Project Summary Exposure to chronic stress is a known risk factor for neuropsychiatric disease and yields structural and molecular changes to the brain and immune system. The nucleus accumbens (NAc), a hub for integrating reward and motivation, exhibits molecular and cellular alterations that are found in postmortem tissue of patients with stress- related disorders, such as major depressive disorder, and drive motivational deficits in rodents. Specifically, dendritic atrophy of dopamine receptor type 1 (D1), but not type 2 (D2) expressing medium spiny neurons (MSNs) is necessary and sufficient for stress-induced negative behavioral outcomes. Emerging evidence implicates microglia as potential mediators of neuronal atrophy and motivational deficits after social stress. Since microglia can interact with neurons and facilitate neuronal dysfunction and cell death, they are prime candidates for investigating the relationship between chronic stress and D1-MSN atrophy. Recent work from the lab shows that chronic social defeat stress (CSDS) reduced overall D1-MSN and microglia contact, but not D2 MSNs, and analyses of individual microglia within the D1-MSN microenvironment revealed reductions in microglia complexity in animals that exhibited negative affective behavior after CSDS. Thus, while it is evident that CSDS alters D1-MSN-microglia contact and morphology, the molecular mechanisms driving these cell-subtype specific, stress-induced changes in the NAc microenvironment remain unclear. Preliminary RNA-seq analysis from the lab points to vitronectin, an extracellular matrix substrate, as a promising molecular messenger mediating D1- MSN and microglia interactions because it plays a driving role in a network of genes altered by chronic stress and is specifically differentially expressed in D1-MSNs. In this proposal, I will virally knock down vitronectin expression in D1-MSNs using a Cre-dependent CRISPR/Cas9 construct in mice before subjecting them to either CSDS or Chronic Witness Defeat Stress (CWDS) to interrogate the role of vitronectin expression in D1-MSN- microglia interactions, D1-MSN and microglia morphology, and stress-induced behavioral outcomes. It is hypothesized that knocking down vitronectin expression in D1-MSNs will rescue D1-MSN stress-induced dendritic atrophy, restore D1-MSN-microglia contact, and prevent negative affective behavior. Understanding the molecular mechanisms driving altered stress-induced behavioral states can shed light on novel therapeutics designed to protect and/or treat the deleterious effects of chronic stress. The proposed training at the University of Maryland Baltimore, School of Medicine will facilitate my transition to a postdoctoral fellowship and allow me to continue to research stress-related neuropsychiatric disease by characterizing neuron, microglia, and extracellular matrix relationships in the context of stress exposure. With the support of my excellent scientific community comprising of my mentor, t...