Maureen V. Martin

Mitochondrial involvement in psychiatric disorders.

Recent findings of mitochondrial abnormalities in brains from subjects with neurological disorders have led to a renewed search for mitochondrial abnormalities in psychiatric disorders. A growing body of evidence suggests that there is mitochondrial dysfunction in schizophrenia, bipolar disorder, and major depressive disorder, including evidence from electron microscopy, imaging, gene expression, genotyping, and sequencing studies. Specific evidence of dysfunction such as increased common deletion and decreased gene expression in mitochondria in psychiatric illnesses suggests that direct examination of mitochondrial DNA from postmortem brain cells may provide further details of mitochondrial alterations in psychiatric disorders.

Mitochondrial Variants in Schizophrenia, Bipolar Disorder, and Major Depressive Disorder

Background Mitochondria provide most of the energy for brain cells by the process of oxidative phosphorylation. Mitochondrial abnormalities and deficiencies in oxidative phosphorylation have been reported in individuals with schizophrenia (SZ), bipolar disorder (BD), and major depressive disorder (MDD) in transcriptomic, proteomic, and metabolomic studies. Several mutations in mitochondrial DNA (mtDNA) sequence have been reported in SZ and BD patients. Methodology/Principal Findings Dorsolateral prefrontal cortex (DLPFC) from a cohort of 77 SZ, BD, and MDD subjects and age-matched controls (C) was studied for mtDNA sequence variations and heteroplasmy levels using Affymetrix mtDNA resequencing arrays. Heteroplasmy levels by microarray were compared to levels obtained with SNaPshot and allele specific real-time PCR. This study examined the association between brain pH and mtDNA alleles. The microarray resequencing of mtDNA was 100% concordant with conventional sequencing results for 103 mtDNA variants. The rate of synonymous base pair substitutions in the coding regions of the mtDNA genome was 22% higher (p = 0.0017) in DLPFC of individuals with SZ compared to controls. The association of brain pH and super haplogroup (U, K, UK) was significant (p = 0.004) and independent of postmortem interval time. Conclusions Focusing on haplogroup and individual susceptibility factors in psychiatric disorders by considering mtDNA variants may lead to innovative treatments to improve mitochondrial health and brain function.

Spatial relationship between synapse loss and β-amyloid deposition in Tg2576 mice

Although there is evidence that β‐amyloid impairs synaptic function, the relationship between β‐amyloid and synapse loss is not well understood. In this study we assessed synapse density within the hippocampus and the entorhinal cortex of Tg2576 mice at 6–18 months of age using stereological methods at both the light and electron microscope levels. Under light microscopy we failed to find overall decreases in the density of synaptophysin‐positive boutons in any brain areas selected, but bouton density was significantly decreased within 200 μm of compact β‐amyloid plaques in the outer molecular layer of the dentate gyrus and Layers II and III of the entorhinal cortex at 15–18 months of age in Tg 2576 mice. Under electron microscopy, we found overall decreases in synapse density in the outer molecular layer of the dentate gyrus at both 6–9 and 15–18 months of age, and in Layers II and III of the entorhinal cortex at 15–18 months of age in Tg 2576 mice. However, we did not find overall changes in synapse density in the stratum radiatum of the CA1 subfield. Furthermore, in the two former brain areas we found a correlation between lower synapse density and greater proximity to β‐amyloid plaques. These results provide the first quantitative morphological evidence at the ultrastructure level of a spatial relationship between β‐amyloid plaques and synapse loss within the hippocampus and the entorhinal cortex of Tg2576 mice. J. Comp. Neurol. 500:311–321, 2007.

Cholinesterase inhibitors ameliorate behavioral deficits induced by MK-801 in mice

