Targeting Homer genes using adeno-associated viral vector: lessons learned from behavioural and neurochemical studies

Molecular mechanisms of alcohol's effects on the human body: A review and update

Alcohol consumption has been linked to numerous negative health outcomes although it has some beneficial effects on moderate dosages, the most severe of which being alcohol-induced hepatitis. The number of people dying from this liver illness has been shown to climb steadily over time, and its prevalence has been increasing. Researchers have found that alcohol consumption primarily affects the brain, leading to a wide range of neurological and psychological diseases. High-alcohol-consumption addicts not only experienced seizures, but also ataxia, aggression, social anxiety, and variceal hemorrhage that ultimately resulted in death, ascites, and schizophrenia. Drugs treating this liver condition are limited and can cause serious side effects like depression. Serine-threonine kinases, cAMP protein kinases, protein kinase C, ERK, RACK 1, Homer 2, and more have all been observed to have their signaling pathways disrupted by alcohol, and alcohol has also been linked to epigenetic changes. In addition, alcohol consumption induces dysbiosis by changing the composition of the microbiome found in the gastrointestinal tract. Although more studies are needed, those that have been done suggest that probiotics aid in keeping the various microbiota concentrations stable. It has been argued that reducing one's alcohol intake may seem less harmful because excessive drinking is a lifestyle disorder.

Frontal cortex genetic ablation of metabotropic glutamate receptor subtype 3 (mGlu3) impairs postsynaptic plasticity and modulates affective behaviors

Clinical and translational studies suggest that prefrontal cortex (PFC) dysregulation is a hallmark feature of several affective disorders. Thus, investigating the mechanisms involved in the regulation of PFC function and synaptic plasticity could aid in developing new medications. In recent years, the mGlu2 and mGlu3 subtypes of metabotropic glutamate (mGlu) receptors have emerged as exciting potential targets for the treatment of affective disorders, as mGlu2/3 antagonists exert antidepressant-like effects across many rodent models. Several recent studies suggest that presynaptic mGlu2 receptors may contribute to these effects by regulating excitatory transmission at synapses from the thalamus to the PFC. Interestingly, we found that mGlu3 receptors also inhibit excitatory drive to the PFC but act by inducing long-term depression (LTD) at amygdala-PFC synapses. It remains unclear, however, whether blockade of presynaptic, postsynaptic, or glial mGlu3 receptors contribute to long-term effects on PFC circuit function and antidepressant-like effects of mGlu2/3 antagonists. To address these outstanding questions, we leveraged transgenic Grm3fl/fl mice and viral-mediated gene transfer to genetically ablate mGlu3 receptors from pyramidal cells in the frontal cortex of adult mice of all sexes. Consistent with a role for mGlu3 in PFC pyramidal cells, mGlu3-dependent amygdala-cortical LTD was eliminated following mGlu3 receptor knockdown. Furthermore, knockdown mice displayed a modest, task-specific anxiolytic phenotype and decreased passive coping behaviors. These studies reveal that postsynaptic mGlu3 receptors are critical for mGlu3-dependent LTD and provide convergent genetic evidence suggesting that modulating cortical mGlu3 receptors may provide a promising new approach for the treatment of mood disorders.

Genomic analyses reveal distinct genetic architectures and selective pressures in buffaloes

BACKGROUND

The domestic buffalo (Bubalus bubalis) is an essential farm animal in tropical and subtropical regions, whose genomic diversity is yet to be fully discovered.

RESULTS

In this study, we describe the demographic events and selective pressures of buffalo by analyzing 121 whole genomes (98 newly reported) from 25 swamp and river buffalo breeds. Both uniparental and biparental markers were investigated to provide the final scenario. The ancestors of swamp and river buffalo diverged ∼0.23 million years ago and then experienced independent demographic histories. They were domesticated in different regions, the swamp buffalo at the border between southwest China and southeast Asia, while the river buffalo in south Asia. The domestic stocks migrated to other regions and further differentiated, as testified by (at least) 2 ancestral components identified in each subspecies. Different signals of selective pressures were also detected in these 2 types of buffalo. The swamp buffalo, historically used as a draft animal, shows selection signatures in genes associated with the nervous system, while in river dairy breeds, genes under selection are related to heat stress and immunity.

CONCLUSIONS

Our findings substantially expand the catalogue of genetic variants in buffalo and reveal new insights into the evolutionary history and distinct selective pressures in river and swamp buffalo.

Transgenic Analyses of Homer2 Function Within Nucleus Accumbens Subregions in the Regulation of Methamphetamine Reward and Reinforcement in Mice

Problems associated with the abuse of amphetamine-type stimulants, including methamphetamine (MA), pose serious health and socioeconomic issues world-wide. While it is well-established that MA's psychopharmacological effects involve interactions with monoamine neurotransmission, accumulating evidence from animal models implicates dysregulated glutamate in MA addiction vulnerability and use disorder. Recently, we discovered an association between genetic vulnerability to MA-taking and increased expression of the glutamate receptor scaffolding protein Homer2 within both the shell and core subregions of the nucleus accumbens (NAC) and demonstrated a necessary role for Homer2 within the shell subregion in MA reward and reinforcement in mice. This report extends our earlier work by interrogating the functional relevance of Homer2 within the NAC core for the conditioned rewarding and reinforcing properties of MA. C57BL/6J mice with a virus-mediated knockdown of Homer2b expression within the NAC core were first tested for the development and expression of a MA-induced conditioned place-preference/CPP (four pairings of 2 mg/kg MA) and then were trained to self-administer oral MA under operant-conditioning procedures (5-80 mg/L). Homer2b knockdown in the NAC core augmented a MA-CPP and shifted the dose-response function for MA-reinforced responding, above control levels. To determine whether Homer2b within NAC subregions played an active role in regulating MA reward and reinforcement, we characterized the MA phenotype of constitutive Homer2 knockout (KO) mice and then assayed the effects of virus-mediated overexpression of Homer2b within the NAC shell and core of wild-type and KO mice. In line with the results of NAC core knockdown, Homer2 deletion potentiated MA-induced CPP, MA-reinforced responding and intake, as well as both cue- and MA-primed reinstatement of MA-seeking following extinction. However, there was no effect of Homer2b overexpression within the NAC core or the shell on the KO phenotype. These data provide new evidence indicating a globally suppressive role for Homer2 in MA-seeking and MA-taking but argue against specific NAC subregions as the neural loci through which Homer2 actively regulates MA addiction-related behaviors.

