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.

The renal proximal tubule reabsorbs 90% of the filtered glucose load through the Na+-coupled glucose transporter SGLT2, and specific inhibitors of SGLT2 are now available to patients with diabetes to increase urinary glucose excretion. Using expression cloning, we identified an accessory protein, 17 kDa membrane-associated protein (MAP17), that increased SGLT2 activity in RNA-injected Xenopus oocytes by two orders of magnitude. Significant stimulation of SGLT2 activity also occurred in opossum kidney cells cotransfected with SGLT2 and MAP17. Notably, transfection with MAP17 did not change the quantity of SGLT2 protein at the cell surface in either cell type. To confirm the physiologic relevance of the MAP17-SGLT2 interaction, we studied a cohort of 60 individuals with familial renal glucosuria. One patient without any identifiable mutation in the SGLT2 coding gene (SLC5A2) displayed homozygosity for a splicing mutation (c.176+1G>A) in the MAP17 coding gene (PDZK1IP1). In the proximal tubule and in other tissues, MAP17 is known to interact with PDZK1, a scaffolding protein linked to other transporters, including Na+/H+ exchanger 3, and to signaling pathways, such as the A-kinase anchor protein 2/protein kinase A pathway. Thus, these results provide the basis for a more thorough characterization of SGLT2 which would include the possible effects of its inhibition on colocalized renal transporters.

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.

Mutations in Aquaporin‐2 (AQP2) induce nephrogenic diabetes insipidus (NDI), a water reabsorption defect of the kidney challenging water homeostasis. While the accepted NDI model claims that recessive (rec) mutations are misfolded and monomeric by nature, the partial activity (Pf= 15‐25% of wt‐AQP2) found with some mutations (D150E, V24A) challenges this notion, and even suggest the presence for mixed wt/rec heteromers in heterozygotes. We have tested this hypothesis in Xenopus oocytes and showed that not only do wt/rec associate (co‐IP assays), but activity of the rec‐AQP2 mutant is shown to be mostly restored (63‐100% of wt‐AQP2) in the process. Also, using bidirectional vectors for dual transfection in cell lines, we find a recovery of targeting for rec‐AQP2 when in presence of wt‐AQP2. Finally, the negative results found when analyzing the strictly monomeric R187C mutant support the notion that the functional recovery of (at least some) rec‐AQP2 mutants is secondary to wt/rec heteromerization. In conclusion, we show that recessive mutations may be functionally recovered through wt/rec oligomerization, contributing to the overall activity in heterozygotes, a notion which could be relevant to all homomeric proteins.

Mutations in Aquaporin‐2 (AQP2) induce nephrogenic diabetes insipidus, a water reabsorption defect of the kidney. Using co‐immunoprecipitation and functional assays, we have recently shown that (at least) some mild recessive (rec) AQP2 mutants can adequately associate to its wild‐type (wt) counterpart to generate fully functional wt/rec heterotetramers. To better characterize actual subunit distributions within wt/rec AQP2 populations, we have developed a dual‐colored fluorescent subunit counting approach in TIRF condition, in order to identify and quantify individual monomers through bleaching of linked fluorophores. Assays performed on both Xenopus oocyte and transfected cell line have confirmed that mild mutations (K228E, D150E and V24A), similar to wt‐AQP2, adequately oligomerize in tetramer, unlike the severe R187C mutant which only displays monomeric structures. In dual transfections (sfGFP‐wt‐AQP2 + RFP‐rec‐AQP2), we also confirm the presence of mixed wt/rec tetramers for mild mutations, but not with R187C, which remains monomeric. Ultimately, we aim at describing subunit distributions within wt/rec tetramers in order to evaluate the properties, limitations and functional outcome of such heteromeric associations.

