The regulation of human choline acetyltransferase

Abstract

Alzheimer's disease, a condition marked by progressive dementia, is due in part to a deficit in cholinergic metabolism in the basal forebrain. The most reliable marker for cholinergic neurons is choline acetyltransferase, abbreviated ChAT, the biosynthetic enzyme for acetylcholine synthesis. Levels of this enzyme are found to be reduced as much as 40 to 90 percent in the cortex of patients who die of Alzheimer's disease. It is with this interest that the following studies on the regulation of choline acetyltransferase are presented.;The regulation of choline acetyltransferase activity was studied in a human cholinergic neuroblastoma cell line, MC-IXC. It was found that cell passage, cell density, retinoic acid and sodium butyrate were able to increase ChAT activity on a per cell or per milligram protein basis. These activating conditions could only act on cell lines which already expressed some ChAT activity. In the cell culture system, retinoic acid and sodium butyrate activated ChAT to varying extents and had differential effects on acetylcholinesterase activity. The effects of retinoic acid and cell density were reversible by trypsinization of the cultures. Whereas other retinoids were able to stimulate ChAT activity, analogs of butyrate, such as gamma-amino butyric acid or beta-hydroxy butyrate were unable to mimic its effects. Serum deprivation and dimethylsulfoxide treatment were unable to stimulate ChAT activity.;Relying on specific activity as a probe for ChAT, the mechanism(s) by which retinoic acid and sodium butyrate was (were) able to stimulate ChAT activity was (were) further examined. Agents which inhibited mRNA or protein synthesis had no effect on the activation process. Retinoic acid and sodium butyrate treatment caused increases in the V{dollar}\sb{lcub}\rm max{rcub}{dollar} of ChAT but had no effect on the K{dollar}\sb{lcub}\rm m{rcub}{dollar} for either of its substrates. Agents which affect the activity of the protein kinase C or cyclic AMP-dependent protein kinase second messenger systems had no effect on the activation process, although retinoic acid treatment did result in a three-fold increase in cAMP levels. The evidence presented here suggests that ChAT activation by retinoic acid and sodium butyrate is post-translational in this neuroblastoma cell line.