Download Pseudomonas aeruginosa ExoT inhibits in vitro lung epithelial

Cellular Microbiology (2001) 3(4), 223±236
Pseudomonas aeruginosa ExoT inhibits in vitro lung
epithelial wound repair
Thomas K. Geiser,1²³ Barbara I. Kazmierczak,2³
Lynne K. Garrity-Ryan,2,3 Michael A. Matthay1 and
Joanne N. Engel1,2,3*
1
Cardiovascular Research Institute, 2Departments of
Medicine, and 3Microbiology and Immunology,
University of California, San Francisco, CA 94143, USA.
Summary
The nosocomial pathogen Pseudomonas aeruginosa
causes clinical infection in the setting of pre-existing
epithelial tissue damage, an association that is
mirrored by the increased ability of P. aeruginosa to
bind, invade and damage injured epithelial cells in
vitro. In this study, we report that P. aeruginosa
inhibits the process of epithelial wound repair in vitro
through the type III-secreted bacterial protein ExoT,
a GTPase-activating protein (GAP) for Rho family
GTPases. This inhibition primarily targets cells at the
edge of the wound, and causes actin cytoskeleton
collapse, cell rounding and cell detachment. ExoTdependent inhibition of wound repair is mediated
through the GAP activity of this bacterial protein, as
mutations in ExoT that alter the conserved arginine
(R149) within the GAP domain abolish the ability of
P. aeruginosa to inhibit wound closure. Because
ExoT can also inhibit P. aeruginosa internalization
by phagocytes and epithelial cells, this protein may
contribute to the in vivo virulence of P. aeruginosa
by allowing organisms both to overcome local host
defences, such as an intact epithelial barrier, and to
evade phagocytosis by immune effector cells.
Introduction
Pseudomonas aeruginosa is an opportunistic Gramnegative pathogen responsible for a large number of
nosocomial infections. This organism commonly causes
disease in the setting of pre-existing epithelial tissue
damage, e.g. superinfection of burns, corneal ulcer
formation at the sites of contact lens associated trauma,
Received 15 August, 2000; revised 13 October, 2000; accepted 16
October, 2000. *For correspondence. E-mail [email protected].
²
edu; Tel. (11) 415 476 7355; Fax (11) 415 476 9364. Present
address: Division of Pulmonary Medicine, University Hospital±
³
Inselspital, CH-3010 Bern, Switzerland. These authors contributed
equally to this work.
Q 2001 Blackwell Science Ltd
cystitis in catheterized patients, and tracheobronchitis and
pneumonia in mechanically ventilated patients (Salyers
and Whitt, 1994). A number of in vitro and ex vivo studies
support the notion that injured epithelium is a preferred
target for P. aeruginosa infection (Yamaguchi and
Yamada, 1991; Zahm et al., 1991; Tsang et al., 1994;
de Bentzmann et al., 1996a), an effect that has been
ascribed to increased levels of a putative pilin receptor,
asialoGM1 (de Bentzmann et al., 1996a, b, c) or to the
upregulation of fibronectin and the integrin a5b1 during
wound repair (Roger et al., 1999). Dedifferentiation and
loss of polarity in repairing cells may also promote P.
aeruginosa adhesion and internalization, as both adhesion and internalization increase in epithelial cells whose
polarity has been pharmacologically disrupted (Fleiszig
et al., 1997; 1998a; Plotkowski et al., 1999).
Despite extensive study of the interactions between P.
aeruginosa and injured epithelium, the effect of bacterial
infection on epithelial wound healing has received little
attention. P. aeruginosa possesses many virulence
factors that might cause epithelial injury or impair
epithelial wound closure. These include: secreted
enzymes, such as the proteases LasA and LasB, which
exhibit elastolytic activity; the diphtheria toxin-like Exotoxin A; several phospholipases; and an alkaline protease
(Salyers and Whitt, 1994). P. aeruginosa also expresses
a type III secretion system capable of translocating
bacterially synthesized effector proteins directly into
eukaryotic target cells. The effector proteins that are
substrates of this system include a potent cytotoxin, ExoU
(Finck-Barbancon et al., 1997; Hauser et al., 1998); an
adenylate cyclase, ExoY (Yahr et al., 1998); and two
related proteins, ExoS and ExoT, which are GTPaseactivating proteins (GAPs) with in vitro specificity for the
Rho-family GTPases Rac, Rho and Cdc42 (Goehring
et al., 1999; Krall et al., 2000). The last two bacterial
proteins might have a significant impact on wound
closure, as Rho and Rac activity are required for cell
migration during wound repair (Santos et al., 1997; Nobes
and Hall, 1999).
In this study, we test the hypothesis that P. aeruginosa
can inhibit the process of wound repair and thus increase
its ability to colonize and damage injured epithelium.
P. aeruginosa strain PA103, a clinical lung isolate,
reverses epithelial wound closure and causes cell loss
by disrupting the actin cytoskeleton through a process
requiring the GAP activity of the type III-secreted effector
224 T. K. Geiser et al.
ExoT. Activation of Rho-family GTPases with Cytotoxic
Necrotizing Factor-1 (CNF-1) prevents cell detachment,
as does mutation and inactivation of the GAP domain
within ExoT. These results help to explain why P.
aeruginosa is such a virulent pathogen in the setting of
epithelial cell injury.
