The structure of CLEC-2: mechanisms of dimerization and higher-order clustering

A role of platelet C-type lectin-like receptor-2 and its ligand podoplanin in vascular biology

PURPOSE OF REVIEW

Platelets are essential for hemostasis and are also vital in lymphatic and lung development and the maintenance of vascular integrity. Platelet activation receptor C-type lectin-like receptor 2 (CLEC-2) and its endogenous ligand podoplanin (PDPN) in lymphatic endothelial cells (LECs) and other cells regulate these processes. This review aims to comprehensively summarize the roles of platelet CLEC-2 and PDPN. This review also focuses on discussing the underlying mechanisms by which platelet CLEC-2 and PDPN mediate blood/lymphatic separation.

FINDINGS

CLEC-2/PDPN-induced platelet activation in the primary lymph sacs, developmental lymphovenous junctions, neonatal mesentery, and the site of tumor lymphangiogenesis prevents blood/lymphatic vessel misconnection. Further, CLEC-2/PDPN-induced platelet activation is essential for lung development. Mice deficient in CLEC-2 or PDPN show blood-filled lymphatics, lung malformations, and cerebrovascular abnormalities. CLEC-2 deletion in steady-state adult mice did not result in blood/lymphatic vessel mixing. In adulthood, CLEC-2 maintains vascular integrity and that of high endothelial venules in lymph nodes. CLEC-2 deletion in adulthood results in hemorrhage under inflammatory conditions, and hemolymph nodes.

SUMMARY

The platelet CLEC-2/LEC PDPN interaction prevents blood/lymphatic vessel mixing at active remodeling sites of the blood/lymphatic system, but not in steady-state adult mice. This interaction also regulates vascular integrity when vascular permeability increases before and after birth.

The podoplanin-CLEC-2 interaction promotes platelet-mediated melanoma pulmonary metastasis

BACKGROUND

Podoplanin (PDPN) expressed on tumour cells interacts with platelet C-type lectin-like receptor 2 (CLEC-2). This study aimed to investigate the role of the PDPN-platelet CLEC-2 interaction in melanoma pulmonary metastasis.

METHODS

Murine melanoma B16-F0 cells, which have two populations that express podoplanin, were sorted by FACS with anti-podoplanin staining to obtain purified PDPN + and PDPN- B16-F0 cells. C57BL/6J mice transplanted with CLEC-2-deficient bone marrow cells were used for in vivo experiments.

RESULTS

The in vivo data showed that the number of metastatic lung nodules in WT mice injected with PDPN + cells was significantly higher than that in WT mice injected with PDPN- cells and in WT or CLEC-2 KO mice injected with PDPN- cells. In addition, our results revealed that the platelet Syk-dependent signalling pathway contributed to platelet aggregation and melanoma metastasis.

CONCLUSIONS

Our study indicates that the PDPN-CLEC-2 interaction promotes experimental pulmonary metastasis in a mouse melanoma model. Tumour cell-induced platelet aggregation mediated by the interaction between PDPN and CLEC-2 is a key factor in melanoma pulmonary metastasis.

C-type lectin-like receptor 2: roles and drug target

C-type lectin-like receptor-2 (CLEC-2) is a member of the C-type lectin superfamily of cell surface receptors. The first confirmed endogenous and exogenous ligands of CLEC-2 are podoplanin and rhodocytin, respectively. CLEC-2 is expressed on the surface of platelets, which participates in platelet activation and aggregation by binding with its ligands. CLEC-2 and its ligands are involved in pathophysiological processes, such as atherosclerosis, cancer, inflammatory thrombus status, maintenance of vascular wall integrity, and cancer-related thrombosis. In the last 5 years, different anti- podoplanin antibody types have been developed for the treatment of cancers, such as glioblastoma and lung cancer. New tests and new diagnostics targeting CLEC-2 are also discussed. CLEC-2 mediates thrombosis in various pathological states, but CLEC-2-specific deletion does not affect normal hemostasis, which would provide a new therapeutic tool for many thromboembolic diseases. The CLEC-2-podoplanin interaction is a target for cancer treatment. CLEC-2 may be applied in clinical practice and play a therapeutic role.

