Increased truncated TrkB receptor expression and decreased BDNF/TrkB signaling in the frontal cortex of reeler mouse model of schizophrenia

New Strategies for the Treatment of Neuropsychiatric Disorders Based on Reelin Dysfunction

Reelin is an extracellular matrix protein that is mainly produced in Cajal-Retzius cells and controls neuronal migration, which is important for the proper formation of cortical layers in the developmental stage of the brain. In the adult brain, Reelin plays a crucial role in the regulation of N-methyl-D-aspartate receptor-dependent synaptic function, and its expression decreases postnatally. Clinical studies showed reductions in Reelin protein and mRNA expression levels in patients with psychiatric disorders; however, the causal relationship remains unclear. Reelin-deficient mice exhibit an abnormal neuronal morphology and behavior, while Reelin supplementation ameliorates learning deficits, synaptic dysfunctions, and spine loss in animal models with Reelin deficiency. These findings suggest that the neuronal deficits and brain dysfunctions associated with the down-regulated expression of Reelin are attenuated by enhancements in its expression and functions in the brain. In this review, we summarize findings on the role of Reelin in neuropsychiatric disorders and discuss potential therapeutic approaches for neuropsychiatric disorders associated with Reelin dysfunctions.

Sustained NMDA receptor hypofunction impairs brain-derived neurotropic factor signalling in the PFC, but not in the hippocampus, and disturbs PFC-dependent cognition in mice

BACKGROUND

Cognitive deficits profoundly impact on the quality of life of patients with schizophrenia. Alterations in brain derived neurotrophic factor (BDNF) signalling, which regulates synaptic function through the activation of full-length tropomyosin-related kinase B receptors (TrkB-FL), are implicated in the aetiology of schizophrenia, as is N-methyl-D-aspartate receptor (NMDA-R) hypofunction. However, whether NMDA-R hypofunction contributes to the disrupted BDNF signalling seen in patients remains unknown.

AIMS

The purpose of this study was to characterise BDNF signalling and function in a preclinical rodent model relevant to schizophrenia induced by prolonged NMDA-R hypofunction.

METHODS

Using the subchronic phencyclidine (PCP) model, we performed electrophysiology approaches, molecular characterisation and behavioural analysis.

RESULTS

The data showed that prolonged NMDA-R antagonism, induced by subchronic PCP treatment, impairs long-term potentiation (LTP) and the facilitatory effect of BDNF upon LTP in the medial prefrontal cortex (PFC) of adult mice. Additionally, TrkB-FL receptor expression is decreased in the PFC of these animals. By contrast, these changes were not present in the hippocampus of PCP-treated mice. Moreover, BDNF levels were not altered in the hippocampus or PFC of PCP-treated mice. Interestingly, these observations are paralleled by impaired performance in PFC-dependent cognitive tests in mice treated with PCP.

CONCLUSIONS

Overall, these data suggest that NMDA-R hypofunction induces dysfunctional BDNF signalling in the PFC, but not in the hippocampus, which may contribute to the PFC-dependent cognitive deficits seen in the subchronic PCP model. Additionally, these data suggest that targeting BDNF signalling may be a mechanism to improve PFC-dependent cognitive dysfunction in schizophrenia.

Galantamine-Memantine Combination as an Antioxidant Treatment for Schizophrenia

PURPOSE OF REVIEW

The objective of this article is to highlight the potential role of the galantamine-memantine combination as a novel antioxidant treatment for schizophrenia.

RECENT FINDINGS

In addition to the well-known mechanisms of action of galantamine and memantine, these medications also have antioxidant activity. Furthermore, an interplay exists between oxidative stress, inflammation (redox-inflammatory hypothesis), and kynurenine pathway metabolites. Also, there is an interaction between brain-derived neurotrophic factor and oxidative stress in schizophrenia. Oxidative stress may be associated with positive, cognitive, and negative symptoms and impairments in white matter integrity in schizophrenia. The antipsychotic-galantamine-memantine combination may provide a novel strategy in schizophrenia to treat positive, cognitive, and negative symptoms.

SUMMARY

A "single antioxidant" may be inadequate to counteract the complex cascade of oxidative stress. The galantamine-memantine combination as "double antioxidants" is promising. Hence, randomized controlled trials are warranted with the antipsychotic-galantamine-memantine combination with oxidative stress and antioxidant biomarkers in schizophrenia.

