P2Y12-dependent activation of hematopoietic stem and progenitor cells promotes emergency hematopoiesis after myocardial infarction.

Emergency hematopoiesis is the driving force of the inflammatory response to myocardial infarction (MI). Increased proliferation of hematopoietic stem and progenitor cells (LSK) after MI enhances cell production in the bone marrow (BM) and replenishes leukocyte supply for local cell recruitment to the infarct. Decoding the regulation of the inflammatory cascade after MI may provide new avenues to improve post-MI remodeling. In this study, we describe the influence of adenosine diphosphate (ADP)-dependent P2Y12-mediated signaling on emergency hematopoiesis and cardiac remodeling after MI. Permanent coronary ligation was performed to induce MI in a murine model. BM activation, inflammatory cell composition and cardiac function were assessed using global and platelet-specific gene knockout and pharmacological inhibition models for P2Y12. Complementary in vitro studies allowed for investigation of ADP-dependent effects on LSK cells. We found that ADP acts as a danger signal for the hematopoietic BM and fosters emergency hematopoiesis by promoting Akt phosphorylation and cell cycle progression. We were able to detect P2Y12 in LSK, implicating a direct effect of ADP on LSK via P2Y12 signaling. P2Y12 knockout and P2Y12 inhibitor treatment with prasugrel reduced emergency hematopoiesis and the excessive inflammatory response to MI, translating to lower numbers of downstream progeny and inflammatory cells in the blood and infarct. Ultimately, P2Y12 inhibition preserved cardiac function and reduced chronic adverse cardiac remodeling after MI. P2Y12-dependent signaling is involved in emergency hematopoiesis after MI and fuels post-ischemic inflammation, proposing a novel, non-canonical value for P2Y12 antagonists beyond inhibition of platelet-mediated atherothrombosis.

P2X4 deficiency reduces atherosclerosis and plaque inflammation in mice.

Extracellular adenosine-5'-triphosphate (ATP) acts as an import signaling molecule mediating inflammation via purinergic P2 receptors. ATP binds to the purinergic receptor P2X4 and promotes inflammation via increased expression of pro-inflammatory cytokines. Because of the central role of inflammation, we assumed a functional contribution of the ATP-P2X4-axis in atherosclerosis. Expression of P2X4 was increased in atherosclerotic aortic arches from low-density lipoprotein receptor-deficient mice being fed a high cholesterol diet as assessed by real-time polymerase chain reaction and immunohistochemistry. To investigate the functional role of P2X4 in atherosclerosis, P2X4-deficient mice were crossed with low-density lipoprotein receptor-deficient mice and fed high cholesterol diet. After 16 weeks, P2X4-deficient mice developed smaller atherosclerotic lesions compared to P2X4-competent mice. Furthermore, intravital microscopy showed reduced ATP-induced leukocyte rolling at the vessel wall in P2X4-deficient mice. Mechanistically, we found a reduced RNA expression of CC chemokine ligand 2 (CCL-2), C-X-C motif chemokine-1 (CXCL-1), C-X-C motif chemokine-2 (CXCL-2), Interleukin-6 (IL-6) and tumor necrosis factor α (TNFα) as well as a decreased nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3)-inflammasome priming in atherosclerotic plaques from P2X4-deficient mice. Moreover, bone marrow derived macrophages isolated from P2X4-deficient mice revealed a reduced ATP-mediated release of CCL-2, CC chemokine ligand 5 (CCL-5), Interleukin-1ß (IL-1ß) and IL-6. Additionally, P2X4-deficient mice shared a lower proportion of pro-inflammatory Ly6Chigh monocytes and a higher proportion of anti-inflammatory Ly6Clow monocytes, and expressend less endothelial VCAM-1. Finally, increased P2X4 expression in human atherosclerotic lesions from carotid endarterectomy was found, indicating the importance of potential implementations of this study's findings for human atherosclerosis. Collectively, P2X4 deficiency reduced experimental atherosclerosis, plaque inflammation and inflammasome priming, pointing to P2X4 as a potential therapeutic target in the fight against atherosclerosis.

P2Y12 Inhibition in Murine Myocarditis Results in Reduced Platelet Infiltration and Preserved Ejection Fraction.

