Abstract Number: PB1735
Meeting: ISTH 2020 Congress
Background: GPIb-IX is a highly expressed platelet receptor complex composed of one GPIbα, one GPIX, and two GPIbβ subunits. Once immobilized to VWF, shear generated by blood flow unfolds a membrane-proximal mechanosensory domain (MSD) in GPIbα, exposing a conserved trigger sequence and activating the receptor. Although it is established that all three subunits of the complex are required for robust surface expression, only GPIbα appears to facilitate activation, containing both the MSD and the binding sites for all known ligands to GPIb-IX.
Aims: Despite being positioned directly adjacent to (and potentially in contact with) the MSD, the roles of GPIbβ and GPIX in signal transduction remain murky. We aim to investigate the effects of antibodies targeting these subunits in activating or allosterically modulating GPIb-IX.
Methods: Using a host of activating ligands for GPIb-IX, we tested the effects of antibodies targeting GPIbβ on shear-induced platelet activation via flow cytometry. We also assessed the modulatory effects of these antibodies on GPIb-IX-induced changes in the morphology of platelets and Chinese hamster ovary (CHO) cells expressing human GPIb-IX via confocal microscopy.
Results: We report the effects of anti-GPIbβ monoclonal antibodies RAM.1 and the novel 3G6. Both antibodies bound to the extracellular domain of GPIbβ with low nanomolar affinity. 3G6 amplified morphology changes in platelets induced by VWF or anti-GPIbα antibodies, leading to a greater proportion of platelets exhibiting a “spread” morphology. 3G6 also amplified platelet degranulation and desialylation downstream of GPIb-IX activation. In contrast, RAM.1 inhibited GPIb-IX signaling, blocking both platelet membrane spreading as well as surface markers of activation triggered by GPIb-IX activation.
Conclusions: These data represent the first report of a positive modulator of GPIb-IX activation, and strongly suggest that changes in the conformation of GPIbβ underlie mechano-transduction via GPIb-IX.
To cite this abstract in AMA style:Quach ME, Chen W, Wang Y, Deckmyn H, Lanza F, Nieswandt B, Li R. Differential Regulation of the Platelet GPIb-IX Complex by Anti-GPIbβ Antibodies [abstract]. Res Pract Thromb Haemost. 2020; 4 (Suppl 1). https://abstracts.isth.org/abstract/differential-regulation-of-the-platelet-gpib-ix-complex-by-anti-gpib%ce%b2-antibodies/. Accessed October 2, 2023.
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