Multiple autism-linked genes mediate synapse elimination via proteasomal degradation of a synaptic scaffold PSD-95.

The activity-dependent transcription factor myocyte enhancer factor 2 (MEF2) induces excitatory synapse elimination in mouse neurons, which requires fragile X mental retardation protein (FMRP), an RNA-binding protein implicated in human cognitive dysfunction and autism. We report here that protocadherin 10 (Pcdh10), an autism-spectrum disorders gene, is necessary for this process. MEF2 and FMRP cooperatively regulate the expression of Pcdh10. Upon MEF2 activation, PSD-95 is ubiquitinated by the ubiquitin E3 ligase murine double minute 2 (Mdm2) and then binds to Pcdh10, which links it to the proteasome for degradation. Blockade of the Pcdh10-proteasome interaction inhibits MEF2-induced PSD-95 degradation and synapse elimination. In FMRP-lacking neurons, elevated protein levels of eukaryotic translation elongation factor 1 α (EF1α), an Mdm2-interacting protein and FMRP target mRNA, sequester Mdm2 and prevent MEF2-induced PSD-95 ubiquitination and synapse elimination. Together, our findings reveal roles for multiple autism-linked genes in activity-dependent synapse elimination.

Pre- and postsynaptic MEF2C promote experience-dependent, input-specific development of cortical Layer 4 to Layer 2/3 excitatory synapses and regulate activity-dependent expression of synaptic cell adhesion molecules.

Experience and activity-dependent transcription is a candidate mechanism to mediate development and refinement of specific cortical circuits. Here we demonstrate that the activity-dependent transcription factor Myocyte-Enhancer Factor 2C (MEF2C) is required in both presynaptic layer 4 (L4) and postsynaptic L2/3 mouse (male and female) somatosensory (S1) cortical neurons for development of this specific synaptic connection. While postsynaptic deletion of Mef2c weakens L4 synaptic inputs, it has no effect on inputs from local L2/3, contralateral S1, or ipsilateral frontal/motor cortex. Similarly, homozygous, or heterozygous deletion of Mef2c in presynaptic L4 neurons weakens L4 to L2/3 excitatory synaptic inputs by decreasing presynaptic release probability. Postsynaptic MEF2C is specifically required during an early postnatal, experience-dependent, period for L4 to L2/3 synapse function and expression of transcriptionally active MEF2C (MEF2C-VP16) rescues weak L4 to L2/3 synaptic strength in sensory deprived mice. Together these results suggest that experience and/or activity-dependent transcriptional activation of MEF2C promotes development of L4 to L2/3 synapses. MEF2C regulated expression of many pre- and postsynaptic genes in postnatal cortical neurons. Interestingly, MEF2C was necessary for activity-dependent expression of many presynaptic genes, including those that function in transsynaptic adhesion and neurotransmitter release. This work provides mechanistic insight into the experience-dependent development of specific cortical circuits.Significance Statement Experience-driven neuronal activity is necessary for the development of synaptic connectivity of specific cortical circuits. Here we demonstrate that the activity-dependent transcription factor MEF2C is necessary for development of a specific synaptic connection between Layer (L)4 and L2/3 neurons in mouse somatosensory cortex. MEF2C is required in both presynaptic L4 and postsynaptic L2/3 neurons during an early postnatal and experience-dependent period for development of their connection. Our results suggest that sensory experience drives transcriptional activation of MEF2C to promote development of the L4 to L2/3 synaptic connection. We identify activity-dependent, MEF2C- regulated presynaptic genes that promote development of specific connections. This work provides insight into the mechanisms by which sensory experience determines development of cortical circuit connectivity.

Developmental trajectory and sex differences in auditory processing in a PTEN-deletion model of autism spectrum disorders.

