INTRODUCTION
Chlamydiae are Gram-negative obligate intracellular bacteria that are the causative
agents of several significant human diseases. Chlamydia trachomatis includes over
15 serologically defined variants, or serovars, associated with different diseases
and tissue tropisms. These include the etiologic agents of trachoma, the most common
cause of infectious blindness, while other serovars cause sexually transmitted diseases
(1). C. pneumoniae is a common cause of community-acquired pneumonia (2). C. muridarum
and C. caviae were isolated from mice and guinea pigs, respectively (3, 4), and are
not known to infect humans.
Chlamydiae are characterized by a biphasic life cycle, alternating between infectious
elementary bodies (EBs) and replicative reticulate bodies (RBs) (5). Following endocytosis
by a host cell, chlamydiae reside within a vacuole termed an inclusion, which is nonfusogenic
with the endosomal/lysosomal pathway but acquires sphingomyelin and cholesterol from
the Golgi apparatus (6–8). Once modified by de novo-synthesized chlamydial proteins,
the nascent inclusion is trafficked to the perinuclear region, where it is typically
observed in close apposition with the host cell centrosomes (9–14). This initial trafficking
is dependent upon the host microtubule network and the minus-end microtubule motor,
dynein (14, 15). Recently, we have shown that active forms of the protein tyrosine
kinases Src and Fyn are recruited to the chlamydial inclusion membrane, where they
colocalize in microdomains that are thought to serve as a platform for interactions
between dynein and the chlamydial inclusion (16).
Src and Fyn belong to the Src family of nonreceptor membrane-associated tyrosine kinases.
Src, Yes, and Fyn are ubiquitously expressed, with some functional redundancy, whereas
the other family members are expressed in subsets of specialized cells. Src and Fyn
are involved in a variety of signaling pathways that govern a broad range of cellular
processes (17).
Here we show that Src-family kinases (SFKs) have multiple functions in C. trachomatis
development, including trafficking of nascent inclusions to the microtubule organizing
center (MTOC) and a necessary role in completion of the C. trachomatis intracellular
developmental cycle. The functional implications of these phenotypes are supported
by the observation that chlamydial species that do not recruit Src-family kinases
to their inclusion membranes (16) do not traffic to the MTOC and show enhanced inclusion
development in cells deficient in Src-family kinases.
RESULTS
SFK activation is upregulated in C. trachomatis-infected cells.
Src-family kinase activity is regulated by the phosphorylation state of tyrosine residues
within the kinase. Enzymatic activity is upregulated in kinases that are phosphorylated
at Tyr419 (human Src numbering) and downregulated when phosphorylated at Tyr530 (17).
Active Src-family kinases are recruited to the C. trachomatis inclusion membrane,
where they are observed in microdomains enriched in cholesterol and specific chlamydial
inclusion membrane proteins (16). The intensity of the active Src kinase signal at
these microdomains indicates a significant enrichment at the inclusion membrane (16).
Src-family kinase activity is upregulated in C. trachomatis-infected cells (Fig. 1A).
This increase does not occur in the presence of chloramphenicol and thus is not simply
a response by the host cell to the challenge of chlamydial infection. C. caviae, a
species that does not recruit active kinase to the inclusion membrane, does not induce
activation of Src-family kinases (Fig. 1B).
FIG 1
Src-family kinase activation in C. trachomatis-infected cells. Immunoblot analysis
of cell lysate 24 h after infection with C. trachomatis L2 (A) or C. caviae GPIC (B)
in the absence or presence of chloramphenicol (CAM) shows that active Src-family kinase
is upregulated after C. trachomatis, but not C. caviae, infection. Upregulation is
dependent upon bacterial protein synthesis, as it is inhibited by chloramphenicol.
The concentrations of total kinase (Fyn and Src) are unchanged. GAPDH is included
as a loading control. MW, molecular weight (in thousands).
Src-family kinases are not required for C. trachomatis attachment or entry.
