Kumiko Hara

Epigenetic Status of Gdnf in the Ventral Striatum Determines Susceptibility and Adaptation to Daily Stressful Events

Stressful events during adulthood are potent adverse environmental factors that can predispose individuals to psychiatric disorders, including depression; however, many individuals exposed to stressful events can adapt and function normally. While stress vulnerability may influence depression, the molecular mechanisms underlying the susceptibility and adaptation to chronic stress within the brain are poorly understood. In this study, two genetically distinct mouse strains that exhibit different behavioral responses to chronic stress were used to demonstrate how the differential epigenetic status of the glial cell-derived neurotrophic factor (Gdnf) gene in the ventral striatum modulates susceptibility and adaptation to chronic stress. Our results suggest that the histone modifications and DNA methylation of the Gdnf promoter have crucial roles in the control of behavioral responses to chronic stress. Our data provide insights into these mechanisms, suggesting that epigenetic modifications of Gdnf, along with genetic and environmental factors, contribute to behavioral responses to stress.

Early Life Stress Enhances Behavioral Vulnerability to Stress through the Activation of REST4-Mediated Gene Transcription in the Medial Prefrontal Cortex of Rodents

There is growing evidence suggesting that early life events have long-term effects on the neuroendocrine and behavioral developments of rodents. However, little is known about the involvement of early life events in the susceptibility to subsequent stress exposure during adulthood. The present study characterized the effect of maternal separation, an animal model of early life adversity, on the behavioral response to repeated restraint stress in adult rats and investigated the molecular mechanism underlying behavioral vulnerability to chronic stress induced by the maternal separation. Rat pups were separated from the dams for 180 min per day from postnatal day 2 through 14 (HMS180 rats). We found that, as young adults, HMS180 rats showed a greater hypothalamic-pituitary-adrenal axis response to acute restraint stress than nonseparated control rats. In addition, repeatedly restrained HMS180 rats showed increased depression-like behavior and an anhedonic response compared with nonrestrained HMS180 rats. Furthermore, HMS180 rats showed increased expression of REST4, a neuron-specific splicing variant of the transcriptional repressor REST (repressor element-1 silencing transcription factor), and a variety of REST target gene mRNAs and microRNAs in the medial prefrontal cortex (mPFC). Finally, REST4 overexpression in the mPFC of neonatal mice via polyethyleneimine-mediated gene transfer enhanced the expression of its target genes as well as behavioral vulnerability to repeated restraint stress. In contrast, REST4 overexpression in the mPFC of adult mice did not affect depression-like behaviors after repeated stress exposure. These results suggest that the activation of REST4-mediated gene regulation in the mPFC during postnatal development is involved in stress vulnerability.

Characterization of the vulnerability to repeated stress in Fischer 344 rats: possible involvement of microRNA-mediated down-regulation of the glucocorticoid receptor.

In the present study, we established and characterized an animal model of vulnerability to repeated stress. We found that control Sprague–Dawley (SD) rats showed a gradual decrease in the HPA axis response following 14 days of repeated restraint stress, whereas Fischer 344 (F344) rats did not show such HPA axis habituation. Similar habituation was observed in the expression of c‐fos mRNA, corticotropin‐releasing hormone hnRNA, and phospho‐CREB and phospho‐ERK proteins in the hypothalamic paraventricular nucleus (PVN) of SD rats, but not in the F344 rats. In addition, repeatedly restrained F344 rats exhibited decreased cell proliferation in the dentate gyrus of the hippocampus and increased anxiety‐related behaviours, while repeatedly restrained SD rats exhibited a selective enhancement of hippocampal cell proliferation in the ventral area. Moreover, we found a lower expression of glucocorticoid receptor (GR) protein, but not mRNA, in the PVN of F344 rats compared to SD rats. We also identified that microRNA (miR)‐18a inhibited translation of GR mRNA in cultured neuronal cells and that increased expression of miR‐18a in the PVN was observed in F344 rats compared with SD rats. These strain differences in GR protein levels were not found in the hippocampus and prefrontal cortex, and the expression of miR‐18a was much lower in these brain regions than in the PVN. Our results suggest that F344 rats could be a useful animal model for studying vulnerability to repeated stress, and that miR‐18a‐mediated down‐regulation of GR translation may be an important factor to be considered in susceptibility to stress‐related disorders.

