Myelin Lipid Abnormalities in the Aspartoacylase-Deficient Tremor Rat

The high concentration of N-acetylaspartate (NAA) in neurons of the central nervous system and its growing clinical use as an indicator of neuronal viability has intensified interest in the biological function of this amino acid derivative. The biomedical relevance of such inquiries is highlighted by the myelin-associated pathology of Canavan disease, an inherited childhood disorder resulting from mutation of aspartoacylase (ASPA), the NAA-hydrolyzing enzyme. This enzyme is known to be localized in oligodendrocytes with bimodal distribution in cytosol and the myelin sheath, and to produce acetyl groups utilized in myelin lipid synthesis. Loss of this acetyl source in Canavan disease and rodent models such as the tremor rat are thought to account for the observed myelin deficit. This study was undertaken to further define and quantify the specific lipid abnormalities that occur as a result of ASPA deficit in the tremor rat. Employing mass spectrometry together with high performance thin-layer chromatography, we found that myelin from 28-day-old animals showed major reduction in cerebrosides (CB) and sulfatides (Sulf) with unsubstituted fatty acids, and equal if not greater changes in myelin from 7-month-old tremors. Cerebrosides with 2-hydroxyfatty acids showed little if any change at either age; Sulf with 2-hydroxyfatty acids showed no significant change at 28 days, but surprisingly a major increase at 7 months. Two species of phosphatidylcholine, 32:0 and 34:1, also showed significant increase, but only at 28 days. One form of phosphatidylethanolamine, PE36:1, was reduced a modest amount at both ages, whereas the plasmalogen form did not change. The dysmyelination that results from inactivation of ASPA is thus characterized by selective decreases as well as some increases in specific lipids.