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1 Department of Pathology and Laboratory Medicine, University of British Columbia, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, Canada
2 Department of Pathology and Laboratory Medicine, University of British Columbia, Department of Cancer Genetics, BC Cancer Research Centre, 601 West 10th Avenue, Vancouver, BC V5Z 1L3, Canada
3 Departments of Paediatrics and Pathology and Laboratory Medicine, University of British Columbia, Division of Infectious and Immunological Diseases, BC Children's Hospital, 950 West 28th Avenue, Vancouver, BC V5Z 4H4, Canada
Correspondence
Richard W. Stokes
rstokes{at}interchange.ubc.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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, bone-marrow-derived macrophages; FRD, fumarate reductase; MNPO, mercaptopyridine-N-oxide; qPCR, quantitative real-time PCR; SDH, succinate dehydrogenase; TB, tuberculosis |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). Current vaccines and chemotherapeutic measures are limited in their efficacy and are failing to prevent the incidence of TB. In the last two decades, the emergence of multi-drug resistant strains of the causative agent, Mycobacterium tuberculosis, and the increased susceptibility of AIDS patients to both infection and reactivation of disease have been observed (Narain et al., 1992; Selwyn et al., 1989). Furthermore, with an increasingly mobile population, it has become easier for tuberculosis to spread to areas with lax preventative measures against the disease. M. tuberculosis is an intracellular pathogen that is able to successfully evade host defences, enabling its survival within the host. However, even with the insight gained recently into mycobacterial gene expression via novel techniques such as SCOTS (Graham & Clark-Curtiss, 1999), subtractive hybridization (Kinger & Tyagi, 1993; Li et al., 2001) and differential display (Mostowy et al., 2004; Rindi et al., 1999; Rivera-Marrero et al., 1998), and especially the profusion of information uncovered by microarray technology (Bacon et al., 2004; Mostowy et al., 2004; Schnappinger et al., 2003; Waddell et al., 2005; Wei et al., 2000), much remains to be understood about the molecular aspects of mycobacterial pathogenesis in relation to the host cell.
The bacterial model applied in this study is that of M. tuberculosis H37Ra and H37Rv, sibling strains derived from the parental H37 (Oatway & Steenken, 1936; Steenken et al., 1934), a laboratory strain originally isolated from the sputum of a pulmonary TB patient in 1906 (Steenken & Gardner, 1946). Strains H37Ra and H37Rv are highly similar, as studies have shown little genomic difference between them (Bhargava et al., 1990; Collins & De Lisle, 1984; Imaeda, 1985), save for the RvD2 region of difference (Brosch et al., 1999; Lari et al., 2001), which has not been conclusively linked to pathogenesis (Lari et al., 2001). However, in spite of this lack of genomic differences between the strains, H37Rv has been shown to display a marked advantage in survival and growth in animal hosts over that of H37Ra (Collins & Smith, 1969; Jung et al., 2002). Thus, dissecting the molecular aspects of the phenotypic differences between these highly related strains may lead to a better understanding of mycobacterial virulence.
Studies comparing the expression differences between broth-grown H37Ra and H37Rv and/or intracellular and broth-grown H37Ra/Rv bacteria have elucidated genes differentially expressed between the strains, some of which have been shown to have roles in mycobacterial virulence (Graham & Clark-Curtiss, 1999; Kinger & Tyagi, 1993; Rindi et al., 1999, 2001; Rivera-Marrero et al., 1998; Wei et al., 2000; Gao et al., 2004). For example, Gao et al. (2004) examined, via microarray, genes involved in the cording and non-cording phenotypes of the respective H37Rv and H37Ra. Graham & Clark-Curtiss (1999) developed the novel SCOTS technique to minimize the noise introduced by rRNA populations and, in their comparisons of the transcriptomes of broth-grown and intracellular M. tuberculosis H37Rv, they were able to isolate several genes upregulated in intracellular M. tuberculosis. One of these genes was pks2, a putative polyketide synthase which has been found to play a role in the synthesis of sulpholipid (Sirakova et al., 2001), an important component of the mycobacterial cell wall. Furthermore, Kinger & Tyagi (1993) examined, via differential display, genes differentially expressed between aerobic cultures of M. tuberculosis H37Ra and H37Rv. From this study, the devR/devS (also known as dosR/dosS) two-component system was identified, which has subsequently been found to have a role in the survival of M. tuberculosis under anoxic conditions (Dasgupta et al., 2000; Guinn et al., 2004; Park et al., 2003; Saini et al., 2004; Wei et al., 2000). These studies highlight the value of the H37Rv/H37Ra model in the search for genes that may have a role in M. tuberculosis pathogenesis.