Results
PA103 infection of mechanically wounded A549
monolayers inhibits epithelial repair
To test whether P. aeruginosa can alter the process of
epithelial wound repair, mechanically wounded monolayers of lung epithelial A549 cells were used; this is a
well-characterized system in which wound closure results
predominantly from cell migration and spreading, rather
than cell proliferation (Kheradmand et al., 1994; Geiser
et al., 2000). Four hours after mechanical wounding,
wound area reproducibly decreased by 30±40%, as
seen in Fig. 1 (see also Figs 3±5). The addition of P.
aeruginosa strain PA103 (MOI of 3) (a human lung isolate
that elaborates the type III-secreted proteins ExoU and
ExoT but lacks the quorum-sensing LasI/LasR system
and fails to produce elastase), induced a rapid increase in
Fig. 1. Infection of wounded A549 monolayers with PA103 strains
inhibits epithelial wound repair in vitro. A549 cells grown as
confluent monolayers were mechanically wounded 4 h before
infection with PA103 (MOI 3) or the isogenic mutants PA103DU
(MOI 3.6) and PA103pscJ::Tn5 (MOI 29). Wound area was
measured at 2 4, 0, 2, 4, 6, 12 and 24 h post wounding as
described in Experimental procedures. Wound measurements were
made on triplicate wells and the area of the wound is expressed as
mean per cent of the initial wound area ^ SD.
the area of the wound (Fig. 1). The bacterially infected
cells appeared shrunken with pyknotic nuclei, and cell
loss was observed throughout the monolayer (Fig. 2C),
consistent with the known cytotoxic effect of this strain on
epithelial cells (Apodaca et al., 1995; Fleiszig et al.,
1998b). Because of the widespread cell damage and cell
loss throughout the monolayer, the original wound could
no longer be clearly demarcated from other areas of cell
loss in the monolayer at $12 h; therefore, no further
measurements were taken.
In contrast, cells infected with the isogenic strain
PA103pscJ::Tn5, which cannot secrete type III effectors
(for PA103, ExoU and ExoT) because of a transposon
insertion in the type III secretion (psc) operon, showed
continued wound closure after infection, resulting in a 60±
70% decrease in wound size after 24 h (Fig. 1). The cell
morphology at the edge of the wound was indistinguishable from uninfected control cells (compare Fig. 2B
and A); however, the uninfected control did achieve a
greater degree of wound closure (90±95%) than the
PA103pscJ::Tn5-infected samples. The difference in
wound closure between uninfected monolayers and
PA103pscJ::Tn5-infected monolayers was consistently
observed at time points beyond 6 h post infection.
Infection of wounded cells with PA103DU, a strain
carrying a large in-frame deletion within the gene
encoding the putative cytotoxin ExoU but otherwise intact
for type III secretion (Garrity-Ryan et al., 2000), had a
particularly interesting effect on epithelial wound repair.
Four hours after infection with PA103DU, the A549 cells at
the edge of the wound showed cytoplasmic retraction and
cell rounding; similar changes were not seen at this time
within portions of the monolayer at a distance from the
wound edge (Fig. 2D). Concomitantly, the area of the
wound increased to $100%, as can be seen in Figs 1, 4
and 5.
We asked whether the phenotypes of the mutants
varied with the number of bacteria applied to the injured
epithelium. However, infection at MOIs up to 30 resulted
only in a more rapid appearance of the phenotypes
associated with each strain. Neither the morphological
changes observed in infected cells nor the final percentage of wound closure measured at 12 or 24 h were
altered (data not shown).
Inhibition of wound healing requires live bacteria and
active protein synthesis
The data presented so far suggest that a type III-secreted
protein may be involved in reversal of wound closure;
however, this phenotype may be caused by another
bacterial product whose synthesis, localization or secretion is co-regulated with the type III secretion system. We
therefore explicitly tested whether inhibition of wound
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P. aeruginosa ExoT inhibits epithelial wound repair
225
Fig. 2. Cells at the edge of the wound show rounding and retraction when infected by PA103DU. A549 cells were grown, mechanically
wounded and infected as described for Fig. 1. At 4 h post infection, the cells were imaged using a Spot CCD camera coupled to a Nikon
Eclipse TE200 inverted microscope (as described in Experimental procedures).
A. Uninfected cells.
B. PA103pscJ::Tn5-infected cells.
C. PA103 infected cells.
D. PA103DU-infected cells.
healing by P. aeruginosa requires the presence of live
bacteria or active bacterial protein synthesis. PA103 or
PA103DU was incubated for 20 min in a 658C water bath;
this treatment reduced bacterial viability by .105-fold.
When the heat-killed bacteria were applied to wounded
A549 cells at an MOI of 5±10, no inhibition of wound
closure was observed (Fig. 3A). Microscopic examination
of the monolayers did not demonstrate cell rounding, cell
shrinkage or pyknotic nuclei (data not shown).
To assess the requirement for active protein synthesis,
bacterial cultures were pretreated with chloramphenicol
(Cm; 50 mg ml21) for 45 min and then applied to the
wounded epithelial monolayer in the continued presence
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of drug. While Cm itself had no effect on wound healing,
Cm-treated PA103 and PA103DU failed to inhibit wound
healing (Fig. 3B) or to cause changes in cell morphology
(data not shown).
The wound closure curves of both heat-killed bacteria
and Cm-treated bacteria were superimposable upon
those obtained for uninfected monolayers over 8 h of
experimental observation. These data suggest that the
type III secretion-independent plateau in wound closure
observed with PA103pscJ::Tn5 requires bacterial protein
synthesis. It was not possible to extend these studies to
24 h as Cm-resistant bacteria appeared during the course
of the experiment (data not shown).
226 T. K. Geiser et al.
Fig. 3. Heat-killed or chloramphenicol-treated bacteria no longer inhibit epithelial wound repair. A549 cells grown as confluent monolayers
were mechanically wounded 4 h before infection with P. aeruginosa. Wound area was measured immediately after wounding, at the time of
bacterial infection, and 2, 4, 6 and 8 h after infection. Each point represents the average of triplicate wells; values are expressed as mean per
cent of initial wound area ^ SD.
A. Heat-inactivated bacteria. Aliquots of PA103 and PA103DU were split, and either heated to 688C or maintained at room temperature for
20 min before infecting wounded monolayers.
B. Chloramphenicol-treated bacteria. Bacteria were diluted to an approximate MOI of 5±10 in tissue culture media and incubated at 378C in
the presence or absence of Cm (50 mg ml21) for 45 min before inoculation of A549 monolayers wounded 4 h previously. Cm was maintained
in the samples at 50 mg ml21 throughout the experiment; this did not alter the rate of wound closure as can be seen by comparing the medium
vs. medium plus Cm curves (P . 0.1 at all time points, Student's two-tailed t-test). At all time points, neither Cm-treated bacterial sample
differed significantly from the Cm-treated uninfected control (P $ 0.08 for all two-way comparisons, Student's two-tailed t-test).