The atypical dopamine receptor agonist SKF 83959 enhances hippocampal and prefrontal cortical neuronal network activity in a rat model of cognitive dysfunction

Deficits in neuronal network synchrony in hippocampus and prefrontal cortex have been widely demonstrated in disorders of cognitive dysfunction, including schizophrenia and Alzheimer's disease. The atypical dopamine agonist SKF 83959 has been shown to increase brain-derived neurotrophic factor signalling and suppress activity of glycogen synthase kinase-3 in PFC, two processes important to learning and memory. The purpose of this study was to therefore evaluate the impact of SKF 83959 on oscillatory deficits in methylazoxymethanol acetate (MAM) rat model of schizophrenia. To achieve this, local field potentials were recorded simultaneously from the hippocampus and prefrontal cortex of anesthetized rats at 15 and 90 min following both acute and repeated administration of SKF 83959 (0.4 mg/kg). In MAM rats, but not controls, repeated SKF 83959 treatment increased signal amplitude in hippocampus and enhanced the spectral power of low frequency delta and theta oscillations in this region. In PFC, SKF 83959 increased delta, theta and gamma spectral power. Increased HIP-PFC theta coherence was also evident following acute and repeated SKF 83959. In apparent contradiction to these oscillatory effects, in MAM rats, SKF 83959 inhibited spatial learning and induced a significant increase in thigmotactic behaviour. These findings have uncovered a previously unknown role for SKF 83959 in the positive regulation of hippocampal-prefrontal cortical oscillatory network activity. As SKF 83959 is known to have affinity for a number of receptors, delineating the receptor mechanisms that mediate the positive drug effects on neuronal oscillations could have significant future implications in disorders associated with cognitive dysfunction.

Synapsins regulate brain-derived neurotrophic factor-mediated synaptic potentiation and axon elongation by acting on membrane rafts

In neurons, intracellular membrane rafts are essential for specific actions of brain-derived neurotrophic factor (BDNF), which include the regulation of axon outgrowth, growth cone turning and synaptic transmission. Virtually, all the actions of BDNF are mediated by binding to its receptor, TrkB. The association of TrkB with the tyrosine kinase, Fyn, is critical for its localization to intracellular membrane rafts. Here, we show that synapsins, a family of highly amphipathic neuronal phosphoproteins, regulate membrane raft lipid composition and consequently, the ability of BDNF to regulate axon/neurite development and potentiate synaptic transmission. In the brains of mice lacking all synapsins, the expression of both BDNF and TrkB were increased, suggesting that BDNF/TrkB-mediated signaling is impaired. Consistent with this finding, synapsin-depleted neurons exhibit altered raft lipid composition, deficient targeting of Fyn to rafts, attenuated TrkB activation, and abrogation of BDNF-stimulated axon outgrowth and synaptic potentiation. Conversely, overexpression of synapsins in neuroblastoma cells results in corresponding reciprocal changes in raft lipid composition, increased localization of Fyn to rafts and promotion of BDNF-stimulated neurite formation. In the presence of synapsins, the ratio of cholesterol to estimated total phospholipids converged to 1, suggesting that synapsins act by regulating the ratio of lipids in intracellular membranes, thereby promoting lipid raft formation. These studies reveal a mechanistic link between BDNF and synapsins, impacting early development and synaptic transmission.

Modulation of schizophrenia-related genes in the forebrain of adolescent and adult rats exposed to maternal immune activation

Maternal immune activation (MIA) is an environmental risk factor for schizophrenia, and may contribute to other developmental disorders including autism and epilepsy. Activation of pro-inflammatory cytokine systems by injection of the synthetic double-stranded RNA polyriboinosinic-polyribocytidilic acid (Poly I:C) mediates important neurochemical and behavioral corollaries of MIA, which have relevance to deficits observed in schizophrenia. We examined the consequences of MIA on forebrain expression of neuregulin-1 (NRG-1), brain-derived neurotrophic factor (BDNF) and their receptors, ErbB4 and trkB, respectively, genes associated with schizophrenia. On gestational day 14, pregnant rats were injected with Poly I:C or vehicle. Utilizing in situ hybridization, expression of NRG-1, ErbB4, BDNF, and trkB was examined in male rat offspring at postnatal day (P) 14, P30 and P60. ErbB4 mRNA expression was significantly increased at P30 in the anterior cingulate (AC Ctx), frontal, and parietal cortices, with increases in AC Ctx expression continuing through P60. ErbB4 expression was also elevated in the prefrontal cortex (PFC) at P14. In contrast, NRG-1 mRNA was decreased in the PFC at P60. Expression of BDNF mRNA was significantly upregulated in the PFC at P60 and decreased in the AC Ctx at P14. Expression of trkB was increased in two regions, the piriform cortex at P14 and the striatum at P60. These findings demonstrate developmentally and regionally selective alterations in the expression of schizophrenia-related genes as a consequence of MIA. Further study is needed to determine contributions of these effects to the development of alterations of relevance to neuropsychiatric diseases.

