Aquaporin-9 (AQP9) is an aquaglyceroporin membrane channel shown biophysically to conduct water, glycerol, and other small solutes. Because the physiological role/s of AQP9 remain undefined and the expression sites of AQP9 remain incomplete and conflicting, we generated AQP9 knockout mice. In the absence of physiological stress, knockout mice did not display any visible behavioral or severe physical abnormalities. Immunohistochemical analyses using multiple antibodies revealed AQP9 specific labeling in hepatocytes, epididymis, vas deferens, and in epidermis of wild type mice, but a complete absence of labeling in AQP9 ؊/؊ mice. In brain, no detectable labeling was observed. Compared with control mice, plasma levels of glycerol and triglycerides were markedly increased in AQP9 ؊/؊ mice, whereas glucose, urea, free fatty acids, alkaline phosphatase, and cholesterol were not significantly different. Oral administration of glycerol to fasted mice resulted in an acute rise in blood glucose levels in both AQP9 ؊/؊ and AQP9 ؉/؊ mice, revealing no defect in utilization of exogenous glycerol as a gluconeogenic substrate and indicating a high gluconeogenic capacity in nonhepatic organs. Obese Lepr db /Lepr db AQP9 ؊/؊ and obese Lepr db /Lepr db AQP9 ؉/؊ mice showed similar body weight, whereas the glycerol levels in obese Lepr db /Lepr db AQP9 ؊/؊ mice were dramatically increased. Consistent with a role of AQP9 in hepatic uptake of glycerol, blood glucose levels were significantly reduced in Lepr db /Lepr db AQP9 ؊/؊ mice compared with Lepr db / Lepr db AQP9 ؉/؊ in response to 3 h of fasting. Thus, AQP9 is important for hepatic glycerol metabolism and may play a role in glycerol and glucose metabolism in diabetes mellitus.aquaglyceroporin ͉ diabetes mellitus ͉ leptin receptor

The discovery of aquaporin water channels by Agre and coworkers answered a long-standing biophysical question of how the majority of water crosses biological membranes. The identification and study of aquaporins have provided insight, at the molecular level, into the fundamental physiology of water balance regulation and the pathophysiology of water balance disorders. In addition to the originally identified classical aquaporins, a second class of aquaporins has been identified. Aquaporins in this latter class, the so-called aquaglyceroporins, transport small uncharged molecules such as glycerol and urea as well as water. Aquaglyceroporins have a wide tissue distribution, and emerging data suggest that several of them may play previously unappreciated physiological or pathophysiological roles. Analyses of transgenic mice have revealed potential roles of aquaglyceroporins in skin elasticity, gastrointestinal function and metabolism, and metabolic diseases such as diabetes mellitus. This review comprehensively discusses the recent discoveries in the field of aquaglyceroporins, alongside a brief overview of the so-called unorthodox aquaporins.

Aquaporin-2 (AQP2) is the predominant vasopressin-regulated water channel in kidney connecting tubule (CNT) and collecting duct (CD) and is essential for renal regulation of body water balance. However, the relative role of AQP2 to urinary concentration in the CNT and CD segments is unknown. To examine this directly, transgenic mice expressing AQP2 selectively in CNT but lacking AQP2 expression in CD (AQP2-CD-KO) and mice lacking AQP2 globally (AQP2-total-KO) were generated by exploiting the Cre͞loxP technology. LoxP sites were inserted into AQP2 introns 2 and 3, and transgenic mice were bred with strains expressing Cre recombinase under the control of CD-specific Hoxb7؊ or global EIIa promoter. Mice lacking AQP2 globally died postnatally (days 5-12). AQP2-CD-KO mice were viable to adulthood and showed decreased body weight, 10-fold increased urine production (0.96 ؎ 0.11 vs. 0.10 ؎ 0.01 ml͞g of body weight), and decreased urinary osmolality (170 ؎ 19 vs. 1,630 ؎ 135 milliosmoles͞kg of H2O). Immunohistochemical staining of AQP2-CD-KO kidneys (n ‫؍‬ 12) revealed sustained, strong AQP2 expression in CNT cells, whereas >95% of CD principal cells were completely AQP2-negative. Water deprivation for 3 hours caused only marginal decreased urine output (87 ؎ 7% of levels when mice had free water access; P ‫؍‬ 0.04) with no change in urine osmolality, revealing an absence of compensatory mechanisms. These results demonstrate that AQP2 in CNT is sufficient for postnatal survival and that AQP2 in CD is essential for regulation of body water balance and cannot be compensated for by other mechanisms.connecting tubule ͉ kidney ͉ vasopressin ͉ water channel A quaporins (AQPs) in the kidney are essential for body water balance regulation. AQP, expressed in the proximal nephron, is essential for reabsorption of the large fraction of water filtered in the glomerulus. Three AQPs are expressed in the collecting duct (CD) and connecting tubule (CNT): AQP2, AQP3, and AQP4. They are involved in the vasopressinregulated water reabsorption for tight control of body water balance. AQP2 is expressed in the apical plasma membrane and vesicles in the connecting tubule cells and in the CD principal cells (1) and is the chief target for regulation of the osmotic water permeability of these segments in response to vasopressin. This water balance regulation occurs by short-term, acute regulated translocation of intracellular AQP2-bearing vesicles to the apical plasma membrane (2-5) and by long-term regulation of the expression (reviewed in ref. 6). This process allows production of concentrated urine and is essential for water balance regulation. The molecular mechanisms involved in AQP2 insertion into the plasma membrane are the subject of intensive research and are not completely understood (recently reviewed in ref. 6).Studies in the rat indicate similar regulation of AQP2 gene expression and intracellular trafficking in the CNT and in the CD under various physiological and pathophysiological conditions (7), indicating that similar water tr...