Impaired hippocampal spinogenesis and neurogenesis and altered affective behavior in mice lacking heat shock factor 1

Aberrant transcriptional regulation in the brain is thought to be one of the key components of the pathogenesis and pathophysiology of neuropsychiatric disorders. Heat shock factors (HSFs) modulate cellular homeostasis through the control of gene expression. However, the roles of HSFs in brain function have yet to be elucidated fully. In the present study, we attempted to clarify the role of HSF1-mediated gene regulation in neuronal and behavioral development using HSF1-deficient (HSF1−/−) mice. We found granule neurons of aberrant morphology and impaired neurogenesis in the dentate gyrus of HSF1−/− mice. In addition, HSF1−/− mice showed aberrant affective behavior, including reduced anxiety and sociability but increased depression-like behavior and aggression. Furthermore, HSF1 deficiency enhanced behavioral vulnerability to repeated exposure to restraint stress. Importantly, rescuing the HSF1 deficiency in the neonatal but not the adult hippocampus reversed the aberrant anxiety and depression-like behaviors. These results indicate a crucial role for hippocampal HSF1 in neuronal and behavioral development. Analysis of the molecular mechanisms revealed that HSF1 directly modulates the expression of polysialyltransferase genes, which then modulate polysialic acid–neural cell adhesion molecule (PSA-NCAM) levels in the hippocampus. Enzymatic removal of PSA from the neonatal hippocampus resulted in aberrant behavior during adulthood, similar to that observed in HSF1−/− mice. Thus, these results suggest that one role of HSF1 is to control hippocampal PSA-NCAM levels through the transcriptional regulation of polysialyltransferases, a process that might be involved in neuronal and behavioral development in mice.

Hippocampal Sirtuin 1 Signaling Mediates Depression-like Behavior

BACKGROUND Although depression is the leading cause of disability worldwide, its pathophysiology is poorly understood. Recent evidence has suggested that sirtuins (SIRTs) play a key role in cognition and synaptic plasticity, yet their role in mood regulation remains controversial. Here, we aimed to investigate whether SIRT function is associated with chronic stress-elicited depression-like behaviors and neuronal atrophy. METHODS We measured SIRT expression and activity in a mouse model of depression. We injected mice with a SIRT1 activator or inhibitor and measured their depression-like behaviors and dendritic spine morphology. To assess the role of SIRT1 directly, we used a viral-mediated gene transfer to overexpress the wild-type SIRT1 or dominant negative SIRT1 and evaluated their depression-like behaviors. Finally, we examined the role of extracellular signal-regulated protein kinases 1 and 2, a potential downstream target of SIRT1, in depression-like behavior. RESULTS We found that chronic stress reduced SIRT1 activity in the dentate gyrus of the hippocampus. Pharmacologic and genetic inhibition of hippocampal SIRT1 function led to an increase in depression-like behaviors. Conversely, SIRT1 activation blocked both the development of depression-related phenotypes and aberrant dendritic structures elicited by chronic stress exposure. Furthermore, hippocampal SIRT1 activation increased the phosphorylation level of extracellular signal-regulated protein kinases 1 and 2 in the stressed condition, and viral-mediated activation and inhibition of hippocampal extracellular signal-regulated protein kinase 2 led to antidepressive and prodepressive behaviors, respectively. CONCLUSIONS Our results suggest that the hippocampal SIRT1 pathway contributes to the chronic stress-elicited depression-related phenotype and aberrant dendritic atrophy.

Hippocampal MicroRNA-124 Enhances Chronic Stress Resilience in Mice.

Chronic stress-induced aberrant gene expression in the brain and subsequent dysfunctional neuronal plasticity have been implicated in the etiology and pathophysiology of mood disorders. In this study, we examined whether altered expression of small, regulatory, noncoding microRNAs (miRNAs) contributes to the depression-like behaviors and aberrant neuronal plasticity associated with chronic stress. Mice exposed to chronic ultra-mild stress (CUMS) exhibited increased depression-like behaviors and reduced hippocampal expression of the brain-enriched miRNA-124 (miR-124). Aberrant behaviors and dysregulated miR-124 expression were blocked by chronic treatment with an antidepressant drug. The depression-like behaviors are likely not conferred directly by miR-124 downregulation because neither viral-mediated hippocampal overexpression nor intrahippocampal infusion of an miR-124 inhibitor affected depression-like behaviors in nonstressed mice. However, viral-mediated miR-124 overexpression in hippocampal neurons conferred behavioral resilience to CUMS, whereas inhibition of miR-124 led to greater behavioral susceptibility to a milder stress paradigm. Moreover, we identified histone deacetylase 4 (HDAC4), HDAC5, and glycogen synthase kinase 3β (GSK3β) as targets for miR-124 and found that intrahippocampal infusion of a selective HDAC4/5 inhibitor or GSK3 inhibitor had antidepressant-like actions on behavior. We propose that miR-124-mediated posttranscriptional controls of HDAC4/5 and GSK3β expressions in the hippocampus have pivotal roles in susceptibility/resilience to chronic stress. SIGNIFICANCE STATEMENT Depressive disorders are a major public health concern worldwide. Although a clear understanding of the etiology of depression is still lacking, chronic stress-elicited aberrant neuronal plasticity has been implicated in the pathophysiology of depression. We show that the hippocampal expression of microRNA-124 (miR-124), an endogenous small, noncoding RNA that represses gene expression posttranscriptionally, controls resilience/susceptibility to chronic stress-induced depression-like behaviors. These effects on depression-like behaviors may be mediated through regulation of the mRNA or protein expression levels of histone deacetylases HDAC4/5 and glycogen synthase kinase 3β, all highly conserved miR-124 targets. Moreover, miR-124 contributes to stress-induced dendritic hypotrophy and reduced spine density of dentate gyrus granule neurons. Modulation of hippocampal miR-124 pathways may have potential antidepressant effects.