Maternal separation produces alterations of forebrain brain-derived neurotrophic factor expression in differently aged rats

Early life adversity, such as postnatal maternal separation (MS), play a central role in the development of psychopathologies during individual ontogeny. In this study, we investigated the effects of repeated MS (4 h per day from postnatal day (PND) 1-21) on the brain-derived neurotrophic factor (BDNF) expression in the medial prefrontal cortex (mPFC), the nucleus accumbens (NAc) and the hippocampus of male and female juvenile (PND 21), adolescent (PND 35) and young adult (PND 56) Wistar rats. The results indicated that MS increased BDNF in the CA1 and the dentate gyrus (DG) of adolescent rats as well as in the DG of young adult rats. However, the expression of BDNF in the mPFC in the young adult rats was decreased by MS. Additionally, in the hippocampus, there was decreased BDNF expression with age in both the MS and non separated rats. However, in the mPFC, the BDNF expression was increased with age in the non separated rats; nevertheless, the BDNF expression was significantly decreased in the MS young adult rats. In the NAc, the BDNF expression was increased with age in the male non-maternal separation (NMS) rats, and the young adult female MS rats had less BDNF expression than the adolescent female MS rats. The present study shows unique age-differently changes on a molecular level induced by MS and advances the use of MS as a valid animal model to detect the underlying neurobiological mechanisms of mental disorders.

Long-Term Effects of Prenatal Hypoxia on Schizophrenia-Like Phenotype in Heterozygous Reeler Mice

Prenatal hypoxia (PHX) is a well-known environmental factor implicated in the pathophysiology of schizophrenia. However, the long-term effects of PHX on schizophrenia-related neuroplasticity are poorly understood. Using behavioral tasks, MRI imaging, and biochemical studies, we examined the long-term effects of PHX in heterozygous reeler mice (HRM; mice deficient for reelin, a candidate gene for schizophrenia). PHX at E17 failed to induce any significant deficits in prepulse inhibition, spatial memory, anxiety-like behavior, or blood flow in wild type (WT) and HRM at 6 months of age. However, PHX induced a significant increase in frontal cortex volume in WT whereas the higher frontal cortical volume found in HRM was significantly reduced by PHX. A significant decrease in reelin levels was observed in frontal cortex of WT and HRM and hippocampus of HRM following PHX. In addition, PHX induced significant reductions in hypoxia inducible factor-1α (HIF-1α) levels in frontal cortex and hippocampus of HRM. Although no significant effect of PHX was observed in vascular endothelial growth factor (VEGF) protein levels in frontal cortex and hippocampus of WT and HRM, serum VEGF levels were found higher in HRM following PHX. Moreover, glucocorticoid receptor (GR) protein levels were significantly lower in frontal cortex of WT and HRM and hippocampus of HRM following PHX. We found a significant reduction in serum corticosterone levels of PHX-treated WT mice. These findings suggest that future experiments addressing gene-environment interaction in schizophrenia should consider age-dependent effects of the environmental factor, in addition to the specificity of the gene of interest.

Decreased mTOR signaling pathway in human idiopathic autism and in rats exposed to valproic acid

Background

The molecular mechanisms underlying autistic behaviors remain to be elucidated. Mutations in genes linked to autism adversely affect molecules regulating dendritic spine formation, function and plasticity, and some increase the mammalian target of rapamycin, mTOR, a regulator of protein synthesis at spines. Here, we investigated whether the Akt/mTOR pathway is disrupted in idiopathic autism and in rats exposed to valproic acid, an animal model exhibiting autistic-like behavior.

Methods

Components of the mTOR pathway were assayed by Western blotting in postmortem fusiform gyrus samples from 11 subjects with idiopathic autism and 13 controls and in valproic acid versus saline-exposed rat neocortex. Additionally, protein levels of brain-derived neurotrophic factor receptor (TrkB) isoforms and the postsynaptic organizing molecule PSD-95 were measured in autistic versus control subjects.

Results

Full-length TrkB, PI3K, Akt, phosphorylated and total mTOR, p70S6 kinase, eIF4B and PSD-95 were reduced in autistic versus control fusiform gyrus. Similarly, phosphorylated and total Akt, mTOR and 4E-BP1 and phosphorylated S6 protein were decreased in valproic acid- versus saline-exposed rats. However, no changes in 4E-BP1 or eIF4E were found in autistic brains.

Conclusions

In contrast to some monogenic disorders with high rates of autism, our data demonstrate down-regulation of the Akt/mTOR pathway, specifically via p70S6K/eIF4B, in idiopathic autism. These findings suggest that disruption of this pathway in either direction is widespread in autism and can have adverse consequences for synaptic function. The use of valproic acid, a histone deacetylase inhibitor, in rats successfully modeled these changes, implicating an epigenetic mechanism in these pathway disruptions.

