Agonist-Evoked Increases in Intra-Platelet Zinc Couple to Functional Responses

Abstract

Background ?Zinc (Zn 2+ ) is an essential trace element that regulates intracellular processes in multiple cell types. While the role of Zn 2+ as a platelet agonist is known, its secondary messenger activity in platelets has not been demonstrated. Objectives ?This article determines whether cytosolic Zn 2+ concentrations ([Zn 2+ ] i ) change in platelets in response to agonist stimulation, in a manner consistent with a secondary messenger, and correlates the effects of [Zn 2+ ] i changes on activation markers. Methods ?Changes in [Zn 2+ ] i were quantified in Fluozin-3 (Fz-3)-loaded washed, human platelets using fluorometry. Increases in [Zn 2+ ] i were modelled using Zn 2+ -specific chelators and ionophores. The influence of [Zn 2+ ] i on platelet function was assessed using platelet aggregometry, flow cytometry and Western blotting. Results ?Increases of intra-platelet Fluozin-3 (Fz-3) fluorescence occurred in response to stimulation by cross-linked collagen-related peptide (CRP-XL) or U46619, consistent with a rise of [Zn 2+ ] i . Fluoresence increases were blocked by Zn 2+ chelators and modulators of the platelet redox state, and were distinct from agonist-evoked [Ca 2+ ] i signals. Stimulation of platelets with the Zn 2+ ionophores clioquinol (Cq) or pyrithione (Py) caused sustained increases of [Zn 2+ ] i , resulting in myosin light chain phosphorylation, and cytoskeletal re-arrangements which were sensitive to cytochalasin-D treatment. Cq stimulation resulted in integrin α IIb β 3 activation and release of dense, but not α, granules. Furthermore, Zn 2+ -ionophores induced externalization of phosphatidylserine. Conclusion ?These data suggest that agonist-evoked fluctuations in intra-platelet Zn 2+ couple to functional responses, in a manner that is consistent with a role as a secondary messenger. Increased intra-platelet Zn 2+ regulates signalling processes, including shape change, α IIb β 3 up-regulation and dense granule release, in a redox-sensitive manner.