1Universitätsmedizin Greifswald Klinik und Poliklinik für Innere Medizin B Greifswald, Deutschland; 2Interfakultäres Institut für Genetik und Funktionelle Genomforschung Greifswald, Deutschland; 3Department for Microbial Physiology and Molecular Biology Greifswald, Deutschland
Background:
The nutrient-sensitive transcription factor EB (TFEB) controls cardiomyocyte energy homeostasis via regulation of fatty acid oxidation (FAO), glucose oxidation (GO) and oxidative phosphorylation (OXPHOS). With adequate nutrition, TFEB is phosphorylated by mTORC1 that causes it to be sequestered inactively in the cytoplasm. During starvation, mTORC1 is inactivated and cannot phosphorylate TFEB that translocates to the nucleus and induces the expression of its target genes. We have previously reported that cardiomyocyte-specific TFEB deletion caused a modest decline in cardiac function and an increase in cardiac remodeling in young mice (11 weeks old). However, if the absence of TFEB from cardiomyocytes would increase the susceptibility of the heart to starvation in aged mice (40 weeks old) was not investigated.
Methods:
We generated cardiomyocyte-specific aged male Tfeb knockout mice (cKO, TfebloxP/loxP; αMHC-CRE) and used wildtype littermates as controls (WT, TfebloxP/loxP) and subjected them to starvation for 48h. WT and cKO mice with free access to food served as controls. We conducted echocardiographic, morphological, and histological analyses. The morphology of mitochondria in the heart were investigated by transmission electron microscopy (TEM). Western blot analyses, mass spectrometry and quantitative real-time PCR were used to quantitate changes in markers of cardiac stress and remodeling, metabolic shift and OXPHOS.
Results:
Cardiac function of aged cKO mice was significantly lower (ejection fraction, EF ≈ 50%) as compared to WT and young cKO mice. Aged cKO mice exhibited cardiac fibrosis when compared to WT mice. Starvation resulted in a further decline in cardiac function (EF ≤ 40%) and increased cardiac remodeling in cKO, compared to WT mice. cKO mice lost more liver and brown adipose tissue weight during starvation. FAO, GO and OXPHOS related genes were all downregulated in hearts of starved cKO compared to starved WT mice. TEM images revealed an increase in mitochondrial cross-sectional area (MiCSA) in starved compared to fed WT mice, which was not observed in starved compared to fed cKO mice. Mass spectrometry analyses of proteins from hearts of all experimental groups confirmed our gene expression data and revealed a reduction of proteins involved in OXPHOS, FAO and TCA cycle.
Conclusions:
Deletion of Tfeb leads to an impaired cardiac performance in aged mice. During starvation, the absence of TFEB leads to extensive cardiac remodeling and a marked deterioration of cardiac function in aged mice. TFEB plays a pivotal role for maintenance of cardiac structure and function during starvation. Further studies are needed to elucidate the molecular basis of this phenotype.