Maternal and genetic factors in stress-resilient and -vulnerable rats: A cross-fostering study

Early environmental factors can modulate the development of the hypothalamic-pituitary-adrenal (HPA) axis response to stress, together with subsequent brain functions and emotional behaviors. Two rat strains, Sprague-Dawley (SD) and Fischer 344 (F344), are known to exhibit differences in HPA axis reactivity and anxiety behavior in response to restraint stress in adulthood. To investigate the contribution of maternal influences in determining HPA axis and behavioral responses to stress, a cross-fostering study was performed using stress-resilient (SD) or stress-susceptible (F344) strains. We found that SD rats adopted by either an SD (in-fostered) or an F344 (cross-fostered) dam and F344 rats adopted by an SD dam (cross-fostered) showed a suppression of the HPA axis response following 14 days of repeated restraint stress. In contrast, F344 rats adopted by an F344 dam (in-fostered) did not show such HPA axis habituation. We also found that F344 rats adopted by an F344 dam showed increased anxiety-related behaviors in social interaction and novelty-suppressed feeding tests as a result of the 14 days of restraint stress, while SD rats adopted by either an SD or an F344 dam and F344 rats adopted by an SD dam showed normal anxiety-related behaviors under the same experimental conditions. These results suggest that while genetic differences between SD and F344 strains account for some of the variations in stress vulnerability, maternal factors also contribute.

Involvement of REST4-mediated gene regulations in stress vulnerability

There is growing evidence suggesting that early life events have long-term effects on the neuroendocrine and behavioral developments of rodents. However, little is known about the involvement of early life events in the susceptibility to subsequent stress exposure during adulthood. The present study characterized the effect of maternal separation, an animal model of early life adversity, on the behavioral response to repeated restraint stress in adult rats and investigated the molecular mechanism underlying behavioral vulnerability to chronic stress induced by the maternal separation. Rat pups were separated from the dams for 180 min per day from postnatal day 2 through 14 (HMS180 rats). We found that adult HMS180 rats showed a greater hypothalamic–pituitary–adrenal axis response to acute restraint stress than non-separated control rats. In addition, repeatedly restrained HMS180 rats showed increased depression-like behavior and an anhedonic response compared with non-restrained HMS180 rats. Furthermore, HMS180 rats showed increased expression of REST4, a neuron-specific splicing variant of the transcriptional repressor REST, and a variety of REST target gene mRNAs and microRNAs in the medial prefrontal cortex (mPFC). Finally, REST4 overexpression in the mPFC of neonatal mice via polyethyleniminemediated gene transfer enhanced the expression of its target genes as well as behavioral vulnerability to repeated restraint stress. In contrast, REST4 overexpression in the mPFC of adult mice did not affect depression-like behaviors after the repeated stress exposure. These results suggest that the activation of REST4-mediated gene regulation in the mPFC during postnatal development is involved in stress vulnerability.

HDAC2-mediated epigenetic repression of the GDNF gene in the striatum modulates behavioral vulnerability to stress

We previously reported that brain thromboxane A2 is positively involved in the central regulation of plasma catecholamine levels in uretrhane-anesthetized rats. In the present study, we pharmacologically analyzed the role of inflammatory mediators in the restraint stress-induced elevation of plasma catecholamines (noradrenaline and adrenaline). The Restraint stress (8 h) elevated plasma catecholamines levels (adrenaline > noradrenaline). The elevations of both catecholamines reached the peak about 30 min after the restraint stress. Intraperitoneal administration of indomethacin (10 mg/kg, a non-selective cyclooxygenase inhibitor), NS-398 (5 mg/kg, a selective cyclooxygenase 2 inhibitor), daltroban (20 mg /kg, a selective thromboxane A2 receptor antagonist), CP-154,526 (20 mg/kg, a selective CRF1 receptor antagonist), PDTC (200 mg/kg, a NF B inhibitor) or MK 801 (2 mg/kg, a NMDA receptor antagonist) reduced the stress-induced elevation of plasma catecholamines. These results suggest that brain inflammatory mediators are important component of the restraint stress-induced elevation of plasma catecholamines.

Involvement of microRNA-mediated downregulation of glucocorticoid receptor in vulnerability to stress

Dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis is associated with vulnerability to depression and anxiety disorders. Daily repeated exposure to a stressor resulted in reduced HPA axis response to the same stressor, that is termed habituation. In this study, we established the animal model of vulnerability to repeated stress. We found that Fischer344 (F344) rats showed the deficit in habituation to repeated restraint stress such as suppressed inhibition of HPA axis and hypothalamic neuronal activity. In addition, F344 rats showed the decreased cell proliferation in hippocampus and increased anxiety behavior. Moreover, we found that microRNA (miR)-18a inhibited translation of glucocorticoid receptor (GR) mRNA in vitro and F344 rats showed the decreased expression of GR protein and increased expression of miR-18a in the hypothalamus. Thus, our results suggest that miR-18a-mediated downregulation of GR could be involved in vulnerability to stress.