Group I Metabotropic Glutamate Receptors Bind to Protein Phosphatase 1C

Protein Networks Associated with Native Metabotropic Glutamate 1 Receptors (mGlu1) in the Mouse Cerebellum

The metabotropic glutamate receptor 1 (mGlu1) plays a pivotal role in synaptic transmission and neuronal plasticity. Despite the fact that several interacting proteins involved in the mGlu1 subcellular trafficking and intracellular transduction mechanisms have been identified, the protein network associated with this receptor in specific brain areas remains largely unknown. To identify novel mGlu1-associated protein complexes in the mouse cerebellum, we used an unbiased tissue-specific proteomic approach, namely co-immunoprecipitation followed by liquid chromatography/tandem mass spectrometry analysis. Many well-known protein complexes as well as novel interactors were identified, including G-proteins, Homer, δ2 glutamate receptor, 14-3-3 proteins, and Na/K-ATPases. A novel putative interactor, KCTD12, was further investigated. Reverse co-immunoprecipitation with anti-KCTD12 antibodies revealed mGlu1 in wild-type but not in KCTD12-knock-out homogenates. Freeze-fracture replica immunogold labeling co-localization experiments showed that KCTD12 and mGlu1 are present in the same nanodomain in Purkinje cell spines, although at a distance that suggests that this interaction is mediated through interposed proteins. Consistently, mGlu1 could not be co-immunoprecipitated with KCTD12 from a recombinant mammalian cell line co-expressing the two proteins. The possibility that this interaction was mediated via GABAB receptors was excluded by showing that mGlu1 and KCTD12 still co-immunoprecipitated from GABAB receptor knock-out tissue. In conclusion, this study identifies tissue-specific mGlu1-associated protein clusters including KCTD12 at Purkinje cell synapses.

Metabotropic Glutamate Receptors at Ribbon Synapses in the Retina and Cochlea

Our senses define our view of the world. They allow us to adapt to environmental stimuli and are essential for communication and social behaviour. For most humans, seeing and hearing are central senses for their daily life. Our eyes and ears respond to an extraordinary broad range of stimuli covering about 12 log units of light intensity or acoustic power, respectively. The cellular basis is represented by sensory cells (photoreceptors in the retina and inner hair cells in the cochlea) that convert sensory inputs into electrical signals. Photoreceptors and inner hair cells have developed a specific pre-synaptic structure, termed synaptic ribbon, that is decorated with numerous vesicles filled with the excitatory neurotransmitter glutamate. At these ribbon synapses, glutamatergic signal transduction is guided by distinct sets of metabotropic glutamate receptors (mGluRs). MGluRs belong to group II and III of the receptor classification can inhibit neuronal activity, thus protecting neurons from overstimulation and subsequent degeneration. Consequently, dysfunction of mGluRs is associated with vision and hearing disorders. In this review, we introduce the principle characteristics of ribbon synapses and describe group II and III mGluRs in these fascinating structures in the retina and cochlea.

Metabotropic Glutamate Receptor Trafficking and its Role in Drug-Induced Neurobehavioral Plasticity

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system that guides developmental and experience-dependent changes in many cellular substrates and brain circuits, through the process collectively referred to as neurobehavioral plasticity. Regulation of cell surface expression and membrane trafficking of glutamate receptors represents an important mechanism that assures optimal excitatory transmission, and at the same time, also allows for fine-tuning neuronal responses to glutamate. On the other hand, there is growing evidence implicating dysregulated glutamate receptor trafficking in the pathophysiology of several neuropsychiatric disorders. This review provides up-to-date information on the molecular determinants regulating trafficking and surface expression of metabotropic glutamate (mGlu) receptors in the rodent and human brain and discusses the role of mGluR trafficking in maladaptive synaptic plasticity produced by addictive drugs. As substantial evidence links glutamatergic dysfunction to the progression and the severity of drug addiction, advances in our understanding of mGluR trafficking may provide opportunities for the development of novel pharmacotherapies of addiction and other neuropsychiatric disorders.

Homodimerization of a proximal region within the C-terminus of the orphan G-protein coupled receptor GPR179

G-protein coupled receptors exhibit numerous biological functions. The orphan G-protein coupled receptor GPR179 is a central component of a 1 Megadalton large signalling complex in the ON-pathway of the mammalian retina that assembles multiple proteins, including the metabotropic glutamate receptor mGluR6. Dimer formation is a hallmark of G-protein coupled receptors and some use intracellular C-termini for dimerization. Here we tested the dimerization properties of the intracellular C-terminal domains of mGluR6 and GPR179. While the C-termini of GPR179 and mGluR6 did not interact, we detected a robust homodimerization of a proximal region in the GPR179 C-terminus. Mapping studies defined a linear stretch of 64 amino acids as dimerization region. Bioinformatic analysis indicated that this dimerization region might adopt an α-helical structure that is predicted to dimerize by forming a coiled-coil. Based on these data, we speculate that homodimerization of GPR179 might contribute to the formation of large signalling complexes in the mammalian retina.

