Aquaporinen

Aquaporinen are a family of integral membrane proteins that form water channels, enabling rapid and selective passage of water molecules across cellular membranes. They are found in many organisms, from bacteria to humans, and play a central role in water homeostasis and osmoregulation. Aquaporinen belong to the major intrinsic protein (MIP) superfamily and include both water-selective channels and aquaglyceroporins that can also transport glycerol and other small solutes.

Structurally, each aquaporin monomer typically contains six transmembrane helices and assembles into tetramers in the plasma membrane. Each monomer forms an individual water-conducting pore. The pore architecture includes conserved NPA motifs and an ar/R constriction region, which together create a selective filter that favors water over ions and other solutes. Some aquaporin channels can be regulated by gating or trafficking of the protein to or from the membrane, allowing cells to adjust water permeability in response to physiological signals.

Physiological roles of aquaporinen are diverse. In the kidneys, certain aquaporins mediate water reabsorption, contributing to urine concentration. In the brain and eyes, they help regulate water movement and volume. In epithelia of glands and airways, they participate in fluid secretion. Aquaporinen expression is tightly regulated during development and in response to hormones; for example, vasopressin promotes the insertion of aquaporin-2 into the apical membrane of kidney collecting duct cells to increase water reabsorption.

Clinical relevance includes inherited mutations that disrupt aquaporin function, leading to disorders such as nephrogenic diabetes insipidus, and alterations in aquaporin expression or function affecting brain edema and other fluid balance conditions. Aquaporins were first purified and characterized in the 1990s, a discovery that earned Peter Agre the Nobel Prize in Chemistry in 2003. The name combines aqua (water) with porin, reflecting their pore-forming, water-transporting role.