Delineating Zinc Influx Mechanisms during Platelet Activation.

Zinc (Zn 2+ ) is released by platelets during a hemostatic response to injury. Extracellular zinc ([Zn 2+ ] o ) initiates platelet activation following influx into the platelet cytosol. However, the mechanisms that permit Zn 2+ influx are unknown. Fluctuations in intracellular zinc ([Zn 2+ ] i ) were measured in fluozin-3-loaded platelets using fluorometry and flow cytometry. Platelet activation was assessed using light transmission aggregometry. The detection of phosphoproteins was performed by Western blotting. [Zn 2+ ] o influx and subsequent platelet activation were abrogated by blocking the sodium/calcium exchanged, TRP channels, and ZIP7. Cation store depletion regulated Zn 2+ influx. [Zn 2+ ] o stimulation resulted in the phosphorylation of PKC substates, MLC, and β3 integrin. Platelet activation via GPVI or Zn 2+ resulted in ZIP7 phosphorylation in a casein kinase 2-dependent manner and initiated elevations of [Zn 2+ ] i that were sensitive to the inhibition of Orai1, ZIP7, or IP 3 R-mediated pathways. These data indicate that platelets detect and respond to changes in [Zn 2+ ] o via influx into the cytosol through TRP channels and the NCX exchanger. Platelet activation results in the externalization of ZIP7, which further regulates Zn 2+ influx. Increases in [Zn 2+ ] i contribute to the activation of cation-dependent enzymes. Sensitivity of Zn 2+ influx to thapsigargin indicates a store-operated pathway that we term store-operated Zn 2+ entry (SOZE). These mechanisms may affect platelet behavior during thrombosis and hemostasis.