Constitutive arrestin-mediated desensitization of a human vasopressin receptor mutant associated with nephrogenic diabetes insipidus

Recent Advances in Endocannabinoid System Targeting for Improved Specificity: Strategic Approaches to Targeted Drug Delivery

Opportunities for developing innovative and intelligent drug delivery technologies by targeting the endocannabinoid system are becoming more apparent. This review provides an overview of strategies to develop targeted drug delivery using the endocannabinoid system (ECS). Recent advances in endocannabinoid system targeting showcase enhanced pharmaceutical therapy specificity while minimizing undesirable side effects and overcoming formulation challenges associated with cannabinoids. This review identifies advances in targeted drug delivery technologies that may permit access to the full pharmacotherapeutic potential of the ECS. The design of optimized nanocarriers that target specific tissues can be improved by understanding the nature of the signaling pathways, distribution in the mammalian body, receptor structure, and enzymatic degradation of the ECS. A closer look at ligand-receptor complexes, endocannabinoid tone, tissue distribution, and G-protein activity leads to a better understanding of the potential of the ECS toolkit for therapeutics. The signal transduction pathways examine the modulation of downstream effector proteins, desensitization, signaling cascades, and biased signaling. An in-depth and overall view of the targeted system is achieved through homology modeling where mutagenesis and ligand binding examine the binding site and allow sequence analysis and the formation of libraries for molecular docking and molecular dynamic simulations. Internalization routes exploring receptor-mediated endocytosis and lipid rafts are also considered for explicit signaling. Furthermore, the review highlights nanotechnology and surface modification aspects as a possible future approach for specific targeting.

Structural basis of sphingosine-1-phosphate receptor 1 activation and biased agonism

Sphingosine-1-phosphate receptor 1 (S1PR1) is a master regulator of lymphocyte egress from the lymph node and an established drug target for multiple sclerosis (MS). Mechanistically, therapeutic S1PR1 modulators activate the receptor yet induce sustained internalization through a potent association with β-arrestin. However, a structural basis of biased agonism remains elusive. Here, we report the cryo-electron microscopy (cryo-EM) structures of G_i-bound S1PR1 in complex with S1P, fingolimod-phosphate (FTY720-P) and siponimod (BAF312). In combination with functional assays and molecular dynamics (MD) studies, we reveal that the β-arrestin-biased ligands direct a distinct activation path in S1PR1 through the extensive interplay between the PIF and the NPxxY motifs. Specifically, the intermediate flipping of W269^6.48 and the retained interaction between F265^6.44 and N307^7.49 are the key features of the β-arrestin bias. We further identify ligand-receptor interactions accounting for the S1PR subtype specificity of BAF312. These structural insights provide a rational basis for designing novel signaling-biased S1PR modulators.

GRK6 Depletion Induces HIF Activity in Lung Adenocarcinoma

G protein-coupled receptor kinase 6 (GRK6) is expressed in various tissues and is involved in the development of several diseases including lung cancer. We previously reported that GRK6 is down-regulated in lung adenocarcinoma patients, which induces cell invasion and metastasis. However, further understanding of the role of GRK6 in lung adenocarcinoma is required. Here we explored the functional consequence of GRK6 inhibition in lung epithelial cells. Analysis of TCGA data was coupled with RNA sequencing (RNA-seq) in alveolar epithelial type II (ATII) cells following depletion of GRK6 with RNA interference (RNAi). Findings were validated in ATII cells followed by tissue microarray analysis. Pathway analysis suggested that one of the Hallmark pathways enriched upon GRK6 inhibition is 'Hallmark_Hypoxia' (FDR = 0.014). We demonstrated that GRK6 depletion induces HIF1α (hypoxia-inducible factor 1 alpha) levels and activity in ATII cells. The findings were further confirmed in lung adenocarcinoma samples, in which GRK6 expression levels negatively and positively correlate with HIF1α expression (P = 0.015) and VHL expression (P < 0.0001), respectively. Mechanistically, we showed the impact of GRK6 on HIF activity could be achieved via regulation of VHL levels. Taken together, targeting the HIF pathway may provide new strategies for therapy in GRK6-depleted lung adenocarcinoma patients.

Cryo-electron microscopy structure of the antidiuretic hormone arginine-vasopressin V2 receptor signaling complex

The antidiuretic hormone arginine-vasopressin (AVP) forms a signaling complex with the V2 receptor (V2R) and the G_s protein, promoting kidney water reabsorption. Molecular mechanisms underlying activation of this critical G protein-coupled receptor (GPCR) signaling system are still unknown. To fill this gap of knowledge, we report here the cryo-electron microscopy structure of the AVP-V2R-G_s complex. Single-particle analysis revealed the presence of three different states. The two best maps were combined with computational and nuclear magnetic resonance spectroscopy constraints to reconstruct two structures of the ternary complex. These structures differ in AVP and G_s binding modes. They reveal an original receptor-G_s interface in which the Gα_s subunit penetrates deep into the active V2R. The structures help to explain how V2R R137H or R137L/C variants can lead to two severe genetic diseases. Our study provides important structural insights into the function of this clinically relevant GPCR signaling complex.

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.

Vasopressin receptor 2 mutations in the nephrogenic syndrome of inappropriate antidiuresis show different mechanisms of constitutive activation for G protein coupled receptors

Vasopressin receptor 2 (V2R) mutations causing the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) can generate two constitutively active receptor phenotypes. One type results from residue substitutions in several V2R domains and is sensitive to vaptan inverse agonists. The other is only caused by Arg 137 replacements and is vaptan resistant. We compared constitutive and agonist-driven interactions of the vaptan-sensitive F229V and vaptan-resistant R137C/L V2R mutations with β-arrestin 1, β-arrestin 2, and Gαs, using null fibroblasts reconstituted with individual versions of the ablated transduction protein genes. F229V displayed very high level of constitutive activation for Gs but not for β-arrestins, and enhanced or normal responsiveness to agonists and inverse agonists. In contrast, R137C/L mutants exhibited maximal levels of constitutive activation for βarrestin 2 and Gs, minimal levels for β-arrestin 1, but a sharp decline of ligands sensitivity at all transducer interactions. The enhanced constitutive activity and reduced ligand sensitivity of R137 mutants on cAMP signaling persisted in cells lacking β-arrestins, indicating that these are intrinsic molecular properties of the mutations, not the consequence of altered receptor trafficking. The results suggest that the two groups of NSIAD mutations represent two distinct molecular mechanisms of constitutive activation in GPCRs.

The Vasopressin Receptor 2 Mutant R137L Linked to the Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD) Signals through an Alternative Pathway that Increases AQP2 Membrane Targeting Independently of S256 Phosphorylation

NSIAD is a rare X-linked condition, caused by activating mutations in the AVPR2 gene coding for the vasopressin V2 receptor (V2R) associated with hyponatremia, despite undetectable plasma vasopressin levels. We have recently provided in vitro evidence that, compared to V2R-wt, expression of activating V2R mutations R137L, R137C and F229V cause a constitutive redistribution of the AQP2 water channel to the plasma membrane, higher basal water permeability and significantly higher basal levels of p256-AQP2 in the F229V mutant but not in R137L or R137C. In this study, V2R mutations were expressed in collecting duct principal cells and the associated signalling was dissected. V2R-R137L and R137C mutants had significantly higher basal pT269-AQP2 levels -independently of S256 and PKA-which were reduced to control by treatment with Rho kinase (ROCK) inhibitor. Interestingly, ROCK activity was found significantly higher in V2R-R137L along with activation of the Gα12/13–Rho–ROCK pathway. Of note, inhibition of ROCK reduced the basal elevated osmotic water permeability to control. To conclude, our data demonstrate for the first time that the gain-of-function mutation of the V2R, R137L causing NSIAD, signals through an alternative PKA-independent pathway that increases AQP2 membrane targeting through ROCK-induced phosphorylation at S/T269 independently of S256 of AQP2.

