The platelet glycoprotein (GP) Ib-IX-V complex is major platelet adhesion receptor encoded by four distinct gene products (11
). The major structural and functional subunit of the receptor, GP Ib, is composed of two disulfide-linked subunits each encoded by a separate gene, GP Ibα and GP Ibβ. The disulfide bridge linking GP Ibα and GP Ibβ is present in the extracytoplasmic portion of each subunit near the single transmembrane spanning region of each polypeptide (). As a separate gene product, the GP IX polypeptide facilitates membrane expression of a GP Ib-IX complex (12
). Expression of the GP V gene and polypeptide is not required for the assembly of a GP Ib-IX complex but does assemble with GP Ib-IX and is clearly related to each subunit of the GP Ib-IX as evidenced by the presence of leucine-rich repeats common to all subunits of the GP Ib-IX-V complex (13
Platelet membrane receptors are depicted that are essential for the initial events of thrombus formation.GP Ib-IX-V,GP IIb/IIIa,GP VI/FcR-γ and α2β1
As with most disorders of hemostasis and thrombosis, the clinical manifestation of an absent GP Ib-IX-V complex was characterized long before recognition of the underlying biochemical defect. French physicians, Jean Bernard and Jean Pierre Soulier, originally described the clinical disorder, the Bernard-Soulier syndrome, characterized by mild thrombocytopenia, giant platelets in a blood smear, and absent ristocetin-induced platelet aggregation (14
). The molecular basis of the Bernard-Soulier syndrome results from a mutation in either of the 3 genes encoding the GP Ib-IX complex. A model recapitulating the human Bernard-Soulier syndrome was produced by a genetic deletion of the mouse gene for GP Ibα (15
). In this case, the mutation reproduced the giant platelet and low circulating platelet count associated with the human syndrome. The characterization of mouse Bernard-Soulier syndrome platelets with ristocetin is not possible as ristocetin-mediated platelet agglutination is species-specific. However, by one criteria of mouse hemostasis, the tail bleeding time assay, the mouse Bernard-Soulier syndrome produces a severe bleeding phenotype (15
). Beyond its defect in hemostasis, the mice representing the murine equivalent of the Bernard-Soulier syndrome have no obvious phenotypic anomalies.
The generation of a mouse model of the Bernard-Soulier syndrome also provided “proof of principle” that an absent of the GP Ib-IX complex directly results in the macrothrombocytopenia. In the addition to the abnormal platelets observed on blood smears, the megakaryocytes present in the marrow of Bernard-Soulier mouse display a disordered demarcation membrane system and a reduced cytoplasmic amount of membrane (16
). The linkage in producing this phenotype is clearly dependent on the cytoplasmic tail of glycoprotein Ibα as a fusion protein composed of an extracytoplasmic sequence of the interleukin-4 receptor fused to the GPIbα transmembrane and cytoplasmic domains ameliorates the macrothrombocytopenic phenotype (17
). Of course, these mice retain their severe bleeding phenotype due to the absence of an amino-terminal domain of GP Ibα and ligand binding sites for von Willebrand factor and thrombin. The same extracytoplasmic region of GP Ibα also interacts with an expanding repertoire of ligands, such as factor XI and Mac-1 (11
), and the physiologic relevance of these interactions may also be contributing to some aspects of the bleeding phenotype.
Prior to the development of mouse models of the Bernard-Soulier syndrome a relatively extensive database of mutations associated with human syndrome had been obtained (http://www.bernard-soulier.org/
). Most mutations are within the gene encoding the GP Ibα subunit while a limited number of mutations have been localized to either the GP Ibβ or GP IX genes. The preponderance of mutations in the GP Ibα gene most likely reflects the larger size of the GP Ibα gene and the possible dominance of GP Ibα as the major functional subunit of the receptor. However, in comparing the mouse Bernard-Soulier syndrome to the described phenotypes of individuals diagnosed with the Bernard-Soulier syndrome it is important to consider the heterogeneity of mutations producing the human Bernard-Soulier phenotype. For example, the mouse model of the Bernard-Soulier syndrome was generated by a complete replacement of the all of the nucleotide sequences that encode GP Ibα In constrast, in the human syndrome many of the mutations produce frame-shifts, or premature stop codons that certainly abrogate surface expression of a GP Ib-IX complex but might allow the partial synthesis of an abnormal subunit. Thus, it would be expected that some subtle differences might exist among individuals with the Bernard-Soulier syndrome and the phenotype found in the murine model.
As an associated subunit of the GP Ib-IX complex, it is worth noting that no human mutations have been found within the GP V that give rise to a Bernard-Soulier phenotype. Indeed, two different genetic knockouts of the mouse GP V gene failed to generate a Bernard-Soulier phenotype (18
). Instead, the deletion of GP V results in a slightly shortened bleeding time face but unable to spread or form thrombi in flowing whole blood (31
). The conclusion from such an experiment is that GP VI is not essential as an adhesion receptor but is an important activation receptor. However, a contradictory conclusion was made by Nieswandt and colleagues examining GP VI-deficient platelets generated by antibody (anti-GP VI) removal of the GP VI from the platelet surface (33
). In the later study, an examination of thrombus formation was performed in vivo with a damaged carotid artery and a major defect in both adhesion and thrombus formation was noted. Either a genetic or immunological removal of GP VI had little impact on the tail bleeding time in mice.
Comparing the genetic versus immunological removal of GP VI raises questions into how critical is GP VI for platelet adhesion (34
). However, a number of differences do exist between the two models. First, is the surprising stability and presence of the FcR-γ subunit in platelets genetically devoid of GP VI. In the antibody-induced depletion of GP VI, a concomitant removal of GP VI and FcR-γ occurs (31
). Whether the FcR-γ subunit can assemble with other platelet receptor complexes in the absence of GP VI is still under investigation. In addition, it is unclear what impact the immuno-depletion of a GP VI/FcR-γ complex would have on platelet function mediated by other receptor complexes. As is true in both cases, the conclusions derived from both systems may be biased due to an under appreciation on the net global platelet effect that either genetic and/or immunological depletion causes. Nevertheless, some of the complexity of the platelet response to vascular injury can be appreciated just by acknowledging the similarities that exist between the GP Ib-IX-V receptor and GP VI. Both are critical for the early events of primary hemostasis. GP Ib-IX-V interacts with vWF, whereas GP VI interacts with collagen. Once you move beyond ligand binding both receptors facilitate platelet activation and both must activate a myriad of signal transduction molecules critical for the hemostatic response. The possibility of GP Ib-IX-V/GP VI cross-talk has been recently supported with data characterizing the signaling effects of the snake lectin, alboaggregin A (35
). The snake venom protein binds to both GP Ib and GP VI and appears to result in similar signaling events for each receptor (35
). A second snake venom protein, Convulxin, was long thought to be a GP VI-specific agonist but has recently been shown to interact with the GP Ib-IX-V complex (36
). Whether Convulxin generates platelet activation via the GP Ib-IX-V complex has yet to be explored but certainly highlights the potential overlap for receptor responses in the initiation of the hemostatic response.