Myocardial glycophagy — A specific glycogen handling response to metabolic stress is accentuated in the female heart

Abstract

Cardiac metabolic stress is a hallmark of many cardiac pathologies, including diabetes. Cardiac glycogen mis-handling is a frequent manifestation of various cardiopathologies. Diabetic females have a higher risk of heart disease than males, yet sex disparities in cardiac metabolic stress settings are not well understood. Oestrogen acts on key glycogen regulatory proteins. The goal of this study was to evaluate sex-specific metabolic stress-triggered cardiac glycogen handling responses. Male and female adult C57Bl/6J mice were fasted for 48h. Cardiac glycogen content, particle size, regulatory enzymes, signalling intermediates and autophagic processes were evaluated. Female hearts exhibited 51% lower basal glycogen content than males associated with lower AMP-activated-kinase (AMPK) activity (35% decrease in pAMPK:AMPK). With fasting, glycogen accumulated in female hearts linked with decreased particle size and upregulation of Akt and AMPK signalling, activation of glycogen synthase and inactivation of glycogen phosphorylase. Fasting did not alter glycogen content or regulatory proteins in male hearts. Expression of glycogen autophagy marker, starch-binding-protein-domain-1 (STBD1), was 63% lower in female hearts than males and increased by 69% with fasting in females only. Macro-autophagy markers, p62 and LC3BII:I ratio, increased with fasting in male and female hearts. This study identifies glycogen autophagy ('glycophagy') as a potentially important component of the response to cardiac metabolic stress. Glycogen autophagy occurs in association with a marked and selective accumulation of glycogen in the female myocardium. Our findings suggest that sex-specific differences in glycogen handling may have cardiopathologic consequences in various settings, including diabetic cardiomyopathy.

Myocardial autophagic energy stress responses—macroautophagy, mitophagy, and glycophagy

“…In cultured neonatal rat cardiomyocytes, protein expression of the glycophagy marker STBD1 was increased by extracellular insulin concentration linked with activation of the insulin-regulated PI3K/Akt signaling pathway (61). Similarly, fasting-induced upregulation of STBD1 and GABARAPL1 protein content was associated with activation of Akt in vivo (76). In cultured cardiomyocytes, it has been demonstrated using chromatin immunoprecipitation that FoxO1 and FoxO3, which are inhibited by Akt, directly bind to the GABARAPL1 promoter in the nucleus and regulate transcription of this glycophagy marker (83).…”

“…Engulfment of selective cargo requires adaptor proteins, which bridge Atg8 proteins and the target. More recently, a new understanding of selective autophagy processing has emerged, with specific protein machinery recognized for autophagic degradation of mitochondria [mitophagy (22,33)], endoplasmic reticulum [reticulophagy or ERphagy (26)], peroxisomes [pexophagy (89)], lipid droplets [lipophagy (52)], and glycogen [glycophagy (39,76)]. In this context, the term macroautophagy is best used to denote the process of general autophagic degradation of heterogeneous macromolecules.…”

“…In diabetes, the intracellular glucose deprivation conferred by impaired glucose transport (albeit in the context of extracellular fuel overabundance) may trigger a pseudo fasted heart phenotype. Energy stress is also a potent activator of cardiac autophagy in vivo and in isolated cardiomyocytes (3,42,76). The role of autophagy in the diabetic heart is emerging as an important research question.…”

“…Recently we have demonstrated that the autophagic/lysosomal glycogen degradation pathway (glycogen-specific autophagy, 'glycophagy') is operational in the adult heart and is upregulated by diabetes in vivo (61,76). Glycophagy is well characterized in the newborn liver and heart where a transient upregulation of lysosomal acid ␣-glucosidase mediates an increase in autophagic glycogen degradation (43).…”

“…In cultured cardiomyocytes, it has been demonstrated using chromatin immunoprecipitation that FoxO1 and FoxO3, which are inhibited by Akt, directly bind to the GABARAPL1 promoter in the nucleus and regulate transcription of this glycophagy marker (83). Thus Akt activation may lead to inhibition of FoxOmediated GABARAPL1 transcription, but such a response is difficult to reconcile with the experimental observations that there is parallel upregulation of glycophagy and Akt activity with fasting (76). A more detailed dissection of the signaling nexus of PI3K/Akt and AMPK pathways in mediating glycophagy induction is required to resolve these contrasting observations, in both physiological and pathophysiological settings.…”