The C-terminal tail of the metabotropic glutamate receptor subtype 7 is necessary but not sufficient for cell surface delivery and polarized targeting in neurons and epithelia

Involvement of the C-terminal domain in cell surface localization and G-protein coupling of mGluR6

Metabotropic glutamate receptor 6, mGluR6, interacts with scaffold proteins and Gβγ subunits via its intracellular C-terminal domain (CTD). The mGluR6 pathway is critically involved in the retinal processing of visual signals. We herein investigated whether the CTD (residues 840-871) was necessary for mGluR6 cell surface localization and G-protein coupling using mGluR6-CTD mutants with immunocytochemistry, surface biotinylation assays, and electrophysiological approaches. We used 293T cells and primary hippocampal neurons as model systems. We examined C-terminally truncated mGluR6 and showed that the removal of up to residue 858 did not affect surface localization or glutamate-induced G-protein-mediated responses, whereas a 15-amino acid deletion (Δ857-871) impaired these functions. However, a 21-amino acid deletion (Δ851-871) restored surface localization and glutamate-dependent responses, which were again attenuated when the entire CTD was removed. The sequence alignment of group III mGluRs showed conserved amino acids resembling an ER retention motif in the CTD. These results suggest that the intracellular CTD is required for the cell surface transportation and receptor function of mGluR6, whereas it may contain regulatory elements for intracellular trafficking and signaling.

Coupling of metabotropic glutamate receptor 8 to N-type Ca2+ channels in rat sympathetic neurons

Group III metabotropic glutamate receptors (mGluRs; mGluR4, 6, 7, and 8) couple to the Galpha(i/o)-containing G protein heterotrimers and act as autoreceptors to regulate glutamate release, probably by inhibiting voltage-gated Ca(2+) channels. Although most mGluRs have been functionally expressed in a variety of systems, few studies have demonstrated robust coupling of mGluR8 to downstream effectors. We therefore tested whether activation of mGluR8 inhibited Ca(2+) channels. Both L-glutamate (L-Glu) and l-2-amino-4-phosphonobutyric acid (L-AP4), a selective agonist for group III mGluRs, inhibited N-type Ca(2+) current in rat superior cervical ganglion neurons previously injected with a cDNA encoding mGluR8a/b. L-AP4 was approximately 100-fold more potent (IC(50) = 0.1 microM) than L-Glu ( approximately 10 microM), but it had efficacy similar to that of L-Glu ( approximately 50% maximal inhibition). The potency and efficacy of L-AP4 and L-Glu were similar for both splice variants. Agonist-induced inhibition was abolished by pretreatment with (R,S)-alpha-cyclopropyl-4-phosphonophenylglycine, a selective group III mGluR antagonist, and pertussis toxin. Deletion of either a calmodulin (CaM) binding motif in the C terminus or the entire C terminus of mGluR8 did not affect mGluR8-mediated response. Our studies indicate that both mGluR8a and 8b are capable of inhibiting N-type Ca(2+) channel, suggesting a role as presynaptic autoreceptors to regulate neuronal excitability. The studies also imply that the potential CaM binding domain is not required for the mGluR8-mediated Ca(2+) channel inhibition and the C terminus of mGluR8a is dispensable for receptor coupling to N-type Ca(2+) channels.

Identification and functional roles of metabotropic glutamate receptor-interacting proteins

In the mammalian brain, a majority of excitatory synapses use glutamate as a neurotransmitter. Glutamate activates ligand-gated channels (ionotropic receptors) and G protein-coupled (metabotropic) receptors. During the past decade, a number of intracellular proteins have been described to interact with these receptors. These proteins not only scaffold the glutamate receptors at the pre- and post-synaptic membranes, but also regulate their subcellular targeting and intracellular signaling. Thus, identification of these proteins has been essential for further understanding the functions of glutamate receptors. Here we will focus on those proteins that interact with the subgroup of metabotropic glutamate (mGlu) receptors, and review the methods used for their identification, as well as their functional roles in neurons.

