ADP receptor P2Y12 is the capstone of the cross-talk between Ca2+ mobilization pathways dependent on Ca2+ ATPases sarcoplasmic/endoplasmic reticulum type 3 and type 2b in platelets

Background

Blood platelet Ca²⁺ stores are regulated by 2 Ca²⁺-ATPases (SERCA2b and SERCA3). On thrombin stimulation, nicotinic acid adenosine dinucleotide phosphate mobilizes SERCA3-dependent stores, inducing early adenosine 5'-diphosphate (ADP) secretion, potentiating later SERCA2b-dependent secretion.

Objectives

The aim of this study was to identify which ADP P2 purinergic receptor (P2Y1 and/or P2Y12) is(are) involved in the amplification of platelet secretion dependent on the SERCA3-dependent Ca²⁺ mobilization pathway (SERCA3 stores mobilization) as triggered by low concentration of thrombin.

Methods

The study used the pharmacologic antagonists MRS2719 and AR-C69931MX, of the P2Y1 and P2Y12, respectively, as well as Serca3 ^-/- mice and mice exhibiting platelet lineage-specific inactivation of the P2Y1 or P2Y12 genes.

Results

We found that in mouse platelets, pharmacological blockade or gene inactivation of P2Y12 but not of P2Y1 led to a marked inhibition of ADP secretion after platelet stimulation with low concentration of thrombin. Likewise, in human platelets, pharmacological inhibition of P2Y12 but not of P2Y1 alters amplification of thrombin-elicited secretion through SERCA2b stores mobilization. Finally, we show that early SERCA3 stores secretion of ADP is a dense granule secretion, based on parallel adenosine triphosphate and serotonin early secretion. Furthermore, early secretion involves a single granule, based on the amount of adenosine triphosphate released.

Conclusion

Altogether, these results show that at low concentrations of thrombin, SERCA3- and SERCA2b-dependent Ca²⁺ mobilization pathways cross-talk via ADP and activation of the P2Y12, and not the P2Y1 ADP receptor. The relevance in hemostasis of the coupling of the SERCA3 and the SERCA2b pathways is reviewed.

NAADP/SERCA3-Dependent Ca2+ Stores Pathway Specifically Controls Early Autocrine ADP Secretion Potentiating Platelet Activation

RATIONALE

Ca²⁺ signaling is a key and ubiquitous actor of cell organization and its modulation controls many cellular responses. SERCAs (sarco-endoplasmic reticulum Ca²⁺-ATPases) pump Ca²⁺ into internal stores that play a major role in the cytosolic Ca²⁺ concentration rise upon cell activation. Platelets exhibit 2 types of SERCAs, SERCA2b and SERCA3 (SERCA3 deficient mice), which may exert specific roles, yet ill-defined. We have recently shown that Ca²⁺ mobilization from SERCA3-dependent stores was required for full platelet activation in weak stimulation conditions.

OBJECTIVE

To uncover the signaling mechanisms associated with Ca²⁺ mobilization from SERCA3-dependent stores leading to ADP secretion.

METHODS AND RESULTS

Using platelets from wild-type or Serca3-deficient mice, we demonstrated that an early (within 5-10 s following stimulation) secretion of ADP specifically dependent on SERCA3 stored Ca²⁺ is exclusively mobilized by nicotinic acid adenosine dinucleotide-phosphate (NAADP): both Ca²⁺ mobilization from SERCA3-dependent stores and primary ADP secretion are blocked by the NAADP receptor antagonist Ned-19, and reciprocally both are stimulated by permeant NAADP. In contrast, Ca²⁺ mobilization from SERCA3-dependent stores and primary ADP secretion were unaffected by inhibition of the production of IP3 (inositol-1,4,5-trisphosphate) by phospholipase-C and accordingly were not stimulated by permeant IP3.

CONCLUSIONS

Upon activation, an NAADP/SERCA3 Ca²⁺ mobilization pathway initiates an early ADP secretion, potentiating platelet activation, and a secondary wave of ADP secretion driven by both an IP3/SERCA2b-dependent Ca²⁺ stores pathway and the NAADP/SERCA3 pathway. This does not exclude that Ca²⁺ mobilized from SERCA3 stores may also enhance platelet global reactivity to agonists. Because of its modulating effect on platelet activation, this NAADP-SERCA3 pathway may be a relevant target for anti-thrombotic therapy. Graphic Abstract: A graphic abstract is available for this article.

Relevance of platelet desialylation and thrombocytopenia in type 2B von Willebrand disease: preclinical and clinical evidence

Patients with type 2B von Willebrand disease (vWD) (caused by gain-of-function mutations in the gene coding for von Willebrand factor) display bleeding to a variable extent and, in some cases, thrombocytopenia. There are several underlying causes of thrombocytopenia in type 2B vWD. It was recently suggested that desialylation-mediated platelet clearance leads to thrombocytopenia in this disease. However, this hypothesis has not been tested in vivo. The relationship between platelet desialylation and the platelet count was probed in 36 patients with type 2B von Willebrand disease (p.R1306Q, p.R1341Q, and p.V1316M mutations) and in a mouse model carrying the severe p.V1316M mutation (the 2B mouse). We observed abnormally high elevated levels of platelet desialylation in both patients with the p.V1316M mutation and the 2B mice. In vitro, we demonstrated that 2B p.V1316M/von Willebrand factor induced more desialylation of normal platelets than wild-type von Willebrand factor did. Furthermore, we found that N-glycans were desialylated and we identified αIIb and β3 as desialylation targets. Treatment of 2B mice with sialidase inhibitors (which correct platelet desialylation) was not associated with the recovery of a normal platelet count. Lastly, we demonstrated that a critical platelet desialylation threshold (not achieved in either 2B patients or 2B mice) was required to induce thrombocytopenia in vivo. In conclusion, in type 2B vWD, platelet desialylation has a minor role and is not sufficient to mediate thrombocytopenia.

Platelet Functions are Decreased in Obesity and Restored after Weight Loss: Evidence for a Role of the SERCA3-Dependent ADP Secretion Pathway

In obesity, platelets are described as hyperactive, mainly based on increased platelet size and presence of pro-thrombotic plasmatic molecules. We explored platelet functions, including calcium signalling in obesity, and the effect of weight loss. We included 40 obese patients (women with body mass index [BMI] of ≥ 35 kg/m²) who were to undergo gastric bypass surgery and 40 healthy lean subjects (women with BMI of < 25 kg/m²) as a control group. Approximately 1 year after surgery, the obese patients lost weight (75% had a BMI < 35 kg/m²). They were explored a second time with the same healthy control for the same platelet experiments. Compared with controls, obese patients' platelets displayed reduced sensitivity to thrombin (aggregation EC₅₀ increased by 1.9 ± 0.3-fold, p = 0.005) and a lower Ca²⁺ response (70 ± 7% decrease, p < 10^-4). In 17 pairs of patients, we performed additional experiments: in obese patients' platelets, thrombin-induced αIIbβ3 activation was significantly lower (p = 0.003) and sarco-endoplasmic reticulum Ca²⁺ATPase (SERCA3) expression was decreased (48 ± 6% decrease, p < 10^-4). These differences were abolished after weight loss. Interestingly, pharmacological inhibition of SERCA3 activity in control group's platelets mimicked similar alterations than in obese patients' platelets and was associated with defective adenosine diphosphate (ADP) secretion. Addition of ADP to agonist restored platelet functions in obese patients and in SERCA3-inhibited control platelets (five experiments) confirming the direct involvement of the SERCA3-dependent ADP secretion pathway. This is the first study demonstrating that platelets from obese patients are hypo-reactive, due to a deficiency of SERCA3-dependent ADP secretion. Weight loss restores SERCA3 activity and subsequent calcium signalling, αIIbβ3 activation, platelet aggregation and ADP secretion.

