Distinct subdomain organization and molecular composition of a tight junction with adherens junction features

Localization of cadherins in the postnatal cochlear epithelium and their relation to space formation

The sensory epithelium of the cochlea, the organ of Corti, has complex cytoarchitecture consisting of mechanosensory hair cells intercalated by epithelial support cells. The support cells provide important trophic and structural support to the hair cells. Thus, the support cells must be stiff yet compliant enough to withstand and modulate vibrations to the hair cells. Once the sensory cells are properly patterned, the support cells undergo significant remodeling from a simple epithelium into a structurally rigid epithelium with fluid-filled spaces in the murine cochlea. Cell adhesion molecules such as cadherins are necessary for sorting and connecting cells in an intact epithelium. To create the fluid-filled spaces, cell adhesion properties of adjoining cell membranes between cells must change to allow the formation of spaces within an epithelium. However, the dynamic localization of cadherins has not been properly analyzed as these spaces are formed. There are three cadherins that are reported to be expressed during the first postnatal week of development when the tunnel of Corti forms in the cochlea. In this study, we characterize the dynamic localization of cadherins that are associated with cytoskeletal remodeling at the contacting membranes of the inner and outer pillar cells flanking the tunnel of Corti.

The zonula adherens matura redefines the apical junction of intestinal epithelia

Cell-cell apical junctions of epithelia consist of multiprotein complexes that organize as belts regulating cell-cell adhesion, permeability, and mechanical tension: the tight junction (zonula occludens), the zonula adherens (ZA), and the macula adherens. The prevailing dogma is that at the ZA, E-cadherin and catenins are lined with F-actin bundles that support and transmit mechanical tension between cells. Using super-resolution microscopy on human intestinal biopsies and Caco-2 cells, we show that two distinct multiprotein belts are basal of the tight junctions as the intestinal epithelia mature. The most apical is populated with nectins/afadin and lined with F-actin; the second is populated with E-cad/catenins. We name this dual-belt architecture the zonula adherens matura. We find that the apical contraction apparatus and the dual-belt organization rely on afadin expression. Our study provides a revised description of epithelial cell-cell junctions and identifies a module regulating the mechanics of epithelia.

Trans-Compartmental Regulation of Tight Junction Barrier Function

Tight junctions (TJs) are the most apical components of junctional complexes in epithelial and endothelial cells. Barrier function is one of the major functions of TJ, which restricts the ions and small water-soluble molecules from passing through the paracellular pathway. Adherens junctions (AJs) play an important role in cell-cell adhesion and cell signaling. Gap junctions (GJs) are intercellular channels regulating electrical and metabolic signals between cells. It is well known that TJ integral membrane proteins, such as claudins and occludins, are the molecular building blocks responsible for TJ barrier function. However, recent studies demonstrate that proteins of other junctional complexes can influence and regulate TJ barrier function. Therefore, the crosstalk between different cell junctions represents a common means to modulate cellular activities. In this review, we will discuss the interactions among TJ, AJ, and GJ by focusing on how AJ and GJ proteins regulate TJ barrier function in different biological systems.

Hearing loss in mice with disruption of auditory epithelial patterning in the cochlea

In the cochlear auditory epithelia, sensory hair and supporting cells are arranged in a checkerboard-like mosaic pattern, which is conserved across a wide range of species. The cell adhesion molecules nectin-1 and nectin-3 are required for this pattern formation. The checkerboard-like pattern is thought to be necessary for auditory function, but has never been examined. Here, we showed the significance of checkerboard-like cellular pattern in the survival and function of sensory hair cells in the cochlear auditory epithelia of nectin-3 knockout (KO) mice. Nectin-3 KO mice showed progressive hearing loss associated with degeneration of aberrantly attached hair cells via apoptosis. Apoptotic hair cell death was due to the disorganization of tight junctions between the hair cells. Our study revealed that the checkerboard-like cellular pattern in the auditory epithelium provides a structural basis for ensuring the survival of cochlear hair cells and hearing function.

EpCAM proteolysis and release of complexed claudin-7 repair and maintain the tight junction barrier

TJs maintain the epithelial barrier by regulating paracellular permeability. Since TJs are under dynamically fluctuating intercellular tension, cells must continuously survey and repair any damage. However, the underlying mechanisms allowing cells to sense TJ damage and repair the barrier are not yet fully understood. Here, we showed that proteinases play an important role in the maintenance of the epithelial barrier. At TJ break sites, EpCAM-claudin-7 complexes on the basolateral membrane become accessible to apical membrane-anchored serine proteinases (MASPs) and the MASPs cleave EpCAM. Biochemical data and imaging analysis suggest that claudin-7 released from EpCAM contributes to the rapid repair of damaged TJs. Knockout (KO) of MASPs drastically reduced barrier function and live-imaging of TJ permeability showed that MASPs-KO cells exhibited increased size, duration, and frequency of leaks. Together, our results reveal a novel mechanism of TJ maintenance through the localized proteolysis of EpCAM at TJ leaks, and provide a better understanding of the dynamic regulation of epithelial permeability.

Cochlear motion across the reticular lamina implies that it is not a stiff plate

Within the cochlea, the basilar membrane (BM) is coupled to the reticular lamina (RL) through three rows of piezo-like outer hair cells (OHCs) and supporting cells that endow mammals with sensitive hearing. Anatomical differences across OHC rows suggest differences in their motion. Using optical coherence tomography, we measured in vivo and postmortem displacements through the gerbil round-window membrane from approximately the 40-47 kHz best-frequency (BF) regions. Our high spatial resolution allowed measurements across the RL surface at the tops of the three rows of individual OHCs and their bottoms, and across the BM. RL motion varied radially; the third-row gain was more than 3 times greater than that of the first row near BF, whereas the OHC-bottom motions remained similar. This implies that the RL mosaic, comprised of OHC and phalangeal-process tops joined together by adhesion molecules, is much more flexible than the Deiters' cells connected to the OHCs at their bottom surfaces. Postmortem, the measured points moved together approximately in phase. These imply that in vivo, the RL does not move as a stiff plate hinging around the pillar-cell heads near the first row as has been assumed, but that its mosaic-like structure may instead bend and/or stretch.

Thyroxine Regulates the Opening of the Organ of Corti through Affecting P-Cadherin and Acetylated Microtubule

Different serum thyroxine levels may influence the morphology of the inner ear during development. A well-developed organ of Corti (OC) is considered to be critical to the function of hearing. In our study, we treated mice with triiodothyronine (T3) and found that the opening of the OC occurred sooner than in control mice. We also observed an increased formation of acetylated microtubules and a decrease in the adhesion junction molecule P-cadherin the during opening of the OC. Our investigation indicates that thyroxin affects P-cadherin expression and microtubule acetylation to influence the opening of the OC.

