Copy number analysis of the murine platelet proteome spanning the complete abundance range

New insights of glycoprotein Ib-IX-V complex organization and glycoprotein Ibα in platelet biogenesis

PURPOSE OF REVIEW

Glycoprotein (GP) Ib-IX-V, a platelet surface receptor that plays a critical role in platelet adhesion and platelet-mediated immune responses, consists of GPIbα, GPIbβ, GPIX, and GPV in a stoichiometry of 2 : 4 : 2 : 1. Forming a complex is essential for GPIb-IX-V to function. GPIb-IX-V also plays an important role in platelet biogenesis by regulating the number and size of platelets. Yet how GPIb-IX-V regulates platelet biogenesis remains elusive. This review will summarize recent findings in the complex organization of GPIb-IX-V and its role in platelet biogenesis.

RECENT FINDINGS

Proteomics studies suggest that GPIbα, GPIbβ, GPIX, and GPV form the complex in a ratio of 1 : 2 : 1 : 1, which is supported by analysis of molecular weight of GPIb-IX-V and GPIb-IX and the structure of entire GPIb-IX-V. To activate platelets, GPIbα requires binding of CLEC-2 to trigger signals. Furthermore, disrupting the GPIbα anchorage to filamin A causes defects in platelet budding away from proplatelets leading to giant platelets and a low platelet count.

SUMMARY

New studies challenge the traditional model for the organization of GPIb-IX-V as a complex and indicate the role of GPIb-IX-V in platelet production. Those studies provide insights for GPIb-IX-V in the regulation of platelet activation and platelet biogenesis.

C-type lectin-like receptor-2 in platelets mediates ferric chloride-induced platelet activation and attenuates ferroptosis of endothelial cells

BACKGROUND

An iron overload status induces ferroptosis, an iron-dependent nonapoptotic cell death, in various pathological conditions. We previously reported that hemin (heme), protoporphyrin-IX with ferric iron, activates platelets via C-type lectin-like receptor-2 (CLEC-2) and glycoprotein VI/FcRγ, but protoporphyrin-IX alone blocks CLEC-2-dependent platelet activation. Therefore, we hypothesized that free iron has the ability to activate platelets.

OBJECTIVES

This study aimed to elucidate platelet activation mechanisms of iron (ferric chloride), including the identification of signaling pathways and receptors, and to examine whether platelets regulate ferroptosis.

METHODS

Platelet aggregometry, platelet activation marker expression, and protein phosphorylation were examined in ferric chloride-stimulated human and murine platelets. Inhibitors of platelet activation signaling pathways and receptor-deleted platelets were utilized to identify the responsible signaling pathway and receptor. The effect of platelets on ferroptosis of endothelial cells was investigated in vitro.

RESULTS

Ferric chloride induced platelet activation dependent on Src family kinase pathways in humans and mice. Ferric chloride-induced platelet aggregation was almost lost in CLEC-2-depleted murine platelets and wild-type platelets preincubated with recombinant CLEC-2 proteins. Furthermore, coculture of wild-type platelets, but not CLEC-2-deficient platelets, attenuated ferroptosis of endothelial cells in vitro.

CONCLUSION

Ferric chloride activates platelets via CLEC-2 and Src family kinase pathways, and platelets have a protective role in the ferroptosis of endothelial cells dependent on CLEC-2.

Identification of the changes in the platelet proteomic profile of elderly individuals

Background

Platelet hyperreactivity is a risk factor for thrombosis in elderly patients with cardiovascular diseases. However, the mechanism of platelet hyperactivation has not been elucidated. This study aims to investigate alterations in the proteomes of platelets and their correlation with platelet hyperreactivity among elderly individuals.

Methods

This study included 10 young (28.1 ± 1.9 years), 10 middle-aged (60.4 ± 2.2 years), and 10 old (74.2 ± 3.0 years) subjects. Washed platelets were used in the present study. Platelet samples were analysed by using data-independent acquisition (DIA) quantitative mass spectrometry (MS).

Results

The results showed that the platelet proteomic profile exhibited high similarity between the young and middle-aged groups. However, there were significant differences in protein expression profiles between the old group and the young group. By exploring the dynamic changes in the platelet proteome with ageing, clusters of proteins that changed significantly with ageing were selected for further investigation. These clusters were related to the initial triggering of complement, phagosome and haemostasis based on enrichment analysis. We found that platelet degranulation was the major characteristic of the differentially expressed proteins between the old and young populations. Moreover, complement activation, the calcium signalling pathway and the nuclear factor-κB (NF-κB) signalling pathway were enriched in differentially expressed proteins.

Conclusions

The present study showed that there are obvious differences in the protein profiles of the elderly compared with young and middle-aged populations. The results provide novel evidence showing changes in platelet hyperactivity and susceptibility to thrombosis in the elderly population.

Differential Role of the RAC1-Binding Proteins FAM49b (CYRI-B) and CYFIP1 in Platelets

Platelet function at vascular injury sites is tightly regulated through the actin cytoskeleton. The Wiskott-Aldrich syndrome protein-family verprolin-homologous protein (WAVE)-regulatory complex (WRC) activates lamellipodia formation via ARP2/3, initiated by GTP-bound RAC1 interacting with the WRC subunit CYFIP1. The protein FAM49b (Family of Unknown Function 49b), also known as CYRI-B (CYFIP-Related RAC Interactor B), has been found to interact with activated RAC1, leading to the negative regulation of the WRC in mammalian cells. To investigate the role of FAM49b in platelet function, we studied platelet-specific Fam49b-/--, Cyfip1-/--, and Cyfip1/Fam49b-/--mice. Platelet counts and activation of Fam49b-/- mice were comparable to those of control mice. On fully fibrinogen-coated surfaces, Fam49b-/--platelets spread faster with an increased mean projected cell area than control platelets, whereas Cyfip1/Fam49b-/--platelets did not form lamellipodia, phenocopying the Cyfip1-/--platelets. However, Fam49b-/--platelets often assumed a polarized shape and were more prone to migrate on fibrinogen-coated surfaces. On 2D structured micropatterns, however, Fam49b-/--platelets displayed reduced spreading, whereas spreading of Cyfip1-/-- and Cyfip1/Fam49b-/--platelets was enhanced. In summary, FAM49b contributes to the regulation of morphology and migration of spread platelets, but to exert its inhibitory effect on actin polymerization, the functional WAVE complex must be present.

The C-Type Lectin Receptor CD93 Regulates Platelet Activation and Surface Expression of the Protease Activated Receptor 4

BACKGROUND

 The C-type lectin receptor CD93 is a single pass type I transmembrane glycoprotein involved in inflammation, immunity, and angiogenesis. This study investigates the role of CD93 in platelet function. CD93 knockout (KO) mice and wild-type (WT) controls were compared in this study.

METHODS

 Platelet activation and aggregation were investigated by flow cytometry and light transmission aggregometry, respectively. Protein expression and phosphorylation were analyzed by immunoblotting. Subcellular localization of membrane receptors was investigated by wide-field and confocal microscopy.

RESULTS

 The lack of CD93 in mice was not associated to any evident bleeding defect and no alterations of platelet activation were observed upon stimulation with thromboxane A2 analogue and convulxin. Conversely, platelet aggregation induced by stimulation of the thrombin receptor PAR4 was significantly reduced in the absence of CD93. This defect was associated with a significant reduction of α-granule secretion, integrin αIIbβ3 activation, and protein kinase C (PKC) stimulation. Resting WT and CD93-deficient platelets expressed comparable amounts of PAR4. However, upon stimulation with a PAR4 activating peptide, a more pronounced clearance of PAR4 from the platelet surface was observed in CD93-deficient platelets compared with WT controls. Confocal microscopy analysis revealed a massive movement of PAR4 in cytosolic compartments of activated platelets lacking CD93. Accordingly, platelet desensitization following PAR4 stimulation was more pronounced in CD93 KO platelets compared with WT controls.

