Glycogen metabolism and PRKAG2 cardiomyopathy

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DESCRIPTION (provided by applicant): AMP-activated protein kinase (AMPK) is a serine/threonine kinase that acts as a cellular energy sensor and a master regulator of metabolism in a variety of cell types including cardiac myocytes. AMPK responds to decreases in cellular AMP/ATP ratio, an ultra-sensitive indicator of impaired energy status, and triggers multiple signaling cascades to restore energy balance by stimulating ATP-generating pathways while inhibiting ATP consuming pathways. AMPK is a heterotrimeric protein consisting of a catalytic subunit (a) and two regulatory subunits (P and v). Each subunit has 2-3 isoforms, all except y3 are expressed in the heart. Mutations in the y2 subunit (encoded by PRKAG2 gene) cause human cardiomyopathy characterized by substantial myocardial glycogen accumulation, pre-excitation syndrome, and cardiac hypertrophy. Studies by several laboratories have shown that the human disease phenotype can be faithfully recapitulated in transgenic mice with cardiac-specific overexpression of the mutant y2 subunit of AMPK. Using the mouse model expressing N488I mutant v2-AMPK in the hearts, we have shown that the mutation causes aberrant activation of AMPK in the absence of energetic deficit, and the disease phenotype can be rescued by introducing an inactive catalytic subunit of AMPK. However, the mechanisms by which altered AMPK activity cause cardiomyopathy remains unknown. While the important role of AMPK in promoting glucose and fatty acids utilization during energetic stress is well established in cardiac and skeletal muscle, the metabolic consequence of inordinate AMPK activity in energetically intact heart is poorly understood. Here we propose that abnormal AMPK activity due to PRKAG2 mutation remodels cardiac metabolism to achieve a new level of energy homeostasis by establishing and turning over a substantially enlarged glycogen pool at the expense of developing glycogen storage cardiomyopathy. To test this hypothesis, we will combine hypothesis-driven and discovery-based approaches to dissect the metabolic pathways leading to glycogen storage in mouse hearts expressing N488I mutant of PRKAG2. We will first characterize the remodeled metabolic profile in mutant mouse hearts using 31P and 13C NMR spectroscopy together with biochemical approaches. Secondly, we will define candidate molecules that mediate glycogen accumulation by identifying target proteins and transcriptional changes that may result directly or indirectly from aberrant AMPK activity. Finally, we will evaluate the role of candidate proteins in cellular metabolism by biochemical and molecular biology analyses of their function in cardiac myocytes and mouse hearts. Results obtained from this study will serve our long-term goal of understanding the cardiac-specific AMPK signaling in physiology and disease.