Chemotactic responses of Dictyostelium discoideum amoebae to a cyclic amp concentration gradient: Evidence to support a spatial mechanism for sensingcyclic AMP

The motile responses of Dictyostelium discoideum amoebae to a cyclic AMP (c AMP) concentration gradient were examined using a novel assay system. In th is system, a cAMP concentration gradient was generated, while the overall c AMP concentration could be either increased or decreased in a chamber conta ining amoebae. The chemotactic responses of amoebae were examined immediate ly after they had been subjected to the cAMP concentration gradient. Amoeba e moving in random directions in a reference solution ascended a cAMP conce ntration gradient after they had been exposed to the gradient irrespective of whether there was an increase or a decrease in the overall cAMP concentr ation. This strongly supports the idea that D. discoideum amoebae can sense a spatial cAMP gradient around them and that this causes their chemoaccumu lation behavior. Ascending locomotion became less conspicuous when the amoe bae were treated with a homogeneous cAMP solution for approximately 8 min b efore exposure to a cAMP gradient. This cAMP pretreatment reduced the sensi tivity of the amoeba to a cAMP concentration gradient. The cAMP concentrati on gradient could be reversed in less than 30s in this assay system, allowi ng the generation of a cAMP wave by accumulating amoebae to be mimicked. Th e ascending amoebae continued to move in the same direction for 1-2 min aft er the gradient had been reversed. This is consistent with the well-known o bservation that reversal of a cAMP concentration gradient experienced by th e amoebae passing through a cAMP wave does not negate their chemotactic mov ement towards the accumulation center.