Binding of the beta2 adrenergic receptor to N-ethylmaleimide-sensitive factor regulates receptor recycling

Function and dynamics of the intrinsically disordered carboxyl terminus of β2 adrenergic receptor

Advances in structural biology have provided important mechanistic insights into signaling by the transmembrane core of G-protein coupled receptors (GPCRs); however, much less is known about intrinsically disordered regions such as the carboxyl terminus (CT), which is highly flexible and not visible in GPCR structures. The β2 adrenergic receptor's (β2AR) 71 amino acid CT is a substrate for GPCR kinases and binds β-arrestins to regulate signaling. Here we show that the β2AR CT directly inhibits basal and agonist-stimulated signaling in cell lines lacking β-arrestins. Combining single-molecule fluorescence resonance energy transfer (FRET), NMR spectroscopy, and molecular dynamics simulations, we reveal that the negatively charged β2AR-CT serves as an autoinhibitory factor via interacting with the positively charged cytoplasmic surface of the receptor to limit access to G-proteins. The stability of this interaction is influenced by agonists and allosteric modulators, emphasizing that the CT plays important role in allosterically regulating GPCR activation.

Intracellular VHHs to monitor and modulate GPCR signaling

Single-domain antibody fragments, also known as VHHs or nanobodies, have opened promising avenues in therapeutics and in exploration of intracellular processes. Because of their unique structural properties, they can reach cryptic regions in their cognate antigen. Intracellular VHHs/antibodies primarily directed against cytosolic proteins or transcription factors have been described. In contrast, few of them target membrane proteins and even less recognize G protein-coupled receptors. These receptors are major therapeutic targets, which reflects their involvement in a plethora of physiological responses. Hence, they elicit a tremendous interest in the scientific community and in the industry. Comprehension of their pharmacology has been obscured by their conformational complexity, that has precluded deciphering their structural properties until the early 2010's. To that respect, intracellular VHHs have been instrumental in stabilizing G protein-coupled receptors in active conformations in order to solve their structure, possibly bound to their primary transducers, G proteins or β-arrestins. In contrast, the modulatory properties of VHHs recognizing the intracellular regions of G protein-coupled receptors on the induced signaling network have been poorly studied. In this review, we will present the advances that the intracellular VHHs have permitted in the field of GPCR signaling and trafficking. We will also discuss the methodological hurdles that linger the discovery of modulatory intracellular VHHs directed against GPCRs, as well as the opportunities they open in drug discovery.

Autistic-Like Behavior and Impairment of Serotonin Transporter and AMPA Receptor Trafficking in N-Ethylmaleimide Sensitive Factor Gene-Deficient Mice

Autism spectrum disorder (ASD), characterized by profound impairment in social interactions and communication skills, is the most common neurodevelopmental disorder. Many studies on the mechanisms underlying the development of ASD have focused on the serotonergic system; however, these studies have failed to completely elucidate the mechanisms. We previously identified N-ethylmaleimide-sensitive factor (NSF) as a new serotonin transporter (SERT)-binding protein and described its importance in SERT membrane trafficking and uptake in vitro. In the present study, we generated Nsf ^+/- mice and investigated their behavioral, neurotransmitter, and neurophysiological phenotypes in vivo. Nsf ^+/- mice exhibited abnormalities in sociability, communication, repetitiveness, and anxiety. Additionally, Nsf loss led to a decrease in membrane SERT expression in the raphe and accumulation of glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors at the synaptic membrane surface in the hippocampal CA1 region. We found that postsynaptic density and long-term depression were impaired in the hippocampal CA1 region of Nsf ^+/- mice. Taken together, these findings demonstrate that NSF plays a role in synaptic plasticity and glutamatergic and serotonergic systems, suggesting a possible mechanism by which the gene is linked to the pathophysiology of autistic behaviors.

Adrenomedullin - Current perspective on a peptide hormone with significant therapeutic potential

The peptide hormone adrenomedullin (ADM) consists of 52 amino acids and plays a pivotal role in the regulation of many physiological processes, particularly those of the cardiovascular and lymphatic system. Like calcitonin (CT), calcitonin gene-related peptide (CGRP), intermedin (IMD) and amylin (AMY), it belongs to the CT/CGRP family of peptide hormones, which despite their low little sequence identity share certain characteristic structural features as well as a complex multicomponent receptor system. ADM, IMD and CGRP exert their biological effects by activation of the calcitonin receptor-like receptor (CLR) as a complex with one of three receptor activity-modifying proteins (RAMP), which alter the ligand affinity. Selectivity within the receptor system is largely mediated by the amidated C-terminus of the peptide hormones, which bind to the extracellular domains of the receptors. This enables their N-terminus consisting of a disulfide-bonded ring structure and a helical segment to bind within the transmembrane region and to induce an active receptor confirmation. ADM is expressed in a variety of tissues in the human body and is fundamentally involved in multitude biological processes. Thus, it is of interest as a diagnostic marker and a promising candidate for therapeutic interventions. In order to fully exploit the potential of ADM, it is necessary to improve its pharmacological profile by increasing the metabolic stability and, ideally, creating receptor subtype-selective analogs. While several successful attempts to prolong the half-life of ADM were recently reported, improving or even retaining receptor selectivity remains challenging.

Disruption of auto-inhibition underlies conformational signaling of ASIC1a to induce neuronal necroptosis

We reported previously that acid-sensing ion channel 1a (ASIC1a) mediates acidic neuronal necroptosis via recruiting receptor-interacting protein kinase 1 (RIPK1) to its C terminus (CT), independent of its ion-conducting function. Here we show that the N-terminus (NT) of ASIC1a interacts with its CT to form an auto-inhibition that prevents RIPK1 recruitment/activation under resting conditions. The interaction involves glutamate residues at distal NT and is disrupted by acidosis. Expression of mutant ASIC1a bearing truncation or glutamate-to-alanine substitutions at distal NT causes constitutive cell death. The NT-CT interaction is further disrupted by N-ethylmaleimide-sensitive fusion ATPase (NSF), which associates with ASIC1a-NT under acidosis, facilitating RIPK1 interaction with ASIC1a-CT. Importantly, a membrane-penetrating synthetic peptide representing the distal 20 ASIC1a NT residues, NT_1–20, reduced neuronal damage in both in vitro model of acidotoxicity and in vivo mouse model of ischemic stroke, demonstrating the therapeutic potential of targeting the auto-inhibition of ASIC1a for neuroprotection against acidotoxicity.

Acid-sensing ion channel 1a (ASIC1a) mediates acidic neuronal necroptosis via recruiting receptor-interacting protein kinase 1 (RIPK1). Here authors show that auto-inhibition of ASICa prevents RIPK1 recruitment and demonstrate that targeting the auto-inhibition has therapeutic potential to prevent acidotoxicity.

