Characterization of three vasopressin receptor 2 variants: an apparent polymorphism (V266A) and two loss-of-function mutations (R181C and M311V)

V2 vasopressin receptor mutations: future personalized therapy based on individual molecular biology

The diluting and concentrating function of the kidney plays a crucial role in regulating the water homeostasis of the body. This function is regulated by the antidiuretic hormone, arginine vasopressin through the type 2 vasopressin receptor (V2R), allowing the body to adapt to periods of water load or water restriction. Loss-of-function mutations of the V2R cause X-linked nephrogenic diabetes insipidus (XNDI), which is characterized by polyuria, polydipsia, and hyposthenuria. Gain-of-function mutations of the V2R lead to nephrogenic syndrome of inappropriate antidiuresis disease (NSIAD), which results in hyponatremia. Various mechanisms may be responsible for the impaired receptor functions, and this review provides an overview of recent findings about the potential therapeutic interventions in the light of the current experimental data.

Gαs and Gαq/11 protein coupling bias of two AVPR2 mutants (R68W and V162A) that cause nephrogenic diabetes insipidus

Loss-of-function mutations of the arginine vasopressin receptor 2 gene (AVPR2) cause Nephrogenic diabetes insipidus (NDI). AVPR2 is a kind of G protein coupled receptor (GPCR) and mainly couples with Gαs protein leading to cAMP accumulation in the cell as a secondary messenger. Recent studies showed that some AVPR2 mutations could cause biased Gαq/11 protein coupling rather than Gαs. Investigation into the characterization of biased receptors may give insights into the relationship between the conformational change of the receptor because of the mutation and related downstream signaling. In this study, R68W and V162A were analyzed to whether they show a bias to Gαs or Gαq/11 proteins. Their functionality in terms of cAMP production via Gαs protein coupling was decreased compared to the wild-type receptor. On the other hand, they showed the ability to couple with Gαq/11 protein and make Ca²⁺ mobilization at different levels in the cell. R68W showed bias to coupling with Gαq/11 protein rather than V162A and wild-type receptor. Studies about the Gα protein coupling bias of mutant AVPR2s may broaden our understanding of the relationship between the changed conformation of the receptor and consequently activated signaling pathways, and also may shed light on the development of more effective new therapeutics.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Agonist-induced formation of unproductive receptor-G12 complexes

G protein-coupled receptors (GPCRs) are targeted by a large fraction of approved drugs and regulate many important cellular processes. Association of GPCRs with heterotrimeric G proteins in response to agonist activation is thought to invariably lead to G protein activation. We find instead that G₁₂ heterotrimers can associate with agonist-bound receptors in a manner that does not lead to activation. These unproductive agonist–receptor-G protein ternary complexes sequester G₁₂ heterotrimers and thus inhibit rather than support G₁₂ signaling. These findings reveal a mechanism whereby agonist activation of GPCRs can inhibit as well as promote G protein signaling.

G proteins are activated when they associate with G protein-coupled receptors (GPCRs), often in response to agonist-mediated receptor activation. It is generally thought that agonist-induced receptor-G protein association necessarily promotes G protein activation and, conversely, that activated GPCRs do not interact with G proteins that they do not activate. Here we show that GPCRs can form agonist-dependent complexes with G proteins that they do not activate. Using cell-based bioluminescence resonance energy transfer (BRET) and luminescence assays we find that vasopressin V₂ receptors (V₂R) associate with both G_s and G₁₂ heterotrimers when stimulated with the agonist arginine vasopressin (AVP). However, unlike V₂R-G_s complexes, V₂R-G₁₂ complexes are not destabilized by guanine nucleotides and do not promote G₁₂ activation. Activating V₂R does not lead to signaling responses downstream of G₁₂ activation, but instead inhibits basal G₁₂-mediated signaling, presumably by sequestering G₁₂ heterotrimers. Overexpressing G₁₂ inhibits G protein receptor kinase (GRK) and arrestin recruitment to V₂R and receptor internalization. Formyl peptide (FPR1 and FPR2) and Smoothened (Smo) receptors also form complexes with G₁₂ that are insensitive to nucleotides, suggesting that unproductive GPCR-G₁₂ complexes are not unique to V₂R. These results indicate that agonist-dependent receptor-G protein association does not always lead to G protein activation and may in fact inhibit G protein activation.

