Elucidating the neural and genetic factors underlying psychiatric illness is hampered by current methods of clinical diagnosis. The identification and investigation of clinical endophenotypes may be one solution, but represents a considerable challenge in human subjects. Here we report that mice heterozygous for a null mutation of the alpha-isoform of calcium/calmodulin-dependent protein kinase II (alpha-CaMKII+/-) have profoundly dysregulated behaviours and impaired neuronal development in the dentate gyrus (DG). The behavioral abnormalities include a severe working memory deficit and an exaggerated infradian rhythm, which are similar to symptoms seen in schizophrenia, bipolar mood disorder and other psychiatric disorders. Transcriptome analysis of the hippocampus of these mutants revealed that the expression levels of more than 2000 genes were significantly changed. Strikingly, among the 20 most downregulated genes, 5 had highly selective expression in the DG. Whereas BrdU incorporated cells in the mutant mouse DG was increased by more than 50 percent, the number of mature neurons in the DG was dramatically decreased. Morphological and physiological features of the DG neurons in the mutants were strikingly similar to those of immature DG neurons in normal rodents. Moreover, c-Fos expression in the DG after electric footshock was almost completely and selectively abolished in the mutants. Statistical clustering of human post-mortem brains using 10 genes differentially-expressed in the mutant mice were used to classify individuals into two clusters, one of which contained 16 of 18 schizophrenic patients. Nearly half of the differentially-expressed probes in the schizophrenia-enriched cluster encoded genes that are involved in neurogenesis or in neuronal migration/maturation, including calbindin, a marker for mature DG neurons. Based on these results, we propose that an "immature DG" in adulthood might induce alterations in behavior and serve as a promising candidate endophenotype of schizophrenia and other human psychiatric disorders.
The alpha-isoform of calcium/calmodulin-dependent protein kinase II (α-CaMKII) is expressed abundantly in the forebrain and is considered to have an essential role in synaptic plasticity and cognitive function. Previously, we reported that mice heterozygous for a null mutation of α-CaMKII (α-CaMKII+/−) have profoundly dysregulated behaviors including a severe working memory deficit, which is an endophenotype of schizophrenia and other psychiatric disorders. In addition, we found that almost all the neurons in the dentate gyrus (DG) of the mutant mice failed to mature at molecular, morphological and electrophysiological levels. In the present study, to identify the brain substrates of the working memory deficit in the mutant mice, we examined the expression of the immediate early genes (IEGs), c-Fos and Arc, in the brain after a working memory version of the eight-arm radial maze test. c-Fos expression was abolished almost completely in the DG and was reduced significantly in neurons in the CA1 and CA3 areas of the hippocampus, central amygdala, and medial prefrontal cortex (mPFC). However, c-Fos expression was intact in the entorhinal and visual cortices. Immunohistochemical studies using arc promoter driven dVenus transgenic mice demonstrated that arc gene activation after the working memory task occurred in mature, but not immature neurons in the DG of wild-type mice. These results suggest crucial insights for the neural circuits underlying spatial mnemonic processing during a working memory task and suggest the involvement of α-CaMKII in the proper maturation and integration of DG neurons into these circuits.
α-CaMKII; working memory; dentate gyrus; schizophrenia; immediate-early genes; c-fos
Adequate maturation of neurons and their integration into the hippocampal circuit is crucial for normal cognitive function and emotional behavior, and disruption of this process could cause disturbances in mental health. Previous reports have shown that mice heterozygous for a null mutation in α-CaMKII, which encodes a key synaptic plasticity molecule, display abnormal behaviors related to schizophrenia and other psychiatric disorders. In these mutants, almost all neurons in the dentate gyrus are arrested at a pseudoimmature state at the molecular and electrophysiological levels, a phenomenon defined as “immature dentate gyrus (iDG).” To date, the iDG phenotype and shared behavioral abnormalities (including working memory deficit and hyperlocomotor activity) have been discovered in Schnurri-2 knockout, mutant SNAP-25 knock-in, and forebrain-specific calcineurin knockout mice. In addition, both chronic fluoxetine treatment and pilocarpine-induced seizures reverse the neuronal maturation, resulting in the iDG phenotype in wild-type mice. Importantly, an iDG-like phenomenon was observed in post-mortem analysis of brains from patients with schizophrenia/bipolar disorder. Based on these observations, we proposed that the iDG is a potential endophenotype shared by certain types of neuropsychiatric disorders. This review summarizes recent data describing this phenotype and discusses the data's potential implication in elucidating the pathophysiology of neuropsychiatric disorders.
Neuregulin 1 (NRG1) and the γ-secretase subunit APH1B have been previously implicated as genetic risk factors for schizophrenia and schizophrenia relevant deficits have been observed in rodent models with loss of function mutations in either gene. Here we show that the Aph1b-γ-secretase is selectively involved in Nrg1 intracellular signalling. We found that Aph1b-deficient mice display a decrease in excitatory synaptic markers. Electrophysiological recordings show that Aph1b is required for excitatory synaptic transmission and plasticity. Furthermore, gain and loss of function and genetic rescue experiments indicate that Nrg1 intracellular signalling promotes dendritic spine formation downstream of Aph1b-γ-secretase in vitro and in vivo. In conclusion, our study sheds light on the physiological role of Aph1b-γ-secretase in brain and provides a new mechanistic perspective on the relevance of NRG1 processing in schizophrenia.
