Increased expression of the astrocytic glutamate transporter GLT-1 in the prefrontal cortex of schizophrenics

New clues for the role of cerebellum in schizophrenia and the associated cognitive impairment

Schizophrenia (SZ) is a complex neuropsychiatric disorder associated with severe cognitive dysfunction. Although research has mainly focused on forebrain abnormalities, emerging results support the involvement of the cerebellum in SZ physiopathology, particularly in Cognitive Impairment Associated with SZ (CIAS). Besides its role in motor learning and control, the cerebellum is implicated in cognition and emotion. Recent research suggests that structural and functional changes in the cerebellum are linked to deficits in various cognitive domains including attention, working memory, and decision-making. Moreover, cerebellar dysfunction is related to altered cerebellar circuit activities and connectivity with brain regions associated with cognitive processing. This review delves into the role of the cerebellum in CIAS. We initially consider the major forebrain alterations in CIAS, addressing impairments in neurotransmitter systems, synaptic plasticity, and connectivity. We then focus on recent findings showing that several mechanisms are also altered in the cerebellum and that cerebellar communication with the forebrain is impaired. This evidence implicates the cerebellum as a key component of circuits underpinning CIAS physiopathology. Further studies addressing cerebellar involvement in SZ and CIAS are warranted and might open new perspectives toward understanding the physiopathology and effective treatment of these disorders.

Molecular and Structural Insight into Adenosine A2A Receptor in Neurodegenerative Disorders: A Significant Target for Efficient Treatment Approach

All biological tissues and bodily fluids include the autacoid adenosine. The P1 class of purinergic receptors includes adenosine receptors. Four distinct G-protein-coupled receptors on the cellular membrane mediate the effects of adenosine, whose cytoplasmic content is regulated by producing/degrading enzymes and nucleoside transporters. A2A receptor has received a great deal of attention in recent years because it has a wide range of potential therapeutic uses. A2B and, more significantly, A2A receptors regulate numerous physiological mechanisms in the central nervous system (CNS). The inferior targetability of A2B receptors towards adenosine points that they might portray a promising medicinal target since they are triggered only under pharmacological circumstances (when adenosine levels rise up to micromolar concentrations). The accessibility of specific ligands for A2B receptors would permit the exploration of such a theory. A2A receptors mediate both potentially neurotoxic and neuroprotective actions. Hence, it is debatable to what extent they play a role in neurodegenerative illnesses. However, A2A receptor blockers have demonstrated clear antiparkinsonian consequences, and a significant attraction exists in the role of A2A receptors in other neurodegenerative disorders. Amyloid peptide extracellular accumulation and tau hyperphosphorylation are the pathogenic components of AD that lead to neuronal cell death, cognitive impairment, and memory loss. Interestingly, in vitro and in vivo research has shown that A2A adenosine receptor antagonists may block each of these clinical symptoms, offering a crucial new approach to combat a condition for which, regrettably, only symptomatic medications are currently available. At least two requirements must be met to determine whether such receptors are a target for diseases of the CNS: a complete understanding of the mechanisms governing A2A-dependent processes and the availability of ligands that can distinguish between the various receptor populations. This review concisely summarises the biological effects mediated by A2A adenosine receptors in neurodegenerative disorders and discusses the chemical characteristics of A2A adenosine receptor antagonists undergoing clinical trials. Selective A2A receptor blocker against neurodegenerative disorders.

The emerging role of exosomes in Schizophrenia

Schizophrenia (SCZ), a serious mental disorder, is one of the leading causes of disease burden worldwide. Exosomes, as a natural nanocarrier, are able to cross the blood-brain barrier (BBB) and play a key bridging role in central nervous system (CNS) communication, participating in important physiological processes such as neural regeneration, prominent plasticity, axonal support, and neuroinflammation. In recent years, exosomes have received widespread attention in the field of neurodegenerative diseases and mental disorders, especially Alzheimer's disease. However, there are few reviews on exosomes and SCZ. Therefore, we conducted a literature search in PubMed and Web of Science using the following search terms: "schizophrenia", "mental disorder", "central system", "exosome", "extracellular vesicles" to identify publications from January 2010 to December 2022. Our review summarized exosomes secreted by different cell types in the CNS and the double-edged role of exosomes in the development of SCZ, and discussed their future potential as biomarkers and therapeutic targets. In conclusion, this article provides an up-to-date overview of the current research on the involvement of exosomes in SCZ, while also highlighting the challenges that are currently faced in this field.

SLC1A2 Gene Polymorphism Influences Methamphetamine-Induced Psychosis

SLC1A2 is a gene encoded for the excitatory amino acid transporter 2 which is responsible for glutamate reuptake from the synaptic cleft in the central nervous system. Recent studies have suggested that polymorphisms on glutamate transporters can affect drug dependence, leading to the development of neurological diseases and psychiatric disorders. Our study investigated the association of rs4755404 single nucleotide polymorphism (SNP) of the SLC1A2 gene with methamphetamine (METH) dependence and METH-induced psychosis and mania in a Malaysian population. The rs4755404 gene polymorphism was genotyped in METH-dependent male subjects (n = 285) and male control subjects (n = 251). The subjects consisted of the four ethnic groups in Malaysia (Malay, Chinese, Kadazan-Dusun, and Bajau). Interestingly, there was a significant association between rs4755404 polymorphism and METH-induced psychosis in the pooled METH-dependent subjects in terms of genotype frequency (p = 0.041). However, there was no significant association between rs4755404 polymorphism and METH dependence. Also, the rs455404 polymorphism was not significantly associated with METH-induced mania for both genotype frequencies and allele frequencies in the METH-dependent subjects, regardless of stratification into the different ethnicities. Our study suggests that the SLC1A2 rs4755404 gene polymorphism confers some susceptibility to METH-induced psychosis, especially for those who carry the GG homozygous genotype.

Astrocyte regulation of synaptic signaling in psychiatric disorders

Over the last 15 years, the field of neuroscience has evolved toward recognizing the critical role of astroglia in shaping neuronal synaptic activity and along with the pre- and postsynapse is now considered an equal partner in tripartite synaptic transmission and plasticity. The relative youth of this recognition and a corresponding deficit in reagents and technologies for quantifying and manipulating astroglia relative to neurons continues to hamper advances in understanding tripartite synaptic physiology. Nonetheless, substantial advances have been made and are reviewed herein. We review the role of astroglia in synaptic function and regulation of behavior with an eye on how tripartite synapses figure into brain pathologies underlying behavioral impairments in psychiatric disorders, both from the perspective of measures in postmortem human brains and more subtle influences on tripartite synaptic regulation of behavior in animal models of psychiatric symptoms. Our goal is to provide the reader a well-referenced state-of-the-art understanding of current knowledge and predict what we may discover with deeper investigation of tripartite synapses using reagents and technologies not yet available.

Beyond NMDA Receptors: Homeostasis at the Glutamate Tripartite Synapse and Its Contributions to Cognitive Dysfunction in Schizophrenia

Cognitive deficits are core symptoms of schizophrenia but remain poorly addressed by dopamine-based antipsychotic medications. Glutamate abnormalities are implicated in schizophrenia-related cognitive deficits. While the role of the NMDA receptor has been extensively studied, less attention was given to other components that control glutamate homeostasis. Glutamate dynamics at the tripartite synapse include presynaptic and postsynaptic components and are tightly regulated by neuron-astrocyte crosstalk. Here, we delineate the role of glutamate homeostasis at the tripartite synapse in schizophrenia-related cognitive dysfunction. We focus on cognitive domains that can be readily measured in humans and rodents, i.e., working memory, recognition memory, cognitive flexibility, and response inhibition. We describe tasks used to measure cognitive function in these domains in humans and rodents, and the relevance of glutamate alterations in these domains. Next, we delve into glutamate tripartite synaptic components and summarize findings that implicate the relevance of these components to specific cognitive domains. These collective findings indicate that neuron-astrocyte crosstalk at the tripartite synapse is essential for cognition, and that pre- and postsynaptic components play a critical role in maintaining glutamate homeostasis and cognitive well-being. The contribution of these components to cognitive function should be considered in order to better understand the role played by glutamate signaling in cognition and develop efficient pharmacological treatment avenues for schizophrenia treatment-resistant symptoms.

