Effect of early stress on hippocampal gray matter is influenced by a functional polymorphism in EAAT2 in bipolar disorder

Glial-Neuronal Interaction in Synapses: A Possible Mechanism of the Pathophysiology of Bipolar Disorder

Bipolar disorder (BD) is a severe and chronic psychiatric disorder that affects approximately 1-4% of the world population and is characterized by recurrent episodes of mania or hypomania and depression. BD is also associated with illnesses marked by immune activation, such as metabolic syndrome, obesity, type 2 diabetes mellitus, and cardiovascular diseases. Indeed, a connection has been suggested between neuroinflammation and peripheral inflammatory markers in the pathophysiology of BD, which can be associated with the modulation of many dysfunctional processes, including synaptic plasticity, neurotransmission, neurogenesis, neuronal survival, apoptosis, and even cognitive/behavioral functioning. Rising evidence suggests that synaptic dysregulations, especially glutamatergic system dysfunction, are directly involved in mood disorders. It is becoming clear that dysregulations in connection and structural changes of glial cells play a central role in the BD pathophysiology. This book chapter highlighted the latest findings that support the theory of synaptic dysfunction in BD, providing an overview of the alterations in neurotransmitters release, astrocytic uptake, and receptor signaling, as well as the role of inflammation on glial cells in mood disorders. Particular emphasis is given to the alterations in presynaptic and postsynaptic neurons and glial cells, all cellular elements of the "tripartite synapse," compromising the neurotransmitters system, excitatory-inhibitory balance, and neurotrophic states of local networks in mood disorders. Together, these studies provide a foundation of knowledge about the exact role of the glial-neuronal interaction in mood disorders.

Going beyond risk factor: Childhood maltreatment and associated modifiable targets to improve life-long outcomes in mood disorders

Childhood maltreatment increases risk for mood disorders and is associated with earlier onset-and more pernicious disease course following onset-of mood disorders. While the majority of studies to date have been cross-sectional, longitudinal studies are emerging and support the devastating role(s) childhood maltreatment has on development of, and illness course in, mood disorders. This manuscript extends prior reviews to emphasize more recent work, highlighting longitudinal data, and discusses treatment studies that provide clues to mechanisms that mediate disease risk, course, relapse, and treatment response. Evidence suggesting systemic inflammation, alterations in hypothalamic-pituitary-adrenal (HPA) axis function and corticotropin-releasing factor (CRF) neural systems, genetic and other familial factors as mechanisms that mediate risk and onset of, and illness course in, mood disorders following childhood maltreatment is discussed. Risky behaviors following maltreatment, e.g., substance use and unhealthy lifestyles, may further exacerbate alterations in the HPA axis, CRF neural systems, and systematic inflammation to contribute to a more pernicious disease course. More research on sex differences and the impact of maltreatment in vulnerable populations is needed. Future research needs to be aimed at leveraging knowledge on modifiable targets, going beyond childhood maltreatment as a risk factor, to inform prevention and treatment strategies and foster trauma-informed care.

EAAT2 as a Research Target in Bipolar Disorder and Unipolar Depression: A Systematic Review

Glutamate is implicated in the neuropathology of both major depressive disorder and bipolar disorder. Excitatory amino acid transporter 2 (EAAT2) is the major glutamate transporter in the mammalian brain, removing glutamate from the synaptic cleft and transporting it into glia for recycling. It is thereby the principal regulator of extracellular glutamate levels and prevents neuronal excitotoxicity. EAAT2 is a promising target for elucidating the mechanisms by which the glutamate-glutamine cycle interacts with neuronal systems in mood disorders. Forty EAAT2 studies (published January 1992-January 2018) were identified via a systematic literature search. The studies demonstrated that chronic stress/steroids were most commonly associated with decreased EAAT2. In rodents, EAAT2 inhibition worsened depressive behaviors. Human EAAT2 expression usually decreased in depression, with some regional brain differences. Fewer data have been collected regarding the roles and regulation of EAAT2 in bipolar disorder. Future directions for research include correlating EAAT2 and glutamate levels in vivo, elucidating genetic variability and epigenetic regulation, clarifying intracellular protein and pharmacologic interactions, and examining EAAT2 in different bipolar mood states. As part of a macromolecular complex within glia, EAAT2 may contribute significantly to intracellular signaling, energy regulation, and cellular homeostasis. An enhanced understanding of this system is needed.

