Different binding motifs in metabotropic glutamate receptor type 7b for filamin A, protein phosphatase 1C, protein interacting with protein kinase C (PICK) 1 and syntenin allow the formation of multimeric protein complexes

Biological Role and Aberrant Overexpression of Syntenin-1 in Cancer: Potential Role as a Biomarker and Therapeutic Target

Syntenin-1 is a 298 amino acid protein codified by the melanoma differentiation-associated gene-9 (MDA-9). Structurally, it is composed of four domains: N-terminal, PDZ1, PDZ2, and C-terminal. The PDZ domains of syntenin-1 are involved in the stability and interaction with other molecules such as proteins, glycoproteins, and lipids. Domains are also associated with several biological functions such as the activation of signaling pathways related to cell-to-cell adhesion, signaling translation, and the traffic of intracellular lipids, among others. The overexpression of syntenin-1 has been reported in glioblastoma, colorectal, melanoma, lung, prostate, and breast cancer, which promotes tumorigenesis by regulating cell migration, invasion, proliferation, angiogenesis, apoptosis, and immune response evasion, and metastasis. The overexpression of syntenin-1 in samples has been associated with worst prognostic and recurrence, whereas the use of inhibitors such as shRNA, siRNA, and PDZli showed a diminution of the tumor size and reduction in metastasis and invasion. Syntenin-1 has been suggested as a potential biomarker and therapeutic target in cancer for developing more effective diagnostic/prognostic tests or passive/active immunotherapies.

Spine morphogenesis and synapse formation in tubular sclerosis complex models

Tuberous sclerosis complex (TSC) is caused by mutations in the Tsc1 or Tsc2 genes, whose products form a complex and inactivate the small G-protein Rheb1. The activation of Rheb1 may cause refractory epilepsy, intellectual disability, and autism, which are the major neuropsychiatric manifestations of TSC. Abnormalities in dendritic spines and altered synaptic structure are hallmarks of epilepsy, intellectual disability, and autism. In addition, spine dysmorphology and aberrant synapse formation are observed in TSC animal models. Therefore, it is important to investigate the molecular mechanism underlying the regulation of spine morphology and synapse formation in neurons to identify therapeutic targets for TSC. In this review, we focus on the representative proteins regulated by Rheb1 activity, mTORC1 and syntenin, which are pivotal downstream factors of Rheb1 in the alteration of spine formation and synapse function in TSC neurons.

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.

Residue-Level Contact Reveals Modular Domain Interactions of PICK1 Are Driven by Both Electrostatic and Hydrophobic Forces

PICK1 is a multi-domain scaffolding protein that is uniquely comprised of both a PDZ domain and a BAR domain. While previous experiments have shown that the PDZ domain and the linker positively regulate the BAR domain and the C-terminus negatively regulates the BAR domain, the details of internal regulation mechanisms are unknown. Molecular dynamics (MD) simulations have been proven to be a useful tool in revealing the intramolecular interactions at atomic-level resolution. PICK1 performs its biological functions in a dimeric form which is extremely computationally demanding to simulate with an all-atom force field. Here, we use coarse-grained MD simulations to expose the key residues and driving forces in the internal regulations of PICK1. While the PDZ and BAR domains do not form a stable complex, our simulations show the PDZ domain preferentially interacting with the concave surface of the BAR domain over other BAR domain regions. Furthermore, our simulations show that the short helix in the linker region can form interactions with the PDZ domain. Our results reveal that the surface of the βB-βC loop, βC strand, and αA-βD loop of the PDZ domain can form a group of hydrophobic interactions surrounding the linker helix. These interactions are driven by hydrophobic forces. In contrast, our simulations reveal a very dynamic C-terminus that most often resides on the convex surface of the BAR domain rather than the previously suspected concave surface. These interactions are driven by a combination of electrostatic and hydrophobic interactions.

Syntenin: PDZ Protein Regulating Signaling Pathways and Cellular Functions

Syntenin is an adaptor-like molecule that has two adjacent tandem postsynaptic density protein 95/Discs large protein/Zonula occludens 1 (PDZ) domains. The PDZ domains of syntenin recognize multiple peptide motifs with low to moderate affinity. Many reports have indicated interactions between syntenin and a plethora of proteins. Through interactions with various proteins, syntenin regulates the architecture of the cell membrane. As a result, increases in syntenin levels induce the metastasis of tumor cells, protrusion along the neurite in neuronal cells, and exosome biogenesis in various cell types. Here, we review the updated data that support various roles for syntenin in the regulation of neuronal synapses, tumor cell invasion, and exosome control.

MDA-9/Syntenin: An emerging global molecular target regulating cancer invasion and metastasis

With few exceptions, metastasis is the terminal stage of cancer with limited therapeutic options. Metastasis consists of numerous phenotypic and genotypic alterations of cells that are directly and indirectly induced by multiple intrinsic (cellular) and extrinsic (micro-environmental) factors. To metastasize, a cancer cell often transitions from an epithelial to mesenchymal morphology (EMT), modifies the extracellular matrix, forms emboli and survives in the circulation, escapes immune surveillance, adheres to sites distant from the initial tumor and finally develops a blood supply (angiogenesis) and colonizes in a secondary niche (a micrometastasis). Scientific advances have greatly enhanced our understanding of the precise molecular and genetic changes, operating independently or collectively, that lead to metastasis. This review focuses on a unique gene, melanoma differentiation associated gene-9 (also known as Syntenin-1; Syndecan Binding Protein (sdcbp); mda-9/syntenin), initially cloned and characterized from metastatic human melanoma and shown to be a pro-metastatic gene. In the last two decades, our comprehension of the diversity of actions of MDA-9/Syntenin on cellular phenotype has emerged. MDA-9/Sytenin plays pivotal regulatory roles in multiple signaling cascades and orchestrates both metastatic and non-metastatic events. Considering the relevance of this gene in controlling cancer invasion and metastasis, approaches have been developed to uniquely and selectively target this gene. We also provide recent updates on strategies that have been successfully employed in targeting MDA-9/Syntenin resulting in profound pre-clinical anti-cancer activity.

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.

