Importance of aquaporin-2 expression levels in genotype -phenotype studies in nephrogenic diabetes insipidus

Further evidence for functional recovery of AQP2 mutations associated with nephrogenic diabetes insipidus

Aquaporin-2 (AQP2) is a homotetrameric water channel responsible for the final water reuptake in the kidney. Disease-causing AQP2 mutations induce nephrogenic diabetes insipidus (NDI), a condition that challenges the bodily water balance by producing large urinary volumes. In this study, we characterize three new AQP2 mutations identified in our lab from NDI patients (A120D, A130V, T179N) along the previously reported A47V variant. Using Xenopus oocytes, we compared the key functional and biochemical features of these mutations against classical recessive (R187C) and dominant (R254Q) forms, and once again found clear functional recovery features (increased protein stability and function) for all mutations under study. This behaviour, attributed to heteromerization to wt-AQP2, challenge the classical model to NDI which often depicts recessive mutations as ill-structured proteins unable to oligomerize. Consequently, we propose a revised model to the cell pathophysiology of AQP2-related NDI which accounts for the functional recovery of recessive AQP2 mutations.

Functional Recovery of AQP2 Recessive Mutations Through Hetero-Oligomerization with Wild-Type Counterpart

Aquaporin-2 (AQP2) is a homotetrameric water channel responsible for the final water reuptake in the kidney. Mutations in the protein induce nephrogenic diabetes insipidus (NDI), which challenges the water balance by producing large urinary volumes. Although recessive AQP2 mutations are believed to generate non-functional and monomeric proteins, the literature identifies several mild mutations which suggest the existence of mixed wt/mut tetramers likely to carry function in heterozygotes. Using Xenopus oocytes, we tested this hypothesis and found that mild mutants (V24A, D150E) can associate with wt-AQP2 in mixed heteromers, providing clear functional gain in the process (62 ± 17% and 63 ± 17% increases, respectively), conversely to the strong monomeric R187C mutant which fails to associate with wt-AQP2. In kidney cells, both V24A and D150E display restored targeting while R187C remains in intracellular stores. Using a collection of mutations to expand recovery analyses, we demonstrate that inter-unit contacts are central to this recovery process. These results not only present the ground data for the functional recovery of recessive AQP2 mutants through heteromerization, which prompt to revisit the accepted NDI model, but more importantly describe a general recovery process that could impact on all multimeric systems where recessive mutations are found.

Genetic forms of nephrogenic diabetes insipidus (NDI): Vasopressin receptor defect (X-linked) and aquaporin defect (autosomal recessive and dominant)

Nephrogenic diabetes insipidus (NDI), which can be inherited or acquired, is characterized by an inability to concentrate urine despite normal or elevated plasma concentrations of the antidiuretic hormone, arginine vasopressin (AVP). Polyuria with hyposthenuria and polydipsia are the cardinal clinical manifestations of the disease. About 90% of patients with congenital NDI are males with X-linked NDI who have mutations in the vasopressin V2 receptor (AVPR2) gene encoding the vasopressin V2 receptor. In less than 10% of the families studied, congenital NDI has an autosomal recessive or autosomal dominant mode of inheritance with mutations in the aquaporin-2 (AQP2) gene. When studied in vitro, most AVPR2 and AQP2 mutations lead to proteins trapped in the endoplasmic reticulum and are unable to reach the plasma membrane. Prior knowledge of AVPR2 or AQP2 mutations in NDI families and perinatal mutation testing is of direct clinical value and can avert the physical and mental retardation associated with repeated episodes of dehydration.

Measuring cation transport by Na,K- and H,K-ATPase in Xenopus oocytes by atomic absorption spectrophotometry: an alternative to radioisotope assays