Gain-of-function mutations of the V2 vasopressin receptor in nephrogenic syndrome of inappropriate antidiuresis (NSIAD): a cell-based assay to assess constitutive water reabsorption

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a recently identified chromosome X-linked disease associated with gain-of-function mutations of the V2 vasopressin receptor (V2R), a G-protein-coupled receptor. It is characterized by inability to excrete a free water load, hyponatremia, and undetectable vasopressin-circulating levels. Hyponatremia can be quite severe in affected male children. To gain a deeper insight into the functional properties of the V2R active mutants and how they might translate into the pathological outcome of NSIAD, in this study, we have expressed the wild-type V2R and three constitutively active V2R mutants associated with NSIAD (R137L, R137C, and the F229V) in MCD4 cells, a cell line derived from renal mouse collecting duct, stably expressing the vasopressin-sensitive water channel aquaporin-2 (AQP2). Our findings indicate that in cells expressing each active mutant, AQP2 was constitutively localized to the apical plasma membrane in the absence of vasopressin stimulation. In line with these observations, under basal conditions, osmotic water permeability in cells expressing the constitutively active mutants was significantly higher compared to that of cells expressing the wild-type V2R. Our findings demonstrate a direct link between activating mutations of the V2R and the perturbation of water balance in NSIAD. In addition, this study provides a useful cell-based assay system to assess the functional consequences of newly discovered activating mutations of the V2R on water permeability in kidney cells and to screen the effect of drugs on the mutated receptors.

New insights into the transcriptional regulation of aquaporin-2 and the treatment of X-linked hereditary nephrogenic diabetes insipidus

The kidney collecting duct (CD) is a tubular segment of the kidney where the osmolality and final flow rate of urine are established, enabling urine concentration and body water homeostasis. Water reabsorption in the CD depends on the action of arginine vasopressin (AVP) and a transepithelial osmotic gradient between the luminal fluid and surrounding interstitium. AVP induces transcellular water reabsorption across CD principal cells through associated signaling pathways after binding to arginine vasopressin receptor 2 (AVPR2). This signaling cascade regulates the water channel protein aquaporin-2 (AQP2). AQP2 is exclusively localized in kidney connecting tubules and CDs. Specifically, AVP stimulates the intracellular translocation of AQP2-containing vesicles to the apical plasma membrane, increasing the osmotic water permeability of CD cells. Moreover, AVP induces transcription of the Aqp2 gene, increasing AQP2 protein abundance. This review provides new insights into the transcriptional regulation of the Aqp2 gene in the kidney CD with an overview of AVP and AQP2. It summarizes current therapeutic approaches for X-linked nephrogenic diabetes insipidus caused by AVPR2 gene mutations.

Nephrogenic Diabetes Insipidus

Body fluid homeostasis is essential for normal life. In the maintenance of water balance, the most important factor and regulated process is the excretory function of the kidneys. The kidneys are capable to compensate not only the daily fluctuations of water intake but also the consequences of fluid loss (respiration, perspiration, sweating, hemorrhage). The final volume and osmolality of the excreted urine is set in the collecting duct via hormonal regulation. The hormone of water conservation is the vasopressin (AVP), and a large volume of urine is produced and excreted in the absence of AVP secretion or if AVP is ineffective in the kidneys. The aquaporin-2 water channel (AQP2) is expressed in the principal cells, and it plays an essential role in the reabsorption of water in the collecting ducts via type 2 vasopressin receptor (V2R)-mediated mechanism. If neural or hormonal regulation fails to operate the normal function of AVP-V2R-AQP2 system, it can result in various diseases such as diabetes insipidus (DI) or nephrogenic syndrome of inappropriate diuresis (NSIAD). The DI is characterized by excessive production of hyposmotic urine ("insipidus" means tasteless) due to the inability of the kidneys to concentrate urine. In this chapter, we focus and discuss the pathophysiology of nephrogenic DI (NDI) and the potential therapeutic interventions in the light of the current experimental data.

Calcium-sensing receptor residues with loss- and gain-of-function mutations are located in regions of conformational change and cause signalling bias

The calcium-sensing receptor (CaSR) is a homodimeric G-protein-coupled receptor that signals via intracellular calcium (Ca2+i) mobilisation and phosphorylation of extracellular signal-regulated kinase 1/2 (ERK) to regulate extracellular calcium (Ca2+e) homeostasis. The central importance of the CaSR in Ca2+e homeostasis has been demonstrated by the identification of loss- or gain-of-function CaSR mutations that lead to familial hypocalciuric hypercalcaemia (FHH) or autosomal dominant hypocalcaemia (ADH), respectively. However, the mechanisms determining whether the CaSR signals via Ca2+i or ERK have not been established, and we hypothesised that some CaSR residues, which are the site of both loss- and gain-of-function mutations, may act as molecular switches to direct signalling through these pathways. An analysis of CaSR mutations identified in >300 hypercalcaemic and hypocalcaemic probands revealed five 'disease-switch' residues (Gln27, Asn178, Ser657, Ser820 and Thr828) that are affected by FHH and ADH mutations. Functional expression studies using HEK293 cells showed disease-switch residue mutations to commonly display signalling bias. For example, two FHH-associated mutations (p.Asn178Asp and p.Ser820Ala) impaired Ca2+i signalling without altering ERK phosphorylation. In contrast, an ADH-associated p.Ser657Cys mutation uncoupled signalling by leading to increased Ca2+i mobilization while decreasing ERK phosphorylation. Structural analysis of these five CaSR disease-switch residues together with four reported disease-switch residues revealed these residues to be located at conformationally active regions of the CaSR such as the extracellular dimer interface and transmembrane domain. Thus, our findings indicate that disease-switch residues are located at sites critical for CaSR activation and play a role in mediating signalling bias.

Integration of GPCR Signaling and Sorting from Very Early Endosomes via Opposing APPL1 Mechanisms

Endocytic trafficking is a critical mechanism for cells to decode complex signaling pathways, including those activated by G-protein-coupled receptors (GPCRs). Heterogeneity in the endosomal network enables GPCR activity to be spatially restricted between early endosomes (EEs) and the recently discovered endosomal compartment, the very early endosome (VEE). However, the molecular machinery driving GPCR activity from the VEE is unknown. Using luteinizing hormone receptor (LHR) as a prototype GPCR for this compartment, along with additional VEE-localized GPCRs, we identify a role for the adaptor protein APPL1 in rapid recycling and endosomal cAMP signaling without impacting the EE-localized β2-adrenergic receptor. LHR recycling is driven by receptor-mediated Gαs/cAMP signaling from the VEE and PKA-dependent phosphorylation of APPL1 at serine 410. Receptor/Gαs endosomal signaling is localized to microdomains of heterogeneous VEE populations and regulated by APPL1 phosphorylation. Our study uncovers a highly integrated inter-endosomal communication system enabling cells to tightly regulate spatially encoded signaling.

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GPCRs that internalize to very early endosomes (VEEs) require APPL1 to recycle

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Receptor recycling is driven by cAMP/PKA to phosphorylate serine 410 on APPL1

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cAMP signaling from GPCRs, such as LHR, occurs from distinct VEE microdomains

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APPL1 limits VEE cAMP signaling via opposing mechanisms required for GPCR sorting

Pepducins as a potential treatment strategy for asthma and COPD

Current therapies to treat asthma and other airway diseases primarily include anti-inflammatory agents and bronchodilators. Anti-inflammatory agents target trafficking and resident immunocytes and structural cells, while bronchodilators act to prevent or reverse shortening of airway smooth muscle (ASM), the pivotal tissue regulating bronchomotor tone. Advances in our understanding of the biology of G protein-coupled receptors (GPCRs) and biased agonism offers unique opportunities to modulate GPCR function that include the use of pepducins and allosteric modulators. Recent evidence suggests that small molecule inhibitors of Gα_q as well as pepducins targeting G_q-coupled receptors can broadly inhibit contractile agonist-induced ASM function. Given these advances, new therapeutic approaches can be leveraged to diminish the global rise in morbidity and mortality associated with asthma and chronic obstructive pulmonary disease.