Impairment in extinction of cued fear memory in syntenin-1 knockout mice

Syntenin-1 is a PDZ domain-containing intracellular scaffold protein involved in exosome production, synapse formation, and synaptic plasticity. We tested whether syntenin-1 can regulate learning and memory through its effects on synaptic plasticity. Specifically, we investigated the role of syntenin-1 in contextual and cued fear conditioning and extinction of conditioned fear using syntenin-1 knockout (KO) mice. Genetic disruption of syntenin-1 had little effect on contextual and cued fear memory. However, syntenin-1 KO mice exhibited selective impairment in cued fear extinction retention. This extinction retention deficit in syntenin-1 KO mice was associated with reduced c-Fos-positive neurons in the basolateral amygdala (BLA) and infralimbic cortex (IL) after extinction training and increased c-Fos-positive neurons in the BLA after an extinction retention test. Our results suggest that syntenin-1 plays an important role in extinction of cued fear memory by modulating neuronal activity in the BLA and IL.

G protein-coupled receptors directly bind filamin A with high affinity and promote filamin phosphorylation

Although interaction of a few G protein-coupled receptors (GPCRs) with Filamin A, a key actin cross-linking and biomechanical signal transducer protein, has been observed, a comprehensive structure-function analysis of this interaction is lacking. Through a systematic sequence-based analysis, we found that a conserved filamin binding motif is present in the cytoplasmic domains of >20% of the 824 GPCRs encoded in the human genome. Direct high-affinity interaction of filamin binding motif peptides of select GPCRs with the Ig domain of Filamin A was confirmed by nuclear magnetic resonance spectroscopy and isothermal titration calorimetric experiments. Engagement of the filamin binding motif with the Filamin A Ig domain induced the phosphorylation of filamin by protein kinase A in vitro. In transfected cells, agonist activation as well as constitutive activation of representative GPCRs dramatically elicited recruitment and phosphorylation of cellular Filamin A, a phenomenon long known to be crucial for regulating the structure and dynamics of the cytoskeleton. Our data suggest a molecular mechanism for direct GPCR-cytoskeleton coupling via filamin. Until now, GPCR signaling to the cytoskeleton was predominantly thought to be indirect, through canonical G protein-mediated signaling cascades involving GTPases, adenylyl cyclases, phospholipases, ion channels, and protein kinases. We propose that the GPCR-induced filamin phosphorylation pathway is a conserved, novel biochemical signaling paradigm.

Rheb activation disrupts spine synapse formation through accumulation of syntenin in tuberous sclerosis complex

Rheb is a small GTP-binding protein and its GTPase activity is activated by the complex of Tsc1 and Tsc2 whose mutations cause tuberous sclerosis complex (TSC). We previously reported that cultured TSC neurons showed impaired spine synapse morphogenesis in an mTORC1-independent manner. Here we show that the PDZ protein syntenin preferentially binds to the GDP-bound form of Rheb. The levels of syntenin are significantly higher in TSC neurons than in wild-type neurons because the Rheb-GDP-syntenin complex is prone to proteasomal degradation. Accumulated syntenin in TSC neurons disrupts spine synapse formation through inhibition of the association between syndecan-2 and calcium/calmodulin-dependent serine protein kinase. Instead, syntenin enhances excitatory shaft synapse formation on dendrites by interacting with ephrinB3. Downregulation of syntenin in TSC neurons restores both spine and shaft synapse densities. These findings suggest that Rheb-syntenin signalling may be a novel therapeutic target for abnormalities in spine and shaft synapses in TSC neurons.

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.

Targeting tumor invasion: the roles of MDA-9/Syntenin

INTRODUCTION

Melanoma differentiation-associated gene - 9 (MDA-9)/Syntenin has become an increasingly popular focus for investigation in numerous cancertypes. Originally implicated in melanoma metastasis, it has diverse cellular roles and is consistently identified as a regulator of tumor invasion and angiogenesis. As a potential target for inhibiting some of the most lethal aspects of cancer progression, further insight into the function of MDA-9/Syntenin is mandatory.

AREAS COVERED

Recent literature and seminal articles were reviewed to summarize the latest collective understanding of MDA-9/Syntenin's role in normal and cancerous settings. Insights into its participation in developmental processes are included, as is the functional significance of the N- and C-terminals and PDZ domains of MDA-9/Syntenin. Current reports highlight the clinical significance of MDA-9/Syntenin expression level in a variety of cancers, often correlating directly with reduced patient survival. Also presented are assessments of roles of MDA-9/Syntenin in cancer progression as well as its functions as an intracellular adapter molecule.

EXPERT OPINION

Multiple studies demonstrate the importance of MDA-9/Syntenin in tumor invasion and progression. Through the use of novel drug design approaches, this protein may provide a worthwhile therapeutic target. As many conventional therapies do not address, or even enhance, tumor invasion, an anti-invasive approach would be a worthwhile addition in cancer therapy.

Schistosome syntenin partially protects vaccinated mice against Schistosoma mansoni infection

Background

Schistosomiasis is a neglected tropical disease caused by several species of trematode of the genus Schistosoma. The disease affects more than 200 million people in the world and causes up to 280,000 deaths per year, besides having high morbidity due to chronic illness that damages internal organs. Current schistosomiasis control strategies are mainly based on chemotherapy, but many researchers believe that the best long-term strategy to control disease is a combination of drug treatment and immunization with an anti-schistosome vaccine. Among the most promising molecules as vaccine candidates are the proteins present in the tegument and digestive tract of the parasite.

Methodology/Principal Findings

In this study, we describe for the first time Schistosoma mansoni syntenin (SmSynt) and we evaluate its potential as a recombinant vaccine. We demonstrate by real-time PCR that syntenin is mainly expressed in intravascular life stages (schistosomula and adult worms) of the parasite life cycle and, by confocal microscopy, we localize it in digestive epithelia in adult worms and schistosomula. Administration of siRNAs targeting SmSynt leads to the knock-down of syntenin gene and protein levels, but this has no demonstrable impact on parasite morphology or viability, suggesting that high SmSynt gene expression is not essential for the parasites in vitro. Mice immunization with rSmSynt, formulated with Freund's adjuvant, induces a Th1-type response, as suggested by the production of IFN-γ and TNF-α by rSmSynt-stimulated cultured splenocytes. The protective effect conferred by vaccination with rSmSynt was demonstrated by 30–37% reduction of worm burden, 38–43% reduction in the number, and 35–37% reduction in the area, of liver granulomas.