Whereas cation transport by the electrogenic membrane transporter Na(+),K(+)-ATPase can be measured by electrophysiology, the electroneutrally operating gastric H(+),K(+)-ATPase is more difficult to investigate. Many transport assays utilize radioisotopes to achieve a sufficient signal-to-noise ratio, however, the necessary security measures impose severe restrictions regarding human exposure or assay design. Furthermore, ion transport across cell membranes is critically influenced by the membrane potential, which is not straightforwardly controlled in cell culture or in proteoliposome preparations. Here, we make use of the outstanding sensitivity of atomic absorption spectrophotometry (AAS) towards trace amounts of chemical elements to measure Rb(+) or Li(+) transport by Na(+),K(+)- or gastric H(+),K(+)-ATPase in single cells. Using Xenopus oocytes as expression system, we determine the amount of Rb(+) (Li(+)) transported into the cells by measuring samples of single-oocyte homogenates in an AAS device equipped with a transversely heated graphite atomizer (THGA) furnace, which is loaded from an autosampler. Since the background of unspecific Rb(+) uptake into control oocytes or during application of ATPase-specific inhibitors is very small, it is possible to implement complex kinetic assay schemes involving a large number of experimental conditions simultaneously, or to compare the transport capacity and kinetics of site-specifically mutated transporters with high precision. Furthermore, since cation uptake is determined on single cells, the flux experiments can be carried out in combination with two-electrode voltage-clamping (TEVC) to achieve accurate control of the membrane potential and current. This allowed e.g. to quantitatively determine the 3Na(+)/2K(+) transport stoichiometry of the Na(+),K(+)-ATPase and enabled for the first time to investigate the voltage dependence of cation transport by the electroneutrally operating gastric H(+),K(+)-ATPase. In principle, the assay is not limited to K(+)-transporting membrane proteins, but it may work equally well to address the activity of heavy or transition metal transporters, or uptake of chemical elements by endocytotic processes.

Congenital nephrogenic diabetes insipidus: the current state of affairs

The anti-diuretic hormone arginine vasopressin (AVP) is released from the pituitary upon hypovolemia or hypernatremia, and regulates water reabsorption in the renal collecting duct principal cells. Binding of AVP to the arginine vasopressin receptor type 2 (AVPR2) in the basolateral membrane leads to translocation of aquaporin 2 (AQP2) water channels to the apical membrane of the collecting duct principal cells, inducing water permeability of the membrane. This results in water reabsorption from the pro-urine into the medullary interstitium following an osmotic gradient. Congenital nephrogenic diabetes insipidus (NDI) is a disorder associated with mutations in either the AVPR2 or AQP2 gene, causing the inability of patients to concentrate their pro-urine, which leads to a high risk of dehydration. This review focuses on the current knowledge regarding the cell biological aspects of congenital X-linked, autosomal-recessive and autosomal-dominant NDI while specifically addressing the latest developments in the field. Based on deepened mechanistic understanding, new therapeutic strategies are currently being explored, which we also discuss here.

Aquaporin-2: new mutations responsible for autosomal-recessive nephrogenic diabetes insipidus-update and epidemiology

It is clinically useful to distinguish between two types of hereditary nephrogenic diabetes insipidus (NDI): a ‘pure’ type characterized by loss of water only and a complex type characterized by loss of water and ions. Patients with congenital NDI bearing mutations in the vasopressin 2 receptor gene, AVPR2, or in the aquaporin-2 gene, AQP2, have a pure NDI phenotype with loss of water but normal conservation of sodium, potassium, chloride and calcium. Patients with hereditary hypokalemic salt-losing tubulopathies have a complex phenotype with loss of water and ions. They have polyhydramnios, hypercalciuria and hypo- or isosthenuria and were found to bear KCNJ1 (ROMK) and SLC12A1 (NKCC2) mutations. Patients with polyhydramnios, profound polyuria, hyponatremia, hypochloremia, metabolic alkalosis and sensorineural deafness were found to bear BSND mutations. These clinical phenotypes demonstrate the critical importance of the proteins ROMK, NKCC2 and Barttin to transfer NaCl in the medullary interstitium and thereby to generate, together with urea, a hypertonic milieu. This editorial describes two new developments: (i) the genomic information provided by the sequencing of the AQP2 gene is key to the routine care of these patients, and, as in other genetic diseases, reduces health costs and provides psychological benefits to patients and families and (ii) the expression of AQP2 mutants in Xenopus oocytes and in polarized renal tubular cells recapitulates the clinical phenotypes and reveals a continuum from severe loss of function with urinary osmolalities <150 mOsm/kg H2O to milder defects with urine osmolalities >200 mOsm/kg H2O.

New autosomal recessive mutations in aquaporin-2 causing nephrogenic diabetes insipidus through deficient targeting display normal expression in Xenopus oocytes

