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G-protein coupled receptors: Metabotropic Glutamate Receptors
Ligand binding and activation of mGluR
- Review on Group I mGluRs, mGluR1 and mGluR5
- cloning of "new type fo GPCR" in 1991: the type 1a mGluR
- Group I mGluR are widely and differentially expressed in neuronal and glial cells in the brain
- receptor activation has been implicated in the control of signaling events including roles in adaptive changes needed for long-term depression or potentiation of neuronal synaptic connectivity
- potential drug target for the treatment of a variety of neurological and psychiatric disorders
- glutamate major excitatory neurotransmitter in the mammalian central nervous system
- well documented role in learning and memory acquisition processes
- can also act as potent neurotoxic agent that plays a critical role in a number of neurological disorders
- glutamate exerts its role via
1. ionotropic glutamate receptors/channels, i.e. NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid) and kainate: [review see Dingledine, R., K. Borges, D. Bowie, and S.F. Traynelis, The glutamate receptor ion channels. Pharmacol Rev, 1999. 51(1): p. 7-61]
2. metabotropic glutamate receptors with mediation by GPCR: search for molecular idetnity for GPCR type glutamate receptor resulted in 2 independent clonings of mGlu1a: [ref. Houamed, K.M., J.L. Kuijper, T.L. Gilbert, B.A. Haldeman, P.J. O'Hara, E.R. Mulvihill, W. Almers, and F.S. Hagen, Cloning, expression, and gene structure of a G protein-coupled glutamate receptor from rat brain. Science, 1991. 252(5010): p. 1318-21. ; Masu, M., Y. Tanabe, K. Tsuchida, R. Shigemoto, and S. Nakanishi, Sequence and expression of a metabotropic glutamate receptor. Nature, 1991. 349(6312): p. 760-5.]
- in 1991-1995: 7 mammalian mGluR with 35-60% sequence identity with mGlu1a were cloned, based on sequence similarity and pharmacological and signaling pathway criteria into the following groups:
parameter group I group II group III
splice variants (length) mGlu1a (h1194,r1199) mGlu2 (h872,r872) mGlu4a (h912,r912)
h=human, r=rat mGlu1b (h906, r906) mGlu3 (h877,r879) mGlu4b (r683)
mGlu1c (h897) mGlu6 (h877, r877)
mGlu1d (h908, r912) mGlu7a (h915,r915)
mGlu5a (h1180, r1171) mGlu7b (h922,r922)
mGlu5b (h1212, r1203) mGlu8a (h908,r908)
mGlu8b (h908,r908)
mGlu8c (h501)
Signaling PLC adenylate cyclase inhibition adenylate cyclase inhibition, phospodiesterase (mGlu6)
selective agonists L-quisqualate, (S)-DHPG DCG-IV, (2R,4R)-APDC L-AP4, L-SOP
CHPG (mGlu5)
Selective antagonists AIDA, LY367385 (mGlu1) MCCG, LY341495, EGLU MeSOP, MAP4, CPPG
CPCCOEt (mGlu1)
MPEP (mGlu5)
Synaptic localization postsynaptic, some presynaptic presynaptic presynaptic
- isoforms of Group I differ in C-terminus only, shortest one is mGlu1c
- mGlu1 (-/-) and mGlu5 (-/-) knock-out mice
- general properties of Class C:
mGluR, GABAB (3 subtypes gamma-aminobutyric acid), extracellular calcium-sensing receptor CaR and a broad multigene family of olfactory, taste and pheromone recpetors
within TM domains family C share 10-15% amino acid identity with family A/B members
all family C have large EC domain (usually more than 500 aa)
- Structure and function of extracellular domain:
- Reviews:
1. Pin, J.P., C. De Colle, A.S. Bessis, and F. Acher, New perspectives for the development of selective metabotropic glutamate receptor ligands. Eur J Pharmacol, 1999. 375(1-3): p. 277-94.
2. Bockaert, J. and J.P. Pin, Molecular tinkering of G protein-coupled receptors: an evolutionary success. Embo J, 1999. 18(7): p. 1723-9.
3. Costantino, G., A. Macchiarulo, and R. Pellicciari, Homology model of the closed, functionally active, form of the amino terminal domain of mGlur1. Bioorg Med Chem, 2001. 9(4): p. 847-52.
