Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

Heptaspanning membrane receptors and cytoskeletal/scaffolding proteins: focus on adenosine, dopamine, and metabotropic glutamate receptor function

Most cellular functions are mediated by multiprotein complexes. In neurons, these complexes are directly involved in the proper neuronal transmission, which is responsible for phenomena like learning, memory, and development. In recent years studies based on two-hybrid screens and proteomic, biochemical, and cell biology approaches have shown that intracellular domains of G protein-coupled receptors (GPCRs) or heptaspanning membrane receptors (HSMRs) interact with intracellular proteins. These interactions are the basis of a protein network associated with these receptors, which includes scaffolding proteins containing one or several PDZ (postsynaptic-density-95/discs-large/zona occludens-1) domains, signaling proteins, and proteins of the cytoskeleton. The present article is focused on the emerging evidence for interactions of adenosine, dopamine, and metabotropic glutamate receptors, with scaffolding and cytoskeletal proteins that play a role in the targeting and anchoring of these receptors to the plasma membrane, thus contributing to neuronal development and plasticity. Finally, given the complexity of neurological disorders such as ischemic stroke, Alzheimer's disease, and epilepsy, exploitation of these HSMR-associated interactions might prove to be efficient in the treatment of such disorders.

Interaction between GABAA receptor subunit intracellular loops: implications for higher order complex formation

The majority of fast inhibitory neurotransmission in the CNS is mediated by the GABA type-A (GABAA) receptor, a ligand-gated chloride channel. Of the approximately 20 different subunits composing the hetero-pentameric GABAA receptor, the gamma2 subunit in particular seems to be important in several aspects of GABAA receptor function, including clustering of the receptor at synapses. In this study, we report that the intracellular loop of the gamma2 subunit interacts with itself as well as with gamma1, gamma3 and beta1-3 subunits, but not with the alpha subunits. We further show that gamma2 subunits interact with photolabeled pentameric GABAA receptors composed of alpha1, beta2/3 and gamma2 subunits, and calculate the dissociation constant to be in the micromolar range. By using deletion constructs of the gamma2 subunit in a yeast two-hybrid assay, we identified a 23-amino acid motif that mediates self-association, residues 389-411. We confirmed this interaction motif by inhibiting the interaction in a glutathione-S-transferase pull-down assay by adding a corresponding gamma2-derived peptide. Using similar approaches, we identified the interaction motif in the gamma2 subunit mediating interaction with the beta2 subunit as a 47-amino acid motif that includes the gamma2 self-interacting motif. The identified gamma2 self-association motif is identical to the interaction motif reported between GABAA receptor and GABAA receptor-associated protein (GABARAP). We propose a model for GABAA receptor clustering based on GABARAP and GABAA receptor subunit-subunit interaction.

Biochemical identification of the binding domain in the GABA(A) receptor-associated protein (GABARAP) mediating dimer formation

The gamma-aminobutyric acid receptor type A (GABA(A)) receptor-associated protein (GABARAP) is a member of a growing family of intracellular membrane trafficking and/or fusion proteins and has been implicated in plasma membrane targeting and clustering of GABA(A) receptors. GABARAP interacts with microtubules and the gamma2 subunit of GABA(A) receptor and modulates channel kinetics. From crystal structures of GABARAP in high salt concentration it has been proposed that oligomerization of GABARAP might take place in a head-to-tail fashion. In this study, we report that GABARAP self-associates and dimerizes in physiological salt concentrations. We find no evidence for higher order complex larger than a dimer. By using deletion constructs of GABARAP we show that interaction takes place between amino acid 36 and 68. We further narrow the interacting domain by inhibiting the self-association, by adding GABARAP-derived synthetic peptides in GST pull-down assays and shows that the interaction specifically takes place in the previously identified GABARAP-GABA(A) receptor interaction domain from amino acid 41-51. The identification of binding domains in GABARAP allows for the study of GABARAP functions, including GABA(A) receptor dynamics.

