Glycoprotein VI interplay with fibrin(ogen) in thrombosis

GPVI-dependent functional competence of buffy coat platelet concentrates (PCs) versus PRP-derived PCs: insights into the effects of biomechanical forces during platelet preparation

Platelet isolation is a decisive stage in the preparation of platelet concentrate (PC), which may affect functional competence of platelets during storage and post-transfusion. Now considering GPVI as a vulnerable receptor that is mainly affected by shear stress and redox state, this study was conducted for the first time to compare preparation methods of Buffy Coat (BC)- and Platelet-Rich Plasma (PRP)-PCs in terms of GPVI-related phenotypic and functional status. BC- and PRP-PCs were subjected to flow cytometry to analyze GPVI expression and intra-platelet ROS generation. Soluble GPVI were determined by ELISA. Platelet aggregation response to collagen and its adhesion to the collagen matrix were examined by aggregometry and fluorescence microscope, respectively. All the parameters were analyzed on days 0, 1, 3, and 5 of storage. Stored-dependent ROS generation showed significantly higher levels in PRP-PCs compared to BC-PCs on days 1 and 3 of storage. GPVI expression decreased in both products, with BC-PCs showing higher levels on 1 and 3 days of storage. However, with a similar trend, PRP-PCs showed higher levels of shedding, which was significant on day 3. Adhesion/spreading to the collagen matrix also decreased during storage, with higher declines observed in PRP-PCs. Platelet aggregation showed the same pattern with significantly lower PRP-PCs responses to collagen than BC-PCs on the third and fifth days of storage. During platelet storage, GPVI-dependent platelet functional capacity in BC-PCs was better preserved than PRP-PCs, suggesting the priority of BC-PCs method for platelet preparation. In this regard, adopting methods such as platelet isolation and storage in optimal additive solutions, especially those containing ROS scavengers, may help maintain the integrity of GPVI in PRP products. However, further studies, particularly using animal models of thrombus formation, are needed to determine whether the enhanced GPVI function in BC-PCs compared with PRP-PCs can translate into superior efficacy after blood transfusion.

Platelets and diseases: signal transduction and advances in targeted therapy

Platelets are essential anucleate blood cells that play pivotal roles in hemostasis, tissue repair, and immune modulation. Originating from megakaryocytes in the bone marrow, platelets are small in size but possess a highly specialized structure that enables them to execute a wide range of physiological functions. The platelet cytoplasm is enriched with functional proteins, organelles, and granules that facilitate their activation and participation in tissue repair processes. Platelet membranes are densely populated with a variety of receptors, which, upon activation, initiate complex intracellular signaling cascades. These signaling pathways govern platelet activation, aggregation, and the release of bioactive molecules, including growth factors, cytokines, and chemokines. Through these mechanisms, platelets are integral to critical physiological processes such as thrombosis, wound healing, and immune surveillance. However, dysregulated platelet function can contribute to pathological conditions, including cancer metastasis, atherosclerosis, and chronic inflammation. Due to their central involvement in both normal physiology and disease, platelets have become prominent targets for therapeutic intervention. Current treatments primarily aim to modulate platelet signaling to prevent thrombosis in cardiovascular diseases or to reduce excessive platelet aggregation in other pathological conditions. Antiplatelet therapies are widely employed in clinical practice to mitigate clot formation in high-risk patients. As platelet biology continues to evolve, emerging therapeutic strategies focus on refining platelet modulation to enhance clinical outcomes and prevent complications associated with platelet dysfunction. This review explores the structure, signaling pathways, biological functions, and therapeutic potential of platelets, highlighting their roles in both physiological and pathological contexts.

Contrasting Effects of Platelet GPVI Deletion Versus Syk Inhibition on Mouse Jugular Vein Puncture Wound Structure

