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Platelets have a fundamental role in mediating hemostasis and thrombosis. However, more recently, a new idea is making headway, highlighting the importance of platelets as significant actors in modulating immune and inflammatory responses. In particular, platelets have an important role in the development of vascular amyloid-b-peptide(ab) deposits, known to play a relevant role in Alzheimer's disease (AD) through accumulation and deposition within the frontal cortex and hippocampus in the brain. The involvement of platelets in the pathogenesis of AD opens up the highly attractive possibility of applying antiplatelet therapy for the treatment and/or prevention of AD, but conclusive results are scarce. Even less is known about the potential role of platelets in mild cognitive impairment (MCI). The aim to this brief review is to summarize current knowledge on this topic and to introduce the new perspectives on the possible role of platelet activation as therapeutic target both in AD and MCI.

Nonalcoholic fatty liver disease (NAFLD) is the main cause of chronic liver disease and ranges from simple steatosis to nonalcoholic steatohepatitis. Recently, a platelet role in NAFLD pathogenesis and progression has been reported in mouse models and in patients. We investigated whether platelets are involved in liver and systemic inflammation processes in NAFLD. In this exploratory study we recruited 24 consecutive patients with biopsy-proven diagnosis of NAFLD and 17 healthy volunteers. We measured plasma levels of inflammatory markers by ELISA. We investigated hemostatic and inflammatory transcripts in circulating platelets and leukocytes from NAFLD patients. We analyzed platelet and neutrophil extracellular traps (NET) accumulations in liver sinusoids using CD42 and H3 citrullinated histones immunohistochemical staining on liver biopsies. NAFLD patients had increased inflammation markers and lipolysaccharides plasma levels. We found significant increase of inflammatory transcripts in circulating platelets and not in leukocytes of NAFLD subjects compared with healthy controls. We demonstrated increased intrahepatic platelet accumulation that correlated with NAFLD activity score (NAS) score and intrahepatic neutrophil extracellular traps (NET) formation in liver biopsies of NAFLD patients. NET formation was higher in livers with higher NAS and inflammation scores. The presence of low-grade systemic inflammation and proinflammatory changes of circulating platelets indicate that platelets participate on systemic inflammatory changes associated with NAFLD. Liver platelet accumulation and liver NET formation, together with low-grade endotoxemia, suggest that platelets may act to protect the liver from invading microorganisms by favoring local NET formation., (Copyright © 2020 Elsevier Inc. All rights reserved.)

The bioactive lipid mediator sphingosine 1-phosphate (S1P) was recently assigned critical roles in platelet biology: whereas S1P 1 receptor-mediated S1P gradient sensing was reported to be essential for directing proplatelet extensions from megakaryocytes (MKs) toward bone marrow sinusoids, MK sphingosine kinase 2 (Sphk2)-derived S1P was reported to further promote platelet shedding through receptor-independent intracellular actions, and platelet aggregation through S1P 1 Yet clinical use of S1P pathway modulators including fingolimod has not been associated with risk of bleeding or thrombosis. We therefore revisited the role of S1P in platelet biology in mice. Surprisingly, no reduction in platelet counts was observed when the vascular S1P gradient was ablated by impairing S1P provision to plasma or S1P degradation in interstitial fluids, nor when gradient sensing was impaired by S1pr1 deletion selectively in MKs. Moreover, S1P 1 expression and signaling were both undetectable in mature MKs in situ, and MK S1pr1 deletion did not affect platelet aggregation or spreading. When S1pr1 deletion was induced in hematopoietic progenitor cells, platelet counts were instead significantly elevated. Isolated global Sphk2 deficiency was associated with thrombocytopenia, but this was not replicated by MK-restricted Sphk2 deletion and was reversed by compound deletion of either Sphk1 or S1pr2 , suggesting that this phenotype arises from increased S1P export and S1P 2 activation secondary to redistribution of sphingosine to Sphk1. Consistent with clinical observations, we thus observe no essential role for S1P 1 in facilitating platelet production or activation. Instead, S1P restricts megakaryopoiesis through S1P 1 , and can further suppress thrombopoiesis through S1P 2 when aberrantly secreted in the hematopoietic niche.

The Gray Platelet Syndrome (GPS) is a rare inherited bleeding disorder characterized by deficiency of platelet α-granules, macrothrombocytopenia and marrow fibrosis. The autosomal recessive form of GPS is linked to loss of function mutations in NBEAL2, which is predicted to regulate granule trafficking in megakaryocytes, the platelet progenitors. We report the first analysis of cultured megakaryocytes from GPS patients with NBEAL2 mutations. Megakaryocytes cultured from peripheral blood or bone marrow hematopoietic progenitor cells from four patients were used to investigate megakaryopoiesis, megakaryocyte morphology and platelet formation. In vitro differentiation of megakaryocytes was normal, whereas we observed deficiency of megakaryocyte α-granule proteins and emperipolesis. Importantly, we first demonstrated that platelet formation by GPS megakaryocytes was severely affected, a defect which might be the major cause of thrombocytopenia in patients. These results demonstrate that cultured megakaryocytes from GPS patients provide a valuable model to understand the pathogenesis of GPS in humans.

