Role of endocytosis in mediating downregulation of G-protein-coupled receptors

Role of delivery and trafficking of delta-opioid peptide receptors in opioid analgesia and tolerance

Changes in the number of receptors on the cell surface lead to modulations of physiological functions and pharmacological responses of neurons. Recent studies show that delta-opioid peptide (DOP) and mu-opioid peptide (MOP) receptors have distinct subcellular localizations in neurons. In nociceptive small neurons in the dorsal root ganglia, DOP receptors are sorted into neuropeptide-containing secretory vesicles, enabling the stimulus-induced cell surface expression of these receptors. MOP receptors are constitutively expressed on the cell surface. The physical interaction between DOP receptors and MOP receptors seems to be an important mechanism for the modulation of receptor functions. Experiments in animals show that MOP-receptor-mediated spinal analgesia is enhanced and morphine tolerance does not develop when DOP receptor functions are pharmacologically or genetically attenuated. Thus, the delivery and trafficking of DOP receptors are crucial processes that modulate opioid analgesia and tolerance.

Agonist-regulated endocytosis and desensitization of the human nociceptin receptor

A series of nociceptin receptor ligands has been investigated in relationship to their capability to promote receptor endocytosis, desensitization (evaluated as inhibition of forskolin-stimulated cAMP production) and compensatory upregulation of adenylyl cyclase activity in CHO-K1 cells expressing the cloned human nociceptin receptor. Nociceptin (NC), [Arg14, Lys15]NC-NH2 and NNC 63-0532 (0.01 nM-10 microM) induce a concentration-dependent endocytosis and recycling of the nociceptin receptor. This mechanism contributes to maintain receptor signaling as it counteracts desensitization development and enhances a compensatory upregulation of adenyl cyclase activity. In contrast, the partial agonists [Phe1,Psi(CH2NH)Gly2]NC(1-13)-NH2, Ac-RYYRIK-NH2 and Ac-RYYRWK-NH2 (up to 100 microM) fail to induce receptor endocytosis and cause a pronounced receptor desensitization that is not influenced by monensin, a blocker of recycling of the internalized receptors.

Accessory proteins for G proteins: partners in signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

Alteration of intracellular histamine H2 receptor cycling precedes antagonist-induced upregulation

Long-term administration of a histamine H2 receptor (H2R) antagonist (inverse agonist) induces upregulation of H2R in parietal cells, which may be relevant to the rebound hypersecretion of gastric acid that occurs after withdrawal of treatment. The mechanisms underlying this effect are unknown. We hypothesized that the H2R upregulation could be related to receptor trafficking and used H2R-green fluorescent protein (H2R-GFP) to test the hypothesis. Human H2R-GFP was generated and functionally expressed in HEK-293 cells. Binding of the H2R antagonist [3H]tiotidine was performed to quantify H2R expression, and H2R-GFP was imaged in living cells by confocal and evanescent wave microscopy. The binding affinity of [3H]tiotidine was not significantly different between H2R-GFP- and wild-type H2R-expressing HEK-293 cells, both of which had constitutive activity of adenylate cyclase. Visualization of H2R-GFP revealed that the agonist-induced H2R internalization and the antagonist-induced recycling of the internalized H2R from the recycling endosome within 2 h. Long exposure to the antagonist increased GFP fluorescence in the plasma membrane and also induced upregulation of H2R-GFP estimated by the binding assay, whereas long exposure to the agonist enhanced degradative trafficking of H2R-GFP. We examined whether the upregulation reflected an increase in receptor synthesis. Treatment with antagonist did not augment H2R mRNA, and subsequent inhibition of protein synthesis by cycloheximide had no effect on H2R upregulation. These findings suggested that upon exposure to an antagonist (inverse agonist), the equilibrium between receptor endocytosis and recycling is altered before H2R upregulation, probably via suppressing H2R degradation.

Qdot nanocrystal conjugates conjugated to bombesin or ANG II label the cognate G protein-coupled receptor in living cells

Quantum dots (Qdot Nanocrystal Conjugates; Quantum Dot, Hayward, CA) exhibit high fluorescence and low photobleaching compared with organic dyes, properties that should enhance their detection at low densities. In view of the properties of Qdots and the biological and pharmaceutical importance of G protein-coupled receptors (GPCRs), we attempted to use Qdots to label GPCRs in a variety of live cell types. An agonist consisting of biotinylated bombesin or ANG II was conjugated to Qdot Nanocrystal Conjugates coated with streptavidin through a biotin-streptavidin linkage (Qdot agonist). Herein we demonstrate that Qdot-bombesin conjugate can label the bombesin-preferring GPCR in living mouse Swiss 3T3 cells and in Rat-1 cells. Similarly, we used the Qdot-ANG II conjugate to label GPCR in intact rat intestinal epithelial cells (IEC)-18 and in a human pancreatic adenocarcinoma cell line of ductal origin, HPAF-II cells. We demonstrate that Qdot-ANG II is brighter and more photostable than agonist labeled with the organic dye Cy3. Our results demonstrate that Qdot technology can be adapted to monitor ligand binding to GPCRs. Combined with the narrow and symmetric emission profile of Qdot Nanocrystal Conjugates, this information suggests the potential for a new multiplex strategy to determine the effect of agonists and/or antagonists on agonist binding to several GPCRs simultaneously in living cells.

Regulatory mechanisms underlying pheromone biosynthesis activating neuropeptide (PBAN)-induced internalization of the Bombyx mori PBAN receptor

Internalization of the Bombyx mori pheromone biosynthesis activating neuropeptide receptor (PBANR) has been attributed to the presence of a 67 amino acid C-terminal extension absent in PBANRs from Helicoverpa. To identify the structural motif(s) responsible for internalization, a series of truncation mutants fused with enhanced green fluorescent protein were constructed and transiently expressed in insect Sf9 cells. Confocal microscopy analyses revealed that truncation at Gly357 severely inhibited internalization while truncation at Gln367 did not, indicating that the PBANR internalization motif resides between Gly357-Gln367. Alanine substitution studies suggest that Tyr360 and Leu363 may constitute a YXXL endosomal targeting motif that facilitates endocytosis, however, this motif does not appear to be the primary determinant; an indication that multiple sites are involved. Furthermore, we determined that internalization of the PBANR proceeds via a clathrin-dependent pathway, is dependent on the influx of extracellular calcium, and likely does not involve a G protein-coupled receptor kinase.

Internalization and trafficking of guanylyl cyclase/natriuretic peptide receptor-A

One of the principal loci involved in the regulatory action of atrial and brain natriuretic peptides (ANP and BNP) is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), whose ligand-binding efficiency and GC catalytic activity vary remarkably in different target cells and tissues. In its mature form, NPRA resides in the plasma membrane and contains an extracellular ligand-binding domain, a single transmembrane region, and the intracellular protein kinase-like homology domain (KHD) and guanylyl cyclase (GC) catalytic domain. NPRA is a dynamic cellular macromolecule that traverses through different compartments of the cell through its lifetime. Binding of ligand to NPRA triggers a complex array of signal transduction events and accelerates the endocytosis. The endocytic transport is important in regulating signal transduction, formation of specialized signaling complexes, and modulation of specific components of internalization events. The present review describes the experiments which reveal the internalization of ligand-receptor complexes of NPRA, receptor trafficking and recycling, and delivery of both ligand-receptor molecules into subcellular compartments. The ligand-receptor complexes of NPRA are finally degraded within the lysosomes. The experimental evidence provides a consensus forum, which establishes the endocytosis, cellular trafficking, sequestration, and metabolic processing of ANP/NPRA complexes in the intact cells. The discussion is afforded to address the experimental insights into the mechanisms that cells utilize in modulating the delivery and metabolic processing of ligand-bound NPRA into the cell interior.

