Npas4: Linking Neuronal Activity to Memory

Expression of immediate early genes in brain reward circuitries: Differential regulation by psychostimulant and opioid drugs

Although some of the clinical manifestations of substance use disorders might be superficially similar, it is highly likely that different classes of abused drugs including opioids (heroin, morphine, and oxycodone, other opioids) and psychostimulants (cocaine and amphetamines) cause different neuroadaptations in various brain regions dependent in the distribution and concentration of their biochemical sites of actions. In fact, different molecular networks are indeed impacted by acute and chronic administration of addictive substances. Some of the genes whose expression is influenced by the administration of these substances are immediate-early genes (IEGs). IEGs include classes of low expression genes that can become very highly induced within seconds or minutes of activation by endogenous or exogenous stimuli. These IEGs might play important roles in activating target genes that regulate adaptations implicated in the behavioral manifestations diagnosed as addiction. Therefore, the purpose of this review is to provide an overview of recent data on the effects of psychostimulants and opioids on IEG expression in the brain. The review documents some contrasting effects of these classes of drugs on gene expression and indicates that further studies are necessary to identify the specific effects of each drug class when trying to predict clinical responses to therapeutic agents.

METTL3-mediated N6-methyladenosine mRNA modification enhances long-term memory consolidation

The formation of long-term memory is critical for learning ability and social behaviors of humans and animals, yet its underlying mechanisms are largely unknown. We found that the efficacy of hippocampus-dependent memory consolidation is regulated by METTL3, an RNA N⁶-methyladenosine (m⁶A) methyltransferase, through promoting the translation of neuronal early-response genes. Such effect is exquisitely dependent on the m⁶A methyltransferase function of METTL3. Depleting METTL3 in mouse hippocampus reduces memory consolidation ability, yet unimpaired learning outcomes can be achieved if adequate training was given or the m⁶A methyltransferase function of METTL3 was restored. The abundance of METTL3 in wild-type mouse hippocampus is positively correlated with learning efficacy, and overexpression of METTL3 significantly enhances long-term memory consolidation. These findings uncover a direct role of RNA m⁶A modification in regulating long-term memory formation, and also indicate that memory efficacy difference among individuals could be compensated by repeated learning.

Fear extinction requires ASIC1a-dependent regulation of hippocampal-prefrontal correlates

Extinction of conditioned fear necessitates the dynamic involvement of hippocampus, medial prefrontal cortex (mPFC), and basolateral amygdala (BLA), but key molecular players that regulate these circuits to achieve fear extinction remain largely unknown. Here, we report that acid-sensing ion channel 1a (ASIC1a) is a crucial molecular regulator of fear extinction, and that this function requires ASIC1a in ventral hippocampus (vHPC), but not dorsal hippocampus, mPFC, or BLA. While genetic disruption or pharmacological inhibition of ASIC1a in vHPC attenuated the extinction of conditioned fear, overexpression of the channel in this area promoted fear extinction. Channelrhodopsin-2-assisted circuit mapping revealed that fear extinction involved an ASIC1a-dependent modification of the long-range hippocampal-prefrontal correlates in a projection-specific manner. Gene expression profiling analysis and validating experiments identified several neuronal activity-regulated and memory-related genes, including Fos, Npas4, and Bdnf, as the potential mediators of ASIC1a regulation of fear extinction. Mechanistically, genetic overexpression of brain-derived neurotrophic factor (BDNF) in vHPC or supplement of BDNF protein in mPFC both rescued the deficiency in fear extinction and the deficits on extinction-driven adaptations of hippocampal-prefrontal correlates caused by the Asic1a gene inactivation in vHPC. Together, these results establish ASIC1a as a critical constituent in fear extinction circuits and thus a promising target for managing adaptive behaviors.

