Clustering of AMPA receptors by the synaptic PDZ domain-containing protein PICK1

A novel protein complex linking the delta 2 glutamate receptor and autophagy: implications for neurodegeneration in lurcher mice

Autophagy is a pathway for bulk degradation of subcellular constituents that is hyperactivated in many neurodegenerative conditions. It has been considered a second form of programmed cell death. Death of cerebellar Purkinje cells in lurcher animals is due to a mutation in GluRdelta2 that results in its constitutive activation. Here we have identified protein interactions between GluRdelta2, a novel isoform of a PDZ domain-containing protein (nPIST) that binds to this receptor, and Beclin1. nPIST and Beclin1 can synergize to induce autophagy. GluRdelta2(Lc), but not GluRdelta2(wt), can also induce autophagy. Furthermore, dying lurcher Purkinje cells contain morphological hallmarks of autophagic death in vivo. These results provide strong evidence that a direct link exists between GluRdelta2(Lc) receptor and stimulation of the autophagic pathway in dying lurcher Purkinje cells.

Neurofilament-M interacts with the D1 dopamine receptor to regulate cell surface expression and desensitization

We used the yeast two-hybrid assay to identify novel proteins that interact with the D(1) dopamine receptor. The third cytoplasmic loop (residues 217-273) of the rat D(1) receptor was used as bait to identify clones encoding interacting proteins from a rat brain cDNA library. This identified two clones encoding the C terminus of rat neurofilament-M (NF-M) (residues 782-846). The NF-M clone did not interact with the third cytoplasmic loops of the rat D(2), D(3), or D(4) receptors, but showed weak interaction with that of the D(5) receptor. Coexpression of full-length NF-M with the D(1) receptor in HEK-293 cells resulted in >50% reduction of receptor binding accompanied by a reduction in D(1) receptor-mediated cAMP accumulation. NF-M had no effect on the expression of other dopamine receptor subtypes. Using a D(1) receptor-green fluorescent protein chimera and confocal fluorescence microscopy, we found that NF-M reduced D(1) receptor expression at the cell surface and promoted accumulation of the receptor in the cytosol. Interestingly, the D(1) receptors that were expressed at the cell surface in the presence of NF-M were resistant to agonist-induced desensitization. Cellular colocalization of NF-M and the D(1) receptor in the rat brain was examined by epifluorescence microscopy. These experiments showed that approximately 50% of medium-sized striatal neurons expressed both proteins. Colocalization was also observed in pyramidal cells and interneurons within the frontal cortex. Similar immunohistochemical analyses using NF-M-deficient mice showed decrements in D(1) receptor expression compared with control mice. These results suggest that NF-M interacts with the D(1) receptor in vivo and may modify its expression and regulation.

Critical postsynaptic density 95/disc large/zonula occludens-1 interactions by glutamate receptor 1 (GluR1) and GluR2 required at different subcellular sites

Interactions between AMPA receptor subunits and proteins containing postsynaptic density 95/disc large/zonula occludens-1 (PDZ) domains have been shown to play critical roles in the proper trafficking of receptors to excitatory synapses. Synaptic accumulation of AMPA receptors containing the glutamate receptor 1 (GluR1) subunit can be driven by calcium/calmodulin-dependent protein kinase II activity or long-term potentiation and requires an interaction between GluR1 and a type I PDZ domain-containing protein. Synaptic incorporation of AMPA receptors with only GluR2 occurs continuously, and this requires an interaction between GluR2 and a type II PDZ domain-containing protein. We used dual-channel, two-photon laser scanning microscopy to provide high-resolution visualization and quantification of green fluorescent protein-tagged AMPA receptors in different subcellular compartments. We showed that mutations on GluR1 or GluR2 AMPA subunit that perturb interactions with PDZ domain proteins lead to the accumulation of these receptors at different subcellular sites. GluR1 mutants accumulate in the dendrite, whereas GluR2 mutants accumulate in dendritic spines. This suggests that the critical PDZ domain interactions are required for entry into spines for GluR1 and for entry into synapses for GluR2.

Association of the kinesin superfamily motor protein KIF1Balpha with postsynaptic density-95 (PSD-95), synapse-associated protein-97, and synaptic scaffolding molecule PSD-95/discs large/zona occludens-1 proteins

Mutation in KIF1B, a kinesin superfamily motor protein, causes a peripheral neuropathy known as Charcot-Marie-Tooth disease type 2A (CMT2A). Little is known, however, about how a defective KIF1B gene leads to CMT2A. Here we report that KIF1Balpha, one of the two splice variants of KIF1B, directly interacts through its C-terminal postsynaptic density-95 (PSD-95)/discs large/zona occludens (PDZ) domain-binding motif with PDZ proteins including PSD-95/synapse-associated protein-90 (SAP90), SAP97, and synaptic scaffolding molecule (S-SCAM)-90 (SAP90). KIF1Balpha selectively interacts with PSD-95, SAP97, and S-SCAM in yeast two-hybrid, pull-down, and in vivo coimmunoprecipitation experiments. KIF1Balpha, SAP97, and S-SCAM are widely distributed to both dendrites and axons of cultured neurons and are enriched in the small membrane fraction of the brain. In the flotation assay, KIF1Balpha cofractionates and coimmunoprecipitates with PSD-95, SAP97, and S-SCAM. These results suggest that the PSD-95 family proteins and S-SCAM have a novel function as KIF1Balpha receptors, linking KIF1Balpha to its specific cargos, and are involved in peripheral neuropathies.

Alpha7 nicotinic acetylcholine receptors occur at postsynaptic densities of AMPA receptor-positive and -negative excitatory synapses in rat sensory cortex

NMDA receptor (NMDAR) activation requires concurrent membrane depolarization, and glutamatergic synapses lacking AMPA receptors (AMPARs) are often considered "silent" in the absence of another source of membrane depolarization. During the second postnatal week, NMDA currents can be enhanced in rat auditory cortex through activation of the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). Electrophysiological results support a mainly presynaptic role for alpha7nAChR at these synapses. However, immunocytochemical evidence that alpha7nAChR is prevalent at postsynaptic sites of glutamatergic synapses in hippocampus and neocortex, along with emerging electrophysiological evidence for postsynaptic nicotinic currents in neocortex and hippocampus, has prompted speculation that alpha7nAChR allows for activation of NMDAR postsynaptically at synapses lacking AMPAR. Here we used dual immunolabeling and electron microscopy to examine the distribution of alpha7nAChR relative to AMPAR (GluR1, GluR2, and GluR3 subunits combined) at excitatory synapses in somatosensory cortex of adult and 1-week-old rats. alpha7nAChR occurred discretely over most of the thick postsynaptic densities in all cortical layers of both age groups. AMPAR immunoreactivity was also detectable at most synapses; its distribution was independent of that of alpha7nAChR. In both age groups, approximately one-quarter of asymmetrical synapses were alpha7nAChR positive and AMPAR negative. The variability of postsynaptic alpha7nAChR labeling density was greater at postnatal day (PD) 7 than in adulthood, and PD 7 neuropil contained a subset of small AMPA receptor-negative synapses with a high density of alpha7nAChR immunoreactivity. These observations support the idea that acetylcholine receptors can aid in activating glutamatergic synapses and work together with AMPA receptors to mediate postsynaptic excitation throughout life.