Autism Spectrum Disorders (ASD) encompass a wide array of debilitating symptoms, including severe sensory deficits and abnormal language development. Sensory deficits early in development may lead to broader symptomatology in adolescents and adults. The mechanistic links between ASD risk genes, sensory processing and language impairment are unclear. There is also a sex bias in ASD diagnosis and symptomatology. The current study aims to identify the developmental trajectory and genotype- and sex-dependent differences in auditory sensitivity and temporal processing in a Pten-deletion (phosphatase and tensin homolog missing on chromosome 10) mouse model of ASD. Auditory temporal processing is crucial for speech recognition and language development and deficits will cause language impairments. However, very little is known about the development of temporal processing in ASD animal models, and if there are sex differences. To address this major gap, we recorded epidural electroencephalography (EEG) signals from the frontal (FC) and auditory (AC) cortex in developing and adult Nse-cre PTEN mice, in which Pten is deleted in specific cortical layers (layers III-V) (PTEN conditional knock-out (cKO). We quantified resting EEG spectral power distribution, auditory event related potentials (ERP) and temporal processing from awake and freely moving male and female mice. Temporal processing is measured using a gap-in-noise-ASSR (auditory steady state response) stimulus paradigm. The experimental manipulation of gap duration and modulation depth allows us to measure cortical entrainment to rapid gaps in sounds. Temporal processing was quantified using inter-trial phase clustering (ITPC) values that account for phase consistency across trials. The results show genotype differences in resting power distribution in PTEN cKO mice throughout development. Male and female cKO mice have significantly increased beta power but decreased high frequency oscillations in the AC and FC. Both male and female PTEN cKO mice show diminished ITPC in their gap-ASSR responses in the AC and FC compared to control mice. Overall, deficits become more prominent in adult (p60) mice, with cKO mice having significantly increased sound evoked power and decreased ITPC compared to controls. While both male and female cKO mice demonstrated severe temporal processing deficits across development, female cKO mice showed increased hypersensitivity compared to males, reflected as increased N1 and P2 amplitudes. These data identify a number of novel sensory processing deficits in a PTEN-ASD mouse model that are present from an early age. Abnormal temporal processing and hypersensitive responses may contribute to abnormal development of language function in ASD.

FMRP phosphorylation and interactions with Cdh1 regulate association with dendritic RNA granules and MEF2-triggered synapse elimination.

Fragile X Messenger Ribonucleoprotein (FMRP) is necessary for experience-dependent, developmental synapse elimination and the loss of this process may underlie the excess dendritic spines and hyperconnectivity of cortical neurons in Fragile X Syndrome, a common inherited form of intellectual disability and autism. Little is known of the signaling pathways that regulate synapse elimination and if or how FMRP is regulated during this process. We have characterized a model of synapse elimination in CA1 neurons of organotypic hippocampal slice cultures that is induced by expression of the active transcription factor Myocyte Enhancer Factor 2 (MEF2) and relies on postsynaptic FMRP. MEF2-induced synapse elimination is deficient in Fmr1 KO CA1 neurons, and is rescued by acute (24 h), postsynaptic and cell autonomous reexpression of FMRP in CA1 neurons. FMRP is an RNA binding protein that suppresses mRNA translation. Derepression is induced by posttranslational mechanisms downstream of metabotropic glutamate receptor signaling. Dephosphorylation of FMRP at S499 triggers ubiquitination and degradation of FMRP which then relieves translation suppression and promotes synthesis of proteins encoded by target mRNAs. Whether this mechanism functions in synapse elimination is not known. Here we demonstrate that phosphorylation and dephosphorylation of FMRP at S499 are both necessary for synapse elimination as well as interaction of FMRP with its E3 ligase for FMRP, APC/Cdh1. Using a bimolecular ubiquitin-mediated fluorescence complementation (UbFC) assay, we demonstrate that MEF2 promotes ubiquitination of FMRP in CA1 neurons that relies on activity and interaction with APC/Cdh1. Our results suggest a model where MEF2 regulates posttranslational modifications of FMRP via APC/Cdh1 to regulate translation of proteins necessary for synapse elimination.

Optimization of ribosome profiling using low-input brain tissue from fragile X syndrome model mice.

