Natalie S. Poulter

Platelet spreading assays are widely used to assess platelet adhesion, cytoskeletal remodeling and receptor‑specific signaling, but current GPVI ligands such as Collagen Related Peptide (CRP‑XL) suffer from non‑defined structure, batch variability, and high cost. Nanobodies targeting GPVI offer an attractive alternative due to their defined stoichiometry and recombinant production. Here, we evaluated whether GPVI‑binding nanobodies can support GPVI‑dependent platelet spreading. Using automated image analysis, we compared spreading on monovalent Nb2, trivalent Nb2‑3 and CRP‑XL. Monovalent Nb2 supported full spreading in only a subset of platelets, with reduced adhesion compared to CRP‑XL. In contrast, trivalent Nb2‑3 induced platelet adhesion and spreading indistinguishable from CRP‑XL, with comparable morphology, actin organization and spread area at concentrations ≥0.1 µg mL^-1. Spreading on Nb2‑3 was inhibited by soluble Nb2 and by Src kinase blockade, confirming GPVI‑dependent signaling, while the non‑blocking nanobody Nb28 had no inhibitory effect. Notably, Nb28 alone supported spreading, demonstrating that ligand‑induced GPVI clustering, rather than binding site location, drives activation. These findings show that Nb2‑3 is a potent and reproducible alternative to CRP‑XL, offering a well‑defined and standardizable reagent for GPVI‑dependent static platelet spreading assays.

Schlafen 14–related (SLFN14-related) thrombocytopenia is a rare bleeding disorder caused by SLFN14 mutations altering hemostasis in patients with platelet dysfunction. SLFN proteins are highly conserved in mammals where SLFN14 is specifically expressed in megakaryocyte (MK) and erythroblast lineages. The role of SLFN14 in megakaryopoiesis and platelet function has not been elucidated. Therefore, we generated a murine model with a platelet- and MK-specific SLFN14 deletion using platelet factor 4 (PF4) Cre-mediated deletion of exons 2 and 3 in Slfn14 ( Slfn14 PF4-Cre ) to decipher the molecular mechanisms driving the bleeding phenotype. Slfn14 PF4-Cre ⁺ platelets displayed reduced platelet signaling to thrombin, reduced thrombin formation, increased bleeding tendency, and delayed thrombus formation as assessed by intravital imaging. Moreover, fewer in situ bone marrow MKs were present compared with controls. RNA-Seq and Gene Ontology analysis of MKs and platelets from Slfn14 PF4-Cre homozygous mice revealed altered pathways of ubiquitination, adenosine triphosphate activity, and cytoskeleton and molecular function. In summary, we investigated how SLFN14 deletion in MKs and platelets leads to platelet dysfunction and alters their transcriptome, explaining the platelet dysfunction and bleeding in humans and mice with SLFN14 mutations.

1 Current microscopy approaches applied to platelet aggregates in both haemostatic and thrombotic settings indicate their structure has important implications in efficient haemostasis and in clinical treatment of thrombosis. However, current fluorescence microscopy approaches are not amenable to volumetric imaging of platelet aggregate structures. This is largely due to the small size of individual platelets and the tight packing of platelets within aggregates, resulting in optical opacity. Here we demonstrate that expansion microscopy, applied to platelet aggregates, can reveal multi-scale information about the structure of platelet aggregates. We produced volumetric images at nanoscale resolution of >700 platelet aggregates under normal and perturbed conditions, stained for cytoskeletal and membrane components. We demonstrate our custom analysis workflow provides quantitative description of platelet numbers, volumes and morphology within entire platelet aggregates. Additionally, we quantitatively describe subcellular organisation of F-actin. By comparing these measurements following treatment with the actin inhibitors, cytochalasin D and latrunculin A, we can robustly detect structural disruptions in platelet aggregates. Together these data provide a workflow to qualitatively and quantitatively describe the architecture of platelet aggregates at a range of scales (whole aggregates down to sub-cellular features within individual platelets).

S100A8/A9, also known as “calprotectin” or “MRP8/14,” is an alarmin primarily secreted by activated myeloid cells with antimicrobial, proinflammatory, and prothrombotic properties. Increased plasma levels of S100A8/A9 in thrombo-inflammatory diseases are associated with thrombotic complications. We assessed the presence of S100A8/A9 in the plasma and lung autopsies from patients with COVID-19 and investigated the molecular mechanism by which S100A8/A9 affects platelet function and thrombosis. S100A8/A9 plasma levels were increased in patients with COVID-19 and sustained high levels during hospitalization correlated with poor outcomes. Heterodimeric S100A8/A9 was mainly detected in neutrophils and deposited on the vessel wall in COVID-19 lung autopsies. Immobilization of S100A8/A9 with collagen accelerated the formation of a fibrin-rich network after perfusion of recalcified blood at venous shear. In vitro, platelets adhered and partially spread on S100A8/A9, leading to the formation of distinct populations of either P-selectin or phosphatidylserine (PS)-positive platelets. By using washed platelets, soluble S100A8/A9 induced PS exposure but failed to induce platelet aggregation, despite GPIIb/IIIa activation and alpha-granule secretion. We identified GPIbα as the receptor for S100A8/A9 on platelets inducing the formation of procoagulant platelets with a supporting role for CD36. The effect of S100A8/A9 on platelets was abolished by recombinant GPIbα ectodomain, platelets from a patient with Bernard-Soulier syndrome with GPIb-IX-V deficiency, and platelets from mice deficient in the extracellular domain of GPIbα. We identified the S100A8/A9-GPIbα axis as a novel targetable prothrombotic pathway inducing procoagulant platelets and fibrin formation, in particular in diseases associated with high levels of S100A8/A9, such as COVID-19.