4. De Blasi, A., P.J. Conn, J. Pin, and F. Nicoletti, Molecular determinants of metabotropic glutamate receptor signaling. Trends Pharmacol Sci, 2001. 22(3): p. 114-20.
- critical role of N-term in ligand binding first demonstrated by mutagenesis and chimeras, final proof by expression of N-term alone which can bind mGlu agonists (ref.
1. Han, G. and D.R. Hampson, Ligand binding to the amino-terminal domain of the mGluR4 subtype of metabotropic glutamate receptor. J Biol Chem, 1999. 274(15): p. 10008-13.
2. Okamoto, T., N. Sekiyama, M. Otsu, Y. Shimada, A. Sato, S. Nakanishi, and H. Jingami, Expression and purification of the extracellular ligand binding region of metabotropic glutamate receptor subtype 1. J Biol Chem, 1998. 273(21): p. 13089-96.)
- similar conclusions of N-term binding to ligands for GABAB and CaR
- similarity of N-term of mGlu1a with bacterial periplasmic aa binding proteins e.g. the leucine/isoleucine/valine binding protein LIVBP which are involved in aa uptake, ref. O'Hara, P.J., P.O. Sheppard, H. Thogersen, D. Venezia, B.A. Haldeman, V. McGrane, K.M. Houamed, C. Thomsen, T.L. Gilbert, and E.R. Mulvihill, The ligand-binding domain in metabotropic glutamate receptors is related to bacterial periplasmic binding proteins. Neuron, 1993. 11(1): p. 41-52.
- review of bacterial periplasmic binding protein structure and function knowledge: Quiocho, F.A. and P.S. Ledvina, Atomic structure and specificity of bacterial periplasmic receptors for active transport and chemotaxis: variation of common themes. Mol Microbiol, 1996. 20(1): p. 17-25
- Model:
2 lobes separated by a cavity where glutamate binds [Costantino, G. and R. Pellicciari, Homology modeling of metabotropic glutamate receptors. (mGluRs) structural motifs affecting binding modes and pharmacological profile of mGluR1 agonists and competitive antagonists. J Med Chem, 1996. 39(20): p. 3998-4006; Costantino, G., A. Macchiarulo, and R. Pellicciari, Homology model of the closed, functionally active, form of the amino terminal domain of mGlur1. Bioorg Med Chem, 2001. 9(4): p. 847-52]
Ser165 and Thr188 in rat mGlu1, conserved in all mGluR subtypes interact with glutamate (ref. O'Hara, P.J., P.O. Sheppard, H. Thogersen, D. Venezia, B.A. Haldeman, V. McGrane, K.M. Houamed, C. Thomsen, T.L. Gilbert, and E.R. Mulvihill, The ligand-binding domain in metabotropic glutamate receptors is related to bacterial periplasmic binding proteins. Neuron, 1993. 11(1): p. 41-520
Arg78 binds to acidic group of mGluR agonists (ref. Jensen, A.A., P.O. Sheppard, P.J. O'Hara, P. Krogsgaard-Larsen, and H. Brauner-Osborne, The role of Arg(78) in the metabotropic glutamate receptor mGlu(1) for agonist binding and selectivity. Eur J Pharmacol, 2000. 397(2-3): p. 247-530)
- crystal structure of mGlu1 plus/minus glutamate supports "Venus flytrap" mechanism, where glutamate binding causes the lobes to close around the ligand) (ref. Kunishima, N., Y. Shimada, Y. Tsuji, T. Sato, M. Yamamoto, T. Kumasaka, S. Nakanishi, H. Jingami, and K. Morikawa, Structural basis of glutamate recognition by a dimeric metabotropic glutamate receptor. Nature, 2000. 407(6807): p. 971-7.) This mechanism also proposed for GABAB receptor (ref. Galvez, T., M.L. Parmentier, C. Joly, B. Malitschek, K. Kaupmann, R. Kuhn, H. Bittiger, W. Froestl, B. Bettler, and J.P. Pin, Mutagenesis and modeling of the GABAB receptor extracellular domain support a venus flytrap mechanism for ligand binding. J Biol Chem, 1999. 274(19): p. 13362-9)
- 2-lobe structure is separated from the TM domain by cysteine-rich domain, proposed to be a flexible spacer to allow displacement of glutamate binding pocket towards the TM domain
- Challenge of activation model for generality in GPCR:
- in Class A, mostly binding of ligands to TM domain, here binding of ligand to EC domain