Platelet glycoprotein (GP)VI is a transmembrane protein that was originally characterized as a collagen receptor supporting platelet adhesion and activation through its association with the Fc receptor γ-chain (FcRγ). The FcRγ subunit contains immunoreceptor tyrosine-based activation motifs (ITAMs) that recruit and activate Syk (spleen tyrosine kinase), a key player in intracellular signaling pathways. The absence or dysfunction of GPVI produces a mild bleeding defect in humans like the impaired hemostasis reported in the murine knockout. Here, we took an ultrastructure approach to examine the impact of ligand binding to GPVI versus the downstream pharmacologic inhibition of the GPVI-dependent ITAM signaling pathway. Clots were generated for analysis following a puncture wound in the mouse external jugular vein. Images were obtained using mice genetically missing GPVI and mice pretreated with the Syk inhibitor, BI 1002494. Our study was designed to test the hypothesis that the predominant contribution of GPVI to hemostasis is mediated by a Syk-dependent signaling cascade. If true, the clot structure observed with a Syk inhibitor versus the GPVI knockout would be similar. If the extracellular domains of the protein had a Syk-independent platelet adhesion role, then significant comparative differences in the thrombus structure would be expected. Our results clearly indicate an important, Syk-independent role of the GPVI extracellular domain in the adherence of platelets within the intravascular crown of a growing venous clot, a site distant from exposed collagen-rich adventitia. In striking contrast, the adventitial proximal role of GPVI was Syk-dependent, with the GPVI knockout and Syk inhibitor giving the same, limited structural outcome of collagen-proximal platelet cytosol loss and a thinned extravascular cap. Consistent with the lesser role of Syk-dependent processes on the thrombus structure, the Syk inhibitor had no detectable effect on jugular puncture wound bleeding times, while the knockout had a statistically significant, but modest effect on bleeding time. Based on this contrast, we suggest that Syk inhibition may be the more selective approach to modulating the role of GPVI in occlusive clotting.

Platelet glycoprotein VI promotes folic acid-induced acute kidney injury through interaction with tubular epithelial cell-derived galectin-3

BACKGROUND

Acute kidney injury (AKI) is defined by a significant reduction in renal function, which subsequently impairs coagulation and activates the inflammatory immune response, ultimately resulting in damage to renal tubular epithelial cells (TECs). Platelets are crucial in mediating both inflammatory and coagulation processes. While it is established that platelet activation contributes to the progression of AKI, the precise mechanisms underlying this relationship remain largely unclear.

METHODS

We investigated platelet function in folic acid-induced acute kidney injury (FA-AKI) and examined the effects of galectin-3, a protein derived from renal tubular epithelial cells (TECs), on its interaction with platelet glycoprotein VI (GPVI). This interaction was assessed through the analysis of monocyte migration, macrophage polarization, and the generation of monocyte-platelet aggregation. Additionally, we utilized platelet GPVI-specific knockout mice in conjunction with TD139, a small-molecule inhibitor of galectin-3, to explore the effects of inhibiting the galectin-3-GPVI interaction on FA-AKI.

RESULTS

In the current study, we observed that mouse platelets displayed hyperactivity in the context of functional acute kidney injury (FA-AKI). This hyperactivity was linked to the interaction between galectin-3, which is derived from damaged renal tubular epithelial cells (TECs), and the glycoprotein VI (GPVI) on platelets. Our findings indicated a heightened interaction between activated platelets and monocytes, along with an increase in monocyte-platelet aggregation (MPA) within the circulation. The increased infiltration of monocytes and platelets in renal tissue was further validated through CD41 and CD68 immunofluorescence techniques. Additionally, the interaction between galectin-3 and platelet GPVI was shown to facilitate monocyte migration, promote M1-type macrophage polarization, and enhance phagocytic activity. The galectin-3 inhibitor TD139 significantly suppressed monocyte-platelet aggregation (MPA), reduced inflammatory responses, and extended the survival of mice with acute kidney injury (AKI).

CONCLUSIONS

These findings suggest that galectin-3, which is released from damaged cells during acute kidney injury (AKI), exacerbates renal inflammation and tissue damage by activating platelets through glycoprotein VI (GPVI). This activation enhances interactions between monocytes and platelets, ultimately leading to the formation of monocyte-platelet aggregates (MPA) and the polarization of M1 macrophages.

Glanzmann Thrombasthenia 10 Years Later: Progress Made and Future Directions

Glanzmann thrombasthenia (GT) is the most common inherited platelet disorder (IPD) with mucocutaneous bleeding and a failure of platelets to aggregate when stimulated. The molecular cause is insufficient or defective αIIbβ3, an integrin encoded by the ITGA2B and ITGB3 genes. On activation αIIbβ3 undergoes conformational changes and binds fibrinogen (Fg) and other proteins to join platelets in the aggregate. The application of next-generation sequencing (NGS) to patients with IPDs has accelerated genotyping for GT; progress accompanied by improved mutation curation. The evaluation by NGS of variants in other hemostasis and vascular genes is a major step toward understanding why bleeding varies so much between patients. The recently discovered role for glycoprotein VI in thrombus formation, through its binding to fibrin and surface-bound Fg, may offer a mechanosensitive back-up for αIIbβ3, especially at sites of inflammation. The setting up of national networks for IPDs and GT is improving patient care. Hematopoietic stem cell therapy provides a long-term cure for severe cases; however, prophylaxis by monoclonal antibodies designed to accelerate fibrin formation at injured sites in the vasculature is a promising development. Gene therapy using lentil-virus vectors remains a future option with CRISPR/Cas9 technologies offering a promising alternative route.

