Expression and localization of two isoforms of AQP10 in human small intestine

Immunolocalisation of aquaporins 3, 7, 9 and 10 in the epididymis of three wild ruminant species (Iberian ibex, mouflon and chamois) and sperm cryoresistance

CONTEXT

In the epididymis, epithelial cells manage changes in the luminal environment for proper sperm maturation. Moreover, aquaglyceroporins, a subgroup of aquaporins (AQP), modulate the transport of water, glycerol and other small molecules in epithelial cells.

AIMS

We aim to characterise the lining epithelium, quantify its cell composition and immunolocalise the aquaglyceroporins AQP3, AQP7, AQP9 and AQP10 alongside the epididymal ductus of three wild ruminant species, and to determine if species-specific differences could be associated with cauda sperm cryoresistance variations.

METHODS

Epididymides from Iberian ibex (n =5), mouflon (n =5) and chamois (n =6) were obtained. Cauda spermatozoa were collected and sperm parameters were analysed before and after freezing. Histology and immunohistochemistry of AQP3, 7, 9, 10 and T-CD3 were performed in the caput, corpus and cauda epididymal regions.

KEY RESULTS

This work first describes the lining epithelium in Iberian ibex, mouflon and chamois epididymis along the three anatomical regions, consisting of principal, basal, apical, clear and halo cells. However, the percentage of each cell type differed in ibex compared to mouflon and chamois. The positive T-CD3 immunolabeling of all the halo cells confirmed their T-lymphocyte nature. Aquaglyceroporin expression patterns were similar among species, except for differences in AQP7 and AQP10 immunolocalisation in ibex. Species-specific differences in epididymal sperm cryoresistance were confirmed.

CONCLUSIONS

The epididymal epithelium of the three wild ruminants differ in their relative number of cell types and AQP immunolocalisation, which ultimately appears to affect cauda epidydimal spermatozoa cryoresistance.

IMPLICATIONS

Our study provides information on the relevance of the quantitative composition and AQP pattern expression in epididymal lining epithelium on sperm cryoresistance.

Aquaporins in Obesity

Obesity is one of the most important metabolic disorders of this century and is associated with a cluster of the most dangerous cardiovascular disease risk factors, such as insulin resistance and diabetes, dyslipidemia, and hypertension, collectively named Metabolic Syndrome. The role of aquaporins (AQP) in glycerol metabolism facilitating glycerol release from the adipose tissue and distribution to various tissues and organs unveils these membrane channels as important players in lipid balance and energy homeostasis and points to their involvement in a variety of pathophysiological mechanisms including insulin resistance, obesity, and diabetes. This review summarizes the physiologic role of aquaglyceroporins in glycerol metabolism and lipid homeostasis, describing their specific tissue distribution, involvement in glycerol balance, and implication in obesity and fat-related metabolic complications. The development of specify pharmacologic modulators able to regulate aquaglyceroporins expression and function, in particular AQP7 in adipose tissue, might constitute a novel approach for controlling obesity and other metabolic disorders.

Aquaporins in Digestive System

In this chapter, we mainly discuss the expression and function of aquaporins (AQPs) expressed in digestive system. AQPs are highly conserved transmembrane protein responsible for water transport across cell membranes. AQPs in gastrointestinal tract include four members of aquaporin subfamily: AQP1, AQP4, AQP5, and AQP8, and three members of aquaglyceroporin subfamily: AQP3, AQP7, and AQP10. In the digestive glands, especially the liver, we discuss four members of aquaporin subfamily: AQP1, AQP4, AQP5, and AQP8, three members of aquaglyceroporin subfamily: AQP7, AQP9, and AQP12. In digestive system, the abnormal expression of AQPs is closely related to the occurrence and development of a variety of diseases. AQP1 is involved in saliva secretion and fat digestion and is closely related to gastric cancer and chronic liver disease; AQP3 is involved in the diarrhea and inflammatory bowel disease; AQP4 regulates gastric acid secretion and is associated with the development of gastric cancer; AQP5 is relevant to gastric carcinoma cell proliferation and migration; AQP7 is the major aquaglyceroporin in pancreatic β cells; AQP8 plays a role in pancreatic juice secretion and may be a potential target for the treatment of diarrhea; AQP9 plays considerable role in glycerol metabolism and hepatocellular carcinoma; Studies on the function of AQP10 and AQP12 are still limited. Further studies are necessary for specific locations and functions of AQPs in digestive system.

Clinical value and molecular mechanism of AQGPs in different tumors

Aquaglyceroporins (AQGPs), including AQP3, AQP7, AQP9, and AQP10, are transmembrane channels that allow small solutes across biological membranes, such as water, glycerol, H₂O₂, and so on. Increasing evidence suggests that they play critical roles in cancer. Overexpression or knockdown of AQGPs can promote or inhibit cancer cell proliferation, migration, invasion, apoptosis, epithelial-mesenchymal transition and metastasis, and the expression levels of AQGPs are closely linked to the prognosis of cancer patients. Here, we provide a comprehensive and detailed review to discuss the expression patterns of AQGPs in different cancers as well as the relationship between the expression patterns and prognosis. Then, we elaborate the relevance between AQGPs and malignant behaviors in cancer as well as the latent upstream regulators and downstream targets or signaling pathways of AQGPs. Finally, we summarize the potential clinical value in cancer treatment. This review will provide us with new ideas and thoughts for subsequent cancer therapy specifically targeting AQGPs.

Expression of Aquaglyceroporins in Spermatozoa from Wild Ruminants Is Influenced by Photoperiod and Thyroxine Concentrations

This work identified the presence of AQPs in frozen-thawed sperm of wild ruminants and assessed the influence of the interaction between photoperiod and thyroxine on AQP expression, and on testosterone secretion. Thyroxine and melatonin were administered to ibexes. In a second experiment, performed in mouflons, circulating thyroxine was reduced via treatment with propylthiouracil (PTU), and an artificial long day (LD) photoperiod established. In the ibexes, the melatonin treatment increased the blood plasma testosterone concentration, reduced the cryoresistance ratio (CR) for sperm viability and the presence of an intact acrosome, and increased the percentage of sperm with AQP7 in the acrosome and of AQP3 and AQP10 in the midpiece. In the mouflons, neither the PTU treatment, the LD, nor the combination of both affected the CR of any sperm variable. The percentage of sperm with AQP3 increased in the post-acrosome region but decreased in the tail in the LD+PTU group. The percentage of sperm with AQP10 in the principal piece and endpiece was lower in the PTU+LD group than in the control and LD groups. The influence of photoperiod/melatonin on AQP expression might be indirectly exerted through changes in the testosterone concentration, and thus ultimately affect sperm cryoresistance.

