Roles of basolateral solute uptake via NKCC1 and of myosin II in vasopressin-induced cell swelling in inner medullary collecting duct

Using CRISPR-Cas9/phosphoproteomics to identify substrates of calcium/calmodulin-dependent kinase 2δ

Ca2+/Calmodulin-dependent protein kinase 2 (CAMK2) family proteins are involved in the regulation of cellular processes in a variety of tissues including brain, heart, liver, and kidney. One member, CAMK2δ (CAMK2D), has been proposed to be involved in vasopressin signaling in the renal collecting duct, which controls water excretion through regulation of the water channel aquaporin-2 (AQP2). To identify CAMK2D target proteins in renal collecting duct cells (mpkCCD), we deleted Camk2d and carried out LC-MS/MS-based quantitative phosphoproteomics. Specifically, we used CRISPR/Cas9 with two different guide RNAs targeting the CAMK2D catalytic domain to create multiple CAMK2D KO cell lines. AQP2 protein abundance was lower in the CAMK2D KO cells than in CAMK2D-intact controls. AQP2 phosphorylation at Ser256 and Ser269 (normalized for total AQP2) was decreased. However, trafficking of AQP2 to and from the apical plasma membrane was sustained. Large-scale quantitative phosphoproteomic analysis (TMT-labeling) in the presence of the vasopressin analog dDAVP (0.1 nM, 30 min) allowed quantification of 11,570 phosphosites of which 169 were significantly decreased, while 206 were increased in abundance in CAMK2D KO clones. These data are available for browsing or download at https://esbl.nhlbi.nih.gov/Databases/CAMK2D-proteome/. Motif analysis of the decreased phosphorylation sites revealed a target preference of -(R/K)-X-X-p(S/T)-X-(D/E), matching the motif identified in previous in vitro phosphorylation studies using recombinant CAMK2D. Thirty five of the significantly downregulated phosphorylation sites in CAMK2D KO cells had exactly this motif and are judged to be likely direct CAMK2D targets. This adds to the list of known CAMK2D target proteins found in prior reductionist studies.

Data resource: vasopressin-regulated protein phosphorylation sites in the collecting duct

Vasopressin controls renal water excretion through actions to regulate aquaporin-2 (AQP2) trafficking, transcription, and degradation. These actions are in part dependent on vasopressin-induced phosphorylation changes in collecting duct cells. Although most efforts have focused on the phosphorylation of AQP2 itself, phosphoproteomic studies have identified many vasopressin-regulated phosphorylation sites in proteins other than AQP2. The goal of this bioinformatics-based review is to create a compendium of vasopressin-regulated phosphorylation sites with a focus on those that are seen in both native rat inner medullary collecting ducts and cultured collecting duct cells from the mouse (mpkCCD), arguing that these sites are the best candidates for roles in AQP2 regulation. This analysis identified 51 vasopressin-regulated phosphorylation sites in 45 proteins. We provide resource web pages at https://esbl.nhlbi.nih.gov/Databases/AVP-Phos/ and https://esbl.nhlbi.nih.gov/AVP-Network/, listing the phosphorylation sites and describing annotated functions of each of the vasopressin-targeted phosphoproteins. Among these sites are 23 consensus protein kinase A (PKA) sites that are increased in response to vasopressin, consistent with a central role for PKA in vasopressin signaling. The remaining sites are predicted to be phosphorylated by other kinases, most notably ERK1/2, which accounts for decreased phosphorylation at sites with a X-p(S/T)-P-X motif. Additional protein kinases that undergo vasopressin-induced changes in phosphorylation are Camkk2, Cdk18, Erbb3, Mink1, and Src, which also may be activated directly or indirectly by PKA. The regulated phosphoproteins are mapped to processes that hypothetically can account for vasopressin-mediated control of AQP2 trafficking, cytoskeletal alterations, and Aqp2 gene expression, providing grist for future studies.NEW & NOTEWORTHY Vasopressin regulates renal water excretion through control of the aquaporin-2 water channel in collecting duct cells. Studies of vasopressin-induced protein phosphorylation have focused mainly on the phosphorylation of aquaporin-2. This study describes 44 phosphoproteins other than aquaporin-2 that undergo vasopressin-mediated phosphorylation changes and summarizes potential physiological roles of each.