Role of the C terminus in neuropeptide Y Y1 receptor desensitization and internalization

We have studied truncation mutants of the rat neuropeptide Y (NPY) Y1 receptor lacking four (Thr361stop, Y1T361*) or eight (Ser352stop, Y1S352*) potential serine/threonine C-terminal phosphorylation sites. NPY-stimulated hemagglutinin-tagged Y1, Y1T361*, and Y1S352* receptors all efficiently activated G proteins in Chinese hamster ovary (CHO) cell membranes, but desensitization after NPY pretreatment was only prevented in the HAY1S352* clone. In transfected colonic carcinoma epithelial layers, functional Y1 and Y1T361* peptide YY responses became more transient as the agonist concentration increased, whereas those mediated by the Y1S352* receptor remained sustained. NPY-stimulated HAY1 receptor phosphorylation was increased by transient overexpression of G protein-coupled receptor kinase 2, and only Ser352stop truncation abolished this response in CHO or human embryonic kidney (HEK) 293 cells. Rapid internalization of cell-surface HAY1 receptors in HEK293 cells was observed in response to agonist, resulting in partial colocalization with transferrin, a marker for clathrin-mediated endocytosis and recycling. It is surprising that both truncated receptors were constitutively internalized, predominantly in transferrin-positive compartments. NPY increased cell-surface localization of HAY1S352* receptors, whereas the distribution of both mutants was unaltered by BIBO3304. Recruitment of green fluorescent protein-tagged beta-arrestin2 to punctate endosomes was observed only for HAY1 and HAY1T361* receptors and solely under NPY-stimulated conditions. Thus, the key C-terminal sequence between Ser352 and Lys360 is a major site for Y1 receptor phosphorylation, is critical for its desensitization, and contributes to the association between the receptor and beta-arrestin proteins. However, additional beta-arrestin-independent mechanisms control Y1 receptor trafficking under basal conditions.

Transport protein trafficking in polarized cells

In order to carry out their physiological functions, ion transport proteins must be targeted to the appropriate domains of cell membranes. Regulation of ion transport activity frequently involves the tightly controlled delivery of intracellular populations of transport proteins to the plasma membrane or the endocytic retrieval of transport proteins from the cell surface. Transport proteins carry signals embedded within their structures that specify their subcellular distributions and endow them with the capacity to participate in regulated membrane trafficking processes. Recently, a great deal has been learned about the biochemical nature of these signals, as well as about the cellular machinery that interprets them and acts upon their messages.

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.

A novel sorting motif in the glutamate transporter excitatory amino acid transporter 3 directs its targeting in Madin-Darby canine kidney cells and hippocampal neurons

The glutamate transporter excitatory amino acid transporter 3 (EAAT3) is polarized to the apical surface in epithelial cells and localized to the dendritic compartment in hippocampal neurons, where it is clustered adjacent to postsynaptic sites. In this study, we analyzed the sequences in EAAT3 that are responsible for its polarized localization in Madin-Darby canine kidney (MDCK) cells and neurons. Confocal microscopy and cell surface biotinylation assays demonstrated that deletion of the EAAT3 C terminus or replacement of the C terminus of EAAT3 with the analogous region in EAAT1 eliminated apical localization in MDCK cells. The C terminus of EAAT3 was sufficient to redirect the basolateral-preferring EAAT1 and the nonpolarized EAAT2 to the apical surface. Using alanine substitution mutants, we identified a short peptide motif in the cytoplasmic C-terminal region of EAAT3 that directs its apical localization in MDCK cells. Mutation of this sequence also impairs dendritic targeting of EAAT3 in hippocampal neurons but does not interfere with the clustering of EAAT3 on dendritic spines and filopodia. These data provide the first evidence that an identical cytoplasmic motif can direct apical targeting in epithelia and somatodendritic targeting in neurons. Moreover, our results demonstrate that the two fundamental features of the localization of EAAT3 in neurons, its restriction to the somatodendritic domain and its clustering near postsynaptic sites, are mediated by distinct molecular mechanisms.