A structural basis for how ligand binding site changes can allosterically regulate GPCR signaling and engender functional selectivity

Signaling bias is the propensity for some agonists to preferentially stimulate G protein-coupled receptor (GPCR) signaling through one intracellular pathway versus another. We previously identified a G protein-biased agonist of the D₂ dopamine receptor (D2R) that results in impaired β-arrestin recruitment. This signaling bias was predicted to arise from unique interactions of the ligand with a hydrophobic pocket at the interface of the second extracellular loop and fifth transmembrane segment of the D2R. Here, we showed that residue Phe189 within this pocket (position 5.38 using Ballesteros-Weinstein numbering) functions as a microswitch for regulating receptor interactions with β-arrestin. This residue is relatively conserved among class A GPCRs, and analogous mutations within other GPCRs similarly impaired β-arrestin recruitment while maintaining G protein signaling. To investigate the mechanism of this signaling bias, we used an active-state structure of the β₂-adrenergic receptor (β2R) to build β2R-WT and β2R-Y199^5.38A models in complex with the full β2R agonist BI-167107 for molecular dynamics simulations. These analyses identified conformational rearrangements in β2R-Y199^5.38A that propagated from the extracellular ligand binding site to the intracellular surface, resulting in a modified orientation of the second intracellular loop in β2R-Y199^5.38A, which is predicted to affect its interactions with β-arrestin. Our findings provide a structural basis for how ligand binding site alterations can allosterically affect GPCR-transducer interactions and result in biased signaling.

Cannabinoid CB1 and CB2 Receptor-Mediated Arrestin Translocation: Species, Subtype, and Agonist-Dependence

Arrestin translocation and signaling have come to the fore of the G protein-coupled receptor molecular pharmacology field. Some receptor–arrestin interactions are relatively well understood and considered responsible for specific therapeutic or adverse outcomes. Coupling of arrestins with cannabinoid receptors 1 (CB₁) and 2 (CB₂) has been reported, though the majority of studies have not systematically characterized the differential ligand dependence of this activity. In addition, many prior studies have utilized bovine (rather than human) arrestins, and the most widely applied assays require reporter-tagged receptors, which prevent meaningful comparison between receptor types. We have employed a bioluminescence resonance energy transfer (BRET) method that does not require the use of tagged receptors and thereby allows comparisons of arrestin translocation between receptor types, as well as with cells lacking the receptor of interest – an important control. The ability of a selection of CB₁ and CB₂ agonists to stimulate cell surface translocation of human and bovine β-arrestin-1 and -2 was assessed. We find that some CB₁ ligands induce moderate β-arrestin-2 translocation in comparison with vasopressin V₂ receptor (a robust arrestin recruiter); however, CB₁ coupling with β-arrestin-1 and CB₂ with either arrestin elicited low relative efficacies. A range of efficacies between ligands was evident for both receptors and arrestins. Endocannabinoid 2-arachidonoylglycerol stood out as a high efficacy ligand for translocation of β-arrestin-2 via CB₁. Δ⁹-tetrahydrocannabinol was generally unable to elicit translocation of either arrestin subtype via CB₁ or CB₂; however, control experiments revealed translocation in cells not expressing CB₁/CB₂, which may assist in explaining some discrepancy with the literature. Overexpression of GRK2 had modest influence on CB₁/CB₂-induced arrestin translocation. Results with bovine and human arrestins were largely analogous, but a few instances of inconsistent rank order potencies/efficacies between bovine and human arrestins raise the possibility that subtle differences in receptor conformation stabilized by these ligands manifest in disparate affinities for the two arrestin species, with important potential consequences for interpretation in ligand bias studies. As well as contributing important information regarding CB₁/CB₂ ligand-dependent arrestin coupling, our study raises a number of points for consideration in the design and interpretation of arrestin recruitment assays.