The Devastating Clinical Consequences of Child Abuse and Neglect: Increased Disease Vulnerability and Poor Treatment Response in Mood Disorders

A large body of evidence has demonstrated that exposure to childhood maltreatment at any stage of development can have long-lasting consequences. It is associated with a marked increase in risk for psychiatric and medical disorders. This review summarizes the literature investigating the effects of childhood maltreatment on disease vulnerability for mood disorders, specifically summarizing cross-sectional and more recent longitudinal studies demonstrating that childhood maltreatment is more prevalent and is associated with increased risk for first mood episode, episode recurrence, greater comorbidities, and increased risk for suicidal ideation and attempts in individuals with mood disorders. It summarizes the persistent alterations associated with childhood maltreatment, including alterations in the hypothalamic-pituitary-adrenal axis and inflammatory cytokines, which may contribute to disease vulnerability and a more pernicious disease course. The authors discuss several candidate genes and environmental factors (for example, substance use) that may alter disease vulnerability and illness course and neurobiological associations that may mediate these relationships following childhood maltreatment. Studies provide insight into modifiable mechanisms and provide direction to improve both treatment and prevention strategies.

The role of astrocytic glutamate transporters GLT-1 and GLAST in neurological disorders: Potential targets for neurotherapeutics

Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS) which initiates rapid signal transmission in the synapse before its re-uptake into the surrounding glia, specifically astrocytes. The astrocytic glutamate transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) and their human homologs excitatory amino acid transporter 1 (EAAT1) and 2 (EAAT2), respectively, are the major transporters which take up synaptic glutamate to maintain optimal extracellular glutamic levels, thus preventing accumulation in the synaptic cleft and ensuing excitotoxicity. Growing evidence has shown that excitotoxicity is associated with various neurological disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), manganism, ischemia, schizophrenia, epilepsy, and autism. While the mechanisms of neurological disorders are not well understood, the dysregulation of GLAST/GLT-1 may play a significant role in excitotoxicity and associated neuropathogenesis. The expression and function of GLAST/GLT-1 may be dysregulated at the genetic, epigenetic, transcriptional or translational levels, leading to high levels of extracellular glutamate and excitotoxicity. Consequently, understanding the regulatory mechanisms of GLAST/GLT-1 has been an area of interest in developing therapeutics for the treatment of neurological disorders. Pharmacological agents including β-lactam antibiotics, estrogen/selective estrogen receptor modulators (SERMs), growth factors, histone deacetylase inhibitors (HDACi), and translational activators have shown significant efficacy in enhancing the expression and function of GLAST/GLT-1 and glutamate uptake both in vitro and in vivo. This comprehensive review will discuss the regulatory mechanisms of GLAST/GLT-1, their association with neurological disorders, and the pharmacological agents which mediate their expression and function. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Association of SLC1A2 and SLC17A7 polymorphisms with major depressive disorder in a Thai population

Background

Major depressive disorder (MDD) is a common psychiatric disorder with high prevalence and high risk of suicide. Genetic variation of glutamate transporters may associate with MDD and suicide attempt.

Objectives

To evaluate polymorphisms of excitatory amino acid transporter 2 gene (SLC1A2; rs752949, rs1885343, rs4755404, and rs4354668) and vesicular glutamate transporter 1 gene (SLC17A7; rs1043558, rs2946848, and rs11669017) in patients with MDD with and without suicide attempt, and determine the association of these polymorphisms with age of onset and severity of MDD.

Methods

DNA was extracted from blood taken from patients with MDD (n = 100; including nonsuicidal [n = 50] and suicidal [n = 50] subgroups) and controls (n = 100). Genotyping was conducted using TaqMan single-nucleotide polymorphism (SNP) genotyping.

Results

We found a significant difference in SLC17A7 rs2946848 genotype distribution between patients in the MDD and control groups (P = 0.016). Moreover, significant differences in SLC1A2 rs752949 (P = 0.022) and SLC17A7 rs2946848 (P = 0.026) genotype distributions were observed between patients in the nonsuicidal MDD and suicidal MDD groups. SLC1A2 rs1885343 A allele carriers showed significantly lower age of onset than GG genotype (P = 0.049). Furthermore, the severity of MDD indicated by the Hamilton Depression Rating Scale (HDRS) score of G allele carriers of SLC1A2 rs4755404 was significantly greater than the CC genotype (P = 0.013).

Conclusions

Polymorphisms of SLC1A2 and SLC17A7 may contribute to the risk of MDD and/or suicide attempt. An association of an SLC1A2 polymorphism with the severity of MDD was apparent.