Schizophrenia affects around 1% of the world's population, with symptoms including hallucinations and delusions, apathy and cognitive impairments. Multiple genes and environmental factors interact to increase the risk of schizophrenia, making the causes of the disease—which can differ between individuals—difficult to disentangle. However, Schizophrenia is known to be associated with a reduction in the number of dendritic spines, the small protrusions that allow brain cells to receive inputs from other brain cells.
One gene that has repeatedly been implicated in schizophrenia is neuregulin 1 (NRG1), which encodes a signalling protein with more than thirty different variants. One of these variants, type III NRG1, is located on the cell membrane. An enzyme called γ-secretase can cleave the 'tail' of this protein, which means that the tail becomes free to move to the nucleus of the cell, where it can alter the expression of genes.
Fazzari et al. have now studied how different γ-secretases interact with type III NRG1 by using genetic techniques to remove a specific part of the enzymes in the brains of mice. The brain cells of these mutant mice contained fewer dendritic spines than mice with normal γ-secretases. However, the number of dendritic spines in the mutant mice could be restored by introducing γ-secretase.
These results are consistent with a model in which mutations that remove the ability of γ-secretases to cleave NRG1 lead to some of the structural and functional changes in the brain that are associated with schizophrenia. An improved understanding of the properties of the various γ-secretases could also lead to the design of safer versions of drugs called γ-secretase modulators that are used to treat Alzheimer's disease.
Aph1b; Gamma-secretase; NRG1; schizophrenia; Alzheimerand's disease; mouse
Brain-derived neurotrophic factor (BDNF) has been implicated in the pathophysiology of schizophrenia, yet its role in the development of specific symptoms is unclear. Methamphetamine (METH) users have an increased risk of psychosis and schizophrenia, and METH-treated animals have been used extensively as a model to study the positive symptoms of schizophrenia. We investigated whether METH treatment in BDNF heterozygous (HET) mutant mice has cumulative effects on sensorimotor gating, including the disruptive effects of psychotropic drugs. BDNF HETs and wildtype (WT) littermates were treated during young adulthood with METH and, following a 2-week break, prepulse inhibition (PPI) was examined. At baseline, BDNF HETs showed reduced PPI compared to WT mice irrespective of METH pre-treatment. An acute challenge with amphetamine (AMPH) disrupted PPI but male BDNF HETs were more sensitive to this effect, irrespective of METH pre-treatment. In contrast, female mice treated with METH were less sensitive to the disruptive effects of AMPH, and there were no effects of BDNF genotype. Similar changes were not observed in the response to an acute apomorphine (APO) or MK-801 challenge. These results show that genetically-induced reduction of BDNF caused changes in a behavioral endophenotype relevant to the positive symptoms of schizophrenia. However, major sex differences were observed in the effects of a psychotropic drug challenge on this behavior. These findings suggest sex differences in the effects of BDNF depletion and METH treatment on the monoamine signaling pathways that regulate PPI. Given that these same pathways are thought to contribute to the expression of positive symptoms in schizophrenia, this work suggests that there may be significant sex differences in the pathophysiology underlying these symptoms. Elucidating these sex differences may be important for our understanding of the neurobiology of schizophrenia and developing better treatments strategies for the disorder.
methamphetamine; prepulse inhibition; schizophrenia; BDNF
Human studies suggest that a variety of prenatal stressors are related to high risk for cognitive and behavioral abnormalities associated with psychiatric illness (Markham and Koenig, 2011). Recently, a down-regulation in the expression of GABAergic genes (i.e., glutamic acid decarboxylase 67 and reelin) associated with DNA methyltransferase (DNMT) overexpression in GABAergic neurons has been regarded as a characteristic phenotypic component of the neuropathology of psychotic disorders (Guidotti et al., 2011).
Here, we characterized mice exposed to prenatal restraint stress (PRS) in order to study neurochemical and behavioral abnormalities related to development of schizophrenia in the adult. Offspring born from non-stressed mothers (control mice) showed high levels of DNMT1 and 3a mRNA expression in the frontal cortex at birth, but these levels progressively decreased at post-natal days (PND) 7, 14, and 60. Offspring born from stressed mothers (PRS mice) showed increased levels of DNMTs compared to controls at all time-points studied including at birth and at PND 60. Using GAD67-GFP transgenic mice, we established that, in both control and PRS mice, high levels of DNMT1 and 3a were preferentially expressed in GABAergic neurons of frontal cortex and hippocampus. Importantly, the overexpression of DNMT in GABAergic neurons was associated with a decrease in reelin and GAD67 expression in PRS mice in early and adult life. PRS mice also showed an increased binding of DNMT1 and MeCP2, and an increase in 5-methylcytosine and 5-hydroxymethylcytosine in specific CpG-rich regions of the reelin and GAD67 promoters. Thus, the epigenetic changes in PRS mice are similar to changes observed in the post-mortem brains of psychiatric patients. Behaviorally, adult PRS mice showed hyperactivity and deficits in social interaction, prepulse inhibition, and fear conditioning that were corrected by administration of valproic acid (a histone deacetylase inhibitor) or clozapine (an atypical antipsychotic with DNA-demethylation activity). Taken together, these data show that prenatal stress in mice induces abnormalities in the DNA methylation network and in behaviors indicative of a schizophrenia-like phenotype. Thus, PRS mice may be a valid model for the investigation of new drugs for schizophrenia treatment targeting DNA methylation.
