Visual experience regulates metabotropic glutamate receptor-mediated plasticity of AMPA receptor synaptic transmission by homer1a induction

Remembering and forgetting in sleep: Selective synaptic plasticity during sleep driven by scaling factors Homer1a and Arc

Sleep is a conserved and essential process that supports learning and memory. Synapses are a major target of sleep function and a locus of sleep need. Evidence in the literature suggests that the need for sleep has a cellular or microcircuit level basis, and that sleep need can accumulate within localized brain regions as a function of waking activity. Activation of sleep promoting kinases and accumulation of synaptic phosphorylation was recently shown to be part of the molecular basis for the localized sleep need. A prominent hypothesis in the field suggests that some benefits of sleep are mediated by a broad but selective weakening, or scaling-down, of synaptic strength during sleep in order to offset increased excitability from synaptic potentiation during wake. The literature also shows that synapses can be strengthened during sleep, raising the question of what molecular mechanisms may allow for selection of synaptic plasticity types during sleep. Here I describe mechanisms of action of the scaling factors Arc and Homer1a in selective plasticity and links with sleep need. Arc and Homer1a are induced in neurons in response to waking neuronal activity and accumulate with time spent awake. I suggest that during sleep, Arc and Homer1a drive broad weakening of synapses through homeostatic scaling-down, but in a manner that is sensitive to the plasticity history of individual synapses, based on patterned phosphorylation of synaptic proteins. Therefore, Arc and Homer1a may offer insights into the intricate links between a cellular basis of sleep need and memory consolidation during sleep.

The Homer1 family of proteins at the crossroad of dopamine-glutamate signaling: An emerging molecular "Lego" in the pathophysiology of psychiatric disorders. A systematic review and translational insight

Once considered only scaffolding proteins at glutamatergic postsynaptic density (PSD), Homer1 proteins are increasingly emerging as multimodal adaptors that integrate different signal transduction pathways within PSD, involved in motor and cognitive functions, with putative implications in psychiatric disorders. Regulation of type I metabotropic glutamate receptor trafficking, modulation of calcium signaling, tuning of long-term potentiation, organization of dendritic spines' growth, as well as meta- and homeostatic plasticity control are only a few of the multiple endocellular and synaptic functions that have been linked to Homer1. Findings from preclinical studies, as well as genetic studies conducted in humans, suggest that both constitutive (Homer1b/c) and inducible (Homer1a) isoforms of Homer1 play a role in the neurobiology of several psychiatric disorders, including psychosis, mood disorders, neurodevelopmental disorders, and addiction. On this background, Homer1 has been proposed as a putative novel target in psychopharmacological treatments. The aim of this review is to summarize and systematize the growing body of evidence on Homer proteins, highlighting the role of Homer1 in the pathophysiology and therapy of mental diseases.

Mechanisms of Plasticity in Subcortical Visual Areas

Visual plasticity is classically considered to occur essentially in the primary and secondary cortical areas. Subcortical visual areas such as the dorsal lateral geniculate nucleus (dLGN) or the superior colliculus (SC) have long been held as basic structures responsible for a stable and defined function. In this model, the dLGN was considered as a relay of visual information travelling from the retina to cortical areas and the SC as a sensory integrator orienting body movements towards visual targets. However, recent findings suggest that both dLGN and SC neurons express functional plasticity, adding unexplored layers of complexity to their previously attributed functions. The existence of neuronal plasticity at the level of visual subcortical areas redefines our approach of the visual system. The aim of this paper is therefore to review the cellular and molecular mechanisms for activity-dependent plasticity of both synaptic transmission and cellular properties in subcortical visual areas.

Regulation and Function of Activity-Dependent Homer in Synaptic Plasticity

Alterations in synaptic signaling and plasticity occur during the refinement of neural circuits over the course of development and the adult processes of learning and memory. Synaptic plasticity requires the rearrangement of protein complexes in the postsynaptic density (PSD), trafficking of receptors and ion channels and the synthesis of new proteins. Activity-induced short Homer proteins, Homer1a and Ania-3, are recruited to active excitatory synapses, where they act as dominant negative regulators of constitutively expressed, longer Homer isoforms. The expression of Homer1a and Ania-3 initiates critical processes of PSD remodeling, the modulation of glutamate receptor-mediated functions, and the regulation of calcium signaling. Together, available data support the view that Homer1a and Ania-3 are responsible for the selective, transient destabilization of postsynaptic signaling complexes to facilitate plasticity of the excitatory synapse. The interruption of activity-dependent Homer proteins disrupts disease-relevant processes and leads to memory impairments, reflecting their likely contribution to neurological disorders.

