Transient phosphorylation of the V1a vasopressin receptor

Membrane phosphoinositides regulate GPCR-β-arrestin complex assembly and dynamics

Binding of arrestin to phosphorylated G protein-coupled receptors (GPCRs) is crucial for modulating signaling. Once internalized, some GPCRs remain complexed with β-arrestins, while others interact only transiently; this difference affects GPCR signaling and recycling. Cell-based and in vitro biophysical assays reveal the role of membrane phosphoinositides (PIPs) in β-arrestin recruitment and GPCR-β-arrestin complex dynamics. We find that GPCRs broadly stratify into two groups, one that requires PIP binding for β-arrestin recruitment and one that does not. Plasma membrane PIPs potentiate an active conformation of β-arrestin and stabilize GPCR-β-arrestin complexes by promoting a fully engaged state of the complex. As allosteric modulators of GPCR-β-arrestin complex dynamics, membrane PIPs allow for additional conformational diversity beyond that imposed by GPCR phosphorylation alone. For GPCRs that require membrane PIP binding for β-arrestin recruitment, this provides a mechanism for β-arrestin release upon translocation of the GPCR to endosomes, allowing for its rapid recycling.

The Biology of Vasopressin

Vasopressins are evolutionarily conserved peptide hormones. Mammalian vasopressin functions systemically as an antidiuretic and regulator of blood and cardiac flow essential for adapting to terrestrial environments. Moreover, vasopressin acts centrally as a neurohormone involved in social and parental behavior and stress response. Vasopressin synthesis in several cell types, storage in intracellular vesicles, and release in response to physiological stimuli are highly regulated and mediated by three distinct G protein coupled receptors. Other receptors may bind or cross-bind vasopressin. Vasopressin is regulated spatially and temporally through transcriptional and post-transcriptional mechanisms, sex, tissue, and cell-specific receptor expression. Anomalies of vasopressin signaling have been observed in polycystic kidney disease, chronic heart failure, and neuropsychiatric conditions. Growing knowledge of the central biological roles of vasopressin has enabled pharmacological advances to treat these conditions by targeting defective systemic or central pathways utilizing specific agonists and antagonists.

Sex-Specific Vasopressin Signaling Buffers Stress-Dependent Synaptic Changes in Female Mice

In many species, social networks provide benefit for both the individual and the collective. In addition to transmitting information to others, social networks provide an emotional buffer for distressed individuals. Our understanding about the cellular mechanisms that contribute to buffering is poor. Stress has consequences for the entire organism, including a robust change in synaptic plasticity at glutamate synapses onto corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN). In females, however, this stress-induced metaplasticity is buffered by the presence of a naive partner. This buffering may be because of discrete behavioral interactions, signals in the context in which the interaction occurs (i.e., olfactory cues), or it may be influenced by local signaling events in the PVN. Here, we show that local vasopressin (VP) signaling in PVN buffers the short-term potentiation (STP) at glutamate synapses after stress. This social buffering of metaplasticity, which requires the presence of another individual, was prevented by pharmacological inhibition of the VP 1a receptor (V1aR) in female mice. Exogenous VP mimicked the effects of social buffering and reduced STP in CRH^PVN neurons from females but not males. These findings implicate VP as a potential mediator of social buffering in female mice.SIGNIFICANCE STATEMENT In many organisms, including rodents and humans, social groups are beneficial to overall health and well-being. Moreover, it is through these social interactions that the harmful effects of stress can be mitigated, a phenomenon known as social buffering. In the present study, we describe a critical role for the neuropeptide vasopressin (VP) in social buffering of synaptic metaplasticity in stress-responsive corticotropin-releasing hormone (CRH) neurons in female mice. These effects of VP do not extend to social buffering of stress behaviors, suggesting this is a very precise and local form of sex-specific neuropeptide signaling.

Detection of activity-dependent vasopressin release from neuronal dendrites and axon terminals using sniffer cells

Our understanding of neuropeptide function within neural networks would be improved by methods allowing dynamic detection of peptide release in living tissue. We examined the usefulness of sniffer cells as biosensors to detect endogenous vasopressin (VP) release in rat hypothalamic slices and from isolated neurohypophyses. Human embryonic kidney cells were transfected to express the human V1a VP receptor (V1aR) and the genetically encoded calcium indicator GCaMP6m. The V1aR couples to Gq₁₁, thus VP binding to this receptor causes an increase in intracellular [Ca²⁺] that can be detected by a rise in GCaMP6 fluorescence. Dose-response analysis showed that VP sniffer cells report ambient VP levels >10 pM (EC₅₀ = 2.6 nM), and this effect could be inhibited by the V1aR antagonist SR 49059. When placed over a coverslip coated with sniffer cells, electrical stimulation of the neurohypophysis provoked a reversible, reproducible, and dose-dependent increase in VP release using as few as 60 pulses delivered at 3 Hz. Suspended sniffer cells gently plated over a slice adhered to the preparation and allowed visualization of VP release in discrete regions. Electrical stimulation of VP neurons in the suprachiasmatic nucleus caused significant local release as well as VP secretion in distant target sites. Finally, action potentials evoked in a single magnocellular neurosecretory cell in the supraoptic nucleus provoked significant VP release from the somatodendritic compartment of the neuron. These results indicate that sniffer cells can be used for the study of VP secretion from various compartments of neurons in living tissue. NEW & NOTEWORTHY The specific functional roles of neuropeptides in neuronal networks are poorly understood due to the absence of methods allowing their real-time detection in living tissue. Here, we show that cultured "sniffer cells" can be engineered to detect endogenous release of vasopressin as an increase in fluorescence.

Fine-tuning somatostatin receptor signalling by agonist-selective phosphorylation and dephosphorylation: IUPHAR Review 5

The biological actions of somatostatin are mediated by a family of five GPCRs, named sst1 to sst5 . Somatostatin receptors exhibit equally high-binding affinities to their natural ligand somatostatin-14 and largely overlapping distributions. The overexpression of somatostatin receptors in human tumours is the molecular basis for diagnostic and therapeutic application of the stable somatostatin analogues octreotide, lanreotide and pasireotide. The efficiency of somatostatin receptor signalling is tightly regulated and ultimately limited by the coordinated phosphorylation and dephosphorylation of intracellular carboxyl-terminal serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific antibodies for human sst2 and sst5 receptors. These phosphosite-specific antibodies are unique tools to monitor the spatial and temporal dynamics of receptors phosphorylation and dephosphorylation. Using a combined approach of phosphosite-specific antibodies and siRNA knock-down screening, relevant kinases and phosphatases were identified. Emerging evidence suggests distinct mechanisms of agonist-selective fine-tuning for individual somatostatin receptors. The recently uncovered differences in phosphorylation and dephosphorylation of these receptors may hence be of physiological significance in mediating responses to acute, persistent or repeated stimuli in a variety of target tissues.

