Arrestin-independent internalization of the m1, m3, and m4 subtypes of muscarinic cholinergic receptors

The biased M3 mAChR ligand PD 102807 mediates qualitatively distinct signaling to regulate airway smooth muscle phenotype

Airway smooth muscle (ASM) cells attain a hypercontractile phenotype during obstructive airway diseases. We recently identified a biased M3 muscarinic acetylcholine receptor (mAChR) ligand, PD 102807, that induces GRK-/arrestin-dependent AMP-activated protein kinase (AMPK) activation to inhibit transforming growth factor-β-induced hypercontractile ASM phenotype. Conversely, the balanced mAChR agonist, methacholine (MCh), activates AMPK yet does not regulate ASM phenotype. In the current study, we demonstrate that PD 102807- and MCh-induced AMPK activation both depend on Ca2+/calmodulin-dependent kinase kinases (CaMKKs). However, MCh-induced AMPK activation is calcium-dependent and mediated by CaMKK1 and CaMKK2 isoforms. In contrast, PD 102807-induced signaling is calcium-independent and mediated by the atypical subtype protein kinase C-iota and the CaMKK1 (but not CaMKK2) isoform. Both MCh- and PD 102807-induced AMPK activation involve the AMPK α1 isoform. PD 102807-induced AMPK α1 (but not AMPK α2) isoform activation mediates inhibition of the mammalian target of rapamycin complex 1 (mTORC1) in ASM cells, as demonstrated by increased Raptor (regulatory-associated protein of mTOR) phosphorylation as well as inhibition of phospho-S6 protein and serum response element-luciferase activity. The mTORC1 inhibitor rapamycin and the AMPK activator metformin both mimic the ability of PD 102807 to attenuate transforming growth factor-β-induced α-smooth muscle actin expression (a marker of hypercontractile ASM). These data indicate that PD 102807 transduces a signaling pathway (AMPK-mediated mTORC1 inhibition) qualitatively distinct from canonical M3 mAChR signaling to prevent pathogenic remodeling of ASM, thus demonstrating PD 102807 is a biased M3 mAChR ligand with therapeutic potential for the management of obstructive airway disease.

Arrestin-Dependent and -Independent Internalization of G Protein-Coupled Receptors: Methods, Mechanisms, and Implications on Cell Signaling

Agonist-induced endocytosis is a key regulatory mechanism for controlling the responsiveness of the cell by changing the density of cell surface receptors. In addition to the role of endocytosis in signal termination, endocytosed G protein-coupled receptors (GPCRs) have been found to signal from intracellular compartments of the cell. Arrestins are generally believed to be the master regulators of GPCR endocytosis by binding to both phosphorylated receptors and adaptor protein 2 (AP-2) or clathrin, thus recruiting receptors to clathrin-coated pits to facilitate the internalization process. However, many other functions have been described for arrestins that do not relate to their role in terminating signaling. Additionally, there are now more than 30 examples of GPCRs that internalize independently of arrestins. Here we review the methods, pharmacological tools, and cellular backgrounds used to determine the role of arrestins in receptor internalization, highlighting their advantages and caveats. We also summarize key examples of arrestin-independent GPCR endocytosis in the literature and their suggested alternative endocytosis pathway (e.g., the caveolae-dependent and fast endophilin-mediated endocytosis pathways). Finally, we consider the possible function of arrestins recruited to GPCRs that are endocytosed independently of arrestins, including the catalytic arrestin activation paradigm. Technological improvements in recent years have advanced the field further, and, combined with the important implications of endocytosis on drug responses, this makes endocytosis an obvious parameter to include in molecular pharmacological characterization of ligand-GPCR interactions. SIGNIFICANCE STATEMENT: G protein-coupled receptor (GPCR) endocytosis is an important means to terminate receptor signaling, and arrestins play a central role in the widely accepted classical paradigm of GPCR endocytosis. In contrast to the canonical arrestin-mediated internalization, an increasing number of GPCRs are found to be endocytosed via alternate pathways, and the process appears more diverse than the previously defined "one pathway fits all."

Knockin' on heaven's door: Molecular mechanisms of neuronal tau uptake

Since aggregates of the microtubule‐binding protein tau were found to be the main component of neurofibrillary tangles more than 30 years ago, their contribution to neurodegeneration in Alzheimer's disease (AD) and tauopathies has become well established. Recent work shows that both tau load and its distribution in the brain of AD patients correlate with cognitive decline more closely compared to amyloid plaque deposition. In addition, the amyloid cascade hypothesis has been recently challenged because of disappointing results of clinical trials designed to treat AD by reducing beta‐amyloid levels, thus fuelling a renewed interest in tau. There is now robust evidence to indicate that tau pathology can spread within the central nervous system via a prion‐like mechanism following a stereotypical pattern, which can be explained by the trans‐synaptic inter‐neuronal transfer of pathological tau. In the receiving neuron, tau has been shown to take multiple routes of internalisation, which are partially dependent on its conformation and aggregation status. Here, we review the emerging mechanisms proposed for the uptake of extracellular tau in neurons and the requirements for the propagation of its pathological conformers, addressing how they gain access to physiological tau monomers in the cytosol. Furthermore, we highlight some of the key mechanistic gaps of the field, which urgently need to be addressed to expand our understanding of tau propagation and lead to the identification of new therapeutic strategies for tauopathies.

Cholino-ncRNAs modulate sex-specific- and age-related acetylcholine signals

Acetylcholine (ACh) signaling orchestrates mammalian movement, mental capacities, and inflammation. Dysregulated ACh signaling associates with many human mental disorders and neurodegeneration in an individual‐, sex‐, and tissue‐related manner. Moreover, aged patients under anticholinergic therapy show increased risk of dementia, but the underlying molecular mechanisms are incompletely understood. Here, we report that certain cholinergic‐targeting noncoding RNAs, named Cholino‐noncoding RNAs (ncRNAs), can modulate ACh signaling, agonistically or antagonistically, via distinct direct and indirect mechanisms and at different timescales. Cholino‐ncRNAs include both small microRNAs (miRNAs) and long noncoding RNAs (lncRNAs). The former may attenuate translation and/or induce destruction of target mRNAs that code for either ACh‐signaling proteins or transcription factors controlling the expression of cholinergic genes. lncRNAs may block miRNAs via ‘sponging’ events or by competitive binding to the cholinergic target mRNAs. Also, single nucleotide polymorphisms in either Cholino‐ncRNAs or in their recognition sites in the ACh‐signaling associated genes may modify ACh signaling‐regulated processes. Taken together, both inherited and acquired changes in the function of Cholino‐ncRNAs impact ACh‐related deficiencies, opening new venues for individual, sex‐related, and age‐specific oriented research, diagnosis, and therapeutics.

Characterization of an A-Type Muscarinic Acetylcholine Receptor and Its Possible Non-neuronal Role in the Oriental Armyworm, Mythimna separata Walker (Lepidoptera: Noctuidae)

Muscarinic acetylcholine receptor (mAChR) regulates many neurophysiological functions in insects. In this report, a full-length cDNA encoding an A-type mAChR was cloned from the oriental armyworm, Mythimna separata. Pharmacological properties studies revealed that nanomolar to micromolar concentrations of carbachol or muscarine induced an increase of intracellular Ca2+ concentration ([Ca2+] i ), with the EC50 values of 124.6 and 388.1 nM, respectively. The increases of [Ca2+] i can be greatly blocked by the antagonist atropine, with an IC50 value of 0.09 nM. The receptor mRNA is expressed in all developmental stages, with great differential expression between male and female adults. The tissue expression analysis identified novel target tissues for this receptor, including ovaries and Malpighian tubules. The distribution of Ms A-type mAChR protein in the male brain may suggest the neurophysiological roles that are mediated by this receptor. However, the receptor protein was found to be distributed on the membranes of oocytes that are not innervated by neurons at all. These results indicate that Ms A-type mAChR selectively mediates intracellular Ca2+ mobilization. And the high level of receptor protein in the membrane of oocytes may indicate a possible non-neuronal role of A-type mAChR in the reproductive system of M. separata.

