Epithelial Myosin Light Chain Kinase Research Articles

ADT-OH improves intestinal barrier function and remodels the gut microbiota in DSS-induced colitis.

Owing to the increasing incidence and prevalence of inflammatory bowel disease (IBD) worldwide, effective and safe treatments for IBD are urgently needed. Hydrogen sulfide (H2S) is an endogenous gasotransmitter and plays an important role in inflammation. To date, H2S-releasing agents are viewed as potential anti-inflammatory drugs. The slow-releasing H2S donor 5-(4-hydroxyphenyl)-3H-1,2-dithiole-3-thione (ADT-OH), known as a potent therapeutic with chemopreventive and cytoprotective properties, has received attention recently. Here, we reported its anti-inflammatory effects on dextran sodium sulfate (DSS)-induced acute (7 days) and chronic (30 days) colitis. We found that ADT-OH effectively reduced the DSS-colitis clinical score and reversed the inflammation-induced shortening of colon length. Moreover, ADT-OH reduced intestinal inflammation by suppressing the nuclear factor kappa-B pathway. In vivo and in vitro results showed that ADT-OH decreased intestinal permeability by increasing the expression of zonula occludens-1 and occludin and blocking increases in myosin II regulatory light chain phosphorylation and epithelial myosin light chain kinase protein expression levels. In addition, ADT-OH restored intestinal microbiota dysbiosis characterized by the significantly increased abundance of Muribaculaceae and Alistipes and markedly decreased abundance of Helicobacter, Mucispirillum, Parasutterella, and Desulfovibrio. Transplanting ADT-OH-modulated microbiota can alleviate DSS-induced colitis and negatively regulate the expression of local and systemic proinflammatory cytokines. Collectively, ADT-OH is safe without any short-term (5 days) or long-term (30 days) toxicological adverse effects and can be used as an alternative therapeutic agent for IBD treatment.

Transepithelial barrier dysfunction drives microbiota dysbiosis to initiate epithelial clock-driven inflammation.

Factors that contribute to inflammatory bowel disease (IBD) pathogenesis include genetic polymorphisms, barrier loss, and microbial dysbiosis. A major knowledge gap exists in the origins of colitogenic microbiome and its relationship with barrier impairment. Epithelial myosin light chain kinase (MLCK) is a critical regulator of the paracellular barrier, but the effects of MLCK activation on intraepithelial bacteria (IEB) and dysbiosis are incompletely understood. We hypothesize that MLCK-dependent bacterial endocytosis promotes pathobiont conversion and shapes a colitogenic microbiome. To explore this, transgenic (Tg) mice with barrier loss induced by intestinal epithelium-specific expression of a constitutively active MLCK were compared to wild-type (WT) mice. When progeny of homozygous MLCK-Tg mice were separated postweaning by genotype (Tg/Tg, Tg/WT, WT/WT), increased IEB numbers associated with dysbiosis and more severe colitis were present in Tg/Tg and Tg/WT mice, relative to WT/WT mice. Cohousing with MLCK-Tg mice induced dysbiosis, increased IEB abundance and exacerbated colitis in WT mice. Conversely, MLCK-Tg mice colonized with WT microbiota at birth displayed increased Escherichia abundance and greater colitis severity by 6 weeks of age. Microarray analysis revealed circadian rhythm disruption in WT mice co-housed with MLCK-Tg mice relative to WT mice housed only with WT mice. This circadian disruption required Rac1/STAT3-dependent microbial invasion but not MLCK activity, and resulted in increased proinflammatory cytokines and glucocorticoid downregulation. In summary, the data demonstrate that barrier dysfunction induces dysbiosis and expansion of invasive microbes that lead to circadian disruption and mucosal inflammation. These results suggest that barrier-protective or bacterium-targeted precision medicine approaches may be of benefit to IBD patients.

Molecular mechanism of the TGF‑β/Smad7 signaling pathway in ulcerative colitis.

