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]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 19, Tight Junctions Form a Selective Permeability Barrier Across Epithelial Cell Sheets
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-based tight junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice by Mikio Furuse and others. 2002, abstract: barrier defect and excessive water loss
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]
Claudin-2 expression induces cation-selective channels in tight junctions of epithelial cells by Salah Amasheh and others. 2002, abstract: transfected monolayers and relative ion permeabilities
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. Articles by others on the same topic
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