Cytoskeletal requirements in Chlamydia trachomatis infection of host cells

Fusion of Chlamydia trachomatis-containing inclusions is inhibited at low temperatures and requires bacterial protein synthesis

The human pathogen Chlamydia trachomatis is an obligate intracellular bacterium with a unique developmental cycle. Within the host cell cytoplasm, it resides within a membrane-bound compartment, the inclusion. A distinguishing characteristic of the C. trachomatis life cycle is the fusion of the chlamydia-containing inclusions with each other in the host cell cytoplasm. We report that fusion of inclusions does not occur at 32 degreesC in multiple mammalian cell lines and with three different serovars of C. trachomatis. The inhibition of fusion was inclusion specific; the fusion with sphingolipid-containing secretory vesicles and the interaction with early endosomes were unaffected by incubation at 32 degreesC. The inhibition of fusion of the inclusions was not primarily the result of delayed maturation of the inclusion, as infectious progeny was produced in host cells incubated at 32 degreesC, and the unfused inclusions remained competent to fuse up to 48 h postinfection. The ability to reverse the inhibition of fusion by shifting the infected cells from 32 to 37 degreesC allowed the measurement of the rate and the time of fusion of the inclusions after entry of the bacteria. Most significantly, we demonstrate that fusion of inclusions with each other requires bacterial protein synthesis and that the required bacterial protein(s) is present, but inactive or not secreted, at 32 degreesC.

Signal transduction during Legionella pneumophila entry into human monocytes

Legionella pneumophila causes Legionnaires' disease by replication in alveolar macrophages and monocytes. The bacteria are internalized most efficiently by opsonin-dependent, CR3-mediated phagocytosis. This investigation focused on determining the role of actin polymerization and phosphorylation signals in this uptake mechanism. Uptake inhibition assays and confocal microscopic analysis indicated that entry of L. pneumophila activated tyrosine kinase (TK) and protein kinase C (PKC) and induced actin polymerization at the site of bacterial entry. Upon L. pneumophila entry, six major cellular proteins (75, 71, 59, 56, 53, and 52 kDa) were TK phosphorylated in soluble fractions of monocytes, and three of these proteins (52, 53, and 56 kDa) were consistently found in insoluble (i.e., cytoskeletal) fractions of monocytes as well. Tyrosine phosphorylation was suppressed when cells were pretreated with the kinase inhibitor genistein, tyrphostin, or staurosporine. A similar tyrosine-phosphorylated protein pattern was observed with CR3-mediated entry of avirulent L. pneumophila, Escherichia coli, or zymosan into monocytes. This study has shown that PKC and TK signals which activate actin polymerization during the process of phagocytosis are induced upon L. pneumophila entry. In addition, CR3 receptor-mediated phagocytosis into monocytes may involve tyrosine phosphorylation of similar proteins, regardless of the particle being phagocytosed. Therefore, the tyrosine-induced phosphorylation observed during opsonized L. pneumophila entry is not a virulence-associated event.

Host cell phospholipids are trafficked to and then modified by Chlamydia trachomatis

There is little information on the trafficking of eukaryotic lipids from a host cell to either the cytoplasmic membrane of or the vacuolar membrane surrounding intracellular pathogens. Purified Chlamydia trachomatis, an obligate intracellular bacterial parasite, contains several eukaryotic glycerophospholipids, yet attempts to demonstrate transfer of these lipids to the chlamydial cell membrane have not been successful. In this report, we demonstrate that eukaryotic glycerophospholipids are trafficked from the host cell to C. trachomatis. Phospholipid trafficking was assessed by monitoring the incorporation of radiolabelled isoleucine, a precursor of C. trachomatis specific branched-chain fatty acids, into host-derived glycerophospholipids and by monitoring the transfer of host phosphatidylserine to chlamydiae and its subsequent decarboxylation to form phosphatidylethanolamine. Phospholipid trafficking to chlamydiae was unaffected by brefeldin A, an inhibitor of Golgi function. Furthermore, no changes in trafficking were observed when C. trachomatis was grown in a mutant cell line with a nonfunctional, nonspecific phospholipid transfer protein. Host glycerophospholipids are modified by C. trachomatis, such that a host-synthesized straight-chain fatty acid is replaced with a chlamydia-synthesized branched-chain fatty acid. We also demonstrate that despite the acquisition of host-derived phospholipids, C. trachomatis is capable of de novo synthesis of phospholipids typically synthesized by prokaryotic cells. Our results provide novel information on chlamydial phospholipid metabolism and eukaryotic cell lipid trafficking, and they increase our understanding of the evolutionary steps leading to the establishment of an intimate metabolic association between an obligate intracellular bacterial parasite and a eukaryotic host cell.

