Cell-cell interactions in the development of Dictyostelium

Dictyostelium discoideum lipids modulate cell-cell cohesion and cyclic AMP signaling

During Dictyostelium discoideum development, cell-cell communication is mediated through cyclic AMP (cAMP)-induced cAMP synthesis and secretion (cAMP signaling) and cell-cell contact. Cell-cell contact elicits cAMP secretion and modulates the magnitude of a subsequent cAMP signaling response (D. R. Fontana and P. L. Price, Differentiation 41:184-192, 1989), demonstrating that cell-cell contact and cAMP signaling are not independent events. To identify components involved in the contact-mediated modulation of cAMP signaling, amoebal membranes were added to aggregation-competent amoebae in suspension. The membranes from aggregation-competent amoebae inhibited cAMP signaling at all concentrations tested, while the membranes from vegetative amoebae exhibited a concentration-dependent enhancement or inhibition of cAMP signaling. Membrane lipids inhibited cAMP signaling at all concentrations tested. The lipids abolished cAMP signaling by blocking cAMP-induced adenylyl cyclase activation. The membrane lipids also inhibited amoeba-amoeba cohesion at concentrations comparable to those which inhibited cAMP signaling. The phospholipids and neutral lipids decreased cohesion and inhibited the cAMP signaling response. The glycolipid/sulfolipid fraction enhanced cohesion and cAMP signaling. Caffeine, a known inhibitor of cAMP-induced adenylyl cyclase activation, inhibited amoeba-amoeba cohesion. These studies demonstrate that endogenous lipids are capable of modulating amoeba-amoeba cohesion and cAMP-induced activation of the adenylyl cyclase. These results suggest that cohesion may modulate cAMP-induced adenylyl cyclase activation. Because the complete elimination of cohesion is accompanied by the complete elimination of cAMP signaling, these results further suggest that cohesion may be necessary for cAMP-induced adenylyl cyclase activation in D. discoideum.

Cell-cell contact mediates cAMP secretion in Dictyostelium discoideum

Cyclic adenosine 3':5' monophosphate (cAMP) and cell-cell contact regulate developmental gene expression in Dictyostelium discoideum. Developing D. discoideum amoebae synthesize and secrete cAMP following the binding of cAMP to their surface cAMP receptor, a response called cAMP signaling. We have demonstrated two responses of developing D. discoideum amoebae to cell-cell contact. Cell-cell contact elicits cAMP secretion and alters the amount of cAMP secreted in a subsequent cAMP signaling response. Depending upon experimental conditions, bacterial-amoebal contact and amoebal-amoebal contact can enhance or diminish the amount of cAMP secreted during a subsequent cAMP signaling response. We have hypothesized that cell-cell contact regulates D. discoideum development by altering cellular and extracellular levels of cAMP. To begin testing this hypothesis, these responses were further characterized. The two responses to cell-cell contact are independent, i.e., they can each occur in the absence of the other. The responses to cell-cell contact also have unique temperature dependences when compared to each other, cAMP signaling, and phagocytosis. This suggests that these four responses have unique steps in their transduction mechanisms. The secretion of cAMP in response to cell-cell contact appears to be a non-specific response; contact between D. discoideum amoebae and Enterobacter aerogenes, latex beads, or other amoebae elicits cAMP secretion. Despite the apparent similarities of the effects of bacterial-amoebal and amoebal-amoebal contact on the cAMP signaling response, this contact-induced response appears to be specific. Latex beads addition does not alter the magnitude of a subsequent cAMP signaling response.(ABSTRACT TRUNCATED AT 250 WORDS)

Dictyostelium discoideum lipids modulate cell-cell cohesion and cyclic AMP signaling

During Dictyostelium discoideum development, cell-cell communication is mediated through cyclic AMP (cAMP)-induced cAMP synthesis and secretion (cAMP signaling) and cell-cell contact. Cell-cell contact elicits cAMP secretion and modulates the magnitude of a subsequent cAMP signaling response (D. R. Fontana and P. L. Price, Differentiation 41:184-192, 1989), demonstrating that cell-cell contact and cAMP signaling are not independent events. To identify components involved in the contact-mediated modulation of cAMP signaling, amoebal membranes were added to aggregation-competent amoebae in suspension. The membranes from aggregation-competent amoebae inhibited cAMP signaling at all concentrations tested, while the membranes from vegetative amoebae exhibited a concentration-dependent enhancement or inhibition of cAMP signaling. Membrane lipids inhibited cAMP signaling at all concentrations tested. The lipids abolished cAMP signaling by blocking cAMP-induced adenylyl cyclase activation. The membrane lipids also inhibited amoeba-amoeba cohesion at concentrations comparable to those which inhibited cAMP signaling. The phospholipids and neutral lipids decreased cohesion and inhibited the cAMP signaling response. The glycolipid/sulfolipid fraction enhanced cohesion and cAMP signaling. Caffeine, a known inhibitor of cAMP-induced adenylyl cyclase activation, inhibited amoeba-amoeba cohesion. These studies demonstrate that endogenous lipids are capable of modulating amoeba-amoeba cohesion and cAMP-induced activation of the adenylyl cyclase. These results suggest that cohesion may modulate cAMP-induced adenylyl cyclase activation. Because the complete elimination of cohesion is accompanied by the complete elimination of cAMP signaling, these results further suggest that cohesion may be necessary for cAMP-induced adenylyl cyclase activation in D. discoideum.