Characterization of aquaporins in brain and in liver

Aquaporins (AQP) are a family of integral membrane proteins that transport water and small solutes. Two aquaporins sub-families were identified: the orthodox aquaporins, which transport only water; and the aquaglyceroporins, which transport water and small solutes such as glycerol and urea. Searching the human genome database, we discovered two novel aquaporins. Comparative sequence alignment indicates that these new aquaporins, AQP11 and AQP12, are possible members of a third sub-family. Unlike the other aquaporins, AQP11 fails to transport water, glycerol or urea. Northern and Western blot studies indicate that AQP11 is expressed in rat brain, liver, kidney and testes. Immunolocalization studies indicate that AQP11 is expressed in pyramidal neurons in the cerebral cortex. When overexpressed in cultured HEK293 cells, AQP11 has a predominantly intracellular distribution.; AQP4 is the most abundant aquaporin expressed in brain. AQP4, an orthodox aquaporin, is localized to astrocyte endfeet abutting capillaries. AQP4 exists in two forms: a full length form, termed M1; and a truncated form lacking the first 22 amino acids, termed M23. It was known that M23 forms large square arrays, crystalline arrangements in the plasma membrane visible only by freeze fracture electron microscopy. We demonstrate that in a heterologous system, M1 fails to form square arrays. When co-expressed with M23, M1 inhibits array formation. The physiological relevance of square arrays is still unknown.; AQP9 is an aquaglyceroporin expressed in liver. We demonstrate that purified AQP9 is a robust channel for glycerol and urea, but a poor water channel. In vivo, AQP9 may facilitate glycerol influx into the liver. During a prolonged fast, AQP9 protein levels increase 2--20 fold. Our studies in cultured hepatoma cells, as well as in vivo studies in rats, indicate that insulin downregulates AQP9 levels. Destroying insulin producing cells in the pancreas causes a constant upregulation of AQP9 levels in liver---similar to the levels found in fasted rats. Insulin administration reverses this trend. We conclude that AQP9 is upregulated during periods of energy depletion, in order to maximize glycerol uptake by the liver and increase glucose production.