This competing renewal proposal probes how early-life experiences induce synaptic `rewiring' of stress-sensitive neurons, and the nature of the resulting molecular and epigenetic mechanisms that promote life-long emotional resilience. Experiences during sensitive periods early in life exert indelible influence on an individual's resilience or vulnerability to stress-related disorders. This occurs, at least in part, by modulation of neuronal function via changes in gene expression programs through epigenetic processes. However, it is not known how neonatal experiences `signal' to specific neuronal populations and how these signals influence the orchestrated programs of gene expression that mediate phenotypic resilience or vulnerability. We have discovered that a resilience-promoting neonatal experience, augmented maternal care (AUG), reduces glutamatergic synapses onto individual stress-sensitive hypothalamic neurons and represses expression of the stress-sensitive gene, Crh. In hypothalamic explants, reduced glutamatergic synapse function sufficed to recapitulate the repressive effects of AUG on Crh in a mechanism requiring the transcriptional repressor REST/NRSF. Here we capitalize on these findings to probe pivotal knowledge gaps that prevent translational application of our discoveries. First, we shall identify the mechanisms by which AUG reduces the number of excitatory synapses onto stress- sensitive hypothalamic CRH-expressing neurons. Because reduced excitatory synapse function suffices to repress Crh, understanding how this reduction takes place is important. We will test the hypothesis that microglia activation by AUG prunes glutamatergic synapses. Second, we shall probe the molecular mechanisms by which reduced excitatory input upregulates NRSF. Enhanced levels and function of NRSF, involved in numerous fundamental developmental processes, are required for the resilience-promoting effects of AUG, but the repressor is a poor molecular target. Therefore, we will identify how NRSF is upregulated, aiming for translatable targets. Capitalizing on pilot data we will test the hypothesis that miR124 repression promotes NRSF upregulation, generating the resilient phenotype. Third, we shall investigate the epigenetic mechanisms involved in the persistent alteration of cellular and behavioral phenotype generated by augmented maternal care. We will employ innovative single-neuron RNA- seq methods in identified neurons together with NRSF-ChIP-seq and multiple levels of analysis to discover both NRSF-dependent and independent gene expression changes that contribute to the resilient phenotype. Fourth, we shall capitalize on tantalizing preliminary data to examine the sex-specificity of the processes and outcomes described above. These studies will have major impact, as they are the first to causally connect neonatal experience with synaptic `rewiring' that promotes epigenetic processes within select neuronal populations. The proposed studies provide a novel mechanistic and `translatable' pathway from early-life experience to phenotypes that govern human vulnerability and resilience to neuropsychiatric diseases.

Experiences during sensitive periods early in life exert indelible influence on an individual's resilience or vulnerability to depression and to some cognitive disorders. There is evidence that neonatal experiences modulate the function of neurons involved in these crucial behaviors by modifying the genes expressed in these neurons in a long-lasting manner. However, it is not known how neonatal experiences `signal' to specific brain cells and how these signals influence the orchestrated programs of gene expression that mediate phenotypic resilience or vulnerability. The studies proposed here will build on robust rodent models and will employ cutting-edge scientific tools to solve these important mysteries. The information that we will gain from these studies can then be used to enhance resilience to depression in individuals who have suffered difficult experiences early life and who are prone to mental disease.