Extracellular BRICK1 drives heart repair after myocardial infarction in mice

Tissue repair after myocardial infarction entails a vigorous angiogenic response that mitigates scarring and worsening of heart function. Angiogenesis in the infarct wound is guided by incompletely defined myeloid cell-endothelial cell interactions. Here, we identify the 75-amino acid microprotein BRICK1 (BRK1) as an indispensable driver of postinfarction angiogenesis in a mouse model of reperfused myocardial infarction. We show that BRK1 is preferentially expressed by myeloid cells and translocates to the extracellular space after myocardial infarction in mice and humans. As a subunit of the intracellular actin-regulatory WAVE complex, BRK1 was not previously known to function outside the cell. We find that BRK1 is not actively secreted but released during myeloid cell death. Cre-<i>loxP</i>-driven myeloid cell-selective genetic deletion of <i>Brk1</i> or antibody-mediated neutralization of extracellular BRK1 impaired microvessel formation in the infarct border zone and resulted in severe postinfarction heart failure in mice. Conversely, treatment with recombinant BRK1 preserved heart function in infarcted mice. Mechanistically, BRK1 induced an angiogenic phenotype in human cardiac endothelial cells by signaling via the small GTPase Ras-related protein Rap-1 and mitogen-activated protein kinases 1 and 3 to promote retinoblastoma protein hyperphosphorylation and E2F transcription factor activation. BRK1 thus emerges as an angiogenic factor linking myeloid cell death to ischemic tissue repair, potentially enabling a protein-based therapy for myocardial infarction.