Differences in the association of Chlamydia trachomatis serovar E and serovar L2 with epithelial cells in vitro may reflect biological differences in vivo

Chlamydia trachomatis serovar E is one of the most common bacterial sexually transmitted pathogens. Since it is an obligate intracellular bacterium, efficient colonization of genital mucosal epithelial cells is crucial to the infectious process. Serovar E elementary bodies (EB) metabolically radiolabeled with 35S-Cys-Met and harvested from microcarrier bead cultures, which significantly improves the infectious EB-to-particle ratio, provided a more accurate picture of the parameters of attachment of EB to human endometrial epithelial cells (HEC-1B) than did less infectious 14C-EB harvested from flask cultures. Binding of serovar E EB was (i) equivalent at 35 and 4 degrees C, (ii) decreased by preexposure of EB to heat or the topical microbicide C31G, (iii) comparable among common eukaryotic cell lines (HeLa, McCoy), and (iv) significantly increased to the apical surfaces of polarized cells versus nonpolarized cells. In parallel experiments with C. trachomatis serovar L2, serovar E attachment was not affected by heparin or heparan sulfate whereas these glucosaminoglycans dramatically reduced serovar L2 attachment. These data were confirmed by competitive inhibition of serovar E binding and infectivity by excess unlabeled live and UV-inactivated serovar E EB but not by excess serovar L2 EB. The noninvasive serovar E strains in the lumen of the genital tract enter and exit the apical domains of target columnar epithelial cells to spread canalicularly in an ascending fashion from the lower to the upper genital tract. In contrast, the invasive serovar L2 strains are primarily submucosal pathogens and likely use the glucosaminoglycans concentrated in the extracellular matrix to colonize the basolateral domains of mucosal epithelia to perpetuate the infectious process.

Origins and functions of the chlamydial inclusion

Chlamydiae dissociate themselves from the endocytic pathway shortly after internalization by actively modifying the vacuole to become fusogenic with sphingomyelin-containing exocytic vesicles. Interaction with this secretory pathway appears to provide a pathogenic mechanism that allows chlamydiae to establish themselves in a site that is not destined to fuse with lysosomes.

Polymerase chain reaction (PCR) detection of porcine Chlamydia trachomatis and ruminant Chlamydia psittaci serovar 1 DNA in formalin-fixed intestinal specimens from swine

In previous studies chlamydiae were detected immunohistologically in the gut of 66 out of 311 pigs. The aim of the present investigation was the classification of these intestinal porcine chlamydiae. For the study, DNA extracted from 52 paraffin-embedded intestinal tissues was amplified in nested polymerase chain reactions (PCRs) with Chlamydia omp1 genus- and species-specific primers. Some of the amplification products were cloned and sequenced. In 45 cases DNA could be amplified with genus-specific primers. Species-specific PCR and sequencing showed that in 42 cases the chlamydial omp1 genotype was Chlamydia trachomatis. Sequenced DNA fragments were 95-99% identical with the porcine strain S45. In three further cases sequencing analysis provided DNA sequences which were 100% identical with Chlamydia psittaci B577 (serovar 1) omp1 genotype. So far as the authors are aware this is the first report on the occurrence of C. psittaci serovar 1 in pigs.

Temporal analysis of the developing Chlamydia psittaci inclusion by use of fluorescence and electron microscopy

The chlamydiae are obligate intracellular parasites that develop and multiply within a vacuole (termed an inclusion) that does not fuse with lysosomes. Inclusion morphology varies dramatically among the different chlamydiae, particularly within the species Chlamydia psittaci. Some strains develop within a single vacuole, while the mature inclusion of other strains consists of several distinct lobes, each filled with chlamydial developmental forms. The development of this lobed structure was investigated in HeLa cells infected with the guinea pig inclusion conjunctivitis (GPIC) strain of C. psittaci. We employed two recently described probes for the chlamydial inclusion to study the development of these unique lobed structures. The novel probes were an antiserum directed at a protein localized to the GPIC inclusion membrane (anti-IncA) and the fluorescent sphingolipid (N-[7-(4-nitrobenzo-2-oxa-1,3-)]) aminocaproyl sphingosine (NBD-ceramide). Lobed inclusions developed in cells infected at very low multiplicities of infection, suggesting that the structure is not a function of infection by more than one elementary body (EB). Double-label fluorescent-antibody analysis with anti-IncA and an antibody directed at a chlamydial outer membrane protein showed that, prior to 18 h postinfection (p.i.), the inclusion membrane and the chlamydial membrane were tightly associated. After 18 to 20 h p.i., the lobes began to expand and fill with developmental forms and the inclusion membrane and chlamydial membrane became distinct. At times from 8 to 48 h p.i., GPIC inclusions were shown to receive fluorescent derivatives of NBD-ceramide and to be localized to the perinuclear region of the host cell. Labeled lectins with affinity for carbohydrate moieties localized to the Golgi apparatus showed that the lobes of mature inclusions surround the Golgi apparatus. Labeling with NBD-ceramide and the Golgi apparatus-specific lectins therefore demonstrated a functional and physical association of the inclusion with the Golgi apparatus throughout the developmental cycle. Collectively, these results lead to a model for the development of the lobed chlamydial inclusion. We propose that the lobed structure is a result of division of inclusions occurring in parallel with the multiplication of reticulate bodies (RB) early in the developmental cycle. The division of inclusions slows or stops in mid-cycle, and dividing RB accumulate within the enlarging lobes. The RB then differentiate to EBs, the inclusion and cell are lysed, and EBs are freed to infect another cell.

