Multi-phased Kinetics and Interaction of Protein Kinase Signaling in Glycoprotein VI-Induced Platelet αIIbß3 Integrin Activation and Degranulation.

BACKGROUND: Platelet glycoprotein VI (GPVI) stimulation activates the tyrosine kinases Syk and Btk, and the effector proteins phospholipase Cγ 2 (PLCγ2) and protein kinase C (PKC). Here, the activation sequence, crosstalk, and downstream effects of this Syk-Btk-PKC signalosome in human platelets were analyzed. METHODS AND RESULTS: Using immunoblotting, we quantified 14 regulated phospho-sites in platelets stimulated by convulxin with and without inhibition of Syk, Btk, or PKC. Convulxin induced fast, reversible tyrosine phosphorylation (pY) of Syk, Btk, LAT, and PLCγ2, followed by reversible serine/threonine phosphorylation (pS/T) of Syk, Btk, and downstream kinases MEK1/2, Erk1/2, p38, and Akt. Syk inhibition by PRT-060318 abolished all phosphorylations, except Syk pY352. Btk inhibition by acalabrutinib strongly decreased Btk pY223/pS180, Syk pS297, PLCγ2 pY759/Y1217, MEK1/2 pS217/221, Erk1/2 pT202/Y204, p38 pT180/Y182, and Akt pT308/S473. PKC inhibition by GF109203X abolished most pS/T phosphorylations except p38 pT180/Y182 and Akt pT308, but enhanced most Y-phosphorylations. Acalabrutinib, but not GF109203X, suppressed convulxin-induced intracellular Ca2+ mobilization, whereas all three protein kinase inhibitors abolished degranulation and αIIbß3 integrin activation assessed by flow cytometry. Inhibition of autocrine ADP effects by AR-C669931 partly diminished convulxin-triggered degranulation. CONCLUSION: Kinetic analysis of GPVI-initiated multisite protein phosphorylation in human platelets demonstrates multiple phases and interactions of tyrosine and serine/threonine kinases with activation-altering feedforward and feedback loops partly involving PKC. The protein kinase inhibitor effects on multisite protein phosphorylation and functional readouts reveal that the signaling network of Syk, Btk, and PKC controls platelet granule exocytosis and αIIbß3 integrin activation.

Cyclophilin A is a ligand for RAGE in thrombo-inflammation.

AIMS: Cyclophilin A (CyPA) induces leucocyte recruitment and platelet activation upon release into the extracellular space. Extracellular CyPA therefore plays a critical role in immuno-inflammatory responses in tissue injury and thrombosis upon platelet activation. To date, CD147 (EMMPRIN) has been described as the primary receptor mediating extracellular effects of CyPA in platelets and leucocytes. The receptor for advanced glycation end products (RAGE) shares inflammatory and prothrombotic properties and has also been found to have similar ligands as CD147. In this study, we investigated the role of RAGE as a previously unknown interaction partner for CyPA. METHODS AND RESULTS: Confocal imaging, proximity ligation, co-immunoprecipitation, and atomic force microscopy were performed and demonstrated an interaction of CyPA with RAGE on the cell surface. Static and dynamic cell adhesion and chemotaxis assays towards extracellular CyPA using human leucocytes and leucocytes from RAGE-deficient Ager-/- mice were conducted. Inhibition of RAGE abrogated CyPA-induced effects on leucocyte adhesion and chemotaxis in vitro. Accordingly, Ager-/- mice showed reduced leucocyte recruitment and endothelial adhesion towards CyPA in vivo. In wild-type mice, we observed a downregulation of RAGE on leucocytes when endogenous extracellular CyPA was reduced. We furthermore evaluated the role of RAGE for platelet activation and thrombus formation upon CyPA stimulation. CyPA-induced activation of platelets was found to be dependent on RAGE, as inhibition of RAGE, as well as platelets from Ager-/- mice showed a diminished activation and thrombus formation upon CyPA stimulation. CyPA-induced signalling through RAGE was found to involve central signalling pathways including the adaptor protein MyD88, intracellular Ca2+ signalling, and NF-κB activation. CONCLUSION: We propose RAGE as a hitherto unknown receptor for CyPA mediating leucocyte as well as platelet activation. The CyPA-RAGE interaction thus represents a novel mechanism in thrombo-inflammation.

ACKR3 regulates platelet activation and ischemia-reperfusion tissue injury.

Platelet activation plays a critical role in thrombosis. Inhibition of platelet activation is a cornerstone in treatment of acute organ ischemia. Platelet ACKR3 surface expression is independently associated with all-cause mortality in CAD patients. In a novel genetic mouse strain, we show that megakaryocyte/platelet-specific deletion of ACKR3 results in enhanced platelet activation and thrombosis in vitro and in vivo. Further, we performed ischemia/reperfusion experiments (transient LAD-ligation and tMCAO) in mice to assess the impact of genetic ACKR3 deficiency in platelets on tissue injury in ischemic myocardium and brain. Loss of platelet ACKR3 enhances tissue injury in ischemic myocardium and brain and aggravates tissue inflammation. Activation of platelet-ACKR3 via specific ACKR3 agonists inhibits platelet activation and thrombus formation and attenuates tissue injury in ischemic myocardium and brain. Here we demonstrate that ACKR3 is a critical regulator of platelet activation, thrombus formation and organ injury following ischemia/reperfusion.

