Tissue extracellular vesicles suppress neonatal cardiac regeneration: a Pak2-Erk1/2-mediated macrophage paracrine signaling.

Myocardial infarction leads to cardiomyocyte loss, and the compromised proliferative capacity of cardiomyocytes after birth hinders the process of heart repair, ultimately culminating in heart failure. Extracellular vesicles (EVs), known as cell-secreted "messengers", play a pivotal role in tissue pathophysiology. Here, we report the novel finding that myocardial tissue-derived vesicles from mice on postnatal day 8 (P8-EVs) possess the potential to modulate cardiomyocyte proliferation. Notably, direct administration of EVs derived from day 1 or day 8 (P1/P8) myocardial tissue does not impact neonatal cardiomyocyte proliferation or myocardial repair in mice with myocardial infarction. However, by leveraging bioinformatics, high-throughput omics, and single-cell analyses, we unveil that P8-EVs are enriched with the key gene p21 activated kinase 2 (Pak2), a regulator of macrophage reparative function. Through single-cell sequencing of P8 myocardial tissue, we identify macrophages as the cell type with the highest Pak2 content, implying a close association between macrophages and P8-EV function. Intriguingly, further investigations reveal that P8-EVs significantly promote M1-like polarization, augment phagocytosis, and affect factor secretion in macrophages. Co-culture experiments demonstrate that P8-EV-treated macrophages strongly suppress neonatal cardiomyocyte proliferation, and this effect is effectively reversed by a Pak2 inhibitor. Additional pathway intervention experiments reveal that P8-EVs activate the downstream Erk1/2 signaling pathway of Pak2. Collectively, our findings indicate that P8-EVs regulate macrophage paracrine activities through the Pak2-Erk1/2 axis, thereby influencing cardiomyocyte proliferation. This finding reveals a potential underlying mechanism for the compromised proliferative capacity of cardiomyocytes in adult mice.