Hippocampal Mechanisms Underlying Impairment in Spatial Learning Long After Establishment of Noise-Induced Hearing Loss in CBA Mice

Sensorineural hearing loss (SNHL) has been demonstrated in many clinical reports as a risk factor that promotes the development of cognitive impairment. However, the underlying neurological mechanisms are not clear. Noise exposure is one of the most common causes of SNHL. Although noise exposure causes relatively less damage to general health as compared with other methods for creating hearing loss (such as ototoxicity), it does impair cognitive function. Many studies have shown that the noise-induced cognitive impairment occur via the oxidative stress induced by the noise. In those studies, the effects of the noise-induced hearing loss induced (NIHL) were not addressed. Previously, we have demonstrated in the CBA/CaJ mouse model that oxidative stress was transient after a brief noise exposure, but the NIHL was permanent. In addition, NIHL was followed by a declined cognitive function and decreased hippocampal neurogenesis that were developed long after the oxidative stress disappeared. Therefore, NIHL can cause cognitive impairment independent of its stress effect and can serve as a model to investigate the relationship between hearing loss and the development of cognitive impairment. In the present study, we further demonstrated that the oxidative stress produced by the brief noise exposure did not damage the stem cell bank of hippocampus that was evaluated shortly after the noise exposure. In addition to the reduction in the rate of cell proliferation in hippocampus that was found previously, we found that the NIHL significantly reduced the promoting effect of learning activity on various stages of hippocampal neurogenesis, accompanied by the reduction in learning-induced expression of immediate early genes (IEGs) in hippocampus. Since the MWM-tested spatial function does not directly require auditory input, the results provide evidence for the maintenance role of auditory input on the cognitive function; the reduction of IEG expression that is required in memory-formation may be the initial step in blocking the effect of learning activity on neurogenesis in subjects with NIHL.

Structural insights and characterization of human Npas4 protein

Npas4 is an activity dependent transcription factor which is responsible for gearing the expression of target genes involved in neuro-transmission. Despite the importance of Npas4 in many neuronal diseases, the tertiary structure of Npas4 protein along with its physico-chemical properties is limited. In the current study, first we perfomed the phylogenetic analysis of Npas4 and determined the content of hydrophobic, flexible and order-disorder promoting amino acids. The protein binding regions, post-translational modifications and crystallization propensity of Npas4 were predicted through different in-silico methods. The three dimensional model of Npas4 was predicted through LOMET, SPARSKS-X, I-Tasser, RaptorX, MUSTER and Pyhre and the best model was selected on the basis of Ramachandran plot, PROSA, and Qmean scores. The best model was then subjected to further refinement though MODREFINER. Finally the interacting partners of Npas4 were identified through STRING database. The phylogenetic analysis showed the human Npas4 gene to be closely related to other primates such as chimpanzees, monkey, gibbon. The physiochemical properties of Npas4 showed that it is an intrinsically disordered protein with N-terminal ordered region. The post-translational modification analyses indicated absence of acetylation and mannosylation sites. Three potential phosphorylation sites (S108, T130 and T136) were found in PAS A domain whilst a single phosphorylation site (S273) was present in PAS B domain. The predicted tertiary structure of Npas4 showed that bHLH domain and PAS domain possess tertiary structures while the rest of the protein exhibited disorder property. Protein-protein interaction analysis revealed NPas4 interaction with various proteins which are mainly involved in nuclear trafficking of proteins to cytoplasm, activity regulated gene transcription and neurodevelopmental disorders. Moreover the analysis also highlighted the direct relation to proteins involved in promoting neuronal survival, plasticity and cAMP responsive element binding protein proteins. The current study helps in understanding the physicochemical properties and reveals the neuro-modulatory role of Npas4 in crucial pathways involved in neuronal survival and neural signalling hemostasis.

Dopaminergic dysregulation and impaired associative learning behavior in zebrafish during chronic dietary exposure to selenium