Neuroendocrine regulation of stress in birds with an emphasis on vasotocin receptors (VTRs)

The neuroendocrine stress response of vertebrates, particularly mammals, comprises at least two types of neuropeptide containing neurons, corticotropin-releasing hormone (CRH) and vasopressin (VP) neurons, and four receptors [CRH receptor one (CRH-R1) and two (CRH-R2) and VP receptor 1a (V1aR) and 1b (V1bR)]. The avian neuropeptide CRH, a 41-amino acid peptide, has been shown to have the same amino acid sequence as humans while nonapeptide neurohormone arginine-vasotocin (AVT) is regarded as highly conserved having a single amino acid substitution compared to mammalian arginine vasopressin. Similar to mammals, birds have two receptor subtypes (CRH-R1 and CRH-R2) for CRH, however, four vasotocin receptors have been identified. Less is known about the functions of the four avian vasotocin receptors compared to homologous ones found in mammals and other vertebrate classes. Recently, chicken vasotocin receptor two (VT2R) and four (VT4R) have been characterized utilizing immunocytochemistry and an imposed stress test. The purpose of this review is to present evidence that the VT2R and VT4R are involved in the avian stress response and that the cephalic lobe of the anterior pituitary appears specialized for this function as it contains the major population of corticotropes and necessary neuroendocrine receptors to respond to stressors impacting avian species.

Arginine vasopressin enhances cell survival via a G protein-coupled receptor kinase 2/β-arrestin1/extracellular-regulated kinase 1/2-dependent pathway in H9c2 cells

Circulating levels of arginine vasopressin (AVP) are elevated during hypovolemia and during cardiac stress. AVP activates arginine vasopressin type 1A (V(1A))/Gα(q)-coupled receptors in the heart and vasculature and V(2)/Gα(s)-coupled receptors in the kidney. However, little is known regarding the signaling pathways that influence the effects of V(1A) receptor (V(1A)R) activation during cellular injury. Using hypoxia-reoxygenation (H/R) as a cell injury model, we evaluated cell survival and caspase 3/7 activity in H9c2 myoblasts after treatment with AVP. Pretreatment of H9c2 cells with AVP significantly reduced H/R-induced cell death and caspase 3/7 activity, effects that were blocked via both selective V(1A)R inhibition and mitogen-activated protein kinase (MEK1/2) inhibition. AVP increased extracellular-regulated kinase 1/2 (ERK1/2) phosphorylation in a concentration-dependent manner that was sensitive to MEK1/2 inhibition and V(1A)R inhibition, but not V(1B)R or V(2)R inhibition. Discrete elements of the V(1A)/Gα(q)-protein kinase C (PKC) and V(1A)/G protein-coupled receptor kinase (GRK)/β-arrestin signaling cascades were inhibited to dissect the pathways responsible for the protective effects of V(1A)R signaling: Gα(q) (overexpression of Gq-I-ires-green fluorescent protein), PKC (administration of Ro 31-82425; 2-[8-(aminomethyl)-6,7,8,9-tetrahydropyrido[1,2-a]indol-3-yl]-3-(1-methyl-1H-indol-3-yl)maleimide, HCl, bisindolylmaleimide X, HCl), GRK2 [C-terminal GRK2 peptide overexpression and small interfering RNA (siRNA) knockdown], GRK5 (siRNA knockdown), and β-arrestin1 (siRNA knockdown). These studies demonstrated that both Gα(q)/PKC- and GRK2/β-arrestin1-dependent V(1A)R signaling were capable of inducing ERK1/2 phosphorylation in response to AVP stimulation. However, AVP-mediated protection against H/R was elicited only via GRK2- and β-arrestin1-dependent signaling. These results suggest that activation of the V(1A)R in H9c2 cells mediates protective signaling via a GRK2/β-arrestin1/ERK1/2-dependent mechanism that leads to decreased caspase 3/7 activity and enhanced survival under conditions of ischemic stress.

Distribution of the Vasotocin Subtype Four Receptor (VT4R) in the Anterior Pituitary Gland of the Chicken, Gallus gallus, and its Possible Role in the Avian Stress Response

The neurohormone arginine vasotocin (AVT) in non mammalian vertebrates is homologous to arginine vasopressin (AVP) in mammals. Its actions are mediated via G protein-coupled receptors that belong to the vasotocin/mesotocin family. Because of the known regulatory effects of nonapeptide hormones on anterior pituitary functions, receptor subtypes in that family have been proposed to be located in anterior pituitary cells. Recently, an avian vasotocin receptor subtype designated VT4R has been cloned, which shares 69% sequence homology with a human vasopressin receptor, the V1aR. In the present study, a polyclonal antibody to the VT4R was developed and validated to confirm its specificity to the VT4R. The antibody was used to test the hypothesis that the VT4R is present in the avian anterior pituitary and is specifically associated with certain cell types, where its expression is modulated by acute stress. Western blotting of membrane protein extracts from pituitary tissue, the use of HeLa cells transfected with the VT4R and peptide competition assays all confirmed the specificity of the antibody to the VT4R. Dual-labelling immunofluorescence microscopy was utilised to identify pituitary cell types that contained immunoreactive VT4R. The receptor was found to be widely distributed throughout the cephalic lobe but not in the caudal lobe of the anterior pituitary. Immunoreactive VT4R was associated with corticotrophs. Approximately 89% of immunolabelled corticotrophs were shown to contain the VT4R. The immunoreactive VT4R was not found in gonadotrophs, somatotrophs or lactotrophs. To determine a possible functional role of the VT4R and previously characterised VT2R, gene expression levels in the anterior pituitary were determined after acute immobilisation stress by quantitative reverse transcriptase-polymerase chain reaction. The results showed a significant increase in plasma corticosterone levels (three- to four-fold), a significant reduction of VT4R mRNA and an increase of VT2R mRNA (P < 0.05) in acutely immobilised chicks compared to controls. The data suggest a role of the VT4R in the avian stress response.

The role of vasopressin V1A receptors in cytotoxic brain edema formation following brain injury

BACKGROUND

The hormone and neuropeptide arginine-vasopressin is designated to the maintenance of osmotic homoeostasis and blood pressure regulation. While experimental data show vasopressin V(1A) receptors to regulate aquaporin (AQP)4 water channel dependent brain water movement, the specific role in vasogenic and cytotoxic edema formation remains unclear. The present study was designed to quantify the V(1A) receptor mediated regional brain edema formation in two clinically relevant experimental models, brain injury combined with secondary insult and focal ischemia.

