Depression affects millions of individuals each year, exerting untold costs on society. Altered serotonergic transmission and presynaptic serotonin (5-hydroxytryptamine, 5-HT) transporter (SERT) expression have long been implicated in the pathophysiology of psychiatric disorders. Drugs that block SERT are antidepressants and are successfully used for the treatment of mental disorders. However, the molecular or cellular basis for SERT dysregulation and the effectiveness of SSRIs in psychiatric disorder treatment and their relationship to the etiologies of psychiatric disorders remain poorly understood. Studies from the PI's laboratory and colleagues have illuminated the dynamic SERT functional regulation through powerful posttranslational mechanisms. A rare single nucleotide polymorphism, isoleucine to valine at amino acid 425 (Val425), in SERT has been implicated in the pathogenesis of OCD. Remarkably, Val425-SERT revealed abnormal functional expression, normal PKG/p38 MAPK-mediated regulation, and normal basal phosphorylation. These findings led us to the novel hypothesis that abnormal functional expression arising from perturbations of the SERT regulatory cascades, which normally regulate SERT function to maintain synaptic 5-HT, leads to compromised 5-HT signaling and consequently 5-HT-linked neurobehavioral disorders. To test this hypothesis, we have generated viable Val425-SERT knock-in mice in C57BL/6N background to profile brain region-specific SERT functional expression, kinase(s)-mediated posttranslational SERT modifications, transcriptome features and to link synaptic 5-HT clearance to behavior. Parallel with human brain SERT imaging profiles from individuals with major depression, Val425-SERT mice exhibit reduced SERT activity in the amygdala, midbrain and elevated SERT activity in the hippocampus, thus providing unique mouse model for exploring the kinase-mediated SERT regulatory pathways that are set points in disrupting normal SERT function in disease-linked human SERT variants. In our current research proposal, we propose to 1) Identify the cellular mechanisms by which SERT functions are altered in amygdala and hippocampus as a consequence of in-vivo Val425-SERT mutation, 2) determine the impact of SERT dysregulation on 5-HT clearance, and subsequent neurotransmission in amygdala and hippocampus, and 3) Determine the impact of dysregulated SERT phosphorylation in the amygdala and hippocampus on the behavioral deficits seen in Val425-SERT mice. We hope to identify the important relationship between differential and brain region-specific SERT dysregulation and psychiatric disorders that arise from naturally occurring rare single amino acid coding variation (Ile425Val) within the SERT and offer important new insights into the brain region abnormalities of serotonergic neurotransmission underlying psychiatric disorders.