Aquaporin-2 protein dynamics within the cell

Quantitative and molecular aspects of water intake in meat-type chickens

Even though water is the most essential nutrient for poultry production, adequate data on individual water intake in broiler chickens and its relationship with other traits of economic importance is scant. Water is provided to chickens in an unrestricted manner in spite of being a finite resource. Climate change continues to affect water sources and efficient bird use of water is long overdue. Understanding the biological basis of water intake is essential for sustainability of the poultry industry. Individual water and feed intake, and growth data was collected on 520 commercial broilers aged 14 to 42 days. We introduced the concepts of water conversion ratio (WCR) and residual water intake (RWI) as parameters that can be used to assess water intake efficiency. Water conversion ratio was defined as the amount of water consumed per unit of body weight gain, and RWI was defined as the difference between the actual water intake (WI) of a given bird and the expected WI by an average bird from the population with the same metabolic body weight, feed intake (FI) and body weight gain (BWG). The correlation between WI and FI was positive (r=0.77; P<0.0001), and the correlation between WI and BWG was positive (r=0.80; P<0.0001). Based on the distribution of RWI, the bottom 5 birds (LRWI) and the top 5 birds (HRWI) for RWI were selected for mRNA expression differences. The average broiler consumed about 7.8 L (± 1L) of water from 14 to 42 days of age. The mRNA expression of arginine vasopressin (AVP) antidiuretic hormone, calcium sensing receptor (CasR), sodium channel epithelial 1 subunit alpha (SCNN1A) and SCNN1D in the hypothalamus was upregulated in the LRWI group compared to the HRWI group. Similarly, kidney aquaporins (AQP) 2, 3, and 4 were upregulated in the LRWI group compared with the HRWI group. Given that water was provided ad libitum, the up-regulation of AVP and AQP gene mRNA expressions seem to indicate that the LRWI birds were more efficient in water reabsorption in the kidney compared to their HRWI counterparts. Increased water reabsorption will reduce the amount of water consumed to attain hydration. The water reabsorption potential was reflected in the excreta moisture levels as the LRWI birds had significantly lower excreta moisture than the HRWI birds. Excreta moisture level require further studies and could be considered as a potential proxy trait for water intake.

Urinary Reference Values and First Insight into the Urinary Proteome of Captive Giraffes

Urinalysis is widely recognized to be a useful tool in routine health investigations, since it can diagnose numerous pathologies. Considering the paucity of knowledge concerning giraffes, urine from 44 giraffes (Giraffa camelopardalis) (18 males and 26 females, from 3 months of age to 21 years of age) underwent routine urinalysis, 1D-electrophoresis, and protein identification using mass spectrometry, with the aim of identifying the urinary reference values and the urine proteome. The urine specific gravity (USG), urine total proteins (uTP), urine creatinine (uCr), and urine protein:creatinine ratio (UPC) reference values, reported as the median, and lower limit (LL) and upper limit (UL), were 1.030 (1006-1.049), 17.58 (4.54-35.31) mg/dL, 154.62 (39.59-357.95) mg/dL, and 0.11 (0.07-0.16), respectively. Mass spectrometry, together with electrophoresis, revealed a pattern of common urinary proteins; albumin, lysozyme C, and ubiquitin were the most represented proteins in the giraffe urine. It has been hypothesized that these proteins could act as a defense against microbes. Moreover, in giraffes, urinalysis could be a valid tool for gauging renal function and physiological status changes.