Aberrant TGF-β/Smad7 signaling has been reported to be an important mechanism underlying the pathogenesis of ulcerative colitis. Therefore, the present study aimed to investigate the effects of a number of potential anti-colitis agents on intestinal epithelial permeability and the TGF-β/Smad7 signaling pathway in an experimental model of colitis. A mouse model of colitis was first established before anti-TNF-α and 5-aminosalicyclic acid (5-ASA) were administered intraperitoneally and orally, respectively. Myeloperoxidase (MPO) activity, histological index (HI) of the colon and the disease activity index (DAI) scores were then detected in each mouse. Transmission electron microscopy (TEM), immunohistochemical and functional tests, including Evans blue (EB) and FITC-dextran (FD-4) staining, were used to evaluate intestinal mucosal permeability. The expression of epithelial phenotype markers E-cadherin, occludin, zona occludens (ZO-1), TGF-β and Smad7 were measured. In addition, epithelial myosin light chain kinase (MLCK) expression and activity were measured. Anti-TNF-α and 5-ASA treatments was both found to effectively reduce the DAI score and HI, whilst decreasing colonic MPO activity, plasma levels of FD-4 and EB permeation of the intestine. Furthermore, anti-TNF-α and 5-ASA treatments decreased MLCK expression and activity, reduced the expression of Smad7 in the small intestine epithelium, but increased the expression of TGF-β. In mice with colitis, TEM revealed partial epithelial injury in the ileum, where the number of intercellular tight junctions and the expression levels of E-cadherin, ZO-1 and occludin were decreased, all of which were alleviated by anti-TNF-α and 5-ASA treatment. In conclusion, anti-TNF-α and 5-ASA both exerted protective effects on intestinal epithelial permeability in an experimental mouse model of colitis. The underlying mechanism may be mediated at least in part by the increase in TGF-β expression and/or the reduction in Smad7 expression, which can inhibit epithelial MLCK activity and in turn reduce mucosal permeability during the pathogenesis of ulcerative colitis.

Myosin light chain kinase mediates intestinal barrier dysfunction following simulated microgravity based on proteomic strategy

Microgravity induces injury of intestinal barrier. However, the underlying mechanism remains unclear. The present study aimed to investigate the pathological change of intestinal mucosa induced by long term simulated microgravity and to explore its etiological mechanism using a proteomic approach. The well accepted tail-suspended rat model was used to simulate microgravity. The damage of rat small intestine was evaluated via histological and molecular test, and a label-free comparative proteomic strategy was used to determine the molecular mechanism. Simulated microgravity for 21 days damaged intestine barrier with decreased numbers of the goblet cells, large intercellular space, and down-regulated adhesion molecules, accompanied by increased intestinal permeability. Proteomic analysis identified 416 differentially expressed proteins and showed simulated microgravity dramatically down-regulated the adhesion molecules and deteriorated several pathways for metabolism, focal adhesion, and regulation of actin cytoskeleton. Western-blot analysis confirmed that myosin regulatory light chain (MLC) 12B was significantly down-regulated, while rho-associated protein kinase, myosin light chain kinase (MLCK), and phosphorylated MLC were dramatically up-regulated. Taken together, these data reveal that down-regulation of adhesion molecules and MLCK dependent up-regulation MLC phosphorylation mediate intestinal barrier dysfunction during simulated microgravity injury. Our results also indicate that regulation of epithelial MLCK is a potential target for the therapeutic treatment of microgravity injury.

Complementary Roles of Nod2 in Hematopoietic and Nonhematopoietic Cells in Preventing Gut Barrier Dysfunction Dependent on MLCK Activity

Crohn's disease (CD) pathogenesis is multifactorial involving genetic and environmental factors. Loss of function mutations in the nucleotide oligomerization domain 2 (NOD2) gene are the main genetic risk factor for CD. Like patients with CD, Nod2 mice are characterized by an enhanced Th1 immune response and a defective mucosal barrier function evidenced by increased intestinal permeability. We previously showed that the latter is related to hematopoietic Nod2 deficiency. Our aim was to explore the mechanisms by which Nod2 expressed in the hematopoietic and in the nonhematopoietic compartments interplay to control epithelial paracellular permeability. Depletion of CD4 T cells in Nod2 mice and treatments with inhibitors were conducted in chimeric mice transplanted with bone marrow cells from Nod2-deficient donors into Nod2-sufficient recipients or vice versa. Caco-2 cells overexpressing a NOD2 gene which did or did not include a CD-associated polymorphism were treated with inhibitors or siRNAs and cocultured with hematopoietic cells from Peyer's patches. In vivo and in vitro Nod2 in hematopoietic cells regulates epithelial paracellular permeability through cytokine production influencing myosin light chain kinase (MLCK) activity. Indeed, tumor necrosis factor-α and interferon-γ secretion by CD4 T cells upregulated expression and activity of epithelial MLCK leading to increased epithelial tight junction opening. When stimulated by muramyl dipeptide, Nod2 in the nonhematopoietic compartment normalized the permeability and T-cell cytokine secretion and regulated MLCK activity. This MLCK regulation is mediated by TAK1 and RICK-dependent mechanisms. Our study demonstrates how hematopoietic and nonhematopoietic Nod2 regulate intestinal barrier function, improving our knowledge on the mechanisms involved in CD pathogenesis.