Identification of Novel Adenylyl Cyclase 5 (AC5) Signaling Networks in D1 and D2 Medium Spiny Neurons using Bimolecular Fluorescence Complementation Screening

Adenylyl cyclase type 5 (AC5), as the principal isoform expressed in striatal medium spiny neurons (MSNs), is essential for the integration of both stimulatory and inhibitory midbrain signals that initiate from dopaminergic G protein-coupled receptor (GPCR) activation. The spatial and temporal control of cAMP signaling is dependent upon the composition of local regulatory protein networks. However, there is little understanding of how adenylyl cyclase protein interaction networks adapt to the multifarious pressures of integrating acute versus chronic and inhibitory vs. stimulatory receptor signaling in striatal MSNs. Here, we presented the development of a novel bimolecular fluorescence complementation (BiFC)-based protein-protein interaction screening methodology to further identify and characterize elements important for homeostatic control of dopamine-modulated AC5 signaling in a neuronal model cell line and striatal MSNs. We identified two novel AC5 modulators: the protein phosphatase 2A (PP2A) catalytic subunit (PPP2CB) and the intracellular trafficking associated protein-NSF (N-ethylmaleimide-sensitive factor) attachment protein alpha (NAPA). The effects of genetic knockdown (KD) of each gene were evaluated in several cellular models, including D₁- and D₂-dopamine receptor-expressing MSNs from CAMPER mice. The knockdown of PPP2CB was associated with a reduction in acute and sensitized adenylyl cyclase activity, implicating PP2A is an important and persistent regulator of adenylyl cyclase activity. In contrast, the effects of NAPA knockdown were more nuanced and appeared to involve an activity-dependent protein interaction network. Taken together, these data represent a novel screening method and workflow for the identification and validation of adenylyl cyclase protein-protein interaction networks under diverse cAMP signaling paradigms.

A brain proteomic investigation of rapamycin effects in the Tsc1+/- mouse model

BACKGROUND

Tuberous sclerosis complex (TSC) is a rare monogenic disorder characterized by benign tumors in multiple organs as well as a high prevalence of epilepsy, intellectual disability and autism. TSC is caused by inactivating mutations in the TSC1 or TSC2 genes. Heterozygocity induces hyperactivation of mTOR which can be inhibited by mTOR inhibitors, such as rapamycin, which have proven efficacy in the treatment of TSC-associated symptoms. The aim of the present study was (1) to identify molecular changes associated with social and cognitive deficits in the brain tissue of Tsc1+/- mice and (2) to investigate the molecular effects of rapamycin treatment, which has been shown to ameliorate genotype-related behavioural deficits.

METHODS

Molecular alterations in the frontal cortex and hippocampus of Tsc1+/- and control mice, with or without rapamycin treatment, were investigated. A quantitative mass spectrometry-based shotgun proteomic approach (LC-MSE) was employed as an unbiased method to detect changes in protein levels. Changes identified in the initial profiling stage were validated using selected reaction monitoring (SRM). Protein Set Enrichment Analysis was employed to identify dysregulated pathways.

RESULTS

LC-MSE analysis of Tsc1+/- mice and controls (n = 30) identified 51 proteins changed in frontal cortex and 108 in the hippocampus. Bioinformatic analysis combined with targeted proteomic validation revealed several dysregulated molecular pathways. Using targeted assays, proteomic alterations in the hippocampus validated the pathways "myelination", "dendrite," and "oxidative stress", an upregulation of ribosomal proteins and the mTOR kinase. LC-MSE analysis was also employed on Tsc1+/- and wildtype mice (n = 34) treated with rapamycin or vehicle. Rapamycin treatment exerted a stronger proteomic effect in Tsc1+/- mice with significant changes (mainly decreased expression) in 231 and 106 proteins, respectively. The cellular pathways "oxidative stress" and "apoptosis" were found to be affected in Tsc1+/- mice and the cellular compartments "myelin sheet" and "neurofilaments" were affected by rapamycin treatment. Thirty-three proteins which were altered in Tsc1+/- mice were normalized following rapamycin treatment, amongst them oxidative stress related proteins, myelin-specific and ribosomal proteins.

CONCLUSIONS

Molecular changes in the Tsc1+/- mouse brain were more prominent in the hippocampus compared to the frontal cortex. Pathways linked to myelination and oxidative stress response were prominently affected and, at least in part, normalized following rapamycin treatment. The results could aid in the identification of novel drug targets for the treatment of cognitive, social and psychiatric symptoms in autism spectrum disorders. Similar pathways have also been implicated in other psychiatric and neurodegenerative disorders and could imply similar disease processes. Thus, the potential efficacy of mTOR inhibitors warrants further investigation not only for autism spectrum disorders but also for other neuropsychiatric and neurodegenerative diseases.

GPCR Signaling and Trafficking: The Long and Short of It

Emerging findings disclose unexpected components of G protein-coupled receptor (GPCR) signaling and cell biology. Select GPCRs exhibit classical signaling, that is restricted to cell membranes, as well as newly described persistent signaling that depends on internalization of the GPCR bound to β-arrestins. Termination of non-canonical endosomal signaling requires intraluminal acidification and sophisticated protein trafficking machineries. Recent studies reveal the structural determinants of the trafficking chaperones. This review summarizes advances in GPCR signaling and trafficking with a focus on the parathyroid hormone receptor (PTHR) as a prototype, and on the actin-sorting nexin 27 (SNX27)-retromer tubule (ASRT) complex, an endosomal sorting hub responsible for recycling and preservation of cell surface receptors. The findings are integrated into a model of PTHR trafficking with implications for signal transduction, bone growth, and mineral ion metabolism.

A Highly Polymorphic Copy Number Variant in the NSF Gene is Associated with Cocaine Dependence

Cocaine dependence is a complex psychiatric disorder involving both genetic and environmental factors. Several neurotransmitter systems mediate cocaine’s effects, dependence and relapse, being the components of the neurotransmitter release machinery good candidates for the disorder. Previously, we identified a risk haplotype for cocaine dependence in the NSF gene, encoding the protein N-Ethylmaleimide-Sensitive Factor essential for synaptic vesicle turnover. Here we examined the possible contribution to cocaine dependence of a large copy number variant (CNV) that encompasses part of the NSF gene. We performed a case-control association study in a discovery sample (359 cases and 356 controls) and identified an association between cocaine dependence and the CNV (P = 0.013), that was confirmed in the replication sample (508 cases and 569 controls, P = 7.1e-03) and in a pooled analysis (P = 1.8e-04), with an over-representation of low number of copies in cases. Subsequently, we studied the functional impact of the CNV on gene expression and found that the levels of two NSF transcripts were significantly increased in peripheral blood mononuclear cells (PBMC) along with the number of copies of the CNV. These results, together with a previous study from our group, support the role of NSF in the susceptibility to cocaine dependence.

N-Ethylmaleimide Dissociates α7 ACh Receptor from a Complex with NSF and Promotes Its Delivery to the Presynaptic Membrane

N-Ethylmaleimide (NEM)-sensitive factor (NSF) associates with soluble NSF attachment protein (SNAP), that binds to SNAP receptors (SNAREs) including syntaxin, SNAP25, and synaptobrevin. The complex of NSF/SNAP/SNAREs plays a critical role in the regulation of vesicular traffic. The present study investigated NEM-regulated α7 ACh receptor translocation. NSF associated with β-SNAP and the SNAREs syntaxin 1 and synaptobrevin 2 in the rat hippocampus. NSF also associated with the α7 ACh receptor subunit, the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluA1 and GluA2, and the γ-aminobutyric acid A (GABAA) receptor γ2 subunit. NEM, an inhibitor of NSF, significantly dissociated the α7 ACh receptor subunit from a complex with NSF and increased cell surface localization of the receptor subunit, but such effect was not obtained with the GluA1, GluA2 or γ2 subunits. NEM, alternatively, dissociated synaptobrevin 2 from an assembly of NSF/β-SNAP/syntaxin 1/synaptobrevin 2. NEM significantly increased the rate of nicotine-triggered AMPA receptor-mediated miniature excitatory postsynaptic currents, without affecting the amplitude, in rat hippocampal slices. The results of the present study indicate that NEM releases the α7 ACh receptor subunit and synaptobrevin 2 from an assembly of α7 ACh receptor subunit/NSF/β-SNAP/syntaxin 1/synaptobrevin 2, thereby promoting delivery of the α7 ACh receptor subunit to presynaptic membrane.

