Madeline L. Pfau

Individual differences in the peripheral immune system promote resilience versus susceptibility to social stress

Significance Depression and anxiety have been linked to increased inflammation. However, we do not know if inflammatory status predates onset of disease or whether it contributes to depression symptomatology. We report preexisting individual differences in the peripheral immune system that predict and promote stress susceptibility. Replacing a stress-naive animal’s peripheral immune system with that of a stressed animal increases susceptibility to social stress including repeated social defeat stress (RSDS) and witness defeat (a purely emotional form of social stress). Depleting the cytokine IL-6 from the whole body or just from leukocytes promotes resilience, as does sequestering IL-6 outside of the brain. These studies demonstrate that the emotional response to stress can be generated or blocked in the periphery, and offer a potential new form of treatment for stress disorders. Depression and anxiety disorders are associated with increased release of peripheral cytokines; however, their functional relevance remains unknown. Using a social stress model in mice, we find preexisting individual differences in the sensitivity of the peripheral immune system that predict and promote vulnerability to social stress. Cytokine profiles were obtained 20 min after the first social stress exposure. Of the cytokines regulated by stress, IL-6 was most highly up-regulated only in mice that ultimately developed a susceptible behavioral phenotype following a subsequent chronic stress, and levels remained elevated for at least 1 mo. We confirmed a similar elevation of serum IL-6 in two separate cohorts of patients with treatment-resistant major depressive disorder. Before any physical contact in mice, we observed individual differences in IL-6 levels from ex vivo stimulated leukocytes that predict susceptibility versus resilience to a subsequent stressor. To shift the sensitivity of the peripheral immune system to a pro- or antidepressant state, bone marrow (BM) chimeras were generated by transplanting hematopoietic progenitor cells from stress-susceptible mice releasing high IL-6 or from IL-6 knockout (IL-6−/−) mice. Stress-susceptible BM chimeras exhibited increased social avoidance behavior after exposure to either subthreshold repeated social defeat stress (RSDS) or a purely emotional stressor termed witness defeat. IL-6−/− BM chimeric and IL-6−/− mice, as well as those treated with a systemic IL-6 monoclonal antibody, were resilient to social stress. These data establish that preexisting differences in stress-responsive IL-6 release from BM-derived leukocytes functionally contribute to social stress-induced behavioral abnormalities.

Sex Differences in Nucleus Accumbens Transcriptome Profiles Associated with Susceptibility versus Resilience to Subchronic Variable Stress

Depression and anxiety disorders are more prevalent in females, but the majority of research in animal models, the first step in finding new treatments, has focused predominantly on males. Here we report that exposure to subchronic variable stress (SCVS) induces depression-associated behaviors in female mice, whereas males are resilient as they do not develop these behavioral abnormalities. In concert with these different behavioral responses, transcriptional analysis of nucleus accumbens (NAc), a major brain reward region, by use of RNA sequencing (RNA-seq) revealed markedly different patterns of stress regulation of gene expression between the sexes. Among the genes displaying sex differences was DNA methyltransferase 3a (Dnmt3a), which shows a greater induction in females after SCVS. Interestingly, Dnmt3a expression levels were increased in the NAc of depressed humans, an effect seen in both males and females. Local overexpression of Dnmt3a in NAc rendered male mice more susceptible to SCVS, whereas Dnmt3a knock-out in this region rendered females more resilient, directly implicating this gene in stress responses. Associated with this enhanced resilience of female mice upon NAc knock-out of Dnmt3a was a partial shift of the NAc female transcriptome toward the male pattern after SCVS. These data indicate that males and females undergo different patterns of transcriptional regulation in response to stress and that a DNA methyltransferase in NAc contributes to sex differences in stress vulnerability. SIGNIFICANCE STATEMENT Women have a higher incidence of depression than men. However, preclinical models, the first step in developing new diagnostics and therapeutics, have been performed mainly on male subjects. Using a stress-based animal model of depression that causes behavioral effects in females but not males, we demonstrate a sex-specific transcriptional profile in brain reward circuitry. This transcriptional profile can be altered by removal of an epigenetic mechanism, which normally suppresses DNA transcription, creating a hybrid male/female transcriptional pattern. Removal of this epigenetic mechanism also induces behavioral resilience to stress in females. These findings shed new light onto molecular factors controlling sex differences in stress response.