A change of rhodocytin's suprastructure turns the agonist into an antagonist of tumor cell induced platelet aggregation

Cancer cells circulating in the blood attach to platelets by direct cell-cell interactions via several receptor-counterreceptor contacts and indirectly by fibrin bridges which connect the two cell types by distinct integrin receptors. In the microenvironment of these tumor cell platelet aggregates (TCPAs), the tumor cells are shielded from the shear stress of the blood flow and from attack by the immune system. This supports hematogenous metastasis and tumor cell induced thrombosis. Platelet activation is triggered by binding of podoplanin on cancer cells to the platelet receptor Clec-2. Therefore, we hypothesize that targeting this initial step will prevent the entire cascade leading to the formation of TCPAs. Rhodocytin, a heterodimeric (αβ)2 C-type lectin from the Malayan pit viper Calloselasma rhodostoma, binds to Clec-2 and thereby induces TCPA formation. Remarkably, mutations in rhodocytin that disturbed formation of oligomers, blocked the podoplanin-Clec-2 axis and prevented platelet activation. Therefore, we used lysine reactive chemicals to modify rhodocytin isolated from the crude snake venom. Blue native gel electrophoresis and far western blotting showed a change of rhodocytin's suprastructure triggered by acetylation and PEGylation. Mass spectrometry analysis of altered lysines suggested that their modifications interfered with the formation of rhodocytin tetramers. When tested in assays for tumor cell induced platelet aggregation, we found that derivatization turned rhodocytin from an agonist into an antagonist. This observation indicates that Clec-2 is a valid target receptor molecule to curb TCPA formation and to prevent hematogenous metastasis and tumor cell induced thrombosis in cancer patients.

Trivalent nanobody-based ligands mediate powerful activation of GPVI, CLEC-2, and PEAR1 in human platelets whereas FcγRIIA requires a tetravalent ligand

BACKGROUND

Clustering of the receptors glycoprotein receptor VI (GPVI), C-type lectin-like receptor 2 (CLEC-2), low-affinity immunoglobulin γ Fc region receptor II-a (FcγRIIA), and platelet endothelial aggregation receptor 1 (PEAR1) leads to powerful activation of platelets through phosphorylation of tyrosine in their cytosolic tails and initiation of downstream signaling cascades. GPVI, CLEC-2, and FcγRIIA signal through YxxL motifs that activate Syk. PEAR1 signals through a YxxM motif that activates phosphoinositide 3-kinase. Current ligands for these receptors have an undefined valency and show significant batch variation and, for some, uncertain specificity.

OBJECTIVES

We have raised nanobodies against each of these receptors and multimerized them to identify the minimum number of epitopes to achieve robust activation of human platelets.

METHODS

Divalent and trivalent nanobodies were generated using a flexible glycine-serine linker. Tetravalent nanobodies utilize a mouse Fc domain (IgG2a, which does not bind to FcγRIIA) to dimerize the divalent nanobody. Ligand affinity measurements were determined by surface plasmon resonance. Platelet aggregation, adenosine triphosphate secretion, and protein phosphorylation were analyzed using standardized methods.

RESULTS

Multimerization of the nanobodies led to a stepwise increase in affinity with divalent and higher-order nanobody oligomers having sub-nanomolar affinity. The trivalent nanobodies to GPVI, CLEC-2, and PEAR1 stimulated powerful and robust platelet aggregation, secretion, and protein phosphorylation at low nanomolar concentrations. A tetravalent nanobody was required to activate FcγRIIA with the concentration-response relationship showing a greater variability and reduced sensitivity compared with the other nanobody-based ligands, despite a sub-nanomolar binding affinity.

CONCLUSION

The multivalent nanobodies represent a series of standardized, potent agonists for platelet glycoprotein receptors. They have applications as research tools and in clinical assays.

C-type lectin-like receptor-2 (CLEC-2) is a key regulator of kappa-carrageenan-induced tail thrombosis model in mice