Nanoarchitecture and molecular interactions of epithelial cell junction proteins revealed by super-resolution microscopy

Epithelial cells are polarized with defined apical tight junctions (TJs), lateral adherens junctions (AJs), and basal integrin-matrix interactions. However, it is increasingly recognized that resident cell junction proteins can be found in varying locations and with previously unrecognized functions. Our study here presents the nanoarchitecture and nanocolocalization of cell junction proteins in culture and tissue by stochastic optical reconstruction microscopy (STORM). The Z-axial view of noncancerous MDCK-II and PZ-HPV-7 cell-cell junctions resolved β-catenin and p120^ctn localizations to TJs and AJs, with p120^ctn apical to β-catenin and colocalizing with TJ protein claudin-7. More basally, p120^ctn and β-catenin become colocalized. This topography was lost in isogenic Ras-transformed MDCK cells and cancerous PC3 cells, where p120^ctn becomes basally localized in relation to β-catenin. Claudin-7 gene conditional knockout (cKO) in mice also have altered polarity of p120^ctn relative to β-catenin, like that seen in normal-to-cancer cell phenotypic transformation. Additionally, claudin-7 cKO resulted in redistribution and relocalization of other cell junction proteins, including claudin-1, zonula occludens-1, integrin α2, epithelial cell adhesion molecule, and focal adhesion kinase (FAK); specifically, integrin α2 and FAK were observed at the apical-lateral compartment. Our data show that STORM reveals regional cellular junction nanoarchitecture previously uncharacterized, providing new insight into potential trans-compartmental modulation of protein functions.

Microscopic Visualization of Cell-Cell Adhesion Complexes at Micro and Nanoscale

Considerable progress has been made in our knowledge of the morphological and functional varieties of anchoring junctions. Cell-cell adhesion contacts consist of discrete junctional structures responsible for the mechanical coupling of cytoskeletons and allow the transmission of mechanical signals across the cell collective. The three main adhesion complexes are adherens junctions, tight junctions, and desmosomes. Microscopy has played a fundamental role in understanding these adhesion complexes on different levels in both physiological and pathological conditions. In this review, we discuss the main light and electron microscopy techniques used to unravel the structure and composition of the three cell-cell contacts in epithelial and endothelial cells. It functions as a guide to pick the appropriate imaging technique(s) for the adhesion complexes of interest. We also point out the latest techniques that have emerged. At the end, we discuss the problems investigators encounter during their cell-cell adhesion research using microscopic techniques.

Claudin-9 constitutes tight junctions of folliculo-stellate cells in the anterior pituitary gland

The anterior pituitary gland regulates growth, metabolism, and reproduction by secreting hormones. Folliculo-stellate (FS) cells are non-endocrine cells located among hormone-producing cells in the anterior pituitary glands. They form follicular lumens, which are sealed by tight junctions (TJs). Although FS cells are hypothesized to contribute to fine-tuning of endocrine cells, little is known about the exact roles of FS cells. Here, we investigated the molecular composition of TJs in FS cells. We demonstrated that occludin is a good marker for TJs in the pituitary gland and examined the structure of the lumens surrounded by FS cells. We also found that claudin-9 is a major component of TJs in the FS cells. In immunoelectron microscopy, claudin-9 was specifically localized at TJs of the FS cells. The expression of claudin-9 was gradually increased in the pituitary gland after birth, suggesting that claudin-9 is developmentally regulated and performs some specific functions on the paracellular barrier of follicles in the pituitary gland. Furthermore, we found that angulin-1, angulin-2, and tricellulin are localized at the tricellular contacts of the FS cells. Our findings provide a first comprehensive molecular profile of TJs in the FS cells, and may lead us towards unveiling the FS cell functions.

Tight Junction Structure and Function Revisited

Tight junctions (TJs) are intercellular junctions critical for building the epithelial barrier and maintaining epithelial polarity. The claudin family of membrane proteins play central roles in TJ structure and function. However, recent findings have uncovered claudin-independent aspects of TJ structure and function, and additional players including junctional adhesion molecules (JAMs), membrane lipids, phase separation of the zonula occludens (ZO) family of scaffolding proteins, and mechanical force have been shown to play important roles in TJ structure and function. In this review, we discuss how these new findings have the potential to transform our understanding of TJ structure and function, and how the intricate network of TJ proteins and membrane lipids dynamically interact to drive TJ assembly.

Striatin Is Required for Hearing and Affects Inner Hair Cells and Ribbon Synapses

Striatin, a subunit of the serine/threonine phosphatase PP2A, is a core member of the conserved striatin-interacting phosphatase and kinase (STRIPAK) complexes. The protein is expressed in the cell junctions between epithelial cells, which play a role in maintaining cell-cell adhesion. Since the cell junctions are crucial for the function of the mammalian inner ear, we examined the localization and function of striatin in the mouse cochlea. Our results show that in neonatal mice, striatin is specifically expressed in the cell-cell junctions of the inner hair cells, the receptor cells in the mammalian cochlea. Auditory brainstem response measurements of striatin-deficient mice indicated a progressive, high-frequency hearing loss, suggesting that striatin is essential for normal hearing. Moreover, scanning electron micrographs of the organ of Corti revealed a moderate degeneration of the outer hair cells in the middle and basal regions, concordant with the high-frequency hearing loss. Additionally, striatin-deficient mice show aberrant ribbon synapse maturation. Loss of the outer hair cells, combined with the aberrant ribbon synapse distribution, may lead to the observed auditory impairment. Together, these results suggest a novel function for striatin in the mammalian auditory system.

Role of Claudin Proteins in Regulating Cancer Stem Cells and Chemoresistance-Potential Implication in Disease Prognosis and Therapy

Claudins are cell–cell adhesion proteins, which are expressed in tight junctions (TJs), the most common apical cell-cell adhesion. Claudin proteins help to regulate defense and barrier functions, as well as differentiation and polarity in epithelial and endothelial cells. A series of studies have now reported dysregulation of claudin proteins in cancers. However, the precise mechanisms are still not well understood. Nonetheless, studies have clearly demonstrated a causal role of multiple claudins in the regulation of epithelial to mesenchymal transition (EMT), a key feature in the acquisition of a cancer stem cell phenotype in cancer cells. In addition, claudin proteins are known to modulate therapy resistance in cancer cells, a feature associated with cancer stem cells. In this review, we have focused primarily on highlighting the causal link between claudins, cancer stem cells, and therapy resistance. We have also contemplated the significance of claudins as novel targets in improving the efficacy of cancer therapy. Overall, this review provides a much-needed understanding of the emerging role of claudin proteins in cancer malignancy and therapeutic management.

