Protein Synthesis-dependent Synaptic Plasticity Research Articles

Impaired hippocampal NMDAR-LTP in a transgenic model of NSUN2-deficiency

Biallelic loss-of-function NSUN2 mutations have recently been associated with cases of Autism Spectrum Condition (ASC), and NSun2-deficiency was also previously shown to cause a severe autosomal recessive intellectually disability disorder syndrome in which patients can sometimes display autistic behaviour. It has been demonstrated that NSUN2 can control protein synthesis rates via direct regulation of RNA methylation, and it is therefore of interest that other studies have suggested protein synthesis-dependent synaptic plasticity dysregulation as a mechanism for learning difficulties in various other autism-expressing conditions and disorders. Here we investigated NMDAR-LTP in a murine transgenic model harbouring loss-of-function mutation in the NSun2 gene and find an impairment of a protein synthesis-dependent form of this synaptic plasticity pathway. Our findings support the idea that NMDAR-LTP mis-regulation may represent a previously underappreciated mechanism associated with autism phenotypes.

Phosphoproteomic basis of neuroplasticity in the antennal lobes influences the olfactory differences between A. mellifera and A. cerana honeybees

The honeybee species A. mellifera and A. cerana have evolved substantial differences in olfactory-driven behaviors and in peripheral olfactory systems. Knowledge of the central nervous system regulating these olfaction differences is limited, however. We compared the phosphoproteome of the antennal lobes (ALs, the primary olfactory neuropil) of A. mellifera and A. cerana, and identified a total of 2812 phosphopeptides carrying 2971 phosphosites from 1265 phosphoproteins. Of these, 76% of the phosphoproteins were shared by both species, which were mainly presynapse and cytoskeleton components, and were involved in signal transduction and neurotransmitter secretion. This finding indicates the fundamental role of protein phosphorylation in regulating signal transduction in the ALs. The mTOR signaling pathway, the phagosome pathway, and the autophagy pathway, which are important in protein metabolism, were enriched, suggesting glomeruli plasticity and olfactory processing are intensively modulated by phosphorylation via these pathways. Compared with A. mellifera, 107 phosphoproteins associated with protein metabolism and transport were uniquely expressed in A. cerana, indicating the protein synthesis-dependent synaptic plasticity is enhanced in A. cerana to facilitate the processing of more complex floral odor clues in mountain foraging areas. This finding is further supported by the significantly upregulated key phosphoproteins of the mTOR signaling pathway in A. cerana ALs. These results provide insights into the phosphoproteomic basis of neuroplasticity that is coupled with the divergent evolution of bees in different environments. SignificanceTo adapt to their own ecological niche, the two major honeybee species, A. mellifera and A. cerana, have developed significant difference in olfactory-driven behaviors, but our understanding of the underlying regulation of the central nervous system is still limitate. Here we performed the first comprehensive phosphoproteomic comparison of antennal lobes (Als) between A. mellifera and A. cerena. A large proportion of the identified phosphosites and phosphoproteins were shared between the two species to serve as a core network in the regulation of signal transduction and glomeruli plasticity of ALs. However, compared with A. mellifera, 107 phosphoproteins associated with protein metabolism and transport were uniquely identified in A. cerana ALs, and also several key phosphoproteins in mTOR signaling pathway were found upregulated in A. cerana. These findings indicate protein phosphorylation enhanced the protein synthesis-dependent synaptic plasticity in A. cerana to facilitate the processing of more complex floral odor clues in mountain foraging areas. Our data provide a valuable insight into phosphoproteome-driven cerebral regulation of honeybee olfactory behaviors, which is potentially useful for further neurobiological investigation in both honeybees and other insects.

The Immediate Early Gene Arc Is Not Required for Hippocampal Long-Term Potentiation.