Genetic variability in scaffolding proteins and risk for schizophrenia and autism-spectrum disorders: a systematic review

Scaffolding proteins represent an evolutionary solution to controlling the specificity of information transfer in intracellular networks. They are highly concentrated in complexes located in specific subcellular locations. One of these complexes is the postsynaptic density of the excitatory synapses. There, scaffolding proteins regulate various processes related to synaptic plasticity, such as glutamate receptor trafficking and signalling, and dendritic structure and function. Most scaffolding proteins can be grouped into 4 main families: discs large (DLG), discs-large-associated protein (DLGAP), Shank and Homer. Owing to the importance of scaffolding proteins in postsynaptic density architecture, it is not surprising that variants in the genes that code for these proteins have been associated with neuropsychiatric diagnoses, including schizophrenia and autism-spectrum disorders. Such evidence, together with the clinical, neurobiological and genetic overlap described between schizophrenia and autism-spectrum disorders, suggest that alteration of scaffolding protein dynamics could be part of the pathophysiology of both. However, despite the potential importance of scaffolding proteins in these psychiatric conditions, no systematic review has integrated the genetic and molecular data from studies conducted in the last decade. This review has the following goals: to systematically analyze the literature in which common and/or rare genetic variants (single nucleotide polymorphisms, single nucleotide variants and copy number variants) in the scaffolding family genes are associated with the risk for either schizophrenia or autism-spectrum disorders; to explore the implications of the reported genetic variants for gene expression and/or protein function; and to discuss the relationship of these genetic variants to the shared genetic, clinical and cognitive traits of schizophrenia and autism-spectrum disorders.

Hippocampal Regulation of Postsynaptic Density Homer1 by Associative Learning

Genes involved in synaptic plasticity, particularly genes encoding postsynaptic density proteins, have been recurrently linked to psychiatric disorders including schizophrenia and autism. Postsynaptic density Homer1 proteins contribute to synaptic plasticity through the competing actions of short and long isoforms. The activity-induced expression of short Homer1 isoforms, Homer1a and Ania-3, is thought to be related to processes of learning and memory. However, the precise regulation of Homer1a and Ania-3 with different components of learning has not been investigated. Here, we used in situ hybridization to quantify short and long Homer1 expression in the hippocampus following consolidation, retrieval, and extinction of associative fear memory, using contextual fear conditioning in rats. Homer1a and Ania-3, but not long Homer1, were regulated by contextual fear learning or novelty detection, although their precise patterns of expression in hippocampal subregions were dependent on the isoform. We also show for the first time that the two short Homer1 isoforms are regulated after the retrieval and extinction of contextual fear memory, albeit with distinct temporal and spatial profiles. These findings support a role of activity-induced Homer1 isoforms in learning and memory processes in discrete hippocampal subregions and suggest that Homer1a and Ania-3 may play separable roles in synaptic plasticity.

Do group I metabotropic glutamate receptors mediate LTD?

Synapses undergo significant structural and functional reorganization in response to varying patterns of stimulation. These forms of plasticity are considered fundamental to cognition and neuronal homeostasis. An increasing number of reports highlight the importance of activity-dependent synaptic strengthening (long term potentiation: LTP) for learning. However, the functional significance of activity-dependent weakening of synapses (long term depression: LTD) remains relatively poorly understood. One form of synaptic weakening, induced by group I metabotropic glutamate receptors (mGluRs), has received significant attention from a mechanistic point of view and because of its augmentation in a murine model of Fragile X Syndrome. Yet, studies of this form of plasticity often yield confusing, contradictory results. These conflicting findings are likely attributable to the bulk stimulation and recording techniques often used to study synaptic plasticity (typically involving evoked extracellular recordings, which represent the summed activity of many synapses). Such studies inherently blur the identity of the synapses undergoing change, thus giving the illusion that synapses per se are being modified when in fact this may only be true of a specific subset of synapses. Indeed, studies employing minimal synaptic activation paint a fundamentally different picture of what is commonly called "mGluR-LTD". Here, I review the evidence in favour of group I mGluRs as mediators of various forms of synaptic downregulation and attempt to explain discrepancies in the literature. I argue that, while multiple forms of synaptic weakening may be triggered by these receptors, the canonical form of group I mGluR-mediated depression, mGluR-LTD, is in fact not a depression of basal synaptic responses. Rather, it is a reversal of established LTP and thus a form of depotentiation. Far from being arbitrary, this distinction has significant implications for the role of group I mGluRs in cognition, both in the healthy brain and in pathological conditions. Further, the differential actions of group I mGluRs at naïve and potentiated synapses suggest these receptors signal in a state-dependent manner to regulate various stages of the learning process.