Detailed Clinical Features of Deafness Caused by a Claudin-14 Variant

Tight junctions are cellular junctions that play a major role in the epithelial barrier function. In the inner ear, claudins, occludin, tricellulin, and angulins form the bicellular or tricellular binding of membrane proteins. In these, one type of claudin gene, CLDN14, was reported to be responsible for human hereditary hearing loss, DFNB29. Until now, nine pathogenic variants have been reported, and most phenotypic features remain unclear. In the present study, genetic screening for 68 previously reported deafness causative genes was carried out to identify CLDN14 variants in a large series of Japanese hearing loss patients, and to clarify the prevalence and clinical characteristics of DFNB29 in the Japanese population. One patient had a homozygous novel variant (c.241C>T: p.Arg81Cys) (0.04%: 1/2549). The patient showed progressive bilateral hearing loss, with post-lingual onset. Pure-tone audiograms indicated a high-frequency hearing loss type, and the deterioration gradually spread to other frequencies. The patient showed normal vestibular function. Cochlear implantation improved the patient's sound field threshold levels, but not speech discrimination scores. This report indicated that claudin-14 is essential for maintaining the inner ear environment and suggested the possible phenotypic expansion of DFNB29. This is the first report of a patient with a tight junction variant receiving a cochlear implantation.

Newly synthesized claudins but not occludin are added to the basal side of the tight junction

A network of claudin strands creates continuous cell–cell contacts to form the intercellular tight junction barrier; a second protein, occludin, is associated along these strands. The physiological barrier remains stable despite protein turnover, which involves removal and replacement of claudins both in the steady state and during junction remodeling. Here we use a pulse–block–pulse labeling protocol with fluorescent ligands to label SNAP/CLIP-tags fused to claudins and occludin to identify their spatial trafficking pathways and kinetics in Madin–Darby canine kidney monolayers. We find that claudins are first delivered to the lateral membrane and, over time, enter the junction strand network from the basal side; this is followed by slow replacement of older claudins in the strands. In contrast, even at early times, newly synthesized occludin is found throughout the network. Taking the results together with our previous documentation of the mechanism for claudin strand assembly in a fibroblast model, we speculate that newly synthesized claudins are added at strand breaks and free ends; these are most common in the basalmost edge of the junction. In contrast, occludin can be added directly within the strand network. We further demonstrate that claudin trafficking and half-life depend on carboxy-terminal sequences and that different claudins compete for tight junction localization.

Carbon replicas reveal double stranded structure of tight junctions in phase-contrast electron microscopy

Replica-based freeze-fracture and freeze-etching electron microscopy methods provide surface topography information, particularly suited to studying membrane protein complexes in their native context. The fidelity and resolution of metal replicas is limited by the inherent property of metal atoms to crystallize. To overcome the limitations of metal replicas, we combined amorphous carbon replicas with phase-contrast electron microscopy. Using this approach, tight junction intramembrane fibrils were shown to have a double stranded morphology.

LMO7 deficiency reveals the significance of the cuticular plate for hearing function

Sensory hair cells, the mechanoreceptors of the auditory and vestibular systems, harbor two specialized elaborations of the apical surface, the hair bundle and the cuticular plate. In contrast to the extensively studied mechanosensory hair bundle, the cuticular plate is not as well understood. It is believed to provide a rigid foundation for stereocilia motion, but specifics about its function, especially the significance of its integrity for long-term maintenance of hair cell mechanotransduction, are not known. We discovered that a hair cell protein called LIM only protein 7 (LMO7) is specifically localized in the cuticular plate and the cell junction. Lmo7 KO mice suffer multiple cuticular plate deficiencies, including reduced filamentous actin density and abnormal stereociliar rootlets. In addition to the cuticular plate defects, older Lmo7 KO mice develop abnormalities in inner hair cell stereocilia. Together, these defects affect cochlear tuning and sensitivity and give rise to late-onset progressive hearing loss.

Tight Junction Proteins and Signaling Pathways in Cancer and Inflammation: A Functional Crosstalk

The ability of epithelial cells to organize through cell–cell adhesion into a functioning epithelium serves the purpose of a tight epithelial protective barrier. Contacts between adjacent cells are made up of tight junctions (TJ), adherens junctions (AJ), and desmosomes with unique cellular functions and a complex molecular composition. These proteins mediate firm mechanical stability, serves as a gatekeeper for the paracellular pathway, and helps in preserving tissue homeostasis. TJ proteins are involved in maintaining cell polarity, in establishing organ-specific apical domains and also in recruiting signaling proteins involved in the regulation of various important cellular functions including proliferation, differentiation, and migration. As a vital component of the epithelial barrier, TJs are under a constant threat from proinflammatory mediators, pathogenic viruses and bacteria, aiding inflammation and the development of disease. Inflammatory bowel disease (IBD) patients reveal loss of TJ barrier function, increased levels of proinflammatory cytokines, and immune dysregulation; yet, the relationship between these events is partly understood. Although TJ barrier defects are inadequate to cause experimental IBD, mucosal immune activation is changed in response to augmented epithelial permeability. Thus, the current studies suggest that altered barrier function may predispose or increase disease progression and therapies targeted to specifically restore the barrier function may provide a substitute or supplement to immunologic-based therapies. This review provides a brief introduction about the TJs, AJs, structure and function of TJ proteins. The link between TJ proteins and key signaling pathways in cell proliferation, transformation, and metastasis is discussed thoroughly. We also discuss the compromised intestinal TJ integrity under inflammatory conditions, and the signaling mechanisms involved that bridge inflammation and cancer.