Evidence for Effective Multiple K+-Current Inhibitions by Tolvaptan, a Non-peptide Antagonist of Vasopressin V2 Receptor

Tolvaptan (TLV), an oral non-peptide antagonist of vasopressin V₂ receptor, has been increasingly used for managements in patients with hyponatremia and/or syndrome of inappropriate antidiuretic hormone secretion. However, none of the studies have thus far been investigated with regard to its possible perturbations on membrane ion currents in endocrine or neuroendocrine cells. In our electrophysiological study, the whole-cell current recordings showed that the presence of TLV effectively and differentially suppressed the amplitude of delayed rectifier K⁺ (I_K(DR)) and M-type K⁺ current (I_K(M)) in pituitary GH₃ cells with an IC₅₀ value of 6.42 and 1.91 μM, respectively. This compound was also capable of shifting the steady-state activation curve of I_K(M) to less depolarized potential without any appreciable change in the gating charge of this current. TLV at a concentration greater than 10 μM also suppressed the amplitude of erg-mediated K⁺ current or the activity of large-conductance Ca²⁺-activated K⁺ channels; however, this compound failed to alter the amplitude of hyperpolarization-activated cation current in GH₃ cells. In vasopressin-preincubated GH₃ cells, TLV-mediated suppression of I_K(M) remained little altered. Under current-clamp condition, we also observed that addition of TLV increased the firing of spontaneous action potentials in GH₃ cells and further addition of flupirtine could reverse TLV-mediated elevation of the firing. In Madin-Darby canine kidney (MDCK) cells, the K⁺ current elicited by long ramp pulse was also effectively subject to inhibition by this compound. Findings from the present study were thus stated as saying that the suppression by TLV of multiple type K⁺ currents could be direct and independent of its antagonism of vasopressin V₂ receptors. Our study also reveals an important aspect that should be considered when assessing aquaretic effect of TLV or its structurally similar compounds.

Standard Curves Are Necessary to Determine Pharmacological Properties for Ligands in Functional Assays Using Competition Binding Technologies

Homogeneous functional assays that utilize competition binding technology are widely used for determining pharmacological properties such as intrinsic activity and potency. One example is time-resolved fluorescence resonance energy transfer (TR-FRET) 3',5'-cyclic adenosine monophosphate (cAMP) assays, where labeled cAMP (tracer) and a labeled anti-cAMP antibody bind together to produce a TR-FRET signal when the two constituents are proximal to each other. This signal is disrupted when unlabeled and cellularly generated cAMP competes with the tracer cAMP for binding to the labeled antibody. It is important that the resulting assay signal, usually expressed as a TR-FRET ratio, be transformed to cAMP concentration using a cAMP standard curve. However, examples are still generated in the literature wherein investigators have used the ratiometric signal (not transformed using a standard curve) to determine values for intrinsic activity and potency of ligands. Untransformed raw data often produce reasonable looking sigmoidal concentration response curves, perhaps tempting investigators to use the raw data instead of the transformed data for applying pharmacological models. In this article, we describe the correct procedure for determining the potency and intrinsic activity of ligands that result in changes in cAMP levels using a lysate dilution assay of GLP-1 (7-36)-mediated TR-FRET cAMP accumulation and simulated data. We also highlight how the inappropriate use of raw signal data can dramatically affect interpretation of intrinsic activity and potency of ligands, and how this can adversely affect drug discovery programs. These findings apply not only to cAMP functional assays but also to other functional cellular signaling assays that utilize competition binding technologies.

