Epigenetic Alterations in Prenatal Stress Mice as an Endophenotype Model for Schizophrenia: Role of Metabotropic Glutamate 2/3 Receptors

Prenatal stress alters transcription of NMDA-type glutamate receptors in the hippocampus

Prenatal stress increases the risk of neurodevelopmental disorders. NMDA-type glutamate receptor (NMDAR) activity plays an important pathophysiological role in the cortico-hippocampal circuit in these disorders. We tested the hypothesis that transcription of NMDAR subunits is modified in the frontal cortex (FCx) and hippocampus after exposure to prenatal restraint stress (PRS) in mice. At 10 weeks of age, male PRS offspring (n = 20) and non-stressed controls (NS, n = 20) were treated with haloperidol (1 mg/kg), clozapine (5 mg/kg) or saline twice daily for 5 days, before measuring social approach (SOC). Saline-treated and haloperidol-treated PRS mice had reduced SOC relative to NS (P < 0.01), but clozapine-treated PRS mice had similar SOC to NS mice. These effects of PRS were associated with increased transcription of NMDAR subunits encoded by GRIN2A and GRIN2B genes in the hippocampus but not FCx. GRIN transcription in FCx correlated positively with SOC, but hippocampal GRIN transcription had negative correlation with SOC. The ratio of GRIN2A/GRIN2B transcription is known to increase during development but was lower in PRS mice. These results suggest that GRIN2A and GRIN2B transcript levels are modified in the hippocampus by PRS, leading to life-long deficits in social behavior. These data have some overlap with the molecular pathophysiology of schizophrenia. Similar to PRS in mice, schizophrenia, has been associated with social withdrawal, with increased GRIN2 expression in the hippocampus, and reduced GRIN2A/GRIN2B expression ratios in the hippocampus. These findings suggest that PRS in mice may have construct validity as a preclinical model for antipsychotic drug development.

Epigenetic regulation of metabotropic glutamate 2/3 receptors: Potential role for ultra-resistant schizophrenia?

Schizophrenia is a severe and debilitating psychiatric disorder characterized by early cognitive deficits, emotional and behavioral abnormalities resulted by a dysfunctional gene x environment interaction. Genetic and epigenetic abnormalities in cortical parvalbumin-positive GABAergic interneurons lead to alterations in glutamate-mediated excitatory neurotransmission, synaptic plasticity, and neuronal development. Epigenetic alterations during pregnancy or early phases of postnatal life are associated with schizophrenia vulnerability as well as inflammatory processes which are at the basis of brain pathology. An epigenetic animal model of schizophrenia showed specific changes in promoter DNA methylation activity of genes related to schizophrenia such as reelin, BDNF and GAD67, and altered expression and function of mGlu2/3 receptors in the frontal cortex. Although antipsychotic medications represent the main treatment for schizophrenia and generally show an optimal efficacy profile for positive symptoms and relatively poor efficacy for negative or cognitive symptoms, a considerable percentage of individuals show poor response, do not achieve a complete remission, and approximately 30 % of patients show treatment-resistance. Here, we explore the potential role of epigenetic abnormalities linked to metabotropic glutamate 2/3 receptors changes in expression and function as key molecular factors underlying the difference in response to antipsychotics.

Understanding translational research in schizophrenia: A novel insight into animal models