Adenosine Receptors in Neuropsychiatric Disorders: Fine Regulators of Neurotransmission and Potential Therapeutic Targets

Adenosine exerts an important role in the modulation of central nervous system (CNS) activity. Through the interaction with four G-protein coupled receptor (GPCR) subtypes, adenosine subtly regulates neurotransmission, interfering with the dopaminergic, glutamatergic, noradrenergic, serotoninergic, and endocannabinoid systems. The inhibitory and facilitating actions of adenosine on neurotransmission are mainly mediated by A₁ and A_2A adenosine receptors (ARs), respectively. Given their role in the CNS, ARs are promising therapeutic targets for neuropsychiatric disorders where altered neurotransmission represents the most likely etiological hypothesis. Activating or blocking ARs with specific pharmacological agents could therefore restore the balance of altered neurotransmitter systems, providing the rationale for the potential treatment of these highly debilitating conditions. In this review, we summarize and discuss the most relevant studies concerning AR modulation in psychotic and mood disorders such as schizophrenia, bipolar disorders, depression, and anxiety, as well as neurodevelopment disorders such as autism spectrum disorder (ASD), fragile X syndrome (FXS), attention-deficit hyperactivity disorder (ADHD), and neuropsychiatric aspects of neurodegenerative disorders.

Astrocytes in Neuropsychiatric Disorders: A Review of Postmortem Evidence

Glial cell types in the central nervous system (CNS) include microglia, oligodendrocytes and the most diverse type, astrocytes. Clinical and experimental evidence suggest critical roles for astrocytes in the pathogenesis of CNS disease. Here, we summarize the extensive morphological heterogeneity and physiological properties of different astrocyte subtypes. We review postmortem studies, discussing astrocyte-related changes found in the brain in subjects diagnosed with the neuropsychiatric disorders schizophrenia, major depressive disorder and bipolar disorder. Finally, we discuss the potential effects of psychotropic medication on these findings. In summary, postmortem studies highlight that the morphology of astrocytes and the expression of functionally important astrocyte markers are altered in the brain in neuropsychiatric disorders and may play a role in the pathophysiology of these serious mental illnesses.

Zinc Supplementation During Pregnancy Alleviates Lipopolysaccharide-Induced Glial Activation and Inflammatory Markers Expression in a Rat Model of Maternal Immune Activation

Maternal immune activation (MIA) model has been profoundly described as a suitable approach to study the pathophysiological mechanisms of neuropsychiatric disorders, including schizophrenia. Our previous study revealed that prenatal exposure to lipopolysaccharide (LPS) induced working memory impairments in only male offspring. Based on the putative role of prefrontal cortex (PFC) in working memory process, the current study was conducted to examine the long-lasting effect of LPS-induced MIA on several neuroinflammatory mediators in the PFC of adult male pups. We also investigated whether maternal zinc supplementation can alleviate LPS-induced alterations in this region. Pregnant rats received intraperitoneal injections of either LPS (0.5 mg/kg) or saline on gestation days 15/16 and supplemented with ZnSO4 (30 mg/kg) throughout pregnancy. At postnatal day 60, the density of both microglia and astrocyte cells and the expression levels of IL-6, IL-1β, iNOS, TNF-α, NF-κB, and GFAP were evaluated in the PFC of male pups. Although maternal LPS treatment increased microglia and astrocyte density, number of neurons in the PFC of adult offspring remained unchanged. These findings were accompanied by the exacerbated mRNA levels of IL-6, IL-1β, iNOS, TNF-α, NF-κB, and GFAP as well. Conversely, prenatal zinc supplementation alleviated the mentioned alterations induced by LPS. These findings support the idea that the deleterious effects of prenatal LPS exposure could be attenuated by zinc supplementation during pregnancy. It is of interest to suggest early therapeutic intervention as a valuable approach to prevent neurodevelopmental deficits, following maternal infection. Schematic diagram describing the experimental timeline. On gestation days (GD) 15 and 16, pregnant dams were administered with intraperitoneal injections of either LPS (0.5 mg/kg) or vehicle and supplemented with ZnSO4 (30 mg/kg) throughout pregnancy by gavage. The resulting offspring were submitted to qPCR, immunostaining, and morphological analysis at PND 60. Maternal zinc supplementation alleviated increased expression levels of inflammatory mediators and microglia and astrocyte density induced by LPS in the PFC of treated offspring. PND postnatal day, PFC prefrontal cortex.

Role of Astrocytes in Major Neuropsychiatric Disorders

Astrocytes are the primary homeostatic cells of the central nervous system, essential for normal neuronal development and function, metabolism and response to injury and inflammation. Here, we review postmortem studies examining changes in astrocytes in subjects diagnosed with the neuropsychiatric disorders schizophrenia (SCZ), major depressive disorder (MDD), and bipolar disorder (BPD). We discuss the astrocyte-related changes described in the brain in these disorders and the potential effects of psychotropic medication on these findings. Finally, we describe emerging tools that can be used to study the role of astrocytes in neuropsychiatric illness.

Not Just a Bystander: The Emerging Role of Astrocytes and Research Tools in Studying Cognitive Dysfunctions in Schizophrenia

Cognitive dysfunction is one of the core symptoms in schizophrenia, and it is predictive of functional outcomes and therefore useful for treatment targets. Rather than improving cognitive deficits, currently available antipsychotics mainly focus on positive symptoms, targeting dopaminergic/serotoninergic neurons and receptors in the brain. Apart from investigating the neural mechanisms underlying schizophrenia, emerging evidence indicates the importance of glial cells in brain structure development and their involvement in cognitive functions. Although the etiopathology of astrocytes in schizophrenia remains unclear, accumulated evidence reveals that alterations in gene expression and astrocyte products have been reported in schizophrenic patients. To further investigate the role of astrocytes in schizophrenia, we highlighted recent progress in the investigation of the effect of astrocytes on abnormalities in glutamate transmission and impairments in the blood–brain barrier. Recent advances in animal models and behavioral methods were introduced to examine schizophrenia-related cognitive deficits and negative symptoms. We also highlighted several experimental tools that further elucidate the role of astrocytes. Instead of focusing on schizophrenia as a neuron-specific disorder, an additional astrocytic perspective provides novel and promising insight into its causal mechanisms and treatment. The involvement of astrocytes in the pathogenesis of schizophrenia and other brain disorders is worth further investigation.

Excitatory amino acid transporter (EAAT)1 and EAAT2 mRNA levels are altered in the prefrontal cortex of subjects with schizophrenia

Excitatory amino acid transporter (EAAT)1 and EAAT2 mediate glutamatergic neurotransmission and prevent excitotoxicity through binding and transportation of glutamate into glia. These EAATs may be regulated by metabotropic glutamate receptor 5 (mGluR5), which is also expressed by glia. Whilst we have data from an Affymetrix™ Human Exon 1.0 ST Array showing higher levels of EAAT1 mRNA (+36%) in Brodmann's are (BA)9 of subjects with schizophrenia, there is evidence that EAAT1 and EAAT2, as well as mGluR5 levels, are altered in the cortex of subjects with the disorder. Hence, we measured mRNA levels of these genes in other cortical regions in subjects with that disorder. EAAT1, EAAT2 and mGluR5 mRNA were measured, in triplicate, using Quantitative PCR in BA10 and BA46 from subjects with schizophrenia (n = 20) and age and sex matched controls (n = 18). Levels of mRNA were normalised to the geometric mean of two reference genes, transcription factor B1, mitochondrial (TFB1M) and S-phase kinase-associated protein 1A (SKP1A), for which mRNA did not vary between diagnostic groups in either region. Normalised levels of EAAT1 and EAAT2 mRNA were significantly higher in BA10 (EAAT1: U = 58, p = 0.0002; EAAT2 U = 70, p = 0.0009), but not BA46 (EAAT1: U = 122, p = 0.09; EAAT2: U = 136, p = 0.21), from subjects with schizophrenia compared to controls. mGluR5 levels in BA10 (U = 173, p=0.85) and BA46 (U = 178, p = 0.96) did not vary by cohort. Our data suggests that region-specific increases in cortical EAAT1 and EAAT2 mRNA are involved in schizophrenia pathophysiology and that disrupted glutamate uptake in schizophrenia may be of particular significance in BA10.