Kappa-carrageenan (KCG), which is used to induce thrombosis in laboratory animals for antithrombotic drug screening, can trigger platelet aggregation. However, the cell-surface receptor and related signaling pathways remain unclear. In this study, we investigated the molecular basis of KCG-induced platelet activation using light-transmittance aggregometry, flow cytometry, western blotting, and surface plasmon resonance assays using platelets from platelet receptor-deficient mice and recombinant proteins. KCG-induced tail thrombosis was also evaluated in mice lacking the platelet receptor. We found that KCG induces platelet aggregation with α-granule secretion, activated integrin αIIbβ3, and phosphatidylserine exposure. As this aggregation was significantly inhibited by the Src family kinase inhibitor and spleen tyrosine kinase (Syk) inhibitor, a tyrosine kinase-dependent pathway is required. Platelets exposed to KCG exhibited intracellular tyrosine phosphorylation of Syk, linker activated T cells, and phospholipase C gamma 2. KCG-induced platelet aggregation was abolished in platelets from C-type lectin-like receptor-2 (CLEC-2)-deficient mice, but not in platelets pre-treated with glycoprotein VI-blocking antibody, JAQ1. Surface plasmon resonance assays showed a direct association between murine/human recombinant CLEC-2 and KCG. KCG-induced thrombosis and thrombocytopenia were significantly inhibited in CLEC-2-deficient mice. Our findings show that KCG induces platelet activation via CLEC-2.

Association of C-type lectin-like receptor 2 and galectin-1 with portal vein system thrombosis in HBV-related liver cirrhosis

Background and aims

Hepatitis B virus (HBV) infection is the most common cause of liver cirrhosis. Portal venous system thrombosis (PVST) is a major complication of liver cirrhosis. Recently, it has been shown that C-type lectin-like receptor 2 (CLEC-2) and galectin-1 participate in the activation and aggregation of platelets, thereby promoting the development of thrombosis. This cross-sectional study aims to evaluate the association of serum CLEC-2 and galectin-1 levels with PVST in patients with HBV-related liver cirrhosis.

Methods

Overall, 65 patients with HBV-related liver cirrhosis were included, of whom 23 had PVST and 42 did not have. Serum CLEC-2 and galectin-1 levels were measured using enzyme-linked immunosorbent assay kits. PVST was assessed by contrast-enhanced computed tomography and/or magnetic resonance imaging scans. Subgroup analyses were conducted according to the degree and location of PVST.

Results

Patients with PVST had significantly higher serum CLEC-2 (p = 0.006) and galectin-1 (p = 0.009) levels than those without. Patients with partial/complete PVST or fibrotic cord (p = 0.007; p = 0.002), but not those with mural PVST (p = 0.199; p = 0.797), had significantly higher serum CLEC-2 and galectin-1 levels than those without PVST. Patients with superior mesenteric vein thrombosis had significantly higher serum CLEC-2 (p = 0.013) and galectin-1 (p = 0.025) levels than those without PVST. Patients with main portal vein thrombosis had higher serum CLEC-2 (p = 0.020) and galectin-1 (p = 0.066) levels than those without PVST, but the difference in serum galectin-1 level was not significant between them.

Conclusion

Serum CLEC-2 and galectin-1 levels may be associated with the presence of PVST in HBV-related cirrhotic patients, but this association should be dependent upon the degree of PVST.

Divalent nanobodies to platelet CLEC-2 can serve as agonists or antagonists

CLEC-2 is a target for a new class of antiplatelet agent. Clustering of CLEC-2 leads to phosphorylation of a cytosolic YxxL and binding of the tandem SH2 domains in Syk, crosslinking two receptors. We have raised 48 nanobodies to CLEC-2 and crosslinked the most potent of these to generate divalent and tetravalent nanobody ligands. Fluorescence correlation spectroscopy (FCS) was used to show that the multivalent nanobodies cluster CLEC-2 in the membrane and that clustering is reduced by inhibition of Syk. Strikingly, the tetravalent nanobody stimulated aggregation of human platelets, whereas the divalent nanobody was an antagonist. In contrast, in human CLEC-2 knock-in mouse platelets, the divalent nanobody stimulated aggregation. Mouse platelets express a higher level of CLEC-2 than human platelets. In line with this, the divalent nanobody was an agonist in high-expressing transfected DT40 cells and an antagonist in low-expressing cells. FCS, stepwise photobleaching and non-detergent membrane extraction show that CLEC-2 is a mixture of monomers and dimers, with the degree of dimerisation increasing with expression thereby favouring crosslinking of CLEC-2 dimers. These results identify ligand valency, receptor expression/dimerisation and Syk as variables that govern activation of CLEC-2 and suggest that divalent ligands should be considered as partial agonists.

