Homer interactions are necessary for metabotropic glutamate receptor-induced long-term depression and translational activation

Changes in mGlu5 receptor-dependent synaptic plasticity and coupling to homer proteins in the hippocampus of Ube3A hemizygous mice modeling angelman syndrome

Angelman syndrome (AS) is caused by the loss of Ube3A, an ubiquitin ligase that commits specific proteins to proteasomal degradation. How this defect causes autism and other pathological phenotypes associated with AS is unknown. Long-term depression (LTD) of excitatory synaptic transmission mediated by type 5 metabotropic glutamate (mGlu5) receptors was enhanced in hippocampal slices of Ube3A(m-/p+) mice, which model AS. No changes were found in NMDA-dependent LTD induced by low-frequency stimulation. mGlu5 receptor-dependent LTD in AS mice was sensitive to the protein synthesis inhibitor anisomycin, and relied on the same signaling pathways as in wild-type mice, e.g., the mitogen-activated protein kinase (MAPK) pathway, the phosphatidylinositol-3-kinase (PI3K)/mammalian target of rapamycine pathway, and protein tyrosine phosphatase. Neither the stimulation of MAPK and PI3K nor the increase in Arc (activity-regulated cytoskeleton-associated protein) levels in response to mGlu5 receptor activation were abnormal in hippocampal slices from AS mice compared with wild-type mice. mGlu5 receptor expression and mGlu1/5 receptor-mediated polyphosphoinositide hydrolysis were also unchanged in the hippocampus of AS mice. In contrast, AS mice showed a reduced expression of the short Homer protein isoform Homer 1a, and an increased coupling of mGlu5 receptors to Homer 1b/c proteins in the hippocampus. These findings support the link between Homer proteins and monogenic autism, and lay the groundwork for the use of mGlu5 receptor antagonists in AS.

Progress toward advanced understanding of metabotropic glutamate receptors: structure, signaling and therapeutic indications

The metabotropic glutamate (mGlu) receptors are a group of Class C seven-transmembrane spanning/G protein-coupled receptors (7TMRs/GPCRs). These receptors are activated by glutamate, one of the standard amino acids and the major excitatory neurotransmitter. By activating G protein-dependent and non-G protein-dependent signaling pathways, mGlus modulate glutamatergic transmission both in the periphery and throughout the central nervous system. Since the discovery of the first mGlu receptor, and especially during the last decade, a great deal of progress has been made in understanding the signaling, structure, pharmacological manipulation and therapeutic indications of the 8 mGlu members.

The eIF2α kinase PERK limits the expression of hippocampal metabotropic glutamate receptor-dependent long-term depression

The proper regulation of translation is required for the expression of long-lasting synaptic plasticity. A major site of translational control involves the phosphorylation of eukaryotic initiation factor 2 α (eIF2α) by PKR-like endoplasmic reticulum (ER) kinase (PERK). To determine the role of PERK in hippocampal synaptic plasticity, we used the Cre-lox expression system to selectively disrupt PERK expression in the adult mouse forebrain. Here, we demonstrate that in hippocampal area CA1, metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) is associated with increased eIF2α phosphorylation, whereas stimulation of early- and late-phase long-term potentiation (E-LTP and L-LTP, respectively) is associated with decreased eIF2α phosphorylation. Interesting, although PERK-deficient mice exhibit exaggerated mGluR-LTD, both E-LTP and L-LTP remained intact. We also found that mGluR-LTD is associated with a PERK-dependent increase in eIF2α phosphorylation. Our findings are consistent with the notion that eIF2α phosphorylation is a key site for the bidirectional control of persistent forms of synaptic LTP and LTD and suggest a distinct role for PERK in mGluR-LTD.

Prenatal cocaine exposure uncouples mGluR1 from Homer1 and Gq Proteins

Cocaine exposure during gestation causes protracted neurobehavioral changes consistent with a compromised glutamatergic system. Although cocaine profoundly disrupts glutamatergic neurotransmission and in utero cocaine exposure negatively affects metabotropic glutamate receptor-type 1 (mGluR1) activity, the effect of prenatal cocaine exposure on mGluR1 signaling and the underlying mechanism responsible for the prenatal cocaine effect remain elusive. Using brains of the 21-day-old (P21) prenatal cocaine-exposed rats, we show that prenatal cocaine exposure uncouples mGluR1s from their associated synaptic anchoring protein, Homer1 and signal transducer, Gq/11 proteins leading to markedly reduced mGluR1-mediated phosphoinositide hydrolysis in frontal cortex (FCX) and hippocampus. This prenatal cocaine-induced effect is the result of a sustained protein kinase C (PKC)-mediated phosphorylation of mGluR1 on the serine residues. In support, phosphatase treatment of prenatal cocaine-exposed tissues restores whereas PKC-mediated phosphorylation of saline-treated synaptic membrane attenuates mGluR1 coupling to both Gq/11 and Homer1. Expression of mGluR1, Homer1 or Gα proteins was not altered by prenatal cocaine exposure. Collectively, these data indicate that prenatal cocaine exposure triggers PKC-mediated hyper-phosphorylation of the mGluR1 leading to uncoupling of mGluR1 from its signaling components. Hence, blockade of excessive PKC activation may alleviate abnormalities in mGluR1 signaling and restores mGluR1-regulated brain functions in prenatal cocaine-exposed brains.

Fragile X syndrome neurobiology translates into rational therapy

Causal genetic defects have been identified for various neurodevelopmental disorders. A key example in this respect is fragile X syndrome, one of the most frequent genetic causes of intellectual disability and autism. Since the discovery of the causal gene, insights into the underlying pathophysiological mechanisms have increased exponentially. Over the past years, defects were discovered in pathways that are potentially amendable by pharmacological treatment. These findings have inspired the initiation of clinical trials in patients. The targeted pathways converge in part with those of related neurodevelopmental disorders raising hopes that the treatments developed for this specific disorder might be more broadly applicable.