Neurodevelopment, GABA system dysfunction, and schizophrenia

The origins of schizophrenia have eluded clinicians and researchers since Kraepelin and Bleuler began documenting their findings. However, large clinical research efforts in recent decades have identified numerous genetic and environmental risk factors for schizophrenia. The combined data strongly support the neurodevelopmental hypothesis of schizophrenia and underscore the importance of the common converging effects of diverse insults. In this review, we discuss the evidence that genetic and environmental risk factors that predispose to schizophrenia disrupt the development and normal functioning of the GABAergic system.

Neurotrophins regulate ApoER2 proteolysis through activation of the Trk signaling pathway

Background

ApoER2 and the neurotrophin receptors Trk and p75^NTR are expressed in the CNS and regulate key functional aspects of neurons, including development, survival, and neuronal function. It is known that both ApoER2 and p75^NTR are processed by metalloproteinases, followed by regulated intramembrane proteolysis. TrkA activation by nerve growth factor (NGF) increases the proteolytic processing of p75^NTR mediated by ADAM17. Reelin induces the sheeding of ApoER2 ectodomain depending on metalloproteinase activity. However, it is not known if there is a common regulation mechanism for processing these receptors.

Results

We found that TrkA activation by NGF in PC12 cells induced ApoER2 processing, which was dependent on TrkA activation and metalloproteinases. NGF-induced ApoER2 proteolysis was independent of mitogen activated protein kinase activity and of phosphatidylinositol-3 kinase activity. In contrast, the basal proteolysis of ApoER2 increased when both kinases were pharmacologically inhibited. The ApoER2 ligand reelin regulated the proteolytic processing of its own receptor but not of p75^NTR. Finally, in primary cortical neurons, which express both ApoER2 and TrkB, we found that the proteolysis of ApoER2 was also regulated by brain-derived growth factor (BDNF).

Conclusions

Our results highlight a novel relationship between neurotrophins and the reelin-ApoER2 system, suggesting that these two pathways might be linked to regulate brain development, neuronal survival, and some pathological conditions.

Adolescent social isolation influences cognitive function in adult rats

Adolescence is a critical period for neurodevelopment. Evidence from animal studies suggests that isolated rearing can exert negative effects on behavioral and brain development. The present study aimed to investigate the effects of adolescent social isolation on latent inhibition and brain-derived neurotrophic factor levels in the forebrain of adult rats. Male Wistar rats were randomly divided into adolescent isolation (isolated housing, 38–51 days of age) and social groups. Latent inhibition was tested at adulthood. Brain-derived neurotrophic factor levels were measured in the medial prefrontal cortex and nucleus accumbens by an enzyme-linked immunosorbent assay. Adolescent social isolation impaired latent inhibition and increased brain-derived neurotrophic factor levels in the medial prefrontal cortex of young adult rats. These data suggest that adolescent social isolation has a profound effect on cognitive function and neurotrophin levels in adult rats and may be used as an animal model of neurodevelopmental disorders.

Corticosterone treatment during adolescence induces down-regulation of reelin and NMDA receptor subunit GLUN2C expression only in male mice: implications for schizophrenia

Stress exposure during adolescence/early adulthood has been shown to increase the risk for psychiatric disorders such as schizophrenia. Reelin plays an essential role in brain development and its levels are decreased in schizophrenia. However, the relationship between stress exposure and reelin expression remains unclear. We therefore treated adolescent reelin heteroyzogous mice (HRM) and wild-type (WT) littermates with the stress hormone, corticosterone (CORT) in their drinking water (25 mg/l) for 3 wk. In adulthood, we measured levels of full-length (FL) reelin and the N-R6 and N-R2 cleavage fragments in the frontal cortex (FC) and dorsal (DH) and ventral (VH) hippocampus. As expected, levels of all reelin forms were approximately 50% lower in HRMs compared to WT. In male mice, CORT treatment significantly decreased FL and N-R2 expression in the FC and N-R2 and N-R6 levels in the DH. This reelin down-regulation was accompanied by significant reductions in downstream N-methyl-D-aspartate (NMDA) GluN2C subunit levels. There were no effects of CORT treatment in the VH of either of the sexes and only subtle changes in female DH. CORT-induced reelin and GluN2C down-regulation in males was not associated with changes in two GABAergic neuron markers, GAD67 and parvalbumin, or glucocorticoids receptors (GR). These results show that CORT treatment causes long-lasting and selective reductions of reelin form levels in male FC and DH accompanied by changes in NMDAR subunit composition. This sex-specific reelin down-regulation in regions implicated in schizophrenia could be involved in the effects of stress in this disease.

Is the mTOR-signalling cascade disrupted in Schizophrenia?