Our study also utilized the H37Rv/H37Ra model; however, we reasoned that gene expression changes that would affect bacterial survival and growth within host cells are more likely to be discovered by focusing on differences between H37Ra and H37Rv expression when both strains are in interaction with macrophages rather than by comparing broth-grown with intracellular bacteria. To assess these transcriptome changes between strains, we used a novel technique – bacterial artificial chromosome fingerprint array (BACFA) – that has been successfully utilized for comparative genomic studies (Brosch et al., 1998, 1999; Gordon et al., 1999a). We also provide evidence that, through use of BACFA, we have identified an enzyme complex, fumarate reductase, that aids the survival of intracellular M. tuberculosis.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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), 10 ml volumes of PB+T cultures inoculated with the respective strains were seeded into 14 ml polystyrene tubes. The tubes were tightly capped and left undisturbed at 37 °C. At designated time points, the tubes were inverted to resuspend the sedimented bacterial pellet and immediately processed for RNA. For broth culture transcriptome studies, bacteria were grown as aerated cultures in roller bottles in either PB+T or 7H9 supplemented with 0.05 % Tween 80 and 10 % OADC (oleic acid/albumin/dextrose-complex). Doubling times were calculated from viable counts collected on days 0 and 5 using the following equation: doubling time (h)=(txlog102)÷(log10Nt–log10N0), where t=time elapsed in hours, Nt=number of bacteria at time t and N0=number of bacteria at time 0 (Madigan et al., 1997b; Monod, 1949).
Isolation and culturing of bone-marrow-derived macrophages (BM-M).
BM-M were obtained from the femora, tibiae and humeri of CD-1 mice as previously described (Rooyakkers & Stokes, 2005; Stokes et al., 1993). Briefly, the bones were dissected out from euthanized, 7–10-week-old female CD-1 mice, and the marrow was flushed from the bones using BM-M medium [RPMI containing 10 % heat-inactivated fetal calf serum, 10 % L-929 cell medium (Rooyakkers & Stokes, 2005), 2 mM L-glutamine and 1 mM sodium pyruvate]. The cell suspension was left to adhere in a 175 cm2 flask for 3 h to deplete non-stem cells. Non-adherent cells (stem cells) were collected and cultured for 7 days in BM-M medium at 37 °C with 5 % CO2 to allow differentiation into macrophages in 150 cm2 tissue culture flasks for RNA studies, or on 13 mm diameter coverslips for c.f.u. determinations (Rooyakkers & Stokes, 2005).
Infection and determination of macrophage-associated growth of M. tuberculosis.
To assess bacterial expression profiles 96 h and 168 h post-infection, monolayers were incubated at an m.o.i. of 10 bacteria to 1 macrophage, which resulted in an average infection rate (average c.f.u. at 4 h per macrophage plated in the well) of 0.1 bacteria per macrophage. After a 4 h incubation with binding medium (138 mM NaCl, 8.1 mM Na2HPO4, 1.5 mM KH2PO4, 2.7 mM KCl, 0.6 mM CaCl2, 1.0 mM MgCl2 and 5.5 mM D-glucose) containing the requisite number of bacteria, the monolayers were washed three times with pre-warmed medium to rinse off unbound bacteria, before being submerged in cRPMI (RPMI 1640 with 10 % fetal calf serum, 2 mM L-glutamate and 1 mM sodium pyruvate) until they were to be processed for RNA at days 4 and 7 post-infection. To assess bacterial expression at 4 h post-infection, monolayers were infected at an m.o.i. of 60 bacteria to 1 macrophage, resulting in an average infection rate of 0.6 bacteria per macrophage. This higher m.o.i. was used to allow for the harvest of a sufficient amount of bacterial RNA and was comparable to the bacterial load in macrophages 4 days post-infection at an m.o.i. of 10 : 1 (data not shown). At each time point, coverslips and supernatants were briefly sonicated (10 s) to release and disperse bacteria, and plated on 7H10 agar plates supplemented with 10 % OADC to determine c.f.u. per ml (Rooyakkers & Stokes, 2005). For all macrophage experiments, three replicate coverslips were assessed at each time point for each of three independent experiments.
Extraction and purification of bacterial RNA.
