Association of GABA(B) receptors and members of the 14-3-3 family of signaling proteins

The intracellular C-terminal domain of mGluR6 contains ER retention motifs

Metabotropic glutamate receptor 6 (mGluR6) predominantly localizes to the postsynaptic sites of retinal ON-bipolar cells, at which it recognizes glutamate released from photoreceptors. The C-terminal domain (CTD) of mGluR6 contains a cluster of basic amino acids resembling motifs for endoplasmic reticulum (ER) retention. We herein investigated whether these basic residues are involved in regulating the subcellular localization of mGluR6 in 293T cells expressing mGluR6 CTD mutants using immunocytochemistry, immunoprecipitation, and flow cytometry. We showed that full-length mGluR6 localized to the ER and cell surface, whereas mGluR6 mutants with 15- and 20-amino acid deletions from the C terminus localized to the ER, but were deficient at the cell surface. We also demonstrated that the cell surface deficiency of mGluR6 mutants was rescued by introducing an alanine substitution at basic residues within the CTD. The surface-deficient mGluR6 mutant still did not localize to the cell surface and was retained in the ER when co-expressed with surface-expressible constructs, including full-length mGluR6, even though surface-deficient and surface-expressible constructs formed heteromeric complexes. The co-expression of the surface-deficient mGluR6 mutant reduced the surface levels of surface-expressible constructs. These results indicate that basic residues in the mGluR6 CTD served as ER retention signals. We suggest that exposed ER retention motifs in the aberrant assembly containing truncated or misfolded mGluR6 prevent these protein complexes from being transported to the cell surface.

The role of GABAB receptors in the subcortical pathways of the mammalian auditory system

GABAB receptors are G-protein coupled receptors for the inhibitory neurotransmitter GABA. Functional GABAB receptors are formed as heteromers of GABAB1 and GABAB2 subunits, which further associate with various regulatory and signaling proteins to provide receptor complexes with distinct pharmacological and physiological properties. GABAB receptors are widely distributed in nervous tissue, where they are involved in a number of processes and in turn are subject to a number of regulatory mechanisms. In this review, we summarize current knowledge of the cellular distribution and function of the receptors in the inner ear and auditory pathway of the mammalian brainstem and midbrain. The findings suggest that in these regions, GABAB receptors are involved in processes essential for proper auditory function, such as cochlear amplifier modulation, regulation of spontaneous activity, binaural and temporal information processing, and predictive coding. Since impaired GABAergic inhibition has been found to be associated with various forms of hearing loss, GABAB dysfunction could also play a role in some pathologies of the auditory system.

GABAB Receptor Chemistry and Pharmacology: Agonists, Antagonists, and Allosteric Modulators

The GABA_B receptors are metabotropic G protein-coupled receptors (GPCRs) that mediate the actions of the primary inhibitory neurotransmitter, γ-aminobutyric acid (GABA). In the CNS, GABA plays an important role in behavior, learning and memory, cognition, and stress. GABA is also located throughout the gastrointestinal (GI) tract and is involved in the autonomic control of the intestine and esophageal reflex. Consequently, dysregulated GABA_B receptor signaling is associated with neurological, mental health, and gastrointestinal disorders; hence, these receptors have been identified as key therapeutic targets and are the focus of multiple drug discovery efforts for indications such as muscle spasticity disorders, schizophrenia, pain, addiction, and gastroesophageal reflex disease (GERD). Numerous agonists, antagonists, and allosteric modulators of the GABA_B receptor have been described; however, Lioresal^® (Baclofen; β-(4-chlorophenyl)-γ-aminobutyric acid) is the only FDA-approved drug that selectively targets GABA_B receptors in clinical use; undesirable side effects, such as sedation, muscle weakness, fatigue, cognitive deficits, seizures, tolerance and potential for abuse, limit their therapeutic use. Here, we review GABA_B receptor chemistry and pharmacology, presenting orthosteric agonists, antagonists, and positive and negative allosteric modulators, and highlight the therapeutic potential of targeting GABA_B receptor modulation for the treatment of various CNS and peripheral disorders.

Mechanisms and Regulation of Neuronal GABAB Receptor-Dependent Signaling

γ-Aminobutyric acid B receptors (GABA_BRs) are broadly expressed throughout the central nervous system where they play an important role in regulating neuronal excitability and synaptic transmission. GABA_BRs are G protein-coupled receptors that mediate slow and sustained inhibitory actions via modulation of several downstream effector enzymes and ion channels. GABA_BRs are obligate heterodimers that associate with diverse arrays of proteins to form modular complexes that carry out distinct physiological functions. GABA_BR-dependent signaling is fine-tuned and regulated through a multitude of mechanisms that are relevant to physiological and pathophysiological states. This review summarizes the current knowledge on GABA_BR signal transduction and discusses key factors that influence the strength and sensitivity of GABA_BR-dependent signaling in neurons.

Spinal Inhibition of GABAB Receptors by the Extracellular Matrix Protein Fibulin-2 in Neuropathic Rats

In the central nervous system, the inhibitory GABAB receptor is the archetype of heterodimeric G protein-coupled receptors (GPCRs). Receptor interaction with partner proteins has emerged as a novel mechanism to alter GPCR signaling in pathophysiological conditions. We propose here that GABAB activity is inhibited through the specific binding of fibulin-2, an extracellular matrix protein, to the B1a subunit in a rat model of neuropathic pain. We demonstrate that fibulin-2 hampers GABAB activation, presumably through decreasing agonist-induced conformational changes. Fibulin-2 regulates the GABAB-mediated presynaptic inhibition of neurotransmitter release and weakens the GABAB-mediated inhibitory effect in neuronal cell culture. In the dorsal spinal cord of neuropathic rats, fibulin-2 is overexpressed and colocalized with B1a. Fibulin-2 may thus interact with presynaptic GABAB receptors, including those on nociceptive afferents. By applying anti-fibulin-2 siRNA in vivo, we enhanced the antinociceptive effect of intrathecal baclofen in neuropathic rats, thus demonstrating that fibulin-2 limits the action of GABAB agonists in vivo. Taken together, our data provide an example of an endogenous regulation of GABAB receptor by extracellular matrix proteins and demonstrate its functional impact on pathophysiological processes of pain sensitization.

The Serotonin Brainstem Hypothesis for the Sudden Infant Death Syndrome

The sudden infant death syndrome (SIDS) is the leading cause of postneonatal infant mortality in the United States today, with an overall rate of 0.39/1000 live births. It is defined as the sudden and unexpected death of an infant <12 months of age that remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. The serotonin brainstem hypothesis has been a leading hypothesis for SIDS over the last 2 decades. Our laboratory has studied this hypothesis over time with a variety of tissue techniques, including tissue receptor autoradiography, high performance liquid chromatography, Western blot analysis, immunocytochemistry, and proteomics. The purpose of this article is to review the progress in our laboratory toward supporting this hypothesis. We conclude that an important subset of SIDS infants has serotonergic abnormalities resulting from a "core lesion" in the medullary reticular formation comprised of nuclei that contain serotonin neurons. This lesion could lead to a failure of protective brainstem responses to homeostatic challenges during sleep in a critical developmental period which cause sleep-related sudden death.