Enhancing cholinergic function has been suggested as a possible strategy for ameliorating the cognitive deficits of schizophrenia. The purpose of this study was to examine the effects of acetylcholinesterase (AChE) inhibitors in mice treated with the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801, which has been suggested as an animal model of the cognitive deficits of schizophrenia. Three separate experiments were conducted to test the effects of physostigmine, donepezil, or galantamine on deficits in learning and memory induced by MK-801. In each experiment, MK-801 (0.05 or 0.10 mg/kg) or saline was administered i.p. 20 min prior to behavioral testing over a total of 12 days. At 30 min prior to administration of MK-801 or saline, one of three doses of the AChE inhibitor (ie physostigmine—0.03, 0.10, or 0.30 mg/kg; donepezil—0.10, 0.30, or 1.00 mg/kg; or galantamine—0.25, 0.50, or 1.00 mg/kg) or saline was administered s.c. Behavioral testing was performed in all experimental animals using the following sequence: (1) spatial reversal learning, (2) locomotion, (3) fear conditioning, and (4) shock sensitivity. Both doses of MK-801 produced impairments in spatial reversal learning and in contextual and cued memory, as well as hyperlocomotion. Physostigmine and donepezil, but not galantamine, ameliorated MK-801-induced deficits in spatial reversal learning and in contextual and cued memory in a dose-dependent manner. Also, physostigmine, but not donepezil or galantamine, reversed MK-801-induced hyperlocomotion. Galantamine, but not physostigmine or donepezil, altered shock sensitivity. These results suggest that AChE inhibitors may differ in their capacity to ameliorate learning and memory deficits produced by MK-801 in mice, which may have relevance for the cognitive effects of cholinomimetic drugs in patients with schizophrenia.

Corticosterone and Related Receptor Expression are Associated With Increased β-Amyloid Plaques in Isolated Tg2576 Mice

Previously, we reported that the stress associated with chronic isolation was associated with increased beta-amyloid (Abeta) plaque deposition and memory deficits in the Tg2576 transgenic animal model of Alzheimers disease (AD) [Dong H, Goico B, Martin M, Csernansky CA, Bertchume A, Csernansky JG (2004) Effects of isolation stress on hippocampal neurogenesis, memory, and amyloid plaque deposition in APP (Tg2576) mutant mice. Neuroscience 127:601-609]. In this study, we investigated the potential mechanisms of stress-accelerated Abeta plaque deposition in this Tg2576 mice by examining the relationship between plasma corticosterone levels, expression of glucocorticoid receptor (GR) and corticotropin-releasing factor receptor-1 (CRFR1) in the brain, brain tissue Abeta levels and Abeta plaque deposition during isolation or group housing from weaning (i.e. 3 weeks of age) until 27 weeks of age. We found that isolation housing significantly increased plasma corticosterone levels as compared with group-housing in both Tg+ mice (which contain and overexpress human amyloid precursor protein (hAPP) gene) and Tg- mice (which do not contain hAPP gene as control). Also, isolated, but not group-housed animals showed increases in the expression of GR in the cortex. Furthermore, the expression of CRFR1 was increased in isolated Tg+ mice, but decreased in isolated Tg- mice in both cortex and hippocampus. Changes in the components of hypothalamic-pituitary-adrenal (HPA) axis were accompanied by increases in brain tissue Abeta levels and Abeta plaque deposition in the hippocampus and overlying cortex in isolated Tg+ mice. These results suggest that isolation stress increases corticosterone levels and GR and CRFR1 expression in conjunction with increases in brain tissue Abeta levels and Abeta plaque deposition in the Tg2576 mouse model of AD.

Loss of nuclear factor E2-related factor 1 in the brain leads to dysregulation of proteasome gene expression and neurodegeneration

The ubiquitin–proteasome pathway plays an important role in the pathogenesis of neurodegeneration, but mechanisms controlling expression of components in this pathway remain poorly understood. Nuclear factor E2-related factor 1 (Nrf1) transcription factor has been shown to regulate expression of antioxidant and cytoprotective genes. To determine the function of Nrf1 in the brain, mice with a late-stage deletion of Nrf1 in neuronal cells were generated. Loss of Nrf1 leads to impaired proteasome function and neurodegeneration. Gene expression profiling and RT-PCR analysis revealed a coordinate down-regulation of various proteasomal genes including PsmB6, which encodes a catalytic subunit of the proteasome. Transcriptional analysis and chromatin immunoprecipitation experiments demonstrated that PsmB6 is an Nrf1 target gene. These findings reveal Nrf1 as a key transcriptional regulator required for the expression of proteasomal genes in neurons and suggest that perturbations of Nrf1 function may contribute to the pathogenesis of neurodegenerative diseases.