The Na+/glucose cotransporter SGLT2, which accounts for over 90% of renal glucose reabsorption, has become a major pharmaceutical target for type 2 diabetes treatment. Unfortunately, functional studies on SGLT2 have been hindered due to its lack of activity when expressed heterologously. Using an expression cloning strategy to identify a required accessory protein for SGLT2 function, we have identified MAP17 (membrane associated protein of 17 kDa) which is specifically expressed in the kidney proximal tubule, along with SGLT2. Using electrophysiology (oocytes) and radioactive uptakes (oocytes/OK cells), we demonstrate that MAP17 increases the activity of SGLT2 in both oocytes (150 fold) and OK cells (15 fold). Furthermore, Western blot and immunofluorescence studies using FLAG‐SGLT2 show that the cell surface density for SGLT2 is independent of MAP17 expression, which suggests that MAP17 stimulates the activity of SGLT2, but not its targeting. Finally, functional assays of disease‐causing mutations to SGLT2 (familial renal glucosuria) expressed in OK cells confirm these mutations to be non‐functional, even in the presence of MAP17. In conclusion, we have identified a cofactor responsible for the activation of SGLT2 which enables its characterization in heterologous systems.

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.

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 (∼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.

Myo-inositol (MI; hexahydroxycyclohexane, C6H6O12) is a small neutral molecule used as a compatible osmolyte in the kidney medulla. At high concentrations, MI appears to act as a chemical chaperone and was shown to promote plasma membrane expression of the impaired cystic fibrosis chloride channel (Δ508-CFTR). In the present study, we measured whether MI could increase expression of two human aquaporin 2 (AQP2) mutants which were recently identified as causing nephrogenic diabetes insipidus (NDI). Both proteins (D150E and G196D) were expressed in Xenopus laevis oocytes, but only D150E displayed an increase in oocyte water permeability (Pf). Adding 5 mM MI to the bathing solution for 24 h produced a 50% increase in the D150E-associated Pf, while it had no effect on noninjected oocytes or on oocytes expressing wt-AQP2 or G196D. Western blots performed on purified plasma membrane preparations confirmed that MI increased the amount of D150E present at the plasma membrane, while G196D was always undetectable. X. laevis oocytes are remarkably impermeable to MI, and the effect of MI on D150E expression does not require the presence of intracellular MI. The effect of external MI was dose-dependent (K0.5 was 130 μM) and specific with respect to other forms of inositols. Further studies on a second group of AQP2 mutants causing NDI showed that K228E activity was similarly stimulated by MI, while V71M, A70D and S256L were not. It is concluded that physiological concentrations of extracellular MI can stimulate the expression of a specific subgroup of AQP2 mutants.

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.

Myo‐inositol (MI) uptakes in cells is essentially performed by two Na+‐coupled transport systems; SMIT1, a basolateral system inducible through hypertonicity and SMIT2, an apical system responsible for MI uptake in the kidney and intestine, amongst other tissues. In diabetes, urinary MI excretion is increased by 1 order of magnitude through a non‐established mechanism. The purpose of this study is thus to investigate the influence of an acute diabetic condition (5‐day streptozotocintreatment) on the expression of both SMIT mRNA's in rat brain, liver, muscle, kidney and intestine through quantitative‐RT‐PCR (qRT‐PCR). Also, SMIT2 activity was determined by measuring MI transport in purified brush border membranes (BBMv) from kidney and intestine. SMIT1 transcripts are mainly found in the kidney where it doubles in diabetic rats. Other tissues show more modest mRNA levels and were not altered in diabetes. SMIT2 transcripts were found in all tested tissues in varying levels but were not modified in diabetes. MI uptakes performed on kidney and intestine BBMv confirmed the qRT‐PCR results showing no variations of transport in STZ rats due to SMIT2. We thus conclude that the increase in MI urinary excretion is not due to a reduction in the apical expression of MI cotransporter SMIT2.