Genetic variant in SLC1A2 is associated with elevated anterior cingulate cortex glutamate and lifetime history of rapid cycling

Glutamatergic dysregulation is implicated in the neurobiology of mood disorders. This study investigated the relationship between the anterior cingulate cortex (AC) glutamate, as measured by proton magnetic resonance spectroscopy (¹H-MRS), and single-nucleotide polymorphisms (SNPs) from four genes (GLUL, SLC1A3, SLC1A2, and SLC1A7) that regulate the extracellular glutamate in 26 depressed patients with major depressive disorder (MDD; n = 15) and bipolar disorder (BD; n = 11). Two SNPs (rs3812778 and rs3829280), in perfect linkage disequilibrium, in the 3' untranslated region of the EAAT2 gene SLC1A2, were associated with AC glutamate, with minor allele carriers having significantly higher glutamate levels (p < 0.001) in comparison with common allele homozygotes. In silico analysis revealed an association of minor allele carriers of rs3812778/rs382920 with an upregulation of the astrocytic marker CD44 localized downstream of SLC1A2 on chromosome 11. Finally, we tested the disease relevance of these SNPs in a large group of depressed patients [MDD (n = 458); BD (n = 1473)] and found that minor allele carriers had a significantly higher risk for rapid cycling (p = 0.006). Further work is encouraged to delineate the functional impact of excitatory amino acid transporter genetic variation on CD44 associated physiology and glutamatergic neurotransmission, specifically glutamate-glutamine cycling, and its contribution to subphenotypes of mood disorders.

Differential SLC1A2 Promoter Methylation in Bipolar Disorder With or Without Addiction

While downregulation of excitatory amino acid transporter 2 (EAAT2), the main transporter removing glutamate from the synapse, has been recognized in bipolar disorder (BD), the underlying mechanisms of downregulation have not been elucidated. BD is influenced by environmental factors, which may, via epigenetic modulation of gene expression, differentially affect illness presentation. This study thus focused on epigenetic DNA methylation regulation of SLC1A2, encoding for EAAT2, in BD with variable environmental influences of addiction. High resolution melting PCR (HRM-PCR) and thymine–adenine (TA) cloning with sequence analysis were conducted to examine methylation of the promoter region of the SLC1A2. DNA was isolated from blood samples drawn from BD patients (N = 150) with or without addiction to alcohol, nicotine, or food, defined as binge eating, and matched controls (N = 32). In comparison to controls, the SLC1A2 promoter region was hypermethylated in BD without addiction but was hypomethylated in BD with addiction. After adjusting for age and sex, the association of methylation levels with nicotine addiction (p = 0.0009) and binge eating (p = 0.0002) remained significant. Consistent with HRM-PCR, direct sequencing revealed increased methylation in CpG site 6 in BD, but decreased methylation in three CpG sites (6, 48, 156) in BD with alcohol and nicotine addictions. These results suggest that individual point methylation within the SLC1A2 promoter region may be modified by exogenous addiction and may have a potential for developing clinically valuable epigenetic biomarkers for BD diagnosis and monitoring.

Genetic Studies on the Tripartite Glutamate Synapse in the Pathophysiology and Therapeutics of Mood Disorders

Both bipolar disorder (BD) and major depressive disorder (MDD) have high morbidity and share a genetic background. Treatment options for these mood disorders are currently suboptimal for many patients; however, specific genetic variables may be involved in both pathophysiology and response to treatment. Agents such as the glutamatergic modulator ketamine are effective in treatment-resistant mood disorders, underscoring the potential importance of the glutamatergic system as a target for improved therapeutics. Here we review genetic studies linking the glutamatergic system to the pathophysiology and therapeutics of mood disorders. We screened 763 original genetic studies of BD or MDD that investigated genes encoding targets of the pathway/mediators related to the so-called tripartite glutamate synapse, including pre- and post-synaptic neurons and glial cells; 60 papers were included in this review. The findings suggest the involvement of glutamate-related genes in risk for mood disorders, treatment response, and phenotypic characteristics, although there was no consistent evidence for a specific gene. Target genes of high interest included GRIA3 and GRIK2 (which likely play a role in emergent suicidal ideation after antidepressant treatment), GRIK4 (which may influence treatment response), and GRM7 (which potentially affects risk for mood disorders). There was stronger evidence that glutamate-related genes influence risk for BD compared with MDD. Taken together, the studies show a preliminary relationship between glutamate-related genes and risk for mood disorders, suicide, and treatment response, particularly with regard to targets on metabotropic and ionotropic receptors.

