Akt1 and Akt2 are the main isoforms of protein kinase B (Akt) expressed in the mammalian heart. Knockout mice for Akt1 and Akt2, respectively, have demonstrated that Akt1 primarily appears to regulate growth whereas Akt2 is rather involved in regulating metabolism. In order to investigate common as well as divergent functions of AKT isoforms in the adult heart, we generated and analyzed tamoxifen inducible, cardiac myocyte specific Akt1 and Akt2 single knockout as well as Akt1/Akt2 double knockout mice. We observed a maximal loss of both isoforms 8 days after start of 5 consecutive IP injections of OH-Txf. Single KO mice were fully viable and showed no functional or structural alteration of the heart. In contrast, deletion of both isoforms induced progressive cardiac atrophy and loss of heart function leading to terminal heart failure 3 to 4 weeks after first injection of OH-Txf. Echocardiography revealed a progressive decline of pump function characterized by a drop of ejection fraction from initially 62 to 37 % on day 10 and 31 % on day 21. Wall thickness of iCM-Akt1/2 KO hearts declined from day 10 to day 21. Concomitantly, cardiac mass dropped by 30 %. This effect was mainly due to a loss of cardiomyocyte size, which progressively declined after Akt1/2 deletion. Increased nuclear localization of the pro-autophagic transcription factor Foxo3A as shown by immunohistochemistry was identified as one possible cellular mechanism contributing to cellular atrophy in Akt1/2 KO hearts. Simultaneously, the expression of Foxo3A-regulated pro-autophagic genes like Gabarapl1, Bnip3 or Sqstm1 was increased early during development of heart failure (d10 after start of OH-Txf) as demonstrated by quantitative real time PCR and western blot analyses. Increased proteasomal (chymotrypsin-like and caspase-like) and calpain activities were detected at a late stage of heart failure (d21) which might further promote the loss of cardiac cell mass. Global cardiac gene expression analysis using Agilent 60 K microarrays demonstrated a concerted downregulation of transcripts for mitochondrial proteins suggesting a compromised ATP generation. Therefore, we further investigated the impact of AKT1/2 deletion on cardiac energetics. Repetitive 31P-NMR spectroscopy performed in anesthetized mice revealed progressive energetic depletion of Akt1/2 KO hearts. Phosphocreatine:ATP ratios (PCr/ATP) declined to values of 1.5 which in humans is associated with serious heart failure. In parallel, free creatine levels considerably increased as shown by ceratine chemical exchange saturation transfer (CrCEST) imaging. Conclusion: Common functions of AKT1 and AKT2 provide essential backup control mechanisms which promote cardiac energy generation and restrict FOXO-mediated degradation processes and thereby mediate the structural and functional integrity of the adult mammalian heart.
Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB045
Poster Communications: Deletion of Akt1 and Akt2 leads to cardiac atrophy and loss of cardiac function
S. Gödecke1, T. Appel1, P. Müller1, F. Möller1, U. Flögel2, K. Köhrer3, A. Heinen1, A. Gödecke1
1. Institut für Herz- und Kreislaufphysiologie, Heirich-Heine-Univerität Düsseldorf, Düsseldorf, Germany. 2. Institut für Molekulare Kardiologie, Heinrich-Heine-Universität, Düsseldorf, Germany. 3. BMFZ, Heinrich-Heine-Universität, Düsseldorf, Germany.
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Where applicable, experiments conform with Society ethical requirements.