title: Acyl coenzyme A-binding protein (ACBP) is phosphorylated and secreted by retinal Müller astrocytes following protein kinase C activation.
abstract: 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.
authors.affiliation: Neurological Sciences Institute, Oregon Health & Science University, Beaverton, Oregon 97006, USA. Qianz@ohsu.edu
authors.forename: Zuyuan
authors.initials: Z
authors.lastname: Qian
authors.forename: Timothy R
authors.initials: TR
authors.lastname: Bilderback
authors.forename: Neal H
authors.initials: NH
authors.lastname: Barmack
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000201:enzymology* / Q000378:metabolism*; D002478:Cells, Cultured; D026261:Diazepam Binding Inhibitor / Q000378:metabolism*; D004789:Enzyme Activation / Q000502:physiology; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D011817:Rabbits; D012160:Retina / Q000201:enzymology* / Q000378:metabolism*
chemical_list: D026261:Diazepam Binding Inhibitor; D011493:Protein Kinase C