Increased Alcohol-Drinking Induced by Manipulations of mGlu5 Phosphorylation within the Bed Nucleus of the Stria Terminalis

The bed nucleus of the stria terminalis (BNST) is part of the limbic-hypothalamic system important for behavioral responses to stress, and glutamate transmission within this region has been implicated in the neurobiology of alcoholism. Herein, we used a combination of immunoblotting, neuropharmacological and transgenic procedures to investigate the role for metabotropic glutamate receptor 5 (mGlu5) signaling within the BNST in excessive drinking. We discovered that mGlu5 signaling in the BNST is linked to excessive alcohol consumption in a manner distinct from behavioral or neuropharmacological endophenotypes that have been previously implicated as triggers for heavy drinking. Our studies demonstrate that, in male mice, a history of chronic binge alcohol-drinking elevates BNST levels of the mGlu5-scaffolding protein Homer2 and activated extracellular signal-regulated kinase (ERK) in an adaptive response to limit alcohol consumption. Male and female transgenic mice expressing a point mutation of mGlu5 that cannot be phosphorylated by ERK exhibit excessive alcohol-drinking, despite greater behavioral signs of alcohol intoxication and reduced anxiety, and are insensitive to local manipulations of signaling in the BNST. These transgenic mice also show selective insensitivity to alcohol-aversion and increased novelty-seeking, which may be relevant to excessive drinking. Further, the insensitivity to alcohol-aversion exhibited by male mice can be mimicked by the local inhibition of ERK signaling within the BNST. Our findings elucidate a novel mGluR5-linked signaling state within BNST that plays a central and unanticipated role in excessive alcohol consumption.SIGNIFICANCE STATEMENT The bed nucleus of the stria terminalis (BNST) is part of the limbic-hypothalamic system important for behavioral responses to stress and alcohol, and glutamate transmission within BNST is implicated in the neurobiology of alcoholism. The present study provides evidence that a history of excessive alcohol drinking increases signaling through the metabotropic glutamate receptor 5 (mGlu5) receptor within the BNST in an adaptive response to limit alcohol consumption. In particular, disruption of mGlu5 phosphorylation by extracellular signal-regulated kinase within this brain region induces excessive alcohol-drinking, which reflects a selective insensitivity to the aversive properties of alcohol intoxication. These data indicate that a specific signaling state of mGlu5 within BNST plays a central and unanticipated role in excessive alcohol consumption.

Homer2 within the central nucleus of the amygdala modulates withdrawal-induced anxiety in a mouse model of binge-drinking

A history of binge-drinking decreases protein expression of the glutamate-related scaffolding protein Homer2 within the central nucleus of the amygdala (CEA), coinciding with behavioral signs of negative affect. To assess the functional relevance of this protein change for withdrawal-induced hyper-anxiety, adult (PND 56) and adolescent (PND 28) male C57BL/6J mice were administered an intra-CEA infusion of an adeno-associated viral vector (AAV) carrying either cDNA to express Homer2 (H2-cDNA) or GFP as control. Mice underwent 14 days of binge-drinking under multi-bottle, limited-access conditions and were assayed for behavioral signs of negative affect during withdrawal using the light-dark box, marble burying, and forced swim tests (FST). Following behavioral testing, all animals experienced 5 days of drinking to evaluate the effects of prior alcohol experience and Homer2 manipulation on subsequent alcohol consumption. During protracted (4 weeks) withdrawal, adolescent alcohol-experienced GFP controls showed increased signs of negative affect across all 3 assays, compared to water-drinking GFP animals, and also showed elevated alcohol consumption during the subsequent drinking period. Homer2-cDNA infusion in adolescent-onset alcohol-drinking animals was anxiolytic and reduced subsequent alcohol consumption. Conversely, Homer2-cDNA was anxiogenic and increased drinking in water-drinking adolescents. Unfortunately, the data from adult-onset alcohol-drinking animals were confounded by low alcohol consumption and negligible behavioral signs of anxiety. Nevertheless, the present results provide novel cause-effect evidence supporting a role for CEA Homer2 in the regulation of both basal anxiety and the time-dependent intensification of negative affective states in individuals with a history of binge-drinking during adolescence.