Dysregulated protein synthesis is a major underlying cause of many neurodevelopmental diseases including fragile X syndrome. In order to capture subtle but biologically significant differences in translation in these disorders, a robust technique is required. One powerful tool to study translational control is ribosome profiling, which is based on deep sequencing of mRNA fragments protected from ribonuclease (RNase) digestion by ribosomes. However, this approach has been mainly applied to rapidly dividing cells where translation is active and large amounts of starting material are readily available. The application of ribosome profiling to low-input brain tissue where translation is modest and gene expression changes between genotypes are expected to be small has not been carefully evaluated. Using hippocampal tissue from wide type and fragile X mental retardation 1 (Fmr1) knockout mice, we show that variable RNase digestion can lead to significant sample batch effects. We also establish GC content and ribosome footprint length as quality control metrics for RNase digestion. We performed RNase titration experiments for low-input samples to identify optimal conditions for this critical step that is often improperly conducted. Our data reveal that optimal RNase digestion is essential to ensure high quality and reproducibility of ribosome profiling for low-input brain tissue.

Audiogenic Seizures in the Fmr1 Knock-Out Mouse Are Induced by Fmr1 Deletion in Subcortical, VGlut2-Expressing Excitatory Neurons and Require Deletion in the Inferior Colliculus.

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and the leading monogenetic cause of autism. One symptom of FXS and autism is sensory hypersensitivity (also called sensory over-responsivity). Perhaps related to this, the audiogenic seizure (AGS) is arguably the most robust behavioral phenotype in the FXS mouse model-the Fmr1 knock-out (KO) mouse. Therefore, the AGS may be considered a mouse model of sensory hypersensitivity. Hyperactive circuits are hypothesized to underlie dysfunction in a number of brain regions in patients with FXS and Fmr1 KO mice, and the AGS may be a result of this. But the specific cell types and brain regions underlying AGSs in the Fmr1 KO are unknown. We used conditional deletion or expression of Fmr1 in different cell populations to determine whether Fmr1 deletion in those cells was sufficient or necessary, respectively, for the AGS phenotype in males. Our data indicate that Fmr1 deletion in glutamatergic neurons that express vesicular glutamate transporter 2 (VGlut2) and are located in subcortical brain regions is sufficient and necessary to cause AGSs. Furthermore, the deletion of Fmr1 in glutamatergic neurons of the inferior colliculus is necessary for AGSs. When we demonstrate necessity, we show that Fmr1 expression in either the larger population of VGlut2-expressing glutamatergic neurons or the smaller population of inferior collicular glutamatergic neurons-in an otherwise Fmr1 KO mouse-eliminates AGSs. Therefore, targeting these neuronal populations in FXS and autism may be part of a therapeutic strategy to alleviate sensory hypersensitivity.SIGNIFICANCE STATEMENT Sensory hypersensitivity in fragile X syndrome (FXS) and autism patients significantly interferes with quality of life. Audiogenic seizures (AGSs) are arguably the most robust behavioral phenotype in the FXS mouse model-the Fmr1 knockout-and may be considered a model of sensory hypersensitivity in FXS. We provide the clearest and most precise genetic evidence to date for the cell types and brain regions involved in causing AGSs in the Fmr1 knockout and, more broadly, for any mouse mutant. The expression of Fmr1 in these same cell types in an otherwise Fmr1 knockout eliminates AGSs indicating possible cellular targets for alleviating sensory hypersensitivity in FXS and other forms of autism.

Roles for Arc in metabotropic glutamate receptor-dependent LTD and synapse elimination: Implications in health and disease.

The Arc gene is robustly transcribed in specific neural ensembles in response to experience-driven activity. Upon induction, Arc mRNA is transported to dendrites, where it can be rapidly and locally translated by activation of metabotropic glutamate receptors (mGluR1/5). mGluR-induced dendritic synthesis of Arc is implicated in weakening or elimination of excitatory synapses by triggering endocytosis of postsynaptic AMPARs in both hippocampal CA1 and cerebellar Purkinje neurons. Importantly, CA1 neurons with experience-induced Arc mRNA are susceptible, or primed for mGluR-induced long-term synaptic depression (mGluR-LTD). Here we review mechanisms and function of Arc in mGluR-LTD and synapse elimination and propose roles for these forms of plasticity in Arc-dependent formation of sparse neural representations of learned experience. We also discuss accumulating evidence linking dysregulation of Arc and mGluR-LTD in human cognitive disorders such as intellectual disability, autism and Alzheimer's disease.

