INTRODUCTION
The pathogen
Chlamydia trachomatis is the leading cause of noncongenital blindness and causative agent of the most common sexually transmitted infection of bacterial origin (
1). As an obligate intracellular pathogen with a small genome (1 Mb),
C. trachomatis is strictly dependent on the host cell to complete its developmental cycle. Upon entering epithelial cells,
C. trachomatis resides within a membranous vacuole, the inclusion. In the lumen of the inclusion, the bacteria undergo a biphasic developmental cycle, alternating between the infectious elementary body (EB) form and the replicative reticulate body (RB) form (
2,
3). After entry, the EBs differentiate into RBs, and the RBs continue to replicate within the inclusion until the asynchronous differentiation of RBs back to EBs begins approximately 24 h postinfection (
3). The EBs are then released from the host cell through extrusion or host cell lysis, allowing for infection of neighboring cells (
4–6).
It is still unclear how much
C. trachomatis relies on the host cell for energy production throughout development. For almost 40 years,
C. trachomatis was thought to be an energy parasite (
7–10). However, sequencing of the
C. trachomatis genome revealed that the bacteria possess components of the electron transport chain and ATP synthase complex, suggesting that
C. trachomatis can drive a minimal electron transport chain to produce ATP through oxidative phosphorylation (
11). In addition,
C. trachomatis was found to have an intact pentose phosphate pathway and a partial citric acid (tricarboxylic acid [TCA]) cycle (
11,
12). Although these findings indicate that
C. trachomatis is not merely an energy parasite, there is evidence to suggest that
C. trachomatis is in part dependent on the host cell for energy production. For example, the nucleotide transporters Npt1 and Npt2 are highly expressed in
Chlamydia RBs (
13,
14), indicating that
Chlamydia is reliant on scavenging ATP and NAD
+ from the host (
15,
16).
In addition to oxidative phosphorylation, glycolysis is another major source of cellular energy. Glycolysis relies on the function of 10 different enzymes to sequentially convert glucose into pyruvate and NAD
+ to NADH, producing ATP in the process (
17). Sequencing of the
C. trachomatis genome identified a nearly full set of glycolytic enzymes, lacking only the gene for hexokinase, the first enzyme of the pathway responsible for converting glucose to glucose-6-phosphate (
11). Instead, glucose-6-phosphate is taken directly from the host cell via the UhpC antiporter produced by
Chlamydia (
18). Heterologous expression of
C. trachomatis glycolytic enzymes in
Escherichia coli confirmed their functionality (
19). However, the
C. trachomatis glycolytic enzymes were proposed to be expendable, as saturated ethyl methanesulfonate mutagenesis resulted in a loss-of-function mutation in bacterial glucose-6-phosphate isomerase (
pgi), the enzyme responsible for shuttling glucose-6-phosphate into the glycolytic pathway (
20).
More recently, a genome-wide RNA interference screen performed by Rother et al. suggested that two host glycolytic enzymes, glucose-6-phosphate isomerase and 6-phosphofructokinase, were potentially involved in
Chlamydia progeny production (
21). However, these results were not validated. The same study also profiled metabolites of central carbon metabolism after
Chlamydia infection and observed elevated levels of pyruvate, lactate, and glutamate. An increase in these metabolites is indicative of Warburg metabolism, a metabolic state commonly observed in cancer cells that is characterized by the increased utilization of glycolysis rather than oxidative phosphorylation, resulting in the increased production of lactate (
22–24). In this metabolic state, the upregulation of glycolysis leads to an accumulation of glycolytic intermediates that can be shuttled into the pentose phosphate pathway and used for ribonucleotide synthesis. Thus, Rother et al. (
21) concluded that, much like what occurs in cancer cells,
Chlamydia is able to shift the host cell into a hypermetabolic state in order to meet the high energetic demand of bacterial replication. However, whether this upregulation of host glycolysis is influenced by the localization of host glycolytic enzymes in relation to the
C. trachomatis inclusion remains unknown.
In the present study, we show that several of the host glycolytic enzymes localized at the inclusion membrane and that the number of positive inclusions increased as the developmental cycle progressed. Moreover, knockdown of the host glycolytic enzyme aldolase A (AldoA) resulted in reduced inclusion size and decreased infectious progeny production, suggesting a role for host glycolysis in bacterial development. Lastly, quantitative PCR (qPCR) analysis of C. trachomatis glycolytic enzymes showed their downregulation throughout the developmental cycle. These novel findings further shed light on host and bacterial metabolism throughout Chlamydia development.