Yeast two-hybrid screening identifies MPZ-1 and PTP-1 as candidate scaffolding proteins of metabotropic glutamate receptors in Caenorhabditis elegans

The metabotropic glutamate receptors (mGluRs) are a class of G-protein-coupled receptor that undergo extensive interactions with scaffolding proteins, and this is intrinsic to their function as an important group of neuromodulators at glutamatergic synapses. The Caenorhabditis elegans nervous system expresses three metabotropic glutamate receptors, MGL-1, MGL-2 and MGL-3. Relatively little is known about how the function and signalling of these receptors is organised in C. elegans. To identify proteins that scaffold the MGL-1 receptor, we have conducted a yeast two-hybrid screen. Three of the interacting proteins, MPZ-1, NRFL-1 and PTP-1, displayed motifs characteristic of mammalian mGluR scaffolding proteins. Using cellular co-expression criterion, we show mpz-1 and ptp-1 exhibited overlapping expression patterns with subsets of mgl-1 neurons. This included neurones in the pharyngeal nervous system that control the feeding organ of the worm. The mGluR agonist L-CCG-I inhibits the activity of this network in wild-type worms, in an MGL-1 and dose-dependent manner. We utilised L-CCG-I to identify if MGL-1 function was disrupted in mutants with deletions in the mpz-1 gene. The mpz-1 mutants displayed a largely wild-type response to L-CCG-I, suggesting MGL-1 signalling is not overtly disrupted consistent with a non-obligatory modulatory function in receptor scaffolding. The selectivity of the protein interactions and overlapping expression identified here warrant further investigation of the functional significance of scaffolding of metabotropic glutamate receptor function.

Metabotropic glutamate receptor trafficking

The metabotropic glutamate receptors (mGlu receptors) are G protein-coupled receptors that bind to the excitatory neurotransmitter glutamate and are important in the modulation of neuronal excitability, synaptic transmission, and plasticity in the central nervous system. Trafficking of mGlu receptors in and out of the synaptic plasma membrane is a fundamental mechanism modulating excitatory synaptic function through regulation of receptor abundance, desensitization, and signaling profiles. In this review, we cover the regulatory mechanisms determining surface expression and endocytosis of mGlu receptors, with particular focus on post-translational modifications and receptor-protein interactions. The literature we review broadens our insight into the precise events defining the expression of functional mGlu receptors at synapses, and will likely contribute to the successful development of novel therapeutic targets for a variety of developmental, neurological, and psychiatric disorders.

The heterotrimeric G protein Gβ1 interacts with the catalytic subunit of protein phosphatase 1 and modulates G protein-coupled receptor signaling in platelets

Thrombosis is caused by the activation of platelets at the site of ruptured atherosclerotic plaques. This activation involves engagement of G protein-coupled receptors (GPCR) on platelets that promote their aggregation. Although it is known that protein kinases and phosphatases modulate GPCR signaling, how serine/threonine phosphatases integrate with G protein signaling pathways is less understood. Because the subcellular localization and substrate specificity of the catalytic subunit of protein phosphatase 1 (PP1c) is dictated by PP1c-interacting proteins, here we sought to identify new PP1c interactors. GPCRs signal via the canonical heterotrimeric Gα and Gβγ subunits. Using a yeast two-hybrid screen, we discovered an interaction between PP1cα and the heterotrimeric G protein Gβ₁ subunit. Co-immunoprecipitation studies with epitope-tagged PP1c and Gβ₁ revealed that Gβ₁ interacts with the PP1c α, β, and γ1 isoforms. Purified PP1c bound to recombinant Gβ₁-GST protein, and PP1c co-immunoprecipitated with Gβ₁ in unstimulated platelets. Thrombin stimulation of platelets induced the dissociation of the PP1c-Gβ₁ complex, which correlated with an association of PP1c with phospholipase C β3 (PLCβ3), along with a concomitant dephosphorylation of the inhibitory Ser¹¹⁰⁵ residue in PLCβ3. siRNA-mediated depletion of GNB1 (encoding Gβ₁) in murine megakaryocytes reduced protease-activated receptor 4, activating peptide-induced soluble fibrinogen binding. Thrombin-induced aggregation was decreased in PP1cα^-/- murine platelets and in human platelets treated with a small-molecule inhibitor of Gβγ. Finally, disruption of PP1c-Gβ₁ complexes with myristoylated Gβ₁ peptides containing the PP1c binding site moderately decreased thrombin-induced human platelet aggregation. These findings suggest that Gβ₁ protein enlists PP1c to modulate GPCR signaling in platelets.

