New molecular and structural determinants involved in beta 2-adrenergic receptor desensitization and sequestration. Delineation using chimeric beta 3/beta 2-adrenergic receptors

Inside the Biology of the β3-Adrenoceptor

Since the first discovery in 1989, the β3-adrenoceptor (β3-AR) has gained great attention because it showed the ability to regulate many physiologic and metabolic activities, such as thermogenesis and lipolysis in brown and white adipose tissue, respectively (BAT, WAT), negative inotropic effects in cardiomyocytes, and relaxation of the blood vessels and the urinary bladder. The β3-AR has been suggested as a potential target for cancer treatment, both in adult and pediatric tumors, since under hypoxia its upregulation in the tumor microenvironment (TME) regulates stromal cell differentiation, tumor growth and metastases, signifying that its agonism/antagonism could be useful for clinical benefits. Promising results in cancer research have proposed the β3-AR being targeted for the treatment of many conditions, with some drugs, at present, undergoing phase II and III clinical trials. In this review, we report the scientific journey followed by the research from the β3-Ars' discovery, with focus on the β3-Ars' role in cancer initiation and progression that elects it an intriguing target for novel antineoplastic approaches. The overview highlights the great potential of the β3-AR, both in physiologic and pathologic conditions, with the intention to display the possible benefits of β3-AR modulation in cancer reality.

Mitochondria-targeted melatonin photorelease supports the presence of melatonin MT1 receptors in mitochondria inhibiting respiration

The presence of signaling-competent G protein-coupled receptors in intracellular compartments is increasingly recognized. Recently, the presence of Gi/o protein-coupled melatonin MT1 receptors in mitochondria has been revealed, in addition to the plasma membrane. Melatonin is highly cell permeant, activating plasma membrane and mitochondrial receptors equally. Here, we present MCS-1145, a melatonin derivative bearing a triphenylphosphonium cation for specific mitochondrial targeting and a photocleavable o-nitrobenzyl group releasing melatonin upon illumination. MCS-1145 displayed low affinity for MT1 and MT2 but spontaneously accumulated in mitochondria, where it was resistant to washout. Uncaged MCS-1145 and exogenous melatonin recruited β-arrestin 2 to MT1 in mitochondria and inhibited oxygen consumption in mitochondria isolated from HEK293 cells only when expressing MT1 and from mouse cerebellum of WT mice but not from MT1-knockout mice. Overall, we developed the first mitochondria-targeted photoactivatable melatonin ligand and demonstrate that melatonin inhibits mitochondrial respiration through mitochondrial MT1 receptors.

Design and Validation of the First Family of Photo-Activatable Ligands for Melatonin Receptors

Melatonin is a neurohormone released in a circadian manner with peak levels at night. Melatonin mediates its effects mainly through G protein-coupled MT₁ and MT₂ receptors. Drugs acting on melatonin receptors are indicated for circadian rhythm- and sleep-related disorders. Tools to study the activation of these receptors with high temporal resolution are lacking. Here, we synthesized a family of light-activatable caged compounds by attaching o-nitrobenzyl (o-NB) or coumarin photocleavable groups to melatonin indolic nitrogen. All caged compounds showed the expected decrease in binding affinity for MT₁ and MT₂. The o-NB derivative MCS-0382 showed the best uncaging and biological properties, with 250-fold increase in affinity and potency upon illumination. Generation of melatonin from MCS-0382 was further demonstrated by its ability to modulate the excitation of SCN neurons in rat brain slices. MCS-0382 is available to study melatonin effects in a temporally controlled manner in cellular and physiological settings.

β3-Adrenoreceptors as ROS Balancer in Hematopoietic Stem Cell Transplantation

In the last decades, the therapeutic potential of hematopoietic stem cell transplantation (HSCT) has acquired a primary role in the management of a broad spectrum of diseases including cancer, hematologic conditions, immune system dysregulations, and inborn errors of metabolism. The different types of HSCT, autologous and allogeneic, include risks of severe complications including acute and chronic graft-versus-host disease (GvHD) complications, hepatic veno-occlusive disease, lung injury, and infections. Despite being a dangerous procedure, it improved patient survival. Hence, its use was extended to treat autoimmune diseases, metabolic disorders, malignant infantile disorders, and hereditary skeletal dysplasia. HSCT is performed to restore or treat various congenital conditions in which immunologic functions are compromised, for instance, by chemo- and radiotherapy, and involves the administration of hematopoietic stem cells (HSCs) in patients with depleted or dysfunctional bone marrow (BM). Since HSCs biology is tightly regulated by oxidative stress (OS), the control of reactive oxygen species (ROS) levels is important to maintain their self-renewal capacity. In quiescent HSCs, low ROS levels are essential for stemness maintenance; however, physiological ROS levels promote HSC proliferation and differentiation. High ROS levels are mainly involved in short-term repopulation, whereas low ROS levels are associated with long-term repopulating ability. In this review, we aim summarize the current state of knowledge about the role of β3-adrenoreceptors (β3-ARs) in regulating HSCs redox homeostasis. β3-ARs play a major role in regulating stromal cell differentiation, and the antagonist SR59230A promotes differentiation of different progenitor cells in hematopoietic tumors, suggesting that β3-ARs agonism and antagonism could be exploited for clinical benefit.

The Regulator of G Protein Signaling Homologous Domain of G Protein-Coupled Receptor Kinase 2 Mediates Short-Term Desensitization of β3-Adrenergic Receptor

G protein coupled receptor (GPCR) kinases (GRKs) are key regulators of GPCR signaling. Canonical mechanism of GPCR desensitization involves receptor phosphorylation by GRKs followed by arrestin recruitment and uncoupling from heterotrimeric G protein. Although β3-adrenergic receptor (β3AR) lacks phosphorylation sites by GRKs, agonist treatment proved to induce β3AR desensitization in many cell types. Here we show that GRK2 mediates short-term desensitization of β3AR by a phosphorylation independent mechanism but mediated by its domain homologous to the regulator of G protein signaling (RGS). HEK293T cells overexpressing human β3AR presented a short-term desensitization of cAMP response stimulated by the β3AR agonist, BRL37344, and not by forskolin. We found that β3AR desensitization was higher in cells co-transfected with GRK2. Similarly, overexpression of the RGS homology domain but not kinase domain of GRK2 increased β3AR desensitization. Consistently, stimulation of β3AR increased interaction between GRK2 and Gαs subunit. Furthermore, in rat cardiomyocytes endogenously expressing β3AR, transfection with dominant negative mutant of RH domain of GRK2 (GRK2/D110A) increased cAMP response to BRL37344 and inhibited receptor desensitization. We expect our study to be a starting point for more sophisticated characterization of the consequences of GRK2 mediated desensitization of the β3AR in heart function and disease.

The role of Ap2a2 in PPARα-mediated regulation of lipolysis in adipose tissue

Adipose tissue plays a major role in the regulation of systemic metabolic homeostasis, with the AP2 adaptor complex being important in clathrin-mediated endocytosis (CME) of various cell surface receptors, including glucose transporter 4, the insulin receptor, and β-adrenergic receptors (ARs). One of the AP2 subunits, adaptor-related protein complex 2, α2 subunit (Ap2a2), has recently been identified as a peroxisome proliferator-activated receptor (PPAR)α target gene. The effects of PPARα on the AP2 adaptor complex and CME are unknown. We generated adipocyte-specific Ap2a2 knockout mice and investigated their metabolism when fed a standard chow or high-fat diet, without and with supplementation with the PPARα-agonist WY-14643 (WY). Although Ap2a2 deletion had only minor effects on glycaemic control, it led to substantial impairment in β-adrenergic activation of lipolysis, as evidenced by a loss of cAMP response, PKA activation, and glycerol/fatty acid release. These differences were related to increased cell surface localization of the β2- and β3-ARs. Lipolytic defects were accompanied by impaired WY-mediated loss of fat mass and whole-body fat oxidation. This study demonstrates a novel role for PPARα in β-adrenergic regulation of adipose tissue lipolysis and for adipose tissue in supplying adequate substrate to other peripheral tissues to accommodate the increase in systemic fatty acid oxidation that occurs upon treatment with PPARα agonists.-Montgomery, M. K., Bayliss, J., Keenan, S., Rhost, S., Ting, S. B., Watt, M. J. The role of Ap2a2 in PPARα-mediated regulation of lipolysis in adipose tissue.

