Abstract
Whether tight junction permeates water is highly controversial. Across both epithelium and endothelium, significant water permeability remains when transcellular water channels - aquaporins are removed from the plasma membrane. Tricellular tight junction is a new concept. Regular bicellular tight junctions cannot seal the tricellular corners. In fact, tricellular tight junction is made of two classes of proteins - tricellulin and angulins, which are different from the components of bicellular tight junction. Unlike the bicellular tight junction that only permeates small molecules with diameter of < 8 Å, the tricellular tight junction can permeate macromolecules with sizes up to 10 nm in diameter. A serendipitous discovery has revealed that removal of angulin-2 from the tricellular tight junction makes the epithelium highly permeable to water without affecting the permeability to ions. This evidence suggests that it is the tricellular tight junction that contains a paracellular water channel. Finally, physiologic and pharmacologic regulation of tricellular tight junction permeability may allow creating a novel, size-independent paracellular pathway for solute and water permeation.
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