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

Assistance for Folding of Disease-Causing Plasma Membrane Proteins

An extensive catalog of plasma membrane (PM) protein mutations related to phenotypic diseases is associated with incorrect protein folding and/or localization. These impairments, in addition to dysfunction, frequently promote protein aggregation, which can be detrimental to cells. Here, we review PM protein processing, from protein synthesis in the endoplasmic reticulum to delivery to the PM, stressing the main repercussions of processing failures and their physiological consequences in pathologies, and we summarize the recent proposed therapeutic strategies to rescue misassembled proteins through different types of chaperones and/or small molecule drugs that safeguard protein quality control and regulate proteostasis.

Heterogeneous localization of muscarinic cholinoceptor M1 in the salivary ducts of adult mice

We hypothesize variation in expression and localization, along the course of the glandular tubule, of muscarinic cholinergic receptor M₁ which plays as a distinct contribution, though minor in comparison with M₃ receptor, in saliva secretion. Localization of the M₁ receptor was examined using immunohistochemistry in three major salivary glands. Although all glandular cells were more or less M₁-immunoreactive, acinar cells were weakly immunoreactive, while ductal cells exhibited substantial M₁-immunoreactivity. Many ductal cells exhibited clear polarity with higher immunoreactivity in their apical/supra-nuclear domain. However, some exhibited indistinct polarity because of additional higher immunoreactivity in their basal/infra-nuclear domain. A small group of cells with intense immunoreactivity was found, mostly located in the intercalated ducts or in portions of the striated ducts close to the intercalated ducts. In immuno-electron microscopy, the immunoreactive materials were mainly in the cytoplasm including various vesicles and vacuoles. Unexpectedly, distinct immunoreactivity on apical and basal plasma membranes was infrequent in most ductal cells. The heterogeneous localization of M₁-immunoreactivity along the gland tubular system is discussed in view of possible modulatory roles of the M₁ receptor in saliva secretion.

Extrasynaptic muscarinic acetylcholine receptors on neuronal cell bodies regulate presynaptic function in Caenorhabditis elegans

Acetylcholine (ACh) is a potent neuromodulator in the brain, and its effects on cognition and memory formation are largely performed through muscarinic acetylcholine receptors (mAChRs). mAChRs are often preferentially distributed on specialized membrane regions in neurons, but the significance of mAChR localization in modulating neuronal function is not known. Here we show that the Caenorhabditis elegans homolog of the M1/M3/M5 family of mAChRs, gar-3, is expressed in cholinergic motor neurons, and GAR-3-GFP fusion proteins localize to cell bodies where they are enriched at extrasynaptic regions that are in contact with the basal lamina. The GAR-3 N-terminal extracellular domain is necessary and sufficient for this asymmetric distribution, and mutation of a predicted N-linked glycosylation site within the N-terminus disrupts GAR-3-GFP localization. In transgenic animals expressing GAR-3 variants that are no longer asymmetrically localized, synaptic transmission at neuromuscular junctions is impaired and there is a reduction in the abundance of the presynaptic protein sphingosine kinase at release sites. Finally, GAR-3 can be activated by endogenously produced ACh released from neurons that do not directly contact cholinergic motor neurons. Together, our results suggest that humoral activation of asymmetrically localized mAChRs by ACh is an evolutionarily conserved mechanism by which ACh modulates neuronal function.

Synthesis, trafficking, and localization of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors are members of the G-protein coupled receptor superfamily that are expressed in and regulate the function of neurons, cardiac and smooth muscle, glands, and many other cell types and tissues. The correct trafficking of membrane proteins to the cell surface and their subsequent localization at appropriate sites in polarized cells are required for normal cellular signaling and physiological responses. This review will summarize work on the synthesis and trafficking of muscarinic receptors to the plasma membrane and their localization at the cell surface.

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.