Everything You Always Wanted to Know about β3-AR * (* But Were Afraid to Ask)

The beta-3 adrenergic receptor (β₃-AR) is by far the least studied isotype of the beta-adrenergic sub-family. Despite its study being long hampered by the lack of suitable animal and cellular models and inter-species differences, a substantial body of literature on the subject has built up in the last three decades and the physiology of β₃-AR is unraveling quickly. As will become evident in this work, β₃-AR is emerging as an appealing target for novel pharmacological approaches in several clinical areas involving metabolic, cardiovascular, urinary, and ocular disease. In this review, we will discuss the most recent advances regarding β₃-AR signaling and function and summarize how these findings translate, or may do so, into current clinical practice highlighting β₃-AR’s great potential as a novel therapeutic target in a wide range of human conditions.

Agonist-induced desensitisation of β3 -adrenoceptors: Where, when, and how?

β₃ -Adrenoceptor agonists have proven useful in the treatment of overactive bladder syndrome, but it is not known whether their efficacy during chronic administration may be limited by receptor-induced desensitisation. Whereas the β₂ -adrenoceptor has phosphorylation sites that are important for desensitisation, the β₃ -adrenoceptor lacks these; therefore, it had been assumed that β₃ -adrenoceptors are largely resistant to agonist-induced desensitisation. While all direct comparative studies demonstrate that β₃ -adrenoceptors are less susceptible to desensitisation than β₂ -adrenoceptors, desensitisation of β₃ -adrenoceptors has been observed in many models and treatment settings. Chimeric β₂ - and β₃ -adrenoceptors have demonstrated that the C-terminal tail of the receptor plays an important role in the relative resistance to desensitisation but is not the only relevant factor. While the evidence from some models, such as transfected CHO cells, is inconsistent, it appears that desensitisation is observed more often after long-term (hours to days) than short-term (minutes to hours) agonist exposure. When it occurs, desensitisation of β₃ -adrenoceptors can involve multiple levels including down-regulation of its mRNA and the receptor protein and alterations in post-receptor signalling events. The relative contributions of these mechanistic factors apparently depend on the cell type under investigation. Which if any of these factors is applicable to the human urinary bladder remains to be determined. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Noradrenaline increases intracellular glutathione in human astrocytoma U-251 MG cells by inducing glutamate-cysteine ligase protein via β3-adrenoceptor stimulation

Glutathione (GSH) plays a critical role in protecting cells from oxidative damage. Since neurons rely on the supply of GSH from astrocytes to maintain optimal intracellular GSH concentrations, the GSH concentration of astrocytes is important for the survival of neighboring neurons against oxidative stress. The neurotransmitter noradrenaline is known to modulate the functions of astrocytes and has been suggested to have neuroprotective properties in neurodegenerative diseases. To elucidate the mechanisms underlying the neuroprotective properties of noradrenaline, in this study, we investigated the effect of noradrenaline on the concentrations of intracellular GSH in human U-251 malignant glioma (MG; astrocytoma) cells. Treatment of the cells with noradrenaline for 24h concentration-dependently increased their intracellular GSH concentration. This increase was inhibited by a non-selective β-adrenoceptor antagonist propranolol and by a selective β3-adrenoceptor antagonist SR59230A, but not by a non-selective α-adrenoceptor antagonist phenoxybenzamine, or by a selective β1-adrenoceptor antagonist atenolol or by a selective β2-adrenoceptor antagonist butoxamine. In addition, the selective β3-adrenoceptor agonist CL316243 increased the intracellular GSH in U-251 MG cells. Treatment of the cells with noradrenaline (10μM) for 24h increased the protein level of the catalytic subunit of glutamate-cysteine ligase (GCLc), the rate-limiting enzyme of GSH synthesis; and this increase was inhibited by SR59230A. These results thus suggest that noradrenaline increased the GSH concentration in astrocytes by inducing GCLc protein in them via β3-adrenoceptor stimulation.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Somatostatin receptor-3 mediated intracellular signaling and apoptosis is regulated by its cytoplasmic terminal

In the present study, we describe the role of cytoplasmic terminal (C-tail) domain in regulating coupling to adenylyl cyclase, signaling, and apoptosis in human embryonic kidney (HEK-293) cells transfected with wild type (wt)-hSSTR3 and C-tail deleted mutants. Cells transfected with wt-hSSTR3 and C-tail mutants show comparable membrane expression; however, display decreased expression in presence of agonist. wt-hSSTR3 exists as preformed homodimer at cell surface in basal conditions and decreases in response to agonist. Cells expressing C-tail mutants also show evidence of homodimerization with the same intensity as wt-hSSTR3. The agonist-dependent inhibition of cyclic adenosine monophosphate (cAMP) was lost in cells expressing C-tail mutants. Agonist treatment in cells expressing wt-hSSTR3 resulted in inhibition of cell proliferation, increased expression of PARP-1, and TUNEL positivity in proliferating cell nuclear antigen (PCNA)-positive cells. The agonist mediated increase in membrane expression of protein tyrosine phosphatase (PTP) seen with wt-hSSTR3 was diminished in C-tail mutants, which was accompanied with the loss of receptor's ability to induce apoptosis. Taken together, our data provide new insights into C-tail-dependent regulation of cell signaling and apoptosis by hSSTR3.

Functional characterization of polymorphic variants for ovine MT1 melatonin receptors: possible implication for seasonal reproduction in sheep

In seasonal breeding species, the gene encoding for the melatonin MT(1) receptor (oMT(1)) is highly polymorphic and numerous data have reported the existence of an association between an allele of the receptor and a marked expression of the seasonality of reproduction in ewes. This allele called "m" (previously named "-" allele) carries a mutation leading to the absence of a MnlI restriction site as opposed to the "M" allele (previously named "+" allele) carrying the MnlI restriction site (previously "+" allele). This allows the determination of the three genotypes "M/M" (+/+), "M/m" (+/-) and "m/m" (-/-). This mutation is conservative and could therefore not be causal. However, it is associated with another mutation introducing the change of a valine to an isoleucine in the fifth transmembrane domain of the receptor. Homozygous "M/M" and "m/m" animals consequently express structurally different receptors respectively named oMT(1) Val(220) and oMT(1) Ile(220). The objective of this study was to test whether these polymorphic variants are functionally different. To achieve this goal, we characterized the binding properties and the transduction pathways associated with both variants of the receptors. Using a pharmacological approach, no variation in binding parameters between the two receptors when transiently expressed in COS-7. In stably transfected HEK293 cells, significant differences were detected in the inhibition of cAMP production whereas receptors internalization processes were not different. In conclusion, the possibility that subtle alterations induced by the non conservative mutation in "m/m" animals might modify the perception of the melatoninergic signal is discussed in the context of melatonin action.

Metabotropic glutamate receptor mGlu2 is resistant to homologous agonist-induced desensitization but undergoes protein kinase C-mediated heterologous desensitization

To investigate the susceptibility of the group II metabotropic glutamate receptor mGlu2 to agonist-induced desensitization, the receptor was stably expressed in Chinese hamster ovary (CHO-mGlu2) or C6 glioma cells (C6-mGlu2). Exposure of CHO-mGlu2 cells to the group II mGlu receptor agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG-1; 10 μM) for up to 15 h did not affect the subsequent ability of LCCG-1 to inhibit forskolin-stimulated cAMP accumulation. Similarly, in C6-mGlu2 cells, prolonged exposure to LCCG-1 also did not affect the subsequent ability of LCCG-1 to inhibit cAMP formation. In contrast, exposure of CHO-mGlu2 cells to the protein kinase C activator phorbol myristate acetate (PMA) suppressed the ability of LCCG-1 to inhibit cAMP formation. Using an in vitro model of group II mGlu receptor activity, the hemisected neonatal rat spinal cord preparation, the ability of the selective group II agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate ((2R,4R)-APDC) to depress the fast component of the dorsal root-evoked ventral root potential (fDR-VRP) did not desensitize when applied for up to 2 h. Together these results indicate that in contrast to most G protein-coupled receptors, the mGlu2 receptor is resistant to agonist-induced homologous desensitization, and that in vitro data suggests that resistance to desensitization is a physiologically relevant property of this mGlu receptor subtype.