Glutamate Transporters: Expression and Function in Oligodendrocytes

Glutamate, the main excitatory neurotransmitter of the vertebrate central nervous system (CNS), is well known as a regulator of neuronal plasticity and neurodevelopment. Such glutamate function is thought to be mediated primarily by signaling through glutamate receptors. Thus, it requires a tight regulation of extracellular glutamate levels and a fine-tuned homeostasis that, when dysregulated, has been associated with a wide range of central pathologies including neuropsychiatric, neurodevelopmental, and neurodegenerative disorders. In the mammalian CNS, extracellular glutamate levels are controlled by a family of sodium-dependent glutamate transporters belonging to the solute carrier family 1 (SLC1) that are also referred to as excitatory amino acid transporters (EAATs). The presumed main function of EAATs has been best described in the context of synaptic transmission where EAATs expressed by astrocytes and neurons effectively regulate extracellular glutamate levels so that synapses can function independently. There is, however, increasing evidence that EAATs are expressed by cells other than astrocytes and neurons, and that they exhibit functions beyond glutamate clearance. In this review, we will focus on the expression and functions of EAATs in the myelinating cells of the CNS, oligodendrocytes. More specifically, we will discuss potential roles of oligodendrocyte-expressed EAATs in contributing to extracellular glutamate homeostasis, and in regulating oligodendrocyte maturation and CNS myelination by exerting signaling functions that have traditionally been associated with glutamate receptors. In addition, we will provide some examples for how dysregulation of oligodendrocyte-expressed EAATs may be involved in the pathophysiology of neurologic diseases.

Fasciculation and Elongation Protein Zeta-1 Expression in Reactive Astrocytes in a Rat Model of Frontal Lobe Injury

There are reports that depression induced by frontal lobe injury (FLI) has a devastating effect on human mental health. We previously reported that fasciculation and elongation protein zeta-1 (FEZ1) was essential for astrocytic protection of dopamine neurons. Studies of glutamate-glutamine cycle in mental illness have been reported, whereas not from the perspective of astrocytes. This study was designed to investigate the roles of astrocytic FEZ1 and glutamate-glutamine cycle after FLI. A model of FLI was established by inserting a blade into the right frontal lobe of rats. Behavioral tests were used to observe the behavioral changes of FLI rats. Neuropathologic examinations, including immunohistochemistry, were conducted. Behavioral tests showed that FLI decreased exploratory activity. Western blot analysis revealed that the expression of astroglial proteins overall decreased in the initial injury stage, as well as FEZ1. Immunohistochemistry showed a shift of FEZ1 localization from neurons in sham-lesioned rats to astrocytes in FLI rats, and showed the expression profile of glutamate transporter 1 and glutamine synthetase (GS) was consistent with Western blot observation. Our results indicate that astrocytic FEZ1 and glutamate-glutamine cycle dysfunction may be involved in the pathogenesis of depression after FLI.

Quantitative Subcellular Proteomics of the Orbitofrontal Cortex of Schizophrenia Patients

Schizophrenia is a chronic disease characterized by the impairment of mental functions with a marked social dysfunction. A quantitative proteomic approach using iTRAQ labeling and SRM, applied to the characterization of mitochondria (MIT), crude nuclear fraction (NUC), and cytoplasm (CYT), can allow the observation of dynamic changes in cell compartments providing valuable insights concerning schizophrenia physiopathology. Mass spectrometry analyses of the orbitofrontal cortex from 12 schizophrenia patients and 8 healthy controls identified 655 protein groups in the MIT fraction, 1500 in NUC, and 1591 in CYT. We found 166 groups of proteins dysregulated among all enriched cellular fractions. Through the quantitative proteomic analysis, we detect as the main biological pathways those related to calcium and glutamate imbalance, cell signaling disruption of CREB activation, axon guidance, and proteins involved in the activation of NF-kB signaling along with the increase of complement protein C3. Based on our data analysis, we suggest the activation of NF-kB as a possible pathway that links the deregulation of glutamate, calcium, apoptosis, and the activation of the immune system in schizophrenia patients. All MS data are available in the ProteomeXchange Repository under the identifier PXD015356 and PXD014350.

Aberrant transcriptomes and DNA methylomes define pathways that drive pathogenesis and loss of brain laterality/asymmetry in schizophrenia and bipolar disorder

Although the loss of brain laterality is one of the most consistent modalities in schizophrenia (SCZ) and bipolar disorder (BD), its molecular basis remains elusive. Our limited previous studies indicated that epigenetic modifications are key to the asymmetric transcriptomes of brain hemispheres. We used whole-genome expression microarrays to profile postmortem brain samples from subjects with SCZ, psychotic BD [BD[+]] or non-psychotic BD [BD(-)], or matched controls (10/group) and performed whole-genome DNA methylation (DNAM) profiling of the same samples (3-4/group) to identify pathways associated with SCZ or BD[+] and genes/sites susceptible to epigenetic regulation. qRT-PCR and quantitative DNAM analysis were employed to validate findings in larger sample sets (35/group). Gene Set Enrichment Analysis (GSEA) demonstrated that BMP signaling and astrocyte and cerebral cortex development are significantly (FDR q < 0.25) coordinately upregulated in both SCZ and BD[+], and glutamate signaling and TGFβ signaling are significantly coordinately upregulated in SCZ. GSEA also indicated that collagens are downregulated in right versus left brain of controls, but not in SCZ or BD[+] patients. Ingenuity Pathway Analysis predicted that TGFB2 is an upstream regulator of these genes (p = .0012). While lateralized expression of TGFB2 in controls (p = .017) is associated with a corresponding change in DNAM (p ≤ .023), lateralized expression and DNAM of TGFB2 are absent in SCZ or BD. Loss of brain laterality in SCZ and BD corresponds to aberrant epigenetic regulation of TGFB2 and changes in TGFβ signaling, indicating potential avenues for disease prevention/treatment.

Neuroglial Involvement in Abnormal Glutamate Transport in the Cochlear Nuclei of the Igf1 -/- Mouse

Insulin-like growth factor 1 (IGF-1) is a powerful regulator of synaptic activity and a deficit in this protein has a profound impact on neurotransmission, mostly on excitatory synapses in both the developing and mature auditory system. Adult Igf1^−/− mice are animal models for the study of human syndromic deafness; they show altered cochlear projection patterns into abnormally developed auditory neurons along with impaired glutamate uptake in the cochlear nuclei, phenomena that probably reflect disruptions in neuronal circuits. To determine the cellular mechanisms that might be involved in regulating excitatory synaptic plasticity in 4-month-old Igf1^−/− mice, modifications to neuroglia, astroglial glutamate transporters (GLTs) and metabotropic glutamate receptors (mGluRs) were assessed in the cochlear nuclei. The Igf1^−/− mice show significant decreases in IBA1 (an ionized calcium-binding adapter) and glial fibrillary acidic protein (GFAP) mRNA expression and protein accumulation, as well as dampened mGluR expression in conjunction with enhanced glutamate transporter 1 (GLT1) expression. By contrast, no differences were observed in the expression of glutamate aspartate transporter (GLAST) between these Igf1^−/− mice and their heterozygous or wildtype littermates. These observations suggest that congenital IGF-1 deficiency may lead to alterations in microglia and astrocytes, an upregulation of GLT1, and the downregulation of groups I, II and III mGluRs. Understanding the molecular, biochemical and morphological mechanisms underlying neuronal plasticity in a mouse model of hearing deficits will give us insight into new therapeutic strategies that could help to maintain or even improve residual hearing when human deafness is related to IGF-1 deficiency.