Human Platelet Antigen 3 (HPA-3): Localization of the Determinant of the Alloantibody Leka (HPA-3a) to the C-Terminus of Platelet Glycoprotein IIb Heavy Chain and Contribution of O-Linked Carbohydrates

The human platelet antigen (HPA) 3 system is expressed on GPIIb, one subunit of GPIIb-IIIa, the platelet fibrinogen receptor. It was recently shown that HPA-3 was associated with an Ile843/Ser polymorphism. To investigate further HPA-3 determinant structure, we localized an HPA-3a determinant, recognized by the alloantiserum Leka, within the last 29 amino acids of GPIIbα. This region encompasses the polymorphic Ile843, which, as expected, is substituted into Ser in Leka-negative individuals, as shown by DNA sequence after polymerase chain reaction on platelet RNA. In addition, contribution of glycosylation to the determinant structure was demonstrated since the Leka antigenicity was strongly decreased after specifically removing nonterminal O-linked sugars, but not terminal sialic acids. We have thus refined the localization of an HPA-3a determinant within the last 29 amino acids, including Ile843, of GPIIb heavy chain, and shown that the Leka HPA-3a determinant is dependent, in part, upon the serine-linked carbohydrates adjacent to Ile/Ser843.

Abnormal Tyrosine Phosphorylation Linked to a Defective Interaction between ADP and Its Receptor on Platelets

ADP, a primary stimulus of platelets, binds to one or more populations of receptors on the platelet surface. These receptors are linked to discrete activation pathways. Both G proteins and tyrosine kinases have been implicated in the cellular responses to this agonist. We have studied a patient with a congenital abnormality of ADP-induced platelet aggregation in an effort to gain information on the signalling pathways used by ADP. Immunoblotting with a broadly reactive rabbit antibody recognizing the GTP-binding domain of G protein α-subunits, and with rabbit antibodies specific for Giαl-3, and Gα12 all showed normal reactivity when tested against the patient‘s platelets. The phosphorylation of proteins was studied using an anti-phosphotyrosine MoAb (4G10) and platelets stimulated in a platelet aggregometer with ADP, a thromboxane A2 mimetic (IBOP), TRAP-14-mer peptide and α-thrombin. With normal platelets, a time-dependent phosphorylation of several bands in the 60 to 130 kDa mol. wt. range was observed with all agonists. For the patient, minimal aggregation and little or no phosphorylation of proteins of 80-85 kDa (cortactin), 100-105 kDa and 125-130 kDa were seen in response to ADP. The aggregation and phosphorylation responses were slightly modified in the presence of low doses of thrombin but were normal with high doses. Aggregation and tyrosine phosphorylation were virtually absent with IBOP, a finding reproduced when normal platelets were incubated with IBOP and the CP/CPK ADP scavenging system, thereby underlining the role of ADP in the response to IBOP. Our results show that the ADP receptor pathway deficient in the patient is linked to a selective tyrosine phosphorylation response.

Gain-of-Function Mutation in Filamin A Potentiates Platelet Integrin αIIbβ3 Activation

OBJECTIVE

Dominant mutations of the X-linked filamin A (FLNA) gene are responsible for filaminopathies A, which are rare disorders including brain periventricular nodular heterotopia, congenital intestinal pseudo-obstruction, cardiac valves or skeleton malformations, and often macrothrombocytopenia.

APPROACH AND RESULTS

We studied a male patient with periventricular nodular heterotopia and congenital intestinal pseudo-obstruction, his unique X-linked FLNA allele carrying a stop codon mutation resulting in a 100-amino acid-long FLNa C-terminal extension (NP_001447.2: p.Ter2648SerextTer101). Platelet counts were normal, with few enlarged platelets. FLNa was detectable in all platelets but at 30% of control levels. Surprisingly, all platelet functions were significantly upregulated, including platelet aggregation and secretion, as induced by ADP, collagen, or von Willebrand factor in the presence of ristocetin, as well as thrombus formation in blood flow on a collagen or on a von Willebrand factor matrix. Most importantly, patient platelets stimulated with ADP exhibited a marked increase in α_IIbβ₃ integrin activation and a parallel increase in talin recruitment to β₃, contrasting with normal Rap1 activation. These results are consistent with the mutant FLNa affecting the last step of α_IIbβ₃ activation. Overexpression of mutant FLNa in the HEL megakaryocytic cell line correlated with an increase (compared with wild-type FLNa) in PMA-induced fibrinogen binding to and in talin and kindlin-3 recruitment by α_IIbβ₃.

CONCLUSIONS

Altogether, our results are consistent with a less binding of mutant FLNa to β₃ and the facilitated recruitment of talin by β₃ on platelet stimulation, explaining the increased α_IIbβ₃ activation and the ensuing gain-of-platelet functions.

LIM kinase/cofilin dysregulation promotes macrothrombocytopenia in severe von Willebrand disease-type 2B

von Willebrand disease type 2B (VWD-type 2B) is characterized by gain-of-function mutations of von Willebrand factor (vWF) that enhance its binding to platelet glycoprotein Ibα and alter the protein's multimeric structure. Patients with VWD-type 2B display variable extents of bleeding associated with macrothrombocytopenia and sometimes with thrombopathy. Here, we addressed the molecular mechanism underlying the severe macrothrombocytopenia both in a knockin murine model for VWD-type 2B by introducing the p.V1316M mutation in the murine Vwf gene and in a patient bearing this mutation. We provide evidence of a profound defect in megakaryocyte (MK) function since: (a) the extent of proplatelet formation was drastically decreased in 2B MKs, with thick proplatelet extensions and large swellings; and (b) 2B MKs presented actin disorganization that was controlled by upregulation of the RhoA/LIM kinase (LIMK)/cofilin pathway. In vitro and in vivo inhibition of the LIMK/cofilin signaling pathway rescued actin turnover and restored normal proplatelet formation, platelet count, and platelet size. These data indicate, to our knowledge for the first time, that the severe macrothrombocytopenia in VWD-type 2B p.V1316M is due to an MK dysfunction that originates from a constitutive activation of the RhoA/LIMK/cofilin pathway and actin disorganization. This suggests a potentially new function of vWF during platelet formation that involves regulation of actin dynamics.

Comparison between electronic cigarette refill liquid and nicotine on metabolic parameters in rats

AIMS

Nicotine is known to promote body weight loss and to disturb glucose homeostasis and lipoprotein metabolism. Electronic cigarettes, as a substitute to nicotine, are becoming increasingly popular, although there is no evidence regarding their safety. Considering the dearth of information about e-cigarette toxicity, the present study was designed to compare nicotine alone to e-liquid with or without nicotine on metabolic parameters in Wistar rats.

MAIN METHODS

For this purpose, e-liquid with or without nicotine and nicotine alone (0.5mg/kg of body weight) were administered intra-peritoneally during 28 days.

KEY FINDINGS

Our results show a significant decrease in food and energy intake after nicotine or e-liquid with nicotine exposure, when compared to control or e-liquid without nicotine. Analysis of lipid status identified a significant decrease in cholesterol and LDL levels in e-cigarette groups, suggesting an improvement in lipid profile. Interestingly, e-liquid without nicotine induced hyperglycemia which is negatively correlated to hepatic glycogen level, acting like nicotine alone. Furthermore, an increase in liver biomarkers was observed in all treated groups. qRT-PCR analysis showed GSK3β up-regulation in e-liquid with nicotine as well as, surprisingly, in e-liquid without nicotine exposure. In contrast, PEPCK genes were only up-regulated in e-liquid with nicotine.

