Platelet C-type Lectin-like Receptor 2 Research Articles

Diphenyl-tetrazol-propanamide Derivatives Act as Dual-Specific Antagonists of Platelet CLEC-2 and Glycoprotein VI.

Platelet C-type lectin-like receptor 2 (CLEC-2) induces platelet activation and aggregation after clustering by its ligand podoplanin (PDPN). PDPN, which is not normally expressed in cells in contact with blood flow, is induced in inflammatory immune cells and some malignant tumor cells, thereby increasing the risk of venous thromboembolism (VTE) and tumor metastasis. Therefore, small-molecule compounds that can interfere with the PDPN-CLEC-2 axis have the potential to become selective antiplatelet agents. Using molecular docking analysis of CLEC-2 and a PDPN-CLEC-2 binding-inhibition assay, we identified a group of diphenyl-tetrazol-propanamide derivatives as novel CLEC-2 inhibitors. A total of 12 hit compounds also inhibited PDPN-induced platelet aggregation in humans and mice. Unexpectedly, these compounds also fit the collagen-binding pocket of the glycoprotein VI molecule, thereby inhibiting collagen interaction. These compounds also inhibited collagen-induced platelet aggregation, and one compound ameliorated collagen-induced thrombocytopenia in mice. For clinical use, these compounds will require a degree of chemical modification to decrease albumin binding. Nonetheless, as dual activation of platelets by collagen and PDPN-positive cells is expected to occur after the rupture of atherosclerotic plaques, these dual antagonists could represent a promising pharmacophore, particularly for arterial thrombosis, in addition to VTE and metastasis.

Deletion of platelet CLEC-2 decreases GPIbα-mediated integrin αIIbβ3 activation and decreases thrombosis in TTP

AbstractMicrovascular thrombosis in patients with thrombotic thrombocytopenic purpura (TTP) is initiated by GPIbα-mediated platelet binding to von Willebrand factor (VWF). Binding of VWF to GPIbα causes activation of the platelet surface integrin αIIbβ3. However, the mechanism of GPIbα-initiated activation of αIIbβ3 and its clinical importance for microvascular thrombosis remain elusive. Deletion of platelet C-type lectin-like receptor 2 (CLEC-2) did not prevent VWF binding to platelets but specifically inhibited platelet aggregation induced by VWF binding in mice. Deletion of platelet CLEC-2 also inhibited αIIbβ3 activation induced by the binding of VWF to GPIbα. Using a mouse model of TTP, which was created by infusion of anti-mouse ADAMTS13 monoclonal antibodies followed by infusion of VWF, we found that deletion of platelet CLEC-2 decreased pulmonary arterial thrombosis and the severity of thrombocytopenia. Importantly, prophylactic oral administration of aspirin, an inhibitor of platelet activation, and therapeutic treatment of the TTP mice with eptifibatide, an integrin αIIbβ3 antagonist, reduced pulmonary arterial thrombosis in the TTP mouse model. Our observations demonstrate that GPIbα-mediated activation of integrin αIIbβ3 plays an important role in the formation of thrombosis in TTP. These observations suggest that prevention of platelet activation with aspirin may reduce the risk for thrombosis in patients with TTP.

Elevated Soluble Podoplanin Associates with Hypercoagulability in Patients with Nephrotic Syndrome.