Metabotropic Glutamate Receptors and Interacting Proteins in Epileptogenesis

Neurotransmitter and receptor systems are involved in different neurological and neuropsychological disorders such as Parkinson's disease, depression, Alzheimer’s disease and epilepsy. Recent advances in studies of signal transduction pathways or interacting proteins of neurotransmitter receptor systems suggest that different receptor systems may share the common signal transduction pathways or interacting proteins which may be better therapeutic targets for development of drugs to effectively control brain diseases. In this paper, we reviewed metabotropic glutamate receptors (mGluRs) and their related signal transduction pathways or interacting proteins in status epilepticus and temporal lobe epilepsy, and proposed some novel therapeutical drug targets for controlling epilepsy and epileptogenesis.

Group 1 metabotropic glutamate receptors 1 and 5 form a protein complex in mouse hippocampus and cortex

The group 1 metabotropic glutamate receptors 1 and 5 (mGluR1/5) have been implicated in mechanisms of synaptic plasticity and may serve as potential therapeutic targets in autism spectrum disorders. The interactome of group 1 mGluRs has remained largely unresolved. Using a knockout‐controlled interaction proteomics strategy we examined the mGluR5 protein complex in two brain regions, hippocampus and cortex, and identified mGluR1 as its major interactor in addition to the well described Homer proteins. We confirmed the presence of mGluR1/5 complex by (i) reverse immunoprecipitation using an mGluR1 antibody to pulldown mGluR5 from hippocampal tissue, (ii) coexpression in HEK293 cells followed by coimmunoprecipitation to reveal the direct interaction of mGluR1 and 5, and (iii) superresolution microscopy imaging of hippocampal primary neurons to show colocalization of the mGluR1/5 in the synapse.

Role of Spinophilin in Group I Metabotropic Glutamate Receptor Endocytosis, Signaling, and Synaptic Plasticity

Activation of Group I metabotropic glutamate receptors (mGluRs) activates signaling cascades, resulting in calcium release from intracellular stores, ERK1/2 activation, and long term changes in synaptic activity that are implicated in learning, memory, and neurodegenerative diseases. As such, elucidating the molecular mechanisms underlying Group I mGluR signaling is important for understanding physiological responses initiated by the activation of these receptors. In the current study, we identify the multifunctional scaffolding protein spinophilin as a novel Group I mGluR-interacting protein. We demonstrate that spinophilin interacts with the C-terminal tail and second intracellular loop of Group I mGluRs. Furthermore, we show that interaction of spinophilin with Group I mGluRs attenuates receptor endocytosis and phosphorylation of ERK1/2, an effect that is dependent upon the interaction of spinophilin with the C-terminal PDZ binding motif encoded by Group I mGluRs. Spinophilin knock-out results in enhanced mGluR5 endocytosis as well as increased ERK1/2, AKT, and Ca(2+) signaling in primary cortical neurons. In addition, the loss of spinophilin expression results in impaired mGluR5-stimulated LTD. Our results indicate that spinophilin plays an important role in regulating the activity of Group I mGluRs as well as their influence on synaptic activity.

Protein phosphatase 1 catalytic isoforms: specificity toward interacting proteins

The coordinated and reciprocal action of serine-threonine protein kinases and protein phosphatases produces transitory phosphorylation, a fundamental regulatory mechanism for many biological processes. Phosphoprotein phosphatase 1 (PPP1), a major serine-threonine phosphatase, in particular, is ubiquitously distributed and regulates a broad range of cellular functions, including glycogen metabolism, cell cycle progression, and muscle relaxation. PPP1 has evolved effective catalytic machinery but in vitro lacks substrate specificity. In vivo, its specificity is achieved not only by the existence of different PPP1 catalytic isoforms, but also by binding of the catalytic moiety to a large number of regulatory or targeting subunits. Here, we will address exhaustively the existence of diverse PPP1 catalytic isoforms and the relevance of their specific partners and consequent functions.

Location-dependent signaling of the group 1 metabotropic glutamate receptor mGlu5

Although G protein-coupled receptors are primarily known for converting extracellular signals into intracellular responses, some receptors, such as the group 1 metabotropic glutamate receptor, mGlu5, are also localized on intracellular membranes where they can mediate both overlapping and unique signaling effects. Thus, besides "ligand bias," whereby a receptor's signaling modality can shift from G protein dependence to independence, canonical mGlu5 receptor signaling can also be influenced by "location bias" (i.e., the particular membrane and/or cell type from which it signals). Because mGlu5 receptors play important roles in both normal development and in disorders such as Fragile X syndrome, autism, epilepsy, addiction, anxiety, schizophrenia, pain, dyskinesias, and melanoma, a large number of drugs are being developed to allosterically target this receptor. Therefore, it is critical to understand how such drugs might be affecting mGlu5 receptor function on different membranes and in different brain regions. Further elucidation of the site(s) of action of these drugs may determine which signal pathways mediate therapeutic efficacy.

