Identification of a region required for subtype-specific agonist-induced sequestration of the m2 muscarinic acetylcholine receptor

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

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

FRET-based detection of M1 muscarinic acetylcholine receptor activation by orthosteric and allosteric agonists

Background and Objective

Muscarinic acetylcholine receptors (mAChRs) are 7-transmembrane, G protein-coupled receptors that regulate a variety of physiological processes and represent potentially important targets for therapeutic intervention. mAChRs can be stimulated by full and partial orthosteric and allosteric agonists, however the relative abilities of such ligands to induce conformational changes in the receptor remain unclear. To gain further insight into the actions of mAChR agonists, we have developed a fluorescently tagged M₁ mAChR that reports ligand-induced conformational changes in real-time by changes in Förster resonance energy transfer (FRET).

Methods

Variants of CFP and YFP were inserted into the third intracellular loop and at the end of the C-terminus of the mouse M₁ mAChR, respectively. The optimized FRET receptor construct (M₁-cam5) was expressed stably in HEK293 cells.

Results

The variant CFP/YFP-receptor chimera expressed predominantly at the plasma membrane of HEK293 cells and displayed ligand-binding affinities comparable with those of the wild-type receptor. It also retained an ability to interact with Gα_q/11 proteins and to stimulate phosphoinositide turnover, ERK1/2 phosphorylation and undergo agonist-dependent internalization. Addition of the full agonist methacholine caused a reversible decrease in M₁ FRET (F_EYFP/F_ECFP) that was prevented by atropine pre-addition and showed concentration-dependent amplitude and kinetics. Partial orthosteric agonists, arecoline and pilocarpine, as well as allosteric agonists, AC-42 and 77-LH-28-1, also caused atropine-sensitive decreases in the FRET signal, which were smaller in amplitude and significantly slower in onset compared to those evoked by methacholine.

Conclusion

The M₁ FRET-based receptor chimera reports that allosteric and orthosteric agonists induce similar conformational changes in the third intracellular loop and/or C-terminus, and should prove to be a valuable molecular reagent for pharmacological and structural investigations of M₁ mAChR activation.

Muscarinic receptor trafficking

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

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

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

The internalization of the M2 and M4 muscarinic acetylcholine receptors involves distinct subsets of small G-proteins

Multiple mechanisms exist for the endocytosis of receptors from the cell surface. While the M1, M3, and M4 subtypes of muscarinic acetylcholine receptor and M4 receptors transduce their signals through the same second messengers but internalize though different pathways, we tested the ability of several small G-proteins to regulate the agonist-induced endocytosis of M2 and M4 in JEG-3 human choriocarcinoma cells. Dominant-negative Rab5 as well as both wild-type and dominant-negative Rab11 inhibited M4 but not M2 endocytosis. In contrast, a dominant-negative Arf6 as well as wild-type Rab22 increased M2 but not M4 endocytosis. We used immunocytochemistry to show that in unstimulated cells, the M2 and M4 receptors co-localize on the cell surface, whereas after stimulation M2 and M4 are in distinct vesicular compartments. In this study, we demonstrate that agonist-induced internalization of the M2 receptor utilizes an Arf6, Rab22 dependent pathway, while the M4 receptor undergoes agonist-induced internalization through a Rab5, Rab11 dependent pathway. Additionally, we show that Rab15 and RhoA are not involved in either pathway in JEG-3 cells.

