The 17 kDa band identified by multiple anti-aquaporin 2 antisera in rat kidney medulla is a histone

Deep proteomic profiling of vasopressin-sensitive collecting duct cells. I. Virtual Western blots and molecular weight distributions

The mouse mpkCCD cell line is a continuous cultured epithelial cell line with characteristics of renal collecting duct principal cells. This line is widely used to study epithelial transport and its regulation. To provide a data resource useful for experimental design and interpretation in studies using mpkCCD cells, we have carried out "deep" proteomic profiling of these cells using three levels of fractionation (differential centrifugation, SDS-PAGE, and HPLC) followed by tandem mass spectrometry to identify and quantify proteins. The analysis of all resulting samples generated 34.6 gigabytes of spectral data. As a result, we identified 6,766 proteins in mpkCCD cells at a high level of stringency. These proteins are expressed over eight orders of magnitude of protein abundance. The data are provided to users as a public data base (https://helixweb.nih.gov/ESBL/Database/mpkFractions/). The mass spectrometry data were mapped back to their gel slices to generate "virtual Western blots" for each protein. For most of the 6,766 proteins, the apparent molecular weight from SDS-PAGE agreed closely with the calculated molecular weight. However, a substantial fraction (>15%) of proteins was found to run aberrantly, with much higher or much lower mobilities than predicted. These proteins were analyzed to identify mechanisms responsible for altered mobility on SDS-PAGE, including high or low isoelectric point, high or low hydrophobicity, physiological cleavage, residence in the lysosome, posttranslational modifications, and expression of alternative isoforms due to alternative exon usage. Additionally, this analysis identified a previously unrecognized isoform of aquaporin-2 with apparent molecular mass <20 kDa.

Quantitative analysis of aquaporin-2 phosphorylation

The action of vasopressin in rodent collecting ducts to regulate water permeability depends in part on increases in phosphorylation of the water channel aquaporin-2 (AQP2) at three sites: Ser256, Ser264, and Ser269. Previous studies of AQP2 phosphorylation have depended largely on qualitative data using protein mass spectrometry and phospho-specific antibodies. Here, we use a new method employing phospho-specific antibodies to determine the percentage of total AQP2 phosphorylated at each site in the presence and absence of the V2-receptor-selective vasopressin analog dDAVP in rat renal inner medullary collecting duct (IMCD) and cultured mpkCCD cells. Phosphorylation of Ser269, a site previously implicated in plasma membrane retention, was found to increase from 3 to 26% of total AQP2 in rat IMCD cells following dDAVP. Quantification of immunogold labeling of the opposite kidneys from the same rats estimated that 11% of total AQP2 is present in the apical plasma membrane (APM) without injection of dDAVP and 25% is present in the APM after dDAVP. Surprisingly, the baseline level of Ser256 phosphorylation was constitutively high, and there was no increase with dDAVP (confirmed in 2 more sets of rats). In general, Ser264 phosphorylation remained below 5% of total. The pattern of response was similar in cultured mpkCCD cells (large increase in Ser269 phosphorylation following dDAVP, but constitutively high levels of Ser256 phosphorylation). We suggest from these studies that Ser269 phosphorylation may be a more consistent indicator of vasopressin action and AQP2 membrane abundance than is Ser256 phosphorylation.

Membrane organization and function of M1 and M23 isoforms of aquaporin-4 in epithelial cells