The role of Src-family kinases at various stages of the chlamydial development cycle
was examined. SYF cells are mouse embryonic fibroblast cells that are null mutants
for the three ubiquitous Src-family kinases (Src, Yes, and Fyn). Src was subsequently
reintroduced to create the SYF + Src cell line (18). Mouse L929 fibroblasts were used
as an additional control. Comparison of the numbers of attached and internalized C. trachomatis
L2 EBs indicates that the SYF cells showed no significant defect in bacterial attachment
or invasion compared to L929 and SYF + Src cells (see Fig. S1 in the supplemental
material). This is in agreement with previous reports showing that inhibition of Src-family
kinases does not reduce C. trachomatis L2 invasion (19, 20).
Nascent C. trachomatis inclusions require SFKs for proper trafficking to the MTOC.
Trafficking of C. trachomatis L2 inclusions to the MTOC was examined in L929, SYF,
and SYF + Src cells (Fig. 2). Nascent inclusions rapidly cluster at the MTOC in L929
cells and SYF + Src cells. This is in contrast to the dispersed inclusions observed
in SYF cells or cultures in which bacterial translation was blocked with chloramphenicol
(Fig. S2). Approximately the same percentages of cells with C. trachomatis EBs aggregated
at the MTOC were observed in L929 and SYF + Src cells, whereas in SYF cells this value
was reduced to the minimal levels seen in chloramphenicol-treated cultures. Src-family
kinase activity is thus required for the dynein-dependent trafficking of the nascent
C. trachomatis inclusion to the MTOC.
FIG 2
Trafficking of nascent C. trachomatis inclusions to the MTOC requires Src-family kinases.
(A) L929, SYF, and SYF + Src cells were infected with C. trachomatis L2 in the absence
or presence of chloramphenicol. Five hours postinfection, samples were fixed and stained
for EBs (green) and γ-tubulin (red). Arrowheads indicate early inclusions converged
at the MTOC. Bar, 10 µm. (B) The percentage of cells with inclusions clustered at
the MTOC is significantly decreased in SYF cells compared to that in L929 and SYF
+ Src cells and approximates the percentage observed in chloramphenicol-treated cells.
One hundred infected cells were counted per sample (n = 3); error bars show standard
deviations.
C. trachomatis serovars L2 and D and C. pneumoniae recruit active Src-family kinases
to the inclusion membrane, whereas C. caviae and C. muridarum do not (16). We investigated
whether Src-family kinase recruitment might correlate with trafficking to the MTOC
in a species-specific manner. Examination of infected L929 cells confirms that C. trachomatis
and C. pneumoniae inclusions consistently traffic to the MTOC, whereas C. caviae and
C. muridarum inclusions do not (Fig. 3). These findings support the conclusion that
Src-family kinases play an essential role in the trafficking of chlamydial inclusions
to the MTOC. Examination of C. caviae and C. muridarum inclusions at later times confirms
a lack of trafficking to the MTOC (Fig. S3).
FIG 3
Trafficking of nascent chlamydial inclusions to the MTOC is species specific. (A)
L929 cells were infected with C. trachomatis L2, C. caviae GPIC, C. muridarum MoPn,
or C. pneumoniae AR39. Five hours postinfection, the cells were fixed and labeled
with anti-EB antisera (green) and anti-γ-tubulin antibodies (red) followed by DyLight
488 and 594 secondary antibodies as well as DRAQ5 DNA stain (blue). Arrowheads indicate
early inclusions converged at the MTOC. Bar, 10 µm. (B) The percentage of infected
cells with EBs clustered at the MTOC was determined using confocal fluorescent microscopy.
C. trachomatis L2 and C. pneumoniae consistently traffic to the centrosome, whereas
C. caviae and C. muridarum do not. One hundred cells were counted per sample (n =
3); error bars show standard deviations. Cpn, C. pneumoniae.
Differential effects of SFK deficiency on chlamydial development.
Although attachment and entry are unaffected, development of C. trachomatis inclusions
is significantly reduced in SYF cells compared to development in L929 or SYF + Src
cells (Fig. S4). Consistent with the observed inhibition of inclusion development,
the number of infectious progeny is decreased in SYF cells to less than 10% of that
obtained from L929 cells (Fig. 4). Reconstitution of Src largely restores infectious
progeny production, although a modest decrease is seen from the SYF + Src cells relative
to L929 cells, suggesting that Fyn or Yes may play specific roles during chlamydial
development for which Src does not provide functional redundancy.