Aquaglyceroporin Modulators as Emergent Pharmacological Molecules for Human Diseases

Aquaglyceroporins, a sub-class of aquaporins that facilitate the diffusion of water, glycerol and other small uncharged solutes across cell membranes, have been recognized for their important role in human physiology and their involvement in multiple disorders, mostly related to disturbed energy homeostasis. Aquaglyceroporins dysfunction in a variety of pathological conditions highlighted their targeting as novel therapeutic strategies, boosting the search for potent and selective modulators with pharmacological properties. The identification of selective inhibitors with potential clinical applications has been challenging, relying on accurate assays to measure membrane glycerol permeability and validate effective functional blockers. Additionally, biologicals such as hormones and natural compounds have been revealed as alternative strategies to modulate aquaglyceroporins via their gene and protein expression. This review summarizes the current knowledge of aquaglyceroporins’ involvement in several pathologies and the experimental approaches used to evaluate glycerol permeability and aquaglyceroporin modulation. In addition, we provide an update on aquaglyceroporins modulators reported to impact disease, unveiling aquaglyceroporin pharmacological targeting as a promising approach for innovative therapeutics.

Aquaporin regulation in metabolic organs

Aquaporins (AQPs) are a family of 13 small trans-membrane proteins, which facilitate shuttling of glycerol, water and urea. The peculiar role of AQPs in glycerol transport makes them attractive targets in metabolic organs since glycerol represents the backbone of triglyceride synthesis. Importantly, AQPs are known to be regulated by various nuclear receptors which in turn govern lipid and glucose metabolism as well as inflammatory cascades. Here, we review the role of AQPs regulation in metabolic organs exploring their physiological impact in health and disease.

Ecological Specialisation and Evolutionary Reticulation in Extant Hyaenidae

During the Miocene, Hyaenidae was a highly diverse family of Carnivora that has since been severely reduced to four species: the bone-cracking spotted, striped, and brown hyenas, and the specialized insectivorous aardwolf. Previous studies investigated the evolutionary histories of the spotted and brown hyenas, but little is known about the remaining two species. Moreover, the genomic underpinnings of scavenging and insectivory, defining traits of the extant species, remain elusive. Here, we generated an aardwolf genome and analyzed it together with the remaining three species to reveal their evolutionary relationships, genomic underpinnings of their scavenging and insectivorous lifestyles, and their respective genetic diversities and demographic histories. High levels of phylogenetic discordance suggest gene flow between the aardwolf lineage and the ancestral brown/striped hyena lineage. Genes related to immunity and digestion in the bone-cracking hyenas and craniofacial development in the aardwolf showed the strongest signals of selection, suggesting putative key adaptations to carrion and termite feeding, respectively. A family-wide expansion in olfactory receptor genes suggests that an acute sense of smell was a key early adaptation. Finally, we report very low levels of genetic diversity within the brown and striped hyenas despite no signs of inbreeding, putatively linked to their similarly slow decline in effective population size over the last ∼2 million years. High levels of genetic diversity and more stable population sizes through time are seen in the spotted hyena and aardwolf. Taken together, our findings highlight how ecological specialization can impact the evolutionary history, demographics, and adaptive genetic changes of an evolutionary lineage.

Human adipose glycerol flux is regulated by a pH gate in AQP10

Obesity is a major threat to global health and metabolically associated with glycerol homeostasis. Here we demonstrate that in human adipocytes, the decreased pH observed during lipolysis (fat burning) correlates with increased glycerol release and stimulation of aquaglyceroporin AQP10. The crystal structure of human AQP10 determined at 2.3 Å resolution unveils the molecular basis for pH modulation-an exceptionally wide selectivity (ar/R) filter and a unique cytoplasmic gate. Structural and functional (in vitro and in vivo) analyses disclose a glycerol-specific pH-dependence and pinpoint pore-lining His80 as the pH-sensor. Molecular dynamics simulations indicate how gate opening is achieved. These findings unravel a unique type of aquaporin regulation important for controlling body fat mass. Thus, targeting the cytoplasmic gate to induce constitutive glycerol secretion may offer an attractive option for treating obesity and related complications.

Nuclear Receptor Regulation of Aquaglyceroporins in Metabolic Organs

Nuclear receptors, such as the farnesoid X receptor (FXR) and the peroxisome proliferator-activated receptors gamma and alpha (PPAR-γ, -α), are major metabolic regulators in adipose tissue and the liver, where they govern lipid, glucose, and bile acid homeostasis, as well as inflammatory cascades. Glycerol and free fatty acids are the end products of lipid droplet catabolism driven by PPARs. Aquaporins (AQPs), a family of 13 small transmembrane proteins, facilitate the shuttling of water, urea, and/or glycerol. The peculiar role of AQPs in glycerol transport makes them pivotal targets in lipid metabolism, especially considering their tissue-specific regulation by the nuclear receptors PPARγ and PPARα. Here, we review the role of nuclear receptors in the regulation of glycerol shuttling in liver and adipose tissue through the function and expression of AQPs.

Endothelial Aquaporins and Hypomethylation: Potential Implications for Atherosclerosis and Cardiovascular Disease

Aquaporins (AQPs) are transmembrane channels that facilitate water and glycerol permeation through cell membranes. Recently, the water channel AQP1 was suggested to contribute to endothelial homeostasis and cardiovascular health. Less is known about endothelial aquaglyceroporins expression and its implication in cardiovascular disease (CVD). We have previously used cultured human endothelial cells under a hypomethylating environment to study endothelial dysfunction and activation, a phenotype implicated in the establishment of atherosclerosis and CVD. Here, we used the same cell model to investigate aquaporin’s expression and function in healthy or pro-atherogenic phenotype. We first confirmed key features of endothelium dysfunction and activation in our cell model, including an augmented endothelial transmigration under hypomethylation. Subsequently, we found AQP1 and AQP3 to be the most predominant AQPs accounting for water and glycerol fluxes, respectively, in the healthy endothelium. Moreover, endothelial hypomethylation led to decreased levels of AQP1 and impaired water permeability without affecting AQP3 and glycerol permeability. Furthermore, TNF-α treatment-induced AQP1 downregulation suggesting that the inflammatory NF-κB signaling pathway mediates AQP1 transcriptional repression in a pro-atherogenic endothelium, a possibility that warrants further investigation. In conclusion, our results add further support to AQP1 as a candidate player in the setting of endothelial dysfunction and CVD.

