1H, 13C, and 15N resonance assignment of the first PDZ domain of mouse ZO-1

Pharmacologic Comparison of High-Dose Hesperetin and Quercetin on MDCK II Cell Viability, Tight Junction Integrity, and Cell Shape

The modulation of tight junction (TJ) integrity with small molecules is important for drug delivery. High-dose baicalin (BLI), baicalein (BLE), quercetin (QUE), and hesperetin (HST) have been shown to open TJs in Madin-Darby canine kidney (MDCK) II cells, but the mechanisms for HST and QUE remain unclear. In this study, we compared the effects of HST and QUE on cell proliferation, morphological changes, and TJ integrity. HST and QUE were found to have opposing effects on the MDCK II cell viability, promotion, and suppression, respectively. Only QUE, but not HST, induced a morphological change in MDCK II into a slenderer cell shape. Both HST and QUE downregulated the subcellular localization of claudin (CLD)-2. However, only QUE, but not HST, downregulated CLD-2 expression. Conversely, only HST was shown to directly bind to the first PDZ domain of ZO-1, a key molecule to promote TJ biogenesis. The TGFβ pathway partially contributed to the HST-induced cell proliferation, since SB431541 ameliorated the effect. In contrast, the MEK pathway was not involved by both the flavonoids, since U0126 did not revert their TJ-opening effect. The results offer insight for using HST or QUE as naturally occurring absorption enhancers through the paracellular route.

Direct Inhibition of the First PDZ Domain of ZO-1 by Glycyrrhizin is a Possible Mechanism of Tight Junction Opening of Caco-2 Cells

Glycyrrhizin (GL) is known to exhibit a variety of useful pharmacological activities, including anti-inflammation, anti-hepatotoxicity, and enhancement of intestinal drug absorption. GL has been reported to modify the assembly of...

NMR-Guided Repositioning of Non-Steroidal Anti-Inflammatory Drugs into Tight Junction Modulators

Bioavailability is a major bottleneck in the clinical application of medium molecular weight therapeutics, including protein and peptide drugs. Paracellular transport of these molecules is hampered by intercellular tight junction (TJ) complexes. Therefore, safe chemical regulators for TJ loosening are desired. Here, we showed a potential application of select non-steroidal anti-inflammatory drugs (NSAIDs) as TJ modulators. Based on our previous observation that diclofenac and flufenamic acid directly bound various PDZ domains with a broad specificity, we applied solution nuclear magnetic resonance techniques to examine the interaction of other NSAIDs and the first PDZ domain (PDZ1) of zonula occludens (ZO)-1, ZO-1(PDZ1). Inhibition of ZO-1(PDZ1) is expected to provide loosening of the epithelial barrier function because the domain plays a crucial role in maintaining TJ integrity. Accordingly, diclofenac and indomethacin were found to decrease the subcellular localization of claudin (CLD)-2 but not occludin and ZO-1 at the apicolateral intercellular compartment of Madin–Darby canine kidney (MDCK) II cells. These NSAIDs exhibited 125–155% improved paracellular efflux of fluorescein isothiocyanate insulin for the Caco-2 cell monolayer. We propose that these NSAIDs can be repurposed as drug absorption enhancers for peptide drugs.

Opposing Effect of Naringenin and Quercetin on the Junctional Compartment of MDCK II Cells to Modulate the Tight Junction