Regulation of metabotropic glutamate receptor 7 (mGluR7) internalization and surface expression by Ser/Thr protein phosphatase 1

The metabotropic glutamate receptor type 7 (mGluR7) is the predominant group III mGluR in the presynaptic active zone, where it serves as an autoreceptor to inhibit neurotransmitter release. Our previous studies show that PKC phosphorylation of mGluR7 on Ser-862 is a key mechanism controlling constitutive and activity-dependent surface expression of mGluR7 by regulating a competitive interaction of calmodulin and protein interacting with C kinase (PICK1). As receptor phosphorylation and dephosphorylation are tightly coordinated through the precise action of protein kinases and phosphatases, dephosphorylation by phosphatases is likely to play an active role in governing the activity-dependent or agonist-induced changes in mGluR7 receptor surface expression. In the present study, we find that the serine/threonine protein phosphatase 1 (PP1) has a crucial role in the constitutive and agonist-induced dephosphorylation of Ser-862 on mGluR7. Treatment of neurons with PP1 inhibitors leads to a robust increase in Ser-862 phosphorylation and increased surface expression of mGluR7. In addition, Ser-862 phosphorylation of both mGluR7a and mGluR7b is a target of PP1. Interestingly, agonist-induced dephosphorylation of mGluR7 is regulated by PP1, whereas NMDA-mediated activity-induced dephosphorylation is not, illustrating there are multiple signaling pathways that affect receptor phosphorylation and trafficking. Importantly, PP1γ1 regulates agonist-dependent Ser-862 dephosphorylation and surface expression of mGluR7.

siRNA screen identifies the phosphatase acting on the G protein-coupled thyrotropin-releasing hormone receptor

G protein-coupled receptors (GPCRs) are an ubiquitously expressed class of transmembrane proteins involved in the signal transduction of neurotransmitters, hormones and various other ligands. Their signaling output is desensitized by mechanisms involving phosphorylation, internalization, and dissociation from G proteins and resensitized by mechanisms involving dephosphorylation, but details about the phosphatases responsible are generally lacking. We describe here the use of an siRNA-based library to knock down expression of specific phosphatase subunits to identify protein phosphatase 1-α (PP1α) as important for the thyrotropin-releasing hormone (TRH) receptor. Inhibition of PP1α synthesis and overexpression of dominant negative PP1α preserved receptor phosphorylation under conditions favoring dephosphorylation, whereas overexpression of PP1α accelerated dephosphorylation. Knockdown of all three PP1 catalytic subunits inhibited TRH receptor phosphorylation much more powerfully than knockdown of PP1α alone, suggesting that different PP1 isoforms function redundantly. Knockdown of a structural subunit of PP2A, a second potential hit in the library screen, was ineffective. Calyculin A, a potent inhibitor of PP1 family phosphatases, strongly inhibited dephosphorylation of transfected TRH receptors and endogenous receptors in pituitary cells, but fostriecin, which is selective for PP2A family phosphatases, did not. We conclude that the PP1 class of phosphatases is essential for TRH receptor dephosphorylation.

Expression, purification, and structural analysis of intracellular C-termini from metabotropic glutamate receptors

Glutamate is the most important excitatory neurotransmitter in the mammalian central nervous system (CNS). Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors (GPCRs) that guide several intracellular signal cascades thereby controlling multiple physiological tasks, such as neuronal excitability, learning, and memory. Consequently, these receptors are discussed in the context of several CNS-associated diseases, including addiction for drugs, Alzheimer's disease, Fragile X syndrome, night blindness, or schizophrenia. Although increasing structural information is available for the extracellular and transmembrane domains of GPCRs, data describing the nature of intracellular receptor domains are largely missing. Indeed, in all available crystal structures of neurotransmitter receptors, their intracellular domains were omitted. Most intracellular mGluR C-termini are alternatively spliced and contain multiple binding sites for interacting proteins. Therefore, analyzing their structure can identify molecular mechanisms of receptor regulation. Recently, we analyzed the conformation of the intracellular C-termini of mGluR6, mGluR7a, and mGluR8a. Here, we describe an array of biochemical, biophysical, and computational techniques suited to elucidate the nature of these highly interesting receptor domains.

Desensitization, trafficking, and resensitization of the pituitary thyrotropin-releasing hormone receptor