Podoplanin: A potential therapeutic target for thrombotic diseases

As a specific lymphatic marker and a key ligand of C-type lectin-like receptor 2 (CLEC-2), podoplanin (Pdpn) is involved in various physiological and pathological processes such as growth and development, respiration, blood coagulation, lymphangiogenesis, angiogenesis, and inflammation. Thrombotic diseases constitute a major cause of disability and mortality in adults, in which thrombosis and inflammation play a crucial role. Recently, increasing evidence demonstrates the distribution and function of this glycoprotein in thrombotic diseases such as atherosclerosis, ischemic stroke, venous thrombosis, ischemic-reperfusion injury (IRI) of kidney and liver, and myocardial infarction. Evidence showed that after ischemia, Pdpn can be acquired over time by a heterogeneous cell population, which may not express Pdpn in normal conditions. In this review, the research progresses in understanding the roles and mechanisms of podoplanin in thromobotic diseases are summarized. The challenges of podoplanin-targeted approaches for disease prognosis and preventions are also discussed.

CLEC-2 Supports Platelet Aggregation in Mouse but not Human Blood at Arterial Shear

C-type lectin-like receptor 2 (CLEC-2) is highly expressed on platelets and a subpopulation of myeloid cells, and is critical in lymphatic development. CLEC-2 has been shown to support thrombus formation at sites of inflammation, but to have a minor/negligible role in hemostasis. This identifies CLEC-2 as a promising therapeutic target in thromboinflammatory disorders, without hemostatic detriment. We utilized a GPIbα-Cre recombinase mouse for more restricted deletion of platelet-CLEC-2 than the previously used PF4-Cre mouse. clec1bfl/flGPIbα-Cre+ mice are born at a Mendelian ratio, with a mild reduction in platelet count, and present with reduced thrombus size post-FeCl3-induced thrombosis, compared to littermates. Antibody-mediated depletion of platelet count in C57BL/6 mice, to match clec1bfl/flGPIbα-Cre+ mice, revealed that the reduced thrombus size post-FeCl3-injury was due to the loss of CLEC-2, and not mild thrombocytopenia. Similarly, clec1bfl/flGPIbα-Cre+ mouse blood replenished with CLEC-2-deficient platelets ex vivo to match littermates had reduced aggregate formation when perfused over collagen at arterial flow rates. In contrast, platelet-rich thrombi formed following perfusion of human blood under flow conditions over collagen types I or III, atherosclerotic plaque, or inflammatory endothelial cells were unaltered in the presence of CLEC-2-blocking antibody, AYP1, or recombinant CLEC-2-Fc. The reduction in platelet aggregation observed in clec1bfl/flGPIbα-Cre+ mice during arterial thrombosis is mediated by the loss of CLEC-2 on mouse platelets. In contrast, CLEC-2 does not support thrombus generation on collagen, atherosclerotic plaque, or inflamed endothelial cells in human at arterial shear.

Experimental validation of computerised models of clustering of platelet glycoprotein receptors that signal via tandem SH2 domain proteins

The clustering of platelet glycoprotein receptors with cytosolic YxxL and YxxM motifs, including GPVI, CLEC-2 and PEAR1, triggers activation via phosphorylation of the conserved tyrosine residues and recruitment of the tandem SH2 (Src homology 2) domain effector proteins, Syk and PI 3-kinase. We have modelled the clustering of these receptors with monovalent, divalent and tetravalent soluble ligands and with transmembrane ligands based on the law of mass action using ordinary differential equations and agent-based modelling. The models were experimentally evaluated in platelets and transfected cell lines using monovalent and multivalent ligands, including novel nanobody-based divalent and tetravalent ligands, by fluorescence correlation spectroscopy. Ligand valency, receptor number, receptor dimerisation, receptor phosphorylation and a cytosolic tandem SH2 domain protein act in synergy to drive receptor clustering. Threshold concentrations of a CLEC-2-blocking antibody and Syk inhibitor act in synergy to block platelet aggregation. This offers a strategy for countering the effect of avidity of multivalent ligands and in limiting off-target effects.

Katacine Is a New Ligand of CLEC-2 that Acts as a Platelet Agonist

BACKGROUND

 CLEC-2 is a platelet receptor with an important role in thromboinflammation but a minor role in hemostasis. Two endogenous ligands of CLEC-2 have been identified, the transmembrane protein podoplanin and iron-containing porphyrin hemin, which is formed following hemolysis from red blood cells. Other exogenous ligands such as rhodocytin have contributed to our understanding of the role of CLEC-2.

