Tight junctions

Various functions within our bodies require the generation and maintenance of compartments with distinct compositions, which in turn necessitate the formation of semipermeable cellular diffusion barriers. For example, the blood–brain barrier protects the brain by allowing only specific molecules to pass through. Another instance is the intestinal barrier, which allows the uptake of essential nutrients, while restricting the passage of pathogenic molecules and bacteria. Breakdown of such barriers causes various pathologies, such as brain or retinal edema, or diarrhoea. Epithelia and endothelia are the most common barrier-forming cells. Individual cells in such barriers are held together by cell–cell adhesion structures — also known as intercellular junctions — that are essential for barrier formation and maintenance. Here, we will focus on the structure and assembly of tight junctions (TJs) and their functions as barriers, but will refer to other adhesive structures crucial for barrier regulation such as adherens junctions (AJs) and focal adhesions to the extracellular matrix (ECM) (Figure 1A,B). We will also discuss additional functions of TJs in cell surface polarity and the regulation of gene expression, cell function, and cell behaviour.