Maintaining tight junction (TJ) integrity is important for epithelial cell barriers. Previously, the enhancement of TJ integrity, induced by citrus-derived flavonoids, naringin (NRG) and hesperidin (HSD), was demonstrated, but the effects of their aglycones naringenin (NAR) and hesperetin (HST), and the mechanisms, have not been systematically investigated. Here we compared three series of flavonoids related to NAR, HST, quercetin (QUE) and their glycosides with the Madin–Darby canine kidney (MDCK) II cell monolayers. The effect of flavonoids on the protein expression level of claudin (CLD)-2 and its subcellular localization were investigated. NAR, NRG, and HSD increased the CLD-2 localization at the TJ compartment, and its protein expression level. QUE and HST showed TJ-mitigating activity. Narirutin (NRT), neohesperidin (NHD) and rutin (RUT) did not affect the TJ. In addition, NAR and QUE induced an increase or decrease of the transepithelial electrical resistance (TEER) values of the MDCK II monolayers. Two known signaling pathways, phosphatidyl-inositol-3 kinase (PI3K) and 5′-AMP-activated protein kinase (AMPK), were further compared with NAR. Two-dimensional polyacrylamide electrophoresis (2D PAGE) analysis of whole-cell proteins treated with NAR, AICA-riboside (AMPK activator) and LY294002 (PI3K inhibitor) showed in both a distinct pattern. This suggests the target of NAR’s CLD-2 or zonula occludens-1 (ZO-1) modulation was unique.

High dose of baicalin or baicalein can reduce tight junction integrity by partly targeting the first PDZ domain of zonula occludens-1 (ZO-1)

The tight junction (TJ) is the apical-most intercellular junction complex, serving as a biological barrier of intercellular spaces between epithelial cells. The TJ's integrity is maintained by a key protein-protein interaction between C-terminal motifs of claudins (CLDs) and the postsynaptic density 95 (PSD-95)/discs large/zonula occludens 1 (ZO-1; PDZ) domains of ZO-1. Weak but direct interaction of baicalin and its aglycon, baicalein-which are pharmacologically active components of Chinese skullcap (Radix scutellariae)-with ZO-1(PDZ1) have been observed in NMR experiments. Next, we observed TJ-mitigating activity of these flavonoids against Madin-Darby canine kidney (MDCK) II cells with the downregulation of subcellular localization of CLD-2 at TJs. Meanwhile, baicalein-but not baicalin-induced a slender morphological change of MDCK cells' shape from their normal cobblestone-like shapes. Since baicalin and baicalein did not induce a localization change of occludin (OCLN), a "partial" epithelial-mesenchymal transition (EMT) induced by these flavonoids was considered. SB431542, an ALK-5 inhibitor, reversed the CLD-2 downregulation of both baicalin and baicalein, while SB431542 did not reverse the slender morphology. In contrast, the MEK/ERK inhibitor U0126 reversed the slender shape change. Thus, in addition to inhibition of the ZO-1-CLD interaction, activation of both transforming growth factor-β (TGF-β) and MEK/ERK signaling pathways have been suggested to be involved in TJ reduction by these flavonoids. Finally, we demonstrated that baicalin enhanced the permeability of fluorescence-labeled insulin via the paracellular pathway of the Caco-2 cell layer. We propose that baicalin, baicalein, and Radix scutellariae extract are useful as drug absorption enhancers.

Using 1 HN amide temperature coefficients to define intrinsically disordered regions: An alternative NMR method

This report describes a cost-effective experimental method for determining an intrinsically disordered protein (IDP) region in a given protein sample. In this area, the most popular (and conventional) means is using the amide (¹ H^N ) NMR signal chemical shift distributed in the range of 7.5-8.5 ppm. For this study, we applied an additional step: analysis of ¹ H^N chemical shift temperature coefficients (¹ H^N -CSTCs) of the signals. We measured ¹ H-¹⁵ N two-dimensional NMR spectra of model IDP samples and ordered samples at four temperatures (288, 293, 298, and 303 K). We derived the ¹ H^N -CSTC threshold deviation, which gives the best correlation of ordered and disordered regions among the proteins examined (below -3.6 ppb/K). By combining these criteria with the newly optimized chemical shift range (7.8-8.5 ppm), the ratios of both true positive and true negative were improved by approximately 19% (62-81%) compared with the conventional "chemical shift-only" method.