- according to ternary complex model: when GPCR is bound to G protein, agonist binds better; if you release G protein, agonist binding is disrupted; the same is true for mGluR, suggesting that there is a dynamic interaction between EC and TM domains (coupling of the domains)
- current belief: TM domain is activated by EC domain (similar to thrombin and other protease activated receptors, for which the activator of the receptor is a N-term region that becomes unmasked following protease cleave and interacts with specific TM domains to cause receptor activation [Coughlin, S.R., How the protease thrombin talks to cells. Proc Natl Acad Sci U S A, 1999. 96(20): p. 11023-7]) by conformational change of EC domain which brings it closer to the TM domain (by analogy with bacterial periplasmic binding protein, ref. Olah, G.A., S. Trakhanov, J. Trewhella, and F.A. Quiocho, Leucine/isoleucine/valine-binding protein contracts upon binding of ligand. J Biol Chem, 1993. 268(22): p. 16241-7)
- there seems to be a lack of specificity in activation of TM domain: chimera mGlu1(1-592)/CaR(613-1078) was able to be activated by glutamate [Hammerland, L.G., K.J. Krapcho, J.E. Garrett, N. Alasti, B.C. Hung, R.T. Simin, C. Levinthal, E.F. Nemeth, and F.H. Fuller, Domains determining ligand specificity for Ca2+ receptors. Mol Pharmacol, 1999. 55(4): p. 642-8]; other references:
1. Brauner-Osborne, H., A.A. Jensen, and P. Krogsgaard-Larsen, Interaction of CPCCOEt with a chimeric mGlu1b and calcium sensing receptor. Neuroreport, 1999. 10(18): p. 3923-5.
2. Jensen, A.A., T.A. Spalding, E.S. Burstein, P.O. Sheppard, P.J. O'Hara, M.R. Brann, P. Krogsgaard-Larsen, and H. Brauner-Osborne, Functional importance of the Ala(116)-Pro(136) region in the calcium-sensing receptor. Constitutive activity and inverse agonism in a family C G-protein-coupled receptor. J Biol Chem, 2000. 275(38): p. 29547-55.
3. Malitschek, B., C. Schweizer, M. Keir, J. Heid, W. Froestl, J. Mosbacher, R. Kuhn, J. Henley, C. Joly, J.P. Pin, K. Kaupmann, and B. Bettler, The N-terminal domain of gamma-aminobutyric Acid(B) receptors is sufficient to specify agonist and antagonist binding. Mol Pharmacol, 1999. 56(2): p. 448-54.
4. Brauner-Osborne, H., A.A. Jensen, P.O. Sheppard, P. O'Hara, and P. Krogsgaard-Larsen, The agonist-binding domain of the calcium-sensing receptor is located at the amino-terminal domain. J Biol Chem, 1999. 274(26): p. 18382-6.
- binding of agonists/antagonists to TM domain in mGluR:
- mGlu subtype selective antagonists CPCCOEt and BAY36-7620 block mGlu1 non-competitively
- and MPEP non-competitive effect on mGlu5
- all agents were shown NOT to bind at the agonist binding site, but in the TM domain [Litschig, S., F. Gasparini, D. Rueegg, N. Stoehr, P.J. Flor, I. Vranesic, L. Prezeau, J.P. Pin, C. Thomsen, and R. Kuhn, CPCCOEt, a noncompetitive metabotropic glutamate receptor 1 antagonist, inhibits receptor signaling without affecting glutamate binding. Mol Pharmacol, 1999. 55(3): p. 453-61; Pagano, A., D. Ruegg, S. Litschig, N. Stoehr, C. Stierlin, M. Heinrich, P. Floersheim, L. Prezeau, F. Carroll, J.P. Pin, A. Cambria, I. Vranesic, P.J. Flor, F. Gasparini, and R. Kuhn, The non-competitive antagonists 2-methyl-6-(phenylethynyl)pyridine and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester interact with overlapping binding pockets in the transmembrane region of group I metabotropic glutamate receptors. J Biol Chem, 2000. 275(43): p. 33750-8; Brauner-Osborne, H., A.A. Jensen, and P. Krogsgaard-Larsen, Interaction of CPCCOEt with a chimeric mGlu1b and calcium sensing receptor. Neuroreport, 1999. 10(18): p. 3923-5]
- note: check these references:
1. Jensen, A.A., J.R. Greenwood, and H. Brauner-Osborne, The dance of the clams: twists and turns in the family C GPCR homodimer. Trends Pharmacol Sci, 2002. 23(11): p. 491-3.