Beta3-adrenergic receptors in cardiac and vascular tissues emerging concepts and therapeutic perspectives

Catecholamines released by the orthosympathetic system play a major role in the short- and long-term regulation of cardiovascular function. Beta1- and beta2-adrenoreceptors (ARs) have classically been considered as mediating most of their effects on cardiac contraction. After their initial cloning and pharmacologic characterization in the late 1980s, beta3-ARs have been mostly thought of as receptors mediating metabolic effects (e.g., lipolysis) in adipocytes. However, definitive evidence for their expression and functional coupling in cardiovascular tissues (including in humans) has recently initiated a re-examination of their implication in the pathophysiology of cardiovascular diseases. Distinctive pharmacodynamic properties of beta3-AR, e.g., their upregulation in disease and resistance to desensitization, suggest that they may be attractive targets for therapeutic intervention. They may substitute efficient vasodilating pathways when beta1/2-ARs are inoperative. In the heart, their contractile effects, which are functionally antipathetic to those of beta1/2-AR, may protect the myocardium against adverse effects of excessive catecholamine stimulation and perhaps mediate additional ancillary effects on key aspects of electrophysiology or remodeling. Longitudinal studies in animals and patients with different stages of heart failure are now needed to identify the optimal therapeutic scheme using specific combinations of agonists or antagonists at all three beta-ARs.

C-tail mediated modulation of somatostatin receptor type-4 homo- and heterodimerizations and signaling

Somatostatin receptors show great diversity in response to agonist mediated receptor-specific homo- and heterodimerizations. Here, using photobleaching-fluorescence resonance energy transfer, immunocytochemistry, western blot and co-immunoprecipitation, we investigated dimerization, trafficking, coupling to adenylyl cyclase and signaling of human somatostatin receptor-4 (hSSTR4) in HEK-293 cells. We also determined the role of the C-tail of hSSTR4 on physiological responses of the cells. wt-hSSTR4 exogenously expressed in HEK-293 cells exhibits constitutive dimerization, inhibits forskolin-stimulated cAMP, and displays agonist dependent changes in pERK1/2 and pERK5 expressions. Upon C-tail deletion, the receptor loses membrane expression and ability to dimerize and inhibition of cAMP and pERK5 however, displays several-fold increases in the expression of pERK1/2. Chimeric hSSTR4 with the C-tail of hSSTR5 functions like wt-hSSTR4, in contrast, with the C-tail of hSSTR1 functions like C-tail deleted hSSTR4. hSSTR4 dimerization and signaling are associated with increased cyclin-dependent-kinase p27(kip1) expression and inhibition of the cell proliferation. We also report heterodimerization between hSSTR4/hSSTR5, but not between hSSTR4/hSSTR1, with significant changes in receptor functions. Taken together, these data define a novel mechanism for the role of hSSTR4 in cell proliferation and modulation of signaling pathways.

Importance of adrenergic receptors in prenatally induced cognitive impairment in the domestic chick

In the domestic chick, mild hypoxia (24h of 14% oxygen) at two stages of embryonic development results in post-hatch memory deficiencies tested using a discriminated bead avoidance task. The nature of the memory loss depends on the gestational age at which the hypoxia occurs. Hypoxia on embryonic day 10 (E10) of a 21 day incubation results in chicks with no short-term memory 10 min after training, whereas hypoxia on day 14 (E14) results in chicks with good labile memory 30 min after training but no consolidation of memory into permanent storage (120 min). Hypoxia at E14 is associated with increased plasma levels of noradrenaline and therefore we suggest that altered catecholamine exposure within the brain contributes to cognitive problems by modifying the responsiveness of brain beta-adrenoceptors. In ovo administration of noradrenaline, or the beta(2)-adrenoceptor agonist formoterol, at E14 had the same effect on memory consolidation as hypoxia. Following hypoxia at E14, memory could be rescued after training by central injection of a beta(3)-adrenoceptor agonist, but not by a beta(2)-adrenoceptor agonist. The differences in the responsiveness of memory processing to beta(2)-adrenoceptor agonists suggests alterations to the receptors or downstream of the receptor activation. However, both types of beta-adrenoceptor agonists rescued memory in E10 treated chicks implying that at this age hypoxia does not affect the receptors. In summary, hypoxia or increased levels of stress hormones during incubation alters beta-adrenoceptor responsiveness; the outcome of the insult depends upon the cellular developmental processes at a given embryonic stage.

Delayed internalization and lack of recycling in a beta2-adrenergic receptor fused to the G protein alpha-subunit

Background

Chimeric proteins obtained by the fusion of a G protein-coupled receptor (GPCR) sequence to the N-terminus of the G protein α-subunit have been extensively used to investigate several aspects of GPCR signalling. Although both the receptor and the G protein generally maintain a fully functional state in such polypeptides, original observations made using a chimera between the β₂-adrenergic receptor (β₂AR) and Gα_s indicated that the fusion to the α-subunit resulted in a marked reduction of receptor desensitization and down-regulation. To further investigate this phenomenon, we have compared the rates of internalization and recycling between wild-type and Gα_s-fused β₂AR.

Results

The rate of agonist-induced internalization, measured as the disappearance of cell surface immunofluorescence in HEK293 cells permanently expressing N-terminus tagged receptors, was reduced three-fold by receptor-G protein fusion. However, both fused and non-fused receptors translocated to the same endocytic compartment, as determined by dual-label confocal analysis of cells co-expressing both proteins and transferrin co-localization.

Receptor recycling, determined as the reversion of surface immunofluorescence following the addition of antagonist to cells that were previously exposed to agonist, markedly differed between wild-type and fused receptors. While most of the internalized β₂AR returned rapidly to the plasma membrane, β₂AR-Gα_s did not recycle, and the observed slow recovery for the fusion protein immunofluorescence was entirely accounted for by protein synthesis.

Conclusion

The covalent linkage between β₂AR and Gα_s does not appear to alter the initial endocytic translocation of the two proteins, although there is reduced efficiency. It does, however, completely disrupt the process of receptor and G protein recycling. We conclude that the physical separation between receptor and Gα is not necessary for the transit to early endosomes, but is an essential requirement for the correct post-endocytic sorting and recycling of the two proteins.

Down-regulation of histamine H1 receptors by beta2-adrenoceptor-mediated inhibition of H1 receptor gene transcription

Histamine H1 receptor (H1R) levels vary under various pathological conditions, and these changes may be responsible for some pathogenesis such as in allergic rhinitis. Several stimulants, including histamine, muscarinic agonists and platelet-activating factor, have now been shown to regulate H1R levels and may have roles in regulating the H1R level in physiological and pathological conditions. Results for beta2-adrenoceptor (beta2AR) stimulation are conflicting, however.beta2AR up-regulated H1R in bovine tracheal smooth muscle, but down-regulated human H1R expressed in Chinese hamster ovary (CHO) cells. It is possible that this discrepancy comes from the differences in the preparations used for each study: the former cell expressed bovine H1R and the latter cell expressed human H1R. Moreover, CHO cells have been shown to be inadequate for studying the effects on H1R gene expression, because the cells express non-endogenous stably transfected H1R under the control of the SV40 promoter. Therefore, in this study, we have investigated the role of beta2AR stimulation in H1R gene regulation using human U373 astrocytoma cells that express endogenous H1R and transfected beta2AR. Stimulation of beta2AR significantly reduced H1R promoter activity and H1R mRNA levels. H1R mRNA stability was slightly reduced by beta2AR stimulation, although this was not significant. The decrease of H1R mRNA by beta2AR stimulation was blocked by the protein kinase A (PKA) inhibitor KT5720, suggesting the involvement of PKA. These results indicate that the beta2AR is involved in the down-regulation of human H1R by inhibiting H1R gene transcription through a PKA-dependent process.

A generic approach for the purification of signaling complexes that specifically interact with the carboxyl-terminal domain of G protein-coupled receptors

G protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors and are major drug targets. Recent progress has shown that GPCRs are part of large protein complexes that regulate their activity. We present here a generic approach for identification of these complexes that is based on the use of receptor subdomains and that overcomes the limitations of currently used genetics and proteomics approaches. Our approach consists of a carefully balanced combination of chemically synthesized His6-tagged baits, immobilized metal affinity chromatography, one- and two-dimensional gel electrophoresis separation and mass spectrometric identification. The carboxyl-terminal tails (C-tails) of the human MT1 and MT2 melatonin receptors, two class A GPCRs, were used as models to purify protein complexes from mouse brain lysates. We identified 32 proteins that interacted with the C-tail of MT1, 14 proteins that interacted with the C-tail of MT2, and eight proteins that interacted with both C-tails. Several randomly selected proteins were validated by Western blotting, and the functional relevance of our data was further confirmed by showing the interaction between the full-length MT1 and the regulator of G protein signaling Z1 in transfected HEK 293 cells and native tissue. Taken together, we have established an integrated and generic purification strategy for the identification of high quality and functionally relevant GPCR-associated protein complexes that significantly widens the repertoire of available techniques.