Actin-Sorting Nexin 27 (SNX27)-Retromer Complex Mediates Rapid Parathyroid Hormone Receptor Recycling

The G protein-coupled parathyroid hormone receptor (PTHR) regulates mineral-ion homeostasis and bone remodeling. Upon parathyroid hormone (PTH) stimulation, the PTHR internalizes into early endosomes and subsequently traffics to the retromer complex, a sorting platform on early endosomes that promotes recycling of surface receptors. The C terminus of the PTHR contains a type I PDZ ligand that binds PDZ domain-containing proteins. Mass spectrometry identified sorting nexin 27 (SNX27) in isolated endosomes as a PTHR binding partner. PTH treatment enriched endosomal PTHR. SNX27 contains a PDZ domain and serves as a cargo selector for the retromer complex. VPS26, VPS29, and VPS35 retromer subunits were isolated with PTHR in endosomes from cells stimulated with PTH. Molecular dynamics and protein binding studies establish that PTHR and SNX27 interactions depend on the PDZ recognition motif in PTHR and the PDZ domain of SNX27. Depletion of either SNX27 or VPS35 or actin depolymerization decreased the rate of PTHR recycling following agonist stimulation. Mutating the PDZ ligand of PTHR abolished the interaction with SNX27 but did not affect the overall rate of recycling, suggesting that PTHR may directly engage the retromer complex. Coimmunoprecipitation and overlay experiments show that both intact and mutated PTHR bind retromer through the VPS26 protomer and sequentially assemble a ternary complex with PTHR and SNX27. SNX27-independent recycling may involve N-ethylmaleimide-sensitive factor, which binds both PDZ intact and mutant PTHRs. We conclude that PTHR recycles rapidly through at least two pathways, one involving the ASRT complex of actin, SNX27, and retromer and another possibly involving N-ethylmaleimide-sensitive factor.

Snapin interacts with G-protein coupled receptor PKR2

Mutations in Prokineticin receptor 2 (PKR2), a G-protein-coupled receptor, have been identified in patients with Kallmann syndrome and/or idiopathic hypogonadotropic hypogonadism, characterized by delayed puberty and infertility. In this study, we performed yeast two-hybrid screening by using PKR2 C-terminus (amino acids 333-384) as a bait, and identified Snapin as a novel interaction partner for PKR2. The interaction of Snapin and PKR2 was confirmed in GST pull-down and co-immunoprecipitation studies. We further demonstrated that two α-helix domains in Snapin are required for the interaction. And the interactive motifs of PKR2 were mapped to YFK (343-345) and HWR (351-353), which shared a similar sequence of two aromatic amino acids followed by a basic amino acid. Disruption of Snapin-PKR2 interaction did not affect PKR2 signaling, but increased the ligand-induced degradation, implying a role of Snapin in the trafficking of PKR2.

Postendocytic Sorting of Adrenergic and Opioid Receptors: New Mechanisms and Functions

The endocytic pathway tightly regulates the activity of G protein-coupled receptors (GPCRs). Much of our understanding of this relationship between GPCR endocytic trafficking and signaling comes from studies done on catecholamine and opioid receptors. After ligand-induced endocytosis, a key sorting step in the endosome determines whether receptors are recycled back to the cell surface, leading to recovery of signaling, or are degraded in the lysosome, leading to desensitization. Recycling of GPCRs, unlike that of many other proteins, is an active process driven by specific sequences on the receptor and proteins that interact with this sequence. Recent data suggest that sequence-dependent recycling plays complex roles in regulating both the timing and location of GPCR signaling. This chapter will describe our current understanding of the mechanisms regulating GPCR sorting in the endosome and discuss emerging ideas on their role in GPCR signaling, focusing on adrenergic and opioid receptors as prototypes.

The OsSec18 complex interacts with P0(P1-P2)2 to regulate vacuolar morphology in rice endosperm cell

BACKGROUND

Sec18p/N-ethylmaleimide-sensitive factor (NSF) is a conserved eukaryotic ATPase, which primarily functions in vesicle membrane fusion from yeast to human. However, the function of the OsSec18 gene, a homologue of NSF in rice, remains unknown.

RESULTS

In the present study, we investigated the function of OsSec18 in rice and found that OsSec18 complements the temperature-sensitive phenotype and interferes with vacuolar morphogenesis in yeast. Overexpression of OsSec18 in rice decreased the plant height and 1000-grain weight and altered the morphology of the protein bodies. Further examination revealed that OsSec18 presented as a 290-kDa complex in rice endosperm cells. Moreover, Os60sP0 was identified a component of this complex, demonstrating that the OsSec18 complex contains another complex of P0(P1-P2)2 in rice endosperm cells. Furthermore, we determined that the N-terminus of OsSec18 can interact with the N- and C-termini of Os60sP0, whereas the C-terminus of OsSec18 can only interact with the C-terminus of Os60sP0.

CONCLUSION

Our results revealed that the OsSec18 regulates vacuolar morphology in both yeast and rice endosperm cell and the OsSec18 interacts with P0(P1-P2)2 complex in rice endosperm cell.

N-ethylmaleimide-sensitive factor interacts with the serotonin transporter and modulates its trafficking: implications for pathophysiology in autism

Background

Changes in serotonin transporter (SERT) function have been implicated in autism. SERT function is influenced by the number of transporter molecules present at the cell surface, which is regulated by various cellular mechanisms including interactions with other proteins. Thus, we searched for novel SERT-binding proteins and investigated whether the expression of one such protein was affected in subjects with autism.

Methods

Novel SERT-binding proteins were examined by a pull-down system. Alterations of SERT function and membrane expression upon knockdown of the novel SERT-binding protein were studied in HEK293-hSERT cells. Endogenous interaction of SERT with the protein was evaluated in mouse brains. Alterations in the mRNA expression of SERT (SLC6A4) and the SERT-binding protein in the post-mortem brains and the lymphocytes of autism patients were compared to nonclinical controls.

Results

N-ethylmaleimide-sensitive factor (NSF) was identified as a novel SERT-binding protein. NSF was co-localized with SERT at the plasma membrane, and NSF knockdown resulted in decreased SERT expression at the cell membranes and decreased SERT uptake function. NSF was endogenously co-localized with SERT and interacted with SERT. While SLC6A4 expression was not significantly changed, NSF expression tended to be reduced in post-mortem brains, and was significantly reduced in lymphocytes of autistic subjects, which correlated with the severity of the clinical symptoms.

Conclusions

These data clearly show that NSF interacts with SERT under physiological conditions and is required for SERT membrane trafficking and uptake function. A possible role for NSF in the pathophysiology of autism through modulation of SERT trafficking, is suggested.

Hippocampal monoamine receptor complex levels linked to spatial memory decline in the aging C57BL/6J

Although a large series of reports on monoamine receptor (MAR) biochemistry and pharmacology in aging are available, work on MAR complexes rather than subunits is limited. It was the aim of the study to determine MAR complexes in hippocampi of three different age groups (3-12 and 18 months) in the mouse and to link MAR changes to spatial memory retrieval in the Morris water maze (MWM). MAR complexes were separated by blue native electrophoresis. Immunohistochemistry was performed in order to show the pattern of dopamine receptors and its colocalizations. D1R, D2R and 5-HT7R containing receptor complex levels were decreasing with age while 5-HT1AR-containing complex levels were increasing with age. D1R, 5-HT7R and 5-HT1AR were significantly correlating with the time spent in the target quadrant, representing retrieval in the MWM. D1R and D2R immunoreactivity was decreasing in an area-dependent pattern and D1R and D2R were colocalized. Individual monoamine receptors are linked to spatial memory retrieval and are modulated by age. The findings are relevant for interpretation of previous and design of future work on brain receptors, spatial memory and aging.