FIG 4
Chlamydial dependency on Src-family kinases for productive infection is species specific.
Confluent monolayers of L929, SYF, and SYF + Src cells were infected with C. trachomatis
L2, C. caviae GPIC, or C. muridarum MoPn. Cultures were lysed at 30 h postinfection,
and the progeny EBs were replated onto confluent monolayers of HeLa cells. At 24 h
postinfection, the number of inclusions per field was determined (10 fields per sample;
n = 3) using fluorescent microscopy. Values reported are relative to those obtained
from L929 cells (L2, 4.7 × 107 IFU/ml; GPIC, 5.3 × 107 IFU/ml; MoPn, 7.4 × 107 IFU/ml).
Error bars show standard deviations.
Neither C. caviae nor C. muridarum displays a decrease in the number of developed
inclusions or in infectious progeny produced from SYF cells. Indeed, C. caviae and
C. muridarum inclusions in SYF cells appear significantly larger than those in L929
or SYF + Src cells (Fig. 5). There was a corresponding increase in the number of infectious
progeny produced in SYF cells, with approximately an 800% and 20% increase over those
produced in L929 cells for C. caviae and C. muridarum, respectively (Fig. 4).
FIG 5
Enhanced development of non-C. trachomatis inclusions in SYF cells. L929, SYF, and
SYF + Src cells were infected with C. caviae GPIC or C. muridarum MoPn. At 24 h postinfection,
samples were visualized using fluorescent confocal microscopy (bar, 10 µm) (A) or
transmission electron microscopy (bar, 5 µm) (B). Inclusions in SYF cells are much
larger than corresponding inclusions in L929 or SYF + Src cells.
Because depletion of Src-family kinases blocks both trafficking of the nascent C. trachomatis
inclusions to the MTOC and normal inclusion development, we asked whether inhibition
of trafficking to the MTOC might be responsible for the lack of inclusion development
in SYF cells. As has been shown previously (14, 21), nocodazole inhibits trafficking
of C. trachomatis inclusions to the MTOC but does not impair subsequent inclusion
development or production of infectious progeny (Fig. S5).
Src-family kinases are not required for Inc microdomain formation.
Although few C. trachomatis inclusions develop in SYF cells, those that do appear
to develop normally (Fig. S6). In those inclusions, Inc microdomains are observed
on the inclusion membrane. This is in agreement with previous observations that Src-family
kinases are not required for Inc microdomain formation (16). Active kinase colocalizes
with Inc microdomains in L929 cells, and the signal intensity is enhanced in cells
overexpressing Src (SYF + Src), but the signal is not detected in cells lacking Src-family
kinases. However, tyrosine-phosphorylated proteins are observed in microdomains at
the inclusion membrane in all three cell lines, although they appear more abundant
in the SYF + Src cells and less abundant in the SYF cells. Active Src and Fyn are
themselves tyrosine phosphorylated, which may account for the reduced phosphotyrosine
signal in SYF cells and the increased signal observed in SYF + Src cells. However,
it is likely that there are tyrosine-phosphorylated proteins other than Src-family
kinases present in the microdomains.
Src-family kinase inhibitors mimic the inhibitory effects observed in SYF cells.
Given the phenotypes observed in SYF cells, we investigated the ability of Src-family
kinase inhibitors to recapitulate these phenotypes. Dynein-dependent trafficking of
C. trachomatis early inclusions to the MTOC was inhibited by the protein tyrosine
kinase inhibitor SU6656 (Fig. S7). The effect of SU6656 on C. trachomatis L2 and C. caviae
progeny production was also examined (Fig. 6A). Consistent with the effects observed
in SYF cells, the production of C. trachomatis progeny EBs was reduced by over 95%
by inhibition of Src-family kinases. Conversely, production of infectious progeny
from C. caviae-infected cells was unaffected by Src-family kinase inhibitors. The
decreasing viability of the host cells in the extended presence of inhibitor prevented
accurate quantitation of inclusion-forming units (IFU) at later time points where
the largest increase in IFU from SYF cells was observed.
FIG 6
Src-family kinase inhibitors inhibit production of C. trachomatis infectious progeny.
