Defective N-acetylaspartate catabolism reduces brain acetate levels and myelin lipid synthesis in Canavan's disease

Abstract

Canavan's disease (CD) is a fatal, hereditary disorder of CNS development that has been linked to mutations in the gene for the enzyme aspartoacylase (ASPA) (EC 3.5.1.15). ASPA acts to hydrolyze N-acetylaspartate (NAA) into l-aspartate and acetate, but the connection between ASPA deficiency and the failure of proper CNS development is unclear. We hypothesize that one function of ASPA is to provide acetate for the increased lipid synthesis that occurs during postnatal CNS myelination. The gene encoding ASPA has been inactivated in the mouse model of CD, and here we show significant decreases in the synthesis of six classes of myelin-associated lipids, as well as reduced acetate levels, in the brains of these mice at the time of peak postnatal CNS myelination. Analysis of the lipid content of white matter from a human CD patient showed decreased cerebroside and sulfatide relative to normal white matter. These results demonstrate that myelin lipid synthesis is significantly compromised in CD and provide direct evidence that defective myelin synthesis, resulting from a deficiency of NAA-derived acetate, is involved in the pathogenesis of CD.

Figures

Fig. 1.

ASPA (A) and NAA (B) immunoreactivity in the rat corpus callosum. Oligodendrocytes are arranged in rows within the corpus callosum and are strongly stained for ASPA but only weakly stained for NAA (arrows in B). Neurons, such as those in the cortex, are stained much more intensely for NAA than are oligodendrocytes. (Scale bar, 120 μm.)

Fig. 2.

Lipid analysis in CD and normal human cerebral white matter (WM). (A) Note the relative paucity of bands comigrating with the cerebroside standards (black arrows) and relative lack of the more mobile sulfatide fraction (white arrow) in the CD patient sample compared with the control. Lane 1, standards mix; lane 2, CD patient white matter lipids; lane 3, control human white matter lipids; lane 4, sulfatides; lane 5, cerebrocides. (B) Note the prominence of the bands (comigrating with cerebroside) in the control specimen and their reduction in the CD patient specimen (white arrows). Note also the prominence of anomalous lipid bands in the CD patient (black arrows).

Fig. 3.

Proposed model for NAA synthesis and degradation. See Discussion for details. OAA, oxaloacetic acid; α-KG, α-ketoglutarate; AAT, aspartate aminotransferase; GDH, glutamate dehydrogenase; PDH, pyruvate dehydrogenase; F.A., fatty acids; Ac-CoA, acetyl CoA.