Platelet heterogeneity in activation-induced glycoprotein shedding: functional effects

Abstract

The platelet receptors glycoprotein Ibα (GPIbα) and GPVI are known to be cleaved by members of a disintegrin and metalloprotease (ADAM) family (ADAM10 and ADAM17), but the mechanisms and consequences of this shedding are not well understood. Our results revealed that (1) glycoprotein shedding is confined to distinct platelet populations showing near-complete shedding, (2) the heterogeneity between (non)shed platelets is independent of agonist type but coincides with exposure of phosphatidylserine (PS), and (3) distinct pathways of shedding are induced by elevated Ca²⁺, low Ca²⁺ protein kinase C (PKC), or apoptotic activation. Furthermore, we found that receptor shedding reduces binding of von Willebrand factor, enhances binding of coagulation factors, and augments fibrin formation. In response to Ca²⁺-increasing agents, shedding of GPIbα was abolished by ADAM10/17 inhibition but not by blockage of calpain. Stimulation of PKC induced shedding of only GPIbα, which was annulled by kinase inhibition. The proapoptotic agent ABT-737 induced shedding, which was caspase dependent. In Scott syndrome platelets that are deficient in Ca²⁺-dependent PS exposure, shedding occurred normally, indicating that PS exposure is not a prerequisite for ADAM activity. In whole-blood thrombus formation, ADAM-dependent glycoprotein shedding enhanced thrombin generation and fibrin formation. Together, these findings indicate that 2 major activation pathways can evoke ADAM-mediated glycoprotein shedding in distinct platelet populations and that shedding modulates platelet function from less adhesive to more procoagulant.

Graphical abstract

Figure 1.

ADAM10/17-mediated glycoprotein shedding upon platelet stimulation with strong agonists. Washed platelets, which were preincubated for 15 minutes with DMSO (vehicle) or ADAM17/10 inhibitor GW280264X (GW; 5 μM), were stimulated with 2MeS-ADP, CRP-XL, thrombin (Thr), or CRP-XL/Thr (GPIbα, n = 8) for 180 minutes or with ionomycin (GPIbα, n = 8) for 60 minutes (all at 37°C). Unstimulated platelets were used as control (Ctrl). Staining for GPIbα (A,D), GPVI (B,E), and GPIX (C,F) was measured with optimized concentrations of FITC anti-GPIbα mAb, PE anti-GPVI mAb, or FITC anti-GPIX mAb using flow cytometry (mean fluorescence intensities). (A-C) Effect of agonist stimulation on glycoprotein expression levels. Data are normalized to those of unstimulated platelets. (D-F) Effect of GW treatment on glycoprotein expression level. Data are mean ± SD, n = 3-5 (≥3 donors). *P < .05 vs unstimulated platelets, ^#P < .05 vs vehicle. n.s., not significant.

Figure 2.

Comparable kinetics of glycoprotein receptor shedding and PS exposure in ionomycin-stimulated platelets. Washed platelets, which were preincubated with vehicle or GW280264X (GW; 5 μM), were left unstimulated (Ctrl) or were stimulated for the indicated times (15 to 300 minutes) with ionomycin (37°C) and evaluated for glycoprotein expression by flow cytometry. (A-B) ADAM-dependent decrease in surface expression of GPIbα and GPVI over time. Mean fluorescence intensities were normalized to those of unstimulated platelets. (C-D) Analysis of platelet populations after dual staining for GPIbα (FITC mAb) or GPVI (PE mAb) and for PS exposure (AF647-annexin A5). Flow cytometric events were separated into 4 platelet populations: GPIbα/VI^high PS⁻, GPIbα/VI^high PS⁺, GPIbα/VI^low PS⁻, and GPIbα/VI^low PS⁺. Shown are representative dot plots and quantification of 4 quadrants. Data are mean ± SD, n = 3-5 (≥3 donors). *P < .05 vs control, ^#P < .05 vs vehicle.

Figure 3.

Glycoprotein receptor shedding following PS exposure in CRP-XL/thrombin–stimulated platelets. Washed platelets, which were preincubated with vehicle or ADAM10/17 inhibitor GW280264X (GW; 5 μM), were left unstimulated (Ctrl) or were stimulated with CRP-XL plus thrombin (Thr) for the indicated times and then evaluated for glycoprotein expression by flow cytometry. (A-B) ADAM-dependent decrease in surface expression of GPIbα and GPVI over time. (C-D) Population analysis of platelets after dual staining for GPIbα (FITC mAb) or GPVI (PE mAb) and for PS exposure (AF647-annexin A5). Four platelet populations are defined, as in Figure 2. Note the gradual increase in the PS-exposing platelet populations and the slower increase in populations with shed GPIbα or GPVI. Data are mean ± SD, n = 3-7 (≥3 donors). *P < .05 vs control, ^#P < .05 vs vehicle.