Confluent monolayers of L929 (A) or HeLa (B) cells were infected with C. trachomatis
L2 or C. caviae GPIC. One hour postinfection, cells were treated with DMSO (gray bars)
or SU6656 (black bars). Cells were lysed, and progeny was replated onto HeLa cell
monolayers. Cells were fixed and fluorescently labeled, and the number of inclusions
per field was counted using fluorescent microscopy. Values reported are relative to
those obtained from mock-treated (DMSO) controls (HeLa cells: L2, 1.1 × 107 IFU/ml;
GPIC, 2.3 × 107 IFU/ml; L929 cells: L2, 1.0 × 108 IFU/ml; GPIC, 6.3 × 106 IFU/ml).
Ten fields per sample (n = 3). Error bars show standard deviations.
To further delineate the requirements for Src-family kinases during C. trachomatis
intracellular trafficking and development, we allowed nascent inclusions to traffic
to the MTOC before inhibiting Src-family kinase activity with SU6656. SU6656 reduced
progeny EB formation to 1.9% ± 0.6% and 3.1% ± 0.6% of untreated control values when
added at 0 and 5 h postinfection, respectively. Complementary to the results above
demonstrating that trafficking to the MTOC is not necessary for C. trachomatis development,
these results indicate that trafficking to the MTOC is not sufficient to promote normal
development in the absence of Src-family kinase activity. The growth inhibition of
C. trachomatis by Src-family kinase inhibition thus appears unrelated to effects on
microtubule trafficking.
Because C. trachomatis and C. caviae have different natural hosts, we examined the
possibility that these species-specific chlamydial phenotypes might be related to
the murine origin of the Src-family kinase-knockout cells. Similar inhibition of C. trachomatis
by SU6566 was observed in the human cells, while C. caviae infectious progeny production
was not disrupted (Fig. 6B). This result indicates that the differential effects of
Src-family kinases on C. trachomatis and C. caviae replication are independent of
the host cell species.
DISCUSSION
Tyrosine phosphorylation of proteins is an important posttranslational regulator of
activity. Src-family kinases comprise a prominent family of protein tyrosine kinases
that play a central role in multiple signaling pathways that regulate a diverse array
of cellular activities, including adhesion, migration, differentiation, cell cycle
progression, apoptosis, transcription, and intracellular trafficking (17). The intimate
relationship of intracellular pathogens with eukaryotic host cells necessitates that
they adapt to and manipulate host cellular processes. Accordingly, intracellular pathogens
subvert cellular signaling pathways at several stages of infection to promote entry,
replication, or release during their life cycles. Src-family kinases have been shown
to play important roles in the pathogenesis of a wide range of intracellular pathogens,
including viruses, parasites, and bacteria (22–31).
C. trachomatis interacts with Src-family kinases at multiple stages during its intracellular
development. The C. trachomatis type III effector Tarp, which is secreted upon attachment
and implicated in the entry process due to its actin-nucleating activity, is tyrosine
phosphorylated by Src-family or other tyrosine kinases upon translocation into the
host cytosol (19, 20, 32, 33). Although the role(s) of Tarp remains to be fully elucidated,
the functions associated with it are independent of chlamydial transcription or translation
(34) and thus can be readily distinguished from events that are dependent upon chlamydial
protein synthesis, such as trafficking to the MTOC (14), fusion with Golgi apparatus-derived
vesicles (13), differentiation of chlamydial developmental forms (35), and activation
of Src-family kinases.