Striatin is a novel modulator of cell adhesion

The adherens junctions (AJs) and tight junctions (TJs) provide critical adhesive contacts between neighboring epithelial cells and are crucial for epithelial adhesion, integrity, and barrier functions in a wide variety of tissues and organisms. The striatin protein family, which are part of the striatin interaction phosphatases and kinases complex, are multidomain scaffolding proteins that play important biologic roles. We have previously shown that striatin colocalizes with the tumor suppressor protein adenomatous polyposis coli in the TJs of epithelial cells. Here we show that striatin affects junction integrity and cell migration, probably through a mechanism that involves the adhesion molecule E-cadherin. Cells engaged in cell-cell adhesion expressed a high MW-modified form of striatin that forms stable associations with detergent-insoluble, membrane-bound cellular fractions. In addition, striatin has recently been suggested to be a target of the poly (ADP-ribose) polymerases Tankyrase 1, and we have found that striatin interacts with Tankyrase 1 and is subsequently poly-ADP-ribosylated. Taken together, our results suggest that striatin is a novel cell-cell junctional protein that functions to maintain correct cell adhesion and may have a role in establishing the relationship between AJs and TJs that is fundamental for epithelial cell-cell adhesion.-Lahav-Ariel, L., Caspi, M., Nadar-Ponniah, P. T., Zelikson, N., Hofmann, I., Hanson, K. K., Franke, W. W., Sklan, E. H., Avraham, K. B., Rosin-Arbesfeld, R. Striatin is a novel modulator of cell adhesion.

Transcriptomes of cochlear inner and outer hair cells from adult mice

Inner hair cells (IHCs) and outer hair cells (OHCs) are the two anatomically and functionally distinct types of mechanosensitive receptor cells in the mammalian cochlea. The molecular mechanisms defining their morphological and functional specializations are largely unclear. As a first step to uncover the underlying mechanisms, we examined the transcriptomes of IHCs and OHCs isolated from adult CBA/J mouse cochleae. One thousand IHCs and OHCs were separately collected using the suction pipette technique. RNA sequencing of IHCs and OHCs was performed and their transcriptomes were analyzed. The results were validated by comparing some IHC and OHC preferentially expressed genes between present study and published microarray-based data as well as by real-time qPCR. Antibody-based immunocytochemistry was used to validate preferential expression of SLC7A14 and DNM3 in IHCs and OHCs. These data are expected to serve as a highly valuable resource for unraveling the molecular mechanisms underlying different biological properties of IHCs and OHCs as well as to provide a road map for future characterization of genes expressed in IHCs and OHCs.

Anatomical basis of drug delivery to the inner ear

The isolated anatomical position and blood-labyrinth barrier hampers systemic drug delivery to the mammalian inner ear. Intratympanic placement of drugs and permeation via the round- and oval window are established methods for local pharmaceutical treatment. Mechanisms of drug uptake and pathways for distribution within the inner ear are hard to predict. The complex microanatomy with fluid-filled spaces separated by tight- and leaky barriers compose various compartments that connect via active and passive transport mechanisms. Here we provide a review on the inner ear architecture at light- and electron microscopy level, relevant for drug delivery. Focus is laid on the human inner ear architecture. Some new data add information on the human inner ear fluid spaces generated with high resolution microcomputed tomography at 15 μm resolution. Perilymphatic spaces are connected with the central modiolus by active transport mechanisms of mesothelial cells that provide access to spiral ganglion neurons. Reports on leaky barriers between scala tympani and the so-called cortilymph compartment likely open the best path for hair cell targeting. The complex barrier system of tight junction proteins such as occludins, claudins and tricellulin isolates the endolymphatic space for most drugs. Comparison of relevant differences of barriers, target cells and cell types involved in drug spread between main animal models and humans shall provide some translational aspects for inner ear drug applications.

Multiple claudin-claudin cis interfaces are required for tight junction strand formation and inherent flexibility

Tight junctions consist of a network of sealing strands that create selective ion permeability barriers between adjoining epithelial or endothelial cells. The current model for tight junction strands consists of paired rows of claudins (Cldn) coupled by a cis interface (X-1) derived from crystalline Cldn15. Here we show that tight junction strands exhibit a broad range of lateral bending, indicating diversity in cis interactions. By combining protein–protein docking, coevolutionary analysis, molecular dynamics, and a mutagenesis screen, we identify a new Cldn–Cldn cis interface (Cis-1) that shares interacting residues with X-1 but has an ~ 17° lateral rotation between monomers. In addition, we found that a missense mutation in a Cldn14 that causes deafness and contributes stronger to Cis-1 than to X-1 prevents strand formation in cultured cells. Our results suggest that Cis-1 contributes to the inherent structural flexibility of tight junction strands and is required for maintaining permeability barrier function and hearing.

Jun Zhao, Evan S. Krystofiak, and colleagues identified a new cis interface (Cis-1) essential for the formation of normal tight junctions. This study suggests that Cis-1 contributes to maintaining structural flexibility of tight junction strands for proper ion balance and hearing.

Cytoskeletal Stability in the Auditory Organ In Vivo: RhoA Is Dispensable for Wound Healing but Essential for Hair Cell Development

Wound healing in the inner ear sensory epithelia is performed by the apical domains of supporting cells (SCs). Junctional F-actin belts of SCs are thin during development but become exceptionally thick during maturation. The functional significance of the thick belts is not fully understood. We have studied the role of F-actin belts during wound healing in the developing and adult cochlea of mice in vivo. We show that the thick belts serve as intracellular scaffolds that preserve the positions of surviving cells in the cochlear sensory epithelium. Junctions associated with the thick F-actin belts did not readily disassemble during wound healing. To compensate for this, basolateral membranes of SCs participated in the closure of surface breach. Because not only neighboring but also distant SCs contributed to wound healing by basolateral protrusions, this event appears to be triggered by contact-independent diffusible signals. In the search for regulators of wound healing, we inactivated RhoA in SCs, which, however, did not limit wound healing. RhoA inactivation in developing outer hair cells (OHCs) caused myosin II delocalization from the perijunctional domain and apical cell-surface enlargement. These abnormalities led to the extrusion of OHCs from the epithelium. These results demonstrate the importance of stability of the apical domain, both in wound repair by SCs and in development of OHCs, and that only this latter function is regulated by RhoA. Because the correct cytoarchitecture of the cochlear sensory epithelium is required for normal hearing, the stability of cell apices should be maintained in regenerative and protective interventions.