PDZ domain interaction controls the endocytic recycling of the cystic fibrosis transmembrane conductance regulator

The C terminus of CFTR contains a PDZ interacting domain that is required for the polarized expression of cystic fibrosis transmembrane conductance regulator (CFTR) in the apical plasma membrane of polarized epithelial cells. To elucidate the mechanism whereby the PDZ interacting domain mediates the polarized expression of CFTR, Madin-Darby canine kidney cells were stably transfected with wild type (wt-CFTR) or C-terminally truncated human CFTR (CFTR-DeltaTRL). We tested the hypothesis that the PDZ interacting domain regulates sorting of CFTR from the Golgi to the apical plasma membrane. Pulse-chase studies in combination with domain-selective cell surface biotinylation revealed that newly synthesized wt-CFTR and CFTR-DeltaTRL were targeted equally to the apical and basolateral membranes in a nonpolarized fashion. Thus, the PDZ interacting domain is not an apical sorting motif. Deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane from approximately 24 to approximately 13 h but had no effect on the half-life of CFTR in the basolateral membrane. Thus, the PDZ interacting domain is an apical membrane retention motif. Next, we examined the hypothesis that the PDZ interacting domain affects the apical membrane half-life of CFTR by altering its endocytosis and/or endocytic recycling. Endocytosis of wt-CFTR and CFTR-DeltaTRL did not differ. However, endocytic recycling of CFTR-DeltaTRL was decreased when compared with wt-CFTR. Thus, deletion of the PDZ interacting domain reduced the half-life of CFTR in the apical membrane by decreasing CFTR endocytic recycling. Our results identify a new role for PDZ proteins in regulating the endocytic recycling of CFTR in polarized epithelial cells.

Intracellular trafficking and membrane targeting mechanisms of the human reduced folate carrier in Mammalian epithelial cells

The major pathway for cellular uptake of the water-soluble vitamin folic acid in mammalian cells is via a plasma membrane protein known as the reduced folate carrier (RFC). The molecular determinants that dictate plasma membrane expression of RFC as well as the cellular mechanisms that deliver RFC to the cell surface remain poorly defined. Therefore, we designed a series of fusion proteins of the human RFC (hRFC) with green fluorescent protein to image the targeting and trafficking dynamics of hRFC in living epithelial cells. We show that, in contrast to many other nutrient transporters, the molecular determinants that dictate hRFC plasma membrane expression reside within the hydrophobic backbone of the polypeptide and not within the cytoplasmic NH(2)- or COOH-terminal domains of the protein. Further, the integrity of the hRFC backbone is critical for export of the polypeptide from the endoplasmic reticulum to the cell surface. This trafficking is critically dependent on intact microtubules because microtubule disruption inhibits motility of hRFC-containing vesicles as well as final expression of hRFC in the plasma membrane. For the first time, these data define the mechanisms that control the intracellular trafficking and cell surface localization of hRFC within mammalian epithelia.

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.

PICK1 is required for the control of synaptic transmission by the metabotropic glutamate receptor 7

Both postsynaptic density and presynaptic active zone are structural matrix containing scaffolding proteins that are involved in the organization of the synapse. Little is known about the functional role of these proteins in the signaling of presynaptic receptors. Here we show that the interaction of the presynaptic metabotropic glutamate (mGlu) receptor subtype, mGlu7a, with the postsynaptic density-95 disc-large zona occludens 1 (PDZ) domain-containing protein, PICK1, is required for specific inhibition of P/Q-type Ca(2+) channels, in cultured cerebellar granule neurons. Furthermore, we show that activation of the presynaptic mGlu7a receptor inhibits synaptic transmission and this effect also requires the presence of PICK1. These results indicate that the scaffolding protein, PICK1, plays an essential role in the control of synaptic transmission by the mGlu7a receptor complex.