Conclusions/Significance

Our report is the first characterization of syntenin in Schistosoma mansoni and our data suggest that this protein is a potential candidate for the development of a multi-antigen vaccine to control schistosomiasis.

Schistosomiasis affects more than 200 million people worldwide and causes up to 280,000 deaths per year. In terms of global mortality and morbidity, this disease is the most important human helminth infection. Current control strategies are based on chemotherapy, but recurrent re-infection of people living in endemic areas makes many researchers, and also the World Health Organization, search for an effective vaccine to provide protection against schistosomiasis. Substantial efforts have been committed to the characterization of new antigens for an anti-schistosome vaccine and, in order to find new targets for vaccine and/or drug development, we searched transcriptomics and proteomics of Schistosoma mansoni and identified the protein syntenin (SmSynt) for analysis. In this study, we characterize SmSynt and evaluate its potential as a vaccine candidate to protect mice against S. mansoni infection. We demonstrate that SmSynt is expressed in schistosomula and adult worms, the intravascular stages of S. mansoni and it is located in the intestinal tract of the worms, an important host/parasite interface. Furthermore, vaccination of mice with rSmSynt confers partial protection against S. mansoni challenge infection and ameliorates parasite-induced liver pathology. Our data suggest that SmSynt is a potential candidate in the development of a vaccine against schistosomiasis.

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.

Expression patterns of MDA-9/syntenin during development of the mouse embryo

Melanoma differentiation associated gene-9 (MDA-9)/syntenin is a PDZ domain-containing adaptor protein involved in multiple diverse cellular processes including organization of protein complexes in the plasma membrane, intracellular trafficking and cell surface targeting, synaptic transmission, and cancer metastasis. In the present study, we analyzed the expression pattern of MDA-9/syntenin during mouse development. MDA-9/syntenin was robustly expressed with tight regulation of its temporal and spatial expression during fetal development in the developing skin, spinal cord, heart, lung and liver, which are regulated by multiple signaling pathways in the process of organogenesis. Recent studies also indicate that MDA-9/syntenin is involved in the signaling pathways crucial during development such as Wnt, Notch and FGF. Taken together, these results suggest that MDA-9/syntenin may play a prominent role during normal mouse development in the context of cell proliferation as well as differentiation through modulating multiple signaling pathways as a crucial adaptor protein. Additionally, temporal regulation of MDA-9/syntenin expression may be required during specific stages and in specific tissues during development.

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.

PDZ binding to the BAR domain of PICK1 is elucidated by coarse-grained molecular dynamics

A key regulator of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor traffic, PICK1 is known to interact with over 40 other proteins, including receptors, transporters and ionic channels, and to be active mostly as a homodimer. The current lack of a complete PICK1 structure determined at atomic resolution hinders the elucidation of its functional mechanisms. Here, we identify interactions between the component PDZ and BAR domains of PICK1 by calculating possible binding sites for the PDZ domain of PICK1 (PICK1-PDZ) to the homology-modeled, crescent-shaped dimer of the PICK1-BAR domain using multiplexed replica-exchange molecular dynamics (MREMD) and canonical molecular dynamics simulations with the coarse-grained UNRES force field. The MREMD results show that the preferred binding site for the single PDZ domain is the concave cavity of the BAR dimer. A second possible binding site is near the N-terminus of the BAR domain that is linked directly to the PDZ domain. Subsequent short canonical molecular dynamics simulations used to determine how the PICK1-PDZ domain moves to the preferred binding site on the BAR domain of PICK1 revealed that initial hydrophobic interactions drive the progress of the simulated binding. Thus, the concave face of the BAR dimer accommodates the PDZ domain first by weak hydrophobic interactions and then the PDZ domain slides to the center of the concave face, where more favorable hydrophobic interactions take over.

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Uncovering molecular biomarkers that correlate cognitive decline with the changes of hippocampus' gene expression profiles in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is characterized by a neurodegenerative progression that alters cognition. On a phenotypical level, cognition is evaluated by means of the MiniMental State Examination (MMSE) and the post-mortem examination of Neurofibrillary Tangle count (NFT) helps to confirm an AD diagnostic. The MMSE evaluates different aspects of cognition including orientation, short-term memory (retention and recall), attention and language. As there is a normal cognitive decline with aging, and death is the final state on which NFT can be counted, the identification of brain gene expression biomarkers from these phenotypical measures has been elusive.

METHODOLOGY/PRINCIPAL FINDINGS

We have reanalysed a microarray dataset contributed in 2004 by Blalock et al. of 31 samples corresponding to hippocampus gene expression from 22 AD subjects of varying degree of severity and 9 controls. Instead of only relying on correlations of gene expression with the associated MMSE and NFT measures, and by using modern bioinformatics methods based on information theory and combinatorial optimization, we uncovered a 1,372-probe gene expression signature that presents a high-consensus with established markers of progression in AD. The signature reveals alterations in calcium, insulin, phosphatidylinositol and wnt-signalling. Among the most correlated gene probes with AD severity we found those linked to synaptic function, neurofilament bundle assembly and neuronal plasticity.

CONCLUSIONS/SIGNIFICANCE

A transcription factors analysis of 1,372-probe signature reveals significant associations with the EGR/KROX family of proteins, MAZ, and E2F1. The gene homologous of EGR1, zif268, Egr-1 or Zenk, together with other members of the EGR family, are consolidating a key role in the neuronal plasticity in the brain. These results indicate a degree of commonality between putative genes involved in AD and prion-induced neurodegenerative processes that warrants further investigation.

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.