CAPA periviscerokinin-mediated activation of MAPK/ERK signaling through Gq-PLC-PKC-dependent cascade and reciprocal ERK activation-dependent internalized kinetics of Bom-CAPA-PVK receptor 2

Bombyx mori neuropeptide G protein-coupled receptor (BNGR)-A27 is a specific receptor for B. mori capability (CAPA) periviscerokinin (PVK), that is, Bom-CAPA-PVK receptor 2. Upon stimulation of Bom-CAPA-PVK-1 or -PVK-2, Bom-CAPA-PVK receptor 2 significantly increases cAMP-response element-controlled luciferase activity and Ca²⁺ mobilization in a Gq inhibitor-sensitive manner. However, the underlying mechanism(s) for CAPA/CAPA receptor system mediation of extracellular signal-regulated kinases1/2 (ERK1/2) activation remains to be explained further. Here, we discovered that Bom-CAPA-PVK receptor 2 stimulated ERK1/2 phosphorylation in a dose- and time-dependent manner in response to Bom-CAPA-PVK-1 or -PVK-2 with similar potencies. Furthermore, ERK1/2 phosphorylation can be inhibited by Gq inhibitor UBO-QIC, PLC inhibitor U73122, protein kinase C (PKC) inhibitor Go 6983, phospholipase D (PLD) inhibitor FIPI and Ca²⁺ chelators EGTA and BAPTA-AM. Moreover, Bom-CAPA-PVK-R2-induced activation of ERK1/2 was significantly attenuated by treatment with the Gβγ-specific inhibitors, phosphatidylinositol 3-kinase (PI3K)-specific inhibitor Wortmannin and Src-specific inhibitor PP2. Our data also demonstrate that receptor tyrosine kinase (RTK) transactivation pathways are involved in the mechanisms of Bom-CAPA-PVK receptor to ERK1/2 phosphorylation. In addition, β-arrestin1/2 is not involved in Bom-CAPA-PVK-R2-mediated ERK1/2 activation but required for the agonist-independent, ERK1/2 activation-dependent internalization of the G protein-coupled receptor (GPCR).

Regulation and trafficking of muscarinic acetylcholine receptors

Fidelity of signal transduction relies on cells expressing the appropriate number of functional receptors. Fluctuation in the total number of muscarinic acetylcholine receptors has been implicated in a range of physiological and pathophysiological processes, and the mechanisms responsible for this regulation represent potential molecular targets for therapeutic intervention. This article will review the current literature on the endocytic trafficking of muscarinic receptors and how knowledge of the trafficking of related receptors might influence future studies. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

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.

Unraveling a molecular determinant for clathrin-independent internalization of the M2 muscarinic acetylcholine receptor

Endocytosis and postendocytic sorting of G-protein-coupled receptors (GPCRs) is important for the regulation of both their cell surface density and signaling profile. Unlike the mechanisms of clathrin-dependent endocytosis (CDE), the mechanisms underlying the control of GPCR signaling by clathrin-independent endocytosis (CIE) remain largely unknown. Among the muscarinic acetylcholine receptors (mAChRs), the M4 mAChR undergoes CDE and recycling, whereas the M2 mAChR is internalized through CIE and targeted to lysosomes. Here we investigated the endocytosis and postendocytic trafficking of M2 mAChR based on a comparative analysis of the third cytoplasmic domain in M2 and M4 mAChRs. For the first time, we identified that the sequence (374)KKKPPPS(380) servers as a sorting signal for the clathrin-independent internalization of M2 mAChR. Switching (374)KKKPPPS(380) to the i3 loop of the M4 mAChR shifted the receptor into lysosomes through the CIE pathway; and therefore away from CDE and recycling. We also found another previously unidentified sequence that guides CDE of the M2 mAChR, (361)VARKIVKMTKQPA(373), which is normally masked in the presence of the downstream sequence (374)KKKPPPS(380). Taken together, our data indicate that endocytosis and postendocytic sorting of GPCRs that undergo CIE could be sequence-dependent.

Revisiting the endocytosis of the m2 muscarinic acetylcholine receptor

The agonist-induced endocytosis of the muscarinic acetylcholine receptor M2 is different from that of the other members of the muscarinic receptor family. The uptake of the M2 receptor involves the adapter proteins of the β-arrestin family and the small GTPase ADP-ribosylation factor 6. However, it has remained inconclusive if M2 endocytosis is dependent on clathrin or the large GTPase dynamin. We here show by means of knocking down the clathrin heavy chain that M2 uptake upon agonist stimulation requires clathrin. The expression of various dominant-negative dynamin-2 mutants and the use of chemical inhibitors of dynamin function revealed that dynamin expression and membrane localization as such appear to be necessary for M2 endocytosis, whereas dynamin GTPase activity is not required for this process. Based on the data from the present and from previous studies, we propose that M2 endocytosis takes place by means of an atypical clathrin-mediated pathway that may involve a specific subset of clathrin-coated pits/vesicles.

Role of the third intracellular loop in the subtype-specific internalization and recycling of muscarinic M2 and M4 receptors

Muscarinic M2, M4, and M2-M4 chimera receptors were transiently expressed in HEK-293 tsA201 cells, and agonist-dependent internalization of these receptors and recycling of internalized receptors were examined by measuring the amount of cell-surface receptors as [3H]N-methylscopolamine (NMS) binding activity. Coexpression of a dominant negative form of dynamin (DN-dynamin,dynamin K44A) greatly reduced the agonist-dependent internalization of M4 receptors but not of M2 receptors, as was reported by Vögler et al. (J Biol Chem 273, 12155-12160, 1998).The agonist-dependent internalization of M2/M4-i3/M2 chimera receptors (M2 receptors with the i3 loop replaced by that of M4 receptors) was greatly reduced by co-expression of DN-dynamin as was the case for M4 receptors, whereas the agonist-dependent internalization of M4/M2-i3/M4 chimera receptors was hardly affected by co-expression of DN-dynamin as was the case for M2 receptors.Internalized M2/M4-i3/M2 receptors as well as internalized M4 receptors were shown to be recycled back to the cell surface after removal of agonists, whereas no recycling was observed for M4/M2-i3/M4 receptors as well as M2 receptors. These results indicate that the i3 loops of M2 and M4 receptors take a major role in their agonist-dependent internalization and recycling.

Allosteric modulation of M1 muscarinic acetylcholine receptor internalization and subcellular trafficking

Allosteric modulators are an attractive approach to achieve receptor subtype-selective targeting of G protein-coupled receptors. Benzyl quinolone carboxylic acid (BQCA) is an unprecedented example of a highly selective positive allosteric modulator of the M1 muscarinic acetylcholine receptor (mAChR). However, despite favorable pharmacological characteristics of BQCA in vitro and in vivo, there is limited evidence of the impact of allosteric modulation on receptor regulatory mechanisms such as β-arrestin recruitment or receptor internalization and endocytic trafficking. In the present study we investigated the impact of BQCA on M1 mAChR regulation. We show that BQCA potentiates agonist-induced β-arrestin recruitment to M1 mAChRs. Using a bioluminescence resonance energy transfer approach to monitor intracellular trafficking of M1 mAChRs, we show that once internalized, M1 mAChRs traffic to early endosomes, recycling endosomes and late endosomes. We also show that BQCA potentiates agonist-induced subcellular trafficking. M1 mAChR internalization is both β-arrestin and G protein-dependent, with the third intracellular loop playing an important role in the dynamics of β-arrestin recruitment. As the global effect of receptor activation ultimately depends on the levels of receptor expression at the cell surface, these results illustrate the need to extend the characterization of novel allosteric modulators of G protein-coupled receptors to encapsulate the consequences of chronic exposure to this family of ligands.

