Exercise, programmed cell death and exhaustion of cardiomyocyte proliferation in aging zebrafish

Abstract

As the human heart ages, the myocardium undergoes fibrotic remodelling, there is declining cardiovascular performance and eventual heart failure. It is suggested that exercise is an important intervention to ameliorate these changes. In this study we establish zebrafish as a laboratory model to understand how aging and exercise affect cardiomyocyte turnover. We show the zebrafish heart does not exhibit indeterminate growth but follows the pattern seen in human aging. In zebrafish, cardiomyocyte proliferation remains constant, but a late increase cell death underlies the pattern of initial cardiac growth and later fibrosis. These anatomical findings are corelated with the human like decline in cardiovascular performance reflected in voluntary swimming activity, critical swimming speed (Ucrit) and biomarkers of cardiac insufficiency. Whilst the vertebrate heart can respond to injury through cardiomyocyte proliferation, it is not known if a proliferative response occurs when the cardiovascular system is exposed to prolonged severe physiological stress, or if this changes with age. To investigate this, young and old adult zebrafish were challenged by 72 hours of enforced swimming in a purpose-built flume at levels close to maximal Ucrit. Whilst young adult fish produced a significant proliferative response older fish had a dramatically impaired response, provided by a smaller proliferative cardiomyocyte population. Finally, we asked if these aging responses could be improved by increased activity throughout adulthood. Whilst there was some improvement in the aged proliferative response the size of the reduced proliferative cardiomyocyte pool remained unchanged and importantly, there was increased myocardial fibrosis. The zebrafish heart thus provides a laboratory model to study cardiomyocyte turnover during aging and physiological stresses, revealing the important trade-off between preserving cardiovascular fitness through exercise and accelerated fibrotic change, whilst the available proliferative pool of cardiomyocytes continues to diminish.