Distinct motifs of neuropeptide Y receptors differentially regulate trafficking and desensitization

Activated human neuropeptide Y Y(1) receptors rapidly desensitize and internalize through clathrin-coated pits and recycle from early and recycling endosomes, unlike Y(2) receptors that neither internalize nor desensitize. To identify motifs implicated in Y(1) receptor desensitization and trafficking, mutants with varying C-terminal truncations or a substituted Y(2) C-terminus were constructed. Point mutations of key putative residues were made in a C-terminal conserved motif [phi-H-(S/T)-(E/D)-V-(S/T)-X-T] that we have identified and in the second intracellular i2 loop. Receptors were analyzed by functional assays, spectrofluorimetric measurements on living cells, flow cytometry, confocal imaging and bioluminescence resonance energy transfer assays for beta-arrestin activation and adaptor protein (AP-2) complex recruitment. Inhibitory GTP-binding protein-dependent signaling of Y(1) receptors to adenylyl cyclase and desensitization was unaffected by C-terminal truncations or mutations, while C-terminal deletion mutants of 42 and 61 amino acids no longer internalized. Substitutions of Thr357, Asp358, Ser360 and Thr362 by Ala in the C-terminus abolished both internalization and beta-arrestin activation but not desensitization. A Pro145 substitution by His in an i2 consensus motif reported to mediate phosphorylation-independent recruitment of beta-arrestins affected neither desensitization, internalization or recycling kinetics of activated Y(1) receptors nor beta-arrestin activation. Interestingly, combining Pro145 substitution by His and C-terminal substitutions significantly attenuates Y(1) desensitization. In the Y(2) receptor, replacement of His155 with Pro at this position in the i2 loop motif promotes agonist-mediated desensitization, beta-arrestin activation, internalization and recycling. Overall, our results indicate that beta-arrestin-mediated desensitization and internalization of Y(1) and Y(2) receptors are differentially regulated by the C-terminal motif and the i2 loop consensus motif.

The function of alpha- and beta-adrenoceptors of the saphenous artery in caveolin-1 knockout and wild-type mice

BACKGROUND AND PURPOSE

Adrenoceptors can associate with cardiac caveolae. To investigate the function of vascular caveolae, adrenoceptor-mediated effects were compared in the saphenous artery of caveolin-1 knockout (cav-1KO) and wild-type (WT) mice.

EXPERIMENTAL APPROACH

Electronmicroscopy was used to detect caveolae. Real-Time quantitative PCR was used for adrenoceptor subtypes. Catecholamine-evoked contractions and relaxations were studied in arterial segments.

KEY RESULTS

Caveolae were found in arterial smooth muscle from WT but not from cav-1KO mice. Arterial mRNA levels for the adrenoceptors alpha1A, alpha1B, alpha1D, beta1, beta2 and beta3 were similar in cav-1KO and WT. (-)-Noradrenaline contracted cav-1KO (-log EC50M=7.1) and WT (-log EC50M=7.3) arteries through prazosin-sensitive receptors. Maximum (-)-noradrenaline-evoked contractions were greater in cav-1KO than WT arteries. (-)-Isoprenaline relaxed WT arteries (-log EC50M=7.3) more potently than cav-1KO arteries (-log EC50M=6.8); the effects were antagonized partially and similarly by the beta2-selective antagonist ICI118551 (50 nM). The (-)-isoprenaline-evoked relaxation was partially antagonized by the beta1-adrenoceptor-selective antagonist CGP20712 (300 nM) in WT but not cav-1KO arteries. The beta3-adrenoceptor-selective antagonist L748337 (100 nM) partially antagonized the relaxant effects of (-)-isoprenaline in cav-1KO but not in WT arteries. BRL37344 partially relaxed arteries through beta3-adrenoceptors in cav-1KO but not WT. The relaxant effects of BRL37344 were decreased by the NO synthase inhibitor OmegaL-nitroarginine.

CONCLUSIONS AND IMPLICATIONS

The function of arterial alpha1- and beta2-adrenoceptors is similar in cav-1KO and WT mice. beta1-adrenoceptor-mediated relaxation in WT is lost in cav-1KO and replaced by the appearance of beta3-adrenoceptors.

Intracrine signaling through lipid mediators and their cognate nuclear G-protein-coupled receptors: a paradigm based on PGE2, PAF, and LPA1 receptors

Prostaglandins (PGs), platelet-activating factor (PAF), and lysophosphatidic acid (LPA) are ubiquitous lipid mediators that play important roles in inflammation, cardiovascular homeostasis, and immunity and are also known to modulate gene expression of specific pro-inflammatory genes. The mechanism of action of these lipids is thought to be primarily dependent on their specific plasma membrane receptors belonging to the superfamily of G-protein-coupled receptors (GPCR). Increasing evidence suggests the existence of a functional intracellular GPCR population. It has been proposed that immediate effects are mediated via cell surface receptors whereas long-term responses are dependent upon intracellular receptor effects. Indeed, receptors for PAF, LPA, and PGE(2) (specifically EP(1), EP(3), and EP(4)) localize at the cell nucleus of cerebral microvascular endothelial cells of newborn pigs, rat hepatocytes, and cells overexpressing each receptor. Stimulation of isolated nuclei with these lipids reveals biological functions including transcriptional regulation of major genes, namely c-fos, cylooxygenase-2, and endothelial as well as inducible nitric oxide synthase. In the present review, we shall focus on the nuclear localization and signaling of GPCRs recognizing PGE(2), PAF, and LPA phospholipids as ligands. Mechanisms on how nuclear PGE2, PAF, and LPA receptors activate gene transcription and nuclear localization pathways are presented. Intracrine signaling for lipid mediators uncover novel pathways to elicit their effects; accordingly, intracellular GPCRs constitute a distinctive mode of action for gene regulation.

An aplysia dopamine1-like receptor: molecular and functional characterization

In Aplysia, the neurotransmitter dopamine is involved in the regulation of various physiological processes and motor functions, like feeding behaviour, and in the siphon-gill withdrawal reflex. In this paper, we report the characterization of the first Aplysia D1-like dopamine receptor (Apdop1) mainly expressed in the CNS, heart and buccal mass. Following expression of the Apdop1 receptor in HEK293 cells, a higher level of cAMP was observed in the absence of the receptor ligand, showing that Apdop1 is constitutively active. This activity was blocked by the inverse agonist flupentixol. Application of dopamine (EC50 of 35 nm) or serotonin (EC50 of 36 microm) to Apdop1-transfected HEK293 cells further increased the level of cAMP, suggesting that the receptor is linked to the stimulatory Gs protein pathway. When expressed in cultured sensory neurons, Apdop1 immunoreactivity was observed in the cell body and neurites. Control sensory neurons responded to dopamine with a decrease in excitability mediated by a pertusis toxin-sensitive G protein. Expression of Apdop1 produced an increase in hyperpolarization in the absence of agonist and an increase in membrane excitability following stimulation by dopamine. In the presence of pertussis toxin to inhibit the Gi protein inhibitory pathway responsible for decrease in excitability mechanism, Stimulation of membrane excitability was observed. Apdop1 sensitivity to dopamine makes it a potential modulator of operant conditioning procedure.

Intracellular signaling of lipid mediators via cognate nuclear G protein-coupled receptors

Platelet-activating factor (PAF) and lysophosphatidic acid (LPA) are ubiquitous lipid mediators that play important roles in inflammation, cardiovascular homeostasis, and immunity and are also known to modulate gene expression of specific proinflammatory genes. The mechanism of action of these phospholipids is thought to be primarily dependent on their specific plasma membrane receptors belonging to the superfamily of G protein-coupled receptors (GPCRs). However, increasing evidence suggests the existence of a functional intracellular GPCR population. It has been suggested that immediate effects are mediated by cell surface receptors, whereas long-term responses are mediated by intracellular receptors. PAF and LPA(1) receptors localize at the cell nucleus of cerebral microvascular endothelial cells of newborn pig, rat hepatocytes, and cells overexpressing each receptor, and stimulation of isolated nuclei reveal biological functions, including transcriptional regulation of major genes, namely cylooxygenase-2 and inducible nitric oxide synthase. This mini review focuses on the nuclear localization and signaling of GPCRs, recognizing PAF and LPA phospholipids as ligands. Theories on how nuclear PAF and LPA1 receptors activate gene transcription and nuclear localization pathways are discussed. Intracrine signaling for lipid mediators uncover novel pathways to elicit their effects; moreover, intracellular GPCRs constitute a distinctive mode of action for gene regulation.