Direct Interaction of PP2A Phosphatase with GABAB Receptors Alters Functional Signaling

Addictive drugs usurp the brain's intrinsic mechanism for reward, leading to compulsive and destructive behaviors. In the ventral tegmental area (VTA), the center of the brain's reward circuit, GABAergic neurons control the excitability of dopamine (DA) projection neurons and are the site of initial psychostimulant-dependent changes in signaling. Previous work established that cocaine/methamphetamine exposure increases protein phosphatase 2A (PP2A) activity, which dephosphorylates the GABA_BR2 subunit, promotes internalization of the GABA_B receptor (GABA_BR) and leads to smaller GABA_BR-activated G-protein-gated inwardly rectifying potassium (GIRK) currents in VTA GABA neurons. How the actions of PP2A become selective for a particular signaling pathway is poorly understood. Here, we demonstrate that PP2A can associate directly with a short peptide sequence in the C terminal domain of the GABA_BR1 subunit, and that GABA_BRs and PP2A are in close proximity in rodent neurons (mouse/rat; mixed sexes). We show that this PP2A-GABA_BR interaction can be regulated by intracellular Ca²⁺ Finally, a peptide that potentially reduces recruitment of PP2A to GABA_BRs and thereby limits receptor dephosphorylation increases the magnitude of baclofen-induced GIRK currents. Thus, limiting PP2A-dependent dephosphorylation of GABA_BRs may be a useful strategy to increase receptor signaling for treating diseases.SIGNIFICANCE STATEMENT Dysregulation of GABA_B receptors (GABA_BRs) underlies altered neurotransmission in many neurological disorders. Protein phosphatase 2A (PP2A) is involved in dephosphorylating and subsequent internalization of GABA_BRs in models of addiction and depression. Here, we provide new evidence that PP2A B55 regulatory subunit interacts directly with a small region of the C-terminal domain of the GABA_BR1 subunit, and that this interaction is sensitive to intracellular Ca²⁺ We demonstrate that a short peptide corresponding to the PP2A interaction site on GABA_BR1 competes for PP2A binding, enhances phosphorylation GABA_BR2 S783, and affects functional signaling through GIRK channels. Our study highlights how targeting PP2A dependent dephosphorylation of GABA_BRs may provide a specific strategy to modulate GABA_BR signaling in disease conditions.

Diversity of structure and function of GABAB receptors: a complexity of GABAB-mediated signaling

γ-aminobutyric acid type B (GABA_B) receptors are broadly expressed in the nervous system and play an important role in neuronal excitability. GABA_B receptors are G protein-coupled receptors that mediate slow and prolonged inhibitory action, via activation of Gαi/o-type proteins. GABA_B receptors mediate their inhibitory action through activating inwardly rectifying K⁺ channels, inactivating voltage-gated Ca²⁺ channels, and inhibiting adenylate cyclase. Functional GABA_B receptors are obligate heterodimers formed by the co-assembly of R1 and R2 subunits. It is well established that GABA_B receptors interact not only with G proteins and effectors but also with various proteins. This review summarizes the structure, subunit isoforms, and function of GABA_B receptors, and discusses the complexity of GABA_B receptors, including how receptors are localized in specific subcellular compartments, the mechanism regulating cell surface expression and mobility of the receptors, and the diversity of receptor signaling through receptor crosstalk and interacting proteins.

Neuronal Chloride Regulation via KCC2 Is Modulated through a GABAB Receptor Protein Complex

GABAB receptors are G-protein-coupled receptors that mediate inhibitory synaptic actions through a series of downstream target proteins. It is increasingly appreciated that the GABAB receptor forms part of larger signaling complexes, which enable the receptor to mediate multiple different effects within neurons. Here we report that GABAB receptors can physically associate with the potassium-chloride cotransporter protein, KCC2, which sets the driving force for the chloride-permeable ionotropic GABAA receptor in mature neurons. Using biochemical, molecular, and functional studies in rodent hippocampus, we show that activation of GABAB receptors results in a decrease in KCC2 function, which is associated with a reduction in the protein at the cell surface. These findings reveal a novel "crosstalk" between the GABA receptor systems, which can be recruited under conditions of high GABA release and which could be important for the regulation of inhibitory synaptic transmission.SIGNIFICANCE STATEMENT Synaptic inhibition in the brain is mediated by ionotropic GABAA receptors (GABAARs) and metabotropic GABAB receptors (GABABRs). To fully appreciate the function and regulation of these neurotransmitter receptors, we must understand their interactions with other proteins. We describe a novel association between the GABABR and the potassium-chloride cotransporter protein, KCC2. This association is significant because KCC2 sets the intracellular chloride concentration found in mature neurons and thereby establishes the driving force for the chloride-permeable GABAAR. We demonstrate that GABABR activation can regulate KCC2 at the cell surface in a manner that alters intracellular chloride and the reversal potential for the GABAAR. Our data therefore support an additional mechanism by which GABABRs are able to modulate fast synaptic inhibition.

G-protein-coupled receptors mediate 14-3-3 signal transduction

G-protein-coupled receptor (GPCR)-interacting proteins likely participate in regulating GPCR signaling by eliciting specific signal transduction cascades, inducing cross-talk with other pathways, and fine tuning the signal. However, except for G-proteins and β-arrestins, other GPCR-interacting proteins are poorly characterized. 14-3-3 proteins are signal adaptors, and their participation in GPCR signaling is not well understood or recognized. Here we demonstrate that GPCR-mediated 14-3-3 signaling is ligand-regulated and is likely to be a more general phenomenon than suggested by the previous reports of 14-3-3 involvement with a few GPCRs. For the first time, we can pharmacologically characterize GPCR/14-3-3 signaling. We have shown that GPCR-mediated 14-3-3 signaling is phosphorylation-dependent, and that the GPCR/14-3-3 interaction likely occurs later than receptor desensitization and internalization. GPCR-mediated 14-3-3 signaling can be β-arrestin-independent, and individual agonists can have different potencies on 14-3-3 and β-arrestin signaling. GPCRs can also mediate the interaction between 14-3-3 and Raf-1. Our work opens up a new broad realm of previously unappreciated GPCR signal transduction. Linking GPCRs to 14-3-3 signal transduction creates the potential for the development of new research directions and provides a new signaling pathway for drug discovery.