2. Jensen, A.A., J.L. Hansen, S.P. Sheikh, and H. Brauner-Osborne, Probing intermolecular protein-protein interactions in the calcium-sensing receptor homodimer using bioluminescence resonance energy transfer (BRET). Eur J Biochem, 2002. 269(20): p. 5076-87.
3. Jensen, A.A., J. Mosbacher, S. Elg, K. Lingenhoehl, T. Lohmann, T.N. Johansen, B. Abrahamsen, J.P. Mattsson, A. Lehmann, B. Bettler, and H. Brauner-Osborne, The anticonvulsant gabapentin (neurontin) does not act through gamma-aminobutyric acid-B receptors. Mol Pharmacol, 2002. 61(6): p. 1377-84.
4. Jensen, A.A., B.E. Madsen, P. Krogsgaard-Larsen, and H. Brauner-Osborne, Pharmacological characterization of homobaclofen on wild type and mutant GABA(B)1b receptors coexpressed with the GABA(B)2 receptor. Eur J Pharmacol, 2001. 417(3): p. 177-80.
5. Brauner-Osborne, H., A.A. Jensen, P.O. Sheppard, B. Brodin, P. Krogsgaard-Larsen, and P. O'Hara, Cloning and characterization of a human orphan family C G-protein coupled receptor GPRC5D. Biochim Biophys Acta, 2001. 1518(3): p. 237-48.
6. Jensen, A.A., P.O. Sheppard, L.B. Jensen, P.J. O'Hara, and H. Brauner-Osborne, Construction of a high affinity zinc binding site in the metabotropic glutamate receptor mGluR1: noncompetitive antagonism originating from the amino-terminal domain of a family C G-protein-coupled receptor. J Biol Chem, 2001. 276(13): p. 10110-8.
7. Jensen, A.A., P.O. Sheppard, P.J. O'Hara, P. Krogsgaard-Larsen, and H. Brauner-Osborne, The role of Arg(78) in the metabotropic glutamate receptor mGlu(1) for agonist binding and selectivity. Eur J Pharmacol, 2000. 397(2-3): p. 247-53.
8. Jensen, A.A., T.A. Spalding, E.S. Burstein, P.O. Sheppard, P.J. O'Hara, M.R. Brann, P. Krogsgaard-Larsen, and H. Brauner-Osborne, Functional importance of the Ala(116)-Pro(136) region in the calcium-sensing receptor. Constitutive activity and inverse agonism in a family C G-protein-coupled receptor. J Biol Chem, 2000. 275(38): p. 29547-55.
9. Brauner-Osborne, H., A.A. Jensen, and P. Krogsgaard-Larsen, Interaction of CPCCOEt with a chimeric mGlu1b and calcium sensing receptor. Neuroreport, 1999. 10(18): p. 3923-5.
10. Brauner-Osborne, H., A.A. Jensen, P.O. Sheppard, P. O'Hara, and P. Krogsgaard-Larsen, The agonist-binding domain of the calcium-sensing receptor is located at the amino-terminal domain. J Biol Chem, 1999. 274(26): p. 18382-6.
