Bifidobacterium bifidum strain BB1 inhibits TNF-α-induced increase in intestinal epithelial tight junction permeability via TLR-2/TLR-6 receptor complex-dependent stimulation of PPAR-γ and suppression of NF-kB p65.

Bifidobacterium bifidum (BB) strain BB1 causes a strain-specific enhancement in intestinal epithelial tight junction (TJ) barrier. TNF-α induces an increase in intestinal epithelial TJ permeability and promotes intestinal inflammation. The major purpose of this study was to delineate the protective effect of BB1 against the TNF-α induced increase in intestinal TJ permeability and to unravel the intracellular mechanisms involved. Previously reported, TNF-α produces an increase in intestinal epithelial TJ permeability in Caco-2 monolayers and in mice. The addition of BB1 inhibited the TNF-α increase in Caco-2 intestinal TJ permeability and mouse intestinal permeability in a strain-specific manner. BB1 inhibited the TNF-α induced increase in intestinal TJ permeability by interfering the with TNF-α induced enterocyte NF-κB p50/p65 and MLCK gene activation. The BB1 protective effect against the TNF-α induced increase in intestinal permeability was mediated by TLR-2/TLR-6 heterodimer complex activation of PPAR-γ and PPAR-γ pathway inhibition of TNF-α induced IKK-α activation, which in turn resulted in a step-wise inhibition of NF-κB p50/p65, MLCK gene, MLCK kinase activity, MLCK-induced opening of the TJ barrier. In conclusion, these studies unravel novel intracellular mechanisms of BB1 protection against the TNF-α induced increase in intestinal TJ permeability. Our data show that BB1 protects against the TNF-α induced increase in intestinal epithelial TJ permeability via a PPAR-γ dependent inhibition of NF-κB p50/p65 and MLCK gene activation.

Role of Bifidobacterium in Modulating the Intestinal Epithelial Tight Junction Barrier: Current Knowledge and Perspectives.

The intestinal tight junction (TJ) barrier is a crucial defense mechanism that prevents the passage of intestinal content into the intestinal wall, tissue, and systemic circulation. A compromised intestinal TJ barrier has been identified as a significant factor in inflammatory bowel disease (IBD), necrotizing enterocolitis, and other gut-related inflammatory conditions. Recent studies have revealed the importance of the probiotic bacterial strains of Bifidobacterium in protecting against intestinal inflammation and IBD pathogenesis via the regulation of intestinal TJ barrier function. Numerous species and strains of Bifidobacterium have been found to regulate TJ proteins and the signaling pathways responsible for maintaining intestinal barrier integrity and permeability. In this review, we provide a summary of recent studies that highlight the regulatory role of Bifidobacterium species and the strain effect on the intestinal TJ barrier. We also discuss the intracellular mechanisms involved in Bifidobacterium modulation of the intestinal barrier and the potential therapeutic efficacy of targeting the barrier function to regulate intestinal inflammation.

Talk about micromanaging! Role of microRNAs in intestinal barrier function.

Defective intestinal tight-junction (TJ) barrier has been implicated in the pathogenesis of inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), and other inflammatory conditions of the gut. The role of microRNAs (miRNA's or miR's) has also been demonstrated in the last two decades in the pathogenesis of IBD and in the regulation of intestinal TJ barrier function. MiRNAs are noncoding regulators of gene expression at the posttranscription level that have an essential role in targeting transcripts encoding proteins of intestinal TJs and their regulators. Many miRNAs have been reported to regulate or deregulate the TJ proteins responsible for the intestinal barrier integrity and intestinal permeability. Many of those miRNAs have been reported to have essential roles in the pathogenesis of IBD. In this mini-review, we summarize the results of studies in the last three years that implicate miRNAs in the defective TJ barrier in relation to IBD. The therapeutic potential of using specific miRNAs to target the intestinal TJ barrier might be of great insight for IBD therapy.

Pore properties of Orai1 calcium channel dimers and their activation by the STIM1 ER calcium sensor.

Store-operated Ca2+ entry signals are mediated by plasma membrane Orai channels activated through intermembrane coupling with Ca2+-sensing STIM proteins in the endoplasmic reticulum (ER). The nature of this elaborate Orai-gating mechanism has remained enigmatic. Based on the Drosophila Orai structure, mammalian Orai1 channels are hexamers comprising three dimeric subunit pairs. We utilized concatenated Orai1 dimers to probe the function of key domains within the channel pore and gating regions. The Orai1-E106Q selectivity-filter mutant, widely considered a dominant pore blocker, was surprisingly nondominant within concatenated heterodimers with Orai1-WT. The Orai1-E106Q/WT heterodimer formed STIM1-activated nonselective cation channels with significantly enlarged apparent pore diameter. Other Glu-106 substitutions entirely blocked the function of heterodimers with Orai1-WT. The hydrophobic pore-lining mutation V102C, which constitutively opens channels, was suppressed by Orai1-WT in the heterodimer. In contrast, the naturally occurring R91W pore-lining mutation associated with human immunodeficiency was completely dominant-negative over Orai-WT in heterodimers. Heterodimers containing the inhibitory K85E mutation extending outward from the pore helix gave an interesting partial effect on both channel activation and STIM1 binding, indicating an important allosteric link between the cytosolic Orai1 domains. The Orai1 C-terminal STIM1-binding domain mutation L273D powerfully blocked STIM1-induced channel activation. The Orai1-L273D/WT heterodimer had drastically impaired STIM1-induced channel gating but, unexpectedly, retained full STIM1 binding. This reveals the critical role of Leu-273 in transducing the STIM1-binding signal into the allosteric conformational change that initiates channel gating. Overall, our results provide important new insights into the role of key functional domains that mediate STIM1-induced gating of the Orai1 channel.