The role of protein interaction motifs in regulating the polarity and clustering of the metabotropic glutamate receptor mGluR1a

When expressed in cultured hippocampal neurons, the metabotropic glutamate receptor mGluR1a is polarized to dendrites and concentrated at postsynaptic sites. We used a mutational analysis to determine how previously identified protein interaction motifs in the C terminus of mGluR1a contribute to its localization. Our results show that the polyproline motif that mediates interaction with Homer family proteins is critical for the synaptic clustering of mGluR1a. A single point mutation in this motif, which prevents the binding of Homer with mGluR1a, reduced its colocalization with a postsynaptic marker to near-chance levels but did not affect its dendritic polarity. In contrast, deleting the PDZ (postsynaptic density-95/Discs large/zona occludens-1) binding domain, which interacts with Tamalin and Shank, had no effect on synaptic localization. Neither of these protein interaction motifs is important for trafficking to the plasma membrane or for polarization to dendrites. Although deleting the entire C terminus of mGluR1a only modestly reduced its dendritic polarity, this domain was sufficient to redirect an unpolarized reporter protein to dendrites. These observations suggest that mGluR1a contains redundant dendritic targeting signals. Together, our results indicate that the localization of mGluR1a involves two distinct steps, one that targets the protein to dendrites and a second that sequesters it at postsynaptic sites; different protein interactions motifs mediate each step.

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.

Localization of AQP5/AQP8 chimeras in MDCK-II cells: exchange of the N- and C-termini

AQP5 and AQP8 possess targeting/retention motifs which mediate their localization to the apical and basolateral membranes, respectively, of polarized MDCK-II cells. As targeting/retention motifs have been localized to the N- or C-termini of other AQPs, we sought the location of such motifs in AQPs 5 and 8 by exchanging their corresponding N- or C-termini and examining the expression, localization, and function of the resultant chimeras. We did not detect the expression of constructs in which the C-terminus of AQP5 was replaced by the C-terminus of AQP8. Substitution of the N-terminus of AQP8 for the N-terminus of AQP5 generated a construct which was trapped intracellularly and did not significantly facilitate transepithelial fluid movement. In contrast, modifications of the N- and C-termini of AQP8 were better tolerated. Substitution of either AQP8 terminus by the corresponding AQP5 terminus generated constructs which localized to basolateral membranes and facilitated transepithelial fluid movement. Our results suggest that, unlike the other AQP targeting/retention signals reported thus far, an AQP8 basolateral targeting/retention motif might reside between the two cytosolic termini.

The apical targeting signal of the P2Y2 receptor is located in its first extracellular loop

P2Y2 and P2Y4 receptors, which have 52% sequence identity, are both expressed at the apical membrane of Madin-Darby canine kidney cells, but the locations of their apical targeting signals are distinctly different. The targeting signal of the P2Y2 receptor is located between the N terminus and 7TM, whereas that of the P2Y4 receptor is present in its C-terminal tail. To identify the apical targeting signal in the P2Y2 receptor, regions of the P2Y2 receptor were progressively substituted with the corresponding regions of the P2Y4 receptor lacking its targeting signal. Characterization of these chimeras and subsequent mutational analysis revealed that four amino acids (Arg95, Gly96, Asp97, and Leu108) in the first extracellular loop play a major role in apical targeting of the P2Y2 receptor. Mutation of RGD to RGE had no effect on P2Y2 receptor targeting, indicating that receptor-integrin interactions are not involved in apical targeting. P2Y2 receptor mutants were localized in a similar manner in Caco-2 colon epithelial cells. This is the first identification of an extracellular protein-based targeting signal in a seven-transmembrane receptor.

Polarized expression of human P2Y receptors in epithelial cells from kidney, lung, and colon

Eight human G protein-coupled P2Y receptors (P2Y(1), P2Y(2), P2Y(4), P2Y(6), P2Y(11), P2Y(12), P2Y(13), and P2Y(14)) that respond to extracellular nucleotides have been molecularly identified and characterized. P2Y receptors are widely expressed in epithelial cells and play an important role in regulating epithelial cell function. Functional studies assessing the capacity of various nucleotides to promote increases in short-circuit current (I(sc)) or Ca(2+) mobilization have suggested that some subtypes of P2Y receptors are polarized with respect to their functional activity, although these results often have been contradictory. To investigate the polarized expression of the family of P2Y receptors, we determined the localization of the entire P2Y family after expression in Madin-Darby canine kidney (MDCK) type II cells. Confocal microscopy of polarized monolayers revealed that P2Y(1), P2Y(11), P2Y(12), and P2Y(14) receptors reside at the basolateral membrane, P2Y(2), P2Y(4), and P2Y(6) receptors are expressed at the apical membrane, and the P2Y(13) receptor is unsorted. Biotinylation studies and I(sc) measurements in response to the appropriate agonists were consistent with the polarized expression observed in confocal microscopy. Expression of the G(q)-coupled P2Y receptors (P2Y(1), P2Y(2), P2Y(4), P2Y(6), and P2Y(11)) in lung and colonic epithelial cells (16HBE14o- and Caco-2 cells, respectively) revealed a targeting profile nearly identical to that observed in MDCK cells, suggesting that polarized targeting of these P2Y receptor subtypes is not a function of the type of epithelial cell in which they are expressed. These experiments highlight the highly polarized expression of P2Y receptors in epithelial cells.