Behavioral and Neurochemical Phenotyping of Mice Incapable of Homer1a Induction

Immediate early and constitutively expressed products of the Homer1 gene regulate the functional assembly of post-synaptic density proteins at glutamatergic synapses to influence excitatory neurotransmission and synaptic plasticity. Earlier studies of Homer1 gene knock-out (KO) mice indicated active, but distinct, roles for IEG and constitutively expressed Homer1 gene products in regulating cognitive, emotional, motivational and sensorimotor processing, as well as behavioral and neurochemical sensitivity to cocaine. More recent characterization of transgenic mice engineered to prevent generation of the IEG form (a.k.a Homer1a KO) pose a critical role for Homer1a in cocaine-induced behavioral and neurochemical sensitization of relevance to drug addiction and related neuropsychiatric disorders. Here, we extend our characterization of the Homer1a KO mouse and report a modest pro-depressant phenotype, but no deleterious effects of the KO upon spatial learning/memory, prepulse inhibition, or cocaine-induced place-conditioning. As we reported previously, Homer1a KO mice did not develop cocaine-induced behavioral or neurochemical sensitization within the nucleus accumbens; however, virus-mediated Homer1a over-expression within the nucleus accumbens reversed the sensitization phenotype of KO mice. We also report several neurochemical abnormalities within the nucleus accumbens of Homer1a KO mice that include: elevated basal dopamine and reduced basal glutamate content, Group1 mGluR agonist-induced glutamate release and high K⁺-stimulated release of dopamine and glutamate within this region. Many of the neurochemical anomalies exhibited by Homer1a KO mice are recapitulated upon deletion of the entire Homer1 gene; however, Homer1 deletion did not affect NAC dopamine or alter K⁺-stimulated neurotransmitter release within this region. These data show that the selective deletion of Homer1a produces a behavioral and neurochemical phenotype that is distinguishable from that produced by deletion of the entire Homer1 gene. Moreover, the data indicate a specific role for Homer1a in regulating cocaine-induced behavioral and neurochemical sensitization of potential relevance to the psychotogenic properties of this drug.

Altered surface mGluR5 dynamics provoke synaptic NMDAR dysfunction and cognitive defects in Fmr1 knockout mice

Metabotropic glutamate receptor subtype 5 (mGluR5) is crucially implicated in the pathophysiology of Fragile X Syndrome (FXS); however, its dysfunction at the sub-cellular level, and related synaptic and cognitive phenotypes are unexplored. Here, we probed the consequences of mGluR5/Homer scaffold disruption for mGluR5 cell-surface mobility, synaptic N-methyl-D-aspartate receptor (NMDAR) function, and behavioral phenotypes in the second-generation Fmr1 knockout (KO) mouse. Using single-molecule tracking, we found that mGluR5 was significantly more mobile at synapses in hippocampal Fmr1 KO neurons, causing an increased synaptic surface co-clustering of mGluR5 and NMDAR. This correlated with a reduced amplitude of synaptic NMDAR currents, a lack of their mGluR5-activated long-term depression, and NMDAR/hippocampus dependent cognitive deficits. These synaptic and behavioral phenomena were reversed by knocking down Homer1a in Fmr1 KO mice. Our study provides a mechanistic link between changes of mGluR5 dynamics and pathological phenotypes of FXS, unveiling novel targets for mGluR5-based therapeutics.

Dysfunction of mGluR5 has been implicated in Fragile X syndrome. Here, using a single-molecule tracking technique, the authors found an increased lateral mobility of mGluR5 at the synaptic site in Fmr1 KO hippocampal neurons, leading to abnormal NMDAR-mediated synaptic plasticity and cognitive deficits.

Methamphetamine Addiction Vulnerability: The Glutamate, the Bad, and the Ugly

BACKGROUND

The high prevalence and severity of methamphetamine (MA) abuse demands greater neurobiological understanding of its etiology.

METHODS

We conducted immunoblotting and in vivo microdialysis procedures in MA high/low drinking mice, as well as in isogenic C57BL/6J mice that varied in their MA preference/taking, to examine the glutamate underpinnings of MA abuse vulnerability. Neuropharmacological and Homer2 knockdown approaches were also used in C57BL/6J mice to confirm the role for nucleus accumbens (NAC) glutamate/Homer2 expression in MA preference/aversion.

RESULTS

We identified a hyperglutamatergic state within the NAC as a biochemical trait corresponding with both genetic and idiopathic vulnerability for high MA preference and taking. We also confirmed that subchronic subtoxic MA experience elicits a hyperglutamatergic state within the NAC during protracted withdrawal, characterized by elevated metabotropic glutamate 1/5 receptor function and Homer2 receptor-scaffolding protein expression. A high MA-preferring phenotype was recapitulated by elevating endogenous glutamate within the NAC shell of mice and we reversed MA preference/taking by lowering endogenous glutamate and/or Homer2 expression within this subregion.

CONCLUSIONS

Our data point to an idiopathic, genetic, or drug-induced hyperglutamatergic state within the NAC as a mediator of MA addiction vulnerability.

Overexpression of Homer1a in the basal and lateral amygdala impairs fear conditioning and induces an autism-like social impairment

Background

Autism spectrum disorders (ASDs) represent a heterogeneous group of disorders with a wide range of behavioral impairments including social and communication deficits. Apart from these core symptoms, a significant number of ASD individuals display higher levels of anxiety, and some studies indicate that a subset of ASD individuals have a reduced ability to be fear conditioned. Deciphering the molecular basis of ASD has been considerably challenging and it currently remains poorly understood. In this study we examined the molecular basis of autism-like impairments in an environmentally induced animal model of ASD, where pregnant rats are exposed to the known teratogen, valproic acid (VPA), on day 12.5 of gestation and the subsequent progeny exhibit ASD-like symptoms. We focused our analysis on the basal and lateral nucleus of the amygdala (BLA), a region of the brain found to be associated with ASD pathology.

Methods

We performed whole genome gene expression analysis on the BLA using DNA microarrays to examine differences in gene expression within the amygdala of VPA-exposed animals. We validated one VPA-dysregulated candidate gene (Homer1a) using both quantitative PCR (qRT-PCR) and western blot. Finally, we overexpressed Homer1a within the basal and lateral amygdala of naïve animals utilizing adeno-associated viruses (AAV) and subsequently examined these animals in a battery of behavioral tests associated with ASD, including auditory fear conditioning, social interaction and open field.