Gatekeepers Controlling GPCR Export and Function

Regulated export of G protein-coupled receptors (GPCRs) from intracellular stores involves chaperones and escort proteins, which promote their progression to the cell surface, and gatekeepers, which retain them in intracellular compartments. Functional γ-aminobutyric acid (GABA)B receptors, the paradigm of this phenomenon, comprise GB1 and GB2 subunits forming a heterodimer. GB1 is retained in the endoplasmic reticulum (ER) in the absence of GB2. A specific ER-resident gatekeeper, prenylated Rab acceptor family 2 (PRAF2), is involved in GB1 retention and prevents its progression into the biosynthetic pathway. GB1 can be released from PRAF2 only on competitive interaction with GB2. PRAF2 is ubiquitous and belongs to a subgroup of the mammalian Ypt-interacting protein (Yip) family. Several other GPCRs are likely to be regulated by Yip proteins, which might be involved in the pathophysiology of human diseases that are associated with impaired receptor targeting to the cell surface.

Modular composition and dynamics of native GABAB receptors identified by high-resolution proteomics

GABAB receptors, the most abundant inhibitory G protein-coupled receptors in the mammalian brain, display pronounced diversity in functional properties, cellular signaling and subcellular distribution. We used high-resolution functional proteomics to identify the building blocks of these receptors in the rodent brain. Our analyses revealed that native GABAB receptors are macromolecular complexes with defined architecture, but marked diversity in subunit composition: the receptor core is assembled from GABAB1a/b, GABAB2, four KCTD proteins and a distinct set of G-protein subunits, whereas the receptor's periphery is mostly formed by transmembrane proteins of different classes. In particular, the periphery-forming constituents include signaling effectors, such as Cav2 and HCN channels, and the proteins AJAP1 and amyloid-β A4, both of which tightly associate with the sushi domains of GABAB1a. Our results unravel the molecular diversity of GABAB receptors and their postnatal assembly dynamics and provide a roadmap for studying the cellular signaling of this inhibitory neurotransmitter receptor.

Rapid antidepressants stimulate the decoupling of GABA(B) receptors from GIRK/Kir3 channels through increased protein stability of 14-3-3η

A single injection of N-methyl-D-aspartate receptor (NMDAR) antagonists produces a rapid antidepressant response. Lasting changes in the synapse structure and composition underlie the effectiveness of these drugs. We recently discovered that rapid antidepressants cause a shift in the γ-aminobutyric acid receptor (GABABR) signaling pathway, such that GABABR activation shifts from opening inwardly rectifiying potassium channels (Kir/GIRK) to increasing resting dendritic calcium signal and mammalian Target of Rapamycin activity. However, little is known about the molecular and biochemical mechanisms that initiate this shift. Herein, we show that GABABR signaling to Kir3 (GIRK) channels decreases with NMDAR blockade. Blocking NMDAR signaling stabilizes the adaptor protein 14-3-3η, which decouples GABABR signaling from Kir3 and is required for the rapid antidepressant efficacy. Consistent with these results, we find that key proteins involved in GABABR signaling bidirectionally change in a depression model and with rapid antidepressants. In socially defeated rodents, a model for depression, GABABR and 14-3-3η levels decrease in the hippocampus. The NMDAR antagonists AP5 and Ro-25-6981, acting as rapid antidepressants, increase GABABR and 14-3-3η expression and decrease Kir3.2. Taken together, these data suggest that the shift in GABABR function requires a loss of GABABR-Kir3 channel activity mediated by 14-3-3η. Our findings support a central role for 14-3-3η in the efficacy of rapid antidepressants and define a critical molecular mechanism for activity-dependent alterations in GABABR signaling.

Isovaline does not activate GABA(B) receptor-coupled potassium currents in GABA(B) expressing AtT-20 cells and cultured rat hippocampal neurons

Isovaline is a non-proteinogenic amino acid that has analgesic properties. R-isovaline is a proposed agonist of the γ-aminobutyric acid type B (GABA_B) receptor in the thalamus and peripheral tissue. Interestingly, the responses to R-isovaline differ from those of the canonical GABA_B receptor agonist R-baclofen, warranting further investigation. Using whole cell recording techniques we explored isovaline actions on GABA_B receptors coupled to rectifying K⁺ channels in cells of recombinant and native receptor preparations. In AtT-20 cells transfected with GABA_B receptor subunits, bath application of the GABA_B receptor agonists, GABA (1 μM) and R-baclofen (5 μM) produced inwardly rectifying currents that reversed approximately at the calculated reversal potential for K⁺ R- isovaline (50 μM to 1 mM) and S-isovaline (500 μM) did not evoke a current. R-isovaline applied either extracellularly (250 μM) or intracellularly (10 μM) did not alter responses to GABA at 1 μM. Co-administration of R-isovaline (250 μM) with a low concentration (10 nM) of GABA did not result in a response. In cultured rat hippocampal neurons that natively express GABA_B receptors, R-baclofen (5 μM) induced GABA_B receptor-dependent inward currents. Under the same conditions R-isovaline (1 or 50 μM) did not evoke a current or significantly alter R-baclofen-induced effects. Therefore, R-isovaline does not interact with recombinant or native GABA_B receptors to open K⁺ channels in these preparations.

GIRK Channels: A Potential Link Between Learning and Addiction

The ability of drug-associated cues to reinitiate drug craving and seeking, even after long periods of abstinence, has led to the hypothesis that addiction represents a form of pathological learning, in which drugs of abuse hijack normal learning and memory processes to support long-term addictive behaviors. In this chapter, we review evidence suggesting that G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels are one mechanism through which numerous drugs of abuse can modulate learning and memory processes. We will examine the role of GIRK channels in two forms of experience-dependent long-term changes in neuronal function: homeostatic plasticity and synaptic plasticity. We will also discuss how drug-induced changes in GIRK-mediated signaling can lead to changes that support the development and maintenance of addiction.

Depletion of 14-3-3γ reduces the surface expression of Transient Receptor Potential Melastatin 4b (TRPM4b) channels and attenuates TRPM4b-mediated glutamate-induced neuronal cell death

BACKGROUND

TRPM4 channels are Ca2+-activated nonselective cation channels which are deeply involved in physiological and pathological conditions. However, their trafficking mechanism and binding partners are still elusive.

RESULTS

We have found the 14-3-3γ as a binding partner for TRPM4b using its N-terminal fragment from the yeast-two hybrid screening. Ser88 at the N-terminus of TRPM4b is critical for 14-3-3γ binding by showing GST pull-down and co-immunoprecipitation. Heterologous overexpression of 14-3-3γ in HEK293T cells increased TRPM4b expression on the plasma membrane which was measured by whole-cell recordings and cell surface biotinylation experiment. Surface expression of TRPM4b was greatly reduced by short hairpin RNA (shRNA) against 14-3-3γ. Next, endogenous TRPM4b-mediated currents were electrophysiologically characterized by application of glutamate and 9-phenanthrol, a TRPM4b specific antagonist in HT-22 cells which originated from mouse hippocampal neurons. Glutamate-induced TRPM4b currents were significantly attenuated by shRNAs against 14-3-3γ or TRPM4b in these cells. Finally, glutamate-induced cell death was greatly prevented by treatment of 9-phenanthrol or 14-3-3γ shRNA.