SIGNIFICANCE

While some features observed in rats may not be observed in human smokers, most of our data are consistent with, e-liquid per se i.e. without nicotine, not being neutral from a metabolic stand point since disrupting glucose homeostasis in rats.

Of von Willebrand factor and platelets

Hemostasis and pathological thrombus formation are dynamic processes that require multiple adhesive receptor-ligand interactions, with blood platelets at the heart of such events. Many studies have contributed to shed light on the importance of von Willebrand factor (VWF) interaction with its platelet receptors, glycoprotein (GP) Ib-IX-V and αIIbβ3 integrin, in promoting primary platelet adhesion and aggregation following vessel injury. This review will recapitulate our current knowledge on the subject from the rheological aspect to the spatio-temporal development of thrombus formation. We will also discuss the signaling events generated by VWF/GPIb-IX-V interaction, leading to platelet activation. Additionally, we will review the growing body of evidence gathered from the recent development of pathological mouse models suggesting that VWF binding to GPIb-IX-V is a promising target in arterial and venous pathological thrombosis. Finally, the pathological aspects of VWF and its impact on platelets will be addressed.

von Willebrand factor mutation promotes thrombocytopathy by inhibiting integrin αIIbβ3

von Willebrand disease type 2B (vWD-type 2B) is characterized by gain-of-function mutations in von Willebrand factor (vWF) that enhance its binding to the glycoprotein Ib-IX-V complex on platelets. Patients with vWD-type 2B have a bleeding tendency that is linked to loss of vWF multimers and/or thrombocytopenia. In this study, we uncovered evidence that platelet dysfunction is a third possible mechanism for bleeding tendency. We found that platelet aggregation, secretion, and spreading were diminished due to inhibition of integrin αIIbβ3 in platelets from mice expressing a vWD-type 2B-associated vWF (vWF/p.V1316M), platelets from a patient with the same mutation, and control platelets pretreated with recombinant vWF/p.V1316M. Impaired platelet function coincided with reduced thrombus growth. Further, αIIbβ3 activation and activation of the small GTPase Rap1 were impaired by vWF/p.V1316M following exposure to platelet agonists (thrombin, ADP, or convulxin). Conversely, thrombin- or ADP-induced Ca2+ store release, which is required for αIIbβ3 activation, was normal, indicating that vWF/p.V1316M acts downstream of Ca2+ release and upstream of Rap1. We found normal Syk phosphorylation and PLCγ2 activation following collagen receptor signaling, further implying that vWF/p.V1316M acts directly on or downstream of Ca2+ release. These data indicate that the vWD-type 2B mutation p.V1316M is associated with severe thrombocytopathy, which likely contributes to the bleeding tendency in vWD-type 2B.

Heterogeneity of Platelet Functional Alterations in Patients With Filamin A Mutations

Objective—

We examined platelet functions in 4 unrelated patients with filaminopathy A caused by dominant mutations of the X-linked filamin A ( FLNA ) gene.

Methods and Results—

Patients P1, P2, and P4 exhibited periventricular nodular heterotopia, heterozygozity for truncating FLNA mutations, and thrombocytopenia (except P2). P3 exhibited isolated thrombocytopenia and heterozygozity for a p.Glu1803Lys FLNA mutation. Truncated FLNa was undetectable by Western blotting of P1, P2, and P4 platelets, but full-length FLNa was detected at 37%, 82%, and 57% of control, respectively. P3 FLNa (p.Glu1803Lys and full-length) was assessed at 79%. All patients exhibited a platelet subpopulation negative for FLNa. Platelet aggregation, secretion, glycoprotein VI signaling, and thrombus growth on collagen were decreased for P1, P3, and P4, but normal for P2. For the 2 patients analyzed (P1 and P4), spreading was enhanced and, more markedly, in FLNa-negative platelets, suggesting that FLNa negatively regulates cytoskeleton reorganization. Platelet adhesion to von Willebrand factor under flow correlated with platelet full-length FLNa content: markedly reduced for P1 and P4 and unchanged for P2. Interestingly, von Willebrand factor flow adhesion was increased for P3, consistent with a gain-of-function effect enhancing glycoprotein Ib-IX-V/von Willebrand factor interaction. These results are consistent with a positive role for FLNa in platelet adhesion under high shear.

Conclusion—

FLNA mutation heterogeneity correlates with different platelet functional impacts and points to opposite regulatory roles of FLNa in spreading and flow adhesion under shear.

Experimental photophysical characterization of fluorophores in the vicinity of gold nanoparticles

We propose an experimental-based tool for dealing with fluorescence modulation close to nanoparticles for application in studies of fluorophores in the vicinity of gold nanoparticles (AuNPs), typically addressed via theoretical models. We performed a photophysical characterization of fluorophores in the vicinity of AuNPs, showing that correct Φ(F) determination suffers from a local pH effect, and address the observed radiative enhancement. Our approach is based on the experimental assurance that the reference fluorophores are in the same optical conditions as those of the AuNP-fluorophore conjugates. We demonstrate the relevance for introducing corrections for the inner filter effect and the reabsorption of the emitted light caused by AuNPs. The proposed approach could circumvent the need for theoretical based corrections and allow for more accurate determination of fluorescence emission in the vicinity of gold nanoparticles.

Thrombopoietin-induced Dami cells as a model for α-granule biogenesis

Megakaryocytic alpha-granules contain secretory proteins relevant to megakaryocyte and platelet functions. Understanding alpha-granule biogenesis is hampered because human primary megakaryocytes are difficult to manipulate. Existing promegakaryocytic cell lines do not spontaneously exhibit mature alpha-granules. Dami cells, transfected with the megakaryocytic platelet factor 4, fused to GFP (PF4-GFP), were induced in the presence of thrombopoietin (TPO), a megakaryocyte cytokine and PMA. Using confocal microscopy, PF4-GFP colocalized with von Willebrand Factor (vWF) in newly formed storage granules. Immunoelectron microscopy demonstrated alpha-granule-like features, including a dense core or parallel tubules and colocalization of PF4-GFP and vWF. Hence, TPO-treated Dami cells are a suitable model to study alpha-granules and their biogenesis.

Involvement of the Mitogen-activated Protein Kinase c-Jun NH2-terminal Kinase 1 in Thrombus Formation

The involvement of the mitogen-activated protein kinase c-Jun NH2-terminal kinase-1 (JNK1) has never been investigated in hemostasis and thrombosis. Using two JNK inhibitors (SP600125 and 6o), we have demonstrated that JNK1 is involved in collagen-induced platelet aggregation dependent on ADP. In these conditions, JNK1 activation requires the coordinated signaling pathways of collagen receptors (alpha2beta1 and glycoprotein (GP)VI) and ADP. In contrast, JNK1 is not required for platelet adhesion on a collagen matrix in static or blood flow conditions (300-1500 s(-1)) involving collagen receptors (alpha2beta1 and GPVI). Importantly, at 1500 s(-1), JNK1 acts on thrombus formation on a collagen matrix dependent on GPIb-von Willebrand factor (vWF) interaction but not ADP receptor activation. This is confirmed by the involvement of JNK1 in shear-induced platelet aggregation at 4000 s(-1). We also provide evidence during rolling and adhesion of platelets to vWF that platelet GPIb-vWF interaction triggers alphaIIbbeta3 activation in a JNK1-dependent manner. This was confirmed with a Glanzmann thrombastenic patient lacking alphaIIbbeta3. Finally, in vivo, JNK1 is involved in arterial but not in venular thrombosis in mice. Overall, our in vitro studies define a new role of JNK1 in thrombus formation in flowing blood that is relevant to thrombus development in vivo.