- use of chimera mGlu1/mGlu5 and CaR/mGlu showed that the pharmacological specificity of these agents depends on aa in TM domain, i.e. Thr815 and Ala818 in TM VII of mGlu1 are essential for antagonist activity of CPCCOEt [Litschig, S., F. Gasparini, D. Rueegg, N. Stoehr, P.J. Flor, I. Vranesic, L. Prezeau, J.P. Pin, C. Thomsen, and R. Kuhn, CPCCOEt, a noncompetitive metabotropic glutamate receptor 1 antagonist, inhibits receptor signaling without affecting glutamate binding. Mol Pharmacol, 1999. 55(3): p. 453-61.] and residues in TM VII and TM III of mGlu5 for MPEP (Pagano, A., D. Ruegg, S. Litschig, N. Stoehr, C. Stierlin, M. Heinrich, P. Floersheim, L. Prezeau, F. Carroll, J.P. Pin, A. Cambria, I. Vranesic, P.J. Flor, F. Gasparini, and R. Kuhn, The non-competitive antagonists 2-methyl-6-(phenylethynyl)pyridine and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester interact with overlapping binding pockets in the transmembrane region of group I metabotropic glutamate receptors. J Biol Chem, 2000. 275(43): p. 33750-8.)
Constitutive activity of mGluR
- reviewed in Hermans and Challis (2001) review
- review of GPCR in general: de Ligt, R.A., A.P.
Kourounakis, and I.J. AP, Inverse agonism
at G protein-coupled receptors: (patho)physiological relevance and implications
for drug discovery. Br J Pharmacol, 2000. 130(1):
p. 1-12
- mGlu1a but not mGlu1b,c in HEK293 cells exhibit constitutive activity with respect to phosphoinositide turnover [ Prezeau, L., J. Gomeza, S. Ahern, S. Mary, T. Galvez, J. Bockaert, and J.P. Pin, Changes in the carboxyl-terminal domain of metabotropic glutamate receptor 1 by alternative splicing generate receptors with differing agonist-independent activity. Mol Pharmacol, 1996. 49(3): p. 422-9]
- constitutive activity can be suppressed by MPEP [Pagano, A., D. Ruegg, S. Litschig, N. Stoehr, C. Stierlin, M. Heinrich, P. Floersheim, L. Prezeau, F. Carroll, J.P. Pin, A. Cambria, I. Vranesic, P.J. Flor, F. Gasparini, and R. Kuhn, The non-competitive antagonists 2-methyl-6-(phenylethynyl)pyridine and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester interact with overlapping binding pockets in the transmembrane region of group I metabotropic glutamate receptors. J Biol Chem, 2000. 275(43): p. 33750-8.)
- in neuronal cells, the constitutive activity of mGlu1/5 is normally suppressed through interactions with cytoskeletal elements, i.e. homer proteins [Ango, F., L. Prezeau, T. Muller, J.C. Tu, B. Xiao, P.F. Worley, J.P. Pin, J. Bockaert, and L. Fagni, Agonist-independent activation of metabotropic glutamate receptors by the intracellular protein Homer. Nature, 2001. 411(6840): p. 962-5]
- some evidence for requirement for extracellular Ca2+ in receptor ligand interactions, supported by crystal structure which contains metal binding site [Kunishima, N., Y. Shimada, Y. Tsuji, T. Sato, M. Yamamoto, T. Kumasaka, S. Nakanishi, H. Jingami, and K. Morikawa, Structural basis of glutamate recognition by a dimeric metabotropic glutamate receptor. Nature, 2000. 407(6807): p. 971-7.]
Dimerization of mGluR
- mGlu1 and mGlu5 by Western blot shown to aggregate to dimers and multimers and mutagenesis has shown that cysteines are involved in the dimerization and it is considered well established that mGlu exist as dimers on the cell surface, but its role is unclear
- possibly role because transfected fibroblasts expressing mGlu5 with reducing agents do not respond as well to agonist, but mutagenesis of the cysteines did not alter agonist biding
- for GABAB it is clearly established that heterodimerization between GABAB reeptor subtypes is required for targeting of functional receptors to the cell surface
- review of signaling pathways in mGluR1/5 by Hermans and Challiss review (2001)
Sites on mGluR that interact with the G protein:
- by chimera work between receptor subtypes that couple to PLC or to inhibit adenylate cyclase [Ref: Gomeza, J., C. Joly, R. Kuhn, T. Knopfel, J. Bockaert, and J.P. Pin, The second intracellular loop of metabotropic glutamate receptor 1 cooperates with the other intracellular domains to control coupling to G-proteins. J Biol Chem, 1996. 271(4): p. 2199-205.; Pin, J.P., C. Joly, S.F. Heinemann, and J. Bockaert, Domains involved in the specificity of G protein activation in phospholipase C-coupled metabotropic glutamate receptors. Embo J, 1994. 13(2): p. 342-8.]:
i2 loop and membrane-proximal C0termala region are responsible for PLC activation
these regions contain several basic residues that have been proposed to form amphipathic alpha-helices, typical of G-protein coupling domains (get references above!!!)