RNA was extracted from broth cultures as previously described (Mangan et al., 1997; Stewart et al., 2002). For intracellular bacteria used to examine M. tuberculosis expression profiles at 96 and 168 h post-infection, 50 ml GTC lysis buffer (Monahan et al., 2001) was added to each flask at the designated time and rocked gently until Schlieren lines disappeared, indicating homogeneous mixing of the solutions. For intracellular bacteria used to examine mycobacterial expression at 4 h post-infection, flasks were washed three times with pre-warmed medium, and 50 ml GTC lysis buffer was poured directly onto the monolayer. The entire contents of flasks were poured into 50 ml conical tubes, and spun at 3700 r.p.m. (3500 g) for 15 min. Pellets were resuspended and pooled with 1 ml GTC lysis buffer. RNA was extracted from the pellets and then purified as previously described (Mangan et al., 1997; Stewart et al., 2002; Wernisch et al., 2003).
Reverse transcription and cDNA synthesis.
For BAC arrays, 1–5 µg total RNA was reverse transcribed with Superscript II (Invitrogen) and a universal arbitrary primer (5'-GCCGGAGCTCTGCAGAATTC-3'), henceforth called Uniprime, was used to generate single-stranded cDNA (Mangan & Butcher, 1998). Eight units of Klenow fragment of DNA polymerase I (Invitrogen) was then used to synthesize second-strand cDNA according to previously published protocols (Mangan & Butcher, 1998).
DIG-labelling of cDNA for use in BACFA.
A 2 µl aliquot of double-stranded cDNA was added to 8 µl water, boiled for 10 min and added to the following: 2 µl 10x PCR buffer (Invitrogen), 2.5 µl 2 mM DIG-dUTP : dTTP (3 : 1 ratio), 2.5 µl 2 mM dNTPs (dATP, dCTP, dGTP), 0.8 µl 50 µM Uniprime stock solution, 0.6 µl 50 mM MgCl2 and 1 U Taq Polymerase (Invitrogen), to a final volume of 20 µl. DIG labelling of cDNA was then done via PCR (94 °C for 2 min followed by 30 cycles of 94 °C, 55 °C and 72 °C for 1, 2 and 3 min, respectively).
Generation of bacterial artificial chromosome fingerprint arrays (BACFAs).
The M. tuberculosis H37Rv BAC library was kindly provided by Drs Stewart Cole and Roland Brosch at the Pasteur Insitute (Paris, France), who used it to isolate and identify genomic differences between H37Ra and H37Rv (Brosch et al., 1998, 1999; Gordon et al., 1999b). This library contains 78 BACs, each containing an average of 68 kb of genomic sequence from H37Rv. For preliminary BACFA assessment of differential expression between H37Ra and H37Rv, all BACs were digested with 10 U each of PvuII and StuI (New England Biolabs) overnight at 37 °C. After preliminary screening, some BACs were digested with StuI and SalI. The BACs of interest were first digested with SalI overnight at 37 °C; the products were cleaned with a Qiagen PCR purification column and eluted with 20 µl water. Eluted volumes were then digested overnight at 37 °C with 10 U StuI.
Digestion products were run on a 1.5 % TAE-agarose gel for 2 h at 80 V. Southern transfer of fragments to nylon membrane was done by the manufacturer's protocol (Roche). Fragments were immobilized onto nylon membranes by UV cross-linking at 260 nm for 5 min followed by baking at 80 °C for 2 h.
Generation of BACFA hybridization profiles.
Nylon membranes were hybridized according to the manufacturer's instructions with 2.5 µl DIG-labelled PCR reaction product (above) per ml hybridization buffer. Membranes were hybridized for 16–18 h at 50 °C. Immunological detection of blots was carried out by the manufacturer's protocols (Roche). Profiles were generated in duplicate for each of three independent pools of cDNA from H37Ra and H37Rv. Differences were only singled out if they appeared in all six profiles.
Quantitative real-time PCR (qPCR) analysis of expression differences observed with BACFA.
As hybridization protocols could potentially allow for non-specific binding resulting in a false positive, all differences identified via BACFA were assessed via qPCR. Using gene-specific primers, this technique allows for unambiguous expression analysis of the candidates. Second-strand cDNA generated with the universal primer was used in qPCRs with SYBR Green as the indicator dye. For qPCRs, primers were designed with PrimerQuest, a web-based program freely available from the IDT website (http://www.idtdna.com/Scitools/Applications/Primerquest/), as well as Primer Software from Molecular Biology Tools. Primers were designed with an annealing temperature of 57 °C, and used at a final concentration of 300 nM in a reaction volume of 20 µl. qPCR cycling conditions were as follows: 95 °C for 10 min followed by 35 cycles of 94 °C for 30 s, 57 °C for 20 s, and 72 °C for 30 s. qPCR data are presented as fold difference of expression in H37Ra over that in H37Rv using the 2–
Ct method (Schmittgen et al., 2000), with rrnAP1(Menendez Mdel et al., 2005) and dnaK used as normalizing genes for the 96 h and 168 h transcripts, and rrnAP1 and 16S used as normalizing genes for 4 h transcripts. 16S was used in addition to rrnAP1 at 4 h post-infection as it was found to be more reliable at the earlier time point compared to the later time point. Two normalizing genes were used for each time point as previous reports have shown that normalizing genes do not necessarily stay constant over time and that more than one is needed for reliable expression analysis (Vandesompele et al., 2002).
