Date Published: August 6, 2015
Publisher: Public Library of Science
Author(s): Neha Dikshit, Pradeep Bist, Shannon N. Fenlon, Niyas Kudukkil Pulloor, Christelle En Lin Chua, Marci A. Scidmore, Jason A. Carlyon, Bor Luen Tang, Swaine L. Chen, Bindu Sukumaran, Isabelle Derré.
Recurrent urinary tract infections (UTIs) caused by uropathogenic E. coli (UPEC) are common and morbid infections with limited therapeutic options. Previous studies have demonstrated that persistent intracellular infection of bladder epithelial cells (BEC) by UPEC contributes to recurrent UTI in mouse models of infection. However, the mechanisms employed by UPEC to survive within BEC are incompletely understood. In this study we aimed to understand the role of host vesicular trafficking proteins in the intracellular survival of UPEC. Using a cell culture model of intracellular UPEC infection, we found that the small GTPase Rab35 facilitates UPEC survival in UPEC-containing vacuoles (UCV) within BEC. Rab35 plays a role in endosomal recycling of transferrin receptor (TfR), the key protein responsible for transferrin–mediated cellular iron uptake. UPEC enhance the expression of both Rab35 and TfR and recruit these proteins to the UCV, thereby supplying UPEC with the essential nutrient iron. Accordingly, Rab35 or TfR depleted cells showed significantly lower intracellular iron levels and reduced ability to support UPEC survival. In the absence of Rab35, UPEC are preferentially trafficked to degradative lysosomes and killed. Furthermore, in an in vivo murine model of persistent intracellular infection, Rab35 also colocalizes with intracellular UPEC. We propose a model in which UPEC subverts two different vesicular trafficking pathways (endosomal recycling and degradative lysosomal fusion) by modulating Rab35, thereby simultaneously enhancing iron acquisition and avoiding lysosomal degradation of the UCV within bladder epithelial cells. Our findings reveal a novel survival mechanism of intracellular UPEC and suggest a potential avenue for therapeutic intervention against recurrent UTI.
Urinary tract infections (UTIs) are one of the most common bacterial infections in humans, affecting at least 50% of women at some point in their lifetime. UTIs constitute significant morbidity and economic burden, accounting for more than 1 million hospitalizations and $2.4 billion in medical expenses in the USA alone annually [1,2]. Most (>80%) UTIs are caused by Escherichia coli, thus the term uropathogenic E. coli (UPEC) . After an initial infection, 25% of patients suffer a recurrence within 6 months, with 68% of these UTIs apparently caused by the original strain, despite appropriate antibiotic therapy [4,5]. Mouse models of UTI have been used by many groups to elucidate mechanisms underlying UPEC pathogenesis [6–8]. Experimentally infected mice also suffer episodes of recurrent UTI subsequent to clearance of bacteriuria following antibiotic therapy . These recurrent infections are due to UPEC that persist within urinary bladder epithelial cells. UPEC have been described to form several types of intracellular populations in vivo, such as intracellular bacterial communities (IBCs) and quiescent intracellular reservoirs (QIRs), that contribute to phenotypic antibiotic resistance and evasion of the host immune response . QIRs, in particular, cause persistent infections lasting for months in mice , while IBCs and other intracellular bacteria have been detected in the urine of both adult and pediatric human UTI patients [12–15]. Therefore, understanding the molecular mechanisms underlying the establishment of the intracellular bacterial reservoirs is critical for the development of efficient therapeutic strategies to control recurrent UTIs.
Numerous studies have described the complexities and consequences of intracellular UPEC infections in mice, humans, and cell culture. In general, studies on intracellular UPEC infections resembling those found in vivo have been limited partly because in vitro models do not seem to recreate the same structures. Several features of in vitro infections of cultured 5637 cells, however, have been validated using mouse models of infection, particularly for early stages of invasion [60,61]. We have used this in vitro system to identify a role for host Rab35 in intracellular UPEC survival.