Cell dispersal by localized degradation of a chemoattractant

Chemotaxis, the guided motion of cells by chemical gradients, plays a crucial role in many biological processes. In the social amoeba Dictyostelium discoideum, chemotaxis is critical for the formation of cell aggregates during starvation. The cells in these aggregates generate a pulse of the chemoattractant, cyclic adenosine 3’,5’-monophosphate (cAMP), every 6 min to 10 min, resulting in surrounding cells moving toward the aggregate. In addition to periodic pulses of cAMP, the cells also secrete phosphodiesterase (PDE), which degrades cAMP and prevents the accumulation of the chemoattractant. Here we show that small aggregates of Dictyostelium can disperse, with cells moving away from instead of toward the aggregate. This surprising behavior often exhibited oscillatory cycles of motion toward and away from the aggregate. Furthermore, the onset of outward cell motion was associated with a doubling of the cAMP signaling period. Computational modeling suggests that this dispersal arises from a competition between secreted cAMP and PDE, creating a cAMP gradient that is directed away from the aggregate, resulting in outward cell motion. The model was able to predict the effect of PDE inhibition as well as global addition of exogenous PDE, and these predictions were subsequently verified in experiments. These results suggest that localized degradation of a chemoattractant is a mechanism for morphogenesis.

Eukaryotic cell motion guided by gradients of diffusible chemoattractants plays a crucial role in wound healing, embryology, the movement of immune cells, and cancer metastasis (1–4). In some cell types, including neutrophils and the social amoeba Dictyostelium discoideum, gradient generation occurs through a relay mechanism where cells stimulated by the chemoattractant secrete additional chemoattractant (5–7). To prevent continuous build-up of chemoattractants, many systems use enzymes to degrade the chemoattractant. These enzymes are either membrane-bound, with their active sites facing the extracellular environment, or can diffuse in the surrounding environment (8, 9). In addition, chemoattractants may be removed by other methods, including receptor uptake or decoy receptors (10, 11).Compared to chemoattraction, relatively little is known about chemorepulsion, exemplified by the dispersal of immune cells out of a tissue during the resolution of inflammation. Under some conditions in chamber-based assays, chemoattractant-degrading enzymes can create a local minimum in chemoattractant concentration, resulting in chemoattractant gradients that point away from areas with high cell density (12–14).In a nutrient-rich environment, Dictyostelium cells grow as separate, independent cells. When deprived of food, these amoebae start to secrete the chemoattractant cyclic adenosine 3′,5′-monophosphate (cAMP) in an oscillatory manner (15, 16). The secreted cAMP rapidly diffuses to neighboring cells, which, in turn, start to secrete cAMP as well. The resulting relay mechanism generates periodic waves that sweep through the cell population with a period of 6 min to 10 min (17). For wave periods smaller than 10 min, cells only respond to incoming waves, ignoring the “back of the wave,” and directionally move toward higher concentrations of cAMP (18, 19), form streams, and eventually aggregate into mounds of up to ∼100,000 cells. Cells within the aggregate subsequently differentiate and form a fruiting body that contains the majority of the original population of cells as a mass of spores held up off of the substrate by a stalk to maximize dispersal of spores (15, 16). In addition to cAMP, Dictyostelium cells also secrete phosphodiesterases (PDEs), which hydrolyze cAMP and which prevent continuous build-up of cAMP (8, 20). The presence of PDE is essential for aggregation, and mutants that cannot secrete PDE fail to form viable aggregation centers (21).We reasoned that the competition between a time-varying chemoattractant signal and an inhibitor can result in guidance that changes direction as a function of time. To test this hypothesis, we examined the aggregation of Dictyostelium cells at low cell densities. We present evidence that aggregates of developing Dictyostelium cells display dispersal behavior in which cells are “repelled” from, rather than attracted to, aggregates. This behavior was only present for small aggregates. Furthermore, we show that, during this dispersal behavior, oscillatory cAMP signaling is still active, but that its period is abruptly increased at the onset of dispersal. We develop a model for cell aggregation and show that periodic signaling of cAMP, together with a spatial profile of PDE, can explain the observed dispersal. Furthermore, the model predicts that the disruption of the PDE profile, either by removal of PDE or by globally adding PDE, will result in the abolishment of dispersal. These predictions were subsequently verified in experiments. Our results suggest that, by modulating the frequency of cAMP signaling, small aggregates can shed their cells, potentially avoiding mounds that would form small and thus relatively ineffective fruiting bodies.