Results

Our microarray data indicated that Homer1a was one of the genes which exhibited a significant upregulation within the amygdala. We observed an increase in Homer1a messenger RNA (mRNA) and protein in multiple cohorts of VPA-exposed animals indicating that dysregulation of Homer1a levels might underlie some of the symptoms exhibited by VPA-exposed animals. To test this hypothesis, we overexpressed Homer1a within BLA neurons utilizing a viral-mediated approach and found that overexpression of Homer1a impaired auditory fear conditioning and reduced social interaction, while having no influence on open-field behavior.

Conclusions

This study indicates that dysregulation of amygdala Homer1a might contribute to some autism-like symptoms induced by VPA exposure. These findings are interesting in part because Homer1a influences the functioning of Shank3, metabotropic glutamate receptors (mGluR5), and Homer1, and these proteins have previously been associated with ASD, indicating that these differing models of ASD may have a similar molecular basis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13229-016-0077-9) contains supplementary material, which is available to authorized users.

Functional regulation of PI3K-associated signaling in the accumbens by binge alcohol drinking in male but not female mice

It is well established that binge alcohol consumption produces alterations in Group 1 metabotropic glutamate receptors (mGlus) and related signaling cascades in the nucleus accumbens (NAC) of adult male mice, but female and adolescent mice have not been examined. Thus, the first set of studies determined whether repeated binge alcohol consumption produced similar alterations in protein and mRNA levels of Group 1 mGlu-associated signaling molecules in the NAC of male and female adult and adolescent mice. The adult (9 weeks) and adolescent (4 weeks) C57BL/6J mice were exposed to 7 binge alcohol sessions every 3rd day while controls drank water. Repeated binge alcohol consumption produced sexually divergent changes in protein levels and mRNA expression for Group 1 mGlus and downstream signaling molecules in the NAC, but there was no effect of age. Binge alcohol intake decreased mGlu5 levels in females, whereas it decreased indices of phosphoinositide 3-kinase (PI3K), mammalian target of rapamycin (mTOR), 4E-binding protein 1, and p70 ribosomal protein S6 kinase in males. Expression of genes encoding mGlu1, mGlu5, the NR2A subunit of the NMDA receptor, and Homer2 were all decreased by binge alcohol consumption in males, while females were relatively resistant (only phosphoinositide-dependent protein kinase 1 was decreased). The functional implication of these differences was investigated in a separate study by inhibiting mTOR in the NAC (via infusions of rapamycin) before binge drinking sessions. Rapamycin (50 and 100 ng/side) significantly decreased binge alcohol consumption in males, while consumption in females was unaffected. Altogether these results highlight that mTOR signaling in the NAC was necessary to maintain binge alcohol consumption only in male mice and that binge drinking recruits sexually divergent signaling cascades downstream of PI3K and presumably, Group 1 mGlus. Importantly, these findings emphasize that sex should be considered in the development of potential pharmacotherapeutic targets.

Ethanol-Associated Changes in Glutamate Reward Neurocircuitry: A Minireview of Clinical and Preclinical Genetic Findings

Herein, we have reviewed the role of glutamate, the major excitatory neurotransmitter in the brain, in a number of neurochemical, -physiological, and -behavioral processes mediating the development of alcohol dependence. The findings discussed include results from both preclinical as well as neuroimaging and postmortem clinical studies. Expression levels for a number of glutamate-associated genes and/or proteins are modulated by alcohol abuse and dependence. These changes in expression include metabotropic receptors and ionotropic receptor subunits as well as different glutamate transporters. Moreover, these changes in gene expression parallel the pharmacologic manipulation of these same receptors and transporters. Some of these gene expression changes may have predated alcohol abuse and dependence because a number of glutamate-associated polymorphisms are related to a genetic predisposition to develop alcohol dependence. Other glutamate-associated polymorphisms are linked to age at the onset of alcohol-dependence and initial level of response/sensitivity to alcohol. Finally, findings of innate and/or ethanol-induced glutamate-associated gene expression differences/changes observed in a genetic animal model of alcoholism, the P rat, are summarized. Overall, the existing literature indicates that changes in glutamate receptors, transporters, enzymes, and scaffolding proteins are crucial for the development of alcohol dependence and there is a substantial genetic component to these effects. This indicates that continued research into the genetic underpinnings of these glutamate-associated effects will provide important novel molecular targets for treating alcohol abuse and dependence.

Homer2 within the nucleus accumbens core bidirectionally regulates alcohol intake by both P and Wistar rats

In murine models of alcoholism, the glutamate receptor scaffolding protein Homer2 bidirectionally regulates alcohol intake. Although chronic alcohol drinking increases Homer2 expression within the core subregion of the nucleus accumbens (NAc) of alcohol-preferring P rats, the relevance of this neuroadaptation for alcohol intake has yet to be determined in rats. Thus, the present study employed an adeno-associated viral vector (AAV) strategy to over-express and knock down the major rodent isoform Homer2b within the NAc of both P and outbred Wistar rats to examine for changes in alcohol preference and intake (0-30% v/v) under continuous-access procedures. The generalization of AAV effects to non-drug, palatable, sweet solutions was also determined in tests of sucrose (0-5% w/v) and saccharin (0-0.125% w/v) intake/preference. No net-flux in vivo microdialysis was conducted for glutamate in the NAc to relate Homer2-dependent changes in alcohol intake to extracellular levels of glutamate. Line differences were noted for sweet solution preference and intake, but these variables were not affected by intra-NAc AAV infusion in either line. In contrast, Homer2b over-expression elevated, while Homer2b knock-down reduced, alcohol intake in both lines, and this effect was greatest at the highest concentration. Strikingly, in P rats there was a direct association between changes in Homer2b expression and NAc extracellular glutamate levels, but this effect was not seen in Wistar rats. These data indicate that NAc Homer2b expression actively regulates alcohol consumption by rats, paralleling this previous observation in mice. Overall, these findings underscore the importance of mesocorticolimbic glutamate activity in alcohol abuse/dependence and suggest that Homer2b and/or its constituents may serve as molecular targets for the treatment of these disorders.

