Extracellular BRICK1 drives heart repair after myocardial infarction

Tissue repair after myocardial infarction entails a vigorous angiogenic response that mitigates scarring and worsening of heart function and may represent a therapeutic target. Angiogenesis in the infarct wound is guided by incompletely defined myeloid cell–endothelial cell interactions. Here we identify the myeloid cell-expressed 75-amino acid microprotein BRICK1 (short name: BRK1) as an indispensable driver of postinfarction angiogenesis. As a subunit of the intracellular actin-regulatory WAVE complex, BRK1 was not previously known to function outside the cell. We show that BRK1 translocates to the extracellular space after myocardial infarction in mice and humans. We find that BRK1 is not actively secreted but released from dying monocytes and macrophages. Cre-loxP-mediated myeloid cell-selective genetic deletion of Brk1 or antibody neutralization of extracellular BRK1 impaired microvessel formation in the infarct border zone and resulted in severe postinfarction heart failure in mice. Treatment with recombinant BRK1, conversely, preserved heart function after myocardial infarction. Mechanistically, BRK1 induced an angiogenic phenotype in human cardiac endothelial cells by signaling via the small GTPase RAP1 and mitogen-activated protein kinases 1 and 3 to promote retinoblastoma protein hyperphosphorylation and E2F transcription factor activation. BRK1 thus emerges as an angiogenic growth factor linking myeloid cell death to ischemic tissue repair and potentially enabling a protein-based therapy for myocardial infarction. Overall design: Bulk-RNA sequencing of human coronary artery endothelial cells cultured for 8 hours in the absence (control) or presence of BRK1, VEGFA, PD98059, or PD plus BRK1.

心肌梗死（myocardial infarction）后的组织修复伴随强烈的血管生成应答，该应答可减轻瘢痕形成与心功能恶化，或可作为治疗靶点。梗死创面中的血管生成由尚未完全阐明的髓系细胞-内皮细胞互作所调控。本研究鉴定出髓系细胞表达的75氨基酸微蛋白BRICK1（简称BRK1）是梗死后血管生成的不可或缺的驱动因子。作为细胞内肌动蛋白调控WAVE复合物的亚基，BRK1此前从未被报道可在细胞外发挥功能。本研究证实，在小鼠与人类体内，心肌梗死后BRK1会转位至细胞外间隙。研究发现BRK1并非主动分泌，而是由凋亡的单核细胞与巨噬细胞释放。通过Cre-loxP介导的髓系细胞选择性Brk1基因敲除，或通过抗体中和细胞外BRK1，均可损伤小鼠梗死边缘区的微血管生成，并引发严重的梗死后心力衰竭。反之，给予重组BRK1治疗则可维持心肌梗死后的心功能。从机制层面来看，BRK1可通过小GTP酶RAP1与丝裂原活化蛋白激酶1、3进行信号转导，促进视网膜母细胞瘤蛋白过度磷酸化与E2F转录因子活化，从而诱导人心脏内皮细胞产生血管生成表型。由此可见，BRK1作为一种血管生成生长因子，将髓系细胞死亡与缺血组织修复联系起来，或可为心肌梗死的蛋白类治疗提供新的策略。实验整体设计：将人冠状动脉内皮细胞分别置于无（对照组）或添加BRK1、血管内皮生长因子A（VEGFA）、PD98059或PD98059联合BRK1的培养基中培养8小时，随后对其进行批量RNA测序（Bulk-RNA sequencing）。