Schizophrenia affects millions of people worldwide and is a major challenge for the scientific community. Like most psychotic diseases, it is also considered a complicated mental disorder caused by an imbalance in neurotransmitters. Due to the complexity of neuropathology, it is always a complicated disorder. The lack of proper understanding of the pathophysiology makes the disorder unmanageable in clinical settings. However, due to recent advances in animal models, we hope we can have better therapeutic approaches with more success in clinical settings. Dopamine, glutamate, GABA, and serotonin are the neurotransmitters involved in the pathophysiology of schizophrenia. Various animal models have been put forward based on these neurotransmitters, including pharmacological, neurodevelopmental, and genetic models. Polymorphism of genes such as dysbindin, DICS1, and NRG1 has also been reported in schizophrenia. Hypothesis based on dopamine, glutamate, and serotonin are considered successful models of schizophrenia on which drug therapies have been designed to date. New targets like the orexin system, muscarinic and nicotinic receptors, and cannabinoid receptors have been approached to alleviate the negative and cognitive symptoms. The non-pharmacological models like the post-weaning social isolation model (maternal deprivation), the isolation rearing model etc. have been also developed to mimic the symptoms of schizophrenia and to create and test new approaches of drug therapy which is a breakthrough at present in psychiatric disorders. Different behavioral tests have been evaluated in these specific models. This review will highlight the currently available animal models and behavioral tests in psychic disorders concerning schizophrenia.

Presynaptic 5-HT2A-mGlu2/3 Receptor–Receptor Crosstalk in the Prefrontal Cortex: Metamodulation of Glutamate Exocytosis

The glutamatergic nerve endings of a rat prefrontal cortex (PFc) possess presynaptic 5-HT_2A heteroreceptors and mGlu2/3 autoreceptors, whose activation inhibits glutamate exocytosis, and is measured as 15 mM KCl-evoked [³H]D-aspartate ([³H]D-asp) release (which mimics glutamate exocytosis). The concomitant activation of the two receptors nulls their inhibitory activities, whereas blockade of the 5-HT_2A heteroreceptors with MDL11,939 (1 μM) strengthens the inhibitory effect elicited by the mGlu2/3 receptor agonist LY329268 (1 μM). 5-HT_2A receptor antagonists (MDL11,939; ketanserin; trazodone) amplify the impact of low (3 nM) LY379268. Clozapine (0.1–10 μM) mimics the 5-HT_2A agonist (±) DOI and inhibits the KCl-evoked [³H]D-asp overflow in a MDL11,939-dependent fashion, but does not modify the (±) DOI-induced effect. mGlu2 and 5-HT_2A proteins do not co-immunoprecipitate from synaptosomal lysates, nor does the incubation of PFc synaptosomes with MDL11,939 (1 μM) or clozapine (10 µM) modify the insertion of mGlu2 subunits in synaptosomal plasma membranes. In conclusion, 5-HT_2A and mGlu2/3 receptors colocalize, but do not physically associate, in PFc glutamatergic terminals, where they functionally interact in an antagonist-like fashion to control glutamate exocytosis. The mGlu2/3-5-HT_2A metamodulation could be relevant to therapy for central neuropsychiatric disorders, including schizophrenia, but also unveil cellular events accounting for their development, which also influence the responsiveness to drugs regimens.

Treatment-resistant schizophrenia: Addressing white matter integrity, intracortical glutamate levels, clinical and cognitive profiles between early- and adult-onset patients

BACKGROUND

Treatment-resistance in schizophrenia is 30-40%. Its neurobiology remains unclear; to explore it, we conducted a combined spectrometry/tractography/cognitive battery and psychopathological rating study on patients with treatment-resistant schizophrenia (TRS), dividing the sample into early-onset (N = 21) and adult-onset TRS (N = 20). Previous studies did not differentiate between early- (onset 13-18 years) and adult-onset (>18 years at formal diagnosis of schizophrenia) TRS.

METHODS

We evaluated cross-sectionally 41 TRS patients (26 male and 15 female) and 20 matched healthy controls (HCs) with psychopathological and cognitive testing prior to participating in brain imaging scanning using magnetic resonance spectroscopy and diffusion tensor imaging to determine the relationship between their symptoms and their glutamate levels and white matter integrity.

RESULTS

TRS patients scored lower than HCs on all cognitive domains; early-onset patients performed better than adult-onset patients only on the Symbol Coding domain. TRS correlated with symptom severity, especially negative symptoms. Glutamate levels and glutamate/creatine were increased in anterior cingulate cortex. Diffusion tensor imaging showed low fractional anisotropy in TRS patients in specific white matter tracts compared to HCs (bilateral anterior thalamic radiation, cortico-spinal tract, forceps minor, inferior fronto-occipital fasciculus, inferior longitudinal fasciculus, superior longitudinal fasciculus, and right uncinate fasciculus).