The PA103 type III-secreted protein ExoT is required to
inhibit epithelial wound repair
The differences in the behaviour of PA103pscJ::Tn5 and
PA103DU-infected monolayers led us to test whether one
of the type III-secreted proteins elaborated by PA103DU,
and absent in PA103pscJ::Tn5, could mediate cell
rounding and inhibition of wound closure. ExoT, a Rhofamily GAP (Krall et al., 2000) that causes cell rounding of
HeLa and Chinese Hamster ovary (CHO) cells (Vallis
et al., 1999; Garrity-Ryan et al., 2000), was the most likely
candidate. We constructed PA103DUDT, a strain that
no longer synthesizes ExoT by virtue of a gentamicin
cassette insertion into the exoT gene. Infection with the
double mutant resulted in a phenotype indistinguishable
from that seen with PA103pscJ::Tn5. There was no
change in cell morphology at the wound edge (data not
shown), and wound closure proceeded at the same rate
for cells infected with either strain (Fig. 4A).
To determine whether ExoT was required for cell
rounding and inhibition of wound healing, the strain
PA103DUDT transformed with pUCP20-ExoT or vector
alone was added to wounded A549 monolayers, and
the effect on wound healing between the mutant and
complemented strains was compared. The PA103DUDTpUCP20 control strain did not increase the area of the
wound, and wound size plateaued at 4±6 h post infection
at approximately 45% wound closure. However, the
complemented strain PA103DUDT-pUCP20-ExoT showed
complete reversal of wound closure by 4±6 h post infection,
similar to PA103DU. Microscopic examination showed
typical morphological changes: the complemented strain
caused cell rounding and cytoplasmic retraction indistinguishable from that observed after infection with PA103DU
(data not shown). These results demonstrate that ExoT is
required for PA103 both to inhibit wound closure and to
induce cell rounding preferentially in cells along the
wound edge.
ExoT-mediated cell detachment can be reversed by CNF-1
Our finding that ExoT-mediated inhibition of wound
closure occurs rapidly (within 4 h of bacterial infection)
and is associated with marked changes in cell shape
suggests that alterations in the actin cytoskeleton affecting cell attachment (rather than changes in cell proliferation) are important in this process. To test this hypothesis,
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P. aeruginosa ExoT inhibits epithelial wound repair
227
Fig. 4. ExoT is necessary for PA103 inhibition of A549 wound closure.
A. PA103DUDT infection allows epithelial wound closure to occur. A549 monolayers were wounded 4 h before infection with PA103DU,
PA103DUDT or PA103pscJ::Tn5. Wound area was measured at indicated time points and is expressed as mean per cent of initial wound
area. Each point represents the mean of triplicate wells ^ SD.
B. Expression of ExoT from a plasmid restores the ability of PA103DUDT to inhibit wound closure. PA103DUDT carrying the plasmid pUCP20ExoT or the empty vector (pUCP20) were used to infect wounded A549 monolayers. All samples, including the uninfected control, contained
carbenicillin (200 mg ml21) to select against plasmid loss during the course of the experiment.
culture supernatants were collected 4 h after addition of
bacteria and detached cells counted. Infection with
PA103DU resulted in significant cell detachment compared with uninfected controls (P ˆ 0.008, Student's twotailed t-test) (Fig. 5). This effect was not observed after
infection with strains that did not inhibit wound closure or
cause cell rounding, such as PA103DUDT or
PA103pscJ::Tn5. It has been reported that inactivation
of the GTPase Rho inhibits wound closure by causing
migrating cells to detach from their substrate (Nobes and
Hall, 1999). We hypothesized that if ExoT was causing
cell rounding and detachment by acting as a GAP for Rho,
then these phenotypes should be prevented by pretreating the cells with an agent that activates Rho, such as
Cytotoxic Necrotizing Factor-1 (CNF-1). As seen in Fig. 5,
cells pretreated with CNF-1 for 15 h before wounding no
longer showed detachment above baseline when infected
by the ExoT-expressing strain, PA103DU (P ˆ 0.14,
Student's two-tailed t-test). Microscopic examination
demonstrated that PA103DU-associated cell rounding
was also attenuated in CNF-1 pretreated cells compared
with untreated infected cells (data not shown). We did not
measure the effect of CNF-1 on wound closure after
infection, however, because both activation and inactivation of Rho result in impaired cell migration (Nobes and
Hall, 1999).
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Fig. 5. CNF-1 treatment of A549 wounded monolayers prevents
cell detachment after PA103DU infection. A549 monolayers were
pretreated with CNF-1 (0.3 nM) 15 h before wounding and infected
with PA103 strains 4 h after wounding. Detached cells were
counted with a haematocytometer 4 h after infection. Bars indicate
the mean number of detached cells per well ^ SD as measured for
triplicate wells in a representative experiment.
228 T. K. Geiser et al.
The conserved residue Arg149 of ExoT is required to
inhibit wound healing
The observation that activation of Rho by CNF-1
pretreatment of cells prevents cell rounding and detachment of PA103DU-infected cells suggests that the GAP
activity of ExoT mediates these phenotypes after bacterial
infection. We tested this hypothesis by constructing
PA103DU strains in which Arg149 within the arginine
finger motif of the GAP domain had been mutated to
either lysine [PA103DU/T(R149K)] or glycine [PA103DU/
T(R149G)], a change that is predicted to abolish the
GAP activity of ExoT. Western blot analysis of secreted
proteins prepared from PA103DU, PA103DU/T(R149G)
and PA103DU/T(R149K) showed that ExoT was secreted
into the media in comparable amounts by all three strains,
suggesting that the wild-type and mutant proteins are
equally competent for secretion (data not shown). In vivo
measurement of Rho-GTP levels using a rhotekin-based
assay (Ren et al., 1999) confirmed that the Rho-GAP
activity observed in HeLa cells transfected with the
wild-type ExoT gene was no longer present in cells
transfected with constructs encoding either ExoT(R149K)
or ExoT(R149G) (B. Kazmierczak and J. Engel, in
preparation). Both point mutants abolish the rapid and
complete reversal of wound closure seen with the parental
strain PA103DU (Fig. 6). Moreover, the wound healing
curves obtained in their presence are indistinguishable
from those seen with the strain completely lacking ExoT.
