Mitochondrial dynamics is a conserved process by which mitochondria undergo repeated cycles of fusion and fission, leading to exchange of mitochondrial genetic content, ions, metabolites, and proteins. Here, we examine the role of the mitochondrial fusion protein optic atrophy 1 (OPA1) in differentiated skeletal muscle by reducing OPA1 gene expression in an inducible manner. OPA1 deficiency in young mice results in non-lethal progressive mitochondrial dysfunction and loss of muscle mass. Mutant mice are resistant to age- and diet-induced weight gain and insulin resistance, by mechanisms that involve activation of ER stress and secretion of fibroblast growth factor 21 (FGF21) from skeletal muscle, resulting in increased metabolic rates and improved whole-body insulin sensitivity. OPA1-elicited mitochondrial dysfunction activates an integrated stress response that locally induces muscle atrophy, but via secretion of FGF21 acts distally to modulate whole-body metabolism.

Rationale Pressure overload cardiac hypertrophy, a risk factor for heart failure, is associated with reduced mitochondrial fatty acid oxidation (FAO) and oxidative phosphorylation (OXPHOS) proteins that correlate in rodents with reduced PGC-1α expression. Objective To determine the role of PGC-1β in maintaining mitochondrial energy metabolism and contractile function in pressure overload hypertrophy. Methods and Results PGC-1β deficient (KO) mice and wildtype controls (WT) were subjected to transverse aortic constriction (TAC). Although LV function was modestly reduced in young KO hearts, there was no further decline with age so that LV function was similar between KO and WT when TAC was performed. WT – TAC mice developed relatively compensated LVH, despite reduced mitochondrial function and repression of OXPHOS and FAO genes. In non-stressed KO hearts, OXPHOS gene expression and palmitoyl-carnitine supported mitochondrial function were reduced to the same extent as banded WT, but FAO gene expression was normal. Following TAC, KO mice progressed more rapidly to heart failure and developed more severe mitochondrial dysfunction, despite a similar overall pattern of repression of OXPHOS and FAO genes as WT – TAC. However, relative to WT TAC, PGC-1β deficient mice exhibited greater degrees of oxidative stress, decreased cardiac efficiency, lower rates of glucose metabolism and repression of hexokinase II protein. Conclusions PGC-1β plays an important role in maintaining baseline mitochondrial function and cardiac contractile function following pressure overload hypertrophy by preserving glucose metabolism and preventing oxidative stress.

The induction of autophagy in the mammalian heart during the perinatal period is an essential adaptation required to survive early neonatal starvation; however, the mechanisms that mediate autophagy suppression once feeding is established are not known. Insulin signaling in the heart is transduced via insulin and IGF-1 receptors (IGF-1Rs). We disrupted insulin and IGF-1R signaling by generating mice with combined cardiomyocyte-specific deletion of Irs1 and Irs2. Here we show that loss of IRS signaling prevented the physiological suppression of autophagy that normally parallels the postnatal increase in circulating insulin. This resulted in unrestrained autophagy in cardiomyocytes, which contributed to myocyte loss, heart failure, and premature death. This process was ameliorated either by activation of mTOR with aa supplementation or by genetic suppression of autophagic activation. Loss of IRS1 and IRS2 signaling also increased apoptosis and precipitated mitochondrial dysfunction, which were not reduced when autophagic flux was normalized. Together, these data indicate that in addition to prosurvival signaling, insulin action in early life mediates the physiological postnatal suppression of autophagy, thereby linking nutrient sensing to postnatal cardiac development.

SUMMARY Autophagy is a homeostatic cellular process involved in the degradation of long-lived or damaged cellular components. The role of autophagy in adipogenesis is well recognized, but its role in mature adipocyte function is largely unknown. We show that the autophagy proteins Atg3 and Atg16L1 are required for proper mitochondrial function in mature adipocytes. In contrast to previous studies, we found that post-developmental ablation of autophagy causes peripheral insulin resistance independently of diet or adiposity. Finally, lack of adipocyte autophagy reveals cross talk between fat and liver, mediated by lipid peroxide-induced Nrf2 signaling. Our data reveal a role for autophagy in preventing lipid peroxide formation and its transfer in insulin-sensitive peripheral tissues.

Objective Impaired endothelial cell autophagy compromises shear-stress induced nitric oxide (NO) generation. We determined the responsible mechanism. Approach and Results Upon autophagy compromise in bovine aortic endothelial cells (ECs) exposed to shear-stress, a decrease in glucose uptake and EC glycolysis attenuated ATP production. We hypothesized that decreased glycolysis-dependent purinergic signaling via P2Y-1 receptors, secondary to impaired autophagy in ECs, prevents shear-induced p-eNOSS1177 and NO generation. Maneuvers that restore glucose transport and glycolysis (e.g., overexpression of GLUT1) or purinergic signaling (e.g., addition of exogenous ADP) rescue shear-induced p-eNOSS1177 and NO production in ECs with impaired autophagy. Conversely, inhibiting glucose-transport via GLUT1 siRNA, blocking purinergic signaling via ectonucleotidase-mediated ATP/ADP degradation (e.g., apyrase), or inhibiting P2Y1 receptors using pharmacological (e.g., MRS2179) or genetic (e.g., P2Y1-R siRNA) procedures, inhibits shear-induced p-eNOSS1177 and NO generation in ECs with intact autophagy. Supporting a central role for PKCδT505 in relaying the autophagy-dependent purinergic-mediated signal to eNOS, we find that: (i) shear-stress induced activating phosphorylation of PKCδT505 is negated by inhibiting autophagy; (ii) shear-induced p-eNOSS1177 and NO generation are restored in autophagy-impaired ECs via pharmacological (e.g., bryostatin) or genetic (e.g., CA-PKCδ) activation of PKCδT505 and (iii) pharmacological (e.g., rottlerin) and genetic (e.g., PKCδ siRNA) PKCδ inhibition prevents shear-induced p-eNOSS1177 and NO generation in ECs with intact autophagy. Key nodes of dysregulation in this pathway upon autophagy compromise were revealed in human arterial endothelial cells. Conclusion Targeted reactivation of purinergic signaling and/or PKCδ has strategic potential to restore compromised NO generation in pathologies associated with suppressed EC autophagy.