METHODS

Male Sprague-Dawley rats were randomly assigned to a continuous infusion of vehicle (1 % DMSO) or the selective non-peptide V(1A) antagonist SR49059 (83nM = 1 mg/kg) starting before controlled cortical impact (CCI) injury plus hypoxia and hypotension (HH, 30 min), or middle cerebral artery (MCA) occlusion (2 h + 2 h reperfusion).

RESULTS

A global analysis of brain water content by the wet/dry weight method allowed optimizing the SR49059 dosage, and demonstrated the down-regulation of brain AQP4 expression by immunoblotting. Microgravimetrical quantification in 64 one mm(3) samples per animal (n = 6 per group) from bregma +2.7 to -6.3 mm analysis demonstrated brain edema to be reduced at 4 h by SR49059 treatment in the injured and contralateral cortex following CCI + HH (p = 0.007, p < 0.001) and in the infarct area following MCA occlusion (p = 0.013, p = 0.002, p = 0.004).

CONCLUSIONS

Our findings demonstrate that an early cytotoxic brain edema component following brain injury plus secondary insult or focal ischemia results from a vasopressin V(1A) receptor mediated response, and occurs most likely through AQP4 up-regulation. The V(1A) antagonist SR49059 offers a new avenue in brain edema treatment and prompts further study into the role of vasopressin following brain injury.

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.

Rapid dephosphorylation of G protein-coupled receptors by protein phosphatase 1β is required for termination of β-arrestin-dependent signaling

Termination of signaling of activated G protein-coupled receptors (GPCRs) is essential for maintenance of cellular homeostasis. It is well established that β-arrestin redistributes to phosphorylated GPCRs and thereby facilitates desensitization of classical G protein-dependent signaling. β-Arrestin in turn serves as a scaffold to initiate a second wave of signaling. Here, we report a molecular mechanism that regulates the termination of unconventional β-arrestin-dependent GPCR signaling. We identify protein phosphatase 1β (PP1β) as a phosphatase for the cluster of phosphorylated threonines ((353)TTETQRT(359)) within the sst(2A) somatostatin receptor carboxyl terminus that mediates β-arrestin binding using siRNA knock-down screening. We show that PP1β-mediated sst(2A) dephosphorylation is initiated directly after receptor activation at or near the plasma membrane. As a functional consequence of diminished PP1β activity, we find that somatostatin- and substance P-induced but not epidermal growth factor-induced ERK activation was aberrantly enhanced and prolonged. Thus, we demonstrate a novel mechanism for fine tuning unconventional β-arrestin-dependent GPCR signaling in that recruitment of PP1β to activated GPCRs facilitates GPCR dephosphorylation and, hence, leads to disruption of the β-arrestin-GPCR complex.

Phosphorylation analysis of G protein-coupled receptor by mass spectrometry: identification of a phosphorylation site in V2 vasopressin receptor

Phosphorylation plays vital roles in the regulation and function of the V2 vasopressin receptor (V2R), a G protein-coupled receptor (GPCR) that is responsible for maintaining water homeostasis in the kidney. Through a combination of immunoaffinity purification, immobilized metal affinity chromatography, and nanoflow liquid chromatography tandem mass spectrometry, we identified a novel phosphorylation site (Ser(255)) in the third intracellular loop of human V2R. We showed that the third intracellular loop could be phosphorylated in vitro by protein kinase A, but not by Akt kinase, although sequence motif analysis predicated otherwise. The analytical procedures and methodologies described in this study should be generally applicable for identifying the endogenous phosphorylation sites in other GPCRs, overcoming the limitations of conventional approaches such as sequence motif analysis and site-directed mutagenesis.

Photoperiodic mechanisms controlling scent marking: interactions of vasopressin and gonadal steroids

Microinjection of arginine vasopressin (Avp) into the rostral hypothalamus of Syrian hamsters induces a form of scent marking known as flank marking. The ability of Avp to stimulate flank marking is mediated by the vasopressin 1a receptor (Avpr1a). In hamsters housed in long 'summer-like' photoperiods, the amount of flank marking and the number of Avpr1a receptors in the rostral hypothalamus are regulated by testosterone. However, hamsters housed in short 'winter-like' photoperiods for 6-8 weeks continue to flank mark at high levels despite significant reductions in the circulating levels of testosterone. In the present study, we compared the effects of gonadal steroids on Avp-induced flank marking and Avpr1a binding and affinity in hamsters housed in short photoperiods and those housed in long photoperiods. In long-photoperiod-housed hamsters, castration significantly reduced the amount of Avp-induced flank marking; however, in short-photoperiod-housed hamsters there were no significant differences between gonadally regressed and castrated hamsters. Surprisingly, Avpr1a receptor binding, but not affinity, in the medial preoptic area and the medial preoptic nucleus was significantly reduced in long-photoperiod-housed castrates as well as short-photoperiod-housed gonadally regressed and castrated hamsters, compared with long-photoperiod-housed gonadally intact hamsters. These data demonstrate that in short photoperiods Avp-induced flank marking is independent of gonadal hormones, despite gonadal steroid-dependent reductions in Avpr1a binding.

Dimerization of the thyrotropin-releasing hormone receptor potentiates hormone-dependent receptor phosphorylation

The G protein-coupled thyrotropin (TSH)-releasing hormone (TRH) receptor forms homodimers. Regulated receptor dimerization increases TRH-induced receptor endocytosis. These studies test whether dimerization increases receptor phosphorylation, which could potentiate internalization. Phosphorylation at residues 355-365, which is critical for internalization, was measured with a highly selective phospho-site-specific antibody. Two strategies were used to drive receptor dimerization. Dimerization of a TRH receptor-FK506-binding protein (FKBP) fusion protein was stimulated by a dimeric FKBP ligand. The chemical dimerizer caused a large increase in TRH-dependent phosphorylation within 1 min, whereas a monomeric FKBP ligand had no effect. The dimerizer did not alter phoshorylation of receptors lacking the FKBP domain. Dimerization of receptors containing an N-terminal HA epitope also was induced with anti-HA antibody. Anti-HA IgG strongly increased TRH-induced phosphorylation, whereas monomeric Fab fragments had no effect. Anti-HA antibody did not alter phosphorylation in receptors lacking an HA tag. Furthermore, two phosphorylation-defective TRH receptors functionally complemented one another and permitted phosphorylation. Receptors with a D71A mutation in the second transmembrane domain do not signal, whereas receptors with four Ala mutations in the 355-365 region signal normally but lack phosphorylation sites. When D71A- and 4Ala-TRH receptors were expressed alone, neither underwent TRH-dependent phosphorylation. When they were expressed together, D71A receptor was phosphorylated by G protein-coupled receptor kinases in response to TRH. These results suggest that the TRH receptor is phosphorylated preferentially when it is in dimers or when preexisting receptor dimers are driven into microaggregates. Increased receptor phosphorylation may amplify desensitization.

