Structure of aquaporin-2 vasopressin water channel

Effects of missense mutations on rat aquaporin-2 in LLC-PK1 porcine kidney cells

BACKGROUND

Mutations in the aquaporin-2 (AQP2) gene have been found in families with nephrogenic diabetes insipidus (NDI), but the pathophysiological mechanisms of how mutant AQP2 causes the disease are still not clear.

METHODS

Wild-type (WT) AQP2 and four mutants-T126M, A147T, R187C, and S216P-were transiently expressed in LLC-PK1 cells. The osmotic water permeability of LLC-PK1 cells expressing AQP2 mutants was determined by stopped-flow light-scattering microphotometry. Cell surface expression, subcellular localization, and effects of vasopressin stimulation were examined by surface biotin labeling and confocal immunohistochemistry.

RESULTS

The osmotic water permeability (Pf) of cells expressing WT increased significantly after vasopressin treatment, whereas the Pf of cells expressing T126M A147T, R187C, and S216P was not significantly different from that of the control even after vasopressin stimulation. Confocal immunohistochemistry demonstrated distribution of WT and A147T in early/recycling endosomal compartments and vasopressin-responsive translocation and surface expression. In contrast, stainings of T126M, R187C, and S216P were similar to that of Grp78, indicating that these mutants were misassembled and retarded in the endoplasmic reticulum.

CONCLUSION

Our results indicated that the intracellular distribution and vasopressin-regulated trafficking of A147T is intact, in contrast to the other three mutants, of which both were impaired. Thus, it is conceivable that the disruption of the AQP2 channel function accounts for the pathogenesis of A147T NDI, whereas trafficking defects account for that of the other types, suggesting that the pathophysiology of AQP2-related NDI is heterogeneous.

Modulation of vasopressin-elicited water transport by trafficking of aquaporin2-containing vesicles

Vasopressin or AVP regulates water reabsorption by the kidney inner medullary collecting duct (IMCD) through the insertion and removal of aquaporin (AQP) 2 water channels into the IMCD apical membrane. AVP-elicited trafficking of AQP2 with the apical membrane occurs via a specialized population of vesicles that resemble synaptic vesicles in neurons. AQP2 vesicles and the IMCD apical membrane contain homologs of vesicle-targeting and signal transduction proteins found in neurons. Expression studies of AQP2, including human AQP2 mutants, suggest that the carboxyl-terminal domain of AQP2 is important in AQP2 trafficking, particularly as a site for cAMP-dependent protein kinase phosphorylation. These present data reveal that IMCD cells possess a complex integrated-signaling and vesicle-trafficking machinery that provides integration of AVP-elicited water transport with many other parameters within the IMCD cell as well as kidney.

Oligomerization state of water channels and glycerol facilitators. Involvement of loop E

The major intrinsic protein (MIP) family includes water channels aquaporins (AQPs) and facilitators for small solutes such as glycerol (GlpFs). Velocity sedimentation on sucrose gradients demonstrates that heterologous AQPcic expressed in yeast or Xenopus oocytes behaves as an homotetramer when extracted by n-octyl beta-D-glucopyranoside (OG) and as a monomer when extracted by SDS. We performed an analysis of GlpF solubilized from membranes of Escherichia coli or of mRNA-injected Xenopus oocytes. The GlpF protein extracted either by SDS or by nondenaturing detergents, OG and Triton X-100, exhibits sedimentation coefficients only compatible with a monomeric form of the protein in micelles. We then substituted in loop E of AQPcic two amino acids predicted to play a role in the functional/structural properties of the MIPs. In two expression systems, yeast and oocytes, the mutant AQPcic-S205D is monomeric in OG and in SDS. The A209K mutation does not modify the tetrameric form of the heterologous protein in OG. This study shows that the serine residue at position 205 is essential for AQPcic tetramerization. Because the serine in this position is highly conserved among aquaporins and systematically replaced by an acid aspartic in GlpFs, we postulate that glycerol facilitators are monomers whereas aquaporins are organized in tetramers. Our data suggest that the role of loop E in MIP properties partly occurs through its ability to allow oligomerization of the proteins.