CONCLUSION

These results showed that the cell surface expression of TRPM4 channels is mediated by 14-3-3γ binding, and the specific inhibition of this trafficking process can be a potential therapeutic target for glutamate-induced neuronal cell death.

Functional characterization of G-protein-coupled receptors: a bioinformatics approach

Complex molecular and cellular mechanisms regulate G protein-coupled receptors (GPCRs). It is suggested that proteins intrinsically disordered regions (IDRs) are to play a role in GPCR's intra and extracellular regions plasticity, due to their potential for post-translational modification and interaction with other proteins. These regions are defined as lacking a stable three-dimensional (3D) structure. They are rich in hydrophilic and charged, amino acids and are capable to assume different conformations which allow them to interact with multiple partners. In this study we analyzed 75 GPCR involved in synaptic transmission using computational tools for sequence-based prediction of IDRs within a protein. We also evaluated putative ligand-binding motifs using receptor sequences. The disorder analysis indicated that neurotransmitter GPCRs have a significant amount of disorder in their N-terminus, third intracellular loop (3IL) and C-terminus. About 31%, 39% and 53% of human GPCR involved in synaptic transmission are disordered in these regions. Thirty-three percent of receptors show at least one predicted PEST motif, this being statistically greater than the estimate for the rest of human GPCRs. About 90% of the receptors had at least one putative site for dimerization in their 3IL or C-terminus. ELM instances sampled in these domains were 14-3-3, SH3, SH2 and PDZ motifs. In conclusion, the increased flexibility observed in GPCRs, added to the enrichment of linear motifs, PEST and heteromerization sites, may be critical for the nervous system's functional plasticity.

Heterodimeric coiled-coil interactions of human GABAB receptor

Metabotropic GABAB receptor is a G protein-coupled receptor that mediates inhibitory neurotransmission in the CNS. It functions as an obligatory heterodimer of GABAB receptor 1 (GBR1) and GABAB receptor 2 (GBR2) subunits. The association between GBR1 and GBR2 masks an endoplasmic reticulum (ER) retention signal in the cytoplasmic region of GBR1 and facilitates cell surface expression of both subunits. Here, we present, to our knowledge, the first crystal structure of an intracellular coiled-coil heterodimer of human GABAB receptor. We found that polar interactions buried within the hydrophobic core determine the specificity of heterodimer pairing. Disruption of the hydrophobic coiled-coil interface with single mutations in either subunit impairs surface expression of GBR1, confirming that the coiled-coil interaction is required to inactivate the adjacent ER retention signal of GBR1. The coiled-coil assembly buries an internalization motif of GBR1 at the heterodimer interface. The ER retention signal of GBR1 is not part of the core coiled-coil structure, suggesting that it is sterically shielded by GBR2 upon heterodimer formation.

Complex GABAB receptor complexes: how to generate multiple functionally distinct units from a single receptor

The main inhibitory neurotransmitter, GABA, acts on both ligand-gated and G protein-coupled receptors, the GABA_A/C and GABA_B receptors, respectively. The later play important roles in modulating many synapses, both at the pre- and post-synaptic levels, and are then still considered as interesting targets to treat a number of brain diseases, including addiction. For many years, several subtypes of GABA_B receptors were expected, but cloning revealed only two genes that work in concert to generate a single type of GABA_B receptor composed of two subunits. Here we will show that the signaling complexity of this unit receptor type can be largely increased through various ways, including receptor stoichiometry, subunit isoforms, cell-surface expression and localization, crosstalk with other receptors, or interacting proteins. These recent data revealed how complexity of a receptor unit can be increased, observation that certainly are not unique to the GABA_B receptor.

14-3-3ζ Protein regulates anterograde transport of the human κ-opioid receptor (hKOPR)

By proteomic analysis, we found that 14-3-3ζ was one of the proteins co-immunoprecipitated with human κ-opioid receptor (hKOPR) from extracts of solubilized Neuro2A cells stably expressing FLAG-hKOPR (N2A-FLAG-hKOPR cells). 14-3-3 proteins are a family of conserved regulatory molecules in eukaryotic cells, where they participate in signal transduction, metabolism, and membrane protein transport. 14-3-3ζ co-localized with the hKOPR in N2A cells. The hKOPR C-tail interacted with 14-3-3ζ in rat brain extracts and bound directly to purified 14-3-3ζ as demonstrated by pulldown techniques. 14-3-3ζ siRNA decreased expression of the hKOPR in N2A-FLAG-hKOPR cells and cultured primary cortical neurons of E19 rats by ~25% as determined by immunoblotting, ligand binding, and flow cytometry. The effect of 14-3-3ζ siRNA was reversed by overexpression of 14-3-3ζ. Expression of the 14-3-3 scavenger protein pGpLI-R18 also decreased hKOPR expression. 14-3-3ζ siRNA did not change expressions of the hDOPR and rMOPR in N2A cells. Pulse-chase study showed that 14-3-3ζ siRNA decreased the amount of mature hKOPR but did not change the rate of maturation or stability of hKOPR protein. Mutations of R354A/S358A in the putative 14-3-3 interaction motif (354)RQSTS(358) in the hKOPR C-tail reduced interaction of the hKOPR with 14-3-3ζ and abolished the effect of 14-3-3ζ knockdown on hKOPR expression. Mutation of the endoplasmic reticulum retention motif (359)RVR adjacent to the 14-3-3 interaction motif in the hKOPR C-tail decreased interaction of coatomer protein I (COPI) with the hKOPR and abolished 14-3-3ζ-mediated regulation of hKOPR expression. 14-3-3ζ knockdown increased association of COPI with the hKOPR. These results suggest that 14-3-3ζ promotes expression of the hKOPR by inhibiting COPI and RVR motif-mediated endoplasmic reticulum localization machinery.

Impairment of GABAB receptor dimer by endogenous 14-3-3ζ in chronic pain conditions

In the central nervous system, the inhibitory GABAB receptor is the archetype of heterodimeric G protein-coupled receptors (GPCRs). However, the regulation of GABAB dimerization, and more generally of GPCR oligomerization, remains largely unknown. We propose a novel mechanism for inhibition of GPCR activity through de-dimerization in pathological conditions. We show here that 14-3-3ζ, a GABAB1-binding protein, dissociates the GABAB heterodimer, resulting in the impairment of GABAB signalling in spinal neurons. In the dorsal spinal cord of neuropathic rats, 14-3-3ζ is overexpressed and weakens GABAB inhibition. Using anti-14-3-3ζ siRNA or competing peptides disrupts 14-3-3ζ/GABAB1 interaction and restores functional GABAB heterodimers in the dorsal horn. Importantly, both strategies greatly enhance the anti-nociceptive effect of intrathecal Baclofen in neuropathic rats. Taken together, our data provide the first example of endogenous regulation of a GPCR oligomeric state and demonstrate its functional impact on the pathophysiological process of neuropathic pain sensitization.