Identification of a novel apical sorting motif and mechanism of targeting of the M2 muscarinic acetylcholine receptor

Previous studies have shown that the M2 receptor is localized at steady state to the apical domain in Madin-Darby canine kidney (MDCK) epithelial cells. In this study, we identify the molecular determinants governing the localization and the route of apical delivery of the M2 receptor. First, by confocal analysis of a transiently transfected glycosylation mutant in which the three putative glycosylation sites were mutated, we determined that N-glycans are not necessary for the apical targeting of the M2 receptor. Next, using a chimeric receptor strategy, we found that two independent sequences within the M2 third intracellular loop can confer apical targeting to the basolaterally targeted M4 receptor, Val270-Lys280 and Lys280-Ser350. Experiments using Triton X-100 extraction followed by OptiPrep density gradient centrifugation and cholera toxin beta-subunit-induced patching demonstrate that apical targeting is not because of association with lipid rafts. 35S-Metabolic labeling experiments with domain-specific surface biotinylation as well as immunocytochemical analysis of the time course of surface appearance of newly transfected confluent MDCK cells expressing FLAG-M2-GFP demonstrate that the M2 receptor achieves its apical localization after first appearing on the basolateral domain. Domain-specific application of tannic acid of newly transfected cells indicates that initial basolateral plasma membrane expression is required for subsequent apical localization. This is the first demonstration that a G-protein-coupled receptor achieves its apical localization in MDCK cells via transcytosis.

Quantitative Analysis of Muscarinic Acetylcholine Receptor Homo- and Heterodimerization in Live Cells

Although previous pharmacological and biochemical data support the notion that muscarinic acetylcholine receptors (mAChR) form homo- and heterodimers, the existence of mAChR oligomers in live cells is still a matter of controversy. Here we used bioluminescence resonance energy transfer to demonstrate that M(1), M(2), and M(3) mAChR can form constitutive homo- and heterodimers in living HEK 293 cells. Quantitative bioluminescence resonance energy transfer analysis has revealed that the cell receptor population in cells expressing a single subtype of M(1), M(2), or M(3) mAChR is predominantly composed of high affinity homodimers. Saturation curve analysis of cells expressing two receptor subtypes demonstrates the existence of high affinity M(1)/M(2), M(2)/M(3), and M(1)/M(3) mAChR heterodimers, although the relative affinity values were slightly lower than those for mAChR homodimers. Short term agonist treatment did not modify the oligomeric status of homo- and heterodimers. When expressed in JEG-3 cells, the M(2) receptor exhibits much higher susceptibility than the M(3) receptor to agonist-induced down-regulation. Coexpression of M(3) mAChR with increasing amounts of the M(2) subtype in JEG-3 cells resulted in an increased agonist-induced down-regulation of M(3), suggesting a novel role of heterodimerization in the mechanism of mAChR long term regulation.

Identification and Structural Determination of the M3 Muscarinic Acetylcholine Receptor Basolateral Sorting Signal

Muscarinic acetylcholine receptors comprise a family of G-protein-coupled receptors that display differential localization in polarized epithelial cells. We identify a seven-residue sequence, Ala(275)-Val(281), in the third intracellular loop of the M(3) muscarinic receptor that mediates dominant, position-independent basolateral targeting in Madin-Darby canine kidney cells. Mutational analyses identify Glu(276), Phe(280), and Val(281) as critical residues within this sorting motif. Phe(280) and Val(281) comprise a novel dihydrophobic sorting signal as mutations of either residue singly or together with leucine do not disrupt basolateral targeting. Conversely, Glu(276) is required and cannot be substituted with alanine or aspartic acid. A 19-amino acid peptide representing the M(3) sorting signal and surrounding sequence was analyzed via two-dimensional nuclear magnetic resonance spectroscopy. Solution structures show that Glu(276) resides in a type IV beta-turn and the dihydrophobic sequence Phe(280)Val(281) adopts either a type I or IV beta-turn.

