Background FUN14 domain containing 1 (FUNDC1) is a highly conserved outer mitochondrial membrane protein. The aim of this study is to examine if FUNDC1 modulates the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), mitochondrial morphology, and function in cardiomyocytes and in 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 (Fundc1f/Y/CreαMyHC+/−), 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, 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. Fundc1f/Y/CreαMyHC+/− mice but not their littermate control mice (Fundc1wt/Y/CreαMyHC+/−) exhibited cardiac dysfunction. The ligation of the left ventricle artery of Fundc1f/Y/CreαMyHC+/− mice caused more severe cardiac dysfunction than those in sham-treated Fundc1f/Y/CreαMyHC+/− mice. Finally, we found that the FUNDC1/MAMs/CREB/Fis1 signaling axis was significantly suppressed in the patients with heart failure. Conclusions We conclude that FUNDC1 binds to IP3R2 to modulate ER Ca2+ release into mitochondria and cytosol and that a disruption of 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.

Background FUN14 domain containing 1 (Fundc1), an outer mitochondrial membrane protein, is important for mitophagy and mitochondria‐associated endoplasmic reticulum (ER) membranes (MAMs). The roles of Fundc1 and MAMs in diabetic hearts remain unknown. The aims of this study therefore, were to determine if the diabetes‐induced Fundc1 expression could increase MAM formation, and whether disruption of MAM formation improves diabetic cardiac function. Methods Levels of FUNDC1 were examined in the hearts from diabetic patients and non‐diabetic donors. Levels of Fundc1‐induced MAMs, and mitochondrial and heart function were examined in mouse neonatal cardiomyocytes exposed to high glucose (HG, 30 mmol/L D‐glucose for 48 h), as well as in streptozotocin (STZ)‐treated cardiac‐specific Fundc1 knockout (KO) mice and cardiac‐specific Fundc1 KO diabetic Akita mice. Results FUNDC1 levels were significantly elevated in cardiac tissues from diabetic patients compared to those in non‐diabetic donors. In cultured mouse neonatal cardiomyocytes, HG conditions increased levels of Fundc1, the inositol 1,4,5‐trisphosphate type 2 receptor (Ip3r2), and MAMs. Genetic downregulation of either Fundc1 or Ip3r2 inhibited MAM formation, reduced ER‐mitochondrial Ca2+ flux, and improved mitochondrial function in HG‐treated cardiomyocytes. Consistently, adenoviral overexpression of Fundc1 promoted MAM formation, mitochondrial Ca2+ overload, and mitochondrial dysfunction in cardiomyocytes exposed to normal glucose (5.5 mmol/L D‐glucose). Compared with non‐diabetic controls, levels of Fundc1, Ip3r2, and MAMs were significantly increased in hearts from STZ‐treated mice and Akita mice. Further, compared with control hearts, diabetes markedly increased co‐immunoprecipitation of Fundc1 and Ip3r2. The binding of Fundc1 to Ip3r2 inhibits Ip3r2 ubiquitination and proteasome‐mediated degradation. Cardiomyocyte‐specific Fundc1 deletion ablated diabetes‐induced MAM formation, prevented mitochondrial Ca2+ overload, mitochondrial fragmentation, and apoptosis with improved mitochondrial functional capacity and cardiac function. In mouse neonatal cardiomyocytes, HG suppressed AMP‐activated protein kinase (Ampk) activity. Furthermore, in cardiomyocytes of Prkaa2 KO mice, expression of Fundc1, MAM formation, and mitochondrial Ca2+ levels were significantly increased. Finally, adenoviral overexpression of a constitutively active mutant Ampk ablated HG‐induced MAM formation, mitochondrial Ca2+ overload, and mitochondrial dysfunction. Conclusions We conclude that HG conditions in diabetes suppress Ampk, initiating Fundc1‐mediated MAM formation, mitochondrial dysfunction, and cardiomyopathy, suggesting that Ampk‐induced Fundc1 suppression is a valid target to treat diabetic cardiomyopathy. Support or Funding Information This study was supported by funding from the following agencies: NHLBI (HL079584, HL080499, HL089920, HL110488, HL128014, HL132500, HL137371, HL142287, and HL140954), NCI (CA213022), NIA (AG047776), and AHA (16GRANT29590003). Dr. Zou is the Eminent Sc...

Endoplasmic reticulum stress is an evolutionarily conserved cell stress response associated with numerous diseases, including cardiac hypertrophy and heart failure. The major endoplasmic reticulum stress signaling pathway causing cardiac hypertrophy involves endoplasmic reticulum stress sensor PERK (protein kinase-like kinase) and eIF2α-ATF4-CHOP signaling. Here, we describe a non-canonical, AGGF1-mediated regulatory system for endoplasmic reticulum stress signaling associated with increased p-eIF2α and ATF4 and decreased sXBP1 and CHOP. Specifically, we see a reduced AGGF1 level consistently associated with induction of endoplasmic reticulum stress signaling in mouse models and human patients with heart failure. Mechanistically, AGGF1 regulates endoplasmic reticulum stress signaling by inhibiting ERK1/2 activation, which reduces the level of transcriptional repressor ZEB1, leading to induced expression of miR-183-5p. miR-183-5p post-transcriptionally downregulates CHOP and inhibits endoplasmic reticulum stress-induced apoptosis. AGGF1 protein therapy and miR-183-5p regulate endoplasmic reticulum stress signaling and block endoplasmic reticulum stress-induced apoptosis, cardiac hypertrophy, and heart failure, providing an attractive paradigm for treatment of cardiac hypertrophy and heart failure.