CONCLUSION

 These results demonstrate that CD93 supports platelet activation triggered by PAR4 stimulation and is required to stabilize the expression of the thrombin receptor on the cell surface.

Platelet mechanosensing as key to understanding platelet function

PURPOSE OF REVIEW

This review highlights how the perception of platelet function is evolving based on recent insights into platelet mechanobiology.

RECENT FINDINGS

The mechanosensitive ion channel Piezo1 mediates activation of free-flowing platelets under conditions of flow acceleration through mechanisms independent of adhesion receptors and classical activation pathways. Interference with the initiation of platelet migration or with the phenotypic switch of migrating platelets to a procoagulant state aggravates inflammatory bleeding. Mechanosensing of biochemical and biophysical microenvironmental cues during thrombus formation feed into platelet contractile force generation. Measurements of single platelet contraction and bulk clot retraction show promise to identify individuals at risk for hemorrhage.

SUMMARY

New findings unravel novel mechanotransduction pathways and effector functions in platelets, establishing mechanobiology as a pivotal component of platelet function. These insights highlight limitations of existing treatments and offer new potential therapeutic approaches and diagnostic avenues based on mechanobiological principles. Further extensive research is required to distinguish between core hemostatic and pathological mechanisms influenced by platelet mechanosensing.

EcoHIV-Infected Mice Show No Signs of Platelet Activation

Platelets express several surface receptors that could interact with different viruses. To understand the mechanisms of HIV-1's interaction with platelets, we chose the EcoHIV model. While EcoHIV is an established model for neuroAIDS, its effects on platelets are ill-defined. Our results indicate that EcoHIV behaves differently from HIV-1 and is cleared from circulation after 48 h post-infection. The EcoHIV course of infection resembles an HIV-1 infection under the effects of combined antiretroviral therapy (cART) since infected mice stayed immunocompetent and the virus was readily detected in the spleen. EcoHIV-infected mice failed to become thrombocytopenic and showed no signs of platelet activation. One explanation is that mouse platelets lack the EcoHIV receptor, murine Cationic Amino acid Transporter-1 (mCAT-1). No mCAT-1 was detected on their surface, nor was any mCAT-1 mRNA detected. Thus, mouse platelets would not bind or become activated by EcoHIV. However, impure virus preparations, generated by Polyethylene Glycol (PEG) precipitation, do activate platelets, suggesting that nonspecific PEG-precipitates may contain other platelet activators (e.g., histones and cell debris). Our data do not support the concept that platelets, through general surface proteins such as DC-SIGN or CLEC-2, have a wide recognition for different viruses and suggest that direct platelet/pathogen interactions are receptor/ligand specific.

Platelet-Rich Plasma in Dermatology: New Insights on the Cellular Mechanism of Skin Repair and Regeneration

The skin's recognised functions may undergo physiological alterations due to ageing, manifesting as varying degrees of facial wrinkles, diminished tautness, density, and volume. Additionally, these functions can be disrupted (patho)physiologically through various physical and chemical injuries, including surgical trauma, accidents, or chronic conditions like ulcers associated with diabetes mellitus, venous insufficiency, or obesity. Advancements in therapeutic interventions that boost the skin's innate regenerative abilities could significantly enhance patient care protocols. The application of Platelet-Rich Plasma (PRP) is widely recognized for its aesthetic and functional benefits to the skin. Yet, the endorsement of PRP's advantages often borders on the dogmatic, with its efficacy commonly ascribed solely to the activation of fibroblasts by the factors contained within platelet granules. PRP therapy is a cornerstone of regenerative medicine which involves the autologous delivery of conditioned plasma enriched by platelets. This is achieved by centrifugation, removing erythrocytes while retaining platelets and their granules. Despite its widespread use, the precise sequences of cellular activation, the specific cellular players, and the molecular machinery that drive PRP-facilitated healing are still enigmatic. There is still a paucity of definitive and robust studies elucidating these mechanisms. In recent years, telocytes (TCs)-a unique dermal cell population-have shown promising potential for tissue regeneration in various organs, including the dermis. TCs' participation in neo-angiogenesis, akin to that attributed to PRP, and their role in tissue remodelling and repair processes within the interstitia of several organs (including the dermis), offer intriguing insights. Their potential to contribute to, or possibly orchestrate, the skin regeneration process following PRP treatment has elicited considerable interest. Therefore, pursuing a comprehensive understanding of the cellular and molecular mechanisms at work, particularly those involving TCs, their temporal involvement in structural recovery following injury, and the interconnected biological events in skin wound healing and regeneration represents a compelling field of study.

Enhancing Single-Cell Western Blotting Sensitivity Using Diffusive Analyte Blotting and Antibody Conjugate Amplification

While there are many techniques to achieve highly sensitive, multiplex detection of RNA and DNA from single cells, detecting protein content often suffers from low limits of detection and throughput. Miniaturized, high-sensitivity Western blots on single cells (scWesterns) are attractive because they do not require advanced instrumentation. By physically separating analytes, scWesterns also uniquely mitigate limitations to target protein multiplexing posed by the affinity reagent performance. However, a fundamental limitation of scWesterns is their limited sensitivity for detecting low-abundance proteins, which arises from transport barriers posed by the separation gel against detection species. Here we address the sensitivity by decoupling the electrophoretic separation medium from the detection medium. We transfer scWestern separations to a nitrocellulose blotting medium with distinct mass transfer advantages over traditional in-gel probing, yielding a 5.9-fold improvement in the limit of detection. We next amplify probing of blotted proteins with enzyme-antibody conjugates, which are incompatible with traditional in-gel probing to achieve further improvement in the limit of detection to 1000 molecules, a 120-fold improvement. This enables us to detect 100% of cells in an EGFP-expressing population using fluorescently tagged and enzyme-conjugated antibodies compared to 84.5% of cells using in-gel detection. These results suggest the compatibility of nitrocellulose-immobilized scWesterns with a variety of affinity reagents─not previously accessible for in-gel use─for further signal amplification and detection of low-abundance targets.

Biochemical characterization of spleen tyrosine kinase (SYK) isoforms in platelets

Alternate splicing is among the regulatory mechanisms imparting functional diversity in proteins. Studying protein isoforms generated through alternative splicing is therefore critical for understanding protein functions in many biological systems. Spleen tyrosine kinase (Syk) plays an essential role in ITAM/hemITAM signaling in many cell types, including platelets. However, the spectrum of Syk isoforms expressed in platelets has not been characterized. Syk has been shown to have a full-length long isoform SykL and a shorter SykS lacking 23 amino acid residues within its interdomain B. Furthermore, putative isoforms lacking another 23 amino acid-long sequence or a combination of the two deletions have been postulated to exist. In this report, we demonstrate that mouse platelets express full-length SykL and the previously described shorter isoform SykS, but lack other shorter isoforms, whereas human platelets express predominantly SykL. These results both indicate a possible role of alternative Syk splicing in the regulation of receptor signaling in mouse platelets and a difference between signaling regulation in mouse and human platelets.

PACSIN2 regulates platelet integrin β1 hemostatic function

BACKGROUND

Upon vessel injury, platelets adhere to exposed matrix constituents via specific membrane receptors, including the von Willebrand factor receptor glycoprotein (GP)Ib-IX-V complex and integrins β1 and β3. In platelets, the Fes/CIP4-homology Bin-Amphiphysin-Rvs protein PACSIN2 associates with the cytoskeletal and scaffolding protein filamin A (FlnA), linking GPIbα and integrins to the cytoskeleton.

