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NATURE MEDICINE VOLUME 13 | NUMBER 5 | MAY 2007 531

Flushing bacteria out of the bladderDanelle S Eto & Matthew A Mulvey

A drug that elevates host cAMP levels forces pathogenic bacteria out of bladder epithelial cells. This process may make urinary tract infections more amenable to treatment with antibiotics (pages 625–630).

With over one hundred million infections annually, urinary tract infections (UTIs) remain among the most common infectious diseases worldwide, despite improved hygiene and widespread antibiotic use1. Even with antibiotic treatment, UTIs can linger and recur in many individuals, such that 25% of women have recurrent symptoms within six months of an initial acute infection2. The rise of antibiotic-resistant strains of uropathogenic bacteria threatens to worsen the situation. Consequently, there is a need to improve cur-rent UTI treatment and prevention protocols. In a study published in this issue, Bishop et al. provide evidence that forskolin, a compound that raises intracellular levels of cyclic adenos-ine monophosphate (cAMP), may be useful for combating UTIs3.

More than 80% of community-acquired UTIs are caused by strains of uropathogenic Escherichia coli (UPEC)4. These pathogens typ-ically produce adhesive hair-like fibers called type 1 pili (or fimbriae) that promote attach-ment to and invasion of bladder epithelial cells. Once in the host cell, UPEC can withstand antibiotic treatments5, potentially serving as a reservoir for recurrent acute infections6,7. If so, elimination of intracellular bacterial reservoirs from the urinary tract may provide a means to limit, or even cure, chronic and recurrent UTIs.

The bladder epithelium is composed of a layer of terminally differentiated superficial umbrella cells covering two or more layers of immature intermediate and basal epithelial cells. UPEC can invade all layers of this strati-fied epithelium, but the umbrella cells lining the lumenal surface of the bladder are by far

the most susceptible to infection6. The apical surfaces of umbrella cells are embedded with plaques of integral membrane proteins known as uroplakins8. These plaques are thought to contribute to the permeability barrier formed by the bladder epithelium, providing structure

and limiting the diffusion of unwanted solutes into the bladder tissue. Nascent uroplakin plaques are assembled into discoidal or fusi-form-shaped vesicles that accumulate beneath the apical surface. As the bladder fills and emp-ties, exocytosis of these vesicles is coordinated

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The authors are in the Division of Cell Biology and

Immunology, Pathology Department, University of

Utah, Salt Lake City, Utah 84112, USA.

e-mail: mulvey@path.utah.edu

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Figure 1 UPEC trafficking within bladder epithelial cells. Within the terminally differentiated umbrella cells of the bladder epithelium, UPEC is trafficked into CD63-positive compartments. Bacteria within these vesicles face one of at least three possible fates. 1. The majority of the bacteria are exocytosed via a cAMP-dependent process. 2. UPEC persists within CD63-positive compartments, forming quiescent intracellular reservoirs (QIRs) that may serve as a source for later recurrent acute infections. QIRs can also be established within the immature cells of the bladder epithelium, and the re-emergence of these bacteria may be triggered as the immature host cells differentiate. 3. UPEC can break out of its membrane-bound compartment and multiply within the cytosol of host umbrella cells, forming large intracellular bacterial communities (IBCs). Bishop et al. report that forskolin, by elevating cAMP levels, promotes the release of UPEC and may consequently be able to diminish both QIR levels and IBC formation3.

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N E W S A N D V I E W S

532 VOLUME 13 | NUMBER 5 | MAY 2007 NATURE MEDICINE

with the endocytosis of older uroplakin plaques, increasing and decreasing the lumenal surface area of the bladder as needed.

Trafficking of the fusiform vesicles to the plasma membrane is regulated in part by intra-cellular cAMP and calcium levels as well as the small GTPase Rab27b8,9. In these respects, fusiform vesicles behave similarly to secretory lysosomes, which store and secrete cellular components and participate in the repair of damaged plasma membrane10. At least one pathogen, Trypanosoma cruzi, exploits these organelles to gain access to the cytoplasm of targeted cells. This parasite can invade cells by inducing the exocytosis of host cell secretory lysosomes, a gradual cAMP-regulated process that provides membrane for the formation of T. cruzi-containing vacuoles11.

Knowing all this, Bishop et al. asked whether the uroplakin vesicles might contain UPEC and spit them out into the lumen of the bladder under the control of cAMP3. Could such a pro-cess contribute to the uptake and re-emergence of UPEC?

In umbrella cells of infected mice, the authors found that UPEC often colocalize with Rab27b and CD63, a marker of lysosomes and late endosomes. In a human bladder epithelial cancer cell line, E. coli stimulated the extracel-lular release of the lysosomal enzyme β-hex-aminidase, while the knockdown of Rab27b expression inhibited type 1 pili–mediated bacterial invasion. On the basis of these and related observations, the authors suggest that UPEC, like T. cruzi, can induce the exocytosis of secretory lysosomes (or fusiform vesicles), allowing UPEC entry into target host cells. Once the bacteria were internalized, most were rapidly cycled through the host cell and externalized—consistent with previous stud-ies6,7. Bishop et al. went on to show that, like uroplakin fusiform vesicles and secretory lyso-somes, the re-emergence of UPEC is regulated by calcium and cAMP levels3. In addition, the authors show that treatment with forskolin, which increases cAMP levels, reduced the lev-els of intracellular bacteria in the bladders of infected mice, presumably by stimulating the exocytosis of UPEC-containing vesicles. With this key observation, the authors discovered a way to possibly target intracellular reservoirs of UPEC: by forcing their release into the extra-cellular space where they can be more easily killed by traditional antibiotic treatments.