Treatment of extracellular and intracellular bacteria with mercaptopyridine-N-oxide (MPNO).
We assessed bacterial viability in the presence of MPNO, a putative inhibitor of fumarate reductase. Frozen aliquots of M. tuberculosis H37Ra and H37Rv were thawed, and diluted to 6x106 ml–1 in 7H9. Aliquots (200 µl) of the bacterial suspension were then dispensed into 96-well plates and incubated at 37 °C for 72 h to produce actively growing bacteria. At 72 h, stock MPNO (10 mM in water) was added to wells to give a final concentration ranging from 0 to 4.8 µM and incubated for a further 72 h. Then 50 µl Alamar Blue reagent was added to each well; the plate was left at 37 °C for 24 h and read at both 550 nm and 570 nm on a Bio-Rad microplate reader. Analysis of Alamar Blue reduction as a measure of bacterial viability was done according to the manufacturer's protocol (TREK Diagnostic Systems, Fisher Biosciences).
To monitor the effects of MPNO on intracellular mycobacteria, monolayers of BM-M were infected as described above. After infection, the monolayers were washed thoroughly to remove unbound bacteria and three replicate coverslips were removed to assess the numbers of bacteria bound/internalized by macrophages. Half of the remaining coverslips were overlaid with cRPMI supplemented with 2.4 µM MPNO whilst the other half were overlaid with unsupplemented cRPMI. Coverslips and supernatants were processed at 96 and 168 h post-infection as described above to monitor intracellular bacterial growth.
Statistics.
Statistical significance of comparisons between H37Rv and H37Ra, or between two time points, was determined with the two-tailed, unpaired Student's t test. P values <0.05 were considered significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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) was not significantly different when their respective doubling times during exponential growth were compared (H37Ra, 23.0±1.1 h; H37Rv, 24.1±2.4 h, P=0.6973; Fig. 1b). However, a significant difference in growth rate was observed between the two strains during intracellular growth in BM-M
(Fig. 1c). Assessing growth of intracellular bacteria at day 7 (168 h) following infection of BM-M the doubling time of H37Rv was 31.2±1.3 h, whereas the doubling time of H37Ra was 51.1±1.4 h (P=0.014). While not significant, a marked difference in growth between strains was observed on day 4 (96 h) post-infection (for H37Ra and for H37Rv, P=0.1058, Fig. 1c). This difference represents an inhibition of growth of the attenuated strain when inside macrophages, but not when grown in broth culture.
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). Presence or absence of bands in the hybridization profiles and, in some cases, marked changes in band intensity were designated expression differences. Only differences seen in all three cDNA populations were chosen as candidates for further investigation. To identify genes in the bands of interest, the program Restriction Site Digest was used (http://www.flintbox.com/technology.asp?Page=3352). This program allows the user to digest sequences in silico with restriction enzymes of interest, and provides both a pictorial image of how an ideal digest should run on an agarose gel, as well as a text file of the sizes and sequences of all restriction digest products.
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). To narrow down the potential candidate genes to be confirmed via qPCR, Restriction Site Digest was used to predict the enzymes needed to cut this fragment. StuI and SalI were used to generate a second set of BACFAs and hybridized to DIG-labelled cDNA probes. Here, a band of 1.49 kb was predicted and then seen to be expressed at a higher level in H37Ra versus H37Rv (Fig. 2b). This fragment was found to contain the genes frdB, frdC and frdD. After BACFA hybridization had been performed with cDNA probes derived from three independent populations of RNA extracted from intracellular H37Ra and H37Rv, 20 genes were selected for subsequent qPCR confirmation (Table 1).
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). Expression trends as seen in BACFA were confirmed for frdB, frdC, frdD, pks2, aceE and Rv1571 (Fig. 3a). frdB, frdC and frdD all encode subunits of the fumarate reductase enzyme (FRD) complex, and are found in the frd operon (Cole et al., 1998). Another component of the operon is frdA, whose gene product, FRD-A, along with FRD-B, constitutes the catalytic domain of the FRD complex. Even though frdA was not identified via BACFA, it was therefore included in the qPCR analysis. frdA was also found to show a higher expression level in H37Ra versus H37Rv (Fig. 3a) at 168 h post-infection.
