Knock-in mice bearing constitutively active αIIb(R990W) mutation develop macrothrombocytopenia with severe platelet dysfunction.

BACKGROUND: To date, several mutations that induce constitutive activation of integrin αIIbß3 have been identified in congenital macrothrombocytopenia. Of these, αIIb(R995W) is the most prevalent mutation observed in Japanese patients with αIIbß3-related congenital macrothrombocytopenia. OBJECTIVE AND METHODS: The present study aimed to explore the effects of constitutive activation of the αIIb(R995W) mutation on platelet production, morphology, and function. We generated αIIb(R990W) knock-in (KI) mice corresponding to human αIIb(R995W). RESULTS: Platelet counts of heterozygous (hetero) and homozygous (homo) KI mice were decreased by ~10% and ~25% relative to those of wild-type (WT) mice, respectively, with increase in platelet size. Decrease in absolute reticulated platelet numbers in steady state, delayed recovery from thrombocytopenia induced by anti-platelet antibody and impaired response to exogenous thrombopoietin administration suggested impaired platelet production in KI mice. WT and KI mice showed no significant differences in the number of megakaryocytes and ploidy of megakaryocytes, whereas proplatelet formation was significantly impaired in homo mice. We observed a slight but significant reduction in platelet lifespan in homo mice. The homo mice showed dramatic reduction in αIIbß3 expression in platelets, which was accompanied by severe in vivo and in vitro platelet dysfunction. CONCLUSION: The αIIb(R990W) KI mice developed macrothrombocytopenia, which was primarily attributed to impaired proplatelet formation. In addition, homo KI mice showed marked downregulation in αIIbß3 expression in platelets with severe impaired platelet function, similar to Glanzmann thrombasthenia.

Trans-fatty acid promotes thrombus formation in mice by aggravating antithrombogenic endothelial functions via Toll-like receptors.

SCOPE: Since excessive intake of trans-fatty acid (TFA) increases the risk of myocardial infarction, we investigated the effects of TFA on thrombus formation using animal and cell culture experiments. METHODS AND RESULTS: C57BL/6 mice were fed a diet containing TFA or cis-fatty acid (5% each of total calories) or a chow diet for 4 weeks, and thrombus formation was induced in the carotid artery by He-Ne laser irradiation. The high-TFA diet significantly promoted thrombus formation in the carotid artery compared to the chow or cis-fatty acid diet. TFA activated the inflammatory signaling pathway in cultured endothelial cells and in mice; aortic gene expression levels of antithrombogenic molecules, including thrombomodulin and tissue factor pathway inhibitor, were decreased, and the expression levels of prothrombogenic molecules were increased in TFA-treated mice. TFA markedly upregulated the prothrombogenic molecules and downregulated the antithrombogenic molecules in endothelial cells. In addition, TFA induced phosphorylation of c-Jun N-terminal kinase, extracellular signal-regulated kinase, and nuclear factor-κB. The TFA-activated signal pathways and prothrombogenic phenotypic changes of endothelial cells were inhibited by genetic or pharmacological inactivation of Toll-like receptors 2 and 4. CONCLUSION: TFA aggravates the antithrombogenic phenotypes of vascular endothelial cells via Toll-like receptors and promotes thrombus formation in mice.

Agonist stimulation, talin-1, and kindlin-3 are crucial for α(IIb)ß(3) activation in a human megakaryoblastic cell line, CMK.