Regulation of the subcellular distribution and gene expression of GABA(A) receptor by microtubules and microfilaments in cultured brain neurons

Mechanisms underlying the intracellular transport of gamma-aminobutyric acid(A) receptor (GABA(A)R) were examined in the cultured neurons derived from chicken embryo brains. In situ trypsinization of the cultures and (3)H-flunitrazepam (FNZ) binding assay were employed to determine the cell surface and intracellular distribution of the receptor. A 3-h treatment of the cells with 1 microM of colchicine, a microtubule depolymerizer, reversibly raised the proportion of intracellular GABA(A)R density by about 36% and decreased that of the cell surface receptors by 18% from respective control values, whereas the 3-h incubation with 2 microM of cytochalasin D, a microfilament disrupter, did not cause significant changes. These treatments failed to alter the total number of the (3)H-FNZ binding sites of the neurons and the affinity of the ligand. Moreover, the exposure to colchicine seemed to produce a stronger cytoplasmic immunostaining of the GABA(A)R alpha subunits in many neurons without affecting the total cellular level of the proteins, in accordance with the increased fraction of intracellular (3)H-FNZ binding. However, in the neurons exposed to cytochalasin D, there was an increase of around 28% in the total content of alpha(1)+51kDa proteins. In addition, the colchicine or cytochalasin D treatment inhibited approximately 21 or 18% of the rate of general protein synthesis in the culture. Notably, in situ hybridization assay showed that the GABA(A)R alpha(1) or alpha(2) mRNA was present in 92 +/- 2% or 94 +/- 2% of the cytochalasin D-treated neurons, both of which were higher than 71 +/- 2-74 +/- 3% of the control and colchicine-treated cells. The data suggest that by regulating the intracellular transport, the microtubular system participates in the maintenance of normal subcellular distribution of GABA(A)R in the neurons. By contrast, the organization of microfilaments may play a role in modulating the gene expression of GABA(A)R subunits.

Immunocytochemical localization of metabotropic glutamate receptor type 1 alpha and tubulin in rat brain

The distribution of mGlu1 alpha receptor and tubulin was immunocytochemically examined in the rat cerebellar cortex and primary rat cortical neurons at both immunofluorescence and electron microscopic level. In cryosections from rat cerebellar cortex mGlu1 alpha receptor immunoreactivity was expressed in cell bodies and dendrites of Purkinje and basket cells of the cerebellar molecular layer. Tubulin immunoreactivity was concentrated in the dendritic tree of the cerebellar molecular layer, as well as in the granule cell layer. In primary rat cortical neurons, both proteins colocalized throughout the proximal and distal dendrites of these cells. At the electron microscopic level, the receptor was present in dendritic shafts and dendritic spines of Purkinje cells at perisynaptic sites of asymmetrical synapses. Immunoreactivity corresponding to tubulin was associated with the plasma membrane of dendritic shafts of Purkinje cells, as well as throughout its cytoplasm as part of the cytoskeletal components. Interestingly, double labeling for both proteins reveals an association of tubulin with mGlu1 alpha receptor at the plasma membrane level of dendritic shafts of Purkinje cells. This suggests that tubulin interacts with mGlu1 alpha receptor and may be involved in the anchoring of the receptor to the plasma membrane.

Metabotropic glutamate receptor type 1alpha and tubulin assemble into dynamic interacting complexes

Metabotropic glutamate receptors (mGlu receptors) are coupled to G-protein second messenger pathways and modulate glutamate neurotransmission in the brain, where they are targeted to specific synaptic locations. Very recently, we identified tubulin as an interacting partner of the mGlu(1alpha) receptor in rat brain. Using BHK-570 cells permanently expressing the receptor we have shown that this interaction occurs predominantly with soluble tubulin, following its translocation to the plasma membrane. In addition, treatment of the cells with the agonist quisqualic acid induce tubulin depolymerization and its translocation to the plasma membrane. Immunofluorescence detection of both the receptor and tubulin in agonist-treated cells reveals a disruption of the microtubule network and an increased clustering of the receptor. Collectively these data demonstrate that the mGlu(1alpha) receptor interacts with soluble tubulin and that this association can take place at the plasma membrane.

