Arginine vasopressin gene regulation in the homozygous Brattleboro rat

Kidney collecting duct-derived vasopressin is not essential for appropriate concentration or dilution of urine

We have previously shown that kidney collecting ducts make vasopressin. However, the physiological role of collecting duct-derived vasopressin is uncertain. We hypothesized that collecting duct-derived vasopressin is required for the appropriate concentration of urine. We developed a vasopressin conditional knockout (KO) mouse model wherein Cre recombinase expression induces deletion of arginine vasopressin (Avp) exon 1 in the distal nephron. We then used age-matched 8- to 12-wk-old Avp fl/fl;Ksp-Cre(-) [wild type (WT)] and Avp fl/fl;Ksp-Cre(+) mice for all experiments. We collected urine, serum, and kidney lysates at baseline. We then challenged both WT and knockout (KO) mice with 24-h water restriction, water loading, and administration of the vasopressin type 2 receptor agonist desmopressin (1 µg/kg ip) followed by the vasopressin type 2 receptor antagonist OPC-31260 (10 mg/kg ip). We performed immunofluorescence and immunoblot analysis at baseline and confirmed vasopressin KO in the collecting duct. We found that urinary osmolality (UOsm), plasma Na+, K+, Cl-, blood urea nitrogen, and copeptin were similar in WT vs. KO mice at baseline. Immunoblots of the vasopressin-regulated proteins Na+-K+-2Cl- cotransporter, NaCl cotransporter, and water channel aquaporin-2 showed no difference in expression or phosphorylation at baseline. Following 24-h water restriction, WT and KO mice had no differences in UOsm, plasma Na+, K+, Cl-, blood urea nitrogen, or copeptin. In addition, there were no differences in the rate of urinary concentration or dilution as in WT and KO mice UOsm was nearly identical after desmopressin and OPC-31260 administration. We conclude that collecting duct-derived vasopressin is not essential to appropriately concentrate or dilute urine.NEW & NOTEWORTHY Hypothalamic vasopressin is required for appropriate urinary concentration. However, whether collecting duct-derived vasopressin is involved remains unknown. We developed a novel transgenic mouse model to induce tissue-specific deletion of vasopressin and showed that collecting duct-derived vasopressin is not required to concentrate or dilute urine.

Role of glucocorticoid hormones in arginine vasopressin gene regulation

The mechanism underlying increased AVP synthesis and release in glucocorticoid deficiency is not known. Therefore, the present study was undertaken to investigate whether the mechanism was at the level of AVP gene transcription. The AVP gene promoter contains a consensus GRE, a CRE, and four AP2 sites. To assess the functional importance of these sites, 5' deletions of the AVP promoter were created and transient transfections were performed. Promoter activity in hypothalamic cells transfected with deletions lacking the GRE or both the GRE and CRE exhibited higher activity when compared to longer constructs containing both sites. In neuroblastoma cells, only the deletion lacking the GRE exhibited increased AVP promoter activity over the longer construct. These results are consistent with the idea that glucocorticoids suppress AVP gene expression by acting on a GRE in the AVP promoter region. Further, dexamethasone inhibited AVP promoter activity by >50% in hypothalamic cells transfected with the GRE-containing construct. In conclusion, the data presented here support a central mechanism to explain, at least in part, the nonosmotic increase in AVP with glucocorticoid deficiency.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Water-losing and water-retaining states: role of water channels and vasopressin receptor antagonists

Alterations in water metabolism are present in conditions such as diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion, cardiac failure, cirrhosis, and pregnancy. Recent advances in molecular biology have enhanced our understanding of disordered water metabolism in these conditions. This review examines the roles of central vasopressin synthesis and release and collecting duct vasopressin V2 receptor and aquaporin-2 water channel regulation in water-losing and water-retaining states.

Vasopressin V(2)-receptor-dependent regulation of AQP2 expression in Brattleboro rats