Cell-specific diversity in the expression and organization of cytoplasmic plaque proteins of apical junctions

Tight and adherens junctions play critical roles in the barrier, adhesion, and signaling functions of epithelial and endothelial cells. How the molecular organization of these junctions is tuned to the widely diverse physiological requirements of each tissue type is not well understood. Here, we address this question by examining the expression, localization, and interactions of major cytoplasmic plaque proteins of tight and adherens junctions in different cultured epithelial and endothelial cell lines. Immunoblotting and immunofluorescence analyses show that the expression profiles of cingulin, paracingulin, ZO-1, ZO-2, ZO-3, PLEKHA7, afadin, PDZD11, p120-catenin, and α-catenin, as well as the transmembrane junctional proteins occludin, E-cadherin, and VE-cadherin, are significantly diverse when comparing kidney cells (MDCK, mCCD), keratinocytes (HaCaT), lung carcinoma (A427, A549), and endothelium-derived cells (bEnd.3, meEC, H5V). Proximity ligation and co-immunoprecipitation assays show that PLEKHA7 and PDZD11 are significantly more associated with the tight junction proteins cingulin and ZO-1 in aortic endothelium-derived (meEC) cells but not kidney collecting duct epithelial (mCCD) cells. These results provide evidence that the cytoplasmic plaques of tight and adherens junctions are diverse in their composition and molecular architecture and establish a conceptual framework by which we can rationally address the mechanisms of tissue-dependent junction physiology and signaling by cytoplasmic junctional proteins.

Intestinal epithelial claudins: expression and regulation in homeostasis and inflammation

The intestinal epithelium forms a highly dynamic and selective barrier that controls absorption of fluid and solutes while restricting pathogen access to underlying tissues. Barrier properties are achieved by intercellular junctions that include an apical tight junction (TJ) and subjacent adherens junctions and desmosomes. The TJ tetraspan claudin proteins form pores between epithelial cells to control paracellular fluid and ion movement. In addition to regulation of barrier function, claudin family members control epithelial homeostasis and are expressed in a spatiotemporal manner in the intestinal crypt-luminal axis. This delicate balance of physiologic differential claudin protein expression is altered during mucosal inflammation. Inflammatory mediators influence transcriptional regulation, as well as endocytic trafficking, targeting, and retention of claudins in the TJ. Increased expression of intestinal epithelial claudin-1, -2, and -18 with downregulation of claudin-3, -4, -5, -7, -8, and -12 has been observed in intestinal inflammatory disorders. Such changes in claudin proteins modify the epithelial barrier function in addition to influencing epithelial and mucosal homeostasis. An improved understanding of the regulatory mechanisms that control epithelial claudin proteins will provide strategies to strengthen the epithelial barrier function and restore mucosal homeostasis in inflammatory disorders.

Survival of human embryonic stem cells implanted in the guinea pig auditory epithelium

Hair cells in the mature cochlea cannot spontaneously regenerate. One potential approach for restoring hair cells is stem cell therapy. However, when cells are transplanted into scala media (SM) of the cochlea, they promptly die due to the high potassium concentration. We previously described a method for conditioning the SM to make it more hospitable to implanted cells and showed that HeLa cells could survive for up to a week using this method. Here, we evaluated the survival of human embryonic stem cells (hESC) constitutively expressing GFP (H9 Cre-LoxP) in deaf guinea pig cochleae that were pre-conditioned to reduce potassium levels. GFP-positive cells could be detected in the cochlea for at least 7 days after the injection. The cells appeared spherical or irregularly shaped, and some were aggregated. Flushing SM with sodium caprate prior to transplantation resulted in a lower proportion of stem cells expressing the pluripotency marker Oct3/4 and increased cell survival. The data demonstrate that conditioning procedures aimed at transiently reducing the concentration of potassium in the SM facilitate survival of hESCs for at least one week. During this time window, additional procedures can be applied to initiate the differentiation of the implanted hESCs into new hair cells.

Cycling hypoxia affects cell invasion and proliferation through direct regulation of claudin1 / claudin7 expression, and indirect regulation of P18 through claudin7

Claudins (CLDNs), the major integral membrane proteins at tight junction, play critical roles in apical cell-to-cell adhesion, maintenance of epithelial polarity, and formation of impermeable barriers between epithelial cells.We investigated in this study the expression of CLDNs- Claudin1 (CLDN1) and Claudin7 (CLDN7), and their relation to tumor progression in nasopharyngeal cancer (NPC). CLDN7, rather than CLDN1, showed higher expression in both undifferentiated tumor tissue and the poorly differentiated CNE2 cells, compared with differentiated tissue and the highly differentiated CNE1 cells. Furthermore, knockdown of CLDN7 dramatically inhibited the metastasis and invasion of CNE2 cells suggesting that CLDN7 could act as a biomarker for NPC metastasis.Cycling hypoxia could induce significant changes in CLDN1 and CLDN7 expression in NPC cells. Genetics analysis demonstrated that CLDN1/CLDN7 were not only regulated directly by HIF1a but also affected each other through a feedback mechanism. CLDN7 acted as a bridge to promote HIF1a-induced P18 expression and cell differentiation. Taken together, our results provide evidence that adjusting the oxygenation time and cycles in NPC might be an effective method to prevent / delay the metastasis of poorly differentiated NPC cells.

Claudin-11 Tight Junctions in Myelin Are a Barrier to Diffusion and Lack Strong Adhesive Properties

The radial component is a network of interlamellar tight junctions (TJs) unique to central nervous system myelin. Ablation of claudin-11, a TJ protein, results in the absence of the radial component and compromises the passive electrical properties of myelin. Although TJs are known to regulate paracellular diffusion, this barrier function has not been directly demonstrated for the radial component, and some evidence suggests that the radial component may also mediate adhesion between myelin membranes. To investigate the physical properties of claudin-11 TJs, we compared fresh, unfixed Claudin 11-null and control nerves using x-ray and neutron diffraction. In Claudin 11-null tissue, we detected no changes in myelin structure, stability, or membrane interactions, which argues against the notion that myelin TJs exhibit significant adhesive properties. Moreover, our osmotic stressing and D2O-H2O exchange experiments demonstrate that myelin lacking claudin-11 is more permeable to water and small osmolytes. Thus, our data indicate that the radial component serves primarily as a diffusion barrier and elucidate the mechanism by which TJs govern myelin function.

