Turnover of myelin lipids in aging brain.

Turnover rates of myelin membrane components in mouse brains were determined by a method using stable isotope-labeling and mass spectrometry. The half-replacement times based on incorporation rates of newly synthesized molecules for young adult mice were 359 days for cholesterol, 20 days for phosphatidylcholine, 25 days for phosphatidylethanolamine, 94 days for cerebroside and 102 days for ganglioside GM1. The turnover rates of half-lives of myelin components were calculated from the decay curves of initially labeled molecules, and they were about the same as the half-replacement times. Individual components were thus revealed to be metabolized at different rates, and their turnover rates were differently affected by aging. As was observed with phospholipids, myelin pools appeared to be compartmentalized into rapidly and slowly exchanging pools. The turnover rates of cerebroside and GM1 decreased between the young and adult periods and slightly increased in senescence. The latter phenomenon may indicate an enhanced myelin turnover in senescence. The present study reveals the dynamic aspects of myelin membrane turnover during the life span of mouse.