Endocytosis and recycling of muscarinic receptors

Evidence of nigericin as a potential therapeutic candidate for cancers: A review

Emerging studies have shown that nigericin, an H⁺, K⁺ and Pb²⁺ ionophore, has exhibited a promising anti-cancer activity in various cancers. However, its anti-cancer mechanisms have not been fully elucidated. In this review, the recent progresses on the use of nigericin in human cancers have been summarized. By exchanging H⁺ and K⁺ across cell membranes, nigericin shows promising anti-cancer activities in in vitro and in vivo as a single agent or in combination with other anti-cancer drugs through decreasing intracellular pH (pHi). The underlying mechanisms of nigericin also include the inactivation of Wnt/β-catenin signals, blockade of Androgen Receptor (AR) signaling, and activation of Stress-Activated Protein Kinase/c-Jun N-terminal Kinase (SAPK/JNK) signaling pathways. In many cancers, nigericin is proved to specifically target putative Cancer Stem Cells (CSCs), and its synergistic effects on photodynamic therapy are also reported. Other mechanisms of nigericin including influencing the mitochondrial membrane potentials, inducing an increase in drug accumulation and autophagy, controlling insulin accumulation in nuclei, and increasing the cytotoxic activity of liposome-entrapped drugs, are also discussed. Notably, the potential adverse effects such as teratogenic effects, insulin resistance and eryptosis shall not be ignored. Taken together, these reports suggest that treatment of cancer cells with nigericin may offer a novel therapeutic strategy and future potential of translation to clinics.

Corticosterone suppresses vasotocin-enhanced clasping behavior in male rough-skinned newts by novel mechanisms interfering with V1a receptor availability and receptor-mediated endocytosis

In rough-skinned newts, Taricha granulosa, exposure to an acute stressor results in the rapid release of corticosterone (CORT), which suppresses the ability of vasotocin (VT) to enhance clasping behavior. CORT also suppresses VT-induced spontaneous activity and sensory responsiveness of clasp-controlling neurons in the rostromedial reticular formation (Rf). The cellular mechanisms underlying this interaction remain unclear. We hypothesized that CORT blocks VT-enhanced clasping by interfering with V1a receptor availability and/or VT-induced endocytosis. We administered a physiologically active fluorescent VT conjugated to Oregon Green (VT-OG) to the fourth ventricle 9 min after an intraperitoneal injection of CORT (0, 10, 40 μg/0.1mL amphibian Ringers). The brains were collected 30 min post-VT-OG, fixed, and imaged with confocal microscopy. CORT diminished the number of endocytosed vesicles, percent area containing VT-OG, sum intensity of VT-OG, and the amount of VT-V1a within each vesicle; indicating that CORT was interfering with V1a receptor availability and VT-V1a receptor-mediated endocytosis. CORT actions were brain location-specific and season-dependent in a manner that is consistent with the natural and context-dependent expression of clasping behavior. Furthermore, the sensitivity of the Rf to CORT was much higher in animals during the breeding season, arguing for ethologically appropriate seasonal variation in CORT's ability to prevent VT-induced endocytosis. Our data are consistent with the time course and interaction effects of CORT and VT on clasping behavior and neurophysiology. CORT interference with VT-induced endocytosis may be a common mechanism employed by hormones across taxa for mediating rapid context- and season-specific behavioral responses.

Non-neuronal functions of the m2 muscarinic acetylcholine receptor

Acetylcholine is an important neurotransmitter whose effects are mediated by two classes of receptors. The nicotinic acetylcholine receptors are ion channels, whereas the muscarinic receptors belong to the large family of G protein coupled seven transmembrane helix receptors. Beyond its function in neuronal systems, it has become evident that acetylcholine also plays an important role in non-neuronal cells such as epithelial and immune cells. Furthermore, many cell types in the periphery are capable of synthesizing acetylcholine and express at least some of the receptors. In this review, we summarize the non-neuronal functions of the muscarinic acetylcholine receptors, especially those of the M₂ muscarinic receptor in epithelial cells. We will review the mechanisms of signaling by the M₂ receptor but also the cellular trafficking and ARF6 mediated endocytosis of this receptor, which play an important role in the regulation of signaling events. In addition, we provide an overview of the M₂ receptor in human pathological conditions such as autoimmune diseases and cancer.