Increased locomotor activity and non-selective attention and impaired learning ability in SD rats after lentiviral vector-mediated RNA interference of Homer 1a in the brain

Our previous studies found that Homer 1a, a scaffolding protein localized at the post-synaptic density (PSD) of glutamatergic excitatory synapses, is significantly down-regulated in the brain of spontaneous hypertensive rats (SHR), an animal model of attention deficit hyperactivity disorder (ADHD). Furthermore, a first-line treatment drug for ADHD, methylphenidate, can up-regulate the expression of Homer 1a. To investigate the possible role of Homer 1a in the etiology and pathogenesis of ADHD, a lentiviral vector containing miRNA specific for Homer 1a was constructed in this study. Intracerebroventricular injection of this vector into the brain of Sprague Dawley (SD) rats significantly decreased Homer 1a mRNA and protein expression levels. Compared to their negative controls, these rats displayed a range of abnormal behaviors, including increased locomotor activity and non-selective attention and impaired learning ability. Our results indicated that Homer 1a down-regulation results in deficits in control over behavioral output and learning similar to ADHD.

Synapse-specific and size-dependent mechanisms of spine structural plasticity accompanying synaptic weakening

Refinement of neural circuits in the mammalian cerebral cortex shapes brain function during development and in the adult. However, the signaling mechanisms underlying the synapse-specific shrinkage and loss of spiny synapses when neural circuits are remodeled remain poorly defined. Here, we show that low-frequency glutamatergic activity at individual dendritic spines leads to synapse-specific synaptic weakening and spine shrinkage on CA1 neurons in the hippocampus. We found that shrinkage of individual spines in response to low-frequency glutamate uncaging is saturable, reversible, and requires NMDA receptor activation. Notably, shrinkage of large spines additionally requires signaling through metabotropic glutamate receptors (mGluRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs), supported by higher levels of mGluR signaling activity in large spines. Our results support a model in which signaling through both NMDA receptors and mGluRs is required to drive activity-dependent synaptic weakening and spine shrinkage at large, mature dendritic spines when neural circuits undergo experience-dependent modification.

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.

Successful cognitive aging in rats: a role for mGluR5 glutamate receptors, homer 1 proteins and downstream signaling pathways

Normal aging is associated with impairments in cognition, especially learning and memory. However, major individual differences are known to exist. Using the classical Morris Water Maze (MWM) task, we discriminated a population of 24-months old Long Evans aged rats in two groups--memory-impaired (AI) and memory-unimpaired (AU) in comparison with 6-months old adult animals. AI rats presented deficits in learning, reverse memory and retention. At the molecular level, an increase in metabotropic glutamate receptors 5 (mGluR5) was observed in post-synaptic densities (PSD) in the hippocampus of AU rats after training. Scaffolding Homer 1b/c proteins binding to group 1 mGluR facilitate coupling with its signaling effectors while Homer 1a reduces it. Both Homer 1a and 1b/c levels were up-regulated in the hippocampus PSD of AU animals following MWM task. Using immunohistochemistry we further demonstrated that mGluR5 as well as Homer 1b/c stainings were enhanced in the CA1 hippocampus sub-field of AU animals. In fact mGluR5 and Homer 1 isoforms were more abundant and co-localized in the hippocampal dendrites in AU rats. However, the ratio of Homer 1a/Homer 1b/c bound to mGluR5 in the PSD was four times lower for AU animals compared to AI rats. Consequently, AU animals presented higher PKCγ, ERK, p70S6K, mTOR and CREB activation. Finally the expression of immediate early gene Arc/Arg3.1 was shown to be higher in AU rats in accordance with its role in spatial memory consolidation. On the basis of these results, a model of successful cognitive aging with a critical role for mGluR5, Homer 1 proteins and downstream signalling pathways is proposed here.

Single-cell electroporation in Xenopus

Single-cell electroporation (SCE) is a versatile technique for delivering electrically charged macromolecules including DNA, RNA, synthetic oligonucleotides, peptides, dyes, and drugs to individual cells within intact tissues. Here, we describe methods for in vivo-targeted electroporation of single tectal neurons within the albino Xenopus laevis tadpole. Focal electroporation is achieved using a pipette electrode filled with a solution of the delivery molecules and with a tip diameter much smaller than the width of the target cell. The small tip allows for localization of an electric field, which restricts pore formation to only the individual cell in direct contact with the tip. Thus, the small tip permits focal delivery of the charged molecules within the pipette into individual cells. Factors affecting the efficiency of SCE, as well as various applications of this technique, are discussed. Particular focus is directed toward combining SCE with in vivo two-photon microscopy for three-dimensional (3D) imaging of neuron growth and cell-autonomous effects of altered protein function.

