Retinal colocalization and in vitro interaction of the glutamate transporter EAAT3 and the serum- and glucocorticoid-inducible kinase SGK1 [correction].
OBJECTIVE
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.
Schniepp
Roman
R
Department of Physiology I, University of Tübingen, Germany.
Kohler
Konrad
K
Ladewig
Thomas
T
Guenther
Elke
E
Henke
Guido
G
Palmada
Monica
M
Boehmer
Christoph
C
Rothstein
Jeffrey D
JD
Bröer
Stefan
S
Lang
Florian
F
D027322:Amino Acid Transport System X-AG / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D050580:Excitatory Amino Acid Transporter 3; D019084:Fluorescent Antibody Technique, Indirect; D005822:Genetic Vectors; D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000378:metabolism; D017874:Immediate-Early Proteins; D009687:Nuclear Proteins; D018408:Patch-Clamp Techniques; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D018075:RNA, Complementary / Q000378:metabolism; D051381:Rats; D011914:Rats, Inbred BN; D012160:Retina / Q000378:metabolism*; D012165:Retinal Ganglion Cells / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D027981:Symporters / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis; D000099184:Serum-Glucocorticoid Regulated Kinases
D027322:Amino Acid Transport System X-AG; D050580:Excitatory Amino Acid Transporter 3; D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid; D017874:Immediate-Early Proteins; D009687:Nuclear Proteins; D017346:Protein Serine-Threonine Kinases; D018075:RNA, Complementary; D012964:Sodium; D027981:Symporters; D000099184:Serum-Glucocorticoid Regulated Kinases; C493398:Slc1a1 protein, rat