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Tight junction

Tight junctions (TJs) are specialized regions of cell-cell contact in epithelial and endothelial tissues that form selective, semipermeable barriers regulating the paracellular diffusion of ions, solutes, and water while maintaining apicobasal polarity of the plasma membrane.^[1] These junctions create a circumferential seal at the apical-most aspect of lateral membranes, just above adherens junctions, appearing as a meshwork of anastomosing strands under freeze-fracture electron microscopy.^[1] First observed in 1963 by Farquhar and Palade using transmission electron microscopy, TJs were initially termed "zonula occludens" due to their belt-like arrangement around epithelial cells.^[1] Structurally, TJs comprise transmembrane proteins embedded in the lipid bilayer, including the claudin family (with at least 27 members in mammals), occludin, and tricellulin, which polymerize to form the paracellular barrier strands.^[1] Claudins are the core components, adopting a tetraspan configuration to create selective ion channels or pores within bicellular TJs (bTJs), while tricellulin and angulins seal tricellular TJs (tTJs) at points where three cells meet.^[1] Cytoplasmic plaque proteins, such as zonula occludens (ZO)-1, ZO-2, ZO-3, and cingulin, anchor these transmembrane elements to the actin cytoskeleton and facilitate signaling.^[2] The strands exhibit dynamic remodeling, influenced by factors like calcium levels and phosphorylation, allowing adaptability in barrier permeability.^[2] Functionally, TJs serve multiple roles beyond barrier formation: they act as a "fence" to restrict the diffusion of membrane lipids and proteins between apical and basolateral domains, ensuring epithelial polarity essential for vectorial transport.^[2] They also function as signaling hubs, recruiting GTPases (e.g., Rap1, RhoA), kinases, and transcription factors to regulate gene expression, cytoskeletal dynamics, and junction assembly.^[1] Paracellular selectivity varies by tissue; for instance, cation-selective pores form via claudin-2 or -15, with pore sizes around 4-10 Å, while leak pathways (~100 Å) handle larger or unregulated flux during stress.^[2] In physiology, TJs are vital for compartmentalizing body fluids, as in the intestinal mucosa, renal tubules, and blood-brain barrier, preventing pathogen invasion and maintaining homeostasis.^[1] Dysregulation contributes to diseases, including inflammatory conditions like Crohn's disease (via claudin loss), cancers (e.g., claudin-11 promoting migration), and genetic disorders such as neonatal ichthyosis (claudin-1 mutations) or nonsyndromic deafness (tricellulin defects).^[1] Recent advances, including super-resolution imaging and liquid-liquid phase separation models for ZO-1, highlight their dynamic assembly and therapeutic potential in barrier restoration.^[2]

Historical Background

Discovery and Early Observations

The initial discovery of tight junctions occurred in 1963 through electron microscopy studies of epithelial tissues, where Marilyn G. Farquhar and George E. Palade observed focal points of apparent fusion between the outer leaflets of adjacent plasma membranes, resembling "kissing points" that formed a continuous seal around the apical region of cells.^[3] These structures were identified as the most apical component of the epithelial junctional complex, located precisely at the border between the apical and lateral domains of polarized epithelial cells, distinguishing them from underlying adherens junctions and desmosomes.^[3] Early terminology for these structures derived from their Latin description as zonula occludens, reflecting their belt-like, occluding arrangement, though Farquhar and Palade introduced the English term "tight junction" in parentheses to emphasize their close membrane apposition.^[3] The term "tight junction" gained prominence in the early 1970s following the application of freeze-fracture electron microscopy, which revealed the structures as intricate networks of intramembranous strands or fibrils spanning the plasma membrane fracture faces, providing a more detailed view of their sealing architecture.^[4] These early observations led to initial hypotheses that tight junctions function as barriers impermeable to extracellular tracers, as demonstrated by experiments using lanthanum nitrate, an electron-dense marker that penetrated intercellular spaces up to the junctional seal but was excluded beyond it in various epithelia. Such findings in renal and intestinal tissues supported the idea of tight junctions as selective gates maintaining epithelial polarity and compartmentalization.