Novel, gel-free proteomics approach identifies RNF5 and JAMP as modulators of GPCR stability

The maturation and folding of G protein-coupled receptors are governed by mechanisms that remain poorly understood. In an effort to characterize these biological events, we optimized a novel, gel-free proteomic approach to identify partners of the β2-adrenergic receptor (β2AR). In addition to a number of known interacting proteins such as heterotrimeric G protein subunits, this allowed us to identify proteins involved in endoplasmic reticulum (ER) QC of the receptor. Among β2AR-associated proteins is Ring finger protein 5 (RNF5), an E3 ubiquitin ligase anchored to the outer membrane of the ER. Coimmunoprecipitation assays confirmed, in a cellular context, the interaction between RNF5 and the β2AR as well as the prostaglandin D2 receptor (DP). Confocal microscopy revealed that DP colocalized with RNF5 at the ER. Coexpression of RNF5 with either receptor increased levels of their expression, whereas small interfering RNA-mediated knockdown of endogenous RNF5 promoted the opposite. RNF5 did not modulate the ubiquitination state of β2AR or DP. Instead, RNF5 ubiquitinated JNK-associated membrane protein (JAMP), a protein that recruits the proteasome to the ER membrane and that is negatively regulated by RNF5-mediated ubiquitination. JAMP coimmunoprecipitated with both β2AR and DP and decreased total receptor protein levels through proteasomal degradation. Expression of DP, a receptor largely retained in the ER, promoted proteasome recruitment by JAMP. Degradation of both receptors via JAMP was increased when RNF5 was depleted. Our data suggest that RNF5 regulates the turnover of specific G protein-coupled receptors by ubiquitinating JAMP and preventing proteasome recruitment.

Identification of a novel protein-protein interaction motif mediating interaction of GPCR-associated sorting proteins with G protein-coupled receptors

GPCR desensitization and down-regulation are considered key molecular events underlying the development of tolerance in vivo. Among the many regulatory proteins that are involved in these complex processes, GASP-1 have been shown to participate to the sorting of several receptors toward the degradation pathway. This protein belongs to the recently identified GPCR-associated sorting proteins (GASPs) family that comprises ten members for which structural and functional details are poorly documented. We present here a detailed structure-function relationship analysis of the molecular interaction between GASPs and a panel of GPCRs. In a first step, GST-pull down experiments revealed that all the tested GASPs display significant interactions with a wide range of GPCRs. Importantly, the different GASP members exhibiting the strongest interaction properties were also characterized by the presence of a small, highly conserved and repeated "GASP motif" of 15 amino acids. We further showed using GST-pull down, surface plasmon resonance and co-immunoprecipitation experiments that the central domain of GASP-1, which contains 22 GASP motifs, is essential for the interaction with GPCRs. We then used site directed mutagenesis and competition experiments with synthetic peptides to demonstrate that the GASP motif, and particularly its highly conserved core sequence SWFW, is critically involved in the interaction with GPCRs. Overall, our data show that several members of the GASP family interact with GPCRs and highlight the presence within GASPs of a novel protein-protein interaction motif that might represent a new target to investigate the involvement of GASPs in the modulation of the activity of GPCRs.

Protein-protein interactions at the adrenergic receptors

The adrenergic receptors are among the best characterized G protein-coupled receptors (GPCRs) and knowledge on this receptor family has provided several important paradigms about GPCR function and regulation. One of the most recent paradigms initially supported by studies on adrenergic receptors is that both βarrestins and G protein-coupled receptors themselves can act as scaffolds binding a variety of proteins and this can result in growing complexity of the receptor-mediated cellular effects. In this review we will briefly summarize the main features of βarrestin binding to the adrenergic receptor subtypes and we will review more in detail the main proteins found to selectively interact with distinct AR subtype. At the end, we will review the main findings on oligomerization of the AR subtypes.

Protein interacting with C kinase 1 (PICK1) reduces reinsertion rates of interaction partners sorted to Rab11-dependent slow recycling pathway

The scaffolding protein PICK1 (protein interacting with C kinase 1) contains an N-terminal PSD-95/Discs large/ZO-1 (PDZ) domain and a central lipid-binding Bin/amphiphysin/Rvs (BAR) domain. PICK1 is thought to regulate trafficking of its PDZ binding partners but different and even opposing functions have been suggested. Here, we apply ELISA-based assays and confocal microscopy in HEK293 cells with inducible PICK1 expression to assess in an isolated system the ability of PICK1 to regulate trafficking of natural and engineered PDZ binding partners. The dopamine transporter (DAT), which primarily sorts to degradation upon internalization, did not form perinuclear clusters with PICK1, and PICK1 did not affect DAT internalization/recycling. However, transfer of the PICK1-binding DAT C terminus to the β(2)-adrenergic receptor, which sorts to recycling upon internalization, led to formation of PICK1 co-clusters in Rab11-positive compartments. Furthermore, PICK1 inhibited Rab11-mediated recycling of the receptor in a BAR and PDZ domain-dependent manner. In contrast, transfer of the DAT C terminus to the δ-opioid receptor, which sorts to degradation, did not result in PICK1 co-clusters or any change in internalization/recycling. Further support for a role of PICK1 determined by its PDZ cargo was obtained for the PICK1 interaction partner prolactin-releasing peptide receptor (GPR10). GPR10 co-localized with Rab11 and clustered with PICK1 upon constitutive internalization but co-localized with the late endosomal marker Rab7 and did not cluster with PICK1 upon agonist-induced internalization. Our data suggest a selective role of PICK1 in clustering and reducing the recycling rates of PDZ domain binding partners sorted to the Rab11-dependent recycling pathway.

Candidate pathway association study in cocaine dependence: the control of neurotransmitter release

OBJECTIVES

Cocaine is the second most used illegal drug in Europe. The transition from use to dependence involves both genetic and environmental factors. Genetic variation in neurotransmitter systems is involved in the susceptibility to cocaine dependence. We examined the possible contribution to cocaine dependence of 16 genes involved in the cellular machinery that controls neurotransmitter release: genes encoding proteins of the SNARE complex (STX1A, SNAP25, VAMP1 and VAMP2), fusion control elements (SYT1, SYT2, CPLX1, CPLX2, CPLX3 and CPLX4) and regulatory elements (STXBP1, SYP, SNPH, NSF, NAPA and RAB3A).

METHODS

We genotyped 121 SNPs, selected according to genetic coverage criteria, in 360 cocaine-dependent patients and 360 controls from Spain.

RESULTS

Single and multiple-marker analyses revealed a strong association between cocaine dependence and the NSF gene, encoding the N-ethylmaleimide-sensitive factor (P = 5.1e-04, OR = 2.44 (1.45-4.00) and P = 0.001, OR = 1.82 (1.28-2.59), respectively). The presence and absence of psychotic symptoms were also studied. Interestingly, when we considered the time between initial consumption and the onset of cocaine dependence, we observed that the association was mainly restricted to the group of patients that rapidly developed drug dependence (≤ 2 years; P = 2.98e-06, OR = 1.33 (1.20-1.47)).

CONCLUSIONS

Our data show preliminary evidence that NSF may predispose not only to cocaine dependence, but also to an early onset of the dependence.