Pharmacoperone rescue of vasopressin 2 receptor mutants reveals unexpected constitutive activity and coupling bias

Pharmacoperones are small molecules that diffuse into cells and rescue misfolded, mistrafficked protein mutants, restoring their function. These substances act with high target specificity, serving as templates to fold (or refold) receptors, enzymes, ion channels or other proteins and enable them to pass the scrutiny of the cellular quality control system ("rescue"). In the present study we demonstrate that a rescued mutant (L83Q) of the vasopressin type 2 receptor (V2R), shows a strong bias for Gs coupling unlike the WT V2 receptor, which couples to both Gs and Gq/11. Failure of the mutant to couple to Gq/11 was not due to a limiting quantity of G-proteins since other Gq/11-coupled receptors (WT V2R, histamine receptor and muscarinic receptor) responded appropriately to their ligands. Transfection with DNA encoding Gq enabled the V2 receptor mutant to couple to this G protein, but only modestly compared with the WT receptor. Fourteen V2R mutant pharmacoperones, of multiple chemical classes, obtained from a high throughput screen of a 660,000 structure library, and one V2R peptidomimetic antagonist rescues L83Q. The rescued mutant shows similar bias with all pharmacoperones identified, suggesting that the bias is intrinsic to the mutant protein's structure, rather than due to the chemical class of the pharmacoperone. In the case of V2R mutant Y128S, rescue with a pharmacoperone revealed constitutive activity, also with bias for Gs, although both IP and cAMP were produced in response to agonist. These results suggest that particular rescued receptor mutants show functional characteristics that differ from the WT receptor; a finding that may be important to consider as pharmacoperones are developed as therapeutic agents.

Genetics of Diabetes Insipidus

Diabetes insipidus is a disease characterized by polyuria and polydipsia due to inadequate release of arginine vasopressin from the posterior pituitary gland (neurohypophyseal diabetes insipidus) or due to arginine vasopressin insensitivity by the renal distal tubule, leading to a deficiency in tubular water reabsorption (nephrogenic diabetes insipidus). This article reviews the genetics of diabetes insipidus in the context of its diagnosis, clinical presentation, and therapy.

Vasopressin regulates the growth of the biliary epithelium in polycystic liver disease

The neurohypophysial hormone arginine vasopressin (AVP) acts by three distinct receptor subtypes: V1a, V1b, and V2. In the liver, AVP is involved in ureogenesis, glycogenolysis, neoglucogenesis and regeneration. No data exist about the presence of AVP in the biliary epithelium. Cholangiocytes are the target cells in a number of animal models of cholestasis, including bile duct ligation (BDL), and in several human pathologies, such as polycystic liver disease characterized by the presence of cysts that bud from the biliary epithelium. In vivo, liver fragments from normal and BDL mice and rats as well as liver samples from normal and ADPKD patients were collected to evaluate: (i) intrahepatic bile duct mass by immunohistochemistry for cytokeratin-19; and (ii) expression of V1a, V1b and V2 by immunohistochemistry, immunofluorescence and real-time PCR. In vitro, small and large mouse cholangiocytes, H69 (non-malignant human cholangiocytes) and LCDE (human cholangiocytes from the cystic epithelium) were stimulated with vasopressin in the absence/presence of AVP antagonists such as OPC-31260 and Tolvaptan, before assessing cellular growth by MTT assay and cAMP levels. Cholangiocytes express V2 receptor that was upregulated following BDL and in ADPKD liver samples. Administration of AVP increased proliferation and cAMP levels of small cholangiocytes and LCDE cells. We found no effect in the proliferation of large mouse cholangiocytes and H69 cells. Increases were blocked by preincubation with the AVP antagonists. These results showed that AVP and its receptors may be important in the modulation of the proliferation rate of the biliary epithelium.