Epithelial Nuclear Factor-κB Activation in Inflammatory Bowel Diseases and Colitis-Associated Carcinogenesis

Prolonged inflammatory bowel diseases (IBD) may lead to colitis-associated carcinogenesis (CAC). Previous studies had shown that nuclear factor-κB (NF-κB) activation in both macrophages and epithelia in inflamed colonic tissue is associated with CAC development. However, the mechanism by which epithelial NF-κB activation leading to CAC development had not previously been rigorously studied. We and others had observed the increased expression of the type 2 receptor for tumor necrosis factor (TNFR2/TNFRSF1b/p75) in IBD models. Myosin light chain kinase (MLCK) is suggested to be associated with epithelial permeability via TNF signaling. Therefore, the relationship between epithelial MLCK expression and NF-κB activation via TNFR2 signaling on CAC development was investigated. Pro-tumorigenic cytokines such as interleukin (IL)-1ß, IL-6 and macrophage inflammatory protein-2 at the lamina propria were increased in the setting of colitis and further increased in tumor tissues with upregulated epithelial TNFR2 and MLCK expressions in an animal model of CAC. The upregulated MLCK expression was also observed in TNF-stimulated colonic epithelial cells in vitro in association with the upregulation of TNFR2 but not TNFR1/TNFRSF1a/p55. Gene silencing of tnfrsf1b, but not tnfrsf1a, resulted in restoration of epithelial tight junction (TJ) associated with decreased MLCK expression. The presence of anti-TNF antibody also resulted in restoration of TJ in association with suppressed MLCK expression, and interestingly, similar results including the suppressed TNFR2 and MLCK expressions were observed by inhibiting MLCK in the epithelial cells. MLCK silencing also led to suppressed TNFR2 expression, suggesting that the restored TJ leads to reduced TNFR2 signaling. Such suppression of MLCK as well as blockade of TNFR2 signaling resulted in reduced CAC development, restored TJ, and decreased pro-tumorigenic cytokines. These imply that TNF-induced NF-κB activation and MLCK expression may be a potential target for the prevention of IBD-associated carcinogenesis.

Beyond Ussing's chambers: contemporary thoughts on integration of transepithelial transport.

In the mid-20th century, Hans Ussing developed a chamber that allowed for the simultaneous measurement of current and labeled probe flux across epithelia. Using frog skin as a model, Ussing used his results to propose mechanisms of transcellular Na(+) and K(+) transport across apical (exterior/luminal) and basolateral (interior) membranes that is essentially unchanged today. Others took advantage of Ussing's chambers to study mucosal tissues, including bladder and intestines. It quickly became clear that, in some tissues, passive paracellular flux, i.e., across the tight junction, was an important component of overall transepithelial transport. Subsequent work demonstrated that activation of the apical Na(+)-glucose cotransporter SGLT1 regulated paracellular permeability such that intestinal paracellular transport could coordinate with and amplify transcellular transport. Intermediates in this process include activation of p38 MAPK, the apical Na(+)/H(+) exchanger NHE3, and myosin light chain kinase (MLCK). Investigators then focused on these processes in disease. They found that TNF induces barrier dysfunction via MLCK activation and downstream caveolin-1-dependent endocytosis of the tight junction protein occludin. TNF also inhibited NHE3, and both barrier loss and PKCα-dependent NHE3 inhibition were required for TNF-induced acute diarrhea, emphasizing the interplay between transcellular and paracellular transport. Finally, studies using immune-mediated inflammatory bowel disease models showed that mice lacking epithelial MLCK were initially protected, but became ill as epithelial damage progressed and provided a tight junction-independent means of barrier loss. None of these advances would have been possible without the insights provided by Ussing and others using Ussing's ingenious, and still useful, chambers.