Examination of the wounded monolayers showed cell
rounding along the edge of the wound 4±6 h post
infection with PA103DU; these changes were absent
from samples infected by either point mutant or by
PA103DUDT. However, the appearance of migrating
cells differed between samples infected by the two point
mutants and PA103DUDT. In A549 monolayers infected
with PA103DU/T(R149G) or PA103DU/T(R149K), many
cells possessed a markedly elongated shape. These
cells appeared to migrate into the wound area as
individual cells rather than as a cell sheet, the pattern
commonly observed with the DUDT double mutant (data
not shown).
The actin cytoskeleton and focal adhesions of infected
cells are altered by bacteria expressing wild-type ExoT or
ExoT mutated at Arg149
Although point mutations in ExoT at Arg149 abolished the
ability of PA103DU to reverse epithelial wound healing,
they did not result in a phenotype indistinguishable from
that of the double mutant strain PA103DUDT. We therefore decided to examine the effects of these different
bacterial strains on the morphology of healing monolayers. Wounded A549 monolayers were fixed at several
Fig. 6. Mutation of Arginine149 in ExoT abolishes the ability of
P. aeruginosa to inhibit epithelial wound closure in vitro. A549
monolayers were wounded 4 h before infection with PA103DU,
PA103DUDT, PA103DU/T(R149K) or PA103DU/T(R149G) at an
MOI of 1±3. Wound area was measured over 24 h in triplicate
wells and is expressed as mean per cent of initial wound area
^ SD. At 12 and 24 h post infection, the wound area in PA103DUinfected samples differs significantly from the wound area observed
in PA103DUDT-infected samples (P # 0.0001, Student's two-tailed
t-test) or in samples infected with either Arg149 point mutant
(P , 0.003, Student's two-tailed t-test). There is no statistically
significant difference at these time points between the Arg149infected samples and samples infected with PA103DUDT (P . 0.9
for all two-way comparisons, Student's two-tailed t-test).
time points after bacterial infection and the actin cytoskeleton visualized by phalloidin staining. Five hours post
infection, migrating A549 cells were clearly visible at the
wound margins; phalloidin staining revealed lamellipodia
at the leading edge of the cells and prominent stress
fibres appearing to terminate in focal adhesions (Fig. 7A
and E). The cells maintained a well-spread morphology
and many cell±cell contacts during migration. Monolayers
infected by PA103DUDT had a similar appearance, with
well-spread cells appearing to migrate away from the
wound margin (Fig. 7C and G). In contrast, PA103DUinfected monolayers showed markedly altered A549 cell
morphology along the wound margin: cells were rounded,
with collapse and retraction of the cytoskeleton (Fig. 7B
and F). No stress fibres were visible, even in cells that
were not yet rounded. Cells migrating into the wound
area in the PA103DU/T(R149K)-infected samples often
appeared more elongated than those infected by
PA103DUDT, and exhibited fewer cell±cell contacts
during migration than control monolayers (Fig. 7D and
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P. aeruginosa ExoT inhibits epithelial wound repair
229
Fig. 7. A549 cells bordering the wound area assume different morphologies after infection with P. aeruginosa depending on the genotype at
exoT. Confluent monolayers of A549 cells grown on fibronectin-coated eight-chamber slides were mechanically wounded 4 h before bacterial
infection (MOI 10). Samples were fixed and stained as described in Experimental procedures, and imaged using a Nikon Eclipse E800
microscope (400). Green staining indicates Syto9 (nuclear) staining of eukaryotic and bacterial DNA; red staining indicates F-actin staining
by phalloidin. Panels E-H are enlarged to show detail. Arrows show stress fibres in control cells (E) and in cells infected with either
PA103DUDT (G) or PA103DU/T(R149K) (H).
A and E. Uninfected control, 5 h post mock-infection.
B and F. PA103DU, 5 h post infection.
C and G. PA103DUDT, 5 h post infection.
D and H. PA103DU/T(R149K), 5 h post infection.
H). Rounded cells were present, though not to the same
extent as in samples infected by PA103DU, and many
cells had visible stress fibres (Fig. 7H). Similar morphology and migration patterns were observed after infection
Q 2001 Blackwell Science Ltd, Cellular Microbiology, 3, 223±236
with the other point mutant, PA103DU/T(R149G) (data not
shown).
We also examined focal adhesions in the wounded
epithelial cell monolayers by fixing cells after infection,
230 T. K. Geiser et al.
Fig. 8. Paxillin staining and focal adhesion appearance are altered by PA103 infection depending on the genotype at exoT. Confluent
monolayers of A549 cells grown on fibronectin-coated eight-chamber slides were mechanically wounded 4 h before bacterial infection (MOI
10). Samples were fixed and stained as described in Experimental procedures 8 h post infection. Anti-paxillin staining (green), F-actin staining
with phalloidin (red) and DAPI staining of nucleic acids (blue) were imaged with a Nikon Eclipse E800 microscope (400). Panels E-H are
enlarged to show detail. The diffuse paxillin staining visible in PA103DU/T(R149K)-infected cells (H) is clearly different from the discrete focal
adhesion staining pattern present in uninfected (E) or PA103DUDT-infected cells (G). Some paxillin staining is visible in areas devoid of cells
(E); this is probably a result of residual paxillin being retained on the coverslip in areas where cell loss occurred.
(A and E). Uninfected control.
(B and F). PA103DU.
(C and G). PA103DUDT.
(D and H). PA103DU/T(R149K).
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P. aeruginosa ExoT inhibits epithelial wound repair
and staining the samples with monoclonal antibodies to
paxillin and focal adhesion kinase (FAK), components of
focal adhesions. As seen in Fig. 8A and E, the majority of
cells at the wound margin showed discrete paxillin
staining at focal adhesions and maintained a well-spread
morphology. The PA103DUDT-infected cells looked very
similar, with paxillin staining predominantly confined to
discrete focal adhesions in the cells at the edge of the
wound (Fig. 8C and G). In contrast, cells infected with
PA103DU/T(R149K) or PA103DU/T(R149G) showed
diffuse paxillin staining with relatively few discrete focal
adhesions (Fig. 8D and H; data not shown). The
elongated cell morphology and absence of cell±cell
contacts were again apparent. Cells in PA103DU-infected
monolayers showed almost no focal adhesions on paxillin
staining (Fig. 8B and F); characteristic cell rounding was
visible, with spindly projections presumably marking
cytoplasmic retraction and cytoskeleton collapse. Staining
of infected cells with antibodies to FAK revealed a similar
focal adhesion staining pattern to that seen with staining
of paxillin (data not shown). Together, these findings
suggest that the GAP domain of ExoT is required for P.
aeruginosa to inhibit epithelial wound healing in vitro, and
is critical for the cell rounding phenotype and detachment
we observed. However, other domains or activities of
ExoT may also contribute to alteration of the cell
cytoskeleton during epithelial wound closure because
the Arg149 mutants clearly alter the distribution of focal
adhesions during cell migration and wound closure.
