Chronic social defeat reduces myelination in the mouse medial prefrontal cortex

Oligodendroglia Are Primed for Antigen Presentation in Response to Chronic Stress-Induced Microglial-Derived Inflammation

Chronic stress is a major contributor to the development of major depressive disorder, one of the leading causes of disability worldwide. Using a model of repeated social defeat stress in mice, we and others have reported that neuroinflammation plays a dynamic role in the development of behavioral deficits consistent with social avoidance and impaired reward responses. Animals susceptible to the model also exhibit hypomyelination in the medial prefrontal cortex, indicative of changes in the differentiation pathway of cells of the oligodendroglial lineage (OLN). We computationally confirmed the presence of immune oligodendrocytes, a population of OLN cells, which express immune markers and myelination deficits. In the current study, we report that microglia are necessary to induce expression of antigen presentation markers (and other immune markers) on oligodendroglia. We further associate the appearance of these markers with changes in the OLN and confirm that microglial changes precede OLN changes. Using co-cultures of microglia and OLN, we show that under inflammatory conditions the processes of phagocytosis and expression of MHCII are linked, suggesting potential priming for antigen presentation by OLN cells. Our findings provide insights into the nature of these OLN cells with immune capabilities, their obligatory interaction with microglia, and identify them as a potential cellular contributor to the pathological manifestations of psychosocial stress.

Clemastine enhances myelin formation in the striatum and medial prefrontal cortex and improves sociability in a neonatal rat hypoxic-ischemic model

Neonatal hypoxia-ischemia (HI) results in gray and white matter injuries, leading to impairments in social behavior and severe neurological deficits. Clemastine treatment has demonstrated efficacy in alleviating behavioral deficits in various neurological disorders by improving myelin formation. It has been suggested that the medial prefrontal cortex (mPFC) and the striatum play a key role in human social behaviors. To test whether clemastine can mitigate sociability deficits by rescuing the myelin damage in these key brain areas, we administered clemastine orally for two weeks following HI insult in neonatal rats. We demonstrated that clemastine successfully ameliorated HI-induced social deficits during adolescence, attenuated hypomyelination and promoted oligodendrocyte maturation in the striatum and mPFC. We also observed that clementine reduced proliferation and apoptosis of oligodendrocyte progenitor cells (OPCs), decreased myelin debris induced by HI in the striatum, and was accompanied by microglia morphological changes in the striatum. Furthermore, our findings revealed a positive correlation between sociability and myelin formation in the striatum and mPFC. In conclusion, our data indicate that clemastine attenuates HI-induced sociability impairments during adolescence, potentially through its role in promoting myelin formation in the striatum and mPFC.

Rapid changes in plasma corticosterone and medial amygdala transcriptome profiles during social status change reveal molecular pathways associated with a major life history transition in mouse dominance hierarchies

Social hierarchies are a common form of social organization across species. Although hierarchies are largely stable across time, animals may socially ascend or descend within hierarchies depending on environmental and social challenges. Here, we develop a novel paradigm to study social ascent and descent within male CD-1 mouse social hierarchies. We show that mice of all social ranks rapidly establish new stable social hierarchies when placed in novel social groups with animals of equivalent social status. Seventy minutes following social hierarchy formation, males that were socially dominant prior to being placed into new social hierarchies exhibit higher increases in plasma corticosterone and vastly greater transcriptional changes in the medial amygdala (MeA), which is central to the regulation of social behavior, compared to males who were socially subordinate prior to being placed into a new hierarchy. Specifically, the loss of social status in a new hierarchy (social descent) is associated with reductions in MeA expression of myelination and oligodendrocyte differentiation genes. Maintaining high social status is associated with high expression of genes related to cholinergic signaling in the MeA. Conversely, gaining social status in a new hierarchy (social ascent) is related to relatively few unique rapid changes in the MeA. We also identify novel genes associated with social transition that show common changes in expression when animals undergo either social descent or social ascent compared to maintaining their status. Two genes, Myosin binding protein C1 (Mybpc1) and μ-Crystallin (Crym), associated with vasoactive intestinal polypeptide (VIP) and thyroid hormone pathways respectively, are highly upregulated in socially transitioning individuals. Further, increases in genes associated with synaptic plasticity, excitatory glutamatergic signaling and learning and memory pathways were observed in transitioning animals suggesting that these processes may support rapid social status changes.

Understanding stress-induced transmission of peripherally derived factors into the brain and responses in non-neuronal cells

Stress is a significant cause of mental disorders, for which effective treatments remain limited due to an insufficient understanding of its pathogenic mechanisms. Recent research has increasingly focused on non-neuronal cells to elucidate the molecular mechanisms underlying psychopathology. In this review, we summarize the current knowledge on how non-neuronal cells in the central nervous system, including microglia, astrocytes, and oligodendrocytes, respond to peripherally derived stress-related factors and how these responses contribute to the development of mental disorders. A more comprehensive understanding of stress-induced alterations, with careful consideration of the type and timing of stress exposure, will provide fundamental insights into the pathogenesis of diverse stress-related mental disorders.

Dietary Intervention with Omega-3 Fatty Acids Mitigates Maternal High-Fat Diet-Induced Behavioral and Myelin-Related Alterations in Adult Offspring

BACKGROUND

Maternal high-fat diet (HFD) during pregnancy and lactation induces depression- like phenotype and provokes myelin-related changes in rat offspring in the prefrontal cortex (PFCTX), which persist even to adulthood.

OBJECTIVE

Due to the plasticity of the developing brain, it was decided to analyze whether depressionlike phenotype and myelin-related changes in the early lifetime induced by maternal HFD (60% energy from fat) could be reversed by the omega-3 fatty acid-enriched diet (Ω3D) given from the postweaning period until adulthood (63rd day of life) in offspring.

METHODS

We analyzed the effect of post-weaning Ω3D on the depressive-like phenotype (assessed by the forced swimming test) and myelin-related changes (measured using RT-qPCR, ELISA, and immunofluorescence staining) in the PFCTX of adult offspring.

RESULTS

Ω3D reversed increased immobility time in adult offspring induced by maternal HFD, without affecting the animals' locomotor activity. Molecularly, Ω3D normalized the reduced expression levels of myelin-oligodendrocyte glycoprotein (MOG), as well as myelin and lymphocyte protein (MAL) in males and MOG in females in the PFCTX, changes initially induced by maternal HFD. Additionally, Ω3D normalized the quantity of oligodendrocyte precursor cells and mature oligodendrocytes in the prelimbic, infralimbic, and cingulate cortex in males, which were reduced following maternal HFD exposure. In females, the Ω3D effect was less pronounced, with normalization of oligodendrocyte precursors occurring only in the infralimbic cortex.

CONCLUSION

These findings suggest that Ω3D may play a significant role in correcting behavioral and neurobiological changes caused by adverse prenatal conditions.

Genetic Tools in Rodents to Study Cannabinoid Functions

During the past 30 years, the endocannabinoid system (ECS) has emerged as a major signalling system in the mammalian brain regulating neurotransmission in numerous brain regions and in various cell populations. Endocannabinoids are able to regulate specific physiological functions and thus modify their behavioural manifestations and allostatic alterations of the ECS linked to different pathological conditions. As discussed in detail in other chapters of this book, endocannabinoids are involved in learning and memory, stress, and anxiety, feeding, energy balance, development, and ageing. Likewise, many CNS disorders (e.g. schizophrenia, epilepsy, substance use disorders, and multiple sclerosis) are associated with dysregulation of the ECS. Discerning the physiological functions of the synthetic and degrading enzymes of endocannabinoids and their receptors is a challenging task because of their distinct and complex expression patterns. Techniques of genetic engineering have been able to shed light on a number of complex ECS-related tasks during the past years. In this chapter, first, we take a critical look at the toolbox available to researchers who would like to investigate cannabinoid effects using genetic engineering techniques, then we comprehensively discuss genetically modified rodent models in various neuronal and non-neuronal cell populations, both within and outside the nervous system.

Effect of Xiaoyaosan on brain volume and microstructure diffusion changes to exert antidepressant-like effects in mice with chronic social defeat stress

Objective

Depression is a prevalent mental disorder characterized by persistent negative mood and loss of pleasure. Although there are various treatment modalities available for depression, the rates of response and remission remain low. Xiaoyaosan (XYS), a traditional Chinese herbal formula with a long history of use in treating depression, has shown promising effects. However, the underlying mechanism of its therapeutic action remains elusive. The aim of this study is to investigate the neuroimaging changes in the brain associated with the antidepressant-like effects of XYS.

Methods

Here, we combined voxel-based morphometry of T2-weighted images and voxel-based analysis on diffusion tensor images to evaluate alterations in brain morphometry and microstructure between chronic social defeat stress (CSDS) model mice and control mice. Additionally, we examined the effect of XYS treatment on structural disruptions in the brains of XYS-treated mice. Furthermore, we explored the therapeutic effect of 18β-glycyrrhetinic acid (18β-GA), which was identified as the primary compound present in the brain following administration of XYS. Significant differences in brain structure were utilized as classification features for distinguishing mice with depression model form the controls using a machine learning method.