The pituitary receptor for thyrotropin-releasing hormone (TRH) is a calcium-mobilizing G protein-coupled receptor (GPCR) that signals through Gq/11, elevating calcium, and activating protein kinase C. TRH receptor signaling is quickly desensitized as a consequence of receptor phosphorylation, arrestin binding, and internalization. Following activation, TRH receptors are phosphorylated at multiple Ser/Thr residues in the cytoplasmic tail. Phosphorylation catalyzed by GPCR kinase 2 (GRK2) takes place rapidly, reaching a maximum within seconds. Arrestins bind to two phosphorylated regions, but only arrestin bound to the proximal region causes desensitization and internalization. Phosphorylation at Thr365 is critical for these responses. TRH receptors internalize in clathrin-coated vesicles with bound arrestin. Following endocytosis, vesicles containing phosphorylated TRH receptors soon merge with rab5-positive vesicles. Over approximately 20 min these form larger endosomes rich in rab4 and rab5, early sorting endosomes. After TRH is removed from the medium, dephosphorylated receptors start to accumulate in rab4-positive, rab5-negative recycling endosomes. The mechanisms responsible for sorting dephosphorylated receptors to recycling endosomes are unknown. TRH receptors from internal pools help repopulate the plasma membrane. Dephosphorylation of TRH receptors begins when TRH is removed from the medium regardless of receptor localization, although dephosphorylation is fastest when the receptor is on the plasma membrane. Protein phosphatase 1 is involved in dephosphorylation but the details of how the enzyme is targeted to the receptor remain obscure. It is likely that future studies will identify biased ligands for the TRH receptor, novel arrestin-dependent signaling pathways, mechanisms responsible for targeting kinases and phosphatases to the receptor, and principles governing receptor trafficking.

Regulation of GPCR activity, trafficking and localization by GPCR-interacting proteins

GPCRs represent the largest family of integral membrane proteins and were first identified as receptor proteins that couple via heterotrimeric G-proteins to regulate a vast variety of effector proteins to modulate cellular function. It is now recognized that GPCRs interact with a myriad of proteins that not only function to attenuate their signalling but also function to couple these receptors to heterotrimeric G-protein-independent signalling pathways. In addition, intracellular and transmembrane proteins associate with GPCRs and regulate their processing in the endoplasmic reticulum, trafficking to the cell surface, compartmentalization to plasma membrane microdomains, endocytosis and trafficking between intracellular membrane compartments. The present review will overview the functional consequence of β-arrestin, receptor activity-modifying proteins (RAMPS), regulators of G-protein signalling (RGS), GPCR-associated sorting proteins (GASPs), Homer, small GTPases, PSD95/Disc Large/Zona Occludens (PDZ), spinophilin, protein phosphatases, calmodulin, optineurin and Src homology 3 (SH3) containing protein interactions with GPCRs.

Structure of metabotropic glutamate receptor C-terminal domains in contact with interacting proteins

Metabotropic glutamate receptors (mGluRs) regulate intracellular signal pathways that control several physiological tasks, including neuronal excitability, learning, and memory. This is achieved by the formation of synaptic signal complexes, in which mGluRs assemble with functionally related proteins such as enzymes, scaffolds, and cytoskeletal anchor proteins. Thus, mGluR associated proteins actively participate in the regulation of glutamatergic neurotransmission. Importantly, dysfunction of mGluRs and interacting proteins may lead to impaired signal transduction and finally result in neurological disorders, e.g., night blindness, addiction, epilepsy, schizophrenia, autism spectrum disorders and Parkinson's disease. In contrast to solved crystal structures of extracellular N-terminal domains of some mGluR types, only a few studies analyzed the conformation of intracellular receptor domains. Intracellular C-termini of most mGluR types are subject to alternative splicing and can be further modified by phosphorylation and SUMOylation. In this way, diverse interaction sites for intracellular proteins that bind to and regulate the glutamate receptors are generated. Indeed, most of the known mGluR binding partners interact with the receptors' C-terminal domains. Within the last years, different laboratories analyzed the structure of these domains and described the geometry of the contact surface between mGluR C-termini and interacting proteins. Here, I will review recent progress in the structure characterization of mGluR C-termini and provide an up-to-date summary of the geometry of these domains in contact with binding partners.

Differential binding of calmodulin to group I metabotropic glutamate receptors regulates receptor trafficking and signaling

Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors that modulate excitatory neurotransmission and synaptic plasticity. The group I mGluRs (mGluR1 and mGluR5) have long intracellular C-terminal domains, which interact with many proteins. Our previous studies identified calmodulin (CaM) as a strong regulator of mGluR5 trafficking and mGluR5-induced calcium signaling. Although it has been accepted that both mGluR1 and mGluR5 interact with CaM, we now show that CaM specifically binds mGluR5 and not mGluR1. We have identified a single critical residue in mGluR5 (L896) that is required for CaM binding. In mGluR1, mutation of the corresponding residue, V909, to leucine is sufficient to confer CaM binding to mGluR1. To investigate the functional effects of CaM binding, we examined the surface expression of mGluR1 and mGluR5 in hippocampal neurons. The mutation in mGluR1 (V909L) that confers CaM binding dramatically increases mGluR1 surface expression, whereas the analogous mutation in mGluR5 that disrupts CaM binding (L896V) decreases mGluR5 surface expression. In addition, the critical residue that alters CaM binding regulates mGluR internalization. Furthermore, we find that mGluR-mediated AMPA receptor endocytosis is enhanced by CaM binding to group I mGluRs. Finally, we show that calcium responses evoked by group I mGluRs are modulated by these mutations, which regulate CaM binding. Our findings elucidate a critical mechanism that specifically affects mGluR5 trafficking and signaling, and distinguishes mGluR1 and mGluR5 regulation.