Platelet collagen receptors and their role in modulating platelet adhesion patterns and activation on alternatively processed collagen substrates

This study examines the roles of platelet collagen receptors glycoprotein VI (GPVI), α2β1, and the GPIb-IX-V complex in platelet activation and thrombus formation on various collagen sources from different species. Type I collagens standardly used in haematology testing, i.e. collagen type I derived from equine tendon (HORM) and rat tail collagen were evaluated. Moreover, acid soluble collagen from human umbilical cord was tested. To inhibit platelet-collagen interactions, combinations of monoclonal antibodies 6B4 and 6F1, targeting GPIbα and α2β1, respectively, were used, along with the therapeutic collagen receptor GPVI antibody glenzocimab. Our findings reveal distinct dependencies on these receptors: platelet aggregation of washed platelets to HORM collagen relied on both α2β1 and GPVI, to acid soluble collagen mainly on GPVI, and to rat tail collagen solely on α2β1, respectively. In whole blood perfusion assays under non-coagulating conditions, the acid soluble collagen surface triggered a more homogenous platelet adhesion when compared to the HORM collagen surface, whilst platelet adhesion on rat tail collagen varied considerably. The GPIb-IX-V complex was shown to play a key role in initial platelet adhesion and activation across all collagen surfaces at a shear rate of 1600 s-1. At 1600 s-1, inhibiting platelet α2β1 interaction with collagen by 6F1 antibody did not affect platelet thrombus formation on acid soluble collagen, while it did reduce platelet surface coverage and P-selectin expression on HORM collagen without changing the overall thrombus morphology or contraction. Inhibiting GPVI interaction with collagen significantly reduced all thrombus parameters and abolished PS exposure and P-selectin expression on all three collagen surfaces, at both 1600 s-1 and 150 s-1. Interestingly, upon investigating combined inhibition of GPIb and α2β1, an additive inhibitor effect of 6F1 was observed on P-selectin expression and PS-exposure on acid soluble collagen but not HORM collagen at 1600s-1, suggesting that the acid soluble collagen is well suited to study reinforcing functions of collagen receptors. Overall, this study highlights the potential advantages of using alternative collagen surfaces beyond the conventional HORM collagen to detect nuanced collagen receptor dependencies, which may prove valuable in evaluating anti-platelet medication.

Soluble glycoprotein VI predicts abdominal aortic aneurysm growth rate and is a novel therapeutic target

ABSTRACT

A common feature in patients with abdominal aortic aneurysms (AAAs) is the formation of a nonocclusive intraluminal thrombus (ILT) in regions of aortic dilation. Platelets are known to maintain hemostasis and propagate thrombosis through several redundant activation mechanisms, yet the role of platelet activation in the pathogenesis of AAA-associated ILT is still poorly understood. Thus, we sought to investigate how platelet activation affects the pathogenesis of AAA. Using RNA sequencing, we identified that the platelet-associated transcripts are significantly enriched in the ILT compared with the adjacent aneurysm wall and healthy control aortas. We found that the platelet-specific receptor glycoprotein VI (GPVI) is among the top enriched genes in AAA ILT and is increased on the platelet surface of patients with AAAs. Examination of a specific indicator of platelet activity, soluble GPVI (sGPVI), in 2 independent cohorts of patients with AAAs is highly predictive of an AAA diagnosis and associates more strongly with aneurysm growth rate than D-dimer in humans. Finally, intervention with the anti-GPVI antibody (JAQ1) in mice with established aneurysms blunted the progression of AAA in 2 independent mouse models. In conclusion, we show that the levels of sGPVI in humans can predict a diagnosis of AAA and AAA growth rate, which may be critical in the identification of high-risk patients. We also identify GPVI as a novel platelet-specific AAA therapeutic target, with minimal risk of adverse bleeding complications, for which none currently exists.