Mu opioid receptor-effector coupling and trafficking in dorsal root ganglia neurons

Morphine induces profound analgesic tolerance in vivo despite inducing little internalization of the mu opioid receptor (muOR). Previously proposed explanations suggest that this lack of internalization could either lead to prolonged signaling and associated compensatory changes in downstream signaling systems, or that the receptor is unable to recycle and resensitize and so loses efficacy, either mechanism resulting in tolerance. We therefore examined, in cultured neurons, the relationship between muOR internalization and desensitization in response to two agonists, D-Ala2, N-MePhe4, Gly5-ol-enkephalin (DAMGO) and morphine. In addition, we studied the chimeric mu/delta opioid receptor (mu/ partial differentialOR) which could affect internalization and desensitization in neurons. Dorsal root ganglia neurons from muOR knockout mice were transduced with an adenovirus expressing either receptor and their respective internalization, desensitization and trafficking profiles determined. Both receptors desensitized equally, measured by Ca2+ current inhibition, during the first 5 min of agonist exposure to DAMGO or morphine treatment, although the mu/partial differentialOR desensitized more extensively. Such rapid desensitization was unrelated to internalization as DAMGO, but not morphine, internalized both receptors after 20 min. In response to DAMGO the mu/partial differentialOR internalized more rapidly than the muOR and was trafficked through Rab4-positive endosomes and lysosomal-associated membrane protein-1-labeled lysosomes whereas the muOR was trafficked through Rab4 and Rab11-positive endosomes. Chronic desensitization of the Ca2+ current response, after 24 h of morphine or DAMGO incubation, was seen in the DAMGO, but not morphine-treated, muOR-expressing cells. Such persistence of signaling after chronic morphine treatment suggests that compensation of downstream signaling systems, rather than loss of efficacy due to poor receptor recycling, is a more likely mechanism of morphine tolerance in vivo. In contrast to the muOR, the mu/partial differentialOR showed equivalent desensitization whether morphine or DAMGO treated, but internalized further with DAMGO than morphine. Such ligand-independent desensitization could be a result of the observed higher rate of synthesis and degradation of this chimeric receptor.

Receptor Signaling and Endocytosis Are Differentially Regulated by Somatostatin Analogs

Upon hormone stimulation, the sst2 somatostatin receptor couples to adenylyl cyclase through G(i/o) proteins and undergoes rapid endocytosis via clathrin-coated pits. In this study, we determined the relationship between the ability of ligands to induce sst2 receptor internalization and inhibit adenylyl cyclase. Immunocytochemical studies demonstrated that peptide agonists [such as somatostatin-14, cortistatin-17, octreotide, vapreotide, KE108 (Tyr0-cyclo[d-diaminobutyric acid-Arg-Phe-Phe-d-Trp-Lys-Thr-Phe]), and SOM230 (cyclo[diaminoethylcarbamoyl-hydroxyproline-phenylglycine-d-Trp-Lys-(4-O-benzyl)-l-Tyr-Phe])] and nonpeptide agonists (such as L-779,976), stimulated the rapid endocytosis of sst2 receptors in human embryonic kidney 293 and CHO-K1 cells. In contrast, two antagonists did not induce receptor endocytosis by themselves and completely blocked agonist stimulation. Using a quantitative enzyme-linked immunosorbent assay to measure sst2 receptor sequestration, we found that peptide agonists varied by more than 100-fold in their potencies but exhibited the same efficacy as somatostatin14. In contrast, L-779,976 did not induce maximal receptor internalization. It is interesting that although betaarrestin-2 was recruited to cell surface sst2 receptors after stimulation with either somatostatin14 or L-779,976, the betaarrestin-receptor complex dissociated earlier in the endocytic pathway with the nonpeptide ligand. Although all agonists, including L-779,976, produced the same maximal inhibition of cyclic AMP, the potency ratio for inhibition of cyclic AMP and stimulation of receptor endocytosis varied by 15-fold. In general, native peptides showed similar potencies for cyclic AMP inhibition and receptor endocytosis, whereas short therapeutic analogs were substantially more potent at inhibiting cyclic AMP synthesis. These results demonstrate that the activity of somatostatin analogs to regulate receptor endocytosis and signaling are not tightly linked and provide compelling evidence for the induction of agonist specific states of the sst2 receptor.