Dopamine D1 receptor interaction with arrestin3 in neostriatal neurons

Dopamine D1 receptor interactions with arrestins have been characterized using heterologously expressed D1 receptor and arrestins. The purpose of this study was to investigate the interaction of the endogenous D1 receptor with endogenous arrestin2 and 3 in neostriatal neurons. Endogenous arrestin2 and 3 in striatal homogenates bound to the C-terminus of the D1 receptor in a glutathione-S-transferase (GST) pulldown assay, with arrestin3 binding more strongly. The D1 C-terminus and, to a lesser extent, the third cytoplasmic loop also bound purified arrestin2 and 3. In neostriatal neurons, 2, 5, and 20 min agonist treatment increased the colocalization of the D1 receptor and arrestin3 immunoreactivity without altering the colocalization of the D1 receptor and arrestin2. Further, agonist treatment for 5 and 20 min caused translocation of arrestin3, but not arrestin2, to the membrane. The binding of arrestin3, but not arrestin2, to the D1 receptor was increased as assessed by coimmunoprecipitation after agonist treatment for 5 and 20 min. Agonist treatment of neurons induced D1 receptor internalization (35-45%) that was maximal within 2-5 min, a time-course similar to that of the increase in colocalization of the D1 receptor with arrestin3. These data indicate that the D1 receptor preferentially interacts with arrestin3 in neostriatal neurons.

Regulation of expression of the receptors controlling gastric acidity

Gastric acid secretion is regulated by the stimulatory effects of gastrin, histamine and acetylcholine and the inhibitory actions of somatostatin on their respective receptors. We proposed that the expression of these receptors could be regulated at the transcription level by agonists and antagonists known to effect acid secretion. A quantitative "real-time" PCR method was used to determine changes in mRNA expression for these receptors. The agonists, pentagastrin and histamine, and the H2 antagonist, ranitidine, were infused over a 6 h period to conscious sheep. Blood, antral and fundic tissue samples were taken for analysis. Both pentagastrin and histamine resulted in elevated plasma somatostatin concentrations during the treatment. Ranitidine stimulated a fourfold increase in plasma gastrin while histamine caused a transient decrease. Except for an increase in antral gastrin following ranitidine infusion, there was no significant change in gastric gastrin and somatostatin concentration. Histamine (H2) receptor mRNA expression in the antrum was significantly increased by pentagastrin and decreased by ranitidine. Pentagastrin also stimulated a significant increase in the level of muscarinic (M3) receptor mRNA in the antrum. Antral somatostatin II receptor mRNA was significantly decreased by histamine. In the fundus, pentagastrin infusion resulted in a significant increase in histamine receptor mRNA and a decrease in the muscarinic receptor mRNA. This work demonstrates that the receptors involved in the regulation of acid secretion can be regulated by local events.

High-affinity neurotensin receptor is involved in phosphoinositide hydrolysis stimulation by carbachol in neonatal rat brain

Ontogenetic studies indicate that inositol phosphate accumulation in rodent brain tissue by cholinergic muscarinic agonists as well as expression of high-affinity neurotensin receptor (NTS1) peak at 7 days after birth. Herein, potential participation of this receptor in such effect was investigated. Cerebral cortex prisms of 7-day-old rats were preloaded with [3H]myoinositol and later incubated during 60 or 20 min in the presence of muscarinic agonist carbachol plus neurotensin and SR 48692, a non-peptide NTS1 antagonist. In 60-min incubation experiments, inositol phosphate accumulation by 10(-3) M carbachol was roughly 320%, an effect which remained unaltered plus 10(-6) M to 10(-4) M neurotensin but partially decreased with equimolar SR 48692 concentration. In 20-min incubation experiments, inositol phosphate accumulation by 10(-3) M carbachol was circa 240%, a value which attained 320-360% plus 10(-7) M neurotensin; this effect was totally blocked by 10(-7) M SR 48692. It was concluded that in inositol phosphate accumulation by carbachol, besides the cholinergic muscarinic receptor, the NTS1 receptor is likewise involved; findings at 60 min are attributable to the effect of endogenous neurotensin whereas those at 20 min most likely involve both endogenous and exogenously added peptide.

Administration of haloperidol with biperiden reduces mRNAs related to the ubiquitin-proteasome system in mice

In order to find molecules affected by administration of an antipsychotic drug with an antimuscarinic drug, which is a common prescription used to prevent extrapyramidal adverse effects caused by the antipsychotic drugs, gene expression profiling in the frontal cortex was studied in mice. After 14 days of administration with 2 mg/kg haloperidol, a typical antipsychotic drug, and 2 mg/kg biperiden, a high-affinity antagonist for muscarinic receptors in the brain, approximately 500 mRNAs related to synaptic function were investigated. The levels of the mRNAs related to the ubiquitin-related systems were significantly reduced after the combined administration. However, the separate administration of either haloperidol or biperiden had little effect on the levels of the mRNAs. This result suggests that coadministration of haloperidol and biperiden specifically affects the ubiquitin-related system.

Free fatty acids regulate gut incretin glucagon-like peptide-1 secretion through GPR120

Diabetes, a disease in which the body does not produce or use insulin properly, is a serious global health problem. Gut polypeptides secreted in response to food intake, such as glucagon-like peptide-1 (GLP-1), are potent incretin hormones that enhance the glucose-dependent secretion of insulin from pancreatic beta cells. Free fatty acids (FFAs) provide an important energy source and also act as signaling molecules in various cellular processes, including the secretion of gut incretin peptides. Here we show that a G-protein-coupled receptor, GPR120, which is abundantly expressed in intestine, functions as a receptor for unsaturated long-chain FFAs. Furthermore, we show that the stimulation of GPR120 by FFAs promotes the secretion of GLP-1 in vitro and in vivo, and increases circulating insulin. Because GLP-1 is the most potent insulinotropic incretin, our results indicate that GPR120-mediated GLP-1 secretion induced by dietary FFAs is important in the treatment of diabetes.

Type I PDZ ligands are sufficient to promote rapid recycling of G Protein-coupled receptors independent of binding to N-ethylmaleimide-sensitive factor

Molecular sorting of G protein-coupled receptors (GPCRs) between divergent recycling and lysosomal pathways determines the functional consequences of agonist-induced endocytosis. The carboxyl-terminal cytoplasmic domain of the beta2 adrenergic receptor (beta2AR) mediates both PDZ binding to Na+/H+ exchanger regulatory factor/ezrin/radixin/moesin-binding phosphoprotein of 50 kDa (NHERF/EBP50) family proteins and non-PDZ binding to the N-ethylmaleimide-sensitive factor (NSF). We have investigated whether PDZ interaction(s) are actually sufficient to promote rapid recycling of endocytosed receptors and, if so, whether PDZ-mediated sorting is restricted to the beta2AR tail or to sequences that bind NHERF/EBP50. The trafficking effects of short (10 residue) sequences differing in PDZ and NSF binding properties were examined using chimeric mutant receptors. The recycling activity of the beta2AR-derived tail sequence was not blocked by a point mutation that selectively disrupts binding to NSF, and naturally occurring PDZ ligand sequences were identified that do not bind detectably to NSF yet function as strong recycling signals. The carboxyl-terminal cytoplasmic domain of the beta1-adrenergic receptor, which does not bind either to NSF or NHERF/EBP50 and interacts selectively with a distinct group of PDZ proteins, promoted rapid recycling of chimeric mutant receptors with efficiency similarly high as that of the beta2AR tail. These results indicate that PDZ domain-mediated protein interactions are sufficient to promote rapid recycling of GPCRs, independent of binding to NSF. They also suggest that PDZ-directed recycling is a rather general mechanism of GPCR regulation, which is not restricted to a single GPCR, and may involve additional PDZ domain-containing protein(s) besides NHERF/EBP50.