schizophrenia; DNA methyltransferase; prenatal stress; epigenetic; antipsychotic; valproic acid
Schnurri-2 (Shn-2), an nuclear factor-κB site-binding protein, tightly binds to the enhancers of major histocompatibility complex class I genes and inflammatory cytokines, which have been shown to harbor common variant single-nucleotide polymorphisms associated with schizophrenia. Although genes related to immunity are implicated in schizophrenia, there has been no study showing that their mutation or knockout (KO) results in schizophrenia. Here, we show that Shn-2 KO mice have behavioral abnormalities that resemble those of schizophrenics. The mutant brain demonstrated multiple schizophrenia-related phenotypes, including transcriptome/proteome changes similar to those of postmortem schizophrenia patients, decreased parvalbumin and GAD67 levels, increased theta power on electroencephalograms, and a thinner cortex. Dentate gyrus granule cells failed to mature in mutants, a previously proposed endophenotype of schizophrenia. Shn-2 KO mice also exhibited mild chronic inflammation of the brain, as evidenced by increased inflammation markers (including GFAP and NADH/NADPH oxidase p22 phox), and genome-wide gene expression patterns similar to various inflammatory conditions. Chronic administration of anti-inflammatory drugs reduced hippocampal GFAP expression, and reversed deficits in working memory and nest-building behaviors in Shn-2 KO mice. These results suggest that genetically induced changes in immune system can be a predisposing factor in schizophrenia.
Animal models; Dentate Gyrus; Endophenotype; Inflammation; Mood/Anxiety/Stress Disorders; Psychiatry & Behavioral Sciences; Schizophrenia/Antipsychotics; animal models; dentate gyrus; endophenotype; inflammation; mood/anxiety/stress disorders; psychiatry & behavioral sciences; schizophrenia/antipsychotics
Schizophrenia is a complex disorder characterized by wide-ranging cognitive impairments, including deficits in learning as well as sensory gating. The causes of schizophrenia are unknown, but alterations in intracellular G-protein signaling pathways are among the molecular changes documented in patients with schizophrenia. Using the CaMKIIα promoter to drive expression in neurons within the forebrain, we have developed transgenic mice that express a constitutively active form of Gsα (Gsα*), the G protein that couples receptors such as the D1 and D5 dopamine receptors to adenylyl cyclase. We have also generated mice in which the CaMKIIα promoter drives expression of a dominant-negative form of protein kinase A, R(AB). Here, we examine startle responses and prepulse inhibition of the startle reflex (PPI) in these Gsα* and R(AB) transgenic mice. Gsα* transgenic mice exhibited selective deficits in PPI, without exhibiting alterations in the startle response, whereas no deficit in startle or PPI was found in the R(AB) transgenic mice. Thus, overstimulation of the cAMP/PKA pathway disrupts PPI, but the cAMP/PKA pathway may not be essential for sensorimotor gating. Gsα* transgenic mice may provide an animal model of certain endophenotypes of schizophrenia, because of the similarities between them and patients with schizophrenia in G-protein function, hippocampus-dependent learning, and sensorimotor gating.
G proteins; protein kinase A; startle response; prepulse inhibition; schizophrenia; animal model
A novel animal model highlights the link between Akt dysfunction, reduced cortical dopamine function, norepinephrine transporters, and schizophrenia-like behaviors.
The mammalian target of rapamycin (mTOR) complex 2 (mTORC2) is a multimeric signaling unit that phosphorylates protein kinase B/Akt following hormonal and growth factor stimulation. Defective Akt phosphorylation at the mTORC2-catalyzed Ser473 site has been linked to schizophrenia. While human imaging and animal studies implicate a fundamental role for Akt signaling in prefrontal dopaminergic networks, the molecular mechanisms linking Akt phosphorylation to specific schizophrenia-related neurotransmission abnormalities have not yet been described. Importantly, current understanding of schizophrenia suggests that cortical decreases in DA neurotransmission and content, defined here as cortical hypodopaminergia, contribute to both the cognitive deficits and the negative symptoms characteristic of this disorder. We sought to identify a mechanism linking aberrant Akt signaling to these hallmarks of schizophrenia. We used conditional gene targeting in mice to eliminate the mTORC2 regulatory protein rictor in neurons, leading to impairments in neuronal Akt Ser473 phosphorylation. Rictor-null (KO) mice exhibit prepulse inhibition (PPI) deficits, a schizophrenia-associated behavior. In addition, they show reduced prefrontal dopamine (DA) content, elevated cortical norepinephrine (NE), unaltered cortical serotonin (5-HT), and enhanced expression of the NE transporter (NET). In the cortex, NET takes up both extracellular NE and DA. Thus, we propose that amplified NET function in rictor KO mice enhances accumulation of both NE and DA within the noradrenergic neuron. This phenomenon leads to conversion of DA to NE and ultimately supports both increased NE tissue content as well as a decrease in DA. In support of this hypothesis, NET blockade in rictor KO mice reversed cortical deficits in DA content and PPI, suggesting that dysregulation of DA homeostasis is driven by alteration in NET expression, which we show is ultimately influenced by Akt phosphorylation status. These data illuminate a molecular link, Akt regulation of NET, between the recognized association of Akt signaling deficits in schizophrenia with a specific mechanism for cortical hypodopaminergia and hypofunction. Additionally, our findings identify Akt as a novel modulator of monoamine homeostasis in the cortex.
