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BBA - Biomembranes (v.1758, #8)

Editorial Board (pp. ii).

Introduction: Special Issue on Aquaporins by Francois Chaumont; Peter M.T. Deen; Christine Delporte; Olivier Devuyst; Bruno Flamion; Norbert Lameire; Paul Steels; Jean-Marc Verbavatz (pp. 975-975).

Molecular mechanisms of aquaporin biogenesis by the endoplasmic reticulum Sec61 translocon by David Pitonzo; William R. Skach (pp. 976-988).

The past decade has witnessed remarkable advances in our understanding of aquaporin (AQP) structure and function. Much, however, remains to be learned regarding how these unique and vitally important molecules are generated in living cells. A major obstacle in this respect is that AQP biogenesis takes place in a highly specialized and relatively inaccessible environment formed by the ribosome, the Sec61 translocon and the ER membrane. This review will contrast the folding pathways of two AQP family members, AQP1 and AQP4, and attempt to explain how six TM helices can be oriented across and integrated into the ER membrane in the context of current (and somewhat conflicting) translocon models. These studies indicate that AQP biogenesis is intimately linked to translocon function and that the ribosome and translocon form a highly dynamic molecular machine that both interprets and is controlled by specific information encoded within the nascent AQP polypeptide. AQP biogenesis thus has wide ranging implications for mechanisms of translocon function and general membrane protein folding pathways.

Keywords: Aquaporin; Biogenesis; Sec61; Endoplasmic reticulum; ER; Translocon; Polytopic protein

Aquaporin subfamily with unusual NPA boxes by Kenichi Ishibashi (pp. 989-993).

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.

Keywords: NPA box; Aquaporin; Subcellular; SIP; Polycystic kidney

Membrane transport of hydrogen peroxide by Gerd P. Bienert; Jan K. Schjoerring; Thomas P. Jahn (pp. 994-1003).

Hydrogen peroxide (H₂O₂) belongs to the reactive oxygen species (ROS), known as oxidants that can react with various cellular targets thereby causing cell damage or even cell death. On the other hand, recent work has demonstrated that H₂O₂ also functions as a signalling molecule controlling different essential processes in plants and mammals. Because of these opposing functions the cellular level of H₂O₂ is likely to be subjected to tight regulation via processes involved in production, distribution and removal. Substantial progress has been made exploring the formation and scavenging of H₂O₂, whereas little is known about how this signal molecule is transported from its site of origin to the place of action or detoxification. From work in yeast and bacteria it is clear that the diffusion of H₂O₂ across membranes is limited. We have now obtained direct evidence that selected aquaporin homologues from plants and mammals have the capacity to channel H₂O₂ across membranes. The main focus of this review is (i) to summarize the most recent evidence for a signalling role of H₂O₂ in various pathways in plants and mammals and (ii) to discuss the relevance of specific transport of H₂O₂.

Keywords: Abbreviations; ABA; Abscisic acid; AQP; Aquaporin; IMM; Inner mitochondrial membrane; MAPK; Mitogen activated protein kinase; NADH/NAD; ⁺; Nicotinamide adenine dinucleotide (reduced/oxidized); NADPH/NADP; ⁺; Nicotinamide adenine dinucleotide phosphate (reduced/oxidized); PTP; Protein-tyrosine phosphatase; ROS; Reactive oxygen species; TIP; Tonoplast intrinsic proteinAquaporin; Hydrogen Peroxide; Oxidative Stress; Reactive Oxygen Species; Signalling; Transport

Aquaporins and glycerol metabolism by Toshiyuki Hibuse; Norikazu Maeda; Azumi Nagasawa; Tohru Funahashi (pp. 1004-1011).

