Cytoskeletal perturbation leads to platelet dysfunction and thrombocytopenia in variant forms of Glanzmann thrombasthenia

Abstract

Several patients have been reported to have variant dominant forms of Glanzmann thrombasthenia, associated with macrothrombocytopenia and caused by gain-of-function mutations of ITGB3 or ITGA2B leading to reduced surface expression and constitutive activation of integrin α IIb β 3 . The mechanisms leading to a bleeding phenotype of these patients have never been addressed. The aim of this study was to unravel the mechanism by which ITGB3 mutations causing activation of α IIb β 3 lead to platelet dysfunction and macrothrombocytopenia. Using platelets from two patients carrying the β 3 del647-686 mutation and Chinese hamster ovary cells expressing different α IIb β 3 -activating mutations, we showed that reduced surface expression of α IIb β 3 is due to receptor internalization. Moreover, we demonstrated that permanent triggering of α IIb β 3 -mediated outside-in signaling causes an impairment of cytoskeletal reorganization arresting actin turnover at the stage of polymerization. The induction of actin polymerization by jasplakinolide, a natural toxin that promotes actin nucleation and prevents depolymerization of stress fibers, in control platelets produced an impairment of platelet function similar to that of patients with variant forms of dominant Glanzmann thrombasthenia. del647-686β 3 -transduced murine megakaryocytes generated proplatelets with a reduced number of large tips and asymmetric barbell-proplatelets, suggesting that impaired cytoskeletal rearrangement is the cause of macrothrombocytopenia. These data show that impaired cytoskeletal remodeling caused by a constitutively activated α IIb β 3 is the main effector of platelet dysfunction and macrothrombocytopenia, and thus of bleeding, in variant forms of dominant Glanzmann thrombasthenia. Cytoskeletal perturbation leads to platelet dysfunction and thrombocytopenia in variant forms of Glanzmann thrombasthenia

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

“…In particular, the formation of extensions and proplatelet shaft bifurcations are clearly actin dependent, and it is not surprising that actin plays a fundamental role in the terminal step of proplatelet production (8). Recently, it has been described that proplatelet formation was impaired and MKs displayed a reduced number of proplatelet tips of altered diameter, while actin polymerization was increased in vitro (36). The signaling cascade that controls actin dynamics involves a variety of proteins, including cofilin.…”

An intracytoplasmic β3 Leu718 deletion in a patient with a novel platelet phenotype

“…22,23 Genetic variants affecting aIIb and b3 cytoplasmic domains that produce MTP show that the mutations result in an altered cytoskeletal architecture and reduced proplatelet formation. 2,13,24 Our ITGB3 L718del (with loss of synchronization between the intracytoplasmic tail of b3 with that of aIIb) resulted in moderate MTP, decreased activation of aIIbb3 at the platelet surface, and abnormally large a-granules and represents a novel phenotype. Large compound granules have recently been described in platelets as part of the exocytosis mechanism, 25 but in our case, the giant granules were not empty.…”

“…1,2 We now report a novel phenotype in a French family with MTP in whom a single amino acid deletion (del) in the b3 cytoplasmic tail is accompanied by platelet anisocytosis, enlarged platelet a-granules, and reduced platelet aggregation.…”

ITGA2B and ITGB3 gene mutations associated with Glanzmann thrombasthenia

“…Also shown in Figure 2 are membrane-proximal or cytoplasmic tail missense mutations or deletions, often affecting single alleles (with possible autosomal dominant inheritance) and provoking limited conformational changes in αIIbβ3 (detected by the activation-dependent monoclonal antibody, PAC-1 but without spontaneous Fg binding) that lead to a moderate macrothrombocytopenia combined with a severe platelet aggregation deficiency [reviewed in 6,16]. This is associated with a reduced capacity of MKs to form proplatelets with release of platelets with anisotrophy and according to a recent report enlarged α-granules [16,17].…”