Amelioration of the macrothrombocytopenia associated with the murine Bernard-Soulier syndrome

Abstract

An absent platelet glycoprotein (GP) Ib-IX receptor results in the Bernard-Soulier syndrome and is characterized by severe bleeding and the laboratory presentation of macrothrombocytopenia. Although the macrothrombocytopenic phenotype is directly linked to an absent GP Ib-IX complex, the disrupted molecular mechanisms that produce the macrothrombocytopenia are unknown. We have utilized a mouse model of the Bernard-Soulier syndrome to engineer platelets expressing an α-subunit of GP Ib (GP Ibα) in which most of the extracytoplasmic sequence has been replaced by an isolated domain of the α-subunit of the human interleukin-4 receptor (IL-4Rα). The IL-4Rα/GP Ibα fusion is membrane expressed in Chinese hamster ovary (CHO) cells, and its expression is facilitated by the presence of human GP IX and the β-subunit of GP Ib. Transgenic animals expressing a chimeric receptor were generated and bred into the murine Bernard-Soulier syndrome–producing animals devoid of mouse GP Ibα but expressing the IL-4Rα/GP Ibα fusion sequence. The characterization of these mice revealed a 2-fold increase in circulating platelet count and a 50% reduction in platelet size when compared with platelets from the mouse model of the Bernard-Soulier syndrome. Immunoprecipitation confirmed that the IL-4Rα/GP Ibα subunit interacts with filamin-1 and 14-3-3ζ, known binding proteins to the GP Ibα cytoplasmic tail. Mice expressing the chimeric receptor retain a severe bleeding phenotype, confirming a critical role for the GP Ibα extracytoplasmic domain in hemostasis. These results provide in vivo insights into the structural elements of the GP Ibα subunit that contribute to normal megakaryocyte maturation and thrombopoiesis.