The discovery of aquaporin (AQP) has made a great impact on life sciences. AQPs are a family of homologous water channels widely distributed in plants, unicellular organisms, invertebrates, and vertebrates. So far, 13 AQPs have been identified in human. AQP3, 7, 9, and 10 are subcategorized as aquaglyceroporins which permeabilize glycerol as well as water. Many investigators have demonstrated that AQPs play a crucial role in maintaining water homeostasis, but the physiological significance of some AQPs as a glycerol channel is not fully understood. Adipose tissue is a major source of glycerol and glycerol is one of substrates for gluconeogenesis. This review focuses on recent studies of glycerol metabolism through aquaglyceroporins, and briefly discusses the importance of glycerol channel in adipose tissues and liver.

Keywords: Abbreviations; AQP; aquaporin; BAT; brown adipose tissue; FFA; free fatty acid; GLUT4; glucose transporter 4; HSL; hormone sensitive lipase; TG; triglyceride; WAT; white adipose tissueAquaporin; Adipocyte; Glycerol; Gluconeogenesis; Obesity; Lipolysis

Dihydroxyacetone and methylglyoxal as permeants of the Plasmodium aquaglyceroporin inhibit parasite proliferation by Slavica Pavlovic-Djuranovic; Jürgen F.J. Kun; Joachim E. Schultz; Eric Beitz (pp. 1012-1017).

The aquaglyceroporin of Plasmodium falciparum (PfAQP) is a bi-functional channel with permeability for water and solutes. Its functions supposedly are in osmotic protection of parasites and in facilitation of glycerol permeation for glycerolipid biosynthesis. Here, we show PfAQP permeability for the glycolysis-related metabolites methylglyoxal, a cytotoxic byproduct, and dihydroxyacetone, a ketotriose. AQP3, the red cell aquaglyceroporin, also passed dihydroxacetone but excluded methylglyoxal. Proliferation of malaria parasites was inhibited by methylglyoxal with an IC₅₀ around 200 μM. Surprisingly, also dihydroxyacetone, which is an energy source in human cells, was antiproliferative in chloroquine-sensitive and resistant strains with an IC₅₀ around 3 mM. We expressed P. falciparum glyceraldehyde 3-phosphate dehydrogenase (PfGAPDH) to examine whether it is inhibited by either carbonyl compound. Methylglyoxal did not affect PfGAPDH on incubation with 2.5 mM for 20 h. Treatment with 2.5 mM dihydroxyacetone, however, abolished PfGAPDH activity within 6 h. Aquaglyceroporin permeability for glycolytic metabolites may thus be of physiological significance.

Keywords: Aquaglyceroporin; Dihydroxyacetone; Methylglyoxal; Glyceraldehyde 3-phosphate dehydrogenase; Malaria; Plasmodia

Water permeability of rat liver mitochondria: A biophysical study by Giuseppe Calamita; Patrizia Gena; Daniela Meleleo; Domenico Ferri; Maria Svelto (pp. 1018-1024).

The movement of water accompanying solutes between the cytoplasm and the mitochondrial spaces is central for mitochondrial volume homeostasis, an important function for mitochondrial activities and for preventing the deleterious effects of excess matrix swelling or contraction. While the discovery of aquaporin water channels in the inner mitochondrial membrane provided valuable insights into the basis of mitochondrial plasticity, questions regarding the identity of mitochondrial water permeability and its regulatory mechanism remain open. Here, we use a stopped flow light scattering approach to define the water permeability and Arrhenius activation energy of the rat liver whole intact mitochondrion and its membrane subcompartments. The water permeabilities of whole brain and testis mitochondria as well as liposome models of the lipid bilayer composing the liver inner mitochondrial membrane are also characterized. Besides finding remarkably high water permeabilities for both mitochondria and their membrane subcompartments, the existence of additional pathways of water movement other than aquaporins are suggested.