Loss of NKCC1 function increases epithelial tight junction permeability by upregulating claudin-2 expression

Conditions that cause the loss of epithelial barrier integrity are often accompanied by dysregulation of tight junction protein expression and/or localization. Recently, we have reported that patients with mutations in SLC12A2, the gene encoding the basolateral Na+-K+-2Cl- cotransporter (NKCC1), suffer from severe gastrointestinal deficits, including chronic gastrointestinal inflammation, gastrointestinal hemorrhage, intestinal obstruction, and constipation. Although the intestinal inflammation observed in patients with loss of NKCC1 function may or may not be due to tight junction dysfunction, we investigated whether the loss of NKCC1 function affects paracellular ion transport and epithelial barrier function. Wild-type HT29-MTX-E12 and CRISPR/Cas9-mediated NKCC1 knockout (KO) HT29 clones were tested for tight junction protein expression and localization. Tightness of epithelial cell monolayer was assessed by measurement of transepithelial electrical resistance and permeability of molecular tracers in transwell filters. Tight junction protein localization was assessed by immunofluorescence. Loss of NKCC1 expression strongly increases the expression of claudin-2 and occludin in epithelial cell monolayers. Loss of NKCC1 significantly reduces the transepithelial electrical resistance (TER) indicating an increase in paracellular ions flux, consistent with upregulation of the cation-selective and channel-forming claudin-2. In addition, NKCC1-KO monolayers showed a significant increase in the paracellular flux of small molecules like fluorescein (0.33 kDa), whereas the permeability of higher molecular weight TRITC-Dextran (4 kDa and 70 kDa) remained unchanged. Thus, NKCC1 regulates tight junction protein expression and loss of NKCC1 function affects epithelial barrier integrity.

Phosphoproteomic Identification of Vasopressin/cAMP/Protein Kinase A-Dependent Signaling in Kidney

Water excretion by the kidney is regulated by the neurohypophyseal peptide hormone vasopressin through actions in renal collecting duct cells to regulate the water channel protein aquaporin-2. Vasopressin signaling is initiated by binding to a G-protein-coupled receptor called V2R, which signals through heterotrimeric G-protein subunit Gs α, adenylyl cyclase 6, and activation of the cAMP-regulated protein kinase (PKA). Signaling events coupling PKA activation and aquaporin-2 regulation were largely unknown until the advent of modern protein mass spectrometry techniques that allow proteome-wide quantification of protein phosphorylation changes (phosphoproteomics). This short review documents phosphoproteomic findings in collecting duct cells describing the response to V2R-selective vasopressin agonists and antagonists, the response to CRISPR-mediated deletion of PKA, results from in vitro phosphorylation studies using recombinant PKA, the response to the broad-spectrum kinase inhibitor H89 (N-[2-p-bromocinnamylamino-ethyl]-5-isoquinolinesulphonamide), and the responses underlying lithium-induced nephrogenic diabetes insipidus. These phosphoproteomic data sets have been made available online for modeling vasopressin signaling and signaling downstream from other G-protein-coupled receptors. SIGNIFICANCE STATEMENT: New developments in protein mass spectrometry are facilitating progress in identification of signaling networks. Using mass spectrometry, it is now possible to identify and quantify thousands of phosphorylation sites in a given cell type (phosphoproteomics). The authors describe the use of phosphoproteomics technology to identify signaling mechanisms downstream from a G-protein-coupled receptor, the vasopressin V2 subtype receptor, and its role of the regulation and dysregulation of water excretion in the kidney. Data from multiple phosphoproteomic data sets are provided as web-based resources.

Purinergic signaling is enhanced in the absence of UT-A1 and UT-A3

ATP is an important paracrine regulator of renal tubular water and urea transport. The activity of P2Y2 , the predominant P2Y receptor of the medullary collecting duct, is mediated by ATP, and modulates urinary concentration. To investigate the role of purinergic signaling in the absence of urea transport in the collecting duct, we studied wild-type (WT) and UT-A1/A3 null (UT-A1/A3 KO) mice in metabolic cages to monitor urine output, and collected tissue samples for analysis. We confirmed that UT-A1/A3 KO mice are polyuric, and concurrently observed lower levels of urinary cAMP as compared to WT, despite elevated serum vasopressin (AVP) levels. Because P2Y2 inhibits AVP-stimulated transport by dampening cAMP synthesis, we suspected that, similar to other models of AVP-resistant polyuria, purinergic signaling is increased in UT-A1/A3 KO mice. In fact, we observed that both urinary ATP and purinergic-mediated prostanoid (PGE2 ) levels were elevated. Collectively, our data suggest that the reduction of medullary osmolality due to the lack of UT-A1 and UT-A3 induces an AVP-resistant polyuria that is possibly exacerbated by, or at least correlated with, enhanced purinergic signaling.