Figure 4.

Normal glycoprotein shedding in Scott syndrome platelets with defective PS exposure. Platelets from a healthy control subject or a Scott patient were stimulated with 10 µM ionomycin (A), 10 µM ABT-737 (B), 100 ng/mL convulxin and 4 nM thrombin (C), or 100 µM CCCP (D) at 37°C. GPIbα expression and PS exposure were measured at the indicated time points. Platelets were separated into 4 populations (GPIbα^high PS⁻, GPIbα^high PS⁺, GPIbα^low PS⁻, and GPIbα^low PS⁺) and quantified. Data are means of 3 replicates.

Figure 5.

Analysis of signaling pathways implicated in agonist-induced glycoprotein shedding. Washed platelets were pretreated with ADAM10/17 GW280264X, the calpain inhibitor MDL-28170, the pan-caspase inhibitor Q-VD-OPh, the general PKC inhibitor Ro-318425, the Ca²⁺ chelator dm-BAPTA AM, or the Ca²⁺ entry inhibitor 2APB. Samples were stimulated with ionomycin/CaCl₂, CRP-XL plus thrombin, PMA, ABT-737, or CCCP, as indicated. Data are normalized to those of unstimulated platelets without any inhibitors. Shown is a heat map representing the effects of agonists on GPIbα shedding (red), as well as relative antagonizing effects of inhibitors (blue). Note the 3 clusters representing a role for Ca²⁺ elevation (i), caspase activation (ii), and PKC activity (iii). See supplemental Table 1 for additional information.

Figure 6.

Selective loss of GPIbα and GPVI in high-Ca²⁺PS-exposing platelets in thrombi. Platelet thrombi were formed in flow chambers on collagen type I using whole-blood perfusion at a wall shear rate of 1000 s⁻¹. The thrombi were postactivated with thrombin, incubated with vehicle or GW280264X (GW) for up to 180 minutes at 37°C, and stained for GPIbα and GPVI expression and PS exposure. (A) Representative confocal fluorescence images of GPIbα and GPVI expression and PS exposure. PS-exposing ballooned platelets are indicated by arrows and arrowheads. (B) Regions of interest corresponding to aggregated platelets and ballooned platelets were analyzed for fluorescence staining. Pixels representing GPIbα⁻ or GPVI⁺ platelets were determined and expressed as percentages of surface area coverage of ballooned or aggregated platelets. Data are mean ± SD, n = 3. *P < .05.

Figure 7.

Glycoprotein shedding increases coagulation factor binding to platelets and enhances the formation of thrombin and fibrin. (A) Washed platelets were preincubated with vehicle, GW280264X (GW; 5 µM, 15 minutes), or 5G6 Fab fragment, which are known to block ADAM17-mediated shedding of GPIbα (10 µg/mL, 15 minutes). After 60 minutes of stimulation (37°C) with ionomycin, ABT-737, or CRP-XL plus SFLLRN, flow cytometry was used to measure platelet binding of VWF with fluorescent anti-VWF mAb (in the presence of ristocetin) (Ai), OG488-prothrombin (Aii), OG488-factor Xa (Aiii), or AF488-factor Va (Aiv). Binding of prothrombin, factor Xa, and factor Va was evaluated for the PS-exposing platelet population, identified with AF647-annexin A5. Data are mean ± SD, n = 3-6. (B) Platelet thrombi were formed on collagen and posttreated with SFLLRN for 60 minutes at 37°C in the presence of vehicle or GW (5 µM). After incubation, the thrombi were stained with OG488-prothrombin or were incubated with recalcified plasma supplemented with fluorogenic substrate Z-Gly-Gly-Arg-AMC to measure thrombin generation. Representative fluorescence images (Bi) and quantification of prothrombin fluorescence (Bii). (Biii) Time course (under stasis) of generation of thrombin from the surface of platelet thrombi formed in the presence or absence of GW. Data are mean ± SD, n = 3. (C) Kinetics of fibrin formation at a layer of adhered platelets on collagen, posttreated with ionomycin for 10 minutes, and inducing maximal shedding, in the presence of vehicle or GW. Fibrin formation was then allowed by perfusion of recalcified normal plasma at 250 s⁻¹, while recording fluorescence images. Shown are times to fibrin formation and representative time traces of fibrin formation (expressed as SAC). Data are mean ± SD, n = 6. *P < .05.