Schizophrenia is a disorder caused by multiple genetic and environmental variables. Despite the disease's heterogeneous causes, current hypotheses suggest that dysfunction of dopamine signaling in the brain is one of the final common pathways involved. One gene that may be involved encodes the protein kinase Akt, which is regulated by hormones, growth factors, and neurotransmitter receptors. In this study, we examined the potential molecular mechanisms linking Akt dysregulation to cortical hypodopaminergia, and ultimately to the pathology of schizophrenia. Using transgenic technology, we generated a mouse model with defective neuronal Akt signaling. Neurochemical and behavioral phenotypes associated with schizophrenia include decreases in prefrontal dopamine signaling and deficits in sensorimotor gating, two phenotypes we observed in our transgenic animals. Further, we observed that impaired cortical Akt activity significantly enhanced norepinephrine transporter function. Interestingly, we found that by blocking this transporter, we could reverse the cortical hypodopaminergia and behavioral deficits seen in our transgenic mice. The norepinephrine transporter is a presynaptic membrane protein that is critical for maintaining both norepinephrine and cortical dopamine homeostasis. Taken together, this work supports the potential for targeting both Akt and the norepinephrine transporter for treating dopamine-related mood disorders.
Aging-related cognitive declines are well documented in humans and animal models. Yet the synaptic and molecular mechanisms responsible for cognitive aging are not well understood. Here we demonstrated age-dependent deficits in long-term synaptic plasticity and loss of dendritic spines in the hippocampus of aged Fisher 344 rats, which were closely associated with reduced histone acetylation, upregulation of histone deacetylase 2 (HDAC2), and decreased expression of a histone acetyltransferase. Further analysis showed that one of the key genes affected by such changes was the brain-derived neurotrophic factor (Bdnf) gene. Age-dependent reductions in H3 and H4 acetylation were detected within multiple promoter regions of the Bdnf gene, leading to a significant decrease in BDNF expression and impairment of downstream signaling in the aged hippocampus. These synaptic and signaling deficits could be rescued by enhancing BDNF and trkB expression via HDAC inhibition or by directly activating trkB receptors with 7, 8-dihydroxyflavone, a newly identified, selective agonist for trkB. Taken together, our findings suggest that age-dependent declines in chromatin histone acetylation and the resulting changes in BDNF expression and signaling are key mechanisms underlying the deterioration of synaptic function and structure in the aging brain. Furthermore, epigenetic or pharmacological enhancement of BDNF-trkB signaling could be a promising strategy for reversing cognitive aging.
One approach to understanding the genetic complexity of schizophrenia is to study associated behavioral and biological phenotypes that may be more directly linked to genetic variation.
To identify single nucleotide polymorphisms associated with general cognitive ability (“g”) in people with schizophrenia and controls.
Genome-wide association study (GWAS), followed by analyses in unaffected siblings and independent schizophrenia samples, functional magnetic resonance imaging studies of brain physiology in vivo, and RNA sequencing in post-mortem brain samples.
The discovery cohort and unaffected siblings were participants in the NIMH Clinical Brain Disorders Branch schizophrenia genetics studies. Additional schizophrenia cohorts were from psychiatric treatment settings in the United States, Japan, and Germany.
The discovery cohort comprised 339 with schizophrenia and 363 community controls. Follow-up analyses studied 147 unaffected siblings of the schizophrenia cases, and independent schizophrenia samples of 279, 95 and 294 participants. Imaging analyses included 87 schizophrenia cases and 397 controls. Brain tissue samples were available for 64 cases and 61 controls.
Main Outcome Measures
We studied genome-wide association with g, by group, in the discovery cohort. We used selected genotypes to test specific associations in unaffected siblings and independent schizophrenia samples. Imaging analyses focused on activation in prefrontal cortex during working memory. Brain tissue studies yielded mRNA expression levels for RefSeq transcripts.
The schizophrenia discovery cohort showed GWAS-significant association of g with polymorphisms in sodium channel gene SCN2A, accounting for 10.4% of g variance (rs10174400, P=9.27×10−10). Controls showed a trend for g/genotype association with reversed allelic directionality. The genotype-by-group interaction was also GWAS-significant (P=1.75×10−9). Siblings showed a genotype association with g parallel to the schizophrenia group, and the same interaction pattern. Parallel, but weaker, associations with cognition were found in independent schizophrenia samples. Imaging analyses showed a similar pattern of genotype associations by group and genotype-by-group interaction. RNA sequencing revealed reduced expression in 2 of 3 SCN2A alternative transcripts in the patient group, with genotype-by-group interaction, that again paralleled the cognition effects.
The findings implicate SCN2A and sodium channel biology in cognitive impairment in schizophrenia cases and unaffected relatives, and may facilitate development of cognition-enhancing treatments.
Increasing evidence suggests that epigenetic factors have critical roles in gene
regulation in neuropsychiatric disorders and in aging, both of which are
typically associated with a wide range of gene expression abnormalities. Here,
we have used chromatin immunoprecipitation-qPCR to measure levels of acetylated
histone H3 at lysines 9/14 (ac-H3K9K14), two epigenetic marks associated
with transcriptionally active chromatin, at the promoter regions of eight
schizophrenia-related genes in n=82 postmortem prefrontal
cortical samples from normal subjects and those with schizophrenia and bipolar
disorder. We find that promoter-associated ac-H3K9K14 levels are correlated with
gene expression levels, as measured by real-time qPCR for several genes,
including, glutamic acid decarboxylase 1 (GAD1), 5-hydroxytryptamine
receptor 2C (HTR2C), translocase of outer mitochondrial membrane 70
homolog A (TOMM70A) and protein phosphatase 1E (PPM1E).