Synaptically targeted narp plays an essential role in the aggregation of AMPA receptors at excitatory synapses in cultured spinal neurons

Neuronal activity regulated pentraxin (Narp) has been implicated in the aggregation of AMPA-type glutamate receptors (GluR) at excitatory synapses. In the present paper, we examine the role of endogenous Narp in excitatory synapse formation by using novel, dominant-negative Narp mutants (dnNarp) that selectively bind endogenous Narp and prevent its accumulation at synapses. Axons from neurons transfected with wild-type Narp showed an increase in their ability to cluster AMPA receptors on spinal neurons, whereas axons from neurons transfected with dnNarp showed a marked decrease in their ability to induce GluR1 clusters on contacted dendrites. Despite their marked effect at excitatory synapses, dnNarp and wild-type Narp had no effect on the postsynaptic clustering of the inhibitory protein gephyrin or the percentage of contacts associated with staining for the presynaptic vesicle proteins GAD or synaptophysin. Use of the dnNarp mutants to suppress endogenous Narp expression by postsynaptic dendrites showed a complementary role for dendritic Narp in the clustering of synaptic AMPA receptors, as well as a reduction in the total number of excitatory synapses on transfected neurons. Together these experiments suggest an important role for Narp in the formation of excitatory synapses in cultured spinal neurons.

Molecular constituents and phosphorylation-dependent regulation of the post-synaptic density

The post-synaptic density (PSD) contains receptors with associated signaling- and scaffolding-proteins that organize signal-transduction pathways near the post-synaptic membrane. The PSD plays an important role in synaptic plasticity, and protein phosphorylation is critical to the regulation of PSD function, including learning and memory. Recently, studies have investigated the protein constituents of the PSD and substrate proteins for various protein kinases by proteomic analysis. The present review focuses on the molecular properties of PSD proteins, and substrates of protein kinases and their regulation by phosphorylation in order to understand the role of PSD in synaptic plasticity.

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.

Dynamic regulation of GABA(A) receptors at synaptic sites

gamma-Aminobutyric acid type A receptors (GABA(A)Rs) mediate fast synaptic inhibition in brain and spinal cord. They are ligand-gated ion channels composed of numerous distinct subunit combinations. For efficient synaptic transmission, GABA(A)Rs need to be localized to and anchored at postsynaptic sites in precise apposition to presynaptic nerve terminals that release the neurotransmitter GABA. Neurons therefore require distinct mechanisms to regulate intracellular vesicular protein traffic, plasma membrane insertion, synaptic clustering and turnover of GABA(A)Rs. The GABA(A) receptor-associated protein GABARAP interacts with the gamma2 subunit of GABA(A)Rs and displays high homology to proteins involved in membrane fusion underlying Golgi transport and autophagic processes. The binding of GABARAP with NSF, microtubules and gephyrin together with its localization at intracellular membranes suggests a role in GABA(A)R targeting and/or degradation. Growth factor tyrosine kinase receptor activation is involved in the control of GABA(A)R levels at the plasma membrane. In particular insulin recruits GABA(A)Rs to the cell surface. Furthermore, the regulation of GABA(A)R surface half-life can also be the consequence of negative modulation at the proteasome level. Plic-1, a ubiquitin-like protein binds to both the proteasome and GABA(A)Rs and the Plic1-GABA(A)R interaction is important for the maintenance of GABA-activated current amplitudes. At synaptic sites, GABA(A)Rs are clustered via gephyrin-dependent and gephyrin-independent mechanisms and may subsequently become internalized via clathrin-mediated endocytosis underlying receptor recycling or degradation processes. This article discusses these recent data in the field of GABA(A)R dynamics.

The metabotropic glutamate receptor mGluR7b binds to the catalytic gamma-subunit of protein phosphatase 1

Correct targeting of enzymes represents an important biological mechanism to control post-translational modifications of neurotransmitter receptors. The metabotropic glutamate receptor type 7 (mGluR7) exists in two splice variants (mGluR7a and mGluR7b), defined by different C-termini that are phosphorylated by protein kinase C (PKC). Recently, the search for mGluR7a binding partners yielded several proteins that interacted with its C-terminus. Here, a yeast two-hybrid screen using the mGluR7b C-terminus identified both variants of the catalytic gamma-subunit of protein phosphatase 1 (PP1gamma1 and PP1gamma2) as binding partners. The minimal interacting region of PP1gamma1/2 contained the core domain and was homologous to a region of PP1alpha that is needed for functional expression. Although this core domain is highly conserved within the protein phosphatase family, PP1alpha1 and PP1beta did not interact with mGluR7b. Binding between PP1gamma1 and mGluR7b might be regulated by alternative splicing, as the variant-specific distal part of the mGluR7b C-terminus mediated the interaction. Within this domain, amino acids involved in the binding to PP1gamma1 were mapped and biochemical assays using recombinant and native proteins verified the proposed interaction. Finally, the expression pattern of PP1gamma1, PP1gamma2 and mGluR7b was analysed in various CNS regions. In summary, these results suggest a regulation of mGluR7b by PP1gamma.

RNA editing at arg607 controls AMPA receptor exit from the endoplasmic reticulum

AMPA-receptor (AMPAR) transport to synapses plays a critical role in the modulation of synaptic strength. We show that the functionally critical GluR2 subunit stably resides in an intracellular pool in the endoplasmic reticulum (ER). GluR2 in this pool is extensively complexed with GluR3 but not with GluR1, which is mainly confined to the cell surface. Mutagenesis revealed that elements in the C terminus including the PDZ motif are required for GluR2 forward-transport from the ER. Surprisingly, ER retention of GluR2 is controlled by Arg607 at the Q/R-editing site. Reversion to Gln (R607Q) resulted in rapid release from the pool and elevated surface expression of GluR2 in neurons. Therefore, Arg607 is a central regulator. In addition to channel gating, it also controls ER exit and may thereby ensure the availability of GluR2 for assembly into AMPARs.

Differential palmitoylation directs the AMPA receptor-binding protein ABP to spines or to intracellular clusters

Long-term changes in excitatory synapse strength are thought to reflect changes in synaptic abundance of AMPA receptors mediated by receptor trafficking. AMPA receptor-binding protein (ABP) and glutamate receptor-interacting protein (GRIP) are two similar PDZ (postsynaptic density 95/Discs large/zona occludens 1) proteins that interact with glutamate receptors 2 and 3 (GluR2 and GluR3) subunits. Both proteins have proposed roles during long-term potentiation and long-term depression in the delivery and anchorage of AMPA receptors at synapses. Here we report a variant of ABP-L (seven PDZ form of ABP) called pABP-L that is palmitoylated at a cysteine residue at position 11 within a novel 18 amino acid N-terminal leader sequence encoded through differential splicing. In cultured hippocampal neurons, nonpalmitoylated ABP-L localizes with internal GluR2 pools expressed from a Sindbis virus vector, whereas pABP-L is membrane targeted and associates with surface-localized GluR2 receptors at the plasma membrane in spines. Mutation of Cys-11 to alanine blocks the palmitoylation of pABP-L and targets the protein to intracellular clusters, confirming that targeting the protein to spines is dependent on palmitoylation. Non-palmitoylated GRIP is primarily intracellular, but a chimera with the pABP-L N-terminal palmitoylation sequence linked to the body of the GRIP protein is targeted to spines. We suggest that pABP-L and ABP-L provide, respectively, synaptic and intracellular sites for the anchorage of AMPA receptors during receptor trafficking to and from the synapse.