A growing body of evidence indicates that exposure to selenium (Se) can cause neurotoxicity, and this can occur because of its interference with several neurotransmitter systems in humans and animals. Dopamine is a critical modulator of a variety of brain functions and a prime target for environmental neurotoxicants. However, effects of environmentally relevant concentrations of Se on dopaminergic system and its neurobehavioral effects are still largely unknown. For this purpose, we exposed zebrafish, a model organism, to different concentrations of dietary l-selenomethionine (control, 3.5, 11.1, 27.4, and 63.4 μg Se/g dry weight) for a period of 60 days. Cognitive performance of fish was evaluated using a plus maze associative learning paradigm. Oxidative stress, as the main driver of Se neurotoxicity, was assessed by measuring the ratio of reduced to oxidized glutathione (GSH:GSSG), lipid peroxidation (LPO) levels, and mRNA expression of several antioxidant enzymes in the zebrafish brain. Dopamine levels in the brain and the expression of genes involved in dopamine synthesis, storage, reuptake, metabolism, and receptor activation were examined. Moreover, transcription of several synaptic plasticity-related immediate-early and late response genes was determined. Overall, fish fed with the two highest concentrations of dietary Se displayed impaired associative learning. Se exposure also induced oxidative stress in the zebrafish brain, as indicated by a reduction in GSH:GSSG ratio, increased LPO levels, and up-regulation of antioxidant genes in fish treated with the two highest concentrations of Se. An increase in brain dopamine levels associated with altered expression of dopaminergic cell markers was evident in different treatment groups. Moreover, Se exposure led to the down-regulation of immediate-early and late response genes in fish that exhibiting learning impairment. Taken together, the results of this study imply that the induction of oxidative stress and dysregulation of dopaminergic neurotransmission may underlie Se-induced impairment of associative learning in zebrafish.

Astrocyte glycogen and lactate: New insights into learning and memory mechanisms

Memory, the ability to retain learned information, is necessary for survival. Thus far, molecular and cellular investigations of memory formation and storage have mainly focused on neuronal mechanisms. In addition to neurons, however, the brain comprises other types of cells and systems, including glia and vasculature. Accordingly, recent experimental work has begun to ask questions about the roles of non-neuronal cells in memory formation. These studies provide evidence that all types of glial cells (astrocytes, oligodendrocytes, and microglia) make important contributions to the processing of encoded information and storing memories. In this review, we summarize and discuss recent findings on the critical role of astrocytes as providers of energy for the long-lasting neuronal changes that are necessary for long-term memory formation. We focus on three main findings: first, the role of glucose metabolism and the learning- and activity-dependent metabolic coupling between astrocytes and neurons in the service of long-term memory formation; second, the role of astrocytic glucose metabolism in arousal, a state that contributes to the formation of very long-lasting and detailed memories; and finally, in light of the high energy demands of the brain during early development, we will discuss the possible role of astrocytic and neuronal glucose metabolisms in the formation of early-life memories. We conclude by proposing future directions and discussing the implications of these findings for brain health and disease. Astrocyte glycogenolysis and lactate play a critical role in memory formation. Emotionally salient experiences form strong memories by recruiting astrocytic β2 adrenergic receptors and astrocyte-generated lactate. Glycogenolysis and astrocyte-neuron metabolic coupling may also play critical roles in memory formation during development, when the energy requirements of brain metabolism are at their peak.

Ptchd1 deficiency induces excitatory synaptic and cognitive dysfunctions in mouse

Synapse development and neuronal activity represent fundamental processes for the establishment of cognitive function. Structural organization as well as signalling pathways from receptor stimulation to gene expression regulation are mediated by synaptic activity and misregulated in neurodevelopmental disorders such as autism spectrum disorder (ASD) and intellectual disability (ID). Deleterious mutations in the PTCHD1 (Patched domain containing 1) gene have been described in male patients with X-linked ID and/or ASD. The structure of PTCHD1 protein is similar to the Patched (PTCH1) receptor; however, the cellular mechanisms and pathways associated with PTCHD1 in the developing brain are poorly determined. Here we show that PTCHD1 displays a C-terminal PDZ-binding motif that binds to the postsynaptic proteins PSD95 and SAP102. We also report that PTCHD1 is unable to rescue the canonical sonic hedgehog (SHH) pathway in cells depleted of PTCH1, suggesting that both proteins are involved in distinct cellular signalling pathways. We find that Ptchd1 deficiency in male mice (Ptchd1^-/y) induces global changes in synaptic gene expression, affects the expression of the immediate-early expression genes Egr1 and Npas4 and finally impairs excitatory synaptic structure and neuronal excitatory activity in the hippocampus, leading to cognitive dysfunction, motor disabilities and hyperactivity. Thus our results support that PTCHD1 deficiency induces a neurodevelopmental disorder causing excitatory synaptic dysfunction.

