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The two calcium- and zinc-binding proteins, S100A9 and S100 A8, abundant in myeloid cells are considered to play important roles in both calcium signalling and zinc homeostasis. Polymorphonuclear neutrophils from S100A9 ko mice are also devoid of S100A8. Therefore, S100A9-deficient neutrophils were used as a model to study the role of the two S100 proteins in the neutrophils's calcium and zinc metabolism. Analysis of the intracellular zinc level upon pyrithione and (+/-)-(E)-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexeneamide (NOR-1) treatment revealed no differences between S100A9-deficient and wildtype neutrophils. Similar, the calcium signals were not distinguishable from S100A9-deficient and wildtype neutrophils upon stimulation with platelet activating factor (PAF), thapsigargin or macrophage inflammatory protein 1 alpha (MIP-1 alpha), indicating despite their massive expression S100A8/A9 do neither serve as calcium nor as zinc buffering proteins in granulocytes. In contrast, stimulation with adenosine-5'-triphosphate (ATP) induces a significant stronger increase of the intracellular free calcium level in S100A9-deficient cells compared to wildtype cells. Moreover, the ATP-induced calcium signal was still different when the cells were incubated in calcium free buffer suggesting that pirinergic receptors of the P(2Y) class could be involved in this signalling pathway.

MRP14 (S100A9) is the major calcium-binding protein of neutrophils and monocytes. Targeted gene disruption reveals an essential role of this S100 protein for transendothelial migration of phagocytes. The underlying molecular mechanism comprises major alterations of cytoskeletal metabolism. MRP14, in complex with its binding partner MRP8 (S100A8), promotes polymerization of microtubules. MRP14 is specifically phosphorylated by p38 mitogen-activated protein kinase (MAPK). This phosphorylation inhibits MRP8/MRP14-induced tubulin polymerization. Phosphorylation of MRP14 is antagonistically regulated by binding of MRP8 and calcium. The biologic relevance of these findings is confirmed by the fact that MAPK p38 fails to stimulate migration of MRP14(-/-) granulocytes in vitro and MRP14(-/-) mice show a diminished recruitment of granulocytes into the granulation tissue during wound healing in vivo. MRP14(-/-) granulocytes contain significantly less polymerized tubulin, which subsequently results in minor activation of Rac1 and Cdc42 after stimulation of p38 MAPK. Thus, the complex of MRP8/MRP14 is the first characterized molecular target integrating MAPK- and calcium-dependent signals during migration of phagocytes.