Early Intraplatelet Signaling Enhances the Release of Human Platelet PAR-1 and -4 Amino-Terminal Peptides in Response to Thrombin

Preliminary Investigation about the Expression of G Protein-Coupled Receptors in Platelets from Patients with Chronic Immune Thrombocytopenic Purpura

OBJECTIVE

The objective of this study was to determine the expression of G protein-coupled receptors (GPCRs) in platelets from adult patients with chronic immune thrombocytopenic purpura (ITP).

METHODS

Peripheral blood samples were collected from 40 patients with chronic ITP in the Second Affiliated Hospital of Shantou University Medical College, and 40 peripheral blood samples from healthy volunteers were collected; expressions of the adenosine diphosphate receptors (P2Y1 and P2Y12), alpha-2A adrenergic receptor (α2A-AR), and thromboxane A2 receptor (TP) in platelets were detected by flow cytometry. Gα protein, protease-activated receptor 1 (PAR1), and protease-activated receptor 4 (PAR4) were analyzed by Western blot and analyzed statistically.

RESULTS

Flow cytometry measurements of mean fluorescence intensities showed platelets from patients with chronic ITP, compared to healthy individuals, had significantly higher levels of P2Y1 (31.4 ± 2.2 vs. 7.8 ± 0.8), P2Y12 (29.6 ± 2.1 vs. 7.2 ± 1.3), α2A-AR (25.8 ± 2.9 vs. 9.8 ± 0.9), and TP (39.8 ± 3.1 vs. 4.7 ± 0.6) (all p < 0.01). Similarly, integrated optical density analysis of Western blots showed that platelets from patients with chronic ITP had significantly higher levels of Gα (1046.3 ± 159.96 vs. 254.49 ± 39.51), PAR1 (832.98 ± 98.81 vs. 203.92 ± 27.47), and PAR4 (1518.80 ± 272.45 vs. 431.27 ± 41.86) (all p < 0.01).

CONCLUSION

Expression of GPCRs is increased in platelets from patients with chronic ITP, suggesting that platelets of chronic ITP may participate in the complicated biological process by means of GPCR-mediated signaling pathways.

Function of Platelet-Induced Epithelial Attachment at Titanium Surfaces Inhibits Microbial Colonization

The aim of this study was to evaluate the barrier function of platelet-induced epithelial sheets on titanium surfaces. The lack of functional peri-implant epithelial sealing with basal lamina (BL) attachment at the interface of the implant and the adjacent epithelium allows for bacterial invasion, which may lead to peri-implantitis. Although various approaches have been reported to combat bacterial infection by surface modifications to titanium, none of these have been successful in a clinical application. In our previous study, surface modification with protease-activated receptor 4–activating peptide (PAR4-AP), which induced platelet activation and aggregation, was successful in demonstrating epithelial attachment via BL and epithelial sheet formation on the titanium surface. We hypothesized that the platelet-induced epithelial sheet on PAR4-AP–modified titanium surfaces would reduce bacterial attachment, penetration, and invasion. Titanium surface was modified with PAR4-AP and incubated with platelet-rich plasma (PRP). The aggregated platelets released collagen IV, a critical BL component, onto the PAR4-AP–modified titanium surface. Then, human gingival epithelial cells were seeded on the modified titanium surface and formed epithelial sheets. Green fluorescent protein (GFP)–expressing Escherichia coli was cultured onto PAR4-AP–modified titanium with and without epithelial sheet formation. While Escherichia coli accumulated densely onto the PAR4-AP titanium lacking epithelial sheet, few Escherichia coli were observed on the epithelial sheet on the PAR4-AP surface. No bacterial invasion into the interface of the epithelial sheet and the titanium surface was observed. These in vitro results indicate the efficacy of a platelet-induced epithelial barrier that functions to prevent bacterial attachment, penetration, and invasion on PAR4-AP–modified titanium.

Establishment of Epithelial Attachment on Titanium Surface Coated with Platelet Activating Peptide

The aim of this study was to produce epithelial attachment on a typical implant abutment surface of smooth titanium. A challenging complication that hinders the success of dental implants is peri-implantitis. A common cause of peri-implantitis may results from the lack of epithelial sealing at the peri-implant collar. Histologically, epithelial sealing is recognized as the attachment of the basement membrane (BM). BM-attachment is promoted by activated platelet aggregates at surgical wound sites. On the other hand, platelets did not aggregate on smooth titanium, the surface typical of the implant abutment. We then hypothesized that epithelial BM-attachment was produced when titanium surface was modified to allow platelet aggregation. Titanium surfaces were coated with a protease activated receptor 4-activating peptide (PAR4-AP). PAR4-AP coating yielded rapid aggregation of platelets on the titanium surface. Platelet aggregates released robust amount of epithelial chemoattractants (IGF-I, TGF-β) and growth factors (EGF, VEGF) on the titanium surface. Human gingival epithelial cells, when they were co-cultured on the platelet aggregates, successfully attached to the PAR4-AP coated titanium surface with spread laminin5 positive BM and consecutive staining of the epithelial tight junction component ZO1, indicating the formation of complete epithelial sheet. These in-vitro results indicate the establishment of epithelial BM-attachment to the titanium surface.