The tandem PDZ protein Syntenin interacts with the aminoacyl tRNA synthetase complex in a lysyl-tRNA synthetase-dependent manner

Syntenin-1 is a tandem PDZ protein that binds a diverse array of signaling molecules that are often associated with cell adhesion and intracellular trafficking. With the use of a MS-based functional proteomics approach, we identified several members of the aminoacyl-tRNA synthetase macromolecular (ARS) complex in a syntenin-1 pull down assay. Interaction of these proteins with syntenin-1 was confirmed by co-immunoprecipitation from cultured cells. We demonstrate a direct interaction of syntenin-1 with lysyl-tRNA synthetase (KRS), which contains a PDZ binding motif at its C-terminus. This motif is important for the interaction of the entire complex with syntenin-1. A point mutation in the PDZ2 domain of syntenin-1 abrogates interaction with KRS. As a result, other components of the ARS complex no longer co-immunoprecipitate with syntenin-1. We further show that syntenin-1 regulates KRS activity. These findings suggest that syntenin-1 is an adaptor modulating the activity of KRS.

Knock-in mice lacking the PDZ-ligand motif of mGluR7a show impaired PKC-dependent autoinhibition of glutamate release, spatial working memory deficits, and increased susceptibility to pentylenetetrazol

The metabotropic glutamate receptor 7 (mGluR7) is widely expressed throughout the brain and primarily localized at presynaptic active zones, where it is thought to regulate neurotransmitter release. Protein interacting with C kinase 1 (PICK1), a postsynaptic density protein-95/disc-large tumor suppressor protein/zonula occludens-1 (PDZ)-domain protein, binds to the three C-terminal amino acids (-LVI) of the predominant mGluR7 splice variant, mGluR7a, and has been implicated in the synaptic clustering of this receptor. Here, we generated knock-in mice in which the C-terminal LVI coding sequence of exon 10 of the mGluR7 gene was replaced by three alanine codons (-AAA). Immunoprecipitation showed that the PICK1-mGluR7a interaction is disrupted in mGluR7a(AAA/AAA) mice. However, the synaptic localization of mGluR7a was not altered in cultured hippocampal neurons and brain sections prepared from the knock-in animals. In cerebellar granule cell cultures, the group III mGluR agonist l-AP-4 decreased the frequency of spontaneous excitatory currents in neurons derived from wild-type but not mGluR7a(AAA/AAA) mice, consistent with the interaction between mGluR7a and PICK1 being required for protein kinase C-mediated inhibition of glutamate release. At the behavioral level, the mGluR7a(AAA/AAA) mice showed no deficits in motor coordination, pain sensitivity, and anxiety but exhibited significant defects in hippocampus-dependent spatial working memory. In addition, they displayed a high susceptibility to the convulsant drug pentylenetetrazole. Together, these results indicate that PICK1 binding to the C-terminal region of mGluR7a is crucial for agonist-triggered presynaptic signaling in vivo.

Corequirement of PICK1 binding and PKC phosphorylation for stable surface expression of the metabotropic glutamate receptor mGluR7

The presynaptic metabotropic glutamate receptor (mGluR) mGluR7 modulates excitatory neurotransmission by regulating neurotransmitter release and plays a critical role in certain forms of synaptic plasticity. Although the dynamic regulation of mGluR7 surface expression governs a form of metaplasticity in the hippocampus, little is known about the molecular mechanisms regulating mGluR7 trafficking. We now show that mGluR7 surface expression is stabilized by both PKC phosphorylation and by receptor binding to the PDZ domain-containing protein PICK1. Phosphorylation of mGluR7 on serine 862 (S862) inhibits CaM binding, thereby increasing mGluR7 surface expression and receptor binding to PICK1. Furthermore, in mice lacking PICK1, PKC-dependent increases in mGluR7 phosphorylation and surface expression are diminished, and mGluR7-dependent plasticity at mossy fiber-interneuron hippocampal synapses is impaired. These data support a model in which PICK1 binding and PKC phosphorylation act together to stabilize mGluR7 on the cell surface in vivo.

mda-9/Syntenin: more than just a simple adapter protein when it comes to cancer metastasis

Cancer is a progressive disease that, in many instances, if untreated, can culminate in metastatic spread of primary tumor cells to distant sites in the body. Metastasis frequently confers virulence and therapy resistance to cancer cells, and defining the molecular events that control metastasis will be mandatory to develop rational, targeted therapies for effective intervention, prevention of recurrence, and the "holy grail" of engendering a cure. Adapter proteins are physiologically pertinent molecules that, through interactions with key regulatory proteins via specific conserved domains, control important cellular events. Melanoma differentiation associated gene-9 (mda-9), also known as syntenin, is a PDZ domain-containing adapter protein that is involved in organization of protein complexes in the plasma membranes, regulation of B-cell development, intracellular trafficking and cell-surface targeting, synaptic transmission, and axonal outgrowth. Recent studies now define a seminal role for mda-9/syntenin in cancer metastasis. The present review provides a current perspective of our understanding of this important aspect of mda-9/syntenin, suggesting that this gene and its encoded protein and interacting protein partners may provide viable targets for intervening in the final and invariably the most lethal stage of cancer progression, namely, cancer metastasis.

The ins and outs of syntenin, a multifunctional intracellular adaptor protein

One of the most challenging issues currently facing cell biologists is how signal specificity and compartmentalization is achieved, allowing extracellular stimulation to result in a unique and pre-defined intracellular outcome. For this to occur, intracellular components must be correctly positioned in both space and time. Adaptor molecules, which contain protein-interaction domains, are often involved in the assembly of multimeric complexes that organize intracellular signal-transduction pathways. One such protein is syntenin, a PDZ-domain-containing molecule that has a surprising variety and diversity of interaction partners. Here we assimilate and discuss current data that support a role for syntenin in regulating transmembrane-receptor trafficking, tumour-cell metastasis and neuronal-synapse function.