Results

Significant changes in brain volume and diffusion metrics were observed in the CSDS model mice, primarily concentrated in the nucleus accumbens (ACB), primary somatosensory area (SSP), thalamus (TH), hypothalamus (HY), basomedical amygdala nucleus (BMA), caudoputamen (CP), and retrosplenial area (RSP). However, both XYS and 18β-GA treatment prevented disruptions in brain volume and diffusion metrics in certain regions, including bilateral HY, right SSP, right ACB, bilateral CP, and left TH. The classification models based on each type of neuroimaging feature achieved high accuracy levels (gray matter volume: 76.39%, AUC=0.83; white matter volume: 76.39%, AUC=0.92; fractional anisotropy: 82.64%, AUC=0.9; radial diffusivity: 76.39%, AUC=0.82). Among these machine learning analyses, the right ACB, right HY, and right CP were identified as the most important brain regions for classification purposes.

Conclusion

These findings suggested that XYS can prevent abnormal changes in brain volume and microstructure within TH, SSP, ACB, and CP to exert prophylactic antidepressant-like effects in CSDS model mice. The neuroimaging features within these regions demonstrate excellent performance for classifying CSDS model mice from controls while providing valuable insights into the antidepressant effects of XYS.

Myelin-associated oligodendrocytic basic protein-dependent myelin repair confers the long-lasting antidepressant effect of ketamine

Ketamine exhibits rapid and sustained antidepressant effects. As decreased myelination has been linked to depression pathology, changes in myelination may be a pivotal mechanism underlying ketamine's long-lasting antidepressant effects. Although ketamine has a long-lasting facilitating effect on myelination, the precise roles of myelination in ketamine's sustained antidepressant effects remain unknown. In this study, we employed spatial transcriptomics (ST) to examine ketamine's lasting effects in the medial prefrontal cortex (mPFC) and hippocampus of mice subjected to chronic social defeat stress and identified several differentially expressed myelin-related genes. Ketamine's ability to restore impaired myelination in the brain by promoting the differentiation of oligodendrocyte precursor cells (OPCs) into mature oligodendrocytes was demonstrated. Moreover, we showed that inhibiting the expression of myelin-associated oligodendrocytic basic protein (Mobp) blocked ketamine's long-lasting antidepressant effects. We also illustrated that α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) signaling mediated ketamine's facilitation on myelination. In addition, we found that the (R)-stereoisomer of ketamine showed stronger effects on myelination than (S)-ketamine, which may explain its longer-lasting antidepressant effects. These findings reveal novel mechanisms underlying the sustained antidepressant effects of ketamine and the differences in antidepressant effects between (R)-ketamine and (S)-ketamine, providing new insights into the role of myelination in antidepressant mechanisms.

General anesthetic agents induce neurotoxicity through oligodendrocytes in the developing brain

General anesthetic agents can impact brain function through interactions with neurons and their effects on glial cells. Oligodendrocytes perform essential roles in the central nervous system, including myelin sheath formation, axonal metabolism, and neuroplasticity regulation. They are particularly vulnerable to the effects of general anesthetic agents resulting in impaired proliferation, differentiation, and apoptosis. Neurologists are increasingly interested in the effects of general anesthetic agents on oligodendrocytes. These agents not only act on the surface receptors of oligodendrocytes to elicit neuroinflammation through modulation of signaling pathways, but also disrupt metabolic processes and alter the expression of genes involved in oligodendrocyte development and function. In this review, we summarize the effects of general anesthetic agents on oligodendrocytes. We anticipate that future research will continue to explore these effects and develop strategies to decrease the incidence of adverse reactions associated with the use of general anesthetic agents.

Genetic background modulates the effect of glucocorticoids on proliferation, differentiation and myelin formation of oligodendrocyte lineage cells

Anxiety disorders are prevalent mental disorders. Their predisposition involves a combination of genetic and environmental risk factors, such as psychosocial stress. Myelin plasticity was recently associated with chronic stress in several mouse models. Furthermore, we found that changes in both myelin thickness and node of Ranvier morphology after chronic social defeat stress are influenced by the genetic background of the mouse strain. To understand cellular and molecular effects of stress-associated myelin plasticity, we established an oligodendrocyte (OL) model consisting of OL primary cell cultures isolated from the C57BL/6NCrl (B6; innately non-anxious and mostly stress-resilient strain) and DBA/2NCrl (D2; innately anxious and mostly stress-susceptible strain) mice. Characterization of naïve cells revealed that D2 cultures contained more pre-myelinating and mature OLs compared with B6 cultures. However, B6 cultures contained more proliferating oligodendrocyte progenitor cells (OPCs) than D2 cultures. Acute exposure to corticosterone, the major stress hormone in mice, reduced OPC proliferation and increased OL maturation and myelin production in D2 cultures compared with vehicle treatment, whereas only OL maturation was reduced in B6 cultures. In contrast, prolonged exposure to the synthetic glucocorticoid dexamethasone reduced OPC proliferation in both D2 and B6 cultures, but only D2 cultures displayed a reduction in OPC differentiation and myelin production. Taken together, our results reveal that genetic factors influence OL sensitivity to glucocorticoids, and this effect is dependent on the cellular maturation stage. Our model provides a novel framework for the identification of cellular and molecular mechanisms underlying stress-associated myelin plasticity.

Repurposing Ketamine in the Therapy of Depression and Depression-Related Disorders: Recent Advances and Future Potential

Depression represents a prevalent and enduring mental disorder of significant concern within the clinical domain. Extensive research indicates that depression is very complex, with many interconnected pathways involved. Most research related to depression focuses on monoamines, neurotrophic factors, the hypothalamic-pituitary-adrenal axis, tryptophan metabolism, energy metabolism, mitochondrial function, the gut-brain axis, glial cell-mediated inflammation, myelination, homeostasis, and brain neural networks. However, recently, Ketamine, an ionotropic N-methyl-D-aspartate (NMDA) receptor antagonist, has been discovered to have rapid antidepressant effects in patients, leading to novel and successful treatment approaches for mood disorders. This review aims to summarize the latest findings and insights into various signaling pathways and systems observed in depression patients and animal models, providing a more comprehensive view of the neurobiology of anxious-depressive-like behavior. Specifically, it highlights the key mechanisms of ketamine as a rapid-acting antidepressant, aiming to enhance the treatment of neuropsychiatric disorders. Moreover, we discuss the potential of ketamine as a prophylactic or therapeutic intervention for stress-related psychiatric disorders.

Pathophysiology in cortico-amygdala circuits and excessive aversion processing: the role of oligodendrocytes and myelination

Stress-related psychiatric illnesses, such as major depressive disorder, anxiety and post-traumatic stress disorder, present with alterations in emotional processing, including excessive processing of negative/aversive stimuli and events. The bidirectional human/primate brain circuit comprising anterior cingulate cortex and amygdala is of fundamental importance in processing emotional stimuli, and in rodents the medial prefrontal cortex-amygdala circuit is to some extent analogous in structure and function. Here, we assess the comparative evidence for: (i) Anterior cingulate/medial prefrontal cortex<->amygdala bidirectional neural circuits as major contributors to aversive stimulus processing; (ii) Structural and functional changes in anterior cingulate cortex<->amygdala circuit associated with excessive aversion processing in stress-related neuropsychiatric disorders, and in medial prefrontal cortex<->amygdala circuit in rodent models of chronic stress-induced increased aversion reactivity; and (iii) Altered status of oligodendrocytes and their oligodendrocyte lineage cells and myelination in anterior cingulate/medial prefrontal cortex<->amygdala circuits in stress-related neuropsychiatric disorders and stress models. The comparative evidence from humans and rodents is that their respective anterior cingulate/medial prefrontal cortex<->amygdala circuits are integral to adaptive aversion processing. However, at the sub-regional level, the anterior cingulate/medial prefrontal cortex structure-function analogy is incomplete, and differences as well as similarities need to be taken into account. Structure-function imaging studies demonstrate that these neural circuits are altered in both human stress-related neuropsychiatric disorders and rodent models of stress-induced increased aversion processing. In both cases, the changes include altered white matter integrity, albeit the current evidence indicates that this is decreased in humans and increased in rodent models. At the cellular-molecular level, in both humans and rodents, the current evidence is that stress disorders do present with changes in oligodendrocyte lineage, oligodendrocytes and/or myelin in these neural circuits, but these changes are often discordant between and even within species. Nonetheless, by integrating the current comparative evidence, this review provides a timely insight into this field and should function to inform future studies-human, monkey and rodent-to ascertain whether or not the oligodendrocyte lineage and myelination are causally involved in the pathophysiology of stress-related neuropsychiatric disorders.

Interaction between Oligodendrocytes and Interneurons in Brain Development and Related Neuropsychiatric Disorders

A variety of neurological and psychiatric disorders have recently been shown to be highly associated with the abnormal development and function of oligodendrocytes (OLs) and interneurons. OLs are the myelin-forming cells in the central nervous system (CNS), while interneurons are important neural types gating the function of excitatory neurons. These two types of cells are of great significance for the establishment and function of neural circuits, and they share similar developmental origins and transcriptional architectures, and interact with each other in multiple ways during development. In this review, we compare the similarities and differences in these two cell types, providing an important reference and further revealing the pathogenesis of related brain disorders.

