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Platelet CD40 ligand (CD40L)--subcellular localization, regulation of expression, and inhibition by clopidogrel
A Hermann1, B H Rauch, M Braun
1Institut für Pharmakologie und Klinische Pharmakologie, Heinrich-Heine-Universität, Düsseldorf, Germany.
Insights
This study reveals that CD40 ligand (CD40L) is expressed on human platelets, but its surface expression is low and regulated differently than other activation markers. Clopidogrel inhibits ADP-induced CD40L expression.
Area of Science:
- Hematology
- Immunology
- Cell Biology
Background:
- Platelet activation is crucial in hemostasis and thrombosis.
- CD40 ligand (CD40L) is a key molecule in immune responses and platelet function.
- Understanding CD40L expression and regulation on platelets is important for therapeutic targeting.
Purpose of the Study:
- To compare the subcellular localization and expression regulation of CD40L with known platelet activation markers CD62P and CD63.
- To investigate the signaling pathways and conditions influencing CD40L surface expression on human platelets.
- To examine the effect of antiplatelet drugs on CD40L expression.
Main Methods:
- Quantitative flow cytometry to measure surface marker expression.
- Immunofluorescence microscopy and Western blotting for subcellular localization.
- Stimulation of washed human platelets with various agonists (e.g., TRAP-6, collagen, ADP).
- Investigation of signaling pathways involving Ca2+ stores, protein kinase C, MAP kinases, and tyrosine kinases.
- Assessment of antiplatelet agents (aspirin, clopidogrel) and PMA treatment on CD40L expression.
Main Results:
- CD40L is expressed on platelets upon stimulation, but at lower levels than CD62P and CD63.
- CD40L exhibits cytoplasmic localization, distinct from filamentous actin and the cytoskeleton.
- Collagen, but not TRAP-6, selectively induced CD40L expression without CD62P/CD63.
- Surface CD40L expression is dependent on internal Ca2+ stores and protein kinase C, independent of ERK/p38 or tyrosine kinases.
- ADP-induced CD40L expression is insensitive to aspirin but completely blocked by clopidogrel.
- Phorbol myristate acetate (PMA) upregulates CD40L in the MEG-01 cell line.
Conclusions:
- CD40L displays unique subcellular localization and differential expression patterns compared to canonical platelet activation markers.
- Platelet CD40L expression is regulated by specific intracellular signaling pathways, including Ca2+ and PKC.
- Clopidogrel effectively inhibits ADP-mediated CD40L expression, suggesting a potential therapeutic target.
- Further research into CD40L's role in platelet function and associated diseases is warranted.
Abstract:
This study compares the subcellular localization and the regulation of expression of the platelet activation markers CD62P and CD63 with CD40 ligand (CD40L) on the surface of washed human platelets. CD40L was expressed upon stimulation with a wide range of platelet activators. However, quantitative flow cytometry demonstrated that, as compared with CD62P and CD63, CD40L expression was low. Upon stimulation with thrombin receptor-activating peptide (TRAP-6), all activation markers were expressed. In contrast, upon stimulation with low concentrations of collagen (1-3 microg/ml), CD40L, but not the granule proteins (CD62P, CD63), were expressed. Using immunofluorescence microscopy, a cytoplasmic staining was observed for CD40L, and cytoplasmic localization of CD40L was verified by Western blotting of subcellular platelet fractions. The staining of CD40L was different from that of filamentous actin and only little association of CD40L with platelet cytoskeleton was found. Surface expression of CD40L was dependent on internal Ca2+ stores and protein kinase C, while the mitogen-activated protein kinases (ERK, p38) or tyrosine kinases were not involved. ADP (30 microM)-induced CD40L expression was not inhibited by aspirin. In contrast, clopidogrel treatment completely abolished ADP-induced expression of CD40L. Finally, the expression level of CD40L was shown to be upregulated by phorbol myristate acetate (PMA) in the promegakaryocytic cell line MEG-01.