Freeze-fracture electron microscopic study of tight junction strands in HEK293 cells and MDCK II cells expressing claudin-1 mutants in the second extracellular loop.

Claudins constitute tight junction (TJ) strands. In order to examine the function of the second extracellular loop (ECL2), we constructed 1CLDeltaFY and 1CLDeltaPL in which highly conserved amino acids, FY or PL, in the ECL2 of mouse claudin-1 were deleted. They were then tagged with either EGFP at the NH(2)-terminus (EGFP1CLDeltaFY and EGFP1CLDeltaPL) or the myc-epitope at the COOH-terminus (1CLDeltaFYmyc and 1CLDeltaPLmyc). The expression of EGFP1CLDeltaFY and EGFP1CLDeltaPL in TJ-free HEK293 cells formed TJ strands resembling those formed by wild-type claudin-1. The expression of 1CLDeltaPLmyc in TJ-bearing MDCK II cells induced aberrant TJ strands in the lateral plasma membranes whose intramembranous particles were almost equally distributed in the P- and E-face. In contrast, 1CLDeltaFYmyc formed aggregates of short continuous strands which were frequently associated with vesicle-like structures. Coculture experiments with MDCK II cells showed that 1CLDeltaPLmyc was localized at heterotypic cell-cell junctions but 1CLDeltaFYmyc was not. These results suggest that changes in the TJ morphology due to the expression of either 1CLDeltaFYmyc or 1CLDeltaPLmyc may be caused by some factors specific to epithelial MDCK II cells including endogenous claudins.

Intracellular events in retinal glial cells exposed to ICG and BBG.

PURPOSE: To investigate the intracellular events in retinal glial cells exposed to indocyanine green (ICG) and brilliant blue G (BBG). METHODS: The human Müller cell line MIO-M1 was exposed to a low dose (0.25 mg/mL) and a clinical dose (2.5 mg/mL) of ICG and a clinical dose (0.25 mg/mL) of BBG for 15 minutes, respectively. To quantify the proliferation and viability of the cells, [(3)H]-thymidine incorporation was measured and cell numbers were counted 24 hours after treatment. Cell morphology was evaluated using phase-contrast microscopy and transmission electron microscopy. The effects of ICG and BBG on phosphorylation of p38 MAPK and cleavage of caspase-9 and caspase-3 were examined by Western blot. RESULTS: ICG and BBG significantly reduced [(3)H]-thymidine incorporation in MIO-M1 cells compared with the vehicle-treated controls (P < 0.01). Cell number significantly decreased after exposure to ICG at 2.5 or 0.25 mg/mL (P < 0.01) but did not decrease after exposure to BBG at 0.25 mg/mL. Transmission electron microscopy revealed apoptotic changes only in the ICG-treated cells. Prominent p38 MAPK phosphorylation was observed in the presence of ICG, even at the low concentration and within a short time exposure; however, no apparent enhancement was observed in the presence of 0.25 mg/mL BBG. Furthermore, ICG, but not BBG, induced the cleavage of caspase-9 and caspase-3, which was inhibited by an inhibitor of p38 MAPK. CONCLUSIONS: ICG is toxic to retinal glial cells because it induces apoptosis, involving induction of the caspase cascade through p38 MAPK phosphorylation. In contrast, BBG does not cause apoptosis and thus could be a safer adjuvant during vitreoretinal surgery.

Formation of aberrant TJ strands by overexpression of claudin-15 in MDCK II cells.