CONCLUSIONS

We identified specific magnetic resonance spectroscopy and diffusion tensor imaging alterations in TRS patients. Adult-onset TRS differed little from early-onset TRS on most measures; this points to alterations being present since the outset of schizophrenia and may constitute a biological signature of treatment-resistance.

Epigenetic mechanisms impacted by chronic stress across the rodent lifespan

Exposures to stress at all stages of development can lead to long-term behavioural effects, in part through changes in the epigenome. This review describes rodent research suggesting that stress in prenatal, postnatal, adolescent and adult stages leads to long-term changes in epigenetic regulation in the brain which have causal impacts on rodent behaviour. We focus on stress-induced epigenetic changes that have been linked to behavioural deficits including poor learning and memory, and increased anxiety-like and depressive-like behaviours. Interestingly, aspects of these stress-induced behavioural changes can be transmitted to offspring across several generations, a phenomenon that has been proposed to result via epigenetic mechanisms in the germline. Here, we also discuss evidence for the differential impact of stress on the epigenome in males and females, conscious of the fact that the majority of published studies have only investigated males. This has led to a limited picture of the epigenetic impact of stress, highlighting the need for future studies to investigate females as well as males.

Dysregulation of schizophrenia-associated genes and genome-wide hypomethylation in neurons overexpressing DNMT1

Aim: To study the effects of DNMT1 overexpression on transcript levels of genes dysregulated in schizophrenia and on genome-wide methylation patterns. Materials & methods: Transcriptome and DNA methylome comparisons were made between R1 (wild-type) and Dnmt1^tet/tet mouse embryonic stem cells and neurons overexpressing DNMT1. Genes dysregulated in both Dnmt1^tet/tet cells and schizophrenia patients were studied further. Results & conclusions: About 50% of dysregulated genes in patients also showed altered transcript levels in Tet/Tet neurons in a DNA methylation-independent manner. These neurons unexpectedly showed genome-wide hypomethylation, increased transcript levels of Tet1 and Apobec 1-3 genes and increased activity and copy number of LINE-1 elements. The observed similarities between Tet/Tet neurons and schizophrenia brain samples reinforce DNMT1 overexpression as a risk factor.

Connecting the Neurobiology of Developmental Brain Injury: Neuronal Arborisation as a Regulator of Dysfunction and Potential Therapeutic Target

Neurodevelopmental disorders can derive from a complex combination of genetic variation and environmental pressures on key developmental processes. Despite this complex aetiology, and the equally complex array of syndromes and conditions diagnosed under the heading of neurodevelopmental disorder, there are parallels in the neuropathology of these conditions that suggest overlapping mechanisms of cellular injury and dysfunction. Neuronal arborisation is a process of dendrite and axon extension that is essential for the connectivity between neurons that underlies normal brain function. Disrupted arborisation and synapse formation are commonly reported in neurodevelopmental disorders. Here, we summarise the evidence for disrupted neuronal arborisation in these conditions, focusing primarily on the cortex and hippocampus. In addition, we explore the developmentally specific mechanisms by which neuronal arborisation is regulated. Finally, we discuss key regulators of neuronal arborisation that could link to neurodevelopmental disease and the potential for pharmacological modification of arborisation and the formation of synaptic connections that may provide therapeutic benefit in the future.

The G Protein-Coupled Glutamate Receptors as Novel Molecular Targets in Schizophrenia Treatment-A Narrative Review

Schizophrenia is a severe neuropsychiatric disease with an unknown etiology. The research into the neurobiology of this disease led to several models aimed at explaining the link between perturbations in brain function and the manifestation of psychotic symptoms. The glutamatergic hypothesis postulates that disrupted glutamate neurotransmission may mediate cognitive and psychosocial impairments by affecting the connections between the cortex and the thalamus. In this regard, the greatest attention has been given to ionotropic NMDA receptor hypofunction. However, converging data indicates metabotropic glutamate receptors as crucial for cognitive and psychomotor function. The distribution of these receptors in the brain regions related to schizophrenia and their regulatory role in glutamate release make them promising molecular targets for novel antipsychotics. This article reviews the progress in the research on the role of metabotropic glutamate receptors in schizophrenia etiopathology.

