Localization of intracellular Ca²⁺ stores in HeLa cells during infection with Chlamydia trachomatis

Chlamydia trachomatis elementary bodies (EBs) enter epithelial cells within membrane-bound endosomes that aggregate with each other in a calcium-regulated process, but avoid fusion with lysosomes. Annexin III but not I translocates to chlamydial aggregates and inclusions. In this study, we localize the intracellular Ca²⁺ stores during the course of infection by analyzing the distribution of three intracellular Ca²⁺ store proteins: calreticulin, type-1 inositol-1,4,5-trisphosphate receptor (IP3-R), and Sarcoplasmic/Endoplasmic Reticulum Ca²⁺ ATPase type 2 (SERCA2) in HeLa cells infected with C. trachomatis serovar L2. In uninfected cells, immunofluorescence staining of the proteins showed a fine granular distributed pattern for all three proteins. After infection with C. trachomatis, calreticulin was found at the periphery of chlamydial aggregates and inclusions from 3 to 48 hours post-infection. In infected cells, SERCA2 was intimately associated with chlamydial inclusions after 3 and 24 hours, but not after 48 hours. Moreover, IP3-R was translocated to and colocalized with EB aggregates and chlamydial inclusions and had a distribution very similar to that of SERCA2. After 24 hours incubation with chlamydiae, there was a local accumulation of [Ca²⁺](i) (105 ± 17 nM) in the proximity of chlamydial inclusions, compared to 50 ± 13 nM in other parts of the cell cytoplasm. In the absence of extracellular Ca²⁺, this local accumulation of Ca²⁺ increased to 295 ± 50 nM after adding 50 μM ATP, and to a similar extent after adding 100 nM thapsigargin (Tg). These data indicate that during infection of HeLa cells with chlamydiae, intracellular Ca²⁺ stores are redistributed, causing local accumulation of Ca²⁺ in the vicinity of chlamydial inclusions. These changes may trigger the association of certain proteins such as annexins with chlamydia-containing vesicles, and thereby regulation of membrane-membrane interaction during endosome aggregation and inclusion formation.