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

Secondary Sites of the C-type Lectin-Like Fold

C-type lectins are a large superfamily of proteins involved in a multitude of biological processes. In particular, their involvement in immunity and homeostasis has rendered them attractive targets for diverse therapeutic interventions. They share a characteristic C-type lectin-like domain whose adaptability enables them to bind a broad spectrum of ligands beyond the originally defined canonical Ca2+-dependent carbohydrate binding. Together with variable domain architecture and high-level conformational plasticity, this enables C-type lectins to meet diverse functional demands. Secondary sites provide another layer of regulation and are often intricately linked to functional diversity. Located remote from the canonical primary binding site, secondary sites can accommodate ligands with other physicochemical properties and alter protein dynamics, thus enhancing selectivity and enabling fine-tuning of the biological response. In this review, we outline the structural determinants allowing C-type lectins to perform a large variety of tasks and to accommodate the ligands associated with it. Using the six well-characterized Ca2+-dependent and Ca2+-independent C-type lectin receptors DC-SIGN, langerin, MGL, dectin-1, CLEC-2 and NKG2D as examples, we focus on the characteristics of non-canonical interactions and secondary sites and their potential use in drug discovery endeavors.

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.

Inflammation-related prognostic markers in resected hepatocellular carcinoma

Hepatocellular carcinoma is usually detected late and therapeutic options are unsatisfactory. Despite marked progress in patient care, HCC remains among the deadliest cancers world-wide. While surgical resection remains a key option for early-stage HCC, the 5-year survival rates after surgical resection are limited. One reason for limited outcomes is the lack of reliable prognostic biomarkers to predict HCC recurrence. HCC prognosis has been shown to correlate with different systemic and pathological markers which are associated with patient survival and HCC recurrence. Liver inflammatory processes offer a large variety of systemic and pathological markers which may be exploited to improve the reliability of prognosis and decision making of liver surgeons and hepatologists. The following review aims to dissect the potential tools, targets and prognostic meaning of inflammatory markers in patients with resectable HCC. We analyze changes in circulant cellular populations and assess inflammatory biomarkers as a surrogate of impaired outcomes and provide an overview on predictive gene expression signatures including inflammatory transcriptional patterns, which are representative of poor survival in these patients.

Luteolin inhibits GPVI-mediated platelet activation, oxidative stress, and thrombosis

Introduction: Luteolin inhibits platelet activation and thrombus formation, but the mechanisms are unclear. This study investigated the effects of luteolin on GPVI-mediated platelet activation in vitro and explored the effect of luteolin on thrombosis, coagulation, and platelet production in vivo. Methods: Washed human platelets were used for aggregation, membrane protein expression, ATP, Ca2+, and LDH release, platelet adhesion/spreading, and clot retraction experiments. Washed human platelets were used to detect collagen and convulxin-induced reactive oxygen species production and endogenous antioxidant effects. C57BL/6 male mice were used for ferric chloride-induced mesenteric thrombosis, collagen-epinephrine induced acute pulmonary embolism, tail bleeding, coagulation function, and luteolin toxicity experiments. The interaction between luteolin and GPVI was analyzed using solid phase binding assay and surface plasmon resonance (SPR). Results: Luteolin inhibited collagen- and convulxin-mediated platelet aggregation, adhesion, and release. Luteolin inhibited collagen- and convulxin-induced platelet ROS production and increased platelet endogenous antioxidant capacity. Luteolin reduced convulxin-induced activation of ITAM and MAPK signaling molecules. Molecular docking simulation showed that luteolin forms hydrogen bonds with GPVI. The solid phase binding assay showed that luteolin inhibited the interaction between collagen and GPVI. Surface plasmon resonance showed that luteolin bonded GPVI. Luteolin inhibited integrin αIIbβ3-mediated platelet activation. Luteolin inhibited mesenteric artery thrombosis and collagen- adrenergic-induced pulmonary thrombosis in mice. Luteolin decreased oxidative stress in vivo. Luteolin did not affect coagulation, hemostasis, or platelet production in mice. Discussion: Luteolin may be an effective and safe antiplatelet agent target for GPVI. A new mechanism (decreased oxidative stress) for the anti-platelet activity of luteolin has been identified.

Characterizing the binding of glycoprotein VI with nanobody 35 reveals a novel monomeric structure of glycoprotein VI where the conformation of D1+D2 is independent of dimerization

BACKGROUND

The platelet-signaling receptor glycoprotein VI (GPVI) is a promising antithrombotic target. We have previously raised a series of high-affinity nanobodies (Nbs) against GPVI and identified Nb2, Nb21, and Nb35 as potent GPVI inhibitors. The Nb2 binding site has been mapped to the D1 domain, which is directly adjacent to the CRP binding site. Ligand-binding complementary determining region 3 has only 15% conservation between all 3 Nbs.