Differential roles for NSF and GRIP/ABP in AMPA receptor cycling

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) stability and movement at synapses are important factors controlling synaptic strength. Here, we study the roles of proteins [N-ethylmaleimide-sensitive fusion protein (NSF), glutamate receptor AMPAR binding protein (ABP)-interacting protein (GRIP)/(ABP), and protein interacting with C-kinase-1 (PICK1) that interact with the GluR2 subunit in the control of the surface expression and cycling of AMPARs. Epitope-tagged GluR2 formed functional receptors that exhibited targeting to synaptic sites. Constructs in which binding to NSF, PDZ proteins (GRIP/ABP and PICK1), or GRIP/ABP alone was eliminated each exhibited normal surface targeting and constitutive cycling. The lack of NSF binding, however, resulted in receptors that were endocytosed to a greater extent than wild-type receptors in response to application of AMPA or N-methyl-d-aspartate (NMDA). Conversely, the behavior of the GluR2 mutants incapable of binding to GRIP/ABP suggests that these PDZ proteins play a role in the stabilization of an intracellular pool of AMPARs that have been internalized on stimulation, thus inhibiting their recycling to the synaptic membrane. These results provide further evidence for distinct functional roles of GluR2-interacting proteins in AMPAR trafficking.

The PDZ proteins PICK1, GRIP, and syntenin bind multiple glutamate receptor subtypes. Analysis of PDZ binding motifs

Using sequence homology searches, yeast two-hybrid assays and glutathione S-transferase (GST)-pull-down approaches we have identified a series of glutamate receptor subunits that interact differentially with the PDZ proteins GRIP, PICK1, and syntenin. GST-pull-down experiments identified more interactions than detected by yeast two-hybrid assays. We report several receptor-protein interactions, strong ones include: (i) GRIP and syntenin with mGluR7a, mGluR4a, and mGluR6; (ii) PICK1 and GRIP with mGluR3; and (iii) syntenin with all forms of GluR1-4 and mGluR7b. We further characterized the novel mGluR7a-GRIP interaction found both in yeast two-hybrid and GST-pull-down assays and observed that mGluR7a localization overlapped with GRIP with in hippocampal neurons. The wide range of targets for PICK1, GRIP, and syntenin suggests they may represent a molecular mechanism that can concentrate and/or regulate a number of different receptors at a common site on a synapse. These data also suggest that the structural determinants involved in PDZ interactions are more complex than originally envisaged.

NSF ATPase and alpha-/beta-SNAPs disassemble the AMPA receptor-PICK1 complex

AMPA receptor (AMPAR) trafficking is crucial for synaptic plasticity that may be important for learning and memory. NSF and PICK1 bind the AMPAR GluR2 subunit and are involved in trafficking of AMPARs. Here, we show that GluR2, PICK1, NSF, and alpha-/beta-SNAPs form a complex in the presence of ATPgammaS. Similar to SNARE complex disassembly, NSF ATPase activity disrupts PICK1-GluR2 interactions in this complex. Alpha- and beta-SNAP have differential effects on this reaction. SNAP overexpression in hippocampal neurons leads to corresponding changes in AMPAR trafficking by acting on GluR2-PICK1 complexes. This demonstrates that the previously reported synaptic stabilization of AMPARs by NSF involves disruption of GluR2-PICK1 interactions. Furthermore, we are reporting a non-SNARE substrate for NSF disassembly activity.

Delphilin: a novel PDZ and formin homology domain-containing protein that synaptically colocalizes and interacts with glutamate receptor delta 2 subunit

The glutamate receptor delta2 (GluRdelta2) subunit is selectively expressed in cerebellar Purkinje cells and plays an important role in cerebellar long-term depression, motor learning, motor coordination, and synapse development. We identified a novel GluRdelta2-interacting protein, named Delphilin, that contains a single PDZ domain and formin homology (FH) domains FH1 and FH2 plus coiled-coil structure. As far as we know, this is the first reported protein that contains both PDZ and FH domains. Yeast two-hybrid and surface plasmon resonance (SPR) analyses indicated that Delphilin interacts with the GluRdelta2 C terminus via its PDZ domain. This was also supported by coimmunoprecipitation experiments using a heterologous expression system in mammalian cells. Yeast cell and SPR analyses also demonstrated the possibility that the FH1 proline-rich region of Delphilin interacts with profilin, an actin-binding protein, and with the Src homology 3 domain of neuronal Src protein tyrosine kinase. In situ hybridization demonstrated the highest expression of Delphilin mRNA in Purkinje cells. Delphilin polypeptide was highly enriched in the synaptosomal membrane fraction of the cerebellum and coimmunoprecipitated with the GluRdelta2 subunit. The post-embedding immunogold technique demonstrated that Delphilin is selectively localized at the postsynaptic junction site of the parallel fiber-Purkinje cell synapse and colocalized with GluRdelta2. Thus, Delphilin is a postsynaptic scaffolding protein at the parallel fiber-Purkinje cell synapse, where it may serve to link GluRdelta2 with actin cytoskeleton and various signaling molecules.

Tamalin, a PDZ domain-containing protein, links a protein complex formation of group 1 metabotropic glutamate receptors and the guanine nucleotide exchange factor cytohesins

In this investigation, we report identification and characterization of a 95 kDa postsynaptic density protein (PSD-95)/discs-large/ZO-1 (PDZ) domain-containing protein termed tamalin, also recently named GRP1-associated scaffold protein (GRASP), that interacts with group 1 metabotropic glutamate receptors (mGluRs). The yeast two-hybrid system and in vitro pull-down assays indicated that the PDZ domain-containing, amino-terminal half of tamalin directly binds to the class I PDZ-binding motif of group 1 mGluRs. The C-terminal half of tamalin also bound to cytohesins, the members of guanine nucleotide exchange factors (GEFs) specific for the ADP-ribosylation factor (ARF) family of small GTP-binding proteins. Tamalin mRNA is expressed predominantly in the telencephalic region and highly overlaps with the expression of group 1 mGluR mRNAs. Both tamalin and cytohesin-2 were enriched and codistributed with mGluR1a in postsynaptic membrane fractions. Importantly, recombinant and native mGluR1a/tamalin/cytohesin-2 complexes were coimmunoprecipitated from transfected COS-7 cells and rat brain tissue, respectively. Transfection of tamalin and mutant tamalin lacking a cytohesin-binding domain caused an increase and decrease in cell-surface expression of mGluR1a in COS-7 cells, respectively. Furthermore, adenovirus-mediated expression of tamalin and dominant-negative tamalin facilitated and reduced the neuritic distribution of endogenous mGluR5 in cultured hippocampal neurons, respectively. The results indicate that tamalin plays a key role in the association of group 1 mGluRs with the ARF-specific GEF proteins and contributes to intracellular trafficking and the macromolecular organization of group 1 mGluRs at synapses.