MATERIALS AND METHODS
Ethics statement.
All genetic manipulations and containment work were approved by the University of Virginia Biosafety Committee and are in compliance with the section III-D-1-a of the NIH guidelines for research involving recombinant DNA molecules.
Cell lines and bacterial strains.
HeLa cells were obtained from the ATCC (CCL-2) and cultured at 37°C with 5% CO
2 in high-glucose Dulbecco modified Eagle medium (Invitrogen) supplemented with 10% heat-inactivated fetal bovine serum (Invitrogen).
C. trachomatis lymphogranuloma venereum (LGV) type II was obtained from the American Type Culture Collection (L2/434/Bu VR-902B).
C. trachomatis propagation and infection were performed as previously described (
63).
C. muridarum was obtained from Michael Starnbach (Harvard Medical School, Boston, MA). mCherry-expressing
C. trachomatis (mCherry CtL2) and
C. muridarum strains were described previously (
57).
Plasmid construction.
Restriction enzymes and T4 DNA ligase were obtained from New England BioLabs (Ipswich, MA). PCR was performed using Herculase DNA polymerase (Stratagene). PCR primers were obtained from Integrated DNA Technologies.
Vectors for expression in mammalian cells.
DNA fragments corresponding to pyruvate kinase, aldolase A, and the R42A and R148A mutants of aldolase A were amplified by PCR and cloned into the BamHI and XhoI restriction sites of pCDNA 4/TO 3×FLAG. Lactate dehydrogenase was cloned into the EcoRI and XhoI restriction sites of pCDNA 4/TO 3×FLAG. HA-aldolase A was cloned into the KpnI and NotI restriction sites of pCDNA3.1+. Pyruvate kinase, aldolase A and lactate dehydrogenase DNA fragments were amplified using uninfected HeLa cell cDNA as the template, and the primers are listed in Table S1 in the supplemental material.
DNA transfection.
DNA transfection was performed using X-tremeGENE 9 DNA transfection reagent (Roche), according to the manufacturer’s recommendations.
Immunofluorescence and confocal microscopy.
All steps were performed at room temperature. At the indicated times, HeLa cells seeded on glass coverslips were fixed with 4% paraformaldehyde in 1× phosphate-buffered saline (PBS) for 30 min. Coverslips were sequentially incubated with primary and secondary antibodies diluted in 0.1% Triton X-100 in 1× PBS for 1 h. Coverslips were washed with 1× PBS and mounted with DABCO antifade-containing mounting media. Imaging was performed using the Leica DMi8 microscope equipped with an Andor iXon ULTRA 888BV EMCCD camera and a CSU-W1 confocal scanner unit and driven by the IQ software. Images were processed using the Imaris software (Bitplane, Belfast, United Kingdom). Line intensity scan analyses presented in
Fig. 1E were performed using ImageJ (NIH).
Antibodies.
The following primary antibodies were used for immunofluorescence microscopy (IF) and immunoblotting (Western blotting): mouse monoclonal anti-FLAG (1:1,000 for IF; Sigma), rabbit polyclonal anti-C. trachomatis IncA (1:200 for IF; kindly provided by T. Hackstadt, Rocky Mountain Laboratories), rabbit polyclonal anti-HA (1:100 for IF; Sigma), mouse monoclonal anti-aldolase A (1:1,000 for Western blotting; Santa Cruz), rabbit polyclonal anti-actin (1:10,000 for Western blotting; Sigma), mouse monoclonal anti-α-tubulin (1:2000 for IF; Sigma), and Alexa Fluor 514-conjugated phalloidin (1:200 for IF; Invitrogen). The following secondary antibodies were used: Alexa Fluor 488-, Alexa Fluor 514-, or Pacific Blue-conjugated goat anti-mouse antibody (1:500 for IF; Molecular Probes), Pacific Blue- or Alexa Fluor 514-conjugated goat anti-rabbit antibody (1:500 for IF; Molecular Probes), peroxidase-conjugated goat anti-mouse IgG (1:10,000 for Western blotting; Jackson ImmunoResearch), and peroxidase-conjugated goat anti-rabbit IgG (1:10,000 for Western blotting; Jackson ImmunoResearch).
Quantification of host glycolytic enzyme enrichment at the inclusion.