Astroglial correlates of neuropsychiatric disease: From astrocytopathy to astrogliosis

Complex roles for astrocytes in health and disease continue to emerge, highlighting this class of cells as integral to function and dysfunction of the nervous system. In particular, escalating evidence strongly implicates a range of changes in astrocyte structure and function associated with neuropsychiatric diseases including major depressive disorder, schizophrenia, and addiction. These changes can range from astrocytopathy, degeneration, and loss of function, to astrogliosis and hypertrophy, and can be either adaptive or maladaptive. Evidence from the literature indicates a myriad of changes observed in astrocytes from both human postmortem studies as well as preclinical animal models, including changes in expression of glial fibrillary protein, as well as changes in astrocyte morphology and astrocyte-mediated regulation of synaptic function. In this review, we seek to provide a comprehensive assessment of these findings and consequently evidence for common themes regarding adaptations in astrocytes associated with neuropsychiatric disease. While results are mixed across conditions and models, general findings indicate decreased astrocyte cellular features and gene expression in depression, chronic stress and anxiety, but increased inflammation in schizophrenia. Changes also vary widely in response to different drugs of abuse, with evidence reflective of features of astrocytopathy to astrogliosis, varying across drug classes, route of administration and length of withdrawal.

Glutamate transporters, EAAT1 and EAAT2, are potentially important in the pathophysiology and treatment of schizophrenia and affective disorders

Glutamate is the predominant excitatory neurotransmitter in the human brain and it has been shown that prolonged activation of the glutamatergic system leads to nerve damage and cell death. Following release from the pre-synaptic neuron and synaptic transmission, glutamate is either taken up into the pre-synaptic neuron or neighbouring glia by transmembrane glutamate transporters. Excitatory amino acid transporter (EAAT) 1 and EAAT2 are Na⁺-dependant glutamate transporters expressed predominantly in glia cells of the central nervous system. As the most abundant glutamate transporters, their primary role is to modulate levels of glutamatergic excitability and prevent spill over of glutamate beyond the synapse. This role is facilitated through the binding and transportation of glutamate into astrocytes and microglia. The function of EAAT1 and EAAT2 is heavily regulated at the levels of gene expression, post-transcriptional splicing, glycosylation states and cell-surface trafficking of the protein. Both glutamatergic dysfunction and glial dysfunction have been proposed to be involved in psychiatric disorder. This review will present an overview of the roles that EAAT1 and EAAT2 play in modulating glutamatergic activity in the human brain, and mount an argument that these two transporters could be involved in the aetiologies of schizophrenia and affective disorders as well as represent potential drug targets for novel therapies for those disorders.

Glutamate homeostasis in the adult rat prefrontal cortex is altered by cortical docosahexaenoic acid accrual during adolescence: An in vivo1H MRS study

Major psychiatric disorders are associated with dysregulated glutamate homeostasis and deficits in the omega-3 fatty acid docosahexaenoic acid (DHA). This study determined the effects of dietary-induced alterations in brain DHA accrual on cortical glutamate homeostasis in the adult rat brain. Adolescent rats were fed a control diet (n = 20), a n-3 fatty acid-deficient diet (DEF, n = 20), or a fish oil-fortified diet containing preformed DHA (FO, n = 20). In adulthood ¹H MRS scans were performed with voxels in the prefrontal cortex (PFC) and thalamus. Compared with controls, erythrocyte, PFC, and thalamus DHA levels were significantly lower in DEF rats and significantly higher in FO rats. In the PFC, but not the thalamus, glutamate was significantly elevated in DEF rats compared with controls and FO rats. Glutamine did not differ between groups and the glutamine/glutamate ratio was lower in DEF rats. No differences were observed for markers of excitotoxicity (NAA, GFAP), or astrocyte glutamate transporter (GLAST, GLT-1) or glutamine synthetase expression. Across diet groups, PFC DHA levels were inversely correlated with PFC glutamate levels and positively correlated with GLAST expression. Together these findings demonstrate that rat cortical DHA accrual during adolescence impacts glutamate homeostasis in the adult PFC.

The Importance of Non-neuronal Cell Types in hiPSC-Based Disease Modeling and Drug Screening

Current applications of human induced pluripotent stem cell (hiPSC) technologies in patient-specific models of neurodegenerative and neuropsychiatric disorders tend to focus on neuronal phenotypes. Here, we review recent efforts toward advancing hiPSCs toward non-neuronal cell types of the central nervous system (CNS) and highlight their potential use for the development of more complex in vitro models of neurodevelopment and disease. We present evidence from previous works in both rodents and humans of the importance of these cell types (oligodendrocytes, microglia, astrocytes) in neurological disease and highlight new hiPSC-based models that have sought to explore these relationships in vitro. Lastly, we summarize efforts toward conducting high-throughput screening experiments with hiPSCs and propose methods by which new screening platforms could be designed to better capture complex relationships between neural cell populations in health and disease.

The role of glutamate transporters in the pathophysiology of neuropsychiatric disorders

Altered glutamate transporter expression is a common feature of many neuropsychiatric conditions, including schizophrenia. Excitatory amino acid transporters (EAATs) are responsible for the reuptake of glutamate, preventing non-physiological spillover from the synapse. Postmortem studies have revealed significant dysregulation of EAAT expression in various brain regions at the cellular and subcellular level. Recent animal studies have also demonstrated a role for glutamate spillover as a mechanism of disease. In this review, we describe current evidence for the role of glutamate transporters in regulating synaptic plasticity and transmission. In neuropsychiatric conditions, EAAT splice variant expression is altered. There are changes in the localization of the transporters and disruption of the metabolic and structural protein network that supports EAAT activity. This results in aberrant neuroplasticity and excitatory signaling, contributing to the symptoms associated with neuropsychiatric disease. Understanding the complex functions of glutamate transporters will clarify the relevance of their role in the pathophysiology of neuropsychiatric disorders.

Behavioral sequelae of astrocyte dysfunction: focus on animal models of schizophrenia

Astrocytes regulate multiple processes in the brain ranging from trophic support of developing neurons to modulation of synaptic neurotransmission and neuroinflammation in adulthood. It is, therefore, understandable that pathogenesis and pathophysiology of major psychiatric disorders involve astrocyte dysfunctions. Until recently, there has been the paucity of experimental approaches to studying the roles of astrocytes in behavioral disease. A new generation of in vivo models allows us to advance our understanding of the roles of astrocytes in psychiatric disorders. This review will evaluate the recent studies that focus on the contribution of astrocyte dysfunction to behavioral alterations pertinent to schizophrenia and will propose the possible solutions of the limitations of the existing approaches.

Astrocytes as new targets to improve cognitive functions

Astrocytes are now viewed as key elements of brain wiring as well as neuronal communication. Indeed, they not only bridge the gap between metabolic supplies by blood vessels and neurons, but also allow fine control of neurotransmission by providing appropriate signaling molecules and insulation through a tight enwrapping of synapses. Recognition that astroglia is essential to neuronal communication is nevertheless fairly recent and the large body of evidence dissecting such role has focused on the synaptic level by identifying neuro- and gliotransmitters uptaken and released at synaptic or extrasynaptic sites. Yet, more integrated research deciphering the impact of astroglial functions on neuronal network activity have led to the reasonable assumption that the role of astrocytes in supervising synaptic activity translates in influencing neuronal processing and cognitive functions. Several investigations using recent genetic tools now support this notion by showing that inactivating or boosting astroglial function directly affects cognitive abilities. Accordingly, brain diseases resulting in impaired cognitive functions have seen their physiopathological mechanisms revisited in light of this primary protagonist of brain processing. We here provide a review of the current knowledge on the role of astrocytes in cognition and in several brain diseases including neurodegenerative disorders, psychiatric illnesses, as well as other conditions such as epilepsy. Potential astroglial therapeutic targets are also discussed.

