MAPKAPK2-mediated LSP1 phosphorylation and FMLP-induced neutrophil polarization.

In neutrophils, the major substrate of MAPKAPK2 (MK2) is an F-actin binding protein LSP1. Studies using mutants of the two potential Serine phosphorylation sites in LSP1 C-terminal F-actin binding region indicated that the major phosphorylation site for MK2 is Ser243 in murine neutrophils (Ser252 in humans). Human phosphoLSP1 antibodies that recognize phosphoSer252 site were prepared and revealed fMLP-induced neutrophil LSP1 phosphorylation. The phosphorylation was inhibited by p38 MAPK (upstream kinase for MK2) inhibitor SB203580. The antibodies also detect LSP1 phosphorylation in murine neutrophils. Immunostaining revealed that in WT murine neutrophils phosphoLSP1 was localized in F-actin enriched lamellipodia and oriented toward the fMLP gradient while non-phosphoLSP1 failed to colocalize with F-actin. In suspension, WT neutrophils exhibited persistent F-actin polarization following fMLP stimulation, while MK2(-/-) neutrophils exhibited transient F-actin polarization. These studies suggest that MK2-regulated LSP1 phosphorylation is involved in stabilization of F-actin polarization during neutrophil chemotaxis.