Vasopressin-induced translocation and proteolysis of protein kinase Cα in an amphibian brain: Modulation by corticosterone

In urodele amphibians, the hypothalamic neuropeptide arginine vasotocin and the adrenal steroid corticosterone interact to regulate reproductive behavior by actions in the brain. The present study investigated signal transduction pathways underlying acute effects of vasotocin and corticosterone, presumably mediated via "non-genomic" steroid action, in an amphibian brain. We used Western blot to examine the effects of corticosterone and the vasotocin receptor agonist arginine vasopressin, alone and in combination, on the subcellular localization and proteolytic processing of protein kinase C-alpha (PKCalpha) in tiger salamander brain tissue. Treatment of whole brain minces with vasopressin or vasotocin led to increases in PKCalpha in membrane fractions and concurrent decreases in PKCalpha in cytosolic fractions. Vasopressin or vasotocin treatment also induced the appearance in membrane and cytosolic fractions of a PKCalpha-immunoreactive band that corresponds to PKMalpha, the proteolytically generated, free catalytic subunit of PKCalpha. Treatment with corticosterone alone had no consistent effect on either PKCalpha or PKMalpha in either fraction. However, pretreatment with corticosterone reliably blocked vasopressin-induced increases in cytosolic PKMalpha. These data provide new information about the cellular mechanisms of action of vasopressin and corticosterone in the vertebrate brain and suggest a cellular mechanism by which the two hormones interact to regulate neuronal physiology and behavior.

Short-photoperiod exposure reduces vasopressin (V1a) receptor binding but not arginine-vasopressin-induced flank marking in male Syrian hamsters

In Syrian hamsters, socially relevant information is communicated with a form of scent marking known as flank marking. There is substantial evidence that arginine-vasopressin acting on V1a vasopressin receptors (V1aR) in the medial preoptic-anterior hypothalamic continuum (MPOA-AH) regulates the expression of flank marking. Previous studies have shown that the expression of flank marking is also influenced by the circulating concentrations of gonadal hormones. In hamsters housed in long 'summer-like' photoperiods (i.e. >12.5 h of light/day), castration reduces flank marking and administration of testosterone restores precastration levels of flank marking. When exposed to short 'winter-like' photoperiods (i.e. <12.5 h of light/day), hamsters undergo gonadal regression and the circulating levels of testosterone decline. Surprisingly, flank marking induced during social encounters is not reduced in hamsters exposed to short photoperiods despite the low circulating concentrations of testosterone. In the present study, it was hypothesized that reductions in testosterone, caused by exposure to short photoperiod, would not reduce the ability of vasopressin to stimulate flank marking by its actions in the MPOA-AH. The amount of flank marking induced by vasopressin injected into the MPOA-AH did not significantly differ between hamsters housed in long and short photoperiods; however, short photoperiod-exposed males had significantly less V1aR binding in the MPOA than long photoperiod-exposed males. These results support the hypothesis that the sensitivity of the MPOA-AH to vasopressin is not reduced in short photoperiod-exposed males, despite decreases in serum testosterone. However, by contrast to our predictions, short photoperiod-exposed males have significantly reduced V1aR binding in the MPOA-AH compared to long photoperiod-exposed males.

Expression levels strongly affect ligand-induced sequestration of B2 bradykinin receptors in transfected cells

Transfection of cells with expression vectors is one of the most important tools used to assess the effects of receptor mutations on ligand-induced receptor sequestration. Most transfection methods give rise to transiently or stably transfected clones with a wide range of receptor expression levels that may also depend on the mutations made. It is, therefore, important to determine how the regulation of the receptors depends on their numbers per cell. In Chinese hamster ovary (CHO) and human embryonic kidney (HEK)-293 cells expressing high levels of B(2) kinin receptors, we observed poor sequestration indicated by <20% reduction in cell surface receptor number after 10 min of stimulation with 1 microM bradykinin (BK) compared with >70% in low-expressing cells. Whereas the rate of [(3)H]BK internalization (internalized [(3)H]BK in percentage of total bound [(3)H]BK) in low-expressing cells was independent of the ligand-concentration used, in high-expressing cells a strong rate decrease was observed with higher (>1 nM) concentrations. Lower ligand concentrations, however, led to internalization rates identical to those obtained in low-expressing cells. Transiently transfected HEK and COS-7 cells showed results similar to those of stably high-expressing cells. Our results demonstrate the difficulty in determining the internalization pattern of (mutated) B(2) kinin receptors, and possibly of G protein-coupled receptors in general, using a sequestration assay in high-expressing cells or transiently transfected cells with high numbers of receptors per transfected cell. However, the receptor (mutant)-specific internalization rate can be measured, provided that the ligand concentrations used are below a threshold at which the internalization rate is still independent of the ligand concentration.

Molecular pharmacology and modeling of vasopressin receptors

AVP receptors represent a logical target for drug development. As a new class of therapeutic agents, orally active AVP analogs could be used to treat several human pathophysiological conditions including neurogenic diabetes insipidus, the syndrome of inappropriate secretion of AVP (SIADH), congestive heart failure, arterial hypertension, liver cirrhosis, nephrotic syndrome, dysmenorrhea, and ocular hypertension. By immunoprecipitation and immunoblotting, we elucidated the phosphorylation pattern of green fluorescent protein-tagged AVP receptors and showed interactions with the specific kinases PKC and GRK5 that are agonist-, time- and receptor subtype-dependent. The tyrosine residue of the NPWIY motif present in the 7th helix of AVP receptors is rapidly and transiently phosphorylated after agonist stimulation. This phosphorylation is instrumental in the genesis of the mitogenic cascade linked to the activation of this receptor, presumably by establishing key intramolecular contacts and by participating in the creation of a scaffold of proteins that produce the activation of downstream kinases. The random screening of chemical entities and optimization of lead compounds recently resulted in the development of orally active non-peptide AVP receptor agonists and antagonists. Furthermore, the identification of the molecular determinants of receptor-ligand interactions should facilitate the development of more potent and very selective orally active compounds via the approach of structure-based drug design. We developed three-dimensional molecular docking models of peptide and non-peptide ligands to the human V1 vascular, V2 renal and V3 pituitary AVP receptors. Docking of the peptide hormone AVP to the receptor ligand binding pockets reflects its dual polar and non-polar structure, but is receptor subtype-specific. The characteristics of non-peptide AVP analogs docking to the receptors are clearly distinct from those of peptide analogs docking. Molecular modeling of the results of site-directed mutagenesis experiments performed in CHO cells stably transfected with the human AVP receptor subtypes revealed that non-peptide antagonists establish key contacts with a few amino acid residues of the receptor subtypes that are different from those involved in agonist binding. Moreover, these interactions are species-specific. These findings provide further understanding of the signal transduction pathways of AVP receptors and new leads for elucidation of drug-receptor interactions and optimization of drug design. NOTE TO THE READER: The recent cloning and molecular characterization of AVP/OT receptor subtypes call for the revision of their nomenclature. For the sake of clarity and reference to their main site of expression, we call the V1a receptor the V1 vascular receptor, the V2 receptor the V2 renal receptor and the V1b or V3 receptor the V3 pituitary receptor in the present review.