Discussion
P. aeruginosa inhibits wound closure
Although it is well known that intact epithelial tissues
present a relative barrier to infection by P. aeruginosa
(Salyers and Whitt, 1994), we demonstrate the novel
finding that P. aeruginosa can perpetuate pre-existing
injury by inhibiting epithelial wound repair through the type
III-secreted protein ExoT. The most striking effect of P.
aeruginosa infection on wounded A549 lung epithelial cell
monolayers was reversal of previously initiated wound
closure. This effect was independent of the type IIIsecreted cytotoxin ExoU, as A549 cells at the wound
margin retracted, rounded up and detached after infection
with PA103DU, resulting in marked increases in epithelial
wound size. Cells distal to the wound edge did not
initially show these morphological changes. These
results suggest that either the bacteria have preferential
access to the cells at the wound edge, or that these
cells are more susceptible to bacterial damage. The
effect of P. aeruginosa infection on epithelial wound
repair was attenuated if bacteria were applied to the
monolayer immediately after wounding (T. Geiser and
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231
B. Kazmierczak, unpublished results). Therefore, the
initiation of wound repair ± which is known to require
the activation of many cell pathways, including those
regulating the actin cytoskeleton ± may make cells more
susceptible to the effects of PA103DU.
We note that PA103 strains that expressed no type IIIsecreted toxins, such as PA103pscJ::Tn5, nonetheless
prevented the complete closure of wounded A549
monolayers. This inhibition required the presence of live
bacteria actively synthesizing proteins, suggesting that a
protein product of PA103 is responsible for this phenotype. Because there was no obvious alteration of cell
morphology or migration pattern in these cells, we cannot
speculate further upon the mechanism responsible for this
arrest of wound closure. It has recently been reported
that, in an in vitro model of airway epithelial cell wound
closure, culture supernatants of elastase-producing P.
aeruginosa strains PAO1 and AK43 can slow cell
migration by a mechanism postulated to involve activation
of the matrix metalloproteinase MMP-2 (de Bentzmann
et al., 2000). Although PA103 (LasR±) does not secrete
elastase, perhaps the activity of one of the other
proteases secreted by this strain in a type III-independent
manner contributes to the slowing and prevention of
complete wound healing that we observe in
PA103pscJ::Tn5-infected monolayers.
ExoT: a Rho-family GTPase-activating protein (GAP)
mediates inhibition of wound closure
ExoT was responsible for both reversing wound closure
and causing cell retraction and rounding. This type IIIsecreted protein has been shown to act as an antiinternalization factor for PA103 (Cowell et al., 2000) and is
capable of inhibiting bacterial internalization in both
epithelial and macrophage-like cell lines (Garrity-Ryan
et al., 2000). ExoT-expressing strains also cause cell
rounding and changes in actin cytoskeleton architecture in
epithelial cells (Vallis et al., 1999; Garrity-Ryan et al.,
2000). Because of the changes we observe in the
migrating A549 cells, the ability of ExoT to modulate the
actin cytoskeleton is probably important for inhibition of
epithelial wound healing.
ExoT has two distinct domains. The carboxyl-terminal
half of ExoT is highly homologous to the ADP-ribosyltransferase domain of the P. aeruginosa protein, ExoS
(Radke et al., 1999); however, ExoT has greatly reduced
in vitro ADP-ribosyltransferase activity as a result of
mutations in the enzymatic active site. The aminoterminal half of ExoT contains a second region of
homology to ExoS, an arginine finger domain motif that
is characteristic of GTPase-activating proteins (GAPs)
found in other bacteria as well as in eukaryotic cells (Fu
and Galan, 1999). The bacterial GAPs described to date
232 T. K. Geiser et al.
inactivate eukaryotic small-GTPases of the Rho-family,
including Rho, Rac and Cdc42, which are master
regulators of the actin cytoskeleton. ExoT has recently
been shown to have in vitro GAP activity towards Rho,
Rac and Cdc42 (Krall et al., 2000). Mutation of a
conserved arginine residue within the arginine finger
motif is sufficient to abolish the in vitro GAP activity of
ExoS (Ganesan et al., 1999), Yersinia enterocolitica YopE
(Black and Bliska, 2000; von Pawel-Rammingen et al.,
2000), and Salmonella typhimurium SptP (Fu and Galan,
1999). Although homologous arginine mutants in ExoT
have not yet been assayed for in vitro loss of GAP activity,
these mutations abrogate the GAP activity of ExoT
towards Rho in an in vivo assay (B. Kazmierczak and J.
Engel, in preparation). Furthermore, bacteria mutated at
Arg149 in ExoT exhibited attenuated anti-internalization
activity and eukaryotic cell rounding when co-cultivated
with epithelial cells (Garrity-Ryan et al., 2000).
In this study, we found that bacteria expressing ExoT
mutated at Arg149 no longer inhibited epithelial wound
closure and failed to induce cell rounding and actin
cytoskeleton collapse, consistent with the loss of ExoT
GAP activity toward Rho. Because Rho activity is required
for migrating cells to adhere to substrate during wound
closure, inhibition of Rho by ExoT would be sufficient to
account for the cell rounding, detachment and consequent
inhibition of wound closure that we observed. Activation of
Rho-family GTPases by CNF-1, which should render Rho
insensitive to the GAP activity of ExoT, prevented cell
detachment, confirming the involvement of this family of
proteins in these processes.
