Heart disease is the leading cause of morbidity and mortality worldwide. In healthy myocardium, the mitochondria utilize oxidative phosphorylation to generate ATP and metabolites to support pumping blood throughout the whole body. Given this critical function of mitochondria in the heart, mutations or reductions of essential mitochondrial factors cause mitochondrial cardiomyopathy (MC) in humans and mice. Mitochondrial dysfunction is also a major pathogenic driver in non-genetic ischemic heart disease such as myocardial infarction (MI). Better understanding of mitochondrial protein functions and pathogenic molecular mechanisms underlying mitochondrial dysfunction will promote the development of therapeutics for MC or MI. Recent RNA-seq coupled with ribosome footprint-seq analyses in mouse hearts reveal Fam210a (family with sequence similarity 210 member A) as a hub gene in cardiac remodeling. Our preliminary data suggests reduced FAM210A expression in mouse MI hearts and human ischemic heart failure. Cardiomyocyte (CM)-specific homozygous (Homo) conditional knockout (cKO) of Fam210a in adult mice led to MC and mortality. Interactome analyses reveal that FAM210A binds to mitochondrial Ca2+/H+ exchanger LETM1 (Leucine zipper and EF-hand containing transmembrane protein 1) and promotes mitochondrial Ca2+ (mCa2+) efflux in vitro and in vivo. Therefore, Fam210a deletion in CMs resulted in an elevated mCa2+ and reactive oxygen species and compromised mitochondrial membrane potential. As a result, the mitochondrial respiratory activity was reduced in Fam210a KO CMs, leading to cardiac dysfunction at a late stage. In addition, persistently activated integrated stress response (ISR) contributed to the disease progression in Fam210a cKO hearts. Moreover, CM-specific heterozygous Fam210a cKO mice exhibited lower FAM210A protein expression and more severe cardiac remodeling than control mice under MI. In contrast, AAV9-mediated overexpression of FAM210A could protect hearts from MI-induced cardiac damage and dysfunction. Our central hypothesis is: FAM210A functions as a mitochondrial Ca2+/H+ antiporter regulator and maintains normal mitochondrial and cardiac function. We will test this hypothesis by pursuing 3 aims. Aim 1. Decipher the molecular mechanism of FAM210A in regulating mCa2+ homeostasis. Aim 2. Elucidate the role of FAM210A in regulating cardiac mitochondrial activity and cardiac function. Aim 3. Determine the effects of FAM210A overexpression on the functional performance of mitochondria, CMs, and the heart under MI. Collectively, our studies provide novel insights into the function and mechanisms of FAM210A in regulating cardiac mitochondrial integrity and thus maintaining the normal physiological function of the heart. This project also suggests that reduced FAM210A level contributes to the MI-induced cardiac pathological remodeling and overexpression of FAM210A has a cardioprotective role in MI treatment.