Casein kinase II sites in the intracellular C-terminal domain of the thyrotropin-releasing hormone receptor and chimeric gonadotropin-releasing hormone receptors contribute to beta-arrestin-dependent internalization

We have previously shown that the mammalian gonadotropin-releasing hormone receptor (GnRHR), a unique G-protein-coupled receptor (GPCR) lacking an intracellular carboxyl tail (C-tail), does not follow a beta-arrestin-dependent internalization pathway. However, internalization of a chimeric GnRHR with the thyrotropin-releasing hormone receptor (TRHR) C-tail does utilize beta-arrestin. Here, we have investigated the sites within the intracellular C-tail domain that are important for conferring beta-arrestin-dependent internalization. In contrast to the chimeric GnRHR with a TRHR C-tail, a chimeric GnRHR with the catfish GnRHR C-tail is not beta-arrestin-dependent. Sequence comparisons between these chimeric receptors show three consensus phosphorylation sites for casein kinase II (CKII) in the TRHR C-tail but none in the catfish GnRHR C-tail. We thus investigated a role for CKII sites in determining GPCR internalization via beta-arrestin. Sequential introduction of three CKII sites into the chimera with the catfish C-tail (H354D,A366E,G371D) resulted in a change in the pattern of receptor phosphorylation and beta-arrestin-dependence, which only occurred when all three sites were introduced. Conversely, mutation of the putative CKII sites (T365A,T371A,S383A) in the C-tail of a beta-arrestin-sensitive GPCR, the TRHR, resulted in decreased receptor phosphorylation and a loss of beta-arrestin-dependence. Mutation of all three CKII sites was necessary before a loss of beta-arrestin-dependence was observed. Visualization of beta-arrestin/GFP redistribution confirmed a loss or gain of beta-arrestin sensitivity for receptor mutants. Internalization of receptors without C-tail CKII sites was promoted by a phosphorylation-independent beta-arrestin mutant (R169E), suggesting that these receptors do not contain the necessary phosphorylation sites required for beta-arrestin-dependent internalization. Apigenin, a specific CKII inhibitor, blocked the increase in receptor internalization by beta-arrestin, thus providing further support for the involvement of CKII. This study presents evidence of a novel role for C-tail CKII consensus sites in targeting these GPCRs to the beta-arrestin-dependent pathway.

Identification of a basolateral sorting signal for the M3 muscarinic acetylcholine receptor in Madin-Darby canine kidney cells

Muscarinic acetylcholine receptors (mAChRs) can be differentially localized in polarized cells. To identify potential sorting signals that mediate mAChR targeting, we examined the sorting of mAChRs in Madin-Darby canine kidney cells, a widely used model system. Expression of FLAG-tagged mAChRs in polarized Madin-Darby canine kidney cells demonstrated that the M(2) subtype is sorted apically, whereas M(3) is targeted basolaterally. Expression of M(2)/M(3) receptor chimeras revealed that a 21-residue sequence, Ser(271)-Ser(291), from the M(3) third intracellular loop contains a basolateral sorting signal. Substitution of sequences containing the M(3) sorting signal into the homologous regions of M(2) was sufficient to confer basolateral localization to this apical receptor. Sequences containing the M(3) sorting signal also conferred basolateral targeting to M(2) when added to either the third intracellular loop or the C-terminal cytoplasmic tail. Furthermore, addition of a sequence containing the M(3) basolateral sorting signal to the cytoplasmic tail of the interleukin-2 receptor alpha-chain caused significant basolateral targeting of this heterologous apical protein. The results indicate that the M(3) basolateral sorting signal is dominant over apical signals in M(2) and acts in a position-independent manner. The M(3) sorting signal represents a novel basolateral targeting motif for G protein-coupled receptors.

Multiple topological domains mediate subtype-specific internalization of the M2 muscarinic acetylcholine receptor