Signaling Modification by GPCR Heteromer and Its Implication on X-Linked Nephrogenic Diabetes Insipidus

The involvement of secretin (SCT) and secretin receptor (SCTR) in regulating body water homeostasis is well established. Identified as one of the vasopressin (Vp)-independent mechanisms in fluid balance, SCT regulates aquaporin 2 (AQP2) in the kidney distal collecting duct cells through activating intracellular cAMP production. This ability to bypass Vp-mediated water reabsorption in kidney implicates SCT’s potential to treat nephrogenic diabetes insipidus (NDI). Research on NDI in the past has largely been focused on the searching for mutations in vasopressin receptor 2 (AVPR2), while the functional relationship between SCTR, AVPR2 and NDI remains unclear. Here, we demonstrate the interaction between SCTR and AVPR2 to modulate cellular signaling in vitro. Interestingly, we show in this report that upon heteromer formation with SCTR, R137H, a NDI-causing AVPR2 mutant that is defective in trafficking to cell surface, can functionally be rescued. Our data may provide an explanation for this clinically mild case of NDI, and insights into the pathological development of NDI in the future.

Mutations of Vasopressin Receptor 2 Including Novel L312S Have Differential Effects on Trafficking

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a genetic disease first described in 2 unrelated male infants with severe symptomatic hyponatremia. Despite undetectable arginine vasopressin levels, patients have inappropriately concentrated urine resulting in hyponatremia, hypoosmolality, and natriuresis. Here, we describe and functionally characterize a novel vasopressin type 2 receptor (V2R) gain-of-function mutation. An L312S substitution in the seventh transmembrane domain was identified in a boy presenting with water-induced hyponatremic seizures at the age of 5.8 years. We show that, compared with wild-type V2R, the L312S mutation results in the constitutive production of cAMP, indicative of the gain-of-function NSIAD profile. Interestingly, like the previously described F229V and I130N NSIAD-causing mutants, this appears to both occur in the absence of notable constitutive β-arrestin2 recruitment and can be reduced by the inverse agonist Tolvaptan. In addition, to understand the effect of various V2R substitutions on the full receptor "life-cycle," we have used and further developed a bioluminescence resonance energy transfer intracellular localization assay using multiple localization markers validated with confocal microscopy. This allowed us to characterize differences in the constitutive and ligand-induced localization and trafficking profiles of the novel L312S mutation as well as for previously described V2R gain-of-function mutants (NSIAD; R137C and R137L), loss-of-function mutants (nephrogenic diabetes insipidus; R137H, R181C, and M311V), and a putative silent V266A V2R polymorphism. In doing so, we describe differences in trafficking between unique V2R substitutions, even at the same amino acid position, therefore highlighting the value of full and thorough characterization of receptor function beyond simple signaling pathway analysis.

Resonance Energy Transfer-Based Approaches to Study GPCRs

Since their discovery, G protein-coupled receptors (GPCRs) constitute one of the most studied proteins leading to important discoveries and perspectives in terms of their biology and implication in physiology and pathophysiology. This is mostly linked to the remarkable advances in the development and application of the biophysical resonance energy transfer (RET)-based approaches, including bioluminescence and fluorescence resonance energy transfer (BRET and FRET, respectively). Indeed, BRET and FRET have been extensively applied to study different aspects of GPCR functioning such as their activation and regulation either statically or dynamically, in real-time and intact cells. Consequently, our view on GPCRs has considerably changed opening new challenges for the study of GPCRs in their native tissues in the aim to get more knowledge on how these receptors control the biological responses. Moreover, the technological aspect of this field of research promises further developments for robust and reliable new RET-based assays that may be compatible with high-throughput screening as well as drug discovery programs.

Novel mutations associated with nephrogenic diabetes insipidus. A clinical-genetic study

Molecular diagnosis is a useful diagnostic tool in primary nephrogenic diabetes insipidus (NDI), an inherited disease characterized by renal inability to concentrate urine. The AVPR2 and AQP2 genes were screened for mutations in a cohort of 25 patients with clinical diagnosis of NDI. Patients presented with dehydration, polyuria-polydipsia, failure to thrive (mean ± SD; Z-height -1.9 ± 2.1 and Z-weight -2.4 ± 1.7), severe hypernatremia (mean ± SD; Na 150 ± 10 mEq/L), increased plasma osmolality (mean ± SD; 311 ± 18 mOsm/Kg), but normal glomerular filtration rate. Genetic diagnosis revealed that 24 male patients were hemizygous for 17 different putative disease-causing mutations in the AVPR2 gene (each one in a different family). Of those, nine had not been previously reported, and eight were recurrent. Moreover, we found those same AVPR2 changes in 12 relatives who were heterozygous carriers. Further, in one female patient, AVPR2 gene study turned out to be negative and she was found to be homozygous for the novel AQP2 p.Ala86Val alteration.