Aquaporin-4 (AQP4) water channels exist as heterotetramers of M1 and M23 splice variants and appear to be present in orthogonal arrays of intramembraneous particles (OAPs) visualized by freeze-fracture microscopy. We report that AQP4 forms OAPs in rat gastric parietal cells but not in parietal cells from the mouse or kangaroo rat. Furthermore, the organization of principal cell OAPs in Brattleboro rat kidney is perturbed by vasopressin (arginine vasopressin). Membranes of LLC-PK1cells expressing M23-AQP4 showed large, abundant OAPs, but none were detectable in cells expressing M1-AQP4. Measurements of osmotic swelling of transfected LLC-PK1cells using videomicroscopy, gave osmotic water permeability coefficient ( Pf) values (in cm/s) of 0.018 (M1-AQP4), 0.019 (M23-AQP4), and 0.003 (control). Quantitative immunoblot and immunofluorescence showed an eightfold greater expression of M1- over M23-AQP4 in the cell lines, suggesting that single-channel pf(cm3/s) is much greater for the M23 variant. Somatic fusion of M1- and M23-AQP4 cells ( Pf= 0.028 cm/s) yielded OAPs that were fewer and smaller than in M23 cells alone, and M1-to-M23 expression ratios (∼1:4) normalized to AQP4 in M1 or M23 cells indicated a reduced single-channel pffor the M23 variant. Expression of an M23-AQP4-Ser111Emutant produced ∼1.5-fold greater single-channel pfand OAPs that were up to 2.5-fold larger than wild-type M23-AQP4 OAPs, suggesting that a putative PKA phosphorylation site Ser111is involved in OAP formation. We conclude that the higher-order organization of AQP4 in OAPs increases single-channel osmotic water permeability by one order of magnitude and that differential cellular expression levels of the two isoforms could regulate this organization.

Identification of a novel A-kinase anchoring protein 18 isoform and evidence for its role in the vasopressin-induced aquaporin-2 shuttle in renal principal cells

Arginine vasopressin (AVP) increases the water permeability of renal collecting duct principal cells by inducing the fusion of vesicles containing the water channel aquaporin-2 (AQP2) with the plasma membrane (AQP2 shuttle). This event is initiated by activation of vasopressin V2 receptors, followed by an elevation of cAMP and the activation of protein kinase A (PKA). The tethering of PKA to subcellular compartments by protein kinase A anchoring proteins (AKAPs) is a prerequisite for the AQP2 shuttle. During the search for AKAP(s) involved in the shuttle, a new splice variant of AKAP18, AKAP18delta, was identified. AKAP18delta functions as an AKAP in vitro and in vivo. In the kidney, it is mainly expressed in principal cells of the inner medullary collecting duct, closely resembling the distribution of AQP2. It is present in both the soluble and particulate fractions derived from renal inner medullary tissue. Within the particulate fraction, AKAP18delta was identified on the same intracellular vesicles as AQP2 and PKA. AVP not only recruited AQP2, but also AKAP18delta to the plasma membrane. The elevation of cAMP caused the dissociation of AKAP18delta and PKA. The data suggest that AKAP18delta is involved in the AQP2 shuttle.

Osmolality and solute composition are strong regulators of AQP2 expression in renal principal cells

The water permeability of the renal collecting duct is regulated by the insertion of aquaporin-2 (AQP2) into the apical plasma membrane of epithelial (principal) cells. Using primary cultured epithelial cells from the inner medulla of rat kidney (IMCD cells), we show that osmolality and solute composition are potent regulators of AQP2 mRNA and protein synthesis, as well as the classical cAMP-dependent pathway, but do not affect the arginine vasopressin-induced AQP2 shuttle. In the presence of the cAMP analog dibutyryl cAMP (DBcAMP, 500 microM), NaCl and sorbitol, but not urea, evoked a robust increase of AQP2 expression in IMCD cells, with NaCl being far more potent than sorbitol. cAMP-responsive element-binding protein phosphorylation increased with DBcAMP concentrations but was not altered by changes in osmolality. In the rat and human AQP2 promoter, we identified a putative tonicity-responsive element. We conclude that, in addition to the arginine vasopressin/cAMP-signaling cascade, a further pathway activated by elevated effective osmolality (tonicity) is crucial for the expression of AQP2 in IMCD cells, and we suggest that the effect is mediated via the tonicity-responsive element.