Activity-dependent transcription of BDNF enhances visual acuity during development

In the developing Xenopus tadpole, conditioning with 20 min of visual stimulation leads to increased proBDNF protein levels in the tectum measured 4 hr later. Following conditioning, the ability to induce direction selectivity in tectal neurons, as well as both retinotectal long-term potentiation and depression, thought to underlie this phenomenon, was strongly facilitated. This facilitation was blocked by knockdown of BDNF expression in tectal neurons. Animals that had been exposed to visual conditioning and subsequently received normal visual input for 7-11 hr exhibited higher spatial frequency thresholds of tectal cell responses to counterphasing gratings than nonconditioned control animals. An improvement in visual acuity was confirmed by enhanced sensitivity to counterphasing gratings in a behavioral test. These results indicate that brief sensory stimulation, by initiating nuclear transcription and de novo protein synthesis of BDNF, can facilitate the refinement of response properties in the developing visual system.

Activity-regulated genes as mediators of neural circuit plasticity

Modifications of neuronal circuits allow the brain to adapt and change with experience. This plasticity manifests during development and throughout life, and can be remarkably long lasting. Evidence has linked activity-regulated gene expression to the long-term structural and electrophysiological adaptations that take place during developmental critical periods, learning and memory, and alterations to sensory map representations in the adult. In all these cases, the cellular response to neuronal activity integrates multiple tightly coordinated mechanisms to precisely orchestrate long-lasting, functional and structural changes in brain circuits. Experience-dependent plasticity is triggered when neuronal excitation activates cellular signaling pathways from the synapse to the nucleus that initiate new programs of gene expression. The protein products of activity-regulated genes then work via a diverse array of cellular mechanisms to modify neuronal functional properties. Synaptic strengthening or weakening can reweight existing circuit connections, while structural changes including synapse addition and elimination create new connections. Posttranscriptional regulatory mechanisms, often also dependent on activity, further modulate activity-regulated gene transcript and protein function. Thus, activity-regulated genes implement varied forms of structural and functional plasticity to fine-tune brain circuit wiring.

Learning to see: patterned visual activity and the development of visual function

To successfully interact with their environments, developing organisms need to correctly process sensory information and generate motor outputs appropriate to their size and structure. Patterned sensory experience has long been known to induce various forms of developmental plasticity that ultimately shape mature neural circuits. These same types of plasticity also allow developing organisms to respond appropriately to the external world by dynamically adapting neural circuit function to ongoing changes in brain circuitry and sensory input. Recent work on the visual systems of frogs and fish has provided an unprecedented view into how visual experience dynamically affects circuit function at many levels, ranging from gene expression to network function, ultimately leading to system-wide functional adaptations.

Early changes in Homer1 proteins in the spinal dorsal horn are associated with loose ligation of the rat sciatic nerve

BACKGROUND

Plasticity in the spinal dorsal horn is thought to underlie, at least in part, pain behavior after peripheral nerve injury. Homer1 proteins play an important role in synaptic plasticity through an activity-dependent remodeling of the postsynaptic density (PSD). In this study, we examined the early consequences of the loose ligation of the sciatic nerve on the levels of Homer1a and Homer1b/c proteins in the PSD of spinal dorsal horn neurons.

METHODS

Male rats were randomly assigned to control, sham-operated, or ligated groups. Four hours after sciatic exposure or ligation, the animals were anesthetized and killed. Dorsal horn ipsilateral and contralateral quadrants were homogenized and centrifuged to obtain a PSD-containing LP1 fraction. Homer1 isoforms were identified in Western immunoblots. In some animals, Homer1 small interfering RNA (siRNA), nontarget siRNA, MK-801, or U01026 was injected intrathecally before surgery to assess the effects of this treatment on the levels of Homer1 isoforms and on 2 signs of injury-associated pain behavior, a shift in weight-bearing distribution and thermal hyperalgesia.

RESULTS

In ligated animals, the protein levels of Homer1a increased and those of Homer1b/c decreased in the ipsilateral LP1 fraction of the spinal dorsal horn. In contrast, no changes were detected in the contralateral LP1 fraction of ligated animals or the ipsilateral or contralateral LP1 fraction of sham-operated animals. Intrathecal injections of Homer1 siRNA, but not nontarget siRNA, 2 h before the ligation prevented the accumulation of Homer1a and loss of Homer1b/c in the ipsilateral LP1 fraction. The same pretreatment with Homer1 siRNA also alleviated both a shift in weight-bearing behavior and thermal hyperalgesia in the ligated animals. Intrathecal injections of MK-801 or U0126 15 min before the ligation similarly prevented the injury-associated changes in Homer1 protein levels and the behavioral signs of pain.