A New Outlook on Mental Illnesses: Glial Involvement Beyond the Glue

Mental illnesses have long been perceived as the exclusive consequence of abnormalities in neuronal functioning. Until recently, the role of glial cells in the pathophysiology of mental diseases has largely been overlooked. However recently, multiple lines of evidence suggest more diverse and significant functions of glia with behavior-altering effects. The newly ascribed roles of astrocytes, oligodendrocytes and microglia have led to their examination in brain pathology and mental illnesses. Indeed, abnormalities in glial function, structure and density have been observed in postmortem brain studies of subjects diagnosed with mental illnesses. In this review, we discuss the newly identified functions of glia and highlight the findings of glial abnormalities in psychiatric disorders. We discuss these preclinical and clinical findings implicating the involvement of glial cells in mental illnesses with the perspective that these cells may represent a new target for treatment.

Deletion of adenosine A2A receptors from astrocytes disrupts glutamate homeostasis leading to psychomotor and cognitive impairment: relevance to schizophrenia

BACKGROUND

Adenosine A2A receptors (A2AR) modulate dopamine and glutamate signaling and thereby may influence some of the psychomotor and cognitive processes associated with schizophrenia. Because astroglial A2AR regulate the availability of glutamate, we hypothesized that they might play an unprecedented role in some of the processes leading to the development of schizophrenia, which we investigated using a mouse line with a selective deletion of A2AR in astrocytes (Gfa2-A2AR knockout [KO] mice].

METHODS

We examined Gfa2-A2AR KO mice for behaviors thought to recapitulate some features of schizophrenia, namely enhanced MK-801 psychomotor response (positive symptoms) and decreased working memory (cognitive symptoms). In addition, we probed for neurochemical alterations in the glutamatergic circuitry, evaluating glutamate uptake and release and the levels of key proteins defining glutamatergic signaling (glutamate transporter-I [GLT-I], N-methyl-D-aspartate receptors [NMDA-R] and α-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors [AMPA-R]) to provide a mechanistic understanding of the phenotype encountered.

RESULTS

We show that Gfa2-A2AR KO mice exhibited enhanced MK-801 psychomotor response and decreased working memory; this was accompanied by a disruption of glutamate homeostasis characterized by aberrant GLT-I activity, increased presynaptic glutamate release, NMDA-R 2B subunit upregulation, and increased internalization of AMPA-R. Accordingly, selective GLT-I inhibition or blockade of GluR1/2 endocytosis prevented the psychomotor and cognitive phenotypes in Gfa2-A2AR KO mice, namely in the nucleus accumbens.

CONCLUSIONS

These results show that the dysfunction of astrocytic A2AR, by controlling GLT-I activity, triggers an astrocyte-to-neuron wave of communication resulting in disrupted glutamate homeostasis, thought to underlie several endophenotypes relevant to schizophrenia.

Do glia drive synaptic and cognitive impairment in disease?

Synaptic dysfunction is a hallmark of many neurodegenerative and psychiatric brain disorders, yet we know little about the mechanisms that underlie synaptic vulnerability. Although neuroinflammation and reactive gliosis are prominent in virtually every CNS disease, glia are largely viewed as passive responders to neuronal damage rather than drivers of synaptic dysfunction. This perspective is changing with the growing realization that glia actively signal with neurons and influence synaptic development, transmission and plasticity through an array of secreted and contact-dependent signals. We propose that disruptions in neuron-glia signaling contribute to synaptic and cognitive impairment in disease. Illuminating the mechanisms by which glia influence synapse function may lead to the development of new therapies and biomarkers for synaptic dysfunction.

Purinergic receptors in psychiatric disorders

Psychiatric disorders describe different mental or behavioral patterns, causing suffering or poor coping of ordinary life with manifold presentations. Multifactorial processes can contribute to their development and progression. Purinergic neurotransmission and neuromodulation in the brain have attracted increasing therapeutic interest in the field of psychiatry. Purine nucleotides and nucleosides are well recognized as signaling molecules mediating cell to cell communication. The actions of ATP are mediated by ionotropic P2X and metabotropic P2Y receptor subfamilies, whilst the actions of adenosine are mediated by P1 (A1 or A2) adenosine receptors. Purinergic mechanisms and specific receptor subtypes have been shown to be linked to the regulation of many aspects of behavior and mood and to dysregulation in pathological processes of brain function. In this review the recent knowledge on the role of purinergic receptors in the two most frequent psychiatric diseases, major depression and schizophrenia, as well as on related animal models is summarized. At present the most promising data for therapeutic strategies derive from investigations of the adenosine system emphasizing a unique function of A2A receptors at neurons and astrocytes in these disorders. Among the P2 receptor family, in particular P2X7 and P2Y1 receptors were related to disturbances in major depression and schizophrenia, respectively. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

[Role of astrocytes in alterations of glutamatergic neurotransmission in schizophrenia]

The glutamatergic hypothesis of schizophrenia based on the hypofunction of the N-methyl-D-aspartate-type glutamate receptors (NMDA-R) is one of the most widely implicated hypothesis that explains the origin of positive and negative symptoms of illness as well as cognitive deficits. The author considered a neuromorphological aspect of this hypothesis related to the glial astrocytes function. The literature on the astrocyte ability to regulate glutamate neurotransmission is reviewed. Astrocyte abnormalities in schizophrenia include the disturbances of glutamate reuptake, recycling and turnover of endogenous NMDA-R ligands. The results of the experimental and clinical studies that target levels of endogenous NMDA-R ligands, their enzymes and transporters for treatment of schizophrenia symptoms are discussed. Further studies studies are needed to develop this strategy.

GLAST But Not Least--Distribution, Function, Genetics and Epigenetics of L-Glutamate Transport in Brain--Focus on GLAST/EAAT1

Synaptically released L-glutamate, the most important excitatory neurotransmitter in the CNS, is removed from extracellular space by fast and efficient transport mediated by several transporters; the most abundant ones are EAAT1/GLAST and EAAT2/GLT1. The review first summarizes their location, functions and basic characteristics. We then look at genetics and epigenetics of EAAT1/GLAST and EAAT2/GLT1 and perform in silico analyses of their promoter regions. There is one CpG island in SLC1A2 (EAAT2/GLT1) gene and none in SLC1A3 (EAAT1/GLAST) suggesting that DNA methylation is not the most important epigenetic mechanism regulating EAAT1/GLAST levels in brain. There are targets for specific miRNA in SLC1A2 (EAAT2/GLT1) gene. We also note that while defects in EAAT2/GLT1 have been associated with various pathological states including chronic neurodegenerative diseases, very little is known on possible contributions of defective or dysfunctional EAAT1/GLAST to any specific brain disease. Finally, we review evidence of EAAT1/GLAST involvement in mechanisms of brain response to alcoholism and present some preliminary data showing that ethanol, at concentrations which may be reached following heavy drinking, can have an effect on the distribution of EAAT1/GLAST in cultured astrocytes; the effect is blocked by baclofen, a GABA-B receptor agonist and a drug potentially useful in the treatment of alcoholism. We argue that more research effort should be focused on EAAT1/GLAST, particularly in relation to alcoholism and drug addiction.

