Fragile X syndrome (FXS) is caused by loss of the FMR1 gene product FMRP, a repressor of mRNA translation. According to the mGluR theory of FXS, excessive protein synthesis downstream of metabotropic glutamate receptor 5 (mGluR5) activation causes the synaptic pathophysiology that underlies multiple aspects of fragile X syndrome (FXS). Here, we utilize an in vitro assay of protein synthesis in the hippocampus of male Fmr1 KO mice to explore the molecular mechanisms involved in this core biochemical phenotype under conditions where aberrant synaptic physiology has been observed. We find that elevated basal protein synthesis in Fmr1 KO mice is selectively reduced to wild type (WT) levels by acute inhibition of mGluR5 or ERK1/2, but not by inhibition of mTOR. The mGluR5-ERK1/2 pathway is not constitutively overactive in the Fmr1 KO, however, suggesting that mRNA translation is hypersensitive to basal ERK1/2 activation in the absence of FMRP. We find that hypersensitivity to ERK1/2 pathway activation also contributes to audiogenic seizure susceptibility in the Fmr1 KO. These results suggest that the ERK1/2 pathway, and other neurotransmitter systems that stimulate protein synthesis via ERK1/2, represent additional therapeutic targets for FXS.

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a leading known cause of autism. It is caused by loss of expression of the fragile X mental retardation protein (FMRP), an RNA-binding protein that negatively regulates protein synthesis. In neurons, multiple lines of evidence suggest that protein synthesis at synapses is triggered by activation of group 1 metabotropic glutamate receptors (Gp1 mGluRs) and that many functional consequences of activating these receptors are altered in the absence of FMRP. These observations have led to the theory that exaggerated protein synthesis downstream of Gp1 mGluRs is a core pathogenic mechanism in FXS. This excess can be corrected by reducing signaling by Gp1 mGluRs, and numerous studies have shown that inhibition of mGluR5, in particular, can ameliorate multiple mutant phenotypes in animal models of FXS. Clinical trials based on this therapeutic strategy are currently under way. FXS is therefore poised to be the first neurobehavioral disorder in which corrective treatments have been developed from the bottom up: from gene identification to pathophysiology in animals to novel therapeutics in humans. The insights gained from FXS and other autism-related single-gene disorders may also assist in identifying molecular mechanisms and potential treatment approaches for idiopathic autism.

Previous studies have reported the presence of migrating and dividing neuronal progenitors in the subventricular zone (SVZ) and rostral migratory stream (RMS) of the postnatal mammalian brain. Although the behaviour of these progenitors is thought to be influenced by local signals, the nature and mode of action of the local signals are largely unknown. One of the signalling molecules known to affect the behaviour of embryonic neurons is the neurotransmitter GABA. In order to determine whether GABA affects neuronal progenitors via the activation of specific receptors, we performed cell‐attached, whole‐cell and gramicidin perforated patch‐clamp recordings of progenitors in postnatal mouse brain slices containing either the SVZ or the RMS. Recorded cells displayed a morphology typical of migrating neuronal progenitors had depolarized zero‐current resting potentials, and lacked action potentials. A subset of progenitors contained GABA and stained positive for glutamic acid decarboxylase 67 (GAD‐67) as shown by immunohistochemistry. In addition, every neuronal progenitor responded to GABA via picrotoxin‐sensitive GABAA receptor (GABAAR) activation. GABAARs displayed an ATP‐dependent rundown and a low sensitivity to Zn2+. GABA responses were sensitive to benzodiazepine agonists, an inverse agonist, as well as a barbiturate agonist. While GABA was hyperpolarizing at the zero‐current resting potentials, it was depolarizing at the cell resting potentials estimated from the reversal potential of K+ currents through a cell‐attached patch. Thus, our study demonstrates that neuronal progenitors of the SVZ/RMS contain GABA and are depolarized by GABA, which may constitute the basis for a paracrine signal among neuronal progenitors to dynamically regulate their proliferation and/or migration.