Biased signalling: from simple switches to allosteric microprocessors

G protein-coupled receptors (GPCRs) are the largest class of receptors in the human genome and some of the most common drug targets. It is now well established that GPCRs can signal through multiple transducers, including heterotrimeric G proteins, GPCR kinases and β-arrestins. While these signalling pathways can be activated or blocked by 'balanced' agonists or antagonists, they can also be selectively activated in a 'biased' response. Biased responses can be induced by biased ligands, biased receptors or system bias, any of which can result in preferential signalling through G proteins or β-arrestins. At many GPCRs, signalling events mediated by G proteins and β-arrestins have been shown to have distinct biochemical and physiological actions from one another, and an accurate evaluation of biased signalling from pharmacology through physiology is crucial for preclinical drug development. Recent structural studies have provided snapshots of GPCR-transducer complexes, which should aid in the structure-based design of novel biased therapies. Our understanding of GPCRs has evolved from that of two-state, on-and-off switches to that of multistate allosteric microprocessors, in which biased ligands transmit distinct structural information that is processed into distinct biological outputs. The development of biased ligands as therapeutics heralds an era of increased drug efficacy with reduced drug side effects.

Structure-Function Relationships of the Follicle-Stimulating Hormone Receptor

The follicle-stimulating hormone receptor (FSHR) plays a crucial role in reproduction. This structurally complex receptor is a member of the G-protein coupled receptor (GPCR) superfamily of membrane receptors. As with the other structurally similar glycoprotein hormone receptors (the thyroid-stimulating hormone and luteinizing hormone-chorionic gonadotropin hormone receptors), the FSHR is characterized by an extensive extracellular domain, where binding to FSH occurs, linked to the signal specificity subdomain or hinge region. This region is involved in ligand-stimulated receptor activation whereas the seven transmembrane domain is associated with receptor activation and transmission of the activation process to the intracellular loops comprised of amino acid sequences, which predicate coupling to effectors, interaction with adapter proteins, and triggering of downstream intracellular signaling. In this review, we describe the most important structural features of the FSHR intimately involved in regulation of FSHR function, including trafficking, dimerization, and oligomerization, ligand binding, agonist-stimulated activation, and signal transduction.

Hereditary Nephrogenic Diabetes Insipidus: Pathophysiology and Possible Treatment. An Update

Under physiological conditions, excessive loss of water through the urine is prevented by the release of the antidiuretic hormone arginine-vasopressin (AVP) from the posterior pituitary. In the kidney, AVP elicits a number of cellular responses, which converge on increasing the osmotic reabsorption of water in the collecting duct. One of the key events triggered by the binding of AVP to its type-2 receptor (AVPR2) is the exocytosis of the water channel aquaporin 2 (AQP2) at the apical membrane the principal cells of the collecting duct. Mutations of either AVPR2 or AQP2 result in a genetic disease known as nephrogenic diabetes insipidus, which is characterized by the lack of responsiveness of the collecting duct to the antidiuretic action of AVP. The affected subject, being incapable of concentrating the urine, presents marked polyuria and compensatory polydipsia and is constantly at risk of severe dehydration. The molecular bases of the disease are fully uncovered, as well as the genetic or clinical tests for a prompt diagnosis of the disease in newborns. A real cure for nephrogenic diabetes insipidus (NDI) is still missing, and the main symptoms of the disease are handled with s continuous supply of water, a restrictive diet, and nonspecific drugs. Unfortunately, the current therapeutic options are limited and only partially beneficial. Further investigation in vitro or using the available animal models of the disease, combined with clinical trials, will eventually lead to the identification of one or more targeted strategies that will improve or replace the current conventional therapy and grant NDI patients a better quality of life. Here we provide an updated overview of the genetic defects causing NDI, the most recent strategies under investigation for rescuing the activity of mutated AVPR2 or AQP2, or for bypassing defective AVPR2 signaling and restoring AQP2 plasma membrane expression.

Differential phosphorylation signals control endocytosis of GPR15

GPR15 undergoes a ligand-independent endocytosis, which requires phosphorylation of a distal C-terminal Ser-357 mediated by multiple basophilic kinases. The functional role of Ser-357 in endocytosis is distinct from that of a conserved Ser/Thr cluster, which is more responsible for the use of GRKs and β-arrestin.

GPR15 is an orphan G protein–coupled receptor (GPCR) that serves for an HIV coreceptor and was also recently found as a novel homing receptor for T-cells implicated in colitis. We show that GPR15 undergoes a constitutive endocytosis in the absence of ligand. The endocytosis was clathrin dependent and partially dependent on β-arrestin in HEK293 cells, and nearly half of the internalized GPR15 receptors were recycled to the plasma membrane. An Ala mutation of the distal C-terminal Arg-354 or Ser-357, which forms a consensus phosphorylation site for basophilic kinases, markedly reduced the endocytosis, whereas phosphomimetic mutation of Ser-357 to Asp did not. Ser-357 was phosphorylated in vitro by multiple kinases, including PKA and PKC, and pharmacological activation of these kinases enhanced both phosphorylation of Ser-357 and endocytosis of GPR15. These results suggested that Ser-357 phosphorylation critically controls the ligand-independent endocytosis of GPR15. The functional role of Ser-357 in endocytosis was distinct from that of a conserved Ser/Thr cluster in the more proximal C-terminus, which was responsible for the β-arrestin– and GPCR kinase–dependent endocytosis of GPR15. Thus phosphorylation signals may differentially control cell surface density of GPR15 through endocytosis.

Constitutive Desensitization of Opioid Receptors in Peripheral Sensory Neurons

Opioid receptors expressed by peripheral pain-sensing neurons are functionally inactive for antinociceptive signaling under most basal conditions; however, tissue damage or exposure to inflammatory mediators (e.g., bradykinin) converts these receptors from a nonresponsive state to a functionally competent state. Here we tested the hypothesis that the basal, nonresponsive state of the mu- and delta-opioid receptors (MOR and DOR, respectively) is the result of constitutive receptor activity that activates desensitization mechanisms, resulting in MOR and DOR receptor systems that are constitutively desensitized. Consistent with our previous findings, under basal conditions, neither the MOR agonist [d-Ala2,N-MePhe4,Gly-ol5]-enkephalin nor the DOR agonist [d-Pen2,5]-enkephalin, inhibited prostaglandin E2 (PGE2)-stimulated cAMP accumulation in peripheral sensory neurons in culture (ex vivo) or inhibited PGE2-stimulated thermal allodynia in the rat hind paw in vivo. Prolonged treatment with naloxone induced MOR and DOR responsiveness both in vivo and ex vivo to a similar magnitude as that produced by bradykinin. Also similar to bradykinin, the effect of naloxone persisted for 60 minutes after washout of the ligand. By contrast, prolonged treatment with 6β-naltrexol, did not induce functional competence of MOR or DOR but blocked the effect of naloxone. Treatment with siRNA for β-arrestin-2, but not β-arrestin-1, also induced MOR and DOR functional competence in cultured peripheral sensory neurons. These data suggest that the lack of responsiveness of MOR and DOR to agonist for antinociceptive signaling in peripheral sensory neurons is due to constitutive desensitization that is likely mediated by β-arrestin-2.

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.

A novel mutation affecting the arginine-137 residue of AVPR2 in dizygous twins leads to nephrogenic diabetes insipidus and attenuated urine exosome aquaporin-2

Mutations in the vasopressin V2 receptor gene AVPR2 may cause X‐linked nephrogenic diabetes insipidus by defective apical insertion of aquaporin‐2 in the renal collecting duct principal cell. Substitution mutations with exchange of arginine at codon 137 can cause nephrogenic syndrome of inappropriate antidiuresis or congenital X‐linked nephrogenic diabetes insipidus. We present a novel mutation in codon 137 within AVPR2 with substitution of glycine for arginine in male dizygotic twins. Nephrogenic diabetes insipidus was demonstrated by water deprivation test and resistance to vasopressin administration. While a similar urine exosome release rate was shown between probands and controls by western blotting for the marker ALIX, there was a selective decrease in exosome aquaporin‐2 versus aquaporin‐1 protein in probands compared to controls.