Keywords: Abbreviations; ANT; adenine nucleotide translocator; AQP; aquaporin; IMM; inner mitochondrial membrane; OMM; outer mitochondrial membrane; PBS; phosphate-buffered saline; PTP; permeability transition pore; VDAC; voltage-dependent anion channelVolume homeostasis; Mitochondria; Outer membrane; Inner membrane; Water transport; Aquaporin

Segmental and cellular expression of aquaporins in the male excurrent duct by Nicolas Da Silva; Christine Piétrement; Dennis Brown; Sylvie Breton (pp. 1025-1033).

The male reproductive tract and accessory glands comprise a complex but interrelated system of tissues that are composed of many distinct cell types, all of which contribute to the ability of spermatozoa to carry out their ultimate function of fertilizing an oocyte. Spermatozoa undergo their final steps of maturation as they pass through the male excurrent duct, which includes efferent ducts, the epididymis and the vas deferens. The composition of the luminal environment in these organs is tightly regulated. Major fluid reabsorption occurs in efferent ducts and in the epididymis, and leads to a significant increase in sperm concentration. In the distal epididymis and vas deferens, fluid secretion controls the final fluidity of the luminal content. Therefore, the process of water movement in the excurrent duct is a crucial step for the establishment of male fertility. Aquaporins contribute to transepithelial water transport in many tissues, including the kidney, the brain, the eye and the respiratory tract. The present article reviews our current knowledge regarding the distribution and function of aquaporins in the male excurrent duct.

Keywords: Water channel; Fluid reabsorption; Fluid secretion; Male fertility

Expression and function of aquaporins in human skin: Is aquaporin-3 just a glycerol transporter? by M. Boury-Jamot; R. Sougrat; M. Tailhardat; B. Le Varlet; F. Bonté; M. Dumas; J.-M. Verbavatz (pp. 1034-1042).

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.

Keywords: Aquaporin; AQP3; Human skin; Epidermis; Glycerol transport; Water channel; Keratinocyte; Aquaglyceroporin

Myocardial water handling and the role of aquaporins by Jonathan R. Egan; Tanya L. Butler; Carol G. Au; Yee Mun Tan; Kathryn N. North; David S. Winlaw (pp. 1043-1052).

Cardiac surgery is performed in approximately 770,000 adults and 30,000 children in the United States of America annually. In this review we outline the mechanistic links between post-operative myocardial stunning and the development of myocardial edema. These interrelated processes cause a decline in myocardial performance that account for significant morbidity and mortality after cardiac surgery. Factors leading to myocardial edema include hemodilution, ischemia and reperfusion as well as osmotic gradients arising from pathological change. Several members of the aquaporin family of water transport proteins have been described in the myocardium although their role in the pathogenesis and resolution of cardiac edema is not established. This review examines evidence for the involvement of aquaporins in myocardial water handling during normal and pathological conditions.

Keywords: Membrane permeability; Myocardium; Cardiopulmonary bypass; Myocardial edema; Aquaporin; Dystrophin associated complex

Aquaporin-5 water channel in lipid rafts of rat parotid glands by Yasuko Ishikawa; Gota Cho; Zhenfang Yuan; Noriko Inoue; Yoshiko Nakae (pp. 1053-1060).

Aquaporin-5 (AQP5), an apical plasma membrane (APM) water channel in salivary glands, lacrimal glands, and airway epithelium, has an important role in fluid secretion. The activation of M3 muscarinic acetylcholine receptors (mAChRs) or α1-adrenoceptors on the salivary glands induces salivary fluid secretion. AQP5 localizes in lipid rafts and activation of the M3 mAChRs or α1-adrenoceptors induced its translocation together with the lipid rafts to the APM in the interlobular ducts of rat parotid glands. This review focuses on the mechanisms of AQP5 translocation together with lipid rafts to the APM in the interlobular duct cells of parotid glands of normal rats and the impairment of AQP5 translocation in diabetes and senescence.

Keywords: Aquaporin-5; Lipid rafts; Parotid glands; Diabetes; Aging; Interlobular ducts

Distribution and roles of aquaporins in salivary glands by Christine Delporte; Serge Steinfeld (pp. 1061-1070).