Ac-H3K9K14 levels of several of the genes tested were significantly negatively
associated with age in normal subjects and those with bipolar disorder, but not
in subjects with schizophrenia, whereby low levels of histone acetylation were
observed in early age and throughout aging. Consistent with this observation,
significant hypoacetylation of H3K9K14 was detected in young subjects with
schizophrenia when compared with age-matched controls. Our results demonstrate
that gene expression changes associated with psychiatric disease and aging
result from epigenetic mechanisms involving histone acetylation. We further find
that treatment with a histone deacetylase (HDAC) inhibitor alters the expression
of several candidate genes for schizophrenia in mouse brain. These findings may
have therapeutic implications for the clinical use of HDAC inhibitors in
bipolar disorder; chromatin; epigenetic; HDAC inhibitor; schizophrenia
Behavioral genetic studies of humans have associated variation in the DTNBP1 gene with schizophrenia and its cognitive deficit phenotypes. The protein coded for by DTNBP1, dysbindin, is expressed within forebrain glutamatergic neurons, where it interacts with proteins involved in vesicular trafficking and exocytosis. In order to further delineate the cellular, physiological and behavioral phenotypes associated with reduced dysbindin expression, we conducted studies in mice carrying a null mutation within the dtnbp1 gene. Dysbindin mutants exhibited impairments of spatial working memory as compared with wild-type controls; heterozygous mice exhibited intermediate levels of cognitive dysfunction. Deep layer pyramidal neurons recorded in the prefrontal cortex of mutant mice exhibited reductions in paired-pulse facilitation, and evoked and miniature excitatory post-synaptic currents, indicating a difference in the function of pre-synaptic glutamatergic terminals, as well as elevated spike thresholds. Taken together, these data indicate that dysbindin potently regulates excitatory transmission in prefrontal cortex, potentially through a pre-synaptic mechanism, and consequently modulates cognitive functions depending upon this brain region, providing new insights into the molecular mechanisms underlying cortical dysfunction in schizophrenia.
working memory; schizophrenia; glutamate; cognition; excitatory; pre-synaptic
Epigenetic regulation of chromatin structure is an essential molecular mechanism that contributes to the formation of synaptic plasticity and long-term memory (LTM). An important regulatory process of chromatin structure is acetylation and deacetylation of histone proteins. Inhibition of histone deacetylase (HDAC) increases acetylation of histone proteins and facilitate learning and memory. Nitric oxide (NO) signaling pathway has a role in synaptic plasticity, LTM and regulation of histone acetylation. We have previously shown that NO signaling pathway is required for contextual fear conditioning. The present study investigated the effects of systemic administration of the HDAC inhibitor sodium butyrate (NaB) on fear conditioning in neuronal nitric oxide synthase (nNOS) knockout (KO) and wild type (WT) mice. The effect of single administration of NaB on total H3 and H4 histone acetylation in hippocampus and amygdala was also investigated. A single administration of NaB prior to fear conditioning a) rescued contextual fear conditioning of nNOS KO mice, and b) had long-term (weeks) facilitatory effect on the extinction of cued fear memory of WT mice. The facilitatory effect of NaB on extinction of cued fear memory of WT mice was confirmed in a study whereupon NaB was administered during extinction. Results suggest that a) the rescue of contextual fear conditioning in nNOS KO mice is associated with NaB-induced increase in H3 histone acetylation, and b) the accelerated extinction of cued fear memory in WT mice is associated with NaB-induced increase in H4 histone acetylation. Hence, a single administration of HDAC inhibitor may rescue NO-dependent cognitive deficits and afford a long-term accelerating effect on extinction of fear memory of WT mice.
histone acetylation; fear conditioning; nitric oxide (NO); extinction
Deficits in auditory processing are among the best documented endophenotypes in schizophrenia, possibly due to loss of excitatory synaptic connections. Dendritic spines, the principal post-synaptic target of excitatory projections, are reduced in schizophrenia. p21-activated kinase 1 (PAK1) regulates both the actin cytoskeleton and dendritic spine density, and is a downstream effector of both kalirin and CDC42, both of which have altered expression in schizophrenia. This study sought to determine if there is decreased auditory cortex PAK1 protein expression in schizophrenia through the use of quantitative western blots of 25 schizophrenia subjects and matched controls. There was no significant change in PAK1 level detected in the schizophrenia subjects in our cohort. PAK1 protein levels within subject pairs correlated positively with prior measures of total kalirin protein in the same pairs. PAK1 level also correlated with levels of a marker of dendritic spines, spinophilin. These latter two findings suggest that the lack of change in PAK1 level in schizophrenia is not due to limited sensitivity of our assay to detect meaningful differences in PAK1 protein expression. Future studies are needed to evaluate whether alterations in PAK1 phosphorylation states, or alterations in protein expression of other members of the PAK family, are present in schizophrenia.