Hippocampal Homer1 levels influence motivational behavior in an operant conditioning task

Loss of motivation and learning impairments are commonly accepted core symptoms of psychiatric disorders such as depression and schizophrenia. Reward-motivated learning is dependent on the hippocampal formation but the molecular mechanisms that lead to functional incentive motivation in this brain region are still largely unknown. Recent evidence implicates neurotransmission via metabotropic glutamate receptors and Homer1, their interaction partner in the postsynaptic density, in drug addiction and motivational learning. As previous reports mainly focused on the prefrontal cortex and the nucleus accumbens, we now investigated the role of hippocampal Homer1 in operant reward learning in the present study. We therefore tested either Homer1 knockout mice or mice that overexpress Homer1 in the hippocampus in an operant conditioning paradigm. Our results show that deletion of Homer1 leads to a diverging phenotype that either displays an inability to perform the task or outstanding hyperactivity in both learning and motivational sessions. Due to the apparent bimodal distribution of this phenotype, the overall effect of Homer1 deletion in this paradigm is not significantly altered. Overexpression of hippocampal Homer1 did not lead to a significantly altered learning performance in any stage of the testing paradigm, yet may subtly contribute to emerging motivational deficits. Our results indicate an involvement of Homer1-mediated signaling in the hippocampus in motivation-based learning tasks and encourage further investigations regarding the specific molecular underpinnings of the phenotypes observed in this study. We also suggest to cautiously interpret the results of this and other studies regarding the phenotype following Homer1 manipulations in animals, since their behavioral phenotype appears to be highly diverse. Future studies would benefit from larger group sizes that would allow splitting the experimental groups in responders and non-responders.

Acid-sensing ion channels activation and hypoxia upregulate Homer1a expression

BACKGROUND

Recent studies have indicated that dynamic alterations in the structure of postsynaptic density (PSD) are involved in the pathogenesis of many central nervous system disorders, including ischemic stroke. Homer is the newly identified scaffolding protein located at PSD and regulates synaptic function. Homer1a, an immediate early gene, has been shown to be induced by several stimulations, such as glutamate, brain-derived neurotrophic factor, and trauma. However, whether acidosis mediated by acid-sensing ion channels (ASICs) and hypoxia during cerebral ischemia can change Homer1a expression remains to be determined.

RESULTS

We investigated that acidosis and hypoxia selectively and rapidly upregulated Homer1a expression, but not Homer1b/c in cultured cortical neurons. We also found that Homer1a exhibited induction expression in brain cortex of the middle cerebral artery occlusion (MCAO) rats. Additionally, acid-evoked Homer1a mRNA induction depended on extracellular signal-regulated kinase1/2 (ERK1/2) and Akt activity, and ASIC1a-mediated calcium influx whereas hypoxia depended only on ERK1/2 activity. Also, we demonstrated that continuous acidosis and hypoxia resulted in pronounced cell injury and Homer1a knockdown with small interfering RNA aggravated this damage induced by 3 h acid and hypoxia incubation in neuro-2a cells.

CONCLUSION

Homer1a might act as an activity-dependent regulator responding to extracellular stimuli during cerebral ischemia.

The role of Homer1c in metabotropic glutamate receptor-dependent long-term potentiation

Group I metabotropic glutamate receptors (mGluR1/5) play a role in synaptic plasticity and they demonstrate direct interactions with the neuronal Homer1c protein. We have previously shown that Homer1c can restore the plasticity deficits in Homer1 knockout mice (H1-KO). Here, we investigated the role of Homer1c in mGluR-dependent synaptic plasticity in wild-type mice, H1-KO, and H1-KO mice overexpressing Homer1c (KO+H1c). We used a form of plasticity induced by activation of mGluR1/5 that transforms short-term potentiaion (STP) induced by a subthreshold theta burst stimulation into long-term potentiation (LTP). We have shown that although acute hippocampal slices from wild-type animals can induce LTP using this stimulation protocol, H1-KO only show STP. Gene delivery of Homer1c into the hippocampus of H1-KO mice rescued LTP to wild-type levels. This form of synaptic plasticity was dependent on mGluR5 but not mGluR1 activation both in wild-type mice and in KO+H1c. mGluR1/5-dependent LTP was blocked with inhibitors of the MEK-ERK and PI3K-mTOR pathways in KO+H1c mice. Moreover, blocking Homer1c-mGluR5 interactions prevented the maintenance of LTP in acute hippocampal slices from KO+H1c. These data indicate that Homer1c-mGluR5 interactions are necessary for mGluR-dependent LTP, and that mGluR1/5-dependent LTP involves PI3K and ERK activation.

c-Jun N-terminal kinase regulates mGluR-dependent expression of post-synaptic FMRP target proteins

Fragile X syndrome (FXS) is caused by the loss of functional fragile X mental retardation protein (FMRP). Loss of FMRP results in an elevated basal protein expression profile of FMRP targeted mRNAs, a loss of local metabotropic glutamate receptor (mGluR)-regulated protein synthesis, exaggerated long-term depression and corresponding learning and behavioral deficits. Evidence shows that blocking mGluR signaling in FXS models ameliorates these deficits. Therefore, understanding the signaling mechanisms downstream of mGluR stimulation may provide additional therapeutic targets for FXS. Kinase cascades are an integral mechanism regulating mGluR-dependent protein translation. The c-Jun N-terminal kinase (JNK) pathway has been shown to regulate mGluR-dependent nuclear transcription; however, the involvement of JNK in local, synaptic signaling has not been explored. Here, we show that JNK is both necessary and sufficient for mGluR-dependent expression of a subset of FMRP target proteins. In addition, JNK activity is basally elevated in fmr1 knockout mouse synapses, and blocking JNK activity reduces the over-expression of post-synaptic proteins in these mice. Together, these data suggest that JNK may be an important signaling mechanism downstream of mGluR stimulation, regulating FMRP-dependent protein synthesis. Furthermore, local, post-synaptic dysregulation of JNK activity may provide a viable target to ameliorate the deficits involved in FXS. Expression of many FMRP target proteins is enhanced in FXS. Here, we evaluated the role of JNKs in FXS. We found that JNK signaling is activated upon mGluR stimulation in wild-type neurons. Conversely, JNK activity is basally elevated in fmr1 knockout. Inhibiting JNK reduced the expression of FMRP target proteins and driving JNK activity increased the expression of these proteins.

Degradation of high affinity HuD targets releases Kv1.1 mRNA from miR-129 repression by mTORC1

Little is known about how a neuron undergoes site-specific changes in intrinsic excitability during neuronal activity. We provide evidence for a novel mechanism for mTORC1 kinase-dependent translational regulation of the voltage-gated potassium channel Kv1.1 messenger RNA (mRNA). We identified a microRNA, miR-129, that repressed Kv1.1 mRNA translation when mTORC1 was active. When mTORC1 was inactive, we found that the RNA-binding protein, HuD, bound to Kv1.1 mRNA and promoted its translation. Unexpectedly, inhibition of mTORC1 activity did not alter levels of miR-129 and HuD to favor binding to Kv1.1 mRNA. However, reduced mTORC1 signaling caused the degradation of high affinity HuD target mRNAs, freeing HuD to bind Kv1.1 mRNA. Hence, mTORC1 activity regulation of mRNA stability and high affinity HuD-target mRNA degradation mediates the bidirectional expression of dendritic Kv1.1 ion channels.