Local cortical circuit correlates of altered EEG in the mouse model of Fragile X syndrome.

Electroencephalogram (EEG) recordings in Fragile X syndrome (FXS) patients have revealed enhanced sensory responses, enhanced resting "gamma frequency" (30-100 Hz) activity, and a decreased ability for sensory stimuli to modulate cortical activity at gamma frequencies. Similar changes are observed in the FXS model mouse - the Fmr1 knockout. These alterations may become effective biomarkers for diagnosis and treatment of FXS. Therefore, it is critical to better understand what circuit properties underlie these changes. We employed Channelrhodopsin2 to optically activate local circuits in the auditory cortical region in brain slices to examine how changes in local circuit function may be related to EEG changes. We focused on layers 2/3 and 5 (L2/3 and L5). In Fmr1 knockout mice, light-driven excitation of L2/3 revealed hyperexcitability and increased gamma frequency power in both local L2/3 and L5 circuits. Moreover, there is increased synchrony in the gamma frequency band between L2/3 and L5. Hyperexcitability and increased gamma power were not observed in L5 with L5 light-driven excitation, indicating that these changes were layer-specific. A component of L2/3 network hyperexcitability is independent of ionotropic receptor mediated synaptic transmission and may be mediated by increased intrinsic excitability of L2/3 neurons. Finally, lovastatin, a candidate therapeutic compound for FXS that targets ERK signaling did not normalize changes in gamma activity. In conclusion, hyperactivity and increased gamma activity in local neocortical circuits, together with increased gamma synchrony between circuits, provide a putative substrate for EEG alterations observed in both FXS patients and the FXS mouse model.

FMRP-dependent Mdm2 dephosphorylation is required for MEF2-induced synapse elimination.

The Myocyte Enhancer Factor 2 (MEF2) transcription factors suppress an excitatory synapse number by promoting degradation of the synaptic scaffold protein, postsynaptic density protein 95 (PSD-95), a process that is deficient in the mouse model of Fragile X Syndrome, Fmr1 KO. How MEF2 activation results in PSD-95 degradation and why this is defective in Fmr1 KO neurons is unknown. Here we report that MEF2 induces a Protein phosphatase 2A (PP2A)-mediated dephosphorylation of murine double minute-2 (Mdm2), the ubiquitin E3 ligase for PSD-95, which results in nuclear export and synaptic accumulation of Mdm2 as well as PSD-95 degradation and synapse elimination. In Fmr1 KO neurons, Mdm2 is hyperphosphorylated, nuclear localized basally, and unaffected by MEF2 activation, which our data suggest due to an enhanced interaction with Eukaryotic Elongation Factor 1α (EF1α), whose protein levels are elevated in Fmr1 KO. Expression of a dephosphomimetic of Mdm2 rescues PSD-95 ubiquitination, degradation and synapse elimination in Fmr1 KO neurons. This work reveals detailed mechanisms of synapse elimination in health and a developmental brain disorder.

Palmitoylation and Membrane Binding of Arc/Arg3.1: A Potential Role in Synaptic Depression.

Activity-regulated cytoskeletal-associated protein (Arc, also known as activity-regulated gene 3.1 or Arg3.1) is induced in neurons in response to salient experience and neural activity and is necessary for activity-induced forms of synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), cellular substrates of learning and memory. The best-characterized function of Arc is enhancement of the endocytic internalization of AMPA receptors in dendritic spines, a process associated with LTD. Arc has also been implicated in the proteolytic processing of amyloid precursor protein on the surface of endosomes. To mediate these activities, Arc must associate with cellular membranes, but it is unclear whether Arc binds directly to the lipid bilayer or requires protein-protein interactions for membrane recruitment. In this study, we show that Arc associates with pure phospholipid vesicles in vitro and undergoes palmitoylation in neurons, a modification that allows it to insert directly into the hydrophobic core of the bilayer. The palmitoylated cysteines are clustered in a motif, 94CLCRC98, located in the N-terminal half of the protein, which has not yet been structurally characterized. Expression of Arc with three mutated cysteines in that motif cannot support synaptic depression induced by the activity-dependent transcription factor, MEF2 (myocyte enhancer factor 2), in contrast to wild-type Arc. Thus, it appears that palmitoylation regulates at least a subset of Arc functions in synaptic plasticity.