OBJECTIVES

 To identify novel CLEC-2 small-molecule ligands to aid therapeutic targeting of CLEC-2.

METHODS

 ALPHA screen technology has been used for the development of a high-throughput screening (HTS) assay recapitulating the podoplanin-CLEC-2 interaction. Light transmission aggregometry was used to evaluate platelet aggregation. Immunoprecipitation and western blot were used to evaluate direct phosphorylation of CLEC-2 and downstream protein phosphorylation. Autodock vina software was used to predict the molecular binding site of katacine and mass spectrometry to determine the polymeric nature of the ligand.

RESULTS AND CONCLUSION

 We developed a CLEC-2-podoplanin interaction assay in a HTS format and screened 5,016 compounds from a European Union-open screen library. We identified katacine, a mixture of polymers of proanthocyanidins, as a novel ligand for CLEC-2 and showed that it induces platelet aggregation and CLEC-2 phosphorylation via Syk and Src kinases. Platelet aggregation induced by katacine is inhibited by the anti-CLEC-2 monoclonal antibody fragment AYP1 F(ab)'2. Katacine is a novel nonprotein ligand of CLEC-2 that could contribute to a better understanding of CLEC-2 activation in human platelets.

Targeting Podoplanin for the Treatment of Osteosarcoma

PURPOSE

Osteosarcoma, the most common bone malignancy in children, has a poor prognosis, especially when the tumor metastasizes to the lungs. Therefore, novel therapeutic strategies targeting both proliferation and metastasis of osteosarcoma are required. Podoplanin (PDPN) is expressed by various tumors and is associated with tumor-induced platelet activation via its interaction with C-type lectin-like receptor 2 (CLEC-2) on platelets. We previously found that PDPN contributed to osteosarcoma growth and metastasis through platelet activation; thus, in this study, we developed an anti-PDPN humanized antibody and evaluated its effect on osteosarcoma growth and metastasis.

EXPERIMENTAL DESIGN

Nine osteosarcoma cell lines and two osteosarcoma patient-derived cells were collected, and we evaluated the efficacy of the anti-DPN-neutralizing antibody PG4D2 and the humanized anti-PDPN antibody AP201, which had IgG4 framework region. The antitumor and antimetastasis effect of PG4D2 and AP201 were examined in vitro and in vivo. In addition, growth signaling by the interaction between PDPN and CLEC-2 was analyzed using phospho-RTK (receptor tyrosine kinase) array, growth assay, or immunoblot analysis under the supression of RTKs by knockout and inhibitor treatment.

RESULTS

We observed that PG4D2 treatment significantly suppressed tumor growth and pulmonary metastasis in osteosarcoma xenograft models highly expressing PDPN. The contribution of PDGFR activation by activated platelet releasates to osteosarcoma cell proliferation was confirmed, and the humanized antibody, AP201, suppressed in vivo osteosarcoma growth and metastasis without significant adverse events.

CONCLUSIONS

Targeting PDPN with a neutralizing antibody against PDPN-CLEC-2 without antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity is a novel therapeutic strategy for PDPN-positive osteosarcoma.

Yeast as carrier for drug delivery and vaccine construction

Yeast has been employed as an effective derived drug carrier as a unicellular microorganism. Many research works have been devoted to the encapsulation of nucleic acid compounds, insoluble small molecule drugs, small molecules, liposomes, polymers, and various nanoparticles in yeast for the treatment of disease. Recombinant yeast-based vaccine carriers (WYV) have played a major role in the development of vaccines. Herein, the latest reports on the application of yeast carriers and the development of related research are summarized, a conceptual description of gastrointestinal absorption of yeast carriers, as well as the various package forms of different drug molecules and nanoparticles in yeast carriers are introduced. In addition, the advantages and development of recombinant yeast vaccine carriers for the disease, veterinary and aquaculture applications are discussed. Moreover, the current challenges and future directions of yeast carriers are proposed.

Galectin-9 activates platelet ITAM receptors glycoprotein VI and C-type lectin-like receptor-2

BACKGROUND

Platelets are multifunctional cellular mediators in many physiological and pathophysiological processes such as thrombosis, angiogenesis, and inflammation. Several members of galectins, a family of carbohydrate-binding proteins with a broad range of immunomodulatory actions, have been reported to activate platelets.