Glutamatergic postsynaptic density protein dysfunctions in synaptic plasticity and dendritic spines morphology: relevance to schizophrenia and other behavioral disorders pathophysiology, and implications for novel therapeutic approaches

Emerging researches point to a relevant role of postsynaptic density (PSD) proteins, such as PSD-95, Homer, Shank, and DISC-1, in the pathophysiology of schizophrenia and autism spectrum disorders. The PSD is a thickness, detectable at electronic microscopy, localized at the postsynaptic membrane of glutamatergic synapses, and made by scaffolding proteins, receptors, and effector proteins; it is considered a structural and functional crossroad where multiple neurotransmitter systems converge, including the dopaminergic, serotonergic, and glutamatergic ones, which are all implicated in the pathophysiology of psychosis. Decreased PSD-95 protein levels have been reported in postmortem brains of schizophrenia patients. Variants of Homer1, a key PSD protein for glutamate signaling, have been associated with schizophrenia symptoms severity and therapeutic response. Mutations in Shank gene have been recognized in autism spectrum disorder patients, as well as reported to be associated to behaviors reminiscent of schizophrenia symptoms when expressed in genetically engineered mice. Here, we provide a critical appraisal of PSD proteins role in the pathophysiology of schizophrenia and autism spectrum disorders. Then, we discuss how antipsychotics may affect PSD proteins in brain regions relevant to psychosis pathophysiology, possibly by controlling synaptic plasticity and dendritic spine rearrangements through the modulation of glutamate-related targets. We finally provide a framework that may explain how PSD proteins might be useful candidates to develop new therapeutic approaches for schizophrenia and related disorders in which there is a need for new biological treatments, especially against some symptom domains, such as negative symptoms, that are poorly affected by current antipsychotics.

Glutamate signaling in benign and malignant disorders: current status, future perspectives, and therapeutic implications

Glutamate, a nonessential amino acid, is the major excitatory neurotransmitter in the central nervous system. As such, glutamate has been shown to play a role in not only neural processes, such as learning and memory, but also in bioenergetics, biosynthetic and metabolic oncogenic pathways. Glutamate has been the target of intense investigation for its involvement not only in the pathogenesis of benign neurodegenerative diseases (NDDs) such as Parkinson's disease, Alzheimer's disease, schizophrenia, multiple sclerosis, and amyotropic lateral sclerosis (ALS), but also in carcinogenesis and progression of malignant diseases. In addition to its intracellular activities, glutamate in secreted form is a phylogenetically conserved cell signaling molecule. Glutamate binding activates multiple major receptor families including the metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors (iGluRs), both of which have been implicated in various signaling pathways in cancer. Inhibition of extracellular glutamate release or glutamate receptor activation via competitive or non-competitive antagonists decreases growth, migration and invasion and induces apoptosis in breast cancer, melanoma, glioma and prostate cancer cells. In this review, we discuss the current state of glutamate signaling research as it relates to benign and malignant diseases. In addition, we provide a synopsis of clinical trials using glutamate antagonists for the treatment of NDD and malignant diseases. We conclude that in addition to its potential role as a metabolic biomarker, glutamate receptors and glutamate-initiated signaling pathways may provide novel therapeutic opportunities for cancer.

Impairment of fragile X mental retardation protein-metabotropic glutamate receptor 5 signaling and its downstream cognates ras-related C3 botulinum toxin substrate 1, amyloid beta A4 precursor protein, striatal-enriched protein tyrosine phosphatase, and homer 1, in autism: a postmortem study in cerebellar vermis and superior frontal cortex

Background

Candidate genes associated with idiopathic forms of autism overlap with other disorders including fragile X syndrome. Our laboratory has previously shown reduction in fragile X mental retardation protein (FMRP) and increase in metabotropic glutamate receptor 5 (mGluR5) in cerebellar vermis and superior frontal cortex (BA9) of individuals with autism.

Methods

In the current study we have investigated expression of four targets of FMRP and mGluR5 signaling - homer 1, amyloid beta A4 precursor protein (APP), ras-related C3 botulinum toxin substrate 1 (RAC1), and striatal-enriched protein tyrosine phosphatase (STEP) - in the cerebellar vermis and superior frontal cortex (BA9) via SDS-PAGE and western blotting. Data were analyzed based on stratification with respect to age (children and adolescents vs. adults), anatomic region of the brain (BA9 vs. cerebellar vermis), and impact of medications (children and adolescents on medications (n = 4) vs. total children and adolescents (n = 12); adults on medications (n = 6) vs. total adults (n = 12)).

Results

There were significant increases in RAC1, APP 120 kDa and APP 80 kDa proteins in BA9 of children with autism vs. healthy controls. None of the same proteins were significantly affected in cerebellar vermis of children with autism. In BA9 of adults with autism there were significant increases in RAC1 and STEP 46 kDa and a significant decrease in homer 1 vs. controls. In the vermis of adult subjects with autism, RAC1 was significantly increased while APP 120, STEP 66 kDa, STEP 27 kDa, and homer 1 were significantly decreased when compared with healthy controls. No changes were observed in vermis of children with autism. There was a significant effect of anticonvulsant use on STEP 46 kDa/β-actin and a potential effect on homer 1/NSE, in BA9 of adults with autism. However, no other significant confound effects were observed in this study.

Conclusions

Our findings provide further evidence of abnormalities in FMRP and mGluR5 signaling partners in brains of individuals with autism and open the door to potential targeted treatments which could help ameliorate the symptoms of autism.

Modulation of behavior by scaffolding proteins of the post-synaptic density

Scaffolding proteins of the neuronal post-synaptic density (PSD) are principal organizers of glutamatergic neurotransmission that bring together glutamate receptors and signaling molecules at discrete synaptic locations. Genetic alterations of individual PSD scaffolds therefore disrupt the function of entire multiprotein modules rather than a single glutamatergic mechanism, and thus induce a range of molecular and structural abnormalities in affected neurons. Despite such broad molecular consequences, knockout, knockdown, or knockin of glutamate receptor scaffolds typically affect a subset of specific behaviors and thereby mold and specialize the actions of the ubiquitous glutamatergic neurotransmitter system. Approaches designed to control the function of neuronal scaffolds may therefore have high potential to restore behavioral morbidities and comorbidities in patients with psychiatric disorders. Here we summarize a series of experiments with genetically modified mice revealing the roles of main N-methyl-d-aspartate (NMDA) and group I metabotropic glutamate (mGluR1/5) receptor scaffolds in behavior, discuss the clinical implications of the findings, and propose future research directions.