The aged hematopoietic system promotes hippocampal-dependent cognitive decline.

The aged systemic milieu promotes cellular and cognitive impairments in the hippocampus. Here, we report that aging of the hematopoietic system directly contributes to the pro-aging effects of old blood on cognition. Using a heterochronic hematopoietic stem cell (HSC) transplantation model (in which the blood of young mice is reconstituted with old HSCs), we find that exposure to an old hematopoietic system inhibits hippocampal neurogenesis, decreases synaptic marker expression, and impairs cognition. We identify a number of factors elevated in the blood of young mice reconstituted with old HSCs, of which cyclophilin A (CyPA) acts as a pro-aging factor. Increased systemic levels of CyPA impair cognition in young mice, while inhibition of CyPA in aged mice improves cognition. Together, these data identify age-related changes in the hematopoietic system as drivers of hippocampal aging.

Platelet surface expression of cyclophilin A is associated with increased mortality in patients with symptomatic coronary artery disease.

BACKGROUND: Cyclophilin A (CyPA) is an important intracellular molecule mediating essential cellular functions such as signaling and protein folding. Enhanced CyPA platelet surface expression is associated with hypertension and hypercholesterolemia in patients with stable coronary artery disease (CAD). In patients with acute myocardial infarction CyPA platelet surface expression is significantly decreased. The aim of this study was to investigate possible associations of CyPA platelet surface expression and a clinically relevant CyPA single-nucleotide polymorphism (CyPA PPIA rs6850) with prognosis in patients with symptomatic cardiovascular disease. MATERIALS AND METHODS: Blood was obtained from 335 consecutive patients with symptomatic CAD. All patients were followed up for 1080 days for endpoints all-cause death, myocardial infarction (MI), ischemic stroke, and bleeding. The primary combined endpoint was defined as a composite of all-cause death and/or MI and/or ischemic stroke. Cyclophilin A platelet surface expression levels less than or equal to the median were significantly associated with a worse prognosis (combined endpoint and all-cause death) when compared to CyPA greater than the median. Genotyping for CyPA PPIA rs6850 was performed in 752 patients with symptomatic CAD. Homozygous carriers of the minor allele showed a significantly worse cumulative event-free survival for both combined endpoint and MI when compared to carriers of the major allele. CONCLUSION: The CyPA platelet surface expression is associated with mortality whereas CyPA PPIA rs6850 is associated with recurrent MI in patients with symptomatic CAD. Cyclophilin A might offer a new biomarker for risk stratification and tailoring therapies in patients with cardiovascular disease.

Circulating MicroRNA-21 Correlates With Left Atrial Low-Voltage Areas and Is Associated With Procedure Outcome in Patients Undergoing Atrial Fibrillation Ablation.

BACKGROUND: Atrial fibrosis is a hallmark of arrhythmogenic structural remodeling in patients with persistent atrial fibrillation (AF) and is negatively correlated with procedure outcome in patients undergoing ablation. However, noninvasive methods to determine the extent of atrial fibrosis are limited. Here, we used microRNA (miRNA) expression analysis to detect markers of left atrial low-voltage areas (LVAs) in patients with persistent AF undergoing catheter ablation. METHODS: We performed 3-dimensional voltage mapping in 102 patients (average age 62.1±13.1 years, CHA2DS2-VASc score of 2.3±1.6, LA size 41.5±5.7 mm) undergoing ablation for persistent AF and determined the extent of left atrial low-voltage. LVAs were defined if bipolar electrogram amplitudes were <0.5 mV during sinus rhythm. Before ablation, we obtained a blood sample, isolated miRNAs, and profiled them on a miRCURY LNA Universal RT microRNA PCR Human panel. RESULTS: Sixty-nine miRNAs were identified in all samples, with an average of 123 miRNAs detectable per sample. We found that the serum concentration of miR-21, a miRNA that has been previously linked to cardiac fibrosis development, was strongly associated with the extent of LVAs determined by voltage mapping. We could confirm that LVAs were negatively correlated with ablation success in a 1-year follow-up. In addition, miR-21 serum levels were associated with AF-free survival after catheter ablation. CONCLUSIONS: Circulating miR-21 correlates with left atrial LVAs and is associated with procedure outcome in patients with persistent AF undergoing ablation.

Extracellular Matrix Metalloproteinase Inducer EMMPRIN (CD147) in Cardiovascular Disease.

