Investigating the connection between cardiac glycogen accumulation and hypertrophy

Abstract

Background/Objective: Hypertrophy and heart failure involve cardiac metabolic changes, including increased glycolysis and flux through accessory glucose pathways which could contribute to hypertrophic growth. The energy sensor adenosine monophosphate-activated protein kinase (AMPK) not only senses AMP levels, but also binds glycogen via the AMPKβ subunit. Our objective was to investigate whether cardiac glycogen regulates signaling pathways for hypertrophic protein synthesis. Hypothesis: Increased cardiac glycogen inactivates AMPK, leading to increased protein synthesis, driving hypertrophy. Methods: 13C-glucose was injected i.p. into wildtype (WT) and mice with cardiac-deletion of the mitochondrial pyruvate carrier (cMPC2-/- mice), hearts excised 30 min later, and 13C enrichment of glucose metabolites measured by LC-MS/MS. Mice were fed either low-fat control or ketogenic diets, and glycogen was measured in hearts from WT and cMPC2-/- hearts, as well as hearts from mice infused with angiotensin-II via Alzet osmotic pump. Western blotting and qPCR were performed with standard procedures. Results: Compared to WT hearts, cMPC2-/- hearts displayed increased 13C-enrichment in glycolytic metabolites as well as UDP-glucose, suggesting increased glycogenesis. Glycogen content was ~3X higher in cMPC2-/- hearts compared to WT low fat-fed mice, yet glycogen was normalized in ketogenic diet-fed cMPC2-/- mouse hearts. Glycogen was also increased in hearts from angiotensin-II-infused mice compared to saline vehicle. Failing cMPC2-/- hearts displayed decreased AMPK phosphorylation, increased mTOR phosphorylation, and hyperphosphorylation of the S6 ribosomal protein, which were all normalized by feeding ketogenic diet. To investigate the connection between glycogen accumulation, AMPK inactivation, and cardiac hypertrophy, we obtained mice with AMPKβ double knock-in (DKI) mutations in the carbohydrate binding module of AMPKβ1 and β2 isoforms, which prevent AMPK-glycogen binding. Hearts from fed or overnight fasted AMPKβ DKI mice were not hypertrophied, displayed normal glycogen levels, and normal mTOR and S6 ribosomal protein phosphorylation compared to WT. AMPKβ DKI hearts displayed the normal cardiac changes with fasting such as pyruvate dehydrogenase phosphorylation and increased fat oxidation gene expression, suggesting no significant alterations to cardiac metabolic flexibility. In future studies, the connection between glycogen accumulation and hypertrophy will be investigated by crossing these AMPKβ DKI mice and cMPC2-/- mice, as well as infusing AMPKβ DKI mice with saline or angiotensin-II. Conclusion: Hypertrophied/failing hearts display increased glycogen, inactivated AMPK, and activated mTOR signaling, which are all normalized by feeding ketogenic diet. Since there is no appreciable cardiac phenotype at baseline, AMPKβ DKI mice appear to be a promising model to study the role of AMPK-glycogen sensing in cardiac hypertrophy. National Institutes of Health and Saint Louis University institutional funding. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.