Homer1 mediates acute stress-induced cognitive deficits in the dorsal hippocampus

In recent years, the glutamatergic system has been implicated in the development and treatment of psychiatric disorders. Glutamate signaling is processed by different receptors, including metabotropic glutamate receptors (mGluRs), which in turn interact with the scaffolding protein Homer1 to modulate downstream Ca(2+) signaling. Stress is a major risk factor for the incidence of psychiatric diseases, yet acute stress episodes may have diverging effects on individuals. Cognitive impairments have often been shown to occur after episodes of stress, however the specific role of mGluR5/Homer1 signaling in the interaction of stress and cognition has not yet been elucidated. In this study we show that a single episode of social defeat stress is sufficient to specifically induce cognitive impairments in mice 8 h after the stressor without affecting the animals' locomotion or anxiety levels. We also demonstrate that Homer1b/c levels as well as mGluR5/Homer1b/c interactions in the dorsal hippocampus are reduced up to 8 h after stress. Blockade of mGluR5 during the occurrence of social stress was able to rescue the cognitive impairments. In addition, a specific overexpression of Homer1b/c in the dorsal hippocampus also reversed the behavioral phenotype, indicating that both mGluR5 and Homer1b/c play a crucial role in the mediation of the stress effects. In summary, we could demonstrate that stress induces a cognitive deficit that is likely mediated by mGluR5/Homer1 signaling in the hippocampus. These findings help to reveal the underlying effects of cognitive impairments in patients suffering from stress-related psychiatric disorders.

The translation of translational control by FMRP: therapeutic targets for FXS

De novo protein synthesis is necessary for long-lasting modifications in synaptic strength and dendritic spine dynamics that underlie cognition. Fragile X syndrome (FXS), characterized by intellectual disability and autistic behaviors, holds promise for revealing the molecular basis for these long-term changes in neuronal function. Loss of function of the fragile X mental retardation protein (FMRP) results in defects in synaptic plasticity and cognition in many models of the disease. FMRP is a polyribosome-associated RNA-binding protein that regulates the synthesis of a set of plasticity-reated proteins by stalling ribosomal translocation on target mRNAs. The recent identification of mRNA targets of FMRP and its upstream regulators, and the use of small molecules to stall ribosomes in the absence of FMRP, have the potential to be translated into new therapeutic avenues for the treatment of FXS.

What's hAPPening at synapses? The role of amyloid β-protein precursor and β-amyloid in neurological disorders

Accumulating evidence suggests that dysregulated levels of amyloid β-protein precursor (APP) and its catabolites contribute to the impaired synaptic plasticity and seizure incidence observed in several neurological disorders, including Alzheimer's disease, fragile X syndrome, Down's syndrome, autism, epilepsy and Parkinson's disease as well as in brain injury. This review article summarizes what is known regarding the synaptic synthesis, processing and function of APP and amyloid-beta (Aβ), as well as discusses how these proteins could contribute to the altered synaptic plasticity and pathology of the aforementioned disorders. In addition, APP and its proteolytic fragments are emerging as biomarkers for neurological health, and pharmacological interventions that modulate their levels, such as secretase inhibitors, passive immunotherapy against Aβ and mGluR5 antagonists, are reviewed.

Gene delivery of Homer1c rescues spatial learning in a rodent model of cognitive aging

Homer1c has been shown to play a role in learning and memory. Overexpression of Homer1c in the hippocampus can improve memory in normal rats and can also rescue spatial learning deficits in Homer1 knockout mice. In a previous study, we found that Homer1c mRNA is upregulated after a spatial learning paradigm in aged rats that successfully learn the task, when compared to aged rats that are learning-impaired (AI). This study was designed to validate the role of Homer1c in successful cognitive aging. In this article, we report that gene delivery of Homer1c into the hippocampus of aged learning-impaired rats significantly improves individual performance on an object location memory task. The learning ability of these rats on the Morris Water Maze was also superior to that of AI control rats. In summary, using 2 independent spatial memory tasks, we demonstrate that Homer1c is sufficient to improve the spatial learning deficits in a rodent model of cognitive aging. These results point to Homer1c as a potential therapeutic target for improving age-related cognitive impairment.

Uncoupling of the endocannabinoid signalling complex in a mouse model of fragile X syndrome

Fragile X syndrome, the most commonly known genetic cause of autism, is due to loss of the fragile X mental retardation protein, which regulates signal transduction at metabotropic glutamate receptor-5 in the brain. Fragile X mental retardation protein deletion in mice enhances metabotropic glutamate receptor-5-dependent long-term depression in the hippocampus and cerebellum. Here we show that a distinct type of metabotropic glutamate receptor-5-dependent long-term depression at excitatory synapses of the ventral striatum and prefrontal cortex, which is mediated by the endocannabinoid 2-arachidonoyl-sn-glycerol, is absent in fragile X mental retardation protein-null mice. In these mutants, the macromolecular complex that links metabotropic glutamate receptor-5 to the 2-arachidonoyl-sn-glycerol-producing enzyme, diacylglycerol lipase-α (endocannabinoid signalosome), is disrupted and metabotropic glutamate receptor-5-dependent 2-arachidonoyl-sn-glycerol formation is compromised. These changes are accompanied by impaired endocannabinoid-dependent long-term depression. Pharmacological enhancement of 2-arachidonoyl-sn-glycerol signalling normalizes this synaptic defect and corrects behavioural abnormalities in fragile X mental retardation protein-deficient mice. The results identify the endocannabinoid signalosome as a molecular substrate for fragile X syndrome, which might be targeted by therapy.

Fragile X syndrome is a major genetic cause of autism and is caused by loss of the fragile X mental retardation protein. In a mouse model of fragile X syndrome, Jung et al. show that an absence of neuronal endocannabinoid signalling is responsible for the neurophysiological and behavioural defects.

Increased metabotropic glutamate receptor subtype 5 availability in human brain after one night without sleep

BACKGROUND

Sleep deprivation (wake therapy) provides rapid clinical relief in many patients with major depressive disorder (MDD). Changes in glutamatergic neurotransmission may contribute to the antidepressant response, yet the exact underlying mechanisms are unknown. Metabotropic glutamate receptors of subtype 5 (mGluR5) are importantly involved in modulating glutamatergic neurotransmission and neuronal plasticity. The density of these receptors is reduced in the brain of patients with MDD, particularly in brain structures involved in regulating wakefulness and sleep. We hypothesized that prolonged wakefulness would increase mGluR5 availability in human brain.