Band 4.1 proteins are expressed in the retina and interact with both isoforms of the metabotropic glutamate receptor type 8

The function of the CNS depends on the correct regulation of neurotransmitter receptors by interacting proteins. Here, we screened a retinal cDNA library for proteins interacting with the intracellular C-terminus of the metabotropic glutamate receptor isoform 8a (mGluR8a). The band 4.1B protein binds to the C-termini of mGluR8a and mGluR8b, co-localizes with these glutamate receptors in transfected mammalian cells, facilitates their cell surface expression and inhibits the mGluR8 mediated reduction of intracellular cAMP concentrations. In contrast, no interaction with 4.1B was observed for other mGluRs tested. Amino acids encoded by exons 19 and 20 of 4.1B and a stretch of four basic amino acids present in the mGluR8 C-termini mediate the protein interaction. Besides binding to 4.1B, mGluR8 isoforms interact with 4.1G, 4.1N, and 4.1R. Because band 4.1 transcripts undergo extensive alternative splicing, we analyzed the splicing pattern of interacting regions and detected a 4.1B isoform expressed specifically in the retina. Within this tissue, mGluR8 and 4.1B, 4.1G, 4.1N, and 4.1R show a comparable distribution, being expressed in both synaptic layers and in somata of the ganglion cell layer. In summary, our studies identified band 4.1 proteins as new players for the mGluR8 mediated signal transduction.

Serine/threonine phosphatases: mechanism through structure

The reversible phosphorylation of proteins is accomplished by opposing activities of kinases and phosphatases. Relatively few protein serine/threonine phosphatases (PSPs) control the specific dephosphorylation of thousands of phosphoprotein substrates. Many PSPs, exemplified by protein phosphatase 1 (PP1) and PP2A, achieve substrate specificity and regulation through combinatorial interactions between conserved catalytic subunits and a large number of regulatory subunits. Other PSPs, represented by PP2C and FCP/SCP, contain both catalytic and regulatory domains within the same polypeptide chain. Here, we discuss biochemical and structural investigations that advance the mechanistic understanding of the three major classes of PSPs, with a focus on PP2A.

Identification of the interaction sites of Inhibitor-3 for protein phosphatase-1

Inhibitor-3 is a potent inhibitor of protein phosphatase-1, with an IC(50) in the nanomolar range for the inhibition of the dephosphorylation of phosphorylase a. Human Inhibitor-3 possesses a putative protein phosphatase-1 binding motif, (39)KKVEW(43). We provide direct evidence that this sequence is involved in PP1 interaction by examining the effects of site-directed mutations of Inhibitor-3 on its ability to inhibit protein phosphatase-1. A second interaction site whose deletion led to loss of inhibitory potency was identified between residues 65 and 77. The existence of two interaction sites is consistent with the high inhibitory potency of Inhibitor-3, and with current models for other inhibitor and targeting proteins that interact with protein phosphatase-1 with high affinity.

Dual neurotoxic and neuroprotective role of metabotropic glutamate receptor 1 in conditions of trophic deprivation - possible role as a dependence receptor

Group I metabotropic glutamate receptors have been often implicated in various models of neuronal toxicity, however, the role played by the individual receptors and their putative mechanisms of action contributing to neurotoxicity or neuroprotection remain unclear. Here, using primary cultures of rat cerebellar granule cells and mouse cortical neurons, we show that conditions of trophic deprivation increased mGlu1 expression which correlated with the developing cell death. The inhibition of mGlu1 expression by specific siRNA attenuated toxicity, while adenovirus-mediated overexpression of mGlu1 resulted in increased cell death, indicating a causal relationship between the level of receptor expression and neuronal survival. In pharmacological experiments selective mGlu1 antagonists failed to protect from mGlu1-induced cell death, instead, neuronal survival was promoted by glutamate acting at mGlu1 receptors. Such properties are characteristics of a novel heterogeneous family of dependence receptors which control neuronal apoptosis. Our findings indicate that increased expression of mGlu1 in neurons creates a state of cellular dependence on the presence of its endogenous agonist glutamate. We propose a new role and a new mechanism for mGlu1 action. This receptor may play a crucial role in determining the fate of individual neurons during the development of the nervous system.

Phosphorylation of group I metabotropic glutamate receptors (mGluR1/5) in vitro and in vivo

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are densely expressed in mammalian brain. They are actively involved in the regulation of normal cellular activity and synaptic plasticity, and are frequently linked to the pathogenesis of various mental illnesses. Like ionotropic glutamate receptors, group I mGluRs are subject to the regulation by protein phosphorylation. Accumulative data demonstrate sufficient phosphorylation of the intracellular mGluR1/5 domains at specific serine/threonine sites by protein kinase C in heterologous cells or neurons, which serves as an important mechanism for regulating the receptor signaling and desensitization. Emerging evidence also shows the significant involvements of G protein-coupled receptor kinases, Ca2+/calmodulin-dependent protein kinase II, tyrosine kinases, and protein phosphatases in controlling the phosphorylation status of group I mGluRs. This review analyzes the recent data concerning group I mGluR phosphorylation and the phosphorylation-dependent regulation of group I mGluR function. Future research directions in this area with newly available high throughput and proteomic approaches are also discussed in the end.