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.

The β-Arrestins: Multifunctional Regulators of G Protein-coupled Receptors

The β-arrestins (βarrs) are versatile, multifunctional adapter proteins that are best known for their ability to desensitize G protein-coupled receptors (GPCRs), but also regulate a diverse array of cellular functions. To signal in such a complex fashion, βarrs adopt multiple conformations and are regulated at multiple levels to differentially activate downstream pathways. Recent structural studies have demonstrated that βarrs have a conserved structure and activation mechanism, with plasticity of their structural fold, allowing them to adopt a wide array of conformations. Novel roles for βarrs continue to be identified, demonstrating the importance of these dynamic regulators of cellular signaling.

A rapid and affordable screening platform for membrane protein trafficking

BACKGROUND

Membrane proteins regulate a diversity of physiological processes and are the most successful class of targets in drug discovery. However, the number of targets adequately explored in chemical space and the limited resources available for screening are significant problems shared by drug-discovery centers and small laboratories. Therefore, a low-cost and universally applicable screen for membrane protein trafficking was developed.

RESULTS

This high-throughput screen (HTS), termed IRFAP-HTS, utilizes the recently described MarsCy1-fluorogen activating protein and the near-infrared and membrane impermeant fluorogen SCi1. The cell surface expression of MarsCy1 epitope-tagged receptors can be visualized by simple addition of SCi1. User-friendly, rapid, and quantitative detection occurs on a standard infrared western-blotting scanner. The reliability and robustness of IRFAP-HTS was validated by confirming human vasopressin-2 receptor and dopamine receptor-2 trafficking in response to agonist or antagonist. The IRFAP-HTS screen was deployed against the leucine-rich G protein-coupled receptor-5 (Lgr5). Lgr5 is expressed in stem cells, modulates Wnt/ß-catenin signaling, and is therefore a promising drug target. However, small molecule modulators have yet to be reported. The constitutive internalization of Lgr5 appears to be one primary mode through which its function is regulated. Therefore, IRFAP-HTS was utilized to screen 11,258 FDA-approved and drug-like small molecules for those that antagonize Lgr5 internalization. Glucocorticoids were found to potently increase Lgr5 expression at the plasma membrane.

CONCLUSION

The IRFAP-HTS platform provides a versatile solution for screening more targets with fewer resources. Using only a standard western-blotting scanner, we were able to screen 5,000 compounds per hour in a robust and quantitative assay. Multi-purposing standardly available laboratory equipment eliminates the need for idiosyncratic and more expensive high-content imaging systems. The modular and user-friendly IRFAP-HTS is a significant departure from current screening platforms. Small laboratories will have unprecedented access to a robust and reliable screening platform and will no longer be limited by the esoteric nature of assay development, data acquisition, and post-screening analysis. The discovery of glucocorticoids as modulators for Lgr5 trafficking confirms that IRFAP-HTS can accelerate drug-discovery and drug-repurposing for even the most obscure targets.

A Single Amino Acid Mutation (R104P) in the E/DRY Motif of GPR40 Impairs Receptor Function

Type 2 Diabetes Mellitus with insulin resistance, pancreatic β cell dysfunction, and hepatic glucose overproduction is increasing in epidemic proportions worldwide. G protein-coupled receptor 40 (GPR40), a clinically proven anti-diabetic drug target, is mainly expressed in pancreatic β cells and insulin-secreting cell lines. Long chain fatty acids (LCFA) increase intracellular calcium concentration and amplify glucose-stimulated insulin secretion by activating GPR40. Here we report that the arginine 104 (R104) is critical for the normal function of GPR40. Mutation of R104 to Proline (R104P) results in complete loss of the receptor function. Linoleic acid, ligand of GPR40, could not elicit calcium increase and ERK phosphorylation in cells expressing this mutant receptor. Further study indicated the R104P mutation reduces cell surface localization of GPR40 without affecting the expression of the protein. The small portion of GPR40 R104P mutant that is still located on the membrane has no physiological function, and does not internalize in response to linoleic acid stimulation. These data demonstrate that R104 in GPR40 is critically involved in the normal receptor functions. Interestingly, R104P is a registered single-nucleotide polymorphism of GPR40. The relationship of this GPR40 variant and type 2 diabetes warrants further investigation.

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.

Trafficking of β-Adrenergic Receptors: Implications in Intracellular Receptor Signaling

β-Adrenergic receptors (βARs), prototypical G-protein-coupled receptors, play a pivotal role in regulating neuronal and cardiovascular responses to catecholamines during stress. Agonist-induced receptor endocytosis is traditionally considered as a primary mechanism to turn off the receptor signaling (or receptor desensitization). However, recent progress suggests that intracellular trafficking of βAR presents a mean to translocate receptor signaling machinery to intracellular organelles/compartments while terminating the signaling at the cell surface. Moreover, the apparent multidimensionality of ligand efficacy in space and time in a cell has forecasted exciting pathophysiological implications, which are just beginning to be explored. As we begin to understand how these pathways impact downstream cellular programs, this will have significant implications for a number of pathophysiological conditions in heart and other systems, that in turn open up new therapeutic opportunities.

G Protein-coupled Receptor Kinases of the GRK4 Protein Subfamily Phosphorylate Inactive G Protein-coupled Receptors (GPCRs)

G protein-coupled receptor (GPCR) kinases (GRKs) play a key role in homologous desensitization of GPCRs. It is widely assumed that most GRKs selectively phosphorylate only active GPCRs. Here, we show that although this seems to be the case for the GRK2/3 subfamily, GRK5/6 effectively phosphorylate inactive forms of several GPCRs, including β2-adrenergic and M2 muscarinic receptors, which are commonly used as representative models for GPCRs. Agonist-independent GPCR phosphorylation cannot be explained by constitutive activity of the receptor or membrane association of the GRK, suggesting that it is an inherent ability of GRK5/6. Importantly, phosphorylation of the inactive β2-adrenergic receptor enhanced its interactions with arrestins. Arrestin-3 was able to discriminate between phosphorylation of the same receptor by GRK2 and GRK5, demonstrating preference for the latter. Arrestin recruitment to inactive phosphorylated GPCRs suggests that not only agonist activation but also the complement of GRKs in the cell regulate formation of the arrestin-receptor complex and thereby G protein-independent signaling.

Phenotypic regulation of the sphingosine 1-phosphate receptor miles apart by G protein-coupled receptor kinase 2

The evolutionarily conserved DRY motif at the end of the third helix of rhodopsin-like, class-A G protein-coupled receptors (GPCRs) is a major regulator of receptor stability, signaling activity, and β-arrestin-mediated internalization. Substitution of the DRY arginine with histidine in the human vasopressin receptor results in a loss-of-function phenotype associated with diabetes insipidus. The analogous R150H substitution of the DRY motif in zebrafish sphingosine-1 phosphate receptor 2 (S1p2) produces a mutation, miles apart m⁹³ (mil^m93), that not only disrupts signaling but also impairs heart field migration. We hypothesized that constitutive S1p2 desensitization is the underlying cause of this strong zebrafish developmental defect. We observed in cell assays that the wild-type S1p2 receptor is at the cell surface whereas in distinct contrast the S1p2 R150H receptor is found in intracellular vesicles, blocking G protein but not arrestin signaling activity. Surface S1p2 R150H expression could be restored by inhibition of G protein-coupled receptor kinase 2 (GRK2). Moreover, we observed that β-arrestin 2 and GRK2 colocalize with S1p2 in developing zebrafish embryos and depletion of GRK2 in the S1p2 R150H miles apart zebrafish partially rescued cardia bifida. The ability of reduced GRK2 activity to reverse a developmental phenotype associated with constitutive desensitization supports efforts to genetically or pharmacologically target this kinase in diseases involving biased GPCR signaling.