Strong selection for behavioural resilience in Australian stock working dogs identified by selective sweep analysis

Background

Working dog handlers and breeders have strong opinions on characteristics that are desirable in the breeds that they use to handle stock. Most of these characteristics are related to conformation or behaviour. This study explored whether the genetics underlying desirable working behaviour traits might be identified by selective sweep analysis; a method that identifies long regions of strong homozygosity combined with allelic divergence from a comparison group. For this analysis, we compared genomic haplotype architecture in two breeds derived from common founder stock but subjected to divergent selective pressures. The breeds studied were the Australian Kelpie, which is registered with the Australian National Kennel Council, and the Australian Working Kelpie, which is registered with the Working Kelpie Council.

Results

A selective sweep spanning 3 megabases on chromosome 3 was identified in the Australian Working Kelpie. This region is the location of genes related to fear-memory formation and pain perception.

Selective sweep loci of similar magnitude were observed in the Australian Kelpie. On chromosome 8 is a locus which may be related to behavioural excitability and on chromosome 30 is a smaller locus which most likely is related to morphology.

Conclusions

Active working stock dogs of the Australian Working Kelpie breed have been bred primarily for gene loci influencing pain perception and fear memory formation. By contrast Australian Kelpies are commonly maintained in urban environments where these characteristics are not required and have been affected by selection for conformation and coat colour. The identified loci may aid in the identification of superior working dogs.

Electronic supplementary material

The online version of this article (doi:10.1186/s40575-015-0017-6) contains supplementary material, which is available to authorized users.

Homer protein-metabotropic glutamate receptor binding regulates endocannabinoid signaling and affects hyperexcitability in a mouse model of fragile X syndrome

The Fmr1 knock-out mouse model of fragile X syndrome (Fmr1(-/y)) has an epileptogenic phenotype that is triggered by group I metabotropic glutamate receptor (mGluR) activation. We found that a membrane-permeable peptide that disrupts mGluR5 interactions with long-form Homers enhanced mGluR-induced epileptiform burst firing in wild-type (WT) animals, replicating the early stages of hyperexcitability in Fmr1(-/y). The peptide enhanced mGluR-evoked endocannabinoid (eCB)-mediated suppression of inhibitory synapses, decreased it at excitatory synapses in WTs, but had no effect on eCB actions in Fmr1(-/y). At a low concentration, the mGluR agonist did not generate eCBs at excitatory synapses but nevertheless induced burst firing in both Fmr1(-/y) and peptide-treated WT slices. This burst firing was suppressed by a cannabinoid receptor antagonist. We suggest that integrity of Homer scaffolds is essential for normal mGluR-eCB functioning and that aberrant eCB signaling resulting from disturbances of this molecular structure contributes to the epileptic phenotype of Fmr1(-/y).

MicroRNAs in Schizophrenia: Implications for Synaptic Plasticity and Dopamine-Glutamate Interaction at the Postsynaptic Density. New Avenues for Antipsychotic Treatment Under a Theranostic Perspective

Despite dopamine-glutamate aberrant interaction that has long been considered a relevant landmark of psychosis pathophysiology, several aspects of these two neurotransmitters reciprocal interaction remain to be defined. The emerging role of postsynaptic density (PSD) proteins at glutamate synapse as a molecular "lego" making a functional hub where different signals converge may add a new piece of information to understand how dopamine-glutamate interaction may work with regard to schizophrenia pathophysiology and treatment. More recently, compelling evidence suggests a relevant role for microRNA (miRNA) as a new class of dopamine and glutamate modulators with regulatory functions in the reciprocal interaction of these two neurotransmitters. Here, we aimed at addressing the following issues: (i) Do miRNAs have a role in schizophrenia pathophysiology in the context of dopamine-glutamate aberrant interaction? (ii) If miRNAs are relevant for dopamine-glutamate interaction, at what level this modulation takes place? (iii) Finally, will this knowledge open the door to innovative diagnostic and therapeutic tools? The biogenesis of miRNAs and their role in synaptic plasticity with relevance to schizophrenia will be considered in the context of dopamine-glutamate interaction, with special focus on miRNA interaction with PSD elements. From this framework, implications both for biomarkers identification and potential innovative interventions will be considered.

Synaptic depression via mGluR1 positive allosteric modulation suppresses cue-induced cocaine craving

Cue-induced cocaine craving is a major cause of relapse in abstinent addicts. In rats, cue-induced craving progressively intensifies (incubates) during withdrawal from extended-access cocaine self-administration. After ~1 month of withdrawal, incubated craving is mediated by Ca²⁺-permeable AMPARs (CP-AMPARs) that accumulate in the nucleus accumbens (NAc). We found that decreased mGluR1 surface expression in the NAc precedes and enables CP-AMPAR accumulation. Thus, restoring mGluR1 tone by administering repeated injections of an mGluR1 positive allosteric modulator (PAM) prevented CP-AMPAR accumulation and incubation, whereas blocking mGluR1 transmission at even earlier withdrawal times accelerated CP-AMPAR accumulation. In studies conducted after prolonged withdrawal, when CP-AMPAR levels and cue-induced craving are high, we found that systemic administration of an mGluR1 PAM attenuated the expression of incubated craving by reducing CP-AMPAR transmission in the NAc to control levels. These results demonstrate a strategy whereby recovering addicts could use a systemically active compound to protect against cue-induced relapse.

