Role of the Leucine-Rich Domain of Platelet GPIbα in Correct Post-translational Processing – The Nancy I Bernard-Soulier Mutation Expressed on CHO Cells

Abstract

The mechanisms governing the biosynthesis and surface expression of platelet adhesive receptors on parent megakaryocytes are as yet poorly understood. In particular, the assembly and processing of the multisubunit glycoprotein (GP) Ib-IX-V complex, a receptor for von Willebrand factor (vWf) is not fully understood. In the present work, these questions were addressed by reproducing a natural mutation of GPIbalpha found in a variant case of Bernard-Soulier syndrome (Nancy I), due to the deletion of leucine 179 in the seventh leucine-rich repeat of the polypeptide. Wild type and mutated GPIbalpha were transfected into CHO cells expressing GPlbbeta and GPIX. Flow cytometry showed surface expression of the three subunits of both GPIb-IX complexes, but GPlbalphadeltaLeu was present at lower levels (20-40%) and was recognized only by a sub class of monoclonal antibodies which epitopes were not modified by the mutation. These properties reproduce the defect found in the patient's platelets, demonstrating the causative nature of the mutation and validate the use of the CHO cells model. Biochemical studies were performed in an attempt to elucidate the mechanism of the conformational change of GPIbalphadeltaLeu. They unexpectedly revealed a major glycosylation deficiency of the mutated GPIbalpha leading to a 40% decrease in molecular weight. The other two subunits of the complex were however normal and present at the plasma membrane. The deletion led to complete functional deficiency with lack of vWf binding of CHOalphadeltaLeu transfected cells in the presence of botrocetin and defective adhesion to a vWf coated surface under static conditions. Finally, in contrast to normal CHOalphabetaIX cells, which displayed rolling and deceleration when perfused over a vWf surface, CHOalphadeltaLeubetaIX cells were unable to roll over or attach to a vWf substratum. These results show that the integrity of the leucine-rich region of GPIbalpha is essential for normal processing and function of the GPIb-IX complex. In addition, these results obtained in a cellular system supported the suspected role of the macroglycopeptide region of GPIbalpha in maintaining a suitable conformation of this multisubunit receptor to perform its adhesive function.