Interestingly, differences exist between wounded
epithelial cells infected with either Arg149 point mutant
compared with those infected by a strain no longer
expressing the ExoT protein. Although the wound healing
curves of the arginine point mutants did not differ
quantitatively from those obtained with the ExoT deletion
strain, the migrating cells assumed an elongated, rather
than broadly spread, morphology and moved into the
wound area as individual cells. This was accompanied
by a marked change in paxillin and FAK staining of
focal adhesions. Uninfected cells and cells infected by
PA103DUDT showed multiple well-defined focal adhesions; in marked contrast, cells infected by the point
mutants PA103DU/T(R149G) or PA103DU/T(R149K)
showed diffuse paxillin and FAK staining and contained
few discrete focal adhesions. It is not unexpected that
cells lacking focal adhesions could continue to migrate;
this has been shown experimentally for primary rat
embryo fibroblasts, which can complete wound closure
in an in vitro wound healing model even when focal
adhesion formation is inhibited with the ROCK inhibitor Y27632 (Nobes and Hall, 1999). However, it is not clear
how infection with the Arg149 point mutants causes this
change in focal adhesions. Focal adhesion formation is
regulated by Rho activity. We have found, however, that
the Arg149 point mutants no longer show GAP activity for
Rho in vivo (B. Kazmierczak and J. Engel, in preparation).
Moreover, stress fibres ± another Rho-dependent structure ± can be visualized in many of the migrating cells,
including those that are intimately associated with
bacteria (see Fig. 7H), again arguing that the Arg149
mutants are not acting as GAPs for Rho. Focal adhesion
assembly and maintenance are also dependent upon
other signal transduction pathways, most notably those
regulating tyrosine phosphorylation of focal adhesion
components. It is possible, therefore, that ExoT interferes
with focal adhesion formation through such a pathway,
either directly or by mediating the interaction of another
protein (bacterial or eukaryotic) with focal adhesion
components.
The roles of ExoT in pathogenesis
The data in this study indicate a novel phenotype of ExoTproducing P. aeruginosa, namely an ability to reverse the
closure of epithelial wounds. The capacity to inhibit wound
closure might be critical for the pathogenesis of P.
aeruginosa in vivo. This pathogen is relatively incapable
of breaching intact epithelial tissues and must either rely
on pre-existing epithelial damage or itself inflict damage
through the action of elaborated cytotoxins in order to
efficiently establish disease (Salyers and Whitt, 1994).
Recent in vivo data have suggested that ExoT plays a role
in virulence in the mouse model of acute pneumonia. In
bacteria lacking the cytotoxin ExoU (and therefore
incapable of eliciting de novo epithelial tissue damage
through this mechanism), the presence or absence of
ExoT influences the ability of bacteria to disseminate to
distant organs from the lung (Garrity-Ryan et al., 2000).
PA103DU strains are four times more likely to be
recovered from the liver than PA103DUDT strains.
Because these bacteria are recovered in equal numbers
from the lung, it is unlikely that a difference in the ability to
survive and proliferate in the host accounts for differential
recovery from the liver. Preferential recovery of PA103DU
from the liver may reflect an advantage conferred by the
anti-internalization activity of ExoT, which would allow
these bacteria to escape phagocytosis by cells of the
immune system (Garrity-Ryan et al., 2000). Alternatively,
the ability of ExoT possessing bacteria to prolong
epithelial tissue damage incurred during lung infection
may permit the bacteria to breach the defence otherwise
provided by an intact or repaired epithelial tissue.
The pathogenesis of P. aeruginosa infections is multifactorial: both host defences and pathogen virulence
factors contribute to this process in a complex fashion.
Based on our results, we propose that ExoT makes critical
Q 2001 Blackwell Science Ltd, Cellular Microbiology, 3, 223±236
P. aeruginosa ExoT inhibits epithelial wound repair
contributions to pathogenesis. As an anti-internalization
factor, ExoT can decrease the likelihood of P. aeruginosa
internalization by both professional phagocytes and
epithelial cells. This property probably increases the
ability of P. aeruginosa to escape local host defences
and to shield it from the immune system. ExoT can also
interfere with Rho-mediated host cell events, such as
those occurring during wound healing, perpetuating tissue
injury and epithelial layer disruption. This role might be
particularly relevant to the ability of this pathogen both to
exploit the opportunities presented by even minor damage
to epithelial tissues during infection and to disseminate.
Together, these observations begin to explain why P.
aeruginosa is such a virulent pathogen in the setting of
epithelial tissue compromise.
Experimental procedures
Bacterial strains and plasmids
Pseudomonas aeruginosa strains were routinely cultured in
Luria±Bertani broth (LB) or Vogel±Bonner minimal medium
(VBM) with antibiotics as needed. The following antibiotic
concentrations were used: ampicillin (Ap), 100 mg ml21, for
Escherichia coli; carbenicillin (Carb), 200 mg ml21, for
P. aeruginosa; gentamicin (Gent), 15 mg ml21, for E. coli;
and 100 mg ml21 for P. aeruginosa. All antibiotics were
purchased from Sigma (St Louis, MO). A list of strains and
constructs used in this study is shown in Table 1. All cloning
steps were carried out in XL2-Blue ultracompetent E. coli
(Stratagene).
Construction of PA103D U/T(R149G) and
PA103D U(R149K)
Standard molecular biology techniques were used to construct PA103DU/T(R149G) and PA103DU/T(R149K) (Maniatis et al., 1982). All restriction enzymes were purchased from
New England Biolabs (NEB) unless otherwise noted. Isolation of DNA from gel-purified fragments was carried out with
Qiaex II Gel Extraction Kit (Qiagen) as per the manufacturer's
instructions. All polymerase chain reaction (PCR) amplifications were carried out in a GeneAmp PCR System 9700
thermocycler (PE Applied Biosystems). PCR primers were
obtained from Operon. Primer sequences are listed in
Table 2; bases shown in lower case are not homologous to
the chromosomal sequence.
The conserved arginine (R149) of the predicted arginine
finger motif found in ExoT was mutated to glycine and lysine
as follows. PA103 genomic DNA (template) was prepared as
described by Chen and Kuo (Chen and Kuo, 1993). PCR
amplification was carried out as 50 ml reactions containing
200 ng of template DNA, 10% dimethyl sulphoxide (DMSO),
10 Pfu buffer, 250 nM primers, 200 nM dNTPs (each). A 1:8
mixture (units) of Pfu: AmpliTaq (PE Roche) was used;
reactions were run for 30 cycles. PCR primers (Operon) were
designed to create a unique restriction enzyme site (BamHI
for R149G; AflII for R149K) at the point of mutation.