METHODS

Metabotropic glutamate receptor subtype 5 binding was quantified with positron emission tomography in 22 young healthy men who completed two experimental blocks separated by 1 week. Two positron emission tomography examinations were conducted in randomized, crossover fashion with the highly selective radioligand, ¹¹C-ABP688, once after 9 hours (sleep control) and once after 33 hours (sleep deprivation) of controlled wakefulness. ¹¹C-ABP688 uptake was quantified in 13 volumes of interest with high mGluR5 expression and presumed involvement in sleep-wake regulation.

RESULTS

Sleep deprivation induced a global increase in mGluR5 binding when compared with sleep control (p<.006). In anterior cingulate cortex, insula, medial temporal lobe, parahippocampal gyrus, striatum, and amygdala, this increase correlated significantly with the sleep deprivation-induced increase in subjective sleepiness.

CONCLUSIONS

This molecular imaging study demonstrates that cerebral functional mGluR5 availability is increased after a single night without sleep. Given that mGluR5 density is reduced in MDD, further research is warranted to examine whether this mechanism is involved in the potent antidepressant effect of wake therapy.

Role of mGluR5 neurotransmission in reinstated cocaine-seeking

In animal models of addiction, reducing glutamate stimulation of the metabotropic glutamate receptor 5 (mGluR5) inhibits drug-seeking. The present study used the reinstatement model of cocaine-seeking to show that blockade of mGluR5 directly in the core subcompartment of the nucleus accumbens (NAcore) prevented both conditioned cue- and cocaine-reinstated drug-seeking. Consistent with this finding, microinjection of the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine into the NAcore produced modest reinstatement of lever pressing when given alone and significantly potentiated cue-induced reinstatement. Homer proteins are contained in the post-synaptic density and regulate mGluR5 intracellular signaling and trafficking to the membrane. Microinjecting a membrane permeable peptide antagonist of Homer binding to mGluR5 into the NAcore also inhibited cue- and cocaine-reinstated lever pressing. However, this peptide did not change the surface expression of mGluR5, indicating that the peptide inhibitor did not alter the surface trafficking of mGluR5. Taken together, these data show that mGluR5 inhibition and stimulation in the NAcore can regulate cocaine-seeking, and demonstrate that one mechanism for this effect is via interactions with Homer proteins.

Dendritic protein synthesis in the normal and diseased brain

Synaptic activity is a spatially limited process that requires a precise, yet dynamic, complement of proteins within the synaptic micro-domain. The maintenance and regulation of these synaptic proteins is regulated, in part, by local mRNA translation in dendrites. Protein synthesis within the postsynaptic compartment allows neurons tight spatial and temporal control of synaptic protein expression, which is critical for proper functioning of synapses and neural circuits. In this review, we discuss the identity of proteins synthesized within dendrites, the receptor-mediated mechanisms regulating their synthesis, and the possible roles for these locally synthesized proteins. We also explore how our current understanding of dendritic protein synthesis in the hippocampus can be applied to new brain regions and to understanding the pathological mechanisms underlying varied neurological diseases.

Fragile X syndrome: from targets to treatments

Fragile X syndrome (FXS) is one of the most prevalent and well-studied monogenetic causes of intellectual disability and autism and, although rare, its high penetrance makes it a desirable model for the study of neurodevelopmental disorders more generally. Indeed recent studies suggest that there is functional convergence of a number of genes that are implicated in intellectual disability and autism indicating that an understanding of the cellular and biochemical dysfunction that occurs in monogenic forms of these disorders are likely to reveal common targets for therapeutic intervention. Fundamental research into FXS has provided a wealth of information about how the loss of function of the fragile X mental retardation protein results in biochemical, anatomical and physiological dysfunction leading to the discovery of interventions that correct many of the core pathological phenotypes associated with animal models of FXS. Most promisingly such strategies have led to development of drugs that are now in clinical trials. This review highlights how progress in understanding disorders such as FXS has led to a new era in which targeted molecular treatment towards neurodevelopmental disorders is becoming a reality. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Metabotropic glutamate receptor 5 in the pathology and treatment of schizophrenia

Metabotropic glutamate receptor 5 (mGluR5) potentiates the NMDA receptor (NMDAR) in brain regions implicated in schizophrenia, making it a viable therapeutic target for the treatment of this disorder. mGluR5 positive allosteric modulators may represent a valuable novel strategy for schizophrenia treatment, given the favourable profile of effects in preclinical paradigms. However it remains unclear whether mGluR5 also plays a causal or epiphenomenal role in NMDAR dysfunction in schizophrenia. Animal and cellular data suggest involvement of mGluR5, whilst post-mortem human studies remain inconclusive. This review will explore the molecular, animal and human data to support and refute the involvement of mGluR5 in the pathology of schizophrenia. Furthermore, this review will discuss the potential of mGluR5 modulators in the therapy of schizophrenia as well as aspects of mGluR5 that require further characterisation.

Synaptic plasticity in mouse models of autism spectrum disorders

Analysis of synaptic plasticity together with behavioral and molecular studies have become a popular approach to model autism spectrum disorders in order to gain insight into the pathosphysiological mechanisms and to find therapeutic targets. Abnormalities of specific types of synaptic plasticity have been revealed in numerous genetically modified mice that have molecular construct validity to human autism spectrum disorders. Constrained by the feasibility of technique, the common regions analyzed in most studies are hippocampus and visual cortex. The relevance of the synaptic defects in these regions to the behavioral abnormalities of autistic like behaviors is still a subject of debate. Because the exact regions or circuits responsible for the core features of autistic behaviors in humans are still poorly understood, investigation using region-specific conditional mutant mice may help to provide the insight into the neuroanatomical basis of autism in the future.

Translational control at the synapse: role of RNA regulators

Translational control of gene expression is instrumental in the regulation of eukaryotic cellular form and function. Neurons in particular rely on this form of control because their numerous synaptic connections need to be independently modulated in an input-specific manner. Brain cytoplasmic (BC) RNAs implement translational control at neuronal synapses. BC RNAs regulate protein synthesis by interacting with eIF4 translation initiation factors. Recent evidence suggests that such regulation is required to control synaptic strength, and that dysregulation of local protein synthesis precipitates neuronal hyperexcitability and a propensity for epileptogenic responses. A similar phenotype results from lack of fragile X mental retardation protein (FMRP), indicating that BC RNAs and FMRP use overlapping and convergent modes of action in neuronal translational regulation.

