Insulin-dependent regulation of GLAST/EAAT1 in Bergmann glial cells

Glutamate is the major excitatory neurotransmitter in the central nervous system. Ionotropic and metabotropic glutamate receptors are present in neurons and glial cells and are involved in gene expression regulation. A family of sodium-dependent glutamate transporters carries out the removal of the neurotransmitter from the synaptic cleft. In the cerebellum, the bulk of glutamate transport is mediated through the excitatory amino acids transporter 1 (EAAT1/GLAST) expressed in Bergmann glial cells. Proper transporter function is critical for glutamate cycling and glucose turnover, as well as prevention of excitotoxic insult to Purkinje cells. In order to gain insight into the regulatory signals that modify this uptake activity, we investigated the effects of insulin exposure. Using the well-defined chick cerebellar Bergmann glial cell culture model, we observed a time and dose-dependent decrease in [³H]-d-aspartate uptake. As expected, this effect is mimicked by the tyrosine phosphatase inhibitor sodium orthovanadate, suggesting a receptor-mediated effect. Equilibrium [³H]-d-aspartate binding experiments as well as a reverse transcriptase/polymerase chain reaction strategy demonstrated that the decrease in the uptake activity is related to reduced numbers of transporter molecules in the plasma membrane. Accordingly, the transcriptional activity of the chick glast promoter diminished upon insulin treatment. The present findings suggest the involvement of insulin in neuronal/glial coupling in the cerebellum.