Selective Disruption of Metabotropic Glutamate Receptor 5-Homer Interactions Mimics Phenotypes of Fragile X Syndrome in Mice.

Altered function of the Gq-coupled, Group 1 metabotropic glutamate receptors, specifically mGlu5, is implicated in multiple mouse models of autism and intellectual disability. mGlu5 dysfunction has been most well characterized in the fragile X syndrome mouse model, the Fmr1 knock-out (KO) mouse, where pharmacological and genetic reduction of mGlu5 reverses many phenotypes. mGlu5 is less associated with its scaffolding protein Homer in Fmr1 KO mice, and restoration of mGlu5-Homer interactions by genetic deletion of a short, dominant negative of Homer, H1a, rescues many phenotypes of Fmr1 KO mice. These results suggested that disruption of mGlu5-Homer leads to phenotypes of FXS. To test this idea, we examined mice with a knockin mutation of mGlu5 (F1128R; mGlu5(R/R)) that abrogates binding to Homer. Although FMRP levels were normal, mGlu5(R/R) mice mimicked multiple phenotypes of Fmr1 KO mice, including reduced mGlu5 association with the postsynaptic density, enhanced constitutive mGlu5 signaling to protein synthesis, deficits in agonist-induced translational control, protein synthesis-independent LTD, neocortical hyperexcitability, audiogenic seizures, and altered behaviors, including anxiety and sensorimotor gating. These results reveal new roles for the Homer scaffolds in regulation of mGlu5 function and implicate a specific molecular mechanism in a complex brain disease. SIGNIFICANCE STATEMENT: Abnormal function of the metabotropic, or Gq-coupled, glutamate receptor 5 (mGlu5) has been implicated in neurodevelopmental disorders, including a genetic cause of intellectual disability and autism called fragile X syndrome. In brains of a mouse model of fragile X, mGlu5 is less associated with its binding partner Homer, a scaffolding protein that regulates mGlu5 localization to synapses and its ability to activate biochemical signaling pathways. Here we show that a mouse expressing a mutant mGlu5 that cannot bind to Homer is sufficient to mimic many of the biochemical, neurophysiological, and behavioral symptoms observed in the fragile X mouse. This work provides strong evidence that Homer-mGlu5 binding contributes to symptoms associated with neurodevelopmental disorders.

Activity-dependent FUS dysregulation disrupts synaptic homeostasis.

The RNA-binding protein fused-in-sarcoma (FUS) has been associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), two neurodegenerative disorders that share similar clinical and pathological features. Both missense mutations and overexpression of wild-type FUS protein can be pathogenic in human patients. To study the molecular and cellular basis by which FUS mutations and overexpression cause disease, we generated novel transgenic mice globally expressing low levels of human wild-type protein (FUS(WT)) and a pathological mutation (FUS(R521G)). FUS(WT) and FUS(R521G) mice that develop severe motor deficits also show neuroinflammation, denervated neuromuscular junctions, and premature death, phenocopying the human diseases. A portion of FUS(R521G) mice escape early lethality; these escapers have modest motor impairments and altered sociability, which correspond with a reduction of dendritic arbors and mature spines. Remarkably, only FUS(R521G) mice show dendritic defects; FUS(WT) mice do not. Activation of metabotropic glutamate receptors 1/5 in neocortical slices and isolated synaptoneurosomes increases endogenous mouse FUS and FUS(WT) protein levels but decreases the FUS(R521G) protein, providing a potential biochemical basis for the dendritic spine differences between FUS(WT) and FUS(R521G) mice.