- distinct roles for i2 and i3 loops in selectivity of G protein coupling demonstrated by single amino acid substitutions [Francesconi, A. and R.M. Duvoisin, Role of the second and third intracellular loops of metabotropic glutamate receptors in mediating dual signal transduction activation. J Biol Chem, 1998. 273(10): p. 5615-24]:
critical residues in i2 interaction in mGlu1a with Gs or Gq and Lys690 in i2 switches coupling to Gi
- summary: i3 in Class A, but i2 in Class C are most important for G protein coupling [Pin, J.P. and R. Duvoisin, The metabotropic glutamate receptors: structure and functions. Neuropharmacology, 1995. 34(1): p. 1-26] (but see above, H8 is also important both for Class A and Class C - this splitting of roles of i2/i3 is not appropriate since the CP domain is a domain made up of the different loops not just loops dangling off the helices)
Role of C-terminal splice forms for receptor signaling:
- all variants of mGluR are splice forms that affect the C-terminus only
- short C-terminus: reduced agonist potency, but overall the effects of C-terminus on signaling are unclear
Proteins interacting with mGluR1/5:
Protein Site of interaction Effect/Role
G-proteins i2,i3 coupling to effectors
Homers Pro-rich domain in C-term adaptor proteins controlling molecular interactions with signaling proteins,
e.g. Ins(1,4,5)P3 receptors [Tu, J.C., B. Xiao, J.P. Yuan, A.A. Lanahan, K. Leoffert, M. Li, D.J. Linden, and P.F. Worley, Homer binds a novel proline-rich motif and links group 1 metabotropic glutamate receptors with IP3 receptors. Neuron, 1998. 21(4): p. 717-26]
ryanodine receptors [Fagni, L., P. Chavis, F. Ango, and J. Bockaert, Complex interactions between mGluRs, intracellular Ca2+ stores and ion channels in neurons. Trends Neurosci, 2000. 23(2): p. 80-8]
receptor trafficking [ Tadokoro, S., T. Tachibana, T. Imanaka, W. Nishida, and K. Sobue, Involvement of unique leucine-zipper motif of PSD-Zip45 (Homer 1c/vesl-1L) in group 1 metabotropic glutamate receptor clustering. Proc Natl Acad Sci U S A, 1999. 96(24): p. 13801-6.; Ciruela, F., M.M. Soloviev, and R.A. McIlhinney, Co-expression of metabotropic glutamate receptor type 1alpha with homer-1a/Vesl-1S increases the cell surface expression of the receptor. Biochem J, 1999. 341 ( Pt 3): p. 795-803. ; Roche, K.W., J.C. Tu, R.S. Petralia, B. Xiao, R.J. Wenthold, and P.F. Worley, Homer 1b regulates the trafficking of group I metabotropic glutamate receptors. J Biol Chem, 1999. 274(36): p. 25953-7.; Roche, K.W., C.D. Ly, R.S. Petralia, Y.X. Wang, A.W. McGee, D.S. Bredt, and R.J. Wenthold, Postsynaptic density-93 interacts with the delta2 glutamate receptor subunit at parallel fiber synapses. J Neurosci, 1999. 19(10): p. 3926-34.; Ango, F., D. Robbe, J.C. Tu, B. Xiao, P.F. Worley, J.P. Pin, J. Bockaert, and L. Fagni, Homer-dependent cell surface expression of metabotropic glutamate receptor type 5 in neurons. Mol Cell Neurosci, 2002. 20(2): p. 323-9.]
