Role of GSK3β in behavioral abnormalities induced by serotonin deficiency

Dysregulation of brain serotonin (5-HT) neurotransmission is thought to underlie mental conditions as diverse as depression, anxiety disorders, bipolar disorder, autism, and schizophrenia. Despite treatment of these conditions with serotonergic drugs, the molecular mechanisms by which 5-HT is involved in the regulation of aberrant emotional behaviors are poorly understood. Here, we generated knockin mice expressing a mutant form of the brain 5-HT synthesis enzyme, tryptophan hydroxylase 2 (Tph2). This mutant is equivalent to a rare human variant (R441H) identified in few individuals with unipolar major depression. Expression of mutant Tph2 in mice results in markedly reduced (approximate to 80%) brain 5-HT production and leads to behavioral abnormalities in tests assessing 5-HT-mediated emotional states. This reduction in brain 5-HT levels is accompanied by activation of glycogen synthase kinase 3 beta (GSK3 beta), a signaling molecule modulated by many psychiatric therapeutic agents. Importantly, inactivation of GSK3 beta in Tph2 knockin mice, using pharmacological or genetic approaches, alleviates the aberrant behaviors produced by 5-HT deficiency. These findings. establish a critical role of Tph2 in the maintenance of brain serotonin homeostasis and identify GSK3 beta signaling as an important pathway through which brain 5-HT deficiency induces abnormal behaviors. Targeting GSK3 beta and related signaling events may afford therapeutic advantages for the management of certain 5-HT-related psychiatric conditions.