Myosin Light Chain Kinase Inhibitor Inhibits Dextran Sulfate Sodium-Induced Colitis in Mice

Myosin light chain kinase (MLCK) plays a central role in the mechanisms of barrier dysfunction, and intestinal epithelial MLCK protein expression is upregulated in active ulcerative colitis (UC). ML-7, a MLCK inhibitor, has been used in many MLCK studies. However, the effect of ML-7 has never been estimated in colitis models. The aim of this study was to determine whether ML-7 can treat UC. Experimental colitis was induced and ML-7 was administered by intraperitoneal injection. The disease activity index (DAI) scores were evaluated and colon tissue was collected for the assessment of histological changes, myeloperoxidase (MPO) activity, and tumor necrosis factor (TNF)-α, interferon (IFN)-γ, interleukin (IL)-13 and interleukin (IL)-17 levels. The small intestinal mucosa was ultrastructurally examined, epithelial MLCK protein expression and enzymatic activity were determined, and intestinal permeability was assayed using FITC-dextran 4000 (FD-4) and Evans blue (EB). ML-7 was found to be significantly effective in reducing the DAI scores and histological index scores, and decreasing MPO activity and TNF-α, IFN-γ, IL-13 and IL-17 levels. The small intestinal epithelial MLCK protein expression and enzymatic activity were downregulated by ML-7. The epithelial cells and intercellular tight junctions were ameliorated, and the amount of FD-4 in blood and EB permeating into the intestine were decreased by ML-7 in colitis mice. ML-7 has a significant anti-colitis effect in colitis mice. It is mainly associated with the inhibition of the epithelial MLCK protein expression, resulting in ameliorated intestinal mucosal permeability.

Enterocyte STAT5 promotes mucosal wound healing via suppression of myosin light chain kinase‐mediated loss of barrier function and inflammation

Epithelial myosin light chain kinase (MLCK)-dependent barrier dysfunction contributes to the pathogenesis of inflammatory bowel diseases (IBD). We reported that epithelial GM-CSF–STAT5 signalling is essential for intestinal homeostatic response to gut injury. However, mechanism, redundancy by STAT5 or cell types involved remained foggy. We here generated intestinal epithelial cell (IEC)-specific STAT5 knockout mice, these mice exhibited a delayed mucosal wound healing and dysfunctional intestinal barrier characterized by elevated levels of NF-κB activation and MLCK, and a reduction of zonula occludens expression in IECs. Deletion of MLCK restored intestinal barrier function in STAT5 knockout mice, and facilitated mucosal wound healing. Consistently, knockdown of stat5 in IEC monolayers led to increased NF-κB DNA binding to MLCK promoter, myosin light chain phosphorylation and tight junction (TJ) permeability, which were potentiated by administration of tumour necrosis factor-α (TNF-α), and prevented by concurrent NF-κB knockdown. Collectively, enterocyte STAT5 signalling protects against TJ barrier dysfunction and promotes intestinal mucosal wound healing via an interaction with NF-κB to suppress MLCK. Targeting IEC STAT5 signalling may be a novel therapeutic approach for treating intestinal barrier dysfunction in IBD.

Role of myosin light chain kinase in intestinal epithelial barrier defects in a rat model of bowel obstruction

BackgroundBowel obstruction is a common cause of abdominal emergency, since the patients are at increased risk of septicemia resulting in high mortality rate. While the compartmentalized changes in enteric microfloral population and augmentation of bacterial translocation (BT) have already been reported using experimental obstruction models, alterations in epithelial permeability of the obstructed guts has not been studied in detail. Myosin light chain kinase (MLCK) is actively involved in the contraction of epithelial perijunctional actinomyosin ring and thereby increases paracellular permeability. In the current study we attempt to investigate the role of MLCK in epithelial barrier defects using a rat model of simple mechanical obstruction.MethodsWistar rats received intraperitoneal injection of ML-7 (a MLCK inhibitor) or vehicle at 24, 12 and 1 hrs before and 12 hrs after intestinal obstruction (IO). The distal small intestine was obstructed with a single ligature placed 10 cm proximal to the ileocecal junction in IO rats for 24 hrs. Sham-operated rats served as controls.ResultsMucosal injury, such as villous blunting and increased crypt/villus ratio, was observed in the distal small intestine of IO rats. Despite massive enterocyte shedding, intestinal villi were covered with a contiguous epithelial layer without cell apoptosis. Increased transmural macromolecular flux was noticed in the distal small intestine and the proximal colon after IO. The bacterial colony forming units in the spleen and liver of IO rats were significantly higher than those of sham controls. Addition of ML-7 ameliorated the IO-triggered epithelial MLC phosphorylation, mucosal injury and macromolecular flux, but not the level of BT.ConclusionsThe results suggest that IO-induced premature enterocytic sloughing and enhanced paracellular antigenic flux were mediated by epithelial MLCK activation. In addition, enteric bacteria may undergo transcytotic routes other than paracellular paths to cross the epithelium.