Endocytosis as a mechanism for tyrosine kinase-dependent suppression of a voltage-gated potassium channel

The voltage-gated potassium channel Kv1.2 undergoes tyrosine phosphorylation-dependent suppression of its ionic current. However, little is known about the physical mechanism behind that process. We have found that the Kv1.2 alpha-subunit protein undergoes endocytosis in response to the same stimuli that evoke suppression of Kv1.2 ionic current. The process is tyrosine phosphorylation-dependent because the same tyrosine to phenylalanine mutation in the N-terminus of Kv1.2 that confers resistance to channel suppression (Y132F) also confers resistance to channel endocytosis. Overexpression of a dominant negative form of dynamin blocked stimulus-induced Kv1.2 endocytosis and also blocked suppression of Kv1.2 ionic current. These data indicate that endocytosis of Kv1.2 from the cell surface is a key mechanism for channel suppression by tyrosine kinases.

M(3) muscarinic acetylcholine receptor plays a critical role in parasympathetic control of salivation in mice

The M(1) and M(3) subtypes are the major muscarinic acetylcholine receptors in the salivary gland and M(3) is reported to be more abundant. However, despite initial reports of salivation abnormalities in M(3)-knockout (M(3)KO) mice, it is still unclear which subtype is functionally relevant in physiological salivation. In the present study, salivary secretory function was examined using mice lacking specific subtype(s) of muscarinic receptor. The carbachol-induced [Ca(2+)](i) increase was markedly impaired in submandibular gland cells from M(3)KO mice and completely absent in those from M(1)/M(3)KO mice. This demonstrates that M(3) and M(1) play major and minor roles, respectively, in the cholinergically induced [Ca(2+)](i) increase. Two-dimensional Ca(2+)-imaging analysis revealed the patchy distribution of M(1) in submandibular gland acini, in contrast to the ubiquitous distribution of M(3). In vivo administration of a high dose of pilocarpine (10 mg kg(-1), s.c.) to M(3)KO mice caused salivation comparable to that in wild-type mice, while no salivation was induced in M(1)/M(3)KO mice, indicating that salivation in M(3)KO mice is caused by an M(1)-mediated [Ca(2+)](i) increase. In contrast, a lower dose of pilocarpine (1 mg kg(-1), s.c.) failed to induce salivation in M(3)KO mice, but induced abundant salivation in wild-type mice, indicating that M(3)-mediated salivation has a lower threshold than M(1)-mediated salivation. In addition, M(3)KO mice, but not M(1)KO mice, had difficulty in eating dry food, as shown by frequent drinking during feeding, suggesting that salivation during eating is mediated by M(3) and that M(1) plays no practical role in it. These results show that the M(3) subtype is essential for parasympathetic control of salivation and a reasonable target for the drug treatment and gene therapy of xerostomia, including Sjögren's syndrome.

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

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

Ligand Specific Up-Regulation of aRenilla reniformisLuciferase-Tagged, Structurally Unstable Muscarinic M3Chimeric G Protein-Coupled Receptor

The rat muscarinic acetylcholine receptor subtype 3 was modified by swapping the third intracellular loop with the corresponding region of a constitutively active mutant human beta2-adrenergic receptor and attaching Renilla reniformis luciferase to its C terminus. The chimeric fusion receptor displayed constitutive Gq- and Gs-coupled activity as demonstrated in nuclear factor of activated T cell and cAMP response element reporter gene assays. The chimeric receptor displayed a pharmacological binding profile comparable with that of the wild-type receptor for agonists, antagonists, and inverse agonists but showed a large decrease in expression in both human embryonic kidney 293 and COS-7 cells. Long-term treatment of cells expressing the chimeric receptor with agonists, antagonists, and inverse agonists resulted in a concentration-dependent up-regulation in the steady-state levels that was not observed for the wild-type receptor. The EC50 of neutral antagonists and inverse agonists was significantly correlated to their binding affinities at the wild-type receptor, whereas agonists demonstrated greater EC50 values for the chimeric receptor. To validate the approach as a means of discovering novel receptor modulators, a cell-based, high-throughput screening assay was developed and used to screen a small molecule compound collection against the chimeric fusion receptor. Several novel hits were identified and confirmed by ligand binding assay and functional assays using the wild-type rat muscarinic acetylcholine receptor subtype 3.