Endocytosis of agonist-activated G protein-coupled receptors (GPCRs) is required for both resensitization and recycling to the cell surface as well as lysosomal degradation. Thus, this process is crucial for regulation of receptor signaling and cellular responsiveness. Although many GPCRs internalize into clathrin-coated vesicles in a dynamin-dependent manner, some receptors, including the M(2) muscarinic acetylcholine receptor (mAChR), can also exhibit dynamin-independent internalization. We have identified five amino acids, located in the sixth and seventh transmembrane domains and the third intracellular loop, that are essential for agonist-induced M(2) mAChR internalization via a dynamin-independent mechanism in JEG-3 choriocarcinoma cells. Substitution of these residues into the M(1) mAChR, which does not internalize in these cells, is sufficient for conversion to the internalization-competent M(2) mAChR phenotype, whereas removal of these residues from the M(2) mAChR blocks internalization. Cotransfection of a dominant-negative isoform of dynamin has no effect on M(2) mAChR internalization. An internalization-incompetent M(2) mutant that lacks a subset of the necessary residues can still internalize via a G protein-coupled receptor kinase-2 and beta-arrestin-dependent pathway. Furthermore, internalization is independent of the signal transduction pathway that is activated. These results identify a novel motif that specifies structural requirements for subtype-specific dynamin-independent internalization of a GPCR.

Phosphorylation of human gp130 at Ser-782 adjacent to the Di-leucine internalization motif. Effects on expression and signaling

The receptor for leukemia inhibitory factor (LIF) consists of two polypeptides, the LIF receptor and gp130. Agonist stimulation has been shown previously to cause phosphorylation of gp130 on serine, threonine, and tyrosine residues. We found that gp130 fusion proteins were phosphorylated exclusively on Ser-782 by LIF- and growth factor-stimulated 3T3-L1 cell extracts. Ser-780 was required for phosphorylation of Ser-782 but was not itself phosphorylated. Ser-782 is located immediately N-terminal to the di-leucine motif of gp130, which regulates internalization of the receptor. Transient expression of chimeric granulocyte colony-stimulating factor receptor (G-CSFR)-gp130(S782A) receptors resulted in increased cell surface expression in COS-7 cells and increased ability to induce vasoactive intestinal peptide gene expression in IMR-32 neuroblastoma cells when compared with expression of chimeric receptors containing wild-type gp130 cytoplasmic domains. These results identify Ser-782 as the major phosphorylated serine residue in human gp130 and indicate that this site regulates cell surface expression of the receptor polypeptide.

Selective recruitment of arrestin-3 to clathrin coated pits upon stimulation of G protein-coupled receptors

Non-visual arrestins (arrestin-2 and arrestin-3) play critical roles in the desensitization and internalization of many G protein-coupled receptors. In vitro experiments have shown that both non-visual arrestins bind with high and approximately comparable affinities to activated, phosphorylated forms of receptors. They also exhibit high affinity binding, again of comparable magnitude, to clathrin. Further, agonist-promoted internalization of many receptors has been found to be stimulated by exogenous over-expression of either arrestin2 or arrestin3. The existence of multiple arrestins raises the question whether stimulated receptors are selective for a specific endogenous arrestin under more physiological conditions. Here we address this question in RBL-2H3 cells, a cell line that expresses comparable levels of endogenous arrestin-2 and arrestin-3. When (beta)(2)-adrenergic receptors are stably expressed in these cells the receptors internalize efficiently following agonist stimulation. However, by immunofluorescence microscopy we determine that only arrestin-3, but not arrestin-2, is rapidly recruited to clathrin coated pits upon receptor stimulation. Similarly, in RBL-2H3 cells that stably express physiological levels of m1AChR, the addition of carbachol selectively induces the localization of arrestin-3, but not arrestin-2, to coated pits. Thus, this work demonstrates coupling of G protein-coupled receptors to a specific non-visual arrestin in an in vivo setting.

The CD3ε Subunit of the TCR Contains Endocytosis Signals

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3γ and CD3δ contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3ε and CDζ display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3ε bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3ε that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3ε displays endocytosis determinants. These data indicate that CD3ε could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3ε subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.