Memory consolidation is thought to occur through protein synthesis-dependent synaptic plasticity mechanisms such as long-term potentiation (LTP). Dynamic changes in gene expression and epigenetic modifications underlie the maintenance of LTP. Similar mechanisms may mediate the storage of memory. Key plasticity genes, such as the immediate early gene Arc, are induced by learning and by LTP induction. Mice that lack Arc have severe deficits in memory consolidation, and Arc has been implicated in numerous other forms of synaptic plasticity, including long-term depression and cell-to-cell signaling. Here, we take a comprehensive approach to determine if Arc is necessary for hippocampal LTP in male and female mice. Using a variety of Arc knock-out (KO) lines, we found that germline Arc KO mice show no deficits in CA1 LTP induced by high-frequency stimulation and enhanced LTP induced by theta-burst stimulation. Temporally restricting the removal of Arc to adult animals and spatially restricting it to the CA1 using Arc conditional KO mice did not have an effect on any form of LTP. Similarly, acute application of Arc antisense oligodeoxynucleotides had no effect on hippocampal CA1 LTP. Finally, the maintenance of in vivo LTP in the dentate gyrus of Arc KO mice was normal. We conclude that Arc is not necessary for hippocampal LTP and may mediate memory consolidation through alternative mechanisms.SIGNIFICANCE STATEMENT The immediate early gene Arc is critical for maintenance of long-term memory. How Arc mediates this process remains unclear, but it has been proposed to sustain Hebbian synaptic potentiation, which is a key component of memory encoding. This form of plasticity is modeled experimentally by induction of LTP, which increases Arc mRNA and protein expression. However, mechanistic data implicates Arc in the endocytosis of AMPA-type glutamate receptors and the weakening of synapses. Here, we took a comprehensive approach to determine if Arc is necessary for hippocampal LTP. We find that Arc is not required for LTP maintenance and may regulate memory storage through alternative mechanisms.

Noncanonical cytoplasmic poly(A) polymerases regulate RNA levels, alternative RNA processing, and synaptic plasticity but not hippocampal-dependent behaviours

ABSTRACT Noncanonical poly(A) polymerases are frequently tethered to mRNA 3ʹ untranslated regions and regulate poly(A) tail length and resulting translation. In the brain, one such poly(A) polymerase is Gld2, which is anchored to mRNA by the RNA-binding protein CPEB1 to control local translation at postsynaptic regions. Depletion of CPEB1 or Gld2 from the mouse hippocampus results in a deficit in long-term potentiation (LTP), but only depletion of CPEB1 alters animal behaviour. To test whether a related enzyme, Gld4, compensates for the lack of Gld2, we separately or simultaneously depleted both proteins from hippocampal area CA1 and again found little change in animal behaviour, but observed a deficit in LTP as well as an increase in long-term depression (LTD), two forms of protein synthesis-dependent synaptic plasticity. RNA-seq data from Gld2, Gld4, and Gld2/Gld4-depleted hippocampus show widespread changes in steady state RNA levels, alternative splicing, and alternative poly(A) site selection. Many of the RNAs subject to these alterations encode proteins that mediate synaptic function, suggesting a molecular foundation for impaired synaptic plasticity.

Genetic removal of eIF2α kinase PERK in mice enables hippocampal L-LTP independent of mTORC1 activity.

Characterization of the molecular signaling pathways underlying protein synthesis-dependent forms of synaptic plasticity, such as late long-term potentiation (L-LTP), can provide insights not only into memory expression/maintenance under physiological conditions but also potential mechanisms associated with the pathogenesis of memory disorders. Here, we report in mice that L-LTP failure induced by the mammalian (mechanistic) target of rapamycin complex 1 (mTORC1) inhibitor rapamycin is reversed by brain-specific genetic deletion of PKR-like ER kinase, PERK (PERK KO), a kinase for eukaryotic initiation factor 2α (eIF2α). In contrast, genetic removal of general control non-derepressible-2, GCN2 (GCN2 KO), another eIF2α kinase, or treatment of hippocampal slices with the PERK inhibitor GSK2606414, does not rescue rapamycin-induced L-LTP failure, suggesting mechanisms independent of eIF2α phosphorylation. Moreover, we demonstrate that phosphorylation of eukaryotic elongation factor 2 (eEF2) is significantly decreased in PERK KO mice but unaltered in GCN2 KO mice or slices treated with the PERK inhibitor. Reduction in eEF2 phosphorylation results in increased general protein synthesis, and thus could contribute to the mTORC1-independent L-LTP in PERK KO mice. We further performed experiments on mutant mice with genetic removal of eEF2K (eEF2K KO), the only known kinase for eEF2, and found that L-LTP in eEF2K KO mice is insensitive to rapamycin. These data, for the first time, connect reduction in PERK activity with the regulation of translation elongation in enabling L-LTP independent of mTORC1. Thus, our findings indicate previously unrecognized levels of complexity in the regulation of protein synthesis-dependent synaptic plasticity. Read the Editorial Highlight for this article on page 119. Cover Image for this issue: doi: 10.1111/jnc.14185.