Dual effect of 17β-estradiol on NMDA-induced neuronal death: involvement of metabotropic glutamate receptor 1

Pretreatment with 10 nm 17β-estradiol (17βE2) or 100 μm of the metabotropic glutamate 1 receptor (mGlu1R) agonist, dihydroxyphenylglycine (DHPG), protected neurons against N-methyl-d-aspartate (NMDA) toxicity. This effect was sensitive to blockade of both estrogen receptors and mGlu1R by their respective antagonists. In contrast, 17βE2 and/or DHPG, added after a low-concentration NMDA pulse (45 μm), produced an opposite effect, i.e. an exacerbation of NMDA toxicity. Again this effect was prevented by both receptor antagonists. In support of an interaction of estrogen receptors and mGlu1R in mediating a neurotoxic response, exacerbation of NMDA toxicity by 17βE2 disappeared when cultures were treated with DHPG prior to NMDA challenge, and conversely, potentiation of NMDA-induced cell death by DHPG was prevented by pretreatment with 17βE2. Addition of calpain III inhibitor (10 μm), 2 h before NMDA, prevented the increased damage induced by the two agonists, an affect that can be secondary to cleavage of mGlu1R by calpain. Accordingly, NMDA stimulation reduced expression of the full-length (140 kDa) mGluR1, an effect partially reversed by calpain inhibitor. Finally, in the presence of NMDA, the ability of 17βE2 to stimulate phosphorylation of AKT and ERK was impaired. Pretreatment with calpain inhibitor prevented the reduction of phosphorylated ERK but had no significant effect on phosphorylated AKT. Accordingly, the inhibition of ERK signaling by U0126 (1 μm) counteracted the effect of calpain inhibition on 17βE2-induced exacerbation of NMDA toxicity. The present data confirm the dual role of estrogens in neurotoxicity/neuroprotection and highlight the role of the timing of exposure to estrogens.

Translational Concepts of mGluR5 in Synaptic Diseases of the Brain

The G-protein coupled receptor family of glutamate receptors, termed metabotropic glutamate receptors (mGluRs), are implicated in numerous cellular mechanisms ranging from neural development to the processing of cognitive, sensory, and motor information. Over the last decade, multiple mGluR-related signal cascades have been identified at excitatory synapses, indicating their potential roles in various forms of synaptic function and dysfunction. This review highlights recent studies investigating mGluR5, a subtype of group I mGluRs, and its association with a number of developmental, psychiatric, and senile synaptic disorders with respect to associated synaptic proteins, with an emphasis on translational pre-clinical studies targeting mGluR5 in a range of synaptic diseases of the brain.

CLC-3 chloride channels moderate long-term potentiation at Schaffer collateral-CA1 synapses

The chloride channel CLC-3 is expressed in the brain on synaptic vesicles and postsynaptic membranes. Although CLC-3 is broadly expressed throughout the brain, the CLC-3 knockout mouse shows complete, selective postnatal neurodegeneration of the hippocampus, suggesting a crucial role for the channel in maintaining normal brain function. CLC-3 channels are functionally linked to NMDA receptors in the hippocampus; NMDA receptor-dependent Ca(2+) entry, activation of Ca(2+)/calmodulin kinase II and subsequent gating of CLC-3 link the channels via a Ca(2+)-mediated feedback loop. We demonstrate that loss of CLC-3 at mature synapses increases long-term potentiation from 135 ± 4% in the wild-type slice preparation to 154 ± 7% above baseline (P < 0.001) in the knockout; therefore, the contribution of CLC-3 is to reduce synaptic potentiation by ∼40%. Using a decoy peptide representing the Ca(2+)/calmodulin kinase II phosphorylation site on CLC-3, we show that phosphorylation of CLC-3 is required for its regulatory function in long-term potentiation. CLC-3 is also expressed on synaptic vesicles; however, our data suggest functionally separable pre- and postsynaptic roles. Thus, CLC-3 confers Cl(-) sensitivity to excitatory synapses, controls the magnitude of long-term potentiation and may provide a protective limit on Ca(2+) influx.

Fluorescent and photo-oxidizing TimeSTAMP tags track protein fates in light and electron microscopy

Protein synthesis is highly regulated throughout nervous system development, plasticity, and regeneration. However, tracking the distributions of specific new protein species has not been possible in living neurons or at the ultrastructural level. Previously we created TimeSTAMP epitope tags, drug-controlled tags for immunohistochemical detection of specific new proteins synthesized at defined times. Here we extend TimeSTAMP to label new protein copies by fluorescence or photo-oxidation. Live microscopy of a fluorescent TimeSTAMP tag reveals that copies of the synaptic protein PSD95 are synthesized in response to local activation of growth factor and neurotransmitter receptors, and preferentially localize to stimulated synapses in rat neurons. Electron microscopy of a photo-oxidizing TimeSTAMP tag reveals new PSD95 at developing dendritic structures of immature neurons and at synapses in differentiated neurons. These results demonstrate the versatility of the TimeSTAMP approach for visualizing newly synthesized proteins in neurons.

Group 1 metabotropic glutamate receptor function and its regulation of learning and memory in the aging brain

Normal aging is generally characterized by a slow decline of cognitive abilities albeit with marked individual differences. Several animal models have been studied to explore the molecular and cellular mechanisms underlying this phenomenon. The excitatory neurotransmitter glutamate and its receptors have been closely linked to spatial learning and hippocampus-dependent memory processes. For decades, ionotropic glutamate receptors have been known to play a critical role in synaptic plasticity, a form of adaptation regulating memory formation. Over the past 10 years, several groups have shown the importance of group 1 metabotropic glutamate receptor (mGluR) in successful cognitive aging. These G-protein-coupled receptors are enriched in the hippocampal formation and interact physically with other proteins in the membrane including glutamate ionotropic receptors. Synaptic plasticity is crucial to maintain cognitive abilities and long-term depression (LTD) induced by group 1 mGluR activation, which has been linked to memory in the aging brain. The translation and synthesis of proteins by mGluR-LTD modulate ionotropic receptor trafficking and expression of immediate early genes related to cognition. Fragile X syndrome, a genetic form of autism characterized by memory deficits, has been associated to mGluR receptor malfunction and aberrant activation of its downstream signaling pathways. Dysfunction of mGluR could also be involved in neurodegenerative disorders like Alzheimer’s disease (AD). Indeed, beta-amyloid, the main component of insoluble senile plaques and one of the hallmarks of AD, occludes mGluR-dependent LTD leading to diminished functional synapses. This review highlights recent findings regarding mGluR signaling, related synaptic plasticity, and their potential involvement in normal aging and neurological disorders.