Potential cellular functions of N-ethylmaleimide sensitive factor in the photoreceptor

Recent work has established potential new functional roles for NSF in the photoreceptor. First, the interaction of Arr1 and NSF is ATP-dependent, and the N-terminal domain of Arr1 interacts with the N and D1 junctional domains of NSF. The Arr1-NSF interactions are greater in the photoreceptor synaptic terminal in the dark. Furthermore, Arr1 enhances the NSF ATPase activity and increases the NSF disassembly activities, which are critical for NSF functions in sustaining a higher rate of exocytosis in the photoreceptor synapses and the compensatory endocytosis to retrieve vesicle membrane and vesicle proteins for vesicle recycling. These data demonstrate the Arr1 and NSF interaction are necessary for both maintenance and modulation of normal photoreceptor synaptic regulation. Second, NSF colocalizes and specifically binds to RP2, especially in the ciliary and synaptic region of the photoreceptor, and NSF-RP2 interaction may play an important role in membrane protein trafficking in the photoreceptor. Inherited retinal degeneration affects about 1 in 2,000-3,000 individuals in the world and is the leading cause of visual loss in young people and accounts for a large proportion of blindness in adult life. These studies accelerate our ability to gain insight into the diverse roles of the NSF in the photoreceptor cells and enable us to understand more precisely the molecular mechanisms underlying night blindness associated with clinically diagnosed Oguchi disease or other forms of retinitis pigmentosa.

Regulation of GPCR trafficking by RAMPs

AM and CGRP receptors undergo differential intracellular receptor trafficking upon ligand stimulation. Intracellular trafficking of CLR/RAMP receptor complexes is regulated by posttranslational modifications and protein-protein interactions that differ for each cell type. Recent evidence is accumulating to suggest that the RAMP isoform in complex with CLR may play a role in determining the intracellular trafficking and fate of ligand-stimulated receptor complexes. In this chapter, we will review the current literature on mechanisms of regulating receptor trafficking and roles that have been demonstrated for RAMPs in this regulation.

Compartmentalization of beta-adrenergic signals in cardiomyocytes

Activation of adrenergic receptors (AR) represents the primary mechanism to increase cardiac performance under stress. Activated βAR couple to Gs protein, leading to adenylyl cyclase-dependent increases in secondary-messenger cyclic adenosine monophosphate (cAMP) to activate protein kinase A. The increased protein kinase A activities promote phosphorylation of diversified substrates, ranging from the receptor and its associated partners to proteins involved in increases in contractility and heart rate. Recent progress with live-cell imaging has drastically advanced our understanding of the βAR-induced cAMP and protein kinase A activities that are precisely regulated in a spatiotemporal fashion in highly differentiated myocytes. Several features stand out: membrane location of βAR and its associated complexes dictates the cellular compartmentalization of signaling; βAR agonist dose-dependent equilibrium between cAMP production and cAMP degradation shapes persistent increases in cAMP signals for sustained cardiac contraction response; and arrestin acts as an agonist dose-dependent master switch to promote cAMP diffusion and propagation into intracellular compartments by sequestrating phosphodiesterase isoforms associated with the βAR signaling cascades. These features and the underlying molecular mechanisms of dynamic regulation of βAR complexes with adenylyl cyclase and phosphodiesterase enzymes and the implication in heart failure are discussed.

Modulation of monoamine receptors by adaptor proteins and lipid rafts: role in some effects of centrally acting drugs and therapeutic agents

The monoamines and their cognate receptors are widespread in the central nervous system and are vital for normal brain function. Dysfunction in these systems underlies several psychiatric and neurological disease states, and consequently monoamines are targets of a host of pharmacotherapies. This review provides an overview on how monoamine receptors are regulated by adaptor proteins and lipid rafts with emphasis on interactions in nerve cells. Monoamine receptors have prominent intracellular loops that provide binding sites for adaptor proteins. Receptor function is further modulated by cholesterol and submembranous microdomains termed lipid rafts. These interactions determine several facets of G protein-coupled receptor (GPCR) function including trafficking, localization, and signaling. Possible roles of adaptor proteins and lipid rafts in disease states and in mediating actions of drugs and therapeutic agents are also discussed.

Role of PDZ proteins in regulating trafficking, signaling, and function of GPCRs: means, motif, and opportunity

PDZ proteins, named for the common structural domain shared by the postsynaptic density protein (PSD95), Drosophila disc large tumor suppressor (DlgA), and zonula occludens-1 protein (ZO-1), constitute a family of 200-300 recognized members. These cytoplasmic adapter proteins are capable of assembling a variety of membrane-associated proteins and signaling molecules in short-lived functional units. Here, we review PDZ proteins that participate in the regulation of signaling, trafficking, and function of G protein-coupled receptors. Salient structural features of PDZ proteins that allow them to recognize targeted GPCRs are considered. Scaffolding proteins harboring PDZ domains may contain single or multiple PDZ modules and may also include other protein-protein interaction modules. PDZ proteins may impact receptor signaling by diverse mechanisms that include retaining the receptor at the cell membrane, thereby increasing the duration of ligand binding, as well as importantly influencing GPCR internalization, trafficking, recycling, and intracellular sorting. PDZ proteins are also capable of modifying the assembled complex of accessory proteins such as β-arrestins that themselves regulate GPCR signaling. Additionally, PDZ proteins may modulate GPCR signaling by altering the G protein to which the receptor binds, or affect other regulatory proteins that impact GTPase activity, protein kinase A, phospholipase C, or modify downstream signaling events. Small molecules targeting the PDZ protein-GPCR interaction are being developed and may become important and selective drug candidates.

Impaired striatum-dependent behavior in GASP-1-knock-out mice

G protein-coupled receptor (GPCR) associated sorting protein-1 (GASP-1) is suspected to play a key role in recycling and degradation of several GPCRs. In a previous study, we have shown that GASP-1-knock-out (GASP-1-KO) mice displayed deficits in acquiring a cocaine self-administration task, associated with an exacerbated down-regulation of striatal dopaminergic and cholinergic receptors. Among several possibilities, GASP-1 deficiency could have impaired memory processes underlying the acquisition of the operant conditioning task. Therefore, the present study investigated cognitive performances of GASP-1-KO mice and their wild-type littermates (WT) in a broad variety of memory tasks. Consistent with a deficit in procedural memory, GASP-1-KO mice showed delayed acquisition of a food-reinforced bar-press task. During water-maze training in hidden- or visible-platform paradigms, mutant and WT mice acquired the tasks at the same rate. However, GASP-1 mice exhibited persistent thigmotaxic swimming, longer distance to the platform, and reduced swim speed. There was no deficit in several tasks requiring simple behavioral responses (Barnes maze, object recognition and passive avoidance tasks). Thus, the ability to acquire and/or express complex responses seems affected in GASP-1-deficient mice. Hippocampal functions were preserved, as the retention of an acquired memory in spatial tasks remained unaffected. The pattern of behavioral deficits observed in GASP-1-KO mice is coherent with current knowledge on the role of striatal GPCRs in acquisition/expression of skilled behavior and in motivation. Together with the previous findings, the so far established phenotype of GASP-1-KO mice makes them a potentially exciting tool to study striatal functions.