Intracellular localization of M1 muscarinic acetylcholine receptor through clathrin-dependent constitutive internalization via a C-terminal tryptophan-based motif

The M1 muscarinic acetylcholine receptor (M1-mAChR) is a G-protein-coupled membrane receptor that is activated by extracellular cholinergic stimuli. Recent investigations have revealed the intracellular localization of M1-mAChRs. In this study, we observed constitutive internalization of M1-mAChRs in mouse neuroblastoma N1E-115 cells without agonist stimulation. Constitutive internalization depends on dynamin, clathrin, and the adaptor protein-2 (AP-2) complex. A WXXI motif in the M1-mAChR C-terminus is essential for its constitutive internalization, since alanine mutations of W442 or I445 abolished constitutive internalization. This WXXI motif resembles YXXΦ, which is the canonical binding motif for the μ2 subunit of the AP-2 complex. The M1-mAChR C-terminal WXXI motif interacted with AP-2 μ2. W442A and I445A mutants of the M1-mAChR C-terminal sequence lost AP-2 μ2 binding activity, while the W442Y mutant bound more effectively than wild-type. Consistent with these results, W442A and I445A M1-mAChR mutants selectively localized to the cell surface. Conversely, the W442Y receptor mutant was found only at intracellular sites. Our data indicate that the cellular distribution of M1-mAChRs is governed by the C-terminal tryptophan-based motif that mediates constitutive internalization.

M1 muscarinic receptor activation mediates cell death in M1-HEK293 cells

HEK293 cells have been used extensively to generate stable cell lines to study G protein-coupled receptors, such as muscarinic acetylcholine receptors (mAChRs). The activation of M1 mAChRs in various cell types in vitro has been shown to be protective. To further investigate M1 mAChR-mediated cell survival, we generated stable HEK293 cell-lines expressing the human M1 mAChR. M1 mAChRs were efficiently expressed at the cell surface and efficiently internalised within 1 h by carbachol. Carbachol also induced early signalling cascades similar to previous reports. Thus, ectopically expressed M1 receptors behaved in a similar fashion to the native receptor over short time periods of analysis. However, substantial cell death was observed in HEK293-M1 cells within 24 h after carbachol application. Death was only observed in HEK cells expressing M1 receptors and fully blocked by M1 antagonists. M1 mAChR-stimulation mediated prolonged activation of the MEK-ERK pathway and resulted in prolonged induction of the transcription factor EGR-1 (>24 h). Blockade of ERK signalling with U0126 did not reduce M1 mAChR-mediated cell-death significantly but inhibited the acute induction of EGR-1. We investigated the time-course of cell death using time-lapse microscopy and xCELLigence technology. Both revealed the M1 mAChR cytotoxicity occurs within several hours of M1 activation. The xCELLigence assay also confirmed that the ERK pathway was not involved in cell-death. Interestingly, the MEK blocker did reduce carbachol-mediated cleaved caspase 3 expression in HEK293-M1 cells. The HEK293 cell line is a widely used pharmacological tool for studying G-protein coupled receptors, including mAChRs. Our results highlight the importance of investigating the longer term fate of these cells in short term signalling studies. Identifying how and why activation of the M1 mAChR signals apoptosis in these cells may lead to a better understanding of how mAChRs regulate cell-fate decisions.

Sphingomyelinase selectively reduces M1 muscarinic receptors in rat hippocampal membranes

Although there is evidence that nicotinic acetylcholine (Ach) receptors are influenced by ceramides, we do not currently know whether or not these sphingolipids can also regulate the muscarinic subtypes of Ach receptors. Using the whole-cell patch technique, we demonstrated that the effectiveness of the muscarinic receptor agonist pilocarpine, in enhancing spontaneous inhibitory postsynaptic currents in CA1 pyramidal cells, was completely abolished in hippocampal slices pre-exposed to the ceramide-generating enzyme sphingomyelinase (SMase). Western blot experiments, performed with biotinylated hippocampal membranes, showed that this electrophysiological defect possibly relies on the loss of M1 muscarinic Ach receptors at the cell surface. However, the effect appears to be relatively specific as the cell-surface expression of M4 muscarinic receptors was not found to be impacted by SMase treatment. Interestingly, we observed that G protein-coupled receptor kinases 2 and β-arrestin1/2 interactions with M1-immunoprecipitated proteins were substantially augmented in SMase-treated slices and that the reduction of cell-surface M1 muscarinic receptor expression generated was completely suppressed by the muscarinic antagonist atropine. Collectively, our data suggest that selective internalization of M1 muscarinic receptors can be accentuated in neurons subjected to high ceramide levels. The potential physiopathological implications of this finding are presented.

Muscarinic receptor trafficking

Knowledge of the mechanisms responsible for the trafficking of neurotransmitter receptors away from the cell surface is of obvious importance in understanding what regulates their expression and function. This chapter will focus on the mechanisms responsible for the internalization and degradation of muscarinic receptors. There are both receptor subtype-specific and cell-type specific differences in muscarinic receptor trafficking. Studies on muscarinic receptor trafficking both in cells in culture and in vivo will be described, and the potential physiological consequences of this trafficking will be discussed.

Comparison of the kinetics and extent of muscarinic M1-M5 receptor internalization, recycling and downregulation in Chinese hamster ovary cells

We characterized agonist-induced internalization, recycling and downregulation of each muscarinic receptor subtype (M(1)-M(5)) stably expressed in Chinese hamster ovary (CHO) cells. The radioligands [(3)H]QNB and [(3)H]NMS were used to measure the total and plasma membrane populations of muscarinic receptors, respectively. Following carbachol treatment (1 mM), the rank orders for the rate of carbachol-induced internalization of the muscarinic subtypes were M(2)>M(4)=M(5)>M(3)=M(1), respectively. Unlike the M(2) receptor, M(1), M(3), M(4) and M(5) receptors recycled back to the plasma membrane after 1 h carbachol treatment. The receptor downregulation elicited to 24h carbachol treatment was similar for M(2), M(3), M(4) and M(5) receptors, whereas that for the M(1) receptor was greater. Our results indicate that there are subtype-specific differences in the rate and extent of agonist-induced muscarinic receptor internalization, recycling and downregulation in CHO cells.

Importance of regions outside the cytoplasmic tail of G-protein-coupled receptors for phosphorylation and dephosphorylation

Two GPCRs (G-protein-coupled receptors), TRHR (thyrotropin-releasing hormone receptor) and beta(2)AR (beta(2)-adrenergic receptor), are regulated in distinct manners. Following agonist binding, TRHR undergoes rapid phosphorylation attributable to GRKs (GPCR kinases); beta(2)AR is phosphorylated by both second messenger-activated PKA (protein kinase A) and GRKs with slower kinetics. TRHR co-internalizes with arrestin, whereas beta(2)AR recruits arrestin, but internalizes without it. Both receptors are dephosphorylated following agonist removal, but TRHR is dephosphorylated much more rapidly while it remains at the plasma membrane. We generated chimaeras swapping the C-terminal domains of these receptors to clarify the role of different receptor regions in phosphorylation, internalization and dephosphorylation. beta(2)AR with a TRHR cytoplasmic tail (beta(2)AR-TRHR) and TRHR with a beta(2)AR tail (TRHR-beta(2)AR) signalled to G-proteins normally. beta(2)AR-TRHR was phosphorylated well at the PKA site in the third intracellular loop, but poorly at GRK sites in the tail, whereas TRHR-beta(2)AR was phosphorylated strongly at GRK sites in the tail (Ser(355)/Ser(356) of the beta(2)AR). Both chimaeric receptors exhibited prolonged, but weak, association with arrestin at the plasma membrane, but high-affinity arrestin interactions and extensive co-internalization of receptor with arrestin required a phosphorylated TRHR tail. In contrast, swapping C-terminal domains did not change the rates of phosphorylation and dephosphorylation or the dependence of TRHR dephosphorylation on the length of agonist exposure. Thus the interactions of GPCRs with GRKs and phosphatases are determined not simply by the amino acid sequences of the substrates, but by regions outside the cytoplasmic tails.