Mitochondrial Mutations and Polymorphisms in Psychiatric Disorders

Mitochondrial deficiencies with unknown causes have been observed in schizophrenia (SZ) and bipolar disorder (BD) in imaging and postmortem studies. Polymorphisms and somatic mutations in mitochondrial DNA (mtDNA) were investigated as potential causes with next generation sequencing of mtDNA (mtDNA-Seq) and genotyping arrays in subjects with SZ, BD, major depressive disorder (MDD), and controls. The common deletion of 4,977 bp in mtDNA was compared between SZ and controls in 11 different vulnerable brain regions and in blood samples, and in dorsolateral prefrontal cortex (DLPFC) of BD, SZ, and controls. In a separate analysis, association of mitochondria SNPs (mtSNPs) with SZ and BD in European ancestry individuals (n = 6,040) was tested using Genetic Association Information Network (GAIN) and Wellcome Trust Case Control Consortium 2 (WTCCC2) datasets. The common deletion levels were highly variable across brain regions, with a 40-fold increase in some regions (nucleus accumbens, caudate nucleus and amygdala), increased with age, and showed little change in blood samples from the same subjects. The common deletion levels were increased in the DLPFC for BD compared to controls, but not in SZ. Full mtDNA genome resequencing of 23 subjects, showed seven novel homoplasmic mutations, five were novel synonymous coding mutations. By logistic regression analysis there were no significant mtSNPs associated with BD or SZ after genome wide correction. However, nominal association of mtSNPs (p < 0.05) to SZ and BD were found in the hypervariable region of mtDNA to T195C and T16519C. The results confirm prior reports that certain brain regions accumulate somatic mutations at higher levels than blood. The study in mtDNA of common polymorphisms, somatic mutations, and rare mutations in larger populations may lead to a better understanding of the pathophysiology of psychiatric disorders.

The First Decade and Beyond of Transcriptional Profiling in Schizophrenia

Gene expression changes in brains of individuals with schizophrenia (SZ) have been hypothesized to reflect possible pathways related to pathophysiology and/or medication. Other factors having robust effects on gene expression profiling in brain and possibly influence the schizophrenia transcriptome such as age and pH are examined. Pathways of curated gene expression or gene correlation networks reported in SZ (white matter, apoptosis, neurogenesis, synaptic plasticity, glutamatergic and GABAergic neurotransmission, immune and stress-response, mitochondrial, and neurodevelopment) are not unique to SZ and have been associated with other psychiatric disorders. Suggestions going forward to improve the next decade of profiling: consider multiple brain regions that are carefully dissected, release large datasets from multiple brain regions in controls to better understand neurocircuitry, integrate genetics and gene expression, measure expression variants on genome wide level, peripheral biomarker studies, and analyze the transcriptome across a developmental series of brains. Gene expression, while an important feature of the genomic landscape, requires further systems biology to advance from control brains to a more precise definition of the schizophrenia interactome.

Coding and Noncoding Gene Expression Biomarkers in Mood Disorders and Schizophrenia

Mood disorders and schizophrenia are common and complex disorders with consistent evidence of genetic and environmental influences on predisposition. It is generally believed that the consequences of disease, gene expression, and allelic heterogeneity may be partly the explanation for the variability observed in treatment response. Correspondingly, while effective treatments are available for some patients, approximately half of the patients fail to respond to current neuropsychiatric treatments. A number of peripheral gene expression studies have been conducted to understand these brain-based disorders and mechanisms of treatment response with the aim of identifying suitable biomarkers and perhaps subgroups of patients based upon molecular fingerprint. In this review, we summarize the results from blood-derived gene expression studies implemented with the aim of discovering biomarkers for treatment response and classification of disorders. We include data from a biomarker study conducted in first-episode subjects with schizophrenia, where the results provide insight into possible individual biological differences that predict antipsychotic response. It is concluded that, while peripheral studies of expression are generating valuable results in pathways involving immune regulation and response, larger studies are required which hopefully will lead to robust biomarkers for treatment response and perhaps underlying variations relevant to these complex disorders.

Independent quantitative trait loci influence ventral and dorsal hippocampal volume in recombinant inbred strains of mice

Anatomical and functional studies support segregation of the hippocampus into ventral and dorsal components along its septotemporal axis. However, it is unknown whether the development of these two components of the hippocampus is influenced by common or separate genetic factors. In this study, we used recombinant inbred strains of mice to determine whether the same or different quantitative trait loci (QTL) influence ventral and dorsal hippocampal volume. Using two sets of strains of recombinant inbred mice (BXD and AXB/BXA), we identified separate QTLs for ventral and dorsal hippocampal volume. In BXD mice, suggestive QTLs for ventral hippocampus were identified on chromosomes 2, 8 and 13, and a significant QTL for dorsal hippocampal volume was identified on chromosome 15. There was also a suggestive QTL for dorsal hippocampal volume on chromosome 13. In AXB/BXA mice, there were no significant or suggestive QTLs for ventral hippocampal volume, but a significant QTL for dorsal hippocampus was identified on chromosome 5. These findings suggest that the development of the ventral and dorsal components of the hippocampus is influenced by separate genetic loci.