Salivary glands are involved in secretion of saliva, which is known to participate in the protection and hydratation of mucosal structures within the oral cavity, oropharynx and oesophagus, the initiation of digestion, some antimicrobial defence, and the protection from chemical and mechanical stress. Saliva secretion is a watery fluid containing electrolytes and a mixture of proteins and can be stimulated by muscarinic and adrenergic agonists. Since water movement is involved in saliva secretion, the expression, localization and function of aquaporins (AQPs) have been studied in salivary glands. This review will focus on the expression, localization and functional roles of the AQPs identified in salivary glands. The presence of AQP1, AQP5 and AQP8 has been generally accepted by many, while the presence of AQP3, AQP4, AQP6 and AQP7 still remains controversial. Functionally, AQP5 seems to be the only AQP thus far to be clearly playing a major role in the salivary secretion process. Modifications in AQPs expression and/or distribution have been reported in xerostomic conditions.

Keywords: Aquaporin; Salivary gland; Review

Transfer of the AQP1 cDNA for the correction of radiation-induced salivary hypofunction by Bruce J. Baum; Changyu Zheng; Ana P. Cotrim; Corinne M. Goldsmith; Jane C. Atkinson; Jaime S. Brahim; John A. Chiorini; Antonis Voutetakis; Rose Anne Leakan; Carter Van Waes; James B. Mitchell; Christine Delporte; Songlin Wang; Stephen M. Kaminsky; Gabor G. Illei (pp. 1071-1077).

The treatment of most patients with head and neck cancer includes ionizing radiation (IR). Salivary glands in the IR field suffer significant and irreversible damage, leading to considerable morbidity. Previously, we reported that adenoviral (Ad)-mediated transfer of the human aquaporin-1 (hAQP1) cDNA to rat [C. Delporte, B.C. O'Connell, X. He, H.E. Lancaster, A.C. O'Connell, P. Agre, B.J. Baum, Increased fluid secretion after adenoviral-mediated transfer of the aquaporin-1 cDNA to irradiated rat salivary glands. Proc. Natl. Acad. Sci. U S A. 94 (1997) 3268-3273] and miniature pig [Z. Shan, J. Li, C. Zheng, X. Liu, Z. Fan, C. Zhang, C.M. Goldsmith, R.B. Wellner, B.J Baum, S. Wang. Increased fluid secretion after adenoviral-mediated transfer of the human aquaporin-1 cDNA to irradiated miniature pig parotid glands. Mol. Ther. 11 (2005) 444-451] salivary glands ∼16 weeks following IR resulted in a dose-dependent increase in salivary flow to ≥80% control levels on day 3. A control Ad vector was without any significant effect on salivary flow. Additionally, after administration of Ad vectors to salivary glands, no significant lasting effects were observed in multiple measured clinical chemistry and hematology values. Taken together, the findings show that localized delivery of AdhAQP1 to IR-damaged salivary glands is useful in transiently increasing salivary secretion in both small and large animal models, without significant general adverse events. Based on these results, we are developing a clinical trial to test if the hAQP1 cDNA transfer strategy will be clinically effective in restoring salivary flow in patients with IR-induced parotid hypofunction.

Keywords: Gene therapy; Adenoviral vector; Aquaporin-1; Salivary gland; Radiation damage; Animal model; Clinical trial

Aquaporin-1 in the peritoneal membrane: Implications for water transport across capillaries and peritoneal dialysis by Olivier Devuyst; Jie Ni (pp. 1078-1084).