MGlu5 positive allosteric modulation normalizes synaptic plasticity defects and motor phenotypes in a mouse model of Rett syndrome.

Rett syndrome (RS) is a neurodevelopmental disorder that shares many symptomatic and pathological commonalities with idiopathic autism. Alterations in protein synthesis-dependent synaptic plasticity (PSDSP) are a hallmark of a number of syndromic forms of autism; in the present work, we explore the consequences of disruption and rescue of PSDSP in a mouse model of RS. We report that expression of a key regulator of synaptic protein synthesis, the metabotropic glutamate receptor 5 (mGlu5) protein, is significantly reduced in both the brains of RS model mice and in the motor cortex of human RS autopsy samples. Furthermore, we demonstrate that reduced mGlu5 expression correlates with attenuated DHPG-induced long-term depression in the hippocampus of RS model mice, and that administration of a novel mGlu5 positive allosteric modulator (PAM), termed VU0462807, can rescue synaptic plasticity defects. Additionally, treatment of Mecp2-deficient mice with VU0462807 improves motor performance (open-field behavior and gait dynamics), corrects repetitive clasping behavior, as well as normalizes cued fear-conditioning defects. Importantly, due to the rationale drug discovery approach used in its development, our novel mGlu5 PAM improves RS phenotypes and synaptic plasticity defects without evoking the overt adverse effects commonly associated with potentiation of mGlu5 signaling (i.e. seizures), or affecting cardiorespiratory defects in RS model mice. These findings provide strong support for the continued development of mGlu5 PAMs as potential therapeutic agents for use in RS, and, more broadly, for utility in idiopathic autism.

REDD1 is essential for stress-induced synaptic loss and depressive behavior.

Major depressive disorder (MDD) affects up to 17% of the population, causing profound personal suffering and economic loss (1). Clinical and pre-clinical studies have revealed that prolonged stress and MDD are associated with neuronal atrophy of cortical and limbic brain regions (2-9), but the molecular mechanisms underlying these morphological alterations have not yet been identified. Here, we show that stress increases levels of REDD1 (regulated in development and DNA damage responses 1), an inhibitor of mTORC1 (mammalian/mechanistic target of rapamycin complex 1) (10), in rat prefrontal cortex (PFC). This is concurrent with a decrease in phosphorylation of signaling targets of mTORC1, which is implicated in protein synthesis-dependent synaptic plasticity. We also found that REDD1 levels are increased in the postmortem PFC of human subjects with MDD relative to matched controls. Mutant mice with a deletion of REDD1 are resilient to the behavioral, synaptic, and mTORC1 signaling deficits caused by chronic unpredictable stress (CUS), while viral-mediated over expression of REDD1 in the rat PFC is sufficient to cause anxiety- and depressive-like behaviors and neuronal atrophy. Taken together, these postmortem and pre-clinical findings identify REDD1 as a critical mediator underlying the atrophy of neurons and depressive behavior caused by chronic stress exposure.

Sleep and protein synthesis-dependent synaptic plasticity: impacts of sleep loss and stress

Sleep has been ascribed a critical role in cognitive functioning. Several lines of evidence implicate sleep in the consolidation of synaptic plasticity and long-term memory. Stress disrupts sleep while impairing synaptic plasticity and cognitive performance. Here, we discuss evidence linking sleep to mechanisms of protein synthesis-dependent synaptic plasticity and synaptic scaling. We then consider how disruption of sleep by acute and chronic stress may impair these mechanisms and degrade sleep function.

