Multiple signalling pathways connect chemoattractant receptors and calcium channels in Dictyostelium.
Dictyostelium mutants expressing aequorin were used to study and compare the roles of heterotrimeric G-proteins and the second messengers IP3 and cGMP in regulating folate- and cAMP receptor-activated [Ca2+]i signals. The calcium responses of vegetative cells to folate were dramatically impaired in Gbeta and Galpha4 null mutants but were restored with altered kinetics and temperature-sensitivity in Gbeta null mutants overexpressing wild type and temperature-sensitive Gbeta isoforms. Folic acid receptors thus mediate changes in [Ca2+]i via a Galpha4betagamma-dependent pathway. Neither folate nor cAMP-induced [Ca2+]i signals were significantly altered in PLC null transformants, but [Ca2+]i changes elicited by both attractants were significantly prolonged in two stmF mutants lacking cGMP-specific phosphodiesterase activity. This confirms an important role of cGMP in regulating receptor-activated Ca2+ uptake and/or extrusion systems. This cGMP-dependent part of the Ca2+ response to cAMP stimuli was developmentally down-regulated and all but disappeared by the time the cells reached full aggregation competence after 8 h of starvation. The results suggest that folate and cAMP receptor-activated [Ca2+]i signals are regulated in a complex manner via multiple signalling pathways, one that is G-protein- and cGMP-dependent (present at the vegetative and early poststarvation stage) and another that is G-protein-independent (dominant in fully aggregation-competent cells at approximately 8 h poststarvation).
Nebl
Thomas
T
Department of Microbiology, La Trobe University, Bundoora, Victoria 3083, Australia.
Kotsifas
Martha
M
Schaap
Pauline
P
Fisher
Paul R
PR
D000818:Animals; D015220:Calcium Channels / Q000502:physiology*; D002628:Chemoreceptor Cells / Q000502:physiology*; D000242:Cyclic AMP / Q000502:physiology; D004023:Dictyostelium / Q000187:drug effects / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D005492:Folic Acid / Q000494:pharmacology; D007700:Kinetics; D016053:RNA, Protozoan / Q000235:genetics; D015290:Second Messenger Systems / Q000502:physiology*
D015220:Calcium Channels; D016053:RNA, Protozoan; D005492:Folic Acid; D000242:Cyclic AMP