Podoplanin (PDPN) promotes platelet aggregation and activation by interacting with C-type lectin-like receptor 2(CLEC-2) on platelets. The interaction between the upregulated PDPN and platelet CLEC-2 stimulates venous thrombosis. PDPN was identified as a risk factor for coagulation and thrombosis in inflammatory processes. Hypercoagulability is defined as the tendency to develop thrombosis according to fibrinogen and/or D dimer levels. Nephrotic syndrome is also considered to be a hypercoagulable state. The aim of this study is to investigate the association of soluble PDPN/CLEC-2 with hypercoagulability in nephrotic syndrome. Thirty-five patients with nephrotic syndrome and twenty-seven healthy volunteers were enrolled. PDPN, CLEC-2 and GPVI concentrations were tested by enzyme-linked immunosorbent assay (ELISA). Patients with nephrotic syndrome showed higher serum levels of PDPN and GPVI in comparison to healthy controls (P < .001, P = .001). PDPN levels in patients with nephrotic syndrome were significantly correlated with GPVI (r = 0.311; P = .025), hypoalbuminemia (r = −0.735; P < .001), hypercholesterolemia (r = 0.665; P < .001), hypertriglyceridemia (r = 0.618; P < .001), fibrinogen (r = 0.606; P < .001) and D-dimer (r = 0.524; P < .001). Area under the curve (AUC) for the prediction of hypercoagulability in nephrotic syndrome using PDPN was 0.886 (95% CI 0.804-0.967, P < .001). Cut-off value for the risk probability was 5.88 ng/ml. The sensitivity of PDPN in predicting hypercoagulability was 0.806, and the specificity was 0.846. When serum PDPN was >5.88 ng/ml, the risk of hypercoagulability was significantly increased in nephrotic syndrome (OR = 22.79, 95% CI 5.92-87.69, P < .001). In conclusion, soluble PDPN levels were correlated with hypercoagulability in nephrotic syndrome. PDPN has the better predictive value of hypercoagulability in nephrotic syndrome as well as was a reliable indicator of hypercoagulable state.

Elevated plasma levels of soluble C-type lectin-like receptor 2 (CLEC2) in patients with thrombotic microangiopathy

BackgroundThrombotic microangiopathy (TMA) is caused by activated platelets. The plasma C-type lectin-like receptor 2 (CLEC2) levels in 58 patients with TMA were examined and compared with those in healthy volunteers and other diseases. Materials and methodsThe plasma levels of soluble platelet surface glycoprotein VI (GPVI) and CLEC2 were measured in patients with TMA. ResultsPlasma CLEC2 levels in patients with DIC and TMA were significantly higher (p < 0.001) than those in thrombocytopenic patients with other hematological diseases, but no significant differences in the plasma CLEC2 levels were observed among patients with thrombotic thrombocytopenic purpura, hemolytic uremic syndrome (HUS), atypical HUS and other TMA. The plasma CLEC2 levels after the remission were significantly lower than those before treatment (p < 0.001). The plasma CLEC2 levels were poorly correlated with the levels of soluble GPVI in the plasma of patients with TMA. The plasma CLEC2 levels were not significantly differ between survivor and non-survivor in TMA patients, but were significantly higher in non-survivor in overall population (p < 0.001). ConclusionThe measurement of the plasma CLEC2 level is considered to be important for the diagnosis and evaluation of TMA.

Platelet glycoprotein VI aids in local immunity during pneumonia-derived sepsis caused by gram-negative bacteria

AbstractPlatelet collagen receptor glycoprotein VI (GPVI) and podoplanin receptor C-type lectin-like receptor 2 (CLEC2) are receptors implicated in platelet activation that both signal via an immunoreceptor tyrosine–based activation motif. Platelets are necessary for host defense and prevention of hemorrhage during sepsis, but the role of platelet GPVI and CLEC2 herein is unknown. To investigate this, we infected mice depleted of platelet GPVI or CLEC2 by antibody treatment or GPVI−/− mice with the common human sepsis pathogen Klebsiella pneumoniae via the airways to induce pneumonia-derived sepsis. The GPVI ligand collagen and the CLEC2 ligand podoplanin were constitutively present in the lung, whereas the GPVI ligands fibrin and histone were induced during pneumonia. During late-stage infection, both mice depleted of GPVI and GPVI−/− mice showed increased bacterial growth in lungs, and GPVI−/− mice also showed increased bacterial growth in distant body sites. Despite higher bacterial loads, GPVI-depleted mice showed reduced platelet numbers, platelet activation, and platelet–leukocyte complex formation in the bronchoalveolar space. Consistently, in human whole blood, GPVI stimulation of platelets increased platelet–leukocyte complex formation and leukocyte activation, which was accompanied by enhanced phagocytosis of Klebsiella. GPVI-depleted mice showed increased lung hemorrhage during infection, but not to the extent observed in platelet-depleted mice, and lung bleeding was not significantly different between GPVI−/− and wild-type mice. CLEC2 depletion did not affect any of the responses during pneumonia. These results suggest that platelet GPVI, but not CLEC2, contributes to local host defense during pneumonia-derived sepsis by enhancing leukocyte function.

