Characterization of T cell mutants with defects in capacitative calcium entry: Genetic evidence for the physiological roles of CRAC channels

Abstract

Prolonged Ca2+ influx is an essential signal for the activation of T lymphocytes by antigen. This influx is thought to occur through highly selective Ca2+ stores. We have isolated mutants of the Jurkat human T cell line NZdipA to explore the molecular mechanisms that underlie capacitative Ca2+ entry and to allow a genetic test of the functions of CRAC channels in T cells. Five mutant cell lines (CJ-1 through CJ-5) were selected based on their failure to express a lethal diphtheria toxin A chain gene and a lacZ reporter gene driven by NF-AT, a Ca2+- and protein kinase C-dependent transcription factor. The rate of Ca2+ influx evoked by thapsigargin was reduced to varying degrees in the mutant cells whereas the dependence of NF- AT/lacZ gene transcription on [Ca2+](i) was unaltered, suggesting that the transcriptional defect in these cells is caused by a reduced level of capacitative Ca2+ entry. We examined several factors that determine the rate of Ca2+ entry, including CRAC channel activity, K+-channel activity, and Ca2+ clearance mechanisms. The only parameter found to be dramatically altered in most of the mutant lines was the amplitude of the Ca2+ current(I(CRAC)), which ranged from 1 to 41% of that seen in parental control cells. In each case, the severity of the I(CRAC) defect was closely correlated with deficits in Ca2+ influx rate and Ca2+-dependent gene transcription. Behavior of the mutant cells provides genetic evidence for several roles of I(CRAC) in T cells. First, mitogenic doses of ionomycin appear to elevate [Ca2+](i) primarily by activating CRAC channels. Second, I(CRAC) promotes the refilling of empty Ca2+ stores. Finally, CRAC channels are solely responsible for the Ca2+ influx that underlies antigen-mediated T cell activation. These mutant cell lines may provide a useful system for isolating, expressing, and exploring the functions of genes involved in capacitative Ca2+ entry.