Phosphorylation of protease-activated receptor-2 differentially regulates desensitization and internalization

Protease-activated receptor 2 (PAR2) is a G protein-coupled receptor irreversibly activated by extracellular proteases. Activated PAR2 couples to multiple heterotrimeric G-protein subtypes including G alpha(q), G alpha(i), and G alpha(12/13). Most activated G protein-coupled receptors are rapidly desensitized and internalized following phosphorylation and beta-arrestin binding. However, the role of phosphorylation in regulation of PAR2 signaling and trafficking is not known. To investigate the function of phosphorylation, we generated a PAR2 mutant in which all serines and threonines in the C-tail were converted to alanines and designated it PAR2 0P. In mammalian cells, the addition of agonist induced a rapid and robust increase in phosphorylation of wild-type PAR2 but not the 0P mutant, suggesting that the major sites of phosphorylation occur within the C-tail domain. Moreover, desensitization of PAR2 0P signaling was markedly impaired compared with the wild-type receptor. Wild-type phosphorylated PAR2 internalized through a canonical dynamin, clathrin- and beta-arrestin-dependent pathway. Strikingly, PAR2 0P mutant internalization proceeded through a dynamin-dependent but clathrin- and beta-arrestin-independent pathway in both a constitutive and agonist-dependent manner. Collectively, our studies show that PAR2 phosphorylation is essential for beta-arrestin binding and uncoupling from heterotrimeric G-protein signaling and that the presence of serine and threonine residues in the PAR2 C-tail hinder constitutive internalization through a non-canonical pathway. Thus, our studies reveal a novel function for phosphorylation that differentially regulates PAR2 desensitization and endocytic trafficking.

M3 muscarinic acetylcholine receptor-mediated signaling is regulated by distinct mechanisms

We have used RNA interference previously to demonstrate that G protein-coupled receptor kinase 2 (GRK2) regulates endogenously expressed H1 histamine receptor in human embryonic kidney 293 cells. In this report, we investigate the regulation of endogenously expressed M(3) muscarinic acetylcholine receptor (M(3) mAChR). We show that knockdown of GRK2, GRK3, or GRK6, but not GRK5, significantly increased carbachol-mediated calcium mobilization. Stable expression of wild-type GRK2 or a kinase-dead mutant (GRK2-K220R) reduced calcium mobilization after receptor activation, whereas GRK2 mutants defective in Galpha(q) binding (GRK2-D110A, GRK2-R106A, and GRK2-R106A/K220R) had no effect on calcium signaling, suggesting that GRK2 primarily regulates G(q) after M(3) mAChR activation. The knockdown of arrestin-2 or arrestin-3 also significantly increased carbachol-mediated calcium mobilization. Knockdown of GRK2 and the arrestins also significantly enhanced carbachol-mediated activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), whereas prolonged ERK1/2 activation was only observed with GRK2 or arrestin-3 knockdown. We also investigated the role of casein kinase-1alpha (CK1alpha) and found that knockdown of CK1alpha increased calcium mobilization but not ERK activation. In summary, our data suggest that multiple proteins dynamically regulate M(3) mAChR-mediated calcium signaling, whereas GRK2 and arrestin-3 play the primary role in regulating ERK activation.

Muscarinic M4 receptor recycling requires a motif in the third intracellular loop

The present study was performed to identify sequence(s) in the third intracellular loop (i3) of the muscarinic acetylcholine receptor M4 subtype (M4 receptor) involved in its internalization and recycling. In transiently transfected human embryonic kidney 293-tsA201 cells, 40 to 50% of cell-surface M4 receptors are internalized in an agonist-dependent manner, and approximately 65% of internalized receptors are recycled back to the cell surface after removal of the agonist. We examined the internalization and recycling of M4 receptor mutants with partial deletion in i3 and found that various mutants (M4del-K(235)-K(240), M4del-T(241)-K(271), and M4del-W(339)-N(372)) showed internalization and cell-surface recycling in a similar manner to the M4 receptor. We also found that the mutant M4del-L(272)-R(338) was internalized to only half the extent of the M4 receptor and was recycled after agonist removal, and the mutant M4del-V(373)-A(393) was also internalized to half the extent of the wild type but was not recycled back to the cell surface after agonist removal. When the sequence corresponding to Val(373)-Ala(393) was grafted onto the i3 portion of a recycling-negative mutant of muscarinic M2 receptor with deletion of almost the whole of the i3 sequence, approximately 40% of the chimeric receptor on the cell surface was internalized, and more than 65% of the internalized receptors were recycled back to the cell surface. These results indicate that the regions including Leu(272)-Arg(338) and Val(373)-Ala(393) are involved in internalization of the M4 receptor, and the region including Val(373)-Ala(393) is indispensable for its recycling, whereas the other regions of i3 are dispensable for internalization and recycling.

Beta-arrestins specifically constrain beta2-adrenergic receptor signaling and function in airway smooth muscle

Chronic use of inhaled beta-agonists by asthmatics is associated with a loss of bronchoprotective effect and deterioration of asthma control. Beta-agonist-promoted desensitization of airway smooth muscle beta-2-adrenergic receptors, mediated by G protein-coupled receptor kinases and arrestins, is presumed to underlie these effects, but such a mechanism has never been demonstrated. Using in vitro, ex vivo, and in vivo murine models, we demonstrate that beta-arrestin-2 gene ablation augments beta-agonist-mediated airway smooth muscle relaxation, while augmenting beta-agonist-stimulated cyclic adenosine monophosphate production. In cultures of human airway smooth muscle, small interfering RNA-mediated knockdown of arrestins also augments beta-agonist-stimulated cyclic adenosine monophosphate production. Interestingly, signaling and function mediated by m2/m3 muscarinic acetylcholine receptors or prostaglandin E(2) receptors were not affected by either beta-arrestin-2 knockout or arrestin knockdown. Thus, arrestins are selective regulators of beta-2-adrenergic receptor signaling and function in airway smooth muscle. These results and our previous findings, which demonstrate a role for arrestins in the development of allergic inflammation in the lung, identify arrestins as potentially important therapeutic targets for obstructive airway diseases.

Function of non-visual arrestins in signaling and endocytosis of the gastrin-releasing peptide receptor (GRP receptor)

Little is known about the role of arrestins in gastrointestinal hormone/neurotransmitter receptor endocytosis. With other G protein-coupled receptors, arrestins induce G protein-uncoupling and receptor endocytosis. In this study, we used arrestin wild-type and dominant-negative mutant constructs to analyze the arrestin dependence of endocytosis and desensitization of the gastrin-releasing peptide receptor (GRP-R). Co-expression of the GRP-R with wild-type arrestin2 and arrestin3 increased not only GRP-R endocytosis but also GRP-R desensitization in arrestin-overexpressing cells. Co-expression of the dominant-negative mutants V53D-arrestin2 or V54D-arrestin3 reduced GRP-R endocytosis. Notably, different trafficking routes for agonist-activated GRP-R-arrestin2 and GRP-R-arrestin3 complexes were found. Arrestin3 internalizes with GRP-R to intracellular vesicles, arrestin2 splits from the GRP-R and localizes to the cell membrane. Also, the recycling pathway of the GRP-R was different if co-expressed with arrestin2 or arrestin3. Using different GRP-R mutants, the C-terminus and the 2nd intracellular loop of the GRP-R were found to be important for the GRP-R-arrestin interaction and for the difference in GRP receptor trafficking with the two arrestin subtypes. Our results show that both non-visual arrestins play an important role in GRP-R internalization and desensitization.

Inverse agonist-induced signaling and down-regulation of the platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator involved in several diseases such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G-protein-coupled receptor family. Following stimulation, PAFR becomes rapidly desensitized; this refractory state is dependent on PAFR phosphorylation, internalization and down-regulation. In this report, we show that the PAFR inverse agonist, WEB2086, can induce phosphorylation and down-regulation of PAFR. Using selective inhibitors, we determined that the agonist, PAF, and WEB2086 could induce phosphorylation of PAFR by PKC. Moreover, dominant-negative (DN) mutant of PKC isoforms beta inhibited WEB2086-stimulated PAFR phosphorylation, whereas PAF-stimulated phosphorylation was inhibited by DN PKCalpha and delta. WEB2086 also induced PAFR down-regulation which could be blocked by PKC inhibitors and by DN PKCbeta. WEB2086-induced down-regulation was dynamin-dependent but arrestin-independent. Unlike PAF, WEB2086-stimulated intracellular trafficking of PAFR was independent of Rab5. Specific inhibitors of lysosomal proteases and of proteasomes were both effective in reducing WEB2086-induced PAFR down-regulation, indicating the importance of receptor targeting to both lysosomes and proteasomes in long-term cell desensitization to WEB2086. These results indicate that although both agonists and inverse agonists induce receptor PAFR down-regulation, this may be accomplished through different signal transduction and trafficking pathways.

