Ca2+ influx shutdown in neutrophils induced by Fas (CD95) cross-linking

In neutrophils, as in most other cell types, Ca2+ signalling is important for a number of cellular activities. Although inositol(1,4,5)trisphosphate-mediated release of Ca2+ from intracellular stores is a necessary prelude, it is the Ca2+ influx that is responsible for many of the neutrophil responses. We report here that although elevations of cytosolic Ca2+ do not accompany Fas-mediated apoptosis in neutrophils, the Ca2+ influx component of the response to N-formyl-methionyl-leucyl-phenylalanine (FMLP) becomes selectively inactived as the neutrophils progress towards accelerated apoptosis induced by Fas (CD95) cross-linking. After 4 hr incubation at 37degrees, untreated neutrophils display an exaggerated Ca2+ influx phase in response to FMLP. This was absent in neutrophils that had been Fas-activated at the same time. No Ca2+ influx component was demonstrable by the removal of extracellular Ca2+ or by Ca2+ channel blockade with Ni2+ and no Mn2+ influx was detectable. The defect could not be attributed to a decrease in receptor sensitivity, receptor coupling or receptor number because the release of stored Ca2+ remained constant during incubation and was unaffected by Fas activation. Ca2+ influx became uncoupled from store release before detectable gross morphological changes or phosphatidyl serine externalization and was also insensitive to caspase 3 and 8 inhibitors. These results suggest a mechanism other than caspase-mediated proteolytic damage to components important for Ca2+ influx.