Postendocytic sorting of constitutively internalized dopamine transporter in cell lines and dopaminergic neurons

The dopamine transporter (DAT) mediates reuptake of released dopamine and is the target for psychostimulants, such as cocaine and amphetamine. DAT undergoes marked constitutive endocytosis, but little is known about the fate and sorting of the endocytosed transporter. To study DAT sorting in cells lines, we fused the one-transmembrane segment protein Tac to DAT, thereby generating a transporter (TacDAT) with an extracellular antibody epitope suited for trafficking studies. TacDAT was functional and endocytosed constitutively in HEK293 cells. According to an ELISA-based assay, TacDAT intracellular accumulation was increased by the lysosomal protease inhibitor leupeptin and by monensin, an inhibitor of lysosomal degradation and recycling. Monensin also reduced TacDAT surface expression consistent with partial recycling. In both HEK293 cells and in the dopaminergic cell line 1Rb3An27, constitutively internalized TacDAT displayed primary co-localization with the late endosomal marker Rab7, less co-localization with the "short loop" recycling marker Rab4, and little co-localization with the marker of "long loop" recycling endosomes, Rab11. Removal by mutation of N-terminal ubiquitination sites did not affect this sorting pattern. The sorting pattern was distinct from a bona fide recycling membrane protein, the beta(2)-adrenergic receptor, that co-localized primarily with Rab11 and Rab4. Constitutively internalized wild type DAT probed with the fluorescently tagged cocaine analogue JHC 1-64, exhibited the same co-localization pattern as TacDAT in 1Rb3An27 cells and in cultured midbrain dopaminergic neurons. We conclude that DAT is constitutively internalized and sorted in a ubiquitination-independent manner to late endosomes/lysosomes and in part to a Rab4 positive short loop recycling pathway.

Regulation of P2Y1 receptor traffic by sorting Nexin 1 is retromer independent

The activity and traffic of G-protein coupled receptors (GPCRs) is tightly controlled. Recent work from our laboratory has shown that P2Y(1) and P2Y(12) responsiveness is rapidly and reversibly modulated in human platelets and that the underlying mechanism requires receptor trafficking as an essential part of this process. However, little is known about the molecular mechanisms underlying P2Y receptor traffic. Sorting nexin 1 (SNX1) has been shown to regulate the endosomal sorting of cell surface receptors either to lysosomes where they are downregulated or back to the cell surface. These functions may in part be due to interactions of SNX1 with the mammalian retromer complex. In this study, we investigated the role of SNX1 in P2Y receptor trafficking. We show that P2Y(1) receptors recycle via a slow recycling pathway that is regulated by SNX1, whereas P2Y(12) receptors return to the cell surface via a rapid route that is SNX1 independent. SNX1 inhibition caused a dramatic increase in the rate of P2Y(1) receptor recycling, whereas inhibition of Vps26 and Vps35 known to be present in retromer had no effect, indicating that SNX1 regulation of P2Y(1) receptor recycling is retromer independent. In addition, inhibition of SNX4, 6 and 17 proteins did not affect P2Y(1) receptor recycling. SNX1 has also been implicated in GPCR degradation; however, we provide evidence that P2Y receptor degradation is SNX1 independent. These data describe a novel function of SNX1 in the regulation of P2Y(1) receptor recycling and suggest that SNX1 plays multiple roles in endocytic trafficking of GPCRs.

Functional modulation of GABAB receptors by protein kinases and receptor trafficking

GABA(B) receptors (GABA(B)R) are heterodimeric G protein-coupled receptors (GPCRs) that mediate slow and prolonged inhibitory signals in the central nervous system. The signaling of GPCRs is under stringent control and is subject to regulation by multiple posttranslational mechanisms. The beta-adrenergic receptor is a prototypic GPCR. Like most GPCRs, prolonged exposure of this receptor to agonist induces phosphorylation of multiple intracellular residues that is largely dependent upon the activity of G protein-coupled receptor kinases (GRKs). Phosphorylation terminates receptor-effector coupling and promotes both interaction with beta-arrestins and removal from the plasma membrane via clathrin-dependent endocytosis. Emerging evidence for GABA(B)Rs suggests that these GPCRs do not conform to this mode of regulation. Studies using both native and recombinant receptor preparations have demonstrated that GABA(B)Rs do not undergo agonist-induced internalization and are not GRK substrates. Moreover, whilst GABA(B)Rs undergo clathrin-dependent constitutive endocytosis, it is generally accepted that their rates of internalization are not modified by prolonged agonist exposure. Biochemical studies have revealed that GABA(B)Rs are phosphorylated on multiple residues within the cytoplasmic domains of both the R1 and R2 subunits by cAMP-dependent protein kinase and 5'AMP-dependent protein kinase (AMPK). Here we discuss the role that this phosphorylation plays in determining GABA(B)R effector coupling and their trafficking within the endocytic pathway and go on to evaluate the significance of GABA(B)R phosphorylation in controlling neuronal excitability under normal and pathological conditions.

Attenuated behavioural responses to acute and chronic cocaine in GASP-1-deficient mice

G protein-coupled receptor (GPCR) associated sorting protein 1 (GASP-1) interacts with GPCRs and is implicated in their postendocytic sorting. Recently, GASP-1 has been shown to regulate dopamine (D(2)) and cannabinoid (CB1) receptor signalling, suggesting that preventing GASP-1 interaction with GPCRs might provide a means to limit the decrease in receptor signalling upon sustained agonist treatment. In order to test this hypothesis, we have generated and behaviourally characterized GASP-1 knockout (KO) mice and have examined the consequences of the absence of GASP-1 on chronic cocaine treatments. GASP-1 KO and wild-type (WT) mice were tested for sensitization to the locomotor effects of cocaine. Additional mice were trained to acquire intravenous self-administration of cocaine on a fixed ratio 1 schedule of reinforcement, and the motivational value of cocaine was then assessed using a progressive ratio schedule of reinforcement. The dopamine and muscarinic receptor densities were quantitatively evaluated in the striatum of WT and KO mice tested for sensitization and self-administration. Acute and sensitized cocaine-locomotor effects were attenuated in KO mice. A decrease in the percentage of animals that acquired cocaine self-administration was also observed in GASP-1-deficient mice, which was associated with pronounced down-regulation of dopamine and muscarinic receptors in the striatum. These data indicate that GASP-1 participates in acute and chronic behavioural responses induced by cocaine and are in agreement with a role of GASP-1 in postendocytic sorting of GPCRs. However, in contrast to previous studies, our data suggest that upon sustained receptor stimulation GASP-1 stimulates recycling rather than receptor degradation.

Essential role of Hrs in endocytic recycling of full-length TrkB receptor but not its isoform TrkB.T1

Brain-derived neurotrophic factor (BDNF) signaling through its receptor, TrkB, modulates survival, differentiation, and synaptic activity of neurons. Both full-length TrkB (TrkB-FL) and its isoform T1 (TrkB.T1) receptors are expressed in neurons; however, whether they follow the same endocytic pathway after BDNF treatment is not known. In this study we report that TrkB-FL and TrkB.T1 receptors traverse divergent endocytic pathways after binding to BDNF. We provide evidence that in neurons TrkB.T1 receptors predominantly recycle back to the cell surface by a "default" mechanism. However, endocytosed TrkB-FL receptors recycle to a lesser extent in a hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs)-dependent manner which relies on its tyrosine kinase activity. The distinct role of Hrs in promoting recycling of internalized TrkB-FL receptors is independent of its ubiquitin-interacting motif. Moreover, Hrs-sensitive TrkB-FL recycling plays a role in BDNF-induced prolonged mitogen-activated protein kinase (MAPK) activation. These observations provide evidence for differential postendocytic sorting of TrkB-FL and TrkB.T1 receptors to alternate intracellular pathways.