Amygdala 14-3-3ζ as a novel modulator of escalating alcohol intake in mice

Alcoholism is a devastating brain disorder that affects millions of people worldwide. The development of alcoholism is caused by alcohol-induced maladaptive changes in neural circuits involved in emotions, motivation, and decision-making. Because of its involvement in these processes, the amygdala is thought to be a key neural structure involved in alcohol addiction. However, the molecular mechanisms that govern the development of alcoholism are incompletely understood. We have previously shown that in a limited access choice paradigm, C57BL/6J mice progressively escalate their alcohol intake and display important behavioral characteristic of alcohol addiction, in that they become insensitive to quinine-induced adulteration of alcohol. This study used the limited access choice paradigm to study gene expression changes in the amygdala during the escalation to high alcohol consumption in C57BL/6J mice. Microarray analysis revealed that changes in gene expression occurred predominantly after one week, i.e. during the initial escalation of alcohol intake. One gene that stood out from our analysis was the adapter protein 14-3-3ζ, which was up-regulated during the transition from low to high alcohol intake. Independent qPCR analysis confirmed the up-regulation of amygdala 14-3-3ζ during the escalation of alcohol intake. Subsequently, we found that local knockdown of 14-3-3ζ in the amygdala, using RNA interference, dramatically augmented alcohol intake. In addition, knockdown of amygdala 14-3-3ζ promoted the development of inflexible alcohol drinking, as apparent from insensitivity to quinine adulteration of alcohol. This study identifies amygdala 14-3-3ζ as a novel key modulator that is engaged during escalation of alcohol use.

Methamphetamine-evoked depression of GABA(B) receptor signaling in GABA neurons of the VTA

Psychostimulants induce neuroadaptations in excitatory and fast inhibitory transmission in the ventral tegmental area (VTA). Mechanisms underlying drug-evoked synaptic plasticity of slow inhibitory transmission mediated by GABA(B) receptors and G protein-gated inwardly rectifying potassium (GIRK/Kir(3)) channels, however, are poorly understood. Here, we show that 1 day after methamphetamine (METH) or cocaine exposure both synaptically evoked and baclofen-activated GABA(B)R-GIRK currents were significantly depressed in VTA GABA neurons and remained depressed for 7 days. Presynaptic inhibition mediated by GABA(B)Rs on GABA terminals was also weakened. Quantitative immunoelectron microscopy revealed internalization of GABA(B1) and GIRK2, which occurred coincident with dephosphorylation of serine 783 (S783) in GABA(B2), a site implicated in regulating GABA(B)R surface expression. Inhibition of protein phosphatases recovered GABA(B)R-GIRK currents in VTA GABA neurons of METH-injected mice. This psychostimulant-evoked impairment in GABA(B)R signaling removes an intrinsic brake on GABA neuron spiking, which may augment GABA transmission in the mesocorticolimbic system.

The adaptor protein 14-3-3 binds to the calcium-sensing receptor and attenuates receptor-mediated Rho kinase signalling

A yeast two-hybrid screen performed to identify binding partners of the CaR (calcium-sensing receptor) intracellular tail identified the adaptor protein 14-3-3θ as a novel binding partner that bound to the proximal membrane region important for CaR expression and signalling. The 14-3-3θ protein directly interacted with the CaR tail in pull-down studies and FLAG-tagged CaR co-immunoprecipitated with EGFP (enhanced green fluorescent protein)-tagged 14-3-3θ when co-expressed in HEK (human embryonic kidney)-293 or COS-1 cells. The interaction between the CaR and 14-3-3θ did not require a putative binding site in the membrane-proximal region of the CaR tail and was independent of PKC (protein kinase C) phosphorylation. Confocal microscopy demonstrated co-localization of the CaR and EGFP–14-3-3θ in the ER (endoplasmic reticulum) of HEK-293 cells that stably expressed the CaR (HEK-293/CaR cells), but 14-3-3θ overexpression had no effect on membrane expression of the CaR. Overexpression of 14-3-3θ in HEK-293/CaR cells attenuated CaR-mediated Rho signalling, but had no effect on ERK (extracellular-signal-regulated kinase) 1/2 signalling. Another isoform identified from the library, 14-3-3ζ, exhibited similar behaviour to that of 14-3-3θ with respect to CaR tail binding, cellular co-localization and impact on receptor-mediated signalling. However, unlike 14-3-3θ, this isoform, when overexpressed, significantly reduced CaR plasma membrane expression. Results indicate that 14-3-3 proteins mediate CaR-dependent Rho signalling and may modulate the plasma membrane expression of the CaR.

14-3-3 proteins in neurodegeneration

Among the first reported functions of 14-3-3 proteins was the regulation of tyrosine hydroxylase (TH) activity suggesting a possible involvement of 14-3-3 proteins in Parkinson's disease. Since then the relevance of 14-3-3 proteins in the pathogenesis of chronic as well as acute neurodegenerative diseases, including Alzheimer's disease, polyglutamine diseases, amyotrophic lateral sclerosis and stroke has been recognized. The reported function of 14-3-3 proteins in this context are as diverse as the mechanism involved in neurodegeneration, reaching from basal cellular processes like apoptosis, over involvement in features common to many neurodegenerative diseases, like protein stabilization and aggregation, to very specific processes responsible for the selective vulnerability of cellular populations in single neurodegenerative diseases. Here, we review what is currently known of the function of 14-3-3 proteins in nervous tissue focussing on the properties of 14-3-3 proteins important in neurodegenerative disease pathogenesis.

Schwann-like adult stem cells derived from bone marrow and adipose tissue express γ-aminobutyric acid type B receptors

γ-Aminobutyric acid type B receptors (GABA-B) are expressed in glial cells of the central and peripheral nervous systems, and recent evidence has shown their importance in modulating physiological parameters of Schwann cell (SC). SC play essential roles in peripheral nerve regeneration, but several drawbacks prevent their use for nerve repair. Adult stem cells from adipose tissue (ASC) or bone marrow (BM-MSC) can be differentiated into an SC-like phenotype and used as SC replacements. The aim of this study was to investigate GABA-B receptor functional expression in differentiated stem cells by assessing the similarity to SC. By means of RT-PCR and Western blot methodologies, BM-MSC and ASC were found to express both GABA-B1 and GABA-B2 receptor subunits. The expression levels of GABA-B1b and GABA-B2 receptors were influenced by SC-like differentiation, as shown by Western blot studies. GABA-B receptor stimulation with baclofen reduced the proliferation rate of SC and differentiated ASC (dASC) but not that of dBM-MSC. In conclusion, both of the subunits that assemble into a functional GABA-B receptor are present in differentiated stem cells. Furthermore, GABA-B receptors in dASC are functionally active, regulating a key process such as proliferation. The presence of functional GABA-B receptors on differentiated stem cells opens new opportunities for a possible pharmacological modulation of their physiology and phenotype.

