Developmental and regional distribution of aspartoacylase in rat brain tissue

Developmental increase of aspartoacylase in oligodendrocytes parallels CNS myelination

Canavan disease, an autosomal-recessive neurogenetic disorder, is caused by mutations in aspartoacylase, an enzyme that deacetylates N-acetylaspartate to generate free acetate in the brain. Earlier studies have shown that aspartoacylase is primarily restricted to myelin synthesizing cells (oligodendroglia) in the CNS. These findings have led us to investigate the developmental expression of aspartoacylase gene in the rat brain in an attempt to shed more light on the role of this enzyme in myelination. In situ hybridization using a 35S riboprobe based on murine aspartoacylase cDNA was used in this study. The probe hybridized mostly to the white matter tracts with different densities depending on the age of the animal and region of the brain examined. Little or no hybridization signals were detected in the 1-day-old rats, whereas the signal was clearly detectable in most of the white matter regions of the CNS in the 11-day-old rats. The signal density markedly increased at postnatal day 17, the peak of myelination. Thereafter, the hybridization signals decreased somewhat but still could be observed in the adult animals. Thus, the developmental expression pattern of aspartoacylase gene in the postnatal brain closely parallels myelination in the CNS. In the CNS, the hybridization signal of ASPA appeared to be restricted primarily to oligodendrocytes, the primary myelin synthesizing cell type in the CNS. However, the signal was not detectable in rat sciatic nerve (Schwann cells) of the peripheral nervous system. These findings indicate that the role of N-acetylaspartate in myelin synthesis is restricted to the CNS. Furthermore, they provide additional support for the acetate deficiency hypothesis of Canavan disease and also make a stronger case for acetate supplementation as an immediate and inexpensive therapy for Canavan disease.

Aspartoacylase is restricted primarily to myelin synthesizing cells in the CNS: therapeutic implications for Canavan disease

Canavan disease is a devastating neurodegenerative childhood disease caused by mutations in aspartoacylase, an enzyme that deacetylates N-acetylaspartate to generate free acetate in the brain. Localization of aspartoacylase in different cell types in the rat brain was examined in an attempt to understand the pathogenesis of Canavan disease. In situ hybridization histochemistry with a riboprobe based on murine aspartoacylase cDNA was used in this study. The hybridization signal was detectable primarily in the myelin-synthesizing cells, namely oligodendroglia. These findings provide strong additional support for insufficient myelin synthesis as the pathogenic basis of Canavan disease and make a compelling case for acetate supplementation as a simple and noninvasive therapy for this fatal disease with no treatment.

Reduced extrahippocampal NAA in mesial temporal lobe epilepsy

PURPOSE

Structural and metabolic abnormalities in the hippocampal region in medial temporal lobe epilepsy (mTLE) are well described; less is known about extrahippocampal changes. This study was designed to characterize extrahippocampal metabolic abnormalities in mTLE with magnetic resonance spectroscopy in combination with tissue segmentation and volumetry of gray and white matter.

METHODS

Multislice magnetic resonance spectroscopic imaging (1H-MRSI) in combination with tissue segmentation was performed on 16 patients with mTLE and 12 age-matched healthy volunteers. The data were analyzed by using a regression-analysis model that estimated the metabolite concentrations in 100% cortical gray and 100% white matter in the frontal lobe and nonfrontal brain. The segmented image was used to calculate the fraction of gray and white matter in these regions.

RESULTS

mTLE had significantly lower N-acetyl aspartate (NAA) in ipsi- and contralateral frontal gray (p = 0.03) and in ipsi- and contralateral nonfrontal white matter (p = 0.008) compared with controls. Although there were no associated volumetric deficits in frontal gray and white matter, ipsilateral nonfrontal gray matter (p = 0.003) was significantly smaller than that in controls.

CONCLUSIONS

mTLE is associated with extrahippocampal metabolic abnormalities and volumetric deficits, but these do not necessarily affect the same regions.

A radiometric assay for aspartoacylase activity in cultured oligodendrocytes

Recent studies have shown that aspartoacylase (ASPA), the defective enzyme in Canavan disease, is detectable in the brain only in the oligodendrocytes. Studying the regulation of ASPA is central to the understanding the pathogenesis of Canavan disease and to the development of therapeutic strategies. Toward this goal, we have developed a sensitive method for the assay of ASPA in cultured oligodendrocytes. The method involves: (a) chemical synthesis of [14C]N-acetylaspartate (NAA) from L-[14C]Asp; (b) use of [14C]NAA as substrate in the assay; and (c) separation and quantitation of the product L-[14C]Asp using a TLC system. This method can detect as low as 10pmol of product and has been optimized for cultured oligodendrocytes. Thus, this method promises to be a valuable tool for understanding the biochemical mechanisms involved in the cell-specific expression and regulation of ASPA in oligodendrocytes.

Clinical protocol. Gene therapy of Canavan disease: AAV-2 vector for neurosurgical delivery of aspartoacylase gene (ASPA) to the human brain

This clinical protocol describes virus-based gene transfer for Canavan disease, a childhood leukodystrophy. Canavan disease, also known as Van Bogaert-Bertrand disease, is a monogeneic, autosomal recessive disease in which the gene coding for the enzyme aspartoacylase (ASPA) is defective. The lack of functional enzyme leads to an increase in the central nervous system of the substrate molecule, N-acetyl-aspartate (NAA), which impairs normal myelination and results in spongiform degeneration of the brain. No effective treatment currently exists; however, virus-based gene transfer has the potential to arrest or reverse the course of this otherwise fatal condition. This procedure involves neurosurgical administration of approximately 900 billion genomic particles (approximately 10 billion infectious particles) of recombinant adeno-associated virus (AAV) containing the aspartoacylase gene (ASPA) directly to affected regions of the brain in each of 21 patients with Canavan disease. Pre- and post-delivery assessments include a battery of noninvasive biochemical, radiological, and neurological tests. This gene transfer study represents the first clinical use of AAV in the human brain and the first instance of viral gene transfer for a neurodegenerative disease.

In vivo magnetic resonance spectroscopy and its application to neuropsychiatric disorders

In vivo magnetic resonance spectroscopy (MRS) is the only noninvasive imaging technique that can directly assess the living biochemistry in localized brain regions. In the past decade, spectroscopy studies have shown biochemical alterations in various neuropsychiatric disorders. These first-generation studies have, in most cases, been exploratory but have provided insightful biochemical information that has furthered our understanding of different brain disorders. This review provides a brief description of spectroscopy, followed by a literature review of key spectroscopy findings in schizophrenia, affective disorders, and autism. In schizophrenia, phosphorus spectroscopy studies have shown altered metabolism of membrane phospholipids (MPL) during the early course of the illness, which is consistent with a neurodevelopmental abnormality around the critical period of adolescence when the illness typically begins. Children and adolescents who are at increased genetic risk for schizophrenia show similar MPL alterations, suggesting that schizophrenia subjects with a genetic predisposition may have a premorbid neurodevelopmental abnormality. Independent of medication status, bipolar subjects in the depressive state tended to have higher MPL precursor levels and a deficit of high-energy phosphate metabolites, which also is consistent with major depression, though these results varied. Further bipolar studies are needed to investigate alterations at the early stage. Lastly, associations between prefrontal metabolism of high-energy phosphate and MPL and neuropsychological performance and reduced N-acetylaspartate in the temporal and cerebellum regions have been reported in individuals with autism. These findings are consistent with developmental alterations in the temporal lobe and in the cerebellum of persons with autism. This paper discusses recent findings of new functions of N-acetylaspartate.