The role of membrane proximal threonine residues conserved among guanine-nucleotide-binding-protein-coupled receptors in internalization of the m4 muscarinic acetylcholine receptor

Modulation of M4 muscarinic acetylcholine receptors by interacting proteins

Protein-protein interactions represent an important mechanism for posttranslational modifications of protein expression and function. In brain cells, surface-expressed and membrane-bound neurotransmitter receptors are common proteins that undergo dynamic protein-protein interactions between their intracellular domains and submembranous regulatory proteins. Recently, the Gα(i/o)-coupled muscarinic M4 receptor (M4R) has been revealed to be one of these receptors. Through direct interaction with the intracellular loops or C-terminal tails of M4Rs, M4R interacting proteins (M4RIPs) vigorously regulate the efficacy of M4R signaling. A synapse-enriched protein kinase, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), exemplifies a prototype model of M4RIPs, and is capable of binding to the second intracellular loop of M4Rs. Through an activity- and phosphorylation-dependent mechanism, CaMKII potentiates the M4R/Gα(i/o)-mediated inhibition of M4R efficacy in inhibiting adenylyl cyclase and cAMP production. In striatal neurons where M4Rs are most abundantly expressed, M4RIPs dynamically control M4R activity to maintain a proper cholinergic tone in these neurons. This is critical for maintaining the acetylcholine-dopamine balance in the basal ganglia, which determines the behavioral responsiveness to dopamine stimulation by psychostimulants.

Mutation in a protein kinase C phosphorylation site of the 5-HT1A receptor preferentially attenuates Ca2+ responses to partial as opposed to higher-efficacy 5-HT1A agonists

The Thr(149)Ala mutation in a putative protein kinase C phosphorylation site of the 5-HT(1A) receptor's second intracellular loop has been shown to affect the closing of Ca(2+) channels and Ca(2+) mobilisation without interfering with the inhibitory cAMP pathway (Mol Pharmacol 52 (1997) 164). Here, the Ca(2+) responses for a series of 5-HT(1A) agonists were compared between the wild-type (wt) and mutant Thr(149)Ala 5-HT(1A) receptor as part of a fusion protein containing a G(alpha)(15) protein. Neither the mutation nor the fusion process modified the [(3)H]WAY 100635-based ligand binding profile of the fusion proteins as compared to the wt 5-HT(1A) receptor protein. Whereas at the wt 5-HT(1A) receptor, 5-HT induced a Ca(2+) response in CHO-K1 cells via endogenous G(i/o) proteins, the Ca(2+) response to 5-HT at the mutant Thr(149)Ala 5-HT(1A) receptor was fully dependent on either the co-expression or the fusion to a recombinant G(alpha)(15) protein. Buspirone, flesinoxan and 8-OH-DPAT produced a graded partial response (26 to 62%) at the wt 5-HT(1A):G(alpha)(15) fusion protein; F 13640, 5-CT and F 14679 behaved as higher-efficacy agonists with maximal Ca(2+) responses similar to 5-HT. The maximal Ca(2+) responses at the mutant Thr(149)Ala 5-HT(1A):G(alpha)(15) fusion protein were significantly attenuated for flesinoxan and 8-OH-DPAT (-45 and -36%, respectively); the response to the other 5-HT agonists was not significantly affected. A similar effect was observed upon treatment with phorbol 12-myristate 13-acetate at the Thr(149)Ala 5-HT(1A):G(alpha)(15) fusion protein. In conclusion, the amplitude of the Ca(2+) responses induced by partial, but not that to fuller 5-HT(1A) receptor agonists, is affected by the Thr(149)Ala mutation of the 5-HT(1A):G(alpha)(15) fusion protein.

Regulation of muscarinic acetylcholine receptor signaling

Multiple mechanisms regulate the signaling of the five members of the family of the guanine nucleotide binding protein (G protein)-coupled muscarinic acetylcholine (ACh) receptors (mAChRs). Following activation by classical or allosteric agonists, mAChRs can be phosphorylated by a variety of receptor kinases and second messenger-regulated kinases. The phosphorylated mAChR subtypes can interact with beta-arrestin and presumably other adaptor proteins as well. As a result, the various mAChR signaling pathways may be differentially altered, leading to short-term or long-term desensitization of a particular signaling pathway, receptor-mediated activation of the mitogen-activated protein kinase pathway downstream of mAChR phosphorylation, as well as long-term potentiation of mAChR-mediated phospholipase C stimulation. Agonist activation of mAChRs may also induce receptor internalization and down-regulation, which proceed in a highly regulated manner, depending on receptor subtype and cell type. In this review, our current understanding of the complex regulatory processes that underlie signaling of mAChR is summarized.

Agonist-induced internalization of the platelet-activating factor receptor is dependent on arrestins but independent of G-protein activation. Role of the C terminus and the (D/N)PXXY motif

As with most G-protein-coupled receptors, repeated agonist stimulation of the platelet-activating factor receptor (PAFR) results in its desensitization, sequestration, and internalization. In this report, we show that agonist-induced PAFR internalization is independent of G-protein activation but is dependent on arrestins and involves the interaction of arrestins with a limited region of the PAFR C terminus. In cotransfected COS-7 cells, both arrestin-2 and arrestin-3 could be coimmunoprecipitated with PAFR, and agonist stimulation of PAFR induced the translocation of both arrestin-2 and arrestin-3. Furthermore, coexpression of arrestin-2 with PAFR potentiated receptor internalization, whereas agonist-induced PAFR internalization was inhibited by a dominant negative mutant of arrestin-2. The coexpression of a minigene encoding the C-terminal segment of the receptor abolished PAF-induced arrestin translocation and inhibited PAFR internalization. Using C terminus deletion mutants, we determined that the association of arrestin-2 with the receptor was dependent on the region between threonine 305 and valine 330 because arrestin-2 could be immunoprecipitated with the mutant PAFRstop330 but not PAFRstop305. Consistently, stop330 could mediate agonist-induced arrestin-2 translocation, whereas stop305 could not. Two other deletion mutants with slightly longer regions of the C terminus, PAFRstop311 and PAFRstop317, also failed to induce arrestin-2 translocation. Finally, the PAFR mutant Y293A, containing a single substitution in the putative internalization motif DPXXY in the seventh transmembrane domain (which we had shown to be able to internalize but not to couple to G-proteins) could efficiently induce arrestin translocation. Taken together, our results indicate that ligand-induced PAFR internalization is dependent on arrestins, that PAFR can associate with both arrestin-2 and -3, and that their translocation involves interaction with the region of residues 318-330 in the PAFR C terminus but is independent of G-protein activation.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