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), normally done by the succinate dehydrogenase (SDH) enzyme complex. Previous studies of sdh mutants in Escherichia coli have suggested that FRD can partially compensate for a lack of SDH activity (Guest, 1981). However, qPCR assessment of sdh in H37Ra and H37Rv indicated that not only is SDH present in both strains, but expression of these genes in the sdh operon (sdhA, sdhB, sdhC and sdhD) did not differ between H37Ra and H37Rv at 168 h post-infection (Fig. 3b).
Assessment of candidate gene expression profiles in broth cultures and at 4 h and 96 h post-infection
The 168 h post-infection time point was chosen primarily because that was the time at which sufficient RNA could be harvested for BACFA analysis. However, there remained the possibility that the significant difference we saw between H37Ra and H37Rv at 168 h post-infection was independent of events at earlier time points. Taking candidate genes identified as having expression differences at 168 h post-infection, the expression profiles of these genes were also assessed at 4 and 96 h post-infection (Fig. 4a). Expression profiles of the genes were also assessed for PB+T broth cultures to determine whether the inter-strain differences are inherent or are more pronounced in interactions with the host macrophage (Fig. 4b, c).
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). However, at 4 and 96 h post-infection these genes are expressed at a higher level in the virulent H37Rv (Fig. 4a). This was reflected in a rapid expression of the frd operon following exposure of H37Rv to the intracellular environment which then gradually declined over the course of the experiment (Fig. 4a, c). qPCR analysis also confirmed the upregulation of pks2, Rv1571 and aceE in intracellular H37Rv compared to H37Ra at 168 h of interaction (Fig. 4a).
Expression of all frd operon genes was elevated in bacteria interacting with macrophages versus bacteria grown in enriched broth. This was manifested as a substantial increase in expression at 4 h that declined over the 168 h period of the experiment for both H37Rv and H37Ra (Fig. 4b, c).
The upregulation of pks2 in macrophage-associated M. tuberculosis over that of broth-grown bacteria was maintained in H37Rv from initial interaction throughout the infection (Fig. 4c). This trend was not detected in macrophage-associated H37Ra at 4 h post-infection, but the expression of pks2 was observed to increase over the duration of the infection (Fig. 4b).
aceE displayed higher levels of expression in macrophage-associated bacteria versus broth-grown bacteria only at 168 h post-infection for both H37Ra and H37Rv (Fig. 4b, c); however, levels of aceE transcript were higher in intracellular H37Rv versus intracellular H37Ra at all time points assessed during the infection (Fig. 4a).
Expression of Rv1571 was upregulated in intracellular H37Rv over H37Ra at both 4 and 168 h post-infection, but in broth, expression was 16-fold higher in H37Ra (Fig. 4a). Interestingly, the expression of Rv1571 was greatly enhanced in both H37Rv and H37Ra on exposure to the intracellular environment (Fig. 4b, c).
Lastly, statistical evaluation of gene expression revealed a significant difference between H37Ra and H37Rv for frdC at 96 h post-infection (P=0.0068). However, statistical significance was not noted for the other genes.
Growth of M. tuberculosis under oxygen-limiting conditions
As FRD is an important enzyme in fumarate respiration, an energy-production pathway relied upon by bacteria exposed to anoxic environments, we assessed the growth of H37Ra and H37Rv under oxygen-limiting conditions similar to those described by Wayne (1976). Doubling times of H37Ra and H37Rv grown under these conditions (40.7±1.3 h for H37Ra; 45.9±7.9 h for H37Rv) differed significantly from either strain grown in aerated roller bottle cultures (23.49±2.4 h for H37Ra P=0.0033; 20.3±1.1 h for H37Rv, P=0.0325). However, the growth rates of the oxygen-limited cultures of H37Ra and H37Rv were not significantly different (P=0.297). All doubling times were calculated at day 5 (120 h).