Aquaporins in Obesity

Obesity is one of the most important metabolic disorders of this century and is associated with a cluster of the most dangerous cardiovascular disease risk factors, such as insulin resistance and diabetes , dyslipidemia and hypertension , collectively named Metabolic Syndrome. The role of aquaporins in glycerol metabolism facilitating glycerol release from the adipose tissue and distribution to various tissues and organs, unveils these membrane channels as important players in lipid balance and energy homeostasis and points to their involvement in a variety of pathophysiological mechanisms including insulin resistance, obesity and diabetes.This review summarizes the physiologic role of aquaglyceroporins in glycerol metabolism and lipid homeostasis, describing their specific tissue distribution, their involvement in glycerol balance and their implication in obesity and fat-related metabolic complications. The development of specify pharmacologic modulators able to regulate aquaglyceroporins expression and function , in particular AQP7 in adipose tissue, might constitute a novel approach for controlling obesity and other metabolic disorders.

Expression, Distribution and Role of Aquaporin Water Channels in Human and Animal Stomach and Intestines

Stomach and intestines are involved in the secretion of gastrointestinal fluids and the absorption of nutrients and fluids, which ensure normal gut functions. Aquaporin water channels (AQPs) represent a major transcellular route for water transport in the gastrointestinal tract. Until now, at least 11 AQPs (AQP1–11) have been found to be present in the stomach, small and large intestines. These AQPs are distributed in different cell types in the stomach and intestines, including gastric epithelial cells, gastric glands cells, absorptive epithelial cells (enterocytes), goblet cells and Paneth cells. AQP1 is abundantly distributed in the endothelial cells of the gastrointestinal tract. AQP3 and AQP4 are mainly distributed in the basolateral membrane of epithelial cells in the stomach and intestines. AQP7, AQP8, AQP10 and AQP11 are distributed in the apical of enterocytes in the small and large intestines. Although AQP-null mice displayed almost no phenotypes in gastrointestinal tracts, the alterations of the expression and localization of these AQPs have been shown to be associated with the pathology of gastrointestinal disorders, which suggests that AQPs play important roles serving as potential therapeutic targets. Therefore, this review provides an overview of the expression, localization and distribution of AQPs in the stomach, small and large intestine of human and animals. Furthermore, this review emphasizes the potential roles of AQPs in the physiology and pathophysiology of stomach and intestines.

Aquaporins in Health and Disease: An Overview Focusing on the Gut of Different Species

Aquaporins (AQPs) play a pivotal role in gut homeostasis since their distribution and function is modulated both in physiological and in pathophysiological conditions. The transport of water and solutes through gut epithelia is essential for osmoregulation and digestive and absorptive functions. This passage is regulated by different AQP isoforms and characterized by their peculiar distribution in the gastrointestinal tract. To date, AQP localization has been identified in the gut and associated organs of several mammalian species by different techniques (immunohistochemical, western blotting, and RT-PCR). The present review describes the modulation of AQP expression, distribution, and function in gut pathophysiology. At the same time, the comparative description of AQP in animal species sheds light on the full range of AQP functions and the screening of their activity as transport modulators, diagnostic biomarkers, and drug targets. Moreover, the phenotype of knockout mice for several AQPs and their compensatory role and the use of specific AQP inhibitors have been also reviewed. The reported data could be useful to design future research in both basic and clinical fields.

Mammalian aquaglyceroporin function in metabolism

Aquaglyceroporins are integral membrane proteins that are permeable to glycerol as well as water. The movement of glycerol from a tissue/organ to the plasma and vice versa requires the presence of different aquaglyceroporins that can regulate the entrance or the exit of glycerol across the plasma membrane. Actually, different aquaglyceroporins have been discovered in the adipose tissue, small intestine, liver, kidney, heart, skeletal muscle, endocrine pancreas and capillary endothelium, and their differential expression could be related to obesity and the type 2 diabetes. Here we describe the expression and function of different aquaglyceroporins in physiological condition and in obesity and type 2 diabetes, suggesting they are potential therapeutic targets for metabolic disorders.

Expression and Localization of Aquaporin 4 and Aquaporin 5 along the Large Intestine of Colostrum-Suckling Buffalo Calves

Aquaporins (AQPs) are membrane channel proteins that play a role in regulating water permeability in many tissues. To date, seven isoforms of AQPs have been reported in the gastrointestinal tract in different mammalian species. In contrast, both tissue distribution and expression of AQPs are unknown in the buffalo. The purpose of this study was to investigate the expression of both AQP4 and AQP5 mRNAs and their relative proteins in the large intestinal tracts of buffalo calves after colostrum suckling using reverse transcriptase polymerase chain reaction (RT-PCR), Western blotting and immunohistochemistry. Our results revealed a diversified tissue AQP4 and AQP5 immunolocalization accompanied by their highest expression in the tissues of colostrum-suckling buffalo calves confirmed by Western blotting. In particular, AQP4 was distributed along the endothelium and enterocytes while AQP5 in the endocrine cells. These findings provide direct evidence for AQP4 and AQP5 expression in the large intestine, suggesting that different AQPs collaborate functionally and distinctively in water handling during intestinal development, especially during the first period after delivery.

Aquaporin10 is a pseudogene in cattle and their relatives

Background

Although AQP10 is mainly expressed in the human GI tract, its physiological role is unclear. In fact, we previously reported that mouse AQP10 is a pseudogene. It is possible that AQP10 is also a pseudogene in other animals.

Methods

Genome databases were searched for AQP10 orthologs and the genomic DNA of each candidate pseudogene was sequenced to confirm its mutations. The expression of the AQP10 mRNA was examined by RT-PCR in the small intestine where human AQP10 is highly expressed.

Results

The genomic database of some mammals had insertions and deletions in the exons of the AQP10 gene, including cattle (Bos taurus), sheep (Ovis aries) and goats (Capra hircus). In the bovine AQP10 gene, exon 1 and 5 had deletions resulting in a frame-shift or a premature termination, respectively, which were confirmed by the direct exon sequencing of the genomic DNA. In the RT-PCR experiments, the PCR primer sets for exon 1/2 and exon 4/5 failed to detect the bands for AQP10 mRNA in the duodenum and jejunum. Similar AQP10 gene mutations were also confirmed in the genomic DNA from sheep and goats. Although these animals were derived from porcine ancestors, the exons of the swine (Sus scrofa) AQP10 gene were complete without mutations. Therefore, AQP10 gene might have turned to a pseudogene around 65 million years before when cattle evolved from porcine ancestors.