Key Milestones in Research

In the 1970s, the application of freeze-fracture electron microscopy provided the first detailed visualization of tight junction ultrastructure, revealing intricate networks of fibril-like intramembrane strands that form the junctional seal.^[5] These observations by Staehelin demonstrated that the strands consist of continuous, anastomosing ridges and grooves in the plasma membrane, varying in complexity across different epithelia.^[5] Subsequent studies correlated the geometrical organization and strand complexity with epithelial barrier permeability, showing that more elaborate networks correspond to tighter seals against paracellular diffusion. This technique shifted research from basic morphology toward understanding the structural basis of barrier function. The 1980s marked a transition to molecular approaches with the identification of the first tight junction-associated protein, ZO-1. In 1984, Stevenson and Goodenough isolated a detergent-insoluble, junction-enriched fraction from mouse liver and used it to generate monoclonal antibodies that specifically labeled tight junctions. This high-molecular-weight polypeptide, ZO-1 (approximately 225 kDa), was confirmed in 1986 to localize exclusively to the cytoplasmic plaque of tight junctions across diverse epithelia, enabling immunofluorescence-based studies of junctional components.^[6] The discovery of ZO-1 facilitated the shift from purely ultrastructural analyses to biochemical and immunological investigations, laying the groundwork for identifying additional plaque proteins.^[6] A pivotal breakthrough in the 1990s came from the cloning of claudins, establishing them as the primary strand-forming proteins of tight junctions. In 1998, the Tsukita laboratory identified claudin-1 and claudin-2 as novel tetraspan integral membrane proteins (20-22 kDa) with no sequence similarity to occludin, localizing specifically to tight junction strands in epithelia.^[7] Functional reconstitution experiments showed that claudins alone could polymerize into strand-like structures and recruit occludin, confirming their central role in forming the physical barrier.^[8] This work, highly cited and influential in cell biology, resolved long-standing questions about the molecular composition of the strands observed decades earlier.^[7] Entering the 2000s, genetic models and advanced imaging techniques elucidated claudin-specific functions and junctional dynamics. Knockout mice for claudin-11, generated in 1999, revealed the complete absence of tight junction strands in Sertoli cells of the testis and oligodendrocytes in CNS myelin, leading to male sterility and impaired neural conduction, thus demonstrating claudins' essential role in strand assembly.^[9] Similarly, claudin-1-deficient mice in 2002 exhibited neonatal lethality due to profound epidermal barrier defects, including increased paracellular permeability and dehydration, underscoring claudin diversity in tissue-specific barrier properties.^[10] Concurrently, live-cell imaging with GFP-tagged junctional proteins captured the dynamic remodeling of tight junctions, showing rapid fusion and fission of strands in response to stimuli, which supports ongoing barrier maintenance and adaptation. Later milestones included the recognition in 2019 that ZO-1 undergoes liquid-liquid phase separation to drive tight junction belt formation, and advances in super-resolution microscopy from the 2010s onward that revealed the nanoscale organization and dynamics of junctional proteins.^[11]^[12] These approaches integrated molecular genetics with real-time visualization and biophysical models, transforming tight junction research into a multidisciplinary field.