Agonist-dependent internalization of the human melanocortin-4 receptors in human embryonic kidney 293 cells

A chimeric protein comprised of melanocortin-4 receptor (MC4R) and the green fluorescent protein (GFP) was created for studying receptor/ligand localization and trafficking. The ligand binding affinities and second messenger stimulation induced by MC4R-GFP closely resembled those of the wild-type receptor, suggesting functional integrity of the chimeric protein. As observed with a confocal microscope, in human embryonic kidney (HEK)-293 cells MC4R/GFP was distributed evenly along the cell membrane. Addition of [Nle4-d-Phe7]-alpha-melanocyte-stimulating hormone (NDP-MSH), a peptide MC4R agonist, induced receptor translocation into intracellular compartments in a time- and concentration-dependent manner. [Ac-Nle-c[Asp-His-d-Nal(2')-Arg-Trp-Lys]-NH2] (SHU9119), a potent MC4R antagonist, completely inhibited NDP-MSH-mediated internalization. MC4R-GFP internalization was unaffected by a protein kinase A inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H89), but was impaired by pretreatment with inhibitors of endocytosis through clathrin-coated pits, hypertonic sucrose, or concanavalin A. Time-dependent colocalization of MC4R-GFP with rhodamine-transferrin, an early endosome marker, and with LysoTraker, a lysosome marker, was observed after short-term (45 min) and prolonged (20 h) agonist exposure, respectively. Rhodamine-[AcNle-c[Asp-His-d-Phe-Arg-Trp-Lys]-NH2] (MTII), a fluorescent derivative of an MC4R agonist, was found to cointernalize with MC4R-GFP into intracellular vesicles. No significant receptor recycling or segregation from the ligand was observed 60 min after removal of the agonist. In contrast, an antagonist rhodamine-Ac-Cys-Glu-His-(d-Nal)-Arg-Trp-Gly-Cys-Pro-Pro-Lys-Asp-NH2 (HS014) bound to and colocalized with MC4R-GFP on the cell surface and did not stimulate receptor internalization. In sum, these results suggest that MC4R is subject to agonist-dependent endocytosis via clathrin-coated pits. Prolonged agonist exposure directs MC4R into lysosomes, possibly for degradation. Receptor and ligand recycling is not efficient for MC4R in HEK-293 cells.

Regulation of muscarinic acetylcholine receptor function in acetylcholinesterase knockout mice

Acetylcholinesterase (AChE) hydrolyzes acetylcholine to terminate cholinergic neurotransmission. Overstimulation of cholinergic receptors by excess acetylcholine is known to be lethal. However, AChE knockout mice live to adulthood, although they have weak muscles, do not eat solid food, and die early from seizures. We wanted to know what compensatory factors allowed these mice to survive. We had previously shown that their butyrylcholinesterase activity was normal and had not increased. In this report, we tested the hypothesis that AChE-/- mice adapted to the absence of AChE by downregulating cholinergic receptors. Receptor downregulation is expected to reduce sensitivity to agonists and to increase sensitivity to antagonists. Physiological response to the muscarinic agonists, oxotremorine (OXO) and pilocarpine, showed that AChE-/- mice were resistant to OXO-induced hypothermia, tremor, salivation, and analgesia, and to pilocarpine-induced seizures. AChE+/- mice had an intermediate response. The muscarinic receptor binding sites measured with [3H]quinuclinyl benzilate, as well as the protein levels of M1, M2, and M4 receptors measured with specific antibodies on Western blots, were reduced to be approximately 50% in AChE-/- brain. However, mRNA levels for muscarinic receptors were unchanged. These results indicate that one adaptation to the absence of AChE is downregulation of muscarinic receptors, thus reducing response to cholinergic stimulation.

