Compromised epithelial barrier function is considered an important pathophysiologic basis for IBD. The increased permeability enhances the transport of microbial flora and other antigenic material from the intestinal lumen into the submucosa, resulting in a recurring cycle of inflammation and mucosal injury (29
). A consequence of excessive activation of the immune system can be the weakening of countering mechanisms critically required to restore normal homeostasis. Here we show that matriptase, an enzyme critical for maintenance of epithelial homeostasis, is a key factor in the restoration and maintenance of intestinal barrier function after injury, and that its down-regulation by inflammatory cytokines generated by activation of the immune system contributes to perpetuation of disease severity and impaired mucosal recovery.
Mouse models of IBD have shown that a compromised epithelium is sufficient to cause intestinal inflammation, and that resident microbial flora is necessary for colitis induction (33
). An initiating event, whether genetic, immune or environmental that enhances barrier permeability can initiate activation of the innate immune system given the exceptional challenge provided by the dense microbial flora unique to the gut. Indeed, complete genetic deficiency of matriptase abrogates epithelial barrier function resulting in acute and severe inflammation along with massive tissue destruction (10
). Although the minimal levels of matriptase expressed by St14
hypomorphic mice weaken barrier function, they do not result in spontaneous disease in the absence of an initiating trigger. Thus the ST14 hypomorph represents a pathologically relevant model of disease susceptibility in patients who have increased intestinal epithelial permeability and are at risk of developing disease.
In the DSS model of colitis, an initiating trigger, e.g.
exposure to DSS, causes damage to the epithelial layer and initiates activation of innate immune responses (25
). Persistent inflammation associated with DSS injury in the ST14 hypomorphic mice was not caused by a more robust inflammatory response but was related to the inability to recover barrier function after the DSS insult was removed. Matriptase down-regulation occurs secondary to the inflammatory response, and in control mice, restoration of normal matriptase levels during the recovery phase promotes colitis resolution. In St14
hypomorphic mice, the inability to restore normal matriptase levels prevents gut barrier recovery and perpetuates the destructive inflammation. These data highlight the critical role of host barrier protective mechanisms in the resolution of colitis, which can become suppressed by excessive activation of the immune system that drives tissue damage.
The specific molecular pathways initiated by matriptase activity in the intestinal epithelium are incompletely understood (11
). Matriptase colocalizes with E-cadherin to apical junctional complexes and evidence suggests that it is required for inter-epithelial junction formation (10
). Matriptase has also been implicated in the control of epithelial-cell turnover by regulating cell-cell and/or cell-substratum adhesions (35
), and in the removal of aged epithelial cells in the small intestine through detachment from the basement membrane component laminin (36
). Substrates that are targeted by matriptase proteolytic activities in other cell systems (37
), pro-urokinase-type plasminogen activator (pro-uPA) and protease activated receptor-2 (PAR-2) do not appear to mediate matriptase intestinal barrier protective activities, since mice deficient in uPA or PAR-2 do not show similar susceptibility to DSS-induced colitis (Figure S6
). Altered pericellular activation of HGF may be involved in this phenotype since matriptase is an activator of pro-HGF (38
) and HGF/c-met signaling is important for repair of injured mucosa (39
). Interestingly, conditional knockout of HAI-1 in the intestine of mice was shown recently to enhance susceptibility to DSS-induced colitis, possibly caused in part by deregulated matriptase activity (41
Increased inflammatory cytokine production is associated with mucosal inflammation, and studies in vitro
and in animal models demonstrate that inflammatory cytokines associated with IBD cause tight junction barrier dysfunction (42
). Matriptase is down-regulated in inflamed intestinal epithelia of IBD patients (), and by the inflammatory cytokines IL-13 and IL-4 in in vitro
cultures (). IL-13 enhances barrier permeability in part by stimulating the synthesis of claudin-2 (20
) and we have shown that matriptase deficiency results in increased claudin-2 expression in both Caco-2 monolayers and in St14
hypomorph intestinal tissues (11
). Endogenous mechanisms that regulate matriptase expression and activation in the gut are not currently known. Nonetheless, in vitro
data suggest that restoration of matriptase expression and activity could interrupt the cytokine-mediated inflammatory cycle to promote epithelial repair and disease resolution.
While dysregulated inflammation induced by immunogen exposure is considered a major cause of mucosal tissue damage and injury in IBD, the present findings highlight the critical importance of host barrier repair mechanisms for restriction of the offending insult and its destructive consequences. Intestinal permeability has been shown to predict and possibly cause relapse in patients suffering from IBD (3
). A better understanding of the role of matriptase in controlling mucosal homeostasis by regulating the integrity and permeability of epithelial barrier function will be important since matriptase-based therapeutic strategies could have application for preventing, treating or altering the natural progression of IBD.