrafish and that loss of function of NCX1h, one of the primary molecules responsible for calcium extrusion in the heart, abolishes synchronized cardiac contraction and leads to chaotic cardiac movements known as cardiac fibrillation. Consistent with the role of NCX1 in calcium homeostasis, we observed abnormal calcium transients in NCX1h null zebrafish embryonic hearts. These observations suggest that the NCX1h mutant zebrafish can serve as a tool for studying the calcium- regulatory networks important for embryonic cardiac function. From a chemical-based suppression screen on the zebrafish tremblor/NCX1h genetic model, we identified a critical component of the gene network governing embryonic cardiac function. We discovered that OK-F7, a novel small molecule suppresses cardiac fibrillation in the tremblor/NCX1 null genetic background, and our biochemical study indicated that the mitochondrial protein VDAC2 is the protein target of OK-F7. Furthermore, over expression of VDAC2 restores rhythmic cardiac contractions in embryos lacking NCX1h activity, suggesting a critical role for VDAC2 and mitochondria in calcium regulation and embryonic cardiac rhythmicity. As the first step toward understanding the role for VDAC2 in embryonic cardiac rhythmicity, we propose to evaluate the requirement of VDAC2 in cardiac development by both gain-of-function and loss-of-function approaches (Aim1). Second, to understand how the interaction of OK-F7 and VDAC modulates calcium homeostasis, we propose to evaluate whether OK-F7 treatment changes VDAC2 channel activity. We will also investigate the impact of OK-F7 on mitochondrial calcium influx. Information obtained from this line of study will provide insight into the mechanism by which OK-F7 and VDAC2 suppress cardiac fibrillation (Aim2). Finally, we will investigate whether OK-F7 treatment can restore rhythmic calcium waves in tremblor and other zebrafish embryos that have calcium-handling defects. We will also determine whether forced expression of other VDAC proteins can restore rhythmic cardiac contractions in embryos lacking NCX1h activity. The success of this line of study will further our understanding of the role for VDAC proteins in embryonic cardiac rhythmicity at the molecular level (Aim3). Our overall goal of this research program is to gain insight into gene networks important for calcium homeostasis and embryonic cardiac rhythmicity through multi-disciplinary studies. Information obtained from this research program will reveal previously unrecognized roles for VDAC and mitochondria in embryonic cardiac function.
Cardiac fibrillation is a form of cardiac arrhythmia that poses serious health threats. We hypothesize that the fundamental mechanisms regulating cardiac rhythmicity are conserved among vertebrates and that the zebrafish tremblor mutant can serve as a good animal model for cardiac fibrillation. The studies in this proposal will help elucidate cellular and molecular mechanisms underlying cardiac fibrillation and ultimately contribute to the development of new preventive and therapeutic approaches.
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