Differential expression of 14-3-3 isoforms in human alcoholic brain

BACKGROUND

Neuropathological damage as a result of chronic alcohol abuse often results in the impairment of cognitive function. The damage is particularly marked in the frontal cortex. The 14-3-3 protein family consists of 7 proteins, β, γ, ε, ζ, η, θ, and σ, encoded by 7 distinct genes. They are highly conserved molecular chaperones with roles in the regulation of metabolism, signal transduction, cell-cycle control, protein trafficking, and apoptosis. They may also play an important role in neurodegeneration in chronic alcoholism.

METHODS

We used real-time PCR to measure the expression of 14-3-3 mRNA transcripts in both the dorsolateral prefrontal cortex and motor cortex of human brains obtained at autopsy.

RESULTS

We found significantly lower 14-3-3β, γ, and θ expression in both cortical areas of alcoholics, but no difference in 14-3-3η expression, and higher expression of 14-3-3σ in both areas. Levels of 14-3-3ζ and ε transcripts were significantly lower only in alcoholic motor cortex.

CONCLUSIONS

Altered 14-3-3 expression could contribute to synaptic dysfunction and altered neurotransmission in chronic alcohol misuse by human subjects.

GABAB receptor coupling to G-proteins and ion channels

GABA(B) receptors have been found to play a key role in regulating membrane excitability and synaptic transmission in the brain. The GABA(B) receptor is a G-protein coupled receptor (GPCR) that associates with a subset of G-proteins (pertussis toxin sensitive Gi/o family), that in turn regulate specific ion channels and trigger cAMP cascades. In this review, we describe the relationships between the GABA(B) receptor, its effectors and associated proteins that mediate GABA(B) receptor function within the brain. We discuss a unique feature of the GABA(B) receptor, the requirement for heterodimerization to produce functional receptors, as well as an increasing body of evidence that suggests GABA(B) receptors comprise a macromolecular signaling heterocomplex, critical for efficient targeting and function of the receptors. Within this complex, GABA(B) receptors associate specifically with Gi/o G-proteins that regulate voltage-gated Ca(2+) (Ca(V)) channels, G-protein activated inwardly rectifying K(+) (GIRK) channels, and adenylyl cyclase. Numerous studies have revealed that lipid rafts, scaffold proteins, targeting motifs in the receptor, and regulators of G-protein signaling (RGS) proteins also contribute to the function of GABA(B) receptors and affect cellular processes such as receptor trafficking and activity-dependent desensitization. This complex regulation of GABA(B) receptors in the brain may provide opportunities for new ways to regulate GABA-dependent inhibition in normal and diseased states of the nervous system.

Binding of alphaII spectrin to 14-3-3beta is involved in NCAM-dependent neurite outgrowth

Members of the 14-3-3 protein family have been implicated in neuronal migration, synaptic plasticity and learning. Using affinity chromatography followed by mass spectrometry analysis, we show here that the cytoskeletal protein alphaII spectrin is a novel ligand of 14-3-3beta. We found that 14-3-3beta interacts with alphaII spectrin via the mode 2 14-3-3 binding motif RLIQS(1302)HP. Binding required phosphorylation of Ser(1302) by casein kinase II and was enhanced in the presence of calmodulin. Co-immunoprecipitation of alphaII spectrin and 14-3-3beta with the neural cell adhesion molecule NCAM suggested that the 14-3-3-spectrin-interaction affects NCAM function. Indeed, disruption of the 14-3-3beta/alphaII spectrin interaction by mutating Ser(1302) to Ala enhanced NCAM-dependent neurite outgrowth. Our results indicate that the phosphorylation-dependent interaction between 14-3-3beta and alphaII spectrin acts as a switch between positive and negative regulation of neurite outgrowth stimulated by NCAM, representing a novel and acute mechanism preventing uncontrolled elongation of neuronal processes.

Identification and characterisation of a Maf1/Macoco protein complex that interacts with GABAA receptors in neurons

The majority of fast inhibitory synaptic transmission in the mammalian nervous system is mediated by GABA(A) receptors (GABA(A)Rs). Here we report a novel interaction between the protein Maf1 and GABA(A)R beta-subunit intracellular domains. We find Maf1 to be highly expressed in brain and enriched in the hippocampus and cortex. In heterologous cells and neurons we show Maf1 co-localises with GABA(A)Rs in intracellular compartments and at the cell surface. In neurons, Maf1 is found localised in the cytoplasm in dendrites, partially overlapping with GABA(A)Rs and inhibitory synapses and in addition is enriched in the neuronal nucleus. We also report that Maf1 interacts with a novel coiled-coil domain containing protein that we have called Macoco (for Maf1 interacting coiled-coil protein). Like Maf1, Macoco can also be found localised to inhibitory synapses and directly interacts with GABA(A)Rs. Expressing Macoco in neurons increases surface GABA(A)R levels. Our results suggest that Maf1 and Macoco are novel GABA(A)R interacting proteins important for regulating GABA(A)R surface expression and GABA(A)R signalling in the brain.

GABA(B) receptor subunit 1 binds to proteins affected in 22q11 deletion syndrome

GABA(B) receptors mediate slow inhibitory effects of the neurotransmitter gamma-aminobutyric acid (GABA) on synaptic transmission in the central nervous system. They function as heterodimeric G-protein-coupled receptors composed of the seven-transmembrane domain proteins GABA(B1) and GABA(B2), which are linked through a coiled-coil interaction. The ligand-binding subunit GABA(B1) is at first retained in the endoplasmic reticulum and is transported to the cell surface only upon assembly with GABA(B2). Here, we report that GABA(B1), via the coiled-coil domain, can also bind to soluble proteins of unknown function, that are affected in 22q11 deletion/DiGeorge syndrome and are therefore referred to as DiGeorge critical region 6 (DGCR6). In transfected neurons the GABA(B1)-DGCR6 association resulted in a redistribution of both proteins into intracellular clusters. Furthermore, the C-terminus of GABA(B2) interfered with the novel interaction, consistent with heterodimer formation overriding transient DGCR6-binding to GABA(B1). Thus, sequential coiled-coil interactions may direct GABA(B1) into functional receptors.

