Cardiomyocyte fusion contributes to cardiac hypertrophy in adult hearts

Cardiac hypertrophy is an adaptive response to increased workload characterized by enlargement of cardiomyocytes. Cardiomyocyte fusion has been reported as a mechanism contributing to cardiac growth early after birth. We hypothesized that cardiomyocyte fusion might also contribute to pathological hypertrophy in adult hearts. To detect fusion events during hypertrophy, we used a fluorescent genetic lineage-tracing system in which cardiomyocytes express an inducible, cardiac-specific Cre recombinase. Administration of a low dose of tamoxifen labelled individual cardiomyocytes with either red or green fluorescence. Cardiac hypertrophy was induced by transverse aortic constriction (TAC). After one month, isolated cardiomyocytes were analyzed for size, color, and ploidy. A small number of yellow cardiomyocytes was observed in sham hearts, likely due to incomplete recombination in multinucleated cells. However, TAC hearts displayed a significantly higher number of double-labelled yellow cardiomyocytes, indicating fusion between red and green cardiomyocytes. These yellow hypertrophic cardiomyocytes were predominantly multinucleated (≥ 3 nuclei). To exclude heterotypic fusion with non-cardiomyocytes, we repeated the experiment using a high dose of tamoxifen, resulting in ~ 100% green cardiomyocytes. Under these conditions, no yellow cells were detected after TAC. Finally, we confirmed the presence of large double-labelled, multinucleated cardiomyocytes in vivo in TAC hearts. Together, these findings suggest that homotypic cardiomyocyte fusion contributes to pressure overload-induced cardiac hypertrophy in adult mammals.