A maternal high-fat diet during pregnancy and lactation induced depression-like behavior in offspring and myelin-related changes in the rat prefrontal cortex

In accordance with the developmental origins of health and disease, early-life environmental exposures, such as maternal diet, can enhance the probability and gravity of health concerns in their offspring in the future. Over the past few years, compelling evidence has emerged suggesting that prenatal exposure to a maternal high-fat diet (HFD) could trigger neuropsychiatric disorders in the offspring, such as depression. The majority of brain development takes place before birth and during lactation. Nevertheless, our understanding of the impact of HFD on myelination in the offspring's brain during both gestation and lactation remains limited. In the present study, we investigated the effects of maternal HFD (60% energy from fat) on depressive-like and myelin-related changes in adolescent and adult rat offspring. Maternal HFD increased immobility time during the forced swimming test in both adolescent and adult offspring. Correspondingly, the depressive-like phenotype in offspring correlated with dysregulation of several genes and proteins in the prefrontal cortex, especially of myelin-oligodendrocyte glycoprotein (MOG), myelin and lymphocyte protein (MAL), 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNPase), kallikrein 6, and transferrin in male offspring, as well as of MOG and kallikrein 6 in female offspring, which persist even into adulthood. Maternal HFD also induced long-lasting adaptations manifested by the reduction of immature and mature oligodendrocytes in the prefrontal cortex in adult offspring. In summary, maternal HFD-induced changes in myelin-related genes are correlated with depressive-like behavior in adolescent offspring, which persists even to adulthood.

Rapamycin Exerts an Antidepressant Effect and Enhances Myelination in the Prefrontal Cortex of Chronic Restraint Stress Mice

Depressive disorder is a psychiatric condition that is characterized by the core symptoms of anhedonia and learned helplessness. Myelination loss was recently found in the prefrontal cortex (PFC) of patients with depression and animal models, but the mechanism of this loss is unclear. In our previous study, chronic restraint stress (CRS) mice showed depressive-like symptoms. In this study, we found that myelin was reduced in the PFC of CRS mice. We also observed increased mammalian target of rapamycin (mTOR) phosphorylation levels in the PFC. Chronic injections of rapamycin, a mTOR complex inhibitor, prevented depressive behavior as shown by the forced swimming test and sucrose preference test. Rapamycin also increased myelination in the PFC of CRS mice. In summary, we found that CRS enhanced mTOR signaling and reduced myelination in the PFC and that rapamycin could prevent it. Our study provides the etiology of reduced myelin in depressive symptoms and suggests that mTOR signaling could be a target for treating depression or improving myelination deficits in depressive disorders.

Axo-glial interactions between midbrain dopamine neurons and oligodendrocyte lineage cells in the anterior corpus callosum

Oligodendrocyte progenitor cells (OPCs) receive synaptic innervation from glutamatergic and GABAergic axons and can be dynamically regulated by neural activity, resulting in activity-dependent changes in patterns of axon myelination. However, it remains unclear to what extent other types of neurons may innervate OPCs. Here, we provide evidence implicating midbrain dopamine neurons in the innervation of oligodendrocyte lineage cells in the anterior corpus callosum and nearby white matter tracts of male and female adult mice. Dopaminergic axon terminals were identified in the corpus callosum of DAT-Cre mice after injection of an eYFP reporter virus into the midbrain. Furthermore, fast-scan cyclic voltammetry revealed monoaminergic transients in the anterior corpus callosum, consistent with the anatomical findings. Using RNAscope, we further demonstrate that ~ 40% of Olig2 + /Pdfgra + cells and ~ 20% of Olig2 + /Pdgfra- cells in the anterior corpus callosum express Drd1 and Drd2 transcripts. These results suggest that oligodendrocyte lineage cells may respond to dopamine released from midbrain dopamine axons, which could affect myelination. Together, this work broadens our understanding of neuron-glia interactions with important implications for myelin plasticity by identifying midbrain dopamine axons as a potential regulator of corpus callosal oligodendrocyte lineage cells.

Node of Ranvier remodeling in chronic psychosocial stress and anxiety

Differential expression of myelin-related genes and changes in myelin thickness have been demonstrated in mice after chronic psychosocial stress, a risk factor for anxiety disorders. To determine whether and how stress affects structural remodeling of nodes of Ranvier, another form of myelin plasticity, we developed a 3D reconstruction analysis of node morphology in C57BL/6NCrl and DBA/2NCrl mice. We identified strain-dependent effects of chronic social defeat stress on node morphology in the medial prefrontal cortex (mPFC) gray matter, including shortening of paranodes in C57BL/6NCrl stress-resilient and shortening of node gaps in DBA/2NCrl stress-susceptible mice compared to controls. Neuronal activity has been associated with changes in myelin thickness. To investigate whether neuronal activation is a mechanism influencing also node of Ranvier morphology, we used DREADDs to repeatedly activate the ventral hippocampus-to-mPFC pathway. We found reduced anxiety-like behavior and shortened paranodes specifically in stimulated, but not in the nearby non-stimulated axons. Altogether, our data demonstrate (1) nodal remodeling of the mPFC gray matter axons after chronic stress and (2) axon-specific regulation of paranodes in response to repeated neuronal activity in an anxiety-associated pathway. Nodal remodeling may thus contribute to aberrant circuit function associated with anxiety disorders.

Influenza A virus infection disrupts oligodendrocyte homeostasis and alters the myelin lipidome in the adult mouse

BACKGROUND

Recent data suggest that myelin may be altered by physiological events occurring outside of the central nervous system, which may cause changes to cognition and behavior. Similarly, peripheral infection by non-neurotropic viruses is also known to evoke changes to cognition and behavior.

METHODS

Mice were inoculated with saline or influenza A virus. Bulk RNA-seq, lipidomics, RT-qPCR, flow cytometry, immunostaining, and western blots were used to determine the effect of infection on OL viability, protein expression and changes to the lipidome. To determine if microglia mediated infection-induced changes to OL homeostasis, mice were treated with GW2580, an inhibitor of microglia activation. Additionally, conditioned medium experiments using primary glial cell cultures were also used to test whether secreted factors from microglia could suppress OL gene expression.

RESULTS

Transcriptomic and RT-qPCR analyses revealed temporal downregulation of OL-specific transcripts with concurrent upregulation of markers characteristic of cellular stress. OLs isolated from infected mice had reduced cellular expression of myelin proteins compared with those from saline-inoculated controls. In contrast, the expression of these proteins within myelin was not different between groups. Similarly, histological and immunoblotting analysis performed on various brain regions indicated that infection did not alter OL viability, but increased expression of a cellular stress marker. Shot-gun lipidomic analysis revealed that infection altered the lipid profile within the prefrontal cortex as well as in purified brain myelin and that these changes persisted after recovery from infection. Treatment with GW2580 during infection suppressed the expression of genes associated with glial activation and partially restored OL-specific transcripts to baseline levels. Finally, conditioned medium from activated microglia reduced OL-gene expression in primary OLs without altering their viability.

CONCLUSIONS

These findings show that peripheral respiratory viral infection with IAV is capable of altering OL homeostasis and indicate that microglia activation is likely involved in the process.

Low myelin-related values in the fornix and thalamus of 7 Tesla MRI of major depressive disorder patients

Introduction

Abnormalities in myelin are believed to be one of the important causes of major depressive disorder, and it is becoming important to more accurately quantify myelin in in vivo magnetic resonance imaging of major depressive disorder patients. We aimed to investigate the difference in myelin concentration in the white matter and subcortical areas using new quantitative myelin-related maps of high-resolution 7 Tesla (7 T) magnetic resonance imaging between patients with major depressive disorder and healthy controls.

Methods

Myelin-related comparisons of the white matter and nearby subcortical regions were conducted between healthy controls (n = 36) and patients with major depressive disorder (n = 34). Smoothed quantitative ratio (sq-Ratio) myelin-related maps were created using the multi-echo magnetization-prepared two rapid gradient echoes (ME-MP2RAGE) sequence of the T1 and T2* images of 7 T magnetic resonance imaging. Differences in the myelin-related values of the regions of interest between the two groups were analyzed using a two-sample t-test, and multiple comparison corrections were performed using the false discovery rate.

Results

The average sq-Ratio myelin-related values were 2.62% higher in the white matter and 2.26% higher in the subcortical regions of the healthy controls group than in the major depressive disorder group. In the group analysis of the healthy control and major depressive disorder groups, the sq-Ratio myelin-related values were significantly different in the fornix area of the white matter (false discovery rate-corrected p = 0.012). In addition, significant differences were observed in both the left (false discovery rate-corrected p = 0.04) and right thalamus (false discovery rate-corrected p = 0.040) among the subcortical regions.

Discussion

The average sq-ratio myelin-related value and sq-ratio myelin-related values in the fornix of the white matter and both thalami were higher in the healthy controls group than in the major depressive disorder group. We look forward to replicating our findings in other populations using larger sample sizes.