Abstract 350: Activation of Platelet CLEC-2 in Hematoma Expansion After Intracerebral Hemorrhage: Finding an Effective Dose

Intracerebral hemorrhage (ICH) is the least treatable and one of the most disabling stroke subtypes, with both the primary and secondary brain injuries mainly due to hematoma expansion (HE) within the first few hours, followed by an inflammatory and oxidative injury. Almost all ICH patients experience some degree of expansion, and ICH volume can be an independent determinant of mortality and functional outcome. Previous studies found that reduced platelet activity is associated with early ICH growth and worse functional outcomes. The platelet C-type lectin-like receptor 2 (CLEC-2) elicits robust platelet activation upon stimulation. Thus, we hypothesized that activation of CLEC-2 would result in activation and aggregation of platelets, reducing HE, and improving functional outcomes after ICH. We administered two CLEC-2 agonists (Fucoidan and the endogenous ligand Podoplanin) at varying doses after murine ICH and measured hemorrhage volume, brain water content, and behavior. We also collected blood from podoplanin-treated mice and conducted hematologic tests. We further tested Rhodocytin, a known CLEC-2 activator, for whole blood clotting times, platelet functions, and soluble fibrin in human blood samples. We found that treatment with Fucoidan did not reduce hemorrhage volume or brain water content and had no effect on neurological function, compared to vehicle. Further, Podoplanin treatment showed a tendency to reduce brain water content but did not affect hematoma volume or behavior. Our in vitro studies showed lower fibrinogen levels with podoplanin treatment, compared to sham and vehicle, but no effect on partial thromboplastin time, prothrombin time, or INR. At higher doses, Rhodocytin resulted in immediate clotting, while at lower doses, it significantly increased soluble fibrin, consistent with initiation of clotting seen in the higher doses. In conclusion, our findings indicate that activation of CLEC-2 at the doses tested does not reduce ICH volume or improve outcomes, but did affect hemostatic parameters. Thus, the agonists may produce favorable outcomes with a different dosing regimen. Further studies are needed to confirm the mechanism by which CLEC-2 is acting in ICH, effective doses, and the effect of altered bleeding parameters.

Abstract 2264: Dual effects of podoplanin on the regulation of cancer cell gene expression and platelet CLEC-2 signaling during cancer metastasis

Abstract Podoplanin (PDPN) is frequently up-regulated in squamous cell carcinoma and is a marker for poor prognosis. PDPN facilitates cancer metastasis and tumor cell-induced platelet aggregation (TCIPA) by modulating cancer cell cytoskeleton reorganization and platelet C-type lectin-like receptor 2 (CLEC-2) activation. In this study, the effects of PDPN on the regulation of cancer cell gene expression and platelet CLEC-2 signaling are investigated. Using xenograft injection of the C6 glioma cells into nude mice as a metastatic model, the cells that colonized at lung tissue were isolated to establish a subline C6-Lung that was found to express high levels of PDPN. Knockdown of PDPN expression in C6-Lung resulted in the abrogation of TCIPA, indicating that PDPN plays a key role in platelet activation and TCIPA. Microarray analyses revealed differential gene expression signatures between low and high PDPN-expressed cells. Several genes were up-regulated (Sncg, Timp1, Mmp3 and Pla1a) and down-regulated (Calcb, Filip1, Agrn and Itga4) significantly in high PDPN-expressed C6-Lung cells. On the other hand, distinct platelet protein tyrosine phosphorylation profiles were revealed when platelets were treated with different CLEC-2 agonists including PDPN, aggretin and fucoidan. In addition to the activation of common CLEC-2 signaling proteins such as Src family kinase, Syk, and phospholipase Cγ2, Akt1/PDK1 and PKCμ were identified as the two novel signaling pathways activated by PDPN, indicating the presence of SFK-independent pathway downstream of CLEC-2. Our data together provide a definitive evidence for the presence of a unique network for PDPN-regulated cancer cell gene expression and platelet CLEC-2 signaling that shed a new insight for the functional roles of PDPN in cancer metastasis. Citation Format: Ching-Ping Tseng, Yao-Wen Chang, Pei-Wen Hsieh, Ju-Chien Cheng. Dual effects of podoplanin on the regulation of cancer cell gene expression and platelet CLEC-2 signaling during cancer metastasis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2264. doi:10.1158/1538-7445.AM2015-2264

