Calcium uptake and gp80 messenger RNA destabilization follows cAMP receptor down regulation in Dictyostelium discoideum

Combining experiments and modelling to understand size regulation in Dictyostelium discoideum

Little is known about how the sizes of specific organs and tissues are regulated. To try to understand these mechanisms, we have been using a combination of modelling and experiments to study the simple system Dictyostelium discoideum, which forms approximately 20000 cell groups. We found that cells secrete a factor, and as the number of cells increases, the concentration of the factor increases. Diffusion calculations indicated that this lets cells sense the local cell density. Computer simulations predicted, and experiments then showed, that this factor decreases cell-cell adhesion and increases random cell motility. In a group, adhesion forces keep cells together, while random motility forces cause cells to pull apart and separate from each other. As the group size increases above a threshold, the factor concentration goes above a threshold and the cells switch from an adhered state to a separated state. This causes excessively large groups to break apart and/or dissipate, creating an upper limit to group size. In this review, we focus on how computer simulations made testable predictions that led the way to understanding the size regulation mechanism mediated by this factor.

Antagonistic effects of signal transduction by intracellular and extracellular cAMP on gene regulation in Dictyostelium

In Dictyostelium, cAMP plays a role as an intracellular second messenger and in addition, as an extracellular first messenger. Both functions are thought to be tightly linked because adenylyl cyclase is coupled via G-proteins to the cell surface cAMP receptor cAR 1. Using the discoidin I gene family as a molecular marker for the first stages of development, we show here that induction of transcription requires the G-protein subunit alpha 2 and thus an as yet unidentified surface receptor, CRAC (cytosolic regulator of adenylyl cyclase), and PKA. Induction can be conferred by an increase in intracellular cAMP. In contrast, transcriptional down-regulation occurs by stimulation of cAR 1 with extracellular cAMP and a subsequent, G-protein-independent Ca2+ influx. In a G alpha 2 gene disruption mutant, discoidin I expression can be efficiently modulated by analogues simulating intracellular cAMP (discoidin induction) and extracellular cAMP (discoidin down-regulation). We thus demonstrate possible antagonistic functions of intra- and extracellular cAMP.