Increased intestinal permeability (IP) has emerged recently as a common underlying mechanism in the pathogenesis of allergic, inflammatory, and autoimmune diseases. The characterization of zonulin, the only physiological mediator known to regulate IP reversibly, has remained elusive. Through proteomic analysis of human sera, we have now identified human zonulin as the precursor for haptoglobin-2 (pre-HP2). Although mature HP is known to scavenge free hemoglobin (Hb) to inhibit its oxidative activity, no function has ever been ascribed to its uncleaved precursor form. We found that the single-chain zonulin contains an EGF-like motif that leads to transactivation of EGF receptor (EGFR) via proteinase-activated receptor 2 (PAR 2) activation. Activation of these 2 receptors was coupled to increased IP. The siRNA-induced silencing of PAR 2 or the use of PAR 2 ؊/؊ mice prevented loss of barrier integrity. Proteolytic cleavage of zonulin into its ␣2-and ␤-subunits neutralized its ability to both activate EGFR and increase IP. Quantitative gene expression revealed that zonulin is overexpressed in the intestinal mucosa of subjects with celiac disease. To our knowledge, this is the initial example of a molecule that exerts a biological activity in its precursor form that is distinct from the function of its mature form. Our results therefore characterize zonulin as a previously undescribed ligand that engages a key signalosome involved in the pathogenesis of human immunemediated diseases that can be targeted for therapeutic interventions.autoimmune diseases ͉ epidermal growth factor receptor ͉ gut permeability ͉ proteinase-activated receptor 2 ͉ celiac disease

Human granzyme B (GrB) released from cytotoxic lymphocytes plays a key role in the induction of target cell apoptosis when internalized in the presence of perforin. Here we demonstrate that GrB also possesses a potent extracellular matrix remodeling activity. Both native and recombinant GrB caused detachment of immortalized and transformed cell lines, primary endothelial cells, and chondrocytes. Cell detachment by GrB induced endothelial cell death (anoikis). GrB also inhibited tumor cell spreading, migration, and invasion in vitro. Investigation into the underlying mechanism revealed that GrB efficiently cleaves three proteins involved in extracellular matrix structure and function: vitronectin, fibronectin, and laminin. In vitronectin, GrB cleaves after an Arg-Lys-Asp (RGD) motif, which is part of the integrin-binding site found in matrix proteins. We propose that targeting of the integrin-extracellular matrix interface by GrB may allow perforin-independent killing of target cells via anoikis, restrict motility of tumor cells, facilitate lymphocyte migration, or directly reduce virus infectivity. It may also contribute to tissue destruction in diseases in which extracellular GrB is evident, such as rheumatoid arthritis and atherosclerosis.

The intestinal epithelium serves as a major protective barrier between the mammalian host and the external environment. Here we show that the transmembrane serine protease matriptase plays a pivotol role in the formation and integrity of the intestinal epithelial barrier. St14 hypomorphic mice, which have a 100-fold reduction in intestinal matriptase mRNA levels, display a 35% reduction in intestinal transepithelial electrical resistance (TEER). Matriptase is expressed during intestinal epithelial differentiation and colocalizes with E-cadherin to apical junctional complexes (AJC) in differentiated polarized Caco-2 monolayers. Inhibition of matriptase activity using a specific peptide inhibitor or by knockdown of matriptase by siRNA disrupts the development of TEER in barrierforming Caco-2 monolayers and increases paracellular permeability to macromolecular FITC-dextran. Loss of matriptase was associated with enhanced expression and incorporation of the permeabilityassociated, "leaky" tight junction protein claudin-2 at intercellular junctions. Knockdown of claudin-2 enhanced the development of TEER in matriptase-silenced Caco-2 monolayers, suggesting that the reduced barrier integrity was caused, at least in part, by an inability to regulate claudin-2 expression and incorporation into junctions. We find that matriptase enhances the rate of claudin-2 protein turnover, and that this is mediated indirectly through an atypical PKCζ-dependent signaling pathway. These results support a key role for matriptase in regulating intestinal epithelial barrier competence, and suggest an intriguing link between pericellular serine protease activity and tight junction assembly in polarized epithelia.claudin-2 | intestinal barrier | St14 | type II transmembrane serine protease | tight junction

Granzyme B (grB) is a serine proteinase released by cytotoxic lymphocytes (CLs) to kill abnormal cells. GrB-mediated apoptotic pathways are conserved in nucleated cells; hence, CLs require mechanisms to protect against ectopic or misdirected grB. The nucleocytoplasmic serpin, proteinase inhibitor 9 (PI-9), is a potent inhibitor of grB that protects cells from grB-mediated apoptosis in model systems. Here we show that PI-9 is present in CD4+ cells, CD8+ T cells, NK cells, and at lower levels in B cells and myeloid cells. PI-9 is up-regulated in response to grB production and degranulation, and associates with grB-containing granules in activated CTLs and NK cells. Intracellular complexes of PI-9 and grB are evident in NK cells, and overexpression of PI-9 enhances CTL potency, suggesting that cytoplasmic grB, which may threaten CL viability, is rapidly inactivated by PI-9. Because dendritic cells (DCs) acquire characteristics similar to those of target cells to activate naive CD8+ T cells and therefore may also require protection against grB, we investigated the expression of PI-9 in DCs. PI-9 is evident in thymic DCs (CD3−, CD4+, CD8−, CD45+), tonsillar DCs, and DC subsets purified from peripheral blood (CD16+ monocytes and CD123+ plasmacytoid DCs). Furthermore, PI-9 is expressed in monocyte-derived DCs and is up-regulated upon TNF-α-induced maturation of monocyte-derived DCs. In conclusion, the presence and subcellular localization of PI-9 in leukocytes and DCs are consistent with a protective role against ectopic or misdirected grB during an immune response.

Granzyme A (GrA) and granzyme B (GrB) play key roles in the induction of target cell death induced by cytotoxic lymphocytes. Whilst these roles have been extensively studied, it is becoming apparent that both granzymes also possess extracellular activities. Soluble granzymes are found extracellularly in normal plasma and are elevated in a number of diseases, ranging from viral and bacterial infections to autoimmune diseases. Here, we discuss the current knowledge of extracellular granzyme substrates, inhibitors and functions; and the pathological consequences of extracellular granzymes in disease. In addition, we provide new evidence for the role of glycosaminoglycan-binding sites of granzymes in extracellular matrix remodeling.