Platelet integrin α(IIb)ß(3) activation is regulated by inside-out signaling via agonist stimulation. However, when α(IIb)ß(3) was exogenously expressed in cell lines such as Chinese hamster ovarian cells, physiological agonists hardly induced α(IIb)ß(3) activation. To overcome this disadvantage, we characterized the functional regulation of endogenously expressed α(IIb)ß(3) in a megakaryoblastic cell line, CMK, employing an initial velocity assay for PAC-1 binding. We firstly demonstrated that protease-activated receptor 1-activating peptide induced robust, but transient α(IIb)ß(3) activation in CMK cells with high glycoprotein-Ib expression. Stable talin-1 or kindlin-3 knockdown cells confirmed that the protease-activated receptor 1-activating peptide-induced α(IIb)ß(3) activation was dependent on talin-1 and kindlin-3 expression. In sharp contrast to exogenously expressed α(IIb)ß(3) in Chinese hamster ovarian cells, transient overexpression of full-length talin (FL-talin) or talin-head domain (THD) alone did not activate α(IIb)ß(3) in CMK cells, but required agonist stimulation. Similarly, kindlin-3 overexpression alone did not induce α(IIb)ß(3) activation, but it significantly augmented agonist-induced α(IIb)ß(3) activation. Several mutants of FL-talin and THD suggested that the head-rod interaction was critical for autoinhibition of talin-1, and the interaction between the THD and the membrane-proximal region of the ß(3) cytoplasmic tail was essential for talin-mediated α(IIb)ß(3) activation. In addition, THD and kindlin-3 cooperatively augmented protease-activated receptor 1-induced α(IIb)ß(3) activation. We proposed that the CMK cell line is an attractive platform for investigating agonist-, talin-1-, and kindlin-3- dependent α(IIb)ß(3) activation.

Demonstration of novel gain-of-function mutations of αIIbß3: association with macrothrombocytopenia and glanzmann thrombasthenia-like phenotype.

Integrin αIIbß3 is indispensable for normal hemostasis, but its role for thrombopoiesis is still controversial. Recently, αIIb and ß3 mutations have been identified in patients with congenital macrothrombocytopenia. We analyzed three unrelated Japanese families with congenital macrothrombocytopenia. Expression and activation state of αIIbß3 in platelets was examined by flow cytometry and immunoblotting. Sequence of whole coding region and exon-intron boundaries of ITGA2B and ITGB3 genes was performed. The effects of mutations on αIIbß3 activation state and phosphorylation of FAK were analyzed in transfected cells. We newly identified three mutations: two mutations in highly conserved Gly-Phe-Phe-Lys-Arg sequence in juxtamembrane region of αIIb, p.Gly991Cys and p.Phe993del, and one donor site mutation of intron 13 of ITGB3 leading to 40 amino acids deletion, p.(Asp621_Glu660del), in the membrane proximal ß-tail domain of ß3. One patient, who showed Glanzmann thrombasthenia-like marked reduction in surface αIIbß3 expression (3-11% of normal control), was a compound heterozygote with ITGA2B p.Gly991Cys and a novel nonsense mutation, ITGA2B p.Arg422*. All three mutations, ITGA2B p.Gly991Cys, ITGA2B p.Phe993del, and ITGB3 p.(Asp621_Glu660del), led to highly activated conformation of αIIbß3 and spontaneous tyrosine phosphorylation of FAK in transfected cells. These results suggest that gain-of-function mutations around membrane region of αIIbß3 lead to abnormal platelet number and morphology with impaired surface αIIbß3 expression.

Recognition of highly restricted regions in the ß-propeller domain of αIIb by platelet-associated anti-αIIbß3 autoantibodies in primary immune thrombocytopenia.

Platelet-associated (PA) IgG autoantibodies play an essential role in primary immune thrombocytopenia (ITP). However, little is known about the epitopes of these Abs. This study aimed to identify critical binding regions for PA anti-αIIbß3 Abs. Because PA anti-αIIbß3 Abs bound poorly to mouse αIIbß3, we created human-mouse chimera constructs. We first examined 76 platelet eluates obtained from patients with primary ITP. Of these, 26 harbored PA anti-αIIbß3 Abs (34%). Further analysis of 15 patients who provided sufficient materials showed that the epitopes of these Abs were mainly localized in the N-terminal half of the ß-propeller domain in αIIb (L1-W235). We could identify 3 main recognition sites in the region; 2 eluates recognized a conformation formed by the W1:1-2 and W2:3-4 loops, 5 recognized W1:2-3, and 4 recognized W3:4-1. The remaining 4 eluates could not be defined by the binding sites. Within these regions, we identified residues critical for binding, including S29 and R32 in W1:1-2; G44 and P45 in W1:2-3; and P135, E136, and R139 in W2:3-4. Of 11 eluates whose recognition sites were identified, 5 clearly showed restricted κ/λ-chain usage. These results suggested that PA anti-αIIbß3 Abs in primary ITP tended to recognize highly restricted regions of αIIb with clonality.