The gray platelet syndrome (GPS) is a rare congenital platelet disorder characterized by mild to moderate bleeding diathesis, macrothrombocytopenia and lack of azurophilic α-granules in platelets. Some platelet and megakaryocyte (MK) abnormalities have been described, but confirmative studies of the defects in larger patient cohorts have not been undertaken. We studied platelet function and bone marrow (BM) features in five GPS patients with NBEAL2 autosomal recessive mutations from four unrelated families. In 3/3 patients, we observed a defect in platelet responses to protease-activated receptor (PAR)1-activating peptide as the most consistent finding, either isolated or combined to defective responses to other agonists. A reduction of PAR1 receptors with normal expression of major glycoproteins on the platelet surface was also found. Thrombin-induced fibrinogen binding to platelets was severely impaired in 2/2 patients. In 4/4 patients, the BM biopsy showed fibrosis (grade 2-3) and extensive emperipolesis, with many (36-65%) MKs containing 2-4 leukocytes engulfed within the cytoplasm. Reduced immunolabeling for platelet factor 4 together with normal immunolabeling for CD63 in MKs of two patients demonstrated that GPS MKs display an alpha granule-specific defect. Increased immunolabeling for P-selectin and decreased immunolabeling for PAR1, PAR4 and c-MPL were also observed in MKs of two patients. Marked emperipolesis, specific defect of MK alpha-granule content and defect of PAR1-mediated platelet responses are present in all GPS patients that we could study in detail. These results help to further characterize the disease.

Abnormalities of platelet size are one of the distinguishing features of inherited thrombocytopenias (ITs), and evaluation of blood films is recommended as an essential step for differential diagnosis of these disorders. Nevertheless, what we presently know about this subject is derived mainly from anecdotal evidence. To improve knowledge in this field, we evaluated platelet size on blood films obtained from 376 patients with all 19 forms of IT identified so far and found that these conditions differ not only in mean platelet diameter, but also in platelet diameter distribution width and the percentage of platelets with increased or reduced diameters. On the basis of these findings, we propose a new classification of ITs according to platelet size. It distinguishes forms with giant platelets, with large platelets, with normal or slightly increased platelet size, and with normal or slightly decreased platelet size. We also measured platelet diameters in 87 patients with immune thrombocytopenia and identified cutoff values for mean platelet diameter and the percentage of platelets with increased or reduced size that have good diagnostic accuracy in differentiating ITs with giant platelets and with normal or slightly decreased platelet size from immune thrombocytopenia and all other forms of IT., (© 2014 by The American Society of Hematology.)

The gray platelet syndrome is a rare inherited bleeding disorder characterized by macrothrombocytopenia and deficiency of alpha (α)-granules in platelets. The genetic defect responsible for gray platelet syndrome was recently identified in biallelic mutations in the NBEAL2 gene. We studied 11 consecutive families with inherited macrothrombocytopenia of unknown origin and α-granule deficiency. All of them underwent NBEAL2 DNA sequencing and evaluation of the platelet phenotype, including a systematic assessment of the α-granule content by immunofluorescence analysis for α-granule secretory proteins. We identified 9 novel mutations hitting the two alleles of NBEAL2 in 4 probands. They included missense, nonsense and frameshift mutations, as well as nucleotide substitutions that altered the splicing mechanisms as determined at the RNA level. All the individuals with NBEAL2 biallelic mutations showed almost complete absence of platelet α-granules. Interestingly, the 13 individuals assumed to be asymptomatic because carriers of a mutated allele had platelet macrocytosis and significant reduction of the α-granule content. However, they were not thrombocytopenic. In the remaining 7 probands, we did not identify any NBEAL2 alterations, suggesting that other genetic defect(s) are responsible for their platelet phenotype. Of note, these patients were characterized by a lower severity of the α-granule deficiency than individuals with two NBEAL2 mutated alleles. Our data extend the spectrum of mutations responsible for gray platelet syndrome and demonstrate that macrothrombocytopenia with α-granule deficiency is a genetic heterogeneous trait. In terms of practical applications, the screening of NBEAL2 is worthwhile only in patients with macrothrombocytopenia and severe reduction of the α-granules. Finally, individuals carrying one NBEAL2 mutated allele have mild laboratory abnormalities, suggesting that even haploinsufficiency has an effect on platelet phenotype.