Compound heterozygosity of novel missense mutations in the gamma-glutamyl-carboxylase gene causes hereditary combined vitamin K-dependent coagulation factor deficiency

Hereditary combined vitamin K-dependent (VKD) coagulation factor deficiency is an autosomal recessive bleeding disorder associated with defects in either the gamma-carboxylase, which carboxylates VKD proteins to render them active, or the vitamin K epoxide reductase (VKORC1), which supplies the reduced vitamin K cofactor required for carboxylation. Such deficiencies are rare, and we report the fourth case resulting from mutations in the carboxylase gene, identified in a Tunisian girl who exhibited impaired function in hemostatic VKD factors that was not restored by vitamin K administration. Sequence analysis of the proposita did not identify any mutations in the VKORC1 gene but, remarkably, revealed 3 heterozygous mutations in the carboxylase gene that caused the substitutions Asp31Asn, Trp157Arg, and Thr591Lys. None of these mutations have previously been reported. Family analysis showed that Asp31Asn and Thr591Lys were coallelic and maternally transmitted while Trp157Arg was transmitted by the father, and a genomic screen of 100 healthy individuals ruled out frequent polymorphisms. Mutational analysis indicated wild-type activity for the Asp31Asn carboxylase. In contrast, the respective Trp157Arg and Thr591Lys activities were 8% and 0% that of wild-type carboxylase, and their compound heterozygosity can therefore account for functional VKD factor deficiency. The implications for carboxylase mechanism are discussed.

Of four mutations in the factor VII gene in Tunisian patients, one novel mutation (Ser339Phe) in three unrelated families abrogates factor X activation

Hereditary factor VII (FVII) deficiency is a rare bleeding disorder. Dysfunctional FVII variants characterized by normal or reduced levels of FVII antigen and discordantly low FVII activity have been described. In this study, seven unrelated Tunisian patients with FVII deficiency were examined. Molecular analysis revealed that three probands harbored a novel Ser339Phe mutation, one proband was inferred to have a novel splice site mutation in intron 2, c.226-2 A>G and three probands had two previously described mutations, Arg304Gln and Cys310Phe. Expression of Ser339Phe in baby hamster kidney cells yielded secretion of FVII antigen at a concentration of 225+/-50 ng/ml, compared with 181+/-47 ng/ml in cells transfected with wild-type FVII but with no demonstrable FVII activity. FVII Ser339Phe bound to tissue factor similarly to the binding of commercial recombinant activated FVII or recombinant wild-type FVII and was normally activated by activated factor X. The major defect of FVII Ser339Phe was its inability to activate factor X in the presence of tissue factor. Modeling predicted that the substitution of Ser339 by Phe abrogated substrate docking.

Evidence for a granule targeting sequence within platelet factor 4

Platelets achieve bleeding arrest at sites of vascular injury via secretion of secretory proteins from their storage granules, termed alpha-granules. We have recently analyzed granule targeting of platelet factor 4 (PF4), a secretory alpha-granule chemokine, and demonstrated that PF4 alpha-granule storage relied upon determinants within PF4 mature sequence. To define these determinants, PF4 mutants fused to the fluorescent reporter protein green fluorescent protein were generated by progressive deletions and site-directed mutagenesis. They were then transfected in AtT20 cells and assessed for granule targeting by colocalization with ACTH-containing granules, using laser scanning confocal microscopy. This strategy identified the amino acid 41-50 (LIATLKNGRK) sequence as most critical for PF4 granule targeting and/or storage; its deletion from PF4 induced a marked decrease in granule storage (from 81 +/- 2% to 17 +/- 3%, p < or = 0.0001). Ala-scanning mutagenesis of LIATLKNGRK narrowed down the targeting motif to LKNG. A direct role for LKNG in alpha-granule targeting was confirmed in the thrombopoietin-induced human megakaryocytic Dami cells, in which the LKNG-green fluorescent protein chimera exhibited an 82.5 +/- 1.8% colocalization with the alpha-granule proteins von Willebrand factor and P-selectin. LKNG is poorly conserved within the chemokine family. However three-dimensional alignments of the human alpha-granule chemokines Nap-2 (neutrophil-activating peptide) and RANTES (Regulated upon Activation Normal T Cell Expressed and Secreted) with PF4 revealed that LKNG, a surface-exposed hydrophilic turn/loop, matched Nap-2 (LKDG) and RANTES (TRKN) peptides with similar features. Moreover Nap-2 and RANTES peptides exhibited the same alpha-granule targeting efficiency than LKNG. We therefore postulate that the three-dimensional and physicochemical characteristics of PF4 LKNG are of general relevance to alpha-granule targeting of chemokines and possibly of other alpha-granule proteins.

Selective dephosphorylation of the threonine(183) residue of ERK2 upon (alpha)llb(beta)3 engagement in platelets

Thrombin-induced extracellular signal-regulated kinase 2 (ERK2) activation is negatively regulated in conditions of all bP3 integrin engagement and platelet aggregation. Here we show by Western blotting with antibodies against mono- and biphosphorylated forms of ERK2 that the dephosphorylation of ERK2 by alpha llb beta 3 engagement affects threonine183 and not tyrosine185. Addition of a potent serine/threonine phosphatase inhibitor, okadaic acid (OA), restored thrombin-induced threonine phosphorylation of ERK2 in conditions of platelet aggregation, whereas OA had no effect in the absence of alpha llb beta 3 engagement. These observations are consistent with alpha llb beta 3 engagement acting via at least one serine/threonine phosphatase,which dephosphorylates the phosphothreonine183 residue of ERK2. Moreover, a small amount (14%) of ERK2 was translocated to the alpha llb beta 3-dependent cytoskeleton, mostly ina monophosphorylated (i.e. inactive) form, suggesting that cytoskeleton-associated ERK2 plays only a minor role, if any. Finally, we show that negative regulation (i.e. dephosphorylation)occurs primarily or totally in the cytosol and that the alpha llb beta 3-dependent ERK2 Thr183-specific phosphatase is different from phosphatase 1 (PP1) or PP2A. We conclude that all alpha llb beta 3 engagement down-regulates ERK2 through selective dephosphorylation of the phosphothreonine183 residue by a cytosolic serine/threonine phosphatase different from known platelet phosphatases.

Fibrin clot retraction by human platelets correlates with alpha(IIb)beta(3) integrin-dependent protein tyrosine dephosphorylation

We have analyzed tyrosine phosphorylation associated with retraction of the fibrin clot by washed platelets in purified fibrinogen. Retraction was dependent on integrin alpha(IIb)beta(3), based on absence of retraction of alpha(IIb)beta(3)-deficient thrombasthenic platelets. However, only a subset of alpha(IIb)beta(3)-blocking antibodies or peptides were able to inhibit retraction, suggesting a differential engagement of alpha(IIb)beta(3) in fibrin clot retraction versus aggregation. Immunoblotting demonstrated a phosphorylated protein pattern comparable with aggregation at early time points. However, as opposed to aggregation, tyrosine phosphorylation decreased rapidly in parallel to retraction (up to 60 min). Dephosphorylation was alpha(IIb)beta(3)-dependent, since it was blocked by alpha(IIb)beta(3)-specific inhibitors and was absent in thrombasthenic platelets. Inhibition of platelet clot retraction by phenyl-arsine oxide and peroxovanadate, suggested a role for tyrosine phosphatases. Cytochalasin D and E (5 microm) blocked fibrin clot retraction and tyrosine dephosphorylation, suggesting regulation by actin cytoskeleton assembly. Tyrosine phosphatase activities were found associated with clot retraction using the "in-gel" tyrosine phosphatase assay; however, none were alpha(IIb)beta(3)-dependent. An 85-kDa protein and to a lesser degree "Src" showed the closest dose-dependent correlation between inhibition of tyrosine dephosphorylation and inhibition of retraction. We thus postulate that alpha(IIb)beta(3) engagement in fibrin clot retraction drives, in an actin cytoskeleton-dependent manner, the interaction of tyrosine phosphatases and of the tyrosine-phosphorylated substrates 85-kDa protein and Src, the dephosphorylation of which regulates the force generation and/or transmission required for full contraction of the fibrin matrix.