Dysregulation of Mammalian Target of Rapamycin Signaling in Mouse Models of Autism.

The mammalian target of rapamycin (mTOR) is a central regulator of a diverse array of cellular processes, including cell growth, proliferation, autophagy, translation, and actin polymerization. Components of the mTOR cascade are present at synapses and influence synaptic plasticity and spine morphogenesis. A prevailing view is that the study of mTOR and its role in autism spectrum disorders (ASDs) will elucidate the molecular mechanisms by which mTOR regulates neuronal function under physiological and pathological conditions. Although many ASDs arise as a result of mutations in genes with multiple molecular functions, they appear to converge on common biological pathways that give rise to autism-relevant behaviors. Dysregulation of mTOR signaling has been identified as a phenotypic feature common to fragile X syndrome, tuberous sclerosis complex 1 and 2, neurofibromatosis 1, phosphatase and tensin homolog, and potentially Rett syndrome. Below are a summary of topics covered in a symposium that presents dysregulation of mTOR as a unifying theme in a subset of ASDs.

Vasoactive intestinal polypeptide (VIP)-expressing neurons in the suprachiasmatic nucleus provide sparse GABAergic outputs to local neurons with circadian regulation occurring distal to the opening of postsynaptic GABAA ionotropic receptors.

GABAergic synaptic transmission plays an important role in resetting and synchronizing circadian rhythms in the suprachiasmatic nucleus (SCN). Although the circadian modulation of intrinsic membrane currents and biochemical signaling have been examined in the SCN, the modulation of specific synaptic pathways within the SCN is unexplored. In addition, little is known about the functional properties of these pathways, including which ones involve GABAA receptors (GABAA-Rs). In brain slices obtained from mice, we examined synaptic responses originating from the SCN neurons expressing vasoactive intestinal peptide (VIP+ neurons). Focusing on the local projection within the ventromedial SCN, we found that VIP+ afferents provided input onto 49% of neurons with a preference for VIP-negative (VIP-) neurons. Responses were mediated by GABAA-Rs. The projection was sparsely connected and preferentially targeted a subset of SCN neurons unrelated to postsynaptic VIP expression. For most aspects of VIP+ network output, there was no circadian regulation. Excitability and spontaneous firing of the presynaptic VIP+ neurons were unchanged between day and night, and their network connectivity and synaptic function up through the evoked synaptic conductance were also unchanged. On the other hand, VIP+ input onto VIP- neurons became less inhibitory at night suggesting a postsynaptic alteration in the coupling of GABAA-R conductances to action potential firing. These data suggest that components of the VIP network and its synaptic output up through GABAA-R opening are invariant during the circadian cycle, but the effect on action potential firing is modulated by postsynaptic processes occurring after GABAA-R channel opening.

Enhancement of dynamin polymerization and GTPase activity by Arc/Arg3.1.

BACKGROUND: The Activity-regulated cytoskeleton-associated protein, Arc, is an immediate-early gene product implicated in various forms of synaptic plasticity. Arc promotes endocytosis of AMPA type glutamate receptors and regulates cytoskeletal assembly in neuronal dendrites. Its role in endocytosis may be mediated by its reported interaction with dynamin 2, a 100 kDa GTPase that polymerizes around the necks of budding vesicles and catalyzes membrane scission. METHODS: Enzymatic and turbidity assays are used in this study to monitor effects of Arc on dynamin activity and polymerization. Arc oligomerization is measured using a combination of approaches, including size exclusion chromatography, sedimentation analysis, dynamic light scattering, fluorescence correlation spectroscopy, and electron microscopy. RESULTS: We present evidence that bacterially-expressed His6-Arc facilitates the polymerization of dynamin 2 and stimulates its GTPase activity under physiologic conditions (37°C and 100mM NaCl). At lower ionic strength Arc also stabilizes pre-formed dynamin 2 polymers against GTP-dependent disassembly, thereby prolonging assembly-dependent GTP hydrolysis catalyzed by dynamin 2. Arc also increases the GTPase activity of dynamin 3, an isoform of implicated in dendrite remodeling, but does not affect the activity of dynamin 1, a neuron-specific isoform involved in synaptic vesicle recycling. We further show in this study that Arc (either His6-tagged or untagged) has a tendency to form large soluble oligomers, which may function as a scaffold for dynamin assembly and activation. CONCLUSIONS AND GENERAL SIGNIFICANCE: The ability of Arc to enhance dynamin polymerization and GTPase activation may provide a mechanism to explain Arc-mediated endocytosis of AMPA receptors and the accompanying effects on synaptic plasticity.

