Claudin-2 upregulation enhances intestinal permeability, immune activation, dysbiosis, and mortality in sepsis

Intestinal epithelial expression of the tight junction protein claudin-2, which forms paracellular cation and water channels, is precisely regulated during development and in disease. Here, we show that small intestinal epithelial claudin-2 expression is selectively upregulated in septic patients. Similar changes occurred in septic mice, where claudin-2 upregulation coincided with increased flux across the paracellular pore pathway. In order to define the significance of these changes, sepsis was induced in claudin-2 knockout (KO) and wild-type (WT) mice. Sepsis-induced increases in pore pathway permeability were prevented by claudin-2 KO. Moreover, claudin-2 deletion reduced interleukin-17 production and T cell activation and limited intestinal damage. These effects were associated with reduced numbers of neutrophils, macrophages, dendritic cells, and bacteria within the peritoneal fluid of septic claudin-2 KO mice. Most strikingly, claudin-2 deletion dramatically enhanced survival in sepsis. Finally, the microbial changes induced by sepsis were less pathogenic in claudin-2 KO mice as survival of healthy WT mice injected with cecal slurry collected from WT mice 24 h after sepsis was far worse than that of healthy WT mice injected with cecal slurry collected from claudin-2 KO mice 24 h after sepsis. Claudin-2 upregulation and increased pore pathway permeability are, therefore, key intermediates that contribute to development of dysbiosis, intestinal damage, inflammation, ineffective pathogen control, and increased mortality in sepsis. The striking impact of claudin-2 deletion on progression of the lethal cascade activated during sepsis suggests that claudin-2 may be an attractive therapeutic target in septic patients.

Tacrolimus-binding protein FKBP8 directs myosin light chain kinase-dependent barrier regulation and is a potential therapeutic target in Crohn's disease

OBJECTIVE

Intestinal barrier loss is a Crohn's disease (CD) risk factor. This may be related to increased expression and enzymatic activation of myosin light chain kinase 1 (MLCK1), which increases intestinal paracellular permeability and correlates with CD severity. Moreover, preclinical studies have shown that MLCK1 recruitment to cell junctions is required for tumour necrosis factor (TNF)-induced barrier loss as well as experimental inflammatory bowel disease progression. We sought to define mechanisms of MLCK1 recruitment and to target this process pharmacologically.

DESIGN

Protein interactions between FK506 binding protein 8 (FKBP8) and MLCK1 were assessed in vitro. Transgenic and knockout intestinal epithelial cell lines, human intestinal organoids, and mice were used as preclinical models. Discoveries were validated in biopsies from patients with CD and control subjects.

RESULTS

MLCK1 interacted specifically with the tacrolimus-binding FKBP8 PPI domain. Knockout or dominant negative FKBP8 expression prevented TNF-induced MLCK1 recruitment and barrier loss in vitro. MLCK1-FKBP8 binding was blocked by tacrolimus, which reversed TNF-induced MLCK1-FKBP8 interactions, MLCK1 recruitment and barrier loss in vitro and in vivo. Biopsies of patient with CD demonstrated increased numbers of MLCK1-FKBP8 interactions at intercellular junctions relative to control subjects.

CONCLUSION

Binding to FKBP8, which can be blocked by tacrolimus, is required for MLCK1 recruitment to intercellular junctions and downstream events leading to immune-mediated barrier loss. The observed increases in MLCK1 activity, MLCK1 localisation at cell junctions and perijunctional MLCK1-FKBP8 interactions in CD suggest that targeting this process may be therapeutic in human disease. These new insights into mechanisms of disease-associated barrier loss provide a critical foundation for therapeutic exploitation of FKBP8-MLCK1 interactions.

Serine 408 phosphorylation is a molecular switch that regulates structure and function of the occludin α-helical bundle

Occludin is a tetramembrane-spanning tight junction protein. The long C-terminal cytoplasmic domain, which represents nearly half of occludin sequence, includes a distal bundle of three α-helices that mediates interactions with other tight junction components. A short unstructured region just proximal to the α-helical bundle is a phosphorylation hotspot within which S408 phosphorylation acts as molecular switch that modifies tight junction protein interactions and barrier function. Here, we used NMR to define the effects of S408 phosphorylation on intramolecular interactions between the unstructured region and the α-helical bundle. S408 pseudophosphorylation affected conformation at hinge sites between the three α-helices. Further studies using paramagnetic relaxation enhancement and microscale thermophoresis indicated that the unstructured region interacts with the α-helical bundle. These interactions between the unstructured domain are enhanced by S408 phosphorylation and allow the unstructured region to obstruct the binding site, thereby reducing affinity of the occludin tail for zonula occludens-1 (ZO-1). Conversely, S408 dephosphorylation attenuates intramolecular interactions, exposes the binding site, and increases the affinity of occludin binding to ZO-1. Consistent with an increase in binding to ZO-1, intravital imaging and fluorescence recovery after photobleaching (FRAP) analyses of transgenic mice demonstrated increased tight junction anchoring of enhanced green fluorescent protein (EGFP)-tagged nonphosphorylatable occludin relative to wild-type EGFP-occludin. Overall, these data define the mechanisms by which S408 phosphorylation modifies occludin tail conformation to regulate tight junction protein interactions and paracellular permeability.

