Physiological roles of glycerol-transporting aquaporins: the aquaglyceroporins

A subclass of aquaporin (AQP) water channels, termed aquaglyceroporins, are also able to transport glycerol and perhaps urea and other small solutes. Although extensive data exist on the physiological roles of aquaporin-facilitated water transport, until recently the biological significance of glycerol transport by the mammalian aquaglyceroporins has been unknown. There is now compelling evidence for involvement of aquaglyceroporin- facilitated glycerol transport in skin hydration and fat cell metabolism. Mice deficient in AQP3 have dry skin, reduced skin elasticity and impaired epidermal biosynthesis. Mice lacking AQP7 manifest progressive adipocyte fat accumulation and hypertrophy. These skin and fat phenotypes are attributable to impaired glycerol transport. A potential implication of these findings is the possibility of modulation of aquaglyceroporin expression or function in the therapy of skin diseases and obesity. Received 20 January 2006; received after revision 21 February 2006; accepted 20 March 2006     

Impairment of angiogenesis and cell migration by targeted aquaporin-1 gene disruption

Aquaporin-1 (AQP1) is a water channel protein expressed widely in vascular endothelia, where it increases cell membrane water permeability. The role of AQP1 in endothelial cell function is unknown. Here we show remarkably impaired tumour growth in AQP1-null mice after subcutaneous or intracranial tumour cell implantation, with reduced tumour vascularity and extensive necrosis. A new mechanism for the impaired angiogenesis was established from cell culture studies. Although adhesion and proliferation were similar in primary cultures of aortic endothelia from wild-type and from AQP1-null mice, cell migration was greatly impaired in AQP1-deficient cells, with abnormal vessel formation in vitro. Stable transfection of non-endothelial cells with AQP1 or with a structurally different water-selective transporter (AQP4) accelerated cell migration and wound healing in vitro. Motile AQP1-expressing cells had prominent membrane ruffles at the leading edge with polarization of AQP1 protein to lamellipodia, where rapid water fluxes occur. Our findings support a fundamental role of water channels in cell migration, which is central to diverse biological phenomena including angiogenesis, wound healing, tumour spread and organ regeneration.     

Protein kinase A regulates chloride conductance in endocytic vesicles from proximal tubule.

Regulation of ion transport by phosphorylation and G proteins occurs in several epithelial and non-epithelial cell plasma membranes1-5. It is not known whether transporters on intracellular membranes are target sites for second messengers. Here we present direct evidence that a chloride conductance in endocytic vesicles from rabbit proximal tubule is activated by phosphorylation through a cyclic AMP-dependent protein kinase. To measure chloride transport, endocytic vesicles were labelled in vivo with a Cl(-)-sensitive fluorescent indicator6-8. It was found that labelled endosomes contained an inward proton pump and a chloride conductance, but no ion-coupled chloride transport, and that the chloride conductance was regulated by protein kinase A. These results, taken together with measurements of chloride effects on ATP-dependent acidification, suggest that endosomal pH can be controlled by phosphorylation of a stilbene-sensitive conductive chloride transporter.     

Endosomes from kidney collecting tubule cells contain the vasopressin-sensitive water channel

The mechanism by which vasopressin rapidly and dramatically increases the water permeability of target epithelial cell membranes is thought to involve a cycle of exo- and endocytosis during which vesicles carrying 'water channels' are successively inserted into, and removed from the apical plasma membrane of epithelial cells. Clusters of intramembranous particles, visible by freeze-fracture electron microscopy and presumed to represent water channels, appear on apical membranes in parallel with increased transepithelial water flow. In the collecting duct, these clusters are located in clathrin-coated pits which are subsequently internalized. There has been no direct evidence, however, that subcellular membranes in vasopressin-sensitive epithelia contain functional water channels. In this report, we have used fluorophores that are sensitive to volume and do not pass through membranes to label and to measure directly the osmotic water permeability of endocytosed vesicles isolated from renal papilla. We present direct evidence that vasopressin induces the appearance of a population of endocytic vesicles whose limiting membranes contain water channels.