Molecular Composition

Transmembrane Proteins

Tight junctions are primarily sealed by integral membrane proteins that span the plasma membrane and interact extracellularly to form paracellular barriers. The core components include members of the claudin family, occludin, junctional adhesion molecules (JAMs), tricellulin, and angulins, each contributing to the architecture of the junctional strands observed in freeze-fracture electron microscopy.^[13] Claudins are the principal tetraspanin proteins responsible for the selective permeability of tight junctions, with 27 members in mammals, 23 of which are identified in humans (claudin-1 to claudin-12 and claudin-14 to claudin-24, excluding claudin-13 which is absent in humans). These proteins feature four transmembrane α-helices, two extracellular loops (ECL1 and ECL2), and intracellular N- and C-termini. ECL1, rich in conserved residues like tryptophan and cysteines forming a disulfide bond, mediates both cis (lateral, within the same membrane) and trans (between opposing membranes) interactions, enabling claudins to polymerize into homotypic or heterotypic pairs that create branched networks of paracellular strands.^[14]^[15] For instance, claudin-1 and claudin-5 primarily form sealing barriers impermeable to ions, while claudin-2 assembles into cation-selective pores with an aperture of approximately 5.7 Å, facilitating paracellular transport of small cations like Na⁺.^[13] Crystal structures, such as that of claudin-15 at 2.4 Å resolution, reveal a five-stranded β-sheet fold in the extracellular domain that supports these interactions, with variable regions allowing specificity in heterotypic binding.^[14] Occludin, another tetraspanin protein, possesses four transmembrane domains and two extracellular loops that contribute to tight junction integrity, though it is not essential for initial barrier formation. Its extracellular loops enable trans interactions with opposing occludin molecules, stabilizing the claudin-based strands and modulating the overall junctional architecture.^[16] The long cytoplasmic C-terminus of occludin features a coiled-coil domain (resolved at 1.45 Å), which facilitates associations with intracellular partners, but its primary role lies in enhancing strand cohesion rather than pore selectivity.^[15] Studies in occludin-knockout models confirm that while tight junctions form without it, barrier properties are compromised, underscoring its supportive function.^[17] Junctional adhesion molecules (JAMs), such as JAM-A, belong to the immunoglobulin-like superfamily and feature a single transmembrane domain with two extracellular Ig-like folds. JAM-A promotes initial cell-cell adhesion through homophilic trans interactions via its N-terminal Ig domain, forming U-shaped dimers as seen in the 2.5 Å crystal structure (PDB: 1F97).^[18] Beyond adhesion, JAMs regulate leukocyte transmigration across epithelia by facilitating transient paracellular gaps without disrupting the overall seal.^[19] Tricellulin, a MARVEL domain-containing tetraspanin, specifically seals tricellular tight junctions where three epithelial cells meet, preventing paracellular leakage of macromolecules. Like occludin, it has four transmembrane helices and two extracellular loops, with a C-terminal coiled-coil domain (2.2 Å structure) enabling homomeric or heteromeric complexes that reinforce the barrier at these vulnerable sites.^[15] Mutations in tricellulin are linked to hereditary deafness due to impaired sealing in inner ear epithelia, highlighting its specialized role.^[20] Angulins, including angulin-1 (lipolysis-stimulated lipoprotein receptor, LSR), angulin-2 (immunoglobulin-like domain containing receptor 1, ILDR1), and angulin-3 (ILDR2), are single-pass transmembrane proteins belonging to the immunoglobulin superfamily. They localize specifically at tricellular tight junctions and are essential for recruiting tricellulin to these sites, thereby sealing the tricellular contacts and maintaining barrier integrity where three cells converge.^[13]^[21] The structural model of tight junctions depicts these transmembrane proteins forming a branched, anastomosing network of strands in the plasma membrane, visible as intramembranous particles in freeze-fracture replicas. Cis interactions within the membrane and trans interactions across the intercellular space, primarily driven by extracellular loops, create this dynamic seal that controls paracellular diffusion. These proteins associate intracellularly with scaffolding elements for anchoring, though the core seal is established extracellularly.^[13]

Cytoplasmic and Scaffolding Proteins

The cytoplasmic plaque underlying tight junctions consists of a multimolecular complex of intracellular proteins that anchors transmembrane components to the actin cytoskeleton and organizes the junctional architecture.^[22] This plaque extends approximately 0.5 μm into the cytoplasm and serves as a scaffolding platform, with zonula occludens-1 (ZO-1) acting as the central hub coordinating interactions among roughly 30 associated proteins.^[22]^[23] Zonula occludens proteins, including ZO-1, ZO-2, and ZO-3, belong to the membrane-associated guanylate kinase (MAGUK) family and function as multi-PDZ domain scaffolds.^[24] These proteins bind to the C-terminal PDZ-binding motifs of transmembrane proteins such as claudins and occludin via their PDZ domains, thereby linking the extracellular seal to intracellular structures.^[1] Additionally, their C-terminal regions contain actin-binding domains that directly connect to the actin cytoskeleton, stabilizing the junction and facilitating mechanical integrity.^[24] ZO-1, the most abundant and first identified member (initially described in 1986), forms homodimers or heterodimers with ZO-2 and ZO-3, exhibiting partial redundancy in their scaffolding roles.^[1]^[25] Beyond the ZO proteins, other plaque components like cingulin and paracingulin contribute to cytoskeletal regulation and broader cellular processes.^[22] Cingulin, a ~140 kDa protein, interacts with ZO-1 through its ZU5-interaction motif and binds non-muscle myosin II, thereby tethering actomyosin filaments to the junction to modulate cytoskeletal dynamics.^[26] Paracingulin (also known as CGNL1), structurally similar to cingulin with a globular head, coiled-coil rod, and tail domains, similarly recruits cytoskeletal elements and can translocate to the nucleus to influence gene expression, such as by regulating RhoA and Rac1 activity that affects tight junction protein transcription.^[27]^[28] These interactions collectively form the dense cytoplasmic plaque, enabling the tight junction to mature by first recruiting ZO-1, which precedes and promotes claudin polymerization into strands.^[25]