This study presents the characterization of MI uptake in rat intestine using purified membrane preparations (BBMv). The two Na+‐coupled myo‐inositol (MI) cotransporters identified (SMIT1 and SMIT2) can be differentiated through inhibition studies using the selective substrates D‐chiro‐inositol (DCI, specific for SMIT2) and L‐fucose (specific for SMIT1). Results show that SMIT2 is exclusively responsible for apical MI transport in rat intestine. Other sugar transport systems present in apical membranes (SGLT1 and GLUT5) lacked any significant contribution to MI uptake. Functional analysis of rat SMIT2 activity determined using electrophysiological studies in Xenopus oocytes, demonstrated similarities to the activities of SMIT2 from species (rabbit and human), displaying high affinities for MI (0.150 ± 0.040 mM), DCI (0.31 ± 0.06 mM) and Pz (0.016 ± 0.007 mM), low affinity for glucose (36 ± 7 mM) and no affinity for L‐fucose. Electrophysiological studies essentially confirmed those found in rat intestinal BBMv with exception of glucose affinity which was about 40‐fold higher in vesicles (Ki = 0.94 ± 0.35 mM) when compared to oocytes. Finally, GLUT2 expressed in oocytes did not mediate any significant radiolabelled MI uptake, indicating that this transport system does not participate in the basolateral exit of MI from small intestine.

Myo-inositol (MI) is a compatible osmolyte used by cells to compensate for changes in the osmolarity of their surrounding milieu. In kidney, the basolateral Na + -MI cotransporter (SMIT1) and apical SMIT2 proteins are homologous cotransporters responsible for cellular uptake of MI. It has been shown in the Madin-Darby canine kidney (MDCK) cell line that SMIT1 expression was under the control of the tonicity-sensitive transcription factor, tonicity-responsive enhancer binding protein (TonEBP). We used an MDCK cell line stably transfected with SMIT2 to determine whether variations in external osmolarity could also affect SMIT2 function. Hyperosmotic conditions (+200 mosM raffinose or NaCl but not urea) generated an increase in SMIT2-specific MI uptake by three- to ninefold in a process that required protein synthesis. Using quantitative RT-PCR, we have determined that hyperosmotic conditions augment both the endogenous SMIT1 and the transfected SMIT2 mRNAs. Transport activities for both SMIT1 and SMIT2 exhibited differences in their respective induction profiles for both their sensitivities to raffinose, as well as in their time course of induction. Application of MG-132, which inhibits nuclear translocation of TonEBP, showed that the effect of osmolarity on transfected SMIT2 was unrelated to TonEBP, unlike the effect observed with SMIT1. Inhibition studies involving the hyperosmolarity-related MAPK suggested that p38 and JNK play a role in the induction of SMIT2. Further studies have shown that hyperosmolarity also upregulates another transfected transporter (Na + -glucose), as well as several endogenously expressed transport systems. This study shows that hyperosmolarity can stimulate transport in a TonEBP-independent manner by increasing the amount of mRNA derived from an exogenous DNA segment.

Over the past two decades, Xenopus laevis oocytes have been widely used as an expression system to investigate both physiological and pathological properties of membrane proteins such as channels and transporters. Past studies have clearly shown the key implications of mistargeting in relation to the pathogenesis of these proteins. To unambiguously determine the plasma membrane targeting of a protein, a thorough purification technique becomes essential. Unfortunately, available techniques are either too cumbersome, technically demanding, or require large amounts of material, all of which are not adequate when using oocytes individually injected with cRNA or DNA. In this article, we present a new technique that permits excellent purification of plasma membranes from X. laevis oocytes. This technique is fast, does not require particular skills such as peeling of vitelline membrane, and permits purification of multiple samples from as few as 10 and up to > 100 oocytes. The procedure combines partial digestion of the vitelline membrane, polymerization of the plasma membrane, and low-speed centrifugations. We have validated this technique essentially with Western blot assays on three plasma membrane proteins [aquaporin (AQP) 2, Na+-glucose cotransporter (SGLT) 1, and transient receptor potential vanilloid (TRPV) 5], using both wild-type and mistargeted forms of the proteins. Purified plasma membrane fractions were easily collected, and samples were found to be adequate for Western blot identification.