CONCLUSION

Genetic analysis presumably confirmed the diagnosis of nephrogenic diabetes insipidus in every patient of the studied cohort. We emphasize that we detected a high presence (50 %) of heterozygous females with clinical NDI symptoms.

WHAT IS KNOWN

• In most cases (90 %), inherited nephrogenic diabetes insipidus (NDI) is an X-linked disease, caused by mutations in the AVPR2 gene. • In rare occasions (10 %), it is caused by mutations in the AQP2 gene. What is new: • In this study, we report 10 novel mutations associated with NDI. • We have detected a high presence (50 %) of heterozygous carriers with clinical NDI symptoms.

Mutation in the V2 vasopressin receptor gene, AVPR2, causes nephrogenic syndrome of inappropriate diuresis

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a recently discovered rare disease caused by gain-of-function mutations of the V2 vasopressin receptor gene, AVPR2. To date, mutations of Phe229 and Arg137 have been identified as gain-of-function in the V2 vasopressin receptor (V2R). These receptor mutations lead to hyponatremia, which may lead to clinical symptoms in infants. Here we present a newly identified I130N substitution in exon 2 of the V2R gene in a family, causing NSIAD. This I130N mutation resulted in constitutive activity of the V2R with constitutive cyclic adenosine monophosphate (cAMP) generation in HEK293 cells. This basal activity could be blocked by the inverse agonist tolvaptan and arginine-vasopressin stimulation enhanced the cAMP production of I130N-V2R. The mutation causes a biased receptor conformation as the basal cAMP generation activity of I130N does not lead to interaction with β-arrestin. The constitutive activity of the mutant receptor caused constitutive dynamin-dependent and β-arrestin-independent internalization. The inhibition of basal internalization using dominant-negative dynamin resulted in an increased cell surface expression. In contrast to the constitutive internalization, agonist-induced endocytosis was β-arrestin dependent. Thus, tolvaptan could be used for treatment of hyponatremia in patients with NSIAD who carry the I130N-V2R mutation.

Functional interaction between angiotensin II receptor type 1 and chemokine (C-C motif) receptor 2 with implications for chronic kidney disease

Understanding functional interactions between G protein-coupled receptors is of great physiological and pathophysiological importance. Heteromerization provides one important potential mechanism for such interaction between different signalling pathways via macromolecular complex formation. Previous studies suggested a functional interplay between angiotensin II receptor type 1 (AT1) and Chemokine (C-C motif) Receptor 2 (CCR2). However the molecular mechanisms are not understood. We investigated AT1-CCR2 functional interaction in vitro using bioluminescence resonance energy transfer in HEK293 cells and in vivo using subtotal-nephrectomized rats as a well-established model for chronic kidney disease. Our data revealed functional heteromers of these receptors resulting in CCR2-Gαi1 coupling being sensitive to AT1 activation, as well as apparent enhanced β-arrestin2 recruitment with agonist co-stimulation that is synergistically reversed by combined antagonist treatment. Moreover, we present in vivo findings where combined treatment with AT1- and CCR2-selective inhibitors was synergistically beneficial in terms of decreasing proteinuria, reducing podocyte loss and preventing renal injury independent of blood pressure in the subtotal-nephrectomized rat model. Our findings further support a role for G protein-coupled receptor functional heteromerization in pathophysiology and provide insights into previous observations indicating the importance of AT1-CCR2 functional interaction in inflammation, renal and hypertensive disorders.