Inhibitory effects of polypeptides derived from a snake venom C‐type lectin, aggretin, on tumor cell‐induced platelet aggregation

Podoplanin, a transmembrane sialoglycoprotein, is expressed by lymphatic endothelial cells and many tumor cells, and is involved in tumor cell-induced platelet aggregation and tumor metastasis. A recent study found that C-type lectin-like receptor2 (CLEC-2) is a physiologic receptor for podoplanin. Previous studies showed that aggretin, a snake venom-derived protein, activates platelets by targeting platelet CLEC-2. We hypothesized that the C-terminal fragment of aggretin may bind to platelet CLEC-2 and displace podoplanin, in turn exerting antitumor metastatic effects. Aggretin α-chain C-terminus (residues106-136; AACT) prolonged the lag phase of platelet aggregation induced by aggretin in human washed platelets, indicating that AACT may target the binding site of CLEC-2. HepG2 cells, which are podoplanin-expressing hepatoma cells, induced platelet aggregation with a lag phase. Pretreatment with AACT inhibited platelet aggregation and prolonged the lag phase induced by HepG2 cells. This inhibitory effect was also found with another hepatocarcinoma cell line, HuH-7. AACT inhibited the interaction between HuH-7 cells and platelets, and a specific binding assay demonstrated that CLEC-2 was the binding site for AACT on platelets. In addition, the invasive ability of HepG2 cells was abolished by AACT in a chick embryo chorioallantoic membrane model. Furthermore, formation of lung metastases after intravenous administration of HuH-7 cells was significantly reduced when mice were treated with AACT. AACT interacts with CLEC-2 of platelets, leading to interference with platelet aggregation and the subsequent metastatic potential of tumor cells. These results suggest that aggretin AACT is a potential candidate for the treatment of tumor metastasis through CLEC-2 blockade.

Platelets regulate lymphatic vascular development through CLEC-2–SLP-76 signaling

AbstractAlthough platelets appear by embryonic day 10.5 in the developing mouse, an embryonic role for these cells has not been identified. The SYK–SLP-76 signaling pathway is required in blood cells to regulate embryonic blood-lymphatic vascular separation, but the cell type and molecular mechanism underlying this regulatory pathway are not known. In the present study we demonstrate that platelets regulate lymphatic vascular development by directly interacting with lymphatic endothelial cells through C-type lectin-like receptor 2 (CLEC-2) receptors. PODOPLANIN (PDPN), a transmembrane protein expressed on the surface of lymphatic endothelial cells, is required in nonhematopoietic cells for blood-lymphatic separation. Genetic loss of the PDPN receptor CLEC-2 ablates PDPN binding by platelets and confers embryonic lymphatic vascular defects like those seen in animals lacking PDPN or SLP-76. Platelet factor 4-Cre–mediated deletion of Slp-76 is sufficient to confer lymphatic vascular defects, identifying platelets as the cell type in which SLP-76 signaling is required to regulate lymphatic vascular development. Consistent with these genetic findings, we observe SLP-76–dependent platelet aggregate formation on the surface of lymphatic endothelial cells in vivo and ex vivo. These studies identify a nonhemostatic pathway in which platelet CLEC-2 receptors bind lymphatic endothelial PDPN and activate SLP-76 signaling to regulate embryonic vascular development.