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A claudin is a member of a family of four-pass membrane proteins that assemble into tight junction strands between neighbouring cells. The choice of claudins helps determine which ions and small solutes can pass through the pathway between epithelial cells. A tight junction can therefore be selectively permeable rather than an absolutely impermeable weld.^[ref]

The structural role was demonstrated by adding claudin-1 or claudin-2 to mouse fibroblasts that lacked tight junctions. Freeze-fracture electron microscopy revealed extended strand networks in the modified cell membranes. Coexpression recruited occludin to the strands. Building recognizable junction strands in this simplified setting supplied stronger evidence for a construction role than finding a protein near an existing junction alone.^[ref]

Claudin-1 is a member of the claudin family of membrane proteins that form tight junction strands. These strands help control passage between neighbouring cells. Expressing claudin-1 in fibroblasts that lack normal tight junctions can reconstitute strand networks.^[ref]

Its physiological importance became especially clear in claudin-1-deficient mice. Their epidermal barrier failed and they lost water rapidly, despite retaining other recognizable skin structures. This showed that a multilayered epidermis still depends on functional tight junctions, rather than deriving all barrier function from the mere presence of many layers.^[ref]

Claudin-11 is a membrane protein in the claudin family that contributes to specialized tight junction strands. It occurs in central-nervous-system myelin and at contacts between Sertoli cells in the testis. A protein shared by those locations can contribute to different tissue functions without making the two tissues anatomically equivalent.^[ref]

In the 1999 null-mouse experiment, freeze-fracture electron microscopy showed loss of the normal strands in myelin and between Sertoli cells. The mice also had slowed nerve conduction, conspicuous hindlimb weakness and male sterility. This connected a defined molecular change with both structural and physiological observations.^[ref]

The comparison separates contact from a functional junction. Two membranes can remain close in an image without possessing the normal strand organization. Conversely, seeing a strand does not supply a direct numerical measurement of permeability to every solute. Structural imaging, tracer passage and nerve conduction ask complementary questions; the knockout results should not be rewritten as if all three measurements were identical.

Claudin-2 is a tight junction membrane protein that can create a pathway selective for small positively charged ions between epithelial cells. Its presence can make a junction more conductive without creating an indiscriminate hole through the epithelial sheet.

Researchers expressed claudin-2 in an epithelial layer that initially had low conductance between cells. Conductivity increased, and comparison of different salts showed preferential passage of cations over anions.^[ref] This is a useful counterexample to treating every claudin as a simple seal: changing the molecular composition of a tight junction can change both how much passes and which substances pass.

Mikio Furuse investigated the molecular construction of tight junctions. In a 1998 study, introducing claudin-1 or claudin-2 into fibroblasts that normally lack tight junctions produced networks of junction-like strands at contacts between cells. Immunofluorescence located the introduced proteins, while freeze-fracture imaging revealed the strand architecture.^[ref]

The experiment demonstrated that either claudin could build a key structural feature in a simplified cellular setting. Recruitment of occludin further distinguished a strand-forming component from another associated junctional protein.

Occludin is an integral membrane protein concentrated at tight junctions, where neighbouring epithelial or endothelial cells regulate passage between them. It has four membrane-spanning segments and a long cytoplasmic tail. Occludin is a junction component rather than a synonym for the entire sealing structure.

In 1993, Mikio Furuse and colleagues raised monoclonal antibodies against a junction-enriched fraction from chick liver. Immunoelectron microscopy placed the recognized protein directly at membrane-contact points of tight junctions; sequence analysis suggested four membrane crossings. These combined biochemical and spatial observations identified occludin.^[ref]

Later, expression of claudin-1 or claudin-2 in fibroblasts reconstituted extensive junction strands and recruited occludin. This separates the capacity to build a strand from the presence of one particular strand-associated protein.^[ref]

A tight junction is a belt-like cell junction near the apical boundary of many epithelial cells. Interacting strands of membrane proteins regulate passage between neighbouring cells, while helping keep apical-basal polarity in membrane components in different domains. A tight junction can be selectively permeable to ions and small solutes; tight does not mean universally impermeable. Its transport function must be measured rather than inferred solely from an apparently continuous contact. ^[ref]

To test strand assembly, investigators expressed claudin-1 or claudin-2 in fibroblasts lacking tight junctions. Freeze-fracture electron microscopy revealed networks of strands at the new contacts, and coexpressed occludin was recruited into the claudin-based structures. This gain-of-function experiment identified major structural components, rather than merely finding proteins near an existing junction. ^[ref]