Eric S. Wohleb, PhD - US grants

DESCRIPTION (provided by applicant): Psychosocial stress can profoundly influence immunity and behavior. In clinical studies stress is associated with inflammatory conditions and earlier mortality. In addition, stress is associated with an increased prevalence of mental health complications including anxiety and depression. Unfortunately, the mechanisms by which stress causes immunological and behavioral consequences are not completely understood. In a particular murine model of social stress, called (Pt.2) repeated social defeat (RSD), the inflammatory potential of peripheral myeloid-lineage cells and monocytes (CD11b+) is enhanced and following immune stimulation these cells produce more pro-inflammatory cytokines. In addition, RSD increased the recruitment of circulating, myeloid-lineage cells to inflamed peripheral tissues causing exaggerated inflammation and pathology. (Pt.1) More recently we published that mice exposed to RSD demonstrate enhanced inflammation in the brain that corresponds to an increase in primed microglia, which are resident CD11b+ cells in the brain. Primed microglia are associated with increased neuroinflammation. Similar to models of aging and neurological disease, RSD initiates microglia activation that leads to an increase in their inflammatory capacity. For example, ex vivo immune stimulation of microglia isolated from RSD mice causes amplified production of inflammatory cytokines and chemokines, such as TNF-¿, CCL2 (MCP-1), and IL-6. In addition, RSD increased the trafficking of Ly6Chigh/CCR2+ macrophages (CD11b+/CD45high) to the brain. Ly6Chigh/CCR2+ macrophages readily traffic to sites of inflammation and initiate or perpetuate immune responses. Furthermore, RSD induces prolonged anxiety-like behavior that is apparent up to 8 days after the cessation of the stressor. Since inflammation and anxiety are intimately linked, it is plausible that RSD primes microglia and recruits macrophages to the brain, which perpetuates inflammation and leads to prolonged anxiety-like behavior. In this application, RSD will be used to test the hypothesis that social stress primes microglia and recruits macrophages to the brain and these inflammatory changes contribute to prolonged anxiety-like behavior. PUBLIC HEALTH RELEVANCE: Psychosocial stress is associated with increased inflammatory conditions, earlier mortality, and psychological disorders, including anxiety. Primed immune cells associated with the brain increase inflammatory mediators, recruit reactive cell populations from the periphery, and are likely an important determinant in prolonged behavioral changes. Understanding how primed immune cells associated with the brain contribute to neuroinflammation can help delineate mechanisms by which psychosocial stress can induce prolonged anxiety and lead to novel therapeutic strategies to alleviate mental health complications caused by stress.

Project Summary/Abstract: Major depressive disorder (MDD) is a recurring psychiatric disease that causes significant disability and socioeconomic burdens. Current therapies for MDD take weeks to months to be effective, and many patients are treatment-resistant reporting no improvement in symptom severity. In this context, it is important for research to be aimed at understanding the neurobiology of depression and to identify novel therapeutic targets. Clinical and basic research indicate that dysfunction of the medial prefrontal cortex (PFC) is a primary pathophysiological feature that contributes to depressive-like behaviors, including despair, reduced sociability, and cognitive impairment. In particular, studies show that depressive-like behaviors and cognitive impairments are associated with reduced synapse number and dendritic atrophy of pyramidal neurons in the medial PFC. It is well- established that brain-derived neurotrophic factor (BDNF) is an important regulator of neuroplasticity. Indeed mice deficient in BDNF signaling have exaggerated stress-induced neuroplasticity deficits and worsened depressive-like behaviors compared to wild-type mice. Consistent with this work, recent studies show that BDNF signaling in the medial PFC is required for rapid antidepressant-like effects following ketamine or scopolamine. While these studies implicate BDNF in the neurobiology of depression and antidepressant treatment, it remains unclear what cell type drives this neurotrophic signaling. Seminal work has shown that microglia, the tissue-resident macrophages in the brain, actively regulate neuroplasticity in physiological and pathological conditions. Notably, recent studies show that microglia-specific BDNF depletion reduced glutamate receptor expression in the motor cortex, which led to impaired synaptic plasticity in response to a motor learning task. Thus, it is plausible that microglial BDNF is a critical mediator of neuroplasticity in chronic stress and antidepressant treatment. In support of this idea, our initial studies showed that chronic stress reduced BDNF expression in purified microglia in the PFC, which corresponded with synaptic deficits and depressive-like behavior. Further studies showed that ketamine administration increased microglial BDNF expression in the PFC, which was associated with increased dendritic spine density and antidepressant- like behavioral responses. To expound on these findings proposed studies will use mice with microglia-specific BDNF depletion (Cx3cr1CreER:Bdnffl/fl) to test two specific aims: 1) Determine if deficient microglial BDNF confers stress susceptibility via increased synapse loss and depressive-like behaviors following stress; and 2) Examine the role of microglial BDNF in neurobiological responses and behavioral effects of rapid-acting antidepressants ketamine or scopolamine. Studies outlined in this application are significant because they will be the first to study the role of microglial BDNF in neurobiological adaptations underlying both stress-induced depressive-like behaviors and antidepressant treatment. We expect to identify a novel neurotrophic role for microglia, which may guide treatment strategies for MDD and other neurological conditions.