Conclusion

AQP10 of ruminantia which regurgitate and rechew their food may have lost its role possibly due to the redundant expression of other aquaglyceroporins.

Role of aquaglyceroporins and caveolins in energy and metabolic homeostasis

Aquaglyceroporins and caveolins are submicroscopic integral membrane proteins that are particularly abundant in many mammalian cells. Aquaglyceroporins (AQP3, AQP7, AQP9 and AQP10) encompass a subfamily of aquaporins that allow the movement of water, but also of small solutes, such as glycerol, across cell membranes. Glycerol constitutes an important metabolite as a substrate for de novo synthesis of triacylglycerols and glucose as well as an energy substrate to produce ATP via the mitochondrial oxidative phosphorylation. In this sense, the control of glycerol influx/efflux in metabolic organs by aquaglyceroporins plays a crucial role with the dysregulation of these glycerol channels being associated with metabolic diseases, such as obesity, insulin resistance, non-alcoholic fatty liver disease and cardiac hypertrophy. On the other hand, caveolae have emerged as relevant plasma membrane sensors implicated in a wide range of cellular functions, including endocytosis, apoptosis, cholesterol homeostasis, proliferation and signal transduction. Caveolae-coating proteins, namely caveolins and cavins, can act as scaffolding proteins within caveolae by concentrating signaling molecules involved in free fatty acid and cholesterol uptake, proliferation, insulin signaling or vasorelaxation, among others. The importance of caveolae in whole-body homeostasis is highlighted by the link between homozygous mutations in genes encoding caveolins and cavins with metabolic diseases, such as lipodystrophy, dyslipidemia, muscular dystrophy and insulin resistance in rodents and humans. The present review focuses on the role of aquaglyceroporins and caveolins on lipid and glucose metabolism, insulin secretion and signaling, energy production and cardiovascular homeostasis, outlining their potential relevance in the development and treatment of metabolic diseases.

Aquaporin expression in the Japanese medaka (Oryzias latipes) in freshwater and seawater: challenging the paradigm of intestinal water transport?

We investigated the salinity-dependent expression dynamics of seven aquaporin paralogs (aqp1a, aqp3a, aqp7, aqp8ab, aqp10a, aqp10b and aqp11a) in several tissues of euryhaline Japanese medaka (Oryzias latipes). All paralogs except aqp7 and aqp10a had a broad tissue distribution, and several were affected by salinity in both osmoregulatory and non-osmoregulatory tissues. In the intestine, aqp1a, aqp7, aqp8ab and aqp10a decreased upon seawater (SW) acclimation in both long-term acclimated fish and during 1-3 days of the transition period. In the gill, aqp3a was lower and aqp10a higher in SW than in freshwater (FW). In the kidney no aqps were affected by salinity. In the skin, aqp1a and aqp3a were lower in SW than in FW. In the liver, aqp8ab and aqp10a were lower in SW than in FW. Furthermore, six Na(+),K(+)-ATPase α-subunit isoform transcripts were analysed in the intestine but none showed a consistent response to salinity, suggesting that water transport is not regulated at this level. In contrast, mRNA of the Na(+),K(+),2Cl(-)-cotransporter type-2 strongly increased in the intestine in SW compared with FW fish. Using custom-made antibodies, Aqp1a, Aqp8ab and Aqp10a were localized in the apical region of enterocytes of FW fish. Apical staining intensity strongly decreased, vanished or moved to subapical regions, when fish were acclimated to SW, supporting the lower mRNA expression in SW. Western blots confirmed the decrease in Aqp1a and Aqp10a in SW. The strong decrease in aquaporin expression in the intestine of SW fish is surprising, and challenges the paradigm for transepithelial intestinal water absorption in SW fishes.

Defensive slime formation in Pacific hagfish requires Ca2+- and aquaporin-mediated swelling of released mucin vesicles

Hagfishes defend themselves from fish predators via the rapid deployment of a fibrous slime that adheres to and clogs gills. The slime transforms from a thick glandular exudate to a fully hydrated product in a fraction of a second through a process that involves the swelling and rupture of numerous mucin vesicles. Here we demonstrate that the vesicle membrane plays an important role in regulating the swelling of mucin granules, and provide evidence that the membrane contains proteins that facilitate the movement of ions and water molecules. By exposing isolated mucin vesicles to varying combinations of inorganic ions, organic compounds and membrane channel inhibitors, we found that the majority of hagfish mucin vesicles require Ca(2+) to rupture. We also show that Ca(2+)-dependent rupture can be pharmacologically inhibited, which suggests a role for Ca(2+)-activated membrane transporters. We demonstrate that the aquaporin inhibitor mercuric chloride reduces the rate of vesicle swelling by an order of magnitude, which suggests that aquaporins facilitate the influx of water during vesicle deployment. Molecular evidence of two aquaporin homologues expressed in the slime glands further supports this idea. We propose a model of hagfish slime mucin vesicle rupture that involves Ca(2+)-activated transporters and aquaporins, and suggest that the presence of these proteins is an adaptation for increasing the speed of vesicle rupture and, consequently, the speed of the sliming response of hagfishes.

Aquaporin3 is required for FGF-2-induced migration of human breast cancers

PURPOSE

The aquaporin (AQP) family consists of a number of small integral membrane proteins that transport water and glycerol. AQPs are critical for trans-epithelial fluid transport. Recent reports demonstrated that AQPs, particularly AQP1 and AQP5, are expressed in high grade tumor cells of a variety of tissue origins, and that AQPs are involved in cell migration and metastasis. Based on this background, we examined whether AQP3, another important member of the AQP family, could facilitate cell migration in human breast cancers.

METHODS

Potential role of AQP3 was examined using two representative breast cancer cell lines (MDA-MB-231 and Bcap-37). Briefly, AQP3 expression was inhibited with a lentivirus construct that stably expressed shRNA against the AQP3 mRNA. AQP3 expression inhibition was verified with Western blot. Cell migration was examined using a wound scratch assay in the presence of fibroblast growth factor-2 (FGF-2). In additional experiments, AQP3 was inhibited by CuSO4. Fibroblast growth factor receptor (FGFR) kinase inhibitor PD173074, PI3K inhibitor LY294002, and MEK1/2 inhibitor PD98059 were used to dissect the molecular mechanism of FGF-2 induced AQP3 expression.