14-3-3 Protein regulates cell adhesion in the seminiferous epithelium of rat testes

Polarity proteins have been implicated in regulating and maintaining tight junction (TJ) and cell polarity in epithelia. Here we report 14-3-3theta, the homolog of Caenorhabditis elegans Par5 in mammalian cells, which is known to confer cell polarity at TJ, is found at the apical ectoplasmic specialization (ES), a testis-specific adherens junction type restricted to the Sertoli cell-elongating spermatid interface, in which TJ is absent. 14-3-3theta was shown to play a critical role in conferring cell adhesion at the apical ES. A loss of 14-3-3theta expression at the apical ES was detected in the seminiferous epithelium before spermiation. Involvement of 14-3-3theta in Sertoli cell adhesion was confirmed by its knockdown by RNA interference in Sertoli cells cultured in vitro with established TJ permeability barrier that mimicked the blood-testis barrier (BTB) in vivo. Mislocalization of N-cadherin and zonula occludens-1, but not alpha- and beta-catenins, was observed after 14-3-3theta knockdown in Sertoli cells, moving from the cell-cell interface to cytosol, indicating a disruption of cell adhesion. Studies by endocytosis assay illustrated that this loss of cell adhesion was mediated by an increase in the kinetics of endocytosis of N-cadherin and junctional adhesion molecule-A at the BTB, which may represent a general mechanism by which polarity proteins regulate cell adhesion. In summary, the testis is using 14-3-3theta to regulate cell adhesion at the apical ES to facilitate spermiation and at the BTB to facilitate the transit of preleptotene spermatocytes at stages VIII-IX of the epithelial cycle. 14-3-3theta may act as a molecular switch that coordinates these two cellular events in the seminiferous epithelium during spermatogenesis.

Enhancement of the surface expression of G protein-coupled receptors

G protein-coupled receptors (GPCRs) mediate physiological responses to a diverse array of stimuli and are the molecular targets for numerous therapeutic drugs. GPCRs primarily signal from the plasma membrane, but when expressed in heterologous cells many GPCRs exhibit poor trafficking to the cell surface. Multiple approaches have been taken to enhance GPCR surface expression in heterologous cells, including addition/deletion of receptor sequences, co-expression with interacting proteins, and treatment with pharmacological chaperones. In addition to providing enhanced surface expression of certain GPCRs in heterologous cells, these approaches have also shed light on the control of GPCR trafficking in vivo and in some cases have led to new therapeutic approaches for treating human diseases that result from defects in GPCR trafficking.

Limiting transport steps and novel interactions of Connexin-43 along the secretory pathway

Connexins are four-transmembrane-domain proteins expressed in all vertebrates which form permeable gap junction channels that connect cells. Here, we analysed Connexin-43 (Cx43) transport to the plasma membrane and studied the effects of small GTPases acting along the secretory pathway. We show that both GTP- and GDP-restricted Sar1 prevents exit of Cx43 from the endoplasmic reticulum (ER), but only GTP-restricted Sar1 arrests Cx43 in COP II-coated ER exit sites and accumulates 14-3-3 proteins in the ER fraction. FRET-FLIM data confirm that already in ER exit sites Cx43 exists in oligomeric form, suggesting an in vivo role for 14-3-3 in Cx43 oligomerization. Exit of Cx43 from the ER can be blocked by other factors—such as expression of the β subunit of the COP I coat or p50/dynamitin that acts on the microtubule-based dynein motor complex. GTP-restricted Arf1 blocks Cx43 in the Golgi. Lastly, we show that GTP-restricted Arf6 removes Cx43 gap junction plaques from the cell–cell interface and targets them to degradation. These data provide a molecular explanation of how small GTPases act to regulate Cx43 transport through the secretory pathway, facilitating or abolishing cell–cell communication through gap junctions.

Dendritic assembly of heteromeric gamma-aminobutyric acid type B receptor subunits in hippocampal neurons

Understanding the mechanisms that control synaptic efficacy through the availability of neurotransmitter receptors depends on uncovering their specific intracellular trafficking routes. gamma-Aminobutyric acid type B (GABA(B)) receptors (GABA(B)Rs) are obligatory heteromers present at dendritic excitatory and inhibitory postsynaptic sites. It is unknown whether synthesis and assembly of GABA(B)Rs occur in the somatic endoplasmic reticulum (ER) followed by vesicular transport to dendrites or whether somatic synthesis is followed by independent transport of the subunits for assembly and ER export throughout the somatodendritic compartment. To discriminate between these possibilities we studied the association of GABA(B)R subunits in dendrites of hippocampal neurons combining live fluorescence microscopy, biochemistry, quantitative colocalization, and bimolecular fluorescent complementation. We demonstrate that GABA(B)R subunits are segregated and differentially mobile in dendritic intracellular compartments and that a high proportion of non-associated intracellular subunits exist in the brain. Assembled heteromers are preferentially located at the plasma membrane, but blockade of ER exit results in their intracellular accumulation in the cell body and dendrites. We propose that GABA(B)R subunits assemble in the ER and are exported from the ER throughout the neuron prior to insertion at the plasma membrane. Our results are consistent with a bulk flow of segregated subunits through the ER and rule out a post-Golgi vesicular transport of preassembled GABA(B)Rs.

[GABA(B) receptors and sensitization to pain]

The GABA(B) receptors belong to the family of class C metabotropic receptors. They are inhibitory receptors forming obligatory heterodimers. Their analgesic role in the dorsal horn of the spinal cord is well established since more than 25 years ago. However, Baclofen, the reference agonist of the GABA(B) receptor, proved to have little efficiency in clinics in neuropathic patients. It seems therefore useful to decipher GABA(B) functions in the nociceptive circuitry, and their regulation in conditions of chronic pain. In the present review, we will focus first on the distribution of the GABA(B) subtypes. Then, we will consider their pre- and post-synaptic functions in the dorsal horn of naïve rats. Finally, we will document the mechanisms that may lead to receptor impairment in neuropathic conditions.

Anterior hippocampus in schizophrenia pathogenesis: molecular evidence from a proteome study

OBJECTIVE

The purpose of the present study was to identify differentially expressed proteins in the anterior and posterior hippocampus of brains of schizophrenia patients compared to neurologically healthy controls.

METHOD

Proteins extracted from fresh frozen post-mortem posterior and anterior hippocampus for nine schizophrenia and nine control individuals, and seven schizophrenia and seven control individuals, respectively, were screened for differential expression using 2-D gel electrophoresis and mass spectrometry.

RESULTS

A significantly larger number of protein spots were differentially expressed in the anterior (n = 43) compared to the posterior (n = 16) hippocampus, representing 34 and 14 unique proteins, respectively. These proteins are involved in cytoskeleton structure and function, neurotransmission and mitochondrial function.

CONCLUSION

Based on the aberrant protein expression profiles, the anterior hippocampus appears to be more involved in schizophrenia pathogenesis than the posterior hippocampus. Furthermore, consistent with previous findings, we found molecular evidence to support abnormal neuronal cytoarchitecture and function, neurotransmission and mitochondrial function in the schizophrenia hippocampus.