Peritoneal dialysis (PD) is an established mode of renal replacement therapy, based on the exchange of fluid and solutes between blood in peritoneal capillaries and a dialysate that has been introduced in the peritoneal cavity. The dialysis involves diffusive and convective transports and osmosis through the highly vascularized peritoneal membrane. Computer simulations predicted that the membrane contains ultrasmall pores (radius < 3 Å) responsible for the transport of solute-free water across the capillary endothelium during crystalloid osmosis. The distribution of the water channel aquaporin-1 (AQP1), as well as its molecular structure ensuring an exquisite selectivity for water perfectly fit with the characteristics of the ultrasmall pore. Treatment with corticosteroids induces the expression of AQP1 in peritoneal capillaries and increases water permeability and ultrafiltration in rats, without affecting the osmotic gradient and the permeability for small solutes. Studies in knockout mice provided further evidence that osmotically-driven water transport across the peritoneal membrane is mediated by AQP1. AQP1 and endothelial NO synthase (eNOS) show a distinct regulation within the endothelium lining peritoneal capillaries. In acute peritonitis, the upregulation of eNOS and increased release of NO dissipate the osmotic gradient and result in ultrafiltration failure, despite the unchanged expression of AQP1. These data illustrate the potential of the peritoneal membrane to investigate the role and regulation of AQP1 in the endothelium. They also emphasize the critical role of AQP1 during peritoneal dialysis and suggest that manipulating AQP1 expression may be used to increase water permeability across the peritoneal membrane.

Keywords: AQP1; Glucocorticoid; Endothelium; Ultrafiltration; Solute transport; Renal failure; Knockout mouse

Three distinct roles of aquaporin-4 in brain function revealed by knockout mice by A.S. Verkman; Devin K. Binder; Orin Bloch; Kurtis Auguste; Marios C. Papadopoulos (pp. 1085-1093).

Aquaporin-4 (AQP4) is expressed in astrocytes throughout the central nervous system, particularly at the blood–brain and brain–cerebrospinal fluid barriers. Phenotype analysis of transgenic mice lacking AQP4 has provided compelling evidence for involvement of AQP4 in cerebral water balance, astrocyte migration, and neural signal transduction. AQP4-null mice have reduced brain swelling and improved neurological outcome in models of (cellular) cytotoxic cerebral edema including water intoxication, focal cerebral ischemia, and bacterial meningitis. However, brain swelling and clinical outcome are worse in AQP4-null mice in models of vasogenic (fluid leak) edema including cortical freeze-injury, brain tumor, brain abscess and hydrocephalus, probably due to impaired AQP4-dependent brain water clearance. AQP4 deficiency or knock-down slows astrocyte migration in response to a chemotactic stimulus in vitro, and AQP4 deletion impairs glial scar progression following injury in vivo. AQP4-null mice also manifest reduced sound- and light-evoked potentials, and increased threshold and prolonged duration of induced seizures. Impaired K⁺ reuptake by astrocytes in AQP4 deficiency may account for the neural signal transduction phenotype. Based on these findings, we propose modulation of AQP4 expression or function as a novel therapeutic strategy for a variety of cerebral disorders including stroke, tumor, infection, hydrocephalus, epilepsy, and traumatic brain injury.

Keywords: AQP4; Water transport; Transgenic mouse; Brain edema; Cell migration; Epilepsy

AQP0-LTR of the Cat^Fr mouse alters water permeability and calcium regulation of wild type AQP0 by Katalin Kalman; Karin L. Németh-Cahalan; Alexandrine Froger; James E. Hall (pp. 1094-1099).