Claudins are one of the transmembrane proteins found at tight junctions (TJs); they constitute the backbone of TJ strands and comprise a multigene family. Claudins share a YV sequence at the COOH-termini, which is considered to be a ZO-binding motif. Overexpression of claudin-15 (15CL) or claudin-15 tagged with enhanced green fluorescent protein at the NH2-terminus (EGFP-15CL) induced aberrant strands in MDCK II cells, even though claudin-15 has the ZO-binding motif. Morphometric analysis by freeze-fracture electron microscopy revealed that the mean number of apical TJ strands increased by 47% in EGFP-1CL-expressing cells, 21% in EGFP-15CL-expressing cells, and 28% in 15CL-expressing cells. The number of free-ended apical strands increased remarkably in EGFP-15CL- and 15CL-expressing cells, but not in EGFP-1CL-expressing cells. When MDCK cells expressing EGFP-1CL, EGFP-15CL or 15CL were co-cultured with parent MDCK cells, which express claudin-1 but not claudin-15, EGFP-15CL and 15CL could not be concentrated at the apical junctional region between the heterotypic cells, though EGFP-1CL could. These results suggest that not only binding to ZO-1, but also head-to-head compatibility between claudin species, is involved in organizing claudin proteins at the apical junctional region.

Differential expression of the tight junction proteins, claudin-1, claudin-4, occludin, ZO-1, and PAR3, in the ameloblasts of rat upper incisors.

Tight junctions (TJs) create a paracellular permeability barrier to restrict the passage of ions, small solutes, and water. Ameloblasts are enamel-forming cells that sequentially differentiate into preameloblasts, secretory, transition, and ruffle-ended and smooth-ended maturation ameloblasts (RAs and SAs). TJs are located at the proximal and distal ends of ameloblasts. TJs at the distal ends of secretory ameloblasts and RAs are well-developed zonula occludens, but other TJs are moderately developed but incomplete zonula occludens (ZO) or less-developed macula occludens. We herein examined the immunofluorescence localization of TJ proteins, 10 claudin isoforms, occludin, ZO-1, and PAR3, a cell polarity-related protein, in ameloblasts of rat upper incisors. ZO-1 and claudin-1 were detected at both ends of all ameloblasts except for the distal ends of SAs. Claudin-4 and occludin were detected at both ends of transition and maturation ameloblasts except for the distal ends of SAs. PAR3 was detected at the proximal TJs of all ameloblasts and faintly at the distal TJs of early RAs. These results indicate that functional zonula occludens formed at the distal ends of the secretory ameloblasts and RAs consisted of different TJ proteins. Therefore, the distal TJs of secretory ameloblasts and RAs may differentially regulate the paracellular permeability to create a microenvironment suitable for enamel deposition and enamel maturation, respectively. In addition, PAR3 may be principally involved in the formation and maintenance of the proximal, but not distal, TJs.

Comparative characterization of mouse rectum CMT93-I and -II cells by expression of claudin isoforms and tight junction morphology and function.

Recent studies suggest that the morphological and physiological properties of tight junctions (TJs) are determined by the combination and mixing ratios of claudin isoforms. In this study, we tried to characterize mouse cell lines by expression of claudin isoforms to use for studying epithelial TJs by overexpression or suppression of claudin(s) in the cells and found that claudin-2 was expressed in a few mouse rectum carcinoma cells, CMT93 cells. We have isolated CMT93-I and -II cells from CMT93 cells by immunohistochemical screening for the presence or absence of claudin-2 expression. Immunofluorescence and RT-PCR analyses showed that expression of claudin-4, -6, -7 and -12 was detected in both cell lines, but claudin-2 was only expressed in CMT93-II cells. There were no differences in paracellular permeability between CMT93-I and -II cells examined by 4 kDa FITC-dextran and fluorescein sodium, or in the number of TJ strands examined by freeze-fracture electron microscopy. However, the transepithelial electrical resistance (TER) of CMT93-I cells was approximately 6.5 times higher than that of CMT93-II cells, suggesting that expression of claudin-2 may be related to decreased TER. Comparative examinations of CMT93-I and -II cells provide a clue how the combination and mixing ratios of claudin isoforms regulate the paracellular permeability.

TEM7 (PLXDC1) in neovascular endothelial cells of fibrovascular membranes from patients with proliferative diabetic retinopathy.