The discovery of candidate susceptibility genes for schizophrenia and the generation of mice lacking proteins that reproduce biochemical processes that are disrupted in this mental illness offer unprecedented opportunities for improved modelling of this complex disorder. Several lines of evidence indicate that obstetrical complications, as well as fetal or neonatal exposure to viral infection, are predisposing events for some forms of schizophrenia. These environmental events can be modelled in animals, resulting in some of the characteristic features of schizophrenia; however, animal models have yet to be developed that encompass both environmental and genetic aspects of this mental illness. A large number of candidate schizophrenia susceptibility genes have been identified that encode proteins implicated in the regulation of synaptic plasticity, neurotransmission, neuronal migration, cell adherence, signal transduction, energy metabolism and neurite outgrowth. In support of the importance of these processes in schizophrenia, mice that have reduced levels or completely lack proteins that control glutamatergic neurotransmission, neuronal migration, cell adherence, signal transduction, neurite outgrowth and synaptic plasticity display many features reminiscent of schizophrenia. In the present review, we discuss strategies for modelling schizophrenia that involve treating mice that bear these mutations in a variety of ways to better model both environmental and genetic factors responsible for this complex mental illness according to a “two-hit hypothesis.” Because rodents are able to perform complex cognitive tasks using odour but not visual or auditory cues, we hypothesize that olfactory-based tests of cognitive performance should be used to search for novel therapeutics that ameliorate the cognitive deficits that are a feature of this devastating mental disorder.
environment; genetics; models, animal; pregnancy complications; schizophrenia
Schizophrenia is a substantially heritable disorder associated with disrupted neural transmission, as well as dysfunction of brain systems involved in higher cognitive processes. Among the several putative candidate genes for schizophrenia, the gene encoding dystrobrevin-binding-protein-1 (aka dysbindin) is associated with cognitive impairments, including memory and attention deficits, in both schizophrenia patients and non-schizophrenic individuals. The mechanism underlying these deficits is thought to be based in changes in glutamatergic and dopaminergic function within corticostriatal networks, circuitry known to be critical for schizophrenia. Recent support for this hypothesis derives from the study of mice with a null mutation in the dysbindin gene that exhibit memory dysfunction and abnormalities in excitatory neurotransmission in prefrontal and hippocampal networks. At a cellular level, dysbindin is thought to mediate pre-synaptic glutamatergic transmission. Here, we investigated whether loss of dysbindin expression also affects postsynaptic NMDA receptor function. We show that decreases in dysbindin are associated with specific decreases in NMDA-evoked currents in prefrontal pyramidal neurons, as well as decreases in expression of the obligatory NMDA receptor subunit (NR1). Furthermore, the degree of NR1 expression directly correlates with performance on a spatial working memory task, providing a mechanistic explanation for cognitive changes previously associated with dysbindin expression. These data show a significant down-regulation of NMDA receptors due to dysbindin deficiency and illuminate molecular mechanisms mediating the association between dysbindin insufficiency and cognitive impairments associated with schizophrenia, encouraging study of the dysbindin/NR1 expression association in humans with and at risk for the disease.
dysbindin; DTNBP1; NMDA; glutamate; schizophrenia; working memory
The role of the cerebellum in coordinating mental activity is supported by its connections with cerebral regions involved in cognitive/affective functioning, with decreased activities on functional neuroimaging observed in the cerebellum of schizophrenia patients performing mental tasks. Brain-derived neurotrophic factor (BDNF)-induced activation of tyrosine kinase B (TrkB) is essential to synaptic plasticity. We hypothesized that alterations in BDNF and TrkB expression in the cerebellum were associated with schizophrenia and affective disorders.
We employed immunohistochemistry and immunoblotting to quantify protein expression of BDNF and TrkB in the cerebellum of patients with schizophrenia, bipolar disorder, and major depression compared to controls (n=15 each).
While TrkB immunoreactivity in each of the molecular and granule-cell layers was reduced in all 3 disease groups (12–34%) compared to the control (P=0.018 and 0.038, respectively, ANOVA), only the reduction in bipolar disorder remained statistically significant upon Tukey-Kramer post hoc analyses (P=0.019 and 0.021, respectively). Apparent decreases in BDNF immunoreactivity in all 3 disease groups (12–30%) compared to the control were not statistically significant. TrkB immunoreactivity was not significantly associated with any of the demographic, clinical, and postmortem variables. Immunoblotting displayed an 85-kDa TrkB-immunoreactive band, consistent with a truncated isoform, in all 60 cases.
On immunoblotting, apparent decreases in 85-kDa-TrkB levels in all 3 disease groups compared to the control were not statistically significant.
Our finding of reduced TrkB expression in bipolar disorder suggests that dysregulation of TrkB-mediated neurotrophin signaling in the cerebellum may play a role in the pathophysiology of this disease.
Bipolar disorder; Brain-derived neurotrophic factor; Cerebellum; Major depression; Schizophrenia; TrkB
There is substantial evidence, both pharmacological and genetic, that hypofunction of the N-methyl-D-aspartate receptor (NMDAR) is a core pathophysiological feature of schizophrenia. There are morphological brain changes associated with schizophrenia, including perturbations in the dendritic morphology of cortical pyramidal neurons and reduction in cortical volume. Our experiments investigated whether these changes in dendritic morphology could be recapitulated in a genetic model of NMDAR hypofunction, the serine racemase knockout (SR−/−) mouse. Pyramidal neurons in primary somatosensory cortex (S1) of SR−/− mice had reductions in the complexity, total length, and spine density of apical and basal dendrites. In accordance with reduced cortical neuropil, SR−/− mice also had reduced cortical volume as compared to wild type mice. Analysis of S1 mRNA by DNA microarray and gene expression analysis revealed gene changes in SR−/− that are associated with psychiatric and neurologic disorders, as well as neurodevelopment. The microarray analysis also identified reduced expression of brain derived neurotrophic factor (BDNF) in SR−/− mice. Follow-up analysis by ELISA confirmed a reduction of BDNF protein levels in the S1 of SR−/− mice. Finally, S1 pyramidal neurons in glycine transporter heterozygote (GlyT1+/−) mutants, which display enhanced NMDAR function, had increased dendritic spine density. These results suggest that proper NMDAR function is important for the arborization and spine density of pyramidal neurons in cortex. Moreover, they suggest that NMDAR hypofunction might, in part, be contributing to the dendritic and synaptic changes observed in schizophrenia and highlight this signaling pathway as a potential target for therapeutic intervention.