C. trachomatis and C. pneumoniae also appear to have evolved a dependency on Src-family
kinases for intracellular trafficking and development. Recently, microdomains on the
C. trachomatis inclusion membrane were identified and shown to be comprised of at
least four inclusion membrane proteins as well as activated Src-family kinases. These
microdomains are thought to participate in the interactions of the chlamydial inclusion
with the microtubule motor complex and function in the trafficking of the nascent
inclusion to the MTOC (16). In SYF cells or cells in which Src-family kinases are
inhibited, nascent C. trachomatis L2 inclusions are not trafficked to the MTOC and
show a reduction in inclusion development with a corresponding decrease in infectious
progeny. The two defects induced by a Src-family kinase deficiency appear to be unrelated,
since the microtubule inhibitor nocodazole effectively blocks trafficking of nascent
C. trachomatis inclusions to the MTOC without negatively affecting inclusion development
or production of infectious progeny. Nascent inclusion trafficking is dependent upon
microtubules and dynein (14). Src-family kinases have been shown to phosphorylate
tubulin and bind to dynein-associated proteins (36–38), suggesting that recruitment
of Src-family kinases by chlamydiae may directly or indirectly mediate linkage of
the inclusion to the microtubule network of the host cell. Those species examined
that do not display active Src-family kinase-enriched microdomains on the inclusion
membrane, C. muridarum and C. caviae, do not exhibit microtubule-dependent trafficking
to or association with the MTOC. Whether Src-family kinase activity is required for
processivity of the dynein motor complex or whether tyrosine phosphorylation contributes
to the recruitment of necessary components remains to be determined. An improved understanding
of C. trachomatis trafficking may help to delineate the role of Src-family kinases
in dynein motor function and microtubule trafficking.
Src-family kinases also appear to serve a separate function later in infection that
is essential for normal C. trachomatis inclusion development. The observation that
those few EBs which differentiate and commit to development do so normally suggests
that this critical initial step of differentiation may be a stochastic process that
does not favor C. trachomatis development in the absence of Src-family kinases. This
developmental defect is not observed in SYF cells infected with C. caviae or C. muridarum.
Indeed, C. caviae and C. muridarum replicate more efficiently in SYF cells. Although
the mechanism of growth enhancement of C. caviae or C. muridarum in cells deficient
in Src-family kinases is unknown, one hypothesis might be that Src-family kinases
play a role in a pathway that restricts growth of these pathogens and that the absence
of this pathway in SYF cells permits unregulated, enhanced growth.
Chlamydial species are well known for their distinct tissue tropisms and species specificity,
despite a high degree of synteny of their genomes (39). Few loci have been associated
with disease or tissue tropism (40–44). Divergence occurs primarily within a hypervariable
replication termination region or plasticity zone (45). A key distinction between
human and murine chlamydial species involves their adaptations to gamma interferon
(IFN-γ) (46–49). In murine cell lines and in vivo, C. muridarum is resistant to the
effects of IFN-γ, whereas C. trachomatis is inhibited. IFN-γ-mediated inhibition of
C. trachomatis in murine cells has been attributed to the presence of p47 or immunity-related
GTPases (IRGs), which are present in mice but largely absent from human cells (47,
49). Resistance of C. muridarum to IFN-γ is thought to be related to the inactivation
of specific IRGs by the cytotoxin that is absent or truncated in C. trachomatis (47).
However, treatment of a human cell line with a Src-family kinase inhibitor led to
a decrease in C. trachomatis replication similar to that observed in murine cells;
thus, the mechanisms of the Src-family kinase appear to be acting though pathways
other than mouse-specific IRGs. IFN-γ treatment was not investigated here; however,
the plasticity zone remains a highly plausible locus for chlamydial genes responsible
for the divergent responses to Src-family kinase depletion.
Early studies of tyrosine phosphorylation during chlamydial infection described two
or more proteins that were tyrosine phosphorylated very early in infection and located
in close apposition to the invading EBs (50, 51). Although the proteins were not identified,
they were presumed to be of host origin. To date, ezrin has been the only host protein
identified that is tyrosine phosphorylated during chlamydial invasion (52). A more
complete accounting of host and chlamydial proteins that are tyrosine phosphorylated
throughout the developmental cycle will undoubtedly refine future studies designed
to understand the mechanisms of growth inhibition of C. trachomatis and growth enhancement
of C. caviae and C. muridarum by depletion of Src-family kinases.
Protein tyrosine kinases are activated and functional at multiple stages of C. trachomatis
intracellular development. By all parameters analyzed, C. caviae and C. muridarum
showed no activation of, recruitment of, or requirement for Src-family kinases and
thus have apparently evolved life-styles that circumvent the need for Src-family kinases.