Internalization of Human 5-HT4a and 5-HT4b Receptors is Splice Variant Dependent

The family of 5-HT4 receptors comprises 16 putative splice variants. We have previously shown that there are differences in signal transduction of the h5-HT4a and h5-HT4b receptors. In the present study, the internalization of these two splice variants following receptor stimulation was investigated with confocal microscopy on living cells. Chimeric receptors, h5-HT4a-GFP and h5-HT4b-GFP were generated by fusing the coding sequence of the 5-HT4 receptor with the coding sequence of the GFP. The agonist stimulation of fluorescent receptors resulted in a time-dependent internalization of the h5-HT4b-GFP receptor, but not of the h5-HT4a-GFP receptor. The h5-HT4b receptor displays a dual coupling to Gαi,o and Gαs proteins, in contrast to the h5-HT4a receptor, which couples to Gαs proteins only. We investigated whether the difference in internalization of the two splice variant receptors was related to their differential coupling. Therefore, we performed agonist-stimulation of the receptor following inhibition of the Gαi,o protein coupling using PTX. The h5-HT4b receptor internalization is PTX insensitive. We co-transfected the fluorescent chimeric receptors with other wild-type variants, which did not produce an alteration of the receptor trafficking. These findings provide the first evidence of differential internalization between the two splice variants, 5-HT4a and 5-HT4b receptors.

Role of G protein-coupled receptor kinase 2 and arrestins in beta-adrenergic receptor internalization

G protein-coupled receptors (GPCRs) mediate the action of messengers that are key modulators of the function, growth, and differentiation of cardiac and vascular cells. A general feature of GPCRs is the existence of complex regulatory mechanisms that modulate receptor responsiveness and underlie important physiologic phenomena such as signal integration and desensitization. The molecular mechanisms of desensitization have been investigated with the beta2-adrenergic receptor (beta2AR) used as the main model system. Rapid regulation of betaAR and other GPCRs appears to involve agonist-promoted receptor phosphorylation by G protein-coupled receptor kinases (GRKs). This is followed by binding of uncoupling proteins termed arrestins and transient receptor internalization, which plays a key role in resensitizing GPCR by allowing its dephosphorylation and recycling. Recent data indicate that, besides the uncoupling function, GRK2 and beta-arrestin also directly participate in beta2AR sequestration, thus providing the trigger for its resensitization. A detailed knowledge of the role of GRKs and arrestins in betaAR internalization would make their physiologic role in the modulation of cellular responses to messengers better understood.

P2Y1 and P2Y12 receptors for ADP desensitize by distinct kinase-dependent mechanisms

Adenosine 5'-diphosphate (ADP) plays a central role in regulating platelet function by the activation of the G protein-coupled receptors P2Y(1) and P2Y(12). Although it is well established that aggregation responses of platelets to ADP desensitize, the underlying mechanisms involved remain unclear. In this study we demonstrate that P2Y(1)- and P2Y(12)-mediated platelet responses desensitize rapidly. Furthermore, we have established that these receptors desensitize by different kinase-dependent mechanisms. G protein-coupled receptor kinase (GRK) 2 and GRK6 are both endogenously expressed in platelets. Transient overexpression of dominant-negative mutants of these kinases or reductions in endogenous GRK expression by the use of specific siRNAs in 1321N1 cells showed that P2Y(12), but not P2Y(1), desensitization is mediated by GRKs. In contrast, desensitization of P2Y(1), but not P2Y(12), is largely dependent on protein kinase C activity. This study is the first to show that both P2Y(1) and P2Y(12) desensitize in human platelets, and it reveals ways in which their sensitivity to ADP may be differentially and independently altered.

Human recombinant membrane-bound aminopeptidase P: production of a soluble form and characterization using novel, internally quenched fluorescent substrates

APP (aminopeptidase P) has the unique ability to cleave the N-terminal amino acid residue from peptides exhibiting a proline at P(1)'. Despite its putative involvement in the processing of bioactive peptides, among them the kinins, little is known about the physiological roles of both human forms of APP. The purpose of the present study is first to engineer and characterize a secreted form of hmAPP (human membrane-bound APP). Our biochemical analysis has shown that the expressed glycosylated protein is fully functional, and exhibits enzymic parameters similar to those described previously for mAPP purified from porcine or bovine lungs or expressed from a porcine clone. This soluble form of hmAPP cross-reacts with a polyclonal antiserum raised against a 469-amino-acid hmAPP fragment produced in Escherichia coli. Secondly, we synthesized three internally quenched fluorescent peptide substrates that exhibit a similar affinity for the enzyme than its natural substrates, the kinins, and a higher affinity compared with the tripeptide Arg-Pro-Pro used until now for the quantification of APP in biological samples. These new substrates represent a helpful analytical tool for rapid and reliable screening of patients susceptible to adverse reactions associated with angiotensin-converting enzyme inhibitors or novel vasopeptidase (mixed angiotensin-converting enzyme/neprilysin) inhibitors.

Endothelin-converting enzyme-like 1 (ECEL1) is present both in the plasma membrane and in the endoplasmic reticulum

Enzymes of the M13 family of zinc-containing endopeptidases are recognized as important regulators of neuropeptide and peptide hormone activity. Peptidases of this family are type II integral-membrane proteins characterized by short cytosolic domains and large extracellular domains containing the active site. The M13 family has, at present, seven members, including ECEL1 (endothelin-converting enzyme-like 1), one of the newest members. ECEL1 is expressed predominantly in the central nervous system. It has been proposed that the enzyme has a role in the nervous regulation of the respiratory system. No physiological substrate has been identified yet. To better understand the function(s) of this enzyme, we have expressed human ECEL1 in cultured cells and monitored its biosynthesis and subcellular localization. Immunoblot and cell-surface biotinylation analysis of transfected cells expressing ECEL1 showed that only a fraction of the protein travelled to the cell surface, while most of the enzyme was present in an intracellular compartment identified by confocal immunofluorescence microscopy and cell fractionation as the ER (endoplasmic reticulum). Pulse-chase experiments showed that ER-localized ECEL1 was stable, with a half-life of more than 3 h. Endogenous ECEL1 from mouse pituitary gland had a similar distribution between the cell surface and the ER. Finally, using domain-swapping experiments with neprilysin, another member of the M13 family, we showed that localization of ECEL1 to the ER requires both the transmembrane and cytoplasmic domains. It thus appears that ECEL1 may have functions both at the cell surface and in the ER.

Hetero-oligomerization between β2- and β3-Adrenergic Receptors Generates a β-Adrenergic Signaling Unit with Distinct Functional Properties

The ability of the closely related beta(2)- and beta(3)-adrenergic receptors (AR) to form hetero-oligomers was assessed by bioluminescence resonance energy transfer. Quantitative bioluminescence resonance energy transfer titration curves revealed that the beta(2)AR has identical propensity to hetero-oligomerize with the beta(3)AR than to form homo-oligomers. To determine the influence of heterooligomerization, a HEK293 cell line stably expressing an excess of beta(3)AR over beta(2)AR was generated so that all beta(2)AR are engaged in hetero-oligomerization with beta(3)AR, providing a tool to study the effect of hetero-oligomerization on beta(2)AR function in the absence of any beta(2)AR homooligomer. The hetero-oligomerization had no effect on the ligand binding properties of various beta(2)AR ligands and did not affect the potency of isoproterenol to stimulate adenylyl cyclase. Despite the unaltered ligand binding properties of the beta(2/3)AR hetero-oligomer, the stable association of the beta(2)AR with the beta(3)AR completely blocked agonist-stimulated internalization of the beta(2)AR. Given that the beta(3)AR is resistant to agonist-promoted endocytosis, the results indicate that the beta(3)AR acted as a dominant negative of the beta(2)AR endocytosis process. Consistent with this notion, the beta(2/3)AR hetero-oligomer displayed a lower propensity to recruit beta-arrestin-2 than the beta(2)AR. The hetero-oligomerization also led to a change in G protein coupling selectivity. Indeed, in contrast to beta(2)AR and beta(3)AR, which regulate adenylyl cyclase and extracellular signal-regulated kinase activity through a coupling to G(s) and G(i/o), no G(i/o) coupling was observed for the beta(2/3)AR hetero-oligomer. Together, these results demonstrate that hetero-oligomerization between beta(2)AR and beta(3)AR forms a beta-adrenergic signaling unit that possesses unique functional properties.

Probing receptor structure/function with chimeric G-protein-coupled receptors

Owing its name to an image borrowed from Greek mythology, a chimera is seen to represent a new entity created as a composite from existing creatures or, in this case, molecules. Making use of various combinations of three basic domains of the receptors (i.e., exofacial, transmembrane, and cytoplasmic segments) that couple agonist binding into activation of effectors through heterotrimeric G-proteins, molecular pharmacology has probed the basic organization, structure/function relationships of this superfamily of heptahelical receptors. Chimeric G-protein-coupled receptors obviate the need for a particular agonist ligand when the ligand is resistant to purification or, in the case of orphan receptors, is not known. Chimeric receptors created from distant members of the heptahelical receptors enable new strategies in understanding how these receptors transduce agonist binding into receptor activation and may be able to offer insights into the evolution of G-protein-coupled receptors from yeast to humans.

