| Abstract | After onset of starvation Dictyostelium discoideum cells initiate formation of cooperative aggregation territories via propagating cAMP waves. Recruitment of a high number of cells is important, and it appears that formation of large territories has been selected for. These cAMP waves are generated by the means of an elaborate cAMP signaling system that makes the whole field of cells excitable. Dictyostelium cells respond chemotactically to these waves, guiding cell aggregation towards a signaling center. A crucial component of the signaling system, the PdsA phosphodiesterase (PDE), an enzyme, that breaks down the external cAMP. can be either membrane bound or secreted. I show that by utilizing both forms of PDE and by fine tuning the ratio, Dictyostelium extends the range of cell densities where cAMP waves can propagate and thus where aggregation can be successful. The membrane bound PDE reduces the likelihood the aggregation territory breaks up into many smaller territories at higher densities, while the secreted PDE is important for wave propagation at low cell densities. These findings have implications for other excitable systems such as the Oregonator model for the Beluzov-Zhabotinskii, Ca++ propagation in cardiac cells and propagation of electrical excitation in nerve axon. With discrete point sources located far apart, wave propagation is not possible if the sink is in the same location as the source. However, when the source and sink are in a different location, wave propagation is possible. |
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