Neurobiology of Rapid Acting Antidepressants: Role of BDNF and GSK-3β

Recent studies demonstrate that a single dose of ketamine, a glutamate NMDA receptor antagonist, produces rapid antidepressant actions in treatment-resistant patients, one of the most significant advances in the field of depression in recent years. Preclinical studies demonstrate that the rapid antidepressant actions of ketamine are mediated by the induction of synaptic proteins and increased number and function of new spine synapses in the prefrontal cortex (PFC; Duman and Aghajanian, 2012). In addition, the behavioral and synaptogenic effects of ketamine require brain-derived neurotrophic factor (BDNF) and stimulation of the mechanistic target of rapamycin complex 1 (mTORC1), which is involved in protein synthesis-dependent synaptic plasticity. Surprisingly, we have also found that scopolamine, a muscarinic receptor antagonist that produces rapid antidepressant effects, also increases mTORC1 and induces synaptogenesis in the PFC (Voleti et al, 2013). Although ketamine is an NMDA receptor antagonist, it paradoxically increases glutamate transmission in the PFC. This is thought to occur via blockade of tonic firing GABAergic interneurons, resulting in disinhibition of glutamate release. The resulting ‘burst of glutamate' contributes to activity-dependent release of BDNF and increased synaptogenesis (Duman and Aghajanian, 2012). A functional BDNF polymorphism, Val66Met, has provided an approach to examine the role of BDNF in depressed patients, as well as rodent models. We have found that the rapid actions of ketamine are blocked in mice with a knockin of the Met allele, which prevents the processing and release of BDNF (Liu et al, 2012). Moreover, depressed patients who are carriers of the Met allele show a significantly decreased response to ketamine, indicating that the Met allele is a marker of nonresponders (Laje et al, 2012). Another interesting update is related to the finding that the actions of ketamine require inhibition of glycogen synthase kinase-3β (GSK-3β) Beurel et al, 2011), a protein kinase implicated in the pathophysiology and treatment of depression and bipolar disorder. We have found that lithium or a selective GSK-3β inhibitor significantly enhances the effects of a low dose of ketamine (Liu et al, 2013), raising the possibility that a low-dose ketamine+lithium combination could be repeated with reduced risk of side effects. The ketamine studies highlight several novel concepts for the development of efficacious, rapid acting, and safe medications. First, ketamine and its acute actions (eg, the burst of glutamate) are transient (1–2 h), which avoids excitoxic damage; this differs from traditional approaches to develop long-lasting pharmacological agents. Second, in contrast to traditional approaches, one of the core pathophysiological changes underlying major depression—a loss of synaptic connectivity—is rapidly addressed. Third, the increased synaptic connections induced by a single dose of ketamine are long-lasting (7–10 days), indicating that it is possible to stimulate therapeutic adaptations that outlast the initial drug actions. In addition, preliminary studies demonstrate that ketamine can enhance other behaviors that require learning and synaptogenesis, notably the extinction of fear conditioning (unpublished). Together these studies demonstrate that ketamine produces rapid remodeling of synapses that control mood and emotion in the treatment of depression, and the feasibility of utilizing similar remodeling approaches for the treatment of other illnesses, such as PTSD.

The mTOR Signaling Pathway Regulates Pain-Related Synaptic Plasticity in Rat Entorhinal-Hippocampal Pathways

BackgroundOur previous work demonstrated that persistent peripheral nociception (PPN) leads to synaptic plasticity and functional changes in the rat hippocampus. The protein kinase mTOR is a critical regulator of protein synthesis-dependent synaptic plasticity in the hippocampus as well as synaptic plasticity associated with central and peripheral pain sensitization. We examined the role of mTOR signaling in pain-associated entorhinal cortex (EC) - hippocampal synaptic plasticity to reveal possible cellular mechanisms underlying the effects of chronic pain on cognition and emotion.ResultsSubcutaneous injection of bee venom (BV) into one hind paw to induce PPN resulted in sustained (> 8 h) mTOR phospho-activation and enhanced phosphorylation of the mTOR target p70 S6 kinase (S6K) in the hippocampus. The magnitude and duration of long-term potentiation (LTP) in both EC - dentate gyrus (DG) and EC - CA1 synaptic pathways were elevated in BV-treated rats as measured by microelectrode array recording. Moreover, the number of potentiated synapses in the hippocampus was markedly upregulated by BV-induced PPN. Both elevated mTOR-S6K signaling and enhanced LTP induced by BV injection were reversed by systemic injection of the mTOR inhibitor rapamycin (RAPA). Rats injected with BV exhibited markedly reduced ambulation and exploratory activity in the open field (signs of depression and anxiety) compared to controls, and these effects were also reversed by RAPA.ConclusionWe suggest that PPN-induced enhancement of synaptic plasticity in EC - hippocampal pathways and the behavioral effects of PPN are dependent on mTOR-S6K signaling.

