Protein Disulfide Isomerase and Extracellular Adherence Protein Cooperatively Potentiate Staphylococcal Invasion into Endothelial Cells

Invasion of host cells is an important feature of Staphylococcus aureus. The main internalization pathway involves binding of the bacteria to host cells, e.g., endothelial cells, via a fibronectin (Fn) bridge between S. aureus Fn binding proteins and α5β1-integrin, followed by phagocytosis. The secreted extracellular adherence protein (Eap) has been shown to promote this cellular uptake pathway of not only S. aureus, but also of bacteria otherwise poorly taken up by host cells, such as Staphylococcus carnosus. The exact mechanisms are still unknown. Previously, we demonstrated that Eap induces platelet activation by stimulation of the protein disulfide isomerase (PDI), a catalyst of thiol-disulfide exchange reactions. Here, we show that Eap promotes PDI activity on the surface of endothelial cells, and that this contributes critically to Eap-driven staphylococcal invasion. PDI-stimulated β1-integrin activation followed by increased Fn binding to host cells likely accounts for the Eap-enhanced uptake of S. aureus into non-professional phagocytes. Additionally, Eap supports the binding of S. carnosus to Fn-α5β1 integrin, thereby allowing its uptake into endothelial cells. To our knowledge, this is the first demonstration that PDI is crucial for the uptake of bacteria into host cells. We describe a hitherto unknown function of Eap-the promotion of an enzymatic activity with subsequent enhancement of bacterial uptake-and thus broaden mechanistic insights into its importance as a driver of bacterial pathogenicity. IMPORTANCE Staphylococcus aureus can invade and persist in non-professional phagocytes, thereby escaping host defense mechanisms and antibiotic treatment. The intracellular lifestyle of S. aureus contributes to the development of infection, e.g., in infective endocarditis or chronic osteomyelitis. The extracellular adherence protein secreted by S. aureus promotes its own internalization as well as that of bacteria that are otherwise poorly taken up by host cells, such as Staphylococcus carnosus. In our study, we demonstrate that staphylococcal uptake by endothelial cells requires catalytic disulfide exchange activity by the cell-surface protein disulfide isomerase, and that this critical enzymatic function is enhanced by Eap. The therapeutic application of PDI inhibitors has previously been investigated in the context of thrombosis and hypercoagulability. Our results add another intriguing possibility: therapeutically targeting PDI, i.e., as a candidate approach to modulate the initiation and/or course of S. aureus infectious diseases.

Reversibility of platelet P2Y12 inhibition by platelet supplementation: ex vivo and in vitro comparisons of prasugrel, clopidogrel and ticagrelor

Essentials Successful outcome of platelet transfusion depends on specific antiplatelet therapy in use. We assessed if ticagrelor, clopidogrel or prasugrel impacts on donor platelet activity ex vivo. Ticagrelor and/or its active metabolite in plasma or bound to platelets can inhibit donor platelets. This might compromise the effectiveness of platelet transfusion therapy.

SUMMARY

Background Platelet transfusion is the conventional approach to restore platelet function during acute bleeds or surgery, but successful outcome depends on the specific antiplatelet therapy. Notably ticagrelor is associated with inadequate recovery of platelet function after platelet transfusion. We examined whether plasma and/or platelets from ticagrelor-treated patients influence donor platelet function, in comparison with clopidogrel and prasugrel. Methods Platelet transfusion was mimicked ex vivo by mixing naïve donor platelet-rich plasma (PRP) or gel-filtered platelets (GFP) in defined proportions with PRP, plasma or GFP from cardiovascular patients receiving standard care including medication with prasugrel, clopidogrel or ticagrelor (n = 20 each). Blood was taken 4 h after the previous dose. HLA2/HLA28 haplotyping let us distinguish net (all platelet) and individual patient/donor platelet reactivity in mixtures of patient/donor platelets, measured by flow cytometry analysis of ADP-induced fibrinogen binding and CD62P expression. Results ADP responsiveness of donor platelets was dramatically reduced by even low (10%) concentrations of PRP or plasma from ticagrelor-treated patients. Clopidogrel and prasugrel were associated with more modest donor platelet inhibition. GFP from ticagrelor-treated patients but not patients receiving clopidogrel or prasugrel also suppressed donor GFP function upon mixing, suggesting the transfer of ticagrelor from patient platelets to donor platelets. This transfer did not lead to recovery of ADP responsiveness of patient's platelets. Conclusion Collectively, these observations support the concept that ticagrelor and/or its active metabolite in plasma or bound to platelets can inhibit donor platelets, which might compromise the effectiveness of platelet transfusion therapy.