Fatty acid-binding protein 7 regulates function of caveolae in astrocytes through expression of caveolin-1

Fatty acid-binding proteins (FABPs) bind and solubilize long-chain fatty acids, controlling intracellular lipid dynamics. FABP7 is expressed by astrocytes in the developing brain, and suggested to be involved in the control of astrocyte lipid homeostasis. In this study, we sought to examine the role of FABP7 in astrocytes, focusing on plasma membrane lipid raft function, which is important for receptor-mediated signal transduction in response to extracellular stimuli. In FABP7-knockout (KO) astrocytes, the ligand-dependent accumulation of Toll-like receptor 4 (TLR4) and glial cell-line-derived neurotrophic factor receptor alpha 1 into lipid raft was decreased, and the activation of mitogen-activated protein kinases and nuclear factor-κB was impaired after lipopolysaccharide (LPS) stimulation when compared with wild-type astrocytes. In addition, the expression of caveolin-1, not cavin-1, 2, 3, caveolin-2, and flotillin-1, was found to be decreased at the protein and transcriptional levels. FABP7 re-expression in FABP7-KO astrocytes rescued the decreased level of caveolin-1. Furthermore, caveolin-1-transfection into FABP7-KO astrocytes significantly increased TLR4 recruitment into lipid raft and tumor necrosis factor-α production after LPS stimulation. Taken together, these data suggest that FABP7 controls lipid raft function through the regulation of caveolin-1 expression and is involved in the response of astrocytes to the external stimuli. GLIA 2015;63:780-794.

An RNA-sequencing transcriptome and splicing database of glia, neurons, and vascular cells of the cerebral cortex

The major cell classes of the brain differ in their developmental processes, metabolism, signaling, and function. To better understand the functions and interactions of the cell types that comprise these classes, we acutely purified representative populations of neurons, astrocytes, oligodendrocyte precursor cells, newly formed oligodendrocytes, myelinating oligodendrocytes, microglia, endothelial cells, and pericytes from mouse cerebral cortex. We generated a transcriptome database for these eight cell types by RNA sequencing and used a sensitive algorithm to detect alternative splicing events in each cell type. Bioinformatic analyses identified thousands of new cell type-enriched genes and splicing isoforms that will provide novel markers for cell identification, tools for genetic manipulation, and insights into the biology of the brain. For example, our data provide clues as to how neurons and astrocytes differ in their ability to dynamically regulate glycolytic flux and lactate generation attributable to unique splicing of PKM2, the gene encoding the glycolytic enzyme pyruvate kinase. This dataset will provide a powerful new resource for understanding the development and function of the brain. To ensure the widespread distribution of these datasets, we have created a user-friendly website (http://web.stanford.edu/group/barres_lab/brain_rnaseq.html) that provides a platform for analyzing and comparing transciption and alternative splicing profiles for various cell classes in the brain.

Antipsychotic treatment modulates glutamate transport and NMDA receptor expression

Schizophrenia patients often suffer from treatment-resistant cognitive and negative symptoms, both of which are influenced by glutamate neurotransmission. Innovative therapeutic strategies such as agonists at metabotropic glutamate receptors or glycin reuptake inhibitors try to modulate the brain's glutamate network. Interactions of amino acids with monoamines have been described on several levels, and first- and second-generation antipsychotic agents (FGAs, SGAs) are known to exert modulatory effects on the glutamatergic system. This review summarizes the current knowledge on effects of FGAs and SGAs on glutamate transport and receptor expression derived from pharmacological studies. Such studies serve as a control for molecular findings in schizophrenia brain tissue and are clinically relevant. Moreover, they may validate animal models for psychosis, foster basic research on antipsychotic substances and finally lead to a better understanding of how monoaminergic and amino acid neurotransmissions are intertwined. In the light of these results, important differences dependent on antipsychotic substances, dosage and duration of treatment became obvious. While some post-mortem findings might be confounded with multifold drug effects, others are unlikely to be influenced by antipsychotic treatment and could represent important markers of schizophrenia pathophysiology. In similarity to the convergence of toxic and psychotomimetic effects of dopaminergic, serotonergic and anti-glutamatergic substances, the therapeutic mechanisms of SGAs might merge on a yet to be defined molecular level. In particular, serotonergic effects of SGAs, such as an agonism at 5HT1A receptors, represent important targets for further clinical research.

Transcriptomic evidence for immaturity of the prefrontal cortex in patients with schizophrenia

BACKGROUND

Schizophrenia, a severe psychiatric disorder, has a lifetime prevalence of 1%. The exact mechanisms underlying this disorder remain unknown, though theories abound. Recent studies suggest that particular cell types and biological processes in the schizophrenic cortex have a pseudo-immature status in which the molecular properties partially resemble those in the normal immature brain. However, genome-wide gene expression patterns in the brains of patients with schizophrenia and those of normal infants have not been directly compared. Here, we show that the gene expression patterns in the schizophrenic prefrontal cortex (PFC) resemble those in the juvenile PFC.

RESULTS

We conducted a gene expression meta-analysis in which, using microarray data derived from different studies, altered expression patterns in the dorsolateral PFC (DLFC) of patients with schizophrenia were compared with those in the DLFC of developing normal human brains, revealing a striking similarity. The results were replicated in a second DLFC data set and a medial PFC (MFC) data set. We also found that about half of the genes representing the transcriptomic immaturity of the schizophrenic PFC were developmentally regulated in fast-spiking interneurons, astrocytes, and oligodendrocytes. Furthermore, to test whether medications, which often confound the results of postmortem analyses, affect on the juvenile-like gene expressions in the schizophrenic PFC, we compared the gene expression patterns showing transcriptomic immaturity in the schizophrenic PFC with those in the PFC of rodents treated with antipsychotic drugs. The results showed no apparent similarities between the two conditions, suggesting that the juvenile-like gene expression patterns observed in the schizophrenic PFC could not be accounted for by medication effects. Moreover, the developing human PFC showed a gene expression pattern similar to that of the PFC of naive Schnurri-2 knockout mice, an animal model of schizophrenia with good face and construct validity. This result also supports the idea that the transcriptomic immaturity of the schizophrenic PFC is not due to medication effects.

CONCLUSIONS

Collectively, our results provide evidence that pseudo-immaturity of the PFC resembling juvenile PFC may be an endophenotype of schizophrenia.

Abnormal partitioning of hexokinase 1 suggests disruption of a glutamate transport protein complex in schizophrenia

Excitatory amino acid transporter 2 (EAAT2) belongs to a family of Na(+) dependent glutamate transporters that maintain a low synaptic concentration of glutamate by removing glutamate from the synaptic cleft into astroglia and neurons. EAAT2 activity depends on Na(+) and K(+) gradients generated by Na(+)/K(+) ATPase and ATP. Hexokinase 1 (HK1), an initial enzyme of glycolysis, binds to mitochondrial outer membrane where it couples cytosolic glycolysis to mitochondrial oxidative phosphorylation, producing ATP utilized by the EAAT2/Na(+)/K(+) ATPase protein complex to facilitate glutamate reuptake. In this study, we hypothesized that the protein complex formed by EAAT2, Na(+)/K(+) ATPase and mitochondrial proteins in human postmortem prefrontal cortex may be disrupted, leading to abnormal glutamate transmission in schizophrenia. We first determined that EAAT2, Na(+)/K(+) ATPase, HK1 and aconitase were found in both EAAT2 and Na(+)/K(+) ATPase interactomes by immunoisolation and mass spectrometry in human postmortem prefrontal cortex. Next, we measured levels of glutamate transport complex proteins in subcellular fractions in the dorsolateral prefrontal cortex and found increases in the EAAT2B isoform of EAAT2 in a fraction containing extrasynaptic membranes and increased aconitase 1 in a mitochondrial fraction. Finally, an increased ratio of HK1 protein in the extrasynaptic membrane/mitochondrial fraction was found in subjects with schizophrenia, suggesting that HK1 protein is abnormally partitioned in this illness. Our findings indicate that the integrity of the glutamate transport protein complex may be disrupted, leading to decreased perisynaptic buffering and reuptake of glutamate, as well as impaired energy metabolism in schizophrenia.