Rab11 regulates the recycling of the beta2-adrenergic receptor through a direct interaction

The beta2ARs (beta(2)-adrenergic receptors) undergo ligand-induced internalization into early endosomes, but then are rapidly and efficiently recycled back to the plasma membrane, restoring the numbers of functional cell-surface receptors. Gathering evidence suggests that, during prolonged exposure to agonist, some beta2ARs also utilize a slow recycling pathway through the perinuclear recycling endosomal compartment regulated by the small GTPase Rab11. In the present study, we demonstrate by co-immunoprecipitation studies that there is a beta2AR-Rab11 association in HEK-293 cells (human embryonic kidney cells). We show using purified His(6)-tagged Rab11 protein and beta2AR intracellular domains fused to GST (glutathione transferase) that Rab11 interacts directly with the C-terminal tail of beta2AR, but not with the other intracellular domains of the receptor. Pull-down and immunoprecipitation assays revealed that the beta2AR interacts preferentially with the GDP-bound form of Rab11. Arg(333) and Lys(348) in the C-terminal tail of the beta2AR were identified as crucial determinants for Rab11 binding. A beta2AR construct with these two residues mutated to alanine, beta2AR RK/AA (R333A/K348A), was generated. Analysis of cell-surface receptors by ELISA revealed that the recycling of beta2AR RK/AA was drastically reduced when compared with wild-type beta2AR after agonist washout, following prolonged receptor stimulation. Confocal microscopy demonstrated that the beta2AR RK/AA mutant failed to co-localize with Rab11 and recycle to the plasma membrane, in contrast with the wild-type receptor. To our knowledge, the present study is the first report of a direct interaction between the beta2AR and a Rab GTPase, which is required for the accurate intracellular trafficking of the receptor.

Dual role of the beta2-adrenergic receptor C terminus for the binding of beta-arrestin and receptor internalization

Homologous desensitization of beta2-adrenergic and other G-protein-coupled receptors is a two-step process. After phosphorylation of agonist-occupied receptors by G-protein-coupled receptor kinases, they bind beta-arrestins, which triggers desensitization and internalization of the receptors. Because it is not known which regions of the receptor are recognized by beta-arrestins, we have investigated beta-arrestin interaction and internalization of a set of mutants of the human beta2-adrenergic receptor. Mutation of the four serine/threonine residues between residues 355 and 364 led to the loss of agonist-induced receptor-beta-arrestin2 interaction as revealed by fluorescence resonance energy transfer (FRET), translocation of beta-arrestin2 to the plasma membrane, and receptor internalization. Mutation of all seven serine/threonine residues distal to residue 381 did not affect agonist-induced receptor internalization and beta-arrestin2 translocation. A beta2-adrenergic receptor truncated distal to residue 381 interacted normally with beta-arrestin2, whereas its ability to internalize in an agonist-dependent manner was compromised. A similar impairment of internalization was observed when only the last eight residues of the C terminus were deleted. Our experiments show that the C terminus distal to residue 381 does not affect the initial interaction between receptor and beta-arrestin, but its last eight amino acids facilitate receptor internalization in concert with beta-arrestin2.

Rapid recycling of beta-adrenergic receptors is dependent on the actin cytoskeleton and myosin Vb

For the beta(2)-adrenergic receptor (beta(2)AR), published evidence suggests that an intact actin cytoskeleton is required for the endocytosis of receptors and their proper sorting to the rapid recycling pathway. We have characterized the role of the actin cytoskeleton in the regulation of beta(2)AR trafficking in human embryonic kidney 293 (HEK293) cells using two distinct actin filament disrupting compounds, cytochalasin D and latrunculin B (LB). In cells pretreated with either drug, beta(2)AR internalization into transferrin-positive vesicles was not altered but both agents significantly decreased the rate at which beta(2)ARs recycled to the cell surface. In LB-treated cells, nonrecycled beta(2)ARs were localized to early embryonic antigen 1-positive endosomes and also accumulated in the recycling endosome (RE), but only a small fraction of receptors localized to LAMP-positive late endosomes and lysosomes. Treatment with LB also markedly enhanced the inhibitory effect of rab11 overexpression on receptor recycling. Dissociating receptors from actin by expression of the myosin Vb tail fragment resulted in missorting of beta(2)ARs to the RE, while the expression of various CART fragments or the depletion of actinin-4 had no detectable effect on beta(2)AR sorting. These results indicate that the actin cytoskeleton is required for the efficient recycling of beta(2)ARs, a process that likely is dependent on myosin Vb.

Phosphorylation state of mu-opioid receptor determines the alternative recycling of receptor via Rab4 or Rab11 pathway

Agonist-induced phosphorylation, internalization, and intracellular trafficking of G protein-coupled receptors are critical in regulating both cellular responsiveness and signal transduction. The current study investigated the role of receptor phosphorylation state in regulation of agonist-induced internalization and intracellular trafficking of mu-opioid receptor (MOR). Our results showed that after agonist stimulation, the recycle of a mutant MOR that lacks the C-terminal residues after Asn(362) (MOR362T) was greatly decreased, whereas a C-terminal phosphorylation sites-mutated MOR (MOR3A), which is deficient in agonist-induced phosphorylation recycled back to the membrane at a level comparable to that of the wild-type receptor, however, interestingly at a slower rate. Inhibition of functions of either Rab4 or Rab11 by dominant-negative mutants and small interfering RNA both significantly impaired the recycling of the wild-type MOR, whereas the recycling of the phosphorylation-deficient mutant was only inhibited by the dominant-negative mutant and small interfering RNA of Rab11, suggesting that the recycling of nonphosphorylated MOR is exclusively via Rab11-mediated pathway. Furthermore, phosphorylated MOR was observed accumulated in Rab5- and Rab4-, but not Rab11-positive vesicles. Our data indicate that both phosphorylated and nonphosphorylated MOR internalize via Rab5-dependent pathway after agonist stimulation, and the phosphorylated and nonphosphorylated MORs recycle through distinct vesicular trafficking pathways mediated by Rab4 and Rab11, respectively, which may ultimately lead to differential cellular responsiveness or downstream signaling.

G protein-coupled receptor sorting to endosomes and lysosomes

The heptahelical G protein-coupled receptors (GPCRs) belong to the largest family of cell surface signaling receptors encoded in the human genome. GPCRs signal to diverse extracellular stimuli and control a vast number of physiological responses, making this receptor class the target of nearly half the drugs currently in use. In addition to rapid desensitization, receptor trafficking is crucial for the temporal and spatial control of GPCR signaling. Sorting signals present in the intracytosolic domains of GPCRs regulate trafficking through the endosomal-lysosomal system. GPCR internalization is mediated by serine and threonine phosphorylation and arrestin binding. Short, linear peptide sequences including tyrosine- and dileucine-based motifs, and PDZ ligands that are recognized by distinct endocytic adaptor proteins also mediate internalization and endosomal sorting of GPCRs. We present new data from bioinformatic searches that reveal the presence of these types of sorting signals in the cytoplasmic tails of many known GPCRs. Several recent studies also indicate that the covalent modification of GPCRs with ubiquitin serves as a signal for internalization and lysosomal sorting, expanding the diversity of mechanisms that control trafficking of mammalian GPCRs.