Parathyroid hormone receptor recycling: role of receptor dephosphorylation and beta-arrestin

The recovery of PTH receptor (PTHR) function after acute homologous receptor desensitization and down-regulation in bone and kidney cells has been attributed to receptor recycling. To determine the role of receptor dephosphorylation in PTHR recycling, we performed morphological and functional assays on human embryonic kidney 293 cells stably expressing wild-type (wt) or mutant PTHRs. Confocal microscopy and ligand binding assays revealed that the wt PTHR is rapidly recycled back to the plasma membrane after removal of the agonist. Receptors that were engineered to either lack the sites of phosphorylation or to resemble constitutively phosphorylated receptors were able to recycle back to the plasma membrane with the same kinetics as the wt PTHR. The PTHR was found to be dephosphorylated by an enzyme apparently distinct from protein phosphatases 1 or 2A. The PTHR and beta-arrestin-2-green fluorescent protein (GFP) were found to stably colocalize during PTHR internalization, whereas after agonist removal and during receptor recycling, the colocalization slowly disappeared. Experiments using phosphorylation-deficient PTHRs and a dominant-negative form of beta-arrestin showed that beta-arrestin does not regulate the efficiency of PTHR recycling. These studies indicate that, unlike many G protein-coupled receptors, PTHR recycling does not require receptor dephosphorylation or its dissociation from beta-arrestin.

Immunocytochemical localization of vasopressin v1a receptors in the rat pituitary gonadotropes

Immunocytochemical labeling using a specific antibody against vasopressin V1a receptor allowed the localization of this receptor within a subset of cells from male rat anterior pituitary. The presence of transcripts of the corresponding gene in the anterior pituitary was confirmed by RT-PCR. Multiple immunocytochemical labeling combined with confocal microscopy allowed the identification of the V1a-labeled cells as gonadotropes. At the subcellular level, the vasopressin V1a receptor was mainly associated with cytoplasmic vesicles dispersed throughout the cell, which were not the secretory granules storing LH or FSH. In addition to effects exerted by vasopressin via central targets involved in the reproductive pathways, the presence of vasopressin V1a receptors on gonadotropes supports the controversial hypothesis of a local direct action of the neuropeptide on this cell type.

Somatostatin-induced paradoxical increase in intracellular Ca2+ concentration and insulin release in the presence of arginine vasopressin in clonal HIT-T15 beta-cells

Somatostatin, a hormone that signals via G(i)/G(o), usually inhibits increases in intracellular calcium concentration ([Ca(2+)](i)) and insulin release from beta-cells. We have found that in the presence of arginine vasopressin (AVP), which signals via G(q), somatostatin increased [Ca(2+)](i), leading to insulin release in HIT-T15 cells. The increase in [Ca(2+)](i) by somatostatin was observed even after 60 min of AVP treatment. Somatostatin alone failed to increase [Ca(2+)](i) and insulin release. Somatostatin induced changes in [Ca(2+)](i) in a biphasic pattern, characterized by a sharp and transient increase followed by a rapid decline to sub-basal levels. Pretreatment with pertussis toxin, which inactivates G(i)/G(o), abolished the effects of somatostatin. U-73122, an inhibitor of phospholipase C, antagonized the somatostatin-induced increase in [Ca(2+)](i). In Ca(2+)-free medium, somatostatin still increased [Ca(2+)](i). Depletion of intracellular Ca(2+) stores with thapsigargin, a microsomal Ca(2+)-ATPase inhibitor, abolished somatostatin's effect. In the presence of bradykinin, another G(q)-coupled receptor agonist, somatostatin also increased [Ca(2+)](i), but not in the presence of isoproterenol (a G(s)-coupled receptor agonist) or medetomidine (a G(i)/G(o)-coupled receptor agonist). Our findings suggest that somatostatin signals through G(i)/G(o), and involves phospholipase C and Ca(2+) release from the endoplasmic reticulum. The increase in [Ca(2+)](i) by somatostatin leads to insulin release. This cross-talk is specific to G(q) and G(i)/G(o), and is not limited to the AVP and somatostatin receptors.

The vasopressin receptors colocalize with vasopressin in the magnocellular neurons of the rat supraoptic nucleus and are modulated by water balance

Activity of the magnocellular neurons that synthesize vasopressin in the supraoptic and paraventricular nuclei of the hypothalamus is modulated by local release of the neuropeptide within the nuclei. V(1a) and V(1b) vasopressin receptor genes are expressed in these cells. The present study reports the localization of V(1a) and V(1b) receptors using multiple labeling immunocytochemistry. Both receptors are mainly located in vasopressinergic magnocellular neurons and colocalized with vasopressin in cytoplasmic vesicles dispersed throughout the cell. Possible functional modifications of the mRNA and protein levels of the V(1a) receptor, the major isoform, were also investigated by semiquantitative in situ hybridization and immunocytochemistry in rats submitted to reduced or increased water intake. V(1a) mRNA and receptor levels varied with water balance. V(1a) mRNA level dropped in rats submitted to high water intake. Conversely, dehydration up-regulated the V(1a) receptor content. These observations suggest that the pathways that regulate the expression of the genes encoding vasopressin and the V(1a) receptor are linked, which fits the present findings that the two partners are colocalized in cytoplasmic vesicles. Colocalization might explain how V(1) autoreceptors are controlled by cell activity and/or local concentration of vasopressin (released locally by the neurons themselves), allowing fine adjustment of magnocellular neuron activity.