Social cognitive neuroscience in the digital age

Human interactions are increasingly taking place from a distance through methods of remote interpersonal communication like video chatting and social media. While remote interpersonal communication has existed for millennia-with the first postal system arising in ∼2400 B.C.-accelerated advances in technology and the recent global COVID-19 pandemic have led to a dramatic increase in remote interpersonal communication use in daily life. Remote interpersonal communication presents a challenge to the field of social-cognitive neuroscience, as researchers seek to understand the implications of various types of remote interpersonal communication for the "social brain." The present paper reviews our current understanding of the social-cognitive neural network and summarizes critical differences between the neural correlates of social cognition in remote vs. face-to-face interactions. In particular, empirical and theoretical work is reviewed that highlight disparities in the neural mechanisms of social perception, evaluation of social stimuli, human motivation, evaluation of social reward, and theory of mind. Potential impacts of remote interpersonal communication on the development of the brain's social-cognitive network are also discussed. Finally, this review closes with future directions for research on social-cognitive neuroscience in our digital technology-connected world and outlines a neural model for social cognition in the context of remote interpersonal communication. For the field of social-cognitive neuroscience to advance alongside of the ever-evolving society, it is crucial for researchers to acknowledge the implications and concepts suggested for future research in this review.

Inhibition of TREM-1 Ameliorates Lipopolysaccharide-induced Depressive-like Behaviors by Alleviating Neuroinflammation in the PFC via PI3K/Akt Signaling Pathway

Neuroinflammation is closely related to depression and is a key pathophysiological process of depression. Triggering receptor expressed on myeloid cells 1 (TREM-1) has been proven to exert proinflammatory effects in various diseases. However, the role of TREM-1 in depression has not been elucidated. Thus, we hypothesized that TREM-1 inhibition might have protective effects in depression. Here, lipopolysaccharide (LPS) was used to induce depressive-like behaviors in mice, LP17 was treated to inhibit TREM-1, and LY294002 was administrated to inhibit phosphatidylinositol 3-kinase (PI3K) which is one of the downstream of TREM-1. Physical and neurobehavioral tests, Western blot analysis, and immunofluorescence staining were performed in this study. We found that LPS caused significant depressive-like behaviors in mice, including body weight decline, anodynia (sucrose preference decrease), lack of locomotor activity, and desperation in tail suspension test (TST) and forced swimming test (FST). Next, we revealed that TREM-1 was expressed on microglia, neurons, and astrocytes in the prefrontal cortex (PFC) after LPS administration. TREM-1 inhibition by LP17 suppressed the expression of TREM-1 in the PFC. In addition, LP17 could alleviate neuroinflammation and microglial activation in the PFC. Meanwhile, LP17 could prevent damage of LPS to neuronal primary cilia and neuronal activity. Finally, we revealed that PI3K/Akt might exert crucial role in the protective effects of TREM-1 inhibition to depressive-like behaviors induced by LPS. Taken together, TREM-1 inhibition by LP17 could alleviate depressive-like behaviors induced by LPS by mitigating neuroinflammation in the PFC via PI3K/Akt signaling pathway. Finally, we demonstrated that TREM-1 might be a promising therapeutic target for treatment of depression.

Prolonged partner separation erodes nucleus accumbens transcriptional signatures of pair bonding in male prairie voles

The loss of a spouse is often cited as the most traumatic event in a person's life. However, for most people, the severity of grief and its maladaptive effects subside over time via an understudied adaptive process. Like humans, socially monogamous prairie voles (Microtus ochrogaster) form opposite-sex pair bonds, and upon partner separation, show stress phenotypes that diminish over time. We test the hypothesis that extended partner separation diminishes pair bond-associated behaviors and causes pair bond transcriptional signatures to erode. Opposite-sex or same-sex paired males were cohoused for 2 weeks and then either remained paired or were separated for 48 hours or 4 weeks before collecting fresh nucleus accumbens tissue for RNAseq. In a separate cohort, we assessed partner-directed affiliation at these time points. We found that these behaviors persist despite prolonged separation in both same-sex and opposite-sex paired voles. Opposite-sex pair bonding led to changes in accumbal transcription that were stably maintained while animals remained paired but eroded following prolonged partner separation. Eroded genes are associated with gliogenesis and myelination, suggesting a previously undescribed role for glia in pair bonding and loss. Further, we pioneered neuron-specific translating ribosomal affinity purification in voles. Neuronally enriched transcriptional changes revealed dopaminergic-, mitochondrial-, and steroid hormone signaling-associated gene clusters sensitive to acute pair bond disruption and loss adaptation. Our results suggest that partner separation erodes transcriptomic signatures of pair bonding despite core behavioral features of the bond remaining intact, revealing potential molecular processes priming a vole to be able to form a new bond.

Formulating treatment of major psychiatric disorders: algorithm targets the dominantly affected brain cell-types

BACKGROUND

Pharmacotherapy for most psychiatric conditions was developed from serendipitous observations of benefit from drugs prescribed for different reasons. An algorithmic approach to formulating pharmacotherapy is proposed, based upon which combination of changed activities by brain cell-types is dominant for any particular condition, because those cell-types contain and surrogate for genetic, metabolic and environmental information, that has affected their function. The algorithm performs because functions of some or all the affected cell-types benefit from several available drugs: clemastine, dantrolene, erythropoietin, fingolimod, fluoxetine, lithium, memantine, minocycline, pioglitazone, piracetam, and riluzole PROCEDURES/FINDINGS: Bipolar disorder, major depressive disorder, schizophrenia, Alzheimer's disease, and post-traumatic stress disorder, illustrate the algorithm; for them, literature reviews show that no single combination of altered cell-types accounts for all cases; but they identify, for each condition, which combination occurs most frequently, i.e., dominates, as compared with other possible combinations. Knowing the dominant combination of altered cell-types in a particular condition, permits formulation of therapy with combinations of drugs taken from the above list. The percentage of patients who might benefit from that therapy, depends upon the frequency with which the dominant combination occurs in patients with that particular condition.

CONCLUSIONS

Knowing the dominant combination of changed cell types in psychiatric conditions, permits an algorithmically formulated, rationally-based treatment. Different studies of the same condition often produce discrepant results; all might be correct, because identical clinical phenotypes result from different combinations of impaired cell-types, thus producing different results. Clinical trials would validate both the proposed concept and choice of drugs.

Chronic social defeat stress caused region-specific oligodendrogenesis impairment in adolescent mice

Introduction

Social stress in adolescents precipitates stress-related emotional disorders. In this study we aimed to investigate oligodendrogenesis in three stress-associated brain regions, medial prefrontal cortex (mPFC), habenula, and amygdala in adolescent mice exposed to social defeat stress.

Methods

Four-week-old adolescent mice were subjected to social defeat for 10 days, followed by behavioral tests and evaluations of oligodendroglial proliferation and differentiation.

Results

Stressed mice showed reduced social interaction, more stretched approach posture, lower sucrose preference, but no changes in the forced swimming test. EdU labeled proliferative cells, newly formed NG2⁺EdU + oligodendrocyte precursor cells (OPCs), and Olig2⁺EdU⁺ oligodendrocyte lineage cells (OLLs) were significantly decreased in the mPFC and the lateral habenula, but not in the amygdala and the medial habenula in socially defeated mice. APC⁺Edu⁺ newly-generated mature oligodendrocytes (OLs) were decreased in the mPFC in stressed mice. However, the total number of NG2⁺ OPCs, APC⁺ mature OLs, and Olig2⁺ OLLs were comparable in all the brain regions examined between stressed and control mice except for a decrease of APC⁺ mature OLs in the prelimbic cortex of stressed mice.

Conclusion

Our findings indicate that adolescent social stress causes emotion-related behavioral changes and region-specific impairment of oligodendrogenesis.

Astrocytes as Context for the Involvement of Myelin and Nodes of Ranvier in the Pathophysiology of Depression and Stress-Related Disorders

Astrocytes, despite some shared features as glial cells supporting neuronal function in gray and white matter, participate and adapt their morphology and neurochemistry in a plethora of distinct regulatory tasks in specific neural environments. In the white matter, a large proportion of the processes branching from the astrocytes' cell bodies establish contacts with oligodendrocytes and the myelin they form, while the tips of many astrocyte branches closely associate with nodes of Ranvier. Stability of myelin has been shown to greatly depend on astrocyte-to-oligodendrocyte communication, while the integrity of action potentials that regenerate at nodes of Ranvier has been shown to depend on extracellular matrix components heavily contributed by astrocytes. Several lines of evidence are starting to show that in human subjects with affective disorders and in animal models of chronic stress there are significant changes in myelin components, white matter astrocytes and nodes of Ranvier that have direct relevance to connectivity alterations in those disorders. Some of these changes involve the expression of connexins supporting astrocyte-to-oligodendrocyte gap junctions, extracellular matrix components produced by astrocytes around nodes of Ranvier, specific types of astrocyte glutamate transporters, and neurotrophic factors secreted by astrocytes that are involved in the development and plasticity of myelin. Future studies should further examine the mechanisms responsible for those changes in white matter astrocytes, their putative contribution to pathological connectivity in affective disorders, and the possibility of leveraging that knowledge to design new therapies for psychiatric disorders.

