Roles of Ca2+ and F-actin in intracellular aggregation of Chlamydia trachomatis in eucaryotic cells

Abstract

The effect of intracellular free Ca2+ ([Ca2+](i)) on the intracellular aggregation of Chlamydia trachomatis serovars L2 and E in McCoy and HeLa cells is investigated. Loading the cells with the Ca2+ chelator MAPT/AM (1,2-bis-5-methyl-amino-phenoxylethane-N,N,n'-tetra-acetoxymethyl acetate), thereby decreasing the [Ca2+](i) from 67 to 19 nM, decreased the number of cells with a local aggregation of chlamydiae in a dose-dependent manner. Neither the attachment nor the uptake of elementary bodies (EBs) was, however, affected after depletion of Ca2+ from the cells. There was no significant difference in the level of measured [Ca2+](i) between infected and uninfected cells. Reducing the [Ca2+](i) also significantly inhibited chlamydial inclusion formation. Differences in the organization of the actin filament network were observed in response to [Ca2+](i) depletion. In Ca2+-depleted cells, where few EB aggregates were formed, few local accumulations of F-actin were observed in the cytosol. These results suggest that the aggregation of EBs in eucaryotic cells requires a normal homeostasis of intracellular Ca2+. By affecting F-actin reorganization and putatively certain Ca2+-binding proteins, [Ca2+](i) plays a vital role in the infectious process of chlamydiae.