A Two-Year Randomized Trial of Interventions to Decrease Stress Hormone Vasopressin Production in Patients with Meniere's Disease-A Pilot Study

Meniere's disease, a common inner ear condition, has an incidence of 15-50 per 100,000. Because mental/physical stress and subsequent increase in the stress hormone vasopressin supposedly trigger Meniere's disease, we set a pilot study to seek new therapeutic interventions, namely management of vasopressin secretion, to treat this disease. We enrolled 297 definite Meniere's patients from 2010 to 2012 in a randomized-controlled and open-label trial, assigning Group-I (control) traditional oral medication, Group-II abundant water intake, Group-III tympanic ventilation tubes and Group-IV sleeping in darkness. Two hundred sixty-three patients completed the planned 2-year-follow-up, which included assessment of vertigo, hearing, plasma vasopressin concentrations and changes in stress/psychological factors. At 2 years, vertigo was completely controlled in 54.3% of patients in Group-I, 81.4% in Group-II, 84.1% in Group-III, and 80.0% in Group-IV (statistically I < II = III = IV). Hearing was improved in 7.1% of patients in Group-I, 35.7% in Group-II, 34.9% in Group-III, and 31.7% in Group-IV (statistically I < II = III = IV). Plasma vasopressin concentrations decreased more in Groups-II, -III, and -IV than in Groups-I (statistically I < II = III = IV), although patients' stress/psychological factors had not changed. Physicians have focused on stress management for Meniere's disease. However, avoidance of stress is unrealistic for patients who live in demanding social environments. Our findings in this pilot study suggest that interventions to decrease vasopressin secretion by abundant water intake, tympanic ventilation tubes and sleeping in darkness is feasible in treating Meniere's disease, even though these therapies did not alter reported mental/physical stress levels.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01099046.

Aquaporins in avian kidneys: function and perspectives

For terrestrial vertebrates, water economy is a prerequisite for survival, and the kidney is their major osmoregulatory organ. Birds are the only vertebrates other than mammals that can concentrate urine in adaptation to terrestrial environments. Aquaporin (AQP) and glyceroporin (GLP) are phylogenetically old molecules and have been found in plants, microbial organisms, invertebrates, and vertebrates. Currently, 13 AQPs/aquaGLPs and isoforms are known to be present in mammals. AQPs 1, 2, 3, 4, 6, 7, 8, and 11 are expressed in the kidney; of these, AQPs 1, 2, 3, 4, and 7 are shown to be involved in fluid homeostasis. In avian kidneys, AQPs 1, 2, 3, and 4 have been identified and characterized. Also, gene and/or amino acid sequences of AQP5, AQP7, AQP8, AQP9, AQP11, and AQP12 have been reported in birds. AQPs 2 and 3 are expressed along cortical and medullary collecting ducts (CDs) and are responsible, respectively, for the water inflow and outflow of CD epithelial cells. While AQP4 plays an important role in water exit in the CD of mammalian kidneys, it is unlikely to participate in water outflow in avian CDs. This review summarizes current knowledge on structure and function of avian AQPs and compares them to those in mammalian and nonmammalian vertebrates. Also, we aim to provide input into, and perspectives on, the role of renal AQPs in body water homeostasis during ontogenic and phylogenetic advancement.

Actin directly interacts with different membrane channel proteins and influences channel activities: AQP2 as a model

The interplay between actin and 10 membrane channel proteins that have been shown to directly bind to actin are reviewed. The 10 membrane channel proteins covered in this review are aquaporin 2 (AQP2), cystic fibrosis transmembrane conductance regulator (CFTR), ClC2, short form of ClC3 (sClC3), chloride intracellular channel 1 (CLIC1), chloride intracellular channel 5 (CLIC5), epithelial sodium channel (ENaC), large-conductance calcium-activated potassium channel (Maxi-K), transient receptor potential vanilloid 4 (TRPV4), and voltage-dependent anion channel (VDAC), with particular attention to AQP2. In regard to AQP2, most reciprocal interactions between actin and AQP2 occur during intracellular trafficking, which are largely mediated through indirect binding. Actin and the actin cytoskeleton work as cables, barriers, stabilizers, and force generators for motility. However, as with ENaC, the effects of actin cytoskeleton on channel gating should be investigated further. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Temporal delays and individual variation in antidiuretic response to desmopressin