Assembly and Regulation

Formation Mechanisms

The formation of tight junctions occurs de novo during epithelial polarization, particularly in model systems like Madin-Darby canine kidney (MDCK) cells plated at confluence. In these cells, assembly is synchronized using the calcium switch assay, where depletion of extracellular calcium dissociates adherens and tight junctions, and readdition of calcium (typically 1.8 mM) triggers rapid reformation over 12-15 hours, requiring de novo protein synthesis. Cadherin-mediated adherens junctions precede tight junction formation, with E-cadherin engagement at nascent cell-cell contacts recruiting the scaffolding protein zonula occludens-1 (ZO-1) through interactions involving α- and β-catenin, establishing an initial platform for junctional maturation.^[29]^[30] Claudins, the principal transmembrane components of tight junction strands, are synthesized in the endoplasmic reticulum and mature in the Golgi apparatus before being packaged into vesicles for transport. These vesicles deliver claudins to the basolateral and lateral plasma membranes, where, upon calcium-induced cell-cell adhesion, the proteins insert into the membrane primarily at the basal aspects of nascent junctions. Subsequent polymerization of claudins into continuous, interwoven strands is driven by lateral associations and stabilization by cytoplasmic partners such as ZO-1 and ZO-2, creating the initial sealing network that restricts paracellular diffusion. Recent findings indicate that the Rho-ROCK pathway liberates sequestered claudins from the EpCAM/TROP2 complex, facilitating rapid incorporation into nascent junctions.^[31]^[32]^[33] Early tight junction contacts manifest as discrete, spot-like aggregates of ZO-1 and claudin clusters at sites of cell-cell apposition, appearing within 15-30 minutes of calcium readdition in MDCK cells. Over the next 1-2 hours, these spots elongate and fuse into a continuous belt encircling the apical perimeter, a process dependent on coordinated actomyosin dynamics. Non-muscle myosin II generates contractile forces along junction-associated actin filaments, promoting strand alignment, tension buildup, and cytoskeletal linkage to consolidate the belt-like morphology.^[34] Once bicellular tight junctions are established, tricellular tight junctions seal the vertices where three epithelial cells converge, preventing leakage through these high-risk sites. Tricellulin, a tetraspan protein related to occludin, is recruited to these tricellular contacts subsequent to bicellular strand formation, guided by angulins (members of the LSR family) that serve as corner landmarks on the plasma membrane. This recruitment, occurring via direct protein-protein interactions, enhances strand complexity and fortifies the barrier against macromolecules up to 10 kDa.^[35]^[36]

Dynamic Regulation

Tight junctions exhibit dynamic regulation through post-translational modifications, particularly phosphorylation, which modulates their assembly and permeability. Protein kinases such as protein kinase C (PKC) and atypical PKC (aPKC) phosphorylate key components like ZO-1 and claudins, thereby altering barrier permeability; for instance, PKC phosphorylates claudin-1 at threonine residues to reduce its localization at junctions, while aPKC targets ZO-1 serines to influence tight junction integrity.^[37] Conversely, dephosphorylation by protein phosphatase 2A (PP2A) stabilizes these proteins; PP2A associates with aPKC at the tight junction complex, promoting dephosphorylation of ZO-1, occludin, and claudin-1 to enhance junctional biogenesis and reduce paracellular leakage.^[38] Calcium and ion signaling further govern tight junction dynamics via Rho GTPases, which control actomyosin contractility. Activation of RhoA strengthens junctions by stimulating Rho-associated kinase (ROCK), which phosphorylates myosin light chain to increase actomyosin tension and seal paracellular pathways.^[39] In contrast, Rac1 and Cdc42 activity loosens junctions; these GTPases promote branched actin polymerization through Arp2/3, facilitating junction disassembly and remodeling by reducing actomyosin-mediated constriction.^[39] Endocytic trafficking provides another layer of regulation, enabling junction disassembly through internalization of transmembrane proteins. Clathrin-mediated endocytosis removes occludin and claudins from the plasma membrane during permeability changes, such as in response to environmental stresses.^[40] This process is modulated by SNARE proteins, like syntaxin 8, which facilitate the trafficking and recycling of claudins to maintain junctional localization, and Rab GTPases, such as Rab13, which disrupt occludin and claudin-1 distribution when hyperactive, leading to barrier breakdown.^[41]^[42] Recent studies have elucidated the role of biomolecular condensates in dynamic regulation. Liquid-liquid phase separation of ZO-1 drives surface condensation at cell-cell contacts, coupled with local actin polymerization, to assemble the tight junction belt and enable remodeling.^[43] In developmental contexts, tight junctions undergo remodeling influenced by signaling pathways like Wnt/β-catenin, which regulates claudin expression to establish epithelial barriers. During epithelial morphogenesis, such as gastrulation, Wnt/β-catenin attenuates to activate transcriptional programs that upregulate claudins, ensuring proper junction formation and tissue compartmentalization.^[44]