Postsynaptic FMRP promotes the pruning of cell-to-cell connections among pyramidal neurons in the L5A neocortical network.

Pruning of structural synapses occurs with development and learning. A deficit in pruning of cortical excitatory synapses and the resulting hyperconnectivity is hypothesized to underlie the etiology of fragile X syndrome (FXS) and related autistic disorders. However, clear evidence for pruning in neocortex and its impairment in FXS remains elusive. Using simultaneous recordings of pyramidal neurons in the layer 5A neocortical network of the wild-type (WT) mouse to observe cell-to-cell connections in isolation, we demonstrate here a specific form of "connection pruning." Connection frequency among pyramidal neurons decreases between the third and fifth postnatal weeks, indicating a period of connection pruning. Over the same interval in the FXS model mouse, the Fmr1 knock-out (KO), connection frequency does not decrease. Therefore, connection frequency in the fifth week is higher in the Fmr1 KO compared with WT, indicating a state of hyperconnectivity. These alterations are due to postsynaptic deletion of Fmr1. At early ages (2 weeks), postsynaptic Fmr1 promoted the maturation of cell-to-cell connections, but not their number. These findings indicate that impaired connection pruning at later ages, and not an excess of connection formation, underlies the hyperconnectivity in the Fmr1 KO mouse. FMRP did not appear to regulate synapses individually, but instead regulated cell-to-cell connectivity in which groups of synapses mediating a single cell-to-cell connection are uniformly removed, retained, and matured. Although we do not link connection pruning directly to the pruning of structurally defined synapses, this study nevertheless provides an important model system for studying altered pruning in FXS.

Postsynaptic mGluR5 promotes evoked AMPAR-mediated synaptic transmission onto neocortical layer 2/3 pyramidal neurons during development.

Both short- and long-term roles for the group I metabotropic glutamate receptor number 5 (mGluR5) have been examined for the regulation of cortical glutamatergic synapses. However, how mGluR5 sculpts neocortical networks during development still remains unclear. Using a single cell deletion strategy, we examined how mGluR5 regulates glutamatergic synaptic pathways in neocortical layer 2/3 (L2/3) during development. Electrophysiological measurements were made in acutely prepared slices to obtain a functional understanding of the effects stemming from loss of mGluR5 in vivo. Loss of postsynaptic mGluR5 results in an increase in the frequency of action potential-independent synaptic events but, paradoxically, results in a decrease in evoked transmission in two separate synaptic pathways providing input to the same pyramidal neurons. Synaptic transmission through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, but not N-methyl-d-aspartate (NMDA) receptors, is specifically decreased. In the local L2/3 pathway, the decrease in evoked transmission appears to be largely due to a decrease in cell-to-cell connectivity and not in the strength of individual cell-to-cell connections. This decrease in evoked transmission correlates with a decrease in the total dendritic length in a region of the dendritic arbor that likely receives substantial input from these two pathways, thereby suggesting a morphological correlate to functional alterations. These changes are accompanied by an increase in intrinsic membrane excitability. Our data indicate that total mGluR5 function, incorporating both short- and long-term processes, promotes the strengthening of AMPA receptor-mediated transmission in multiple neocortical pathways.

A target cell-specific role for presynaptic Fmr1 in regulating glutamate release onto neocortical fast-spiking inhibitory neurons.