This study aimed to estimate the relationship between pharmacokinetics and the antidiuretic effect of desmopressin. In the investigator-blind, randomized, parallel group study, 5 dose groups and 1 placebo group, each consisting of 12 healthy, overhydrated, nonsmoking male subjects 18–55 yr of age were infused intravenously over 2 h with placebo or 30, 60, 125, 250, and 500 ng desmopressin in 50 ml of normal saline. Plasma desmopressin and urine osmolality rose by variable amounts during the infusions of 60, 125, 250, and 500 ng desmopressin. Plotting mean urine osmolality against the concurrent mean plasma desmopressin yielded a temporal delay between pharmacokinetic (PK) and -dynamic (PD) responses in all dose groups. Using simulation from the indirect-response model, assuming a constant (4 ng/ml) desmopressin concentration, this delay between PK and PD was estimated at 4 h (10th-90th percentile: 1.8–8.1). Within each group, however, there were large individual variations (2- to 10-fold) in the magnitude and duration of the antidiuretic effect. The antidiuretic effect of intravenous desmopressin in water-loaded healthy adults varies considerably due largely to factors other than individual differences in pharmacokinetics. The antidiuretic effect is time as well as dose dependent and may be self-amplifying. The most likely explanation for these findings is that the time required for a given level of plasma desmopressin to exert its maximum antidiuretic effect varies markedly from person to person due to individual differences in the kinetics of one or more of the intracellular mechanisms that promote the reabsorption of solute-free water by principal cells in renal collecting tubules.

Heterologous downregulation of vasopressin type 2 receptor is induced by transferrin

Vasopressin (VP) binds to the vasopressin type 2 receptor (V2R) to trigger physiological effects including body fluid homeostasis and blood pressure regulation. Signaling is terminated by receptor downregulation involving clathrin-mediated endocytosis and V2R degradation. We report here that both native and epitope-tagged V2R are internalized from the plasma membrane of LLC-PK1 kidney epithelial cells in the presence of another ligand, transferrin (Tf). The presence of iron-saturated Tf (holo-Tf; 4 h) reduced V2R binding sites at the cell surface by up to 33% while iron-free (apo-Tf) had no effect. However, no change in green fluorescent protein-tagged V2R distribution was observed in the presence of bovine serum albumin, atrial natriuretic peptide, or ANG II. Conversely, holo-Tf did not induce the internalization of another G protein-coupled receptor, the parathyroid hormone receptor. In contrast to the effect of VP, Tf did not increase intracellular cAMP or modify aquaporin-2 distribution in these cells, although addition of VP and Tf together augmented VP-induced V2R internalization. Tf receptor coimmunoprecipitated with V2R, suggesting that they interact closely, which may explain the additive effect of VP and Tf on V2R endocytosis. Furthermore, Tf-induced V2R internalization was abolished in cells expressing a dominant negative dynamin (K44A) mutant, indicating the involvement of clathrin-coated pits. We conclude that Tf can induce heterologous downregulation of the V2R and this might desensitize VP target cells without activating downstream V2R signaling events. It also provides new insights into urine-concentrating defects observed in rat models of hemochromatosis.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Basolateral targeting and microtubule-dependent transcytosis of the aquaporin-2 water channel