Stress and the brain transcriptome: Identifying commonalities and clusters in standardized data from published experiments

Interpretation of transcriptomic experiments is hindered by many problems including false positives/negatives inherent to big-data methods and changes in gene nomenclature. To find the most consistent effect of stress on brain transcriptome, we retrieved data from 79 studies applying animal models and 3 human studies investigating post-traumatic stress disorder (PTSD). The analyzed data were obtained either with microarrays or RNA sequencing applied to samples collected from more than 1887 laboratory animals and from 121 human subjects. Based on the initial database containing a quarter million differential expression effect sizes representing transcripts in three species, we identified the most frequently reported genes in 223 stress-control comparisons. Additionally, the analysis considers sex, individual vulnerability and contribution of glucocorticoids. We also found an overlap between gene expression in PTSD patients and animals which indicates relevance of laboratory models for human stress response. Our analysis points to genes that, as far as we know, were not specifically tested for their role in stress response (Pllp, Arrdc2, Midn, Mfsd2a, Ccn1, Htra1, Csrnp1, Tenm4, Tnfrsf25, Sema3b, Fmo2, Adamts4, Gjb1, Errfi1, Fgf18, Galnt6, Slc25a42, Ifi30, Slc4a1, Cemip, Klf10, Tom1, Dcdc2c, Fancd2, Luzp2, Trpm1, Abcc12, Osbpl1a, Ptp4a2). Provided transcriptomic resource will be useful for guiding the new research.

The long noncoding RNA FEDORA is a cell type- and sex-specific regulator of depression

Women suffer from depression at twice the rate of men, but the underlying molecular mechanisms are poorly understood. Here, we identify marked baseline sex differences in the expression of long noncoding RNAs (lncRNAs), a class of regulatory transcripts, in human postmortem brain tissue that are profoundly lost in depression. One such human lncRNA, RP11-298D21.1 (which we termed FEDORA), is enriched in oligodendrocytes and neurons and up-regulated in the prefrontal cortex (PFC) of depressed females only. We found that virally expressing FEDORA selectively either in neurons or in oligodendrocytes of PFC promoted depression-like behavioral abnormalities in female mice only, changes associated with cell type-specific regulation of synaptic properties, myelin thickness, and gene expression. We also found that blood FEDORA levels have diagnostic implications for depressed women and are associated with clinical response to ketamine. These findings demonstrate the important role played by lncRNAs, and FEDORA in particular, in shaping the sex-specific landscape of the brain and contributing to sex differences in depression.

Alteration in NMDAR subunits in different brain regions of chronic unpredictable mild stress (CUMS) rat model

N-Methyl-d-aspartate receptor (NMDAR) signaling pathway has been implicated in the pathogenesis and treatment of depression. However, the role of NMDAR subunits in depression is still unclear. In this study, alteration in all seven NMDAR subunits in several brain areas of rats exposed to chronic unpredictable mild stress (CUMS), an animal model of depression, was detected. Our findings demonstrated that: (1) CUMS could induce a reduction in sucrose preference, an indicator of typical depression-like behaviors; (2) CUMS significantly reduced the NMDAR subunits of GluN2B and GluN3 in the medial prefrontal cortex (mPFC), but not altered all seven NMDAR subunits in hippocampus and corpus callosum of rats; (3) subunit composition of NMDARs in corpus callosum was different from that in mPFC, PFC and hippocampus; and (4) the mRNA expressions of GluN2B, GluN3A and GluN3B in mPFC as well as mRNA expression of GluN2C in corpus callosum were correlated to sucrose preference in rats. These findings suggested that GluN2B and GluN3 in mPFC may contribute to the pathophysiology of depression.

Activation of liver X receptors protects oligodendrocytes in CA3 of stress-induced mice

Depression is a complex disorder that is associated with various structural abnormalities. Oligodendrocyte (OL) dysfunction is associated with the pathogenesis of depression and the promotion of hippocampal oligodendrocyte maturation and myelination could be a novel therapeutic strategy for ameliorating depressive behaviors. Recent studies have shown that activation of liver X receptors (LXRs) by GW3965 improves depressive phenotypes, but the effects of GW3965 on OL function and myelination in the hippocampus of depression remain relatively unclear. To address this issue, we investigated the effects of GW3965 on mature OL in the hippocampus and on the myelin sheaths of mice subjected to chronic unpredictable stress (CUS). Behavioral tests were performed to assess depressive behaviors. Then, the number of mature OLs (CC1⁺) in each hippocampal subregion was precisely quantified with immunohistochemical and stereological methods, and the density of newborn mature OLs (BrdU⁺/Olig2⁺/CC1⁺ cells) in each hippocampal subregion was quantified with immunofluorescence. In addition, myelin basic protein (MBP) staining intensity in the cornu ammonis 3 (CA3) region was assessed by using immunofluorescence. We found that both the number of CC1⁺ OLs and the density of BrdU⁺/Olig2⁺/CC1⁺ cells were obviously decreased in each hippocampal subregion of mice subjected to CUS, and 4 weeks of GW3965 treatment reversed these effects only in the CA3 region. Furthermore, the decreased MBP expression in the CA3 region of mice subjected to CUS was ameliorated by GW3965 treatment. Collectively, these results suggested that improvement of OL maturation and enhancement of myelination may be structural mechanisms underlying the antidepressant effects of LXR agonists.

Corticotropin-releasing factor receptor 1 in infralimbic cortex modulates social stress-altered decision-making

Chronic stress could lead to a bias in behavioral strategies toward habits. However, it remains unclear which neuronal system modulates stress-induced behavioral abnormality during decision making. The corticotropin-releasing factor (CRF) system in the medial prefrontal cortex (mPFC), which has been implicated in governing strategy choice, is involved in the response to stress. The present study aimed to clarify whether altered function in cortical CRF receptors is linked to abnormal behaviors after chronic stress. In results, mice subjected to a 10-day social defeat preferred to use a habitual strategy. The infralimbic cortex (IL), but not the prelimbic cortex (PL) or anterior cingulate cortex (ACC), showed higher cFos expression in stress-subjected mice than in control mice, which may be associated with habitual behavior choice. Furthermore, CRF receptor 1 (CRFR1) agonist and antagonist infusion in IL during behavioral training mimicked and rescued stress-caused behavioral change in the decision-making assessment, respectively. An electrophysiological approach showed that the frequencies of both spontaneous IPSC and spontaneous EPSC, but not their amplitude, increased after stress and were modulated by CRFR1 agents. Further recordings revealed that an increased ratio of excitation to inhibition (E/I ratio) of IL by stress was rescued under conditions with CRFR1 antagonist. Collectively, these data indicate that CRFR1 plays a critical role in stress-permitted or enhanced glutamatergic and GABAergic presynaptic transmission in direct or indirect ways, as well as the modulation for E/I ratio in the IL. Thus, CRFR1 in the mPFC may be a proper target for treating cases of chronic stress-altered behavior.

Chronic stress disrupts the homeostasis and progeny progression of oligodendroglial lineage cells, associating immune oligodendrocytes with prefrontal cortex hypomyelination

Major depressive disorder (MDD) is a chronic debilitating illness affecting yearly 300 million people worldwide. Oligodendrocyte-lineage cells have emerged as important neuromodulators in synaptic plasticity and crucial components of MDD pathophysiology. Using the repeated social defeat (RSDS) mouse model, we demonstrate that chronic psychosocial stress induces long-lasting losses and transient proliferation of oligodendrocyte-precursor cells (OPCs), aberrant differentiation into oligodendrocytes, and severe hypomyelination in the prefrontal cortex. Exposure to chronic stress results in OPC morphological impairments, excessive oxidative stress, and oligodendroglial apoptosis, implicating integrative-stress responses in depression. Analysis of single-nucleus transcriptomic data from MDD patients revealed oligodendroglial-lineage dysregulation and the presence of immune-oligodendrocytes (Im-OL), a novel population of cells with immune properties and myelination deficits. Im-OL were also identified in mice after RSDS, where oligodendrocyte-lineage cells expressed immune-related markers. Our findings demonstrate cellular and molecular changes in the oligodendroglial lineage in response to chronic stress and associate hypomyelination with Im-OL emergence during depression.

Single-Cell Transcriptomic Analysis Demonstrates the Regulation of Peach Polysaccharides on Circadian Rhythm Disturbance

SCOPE

Plant polysaccharides are thought to have a prebiotic effect, promoting the growth of probiotics, which may regulate circadian rhythms. This study evaluates the regulation of peach polysaccharides (PPS) on circadian rhythm disturbance through intestinal microbiota by a mouse model.

METHODS AND RESULTS

PPS is administered to mice with circadian rhythm disturbance for 4 weeks. The study finds that PPS ameliorated the structural disorder of intestinal microbiota induced by continuous darkness, decreasing the ratio of Firmicutes/Bacteroidetes (F/B), thereby regulating furfural degradation, penicillin and cephalosporin biosynthesis, and antibiotic biosynthesis. Single-cell transcriptomics is used to determine the type of hypothalamus cells and the expression of clock genes in mice, showing that the number of astrocytes and oligoendrocytes cells in the hypothalamus of the transplanted mice is up-regulated, and the expression of neuroprotective genes such as Sox9 and Mobp increased. In addition, clock genes such as Cry2 and Per3 show significant callback.

CONCLUSION

This study shows that PPS can ameliorate the imbalance of intestinal microbiota and cell dysfunction caused by circadian rhythm disorder, suggesting that PPS is a feasible strategy for the prevention and treatment of circadian rhythm disorder and related cognitive impairment.