G Protein and β-arrestin signaling bias at the ghrelin receptor

The G protein-coupled ghrelin receptor GHSR1a is a potential pharmacological target for treating obesity and addiction because of the critical role ghrelin plays in energy homeostasis and dopamine-dependent reward. GHSR1a enhances growth hormone release, appetite, and dopamine signaling through G(q/11), G(i/o), and G(12/13) as well as β-arrestin-based scaffolds. However, the contribution of individual G protein and β-arrestin pathways to the diverse physiological responses mediated by ghrelin remains unknown. To characterize whether a signaling bias occurs for GHSR1a, we investigated ghrelin signaling in a number of cell-based assays, including Ca(2+) mobilization, serum response factor response element, stress fiber formation, ERK1/2 phosphorylation, and β-arrestin translocation, utilizing intracellular second loop and C-tail mutants of GHSR1a. We observed that GHSR1a and β-arrestin rapidly form metastable plasma membrane complexes following exposure to an agonist, but replacement of the GHSR1a C-tail by the tail of the vasopressin 2 receptor greatly stabilizes them, producing complexes observable on the plasma membrane and also in endocytic vesicles. Mutations of the contiguous conserved amino acids Pro-148 and Leu-149 in the GHSR1a intracellular second loop generate receptors with a strong bias to G protein and β-arrestin, respectively, supporting a role for conformation-dependent signaling bias in the wild-type receptor. Our results demonstrate more balance in GHSR1a-mediated ERK signaling from G proteins and β-arrestin but uncover an important role for β-arrestin in RhoA activation and stress fiber formation. These findings suggest an avenue for modulating drug abuse-associated changes in synaptic plasticity via GHSR1a and indicate the development of GHSR1a-biased ligands as a promising strategy for selectively targeting downstream signaling events.

Ligand-specific endocytic dwell times control functional selectivity of the cannabinoid receptor 1

G protein-coupled receptors (GPCRs) are the major transducers of external stimuli and key therapeutic targets in many pathological conditions. When activated by different ligands, one receptor can elicit multiple signalling cascades that are mediated by G proteins or β-arrestin, a process defined as functional selectivity or ligand bias. However, the dynamic mechanisms underlying β-arrestin signalling remain unknown. Here by studying the cannabinoid receptor 1 (CB1R), we identify ligand-specific endocytic dwell times, that is, the time during which receptors are clustered into clathrin pits together with β-arrestins before endocytosis, as the mechanism controlling β-arrestin signalling. Agonists inducing short endocytic dwell times produce little or no β-arrestin signalling, whereas those eliciting prolonged dwell times induce robust signalling. Remarkably, extending CB1R dwell times by preventing endocytosis substantially increased β-arrestin signalling. These studies reveal how receptor activation translates into β-arrestin signalling and identify a mechanism to control this pathway.

G-protein coupled receptors can signal through G-proteins or through β-arrestin, however mechanisms determining pathway selection remain unclear. Here the authors show that the duration of cannabinoid receptor clustering in clathrin coated pits prior to endocytosis determines the strength of β-arrestin signalling.

Altered agonist sensitivity of a mutant v2 receptor suggests a novel therapeutic strategy for nephrogenic diabetes insipidus

Loss-of-function mutations of the type 2 vasopressin receptor (V2R) in kidney can lead to nephrogenic diabetes insipidus (NDI). We studied a previously described, but uncharacterized, mutation of the V2R (N321K missense mutation) of a patient with NDI. The properties of the mutant receptor were evaluated. We constructed a highly sensitive Epac-based bioluminescence resonance energy transfer biosensor to perform real-time cAMP measurements after agonist stimulation of transiently transfected HEK293 cells with V2Rs. β-Arrestin binding of the activated receptors was examined with luciferase-tagged β-arrestin and mVenus-tagged V2Rs using the bioluminescence resonance energy transfer technique. Cell surface expression levels of hemagglutinin-tagged receptors were determined with flow cytometry using anti-hemagglutinin-Alexa 488 antibodies. Cellular localization examinations were implemented with fluorescent tagged receptors visualized with confocal laser scanning microscopy. The effect of various vasopressin analogs on the type 1 vasopressin receptor (V1R) was tested on mouse arteries by wire myography. The N321K mutant V2R showed normal cell surface expression, but the potency of arginine vasopressin for cAMP generation was low, whereas the clinically used desmopressin was not efficient. The β-arrestin binding and internalization properties of the mutant receptor were also different than those for the wild type. The function of the mutant receptor can be rescued with administration of the V2R agonist Val(4)-desmopressin, which had no detectable side effects on V1R in the effective cAMP generating concentrations. Based on these findings we propose a therapeutic strategy for patients with NDI carrying the N321K mutation, as our in vivo experiments suggest that Val(4)-desmopressin could rescue the function of the N321K-V2R without significant side effects on the V1R.

Multiple functions of G protein-coupled receptor kinases

Desensitization is a physiological feedback mechanism that blocks detrimental effects of persistent stimulation. G protein-coupled receptor kinase 2 (GRK2) was originally identified as the kinase that mediates G protein-coupled receptor (GPCR) desensitization. Subsequent studies revealed that GRK is a family composed of seven isoforms (GRK1–GRK7). Each GRK shows a differential expression pattern. GRK1, GRK4, and GRK7 are expressed in limited tissues. In contrast, GRK2, GRK3, GRK5, and GRK6 are ubiquitously expressed throughout the body. The roles of GRKs in GPCR desensitization are well established. When GPCRs are activated by their agonists, GRKs phosphorylate serine/threonine residues in the intracellular loops and the carboxyl-termini of GPCRs. Phosphorylation promotes translocation of β-arrestins to the receptors and inhibits further G protein activation by interrupting receptor-G protein coupling. The binding of β-arrestins to the receptors also helps to promote receptor internalization by clathrin-coated pits. Thus, the GRK-catalyzed phosphorylation and subsequent binding of β-arrestin to GPCRs are believed to be the common mechanism of GPCR desensitization and internalization. Recent studies have revealed that GRKs are also involved in the β-arrestin-mediated signaling pathway. The GRK-mediated phosphorylation of the receptors plays opposite roles in conventional G protein- and β-arrestin-mediated signaling. The GRK-catalyzed phosphorylation of the receptors results in decreased G protein-mediated signaling, but it is necessary for β-arrestin-mediated signaling. Agonists that selectively activate GRK/β-arrestin-dependent signaling without affecting G protein signaling are known as β-arrestin-biased agonists. Biased agonists are expected to have potential therapeutic benefits for various diseases due to their selective activation of favorable physiological responses or avoidance of the side effects of drugs. Furthermore, GRKs are recognized as signaling mediators that are independent of either G protein- or β-arrestin-mediated pathways. GRKs can phosphorylate non-GPCR substrates, and this is found to be involved in various physiological responses, such as cell motility, development, and inflammation. In addition to these effects, our group revealed that GRK6 expressed in macrophages mediates the removal of apoptotic cells (engulfment) in a kinase activity-dependent manner. These studies revealed that GRKs block excess stimulus and also induce cellular responses. Here, we summarized the involvement of GRKs in β-arrestin-mediated and G protein-independent signaling pathways.

Minireview: More than just a hammer: ligand "bias" and pharmaceutical discovery

Conventional orthosteric drug development programs targeting G protein-coupled receptors (GPCRs) have focused on the concepts of agonism and antagonism, in which receptor structure determines the nature of the downstream signal and ligand efficacy determines its intensity. Over the past decade, the emerging paradigms of "pluridimensional efficacy" and "functional selectivity" have revealed that GPCR signaling is not monolithic, and that ligand structure can "bias" signal output by stabilizing active receptor states in different proportions than the native ligand. Biased ligands are novel pharmacologic entities that possess the unique ability to qualitatively change GPCR signaling, in effect creating "new receptors" with distinct efficacy profiles driven by ligand structure. The promise of biased agonism lies in this ability to engender "mixed" effects not attainable using conventional agonists or antagonists, promoting therapeutically beneficial signals while antagonizing deleterious ones. Indeed, arrestin pathway-selective agonists for the type 1 parathyroid hormone and angiotensin AT1 receptors, and G protein pathway-selective agonists for the GPR109A nicotinic acid and μ-opioid receptors, have demonstrated unique, and potentially therapeutic, efficacy in cell-based assays and preclinical animal models. Conversely, activating GPCRs in "unnatural" ways may lead to downstream biological consequences that cannot be predicted from prior knowledge of the actions of the native ligand, especially in the case of ligands that selectively activate as-yet poorly characterized G protein-independent signaling networks mediated via arrestins. Although much needs to be done to realize the clinical potential of functional selectivity, biased GPCR ligands nonetheless appear to be important new additions to the pharmacologic toolbox.