A novel C-terminal motif is necessary for the export of the vasopressin V1b/V3 receptor to the plasma membrane

Little is known about endoplasmic reticulum (ER) export signals, particularly those of members of the G-protein-coupled receptor family. We investigated the structural motifs involved in membrane export of the human pituitary vasopressin V1b/V3 receptor. A series of V3 receptors carrying deletions and point mutations were expressed in AtT20 corticotroph cells. We analyzed the export of these receptors by monitoring radioligand binding and by analysis of a V3 receptor tagged with both green fluorescent protein and Myc epitopes by a novel flow cytometry-based method. This novel method allowed us to quantify total and membrane-bound receptor expression. Receptors lacking the C terminus were not expressed at the cell surface, suggesting the presence of an export motif in this domain. The distal C terminus contains two di-acidic (DXE) ER export motifs; however, mutating both these motifs had no effect on the V3 receptor export. The proximal C terminus contains a di-leucine (345)LL(346) motif surrounded by the hydrophobic residues Phe(341), Asn(342), and Leu(350). The mutation of one or more of these five residues abolished up to 100% of the receptor export. In addition, these mutants colocalized with calnexin, demonstrating that they were retained in the ER. Finally, this motif was sufficient to confer export properties on a CD8alpha glycoprotein-V3 receptor chimera. In conclusion, we have identified a novel export motif, FN(X)(2)LL(X)(3)L, in the C terminus of the V3 receptor.

Neural stem cells express melatonin receptors and neurotrophic factors: colocalization of the MT1 receptor with neuronal and glial markers

Background

In order to optimize the potential benefits of neural stem cell (NSC) transplantation for the treatment of neurodegenerative disorders, it is necessary to understand their biological characteristics. Although neurotrophin transduction strategies are promising, alternative approaches such as the modulation of intrinsic neurotrophin expression by NSCs, could also be beneficial. Therefore, utilizing the C17.2 neural stem cell line, we have examined the expression of selected neurotrophic factors under different in vitro conditions. In view of recent evidence suggesting a role for the pineal hormone melatonin in vertebrate development, it was also of interest to determine whether its G protein-coupled MT₁ and MT₂ receptors are expressed in NSCs.

Results

RT-PCR analysis revealed robust expression of glial cell-line derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in undifferentiated cells maintained for two days in culture. After one week, differentiating cells continued to exhibit high expression of BDNF and NGF, but GDNF expression was lower or absent, depending on the culture conditions utilized. Melatonin MT₁ receptor mRNA was detected in NSCs maintained for two days in culture, but the MT₂ receptor was not seen. An immature MT₁ receptor of about 30 kDa was detected by western blotting in NSCs cultured for two days, whereas a mature receptor of about 40 – 45 kDa was present in cells maintained for longer periods. Immunocytochemical studies demonstrated that the MT₁ receptor is expressed in both neural (β-tubulin III positive) and glial (GFAP positive) progenitor cells. An examination of the effects of melatonin on neurotrophin expression revealed that low physiological concentrations of this hormone caused a significant induction of GDNF mRNA expression in NSCs following treatment for 24 hours.

Conclusions

The phenotypic characteristics of C17.2 cells suggest that they are a heterogeneous population of NSCs including both neural and glial progenitors, as observed under the cell culture conditions used in this study. These NSCs have an intrinsic ability to express neurotrophic factors, with an apparent suppression of GDNF expression after several days in culture. The detection of melatonin receptors in neural stem/progenitor cells suggests involvement of this pleiotropic hormone in mammalian neurodevelopment. Moreover, the ability of melatonin to induce GDNF expression in C17.2 cells supports a functional role for the MT₁ receptor expressed in these NSCs. In view of the potency of GDNF in promoting the survival of dopaminergic neurons, these novel findings have implications for the utilization of melatonin in neuroprotective strategies, especially in Parkinson's disease.

Resistance of the human beta1-adrenergic receptor to agonist-induced ubiquitination: a mechanism for impaired receptor degradation

Down-regulation is a classic response of most G protein-coupled receptors to prolonged agonist stimulation. We recently showed that when expressed in baby hamster kidney cells, the human beta1-but not the beta2-adrenergic receptor (AR) is totally resistant to agonist-mediated down-regulation, whereas both have similar rates of basal degradation (Liang, W., Austin, S., Hoang, Q., and Fishman, P. H. (2003) J. Biol. Chem. 278, 39773-39781). To identify the underlying mechanism(s) for this resistance, we investigated the role of proteasomes, lysosomes, and ubiquitination in the degradation of beta1AR expressed in baby hamster kidney and human embryonic kidney 293 cells. Both lysosomal and proteasomal inhibitors reduced beta1AR degradation in agonist-stimulated cells but were less effective on basal degradation. To determine whether beta1AR trafficked to lysosomes we used confocal fluorescence microscopy. We observed some colocalization of beta1AR and lysosomal markers in agonist-treated cells but much less than that of beta2AR even in cells co-transfected with arrestin-2, which increases beta1AR internalization. Ubiquitination of beta2AR readily occurred in agonist-stimulated cells, whereas ubiquitination of beta1AR was not detectable even under conditions optimal for that of beta2AR. Moreover, in cells expressing betaAR chimeras in which the C termini have been switched, the chimeric beta1AR with beta2AR C-tail underwent ubiquitination and down-regulation, but the chimeric beta2AR with beta1AR C-tail did not. Our results demonstrate for the first time that beta1AR and beta2AR differ in the ability to be ubiquitinated. Because ubiquitin serves as a signal for sorting membrane receptors to lysosomes, the lack of agonist-mediated ubiquitination of beta1AR may prevent its extensive trafficking to lysosomes and, thus, account for its resistance to down-regulation.

Sorting nexin 16 regulates EGF receptor trafficking by phosphatidylinositol-3-phosphate interaction with the Phox domain

Sorting nexins (SNXs) containing the Phox (PX) domain are implicated in the regulation of membrane trafficking and sorting processes of epithelial growth factor receptor (EGFR). In this study, we investigated whether SNX16 regulates EGF-induced cell signaling by regulating EGFR trafficking. SNX16 is localized in early and recycling endosomes via its PX domain. Mutation of the PX domain disrupted the association between SNX16 and phosphatidylinositol 3-phosphate [PtdIns(3)P]. Treatment with wortmannin, a PtdIns 3-kinase inhibitor, abolished the endosomal localization of SNX16, suggesting that the intracellular localization of SNX16 is regulated by PtdIns 3-kinase activity. SNX16 was found to associate with EGFR after stimulation with EGF in COS-7 cells. Moreover, overexpression of SNX16 increased the rate of EGF-induced EGFR degradation and inhibited the EGF-induced up-regulation of ERK and serum response element (SRE). In addition, mutation in the PX domain significantly blocked the inhibitory effect of SNX16 on EGF-induced activation of ERK and SRE. From these results, we suggest that SNX16 directs the sorting of EGFR to the endosomal compartment and thus regulates EGF-induced cell signaling.

Agonist-induced desensitization and endocytosis of heterodimeric GABAB receptors in CHO-K1 cells

gamma-Aminobutyric acid B (GABA(B)) receptor is the first discovered G protein-coupled receptor that requires two subunits, GB1 and GB2, to form a functional receptor. Whereas the molecular and functional characteristics of GABA(B) receptors have been recently extensively studied, the mechanisms underlying receptor desensitization and endocytosis are still poorly understood. We have investigated the effect of continuous agonist exposure on the human GABA(B) receptor functional response and redistribution when expressed in Chinese hamster ovary (CHO-K1) cells. The wild-type GABA(B) receptor-mediated inhibition of the adenylate cyclase activity appeared desensitized after 2 h in the presence of GABA (100 microM). Fusion proteins were generated by attachment of cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) to GB1 and GB2, respectively, and confocal microscopy experiments in intact living cells semi-stably expressing the constructs were performed. Incubation of co-expressing CFP-GB1 and YFP-GB2 cells in the presence of GABA (100 microM) for 2 h induced a profound receptor internalization, and CFP-GB1 and YFP-GB2 appeared co-localized in the endosome (labelled with Cy3-transferrin). The internalization was blocked by a selective GABA(B) receptor antagonist. These results represent the first clear visualization of agonist-induced internalization of the unique heterodimeric GABA(B) receptor.