Effects of chronic social stress on oligodendrocyte proliferation-maturation and myelin status in prefrontal cortex and amygdala in adult mice

Stress-related neuropsychiatric disorders present with excessive processing of aversive stimuli. Whilst underlying pathophysiology remains poorly understood, within- and between-regional changes in oligodendrocyte (OL)-myelination status in anterior cingulate cortex and amygdala (ACC-AMY network) could be important. In adult mice, a 15-day chronic social stress (CSS) protocol leads to increased aversion responsiveness, accompanied by increased resting-state functional connectivity between, and reduced oligodendrocyte- and myelin-related transcript expression within, medial prefrontal cortex and amygdala (mPFC-AMY network), the analog of the human ACC-AMY network. In the current study, young-adult male C57BL/6 mice underwent CSS or control handling (CON). To assess OL proliferation-maturation, mice received 5-ethynyl-2'-deoxyuridine via drinking water across CSS/CON and brains were collected on day 16 or 31. In mPFC, CSS decreased the density of proliferative OL precursor cells (OPCs) at days 16 and 31. CSS increased mPFC myelin basic protein (MBP) integrated density at day 31, as well as increasing myelin thickness as determined using transmission electron microscopy, at day 16. In AMY, CSS increased the densities of total CC1+ OLs (day 31) and CC1+/ASPA+ OLs (days 16 and 31), whilst decreasing the density of proliferative OPCs at days 16 and 31. CSS was without effect on AMY MBP content and myelin thickness, at days 16 and 31. Therefore, CSS impacts on the OL lineage in mPFC and AMY and to an extent that, in mPFC at least, leads to increased myelination. This increased myelination could contribute to the excessive aversion learning and memory that occur in CSS mice and, indeed, human stress-related neuropsychiatric disorders.

Beyond the neuron: Role of non-neuronal cells in stress disorders

Stress disorders are leading causes of disease burden in the U.S. and worldwide, yet available therapies are fully effective in less than half of all individuals with these disorders. Although to date, much of the focus has been on neuron-intrinsic mechanisms, emerging evidence suggests that chronic stress can affect a wide range of cell types in the brain and periphery, which are linked to maladaptive behavioral outcomes. Here, we synthesize emerging literature and discuss mechanisms of how non-neuronal cells in limbic regions of brain interface at synapses, the neurovascular unit, and other sites of intercellular communication to mediate the deleterious, or adaptive (i.e., pro-resilient), effects of chronic stress in rodent models and in human stress-related disorders. We believe that such an approach may one day allow us to adopt a holistic "whole body" approach to stress disorder research, which could lead to more precise diagnostic tests and personalized treatment strategies. Stress is a major risk factor for many psychiatric disorders. Cathomas et al. review new insight into how non-neuronal cells mediate the deleterious effects, as well as the adaptive, protective effects, of stress in rodent models and human stress-related disorders.

Antidepressant effects of Enterococcus faecalis 2001 through the regulation of prefrontal cortical myelination via the enhancement of CREB/BDNF and NF-κB p65/LIF/STAT3 pathways in olfactory bulbectomized mice

A therapeutic strategy through the gut-brain axis has been proven to be effective in treatment for depression. In our previous study, we demonstrated that Enterococcus faecalis 2001 (EF-2001) prevents colitis-induced depressive-like behavior through the gut-brain axis in mice. More recently, we found that demyelination in the prefrontal cortex (PFC) was associated with depressive-like behavior in an animal model of major depressive disorder, olfactory bulbectomized (OBX) mice. The present study investigated the effects of EF-2001 on depressive-like behaviors in OBX mice and the underlying molecular mechanisms from the perspective of myelination in the PFC. OBX mice exhibited depressive-like behaviors in the tail-suspension, splash, and sucrose preference tests, and decreased myelin and paranodal proteins along with mature oligodendrocytes in the PFC. These behavioral and biochemical changes were all prevented by treatment with EF-2001. Further, EF-2001 treatment increased brain-derived neurotrophic factor (BDNF) and leukemia inhibitory factor (LIF) in the PFC. Interestingly, an immunohistochemical analysis revealed enhanced phospho (p) -cAMP-responsive element binding protein (CREB) expression in neurons, p-nuclear factor-kappa B (NFκB) p65 (Ser536) expression in astrocytes, and p-signal transducer and activator of transcription 3 (STAT3) (Ty705) expression in mature oligodendrocytes in the PFC of OBX mice. From these results, we suggest that EF-2001 administration prevents depressive-like behaviors by regulating prefrontal cortical myelination via the enhancement of CREB/BDNF and NFκB p65/LIF/STAT3 pathways. Our findings strongly support the idea that a therapeutic strategy involving the gut microbiota may be a promising alternative treatment for alleviating symptoms of depression.

Ermin deficiency leads to compromised myelin, inflammatory milieu, and susceptibility to demyelinating insult

Ermin is an actin-binding protein found almost exclusively in the central nervous system (CNS) as a component of myelin sheaths. Although Ermin has been predicted to play a role in the formation and stability of myelin sheaths, this has not been directly examined in vivo. Here, we show that Ermin is essential for myelin sheath integrity and normal saltatory conduction. Loss of Ermin in mice caused de-compacted and fragmented myelin sheaths and led to slower conduction along with progressive neurological deficits. RNA sequencing of the corpus callosum, the largest white matter structure in the CNS, pointed to inflammatory activation in aged Ermin-deficient mice, which was corroborated by increased levels of microgliosis and astrogliosis. The inflammatory milieu and myelin abnormalities were further associated with increased susceptibility to immune-mediated demyelination insult in Ermin knockout mice. Supporting a possible role of Ermin deficiency in inflammatory white matter disorders, a rare inactivating mutation in the ERMN gene was identified in multiple sclerosis patients. Our findings demonstrate a critical role for Ermin in maintaining myelin integrity. Given its near-exclusive expression in myelinating oligodendrocytes, Ermin deficiency represents a compelling "inside-out" model of inflammatory dysmyelination and may offer a new paradigm for the development of myelin stability-targeted therapies.

Histone Acetylation and Methylation Underlie Oligodendroglial and Myelin Susceptibility in Schizophrenia

Schizophrenia is a complex neuropsychiatric disorder affected by both genetic and epigenetic factors. Except for neuronal dysfunction, oligodendroglial abnormalities also contribute to the disease pathogenesis, characterized by a robust dysregulation of oligodendrocyte and myelin related genes. Accumulating evidence shows that histone modifications play important roles in transcriptional regulation of the genes crucial for oligodendrocyte differentiation and myelination. Specifically, the histone acetylation and methylation were two well-recognized histone modification abnormalities in the schizophrenic brains. In this mini-review, we will describe the dynamic changes of histone acetylation and methylation in schizophrenia, which may coordinate and induce deleterious epigenetic memory in oligodendroglial cells, and further lead to oligodendrocyte and myelin deficits. Precise modulation of histone modification status in oligodendroglial cells needs to secure the balance of epigenetic marks, which may revise the therapeutic strategy for the white matter etiology of neuropsychiatric disorders.

CCR2 monocytes repair cerebrovascular damage caused by chronic social defeat stress

Immune surveillance of the brain plays an important role in health and disease. Peripheral leukocytes patrol blood-brain barrier interfaces, and after injury, monocytes cross the cerebrovasculature and follow a pattern of pro- and anti-inflammatory activity leading to tissue repair. We have shown that chronic social defeat (CSD) causes scattered vasculature disruptions. Here, we assessed CCR2⁺ monocyte trafficking to the vascular injury sites in Ccr2^wt/rfp reporter mice both during CSD and one week following CSD cessation. We found that CSD for 14 days induced microhemorrhages where plasma fibrinogen leaked into perivascular spaces, but it did not affect the distribution or density of CCR2^rfp+ monocytes in the brain. However, after recovery from CSD, many vascularly adhered CCR2⁺ cells were detected, and gene expression of the CCR2 chemokine receptor ligands CCL7 and CCL12, but not CCL2, was elevated in endothelial cells. Adhered CCR2⁺ cells were mostly the non-classical, anti-inflammatory Ly6C^lo type, and they phagocytosed fibrinogen in perivascular spaces. In CCR2-deficient Ccr2^rfp/rfp mice, fibrinogen levels remained elevated in recovery. Fibrinogen infused intracerebroventricularly induced CCR2⁺ cells to adhere to the vasculature and phagocytose perivascular fibrinogen in Ccr2^wt/rfp but not Ccr2^rfp/rfp mice. Depletion of monocytes with clodronate liposomes during CSD recovery prevented fibrinogen clearance and blocked behavioral recovery. We hypothesize that peripheral CCR2⁺ monocytes are not elevated in the brain on day 14 at the end of CSD and do not contribute to its behavioral effects at that time, but in recovery following cessation of stress, they enter the brain and exert restorative functions mediating vascular repair and normalization of behavior.