Binding of the GABA(A) receptor-associated protein (GABARAP) to microtubules and microfilaments suggests involvement of the cytoskeleton in GABARAPGABA(A) receptor interaction

GABA(A) receptor-associated protein (GABARAP) was isolated on the basis of its interaction with the gamma2 subunit of GABA(A) receptors. It has sequence similarity to light chain 3 (LC3) of microtubule-associated proteins 1A and 1B. This suggests that GABARAP may link GABA(A) receptors to the cytoskeleton. GABARAP associates with tubulin in vitro. However, little is known about the mechanism for the interaction, and it is not clear whether the interaction occurs in vivo. Here, we report that GABARAP interacts directly with both tubulin and microtubules in a salt-sensitive manner, indicating the association is mediated by ionic interactions. GABARAP coimmunoprecipitates with tubulin and associates with both microtubules and microfilaments in intact cells. The cellular distribution is altered by treatment with taxol, nocodazole, and cytochalasin D. The tubulin binding domain was located at the N terminus of GABARAP by using synthetic peptides and deletion constructs and is marked by a specific arrangement of basic amino acids. The interaction between GABARAP and actin might be mediated by other proteins. These results demonstrate the GABARAP interacts with the cytoskeleton both in vitro and in cells and suggest a role of GABARAP in the interaction between GABA(A) receptors and the cytoskeleton. Such interactions are presumably needed for receptor trafficking, anchoring, and/or synaptic clustering. The structural arrangement of the basic amino acids present in the tubulin binding domain of GABARAP may aid in recognition of the potential of tubulin binding activity in other known proteins.

Association of a lower molecular weight protein to the mu-opioid receptor demonstrated by (125)I-beta-endorphin cross-linking studies

Cross-linking experiments using the (125)I-beta-endorphin revealed the presence of several receptor-related species in cell lines expressing endogenous opioid receptors, including a small molecular mass protein (approximately 22 kDa). Previous reports have suggested that this 22-kDa (125)I-beta-endorphin cross-linked protein could be the degradative product from a higher molecular mass species, i.e., a fragment of the receptor. To determine if this protein is indeed a degraded receptor fragment, (125)I-beta-endorphin was cross-linked to the (His)(6) epitope-tagged mu-opioid receptor (His-mu) stably expressed in the murine neuroblastoma Neuro(2A) cells. Similar to earlier reports with cell lines expressing endogenous receptors, two major bands of 72- and 25-kDa proteins were specifically cross-linked. Initial cross-linking experiments indicated the absolute requirement of the high-affinity (125)I-beta-endorphin binding to the mu-opioid receptor prior to the appearance of the low molecular weight species, suggesting that the 22-kDa protein could be a degraded fragment of the receptor. However, variations in the ratios of these protein bands being cross-linked by several homo- or heterobifunctional cross-linking agents were observed. Although neither the carboxyl terminus mu-opioid receptor-specific antibodies nor the antibodies against the epitope at the amino terminus of the receptor could recognize the 22-kDa protein, this (125)I-beta-endorphin cross-linked species could be coimmunoprecipitated with the receptor antibodies or could be isolated with a nickel resin affinity chromatography. The direct physical association of the 22-kDa protein with the receptor was demonstrated also by the observation that the 22-kDa protein could not bind to the nickel resin alone, but that its binding to the nickel resin was restored in the presence of the His-mu. Taken together, these results suggest that the 22-kDa protein cross-linked by (125)I-beta-endorphin is not a degradative product, but a protein located within the proximity of the mu-opioid receptor, and that it is tightly associated with the receptor.

GABA(A)-receptor-associated protein links GABA(A) receptors and the cytoskeleton

Type-A receptors for the neurotransmitter GABA (gamma-aminobutyric acid) are ligand-gated chloride channels that mediate inhibitory neurotransmission. Each subunit of the pentameric receptor protein has ligand-binding sites in the amino-terminal extracellular domain and four membrane-spanning regions, one of which forms a wall of the ion channel. Each subunit also has a large intracellular loop that may be a target for protein kinases and be required for subcellular targeting and membrane clustering of the receptor, perhaps by anchoring the receptor to the cytoskeleton. Neurotransmitter receptors need to be positioned in high density in the cell membrane at sites postsynaptic to nerve terminals releasing that neurotransmitter. Other members of the superfamily of ligand-gated ion-channel receptors associate in postsynaptic-membrane clusters by binding to the proteins rapsyn or gephyrin. Here we identify a new cellular protein, GABA(A)-receptor-associated protein (GABARAP), which can interact with the gamma2 subunit of GABA(A) receptors. GABARAP binds to GABA(A) receptors both in vitro and in vivo, and co-localizes with the punctate staining of GABA(A) receptors on cultured cortical neurons. Sequence analysis shows similarity between GABARAP and light chain-3 of microtubule-associated proteins 1A and 1B. Moreover, the N terminus of GABARAP is highly positively charged and features a putative tubulin-binding motif. The interactions among GABA(A) receptors, GABARAP and tubulin suggest a mechanism for the targeting and clustering of GABA(A) receptors.

