Remodeling the Host Environment: Modulation of the Gastric Epithelium by the Helicobacter pylori vacuolating toxin (VacA)

REVIEW ARTICLE published: 27 March 2012 doi: 10.3389/fcimb.2012.00037 CELLULAR AND INFECTION MICROBIOLOGY Remodeling the host environment: modulation of the gastric epithelium by the Helicobacter pylori vacuolating toxin (VacA) Ik-Jung Kim and Steven R. Blanke* Department of Microbiology, Institute for Genomic Biology, University of Illinois, Urbana, IL, USA Edited by: D. Scott Merrell, Uniformed Services University, USA Reviewed by: Richard Peek, Vanderbilt University Medical Center, USA Timothy Cover, Vanderbilt University School of Medicine, USA *Correspondence: Steven R. Blanke, Department of Microbiology, Institute for Genomic Biology, University of Illinois, 302A Burrill Hall, 601 S. Goodwin Ave., Urbana, IL 61801, USA. e-mail: Virulence mechanisms underlying Helicobacter pylori persistence and disease remain poorly understood, in part, because the factors underlying disease risk are multifactorial and complex. Among the bacterial factors that contribute to the cumulative pathophysiology associated with H. pylori infections, the vacuolating cytotoxin (VacA) is one of the most important. Analogous to a number of H. pylori genes, the vacA gene exhibits allelic mosaicism, and human epidemiological studies have revealed that several families of toxin alleles are predictive of more severe disease. Animal model studies suggest that VacA may contribute to pathogenesis in several ways. VacA functions as an intracellular-acting protein exotoxin. However, VacA does not fit the current prototype of AB intracellular-acting bacterial toxins, which elaborate modulatory effects through the action of an enzymatic domain translocated inside host cells. Rather, VacA may represent an alternative prototype for AB intracellular acting toxins that modulate cellular homeostasis by forming ion-conducting intracellular membrane channels. Although VacA seems to form channels in several different membranes, one of the most important target sites is the mitochondrial inner membrane. VacA apparently take advantage of an unusual intracellular trafficking pathway to mitochondria, where the toxin is imported and depolarizes the inner membrane to disrupt mitochondrial dynamics and cellular energy homeostasis as a mechanism for engaging the apoptotic machinery within host cells. VacA remodeling of the gastric environment appears to be fine-tuned through the action of the Type IV effector protein CagA which, in part, limits the cytotoxic effects of VacA in cells colonized by H. pylori. Keywords: Helicobacter pylori, VacA, vacuolation, mitochondria, apoptosis Helicobacter pylori AND VacA Virulence mechanisms underlying Helicobacter pylori-mediated gastric maladies have remained enigmatic, in part, because disease risk is multifactorial and complex (Compare et al., 2010; Polk and Peek, 2010). However, several bacterial factors clearly contribute to the pathophysiology associated with H. pylori infections (Fischer et al., 2009), and one of the most important is the vacuolating cytotoxin (VacA) (Cover and Blanke, 2005). Since the discovery of VacA nearly 25 years ago as the proteinacious factor within H. pylori culture filtrates that intoxicates epithelial cells and induces vacuole biogenesis (Leunk et al., 1988), the study of this toxin has been challenging in part, because the toxin possesses a number of surprising and unusual characteristics that don’t fit neatly into current concepts of bacterial toxins. Nonetheless, several interesting and important properties of VacA have become apparent. First, the gene encoding VacA (vacA) is characterized by a high degree of genetic variation; strains with specific allelic variants of vacA that exhibit greater levels of VacAmediated cytotoxic activity in vitro are associated with a greater risk of gastric disease in H. pylori-infected humans. Moreover, experimental evidence supports the idea that VacA may promote Frontiers in Cellular and Infection Microbiology H. pylori colonization, persistence, and infection-associated disease pathophysiology. STRUCTURAL PROPERTIES OF VacA H. pylori synthesize VacA as an approximately 140 kDa preprotoxin (Figure 1), which undergoes sequential proteolytic processing during Type Va secretion as an auto-transporter protein (Fischer et al., 2001). The secreted mature form of VacA is a 88 kDa monomer (Cover and Blaser, 1992), that is purified from H. pylori growth medium (Gonzalez-Rivera et al., 2010) as watersoluble hexameric or heptameric rings (Figure 1) in single or bilayered structures (Figure 1) (Lupetti et al., 1996; Cover et al., 1997; Lanzavecchia et al., 1998; Czajkowsky et al., 1999; Adrian et al., 2002; El-Bez et al., 2005). Acidic or alkaline pH promotes dissociation of VacA oligomeric complexes into monomers (Cover et al., 1997; Molinari et al., 1997; Yahiro et al., 1997), which is likely the form of the toxin to bind host cells during infection (Gonzalez-Rivera et al., 2010). The mature 88 kDa form of the toxin is sometimes detected as a proteolytically-nicked protein, comprising two domains designated p33 (residues 1–311) and p55 (residues 312–821), that www.frontiersin.org March 2012 | Volume 2 | Article 37 | 1 Kim and Blanke The unusual properties of VacA B A -33 1 p33 sufficient for channel activity 311 312 -33 1 1 hydrophobic region 32 422 s2 s1 i1 223 p55 vacuolation ++ 190 minimal vacuolating domain (1-422) top view C signal peptide + i2 i3 biochemical function unknown p55 vacuolation ++ + p33 p33 n.d. p33 509 p55 m1 m2 host cell tropism p55 vacuolation (AZ-521, HeLa cells) ++ p33 p33 p33 p55 p55 752 821 822 p55 - p55 AT forms autotransporter channel p33 p33 p55 side/cross-sectional view 1287 FIGURE 1 | VacA structure. (A) Schematic VacA structure. Each domain is denoted by a different color and by the first and last amino residue of that particular domain. The name of each domain is denoted in bold, and its function (if known) is described. (B) The polymorphic nature of the vacA gene is emphasized by highlighting the three major allele families, which are located in the signal region (s region), the intermediate region (i region), and the mid-region (m region). (C) The proposed structure of the VacA oligomeric assembly, based on the crystal structure of a portion of p55 [Gangwer et al. (2007)] and electron microscopy imaging of VacA oligomers [El-Bez et al. (2005)]. remain non-covalently associated (Figure 1A) (Telford et al., 1994; Cover et al., 1997; Ye et al., 1999; Nguyen et al., 2001; Willhite et al., 2002; Torres et al., 2004, 2005). Recently, a crystal has been solved for p55 (Gangwer et al., 2007), revealing a predominantly right-handed parallel beta-helix structure, which is typical for autotransporter passenger domains. High-resolution structural data for p33 are not yet available. Proteolytic cleavage into discrete functional domains is a characteristic of a number of so-called intracellular-acting AB toxins (Blanke, 2005). Howe (...truncated)