The aquaporin-2 (AQP2) water channel relocates mainly to the apical plasma membrane of collecting duct principal cells after vasopressin (VP) stimulation. AQP2 transport to this membrane domain is assumed to be a direct route involving recycling of intracellular vesicles. However, basolateral plasma membrane expression of AQP2 is observed in vivo in principal cells. Here, we asked whether there is a transcytotic pathway of AQP2 trafficking between apical and basolateral membranes. We used MDCK cells in which AQP2 normally accumulates apically after VP exposure. In contrast, both site-specific biotinylation and immunofluorescence showed that AQP2 is strongly accumulated in the basolateral membrane, along with the endocytic protein clathrin, after a brief cold shock (4°C). This suggests that AQP2 may be constitutively targeted to basolateral membranes and then retrieved by clathrin-mediated endocytosis at physiological temperatures. Rab11 does not accumulate in basolateral membranes after cold shock, suggesting that the AQP2 in this location is not associated with Rab11-positive vesicles. After rewarming (37°C), basolateral AQP2 staining is diminished and it subsequently accumulates at the apical membrane in the presence of VP/forskolin, suggesting that transcytosis can be followed by apical insertion of AQP2. This process is inhibited by treatment with colchicine. Our data suggest that the cold shock procedure reveals the presence of microtubule-dependent AQP2 transcytosis, which represents an indirect pathway of apical AQP2 delivery in these cells. Furthermore, our data indicate that protein polarity data obtained from biotinylation assays, which require cells to be cooled to 4°C during the labeling procedure, should be interpreted with caution.

Fluvastatin modulates renal water reabsorption in vivo through increased AQP2 availability at the apical plasma membrane of collecting duct cells

X-linked nephrogenic diabetes insipidus (XNDI), a severe pathological condition characterized by greatly impaired urine-concentrating ability of the kidney, is caused by inactivating mutations in the V2 vasopressin receptor (V2R) gene. The lack of functional V2Rs prevents vasopressin-induced shuttling of aquaporin-2 (AQP2) water channels to the apical plasma membrane of kidney collecting duct principal cells, thus promoting water reabsorption from urine to the interstitium. At present, no specific pharmacological therapy exists for the treatment of XNDI. We have previously reported that the cholesterol-lowering drug lovastatin increases AQP2 membrane expression in renal cells in vitro. Here we report the novel finding that fluvastatin, another member of the statins family, greatly increases kidney water reabsorption in vivo in mice in a vasopressin-independent fashion. Consistent with this observation, fluvastatin is able to increase AQP2 membrane expression in the collecting duct of treated mice. Additional in vivo and in vitro experiments indicate that these effects of fluvastatin are most likely caused by fluvastatin-dependent changes in the prenylation status of key proteins regulating AQP2 trafficking in collecting duct cells. We identified members of the Rho and Rab families of proteins as possible candidates whose reduced prenylation might result in the accumulation of AQP2 at the plasma membrane. In conclusion, these results strongly suggest that fluvastatin, or other drugs of the statin family, may prove useful in the therapy of XNDI.

Transpiration from shoots triggers diurnal changes in root aquaporin expression

Root hydraulic conductivity (Lp(r)) and aquaporin amounts change diurnally. Previously, these changes were considered to be spontaneously driven by a circadian rhythm. Here, we evaluated the new hypothesis that diurnal changes could be triggered and enhanced by transpirational demand from shoots. When rice plants were grown under a 12h light/12h dark regime, Lp(r) was low in the dark and high in the light period. Root aquaporin mRNA levels also changed diurnally, but the amplitudes differed among aquaporin isoforms. Aquaporins, such as OsPIP2;1, showed moderate changes, whereas root-specific aquaporins, such as OsPIP2;5, showed temporal and dramatic induction around 2h after light initiation. When darkness was extended for 12h after the usual dark period, no such induction was observed. Furthermore, plants under 100% relative humidity (RH) showed no induction even in the presence of light. These results suggest that transpirational demand triggers a dramatic increase in gene expressions such as OsPIP2;5. Immunocytochemistry showed that OsPIP2;5 accumulated on the proximal end of the endodermis and of the cell surface around xylem. The strong induction by transpirational demand and the polar localization suggest that OsPIP2;5 contributes to fine adjustment of radial water transport in roots to sustain high Lp(r) during the day.

