Understanding Claudin Family Proteins and Their Roles in Disease

General Characteristics of Claudin

Claudin is part of the tight junction membrane protein family. The claudin family member has four transmembrane domains, two extracellular loops (ECLs), cytoplasmic N- and C-terminal domains, as well as a short intracellular turn. On the same cell, Claudins form cis dimers or higher order oligomers with other Claudins. These then perform trans interactions between Claudins on other cells to organize Claudin oligomers into tight junction strands. The human Claudin family comprises 27 members, widely distributed in epithelial cells with expression patterns specific to epithelial cell types. Claudins are grouped into two functionally different classes. Claudins in the first group build the paracellular barrier that limits the movement of molecules between the epithelial cells. Claudins in the second group build selective paracellular channels that allow the passage of specific molecules between the epithelial cells. According to sequence homology and functional features, claudins are further grouped into classic members and non-classic members. Of the two extracellular loops, the ECL1, about 50 amino acids long, is important for conferring the selective permeability of tight junctions. ECL2 is smaller (~16 amino acids) and forms a helix-turn-helix motif. Since tight junction formation between adjacent cells requires interactions of surface Claudins, both ECL domains are crucial for tight junction assembly.

Figure 2. CLDN proteins at tight junctions
(Source: Katoh M, et al. 2024)

Research has demonstrated that Claudin proteins determine the size and charge-selective conductance properties of paracellular pathways. Based on whether Claudin expression increases or decreases transepithelial resistance, Claudins are classified as barrier Claudins and pore-forming Claudins. The functional classification of Claudins depends on their own conformation on one hand, and on the ion permeability characteristics of their expression sites on the other hand. In nephrons, different types of Claudins have been observed to be associated with tubular segments with different permeability properties. The functional classification of Claudins depends on their own conformation on one hand, and on the ion permeability characteristics of their expression sites on the other hand. In nephrons, different types of Claudins have been observed to be associated with tubular segments with different permeability properties.

Different Claudins exhibit varying affinities for cations and anions, with the electrostatic potential in EL1 determining their selectivity for cations and anions. Combining the barrier and pore functions of Claudins, the Claudin family can be further subdivided into cation channel Claudins, anion channel Claudins, cation barrier Claudins, and anion barrier Claudins.

Claudin 1

Claudin-1 is one of the most investigated members of the TJ protein family, and has been found to be aberrantly expressed in several tumors. The extracellular loops of Claudin-1 can be divided into three domains, Cldn-1 31–53, Cldn-1 53–80 and Cldn-1 146–160. The Cldn-1 53–80 domain possesses a key motif that regulates the epithelial barrier integrity and permeability by modulating the structure and function of tight junctions, which is also capable of binding to other tight junctional proteins. Claudin-1 has been shown to be a target gene of HIF-1α, and mediates the epithelial barrier protective effect of HIF-1α by regulating Claudin-1 expression.

Claudin-1 is aberrantly expressed in digestive tract tumors and plays an important role in tumor initiation and progression. In colon tumor development, Claudin-1 is always found to translocate from the cell membrane to cytoplasm. This may cause alteration of the cell membrane, abnormal intracellular and extracellular signaling, and finally tumor metastasis. Claudin-1 was also a target gene of Wnt signaling pathway. Claudin-1 overexpression in colon cancer cells led to increase the transcriptional repressor ZEB-1 of E-cadherin, lost E-cadherin expression, and nuclear translocation of β-catenin to activate Wnt signaling. The activated Wnt signaling also promoted the binding of activated β-catenin to the ZEB-1 promoter, leading to increased transcription of ZEB-1 mRNA and inducing invasion of tumor cells. High Claudin-1 expression also upregulates Wnt target genes such as oncogenes cyclin D1 and c-myc, contributing to tumorigenesis.

Claudin-1 also promotes gastric cancer tumor growth and metastasis by preventing anoikis. Claudin-1 achieves this anti-apoptotic effect through upregulation and activation of β-catenin expression, in addition to the Akt and Src signaling pathways. Claudin-1 facilitates the activation of MMP-2 in association with MMP-14, degrading extracellular matrix and basement membrane components, which favors the migration and invasion of cancer cells. Transfection of Claudin-1 siRNA into gastric cancer cells significantly impairs their proliferation, migration, and invasion capacity, induces apoptosis, and suppresses TNF-α-induced cell motility.

Claudin 10

Claudin-10 has two isoforms: Claudin-10a and Claudin-10b, which are created by the use of alternative first exons. In human and mouse, the first exon of Claudin-10a and Claudin-10b is 214 bp and 220 bp in length, respectively. Claudin-10a contains two fewer amino acids in the protein sequence compared to Claudin-10b. While the two isoforms have nearly identical lengths, they show a large difference in ion charge selectivity in the first extracellular loop. Claudin-10a has been described as an anion channel with a selectivity for chloride ions and Claudin-10b as a cation channel selective for sodium ions. A number of diseases have been associated with aberrant expression or malfunction of Claudin-10, the most common of which is the HELIX syndrome caused by mutations in Claudin-10. Patients with mutations in Claudin-10 have been reported to exhibit hypokalemic metabolic alkalosis and hypermagnesemia, sweat gland, salivary gland and lacrimal gland dysfunction, and hypermagnesemia/hypokalemia.