CONCLUSION

The ligation-associated changes in the protein levels of Homer1a and Homer1b/c in the ipsilateral PSD of spinal dorsal horn neurons may be an important early reflection of the injury-associated plasticity that in time leads to the development of persistent pain.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

From drugs to deprivation: a Bayesian framework for understanding models of psychosis

INTRODUCTION

Various experimental manipulations, usually involving drug administration, have been used to produce symptoms of psychosis in healthy volunteers. Different drugs produce both common and distinct symptoms. A challenge is to understand how apparently different manipulations can produce overlapping symptoms. We suggest that current Bayesian formulations of information processing in the brain provide a framework that maps onto neural circuitry and gives us a context within which we can relate the symptoms of psychosis to their underlying causes. This helps us to understand the similarities and differences across the common models of psychosis.

MATERIALS AND METHODS

The Bayesian approach emphasises processing of information in terms of both prior expectancies and current inputs. A mismatch between these leads us to update inferences about the world and to generate new predictions for the future. According to this model, what we experience shapes what we learn, and what we learn modifies how we experience things.

DISCUSSION

This simple idea gives us a powerful and flexible way of understanding the symptoms of psychosis where perception, learning and inference are deranged. We examine the predictions of the cognitive model in light of what we understand about the neuropharmacology of psychotomimetic drugs and thereby attempt to account for the common and the distinctive effects of NMDA receptor antagonists, serotonergic hallucinogens, cannabinoids and dopamine agonists.

CONCLUSION

By acknowledging the importance of perception and perceptual aberration in mediating the positive symptoms of psychosis, the model also provides a useful setting in which to consider an under-researched model of psychosis-sensory deprivation.

Homer proteins shape Xenopus optic tectal cell dendritic arbor development in vivo

Considerable evidence suggests that the Homer family of scaffolding proteins contributes to synaptic organization and function. We investigated the role of both Homer 1b, the constitutively expressed, and developmentally regulated form of Homer, and Homer 1a, the activity-induced immediate early gene, in dendritic arbor elaboration and synaptic function of developing Xenopus optic tectal neurons. We expressed exogenous Homer 1a or Homer 1b in developing Xenopus tectal neurons. By collecting in vivo time lapse images of individual, EGFP-labeled and Homer-expressing neurons over 3 days, we found that Homer 1b leads to a significant decrease in dendritic arbor growth rate and arbor size. Synaptic transmission was also altered in developing neurons transfected with Homer 1b. Cells expressing exogenous Homer 1b over 3 days had a significantly greater AMPA to NMDA ratios, and increased AMPA mEPSC frequency. These data suggest that increasing Homer 1b expression increases excitatory synaptic inputs, increases synaptic maturation, and slows dendritic arbor growth rate. Exogenous Homer 1a expression increases AMPA mEPSC frequency, but did not significantly affect tectal cell dendritic arbor development. Changes in the ratio of Homer 1a to Homer 1b may signal the neuron that overall activity levels in the cell have changed, and this in turn could affect protein interactions at the synapse, synaptic transmission, and structural development of the dendritic arbor.

Disassembly of shank and homer synaptic clusters is driven by soluble beta-amyloid(1-40) through divergent NMDAR-dependent signalling pathways

Disruption of the postsynaptic density (PSD), a network of scaffold proteins located in dendritic spines, is thought to be responsible for synaptic dysfunction and loss in early-stage Alzheimer's disease (AD). Extending our previous demonstration that derangement of the PSD by soluble amyloid-β (Aβ) involves proteasomal degradation of PSD-95, a protein important for ionotropic glutamate receptor trafficking, we now show that Aβ also disrupts two other scaffold proteins, Homer1b and Shank1, that couple PSD-95 with ionotropic and metabotropic glutamate receptors. Treatment of fronto-cortical neurons with soluble Aβ results in rapid (within 1 h) and significant thinning of the PSD, decreased synaptic levels of Homer1b and Shank1, and reduced synaptic mGluR1 levels. We show that de novo protein synthesis is required for the declustering effects of Aβ on Homer1b (but not Shank1) and that, in contrast to PSD-95, Aβ-induced Homer1b and Shank1 cluster disassembly does not depend on proteasome activity. The regulation of Homer1b and Shank1 by Aβ diverges in two other respects: i) whereas the activity of both NMDAR and VDCC is required for Aβ-induced declustering of Homer1b, Aβ-induced declustering of Shank1 only requires NMDAR activity; and ii) whereas the effects of Aβ on Homer1b involve engagement of the PI-3K pathway and calcineurin phosphatase (PP2B) activity, those on Shank1 involve activation of the ERK pathway. In summary, soluble Aβ recruits discrete signalling pathways to rapidly reduce the synaptic localization of major components of the PSD and to regulate the availability of mGluR1 in the synapse.