Physiological Functions

Barrier and Seal Functions

Tight junctions serve as a critical paracellular barrier in epithelial and endothelial cells, selectively regulating the diffusion of solutes between the apical and basolateral compartments to maintain tissue homeostasis. This barrier is size- and charge-selective, preventing passive paracellular transport of solutes larger than approximately 0.4 nm (4 Å), such as ions and water, while allowing controlled flux of smaller molecules.^[45] The integrity and permeability of this seal are quantitatively assessed using transepithelial electrical resistance (TER), which measures the electrical conductance across the epithelial monolayer; higher TER values indicate tighter barriers with reduced paracellular ion flux.^[46] The selective permeability of tight junctions is primarily determined by the composition of claudin proteins, which form the backbone of the paracellular pore. Pore-forming claudins, such as claudin-2, create cation-selective channels that facilitate paracellular flux of small ions like Na⁺, resulting in leaky epithelia with low TER values around 100 Ω·cm², as observed in proximal tubules or small intestine.^[45] In contrast, sealing claudins like claudin-1 polymerize to form a robust barrier that restricts ion and solute passage, yielding high TER values exceeding 1000 Ω·cm², typical of tight epithelia such as those in the distal nephron.^[45] These functional differences arise from the extracellular loops of claudins, which interact to modulate pore size and charge selectivity.^[46] In endothelial cells, particularly at the blood-brain barrier (BBB), tight junctions exhibit enhanced sealing properties dominated by claudin-5, which confers near-impermeability to macromolecules larger than 400 Da while maintaining essential nutrient transport.^[46] This contrasts with epithelial tight junctions, where claudin diversity allows greater variability in permeability to support functions like absorption in the gut.^[45] Claudin-5 knockout studies in mice demonstrate size-selective loosening of the BBB, with increased paracellular diffusion of molecules up to 800 Da but preserved restriction of larger ones, underscoring its pivotal role in neuroprotection.^[46] Paracellular flux across tight junctions occurs via two distinct pathways: the pore pathway and the leak pathway. The pore pathway, mediated by claudins, provides a high-capacity, charge- and size-selective route for small ions and solutes (<0.4 nm), with permeability finely tuned by claudin isoform composition.^[45] The leak pathway, involving transient discontinuities regulated by proteins like ZO-1 and occludin, offers a low-capacity, non-selective route for larger molecules and is more prominent under physiological stress or injury, though it remains minimal in healthy barriers.^[45] This dual model explains the dynamic yet controlled nature of paracellular transport.^[46]