The role of AVP-V(2) receptor (AVP-V(2)R)-dependent regulation of aquaporin-2 (AQP2) expression was evaluated in vasopressin-deficient Brattleboro (BB) rats. AQP2 levels were relatively high in BB rats (52 +/- 8% of levels in Wistar rats), and treatment with the AVP-V(2)R antagonist SR-121463A (0.8 mg/day) for 48 h was associated with 1) increased urine output (170 +/- 9%), 2), reduced AQP2 protein levels (42 +/- 10% in whole kidney and 53 +/- 8% in inner medulla), and 3) reduced AQP2 mRNA levels (36 +/- 7%). In addition, the levels of AQP2 phosphorylated in the protein kinase A (PKA) consensus site (Ser(256) of AQP2) was reduced to 3 +/- 1% of control levels. Lithium (Li) treatment of BB rats for 1 mo, known to reduce adenylyl cyclase (AC) activity, downregulated AQP2 protein levels (15 +/- 6%) and increased urine output (220%). Downregulation of AQP2 expression in response to SR-121463A or Li treatment indicates that AQP2 expression in BB rats depends in part on activation of AVP-V(2)Rs and that the signaling cascade(s) involves AC and hence cAMP. Complete water restriction of BB rats produced only a small increase in AQP2 mRNA (235 +/- 33%) and AQP2 protein (156 +/- 22%) levels. Immunoelectron microscopy confirmed the increase in AQP2 abundance but revealed no change in AQP2 apical plasma membrane labeling in response to thirsting. In conclusion, the expression and phosphorylation of AQP2 in BB rats are in part dependent on AVP-V(2)R signaling, and AVP-V(2)-mediated regulation of AQP2 trafficking and expression is effectively decoupled in BB rats, indicating differences in AVP-V(2)R-mediated regulation of AQP2 trafficking and expression.

Kappa opiate agonist RU 51599 inhibits vasopressin gene expression and osmotically-induced vasopressin secretion in vitro

Kappa (kappa) opioid agonists induce a water diuresis and inhibit vasopressin (AVP) secretion. Hypothalamic and neurohypophysial sites have both been implicated in the response. The present study was designed to ascertain if kappa-agonist inhibition of osmotically-stimulated AVP secretion is associated with parallel changes in AVP gene expression. Experiments were performed using the selective kappa-agonist RU 51599 (RU) in compartmentalized hypothalamo-neurohypophysial explants. When added to either the hypothalamus or the neural lobe, RU dose dependently inhibited osmotically-induced AVP secretion that was reversed by the highly selective kappa-antagonist nor-binaltorphimine (nor-BNI) only at the hypothalamic, not the neurohypophysial level. AVP mRNA content paralleled the changes in AVP secretory rate induced by hypothalamic kappa-agonism. AVP mRNA levels were unaltered when RU was applied to the neural lobe. Neurohypophysial AVP content did not change. These data indicate that hypothalamic kappa-agonism inhibits osmotically induced AVP secretion and that a non-kappa1 opiate receptor mediates posterior pituitary opioid inhibition of AVP release. Neural or receptor inputs to the hypothalamus or magnocellular cell body may downwardly modulate AVP mRNA content by altering AVP gene transcription and/or message stability. Inhibition of AVP release directly at the neurohypophysis can be uncoupled from the cellular mechanisms that generate changes in AVP mRNA content.

Quantitative analysis of oxytocin and vasopressin messenger ribonucleic acids in single magnocellular neurons isolated from supraoptic nucleus of rat hypothalamus

Oxytocin (OT) and vasopressin (VP) are peptide hormones that are derived from genes predominantly expressed in distinct magnocellular neurons in the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Recent evidence suggests that some magnocellular neurons coexpress both peptides. Our qualitative RT-PCR experiments on single cells show that the majority of magnocellular neurons coexpress both peptide messenger RNAs (mRNAs) in varying amounts. Using a competitive RT-PCR method combined with a standard calibration curve, we quantitatively determined OT and VP mRNA in single magnocellular neurons from the normal female rat SON, with a detection sensitivity of less than 30 mRNA molecules/cell. We defined the phenotypes of the single magnocellular neurons according to their ratios of these two peptide mRNAs. Using this approach, we identified three major phenotypes: oxytocin neurons, where the average OT to VP mRNA ratio is about 256; vasopressin neurons, where the average VP to OT mRNA ratio is about 182; and one oxytocin/vasopressin coexisting neuron, where the OT/VP mRNA ratio is 2. Thus, there is some OT and VP mRNA coexpression in virtually all of the magnocellular neurons in supraoptic nuclei of hypothalamus. However, clear phenotypes are identifiable by considering quantitative as opposed to qualitative differences.