Hematopoietic differentiation of embryonic stem cells: an in vitro model to study gene regulation during megakaryocytopoiesis

We are interested in the regulation of the tissue specificity of the megakaryocyte-specific platelet glycoprotein IIb gene. The murine embryonic stem (ES) cells are able to differentiate into erythroid, mast and granulomonocytic cells in appropriate culture conditions. Our goal is to optimize the production of myeloid cells including megakaryocytes (MKs) by ES cells. We have found that coculture with MS-5 stromal cells and the presence of a cocktail of hematopoietic growth factors (HGFs) [stem cell factor, interleukin 3 (IL-3), IL-6, IL-11, G-CSF and erythropoietin] had a high synergistic activity on differentiation of ES cells into pure and MK-containing myeloid colonies from day 12 embryoid bodies. Thrombopoietin increased the number of MKs only when added to the HGF cocktail in the presence of MS-5 cells. Interestingly, many MKs exhibited a "hairy" appearance evocative of pseudopodial proplatelet formation. Expression of genes specific for the megakaryocytic lineage, GPIIb, PF4, mpl and GPIIIa, was detected by reverse transcriptase-polymerase chain reaction (RT-PCR) during differentiation of ES cells, and their relative time course was evaluated. This demonstrates that optimized culture conditions for the differentiation of ES cells into the MK lineage provide a useful tool for the study of the regulation of expression of genes during megakaryocytopoiesis.

Identification of a GATA-overlapping sequence within the enhancer of the murine GPIIb promoter that induces transcriptional deregulation in human K562 cells

The human and the murine glycoprotein platelet IIb (GPIIb) promoters are megakaryocyte specific in human and murine cell systems, respectively. Here we show that the murine promoter is, however, highly active when transfected in K562 human cells in which the human promoter is almost inactive. A murine promoter, in which the enhancer element was replaced by the human, retrieves its megakaryocytic specificity in human cell lines. The human and murine GATA-binding sites located in the enhancer region display slight sequence divergence next to the consensus GATA core sequence. Gel shift experiments show that, although the murine and the human GATA sequences both bind GATA-1, the murine sequence alone forms an additional complex (B) not detected with the human sequence. When the murine GATA-containing region is replaced by the human in the context of the murine GPIIb promoter, megakaryocyte specificity is restored in the human cell lines. A G nucleotide 3 to GATA appears crucial because its substitution abrogates B but not GATA-1 binding and restores megakaryocyte specificity to the murine promoter. Conversely, substitution of the human GATA-1 binding sequence by its murine homologue that binds both GATA-1 and complex B induces an abnormal activity for the human promoter in K562 cells. Altogether, our data suggest that limited changes in the GATA-containing enhancer of the GPIIb promoter can induce the recruitment of accessory proteins that could be involved in alteration of a megakaryocyte-restricted gene activation program. (Blood. 2000;96:1348-1357)

Regulation of c-jun-NH2 terminal kinase and extracellular-signal regulated kinase in human platelets

Platelets are an interesting model for studying the relationship betwen adhesion and mitogen-activated protein (MAP) kinase activation. We have recently shown that in platelets, ERK2 was activated by thrombin and downregulated by alpha(IIb)beta(3) integrin engagement. Here we focused our attention on the c-Jun NH2-terminal kinases (JNKs) and their activation in conditions of platelet aggregation. We found that JNK1 was present in human platelets and was activated after thrombin induction. JNK1 phosphorylation was detected with low concentrations of thrombin (0. 02 U/mL) and after 1 minute of thrombin-induced platelet aggregation. JNK1 activation was increased (fivefold) when fibrinogen binding to alpha(IIb)beta(3) integrin was inhibited by the Arg-Gly-Asp-Ser (RGDS) peptide or (Fab')(2) fragments of a monoclonal antibody specific for alpha(IIb)beta(3), demonstrating that, like ERK2, alpha(IIb)beta(3) integrin engagement negatively regulates JNK1 activation. Comparison of JNK1 activation by thrombin in stirred and unstirred platelets in the presence of RGDS peptide showed a positive regulation by stirring itself, independently of alpha(IIb)beta(3) integrin engagement, which was confirmed in a thrombasthenic patient lacking platelet alpha(IIb)beta(3). The same positive regulation by stirring was found for ERK2. These results suggest that MAP kinases (JNK1 and ERK2) are activated positively by thrombin and stirring. In conclusion, we found that JNK1 is present in platelets and can be activated after thrombin induction. Moreover, this is the first report showing that two different MAP kinases (ERK2 and JNK1) are regulated negatively by alpha(IIb)beta(3) engagement and positively by mechanical forces in platelets.

Tyrosine phosphorylation of cortactin associated with Syk accompanies thromboxane analogue-induced platelet shape change

Thromboxane A(2) (TxA(2)) is a potent vasoconstrictor and platelet agonist. Pharmacological studies have defined two classes of thromboxane receptors (TPs) in human platelets; sites that bind the agonist 1S-(1,2(5Z),3-(1E,3S),4)-7- 3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxabicyclo-2.2. 1-heptan-2-yl-5-heptenoic acid (I-BOP) with high affinity support platelet shape change, whereas low affinity sites that bind irreversibly the antagonist GR 32191 transduce platelet aggregation. As the mechanisms of signal transduction involved in platelet aggregation begin to be elucidated, few results concern those involved in platelet shape change, which is independent of the engagement of GPIIb/IIIa. To elucidate the respective role of the two classes of pharmacological binding sites of TPs in shape change, platelets were incubated with I-BOP at low concentrations or stimulated by I-BOP at high concentrations after pretreatment with GR 32191 or activated with low concentrations of 8-epi-prostaglandin F(2)alpha. Under these three conditions, there is a rapid stimulation of protein tyrosine phosphorylation of the 80/85-kDa doublet identified as the cytoskeletal protein cortactin. Tyrosine phosphorylation of cortactin is kinetically correlated with the occurrence of shape change. These biochemical and morphological events are both inhibited by SQ 29548, a TP antagonist, indicating the specificity of the signal. Since tyrosine kinase Syk was activated early during platelet activation, we examined the possibility that cortactin is a potential substrate of Syk in TxA(2)-induced platelet shape change. p72 Syk phosphorylation and kinase activity took place during the period when platelets were changing shape upon low concentrations of I-BOP stimulation. Furthermore, cortactin was associated with Syk, and this association increases along with the level of phosphorylation. These data suggest a novel pathway for a G protein-coupled TxA(2) high affinity receptor to the protein-tyrosine kinase Syk, which is associated with cortactin in the very early steps of platelet activation.

