Cyclic-AMP-induced elevation of intracellular pH precedes, but does not mediate, the induction of prespore differentiation in Dictyostelium discoideum

Prespore gene expression in Dictyostelium is induced by the interaction of cAMP with cell surface cAMP receptors. We investigated whether intracellular pH (pHi) changes mediate the induction of prespore gene expression by cAMP. It was found that cAMP induces a 0.15 unit increase in pHi within 45 min after stimulation. The cAMP-induced pHi increase precedes the induction of prespore gene expression, measured by in vitro transcription, by about 15–30 min. Cyclic-AMP-induced pHi changes can be bypassed or clamped by addition of, respectively, the weak base methylamine, which increases pHi, or the weak acid 5,5-dimethyl-2,4-oxazolidinedione (DMO), which decreases pHi. Bypass of the cAMP-induced increase of pHi with methylamine does not induce the expression of prespore genes, while inhibition of the pHi increase with DMO does not inhibit the induction of prespore gene expression. Cyclic-AMP-induced prespore protein synthesis and the proportion of prespore cells in multicellular aggregates are also not affected by bypassing or inhibiting the cAMP-induced pHi increase. These results show that although a morphogen-induced pHi increase precedes the induction of prespore gene expression, this increase does not mediate the effects of the extracellular cAMP signal on the transcription or translation of prespore genes in Dictyostelium discoideum.