Dopamine denervation of the prefrontal cortex increases expression of the astrocytic glutamate transporter GLT-1

Both dopamine and glutamate are critically involved in cognitive processes such as working memory. Astrocytes, which express dopamine receptors, are essential elements in the termination of glutamatergic signaling: the astrocytic glutamate transporter GLT-1 is responsible for > 90% of cortical glutamate uptake. The effect of dopamine depletion on glutamate transporters in the prefrontal cortex (PFC) remains unknown. In an effort to determine if astrocytes are a locus of cortical dopamine-glutamate interactions, we examined the effects of chronic dopamine denervation on PFC protein and mRNA levels of glutamate transporters. PFC dopamine denervation elicited a marked increase in GLT-1 protein levels, but had no effect on levels of other glutamate transporters; high-affinity glutamate transport was positively correlated with the extent of dopamine depletion. GLT-1 gene expression was not altered. Our data suggest that dopamine depletion may lead to post-translational modifications that result in increased expression and activity of GLT-1 in PFC astrocytes. The glutamate transporter GLT-1 is expressed by astrocytes, which also express dopamine receptors. Regulation of prefrontal cortical (PFC) GLT-1 potentially offers a novel treatment approach to the cognitive deficits of schizophrenia. Partial PFC dopamine deafferentation increased membrane expression of GLT-1 protein and glutamate uptake, but did not alter levels of the other two neocortical glutamate transporters, GLAST and EAAC1.

Increased glutamine in patients undergoing long-term treatment for schizophrenia: a proton magnetic resonance spectroscopy study at 3 T

IMPORTANCE

The N-methyl-d-aspartic acid receptor hypofunction model of schizophrenia predicts a paradoxical increase in synaptic glutamate release. In vivo measurement of glutamatergic neurotransmission in humans is challenging, but glutamine, the principal metabolite of synaptic glutamate, can be quantified with proton magnetic resonance spectroscopy (1H-MRS). Although a few studies have measured glutamate, glutamine, and glutamine to glutamate ratio, it is not clear which of these 1H-MRS indices of glutamatergic neurotransmission is altered in schizophrenia.

OBJECTIVE

To examine glutamine, glutamate, and glutamine to glutamate ratio in the dorsal anterior cingulate, as well as their relationships with symptoms and cognition in schizophrenia.

DESIGN, SETTING, AND PARTICIPANTS

Cross-sectional design using 3-T 1H-MRS in participants recruited from university-based psychiatric outpatient clinics who underwent neuroimaging at an affiliated research facility. Participants were 84 patients with a DSM-IV-TR diagnosis of schizophrenia and 81 psychiatrically healthy volunteers, matched in age, sex, ethnicity, and occupational level to the head of household of family of origin.

MAIN OUTCOMES AND MEASURES

Glutamine, glutamate, and glutamine to glutamate ratio. Also symptoms and cognition.

RESULTS

Glutamine was increased in the schizophrenia group (P = .01) as well as the glutamine to glutamate ratio (P = .007) but not glutamate (P = .89). Glutamine levels were positively correlated with severity of psychotic symptoms (P = .02). Choline was also increased in schizophrenia (P = .002).

CONCLUSIONS AND RELEVANCE

Elevated glutamine, which was directly related to psychotic symptoms, is consistent with increased glutamatergic synaptic release in schizophrenia, as predicted by the N-methyl-d-aspartic acid receptor hypofunction model. Further understanding the underlying mechanism of glutamatergic dysfunction in schizophrenia may lead to new pharmacological strategies to treat psychosis.

Adenosine receptor control of cognition in normal and disease

Adenosine and adenosine receptors (ARs) are increasingly recognized as important therapeutic targets for controlling cognition under normal and disease conditions for its dual roles of neuromodulation as well as of homeostatic function in the brain. This chapter first presents the unique ability of adenosine, by acting on the inhibitory A1 and facilitating A2A receptor, to integrate dopamine, glutamate, and BNDF signaling and to modulate synaptic plasticity (e.g., long-term potentiation and long-term depression) in brain regions relevant to learning and memory, providing the molecular and cellular bases for adenosine receptor (AR) control of cognition. This led to the demonstration of AR modulation of social recognition memory, working memory, reference memory, reversal learning, goal-directed behavior/habit formation, Pavlovian fear conditioning, and effort-related behavior. Furthermore, human and animal studies support that AR activity can also, through cognitive enhancement and neuroprotection, reverse cognitive impairments in animal models of Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and schizophrenia. Lastly, epidemiological evidence indicates that regular human consumption of caffeine, the most widely used psychoactive drug and nonselective AR antagonists, is associated with the reduced cognitive decline in aging and AD patients, and with the reduced risk in developing PD. Thus, there is a convergence of the molecular studies revealing AR as molecular targets for integrating neurotransmitter signaling and controlling synaptic plasticity, with animal studies demonstrating the strong procognitive impact upon AR antagonism in normal and disease brains and with epidemiological and clinical evidences in support of caffeine and AR drugs for therapeutic modulation of cognition. Since some of adenosine A2A receptor antagonists are already in phase III clinical trials for motor benefits in PD patients with remarkable safety profiles, additional animal and human studies to better understand the mechanism underlying the AR-mediated control of cognition under normal and disease conditions will provide the required rationale to stimulate the necessary clinical investigation to rapidly translate adenosine and AR drug as a novel strategy to control memory impairment in neuropsychiatric disorders.

Astrogliopathology: a central element of neuropsychiatric diseases?

Astroglia are the homoeostatic cells of the central nervous system that control a normal function of synaptically connected neuronal networks and contribute to brain defense. Recent advances in comprehension of pathological potential of astroglia indicate that astrocytes are fundamental for most (if not all) neurological diseases. Neuropathological and neuroimaging studies demonstrate prominent astroglial atrophy and astroglial asthenia occurring in most of neuropsychiatric illnesses. In chronic diseases such as schizophrenia and major depression, decrease in astroglial numbers and functional capabilities are, arguably, fundamental for pathological developments being responsible for neurotransmitter disbalance and failures in connectivity within neural networks. In neurodegenerative diseases atrophic changes in astrocytes are complemented by astrogliosis triggered by specific lesions such as senile plaques or dying neurons, these two processes contributing to cognitive decline and ultimately neuronal death. It is therefore possible to hypothesize that neuropsychiatric diseases represent a chronic astrogliopathology, which compromises glial homeostatic and defensive capabilities, and the degree and the alacrity of gliodegenerative changes define the progression and outcome of these disorders.

Effect of glutamate transporter EAAT2 gene variants and gray matter deficits on working memory in schizophrenia

Glutamate is the major excitatory neurotransmitter in the brain, with up to 40% of all synapses being glutamatergic. An altered glutamatergic transmission could play a critical role in working memory deficts observed in schizophrenia and could underline progressive changes such as grey matter loss throughout the brain. The aim of the study was to investigate if gray matter volume and working memory could be modulated by a genetic polymorphism related to glutamatergic function. Fifty schizophrenia patients underwent magnetic resonance and working memory testing outside of the scanner and were genotyped for rs4354668 EAAT2 polymorphism. Carriers of the G allele had lower gray matter volumes than T/T homozygote and worse working memory performance. Poor working memory performance was associated with gray matter reduction. Differences between the three genotypes are more relevant among patients showing poor performance at the 2-back task. Since glutamate abnormalities are known to be involved in excitotoxic processes, the decrease in cortical thickness observed in schizophrenia patients could be linked to an excess of extracellular glutamate. The differential effect of EAAT2 observed between good and poor performers suggests that the effect of EEAT2 on gray matter might reveal in the presence of a pathological process affecting gray matter.