Src-family kinases therefore play an important, but species-specific, role in the
chlamydial developmental cycle. It is apparent that different combinations of requirements
occur between species. For example, C. pneumoniae Tarp is not phosphorylated (32),
and yet its inclusions contain tyrosine-phosphorylated microdomains (16) and are trafficked
to the MTOC. It is likely that additional functions regulated by Src-family kinases
will be identified in chlamydia-infected cells. A more complete understanding of host
and chlamydial tyrosine-phosphorylated proteins and the pathways associated with them
should help elucidate significant questions in chlamydial biology and pathogenesis.
MATERIALS AND METHODS
Organisms and cell culture.
Chlamydia trachomatis serovar L2 (LGV 434) and serovar D (UW-3-Cx), Chlamydia muridarum
Nigg, Chlamydophila caviae GPIC, and Chlamydia pneumoniae AR-39 were propagated in
HeLa 229 cells and purified by Renografin density gradient centrifugation as previously
described (53). L929, SYF, and SYF + Src cells (American Type Culture Collection [ATCC],
Manassas, VA) were grown in Dulbecco’s modified Eagle’s medium (DMEM; ATCC) with 10%
fetal bovine serum (FBS; ATCC) and gentamicin (Gibco, Carlsbad, CA). The SYF cells
used were between passage numbers 22 and 30.
Antibodies.
The antichlamydia antibodies used were polyclonal rabbit anti-C. trachomatis L2 EB,
anti-C. pneumoniae EB, and anti-C. caviae EB. Monoclonal antibody L2-i-45 against
C. trachomatis L2 major outer membrane protein (MOMP) was kindly provided by H. D.
Caldwell. Rabbit anti-Inc101 was previously described (16). Anti-phospho-Src-family
Tyr416 clone 9A6 (Millipore, Billerica, MA) was used to detect active Src-family kinases.
Monoclonal antibody 4G10 (Millipore) was used to detect phosphotyrosine. Centrosomes
were detected using mouse anti-γ-tubulin, clone GTU-88 (Sigma, St. Louis, MO). Anti-mouse
or anti-goat DyLight 594 and anti-rabbit DyLight 488 (Jackson ImmunoResearch Laboratories,
West Grove, PA) were used as secondary antibodies for indirect immunofluorescence.
Anti-Src (Cell Signaling, Danvers, MA), anti-Fyn (BD Biosciences, San Diego, CA),
anti-glyceraldehyde-3-phosphate dehydrogenase (anti-GAPDH; Sigma), and horseradish
peroxidase (HRP)-conjugated donkey anti-rabbit IgG and HRP-conjugated donkey anti-mouse
IgG (Jackson ImmunoResearch Laboratories) were used for immunoblotting.
Western blotting.
HeLa cells were mock infected or infected with C. trachomatis or C. caviae at a multiplicity
of infection (MOI) of 1 or 20. At 24 h postinfection, the cells were incubated with
1 mM sodium vanadate for 30 minutes; rinsed with 50 mM NaPO4, 150 mM NaCl, pH 7.4
(phosphate-buffered saline [PBS]); and lysed in Laemmli buffer (54) with 5% beta-mercaptoethanol
and 1 mM sodium vanadate. The lysates were separated by SDS-PAGE for immunoblotting
as previously described (16).
Attachment and invasion assays.
Monolayers of L929, SYF, and SYF + Src cells on glass coverslips in 24-well plates
were chilled to 4°C on ice for 30 minutes prior to addition of C. trachomatis L2 at
an MOI of ~20 followed by a 30-minute incubation at 4°C. For attachment assays, cells
were rinsed once in cold Hanks’ balanced salt solution (HBSS) and fixed with methanol
before immunostaining. Images of attached chlamydiae were obtained using a Zeiss LSM
510 Meta laser confocal scanning microscope using a 20×, 0.8-numerical-aperture (NA)
objective (Carl Zeiss MicroImaging, Inc., Maple Grove, MN), and the number of attached
bacteria was determined for 10 fields per sample. For invasion assays, cells were
inoculated as described above, prewarmed medium was added, and cultures were incubated
at 37°C for 30 minutes prior to fixation with 4% paraformaldehyde (Electron Microscopy
Sciences, Hatfield, PA) in PBS. Intracellular and extracellular bacteria were differentially
stained by indirect immunofluorescence, and the percentage of internalized bacteria
was calculated by subtracting the number of extracellular bacteria from the total
number of bacteria and dividing by the total number of bacteria as previously described
(55). A minimum of 10 cells and 100 bacteria were counted for each sample.