serine racemase; glycine; NMDA receptor; somatosensory cortex; BDNF; dendritic spines; schizophrenia
Recent genetic linkage analysis has shown that LRRTM1 (Leucine rich repeat transmembrane neuronal 1) is associated with schizophrenia. Here, we characterized Lrrtm1 knockout mice behaviorally and morphologically. Systematic behavioral analysis revealed reduced locomotor activity in the early dark phase, altered behavioral responses to novel environments (open-field box, light-dark box, elevated plus maze, and hole board), avoidance of approach to large inanimate objects, social discrimination deficit, and spatial memory deficit. Upon administration of the NMDA receptor antagonist MK-801, Lrrtm1 knockout mice showed both locomotive activities in the open-field box and responses to the inanimate object that were distinct from those of wild-type mice, suggesting that altered glutamatergic transmission underlay the behavioral abnormalities. Furthermore, administration of a selective serotonin reuptake inhibitor (fluoxetine) rescued the abnormality in the elevated plus maze. Morphologically, the brains of Lrrtm1 knockout mice showed reduction in total hippocampus size and reduced synaptic density. The hippocampal synapses were characterized by elongated spines and diffusely distributed synaptic vesicles, indicating the role of Lrrtm1 in maintaining synaptic integrity. Although the pharmacobehavioral phenotype was not entirely characteristic of those of schizophrenia model animals, the impaired cognitive function may warrant the further study of LRRTM1 in relevance to schizophrenia.
Long-term memory formation is known to be critically dependent upon de novo gene expression in the brain. As a consequence, pharmacological enhancement of the transcriptional processes mediating long-term memory formation provides a potential therapeutic strategy for cognitive disorders involving aberrant neuroplasticity. Here we focus on the identification and characterization of small molecule inhibitors of histone deacetylases (HDACs) as enhancers of CREB (cAMP response element-binding protein)-regulated transcription and modulators of chromatin-mediated neuroplasticity. Using a CREB reporter gene cell line, we screened a library of small molecules structurally related to known HDAC inhibitors leading to the identification of a probe we termed crebinostat that produced robust activation of CREB-mediated transcription. Further characterization of crebinostat revealed its potent inhibition of the deacetylase activity of recombinant class I HDACs 1, 2, 3, and class IIb HDAC6, with weaker inhibition of the class I HDAC8 and no significant inhibition of the class IIa HDACs 4, 5, 7, and 9. In cultured mouse primary neurons, crebinostat potently induced acetylation of both histone H3 and histone H4 as well as enhanced the expression of the CREB target gene Egr1 (early growth response 1). Using a hippocampus-dependent, contextual fear conditioning paradigm, mice systemically administered crebinostat for a ten day time period exhibited enhanced memory. To gain insight into the molecular mechanisms of memory enhancement by HDAC inhibitors, whole genome transcriptome profiling of cultured mouse primary neurons treated with crebinostat, combined with bioinformatic analyses of CREB-target genes, was performed revealing a highly connected protein-protein interaction network reflecting modules of genes important to synaptic structure and plasticity. Consistent with these findings, crebinostat treatment increased the density of synapsin-1 punctae along dendrites in cultured neurons. Finally, crebinostat treatment of cultured mouse primary neurons was found to upregulate Bdnf (brain-derived neurotrophic factor) and Grn (granulin) and downregulate Mapt (tau) gene expression—genes implicated in aging-related cognitive decline and cognitive disorders. Taken together, these results demonstrate that crebinostat provides a novel probe to modulate chromatin-mediated neuroplasticity and further suggests that pharmacological optimization of selective of HDAC inhibitors may provide an effective therapeutic approach for human cognitive disorders.
Cognitive enhancer; histone deacetylases; epigenetic; chromatin; acetylation; CREB
There is substantial evidence implicating N-methyl-d-aspartate receptors (NMDARs) in memory and cognition. It has also been suggested that NMDAR hypofunction might underlie the cognitive deficits observed in schizophrenia since morphological changes, including alterations in the dendritic architecture of pyramidal neurons in the prefrontal cortex (PFC), have been reported in the schizophrenic brain post mortem. Here, we used a genetic model of NMDAR hypofunction, a serine racemase knockout (SR−/−) mouse in which the first coding exon of the mouse serine racemase gene has been deleted, to explore the role of d-serine in regulating cognitive functions as well as dendritic architecture. SR −/− mice exhibited a significantly disrupted representation of the order of events in distinct experiences as revealed by object recognition and odor sequence tests; however, SR −/− animals were unimpaired in the detection of novel objects and in spatial displacement, and showed intact relational memory in a test of transitive inference. In addition, SR −/− mice exhibited normal sociability and preference for social novelty. Neurons in the medial PFC of SR−/− mice displayed reductions in the complexity, total length, and spine density of apical dendrites. These findings demonstrate that d-serine is important for specific aspects of cognition, as well as in regulating dendritic morphology of pyramidal neurons in the mPFC. Moreover, they suggest that NMDAR hypofunction might, in part, be responsible for the cognitive deficits and synaptic changes associated with schizophrenia, and highlight this signaling pathway as a potential target for therapeutic intervention.