The mammalian target of rapamycin (mTOR) signalling cascade is involved in the intracellular regulation of protein synthesis, specifically for proteins involved in controlling neuronal morphology and facilitating synaptic plasticity. Research has revealed that the activity of the mTOR cascade is influenced by several extracellular and environmental factors that have been implicated in schizophrenia. Therefore, there is reason to believe that one of the downstream consequences of dysfunction or hypofunction of these factors in schizophrenia is disrupted mTOR signalling and hence impaired protein synthesis. This results in abnormal neurodevelopment and deficient synaptic plasticity, outcomes which could underlie some of the positive, negative and cognitive symptoms of schizophrenia. This review will discuss the functional roles of the mTOR cascade and present evidence in support of a novel mTOR-based hypothesis of the neuropathology of schizophrenia. During neurodevelopment, genetic and epigenetic factors can disrupt mTOR signalling which affects synthesis of proteins essential for correct neuronal growth and network connectivity. This renders the CNS particularly vulnerable to the effects of secondary factors during adolescence which increases the risk of developing schizophrenia in adulthood. This review discusses the functional roles of the mTOR cascade and presents evidence in support of a novel mTOR-based hypothesis of the neuropathology of schizophrenia. Testing this hypothesis will advance our understanding of the aetiology of this illness and reveal novel therapeutic targets.

A physiological role for the dopamine D5 receptor as a regulator of BDNF and Akt signalling in rodent prefrontal cortex

The dopamine D5 receptor (D5R) exhibits a wide distribution in prefrontal cortex (PFC) but its role in this region has not yet been elucidated. In the present study, we identified a novel physiological function for the D(5)R as a regulator of brain-derived neurotrophic factor (BDNF) and Akt signalling in PFC. Specifically, acute activation of the D(5)R by the dopamine agonist SKF 83959 enhanced BDNF expression and signalling through its receptor, tropomyosin receptor kinase B (TrkB), in rats and in mice gene-deleted for the D1 receptor but not the D(5)R. These changes were concomitant with increased expression of GAD67, a protein whose down-regulation has been implicated in the aetiology of schizophrenia. Furthermore, D(5)R activation increased phosphorylation of Akt at the Ser(473) site, consequently decreasing the activity of its substrate GSK-3β. These findings could have wide-reaching implications given evidence showing activation of these pathways in PFC has therapeutic effects in neuropsychiatric disorders such as drug addiction, schizophrenia and depression.

From molecular to nanotechnology strategies for delivery of neurotrophins: emphasis on brain-derived neurotrophic factor (BDNF)

Neurodegenerative diseases represent a major public health problem, but beneficial clinical treatment with neurotrophic factors has not been established yet. The therapeutic use of neurotrophins has been restrained by their instability and rapid degradation in biological medium. A variety of strategies has been proposed for the administration of these leading therapeutic candidates, which are essential for the development, survival and function of human neurons. In this review, we describe the existing approaches for delivery of brain-derived neurotrophic factor (BDNF), which is the most abundant neurotrophin in the mammalian central nervous system (CNS). Biomimetic peptides of BDNF have emerged as a promising therapy against neurodegenerative disorders. Polymer-based carriers have provided sustained neurotrophin delivery, whereas lipid-based particles have contributed also to potentiation of the BDNF action. Nanotechnology offers new possibilities for the design of vehicles for neuroprotection and neuroregeneration. Recent developments in nanoscale carriers for encapsulation and transport of BDNF are highlighted.

Nicotine restores Wt-like levels of reelin and GAD67 gene expression in brain of heterozygous reeler mice

Important reduction of reelin, a neural development- and plasticity-associated protein, and glutamic acid decarboxylase (GAD67) are reported in brains of schizophrenic patients. These individuals are consistently engaged in tobacco smoking and nicotine is thought to alleviate negative behavioral symptoms or cognitive alterations. In mouse brain, nicotine has been shown to reduce GAD67 promoter methylation and increase its transcription. We assessed the effects of administration of nicotine (1 mg/kg s.c.) for 6 days, in male mice heterozygous for reelin (HRM), a putative model for symptoms related to schizophrenia. Expression of reelin, GAD67 and brain-derived neurotrophic factor (BDNF) was measured in different brain areas. RNA expression analysis evidenced genotype-related changes, with a marked reduction in reelin and GAD67 gene expression in prefrontal cortex, hippocampus, cerebellum, and striatum from HRM. Nicotine treatment selectively reversed the HRM-related phenotype in most brain areas and increased BDNF gene expression in cortex and hippocampus of both genotypes. Locomotor performance in their home cage revealed that HRM subjects were characterized by general hyperactivity; with nicotine administration restoring WT-like levels of locomotion. These findings are interpreted within the hypothesis of pre-existing vulnerability (based on haploinsufficiency of reelin) to brain and behavioral disorders and regulative effects associated with nicotine exposure.