HeLa cells were transfected with the indicated 3×FLAG-tagged enzyme construct 18 h before infection with the mCherry CtL2 strain. The samples were processed for confocal microscopy and analyzed using Imaris imaging software. For each inclusion, the quantification was performed on a 1-μm slice located in the middle of the inclusion. Three-dimensional reconstructions of the raw signal corresponding to the enzyme signal at the inclusion and to the enzyme signal in the surrounding cytosol were generated using the Imaris imaging software. The average intensity of these three-dimensional objects was calculated using Imaris imaging software. The average intensity of the enzyme signal at the inclusion divided by the average intensity of the surrounding cytosol was used to determine the percent enrichment of the enzyme at the inclusion. Each experiment was performed in triplicate. Fifteen to thirty inclusions were analyzed per condition. The graphs were generated using GraphPad Prism. Averages and standard deviations (SD) are shown. A Student t test was performed, and statistical significance was set to P < 0.05.
Quantification of the percentage of inclusions positive for a given enzyme.
HeLa cells were transfected with the indicated 3×FLAG-tagged enzyme construct 18 h before infection with the mCherry CtL2 strain. The samples were processed for immunofluorescence microscopy and analyzed using an epifluorescence microscope. Transfected cells were scored for the presence (positive) or absence (negative) of a ring of enzyme enrichment at the inclusion (as seen in
Fig. 1A). One hundred inclusions were analyzed per condition. The average percent positive inclusions and SD from three replicate experiments are presented. A Student
t test was performed. and statistical significance was set to
P < 0.05.
Quantification of the graph presented in
Fig. 2B was performed as described above, except that HeLa cells were cotransfected with HA-aldolase A and the indicated 3×FLAG-tagged enzyme construct 18 h before infection with the mCherry CtL2 strain. Inclusions in cotransfected cells were scored for the presence or absence the HA- and 3×FLAG-tagged constructs. Quantification of the graph presented in
Fig. 4B was performed as described above, except that the cells were transfected with the R42A, R148A, or WT aldolase A. Quantification of the graph presented in
Fig. 4C was performed as described above, except that the cells were infected the
inaC::
aadA mutant or corresponding WT
C. trachomatis strain.
Inhibition of eukaryotic protein synthesis.
HeLa cells were transfected with the 3×FLAG-tagged aldolase A construct 18 h before infection with the mCherry CtL2 strain. At the time of infection, cells were incubated in the presence or absence of 1 μg/ml cycloheximide for 18, 24, or 32 h. At the indicated time points, the samples were processed for confocal microscopy, and the percentage of positive inclusions was determined. One hundred inclusions were analyzed per condition. The average percentage of positive inclusions and SD from three replicate experiments are presented. A one-way analysis of variance (ANOVA) with multiple comparisons was performed, and statistical significance was set to P < 0.05.
Inhibition of C. trachomatis protein synthesis.
HeLa cells were transfected with the 3×FLAG-tagged aldolase A construct 18 h before infection with the mCherry CtL2 strain. At 24 h postinfection, infected cells were incubated in the presence of 40 μg/ml of chloramphenicol for 8 h. At 32 h postinfection, the samples were processed for confocal microscopy, and the percentage of positive inclusions was determined. One hundred inclusions were analyzed per condition. The average percentage of positive inclusions and SD from three replicate experiments are presented. A Student t test was performed, and statistical significance was set to P < 0.05.
Drug treatments.
HeLa cells were transfected with the indicated 3×FLAG-tagged enzyme construct 18 h before infection with the mCherry CtL2 strain. For cytochalasin D and nocodazole treatment, at 23.5 h postinfection, infected cells were incubated in the presence of 1 μM cytochalasin D or 33 μM nocodazole for 30 min. For brefeldin A treatment, at 6 h postinfection, infected cells were incubated in the presence of 1 μg/ml brefeldin A overnight. At 24 h postinfection, the samples for all drug treatments were processed for confocal microscopy, and the percentage of positive inclusions was determined. One hundred inclusions were analyzed per condition. The average percentage of positive inclusions and SD from three replicate experiments are presented. A Student t test was performed, and statistical significance was set to P < 0.05.
inaC mutant generation.
An
inaC::
aadA mutant was generated in our lab
C. trachomatis LGV L2 strain background using TargeTron, as described by previous studies (
20,
30). Using PCR, the GrpII intron was retargeted for
C. trachomatis 434/Bu
inaC using the primers CTL0184 129 130 IBS1/2, CTL0184 129 130 EBS1/delta, and CTL0184 129 130 EBS2 designed by the TargeTron computer algorithm (TargeTronics) (the primer sequences are listed in Table S1 in the supplemental material). The resulting PCR product was digested with BsrGI and HindIII and cloned into the BsrGI/HindIII site of the pDFTT3-
aadA suicide vector (
64).