Cellular Polarity and Signaling

Tight junctions play a crucial role in maintaining cellular polarity by functioning as a diffusion barrier, known as the fence function, that separates the apical and basolateral membrane domains of epithelial cells. This barrier restricts the lateral mobility of lipids and proteins, preventing their diffusion between the apical surface, which faces the external environment or lumen, and the basolateral surface, which interacts with the underlying basement membrane and neighboring cells. For instance, zonula occludens-1 (ZO-1) contributes to this restriction by tethering transmembrane proteins and cytoskeletal elements, such as actin filaments, to the junctional complex, thereby stabilizing domain boundaries and ensuring functional asymmetry in ion transport, receptor distribution, and signaling cascades.^[47]^[48] In establishing epithelial polarity, tight junctions interact with key polarity complexes to define the boundaries between membrane domains. At the apical side, tight junctions engage the Crumbs-PALS1-aPKC complex, which promotes apical membrane specification and excludes basolateral determinants, while on the lateral side, they coordinate with the Par3-aPKC complex to reinforce segregation and prevent intermixing of apical and basolateral components. These interactions are essential for the proper assembly of the apical junctional complex, where tight junctions act as a demarcation line that polarizes the distribution of transporters and adhesion molecules, thereby supporting tissue organization in structures like the intestinal epithelium.^[49]^[32]^[50] Beyond structural roles, tight junctions serve as signaling hubs that influence gene expression and cell fate decisions. ZO-1 and ZO-2 can translocate to the nucleus, where they act as transcriptional regulators by interacting with components of the Hippo pathway, such as promoting the activation of genes involved in proliferation control and tissue homeostasis through modulation of YAP/TAZ activity. Similarly, junctional adhesion molecules (JAMs), integral tight junction components, participate in Notch signaling by facilitating ligand-receptor interactions that dictate cell fate, particularly in proliferative tissues where precise boundary formation is required.^[51]^[52] The mechanisms underlying tight junction-mediated polarity and signaling exhibit evolutionary conservation across metazoans, underscoring their fundamental role in multicellularity. Tight junctions, or analogous structures, are present in diverse phyla, enabling compartmentalized tissues such as the gut for nutrient absorption and the kidney for filtration, where polarity ensures directional transport and barrier integrity essential for organismal physiology. This conservation highlights tight junctions as an ancient innovation that facilitated the evolution of complex epithelial barriers in early metazoans.^[53]^[54]^[55]

Pathological and Clinical Aspects

Associated Diseases

Mutations in the CLDN16 gene, which encodes the tight junction protein claudin-16, cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), an autosomal recessive renal disorder. These mutations disrupt the paracellular reabsorption of magnesium ions (Mg²⁺) in the thick ascending limb of the loop of Henle, resulting in severe hypomagnesemia, hypercalciuria, nephrocalcinosis, and progressive chronic kidney disease. Dysfunction of occludin, including genetic variations and altered expression, contributes to barrier defects in inflammatory bowel disease (IBD), promoting increased intestinal permeability, gut leakiness, and bacterial translocation that exacerbate inflammation in conditions like Crohn's disease and ulcerative colitis.^[56]^[57] In multiple sclerosis (MS), reduced expression of claudin-5 at the blood-brain barrier (BBB) leads to its breakdown, facilitating immune cell infiltration into the central nervous system and contributing to demyelination and neurodegeneration. This claudin-5 deficiency is observed in MS lesions and animal models of experimental autoimmune encephalomyelitis, highlighting its role in BBB integrity.^[58] Downregulation of claudin-1 in breast cancer promotes tumor cell invasion and metastasis by compromising epithelial barrier function and facilitating epithelial-mesenchymal transition; claudin-1 acts as a tumor suppressor, with low expression correlating to poor prognosis and increased metastatic potential.^[59]