Metabotropic glutamate receptor 5 (mGluR5) antagonists attenuate cocaine priming- and cue-induced reinstatement of cocaine seeking

Accumulating evidence suggests that metabotropic glutamate receptors (mGluRs) are involved in both cocaine reinforcement and the reinstatement of cocaine-seeking behavior. In the present experiments, rats were trained to self-administer cocaine under fixed ratio (for cocaine priming-induced reinstatement) or second-order (for cocaine cue-induced reinstatement) schedules of reinforcement. Lever pressing was then extinguished followed by a reinstatement phase where operant responding was promoted by either cocaine itself or cocaine-associated light cues. Results indicated that systemic administration of the mGluR5 antagonists 2-methyl-6-(phenylethynyl)pyridine (MPEP: 1 and 3mg/kg i.p.) or 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP: 0.1 and 1mg/kg i.p.) dose-dependently attenuated reinstatement of drug seeking induced by a systemic priming injection of 10mg/kg cocaine. Systemic administration of MTEP (0.1 and 1mg/kg i.p.) also dose-dependently attenuated cocaine cue-induced reinstatement of drug seeking. Systemic administration of neither MPEP nor MTEP influenced the reinstatement of sucrose seeking, which indicates that the effects of these compounds on cocaine seeking were reinforcer specific. Additionally, administration of MPEP (1microg/0.5microl) into the nucleus accumbens shell, a brain region that plays a critical role in cocaine seeking, attenuated cocaine priming-induced reinstatement of drug seeking. These results add to a growing literature indicating that mGluR antagonists attenuate the reinstatement of cocaine seeking. Importantly, the current findings also suggest that activation of mGluR5s specifically in the nucleus accumbens shell may promote the reinstatement of cocaine seeking.

The serum response factor and a putative novel transcription factor regulate expression of the immediate-early gene Arc/Arg3.1 in neurons

The immediate-early effector gene Arc/Arg3.1 is robustly upregulated by synaptic activity associated with learning and memory. Here we show in primary cortical neuron culture that diverse stimuli induce Arc expression through new transcription. Searching for regulatory regions important for Arc transcription, we found nine DNaseI-sensitive nucleosome-depleted sites at this genomic locus. A reporter gene encompassing these sites responded to synaptic activity in an NMDA receptor-dependent manner, consistent with endogenous Arc mRNA. Responsiveness mapped to two enhancer regions approximately 6.5 kb and approximately 1.4 kb upstream of Arc. We dissected these regions further and found that the proximal enhancer contains a functional and conserved "Zeste-like" response element that binds a putative novel nuclear protein in neurons. Therefore, activity regulates Arc transcription partly by a novel signaling pathway. We also found that the distal enhancer has a functional and highly conserved serum response element. This element binds serum response factor, which is recruited by synaptic activity to regulate Arc. Thus, Arc is the first target of serum response factor that functions at synapses to mediate plasticity.

The RNA binding protein CPEB regulates dendrite morphogenesis and neuronal circuit assembly in vivo

Visual system development requires experience-dependent mechanisms that regulate neuronal structure and function, including dendritic arbor growth, synapse formation, and stabilization. Although RNA binding proteins have been shown to affect some forms of synaptic plasticity in adult animals, their role in the development of neuronal structure and functional circuitry is not clear. Using two-photon time-lapse in vivo imaging and electrophysiology combined with morpholino-mediated knockdown and expression of functional deletion mutants, we demonstrate that the mRNA binding protein, cytoplasmic polyadenylation element binding protein1 (CPEB1), affects experience-dependent neuronal development and circuit formation in the visual system of Xenopus laevis. These data indicate that sensory experience controls circuit development by regulating translational activity of mRNAs.

Developmental roles for Homer: more than just a pretty scaffold

Homer proteins are best known as scaffold proteins at the post-synaptic density where they facilitate synaptic signalling and are thought to be required for learning and memory. Evidence implicating Homer proteins in the development of the nervous system is also steadily accumulating. Homer is highly conserved and is expressed at key developmental time points in the nervous system of several species. Homer regulates intracellular calcium homeostasis, clustering and trafficking of receptors and proteins at the cytosolic surface of the plasma membrane, transcription and translation, and cytoskeletal organization. Each of these functions has obvious potential to regulate neuronal development, and indeed Homer is implicated in several pathologies associated with the developing nervous system. Current data justify more critical experimental approaches to the role of Homer in the developing nervous system and related neurological disorders.

