Septins Arrange F-Actin-Containing Fibers on the Chlamydia trachomatis Inclusion and Are Required for Normal Release of the Inclusion by Extrusion

Abstract

Chlamydia trachomatis is an obligate intracellular human pathogen that grows inside a membranous, cytosolic vacuole termed an inclusion. Septins are a group of 13 GTP-binding proteins that assemble into oligomeric complexes and that can form higher-order filaments. We report here that the septins SEPT2, -9, -11, and probably -7 form fibrillar structures around the chlamydial inclusion. Colocalization studies suggest that these septins combine with F actin into fibers that encase the inclusion. Targeting the expression of individual septins by RNA interference (RNAi) prevented the formation of septin fibers as well as the recruitment of actin to the inclusion. At the end of the developmental cycle of C. trachomatis, newly formed, infectious elementary bodies are released, and this release occurs at least in part through the organized extrusion of intact inclusions. RNAi against SEPT9 or against the combination of SEPT2/7/9 substantially reduced the number of extrusions from a culture of infected HeLa cells. The data suggest that a higher-order structure of four septins is involved in the recruitment or stabilization of the actin coat around the chlamydial inclusion and that this actin recruitment by septins is instrumental for the coordinated egress of C. trachomatis from human cells. The organization of F actin around parasite-containing vacuoles may be a broader response mechanism of mammalian cells to the infection by intracellular, vacuole-dwelling pathogens.

In Contrast to Chlamydia trachomatis, Waddlia chondrophila Grows in Human Cells without Inhibiting Apoptosis, Fragmenting the Golgi Apparatus, or Diverting Post-Golgi Sphingomyelin Transport

“…3C]. ) We recently reported that septins are arranged around the chlamydial inclusion and play a role in the arrangement of F-actin fibers (24). In contrast, no such coating with septins was visible on Waddlia inclusions (Fig.…”

Chlamydia cell biology and pathogenesis

“…Consistent with this notion, the growing inclusion becomes encased in F-actin and intermediate filaments that form a dynamic scaffold, which provides structural stability and limits the access of bacterial products to the host cytosol 7,49 . The recruitment and assembly of F-actin involves RHO-family GTPases 49 , septins 94 , EGFR signalling 95 and at least one bacterial effector, InaC 81 . Microtubules are also actively reorganized around the inclusion by IPAM in C. trachomatis , which hijacks a centrosome protein, the centrosomal protein of 170 kDa (CEP170) 96 .…”

Expansion of the Chlamydia trachomatis inclusion does not require bacterial replication

“…The inclusion expansion has been suggested to be dependent on host cell lipid metabolism (Derre et al, 2011;Robertson et al, 2009), host cell cytoskeletal proteins, vimentin and actin (Kumar and Valdivia, 2008a), and secreted bacterial effector proteins (Jorgensen and Valdivia, 2008;Mital et al, 2013), which may manipulate processes in the host cytoplasm that are required for optimal inclusion expansion. It is possible that expansion of the chlamydial inclusion is linked to bacterial replication (Fields and Hackstadt, 2002;Kumar and Valdivia, 2008b), and in fact, inclusion area has been used to indirectly measure chlamydial replication and growth (Engström et al, 2013;Nguyen et al, 2011;Tietzel et al, 2009;Volceanov et al, 2014). However, a potential link between inclusion expansion and bacterial replication has not been proven experimentally or characterized under different growth conditions.…”