Dysmyelination by Oligodendrocyte-Specific Ablation of Ninj2 Contributes to Depressive-Like Behaviors

Depression is a mental disorder affecting more than 300 million people in the world. Abnormalities in white matter are associated with the development of depression. Here, the authors show that mice with oligodendrocyte-specific deletion of Nerve injury-induced protein 2 （Ninj2) exhibit depressive-like behaviors. Loss of Ninj2 in oligodendrocytes inhibits oligodendrocyte development and myelination, and impairs neuronal structure and activities. Ninj2 competitively inhibits TNFα/TNFR1 signaling pathway by directly binding to TNFR1 in oligodendrocytes. Loss of Ninj2 activates TNFα-induced necroptosis, and increases C-C Motif Chemokine Ligand 2 (Ccl2) production, which might mediate the signal transduction from oligodendrocyte to neurons. Inhibition of necroptosis by Nec-1s administration synchronously restores oligodendrocyte development, improves neuronal excitability, and alleviates depressive-like behaviors. This study thus illustrates the role of Ninj2 in the development of depression and myelination, reveals the relationship between oligodendrocytes and neurons, and provides a potential therapeutic target for depression.

White matter diffusivity and its correlations to state measures of psychopathology in male refugees with posttraumatic stress disorder

Post-traumatic stress disorder (PTSD) is a heterogenous condition and the underlying neurobiology is still poorly understood. In this study, we tested the hypothesis that PTSD is associated with microstructural changes in white matter (WM) fibre tracts that connect regions involved in emotional processing, memory, attention, and language. Furthermore, we examined how different response patterns to individualized trauma-provoking stimuli related to underlying WM microstructure. Sixty-nine trauma-affected male refugees with PTSD (N = 38) or without PTSD (N = 31) underwent clinical assessments and diffusion-weighted magnetic resonance imaging (DWI) of the whole brain at 3 Tesla. Diffusion tensor metrics were computed from DWI data and used to characterize regional white-matter microstructure. An automated tract segmentation method was used to extract diffusion tensor metrics from subject-based reconstructions of tract segments (ROI), including uncinate fasciculus (UF), cingulum bundle (CB), superior longitudinal fasciculus (SLF) in three subdivisions (SLF I - III), and fibre bundles connecting orbito-frontal cortex to striatum (OF-ST). Outside the scanner we obtained measures of immediate (state) arousal, avoidance and dissociation symptoms assessed in response to auditory exposure to a personal traumatic memory. Using mean FA of the middle part of each ROI, mixed ANOVA revealed a significant interaction between group, ROI and hemisphere. Post-hoc comparisons showed that, relative to refugees without PTSD, refugees with PTSD had lower FA in right CB, left SLF-I, bilateral OF-ST and bilateral SLF-II. Mean FA scaled negatively with avoidance in right CB while mean FA in bilateral UF scaled positively with individual scores reflecting dissociation symptoms. The results support a pathophysiological model of PTSD that implicates limbic structures, prefrontal cortex and striatum. The results also emphasize the need to consider PTSD's multifaceted manifestations when searching for functional-structural relationships.

Regional gray matter oligodendrocyte- and myelin-related measures are associated with differential susceptibility to stress-induced behavior in rats and humans

Individual reactions to traumatic stress vary dramatically, yet the biological basis of this variation remains poorly understood. Recent studies demonstrate the surprising plasticity of oligodendrocytes and myelin with stress and experience, providing a potential mechanism by which trauma induces aberrant structural and functional changes in the adult brain. In this study, we utilized a translational approach to test the hypothesis that gray matter oligodendrocytes contribute to traumatic-stress-induced behavioral variation in both rats and humans. We exposed adult, male rats to a single, severe stressor and used a multimodal approach to characterize avoidance, startle, and fear-learning behavior, as well as oligodendrocyte and myelin basic protein (MBP) content in multiple brain areas. We found that oligodendrocyte cell density and MBP were correlated with behavioral outcomes in a region-specific manner. Specifically, stress-induced avoidance positively correlated with hippocampal dentate gyrus oligodendrocytes and MBP. Viral overexpression of the oligodendrogenic factor Olig1 in the dentate gyrus was sufficient to induce an anxiety-like behavioral phenotype. In contrast, contextual fear learning positively correlated with MBP in the amygdala and spatial-processing regions of the hippocampus. In a group of trauma-exposed US veterans, T1-/T2-weighted magnetic resonance imaging estimates of hippocampal and amygdala myelin associated with symptom profiles in a region-specific manner that mirrored the findings in rats. These results demonstrate a species-independent relationship between region-specific, gray matter oligodendrocytes and differential behavioral phenotypes following traumatic stress exposure. This study suggests a novel mechanism for brain plasticity that underlies individual variance in sensitivity to traumatic stress.

Activation of 5-HT1A receptor reduces abnormal emotionality in stress-maladaptive mice by alleviating decreased myelin protein in the ventral hippocampus

We previously reported that abnormal emotionality in stress-maladaptive mice was ameliorated by chronic treatment with flesinoxan, a 5-HT_1A receptor agonist. Furthermore, the maintenance of hippocampal myelination appeared to contribute to the development of stress adaptation in mice. However, the effects of 5-HT_1A receptor activation on myelination under the stress-maladaptive situations and the underlying mechanisms remain unknown. In the present study, we examined using flesinoxan whether activation of 5-HT_1A receptor can reduce an abnormal emotional response by acting on oligodendrocytes to preserve myelin proteins in stress-maladaptive mice. Mice were exposed to repeated restraint stress for 4 h/day for 14 days as a stress-maladaptive model. Flesinoxan was given intraperitoneally immediately after the daily exposure to restraint stress. After the final exposure to restraint stress, the emotionality of mice was evaluated by the hole-board test. The expression levels of brain-derived neurotrophic factor (BDNF), phosphorylated-extracellular signal-regulated kinase (p-ERK), phosphorylated-cAMP response element-binding protein (p-CREB), myelin-associated glycoprotein (MAG), myelin basic protein (MBP) and oligodendrocyte transcription factor 2 (olig2) in the hippocampus was assessed by western blotting. Hippocampal oligodendrogenesis were examined by immunohistochemistry. Chronic treatment with flesinoxan suppressed the decrease in head-dipping behaviors in stress-maladaptive mice in the hole-board test. Under this condition, the decreases in MAG and MBP in the hippocampus recovered with increase in BDNF, p-ERK, p-CREB, and olig2. Furthermore, hippocampal oligodendrogenesis in stress-maladaptive mice was promoted by chronic treatment with flesinoxan. These findings suggest that 5-HT_1A receptor activation may promote oligodendrogenesis and myelination via an ERK/CREB/BDNF signaling pathway in the hippocampus and reduces abnormal emotionality due to maladaptation to excessive stress.

B-cells are abnormal in psychosocial stress and regulate meningeal myeloid cell activation

There is increasing interest in how immune cells, including those within the meninges at the blood-brain interface, influence brain function and mood disorders, but little data on humoral immunity in this context. Here, we show that in mice exposed to psychosocial stress, there is increased splenic B cell activation and secretion of the immunoregulatory cytokine interleukin (IL)-10. Meningeal B cells were prevalent in homeostasis but substantially decreased following stress, whereas Ly6Chi monocytes increased, and meningeal myeloid cells showed augmented expression of activation markers. Single-cell RNA sequencing of meningeal B cells demonstrated the induction of innate immune transcriptional programmes following stress, including genes encoding antimicrobial peptides that are known to alter myeloid cell activation. Cd19-/- mice, that have reduced B cells, showed baseline meningeal myeloid cell activation and decreased exploratory behaviour. Together, these data suggest that B cells may influence behaviour by regulating meningeal myeloid cell activation.

White Matter Changes Following Chronic Restraint Stress and Neuromodulation: A Diffusion Magnetic Resonance Imaging Study in Young Male Rats

Background

Repetitive transcranial magnetic stimulation (rTMS), a noninvasive neuromodulation technique, is an effective treatment for depression. However, few studies have used diffusion magnetic resonance imaging to investigate the longitudinal effects of rTMS on the abnormal brain white matter (WM) described in depression.

Methods

In this study, we acquired diffusion magnetic resonance imaging from young adult male Sprague Dawley rats to investigate 1) the longitudinal effects of 10- and 1-Hz low-intensity rTMS (LI-rTMS) in healthy animals; 2) the effect of chronic restraint stress (CRS), an animal model of depression; and 3) the effect of 10 Hz LI-rTMS in CRS animals. Diffusion magnetic resonance imaging data were analyzed using tract-based spatial statistics and fixel-based analysis.

Results

Similar changes in diffusion and kurtosis fractional anisotropy were induced by 10- and 1-Hz stimulation in healthy animals, although changes induced by 10-Hz stimulation were detected earlier than those following 1-Hz stimulation. Additionally, 10-Hz stimulation increased axial and mean kurtosis within the external capsule, suggesting that the two protocols may act via different underlying mechanisms. Brain maturation–related changes in WM, such as increased corpus callosum, fimbria, and external and internal capsule fiber cross-section, were compromised in CRS animals compared with healthy control animals and were rescued by 10-Hz LI-rTMS. Immunohistochemistry revealed increased myelination within the corpus callosum in LI-rTMS–treated CRS animals compared with those that received sham or no stimulation.

Conclusions

Overall, decreased WM connectivity and integrity in the CRS model corroborate findings in patients experiencing depression with high anxiety, and the observed LI-rTMS–induced effects on WM structure suggest that LI-rTMS might rescue abnormal WM by increasing myelination.