The PDZ domain protein PICK1 and the sodium channel BNaC1 interact and localize at mechanosensory terminals of dorsal root ganglion neurons and dendrites of central neurons

Members of the BNaC/ASIC family of ion channels have been implicated in mechanotransduction and nociception mediated by dorsal root ganglion (DRG) neurons. These ion channels are also expressed in the CNS. We identified the PDZ domain protein PICK1 as an interactor of BNaC1(ASIC2) in a yeast two-hybrid screen. We show by two-hybrid assays, glutathione S-transferase pull-down assays, and coimmunoprecipitations that the BNaC1-PICK1 interaction is specific, and that coexpression of both proteins leads to their clustering in intracellular compartments. The interaction between BNaC1 and PICK1 requires the PDZ domain of PICK1 and the last four amino acids of BNaC1. BNaC1 is similar to two other BNaC/ASIC family members, BNaC2 (ASIC1) and ASIC4, at its extreme C terminus, and we show that PICK1 also interacts with BNaC2. We found that PICK1, like BNaC1 and BNaC2, is expressed by DRG neurons and, like the BNaC1alpha isoform, is present at their peripheral mechanosensory endings. Both PICK1 and BNaC1alpha are also coexpressed by some pyramidal neurons of the cortex, by pyramidal neurons of the CA3 region of hippocampus, and by cerebellar Purkinje neurons, localizing to their dendrites and cell bodies. Therefore, PICK1 interacts with BNaC/ASIC channels and may regulate their subcellular distribution or function in both peripheral and central neurons.

A Golgi-associated PDZ domain protein modulates cystic fibrosis transmembrane regulator plasma membrane expression

We identified a novel cystic fibrosis transmembrane conductance regulator (CFTR)-associating, PDZ domain-containing protein, CAL (CFTR associated ligand) containing two predicted coiled-coiled domains and one PDZ domain. The PDZ domain of CAL binds to the C terminus of CFTR. Although CAL does not have any predicted transmembrane domains, CAL is associated with membranes mediated by a region containing the coiled-coil domains. CAL is located primarily at the Golgi apparatus, co-localizing with trans-Golgi markers and is sensitive to Brefeldin A treatment. Immunoprecipitation experiments suggest that CAL exists as a multimer. Overexpression of CAL reduces CFTR chloride currents in mammalian cells and decreases expression, rate of insertion and half-life of CFTR in the plasma membrane. The Na(+)/H(+) exchanger regulatory factor, NHE-RF, a subplasma membrane PDZ domain protein, restores cell surface expression of CFTR and chloride currents. In addition, NHE-RF inhibits the binding of CAL to CFTR. CAL modulates the surface expression of CFTR. CAL favors retention of CFTR within the cell, whereas NHE-RF favors surface expression by competing with CAL for the binding of CFTR. Thus, the regulation of CFTR in the plasma membrane involves the dynamic interaction between at least two PDZ domain proteins.

Interaction of the synaptic protein PICK1 (protein interacting with C kinase 1) with the non-voltage gated sodium channels BNC1 (brain Na+ channel 1) and ASIC (acid-sensing ion channel)

Neuronal members of the degenerin/epithelial Na(+) channel (DEG/ENaC) family of cation channels include the mammalian brain Na(+) channel 1 (BNC1), acid-sensing ion channel (ASIC) and dorsal-root acid-sensing ion channel (DRASIC). Their response to acidic pH, their sequence similarity to nematode proteins involved in mechanotransduction and their modulation by neuropeptides suggest that they may function as receptors for a number of different stimuli. Using the yeast two-hybrid assay, we found that the PDZ domain-containing protein PICK1 (protein interacting with C kinase) interacts specifically with the C-termini of BNC1 and ASIC, but not DRASIC or the related alphaENaC or betaENaC. In both the yeast two-hybrid system and mammalian cells, deletion of the BNC1 and ASIC C-termini abolished the interaction with PICK1. Likewise, mutating the PDZ domain in PICK1 abolished its interaction with BNC1 and ASIC. In addition, in a heterologous expression system PICK1 altered the distribution of BNC1 channels; this effect was dependent on the PDZ domain of PICK1 and the C-terminus of BNC1. We found crude synaptosomal fractions of brain to be enriched in ASIC, suggesting a possible synaptic localization. Moreover, in transfected hippocampal neurons ASIC co-localized with PICK1 in a punctate pattern at synapses. These data suggest that PICK1 binds ASIC and BNC1 via its PDZ domain. This interaction may be important for the localization and/or function of these channels in both the central and peripheral nervous systems.

Disparate cell types use a shared complex of PDZ proteins for polarized protein localization

Based on their morphology and function, epithelial cells and neurons appear to have very little in common; however, growing evidence indicates that these two disparate cell types share an underlying polarization pathway responsible for sorting proteins to specific subcellular sites. An evolutionarily conserved complex of PDZ domain-containing proteins thought to be responsible for polarized protein localization has been identified from both brain and epithelial tissue, both from mammals and from the nematode C. elegans. Some of the most recent data on PDZ proteins and the proteins with which they interact are summarized. In particular, some of the more recently proposed models for their function in cells, and the in vivo and in vitro data that support these models are focussed upon.

Interdomain chaperoning between PSD-95, Dlg, and Zo-1 (PDZ) domains of glutamate receptor-interacting proteins

The multiple PSD-95, Dlg, and Zo-1 (PDZ) domain protein, glutamate receptor-interacting protein (GRIP), is involved in the clustering and trafficking of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor by directly binding to the cytoplasmic tail of the receptor's GluR2 subunit. Both the forth and fifth PDZ domains (PDZ4 and PDZ5) of GRIP are required for effective binding to the receptor. Using NMR and circular dichroism spectroscopic techniques, we show that PDZ5 is completely unstructured in solution. Freshly prepared PDZ4 is largely folded, but the domain can spontaneously unfold. Neither PDZ4 nor PDZ5 binds to GluR2 in solution. Unexpectedly, when PDZ4 and PDZ5 are covalently connected (i.e. PDZ45), both PDZ domains become well folded and stable in solution. The covalent linkage of the two PDZ domains is essential for proper folding of the tandem PDZ domains and its effective binding to GluR2. The interdomain chaperoning effect observed in the PDZ domains of GRIP represents a previously uncharacterized function of PDZ domains.

Biochemical and morphological characterization of an intracellular membrane compartment containing AMPA receptors

AMPA receptors cycle rapidly in and out of the postsynaptic membrane, while NMDA receptors are relatively immobile. Changing the distribution of AMPA receptors between intracellular and surface synaptic pools is an important means of controlling synaptic strength. However, little is known about the intracellular membrane compartments of neurons that contain AMPA receptors. Here we describe biochemical and morphological characteristics of an intracellular pool of AMPA receptors in rat brain. By velocity gradient centrifugation of microsomal light membranes from rat brain, we identified a membrane fraction enriched for AMPA receptor subunits GluR2/3 but lacking NMDA receptors. This membrane compartment sedimented more slowly than synaptosomes but faster than synaptic vesicles and cofractionated with GRIP, PICK-1 and syntaxin-13. Morphological examination of this fraction revealed round and tubular vesicles ranging from approximately 50 to 300 nm in diameter. Immunocytochemistry of cultured hippocampal neurons showed that a significant portion of AMPA receptors colocalized with syntaxin-13 (a SNARE protein associated with tubulovesicular recycling endosomes) and with transferrin receptors. Taken together, these results suggest that a pool of intracellular GluR2/3 resides in a syntaxin 13-positive tubulovesicular membrane compartment, which might serve as a reservoir for the dendritic recycling of AMPA receptors.

