Physiological, pathological, and therapeutic implications of zonulin-mediated intestinal barrier modulation: living life on the edge of the wall

Abstract

The anatomical and functional arrangement of the gastrointestinal tract suggests that this organ, beside its digestive and absorptive functions, regulates the trafficking of macromolecules between the environment and the host through a barrier mechanism. Under physiological circumstances, this trafficking is safeguarded by the competency of intercellular tight junctions, structures whose physiological modulation is mediated by, among others, the recently described protein zonulin. To prevent harm and minimize inflammation, the same paracellular pathway, in concert with the gut-associated lymphoid tissue and the neuroendocrine network, controls the equilibrium between tolerance and immunity to nonself antigens. The zonulin pathway has been exploited to deliver drugs, macromolecules, or vaccines that normally would not be absorbed through the gastrointestinal mucosal barrier. However, if the tightly regulated trafficking of macromolecules is jeopardized secondary to prolonged zonulin up-regulation, the excessive flow of nonself antigens in the intestinal submucosa can cause both intestinal and extraintestinal autoimmune disorders in genetically susceptible individuals. This new paradigm subverts traditional theories underlying the development of autoimmunity, which are based on molecular mimicry and/or the bystander effect, and suggests that the autoimmune process can be arrested if the interplay between genes and environmental triggers is prevented by re-establishing intestinal barrier competency. Understanding the role of zonulin-dependent intestinal barrier dysfunction in the pathogenesis of autoimmune diseases is an area of translational research that encompasses many fields.

Figures

Figure 1

Composition of intercellular TJs. TJs are composed by integral membrane proteins, including occludin, the claudin family, and JAM; the scaffold proteins, including ZO-1, ZO-2, ZO-3, symplekin, cingulin, and 7H6, and the cell cytoskeleton. This figure has been reused with permission from Advanced Drug Delivery Reviews (2004, 56:795–807).

Figure 2

Immunofluorescence microscopy of mouse small intestine (ileum) cross section. The confocal image shows the anatomical arrangement of epithelial cells (stained in red) and immune cells (dendritic cells stained in green, B cells stained in blue, and T cells stained in white). This arrangement suggests functional interactions between epithelial and immune cells for handling nonself antigens and microbiota present in the intestinal lumen. (Image provided by Marcello Chieppa, Lymphocyte Biology Section, Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health).

Figure 3

TLR. Currently, at least 10 different TLRs have been described, recognizing different ligands that activate the innate immune system. TLRs 3, 7, 8, and 9 are endosomal TLRs, whereas the remaining TLRs are surface, extracellular receptors.

Figure 4

Proposed zonulin intracellular signaling leading to the opening of intestinal TJ. Zonulin interacts with a specific surface receptor (1) whose distribution within the intestine varies. The protein then activates phospholipase C (2) that hydrolyzes phosphatidyl inositol (3) to release inositol 1,4,5-Tris phosphate (PPI-3) and diacylglycerol (DAG) (4). PKC-α is then activated (5), either directly (via DAG) (4) or through the release of intracellular Ca2+ (via PPI-3) (4a). Membrane-associated, activated PKC-α (6) catalyzes the phosphorylation of target protein(s), with subsequent polymerization of soluble G-actin in F-actin (7). This polymerization causes the rearrangement of the filaments of actin and the subsequent displacement of proteins (including ZO-1) from the junctional complex (8). As a result, intestinal TJs become looser (see freeze fracture electron microscopy). Once zonulin signaling is over, the TJs resume their baseline steady state. Portions of this figure have been reused with permission from Proceedings of the National Academy of Sciences U.S.A. (1991, 88:5242–5246) and Molecular Genetics and Metabolism (1998, 64:12–18).

Figure 5

Proposed role of abnormal intestinal permeability in the pathogenesis of CD. Gliadin and its immunomodulatory/inflammatory fragments are present in the intestinal lumen (1), inducing MyD88-dependent zonulin release (2) that causes opening of TJs (2a) and gliadin passage across the TJ barriers in patients with dysregulation of the zonulin system (2a). After TTG deamidation (3), gliadin peptides bind to HLA receptors present on the surface of antigen-presenting cells (APCs) (4). Alternatively, gliadin can act directly on APC (3a) causing MyD88-dependent release of both zonulin and cytokines (5). Gliadin peptides are also presented to T lymphocytes (6), followed by an aberrant immune response, both humoral (7) and cell-mediated (8) in genetically susceptible individuals. This interplay between innate and adaptive immunity is ultimately responsible for the autoimmune process targeting intestinal epithelial cells, leading to the intestinal damage typical of celiac disease (9). This figure has been redrawn with permission from Nature Clinical Practice Gastroenterology & Hepatology (2005, 2:416–422).