Background: Bone marrow (BM) cells possess broad differentiation potential and can form various cell lineages in response to pathophysiological cues. The present study investigated whether BM-derived cells contribute to the pathogenesis of cardiac hypertrophy, as well as the possible cellular mechanisms involved in such a role.
Methods and results: Lethally irradiated wild-type mice were transplanted with BM cells from enhanced green fluorescent protein-transgenic mice. The chimeric mice were subjected to either prolonged hypoxia or transverse aortic constriction. BM-derived enhanced green fluorescent protein-expressing cardiomyocytes increased in number over time, emerging predominantly in the pressure-overloaded ventricular myocardium, although they constituted <0.01% of recipient cardiomyocytes. To determine whether BM-derived cardiomyocytes were derived from cell fusion or transdifferentiation at the single-cell level, lethally irradiated Cre mice were transplanted with BM cells from the double-conditional Cre reporter mouse line Z/EG. BM-derived cardiomyocytes were shown to arise from both cell fusion and transdifferentiation. Interestingly, BM-derived myofibroblasts expressing both vimentin and alpha-smooth muscle actin were concentrated in the perivascular fibrotic area. These cells initially expressed MAC-1/CD14 but lost expression of these markers during the chronic phase, which suggests that they were derived from monocytes. A similar phenomenon occurred in cultured human monocytes, most of which ultimately expressed vimentin and alpha-smooth muscle actin.
Conclusions: We found that BM-derived cells were involved in the pathogenesis of cardiac hypertrophy via the dual mechanisms of cell fusion and transdifferentiation. Moreover, the present results suggest that BM-derived monocytes accumulating in the perivascular space might play an important role in the formation of perivascular fibrosis via direct differentiation into myofibroblasts.