Physiology of the Aquaporins

Department of Physiology, Faculty of Medicine, Al-Azhar University, Assiut ,Egypt

^*Corresponding Author

Khaled A. Abdel-Sater

Department of Physiology,

Faculty of Medicine,

Al-Azhar University,Assiut,

Egypt

Mobile: +20167970804

E-mail: Khaled_71111@yahoo.com

Abstract

Aquaporins (AQPs) form pores in the membranes of cells and selectively conduct water molecules through the membrane, while preventing the passage of ions such as sodium and potassium and other small molecules. The water movement through AQPs is considered to be facilitated simply dependent on the osmotic gradient. There are subtypes of AQPs; classical aquaporins or orthodox AQPs (AQP0, AQP1, AQP2, AQP4, AQP5) permeable only to water molecules ; aquaglyceroporins (AQP3, 7, 9 and 10) permeable to uncharged solutes, such as glycerol, CO2, ammonia and urea in addition to water and unorthodox AQPs, (AQP6, AQP8, AQP11 and AQP12) with unknown functions. The specific distribution of AQP in certain cell types of an organ often reflects a precise function. AQP0 is present in the eye lens for maintaining its transparency. AQP1 is widely distributed water channel in the body. It is mostly expressed in kidneys, lungs, red blood cells, liver, skin, intervertebral disc peripheral and central nervous system. It is involved in angiogenesis, cell migration, cell growth and countercurrent concentration. Its defect shows a protective action against edema in the lungs. AQP2 is expressed in kidney collecting duct and inner ear for water transport in presence of vasopressin; its mutations in kidney can cause nephrogenic diabetes insipidus and its mutation in inner ear provokes Menieres disease. AQP3 is the most abundant skin aquaglyceroporin, where AQP3 facilitated water and glycerol transport plays an important role in hydration of mammalian skin epidermis and proliferation and differentiation of keratinocytes. It is also found in kidney collecting duct, conjunctiva of the eye, oesophagus, colon, spleen, stomach, small intestine, intervertebral disc and respiratory tract airway epithelium. AQP4 is found in astroglial cells at blood–brain barrier and spinal cord, kidney collecting duct, glandular epithelia, airways, skeletal muscle, stomach and retina. AQP4 null mice showed altered cerebral water balance with protection from brain edema. AQP5 helps glandular water secretion so, it expressed in glandular epithelia, corneal epithelium, alveolar epithelium and gastrointestinal tract. AQP6 is expressed in kidney collecting duct intercalated cells, retina, parotid gland acinar cells, inner ear, and brain synaptic vesicles. It is involved in chloride, urea and nitrate permeability. AQP6 may functionally interact with H+-ATPase in the vesicles to regulate intra - vesicle pH and acid - base balance.AQP7 is found mainly in fat tissue, and in testis, heart, skeletal muscle and kidney proximal tubule. In adipocytes, AQP7 is known to facilitate the secretion of glycerol. AQP8 is expressed in liver, pancreas intestine, salivary gland, testis, ovary and heart. In the kidney AQP8 was demonstrated to play an important role in the adaptive response of proximal tubule to acidosis. It may also facilitate the diffusion of hydrogen peroxide across membranes of mitochondrial in situations when reactive oxygen species are generated. AQP9 is found in liver, white blood cells, testis and brain, and is involved in water and small solutes permeability. AQP9 is supposed to be a glycerol channel in liver cells. The presence of AQP9 in the brain may play a role in energy metabolism. AQP10 is expressed in skin and small intestine for transport of water and glycerol. AQP11 is expressed in kidney, testis, liver, brain, intestine, heart, and adipose tissue, but its function is still unknown. AQP12 seems to be expressed specifically in pancreatic acinar cells but its function is still unknown and may be implicated in exocrine water execration.

Keywords:Aquaporins, Water channels, Water reabsorption and secretion, Glycerol transport, Cell migration

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