AQP-4 and its role in maintaining the hydric balance in the brain

Abstract

Aquaporins are a vast family of channel proteins whose main role is the bidirectional transport, depending on the osmotic gradient, of water across the lipid membranes, which have low permeability for this solvent. The aquaporins are also involved in the lipid metabolism, the cell proliferation and migration processes, the transport of glycerol, neuroexcitation and epithelial fluids secretion, having numerous roles such as ensuring the water transport in the central nervous system, the production of CSF, aqueous humor and saliva, epithelial hydration, urine concentration and nervous impulse transmission. Up to now, in mammals there have been identified 13 types of aquaporins, each of them annotated from 0 to 12 (Aqp 0 – Aqp 12). Of these, aquaporin-4 (AQP-4) – located in the astroglia - is the most abundant aquaporin in the brain. Although this type of aquaporin is also present inside the brain parenchyma, especially in the astroglial processes lining the neuronal synapses, AQP-4 is mainly located in the astroglial end-feet adjacent to the ependymocytes or endothelial cells, where its main role is to ensure the bidirectional transport of water between the astroglia and the cerebrospinal fluid (CSF) or blood vessels. The presence of AQP-4 on the surface of astroglial processes promotes, in the initial stages of ischemia, the formation of the cytotoxic oedema, but has a protective role against vasogenic oedema. This review aims to describe the roles of AQP-4, and especially the role that this protein has in maintaining the hydric balance in the brain.

Keywords

• aqp-4
• hydric balance
• astroglia