Synaptic and circuit development of the primary sensory cortex

Animals, including humans, optimize their primary sensory cortex through the use of input signals, which allow them to adapt to the external environment and survive. The time window at the beginning of life in which external input signals are connected sensitively and strongly to neural circuit optimization is called the critical period. The critical period has attracted the attention of many neuroscientists due to the rapid activity-/experience-dependent circuit development that occurs, which is clearly differentiated from other developmental time periods and brain areas. This process involves various types of GABAergic inhibitory neurons, the extracellular matrix, neuromodulators, transcription factors, and neurodevelopmental factors. In this review, I discuss recent progress regarding the biological nature of the critical period that contribute to a better understanding of brain development.

Synapse development organized by neuronal activity-regulated immediate-early genes

Classical studies have shown that neuronal immediate-early genes (IEGs) play important roles in synaptic processes critical for key brain functions. IEGs are transiently activated and rapidly upregulated in discrete neurons in response to a wide variety of cellular stimuli, and they are uniquely involved in various aspects of synapse development. In this review, we summarize recent studies of a subset of neuronal IEGs in regulating synapse formation, transmission, and plasticity. We also discuss how the dysregulation of neuronal IEGs is associated with the onset of various brain disorders and pinpoint key outstanding questions that should be addressed in this field.

Immediate-early genes (IEGs), genes that are rapidly and transiently activated by cellular stimuli, regulate the interactions between neurons and key brain functions. Ji Won Um and colleagues at Daegu Gyeongbuk Institute of Science and Technology in South Korea review recent studies on three IEGs that are activated by neuronal activity and highlight their contribution to neuronal excitability and cognitive behaviors. These genes rely on different molecular mechanisms to regulate neuronal receptors and the structure of synapses. Research in mice lacking any one of these IEGs reveals their contribution to learning and memory as well as to some behavioral abnormalities associated with neuropsychiatric disorders. Further research into the activity of IEGs will advance our understanding of how a neuron’s environment influences brain development and disease.

Growth of Triple Negative and Progesterone Positive Breast Cancer Causes Oxidative Stress and Down-Regulates Neuroprotective Transcription Factor NPAS4 and NPAS4-Regulated Genes in Hippocampal Tissues of TumorGraft Mice-an Aging Connection

While the refinement of existing and the development of new chemotherapeutic regimens has significantly improved cancer treatment outcomes and patient survival, chemotherapy still causes many persistent side effects. Central nervous system (CNS) toxicity is of particular concern, as cancer patients experience significant deficits in memory, learning, cognition, and decision-making. These chemotherapy-induced cognitive changes are termed chemo brain, and manifest in more than half of cancer survivors. Moreover, recent studies have emerged suggesting that neurocognitive deficits manifest prior to cancer diagnosis and treatment, and thus may be associated with tumor presence, a phenomenon recently termed “tumor brain.” To dissect the molecular mechanisms of tumor brain, we used TumorGraft^TM models, wherein part of a patient's tumor is grafted into immune-deficient mice. Here, we analyzed molecular changes in the hippocampal tissues of mice carrying triple negative (TNBC) or progesterone receptor positive (PR+BC) xenografts. TNBC growth led to increased oxidative damage, as detected by elevated levels of 4-hydroxy-2-nonenal, a product of lipid peroxidation. Furthermore, the growth of TNBC and PR+BC tumors altered global gene expression in the murine hippocampus and affected multiple pathways implicated in PI3K-Akt and MAPK signaling, as well as other pathways crucial for the proper functioning of hippocampal neurons. TNBC and PR+BC tumor growth also led to a significant decrease in the levels of neuronal transcription factor NPAS4, a regulator that governs the expression of brain-derived neurotrophic factor (BDNF), and several other key brain neurotrophic factors and pro-survival molecules. The decreased expression of ERK1/2, NPAS4, and BDNF are also seen in neurodegenerative conditions and aging, and may constitute an important tumor brain mechanism.