Metabotropic glutamate receptors: phosphorylation and receptor signaling

Metabotropic glutamate receptors (mGluRs) play important roles in neurotransmission, neuronal development, synaptic plasticity, and neurological disorders. Recent studies have revealed a sophisticated interplay between mGluRs and protein kinases: activation of mGluRs regulates the activity of a number of kinases, and direct phosphorylation of mGluRs affects receptor signaling, trafficking, and desensitization. Here we review the emerging literature on mGluR phosphorylation, signaling, and synaptic function.

The trick of the tail: protein-protein interactions of metabotropic glutamate receptors

It was initially believed that G-protein-coupled receptors, such as metabotropic glutamate receptors, could simply be described as individual proteins that are associated with intracellular signal cascades via G-proteins. This view is no longer tenable. Today we know that metabotropic glutamate receptors (mGluRs) can dimerize and bind to a variety of proteins in addition to trimeric G-proteins. These newly identified protein interactions led to the discovery of new regulatory mechanisms that are independent of and sometimes synergistic with the classical G-protein-coupled second messenger pathways. Notably, several of these mechanisms connect mGluR-mediated signaling to other receptor classes, thereby creating a network of different receptor types and associated signal cascades. The intracellular C-termini of mGluRs play a key role in the regulation of these networks, and various new protein interactions of these domains were described recently. Because mGluRs are involved in a variety of physiological and pathophysiological processes, some of the proteins interacting with this receptor class have potential as valuable pharmaceutical targets. This review will give a comprehensive overview of proteins interacting with mGluR C-termini, highlight new evolving regulatory mechanisms for glutamatergic signal transduction and discuss possibilities for future drug development.

In Vivo Regulation of Homer1a Expression in the Striatum by Cocaine

The glutamate receptor adaptor protein Homer is concentrated in the postsynaptic density of excitatory synapses and is critical for normal operation of synaptic transmission. In this study, we investigated the responsiveness of Homer family proteins to dopamine stimulation with the psychostimulant cocaine in rat striatal neurons both in vivo and in vitro. We found that a single dose of cocaine specifically induced a rapid and transient increase in protein levels of the Homer1a, but not Homer1b/c and Homer2a/b, isoforms in the striatum. This selective Homer1a induction was mediated primarily through activation of dopamine D1, but not D2, receptors. Both protein kinase A and Ca(2+)/calmodulin-dependent protein kinases are important for mediating the cocaine stimulation of Homer1a expression. At the transcriptional level, cAMP response element-binding protein serves as a prime transcription factor transmitting the signals derived from D1 receptors and associative pathways to the CaCRE sites within the Homer1a promoter. From a functional perspective, non-cross-linking Homer1a, once induced, competed with the cross-linking isoforms of Homer proteins (Homer1b/c and Homer2a/b) to uncouple the connection of group I metabotropic glutamate receptors (mGluRs) with inositol-1,4,5-triphosphate receptors. These results indicate that cocaine possesses the ability to stimulate Homer1a expression in striatal neurons through a specific synapse-to-nucleus pathway. Moreover, inducible Homer1a expression may represent a transcription-dependent mechanism underlying the dynamic regulation of submembranous macromolecular complex formation between group I mGluRs and their anchoring proteins.

Tax1-binding protein 1 is expressed in the retina and interacts with the GABA(C) receptor rho1 subunit

Macromolecular signalling complexes that link neurotransmitter receptors to functionally and structurally associated proteins play an important role in the regulation of neurotransmission. Thus the identification of proteins binding to neurotransmitter receptors describes molecular mechanisms of synaptic signal transduction. To identify interacting proteins of GABA(C) (where GABA is gamma-aminobutyric acid) receptors in the retina, we used antibodies specific for GABA(C) receptor rho1-3 subunits. Analysis of immunoprecipitated proteins by MALDI-TOF MS (matrix-assisted laser-desorption ionization-time-of-flight MS) identified the liver regeneration-related protein 2 that is identical with amino acids 253-813 of the Tax1BP1 (Tax1-binding protein 1). A C-terminal region of Tax1BP1 bound to an intracellular domain of the rho1 subunit, but not to other subunits of GABA(C), GABA(A) or glycine receptors. Confocal laser-scanning microscopy demonstrated co-localization of Tax1BP1 and rho1 in clusters at the cell membrane of transfected cells. Furthermore, Tax1BP1 and GABA(C) receptors were co-expressed in both synaptic layers of the retina, indicating that Tax1BP1 is a component of GABA(C) receptor-containing signal complexes.