PSD proteins [Tu, J.C., B. Xiao, S. Naisbitt, J.P. Yuan, R.S. Petralia, P. Brakeman, A. Doan, V.K. Aakalu, A.A. Lanahan, M. Sheng, and P.F. Worley, Coupling of mGluR/Homer and PSD-95 complexes by the Shank family of postsynaptic density proteins. Neuron, 1999. 23(3): p. 583-92]
and other cytoskeletal elements [Shiraishi, Y., A. Mizutani, H. Bito, K. Fujisawa, S. Narumiya, K. Mikoshiba, and T. Furuichi, Cupidin, an isoform of Homer/Vesl, interacts with the actin cytoskeleton and activated rho family small GTPases and is expressed in developing mouse cerebellar granule cells. J Neurosci, 1999. 19(19): p. 8389-400]
and in modulation of the functional coupling of the receptor with ion channels [Kammermeier, P.J., B. Xiao, J.C. Tu, P.F. Worley, and S.R. Ikeda, Homer proteins regulate coupling of group I metabotropic glutamate receptors to N-type calcium and M-type potassium channels. J Neurosci, 2000. 20(19): p. 7238-45]
Calmodulin membrane proximal C-term interferes with phosphorylation of mGluR5 [Minakami, R., N. Jinnai, and H. Sugiyama, Phosphorylation and calmodulin binding of the metabotropic glutamate receptor subtype 5 (mGluR5) are antagonistic in vitro. J Biol Chem, 1997. 272(32): p. 20291-8.]
Tubulin C-term of GluR1 mGluR clustering [Ciruela, F., M.J. Robbins, A.C. Willis, and R.A. McIlhinney, Interactions of the C terminus of metabotropic glutamate receptor type 1alpha with rat brain proteins: evidence for a direct interaction with tubulin. J Neurochem, 1999. 72(1): p. 346-54.; Ciruela, F. and R.A. McIlhinney, Metabotropic glutamate receptor type 1alpha and tubulin assemble into dynamic interacting complexes. J Neurochem, 2001. 76(3): p. 750-7.]
anchoring to membrane [Lujan, R. and F. Ciruela, Immunocytochemical localization of metabotropic glutamate receptor type 1 alpha and tubulin in rat brain. Neuroreport, 2001. 12(6): p. 1285-91.]
Src-family protein tyrosine kinase Pro-rich domain of C-term in long isoforms G-protein independent intracellular signaling [Heuss, C. and U. Gerber, G-protein-independent signaling by G-protein-coupled receptors. Trends Neurosci, 2000. 23(10): p. 469-75.; Heuss, C., M. Scanziani, B.H. Gahwiler, and U. Gerber, G-protein-independent signaling mediated by metabotropic glutamate receptors. Nat Neurosci, 1999. 2(12): p. 1070-7.]
arrestins not determined desensitization after phosphorylation [Mundell, S.J., A.L. Matharu, G. Pula, P.J. Roberts, and E. Kelly, Agonist-induced internalization of the metabotropic glutamate receptor 1a is arrestin- and dynamin-dependent. J Neurochem, 2001. 78(3): p. 546-51]
G-protein independent signaling of mGluR
is reviewed in Hermans and Challiss (2001) review
Interaction of mGluR with PDZ domains --> Homer proteins
- reviewed in Hermans and Challiss (2001) review
- PDZ = postsynaptic density PSD-95 Discs-large, Zona occludens-1allow protein protein interactions to occurs in general with the specified motif
- in neurons, proteins having PDZ domains are often localized to the PSD, where they are believed to assebmle supramolecular signalling complesxes between surface receptors, ion channels and signaling elements [ for review see Scannevin, R.H. and R.L. Huganir, Postsynaptic organization and regulation of excitatory synapses. Nat Rev Neurosci, 2000. 1(2): p. 133-41]
- iGluR (NMDA, AMPA and kainate) have all been shown to interact with PDZ domain containing proteins
- like iGluR, mGluR group I also display localization to postsynaptic glutamatergic membrane, they form an annulus around a central core of iGluR
- this organization is achieved via protein called Homer, which contains PDZ domain and EVH1 (Ena/VASP homology)-like domain
- EVH1 interacts with C-term of mGlu1/5 (Pro-rich region)
- this interaction between mGluR and Homer is complicated (for details see Hermans and Challiss Review (2001))