Tubular cell-enriched subpopulation of primary renal cells improves survival and augments kidney function in rodent model of chronic kidney disease

Established chronic kidney disease (CKD) may be identified by severely impaired renal filtration that ultimately leads to the need for dialysis or kidney transplant. Dialysis addresses only some of the sequelae of CKD, and a significant gap persists between patients needing transplant and available organs, providing impetus for development of new CKD treatment modalities. Some postulate that CKD develops from a progressive imbalance between tissue damage and the kidney's intrinsic repair and regeneration processes. In this study we evaluated the effect of kidney cells, delivered orthotopically by intraparenchymal injection to rodents 4-7 wk after CKD was established by two-step 5/6 renal mass reduction (NX), on the regeneration of kidney function and architecture as assessed by physiological, tissue, and molecular markers. A proof of concept for the model, cell delivery, and systemic effect was demonstrated with a heterogeneous population of renal cells (UNFX) that contained cells from all major compartments of the kidney. Tubular cells are known contributors to kidney regeneration in situ following acute injury. Initially tested as a control, a tubular cell-enriched subpopulation of UNFX (B2) surprisingly outperformed UNFX. Two independent studies (3 and 6 mo in duration) with B2 confirmed that B2 significantly extended survival and improved renal filtration (serum creatinine and blood urea nitrogen). The specificity of B2 effects was verified by direct comparison to cell-free vehicle controls and an equivalent dose of non-B2 cells. Quantitative histological evaluation of kidneys at 6 mo after treatment confirmed that B2 treatment reduced severity of kidney tissue pathology. Treatment-associated reduction of transforming growth factor (TGF)-β1, plasminogen activator inhibitor (PAI)-1, and fibronectin (FN) provided evidence that B2 cells attenuated canonical pathways of profibrotic extracellular matrix production.

Protein kinase C induces endocytosis of the sodium taurocholate cotransporting polypeptide

Bile salts influence signaling and metabolic pathways. In hepatocytes, the sodium taurocholate cotransporting polypeptide (Ntcp) is a major determinant of intracellular bile salt levels. Short-term downregulation of Ntcp is not well characterized to date. FLAG and enhanced green fluorescent protein (EGFP) tags were cloned to the extra- and intracellular termini of Ntcp. Endocytosis of Ntcp in transfected HepG2 cells was visualized by fluorescence of EGFP, and membrane surface expression of Ntcp was quantified by flow cytometry with fluorochrome-labeled FLAG antibodies. Activation of protein kinase C (PKC) by phorbolester or thymeleatoxin an activator of Ca(2+)-dependent conventional PKCs (cPKCs), induced endocytosis of Ntcp, whereas the Na(+)-K(+)-ATPase remained in the plasma membrane. The PKC inhibitor BIM I and the cPKC-selective inhibitor Gö6976 abolished PMA-induced endocytosis. Because of this internalization, cell surface expression of Ntcp was reduced by 36 +/- 7%, bile salt uptake was decreased by 25%, and taurolithocholate sulfate-induced cell toxicity was prevented. In conclusion, Ca(2+)-dependent PKCs induce vesicular retrieval of Ntcp, thereby reducing bile salt uptake. This mechanism may protect hepatocytes from toxic intracellular bile salt concentrations.

Molecular diversity of vasotocin-dependent aquaporins closely associated with water adaptation strategy in anuran amphibians