PTPN2 mutations cause epithelium-intrinsic barrier loss that synergizes with mucosal immune hyperactivation

It is clear that excessive mucosal immune activation and intestinal barrier dysfunction both contribute to inflammatory bowel disease (IBD) pathogenesis. T cell protein tyrosine phosphatase (TCPTP), which extinguishes signaling in immune cells, is linked to IBD and other immune-mediated diseases. In this issue of the JCI, Marchelletta and Krishnan et al. demonstrate that, in intestinal epithelial cells, TCPTP regulates tight junction permeability in vivo. Intestinal epithelial TCPTP loss potentiated cytokine-induced barrier loss, and this synergized with effects of TCPTP loss in immune cells. This work implicates a single mutation as the cause of distinct functional aberrations in diverse cell types and demonstrates how one genetic defect can drive multihit disease pathogenesis.

Gene-environment interaction in molar-incisor hypomineralization

Molar incisor hypomineralization (MIH) is an enamel condition characterized by lesions ranging in color from white to brown which present rapid caries progression, and mainly affects permanent first molars and incisors. These enamel defects usually occur when there are disturbances during the mineralization or maturation stage of amelogenesis. Both genetic and environmental factors have been suggested to play roles in MIH’s development, but no conclusive risk factors have shown the source of the disease. During head and neck development, the interferon regulatory factor 6 (IRF6) gene is involved in the structure formation of the oral and maxillofacial regions, and the transforming growth factor alpha (TGFA) is an essential cell regulator, acting during proliferation, differentiation, migration and apoptosis. In this present study, it was hypothesized that these genes interact and contribute to predisposition of MIH. Environmental factors affecting children that were 3 years of age or older were also hypothesized to play a role in the disease etiology. Those factors included respiratory issues, malnutrition, food intolerance, infection of any sort and medication intake. A total of 1,065 salivary samples from four different cohorts were obtained, and DNA was extracted from each sample and genotyped for nine different single nucleotide polymorphisms. Association tests and logistic regression implemented in PLINK were used for analyses. A potential interaction between TGFA rs930655 with all markers tested in the cohort from Turkey was identified. These interactions were not identified in the remaining cohorts. Associations (p<0.05) between the use of medication after three years of age and MIH were also found, suggesting that conditions acquired at the age children start to socialize might contribute to the development of MIH.

Sox2 Controls Periderm and Rugae Development to Inhibit Oral Adhesions

In humans, ankyloglossia and cleft palate are common congenital craniofacial anomalies, and these are regulated by a complex gene regulatory network. Understanding the genetic underpinnings of ankyloglossia and cleft palate will be an important step toward rational treatment of these complex anomalies. We inactivated the Sry (sex-determining region Y)-box 2 (Sox2) gene in the developing oral epithelium, including the periderm, a transient structure that prevents abnormal oral adhesions during development. This resulted in ankyloglossia and cleft palate with 100% penetrance in embryos examined after embryonic day 14.5. In Sox2 conditional knockout embryos, the oral epithelium failed to differentiate, as demonstrated by the lack of keratin 6, a marker of the periderm. Further examination revealed that the adhesion of the tongue and mandible expressed the epithelial markers E-Cad and P63. The expanded epithelia are Sox9-, Pitx2-, and Tbx1-positive cells, which are markers of the dental epithelium; thus, the dental epithelium contributes to the development of oral adhesions. Furthermore, we found that Sox2 is required for palatal shelf extension, as well as for the formation of palatal rugae, which are signaling centers that regulate palatogenesis. In conclusion, the deletion of Sox2 in oral epithelium disrupts palatal shelf extension, palatal rugae formation, tooth development, and periderm formation. The periderm is required to inhibit oral adhesions and ankyloglossia, which is regulated by Sox2. In addition, oral adhesions occur through an expanded dental epithelial layer that inhibits epithelial invagination and incisor development. This process may contribute to dental anomalies due to ankyloglossia.

Ectodermal Organ Development Is Regulated by a microRNA-26b-Lef-1-Wnt Signaling Axis

The developmental role of Lef-1 in ectodermal organs has been characterized using Lef-1 murine knockout models. We generated a Lef-1 conditional over-expression (COEL) mouse to determine the role of Lef-1 expression in epithelial structures at later stages of development after endogenous expression switches to the mesenchyme. Lef-1 over expression (OE) in the oral epithelium creates a new dental epithelial stem cell niche that significantly increases incisor growth. These data indicate that Lef-1 expression is switched off in the dental epithelial at early stages to maintain the stem cell niche and regulate incisor growth. Bioinformatics analyses indicated that miR-26b expression increased coinciding with decreased Lef-1 expression in the dental epithelium. We generated a murine model over-expressing miR-26b that targets endogenous Lef-1 expression and Lef-1-related developmental mechanisms. miR-26b OE mice have ectodermal organ defects including a lack of incisors, molars, and hair similar to the Lef-1 null mice. miR-26b OE rescues the Lef-1 OE phenotype demonstrating a critical genetic and developmental role for miR-26b in the temporal and spatial expression of Lef-1 in epithelial tissues. Lef-1 expression regulates Wnt signaling and Wnt target genes as well as cell proliferation mechanisms, while miR-26b OE reduced the levels of Wnt target gene expression. The extra stem cell compartment in the COEL mice expressed Lef-1 suggesting that Lef-1 is a stem cell factor, which was absent in the miR-26b OE/COEL rescue mice. This is the first demonstration of a microRNA OE mouse model that has ectodermal organ defects. These findings demonstrate that the levels of Lef-1 are critical for development and establish a role for miR-26b in the regulation of ectodermal organ development through the control of Lef-1 expression and an endogenous stem cell niche.