Cyclic AMP, Cyclic AMP Phosphodiesterase, and the Duration of the Interphase in Dictyostelium discoideum

Abstract

Summary After feeding has ended, amoebae of the cellular slime mold, Dictyostelium discoideum, enter a quiescent period (the interphase) before aggregation. Many characteristics required for aggregation develop in the amoebae during the interphase period, and the length of the interphase is affected by numerous factors including the density of the developing cell population. It was previously shown that the fewer the cells present, the longer the interphase [12, 13]. The present study is an investigation of the cell density dependence of the length of the interphase. It is shown that the interphase can be shortened for cell populations of low density by reducing the external cyclic AMP levels surrounding the amoebae. The same effect is obtained whether the cyclic AMP level is reduced by dilution into salt solution below the support medium, by absorption on to charcoal, or by destruction with the enzyme phosphodiesterase. Additional cyclic AMP in the support medium lengthens the interphase. Measurements of the cyclic AMP phophodiesterase levels produced by amoebae show that, under the condition of a reduced external cyclic AMP level, populations of low cell density not only aggregate earlier, but also aggregate at a lower level of cell-bound phosphodiesterase. At the time of aggregation, external cyclic AMP levels and cell-bound phosphodiesterase levels are seen to reach a specific ratio for lower cell densities. The relationship of cyclic AMP and cell-bound phosphodiesterase at aggregation is interpreted as important in the detection of a cyclic AMP gradient by the receptor system on the surface of the amoebae. It is suggested that low cell densities require a longer period of time to achieve the appropriate relationship between external cyclic AMP and cell-bound phosphodiesterase and therefore have a longer interphase.