Aquaporin-2 (AQP2), located at the luminal side of the collecting duct principal cells, is a water channel responsible for the final concentration of urine. Lack of function, often occurring through mistargeting of mutated proteins, induces nephrogenic diabetes insipidus (NDI), a condition characterized by large urinary volumes. In the present study, two new mutations (K228E and V24A) identified in NDI-affected individuals from distinct families along with the already reported R187C were analysed in comparison to the wild-type protein (AQP2-wt) using Xenopus laevis oocytes and a mouse collecting duct cell-line (mIMCD-3). Initial data in oocytes showed that all mutations were adequately expressed at reduced levels when compared to AQP2-wt. K228E and V24A were found to be properly targeted at the plasma membrane and exhibited adequate functionality similar to AQP2-wt, as opposed to R187C which was retained in internal stores and was thus inactive. In coexpression studies using oocytes, R187C impeded the functionality of all other AQP2 variants while combinations with K228E, V24A and AQP2-wt only showed additive functionalities. When expressed in mIMCD-3 cells, forskolin treatment efficiently promoted the targeting of AQP2-wt at the plasma membrane (>90%) while K228E only weakly responded to the same treatment (approximately 20%) and both V24A and R187C remained completely insensitive to the treatment. We concluded that both V24A and K228E are intrinsically functional water channels that lack a proper response to vasopressin, which leads to NDI as found in both compound mutations studied (K228E + R187C and V24A + R187C). The discrepancies in plasma membrane targeting response found in both expression systems stress the need to evaluate such data using mammalian cell systems.

Characterization of D150E and G196D aquaporin-2 mutations responsible for nephrogenic diabetes insipidus: importance of a mild phenotype

Aquaporin-2 (AQP2) is a water channel responsible for the final water reabsorption in renal collecting ducts. Alterations in AQP2 function induce nephrogenic diabetes insipidus (NDI), a condition characterized by severe polyuria and polydipsia. Three patients affected with severe NDI, who were compound heterozygous for the AQP2 mutations D150E and G196D, are presented here along with a mildly affected D150E homozygous patient from another family. Using Xenopus oocytes as an expression system, these two mutations (G196D and D150E) were compared with the wild-type protein (AQP2-wt) for functional activity (water flux analysis), protein maturation, and plasma membrane targeting. AQP2-wt induces a major increase in water permeability (P(f) = 47.4 +/- 12.2 x 10(-4) cm/s) whereas D150E displays intermediate P(f) values (P(f) = 12.5 +/- 3.0 x 10(-4) cm/s) and G196D presents no specific water flux, similar to controls (P(f) = 2.1 +/- 0.8 x 10(-4) cm/s and 2.2 +/- 0.7 x 10(-4) cm/s, respectively). Western blot and immunocytochemical evaluations show protein targeting that parallels activity levels with AQP2-wt adequately targeted to the plasma membrane, partial targeting for D150E, and complete sequestration of G196D within intracellular compartments. When coinjecting AQP2-wt with mutants, no (AQP2-wt + D150E) or partial (AQP2-wt + G196D) reduction of water flux were observed compared with AQP2-wt alone, whereas complete loss of function was found when both mutants were coinjected. These results essentially recapitulate the clinical profiles of the family members, showing a typical dominant negative effect when G196D is coinjected with either AQP2-wt or D150E but not between AQP2-wt and D150E mutant.

Aquaporin 2 mutations in nephrogenic diabetes insipidus

Water reabsorption in the renal collecting duct is regulated by the antidiuretic hormone vasopressin (AVP). When the vasopressin V2 receptor, present on the basolateral site of the renal principal cell, becomes activated by AVP, aquaporin-2 (AQP2) water channels will be inserted in the apical membrane, and in this fashion, water can be reabsorbed from the pro-urine into the interstitium. The essential role of the vasopressin V2 receptor and AQP2 in the maintenance of body water homeostasis became clear when it was shown that mutations in their genes cause nephrogenic diabetes insipidus, a disorder in which the kidney is unable to concentrate urine in response to AVP. This review describes the current knowledge on AQP2 mutations in nephrogenic diabetes insipidus.

Polarisation, key to good localisation

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.

A novel mechanism in recessive nephrogenic diabetes insipidus: wild-type aquaporin-2 rescues the apical membrane expression of intracellularly retained AQP2-P262L

Vasopressin regulates water homeostasis through insertion of homotetrameric aquaporin-2 (AQP2) water channels in the apical plasma membrane of renal cells. AQP2 mutations cause recessive and dominant nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin. Until now, all AQP2 mutants in recessive NDI were shown to be misfolded, retained in the endoplasmic reticulum (ER) and unable to interact with wild-type (wt)-AQP2, whereas AQP2 mutants in dominant NDI are properly folded and interact with wt-AQP2, but, due to the mutation, cause missorting of the wt-AQP2/mutant complex. Here, patients of two families with recessive NDI appeared compound heterozygotes for AQP2-A190T or AQP2-R187C mutants, together with AQP2-P262L. As mutations in the AQP2 C-tail, where P262 resides, usually cause dominant NDI, the underlying cell-biological mechanism was investigated. Upon expression in oocytes, AQP2-P262L was a properly folded and functional aquaporin in contrast to the classical mutants, AQP2-R187C and AQP2-A190T. Expressed in polarized cells, AQP2-P262L was retained in intracellular vesicles and did not localize to the ER. Upon co-expression, however, AQP2-P262L interacted with wt-AQP2, but not with AQP2-R187C, resulting in a rescued apical membrane expression of AQP2-P262L. In conclusion, our study reveals a novel cellular phenotype in recessive NDI in that AQP2-P262L acts as a mutant in dominant NDI, except for that its missorting is overruled by apical sorting of wt-AQP2. Also, it demonstrates for the first time that the recessive inheritance of a disease involving a channel can be due to two cell-biological mechanisms.

