Cyclic AMP relay and cyclic AMP-induced cyclic GMP accumulation during development ofDictyostelium discoideum

A spatially restricted increase in receptor mobility is involved in directional sensing during Dictyostelium discoideum chemotaxis

The directed cell migration towards a chemotactic source, chemotaxis, involves three complex and interrelated processes: directional sensing, cell polarization and motility. Directional sensing allows migrating eukaryotic cells to chemotax in extremely shallow gradients (<2% across the cell body) of the chemoattractant. Although directional sensing has been observed as spatially restricted responses along the plasma membrane, our understanding of the ;compass' of the cell that controls the gradient-induced translocation of proteins during chemotactic movements is still largely lacking. Until now, the dynamical behaviour and mobility of the chemoattractant-receptor molecule has been neglected in models describing the directional sensing mechanisms. Here, we show by single-molecule microscopy an agonist-induced increase in the mobile fraction of cAMP-receptor at the leading edge of chemotacting Dictyostelium discoideum cells. The onset of receptor mobility was correlated to the uncoupling and activation of the Galpha2-protein. A finite-element simulation showed that the increase in mobile fraction of the activated receptor enabled the amplified generation of activated Gbetagamma-dimers at the leading edge of the cell, faithfully representing a primary linear amplification step in directional sensing. We propose here that modulation of the receptor mobility is directly involved in directional sensing and provides a new mechanistic basis for the primary amplification step in current theoretical models that describe directional sensing.

Integration of signaling information in controlling cell-fate decisions in Dictyostelium

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Involvement of cyclic AMP cell surface receptors and G-proteins in signal transduction during slug migration of Dictyostelium discoideum

The presence of G-proteins, interacting with cAMP surface receptors, was investigated in vegetative cells, aggregation-competent cells, and migrating slugs of Dictyostelium discoideum. Our results indicate that G-proteins are present in all stages. In vegetative cells there is a limited number of cAMP receptors but no effect of GTP tau S on cAMP binding could be detected; in addition, no effect of cAMP on GTP tau S binding or GTPase activity was observed. In both aggregation-competent cells and slugs GTP tau S inhibits cAMP binding, while cAMP stimulates GTP tau S binding and high-affinity GTPase. Since the presence of G-proteins coupled to cAMP receptors could be demonstrated in slugs, the involvement of the effector enzymes adenylate cyclase and phospholipase C was investigated. The results show that adenylate cyclase activity is stimulated by GTP tau S in both stages and that in cells from migrating slugs the Ins(1,4,5)P3 production is increased upon stimulation with cAMP. The possible involvement of G-proteins in signal transduction during the slug stage of D. discoideum is discussed.

Production and turnover of cAMP signals by prestalk and prespore cells in Dictyostelium discoideum cell aggregates

Dictyostelium discoideum prestalk cells and prespore cells from migrating slugs and culminating cell aggregates were isolated by Percoll density centrifugation. Several activities relevant to the generation, detection, and turnover of extracellular cyclic AMP (cAMP) signals were determined. It was found that: the two cell types have the same basal adenylate cyclase activity; prespore cells and prestalk cells are able to relay the extracellular cAMP signal equally well; intact prestalk cells show a threefold higher cAMP phosphodiesterase activity on the cell surface than prespore cells, whereas their cytosolic activity is the same; intact prestalk cells bind three to four times more cAMP than prespore cells; no large differences in cAMP metabolism and detection were observed between cells derived from migrating slugs and culminating aggregates. The results are discussed in relation to the possible morphogenetic role of extracellular cAMP in Dictyostelium cell aggregates. On the basis of the properties of the isolated cells we assume that a gradient of extracellular cAMP exists in Dictyostelium aggregates. This gradient appears to be involved in the formation and stabilization of the prestalk-prespore cell pattern.

Postaggregative differentiation induction by cyclic AMP in Dictyostelium: intracellular transduction pathway and requirement for additional stimuli

Cyclic AMP induces postaggregative differentiation in aggregation competent cells of Dictyostelium by interacting with cell surface cAMP receptors. We investigated the transduction pathway of this response and additional requirements for the induction of postaggregative differentiation. Optimal induction of postaggregative gene expression requires that vegetative cells are first exposed to 2-4 hr of nanomolar cAMP pulses, and subsequently for 4-6 hr to steady-state cAMP concentrations in the micromolar range. Cyclic AMP pulses, which are endogenously produced before and during aggregation, induce full responsiveness to cAMP as a morphogen. The transduction pathway from the cell surface cAMP receptor to postaggregative gene expression may involve Ca2+ ions as intracellular messengers. A cAMP-induced increase in intracellular cAMP or cGMP levels is not involved in the transduction pathway.