Abnormal tyrosine phosphorylation linked to a defective interaction between ADP and its receptor on platelets

ADP, a primary stimulus of platelets, binds to one or more populations of receptors on the platelet surface. These receptors are linked to discrete activation pathways. Both G proteins and tyrosine kinases have been implicated in the cellular responses to this agonist. We have studied a patient with a congenital abnormality of ADP-induced platelet aggregation in an effort to gain information on the signalling pathways used by ADP. Immunoblotting with a broadly reactive rabbit antibody recognizing the GTP-binding domain of G protein alpha-subunits, and with rabbit antibodies specific for Gialpha1-3, and Galpha12 all showed normal reactivity when tested against the patient's platelets. The phosphorylation of proteins was studied using an anti-phosphotyrosine MoAb (4G10) and platelets stimulated in a platelet aggregometer with ADP, a thromboxane A2 mimetic (IBOP), TRAP-14-mer peptide and alpha-thrombin. With normal platelets, a time-dependent phosphorylation of several bands in the 60 to 130 kDa mol. wt. range was observed with all agonists. For the patient, minimal aggregation and little or no phosphorylation of proteins of 80-85 kDa (cortactin), 100-105 kDa and 125-130 kDa were seen in response to ADP. The aggregation and phosphorylation responses were slightly modified in the presence of low doses of thrombin but were normal with high doses. Aggregation and tyrosine phosphorylation were virtually absent with IBOP, a finding reproduced when normal platelets were incubated with IBOP and the CP/CPK ADP scavenging system, thereby underlining the role of ADP in the response to IBOP. Our results show that the ADP receptor pathway deficient in the patient is linked to a selective tyrosine phosphorylation response.

Negative regulation of mitogen-activated protein kinase activation by integrin alphaIIbbeta3 in platelets

Activation of the mitogen-activated protein (MAP) kinase pathway in nucleated cells is dependent on both growth factor receptors and integrins engaged in cell adhesion. Human platelets are an interesting model for studying cell adhesion and the involvement of integrin engagement on extracellular signal-regulated kinase (ERK) activation, independently from the nuclear-DNA signal pathway. Maximal phosphorylation and activity of ERK2 occurred late during thrombin-induced platelet aggregation (90 s and later), an alphaIIbbeta3 integrin-dependent event. Surprisingly, alphaIIbbeta3 inhibition by the RGDS ligand peptide, or (Fab')2 fragments of the AP-2 monoclonal antibody, resulted in a 2-fold enhancement in ERK2 phosphorylation and activity. A similar 2-fold enhancement of ERK2 activation was observed in thrombasthenic platelets which are defective in alphaIIbbeta3 and do not aggregate. This suggests that ERK2 activation in thrombin-induced platelet aggregation is dependent on thrombin rather than on alphaIIbbeta3 and is down-regulated by alphaIIbbeta3 engaged in ligand (fibrinogen) binding and/or aggregation. Finally, in the absence of stirring which allows fibrinogen binding to alphaIIbbeta3 but prevents aggregation, ERK2 was again overactivated. This overactivation appears to be consecutive to inhibition of aggregation itself and to alphaIIbbeta3 ligand binding. We conclude that in platelets, alphaIIbbeta3 engaged in aggregation down-regulates thrombin-induced ERK2 activation. To our knowledge, this is the first report of a down-regulation of the MAP kinase pathway by integrin engagement.

Reassessment of protein tyrosine phosphorylation in thrombasthenic platelets: evidence that phosphorylation of cortactin and a 64-kD protein is dependent on thrombin activation and integrin alphaIIb beta3

Tyrosine phosphorylation of a number of platelet proteins is dependent on platelet integrin alphaIIb beta3 (also termed GPIIb-IIIa) and its engagement in aggregation. For instance, in type I thrombasthenic platelets, which lack alphaIIb beta3 and do not aggregate, several substrates are either poorly or not phosphorylated. We have compared thrombasthenic platelets of type I, type II (15% alphaIIb beta3, functional), and variant type (50% alphaIIb beta3, no fibrinogen binding). The platelets from the three patients exhibited the same low tyrosine phosphorylation profiles, confirming the key role of functional alphaIIb beta3 in initiating protein tyrosine phosphorylation. We noted that in addition to the characteristic absence of the 100 to 105 kD doublet, a 77 to 80 kD doublet and to a lesser extent a 64-kD band, exhibited low phosphorylation kinetics, but with normal initial phosphorylation rates (up to 60 seconds). Similar results were obtained by inhibition of thrombin aggregation of control platelets by alphaIIb beta3 antagonists (the RGDS peptide or the monoclonal antibody 10E5), or in the absence of stirring (fibrinogen binding, but no aggregation). These results suggest that tyrosine phosphorylation of the 77 to 80 kD doublet, identified by immunoprecipitation as the cytoskeletal protein cortactin, and the 64 kD band are dependent both on thrombin activation during early steps and on the late steps of alphaIIb beta3 engagement in aggregation. Implications as to involvement of step-specific kinase and/or phosphatase activities are discussed.

Platelet-vessel wall interaction: from the bedside to molecules

This paper is an overview of the progress made in the field of platelet-vessel wall interaction during the past three quarters of a century. (I) "Prehistoric" era (1918-1948): description of Thrombasthenia by 3 European clinicians (Glanzmann, Switzerland, 1918), Hereditary Pseudohemophila (von Willebrand, Finland 1926) and Congenital Hereditary Platelet Dystrophy (Bernard and Soulier, France 1948). (II) Physiopathological era (1957-1972) based on the understanding of these 3 disorders: abnormality of platelet adhesion to the vessel wall in von Willebrand (vW) and Bernard Soulier syndrome (BSS), abnormality of platelet aggregation in thrombasthenias (GT). Breakthrough was made by I.M. Nilsson et al in vWD in Aåland Islands with the recognition of a plasmatic anomaly, later confirmed by Cornu et al to be different from factor VIII deficiency. (III) The third and crucial development (glycoprotein era) (1974-1981) came from the discovery by Nurden & Caen (1974-1975) of GPIIb-IIIa defect in GT and of GPIb-IX in BSS. Polyclonal antibodies against GPIIb-IIIa and GPIb-IX inhibit clearly ex vivo platelet aggregation and adhesion respectively. On everted rabbit subendothelium platelet, adhesion was abnormal in BSS whichever shear stress and at high shear stress in vWD. (IV) The molecular biology era (1984-1995) with the exquisite recognition of gene or protein anomalies in the above mentioned disorders together with the cloning of the 3 genes [for vW factor (vWF) (GPIb-IX, GPIIb, and GPIIIa]. (V) We are at the dawn of the more crucial era, the antithrombotic therapy acting either on the GPIb-vWF complex or on the GPIIb-IIIa.(ABSTRACT TRUNCATED AT 250 WORDS)

A point mutation in the integrin beta 3 cytoplasmic domain (S752-->P) impairs bidirectional signaling through alpha IIb beta 3 (platelet glycoprotein IIb-IIIa)

Agonist-induced inside-out signaling results in an increased affinity of integrin alpha IIb beta 3 (platelet glycoprotein IIb-IIIa) for soluble ligands (fibrinogen [Fg] and PAC1). Ligand binding to integrins initiates outside-in signaling that leads to cellular responses such as cell spreading and focal adhesion formation. A point mutation in the beta 3 cytoplasmic domain (S752-->P) is associated with blocked inside-out alpha IIb beta 3 signaling in a variant Glanzmann's thrombasthenia. This mutation was introduced into beta 3 and cotransfected into Chinese hamster ovary cells with a chimeric alpha subunit consisting of the alpha IIb extracellular and transmembrane domains and the alpha 6B cytoplasmic domain. The substitution of the alpha IIb cytoplasmic domain with that of alpha 6 led to activation of alpha IIb beta 3 to bind PAC1, mimicking inside-out signaling. This effect was reversed by the S752-->P mutation, indicating a disruption of inside-out signaling by the mutation. In addition, transfectants expressing this beta 3 variant showed reduced alpha IIb beta 3-mediated cell spreading on immobilized Fg, focal adhesion, and fibrin clot retraction, suggesting an impairment in outside-in alpha IIb beta 3 signaling. Therefore, a single point mutation in the beta 3 cytoplasmic domain impaired bidirectional signaling through alpha IIb beta 3.