Aquaporin 0 (AQP0) is the major intrinsic protein of the lens and its water permeability can be modulated by changes in pH and Ca²⁺. The Cataract Fraser (Cat^Fr) mouse accumulates an aberrant AQP0 (AQP0-LTR) in sub-cellular compartments resulting in a congenital cataract. We investigated the interference of AQP0-LTR with normal function of AQP0 in three systems. First, we created a transgenic mouse expressing AQP0 and AQP0-LTR in the lens. Expression of AQP0 did not prevent the congenital cataract but improved the size and transparency of the lens. Second, we measured water permeability of AQP0 co-expressed with AQP0-LTR in Xenopus oocytes. A low expression level of AQP0-LTR decreased the water permeability of AQP0, and a high expression level eliminated its calcium regulation. Third, we studied trafficking of AQP0 and AQP0-LTR in transfected lens epithelial cells. At low expression level, AQP0-LTR migrated with AQP0 toward the cell membrane, but at high expression level, it accumulated in sub-cellular compartments. The deleterious effect of AQP0-LTR on lens development may be explained by lowering water permeability and abolishing calcium regulation of AQP0. This study provides the first evidence that calcium regulation of AQP0 water permeability may be crucial for maintaining normal lens homeostasis and development.

Keywords: Aquaporin; Cataract; LTR; Xenopus; oocyte; Transgenic mouse; Lens epithelial cell

cAMP regulates vasopressin-induced AQP2 expression via protein kinase A-independent pathway by Fuminori Umenishi; Takefumi Narikiyo; Alain Vandewalle; Robert W. Schrier (pp. 1100-1105).

The regulation of AVP-induced AQP2 expression was investigated in the present study. AVP administration induced AQP2 expression in a dose-dependent manner in association with an increase in intracellular cAMP concentration. PKA activity was stimulated by AVP but PKA inhibitors did not block the upregulation of AQP2 expression. However, AVP also activated both ERK and CREB pathways, and ERK inhibitor attenuated the upregulation of AQP2 expression. These results therefore indicate that the effect of AVP stimulation to upregulate AQP2 expression involves a PKA-independent pathway.

Keywords: Aquaporin-2; Arginine vasopressin; cAMP; ERK; CREB; PKA

Physiological roles of AQP7 in the kidney: Lessons from AQP7 knockout mice by Eisei Sohara; Tatemitsu Rai; Sei Sasaki; Shinichi Uchida (pp. 1106-1110).

The aquaporin7 (AQP7) water channel is known to be a member of the aquaglyceroporins, which allow the rapid transport of glycerol and water. AQP7 is abundantly present at the apical membrane of the proximal straight tubules in the kidney. In this paper, we review the physiological functions of AQP7 in the kidney. To investigate this, we generated AQP7 knockout mice. The water permeability of the proximal straight tubule brush border membrane measured by the stopped flow method was reduced in AQP7 knockout mice compared to wild-type mice (AQP7, 18.0±0.4×10⁻³cm/s vs. wild-type, 20.0±0.3×10⁻³ cm/s). Although AQP7 solo knockout mice did not show a urinary concentrating defect, AQP1/AQP7 double knockout mice showed reduced urinary concentrating ability compared to AQP1 solo knockout mice, indicating that the contribution of AQP7 to water reabsorption in the proximal straight tubules is physiologically substantial. On the other hand, AQP7 knockout mice showed marked glycerol in their urine (AQP7, 1.7±0.34 mg/ml vs. wild-type, 0.005±0.002 mg/ml). This finding identified a novel pathway of glycerol reabsorption that occurs in the proximal straight tubules. In two mouse models of proximal straight tubule injury, the cisplatin-induced acute renal failure (ARF) model and the ischemic–reperfusion ARF model, an increase of urine glycerol was observed (pre-treatment, 0.007±0.005 mg/ml; cisplatin, 0.063±0.043 mg/ml; ischemia, 0.076±0.02 mg/ml), suggesting that urine glycerol could be used as a new biomarker for detecting proximal straight tubule injury.

Keywords: Aquaporin7; Glycerol; Water; Urea; Kidney; Proximal straight tubules

Water immersion is associated with an increase in aquaporin-2 excretion in healthy volunteers by G. Valenti; W. Fraszl; F. Addabbo; G. Tamma; G. Procino; E. Satta; M. Cirillo; N.G. De Santo; C. Drummer; L. Bellini; R. Kowoll; M. Schlemmer; S. Vogler; K.A. Kirsch; M. Svelto; H.C. Gunga (pp. 1111-1116).