Regulation of Presynaptic Neurotransmission by Macroautophagy

mTOR is a regulator of cell growth and survival, protein synthesis-dependent synaptic plasticity, and autophagic degradation of cellular components. When triggered by mTOR inactivation, macroautophagy degrades long-lived proteins and organelles viasequestration into autophagic vacuoles. mTOR further regulates synaptic plasticity, and neurodegeneration occurs when macroautophagy is deficient. It is nevertheless unknown whether macroautophagy modulates presynaptic function. We find that the mTOR inhibitor rapamycin induces formation of autophagic vacuoles in prejunctional dopaminergic axons with associated decreased axonal profile volumes, synaptic vesicle numbers, and evoked dopamine release. Evoked dopamine secretion was enhanced and recovery was accelerated intransgenic mice in which macroautophagy deficiency was restricted to dopaminergic neurons; rapamycin failed to decrease evoked dopamine release in the striatum of these mice. Macroautophagy that follows mTOR inhibition in presynaptic terminals, therefore, rapidly alters presynaptic structure and neurotransmission.

Against addiction: light at the end of the tunnel?

Drug addiction is the among the most costly brain diseases, totaling 57 billion euros in direct and indirect costs in Europe alone. It is a disease without cure. Although it is impossible to say how far off we are from seeing the light at the end of the tunnel, substantial advances have been made over the last decade. The emerging model that addiction is a disease of ‘learning gone wrong’ is now widely accepted (Redish et al. 2008). Of particular interest in this context is the observation that all addictive drugs, and only addictive drugs, drive specific forms of synaptic plasticity in the mesolimbic system. Sometimes also referred to as the reward system, the mesolimbic system originates in the ventral tegmental area and projects to the nucleus accumbens (NAc). In a paper in a recent issue of The Journal of PhysiologyD’Ascenzo et al. (2009) point to an interesting mechanism for how the excitability of neurons in the NAc is enhanced by type 5 metabotropic glutamate receptors (mGluR5). These receptors are distinct from the fast ionotropic AMPA and NMDA receptors, as they activate Gq-proteins and the enzyme phospholipase C. As a result, a calcium signalling pathway is engaged, which recruits a sodium conductance that then changes the waveform of the action potential of these neurons. What might initially read like a story for specialists, suddenly becomes really exciting when the authors go on to show that this leads to an increase in the firing rates of neurons in the NAc. The excitability of neurons of the NAc has been shown to change in association with several drug treatments. It is also thought to drive behaviours that mimic core components of addiction such as the preference for the environment where drugs have been administered or cue-induced drug seeking, which is a model for relapse. The seminal observation that the mutant mice lacking mGluR5 are insensitive to cocaine reward (Chiamulera et al. 2001) suggests that these receptors are involved in drug addiction. However, the role of mGluR5s in the NAc, and therefore addiction, is likely to be more complex. In addition to enhancing the excitability of NAc neurons, they also drive the release of endocannabinoids. These lipophilic substances regulate, in turn, the amount of transmitter that is released by afferent neurons. The same mGluR5s also tap into the protein synthesis machinery to permanently change synaptic efficacy, a process referred to as protein synthesis-dependent synaptic plasticity. Regardless of the admittedly complicated functions of the mGluR5s, one might ask whether mGluRs could be good therapeutic targets to treat drug addiction. In fact, several experimental compounds are available. A handful of reports suggest that mGluR5 antagonists might reduce symptoms of drug addiction when directly applied to the NAc. However, when given systemically, mGluR5s are also inhibited elsewhere, which may have confounding effects. Moreover, mGluR1, a receptor of the same group of metabotropic receptors, seems to have the opposite effect. In fact, several studies suggest that mGluR1 enhancers may reverse cocaine-evoked synaptic plasticity (Mameli et al. 2007). These receptors therefore might have a protective role against addiction. Even if mGluR5s drive neuron excitability as suggested here, this may not explain how behavioural changes come about. A very recent study using sophisticated opto-genetic tools concludes that the main effect depends on Gq signalling (Airan et al. 2009). The authors combined the light-sensitive rhodopsin with the cytoplasmic portion of G-protein coupled receptors and subsequently transfected this chimeric protein into neurons of the NAc using viral vectors. When they then used fibre optics to shine blue light on the neurons of the NAc in a freely moving mouse in order to excite the fusion protein, it was the activation of the Gq pathway rather than increased neuronal firing per se that appeared to drive the addiction-related behaviour. These are, without doubt, exciting times and the (blue) light at the end of the tunnel may appear quicker than we think.