Two threonine residues and two serine residues in the second and third intracellular loops are both involved in histamine H1receptor downregulation

Human histamine H1 receptor (H1R) contains five possible phosphorylation residues (Thr140, Thr142, Ser396, Ser398 and Thr478) and the substitution of all these five residues to alanine completely impairs agonist-induced receptor downregulation. In the present study, to determine which residue(s) are responsible for receptor downregulation, we used mutant H1Rs in which single or multiple residues were substituted with alanine. The results suggested that two groups, i.e., residues Thr140 and Thr142, and residues Ser396 and Ser398, independently contributed to H1R downregulation. Thr140 and Ser398 mainly contributed to downregulation, and Thr142 or Ser396 had a slight inhibitory effect on Thr140- or Ser398-mediated process, respectively.

Novel roles for arrestins in the post-endocytic trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane signaling molecules in the human genome. As such, they interact with numerous intracellular molecules, which can act either to propagate or curtail signaling from the receptor. Their primary mode of cellular activation occurs through heterotrimeric G proteins, which in turn can activate a wide spectrum of effector molecules, including phosphodiesterases, phospholipases, adenylyl cyclases and ion channels. Active GPCRs are also the target of G protein-coupled receptor kinases, which phosphorylate the receptors culminating in the binding of the protein arrestin. This results in rapid desensitization through inhibition of G protein binding, as well as novel mechanisms of cellular activation that involve the scaffolding of cellular kinases to GPCR-arrestin complexes. Arrestins can also serve to mediate the internalization of certain GPCRs, a process which plays an important role in regulating cellular activity both by mediating long-term desensitization through down regulation (degradation) of receptors and by recycling desensitized receptors back to the cell surface to initiate additional rounds of signaling. The mechanisms that regulate the subsequent intracellular trafficking of GPCRs following internalization are largely unknown. Recently however, it has become clear that the pattern of receptor phosphorylation and subsequent binding of arrestin play a critical role in the intracellular trafficking of internalized receptors, thereby dictating the ultimate fate of the receptor. In addition, arrestins have now been shown to be required for the recycling of GPCRs that are capable of internalizing through arrestin-independent mechanisms. This review will summarize recent advances in our understanding of the roles of arrestins in post-endocytic GPCR trafficking.

Constitutive endocytic cycle of the CB1 cannabinoid receptor

The CB1 cannabinoid receptor (CB1R) displays a significant level of ligand-independent (i.e. constitutive) activity, either when heterologously expressed in nonneuronal cells or in neurons where CB1Rs are endogenous. The present study investigates the consequences of constitutive activity on the intracellular trafficking of CB1R. When transfected in HEK-293 cells, CB1R is present at the plasma membrane, but a substantial proportion ( approximately 85%) of receptors is localized in intracellular vesicles. Detailed analysis of CB1-EGFP expressed in HEK-293 cells shows that the intracellular CB1R population is mostly of endocytic origin and that treatment with inverse agonist AM281 traps CB1R at the plasma membrane through a monensin-sensitive recycling pathway. Co-transfection with dominant positive or dominant negative mutants of the small GTPases Rab5 and Rab4, but not Rab11, profoundly modifies the steady-state and ligand-induced intracellular distribution of CB1R, indicating that constitutive endocytosis is Rab5-dependent, whereas constitutive recycling is mediated by Rab4. In conclusion, our results indicate that, due to its natural constitutive activity, CB1R permanently and constitutively cycles between plasma membrane and endosomes, leading to a predominantly intracellular localization at steady state.

Fluorescent Vasotocin Conjugate for Identification of the Target Cells for Brain Actions of Vasotocin

The effects of neuropeptides on the brain are a major focus of neuroendocrine research, and little progress has been made in the identification of the target neurons for many neuropeptides. Arginine8-vasotocin (AVT) is a neurohypophyseal peptide present in nonmammalian vertebrates that controls many neural and behavioral functions. Here we describe synthesis and functional characterization of an AVT-Oregon green conjugate 1 (AVT-OG 1) that can be used in vivo to identify AVT target neurons. Application of AVT-OG 1 to the brainstem of an amphibian produces rapid, endosome-like internalization together with typical AVT-like neurophysiological effects. Thus, preparation of AVT-OG 1, which preserves the peptide's neurophysiological effects, is useful as a fluorescent marker for AVT target neurons. Consequently, AVT-OG 1 conjugate will have considerable utility for analyzing the neural actions of AVT in the intact brain.

Microanatomy of dendritic spines: emerging principles of synaptic pathology in psychiatric and neurological disease

Psychiatric and neurologic disorders ranging from mental retardation to addiction are accompanied by structural and functional alterations of synaptic connections in the brain. Such alterations include abnormal density and morphology of dendritic spines, synapse loss, and aberrant synaptic signaling and plasticity. Recent work is revealing an unexpectedly complex biochemical and subcellular organization of dendritic spines. In this review, we highlight the molecular interplay between functional domains of the spine, including the postsynaptic density, the actin cytoskeleton, and membrane trafficking domains. This research points to an emerging level of analysis--a microanatomical understanding of synaptic physiology--that will be critical for discerning how synapses operate in normal physiologic states and for identifying and reversing microscopic changes in psychiatric and neurologic disease.

pH-Independent Endocytic Cycling of the Chemokine Receptor CCR5

Following agonist activation, the chemokine receptor CCR5 is internalised through clathrin-coated pits and delivered to recycling endosomes. Subsequently, ligand- free and resensitised receptors are recycled to the cell surface. Currently little is known of the mechanisms regulating resensitisation and recycling of this G-protein coupled receptor. Here we show that raising the pH of endocytic compartments, using bafilomycin A, monensin or NH(4)Cl, does not significantly affect CCR5 endocytosis, recycling or dephosphorylation. By contrast, these reagents inhibited recycling of another well-characterised G protein coupled receptor, the beta(2)-adrenergic receptor, following agonist-induced internalisation. CCR5-bound RANTES (CCL5) and MIP-1beta (CCL4) only exhibit pH-dependent dissociation at pH < 4.0, below the values normally found in endocytic organelles. Although receptor-agonist dissociation is not dependent on low pH, the subsequent degradation of released chemokine is inhibited in the presence of reagents that raise endosomal pH. Our data show that exposure to low pH is not required for RANTES or MIP-1beta dissociation from CCR5, or for recycling of internalised CCR5 to the cell surface.