Enhanced phosphorylation-independent arrestins and gene therapy

A variety of heritable and acquired disorders is associated with excessive signaling by mutant or overstimulated GPCRs. Since any conceivable treatment of diseases caused by gain-of-function mutations requires gene transfer, one possible approach is functional compensation. Several structurally distinct forms of enhanced arrestins that bind phosphorylated and even non-phosphorylated active GPCRs with much higher affinity than parental wild-type proteins have the ability to dampen the signaling by hyperactive GPCR, pushing the balance closer to normal. In vivo this approach was so far tested only in rod photoreceptors deficient in rhodopsin phosphorylation, where enhanced arrestin improved the morphology and light sensitivity of rods, prolonged their survival, and accelerated photoresponse recovery. Considering that rods harbor the fastest, as well as the most demanding and sensitive GPCR-driven signaling cascade, even partial success of functional compensation of defect in rhodopsin phosphorylation by enhanced arrestin demonstrates the feasibility of this strategy and its therapeutic potential.

Targeting individual GPCRs with redesigned nonvisual arrestins

Numerous human diseases are caused by excessive signaling of mutant G protein-coupled receptors (GPCRs) or receptors that are overstimulated due to upstream signaling imbalances. The feasibility of functional compensation by arrestins with enhanced ability to quench receptor signaling was recently tested in the visual system. The results showed that even in this extremely demanding situation of rods that have no ability to phosphorylate rhodopsin, enhanced arrestin improved rod morphology, light sensitivity, survival, and accelerated photoresponse recovery. Structurally distinct enhanced mutants of arrestins that bind phosphorylated and non-phosphorylated active GPCRs with much higher affinity than parental wild-type (WT) proteins have been constructed. These "super-arrestins" are likely to have the power to dampen the signaling by hyperactive GPCRs. However, most cells express 5-20 GPCR subtypes, only one of which would be overactive, while nonvisual arrestins are remarkably promiscuous, binding hundreds of different GPCRs. Thus, to be therapeutically useful, enhanced versions of nonvisual arrestins must be made fairly specific for particular receptors. Recent identification of very few arrestin residues as key receptor discriminators paves the way to the construction of receptor subtype-specific nonvisual arrestins.

The stem cell-expressed receptor Lgr5 possesses canonical and functionally active molecular determinants critical to β-arrestin-2 recruitment

Lgr5 is a membrane protein related to G protein-coupled receptors (GPCR)s whose expression identifies stem cells in multiple tissues and is strongly correlated with cancer. Despite the recent identification of endogenous ligands for Lgr5, its mode of signaling remains enigmatic. The ability to couple to G proteins and βarrestins are classical molecular behaviors of GPCRs that have yet to be observed for Lgr5. Therefore, the goal of this study was to determine if Lgr5 can engage a classical GPCR behavior and elucidate the molecular determinants of this process. Structural analysis of Lgr5 revealed several motifs consistent with its ability to recruit βarr2. Among them, a "SSS" serine cluster located at amino acid position 873-875 within the C-terminal tail (C-tail), is in a region consistent with other GPCRs that bind βarr2 with high-affinity. To test its functionality, a ligand-independent βarr2 translocation assay was implemented. We show that Lgr5 recruits βarr2 and that the "SSS" amino acids (873-875) are absolutely critical to this process. We also demonstrate that for full efficacy, this cluster requires other Lgr5 C-tail serines that were previously shown to be important for constitutive and βarr2 independent internalization of Lgr5. These data are proof of principle that a classical GPCR behavior can be manifested by Lgr5. The existence of alternative ligands or missing effectors of Lgr5 that scaffold this classical GPCR behavior and the downstream signaling pathways engaged should be considered. Characterizing Lgr5 signaling will be invaluable for assessing its role in tissue maintenance, repair, and disease.

The chemokine receptor CCR1 is constitutively active, which leads to G protein-independent, β-arrestin-mediated internalization

Activation of G protein-coupled receptors by their associated ligands has been extensively studied, and increasing structural information about the molecular mechanisms underlying ligand-dependent receptor activation is beginning to emerge with the recent expansion in GPCR crystal structures. However, some GPCRs are also able to adopt active conformations in the absence of agonist binding that result in the initiation of signal transduction and receptor down-modulation. In this report, we show that the CC-type chemokine receptor 1 (CCR1) exhibits significant constitutive activity leading to a variety of cellular responses. CCR1 expression is sufficient to induce inhibition of cAMP formation, increased F-actin content, and basal migration of human and murine leukocytes. The constitutive activity leads to basal phosphorylation of the receptor, recruitment of β-arrestin-2, and subsequent receptor internalization. CCR1 concurrently engages Gαi and β-arrestin-2 in a multiprotein complex, which may be accommodated by homo-oligomerization or receptor clustering. The data suggest the presence of two functional states for CCR1; whereas receptor coupled to Gαi functions as a canonical GPCR, albeit with high constitutive activity, the CCR1·β-arrestin-2 complex is required for G protein-independent constitutive receptor internalization. The pertussis toxin-insensitive uptake of chemokine by the receptor suggests that the CCR1·β-arrestin-2 complex may be related to a potential scavenging function of the receptor, which may be important for maintenance of chemokine gradients and receptor responsiveness in complex fields of chemokines during inflammation.

Triphenylmethane dye activation of beta-arrestin

β-Arrestins regulate G protein-coupled receptor signaling as competitive inhibitors and protein adaptors. Low molecular weight biased ligands that bind receptors and discriminate between the G protein dependent arm and β-arrestin, clathrin-associated arm of receptor signaling are considered therapeutically valuable as a result of this distinctive pharmacological behavior. Other than receptor agonists, compounds that activate β-arrestins are not available. We show that within minutes of exposure to the cationic triphenylmethane dyes malachite green and brilliant green, tissue culture cells recruit β-arrestins to clathrin scaffolds in a receptor-activation independent manner. In the presence of these compounds, G protein signaling is inhibited, ERK and GSK3β signaling are preserved, and the recruitment of the beta2-adaptin, AP2 adaptor complex to clathrin as well as transferrin internalization is reduced. Moreover, malachite green binds β-arrestin2-GFP coated immunotrap beads relative to GFP only coated beads. Triphenylmethane dyes are FDA approved for topical use on newborns as components of triple-dye preparations and are not approved but used effectively as aqueous antibiotics in fish husbandry. As possible carcinogens, their chronic ingestion in food preparations, particularly through farmed fish, is discouraged in the U.S. and Europe. Our results indicate triphenylmethane dyes as a result of novel pharmacology may have additional roles as β-arrestin/clathrin pathway signaling modulators in both pharmacology research and clinical therapy.

Constitutive internalization of the leucine-rich G protein-coupled receptor-5 (LGR5) to the trans-Golgi network

LGR5 is a Wnt pathway associated G protein-coupled receptor (GPCR) that serves as a molecular determinant of stem cells in numerous tissues including the intestine, stomach, hair follicle, eye, and mammary gland. Despite its importance as a marker for this critical niche, little is known about LGR5 signaling nor the biochemical mechanisms and receptor determinants that regulate LGR5 membrane expression and intracellular trafficking. Most importantly, in cells LGR5 is predominantly intracellular, yet the mechanisms underlying this behavior have not been determined. In this work we elucidate a precise trafficking program for LGR5 and identify the motif at its C terminus that is responsible for the observed constitutive internalization. We show that this process is dependent upon dynamin GTPase activity and find that wild-type full-length LGR5 rapidly internalizes into EEA1- and Rab5-positive endosomes. However, LGR5 fails to rapidly recycle to the plasmid membrane through Rab4-positive vesicles, as is common for other GPCRs. Rather, internalized LGR5 transits through Rab7- and Rab9-positive vesicles, co-localizes in vesicles with Vps26, a retromer complex component that regulates retrograde trafficking to the trans-Golgi network (TGN) and reaches a steady-state distribution in the TGN within 2 h. Using mutagenesis, particularly of putative phosphorylation sites, we show that the amino acid pair, serine 861 and 864, is the principal C-tail determinant that mediates LGR5 constitutive internalization. The constitutive internalization of LGR5 to the TGN suggests the existence of novel biochemical roles for its Wnt pathway related, but ill defined signaling program.

