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 ependymocyes 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.
Highlights
The aquaporins are a numerous channel protein family expressed in organisms from all kingdoms, up to the present time having been discovered, in plants, microorganisms, animals and humans, approximately 450 aquaporins [1,2]
The aquaporins are small proteins, with a molecular weight of @ 30 kDa (23481483) and 300 aminoacids in lenght [6], and have a common structure with 6 a-helical transmembrane domains [7] and 2 a-helical domains that don’t fully pass through the cell membrane [5], domains which contain a consensus sequence composed of asparagine-proline-alanine (NPA) which enters the structure of a pore of @ 28 A (1A = 0,1 nm) [8,9], the amino- and carboxiterminal ends of these proteins being intracellular. [7]
(10) AQP-4’s selectivity for water is given by 3 specializations present along the monomeric AQP4 channel: the narrowing of the pore to @ 28 A – to prevent the entrance of molecules smaller than water, the presence of an arginine residue – which serves to block the entrance of protonated water and other cations, and the presence of positively charged dipoles in the middle portion on the channel – which are involved in preventing the flux of protons and in reorientating the water molecules [10]
Summary
The aquaporins are a numerous channel protein family expressed in organisms from all kingdoms, up to the present time having been discovered, in plants, microorganisms, animals and humans, approximately 450 aquaporins [1,2]. Various studies using immunohistochemistry have shown that, in the CNS, AQP-4 is predominantly located in the brain astrocytes, with the biggest density in the astrocitary membranes which limit the cerebral microvessels or the pial layer (pia mater) on the surface of the brain, but its presence – albeit in smaller quantities – in the perisynaptic astrocyte processes reinforces the polarized distribution of this channel-protein in astrocytes [10].
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