Unique structural features of claudin‐5 and claudin‐15 lead to functionally distinct tight junction strand architecture

Biophysics of claudin proteins in tight junction architecture: Three decades of progress

Tight junctions are cell-cell adhesion complexes that act as gatekeepers of the paracellular space. Formed by several transmembrane proteins, the claudin family performs the primary gate-keeping function. The claudin proteins form charge and size-selective diffusion barriers to maintain homeostasis across endothelial and epithelial tissue. Of the 27 known claudins in mammals, some are known to seal the paracellular space, while others provide selective permeability. The differences in permeability arise due to the varying expression levels of claudins in each tissue. The tight junctions are observed as strands in freeze-fracture electron monographs; however, at the molecular level, tight junction strands form when multiple claudin proteins assemble laterally (cis assembly) within a cell and head-on (trans assembly) with claudins of the adjacent cell in a zipper-like architecture, closing the gap between the neighboring cells. The disruption of tight junctions caused by changing claudin expression levels or mutations can lead to diseases. Therefore, knowledge of the molecular architecture of the tight junctions and how that is tied to tissue-specific function is critical for fighting diseases. Here, we review the current understanding of the tight junctions accrued over the last three decades from experimental and computational biophysics perspectives.

Claudins in Cancer: A Current and Future Therapeutic Target

Claudins are a family of 27 proteins that have an important role in the formation of tight junctions. They also have an important function in ion exchange, cell mobility, and the epithelial-to-mesenchymal transition, the latter being very important in cancer invasion and metastasis. Therapeutic targeting of claudins has been investigated to improve cancer outcomes. Recent evidence shows improved outcomes when combining monoclonal antibodies against claudin 18.2 with chemotherapy for patients with gastroesophageal junction cancer. Currently, chimeric antigen receptor T-cells targeting claudin 18 are under investigation. In this review, we will discuss the major functions of claudins, their distribution in the normal as well as cancerous tissues, and their effect in cancer metastasis, with a special focus on the therapeutic targeting of claudins to improve cancer outcomes.

The impact of pathogenic and artificial mutations on Claudin-5 selectivity from molecular dynamics simulations

Tight-junctions (TJs) are multi-protein complexes between adjacent endothelial or epithelial cells. In the blood-brain-barrier (BBB), they seal the paracellular space and the Claudin-5 (Cldn5) protein forms their backbone. Despite the fundamental role in brain homeostasis, little is known on Cldn5-based TJ assemblies. Different structural models were suggested, with Cldn5 protomers generating paracellular pores that restrict the passage of ions and small molecules. Recently, the first Cldn5 pathogenic mutation, G60R, was identified and shown to induce Cl^--selective channels and Na⁺ barriers in BBB TJs, providing an excellent opportunity to validate the structural models. Here, we used molecular dynamics to study the permeation of ions and water through two distinct G60R-Cldn5 paracellular architectures. Only the so-called Pore I reproduces the functional modification observed in experiments, displaying a free energy (FE) minimum for Cl^- and a barrier for Na⁺ consistent with anionic selectivity. We also studied the artificial Q57D and Q63D mutations in the constriction region, Q57 being conserved in Cldns except for cation permeable homologs. In both cases, we obtain FE profiles consistent with facilitated passage of cations. Our calculations provide the first in-silico description of a Cldn5 pathogenic mutation, further assessing the TJ Pore I model and yielding new insight on BBB's paracellular selectivity.

Claudin-10b cation channels in tight junction strands: Octameric-interlocked pore barrels constitute paracellular channels with low water permeability

Claudin proteins constitute the backbone of tight junctions (TJs) regulating paracellular permeability for solutes and water. The molecular mechanism of claudin polymerization and paracellular channel formation is unclear. However, a joined double-rows architecture of claudin strands has been supported by experimental and modeling data. Here, we compared two variants of this architectural model for the related but functionally distinct cation channel-forming claudin-10b and claudin-15: tetrameric-locked-barrel vs octameric-interlocked-barrels model. Homology modeling and molecular dynamics simulations of double-membrane embedded dodecamers indicate that claudin-10b and claudin-15 share the same joined double-rows architecture of TJ-strands. For both, the results indicate octameric-interlocked-barrels: Sidewise unsealed tetrameric pore scaffolds interlocked with adjacent pores via the β1β2 loop of the extracellular segment (ECS) 1. This loop mediates hydrophobic clustering and, together with ECS2, cis- and trans-interaction between claudins of the adjacent tetrameric pore scaffolds. In addition, the β1β2 loop contributes to lining of the ion conduction pathway. The charge-distribution along the pore differs between claudin-10b and claudin-15 and is suggested to be a key determinant for the cation- and water permeabilities that differ between the two claudins. In the claudin-10b simulations, similar as for claudin-15, the conserved D56 in the pore center is the main cation interaction site. In contrast to claudin-15 channels, the claudin-10b-specific D36, K64 and E153 are suggested to cause jamming of cations that prevents efficient water passage. In sum, we provide novel mechanistic information about polymerization of classic claudins, formation of embedded channels and thus regulation of paracellular transport across epithelia.