Mitochondrial ATP generation in stimulated platelets is essential for granule secretion but dispensable for aggregation and procoagulant activity

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Introducing a Novel Biophysical Platelet Function Panel to Investigate Disorders of Primary Hemostasis and Bleeding of Unknown Cause

A subset of patients with chronic bleeding remain undiagnosed even after extensive diagnostic evaluation are labeled as "bleeding of unknown cause" (BUC). The key barrier to treating these patients is that they have a clinical bleeding tendency in the presence of normal diagnostic tests, and optimal methods for monitoring and treating patients with BUC remain unknown. While patients with BUC have symptoms of a primary hemostatic disorder, there is no diagnostic test or biomarker that can accurately identify which patients are at risk for bleeding such as those with mild Von Willebrand Disease (VWD) which comprise a broad spectrum of patients with varying degrees of bleeding. In order to fill this diagnostic gap in disorders of primary hemostasis, there is a clinical need for more assays of platelet function. To that end, we have engineered multiple new biophysical assays to assess disorders of primary hemostasis and apply this panel of platelet function testing to potentially define new bleeding disorders, characterize platelet phenotypes in patients with BUC, and refine the definition of mild VWD. Our panel of platelet function tests (Fig 1) collectively enables us to simultaneously assess different facets of primary hemostasis from the microscopic level of single-platelet physiology to hemostatic plug formation, thereby capturing various aspects of platelet function with a single blood sample. Our platelet function panel ranges from platelet adhesion and bulk clot contraction assays to spatially-regulated platelet granule secretion assay, single-platelet contraction cytometry, microfluidics, and a microengineered vascularized bleeding model. As such, we are leveraging these biophysical assays to correlate platelet function with bleeding phenotype severity and establish the dynamic range of this diagnostic panel. We have now established that our assays can be utilized to study blood samples from patients with disorders including hemophilia A, Hermansky-Pudlak Syndrome, FLI-1 mutation, and sickle cell disease among others (Fig 2), demonstrating the clinical utility of our platelet function panel. Our panel can also be used to assess the effects of novel therapeutics on different aspects of platelet function simultaneously. To investigate how crizanlizumab (p-selectin inhibitor) affects hemostatic plug formation, healthy human blood was treated with crizanlizumab. Platelet α-granule secretion enables exposure of P-selectin, and with crizanlizumab we observed restricted platelet filopodial extension and diminished α-granule exocytosis, and an overall decrease in adhered platelets to the fibrinogen micropattern (Fig 3A). The adhesion assay demonstrated a decrease in spreading and adhesion of platelets to collagen and fibrinogen with treatment (Fig 3B). Using the bleeding model, hemostasis was achieved within the normal established range and platelets contracted normally. This suggests that p-selectin has a limited role in the setting of minor injury. Utilizing the bulk contraction assay, we exhibited increased contraction early in clot formation, however over time the treated platelets contracted similarly to the control (Fig 3C). Interestingly, the effect of crizanlizumab-induced restriction of filopodial extension did not correlate with impaired bulk clot contraction or time to form hemostatic plug. Our work suggests crizanlizumab affects platelet spreading at the single-cell level but does not impair platelet function in achieving primary hemostasis at the whole blood level. Here we demonstrate the translational utility of our platelet function panel in providing a deeper understanding of platelet biophysics as it relates to hematologic conditions, with implications for investigation to include pharmaceutical applications. The versatility of this novel panel in capturing platelet function from single-platelet contraction to providing in vitro models with the bleeding device provides multiple dimensions to platelet investigation for primary hemostatic disorders and BUC that have not yet been elucidated. Ongoing research is being conducted using our comprehensive platelet function panel to investigate platelet properties in BUC and mild VWD and correlate these biophysics with bleeding phenotypes. Using this approach we aim to provide novel diagnostic testing with clinical relevance for disorders that have been incompletely characterized until now. Figure 1 Figure 1. Disclosures Meeks: National Institutes of Health: Research Funding; Hemophilia of Georgia: Research Funding; National Hemophilia Foundation: Research Funding; Spark Therapeutics: Consultancy; Sangamo Therapeutics: Consultancy; Pfizer: Consultancy; Sanofi: Consultancy; CSL Behring: Consultancy; Genentech: Consultancy; Takeda: Consultancy. Lam: Sanguina, Inc.: Current holder of individual stocks in a privately-held company.