PICKing on transporters

Plasma membrane neurotransmitter transporters are regulators of extracellular transmitter levels in brain and are the primary sites of action for several drugs of abuse and therapy. Studies are beginning to reveal how neurons use synaptic machinery to modulate these regulators.

The carboxyl terminus of the prolactin-releasing peptide receptor interacts with PDZ domain proteins involved in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor clustering

PDZ domain proteins use the PDZ domain binding motif to bind to the C-terminal sequence of membrane proteins to help scaffold them and spatially organize the components of the intracellular signaling machinery. We have identified a sequence at the C terminus of a G protein-coupled receptor, the PrRP receptor, that shares similarities with the C-terminal sequence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R) subunits that interact with PDZ domain proteins. When coexpressed in human embryonic kidney 293 cells, PrRP receptor was able to coimmunoprecipitate the three PDZ domain proteins known to interact with AMPA receptors: glutamate receptor interacting protein (GRIP), AMPA binding protein (ABP), and protein that interacts with C-kinase (PICK1), but not the PDZ domain protein PSD-95, which does not interact with AMPA receptors. These interactions are sequence-selective as determined by mutagenesis. Furthermore, we show that PrRP receptor forms intracellular clusters when coexpressed with PICK1, and that this clustering effect is dependent on the interaction between the PICK1 PDZ domain and the last four amino acids of PrRP receptor. We found that PrRP receptor interaction with GRIP is not protein kinase C-regulated but may be regulated by other unidentified kinase because okadaic acid dramatically reduced GRIP interaction. By in situ hybridization, we show that the PrRP receptor is expressed in neurons that also express these PDZ domain proteins. We thus demonstrate that PrRP receptor interacts with the same PDZ domain proteins as the AMPA-Rs, raising the possibility that these two proteins could be scaffolded together at the synapse. These results may help to gain important insights into PrRP functions within the central nervous system.

Identification of protein kinase C phosphorylation sites within the AMPA receptor GluR2 subunit

Phosphorylation of AMPA receptor subunits is believed to regulate channel function and synaptic plasticity. Extensive biochemical and molecular studies have identified sites of PKA, PKC and CamKII phosphorylation in the C-termini of the GluR1 and 4 subunits. Recent studies have shown GluR1 phosphorylation to be bidirectionally altered during long-term potentiation (LTP) and long-term depression (LTD) in the hippocampus. The majority of AMPA receptors in the brain are believed to contain the GluR2 subunit that also contains potential sites for protein phosphorylation. Here we characterize PKC phosphorylation on the GluR2 subunit using biochemical and molecular techniques. Site-directed mutagenesis confirmed that this phosphorylation occurs on Serine 863 and Serine 880 of the GluR2 subunit C-terminus. Site identification allowed the generation of phosphorylation site-specific antibodies to facilitate the examination of GluR2 modification in primary neuronal culture. These studies confirmed that GluR2 is modified in response to the activation of PKC and suggests that phosphorylation of the ubiquitous GluR2 subunit may be important in the regulation of excitatory synaptic transmission.

N-methyl-D-aspartate-induced alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor down-regulation involves interaction of the carboxyl terminus of GluR2/3 with Pick1. Ligand-binding studies using Sindbis vectors carrying AMPA receptor decoys

The dynamics of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)-type glutamate receptors, as represented by their exocytosis, endocytosis and cytoskeletal linkage, has often been implicated in N-methyl-d-aspartate (NMDA)-dependent synaptic plasticity. To explore the molecular mechanisms underlying the AMPA receptor dynamics, cultured hippocampal neurons were stimulated with 100 microm NMDA, and the biochemical and pharmacological changes in the ligand binding activity of AMPA receptor complexes and its subunits, GluR1 and GluR2/3, were investigated. The NMDA treatment reduced the total amount of bound [(3)H]AMPA on the surface of the neurons but not in their total membrane fraction. This process was mimicked by a protein kinase C activator, phorbol ester, but blocked by an inhibitor of the same kinase, calphostin C. The NMDA-induced down-regulation of the ligand binding activity was also reflected by the decreased AMPA-triggered channel activity as well as by the cells' reduced immunoreactivity for GluR1. In parallel, the NMDA treatment markedly altered the interaction between the AMPA receptor subunits and their associating molecule(s); the association of PDZ molecules, including Pick1, with GluR2/3 was enhanced in a protein-kinase-C-dependent manner. Viral expression vectors carrying GluR1 and GluR2 C-terminal decoys, both fused to enhanced green fluorescent protein, were transfected into hippocampal neurons to disrupt their interactions. The overexpression of the C-terminal decoy for GluR2 specifically and significantly blocked the NMDA-triggered reduction in [(3)H]AMPA binding, whereas that for GluR1 had no effects. Co-immunoprecipitation using anti-Pick1 antibodies revealed that the overexpressed GluR2 C-terminal decoy indeed prevented Pick1 from interacting with the endogenous GluR2/3. Therefore, these observations suggest that the NMDA-induced down-regulation of the functional AMPA receptors involves the interaction between GluR2/3 subunits and Pick1.

Synapse-associated protein 97 selectively associates with a subset of AMPA receptors early in their biosynthetic pathway

The regulation of AMPA receptors at the postsynaptic membrane is a fundamental component of synaptic plasticity. In the hippocampus, the induction of long-term potentiation increases the delivery of GluR1, a major AMPA receptor subunit in hippocampal pyramidal neurons, to the synaptic plasma membrane through a mechanism that requires the PDZ binding domain of GluR1. Synapse-associated protein 97 (SAP97), a member of the membrane-associated guanylate kinase family, is believed to associate with AMPA receptors (AMPARs) containing the GluR1 subunit, but the functional significance of these interactions is unclear. We investigated the interaction of GluR1 with SAP97, the only PDZ protein known to interact with GluR1. We find that interactions involving SAP97 and GluR1 occur early in the secretory pathway, while the receptors are in the endoplasmic reticulum or cis-Golgi. In contrast, few synaptic receptors associate with SAP97, suggesting that SAP97 dissociates from the receptor complex at the plasma membrane. We also show that internalization of GluR1, as triggered by NMDAR activation, does not require SAP97. These results implicate GluR1-SAP97 interactions in mechanisms underlying AMPA receptor targeting.