Previous mouse studies have shown the increased presence of platelets in the myocardium during early stages of myocarditis and their selective detection by MRI. Here, we aimed to depict early myocarditis using molecular contrast-enhanced ultrasound of activated platelets, and to evaluate the impact of a P2Y12 receptor platelet inhibition. Experimental autoimmune myocarditis was induced in BALB/c mice by subcutaneous injection of porcine cardiac myosin and complete Freund adjuvant (CFA). Activated platelets were targeted with microbubbles (MB) coupled to a single-chain antibody that binds to the "ligand-induced binding sites" of the GPIIb/IIIa-receptor (=LIBS-MB). Alongside myocarditis induction, a group of mice received a daily dose of 100 g prasugrel for 1 month. Mice injected with myosin and CFA had a significantly deteriorated ejection fraction and histological inflammation on day 28 compared to mice only injected with myosin. Platelets infiltrated the myocardium before reduction in ejection fraction could be detected by echocardiography. No selective binding of the LIBS-MB contrast agent could be detected by either ultrasound or histology. Prasugrel therapy preserved ejection fraction and significantly reduced platelet aggregates in the myocardium compared to mice without prasugrel therapy. Therefore, P2Y12 inhibition could be a promising early therapeutic target in myocarditis, requiring further investigation.

Deficiency of Endothelial CD40 Induces a Stable Plaque Phenotype and Limits Inflammatory Cell Recruitment to Atherosclerotic Lesions in Mice.

OBJECTIVES: The co-stimulatory CD40L-CD40 dyad exerts a critical role in atherosclerosis by modulating leukocyte accumulation into developing atherosclerotic plaques. The requirement for cell-type specific expression of both molecules, however, remains elusive. Here, we evaluate the contribution of CD40 expressed on endothelial cells (ECs) in a mouse model of atherosclerosis. METHODS AND RESULTS: Atherosclerotic plaques of apolipoprotein E-deficient (Apoe -/- ) mice and humans displayed increased expression of CD40 on ECs compared with controls. To interrogate the role of CD40 on ECs in atherosclerosis, we induced EC-specific (BmxCreERT2-driven) deficiency of CD40 in Apoe -/- mice. After feeding a chow diet for 25 weeks, EC-specific deletion of CD40 (iEC-CD40) ameliorated plaque lipid deposition and lesional macrophage accumulation but increased intimal smooth muscle cell and collagen content, while atherosclerotic lesion size did not change. Leukocyte adhesion to the vessel wall was impaired in iEC-CD40-deficient mice as demonstrated by intravital microscopy. In accord, expression of vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1) in the vascular endothelium declined after deletion of CD40. In vitro, antibody-mediated inhibition of human endothelial CD40 significantly abated monocyte adhesion on ECs. CONCLUSION: Endothelial deficiency of CD40 in mice promotes structural features associated with a stable plaque phenotype in humans and decreases leukocyte adhesion. These results suggest that endothelial-expressed CD40 contributes to inflammatory cell migration and consecutive plaque formation in atherogenesis.

Molecular magnetic resonance imaging of activated platelets allows noninvasive detection of early myocarditis in mice.

MRI sensitivity for diagnosis and localization of early myocarditis is limited, although it is of central clinical interest. The aim of this project was to test a contrast agent targeting activated platelets consisting of microparticles of iron oxide (MPIO) conjugated to a single-chain antibody directed against ligand-induced binding sites (LIBS) of activated glycoprotein IIb/IIIa (= LIBS-MPIO). Myocarditis was induced by subcutaneous injection of an emulsion of porcine cardiac myosin and complete Freund's adjuvant in mice. 3D 7 T in-vivo MRI showed focal signal effects in LIBS-MPIO injected mice 2 days after induction of myocarditis, whereas in control-MPIO injected mice no signal was detectable. Histology confirmed CD41-positive staining, indicating platelet involvement in myocarditis in mice as well as in human specimens with significantly higher LIBS-MPIO binding compared to control-MPIO in both species. Quantification of the myocardial MRI signal confirmed a signal decrease after LIBS-MPIO injection and significant less signal in comparison to control-MPIO injection. These data show, that platelets are involved in inflammation during the course of myocarditis in mice and humans. They can be imaged non-invasively with LIBS-MPIO by molecular MRI at an early time point of the inflammation in mice, which is a valuable approach for preclinical models and of interest for both diagnostic and prognostic purposes.