Platelet activation by thrombin is relevant to arterial thrombosis, therefore it is an attractive target for the development of new antithrombotic drugs. In the 1970s the platelet membrane complex glycoprotein (GP) Ib-V-IX was shown to have a high affinity binding site for thrombin on GPIbα and a substrate cleaved by thrombin, GPV. For several years it was considered to be involved in platelet activation by thrombin. The discovery of the protease activated receptors (PARs) in 1991 was a major breakthrough in the field. The first member of this family of receptors to be discovered was PAR1, a seven transmembrane G-protein coupled receptor which, upon cleavage by thrombin, unmasks a new amino-terminus able to bind intramolecularly to PAR1 itself thus inducing signaling. On human platelets PAR1 and, later PAR4, were demonstrated to mediate most of the platelet responses to thrombin. However, after the discovery of PARs, different groups demonstrated that GPIbα is required to stimulate a full platelet activation by thrombin. A model where thrombin binds to the GPIb receptor prior to proteolysis of the PAR receptors was supported by several lines of evidence. A role for GPV as inhibitor of GPIbα signaling has been shown by using GPV knock-out mice. Crystallographic data suggested that thrombin bound to GPIbα might be able to interact with other GPIbα molecules on the same or other platelets, shedding light on a new role for thrombin binding to GPIbα. Finally, anti-PAR1 molecules were developed which are now in phase II and III clinical studies as antithrombotic drugs., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Background: We report a novel case of gray platelet syndrome (GPS). A 14-year-old boy had bleeding diathesis, mild thrombocytopenia, giant platelets with severe defect of alpha-granule secretory proteins, myelofibrosis and splenomegaly., Methods and Results: Platelet function studies showed a marked reduction of aggregation and Ca(2+) mobilization by thrombin, protease-activated receptor 1 (PAR1)-activating peptide (AP) and PAR4-AP, PAR1 expression at 55% of normal levels, and a more than two hundred fold reduction of in vitro whole-blood thromboxane B(2) (TXB(2)) production. Sequencing of coding regions of the PAR1 gene failed to show abnormalities. This patient was initially classified as a sporadic case of GPS, as electron microscopy failed to identify giant platelets and/or alpha-granule deficiency in his relatives. However, further studies on the father and three other relatives showed a relative lack of platelet alpha-granule proteins by immunofluorescence microscopy, a defective platelet response to PAR4-AP, and severely reduced in vitro whole-blood TXB(2) production. On this basis, we suggest that in this family, GPS was transmitted in a dominant fashion with highly variable penetrance., Conclusions: Our study suggests that current diagnostic criteria fail to identify some patients with a mild GPS phenotype and that such patients might be identified by the methods cited above. It also better characterizes the pathogenesis of defective platelet responses to thrombin, and raises interesting questions on the correlation between abnormal PAR function and the lack of alpha-granule content in GPS.

An experimental strategy based on solution viscosity perturbation allowed us to study the energetics of amide-substrates, p-aminobenzamidine (p-ABZ) and proflavin binding to the catalytic site of two proteolyzed forms of alpha-thrombin, i.e. zeta- and gamma T-thrombin. These thrombin derivatives are cleaved at the Leu144-Gly150 loop and at the fibrinogen recognition exosite (FRS), respectively. A phenomenological analysis of thermodynamic data showed that the amide substrates and p-ABZ interactions with zeta-thrombin were respectively, associated with a chemical compensation (i.e. the linear relationship between entropy and enthalpy of binding) and a hydrophobic phenomenon (i.e. a change in the standard heat capacity). The latter was slightly lower than that previously observed for a alpha-thrombin (0.78 +/- 0.25 versus 1.01 +/- 0.17 kcal/mol K). Both phenomenon were absent in gamma T-thrombin. The interaction of a alpha-, zeta- and gamma T-thrombin with macromolecular substrates that "bridge-bind" to both the catalytic site (CS) and fibrinogen recognition exosite (FRS), such as fibrinogen and the cleavable platelet receptor (CPR), was also evaluated. These interactions were studied by following fibrinopeptide A (FpA) release and by measuring intraplatelet Ca2+ changes induced by thrombin-CPR interaction. It was found that the free energy of activation (RT ln Kcat/Km) for both fibrinogen and CPR hydrolysis followed the same hierarchy, i.e. alpha > zeta > gamma. Moreover, the values of delta Cp for alpha-, zeta- and gamma T-thrombin interaction with p-ABZ were found to be linearly correlated to the free energy of activation for both fibrinogen and CPR cleavage. In conclusion, these data demonstrate that: (1) the Leu144-Gly150 loop and the FRS are both involved in the conformational transition linked to the binding of p-aminobenzamidine to the thrombin active site; (2) the extent of thrombin's capacity to undergo conformational transitions in alpha-, zeta- and gamma T forms is positively correlated to the free energy of activation for hydrolysis of macromolecular substrates interacting with both the catalytic domain and the FRS.