OBJECTIVES

Here we investigated the role of PACSIN2 in platelet function.

METHODS

Platelet parameters were evaluated in mice lacking PACSIN2 and platelet integrin β1.

RESULTS

Pacsin2-/- mice displayed mild thrombocytopenia, prolonged bleeding time, and delayed thrombus formation in a ferric chloride-mediated carotid artery injury model, which was normalized by injection of control platelets. Pacsin2-/- platelets formed unstable thrombi that embolized abruptly in a laser-induced cremaster muscle injury model. Pacsin2-/- platelets had hyperactive integrin β1, as evidenced by increased spreading onto surfaces coated with the collagen receptor α2β1-specific peptide GFOGER and increased binding of the antibody 9EG7 directed against active integrin β1. By contrast, Pacsin2-/- platelets had normal integrin αIIbβ3 function and expressed P-selectin normally following stimulation through the collagen receptor GPVI or with thrombin. Deletion of platelet integrin β1 in Pacsin2-/- mice normalized platelet count, hemostasis, and thrombus formation. A PACSIN2 peptide mimicking the FlnA-binding site mediated the pull-down of a FlnA rod 2 construct by integrin β7, a model for integrin β-subunits.

CONCLUSIONS

Pacsin2-/- mice displayed severe thrombus formation defects due to hyperactive platelet integrin β1. The data suggest that PACSIN2 binding to FlnA negatively regulates platelet integrin β1 hemostatic function.

α-Synuclein is the major platelet isoform but is dispensable for activation, secretion, and thrombosis

Platelets play many roles in the vasculature ensuring proper hemostasis and maintaining integrity. These roles are facilitated, in part, by cargo molecules released from platelet granules via Soluble NSF Attachment Protein Receptor (SNARE) mediated membrane fusion, which is controlled by several protein-protein interactions. Chaperones have been characterized for t-SNAREs (i.e. Munc18b for Syntaxin-11), but none have been clearly identified for v-SNAREs. α-Synuclein has been proposed as a v-SNARE chaperone which may affect SNARE-complex assembly, fusion pore opening, and thus secretion. Despite its abundance and that it is the only isoform present, α-synuclein's role in platelet secretion is uncharacterized. In this study, immunofluorescence showed that α-synuclein was present on punctate structures that co-stained with markers for α-granules and lysosomes and in a cytoplasmic pool. We analyzed the phenotype of α-synuclein-/- mice and their platelets. Platelets from knockout mice had a mild, agonist-dependent secretion defect but aggregation and spreading in vitro were unaffected. Consistently, thrombosis/hemostasis were unaffected in the tail-bleeding, FeCl3 carotid injury and jugular vein puncture models. None of the platelet secretory machinery examined, e.g. the v-SNAREs, were affected by α-synuclein's loss. The results indicate that, despite its abundance, α-synuclein has only a limited role in platelet function and thrombosis.

Changes in inflammatory responses and autophagy during apheresis platelet preservation and their correlation with platelet transfusion refractoriness in patients with acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is a common hematopoietic malignancy, and platelet transfusion plays a crucial role in its treatment. This study aimed to investigate the changes in inflammatory response and autophagy during the preservation of apheresis platelets (AP) and their correlation with platelet transfusion refractoriness (PTR) in ALL. ALL patients were included, and APs were categorized based on the preservation period (day 0, day 1, days 2-3, and days 4-5). The activation factors procaspase-activating compound 1 (PAC-1) and P-selectin (CD62P), AP aggregation function, inflammation levels (interleukin 1 beta [IL-1β], interleukin 6 [IL-6], tumor necrosis factor alpha [TNF-α] and NOD-like receptor thermal protein domain associated protein 3 [NLRP3]), and autophagy-related genes (p62) during AP preservation were assessed. Following co-culturing APs with peripheral blood mononuclear cells (PBMCs), specific activation markers were studied to observe APs influence on immune cells activation. The effectiveness of platelet transfusion was assessed, and risk factors for PTR were analyzed. As the storage duration of AP increased, the activation factors, coagulation factor activity, inflammation levels, and the activation of immune cells in AP increased, while fibrinogen levels and AP aggregation function decreased. The expression levels of autophagy-related genes (the autophagy marker light chain 3B gene [LC3B] and Beclin 1 gene) decreased with prolongation preservation. The effective rate of AP transfusion in ALL patients was 68.21%. AP preservation time, IL-6, p62, and Beclin 1 were identified as independent risk factors affecting PTR in ALL patients. In conclusion, during AP preservation, inflammation, autophagy, and activation of immune cells were observed to increase. AP preservation time, IL-6, p62, and Beclin 1 were independent risk factors for PTR.

What do we know about platelets in myocardial ischemia-reperfusion injury and why is it important?

Myocardial ischemia-reperfusion injury (MIRI), the joint result of ischemic injury and reperfusion injury, is associated with poor outcomes in patients with acute myocardial infarction undergoing primary percutaneous coronary intervention. Accumulating evidence demonstrates that activated platelets directly contribute to the pathogenesis of MIRI through participating in the formation of microthrombi, interaction with leukocytes, secretion of active substances, constriction of microvasculature, and activation of spinal afferent nerves. The molecular mechanisms underlying the above detrimental effects of activated platelets include the homotypic and heterotypic interactions through surface receptors, transduction of intracellular signals, and secretion of active substances. Revealing the roles of platelet activation in MIRI and the associated mechanisms would provide potential targets/strategies for the clinical evaluation and treatment of MIRI. Further studies are needed to characterize the temporal (ischemia phase vs. reperfusion phase) and spatial (systemic vs. local) distributions of platelet activation in MIRI by multi-omics strategies. To improve the likelihood of translating novel cardioprotective interventions into clinical practice, basic researches maximally replicating the complexity of clinical scenarios would be necessary.

Platelet-derived exerkine CXCL4/platelet factor 4 rejuvenates hippocampal neurogenesis and restores cognitive function in aged mice

The beneficial effects of physical activity on brain ageing are well recognised, with exerkines, factors that are secreted into the circulation in response to exercise, emerging as likely mediators of this response. However, the source and identity of these exerkines remain unclear. Here we provide evidence that an anti-geronic exerkine is secreted by platelets. We show that platelets are activated by exercise and are required for the exercise-induced increase in hippocampal precursor cell proliferation in aged mice. We also demonstrate that increasing the systemic levels of the platelet-derived exerkine CXCL4/platelet factor 4 (PF4) ameliorates age-related regenerative and cognitive impairments in a hippocampal neurogenesis-dependent manner. Together these findings highlight the role of platelets in mediating the rejuvenating effects of exercise during physiological brain ageing.

Matrix stiffness controls megakaryocyte adhesion, fibronectin fibrillogenesis, and proplatelet formation through Itgβ3

Megakaryocytes (MKs) are the precursor cells of platelets, located in the bone marrow (BM). Once mature, they extend elongated projections named proplatelets through sinusoid vessels, emerging from the marrow stroma into the circulating blood. Not all signals from the microenvironment that regulate proplatelet formation are understood, particularly those from the BM biomechanics. We sought to investigate how MKs perceive and adapt to modifications of the stiffness of their environment. Although the BM is one of the softest tissue of the body, its rigidification results from excess fibronectin (FN), and other matrix protein deposition occur upon myelofibrosis. Here, we have shown that mouse MKs are able to detect the stiffness of a FN-coated substrate and adapt their morphology accordingly. Using a polydimethylsiloxane substrate with stiffness varying from physiological to pathological marrow, we found that a stiff matrix favors spreading, intracellular contractility, and FN fibrils assembly at the expense of proplatelet formation. Itgb3, but not Itgb1, is required for stiffness sensing, whereas both integrins are involved in fibrils assembly. In contrast, soft substrates promote proplatelet formation in an Itgb3-dependent manner, consistent with the ex vivo decrease in proplatelet formation and the in vivo decrease in platelet number in Itgb3-deficient mice. Our findings demonstrate the importance of environmental stiffness for MK functions with potential pathophysiological implications during pathologies that deregulate FN deposition and modulate stiffness in the marrow.