Imbalances in prefrontal cortex CC-Homer1 versus CC-Homer2 expression promote cocaine preference

Homer postsynaptic scaffolding proteins regulate forebrain glutamate transmission and thus, are likely molecular candidates mediating hypofrontality in addiction. Protracted withdrawal from cocaine experience increases the relative expression of Homer2 versus Homer1 isoforms within medial prefrontal cortex (mPFC). Thus, this study used virus-mediated gene transfer strategies to investigate the functional relevance of an imbalance in mPFC Homer1/2 expression as it relates to various measures of sensorimotor, cognitive, emotional and motivational processing, as well as accompanying alterations in extracellular glutamate in C57BL/6J mice. mPFC Homer2b overexpression elevated basal glutamate content and blunted cocaine-induced glutamate release within the mPFC, whereas Homer2b knockdown produced the opposite effects. Despite altering mPFC glutamate, Homer2b knockdown failed to influence cocaine-elicited conditioned place preferences, nor did it produce consistent effects on any other behavioral measures. In contrast, elevating the relative expression of Homer2b versus Homer1 within mPFC, by overexpressing Homer2b or knocking down Homer1c, shifted the dose-response function for cocaine-conditioned reward to the left, without affecting cocaine locomotion or sensitization. Intriguingly, both these transgenic manipulations produced glutamate anomalies within the nucleus accumbens (NAC) of cocaine-naive animals that are reminiscent of those observed in cocaine experienced animals, including reduced basal extracellular glutamate content, reduced Homer1/2 and glutamate receptor expression, and augmented cocaine-elicited glutamate release. Together, these data provide novel evidence in support of opposing roles for constitutively expressed Homer1 and Homer2 isoforms in regulating mPFC glutamate transmission in vivo and support the hypothesis that cocaine-elicited increases in the relative amount of mPFC Homer2 versus Homer1 signaling produces abnormalities in NAC glutamate transmission that enhance vulnerability to cocaine reward.

Nerve injury-induced changes in Homer/glutamate receptor signaling contribute to the development and maintenance of neuropathic pain

While group 1 metabotropic glutamate receptors (mGluRs) and ionotropic N-methyl-d-aspartate (NMDA) receptors regulate nociception, the precise molecular mechanism(s) contributing to glutamate signaling in chronic pain remain unclear. Here we not only confirmed the key involvement of Homer proteins in neuropathic pain, but also distinguished between the functional roles for different Homer family members and isoforms. Chronic constriction injury (CCI) of the sciatic nerve induced long-lasting, time-dependent increases in the postsynaptic density expression of the constitutively expressed (CC) isoforms Homer1b/c and/or Homer2a/b in the spinal dorsal horn and supraspinal structures involved in nociception (prefrontal cortex, thalamus), that co-occurred with increases in their associated mGluRs, NR2 subunits of the NMDA receptor, and the activation of downstream kinases. Virus-mediated overexpression of Homer1c and Homer2b after spinal (intrathecal) virus injection exacerbated CCI-induced mechanical and cold hypersensitivity, however, Homer1 and Homer2 gene knockout (KO) mice displayed no changes in their neuropathic phenotype. In contrast, overexpression of the immediate early gene (IEG) Homer1a isoform reduced, while KO of Homer1a gene potentiated neuropathic pain hypersensitivity. Thus, nerve injury-induced increases in CC-Homers expression promote pain in pathological states, but IEG-Homer induction protects against both the development and maintenance of neuropathy. Additionally, exacerbated pain hypersensitivity in transgenic mice with reduced Homer binding to mGluR5 supports also an inhibitory role for Homer interactions with mGluR5 in mediating neuropathy. Such data indicate that nerve injury-induced changes in glutamate receptor/Homer signaling contribute in dynamic but distinct ways to neuropathic pain processing, which has relevance for the etiology of chronic pain symptoms and its treatment.

Homers at the Interface between Reward and Pain

Pain alters opioid reinforcement, presumably via neuroadaptations within ascending pain pathways interacting with the limbic system. Nerve injury increases expression of glutamate receptors and their associated Homer scaffolding proteins throughout the pain processing pathway. Homer proteins, and their associated glutamate receptors, regulate behavioral sensitivity to various addictive drugs. Thus, we investigated a potential role for Homers in the interactions between pain and drug reward in mice. Chronic constriction injury (CCI) of the sciatic nerve elevated Homer1b/c and/or Homer2a/b expression within all mesolimbic structures examined and for the most part, the Homer increases coincided with elevated mGluR5, GluN2A/B, and the activational state of various down-stream kinases. Behaviorally, CCI mice showed pain hypersensitivity and a conditioned place-aversion (CPA) at a low heroin dose that supported conditioned place-preference (CPP) in naïve controls. Null mutations of Homer1a, Homer1, and Homer2, as well as transgenic disruption of mGluR5-Homer interactions, either attenuated or completely blocked low-dose heroin CPP, and none of the CCI mutant strains exhibited heroin-induced CPA. However, heroin CPP did not depend upon full Homer1c expression within the nucleus accumbens (NAC), as CPP occurred in controls infused locally with small hairpin RNA-Homer1c, although intra-NAC and/or intrathecal cDNA-Homer1c, -Homer1a, and -Homer2b infusions (to best mimic CCI's effects) were sufficient to blunt heroin CPP in uninjured mice. However, arguing against a simple role for CCI-induced increases in either spinal or NAC Homer expression for heroin CPA, cDNA infusion of our various cDNA constructs either did not affect (intrathecal) or attenuated (NAC) heroin CPA. Together, these data implicate increases in glutamate receptor/Homer/kinase activity within limbic structures, perhaps outside the NAC, as possibly critical for switching the incentive motivational properties of heroin following nerve injury, which has relevance for opioid psychopharmacology in individuals suffering from neuropathic pain.