Regulation of GPCRs by endocytic membrane trafficking and its potential implications

The endocytic pathway tightly controls the activity of G protein-coupled receptors (GPCRs). Ligand-induced endocytosis can drive receptors into divergent lysosomal and recycling pathways, producing essentially opposite effects on the strength and duration of cellular signaling via heterotrimeric G proteins, and may also promote distinct signaling events from intracellular membranes. This chapter reviews recent developments toward understanding the molecular machinery and functional implications of GPCR sorting in the endocytic pathway, focusing on mammalian GPCRs whose ligand-induced endocytosis is mediated primarily by clathrin-coated pits. Lysosomal sorting of a number of GPCRs occurs via a highly conserved mechanism requiring covalent tagging of receptors with ubiquitin. There is increasing evidence that additional, noncovalent mechanisms control the sorting of endocytosed GPCRs to lysosomes in mammalian cells. Recycling of several GPCRs to the plasma membrane is also specifically sorted, via a mechanism requiring both receptor-specific and shared sorting proteins. The current data reveal an unprecedented degree of specificity and plasticity in the cellular regulation of mammalian GPCRs by endocytic membrane trafficking. These developments have fundamental implications for GPCR pharmacology, and suggest new mechanisms that could be exploited in GPCR-directed pharmacotherapy.

Intracellular complexes of the beta2 subunit of the nicotinic acetylcholine receptor in brain identified by proteomics

Nicotine acetylcholine receptors (nAChRs) comprise a family of ligand-gated channels widely expressed in the mammalian brain. The beta2 subunit is an abundant protein subunit critically involved in the cognitive and behavioral properties of nicotine as well as in the mechanisms of nicotine addiction. In this work, we used matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry (MALDI-TOF-TOF MS/MS) to uncover protein interactions of the intracellular loop of the beta2 subunit and components of immunoprecipitated beta2-nAChR complexes from mouse brain. Using the beta2-knockout mouse to exclude nonspecific binding to the beta2 antibody, we identify 21 nAChR-interacting proteins (NIPs) expressed in brain. Western blot analysis confirmed the association between the beta2 subunit and candidate NIPs. Based on their functional profiles, the hypothesis is suggested that the identified NIPs can regulate the trafficking and signaling of the beta2-nAChR. Interactions of the beta2 subunit with NIPs such as G protein alpha, G protein-regulated inducer of neurite outgrowth 1, and G protein-activated K(+) channel 1 suggest a link between nAChRs and cellular G protein pathways. These findings reveal intracellular interactions of the beta2 subunit and may contribute to the understanding of the mechanisms of nAChR signaling and trafficking in neurons.

A lysophosphatidic acid receptor lacking the PDZ-binding domain is constitutively active and stimulates cell proliferation

Lysophosphatidic acid (LPA) is an extracellular signaling lipid that regulates cell proliferation, survival, and motility of normal and cancer cells. These effects are produced through G protein-coupled LPA receptors, LPA(1) to LPA(5). We generated an LPA(1) mutant lacking the SerValVal sequence of the C-terminal PDZ-binding domain to examine the role of this domain in intracellular signaling and other cellular functions. B103 neuroblastoma cells expressing the mutant LPA(1) showed rapid cell proliferation and tended to form colonies under serum-free conditions. The enhanced cell proliferation of the mutant cells was inhibited by exogenous expression of the plasmids inhibiting G proteins including G(betagamma), G(alphai) and G(alphaq) or G(alpha12/13), or treatment with pertussis toxin, phosphoinositide 3-kinase (PI3K) inhibitors or a Rho inhibitor. We confirmed that the PI3K-Akt and Rho pathways were intrinsically activated in mutant cells by detecting increases in phosphorylated Akt in western blot analyses or by directly measuring Rho activity. Interestingly, expression of the mutant LPA(1) in non-tumor mouse fibroblasts induced colony formation in a clonogenic soft agar assay, indicating that oncogenic pathways were activated. Taken together, these observations suggest that the mutant LPA(1) constitutively activates the G protein signaling leading to PI3K-Akt and Rho pathways, resulting in enhanced cell proliferation.

Trafficking of central opioid receptors and descending pain inhibition

The delta-opioid receptor (DOR) belongs to the superfamily of G-protein-coupled receptors (GPCRs) with seven transmembrane domains, and its membrane trafficking is regulated by intracellular sorting processes involving its C-tail motifs, intracellular sorting proteins, and several intracellular signaling pathways. In the quiescent state, DOR is generally located in the intracellular compartments in central neurons. However, chronic stimulation, such as chronic pain and sustained opioid exposure, may induce membrane trafficking of DOR and its translocation to surface membrane. The emerged functional DOR on cell membrane is actively involved in pain modulation and opioid analgesia. This article reviews current understanding of the mechanisms underlying GPCRs and DOR membrane trafficking, and the analgesic function of emerged DOR through membrane trafficking under certain pathophysiological circumstances.

Identification of a postendocytic sorting sequence in CCR5

The chemokine receptor 5 (CCR5), a member of the G protein-coupled receptor family (GPCR), is used by human immunodeficiency virus type 1 (HIV-1) with a R5 tropism as an entry receptor in addition to CD4. It is a key target for an antiviral action aiming at inhibiting the HIV-1 entry process. Only few data are available today regarding the mechanism involved in the intracellular trafficking process of CCR5. Understanding how CCR5 cell surface expression is regulated is particularly important with regard to HIV-1 entry inhibition. We set out to investigate whether CCR5 molecular determinants were involved in the postendocytic recycling and degradative pathways. We constructed progressive deletion mutants of the C-terminal domain of CCR5 that we stably expressed in HEK293 cells. All of the deletion mutants were expressed at the cell surface and were functional HIV-1 receptors. The deletion mutants were internalized after stimulation, but they lost their ability to recycle to the plasma membrane. They were rerouted toward a lysosomal degradative pathway. We identified here a sequence of four amino acids, present at the extreme C terminus of CCR5, that is necessary for the recycling of the internalized receptor, independently of its phosphorylation. A detailed analysis of this sequence indicated that the four amino acids acted as a postsynaptic density 95/discs-large/zona occludens (PDZ) interacting sequence. These results show that the CCR5 cytoplasmic domain bears a sequence similar to the "recycling signals" previously identified in other GPCRs. Drugs able to disrupt the recycling pathway of CCR5 may constitute promising tools for therapeutic treatment.

Alveolar epithelial beta2-adrenergic receptors

beta(2)-adrenergic receptors are present throughout the lung, including the alveolar airspace, where they play an important role for regulation of the active Na(+) transport needed for clearance of excess fluid out of alveolar airspace. beta(2)-adrenergic receptor signaling is required for up-regulation of alveolar epithelial active ion transport in the setting of excess alveolar edema. The positive, protective effects of beta(2)-adrenergic receptor signaling on alveolar active Na(+) transport in normal and injured lungs provide substantial support for the use of beta-adrenergic agonists to accelerate alveolar fluid clearance in patients with cardiogenic and noncardiogenic pulmonary edema. In this review, we summarize the role of beta(2)-adrenergic receptors in the alveolar epithelium with emphasis on their role in the regulation of alveolar active Na(+) transport in normal and injured lungs.

Changes in G protein-coupled receptor sorting protein affinity regulate postendocytic targeting of G protein-coupled receptors

After activation, most G protein-coupled receptors (GPCRs) are regulated by a cascade of events involving desensitization and endocytosis. Internalized receptors can then be recycled to the plasma membrane, retained in an endosomal compartment, or targeted for degradation. The GPCR-associated sorting protein, GASP, has been shown to preferentially sort a number of native GPCRs to the lysosome for degradation after endocytosis. Here we show that a mutant beta(2) adrenergic receptor and a mutant mu opioid receptor that have previously been described as lacking "recycling signals" due to mutations in their C termini in fact bind to GASP and are targeted for degradation. We also show that a mutant dopamine D1 receptor, which has likewise been described as lacking a recycling signal, does not bind to GASP and is therefore not targeted for degradation. Together, these results indicate that alteration of receptors in their C termini can expose determinants with affinity for GASP binding and consequently target receptors for degradation.