Synapse-associated protein 97 selectively associates with a subset of AMPA receptors early in their biosynthetic pathway

The regulation of AMPA receptors at the postsynaptic membrane is a fundamental component of synaptic plasticity. In the hippocampus, the induction of long-term potentiation increases the delivery of GluR1, a major AMPA receptor subunit in hippocampal pyramidal neurons, to the synaptic plasma membrane through a mechanism that requires the PDZ binding domain of GluR1. Synapse-associated protein 97 (SAP97), a member of the membrane-associated guanylate kinase family, is believed to associate with AMPA receptors (AMPARs) containing the GluR1 subunit, but the functional significance of these interactions is unclear. We investigated the interaction of GluR1 with SAP97, the only PDZ protein known to interact with GluR1. We find that interactions involving SAP97 and GluR1 occur early in the secretory pathway, while the receptors are in the endoplasmic reticulum or cis-Golgi. In contrast, few synaptic receptors associate with SAP97, suggesting that SAP97 dissociates from the receptor complex at the plasma membrane. We also show that internalization of GluR1, as triggered by NMDAR activation, does not require SAP97. These results implicate GluR1-SAP97 interactions in mechanisms underlying AMPA receptor targeting.

In vivo requirement of the alpha-syntrophin PDZ domain for the sarcolemmal localization of nNOS and aquaporin-4

alpha-Syntrophin is a scaffolding adapter protein expressed primarily on the sarcolemma of skeletal muscle. The COOH-terminal half of alpha-syntrophin binds to dystrophin and related proteins, leaving the PSD-95, discs-large, ZO-1 (PDZ) domain free to recruit other proteins to the dystrophin complex. We investigated the function of the PDZ domain of alpha-syntrophin in vivo by generating transgenic mouse lines expressing full-length alpha-syntrophin or a mutated alpha-syntrophin lacking the PDZ domain (Delta PDZ). The Delta PDZ alpha-syntrophin displaced endogenous alpha- and beta 1-syntrophin from the sarcolemma and resulted in sarcolemma containing little or no syntrophin PDZ domain. As a consequence, neuronal nitric oxide synthase (nNOS) and aquaporin-4 were absent from the sarcolemma. However, the sarcolemmal expression and distribution of muscle sodium channels, which bind the alpha-syntrophin PDZ domain in vitro, were not altered. Both transgenic mouse lines were bred with an alpha-syntrophin-null mouse which lacks sarcolemmal nNOS and aquaporin-4. The full-length alpha-syntrophin, not the Delta PDZ form, reestablished nNOS and aquaporin-4 at the sarcolemma of these mice. Genetic crosses with the mdx mouse showed that neither transgenic syntrophin could associate with the sarcolemma in the absence of dystrophin. Together, these data show that the sarcolemmal localization of nNOS and aquaporin-4 in vivo depends on the presence of a dystrophin-bound alpha-syntrophin PDZ domain.

Interaction of the AMPA receptor subunit GluR2/3 with PDZ domains regulates hippocampal long-term depression

The interaction of PDZ domain-containing proteins with the C termini of alpha-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) receptors has been suggested to be important in the regulation of receptor targeting to excitatory synapses. Recent studies have shown that the rapid internalization of AMPA receptors at synapses may mediate, at least in part, the expression of long-term depression (LTD). We have previously shown that phosphorylation of Ser-880 on the AMPA receptor GluR2 subunit differentially regulated the interaction of GluR2 with the PDZ domain-containing proteins GRIP1 and PICK1. Here, we show that induction of LTD in hippocampal slices increases phosphorylation of Ser-880 within the GluR2 C-terminal PDZ ligand, suggesting that the modulation of GluR2 interaction with GRIP1 and PICK1 may regulate AMPA receptor internalization during LTD. Moreover, postsynaptic intracellular perfusion of GluR2 C-terminal peptides that disrupt GluR2 interaction with PICK1 inhibit the expression of hippocampal LTD. These results suggest that the interaction of GluR2 with PICK1 may play a regulatory role in the expression of LTD in the hippocampus.

PDZ domains and the organization of supramolecular complexes

PDZ domains are modular protein interaction domains that bind in a sequence-specific fashion to short C-terminal peptides or internal peptides that fold in a beta-finger. The diversity of PDZ binding specificities can be explained by variable amino acids lining the peptide-binding groove of the PDZ domain. Abundantly represented in Caenorhabditis elegans, Drosophila melanogaster, and mammalian genomes, PDZ domains are frequently found in multiple copies or are associated with other protein-binding motifs in multidomain scaffold proteins. PDZ-containing proteins are typically involved in the assembly of supramolecular complexes that perform localized signaling functions at particular subcellular locations. Organization around a PDZ-based scaffold allows the stable localization of interacting proteins and enhances the rate and fidelity of signal transduction within the complex. Some PDZ-containing proteins are more dynamically regulated in distribution and may also be involved in the trafficking of interacting proteins within the cell.

PICK1 targets activated protein kinase Calpha to AMPA receptor clusters in spines of hippocampal neurons and reduces surface levels of the AMPA-type glutamate receptor subunit 2

The PICK1 protein interacts in neurons with the AMPA-type glutamate receptor subunit 2 (GluR2) and with several other membrane receptors via its single PDZ domain. We show that PICK1 also binds in neurons and in heterologous cells to protein kinase Calpha (PKCalpha) and that the interaction is highly dependent on the activation of the kinase. The formation of PICK1-PKCalpha complexes is strongly induced by TPA, and PICK1-PKCalpha complexes are cotargeted with PICK1-GluR2 complexes to spines, where GluR2 is found to be phosphorylated by PKC on serine 880. PICK1 also reduces the plasma membrane levels of the GluR2 subunit, consistent with a targeting function of PICK1 and a PKC-facilitated release of GluR2 from the synaptic anchoring proteins ABP and GRIP. This work indicates that PICK1 functions as a targeting and transport protein that directs the activated form of PKCalpha to GluR2 in spines, leading to the activity-dependent release of GluR2 from synaptic anchor proteins and the PICK1-dependent transport of GluR2 from the synaptic membrane.

Neurotransmitter receptor trafficking and the regulation of synaptic strength

Modulation of the strength of synapses is thought to be one of the mechanisms that underlies learning and memory and is also likely to be important in processes of neuropathology and drug tolerance. This review focuses on the emerging role of postsynaptic neurotransmitter receptor trafficking as an essential mechanism underlying the dynamic regulation of synaptic strength.

Regulation of mglu(7) receptors by proteins that interact with the intracellular C-terminus

The metabotropic glutamate type 7 (mglu(7)) receptor is a widely distributed, mainly presynaptic Group III mglu receptor that can regulate glutamate release. Recently, largely as a result of the identification of specific proteins that interact with the C-terminal domain of this receptor, considerable progress has been made towards understanding some of the mechanisms that underlie the regulation, signal transduction pathways and targeting of mglu(7) receptors. This has led to the proposal that there are three distinct functionally relevant domains present in the intracellular C-terminus of this receptor: (1) a proximal intracellular signalling domain that interacts with G-protein betagamma-subunits and the Ca(2+) sensor Ca(2+)-calmodulin, and is phosphorylated by protein kinase; (2) a central domain thought to provide a signal for axonal targeting; and (3) an extreme PDZ-binding motif that interacts with the protein kinase C interacting protein, PICK1.