Structural analysis of the protein phosphatase 1 docking motif: molecular description of binding specificities identifies interacting proteins

The interplay between kinases and phosphatases represents a fundamental regulatory mechanism in biological systems. Being less numerous than kinases, phosphatases increase their diversity by the acquisition of a variety of binding partners, thereby forming a large number of holoenzymes. Proteins interacting with protein phosphatase 1 (PP1) often bind via a so-called docking motif to regulate its enzymatic activity, substrate specificity, and subcellular localization. Here, we systematically determined structural elements that mediate the binding specificity of PP1 interacting proteins, and propose a refined consensus sequence for high-affinity PP1 ligands. Applying this pattern to database searches, we predicted and experimentally confirmed several previously unknown PP1 interactors. Thus, the suggested PP1 docking motif enables a highly specific prediction of PP1 binding partners, thereby facilitating the genome-wide identification of PP1 interactors.

Metabotropic glutamate receptors and dopamine receptors cooperate to enhance extracellular signal-regulated kinase phosphorylation in striatal neurons

Striatal medium spiny neurons are an important site of convergence for signaling mediated by the neurotransmitters dopamine and glutamate. We report that in striatal neurons in primary culture, signaling through group I metabotropic glutamate receptors (mGluRs) 1/5 and the D1 class of dopamine receptors (DRs) 1/5 converges to increase phosphorylation of the mitogen-activated protein kinase ERK2 (extracellular signal-regulated kinase 2). Induction of mitogen-activated protein kinase kinase-dependent signaling cascades by either mGluR1/5 or DR1/5 gave rise to increases in phosphorylation of ERK2. Coactivation of mGluR1/5 and DR1/5 with (S)-3,5-dihydroxyphenylglycine and (+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride enhanced the phosphorylation of ERK2. This interaction between mGluR1/5 and DR1/5 required protein kinase C (PKC), because the PKC inhibitors calphostin C, bisindolylmaleimide I, and Gö6976 blocked DR1/5-enhanced phosphorylation of ERK2. Use of the phosphatase inhibitors calyculin and okadaic acid indicated that inhibition of protein phosphatases 1 and 2A dramatically enhanced ERK2 phosphorylation by mGluR1/5. Coactivation of mGluR1/5 and DR1/5 also enhanced cAMP-response element binding protein (CREB) phosphorylation (compared with each receptor agonist alone) but did not enhance CREB-mediated transcriptional activity. Thus, signal transduction pathways activated by DR1/5 and mGluR5 interact to modify downstream events in striatal neurons while retaining numerous regulatory checkpoints.

Cellular and subcellular distribution of spinophilin, a PP1 regulatory protein that bundles F-actin in dendritic spines

Spinophilin is an actin binding protein that positions protein phosphatase 1 next to its substrates in dendritic spines. It contains a single PDZ domain and has the biochemical characteristics of a cytoskeletal scaffolding protein. Previous studies suggest that spinophilin is present in most spines, but the concentration of spinophilin varies from brain region to region in a manner that does not simply reflect differences in spine density. Here, we show that spinophilin is enriched in the great majority of dendritic spines in cerebral cortex, caudatoputamen, hippocampal formation, and cerebellum, irrespective of regional differences in spinophilin concentration. In addition, spinophilin is present postsynaptic to asymmetrical contacts on interneuronal dendritic shafts. We further show that, in hippocampus and ventral pallidum, spinophilin is occasionally present in dendritic shafts adjacent to gamma-aminobutyric acid-containing contacts. Thus, the functional role of spinophilin may not be exclusively restricted to excitatory synapses and may be significant at a small fraction of inhibitory contacts. These data also suggest that the concentration of spinophilin per spine is variable and is likely regulated by local physiological factors and/or regional influences.

Proteomic analysis of native metabotropic glutamate receptor 5 protein complexes reveals novel molecular constituents

We used a proteomic approach to identify novel proteins that may regulate metabotropic glutamate receptor 5 (mGluR5) responses by direct or indirect protein interactions. This approach does not rely on the heterologous expression of proteins and offers the advantage of identifying protein interactions in a native environment. The mGluR5 protein was immunoprecipitated from rat brain lysates; co-immunoprecipitating proteins were analyzed by mass spectrometry and identified peptides were matched to protein databases to determine the correlating parent proteins. This proteomic approach revealed the interaction of mGluR5 with known regulatory proteins, as well as novel proteins that reflect previously unidentified molecular constituents of the mGluR5-signaling complex. Immunoblot analysis confirmed the interaction of high confidence proteins, such as phosphofurin acidic cluster sorting protein 1, microtubule-associated protein 2a and dynamin 1, as mGluR5-interacting proteins. These studies show that a proteomic approach can be used to identify candidate interacting proteins. This approach may be particularly useful for neurobiology applications where distinct protein interactions within a signaling complex can dramatically alter the outcome of the response to neurotransmitter release, or the disruption of normal protein interactions can lead to severe neurological and psychiatric disorders.