Molecular analysis of the mouse brain exposed to chronic mild stress: The influence of hepatocyte nuclear factor 4α on physiological homeostasis

Transthyretin orchestrates vitamin B12-induced stress resilience

BACKGROUND

Chronic stress significantly contributes to mood- and anxiety disorders. Previous data suggest a correlative connection between vitamin B12 supplementation, depression, and stress resilience. However, the underlying mechanisms are still poorly understood.

METHODS

Using the chronic variable stress mouse model coupled with RNA-sequencing, we determined vitamin B12-induced transcriptional changes related to stress resilience. By viral-mediated gene transfer and in vivo epigenome editing, we reveal a functional pathway linking vitamin B12, DNA methylation, and depressive-like symptoms.

RESULTS

We identified Transthyretin (Ttr) as a sex-specific key target of vitamin B12 in chronic stress. Accordingly, TTR expression was increased postmortem in the prefrontal cortex of male, but not female, depressed patients. Virally altered Ttr in the prefrontal cortex functionally contributed to stress- and depression-related behaviors, changes in dendritic spine morphology, and gene expression. In stressed mice, vitamin B12 reduced DNAme in the Ttr promoter region. Importantly, using in vivo epigenome editing to alter DNAme in the brains of living mice for the first time, we establish a direct causal link between DNAme on Ttr and stress-associated behaviors.

DISCUSSION

Using state-of-the-art techniques, this study uncovers a mechanistic link between vitamin B12 supplementation, Ttr, and markers of chronic stress and depression, encouraging further studies into dietary interventions for mood disorders.

Molecular Mechanisms of IL18 in Disease

Interleukin 18 (IL18) was originally identified as an inflammation-induced cytokine that is secreted by immune cells. An increasing number of studies have focused on its non-immunological functions, with demonstrated functions for IL18 in energy homeostasis and neural stability. IL18 is reportedly required for lipid metabolism in the liver and brown adipose tissue. Furthermore, IL18 (Il18) deficiency in mice leads to mitochondrial dysfunction in hippocampal cells, resulting in depressive-like symptoms and cognitive impairment. Microarray analyses of Il18-/- mice have revealed a set of genes with differential expression in liver, brown adipose tissue, and brain; however, the impact of IL18 deficiency in these tissues remains uncertain. In this review article, we discuss these genes, with a focus on their relationships with the phenotypic disease traits of Il18-/- mice.

Pathophysiological features in the brains of female Spontaneously Diabetic Torii (SDT) fatty rats

Diabetes mellitus (DM) and obesity are associated with neurodegenerative diseases such as Alzheimer's disease and psychiatric disorders such as major depression. In this study, we investigated pathophysiological changes in the brains of female Spontaneously Diabetic Torii (SDT) fatty rats with diabetes and obesity. Brains of Sprague-Dawley (SD), SDT and SDT fatty rats were collected at 58 weeks of age. The parietal cortical thickness was measured and the number of pyramidal cells in the hippocampal cornu ammonis 1 and 3 (CA1 and CA3) and the number of granule cells in the dentate gyrus (DG) regions were counted. The area of glial fibrillary acidic protein (GFAP) positivity in CA1, CA3 and DG regions were measured. The parietal cortical thickness and the number of cells in CA3 and DG regions of SDT and SDT fatty rats did not show obvious changes. On the other hand, in the CA1 region, the number of cells in SDT rats and SDT fatty rats was significantly lower than that in SD rats, and that in SDT fatty rats was significantly lower than that in SDT rats. The GFAP-positive area in SDT fatty rats was significantly reduced compared to that in SD rats only in the DG region. Preliminarily result showed that the expression of S100a9, an inflammation-related gene, was increased in the brains of SDT fatty rats. These results suggest that female SDT fatty rat may exhibit central nervous system diseases due to obesity and DM.

Exploring Molecular Mechanisms Involved in the Development of the Depression-Like Phenotype in Interleukin-18-Deficient Mice

Interleukin-18 (IL-18) is an inflammatory cytokine that has been linked to energy homeostasis and psychiatric symptoms such as depression and cognitive impairment. We previously revealed that deficiency in IL-18 led to hippocampal abnormalities and resulted in depression-like symptoms. However, the impact of IL-18 deficiency on other brain regions remains to be clarified. In this study, we first sought to confirm that IL-18 expression in neural cells can be found in human brain tissue. Subsequently, we examined the expression of genes in the prefrontal cortex of Il18^−/− mice and compared it with gene expression in mice subjected to a chronic mild stress model of depression. Extracted genes were further analyzed using Ingenuity® Pathway Analysis, in which 18 genes common to both the chronic mild stressed model and Il18^−/− mice were identified. Of those, 16 were significantly differentially expressed between Il18^+/+ and Il18^−/− mice. We additionally measured protein expression of α-2-HS-glycoprotein (AHSG) and transthyretin (TTR) in serum and the brain. In the prefrontal cortex of Il18^−/− mice, TTR but not AHSG was significantly decreased. Conversely, in the serum of Il18^−/− mice, AHSG was significantly increased but not TTR. Therefore, our results suggest that in IL-18-deficit conditions, TTR in the brain is one of the mediators causally related to depression, and AHSG in peripheral organs is one of the regulators inducing energy imbalance. Moreover, this study suggests a possible “signpost” to clarify the molecular mechanisms commonly underlying the immune system, energy metabolism, neural function, and depressive disorders.

Analysis of genes linked to depressive-like behaviors in interleukin-18-deficient mice: Gene expression profiles in the brain

Interleukin (IL)-18 is an interferon γ-inducing inflammatory cytokine associated with function of the immune system and other physiological functions. IL-18-deficient (Il18 ^-/-) mice exhibit obesity, dyslipidemia, non-alcoholic steatohepatitis and depressive-like behavioral changes. Therefore, IL-18 has a number of important roles associated with immunity, energy homeostasis and psychiatric conditions. In the present study, gene expression in the brains of Il18 ^-/- mice was analyzed to identify genes associated with the depressive-like behaviors and other impairments displayed by Il18 ^-/- mice. Using whole genome microarray analysis, gene expression patterns in the brains of Il18 ^+/+ and Il18 ^-/- mice at 6 and 12 weeks of age were examined and compared. Subsequently, genes were categorized using Ingenuity^® Pathway Analysis (IPA). At 12 weeks of age, 2,805 genes were identified using microarray analysis. Genes related to 'Major depression' and 'Depressive disorders' were identified by IPA core analysis, and 13 genes associated with depression were isolated. Among these genes, fibroblast growth factor receptor 1 (Fgfr1); protein tyrosine phosphatase, non-receptor type 1 (Ptpn1); and urocortin 3 (Ucn3) were classed as depression-inducing and the other genes were considered depression-suppressing genes. Subsequently, the interactions between the microarray results at 6 weeks of age and the above three depression-inducing genes were analyzed to search for effector genes of depression at 12 weeks of age. This analysis identified cyclin D1 (Ccnd1) and NADPH oxidase 4 (Nox4). The microarray analysis results were correlated with the results of reverse transcription-quantitative PCR (RT-qPCR). Overall, the results suggest that Fgfr1, Ptpn1 and Ucn3 may be involved in depression-like changes and Ccnd1 and Nox4 regulate these three genes in IL-18-deficient mice.