RESULTS

FGF-2 treatment increased AQP3 expression and induced cell migration in a dose dependent manner. Silencing AQP3 expression by a lentiviral shRNA inhibited FGF-2 induced cell migration. CuSO4, a water transport inhibitor selective for AQP3, also suppressed FGF-2-induced cell migration. The FGFR kinase inhibitor PD173074, significantly inhibited FGF-2-induced AQP3 expression and cell migration. The PI3K inhibitor LY294002 and MEK1/2 inhibitor PD98059 inhibited, but not fully blocked, FGF-2-induced AQP3 expression and cell migration.

CONCLUSIONS

AQP3 is required for FGF-2-induced cell migration in cultured human breast cancer cells. Our findings also suggest the importance of FGFR-PI3K and FGFR-ERK signaling in FGF-2-induced AQP3 expression. In summary, our findings suggest a novel function of AQP3 in cell migration and metastasis of breast cancers.

Aquaporin-10 represents an alternative pathway for glycerol efflux from human adipocytes

Background

Glycerol outflow from adipocytes has been considered for a decade to be mediated by aquaporin-7, an aquaglyceroporin highly expressed in the adipose tissue. Its involvement in glycerol metabolism has been widely studied also in humans. Recent studies in different aquaporin-7 KO mice models pose two different questions 1) the exact localization of aquaporin-7 in human white adipose tissue; 2) the existence of other aquaglyceroporins that work with aquaporin-7 to guarantee glycerol efflux and thus a normal adiposity in humans. To this purpose we investigated the expression, the localization and the functioning of aquaglyceroporin-10 in subcutaneous white adipose tissue, in isolated and cultured differentiated adipocytes.

Methodology/Principal Findings

Aquaporin-7 and -10 were expressed in the white adipose tissue both at mRNA and at protein level. Immunofluorescence revealed aquaporin-7 and -10 labelling in the human adipose tissue both to the plasma membrane and to a thin rim of cytoplasm of adipocytes. Aquaporin-7, but not aquaporin-10, colocalized with the endothelial marker CD34. Human cultured differentiated adipocytes showed an aquaporin-7 and -10 labelling mainly in the cytoplasm and in the lipid droplets with insulin reinforcing the lipid droplets staining and isoproterenol inducing its translocation to the plasma membrane compartment. Water and glycerol permeability measurements using adipocytes and adipose membrane vesicles confirmed the presence of functioning aquaglyceroporins. Aquaporin-10 silencing in human differentiated adipocytes resulted in a 50% decrease of glycerol and osmotic water permeability.

Conclusions/Significance

The results indicate that aquaporin-7, differently from mice, is present in both adipocyte and capillary plasma membranes of human adipose tissue. Aquaporin-10, on the contrary, is expressed exclusively in the adipocytes. The expression of two aquaglyceroporins in human adipose tissue is particularly important for the maintenance of normal or low glycerol contents inside the adipocyte, thus protecting humans from obesity.

Water channel proteins in the gastrointestinal tract

Water transport through the human digestive system is physiologically crucial for maintaining body water homeostasis and ensure digestive and absorptive functions. Within the gastrointestinal tract, water recirculates, being secreted with the digestive juices and then almost entirely absorbed by the small and large intestine. The importance of aquaporins (AQPs), transmembrane water channel proteins, in the rapid passage of water across plasma membranes in the gastrointestinal tract appears immediately evident. Several AQP isoforms are found in gastrointestinal epithelia, with AQP1, 3, 7, 10 and 11 being the most abundantly expressed in the whole gut. On the other hand, AQP4 and 8 are located selectively in the stomach and colon, respectively. Here we review AQP expression and localization at the tissue, cellular and subcellular level in gastrointestinal epithelia, and their modification in various gut diseases.

c-Met upregulates aquaporin 3 expression in human gastric carcinoma cells via the ERK signalling pathway

Aquaporin 3 (AQP3) and c-Met are both overexpressed in human gastric carcinoma and highly associated with its metastasis and invasion. However, it still remains unknown whether c-Met and AQP3 correlate with each other. Herein, we demonstrated that c-Met expression in gastric cancer tissues significantly correlated with differentiation, lymph node metastasis and lymphovascular invasion, and c-Met exhibited marked association with AQP3 expression. Immunoblotting assays showed that hHGF phosphorylated c-Met in SGC7901 and AGS cells and upregulated AQP3 expression in a dose- or time-dependent way. RNAi against c-Met reduced total c-Met levels by about two thirds in both AGS and SGC7901 cells and attenuated hHGF-induced AQP3 expression significantly. In vitro migration and proliferation assays showed that siRNA against AQP3 noticeably restrained HGF-promoted migration and proliferation of these cells. Furthermore, Immunoblotting studies revealed that HGF induced phosphorylation of ERK, and pre-treatment with U0126, a MAPK/ERK inhibitor, partially inhibited hHGF-induced increase in AQP3 expression. Together, these data provide initial evidence that c-Met regulates the expression of AQP3 via the ERK signalling pathway in gastric carcinoma. These findings assist in understanding the mechanism of growth and invasion of gastric carcinoma, and provide a possible strategy for the inhibition of gastric tumor metastasis.

Glycosylation increases the thermostability of human aquaporin 10 protein

Human aquaporin10 (hAQP10) is a transmembrane facilitator of both water and glycerol transport in the small intestine. This aquaglyceroporin is located in the apical membrane of enterocytes and is believed to contribute to the passage of water and glycerol through these intestinal absorptive cells. Here we overproduced hAQP10 in the yeast Pichia pastoris and observed that the protein is glycosylated at Asn-133 in the extracellular loop C. This finding confirms one of three predicted glycosylation sites for hAQP10, and its glycosylation is unique for the human aquaporins overproduced in this host. Nonglycosylated protein was isolated using both glycan affinity chromatography and through mutating asparagine 133 to a glutamine. All three forms of hAQP10 where found to facilitate the transport of water, glycerol, erythritol, and xylitol, and glycosylation had little effect on functionality. In contrast, glycosylated hAQP10 showed increased thermostability of 3-6 °C compared with the nonglycosylated protein, suggesting a stabilizing effect of the N-linked glycan. Because only one third of hAQP10 was glycosylated yet the thermostability titration was mono-modal, we suggest that the presence of at least one glycosylated protein within each tetramer is sufficient to convey an enhanced structural stability to the remaining hAQP10 protomers of the tetramer.