C. trachomatis serovar L2 was transformed with pDFTT3-
aadA-inaC according to our calcium-based transformation protocol as previously described (
57). After three passages, transformants were plaque purified and amplified. Genomic DNA was prepared using an illustra bacteria genomicPrep Mini Spin kit (GE Healthcare), according to the manufacturer’s recommendations. The
inaC open reading frame was amplified from WT or
inaC::
aadA mutant genomic DNA by PCR using the primers CTL0184 Up and CTL0184 Dwn (see Table S1). The resulting PCR products were analyzed by DNA gel electrophoresis and sequenced to verify proper insertion of the group II intron (see Fig. S6).
Aldolase A depletion.
The protocol for siRNA transfection has been described previously (
63). Aldolase A depletion was performed by transfection of four independent siRNA duplexes. The sequences of the Aldolase A siRNA duplexes were: duplex 1 (GGACAAAUGGCGAGACUAC), duplex 2 (UUGAAGCGCUGCCAGUAUG), duplex 3 (GGCGUUGUGUGCUGAAGAU), and duplex 4 (UGACAUCGCUCACCGCAUC).
Immunoblotting.
Protein samples were separated by SDS-PAGE and transferred to nitrocellulose membranes. The membranes were blocked for 1 h at room temperature in 1× PBS containing 0.05% Tween and 5% fat-free milk. Primary and horseradish peroxidase-conjugated secondary antibodies were diluted in 1× PBS containing 0.05% Tween and 5% fat-free milk and incubated overnight at 4°C and 1 h at room temperature, respectively. Proteins were detected using the Amersham ECL immunoblotting detection reagent, according to the manufacturer’s recommendation, and a Bio-Rad ChemiDoc imaging system. Immunoblots were quantified using ImageJ (NIH) software.
Inclusion size quantification and computer-assisted image analysis.
HeLa cells were transfected with the indicated 3×FLAG-tagged enzyme construct 18 to 24 h before infection with the mCherry CtL2 strain. The samples were processed for immunofluorescence microscopy and imaged using an epifluorescence microscope. Computer-assisted image analysis, using the analytical tools of the MetaExpress software, was used to determine the area of each aldolase A-positive and -negative inclusion. One hundred inclusions were analyzed per condition.
For quantification of the graph presented in
Fig. 5C, siRNA-treated cells were infected with the mCherry CtL2 strain and fixed at 32 h postinfection. The nuclei were labeled with the DNA dye Hoechst. The cells were subjected to automated fluorescence microscopy using an ImageXpress automated system to capture images corresponding to the cell nuclei and the inclusion. Computer-assisted image analysis, using the analytical tools of the MetaExpress software, was used to determine the number of nuclei and the total area of each inclusion.
Infectious progeny production.
HeLa cells incubated with the indicated siRNA duplexes for 3 days were collected 48 h postinfection and lysed with water, and dilutions of the lysate were used to infect fresh HeLa cells. The cells were fixed 24 h postinfection and the number of inclusion-forming units (IFU) was determined after assessment of the number of infected cells by automated imaging using an ImageXpress automated system.
Real-time PCR analysis of bacterial glycolytic gene expression.
At the indicated time point, infected cells were homogenized with TRIzol (Thermo Fisher Scientific) to extract RNA from infected cells. Each RNA sample was treated with DNase according to the Turbo DNA-free kit protocol (Thermo Fisher Scientific). Complementary DNA (cDNA) was synthesized using SuperScript II reverse transcriptase (Thermo Fisher Scientific) according to the manufacturer’s protocol. Samples were primed using random primers (Thermo Fisher Scientific). mRNA levels were determined by quantitative real-time PCR using the Universal Probe Library (Roche Biochemicals, Indianapolis, IN) and Luna Universal qPCR master mix (New England Biolabs). Thermal cycling was carried out using a LightCycler 96 instrument (Roche Diagnostics) under the following conditions: 95°C for 5 min, followed by 45 cycles of 95°C for 10 s and 56°C for 25 s. Quantification cycle (
Cq) values were derived using the LightCycler 96 software, and fold changes were calculated using threshold cycle (
CT) 16S rRNA for normalization (
65,
66). Statistical analysis was performed by using the Student
t test, and statistical significance was set to
P < 0.05. PCR primers used for quantitative real-time PCR were obtained from Integrated DNA Technologies. Primer sequences and corresponding probes are listed in Table S2 in the supplemental material.