Platelet Metabolic Flexibility: A Matter of Substrate and Location

Platelets are cellular elements that are physiologically involved in hemostasis, inflammation, thrombotic events, and various human diseases. There is a link between the activation of platelets and their metabolism. Platelets possess considerable metabolic versatility. Although the role of platelets in hemostasis and inflammation is known, our current understanding of platelet metabolism in terms of substrate preference is limited. Platelet activation triggers an oxidative metabolism increase to sustain energy requirements better than aerobic glycolysis alone. In addition, platelets possess extra-mitochondrial oxidative phosphorylation, which could be one of the sources of chemical energy required for platelet activation. This review aims to provide an overview of flexible platelet metabolism, focusing on the role of metabolic compartmentalization in substrate preference, since the metabolic flexibility of stimulated platelets could depend on subcellular localization and functional timing. Thus, developing a detailed understanding of the link between platelet activation and metabolic changes is crucial for improving human health.

The WASH-complex subunit Strumpellin regulates integrin αIIbβ3 trafficking in murine platelets

The platelet specific integrin αIIbβ3 mediates platelet adhesion, aggregation and plays a central role in thrombosis and hemostasis. In resting platelets, αIIbβ3 is expressed on the membrane surface and in intracellular compartments. Upon activation, the number of surface-expressed αIIbβ3 is increased by the translocation of internal granule pools to the plasma membrane. The WASH complex is the major endosomal actin polymerization-promoting complex and has been implicated in the generation of actin networks involved in endocytic trafficking of integrins in other cell types. The role of the WASH complex and its subunit Strumpellin in platelet function is still unknown. Here, we report that Strumpellin-deficient murine platelets display an approximately 20% reduction in integrin αIIbβ3 surface expression. While exposure of the internal αIIbβ3 pool after platelet activation was unaffected, the uptake of the αIIbβ3 ligand fibrinogen was delayed. The number of platelet α-granules was slightly but significantly increased in Strumpellin-deficient platelets. Quantitative proteome analysis of isolated αIIbβ3-positive vesicular structures revealed an enrichment of protein markers, which are associated with the endoplasmic reticulum, Golgi complex and early endosomes in Strumpellin-deficient platelets. These results point to a so far unidentified role of the WASH complex subunit Strumpellin in integrin αIIbβ3 trafficking in murine platelets.

TULA Proteins in Men, Mice, Hens, and Lice: Welcome to the Family

The two members of the UBASH3/STS/TULA protein family have been shown to critically regulate key biological functions, including immunity and hemostasis, in mammalian biological systems. Negative regulation of signaling through immune receptor tyrosine-based activation motif (ITAM)- and hemITAM-bearing receptors mediated by Syk-family protein tyrosine kinases appears to be a major molecular mechanism of the down-regulatory effect of TULA-family proteins, which possess protein tyrosine phosphatase (PTP) activity. However, these proteins are likely to carry out some PTP-independent functions as well. Whereas the effects of TULA-family proteins overlap, their characteristics and their individual contributions to cellular regulation also demonstrate clearly distinct features. Protein structure, enzymatic activity, molecular mechanisms of regulation, and biological functions of TULA-family proteins are discussed in this review. In particular, the usefulness of the comparative analysis of TULA proteins in various metazoan taxa, for identifying potential roles of TULA-family proteins outside of their functions already established in mammalian systems, is examined.

Platelet-derived chemokines promote skeletal muscle regeneration by guiding neutrophil recruitment to injured muscles

Skeletal muscle regeneration involves coordinated interactions between different cell types. Injection of platelet-rich plasma is circumstantially considered an aid to muscle repair but whether platelets promote regeneration beyond their role in hemostasis remains unexplored. Here, we find that signaling via platelet-released chemokines is an early event necessary for muscle repair in mice. Platelet depletion reduces the levels of the platelet-secreted neutrophil chemoattractants CXCL5 and CXCL7/PPBP. Consequently, early-phase neutrophil infiltration to injured muscles is impaired whereas later inflammation is exacerbated. Consistent with this model, neutrophil infiltration to injured muscles is compromised in male mice with Cxcl7-knockout platelets. Moreover, neo-angiogenesis and the re-establishment of myofiber size and muscle strength occurs optimally in control mice post-injury but not in Cxcl7ko mice and in neutrophil-depleted mice. Altogether, these findings indicate that platelet-secreted CXCL7 promotes regeneration by recruiting neutrophils to injured muscles, and that this signaling axis could be utilized therapeutically to boost muscle regeneration.

JCAD promotes arterial thrombosis through PI3K/Akt modulation: a translational study

AIMS

Variants of the junctional cadherin 5 associated (JCAD) locus associate with acute coronary syndromes. JCAD promotes experimental atherosclerosis through the large tumor suppressor kinase 2 (LATS2)/Hippo pathway. This study investigates the role of JCAD in arterial thrombosis.

METHODS AND RESULTS

JCAD knockout (Jcad-/-) mice underwent photochemically induced endothelial injury to trigger arterial thrombosis. Primary human aortic endothelial cells (HAECs) treated with JCAD small interfering RNA (siJCAD), LATS2 small interfering RNA (siLATS2) or control siRNA (siSCR) were employed for in vitro assays. Plasma JCAD was measured in patients with chronic coronary syndrome or ST-elevation myocardial infarction (STEMI). Jcad-/- mice displayed reduced thrombogenicity as reflected by delayed time to carotid occlusion. Mechanisms include reduced activation of the coagulation cascade [reduced tissue factor (TF) expression and activity] and increased fibrinolysis [higher thrombus embolization episodes and D-dimer levels, reduced vascular plasminogen activator inhibitor (PAI)-1 expression]. In vitro, JCAD silencing inhibited TF and PAI-1 expression in HAECs. JCAD-silenced HAECs (siJCAD) displayed increased levels of LATS2 kinase. Yet, double JCAD and LATS2 silencing did not restore the control phenotype. si-JCAD HAECs showed increased levels of phosphoinositide 3-kinases (PI3K)/ proteinkinase B (Akt) activation, known to downregulate procoagulant expression. The PI3K/Akt pathway inhibitor-wortmannin-prevented the effect of JCAD silencing on TF and PAI-1, indicating a causative role. Also, co-immunoprecipitation unveiled a direct interaction between JCAD and Akt. Confirming in vitro findings, PI3K/Akt and P-yes-associated protein levels were higher in Jcad-/- animals. Lastly, as compared with chronic coronary syndrome, STEMI patients showed higher plasma JCAD, which notably correlated positively with both TF and PAI-1 levels.

CONCLUSIONS

JCAD promotes arterial thrombosis by modulating coagulation and fibrinolysis. Herein, reported translational data suggest JCAD as a potential therapeutic target for atherothrombosis.

Brain-derived neurotrophic factor measurements in mouse serum and plasma using a sensitive and specific enzyme-linked immunosorbent assay

This study is about the quantification and validation of BDNF levels in mouse serum and plasma using a sensitive immunoassay. While BDNF levels are readily detectable in human serum, the functional implications of these measurements are unclear as BDNF released from human blood platelets is the main contributor to the serum levels of BDNF. As mouse platelets do not contain BDNF, this confounding factor is absent in the mouse. Accordingly, BDNF levels in mouse serum and plasma were found to be indistinguishable at 9.92 ± 1.97 pg/mL for serum and 10.58 ± 2.43 pg/mL for plasma (p = 0.473). These levels are approximately a thousand times lower than those measured in human serum and pre-adsorption with anti-BDNF, but not with anti-NGF or anti-NT3 monoclonal antibodies, markedly reduced the BDNF signal. These results open the possibility to explore the relevance of BDNF levels as a biomarker in accessible body fluids using existing mouse models mimicking human pathological conditions.