OBJECTIVES

To map the binding sites of Nb21 and Nb35 on GPVI.

METHODS

We determined the X-ray crystal structure of the D1 and D2 extracellular domains of the GPVI-Nb35 complex. We then looked at the effects of various GPVI mutations on the ability of Nbs to inhibit collagen binding and GPVI signaling using surface binding assays and transfected cell lines.

RESULTS

The crystal structure of GPVI bound to Nb35 was solved. GPVI was present as a monomer, and the D1+D2 conformation was comparable to that in the dimeric structure. Arg46, Tyr47, and Ala57 are common residues on GPVI targeted by both Nb2 and Nb35. Mutating Arg46 to an Ala abrogated the ability of Nb2, Nb21, and Nb35 to inhibit collagen-induced GPVI signaling and blocked the binding of all 3 Nbs. In addition, Arg60 was found to reduce Nb21 inhibition but not the inhibition Nb2 or Nb35.

CONCLUSIONS

These findings reveal key residues involved in the high-affinity binding of GPVI inhibitors and negate the idea that GPVI dimerization induces a conformational change required for ligand binding.

Antioxidant and antithrombotic properties of fruit, leaf, and seed extracts of the Halhalı olive (Olea europaea L.) native to the Hatay region in Turkey

The olive (Olea europaea L.) is one of the most important plants grown in many Mediterranean countries that has a high economic value. Olives, which are specific to each region, have different bioactive components. In this study, we investigated the phenolic/flavonoid contents, as well as antioxidant, antimicrobial, and antithrombotic activities of the fruit, leaf, and seed extracts obtained from the Halhalı olive grown in Arsuz district of Hatay, Turkey. Antioxidant activities of the phenolic compounds found in the olive fruit, seed, and leaf extracts were determined by employing established in vitro systems. Total phenolics were determined as gallic acid equivalents, while total flavonoids were determined as quercetin equivalents. Also, we evaluated a possible interaction between oleuropein and aggregation-related glycoproteins of the platelet surface via docking studies. The extracts showed effective antioxidant activity. The seed extract had the highest phenolic content of 317.24 μg GAE, while the fruit extract had the highest flavonoid content of 4.43 μg. The highest potential for metal chelating activity was found in the leaf extract, with an IC50 value of 13.33 mg/mL. Also, the leaf extract showed higher levels of antioxidant, antithrombotic, and antimicrobial activity, compared to the fruit and seed extracts. The docking scores of oleuropein against the target molecules GPVI, α2β1, and GPIbα were calculated as –3.798, –4.315, and –6.464 kcal/mol, respectively. The olive fruit, leaf, and seed extracts used as experimental material in our study have remarkable antioxidant, antimicrobial, and antithrombotic potential.

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.

A Comparative Assessment Study of Known Small-molecule GPVI Modulators

The GPVI platelet receptor was recently validated as a safe antiplatelet target for the treatment of thrombosis using several peptidic modulators. In contrast, few weakly potent small-molecule GPVI antagonists have been reported. Those that have been published often lack evidence for target engagement, and their biological efficacy cannot be compared because of the natural donor variability associated with the assays implemented. Herein, we present the first side-by-side assessment of the reported GPVI small-molecule modulators. We have characterized their functional activities on platelet activation and aggregation using flow cytometry as well as light transmission and electrical impedance aggregometry. We also utilized microscale thermophoresis (MST) and saturation transfer difference (STD) NMR to validate GPVI binding and have used this along with molecular modeling to suggest potential binding interactions. We conclude that of the compounds examined, losartan and compound 5 are currently the most viable GPVI modulators.

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.

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

The platelet C-type lectin-like receptor CLEC-2 drives inflammation-driven venous thrombosis in mouse models of thrombo-inflammatory disease with a minimal effect on hemostasis identifying it as a target for a new class of antiplatelet agent. Here, we discuss how the protein structure and dynamic arrangement of CLEC-2 on the platelet membrane helps the receptor, which has a single YxxL motif (known as a hemITAM), to trigger intracellular signaling. CLEC-2 exists as a monomer and homo-dimer within resting platelets and forms higher-order oligomers following ligand activation, a process that is mediated by the multivalent nature of its ligands and the binding of the tandem SH2 domains of Syk to the phosphorylated hemITAM and concomitantly to PIP2 or PIP3 to localize it to the membrane. We propose that a low level of active Syk is present at the membrane in resting platelets due to phosphorylation by Src family kinases and that clustering of receptors disturbs the equilibrium between kinases and phosphatases, triggering phosphorylation of the CLEC-2 hemITAM and recruitment of Syk. Knowledge of the structure of CLEC-2 and the mechanism of platelet activation has important implications for development of therapeutics.