Brevianamide F Exerts Antithrombotic Effects by Modulating the MAPK Signaling Pathway and Coagulation Cascade

Existing antithrombotic drugs have side effects such as bleeding, and there is an urgent need to discover antithrombotic drugs with better efficacy and fewer side effects. In this study, a zebrafish thrombosis model was used to evaluate the antithrombotic activity and mechanism of Brevianamide F, a deep-sea natural product, with transcriptome sequencing analysis, RT-qPCR analysis, and molecular docking. The results revealed that Brevianamide F significantly attenuated the degree of platelet aggregation in the thrombus model zebrafish, leading to an increase in the number of circulating platelets, an augmentation in the return of blood to the heart, an elevated heart rate, and a significant restoration of caudal blood flow velocity. Transcriptome sequencing and RT-qPCR validation revealed that Brevianamide F may exert antithrombotic effects through the modulation of the MAPK signaling pathway and the coagulation cascade reaction. Molecular docking analysis further confirmed this result. This study provides a reference for the development of therapeutic drugs for thrombosis.

Affimer reagents as tool molecules to modulate platelet GPVI-ligand interactions and specifically bind GPVI dimer

ABSTRACT

Glycoprotein VI (GPVI) plays a key role in collagen-induced platelet aggregation. Affimers are engineered binding protein alternatives to antibodies. We screened and characterized GPVI-binding Affimers as novel tools to probe GPVI function. Among the positive clones, M17, D22, and D18 bound GPVI with the highest affinities (dissociation constant (KD) in the nanomolar range). These Affimers inhibited GPVI-collagen-related peptide (CRP)-XL/collagen interactions, CRP-XL/collagen-induced platelet aggregation, and D22 also inhibited in vitro thrombus formation on a collagen surface under flow. D18 bound GPVI dimer but not monomer. GPVI binding was increased for D18 but not M17/D22 upon platelet activation by CRP-XL and adenosine 5'-diphosphate. D22 but not M17/D18 displaced nanobody 2 (Nb2) binding to GPVI, indicating similar epitopes for D22 with Nb2 but not for M17/D18. Mapping of binding sites revealed that D22 binds a site that overlaps with Nb2 on the D1 domain, whereas M17 targets a site on the D2 domain, overlapping in part with the glenzocimab binding site, a humanized GPVI antibody fragment antigen-binding fragment. D18 targets a new region on the D2 domain. We found that D18 is a stable noncovalent dimer and forms a stable complex with dimeric GPVI with 1:1 stoichiometry. Taken together, our data demonstrate that Affimers modulate GPVI-ligand interactions and bind different sites on GPVI D1/D2 domains. D18 is dimer-specific and could be used as a tool to detect GPVI dimerization or clustering in platelets. A dimeric epitope regulating ligand binding was identified on the GPVI D2 domain, which could be used for the development of novel bivalent antithrombotic agents selectively targeting GPVI dimer on platelets.

A novel, quantitative clot retraction assay to evaluate platelet function

Blood platelets are crucial to prevent excessive bleeding following injury to blood vessels. Platelets are crucial for the formation of clots and for clot strength. Platelet activation involves aggregation, attachment to fibrin and clot retraction. Most assays that address platelet function measure platelet aggregation, not clot retraction. Here, we describe a 96-well-based clot retraction assay that requires a relatively short runtime and small sample volume. The assay involves continuous optical density monitoring of platelet-rich plasma that is activated with thrombin. The data can be analyzed using time-series analytical tools to generate quantitative information about different phases of clot formation and clot retraction. The assay demonstrated good repeatability and reproducibility and was robust to different calcium concentrations. Impairment of platelet bioenergetics, actin polymerization, fibrin interaction, and signaling significantly affected clot retraction and was detected and showed good agreement with light transmission aggregometry, suggesting that clot retraction is predictive of platelet function. Using this microplate clot retraction assay, we showed a significant difference in platelets stored in autologous plasma compared with platelet additive solution after 7 days of room temperature storage.

Research and Clinical Approaches to Assess Platelet Function in Flowing Blood

Platelet adhesion and activation is fundamental to the formation of a hemostatic response to limit loss of blood and instigate wound repair to seal a site of vascular injury. The process of platelet aggregate formation is supported by the coagulation system driving injury-proximal formation of thrombin, which converts fibrinogen to insoluble fibrin. This highly coordinated series of molecular and membranous events must be routinely achieved in flowing blood, at vascular fluid shear rates that place significant strain on molecular and cellular interactions. Platelets have long been recognized to be able to slow down and adhere to sites of vascular injury and then activate and recruit more platelets that forge and strengthen adhesive ties with the vascular wall under these conditions. It has been a major challenge for the Platelet Research Community to construct experimental conditions that allow precise definition of the molecular steps occurring under flow. This brief review will discuss work to date from our group, as well as others that has furthered our understanding of platelet function in flowing blood.