Membrane trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) modulate diverse physiological and behavioral signaling pathways by virtue of changes in receptor activation and inactivation states. Functional changes in receptor properties include dynamic interactions with regulatory molecules and trafficking to various cellular compartments at various stages of the life cycle of a GPCR. This review focuses on trafficking of GPCRs to the cell surface, stabilization there, and agonist-regulated turnover. GPCR interactions with a variety of newly revealed partners also are reviewed with the intention of provoking further analysis of the relevance of these interactions in GPCR trafficking, signaling, or both. The disease consequences of mislocalization of GPCRs also are described.

Isoforms of amino acid transporters in placental syncytiotrophoblast: plasma membrane localization and potential role in maternal/fetal transport

Many cell proteins exist as isoforms arising either from gene duplication or alternate RNA splicing. There is growing evidence that isoforms with different, but closely related, functional characteristics are often directed to discrete cellular locations. Thus, specialized functions may be carried out by proteins of similar evolutionary origin in different membrane compartments. In polarized epithelial cells, this mechanism allows the cell to control amino acid transport independently at each of its specialized apical and basolateral plasma membrane domains. Investigations of isoform localization in these membranes have generally been performed in epithelia other than the placental trophoblast.This review of placental amino acid transporter isoforms first provides an overview of their properties and preliminary plasma membrane localization. We then discuss studies suggesting various roles of isoform localization in trophoblast function. To provide insights into the molecular basis of this localization in trophoblast, we present a review of current knowledge of plasma membrane protein localization as derived from investigations with a widely used epithelial model cell line. Finally, we discuss a potential approach using cultured trophoblast-derived cells for studies of transporter isoform localization and function. We hope that this review will stimulate investigation of the properties of trophoblast transporter isoforms, their membrane localization and their contribution to the cellular mechanism of maternal-fetal nutrient transport.

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.

Lateral membrane LXA4 receptors mediate LXA4's anti-inflammatory actions on intestinal epithelium

Lipoxin A(4) (LXA(4)) and its stable analogs downregulate chemokine secretion in polarized epithelia. This anti-inflammatory effect has been suggested to be mediated by the LXA(4) receptor (LXA(4)R), a G protein-coupled receptor. To determine whether LXA(4)R is expressed on the apical, basolateral, or both poles of intestinal epithelia, an NH(2)-terminal c-myc epitope tag was added to the human LXA(4)R cDNA and recombinant retroviruses were used to transduce polarized epithelial cells. In polarized T84 intestinal epithelial cells, c-myc-LXA(4)R was preferentially expressed on the basolateral surface as indicated by cell surface-selective biotinylation and confocal microscopy. Furthermore, expression of c-myc-LXA(4)R and a truncation mutant lacking the cytoplasmic terminus was primarily confined to the lateral subdomain. We also observed that the expression of myc-LXA(4) conferred enhanced downregulation of IL-8 expression in response to LXA(4) analog and that blockade of the CysLT1 receptor by montelukast did not prevent this response to LXA(4) analog. Thus LXA(4) generated in or near the paracellular space via neutrophil-epithelial interactions can rapidly act on epithelial LXA(4)R to downregulate epithelial promotion of intestinal inflammation.

Targeted trafficking of neurotransmitter receptors to synaptic sites

Emerging data are sheding light on the critical task for synapses to locally control the production of neurotransmitter receptors ultimately leading to receptor accumulation and modulation at postsynaptic sites. By analogy with the epithelial-cell paradigm, the postsynaptic compartment may be regarded as a polarized domain favoring the selective recruitment and retention of newly delivered receptors at synaptic sites. Targeted delivery of receptors to synaptic sites is facilitated by a local organization of the exocytic pathway, likely resulting from spatial cues triggered by the nerve. This review focuses on the various mechanisms responsible for regulation of receptor assembly and trafficking. A particular emphasis is given to the role of synaptic anchoring and scaffolding proteins in the sorting and routing of their receptor companion along the exocytic pathway. Other cellular components such as lipidic microdomains, the docking and fusion machinery, and the cytoskeleton also contribute to the dynamics of receptor trafficking at the synapse.