Palmitoylation of the vasopressin V1a receptor reveals different conformational requirements for signaling, agonist-induced receptor phosphorylation, and sequestration

In this study, we establish that the V1a vasopressin receptor (V1aR) is palmitoylated, and we show that this modification has an important functional role. Palmitoylation of the V1aR occurs within the Cys371/Cys372 couplet located in the proximal C-terminal tail domain. Substitution of these residues in a [C371G/C372G]V1aR construct effectively disrupted receptor palmitoylation. Our data also indicate an additional palmitoylation site at another locus in the receptor, as yet undefined. [3H]Palmitate incorporation was agonist-sensitive and increased following exposure to [Arg8]vasopressin (AVP). Given the hydrophobic nature of the acyl chain, palmitoylation of the C terminus of G-protein-coupled receptors has been proposed to form an additional intracellular loop. Consequently, palmitoylation/depalmitoylation will have a profound effect on the local conformation of this domain. The V1aR palmitoylation status regulated both phosphorylation and sequestration of the receptor, and furthermore, palmitoylation, phosphorylation, and sequestration were all regulated by AVP. The palmitoylation-defective construct [C371G/C372G]V1aR exhibited decreased phosphorylation compared to wild-type V1aR, under both basal and AVP-stimulated conditions, and was sequestered at a faster rate. In contrast, the binding of four different classes of ligand and intracellular signaling were not affected by palmitoylation. This study therefore establishes that there are different conformational requirements for signaling, agonist-induced phosphorylation, and sequestration of the V1aR.

A growth factor antagonist as a targeting agent for sterically stabilized liposomes in human small cell lung cancer

The ability of a growth factor antagonist, [D-Arg(6),D-Trp(7,9)-N(me)Phe(8)]-substance P(6-11), named antagonist G, to selectively target polyethylene glycol-grafted liposomes (known as sterically stabilized liposomes) to a human classical small cell lung cancer (SCLC) cell line, H69, was examined. Our results showed that radiolabeled antagonist G-targeted sterically stabilized liposomes (SLG) bound to H69 cells with higher avidity than free antagonist G and were internalized (reaching a maximum of 13000 SLG/cell), mainly through a receptor-mediated process, likely involving clathrin-coated pits. This interaction was confirmed by confocal microscopy to be peptide- and cell-specific. Moreover, it was shown that SLG significantly improved the nuclear delivery of encapsulated doxorubicin to the target cells, increasing the cytotoxic activity of the drug over non-targeted liposomes. In mice, [(125)I]tyraminylinulin-containing SLG were long circulating, with a half-life of 13 h. Use of peptides like antagonist G to promote binding and internalization of sterically stabilized liposomes, with their accompanying drug loads, i.e., anticancer drugs, genes or antisense oligonucleotides, into target cells has the potential to improve therapy of SCLC.

Role of the human V1 vasopressin receptor COOH terminus in internalization and mitogenic signal transduction

We studied the role played by the intracellular COOH-terminal region of the human arginine vasopressin (AVP) V1-vascular receptor (V1R) in ligand binding, trafficking, and mitogenic signal transduction in Chinese hamster ovary cells stably transfected with the human AVP receptor cDNA clones that we had isolated previously. Truncations, mutations, or chimeric alterations of the V1R COOH terminus did not alter ligand binding, but agonist-induced V1R internalization and recycling were reduced in the absence of the proximal region of the V(1)R COOH terminus. Coupling to phospholipase C was altered as a function of the COOH-terminal length. Deletion of the proximal portion of the V1R COOH terminus or its replacement by the V2-renal receptor COOH terminus prevented AVP stimulation of DNA synthesis and progression through the cell cycle. Mutation of a kinase consensus motif in the proximal region of the V1R COOH terminus also abolished the mitogenic response. Thus the V1R cytoplasmic COOH terminus is not involved in ligand specificity but is instrumental in receptor trafficking and facilitates the interaction between the intracellular loops of the receptor, G protein, and phospholipase C. It is absolutely required for transmission of the mitogenic action of AVP, probably via a specific kinase phosphorylation site.

The basic and clinical pharmacology of nonpeptide vasopressin receptor antagonists

The neurohypophysial hormone arginine vasopressin (AVP) is a cyclic nonpeptide whose actions are mediated by the stimulation of specific G protein--coupled membrane receptors pharmacologically classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) AVP receptor subtypes. The random screening of chemical compounds and optimization of lead compounds recently resulted in the development of orally active nonpeptide AVP receptor antagonists. Potential therapeutic uses of AVP receptor antagonists include (a) the blockade of V1-vascular AVP receptors in arterial hypertension, congestive heart failure, and peripheral vascular disease; (b) the blockade of V2-renal AVP receptors in the syndrome of inappropriate vasopressin secretion, congestive heart failure, liver cirrhosis, nephrotic syndrome and any state of excessive retention of free water and subsequent dilutional hyponatremia; (c) the blockade of V3-pituitary AVP receptors in adrenocorticotropin-secreting tumors. The pharmacological and clinical profile of orally active nonpeptide vasopressin receptor antagonists is reviewed here.

The long and the short cycle. Alternative intracellular routes for trafficking of G-protein-coupled receptors

The C terminus of the human V2 vasopressin receptor contains multiple phosphorylation sites including a cluster of amino acids that when phosphorylated prevents the return of the internalized receptor to the cell surface. To identify the step where the recycling process was interrupted, the trafficking of the V2 receptor was compared with that of the recycling V1a receptor after exposure to ligand. Initially, both receptors internalized in small peripheral endosomes, but a physical separation of their endocytic pathways was subsequently detected. The V1a receptor remained evenly distributed throughout the cytosol, whereas the V2 receptor accumulated in a large aggregation of vesicles in the proximity of the nucleus where it colocalized with the transferrin receptor and Rab11, a small GTP-binding protein that is concentrated in the perinuclear recycling compartment; only marginal colocalization of Rab11 with the V1a receptor was observed. Thus, the V2 receptor was sequestered in the perinuclear recycling compartment. Targeting to the perinuclear recycling compartment was determined by the receptor subtype and not by the inability to recycle, since the mutation S363A in the phosphorylation-dependent retention signal generated a V2 receptor that was recycled via the same compartment. The perinuclear recycling compartment was enriched in beta-arrestin after internalization of either wild type V2 receptor or its recycling mutant, indicating that long term interaction between the receptors and arrestin was not responsible for the intracellular retention. Thus, the fully phosphorylated retention domain overrides the natural tendency of the V2 receptor to recycle and, by preventing its exit from the perinuclear recycling compartment, interrupts its transit via the "long cycle." The data suggest that the inactivation of the domain, possibly by dephosphorylation, triggers the return of the receptor from the perinuclear compartment to the plasma membrane.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

The role of phosphorylation/dephosphorylation in agonist-induced desensitization of D1 dopamine receptor function: evidence for a novel pathway for receptor dephosphorylation