In the mouse model of Fragile X syndrome, the Fmr1 knock-out, local excitation of layer 4 fast-spiking (FS) inhibitory neurons is robustly decreased by 50%, but the mechanisms mediating this change are unknown. Here, we performed recordings in acutely prepared slices obtained from Fmr1 "mosaic" mice, where Fmr1 is deleted in about half of all neurons, and we found that loss of presynaptic, but not postsynaptic, Fmr1 fully recapitulates the deficit. The change in connection strength is primarily due to a decrease in release probability indicating that FMRP normally positively regulates these processes. This change in presynaptic neurotransmitter release is observed both in the mosaic mice and in the constitutive Fmr1 knock-out mice. Manipulations in release probability enabled both the mimic and rescue of the impaired function in this synaptic pathway. Loss of presynaptic Fmr1 has no effect on excitatory synapses onto excitatory neurons, indicating a target cell-specific function for presynaptic FMRP. Finally, we demonstrate that the excitation decrement onto FS neurons also exists in layer 5 of the Fmr1 knock-out, suggesting a widespread role for presynaptic Fmr1 in the excitation of inhibitory neurons. In summary, we identify a novel function for presynaptic FMRP in promoting presynaptic neurotransmitter release, and we show that loss of this function accounts for impaired excitation of neocortical FS inhibitory neurons. These changes may contribute to the cognitive dysfunction and circuit hyperexcitability associated with Fragile X syndrome, including patients with complete deletion of FMRP and those with mosaic expression of FMRP.

Postsynaptic FMRP bidirectionally regulates excitatory synapses as a function of developmental age and MEF2 activity.

Rates of synapse formation and elimination change over the course of postnatal development, but little is known of molecular mechanisms that mediate this developmental switch. Here, we report that the dendritic RNA-binding protein fragile X mental retardation protein (FMRP) bidirectionally and cell autonomously regulates excitatory synaptic function, which depends on developmental age as well as function of the activity-dependent transcription factor myocyte enhancer factor 2 (MEF2). The acute postsynaptic expression of FMRP in CA1 neurons of hippocampal slice cultures (during the first postnatal week, P6-P7) promotes synapse function and maturation. In contrast, the acute expression of FMRP or endogenous FMRP in more mature neurons (during the second postnatal week; P13-P16) suppresses synapse number. The ability of neuronal depolarization to stimulate MEF2 transcriptional activity increases over this same developmental period. Knockout of endogenous MEF2 isoforms causes acute postsynaptic FMRP expression to promote, instead of eliminate, synapses onto 2-week-old neurons. Conversely, the expression of active MEF2 in neonatal neurons results in a precocious FMRP-dependent synapse elimination. Our findings suggest that FMRP and MEF2 function together to fine tune synapse formation and elimination rates in response to neuronal activity levels over the course of postnatal development.

Female specific dysfunction of sensory neocortical circuits in a mouse model of autism mediated by mGluR5 and Estrogen Receptor α.

Autism manifests differently in males and females and the brain mechanisms that mediate these sex-dependent differences are unknown. Here, we demonstrate that deletion of the ASD-risk gene, Pten, in neocortical pyramidal neurons (NSE Pten KO) results in robust hyperexcitability of local neocortical circuits in female, but not male, mice, observed as prolonged, spontaneous persistent activity states (UP states). Circuit hyperexcitability in NSE Pten KO mice is mediated by enhanced and/or altered signaling of metabotropic glutamate receptor 5 (mGluR5) and estrogen receptor α (ERα) to ERK and protein synthesis selectively in Pten deleted female neurons. In support of this idea, Pten deleted Layer 5 cortical neurons have female-specific increases in mGluR5 and mGluR5-driven protein synthesis. In addition, mGluR5-ERα complexes are elevated in female cortex and genetic reduction of ERα in Pten KO cortical neurons rescues circuit excitability, protein synthesis and enlarged neurons selectively in females. Abnormal timing and hyperexcitability of neocortical circuits in female NSE Pten KO mice are associated with deficits in temporal processing of sensory stimuli and social behaviors as well as mGluR5-dependent seizures. Female-specific cortical hyperexcitability and mGluR5-dependent seizures are also observed in a human disease relevant mouse model, germline Pten +/- mice. Our results reveal molecular mechanisms by which sex and a high impact ASD-risk gene interact to affect brain function and behavior.