Effects of suboptimal levels of extracellular calcium on the regulation of the cyclic AMP phosphodiesterase-inhibitor system and membrane differentiation in Dictyostelium discoideum.
When starved wild-type amoebae of Dictyostelium discoideum were washed and incubated in 1 mM-EGTA, their ability to induce soluble cyclic AMP phosphodiesterase (PD) activity in response to either millimolar cyclic AMP or a series of nanomolar cyclic AMP pulses was reduced by 55-75%. Supplementation of EGTA-treated cells with exogenous Ca2+ stimulated PD induction in a dose-dependent fashion (EC50 = 100-200 nM free extracellular Ca2+), and enzyme production was maximal at about 1 microM free Ca2+. Ca2+ depletion also strongly impaired production of the phosphodiesterase inhibitor (PDI). In contrast, other than delaying their appearance by about 1 h, EGTA had little effect on the induction by cyclic AMP pulses of cell surface markers such as contact sites A and membrane-bound PD activity. Similar changes in both the soluble and membrane activities were observed with strain NP368, a mutant that overproduces cyclic GMP when stimulated by cyclic AMP. Thus, Ca2+ depletion does not appear to inhibit PD and PDI production by reducing intracellular cyclic GMP. To determine whether Ca2+ depletion alters signal transduction, two mutants that produce the soluble PD activities constitutively were examined. Suboptimal concentrations of free extracellular Ca2+ were found to inhibit PD production in these cells to the same degree and with the same concentration dependence as low Ca2+ inhibited PD induction by cyclic AMP in wild-type cells. These results suggest that Ca2+ depletion by EGTA probably inhibits PD and PDI production indirectly by perturbing an intracellular Ca2+ pool(s) rather than by altering a surface cyclic AMP-receptor-mediated process.
Coukell
M B
MB
Department of Biology, York University, Ontario, Canada.
Cameron
A M
AM
D015105:3',5'-Cyclic-AMP Phosphodiesterases / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000494:pharmacology; D004023:Dictyostelium / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology
D004533:Egtazic Acid; D000242:Cyclic AMP; D015105:3',5'-Cyclic-AMP Phosphodiesterases; D002118:Calcium