Candida albicans and its metabolite gliotoxin inhibit platelet function via interaction with thiols

Platelets bind to Candida albicans, the major cause of candidiasis. But in contrast to other microorganisms the fungus does not aggregate platelets. Gliotoxin (GT), which possesses immunosuppressive properties, is produced by various fungi, including the opportunistic pathogens Aspergillus fumigatus and C. albicans. Its mode of action involves the formation of mixed disulfides with host proteins. Disulfide exchanges play an important role in platelet activation. Therefore, the effect of C. albicans and GT on platelet function was tested. C. albicans yeast cells (5,000–10,000 cells/μl) and GT, in pathophysiologically relevant concentrations (0.05–0.5 μM), inhibited platelet fibrinogen binding, anti gp IIb/IIIa antibody PAC-1 binding, aggregation and procoagulant activity in a dose-dependent manner. Alpha granule release, measured via CD62P surface expression, was not affected. Addition of reduced glutathione partially counteracted the effect of C. albicans and GT on platelet fibrinogen binding and platelet aggregation. The C. albicans metabolite GT features antithrombotic properties in addition to its immunosuppressive functions. Since treatment with reduced glutathione partially counteracted the inhibitory effect of C. albicans yeast cells and GT on platelet fibrinogen binding, the antithrombotic activity is likely to depend on the disulfide bridge of this mycotoxin. GT production by C. albicans could contribute to its survival in the blood stream during vascular infections. The knowledge of the underlying mechanisms of the antithrombotic properties might help to treat fungal infections as well as thrombosis.

Components in Plasma-Derived Factor VIII, But Not in Recombinant Factor VIII Downregulate Anti-Inflammatory Surface Marker CD163 in Human Macrophages through Release of CXCL4 (Platelet Factor 4)

BACKGROUND

Hemarthrosis, or bleeding into the joints, is a hallmark of hemophilia. Heme triggers oxidative stress, inflammation, and destruction of cartilage and bone. The haptoglobin-CD163-heme oxygenase-1 (HO-1) pathway circumvents heme toxicity through enzymatic degradation of heme and transcription of antioxidant genes. Plasma-derived factor concentrates contain many proteins that might impact on cellular pathways in joints, blood, and vessels.

METHODS

Activation of platelets from healthy volunteers was assessed by flow cytometry analysis of fibrinogen binding and CD62P expression. Platelet CXCL4 release was measured by ELISA. Human peripheral blood mononuclear cells were exposed to CXCL4 or platelet supernatants (untreated or pre-stimulated with factor VIII (FVIII) products) during their differentiation to macrophages and analyzed for CD163 expression. Some macrophage cultures were additionally incubated with autologous hemoglobin for 18 h for analysis of HO-1 expression.

RESULTS

Platelet CXCL4 release was increased by all 8 tested plasma-derived FVIII products but not the 3 recombinant products. Macrophages exposed to supernatant from platelets treated with some plasma-derived FVIII products downregulated CD163 surface expression and failed to upregulate the athero- and joint protective enzyme HO-1 in response to hemoglobin.

CONCLUSION

Plasma-derived FVIII products might promote bleeding-induced joint injury via generation of macrophages that are unable to counteract redox stress.

Staphylococcal extracellular adherence protein induces platelet activation by stimulation of thiol isomerases

OBJECTIVE

Staphylococcus aureus can induce platelet aggregation. The rapidity and degree of this correlates with the severity of disseminated intravascular coagulation, and depends on platelet peptidoglycans. Surface-located thiol isomerases play an important role in platelet activation. The staphylococcal extracellular adherence protein (Eap) functions as an adhesin for host plasma proteins. Therefore we tested the effect of Eap on platelets.

METHODS AND RESULTS

We found a strong stimulation of the platelet-surface thiol isomerases protein disulfide isomerase and endoplasmic reticulum stress proteins 57 and 72 by Eap. Eap induced thiol isomerase-dependent glycoprotein IIb/IIIa activation, granule secretion, and platelet aggregation. Treatment of platelets with thiol blockers, bacitracin, and anti-protein disulfide isomerase antibody inhibited Eap-induced platelet activation. The effect of Eap on platelets and protein disulfide isomerase activity was completely blocked by glycosaminoglycans. Inhibition by the hydrophobic probe bis(1-anilinonaphthalene 8-sulfonate) suggested the involvement of hydrophobic sites in protein disulfide isomerase and platelet activation by Eap.

CONCLUSIONS

In the present study, we found an additional and yet unknown mechanism of platelet activation by a bacterial adhesin, involving stimulation of thiol isomerases. The thiol isomerase stimulatory and prothrombotic features of a microbial secreted protein are probably not restricted to S aureus and Eap. Because many microorganisms are coated with amyloidogenic proteins, it is likely that the observed mechanism is a more general one.

Human neutrophil alpha-defensins induce formation of fibrinogen and thrombospondin-1 amyloid-like structures and activate platelets via glycoprotein IIb/IIIa

BACKGROUND

Human neutrophil α-defensins (HNPs) are important constituents of the innate immune system. Beyond their antimicrobial properties, HNPs also have pro-inflammatory features. While HNPs in plasma from healthy individuals are barely detectable, their level is strongly elevated in septic plasma and plasma from patients with acute coronary syndromes.

OBJECTIVES

As thrombosis and inflammation are intertwined processes and activation of human polymorphonuclear leukocytes (PMNL) and subsequent degranulation is associated with full activation of surrounding platelets, we studied the effect of HNPs on platelet function.

METHODS

The effect of HNPs on platelet activation parameters and apoptosis was investigated via aggregometry, flow cytometry, confocal microscopy and the ELISA technique.

