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Acyl coenzyme A‐binding protein (ACBP) is phosphorylated and secreted by retinal Müller astrocytes following protein kinase C activation
Author(s) -
Qian Zuyuan,
Bilderback Timothy R.,
Barmack Neal H.
Publication year - 2008
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.2008.05229.x
Subject(s) - phosphorylation , threonine , biology , protein kinase c , biochemistry , kinase , protein kinase a , protein phosphorylation , autophosphorylation , microbiology and biotechnology , serine
Horizontal optokinetic stimulation of rabbit retina in vivo evokes increased expression of acyl coenzyme A‐binding protein (ACBP), also known as ‘diazepam binding inhibitor,’ from retinal Müller cells. If the expressed ACBP were also secreted by Müller cells, then stimulus‐evoked secretion of ACBP could influence the activity of GABA A receptor‐expressing retinal neurons. In this study, we examine in vitro whether ACBP is secreted by Müller glial cells and Müller‐like QNR/K2 cells following stimulation with elevated levels of KCl and phorbol myristic acetate (PMA). KCl and PMA stimulation evoked secretion of threonine‐phosphorylated ACBP. A sequence analysis of ACBP shows that it has five potential phosphorylation sites: Two threonine sites fit a protein kinase C phosphorylation pattern. Two threonine sites fit a casein kinase II (CK2) pattern. One serine site fits a CK2 pattern. As CK2 is not expressed in QNR/K2 cells, it is probable that protein kinase C accounts for the phosphorylation of ACBP in these cells and for the PMA‐evoked secretion of ACBP. Serine phosphorylation was constitutive. Horizontal optokinetic stimulation increased threonine‐phosphorylated ACBP in rabbit retina. Phosphorylation of ACBP may influence its target affinity. We used a proteolytic fragment of ACBP, octadecaneuropeptide (ODN), to investigate how threonine phosphorylation influences its affinity for GABA A receptors. Threonine‐phosphorylated ODN had a stronger affinity for GABA A receptors than did unphosphorylated ODN or unphosphorylated ACBP. We conclude that stimulus‐induced Müller cell secretion of phosphorylated ACBP could influence the GABAergic transmission in neighboring retinal neurons.