Novel mutations in aquaporin-2 gene in female siblings with nephrogenic diabetes insipidus: evidence of disrupted water channel function

Novel mutations of the aquaporin-2 (AQP2) gene have been detected in Japanese female siblings with autosomal-recessive nephrogenic diabetes insipidus. The patients were compound heterozygote for point mutations at nucleotide position 374 (C374T) and at position 523 (G523A) in exon 2 of the AQP2 gene, resulting in substitution of methionine for threonine at codon 125 (T125M) and arginine for glycine at codon 175 (G175R). The water permeability (Pf) of oocytes injected with wild-type complementary RNA increased 9.0-fold compared with the Pf of water-injected oocytes, whereas the increases in the Pf of oocytes injected with T125M and G175R complementary RNA were only 1.7-fold and 1.5-fold, respectively. Immunoblot and immunocytochemistry indicated that the plasma membrane expressions of T125M and G175R AQP2 proteins were comparable to that of the wild-type, suggesting that although neither the T125M nor G175R mutation had a significant effect on plasma membrane expression, they both distorted the structure and function of the aqueous pore of AQP2. These results provide evidence that the nephrogenic diabetes insipidus in patients with T125M and G175R mutations is attributable not to the misrouting of AQP2, but to the disrupted water channel function.

Prediction of functional residues in water channels and related proteins

In this paper, we present an updated classification of the ubiquitous MIP (Major Intrinsic Protein) family proteins, including 153 fully or partially sequenced members available in public databases. Presently, about 30 of these proteins have been functionally characterized, exhibiting essentially two distinct types of channel properties: (1) specific water transport by the aquaporins, and (2) small neutral solutes transport, such as glycerol by the glycerol facilitators. Sequence alignments were used to predict amino acids and motifs discriminant in channel specificity. The protein sequences were also analyzed using statistical tools (comparisons of means and correspondence analysis). Five key positions were clearly identified where the residues are specific for each functional subgroup and exhibit high dissimilar physico-chemical properties. Moreover, we have found that the putative channels for small neutral solutes clearly differ from the aquaporins by the amino acid content and the length of predicted loop regions, suggesting a substrate filter function for these loops. From these results, we propose a signature pattern for water transport.

The perinatal expression of aquaporin-2 and aquaporin-3 in developing kidney

The kidney provides an important contribution to permit the fetus to successfully transition to an independent existence by production of urine with significantly different osmolality compared with plasma. Although recent work has uncovered many aspects of the maturation and regulation of the renal concentrating and diluting mechanism, understanding of how alterations in the expression of aquaporin (AQP) water channels contribute to the formation of urine in the perinatal period is incomplete. Here, we report that both AQP-2 and -3 are expressed during fetal life as early as embryonic d 18 in ureteric buds of rat kidneys, where each is localized to the apical and basolateral membranes of epithelial cells, respectively. Northern analyses demonstrate that the 1.9-kb AQP-2 transcript is present in fetal and postnatal rat kidneys similar to that observed in adults. AQP-2 mRNA expression increases after d 3 of postnatal life. Immunoblotting reveals an increase in total kidney AQP-2 protein particularly with respect to its glycosylated form after postnatal d 3. AQP-3 protein also exhibits a similar alteration likely due to a similar increase in its glycosylation state. Both AQP-2 and AQP-3 display a distribution in the collecting ducts of human postnatal infants and adults identical to that exhibited in rat kidneys. These data show that both AQP-2 and -3 are present in collecting duct epithelia of fetal and postnatal kidneys. Thus, the reduced AVP-responsiveness and decreased urinary concentrating ability of the kidney during the fetal and immediate postnatal period does not appear to be caused by lack of AQP-2 or AQP-3 proteins.

