Intestinal epithelial tight junction barrier regulation by autophagy-related protein ATG6/beclin 1

A defective tight junction (TJ) barrier is a key pathogenic factor for inflammatory bowel disease. Previously, we have shown that autophagy, a cell survival mechanism, enhances intestinal epithelial TJ barrier function. Autophagy-related protein-6 (ATG6/beclin 1), a key protein in the autophagy pathway, also plays a role in the endocytic pathway. The constitutive role of beclin 1 in the intestinal TJ barrier is not known. In Caco-2 cells, beclin 1 was found to be coimmunoprecipitated with the TJ protein occludin and colocalized with occludin on the membrane. Treatment of Caco-2 cells with beclin 1 peptide [transactivating regulatory protein (Tat)-beclin 1] reduced TJ barrier function. Activation of beclin 1 increased occludin endocytosis and reduced total occludin protein level. In contrast, beclin 1 siRNA transfection enhanced Caco-2 TJ barrier function. In pharmacologic and genetic autophagy inhibition studies, the constitutive function of beclin 1 in the TJ barrier was found to be autophagy independent. However, de novo induction of autophagy with starvation or rapamycin prevented Tat-beclin 1-induced increase in TJ permeability and reduction in occludin level. Induction of autophagy also resulted in reduced beclin 1-occludin association. In mouse colon, beclin 1 colocalized with occludin on the epithelial membrane. Perfusion of mouse colon with beclin 1 peptide caused an increase in colonic TJ permeability that was prevented by in vivo induction of autophagy. These findings show that beclin 1 plays a constitutive, autophagy-independent role in the regulation of intestinal TJ barrier function via endocytosis of occludin. Autophagy terminates constitutive beclin 1 function in the TJ barrier and enhances the TJ barrier.</jats:p

Matrix Metalloproteinase MMP-12 Promotes Macrophage Transmigration Across Intestinal Epithelial Tight Junctions and Increases Severity of Experimental Colitis

Abstract Background and Aims Matrix metalloproteinases [MMPs] play an important role in extracellular matrix regulation during cell growth and wound healing. Increased expression of MMP-12 [human macrophage elastase] has been reported in inflammatory bowel disease [IBD] which is characterised by the loss of epithelial tight junction [TJ] barrier function and an excessive inflammatory response. The aim of this study was to investigate the role of MMP-12 in intestinal TJ barrier function and inflammation. Methods Wild type [WT] and MMP-12-/- mice were subjected to experimental acute or chronic dextran sodium sulphate [DSS] colitis. The mouse colonic permeability was measured in vivo by recycling perfusion of the entire colon and ex vivo by Ussing chamber studies. Results DSS administration increased colonic permeability through modulation of TJ proteins and also increased MMP-12 expression in the colonic mucosa of WT mice. The acute as well as chronic DSS-induced increase in colonic TJ permeability and the severity of DSS colitis was found to be markedly attenuated in MMP-12-/- mice. The resistance of MMP-12-/- mice to DSS colitis was characterised by reduced macrophage infiltration and transmigration, and reduced basement membrane laminin degradation. Further in vitro and in vivo studies show that macrophage transmigration across the epithelial layer is MMP-12 dependent and the epithelial TJ barrier is compromised during macrophage transmigration. Conclusions: Together, these data demonstrate that MMP-12 mediated degradation of basement membrane laminin, macrophage transmigration, and associated loss of intestinal TJ barrier are key pathogenic factors for intestinal inflammation. </jats:sec

Autophagy Increases Occludin Levels to Enhance Intestinal Paracellular Tight Junction Barrier

AbstractBackground and AimFunctional loss of paracellular tight junction (TJ) barrier of the gut epithelium and mutations in autophagy genes are factors potentiating inflammatory bowel disease (IBD). Previously we showed the role of autophagy in enhancing the TJ barrier via claudin-2 degradation, however, its role in the regulation of the barrier-forming protein occludin remains unknown. Here, we investigate the role of autophagy in the regulation of occludin and its role in inflammation-mediated TJ barrier loss.MethodsPharmacological and genetic tools were used to study the effect of autophagy on occludin levels and localization, and the role of the MAPK pathway.ResultsAutophagy induction using pharmacological activators and nutrient starvation increased total occludin levels in different epithelial cells. Starvation enriched membrane occludin levels and reduced paracellular inulin flux in Caco-2 cells. Starvation-induced TJ barrier enhancement was contingent on the presence of occludin as OCLN-/- nullified its TJ barrier enhancing effect. Autophagy inhibited the constitutive degradation of occludin and protected against inflammation-induced TJ barrier loss. Starvation-induced TJ barrier enhancement was prevented by inhibition of autophagy. Autophagy enhanced the phosphorylation of ERK-1/2. Inhibition of these kinases in Caco-2 cells and human intestinal mucosa inhibited the protective effects of autophagy. In-vivo, autophagy induction by rapamycin increased occludin levels in mouse intestines and protected against LPS and TNF-α-induced TJ barrier loss. Additionally, acute Atg7 knockout in adult mice decreased intestinal occludin levels, increasing baseline colonic TJ-permeability and exacerbating the effect of DSS-induced colitis.ConclusionOur data suggest a novel role of autophagy in promoting the intestinal TJ barrier by increasing occludin levels in an ERK1/2 MAPK-dependent mechanism.Graphical abstract</jats:sec