The evolutionary aspects of aquaporin family

Aquaporins (AQPs) were originally identified as channels facilitating water transport across the plasma membrane. They have a pair of highly conserved signature sequences, asparagine-proline-alanine (NPA) boxes, to form a pore. However, some have little conserved amino acid sequences around the NPA boxes unclassifiable to two previous AQP subfamilies, classical AQPs and aquaglyceroporins. These will be called unorthodox AQPs in this review. Interestingly, these unorthodox AQPs have a highly conserved cysteine residue downstream of the second NPA box. AQPs also have a diversity of functions: some related to water transport such as fluid secretion, fluid absorption, and cell volume regulation, and the others not directly related to water transport such as cell adhesion, cell migration, cell proliferation, and cell differentiation. Some AQPs even permeate nonionic small molecules, ions, metals, and possibly gasses. AQP gene disruption studies have revealed their physiological roles: water transport in the kidney and exocrine glands, glycerol transport in fat metabolism and in skin moisture, and nutrient uptakes in plants. Furthermore, AQPs are also present at intracellular organelles, including tonoplasts, mitochondria, and the endoplasmic reticulum. This review focuses on the evolutionary aspects of AQPs from bacteria to humans in view of the structural and functional diversities of AQPs.

Solute transporters and aquaporins are impaired in celiac disease

BACKGROUND INFORMATION

Celiac disease is a chronic inflammatory disorder of the small bowel induced in genetically susceptible subjects by gluten ingestion. Diarrhoea, weight loss and malabsorption represent the major clinical presentation of the disease. Here we examined the possible alteration in the expression and localization of water channels [AQPs (aquaporins)] and some solute transporters in duodenal mucosa of celiac disease patients. Duodenal biopsies from untreated celiacs, treated celiacs, healthy controls and disease controls were considered in the present study. The expressions of some AQPs and transporter mRNAs in human duodenal biopsies were determined by semi-quantitative RT-PCR (reverse transcription PCR) and real-time RT-PCR. The localization of AQPs 3, 7 and 10 and of SGLT1 (Na+/glucose co-transporter 1), PEPT1 (H+/oligopeptide transporter 1) and NHE3 (Na+/H+ exchanger 3) was evaluated by immunohistochemistry.

RESULTS

AQPs 3, 7, 10 and 11, SGLT, PEPT and NHE, CFTR (cystic fibrosis transmembrane conductance regulator) and NKCC (Na-K-2Cl co-transporter) mRNAs were expressed in duodenal biopsies of healthy controls, treated celiac patients and disease controls. The expression of transcripts was virtually absent in duodenal biopsies of untreated celiac disease patients except for CFTR and NKCC. In healthy controls, immunohistochemistry revealed a labelling in the apical membrane of surface epithelial cells of the duodenum. The immunolabelling was heavily reduced or absent in untreated celiac patients, while it was normal in patients consuming a gluten-free diet for at least 12 months.

CONCLUSIONS

Our results indicate that the main routes for water and solute absorption are deficient in celiac disease and may play a role in the onset of malabsorption symptoms.

Expression profile of multiple aquaporins in human gastric carcinoma and its clinical significance

BACKGROUND

New evidence for involvement of aquaporins (AQPs) in cell migration and proliferation adds AQPs to an expanding list of effectors in tumor biology. But there is few report concerning the expression and role of AQPs in human gastric carcinogenesis so far. The aim of this current study was to investigate the expression profile of AQPs in human gastric carcinoma and its significance.

METHODS

We screened the expression profile of AQP0 approximately AQP12 in gastric adenocarcinoma tissues and corresponding normal mucosa from 89 patients with gastric cancer by reverse transcriptase polymerase chain reaction (RT-PCR), Western blot analysis and immunochemical assay. The relationship between AQPs expression and clinicopathologic characteristics of patients was evaluated.

RESULTS

Based on RT-PCR of 13 AQPs examined, AQP1, 3, 4, 5 and 11 were expressed in human gastric cancers or normal gastric tissues, and AQP3, 4 and 5 exhibited differential expression between human gastric carcinomas and corresponding normal tissues, which was confirmed by Western blot analyses. Immunohistochemical assay showed that AQP4 protein was expressed mainly in the membrane of parietal cell and chief cell in the normal gastric mucosa, and absent in carcinoma tissues. AQP3 and AQP5 were detected remarkably stronger in the carcinoma tissues than that in normal mucosa by immunofluorescence. AQP3 expression in cases with undifferentiated tumor was more than that in cases with well-differentiated tumor. Both AQP3 and AQP5 expression were associated with lymph node metastasis and lymphovascular invasion in patients.

CONCLUSIONS

These findings of differential expressions of AQPs and their correlation with clinicopathologic characteristics implicated AQPs might play a role in human gastric carcinogenesis.

Aquaporin-6 is expressed along the rat gastrointestinal tract and upregulated by feeding in the small intestine

Background

Several aquaporins (a family of integral membrane proteins) have been recently identified in the mammalian gastrointestinal tract, and their involvement in the movement of fluid and small solutes has been suggested. In this direction we investigated, in some regions of the rat gastrointestinal tract, the presence and localization of aquaporin-6, given its peculiar function as an ion selective channel.

Results

RT-PCR and immunoblotting experiments showed that aquaporin-6 was expressed in all the investigated portions of the rat gastrointestinal tract. The RT-PCR experiments showed that aquaporin-6 transcript was highly expressed in small intestine and rectum, and less in stomach, caecum and colon. In addition, jejunal mRNA expression was specifically stimulated by feeding.

Immunoblotting analysis showed a major band with a molecular weight of about 55 kDa corresponding to the aquaporin-6 protein dimer; this band was stronger in the stomach and large intestine than in the small intestine. Immunoblotting analysis of brush border membrane vesicle preparations showed an intense signal for aquaporin-6 protein.

The results of in situ hybridization experiments demonstrate that aquaporin-6 transcript is present in the isthmus, neck and basal regions of the stomach lining, and throughout the crypt-villus axis in both small and large intestine. In the latter regions, immunohistochemistry revealed strong aquaporin-6 labelling in the apical membrane of the surface epithelial cells, while weak or no labelling was observed in the crypt cells. In the stomach, an intense staining was observed in mucous neck cells and lower signal in principal cells and some parietal cells.

Conclusion

The results indicate that aquaporin-6 is distributed throughout the gastrointestinal tract. Aquaporin-6 localization at the apical pole of the superficial epithelial cells and its upregulation by feeding suggest that it may be involved in movements of water and anions through the epithelium of the villi.