Bidirectional and long-lasting control of alcohol-seeking behavior by corticostriatal LTP and LTD

Addiction is proposed to arise from alterations in synaptic strength via mechanisms of long-term potentiation (LTP) and depression (LTD). However, the causality between these synaptic processes and addictive behaviors is difficult to demonstrate. Here we report that LTP and LTD induction altered operant alcohol self-administration, a motivated drug-seeking behavior. We first induced LTP by pairing presynaptic glutamatergic stimulation with optogenetic postsynaptic depolarization in the dorsomedial striatum, a brain region known to control goal-directed behavior. Blockade of this LTP by NMDA-receptor inhibition unmasked an endocannabinoid-dependent LTD. In vivo application of the LTP-inducing protocol caused a long-lasting increase in alcohol-seeking behavior, while the LTD protocol decreased this behavior. We further identified that optogenetic LTP and LTD induction at cortical inputs onto striatal dopamine D1 receptor-expressing neurons controlled these behavioral changes. Our results demonstrate a causal link between synaptic plasticity and alcohol-seeking behavior and suggest that modulation of this plasticity may inspire a therapeutic strategy for addiction.

Pronounced Hyperactivity, Cognitive Dysfunctions, and BDNF Dysregulation in Dopamine Transporter Knock-out Rats

Dopamine (DA) controls many vital physiological functions and is critically involved in several neuropsychiatric disorders such as schizophrenia and attention deficit hyperactivity disorder. The major function of the plasma membrane dopamine transporter (DAT) is the rapid uptake of released DA into presynaptic nerve terminals leading to control of both the extracellular levels of DA and the intracellular stores of DA. Here, we present a newly developed strain of rats in which the gene encoding DAT knockout Rats (DAT-KO) has been disrupted by using zinc finger nuclease technology. Male and female DAT-KO rats develop normally but weigh less than heterozygote and wild-type rats and demonstrate pronounced spontaneous locomotor hyperactivity. While striatal extracellular DA lifetime and concentrations are significantly increased, the total tissue content of DA is markedly decreased demonstrating the key role of DAT in the control of DA neurotransmission. Hyperactivity of DAT-KO rats can be counteracted by amphetamine, methylphenidate, the partial Trace Amine-Associated Receptor 1 (TAAR1) agonist RO5203648 ((S)-4-(3,4-Dichloro-phenyl)-4,5-dihydro-oxazol-2-ylamine) and haloperidol. DAT-KO rats also demonstrate a deficit in working memory and sensorimotor gating tests, less propensity to develop obsessive behaviors and show strong dysregulation in frontostriatal BDNF function. DAT-KO rats could provide a novel translational model for human diseases involving aberrant DA function and/or mutations affecting DAT or related regulatory mechanisms.SIGNIFICANCE STATEMENT Here, we present a newly developed strain of rats in which the gene encoding the dopamine transporter (DAT) has been disrupted (Dopamine Transporter Knockout rats [DAT-KO rats]). DAT-KO rats display functional hyperdopaminergia accompanied by pronounced spontaneous locomotor hyperactivity. Hyperactivity of DAT-KO rats can be counteracted by amphetamine, methylphenidate, and a few other compounds exerting inhibitory action on dopamine-dependent hyperactivity. DAT-KO rats also demonstrate cognitive deficits in working memory and sensorimotor gating tests, less propensity to develop compulsive behaviors, and strong dysregulation in frontostriatal BDNF function. These observations highlight the key role of DAT in the control of brain dopaminergic transmission. DAT-KO rats could provide a novel translational model for human diseases involving aberrant dopamine functions.