Role for the third intracellular loop in cell surface stabilization of the alpha2A-adrenergic receptor

Previous studies have shown that alpha2A-adrenergic receptor (alpha2A-AR) retention at the basolateral surface of polarized MDCKII cells involves its third intracellular (3i loop). The present studies examining mutant alpha2A-ARs possessing short deletions of the 3i loop indicate that no single region can completely account for the accelerated surface turnover of the Delta3ialpha2A-AR, suggesting that the entire 3i loop is involved in basolateral retention. Both wild-type and Delta3i loop alpha2A-ARs are extracted from polarized Madin-Darby canine kidney (MDCK) cells with 0.2% Triton X-100 and with a similar concentration/response profile, suggesting that Triton X-100-resistant interactions of the alpha2A-AR with cytoskeletal proteins are not involved in receptor retention on the basolateral surface. The indistinguishable basolateral t(1)/(2) for either the wild-type or nonsense 3i loop alpha2A-AR suggests that the stabilizing properties of the alpha2A-AR 3i loop are not uniquely dependent on a specific sequence of amino acids. The accelerated turnover of Delta3i alpha2A-AR cannot be attributed to alteration in agonist-elicited alpha2A-AR redistribution, because alpha2A-ARs are not down-regulated in response to agonist. Taken together, the present studies show that stabilization of the alpha2A-AR on the basolateral surface of MDCKII cells involves multiple mechanisms, with the third intracellular loop playing a central role in regulating these processes.

Bradykinin-induced internalization of the human B2 receptor requires phosphorylation of three serine and two threonine residues at its carboxyl tail

The binding of bradykinin (BK) to B2 receptor triggers the internalization of the agonist-receptor complex. To investigate the mechanisms and the receptor structures involved in this fundamental process of receptor regulation, the human B2 receptor was mutated within its cytoplasmic tail by complementary strategies of truncation, deletion, and amino acid substitution. Ligand binding, signal transduction, internalization as well as phosphorylation were studied for the mutated receptors expressed in COS, CHO, and HEK 293 cells. Truncation of 44 out of 55 amino acid residues of the receptor's cytoplasmic tail corresponding to positions 321-364 did not alter the kinetics of BK binding and the receptor coupling to phospholipase C and phospholipase A2. By contrast, truncations after positions 320 and 334, deletions within the segment covering positions 335-351, as well as alanine substitution of serine and threonine residues within segment 335-351 diminished the internalization capacity of the mutant receptors. Mutants with a markedly reduced internalization potential failed to produce BK-induced receptor phosphorylation suggesting that phosphorylation may be involved in receptor internalization. The mutagenesis approaches converged at the conclusion that three serines in positions 339, 346, and 348 and two threonines in positions 342 and 345, contained in a sequence segment that is highly conserved between species, have a critical role in the ligand-dependent internalization and phosphorylation of kinin receptors and can intervene in these processes in an alternative manner. However, mutants lacking these residues were still sensitive to dominant-negative forms of beta-arrestin and dynamin, suggesting the existence of additional receptor structure(s) involved in the receptor sequestration through clathrin-coated vesicles.

Structural correlates for down-regulation of m1 and m2 muscarinic receptor subtypes

Three chimeric receptors stably expressed in murine fibroblast (B82) cells were used to examine how different parts of the rat muscarinic m1 and m2 receptors contribute to the down-regulation process. The MCH7 chimeric m2 receptor contained a fragment between VIth TM and C-terminal end derived from the m1 receptor. The MCH3 and MCH5 receptors have exchanged N-terminal and third intracellular loop regions of the MCH7 receptor. Fibroblast cells stably expressing individual muscarinic wild type (m1, m2) or chimeric (MCH3, MCH5, or MCH7) receptors were treated with plain medium (control) or medium containing carbachol for 24 h. Receptor density changes were measured by [3H](-)1-N-methyl-3-quinuclidinyl benzilate ([3H](-)MQNB) saturation binding studies. There was a significant loss of receptor density, different for each receptor studied, following carbachol treatment relative to control cells. We related this loss of [3H](-)MQNB binding to the number of amino acids derived from m1 or m2 receptors for each constructed chimera and to the affinity of carbachol to the receptors studied. We demonstrate that: 1) the region from the VIth TMD to the end of C-terminal controls the extent of m1 and m2 receptor down-regulation; 2) the overall receptor conformation and the interaction between intracellular portions of the receptor influence the extent of receptor down-regulation; and 3) resistance to down-regulation by carbachol correlates with the affinity of carbachol to the muscarinic receptor construct.