Neuropsychiatric autoimmune encephalitis without VGKC-complex, NMDAR, and GAD autoantibodies: case report and literature review

We report a patient with a seronegative autoimmune panencephalitis, adding a subtype to the emerging spectrum of seronegative autoimmune encephalitis, and we review the sparse literature on isolated psychiatric presentations of autoimmune encephalitis. (A PubMed search for "seronegative autoimmune encephalitis," "nonvasculitic autoimmune inflammatory meningoencephalitis," and related terms revealed <25 cases.) A 15-year-old girl developed an acute-onset isolated psychosis with prominent negative symptoms and intermittent encephalopathy. Despite clinical worsening, her brain magnetic resonance imaging (MRI) scans remained normal for 7 years. Serology was negative for voltage-gated potassium channel (VGKC)-complex, N-methyl-D-aspartate receptor (NMDAR), and glutamic acid decarboxylase (GAD) autoantibodies. We excluded genetic, metabolic, paraneoplastic, degenerative, and infectious etiologies. The patient's symptoms remitted fully with immune therapy, but recurred in association with widespread bihemispheric brain lesions. Brain biopsy revealed mild nonvasculitic inflammation and prominent vascular hyalinization. Immune therapy with plasma exchanges cleared the MRI abnormalities but, 10 years after onset, the patient still suffers neuropsychiatric sequelae. We conclude that autoimmune panencephalitis seronegative for VGKC-complex, NMDAR, and GAD autoantibodies is a subtype of autoimmune encephalitis that can present with pure neuropsychiatric features and a normal brain MRI. Immunologic mechanisms may account for psychiatric symptoms in a subset of patients now diagnosed with classical psychotic disorders. Delay in starting immune therapy can lead to permanent neuropsychiatric sequelae. We propose a standardized classification system for the autoimmune encephalitides, integrating earlier pathology-oriented terms with more recently defined serologic and clinical phenotypes.

Gene expression of glutamate transporters SLC1A1, SLC1A3 and SLC1A6 in the cerebellar subregions of elderly schizophrenia patients and effects of antipsychotic treatment

OBJECTIVES

The glutamatergic hypothesis of schizophrenia proposes alterations of excitatory amino acid transporters (solute carrier family, SLCs) expression and cerebellar dysfunctions. The influence of the neuregulin-1 (NRG1) risk genotype or effects of antipsychotics on expression of EAATs are unknown.

METHODS

We compared post-mortem samples from the cerebellar hemispheres and vermis of 10 schizophrenia patients with nine normal subjects by investigating gene expression of SLC1A3, SLC1A1 and SLC1A6 by in-situ hybridization. We further assessed the allelic composition regarding the polymorphism rs35753505 (SNP8NRG221533) near the NRG1 gene. To control for effects due to antipsychotic treatment, we chronically treated rats with the antipsychotics haloperidol or clozapine and assessed gene expression of SLCs.

RESULTS

Schizophrenia patients showed increased expression of SLC1A3 in the molecular layer of the vermis. Individuals carrying at least one C allele of rs35753505 (SNP8NRG221533) showed decreased expression of SLC1A6 in the molecular layer of both hemispheres, compared to individuals homozygous for the T allele. The animal model revealed suppression of SLC1A6 by clozapine.

CONCLUSIONS

Increased SLC1A3 expression indicates facilitated transport and may result in reduced glutamate neurotransmission. Decreased SLC1A6 expression in NRG1 risk variant may be an adaptive effect to restore glutamate signalling, but treatment effects cannot be excluded.

Neuroinflammation and psychiatric illness

Multiple lines of evidence support the pathogenic role of neuroinflammation in psychiatric illness. While systemic autoimmune diseases are well-documented causes of neuropsychiatric disorders, synaptic autoimmune encephalitides with psychotic symptoms often go under-recognized. Parallel to the link between psychiatric symptoms and autoimmunity in autoimmune diseases, neuroimmunological abnormalities occur in classical psychiatric disorders (for example, major depressive, bipolar, schizophrenia, and obsessive-compulsive disorders). Investigations into the pathophysiology of these conditions traditionally stressed dysregulation of the glutamatergic and monoaminergic systems, but the mechanisms causing these neurotransmitter abnormalities remained elusive. We review the link between autoimmunity and neuropsychiatric disorders, and the human and experimental evidence supporting the pathogenic role of neuroinflammation in selected classical psychiatric disorders. Understanding how psychosocial, genetic, immunological and neurotransmitter systems interact can reveal pathogenic clues and help target new preventive and symptomatic therapies.

Abnormal expression of glutamate transporters in temporal lobe areas in elderly patients with schizophrenia

Glutamate transporters facilitate the buffering, clearance and cycling of glutamate and play an important role in maintaining synaptic and extrasynaptic glutamate levels. Alterations in glutamate transporter expression may lead to abnormal glutamate neurotransmission contributing to the pathophysiology of schizophrenia. In addition, alterations in the architecture of the superior temporal gyrus and hippocampus have been implicated in this illness, suggesting that synapses in these regions may be remodeled from a lifetime of severe mental illness and antipsychotic treatment. Thus, we hypothesize that glutamate neurotransmission may be abnormal in the superior temporal gyrus and hippocampus in schizophrenia. To test this hypothesis, we examined protein expression of excitatory amino acid transporter 1-3 and vesicular glutamate transporter 1 and 2 in subjects with schizophrenia (n=23) and a comparison group (n=27). We found decreased expression of EAAT1 and EAAT2 protein in the superior temporal gyrus, and decreased EAAT2 protein in the hippocampus in schizophrenia. We didn't find any changes in expression of the neuronal transporter EAAT3 or the presynaptic vesicular glutamate transporters VGLUT1-2. In addition, we did not detect an effect of antipsychotic medication on expression of EAAT1 and EAAT2 proteins in the temporal association cortex or hippocampus in rats treated with haloperidol for 9 months. Our findings suggest that buffering and reuptake, but not presynaptic release, of glutamate is altered in glutamate synapses in the temporal lobe in schizophrenia.

RNA-binding protein QKI regulates Glial fibrillary acidic protein expression in human astrocytes

Linkage, association and expression studies previously pointed to the human QKI, KH domain containing, RNA-binding (QKI) as a candidate gene for schizophrenia. Functional studies of the mouse orthologue Qk focused mainly on its role in oligodendrocyte development and myelination, while its function in astroglia remained unexplored. Here, we show that QKI is highly expressed in human primary astrocytes and that its splice forms encode proteins targeting different subcellular localizations. Uncovering the role of QKI in astrocytes is of interest in light of growing evidence implicating astrocyte dysfunction in the pathogenesis of several disorders of the central nervous system. We selectively silenced QKI splice variants in human primary astrocytes and used RNA sequencing to identify differential expression and splice variant composition at the genome-wide level. We found that an mRNA expression of Glial fibrillary acidic protein (GFAP), encoding a major component of astrocyte intermediate filaments, was down-regulated after QKI7 splice variant silencing. Moreover, we identified a potential QKI-binding site within the 3' untranslated region of human GFAP. This sequence was not conserved between mice and humans, raising the possibility that GFAP is a target for QKI in humans but not rodents. Haloperidol treatment of primary astrocytes resulted in coordinated increases in QKI7 and GFAP expression. Taken together, our results provide the first link between QKI and GFAP, two genes with alterations previously observed independently in schizophrenic patients. Our findings for QKI, together with its well-known role in myelination, suggest that QKI is a hub regulator of glia function in humans.

Roles of glial cells in schizophrenia: possible targets for therapeutic approaches

Glial cells consisting of oligodendrocytes, astrocytes, microglia, and NG2 positive cells are major cell populations in the central nervous system, number-wise. They function as effectors and modulators of neurodevelopment through a wide variety of neuron-glial cell interactions in brain development and functions. Glial cells can be affected by both genetic and environmental factors, leading to their dysfunctions in supporting neuronal development and functions. These in turn can affect neuronal cells, causing alterations at the circuitry level that manifest as behavioral characteristics associated with schizophrenia in late teens-early twenties. Glial cells are also involved in neuroinflammatory processes, which sometimes have deleterious effects on the normal brain development. If the glial involvement plays significant roles in schizophrenia, the processes involving glial cells can become possible therapeutic targets for schizophrenia. A number of known antipsychotics are shown to have beneficial effects on glial cells, but other drugs targeting glial cell functions may also have therapeutic effects on schizophrenia. The latter can be taken into consideration for future drug development for schizophrenia.