The TGAC8 Mouse, the mouse that roars

National Institute On Aging

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Abstract

Mice harboring cardiac specific over expression of Adenylyl Cyclase (AC) Type VIII (TGAC8) at 3 mo. of age manifest marked increases in heart rate (HR) and left ventricular (LV) ejection fraction (EF) on a chronic basis, driven by a markedly increased AC activity. We hypothesized that this high intrinsic cardiac sympathetic-like stimulation must elicit conciliant activation of numerous signaling pathways, not only to sustain the chronically enhanced heart performance but also to ensure heart survival. To this end we employed unbiased transcriptomic and proteomic analysis to deduce mechanisms that sustain this remarkable cardiac phenotype. Of nearly 12,000 transcripts identified in RNA SEQ approximately 20% differed by genotype and of nearly 7,000 proteins identified in proteomic analyses about 30% differed by genotype. Five hundred and forty-four transcripts and proteins that significantly and concordantly differed in expression in TGAC8 vs WT. These differences in transcript abundance and protein changed expression concordantly accounted for enrichment or activation of 134 canonical signaling pathways in TGAC 8, suggesting concomitant catabolism and anabolism, including activation of nutrient sensing pathways, an increase in glycolysis, reduction in fatty acid Beta oxidation, PI3, and ras, raf, Map kinase pathways, increased protein synthesis, degradation and quality control, including autophagy ubiquitination, NAD + NADP+ salvage pathways, inflammation, NRF signaling and ROS scavenging, and mechanisms to limit the stress of markedly enhanced cAMP signaling. Omics-driven deeper phenotypic validation indicated that basal steady state levels of ATP and phosphocreatine were preserved and that the TGAC8 heart is not hypertrophied compared to WT due to a smaller LV cavity size but thicker LV walls with an increased number of small cardiac myocytes. Western blot analysis confirmed the absence of biomarkers of either physiologic or pathologic hypertrophy. In summary, the three-month TGAC8 mouse indeed harbors a unique heart. It would appear as if cardiac specific over expression of a single gene, Adenylyl Cyclase Type VIII, in co-opting this extremely broad panorama of cell functions, mimics the impact of a virus, cancer, or embryonic cardiac development. The consilience of emergent adaptive mechanisms within the TGAC8 heart, allows it to cope with the chronic stress driven by chronic incessant augmented AC-cAMP-PKA-Ca signaling insuring its survival for at least up to a year. Interestingly cancer was the foremost human clinical disease category predicted by IPA analysis of the TGAC8 LV transcriptome and proteome profiles. Metabolomics analyses of young (3 mo) TGAC8 heart: We hypothesized that the sustained workload in the TGAC8 heart will be matched by metabolic remodeling leading to enhanced energy generation, redox stress elicited by sustained metabolic activity, and activation of nutrient sensing and signaling pathways to respond to those challenges. To test our hypothesis, we performed untargeted metabolomics in the LV of hearts from 3 mo old WT and TGAC8 mice and subsequent joint pathway analysis of LV proteome and metabolome profiling data, and found activation of glucose metabolism, and a preferred flux redirection towards glycolysis as validated by direct measurement of enzymatic activities. Other metabolic modification in the TGAC8 include an increase in amino acids, a relative depletion of fatty acids, an abundance of ketone bodies, enhanced redox metabolism, and amino acid precursors of glutathione synthesis. Together, the data are consistent with underlying LV metabolic remodeling comprising enhancement of glucose, lipid and amino acids metabolism. Several redox balance-keeping pathways appear to be activated to counteract the increased ROS generation under intense metabolic activity in response to sustained energy demand.