Signaling pathways mediating alcohol effects

Ethanol's effects on intracellular signaling pathways contribute to acute effects of ethanol as well as to neuroadaptive responses to repeated ethanol exposure. In this chapter we review recent discoveries that demonstrate how ethanol alters signaling pathways involving several receptor tyrosine kinases and intracellular tyrosine and serine-threonine kinases, with consequences for regulation of cell surface receptor function, gene expression, protein translation, neuronal excitability and animal behavior. We also describe recent work that demonstrates a key role for ethanol in regulating the function of scaffolding proteins that organize signaling complexes into functional units. Finally, we review recent exciting studies demonstrating ethanol modulation of DNA and histone modification and the expression of microRNAs, indicating epigenetic mechanisms by which ethanol regulates neuronal gene expression and addictive behaviors.

Virogenetic and optogenetic mechanisms to define potential therapeutic targets in psychiatric disorders

A continuously increasing body of knowledge shows that the brain is an extremely complex neural network and single neurons possess their own complicated interactive signaling pathways. Such complexity of the nervous system makes it increasingly difficult to investigate the functions of specific neural components such as genes, proteins, transcription factors, neurons and nuclei in the brain. Technically, it has been even more of a significant challenge to identify the molecular and cellular adaptations that are both sufficient and necessary to underlie behavioral functions in health and disease states. Defining such neural adaptations is a critical step to identify the potential therapeutic targets within the complex neural network that are beneficial to treat psychiatric disorders. Recently, the new development and extensive application of in vivo viral-mediated gene transfer (virogenetics) and optical manipulation of specific neurons or selective neural circuits in freely-moving animals (optogenetics) make it feasible, through loss- and gain-of-function approaches, to reliably define sufficient and necessary neuroadaptations in the behavioral models of psychiatric disorders, including drug addiction, depression, anxiety and bipolar disorders. In this article, we focus on recent studies that successfully employ these advanced virogenetic and optogenetic techniques as a powerful tool to identify potential targets in the brain, and to provide highly useful information in the development of novel therapeutic strategies for psychiatric disorders. This article is part of a Special Issue entitled 'Anxiety and Depression'.

AAV-Mediated Overexpression of the CB1 Receptor in the mPFC of Adult Rats Alters Cognitive Flexibility, Social Behavior, and Emotional Reactivity

The endocannabinoid (ECB) system is strongly involved in the regulation of cognitive processing and emotional behavior and evidence indicates that ECB signaling might affect these behavioral abilities by modulations of prefrontal cortical functions. The aim of the present study was to examine the role of the CB1 receptor in the medial prefrontal cortex (mPFC) on cognitive flexibility and emotional behavior. Therefore, the CB1 receptor was overexpressed by adeno-associated virus vector-mediated gene transfer specifically in the mPFC of adult Wistar rats. Animals were then tested in different anxiety-related paradigms for emotional reactivity [e.g., elevated plus maze (EPM), light/dark emergence test (EMT), social interaction] and the attentional set shift task (ASST) – an adaptation of the human Wisconsin card sorting test – for cognitive abilities and behavioral flexibility. A subtle increase in exploratory behavior was found in CB1 receptor overexpressing animals (CB1-R) compared to Empty vector injected controls (Empty) in the EMT and EPM, although general locomotor activity did not differ between the groups. During social interaction testing, social contact behavior toward the unknown conspecific was found to be decreased, whereas social withdrawal was increased in CB1-R animals and they showed an inadequate increase in exploratory behavior compared to control animals. In the ASST, impaired reversal learning abilities were detected in CB1-R animals compared to controls, indicating reduced behavioral flexibility. In conclusion, upregulation of the CB1 receptor specifically in the rat mPFC induces alterations in emotional reactivity, leads to inadequate social behavior, and impairs cognitive flexibility. These findings might be relevant for neuropsychiatric disorders, since higher cortical CB1 receptor expression levels as well as similar behavioral impairments as observed in the present study have been described in schizophrenic patients.

Cannabinoid exposure in pubertal rats increases spontaneous ethanol consumption and NMDA receptor associated protein levels

Recent evidence suggests an involvement of the endocannabinoid system in the regulation of emotional behaviour and ethanol intake. Here we investigated age-specific acute behavioural effects of the cannabinoid receptor agonist WIN 55,212-2 (WIN) on anxiety-related behaviour and voluntary ethanol consumption in rats. Animals were treated with WIN (1.2 mg/kg)/vehicle at puberty onset on postnatal day (PD) 40, or at adulthood (PD 100). Animals were tested in the elevated plus-maze (EPM) and the light/dark emergence test (EMT) and for the initial response to alcohol in a free-choice ethanol consumption paradigm. Acute WIN treatment increased anxiety-related behaviours, and this effect was found to be partially more pronounced in pubertal than adult rats. Additionally, increased intake of higher ethanol solutions after cannabinoid treatment was only observed in pubertal rats. These drug-induced behavioural changes during puberty are paralleled by induction of the NR1 subunit of the NMDA receptor in the medial prefrontal cortex and the striatum. Moreover, pubertal but not adult WIN administration increased the levels of the scaffold protein Homer in these brain regions. Enhanced CB₁ receptor levels in the reinforcement system were also observed in pubertal compared to adult rats. These data support the notion that puberty is a highly vulnerable period for the aversive effects of cannabinoid exposure. In particular, augmented ethanol intake in pubertal cannabinoid-exposed animals might be related to some extent to increased emotional behaviour and in particular to enhanced NMDA and CB₁ receptor signalling.