OBJECTIVE

In this study, we investigated the role of galectin-9 (Gal-9) as a novel ligand for platelet glycoprotein VI (GPVI) and C-type lectin-like receptor 2 (CLEC-2).

METHODS

Platelet spreading, aggregation, and P-selectin expression in response to Gal-9 were measured in washed platelet suspensions via static adhesion assay, light transmission aggregometry, and flow cytometry, respectively. Solid-phase binding assay and protein phosphorylation studies were utilized to validate the interaction between Gal-9 and GPVI, and immunoprecipitation for detecting CLEC-2 phosphorylation. Wild-type (WT), GPVI-knockout (Gp6-/- ), and GPVI and CLEC-2-double knockout (Gp6-/- /Gp1ba-Cre-Clec1bfl/fl ) mice were used.

RESULTS

We have shown that recombinant Gal-9 stimulates aggregation in human and mouse washed platelets dose-dependently. Platelets from both species adhere and spread on immobilized Gal-9 and express P-selectin. Gal-9 competitively inhibited the binding of human recombinant D1 and D2 domains of GPVI to collagen. Gal-9 stimulated tyrosine phosphorylation of CLEC-2 and proteins known to lie downstream of GPVI and CLEC-2 including spleen tyrosine kinase and linker of activated T cells in human platelets. GPVI-deficient murine platelets exhibited significantly impaired aggregation in response to Gal-9, which was further abrogated in GPVI and CLEC-2-double-deficient platelets.

CONCLUSIONS

We have identified Gal-9 as a novel platelet agonist that induces activation through interaction with GPVI and CLEC-2.

Thrombosis and Inflammation—A Dynamic Interplay and the Role of Glycosaminoglycans and Activated Protein C

Hemostasis, thrombosis, and inflammation are tightly interconnected processes which may give rise to thrombo-inflammation, involved in infectious and non-infectious acute and chronic diseases, including cardiovascular diseases (CVD). Traditionally, due to its hemostatic role, blood coagulation is isolated from the inflammation, and its critical contribution in the progressing CVD is underrated, until the full occlusion of a critical vessel occurs. Underlying vascular injury exposes extracellular matrix to deposit platelets and inflammatory cells. Platelets being key effector cells, bridge all the three key processes (hemostasis, thrombosis, and inflammation) associated with thrombo-inflammation. Under physiological conditions, platelets remain in an inert state despite the proximity to the endothelium and other cells which are decorated with glycosaminoglycan (GAG)-rich glycocalyx (GAGs). A pathological insult to the endothelium results in an imbalanced blood coagulation system hallmarked by increased thrombin generation due to losses of anticoagulant and cytoprotective mechanisms, i.e., the endothelial GAGs enhancing antithrombin, tissue factor pathway-inhibitor (TFPI) and thrombomodulin-protein C system. Moreover, the loss of GAGs promotes the release of mediators, such as von Willebrand factor (VWF), platelet factor 4 (PF4), and P-selectin, both locally on vascular surfaces and to circulation, further enhancing the adhesion of platelets to the affected sites. Platelet-neutrophil interaction and formation of neutrophil extracellular traps foster thrombo-inflammatory mechanisms exacerbating the cardiovascular disease course. Therefore, therapies which not only target the clotting mechanisms but simultaneously or independently convey potent cytoprotective effects hemming the inflammatory mechanisms are expected to provide clinical benefits. In this regard, we review the cytoprotective protease activated protein C (aPC) and its strong anti-inflammatory effects thereby preventing the ensuing thrombotic complications in CVD. Furthermore, restoring GAG-like vasculo-protection, such as providing heparin-proteoglycan mimetics to improve regulation of platelet and coagulation activity and to suppress of endothelial perturbance and leukocyte-derived pro-inflammatory cytokines, may provide a path to alleviate thrombo-inflammatory disorders in the future. The vascular tissue-modeled heparin proteoglycan mimic, antiplatelet and anticoagulant compound (APAC), dual antiplatelet and anticoagulant, is an injury-targeting and locally acting arterial antithrombotic which downplays collagen- and thrombin-induced and complement-induced activation and protects from organ injury.

Clustering extent-dependent differential signaling by CLEC-2 receptors in platelets

Background

C-type lectin receptor family members play a role in many cells including platelets, where they are crucial in the separation of lymphatic and blood vessels during development. The C-type lectin-like receptor 2 (CLEC-2) receptor contains the canonical intracellular hemITAM motif through which it signals to activate Syk.