qPCR analysis of frd transcripts in oxygen-limited cultures of M. tuberculosis
Concurrent with growth rate assessments of oxygen-limited cultures, RNA from these and their aerated culture counterparts were also harvested and levels of the frd transcripts were assessed. Transcripts from static cultures used to inoculate the low-oxygen and aerophilic cultures –henceforth referred to as seed cultures – were also assessed to determine if changes after inoculation were pre-existing and strain specific. As seen in the previous experiment (Fig. 4a), seed cultures of M. tuberculosis H37Ra displayed higher levels of the frd transcripts compared to seed cultures of H37Rv (Fig. 5a). Comparable to the results with intracellular bacteria, frdA and frdB were expressed at a higher level in H37Rv shortly after the oxygen constraints were initiated but, thereafter, H37Ra expression of these genes increased to levels higher than in H37Rv (Fig. 5a). Unlike the results obtained for intracellular bacteria, transcription of frdC and frdD was higher in H37Ra over the period of this study (Fig. 5a). Comparing the frd transcripts in oxygen-limited cultures versus their respective aerated culture counterparts, frd genes were expressed rapidly in oxygen-limited cultures of the virulent H37Rv, with a subsequent fall-off in expression levels over time (Fig. 5c). Expression of frdC in H37Ra followed a similar pattern but, in contrast, expression of frdA, B and D was initially low, gradually increasing over the 96 h of the experiment. Using Student's t-test, only the expression of frdA and frdB at 4 h was significantly different between strains (P=0.036, 0.013, respectively) whereas all other transcripts were not significantly different between strains.
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). In our hands, an inhibitory effect of MPNO on the growth of M. tuberculosis H37Ra and H37Rv in 7H9 broth as measured by the reduction of AlamarBlue was observed over all concentrations tested (Fig. 6a). Interestingly, at the lowest concentration tested (0.6 µM) inhibition of H37Ra was greater than that seen for H37Rv (P=0.009).
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). This concentration was used as it was effective against both strains in broth and had previously been shown to be effective against intracellular protozoa while not affecting viability of host cells (Turrens et al., 1999). The growth of H37Ra and H37Rv in untreated infected macrophages was similar to that obtained previously (Fig. 1c), with doubling times significantly different between the two strains at both 96 h (H37Ra, 51.5±1.7; H37Rv, 32.8±0.9; P=0.0007) and 168 h post-infection (H37Ra, 42.1±1.6, H37Rv, 31.3±0.8; P=0.0041). There was a significant reduction of H37Ra and H37Rv c.f.u. in macrophages treated with MPNO with respect to their untreated counterparts at 96 and 168 h post-infection (P<0.0001 for all populations compared: Fig. 6). Calculating death rates as previously described (Jannasch, 1969; Rahn, 1930) revealed a higher rate of killing of intracellular H37Ra compared to H37Rv in MPNO-treated macrophages at both 96 h (H37Ra death rate, 0.025±0.001; H37Rv death rate, 0.02±0.001) and 168 h (H37Ra death rate, 0.013±0.001; H37Rv death rate, 0.009±0.001). However, this did not reach significance levels (96 h, P=0.051; 168 h, P=0.054). |
DISCUSSION |
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Firstly, the primer selected for the generation of DIG-labelled probes was of a defined sequence, which has the potential to limit the differences detected. However, in our experience random primers that would enable the screening of a more complex pool of transcripts did not allow for the generation of probes that could be used in expression analysis. Thus a compromise would be to use a defined primer of similar length, but different sequence, allowing for a different snapshot of the transcriptome. A further possibility would be to use genome-directed primers (Talaat et al., 2000) in separate RT and PCR applications, with pools of these probes enabling a more complex analysis of the differences between strains. For the present study, the arbitrary primers were used to elucidate the ability to generate meaningful BACFAs for M. tuberculosis; subsequent studies will make use of genome-directed primers.
A second consideration that affects the use of BACFAs in expression analyses is that one signal may be the result of several genes. It is this complication that will require resolution by generating alternative BACFAs with different sets of enzymes, which may be time-consuming. Conversely, this limitation is also an advantage of the BACFA method as it will allow for the identification of genes present on the same fragment that may be co-regulated. As in the case of the frd operon isolated in this study, each gene on its own may not have given a signal intense enough to warrant further analysis. However, as three genes of the upregulated operon were present in one fragment, their combined signals led to the isolation of this operon. Ultimately, BACFA analyses can be a viable alternative to, if not a substitute for, other expression analysis techniques, particularly if cost and specialized facilities are of concern.
Using the BACFA technique, both previously reported and novel differences were found. Differences in expression of pks2 (a gene whose product is thought to be involved in sulpholipid synthesis as well as other lipid synthesis roles: Sirakova et al., 2001) have been previously reported in transcriptome comparisons of broth-grown versus macrophage-associated H37Rv (Graham & Clark-Curtiss, 1999). Similarly, we found that pks2 expression was upregulated in intracellular H37Rv and H37Ra versus their broth-grown counterparts. Additionally, we report, apparently for the first time, that pks2 expression is higher in intracellular H37Rv versus H37Ra.