Accumbens Homer2-mediated signaling: a factor contributing to mouse strain differences in alcohol drinking?

Alcohol-induced increases in nucleus accumbens glutamate actively regulate alcohol consumption, and the alcohol responsiveness of corticoaccumbens glutamate systems relates to genetic variance in alcohol reward. Here, we extend earlier data for inbred mouse strain differences in accumbens glutamate by examining for differences in basal and alcohol-induced changes in the striatal expression of glutamate-related signaling molecules between inbred C57BL/6J and DBA2/J mice. Repeated alcohol treatment (8 × 2 g/kg) increased the expression of Group1 metabotropic glutamate receptors, the NR2a/b subunits of the N-methyl-D-aspartate receptor, Homer2a/b, as well as the activated forms of protein kinase C (PKC) epsilon and phosphoinositol-3-kinase within ventral, but not dorsal, striatum. Regardless of prior alcohol experience, C57BL/6J mice exhibited higher accumbens levels of mGluR1/5, Homer2a/b, NR2a and activated kinases vs. DBA2/J mice, whereas an alcohol-induced rise in dorsal striatum mGluR1/5 expression was observed only in C57BL/6J mice. We next employed virus-mediated gene transfer approaches to ascertain the functional relevance of the observed strain difference in accumbens Homer2 expression for B6/D2 differences in alcohol-induced glutamate sensitization, as well as alcohol preference/intake. Manipulating nucleus accumbens shell Homer2b expression actively regulated these measures in C57BL/6J mice, whereas DBA2/J mice were relatively insensitive to the neurochemical and behavioral effects of virus-mediated changes in Homer2 expression. These data support the over-arching hypothesis that augmented accumbens Homer2-mediated glutamate signaling may be an endophenotype related to genetic variance in alcohol consumption. If relevant to humans, such data pose polymorphisms affecting glutamate receptor/Homer2 signaling in the etiology of alcoholism.

An integrated genome research network for studying the genetics of alcohol addiction

Alcohol drinking is highly prevalent in many cultures and contributes to the global burden of disease. In fact, it was shown that alcohol constitutes 3.2% of all worldwide deaths in the year 2006 and is linked to more than 60 diseases, including cancers, cardiovascular diseases, liver cirrhosis, neuropsychiatric disorders, injuries and foetal alcohol syndrome. Alcoholism, which has been proven to have a high genetic load, is one potentially fatal consequence of chronic heavy alcohol consumption, and may be regarded as one of the most prevalent neuropsychiatric diseases afflicting our society today. The aim of the integrated genome research network 'Genetics of Alcohol Addiction'--which is a German inter-/trans-disciplinary life science consortium consisting of molecular biologists, behavioural pharmacologists, system biologists with mathematicians, human geneticists and clinicians--is to better understand the genetics of alcohol addiction by identifying and validating candidate genes and molecular networks involved in the aetiology of this pathology. For comparison, addictive behaviour to other drugs of abuse (e.g. cocaine) is studied as well. Here, we present an overview of our research consortium, the current state of the art on genetic research in the alcohol field, and list finally several of our recently published research highlights. As a result of our scientific efforts, better insights into the molecular and physiological processes underlying addictive behaviour will be obtained, new targets and target networks in the addicted brain will be defined, and subsequently, novel and individualized treatment strategies for our patients will be delivered.

Binge drinking upregulates accumbens mGluR5-Homer2-PI3K signaling: functional implications for alcoholism

The glutamate receptor-associated protein Homer2 regulates alcohol-induced neuroplasticity within the nucleus accumbens (NAC), but the precise intracellular signaling cascades involved are not known. This study examined the role for NAC metabotropic glutamate receptor (mGluR)-Homer2-phosphatidylinositol 3-kinase (PI3K) signaling in regulating excessive alcohol consumption within the context of the scheduled high alcohol consumption (SHAC) model of binge alcohol drinking. Repeated bouts of binge drinking ( approximately 1.5 g/kg per 30 min) elevated NAC Homer2a/b expression and increased PI3K activity in this region. Virus-mediated knockdown of NAC Homer2b expression attenuated alcohol intake, as did an intra-NAC infusion of the mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine hydrochloride] (0.1-1 microg/side) and the PI3K antagonist wortmannin (50 ng/side), supporting necessary roles for mGluR5/Homer2/PI3K in binge alcohol drinking. Moreover, when compared with wild-type littermates, transgenic mice with an F1128R point mutation in mGluR5 that markedly reduces Homer binding exhibited a 50% reduction in binge alcohol drinking, which was related to reduced NAC basal PI3K activity. Consistent with the hypothesis that mGluR5-Homer-PI3K signaling may be a mechanism governing excessive alcohol intake, the "anti-binge" effects of MPEP and wortmannin were not additive, nor were they observed in the mGluR5(F1128R) transgenic mice. Finally, mice genetically selected for a high versus low SHAC phenotype differed in NAC mGluR, Homer2, and PI3K activity, consistent with the hypothesis that augmented NAC mGluR5-Homer2-PI3K signaling predisposes a high binge alcohol-drinking phenotype. Together, these data point to an important role for NAC mGluR5-Homer2-PI3K signaling in regulating binge-like alcohol consumption that has relevance for our understanding of the neurobiology of alcoholism and its pharmacotherapy.