Functional polymorphisms affecting the clinically important arginine-137 residue of AVPR2 do not influence serum sodium concentration at the population level

The protein product of the AVPR2 gene, coding for the arginine vasopressin receptor type 2, is essential for vasopressin-dependent concentration of the urine. The arginine residue at position 137 in the protein product of this gene is uniquely pivotal for function. The R137H mutant inactivates the receptor conferring congenital nephrogenic diabetes insipidus, whereas activating mutations at this same residue (i.e., R137C and R137L) confer pathological water retention in the nephrogenic syndrome of inappropriate antidiuresis. These mutations were discovered in human subjects with conspicuous phenotypes in clinical water balance. Prevalence of these polymorphisms among asymptomatic individuals has not been assessed, nor has their contribution to broad interindividual variation in serum sodium concentration; no data addressing minor allele frequency are available. We genotyped two large cohorts using a validated high-throughput Pyrosequencing-based assay that we designed to capture the totality of pathological variation at this important residue. In the Osteoporotic Fractures in Men (MrOS) Study, all participants were male (i.e., hemizygous for AVPR2 gene on the X-chromosome), and participants were oversampled at the extremes of the population distribution for serum sodium concentration. In the Offspring Cohort of the Framingham Heart Study, male and female participants were genotyped. No pathological variants affecting R137 were detected among the 5,142 AVPR2 alleles successfully genotyped. Even at the population extremes of serum sodium distribution, we estimate minor allele frequency < 0.06%. We conclude that these disease-associated variants are exceedingly uncommon and do not contribute broadly to interindividual variability in serum sodium concentration or to its heritability.

Engineering visual arrestin-1 with special functional characteristics

Arrestin-1 preferentially binds active phosphorylated rhodopsin. Previously, a mutant with enhanced binding to unphosphorylated active rhodopsin (Rh*) was shown to partially compensate for lack of rhodopsin phosphorylation in vivo. Here we showed that reengineering of the receptor binding surface of arrestin-1 further improves the binding to Rh* while preserving protein stability. In mammals, arrestin-1 readily self-associates at physiological concentrations. The biological role of this phenomenon can only be elucidated by replacing wild type arrestin-1 in living animals with a non-oligomerizing mutant retaining all other functions. We demonstrate that constitutively monomeric forms of arrestin-1 are sufficiently stable for in vivo expression. We also tested the idea that individual functions of arrestin-1 can be independently manipulated to generate mutants with the desired combinations of functional characteristics. Here we showed that this approach is feasible; stable forms of arrestin-1 with high Rh* binding can be generated with or without the ability to self-associate. These novel molecular tools open the possibility of testing of the biological role of arrestin-1 self-association and pave the way to elucidation of full potential of compensational approach to gene therapy of gain-of-function receptor mutations.

Congenital nephrogenic diabetes insipidus: the current state of affairs

The anti-diuretic hormone arginine vasopressin (AVP) is released from the pituitary upon hypovolemia or hypernatremia, and regulates water reabsorption in the renal collecting duct principal cells. Binding of AVP to the arginine vasopressin receptor type 2 (AVPR2) in the basolateral membrane leads to translocation of aquaporin 2 (AQP2) water channels to the apical membrane of the collecting duct principal cells, inducing water permeability of the membrane. This results in water reabsorption from the pro-urine into the medullary interstitium following an osmotic gradient. Congenital nephrogenic diabetes insipidus (NDI) is a disorder associated with mutations in either the AVPR2 or AQP2 gene, causing the inability of patients to concentrate their pro-urine, which leads to a high risk of dehydration. This review focuses on the current knowledge regarding the cell biological aspects of congenital X-linked, autosomal-recessive and autosomal-dominant NDI while specifically addressing the latest developments in the field. Based on deepened mechanistic understanding, new therapeutic strategies are currently being explored, which we also discuss here.

Pharmacogenomics of the heptahelical receptor regulators G-protein-coupled receptor kinases and arrestins: the known and the unknown

Heptahelical G-protein-coupled receptors are the most diverse and therapeutically important family of receptors, playing major roles in the physiology of various organs and tissues. They couple their ligand binding to G-protein activation, which then transmits intracellular signals. G-protein signaling is terminated by phosphorylation of the receptor by the family of G-protein-coupled receptor kinases (GRKs), followed by arrestin (Arr) binding, which uncouples the phosphorylated receptor from the G-protein and subsequently targets the receptor for internalization. Moreover, Arrs can transmit signals in their own right during receptor internalization. Genetic polymorphisms in receptors, as well as in GRK and Arr family members per se, which affect regulation of receptor signaling and function, have just started being identified and characterized. The present review will discuss what is known so far in this evolving field of GRK/Arr pharmacogenomics, as well as highlight important areas likely to produce invaluable information in the future.

Engagement of β-arrestin by transactivated insulin-like growth factor receptor is needed for V2 vasopressin receptor-stimulated ERK1/2 activation

G protein-coupled receptors (GPCRs) have been shown to activate the mitogen-activated protein kinases, ERK1/2, through both G protein-dependent and -independent mechanisms. Here, we describe a G protein-independent mechanism that unravels an unanticipated role for β-arrestins. Stimulation of the V2 vasopressin receptor (V2R) in cultured cells or in vivo in rat kidney medullar collecting ducts led to the activation of ERK1/2 through the metalloproteinase-mediated shedding of a factor activating the insulin-like growth factor receptor (IGFR). This process was found to be both Src- and β-arrestin-dependent. Whereas Src was found to act upstream of the metalloproteinase activation and be required for the release of the IGFR-activating factor, β-arrestins were found to act downstream of the IGFR transactivation. Unexpectedly, the engagement of β-arrestins by the IGFR but not by the V2R was needed to promote the vasopressin-stimulated ERK1/2 activation, indicating that a pool of β-arrestins distinct from those β-arrestins recruited to the V2R acts downstream of the receptor tyrosine kinase to activate ERK1/2. Such a dual site of action for β-arrestins helps explain the pleiotropic actions of this scaffolding protein. Given the role that V2R-stimulated ERK1/2 plays in kidney cell proliferation, this transactivation mechanism may have important implications for renal pathophysiology. Still, the role of β-arrestins downstream of a transactivation event is not limited to the V2R, because we observed a similar involvement for an unrelated GPCR (the platelet-activating factor receptor), indicating that it may be a general mechanism shared among GPCRs.

V2 vasopressin receptor (V2R) mutations in partial nephrogenic diabetes insipidus highlight protean agonism of V2R antagonists

Inactivating mutations of the V2 vasopressin receptor (V2R) cause cross-linked congenital nephrogenic diabetes insipidus (NDI), resulting in renal resistance to the antidiuretic hormone AVP. In two families showing partial NDI, characterized by an apparently normal response to diagnostic tests and an increase in the basal ADH levels suggesting AVP resistance, we have identified two V2R mutations, Ser-333del and Y128S. Both mutant V2Rs, when expressed in COS-7 cells, show partial defects in vasopressin-stimulated cAMP accumulation and intracellular localization. The inhibition of internalization does not rescue their localization. In contrast, the non-peptide V2R antagonists OPC41061 and OPC31260 partially rescue the membrane localization and basal function of these V2R mutants, whereas they inhibit the basal activity of the wild-type V2R. These results indicate that a partial loss of function of Ser-333del and Y128S mutant V2Rs results from defective membrane trafficking. These findings further indicate that V2R antagonists can act as protean agonists, serving as pharmacological chaperones for inactivating V2R mutants and also as inverse agonists of wild-type receptors. We speculate that this protean agonism could underlie the possible dual beneficial effects of the V2R antagonist: improvement of hyponatremia with heart failure or polycystic kidney disease and potential rescue of NDI.