Filamin A mutant lacking actin-binding domain restores mu opioid receptor regulation in melanoma cells

We have previously reported that the protein filamin A (FLA) binds to the carboxyl tail of the mu opioid receptor (MOPr). Using human melanoma cells, which do not express filamin A, we showed that receptor down-regulation, functional desensitization and trafficking are deficient in the absence of FLA (Onoprishvili et al. Mol Pharmacol 64:1092-1100, 2003). Since FLA has a binding domain for actin and is a member of the family of actin cytoskeleton proteins, it is usually assumed that FLA functions via the actin cytoskeleton. We decided to test this hypothesis by preparing cDNA coding for mutant FLA lacking the actin binding domain (FLA-ABD) and expressing FLA-ABD in the human melanoma cell line M2 (M2-ABD cell line). We report here that this mutant is capable of restoring almost as well as full length FLA the down-regulation of the human MOPr. It is similarly very effective in restoring functional desensitization of MOPr, as assessed by the decrease in G-protein activation after chronic exposure of M2-ABD cells to the mu agonist DAMGO. We also found that A7 cells, expressing wild type FLA, exhibit rapid activation of the MAP kinases, ERK 1 and 2, by DAMGO, as shown by a rise in the level of phospho-ERK 1 and 2. This is followed by rapid dephosphorylation (inactivation), which reaches basal level between 30 and 60 min after DAMGO treatment. M2 cells show normal activation of ERK 1 and 2 in the presence of DAMGO, but very slow inactivation. The rapid rate of MAPK inactivation is partially restored by FLA-ABD. We conclude that some functions of FLA do not act via the actin cytoskeleton. It is likely that other functions, not studied here, may require functional binding of the MOPr-FLA complex to actin.

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.

Filamins in cardiovascular development

Filamins are classically recognized as large cytoplasmic proteins that cross-link cortical actin into dynamic 3-dimensional structures and transmit extracellular signals through integrin receptors into the cytoplasm. However, recent reports indicate that filamins interact with a large number of other proteins with diverse functions, including transcriptional factors and cellular molecules involved in signaling, adhesion, and cellular motility, and are also present in the cell nucleus. In addition, genetic mutations in filamins have been linked to a wide range of human genetic disorders, including skeletal, central nervous system, and cardiovascular malformations, highlighting distinct filamin interactions. Here, we update the cardiovascular phenotypes of patients with mutations in filamin genes and mice deficient in filamins and filamin-interacting proteins.

Target-cell-dependent plasticity within the mossy fibre-CA3 circuit reveals compartmentalized regulation of presynaptic function at divergent release sites

Individual axons of central neurons innervate a large number of distinct postsynaptic targets belonging to divergent functional categories such as glutamatergic principal cells and inhibitory interneurons. While each bouton along a common axon should experience the same activity pattern in response to action potential firing within the parent presynaptic neuron, accumulating evidence suggests that neighbouring boutons contacting functionally distinct postsynaptic targets regulate their release properties independently, despite being separated by only a few microns. This target-cell-specific autonomy of presynaptic function can greatly expand the computational prowess of central axons to allow for precise coordination of large neuronal ensembles within a given circuit. An excellent example of target-cell-specific presynaptic mechanisms occurs in the CA3 hippocampus where mossy fibre (MF) axons of dentate gyrus granule cells target both principal cells and local circuit inhibitory interneurons via both anatomically and functionally specialized terminals. Of particular interest, mechanisms of both short- and long-term plasticity remain autonomous at these divergent release sites due to an anatomical and biochemical segregation of discrete molecular signalling cascades. Here we review roughly a decades worth of research on the MF-CA3 pathway to showcase the target-cell dependence of presynaptically expressed NMDA receptor-independent synaptic plasticity.

Sumoylation in neurons: nuclear and synaptic roles?

Sumoylation is a post-translational modification that was originally thought to only target nuclear proteins. Evidence has emerged, however, that the role of sumoylation is much more diverse: three plasma membrane proteins belonging to different protein families (glucose transporters, K(+) channels and metabotropic glutamate receptors) have been shown to be sumoylated. In addition, sumoylation of transcription factors, such as myocyte enhancer factor 2 (MEF2), was found to regulate synapse formation. A major role of sumoylation in other systems is to modify protein-protein interactions, and because protein interactions are particularly elaborate in the nervous system and crucial for synapse formation and function, sumoylation could constitute a major regulatory mechanism in neurons. In this review, we evaluate the available data and discuss possible roles for sumoylation in the regulation of crucial neurobiological processes, such as neuronal development and synaptic transmission.

PICK1 is not a susceptibility gene for schizophrenia in a Japanese population: association study in a large case-control population

The protein interacting with C-kinase 1 (PICK1) has been implicated in the susceptibility to schizophrenia. PICK1 interacts with enzymes and receptors that play roles in the pathogenesis of schizophrenia via glutamatergic dysfunction. Recently, two studies reported associations between schizophrenia and two PICK1 gene polymorphisms, rs3952 in Chinese and Japanese populations and rs2076369 in a Japanese population. We attempted to confirm these associations in a case-control study of 1765 Japanese patients with schizophrenia and 1851 Japanese control subjects. Neither polymorphism was associated with schizophrenia (rs3952, p=0.755; rs2076369, p=0.997). A haplotype block with these polymorphisms spanning the 5' region of the PICK1 gene showed high linkage disequilibrium in the Japanese population (D'=0.98, r(2)=0.34); however, neither haplotype was significantly associated with schizophrenia. We conclude that the common haplotypes and polymorphisms of the PICK1 gene identified thus far are unlikely to contribute to genetic susceptibility to schizophrenia in the Japanese population.

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.

Structure and function of PICK1

PICK1 is a peripheral membrane protein conserved from Caenorhabditis elegans to the human. It is expressed in many tissues with high levels in brain and testis. Inside cells, PICK1 is localized at the perinuclear region as well as specialized structures such as synapses of neurons. PICK1 contains a PDZ domain and a BAR domain. The PDZ domain of PICK1 binds to a large number of membrane proteins, especially proteins with C-terminal type II PDZ-binding motifs. The BAR domain of PICK1 binds to lipid molecules, mainly phosphoinositides. While the PDZ domain and the linker region of PICK1 enhance BAR domain's lipid binding, the C-terminal region of PICK1 inhibits its lipid binding. PICK1 regulates the subcellular localization and surface expression of its PDZ-binding partners. Lipid binding of PICK1's BAR domain is important for this regulation. With its PDZ domain interacting with membrane proteins and its BAR domain binding to lipids, the unique structure of PICK1 enables it to couple membrane proteins to protein-trafficking machinery.

