Retinal colocalization and in vitro interaction of the glutamate receptor EAAT3 and the serum- and glucocorticoid-inducible kinase SGK1

PURPOSE. The serum- and glucocorticoid-inducible kinase SGK1 regulates several epithelial channels and transporters, the related protein kinase B (PKB) regulates glucose transport. SGK1 is expressed in the brain and could thus regulate glial and/or neuronal transport processes. The present study explores whether SGK1 is expressed in the retina and whether it regulates EAAT3, a Na⁺-coupled glutamate transporter. EAAT3 is expressed in retinal ganglion cells and accomplishes the clearance of glutamate from synaptic clefts. METHODS. Immunohistochemistry was performed to test for retinal SGK1 expression. For functional analysis, cRNA encoding EAAT3 was injected into Xenopus oocytes with or without additional injection of wild-type SGK1, constitutively active ^S422DSGK1, inactive ^K127NSGK1, and/or constitutively active ^T308D,S473DPKB. Glutamate induced current (I_GLU) was taken as a measure for transport. RESULTS. SGK1 is indeed expressed in several retinal cells including retinal ganglion cells where it is colocalized with EAAT3. In EAAT3-expressing Xenopus oocytes, glutamate-induced current was stimulated by coexpression of wild-type SGK1, constitutively active ^S422DSGK1, and constitutively active ^T308D,S473DPKB, but not by inactive ^K127NSGK1. CONCLUSIONS. SGK1 and EAAT3 are coexpressed in retinal neurons, and SGK1 serves to stimulate EAAT3. This function is shared by protein kinase B (PKB). The experiments reveal a novel mechanism regulating EAAT3, which may be essential for the function of the retinal ganglion cells.