A yeast recombinant aquaporin mutant that is not expressed or mistargeted in Xenopus oocyte can be functionally analyzed in reconstituted proteoliposomes

We have recently identified AQPcic (for aquaporin cicadella), an insect aquaporin found in the digestive tract of homopteran insects and involved in the elimination of water ingested in excess with the dietary sap (Le Cahérec, F., Deschamps, S., Delamarche, C., Pellerin, I., Bonnec, G., Guillam, M. T., Gouranton, J., Thomas, D., and Hubert, J. F. (1996) Eur. J. Biochem. 241, 707-715). Like many other aquaporins, AQPcic is inhibited by mercury reagents. In this study, we have demonstrated that residue Cys82 is essential for mercury inhibition. Another mutant version of AQPcic (AQP-C134S), expression of which in Xenopus laevis failed to produce an active molecule, was successfully expressed in Saccharomyces cerevisiae. Using stopped-flow analysis of reconstituted proteoliposomes, we demonstrated that the biological activity and Hg sensitivity of yeast-expressed wild type and mutant type AQPcic was readily assessed. Therefore, we propose that the yeast system is a valid alternative to Xenopus oocytes for studying particular mutants of aquaporin.

Aquaporin-2 and -3: representatives of two subgroups of the aquaporin family colocalized in the kidney collecting duct

Since the molecular identification of the first aquaporin in 1992, the number of proteins known to belong to this family has been rapidly increasing. These members may be separated into two subgroups based on gene structure, sequence homology, and function. Regulation of the water permeability of the collecting ducts of the kidney is essential for urinary concentration. Aquaporin-2 and -3, which are representative of these subgroups, are colocalized in the collecting ducts. Understanding these subgroups will elucidate the differences between aquaporin-2 and -3. Aquaporin-2 is a vasopressin-regulated water channel located in the apical membrane, and aquaporin-3 is a constitutive water channel located in the basolateral membrane. In contrast to aquaporin-3, which appears to be less well regulated, many studies have now identified multiple regulational mechanisms at the gene, protein, and cell levels for aquaporin-2, thus reflecting its physiological importance. Evidence of the participation of aquaporin-2 in the pathophysiology of water-balance disorders is accumulating.

Connexin membrane protein biosynthesis is influenced by polypeptide positioning within the translocon and signal peptidase access

We reported previously (Falk, M. M., Kumar, N. M., and Gilula, N. B. (1994) J. Cell Biol. 127, 343-355) that the membrane integration of polytopic connexin polypeptides can be accompanied by an inappropriate cleavage that generates amino-terminal truncated connexins. While this cleavage was not detected in vivo, translation in standard cell-free translation/translocation systems resulted in the complete cleavage of all newly integrated connexins. Partial cleavage occurred in heterologous expression systems that correlated with the expression level. Here we report that the transmembrane topology of connexins generated in microsomal membranes was identical to the topology of functional connexins in plasma membranes. Characterization of the cleavage site and reaction showed that the connexins were processed by signal peptidase immediately downstream of their first transmembrane domain in a reaction similar to the removal of signal peptides from pre-proteins. Increasing the length and hydrophobic character of the signal anchor sequence of connexins completely prevented the aberrant cleavage. This result indicates that their signal anchor sequence was falsely recognized and positioned as a cleavable signal peptide within the endoplasmic reticulum translocon, and that this mispositioning enabled signal peptidase to access the cleavage sites. The results provide direct evidence for the involvement of unknown cellular factors in the membrane integration process of connexins.