Protein synthesis dependent synaptic plasticity is competitive and dendritically compartmentalized

Event Abstract Back to Event Protein synthesis dependent synaptic plasticity is competitive and dendritically compartmentalized Inbal Israely1*, Arvind Govindarajan1, Shu-Ying Huang1 and Susumu Tonegawa1 1 Massachusetts Institute of Technology, The Picower Institute for Learning and Memory, RIKEN-MIT-Picower Center for Neural Circuit Genetics, Department of Biology and Department of Brain and Cognitive Sciences, United States Long-term memory (LTM) and a cellular correlate of LTM, the late-phase of long-term potentiation (L-LTP) require protein synthesis at the time of training and stimulation, respectively. While it is known that L-LTP induced at some synapses can facilitate L-LTP expression at other synapses, it is not know what the spatial and temporal limits are over which this process can occur at the single spine level, a fact that is crucial in determining if stable memory engrams are stored at synapses spatially clustered within a dendritic branch or dispersed throughout the dendritic arbor of a neuron. Here, using glutamate uncaging and two-photon imaging, we found that L-LTP at one spine facilitated L-LTP at neighboring spines; furthermore, this facilitative effect decreases with increasing time between stimulations, increasing distance between stimulated spines and with the spines being on different dendritic branches. Paradoxically, stimulated spines can also compete for L-LTP expression if stimulated too closely together in time. These local characteristics of L-LTP bias neurons towards the formation of memories in a clustered fashion. Conference: 11th Meeting of the Portuguese Society for Neuroscience, Braga, Portugal, 4 Jun - 6 Jun, 2009. Presentation Type: Poster Presentation Topic: Neuronal Communication Citation: Israely I, Govindarajan A, Huang S and Tonegawa S (2009). Protein synthesis dependent synaptic plasticity is competitive and dendritically compartmentalized. Front. Neurosci. Conference Abstract: 11th Meeting of the Portuguese Society for Neuroscience. doi: 10.3389/conf.neuro.01.2009.11.158 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 12 Aug 2009; Published Online: 12 Aug 2009. * Correspondence: Inbal Israely, Massachusetts Institute of Technology, The Picower Institute for Learning and Memory, RIKEN-MIT-Picower Center for Neural Circuit Genetics, Department of Biology and Department of Brain and Cognitive Sciences, Cambridge, MA, United States, inbal@mit.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Inbal Israely Arvind Govindarajan Shu-Ying Huang Susumu Tonegawa Google Inbal Israely Arvind Govindarajan Shu-Ying Huang Susumu Tonegawa Google Scholar Inbal Israely Arvind Govindarajan Shu-Ying Huang Susumu Tonegawa PubMed Inbal Israely Arvind Govindarajan Shu-Ying Huang Susumu Tonegawa Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

S6K1 Phosphorylates and Regulates Fragile X Mental Retardation Protein (FMRP) with the Neuronal Protein Synthesis-dependent Mammalian Target of Rapamycin (mTOR) Signaling Cascade

Fragile X syndrome is a common form of cognitive deficit caused by the functional absence of fragile X mental retardation protein (FMRP), a dendritic RNA-binding protein that represses translation of specific messages. Although FMRP is phosphorylated in a group I metabotropic glutamate receptor (mGluR) activity-dependent manner following brief protein phosphatase 2A (PP2A)-mediated dephosphorylation, the kinase regulating FMRP function in neuronal protein synthesis is unclear. Here we identify ribosomal protein S6 kinase (S6K1) as a major FMRP kinase in the mouse hippocampus, finding that activity-dependent phosphorylation of FMRP by S6K1 requires signaling inputs from mammalian target of rapamycin (mTOR), ERK1/2, and PP2A. Further, the loss of hippocampal S6K1 and the subsequent absence of phospho-FMRP mimic FMRP loss in the increased expression of SAPAP3, a synapse-associated FMRP target mRNA. Together these data reveal a S6K1-PP2A signaling module regulating FMRP function and place FMRP phosphorylation in the mGluR-triggered signaling cascade required for protein-synthesis-dependent synaptic plasticity.

Rescue of behavioral phenotype and neuronal protrusion morphology in Fmr1 KO mice

Lack of fragile X mental retardation protein (FMRP) causes Fragile X Syndrome, the most common form of inherited mental retardation. FMRP is an RNA-binding protein and is a component of messenger ribonucleoprotein complexes, associated with brain polyribosomes, including dendritic polysomes. FMRP is therefore thought to be involved in translational control of specific mRNAs at synaptic sites. In mice lacking FMRP, protein synthesis-dependent synaptic plasticity is altered and structural malformations of dendritic protrusions occur. One hypothesized cause of the disease mechanism is based on exaggerated group I mGluR receptor activation. In this study, we examined the effect of the mGluR5 antagonist MPEP on Fragile X related behavior in Fmr1 KO mice. Our results demonstrate a clear defect in prepulse inhibition of startle in Fmr1 KO mice, that could be rescued by MPEP. Moreover, we show for the first time a structural rescue of Fragile X related protrusion morphology with two independent mGluR5 antagonists.