Clathrin-dependent mechanisms of G protein-coupled receptor endocytosis

The heptahelical G protein-coupled receptor (GPCR) family includes approximately 900 members and is the largest family of signaling receptors encoded in the mammalian genome. G protein-coupled receptors elicit cellular responses to diverse extracellular stimuli at the plasma membrane and some internalized receptors continue to signal from intracellular compartments. In addition to rapid desensitization, receptor trafficking is critical for regulation of the temporal and spatial aspects of GPCR signaling. Indeed, GPCR internalization functions to control signal termination and propagation as well as receptor resensitization. Our knowledge of the mechanisms that regulate mammalian GPCR endocytosis is based predominantly on arrestin regulation of receptors through a clathrin- and dynamin-dependent pathway. However, multiple clathrin adaptors, which recognize distinct endocytic signals, are now known to function in clathrin-mediated endocytosis of diverse cargo. Given the vast number and diversity of GPCRs, the complexity of clathrin-mediated endocytosis and the discovery of multiple clathrin adaptors, a single universal mechanism controlling endocytosis of all mammalian GPCRs is unlikely. Indeed, several recent studies now suggest that endocytosis of different GPCRs is regulated by distinct mechanisms and clathrin adaptors. In this review, we discuss the diverse mechanisms that regulate clathrin-dependent GPCR endocytosis.

Caveolin 2 regulates endocytosis and trafficking of the M1 muscarinic receptor in MDCK epithelial cells

Clathrin and caveolins are known for their involvement in the internalization of numerous receptors. Here we show that in polarized epithelial Madin-Darby canine kidney cells, both the clathrin machinery and caveolins are involved in the endocytosis and delivery to the plasma membrane (PM) of the M1 muscarinic acetylcholine receptor (mAChR). We initially localized this receptor to the lateral membrane, where it accumulates proximal to the tight junctions. From there it is internalized through the clathrin-mediated pathway. In addition, the receptor may associate on the PM with caveolin (cav) 2 or in intracellular compartments with either cav 2, or monomeric or oligomeric cav 1. Association of the PM M1 mAChR with cav 2 inhibits receptor endocytosis through the clathrin-mediated pathway or retains the receptor in an intracellular compartment. This intracellular association attenuates receptor trafficking. Expression of cav 1 with cav 2 rescues the latter's inhibitory effect. The caveolins stimulate M1 mAChR oligomerization thus maintaining a constant amount of monomeric receptor. These results provide evidence that caveolins play a role in the attenuation of the M1 muscarinic receptor's intracellular trafficking to and from the PM.

Distinct clathrin-coated pits sort different G protein-coupled receptor cargo

Upon activation, many G protein-coupled receptors (GPCRs) internalize by clathrin-mediated endocytosis and are subsequently sorted to undergo recycling or lysosomal degradation. Here we observe that sorting can take place much earlier than previously thought, by entry of different GPCRs into distinct populations of clathrin-coated pit (CCP). These distinct populations were revealed by analysis of two purinergic GPCRs, P2Y(1) and P2Y(12), which enter two populations of CCPs in a mutually exclusive manner. The mechanisms underlying early GPCR sorting involve differential kinase-dependent processes because internalization of P2Y(12) is mediated by GPCR kinases (GRKs) and arrestin, whereas P2Y(1) internalization is GRK- and arrestin-independent but requires protein kinase C. Importantly, the beta(2) adrenoceptor which also internalizes in a GRK-dependent manner also traffics exclusively to P2Y(12)-containing CCPs. Our data therefore reveal distinct populations of CCPs that sort GPCR cargo at the plasma membrane using different kinase-dependent mechanisms.

Agonist-induced Internalization of the Caenorhabditis elegans Muscarinic Acetylcholine Receptor GAR-3 in Chinese Hamster Ovary Cells

Many membrane-bound neurotransmitter receptors are known to be internalized by exposure to agonist. This agonist-induced receptor internalization is considered to play important roles in receptor-mediated signaling. Here we investigated the internalization of GAR-3, a Caenorhabditis elegans muscarinic acetylcholine receptor, using cultured mammalian cells. When Chinese hamster ovary cells stably expressing GAR-3 were treated with carbachol, GAR-3 was internalized in a dose- and time-dependent manner. Approximately 60% of the cell surface receptor was internalized by exposure to 1 mM carbachol for 1 h. Carbachol-induced GAR-3 internalization was suppressed by treatment with hypertonic sucrose, which blocks the formation of clathrin-coated pits. Overexpression of a dominant-negative dynamin mutant (DynK44A), but not of a dominant-negative beta-arrestin mutant (Arr319-418), substantially inhibited carbachol-induced internalization of GAR-3. Thus, these data suggest that GAR-3 undergoes agonist-induced internalization via a clathrin- and dynamin-dependent but beta-arrestin-independent pathway. Depletion of Ca2+ by simultaneous treatment of the cells with BAPTA/AM (Ca2+ mobilization blocker) and EGTA (Ca2+ influx blocker) almost completely blocked agonist-induced GAR-3 internalization. Moreover, treatment of the cells with the Ca2+ ionophore A23187 led to GAR-3 internalization in the absence of agonist. These results indicate that Ca2+ plays a critical role in GAR-3 internalization. We tested whether the third intracellular (i3) loop of GAR-3 is involved in agonist-stimulated receptor internalization. A GAR-3 deletion mutant lacking a large central portion of the i3 loop exhibited an internalization pattern comparable to that of the wild type, suggesting that the central i3 loop is not required for the internalization of GAR-3.

The Kv2.1 C terminus can autonomously transfer Kv2.1-like phosphorylation-dependent localization, voltage-dependent gating, and muscarinic modulation to diverse Kv channels

Modulation of K+ channels is widely used to dynamically regulate neuronal membrane excitability. The voltage-gated K+ channel Kv2.1 is an abundant delayed rectifier K+ (IK) channel expressed at high levels in many types of mammalian central neurons where it regulates diverse aspects of membrane excitability. Neuronal Kv2.1 is constitutively phosphorylated, localized in high-density somatodendritic clusters, and has a relatively depolarized voltage dependence of activation. Here, we show that the clustering and voltage-dependent gating of endogenous Kv2.1 in cultured rat hippocampal neurons are modulated by cholinergic stimulation, a common form of neuromodulation. The properties of neuronal Kv2.1 are recapitulated in recombinant Kv2.1 expressed in human embryonic kidney 293 (HEK293) cells, but not COS-1 cells, because of cell background-specific differences in Kv2.1 phosphorylation. As in neurons, Kv2.1 in HEK293 cells is dynamically regulated by cholinergic stimulation, which leads to Ca2+/calcineurin-dependent dephosphorylation of Kv2.1, dispersion of channel clusters, and hyperpolarizing shifts in the voltage-dependent gating properties of the channel. Immunocytochemical, biochemical, and biophysical analyses of chimeric Kv channels show that the Kv2.1 cytoplasmic C-terminal domain can act as an autonomous domain sufficient to transfer Kv2.1-like clustering, voltage-dependent activation, and cholinergic modulation to diverse Kv channels. These findings provide novel mechanistic insights into cholinergic modulation of ion channels and regulation of the localization and voltage-dependent gating properties of the abundant neuronal Kv2.1 channel by cholinergic and other neuromodulatory stimuli.