Q 2001 Blackwell Science Ltd, Cellular Microbiology, 3, 223±236
233
The R149G mutation was introduced by amplifying PA103
ExoT with BAKA 22 and BAKA 24 (625 bp product) and
BAKA 23 and BAKA 25 (934 bp product), gel-purifying the
products, digesting them with BamHI (Gibco BRL), ligating
the two products together using T4 DNA ligase (NEB), and
gel-purifying the fragment corresponding to the predicted
1559 bp product. This fragment was subsequently amplified
using the internal primers BAKA 32 and BAKA 33, and the
resulting 1075-bp fragment was gel purified before being Atailed with AmpliTaq polymerase and subcloned into pGEM-T
(Promega). The sequence of the insert was confirmed at this
point. A NsiI±NgoMIV fragment comprising the first 1024 bp
of the ExoT coding sequence and containing the R149G
mutation replaced the corresponding fragment of pUCP20ExoT, a plasmid containing the wild-type PA103 ExoT gene
under control of its native promoter, to create pBK162.
To introduce the R149G point mutation into the PA103DU
chromosome, an internal 939 bp XmaI fragment of the ExoT
gene containing the mutation was subcloned from pBK162
into the unique XmaI site of pEX100T to create pBK163. This
plasmid was transformed into E. coli S17.1 to enable it to be
mated into PA103DU. Exconjugants were selected on VBM
plus carbenicillin (200 mg ml21), then streaked to VBM plus
5% sucrose to select for loss of vector sequences through a
second recombination event. Sucrose-resistant, carbenicillinsensitive colonies were screened for the presence of the
R149G point mutation by PCR amplification of colonies with
BAKA 32 and BAKA 33; the PCR product from colonies
carrying the mutation could be restricted with BamHI. The
presence of the mutation was confirmed by sequencing the
BAKA 32/BAKA 33 PCR product from PA103DU/T(R149G)
(data not shown).
A similar strategy was used to create PA103DU/T(R149K).
The initial PCR was performed using primer pairs BAKA 22
and BAKA 26, and BAKA 23 and BAKA 27; the resulting
fragments were restricted with AflII before ligation. The
correct ligation product was amplified with BAKA 32 and
BAKA 33, A-tailed, subcloned into pGEM-T and sequenced.
Of note, the subcloned PCR product contains a second
mutation, A211V, which was judged to be a conservative
change; this mutation persists in pBK151 and is also found in
the chromosome of PA103DU/T(R149K) (data not shown).
Preparation of A549 alveolar epithelial-like cells
A549 human alveolar epithelial-like cells (American Type
Culture Collection, Rockville, MD) were grown to confluency
in Dulbecco's modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 2 mM L-glutamine, 50
IU ml21 penicillin and 50 IU ml21 streptomycin. Cells were
plated to 24-well tissue culture dishes (Falcon) for wounding
assays at a density of 1.0±1.2 105 cells per well. A
confluent monolayer was obtained after 24 h. All media
components were purchased from the UCSF Tissue Culture
Facility (San Francisco, CA). All tissue culture was carried out
at 378C in a humidified atmosphere containing 5% CO2.
Wound healing assay
Wound healing in A549 epithelial cells was measured as
234 T. K. Geiser et al.
Table 1. Strains and plasmids used in this study.
Strains and plasmids
Relevant characteristics
Source or reference
PA103
Virulent lung isolate of P. aeruginosa,
known type III-secreted effector
proteins are ExoT and ExoU
Tn5 Gentr inserted into pscJ,
defective in type III secretion
PA103 with an in-frame deletion
within exoU corresponding to amino
acids 330±571
PA103DU with a axylE/aacC1
cassette replacing amino acids 36±348
of exoT, Gentr
PA103DU with a point mutation
in ExoT (R149G)
PA103DU with a point mutation
in ExoT (R149K)
E. coli strain used for mating constructs
into P. aeruginosa thi pro hsdR recA RP4-2
(Tet::Mu) (Kan::Tn7)
recA1 endA1 gyrA96 thi-1 hsdR17
supE44 relA1 lacF 0 proAB
lacIqZDM15 Tn10 (Tetr) Amy Cmr
pUCP20-ExoT(R149K)
pEX100T with XmaI
fragment from pBK151
pUCP20-ExoT(R149G)
pEX100T with XmaI fragment from pBK162
Allelic replacement suicide plasmid;
Apr(Carbr) sacB oriT
Cloning vector, Apr
P. aeruginosa expression vector, Apr(Carbr)
pUCP20 with a 1666 bp
NcoI/NdeI fragment containing exoT
Apodaca et al. (1995; Liu, 1966)
PA103pscJ::Tn5
PA103DU
PA103DUDT
PA103DU/T(R149G)
PA103DU/T(R149K)
S17.1
E. coli XL2-Blue
pBK151
pBK152
pBK162
pBK163
pEX100T
pGEM-T
pUCP20
pUCP20-ExoT
Kang et al. (1997)
Garrity-Ryan et al. (2000)
Garrity-Ryan et al. (2000)
This study
This study
Simon et al. (1983)
Stratagene
This study
This study
This study
This study
Schweizer and Hoang (1995)
Promega
West et al. (1994)
Garrity-Ryan et al. (2000)
Gentr, gentamicin resistance; Tetr, tetracycline resistance; Cmr, chloramphenicol resistance; Apr, ampicillin resistance in E. coli; Carbr, carbenicillin
resistance in P. aeruginosa.
described previously (Kheradmand et al., 1994; Geiser
et al., 2000). Experiments were carried out in triplicate.
Confluent cell monolayers of A549 epithelial cells were
washed three times with MEM Eagle's to remove serum
and antibiotics, and a linear wound was made with a plastic
pipette tip. After wounding, the cells were washed three
times to remove the cell debris. The wounded cells were
then incubated in MEM Eagle's plus 10% FBS for 4 h at
378C and 5% CO2. Preliminary experiments indicated that
the effect of P. aeruginosa infection was most consistent if
the cells were infected 4 h after wounding (data not
shown). After this time, cells were infected with PA103
strains grown overnight at 378C in LB as standing cultures.