Identification and characterization of aquaporin-2 water channel mutations causing nephrogenic diabetes insipidus with partial vasopressin response

Congenital nephrogenic diabetes insipidus (NDI) is a rare disease caused most often by mutations in the vasopressin V2 receptor (AVPR2). We studied a family which included a female patient with NDI with symptoms dating from infancy. The patient responded to large doses of desmopressin (dDAVP) which decreased urine volume from 10 to 4 I/day. Neither the parents nor the three sisters were polyuric. The patient was found to be a compound heterozygote for two novel recessive point mutations in the aquaporin-2 (AQP2) gene: L22V in exon 1 and C181W in exon 3. Residue Cys181 in AQP2 is the site for inhibition of water permeation by mercurial compounds and is located near to the NPA motif conserved in all aquaporins. Osmotic water permeability (Pf) in Xenopus oocytes injected with cRNA encoding C181W-AQP2 was not increased over water control, while expression of L22V cRNA increased the Pf to approximately 60% of that for wild-type AQP2. Co-injection of the mutant cRNAs with the wild-type cRNA did not affect the function of the wild-type AQP2. Immunolocalization of AQP2-transfected CHO cells showed that the C181W mutant had an endoplasmic reticulum-like intracellular distribution, whereas L22V and wild-type AQP2 showed endosome and plasma membrane staining. Water permeability assays showed a high Pf in cells expressing wild-type and L22V AQP2. This study indicates that AQP2 mutations can confer partially responsive NDI.

Phosphorylation of serine 256 is required for cAMP-dependent regulatory exocytosis of the aquaporin-2 water channel

The aquaporin-2 (AQP2) vasopressin water channel is translocated to the apical membrane upon vasopressin stimulation. Phosphorylation of serine 256 of AQP2 by cAMP-dependent protein kinase has been shown, but its relation to vasopressin-regulated translocation has not been elucidated. To address this question, wild type (WT) AQP2 and a mutant with alanine in place of serine 256 of AQP2 (S256A) were expressed in LLC-PK1 cells by electroporation. Measurements by a stopped-flow light-scattering method revealed that the osmotic water permeability (Pf) of LLC-PK1 cells transfected with WT was 69.6 +/- 6.5 microm/s (24.8 +/- 2.2 microm/s for mock-transfected), and stimulation by 500 microM 8-(4-chlorophenylthio)-cAMP increased the Pf by 85 +/- 12%. When S256A AQP2 was transfected, the cAMP-dependent increase in the Pf was only 8 +/- 5%. After cAMP stimulation, the increase in surface expression of AQP2 determined by surface biotin labeling was 4 +/- 10%, significantly less than that for WT (88 +/- 5%). In addition, an in vivo [32P]orthophosphate labeling assay demonstrated significant phosphorylation of WT AQP2 and only minimal phosphorylation of S256A AQP2 in LLC-PK1 cells. Our results indicated that serine 256 of AQP2 is necessary for regulatory exocytosis and that cAMP-responsive redistribution of AQP2 may be regulated by phosphorylation of AQP2.

AQUAPORINS AND WATER PERMEABILITY OF PLANT MEMBRANES

The mechanisms of plant membrane water permeability have remained elusive until the recent discovery in both vacuolar and plasma membranes of a class of water channel proteins named aquaporins. Similar to their animal counterparts, plant aquaporins have six membrane-spanning domains and belong to the MIP superfamily of transmembrane channel proteins. Their very high efficiency and selectivity in transporting water molecules have been mostly characterized using heterologous expression in Xenopus oocytes. However, techniques set up to measure the osmotic water permeability of plant membranes such as transcellular osmosis, pressure probe measurements, or stopped-flow spectrophotometry are now being used to analyze the function of plant aquaporins in their native membranes. Multiple mechanisms, at the transcriptional and posttranslational levels, control the expression and activity of the numerous aquaporin isoforms found in plants. These studies suggest a general role for aquaporins in regulating transmembrane water transport during the growth, development, and stress responses of plants. Future research will investigate the integrated function of aquaporins in long-distance water transport and cellular osmoregulation.

