Aquaporin‐4 orthogonal arrays of particles from a physiological and pathophysiological point of view

Aquaporin‐4 (AQP4) is the neuromuscular water channel that is also expressed at the basolateral membranes of other cell types in kidney, stomach, and lung. In skeletal muscle, AQP4 is found at the sarcolemma of fast‐twitch fibers and its function is strictly correlated with the glycolytic metabolism. In the central nervous system, AQP4 is expressed at the basolateral membranes of ependymal cells, and is highly concentrated at the glial end‐foot processes surrounding blood vessels and forming the glia limitans, as well as at the nonend‐foot glial processes of the granule cell layer in the cerebellum. AQP4 plasma membrane organization is different from other aquaporins (AQPs). AQP4 is expressed as two major polypeptides called M1 and M23. These two isoforms form heterotetramers appearing in the plasma membrane as intramembrane particles (IMPs) observable by freeze‐fracture electron microscopy. Such tetrameric organization is common to all other AQPs. In the case of AQP4, however, multiple IMPs further aggregate to form structures called orthogonal arrays of particles (OAPs). The relative abundance of M23 and M1 in vivo is the major determinant for the formation of OAPs of different sizes. The function of AQP4 aggregation into OAPs under normal conditions is still not completely understood. Interestingly, there are several reports indicating that OAPs are involved in different neuromuscular diseases. In particular, the OAP‐related diseases that have attracted more attention are Duchenne muscular distrophy and, more recently, neuromyelitis optica, the two pathological conditions in which OAPs are involved in completely different ways.