PROJECT SUMMARY/ ABSTRACT: Clinical and preclinical studies have linked synapse loss and impaired prefrontal cortex (PFC) function to behavioral and cognitive symptoms of psychiatric diseases, such as major depressive disorder (MDD). Preclinical models, such as chronic unpredictable stress (CUS), are important tools to study these pathophysiological mechanisms as they recapitulate key neurobiological (i.e., synapse loss in PFC) and behavioral (i.e., anhedonia, working memory impairment) aspects of MDD. This is significant because exposure to psychosocial or environmental stressors increases risk of development and recurrence of psychiatric disease. Accumulating evidence shows that the brain-resident macrophages, microglia, have an active role in regulating neuroplasticity in physiological and pathological conditions. In support of this work, research in our lab indicates that dynamic neuron-microglia interactions contribute to neurobiological and behavioral consequences following chronic stress. In particular, CUS increases neuronal colony stimulating factor-1 (CSF1) signaling in the medial PFC, which provokes microglia-mediated neuronal remodeling that contributes to synaptic deficits and behavioral and cognitive consequences. Stress-induced release of glucocorticoids are implicated in the pathophysiology of psychiatric diseases. The actions of glucocorticoids are mediated by glucocorticoid receptors (GR), which regulate gene transcription. Indeed prior work shows that GR signaling alters gene networks that drive structural remodeling and synapse loss on pyramidal neurons in the PFC. Our recent studies indicate that GR signaling induces neuronal CSF1 signaling in the PFC and provokes microglia-mediated neuronal remodeling in the PFC, which contributes to development of depressive behaviors after CUS. This work also revealed that GR signaling regulates specific molecular pathways in neurons (REDD1; regulated in development and DNA damage response 1) and microglia (TNF?; tumor necrosis factor-?). These findings are relevant because both neuronal REDD1 and microglial TNF? have critical roles in regulating synaptic plasticity. Studies in this application will determine the contributions of neuronal or microglial GR signaling and respective downstream mediators in the pathophysiology underlying behavioral consequences of chronic stress. Here we will use brain region- and cell type-specific genetic and pharmacological manipulations to test two specific aims: 1) Define the role of neuronal GR signaling and downstream REDD1 in stress-induced CSF1 signaling, microglia-mediated neuronal remodeling, and associated behavioral consequences; and 2) Examine the role of microglial GR signaling and downstream TNF? in stress- induced microglia-mediated neuronal remodeling, synaptic deficits, and associated behavioral consequences. These studies are significant because they will identify molecular and cellular adaptations that initiate stress- induced synapse loss in the PFC. We expect to generate novel insight into cell type-specific pathways that drive the neurobiology of stress, which may guide treatment strategies for psychiatric disease.