Previously unreported differences in gene expression identified using BACFA and subsequently confirmed via qPCR analyses were those of the genes Rv1571, aceE and frdD. aceE, seen to be expressed more readily in the virulent M. tuberculosis H37Rv throughout the course of infection, encodes pyruvate decarboxylase, otherwise known as the E1 subunit for the pyruvate dehydrogenase complex (PDC). The PDC catalyses the conversion of pyruvate to acetyl-CoA, which feeds into the tricarboxylic acid cycle, one of the main pathways of cellular respiration and biosynthesis in both eukaryotes and prokaryotes (Madigan et al., 1997a). However, this E1 subunit can act independently of the PDC in glycolysis, converting pyruvate into acetaldehyde and a molecule of CO2 (Madigan et al., 1997a). Previous studies have shown that H37Rv displays a higher respiration rate than H37Ra (Heplar et al., 1954) and that H37Ra and H37Rv rely on both glycolytic and oxidative means of glucose metabolism, although H37Rv was seen to rely more heavily on glycolytic pathways (Ramakrishnan et al., 1962). aceE upregulation could help to support both aerobic and glycolytic pathways, acting either in concert with the rest of the PDC in aerobic respiration or independently as pyruvate decarboxylase under low-oxygen growth.
The dramatic increase in Rv1571 expression on transfer of both H37Rv and H37Ra from broth to the intracellular environment is of interest and warrants further investigation. However, there was no consistent pattern of expression differences for this gene between intracellular H37Rv and H37Ra.
An additional difference found with BACFA analysis and confirmed with subsequent qPCR analysis was that of the components of the fumarate reductase (FRD) complex (frdA, B, C and D). This enzyme complex catalyses the conversion of fumarate into succinate, and is composed of four subunits: FRD-A and FRD-B, which constitute the catalytic domain, and FRD-C and FRD-D, which constitute the anchoring domain. Fumarate respiration is an alternative means to acquire energy by utilizing fumarate as the terminal electron acceptor when oxygen or 
is absent. BACFA analysis of 168 h post-infection transcripts revealed an upregulation of frdB, frdC and frdD in H37Ra versus H37Rv. It was previously found in E. coli that FRD can partially compensate for missing succinate dehydrogenase (SDH) activity (Guest, 1981), a critical enzyme of the tricarboxylic acid cycle; however, PCR analysis of the sdh operon in both H37Ra and H37Rv showed no genomic or expression differences between strains (Fig. 3b).
In mycobacterial studies, frdA has been found to be upregulated in M. tuberculosis interacting with the macrophage versus M. tuberculosis grown in broth cultures (Schnappinger et al., 2003) as well as in studies that examined the behaviour of M. tuberculosis grown under carbon starvation (Betts et al., 2002). Microarray studies examining the transcriptome in stationary-phase M. tuberculosis H37Rv cultures found an increase of frdB and frdC transcripts versus bacteria in exponential growth (Raman et al., 2001). Additionally, investigations into the respiratory behaviour of Mycobacterium phlei found that FRD activity increased fourfold when bacteria were grown under low-oxygen conditions (Gillespie et al., 1988). Our studies have taken these initial observations further by including a direct comparison between H37Rv and H37Ra in the intracellular environment as well as in broth culture. The recently published genomic sequence of M. tuberculosis H37Ra allowed the direct comparison of frd gene sequences of the two strains (Wang et al., 2007). However, no differences were noted in either the gene or promoter sequences of the frd operon in H37Ra and H37Rv (Cole et al., 1998; Wang et al., 2007). When we examined the expression of the frd operon over the duration of the infection period at 4, 96 and 168 h post-infection, the trend observed was one where expression was initiated and climaxed earlier in the virulent H37Rv strain versus that in the attenuated H37Ra. These results support a hypothesis where H37Rv is able to respond more quickly to the challenge of the intracellular environment – possibly in response to the oxygen-limited conditions encountered inside the macrophage (Schnappinger et al., 2003), thereby resulting in enhanced replication within the macrophage.
To address the role of FRD under anoxic conditions, we sought to characterize FRD activity and frd transcripts in cultures of M. tuberculosis H37Rv and H37Ra grown as both unaerated static (oxygen-limited) and aerated roller-bottle cultures. The attenuated strain grown under low-oxygen conditions displayed a lag in gene expression with regard to the catalytic domain of FRD (frdA, frdB) but not the anchoring domain (frdC, frdD). The data do not exactly reproduce the trends seen with the intracellular bacteria, probably due to the multiple challenges encountered by the bacterium inside host cells. As such, although H37Ra is observed to transcribe the genes encoding the anchoring domain at even a higher level than H37Rv, the lag in transcription of the catalytic subdomain could limit the function of the complex as a whole.
