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Structure of aspartoacylase, the brain enzyme impaired in Canavan disease.

ABSTRACT: Aspartoacylase catalyzes hydrolysis of N-acetyl-l-aspartate to aspartate and acetate in the vertebrate brain. Deficiency in this activity leads to spongiform degeneration of the white matter of the brain and is the established cause of Canavan disease, a fatal progressive leukodystrophy affecting young children. We present crystal structures of recombinant human and rat aspartoacylase refined to 2.8- and 1.8-A resolution, respectively. The structures revealed that the N-terminal domain of aspartoacylase adopts a protein fold similar to that of zinc-dependent hydrolases related to carboxypeptidases A. The catalytic site of aspartoacylase shows close structural similarity to those of carboxypeptidases despite only 10-13% sequence identity between these proteins. About 100 C-terminal residues of aspartoacylase form a globular domain with a two-stranded beta-sheet linker that wraps around the N-terminal domain. The long channel leading to the active site is formed by the interface of the N- and C-terminal domains. The C-terminal domain is positioned in a way that prevents productive binding of polypeptides in the active site. The structures revealed that residues 158-164 may undergo a conformational change that results in opening and partial closing of the channel entrance. We hypothesize that the catalytic mechanism of aspartoacylase is closely analogous to that of carboxypeptidases. We identify residues involved in zinc coordination, and propose which residues may be involved in substrate binding and catalysis. The structures also provide a structural framework necessary for understanding the deleterious effects of many missense mutations of human aspartoacylase.

SUBMITTER: Bitto E

PROVIDER: S-EPMC1766406 | biostudies-literature | 2007 Jan

REPOSITORIES: biostudies-literature

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Structure of aspartoacylase, the brain enzyme impaired in Canavan disease.

Bitto Eduard E Bingman Craig A CA Wesenberg Gary E GE McCoy Jason G JG Phillips George N GN

Proceedings of the National Academy of Sciences of the United States of America 20061228 2

Aspartoacylase catalyzes hydrolysis of N-acetyl-l-aspartate to aspartate and acetate in the vertebrate brain. Deficiency in this activity leads to spongiform degeneration of the white matter of the brain and is the established cause of Canavan disease, a fatal progressive leukodystrophy affecting young children. We present crystal structures of recombinant human and rat aspartoacylase refined to 2.8- and 1.8-A resolution, respectively. The structures revealed that the N-terminal domain of aspart ...[more]

PMID: 17194761

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Project description:N-Acetylaspartate (NAA) is the second most abundant organic metabolite in the brain, but its physiological significance remains enigmatic. Toxic NAA accumulation appears to be the key factor for neurological decline in Canavan disease-a fatal neurometabolic disorder caused by deficiency in the NAA-degrading enzyme aspartoacylase. To date clinical outcome of gene replacement therapy for this spongiform leukodystrophy has not met expectations. To identify the target tissue and cells for maximum anticipated treatment benefit, we employed comprehensive phenotyping of novel mouse models to assess cell type-specific consequences of NAA depletion or elevation. We show that NAA-deficiency causes neurological deficits affecting unconscious defensive reactions aimed at protecting the body from external threat. This finding suggests, while NAA reduction is pivotal to treat Canavan disease, abrogating NAA synthesis should be avoided. At the other end of the spectrum, while predicting pathological severity in Canavan disease mice, increased brain NAA levels are not neurotoxic per se. In fact, in transgenic mice overexpressing the NAA synthesising enzyme Nat8l in neurons, supra-physiological NAA levels were uncoupled from neurological deficits. In contrast, elimination of aspartoacylase expression exclusively in oligodendrocytes elicited Canavan disease like pathology. Although conditional aspartoacylase deletion in oligodendrocytes abolished expression in the entire CNS, the remaining aspartoacylase in peripheral organs was sufficient to lower NAA levels, delay disease onset and ameliorate histopathology. However, comparable endpoints of the conditional and complete aspartoacylase knockout indicate that optimal Canavan disease gene replacement therapies should restore aspartoacylase expression in oligodendrocytes. On the basis of these findings we executed an ASPA gene replacement therapy targeting oligodendrocytes in Canavan disease mice resulting in reversal of pre-existing CNS pathology and lasting neurological benefits. This finding signifies the first successful post-symptomatic treatment of a white matter disorder using an adeno-associated virus vector tailored towards oligodendroglial-restricted transgene expression.

Dataset Information

Structure of aspartoacylase, the brain enzyme impaired in Canavan disease.

Publications

Structure of aspartoacylase, the brain enzyme impaired in Canavan disease.

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