Cross-linking of Orai1 channels by STIM proteins.

The transmembrane docking of endoplasmic reticulum (ER) Ca2+-sensing STIM proteins with plasma membrane (PM) Orai Ca2+ channels is a critical but poorly understood step in Ca2+ signal generation. STIM1 protein dimers unfold to expose a discrete STIM-Orai activating region (SOAR1) that tethers and activates Orai1 channels within discrete ER-PM junctions. We reveal that each monomer within the SOAR dimer interacts independently with single Orai1 subunits to mediate cross-linking between Orai1 channels. Superresolution imaging and mobility measured by fluorescence recovery after photobleaching reveal that SOAR dimer cross-linking leads to substantial Orai1 channel clustering, resulting in increased efficacy and cooperativity of Orai1 channel function. A concatenated SOAR1 heterodimer containing one monomer point mutated at its critical Orai1 binding residue (F394H), although fully activating Orai channels, is completely defective in cross-linking Orai1 channels. Importantly, the naturally occurring STIM2 variant, STIM2.1, has an eight-amino acid insert in its SOAR unit that renders it functionally identical to the F394H mutant in SOAR1. Contrary to earlier predictions, the SOAR1-SOAR2.1 heterodimer fully activates Orai1 channels but prevents cross-linking and clustering of channels. Interestingly, combined expression of full-length STIM1 with STIM2.1 in a 5:1 ratio causes suppression of sustained agonist-induced Ca2+ oscillations and protects cells from Ca2+ overload, resulting from high agonist-induced Ca2+ release. Thus, STIM2.1 exerts a powerful regulatory effect on signal generation likely through preventing Orai1 channel cross-linking. Overall, STIM-mediated cross-linking of Orai1 channels is a hitherto unrecognized functional paradigm that likely provides an organizational microenvironment within ER-PM junctions with important functional impact on Ca2+ signal generation.

A Diatom Ferritin Optimized for Iron Oxidation but Not Iron Storage.

Ferritin from the marine pennate diatom Pseudo-nitzschia multiseries (PmFTN) plays a key role in sustaining growth in iron-limited ocean environments. The di-iron catalytic ferroxidase center of PmFTN (sites A and B) has a nearby third iron site (site C) in an arrangement typically observed in prokaryotic ferritins. Here we demonstrate that Glu-44, a site C ligand, and Glu-130, a residue that bridges iron bound at sites B and C, limit the rate of post-oxidation reorganization of iron coordination and the rate at which Fe(3+) exits the ferroxidase center for storage within the mineral core. The latter, in particular, severely limits the overall rate of iron mineralization. Thus, the diatom ferritin is optimized for initial Fe(2+) oxidation but not for mineralization, pointing to a role for this protein in buffering iron availability and facilitating iron-sparing rather than only long-term iron storage.

Mechanism of ferrous iron binding and oxidation by ferritin from a pennate diatom.

A novel ferritin was recently found in Pseudo-nitzschia multiseries (PmFTN), a marine pennate diatom that plays a major role in global primary production and carbon sequestration into the deep ocean. Crystals of recombinant PmFTN were soaked in iron and zinc solutions, and the structures were solved to 1.65-2.2-Å resolution. Three distinct iron binding sites were identified as determined from anomalous dispersion data from aerobically grown ferrous soaked crystals. Sites A and B comprise the conserved ferroxidase active site, and site C forms a pathway leading toward the central cavity where iron storage occurs. In contrast, crystal structures derived from anaerobically grown and ferrous soaked crystals revealed only one ferrous iron in the active site occupying site A. In the presence of dioxygen, zinc is observed bound to all three sites. Iron oxidation experiments using stopped-flow absorbance spectroscopy revealed an extremely rapid phase corresponding to Fe(II) oxidation at the ferroxidase site, which is saturated after adding 48 ferrous iron to apo-PmFTN (two ferrous iron per subunit), and a much slower phase due to iron core formation. These results suggest an ordered stepwise binding of ferrous iron and dioxygen to the ferroxidase site in preparation for catalysis and a partial mobilization of iron from the site following oxidation.

Fe-haem bound to Escherichia coli bacterioferritin accelerates iron core formation by an electron transfer mechanism.

BFR (bacterioferritin) is an iron storage and detoxification protein that differs from other ferritins by its ability to bind haem cofactors. Haem bound to BFR is believed to be involved in iron release and was previously thought not to play a role in iron core formation. Investigation of the effect of bound haem on formation of the iron core has been enabled in the present work by development of a method for reconstitution of BFR from Escherichia coli with exogenously added haem at elevated temperature in the presence of a relatively high concentration of sodium chloride. Kinetic analysis of iron oxidation by E. coli BFR preparations containing various amounts of haem revealed that haem bound to BFR decreases the rate of iron oxidation at the dinuclear iron ferroxidase sites but increases the rate of iron core formation. Similar kinetic analysis of BFR reconstituted with cobalt-haem revealed that this haem derivative has no influence on the rate of iron core formation. These observations argue that haem bound to E. coli BFR accelerates iron core formation by an electron-transfer-based mechanism.