Role of the Cyclic AMP-dependent Protein Kinase in Homologous Resensitization of the β1-Adrenergic Receptor

A fundamental question in biology is how the various motifs in G protein-coupled receptors participate in the divergent functions orchestrated by these molecules. Here we describe a fundamental role for a serine residue at position 312 in the third intracellular loop of the human beta(1)-adrenergic receptor (beta(1)-AR) in endocytic recycling of the agonist-internalized receptor. In receptor recycling experiments that were monitored by confocal microscopy, the agonist-internalized wild-type (WT) beta(1)-AR recycled with a t(0.5) of 14 +/- 3 min. Mutagenesis of Ser(312) to alanine (Ser(312) --> Ala beta(1)-AR) or to the phosphoserine mimic aspartic acid (Ser(312) --> Asp beta(1)-AR) resulted in beta(1)-AR constructs that were pharmacologically indistinguishable from the WT beta(1)-AR. The internalized Ser(312) --> Asp beta(1)-AR recycled efficiently with a t(0.5) of 11 +/- 3 min, whereas the internalized Ser(312) --> Ala beta(1)-AR was not recycled or functionally resensitized through the endosomal pathway. Because this serine is a putative residue for phosphorylation by the cyclic AMP-dependent protein kinase (PKA), we examined the role of this kinase in recycling of the internalized beta(1)-AR. Inhibition of PKA biochemically or genetically using a dominant negative PKA construct blocked the recycling of the internalized WT beta(1)-AR. Phosphorylation studies revealed that the beta(1)-AR is partially phosphorylated by PKA and that phosphorylation of the beta(1)-AR by the catalytic subunit of PKA occurs exclusively at Ser(312). Our results identify a new signaling paradigm in which homologous activation of a kinase provides a reversible modification that shifts the itinerary of the internalized receptor toward recycling and resensitization. Therefore, PKA-mediated phosphorylation of G protein-coupled receptors might result in motif-dependent desensitization or resensitization.

The chemokine receptor D6 constitutively traffics to and from the cell surface to internalize and degrade chemokines

The D6 heptahelical membrane protein, expressed by lymphatic endothelial cells, is able to bind with high affinity to multiple proinflammatory CC chemokines. However, this binding does not allow D6 to couple to the signaling pathways activated by typical chemokine receptors such as CC-chemokine receptor-5 (CCR5). Here, we show that D6, like CCR5, can rapidly internalize chemokines. However, D6-internalized chemokines are more effectively retained intracellularly because they more readily dissociate from the receptor during vesicle acidification. These chemokines are then degraded while the receptor recycles to the cell surface. Interestingly, D6-mediated chemokine internalization occurs without bringing about a reduction in cell surface D6 levels. This is possible because unlike CCR5, D6 is predominantly localized in recycling endosomes capable of trafficking to and from the cell surface in the absence of ligand. When chemokine is present, it can enter the cells associated with D6 already destined for internalization. By this mechanism, D6 can target chemokines for degradation without the necessity for cell signaling, and without desensitizing the cell to subsequent chemokine exposure.

Important role for the V-type H(+)-ATPase and the Golgi apparatus in the recycling of PTH/PTHrP receptor

Our previous studies demonstrated that a green fluorescent protein-tagged parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor stably expressed in LLCPK-1 cells undergoes agonist-dependent internalization into clathrin-coated pits. The subcellular localization of the internalized PTH/PTHrP receptor is not known. In the present study, we explored the intracellular pathways of the internalized PTH/PTHrP receptor. Using immunofluorescence and confocal microscopy, we show that the internalized receptors localize at a juxtanuclear compartment identified as the Golgi apparatus. The receptors do not colocalize with lysosomes. Furthermore, whereas the internalized receptors exhibit rapid recycling, treatment with proton pump inhibitors (bafilomycin-A1 and concanamycin A) or brefeldin A, Golgi disrupting agents, reduces PTH/PTHrP receptor recycling. Together, these data indicate an important role for the vacuolar-type hydrogen-ATPase and the Golgi apparatus in postendocytic PTH/PTHrP receptor recovery.

Plasticity of somatostatin and somatostatin sst2A receptors in the rat dentate gyrus during kindling epileptogenesis

Increasing evidence suggests that somatostatin may control neuronal excitability during epileptogenesis. In the hippocampus, sst2A receptors are likely to mediate somatostatin inhibitory actions but little is known about their status in kindled tissues. In the present study, sst2A receptor and somatostatin immunoreactivity were examined by confocal microscopy in the hippocampus during and after kindling acquisition. In control rats, somatostatin-positive axon terminals were mainly found in the stratum lacunosum moleculare of CA1 area and in the outer molecular layer of the dentate gyrus. sst2A receptor immunoreactivity was diffusely distributed in the strata radiatum and oriens of CA1 and in the stratum moleculare of the dentate gyrus. Immunogold electron microscopy revealed that sst2A receptors were predominantly localized postsynaptically, at the plasma membrane of dendritic shafts and spines of principal neurons. During kindling epileptogenesis, qualitative and semiquantitative analysis revealed a progressive decrease of sst2A immunoreactivity in the outer molecular layer, which was spatially associated with an increase in somatostatin immunoreactivity. No obvious changes in sst2A receptor immunoreactivity were observed in other hippocampal subfields. These results suggest that the decrease of sst2A receptor immunoreactivity in the outer molecular layer reflects receptor down-regulation in distal dendrites of granule cells in response to chronic somatostatin release. Because the sst2A receptor appears to mediate anticonvulsant and antiepileptogenic effects of somatostatin, this may represent a pivotal mechanism contributing to epileptogenesis.

Role of mammalian vacuolar protein-sorting proteins in endocytic trafficking of a non-ubiquitinated G protein-coupled receptor to lysosomes

Many signaling receptors require covalent modification by ubiquitin for agonist-induced down-regulation via endocytic trafficking to lysosomes, a process that is mediated by a conserved set of endosome-associating proteins also required for vacuolar protein-sorting (VPS) in yeast. The delta opioid receptor (DOR) is a G protein-coupled receptor that can undergo agonist-induced proteolysis via endocytic trafficking to lysosomes but does not require covalent modification by ubiquitin to do so. This raises the question of whether lysosomal down-regulation of this "ubiquitination-independent" GPCR is mediated by a completely distinct biochemical mechanism or if similar VPS machinery is involved. Agonist-induced proteolysis of DOR was significantly inhibited by dominant negative mutant versions of Vps4/Skd1, an AAA-family ATPase required for a late step in lysosomal sorting of ubiquitinated membrane cargo. Furthermore, overexpression and interfering RNA-mediated knockdown indicated that lysosomal trafficking of opioid receptors is also dependent on Hrs, a VPS protein that mediates an early step in lysosomal sorting of ubiquitinated cargo. However, interfering RNA-mediated knockdown of Tsg101, a VPS protein that is essential for an intermediate step of the conserved lysosomal sorting mechanism, did not detectably affect agonist-induced proteolysis of DOR in the same cells in which (ubiquitination-dependent) lysosomal trafficking of epidermal growth factor receptors was clearly inhibited. These results indicate that opioid receptors, despite their ability to undergo efficient agonist-induced trafficking to lysosomes in the absence of covalent modification by ubiquitin, utilize some (Vps4 and Hrs) but perhaps not all (Tsg101) of the VPS machinery required for lysosomal sorting of ubiquitinated membrane cargo.

Functional interaction among opioid receptor types: up-regulation of μ- and δ-opioid receptor functions after repeated stimulation of κ-opioid receptors

It has been widely accepted that repeated administration of kappa-opioid receptor agonists leads to the development of antinociceptive tolerance. The present study was designed to investigate the effect of repeated administration of a selective kappa-opioid receptor agonist (1S-trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide hydrochloride ((-)U-50,488H) on the mu- and delta-opioid receptor agonist-induced antinociception and G-protein activation in mice. The mice were injected either subcutaneously (s.c.) or intracerebroventricularly (i.c.v.) pretreated with saline or (-)U-50,488H once a day for seven consecutive days. Two hours after the last injection, the mice were challenged by either mu- or delta-opioid receptor agonist for the antinociceptive assay. Repeated treatment with (-)U-50,488H (s.c. or i.c.v.) significantly enhanced antinociceptive effect of both mu-opioid receptor agonist (morphine) and delta-opioid receptor agonists ([d-Ala2]deltorphin (DELT) and (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dime thyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80) compared to saline-treated groups. Under these conditions, repeated s.c. injection of (-)U-50,488H significantly enhanced both mu- and delta-opioid receptor agonist-stimulated [35S]GTPgammaS binding in the membrane of the thalamus. On the contrary, either repeated administration of morphine (s.c. or i.c.v.) or SNC-80 failed to affect the kappa-opioid receptor agonist-induced antinociception and G-protein activation. Taken together, these results suggest that repeated stimulation of kappa-opioid receptor markedly increases the functional mu- and delta-opioid receptors, whereas repeated stimulation of either mu- or delta-opioid receptor had no direct effect on kappa-opioidergic function in mice.

