Additive regulation of GluR1 by stargazin and serum- and glucocorticoid-inducible kinase isoform SGK3

The serum- and glucocorticoid-inducible kinase isoform 3 (SGK3) and stargazin have both been shown to enhance the synaptic expression level of GluR1. The present study was performed to elucidate whether SGK3 and stargazin interact or are effective through different pathways in the regulation of GluR1. Proteins were expressed in Xenopus oocytes by injection of complementary RNA (cRNA) encoding GluR1, SGK isoforms, and/or stargazin. In oocytes expressing GluR1 6 days after cRNA injection, glutamate induced an inward current (IGlu), which was increased approximately fourfold following coexpression of SGK3. Coexpression of stargazin similarly enhanced IGlu. Coexpression of both SGK3 and stargazin stimulated the current by a factor of 15.5. Replacement of the serine by alanine at the only SGK consensus sequence (RXRXXS/T) in stargazin enhanced the efficacy of stargazin but did not prevent further stimulation of IGlu by additional coexpression of SGK3. Western blotting showed that stargazin accelerated membrane insertion of GluR1 protein leading to enhanced GluR1 plasma membrane protein abundance 2 days, but not 6 days, after cRNA injection, while SGK3 increased plasma membrane protein abundance 6 days after cRNA injection. In conclusion, SGK3 and stargazin regulate GluR1 independently, and thus, their effects on glutamate-induced currents are additive.