Bacteria were diluted with MEM Eagle's plus 10% FBS and
the MOI determined by titering the dilution; an inoculum of
100 ml of bacteria per well (24-well plate) at an OD600 of
0.03±0.04 corresponded to an MOI of < 10. The infected
cells were maintained at 378C in 5% CO2 during the
experiment. Experiments using plasmid-bearing strains
were carried out in the presence of carbenicillin
(200 mg ml21) to select against plasmid loss; as can be
seen in Fig. 4B, the presence of antibiotic did not itself affect
wound healing.
The area of the denuded surface was measured immediately after wounding and before infecting the cells, followed
by measurements 2, 4, 6, 8 and 24 h after infection. The cells
were imaged using an inverted microscope (Axiovert 35,
Zeiss, Thornwood) connected to a charge-coupled device
camera (C 2400; NEC, Hawthorne, CA). The image was
subsequently captured by an image-analysing frame-grabber
card (LG-3 Scientific Frame Grabber; Scion, Frederick, MD)
and analysed using an image analysis software (NIH IMAGE
1.55). Wound repair was expressed as percentage of initial
wound area in each well. Data from a single experiment
representative of three or more independent experiments are
shown.
Table 2. Primer sequences used in this study.
Primer
BAKA
BAKA
BAKA
BAKA
BAKA
BAKA
BAKA
BAKA
Sequence
22
23
24
25
26
27
32
33
5 0 -CGGTGTAGGCGCACGGGAG-3 0
5 0 -CGTCGACCGGTCAGGCCAG-3 0
5 0 -GGCCAGgGAtCcCAGtGCGCCGTCGCCGCTG-3 0
5 0 -GCaCTGgGaTCcCTGGCCACCGCCCTGGTCG-3 0
5 0 -GGCCAGCGActtaAGtGCGCCGTCGCCGCTG-3 0
5 0 -GCaCTtaagTCGCTGGCCACCGCCCTGG-3 0
5 0 -CTGGCGGGGAAACATCAGG-3 0
5 0 -AGGTGGAGAGATAGCCGGC-3 0
Bases shown in lower case are not homologous to chromosomal
sequences.
Q 2001 Blackwell Science Ltd, Cellular Microbiology, 3, 223±236
P. aeruginosa ExoT inhibits epithelial wound repair
235
Cell detachment assay
Acknowledgements
A549 cells were plated in triplicate as described for wound
healing assays. Fifteen to 18 h before wounding, CNF-1 (a
generous gift of Peter Flynn and David Stokoe, University of
California, San Francisco Cancer Center) was added to cells
at a final concentration of 0.3 nM as indicated. Cells were
wounded as described above. Four hours after wounding,
wells were washed three times with MEM Eagle's plus 10%
FBS to remove unattached cells and then infected with
bacteria (MOI 1±3). Four hours post infection the media from
each well were collected; the wells were rinsed once with
fresh media, and these were then combined and spun 5 min
at 1000 r.p.m. in a Sorvall RT6000B table top centrifuge
to pellet detached cells. The pellets were resuspended in
100 ml of phosphate-buffered saline (PBS) and the number
of cells present in each determined by counting with a
haematocytometer.
We thank Eric Brown and Keith Mostov for critical reading of this
manuscript, and Peter Flynn and David Stokoe for their generous
gift of Cytotoxic Necrotizing Factor-1. This work was supported
by The Swiss Foundation of Medical-Biological Grants (T.K.G.),
the Howard Hughes Medical Institute (B.I.K.), the Bank of
America Giannini Foundation (L.K.G-R.), the American Lung
Association (J.N.E.) and by NIH grants K08 AI 01636 (B.I.K.),
R01 HL51854 (M.M.), R01 HL51856 (M.M.) and R01 AI42806
(J.N.E.). J.N.E. is an established investigator of the American
Lung Association.
Indirect immunofluorescence
A549 cells were plated on fibronectin-coated eight chamber
slides (Lab-Tek Chamber Slides, Nalge Nunc International,
Naperville, IL). When confluent, the cells were mechanically
wounded as above, then infected 4 h later with bacteria at a
final MOI of < 10. The slides were incubated at 378C in 5%
CO2 for the indicated intervals; they were then removed,
washed three times with ice-cold PBS and fixed for 30 min at
room temperature with 4% paraformaldehyde (Sigma) in
PBS. The fixation was quenched with 75 mM NH4Cl/20 mM
glycine in PBS for 10 min. Slides were blocked and
permeabilized in PBS/0.7% Fish Scale Gelatin (Sigma)/
0.025% Saponin (PFS) plus RNAse (100 mg ml21) for 15±
30 min at 378C. Incubations with primary antibodies were
carried out in PFS for 60±90 min at 378C and followed by four
washes with PFS. Anti-paxillin antibody (Transduction Labs,
#P13520) was used at 1:250, as was anti-FAK antibody
(Transduction Labs, #F15020). Incubations with Alexa 488
anti-mouse IgG (Molecular Probes, OR, 1:500) were carried
out in PFS for 30±45 min at 378C. Samples were also
routinely stained with Syto9 (1:500) (Molecular Probes) or
DAPI (1 mg ml21, Sigma) and Texas red phalloidin (1 : 200)
(Molecular Probes) during incubation with secondary antibody. Samples were then washed twice with PFS followed by
two brief washes with 0.1% Triton X-100 (Sigma) in PBS.
Coverslips were mounted with Prolong Antifade (Molecular
Probes). Samples were imaged on a Nikon Eclipse E800
microscope with a Nikon 40 Plan fluor objective, captured
with a Spot CCD camera (Diagnostic Instruments) using SPOT
ADVANCED 3.0.4 software and pseudocoloured and assembled
into figures using ADOBE PHOTOSHOP 5.0.
Statistics
Data are presented as mean ^ SD. Statistical analysis
was done using the unpaired Student's t-test or ANOVA,
where appropriate. The results were considered statistically
significant if P , 0.05.
Q 2001 Blackwell Science Ltd, Cellular Microbiology, 3, 223±236
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