Effect of Calcium Flux on Filopodia of Epithelial Cells

Abstract

Filopodia are the sensors of a cell and are responsible for directing cell motility. Like “antennae,” they receive signals from both soluble ligands in the extracellular mileau and those attached to the substrate. In the nerve axon, these two aspects of signaling are integrated so as to modulate chemotactic signaling according to the composition of the substrate. The role of filopodia in the modulation is not clear. Although epithelial cells differ from nerve cells in their organization, the same second messengers mediate the filopodia's sensory function. In both cell types, activation of a PKC (protein kinase C) with a tumor promoter inhibited filopodia or caused their dissolution. Because Ca++ selectively activates one subset of PKCs, local Ca++ elevation could activate these isozymes and also thereby decrease filopodia. Previous reports showed contradictory results for Ca++ second messenger, however, in the axon, i.e. positive or negative effects on filopodia. Here, we investigate how filopodia dynamics are affected by calcium. We measured percentage of the cell periphery covered with filopodia and percentage of cells showing filopodia. There is an inhibitor of filopodia in culture media, so that replacing the medium with a buffer increased filopodia. This occurred regardless of whether calcium was present in the buffer or absent. When cells remained in the medium, however, inhibitors of calcium transport caused a decrease in filopodia. Cyclopiazonic acid in Ca++-free buffer typically blocks uptake of Ca++ into the endoplasmic reticulum (ER) and causes net Ca++ efflux. This enhanced filopodia. Restoring extracellular Ca++ in the extracellular buffer after depleting the ER store also enhanced filopodia production. We conclude that filopodia display was sensitive to calcium flux but not to absolute physiologically relevant calcium concentrations. It remains to be determined how calcium flux is detected in epithelial cells.