Perhaps the most important feature of neutrophils is their ability to rapidly change shape. In the bloodstream, the neutrophils circulate as almost spherical cells, with the ability to deform in order to pass along narrower capillaries. Upon receiving the signal to extravasate, they are able to transform their morphology and flatten onto the endothelium surface. This transition, from a spherical to a flattened morphology, is the first key step which neutrophils undergo before moving out of the blood and into the extravascular tissue space. Once they have migrated through tissues towards sites of infection, neutrophils carry out their primary role—killing infecting microbes by performing phagocytosis and producing toxic reactive oxygen species within the microbe-containing phagosome. Phagocytosis involves the second key morphology change that neutrophils undergo, with the formation of pseudopodia which capture the microbe within an internal vesicle. Both the spherical to flattened stage and the phagocytic capture stage are rapid, each being completed within 100 s. Knowing how these rapid cell shape changes occur in neutrophils is thus fundamental to understanding neutrophil behaviour. This article will discuss advances in our current knowledge of this process, and also identify an important regulated molecular event which may represent an important target for anti-inflammatory therapy.

The intracellular events underlying phagocytosis, a crucial event for innate immunity, are still unresolved. In order to test whether the reservoir of membrane required for the formation of the phagocytic pseudopodia is maintained by cortical ezrin, and that its cleavage is a key step in releasing this membrane, the cleavage of cortical ezrin was monitored within living phagocytes (the phagocytically competent cell line RAW264.7) through expressing two ezrin constructs with fluorescent protein tags located either inside the FERM or at the actin-binding domains. When ezrin is cleaved in the linker region by the Ca 2+-activated protease calpain, separation of the two fluorophores would result. Experimentally induced Ca 2+ influx triggered cleavage of peripherally located ezrin, which was temporally associated with cell expansion. Ezrin cleavage was also observed in the phagocytic pseudopodia during phagocytosis. Thus, our data demonstrates that peripheral ezrin is cleaved during Ca 2+-influxinduced membrane expansion and locally within the extending pseudopodia during phagocytosis. This is consistent with a role for intact ezrin in maintaining folded membrane on the cell surface, which then becomes available for cell spreading and phagocytosis.

The susceptibility of prolonged chemotaxis to inhibition by NSC668394 may point to a useful target for anti-inflammatory therapy. Inhibition of neutrophil chemotaxis towards chronically inflamed sites without compromising their ability to undergo phagocytosis is a much sought after the effect of anti-neutrophil therapy.

No abstract

Highlights• A low Ca 2+ affinity GECI-fused to ezrin, EPIC3, was developed to monitor [Ca 2+ ] at the cell cortex • EPIC3 revealed [Ca 2+ ] elevations in the 30-80 µM range at the cell cortex upon Ca 2+ influx. • Similar high [Ca 2+ ] was observed at the leading edge of motile cells and in phagocytic pseudopodia • Micro-domains of high [Ca 2+ ] were also evident near the closed phagosome • The release of ezrin by calpain-1 from cell surface wrinkles occurred at >20 µM [Ca 2+ ].