Recruitment of a nicotinic acetylcholine receptor mutant lacking cytoplasmic tyrosine residues in its beta subunit into agrin-induced aggregates

During synaptogenesis at the vertebrate skeletal neuromuscular junction, acetylcholine receptors (AChRs) form high-density aggregates opposite the presynaptic terminal in response to nerve-derived agrin. Agrin has been shown to stimulate tyrosine phosphorylation of a muscle-specific receptor tyrosine kinase MuSK and of the AChR beta subunit, and tyrosine kinase inhibitors and a tyrosine kinase-deficient mutant of MuSK prevent AChR aggregation. To evaluate the role of tyrosine phosphorylation of the AChR beta subunit in receptor aggregation, we replaced all three putative cytoplasmic tyrosine residues of the AChR beta subunit with phenylalanine residues and expressed the mutant receptors in cultured myotubes. Upon agrin treatment, transfected myotubes formed AChR aggregates that contained receptors with mutant beta subunits. Thus, AChRs can be recruited into agrin-induced specializations by protein-protein interactions that do not depend on tyrosine phosphorylation of the AChR beta subunit.

Regulation of acetylcholine receptor gene expression in human myasthenia gravis muscles. Evidences for a compensatory mechanism triggered by receptor loss

Myasthenia gravis (MG) is a neuromuscular disorder mediated by antibodies directed against the acetylcholine receptor (nAChR) resulting in a functional nAChR loss. To analyze the molecular mechanisms involved at the muscular target site, we studied the expression of nAChR subunits in muscle biopsy specimens from MG patients. By using quantitative PCR with an internal standard for each subunit, we found that the levels of beta-, delta-, and epsilon-subunit mRNA coding for the adult nAChR were increased in severely affected MG patients, matching our previous data on the alpha-subunit. Messenger levels were highly variable in MG patients but not in controls, pointing to individual factors involved in the regulation of nAChR genes. The fetal subunit (gamma-chain) transcripts were almost undetectable in the extrajunctional region of MG muscle, suggesting that gene regulation in MG differs from that in the denervation model, in which nAChR gamma-subunit mRNA is reexpressed. Nicotinic AChR loss mediated by monoclonal anti-nAChR antibodies in both the TE671 muscle cell line and cultured normal human myotubes induces a similar increase in beta- alphand delta-subunit mRNA levels, suggesting the existence of a new muscular signaling pathway system coupled to nAChR internalization and independent of muscle electrical activity. These data demonstrate the existence of a compensatory mechanism regulating the expression of the genes coding for the adult nAChR in patients with MG.

Expression of glycine receptor alpha subunits and gephyrin in cultured spinal neurons

The inhibitory glycine receptor is a pentameric membrane protein composed of alpha and beta subunits. In the postsynaptic membrane, the glycine receptor and the copurifying peripheral membrane protein gephyrin are clustered underneath glycine-releasing nerve terminals. Here, we describe the expression of gephyrin and the neonatal and adult glycine receptor alpha subunit isoforms alpha1 and alpha2 during in vitro differentiation of rat spinal neurons. Analysis by immunoassays and the reverse transcriptase-polymerase chain reaction showed that gephyrin and alpha subunit mRNA and protein levels exhibited a marked increase from 1 to 5 days in vitro, i.e. prior to the formation of functional synaptic contacts. Using confocal and standard immunofluorescence, we determined the number of immunoreactive cells and the cellular localization of the alpha subunits and gephyrin. At 3 days in vitro, glycine receptor immunoreactivity revealed by the monoclonal antibody mAb4a was found in < 10% of cells and was mainly localized intracellularly; in contrast, gephyrin was detected in in vitro, gephyrin was essentially localized at the neuronal surface. At this stage, the number of glycine receptor-positive cells approached that of gephyrin-containing neurons (50%), and glycine receptor antigen was found both intracellularly and at the periphery of the cells. The antibody mAb2b, which binds exclusively to the alpha1 subunit, revealed aggregates at the surface of a few neurons. At 10 days in vitro, glycine receptor and gephyrin staining was localized in clusters at the periphery of the soma and the neurites. This quantitative analysis corroborates temporal differences in the cellular distribution of gephyrin and glycine receptor alpha subunits, the former being accumulated first at the neuronal surface.