The intracellular and plasma membrane pools of phosphatidylinositol-4-monophosphate control megakaryocyte maturation and proplatelet formation

Background

Megakaryocytes (MKs) develop from hematopoietic stem cells after stimulation by the cytokine thrombopoietin. During megakaryopoiesis, MKs enlarge, undergo the process of endomitosis, and develop intracellular membranes (demarcation membrane system, DMS). During DMS formation, there is active transport of proteins, lipids, and membranes from the Golgi apparatus to the DMS. The most important phosphoinositide that controls anterograde transport from the Golgi apparatus to the plasma membrane (PM) is phosphatidylinositol-4-monophosphate (PI4P), whose levels are controlled by suppressor of actin mutations 1-like protein (Sac1) phosphatase at the Golgi and endoplasmic reticulum.

Objectives

Here we investigated the role of Sac1 and PI4P in megakaryopoiesis.

Methods

We analyzed Sac1 and PI4P localization in primary MKs derived from fetal liver or bone marrow and in the DAMI cell line by immunofluorescence. The intracellular and PM pools of PI4P in primary MKs were modulated by expression of Sac1 constructs from retroviral vector and inhibition of PI4 kinase IIIα, respectively.

Results

We showed that in primary mouse MKs, PI4P is mostly found in the Golgi apparatus and the PM in immature MKs, while in mature MKs, it is found in the cell periphery and at the PM. The exogenous expression of wild-type but not C389S mutant (catalytically dead) Sac1 results in the perinuclear retention of the Golgi apparatus resembling immature MKs, with decreased ability to form proplatelets. The pharmacologic inhibition of PI4P production specifically at the PM also resulted in a significant decrease in MKs that form proplatelets.

Conclusion

These results indicate that both intracellular and PM pools of PI4P mediate MK maturation and proplatelet formation.

Immobility-associated thromboprotection is conserved across mammalian species from bear to human

Venous thromboembolism (VTE) comprising deep venous thrombosis and pulmonary embolism is a major cause of morbidity and mortality. Short-term immobility-related conditions are a major risk factor for the development of VTE. Paradoxically, long-term immobilized free-ranging hibernating brown bears and paralyzed spinal cord injury (SCI) patients are protected from VTE. We aimed to identify mechanisms of immobility-associated VTE protection in a cross-species approach. Mass spectrometry-based proteomics revealed an antithrombotic signature in platelets of hibernating brown bears with heat shock protein 47 (HSP47) as the most substantially reduced protein. HSP47 down-regulation or ablation attenuated immune cell activation and neutrophil extracellular trap formation, contributing to thromboprotection in bears, SCI patients, and mice. This cross-species conserved platelet signature may give rise to antithrombotic therapeutics and prognostic markers beyond immobility-associated VTE.

High-throughput assessment identifying major platelet Ca2+ entry pathways via tyrosine kinase-linked and G protein-coupled receptors

In platelets, elevated cytosolic Ca²⁺ is a crucial second messenger, involved in most functional responses, including shape change, secretion, aggregation and procoagulant activity. The platelet Ca²⁺ response consists of Ca²⁺ mobilization from endoplasmic reticulum stores, complemented with store-operated or receptor-operated Ca²⁺ entry pathways. Several channels can contribute to the Ca²⁺ entry, but their relative contribution is unclear upon stimulation of ITAM-linked receptors such as glycoprotein VI (GPVI) and G-protein coupled receptors such as the protease-activated receptors (PAR) for thrombin. We employed a 96-well plate high-throughput assay with Fura-2-loaded human platelets to perform parallel [Ca²⁺]_i measurements in the presence of EGTA or CaCl₂. Per agonist condition, this resulted in sets of EGTA, CaCl₂ and Ca²⁺ entry ratio curves, defined by six parameters, reflecting different Ca²⁺ ion fluxes. We report that threshold stimulation of GPVI or PAR, with a variable contribution of secondary mediators, induces a maximal Ca²⁺ entry ratio of 3-7. Strikingly, in combination with Ca²⁺-ATPase inhibition by thapsigargin, the maximal Ca²⁺ entry ratio increased to 400 (GPVI) or 40 (PAR), pointing to a strong receptor-dependent enhancement of store-operated Ca²⁺ entry. By pharmacological blockage of specific Ca²⁺ channels in platelets, we found that, regardless of GPVI or PAR stimulation, the Ca²⁺ entry ratio was strongest affected by inhibition of ORAI1 (2-APB, Synta66) > Na⁺/Ca²⁺ exchange (NCE) > P2×₁ (only initial). In contrast, inhibition of TRPC6, Piezo1/2 or STIM1 was without effect. Together, these data reveal ORAI1 and NCE as dominating Ca²⁺ carriers regulating GPVI- and PAR-induced Ca²⁺ entry in human platelets.

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.

Oculocerebrorenal syndrome of Lowe protein controls cytoskeletal reorganisation during human platelet spreading

Lowe syndrome (LS) is a rare, X-linked disorder characterised by numerous symptoms affecting the brain, the eyes, and the kidneys. It is caused by mutations in the oculocerebrorenal syndrome of Lowe (OCRL) protein, a 5-phosphatase localised in different cellular compartments that dephosphorylates phosphatidylinositol-4,5-bisphosphate into phosphatidylinositol-4-monophosphate. Some patients with LS also have bleeding disorders, with normal to low platelet (PLT) count and impaired PLT function. However, the mechanism of PLT dysfunction in patients with LS is not completely understood. The main function of PLTs is to activate upon vessel wall injury and stop the bleeding by clot formation. PLT activation is accompanied by a shape change that is a result of massive cytoskeletal rearrangements. Here, we show that OCRL-inhibited human PLTs do not fully spread, form mostly filopodia, and accumulate actin nodules. These nodules co-localise with ARP2/3 subunit p34, vinculin, and sorting nexin 9. Furthermore, OCRL-inhibited PLTs have a retained microtubular coil with high levels of acetylated tubulin. Also, myosin light chain phosphorylation is decreased upon OCRL inhibition, without impaired degranulation or integrin activation. Taken together, these results suggest that OCRL contributes to cytoskeletal rearrangements during PLT activation that could explain mild bleeding problems in patients with LS.

Heparin and heparin proteoglycan-mimetics activate platelets via PEAR1 and PI3Kβ

BACKGROUND

Platelet endothelial aggregation receptor 1 (PEAR1) is a single-transmembrane orphan receptor primarily expressed on platelets and endothelial cells. Genetic variants of PEAR1 have repeatedly and independently been identified to be associated with cardiovascular diseases, including coronary artery disease.

OBJECTIVES

We have identified sulfated fucoidans and their mimetics as ligands for PEAR1 and proposed that its endogenous ligand is a sulfated proteoglycan. The aim of this study was to test this hypothesis.

METHODS

A heparin proteoglycan-mimetic (HPGM) was created by linking unfractionated heparin (UFH) to albumin. The ability of the HPGM, UFH and selectively desulfated heparins to stimulate platelet aggregation and protein phosphorylation was investigated. Nanobodies against the 12th to 13th epidermal growth factor-like repeat of PEAR1 and phosphoinositide 3-kinase (PI3K) isoform-selective inhibitors were tested for the inhibition of platelet activation.