CRISPR-Cas9/phosphoproteomics identifies multiple noncanonical targets of myosin light chain kinase

Prior studies have implicated myosin light chain kinase (MLCK) in the regulation of aquaporin-2 (AQP2) in the renal collecting duct. To discover signaling targets of MLCK, we used CRISPR-Cas9 to delete the MLCK gene (Mylk) to obtain MLCK-null mpkCCD cells and carried out comprehensive phosphoproteomics using stable isotope labeling with amino acids in cell culture for quantification. Immunocytochemistry and electron microscopy demonstrated a defect in the processing of AQP2-containing early endosomes to late endosomes. The phosphoproteomics experiments revealed that, of the 1,743 phosphopeptides quantified over multiple replicates, 107 were changed in abundance by MLCK deletion (29 decreased and 78 increased). One of the decreased phosphopeptides corresponded to the canonical target site in myosin regulatory light chain. Network analysis indicated that targeted phosphoproteins clustered into distinct structural/functional groups: actomyosin, signaling, nuclear envelope, gene transcription, mRNA processing, energy metabolism, intermediate filaments, adherens junctions, and tight junctions. There was significant overlap between the derived MLCK signaling network and a previously determined PKA signaling network. The presence of multiple proteins in the actomyosin category prompted experiments showing that MLCK deletion inhibits the normal effect of vasopressin to depolymerize F-actin, providing a potential explanation for the AQP2 trafficking defect. Changes in phosphorylation of multiple proteins in the nuclear envelope prompted measurement of nuclear size, showing a significant increase in average nuclear volume. We conclude that MLCK is part of a multicomponent signaling pathway in both the cytoplasm and nucleus that includes much more than simple regulation of conventional nonmuscle myosins through myosin regulatory light chain phosphorylation.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Roflumilast and aquaporin-2 regulation in rat renal inner medullary collecting duct

Roflumilast is a cyclic nucleotide phosphodiesterase inhibitor that is FDA‐approved for treatment of chronic obstructive pulmonary disease. With a view toward possible use for treatment of patients with X‐linked nephrogenic diabetes insipidus (NDI) due to hemizygous mutations in the V2 vasopressin receptor, this study sought to determine the effect of roflumilast on aquaporin‐2 (AQP2) phosphorylation, AQP2 trafficking, and water permeability in the rat inner medullary collecting duct (IMCD). In the presence of the vasopressin analog dDAVP (0.1 nmol/L), both roflumilast and its active metabolite roflumilast N‐oxide (RNO) significantly increased phosphorylation at S256, S264, and S269, and decreased phosphorylation at S261 (immunoblotting) in IMCD suspensions in a dose‐dependent manner (3–3000 nmol/L). Another commonly used phosphodiesterase inhibitor, IBMX, affected phosphorylation only at the highest concentration in this range. However, neither roflumilast nor RNO had an effect on AQP2 phosphorylation in the absence of vasopressin. Furthermore, roflumilast alone did not increase AQP2 trafficking to the plasma membrane (immunofluorescence) or increase water permeability in freshly microdissected perfused IMCD segments. We conclude that roflumilast can be used to enhance vasopressin's action on AQP2 activity in the renal collecting duct, but has no detectable effect in the absence of vasopressin. These findings suggest that roflumilast may not have a beneficial effect in X‐linked NDI, but could find useful application in acquired NDI.

Vasopressin-induced serine 269 phosphorylation reduces Sipa1l1 (signal-induced proliferation-associated 1 like 1)-mediated aquaporin-2 endocytosis

The abundance of integral membrane proteins in the plasma membrane is determined by a dynamic balance between exocytosis and endocytosis, which can often be regulated by physiological stimuli. Here, we describe a mechanism that accounts for the ability of the peptide hormone vasopressin to regulate water excretion via a phosphorylation-dependent modulation of the PDZ domain-ligand interaction involving the water channel protein aquaporin-2. We discovered that the PDZ domain-containing protein Sipa1l1 (signal-induced proliferation-associated 1 like 1) binds to the cytoplasmic PDZ-ligand motif of aquaporin-2 and accelerates its endocytosis in the absence of vasopressin. Vasopressin-induced aquaporin-2 phosphorylation within the type I PDZ-ligand motif disrupted the interaction, in association with reduced aquaporin-2 endocytosis and prolonged plasma membrane aquaporin-2 retention. This phosphorylation-dependent alteration in the PDZ domain-ligand interaction was explained by 3D structural models, which showed a hormone-regulated mechanism that controls osmotic water transport and systemic water balance in mammals.