Acetylcholine receptors and the cytoskeletal connection

1. The post-synaptic membranes of neurons and muscle cells are characterized by clusters of transmitter receptors, the number and type of which help to determine synaptic efficacy. Here I briefly review what is known of the mechanism of clustering of nicotinic acetylcholine receptors (AChR) at neuromuscular synapses. 2. The extracellular protein agrin is thought to be secreted by the motor nerve terminal and trigger localized clustering of AChR in the post-synaptic membrane of the skeletal muscle cell. 3. Binding of agrin to its receptor, alpha-dystroglycan, is followed by rearrangements of the muscle membrane cytoskeleton with localized replacement of dystrophin by utrophin. It remains unclear how these changes relate to the clustering of AChR. 4. In separate studies, RAPsyn/43k protein, a protein associated with the inner face of the post-synaptic membrane was shown to be able to cluster AChR and link them to the cytoskeleton when both proteins were co-transfected into fibroblasts. 5. Mutational studies on RAPsyn identified putative binding domains for AChR and for the cytoskeleton within the RAPsyn primary structure. Targeted disruption of the RAPsyn gene in mice prevented post-synaptic AChR clustering and led to neonatal lethality. Thus RAPsyn might be the final link in the pathway that leads to AChR immobilization in the post-synaptic membrane. 6. The recent observation that active forms of agrin are not restricted to cholinergic regions of the brain suggests that analogous pathways may exist for clustering other receptor types.

Clustering of the acetylcholine receptor by the 43-kD protein: involvement of the zinc finger domain

A postsynaptic membrane-associated protein of M(r) 43,000 (43-kD protein) is involved in clustering of the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Previous studies have shown that recombinant mouse 43-kD protein forms membrane-associated clusters when expressed in Xenopus oocytes. Coexpression with the AChR results in colocalization of the receptor with the 43-kD protein clusters (Froehner, S. C., C. W. Luetje, P. B. Scotland, and J. Patrick, 1990. Neuron. 5:403-410). To understand the mechanism of this clustering, we have studied the role of the carboxy-terminal region of the 43-kD protein. The amino acid sequence of this region predicts two tandem zinc finger structures followed by a serine phosphorylation site. Both Torpedo 43-kD protein and the carboxy-terminal region of the mouse 43-kD protein bind radioisotopic zinc. Mutation of two histidine residues in this predicted domain greatly attenuates zinc binding, lending support to the proposal that this region forms zinc fingers. When expressed in oocytes, the ability of this mutant 43-kD protein to form clusters is greatly reduced. Its ability to interact with AChR, however, is retained. In contrast, a mutation that eliminates the potential serine phosphorylation site has no effect on clustering of the 43-kD protein or on interaction with the AChR. These findings suggest that protein interactions via the zinc finger domain of the 43-kD protein may be important for AChR clustering at the synapse.

Receptor-receptor interactions as an integrative mechanism in nerve cells

Several lines of evidence indicate that interactions among transmission lines can take place at the level of the cell membrane via interactions among macromolecules, integral or associated to the cell membrane, involved in signal recognition and transduction. The present view will focus on this last subject, i.e., on the interactions between receptors for chemical signals at the level of the neuronal membrane (receptor-receptor interaction). By receptor-receptor interaction we mean that a neurotransmitter or modulator, by binding to its receptor, modifies the characteristics of the receptor for another transmitter or modulator. Four types of interactions among transmission lines may be considered, but mainly intramembrane receptor-receptor interactions have been dealt with in this article, exemplified by the heteroregulation of D2 receptors via neuropeptide receptors and A2 receptors. The role of receptor-receptor interactions in the integration of signals is discussed, especially in terms of filtration of incoming signals, of integration of coincident signals, and of neuronal plasticity.