Agonist-induced internalization of the platelet-activating factor receptor is dependent on arrestins but independent of G-protein activation. Role of the C terminus and the (D/N)PXXY motif

As with most G-protein-coupled receptors, repeated agonist stimulation of the platelet-activating factor receptor (PAFR) results in its desensitization, sequestration, and internalization. In this report, we show that agonist-induced PAFR internalization is independent of G-protein activation but is dependent on arrestins and involves the interaction of arrestins with a limited region of the PAFR C terminus. In cotransfected COS-7 cells, both arrestin-2 and arrestin-3 could be coimmunoprecipitated with PAFR, and agonist stimulation of PAFR induced the translocation of both arrestin-2 and arrestin-3. Furthermore, coexpression of arrestin-2 with PAFR potentiated receptor internalization, whereas agonist-induced PAFR internalization was inhibited by a dominant negative mutant of arrestin-2. The coexpression of a minigene encoding the C-terminal segment of the receptor abolished PAF-induced arrestin translocation and inhibited PAFR internalization. Using C terminus deletion mutants, we determined that the association of arrestin-2 with the receptor was dependent on the region between threonine 305 and valine 330 because arrestin-2 could be immunoprecipitated with the mutant PAFRstop330 but not PAFRstop305. Consistently, stop330 could mediate agonist-induced arrestin-2 translocation, whereas stop305 could not. Two other deletion mutants with slightly longer regions of the C terminus, PAFRstop311 and PAFRstop317, also failed to induce arrestin-2 translocation. Finally, the PAFR mutant Y293A, containing a single substitution in the putative internalization motif DPXXY in the seventh transmembrane domain (which we had shown to be able to internalize but not to couple to G-proteins) could efficiently induce arrestin translocation. Taken together, our results indicate that ligand-induced PAFR internalization is dependent on arrestins, that PAFR can associate with both arrestin-2 and -3, and that their translocation involves interaction with the region of residues 318-330 in the PAFR C terminus but is independent of G-protein activation.

Agonist-stimulated and tonic internalization of metabotropic glutamate receptor 1a in human embryonic kidney 293 cells: agonist-stimulated endocytosis is beta-arrestin1 isoform-specific

Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors (GPCRs) that contribute to the regulation of integrative brain functions such as cognition, motor control, and neural development. Metabotropic glutamate receptors are members of a unique class of GPCRs (class III) that include the calcium sensing and gamma-aminobutyric acid type B receptors. Although mGluRs bear little sequence homology to well-characterized members of the GPCR superfamily, both second messenger-dependent protein kinases and G protein-coupled receptor kinases (GRKs) contribute to mGluR desensitization. Therefore, in the present study, we examined whether beta-arrestins, regulators of GPCR desensitization and endocytosis, are required for mGluR1a desensitization and internalization in human embryonic kidney (HEK) 293 cells. Unlike what has been reported for other GPCRs, we find that in response to agonist stimulation, mGluR1a internalization is selectively mediated by beta-arrestin1 in HEK 293 cells. However, even though beta-arrestin1 binds directly to the carboxyl-terminal tail of mGluR1a and redistributes with mGluR1a to endosomes, neither beta-arrestin1 nor beta-arrestin2 seems to contribute to mGluR1a desensitization in HEK 293 cells. We also observed extensive tonic mGluR1a internalization via clathrin-coated vesicles in the absence of agonist. The tonic internalization of mGluR1a is insensitive to antagonist treatment, dominant-negative mutants of GRK2, beta-arrestin1, and dynamin as well as treatments that disrupt caveolae, but is blocked by hypertonic sucrose and concanavalin A treatment. Internalized mGluR1a is colocalized with clathrin, transferrin receptor, beta2-adrenergic receptor, and Rab5 GTPase in endocytic vesicles. Therefore, although mGluR1a internalizes with beta-arrestin in response to agonist, the agonist-independent internalization of mGluR1a involves the beta-arrestin-independent targeting of mGluR1a to clathrin-coated vesicles.

Characterization of PHEX endopeptidase catalytic activity: identification of parathyroid-hormone-related peptide107-139 as a substrate and osteocalcin, PPi and phosphate as inhibitors

Mutations in the PHEX gene (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) are responsible for X-linked hypophosphataemia, and studies in the Hyp mouse model of the human disease implicate the gene product in the regulation of renal phosphate (P(i)) reabsorption and bone mineralization. Although the mechanism for PHEX action is unknown, structural homologies with members of the M13 family of endopeptidases suggest a function for PHEX protein in the activation or degradation of peptide factors involved in the control of renal P(i) transport and matrix mineralization. To determine whether PHEX has endopeptidase activity, we generated a recombinant soluble, secreted form of human PHEX (secPHEX) and tested the activity of the purified protein with several peptide substrates, including a variety of bone-related peptides. We found that parathyroid-hormone-related peptide(107-139) is a substrate for secPHEX and that the enzyme cleaves at three positions within the peptide, all located at the N-terminus of aspartate residues. Furthermore, we show that osteocalcin, PP(i) and P(i), all of which are abundant in bone, are inhibitors of secPHEX activity. Inhibition of secPHEX activity by osteocalcin was abolished in the presence of Ca(2+). We suggest that PHEX activity and mineralization may be controlled in vivo by PP(i)/P(i) and Ca(2+) and, in the latter case, the regulation requires the participation of osteocalcin.

Interaction with beta-arrestin determines the difference in internalization behavor between beta1- and beta2-adrenergic receptors

The beta(1)-adrenergic receptor (beta(1)AR) shows the resistance to agonist-induced internalization. As beta-arrestin is important for internalization, we examine the interaction of beta-arrestin with beta(1)AR with three different methods: intracellular trafficking of beta-arrestin, binding of in vitro translated beta-arrestin to intracellular domains of beta(1)- and beta(2)ARs, and inhibition of betaAR-stimulated adenylyl cyclase activities by beta-arrestin. The green fluorescent protein-tagged beta-arrestin 2 translocates to and stays at the plasma membrane by beta(2)AR stimulation. Although green fluorescent protein-tagged beta-arrestin 2 also translocates to the plasma membrane, it returns to the cytoplasm 10-30 min after beta(1)AR stimulation. The binding of in vitro translated beta-arrestin 1 and beta-arrestin 2 to the third intracellular loop and the carboxyl tail of beta(1)AR is lower than that of beta(2)AR. The fusion protein of beta-arrestin 1 with glutathione S-transferase inhibits the beta(1)- and beta(2)AR-stimulated adenylyl cyclase activities, although inhibition of the beta(1)AR-stimulated activity requires a higher concentration of the fusion protein than that of the beta(2)AR-stimulated activity. These results suggest that weak interaction of beta(1)AR with beta-arrestins explains the resistance to agonist-induced internalization. This is further supported by the finding that beta-arrestin can induce internalization of beta(1)AR when beta-arrestin 1 does not dissociate from beta(1)AR by fusing to the carboxyl tail of beta(1)AR.

A novel murine Staufen isoform modulates the RNA content of Staufen complexes

Mouse Staufen (mStau) is a double-stranded RNA-binding protein associated with polysomes and the rough endoplasmic reticulum (RER). We describe a novel endogenous isoform of mStau (termed mStau(i)) which has an insertion of six amino acids within dsRBD3, the major double-stranded RNA (dsRNA)-binding domain. With a structural change of the RNA-binding domain, this conserved and widely distributed isoform showed strongly impaired dsRNA-binding ability. In transfected cells, mStau(i) exhibited the same tubulovesicular distribution (RER) as mStau when weakly expressed; however, when overexpressed, mStau(i) was found in large cytoplasmic granules. Markers of the RER colocalized with mStau(i)-containing granules, showing that overexpressed mStau(i) could still be associated with the RER. Cotransfection of mStau(i) with mStau relocalized overexpressed mStau(i) to the reticular RER, suggesting that they can form a complex on the RER and that a balance between these isoforms is important to achieve proper localization. Coimmunoprecipitation demonstrated that the two mStau isoforms are components of the same complex in vivo. Analysis of the immunoprecipitates showed that mStau is a component of an RNA-protein complex and that the association with mStau(i) drastically reduces the RNA content of the complex. We propose that this new isoform, by forming a multiple-isoform complex, regulates the amount of RNA in mStau complexes in mammalian cells.