Cooperative regulation of extracellular signal-regulated kinase activation and cell shape change by filamin A and beta-arrestins

beta-Arrestins (betaarr) are multifunctional adaptor proteins that can act as scaffolds for G protein-coupled receptor activation of mitogen-activated protein kinases (MAPK). Here, we identify the actin-binding and scaffolding protein filamin A (FLNA) as a betaarr-binding partner using Son of sevenless recruitment system screening, a classical yeast two-hybrid system, coimmunoprecipitation analyses, and direct binding in vitro. In FLNA, the betaarr-binding site involves tandem repeat 22 in the carboxyl terminus. betaarr binds FLNA through both its N- and C-terminal domains, indicating the presence of multiple binding sites. We demonstrate that betaarr and FLNA act cooperatively to activate the MAPK extracellular signal-regulated kinase (ERK) downstream of activated muscarinic M1 (M1MR) and angiotensin II type 1a (AT1AR) receptors and provide experimental evidence indicating that this phenomenon is due to the facilitation of betaarr-ERK2 complex formation by FLNA. In Hep2 cells, stimulation of M1MR or AT1AR results in the colocalization of receptor, betaarr, FLNA, and active ERK in membrane ruffles. Reduction of endogenous levels of betaarr or FLNA and a catalytically inactive dominant negative MEK1, which prevents ERK activation, inhibit membrane ruffle formation, indicating the functional requirement for betaarr, FLNA, and active ERK in this process. Our results indicate that betaarr and FLNA cooperate to regulate ERK activation and actin cytoskeleton reorganization.

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.

Organization of the presynaptic active zone by ERC2/CAST1-dependent clustering of the tandem PDZ protein syntenin-1

Presynaptic active zones contain a cytoskeletal matrix called the CAZ, which is thought to play a critical role in the regulation of active zone formation and neurotransmitter release. Recent studies have identified several CAZ components, but little is known about how they contribute to the molecular organization of active zones. Here, we report a novel PDZ [postsynaptic density-95/Discs large/zona occludens-1] interaction between the CAZ protein ERC2/CAST1 and the tandem PDZ protein syntenin-1, which is known to associate with diverse synaptic proteins, including glutamate receptor subunits, SynCAM, and beta-neurexin. This interaction promotes the localization of syntenin-1 at presynaptic ERC2 clusters. In addition to the PDZ interaction, multimerization of both ERC2 and syntenin-1 mediates syntenin-1 clustering. These results suggest that ERC2 promotes presynaptic syntenin-1 clustering by two distinct mechanisms and that syntenin-1 may contribute to the molecular organization of active zones by linking ERC2 and other CAZ components to diverse syntenin-1-associated synaptic proteins.

Pias1 Interaction and Sumoylation of Metabotropic Glutamate Receptor 8

Group III presynaptic metabotropic glutamate receptors (mGluRs) play a central role in regulating presynaptic activity through G-protein effects on ion channels and signal transducing enzymes. Like all Class C G-protein-coupled receptors, mGluR8 has an extended intracellular C-terminal domain (CTD) presumed to allow for modulation of downstream signaling. In a yeast two-hybrid screen of an adult rat brain cDNA library with the CTDs of mGluR8a and 8b (mGluR8-C) as baits, we identified sumo1 and four different components of the sumoylation cascade (ube2a, Pias1, Piasgamma, Piasxbeta) as interacting proteins. Binding assays using recombinant GST fusion proteins confirmed that Pias1 interacts not only with mGluR8-C but also with all group III mGluR CTDs. Pias1 binding to mGluR8-C required a region N-terminal to a consensus sumoylation motif and was not affected by arginine substitution of the conserved lysine 882 within this motif. Co-transfection of fluorescently tagged mGluR8a-C, sumo1, and enzymes of the sumoylation cascade into HEK293 cells showed that mGluR8a-C can be sumoylated in vivo. Arginine substitution of lysine 882 within the consensus sumoylation motif, but not other conserved lysines within the CTD, abolished in vivo sumoylation. Our results are consistent with post-translational sumoylation providing a novel mechanism of group III mGluR regulation.

The lymphocyte receptor CD6 interacts with syntenin-1, a scaffolding protein containing PDZ domains

CD6 is a type I membrane glycoprotein expressed on thymocytes, mature T and B1a lymphocytes, and CNS cells. CD6 binds to activated leukocyte cell adhesion molecule (CD166), and is considered as a costimulatory molecule involved in lymphocyte activation and thymocyte development. Accordingly, CD6 partially associates with the TCR/CD3 complex and colocalizes with it at the center of the mature immunological synapse (IS) on T lymphocytes. However, the signaling pathway used by CD6 is still mostly unknown. The yeast two-hybrid system has allowed us the identification of syntenin-1 as an interacting protein with the cytoplasmic tail of CD6. Syntenin-1 is a PDZ (postsynaptic density protein-95, postsynaptic discs large, and zona occludens-1) domain-containing protein, which functions as an adaptor protein able to bind cytoskeletal proteins and signal transduction effectors. Mutational analyses showed that certain amino acids of the most C-terminal sequence of CD6 (-YDDISAA) and the two postsynaptic density protein-95, postsynaptic discs large, and zona occludens-1 domains of syntenin-1 are relevant to the interaction. Further confirmation of the CD6-syntenin-1 interaction was obtained from pull-down and co-immunoprecipitation assays in mammalian cells. Image analyses also showed that syntenin-1 accumulates at CD6 caps and at the IS. Therefore, we propose that syntenin-1 may function as a scaffolding protein coupling CD6 and most likely other lymphocyte receptors to cytoskeleton and/or signaling effectors during IS maturation.