Myo-inositol (MI) is involved in several important aspects of cell physiology including cell signaling and the control of intracellular osmolarity i.e. by serving as a "compatible osmolyte". Currently, three MI cotransporters have been identified: two are Na+-dependent (SMIT1 and SMIT2) and one is H+-dependent (HMIT) and predominantly expressed in the brain. The goal of this study was to characterize the expression of SMIT2 in rabbit kidney and to compare it to SMIT1. First, we quantified mRNA levels for both transporters using quantitative real-time PCR and found that SMIT1 was predominantly expressed in the medulla while SMIT2 was mainly in the cortex. This distribution of SMIT2 was confirmed on Western blots where an antibody raised against a SMIT2 epitope specifically detected a 75 kDa protein in both tissues. Characterization of MI transport in brush-border membrane vesicles (BBMV), in the presence of d-chiro-inositol and l-fucose to separately identify SMIT1 and SMIT2 activities, showed that only SMIT2 is expressed at the luminal side of proximal convoluted tubules. We thus conclude that, in the rabbit kidney, SMIT2 is predominantly expressed in the cortex where it is probably responsible for the apical transport of MI into the proximal tubule.

The Na+/ glucose cotransporter (SGLT1) is an archetype for the SLC5 family, which is comprised of Na+-coupled transporters for sugars, myo-inositol, choline, and organic anions. Application of the reducing agent dithriothreitol (DTT, 10 mM) to oocytes expressing human SGLT1 affects the protein's presteady-state currents. Integration of these currents at different membrane potentials (V-m) produces a Q-V curve, whose form was shifted by +25 mV due to DTT. The role of the 15 endogenous cysteine residues was investigated by expressing SGLT1 constructs, each bearing a single mutation for an individual cysteine, in Xenopus oocytes, using two-microelectrode voltage-clamp electrophysiology and fluorescent labeling. 12 of the 15 mutants were functional and could be separated into three distinct groups based on the effect of the mutation on the Q-V curve: four mutants did not perturb the transferred charge, six mutants shifted the Q-V curve towards negative potentials, and two mutants (C255A and C511A) produced a shift in the positive direction that was identical to the shift produced by DTT on the wild-type (wt) SGLT1. The double mutant C(255,511)A confirms that the effects of each single mutant on the Q-V curve were not additive. With respect to wt SGLT1, the apparent affinities for alpha-methylglucose (alpha MG) were increased in a similar manner for the single mutants C255A and C511A, the double mutant C(255,511)A as well as for wt SGLT1 treated with DTT. When exposed to a maleimide-based fluorescent probe, wt SGLT1 was not significantly labeled but mutants C255A and C511A could be clearly labeled, indicating an accessible cysteine residue. These residues are presumed to be C511 and C255, respectively, as the double mutant C(255,511)A could not be labeled. These results strongly support the hypothesis that C255 and C511 form a disulfide bridge in human SGLT1 and that this disulfide bridge is involved in the conformational change of the free carrier.

The putative hinge point revealed by the crystal structure of the MthK potassium channel is a glycine residue that is conserved in many ion channels. In high voltage-activated (HVA) Ca-V channels, the mid-S6 glycine residue is only present in IS6 and IIS6, corresponding to G422 and G770 in Ca(V)1.2. Two additional glycine residues are found in the distal portion of IS6 (Gly(432) and Gly(436) in CaV1.2) to form a triglycine motif unique to HVA CaV channels. Lethal arrhythmias are associated with mutations of glycine residues in the human L-type Ca2+ channel. Hence, we undertook a mutational analysis to investigate the role of S6 glycine residues in channel gating. In Ca(V)1.2, alpha-helix-breaking proline mutants (G422P and G432P) as well as the double G422A/G432A channel did not produce functional channels. The macroscopic inactivation kinetics were significantly decreased with Ca(V)1.2 wild type > G770A > G422A congruent to G436A >> G432A ( from the fastest to the slowest). Mutations at position Gly432 produced mostly nonfunctional mutants. Macroscopic inactivation kinetics were markedly reduced by mutations of Gly(436) to Ala, Pro, Tyr, Glu, Arg, His, Lys, or Asp residues with stronger effects obtained with charged and polar residues. Mutations within the distal GX(3)G residues blunted Ca2+-dependent inactivation kinetics and prevented the increased voltage-dependent inactivation kinetics brought by positively charged residues in the I-II linker. In Ca(V)2.3, mutation of the distal glycine Gly(352) impacted significantly on the inactivation gating. Altogether, these data highlight the role of the GX(3)G motif in the voltage-dependent activation and inactivation gating of HVA CaV channels with the distal glycine residue being mostly involved in the inactivation gating.