Hyperglycemia-Driven Inhibition of AMP-Activated Protein Kinase α2 Induces Diabetic Cardiomyopathy by Promoting Mitochondria-Associated Endoplasmic Reticulum Membranes In Vivo

Abstract

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...

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“…Moreover, involvement of MAMs (where FUNDC1 may reside) adds the complexity of FUNDC1 biological function in different disease settings. MAMs display distinct roles in various pathological settings, with enrichment of MAMs being detrimental in metabolic diseases ( 20 , 34 ), whereas MAMs exert cardioprotection in heart failure ( 16 ).…”

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“…More interestingly, the expression of FUN14 domain containing 1 (Funcd1), an outer mitochondrial membrane protein important for mitophagy and MAMs, was increased in cardiac tissue from diabetic patients. In STZ-induced diabetic mice, increased Funcd1 induced MAM formation, increased mitochondrial Ca 2+ influx and, subsequently, mitochondrial dysfunction, resulting in diabetic cardiomyopathy [89]. These observations suggest that the structural and functional interactions between the ER and mitochondria have a fine line between being beneficial and being detrimental and should, therefore, be precisely regulated.…”

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“…Previous findings demonstrated that mitochondrial structure and function are significantly impaired in the islets and muscles of diabetic animals. [28][29][30] Mitochondria also exhibited diminished biogenesis; defective oxidative phosphorylation (OXPHOS); and reduced expression of complexes I, III, and V of the electron transport chain in STZ-treated myocardium tissue or HG-induced cardiomyocytes, [30][31][32] suggesting the impairment and dysfunction of mitochondria in hyperglycemic hearts. Based on TEM analysis, we found that the mitochondrial volume density was not remarkably altered, but that the mitochondria from HG-induced H9c2 cells exhibited significant morphological defects, including mitochondrial vacuolization and destroyed cristae and membranes ( Figure 1E).…”