Internalization of the TXA2 receptor alpha and beta isoforms. Role of the differentially spliced cooh terminus in agonist-promoted receptor internalization

Thromboxane A2 (TXA2) potently stimulates platelet aggregation and smooth muscle constriction and is thought to play a role in myocardial infarction, atherosclerosis, and bronchial asthma. The TXA2 receptor (TXA2R) is a member of the G protein-coupled receptor family and is found as two alternatively spliced isoforms, alpha (343 residues) and beta (407 residues), which share the first 328 residues. In the present report, we demonstrate by enzyme-linked immunosorbent assay and immunofluorescence microscopy that the TXA2Rbeta, but not the TXA2Ralpha, undergoes agonist-induced internalization when expressed in HEK293 cells as well as several other cell types. Various dominant negative mutants were used to demonstrate that the internalization of the TXA2Rbeta is dynamin-, GRK-, and arrestin-dependent in HEK293 cells, suggesting the involvement of receptor phosphorylation and clathrin-coated pits in this process. Interestingly, the agonist-stimulated internalization of both the alpha and beta isoforms, but not of a mutant truncated after residue 328, can be promoted by overexpression of arrestin-3, identifying the C-tails of both receptors as necessary in arrestin-3 interaction. Simultaneous mutation of two dileucine motifs in the C-tail of TXA2Rbeta did not affect agonist-promoted internalization. Analysis of various C-tail deletion mutants revealed that a region between residues 355 and 366 of the TXA2Rbeta is essential for agonist-promoted internalization. These data demonstrate that alternative splicing of the TXA2R plays a critical role in regulating arrestin binding and subsequent receptor internalization.

Structural and functional requirements for agonist-induced internalization of the human platelet-activating factor receptor

The receptor for platelet-activating factor (PAF) is a member of the G-protein-coupled receptor family. To study the structural elements and mechanisms involved in the internalization of human PAF receptor (hPAFR), we used the following mutants: a truncated mutant in the C-terminal tail of the receptor (Cys317 --> Stop) and mutations in the (D/N)P(X)2,3Y motif (Asp289 --> Asn,Ala and Tyr293 --> Phe,Ala). Chinese hamster ovary cells expressing the Cys317 --> Stop mutant exhibited a marked reduction in their capacity to internalize PAF, suggesting the existence of determinants important for endocytosis in the last 26 amino acids of the cytoplasmic tail. Substitution of Asp289 to alanine abolished both internalization and G-protein coupling, whereas substitution of Tyr293 to alanine abolished coupling but not internalization. Inhibition or activation of protein kinase C did not significantly affect the internalization process. Receptor sequestration and ligand uptake was, at least in part, blocked by concanavalin A and blockers of endocytosis mediated by clathrin-coated pits. Our data suggest that the internalization of a G-protein-coupled receptor and coupling to a G-protein can be two independent events. Moreover, the C terminus tail of hPAFR, but not the putative internalization motifs, may be involved in the internalization of hPAFR.

Muscarinic acetylcholine receptor trafficking in streptolysin O-permeabilized MDCK cells

We investigated the validity of streptolysin O (SLO)-permeabilized Madin-Darbin canine kidney (MDCK) cells which express muscarinic acetylcholine receptors (mAChRs) coupled to pertussis toxin-sensitive guanine nucleotide-binding proteins (G proteins) for the study of the molecular machinery that regulated mAChR internalization and recycling. Exposure of SLO-permeabilized cells to carbachol-reduced cell surface receptor number by up to 40% without changing total receptor number. The kinetics and maximal extent of receptor internalization as well as the potency of carbachol to induce receptor internalization were almost identical in SLO-permeabilized and non-permeabilized cells. Using this semi-intact cell system, we studied the effect of various agents affecting components potentially involved in receptor trafficking. Internalization was prevented by treatment of the SLO-permeabilized MDCK cells with (i) the stable ATP analogues, adenosine 5'-O-(3-thiotriphosphate) and adenylylimidodiphosphate, to block ATP-dependent processes, and (ii) heparin to block G protein-coupled receptor kinases. Inclusion of the stable GTP analogue, guanosine 5'-O-(3-thiotriphosphate), increased the rate but not the extent of receptor internalization. None of the membrane-impermeant agents affected receptor internalization in intact MDCK cells. This model system also allowed recycling of internalized receptors back to the plasma membrane. After removal of the agonist, cell surface receptor number in SLO-permeabilized cells returned to control values within 90 min with the same kinetics as seen in intact cells. Inclusion of guanosine 5'O-(3-thiotriphosphate) shortened the recovery time. These data suggest that both ATP-dependent kinases including G protein-coupled receptor kinases and G proteins participate in receptor internalization and recycling. In summary, the SLO-permeabilized MDCK cell is a feasible model system for the study of mAChR internalization and recycling and allows manipulation of the intracellular milieu with membrane-impermeable macromolecules.