Spatial Overlap of Claudin- and Phosphatidylinositol Phosphate-Binding Sites on the First PDZ Domain of Zonula Occludens 1 Studied by NMR

Background: The tight junction is an intercellular adhesion complex composed of claudins (CLDs), occludin, and the scaffolding proteins zonula occludens 1 (ZO-1) and its two paralogs ZO-2 and ZO-3. ZO-1 is a multifunctional protein that contains three PSD95/Discs large/ZO-1(PDZ) domains. A key functional domain of ZO-1 is the first PDZ domain (ZO-1(PDZ1)) that recognizes the conserved C-termini of CLDs. Methods: In this study, we confirmed that phosphoinositides bound directly to ZO-1(PDZ1) by biochemical and solution NMR experiments. We further determined the solution structure of mouse ZO-1(PDZ1) by NMR and mapped the phosphoinositide binding site onto its molecular surface. Results: The phosphoinositide binding site was spatially overlapped with the CLD-binding site of ZO-1(PDZ1). Accordingly, inositol-hexaphosphate (phytic acid), an analog of the phosphoinositide head group, competed with ZO-1(PDZ)-CLD interaction. Conclusions: The results suggested that the PDZ domain–phosphoinositide interaction plays a regulatory role in biogenesis and homeostasis of the tight junction.

Discovery of Potent Disheveled/Dvl Inhibitors Using Virtual Screening Optimized With NMR-Based Docking Performance Index

Most solid tumors have their own cancer stem cells (CSCs), which are resistant to standard chemo-therapies. Recent reports have described that Wnt pathway plays a key role in self-renewal and tumorigenesis of CSCs. Regarding the Wnt/β-catenin pathway, Dvl (mammalian Disheveled) is an attractive target of drug discovery. After analyzing the PDZ domain of human Dvl1 (Dvl1-PDZ) using NMR, we subjected it to preliminary NMR titration studies with 17 potential PDZ-binding molecules including CalBioChem-322338, a commercially available Dvl PDZ domain inhibitor. Next, we performed virtual screening (VS) using the program GOLD with nine parameter sets. Results were evaluated using the NMR-derived docking performance index (NMR-DPI). One parameter set of GOLD docking showing the best NMR-DPI was selected and used for the second VS against 5,135 compounds. The second docking trial identified more than 1,700 compounds that exhibited higher scores than CalBioChem-322338. Subsequent NMR titration experiments with five new candidate molecules (NPL-4001, 4004, 4011, 4012, and 4013), Dvl1-PDZ revealed larger chemical shift changes than those of CalBioChem-322338. Finally, these compounds showed partial proliferation inhibition activity against BT-20, a triple negative breast cancer (TNBC) cell. These compounds are promising Wnt pathway inhibitors that are potentially useful for anti-TNBC therapy.

Involvement of the p38 MAPK signaling pathway in overexpression of matrix metalloproteinase-9 during the course of brain edema in 1,2-dichloroethane-intoxicated mice

Accumulated data have revealed that subacute poisoning of 1,2-dichloroethane (1,2-DCE), an industrial solvent used in some countries can cause encephalopathy, in which brain edema is the main pathological change. However, the underlying mechanisms are unclear. In the present study, we hypothesized that the p38 MAPK (p38) signaling pathway could be activated in 1,2-DCE-intoxicated mice, which in turn stimulates transcription factors, such as nuclear factor-κB (NF-κB) and activator protein-1 (AP-1), and then enhances the expression of proinflammatory factors, including matrix metalloproteinase-9 (MMP-9), finally leading to blood-brain barrier (BBB) disruption and brain edema formation. Our results revealed that brain water content and BBB permeability increased significantly in the intoxicated mice. Meanwhile, the levels of phosphorylated p38 (p-p38) and inhibitory κBα (p-IκB), as well as the expression levels of MMP-9, c-jun, c-fos, and p65, also increased markedly in the brains of intoxicated mice. Conversely, the protein levels of ZO-1, occludin and claudin-5 in these mice decreased markedly, but their JAM-1 protein levels increased dramatically. Our results revealed that p-p38 levels in the brains of intoxicated mice were suppressed by pretreatment with a p38 inhibitor. In response to suppressed p-p38 levels, the brain water contents and DNA binding activities of NF-κB and AP-1, as well as the expression levels of MMP-9, c-jun, c-fos, p65, p-IκB and JAM-1, decreased, whereas the protein levels of ZO-1, occludin and claudin-5 increased markedly. Taken together, our findings indicated that the p38 signaling pathway might be activated and involved in the course of brain edema in 1,2-DCE-intoxicated mice.