RESULTS

We show that HPGM, heparin conjugated to an albumin protein core, stimulates aggregation and phosphorylation of PEAR1 in washed platelets. Platelet aggregation was abolished by an anti-PEAR1 nanobody, Nb138. UFH stimulated platelet aggregation in washed platelets, but desulfated UFH did not. Furthermore, HPGM, but not UFH, stimulated maximal aggregation in platelet-rich plasma. However, both HPGM and UFH increased integrin αIIbβ3 activation in whole blood. By using PI3K isoform-selective inhibitors, we show that PEAR1 activates PI3Kβ, leading to Akt phosphorylation.

CONCLUSION

Our findings reveal that PEAR1 is a receptor for heparin and HPGM and that PI3Kβ is a key signaling molecule downstream of PEAR1 in platelets. These findings may have important implications for our understanding of the role of PEAR1 in cardiovascular disease.

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.

TULA-Family Regulators of Platelet Activation

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

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.

A gain-of-function filamin A mutation in mouse platelets induces thrombus instability

BACKGROUND

Filaminopathies A are rare disorders affecting the brain, intestine, or skeleton, characterized by dominant X-linked filamin A (FLNA) gene mutations. Macrothrombocytopenia with functionally defective platelets is frequent. We have described a filaminopathy A male patient, exhibiting a C-terminal frame-shift FLNa mutation (Berrou et al., Arterioscler Thromb Vasc Biol. 2017;37:1087-1097). Contrasting with female patients, this male patient exhibited gain of platelet functions, including increased platelet aggregation, integrin αIIbβ3 activation, and secretion at low agonist concentration, raising the issue of thrombosis risk.

OBJECTIVES

Our goal is to assess the thrombotic potential of the patient FLNa mutation in an in vivo model.

METHODS

We have established a mutant FlnA knock-in mouse model.

RESULTS

The mutant FlnA mouse platelets phenocopied patient platelets, showing normal platelet count, lower expression level of mutant FlnA, and gain of platelet functions: increased platelet aggregation, secretion, and αIIbβ3 activation, as well as increased spreading and clot retraction. Surprisingly, mutant FlnA mice exhibited a normal bleeding time, but with increased re-bleeding (77%) compared to wild type (WT) FlnA mice (27%), reflecting hemostatic plug instability. Again, in an in vivo thrombosis model, the occlusion time was not altered by the FlnA mutation, but arteriolar embolies were increased (7-fold more frequent in mutant FlnA mice versus WT mice), confirming thrombus instability.

CONCLUSIONS

This study shows that the FlnA mutation found in the male patient induced gain of platelet functions in vitro, but thrombus instability in vivo. Implications for the role of FLNa in physiology of thrombus formation are discussed.

Optogenetic manipulation of cyclic guanosine monophosphate to probe phosphodiesterase activities in megakaryocytes

Cyclic guanosine monophosphate (cGMP) signalling plays a fundamental role in many cell types, including platelets. cGMP has been implicated in platelet formation, but mechanistic detail about its spatio-temporal regulation in megakaryocytes (MKs) is lacking. Optogenetics is a technique which allows spatio-temporal manipulation of molecular events in living cells or organisms. We took advantage of this method and expressed a photo-activated guanylyl cyclase, Blastocladiella emersonii Cyclase opsin (BeCyclop), after viral-mediated gene transfer in bone marrow (BM)-derived MKs to precisely light-modulate cGMP levels. BeCyclop-MKs showed a significantly increased cGMP concentration after illumination, which was strongly dependent on phosphodiesterase (PDE) 5 activity. This finding was corroborated by real-time imaging of cGMP signals which revealed that pharmacological PDE5 inhibition also potentiated nitric oxide-triggered cGMP generation in BM MKs. In summary, we established for the first-time optogenetics in primary MKs and show that PDE5 is the predominant PDE regulating cGMP levels in MKs. These findings also demonstrate that optogenetics allows for the precise manipulation of MK biology.

Impaired microtubule dynamics contribute to microthrombocytopenia in RhoB-deficient mice

RhoB-deficient mice display microthrombocytopenia.

RhoB exerts nonredundant functions in the megakaryocyte lineage compared with RhoA and regulates microtubule dynamics.

Megakaryocytes are large cells in the bone marrow that give rise to blood platelets. Platelet biogenesis involves megakaryocyte maturation, the localization of the mature cells in close proximity to bone marrow sinusoids, and the formation of protrusions, which are elongated and shed within the circulation. Rho GTPases play important roles in platelet biogenesis and function. RhoA-deficient mice display macrothrombocytopenia and a striking mislocalization of megakaryocytes into bone marrow sinusoids and a specific defect in G-protein signaling in platelets. However, the role of the closely related protein RhoB in megakaryocytes or platelets remains unknown. In this study, we show that, in contrast to RhoA deficiency, genetic ablation of RhoB in mice results in microthrombocytopenia (decreased platelet count and size). RhoB-deficient platelets displayed mild functional defects predominantly upon induction of the collagen/glycoprotein VI pathway. Megakaryocyte maturation and localization within the bone marrow, as well as actin dynamics, were not affected in the absence of RhoB. However, in vitro–generated proplatelets revealed pronouncedly impaired microtubule organization. Furthermore, RhoB-deficient platelets and megakaryocytes displayed selective defects in microtubule dynamics/stability, correlating with reduced levels of acetylated α-tubulin. Our findings imply that the reduction of this tubulin posttranslational modification results in impaired microtubule dynamics, which might contribute to microthrombocytopenia in RhoB-deficient mice. Importantly, we demonstrate that RhoA and RhoB are localized differently and have selective, nonredundant functions in the megakaryocyte lineage.

GRK2 regulates ADP signaling in platelets via P2Y1 and P2Y12

The critical role of GRK2 in regulating cardiac function has been well documented for over three decades. Therefore, targeting GRK2 has been extensively studied as a novel approach to treat cardiovascular disease. However, little is known about its role in hemostasis and thrombosis. Here we provide the first evidence that GRK2 limits platelet activation and regulates the hemostatic response to injury. Deletion of GRK2 in mouse platelets causes increased platelet accumulation following laser-induced injury in cremaster muscle arterioles, shortens tail bleeding time, and enhances thrombosis in ADP-induced pulmonary thromboembolism and in FeCl3-induced carotid injury. GRK2-/- platelets have increased integrin activation, P-selectin exposure, and platelet aggregation in response to ADP stimulation. Furthermore, GRK2-/- platelets retain the ability to aggregate in response to ADP re-stimulation, indicating that GRK2 contributes to ADP receptor desensitization. Underlying these changes in GRK2-/- platelets is an increase in Ca2+ mobilization, RAP1 activation, and Akt phosphorylation stimulated by ADP, and an attenuated rise of cAMP levels in response to ADP in the presence of prostaglandin I2. P2Y12 antagonist treatment eliminates the phenotypic difference in platelet accumulation between WT and GRK2-/- mice at the site of injury. Pharmacologic inhibition of GRK2 activity in human platelets increases platelet activation in response to ADP. Finally, we show that GRK2 binds to endogenous Gβγ subunits during platelet activation. Collectively, these results show that GRK2 regulates ADP signaling via P2Y1 and P2Y12, interacts with Gβγ, and functions as a signaling hub in platelets for modulating the hemostatic response to injury.

The Connection Between Rap1 and Talin1 in the Activation of Integrins in Blood Cells

Integrins regulate the adhesion and migration of blood cells to ensure the proper positioning of these cells in the environment. Integrins detect physical and chemical stimuli in the extracellular matrix and regulate signaling pathways in blood cells that mediate their functions. Integrins are usually in a resting state in blood cells until agonist stimulation results in a high-affinity conformation ("integrin activation"), which is central to integrins' contribution to blood cells' trafficking and functions. In this review, we summarize the mechanisms of integrin activation in blood cells with a focus on recent advances understanding of mechanisms whereby Rap1 regulates talin1-integrin interaction to trigger integrin activation in lymphocytes, platelets, and neutrophils.

