Abstract 3777: Mammalian Target of Rapamycin (mTOR) is Essential for Murine Heart Development
Mammalian target of rapamycin (mTOR) plays an important role in cardiac hypertrophy in part via activation of S6 Kinase. The rictor-mTOR complex (TORC2) also been shown to mediate Ser 473 phosphorylation of Akt. Germline deletion of mTOR in mice disrupts embryonic inner cell mass proliferation leading to lethality shortly after implantation. Thus the role of mTOR in cardiac development is unknown. We therefore generated cardiomyocyte-restricted knockouts of mTOR by crossing mice that were homozygous for a floxed mTOR allele with alpha-MHC Cre mice that were heterozygous for the floxed mTOR allele. Assuming Mendelian inheritance, 25% of offspring should be homozygous for the floxed mTOR allele and also express Cre (mTOR−/−). Of 389 weaned offspring only 2.3% were mTOR−/−, indicating embryonic lethality. We therefore performed timed pregnancies and determined that 26% of embryos were mTOR−/−, and that most died between E14.5 and E17.5. By E15.5 40% of mTOR−/− embryos were already dead. Histological analysis of surviving mTOR−/− embryos revealed approximately 30% reduction in ventricular cardiomyocyte number and remaining myocytes exhibited markedly reduced myofibrillar content. Cardiac volume was reduced, and ventricular walls markedly thinned as evidenced by an 80% increase in the ratio of chamber volume to total cardiac volume. Of the small number of mTOR−/− mice that survived to weaning, all died between 5–10 weeks of age. Cardiac function (evaluated by echocardiography) was relatively preserved in 3-week-old mice but fractional shortening rapidly declined by 40 –70% prior to death. Heart weight/body weight ratios of mTOR−/− mice at the time of demise were increased by 1.92±0.49 fold compared to controls (p<0.05, n=6) and histology revealed increased perivascular and focal fibrosis. As expected, S6 phosphorylation was absent in mTOR−/− hearts. Interestingly, the phosphorylation level of Akt at Ser473 was not reduced. Thus mTOR plays an essential role in embryonic cardiomyocyte survival and persistent phosphorylation of Akt at Ser473 suggests that phosphorylation of Akt at Ser 473 can occur in the heart in the absence of the mTORC2 complex.