Bioluminescence Resonance Energy Transfer Approaches to Discover Bias in GPCR Signaling

Bioluminescence resonance energy transfer (BRET) is a well-established technique for investigating G protein-coupled receptor (GPCR) pharmacology. BRET enables the monitoring of molecular proximity through the use of heterologously expressed proteins of interest and/or fluorophore-labeled ligands. Fusion to a donor luciferase enzyme or an acceptor fluorophore and subsequent detection of resonance energy transfer indicate the close proximity of the molecules of interest. As BRET is readily applied to the study of numerous GPCR signaling and regulatory paths, it is an ideal technique for investigating the pharmacology of biased ligands and receptors.

Homogeneous time-resolved fluorescence-based assay to monitor extracellular signal-regulated kinase signaling in a high-throughput format

The extracellular signal-regulated kinases (ERKs) are key components of multiple important cell signaling pathways regulating diverse biological responses. This signaling is characterized by phosphorylation cascades leading to ERK1/2 activation and promoted by various cell surface receptors including G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). We report the development of a new cell-based Phospho-ERK1/2 assay (designated Phospho-ERK), which is a sandwich proximity-based assay using the homogeneous time-resolved fluorescence technology. We have validated the assay on endogenously expressed ERK1/2 activated by the epidermal growth factor as a prototypical RTK, as well as various GPCRs belonging to different classes and coupling to different heterotrimeric G proteins. The assay was successfully miniaturized in 384-well plates using various cell lines endogenously, transiently, or stably expressing the different receptors. The validation was performed for agonists, antagonists, and inhibitors in dose-response as well as kinetic analysis, and the signaling and pharmacological properties of the different receptors were reproduced. Furthermore, the determination of a Z'-factor value of 0.7 indicates the potential of the Phospho-ERK assay for high-throughput screening of compounds that may modulate ERK1/2 signaling. Finally, our study is of great interest in the current context of investigating ERK1/2 signaling with respect to the emerging concepts of biased ligands, G protein-dependent/independent ERK1/2 activation, and functional transactivation between GPCRs and RTKs, illustrating the importance of considering the ERK1/2 pathway in cell signaling.

[Nephrogenic diabetes insipidus]

The anti-diuretic hormone arginine-vasopressin (AVP) is released from the pituitary and regulates water reabsorption in the principal cells of the kidney collecting duct. Binding of AVP to the arginine-vasopressin receptor type-2 in the basolateral membrane leads to translocation of aquaporin-2 water channels to the apical membrane of the principal cells of the collecting duct, inducing water permeability of the membrane. This results in water reabsorption in the collecting duct of the nephron following an osmotic gradient. Nephrogenic diabetes insipidus is caused by partial or complete renal resistance to the effects of AVP. Congenital nephrogenic diabetes insipidus is a disorder associated with mutations in either the AVPR2 or AQP2 gene, causing the inability of patients to concentrate their urine. Acquired nephrogenic diabetes insipidus can be caused by electrolyte imbalances (e.g., hypercalcemia, hypokalemia), renal/extra-renal diseases and drugs (e.g., lithium toxicity). This article reviews the causes, clinical manifestations, diagnosis and treatment of patients with nephrogenic diabetes insipidus. Based on more in-depth mechanistic understanding, new therapeutic strategies are current being explored.

Biophysical Detection of Diversity and Bias in GPCR Function

Guanine nucleotide binding protein (G protein)-coupled receptors (GPCRs) function in complexes with a range of molecules and proteins including ligands, G proteins, arrestins, ubiquitin, and other receptors. Elements of these complexes may interact constitutively or dynamically, dependent upon factors such as ligand binding, phosphorylation, and dephosphorylation. They may also be allosterically modulated by other proteins in a manner that changes temporally and spatially within the cell. Elucidating how these complexes function has been greatly enhanced by biophysical technologies that are able to monitor proximity and/or binding, often in real time and in live cells. These include resonance energy transfer approaches such as bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET). Furthermore, the use of fluorescent ligands has enabled novel insights into allosteric interactions between GPCRs. Consequently, biophysical approaches are helping to unlock the amazing diversity and bias in G protein-coupled receptor signaling.