Subcellular localization of calcium-permeable AMPA receptors in spinal motoneurons

Activation of Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors has been linked to potent effects on survival and dendritic outgrowth of spinal motoneurons. Ca(2+) permeability of AMPA receptors is controlled by the GluR2 subunit. Whole-cell electrophysiological studies have suggested that GluR2-containing and GluR2-lacking AMPA receptors may coexist in individual motoneurons. However, there has not been a direct demonstration of heterogeneity in AMPA receptor subunit composition in single motoneurons, nor of distinct subcellular distributions of GluR2-containing and GluR2-lacking receptors. In the present study, we have used confocal microscopy, immunocytochemistry and Ca(2+) imaging to characterize the subcellular localization of AMPA receptors in cultured rat spinal motoneurons. Immunoreactivity for GluR2 and GluR4 was concentrated in clusters, the vast majority of which were found in dendrites at synapses. Double-labelling for GluR2 and GluR4 revealed variability in relative expression of GluR2 and GluR4 between clusters within individual motoneurons; most AMPA receptor clusters were immunoreactive for both GluR2 and GluR4, but a significant minority of clusters were immunoreactive for GluR2 only or for GluR4 only. The majority of GluR2-immunonegative AMPA receptor clusters was present in dendrites, but the relative proportion of GluR2-immunonegative and GluR2-immunopositive clusters was similar in dendrites and soma. Imaging of [Ca(2+)](i) rises triggered by AMPA receptor activation confirmed Ca(2+) influx in motoneuron dendrites. These findings strongly support a model in which GluR2-containing and GluR2-lacking AMPA receptors coexist in motoneurons, clustered at synapses, and mixed in a relative proportion that varies considerably between cell membrane microdomains.

The influence of glutamate receptor 2 expression on excitotoxicity in Glur2 null mutant mice

AMPA receptor (AMPAR)-mediated ionic currents that govern gene expression, synaptic strength, and plasticity also can trigger excitotoxicity. However, native AMPARs exhibit heterogeneous pharmacological, biochemical, and ionic permeability characteristics, which are governed partly by receptor subunit composition. Consequently, the mechanisms governing AMPAR-mediated excitotoxicity have been difficult to elucidate. The GluR2 subunit is of particular interest because it influences AMPAR pharmacology, Ca(2+) permeability, and AMPAR interactions with intracellular proteins. In this paper we used mutant mice lacking the AMPAR subunit GluR2 to study AMPAR-mediated excitotoxicity in cultured cortical neurons and in hippocampal neurons in vivo. We examined the hypothesis that in these mice the level of GluR2 expression governs the vulnerability of neurons to excitotoxicity and further examined the ionic mechanisms that are involved. In cortical neuronal cultures AMPAR-mediated neurotoxicity paralleled the magnitude of kainate-evoked AMPAR-mediated currents, which were increased in neurons lacking GluR2. Ca(2+) permeability, although elevated in GluR2-deficient neurons, did not correlate with excitotoxicity. However, toxicity was reduced by removal of extracellular Na(+), the main charge carrier of AMPAR-mediated currents. In vivo, the vulnerability of CA1 hippocampal neurons to stereotactic kainate injections and of CA3 neurons to intraperitoneal kainate administration was independent of GluR2 level. Neurons lacking the GluR2 subunit did not demonstrate compensatory changes in the distribution, expression, or function of AMPARs or of Ca(2+)-buffering proteins. Thus GluR2 level may influence excitotoxicity by effects additional to those on Ca(2+) permeability, such as effects on agonist potency, ionic currents, and synaptic reorganization.

Molecular organization of the postsynaptic specialization

A specific set of molecules including glutamate receptors is targeted to the postsynaptic specialization of excitatory synapses in the brain, gathering in a structure known as the postsynaptic density (PSD). Synaptic targeting of glutamate receptors depends on interactions between the C-terminal tails of receptor subunits and specific PDZ domain-containing scaffold proteins in the PSD. These scaffold proteins assemble a specialized protein complex around each class of glutamate receptor that functions in signal transduction, cytoskeletal anchoring, and trafficking of the receptors. Among the glutamate receptor subtypes, the N-methyl-d-aspartate receptor is relatively stably integrated in the PSD, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor moves in and out of the postsynaptic membrane in highly dynamic fashion. The distinctive cell biological behaviors of N-methyl-d-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors can be explained by their differential interactions with cytoplasmic proteins.

Molecular heterogeneity of central synapses: afferent and target regulation

Electrophysiological recordings show a functional spectrum even within a single class of synapse, with individual synapses ranging widely in fundamental properties, including release probability, unitary response and effects of previous stimulation on subsequent response. Molecular and cellular biological approaches have shown a corresponding diversity in the complement of ion channels, receptors, scaffolds and signal transducing proteins that make up individual synapses. Indeed, we believe that each individual synapse is unique, a function of presynaptic cell type, postsynaptic cell type, environment, developmental stage and history of activity. We review here the molecular diversity of glutamatergic and GABAergic synapses in the mammalian brain in the context of potential cell biological mechanisms that may explain how individual cells develop and maintain such a mosaic of synaptic connections.

Subunit-specific rules governing AMPA receptor trafficking to synapses in hippocampal pyramidal neurons

AMPA-type glutamate receptors (AMPA-Rs) mediate a majority of excitatory synaptic transmission in the brain. In hippocampus, most AMPA-Rs are hetero-oligomers composed of GluR1/GluR2 or GluR2/GluR3 subunits. Here we show that these AMPA-R forms display different synaptic delivery mechanisms. GluR1/GluR2 receptors are added to synapses during plasticity; this requires interactions between GluR1 and group I PDZ domain proteins. In contrast, GluR2/GluR3 receptors replace existing synaptic receptors continuously; this occurs only at synapses that already have AMPA-Rs and requires interactions by GluR2 with NSF and group II PDZ domain proteins. The combination of regulated addition and continuous replacement of synaptic receptors can stabilize long-term changes in synaptic efficacy and may serve as a general model for how surface receptor number is established and maintained.

Molecular determinants for PICK1 synaptic aggregation and mGluR7a receptor coclustering: role of the PDZ, coiled-coil, and acidic domains

PSD-95/Disc-large/ZO-1 (PDZ) domain-containing proteins play a central role in synaptic organization by their involvement in neurotransmitter receptor clustering and signaling complex assembly. The protein interacting with protein kinase C (PICK1), a synaptic PDZ domain protein that also contains a coiled-coil and acidic domain, binds to several synaptic components including the metabotropic glutamate receptor mGluR7a. Coexpression of PICK1 and mGluR7a in heterologous cells induces coclustering of these two proteins. To examine the role of the different structural motifs of PICK1 in synaptic aggregation of PICK1 and mGluR7a coclustering, several PICK1 mutants were generated to analyze their distribution in transfected hippocampal cultured neurons and to test their ability to induce coclusters with mGluR7a when coexpressed in fibroblast cells. The PDZ and coiled-coil domains are both required, whereas the acidic region plays an inhibitory role in these processes. Our data suggest that synaptic aggregation and receptor coclustering depend on PICK1 binding to a target membrane receptor, e.g. mGluR7a, by a PDZ-mediated interaction and on PICK1 oligomerization through the coiled-coil domain. This study defined three structural signals within PICK1 regulating its synaptic localization and receptor coclustering activity, which could represent molecular substrates involved in synaptic development and plasticity.