Basal forebrain projections to the lateral habenula modulate aggression reward

Maladaptive aggressive behaviour is associated with a number of neuropsychiatric disorders and is thought to result partly from the inappropriate activation of brain reward systems in response to aggressive or violent social stimuli. Nuclei within the ventromedial hypothalamus, extended amygdala and limbic circuits are known to encode initiation of aggression; however, little is known about the neural mechanisms that directly modulate the motivational component of aggressive behaviour. Here we established a mouse model to measure the valence of aggressive inter-male social interaction with a smaller subordinate intruder as reinforcement for the development of conditioned place preference (CPP). Aggressors develop a CPP, whereas non-aggressors develop a conditioned place aversion to the intruder-paired context. Furthermore, we identify a functional GABAergic projection from the basal forebrain (BF) to the lateral habenula (lHb) that bi-directionally controls the valence of aggressive interactions. Circuit-specific silencing of GABAergic BF–lHb terminals of aggressors with halorhodopsin (NpHR3.0) increases lHb neuronal firing and abolishes CPP to the intruder-paired context. Activation of GABAergic BF–lHb terminals of non-aggressors with channelrhodopsin (ChR2) decreases lHb neuronal firing and promotes CPP to the intruder-paired context. Finally, we show that altering inhibitory transmission at BF–lHb terminals does not control the initiation of aggressive behaviour. These results demonstrate that the BF–lHb circuit has a critical role in regulating the valence of inter-male aggressive behaviour and provide novel mechanistic insight into the neural circuits modulating aggression reward processing.

Social stress induces neurovascular pathology promoting depression

Studies suggest that heightened peripheral inflammation contributes to the pathogenesis of major depressive disorder. We investigated the effect of chronic social defeat stress, a mouse model of depression, on blood–brain barrier (BBB) permeability and infiltration of peripheral immune signals. We found reduced expression of the endothelial cell tight junction protein claudin-5 (Cldn5) and abnormal blood vessel morphology in nucleus accumbens (NAc) of stress-susceptible but not resilient mice. CLDN5 expression was also decreased in NAc of depressed patients. Cldn5 downregulation was sufficient to induce depression-like behaviors following subthreshold social stress whereas chronic antidepressant treatment rescued Cldn5 loss and promoted resilience. Reduced BBB integrity in NAc of stress-susceptible or mice injected with adeno-associated virus expressing shRNA against Cldn5 caused infiltration of the peripheral cytokine interleukin-6 (IL-6) into brain parenchyma and subsequent expression of depression-like behaviors. These findings suggest that chronic social stress alters BBB integrity through loss of tight junction protein Cldn5, promoting peripheral IL-6 passage across the BBB and depression.Chronic social defeat stress induces loss of protein claudin-5, leading to abnormalities in blood vessel morphology, increased blood brain barrier permeability, infiltration of immune signals and depression-like behaviors.

Peripheral and Central Mechanisms of Stress Resilience.

Viable new treatments for depression and anxiety have been slow to emerge, likely owing to the complex and incompletely understood etiology of these disorders. A budding area of research with great therapeutic promise involves the study of resilience, the adaptive maintenance of normal physiology and behavior despite exposure to marked psychological stress. This phenomenon, documented in both humans and animal models, involves coordinated biological mechanisms in numerous bodily systems, both peripheral and central. In this review, we provide an overview of resilience mechanisms throughout the body, discussing current research in animal models investigating the roles of the neuroendocrine, immune, and central nervous systems in behavioral resilience to stress.

Excitatory transmission at thalamo-striatal synapses mediates susceptibility to social stress

Postsynaptic remodeling of glutamatergic synapses on ventral striatum (vSTR) medium spiny neurons (MSNs) is critical for shaping stress responses. However, it is unclear which presynaptic inputs are involved. Susceptible mice exhibited increased synaptic strength at intralaminar thalamus (ILT), but not prefrontal cortex (PFC), inputs to vSTR MSNs following chronic social stress. Modulation of ILT-vSTR versus PFC-vSTR neuronal activity differentially regulated dendritic spine plasticity and social avoidance.

Sex-specific transcriptional signatures in human depression

Major depressive disorder (MDD) is a leading cause of disease burden worldwide. While the incidence, symptoms and treatment of MDD all point toward major sex differences, the molecular mechanisms underlying this sexual dimorphism remain largely unknown. Here, combining differential expression and gene coexpression network analyses, we provide a comprehensive characterization of male and female transcriptional profiles associated with MDD across six brain regions. We overlap our human profiles with those from a mouse model, chronic variable stress, and capitalize on converging pathways to define molecular and physiological mechanisms underlying the expression of stress susceptibility in males and females. Our results show a major rearrangement of transcriptional patterns in MDD, with limited overlap between males and females, an effect seen in both depressed humans and stressed mice. We identify key regulators of sex-specific gene networks underlying MDD and confirm their sex-specific impact as mediators of stress susceptibility. For example, downregulation of the female-specific hub gene Dusp6 in mouse prefrontal cortex mimicked stress susceptibility in females, but not males, by increasing ERK signaling and pyramidal neuron excitability. Such Dusp6 downregulation also recapitulated the transcriptional remodeling that occurs in prefrontal cortex of depressed females. Together our findings reveal marked sexual dimorphism at the transcriptional level in MDD and highlight the importance of studying sex-specific treatments for this disorder.

