Influx of extracellular Ca²⁺ is necessary for electrotaxis in Dictyostelium

Intracellular free Ca²⁺ ([Ca²⁺]_i) is a pivotal signalling element in cell migration and is thought to be required for chemotaxis of Dictyostelium. Ca²⁺ signalling may also be important for electrotaxis. However this suggestion has been controversial. We show that electric fields direct Dictyostelium cells to migrate cathodally and increase [Ca²⁺]_i in Dictyostelium cells, as determined by Fluo-3 AM imaging and ⁴⁵Ca²⁺ uptake. Omission of extracellular Ca²⁺([Ca²⁺]_e) and incubation with EGTA abolished the electric-field-stimulated [Ca²⁺]_i rise and directional cell migration. This suggests a requirement for [Ca²⁺]_e in the electrotactic response. Deletion of iplA, a gene responsible for chemoattractant-induced [Ca²⁺]_i increase, had only a minor effect on the electric-field-induced [Ca²⁺]_i rise. Moreover, iplA-null Dictyostelium cells showed the same electrotactic response as wild-type cells. Therefore, iplA-independent Ca²⁺ influx is necessary for electrotactic cell migration. These results suggest that the [Ca²⁺]_i regulatory mechanisms induced by electric fields are different from those induced by cAMP and folic acid in Dictyostelium cells. Different roles of the iplA gene in chemoattractant-induced and electrically induced Ca²⁺ signalling, and different effects of [Ca²⁺]_i elevation on chemotaxis and electrotaxis indicate that the chemoattractant and electric cues activate distinctive initial signalling elements.