The Role of Insulin Signaling on Dopamine Transporter Trafficking
Speed, Nicole Kathryn
Dopaminergic neurotransmission is a critical component in the regulation of several behaviors, including cognition, motor function, motivation, and reward. Abnormalities in dopamine (DA) signaling have been linked to several disorders, including schizophrenia, Parkinson’s Disease, drug addiction, and eating disorders. An important component in regulating DA neurotransmission is the DA transporter (DAT), which controls the inactivation of DA signaling by uptake of DA from the synapse into the presynaptic bouton. This action depends upon the number of functional transporters expressed at the plasma membrane. As such, regulation of DAT cell surface expression is an important mechanism to modify DA neurotransmission. A growing body of literature indicates that insulin signaling, including the downstream effector protein kinase B (Akt), is an important regulator of DAT function by virtue of fine tuning the transporter’s cell surface expression. In this dissertation, I first sought to further define the insulin signaling pathway regulating DAT by identifying the isoform of Akt responsible for modulating DAT cell surface expression. The data shown here support that Akt2, the isoform believed to mediate insulin signaling, regulates DAT cell surface expression. Furthermore, we sought to understand whether high fat feeding, diet induced obesity (DIO) would cause a reduction in Akt phosphorylation in brain, and as a consequence, alter DAT function. Here I report that a high fat diet alters insulin signaling, namely Akt activity, and results in a reduction of DAT cell surface expression in the striatum. In addition, I demonstrate a functional reduction in DA clearance in vivo and a reduction in AMPH-induced locomotor activity. Upon viral rescue of insulin signaling via expression of insulin receptor substrate 2 (IRS2), DAT cell surface expression and AMPH-induced locomotor activity are restored. Collectively, these data demonstrate that insulin, by signaling through Akt2, regulates DAT function, and that a high fat diet leads to improper insulin signaling, thus altering DAT cell surface expression. Therefore, dysregulation of DA tone by alterations of DAT function is an important concern for individuals who have developed insulin resistance, and may have mechanistic implications for the co-morbid nature of obesity and neuropsychiatric disorders.