Inclusion development and infectious progeny (IFU) assays.
Confluent monolayers of L929, SYF, and SYF + Src cells on glass coverslips or Costar
Cellbind (Corning, Lowell, MA) 24-well plates were infected with C. trachomatis serovar
L2 or D, C. muridarum, or C. caviae as previously described (16) at an MOI of ~0.5.
At the indicated times postinfection, the cultures were fixed in methanol or rinsed
once and lysed in water and disrupted by passage through a 26-gauge needle, before
dilution in HBSS and replating on HeLa cell monolayers for determination of inclusion-forming
units (IFU). Where indicated, nocodazole (Sigma) was added to 400 ng/ml at the time
of infection. For Src inhibitor experiments, cells were treated with dimethyl sulfoxide
(DMSO; Sigma) or 10 µM SU6656 (Sigma) 1 h after the initial infection and maintained
in the continuous presence of the drug.
Centrosomal clustering assays.
Subconfluent monolayers of L929, SYF, or SYF + Src cells were infected with C. trachomatis
serovar L2 or D, C. pneumoniae, C. muridarum, or C. caviae at an MOI of ~20. At 5 h
postinfection, cells were rinsed and fixed with methanol prior to labeling with rabbit
anti-EB and mouse anti-γ-tubulin (Sigma). The specimens were counterstained with goat
anti-rabbit IgG-DyLight 488 and goat anti-mouse IgG-DyLight 594. The number of infected
cells that had a minimum of two EBs adjacent to at least one host centrosome was determined
by fluorescent microscopy using a Zeiss LSM 510 Meta laser confocal scanning microscope
with a 63×, 1.4-NA oil objective. A minimum of 100 infected cells were counted for
each sample, and experiments were performed in triplicate. Cells were untreated or
treated with DMSO, nocodazole (400 ng/ml), or chloramphenicol (50 µg/ml) concurrently
with infection. For the Src inhibitor experiments, L929 cells were pretreated with
DMSO or 10 µM SU6656 for 1 h prior to infection and maintained in the presence of
inhibitor.
Electron microscopy.
L929, SYF, and SYF + Src monolayers on Thermanox coverslips were infected with C. caviae.
At 24 h postinfection, cells were fixed with 2.5% glutaraldehyde in 0.1 M sodium cacodylate
buffer and processed for transmission electron microscopy as previously described
(16).
SUPPLEMENTAL MATERIAL
FIG S1 Src-family kinases are not required for C. trachomatis attachment or entry.
Monolayers of L929, SYF, and SYF + Src cells were incubated with C. trachomatis L2
EBs. For the attachment assays (gray bars), cells were rinsed once and attached EBs
were stained for immunofluorescence and counted. Values presented are relative to
that observed on L929 cells (6.2 × 103 EBs/mm2). For the invasion assay, extracellular
and intracellular EBs were differentially labeled before and after permeabilization.
The percentage of internalized EBs (black bars) was calculated [(total EBs − extracellular
EBs)/total EBs]. C. trachomatis L2 attaches to and invades all three cell lines equivalently.