Forskolin is the main active ingredient of the Ayurvedic herb Coleus forskohlii, a member of the mint family found in subtropical regions in Asia. Owing to its capacity to raise intracellular levels of cAMP, forskolin is used to treat several ailments, including allergies, respiratory prob-lems such as asthma, glaucoma and cardiovas-

cular diseases. Although the use of forskolin as a potential therapeutic for UTIs is intrigu-ing, several questions remain. Among these is whether or not forskolin treatment alone or in combination with antibiotics can reduce bacte-rial titers within infected bladder tissue more effectively than antibiotics alone.

In addition, UPEC can follow at least three paths within host cells, not all of which may be affected by forskolin treatment (Fig. 1). In the first pathway, highlighted by Bishop et al., many of the internalized UPEC are released back into the extracellular space via a cAMP-mediated, forskolin-stimulated process3. Alternately, UPEC can become sequestered within late endosome/lysosome-like compartments where the pathogens can persist in a nonreplicating state, forming quiescent intracellular reservoirs from which UPEC may eventually re-emerge to cause recurrent acute infections (Fig. 1 and ref. 6). Finally, microbes that manage to enter the cytoplasm of umbrella cells can rapidly multiply, forming large intracellular inclusions (Fig. 1 and refs. 7,12). The effects of forskolin treatment on such reservoirs and inclusions, and the persistence and recurrence of UTIs, remain to be tested.

Bishop et al. offer a model of a dynamic UPEC–host cell relationship that depends on bladder cell physiology, promoting the idea that UPEC essentially hitchhike along with uropla-kin plaques as they are internalized when the bladder empties3. The authors also propose that the re-emergence of UPEC can occur when internalized bacteria-containing fusi-

form vesicles are trafficked back to the bladder lumenal surface as the bladder distends. This scenario suggests that endocytosed uroplakin plaques are recycled rather than degraded in umbrella cell lysosomes, as most studies indi-cate8. Finally, it is unclear how the model put forth by Bishop et al.3 relates to the invasion and subsequent re-emergence of UPEC from immature bladder epithelial cells that lack uroplakin plaques and fusiform vesicles6,7,13. Future studies aimed at further defining how UPEC invades and traffics through target host cells should help resolve these issues.

COMPETING INTERESTS STATEMENTThe authors declare no competing financial interests.

1. Russo, T.A. & Johnson, J.R. Microbes Infect. 5, 449–456 (2003).

2. Foxman, B. Am. J. Med. 113 (suppl, 1A), 5S–13S (2002).

3. Bishop, B.L. et al. Nat. Med. 13, 625–630 (2007).4. Ronald, A. Am. J. Med. 113 (suppl. 1A), 14S–19S

(2002).5. Bower, J.M., Eto, D.S. & Mulvey, M.A. Traffic 6, 18–31

(2005).6. Eto, D.S., Sundsbak, J.L. & Mulvey, M.A. Cell.

Microbiol. 8, 704–717 (2006).7. Mulvey, M.A., Schilling, J.D. & Hultgren, S.J. Infect.

Immun. 69, 4572–4579 (2001).8. Apodaca, G. Traffic 5, 117–128 (2004).9. Chen, Y. et al. Proc. Natl. Acad. Sci. USA 100, 14012–

14017 (2003).10. Huynh, C., Roth, D., Ward, D.M., Kaplan, J. & Andrews,

N.W. Proc. Natl. Acad. Sci. USA 101, 16795–16800 (2004).

11. Andrade, L.O. & Andrews, N.W. Nat. Rev. Microbiol. 3, 819–823 (2005).

12. Justice, S.S. et al. Proc. Natl. Acad. Sci. USA 101, 1333–1338 (2004).

13. Martinez, J.J., Mulvey, M.A., Schilling, J.D., Pinkner, J.S. & Hultgren, S.J. EMBO J. 19, 2803–2812 (2000).

The popcorn plaquesJagat Narula & H William Strauss

Imaging of inflammation in atherosclerotic lesions may identify plaques vulnerable to rupture, and lead to better assessment of individuals at risk for acute coronary events such as myocardial infarction (pages 636–641).

Two-thirds of acute coronary events, such as myocardial infarction, unstable angina and sudden death, result from the rupture of atherosclerotic plaques in coronary arteries1.

When these plaques pop open, their thrombo-genic interior is exposed to the luminal blood, leading to clot formation and the acute event. Because up to 1.2 million individuals have an acute coronary event every year in the United States alone, and up to 325,000 patients die even before reaching a hospital2, prevention of such episodes seems to be the only effective management strategy. Clinical practice cur-rently relies on known risk factors, including circulating biomarkers, to predict the likeli-hood of acute events; however, noninvasive

Jagat Narula is at the University of California Irvine

School of Medicine, Irvine, California 92705,

USA. H. William Strauss is at the Memorial Sloan-

Kettering Cancer Center, New York, New York

10021, USA.

e-mail: narula@uci.edu

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