The neurodevelopmental hypothesis of schizophrenia: convergent clues from epidemiology and neuropathology

The neurodevelopmental hypothesis of schizophrenia suggests that the disruption of early brain development increases the risk of later developing schizophrenia. This hypothesis focuses attention on critical periods of early brain development. From an epidemiologic perspective, various prenatal and perinatal risk factors have been linked to schizophrenia, including exposures related to infection, nutrition, and obstetric complications. From a genetic perspective, candidate genes have also been linked to altered brain development. In recent decades evidence from neuropathology has provided support for the neurodevelopmental hypothesis. Animal models involving early life exposures have been linked to changes in these same brain systems, providing convergent evidence for this long-standing hypothesis.

Convergent functional genomics of schizophrenia: from comprehensive understanding to genetic risk prediction

We have used a translational convergent functional genomics (CFG) approach to identify and prioritize genes involved in schizophrenia, by gene-level integration of genome-wide association study data with other genetic and gene expression studies in humans and animal models. Using this polyevidence scoring and pathway analyses, we identify top genes (DISC1, TCF4, MBP, MOBP, NCAM1, NRCAM, NDUFV2, RAB18, as well as ADCYAP1, BDNF, CNR1, COMT, DRD2, DTNBP1, GAD1, GRIA1, GRIN2B, HTR2A, NRG1, RELN, SNAP-25, TNIK), brain development, myelination, cell adhesion, glutamate receptor signaling, G-protein-coupled receptor signaling and cAMP-mediated signaling as key to pathophysiology and as targets for therapeutic intervention. Overall, the data are consistent with a model of disrupted connectivity in schizophrenia, resulting from the effects of neurodevelopmental environmental stress on a background of genetic vulnerability. In addition, we show how the top candidate genes identified by CFG can be used to generate a genetic risk prediction score (GRPS) to aid schizophrenia diagnostics, with predictive ability in independent cohorts. The GRPS also differentiates classic age of onset schizophrenia from early onset and late-onset disease. We also show, in three independent cohorts, two European American and one African American, increasing overlap, reproducibility and consistency of findings from single-nucleotide polymorphisms to genes, then genes prioritized by CFG, and ultimately at the level of biological pathways and mechanisms. Finally, we compared our top candidate genes for schizophrenia from this analysis with top candidate genes for bipolar disorder and anxiety disorders from previous CFG analyses conducted by us, as well as findings from the fields of autism and Alzheimer. Overall, our work maps the genomic and biological landscape for schizophrenia, providing leads towards a better understanding of illness, diagnostics and therapeutics. It also reveals the significant genetic overlap with other major psychiatric disorder domains, suggesting the need for improved nosology.

Cysteamine treatment ameliorates alterations in GAD67 expression and spatial memory in heterozygous reeler mice

Brain-derived neurotrophic factor (BDNF) signalling through its receptor, TrkB is known to regulate GABAergic function and glutamic acid decarboxylase (GAD) 67 expression in neurons. Alterations in BDNF signalling have been implicated in the pathophysiology of schizophrenia and as a result, they are a potential therapeutic target. Interestingly, heterozygous reeler mice (HRM) have decreased GAD67 expression in the frontal cortex and hippocampus and they exhibit many behavioural and neurochemical abnormalities similar to schizophrenia. In this study, we evaluated the potential of cysteamine, a neuroprotective compound to improve the deficits in GAD67 expression and cognitive function in HRM. We found that cysteamine administration (150 mg/kg.d, through drinking water) for 30 d significantly ameliorated the decreases in GAD67, mature BDNF and full-length TrkB protein levels found in frontal cortex and hippocampus of HRM. A significant attenuation of the increased levels of truncated BDNF in frontal cortex and hippocampus, as well as truncated TrkB in frontal cortex of HRM was also observed following cysteamine treatment. In behavioural studies, HRM were impaired in a Y-maze spatial recognition memory task, but not in a spontaneous alternation task or a sensorimotor, prepulse inhibition (PPI) procedure. Cysteamine improved Y-maze spatial recognition in HRM to the level of wide-type controls and it improved PPI in both wild-type and HRM. Finally, mice deficient in TrkB, showed a reduced response to cysteamine in GAD67 expression suggesting that TrkB signalling plays an important role in GAD67 regulation by cysteamine.

Modulatory effects of sex steroid hormones on brain-derived neurotrophic factor-tyrosine kinase B expression during adolescent development in C57Bl/6 mice