Caveolin-1–dependent occludin endocytosis is required for TNF-induced tight junction regulation in vivo

Epithelial paracellular barrier function, determined primarily by tight junction permeability, is frequently disrupted in disease. In the intestine, barrier loss can be mediated by tumor necrosis factor (alpha) (TNF) signaling and epithelial myosin light chain kinase (MLCK) activation. However, TNF induces only limited alteration of tight junction morphology, and the events that couple structural reorganization to barrier regulation have not been defined. We have used in vivo imaging and transgenic mice expressing fluorescent-tagged occludin and ZO-1 fusion proteins to link occludin endocytosis to TNF-induced tight junction regulation. This endocytosis requires caveolin-1 and is essential for structural and functional tight junction regulation. These data demonstrate that MLCK activation triggers caveolin-1-dependent endocytosis of occludin to effect structural and functional tight junction regulation.

Targeted Epithelial Tight Junction Dysfunction Causes Immune Activation and Contributes to Development of Experimental Colitis

Inflammatory bowel disease (IBD) is a multifactorial disease thought to be caused by alterations in epithelial function, innate and adaptive immunity, and luminal microbiota. The specific role of epithelial barrier function remains undefined, although increased activity of intestinal epithelial myosin light chain kinase (MLCK), which is the primary mechanism of tumor necrosis factor-induced barrier dysfunction, occurs in human IBD. Our aim was to determine whether, in an intact epithelium, primary dysregulation of the intestinal epithelial barrier by pathophysiologically relevant mechanisms can contribute to development of colitis. We developed transgenic (Tg) mice that express constitutively active MLCK (CA-MLCK) specifically within intestinal epithelia. Their physiology, immune status, and susceptibility to disease were assessed and compared with non-Tg littermate controls. CA-MLCK Tg mice demonstrated significant barrier loss but grew and gained weight normally and did not develop spontaneous disease. CA-MLCK Tg mice did, however, develop mucosal immune activation demonstrated by increased numbers of lamina propria CD4(+)lymphocytes, redistribution of CD11c+cells, increased production of interferon-gamma and tumor necrosis factor, as well as increased expression of epithelial major histocompatibility complex class I. When challenged with CD4+CD45+Rb(hi) lymphocytes, Tg mice developed an accelerated and more severe form of colitis and had shorter survival times than non-Tg littermates. Primary pathophysiologically relevant intestinal epithelial barrier dysfunction is insufficient to cause experimental intestinal disease but can broadly activate mucosal immune responses and accelerate the onset and severity of immune-mediated colitis.

LIGHT Signals Directly to Intestinal Epithelia to Cause Barrier Dysfunction via Cytoskeletal and Endocytic Mechanisms