Water channels and urea transporters

1. It now appears that when water crosses an endothelium which is not fenestrated, or an epithelium with tight junctions, it does so rapidly, and with low energy cost, only if the cell membrane contains an adequate number of specific water channels, encoded by one of at least six different genes. 2. The water channel genes so far cloned encode a series of integral membrane proteins called aquaporins, all of approximately 30 kDa (265-282 amino acids), in the unglycosylated state. All but one (AQP3) are specific water channels and all but one (AQP4) are inactivated by mercurial compounds. 3. Aquaporin 0 is the major (60%) intrinsic protein (MIP) of lens fibre cells of the eye. Mutations in this gene are associated with cataract formation in mice. 4. Aquaporin 1, also called CHIP-28, exists in the membrane as a homotetramer, and is present in red blood cells, the choroid plexus, the proximal tubule and descending limb of the loop of Henle in the kidney as well as in many other sites. Surprisingly, no pathological consequence is known in patients lacking a functional AQP1 gene. 5. Aquaporin 2, also called WCH-CD, is the water channel of the principal cell of the cortical and medullary collecting duct, and is located in cytoplasmic vesicles unless arginine vasopressin is acting, when it is translocated to the apical membrane by synaptobrevins or vesicle associated membrane protein 2 (VAMP2). Lack of a functional AQP2 gene leads to a rare form of nephrogenic diabetes insipidus. 6. Aquaporins 3, 4, and 5 are located in many tissues-AQP3 and AQP4 being in the basolateral membrane of the renal cortical and medullary principal cell, as well as in the gastrointestinal tract (AQP3) and the brain (AQP4). 7. Four sequences are known for urea transporters HUT11-the urea transporter of the human red cell membrane, and HUT2, rUT2, rbUT2-the arginine vasopressin inducible urea transporters of the human, rat and rabbit kidney. They are specifically permeable to urea, not to water, and are claimed to be inhibited by phloretin. 8. The water channel proteins contain six membrane-spanning regions, whilst the urea transporters are thought to contain at least 10 membrane spanning segments. 9. Very little work has examined the ontogeny of these proteins, except in the rat, and virtually nothing is known of the expression of these genes in pregnancy or in any disorder of fluid balance in the mother or foetus.

Molecular cloning and characterization of an insect aquaporin functional comparison with aquaporin 1

We previously described the structural organization of P25, a member of the major-intrinsic-protein family found in the digestive tract of homopteran sap-sucking insects [Beuron, F., Le Cahérec, F., Guillam, M. T., Cavalier, A., Garret, A., Tassan, J. P., Delamarche, C., Schultz, P., Mallouh, V., Rolland, J. P., Hubert, J.F., Gouranton, J. & Thomas, D. (1995) J. Biol. Chem. 270, 17414-17422]. We demonstrated, by means of introducing P25 tetramers into the membranes of Xenopus oocytes, that this protein exhibits functional properties similar to those of aquaporin 1, the archetypal water channel [Le Cahérec, F., Bron, P., Verbavatz, J. M., Garret, A., Morel, G., Cavalier, A., Bonnec, G., Thomas, D., Gouranton, J. & Hubert, J.F. (1996) J. Cell Sci. 109, 1285-1295]. In the present work, we cloned a full-length cDNA from a Cicadella viridis library with an open reading frame of 765 bp that encoded a 26-kDa protein whose sequence was 43, 40, 36 and 36% identical to aquaporins 1, 2, z and tonoplast intrinsic protein gamma, respectively. Translation of the corresponding RNA in Xenopus oocytes generated a polypeptide that was specifically recognized by polyclonal antibodies raised against native P25. Expression of the protein in Xenopus oocyte membranes was assessed by immunocytochemistry and led to a 15-fold increase of osmotic membrane water permeability. This increase was inhibited by HgCl2. The permeability had an Arrhenius activation energy of 11.7 kJ/mol. We called this protein Cicadella aquaporin (AQPcic). The oocytes expressing Cicadella aquaporin were less sensitive to HgCl2 than oocytes expressing aquaporin 1. In the Xenopus oocyte system, Cicadella aquaporin failed to transport glycerol, urea and ions. It exhibited permeabilities to ethylene glycol and formamide similar to those measured for aquaporin 1 under the same conditions.