LTP induction translocates cortactin at distant synapses in wild-type but not Fmr1 knock-out mice

Stabilization of long-term potentiation (LTP) depends on reorganization of the dendritic spine actin cytoskeleton. The present study tested whether this involves activity-driven effects on the actin-regulatory protein cortactin, and whether such effects are disturbed in the Fmr1 knock-out (KO) model of fragile X syndrome, in which stabilization of both actin filaments and LTP is impaired. LTP induced by theta burst stimulation (TBS) in hippocampal slices from wild-type mice was associated with rapid, broadly distributed, and NMDA receptor-dependent decreases in synapse-associated cortactin. The reduction in cortactin content was blocked by blebbistatin, while basal levels were reduced by nocodazole, indicating that cortactin's movements into and away from synapses are regulated by microtubule and actomyosin motors, respectively. These results further suggest that synapse-specific LTP influences cytoskeletal elements at distant connections. The rapid effects of TBS on synaptic cortactin content were absent in Fmr1 KOs as was evidence for activity-driven phosphorylation of the protein or its upstream kinase, ERK1/2. Phosphorylation regulates cortactin's interactions with actin, and coprecipitation of the two proteins was reduced in the KOs. We propose that, in the KOs, excessive basal phosphorylation of ERK1/2 disrupts its interactions with cortactin, thereby blocking the latter protein's use of actomyosin transport systems. These impairments are predicted to compromise the response of the subsynaptic cytoskeleton to learning-related afferent activity, both locally and at distant sites.

Dynamic remodeling of scaffold interactions in dendritic spines controls synaptic excitability

Synaptic activity–dependent remodeling of the glutamate receptor scaffold complex generates a negative feedback loop that limits further NMDA receptor activation.

Scaffolding proteins interact with membrane receptors to control signaling pathways and cellular functions. However, the dynamics and specific roles of interactions between different components of scaffold complexes are poorly understood because of the dearth of methods available to monitor binding interactions. Using a unique combination of single-cell bioluminescence resonance energy transfer imaging in living neurons and electrophysiological recordings, in this paper, we depict the role of glutamate receptor scaffold complex remodeling in space and time to control synaptic transmission. Despite a broad colocalization of the proteins in neurons, we show that spine-confined assembly/disassembly of this scaffold complex, physiologically triggered by sustained activation of synaptic NMDA (N-methyl-d-aspartate) receptors, induces physical association between ionotropic (NMDA) and metabotropic (mGlu5a) synaptic glutamate receptors. This physical interaction results in an mGlu5a receptor–mediated inhibition of NMDA currents, providing an activity-dependent negative feedback loop on NMDA receptor activity. Such protein scaffold remodeling represents a form of homeostatic control of synaptic excitability.

Synaptic plasticity and learning in animal models of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is caused by a mutation of either the Tsc1 or Tsc2 gene. As these genes work in concert to negatively regulate the mammalian target of rapamycin (mTOR) kinase which is involved in protein translation, mutations of these genes lead to a disinhibited mTOR activity. Both the clinical appearance of this condition including tumors, cognitive decline, and epileptic seizures and the molecular understanding of the mTOR signaling pathway, not only involved in cell growth, but also in neuronal functioning, have inspired numerous studies on learning behavior as well as on synaptic plasticity which is the key molecular mechanism of information storage in the brain. A couple of interesting animal models have been established, and the data obtained in these animals will be discussed. A special focus will be laid on differences among these models, which may be in part due to different background strains, but also may indicate pathophysiological variation in different mutations.

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

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

Chronic treatment with lithium or valproate modulates the expression of Homer1b/c and its related genes Shank and Inositol 1,4,5-trisphosphate receptor

Homer proteins are associated with both dopaminergic and glutamatergic function. In addition, these proteins are implicated in many signal transduction pathways that are also putative targets of the mood stabilizers lithium and valproate (VPA). This study investigated the effect of in vivo chronic administration of therapeutically-relevant doses of lithium and VPA on the expression of the inducible (Homer1a and ania-3) and constitutive (Homer1b/c) isoforms of the Homer1 gene in rat brain, and of two other Homer-related genes: Inositol 1,4,5 trisphosphate receptor (IP3R) and Shank. Homer1b/c was significantly decreased in cortex by VPA, and in striatal and accumbal subregions by both lithium and VPA. Both mood stabilizers reduced Homer1b/c expression in the dorsolateral caudate-putamen, while only VPA decreased gene expression in all other striatal subregions. Shank and IP3R were downregulated by both mood stabilizers in the cortex. Neither chronic lithium nor VPA affected Homer immediate-early genes. These results suggest that lithium and VPA similarly modulate the expression of structural postsynaptic genes with topographic specificity in cortical and subcortical regions. Thus, Homer may represent an additional molecular substrate for mood stabilizers, and a potential link with dopaminergic function.

Scaffold protein Homer 1: implications for neurological diseases

Homer proteins are commonly known as scaffold proteins at postsynaptic density. Homer 1 is a widely studied member of the Homer protein family, comprising both synaptic structure and mediating postsynaptic signaling transduction. Both an immediate-early gene encoding a Homer 1 variant and a constitutively expressed Homer 1 variant regulate receptor clustering and trafficking, intracellular calcium homeostasis, and intracellular molecule complex formation. Substantial preclinical investigations have implicated that each of these Homer 1 variants are associated with the etiology of many neurological diseases, such as pain, mental retardation syndromes, Alzheimer's disease, schizophrenia, drug-induced addiction, and traumatic brain injury.

RGS4 overexpression in the rat dorsal striatum modulates mGluR5- and amphetamine-mediated behavior and signaling

RATIONALE

Regulator of G-protein signaling 4 (RGS4) is a brain-enriched negative modulator of G-protein-coupled receptor signaling. Decreased availability of RGS4 in the frontal cortex and striatum has been described in animal models of schizophrenia and drug addiction. However, cellular and behavioral consequences of dysregulated RGS4-dependent receptor signaling in the brain remain poorly understood.

OBJECTIVE

This study aims to investigate whether RGS4, through inhibiting the function of mGluR5 receptors in the dorsal striatum (dSTR), regulates cellular and behavioral responses to acute amphetamine.