Anuran amphibians represent the first vertebrates that adapted to terrestrial environments, and are successfully distributed around the world, even to forests and arid deserts. Many adult anurans have specialised osmoregulatory organs, in addition to the kidney (i.e. the ventral pelvic skin to absorb water from the external environments and a urinary bladder that stores water and reabsorbs it in times of need). Aquaporin (AQP), a water channel protein, plays a fundamental role in these water absorption/reabsorption processes. The anuran AQP family consists of at least AQP0-AQP5, AQP7-AQP10 and two anuran-specific types, designated as AQPa1 and AQPa2. For the three osmoregulatory organs, AQP3 is constitutively located in the basolateral membrane of the tight-junctioned epithelial cells, allowing water transport between the cytoplasm of these cells and the neighbouring tissue fluid at all times. On the other hand, AQPs at the apical side of the tight epithelial cells are different among these organs, and are named kidney-type AQP2, ventral pelvic skin-type AQPa2 and urinary bladder-type AQPa2. All of them show translocation from the cytoplasmic pool to the apical plasma membrane in response to arginine vasotocin, thereby regulating water transport independently in each osmoregulatory organ. It was further revealed that, in terrestrial and arboreal anurans, the bladder-type AQPa2 is expressed in the pelvic skin, together with the pelvic skin-type AQPa2, potentially facilitating water absorption from the pelvic skin. By contrast, Xenopus has lost the ability to efficiently produce pelvic skin-type AQPa2 (AQP-x3) because Cys-273 of AQP-x3 and/or Cys-273-coding region of AQPx3 mRNA attenuate gene expression at a post-transcriptional step, presumably leading to the prevention of excessive water influx in this aquatic species. Collectively, the acquisition of two forms of AQPa2 and the diversified regulation of their gene expression appears to provide the necessary mechanisms for the evolutionary adaptation of anurans to a wide variety of ecological environments.

GSK3beta mediates renal response to vasopressin by modulating adenylate cyclase activity

Glycogen synthase kinase 3beta (GSK3beta), a serine/threonine protein kinase, is a key target of drug discovery in several diseases, including diabetes and Alzheimer disease. Because lithium, a potent inhibitor of GSK3beta, causes nephrogenic diabetes insipidus, GSK3beta may play a crucial role in regulating water homeostasis. We developed renal collecting duct-specific GSK3beta knockout mice to determine whether deletion of GSK3beta affects arginine vasopressin-dependent renal water reabsorption. Although only mildly polyuric under normal conditions, knockout mice exhibited an impaired urinary concentrating ability in response to water deprivation or treatment with a vasopressin analogue. The knockout mice had reduced levels of mRNA, protein, and membrane localization of the vasopressin-responsive water channel aquaporin 2 compared with wild-type mice. The knockout mice also expressed lower levels of pS256-AQP2, a phosphorylated form crucial for membrane trafficking. Levels of cAMP, a major regulator of aquaporin 2 expression and trafficking, were also lower in the knockout mice. Both GSK3beta gene deletion and pharmacologic inhibition of GSK3beta reduced adenylate cyclase activity. In summary, GSK3beta inactivation or deletion reduces aquaporin 2 expression by modulating adenylate cyclase activity and cAMP generation, thereby impairing responses to vasopressin in the renal collecting duct.

FAPP2 is required for aquaporin-2 apical sorting at trans-Golgi network in polarized MDCK cells

FAPP2 is an adaptor protein of phosphatidylinositol-4-phosphate and is involved in the transport of some apical cargos from the trans-Golgi network (TGN). To investigate whether the regulated apical transport of aquaporin-2 (AQP2) is involved in the FAPP2-dependent apical protein-sorting machinery, we measured apical sorting of AQP2 in Madin-Darby canine kidney (MDCK) cells with or without FAPP2 knockdown. We established MDCK cell lines that stably express rat AQP2 without any tag sequence. Then, FAPP2-deficient stable cell lines were established from the AQP2-expressing cell lines by a retrovirus-mediated RNA interference system. In the established cell lines, AQP2 was detected in both apical and basolateral membranes. Forskolin increased only the apical localization of AQP2, which was not affected by basolateral treatment with 0.5% tannic acid, indicating that the forskolin-induced apical transport of AQP2 did not include the transcytotic pathway from basolateral to apical membranes but is a direct transport from TGN to the apical membranes. Using these cell lines, we tested the effect of FAPP2 knockdown on the polarized AQP2 transport to plasma membranes and found that the forskolin-induced apical transport of AQP2 was completely abolished by FAPP2 knockdown. By contrast, the basolateral localization of AQP2 was not affected by FAPP2 knockdown. AQP2 phosphorylation by forskolin was also impaired in FAPP2 knockdown MDCK cells. These results suggest that FAPP2 is necessary to generate AQP2-bearing vesicles at trans-Golgi that will undergo phosphorylation by PKA in subapical regions.

