Post‐translational regulation of EAAT2 function by co‐expressed ubiquitin ligase Nedd4‐2 is impacted by SGK kinases

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Abstract
The human excitatory amino acid transporter (EAAT)2 is the major glutamate carrier in the mammalian CNS. Defective expression of the transporter results in neuroexcitotoxicity that may contribute to neuronal disorders such as amyotrophic lateral sclerosis (ALS). The serum and glucocorticoid inducible kinase (SGK) 1 is expressed in the brain and is known to interact with the ubiquitin ligase Nedd4‐2 to modulate membrane transporters and ion channels. The present study aimed to investigate whether SGK isoforms and the related kinase, protein kinase B (PKB), regulate EAAT2. Expression studies in Xenopus oocytes demonstrated that glutamate‐induced inward current (IGLU) was stimulated by co‐expression of SGK1, SGK2, SGK3 or PKB. IGLU is virtually abolished by Nedd4‐2, an effect abrogated by additional co‐expression of either kinase. The kinases diminish the effect through Nedd4‐2 phosphorylation without altering Nedd4‐2 protein abundance. SGKs increase the transporter maximal velocity without significantly affecting substrate affinity. Similar to glutamate‐induced currents, [3H] glutamate uptake and cell surface abundance of the transporter were increased by the SGK isoforms and down‐regulated by the ubiquitin ligase Nedd4‐2. In conclusion, all three SGK isoforms and PKB increase EAAT2 activity and plasma membrane expression and thus, may participate in the regulation of neuroexcitability.