Reduced N-Acetylaspartate Levels in Mice Lacking Aralar, a Brain- and Muscle-type Mitochondrial Aspartate-glutamate Carrier

Abstract

Aralar is a mitochondrial calcium-regulated aspartate-glutamate carrier mainly distributed in brain and skeletal muscle, involved in the transport of aspartate from mitochondria to cytosol, and in the transfer of cytosolic reducing equivalents into mitochondria as a member of the malate-aspartate NADH shuttle. In the present study, we describe the characteristics of aralardeficient (Aralar ؊/؊ ) mice, generated by a gene-trap method, showing no aralar mRNA and protein, and no detectable malate-aspartate shuttle activity in skeletal muscle and brain mitochondria. Aralar ؊/؊ mice were growth-retarded, exhibited generalized tremoring, and had pronounced motor coordination defects along with an impaired myelination in the central nervous system. Analysis of lipid components showed a marked decrease in the myelin lipid galactosyl cerebroside. The content of the myelin lipid precursor, N-acetylaspartate, and that of aspartate are drastically decreased in the brain of Aralar ؊/؊ mice. The defect in N-acetylaspartate production was also observed in cell extracts from primary neuronal cultures derived from Aralar ؊/؊ mouse embryos. These results show that aralar plays an important role in myelin formation by providing aspartate for the synthesis of N-acetylaspartate in neuronal cells.

N-Acetylaspartate in the CNS: From neurodiagnostics to neurobiology

“…Aralar1 moves aspartate out of mitochondria, and glutamate into mitochondria, and works in conjunction with the enzyme aspartate aminotransferase, which is another integral part of the malate-aspartate shuttle. Results with aralar1 −/− mice also provide support for the bioenergetic role for NAA linked to the aspartatemalate shuttle (Jalil et al, 2005). Aralar1 is the only mitochondrial aspartate-glutamate carrier in the brain, and aralar1 −/− mice lack malate-aspartate shuttle activity, have dramatically reduced brain NAA and aspartate levels, and show reduced neuronal respiration on glutamate plus malate (Jalil et al, 2005).…”

“…It has also been shown that the gene for ASPA, which is expressed strongly in oligodendrocytes in the CNS, is not expressed significantly in the peripheral nervous system (Kirmani et al, 2003), suggesting a specific role for the NAA degrading enzyme in CNS myelination, but not in peripheral myelination mediated by Schwann cells. It is interesting in this regard that aralar1 −/− mice showed reduced levels of galactocerebrocides in the CNS, but not in the PNS, providing further support for a role of NAA-derived acetate in synthesis of specific lipids in the brain, but not in peripheral nerves (Jalil et al, 2005). Further, connexin Cx32 is a gap junction protein expressed in both oligodendrocytes and Schwann cells, and Cx32 mutations are associated with X-linked Charcot-Marie-Tooth disease, a peripheral demyelinating disease in which central myelination is unaffected (Menichella et al, 2003).…”

“…A recent study has shown that mice lacking the gene for aralar1, a mitochondrial aspartateglutamate transporter, demonstrated pathologies similar to ASPA −/− mice (Jalil et al, 2005), including dysmyelination ( Figure 6). The authors relate the dysmyelination observed in these mice to a lack of synthesis of aspartate, and resultant drastic reductions in NAA levels in the brain.…”

“…However, much like ASPA −/− mice, ACS1 knockout mice may display postnatal symptoms similar to Canavan disease due to reduced acetyl CoA availability for myelin-related lipogenesis. It is noteworthy that aralar −/− mice, which lack aspartate-malate shuttle activity, show large reductions in galactocerebrocide levels in the CNS, while many other lipid classes appear unaffected (Jalil et al, 2005).…”

Myelin Lipid Abnormalities in the Aspartoacylase-Deficient Tremor Rat

“…Both sources of lipid are apparently necessary for achieving the highly regulated lipid composition of myelin upon which optimal function depends. Additional evidence for NAA/ASPA participation in CNS myelinogenesis came from a recent study [33] showing that aralar (−/−) mice lacking the mitochondrial Asp-glutamate carrier have marked reduction of brain NAA along with reduced myelinogenesis. It will be of interest to explore how NAA deficits of the kind reported in several other neurological disorders [34] affect the chemical composition and fine structure of CNS myelin over time.…”