Effect of kappa opioid agonist RU 51599 on osmotic and non-osmotic stimulated arginine vasopressin release and gene regulation in small cell lung carcinoma cells

Arginine vasopressin (AVP) is synthesized in the hypothalamus, stored in the posterior pituitary, and osmotic and non-osmotic stimuli release AVP into the circulation for antidiuretic and vascular actions on target tissue. The kappa-opioid agonist, RU 51599, exhibits a potent diuretic activity in both experimental animals and humans. This diuretic activity is characterized by a water diuresis without an associated increase in electrolyte excretion. Studies with cultured rat hypothalamo-neurohypophysial system explant showed that AVP mRNA level changed in parallel to the RU 51599-induced changes in AVP secretory rate. There are, however, no hypothalamic neuronal cell lines to study AVP gene regulation system, and it is not known whether RU 51599, regulates AVP secretion and biosynthesis under osmotic and non-osmotic stimulatory conditions of AVP release. The effect of RU 51599 on AVP release, AVP mRNA, and AVP gene promoter activity in osmotic and non-osmotic conditions was therefore studied using cultured small cell lung carcinoma (SCLC) cell lines. RU 51599 significantly inhibited AVP release by osmotic stimulation (330 mOsm) and non-osmotic stimulators, angiotensin II (AII) and endothelin 3 (ET3). However, RU 51599 did not show any effect on the AVP mRNA and AVP gene promoter activity stimulated by high osmolality and ET3. These results indicate, therefore, that RU 51599 suppresses AVP secretion by inhibition at the step of AVP release during osmotic and non-osmotic stimulation but does not affect the AVP gene transcription level in the SCLC cells.

A sensitive reverse transcriptase polymerase chain reaction assay for measuring the effects of dehydration and gestation on rat amounts of vasopressin and ocytocin mRNAs

This study describes a competitive reverse transcriptase polymerase chain reaction (RT-PCR) method for assaying the amounts of vasopressin (AVP) and ocytocin (OT) mRNAs in the rat hypothalamus and uterus. Despite the low concentrations of these mRNAs, the RT-PCR method readily measured both AVP and OT mRNAs in the same sample. A common internal standard for both reactions was designed to quantify the reaction. Both AVP and OT mRNAs were readily quantified in a 75 ng sample of total RNA from the hypothalamus. Water deprivation stimulated AVP mRNA production 3-fold and OT mRNA production 1.7-fold in the hypothalamus. Gestation only influenced the amount of OT mRNA in the hypothalamus (3-fold increase) and uterus (38-fold increase). The amount of AVP mRNA in the hypothalamus remained unchanged and no AVP mRNA was detected in the uteri of either non-pregnant or pregnant rats. This competitive RT-PCR is a powerful tool that provides rapid and precise assays of AVP and OT mRNAs.

Osmotic and non-osmotic regulation of arginine vasopressin (AVP) release, mRNA, and promoter activity in small cell lung carcinoma (SCLC) cells

Arginine vasopression (AVP) is synthesized in the magnocellular neurons of the hypothalamus and stored in the posterior pituitary. It has been shown that hypothalamic AVP mRNA is increased during experimental stimulation of osmotic and non-osmotic stimulation of AVP release. The mechanisms underlying the stimulation of AVP biosynthesis in these conditions are not known. The present study was, therefore, performed to measure AVP release, AVP mRNA level, and AVP gene promoter activity during osmotic and non-osmotic stimulation of AVP secretion in the small cell lung carcinoma (SCLC) cells. AVP release was measured by radioimmunoassay, steady state levels of AVP mRNA by solution hybridization, and AVP gene promoter activity exhibited by a 1.5 kb 5'-flanking AVP gene fragment fused to a luciferase reporter after SCLC cells were subjected to osmotic or non-osmotic conditions. High media osmolality (330 mOsm) significantly increased AVP release (control (C) 1.42 +/- 0.27 vs. High Osm 3.67 +/- 0.39 pg/2 x 10(6) cells, N = 9, P < 0.002); AVP mRNA (C 173.6 +/- 16.8 vs. High Osm 280.1 +/- 19.4 pg/2 x 10(6) cells, N = 7, P < 0.001); and AVP gene promoter activity (C 1353 +/- 99 vs. High Osm 2026 +/- 134 L.U./10(-4) U beta-gal, N = 8, P < 0.001). Non-osmotic stimulators. 0.1 microM endothelin 3 (ET3), 1 microM angiotensin II (AII), and 10 microM acetylcholine (Ach) significantly increased AVP release; ET3 (C 1.78 +/- 0.20 vs. ET3 6.85 +/- 1.86 pg/2 x 10(6) cells, N = 8, P < 0.02); AII (C 1.29 +/- 0.38 vs. AII 27.80 +/- 7.09 pg/2 x 10(6) cells, N = 5, P < 0.05) and Ach (C 1.14 +/- 0.33 vs. Ach 2.68 +/- 0.58 pg/2 x x10(6) cells, N = 6, P < 0.05). However, only ET3 significantly increased AVP mRNA (C 166.6 +/- 19.6 vs. ET3 254.4 +/- 25.6 pg/p x 10(6) cells, N = 5, P < 0.05) and AVP promoter activity (C 1515 +/- 163 vs. ET3 2389 +/- 342 L.U./10(-4) U beta-gal, N = 6, P < 0.05). To localize the region of the AVP promoter that mediates the osmotic stimulation and the effect of ET3, 5' deletions of the AVP promoter fragments terminating at -532, -211, and -102, was assessed. Only the promoter activity of the 1.5 kb construct, but not the deletion constructs, was significantly increased by ET3 or high osmolality. These results suggest that modulation of AVP gene transcription is, at least in part, responsible for increased AVP synthesis and release in response to osmotic and non-osmotic stimulation, and that the region of 5' flanking sequence between -1500 and -532 contains the elements responsible for the effects.