Aquaporin (AQP7) is expressed in proximal tubules and is involved in glycerol uptake. The cellular expression and physiological function in other organs remain largely undefined. AQP7 knockout (KO) mice were generated and used for immunohistochemical analyses to define the organ and cellular expression of AQP7. AQP7 labeling was found in kidney proximal tubule, heart, skeletal muscle, testis, epididymis, as well as in white and brown adipose tissue (WAT and BAT) of wild-type mice. Importantly, immunoreactivity was completely absent from these tissues in AQP7 KO mice. At the cellular level, the capillary endothelium WAT and BAT displayed prominent staining, whereas AQP7 labeling in adipocyte membranes was undetectable. Double-labeling confocal microscopy revealed coexpression of AQP7 with capillary AQP1 but not with adipocyte GLUT4. Moreover, immunoelectron microscopy and RT-PCR of isolated microvessels confirmed the vascular AQP7 expression. Distinct immunolabeling of the capillary endothelium was also observed in both skeletal and heart muscle with no apparent staining of skeletal or cardiac myocytes. As previously reported, specific immunolabeling was confined to brush border in segment 3 renal proximal tubules and to spermatids and spermatozoa in male reproductive tract. The expression of AQP7 was induced up to 2.2-fold in WAT of mice with streptozotocin-induced diabetes mellitus (S-DM) compared with controls and fasting for 72 h (but not 24 h) induced significant increase in AQP7 expression. In conclusion, AQP7 is expressed in capillary endothelia of adipose tissue (and cardiac and striated muscle) and is upregulated in WAT in response to S-DM supporting its role in glycerol metabolism.

Abstract-Although the biophysical fingerprints (ion selectivity, voltage-dependence, kinetics, etc) of Ca 2ϩ -activated Cl Ϫ currents are well established, their molecular identity is still controversial. Several molecular candidates have been suggested; however, none of them has been fully accepted. We have recently characterized a cGMP-dependent Ca 2ϩ -activated Cl Ϫ current with unique characteristics in smooth muscle cells. This novel current has been shown to coexist with a "classic" (cGMP-independent) Ca 2ϩ -activated Cl Ϫ current and to have characteristics distinct from those previously known for Ca 2ϩ -activated Cl Ϫ currents. Here, we suggest that a bestrophin, a product of the Best gene family, is responsible for the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current based on similarities between the membrane currents produced by heterologous expressions of bestrophins and the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current. This is supported by similarities in the distribution pattern of the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current and bestrophin-3 (the product of Best-3 gene) expression in different smooth muscle. Furthermore, downregulation of Best-3 gene expression with small interfering RNA both in cultured cells and in vascular smooth muscle cells in vivo was associated with a significant reduction of the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current, whereas the magnitude of the classic Ca 2ϩ -activated Cl Ϫ current was not affected. Ϫ channel, which results in depolarization in vascular smooth muscle. Furthermore, the current is of similar magnitude as "classic" Ca 2ϩ -activated Cl Ϫ currents in most vascular beds and even larger in some vascular smooth muscles. 3 It is, therefore, highly desirable to know the molecular structure of the channel responsible for this current because it is likely to play an important role in smooth muscle function.Although their biophysical fingerprints (ion selectivity, voltage-dependence, kinetics, etc) are well established, 4 -6 the molecular identity of Ca 2ϩ -sensitive Cl Ϫ channels is still controversial. 7 Recently, the gene responsible for vitelliform macular dystrophy 8 and its homologs that code for bestrophin proteins have been suggested as candidates. 9,10 Four bestrophin family members in the mammalian genome and many homologues in genomes of invertebrates and even prokaryotes have been identified. 11-13 Two different nomenclatures for mammalian bestrophins were previously devel- The majority of suggestions that bestrophins function as Cl Ϫ channels are based on the findings that expression of the gene in different cell types leads to the appearance of a Cl Ϫ conductance 9,10 and that mutations or chemical modifications of the predicted channel pore change this conductance. [15][16][17][18] Although downregulation by small interfering (si)RNA in recent studies demonstrated a direct association between the endogenous Cl Ϫ current in epithelial cell culture and Best-1 expression, 19 -21 the exact role of the bestrophins in native tissues remains questionable...