A knowledge base of vasopressin actions in the kidney

Biological information is growing at a rapid pace, making it difficult for individual investigators to be familiar with all information that is relevant to their own research. Computers are beginning to be used to extract and curate biological information; however, the complexity of human language used in research papers continues to be a critical barrier to full automation of knowledge extraction. Here, we report a manually curated knowledge base of vasopressin actions in renal epithelial cells that is designed to be readable either by humans or by computer programs using natural language processing algorithms. The knowledge base consists of three related databases accessible at https://helixweb.nih.gov/ESBL/TinyUrls/Vaso_portal.html. One of the component databases reports vasopressin actions on individual proteins expressed in renal epithelia, including effects on phosphorylation, protein abundances, protein translocation from one subcellular compartment to another, protein-protein binding interactions, etc. The second database reports vasopressin actions on physiological measures in renal epithelia, and the third reports specific mRNA species whose abundances change in response to vasopressin. We illustrate the application of the knowledge base by using it to generate a protein kinase network that connects vasopressin binding in collecting duct cells to physiological effects to regulate the water channel protein aquaporin-2.

Ultrastructural correlates of antidiuretic response of rat kidney

We studied ultrastructural features of epithelial cells of the inner medullary collecting tubules in Brattleboro and Wistar under the action of desmopressin (dDAVP, 5 μg/100 g body weight for 2 days). Intracellular reorganization of transepithelial barrier for osmotic water transport depended on the capacity of rats to the synthesis of endogenous vasopressin.

Vasopressin and the regulation of aquaporin-2

Water excretion is regulated in large part through the regulation of osmotic water permeability of the renal collecting duct epithelium. Water permeability is controlled by vasopressin through regulation of the water channel, aquaporin-2 (AQP2). Two processes contribute: (1) regulation of AQP2 trafficking to the apical plasma membrane; and (2) regulation of the total amount of the AQP2 protein in the cells. Regulation of AQP2 abundance is defective in several water-balance disorders, including many polyuric disorders and the syndrome of inappropriate antidiuresis. Here we review vasopressin signaling in the renal collecting duct that is relevant to the two modes of water permeability regulation.

Regulatory volume decrease of rat kidney principal cells after successive hypo-osmotic shocks

Outer Medullary Collecting Duct (OMCD) principal cells are exposed to significant changes of the extracellular osmolarity and thus the analysis of their regulatory volume decrease (RVD) function is of great importance in order to avoid cell membrane rupture and subsequent death. In this paper we provide a sub-second temporal analysis of RVD events occurring after two successive hypo-osmotic challenges in rat kidney OMCD principal cells. We performed experimental cell volume measurements and created a mathematical model based on our experimental results. As a consequence of RVD the cell expels part of intracellular osmolytes and reduces the permeability of the plasma membrane to water. The next osmotic challenge does not cause significant RVD if it occurs within a minute after the primary shock. In such a case the cell reacts as an ideal osmometer. Through our model we provide the basis for further detailed studies on RVD dynamical modeling.

Morphological characteristics of the inner medullary zone in the kidneys of Brattleboro and Wistar rats during blockade of prostaglandin synthesis

Morphological characteristics of the inner medullary zone in the kidneys in Brattleboro and Wistar rats were studied during blockade of prostaglandin synthesis. The absence of a diuretic effect of prostaglandins on the kidneys was accompanied by structural changes in the transepithelial and interstitial barriers for an osmotic flow of water.

Monitoring single-channel water permeability in polarized cells

So far the determination of unitary permeability (p_f) of water channels that are expressed in polarized cells is subject to large errors because the opening of a single water channel does not noticeably increase the water permeability of a membrane patch above the background. That is, in contrast to the patch clamp technique, where the single ion channel conductance may be derived from a single experiment, two experiments separated in time and/or space are required to obtain the single-channel water permeability p_f as a function of the incremental water permeability (P_f,c) and the number (n) of water channels that contributed to P_f,c. Although the unitary conductance of ion channels is measured in the native environment of the channel, p_f is so far derived from reconstituted channels or channels expressed in oocytes. To determine the p_f of channels from live epithelial monolayers, we exploit the fact that osmotic volume flow alters the concentration of aqueous reporter dyes adjacent to the epithelia. We measure these changes by fluorescence correlation spectroscopy, which allows the calculation of both P_f,c and osmolyte dilution within the unstirred layer. Shifting the focus of the laser from the aqueous solution to the apical and basolateral membranes allowed the FCS-based determination of n. Here we validate the new technique by determining the p_f of aquaporin 5 in Madin-Darby canine kidney cell monolayers. Because inhibition and subsequent activity rescue are monitored on the same sample, drug effects on exocytosis or endocytosis can be dissected from those on p_f.

