Intestinal permeability and its regulation by zonulin: diagnostic and therapeutic implications

Abstract

One of the most important and overlooked functions of the gastrointestinal tract is to provide a dynamic barrier to tightly controlled antigen trafficking through both the transcellular and paracellular pathways. Intercellular tight junctions (TJ) are the key structures regulating paracellular trafficking of macromolecules. Although steady progress has been made in understanding TJ ultrastructure, relatively little is known about their pathophysiological regulation. Our discovery of zonulin, the only known physiological modulator of intercellular TJ described so far, increased understanding of the intricate mechanisms that regulate gut permeability and led us to appreciate that its up-regulation in genetically susceptible individuals may lead to immune-mediated diseases. This information has translational implications, because the zonulin pathway is currently exploited to develop both diagnostic and therapeutic applications pertinent to a variety of immune-mediated diseases.

Figure 1

Schematic representation of the zonulin mechanism of action. A. Resting state: During the resting state TJ proteins are angeged in both homophilic and heterophilic protein-protein interactions that keep TJ in a competent state (TJ) closed as reflected by the complexity of TJ meshwork shown in the freeze fracture electron microscopy photograph. B. Following zonulin pathway activation: Zonulin transactivates EGFR through PAR₂ (1). The protein then activates phospholipase C (2) that hydrolyzes phosphatidyl inositol (PPI) (3) to release inositol 1,4,5-tris hosphate (IP-3) and diacylglycerol (DAG) (4). PKCα is then activated (5), either directly (via DAG) (4) or through the release of intracellular Ca²⁺ (via IP-3) (4a). Membrane-associated, activated PKCα (6) catalyzes the phosphorylation of target protein(s), including ZO1 and myosin 1C, as well as polymerization of soluble G-actin in F-actin (7). The combination of tight junction protein phosphorylation and actin 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 TJ become looser (see freeze fracture electron microscopy). Once the zonulin signaling is over, the TJ resume their baseline steady state.