Mechanism of integrin activation by talin and its cooperation with kindlin

Talin-induced integrin binding to extracellular matrix ligands (integrin activation) is the key step to trigger many fundamental cellular processes including cell adhesion, cell migration, and spreading. Talin is widely known to use its N-terminal head domain (talin-H) to bind and activate integrin, but how talin-H operates in the context of full-length talin and its surrounding remains unknown. Here we show that while being capable of inducing integrin activation, talin-H alone exhibits unexpectedly low potency versus a constitutively activated full-length talin. We find that the large C-terminal rod domain of talin (talin-R), which otherwise masks the integrin binding site on talin-H in inactive talin, dramatically enhances the talin-H potency by dimerizing activated talin and bridging it to the integrin co-activator kindlin-2 via the adaptor protein paxillin. These data provide crucial insight into the mechanism of talin and its cooperation with kindlin to promote potent integrin activation, cell adhesion, and signaling.

Deletion of platelet CLEC-2 decreases GPIbα-mediated integrin αIIbβ3 activation and decreases thrombosis in TTP

Microvascular thrombosis in patients with thrombotic thrombocytopenic purpura (TTP) is initiated by GPIbα-mediated platelet binding to von Willebrand factor (VWF). Binding of VWF to GPIbα causes activation of the platelet surface integrin αIIbβ3. However, the mechanism of GPIbα-initiated activation of αIIbβ3 and its clinical importance for microvascular thrombosis remain elusive. Deletion of platelet C-type lectin-like receptor 2 (CLEC-2) did not prevent VWF binding to platelets but specifically inhibited platelet aggregation induced by VWF binding in mice. Deletion of platelet CLEC-2 also inhibited αIIbβ3 activation induced by the binding of VWF to GPIbα. Using a mouse model of TTP, which was created by infusion of anti-mouse ADAMTS13 monoclonal antibodies followed by infusion of VWF, we found that deletion of platelet CLEC-2 decreased pulmonary arterial thrombosis and the severity of thrombocytopenia. Importantly, prophylactic oral administration of aspirin, an inhibitor of platelet activation, and therapeutic treatment of the TTP mice with eptifibatide, an integrin αIIbβ3 antagonist, reduced pulmonary arterial thrombosis in the TTP mouse model. Our observations demonstrate that GPIbα-mediated activation of integrin αIIbβ3 plays an important role in the formation of thrombosis in TTP. These observations suggest that prevention of platelet activation with aspirin may reduce the risk for thrombosis in patients with TTP.

Absolute Proteome Quantification in the Gas-Fermenting Acetogen Clostridium autoethanogenum

Microbes that can recycle one-carbon (C₁) greenhouse gases into fuels and chemicals are vital for the biosustainability of future industries. Acetogens are the most efficient known microbes for fixing carbon oxides CO₂ and CO. Understanding proteome allocation is important for metabolic engineering as it dictates metabolic fitness. Here, we use absolute proteomics to quantify intracellular concentrations for >1,000 proteins in the model acetogen Clostridium autoethanogenum grown autotrophically on three gas mixtures (CO, CO+H₂, or CO+CO₂+H₂). We detect the prioritization of proteome allocation for C₁ fixation and the significant expression of proteins involved in the production of acetate and ethanol as well as proteins with unclear functions. The data also revealed which isoenzymes are likely relevant in vivo for CO oxidation, H₂ metabolism, and ethanol production. The integration of proteomic and metabolic flux data demonstrated that enzymes catalyze high fluxes with high concentrations and high in vivo catalytic rates. We show that flux adjustments were dominantly accompanied by changing enzyme catalytic rates rather than concentrations.

IMPORTANCE Acetogen bacteria are important for maintaining biosustainability as they can recycle gaseous C₁ waste feedstocks (e.g., industrial waste gases and syngas from gasified biomass or municipal solid waste) into fuels and chemicals. Notably, the acetogen Clostridium autoethanogenum is being used as a cell factory in industrial-scale gas fermentation. Here, we perform reliable absolute proteome quantification for the first time in an acetogen. This is important as our work advances both rational metabolic engineering of acetogen cell factories and accurate in silico reconstruction of their phenotypes. Furthermore, this absolute proteomics data set serves as a reference toward a better systems-level understanding of the ancient metabolism of acetogens.

[Effect of ethanol on platelet biology]

In recent years, interest in the study of platelets, significantly increased due to recent discoveries providing convincing evidence that their functions by are not limited to their participation in the blood coagulation mechanism. Many works are devoted to the study of the functional state of platelets under conditions of acute and chronic alcohol exposure. The results of such studies can be useful for the development of new markers of the degree of alcohol intoxication of the body for the subsequent choice of the method drug correction of disorders caused by acute or chronic alcohol effects. The review summarizes results in vivo and in vitro of studies performed during more than 60 years on the effect of ethanol on the biogenesis, number, morphology and biochemistry of platelets.

Platelets drive fibronectin fibrillogenesis using integrin αIIbβ3

Platelets interact with multiple adhesion proteins during thrombogenesis, yet little is known about their ability to assemble fibronectin matrix. In vitro three-dimensional superresolution microscopy complemented by biophysical and biochemical methods revealed fundamental insights into how platelet contractility drives fibronectin fibrillogenesis. Platelets adhering to thrombus proteins (fibronectin and fibrin) versus basement membrane components (laminin and collagen IV) pull fibronectin fibrils along their apical membrane versus underneath their basal membrane, respectively. In contrast to other cell types, platelets assemble fibronectin nanofibrils using αIIbβ3 rather than α5β1 integrins. Apical fibrillogenesis correlated with a stronger activation of integrin-linked kinase, higher platelet traction forces, and a larger tension in fibrillar-like adhesions compared to basal fibrillogenesis. Our findings have potential implications for how mechanical thrombus integrity might be maintained during remodeling and vascular repair.

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.

Early Endosomal GTPase Rab5 (Ras-Related Protein in Brain 5) Regulates GPIbβ (Glycoprotein Ib Subunit β) Trafficking and Platelet Production In Vitro

OBJECTIVE

Maturation of megakaryocytes culminates with extensive membrane rearrangements necessary for proplatelet formation. Mechanisms required for proplatelet extension and origin of membranes are still poorly understood. GTPase Rab5 (Ras-related protein in brain 5) regulates endocytic uptake and homotypic fusion of early endosomes and regulates phosphatidylinositol 3-monophosphate production important for binding of effector proteins during early-to-late endosomal/lysosomal maturation. Approach and Results: To investigate the role of Rab5 in megakaryocytes, we expressed GFP (green fluorescent protein)-coupled Rab5 wild type and its point mutants Q79L (active) and N133L (inactive) in primary murine fetal liver-derived megakaryocytes. Active Rab5 Q79L induced the formation of enlarged early endosomes, while inactive Rab5 N133L caused endosomal fragmentation. Consistently, an increased amount of transferrin internalization in Rab5 Q79L was impaired in Rab5 N133L expressing megakaryocytes, when compared with GFP or Rab5 wild type. Moreover, trafficking of GPIbβ (glycoprotein Ib subunit beta), a subunit of major megakaryocytes receptor and membrane marker, was found to be mediated by Rab5 activity. While GPIbβ was mostly present along the plasma membrane, and within cytoplasmic vesicles in Rab5 wild type megakaryocytes, it accumulated in the majority of Rab5 Q79L enlarged endosomes. Conversely, Rab5 N133L caused mostly GPIbβ plasma membrane retention. Furthermore, Rab5 Q79L expression increased incorporation of the membrane dye (PKH26), indicating higher membrane content. Finally, while Rab5 Q79L increased proplatelet production, inactive Rab5 N133L strongly inhibited it and was coupled with a decrease in late endosomes/lysosomes. Localization of GPIbβ in enlarged endosomes was phosphatidylinositol 3-monophosphate dependent.