Differential expression of the immediate early genes c-Fos, Arc, Egr-1, and Npas4 during long-term memory formation in the context preexposure facilitation effect (CPFE)

The context preexposure facilitation effect (CPFE) is a contextual fear conditioning paradigm in which learning about the context, acquiring the context-shock association, and retrieving/expressing contextual fear are temporally dissociated into three distinct phases (context preexposure, immediate-shock training, and retention). The current study examined changes in the expression of plasticity-associated immediate early genes (IEGs) during context and contextual fear memory formation on the preexposure and training days of the CPFE, respectively. Using adolescent Long-Evans rats, preexposure and training day expression of the IEGs c-Fos, Arc, Egr-1, and Npas4 in the medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and basolateral amygdala (BLA) was analyzed using qPCR as an extension of previous studies from our lab examining Egr-1 via in situ hybridization (Asok, Schreiber, Jablonski, Rosen, & Stanton, 2013; Schreiber, Asok, Jablonski, Rosen, & Stanton, 2014). In Expt. 1, context preexposure induced expression of c-Fos, Arc, Egr-1 and Npas4 significantly above that of home-cage (HC) controls in all three regions. In Expt. 2, immediate-shock was followed by a post-shock freezing test, resulting in increased mPFC c-Fos expression in a group preexposed to the training context but not a control group preexposed to an alternate context, indicating expression related to associative learning. This was not seen with other IEGs in mPFC or with any IEG in dHPC or BLA. Finally, when the post-shock freezing test was omitted in Expt. 3, training-related increases were observed in prefrontal c-Fos, Arc, Egr-1, and Npas4, hippocampal c-Fos, and amygdalar Egr-1 expression. These results indicate that context exposure in a post-shock freezing test re-engages IEG expression that may obscure associatively-induced expression during contextual fear conditioning. Additionally, these studies suggest a key role for long-term synaptic plasticity in the mPFC in supporting the CPFE.

Npas4 deficiency and prenatal stress interact to affect social recognition in mice

Neurodevelopmental disorders such as autism spectrum disorders and schizophrenia have an expansive array of reported genetic and environmental contributing factors. However, none of these factors alone can account for a substantial proportion of cases of either disorder. Instead, many gene-by-environment interactions are responsible for neurodevelopmental disturbances that lead to these disorders. The current experiment used heterozygous knock-out mice to examine a potential interaction between 2 factors commonly linked to neurodevelopmental disorders and cognitive deficit: imbalanced excitatory/inhibitory signaling in the cortex and prenatal stress (PNS) exposure. Both of these factors have been linked to disrupt GABAergic signaling in the prefrontal cortex (PFC), a common feature of neurodevelopmental disorders. The neuronal PAS domain protein 4 (Npas4) gene is instrumental in regulation of the excitatory/inhibitory balance in the cortex and hippocampus in response to activation. Npas4 heterozygous and wild-type male and female mice were exposed to either PNS or standard gestation, then evaluated during adulthood in social and anxiety behavioral measures. The combination of PNS and Npas4 deficiency in male mice impaired social recognition. This behavioral deficit was associated with decreased parvalbumin and cFos protein expression in the infralimbic region of the PFC following social stimulation in Npas4 heterozygous males. In contrast, females displayed fewer behavioral effects and molecular changes in PFC in response to PNS and decreased Npas4.

Cognitive appraisal of environmental stimuli induces emotion-like states in fish

The occurrence of emotions in non-human animals has been the focus of debate over the years. Recently, an interest in expanding this debate to non-tetrapod vertebrates and to invertebrates has emerged. Within vertebrates, the study of emotion in teleosts is particularly interesting since they represent a divergent evolutionary radiation from that of tetrapods, and thus they provide an insight into the evolution of the biological mechanisms of emotion. We report that Sea Bream exposed to stimuli that vary according to valence (positive, negative) and salience (predictable, unpredictable) exhibit different behavioural, physiological and neuromolecular states. Since according to the dimensional theory of emotion valence and salience define a two-dimensional affective space, our data can be interpreted as evidence for the occurrence of distinctive affective states in fish corresponding to each the four quadrants of the core affective space. Moreover, the fact that the same stimuli presented in a predictable vs. unpredictable way elicited different behavioural, physiological and neuromolecular states, suggests that stimulus appraisal by the individual, rather than an intrinsic characteristic of the stimulus, has triggered the observed responses. Therefore, our data supports the occurrence of emotion-like states in fish that are regulated by the individual's perception of environmental stimuli.

