Myeloid cells release the intracellular microprotein BRICK1 to promote angiogenesis and tissue repair after myocardial infarction

We conducted a bioinformatic secretome analysis in bone marrow cells from patients with acute myocardial infarction (MI) to discover previously uncharacterized growth factors driving infarct repair. In this screen, we identified the 75‑amino acid microprotein BRICK1 (BRK1). BRK1 is a component of the intracellular actin polymerization-promoting WAVE regulatory complex. While BRK1 was not previously known as a secreted protein, the SecretomeP 2.0 algorithm predicted BRK1 to be released from cells by an unconventional mechanism. BRK1 was weakly expressed in the mouse myocardium under sham-operated baseline conditions but strongly upregulated in the infarct region after transient coronary artery ligation-induced MI (16-fold, peak on day 3, P<0.001). BRK1 was also expressed in left ventricular (LV) tissue samples from patients who had died of acute MI; expression levels were higher than in patients who had died from noncardiac causes (3-fold, P<0.001). As shown by targeted mass spectrometry, BRK1 plasma concentrations increased after MI in mice and patients, indicating that BRK1 can indeed be secreted. Using confocal immunofluorescence microscopy, RT-qPCR, and scRNA-seq, we identified myeloid cells (monocytes, macrophages, and neutrophils) as the main BRK1-expressing cell types in the infarct region. BRK1 plasma concentrations after MI were 72% lower in mice with a selective deletion of Brk1 in myeloid cells (Brk1^fl/fl LysM^Cre/+; mcKO) compared with wild-type control mice (Brk1^fl/fl; mcWT), establishing that most plasma BRK1 is myeloid cell-derived. Monocyte and macrophage turnover in the infarct region is characterized by high rates of cell recruitment and cell death, raising the possibility that dying myeloid cells release BRK1. Indeed, BRK1 was readily released from human monocytic THP‑1 cells treated with cytotoxic agents in vitro. Similarly, inducing macrophage cell death in vivo by injecting liposome-encapsulated clodronate resulted in a significant increase in BRK1 plasma concentrations. Exploring BRK1’s function after MI, we observed that mcKO mice had no apparent phenotype under baseline conditions but developed larger infarct scars 28 days after MI (30 ± 4 vs. 20 ± 3% of the left ventricle; P<0.05) with more pronounced LV systolic dysfunction than mcWT mice (echocardiographic fractional area change, 11 ± 2 vs. 25 ± 2%; P<0.001). Adverse post-MI remodeling in mcKO mice was associated with impaired de novo microvessel formation in the infarct border zone. Treating infarcted wild-type mice with a recombinant monoclonal BRK1-neutralizing antibody replicated the mcKO phenotype, indicating that extracellular BRK1 mediates angiogenesis and functional adaptation after MI. Along this line, recombinant BRK1 promoted human coronary artery endothelial cell migration and proliferation in vitro, confirming that BRK1 can act as an angiogenic growth factor in the extracellular space. Finally, treating wild-type mice with recombinant BRK1 (bolus injection at the time of reperfusion followed by a subcutaneous infusion for 7 days) improved border zone angiogenesis and attenuated adverse post-MI remodeling. In conclusion, the intracellular microprotein BRK1 is released from dying myeloid cells to promote angiogenesis and tissue repair after MI.