Sensing, signaling and sorting events in kidney epithelial cell physiology

The kidney regulates body fluid, ion and acid/base homeostasis through the interaction of a host of channels, transporters and pumps within specific tubule segments, specific cell types and specific plasma membrane domains. Furthermore, renal epithelial cells have adapted to function in an often unique and challenging environment that includes high medullary osmolality, acidic pHs, variable blood flow and constantly changing apical and basolateral 'bathing' solutions. In this review, we focus on selected protein trafficking events by which kidney epithelial cells regulate body fluid, ion and acid-base homeostasis in response to changes in physiological conditions. We discuss aquaporin 2 and G-protein-coupled receptors in fluid and ion balance, the vacuolar H(+)-adenosine triphosphatase (V-ATPase) and intercalated cells in acid/base regulation and acidification events in the proximal tubule degradation pathway. Finally, in view of its direct role in vesicle trafficking that we outline in this study, we propose that the V-ATPase itself should, under some circumstances, be considered a fourth category of vesicle 'coat' protein (COP), alongside clathrin, caveolin and COPs.

Phosphorylation events and the modulation of aquaporin 2 cell surface expression

PURPOSE OF REVIEW

This review highlights the role of phosphorylation in the trafficking and targeting of aquaporin 2. Current knowledge will be put into the context of modulating the cell surface expression of aquaporin 2 by vasopressin in renal epithelial cells, which is critical for regulation of urinary concentration and control of fluid and electrolyte homeostasis.

RECENT FINDINGS

In addition to previously identified phosphorylation sites on aquaporin 2, new data have revealed three other serine residues in the C-terminus whose phosphorylation is altered by vasopressin. Several steps in aquaporin 2 recycling, including exocytosis and endocytosis, are coordinated by phosphorylation and dephosphorylation to regulate cell surface accumulation. Aquaporin 2 phosphorylation on serine 256 regulates aquaporin 2 association with proteins that are involved in trafficking, including hsc/hsp70 and myelin and lymphocyte-associated protein.

SUMMARY

Aquaporin 2 trafficking is regulated by phosphorylation of serine 256 and other amino acid residues in its cytoplasmic domain. These events increase or decrease interaction of aquaporin 2 with key regulatory proteins to determine the cellular distribution and fate of aquaporin 2, both after vasopressin addition and under baseline conditions. Better understanding of these mechanisms may provide new therapeutic avenues for patients with X-linked nephrogenic diabetes insipidus, as well as providing basic cell biological information relevant to membrane trafficking processes in general.

Deficiency of the tetraspanin CD63 associated with kidney pathology but normal lysosomal function

CD63 is a member of the tetraspanin superfamily that constitutes a main component of the lysosomal membrane. In mice, two CD63 gene loci are present, with only one of these two being functional. We generated and analyzed mice deficient for active CD63. Disruption of CD63 results in a complete loss of CD63 protein expression. Despite its abundance in late endosomes/lysosomes, the lack of CD63 does not cause obvious endosomal/lysosomal abnormalities. CD63 knockout mice are viable and fertile without gross morphological abnormalities in the majority of tissues. No alterations in the populations of immune cells and only minor differences in platelet function were observed. This suggests that the lack of CD63 could be successfully compensated for, most likely by other tetraspanins. However, CD63 deficiency leads to an altered water balance. CD63 knockout mice show an increased urinary flow, water intake, reduced urine osmolality, and a higher fecal water content. In principle cells of the collecting duct of CD63-deficient mice, abnormal intracellular lamellar inclusions were observed. This indicates that the sorting of apical transport proteins might be impaired in these cells. CD63 knockout mice provide an important tool for analyzing the various postulated functions of CD63 in vivo.