Transmigration route of Campylobacter jejuni across polarized intestinal epithelial cells: paracellular, transcellular or both?

Abstract

Intact intercellular junctions and cellular matrix contacts are crucial structural components for the formation and maintenance of epithelial barrier functions in humans to control the commensal flora and protect against intruding microbes. Campylobacter jejuni is one of the most important zoonotic pathogens causing food-borne gastroenteritis and potentially more severe diseases such as reactive arthritis or Guillain-Barré syndrome. Crossing the intestinal epithelial barrier and host cell invasion by C. jejuni are considered to represent the primary reasons of gut tissue damage in humans and various animal model systems including monkeys, piglets, rabbits, hamsters and ferrets. C. jejuni is also able to invade underlying tissues such as the lamina propria, can enter the bloodstream, and possibly reach distinct organs such as spleen, liver or mesenteric lymph nodes. However, the molecular mechanisms as well as major bacterial and host cell factors involved in these activities are poorly understood. Various models exist by which the pathogen can trigger its own transmigration across polarized intestinal epithelial cells in vitro, the paracellular and/or transcellular mechanism. Recent studies suggest that bacterial factors such as flagellum, serine protease HtrA and lipooligosaccharide LOS may play an active role in bacterial transmigration. Here we review our knowledge on transmigration of C. jejuni as well as some other Campylobacter species, and discuss the pros and cons for the route(s) taken to travel across polarized epithelial cell monolayers. These studies provide fresh insights into the infection strategies employed by this important pathogen.

Figure 1

A schematic presentation of non-polarised and polarized intestinal cell epithelial cells under non-infective conditions or during infection with C. jejuni. (A) Cultured non-polarised intestinal epithelial cells such as INT-407 do not express typical cell-to cell junctions. Thus, basolateral receptors such as focal adhesion structures are accessible and not protected by tight or adherens junctions. (B) Polarised intestinal epithelial cells such as mucin-producing HT29-MTX-E12 cells express the different types of intercellular junctions including the tight junctions (orange), adherens junctions (light blue), focal adhesions (dark blue) and hemidesmosomes (green) which exhibit specific localization in the lateral or basal membranes as indicated. GAP junctions and desmosomes are other examples which are not discussed in this review article. (C,D)C. jejuni is able to infect both cell variants in vitro. This pathogen encodes numerous described pathogenicity-associated factors involved in important processes including bacterial adhesion to, transmigration across, invasion into and intracellular survival within intestinal epithelial cells. For more details see text.

Figure 2

Composition of major intercellular junctions in the polarized intestinal epithelium. Schematic presentation of specific junctional complexes and associated signaling pathways. (A) Tight junctions (TJs) contain at least four major groups of transmembrane proteins: the JAMs, claudins, occludin and a number of cytoplasmic peripheral proteins. While the transmembrane proteins mediate cell-to-cell adhesion, the cytosolic TJ complex connects to different factors (e.g. ZO-1/-2/-3, MUPP1 or cingulin) that link the involved transmembrane proteins to the actin-cytoskeleton. The integrity of TJs is maintained by a regulatory complex including atypical PKC (aPKC), Rac1, Cdc42, Par6 and Par3. (B) The calcium-dependent integrity of adherens junctions (AJs) is stabilized by binding of E-cadherin to the intracellular catenins. The carboxy-terminal domain of E-cadherin binds to the cytoplasmic protein β-catenin. p120-catenin binds to the juxtamembrane part of E-cadherin and stabilizes the AJ complex. The E-cadherin-β-catenin structure is connected to the actin-cytoskeleton via binding to α-catenin and Eplin. When the E-cadherin complex is disrupted, β-catenin can translocate into the nucleus and activate Tcf/LEF transcription factors. (C) Focal adhesions (FAs) are structural complexes that link the extracellular matrix (ECM) to the intracellular actin-cytoskeleton. They contain various integrin heterodimers which are transmembrane receptors composed of α and β chains. The extracellular integrin tail directly binds to ECM proteins such as fibronectin, while the cytoplasmic domain is linked to the actin-cytoskeleton via a large number of indicated adapter/signaling proteins to transmit signaling. (D) Hemidesmosomes are also located at the basal side of epithelial cells where they link laminins to the intracellular intermediate filament network. Thus, hemidesmosomes provide stable adhesion of epithelial cell layers to the basement tissue. They consist of integrin α6β4, CD151 and BP180 which are transmembrane proteins, while plectin and BPAG1e are located in the cytoplasm. Plectin mediates linkage of hemidesmosomes to the cytokeratin network and not to F-actin filaments.

Figure 3

Models for transepithelial migration across polarised epithelial cells by C. jejuni. Simplified schematic diagram depicting cell junctions and two considered routes of bacterial travel across a polarized epithelium. The apical surface of the epithelial monolayer faces the external environment to the gut and forms the first barrier for C. jejuni invasion. Cell junctions important for the structural stability of a polarized epithelium include the tight junctions, adherens junctions, and matrix receptors as indicated. Various routes for C. jejuni transmigration have been proposed. (A) The transcellular route is characterized by pathogens crossing the epithelial barrier through entering the cells at the apical surface and exiting the cells at the basal membrane. (B) The paracellular route is taken by the bacteria entering the epithelium between two neighboring cells, thus crossing cells through the tight and adherens junctions. Opening of the cell-to-cell junctions maybe a temporal process and potentially close again after C. jejuni have passed. Basal exiting C. jejuni express the adhesin CadF which can bind to the fibronectin→integrin complex utilized for invasion from the bottom of epithelial cells.