Here, we report the alterations in renal water handling in healthy volunteers during a 6 h thermoneutral water immersion at 34 to 36 °C. We found that water immersion is associated with a reversible increase in total urinary AQP2 excretion.

Regulation of aquaporin-2 trafficking and its binding protein complex by Yumi Noda; Sei Sasaki (pp. 1117-1125).

Trafficking of water channel aquaporin-2 (AQP2) to the apical membrane is critical to water reabsorption in renal collecting ducts and its regulation maintains body water homeostasis. However, exact molecular mechanisms which recruit AQP2 are unknown. Recent studies highlighted a key role for spatial and temporal regulation of actin dynamics in AQP2 trafficking. We have recently identified AQP2-binding proteins which directly regulate this trafficking: SPA-1, a GTPase-activating protein (GAP) for Rap1, and cytoskeletal protein actin. In addition, a multiprotein “force generator�? complex which directly binds to AQP2 has been discovered. This review summarizes recent advances related to the mechanism for AQP2 trafficking.

Keywords: PKA phosphorylation; Channel protein; Rho; Cytoskeleton; Vasopressin

Polarisation, key to good localisation by Moniek van Beest; Joris H. Robben; Paul J.M. Savelkoul; Giel Hendriks; Mark A.J. Devonald; Irene B.M. Konings; Anne K. Lagendijk; Fiona Karet; Peter M.T. Deen (pp. 1126-1133).

Polarisation of cells is crucial for vectorial transport of ions and solutes. In literature, however, proteins specifically targeted to the apical or basolateral membrane are often studied in non-polarised cells. To investigate whether these data can be extrapolated to expression in polarised cells, we studied several membrane-specific proteins. In polarised MDCK cells, the Aquaporin-2 water channel resides in intracellular vesicles and apical membrane, while the vasopressin-type 2 receptor, anion-exchanger 1 (AE1) protein and E-Cadherin mainly localise to the basolateral membrane. In non-polarised MDCK cells, however, Aquaporin-2 localises, besides plasma membrane, mainly in the Golgi complex, while the others show a dispersed staining throughout the cell. Moreover, while AQP2 mutants in dominant nephrogenic diabetes insipidus are missorted to different organelles in polarised cells, they all predominantly localise to the Golgi complex in non-polarised MDCK cells. Additionally, the maturation of V2R, and likely its missorting, is affected in transiently-transfected compared to stably-transfected cells. In conclusion, we show that the use of stably-transfected polarised cells is crucial in interpreting the processing and the localisation of membrane targeted proteins.

Keywords: Polarisation; Aquaporin-2; V2R; Principal cell; Kidney

Functional aquaporin diversity in plants by Ralf Kaldenhoff; Matthias Fischer (pp. 1134-1141).

Due to the fact that most plants are immobile, a rapid response of physiological processes to changing environmental conditions is essential for their survival. Thus, in comparison to many other organisms, plants might need a more sophisticated tuning of water balance. Among others, this is reflected by the comparable large amount of aquaporin genes in plant genomes. So far, aquaporins were shown to be involved in many physiological processes like root water uptake, reproduction or photosynthesis. Their classification as simple water pores has changed according to their molecular function into channels permeable for water, small solutes and/or gases. An adjustment of the corresponding physiological process could be achieved by regulation mechanisms. Concerning aquaporins these range from posttranslational modification, molecular trafficking to heteromerization of aquaporin isoforms. The aim of this review is to underline the function of the four plant aquaporin family subclasses with regard to the substrate specificity, regulation and physiological relevance.

Keywords: Plant; Aquaporin; Water channel; Gas transport

Modulating the expression of aquaporin genes in planta: A key to understand their physiological functions? by Charles Hachez; Enric Zelazny; François Chaumont (pp. 1142-1156).