Astroglial glutamate transporters in the brain: Regulating neurotransmitter homeostasis and synaptic transmission

Astrocytes, the major glial cell type in the central nervous system (CNS), are critical for brain function and have been implicated in various disorders of the central nervous system. These cells are involved in a wide range of cerebral processes including brain metabolism, control of central blood flow, ionic homeostasis, fine-tuning synaptic transmission, and neurotransmitter clearance. Such varied roles can be efficiently carried out due to the intimate interactions astrocytes maintain with neurons, the vasculature, as well as with other glial cells. Arguably, one of the most important functions of astrocytes in the brain is their control of neurotransmitter clearance. This is particularly true for glutamate whose timecourse in the synaptic cleft needs to be controlled tightly under physiological conditions to maintain point-to-point excitatory transmission, thereby limiting spillover and activation of more receptors. Most importantly, accumulation of glutamate in the extracellular space can trigger excessive activation of glutamatergic receptors and lead to excitotoxicity, a trademark of many neurodegenerative diseases. It is thus of utmost importance for both physiological and pathophysiological reasons to understand the processes that control glutamate time course within the synaptic cleft and regulate its concentrations in the extracellular space. © 2017 Wiley Periodicals, Inc.

Adverse childhood experiences associate to reduced glutamate levels in the hippocampus of patients affected by mood disorders

Adverse childhood experiences (ACE) can possibly permanently alter the stress response system, affect the glutamatergic system and influence hippocampal volume in mood disorders. The aim of the study is to investigate the association between glutamate levels in the hippocampus, measured through single proton magnetic resonance spectroscopy (1H-MRS), and ACE in patients affected by mood disorders and healthy controls. Higher levels of early stress associate to reduced levels of Glx/Cr in the hippocampus in depressed patients but not in healthy controls. Exposure to stress during early life could lead to a hypofunctionality of the glutamatergic system in the hippocampus of depressed patients. Abnormalities of glutamatergic signaling could then possibly underpin the structural and functional abnormalities observed in patients affected by mood disorders.

Adverse childhood experiences influence white matter microstructure in patients with schizophrenia

Integrity of brain white matter (WM) tracts in adulthood could be detrimentally affected by exposure to adverse childhood experiences (ACE). Changes of diffusion tensor imaging (DTI) measures suggesting WM disruption have been reported in patients with schizophrenia together with a history of childhood maltreatment. We therefore hypothesized that ACE could be associated with altered DTI measures of WM integrity in patients with schizophrenia. We tested this hypothesis in 83 schizophrenia patients using whole brain tract-based spatial statistics in the WM skeleton with threshold-free cluster enhancement of DTI measures of WM microstructure: axial, radial, and mean diffusivity (MD), and fractional anisotropy (FA). We observed an inverse correlation between severity of ACE and DTI measures of FA, and a positive correlation with MD in several WM tracts including corona radiata, thalamic radiations, corpus callosum, cingulum bundle, superior longitudinal fasciculus, inferior fronto-occipital fasciculus, uncinate fasciculus. Lower FA and higher MD are indexes of a reduction in fibre coherence and integrity. The association of ACE to reduced FA and increased MD in key WM tracts contributing to the functional integrity of the brain suggests that ACE might contribute to the pathophysiology of schizophrenia through a detrimental action on structural connectivity in critical cortico-limbic networks.

Association of rare variation in the glutamate receptor gene SLC1A2 with susceptibility to bipolar disorder and schizophrenia

The SLC1A2 gene encodes the excitatory amino acid transporter 2 (EAAT2). Glutamate is the major mediator of excitatory neurotransmission and EAAT2 is responsible for clearing the neurotransmitter from the synaptic cleft. Genetic variation in SLC1A2 has been implicated in a range of neurological and neuropsychiatric conditions including schizophrenia (SZ), autism and in core phenotypes of bipolar disorder (BD). The coding and putative regulatory regions of SLC1A2 gene were screened for variants using high resolution melting or sequenced in 1099 or in 32 BD subjects. Thirty-two variants were detected in the SLC1A2 gene. Fifteen potentially etiological variants were selected for genotyping in 1099 BD and 1095 control samples. Five amino acid changing variants were also genotyped in 630 participants suffering from SZ. None of the variants were found to be associated with BD or SZ or with the two diseases combined. However, two recurrent missense variants (rs145827578:G>A, p.(G6S); rs199599866:G>A, p.(R31Q)) and one recurrent 5′-untranslated region (UTR) variant (ss825678885:G>T) were detected in cases only. Combined analysis of the recurrent-case-only missense variants and of the case-only missense and 5′-UTR variants showed nominal evidence for association with the combined diseases (Fisher's P=0.019 and 0.0076). These findings are exploratory in nature and await replication in larger cohorts, however, they provide intriguing evidence that potentially functional rare variants in the SLC1A2 gene may confer susceptibility to psychotic disorders.