Rationale Ventricular arrhythmias remain the leading cause of death in patients suffering myocardial ischemia. Myeloperoxidase (MPO), a heme-enzyme released by polymorphonuclear neutrophils, accumulates within ischemic myocardium and has been linked to adverse left ventricular remodeling. Objective To reveal the role of MPO for the development of ventricular arrhythmias. Methods and Results In different murine models of myocardial ischemia MPO deficiency profoundly decreased vulnerability for ventricular tachycardia (VT) upon programmed right ventricular and burst stimulation and spontaneously as assessed by ECG telemetry following isoproterenol injection. Experiments employing CD11b/CD18-integrin-deficient (CD11b-/-) mice and intravenous MPO infusion revealed that neutrophil infiltration is a prerequisite for myocardial MPO accumulation. Ventricles from MPO-deficient (Mpo-/-) mice showed less pronounced slowing and decreased heterogeneity of electrical conduction in the periinfarct zone than WT mice. Expression of the redox sensitive gap-junctional protein connexin43 (Cx43) was reduced in the periinfarct area of WT compared to Mpo-/- mice. In isolated WT cardiomyocytes, Cx43 protein content decreased upon MPO/H2O2-incubation. Mapping of induced pluripotent stem-cell-derived cardiomyocyte (iPSCM) networks and in vivo investigations linked Cx43 breakdown to MPO-dependent activation of matrix-metalloproteinase 7. Moreover, Mpo-/- mice showed decreased ventricular postischemic fibrosis reflecting reduced accumulation of myofibroblasts. Ex vivo, MPO was demonstrated to induce fibroblast-to-myofibroblast transdifferentiation by activation of p38 mitogen-activated protein kinases (MAPK) resulting in upregulated collagen generation. In support of our experimental findings, baseline MPO plasma levels were independently associated with a history of ventricular arrhythmias, sudden cardiac death, or implantable cardioverter defibrillator implantation in a cohort of 2622 stable patients with an ejection fraction above 35% undergoing elective diagnostic cardiac evaluation. Conclusions MPO emerges as a crucial mediator of post-ischemic myocardial remodeling, and may evolve as a novel pharmacological target for secondary disease prevention following myocardial ischemia.

Objective- The leukocyte heme-enzyme MPO (myeloperoxidase) exerts proinflammatory effects on the vascular system primarily linked to its catalytic properties. Recent studies have shown that MPO, depending on its cationic charge, mediates neutrophil recruitment and activation. Here, we further investigated MPO's extracatalytic properties and its effect on endothelial glycocalyx (EG) integrity. Approach and Results- In vivo staining of murine cremaster muscle vessels with Alcian Blue 8GX provided evidence of an MPO-dependent decrease in anionic charge of the EG. MPO binding to the glycocalyx was further characterized using Chinese hamster ovary cells and its glycosaminoglycan mutants-pgsA-745 (mutant Chinese hamster ovary cells lacking heparan sulfate and chondroitin sulfate glycosaminoglycan) and pgsD-677 (mutant Chinese hamster ovary cells lacking heparan sulfate glycosaminoglycan), which revealed heparan sulfate as the main mediator of MPO binding. Further, EG integrity was assessed in terms of thickness using intravital microscopy of murine cremaster muscle. A significant reduction in EG thickness was observed on infusion of catalytically active MPO, as well as mutant inactive MPO and cationic polymer polylysine. Similar effects were also observed in wild-type mice after a local inflammatory stimulus but not in MPO-knockout mice. The reduction in EG thickness was reversed after removal of vessel-bound MPO, suggesting a possible physical collapse of the EG. Last, experiments with in vivo neutrophil depletion revealed that MPO also induced neutrophil-mediated shedding of the EG core protein, Sdc1 (syndecan-1). Conclusions- These findings provide evidence that MPO, via ionic interaction with heparan sulfate side chains, can cause neutrophil-dependent Sdc1 shedding and collapse of the EG structure.

Pulmonary arterial hypertension (PAH) remains a disease with limited therapeutic options and dismal prognosis. Despite its etiologic heterogeneity, the underlying unifying pathophysiology is characterized by increased vascular tone and adverse remodeling of the pulmonary circulation. Myeloperoxidase (MPO), an enzyme abundantly expressed in neutrophils, has potent vasoconstrictive and profibrotic properties, thus qualifying as a potential contributor to this disease. Here, we sought to investigate whether MPO is causally linked to the pathophysiology of PAH. Investigation of 2 independent clinical cohorts revealed that MPO plasma levels were elevated in subjects with PAH and predicted adverse outcome. Experimental analyses showed that, upon hypoxia, right ventricular pressure was less increased in Mpo-/- than in WT mice. The hypoxia-induced activation of the Rho-kinase pathway, a critical subcellular signaling pathway yielding vasoconstriction and structural vascular remodeling, was blunted in Mpo-/- mice. Mice subjected to i.v. infusion of MPO revealed activation of Rho-kinase and increased right ventricular pressure, which was prevented by coinfusion of the Rho-kinase inhibitor Y-27632. In the Sugen5416/hypoxia rat model, PAH was attenuated by the MPO inhibitor AZM198. The current data demonstrate a tight mechanistic link between MPO, the activation of Rho-kinase, and adverse pulmonary vascular function, thus pointing toward a potentially novel avenue of treatment.