AQP2 is a substrate for endogenous PP2B activity within an inner medullary AKAP-signaling complex

We have demonstrated that inner medullary collecting duct (IMCD) heavy endosomes purified from rat kidney IMCD contain the type II protein kinase A (PKA) regulatory subunit (RII), protein phosphatase (PP)2B, PKCzeta, and an RII-binding protein (relative molecular mass ~90 kDa) representing a putative A kinase anchoring protein (AKAP). Affinity chromatography of detergent-solubilized endosomes on cAMP-agarose permits recovery of a protein complex consisting of the 90-kDa AKAP, RII, PP2B, and PKCzeta. With the use of small-particle flow cytometry, RII and PKCzeta were localized to an identical population of endosomes, suggesting that these proteins are components of an endosomal multiprotein complex. (32)P-labeled aquaporin-2 (AQP2) present in these PKA-phosphorylated endosomes was dephosphorylated in vitro by either addition of exogenous PP2B or by an endogenous endosomal phosphatase that was inhibited by the PP2B inhibitors EDTA and the cyclophilin-cyclosporin A complex. We conclude that IMCD heavy endosomes possess an AKAP multiprotein-signaling complex similar to that described previously in hippocampal neurons. This signaling complex potentially mediates the phosphorylation of AQP2 to regulate its trafficking into the IMCD apical membrane. In addition, the PP2B component of the AKAP-signaling complex could also dephosphorylate AQP2 in vivo.

Aquaporin-2 expression in primary cultured rat inner medullary collecting duct cells

Cultured renal epithelial cells rapidly downregulate expression of the vasopressin-regulated water channel aquaporin-2 (AQP-2). Our aim was to define conditions that favor maintenance of AQP-2 expression in vitro without genetic manipulation. We show here that primary cultures of rat inner medullary collecting duct (IMCD) cells retain AQP-2 expression for at least 6 days when grown with dibutyryl cAMP (DBcAMP) supplementation. We also found that coating the culture dishes with type IV collagen, rather than rat-tail collagen, retards AQP-2 downregulation. Immunofluorescence and biochemical studies indicate a shuttling of AQP-2-bearing vesicles after stimulation with vasopressin or forskolin. Rab3 proteins, known to be involved in regulated exocytosis, were detected only in cells grown in the presence of DBcAMP. Using the adenylyl cyclase assay, we confirmed the functional integrity of the vasopressin V2 receptor in a broken cell preparation. Our data show that cAMP supplementation is sufficient for the maintenance of AQP-2 expression in primary cultured cells. The model system established here allows the study of the regulation of genes encoding the antidiuretic machinery at the cellular level.

Vasopressin-elicited water and urea permeabilities are altered in IMCD in hypercalcemic rats

To investigate how hypercalcemia blunts renal concentrating ability, alterations in basal and arginine vasopressin (AVP)-elicited osmotic water (Pf) and urea (Purea) permeabilities were measured in isolated perfused terminal inner medullary collecting ducts (IMCD) from control and chronically hypercalcemic rats after dihydrotachysterol (DHT) (M. Levi, L. Peterson, and T. Berl. Kidney Int. 23: 489-497, 1983) treatment. The IMCD Pf of DHT-treated rats did not increase significantly after AVP and was accompanied by a significant 87 +/- 4% reduction in aquaporin-2 (AQP-2) protein but not mRNA. In contrast, both basal and AVP-elicited IMCD Purea from DHT rats were significantly increased and accompanied by a significant 41 +/- 11% increase in AVP-regulated urea transporter protein content. Immunoblotting with anti-calcium/polyvalent cation-sensing receptor protein (CaR) antiserum revealed specific alterations in CaR bands in endosomes purified from the apical membranes of inner medulla of DHT rats. These data are the first detailed analyses of hypercalcemia-induced alterations in AVP-regulated permeabilities and membrane transporters in IMCD. We conclude that selective alterations in IMCD transport occur in hypercalcemia, permitting the body to dispose of excess calcium without forming calcium-containing renal stones.