RESULTS

It was found that HNPs activate platelets in pathophysiologically relevant doses, inducing fibrinogen and thrombospondin-1 binding, aggregation, granule secretion, sCD40L shedding, and procoagulant activity. HNPs bound directly to the platelet membrane, induced membrane pore formation, microparticle formation, mitochondrial membrane depolarization and caspase-3-activity. Confocal microscopy revealed the HNP-induced formation of polymeric fibrinogen and thrombospondin-1 amyloid-like structures, which bound microorganisms. Platelets adhered to these structures and formed aggregates. Blocking of glycoprotein IIb/IIIa (GPIIb/IIIa) markedly inhibited HNP-induced platelet activation. In addition, heparin, heparinoid, serpins and α(2)-macroglobulin, which all bind to HNPs, blocked HNP-1-induced platelet activation in contrast to direct thrombin inhibitors such as hirudin.

CONCLUSIONS

HNPs activate platelets and induce platelet apoptosis by formation of amyloid-like proteins. As these structures entrapped bacteria and fungi, they might reflect an additional function of HNPs in host defense. The described mechanism links again thrombosis and infection.

Molecular characterization of a novel Staphylococcus aureus surface protein (SasC) involved in cell aggregation and biofilm accumulation

Background

Staphylococci belong to the most important pathogens causing implant-associated infections. Colonization of the implanted medical devices by the formation of a three-dimensional structure made of bacteria and host material called biofilm is considered the most critical factor in these infections. To form a biofilm, bacteria first attach to the surface of the medical device, and then proliferate and accumulate into multilayered cell clusters. Biofilm accumulation may be mediated by polysaccharide and protein factors.

Methology/Principal Findings

The information on Staphylococcus aureus protein factors involved in biofilm accumulation is limited, therefore, we searched the S. aureus Col genome for LPXTG-motif containing potential surface proteins and chose the so far uncharacterized S. aureus surface protein C (SasC) for further investigation. The deduced SasC sequence consists of 2186 amino acids with a molecular mass of 238 kDa and has features typical of Gram-positive surface proteins, such as an N-terminal signal peptide, a C-terminal LPXTG cell wall anchorage motif, and a repeat region consisting of 17 repeats similar to the domain of unknown function 1542 (DUF1542). We heterologously expressed sasC in Staphylococcus carnosus, which led to the formation of huge cell aggregates indicative of intercellular adhesion and biofilm accumulation. To localize the domain conferring cell aggregation, we expressed two subclones of sasC encoding either the N-terminal domain including a motif that is found in various architectures (FIVAR) or 8 of the DUF1542 repeats. SasC or its N-terminal domain, but not the DUF1542 repeat region conferred production of huge cell aggregates, higher attachment to polystyrene, and enhanced biofilm formation to S. carnosus and S. aureus. SasC does not mediate binding to fibrinogen, thrombospondin-1, von Willebrand factor, or platelets as determined by flow cytometry.

Conclusions/Significance

Thus, SasC represents a novel S. aureus protein factor involved in cell aggregation and biofilm formation, which may play an important role in colonization during infection with this important pathogen.

Metabolism of surfactant phosphatidylcholine molecular species in cftr(tm1HGU/tm1HGU) mice compared to MF-1 mice

In cftr(tmIHGU/m1HGU) mice, an animal model designed to study pathophysiologic alterations due to the CFTR defect found in cysticfibrosis, surfactant phospholipids of bronchoalveolar lavage fluid (BALF) are increased. To study the metabolical basis of such increases, we intraperitoneally injected cft(tm1HGU/tm1HGU) mice [methyl-3H]choline and measured [methyl-3H]choline incorporation into phosphatidylcholine (PC) molecular species of lung tissue and BALF after 1.5 to 24 hours. MF1 and MF1 x cftr(tm1HGU/tm1HGU) hybrid mice served as controls. In tissue [methyl-3H]choline incorporation into total PC was constant for 24 hours and identical in control and cftr(tmIHGU/m1HGU) mice. However, from 7.5 to 24 hours there was a shift of [methyl-3H]choline incorporation from palmitoyloleoyl-PC and palmitoyllinoleoyl-PC towards PC species enriched in surfactant, dipalmitoyl-PC, palmitoylmyristoyl-PC, and palmitoylpalmitoleoyl-PC. The relative and absolute 3H-labels of PC species were identical for cftr(tmIHGU/m1HGU) compared to control mice. In BALF [methyl-3H]choline of total PC increased from 1.5 to 24 hours (R2 > .98), mainly due to [methyl-3H]choline-labelled dipalmitoyl-PC, in all experimental groups. In BALF from cftr(tmIHGU/m1HGU) mice, the [methyl-3H]choline label of total PC and individual PC species was significantly increased over control values after 24 hours, but not after 1.5 to 6 hours. Numbers and composition of BALF cells were not different between controls and cftr(tmIHGU/m1HGU) mice. We, conclude that increased alveolar phospholipid in cftr(tmIHGU/m1HGU) mice is likely due to decreased reuptake of surfactant.