Journal
JOURNAL OF NEUROCHEMISTRY
Volume 105, Issue 4, Pages 1287-1299Publisher
BLACKWELL PUBLISHING
DOI: 10.1111/j.1471-4159.2008.05229.x
Keywords
acyl coenzyme A-binding protein secretion; astrocytes; direction selectivity; glial cells; Muller; QNR/K2 cells; retina; threonine phosphorylation
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Funding
- NEI NIH HHS [EY018561, EY04778] Funding Source: Medline
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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 Muller cells. If the expressed ACBP were also secreted by Muller 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 Muller glial cells and Muller-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 Muller cell secretion of phosphorylated ACBP could influence the GABAergic transmission in neighboring retinal neurons.
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