Osteopontin stimulates cell cycle in neonatal cardiomyocytes and improves infarct repair

Introduction: Studies into the macrophage cardiac regenerative mechanisms could identify specific molecules able to regulate neonatal myocardial plasticity after injury. We found that osteopontin (OPN), a macrophage-derived matricellular protein, is upregulated following myocardial injury. However, the role of OPN in myocardial regeneration is unknown. Therefore, we sought to determine the role of OPN in myocardial regeneration and repair.

Material and method: Mouse neonatal cardiomyocytes were cultured in a serum-free medium for 24 hours with or without OPN. We assessed cardiomyocyte growth and function by immunofluorescent staining, MTT colorimetric assays, immunoblotting, gene expression, and scratch assay. Then, we used echocardiography and postmortem histology to determine the effect of OPN on left ventricular (LV) remodeling and function after myocardial infarction (MI) in adult mice.

Results: OPN increased the expression of phosphohistone-3 (pH3), a marker of nuclei division, in cardiomyocytes and non-cardiomyocytes cells. MTT colorimetric assays showed that OPN preserved the number of cardiomyocytes under stress conditions. This effect disappeared by inhibition of the OPN receptor CD44. Immunoblotting analysis revealed that OPN phosphorylated the cytoplasmatic YAP1, a transcriptional regulator of cell proliferation (Figure A). OPN also translocated cytoplasmatic YAP1 into the nucleus (Figure B), an essential step in cell proliferation. Gene expression analysis revealed that OPN upregulated genes related to YAP1 activity and the mitotic phase. Interestingly, OPN stimulates cardiomyocyte proliferation and migration, by a scratch (wound healing) assay. Finally, postmortem histology and echocardiography showed that a single injection of OPN to adult mice after MI prevented the infarct thinning (Figure C) and improved LV remodeling and function (Figure D).

Conclusions: OPN stimulates cell-cycle activity in neonatal cardiomyocytes via the CD44 receptor and the YAP1 pathway. Acute administration of OPN attenuates adverse LV remodeling and dysfunction after MI in the mouse. Our findings suggest a new cell-free therapy for heart repair after MI.