Extinction of an instrumental response: a cognitive behavioral assay in Fmr1 knockout mice

Fragile X (FX) is the most common genetic cause of intellectual disability and autism. Previous studies have shown that partial inhibition of metabotropic glutamate receptor signaling is sufficient to correct behavioral phenotypes in a mouse model of FX, including audiogenic seizures, open-field hyperactivity and social behavior. These phenotypes model well the epilepsy (15%), hyperactivity (20%) and autism (30%) that are comorbid with FX in human patients. Identifying reliable and robust mouse phenotypes to model cognitive impairments is critical considering the 90% comorbidity of FX and intellectual disability. Recent work characterized a five-choice visuospatial discrimination assay testing cognitive flexibility, in which FX model mice show impairments associated with decreases in synaptic proteins in prefrontal cortex (PFC). In this study, we sought to determine whether instrumental extinction, another process requiring PFC, is altered in FX model mice, and whether downregulation of metabotropic glutamate receptor signaling pathways is sufficient to correct both visuospatial discrimination and extinction phenotypes. We report that instrumental extinction is consistently exaggerated in FX model mice. However, neither the extinction phenotype nor the visuospatial discrimination phenotype is corrected by approaches targeting metabotropic glutamate receptor signaling. This work describes a novel behavioral extinction assay to model impaired cognition in mouse models of neurodevelopmental disorders, provides evidence that extinction is exaggerated in the FX mouse model and suggests possible limitations of metabotropic glutamate receptor-based pharmacotherapy.

Neurobiology of autism gene products: towards pathogenesis and drug targets

RATIONALE

The genetic heterogeneity of autism spectrum disorders (ASDs) is enormous, and the neurobiology of proteins encoded by genes associated with ASD is very diverse. Revealing the mechanisms on which different neurobiological pathways in ASD pathogenesis converge may lead to the identification of drug targets.

OBJECTIVE

The main objective is firstly to outline the main molecular networks and neuronal mechanisms in which ASD gene products participate and secondly to answer the question how these converge. Finally, we aim to pinpoint drug targets within these mechanisms.

METHOD

Literature review of the neurobiological properties of ASD gene products with a special focus on the developmental consequences of genetic defects and the possibility to reverse these by genetic or pharmacological interventions.

RESULTS

The regulation of activity-dependent protein synthesis appears central in the pathogenesis of ASD. Through sequential consequences for axodendritic function, neuronal disabilities arise expressed as behavioral abnormalities and autistic symptoms in ASD patients. Several known ASD gene products have their effect on this central process by affecting protein synthesis intrinsically, e.g., through enhancing the mammalian target of rapamycin (mTOR) signal transduction pathway or through impairing synaptic function in general. These are interrelated processes and can be targeted by compounds from various directions: inhibition of protein synthesis through Lovastatin, mTOR inhibition using rapamycin, or mGluR-related modulation of synaptic activity.

CONCLUSIONS

ASD gene products may all feed into a central process of translational control that is important for adequate glutamatergic regulation of dendritic properties. This process can be modulated by available compounds but may also be targeted by yet unexplored routes.

Evidence for a common endocannabinoid-related pathomechanism in autism spectrum disorders

In this issue of Neuron, Földy et al. (2013) report that endocannabinoid-mediated signaling at inhibitory synapses is dysregulated in mouse models of autism-associated Neuroligin-3 mutations. These findings carry implications regarding the pathophysiology of autism spectrum disorders and the development of treatment strategies.

Toward fulfilling the promise of molecular medicine in fragile X syndrome

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.

Activation of mGluR5 induces rapid and long-lasting protein kinase D phosphorylation in hippocampal neurons

Metabotropic glutamate receptors (mGluRs), including mGluR5, play a central role in regulating the strength and plasticity of synaptic connections in the brain. However, the signaling pathways that connect mGluRs to their downstream effectors are not yet fully understood. Here, we report that stimulation of mGluR5 in hippocampal cultures and slices results in phosphorylation of protein kinase D (PKD) at the autophosphorylation site Ser-916. This phosphorylation event occurs within 30 s of stimulation, persists for at least 24 h, and is dependent on activation of phospholipase C and protein kinase C. Our data suggest that activation of PKD may represent a novel signaling pathway linking mGluR5 to its downstream targets. These findings have important implications for the study of the molecular mechanisms underlying mGluR-dependent synaptic plasticity.

Expression of PKC substrate proteins, GAP-43 and neurogranin, is downregulated by cAMP signaling and alterations in synaptic activity

Growth-associated protein 43 (GAP-43) and neurogranin are protein kinase C substrate proteins that are thought to play an important role in synaptic plasticity, but little is currently known about the mechanisms that may regulate their function at the synapse. In this study, we show that long-term elevation of intracellular cAMP levels in rat primary cortical cultures results in a persistent downregulation of GAP-43 and neurogranin, most likely at the transcriptional level. This effect may be at least partially mediated by protein kinase A, but is independent of protein kinase C activation. Moreover, it is mimicked and occluded by manipulations that alter the levels of spontaneous synaptic activity in primary cultures, such as bicuculline and tetrodotoxin. These data suggest that levels of GAP-43 and neurogranin are regulated by factors known to modulate synaptic strength, thus providing a potential mechanism by which protein kinase C signaling pathways and their substrates might contribute to synaptic function and/or plasticity.