Differences in endosomal targeting of human (beta)1- and (beta)2-adrenergic receptors following clathrin-mediated endocytosis

The beta(2)-adrenergic receptor (beta(2)AR) undergoes agonist-mediated endocytosis via clathrin-coated pits by a process dependent on both arrestins and dynamin. Internalization of some G protein-coupled receptors, however, is independent of arrestins and/or dynamin and through other membrane microdomains such as caveolae or lipid rafts. The human beta(1)AR is less susceptible to agonist-mediated internalization than the beta(2)-subtype, and its endocytic route, which is unknown, may be different. We have found that (i) co-expression of arrestin-2 or -3 enhanced the internalization of both subtypes whereas co-expression of dominant-negative mutants of arrestin-2 or dynamin impaired their internalization, as did inhibitors of clathrin-mediated endocytosis. (ii) Agonist stimulation increased the phosphorylation of beta(2)AR but not beta(1)AR. (iii) In response to agonist, each subtype redistributed from the cell surface to a distinct population of cytoplasmic vesicles; those containing beta(1)AR were smaller and closer to the plasma membrane whereas those containing beta(2)AR were larger and more perinuclear. (iv) When subcellular fractions from agonist-treated cells were separated by sucrose density gradient centrifugation, all of the internalized beta(2)AR appeared in the lighter endosomal-containing fractions whereas some of the internalized beta(1)AR remained in the denser plasma membrane-containing fractions. (v) Both subtypes recycled with similar kinetics back to the cell surface upon removal of agonist; however, recycling of beta(2)AR but not beta(1)AR was inhibited by monensin. Based on these results, we propose that the internalization of beta(1)AR is both arrestin- and dynamin-dependent and follows the same clathrin-mediated endocytic pathway as beta(2)AR. But during or after endocytosis, beta(1)AR and beta(2)AR are sorted into different endosomal compartments.

Phosphorylation and chronic agonist treatment atypically modulate GABAB receptor cell surface stability

GABA(B) receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. The dynamic control of the cell surface stability of GABA(B) receptors is likely to be of fundamental importance in the modulation of receptor signaling. Presently, however, this process is poorly understood. Here we demonstrate that GABA(B) receptors are remarkably stable at the plasma membrane showing little basal endocytosis in cultured cortical and hippocampal neurons. In addition, we show that exposure to baclofen, a well characterized GABA(B) receptor agonist, fails to enhance GABA(B) receptor endocytosis. Lack of receptor internalization in neurons correlates with an absence of agonist-induced phosphorylation and lack of arrestin recruitment in heterologous systems. We also demonstrate that chronic exposure to baclofen selectively promotes endocytosis-independent GABA(B) receptor degradation. The effect of baclofen can be attenuated by activation of cAMP-dependent protein kinase or co-stimulation of beta-adrenergic receptors. Furthermore, we show that increased degradation rates are correlated with reduced receptor phosphorylation at serine 892 in GABA(B)R2. Our results support a model in which GABA(B)R2 phosphorylation specifically stabilizes surface GABA(B) receptors in neurons. We propose that signaling pathways that regulate cAMP levels in neurons may have profound effects on the tonic synaptic inhibition by modulating the availability of GABA(B) receptors.

Activation of μ opioid receptors in the medial preoptic area following copulation in male rats

The current study tested the hypothesis that sexual behavior is a biological stimulus for release of endogenous opioid peptides. In particular, activation of mu opioid receptors (MOR) in the medial preoptic area (MPOA), a key area for regulation of male sexual behavior, was studied in male rats. MOR endocytosis or internalization was used as a marker for ligand-induced receptor activation, utilizing confocal, electron, and bright microscopic analysis. Indeed, mating including one ejaculation induced receptor activation in the MPOA, demonstrated by increased immunoreactivity for MOR, increased numbers of endosome-like particles immunoreactive for MOR inside the cytoplasm of neurons, and increased percentage of neurons with three or more endosome-like particles inside the cytosol. Moreover, it was demonstrated that MOR activation occurred within 30 min following mating and was still evident after 6 h. Mating-induced internalization was prevented by treatment with the opioid receptor antagonist naloxone before mating, suggesting that mating-induced receptor activation is a result of action of endogenous MOR ligands. i.c.v. injections of MOR ligand [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin resulted in internalization of the MOR in a similar manner observed following mating. Finally, mating induced Fos expression in MOR containing neurons in the MPOA. However, naloxone pretreatment did not prevent Fos activation of MOR neurons, suggesting that Fos induction was not the result of MOR activation. In summary, these results provide further evidence that endogenous opioid peptides are released in the MPOA during male sexual behavior.

Identification of Amino Acid Residues Responsible for Agonist-Induced Down-Regulation of Histamine H1 Receptors

The histamine H(1) receptor (H1R) level is dynamically regulated in vivo under various physiological and pathological conditions. The H1R regulation may consist of various processes, and this study focused on the process of receptor trafficking, that is, receptor internalization to endosomes and the following receptor degradation. First, we identified five possible phosphorylation residues of human H1R, Thr(140), Thr(142), Ser(396), Ser(398), and Thr(478), based on in vitro phosphorylation studies. Then to determine the role of these residues, we constructed a mutant H1R in which all of these five residues were substituted with alanine. Both wild-type and the mutant receptors expressed in Chinese hamster ovary (CHO) cells had similar values of K(d) for [(3)H]mepyramine binding and K(i) for histamine, and these cells showed similar levels of histamine-stimulated inositol phosphate formation. Both types of H1Rs were internalized essentially in the same way upon stimulation with histamine (100 microM) for 30 min. However, down-regulation of the mutant H1R was completely impaired, whereas that of wild-type H1R occurred by approximately 60% by the treatment with 100 microM histamine for 24 h. These results suggest that these residues are responsible for receptor down-regulation but not for receptor internalization. Possibly, phosphorylation of the residues is required for receptor transport from endosomes to lysosomes.

A cell biologist’s perspective on physiological adaptation to opiate drugs

Opiate drugs such as morphine and heroin are among the most effective analgesics known but are also highly addictive. The clinical utility of opiates is limited by adaptive changes in the nervous system occurring after prolonged or repeated drug administration. These adaptations are believed to play an important role in the development of physiological tolerance and dependence to opiates, and to contribute to additional changes underlying the complex neurobehavioral syndrome of drug addiction. All of these adaptive changes are initiated by the binding of opiate drugs to a subfamily of G protein-coupled receptors that are also activated by endogenously produced opioid neuropeptides. It is increasingly evident that opiate-induced adaptations occur at multiple levels in the nervous system, beginning with regulation of opioid receptors themselves and extending to a complex network of direct and indirect modifications of "downstream" signaling machinery. Efforts in my laboratory are directed at understanding the biochemical and cell biological basis of opiate adaptations. So far, we have focused primarily on adaptations occurring at the level of opioid receptors themselves. These studies have contributed to defining a set of membrane trafficking mechanisms by which the number and functional activity of opioid receptors are controlled. The role of these mechanisms in affecting adaptation of "downstream" neurobiological substrates, and in mediating opiate-induced changes in whole-animal physiology and behavior, are exciting questions that are only beginning to be explored.

