PROJECT SUMMARY/ABSTRACT Macrophages are well-characterized as sentinel immune cells that coordinate cellular responses to injury and infection. However, emerging evidence demonstrates that macrophages have novel, non-canonical functions critical for developmental regulation, tissue homeostasis, and regeneration. Likewise, cardiac macrophages have essential functions in patterning the coronary vasculature, valvular remodeling, and in modulating adult heart conduction. Considering the substantial morbidity and economic burden associated with abnormal cardiogenesis and arrhythmias, further elucidating the roles of cardiac macrophages in normal heart development and function is critical for devising novel therapeutic strategies. In adult mammalian hearts, macrophages electrically couple to cardiomyocytes at the atrioventricular node via Connexin 43, a gap junction protein. These macrophages directly modulate electrical activity of nodal cardiomyocytes, and thus cardiac conduction. However, it is not known if macrophages (1) are required during embryogenesis to establish proper conduction or (2) if macrophage-derived signals modulate heart function in the developing heart. Therefore, this proposal will address a critical knowledge gap in the field of cardiac development in two aims. In Aim 1, I will establish the electrical potential of embryonic macrophages to modulate fetal cardiac conduction. In Aim 2, I will determine how loss of embryonic macrophage affects conduction and adult heart health. In my approach, I utilize zebrafish, a well-established developmental model whose salient features include rapid ex vivo development, optical transparency, and high amenability to genetic manipulation. This proposal will be carried out at Brown University, an exemplar academic research institution with extensive access to advanced instrumentation. With the guidance and mentorship of her sponsor, Dr. Jessica Plavicki, and co-sponsor, Dr. Chris Moore, the applicant is prepared and equipped to carryout this fellowship. My exciting preliminary data reveal previously undescribed roles of macrophages in embryonic cardiac conduction and cardiogenesis. Using transgenic zebrafish with macrophage-specific expression of a genetically encoded calcium indicator, GCaMP6s, I found that seeded macrophages have synchronous bursts of calcium activity in time with ventricular beating. In optogenetic experiments, I show that larval heart rate can be modulated by stimulating or silencing macrophage electrical activity via macrophage-specific expression of light-gated ion channels. This poses an intriguing question of whether developmental arrythmias could be corrected by specifically targeting macrophages. Loss of embryonic macrophages, via drug-inducible ablation or in embryonic macrophage mutants, altered ventricular chamber formation and function in embryonic zebrafish, as well as adult cardiomyocyte compaction and gross heart morphology. Successful completion of this fell...