Deletion of Brg1 causes abnormal hair cell planer polarity, hair cell anchorage, and scar formation in mouse cochlea

Hair cells (HCs) are mechanosensors that play crucial roles in perceiving sound, acceleration, and fluid motion. The precise architecture of the auditory epithelium and its repair after HC loss is indispensable to the function of organ of Corti (OC). In this study, we showed that Brg1 was highly expressed in auditory HCs. Specific deletion of Brg1 in postnatal HCs resulted in rapid HC degeneration and profound deafness in mice. Further experiments showed that cell-intrinsic polarity of HCs was abolished, docking of outer hair cells (OHCs) by Deiter’s cells (DCs) failed, and scar formation in the reticular lamina was deficient. We demonstrated that Brg1 ablation disrupted the Gαi/Insc/LGN and aPKC asymmetric distributions, without overt effects on the core planer cell polarity (PCP) pathway. We also demonstrated that Brg1-deficient HCs underwent apoptosis, and that leakage in the reticular lamina caused by deficient scar formation shifted the mode of OHC death from apoptosis to necrosis. Together, these data demonstrated a requirement for Brg1 activity in HC development and suggested a role for Brg1 in the proper cellular structure formation of HCs.

Tricellular Tight Junctions in the Inner Ear

Tight junctions (TJs) are structures that seal the space between the epithelial cell sheets. In the inner ear, the barrier function of TJs is indispensable for the separation of the endolymphatic and perilymphatic spaces, which is essential for the generation and maintenance of the endocochlear potential (EP). TJs are formed by the intercellular binding of membrane proteins, known as claudins, and mutations in these proteins cause deafness in humans and mice. Within the epithelial cell sheet, however, a bound structure is present at the site where the corners of three cells meet (tricellular tight junctions (tTJs)), and the maintenance of the barrier function at this location cannot be explained by the claudins alone. Tricellulin and the angulin family of proteins (angulin-1/LSR, angulin-2/ILDR1, and angulin-3/ILDR2) have been identified as tTJ-associated proteins. Tricellulin and ILDR1 are localized at the tTJ and alterations in these proteins have been reported to be involved in deafness. In this review, we will present the current state of knowledge for tTJs.

Genetic Correction of Induced Pluripotent Stem Cells From a Deaf Patient With MYO7A Mutation Results in Morphologic and Functional Recovery of the Derived Hair Cell-Like Cells

The genetic correction of induced pluripotent stem cells (iPSCs) induced from somatic cells of patients with sensorineural hearing loss (caused by hereditary factors) is a promising method for its treatment. The correction of gene mutations in iPSCs could restore the normal function of cells and provide a rich source of cells for transplantation. In the present study, iPSCs were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T; P-iPSCs), the asymptomatic father of the patient (MYO7A c.1184G>A mutation; CF-iPSCs), and a normal donor (MYO7A(WT/WT); C-iPSCs). One of MYO7A mutation sites (c.4118C>T) in the P-iPSCs was corrected using CRISPR/Cas9. The corrected iPSCs (CP-iPSCs) retained cell pluripotency and normal karyotypes. Hair cell-like cells induced from CP-iPSCs showed restored organization of stereocilia-like protrusions; moreover, the electrophysiological function of these cells was similar to that of cells induced from C-iPSCs and CF-iPSCs. These results might facilitate the development of iPSC-based gene therapy for genetic disorders.

SIGNIFICANCE

Induced pluripotent stem cells (iPSCs) were generated from a deaf patient with compound heterozygous MYO7A mutations (c.1184G>A and c.4118C>T). One of the MYO7A mutation sites (c.4118C>T) in the iPSCs was corrected using CRISPR/Cas9. The genetic correction of MYO7A mutation resulted in morphologic and functional recovery of hair cell-like cells derived from iPSCs. These findings confirm the hypothesis that MYO7A plays an important role in the assembly of stereocilia into stereociliary bundles. Thus, the present study might provide further insight into the pathogenesis of sensorineural hearing loss and facilitate the development of therapeutic strategies against monogenic disease through the genetic repair of patient-specific iPSCs.

Progressive Hearing Loss in Mice Carrying a Mutation in Usp53

Disordered protein ubiquitination has been linked to neurodegenerative disease, yet its role in inner ear homeostasis and hearing loss is essentially unknown. Here we show that progressive hearing loss in the ethylnitrosourea-generated mambo mouse line is caused by a mutation in Usp53, a member of the deubiquitinating enzyme family. USP53 contains a catalytically inactive ubiquitin-specific protease domain and is expressed in cochlear hair cells and a subset of supporting cells. Although hair cell differentiation is unaffected in mambo mice, outer hair cells degenerate rapidly after the first postnatal week. USP53 colocalizes and interacts with the tight junction scaffolding proteins TJP1 and TJP2 in polarized epithelial cells, suggesting that USP53 is part of the tight junction complex. The barrier properties of tight junctions of the stria vascularis appeared intact in a biotin tracer assay, but the endocochlear potential is reduced in adult mambo mice. Hair cell degeneration in mambo mice precedes endocochlear potential decline and is rescued in cochlear organotypic cultures in low potassium milieu, indicating that hair cell loss is triggered by extracellular factors. Remarkably, heterozygous mambo mice show increased susceptibility to noise injury at high frequencies. We conclude that USP53 is a novel tight junction-associated protein that is essential for the survival of auditory hair cells and normal hearing in mice, possibly by modulating the barrier properties and mechanical stability of tight junctions.

SIGNIFICANCE STATEMENT

Hereditary hearing loss is extremely prevalent in the human population, but many genes linked to hearing loss remain to be discovered. Forward genetics screens in mice have facilitated the identification of genes involved in sensory perception and provided valuable animal models for hearing loss in humans. This involves introducing random mutations in mice, screening the mice for hearing defects, and mapping the causative mutation. Here, we have identified a mutation in the Usp53 gene that causes progressive hearing loss in the mambo mouse line. We demonstrate that USP53 is a catalytically inactive deubiquitinating enzyme and a novel component of tight junctions that is necessary for sensory hair cell survival and inner ear homeostasis.

Deletion of Tricellulin Causes Progressive Hearing Loss Associated with Degeneration of Cochlear Hair Cells

Tricellulin (also known as MARVELD2) is considered as a central component of tricellular tight junctions and is distributed among various epithelial tissues. Although mutations in the gene encoding tricellulin are known to cause deafness in humans (DFNB49) and mice, the influence of its systemic deletion in vivo remains unknown. When we generated tricellulin-knockout mice (Tric^−/−), we found an early-onset rapidly progressive hearing loss associated with the degeneration of hair cells (HCs); however, their body size and overall appearance were normal. Tric^−/− mice did not show any morphological change pertaining to other organs such as the gastrointestinal tract, liver, kidney, thyroid gland and heart. The endocochlear potential (EP) was normal in Tric^−/− mice, suggesting that the tight junction barrier is maintained in the stria vascularis, where EP is generated. The degeneration of HCs, which occurred after the maturation of EP, was prevented in the culture medium with an ion concentration similar to that of the perilymph. These data demonstrate the specific requirement of tricellulin for maintaining ion homeostasis around cochlear HCs to ensure their survival. The Tric^−/− mouse provides a new model for understanding the distinct roles of tricellulin in different epithelial systems as well as in the pathogenesis of DFNB49.

