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

ABSTRACT 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 Chlamydia l 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 Chlamydia l aggregates and inclusions from 3 to 48 hours post-infection. In infected cells, SERCA2 was intimately associated with Chlamydia l inclusions after 3 and 24 hours, but not after 48 hours. Moreover, IP3-R was translocated to and colocalized with EB aggregates and Chlamydia l inclusions and had a distribution very similar to that of SERCA 2. After 24 hours incubation with Chlamydia e, there was a local accumulation of [Ca2+]i (105±17 nM) in the proximity of Chlamydia l 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 Chlamydia e, intracellular Ca2+ stores are redistributed, causing local accumulation of Ca2+ in the vicinity of Chlamydia l 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. Key words: Intracellular Ca2+ store protein, Chlamydia, Thapsigargin, Elementary body, HeLa cell