Amebae of Dictyostelium discoideum respond to an increasing temporal gradient of the chemoattractant cAMP with a reduced frequency of turning: Evidence for a temporal mechanism in ameboid chemotaxis

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Abstract
In an aggregation territory of Dictyostelium discoideum, outwardly moving, nondissipating waves of the chemoattractant cAMP sweep across each ameba. At the front of each wave, an ameba experiences an increasing temporal and a positive spatial gradient of cAMP. At the back of a wave, an ameba experiences a decreasing temporal and a negative spatial gradient of cAMP. Employing a perfusion chamber, we have mimicked the temporal dynamics of these waves in the absence of a spatial gradient and demonstrated that the frequency of lateral pseudopod formation and the frequency of turning are dramatically affected by the direction and dynamics of the temporal gradient. In addition, since an ameba will move in a directed fashion up a shallow, nonpulsatile gradient of cAMP, we also mimicked the increasing temporal gradient generated by an ameba moving up a shallow spatial gradient. The frequency of lateral pseudopod formation and the frequency of turning were depressed. Together, these results demonstrate that amebae can assess the direction of a temporal gradient of chemoattractant in the absence of a spatial gradient and alter both the frequency of pseudopod extension and turning, accordingly. Although these results do not rule out the involvement of a spatial mechanism in assessing a spatial gradient, they strongly suggest that the temporal dynamics of a cAMP wave or the temporal gradient generated by an ameba moving through a spatial gradient may play a major role in chemotaxis.