Platelet-GARP: TGFB1 controls post-myocardial infarction inflammation and repair

Cardiac healing after myocardial infarction (MI) encompasses inflammatory and reparative processes. Platelets infiltrate the infarcted myocardium where they can directly interact with neighboring cells and secrete a plethora of molecules, especially TGFβ1. Moreover, platelets are able to activate TGFβ1 through Glycoprotein A Repetitions Predominant (GARP) expressed on their surface. Considering the key anti-inflammatory and pro-fibrotic functions of TGFβ1, we aimed to determine the role of platelet-GARP in active TGFβ1 secretion and its impact on cardiac healing after MI.

The present study sought to determine the contribution of platelet-GARP in TGFβ1 activation and the subsequent regulation of inflammatory and reparative responses after MI.

We generated a Cre transgenic mouse strain allowing megakaryocyte/platelet specific deletion of GARP (GpIba-Cre x GARPfl/fl). Platelet function and production of active TGFβ1 were assessed by FACS, ELISA, and Western blot. Platelet-specific GARP knockout (pKO) and wild-type (WT) mice were subjected to permanent coronary artery ligation to induce MI, and cardiac function was assessed by echocardiography. Post-MI inflammatory and reparative responses were evaluated by RNA sequencing, RT-qPCR, immunofluorescence, and histological staining. Molecular mechanisms were further investigated in vitro, in Human Umbilical Vein Endothelial Cells (HUVECs).

We show that platelet-GARP deficiency drastically reduces systemic active TGFβ1 without affecting platelet activation. Following permanent coronary ligation, platelets infiltrate the myocardium between 1- and 3 days after MI, similarly in both genotypes. While pKO and WT mice display similar cardiomyocyte necrosis, pKO mice are characterized by increased mortality in the first week following MI due to ventricular rupture. Surviving pKO animals present an exacerbated left ventricular dilatation 14 days after MI. Gene Set Enrichment Analysis of our transcriptomic data highlight significant enrichment of pathways related to cytokine synthesis, leukocyte recruitment, and endothelial cell activation in pKO infarcted hearts. Consistently, pKO hearts showed excessive neutrophil and macrophage accumulation, reduced induction of anti-inflammatory cytokines, and impaired cardiac fibroblast proliferation and myofibroblast differentiation. Early extracellular matrix synthesis, including fibronectin and collagen deposition, was strongly diminished. Mechanistically, we found that loss of platelet-GARP selectively impaired TGFβ-Smad3 signaling in endothelial cells, resulting in increased adhesion molecule expression and amplified immune cell recruitment.

Our study identifies for the first time platelet-GARP as a critical regulator of TGFβ1 activation after MI. Moreover, our data demonstrate that platelet-GARP:TGFβ1 axis plays a central role in the post-MI response by modulating endothelial cell activation.