PURPOSE: Proliferative diabetic retinopathy (PDR) results from the formation of fibrovascular membranes (FVMs) in the posterior fundus that can lead to a severe decrease of vision. Tumor endothelial marker 7 (TEM7) is a protein that is highly expressed in the endothelial cells of tumors, but whether it plays a role in FVMs is unknown. The purpose of this study was to determine whether TEM7 is associated with the formation of FVMs. METHODS: FVMs were obtained during vitrectomy from patients with PDR. RT-PCR was performed to determine the level of expression of the mRNA of TEM7. The splice variants of TEM7 were identified by direct sequencing. Immunohistochemical analyses and in situ hybridization was performed to determine the sites of TEM7 in the FVMs. RESULTS: The level of the mRNA of TEM7 was high in 10 of 10 FVMs but was barely detectable in the five idiopathic epiretinal membranes. Direct sequencing of subcloned TEM7 PCR products revealed several splice variants (intracellular, secreted, and membrane-bound forms of TEM7) in the FVMs. Immunohistochemical analysis showed a colocalization of TEM7 and CD34, an endothelial cell marker, in most of the neovascular endothelial cells in the FVMs. Immunoelectron microscopy revealed that membrane-bound TEM7 was expressed on the luminal surfaces of the vascular endothelial cells of FVMs. CONCLUSIONS: This study indicates that TEM7 may play a significant role in the proliferation and maintenance of neovascular endothelial cells in the FVMs. If correct, TEM7 may be a molecular target for new diagnostic and therapeutic strategies for PDR.

Claudin-7 expressed on lateral membrane of rat epididymal epithelium does not form aberrant tight junction strands.

Claudins are integral membrane proteins at tight junctions (TJs) and form TJ strands. In the present study, we found that claudin-7 was localized along the entire lateral membranes of epididymal epithelium, including the apical junctional region throughout the epididymis, but claudin-8 was restricted to the apical junctional region. This finding raises the possibility that aberrant TJ strands may be formed on lateral membranes. Thus, we focused on examining whether TJ strands exist on lateral membranes of epididymal epithelium. Freeze-fracture electron microscopy showed that aberrant TJ strands were observed in only a few principal cells in all segments of the epididymis except for the initial segment, indicating that the occurrence of aberrant strands is very rare. Aberrant TJ strands were smooth and not subdivided into individual particles in the protoplasmic face, and complementary grooves in the extracellular face were almost free of particles. Aberrant TJ strands in the distal caput and corpus epididymis were accompanied by many vesicle-like structures but those in the proximal caput and cauda epididymis were not. These results suggest that most of claudin-7 in lateral membranes may exist in a nonpolymerized form and may play some different roles other than the formation of TJ strands, for example, in the formation of a pool of claudin proteins or in the reinforcement of cell adhesion.

Heterogeneity in expression and subcellular localization of tight junction proteins, claudin-10 and -15, examined by RT-PCR and immunofluorescence microscopy.

Tight junctions regulate paracellular permeability, create the luminal fluid microenvironment of blood vessels and the digestive tract, and also form the protective barrier in the stratified epithelium including the epidermis. Claudins are the integral membrane proteins at tight junctions and form a multigene family composed of at least 24 members, but knowledge of the subcellular localization of each claudin is still fragmentary. We performed RT-PCR for fifteen claudin species to examine the mRNA expression in various mouse tissues, and focused on investigating the subcellular localization of claudin-10 and -15 by immunofluorescence microscopy in various rat tissues. Neither claudin-10 nor -15 was detected in vascular endothelial cells in most tissues, and these claudins were restricted to the vasa recta in the kidney medulla. Both claudins were also detected at apical tight junctions in the epithelium of the jejunum with no intensity gradients along the crypt-to-villus axis. However, both claudins were expressed only in the basal half of the crypt epithelium in the colon, showing obvious gradients along crypt-to-surface axis. Moreover, claudin-10 showed the ectopic subcellular localization where tight junction strands do not exist. Claudin-10 was detected along the entire lateral membranes of acinar cells in addition to the apical tight junctions in exocrine glands, and in the cytoplasm of basal cells in the stratified epithelium including the dorsal skin and cutaneous stomach. These heterogeneous distributions of claudin-10 and -15 in tissues may be related to the differences in paracellular permeability among tissues.