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Among the hallmark phenotypes reported in individuals with fragile X syndrome (FXS) are deficits in attentional function, inhibitory control, and cognitive flexibility, a set of cognitive skills thought to be associated with the prefrontal cortex (PFC). However, despite substantial clinical research into these core deficits, the PFC has received surprisingly little attention in preclinical research, particularly in animal models of FXS. In this study, we sought to investigate the molecular, cellular, and behavioral consequences of the loss of the fragile X mental retardation protein in the PFC of Fmr1 KO mice, a mouse model of FXS. We identify a robust cognitive impairment in these mice that may be related to the deficits in cognitive flexibility observed in individuals with FXS. In addition, we report that levels of proteins involved in synaptic function, including the NMDA receptor subunits NR1, NR2A, and NR2B; the scaffolding proteins PSD-95 and SAPAP3; and the plasticity-related gene Arc, are decreased in the prefrontal cortex of Fmr1 KO mice and are partly correlated with behavioral performance. Finally, we report that expression of c-Fos, a marker of neuronal activity, is decreased in the PFC of Fmr1 KO mice. Together, these data suggest that Fmr1 KO mice may represent a valuable animal model for the PFC-associated molecular, cellular, and behavioral abnormalities in FXS and that this model may be useful for testing the efficacy of therapeutic strategies aimed at treating the cognitive impairments in FXS.is the most common form of inherited mental retardation and a leading known cause of autism (1). It is caused by loss of the Fmr1 gene product fragile X mental retardation protein (FMRP), an mRNA-binding protein involved in translational regulation (2, 3). FMRP is thought to repress the synthesis of proteins required for protein synthesisdependent synaptic plasticity (4, 5). In FXS, the absence of FMRP is hypothesized to result in unrestricted synthesis of plasticity-related proteins (6, 7), impairing the ability of synapses to appropriately undergo plasticity in an activity-dependent and stimulus-specific manner. In support of this hypothesis, mice with a deletion in the Fmr1 gene (Fmr1 KO mice) display aberrant forms of plasticity (4) and an increase in immature dendritic spines that presumably reflects an abnormal synaptic connectivity (8). Together, these synaptic alterations are thought to underlie the cognitive and behavioral phenotypes that are the hallmark features of FXS.Among the most common symptoms reported in FXS are deficits in attentional function, inhibitory control, and cognitive flexibility (9), cognitive skills that have all been linked to the prefrontal cortex (PFC) and associated fronto-striatal networks (10, 11). Anatomical and imaging studies of individuals with FXS have identified structural alterations in PFC, and numerous fMRI studies have shown aberrant patterns of neural activity in fronto-striatal pathways during cognitive tasks (12). Together, all of these results suggest that...

IgSF9b forms a novel subsynaptic domain for adhesion that links to the gephyrin- and GABAA receptor–containing domain to promote inhibitory synaptic development.

Previous studies have reported the presence of neuronal progenitors in the subventricular zone (SVZ) and rostral migratory stream (RMS) of the postnatal mammalian brain. Although many studies have examined the survival and migration of progenitors after transplantation and the factors influencing their proliferation or differentiation, no information is available on the electrophysiological properties of these progenitors in a near-intact environment. Thus we performed whole cell and cell-attached patch-clamp recordings of progenitors in brain slices containing either the SVZ or the RMS from postnatal day 15 to day 25 mice. Both regions displayed strong immunoreactivity for nestin and neuron-specific class III beta-tubulin, and recorded cells displayed a morphology typical of the neuronal progenitors known to migrate throughout the SVZ and RMS to the olfactory bulb. Recorded progenitors had depolarized zero-current resting potentials (mean more depolarized than -28 mV), very high input resistances (about 4 GOmega), and lacked action potentials. Using the reversal potential of K+ currents through a cell-attached patch a mean resting potential of -59 mV was estimated. Recorded progenitors displayed Ca2+-dependent K+ currents and TEA-sensitive-delayed rectifying K+ (KDR) currents, but lacked inward K+ currents and transient outward K+ currents. KDR currents displayed classical kinetics and were also sensitive to 4-aminopyridine and alpha-dendrotoxin, a blocker of Kv1 channels. Na+ currents were found in about 60% of the SVZ neuronal progenitors. No developmental changes were observed in the passive membrane properties and current profile of neuronal progenitors. Together these data suggest that SVZ neuronal progenitors display passive membrane properties and an ionic signature distinct from that of cultured SVZ neuronal progenitors and mature neurons.

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