**3. Epithelial barrier**

The epithelial barrier is a fence composed by intercellular structures termed tight junctions, located at the apical border between gastric epithelial cells, formed by four different transmembrane proteins [occludin, claudins, junction-adhesionmolecules, and CAR –Coxsackievirus and Adenovirus Receptor- proteins] anchored to actin filaments and myosin light chains (MLC) by the actin cytoskeleton and linker proteins zonula occludens ZO-1, ZO-2 and ZO-3 which are members of the membrane-associated guanylate kinase cytoplasmic adaptors. Other highly important members of the barrier are the Adherens Junctions, the Desmosome, the Gap junctions and the Hemidesmosomes. Occludin and claudins interact with adjacent cells through their extracellular loops, whereas JAMs and CAR contain extracellular IgG-like domains [15, 16]. Different proteins form the regulatory complex (Rac, Cdc42, Par3, Par6, PKC). **Figure 2** shows the structural conformation of tight junctions1 . Claudins, a family of 27 different proteins, are essential to establish and maintain the barrier function as they regulate paracellular permeability [18] whereas occludin is important for epithelial differentiation but not for establishing

<sup>1</sup> A profound review of the gastric epithelal barrier can be found at Tegtmeyer and Backert [17].

*Effect of* Helicobacter pylori *on Tight Junctions in Gastric Epithelia DOI: http://dx.doi.org/10.5772/intechopen.96607*

**Figure 2.** *Gastric epithelia tight junction structure.*

the barrier [19]. Paracellular transport across the tight junctions is achieved through the leak pathway which is size-dependent and/or the pore pathway which is size and charge-dependent; size-dependance enables transportation of proteins and lipopolysaccharides and it is controlled by MLC kinase and occludin [20] whereas the pore pathway, controlled by claudins, enables the permeability of cations and anions across different epithelia and exclude molecules larger than 4A [21].

Claudins are responsible for watertight stability and transit of cations and anions. Claudins expression and regulation is tissue specific and their physiological and regulatory function varies according to the organ where they are being expressed [22, 23]. As an example, claudin-4 in ovarian cancer has a proangiogenic function whereas in pancreatic cancer it suppresses invasion [24, 25]. The expression of claudins is dysregulated in various cancers, and in gastric tissue the expression of claudin-1, −4, −6 and − 17 is modified when cancer develops but many other claudins such as −3, −5, −7 and − 18 have also been implicated; the loss or gain of claudins is linked to inflammation and inflammatory cytokines such as IFNy, IL-1, IL-6, IL-10, IL-17, IL-22, EGF, TGFb and TNF [26], as well as to several malignancies, drugs, antibiotics, toxins, pesticides, chemicals, microbiota imbalance and stress [27]. The integrity or modifications in tight junctions that affect claudin distribution is via the MAPK/ERK1/2 pathway [28–30]. It has been postulated that in *G. lamblia* infection the loss of epithelial barrier function could be caspase-3 dependent [31] but it does not seem the case in *H. pylori* infection.

The effect of the secretory molecules released by of *H. pylori* known to affect gastric mucosa tight junctions is discussed.

## **4. VacA**

Amongst the major toxins that *H. pylori* possesses, the vacuolating cytotoxin A (VacA) contributes to host-pathogen interactions. After the 140 kDa VacA protein is translated, an active toxin of 88 kDa emerges after cleavage [32]. The toxin is conformed by two domains and three distinct segments: the signal region with

**Figure 3.** *Organization of VacA p88 protein. From Su et al. [33] and Foegeding et al. [34].*

two allelic variations (s1, s2), the intermediate region, and the mid-region with two alleles (m1, m2) (**Figure 3**) [35, 36]; mosaicism has been reported for all the alleles (s1a, s1b, m1a, m1b) [34]. The relevance of these toxin components lays in the fact that s1 causes vacuolation of mammalian cells whereas s2 do not [37]; the discrepancy may be attributed to differences in channel-forming properties [38]. The combination of different VacA alleles is associated with more virulent strains and severe gastric disease: s1a/m2 strains are found in 87.5% of patients with peptic ulcer and in 93% of patients with gastric carcinoma [39]; other highly pathogenic associations include s1a/m1b, s1b/m1b, and s2/m2 [33]. VacA is involved in bacterial colonization of epithelial cells of the gastric mucosa via formation of low conductance membrane pores that are selective for anions over cations [40], and the induction of vacuole formation [41]. These vacuoles, once inside the epithelial cells, alter the transepithelial resistance but do not alter the localization or abundance of ZO-1 and occludin [42]. VacA exert other effects, mainly: endosomal, mitochondrial and epithelial barrier alterations, autophagy, atypical cell signaling and induction of apoptosis in epithelial cells [34]. AGS cells treated with *H. pylori* culture supernatants show rearrangement and disruption of the actin cytoskeleton due to a lack of actin stress fibers; these changes were not VacA dependent [43].