Arrestin-Independent Internalization and Recycling of the Urotensin Receptor Contribute to Long-Lasting Urotensin II–Mediated Vasoconstriction

Urotensin II (UII), which acts on the G protein-coupled urotensin (UT) receptor, elicits long-lasting vasoconstriction. The role of UT receptor internalization and intracellular trafficking in vasoconstriction has yet not been analyzed. Therefore, UII-mediated contractile responses of aortic ring preparations in wire myography and rat UT (rUT) receptor internalization and intracellular trafficking in binding and imaging analyses were compared. UII elicited a concentration-dependent vasoconstriction of rat aorta (-log EC50, mol/L:9.0+/-0.1). A second application of UII after 30 minutes elicited a reduced contraction (36+/-4% of the initial response), but when applied after 60 minutes elicited a full contraction. In internalization experiments with radioactive labeled VII ((125)I-UII), approximately 70% of rUT receptors expressed on the cell surface of human embryonic kidney 293 cells were sequestered within 30 minutes (half life [t(h)]: 5.6+/-0.2 minutes), but recycled quantitatively within 60 minutes (t(h) 31.9+/-2.6 minutes). UII-bound rUT receptors were sorted to early and recycling endosomes, as evidenced by colocalization of rUT receptors with the early endosomal antigen and the transferrin receptor. Real-time imaging with a newly developed fluorescent UII (Cy3-UII) revealed that rUT receptors recruited arrestin3 green fluorescent protein to the plasma membrane. Arrestin3 was not required for the endocytosis of the rUT receptor, however, as internalization of Cy3-UII was not altered in mouse embryonic fibroblasts lacking endogenous arrestin2/arrestin3 expression. The data demonstrate that the rUT receptor internalizes arrestin independently and recycles quantitatively. The continuous externalization of rUT receptors provides the basis for repetitive and lasting UII-mediated vasoconstriction.

Effects of synaptotagmin reveal two distinct mechanisms of agonist-stimulated internalization of the M4 muscarinic acetylcholine receptor

1. Synaptotagmin has been reported to function in clathrin-mediated endocytosis. Here, we investigated its involvement in agonist-stimulated internalization of M4 muscarinic acetylcholine receptors exogenously expressed in human embryonic kidney (HEK-293 tsA201) cells. 2. Synaptotagmin I was present at low levels in these cells, and when overexpressed resided at the plasma membrane. 3. Synaptotagmin overexpression alone did not affect receptor internalization, but 'rescued' internalization that had been inhibited by either dominant-negative dynamin-1 or dominant-negative arrestin-2. Both normal and 'rescued' internalization were sensitive to inhibitors of clathrin-mediated endocytosis, but not to inhibitors of the function of caveolae. 4. There was no increase in AP-2 recruitment to the plasma membrane in cells overexpressing synaptotagmin. However, a mutant form of the receptor lacking a potential AP-2 recruitment motif, while being internalized normally in response to agonist stimulation, was not rescued by synaptotagmin in cells expressing dominant-negative dynamin or arrestin. 5. A mutant form of synaptotagmin (K326,327A), which binds phosphatidylinositol-4,5-bisphosphate (PIP2) much more weakly than the wild-type protein, did not rescue internalization. Furthermore, internalization was inhibited by the PH domain of phospholipase C-delta1, which sequesters PIP2, and synaptotagmin was now unable to rescue. 6. We propose that AP-2 binding to the C-terminal tail of the receptor is not normally required for its endocytosis, but that the synaptotagmin-mediated rescue involves the formation of a ternary complex with the receptor and AP-2. PIP2 might play a role as an intermediary in the formation of this complex.

Roles of phosphorylation-dependent and -independent mechanisms in the regulation of M1 muscarinic acetylcholine receptors by G protein-coupled receptor kinase 2 in hippocampal neurons

When co-expressed with the inositol 1,4,5-trisphosphate biosensor eGFP-PH(PLC delta), G protein-coupled receptor kinase 2 (GRK2) can suppress M1 muscarinic acetylcholine (mACh) receptor-mediated phospholipase C signaling in hippocampal neurons through a phosphorylation-independent mechanism, most likely involving the direct binding of the RGS homology domain of GRK2 to G alpha(q/11). To define the importance of this mechanism in comparison with classical, phosphorylation-dependent receptor regulation by GRKs, we have examined M1 mACh receptor signaling in hippocampal neurons following depletion of GRK2 and also in the presence of non-G alpha(q/11)-binding GRK2 mutants. Depletion of neuronal GRK2 using an antisense strategy almost completely inhibited M1 mACh receptor desensitization without enhancing acute agonist-stimulated phospholipase C activity. By stimulating neurons with a submaximal agonist concentration before (R1) and after (R2) a period of exposure to a maximal agonist concentration, an index (R2/R1) of agonist-induced desensitization of signaling could be obtained. Co-transfection of neurons with either a non-G alpha(q/11)-binding (D110A) GRK2 mutant or the catalytically inactive (D110A,K220R)GRK2 did not suppress acute M1 mACh receptor-stimulated inositol 1,4,5-trisphosphate production. However, using the desensitization (R2/R1) protocol, it could be shown that expression of (D110A)GRK2 enhanced, whereas (D110A,K220R)GRK2 inhibited, agonist-induced M1 mACh receptor desensitization. In Chinese hamster ovary cells, the loss of G alpha(q/11) binding did not affect the ability of the (D110A)GRK2 mutant to phosphorylate M1 mACh receptors, whereas expression of (D110A,K220R)GRK2 had no effect on receptor phosphorylation. These data indicate that in hippocampal neurons endogenous GRK2 is a key regulator of M1 mACh receptor signaling and that the regulatory process involves both phosphorylation-dependent and -independent mechanisms.

HCMV-encoded chemokine receptor US28 employs multiple routes for internalization

The human cytomegalovirus-encoded G protein-coupled receptor homologue US28 binds inflammatory chemokines and sequesters them from the environment of infected cells. Low surface deposition and endocytosis are dependent on constitutive C-terminal phosphorylation, suggesting a requirement for beta-arrestin binding in receptor internalization. In this report, a US28-dependent redistribution of beta-arrestin into vesicular structures occurred, although internalization of US28 was independent of beta-arrestin. Internalization of US28 was dynamin-dependent, and US28 partially partitioned into the detergent-resistant membrane fraction. Endocytosis was diminished by cholesterol depletion, yet sucrose inhibition was even stronger. The relevance of the clathrin-coated pit pathway was supported by colocalization of beta(2)-adaptin and US28 in endocytic compartments. Exchange of the C-terminal dileucine endocytosis motif inhibited rapid endocytosis, indicating a direct interaction of US28 with the AP-2 adaptor complex. We suggest that the arrestin-independent, dynamin-dependent internalization of US28 reveals a differential sorting of beta-arrestins and the virally encoded chemokine receptor homologue.

Clathrin-mediated endocytosis of m3 muscarinic receptors. Roles for Gbetagamma and tubulin

Receptors as well as some G protein subunits internalize after agonist stimulation. It is not clear whether Galpha(q) or Gbetagamma undergo such regulated translocation. Recent studies demonstrate that m3 muscarinic receptor activation in SK-N-SH neuroblastoma cells causes recruitment of tubulin to the plasma membrane. This subsequently transactivates Galpha(q) and activates phospholipase Cbeta1. Interaction of tubulin-GDP with Gbetagamma at the offset of phospholipase Cbeta1 signaling appears involved in translocation of tubulin and Gbetagamma to vesicle-like structures in the cytosol (Popova, J. S., and Rasenick, M. M. (2003) J. Biol. Chem. 278, 34299-34308). The relationship of this internalization to the clathrin-mediated endocytosis of the activated m3 muscarinic receptors or Galpha(q) involvement in this process has not been clarified. To test this, SK-N-SH cells were treated with carbachol, and localization of Galpha(q), Gbetagamma, tubulin, clathrin, and m3 receptors were analyzed by both cellular imaging and biochemical techniques. Upon agonist stimulation both tubulin and clathrin translocated to the plasma membrane and co-localized with receptors, Galpha(q) and Gbetagamma. Fifteen minutes later receptors, Gbetagamma and tubulin, but not Galpha(q), internalized with the clathrin-coated vesicles. Coimmunoprecipitation of m3 receptors with Gbetagamma, tubulin, and clathrin from the cytosol of carbachol-treated cells was readily observed. These data suggested that Gbetagamma subunits might organize the formation of a multiprotein complex linking m3 receptors to tubulin since they interacted with both proteins. Such protein assemblies might explain the dynamin-dependent but beta-arrestin-independent endocytosis of m3 muscarinic receptors since tubulin interaction with dynamin might guide or insert the complex into clathrin-coated pits. This novel mechanism of internalization might prove important for other beta-arrestin-independent endocytic pathways. It also suggests cross-regulation between G protein-mediated signaling and the dynamics of the microtubule cytoskeleton.

