Synthesis of acetylcholine from choline derived from phosphatidylcholine in a human neuronal cell line.

Cholinergic neurons are unique among cells since they alone utilize choline not only as a component of major membrane phospholipids, such as phosphatidylcholine (Ptd-Cho), but also as a precursor of their neurotransmitter acetylcholine (AcCho). It has been hypothesized that choline-phospholipids might serve as a storage pool of choline for AcCho synthesis. The selective vulnerability of cholinergic neurons in certain neurodegenerative diseases (e.g., Alzheimer disease, motor neuron disorders) might result from the abnormally accelerated liberation of choline (to be used as precursor of AcCho) from membrane phospholipids, resulting in altered membrane composition and function and compromised neuronal viability. However, the proposed metabolic link between membrane turnover and AcCho synthesis has been difficult to demonstrate because of the heterogeneity of the preparations used. Here we used a population of purely cholinergic cells (human neuroblastoma, LA-N-2), incubated in the presence of [methyl-3H]methionine to selectively label PtdCho synthesized by methylation of phosphatidylethanolamine, the only pathway of de novo choline synthesis. PtdCho, purified by thin-layer chromatography, contained 90% of the label incorporated into lipids, demonstrating that LA-N-2 cells contained phosphatidylethanolamine N-methyltransferase. Three peaks of radioactive material that cochromatographed with authentic Ac-Cho, choline, and phosphocholine were observed when the water-soluble metabolites of the [3H]PtdCho were purified by high-performance liquid chromatography. Their identities were ascertained by subjecting them to enzymatic modifications with acetylcholinesterase, choline oxidase, and alkaline phosphatase, respectively. The results demonstrate that AcCho can be synthesized from choline derived from the degradation of endogenous PtdCho formed de novo by methylation of phosphatidylethanolamine.