Immune and Neuroendocrine Mechanisms of Stress Vulnerability and Resilience.

Diagnostic criteria for mood disorders including major depressive disorder (MDD) largely ignore biological factors in favor of behavioral symptoms. Compounding this paucity of psychiatric biomarkers is a need for therapeutics to adequately treat the 30–50% of MDD patients who are unresponsive to traditional antidepressant medications. Interestingly, MDD is highly prevalent in patients suffering from chronic inflammatory conditions, and MDD patients exhibit higher levels of circulating pro-inflammatory cytokines. Together, these clinical findings suggest a role for the immune system in vulnerability to stress-related psychiatric illness. A growing body of literature also implicates the immune system in stress resilience and coping. In this review, we discuss the mechanisms by which peripheral and central immune cells act on the brain to affect stress-related neurobiological and neuroendocrine responses. We specifically focus on the roles of pro-inflammatory cytokine signaling, peripheral monocyte infiltration, microglial activation, and hypothalamic-pituitary-adrenal axis hyperactivity in stress vulnerability. We also highlight recent evidence suggesting that adaptive immune responses and treatment with immune modulators (exogenous glucocorticoids, humanized antibodies against cytokines) may decrease depressive symptoms and thus represent an attractive alternative to the current antidepressant treatments.

Establishment of a repeated social defeat stress model in female mice

Numerous studies have employed repeated social defeat stress (RSDS) to study the neurobiological mechanisms of depression in rodents. An important limitation of RSDS studies to date is that they have been conducted exclusively in male mice due to the difficulty of initiating attack behavior directed toward female mice. Here, we establish a female mouse model of RSDS by inducing male aggression toward females through chemogenetic activation of the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl). We demonstrate that females susceptible to RSDS display social avoidance, anxiety-like behavior, reduction of body weight, and elevated levels of circulating interleukin 6. In contrast, a subset of mice we term resilient only display anxiety-like behaviors after RSDS. This model allows for investigation of sex differences in the neurobiological mechanisms of defeat‒induced depression‒like behaviors. A robust female social defeat model is a critical first step in the identification and development of novel therapeutic compounds to treat depression and anxiety disorders in women.

Threonine 149 Phosphorylation Enhances ΔFosB Transcriptional Activity to Control Psychomotor Responses to Cocaine

Stable changes in neuronal gene expression have been studied as mediators of addicted states. Of particular interest is the transcription factor ΔFosB, a truncated and stable FosB gene product whose expression in nucleus accumbens (NAc), a key reward region, is induced by chronic exposure to virtually all drugs of abuse and regulates their psychomotor and rewarding effects. Phosphorylation at Ser27 contributes to ΔFosBs stability and accumulation following repeated exposure to drugs, and our recent work demonstrates that the protein kinase CaMKIIα phosphorylates ΔFosB at Ser27 and regulates its stability in vivo. Here, we identify two additional sites on ΔFosB that are phosphorylated in vitro by CaMKIIα, Thr149 and Thr180, and demonstrate their regulation in vivo by chronic cocaine. We show that phosphomimetic mutation of Thr149 (T149D) dramatically increases AP-1 transcriptional activity while alanine mutation does not affect transcriptional activity when compared with wild-type (WT) ΔFosB. Using in vivo viral-mediated gene transfer of ΔFosB-T149D or ΔFosB-T149A in mouse NAc, we determined that overexpression of ΔFosB-T149D in NAc leads to greater locomotor activity in response to an initial low dose of cocaine than does WT ΔFosB, while overexpression of ΔFosB-T149A does not produce the psychomotor sensitization to chronic low-dose cocaine seen after overexpression of WT ΔFosB and abrogates the sensitization seen in control animals at higher cocaine doses. We further demonstrate that mutation of Thr149 does not affect the stability of ΔFosB overexpressed in mouse NAc, suggesting that the behavioral effects of these mutations are driven by their altered transcriptional properties.