Structural requirements for the apical sorting of human multidrug resistance protein 2 (ABCC2)

The human multidrug resistance protein 2 (MRP2, symbol ABCC2) is a polytopic membrane glycoprotein of 1545 amino acids which exports anionic conjugates across the apical membrane of polarized cells. A chimeric protein composed of C-proximal MRP2 and N-proximal MRP1 localized to the apical membrane of polarized Madin-Darby canine kidney cells (MDCKII) indicating involvement of the carboxy-proximal part of human MRP2 in apical sorting. When compared to other MRP family members, MRP2 has a seven-amino-acid extension at its C-terminus with the last three amino acids (TKF) comprising a PDZ-interacting motif. In order to analyze whether this extension is required for apical sorting of MRP2, we generated MRP2 constructs mutated and stepwise truncated at their C-termini. These constructs were fused via their N-termini to green fluorescent protein (GFP) and were transiently transfected into polarized, liver-derived human HepG2 cells. Quantitative analysis showed that full-length GFP-MRP2 was localized to the apical membrane in 73% of transfected, polarized cells, whereas it remained on intracellular membranes in 27% of cells. Removal of the C-terminal TKF peptide and stepwise deletion of up to 11 amino acids did not change this predominant apical distribution. However, apical localization was largely impaired when GFP-MRP2 was C-terminally truncated by 15 or more amino acids. Thus, neither the PDZ-interacting TKF motif nor the full seven-amino-acid extension were necessary for apical sorting of MRP2. Instead, our data indicate that a deletion of at least 15 C-terminal amino acids impairs the localization of MRP2 to the apical membrane of polarized cells.

The actin-binding protein Filamin-A interacts with the metabotropic glutamate receptor type 7

A yeast two-hybrid screen identified Filamin-A as a binding partner of the metabotropic glutamate receptor type 7b (mGluR7b) splice variant. In addition, Filamin-A interacted with mGluR4a, mGluR5a, mGluR5b, mGluR7a and mGluR8a. Domain mapping revealed that alternative splicing of mGluR4, mGluR7 and mGluR8 C-termini regulated the interaction. A conserved tyrosine within mGluR C-termini was identified to mediate the binding to Filamin-A. Protein interactions were verified in biochemical assays using recombinant and native proteins. Finally, co-expression of Filamin-A and mGluR7 splice variants was shown in brain regions. These findings suggest that Filamin-A may physically link metabotropic glutamate receptors to the actin cytoskeleton.

The proximal and distal C-terminal tail domains of the CB1 cannabinoid receptor mediate G protein coupling

The human CB1 cannabinoid receptor couples to G(i/o) proteins and inhibits neuronal voltage-gated Ca2+ channels. The role of the C-terminal tail of the CB1 cannabinoid receptor in G(i/o) protein coupling was examined using the superior cervical ganglion neuronal expression system. Deletion of the distal intracellular C-terminal tail (amino acids 418-472) slowed the kinetics and reduced the magnitude of Ca2+ channel inhibition. Deletion of the entire intracellular C-terminal tail (amino acids 401-472) abolished Ca2+ channel inhibition demonstrating the critical role of the proximal amino acids 401-417 of the C-terminal tail in G protein signaling. Expression of the C-terminal truncated receptors on the cell surface was examined using an N-terminal CB1 antibody. Both the C-terminal truncated receptors were expressed on the cell surface and were no different from wild type CB1 cannabinoid receptors. This study establishes that the proximal CB1 cannabinoid receptor intracellular C-terminal tail domain (amino acids 401-417) is critical for G(i/o) protein coupling and that the distal C-terminal tail domain (amino acids 418-472) profoundly modulates both the magnitude and kinetics of signal transduction. Thus, the C-terminal tail of the CB1 cannabinoid receptor has a wider role in G protein coupling than was previously thought.