A3 adenosine receptors in human astrocytoma cells: agonist-mediated desensitization, internalization, and down-regulation

A(3) adenosine receptor activation has been previously demonstrated to result in both neuroprotective and neurodegenerative effects, depending upon specific pathophysiological conditions. This dual effect may depend on receptor regulation mechanisms that are able to change receptor availability and/or function. In the present study, we investigated desensitization, internalization, and down-regulation of native A(3) adenosine receptors in human astrocytoma cells after exposure to the agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (Cl-IBMECA). Cl-IBMECA induced a concentration-dependent inhibition of adenylyl cyclase activity with an EC(50) value of 2.9 +/- 0.1 nM. The effect was suggested to be mediated by A(3) adenosine receptor subtype by the use of selective adenosine receptor antagonists. Cell treatment with pertussis toxin abolished Cl-IBMECA-mediated inhibition of adenylyl cyclase activity, evidencing an A(3) receptor coupling to inhibitory G protein. Short-term exposure to the agonist Cl-IBMECA (100 nM) caused rapid receptor desensitization, within 15 min. Agonist-induced desensitization was accompanied by receptor internalization: A(3) adenosine receptor internalized with rapid kinetics, within 30 min, after cell exposure to 100 nM Cl-IBMECA. The localization of A(3) adenosine receptors on the plasma membrane and in intracellular compartments was directly revealed by immunogold electron microscopy. After desensitization, the removal of agonist led to the restoration of A(3) adenosine receptor functioning through receptor recycling to the cell surface within 120 min. Prolonged agonist exposure (1-24 h) resulted in a marked down-regulation of A(3) adenosine receptors that reached 21.9 +/- 2.88% of control value after 24 h. After down-regulation, the recovery of receptor functioning was slow (24 h) and associated with the restoration of receptor levels close to control values. In conclusion, our results demonstrated that A(3) receptors, in astrocytoma cells, are regulated after short- and long-term agonist exposure.

Nitric oxide induces resensitization of P2Y nucleotide receptors in cultured rat mesangial cells

Receptor desensitization of G protein-coupled receptors (GPCRs), which occurs during short-term (seconds to minutes) exposure of cells to agonists, is mediated by phosphorylation and receptor endocytosis. Recycling of the receptors is a requisite for resensitization of the response. The mechanisms that attenuate signaling by GPCRs are of considerable importance to regulation of intercellular signaling and maintenance of their ability to respond to agonists over time. This study evaluates the effect of nitric oxide (NO) on P2Y nucleotide receptor resensitization in cultured rat glomerular mesangial cells. The NO production in cultured mesangial cells was measured by using confocal microscopy and the fluorescence NO indicator 4,5-diaminofluorescein diacetate (DAF-2 DA). L-arginine increased and Nomega-nitro-L-arginine methyl ester (L-NAME) decreased NO production significantly (P < 0.05). Calcium responses to ATP were measured with fura-2 and imaging techniques. Repeated stimulation with ATP results in receptor desensitization that is characterized by lower calcium peak amplitude. Desensitization was induced by challenging mesangial cells with four consecutive 2-min pulses of ATP (0.1 mM) separated by 4.5-min control perfusions. Intracellular calcium concentration ([Ca2+]i) increase evoked by second, third, and fourth ATP challenges were about 40%, 26%, and 18% of the first one. The NO precursor, L-arginine (10 mM), and the NO donors, spermine-NONOate (500 microM) and sodium nitroprusside (SNP) (1 mM), were added before and during a fourth ATP challenge. Spermine-NONOate and L-arginine induced a recovery of the [Ca2+]i response to the fourth ATP challenge (P < 0.01 and 0.05, respectively). The NO synthase inhibitor, L-NAME (5 mM), applied along with ATP, was shown to enhance desensitization. 1H-(1,2,4)oxadiazolo(4,3-alpha)quinoxalin-1-one (ODQ, 30 microM), an inhibitor of guanylate cyclase, was used along with L-arginine, SNP, or spermine-NONOate. There was no significant difference with or without ODQ. Neither ODQ nor 8-Br-cGMP, an analog of cGMP, at different concentrations showed effects on ATP-stimulated [Ca2+]i. There was no elevation of [Ca2+]i when the cells were challenged by different concentrations (1 microM, 100 microM, 1 mM, 20 mM, and 30 mM) of caffeine, caffeine plus ATP (0.1 mM), and 4-chloro-3-ethylphenol (100 microM, 500 microM, and 1 mM), a new agonist of ryanodine receptors. The results indicate that NO can increase the P2Y receptor resensitization in rat glomerular mesangial cells by acting through a cGMP-independent pathway. No evidence was found for the existence of ryanodine-sensitive intracellular calcium stores in rat mesangial cells.