Mechanisms regulating membrane trafficking of G protein-coupled receptors in the endocytic pathway

Endocytic membrane trafficking plays multiple roles in GPCR signaling and regulation. In the past several years much has been learned about molecular mechanisms that mediate and regulate endocytic trafficking of cloned GPCRs expressed in transfected cell lines, and there is accelerating progress toward elucidating the membrane trafficking of GPCRs in native tissues. Current views regarding ligand-induced endocytosis of adrenergic catecholamine and opioid neuropeptide receptors will be reviewed, focusing on recent data suggesting the existence of additional machinery controlling the endocytosis of specific GPCRs via clathrin-coated pits. Evidence that GPCRs are selectively 'sorted' between divergent downstream pathways after endocytosis will be discussed, focusing on recent insight to mechanisms controlling receptor sorting between distinct recycling and non-recycling membrane pathways.

ADP-ribosylation factor-dependent phospholipase D activation by the M3 muscarinic receptor

G protein-coupled receptors can potentially activate phospholipase D (PLD) by a number of routes. We show here that the native M3 muscarinic receptor in 1321N1 cells and an epitope-tagged M3 receptor expressed in COS7 cells substantially utilize an ADP-ribosylation factor (ARF)-dependent route of PLD activation. This pathway is activated at the plasma membrane but appears to be largely independent of G, phospholipase C, Ca2+ q/11, protein kinase C, tyrosine kinases, and phosphatidyl inositol 3-kinase. We report instead that it involves physical association of ARF with the M3 receptor as demonstrated by co-immunoprecipitation and by in vitro interaction with a glutathione S-transferase fusion protein of the receptor's third intracellular loop domain. Experiments with mutant constructs of ARF1/6 and PLD1/2 indicate that the M3 receptor displays a major ARF1-dependent route of PLD1 activation with an additional ARF6-dependent pathway to PLD1 or PLD2. Examples of other G protein-coupled receptors assessed in comparison display alternative pathways of protein kinase C- or ARF6-dependent activation of PLD2.

Multiple determinants for rapid agonist-induced internalization of a nonmammalian gonadotropin-releasing hormone receptor: a putative palmitoylation site and threonine doublet within the carboxyl-terminal tail Are critical

The chicken GnRH receptor (cGnRH-R) differs from all mammalian GnRH-Rs in possessing a cytoplasmic carboxyl-terminal tail. We have previously demonstrated that the cGnRH-R undergoes more rapid agonist-induced internalization than the mammalian GnRH-Rs and requires the carboxyl-terminal tail for this process. To investigate the structural determinants mediating this rapid internalization, a series of mutant receptors was generated, including progressive truncations of the tail and substitution of serine and threonine residues with alanine. Truncation of the carboxyl-terminal tail to position 366 and then to position 356 resulted in a progressive attenuation of the rate and total extent of receptor internalization. However, truncation between positions 356 and 346 did not alter the kinetics of internalization further, whereas a further truncation to position 337 resulted in an additional marked reduction of internalization. We show that the membrane-proximal Cys(328) and the Thr(369)Thr(370) doublet located in the distal carboxyl terminus play a critical role in mediating rapid internalization. We demonstrate that the cGnRH-R, when expressed in both COS-7 and HEK 293 cells, preferentially undergoes rapid agonist-induced internalization in a caveolae-like, dynamin-dependent manner. These conclusions are based on our observation that pretreatments with filipin and methyl-beta-cyclodextrin, agents that disrupt lipid rafts such as caveolae, and coexpression of dominant-negative dynamin-1 (K44A) and caveolin-1 (Delta 1-81) mutants, effectively inhibited rapid agonist-induced internalization. Furthermore, cGnRH-Rs appeared to be mobilized to the beta-arrestin- and clathrin-coated, vesicle-mediated endocytic pathway upon beta-arrestin overexpression.

The interaction of a constitutively active arrestin with the arrestin-insensitive 5-HT(2A) receptor induces agonist-independent internalization

5-HT(2A) serotonin receptors are unusual among G-protein coupled receptors in that they can be internalized and desensitized, in some cell types, in an arrestin-independent manner. The molecular basis of the arrestin-insensitivity of 5-HT(2A) receptors is unknown but is probably caused, in part, by the apparent lack of agonist-induced 5-HT(2A) receptor phosphorylation. Because the arrestin-insensitivity of 5-HT(2A) receptors is cell-type selective, we used a "constitutively active" arrestin mutant that can interact with agonist-activated but nonphosphorylated receptors. We show here that this "constitutively active" arrestin mutant (Arr2-R169E) can force 5-HT(2A) receptors to be regulated by arrestins. Cotransfection of 5-HT(2A) receptors with Arr2-R169E induced agonist-independent 5-HT(2A) receptor internalization, and a constitutive translocation of the Arr2-R169E mutant to the plasma membrane, whereas wild-type Arrestin-2 had no effect. Additionally, Arr2-R169E, unlike wild-type arrestin-2, induced a significant decrease in efficacy of agonist-induced phosphoinositide hydrolysis with an unexpected increase in agonist potency. Radioligand binding assays demonstrated that the fraction of receptors in the high-affinity agonist binding-state increased with expression of Arr2-R169E, indicating that Arr2-R169E stabilizes the agonist-high affinity state of the 5-HT(2A) receptor (R*). Intriguingly, the agonist-independent interaction of Arr2-R169E with 5-HT(2A) receptors was inhibited by inverse agonist treatment and is thus probably caused by the high level of 5-HT(2A) receptor constitutive activity. This is the first demonstration that a constitutively active arrestin mutant can both induce agonist-independent internalization and stabilize the agonist-high affinity state of an arrestin-insensitive G protein coupled receptor.

Arrestin: roles in the life and death of retinal neurons

G protein-coupled receptors are a large family of signaling molecules that respond to a wide variety of extracellular stimuli. The receptors relay the information encoded by the ligand through the activation of heterotrimeric G proteins and intracellular effector molecules. To ensure the appropriate regulation of the signaling cascade, it is vital to properly inactivate the receptor. This inactivation is achieved, in part, by the binding of a soluble protein, arrestin, which uncouples the receptor from the downstream G protein. In addition to the inactivation of G protein-coupled receptors, arrestins have also been implicated in the endocytosis of receptors and cross talk with other signaling pathways. Due to its central role in cellular signaling, misregulation or misfunction of arrestin can have dramatic affects on cell viability and have direct implications in human disease.

Beta-arrestin is involved in the desensitization but not in the internalization of the somatostatin receptor 2A expressed in CHO cells

The interaction of beta-arrestin-1 with the somatostatin receptor type 2A (sst2A) was monitored using both biochemical and confocal imaging approaches. We show that, using transient transfection of either beta-arrestin-1 or its dominant negative Delta-arrestin-1 in CHO cells stably transfected with the sst2A, beta-arrestin-1 is colocalized with the receptor in endosomal vesicles after somatostatin-induced sequestration. However, this interaction leads to a role of beta-arrestin-1 in the desensitization of the sst2A rather than in the internalization process of the receptor-ligand complex.

