Boundary-Driven Oscillations Rescue PdsA- cells

Abstract

Dictyostelium discoideum amoeba aggregate if deprived of nutrients, producing cAMP waves at precisely timed intervals. Degradation of extracellular cAMP by the enzyme phosphodiesterase PdsA is fundamental to successfully producing waves, regulating the external cAMP gradient field and preventing the accumulation of cAMP. The knockout mutant PdsA- produces no or a greatly reduced amount of main extracellular phosphodiesterase, therefore failing to relay cAMP waves and aggregate under starvation conditions. Using a microfluidic channel, we show how an advective flow can partially recover signaling in a population of starving PdsA- cells. Above a minimum flow velocity, decaying waves are induced, with a decay length that increases with the imposed flow velocity. Interestingly, after stopping the advecting flow, the cells continue to signal, showing wave propagation and aggregation, although with a wave period much higher than in wild type cells. We performed extensive numerical simulations and showed that these waves have a boundary-driven origin, where the lack of cAMP in the upstream flow destabilizes the system. We explored the properties of these waves and the parameter region where they exist, with good agreement with our experimental observations. These boundary-driven waves dominate the system dynamics in the velocity range where they exist, while at higher flow velocities the natural wave period of 6 min recovers. These results provide experimental confirmation of the destabilizing effect of the upstream boundary in an otherwise stable reaction-diffusion system. We expect this mechanism to be relevant for wave creation in other oscillatory or excitable systems that are incapable of normal pattern formation.\n\nSIGNIFICANCE STATEMENTWe present experimental evidence for the existence of boundary-driven instabilities in a reaction-diffusion-advection system. In our theoretical prediction (1), we have shown that imposing an absorbing boundary condition on the upstream end of a flow-through channel filled with signaling cells creates an instability capable of periodically producing wave trains which are advected downstream. Under starvation, these cells secret the signaling molecule cAMP as well as the degrading agent phosphodiestrase that degrades cAMP. This instability was predicted to exist at lower degradation rates of cAMP and thus was expected to provide a mechanism for wave creation in phosphodiesterase deficient systems, such as PdsA- cells. Our experiments confirm the importance of the upstream boundary condition and show that boundary-driven oscillations are relevant in reaction-diffusion systems.