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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., (Copyright © 2016 the authors 0270-6474/16/362131-17$15.00/0.)

Enhanced metabotropic glutamate receptor subunit 5 (mGluR5) function is causally associated with the pathophysiology of fragile X syndrome, a leading inherited cause of intellectual disability and autism. Here we provide evidence that altered mGluR5-Homer scaffolds contribute to mGluR5 dysfunction and phenotypes in the fragile X syndrome mouse model, Fmr1 knockout (Fmr1(-/y)). In Fmr1(-/y) mice, mGluR5 was less associated with long Homer isoforms but more associated with the short Homer1a. Genetic deletion of Homer1a restored mGluR5-long Homer scaffolds and corrected several phenotypes in Fmr1(-/y) mice, including altered mGluR5 signaling, neocortical circuit dysfunction and behavior. Acute, peptide-mediated disruption of mGluR5-Homer scaffolds in wild-type mice mimicked many Fmr1(-/y) phenotypes. In contrast, Homer1a deletion did not rescue altered mGluR-dependent long-term synaptic depression or translational control of target mRNAs of fragile X mental retardation protein, the gene product of Fmr1. Our findings reveal new functions for mGluR5-Homer interactions in the brain and delineate distinct mechanisms of mGluR5 dysfunction in a mouse model of cognitive dysfunction and autism.

Despite the pronounced neurological deficits associated with mental retardation and autism, it is unknown if altered neocortical circuit function occurs in these prevalent disorders. Here we demonstrate specific alterations in local synaptic connections, membrane excitability, and circuit activity of defined neuron types in sensory neocortex of the mouse model of Fragile X Syndrome-the Fmr1 knockout (KO). Overall, these alterations result in hyperexcitability of neocortical circuits in the Fmr1 KO. Specifically, we observe a substantial deficit in local excitatory drive ( approximately 50%) targeting fast-spiking (FS) inhibitory neurons in layer 4 of somatosensory, barrel cortex. This persists until at least 4 wk of age suggesting it may be permanent. In contrast, monosynaptic GABAergic synaptic transmission was unaffected. Overall, these changes indicate that local feedback inhibition in neocortical layer 4 is severely impaired in the Fmr1 KO mouse. An increase in the intrinsic membrane excitability of excitatory neurons may further contribute to hyperexcitability of cortical networks. In support of this idea, persistent neocortical circuit activity, or UP states, elicited by thalamic stimulation was longer in duration in the Fmr1 KO mouse. In addition, network inhibition during the UP state was less synchronous, including a 14% decrease in synchrony in the gamma frequency range (30-80 Hz). These circuit changes may be involved in sensory stimulus hypersensitivity, epilepsy, and cognitive impairment associated with Fragile X and autism.

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 Fmrl 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 Fmrl KO mice, and the AGS may be a result of this. But the specific cell types and brain regions underlying AGSs in the Fmrl KO are unknown. We used conditional deletion or expression of Fmrl in different cell populations to determine whether Fmrl deletion in those cells was sufficient or necessary, respectively, for the AGS phenotype in males. Our data indicate that Fmrl 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 Fmrl in glutamatergic neurons of the inferior colliculus is necessary for AGSs. When we demonstrate necessity, we show that Fmrl expression in either the larger population of VGlut2-expressing glutamatergic neurons or the smaller population of inferior collicular glutamatergic neurons--in an otherwise Fmrl KO mouse-- eliminates AGSs. Therefore, targeting these neuronal populations in FXS and autism may be part of a therapeutic strategy to alleviate sensory hypersensitivity. [ABSTRACT FROM AUTHOR]

Amyloid-beta protein precursor (APP) and metabolite levels are altered in fragile X syndrome (FXS) patients and in the mouse model of the disorder, Fmr1KO mice. Normalization of APP levels in Fmr1KO mice (Fmr1KO/APPHET mice) rescues many disease phenotypes. Thus, APP is a potential biomarker as well as therapeutic target for FXS. Hyperexcitability is a key phenotype of FXS. Herein, we determine the effects of APP levels on hyperexcitability in Fmr1KO brain slices. Fmr1KO/APPHET slices exhibit complete rescue of UP states in a neocortical hyperexcitability model and reduced duration of ictal discharges in a CA3 hippocampal model. These data demonstrate that APP plays a pivotal role in maintaining an appropriate balance of excitation and inhibition (E/I) in neural circuits. A model is proposed whereby APP acts as a rheostat in a molecular circuit that modulates hyperexcitability through mGluR5 and FMRP. Both over- and under-expression of APP in the context of the Fmr1KO increases seizure propensity suggesting that an APP rheostat maintains appropriate E/I levels but is overloaded by mGluR5-mediated excitation in the absence of FMRP. These findings are discussed in relation to novel treatment approaches to restore APP homeostasis in FXS. [ABSTRACT FROM AUTHOR]

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 Fmrl knock-out (KO) mouse, where pharmacological and genetic reduction of mGlu5 reverses many phenotypes. mGlu5 is less associated with its scaffolding protein Homer in Fmrl KO mice, and restoration of mGlu5-Homer interactions by genetic deletion of a short, dominant negative of Homer, Hla, rescues many phenotypes of Fmrl 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) that abrogates binding to Homer. Although FMRP levels were normal, mGlu5R/,i mice mimicked multiple phenotypes of Fmrl 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. [ABSTRACT FROM AUTHOR]

In the mouse model of Fragile X syndrome, the Fmrl 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 Fmrl "mosaic" mice, where Fmrl is deleted in about half of all neurons, and we found that loss of presynaptic, but not postsynaptic, Fmrl 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 Fmrl knock-out mice. Manipulations in release probability enabled both the mimic and rescue of the impaired function in this synaptic pathway. Loss of presynaptic Fmrl 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 Fmrl knock-out, suggesting a widespread role for presynaptic Fmrl 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. [ABSTRACT FROM AUTHOR]