Enhancers regulate anaerobic glycolysis in old cardiomyocytes

Heart failure is a typical age-associated pathology with high rates of mortality and morbidity. Age-related cardiac malfunctioning is multifactorial and cardiac remodelling contributes to this. A major event in aging-associated-cardiac remodelling is a metabolic shift in cardiomyocytes so that glycolysis increases at the expense of mitochondrial oxidative phosphorylation. This metabolism modification causes an “energy deficiency” that contributes in the elderly to an impairment of cardiac contractility. Gene expression analysis in cardiac aging models shows a general down-regulation of genes involved in energy metabolism, including those associated with mitochondrial function (e.g., fatty-acid metabolism) and turnover. This finding thus supports the notion that cardiac aging-related metabolic remodelling is caused by changes in the expression of metabolic genes. Despite this, the molecular mechanisms causing these changes are not, as yet, fully unravelled. Enhancers play a key role in defining the gene expression program of heart development and cardiac hypertrophy. Therefore, alteration of the activity of enhancers could contribute in defining the gene expression changes responsible for metabolic remodelling in old cardiomyocytes. To test this hypothesis, we investigated the activity state of enhancers in relation to gene expression and metabolic changes occurring during cardiac aging. To this end, we integrated metabolic data with RNA-seq and ChIP-seq data for H3K27ac and H3K27me3 (two histone markers that define active and repressed enhancers, respectively) obtained from ventricular cardiomyocytes purified from the heart of mice at different ages (8 weeks, and 6 and 18 months old, corresponding to young, adult and old-age mice). Results show that a large fraction of enhancers undergo a change in activity, an event associated with variation in transcription of neighbouring genes: a set of enhancers switched from a state of little or no activity to an active state, such as those associated with genes involved in glycolysis, hypertrophy and dilated cardiomyopathy; in contrast, enhancers that switched from an active state to an inactive state were involved in cytoskeleton organization. By comparing the enhancer dataset with metabolic profiles, we found that the activation of enhancers neighbouring glycolytic genes was associated with an increase of metabolites of anaerobic glycolysis. Gene ontology analysis showed that the dataset of modulated enhancers overlapped partially with datasets identified for heart development, cardiomyocyte differentiation and cardiac hypertrophy, indicating the involvement during aging of enhancers implicated in heart development and disease. Interestingly, some of these enhancers were listed in Vista, a dataset of human and mouse enhancers validated in vivo. Together, these results demonstrate that enhancers are involved in promoting the gene expression changes – in particular, activation of anaerobic glycolysis – occurring in the cardiomyocyte during aging. These results suggest the possibility of modifying cardiac function during aging by modulating the activity of enhancers through epigenetic drugs.