Expression and localization of aquaporin-1 on the apical membrane of enterocytes in the small intestine of bottlenose dolphins

The small and large intestines are primary sites for water intake in mammals. To reveal how water is absorbed in the intestines of cetaceans, histological and molecular-biological studies were performed on the small intestine of the bottlenose dolphin, Tursiops truncatus. In histological studies using fresh specimens, obvious villi and deep crypts of Lieberkühn, lined by abundant enterocytes with microvilli and goblet cells, were observed in the mucosa. Expressions and immunolocalizations of aquaporin-1 (AQP1), a member of the water-selective channel termed AQP, were also investigated in the intestine. By reverse transcriptional polymerase chain reaction and rapid amplification of cDNA ends using RNA extracted from the dolphins' small intestines, the full length of mRNA for AQP1 was sequenced. The deductive amino acid sequence for an open reading frame showed high homologies with other mammals' AQP1, and water permeability of the protein was certified by cRNA injection to Xenopus oocytes. Immunohistochemistry showed AQP1 distribution on the apical membrane of the enterocytes, especially in the crypts. These data suggest that AQP1 is a channel protein responsible for water absorption in the small intestine of dolphins.

Integrative sequence and tissue expression profiling of chicken and mammalian aquaporins

BACKGROUND

Proteins that selectively transport water across the membranes of cells are recognized as important in the normal functioning of the body systems of vertebrates. There are 13 known mammalian aquaporins (AQP0 to AQP12), some of which have been shown to have unexpected cellular roles beyond transmembrane water transport. The availability of non-mammalian vertebrate animal models has the potential to provide insight into the emergence of diverse function in the aquaporins. The domesticated chicken (Gallus gallus) is the premier avian model for biological research; however, only a limited number of studies have compared chicken and mammalian aquaporins. The identification of aquaporins that share functional motifs or are expressed in the same tissues in human and chicken could allow the further functional analyses of homologous aquaporins in both species. We hypothesize that integrative analyses of protein sequences and body site expression of human, mouse, rat and chicken aquaporins has the potential to yield novel biological hypotheses about the unexpected cellular roles of aquaporins beyond transmembrane water transport.

RESULTS

A total of 76 aquaporin transcript models derived from 47 aquaporin genes were obtained for human, mouse, rat and chicken. Eleven body sites (brain, connective tissue, head, heart, liver, muscle, ovary, pancreas, small intestine, spleen and testis) were identified in which there is suggested expression of at least one mammalian and one chicken aquaporin. This study demonstrates that modern on-line analysis tools, a novel matrix integration technique, and the availability of the chicken genome for comparative genomics and expression analysis enables hypothesis generation in several important areas including: (i) alternative transcription and speciation effects on the conservation of functional motifs in vertebrate aquaporins; (ii) the emergence of basolateral targeting in mammalian species; (iii) the potential of the cysteine-rich AQP11 as a possible target in the pathophysiology of neurodegenerative disorders such as autism that involve Purkinje cells; and (iv) possible impairment of function of pancreas-expressed AQP12 during pancreatotropic necrosis in avian influenza virus infection.

CONCLUSION

The investigation of aquaporin function in chicken and mammalian species has the potential to accelerate the discovery of novel knowledge of aquaporins in both avian and mammalian species.

New members of mammalian aquaporins: AQP10-AQP12

Thirteen members of mammalian aquaporins have been identified. Initial ten members (AQP-AQP9) are relatively well detailed and their roles are clarified. However, the last three members, AQP10-AQP12, are poorly characterized and little is known about their roles though they were cloned 6 years ago. In this review, we focus on these three AQPs. AQP10 is an aquaglyceroporin while AQP11 and AQP12 belong to a new subfamily, superaquaporins. Knockout mice for these aquaporins are now available. The AQP11 null mouse has a remarkable phenotype, polycystic kidneys, which is neonatally fatal. On the other hand, the absence of the other two affected little. In some animals, AQP10 is even a pseudogene. This review summarizes the current knowledge on these AQPs and will hopefully stimulate future research on the subject.

Aquaporins as potential drug targets for Meniere's disease and its related diseases

The homeostasis of water in the inner ear is essential for maintaining function of hearing and equilibrium. Since the discovery of aquaporin water channels, it has become clear that these channels play a crucial role in inner ear fluid homeostasis. Indeed, proteins or mRNAs of AQP1, AQP2, AQP3, AQP4, AQP5, AQP6, AQP7 and AQP9 are expressed in the inner ear. Many of them are expressed mainly in the stria vascularis and the endolymphatic sac, which are the main sites of secretion and/or absorption of endolymph. Vasopressin type2 receptor is also expressed there. Water homeostasis of the inner ear is regulated in part via the arginine vasopressin-AQP2 system in the same fashion as in the kidney, and endolymphatic hydrops, a morphological characteristic of Meniere's disease, is thought to be caused by mal-regulation of this system. Therefore, aquaporins appear to be important for the development of novel drug therapies for Meniere's disease and related disorders.

Aquaporin water channels in mammals

Water channels, aquaporins (AQPs), are a family of small integral plasma membrane proteins that primarily transport water across the plasma membrane. There are 13 members (AQP0-12) in humans. This number is final as the human genome project has been completed. They are divided into three subgroups based on the primary sequences: water selective AQPs (AQP0, 1, 2, 4, 5, 6, 8), aquaglyceroporins (AQP3, 7, 9, 10), and superaquaporins (AQP11, 12). Since no specific inhibitors are yet available, functional roles of AQPs are suggested by AQP null mice and humans. Abnormal water metabolism was shown with AQP1, 2, 3, 4, 5 null mice, especially with AQP2 null mice: fatal at neonate due to diabetes insipidus. Abnormal glycerol transport was shown with AQP3, 7, 9 null mice, although they appeared normal. AQP0 null mice suffer from cataracts, although the pathogenesis is not clear. Unexpectedly, AQP11 null mice die from uremia as a result of polycystic kidneys. Interestingly, AQP6, 8, 10, 12 null mice are almost normal. AQP null humans have been reported with AQP0, 1, 2, 3, 7: only AQP2 null humans show an outstanding phenotype, diabetes insipidus. This review summarizes the current knowledge on all mammalian AQPs and hopefully will stimulate future research in both clinical and basic fields.