A new alternative transcript encodes a 60 kDa truncated form of integrin beta 3

A cDNA for integrin beta 3 isolated from a human erythroleukaemia (HEL) cell library contained a 340 bp insert at position 1281. This mRNA, termed beta 3c, results from the use of a cryptic AG donor splice site in intron 8 of the beta 3 gene, and is different from a previously described alternative beta 3 mRNA. The predicted open reading frame of beta 3C stops at a TAG stop codon 69 bp downstream from position 1281. It starts with the signal peptide and the 404 N-terminal extracellular residues of beta 3, encompassing the ligand binding sites, followed by 23 C-terminal intron-derived residues, corresponding to a truncated form of beta 3 lacking the cysteine-rich, transmembrane and cytoplasmic domains. Expression of beta 3C mRNA was demonstrated in human platelets, megakaryocytes, endothelial cells and HEL cells by reverse transcriptase/PCR. The beta 3C transcript was also demonstrated in the mouse, suggesting its conservation through evolution. Finally, a 60 kDa polypeptide corresponding to the beta 3C alternative transcript was demonstrated in platelets by Western blotting using a polyclonal antibody raised against a synthetic peptide designed from the beta 3C intronic sequence. Taken together, these results suggest a biological role for beta 3C, the first alternative transcript showing an altered extracellular domain of a beta integrin.

A comparative analysis of cDNA-derived sequences for rat and mouse beta 3 integrins (GPIIIA) with their human counterpart

Alpha IIb beta 3 (GPIIb-IIIa), the platelet receptor for fibrinogen, is a member of the integrin superfamily. We have now cloned the mouse and rat beta 3 cDNAs. These data represent the first available non-human beta 3 sequences, allowing important comparative analyses. Both beta 3 sequences are highly homologous with human beta 3, well above average rodent-human protein homology of 79%. The ligand binding domains (aa) 109-171 and 204-229) are, respectively, 90% and 100% homologous. The beta 3 transmembrane and the cytoplasmic tail are surprisingly highly conserved, being 97% and 100% homologous, respectively, but share little homology with beta 1, or beta 2. This latter difference argues strongly in favor for a crucial beta 3-specific function for these domains. In conclusion, we present the first comparative analysis of beta 3 chains and demonstrate high overall homology. The biological implications of these comparisons are discussed.

Human platelet antigen 3 (HPA-3): localization of the determinant of the alloantibody Lek(a) (HPA-3a) to the C-terminus of platelet glycoprotein IIb heavy chain and contribution of O-linked carbohydrates

The human platelet antigen (HPA) 3 system is expressed on GPIIb, one subunit of GPIIb-IIIa, the platelet fibrinogen receptor. It was recently shown that HPA-3 was associated with an Ile843/Ser polymorphism. To investigate further HPA-3 determinant structure, we localized an HPA-3a determinant, recognized by the alloantiserum Lek(a), within the last 29 amino acids of GPIIb alpha. This region encompasses the polymorphic Ile843, which, as expected, is substituted into Ser in Lek(a)-negative individuals, as shown by DNA sequence after polymerase chain reaction on platelet RNA. In addition, contribution of glycosylation to the determinant structure was demonstrated since the Lek(a) antigenicity was strongly decreased after specifically removing non-terminal O-linked sugars, but not terminal sialic acids. We have thus refined the localization of an HPA-3a determinant within the last 29 amino acids, including Ile843, of GPIIb heavy chain, and shown that the Lek(a) HPA-3a determinant is dependent, in part, upon the serine-linked carbohydrates adjacent to Ile/Ser843.

Ser-752-->Pro mutation in the cytoplasmic domain of integrin beta 3 subunit and defective activation of platelet integrin alpha IIb beta 3 (glycoprotein IIb-IIIa) in a variant of Glanzmann thrombasthenia

Integrins are membrane receptors which mediate cell-cell or cell-matrix adhesion. Integrin alpha IIb beta 3 (glycoprotein IIb-IIIa) acts as a fibrinogen receptor of platelets and mediates platelet aggregation. Platelet activation is required for alpha IIb beta 3 to shift from noncompetent to competent for binding soluble fibrinogen. The steps involved in this transition are poorly understood. We have studied a variant of Glanzmann thrombasthenia, a congenital bleeding disorder characterized by absence of platelet aggregation and fibrinogen binding. The patient's platelets did not bind fibrinogen after platelet activation by ADP or thrombin, though his platelets contained alpha IIb beta 3. However, isolated alpha IIb beta 3 was able to bind to an Arg-Gly-Asp-Ser affinity column, and binding of soluble fibrinogen to the patient's platelets could be triggered by modulators of alpha IIb beta 3 conformation such as the Arg-Gly-Asp-Ser peptide and alpha-chymotrypsin. These data suggested that a functional Arg-Gly-Asp binding site was present within alpha IIb beta 3 and that the patient's defect was not secondary to a blockade of alpha IIb beta 3 in a noncompetent conformational state. This was evocative of a defect in the coupling between platelet activation and alpha IIb beta 3 up-regulation. We therefore sequenced the cytoplasmic domain of beta 3, following polymerase chain reaction (PCR) on platelet RNA, and found a T-->C mutation at nucleotide 2259, corresponding to a Ser-752-->Pro substitution. This mutation is likely to be responsible for the uncoupling of alpha IIb beta 3 from cellular activation because (i) it is not a polymorphism, (ii) it is the only mutation in the entire alpha IIb beta 3 sequence, and (iii) genetic analysis of the family showed that absence of the Pro-752 beta 3 allele was associated with the normal phenotype. Our data thus identify the C-terminal portion of the cytoplasmic domain of beta 3 as an intrinsic element in the coupling between alpha IIb beta 3 and platelet activation.

Human platelets express the SERCA2-b isoform of Ca(2+)-transport ATPase

Previous biochemical studies suggested that the human platelet Ca2+ATPase system may be cell-specific. To test this hypothesis, we first undertook the molecular cloning of Ca2+ATPase from human erythroleukaemia (HEL) cells, because this human cell line exhibits megakaryocytic features and expresses a Ca2+ATPase that cross-reacts with platelet Ca(2+)-ATPase. For this cloning, an HEL-cell cDNA library was screened with a rat cardiac Ca2+ATPase cDNA probe. The insert of the longest clone isolated was 3.9 kb and its sequence displayed a 100% identity with that of the non-muscle human Ca2+ATPase 2-b isoform, termed SERCA2-b (sarco-endoplasmic-reticulum Ca2+ATPase). The 3.9 kb cDNA covered a subtotal coding region and part of the 3' non-coding end of the SERCA2-b mRNA. It cross-hybridized with the 4 kb transcript species of cardiac SERCA2-a and with non-muscle SERCA2-b mRNAs, but not with fast-skeletal-muscle SERCA1 mRNA. We next confirmed that SERCA2-b was a component of the platelet Ca2+ATPase system because (1) the platelet clones isolated from a platelet cDNA library exhibited a 100% homology with HEL-cell cDNA; (2) SERCA2-b mRNA was amplified by PCR on total platelet RNA and (3) platelet Ca2+ATPase cross-reacted with a polyclonal SERCA2-b-specific antiserum. Platelets therefore contain a Ca2+ATPase definitely identified as the SERCA2-b isoform of Ca2+ATPase, thus eliminating the possibility that they only contain a single specific Ca2+ATPase.