Dopamine receptor downregulation: an alternative strategy for schizophrenia treatment

Schizophrenia is a common and devastating illness. The cause of schizophrenia is still unknown and the simplest formulation of the "Dopamine hypothesis" posits that schizophrenia results from dopaminergic hyperactivity. Under the hypothesis of dopaminergic hyperactivity in schizophrenia, antipsychotics blocking the dopamine D2 receptor (DRD2) and other approaches to reduce dopamine (DA) transmission have been used to treat schizophrenia. I propose that dopamine receptor (DR) downregulation could be an alternative strategy to compromise dopaminergic overactivity implicated in the pathogenesis of schizophrenia. Agonist-induced receptor downregulation includes receptor proteolysis, modulation of receptor gene transcription and affecting of RNA stability. These processes cause a decrease of existing receptors and reduction of receptor synthesis. This hypothesis could explain the antipsychotic mechanisms of DA agonists or partial agonists, like aripiprazole. It is suggested that the development of agents that increase DR downregulation could be an alternative strategy for schizophrenia treatment.

Environmental influences on signal transduction through membranes: a retrospective mini-review

This mini-review is addressed to the question how the membranous environment may affect traffic of receptors and signalling from membrane-anchored receptors on the outside of cells to transducers and targets on the inside. Signal transduction by membrane-anchored receptors to the interior of the cell and eventually to the genome is a central issue in cellular regulation. In this context the role of membrane fluidity and of the cytoskeleton in restricting the mobility of proteins are discussed and the evidence for a structural order in membranes which could limit the mobility of proteins is scrutinised.

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.

Postendocytotic trafficking of the follicle-stimulating hormone (FSH)-FSH receptor complex

Although the fates of the internalized hormone-receptor complexes formed by the lutropin/choriogonadotropin and the TSH receptors have been examined in some detail, much less is known about the fate of the internalized FSH-FSH receptor (FSHR) complex. Using biochemical and imaging approaches we show here that the majority of the internalized FSH-FSHR complex accumulates in endosomes and subsequently recycles back to the cell surface where the bound, intact hormone dissociates back into the medium. Only small amounts of FSH and the FSHR are routed to a lysosomal degradation pathway, and the extent of FSH-induced down-regulation of the cell surface and total FSHR is minimal. This pathway was detected in heterologous (human kidney 293T) cells transfected with the rat (r) or human (h) FSHR as well as in a mouse Sertoli cell line (MSC-1) or a mouse granulosa cell line (KK-1) transfected with the rFSHR.Additional experiments using a series of C-terminal deletions of the rFSHR and the hFSHR showed that the recycling of the internalized FSH-FSHR complex and the extent of hFSH-induced down-regulation is dictated by a short stretch of amino acids present at the extreme C-terminal end of the receptor.We conclude that most of the internalized FSH-FSHR complex is recycled back to the cell surface, that this recycling pathway is highly dependent on amino acid residues present near the C terminus of the FSHR, and that it is an important determinant of the extent of down-regulation of the FSHR.

Resistance of the human beta 1-adrenergic receptor to agonist-mediated down-regulation. Role of the C terminus in determining beta-subtype degradation

Prolonged agonist stimulation results in down-regulation of most G protein-coupled receptors. When we exposed baby hamster kidney cells stably expressing the human beta1-adrenergic receptor (beta 1AR) to agonist over a 24-h period, we instead observed an increase of approximately 30% in both beta 1AR binding activity and immune-detected receptors. In contrast, beta 2AR expressed in these cells exhibited a decrease of > or =50%. We determined that the basal turnover rates of the two subtypes were similar (t(1/2) approximately 7 h) and that agonist stimulation increased beta 2AR but not beta 1AR turnover. Blocking receptor trafficking to lysosomes with bafilomycin A1 had no effect on basal turnover of either subtype but blocked agonist-stimulated beta 2AR turnover. As beta 1AR mRNA levels increased in agonist-stimulated cells, beta 1AR up-regulation appeared to result from increased synthesis with no change in degradation. To explore the basis for the subtype differences, we expressed chimeras in which the C termini had been exchanged. Each chimera responded to persistent agonist stimulation based on the source of its C-tail; beta 1AR with a beta 2AR C-tail underwent down-regulation, and beta 2AR with a beta 1AR C-tail underwent up-regulation. The C-tails had a corresponding effect on agonist-stimulated receptor phosphorylation and internalization with the order being beta 2AR > beta 1AR with beta 2AR C-tail > beta 2AR with a beta 1AR C-tail > beta 1AR. As internalization may be a prerequisite for down-regulation, we addressed this possibility by co-expressing each subtype with arrestin-2. Although beta 1AR internalization was increased to that of beta 2AR, down-regulation still did not occur. Instead, beta 1AR accumulated inside the cells. We conclude that in unstimulated cells, both subtypes appear to be turned over by the same mechanism. Upon agonist stimulation, both subtypes are internalized, and beta 2AR but not beta 1AR undergoes lysosomal degradation, the fate of each subtype being regulated by determinants in its C-tail.

Locus-specific rescue of GluRepsilon1 NMDA receptors in mutant mice identifies the brain regions important for morphine tolerance and dependence

Tolerance and physical dependence caused by chronic treatment of narcotics are good models to study basic neuronal plasticity. Activation of the NMDA subtype of the glutamate receptor has been implicated as an anti-opioid system in the development of morphine analgesic tolerance and dependence. The present study examines the specific role of the epsilon1 subunit of the NMDA receptor using mice lacking the gene encoding epsilon1 subunit of the NMDA receptor (GluRepsilon1-/- mice). GluRepsilon1-/- mice showed significant enhancement and prolongation of morphine anti-nociception, compared with wild-type GluRepsilon1+/+ mice. GluRepsilon1-/- mice also showed a marked loss of the analgesic tolerance after repeated morphine treatments. In C57BL/6J mice treated with chronic morphine after tolerance paradigm, the GluRepsilon1 protein expression significantly increased in periaqueductal gray matter (PAG), ventral tegmental area (VTA) and nucleus accumbens (NAc), but not amygdala or hippocampus. The rescue of GluRepsilon1 protein by electroporation into the PAG and VTA, but not NAc of GluRepsilon1-/- mice significantly reversed morphine analgesic tolerance liability. Similar attempts were also performed in the naloxone-precipitated physical dependence paradigm. GluRepsilon1-/- mice showed marked loss of typical withdrawal abstinence behaviors, and significant enhancement of GluRepsilon1 protein expression was only observed in NAc by chronic morphine treatments after dependence paradigm. The rescue of GluRepsilon1 protein by electroporation into the NAc of GluRepsilon1-/- mice significantly reversed the loss of abstinence behaviors. These findings suggest that GluRepsilon1 has locus-specific roles in the development of morphine analgesic tolerance and physical dependence.

Multiple mechanisms of CB1 cannabinoid receptors regulation

Agonist-induced regulation of cannabinoid CB1 receptors was examined in HEK-293 cells transfected with CB1 receptors and in neuroblastoma N18TG2 cells that naturally express CB1 receptors. In HEK-293 cells, CB1 receptors internalization proceeded, in parallel, via clathrin-coated pits and caveolae. Simultaneous disruption of both pathways induced compensatory endocytic mechanism(s). In N18TG2 cells, endocytosis was not mediated by caveolae-like membrane domains. Heterologous, opioid-induced, downregulation of CB1 receptors was evident in HEK-293 but not N18TG2 cells. The data demonstrate the existence of multiple pathways of CB1 receptors regulation.