The receptor EMMPRIN is involved in the development and progression of cardiovascular diseases and in the pathogenesis of myocardial infarction. There are several binding partners of EMMPRIN mediating the effects of EMMPRIN in cardiovascular diseases. EMMPRIN interaction with most binding partners leads to disease progression by mediating cytokine or chemokine release, the activation of platelets and monocytes, as well as the formation of monocyte-platelet aggregates (MPAs). EMMPRIN is also involved in atherosclerosis by mediating the infiltration of pro-inflammatory cells. There is also evidence that EMMPRIN controls energy metabolism of cells and that EMMPRIN binding partners modulate intracellular glycosylation and trafficking of EMMPRIN towards the cell membrane. In this review, we systematically discuss these multifaceted roles of EMMPRIN and its interaction partners, such as Cyclophilins, in cardiovascular disease.

Extracellular Cyclophilin A Augments Platelet-Dependent Thrombosis and Thromboinflammation.

Cyclophilin A (CyPA) is involved in the pathophysiology of several inflammatory and cardiovascular diseases. To our knowledge, there is no specific inhibitor targeting extracellular CyPA without affecting other extracellular cyclophilins or intracellular CyPA functions. In this study, we developed an antibody-based inhibitor of extracellular CyPA and analysed its effects in vitro and in vivo. To generate a specific antibody, mice and rats were immunized with a peptide containing the extracellular matrix metalloproteinase inducer binding site and various antibody clones were selected and purified. At first, antibodies were tested for their binding capacity to recombinant CyPA and their functional activity. The clone 8H7-mAb was chosen for further experiments. 8H7-mAb reduced the CyPA-induced migration of inflammatory cells in vitro and in vivo. Furthermore, 8H7-mAb revealed strong antithrombotic effects by inhibiting CyPA-dependent activation of platelets and thrombus formation in vitro and in vivo. Surprisingly, 8H7-mAb did not influence in vivo tail bleeding time or in vitro whole blood coagulation parameters. Our study provides first evidence that antibody-based inhibition of extracellular CyPA inhibits thrombosis and thromboinflammation without affecting blood homeostasis. Thus, 8H7-mAb may be a promising compound for thrombi modulation in inflammatory diseases to prevent organ dysfunction.

Protective Pleiotropic Effect of Flavonoids on NAD⁺ Levels in Endothelial Cells Exposed to High Glucose.

NAD(+) is important for oxidative metabolism by serving as an electron transporter. Hyperglycemia decreases NAD(+) levels by activation of the polyol pathway and by overactivation of poly(ADP-ribose)-polymerase (PARP). We examined the protective role of three structurally related flavonoids (rutin, quercetin, and flavone) during high glucose conditions in an in vitro model using human umbilical vein endothelial cells (HUVECs). Additionally we assessed the ability of these flavonoids to inhibit aldose reductase enzyme activity. We have previously shown that flavonoids can inhibit PARP activation. Extending these studies, we here provide evidence that flavonoids are also able to protect endothelial cells against a high glucose induced decrease in NAD(+). In addition, we established that flavonoids are able to inhibit aldose reductase, the key enzyme in the polyol pathway. We conclude that this protective effect of flavonoids on NAD(+) levels is a combination of the flavonoids ability to inhibit both PARP activation and aldose reductase enzyme activity. This study shows that flavonoids, by a combination of effects, maintain the redox state of the cell during hyperglycemia. This mode of action enables flavonoids to ameliorate diabetic complications.

The Novel Extracellular Cyclophilin A (CyPA) - Inhibitor MM284 Reduces Myocardial Inflammation and Remodeling in a Mouse Model of Troponin I -Induced Myocarditis.

Cyclophilins are a group of highly conserved cytosolic enzymes that have a peptidylprolyl cis/trans isomerase activity. Cyclophilin A (CyPA) can be secreted in the extracellular space by inflammatory cells and upon cell death. The presence of CyPA in patients with non-ischemic cardiomyopathy is associated with poor clinical prognosis. Here, we investigated the inhibition of extracellular CyPA in a mouse model of troponin I-induced autoimmune myocarditis using the strictly extracellular CyPA-inhibitor MM284. Since A/J mice develop severe inflammation and fibrosis after immunization with murine cardiac troponin I (mcTn I), we used this model to analyze the effects of an extracellular CyPA inhibition. As extracellular CyPA-inhibitor we used the recently described CsA-derivate MM284. In vitro studies confirmed that MM284 inhibits CyPA-induced monocytic migration and adhesion. A/J mice immunized with mcTnI were treated with MM284 or vehicle every second day. After 28 days, we found a considerable reduction of myocardial injury and fibrosis. Further analysis revealed a reduced myocardial presence of T-cells and macrophages compared to control treated animals. Whereas MMP-9 expression was reduced significantly by MM284, we observed no significant reduction of inflammatory cytokines such as IL-6 or TNFα. Extracellular CyPA plays an important role in autoimmune myocarditis for myocardial damage and fibrosis. Our data suggest a new pharmacological approach for the treatment of myocardial inflammation and reduction of cardiac fibrosis by inhibition of extracellular CyPA.