Therapeutic and Research Implications

Therapeutic strategies targeting tight junctions focus on modulating their permeability to improve drug delivery or restore barrier integrity in disease states. One approach involves enhancing paracellular transport across the intestinal epithelium for macromolecules like insulin, which are typically degraded or poorly absorbed orally. Chelating agents such as ethylenediaminetetraacetic acid (EDTA) reversibly disrupt tight junctions by binding calcium ions essential for their stability, thereby increasing permeability; in experimental models, EDTA has enhanced the absorption of markers like 10 kDa dextran by up to 21-fold, facilitating oral delivery of peptides including insulin.^[60] Similarly, zonula occludens toxin (ZOT), derived from Vibrio cholerae, specifically interacts with tight junction proteins to open paracellular pathways; animal studies demonstrate that ZOT enables a 10-fold increase in insulin bioavailability in the rabbit jejunum and ileum without causing long-term damage to the barrier.^[61] In anti-inflammatory therapies, agents that stabilize tight junctions aim to prevent antigen leakage and subsequent immune activation. Larazotide acetate, a synthetic peptide antagonist of zonulin, promotes tight junction assembly by inhibiting actin rearrangements that lead to claudin destabilization, thereby reducing intestinal permeability induced by gluten in celiac disease. It was orally administered in phase III clinical trials as an adjunct to a gluten-free diet to alleviate symptoms like abdominal pain and diarrhea; however, the trials were discontinued in 2022 after interim analysis showed insufficient efficacy in meeting primary endpoints, though the drug was well-tolerated with no increase in adverse events compared to placebo.^[62]^[63] As of 2025, larazotide acetate remains under development as a therapeutic candidate for celiac disease and has demonstrated safety and efficacy in small clinical studies for multisystem inflammatory syndrome in children (MIS-C) post-COVID-19, accelerating gastrointestinal symptom resolution and viral antigen clearance, with ongoing investigations for Long COVID.^[64]^[65] Emerging research highlights the interplay between tight junctions and the gut microbiome, as well as gene-editing technologies for barrier repair. The probiotic bacterium Akkermansia muciniphila upregulates tight junction proteins such as occludin and ZO-1 through its extracellular vesicles, which activate AMPK signaling to enhance epithelial integrity; in high-fat diet-fed mouse models, oral administration of these vesicles reduced gut permeability (lowered serum FITC-dextran levels) and improved glucose tolerance, suggesting therapeutic potential in microbiome dysbiosis-related barrier dysfunction.^[66] Additionally, CRISPR/Cas9-mediated editing of claudin genes has been explored in preclinical models to restore tight junction barrier function; for instance, knockout studies of claudins combined with JAM-A demonstrate altered mechanical stiffness and permeability, providing a foundation for targeted restoration in inflammatory conditions like sepsis, where claudin-2 upregulation contributes to hyperpermeability.^[67]^[68] Recent advances as of 2025 include AI-based prediction of drug-gene interactions to identify novel TJ modulators, microRNA regulation of TJs for IBD therapy, and detailed functional mapping of the 27 claudin family members, linking deficiencies to cancers, inflammation, and neurological disorders.^[69]^[70]^[71] Despite these advances, significant gaps persist in understanding tight junction biology for therapeutic translation. The role of tricellular tight junctions—formed at points where three cells meet and sealed by proteins like tricellulin and angulin-1—remains poorly characterized in cancer, where their dysregulation promotes epithelial-to-mesenchymal transition and metastasis, yet specific mechanisms and targeting strategies are underexplored.^[72] Furthermore, while post-2020 advancements in 3D intestinal organoid models have enabled dynamic studies of tight junction formation and response to microbial or inflammatory cues, recapitulating in vivo complexity requires further refinement to address limitations in vascularization and immune interactions.^[73]