Receptor subtype-specific regulation of muscarinic acetylcholine receptor sequestration by dynamin. Distinct sequestration of m2 receptors

Sustained stimulation of muscarinic acetylcholine receptors (mAChRs) and other G protein-coupled receptors usually leads to a loss of receptor binding sites from the plasma membrane, referred to as receptor sequestration. Receptor sequestration can occur via endocytosis of clathrin-coated vesicles that bud from the plasma membrane into the cell but may also be accomplished by other, as yet ill-defined, mechanisms. Previous work has indicated that the monomeric GTPase dynamin controls the endocytosis of plasma membrane receptors via clathrin-coated vesicles. To investigate whether mAChRs sequester in a receptor subtype-specific manner via dynamin-dependent clathrin-coated vesicles, we tested the effect of overexpressing the dominant-negative dynamin mutant K44A on m1, m2, m3, and m4 mAChR sequestration in HEK-293 cells. The m1, m2, m3, and m4 mAChRs sequestered rapidly in HEK-293 cells following agonist exposure but displayed dissimilar sequestration pathways. Overexpression of dynamin K44A mutant fully blocked m1 and m3 mAChR sequestration, whereas m2 mAChR sequestration was not affected. Also, m4 mAChRs, which like m2 mAChRs preferentially couple to pertussis toxin-sensitive G proteins, sequestered in a completely dynamin-dependent manner. Following agonist removal, sequestered m1 mAChRs fully reappeared on the cell surface, whereas sequestered m2 mAChRs did not. The distinct sequestration of m2 mAChRs was also apparent in COS-7 and Chinese hamster ovary cells. We conclude that the m2 mAChR displays unique subtype-specific sequestration that distinguishes this receptor from the m1, m3, and m4 subtypes. These results are the first to demonstrate that receptor sequestration represents a new type of receptor subtype-specific regulation within the family of mAChRs.

Gonadotropin-releasing hormone receptors with intracellular carboxyl-terminal tails undergo acute desensitization of total inositol phosphate production and exhibit accelerated internalization kinetics

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is the only G-protein-coupled receptor (GPCR) in which the intracellular C-terminal tail is completely absent. In contrast to other GPCRs, the GnRH-R does not show rapid desensitization of total inositol (IP) production, and the rates of internalization are exceptionally slow. We investigated whether the incorporation of a cytoplasmic tail into the C terminus of the GnRH-R affects desensitization events and receptor internalization rates. A GnRH-R/TRH-R chimera was created where the intracellular tail of the rat thyrotropin-releasing hormone receptor (TRH-R) was engineered into the C terminus of the rat GnRH-R. Three different rat GnRH-R cDNA stop codon mutations (one for each reading frame) were also made. The GnRH-stimulated IP production of the wild-type rat GnRH-R expressed in either COS-7 or HEK 293 cells did not desensitize even after prolonged stimulation with GnRH. In contrast, the catfish GnRH-R (which does possess an intracellular tail) and the TRH-R rapidly (<10 min) desensitized following agonist stimulation. The GnRH-R/TRH-R chimera also desensitized following treatment with GnRH, resembling the pattern shown by the TRH-R and the catfish GnRH-R. Two of the stop codon mutants did not show desensitization of IP production, and the third mutant with the longest tail was not functional. Internalization experiments showed that the rat GnRH-R had the slowest endocytosis and recycling rates compared with the TRH-R, the catfish GnRH-R, and the chimeric GnRH/TRH-R. This study demonstrates that the addition of a functional intracellular C-terminal tail to the GnRH-R produces rapid desensitization of IP production and significantly increases internalization rates.