The Predominant Role of Arrestin3 in General GPCR Desensitization in Platelets

Arrestins in concert with GPCR kinases (GRKs) function in G protein-coupled receptor (GPCR) desensitization in various cells. Therefore, we characterized the functional differences of arrestin3 versus arrestin2 in the regulation of GPCR signaling and its desensitization in platelets using mice lacking arrestin3 and arrestin2. In contrast to arrestin2, platelet aggregation and dense granule secretion induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in arrestin3-deficient platelets compared to wild-type (WT) platelets, while non-GPCR agonist CRP-induced platelet aggregation and secretion were not affected. Surprisingly, in contrast to GRK6, platelet aggregation induced by the co-stimulation of serotonin and epinephrine was significantly potentiated in arrestin3-deficient platelets, suggesting the central role of arrestin3 in general GPCR desensitization in platelets. In addition, the second challenge of ADP and AYPGKF restored platelet aggregation in arrestin3-deficient platelets but failed to do so in WT and arrestin2-deficient platelets, confirming that arrestin3 contributes to GPCR desensitization. Furthermore, ADP- and AYPGKF-induced Akt and ERK phosphorylation were significantly increased in arrestin3-deficient platelets. Finally, we found that arrestin3 is critical for thrombus formation in vivo. In conclusion, arrestin3, not arrestin2, plays a central role in the regulation of platelet functional responses and thrombus formation through general GPCR desensitization in platelets.

Critical role of peroxisome proliferator-activated receptor α in promoting platelet hyperreactivity and thrombosis under hyperlipidemia

Platelet hyperreactivity and increased atherothrombotic risk are specifically associated with dyslipidemia. Peroxisome proliferator-activated receptor alpha (PPARα) is an important regulator of lipid metabolism. It was suggested to affect both thrombosis and hemostasis, yet the underlying mechanisms are not well understood. In this study, the role and mechanism of PPARα in platelet activation and thrombosis related to dyslipidemia were examined. Employing mice with deletion of PPARα (Pparα -/-), we demonstrated that PPARα is required for platelet activation and thrombus formation. The effect of PPARα is critically dependent on platelet dense granule secretion, and is contributed by p38MAPK/Akt, fatty acid β- oxidation, and NAD(P)H oxidase (NOX) pathways. Importantly, PPARα and the associated pathways mediated a prothrombotic state induced by high-fat diet (HFD) and platelet hyperactivity provoked by oxidized low density lipoproteins (oxLDL). Platelet reactivities were positively correlated with the expression levels of PPARα, as revealed by data from wild-type (WT), chimeric (Pparα +/-), and Pparα -/- mice. This positive correlation was recapitulated in platelets from hyperlipidemic patients. In a lipid-treated megakaryocytic cell line, lipid-induced reactive oxygen species (ROS)-NF-κB pathway was revealed to upregulate platelet PPARα in hyperlipidemia. These data suggested platelet PPARα critically mediates platelet activation and contributes to prothrombotic status under hyperlipidemia.

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.

Early platelet dysfunction in patients receiving extracorporeal membrane oxygenation is associated with mortality

Extracorporeal membrane oxygenation (ECMO) is used for patients with cardiopulmonary failure and is associated with severe bleeding and poor outcome. Platelet dysfunction may be a contributing factor. The aim of this prospective observational study was to characterize platelet dysfunction and its relation to outcome in ECMO patients. Blood was sampled from thirty ECMO patients at three timepoints. Expression of CD62P, CD63, activated GPIIb/IIIa, GPVI, GPIbα and formation platelet-leukocyte aggregates (PLA) were analyzed at rest and in response to stimulation. Delta granule storage-pool deficiency and secretion defects were also investigated. Fifteen healthy volunteers and ten patients with coronary artery disease served as controls. Results were also compared between survivors and non-survivors. Compared to controls, expression of platelet surface markers, delta granule secretion and formation of PLA was reduced, particularly in response to stimulation. Baseline CD63 expression was higher and activated GPIIb/IIIa expression in response to stimulation was lower in non-survivors on day 1 of ECMO. Logistic regression analysis revealed that these markers were associated with mortality. In conclusion, platelets from ECMO patients are severely dysfunctional predisposing patients to bleeding complications and poor outcome. Platelet dysfunction on day 1 of ECMO detected by the platelet surface markers CD63 and activated GPIIb/IIIa is associated with mortality. CD63 and activated GPIIb/IIIa may therefore serve as novel prognostic biomarkers, but future studies are required to determine their true potential.

