miR-499 in Platelet-Derived Extracellular Vesicles Augments Inflammatory Cell Generation and Cardiac Remodeling After Myocardial Infarction

BACKGROUND: Emergency myelopoiesis by bone marrow hematopoietic stem and progenitor cells (HSPCs) exacerbates disease pathology in various chronic diseases, including myocardial infarction (MI) and atherosclerosis. However, the mechanisms triggering myelopoiesis in the bone marrow after a distant organ injury, such as MI, remain unknown. METHODS: We ligated the left descending coronary artery to induce MI in mice. Platelet-derived extracellular vesicles (pEVs) were detected and enumerated in mice and patients with MI using NanoSight, ImageStream, and flow cytometry. microRNA in pEVs was quantified using a microRNA array. We used parabiosis, flow cytometry, adoptive transfer experiments, and transgenic mice to assess the effects of pEVs and microRNA on HSPC lineage commitment and inflammatory cell generation. In addition, we carried out RNA sequencing, luciferase assay, lentivirus-mediated in vivo gene overexpression, and echocardiography to evaluate the merit of lactoferrin/lactotransferrin in post-MI pathogenesis. RESULTS: In this study, we demonstrate that patients and mice with MI and mice with hindlimb ischemia exhibit an increased number of circulating pEVs, which, in turn, augment HSPC number and proliferation in the bone marrow, leading to inflammatory myeloid cell generation and disease progression. S100A8/9 (S100 calcium-binding protein A8/A9), an alarmin complex produced by cardiomyocytes after MI, induced pEV secretion. In vivo and in vitro inhibition of platelet activation and exocytosis, and HSPC endocytosis, markedly lessened the production of pEV, HSPC proliferation, and myeloid cell generation in emergency hematopoiesis. A microRNA array revealed that pEVs isolated after MI had elevated cargo levels of miR-499 and miR-184, which were enriched in reticulated platelets after MI. miR-499 and miR-184 overexpression in mouse and human HSPCs resulted in enhanced hematopoiesis and myelopoiesis. miR-499-deficient pEVs were inefficient in stimulating emergency myelopoiesis and inducing cardiac remodeling after MI. RNA sequencing analysis, luciferase assay, and lentivirus-mediated in vivo gene overexpression demonstrated that miR-499 bound to the 3' region of lactoferrin/lactotransferrin in HSPC to downregulate this gene, promoting myelopoiesis and unleashing inflammation. CONCLUSIONS: Our study suggests that pEVs generated after MI induce HSPC proliferation and inflammatory cell generation. These discoveries uncover several therapeutic targets to reduce cardiac inflammation and remodeling after MI.