Visual Experience Regulates Transient Expression and Dendritic Localization of Fragile X Mental Retardation Protein

Fragile X syndrome is the most common form of inherited mental retardation and is caused by the loss of function of the Fragile X mental retardation protein (FMRP). FMRP is an RNA binding protein thought to play a key role in protein synthesis-dependent synaptic plasticity. The regulation of FMRP expression itself is also likely to be an important control point in this process. Here we used dark-reared/light-exposed rats to determine the role of experience in regulating FMRP levels in the visual cortex. We find that FMRP levels increase in the cell bodies and dendrites of visual cortical neurons after as little as 15 min of light exposure. Remarkably, FMRP expression in these neurons returns to baseline levels by 30 min of light exposure. These changes were post-transcriptional because the FMR1 mRNA levels remained constant over this time period. A transient increase in FMRP levels was also observed in synaptic fractions prepared from visual cortices of light-exposed animals. In contrast, alpha-calcium/calmodulin-dependent kinase II expression showed a sustained upregulation under these conditions. Finally, the increase in FMRP expression was inhibited by blockade of NMDA receptors. This tight temporal-spatial regulation suggests that FMRP plays a dynamic role in a distinct epoch of experience-dependent synaptic plasticity.

Time-restricted role for dendritic activation of the mTOR-p70S6K pathway in the induction of late-phase long-term potentiation in the CA1.

Mammalian target of rapamycin (mTOR) is a key regulator of translational capacity. The mTOR inhibitor rapamycin can prevent forms of protein synthesis-dependent synaptic plasticity such as long-term facilitation in Aplysia and late-phase long-term potentiation (L-LTP) in the hippocampal CA1 region of rodents. In the latter model, two issues remain to be addressed: defining the L-LTP phase sensitive to rapamycin and identifying the site of rapamycin-sensitive protein synthesis. Here, we show that L-LTP is sensitive to application of rapamycin only during the induction paradigm, whereas rapamycin application after the establishment of L-LTP was ineffective. Second, we observed that Thr-389-phosphorylated p70 S6 kinase (p70S6K), the main active phosphoform of the mTOR effector p70S6K, was induced in an N-methyl-D-aspartate and phosphatidylinositol 3-kinase-dependent manner throughout the dendrites but not in the cell bodies of CA1 neurons in hippocampal slices after L-LTP induction. A similar dendrite-wide activation of p70S6K was induced in primary hippocampal neurons by depolarization with KCL or glutamate. In primary hippocampal neurons, the sites of dendritic activation of p70S6K appeared as discrete compartments along dendritic shafts like the hotspots for fast dendritic translation. Conversely, only a subset of dendritic spines also displayed activated p70S6K. Taken together, the present data suggest that the N-methyl-d-aspartate-, phosphatidylinositol 3-kinase-dependent dendritic activation of the mTOR-p70S6K pathway is necessary for the induction phase of protein synthesis-dependent synaptic plasticity. Newly synthesized proteins in dendritic shafts could be targeted selectively to activity-tagged synapses. Thus, coordinated activation of dendrite-wide translation and synaptic-specific activation is likely to be necessary for long-term synaptic plasticity.

Altered synaptic plasticity in a mouse model of fragile X mental retardation.

Fragile X syndrome, the most common inherited form of human mental retardation, is caused by mutations of the Fmr1 gene that encodes the fragile X mental retardation protein (FMRP). Biochemical evidence indicates that FMRP binds a subset of mRNAs and acts as a regulator of translation. However, the consequences of FMRP loss on neuronal function in mammals remain unknown. Here we show that a form of protein synthesis-dependent synaptic plasticity, long-term depression triggered by activation of metabotropic glutamate receptors, is selectively enhanced in the hippocampus of mutant mice lacking FMRP. This finding indicates that FMRP plays an important functional role in regulating activity-dependent synaptic plasticity in the brain and suggests new therapeutic approaches for fragile X syndrome.