Reversed polarized delivery of an aquaporin-2 mutant causes dominant nephrogenic diabetes insipidus

Vasopressin regulates body water conservation by redistributing aquaporin-2 (AQP2) water channels from intracellular vesicles to the apical surface of renal collecting ducts, resulting in water reabsorption from urine. Mutations in AQP2 cause autosomal nephrogenic diabetes insipidus (NDI), a disease characterized by the inability to concentrate urine. Here, we report a frame-shift mutation in AQP2 causing dominant NDI. This AQP2 mutant is a functional water channel when expressed in Xenopus oocytes. However, expressed in polarized renal cells, it is misrouted to the basolateral instead of apical plasma membrane. Additionally, this mutant forms heterotetramers with wild-type AQP2 and redirects this complex to the basolateral surface. The frame shift induces a change in the COOH terminus of AQP2, creating both a leucine- and a tyrosine-based motif, which cause the reversed sorting of AQP2. Our data reveal a novel cellular phenotype in dominant NDI and show that dominance of basolateral sorting motifs in a mutant subunit can be the molecular basis for disease.

Functional Expression of the Human Thiazide-Sensitive NaCl Cotransporter in Madin-Darby Canine Kidney Cells

ABSTRACT. The thiazide-sensitive Na+-Cl−cotransporter (NCC), which is expressed on the apical membrane of epithelial cells lining the distal convoluted tubule, is responsible for the reabsorption of 5% to 10% of filtered Na+and Cl−. To date, functional studies on the structural and regulatory requirements for localized trafficking and ion-transporting activity of NCC have been hampered by lack of a suitable cell system expressing this cotransporter. Reported here is the functional expression of human NCC (hNCC) in a polarized mammalian cell of renal origin—that is, the high-resistance Madin-Darby canine kidney (MDCK) cell. Western blot testing revealed that the cells predominantly expressed the complex glycosylated (approximately 140 kD) form of hNCC. hNCC was present primarily in the apical part of the cell. The functionality of hNCC was demonstrated by the gain of thiazide-sensitive Na+uptake and transepithelial transport activity. Na+uptake was significantly increased after short-term (15 min) treatment with forskolin, whereas cyclic guanosine monophosphate, wortmannin, phorbol 12-myriatate 13-acetate, and staurosporine were without effect. This indicates that hNCC activity is regulated through cyclic adenosine monophosphate, rather than via cyclic guanosine monophosphate, phospho-inositide 3-kinases or protein kinase C. Aldosterone did not alter Na+uptake in the short term (15 min) but significantly increased the transport activity in the long term (16 h). The latter effect of aldosterone was due to an effect on the cytomegalovirus promoter/enhancer driving the expression of hNCC. hNCC-MDCK cells are a good model for the study of the regulation of apical trafficking and ion-transporting activity of hNCC. E-mail r.bindels@ncmls.kun.nl

The role of regulated CFTR trafficking in epithelial secretion

The focus of this review is the regulated trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) in distal compartments of the protein secretory pathway and the question of how changes in CFTR cellular distribution may impact on the functions of polarized epithelial cells. We summarize data concerning the cellular localization and activity of CFTR and attempt to synthesize often conflicting results from functional studies of regulated endocytosis and exocytosis in CFTR-expressing cells. In some instances, findings that are inconsistent with regulated CFTR trafficking may result from the use of overexpression systems or nonphysiological experimental conditions. Nevertheless, judging from data on other transporters, an appropriate cellular context is necessary to support regulated CFTR trafficking, even in epithelial cells. The discovery that disease mutations can influence CFTR trafficking in distal secretory and recycling compartments provides support for the concept that regulated CFTR recycling contributes to normal epithelial function, including the control of apical CFTR channel density and epithelial protein secretion. Finally, we propose molecular mechanisms for regulated CFTR endocytosis and exocytosis that are based on CFTR interactions with other proteins, particularly those whose primary function is membrane trafficking. These models provide testable hypotheses that may lead to elucidation of CFTR trafficking mechanisms and permit their experimental manipulation in polarized epithelial cells.