Expression of autocrine motility factor/phosphohexose isomerase in Cos7 cells

Autocrine motility factor (AMF) is identical to the glycolytic enzyme phosphohexose isomerase (PHI) and overexpression of AMF/PHI is associated with tumor malignancy. In order to study the overexpression of AMF/PHI, an HA-tagged AMF construct was transiently transfected into Cos7 cells. Expression of a tagged AMF-HA allowed us to determine that over a period of 16 hours only a small amount (0.1-1%) of total cellular AMF-HA was secreted into the cell medium. Cell-associated AMF-HA was exclusively cytosolic as it could be completely extracted with Triton X-100 and concentrated within actin rich pseudopodial domains. Treatment of the cells with the glycolysis inhibitor oxamate disrupted the association of AMF-HA with actin concentrations demonstrating that glycolysis regulates the formation of these AMF/PHI-associated actin-rich protrusions. AMF/PHI is a well-characterized tumor cell secreted cytokine and we identify here an alternate intracellular function for this glycolytic enzyme/cytokine in cell motility.

Molecular cloning and biochemical characterization of a new mouse testis soluble-zinc-metallopeptidase of the neprilysin family

Because of their roles in controlling the activity of several bio-active peptides, members of the neprilysin family of zinc metallopeptidases have been identified as putative targets for the design of therapeutic agents. Presently, six members have been reported, these are: neprilysin, endothelin-converting enzyme (ECE)-1 and ECE-2, the Kell blood group protein, PHEX (product of the phosphate-regulating gene with homologies to endopeptidase on the X chromosome) and X-converting enzyme (XCE). In order to identify new members of this important family of peptidases, we designed a reverse transcriptase-PCR strategy based on conserved amino acid sequences of neprilysin, ECE-1 and PHEX. We now report the cloning from mouse testis of a novel neprilysin-like peptidase that we called NL1. NL1 is a glycoprotein that, among the members of the family, shows the strongest sequence identity with neprilysin. However, in contrast with neprilysin and other members of the family which are type II integral membrane proteins, NL1 was secreted when expressed in cultured mammalian cells, likely due to cleavage by a subtilisin-like convertase at a furin-like site located 22 amino acid residues in the C-terminus of the transmembrane domain. The recombinant enzyme exhibited neprilysin-like peptidase activity and was efficiently inhibited by phosphoramidon and thiorphan, two inhibitors of neprilysin. Northern blot analysis and in situ hybridization showed that NL1 mRNA was found predominantly in testis, specifically in round and elongated spermatids. This distribution of NL1 mRNA suggests that it could be involved in sperm formation or other processes related to fertility.

Subtype-specific trafficking of endothelin receptors

We investigated the subcellular localization of two endothelin receptors (ET(A)R and ET(B)R). To visualize these receptors directly, the C terminus of each receptor was fused to the N terminus of enhanced green fluorescent protein (designated as ETR-EGFP). When transiently expressed in various mammalian cell lines, ET(A)R-EGFP was predominantly localized on the plasma membrane. By contrast, ET(B)R-EGFP was, independent of ligand stimulation, predominantly localized on the intracellular vesicular structures containing Lamp-1. Immunoblot analyses revealed that at steady state ET(B)R-EGFP was highly degraded, and its degradation was inhibited by bafilomycin A(1). Antibody uptake experiments suggested that the ET(B)R-EGFP molecules were internalized from the plasma membrane. It is therefore likely that ET(B)R is first transported to the plasma membrane and then internalized, irrespective of ligand stimulation, to lysosomes where it undergoes proteolytic degradation. Exchanging the C-terminal cytoplasmic tails of the two ETRs revealed that the cytoplasmic tail is responsible for both the intracellular localization and the degradation of the receptors. Deletion of the extreme C-terminal 35 amino acids from both receptors allowed the receptor proteins to localize predominantly in the intracellular vesicles and to degrade. These observations indicate that the cytoplasmic tail of ET(A)R determines its plasma membrane localization. Stimulation with endothelin-1 increased the amount of intact ETR-EGFP fusion proteins without increasing their de novo synthesis, suggesting that binding of endothelin-1 stabilizes the ETRs.

Palmitoylation occurs at cysteine 347 and cysteine 351 of the dopamine D(1) receptor

To determine the palmitoylation sites in the human dopamine D(1) receptor, we expressed wild type and mutant receptors in which candidate cysteines in the carboxyl tail were substituted by alanines both individually (A347, A351) and together (AA). Our results showed that palmitoylation levels of A347 and A351 were reduced substantially and that AA had no detectable signal of palmitoylation. These data indicate that cysteines 347 and 351 were both palmitoylated and that they were the only sites of palmitoylation. We introduced a cAMP-dependent protein kinase site encompassing the position 351. We predicted that a functional cAMP-dependent protein kinase site would impair receptor-G protein coupling if it is not occluded by palmitoylation. Our results demonstrated that indeed, the introduction of the cAMP-dependent protein kinase site caused reduced potency of dopamine stimulation of adenylyl cyclase, and thus confirmed that when unoccluded, the cAMP-dependent protein kinase site introduced to position 351 of dopamine D(1) receptor could confer constitutive desensitization.

Dual signaling of human Mel1a melatonin receptors via G(i2), G(i3), and G(q/11) proteins

Mel 1a melatonin receptors belong to the super-family of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. So far, interest in Mel 1a receptor signaling has focused mainly on the modulation of the adenylyl cyclase pathway via pertussis toxin (PTX)-sensitive G proteins. To further investigate signaling of the human Mel 1a receptor, we have developed an antibody directed against the C terminus of this receptor. This antibody detected the Mel 1a receptor as a protein with an apparent molecular mass of approximately 60 kDa in immunoblots after separation by SDS-PAGE. It also specifically precipitated the 2-[125I]iodomelatonin (125I-Mel)-labeled receptor from Mel 1a-transfected HEK 293 cells. Coprecipitation experiments showed that G(i2), G(i3), and G(q/11) proteins couple to the Mel 1a receptor in an agonist-dependent and guanine nucleotide-sensitive manner. Coupling was selective since other G proteins present in HEK 293 cells, (G(i1), G(o), G(s), G(z), and G12) were not detected in receptor complexes. Coupling of the Mel 1a receptor to G(i) and G(q) was confirmed by inhibition of high-affinity 125I-Mel binding to receptors with subtype-selective G protein alpha-subunit antibodies. G(i2) and/or G(i3) mediated adenylyl cyclase inhibition while G(q/11) induced a transient elevation in cytosolic calcium concentrations in HEK 293 cells stably expressing Mel 1a receptors. Melatonin-induced cytosolic calcium mobilization via PTX-insensitive G proteins was confirmed in primary cultures of ovine pars tuberalis cells endogenously expressing Mel 1a receptors. In conclusion, we report the development of the first antibody recognizing the cloned human Mel 1a melatonin receptor protein. We show that Mel 1a receptors functionally couple to both PTX-sensitive and PTX-insensitive G proteins. The previously unknown signaling of Mel 1a receptors through G(q/11) widens the spectrum of potential targets for melatonin.

Beta(2)-adrenergic receptor down-regulation. Evidence for a pathway that does not require endocytosis

Sustained activation of most G protein-coupled receptors causes a time-dependent reduction of receptor density in intact cells. This phenomenon, known as down-regulation, is believed to depend on a ligand-promoted change of receptor sorting from the default endosome-plasma membrane recycling pathway to the endosome-lysosome degradation pathway. This model is based on previous studies of epidermal growth factor (EGF) receptor degradation and implies that receptors need to be endocytosed to be down-regulated. In stable clones of L cells expressing beta(2)-adrenergic receptors (beta(2)ARs), sustained agonist treatment caused a time-dependant decrease in both beta(2)AR binding sites and immuno-detectable receptor. Blocking beta(2)AR endocytosis with chemical treatments or by expressing a dominant negative mutant of dynamin could not prevent this phenomenon. Specific blockers of the two main intracellular degradation pathways, lysosomal and proteasome-associated, were ineffective in preventing beta(2)AR down-regulation. Further evidence for an endocytosis-independent pathway of beta(2)AR down-regulation was provided by studies in A431 cells, a cell line expressing both endogenous beta(2)AR and EGF receptors. In these cells, inhibition of endocytosis and inactivation of the lysosomal degradation pathway did not block beta(2)AR down-regulation, whereas EGF degradation was inhibited. These data indicate that, contrary to what is currently postulated, receptor endocytosis is not a necessary prerequisite for beta(2)AR down-regulation and that the inactivation of beta(2)ARs, leading to a reduction in binding sites, may occur at the plasma membrane.