Sex steroid hormones and neurotrophic factors are involved in pruning and shaping the adolescent brain and have been implicated in the pathogenesis of neurodevelopmental disorders, including mental illness. We aimed to determine the association between altered levels of sex steroid hormones during adolescent development and neurotrophic signalling in the C57Bl/6 mouse. We first performed a week by week analysis from pre-pubescence to adulthood in male and female C57Bl/6 mice, measuring serum levels of testosterone and oestradiol in conjunction with western blot analysis of neurotrophin expression in the forebrain and hippocampal regions. Second, we manipulated adolescent sex steroid hormone levels by gonadectomy and hormone replacement at the pre-pubescent age of 5 weeks. Young-adult forebrain and hippocampal neurotrophin expression was then determined. Male mice showed significant changes in brain-derived neurotrophic factor (BDNF) expression in the forebrain regions during weeks 7-10, which corresponded significantly with a surge in serum testosterone. Castration and testosterone or di-hydrotestosterone replacement experiments revealed an androgen receptor-dependent effect on BDNF-tyrosine kinase (Trk) B signalling in the forebrain and hippocampal regions during adolescence. Female mice showed changes in BDNF-TrkB signalling at a much earlier time point (weeks 4-8) in the forebrain and hippocampal regions and these did not correspond with changes in serum oestradiol. Ovariectomy actually increased BDNF expression but decreased TrkB phosphorylation in the forebrain regions. 17β-Oestradiol replacement had no effect, suggesting a role for other ovarian hormones in regulating BDNF-TrkB signalling in the adolescent female mouse brain. These results suggest the differential actions of sex steroid hormones in modulating BDNF-TrkB signalling during adolescence. These data provide insight into how the male and female brain changes in response to altered levels of circulating sex steroid hormones and could help to explain some of the developmental sex differences in the pathogenesis of neurodevelopmental disorders, including mental illness.

Dopamine D1-D2 receptor heteromer in dual phenotype GABA/glutamate-coexpressing striatal medium spiny neurons: regulation of BDNF, GAD67 and VGLUT1/2

In basal ganglia a significant subset of GABAergic medium spiny neurons (MSNs) coexpress D1 and D2 receptors (D1R and D2R) along with the neuropeptides dynorphin (DYN) and enkephalin (ENK). These coexpressing neurons have been recently shown to have a region-specific distribution throughout the mesolimbic and basal ganglia circuits. While the functional relevance of these MSNs remains relatively unexplored, they have been shown to exhibit the unique property of expressing the dopamine D1–D2 receptor heteromer, a novel receptor complex with distinct pharmacology and cell signaling properties. Here we showed that MSNs coexpressing the D1R and D2R also exhibited a dual GABA/glutamate phenotype. Activation of the D1R–D2R heteromer in these neurons resulted in the simultaneous, but differential regulation of proteins involved in GABA and glutamate production or vesicular uptake in the nucleus accumbens (NAc), ventral tegmental area (VTA), caudate putamen and substantia nigra (SN). Additionally, activation of the D1R–D2R heteromer in NAc shell, but not NAc core, differentially altered protein expression in VTA and SN, regions rich in dopamine cell bodies. The identification of a MSN with dual inhibitory and excitatory intrinsic functions provides new insights into the neuroanatomy of the basal ganglia and demonstrates a novel source of glutamate in this circuit. Furthermore, the demonstration of a dopamine receptor complex with the potential to differentially regulate the expression of proteins directly involved in GABAergic inhibitory or glutamatergic excitatory activation in VTA and SN may potentially provide new insights into the regulation of dopamine neuron activity. This could have broad implications in understanding how dysregulation of neurotransmission within basal ganglia contributes to dopamine neuronal dysfunction.

Cysteamine attenuates the decreases in TrkB protein levels and the anxiety/depression-like behaviors in mice induced by corticosterone treatment

OBJECTIVE

Stress and glucocorticoid hormones, which are released into the circulation following stressful experiences, have been shown to contribute significantly to the manifestation of anxiety-like behaviors observed in many neuropsychiatric disorders. Brain-derived neurotrophic factor (BDNF) signaling through its receptor TrkB plays an important role in stress-mediated changes in structural as well as functional neuroplasticity. Studies designed to elucidate the mechanisms whereby TrkB signaling is regulated in chronic stress might provide valuable information for the development of new therapeutic strategies for several stress-related psychiatric disorders.

MATERIALS AND METHODS

We examined the potential of cysteamine, a neuroprotective compound to attenuate anxiety and depression like behaviors in a mouse model of anxiety/depression induced by chronic corticosterone exposure.

RESULTS

Cysteamine administration (150 mg/kg/day, through drinking water) for 21 days significantly ameliorated chronic corticosterone-induced decreases in TrkB protein levels in frontal cortex and hippocampus. Furthermore, cysteamine treatment reversed the anxiety and depression like behavioral abnormalities induced by chronic corticosterone treatment. Finally, mice deficient in TrkB, showed a reduced response to cysteamine in behavioral tests, suggesting that TrkB signaling plays an important role in the antidepressant effects of cysteamine.

CONCLUSIONS

The animal studies described here highlight the potential use of cysteamine as a novel therapeutic strategy for glucocorticoid-related symptoms of psychiatric disorders.

