The ubiquitin ligase WWP1 contributes to shifts in matrix proteolytic profiles and a myocardial aging phenotype with diastolic heart

Heart failure (HF) with a preserved ejection fraction (HFpEF) is a growing cause of HF and commonly afflicts the elderly. Milestones for HFpEF include diastolic dysfunction and an abnormal extracellular matrix (ECM). The ubiquitin ligases, such as WWP1, change with aging and regulate critical protein turnover/stability processes, such as the ECM. The present study demonstrated that induction of WWP1 in mice induced LV hypertrophy, diastolic dysfunction, and ECM accumulation, consistent with the HFpEF phenotype, and thus may identify a new therapeutic pathway.

Abstract 254: Defects in c-kit Receptor Function Affect Myocyte Turnover Resulting in Premature Myocardial Aging in the W/WV Mouse

The objective of this study was to determine whether attenuation of cardiac stem cell (CSC) growth in the adult heart interferes with physiologic cell turnover leading to premature accumulation of senescent myocytes and myocardial aging. For this purpose, W/W V mice, which carry a mutated non-functional c-kit receptor, were studied. The W/W V mouse is a compound heterozygous mutant mouse. The W mutation corresponds to a large amino acid deletion that includes the transmembrane domain of the c-kit receptor while the W V mutation is a point mutation in the kinase domain of this receptor. These genetic modifications reduce c-kit kinase activity by approximately 80-90% affecting hematopoiesis, gametogenesis and melanogenesis. Echocardiographically, at 2-3 months of age, LV diameters and ejection fraction were similar in controls and W/W V mice. However, at 5-6 months and 8-9 months, diastolic and systolic LVDs were larger in W/W V mice than in WT. Ejection fraction decreased with age in W/W V mice. These various aspects of negative ventricular remodeling typically occur with the development of the aging myopathy in mice 24 month-old and older. Moreover, ventricular dilation is commonly associated with wall thinning, both critical parameters of diastolic and systolic wall stress. Importantly, these anatomical and functional changes in W/W V mice were associated with a marked increase in the percentage of CSCs and myocytes expressing the senescence-associated marker p16 INK4a . At two months of age, wild-type mice and W/W V mice had the same number of LV myocytes which were essentially identical in volume. However, in comparison with wild-type mice, W/W V mice at 4 months of age had a decreased number of LV myocytes which were larger in volume. The magnitude of myocyte proliferation at the time of sacrifice was assessed by two markers of the cell cycle, Ki67 and MCM5. On average, cycling myocytes were 3-4-fold higher in wild type mice than in W/W V mice. In contrast, apoptotic myocytes, evaluated by the TdT assay and hairpin 1 in situ ligation, were significantly more frequent in W/W V mice than in WT mice. In conclusion, mutation of the c-kit receptor in CSCs impairs new myocyte formation and promotes accumulation of senescent myocytes.

Abstract 19: Myocyte Turnover in the Aging Human Heart

The controversy on the growth reserve of the adult human heart has not been resolved and the extent of myocyte renewal reported by different groups varies significantly. Additionally, myocyte regeneration has been claimed to decrease with aging, although cell death is markedly enhanced in the old myocardium. Thus, the effects of age and gender on the magnitude of myocyte turnover were determined. Myocyte replication, senescence and apoptosis were measured in normal female and male human hearts collected from patients 19 to 104 years of age who died from causes other than cardiovascular diseases. Myocardial aging was characterized by a time-dependent increase in the generation of amplifying cardiomyocytes in women and men. Levels of Ki67 and phospho-H3 were comparable in the young female and male heart but differed later in life. As a function of age, the pool of amplifying myocytes was 2-fold higher in women than men, pointing to enhanced myocyte renewal in the female heart. The frequency of p16 INK4a -positive myocytes was higher in men than in women. From 19 to 104 years of age, the time-dependent increase in senescent myocytes was 0.68% per year in women and 0.89% per year in men; the 31% higher rate of accumulation of old myocytes in the aging male heart was significant. Myocyte apoptosis occurred only in p16 INK4a -positive cells and was consistently higher in men than in women at all age intervals. However, the increase in myocyte apoptosis with age did not differ with gender. Based on these parameters, we measured the average age of cardiomyocytes, their age distribution, turnover rate and time to acquire the senescent phenotype to define the biology of myocardial aging as a function of lifespan. In the female heart, myocyte turnover occurs at a rate of 10%, 15% and 40% per year at 20, 60 and 100 years of age, respectively. Corresponding values in the male heart are 7%, 12% and 32% per year, documenting that cardiomyogenesis involves a large and progressively increasing number of parenchymal cells with aging. In conclusion, the human heart is a highly dynamic organ in which progressive myocyte loss is at least in part counteracted by enhanced myocyte renewal. Myocyte regeneration in the physiologically aging heart takes place at previously unexpected levels in both women and men.