CD226 Is Required to Maintain Megakaryocytes/Platelets Homeostasis in the Treatment of Knee Osteoarthritis With Platelet-Rich Plasma in Mice

Platelet-rich plasma (PRP) is a platelet-based application used to treat osteoarthritis (OA) clinically. The co-stimulatory molecule CD226 is expressed in T cells, NK cells, and also platelets. However, exact effects of CD226 on platelets and whether its expression level influences PRP efficacy are largely unknown. Here, CD226fl/flPF4-Cre mice were obtained from mating CD226 fl/fl mice with PF4-Cre mice. Blood samples and washed platelets were collected from the mice eyeballs to undergo routine blood tests and transmission electron microscopy. Differentially expressed proteins were detected by iTRAQ-based proteomics analysis. Animal OA models were established through surgical destabilization of the medial meniscus (DMM) for C57BL/6 wildtype mice, followed by PRP injection to evaluate the effects of platelet CD226 on PRP efficacy. The results showed that deletion of platelet CD226 increased the number of megakaryocytes (MKs) in bone marrow (BM) but reduced MKs in spleen, combined with significantly decreased platelet amounts, α-granule secretion, and reduced immature platelets; indicating that absence of platelet CD226 may disrupt MK/platelet homeostasis and arrested platelet release from MKs. Sequencing analysis showed abnormal ribosomal functions and much downregulated proteins in the absence of platelet CD226. Autophagy-related proteins were also reduced in the CD226-absent MKs/platelets. Moreover, deletion of platelet CD226 diminished the protective effects of PRP on DMM-induced cartilage lesions in mice, and PDGF restored it. Therefore, deficiency of platelet CD226 inhibited platelet maturation, secretion, and normal ribosomal functions, which may lead to depressed PRP efficacy on OA, suggesting that CD226 is required to regulate platelet growth, functions, and its application.

Tetrahydrocurcumin Downregulates MAPKs/cPLA2 Signaling and Attenuates Platelet Thromboxane A2 Generation, Granule Secretion, and Thrombus Growth

Platelet granule secretion plays a key role in atherothrombosis. Curcumin, a natural polyphenol compound derived from turmeric, exerts multiple biological activities. The current study sought to investigate the efficacy of tetrahydrocurcumin (THC, the major active metabolite of curcumin) on platelet granule secretion in vitro and thrombus formation in vivo. We found that THC significantly attenuated agonist-induced granule secretion in human gel-filtered platelets in vitro, including CD62P and CD63 expression and platelet factor 4, CCL5, and adenosine triphosphate release. These inhibitory effects of THC were partially mediated by the attenuation of cytosolic phospholipase A2 (cPLA2) phosphorylation, leading to a decrease in thromboxane A2 (TxA2) generation. Moreover, the MAPK (Erk1/2, JNK1/2, and p38 MAPK) signaling pathways were downregulated by THC treatment, resulting in reduced cPLA2 activation, TxA2 generation, and granule secretion. Additionally, THC and curcumin attenuated murine thrombus growth in a FeCl3-induced mesenteric arteriole thrombosis model in C57BL/6J mice without prolonging the tail bleeding time. THC exerted more potent inhibitory effects on thrombosis formation than curcumin. Through blocking cyclooxygenase-1 activity and thus inhibiting platelet TxA2 synthesis and granule secretion with aspirin, we found that THC did not further decrease the inhibitory effects of aspirin on thrombosis formation. Thus, through inhibiting MAPKs/cPLA2 signaling, and attenuating platelet TxA2 generation, granule secretion, and thrombus formation, THC may be a potent cardioprotective agent.

Platelet dysfunction in platelet-type von Willebrand disease due to the constitutive triggering of the Lyn-PECAM1 inhibitory pathway

Platelet-type von Willebrand disease (PT-VWD) is an inherited platelet disorder characterized by macrothrombocytopenia and mucocutaneous bleeding, of variable severity, due to gain-of-function variants of GP1BA conferring to glycoprotein Ibα (GPIbα) enhanced affinity for von Willebrand factor (VWF). The bleeding tendency is conventionally attributed to thrombocytopenia and large VWF-multimers depletion. Some clues, however, suggest that platelet dysfunction may contribute to the bleeding phenotype but no information on its characteristics and causes are available. Aim of the present study was to characterize platelet dysfunction in PT-VWD and shed light on its mechanism. Platelets from a PT-VWD patient carrying the p.M239V variant and from PT-VWD mice carrying the p.G233V variant showed a remarkable platelet function defect, with impaired aggregation, defective granule secretion and reduced adhesion under static and flow conditions. VWF-binding to GPIbα is known to trigger intracellular signaling involving Src-family kinases (SFKs). We found that constitutive phosphorylation of the platelet SFK Lyn induces a negative-feedback loop downregulating platelet activation through phosphorylation of PECAM1 on Tyr686 and that this is triggered by the constitutive binding of VWF to GPIbα binding. These data show for the first time that the abnormal triggering of inhibitory signals mediated by Lyn and PECAM1 may lead to platelet dysfunction.In conclusion, our study unravels the mechanism of platelet dysfunction in PT-VWD caused by deranged inhibitory signaling triggered by the constitutive binding of VWF to GPIbα which may significantly contribute to the bleeding phenotype of these patients.