Mechanisms of ligand-induced desensitization of the 5-hydroxytryptamine(2A) receptor

We have examined the cellular processes underlying the desensitization of the 5-hydroxytryptamine (5-HT)(2A) receptor induced by agonist or antagonist exposure. Treatment of C6 glioma cells with either 5-HT or the 5-HT(2A) receptor antagonist ketanserin resulted in an attenuation in 5-HT(2A) receptor function, specifically the accumulation of inositol phosphates stimulated by the partial agonist quipazine. 5-HT-induced desensitization of the 5-HT(2A) receptor involved receptor internalization through a clathrin- and dynamin-dependent process because it was prevented by concanavalin A, monodansylcadaverine, and by expression of the dominant negative mutants beta-arrestin (319-418) and dynamin K44A. Although short-term (i.e., 10 min) 5-HT and ketanserin exposure resulted in the same degree of desensitization, ketanserin-induced desensitization was not prevented by these agents and did not involve receptor internalization. In contrast, prolonged ketanserin exposure (i.e., 2 h) resulted in 5-HT(2A) receptor internalization through a clathrin- and dynamin-dependent process, as was observed after agonist treatment. Inhibitors of protein kinase C or calcium-calmodulin kinase II did not attenuate or prevent 5-HT-induced desensitization of the receptor. 5-HT(2A) receptor desensitization induced by 5-HT and prolonged ketanserin treatment, but not by short-term ketanserin treatment, was prevented by the expression of the dominant negative mutant of G protein-coupled receptor kinase (GRK)2, GRK2-K220R, and by an anti-GRK2/3 antibody. Our data indicate a dual mechanism of early and late desensitization by the antagonist ketanserin. Short-term ketanserin treatment reduced the specific binding of the agonist radioligand [(125)I](+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ([(125)I]DOI) and the ability of 5'-guanylylimidodiphosphate to attenuate this binding, suggesting that at the early stage of antagonist-induced desensitization the capacity of the 5-HT(2A) receptor to couple to G protein is impaired.

Differential internalization of mammalian and non-mammalian gonadotropin-releasing hormone receptors. Uncoupling of dynamin-dependent internalization from mitogen-activated protein kinase signaling

Desensitization and internalization of G-protein-coupled receptors can reflect receptor phosphorylation-dependent binding of beta-arrestin, which prevents G-protein activation and targets receptors for internalization via clathrin-coated vesicles. These can be pinched off by a dynamin collar, and proteins controlling receptor internalization can also mediate mitogen-activated protein kinase signaling. Gonadotropin-releasing hormone (GnRH) stimulates internalization of its receptors via clathrin-coated vesicles. Mammalian GnRH receptors (GnRH-Rs) are unique in that they lack C-terminal tails and do not rapidly desensitize, whereas non-mammalian GnRH-R have C-terminal tails and, where investigated, do rapidly desensitize and internalize. Using recombinant adenovirus expressing human and Xenopus GnRH-Rs we have explored the relationship between receptor internalization and mitogen-activated protein kinase signaling in HeLa cells with regulated tetracycline-controlled expression of wild-type or a dominant negative mutant (K44A) of dynamin. These receptors were phospholipase C-coupled and had appropriate ligand affinity and specificity. K44A dynamin expression did not alter human GnRH-R internalization but dramatically reduced internalization of Xenopus GnRH-R (and epidermal growth factor (EGF) receptor). Blockade of clathrin-mediated internalization (sucrose) abolished internalization of all three receptors. Both GnRH-Rs also mediated phosphorylation of ERK 2 and for both receptors, this was inhibited by K44A dynamin. The same was true for EGF- and protein kinase C-mediated ERK 2 phosphorylation. ERK 2 phosphorylation was also inhibited by a protein kinase C inhibitor but not affected by an EGF receptor tyrosine kinase inhibitor. We conclude that a) desensitizing and non-desensitizing GnRH-Rs are targeted for clathrin-coated vesicle-mediated internalization by functionally distinct mechanisms, b) GnRH-R signaling to ERK 2 is dynamin-dependent and c) this does not reflect a dependence on dynamin-dependent GnRH-R internalization.

Apparent capacity of cardiac muscarinic receptors for different radiolabeled antagonists

Muscarinic receptors in sarcolemmal membranes, digitonin-solubilized extracts, and purified preparations from porcine atria have revealed a shortfall in the apparent capacity for N-[3H]methylscopolamine, which was only about 75% of that for [3H]quinuclidinylbenzilate. Since binding at near-saturating concentrations of [3H]quinuclidinylbenzilate was inhibited fully at comparatively low concentrations of unlabeled N-methylscopolamine, the data are inconsistent with the notion that [3H]quinuclidinylbenzilate binds selectively to a subclass of distinct, non-interconverting, and mutually independent sites. The discrepancy is resolved by adjusting the specific activity of N-[3H]methylscopolamine to account for unlabeled scopolamine that was identified in some batches of the radioligand. Also, there was no shortfall in capacity when N-[3H]methylscopolamine was devoid of scopolamine, and the predicted effect was obtained when pure N-[3H]methylscopolamine was supplemented with known amounts of scopolamine. A small discrepancy in the levels of scopolamine estimated pharmacologically and by mass spectrometry can be attributed largely to a difference in the efficiency of ionization between scopolamine and N-methylscopolamine. Different capacities for different radioligands are not uncommon with muscarinic and other G protein-coupled receptors, and in some cases the effect may have been due wholly or in part to an unlabeled impurity. Binding data can be mechanistically ambiguous, particularly when acquired only at graded concentrations of the radioligand. The predicted effects of an unlabeled impurity mimic or resemble those of alternative scenarios such as sequestration behind a hydrophobic barrier, a nucleotide-regulated interconversion from one state of affinity to another, and cooperativity between interacting sites.

Clathrin box in G protein-coupled receptor kinase 2

beta(1)-Adrenergic receptor (beta(1)AR) shows the resistance to agonist-induced internalization. However, beta(1)AR can internalize as G protein-coupled receptor kinase 2 (GRK2) is fused to its carboxyl terminus. Internalization of the beta(1)AR and GRK2 fusion protein (beta(1)AR/GRK2) is dependent on dynamin but independent of beta-arrestin and phosphorylation. The beta(1)AR/GRK2 fusion protein internalizes via clathrin-coated pits and is found to co-localize with the endosome that contains transferrin. The fusion proteins consisting of beta(1)AR and various portions of GRK2 reveal that the residues 498-502 in the carboxyl-terminal domain of GRK2 are critical to promote internalization of the fusion proteins. This domain contains a consensus sequence of a clathrin-binding motif defined as a clathrin box. In vitro binding assays show that the residues 498-502 of GRK2 bind the amino-terminal domain of clathrin heavy chain to almost the same extent as beta-arrestin1. The mutation of the clathrin box in the carboxyl-terminal domain of GRK2 results in the loss of the ability to promote internalization of the fusion protein. GRK2 activity increases and then decreases as the concentration of clathrin heavy chain increases. Taken together, these results imply that GRK2 contains a functional clathrin box and directly interacts with clathrin to modulate its function.

Dynamin and Rab5a-dependent trafficking and signaling of the neurokinin 1 receptor

Understanding the molecular mechanisms of agonist-induced trafficking of G-protein-coupled receptors is important because of the essential role of trafficking in signal transduction. We examined the role of the GTPases dynamin 1 and Rab5a in substance P (SP)-induced trafficking and signaling of the neurokinin 1 receptor (NK1R), an important mediator of pain, depression, and inflammation, by studying transfected cells and enteric neurons that naturally express the NK1R. In unstimulated cells, the NK1R colocalized with dynamin at the plasma membrane, and Rab5a was detected in endosomes. SP induced translocation of the receptor into endosomes containing Rab5a immediately beneath the plasma membrane and then in a perinuclear location. Expression of the dominant negative mutants dynamin 1 K44E and Rab5aS34N inhibited endocytosis of SP by 45 and 32%, respectively. Dynamin K44E caused membrane retention of the NK1R, whereas Rab5aS34N also impeded the translocation of the receptor from superficially located to perinuclear endosomes. Both dynamin K44E and Rab5aS34N strongly inhibited resensitization of SP-induced Ca(2+) mobilization by 60 and 85%, respectively, but had no effect on NK1R desensitization. Dynamin K44E but not Rab5aS34N markedly reduced SP-induced phosphorylation of extracellular signal regulated kinases 1 and 2. Thus, dynamin mediates the formation of endosomes containing the NK1R, and Rab5a mediates both endosomal formation and their translocation from a superficial to a perinuclear location. Dynamin and Rab5a-dependent trafficking is essential for NK1R resensitization but is not necessary for desensitization of signaling. Dynamin-dependent but not Rab5a-dependent trafficking is required for coupling of the NK1R to the mitogen-activated protein kinase cascade. These processes may regulate the nociceptive, depressive, and proinflammatory effects of SP.