None of the frd genes were found in the TraSH (transposon site hybridization) screens used to identify genes essential for survival of the bacterium in macrophages and in mice (Rengarajan et al., 2005; Sassetti & Rubin, 2003). However, as the authors state when comparing their results to expression analysis, their findings ‘emphasize the complementary nature of the two experimental approaches’ and that TraSH may fail to pick up ‘mutants that are complemented in trans by either bacterial or host factors’ (Rengarajan et al., 2005). In addition, TraSH would not identify differences such as we found, where differences in the kinetics of frd gene expression correlated with differences between H37Ra and H37Rv in their intracellular growth and virulence. Thus, the absence of the frd genes in the TraSH screens does not minimize the importance of FRD in M. tuberculosis infections. Studies of directed knockout mutants of all frd genes in addition to knockout mutants of individual frd genes have been initiated and are ongoing.
The transcriptomic data from the infection study suggested a role for FRD in the intracellular survival of M. tuberculosis. We therefore investigated the effect of a putative FRD inhibitor, MPNO, on the growth of intracellular M. tuberculosis. The FRD enzyme complex is not found in mammalian cells (Ge, 2002; Phillips et al., 1987; Van Hellemond & Tielens, 1994), so FRD inhibitors are expected to have low to negligible adverse effects upon the viability of mammalian cells (Ge, 2002). With regard to other pathogenic organisms, FRD has been considered a target in the treatment of Helicobacter pylori, as it was found to be an essential gene in the establishment of H. pylori colonization of the mouse stomach (Birkholz et al., 1994; Ge et al., 2000; Wang et al., 2000). Furthermore, FRD has been reported to be a successful target in the treatment of protozoan and helminth infections using a variety of compounds (Bryant & Bennet, 1983; Chen et al., 2001; Omura et al., 2001; Prichard, 1973; Turrens et al., 1996, 1999), although it should be emphasized that the specificity of these compounds for FRD has not been established and the mode of action of at least some of the compounds is unclear.
MPNO is a common antimicrobial and antifungal ingredient in household products. It has been characterized to be an inhibitor of protozoa and is the precursor to L921-021, a drug developed and implemented for the treatment of protozoan infections. However, it should be stressed that although MPNO has been reported to inhibit FRD activity (Turrens et al., 1999), its mode of action is not understood and the specificity of MPNO for FRD has not been demonstrated.
For our purposes of examining MPNO effects on the intracellular growth of M. tuberculosis H37Ra and H37Rv, we used the concentration of the inhibitor previously used to inhibit the growth of the intracellular parasite Trypanasoma cruzi without adverse effects on mammalian cells (Turrens et al., 1999). The significant reduction in intracellular bacteria in MPNO-treated macrophages throughout the study was possibly due to inhibitory effects of the inhibitor on bacterial FRD. However, as the literature provides no real understanding of the mode of action of MPNO, it is not yet possible to determine whether the MPNO was acting on other bacterial systems to produce its mycobactericidal effect. Indeed, the specificity of MPNO and other reported FRD inhibitors has not been demonstrated (Bryant & Bennet, 1983; Chen et al., 2001; Omura et al., 2001; Prichard, 1973; Turrens et al., 1996, 1999). Further work will be necessary before we can be sure that other putative FRD inhibitors have activity against extracellular and intracellular M. tuberculosis and that any observed activity is indeed due solely to inhibition of FRD. Nevertheless, our observations indicate that MPNO and other putative FRD inhibitors warrant further investigation as anti-mycobacterials.
In this study we have outlined an alternative means to study transcriptomes of intracellular bacteria. Using BACFA, we have identified differences between virulent and attenuated strains of M. tuberculosis that may ultimately help to explain why these highly related bacteria have such different phenotypes when interacting with the host. We have also characterized the expression profile of the genes encoding the FRD enzyme complex, which in other organisms provides energy and substrates under suboptimal conditions, and which appears to play a supportive role in the survival of M. tuberculosis in macrophages.
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ACKNOWLEDGEMENTS |
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Edited by: S. V. Gordon
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REFERENCES |
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Received 4 January 2008;
revised 16 April 2008;
accepted 16 April 2008.
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, dashed lines) and H37Rv (
, full lines) in PB+T monitored via OD580 (a) and via c.f.u. ml–1 (b), and inside murine BM-M
3'.