Endogenous homer proteins regulate metabotropic glutamate receptor signaling in neurons

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are important neuronal mediators of postsynaptic signaling that influence synaptic strength, plasticity, and other factors. Regulation of group I mGluR localization and function by Homer proteins appears to be a viable means for neurons to fine-tune these processes. The presence of different Homer isoforms can act as a switch to reprioritize mGluR1 and mGluR5 signaling at the point of IP(3) receptor activation by promoting or reducing activation of specific downstream effectors. Furthermore, these Homer-dependent effects on mGluR signaling may mechanistically underlie many of the long-term changes in neuronal function associated with changes in Homer protein expression described in the recent literature. However, most studies focusing on mGluR regulation by Homer proteins used relatively long-term overexpression. Thus, a definitive demonstration of mGluR1/5 signal regulation by natively expressed Homer proteins has been elusive. I examined the ability of endogenous Homer 1a to alter mGluR signaling in rat sympathetic neurons and hippocampal autapses using pituitary adenylate cyclase activating peptide (PACAP) to induce native Homer 1a expression. In sympathetic neurons, both Homer 1a overexpression and PACAP treatment reversed the decrease in mGluR1-mediated calcium current modulation associated with Homer 2b expression. In hippocampal autapses, PACAP treatment uncoupled postsynaptic mGluR5 from EPSC inhibition, similar to the effect of Homer 1a overexpression. In both cases, RNA silencing of Homer 1a but not control RNA interference treatment prevented the PACAP effect, suggesting that it resulted specifically from native Homer 1a expression.

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

Group I metabotropic glutamate receptors (mGluRs) induce a form of long-term synaptic depression (mGluR-LTD) in area CA1 of the hippocampus that requires rapid protein synthesis. Although much is known about the mechanisms underlying mGluR-LTD, it is unclear how mGluRs couple to the effectors necessary for translation initiation. A clue comes from work in the mouse model of Fragile X syndrome [Fmr1 knock-out (KO) mice], where group 1 mGluR stimulation of protein synthesis is absent and mGluRs are less associated with the postsynaptic scaffolding protein Homer (Giuffrida et al., 2005). Here, we examined the role of Homer interactions in mGluR-LTD and mGluR signaling to protein synthesis machinery in wild-type and Fmr1 KO animals. A peptide that mimics the C-terminal tail of mGluR5 (mGluR5ct), shown previously to disrupt Homer interactions with mGluRs, blocks mGluR-LTD and mGluR-signaling to protein synthesis initiation in wild-type animals. Disruption of mGluR-Homer interactions selectively blocks mGluR activation of the phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR), but not ERK (extracellular signal-regulated kinase), pathway and translation of a 5' terminal oligopyrimidine tract containing mRNA, Elongation factor 1alpha. In Fmr1 KO mice, mGluR-LTD is insensitive to disruption of Homer interactions and mGluR activation of PI3K-mTOR is lost. Our results find specific roles for Homer in mGluR signaling and plasticity and suggest that reduced mGluR-Homer interactions in Fmr1 KO mice lead to a deficit in mGluR stimulation of translation initiation.

Homer 1a suppresses neocortex long-term depression in a cortical layer-specific manner

Homer1a/Vesl-1S is an activity-dependently induced member of the scaffold protein family Homer/Vesl, which is known to link group I metabotropic glutamate receptors (mGluRs) to endoplasmic calcium release channels and to regulate them. Here we studied roles of Homer 1a in inducing long-term depression (LTD) in rat visual cortex slices. Homer 1a protein was injected by diffusion from whole cell patch pipettes. In layer VI pyramidal cells, LTD was reduced in magnitude with Homer 1a. LTD in layer VI was suppressed by applying antagonists of mGluR5, a subtype of group I mGluRs expressed with higher density than mGluR1 in neocortex pyramidal cells, or inositol-1,4,5-triphosphate receptors (IP3Rs) but not that against N-methyl-d-aspartate receptors (NMDARs). In layer II/III or layer V, Homer 1a injection was unable to affect LTD, which is mostly dependent on NMDARs and not on group I mGluRs in these layers. To examine the effects of endogenous Homer 1a, electroconvulsive shock (ECS) was applied. Homer 1a thereby induced, as did Homer 1a injection, reduced LTD magnitude in layer VI pyramidal cells and failed to do so in layer II/III or layer V pyramidal cells. These results indicate that both exo- and endogenous Homer 1a suppressed LTD in a cortical layer-specific manner, and its layer-specificity may be explained by the high affinity of Homer 1a to group I mGluRs.