Structural connectivity and subcellular changes after antidepressant doses of ketamine and Ro 25-6981 in the rat: an MRI and immuno-labeling study

Ketamine has rapid and robust antidepressant effects. However, unwanted psychotomimetic effects limit its widespread use. Hence, several studies examined whether GluN2B-subunit selective NMDA antagonists would exhibit a better therapeutic profile. Although preclinical work has revealed some of the mechanisms of action of ketamine at cellular and molecular levels, the impact on brain circuitry is poorly understood. Several neuroimaging studies have examined the functional changes in the brain induced by acute administration of ketamine and Ro 25-6981 (a GluN2B-subunit selective antagonist), but the changes in the microstructure of gray and white matter have received less attention. Here, the effects of ketamine and Ro 25-6981 on gray and white matter integrity in male Sprague-Dawley rats were determined using diffusion-weighted magnetic resonance imaging (DWI). In addition, DWI-based structural brain networks were estimated and connectivity metrics were computed at the regional level. Immunohistochemical analyses were also performed to determine whether changes in myelin basic protein (MBP) and neurofilament heavy-chain protein (NF200) may underlie connectivity changes. In general, ketamine and Ro 25-6981 showed some opposite structural alterations, but both compounds coincided only in increasing the fractional anisotropy in infralimbic prefrontal cortex and dorsal raphe nucleus. These changes were associated with increments of NF200 in deep layers of the infralimbic cortex (together with increased MBP) and the dorsal raphe nucleus. Our results suggest that the synthesis of NF200 and MBP may contribute to the formation of new dendritic spines and myelination, respectively. We also suggest that the increase of fractional anisotropy of the infralimbic and dorsal raphe nucleus areas could represent a biomarker of a rapid antidepressant response.

Disturbance of prefrontal cortical myelination in olfactory bulbectomized mice is associated with depressive-like behavior

Recent studies have reported that demyelination is associated with the development of depression. Olfactory bulbectomized (OBX) rodents are a useful experimental animal model for depressive disorder. However, little is known about the change in myelination in the brain of OBX mice. To address this question, we observed depressive-like behavior of OBX mice in the tail-suspension test, and determined the quantity of myelin proteins in the prefrontal cortex (PFC), striatum and hippocampus on day 14 or 21 after surgery. The number of nodes of Ranvier paired with the paranodal marker contactin-associated protein (Caspr), as well as the numbers of immature and mature oligodendrocytes in the PFC, were also measured on day 21 after surgery. We examined whether these behavioral and neurochemical changes observed in OBX mice were reversed by chronic administration of imipramine. OBX mice showed depressive-like behavior in the tail-suspension test together with a decrease in the levels of myelin proteins such as myelin basic protein, myelin-associated glycoprotein and cyclicnucleotide phosphodiesterase in the PFC on day 21 after surgery. The number of nodes of Ranvier and mature oligodendrocytes were also decreased in the PFC of OBX mice, while the number of immature oligodendrocytes was increased on day 21 after surgery. However, the number of immature oligodendrocytes in the PFC of OBX mice was decreased on day 35 after surgery. Administration of imipramine (20 mg/kg) for 2 weeks from day 21 after surgery improved OBX-induced depressive-like behavior and abnormal myelination in the PFC. The present findings suggest that the disturbance of myelin function in the PFC may contribute to the pathophysiology of depression, and further support the notion that it plays an important role in the psychological state.

Molecular laterality encodes stress susceptibility in the medial prefrontal cortex

Functional lateralization of the prefrontal cortex has been implicated in stress and emotional disorders, yet underlying gene expression changes remains unknown. Here, we report molecular signatures lateralized by chronic social defeats between the two medial prefrontal cortices (mPFCs). Stressed mice show 526 asymmetrically expressed genes between the mPFCs. This cortical asymmetry selectively occurs in stressed mice with depressed social activity, but not in resilient mice with normal behavior. We have isolated highly asymmetric genes including connective tissue growth factor (CTGF), a molecule that modulates wound healing at the periphery. Knockdown of CTGF gene in the right mPFC by shRNA led to a stress-resistant behavioral phenotype. Overexpression of CTGF in the right mPFC using viral transduction induces social avoidance while the left mPFC thereof prevent stress-induced social avoidance. Our study provides a molecular window into the mechanism of stress-induced socioemotional disorders, which can pave the way for new interventions by targeting cortical asymmetry.

Acute stress reveals different impacts in male and female Zdhhc7-deficient mice

Numerous processes of neuronal development and synaptic plasticity in the brain rely on the palmitoyl acyltransferase ZDHHC7, as it palmitoylates various synaptic and extrasynaptic proteins such as neural cell adhesion molecule (NCAM) or gamma-aminobutyric acid (GABAA) receptors. In addition, ZDHHC7 palmitoylates sex steroid hormone receptors and is, therefore, indirectly linked to mental disorders that often occur because of or in conjunction with stress. In this work, we investigated how ZDHHC7 affects stress responses in mice. For this purpose, genetically modified mice with a knockout of the Zdhhc7 gene (KO) and wild-type (WT) littermates of both sexes were exposed to acute stressors or control conditions and examined with regard to their behavior, brain microstructure, gene expression, and synaptic plasticity. While no behavioral effects of acute stress were found, we did find that acute stress caused reduced mRNA levels of Esr1 and Esr2 coding for estrogen receptor α and β in the medial prefrontal cortex of male WT and KO mice. Strikingly, after acute stress only male KO mice showed reduced mean fiber lengths of the medioventral hippocampus. Furthermore, Zdhhc7-deficiency impaired synaptic plasticity in mice of both sexes, while acute stress improved it in females, but not in male mice. Taken together, our findings suggest that ZDHHC7 plays a modulatory role in the brain that leads to sex-specific stress responses, possibly due to estrogen receptor-mediated signaling pathways.

Juvenile exposure to acute traumatic stress leads to long-lasting alterations in grey matter myelination in adult female but not male rats

Stress early in life can have a major impact on brain development, and there is increasing evidence that childhood stress confers vulnerability for later developing psychiatric disorders. In particular, during peri-adolescence, brain regions crucial for emotional regulation, such as the prefrontal cortex (PFC), amygdala (AMY) and hippocampus (HPC), are still developing and are highly sensitive to stress. Changes in myelin levels have been implicated in mental illnesses and stress effects on myelin and oligodendrocytes (OLs) are beginning to be explored as a novel and underappreciated mechanism underlying psychopathologies. Yet there is little research on the effects of acute stress on myelin during peri-adolescence, and even less work exploring sex-differences. Here, we used a rodent model to test the hypothesis that exposure to acute traumatic stress as a juvenile would induce changes in OLs and myelin content across limbic brain regions. Male and female juvenile rats underwent 3 h of restraint stress with exposure to a predator odor on postnatal day (p) 28. Acute stress induced a physiological response, increasing corticosterone release and reducing weight gain in stress-exposed animals. Brain sections containing the PFC, AMY and HPC were taken either in adolescence (p40), or in adulthood (p95) and stained for markers of OLs and myelin. We found that acute stress induced sex-specific changes in grey matter (GM) myelination and OLs in both the short- and long-term. Exposure to a single stressor as a juvenile increased GM myelin content in the AMY and HPC in p40 males, compared to the respective control group. At p40, corticosterone release during stress exposure was also positively correlated with GM myelin content in the AMY of male rats. Single exposure to juvenile stress also led to long-term effects exclusively in female rats. Compared to controls, stress-exposed females showed reduced GM myelin content in all three brain regions. Acute stress exposure decreased PFC and HPC OL density in p40 females, perhaps contributing towards this observed long-term decrease in myelin content. Overall, our findings suggest that the juvenile brain is vulnerable to exposure to a brief severe stressor. Exposure to a single short traumatic event during peri-adolescence produces long-lasting changes in GM myelin content in the adult brain of female, but not male, rats. These findings highlight myelin plasticity as a potential contributor to sex-specific sensitivity to perturbation during a critical window of development.

The GABA(B1) receptor within the infralimbic cortex is implicated in stress resilience and vulnerability in mice

Resilience is the capacity to maintain normal psychological and physical functions in the face of stress and adversity. Understanding how one can develop and enhance resilience is of great relevance to not only promoting coping mechanisms but also mitigating maladaptive stress responses in psychiatric illnesses such as depression. Preclinical studies suggest that GABA(B) receptors (GABA(B1) and GABA(B2)) are potential targets for the treatment of major depression. In this study, we assessed the functional role of GABA(B) receptors in stress resilience and vulnerability by using a chronic unpredictable stress (CUS) model in mice. As the medial prefrontal cortex (mPFC) plays a key role in the top-down modulation of stress responses, we focused our study on this brain structure. Our results showed that only approximately 41.9% of subjects exhibited anxiety- or despair-like behaviors after exposure to CUS. The vulnerable mice showed higher c-Fos expression in the infralimbic cortex (IL) subregion of the mPFC when exposed to a social stressor. Moreover, the expression of GABA(B1) but not GABA(B2) receptors was significantly downregulated in IL subregion of susceptible mice. Finally, we found that intra-IL administration of baclofen, a GABA(B) receptor agonist, rapidly relieved the social avoidance symptoms of the "stress-susceptible" mice. Taken together, our results show that the GABA(B1) receptor within the IL may play an important role in stress resilience and vulnerability, and thus open an avenue to develop novel, personalized approaches to promote stress resilience and treat stress-related psychiatric disorders.