Objectives

One proposed hypothesis for signaling cascade is that Syk bridges two receptors through phosphorylated hemITAM motifs. We demonstrated that the phosphorylated hemITAM stimulates PI3 kinase/Btk pathways to activate Syk. To address this controversy, we used a CLEC-2 selective agonist and studied the role of Btk in platelet activation.

Results and Conclusions

Platelet activation and downstream signaling were abolished in murine and human platelets in the presence of the Btk inhibitors ibrutinib or acalabrutinib when a low concentration of a CLEC-2 antibody was used to crosslink CLEC-2 receptors. This inhibition was overcome by increasing concentrations of the CLEC-2 antibody. Similar results were obtained in X-linked immunodeficient mouse platelets, with an inactivating mutation in Btk or in Lyn null platelets. We conclude that at low crosslinking conditions of CLEC-2, Btk plays an important role in the activation of Syk, but at higher crosslinking conditions their role becomes less important and other mechanisms take over to activate Syk.

CLEC14A was up-regulated in hepatocellular carcinoma and may function as a potential diagnostic biomarker

OBJECTIVE

The current work aimed to investigate the expression and potential clinical significance of C-type Lectin domain family 14 (CLEC14A) in hepatocellular carcinoma.

METHODS

The relative expressions of CLEC14A in the Hepatocellular Carcinoma (HCC) tissue and adjacent normal tissue of 105 HCC patients were examined using RT-qPCR methods. Furthermore, Receiver Operating Characteristic (ROC) curve was drawn for exploring the diagnostic value of CLEC14A. Next, the expressions of CLEC14A in HCC cell lines and normal liver epithelial cells were compared, and the effects of knockdown of CLEC14A on the growth and apoptosis of HCC cells were examined.

RESULTS

The authors found that the expression of CLEC14A was markedly increased in hepatocellular carcinoma tumors in comparison with the adjacent tissue, and the expression level of CLEC14A was positively correlated with the size and differentiation of the tumor. Moreover, results of ROC analysis showed CLEC14A might function as a sensitive diagnostic biomarker for HCC. Furthermore, CLEC14A was up-regulated in HCC cell lines, and transient over-expression of CLEC14A decreased the proliferation and increased the apoptosis of HCC cells in vitro.

CONCLUSIONS

Our results suggested that CLEC14A was up-regulated in HCC and might function as a potential diagnostic marker.

Platelet activation by charged ligands and nanoparticles: platelet glycoprotein receptors as pattern recognition receptors

Charge interactions play a critical role in the activation of the innate immune system by damage- and pathogen-associated molecular pattern receptors. The ability of these receptors to recognize a wide spectrum of ligands through a common mechanism is critical in host defense. In this article, we argue that platelet glycoprotein receptors that signal through conserved tyrosine-based motifs function as pattern recognition receptors (PRRs) for charged endogenous and exogenous ligands, including sulfated polysaccharides, charged proteins and nanoparticles. This is exemplified by GPVI, CLEC-2 and PEAR1 which are activated by a wide spectrum of endogenous and exogenous ligands, including diesel exhaust particles, sulfated polysaccharides and charged surfaces. We propose that this mechanism has evolved to drive rapid activation of platelets at sites of injury, but that under some conditions it can drive occlusive thrombosis, for example, when blood comes into contact with infectious agents or toxins. In this Opinion Article, we discuss mechanisms behind charge-mediated platelet activation and opportunities for designing nanoparticles and related agents such as dendrimers as novel antithrombotics.

Overcoming challenges in developing small molecule inhibitors for GPVI and CLEC-2

GPVI and CLEC-2 have emerged as promising targets for long-term prevention of both arterial thrombosis and thrombo-inflammation with a decreased bleeding risk relative to current drugs. However, while there are potent blocking antibodies of both receptors, their protein nature comes with decreased bioavailability, making formulation for oral medication challenging. Small molecules are able to overcome these limitations, but there are many challenges in developing antagonists of nanomolar potency, which is necessary when considering the structural features that underlie the interaction of CLEC-2 and GPVI with their protein ligands. In this review, we describe current small-molecule inhibitors for both receptors and strategies to overcome such limitations, including considerations when it comes to in silico drug design and the importance of complex compound library selection.