CONCLUSIONS

Taken together, our results demonstrate that Rab5-dependent endocytosis plays an important role in megakaryocytes receptor trafficking, membrane formation, and thrombopoiesis.

Platelet PD-L1 reflects collective intratumoral PD-L1 expression and predicts immunotherapy response in non-small cell lung cancer

Immune-checkpoint inhibitors (ICI) have transformed oncological therapy. Up to 20% of all non-small cell lung cancers (NSCLCs) show durable responses upon treatment with ICI, however, robust markers to predict therapy response are missing. Here we show that blood platelets interact with lung cancer cells and that PD-L1 protein is transferred from tumor cells to platelets in a fibronectin 1, integrin α5β1 and GPIbα-dependent manner. Platelets from NSCLC patients are found to express PD-L1 and platelet PD-L1 possess the ability to inhibit CD4 and CD8 T-cells. An algorithm is developed to calculate the activation independent adjusted PD-L1 payload of platelets (pPD-L1^Adj.), which is found to be superior in predicting the response towards ICI as compared to standard histological PD-L1 quantification on tumor biopsies. Our data suggest that platelet PD-L1 reflects the collective tumor PD-L1 expression, plays important roles in tumor immune evasion and overcomes limitations of histological quantification of often heterogeneous intratumoral PD-L1 expression.

Successful prenatal therapy for anti-CD36-mediated severe FNAIT by deglycosylated antibodies in a novel murine model

Recent studies have shown that maternal anti-CD36 antibodies represent a frequent cause of fetal/neonatal alloimmune thrombocytopenia (FNAIT) in Asian and African populations. However, little is known about the pathomechanism and antenatal treatment of anti-CD36-mediated FNAIT. Here, we established a novel animal model to examine the clinical features of pups from immunized Cd36-/- female mice after breeding with wild-type male mice. Mild thrombocytopenia was observed, but high pup mortality was also documented (40.26%). Administration of intravenous immunoglobulin (IVIG) (1 g/kg) on days 7, 12, and 17 to immunized Cd36-/- mothers after breeding reduced fetal death (12.70%). However, delaying the IVIG administration series on days 10, 15, and 20 did not reduce fetal death (40.00%). In contrast, injection of deglycosylated anti-CD36 (deg-anti-CD36) polyclonal antibodies (5 mg/kg) on days 10, 15, and 20 significantly reduced fetal death (5.26%). Subsequently, monoclonal antibodies (mAbs) against mouse CD36 were developed, and one clone producing high-affinity anti-CD36 (termed 32-106) effectively inhibited maternal antibody binding and was therefore selected. Using the same approach of deg-anti-CD36, the administration of deg-32-106 significantly reduced fetal death (2.17%). Furthermore, immunized Cd36-/- mothers exhibited placental deficiency. Accordingly, maternal anti-CD36 antibodies inhibited angiogenesis of placenta endothelial cells, which could be restored by deg-32-106. In summary, maternal anti-CD36 antibodies caused a high frequency of fetal death in our animal model, associated with placental dysfunction. This deleterious effect could be diminished by the antenatal administration of IVIG and deg-mAb 32-106. Interestingly, treatment with deg-32-106 seems more beneficial considering the lower dose, later start of treatment, and therapy success.

Platelet proteome dynamics in hibernating 13-lined ground squirrels

Hibernating mammals undergo a dramatic drop in temperature and blood flow during torpor, yet avoid stasis blood clotting through mechanisms that remain unspecified. The effects of hibernation on hemostasis are especially complex, as cold temperatures generally activate platelets, resulting in platelet clearance and cold storage lesions in the context of blood transfusion. With a hibernating body temperature of 4°C-8°C, 13-lined ground squirrels (Ictidomys tridecemlineatus) provide a model to study hemostasis as well as platelet cold storage lesion resistance during hibernation. Here, we quantified and systematically compared proteomes of platelets collected from ground squirrels at summer (active), fall (entrance), and winter (topor) to elucidate how molecular-level changes in platelets may support hemostatic adaptations in torpor. Platelets were isolated from a total of 11 squirrels in June, October, and January. Platelet lysates from each animal were digested with trypsin prior to 11-plex tandem mass tag (TMT) labeling, followed by LC-MS/MS analysis for relative protein quantification. We measured >700 proteins with significant variations in abundance in platelets over the course of entrance, torpor, and activity-including systems of proteins regulating translation, secretion, metabolism, complement, and coagulation cascades. We also noted species-specific differences in levels of hemostatic, secretory, and inflammatory regulators in ground squirrel platelets relative to human platelets. Altogether, we provide the first ever proteomic characterization of platelets from hibernating animals, where systematic changes in metabolic, hemostatic, and other proteins may account for physiological adaptations in torpor and also inform translational effort to improve cold storage of human platelets for transfusion.

Determination of PAR4 numbers on the surface of human platelets: no effect of the single nucleotide polymorphism rs773902

The thrombin receptor, protease-activated receptor 4 (PAR4), is important for platelet activation and is the target of emerging anti-thrombotic drugs. A frequently occurring single nucleotide polymorphism (SNP; rs773902) causes a function-altering PAR4 sequence variant (NC_000019.10:p.Ala120Thr), whereby platelets from Thr120-expressing individuals are hyper-responsive to PAR4 agonists and hypo-responsive to some PAR4 antagonists than platelets from Ala120-expressing individuals. This altered pharmacology may impact PAR4 inhibitor development, yet the underlying mechanism(s) remain unknown. We tested whether PAR4 surface expression contributes to the altered receptor function. Quantitative flow cytometry was used to determine the absolute number of PAR4 on platelets from individuals subsequently genotyped at rs773902. We detected 539 ± 311 PAR4 per platelet (mean ± SD, n = 84). This number was not different across rs773902 genotypes. This first determination of cellular PAR4 numbers indicates variations in platelet surface expression do not explain the altered pharmacology of the rs773902 PAR4 sequence variant.

Opposing Roles of GSK3α and GSK3β Phosphorylation in Platelet Function and Thrombosis

One of the mechanisms by which PI3 kinase can regulate platelet function is through phosphorylation of downstream substrates, including glycogen synthase kinase-3 (GSK3)α and GSK3β. Platelet activation results in the phosphorylation of an N-terminal serine residue in GSK3α (Ser21) and GSK3β (Ser9), which competitively inhibits substrate phosphorylation. However, the role of phosphorylation of these paralogs is still largely unknown. Here, we employed GSK3α/β phosphorylation-resistant mouse models to explore the role of this inhibitory phosphorylation in regulating platelet activation. Expression of phosphorylation-resistant GSK3α/β reduced thrombin-mediated platelet aggregation, integrin αIIbβ3 activation, and α-granule secretion, whereas platelet responses to the GPVI agonist collagen-related peptide (CRP-XL) were significantly enhanced. GSK3 single knock-in lines revealed that this divergence is due to differential roles of GSK3α and GSK3β phosphorylation in regulating platelet function. Expression of phosphorylation-resistant GSK3α resulted in enhanced GPVI-mediated platelet activation, whereas expression of phosphorylation-resistant GSK3β resulted in a reduction in PAR-mediated platelet activation and impaired in vitro thrombus formation under flow. Interestingly, the latter was normalised in double GSK3α/β KI mice, indicating that GSK3α KI can compensate for the impairment in thrombosis caused by GSK3β KI. In conclusion, our data indicate that GSK3α and GSK3β have differential roles in regulating platelet function.