Quantitative isolation of RNA from human platelets

We have adapted the acid-guanidinium-phenol-chloroform extraction procedure of Chomczinsky and Sacchi to achieve efficient rapid recovery of total RNA from human platelets. Sufficient platelet RNA (20 micrograms of total RNA per 30 ml of whole blood) can be recovered from relatively small individual samples to perform Northern blot analysis on individual donors and detect the mRNAs for glycoproteins IIb1(GP IIb) and IIIa1(GP IIIa), 3.4 kb and 6.2 kb, respectively. Platelet GP IIb and GP IIIa mRNAs could also be reverse transcribed, and amplified in vitro by the polymerase chain reaction (PCR). Thus, our technique allows simultaneous Northern blotting and PCR, and therefore should be of great help to the characterization of inherited platelet disorders such as Glanzmann's thrombasthenia.

Processing and assembly of the integrin, glycoprotein IIb-IIIa, in HEL cells

We examined the biosynthetic processing and assembly of the platelet glycoprotein (GP) IIb-IIIa complex in [35S]methionine-labeled HEL cells, a human cell line with features of megakaryocytes. Both GPIIb and GPIIIa were synthesized as single-chain precursors to which high mannose N-linked oligosaccharides were added in the endoplasmic reticulum (ER). A 5-fold excess of the major IIb precursor, preIIb, was synthesized relative to GPIIIa. Two smaller proteins immunologically related to GPIIb were synthesized in smaller amounts. Assembly of the GPIIb and GPIIIa precursors required 4-6 h for completion. All GPIIIa molecules were eventually assembled; the excess GPIIb precursors were degraded without reaching the cell surface. Following assembly, preIIb-IIIa complexes were rapidly transported to the Golgi apparatus where preIIb underwent modification of high mannose chains into complex oligosaccharides and proteolytic cleavage to yield disulfide-linked heavy and light chains. Pretreating cells with the ionophore monensin blocked cleavage of preIIb but not its carbohydrate modification or its assembly with GPIIIa. These studies suggest that 1) assembly of the precursors of GPIIb and GPIIIa in the ER is a slow process requiring conformational maturation of one or both subunits, and 2) only heterodimers assembled in the ER are transported to the Golgi apparatus for additional processing and, ultimately, expression on the cell surface.

Physical linkage of the genes for platelet membrane glycoproteins IIb and IIIa

The fibrinogen receptor on human platelets is a prototypic member of the integrin family and is composed of subunit glycoproteins IIb (gpIIb) and IIIa (gpIIIa) in a 1:1 stoichiometric ratio. We have isolated cDNA clones for gpIIb and gpIIIa and localized both genes to chromosome 17. In the current study, several approaches were used to localize and map the genes for gpIIb and gpIIIa. A preliminary evaluation of subchromosomal localization was performed by using a panel of mouse-human somatic cell hybrids that contain different amounts of the long arm of human chromosome 17. Southern hybridization to the DNA of these hybrids shows that both genes map near the thymidine kinase gene. In situ hybridization to intact human chromosomes localized both genes to the 17q21-22 region. To better define the physical distance between the two genes, we examined the genomic hybridization pattern of each cDNA probe to high molecular weight restriction fragments separated by pulsed-field gel electrophoresis. Serial hybridizations of the same filter have allowed construction of long-range Mlu I and Sfi I restriction maps spanning more than 500 kilobases. Finally, nonoverlapping portions of the cDNAs for both gpIIb and gpIIIa were used to probe Sfi I digests of genomic DNA separated by field-inversion gels. This confirmed that the genes are physically linked within the same 260-kilobase Sfi I fragment and suggests that the gene for gpIIb is located on the 3' side of the gene for gpIIIa. These results suggest that coordinate expression of gpIIb and gpIIIa may depend on physical proximity.

Complete amino acid sequence of a human platelet proteoglycan

The primary structure of a human platelet proteoglycan (P.PG) core was established by a combination of amino acid sequence analysis and cDNA cloning. The deduced 131 amino acid long protein contains eight Ser-Gly repeats. The significance of homologies observed between P.PG and promyelocytic leukemia cell line proteoglycans is discussed.

Cloning of glycoprotein IIIa cDNA from human erythroleukemia cells and localization of the gene to chromosome 17

Platelet aggregation requires the binding of adhesive proteins such as fibrinogen to the heterodimer of membrane glycoproteins IIb (GPIIb) and IIIa (GPIIIa). Human erythroleukemia (HEL) cells synthesize both GPIIb and GPIIIa. Using poly(A+) RNA purified from HEL cells, we constructed a cDNA library in the lambda gt10 phage vector. This library was screened with a 38mer oligonucleotide derived from a platelet GPIIIa peptide, and three overlapping cDNAs were isolated. The three inserts encompassed 3.5 kilobases (kb), including the entire coding region of mature GPIIIa (2,286 basepairs, bp) and 1.3 kb of 3' untranslated sequence. All 222 residues determined directly from platelet GPIIIa tryptic peptides exactly matched the HEL cell-deduced amino acid sequence. The HEL cell sequence matched a previously reported endothelial cell cDNA sequence except for eight nucleotides. Five of these nucleotide differences were silent changes consistent with genetic polymorphisms. The other three differences resulted in changes in the deduced amino acid sequence of GPIIIa; reexamination of the endothelial cell cDNA sequence in these three areas revealed that it is actually identical to the HEL cell sequence. The virtual identity of the endothelial and HEL cell cDNA sequences provides direct evidence that GPIIIa is a subunit common to cell-adhesion receptors present in more than one cell type. We localized the gene for GPIIIa to chromosome 17, the same chromosome to which we had previously mapped the gene for GPIIb.

Platelet glycoprotein IIb. Chromosomal localization and tissue expression

The GPIIb-IIIa complex functions as a receptor for cytoadhesive proteins on the platelet surface. Both GPIIb and GPIIIa are synthesized by a human erythroleukemia (HEL) cell line. We isolated several cDNA clones by screening a HEL cell cDNA library with an oligonucleotide derived from amino acid sequence of GPIIb. Nucleotide and amino acid sequences were determined from 703 bp of one of these clones. Amino acid sequence of purified platelet GPIIb peptides confirmed the identity of the clone. The cDNA encodes the carboxyl terminus of the large (alpha) subunit of GPIIb and all of the smaller (beta) subunit of GPIIb. By hybridizing the cDNA directly to chromosomes separated by dual laser chromosome sorting, the gene for GPIIb was mapped to chromosome 17. Northern blot analysis showed a approximately 3.4-kb GPIIb mRNA in HEL cells. We also compared the amino acid sequences determined from eight additional platelet GPIIb peptides with the derived amino acids from a published HEL cell GPIIb cDNA, and the platelet and HEL cell proteins appear to be the same. Despite previous reports that vascular endothelial cells and monocytes contain GPIIb, no GPIIb mRNA was observed in either type of cell. Thus, GPIIb appears to be specific for the platelet-megakaryocyte membrane and is distinct from the alpha subunits of the adhesion receptors in other normal tissues.

Gray platelet syndrome. Demonstration of alpha granule membranes that can fuse with the cell surface

Platelets from patients with the gray platelet syndrome have decreased recognizable alpha granules and are markedly deficient in some alpha-granule secretory proteins. Using immunocytochemical techniques with antibodies to an alpha-granule membrane protein, GMP-140, we identified the membranes of intracellular vesicles in gray platelets as alpha-granule membranes. Gray platelets contained normal amounts of GMP-140 as measured by electroimmunoassay. The activation of gray platelets with thrombin caused GMP-140 to be redistributed to the plasma membrane surface, as in normal platelets. In agreement with previous studies, an endogenously synthesized secretory protein, platelet factor 4, was undetectable in gray platelets. However, the alpha-granule proteins albumin and IgG, which are thought to be derived from endocytosis of plasma proteins into megakaryocytes, were present in substantial quantities and were secreted efficiently from gray platelets. Therefore, the fundamental defect in the gray platelet syndrome may be in the targeting of endogenously synthesized secretory proteins to developing alpha granules in megakaryocytes.