LIGHT (lymphotoxin-like inducible protein that competes with glycoprotein D for herpes virus entry on T cells) is a tumor necrosis factor core family member that regulates T-cell activation and causes experimental inflammatory bowel disease. Additional data suggest that LIGHT may be involved in the pathogenesis of human inflammatory bowel disease. The aim of this study was to determine if LIGHT is capable of signaling directly to intestinal epithelia and to define the mechanisms and consequences of such signaling. The effects of LIGHT and interferon-gamma on barrier function, cytoskeletal regulation, and tight junction structure were assessed in mice and intestinal epithelial monolayers. LIGHT induced barrier loss in cultured epithelia via myosin II regulatory light chain (MLC) phosphorylation; both barrier loss and MLC phosphorylation were reversed by MLC kinase (MLCK) inhibition. Pretreatment with interferon-gamma, which induced lymphotoxin beta receptor (LT beta R) expression, was required for these effects, and neither barrier dysfunction nor intestinal epithelial MLC phosphorylation occurred in LT beta R knockout mice. In cultured monolayers, endocytosis of the tight junction protein occludin correlated with barrier loss. Internalized occludin colocalized with caveolin-1. LIGHT-induced occludin endocytosis and barrier loss were both prevented by inhibition of caveolar endocytosis. T cell-derived LIGHT activates intestinal epithelial LT beta R to disrupt barrier function. This requires MLCK activation and caveolar endocytosis. These data suggest a novel role for LIGHT in disease pathogenesis and suggest that inhibition of MLCK-dependent caveolar endocytosis may represent an approach to restoring barrier function in inflammatory bowel disease.

Keeping harmful intruders at bay: Insights into the mechanism of barrier dysfunction during inflammatory bowel diseases

Blair SA, Kane SV, Clayburgh DR, Turner J. (Department of Pathology, The University of Chicago, Chicago, Illinois). Epithelial myosin light chain kinase expression and activity are upregulated in inflammatory bowel disease. Lab Invest 2006;86:191–201.

Reply

We greatly appreciate these comments regarding our recent work and share the enthusiasm of Drs Theiss, Merlin, and Sitaraman regarding the long-term potential of these mechanistic insights to lead to improved therapeutic approaches. As we see it, there are several critical issues that must be addressed before we can move forward into clinical trials. For example, the potential side effects of MLCK inhibition may be significant; MLCK is central to many critical biological functions, including intestinal epithelial wound closure (Gastroenterology 2005;28:987–1001). It will also be important to dissect the role of MLCK in chronic disease. Although our work and that of others has shown that MLCK inhibition can acutely restore intestinal epithelial barrier function, these studies have mostly been performed in vitro using cultured cell lines (Am J Physiol Gastrointest Liver Physiol 2006;290:G496–G504; Gastroenterology 1997;113:1873–1882; Gastroenterology 2002;123:163–172) and, even when tissue or in vivo systems were used, analysis was limited to acute effects over several hours (J Clin Invest 2005;115:2702–2715). Other than our study (Lab Invest 2006;86:191–201), no previous work has suggested a role for MLCK in chronic disease. Although increased MLCK expression and activity in active disease is striking, it should be emphasized that, at present, this is a correlative observation; the question of whether increased MLCK expression and activity is a cause or result of inflammation remains unanswered. We have proposed that either can come first, depending on the initial stimulus, but that they are linked in a self-amplifying cycle of disease (Lab Invest 2004;84:282–291). Because we found that, in patients, increases in MLCK expression and activity paralleled disease activity, one might conclude that inflammation causes these increases. This is consistent with the observation that TNF can increase MLCK expression (Am J Pathol 2005;166:409–419; J Biol Chem 2006;281:26205–26215; Am J Physiol Gastrointest Liver Physiol 2005;288:G422–G430). Alternatively, the observation that MLCK expression was modestly increased in inactive disease suggests that some degree of increased MLCK expression may precede active inflammatory disease. Thus, unlike the actual “chicken and egg” question, which has apparently been solved in favor of the egg (BBC NEWS 2006; available: news.bbc.co.uk/go/pr/fr/-/2/hi/uk_news/england/nottinghamshire/5019682.stm), the relative primacy of active intestinal inflammation and increased epithelial MLCK expression remains to be determined. Keeping harmful intruders at bay: Insights into the mechanism of barrier dysfunction during inflammatory bowel diseasesGastroenterologyVol. 132Issue 1PreviewBlair SA, Kane SV, Clayburgh DR, Turner J. (Department of Pathology, The University of Chicago, Chicago, Illinois). Epithelial myosin light chain kinase expression and activity are upregulated in inflammatory bowel disease. Lab Invest 2006;86:191–201. Full-Text PDF

IFN-γ-Induced TNFR2 Expression Is Required for TNF-Dependent Intestinal Epithelial Barrier Dysfunction