Endocytic clathrin-coated pit formation is independent of receptor internalization signal levels

The mechanisms responsible for coated pit formation in cells remain unknown, but indirect evidence has argued both for and against a critical role of receptor cytoplasmic domains in the process. If the endocytic motifs of receptors are responsible for recruiting AP2 to the plasma membrane, thereby driving coated pit formation, then the level of constitutively internalized receptors at the membrane would be expected to govern the steady-state level of coated pits in cells. Here we directly test this hypothesis for broad classes of receptors containing three distinct constitutive internalization signals. Chimeric proteins consisting of an integral membrane reporter protein (Tac) coupled to cytoplasmic domains bearing tyrosine-, di-leucine-, or acidic cluster/casein kinase II-based internalization signals were overexpressed to levels that saturated the internalization pathway. Quantitative confocal immunofluorescence microscopy indicated that the number of plasma membrane clathrin-coated pits and the concentration of their structural components were invariant when comparing cells expressing saturating levels of the chimeric receptors to nonexpressing cells or to cells expressing only the Tac reporter lacking cytoplasmic internalization signals. Biochemical analysis showed that the distribution of coat proteins between assembled coated pits and soluble pools was also not altered by receptor overexpression. Finally, the cellular localizations of AP2 and AP1 were similarly unaffected. These results provide a clear indication that receptor endocytic signals do not determine coated pit levels by directly recruiting AP2 molecules. Rather, the findings support a model in which coated pit formation proceeds through recruitment and activation of AP2, likely through a limited number of regulated docking sites that act independently of endocytic signals.

Internalization of the m2 muscarinic acetylcholine receptor. Arrestin-independent and -dependent pathways

Recent studies have identified agonist-dependent phosphorylation as a critical event in the rapid uncoupling of the m2 muscarinic cholinergic receptors (mAChR) from G-proteins and sequestration of the receptors away from the cell surface. However, mutant m2 mAChRs were identified that were phosphorylated but unable to desensitize in adenylyl cyclase assays, while they internalized like wild type (WT) mAChRs. We have tested whether these properties might stem from differences in the abilities of the WT and mutant mAChR to bind arrestins, proteins implicated in both receptor/G-protein uncoupling and internalization. We have determined that arrestin binding requires phosphorylation at a cluster of Ser/Thr residues in amino acids 307-311 in the m2 mAChR. A strong correlation was found between the ability of WT and mutant receptors to bind arrestins in vitro or in vivo and to desensitize in adenylyl cyclase assays. However, the phosphorylation-dependent internalization of the m2 mAChR in HEK-tsA201 cells did not require arrestins and did not proceed via clathrin-mediated endocytosis. While the m2 mAChR was able to enter a clathrin- and arrestin-dependent pathway when arrestin 2 or arrestin 3 was significantly overexpressed, the preferred pathway of internalization of WT and certain mutant m2 mAChR in HEK-tsA201 cells did not involve participation of arrestins. The results suggest that the phosphorylation-mediated regulation of the m2 mAChR may involve arrestin-dependent and -independent events.

Synergistic regulation of m2 muscarinic acetylcholine receptor desensitization and sequestration by G protein-coupled receptor kinase-2 and beta-arrestin-1

The m2 muscarinic acetylcholine receptor (m2 mAChR) belongs to the superfamily of G protein-coupled receptors and is regulated by many processes that attenuate signaling following prolonged stimulation by agonist. We used a heterologous expression system to examine the ability of G protein-coupled receptor kinase-2 (GRK2) and beta-arrestin-1 to regulate the phosphorylation state and to promote desensitization and sequestration of the m2 mAChR. Treatment of JEG-3 cells transiently expressing the m2 mAChR with a muscarinic agonist induced an approximately 4- or 8-fold increase in receptor phosphorylation in the absence or presence of cotransfected GRK2, respectively, compared with untreated cells transfected with receptor alone. Using the expression of a cAMP-regulated reporter gene to measure receptor function, we found that transiently transfected m2 mAChRs underwent functional desensitization following exposure to agonist. Transfected GRK2 enhanced agonist-induced functional desensitization in a manner that was synergistically enhanced by cotransfection of beta-arrestin-1, which had no effect on m2 mAChR function when coexpressed in the absence of GRK2. Finally, GRK2 and beta-arrestin-1 synergistically enhanced both the rate and extent of agonist-induced m2 mAChR sequestration. These results are the first to demonstrate that agonist-induced desensitization and sequestration of the m2 mAChR in the intact cell can be enhanced by the presence of GRK2 and beta-arrestin-1 and show that these molecules have multiple actions on the m2 mAChR.