A potential role for α-actinin in inside-out αIIbß3 signaling.

Many different biochemical signaling pathways regulate integrin activation through the integrin cytoplasmic tail. Here, we describe a new role for α-actinin in inside-out integrin activation. In resting human platelets, α-actinin was associated with αIIbß3, whereas inside-out signaling (αIIbß3 activation signals) from protease-activated receptors (PARs) dephosphorylated and dissociated α-actinin from αIIbß3. We evaluated the time-dependent changes of the αIIbß3 activation state by measuring PAC-1 binding velocity. The initial velocity analysis clearly showed that PAR1-activating peptide stimulation induced only transient αIIbß3 activation, whereas PAR4-activating peptide induced long-lasting αIIbß3 activation. When αIIbß3 activation signaling dwindled, α-actinin became rephosphorylated and reassociated with αIIbß3. Compared with control platelets, the dissociation of α-actinin from αIIbß3 was only transient in PAR4-stimulated P2Y(12)-deficient platelets in which the sustained αIIbß3 activation was markedly impaired. Overexpression of wild-type α-actinin, but not the mutant Y12F α-actinin, increased its binding to αIIbß3 and inhibited PAR1-induced initial αIIbß3 activation in the human megakaryoblastic cell line, CMK. In contrast, knockdown of α-actinin augmented PAR-induced αIIbß3 activation in CMK. These observations suggest that α-actinin might play a potential role in setting integrins to a default low-affinity ligand-binding state in resting platelets and regulating αIIbß3 activation by inside-out signaling.

Molecular analysis of a patient with type I Glanzmann thrombasthenia and clinical impact of the presence of anti-αIIbß3 alloantibodies.

The occurrence of transfusion-related alloimmunization against αIIbß3 is a major concern in patients with Glanzmann thrombasthenia (GT). However, few data are available about molecular defects of GT patients with anti-αIIbß3 alloantibodies as well as clinical impact of these antibodies on platelet transfusion. Here, we report a case of type I GT with anti-HLA and anti-αIIbß3 alloantibodies, who underwent laparoscopic total gastrectomy due to gastric cancer. We found a novel ß3 nonsense mutation, 892C > T (Arg272X), and the patient was homozygous for the mutation. Laparoscopic gastrectomy was successfully performed with continuous infusion of HLA-matched platelet concentrates and bolus injection of recombinant factor VIIa at 2 h intervals. Total bleeding was 370 mL and no red-cell transfusion was necessary. Flow cytometric analysis employing anti-αIIbß3 monoclonal antibody revealed that the transfused platelet count was maintained around 20-30 × 109/L during the operation and 10 × 109/L on the following day. Flow cytometric analysis also showed that transfused platelets retained normal reactivity to ADP stimulation. These results indicate that flow cytometry is useful to assess survival and function of transfused platelets in GT patients with anti-αIIbß3 antibodies.

Fulminant septicemia of Bacillus cereus resistant to carbapenem in a patient with biphenotypic acute leukemia.

We report a case of fulminant septicemia with Bacillus cereus resistant to carbapenem. A 33-year-old man was suffering from febrile neutropenia (FN) on day 15 after the start of remission-induction therapy for biphenotypic acute leukemia under gut decontamination with polymyxin B and nystatin. Meropenem, a carbapenem, was administered according to the guideline for FN. Two days later (on day 17), he complained of severe abdominal pain, lost consciousness, went into sudden cardiopulmonary arrest, and died. Autopsy showed multiple spots of hemorrhage and necrosis caused by bacterial plaque in the brain, lungs, and liver. B. cereus was isolated from a blood sample obtained in the morning on day 17 and it was after his death that the isolated B. cereus was revealed to be resistant to carbapenem. B. cereus obtained from blood samples has been reported to be usually sensitive to carbapenem and also to vancomycin, new quinolones, and clindamycin. If B. cereus resistant to carbapem increases, our method of gut decontamination with polymyxin B and nystatin may have to be changed to one containing a new quinolone for the prevention of septicemia. Careful watching to determine whether B. cereus resistant to carbapem increases may be also important for empiric therapy, because carbapenem is often selected as the initial therapy for FN in patients with severe neutropenia.