Arginine vasopressin gene expression in rats with puromycin-induced nephrotic syndrome

Nephrotic syndrome is characterized by water and sodium retention, which leads to edema formation. The nonosmotic stimulation of arginine vasopressin (AVP) release from the pituitary gland has been implicated to be one of the important factors of abnormal water retention in patients with nephrotic syndrome. It is not known, however, whether nephrotic syndrome is associated with stimulation of hypothalamic vasopressin gene expression. Puromycin aminonucleoside is known to cause altered glomerular permeability, which results in experimental nephrotic syndrome in rats. In the present study, therefore, AVP gene expression has been studied in the hypothalamus of rats with puromycin aminonucleoside-induced nephrotic syndrome (PNS). Nephrotic syndrome was induced by a single intravenous injection of puromycin aminonucleoside (50 mg/kg body weight). Nephrotic syndrome was confirmed by urinary protein excretion (control 20.8 +/- 3.5 mg/24 hr v PNS 273.9 +/- 41.4 mg/24 hr; P < 0.0001, n = 6) and serum albumin concentrations (control 4.52 +/- 0.07 g/dL v PNS 2.96 +/- 0.22 g/dL; P < 0.001, n = 6). In PNS rats, plasma AVP was significantly higher than in control rats (control 0.77 +/- 0.10 pg/mL v PNS 2.13 +/- 0.42 pg/mL; P < 0.005, n = 12), even though there were no differences in plasma osmolality (control 292.0 +/- 2.0 mOsm/kg H2O v PNS 290.3 +/- 2.5 mOsm/kg H2O; P = NS, n = 12) or serum sodium concentration (control 142.7 +/- 0.7 v PNS 142.1 +/- 1.1; PNS, n = 12).(ABSTRACT TRUNCATED AT 250 WORDS)

Short-term hypothyroidism and vasopressin gene expression in the rat

Hypothyroidism is associated with abnormalities in renal water handling, which include a delay in excretion of an acute water load, decreased urinary concentrating ability, and increased urine volume. In the present study, we investigated the role of vasopressin in aminotriazole-induced hypothyroidism by measuring vasopressin concentration in the plasma and pituitary along with vasopressin mRNA levels in the hypothalamus. After 5 weeks of aminotriazole treatment, L-thyroxine levels were significantly lower in the experimental animals (122 +/- 8 v 26 +/- 1 nmol/L [9.5 +/- 0.6 v 2.0 +/- 0.1 micrograms/dL]; P less than 0.001). Serum sodium (148 +/- 0.5 v 144 +/- 1.2 mmol/L [mEq/L]; P less than 0.01), and plasma osmolality (311 +/- 2.5 v 304 +/- 1.8 mmol/kg [mOsm/kg] H2O; P less than 0.05) were also lower in the experimental animals. There were no differences in plasma (1.9 +/- 0.4 v 1.5 +/- 0.2 pg/mL) or pituitary (1.5 +/- 0.4 v 1.5 +/- 0.2 microgram/pituitary) vasopressin levels. In addition, steady-state vasopressin mRNA levels were not different between the two groups (1,286 +/- 210 v 1,093 +/- 138 pg/hypothalamus). One week of L-thyroxine replacement resulted in significant increases in serum thyroxine levels without changes in the other variables measured. These results indicate that short-term hypothyroidism, which has been shown to exert substantial effects on renal function, causes only a modest central alteration in the plasma vasopressin-osmolality relationship, which occurs in the absence of detectable changes in vasopressin synthesis.