Aquaporins (AQPs) are believed to act as “cellular plumbers�?, allowing plants to rapidly alter their membrane water permeability in response to environmental cues. This study of AQP regulation at both the RNA and protein levels has revealed a large number of possible mechanisms. Currently, modulation of AQP expression in planta is considered the strategy of choice for elucidating the role of AQPs in plant physiology. This review highlights the fact that this strategy is complicated by many factors, such as the incomplete characterization of transport selectivity of the targeted AQP, the fact that AQPs might act as multifunctional channels with multiple physiological roles, and the number of post-translational regulation mechanisms. The classification of AQPs as constitutive or stress-responsive isoforms is also proposed.

Keywords: Aquaporin; Water relation; Membrane permeability; Hydraulic conductivity

Purification and characterization of two protein kinases acting on the aquaporin SoPIP2;1 by Sara Sjövall-Larsen; Erik Alexandersson; Ingela Johansson; Maria Karlsson; Urban Johanson; Per Kjellbom (pp. 1157-1164).

Aquaporins are water channel proteins that facilitate the movement of water and other small solutes across biological membranes. Plants usually have large aquaporin families, providing them with many ways to regulate the water transport. Some aquaporins are regulated post-translationally by phosphorylation. We have previously shown that the water channel activity of SoPIP2;1, an aquaporin in the plasma membrane of spinach leaves, was enhanced by phosphorylation at Ser115 and Ser274. These two serine residues are highly conserved in all plasma membrane aquaporins of the PIP2 subgroup. In this study we have purified and characterized two protein kinases phosphorylating Ser115 and Ser274 in SoPIP2;1. By anion exchange chromatography, the Ser115 kinase was purified from the soluble protein fraction isolated from spinach leaves. The Ca²⁺-dependent Ser274 kinase was purified by peptide affinity chromatography using plasma membranes isolated from spinach leaves. When characterized, the Ser115 kinase was Mg²⁺-dependent, Ca²⁺-independent and had a pH-optimum at 6.5. In accordance with previous studies using the oocyte expression system, site-directed mutagenesis and kinase and phosphatase inhibitors, the phosphorylation of Ser274, but not of Ser115, was increased in the presence of phosphatase inhibitors while kinase inhibitors decreased the phosphorylation of both Ser274 and Ser115. The molecular weight of the Ser274 kinase was approximately 50 kDa. The identification and characterization of these two protein kinases is an important step towards elucidating the signal transduction pathway for gating of the aquaporin SoPIP2;1.

Keywords: Aquaporin; PIP2; SoPIP2;1; Phosphorylation; Protein kinase

The structure, function and regulation of the nodulin 26-like intrinsic protein family of plant aquaglyceroporins by Ian S. Wallace; Won-Gyu Choi; Daniel M. Roberts (pp. 1165-1175).

The nodulin 26-like intrinsic protein family is a group of highly conserved multifunctional major intrinsic proteins that are unique to plants, and which transport a variety of uncharged solutes ranging from water to ammonia to glycerol. Based on structure–function studies, the NIP family can be subdivided into two subgroups (I and II) based on the identity of the amino acids in the selectivity-determining filter (ar/R region) of the transport pore. Both subgroups appear to contain multifunctional transporters with low to no water permeability and the ability to flux multiple uncharged solutes of varying sizes depending upon the composition of the residues of the ar/R filter. NIPs are subject to posttranslational phosphorylation by calcium-dependent protein kinases. In the case of the family archetype, soybean nodulin 26, phosphorylation has been shown to stimulate its transport activity and to be regulated in response to developmental as well as environmental cues, including osmotic stresses. NIPs tend to be expressed at low levels in the plant compared to other MIPs, and several exhibit cell or tissue specific expression that is subject to spatial and temporal regulation during development.

Keywords: Nodulin-26; NIP; Nitrogen fixation; Symbiosis; Aquaporin

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BBA - Biomembranes (v.1758, #8)