Gonococcal opacity protein promotes bacterial entry-associated rearrangements of the epithelial cell actin cytoskeleton

Neisseria gonorrhoeae enters cultured human mucosal cells following binding of a distinct gonococcal opacity (Opa) outer membrane protein to cell surface proteoglycan receptors. We examined the route of internalization that is activated by Opa-expressing gonococci (strain VP1). Microscopy of infected Chang epithelial cells showed that gonococcal uptake was insensitive to monodansylcadaverine (150 microM), which interferes with clathrin-mediated endocytosis. Similarly, indirect immunofluorescence staining for clathrin in infected cells showed distribution of cellular clathrin unaltered from the distribution in noninfected cells. The microtubule inhibitors colchicine (50 microM) and nocodazole (20 microM) but not the microtubule-stabilizing agent taxol (10 microM) caused a moderate (30 to 50%) reduction in gonococcal entry without affecting bacterial adherence. The most dramatic effects were obtained with the microfilament-disrupting agent cytochalasin D (3 microM), which totally blocked bacterial entry into the cells. Double immunofluorescence staining of gonococci and actin filaments in infected cells demonstrated bacterium-associated accumulations of F-actin as an early signal of bacterial entry. The recruitment of F-actin was transient and disappeared once the bacteria were inside the cells. Cytochalasin D disrupted the actin cytoskeleton architecture but did not prevent the recruitment of F-actin by the bacteria. Adherent, noninvasive gonococcal Opa variants lacked the ability to mobilize F-actin. Recombinant Escherichia coli expressing the gonococcal invasion-promoting Opa of gonococcal strain MS11 (Opa50) adhered to the epithelial cells in an Opa-dependent fashion but was not internalized and did not recruit detectable amounts of F-actin. Coinfection with the E. coli recombinant strain and gonococci resulted in specific entry of the diplococci, despite the presence of large numbers of adherent E. coli cells. Together, our results indicate that Opa-mediated gonococcal entry into Chang cells resembles phagocytosis rather than macropinocytosis reported for Salmonella spp. and sequentially involves gonococcal adherence to the cell surface, Opa-dependent and cytochalasin-insensitive recruitment of F-actin, and cytochalasin D-sensitive bacterial internalization.

Vesicles containing Chlamydia trachomatis serovar L2 remain above pH 6 within HEC-1B cells

Chlamydia trachomatis serovar L2 is an obligate intracellular bacterium which is internalized in target epithelial cells by endocytosis and resides within a membrane-bound vesicle. Over the next several hours following entry, individual serovar L2-containing vesicles fuse with one another to form a single membrane-bound vesicle (or inclusion) within which the microcolony develops. The experiments reported here directly examined the pH of vesicles containing chlamydiae. The pH was determined by measuring emission ratios of the fluorescent, pH-sensitive probe SNAFL (5-[and 6-]-carboxyseminaphthofluorescein-1, succinimidyl ester) conjugated to chlamydiae. The pH remained above 6.0 at 2, 4, and 12 h after infection, while the pH of vesicles contained heat-killed organisms fell 5.3. In the presence of amines, which raise the pH of acidic compartments, C. trachomatis inclusion formation was unaffected. Inactivation of Na+,K+ -ATPases, the ion pumps responsible for maintaining a pH above 6 within early endocytic vesicles, inhibited the growth of C. trachomatis within epithelial cells. Preventing vesicular acidification by inhibiting the vacuolar proton ATPase did not affect chlamydial growth. Thus, chlamydiae do not reside within highly acidic vesicles and avoid the pathway leading to lysosomes.

Adherence, fibronectin binding, and induction of cytoskeleton reorganization in cultured human cells by Mycoplasma penetrans

Mycoplasma penetrans adhered to cultured human cells, forming clusters that localized to specific areas of the host cell surface. Adherence and cluster formation were inhibited by anti-M. penetrans antibodies, suggesting the involvement of specific adhesin-receptor interactions. Ultrastructural studies showed that after 2 h of infection, mycoplasmas attach to and penetrate the host cell surface. M. penetrans bound selectively to immobilized fibronectin, an interaction which was not inhibited by a 70-kDa fragment containing a heparin-gelatin-binding domain of fibronectin, other matrix glycoproteins, or an RGD tripeptide, suggesting the recognition of other specific binding sites on the fibronectin molecule. A ca. 65-kDa fibronectin-binding protein of M. penetrans was eluted following Sepharose-fibronectin affinity chromatography. Confocal, light, and immunofluorescence microscopy demonstrated that the interaction of M. penetrans with target cells triggers a signal that causes recruitment of several cytoskeletal components, including tubulin and alpha-actinin, and aggregation of phosphorylated proteins. Detergent-soluble mycoplasma proteins with apparent molecular masses of 18, 28, 32, 36, 39, and 41 kDa selectively bound to glutaraldehyde-fixed HEp-2 cells. Our findings offer new insights into understanding the interaction of this human mycoplasma with host target cells.