Discovery of Cryoprotective Activity in Human Genome-Derived Intrinsically Disordered Proteins

Intrinsically disordered proteins (IDPs) are an emerging phenomenon. They may have a high degree of flexibility in their polypeptide chains, which lack a stable 3D structure. Although several biological functions of IDPs have been proposed, their general function is not known. The only finding related to their function is the genetically conserved YSK₂ motif present in plant dehydrins. These proteins were shown to be IDPs with the YSK₂ motif serving as a core region for the dehydrins’ cryoprotective activity. Here we examined the cryoprotective activity of randomly selected IDPs toward the model enzyme lactate dehydrogenase (LDH). All five IDPs that were examined were in the range of 35–45 amino acid residues in length and were equally potent at a concentration of 50 μg/mL, whereas folded proteins, the PSD-95/Dlg/ZO-1 (PDZ) domain, and lysozymes had no potency. We further examined their cryoprotective activity toward glutathione S-transferase as an example of the other enzyme, and toward enhanced green fluorescent protein as a non-enzyme protein example. We further examined the lyophilization protective activity of the peptides toward LDH, which revealed that some IDPs showed a higher activity than that of bovine serum albumin (BSA). Based on these observations, we propose that cryoprotection is a general feature of IDPs. Our findings may become a clue to various industrial applications of IDPs in the future.

Biophysical characterization of interactions between the C-termini of peripheral nerve claudins and the PDZ₁ domain of zonula occludens

Our recent study has shown that cellular junctions in myelin and in the epi-/perineruium that encase nerve fibers regulate the permeability of the peripheral nerves. This permeability may affect propagation of the action potential. Direct interactions between the PDZ₁ domain of zonula occludens (ZO₁ or ZO₂) and the C-termini of claudins are known to be crucial for the formation of tight junctions. Using the purified PDZ₁ domain of ZO₂ and a variety of C-terminal mutants of peripheral nerve claudins (claudin-1, claudin-2, claudin-3, claudin-5 in epi-/perineurium; claudin-19 in myelin), we have utilized NMR spectroscopy to determine specific roles of the 3 C-terminal claudin residues (position -2, -1, 0) for their interactions with PDZ₁ of ZO₂. In contrast to the canonical model that emphasizes the importance of residues at the -2 and 0 positions, our results demonstrate that, for peripheral nerve claudins, the residue at position -1 plays a critical role in association with PDZ₁, while the side-chain of residue 0 plays a significant but lesser role. Surprisingly, claudin-19, the most abundant claudin in myelin, exhibited no binding to ZO₂. These findings reveal that the binding mechanism of claudin/ZO in epi-/perineurium is distinct from the canonical interactions between non-ZO PDZ-containing proteins with their ligands. This observation provides the molecular basis for a strategy to develop drugs that target tight junctions in the epi-/perineurium of peripheral nerves.

Accidental interaction between PDZ domains and diclofenac revealed by NMR-assisted virtual screening

In silico approaches have become indispensable for drug discovery as well as drug repositioning and adverse effect prediction. We have developed the eF-seek program to predict protein–ligand interactions based on the surface structure of proteins using a clique search algorithm. We have also developed a special protein structure prediction pipeline and accumulated predicted 3D models in the Structural Atlas of the Human Genome (SAHG) database. Using this database, genome-wide prediction of non-peptide ligands for proteins in the human genome was performed, and a subset of predicted interactions including 14 PDZ domains was then confirmed by NMR titration. Surprisingly, diclofenac, a non-steroidal anti-inflammatory drug, was found to be a non-peptide PDZ domain ligand, which bound to 5 of 15 tested PDZ domains. The critical residues for the PDZ–diclofenac interaction were also determined. Pharmacological implications of the accidental PDZ–diclofenac interaction are further discussed.