Characterization of the transcriptome of nascent hair cells and identification of direct targets of the Atoh1 transcription factor

Hair cells are sensory receptors for the auditory and vestibular system in vertebrates. The transcription factor Atoh1 is both necessary and sufficient for the differentiation of hair cells, and is strongly upregulated during hair-cell regeneration in nonmammalian vertebrates. To identify genes involved in hair cell development and function, we performed RNA-seq profiling of purified Atoh1-expressing hair cells from the neonatal mouse cochlea. We identified >600 enriched transcripts in cochlear hair cells, of which 90% have not been previously shown to be expressed in hair cells. We identified 233 of these hair cell genes as candidates to be directly regulated by Atoh1 based on the presence of Atoh1 binding sites in their regulatory regions and by analyzing Atoh1 ChIP-seq datasets from the cerebellum and small intestine. We confirmed 10 of these genes as being direct Atoh1 targets in the cochlea by ChIP-PCR. The identification of candidate Atoh1 target genes is a first step in identifying gene regulatory networks for hair-cell development and may inform future studies on the potential role of Atoh1 in mammalian hair cell regeneration.

A Chromosome 17 Locus Engenders Frequency-Specific Non-Progressive Hearing Loss that Contributes to Age-Related Hearing Loss in Mice

The 129S6/SvEvTac (129S6) inbred mouse is known for its resistance to noise-induced hearing loss (NIHL). However, less is understood of its unique age-related hearing loss (AHL) phenotype and its potential relationship with the resistance to NIHL. Here, we studied the physiological characteristics of hearing loss in 129S6 and asked if noise resistance (NR) and AHL are genetically linked to the same chromosomal region. We used auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE) to examine hearing sensitivity between 1 and 13 months of age of recombinant-inbred (congenic) mice with an NR phenotype. We identified a region of proximal chromosome (Chr) 17 (D17Mit143-D17Mit100) that contributes to a sensory, non-progressive hearing loss (NPHL) affecting exclusively the high-frequencies (>24 kHz) and maps to the nr1 locus on Chr 17. ABR experiments showed that 129S6 and CBA/CaJ F1 (CBACa) hybrid mice exhibit normal hearing, indicating that the hearing loss in 129S6 mice is inherited recessively. An allelic complementation test between the 129S6 and 101/H (101H) strains did not rescue hearing loss, suggesting genetic allelism between the nphl and phl1 loci of these strains, respectively. The hybrids had a milder hearing loss than either parental strain, which indicate a possible interaction with other genes in the mouse background or a digenic interaction between different genes that reside in the same genomic region. Our study defines a locus for nphl on Chr 17 affecting frequencies greater than 24 kHz.

Using injectoporation to deliver genes to mechanosensory hair cells

Mechanosensation, the transduction of mechanical force into electrochemical signals, allows organisms to detect touch and sound, to register movement and gravity, and to sense changes in cell volume and shape. The hair cells of the mammalian inner ear are the mechanosensors for the detection of sound and head movement. The analysis of gene function in hair cells has been hampered by the lack of an efficient gene transfer method. Here we describe a method termed injectoporation that combines tissue microinjection with electroporation to express cDNAs and shRNAs in mouse cochlear hair cells. Injectoporation allows for gene transfer into dozens of hair cells, and it is compatible with the analysis of hair cell function using imaging approaches and electrophysiology. Tissue dissection and injectoporation can be carried out within a few hours, and the tissue can be cultured for days for subsequent functional analyses.

ILDR1 null mice, a model of human deafness DFNB42, show structural aberrations of tricellular tight junctions and degeneration of auditory hair cells

In the mammalian inner ear, bicellular and tricellular tight junctions (tTJs) seal the paracellular space between epithelial cells. Tricellulin and immunoglobulin-like (Ig-like) domain containing receptor 1 (ILDR1, also referred to as angulin-2) localize to tTJs of the sensory and non-sensory epithelia in the organ of Corti and vestibular end organs. Recessive mutations of TRIC (DFNB49) encoding tricellulin and ILDR1 (DFNB42) cause human nonsyndromic deafness. However, the pathophysiology of DFNB42 deafness remains unknown. ILDR1 was recently reported to be a lipoprotein receptor mediating the secretion of the fat-stimulated cholecystokinin (CCK) hormone in the small intestine, while ILDR1 in EpH4 mouse mammary epithelial cells in vitro was shown to recruit tricellulin to tTJs. Here we show that two different mouse Ildr1 mutant alleles have early-onset severe deafness associated with a rapid degeneration of cochlear hair cells (HCs) but have a normal endocochlear potential. ILDR1 is not required for recruitment of tricellulin to tTJs in the cochlea in vivo; however, tricellulin becomes mislocalized in the inner ear sensory epithelia of ILDR1 null mice after the first postnatal week. As revealed by freeze-fracture electron microscopy, ILDR1 contributes to the ultrastructure of inner ear tTJs. Taken together, our data provide insight into the pathophysiology of human DFNB42 deafness and demonstrate that ILDR1 is crucial for normal hearing by maintaining the structural and functional integrity of tTJs, which are critical for the survival of auditory neurosensory HCs.

Architecture of tight junctions and principles of molecular composition

The tight junction creates an intercellular barrier limiting paracellular movement of solutes and material across epithelia. Currently many proteins have been identified as components of the tight junction and understanding their architectural organization and interactions is critical to understanding the biology of the barrier. In general the architecture can be conceptualized into compartments with the transmembrane barrier proteins (claudins, occludin, JAM-A, etc.), linked to peripheral scaffolding proteins (such as ZO-1, afadin, MAGI1, etc.) which are in turned linked to actin and microtubules through numerous linkers (cingulin, myosins, protein 4.1, etc.). Within this complex network are associated many signaling proteins that affect the barrier and broader cell functions. The PDZ domain is a commonly used motif to specifically link individual junction protein pairs. Here we review some of the key proteins defining the tight junction and general themes of their organization with the perspective that much will be learned about function by characterizing the detailed architecture and subcompartments within the junction.