Tumor necrosis factor (TNF) plays a critical role in intestinal disease. In intestinal epithelia, TNF causes tight junction disruption and epithelial barrier loss by up-regulating myosin light chain kinase (MLCK) activity and expression. The aim of this study was to determine the signaling pathways by which TNF causes intestinal epithelial barrier loss. Caco-2 cells that were either nontransfected or stably transfected with human TNF receptor 1 (TNFR1) or TNFR2 and mouse colonocytes were used for physiologic, morphologic, and biochemical analyses. Colitis induced in vivo by adoptive transfer of CD4(+)CD45RB(hi) T cells was associated with increased epithelial MLCK expression and myosin II regulatory light chain (MLC) phosphorylation as well as morphologic tight junction disruption. In vitro studies showed that TNF caused similar increases in MLCK expression and MLC phosphorylation, as well as barrier dysfunction, in Caco-2 monolayers only after interferon (IFN)-gamma pretreatment. This reductionist model was therefore used to determine the molecular mechanism by which IFN-gamma and TNF synergize to cause intestinal epithelial barrier loss. IFN-gamma priming increased TNFR1 and TNFR2 expression, and blocking antibody studies showed that TNFR2, but not TNFR1, was required for TNF-induced barrier dysfunction. Transgenic TNFR2, but not TNFR1, expression allowed IFN-gamma-independent TNF responses. IFN-gamma primes intestinal epithelia to respond to TNF by inducing TNFR2 expression, which in turn mediates TNF-induced MLCK-dependent barrier dysfunction. The data further suggest that epithelial TNFR2 blockade may be a novel approach to restore barrier function in intestinal disease.

Epithelial myosin light chain kinase expression and activity are upregulated in inflammatory bowel disease

The intestinal epithelial barrier is frequently disrupted in inflammatory bowel disease (IBD) and this has been proposed to play a role in disease pathogenesis and reactivation. In vitro studies show that cytokine-induced epithelial barrier dysfunction can be mediated by increased myosin light chain kinase (MLCK) expression and subsequent myosin II regulatory light chain (MLC) phosphorylation. However, this has never been examined in human disease. The aim of these studies, therefore, was to determine whether MLCK is upregulated in the intestinal epithelium of IBD patients. MLCK expression and MLC phosphorylation in human intestinal resection and biopsy specimens were determined by quantitative immunofluorescence microscopy and correlated with clinical and histopathological data. The data show that ileal epithelial MLCK expression was mildly upregulated in inactive IBD. Expression increased further in active disease, with progressive increases in MLCK expression correlating positively with histological disease activity. This correlation between activity and MLCK expression was also seen in individual patients where areas of differing disease activity were analyzed. Colonic epithelial MLCK expression was similarly increased in active IBD and these increases also correlated positively with disease activity, both in individual patients and the overall study group. To evaluate MLCK enzymatic activity, MLC phosphorylation was assessed in snap-frozen colon biopsies. MLC phosphorylation was significantly increased in biopsies with active, but not inactive, IBD. Therefore, these data show that MLCK expression and enzymatic activity are increased in IBD. Moreover, the correlation with disease activity suggests that MLCK upregulation may contribute to barrier dysfunction and IBD pathogenesis.

Epithelial myosin light chain kinase-dependent barrier dysfunction mediates T cell activation-induced diarrhea in vivo

Disruption of the intestinal epithelial barrier occurs in many intestinal diseases, but neither the mechanisms nor the contribution of barrier dysfunction to disease pathogenesis have been defined. We utilized a murine model of T cell-mediated acute diarrhea to investigate the role of the epithelial barrier in diarrheal disease. We show that epithelial barrier dysfunction is required for the development of diarrhea. This diarrhea is characterized by reversal of net water flux, from absorption to secretion; increased leak of serum protein into the intestinal lumen; and altered tight junction structure. Phosphorylation of epithelial myosin II regulatory light chain (MLC), which has been correlated with tight junction regulation in vitro, increased abruptly after T cell activation and coincided with the development of diarrhea. Genetic knockout of long myosin light chain kinase (MLCK) or treatment of wild-type mice with a highly specific peptide MLCK inhibitor prevented epithelial MLC phosphorylation, tight junction disruption, protein leak, and diarrhea following T cell activation. These data show that epithelial MLCK is essential for intestinal barrier dysfunction and that this barrier dysfunction is critical to pathogenesis of diarrheal disease. The data also indicate that inhibition of epithelial MLCK may be an effective non-immunosuppressive therapy for treatment of immune-mediated intestinal disease.