Regulation of renal NaCl and water transport by the ATP/UTP/P2Y2 receptor system

Extracellular nucleotides (e.g., ATP) activate ionotropic P2X and metabotropic P2Y receptors in the plasma membrane to regulate and maintain cell function and integrity. This includes the renal tubular and collecting duct system, where the locally released nucleotides act in a paracrine and autocrine way to regulate transport of electrolytes and water and maintain cell volume. A prominent role has been assigned to Gq-coupled P2Y(2) receptors, which are typically activated by both ATP and UTP. Studies in gene knockout mice revealed an antihypertensive activity of P2Y(2) receptors that is linked to vasodilation and an inhibitory influence on renal salt reabsorption. Flow induces apical ATP release in the thick ascending limb, and first evidence indicates an inhibitory influence of P2Y(2) receptor tone on the expression and activity of the Na-K-2Cl cotransporter NKCC2 in this segment. The apical ATP/UTP/P2Y(2) receptor system in the connecting tubule/cortical collecting duct mediates the inhibitory effect of dietary salt on the open probability of the epithelial sodium channel ENaC and inhibits ENaC activity during aldosterone escape. Connexin 30 has been implicated in the luminal release of the ATP involved in the regulation of ENaC. An increase in collecting duct cell volume in response to manipulating water homeostasis increases ATP release. The subsequent activation of P2Y(2) receptors inhibits vasopressin-induced cAMP formation and water reabsorption, which facilitates water excretion and stabilizes cell volume. Thus recent studies have established the ATP/UTP/P2Y(2) receptor system as a relevant regulator of renal salt and water homeostasis and blood pressure regulation. The pathophysiological relevance and therapeutic potential remains to be determined, but dual effects of P2Y(2) receptor activation on both the vasculature and renal salt reabsorption implicate these receptors as potential therapeutic targets in hypertension.

Quantitative protein and mRNA profiling shows selective post-transcriptional control of protein expression by vasopressin in kidney cells

Previous studies in yeast have supported the view that post-transcriptional regulation of protein abundances may be more important than previously believed. Here we ask the question: "In a physiological regulatory process (the response of mammalian kidney cells to the hormone vasopressin), what fraction of the expressed proteome undergoes a change in abundance and what fraction of the regulated proteins have corresponding changes in mRNA levels?" In humans and other mammals, vasopressin fulfills a vital homeostatic role (viz. regulation of renal water excretion) by regulating the water channel aquaporin-2 in collecting duct cells. To address the question posed, we utilized large-scale quantitative protein mass spectrometry (LC-MS/MS) employing stable isotopic labeling in cultured mpkCCD cells ('SILAC') coupled with transcriptomic profiling using oligonucleotide expression arrays (Affymetrix). Preliminary studies analyzing two nominally identical control samples by SILAC LC-MS/MS yielded a relative S.D. of 13% (for ratios), establishing the precision of the SILAC approach in our hands. We quantified nearly 3000 proteins with nontargeted SILAC LC-MS/MS, comparing vasopressin- versus vehicle-treated samples. Of these proteins 786 of them were quantified in each of 3 experiments, allowing statistical analysis and 188 of these showed significant vasopressin-induced changes in abundance, including aquaporin-2 (20-fold increase). Among the proteins with statistically significant abundance changes, a large fraction (at least one-third) was found to lack changes in the corresponding mRNA species (despite sufficient statistical power), indicating that post-transcriptional regulation of protein abundance plays an important role in the vasopressin response. Bioinformatic analysis of the regulated proteins (versus all transcripts) shows enrichment of glutathione S-transferase isoforms as well as proteins involved in organization of the actin cytoskeleton. The latter suggests that long-term regulatory processes may contribute to actomyosin-dependent trafficking of the water channel aquaporin-2. The results provide impetus for increased focus on translational regulation and regulation of protein degradation in physiological control in mammalian epithelial cells.