Orbitofrontal cortex and cognitive-motivational impairments in psychostimulant addiction: evidence from experiments in the non-human primate

Addiction is characterized by compulsive drug use despite adverse consequences. The precise psychobiological changes that underlie the progression from casual use to loss of control over drug-seeking and drug-taking behavior are not well understood. Here we report that short-term cocaine exposure in monkeys is sufficient to produce both selective deficits in cognitive functions dependent on the orbitofrontal cortex (OFC) concurrent with enhancements in motivational processes involving limbic-striatal regions. Additional findings from behavioral studies and analyses of the synaptic proteome provide new behavioral and biochemical evidence that cocaine-induced neuroadaptations in cortical and subcortical brain regions result in dysfunctional decision-making abilities and loss of impulse control that in combination with enhancements of incentive motivation may contribute to the development of compulsive behavior in addiction.

Recent advances in neuroproteomics

The last few years have seen a rapid growth in the use of proteomic methods to study normal brain function. In addition, such methods have been used to analyze changes in protein expression associated with the onset and progression of neuronal disease. The field of neuroproteomics faces special challenges given the complex cellular and sub-cellular architecture of the central nervous system. This article presents a review of recent progress in studies of neuroproteomics, and highlights the strengths and limitations of current proteomic profiling technologies used in studies of neuronal protein expression.

Assessment of cognitive function in the heterozygous reeler mouse

RATIONALE

The heterozygous reeler mouse has been proposed as a genetic mouse model of schizophrenia based on several neuroanatomical and behavioral similarities between these mice and patients with schizophrenia. However, the effect of reelin haploinsufficiency on one of the cardinal symptoms of schizophrenia, the impairment of prefrontal-cortex-dependent cognitive function, has yet to be determined.

OBJECTIVE

Here, we investigated multiple aspects of cognitive function in heterozygous reeler mice that are known to be impaired in schizophrenic patients.

METHODS

Heterozygous reeler mice were assessed for (1) cognitive flexibility in an instrumental reversal learning task, (2) impulsivity in an inhibitory control task, (3) attentional function in a three-choice serial reaction time task, and (4) working memory in a delayed matching-to-position task.

RESULTS

No differences were found between heterozygous reeler mice and wild-type littermate controls in any prefrontal-related cognitive measures. However, heterozygous reeler mice showed deficits in the acquisition of two operant tasks, consistent with a role for reelin in certain forms of learning.

CONCLUSIONS

These findings suggest that heterozygous reeler mice may not be an appropriate model for the core prefrontal-dependent cognitive deficits observed in schizophrenia, but may model more general learning deficits that are associated with many psychiatric disorders.

Identification of Multiple HLA-DR-Restricted Epitopes of the Tumor-Associated Antigen CAMEL by CD4+Th1/Th2 Lymphocytes

CD4(+) Th cells play an important role in the induction and maintenance of adequate CD8(+) T cell-mediated antitumor responses. Therefore, identification of MHC class II-restricted tumor antigenic epitopes is of major importance for the development of effective immunotherapies with synthetic peptides. CAMEL and NY-ESO-ORF2 are tumor Ags translated in an alternative open reading frame from the highly homologous LAGE-1 and NY-ESO-1 genes, respectively. In this study, we investigated whether CD4(+) T cell responses could be induced in vitro by autologous, mature dendritic cells pulsed with recombinant CAMEL protein. The data show efficient induction of CAMEL-specific CD4(+) T cells with mixed Th1/Th2 phenotype in two healthy donors. Isolation of CD4(+) T cell clones from the T cell cultures of both donors led to the identification of four naturally processed HLA-DR-binding CAMEL epitopes: CAMEL(1-20), CAMEL(14-33), CAMEL(46-65), and CAMEL(81-102). Two peptides (CAMEL(1-20) and CAMEL(14-33)) also contain previously identified HLA class I-binding CD8(+) T cell epitopes shared by CAMEL and NY-ESO-ORF2 and are therefore interesting tools to explore for immunotherapy. Furthermore, two CD4(+) T cell clones that recognized the CAMEL(14-33) peptide with similar affinities were shown to differ in recognition of tumor cells. These CD4(+) T cell clones recognized the same minimal epitope and expressed similar levels of adhesion, costimulatory, and inhibitory molecules. TCR analysis demonstrated that these clones expressed identical TCR beta-chains, but different complementarity-determining region 3 loops of the TCR alpha-chains. Introduction of the TCRs into proper recipient cells should reveal whether the different complementarity-determining region 3 alpha loops are important for tumor cell recognition.

Biophysical properties and ionic signature of neuronal progenitors of the postnatal subventricular zone in situ

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.

GABA depolarizes neuronal progenitors of the postnatal subventricular zone via GABAA receptor activation

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.