A new aspect of the TrkB signaling pathway in neural plasticity

In the central nervous system (CNS), the expression of molecules is strictly regulated during development. Control of the spatiotemporal expression of molecules is a mechanism not only to construct the functional neuronal network but also to adjust the network in response to new information from outside of the individual, i.e., through learning and memory. Among the functional molecules in the CNS, one of the best-studied groups is the neurotrophins, which are nerve growth factor (NGF)-related gene family molecules. Neurotrophins include NGF, brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4/5 in the mammal. Among neurotrophins and their receptors, BDNF and tropomyosin-related kinases B (TrkB) are enriched in the CNS. In the CNS, the BDNF-TrkB signaling pathway fulfills a wide variety of functions throughout life, such as cell survival, migration, outgrowth of axons and dendrites, synaptogenesis, synaptic transmission, and remodeling of synapses. Although the same ligand and receptor, BDNF and TrkB, act in these various developmental events, we do not yet understand what kind of mechanism provokes the functional multiplicity of the BDNF-TrkB signaling pathway. In this review, we discuss the mechanism that elicits the variety of functions performed by the BDNF-TrkB signaling pathway in the CNS as a tool of pharmacological therapy.

Full length TrkB potentiates estrogen receptor alpha mediated transcription suggesting convergence of susceptibility pathways in schizophrenia

In this study, we determined if estrogen receptor alpha (ERα) can interact with the full length tropomyosin receptor kinase B (TrkB-TK+), both of which are implicated in schizophrenia pathogenesis. Using neuronal (SHSY5Y) and non-neuronal (CHOK1) cell-lines, we showed that TrkB-TK+ can increase transcription at estrogen response elements (EREs) with and without exogenous estrogen treatment. In the presence of estrogen, TrkB-TK+ further potentiated the effect of estrogen stimulation on ERα-mediated transcription. This synergistic effect of TrkB-TK+ on ERα-mediated transcription was not due to direct effects of TrkB-TK+ in the nucleus, but occurred through cytoplasmic signaling of TrkB-TK+ via the MAPK/ERK pathway to phosphorylate ERα, leading to an induction in ERα-mediated transcription. When we examined the PI3K/AKT pathway, we found that PI3K/AKT activity constitutively inhibited baseline transcription at EREs. Furthermore, we showed that signaling via PI3K/AKT inhibited TrkB-TK+-dependent transcriptional potentiation at EREs. Our findings suggest that TrkB-TK+-linked second messenger signaling pathways can reciprocally regulate ERα-mediated transcription at EREs. Considering that both ERα and TrkB-TK+ expression are reduced in schizophrenia, our findings suggest that dysfunction in TrkB-TK+ signaling may occur upstream of, or in conjunction with a dysfunction in ERα, and that transcriptional regulation by ERα may be decreased by reductions in TrkB-TK+.

L-methionine decreases dendritic spine density in mouse frontal cortex

Schizophrenia postmortem brain is characterized by gamma aminobutyric acid downregulation and by decreased dendritic spine density in frontal cortex. Protracted L-methionine treatment exacerbates schizophrenia symptoms, and our earlier work (Tremolizzo et al. and Dong et al.) has shown that L-methionine decreases reelin and GAD67 transcription in mice which is prevented by co-administration of valproate. In this study, we observed a decrease in spine density following L-methionine treatment, which was prevented by co-administration of valproate. Together with our earlier findings conducted under the same experimental conditions, we suggest that downregulation of spine density in L-methionine-treated mice may be because of the decreased expression of reelin and that valproate may prevent spine downregulation by inhibiting the methylation induced decrease in reelin.

Lower number of cerebellar Purkinje neurons in psychosis is associated with reduced reelin expression

Reelin is an extracellular matrix protein synthesized in cerebellar granule cells that plays an important role in Purkinje cell positioning during cerebellar development and in modulating adult synaptic function. In the cerebellum of schizophrenia (SZ) and bipolar (BP) disorder patients, there is a marked decrease ( approximately 50%) of reelin expression. In this study we measured Purkinje neuron density in the Purkinje cell layer of cerebella of 13 SZ and 17 BP disorder patients from the McLean 66 Cohort Collection, Harvard Brain Tissue Resource Center. The mean number of Purkinje neurons (linear density, neurons per millimeter) was 20% lower in SZ and BP disorder patients compared with nonpsychiatric subjects (NPS; n = 24). This decrease of Purkinje neuron linear density was unrelated to postmortem interval, pH, drugs of abuse, or to the presence, dose, or duration of antipsychotic medications. A comparative study in the cerebella of heterozygous reeler mice (HRM), in which reelin expression is down-regulated by approximately 50%, showed a significant loss in the number of Purkinje cells in HRM (10-15%) compared with age-matched (3-9 months) wild-type mice. This finding suggests that lack of reelin impairs GABAergic Purkinje neuron expression and/or positioning during cerebellar development.