Electrophysiological study of luminal and basolateral vasopressin in rabbit cortical collecting duct

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 G_s 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.

Vasopressin Increases Urinary Acidification via V1a Receptors in Collecting Duct Intercalated Cells

BACKGROUND

Antagonists of the V1a vasopressin receptor (V1aR) are emerging as a strategy for slowing progression of CKD. Physiologically, V1aR signaling has been linked with acid-base homeostasis, but more detailed information is needed about renal V1aR distribution and function.

METHODS

We used a new anti-V1aR antibody and high-resolution microscopy to investigate Va1R distribution in rodent and human kidneys. To investigate whether V1aR activation promotes urinary H⁺ secretion, we used a V1aR agonist or antagonist to evaluate V1aR function in vasopressin-deficient Brattleboro rats, bladder-catheterized mice, isolated collecting ducts, and cultured inner medullary collecting duct (IMCD) cells.

RESULTS

Localization of V1aR in rodent and human kidneys produced a basolateral signal in type A intercalated cells (A-ICs) and a perinuclear to subapical signal in type B intercalated cells of connecting tubules and collecting ducts. Treating vasopressin-deficient Brattleboro rats with a V1aR agonist decreased urinary pH and tripled net acid excretion; we observed a similar response in C57BL/6J mice. In contrast, V1aR antagonist did not affect urinary pH in normal or acid-loaded mice. In ex vivo settings, basolateral treatment of isolated perfused medullary collecting ducts with the V1aR agonist or vasopressin increased intracellular calcium levels in ICs and decreased luminal pH, suggesting V1aR-dependent calcium release and stimulation of proton-secreting proteins. Basolateral treatment of IMCD cells with the V1aR agonist increased apical abundance of vacuolar H⁺-ATPase in A-ICs.

CONCLUSIONS

Our results show that activation of V1aR contributes to urinary acidification via H⁺ secretion by A-ICs, which may have clinical implications for pharmacologic targeting of V1aR.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Vasopressin V1a receptor is required for nucleocytoplasmic transport of mineralocorticoid receptor

We previously reported that a deficiency in the vasopressin V1a receptor (V1aR) results in type 4 renal tubular acidosis, which suggests that vasopressin exerts direct effects on the physiological actions of aldosterone. We investigated the role of vasopressin for nucleocytoplasmic transport of mineralocorticoid receptor (MR) in the intercalated cells. Vasopressin V1aR-deficient (V1aR(-/-)) mice showed largely decreased expression of MR and 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) in the medulla of the kidney, which was partially ameliorated by fludrocortisone treatment. The incubation of IN-IC cells, an intercalated cell line established from temperature-sensitive SV40 large T antigen-expressing rats, with aldosterone or vasopressin increased the nuclear-to-cytoplasmic ratio of the MR from 11.2 to 47.2% and from 18.7 to 61.2%, respectively, in 30 min without any changes in MR expression from the whole cell extract. The immunohistochemistry analysis of the IN-IC cells revealed the nuclear accumulation of MRs after a 30-min incubation with aldosterone or vasopressin. These effects were accompanied by an increase in regulator of chromosome condensation-1 (RCC-1) due to aldosterone and a decrease in Ran GTPase-activating protein 1 (Ran Gap1) due to vasopressin. RNA interference against V1aR abolished the nuclear accumulation of MR induced by aldosterone or vasopressin. Vasopressin increased PKCα and -β(1) expression, and aldosterone increased PKCδ and -ζ expression, but these effects were abolished with a V1aR knockdown. These results suggest that vasopressin directly regulates the nucleocytoplasmic transport of MRs via the V1aR in the intercalated cells of the collecting ducts.