The stability of the transcriptome during the estrous cycle in four regions of the mouse brain

We analyzed the transcriptome of the C57BL/6J mouse hypothalamus, hippocampus, neocortex, and cerebellum to determine estrous cycle-specific changes in these four brain regions. We found almost 16,000 genes are present in one or more of the brain areas but only 210 genes, ∼1.3%, are significantly changed as a result of the estrous cycle. The hippocampus has the largest number of differentially expressed genes (DEGs) (82), followed by the neocortex (76), hypothalamus (63), and cerebellum (26). Most of these DEGs (186/210) are differentially expressed in only one of the four brain regions. A key finding is the unique expression pattern of growth hormone (Gh) and prolactin (Prl). Gh and Prl are the only DEGs to be expressed during only one stage of the estrous cycle (metestrus). To gain insight into the function of the DEGs, we examined gene ontology and phenotype enrichment and found significant enrichment for genes associated with myelination, hormone stimulus, and abnormal hormone levels. Additionally, 61 of the 210 DEGs are known to change in response to estrogen in the brain. 50 of the 210 genes differentially expressed as a result of the estrous cycle are related to myelin and oligodendrocytes and 12 of the 63 DEGs in the hypothalamus are oligodendrocyte- and myelin-specific genes. This transcriptomic analysis reveals that gene expression in the female mouse brain is remarkably stable during the estrous cycle and demonstrates that the genes that do fluctuate are functionally related.

SRF modulates seizure occurrence, activity induced gene transcription and hippocampal circuit reorganization in the mouse pilocarpine epilepsy model

A hallmark of temporal lobe epilepsy (TLE) is hippocampal neuronal demise and aberrant mossy fiber sprouting. In addition, unrestrained neuronal activity in TLE patients induces gene expression including immediate early genes (IEGs) such as Fos and Egr1.

We employed the mouse pilocarpine model to analyze the transcription factor (TF) serum response factor (SRF) in epileptogenesis, seizure induced histopathology and IEG induction. SRF is a neuronal activity regulated TF stimulating IEG expression as well as nerve fiber growth and guidance. Adult conditional SRF deficient mice (Srf^CaMKCreERT2) were more refractory to initial status epilepticus (SE) acquisition. Further, SRF deficient mice developed more spontaneous recurrent seizures (SRS). Genome-wide transcriptomic analysis uncovered a requirement of SRF for SE and SRS induced IEG induction (e.g. Fos, Egr1, Arc, Npas4, Btg2, Atf3). SRF was required for epilepsy associated neurodegeneration, mossy fiber sprouting and inflammation. We uncovered MAP kinase signaling as SRF target during epilepsy. Upon SRF ablation, seizure evoked induction of dual specific phosphatases (Dusp5 and Dusp6) was reduced. Lower expression of these negative ERK kinase regulators correlated with altered P-ERK levels in epileptic Srf mutant animals.

Overall, this study uncovered an SRF contribution to several processes of epileptogenesis in the pilocarpine model.

Genetic strategies to access activated neurons

A major goal of modern neuroscience is to understand how ensembles of neurons participate in neural circuits underlying behavior. The recent explosion of genetically-encoded circuit analysis tools has allowed neuroscientists to characterize molecularly-defined neuronal types with unprecedented detail. However, since neurons defined by molecular expression can be functionally heterogeneous, targeting circuit analysis tools to neurons based on their activity is critical to elucidating the neural basis of behavior. Here we review genetic strategies to access activated neurons and characterize their functional properties, molecular profiles, connectivity, and causal roles in sensory-coding, memory, and valence-encoding. We also discuss future possibilities for improving these strategies and using them to screen brain-wide activity patterns underlying adaptive and maladaptive behaviors.