Dynamic targeting of the agonist-stimulated m2 muscarinic acetylcholine receptor to caveolae in cardiac myocytes

In cardiac myocytes, as well as specialized conduction and pacemaker cells, agonist binding to muscarinic acetylcholine receptors (mAchRs) results in the activation of several signal transduction cascades including the endothelial isoform of nitric-oxide synthase (eNOS) expressed in these cells. Recent evidence indicates that, as in endothelial cells, eNOS in cardiac myocytes is localized to plasmalemma caveolae, specialized lipid microdomains that contain caveolin-3, a muscle-specific isoform of the scaffolding protein caveolin. In this report, using a detergent-free method for isolation of sarcolemmal caveolae from primary cultures of adult rat ventricular myocytes, we demonstrated that the muscarinic cholinergic agonist carbachol promotes the translocation of mAchR into low density gradient fractions containing most myocyte caveolin-3 and eNOS. Following isopycnic centrifugation, the different gradient fractions were exposed to the muscarinic radioligand [3H]quinuclidinyl benzilate (QNB), and binding was determined after membrane filtration or immunoprecipitation. In a direct radioligand binding assay, we found that [3H]QNB binding can be detected in caveolin-enriched fractions only when cardiac myocytes have been previously exposed to carbachol. Furthermore, most of this [3H]QNB binding can be specifically immunoprecipitated by an antibody to the m2 mAchR, indicating that the translocation of this receptor subtype is responsible for the [3H]QNB binding detected in the low density fractions. Moreover, the [3H]QNB binding could be quantitatively immunoprecipitated from the light membrane fractions with a caveolin-3 antibody (but not a control IgG1 antibody), confirming that the m2 mAchR is targeted to caveolae after carbachol treatment. Importantly, atropine, a muscarinic cholinergic antagonist, did not induce translocation of m2 mAchR to caveolae and prevented receptor translocation in response to the agonist carbachol. Thus, dynamic targeting of sarcolemmal m2 mAchR to caveolae following agonist binding may be essential to initiate specific downstream signaling cascades in these cells.

In vivo downregulation of M2 receptors revealed by measurement of muscarinic K+ current in cultured guinea-pig atrial myocytes

1. Muscarinic K+ current (IK(ACh)) elicited by acetylcholine (ACh) was measured in guinea-pig atrial myocytes, which were either freshly isolated or cultured for up to 8 days. 2. The half-time of activation of inward IK(ACh) by a saturating concentration (10 microM) of ACh decreased from approximately 400 ms (in freshly isolated cells) to 250 ms after 6 days in culture. This was paralleled by an increase in the fast desensitizing component of IK(ACh). The density of steady-state currents was not changed. Downregulation of M2 receptors by long-term treatment of isolated myocytes with carbachol in vitro had opposite effects. 3. The EC50 of ACh for the activation of steady-state IK(ACh) was reduced from 5 x 10(-7) M (day 0) to 8 x 10(-8) M (day 6). The shift in EC50 occurred with a half-time of about 2 days, similar to the recovery from downregulation induced by treating atrial myocytes with carbachol in vitro. 4. The increase in sensitivity to ACh can be accounted for by an approximately 6-fold increase in the density of M2 receptors. 5. It is concluded that sensitization in culture reflects recovery from downregulation of M2 receptors due to the tonic vagal input to the heart in the intact animal.