Regulation of V2R transcription by hypertonicity and V1aR-V2R signal interaction

Arginine vasopressin (AVP) and hypertonicity in the renal medulla play a major role in the urine concentration mechanism. Previously, we showed that rat vasopressin V2 receptor (rV2R) promoter activity was increased by vasopressin V2R stimulation and decreased by vasopressin V1a receptor (V1aR) stimulation in a LLC-PK1 cell line stably expressing rat V1aR (LLC-PK1/rV1aR). In the present study, we investigated the effects of hypertonicity on the rV2R promoter activity and on the suppression of rV2R promoter activity by V1aR stimulation in LLC-PK1/rV1aR cells. rV2R promoter activity was increased in NaCl- or mannitol-induced hypertonicity. The hypertonicity-responsive site in the rV2R promoter region was limited to 10 bp, including the Sp1 motif. The increase of V2R promoter activity by hypertonicity was significantly inhibited by a JNK inhibitor (SP600125) and PKA inhibitor (H89). In contrast, rV2R promoter activity was remarkably suppressed by V1aR stimulation in the hypertonic condition rather than in the isotonic condition. The AVP-stimulated intracellular Ca2+ concentration was increased in the hypertonic condition, suggesting the functional activation of V1aR by hypertonicity. In conclusion, 1) V2R promoter activity is increased by hypertonicity via the JNK and PKA pathways, 2) suppression of V2R expression by the V1aR-Ca2+ pathway is enhanced by hypertonicity, and 3) hypertonicity enhances the V1aR-Ca2+ pathway. The counteractivity of V2R and V1aR could be required to maintain minimum urine volume in the dehydrated state.

Downregulation of vasopressin V2 receptor promoter activity via V1a receptor pathway

Vasopressin V(1a) and V(2) receptors (V(1a)R and V(2)R, respectively) distribute in the collecting duct of the kidney. Although the function of V(2)R mediating the antidiuretic effect of AVP has been investigated in detail, the role of V(1a)R in the collecting ducts has not been elucidated. In the present study, we have investigated the role of the V(1a)R pathway in V(2)R promoter activity. We cloned the 5'-flanking region of rat V(2)R (rV(2)R) and investigated rV(2)R promoter activity in the LLC-PK(1) cell line transfected to express rat V(1a)R (rV(1a)R) dominantly (LLC-PK(1)/rV(1a)R). AVP induced a transient increase, followed by a sustained decrease, of rV(2)R promoter activity in these cells. This AVP-induced decrease of rV(2)R promoter activity was inhibited by V(1a)R, but not V(2)R, antagonist. PMA mimicked this decrease of rV(2)R promoter activity. On the contrary, 8-(4-chlorophenylthio)-cAMP increased rV(2)R promoter activity. These PMA- and 8-(4-chlorophenylthio)-cAMP-induced effects were not observed on the deletion segment of the 5'-flanking region lacking CAAT and SP1 sites. In conclusion, 1) expression of the V(2)R is downregulated via the V(1a)R pathway in LLC-PK(1)/rV(1a)R cells, and 2) expression of the V(2)R is downregulated by the PMA-induced PKC pathway and upregulated by the cAMP-PKA pathway. These opposite effects of PKC and PKA appear to be regulated by the same promoter region of CAAT and SP1.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

V(1) receptors in luminal action of vasopressin on distal K(+) secretion

Luminal perfusion with collected proximal fluid increases distal K(+) secretion compared with artificial solutions. Arginine vasopressin (AVP), present in luminal fluid, might be responsible for this observation. K(+) secretion rate (J(K)) was measured by K(+)-sensitive microelectrodes during paired luminal stationary microperfusion with control and AVP-containing 0.5 mM K(+) solutions. J(K) was 1.34 +/- 0.35 (n = 24 tubules) nmol x cm(-2) x s(-1) during perfusion with 10(-9) M AVP, against 0.90+/-0.12 nmol x cm(-2) x s(-1) (n = 21) in control (P<0.02). With 10(-9) M AVP+10(-6) M beta-mercapto-beta-beta-cyclopenta-methylenepropionyl(1), O-Me-Tyr(2)-Arg(8) vasopressin (MCMV), a specific peptide V(1)-receptor antagonist, J(K) was 0.36+/-0.067 against 0.77+/-0.10 (control; n = 9) nmol x cm(-2) x s(-1) (P<0.01). With 10(-6) M MCMV alone, J(K) was 0.37+/-0.04 against a control of 0.62+/-0.06 (n = 19) nmol. cm(-2). s(-1) (P<0.01). A peptide V(2) antagonist had no such effect. In Brattleboro rats, which do not produce endogenous AVP, MCMV had no effect when given alone, although AVP still stimulated J(K). In conclusion, luminal AVP stimulates distal J(K) significantly. The V(1) antagonist MCMV inhibits the effect of AVP but also reduces J(K) when given alone. This suggests that AVP acts luminally via V(1) receptors but also that there appears to be a background effect of endogenous AVP blocked by the antagonist.