Target identification for CNS diseases by transcriptional profiling

Gene expression changes in neuropsychiatric and neurodegenerative disorders, and gene responses to therapeutic drugs, provide new ways to identify central nervous system (CNS) targets for drug discovery. This review summarizes gene and pathway targets replicated in expression profiling of human postmortem brain, animal models, and cell culture studies. Analysis of isolated human neurons implicates targets for Alzheimer's disease and the cognitive decline associated with normal aging and mild cognitive impairment. In addition to tau, amyloid-beta precursor protein, and amyloid-beta peptides (Abeta), these targets include all three high-affinity neurotrophin receptors and the fibroblast growth factor (FGF) system, synapse markers, glutamate receptors (GluRs) and transporters, and dopamine (DA) receptors, particularly the D2 subtype. Gene-based candidates for Parkinson's disease (PD) include the ubiquitin-proteosome system, scavengers of reactive oxygen species, brain-derived neurotrophic factor (BDNF), its receptor, TrkB, and downstream target early growth response 1, Nurr-1, and signaling through protein kinase C and RAS pathways. Increasing variability and decreases in brain mRNA production from middle age to old age suggest that cognitive impairments during normal aging may be addressed by drugs that restore antioxidant, DNA repair, and synaptic functions including those of DA to levels of younger adults. Studies in schizophrenia identify robust decreases in genes for GABA function, including glutamic acid decarboxylase, HINT1, glutamate transport and GluRs, BDNF and TrkB, numerous 14-3-3 protein family members, and decreases in genes for CNS synaptic and metabolic functions, particularly glycolysis and ATP generation. Many of these metabolic genes are increased by insulin and muscarinic agonism, both of which are therapeutic in psychosis. Differential genomic signals are relatively sparse in bipolar disorder, but include deficiencies in the expression of 14-3-3 protein members, implicating these chaperone proteins and the neurotransmitter pathways they support as possible drug targets. Brains from persons with major depressive disorder reveal decreased expression for genes in glutamate transport and metabolism, neurotrophic signaling (eg, FGF, BDNF and VGF), and MAP kinase pathways. Increases in these pathways in the brains of animals exposed to electroconvulsive shock and antidepressant treatments identify neurotrophic and angiogenic growth factors and second messenger stimulation as therapeutic approaches for the treatment of depression.

Metabotropic receptors for glutamate and GABA in pain

Glutamate and gamma-amino butyric acid (GABA) are respectively two major excitatory and inhibitory neurotransmitters of the adult mammalian central nervous system. These neurotransmitters exert their action through two types of receptors: ionotropic and metabotropic receptors. While ionotropic receptors are ligand gated ion channels involved in fast synaptic transmission, metabotropic receptors belong to the superfamily of G-protein coupled receptors (GPCRs) and are responsible for the neuromodulatory effect of glutamate and GABA. Metabotropic glutamate receptors (mGluRs) and metabotropic GABA receptors (GABA-B) are present at different levels of the pain neuraxis where they regulate nociceptive transmission and pain. The present review will focus on the role of these receptors in the modulation of pain perception.

Changes in the properties of allosteric and orthosteric GABAB receptor ligands after a continuous, desensitizing agonist pretreatment

It has been estimated that only 15% of the compounds classified as silent G protein-coupled receptor antagonists are indeed devoid of either positive or negative intrinsic efficacy. Considering that 40% of all drugs on the market target G protein-coupled receptors mainly as orthosteric ligands, elucidating their intrinsic properties is becoming increasingly important. While agonism can be demonstrated using appropriately sensitive experimental setups, the detection of inverse agonism can be limited by a low degree of constitutive activity in many assay systems. In this study, changes in ligand behavior upon a lasting pretreatment with gamma-aminobutyric acid (GABA), that induced receptor desensitization, were observed, measuring the second messenger cyclic AMP (cAMP) in a GABA(B) receptor-expressing recombinant cell line. The GABA(B) receptor partial agonist 2-OH-saclofen lost its ability to inhibit 7beta-forskolin-induced cAMP production upon GABA-pretreatment. The "silent" receptor antagonists CGP62349, CGP52432, CGP56999 and SCH50911, on the other hand, stimulated 7beta-forskolin-induced cAMP production under these conditions. The inverse agonism of CGP56999 was inhibited by the efficacy-deficient 2-OH-saclofen, proving it was truly mediated through the orthosteric site of the GABA(B) receptor. Finally, the positive allosteric modulator GS39783, which previously only marginally inhibited cAMP production, suppressed it by 60% both alone and in the presence of the competitive receptor antagonist 2-OH-saclofen, thus GS39783 became an allosteric receptor agonist at desensitized GABA(B) receptors. These changes likely reflect adaptations in the mechanisms of GABA(B) receptor function following desensitization and may be important in the elucidation of intrinsic ligand efficacies as well as for the consequences of continuous drug treatment.

Marlin-1 is expressed in testis and associates to the cytoskeleton and GABAB receptors

Marlin-1 is a GABA(B) receptor and Jak tyrosine kinase-binding protein that also associates with RNA and microtubules. In humans and rodents, expression of Marlin-1 is predominantly restricted to the brain, but expression in lymphoid cells has also been reported. Here, we have studied the distribution of Marlin-1 in testis and spermatozoa. Our results indicate that Marlin-1 is highly expressed in testis. The protein is abundant in spermatogonia, spermatocytes, spermatozoa, and Sertoli cells. We also have studied the subcellular distribution in spermatozoa. Marlin-1 is present in the tail and to a lesser degree in the head of the sperm cell. Finally, we have explored two protein interactions. Our findings demonstrate that Marlin-1 associates with a microtubule fraction and with GABA(B) receptors in testis suggesting that the set of protein interactions of Marlin-1 are conserved in different tissues.

Adenomatous polyposis coli plays a key role, in vivo, in coordinating assembly of the neuronal nicotinic postsynaptic complex

The neuronal nicotinic synapse plays a central role in normal cognitive and autonomic function. Molecular mechanisms that direct the assembly of this synapse remain poorly defined, however. We show here that adenomatous polyposis coli (APC) organizes a multi-molecular complex that is essential for targeting alpha3(*)nAChRs to synapses. APC interaction with microtubule plus-end binding protein EB1 is required for alpha3(*)nAChR surface membrane insertion and stabilization. APC brings together EB1, the key cytoskeletal regulators macrophin and IQGAP1, and 14-3-3 adapter protein at nicotinic synapses. 14-3-3, in turn, links the alpha3-subunit to APC. This multi-molecular APC complex stabilizes the local microtubule and F-actin cytoskeleton and links postsynaptic components to the cytoskeleton--essential functions for controlling the molecular composition and stability of synapses. This work identifies macrophin, IQGAP1 and 14-3-3 as novel nicotinic synapse components and defines a new role for APC as an in vivo coordinator of nicotinic postsynaptic assembly in vertebrate neurons.