Binding of FUN14 Domain Containing 1 with Inositol 1,4,5-Trisphosphate Receptor in Mitochondria-Associated Endoplasmic Reticulum Membranes Maintains Mitochondrial Dynamics and Function in Hearts in Vivo

Background: FUN14 domain containing 1 (FUNDC1) is a highly conserved outer mitochondrial membrane protein. The aim of this study is to examine whether FUNDC1 modulates the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), mitochondrial morphology, and function in cardiomyocytes and intact hearts. Methods: The impacts of FUNDC1 on MAMs formation and cardiac functions were studied in mouse neonatal cardiomyocytes, in mice with cardiomyocyte-specific Fundc1 gene knockout (Fundc1^f/Y/Cre^αMyHC+/-), and in the cardiac tissues of the patients with heart failure. Results: In mouse neonatal cardiomyocytes and intact hearts, FUNDC1 was localized in MAMs by binding to ER-resided inositol 1,4,5-trisphosphate type 2 receptor (IP³R2). Fundc1 ablation disrupted MAMs and reduced the levels of IP³R2 and Ca²⁺ in both mitochondria and cytosol, whereas overexpression of Fundc1 increased the levels of IP³R2 and Ca²⁺ in both mitochondria and cytosol. Consistently, Fundc1 ablation increased Ca²⁺ levels in ER, whereas Fundc1 overexpression lowered ER Ca²⁺ levels. Further, Fundc1 ablation in cardiomyocytes elongated mitochondria and compromised mitochondrial functions. Mechanistically, we found that Fundc1 ablation-induced reduction of intracellular Ca²⁺ levels suppressed mitochondrial fission 1 protein (Fis1) expression and mitochondrial fission by reducing the binding of the cAMP response element binding protein (CREB) in the Fis1 promoter. Fundc1^f/Y/Cre^αMyHC+/- mice but not their littermate control mice (Fundc1^wt/Y/Cre^αMyHC+/-) exhibited cardiac dysfunction. The ligation of the left ventricle artery of Fundc1^f/Y/Cre^αMyHC+/- mice caused more severe cardiac dysfunction than those in sham-treated Fundc1^f/Y/Cre^αMyHC+/- mice. Finally, we found that the FUNDC1/MAMs/CREB/Fis1 signaling axis was significantly suppressed in patients with heart failure. Conclusions: We conclude that FUNDC1 binds to IP³R2 to modulate ER Ca²⁺ release into mitochondria and cytosol. Further, a disruption of the FUNDC1 and IP³R2 interaction lowers the levels of Ca²⁺ in mitochondria and cytosol, both of which instigate aberrant mitochondrial fission, mitochondrial dysfunction, cardiac dysfunction, and heart failure.