Differential palmitoylation of two mouse glutamate receptor interacting protein 1 forms with different N-terminal sequences

Glutamate receptor interacting protein (GRIP) is a member of the PDZ domain-containing protein family that is localized in the postsynaptic density area. This protein has been reported to interact specifically with the C-termini of AMPA-selective glutamate receptor channel subunits, GluRalpha2 and GluRalpha3 through its PDZ domains. To clarify the physiological functions of GRIP, we cloned mouse GRIP1, and found that there are three sites for alternative splicing and two putative translational start codons by characterizing GRIP1 cDNA clones and reverse transcription-polymerase chain reaction products. Metabolic labeling of COS-7 cells expressing two N-terminal GRIP1 proteins demonstrated that these proteins differed in their pattern of palmitoylation. These findings suggested that the molecular diversity of GRIP1 underlies the localization and functional heterogeneity of this protein.

The ERBB2/HER2 receptor differentially interacts with ERBIN and PICK1 PSD-95/DLG/ZO-1 domain proteins

Identification of protein complexes associated with the ERBB2/HER2 receptor may help unravel the mechanisms of its activation and regulation in normal and pathological situations. Interactions between ERBB2/HER2 and Src homology 2 or phosphotyrosine binding domain signaling proteins have been extensively studied. We have identified ERBIN and PICK1 as new binding partners for ERBB2/HER2 that associate with its carboxyl-terminal sequence through a PDZ (PSD-95/DLG/ZO-1) domain. This peptide sequence acts as a dominant retention or targeting basolateral signal for receptors in epithelial cells. ERBIN belongs to the newly described LAP (LRR and PDZ) protein family, whose function is crucial in non vertebrates for epithelial homeostasis. Whereas ERBIN appears to locate ERBB2/HER2 to the basolateral epithelium, PICK1 is thought to be involved in the clustering of receptors. We show here that ERBIN and PICK1 bind to ERBB2/HER2 with different mechanisms, and we propose that these interactions are regulated in cells. Since ERBIN and PICK1 tend to oligomerize, further complexity of protein networks may participate in ERBB2/HER2 functions and specificity.

Biochemical evidence for localization of AMPA-type glutamate receptor subunits in the dendritic raft

A low density Triton-insoluble fraction with characteristic lipid composition was prepared from synaptic plasma membrane from the rat forebrain. The fraction was named dendritic raft based on its absence of the presynaptic marker synaptophysin, the presence of postsynaptic Glutamate receptor (GluR) subunits, and its resemblance to raft, caveolae-like structure. We found a differential distribution of NMDA-type and AMPA-type GluR subunits in the dendritic raft and postsynaptic density (PSD) fractions; the latter type GluR subunits were localized to the dendritic raft as well as PSD fraction, whereas the former type was mostly localized to the PSD fraction. We also found the differential distribution of the components of ras/mitogen-activated protein kinase (MAPK) pathway to the dendritic raft and PSD fractions. Dendritic raft and PSD may possibly interact at the postsynaptic sites for efficient signal processing that is required for expression of synaptic plasticity.

Membrane lipid rafts are necessary for the maintenance of the (alpha)7 nicotinic acetylcholine receptor in somatic spines of ciliary neurons

Calcium-permeable neurotransmitter receptors are concentrated into structurally and biochemically isolated cellular compartments to localize calcium-mediated events during neurotransmission. The cytoplasmic membrane contains lipid microdomains called lipid rafts, which can gather into microscopically visible clusters, and thus the association of a particular protein with lipid rafts can result in its redistribution on the cell surface. The present study asks whether lipid rafts participate in the formation and maintenance of the calcium-permeable alpha7-subunit nicotinic acetylcholine receptor (alpha7nAChR) clusters found in somatic spines of ciliary neurons. Lipid rafts and alpha7nAChR become progressively colocalized within somatic spines during synaptogenesis. To determine whether these rafts are required for the maintenance of alpha7nAChR aggregates, cholesterol was extracted from dissociated ciliary neurons by treatment with methyl-beta-cyclodextrin. This treatment caused the dispersion of lipid rafts and the redistribution of alpha7nAChR into small clusters over the cell surface, suggesting that the integrity of lipid rafts is required to maintain the receptor clustering. However, lipid raft dispersion also caused the depolymerization of the F-actin cytoskeleton, which can also tether the receptor at specific sites. To assess whether interaction between rafts and alpha7nAChR is independent of F-actin filaments, the lipid raft patches were stabilized with a combination of the cholera toxin B subunit (CTX), which specifically binds to the raft component ganglioside GM1, and an antibody against CTX. The stabilized rafts were then treated with latrunculin-A to depolymerize F-actin. Under these conditions, large patches of CTX persisted and were colocalized with alpha7nAChR, indicating that the aggregates of receptors can be maintained independently of the underlying F-actin cytoskeleton. Moreover, it was found that the alpha7nAChR is resistant to detergent extraction at 4 degrees C and floats with the caveolin-containing lipid-rich fraction during density gradient centrifugation, properties that are consistent with a direct association between the receptor and the membrane microdomains.

Functional interaction between monoamine plasma membrane transporters and the synaptic PDZ domain-containing protein PICK1

PDZ domain-containing proteins play an important role in the targeting and localization of synaptic membrane proteins. Here, we report an interaction between the PDZ domain-containing protein PICK1 and monoamine neurotransmitter transporters in vitro and in vivo. In dopaminergic neurons, PICK1 colocalizes with the dopamine transporter (DAT) and forms a stable protein complex. Coexpression of PICK1 with DAT in mammalian cells and neurons in culture results in colocalization of the two proteins in a cluster pattern and an enhancement of DAT uptake activity through an increase in the number of plasma membrane DAT. Deletion of the PDZ binding site at the carboxyl terminus of DAT abolishes its association with PICK1 and impairs the localization of the transporter in neurons. These findings indicate a role for PDZ-mediated protein interactions in the localization, expression, and function of monoamine transporters.

Interferon-gamma-induced changes in synaptic activity and AMPA receptor clustering in hippocampal cultures

Extended release of interferon-gamma (IFN-gamma) in the nervous system during immunological and infectious conditions may trigger demyelinating disorders and cause disturbances in brain function. The aim of this study was to examine the effects of IFN-gamma on neuronal function in rat hippocampal cell cultures by using whole cell patch clamp analysis together with quantitative immunocytochemistry. Acute application of IFN-gamma to differentiated neurons in culture caused no immediate neurophysiological responses, but recordings after 48 h of incubation displayed an increase in frequency of AMPA receptor (AMPAR)-mediated spontaneous excitatory postsynaptic currents (EPSCs). Quantitative immunocytochemistry for the AMPAR subunit GluR1 showed no alteration in receptor clustering at this time point. However, prolonged treatment with IFN-gamma for 2 weeks resulted in a significant reduction in AMPAR clustering on dendrites but no marked differences in EPSC frequency between treated neurons and controls could be observed. On the other hand, treatment of hippocampal neurons for 4 weeks, instituted at an immature stage (1 day in culture), caused a significant reduction in spontaneous EPSC frequency. These neurons developed with no overt alterations in dendritic arborization or in the appearance of dendritic spines as visualized by alpha-actinin immunocytochemistry. Nonetheless, there was a marked reduction in AMPAR clustering on dendrites. These observations show that a key immunomodulatory molecule, IFN-gamma, can cause long-term modifications of synaptic activity and perturb glutamate receptor clustering.