Ten fields per sample (n = 2). Error bars show standard deviations. Download
FIG S2 Trafficking of nascent C. trachomatis inclusions to the MTOC requires Src-family
kinases. (A) L929, SYF, and SYF + Src cells were infected with C. trachomatis L2 in
the presence of chloramphenicol. Five hours postinfection, samples were fixed and
stained with anti-EB (green) and anti-γ-tubulin (red) followed by DyLight 488 and
594 secondary antibodies and examined by confocal fluorescent microscopy. In the presence
of chloramphenicol to inhibit chlamydial protein synthesis, nascent inclusions remain
dispersed throughout the cell. Bar, 10 µm. Download
FIG S3 Inclusion trafficking to the MTOC is unchanged later in infection. L929 cells
were infected with C. trachomatis L2, C. caviae GPIC, or C. muridarum MoPn. Twelve hours
postinfection, cells were fixed, stained with anti-EB and anti-γ-tubulin, and visualized
by confocal microscopy. Images were captured, and the distance from the nearest edge
of the inclusion to the closest centrosome was measured. The histogram represents
the percentages of inclusions at a given distance from the MTOC. Similar to what is
observed at 5 hours postinfection, a majority of C. trachomatis inclusions are in
close proximity to the MTOC at 12 hours postinfection, whereas C. caviae and C. muridarum
inclusions are significantly further from the MTOC. At least 30 inclusions were counted
per sample. Download
FIG S4 Chlamydial dependency on Src-family kinases for productive infection is species
specific. Confluent monolayers of L929, SYF, and SYF + Src cells were infected with
C. trachomatis L2, C. caviae GPIC, or C. muridarum MoPn. Cultures were fixed 24 h
postinfection, and the number of inclusions per field was determined using fluorescent
microscopy (10 fields per sample; n ≥ 3). Values reported are relative to those obtained
in L929 cells (L2, 6.7 × 102 IFU/mm2; GPIC, 2.7 × 102 IFU/mm2; MoPn, 1.1 × 103 IFU/mm2).
Error bars show standard deviations. Download
FIG S5 Nascent inclusion trafficking can be inhibited without affecting inclusion
development. L929 cells were treated with DMSO as a carrier control or 400 ng/ml nocodazole
at the time of infection with C. trachomatis L2. (A) Cells were fixed 5 h postinfection
and labeled with an anti-EB antiserum (green) and anti-γ-tubulin (red), followed by
fluorescent secondary antibodies and DRAQ5 as a DNA stain (blue). Nocodazole inhibits
trafficking of nascent inclusions to the MTOC. Bar, 10 µm. (B) For inclusion development
assays (gray bars), cells were treated with DMSO or nocodazole and fixed 24 h after
infection. For IFU assays to quantify progeny EBs (black bars), infected DMSO- or
nocodazole-treated cells were lysed and replated onto HeLa cell monolayers. The number
of inclusions per field was determined using fluorescent microscopy. Inclusion numbers
were normalized to DMSO-treated samples (inclusion development, 8.8 × 102/mm2; infectious
progeny, 7.3 × 107 ml). Ten fields per sample (n = 3). Error bars show standard deviations.
(C) Confocal microscopy images of representative inclusions grown in the absence (DMSO)
or presence of nocodazole (Noc). Bar, 10 µm. Download
FIG S6 Characterization of inclusion microdomains in SYF cells. L929, SYF, and SYF
+ Src cells were infected with C. trachomatis L2. At 24 h postinfection, cells were
fixed and labeled with anti-Inc101 (green) and either anti-active Src-family kinase
(A) or antiphosphotyrosine (red) and DRAQ5 DNA stain (blue) (B). The rare inclusions
that develop in SYF cells were examined. Inc microdomains appear to form normally
in SYF cells in the absence of active kinase. Active kinase is not detected in Inc
microdomains in SYF cells but is readily observed colocalizing with Inc microdomains
in L929 cells, and the relative intensity is increased in SYF + Src cells. Antiphosphotyrosine
(pTyr) staining colocalizes with Inc microdomains in all three cell lines, suggesting
that phosphotyrosine proteins, other than active SFKs, are present in the inclusion
membrane microdomains. Exposures were fixed to permit comparison of relative intensities.
C. trachomatis inclusions are denoted by “I.” Bar, 10 µm. Download
FIG S7 Src-family kinase inhibitors inhibit microtubule-dependent trafficking of
C. trachomatis. Monolayers of L929 cells were infected with C. trachomatis L2 in the
presence or absence of DMSO or 10 µM SU6656. (A) Cells were fixed 5 h postinfection
and labeled with an anti-EB antiserum (green) and anti-γ-tubulin (red), followed by
fluorescent secondary antibodies and DRAQ5 as a DNA stain (blue). The arrowhead indicates
early inclusions converged at the MTOC. Bar, 10 µm. (B) The percentage of infected
cells with EBs clustered at the MTOC was determined using confocal fluorescent microscopy.
C. trachomatis L2 consistently traffics to the centrosome, whereas C. caviae and C. muridarum
do not. One hundred cells were counted per sample (n = 3). Error bars show standard
deviations. Download