Extracellular cyclic AMP during development of the cellular slime mold Polysphondylium violaceum: comparison of accumulation in the wild type and an aggregation-defective mutant

Abstract

Cyclic AMP was synthesized by Polysphondylium violaceum after starvation and during the preaggregation stage of development. Most of the newly synthesized cyclic AMP accumulated in the extracellular medium, with very little change in the intracellular cyclic AMP concentration. The addition of 10-3 to 10-6 M exogenous cyclic AMP to starved amoebae caused a 20 to 50% decrease in the number of aggregation centers formed compared with untreated controls. An aggregation-defective mutant of P. violaceum (strain aggA586) excreted or accumulated very little cyclic AMP. Strain aggA586 aggregated normally in the presence of a dialyzable, excreted product (D factor) produced by wild-type amoebae. When the mutant was incubated with D factor, cyclic AMP accumulated in the medium, and the amount accumulated depended on the amount of D factor added to the mutant amoebae.Cyclic AMP is involved in controlling development in the cellular slime molds (6,17). In Dictyostelium discoideum cyclic AMP is used as the chemoattractant for aggregate formation (13,14), affects stalk and spore cell differentiation (8), and is involved in gene regulation at the level of transcription (4,15,16,24). In Polysphondylium species the role of cyclic AMP is less well understood. Cyclic AMP is not the chemoattractant for aggregate formation (25). The Polysphondylium violaceum chemoattractant, glorin, has recently been isolated and identified as a substituted dipeptide (glutamyl-ornithine) (21). Polysphondylium species do produce and excrete cyclic AMP during the early stages of development (2, 7, 10), but aggregating amoebae do not show enhanced chemotactic sensitivity toward cyclic AMP (11, 12) as they do toward glorin (J. Bonner, personal communication). High concentrations of externally added cyclic AMP have been shown to delay slightly aggregation of P. violaceum (12) and to cause aberrant aggregation, stalk cell formation, and disruption of slugs in Polysphondylium pallidum (11). In this paper we show that the cyclic AMP synthesized by P. violaceum is rapidly excreted and that high extracellular concentrations of cyclic AMP lead to a decrease in the number of aggregation centers formed.In contrast to the effect of cyclic AMP on aggregation, an aggregation-stimulating factor, called D factor, is produced during the preaggregation period of development in the cellular slime molds (9). In P. violaceum and in D. discoideum production of D factor occurs just before synthesis and excretion of cyclic AMP (9, 10). Thus, in P. violaceum, both stimulatory and inhibitory chemicals are present during aggregation. A well-defined class of aggregation-defective mutants in P. violaceum (aggA) aggregate only in response to an external source of D factor (9). There is no morphological development by the aggA mutants in the absence of D factor, whereas in the presence of D factor aggregation and further development seem to be normal (9). As we show here, aggA mutants, in contrast to wild-type P. violaceum, synthesize and excrete little, if any, cyclic AMP. However, ...

gp36

“…Single-celled organisms may respond to environmental stimuli through the activation of signal transduction pathways. For example, slime molds secrete cyclic adenosine monophosphate upon starvation, stimulating individual cells in the immediate environment to aggregate (Hanna et al 1984), and yeast cells use mating factors to determine the mating types of other cells and participate in sexual reproduction (Sprague 1991).…”