Several "in vitro" and "in vivo" studies indicate that heparin administration may affect platelet function. In this study we investigated the effects of prophylactic heparin on thromboxane (Tx)A2 biosynthesis "in vivo", as assessed by the urinary excretion of major enzymatic metabolites 11-dehydro-TxB2 and 2,3-dinor-TxB2. Twenty-four patients who were candidates for cholecystectomy because of uncomplicated lithiasis were randomly assigned to receive placebo, unfractionated heparin, low molecular weight heparin or unfractionated heparin plus 100 mg aspirin. Measurements of daily excretion of Tx metabolites were performed before and during the treatment. In the groups assigned to placebo and to low molecular weight heparin there was no statistically significant modification of Tx metabolite excretion while patients receiving unfractionated heparin had a significant increase of both metabolites (11-dehydro-TxB2: 3844 +/- 1388 vs 2092 +/- 777, p < 0.05; 2,3-dinor-TxB2: 2737 +/- 808 vs 1535 +/- 771 pg/mg creatinine, p < 0.05). In patients randomized to receive low-dose aspirin plus unfractionated heparin the excretion of the two metabolites was largely suppressed thus suggesting that platelets are the primary source of enhanced thromboxane biosynthesis associated with heparin administration. These data indicate that unfractionated heparin causes platelet activation "in vivo" and suggest that the use of low molecular weight heparin may avoid this complication.

The binding of fibrinogen to platelets occurs according to the law of mass action. The platelet receptor binds reversibly a single fibrinogen molecule and undergoes a conformational transition between two allosteric states, T and R, that differ in their affinity for fibrinogen. The equilibrium between the two forms is shifted by ADP toward the R (high-affinity) state, thus promoting the aggregation process. This model opens the way to consideration of allosteric modulation of the binding of fibrinogen to its platelet receptor.

Patients with myeloproliferative disorders (MPD) are known to have some abnormalities of platelet glycoproteins (Gp). Quantitative changes of the Gp Ib, IIb-IIIa, and/or their glucidic content have been reported. Since the Gp IIb-IIIa complex plays a major role in fibrinogen binding by activated platelets, we measured the platelet fibrinogen affinity in nine patients with polycythemia vera (PV) and one subject with chronic myeloid leukemia (CML) by the aggregometric method of Marguerie. In all patients the Kd of the platelet fibrinogen reaction was significantly decreased as compared to controls, with evidence in two cases with PV of a heterogeneity of platelet-fibrinogen receptor sites. The measurement of 125I-labeled fibrinogen-platelet binding, performed in seven patients (five PV and two CML), showed receptor populations with increased (Kd1 = 0.58 + 0.3 X 10(7) mol/L) and normal affinity (Kd2 = 5.12 + 3.1 X 10(7) mol/L). These results demonstrate a heterogeneity of platelet-fibrinogen receptors in these patients and may explain the thrombotic diathesis of MPD subjects.

Binding of ADP to platelets enhances the binding of fibrinogen to Gp IIb-IIIa, the specific platelet receptor for adhesive proteins. The linkage between ADP and fibrinogen binding is indirect since ADP does not bind to the same receptor as fibrinogen. We have recently proposed that a third component, once affected by ADP binding, induces a conformational transition of the fibrinogen receptor from the low to the high affinity state, which is responsible for platelet aggregation [De Cristofaro, R., Landolfi, R., Castagnola, M., De Candia, E., Di Cera, E., & Wyman, J. (1988) Proc. Natl. Acad. Sci. USA 85, 8473-8476]. In the present study we provide evidence that this component should be identified with the platelet Na+/H+ antiport. Inhibition of the antiport by pharmacological agents such as amiloride, or else by decreasing extracellular Na+, results in a marked decrease of fibrinogen binding to platelets.

Abstract: The optimal management of bleeding or its prophylaxis in patients with disorders of platelet count or function is controversial. The bleeding diathesis of these patients is usually mild to moderate: therefore, transfusion of platelet concentrates may be inappropriate, as potential adverse effects might outweigh its benefit. The availability of several anti-hemorrhagic drugs further compounds this problem, mainly because the efficacy/suitability of the various treatment options in different clinical manifestations is not well defined. In these guidelines, promoted by the Italian Society for Studies on Haemostasis and Thrombosis (Società Italiana per lo Studio dell''Emostasi e della Trombosi [SISET]), we aim at offering the best available evidence to help the physicians involved in the management of patients with disorders of platelet count or function. Literature review and appraisal of available evidence are discussed for different clinical settings and for different available treatments, including platelet concentrates (PC), recombinant activated factor VII, desmopressin, antifibrinolytics, aprotinin and local hemostatic agents. [Copyright &y& Elsevier]