Syntenin is involved in the developmental regulation of neuronal membrane architecture

Syntenin is a approximately 33 kDa scaffolding protein that we have shown previously to bind to kainate receptor subunits via a PDZ interaction. Here we show that syntenin has a tightly regulated developmental profile in neurons and is most abundant in the period of intense growth and synapse formation and stabilization. There is extensive colocalization of syntenin and kainate receptors with particularly intense labeling for both proteins at growth cones. Overexpression of GFP-syntenin in both young and mature neurons evokes marked changes in neuronal morphology by increasing the number of dendritic protrusions. These results are consistent with the involvement of syntenin in controlling membrane organization and suggest that by interaction with kainate receptors it may play a role in determining the formation and maturation of synapses.

mGluR7 is a metaplastic switch controlling bidirectional plasticity of feedforward inhibition

Plasticity of feedforward inhibition in the hippocampal mossy fiber (MF) pathway can dramatically influence dentate gyrus-CA3 dialog. Interestingly, MF inputs to CA3 stratum lucidum interneurons (SLINs) undergo long-term depression (LTD) following high-frequency stimulation (HFS), in contrast to MF-pyramid (PYR) synapses, where long-term potentiation (LTP) occurs. Furthermore, activity-induced potentiation of MF-SLIN transmission has not previously been observed. Here we report that metabotropic glutamate receptor subtype 7 (mGluR7) is a metaplastic switch at MF-SLIN synapses, whose activation and surface expression governs the direction of plasticity. In naive slices, mGluR7 activation during HFS generates MF-SLIN LTD, depressing presynaptic release through a PKC-dependent mechanism. Following agonist exposure, mGluR7 undergoes internalization, unmasking the ability of MF-SLIN synapses to undergo presynaptic potentiation in response to the same HFS that induces LTD in naive slices. Thus, selective mGluR7 targeting to MF terminals contacting SLINs and not PYRs provides cell target-specific plasticity and bidirectional control of feedforward inhibition.

Identification of a unique filamin A binding region within the cytoplasmic domain of glycoprotein Ibalpha

Binding of the platelet GPIb/V/IX (glycoprotein Ib/V/IX) receptor to von Willebrand factor is critical for platelet adhesion and aggregation under conditions of rapid blood flow. The adhesive function of GPIbalpha is regulated by its anchorage to the membrane skeleton through a specific interaction with filamin A. In the present study, we examined the amino acid residues within the cytoplasmic tail of GPIbalpha, which are critical for association with filamin A, using a series of 25-mer synthetic peptides that mimic the cytoplasmic tail sequences of wild-type and mutant forms of GPIbalpha. Peptide binding studies of purified human filamin A have demonstrated a major role for the conserved hydrophobic stretch L567FLWV571 in mediating this interaction. Progressive alanine substitutions of triple, double and single amino acid residues within the Pro561-Arg572 region suggested an important role for Trp570 and Phe568 in promoting GPIbalpha binding to filamin A. The importance of these two residues in promoting filamin A binding to GPIbalpha in vivo was confirmed from the study of Chinese-hamster ovary cells expressing GPIbalpha Trp570-->Ala and Phe568-->Ala substitutions. Phenotypic analysis of these cell lines in flow-based adhesion studies revealed a critical role for these residues in maintaining receptor anchorage to the membrane skeleton and in maintaining cell adhesion to a von Willebrand factor matrix under high-shear conditions. These studies demonstrate a novel filamin A binding motif in the cytoplasmic tail of GPIbalpha, which is critically dependent on both Trp570 and Phe568.

Role of Protein Phosphatase 2A in mGluR5-regulated MEK/ERK Phosphorylation in Neurons

The regulation of protein phosphorylation requires coordinated interaction between protein kinases and protein phosphatases (PPs). Recent evidence has shown that the Galphaq-protein-coupled metabotropic glutamate receptor (mGluR) 5 up-regulates phosphorylation of MAPK/ERK1/2. However, signaling mechanisms linking mGluR5 to ERK are poorly understood. In this study, roles of a major serine/threonine PP, PP2A, in this event were evaluated in cultured neurons. We found that the PP1/2A inhibitors okadaic acid and calyculin A mimicked the effect of the mGluR5 agonists (RS)-3,5-dihydroxyphenylglycine and (RS)-2-chloro-5-hydroxyphenylglycine in facilitating phosphorylation of ERK1/2 and its upstream kinase, MEK1/2, in a PP2A-dependent but not PP1-dependent manner. Co-administration of either inhibitor with an mGluR5 agonist produced additive phosphorylation of ERK1/2. Enzymatic assays showed a basal level of phosphatase activity of PP2A under normal conditions, and activation of mGluR5 selectively inhibited PP2A, but not PP1, activity. In addition, a physical association of the cytoplasmic C terminus of mGluR5 with PP2A was observed, and ligand activation of mGluR5 reduced mGluR5-PP2A binding. Additional mechanistic studies revealed that mGluR5 activation increased tyrosine (Tyr307) phosphorylation of PP2A, which was dependent on activation of a p60c-Src family tyrosine kinase, but not the epidermal growth factor receptor tyrosine kinase and resulted in dissociation of PP2A from mGluR5 and reduced PP2A activity. Together, we have identified a novel, mGluR5-triggered signaling mechanism involving use- and Src-dependent inactivation of PP2A, which contributes to mGluR5 activation of MEK1/2 and ERK1/2.

High Affinity Interaction with Filamin A Protects against Calcium-sensing Receptor Degradation

Calcium-sensing receptors (CaR) regulate cell proliferation, differentiation, and apoptosis through the MAPK pathway. MAPK pathway activation requires the cytoskeletal scaffold protein filamin A. Here we examine the interactions of CaR with filamin A in HEK-293 and M2 or A7 melanoma cells to determine how interactions with filamin A facilitate signaling. Filamin A interacts with CaR through two predicted beta-strands from residues 962 to 981; interactions between filamin A and CaR are greatly enhanced by exposure to 5 mM Ca2+. Truncations or deletions (from 972 to 997 or 962 to 981) of the CaR carboxyl terminus eliminate high affinity interactions with filamin A, but CaR-mediated MAPK pathway activation still occurs. CaR-mediated ERK phosphorylation can be localized to a predicted alpha-helix proximal to the membrane, which has been shown to be important for G protein-mediated signaling (residues 868-879). In M2 cells (-filamin A), CaR expression levels are very low; cotransfection of CaR with filamin A increases total cellular CaR and increases plasma membrane localization of CaR, facilitating CaR signaling to the MAPK pathway; similar results were obtained in HEK-293 cells. Interaction with filamin A increases cellular CaR by preventing CaR degradation, thereby facilitating CaR signaling. In addition, filamin A facilitates signaling to the MAPK pathway even by CaR truncations or deletion mutants that cannot engage in high affinity interactions with filamin A, suggesting the targeting of critical signaling proteins to CaR.

