A pull-down and slot blot-based screening system for inhibitor compounds of the podoplanin-CLEC-2 interaction

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.

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

BACKGROUND

 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.

METHODS AND RESULTS

 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.

CONCLUSION

 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.

C-type lectin-like receptor (CLEC)-2, the ligand of podoplanin, induces morphological changes in podocytes

Podoplanin (PDPN) is intensely expressed on the podocyte membrane in an evolutionally conserved manner. CLEC-2, the endogenous ligand of PDPN, is highly expressed in platelets and also exists in a soluble form in plasma. Normally, podocytes are sequestered from CLEC-2, but when the glomerular barrier is injured, podocytes gain access to CLEC-2. We tested the effects of CLEC-2 in podocytes in vitro and in vivo. Cultured podocytes treated with Fc-CLEC-2 demonstrated that CLEC-2 induced the dephosphorylation of ezrin, radixin, and moesin (ERM) proteins. Podocytes treated with Fc-CLEC-2 also showed the dissociation of F-actin filaments from PDPN, F-actin degradation, detachment, and round morphology. Next, we perfused normal mouse kidney in vivo with FLAG-CLEC-2. CLEC-2 induced dephosphorylation of ERM and widening of the foot processes of podocytes. Platelets were detected by immunostaining for CD41 in the urine of mice with podocyte injury, indicating that podocytes can encounter platelets when glomeruli are injured. Collectively, these observations suggest that when platelets leak through the injured glomeruli, CLEC-2 from the platelets acts on PDPN in podocytes and induces morphological change and detachment, which may further aggravate podocyte injury. Thus, PDPN on podocytes may work as a leaked-platelet sensor.

Fc-conjugated C-type lectin receptors: Tools for understanding host-pathogen interactions

The use of soluble fusion proteins of pattern recognition receptors (PRRs) used in the detection of exogenous and endogenous ligands has helped resolve the roles of PRRs in the innate immune response to pathogens, how they shape the adaptive immune response, and function in maintaining homeostasis. Using the immunoglobulin (Ig) crystallizable fragment (Fc) domain as a fusion partner, the PRR fusion proteins are soluble, stable, easily purified, have increased affinity due to the Fc homodimerization properties, and consequently have been used in a wide range of applications such as flow cytometry, screening of protein and glycan arrays, and immunofluorescent microscopy. This review will predominantly focus on the recognition of pathogens by the cell membrane-expressed glycan-binding proteins of the C-type lectin receptor (CLR) subgroup of PRRs. PRRs bind to conserved pathogen-associated molecular patterns (PAMPs), such as glycans, usually located within or on the outer surface of the pathogen. Significantly, many glycans structures are identical on both host and pathogen (e.g. the Lewis (Le) X glycan), allowing the use of Fc CLR fusion proteins with known endogenous and/or exogenous ligands as tools to identify pathogen structures that are able to interact with the immune system. Screens of highly purified pathogen-derived cell wall components have enabled identification of many unique PAMP structures recognized by CLRs. This review highlights studies using Fc CLR fusion proteins, with emphasis on the PAMPs found in fungi, bacteria, viruses, and parasites. The structure and unique features of the different CLR families is presented using examples from a broad range of microbes whenever possible.