Intrarenal purinergic signaling in the control of renal tubular transport

Renal tubular epithelial cells receive hormonal input that regulates volume and electrolyte homeostasis. In addition, numerous intrarenal, local signaling agonists have appeared on the stage of renal physiology. One such system is that of intrarenal purinergic signaling. This system involves all the elements necessary for agonist-mediated intercellular communication. ATP is released from epithelial cells, which activates P2 receptors in the apical and basolateral membrane and thereby modulates tubular transport. Termination of the signal is conducted via the breakdown of ATP to adenosine. Recent far-reaching advances indicate that ATP is often used as a local transmitter for classical sensory transduction. This transmission apparently also applies to sensory functions in the kidney. Locally released ATP is involved in sensing of renal tubular flow or in detecting the distal tubular load of NaCl at the macula densa. This review describes the relevant aspects of local, intrarenal purinergic signaling and outlines its integrative concepts.

Aquaporin-2 in the "-omics" era

Vasopressin controls renal water excretion largely through actions to regulate the water channel aquaporin-2 in collecting duct principal cells. Our knowledge of the mechanisms involved has increased markedly in recent years with the advent of methods for large-scale systems-level profiling such as protein mass spectrometry, yeast two-hybrid analysis, and oligonucleotide microarrays. Here we review this progress.

Vasopressin and hyperosmolality regulate NKCC1 expression in rat OMCD

Secretory-type Na-K-2Cl cotransporter (NKCC1) is known to play roles in both acid and sodium excretion, and is more abundant in dehydration. To determine the mechanisms by which dehydration stimulates NKCC1 expression, the effects of vasopressin, oxytocin and hyperosmolality on NKCC1 mRNA and protein expressions in the outer medullary collecting duct (OMCD) of rats were investigated using RT-competitive PCR and western blot analysis. Microdissected OMCD was incubated in isotonic or hypertonic solution, or with AVP or oxytocin for 60 min at 37 degrees C. Hyperosmolality induced by NaCl, mannitol or raffinose increased NKCC1 mRNA expression in OMCD by 130-240% in vitro. The stimulation of NKCC1 mRNA expression by NaCl was highest at 690 mosmol kg(-1) H(2)O and gradually decreased at higher osmolalities. The incubation of OMCD with AVP (10(-7) M) for 60 min increased NKCC1 mRNA expression by 100%. The administration of AVP to rats for 4 days using an osmotic mini-pump also increased NKCC1 mRNA and protein expressions in OMCD by 130%. In contrast, oxytocin (10(-7) M) did not stimulate the NKCC1 mRNA expression in OMCD in vitro. Chronic injection of oxytocin increased the NKCC1 mRNA expression by 36%. These data showed that hyperosmolality and vasopressin stimulate NKCC1 mRNA and protein expressions in rat OMCD. It is concluded that NKCC1 expression is regulated directly and indirectly by vasopressin.

Large-scale quantitative LC-MS/MS analysis of detergent-resistant membrane proteins from rat renal collecting duct

In the renal collecting duct, vasopressin controls transport of water and solutes via regulation of membrane transporters such as aquaporin-2 (AQP2) and the epithelial urea transporter UT-A. To discover proteins potentially involved in vasopressin action in rat kidney collecting ducts, we enriched membrane "raft" proteins by harvesting detergent-resistant membranes (DRMs) of the inner medullary collecting duct (IMCD) cells. Proteins were identified and quantified with LC-MS/MS. A total of 814 proteins were identified in the DRM fractions. Of these, 186, including several characteristic raft proteins, were enriched in the DRMs. Immunoblotting confirmed DRM enrichment of representative proteins. Immunofluorescence confocal microscopy of rat IMCDs with antibodies to DRM proteins demonstrated heterogeneity of raft subdomains: MAL2 (apical region), RalA (predominant basolateral labeling), caveolin-2 (punctate labeling distributed throughout the cells), and flotillin-1 (discrete labeling of large intracellular structures). The DRM proteome included GPI-anchored, doubly acylated, singly acylated, cholesterol-binding, and integral membrane proteins (IMPs). The IMPs were, on average, much smaller and more hydrophobic than IMPs identified in non-DRM-enriched IMCD. The content of serine 256-phosphorylated AQP2 was greater in DRM than in non-DRM fractions. Vasopressin did not change the DRM-to-non-DRM ratio of most proteins, whether quantified by tandem mass spectrometry (LC-MS/MS, n=22) or immunoblotting (n=6). However, Rab7 and annexin-2 showed small increases in the DRM fraction in response to vasopressin. In accord with the long-term goal of creating a systems-level analysis of transport regulation, this study has identified a large number of membrane-associated proteins expressed in the IMCD that have potential roles in vasopressin action.