Genetics of auditory mechano-electrical transduction

The hair bundles of cochlear hair cells play a central role in the auditory mechano-electrical transduction (MET) process. The identification of MET components and of associated molecular complexes by biochemical approaches is impeded by the very small number of hair cells within the cochlea. In contrast, human and mouse genetics have proven to be particularly powerful. The study of inherited forms of deafness led to the discovery of several essential proteins of the MET machinery, which are currently used as entry points to decipher the associated molecular networks. Notably, MET relies not only on the MET machinery but also on several elements ensuring the proper sound-induced oscillation of the hair bundle or the ionic environment necessary to drive the MET current. Here, we review the most significant advances in the molecular bases of the MET process that emerged from the genetics of hearing.

Conditioning the cochlea to facilitate survival and integration of exogenous cells into the auditory epithelium

The mammalian auditory epithelium (AE) cannot replace supporting cells and hair cells once they are lost. Therefore, sensorineural hearing loss associated with missing cells is permanent. This inability to regenerate critical cell types makes the AE a potential target for cell replacement therapies such as stem cell transplantation. Inserting stem cells into the AE of deaf ears is a complicated task due to the hostile, high potassium environment of the scala media in the cochlea, and the robust junctional complexes between cells in the AE that resist stem cell integration. Here, we evaluate whether temporarily reducing potassium levels in the scala media and disrupting the junctions in the AE make the cochlear environment more receptive and facilitate survival and integration of transplanted cells. We used sodium caprate to transiently disrupt the AE junctions, replaced endolymph with perilymph, and blocked stria vascularis pumps with furosemide. We determined that these three steps facilitated survival of HeLa cells in the scala media for at least 7 days and that some of the implanted cells formed a junctional contact with native AE cells. The data suggest that manipulation of the cochlear environment facilitates survival and integration of exogenously transplanted HeLa cells in the scala media.

Maintenance of stereocilia and apical junctional complexes by Cdc42 in cochlear hair cells

Cdc42 is a key regulator of dynamic actin organization. However, little is known about how Cdc42-dependent actin regulation influences steady-state actin structures in differentiated epithelia. We employed inner ear hair-cell-specific conditional knockout to analyze the role of Cdc42 in hair cells possessing highly elaborate stable actin protrusions (stereocilia). Hair cells of Atoh1-Cre;Cdc42(flox/flox) mice developed normally but progressively degenerated after maturation, resulting in progressive hearing loss particularly at high frequencies. Cochlear hair cell degeneration was more robust in inner hair cells than in outer hair cells, and began as stereocilia fusion and depletion, accompanied by a thinning and waving circumferential actin belt at apical junctional complexes (AJCs). Adenovirus-encoded GFP-Cdc42 expression in hair cells and fluorescence resonance energy transfer (FRET) imaging of hair cells from transgenic mice expressing a Cdc42-FRET biosensor indicated Cdc42 presence and activation at stereociliary membranes and AJCs in cochlear hair cells. Cdc42-knockdown in MDCK cells produced phenotypes similar to those of Cdc42-deleted hair cells, including abnormal microvilli and disrupted AJCs, and downregulated actin turnover represented by enhanced levels of phosphorylated cofilin. Thus, Cdc42 influenced the maintenance of stable actin structures through elaborate tuning of actin turnover, and maintained function and viability of cochlear hair cells.

Marshalin, a microtubule minus-end binding protein, regulates cytoskeletal structure in the organ of Corti

Dramatic structural changes in microtubules (MT) and the assembly of complicated intercellular connections are seen during the development of the cellular matrix of the sense organ for hearing, the organ of Corti. This report examines the expression of marshalin, a minus-end binding protein, during this process of cochlear development. We discovered that marshalin is abundantly expressed in both sensory hair cells and supporting cells. In the adult, prominent marshalin expression is observed in the cuticular plates of hair cells and in the noncentrosomal MT organization centers (MTOC) of Deiters' and pillar cells. Based upon differences in marshalin expression patterns seen in the organ of Corti, we identified eight isoforms ranging from 863 to 1280 amino acids. mRNAs/proteins associated with marshalin's isoforms are detected at different times during development. These isoforms carry various protein-protein interacting domains, including coiled-coil (CC), calponin homology (CH), proline-rich (PR), and MT-binding domains, referred to as CKK. We, therefore, examined membranous organelles and structural changes in the cytoskeleton induced by expressing two of these marshalin isoforms in vitro. Long forms containing CC and PR domains induce thick, spindle-shaped bundles, whereas short isoforms lacking CC and PR induce more slender variants that develop into densely woven networks. Together, these data suggest that marshalin is closely associated with noncentrosomal MTOCs, and may be involved in MT bundle formation in supporting cells. As a scaffolding protein with multiple isoforms, marshalin is capable of modifying cytoskeletal networks, and consequently organelle positioning, through interactions with various protein partners present in different cells.

A "Tric" to tighten cell-cell junctions in the cochlea for hearing

Tricellulin is a tricellular tight junction-associated membrane protein that controls movement of solutes at these specialized cell intersections. Mutations in the gene encoding tricellulin, TRIC, lead to nonsyndromic deafness. In this issue of the JCI, Nayak et al. created a gene-targeted knockin mouse in order to mimic the pathology of a human TRIC mutation. Deafness appears to be caused either by an increase in the K+ ion concentration around the basolateral surfaces of the outer hair cells or, alternatively, by an increase in small molecules such as ATP around the hair bundle, leading to cellular dysfunction and degeneration. Furthermore, the mice have features suggestive of syndromic hearing loss, which may have implications for care and treatment of patients harboring TRIC mutations.

Claudin interactions in and out of the tight junction

Claudins form the paracellular tight junction seal in epithelial tissues. Although there is still limited information on how these proteins are organized at the junction, a number of recent studies have provided useful insights both into claudin-claudin interactions and into interactions between claudins and other proteins. The focus of this review is to summarize recent information about claudin interactions and to identify critical unanswered questions about claudin organization and tight junction structure which will be required to understand claudin function.

Differential pathways of claudin oligomerization and integration into tight junctions

Tight junctions are the critical intercellular structure required to establish an epithelial barrier. Among the several classes of proteins required to form tight junctions are the tetraspan transmembrane proteins known as claudins that directly determine paracellular permeability. Considerable progress has been made in understanding how incorporation of different claudins into tight junctions increase or decrease paracellular permeability and ion selectivity. However, it has proven difficult to identify discrete steps in claudin assembly and whether claudins exist in distinct oligomerization states prior to tight junction assembly. Studies of homomeric and heteromeric claudin-claudin interactions using complementary techniques suggest a diversity of pathways used by different claudins to oligomerize and integrate into tight junctions.