The role of platelet and endothelial GARP in thrombosis and hemostasis

Abstract

BackgroundGlycoprotein-A Repetitions Predominant protein (GARP or LRRC32) is present on among others human platelets and endothelial cells. Evidence for its involvement in thrombus formation was suggested by full knockout of GARP in zebrafish.ObjectivesTo evaluate the role of GARP in platelet physiology and in thrombus formation using platelet and endothelial conditional GARP knock out mice.MethodsPlatelet and endothelial specific GARP knockout mice were generated using the Cre-loxP recombination system. The function of platelets without GARP was measured by flow cytometry, spreading analysis and aggregometry using PAR4-activating peptide and collagen related peptide. Additionally, clot retraction and collagen-induced platelet adhesion and aggregation under flow were analyzed. Finally, in vivo tail bleeding time, occlusion time of the mesenteric and carotid artery after FeCl3-induced thrombosis were determined in platelet and endothelial specific GARP knock out mice.ResultsPlatelet specific GARP knockout mice had normal surface GPIb, GPVI and integrin αIIb glycoprotein expression. Although GARP expression was increased upon platelet activation, platelets without GARP displayed normal agonist induced activation, spreading on fibrinogen and aggregation responses. Furthermore, absence of GARP on platelets did not influence clot retraction and had no impact on thrombus formation on collagen-coated surfaces under flow. In line with this, neither the tail bleeding time nor the occlusion time in the carotid- and mesenteric artery after FeCl3-induced thrombus formation in platelet or endothelial specific GARP knock out mice were affected.ConclusionsEvidence is provided that platelet and endothelial GARP are not important in hemostasis and thrombosis in mice.

Glycoprotein A repetitions predominant (GARP) positively regulates transforming growth factor (TGF) β3 and is essential for mouse palatogenesis

“…Interestingly, a recent study observed no pups for CMV-cre/ GARP fl/fl mice and suggested an "embryonic lethal phenotype" (39). As neither embryos nor dead pups have been analyzed for defects of CMV-cre/GARP fl/fl mice/embryos, whether the two kinds of GARP KO mice have the same lethal defects need to be further determined by analyzing CMV-cre/GARP fl/fl mice.…”

Immunoregulatory functions and the therapeutic implications of GARP-TGF-β in inflammation and cancer

“…Tregs are not the only cell population that experiences GARP-FoxP3 co-regulation; human and murine megakaryocytes and platelets constitutively express both FoxP3 and the surface GARP/LAP complex. Interestingly, upon activation, platelets upregulate both GARP and FoxP3: protease-activated receptor 4 activating peptide (PAR4-AP) increases surface GARP, while phorbol ester myristate acetate upregulates FoxP3 expression [ 15 , 35 , 36 ]. Although the simultaneous upregulation of GARP and FoxP3 needs to be demonstrated, these findings suggest that platelets are another subset of cellular entities where GARP and FoxP3 interdependence might occur.…”

“…A second study, performed in a genetic mouse model where Lrrc32 is specifically knocked out from platelets and megakaryocytes, shows that GARP is not necessary for thrombus formation and clot retraction, which is also confirmed by our own unpublished observations. Interestingly this last study shows that ex vivo platelet activation triggers increase in GARP surface expression, indicating that GARP might play a role in activating platelets [ 35 ]. Using the same platelet-specific GARP knockout mouse model, we recently demonstrated that GARP enhances the activation of latent TGF-β released by platelets [ 24 ].…”

Correlations of inhaled NO with the cTnI levels and the plasma clotting factor in rabbits with acute massive pulmonary embolism

“…In the study, cardiac troponin levels increased and reached a peak concentration of 0.46 ± 0.10 µg/L, meanwhile plasma vWF, GP IIb/IIIa, and GMP-140 increased in rabbits after massive area pulmonary thromboembolism, and the peak concentrations of plasma cardiac troponin with vWF, GP IIb/IIIa, GMP-140, were significantly positively correlated. The increased vWF indicates that platelet adhesion increased and the pulmonary vascular endothelium was impaired 10,11 , which activated platelets when exposed to blood vessels via vWF 12,19 and aggravated thrombosis and increased vWF 20,21 . GP IIb/IIIa is the main glycoprotein in the platelet membrane 22 and mediates platelet and fibrinogen combination, which can be activated by collagen, causing endothelial cell damage by pulmonary thromboembolism, hypoxia, and pulmonary hypertension 22 .…”