Neuronal Lhx1 expression is regulated by DNMT1-dependent modulation of histone marks

Apart from the conventional view of repressive promoter methylation, the DNA methyltransferase 1 (DNMT1) was recently described to modulate gene expression through a variety of interactions with diverse epigenetic key players. We here investigated the DNMT1-dependent transcriptional control of the homeobox transcription factor LHX1, which we previously identified as an important regulator in cortical interneuron development. We found that LHX1 expression in embryonic interneurons originating in the embryonic pre-optic area (POA) is regulated by non-canonic DNMT1 function. Analysis of histone methylation and acetylation revealed that both epigenetic modifications seem to be implicated in the control of Lhx1 gene activity and that DNMT1 contributes to their proper establishment. This study sheds further light on the regulatory network of cortical interneuron development including the complex interplay of epigenetic mechanisms.

High polygenic burden is associated with blood DNA methylation changes in individuals with suicidal behavior

Suicidal behavior is result of the interaction of several contributors, including genetic and environmental factors. The integration of approaches considering the polygenic component of suicidal behavior, such as polygenic risk scores (PRS) and DNA methylation is promising for improving our understanding of the complex interplay between genetic and environmental factors in this behavior. The aim of this study was the evaluation of DNA methylation differences between individuals with high and low genetic burden for suicidality. The present study was divided into two phases. In the first phase, genotyping with the Psycharray chip was performed in a discovery sample of 568 Mexican individuals, of which 149 had suicidal behavior (64 individuals with suicidal ideation, 50 with suicide attempt and 35 with completed suicide). Then, a PRS analysis based on summary statistics from the Psychiatric Genomic Consortium was performed in the discovery sample. In a second phase, we evaluated DNA methylation differences between individuals with high and low genetic burden for suicidality in a sub-sample of the discovery sample (target sample) of 94 subjects. We identified 153 differentially methylated sites between individuals with low and high-PRS. Among genes mapped to differentially methylated sites, we found genes involved in neurodevelopment (CHD7, RFX4, KCNA1, PLCB1, PITX1, NUMBL) and ATP binding (KIF7, NUBP2, KIF6, ATP8B1, ATP11A, CLCN7, MYLK, MAP2K5). Our results suggest that genetic variants might increase the predisposition to epigenetic variations in genes involved in neurodevelopment. This study highlights the possible implication of polygenic burden in the alteration of epigenetic changes in suicidal behavior.

Oxidative-Antioxidant Imbalance and Impaired Glucose Metabolism in Schizophrenia

Schizophrenia is a neurodevelopmental disorder featuring chronic, complex neuropsychiatric features. The etiology and pathogenesis of schizophrenia are not fully understood. Oxidative-antioxidant imbalance is a potential determinant of schizophrenia. Oxidative, nitrosative, or sulfuric damage to enzymes of glycolysis and tricarboxylic acid cycle, as well as calcium transport and ATP biosynthesis might cause impaired bioenergetics function in the brain. This could explain the initial symptoms, such as the first psychotic episode and mild cognitive impairment. Another concept of the etiopathogenesis of schizophrenia is associated with impaired glucose metabolism and insulin resistance with the activation of the mTOR mitochondrial pathway, which may contribute to impaired neuronal development. Consequently, cognitive processes requiring ATP are compromised and dysfunctions in synaptic transmission lead to neuronal death, preceding changes in key brain areas. This review summarizes the role and mutual interactions of oxidative damage and impaired glucose metabolism as key factors affecting metabolic complications in schizophrenia. These observations may be a premise for novel potential therapeutic targets that will delay not only the onset of first symptoms but also the progression of schizophrenia and its complications.