The dopamine transporter constitutively internalizes and recycles in a protein kinase C-regulated manner in stably transfected PC12 cell lines

The dopamine transporter (DAT) removes dopamine from the extracellular milieu and is potently inhibited by number of psychoactive drugs, including cocaine, amphetamines, and methylphenidate (Ritalin). Multiple lines of evidence demonstrate that protein kinase C (PKC) down-regulates dopamine transport, primarily by redistributing DAT from the plasma membrane to endosomal compartments, although the mechanisms facilitating transporter sequestration are not defined. Here, we demonstrate that DAT constitutively internalizes and recycles in rat pheochromocytoma (PC12) cells. Temperature blockades demonstrated basal internalization and reliance on recycling to maintain DAT cell surface levels. In contrast, recycling blockade with bafilomycin A1 significantly decreased transferrin receptor (TfR) surface expression but had no effect on DAT surface levels, suggesting that DAT and TfR traffic via distinct endosomal mechanisms. Kinetic analyses reveal robust constitutive DAT cycling to and from the plasma membrane, independent of transporter expression levels. In contrast, phorbol ester-mediated PKC activation accelerated DAT endocytosis and attenuated transporter recycling in a manner sensitive to DAT expression levels. These data demonstrate constitutive DAT trafficking and that PKC-mediated DAT sequestration is achieved by a combination of accelerated internalization and reduced recycling. Additionally, the differential sensitivity to expression level exhibited by constitutive and regulated DAT trafficking suggests that these two processes are mediated by independent cellular mechanisms.

A direct interaction of PSD-95 with 5-HT2A serotonin receptors regulates receptor trafficking and signal transduction

The serotonin (5-hydroxytryptamine) 2A receptor (5-HT2A) is an important G protein-coupled receptor (GPCR) that mediates the effects of hallucinogens and is the target of a number of commonly prescribed medications including atypical antipsychotics, antidepressants, and anxiolytics. The 5-HT2A receptor possesses a canonical Type I PDZ-binding domain (X-Ser/Thr-X-Phi) at the carboxyl terminus and has been predicted, but never demonstrated, to interact with PDZ domain-containing proteins. We discovered that PSD-95, a prototypic PDZ domain-containing protein, directly associates with the 5-HT2A receptor and regulates 5-HT2A receptor-mediated signaling and trafficking in HEK-293 cells. Co-immunoprecipitation studies revealed that the native 5-HT2A receptor, but not a mutant lacking the PDZ-binding domain, interacted directly with PSD-95. The association with PSD-95 enhanced 5-HT2A receptor-mediated signal transduction, a novel action of PSD-95 on GPCRs. The augmentation of 5-HT2A receptor signaling by PSD-95 was not accompanied by alteration in the kinetics of 5-HT2A receptor desensitization but was associated with the inhibition of agonist-induced 5-HT2A receptor internalization. Additional studies demonstrated that 5-HT2A receptor and PSD-95 were co-localized in clusters on the cell surface of HEK-293 cells. Taken together, the present work elucidates novel roles for PSD-95 in regulating the functional activity and intracellular trafficking of 5-HT2A receptors and possibly other GPCRs.

Trafficking of the muscarinic m2 autoreceptor in cholinergic basalocortical neurons in vivo: differential regulation of plasma membrane receptor availability and intraneuronal localization in acetylcholinesterase-deficient and -inhibited mice

In vivo, the abundance of receptors at the neuronal plasma membrane may be critical in the mediation of pre- and postsynaptic responses. Thus, we have studied the membrane availability and intraneuronal distribution of the m2 muscarinic autoreceptor (m2R) in cholinergic neurons of the nucleus basalis magnocellularis (NBM) projecting to the frontal cortex (FC). We have studied the subcellular compartmentalization of m2R at somatodendritic postsynaptic and axonal presynaptic sites in control animals (AChE +/+) and in two animal models: mice displaying acute acetylcholinesterase (AChE) inhibition by treatment with metrifonate, and AChE-deficient mice (AChE -/-). In control animals, m2R was mainly located at the plasma membrane in the somatodendritic field of NBM and in cortical varicosities. Acute AChE inhibition and chronic AChE deficiency induced a dramatic decrease of cell surface m2R in the somatodendritic compartment. This finding was associated with two different intracytoplasmic events: (1). internalization of m2R in endosomes after acute AChE inhibition, (2). exaggerated storage of m2R in the endoplasmic reticulum and Golgi complex in AChE -/- mice. In contrast, the m2R density was higher at the membrane of cortical varicosities in AChE -/- mice but unchanged in acutely AChE-inhibited mice. Our data demonstrate that acute and chronic stimulation provoke, in vivo, depletion of the membrane store of somatodendritic m2R through different intracellular mechanisms: endocytosis of receptors from the plasma membrane to the cytoplasm (acute) or regulation of their delivery from intracytoplasmic stores to the plasma membrane (chronic). The increase of m2R at the membrane of axonal varicosities after chronic stimulation suggest modulation of presynaptic cholinergic activity, including neurotransmitter release.

Galphaq-coupled receptor internalization specifically induced by Galphaq signaling. Regulation by EBP50

In the present report, we investigated the effect of ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) expression on the agonist-induced internalization of the thromboxane A(2) beta receptor (TPbeta receptor). Interestingly, we found that EBP50 almost completely blocked TPbeta receptor internalization, which could not be reversed by overexpression of G protein-coupled receptor (GPCR) kinases and arrestins. Because we recently demonstrated that EBP50 can bind to and inhibit Galpha(q), we next studied whether Galpha(q) signaling could induce TPbeta receptor internalization, addressing the long standing question about the relationship between GPCR signaling and their internalization. Expression of a constitutively active Galpha(q) mutant (Galpha(q)-R183C) resulted in a robust internalization of the TPbeta receptor, which was unaffected by expression of dominant negative mutants of arrestin-2 and -3, but inhibited by expression of EBP50 or dynamin-K44A, a dominant negative mutant of dynamin. Phospholipase Cbeta and protein kinase C did not appear to significantly contribute to internalization of the TPbeta receptor, suggesting that Galpha(q) induces receptor internalization through a phospholipase Cbeta- and protein kinase C-independent pathway. Surprisingly, there appears to be specificity in Galpha protein-mediated GPCR internalization. Galpha(q)-R183C also induced the internalization of CXCR4 (Galpha(q)-coupled), whereas it failed to do so for the beta(2)-adrenergic receptor (Galpha(s)-coupled). Moreover, Galpha(s)-R201C, a constitutively active form of Galpha(s), had no effect on internalization of the TPbeta, CXCR4, and beta(2)-adrenergic receptors. Thus, we showed that Galpha protein signaling can lead to internalization of GPCRs, with specificity in both the Galpha proteins and GPCRs that are involved. Furthermore, a new function has been described for EBP50 in its capacity to inhibit receptor endocytosis.

Dramatic depletion of cell surface m2 muscarinic receptor due to limited delivery from intracytoplasmic stores in neurons of acetylcholinesterase-deficient mice

We have studied the consequences of the constitutive acetylcholinesterase (AChE) deficiency in knockout mice for the AChE gene on the subcellular localization of the m2 receptor (m2R) and the regulation of its intraneuronal compartmentalization by the cholinergic environment, using immunohistochemistry at light and electron microscopic levels. (1) In AChE +/+ mice in vivo, m2R is mainly located at the neuronal membrane in striatum, hippocampus, and cortex. In AChE -/- mice, m2R is almost absent at the membrane but is accumulated in the endoplasmic reticulum and Golgi complex. (2) In vivo and in vitro (organotypic culture) dynamic studies demonstrate that the balance between membrane and intracytoplasmic m2R can be regulated by the cholinergic influence: In AChE -/- mice, m2R is translocated from the cytoplasm to the cell surface after (1) blockade of muscarinic receptors by atropine, (2) supplementation of AChE -/- neurons with AChE in vitro, and (3) disruption of the cortical and hippocampal cholinergic afferents in vitro. Our results suggest that the neurochemical environment may contribute to the control of the abundance and availability of cell surface receptors, and consequently to the control of neuronal sensitivity to neurotransmitters or drugs, by regulating their delivery from the endoplasmic reticulum and Golgi complex.