Exposure of D1 dopamine receptors to agonists results in rapid desensitization of the receptor-stimulated accumulation of cAMP. It is believed that agonist-induced phosphorylation of the receptor plays a critical role in the processes that underlie this phenomenon. To investigate the role of agonist-induced receptor phosphorylation, a FLAG epitope was added to the amino terminus of the rat D1 dopamine receptor and this construct was stably expressed in C6 glioma cells. It was found that the D1 receptor was stoichiometrically phosphorylated under basal conditions and that its phosphorylation state was increased by 2- to 3-fold upon exposure of the cells to dopamine for 10 min. The dopamine-induced receptor phosphorylation could be blocked by D1-selective antagonists but was unaffected by inhibitors of either protein kinase A or protein kinase C. The incorporation of phosphate into the receptor was rapid but transient, despite the continued presence of dopamine. A comparison of the rates of receptor phosphorylation approximately ion (t(1/2) < 1 min) and dopamine-induced desensitization (t(1/2) approximately 7 min) revealed that receptor phosphorylation was not the rate limiting step for receptor desensitization. Upon removal of dopamine, the receptor was rapidly dephosphorylated (t(1/2) approximately 10 min) and this was not blocked by agents (i.e., concanavalin A or hypertonic sucrose) that inhibit D1 receptor internalization. Using specific inhibitors, the phosphatase involved in D1 receptor dephosphorylation was shown not to correlate with the recently identified "G protein-coupled receptor phosphatase" (Proc Natl Acad Sci USA 92:8343-8347, 1995). These results suggest that the phosphorylated D(1) receptor is processed through a novel recovery pathway and that internalization is not required for receptor dephosphorylation.

Vasopressin receptors

The biological effects of arginine vasopressin (AVP) are mediated by three receptor subtypes: the V1a and V1b receptors that activate phospholipases via Gq/11, and the V2 receptor that activates adenylyl cyclase by interacting with Gs. Isolation of the cDNAs encoding the V1a and V1b receptor subtypes explained the tissue variability of V1 antagonist binding, whereas identification of the cDNA and gene encoding the V2 receptor provided the information to identify the mutations responsible for X-linked nephrogenic diabetes insipidus. Mutations that abrogate the production and/or release of AVP from the pituitary have diabetes insipidus as their most dramatic manifestation, indicating that the maintenance of water homeostasis is the most important physiological role of this neuropeptide. Evidence for a significant role of AVP in blood pressure control, although actively sought, has been scant.

Cell cycle-dependent coupling of the vasopressin V1a receptor to different G proteins

Arginine vasopressin (AVP) regulates biological processes by binding to G protein-coupled receptors. In Swiss 3T3 fibroblasts, expressing the V(1a) subtype of vasopressin receptors, AVP mobilizes calcium from intracellular stores. In proliferating cells, the AVP-induced increase in intracellular calcium concentration ([Ca(2+)](i)) was mediated by G proteins of the G(q) family, which are insensitive to pertussis toxin (PTX) pretreatment of the cells. In quiescent cells, the AVP-induced increase in [Ca(2+)](i) was partially PTX-sensitive, suggesting an involvement of G(i) proteins. We confirmed this by photoaffinity labeling of G proteins in Swiss 3T3 cell membranes activated by AVP. In Swiss 3T3 cells arrested in the G(0)/G(1) phase of the cell cycle, the AVP-induced increase in [Ca(2+)](i) was also partially PTX-sensitive but was PTX-insensitive in cells arrested in other phases of the cell cycles. The blocking effect of PTX pretreatment in G(0)/G(1) cells was mimicked by microinjection of antisense oligonucleotides suppressing the expression of the Galpha(i3) subunits. These results were confirmed by microinjection of antibodies directed against the C terminus of G protein alpha-subunits. The data presented indicate that in Swiss 3T3 fibroblasts synchronized in the G(0)/G(1) phase of the cell cycle the V(1a) receptor couples to G(q/11) and G(i3) to activate the phospholipase C-beta, leading to release of intracellular calcium.

Vasopressin V1a receptor signaling in a rat choroid plexus cell line

A new cell line was derived from primary culture of rat choroid plexus (RCP) by immortalization with the TSOri minus adenovirus. The selected clone expressed vasopressin V1a receptors at a density of 64,000 sites per cell, and a K(d) of 7.2 nM. Addition of vasopressin to the RCP cells induced a transient calcium peak comparable to V1a receptor signalling in different expression systems. This [Ca(2+)](i) increase was dose-dependent with an EC(50) of 22 nM vasopressin. Similar [Ca(2+)](i) increase was elicited by addition of serotonin, angiotensin II, endothelin-1, and bradykinin. Heterologous desensitization of V1a receptor was observed in RCP cells exposed to the phorbol ester PMA or following stimulation of other receptors coupled to the phosphoinositide pathway. Positive immunolabelling with Factor VIII, Flt1 and CD 34 antibodies suggests that this new RCP cell line originated from endothelial cells of rat choroid plexus.

Association of beta-arrestin with G protein-coupled receptors during clathrin-mediated endocytosis dictates the profile of receptor resensitization

Resensitization of G protein-coupled receptors (GPCRs) following agonist-mediated desensitization is a necessary step for maintaining physiological responsiveness. However, the molecular mechanisms governing the nature of GPCR resensitization are poorly understood. Here, we examine the role of beta-arrestin in the resensitization of the beta(2) adrenergic receptor (beta(2)AR), known to recycle and resensitize rapidly, and the vasopressin V2 receptor (V2R), known to recycle and resensitize slowly. Upon agonist activation, both receptors recruit beta-arrestin to the plasma membrane and internalize in a beta-arrestin- and clathrin-dependent manner. However, whereas beta-arrestin dissociates from the beta(2)AR at the plasma membrane, it internalizes with the V2R into endosomes. The differential trafficking of beta-arrestin and the ability of these two receptors to dephosphorylate, recycle, and resensitize is completely reversed when the carboxyl-terminal tails of these two receptors are switched. Moreover, the ability of beta-arrestin to remain associated with desensitized GPCRs during clathrin-mediated endocytosis is mediated by a specific cluster of phosphorylated serine residues in the receptor carboxyl-terminal tail. These results demonstrate that the interaction of beta-arrestin with a specific motif in the GPCR carboxyl-terminal tail dictates the rate of receptor dephosphorylation, recycling, and resensitization, and thus provide direct evidence for a novel mechanism by which beta-arrestins regulate the reestablishment of GPCR responsiveness.

Role of the carboxyl-terminal region, di-leucine motif and cysteine residues in signalling and internalization of vasopressin V1a receptor

The structural requirements for internalization and signalling of the vasopressin V1a receptor were investigated in stably transfected HEK-293 cells. Removal of the 51 C-terminal amino acids did not affect vasopressin binding, calcium signalling, heterologous desensitization or internalization of the receptor. Deletion of 14 additional amino acids reduced vasopressin-dependent calcium increase and impaired receptor internalization. Substitution of cysteines 371-372 did not affect intracellular signalling, but decreased endocytosis by 26%. Substitution of the 361-362 leucine by alanine residues reduced by 56% V1a receptor sequestration without affecting calcium signalling. These results indicate that di-cysteine and mostly di-leucine motifs present in the C-terminal region of the V1a receptor are involved in its internalization.