Carbachol induces homologous steroidogenic refractoriness of bovine fasciculata-reticularis cells

Recently it was shown that the cholinergic agonist carbachol stimulates cortisol production in bovine ZFR cells via muscarinic receptor M(3). In the present study, we investigated the effect of chronic cholinergic stimulation on steroidogenic response and muscarinic receptor regulation in ZFR cells. Cortisol secretion of ZFR cells treated with 10(-4) M of carbachol decreased in a dose- and time-dependent manner. The carbachol-elicited loss of response was associated with a decrease in M(3) receptor number, which was also time- and dose-dependent. The down-regulation of the receptors was not associated with the decrease of M(3) receptor mRNA level. The marked steroidogenic desensitization caused by pretreatment of carbachol did not alter ACTH or angiotensin II activated steroid response. Northern blot analysis showed that carbachol pretreatment did not change the gene expression of P450scc, P450cl7, 3betaHSD and StAR mRNAs. These results suggest that carbachol induces homologous steroidogenic refractoriness of ZFR cells.

Acetylcholinesterase inhibition increases in vivo N-(2-[18F]fluoroethyl)-4-piperidyl benzilate binding to muscarinic acetylcholine receptors

Although the inhibition of acetylcholinesterase remains the primary treatment of Alzheimer's disease, little is known of the results of increased acetylcholine levels on muscarinic receptor occupancy or function. Using N-(2-[18F]fluoroethyl)-4-piperidyl benzilate ([18F]FEPB), a moderate affinity (Ki = 1.7 nmol/L) nonsubtype-selective muscarinic receptor antagonist, the authors examined the sensitivity of equilibrium in vivo radioligand binding in rat brain with changes in endogenous acetylcholine levels produced by treatments with acetylcholinesterase inhibitors. Phenserine administration 30 minutes before resulted in a dose-dependent into muscarinic cholinergic receptors, reaching a maximum increase of 90% in the striatum at a dose of 5 mg/kg intraperitoneally. Constant infusion of physostigmine at a dosage of 250 microg/kg/min produced an identical increase in radioligand binding. This agonist-induced increase of in vivo mAChR radioligand binding offers a new method for monitoring of the efficacy of acetylcholinesterase inhibitors or other drugs to enhance acetylcholine actions at the muscarinic receptors.

Regulation and intracellular trafficking pathways of the endothelin receptors

The effects of endothelin (ET) are mediated via the G protein-coupled receptors ET(A) and ET(B). However, the mechanisms of ET receptor desensitization, internalization, and intracellular trafficking are poorly understood. The aim of the present study was to investigate the molecular mechanisms of ET receptor regulation and to characterize the intracellular pathways of ET-stimulated ET(A) and ET(B) receptors. By analysis of ET(A) and ET(B) receptor internalization in transfected Chinese hamster ovary cells in the presence of overexpressed betaARK, beta-arrestin-1, beta-arrestin-2, or dynamin as well as dominant negative mutants of these regulators, we have demonstrated that both ET receptor subtypes follow an arrestin- and dynamin/clathrin-dependent mechanism of internalization. Fluorescence microscopy of Chinese hamster ovary and COS cells expressing green fluorescent protein (GFP)-tagged ET receptors revealed that the ET(A) and ET(B) subtypes were targeted to different intracellular routes after ET stimulation. While ET(A)-GFP followed a recycling pathway and colocalized with transferrin in the pericentriolar recycling compartment, ET(B)-GFP was targeted to lysosomes after ET-induced internalization. Both receptor subtypes colocalized with Rab5 in classical early endosomes, indicating that this compartment is a common early intermediate for the two ET receptors during intracellular transport. The distinct intracellular routes of ET-stimulated ET(A) and ET(B) receptors may explain the persistent signal response through the ET(A) receptor and the transient response through the ET(B) receptor. Furthermore, lysosomal targeting of the ET(B) receptor could serve as a biochemical mechanism for clearance of plasma endothelin via this subtype.