Agonist-induced, G protein-dependent and -independent down-regulation of the mu opioid receptor. The receptor is a direct substrate for protein-tyrosine kinase

The mu opioid receptor (MOR) has been shown to desensitize after 1 h of exposure to the opioid peptide, [D-Ala(2), N-MePhe(4), Gly-ol(5)]enkephalin (DAMGO), largely by the loss of receptors from the cell surface and receptor down-regulation. We have previously shown that the Thr(394) in the carboxyl tail is essential for agonist-induced early desensitization, presumably by serving as a primary phosphorylation site for G protein-coupled receptor kinase. Using a T394A mutant receptor, we determined that Thr(394) was also responsible for mu opioid receptor down-regulation. The T394A mutant receptor displayed 50% reduction of receptor down-regulation (14.8%) compared with wild type receptor (34%) upon 1 h of exposure to DAMGO. Agonist-induced T394A receptor down-regulation was unaffected by pertussis toxin treatment, indicating involvement of a mechanism independent of G protein function. Interestingly, pertussis toxin-insensitive T394A receptor down-regulation was completely inhibited by a tyrosine kinase inhibitor, genistein. Tyrosine kinase inhibition blocked wild type MOR down-regulation by 50%, and the genistein-resistant wild type MOR down-regulation was completely pertussis toxin-sensitive. Following DAMGO stimulation, MOR was shown to be phosphorylated at tyrosine residue(s), indicating that the receptor was a direct substrate for tyrosine kinase action. Mutagenesis of the four intracellular tyrosine residues resulted in complete inhibition of the G protein-insensitive MOR internalization. Therefore, agonist-induced MOR down-regulation appears to be mediated by two distinct cellular signal transduction pathways. One is G protein-dependent and GRK-dependent, which can be abolished by pertussis toxin treatment of wild type MOR or by mutagenesis of Thr(394). The other novel pathway is G protein-independent but tyrosine kinase-dependent, blocked by genistein treatment, and one in which Thr(394) has no regulatory role but phosphorylation of tyrosine residues appears essential.

Analysis of domain responsible for desensitization of beta1-adrenergic receptor

When the wild type beta1-adrenergic receptor (WT-beta1AR) was expressed in Sf9 cells, the beta1AR-stimulated adenylyl cyclase activities were desensitized by prior treatment with isoproterenol. The extent of beta1AR desensitization was not modified, and the onset was not promoted by the overexpression of G protein-coupled receptor kinase 2 (GRK2), GRK5 or GRK6. However, overexpression of the dominant negative mutant of GRK2 appeared to inhibit desensitization of the beta1AR. The change of the potential protein kinase A phosphorylation site located at the intracellular third loop did not affect beta1AR desensitization. Desensitization of the truncated mutant, in which nearly all of the serine and threonine residues from the carboxyl terminus were eliminated, was the same as that of the WT-beta1AR. A deletion mutant that lacked serine and threonine residues of the intracellular third loop was also desensitized by isoproterenol stimulation. Furthermore, the deletion of serine and threonine residues from both the intracellular third loop and carboxyl terminus did not affect desensitization of the beta1AR. These results suggested that phosphorylation by endogenous GRKs in Sf9 cells contributed to desensitization of the beta1AR and that the regions other than third intracellular loop and carboxyl terminus may be responsible for beta1AR desensitization.

Alternative splicing generates two isoforms of the beta3-adrenoceptor which are differentially expressed in mouse tissues

The beta3-adrenoceptor (AR) differs from the beta1-AR and beta2-ARs in having introns within and downstream of the coding block. This study demonstrates two splice variants of the mouse beta3-AR which differ within the coding region. Reverse transcription/polymerase chain reaction with intron-spanning primers was used to demonstrate the splice variant of the mouse beta3-adrenoceptor. The novel beta3b-AR has 17 amino acids encoded by exon 2 (SSLLREPRHLYTCLGYP) which differ from the 13 in the known beta3a-AR (RFDGYEGARPFPT). Beta3b-AR mRNA is differentially expressed in mouse tissues, with levels relative to beta3a-AR mRNA highest in hypothalamus, cortex and white adipose tissue, and lower in ileum smooth muscle and brown adipose tissue.

Cellular trafficking of G protein-coupled receptor/beta-arrestin endocytic complexes

beta-Arrestins are multifunctional proteins identified on the basis of their ability to bind and uncouple G protein-coupled receptors (GPCR) from heterotrimeric G proteins. In addition, beta-arrestins play a central role in mediating GPCR endocytosis, a key regulatory step in receptor resensitization. In this study, we visualize the intracellular trafficking of beta-arrestin2 in response to activation of several distinct GPCRs including the beta2-adrenergic receptor (beta2AR), angiotensin II type 1A receptor (AT1AR), dopamine D1A receptor (D1AR), endothelin type A receptor (ETAR), and neurotensin receptor (NTR). Our results reveal that in response to beta2AR activation, beta-arrestin2 translocation to the plasma membrane shares the same pharmacological profile as described for receptor activation and sequestration, consistent with a role for beta-arrestin as the agonist-driven switch initiating receptor endocytosis. Whereas redistributed beta-arrestins are confined to the periphery of cells and do not traffic along with activated beta2AR, D1AR, and ETAR in endocytic vesicles, activation of AT1AR and NTR triggers a clear time-dependent redistribution of beta-arrestins to intracellular vesicular compartments where they colocalize with internalized receptors. Activation of a chimeric AT1AR with the beta2AR carboxyl-terminal tail results in a beta-arrestin membrane localization pattern similar to that observed in response to beta2AR activation. In contrast, the corresponding chimeric beta2AR with the AT1AR carboxyl-terminal tail gains the ability to translocate beta-arrestin to intracellular vesicles. These results demonstrate that the cellular trafficking of beta-arrestin proteins is differentially regulated by the activation of distinct GPCRs. Furthermore, they suggest that the carboxyl-tail of the receptors might be involved in determining the stability of receptor/betaarrestin complexes and cellular distribution of beta-arrestins.

Structural features of heterotrimeric G-protein-coupled receptors and their modulatory proteins

Over the past 20 years, the general mechanism for signaling through 7-transmembrane helix receptors coupled to GTP hydrolysis has been worked out. Although similar in overall organization, subtype variability and subcellular localization of components have built in considerable signaling specificity. Atomic resolution structures for many of the components have delineated the domain organization of these complex proteins and have given physical form to the idea of subtype specificity. This review describes what is known about the physical structures of the 7-transmembrane helix receptors, the heterotrimeric GTP binding coupling proteins, the adenylate cyclase and phospholipase C effector proteins, and signaling modulatory proteins, such as arrestin, phosducin, recoverin-type myristoyl switch proteins, and the pleckstrin homology domain of G-protein receptor kinase-2. These images allow experimenters to contemplate the details of the supramolecular organization of the multiprotein complexes involved in the transmission of signals across the cellular lipid bilayer.

Internalization of the TXA2 receptor alpha and beta isoforms. Role of the differentially spliced cooh terminus in agonist-promoted receptor internalization

Thromboxane A2 (TXA2) potently stimulates platelet aggregation and smooth muscle constriction and is thought to play a role in myocardial infarction, atherosclerosis, and bronchial asthma. The TXA2 receptor (TXA2R) is a member of the G protein-coupled receptor family and is found as two alternatively spliced isoforms, alpha (343 residues) and beta (407 residues), which share the first 328 residues. In the present report, we demonstrate by enzyme-linked immunosorbent assay and immunofluorescence microscopy that the TXA2Rbeta, but not the TXA2Ralpha, undergoes agonist-induced internalization when expressed in HEK293 cells as well as several other cell types. Various dominant negative mutants were used to demonstrate that the internalization of the TXA2Rbeta is dynamin-, GRK-, and arrestin-dependent in HEK293 cells, suggesting the involvement of receptor phosphorylation and clathrin-coated pits in this process. Interestingly, the agonist-stimulated internalization of both the alpha and beta isoforms, but not of a mutant truncated after residue 328, can be promoted by overexpression of arrestin-3, identifying the C-tails of both receptors as necessary in arrestin-3 interaction. Simultaneous mutation of two dileucine motifs in the C-tail of TXA2Rbeta did not affect agonist-promoted internalization. Analysis of various C-tail deletion mutants revealed that a region between residues 355 and 366 of the TXA2Rbeta is essential for agonist-promoted internalization. These data demonstrate that alternative splicing of the TXA2R plays a critical role in regulating arrestin binding and subsequent receptor internalization.