Phosphorylation is required for alteration of kv1.5 K(+) channel function by the Kvbeta1.3 subunit

The Kv1.5 K(+) channel is functionally altered by coassembly with the Kvbeta1.3 subunit, which induces fast inactivation and a hyperpolarizing shift in the activation curve. Here we examine kinase regulation of Kv1.5/Kvbeta1.3 interaction after coexpression in human embryonic kidney 293 cells. The protein kinase C inhibitor calphostin C (3 microM) removed the fast inactivation (66 +/- 1.9 versus 11 +/- 0.25%, steady state/peak current) and the beta-induced hyperpolarizing voltage shift in the activation midpoint (V(1/2)) (-21.9 +/- 1.4 versus -4.3 +/- 2.0 mV). Calphostin C had no effect on Kv1.5 alone with respect to inactivation kinetics and V(1/2). Okadaic acid, but not the inactive derivative, blunted both calphostin C effects (V(1/2) = -17.6 +/- 2.2 mV, 38 +/- 1.8% inactivation), consistent with dephosphorylation being required for calphostin C action. Calphostin C also removed the fast inactivation (57 +/- 2.6 versus 16 +/- 0.6%) and the shift in V(1/2) (-22.1 +/- 1.4 versus -2.1 +/- 2.0 mV) conferred onto Kv1.5 by the Kvbeta1.2 subunit, which shares only C terminus sequence identity with Kvbeta1. 3. In contrast, modulation of Kv1.5 by the Kvbeta2.1 subunit was unaffected by calphostin C. These data suggest that Kvbeta1.2 and Kvbeta1.3 subunit modification of Kv1.5 inactivation and voltage sensitivity require phosphorylation by protein kinase C or a related kinase.

G-protein coupled receptor kinases as modulators of G-protein signalling

G-protein coupled receptors (GPCRs) comprise one of the largest classes of signalling molecules. A wide diversity of activating ligands induce the active conformation of GPCRs and lead to signalling via heterotrimeric G-proteins and downstream effectors. In addition, a complex series of reactions participate in the 'turn-off' of GPCRs in both physiological and pharmacological settings. Some key players in the inactivation or 'desensitization' of GPCRs have been identified, whereas others remain the target of ongoing studies. G-protein coupled receptor kinases (GRKs) specifically phosphorylate activated GPCRs and initiate homologous desensitization. Uncoupling proteins, such as members of the arrestin family, bind to the phosphorylated and activated GPCRs and cause desensitization by precluding further interactions of the GPCRs and G-proteins. Adaptor proteins, including arrestins, and endocytic machinery participate in the internalization of GPCRs away from their normal signalling milieu. In this review we discuss the roles of these regulatory molecules as modulators of GPCR signalling.

Desensitization of G-protein-coupled receptors in the cardiovascular system

Multiple mechanisms exist to control the signaling and density of G-protein-coupled receptors (GPRs). Upon agonist binding and receptor activation, a series of reactions participate in the turn off or desensitization of GPRs. Many GPRs are phosphorylated by protein kinases and consequently uncoupled from G proteins. In addition, many GPRs are sequestered from the cell surface and become inaccessible to their activating ligands. Both receptor:G protein uncoupling and receptor sequestration may involve the participation of arrestins or other proteins. A model for receptor regulation has been developed from studies of the beta-adrenergic receptor. However, recent studies suggest that other GPRs important in the cardiovascular system, such as the muscarinic cholinergic receptors that regulate heart rate, might be regulated by mechanisms other than those that regulate the beta-adrenergic receptors. This review summarizes our current understanding of the processes involved in the desensitization of GPRs.

Phosphorylation and recycling kinetics of G protein-coupled receptors

The rate of ligand-induced phosphorylation of the V2 and V1a vasopressin receptors was characterized in HEK 293 cells. Both receptors were phosphorylated predominantly by GRKs, and the V1a receptor was also phosphorylated by protein kinase C regardless of the presence or absence of ligand. Phosphorylation of the V1aR catalyzed by GRKs reached maximal values at the shortest measured time: 15 seconds, and decayed rapidly with a t1/2 of 6 min in the continuous presence of AVP. In agreement with the hypothesis that dephosphorylation must precede receptor recycling to the cell surface, the V1aR returned rapidly to the cell surface after removal of the hormone from the medium. Phosphate incorporation into the V2R proceeded at a slower pace, and the internalized phosphorylated receptor failed to recycle to the cell surface and retained its phosphate for a long time in the presence or absence of ligand. A single mutation in the carboxy terminus of the V2R accelerated de-phosphorylation of the protein and conferred recycling properties to the V2R. These experiments provided molecular evidence for the hypothesis that internalization is required for de-phosphorylation and recycling of reactivated G protein coupled receptors to the cell surface.

A serine cluster prevents recycling of the V2 vasopressin receptor

Receptor recycling plays a critical role in the regulation of cellular responsiveness to environmental stimuli. Agonist-promoted phosphorylation of G protein-coupled receptors has been related to their desensitization, internalization, and sequestration. Dephosphorylation of internalized G protein-coupled receptors by cytoplasmic phosphatases has been shown to be pH-dependent, and it has been postulated to be necessary for receptors to recycle to the cell surface. The internalized V2 vasopressin receptor (V2R) expressed in HEK 293 cells is an exception to this hypothesis because it does not recycle to the plasma membrane for hours after removal of the ligand. Because this receptor is phosphorylated only by G protein-coupled receptor kinases (GRKs), the relationship between recycling and GRK-mediated phosphorylation was examined. A nonphosphorylated V2R, truncated upstream of the GRK phosphorylation sites, rapidly returned to the cell surface after removal of vasopressin. Less-drastic truncations of V2R revealed the presence of multiple phosphorylation sites and suggested a key role for a serine cluster present at the C terminus. Replacement of any one of Ser-362, Ser-363, or Ser-364 with Ala allowed quantitative recycling of full-length V2R without affecting the extent of internalization. Examination of the stability of phosphate groups incorporated into the recycling S363A mutant V2Rs revealed that the recycling receptor was dephosphorylated after hormone withdrawal, whereas the wild-type V2R was not, providing molecular evidence for the hypothesis that GRK sites must be dephosphorylated prior to receptor recycling. These experiments uncovered a role for GRK phosphorylation in intracellular sorting and revealed a GRK-dependent anchoring domain that blocks V2R recycling.