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Regulation of Cancer Cell Behavior by the Small GTPase Rab13
Furthermore, active Rab13 mutants disrupt claudin-1 and occludin at tight junctions, which suggests that Rab13 activity disrupts tight junction formation ...
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Identification of a core transcriptional program driving the human ...
Tight junction components claudin 7 (CLDN7), claudin 3 (CLDN3) ... Wnt/beta-catenin signaling regulates nephron induction during mouse kidney development.
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Molecular aspects of tight junction barrier function - PMC - NIH
Aug 14, 2014 · Tight junctions establish the rate limiting step for paracellular flux between epithelial cells. (a) The barrier is established by multiple interactions ...
[39]
Physiology and Function of the Tight Junction - PMC - PubMed Central
Tight junctions form the continuous intercellular barrier between epithelial cells, which is required to separate tissue spaces and regulate selective movement ...The Claudin Family · Claudin Cell--Cell Adhesion... · Permselectivity And The Two...<|control11|><|separator|>
[40]
Tight Junctions/Adherens Junctions: Basic Structure and Function
Localization of ZO-1 to tight junctions requires its actin-binding ... separation of the apical and basolateral membrane domains (the fence func- tion).
[41]
ZOning out Tight Junctions - ScienceDirect.com
The tight junction is an intricate seal between adjoining epithelial cells that also separates the apical and basolateral membranes within these cells.
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Multiple regions of Crumbs3 are required for tight junction formation ...
Jul 1, 2005 · The Crumbs3/PALS1/PATJ protein complex localizes at epithelial tight junctions and interacts with the polarity-regulating protein complex of ...
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Loss of PALS1 Expression Leads to Tight Junction and Polarity ...
Our results also support the hypothesis that aPKC is recruited to the tight junction through Par6 and its interaction with PALS1 and not through Par3, which ...
[44]
Tight junction protein ZO-2 modulates the nuclear accumulation of ...
Jul 15, 2021 · The presence of tight junction protein zonula occludens 2 (ZO-2) at the nucleus inhibits the transcription of genes regulated by TEAD ...
[45]
Contact and communication: ZO‐2 and the Hippo pathway
Feb 5, 2025 · ZO-2 is defined as a tight junction (TJ) protein, but is also known to have a role in the maintenance of cellular apicobasal polarity and to function as a ...
[46]
A short guide to the tight junction | Journal of Cell Science
May 7, 2024 · Most TJ scaffolding proteins [ZO-1, ZO-2, ZO-3, MUPP1 (MPDZ), MAGI-1 and a few others] contain PDZ domains that mediate their binding to the ...
[47]
Evolution of metazoan cell junction proteins: the scaffold ... - PubMed
In this study two genes related to the tight junctions are characterized from the demosponge Suberites domuncula: tetraspanin (SDTM4SF), a cell surface receptor ...
[48]
Evolution of Metazoan Cell Junction Proteins
Polarization of epithelial cells occurs at the tight junctions by the assembly of protein scaffolds that cluster transmembrane proteins and anchor them to the ...
[49]
IBD Candidate Genes and Intestinal Barrier Regulation - PMC - NIH
The role of occludin in tight junction regulation is less clearly understood as occludin knockout (KO) mice show no obvious defect in barrier function. However, ...
[50]
Intestinal Barrier in Inflammatory Bowel Disease: Mechanisms and ...
Clinical studies have found that downregulation of the expression of relevant proteins such as occludin and ZO-1 can be observed in patients with ulcerative ...Gut Barrier Composition · Butyrate Affects Intestinal... · Cellular Autophagy And The...<|separator|>
[51]
Claudin-5: gatekeeper of neurological function - PMC
Jan 29, 2019 · At the blood–brain barrier, claudin-5 is the most enriched tight junction protein and its dysfunction has been implicated in neurodegenerative ...
[52]
Claudin 1 in Breast Cancer: New Insights - PMC - PubMed Central
In the latter, a down-regulation of claudin 1 has been associated with poor differentiation, portal invasion and low patient survival rates [63]. Conversely, ...
[53]
Application of Permeation Enhancers in Oral Delivery of ...
Jan 19, 2019 · The application of permeation enhancers (PEs) to improve transport of poorly absorbed active pharmaceutical ingredients across the intestinal epithelium is a ...
[54]
Modulation of intestinal tight junctions by Zonula occludens toxin ...
Modulation of intestinal tight junctions by Zonula occludens toxin permits enteral administration of insulin and other macromolecules in an animal model. A ...
[55]
Larazotide acetate: a pharmacological peptide approach to tight ...
LA is currently being studied in phase III clinical trials and is orally administered to adult patients with celiac disease as an adjunct therapeutic to enhance ...
[56]
9 Meters Discontinues Phase 3 Clinical Trial for Potential Celiac ...
Jun 21, 2022 · 9 Meters Discontinues Phase 3 Clinical Trial for Potential Celiac Disease Drug Larazotide. An analysis by an independent statistician of the ...
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Akkermansia muciniphila-derived extracellular vesicles influence ...
Feb 23, 2018 · Akkermansia muciniphila-derived extracellular vesicles influence gut permeability through the regulation of tight junctions. Chaithanya ...
[58]
Tight junction membrane proteins regulate the mechanical ...
For this, we generated claudin/JAM-A/occludin KO cells and claudin/JAM-A/CAR KO cells by CRISPR/Cas9-mediated genome editing and analyzed their phenotypes (Fig.
[59]
Claudin-2 upregulation enhances intestinal permeability, immune ...
Feb 27, 2024 · In this study, we found that small intestinal epithelial claudin-2 expression is selectively upregulated in septic patients.
[60]
Role of tight junctions in the epithelial-to-mesenchymal transition of ...
Here we will review the role of bicellular and tricellular tight junction proteins in the EMT of cancer cells and discuss their potential as novel diagnostic ...
[61]
Advances and applications of gut organoids: modeling intestinal ...
This review provides an in-depth exploration of technological advancements and applications of gut organoids, with a focus on their construction methods.Intestinal Organoids And... · Intestinal Organoid... · Organoid TransplantationMissing: post- | Show results with:post-