Transcriptional regulation of AQP-8, a Caenorhabditis elegans aquaporin exclusively expressed in the excretory system, by the POU homeobox transcription factor CEH-6

Due to the ever changing environmental conditions in soil, regulation of osmotic homeostasis in the soil-dwelling nematode Caenorhabditis elegans is critical. AQP-8 is a C. elegans aquaporin that is expressed in the excretory cell, a renal equivalent tissue, where the protein participates in maintaining water balance. To better understand the regulation of AQP-8, we undertook a promoter analysis to identify the aqp-8 cis-regulatory elements. Using progressive 5' deletions of upstream sequence, we have mapped an essential regulatory region to roughly 300 bp upstream of the translational start site of aqp-8. Analysis of this region revealed a sequence corresponding to a known DNA functional element (octamer motif), which interacts with POU homeobox transcription factors. Phylogenetic footprinting showed that this site is perfectly conserved in four nematode species. The octamer site's function was further confirmed by deletion analyses, mutagenesis, functional studies, and electrophoretic mobility shift assays. Of the three POU homeobox proteins encoded in the C. elegans genome, CEH-6 is the only member that is expressed in the excretory cell. We show that expression of AQP-8 is regulated by CEH-6 by performing RNA interference experiments. CEH-6's mammalian ortholog, Brn1, is expressed both in the kidney and the central nervous system and binds to the same octamer consensus binding site to drive gene expression. These parallels in transcriptional control between Brn1 and CEH-6 suggest that C. elegans may well be an appropriate model for determining gene-regulatory networks in the developing vertebrate kidney.

Differential expression of intestinal membrane transporters in cholera patients

Vibrio cholerae causes the cholera disease through secretion of cholera toxin (CT), resulting in severe diarrhoea by modulation of membrane transporters in the intestinal epithelium. Genes encoding membrane-spanning transporters identified as being differentially expressed during cholera disease in a microarray screening were studied by real-time PCR, immunohistochemistry and in a CaCo-2 cell model. Two amino acid transporters, SLC7A11 and SLC6A14, were upregulated in acute cholera patients compared to convalescence. Five other transporters were downregulated; aquaporin 10, SLC6A4, TRPM6, SLC23A1 and SLC30A4, which have specificity for water, serotonin (5-HT), magnesium, vitamin C and zinc, respectively. The majority of these changes appear to be attempts of the host to counteract the secretory response. Our results also support the concept that epithelial cells are involved in 5-HT signalling during acute cholera.

Aquaporins: a promising target for drug development

Aquaporins (AQPs) are a family of small hydrophobic, integral membrane proteins that are expressed in all living organisms and play critical roles in controlling the water flow into and out of cells. So far, 13 different AQPs have been identified in mammals (AQP 0-12). AQPs have recently been implicated in various diseases such as cancer, cataract, brain oedema, gallstone disease and nephrogenic diabetes insipidus, as well as in the development of obesity and polycystic kidney disease. Interfering with the expression of AQPs will undoubtedly have therapeutic applications. Hence, in this review, the authors look at each AQP and its association with various pathological conditions in humans and demonstrate that they form potential targets for the treatment of such diseases.

AQP7 is localized in capillaries of adipose tissue, cardiac and striated muscle: implications in glycerol metabolism

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.

Proteomic knowledge of human aquaporins

Aquaporins (AQPs) are an ubiquitous family of proteins characterized by sequence similarity and the presence of two NPA (Asp-Pro-Ala) motifs. At present, 13 human AQPs are known and they are divided into two subgroups according to their ability to transport only water molecules (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, and AQP8), or also glycerol and other small solutes (AQP3, AQP7, AQP9, AQP10, AQP12). The genomic, structural, and functional aspects of this family are briefly described. In particular, proteomic approaches to identify and characterize the most studied AQPs, mainly through SDS-PAGE followed by MS analysis, are discussed. Moreover, the clinical importance of the best studied aquaporin (AQP1) in human diseases is also provided.

Expression and function of aquaporins in human skin: Is aquaporin-3 just a glycerol transporter?

The aquaporins (AQPs) are a family of transmembrane proteins forming water channels. In mammals, water transport through AQPs is important in kidney and other tissues involved in water transport. Some AQPs (aquaglyceroporins) also exhibit glycerol and urea permeability. Skin is the limiting tissue of the body and within skin, the stratum corneum (SC) of the epidermis is the limiting barrier to water loss by evaporation. The aquaglyceroporin AQP3 is abundantly expressed in keratinocytes of mammalian skin epidermis. Mice lacking AQP3 have dry skin and reduced SC hydration. Interestingly, however, results suggested that impaired glycerol, rather than water transport was responsible for this phenotype. In the present work, we examined the overall expression of AQPs in cells from human skin and we reviewed data on the functional role of AQPs in skin, particularly in the epidermis. By RT-PCR on primary cell cultures, we found that up to 6 different AQPs (AQP1, 3, 5, 7, 9 and 10) may be selectively expressed in various cells from human skin. AQP1, 5 are strictly water channels. But in keratinocytes, the major cell type of the epidermis, only the aquaglyceroporins AQP3, 10 were found. To understand the role of aquaglyceroporins in skin, we examined the relevance to human skin of the conclusion, from studies on mice, that skin AQP3 is only important for glycerol transport. In particular, we find a correlation between the absence of AQP3 and intercellular edema in the epidermis in two different experimental models: eczema and hyperplastic epidermis. In conclusion, we suggest that in addition to glycerol, AQP3 may be important for water transport and hydration in human skin epidermis.

Aquaporin subfamily with unusual NPA boxes

Aquaporins have been identified based on highly conserved two asparagine-proline-alanine (NPA) boxes that are important for the formation of a water-permeating pore. Some aquaporin-like sequences, however, have less conserved NPA boxes. Although they have lower homology with conventional aquaporins, they should be included in aquaporin family based on their conserved six transmembrane domains and hydrophobic NPA box-like repeats. They are widely distributed in multicellular organisms. Only SIPs from plants and AQP11/12 from mammals were examined previously and found to be localized inside the cell. Intracellular localization will be a common feature of these aquaporin-like proteins since most of them have positively charged amino acid clusters at the carboxy-termini similar to di-lysine motif (-KKXX) for an endoplasmic reticulum retention signal. Accordingly, they are tentatively named subcellular-aquaporins in this review. Currently, studies on their functions and biological roles are limited. SIPs were shown to function as water channels and the disruption of AQP11 produced neonatally fatal polycystic kidneys. Further works on subcellular-aquaporins will reveal new insights into the roles of aquaporins.