METHODS

After herpes simplex virus-RGS4 was infused into the dSTR, RGS4 overexpression as well as binding of recombinant RGS4 to mGluR5 was assessed. The effect of RGS4 overexpression on behavioral activity induced by the intrastriatal mGluR5 agonist, DHPG, or amphetamine was recorded. Activation of extracellular signal-regulated kinase (ERK) and Akt (protein kinase B) was measured in the dSTR tissue at the end of each behavioral experiment.

RESULTS

RGS4 overexpressed in the dSTR coimmunoprecipitated with mGluR5 receptors and suppressed both behavioral activity and phospho-ERK levels induced by DHPG. RGS4 overexpression or the mGluR5 antagonist, 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (MTEP), attenuated amphetamine-induced phospho-ERK (but not phospho-Akt) levels. RGS4 suppressed amphetamine-induced vertical activity and augmented horizontal activity over 90 min. Similarly, MTEP augmented amphetamine-induced horizontal activity, but did not affect vertical activity.

CONCLUSIONS

The present data demonstrate that RGS4 in the dSTR attenuates amphetamine-induced ERK signaling and decreases the behavioral efficacy of acute amphetamine likely by limiting mGluR5 function.

The interaction between mGluR1 and the calcium channel Cav₂.₁ preserves coupling in the presence of long Homer proteins

Group I metabotropic glutamate receptors (mGluR1 and 5) are G protein coupled receptors that regulate neuronal activity in a number of ways. Some of the most well studied functions of group I mGluRs, such as initiation of multiple forms of mGluR-dependent long-term depression, require receptor localization near the post-synaptic density (PSD). This localization is in turn dependent on the Homer family of scaffolding proteins which bind to a small motif on the distal C-termini of mGluR1 and 5, localize the receptors near the PSD, strengthen coupling to post-synaptic effectors and simultaneously uncouple the mGluRs from extra-synaptic effectors such as voltage dependent ion channels. Here the selectivity of this uncoupling process was examined by testing the ability of Homer-2b to uncouple mGluR1 from multiple voltage dependent calcium channels including Ca(V2.2) (N-type), Ca(V3.2) (T-type), and Ca(V2.1) (P/Q-type) expressed in rat sympathetic neurons from the superior cervical ganglion (SCG). Of these, only the mGluR1-Ca(V2.1) modulatory pathway was insensitive to Homer-2b expression. Uncoupling from this channel was achieved by co-expression of an mGluR1 C-terminal protein designed to disrupt a previously described direct interaction between these two proteins, suggesting that this interaction allows incorporation of Ca(V2.1) into the mGluR1/Homer signaling complex, thereby preserving modulation in the presence of scaffolding Homer proteins. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

High-throughput sequencing of mGluR signaling pathway genes reveals enrichment of rare variants in autism

Identification of common molecular pathways affected by genetic variation in autism is important for understanding disease pathogenesis and devising effective therapies. Here, we test the hypothesis that rare genetic variation in the metabotropic glutamate-receptor (mGluR) signaling pathway contributes to autism susceptibility. Single-nucleotide variants in genes encoding components of the mGluR signaling pathway were identified by high-throughput multiplex sequencing of pooled samples from 290 non-syndromic autism cases and 300 ethnically matched controls on two independent next-generation platforms. This analysis revealed significant enrichment of rare functional variants in the mGluR pathway in autism cases. Higher burdens of rare, potentially deleterious variants were identified in autism cases for three pathway genes previously implicated in syndromic autism spectrum disorder, TSC1, TSC2, and SHANK3, suggesting that genetic variation in these genes also contributes to risk for non-syndromic autism. In addition, our analysis identified HOMER1, which encodes a postsynaptic density-localized scaffolding protein that interacts with Shank3 to regulate mGluR activity, as a novel autism-risk gene. Rare, potentially deleterious HOMER1 variants identified uniquely in the autism population affected functionally important protein regions or regulatory sequences and co-segregated closely with autism among children of affected families. We also identified rare ASD-associated coding variants predicted to have damaging effects on components of the Ras/MAPK cascade. Collectively, these findings suggest that altered signaling downstream of mGluRs contributes to the pathogenesis of non-syndromic autism.

The pathophysiology of fragile X (and what it teaches us about synapses)

Fragile X is the most common known inherited cause of intellectual disability and autism, and it typically results from transcriptional silencing of FMR1 and loss of the encoded protein, FMRP (fragile X mental retardation protein). FMRP is an mRNA-binding protein that functions at many synapses to inhibit local translation stimulated by metabotropic glutamate receptors (mGluRs) 1 and 5. Recent studies on the biology of FMRP and the signaling pathways downstream of mGluR1/5 have yielded deeper insight into how synaptic protein synthesis and plasticity are regulated by experience. This new knowledge has also suggested ways that altered signaling and synaptic function can be corrected in fragile X, and human clinical trials based on this information are under way.

An emerging role for the Mammalian target of rapamycin in "pathological" protein translation: relevance to cocaine addiction

Complex neuroadaptations within key nodes of the brain's "reward circuitry" are thought to underpin long-term vulnerability to relapse. A more comprehensive understanding of the molecular and cellular signaling events that subserve relapse vulnerability may lead to pharmacological treatments that could improve treatment outcomes for psychostimulant-addicted individuals. Recent advances in this regard include findings that drug-induced perturbations to neurotrophin, metabotropic glutamate receptor, and dopamine receptor signaling pathways perpetuate plasticity impairments at excitatory glutamatergic synapses on ventral tegmental area and nucleus accumbens neurons. In the context of addiction, much previous work, in terms of downstream effectors to these receptor systems, has centered on the extracellular-regulated MAP kinase signaling pathway. The purpose of the present review is to highlight the evidence of an emerging role for another downstream effector of these addiction-relevant receptor systems - the mammalian target of rapamycin complex 1 (mTORC1). mTORC1 functions to regulate synaptic protein translation and is a potential critical link in our understanding of the neurobiological processes that drive addiction and relapse behavior. The precise cellular and molecular changes that are regulated by mTORC1 and contribute to relapse vulnerability are only just coming to light. Therefore, we aim to highlight evidence that mTORC1 signaling may be dysregulated by drug exposure and that these changes may contribute to aberrant translation of synaptic proteins that appear critical to increased relapse vulnerability, including AMPARs. The importance of understanding the role of this signaling pathway in the development of addiction vulnerability is underscored by the fact that the mTORC1 inhibitor rapamycin reduces drug-seeking in pre-clinical models and preliminary evidence indicating that rapamycin suppresses drug craving in humans.