D-serine; NMDA receptors; dendritic morphology; temporal order memory; sequence memory; schizophrenia
The alpha-calcium/calmodulin-dependent protein kinase II (αCaMKII) is a serine/threonine protein kinase predominantly expressed in the forebrain, especially in the postsynaptic density, and plays a key role in synaptic plasticity, learning and memory. αCaMKII heterozygous knockout (HKO) mice exhibit abnormal emotional and aggressive behaviors and cognitive impairments and have been proposed as an animal model of psychiatric illness. Our previous studies have shown that the expression of immediate early genes (IEGs) after exposure to electric foot shock or after performing a working memory task is decreased in the hippocampus, central amygdala, and medial prefrontal cortex of mutant mice. These changes could be caused by disturbances in neuronal signal transduction; however, it is still unclear whether neuronal activity is reduced in these regions. In this study, we performed in vivo manganese-enhanced magnetic resonance imaging (MEMRI) to assess the regional cellular activity in the brains of αCaMKII HKO mice. The signal intensity of MEMRI 24 h after systemic MnCl2 administration reflects functional increases of Mn2+ influx into neurons and glia via transport mechanisms, such as voltage-gated and/or ligand-gated Ca2+ channels. αCaMKII HKO mice demonstrated a low signal intensity of MEMRI in the dentate gyrus (DG), in which almost all neurons were at immature status at the molecular, morphological, and electrophysiological levels. In contrast, analysis of the signal intensity in these mutant mice revealed increased activity in the CA1 area of the hippocampus, a region crucial for cognitive function. The signal intensity was also increased in the bed nucleus of the stria terminalis (BNST), which is involved in anxiety. These changes in the mutant mice may be responsible for the observed dysregulated behaviors, such as cognitive deficit and abnormal anxiety-like behavior, which are similar to symptoms seen in human psychiatric disorders.
αCaMKII; manganese-enhanced MRI; immature; dentate gyrus; hippocampus; bed nucleus of stria terminalis; schizophrenia; psychiatric disorder
Solid evidence links schizophrenia (SZ) susceptibility to neurodevelopmental processes involving tyrosine phosphorylation-mediated signaling. Mouse studies implicate the Ptpra gene, encoding protein tyrosine phosphatase RPTPα, in the control of radial neuronal migration, cortical cytoarchitecture, and oligodendrocyte differentiation. The human gene encoding RPTPα, PTPRA, maps to a chromosomal region (20p13) associated with susceptibility to psychotic illness.
We characterized neurobehavioral parameters, as well as gene expression in the central nervous system, of mice with a null mutation in the Ptpra gene. We searched for genetic association between polymorphisms in PTPRA and schizophrenia risk (2 independent cohorts; total of 1420 cases and 1377 controls), and we monitored PTPRA expression in prefrontal dorsolateral cortex of SZ patients (35 cases, 2 control groups of 35 cases)
We find that Ptpra−/− mice reproduce neurobehavioral endophenotypes of human SZ: sensitization to metamphetamine-induced hyperactivity, defective sensorimotor gating, and defective habituation to a startle response. Ptpra loss of function also leads to reduced expression of multiple myelination genes, mimicking the hypomyelination-associated changes in gene expression observed in post mortem patient brains. We further report that a polymorphism at the PTPRA locus is genetically associated with SZ, and that PTPRA mRNA levels are reduced in post mortem dorsolateral prefrontal cortex of subjects with SZ.
The implication of this well-studied signaling protein in SZ risk and endophenotype manifestation provides novel entry points into the etiopathology of this disease.
schizophrenia; tyrosine phosphatase; myelination; mouse model; RPTPα; PTPRA
Schizophrenia is a complex genetic disorder caused by multiple genetic and environmental factors. The dystrobrevin-binding protein 1 (DTNBP1: dysbindin-1) gene is a major susceptibility gene for schizophrenia. Genetic variations in DTNBP1 are associated with cognitive functions, general cognitive ability and memory function, and clinical features of patients with schizophrenia including negative symptoms and cognitive decline. Since reduced expression of dysbindin-1 has been observed in postmortem brains of patients with schizophrenia, the sandy (sdy) mouse, which has a deletion in the Dtnbp1 gene and expresses no dysbindin-1 protein, could be an animal model of schizophrenia. To address this issue, we have carried out a comprehensive behavioral analysis of the sdy mouse in this study.
In a rotarod test, sdy mice did not exhibit motor learning whilst the wild type mice did. In a Barnes circular maze test both sdy mice and wild type mice learned to selectively locate the escape hole during the course of the training period and in the probe trial conducted 24 hours after last training. However, sdy mice did not locate the correct hole in the retention probe tests 7 days after the last training trial, whereas wild type mice did, indicating impaired long-term memory retention. A T-maze forced alternation task, a task of working memory, revealed no effect of training in sdy mice despite the obvious effect of training in wild type mice, suggesting a working memory deficit.
Sdy mouse showed impaired long-term memory retention and working memory. Since genetic variation in DTNBP1 is associated with both schizophrenia and memory function, and memory function is compromised in patients with schizophrenia, the sdy mouse may represent a useful animal model to investigate the mechanisms of memory dysfunction in the disorder.