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  • Title: Retinal colocalization and in vitro interaction of the glutamate transporter EAAT3 and the serum- and glucocorticoid-inducible kinase SGK1 [correction].
    Author: Schniepp R, Kohler K, Ladewig T, Guenther E, Henke G, Palmada M, Boehmer C, Rothstein JD, Bröer S, Lang F.
    Journal: Invest Ophthalmol Vis Sci; 2004 May; 45(5):1442-9. PubMed ID: 15111600.
    Abstract:
    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 (S422D)SGK1, inactive (K127N)SGK1, and/or constitutively active (T308D,S473D)PKB. 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 (S422D)SGK1, and constitutively active (T308D,S473D)PKB, but not by inactive (K127N)SGK1. 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.
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