Parathyroid hormone signaling via Gαs is selectively inhibited by an NH(2)-terminally truncated Gαs: implications for pseudohypoparathyroidism

Pseudohypoparathyroid patients have resistance predominantly to parathyroid hormone (PTH), and here we have examined the ability of an alternative Gαs-related protein to inhibit Gαs activity in a hormone-selective manner. We tested whether the GNAS exon A/B-derived NH(2)-terminally truncated (Tr) αs protein alters stimulation of adenylate cyclase by the PTH receptor (PTHR1), the thyroid-stimulating hormone (TSH) receptor (TSHR), the β(2)-adrenergic receptor (β(2)AR), or the AVP receptor (V2R). HEK293 cells cotransfected with receptor and full-length (FL) Gαs ± Tr αs protein expression vectors were stimulated with agonists (PTH [10(-7) to 10(-9)  M], TSH [1 to 100 mU], isoproterenol [10(-6) to 10(-8)  M], or AVP [10(-6) to 10(-8)  M]). Following PTH stimulation, HEK293 cells cotransfected with PTHR1 + FL Gαs + Tr αs had a significantly lower cAMP response than those transfected with only PTHR1 + FL Gαs. Tr αs also exerted an inhibitory effect on the cAMP levels stimulated by TSH via the TSHR but had little or no effect on isoproterenol or AVP acting via β(2)AR or V2R, respectively. These differences mimic the spectrum of hormone resistance in pseudohypoparathyroidism type 1a (PHP-1a) and type 1b (PHP-1b) patients. In opossum kidney (OK) cells, endogenously expressing the PTHR1 and β(2)AR, the exogenous expression of Tr αs at a level similar to endogenous FL Gαs resulted in blunting of the cAMP response to PTH, whereas that to isoproterenol was unaltered. A pseudopseudohypoparathyroid patient with Albright hereditary osteodystrophy harbored a de novo paternally inherited M1I Gαs mutation. Similar maternally inherited mutations at the initiation codon have been identified previously in PHP-1a patients. The M1I αs mutant (lacking the first 59 amino acids of Gαs) blunted the increase in cAMP levels stimulated via the PTHR1 in both HEK293 and OK cells similar to the Tr αs protein. Thus NH(2)-terminally truncated forms of Gαs may contribute to the pathogenesis of pseudohypoparathyroidism by inhibiting the activity of Gαs itself in a GPCR selective manner.

Visualizing microtubule-dependent vasopressin type 2 receptor trafficking using a new high-affinity fluorescent vasopressin ligand

The vasopressin receptor type 2 (V2R) is the major target of vasopressin (VP) in renal epithelial cells. Although it is known that VP induces V2R internalization, accumulation in the perinuclear area, and degradation, the V2R intracellular trafficking pathways remain elusive. We visualized this process by developing a new fluorescent VP analog tagged by tetramethylrhodamine (TMR)-[Lys-(PEG)(2)-Suc-TMR(8)]VP or (VP(TMR)). This ligand is fully functional as revealed by its high binding affinity toward V2R [(K(d)) =157 ± 52 nM] and ability to increase intracellular cAMP 32-fold. VP(TMR) induced V2R internalization in LLC-PK1 cells expressing either a FLAG-tagged receptor (FLAG-V2R) or V2R C-terminally tagged with green fluorescent protein (GFP) (V2R-GFP). After internalization, VP(TMR) and V2R-GFP colocalized in the perinuclear area, suggesting that the hormone and receptor traffic along the same pathway. VP(TMR) and V2R colocalized initially with the early endosome markers EEA1 and Rab5, and later with the recycling and late endosome markers Rab11 and Rab25. Epifluorescence microscopy of LLC-PK1 cells expressing GFP-tagged microtubules (MT) showed that VP(TMR)-containing vesicles travel along the MT network, and even remain attached to MT during the metaphase and anaphase of mitosis. Colchicine, a MT-depolymerizing agent, abolished perinuclear accumulation of VP(TMR), and Western blot analysis showed that VP-induced V2R-GFP degradation is markedly retarded, but not abolished, by colchicine (10 μM). We conclude that the new VP(TMR) ligand is suitable for dissecting V2R and VP internalization and trafficking in cells, and that V2R trafficking and down-regulation is an MT-dependent mechanism.

Therapeutic potential of β-arrestin- and G protein-biased agonists

Members of the seven-transmembrane receptor (7TMR), or G protein-coupled receptor (GPCR), superfamily represent some of the most successful targets of modern drug therapy, with proven efficacy in the treatment of a broad range of human conditions and disease processes. It is now appreciated that β-arrestins, once viewed simply as negative regulators of traditional 7TMR-stimulated G protein signaling, act as multifunctional adapter proteins that regulate 7TMR desensitization and trafficking and promote distinct intracellular signals in their own right. Moreover, several 7TMR biased agonists, which selectively activate these divergent signaling pathways, have been identified. Here we highlight the diversity of G protein- and β-arrestin-mediated functions and the therapeutic potential of selective targeting of these in disease states.

Quality control for unfolded proteins at the plasma membrane

A temperature-sensitive chimeric transmembrane protein reveals a mechanism for disposing misfolded proteins that make it to the plasma membrane.

Cellular protein homeostasis profoundly depends on the disposal of terminally damaged polypeptides. To demonstrate the operation and elucidate the molecular basis of quality control of conformationally impaired plasma membrane (PM) proteins, we constructed CD4 chimeras containing the wild type or a temperature-sensitive bacteriophage λ domain in their cytoplasmic region. Using proteomic, biochemical, and genetic approaches, we showed that thermal unfolding of the λ domain at the PM provoked the recruitment of Hsp40/Hsc70/Hsp90 chaperones and the E2–E3 complex. Mixed-chain polyubiquitination, monitored by bioluminescence resonance energy transfer and immunoblotting, is responsible for the nonnative chimera–accelerated internalization, impaired recycling, and endosomal sorting complex required for transport–dependent lysosomal degradation. A similar paradigm prevails for mutant dopamine D4.4 and vasopressin V2 receptor removal from the PM. These results outline a peripheral proteostatic mechanism in higher eukaryotes and its potential contribution to the pathogenesis of a subset of conformational diseases.

Beyond desensitization: physiological relevance of arrestin-dependent signaling

Heptahelical G protein-coupled receptors are the most diverse and therapeutically important family of receptors in the human genome. Ligand binding activates heterotrimeric G proteins that transmit intracellular signals by regulating effector enzymes or ion channels. G protein signaling is terminated, in large part, by arrestin binding, which uncouples the receptor and G protein and targets the receptor for internalization. It is clear, however, that heptahelical receptor signaling does not end with desensitization. Arrestins bind a host of catalytically active proteins and serve as ligand-regulated scaffolds that recruit protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into the receptor-arrestin complex. Although many of these arrestin-bound effectors serve to modulate G protein signaling, degrading second messengers and regulating endocytosis and trafficking, other signals seem to extend beyond the receptor-arrestin complex to regulate such processes as protein translation and gene transcription. Although these findings have led to a re-envisioning of heptahelical receptor signaling, little is known about the physiological roles of arrestin-dependent signaling. In vivo, the duality of arrestin function makes it difficult to dissociate the consequences of arrestin-dependent desensitization from those that might be ascribed to arrestin-mediated signaling. Nonetheless, recent evidence generated using arrestin knockouts, G protein-uncoupled receptor mutants, and arrestin pathway-selective "biased agonists" is beginning to reveal that arrestin signaling plays important roles in the retina, central nervous system, cardiovascular system, bone remodeling, immune system, and cancer. Understanding the signaling roles of arrestins may foster the development of pathway-selective drugs that exploit these pathways for therapeutic benefit.