Select overexpression of homer1a in dorsal hippocampus impairs spatial working memory

Long Homer proteins forge assemblies of signaling components involved in glutamate receptor signaling in postsynaptic excitatory neurons, including those underlying synaptic transmission and plasticity. The short immediate-early gene (IEG) Homer1a can dynamically uncouple these physical associations by functional competition with long Homer isoforms. To examine the consequences of Homer1a-mediated "uncoupling" for synaptic plasticity and behavior, we generated forebrain-specific tetracycline (tet) controlled expression of Venus-tagged Homer1a (H1aV) in mice. We report that sustained overexpression of H1aV impaired spatial working but not reference memory. Most notably, a similar impairment was observed when H1aV expression was restricted to the dorsal hippocampus (HP), which identifies this structure as the principal cortical area for spatial working memory. Interestingly, H1aV overexpression also abolished maintenance of CA3-CA1 long-term potentiation (LTP). These impairments, generated by sustained high Homer1a levels, identify a requirement for long Homer forms in synaptic plasticity and temporal encoding of spatial memory.

Food-associated cues alter forebrain functional connectivity as assessed with immediate early gene and proenkephalin expression

Background

Cues predictive of food availability are powerful modulators of appetite as well as food-seeking and ingestive behaviors. The neurobiological underpinnings of these conditioned responses are not well understood. Monitoring regional immediate early gene expression is a method used to assess alterations in neuronal metabolism resulting from upstream intracellular and extracellular signaling. Furthermore, assessing the expression of multiple immediate early genes offers a window onto the possible sequelae of exposure to food cues, since the function of each gene differs. We used immediate early gene and proenkephalin expression as a means of assessing food cue-elicited regional activation and alterations in functional connectivity within the forebrain.

Results

Contextual cues associated with palatable food elicited conditioned motor activation and corticosterone release in rats. This motivational state was associated with increased transcription of the activity-regulated genes homer1a, arc, zif268, ngfi-b and c-fos in corticolimbic, thalamic and hypothalamic areas and of proenkephalin within striatal regions. Furthermore, the functional connectivity elicited by food cues, as assessed by an inter-regional multigene-expression correlation method, differed substantially from that elicited by neutral cues. Specifically, food cues increased cortical engagement of the striatum, and within the nucleus accumbens, shifted correlations away from the shell towards the core. Exposure to the food-associated context also induced correlated gene expression between corticostriatal networks and the basolateral amygdala, an area critical for learning and responding to the incentive value of sensory stimuli. This increased corticostriatal-amygdalar functional connectivity was absent in the control group exposed to innocuous cues.

Conclusion

The results implicate correlated activity between the cortex and the striatum, especially the nucleus accumbens core and the basolateral amygdala, in the generation of a conditioned motivated state that may promote excessive food intake. The upregulation of a number of genes in unique patterns within corticostriatal, thalamic, and hypothalamic networks suggests that food cues are capable of powerfully altering neuronal processing in areas mediating the integration of emotion, cognition, arousal, and the regulation of energy balance. As many of these genes play a role in plasticity, their upregulation within these circuits may also indicate the neuroanatomic and transcriptional correlates of extinction learning.

Homer 1a uncouples metabotropic glutamate receptor 5 from postsynaptic effectors

Metabotropic glutamate receptors (mGluRs) and Homer proteins play critical roles in neuronal functions including plasticity, nociception, epilepsy, and drug addiction. Furthermore, Homer proteins regulate mGluR1/5 function by acting as adapters and facilitating coupling to effectors such as the inositol triphosphate receptor. However, although Homer proteins and their interaction with mGluRs have been the subject of intense study, direct measurements of Homer-induced changes in postsynaptic mGluR-effector coupling have not been reported. This question was addressed here by examining glutamatergic excitatory postsynaptic currents (EPSCs) in rat autaptic hippocampal cultures. In most neurons, the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine strongly inhibited the EPSC acutely. This modulation occurred postsynaptically, was mediated primarily by mGluR5, and was inositol triphosphate receptor-dependent. Expression of the dominant negative, immediate early form of Homer, Homer 1a, strongly reduced EPSC modulation, but the W24A mutant of Homer 1a, which cannot bind mGluRs, had no effect. (S)-3,5-dihydroxyphenylglycine-mediated intracellular calcium responses in the processes of Homer 1a-expressing neurons were reduced compared with those in Homer 1a W24A-expressing cells. However, neither the distribution of mGluR5 nor the modulation of somatic calcium channels was altered by Homer 1a expression. These data demonstrate that Homer 1a can reduce mGluR5 coupling to postsynaptic effectors without relying on large changes in the subcellular distribution of the receptor. Thus, alteration of mGluR signaling by changes in Homer protein expression may represent a viable mechanism for fine-tuning synaptic strength in neurons.