Cell-biologic and functional analyses of five new Aquaporin-2 missense mutations that cause recessive nephrogenic diabetes insipidus

Mutations in the Aquaporin-2 gene, which encodes a renal water channel, have been shown to cause autosomal nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin. Most AQP2 missense mutants in recessive NDI are retained in the endoplasmic reticulum (ER), but AQP2-T125M and AQP2-G175R were reported to be nonfunctional channels unimpaired in their routing to the plasma membrane. In five families, seven novel AQP2 gene mutations were identified and their cell-biologic basis for causing recessive NDI was analyzed. The patients in four families were homozygous for mutations, encoding AQP2-L28P, AQP2-A47V, AQP2-V71M, or AQP2-P185A. Expression in oocytes revealed that all these mutants, and also AQP2-T125M and AQP2-G175R, conferred a reduced water permeability compared with wt-AQP2, which was due to ER retardation. The patient in the fifth family had a G>A nucleotide substitution in the splice donor site of one allele that results in an out-of-frame protein. The other allele has a nucleotide deletion (c652delC) and a missense mutation (V194I). The routing and function of AQP2-V194I in oocytes was not different from wt-AQP2; it was therefore concluded that c652delC, which leads to an out-of-frame protein, is the NDI-causing mutation of the second allele. This study indicates that misfolding and ER retention is the main, and possibly only, cell-biologic basis for recessive NDI caused by missense AQP2 proteins. In addition, the reduced single channel water permeability of AQP2-A47V (40%) and AQP2-T125M (25%) might become of therapeutic value when chemical chaperones can be found that restore their routing to the plasma membrane.

Human cortical distal nephron: distribution of electrolyte and water transport pathways

The exact distributions of the different salt transport systems along the human cortical distal nephron are unknown. Immunohistochemistry was performed on serial cryostat sections of healthy parts of tumor nephrectomized human kidneys to study the distributions in the distal convolution of the thiazide-sensitive Na-Cl cotransporter (NCC), the beta subunit of the amiloride-sensitive epithelial Na channel (ENaC), the vasopressin-sensitive water channel aquaporin 2 (AQP2), and aquaporin 3 (AQP3), the H(+) ATPase, the Na-Ca exchanger (NCX), plasma membrane calcium-ATPase, and calbindin-D28k (CaBP). The entire human distal convolution and the cortical collecting duct (CCD) display calbindin-D28k, although in variable amounts. Approximately 30% of the distal convolution profiles reveal NCC, characterizing the distal convoluted tubule. NCC overlaps with ENaC in a short portion at the end of the distal convoluted tubule. ENaC is displayed all along the connecting tubule (70% of the distal convolution) and the CCD. The major part of the connecting tubule and the CCD coexpress aquaporin 2 with ENaC. Intercalated cells, undetected in the first 20% of the distal convolution, were interspersed among the segment-specific cells of the remainder of the distal convolution, and of the CCD. The basolateral calcium extruding proteins, Na-Ca exchanger (NCX), and the plasma membrane Ca(2+)-ATPase were found all along the distal convolution, and, in contrast to other species, along the CCD, although in varying amounts. The knowledge regarding the precise distribution patterns of transport proteins in the human distal nephron and the knowledge regarding the differences from that in laboratory animals may be helpful for diagnostic purposes and may also help refine the therapeutic management of electrolyte disorders.

Detection of aquaporin-2 in the plasma membranes of oocytes: a novel isolation method with improved yield and purity

Aquaporin-2 (AQP2) water channel mutations cause autosomal recessive and dominant nephrogenic diabetes insipidus (NDI). Expressed in oocytes, a mutant in dominant (AQP2-E258K), but not in recessive (AQP2-R187C), NDI conferred a specific dominant-negative effect on wild-type (wt) AQP2 water permeability (Pf) only at low expression levels. Since at these levels, the yield of conventional-isolated plasma membranes was too low, an improved technique to semiquantify AQP2 in the plasma membrane was needed. Antibodies against the C-loop of AQP2 were not applicable since they were unspecific and introduction of a tag into this loop caused misfolding and ER retardation. Membrane-impermeable biotin analogues turned out to label intracellular AQP2 proteins. Therefore, a method has been developed which generates a high yield of nearly pure plasma membranes, which enables semiquantification of plasma membrane proteins expressed at low levels in oocytes. Our new method allows for phenotype-genotype correlation studies in a wide range of channelopathies.