Upregulation of neurotrophins by S 47445, a novel positive allosteric modulator of AMPA receptors in aged rats

At molecular levels, it has been shown that aging is associated with alterations in neuroplastic mechanisms. In this study, it was examined if the altered expression of neurotrophins observed in aged rats could be corrected by a chronic treatment with S 47445 (1-3-10mg/kg, p.o.), a novel selective positive allosteric modulator of the AMPA receptors. Both the mRNA and the protein levels of the neurotrophins Bdnf, NT-3 and Ngf were specifically measured in the prefrontal cortex and hippocampus (ventral and dorsal) of aged rats. It was found that 2-week-treatment with S 47445 corrected the age-related deficits of these neurotrophins and/or positively modulated their expression in comparison to vehicle aged rats in the range of procognitive and antidepressant active doses in rodents. Collectively, the ability of S 47445 to modulate various neurotrophins demonstrated its neurotrophic properties in two major brain structures involved in cognition and mood regulation suggesting its therapeutic potential for improving several diseases such as Alzheimer's disease and/or Major Depressive Disorders.

Genetic Feedback Regulation of Frontal Cortical Neuronal Ensembles Through Activity-Dependent Arc Expression and Dopaminergic Input

Mental functions involve coordinated activities of specific neuronal ensembles that are embedded in complex brain circuits. Aberrant neuronal ensemble dynamics is thought to form the neurobiological basis of mental disorders. A major challenge in mental health research is to identify these cellular ensembles and determine what molecular mechanisms constrain their emergence and consolidation during development and learning. Here, we provide a perspective based on recent studies that use activity-dependent gene Arc/Arg3.1 as a cellular marker to identify neuronal ensembles and a molecular probe to modulate circuit functions. These studies have demonstrated that the transcription of Arc is activated in selective groups of frontal cortical neurons in response to specific behavioral tasks. Arc expression regulates the persistent firing of individual neurons and predicts the consolidation of neuronal ensembles during repeated learning. Therefore, the Arc pathway represents a prototypical example of activity-dependent genetic feedback regulation of neuronal ensembles. The activation of this pathway in the frontal cortex starts during early postnatal development and requires dopaminergic (DA) input. Conversely, genetic disruption of Arc leads to a hypoactive mesofrontal dopamine circuit and its related cognitive deficit. This mutual interaction suggests an auto-regulatory mechanism to amplify the impact of neuromodulators and activity-regulated genes during postnatal development. Such a mechanism may contribute to the association of mutations in dopamine and Arc pathways with neurodevelopmental psychiatric disorders. As the mesofrontal dopamine circuit shows extensive activity-dependent developmental plasticity, activity-guided modulation of DA projections or Arc ensembles during development may help to repair circuit deficits related to neuropsychiatric disorders.

Structural characterization of mammalian bHLH-PAS transcription factors

The mammalian basic helix-loop-helix-PER-ARNT-SIM (bHLH-PAS) transcription factors share common architectural features that include a bHLH DNA-binding domain and tandemly positioned PAS domains. The sixteen members of this family include the hypoxia-inducible factors (HIF-1α and HIF-2α), ARNT (also known as HIF-1β), CLOCK and BMAL1. Most bHLH-PAS proteins have been genetically linked to variety of diseases in humans, including cancers, metabolic syndromes and psychiatric conditions. To function as transcription factors, the bHLH-PAS proteins must form heterodimeric complexes. Recent crystallographic studies of HIF-α-ARNT and CLOCK-BMAL1 complexes have unveiled the organization of their multi-domain bHLH-PAS-A-PAS-B segments, revealing how these architectures can give rise to unique patterns of heterodimerization. As our structural understanding becomes better integrated with ligand-discovery and target gene identification, a more comprehensive picture of their architectural and functional properties will emerge.

A robust activity marking system for exploring active neuronal ensembles

Understanding how the brain captures transient experience and converts it into long lasting changes in neural circuits requires the identification and investigation of the specific ensembles of neurons that are responsible for the encoding of each experience. We have developed a Robust Activity Marking (RAM) system that allows for the identification and interrogation of ensembles of neurons. The RAM system provides unprecedented high sensitivity and selectivity through the use of an optimized synthetic activity-regulated promoter that is strongly induced by neuronal activity and a modified Tet-Off system that achieves improved temporal control. Due to its compact design, RAM can be packaged into a single adeno-associated virus (AAV), providing great versatility and ease of use, including application to mice, rats, flies, and potentially many other species. Cre-dependent RAM, CRAM, allows for the study of active ensembles of a specific cell type and anatomical connectivity, further expanding the RAM system's versatility.