Making protein interactions druggable: targeting PDZ domains

Modulating protein-protein interactions involved in disease pathways is an attractive strategy for developing drugs, but remains a challenge to achieve. One approach is to target certain domains within proteins that mediate these interactions. One example of such a domain is the PDZ domain, which is involved in interactions between many different proteins in a variety of cellular contexts. Because PDZ domains have well-defined binding sites, they are promising targets for drug discovery. However, there is still much to learn about the function of these domains before drugs targeting PDZ interactions can become a reality.

Molecular mechanisms of ligand binding, signaling, and regulation within the superfamily of G-protein-coupled receptors: molecular modeling and mutagenesis approaches to receptor structure and function

The superfamily of G-protein-coupled receptors (GPCRs) could be subclassified into 7 families (A, B, large N-terminal family B-7 transmembrane helix, C, Frizzled/Smoothened, taste 2, and vomeronasal 1 receptors) among mammalian species. Cloning and functional studies of GPCRs have revealed that the superfamily of GPCRs comprises receptors for chemically diverse native ligands including (1) endogenous compounds like amines, peptides, and Wnt proteins (i.e., secreted proteins activating Frizzled receptors); (2) endogenous cell surface adhesion molecules; and (3) photons and exogenous compounds like odorants. The combined use of site-directed mutagenesis and molecular modeling approaches have provided detailed insight into molecular mechanisms of ligand binding, receptor folding, receptor activation, G-protein coupling, and regulation of GPCRs. The vast majority of family A, B, C, vomeronasal 1, and taste 2 receptors are able to transduce signals into cells through G-protein coupling. However, G-protein-independent signaling mechanisms have also been reported for many GPCRs. Specific interaction motifs in the intracellular parts of these receptors allow them to interact with scaffold proteins. Protein engineering techniques have provided information on molecular mechanisms of GPCR-accessory protein, GPCR-GPCR, and GPCR-scaffold protein interactions. Site-directed mutagenesis and molecular dynamics simulations have revealed that the inactive state conformations are stabilized by specific interhelical and intrahelical salt bridge interactions and hydrophobic-type interactions. Constitutively activating mutations or agonist binding disrupts such constraining interactions leading to receptor conformations that associates with and activate G-proteins.

A Protein Phosphatase-1γ1 Isoform Selectivity Determinant in Dendritic Spine-associated Neurabin

Protein phosphatase-1 (PP1) catalytic subunit isoforms interact with diverse proteins, typically containing a canonical (R/K)(V/I)XF motif. Despite sharing approximately 90% amino acid sequence identity, PP1beta and PP1gamma1 have distinct subcellular localizations that may be determined by selective interactions with PP1-binding proteins. Immunoprecipitation studies from brain and muscle extracts demonstrated that PP1gamma1 selectively interacts with spinophilin and neurabin, F-actin-targeting proteins, whereas PP1beta selectively interacted with G(M)/R(GL), the striated-muscle glycogen-targeting subunit. Glutathione S-transferase (GST) fusion proteins containing residues 146-493 of neurabin (GST-Nb-(146-493)) or residues 1-240 of G(M)/R(GL) (GST-G(M)-(1-240)) recapitulated these isoform selectivities in binding and phosphatase activity inhibition assays. Site-directed mutagenesis indicated that this isoform selectivity was not due to sequence differences between the canonical PP1-binding motifs (neurabin, (457)KIKF(460); G(M)/R(GL), (65)RVSF(68)). A chimeric GST fusion protein containing residues 1-64 of G(M)/R(GL) fused to residues 457-493 of neurabin (GST-G(M)/Nb) selectively bound to and inhibited PP1gamma1, whereas a GST-Nb/G(M) chimera containing Nb-(146-460) fused to G(M)-(69-240) selectively interacted with and weakly inhibited PP1beta, implicating domain(s) C-terminal to the (R/K)(V/I)XF motif as determinants of PP1 isoform selectivity. Deletion of Pro(464) and Ile(465) in neurabin (deltaPI) to equally space a conserved cluster of amino acids from the (R/K)(V/I)XF motif as in G(M)/R(GL) severely compromised the ability of neurabin to bind and inhibit both isoforms but did not affect PP1gamma1 selectivity. Further analysis of a series of C-terminal truncated GST-Nb-(146-493) proteins identified residues 473-479 of neurabin as containing a crucial PP1gamma1-selectivity determinant. In combination, these data identify a novel PP1gamma1-selective interaction domain in neurabin that may allow for selective regulation and/or subcellular targeting of PP1 isoforms.

Group I Metabotropic Glutamate Receptors Bind to Protein Phosphatase 1C

The modulation of neurotransmitter receptors by kinases and phosphatases represents a key mechanism in controlling synaptic signal transduction. However, molecular determinants involved in the specific targeting and interactions of these enzymes are largely unknown. Here, we identified both catalytic gamma-isoforms of protein phosphatase 1C (PP1gamma1 and PP1gamma2) as binding partners of the group I metabotropic glutamate receptors type 1a, 5a, and 5b in yeast cells and pull-down assays, using recombinant and native protein preparations. The tissue distribution of interacting proteins was compared, and protein phosphatase 1C was detected in dendrites of retinal bipolar cells expressing the respective interacting glutamate receptors. We mapped interacting domains within binding partners and identified five amino acids in the intracellular C termini of the metabotropic glutamate receptors type 1a, 5a, 5b, and 7b being both necessary and sufficient to bind protein phosphatase 1C. Furthermore, we show a dose-dependent competition of these C termini in binding the enzyme. Based on our data, we investigated the structure of the identified amino acids bound to protein phosphatase 1C by homology-based molecular modeling. In summary, these results provide a molecular description of the interaction between protein phosphatase 1C and metabotropic glutamate receptors and thereby increase our understanding of glutamatergic signal transduction.