The Tight Junction and Its Proteins: More Than Just a Barrier

For a long time the tight junction (TJ) was known to form and regulate the paracellular barrier between epithelia and endothelial cell sheets. Starting shortly after the discovery of the proteins forming the TJ-mainly the two families of claudins and TAMPs-several other functions have been discovered a striking one being the surprising finding that some claudins form paracellular channels for small ions and/or water. This Special Issue includes 43 articles covering numerous dedicated topics including pathogens affecting the TJ barrier TJ regulation via immune cells the TJ as a therapeutic target TJ and cell polarity function and regulation by proteins of the tricellular TJ TJ as a regulator of cellular processes organ- and tissue-specific functions TJ as sensors and reacting to environmental conditions and last but not least TJ proteins and cancer.