Bimodal occurrence of aspartoacylase in myelin and cytosol of brain

Short-term cuprizone feeding verifies N-acetylaspartate quantification as a marker of neurodegeneration

Proton magnetic resonance spectroscopy (1H-MRS) is a quantitative MR imaging technique often used to complement conventional MR imaging with specific metabolic information. A key metabolite is the amino acid derivative N-Acetylaspartate (NAA) which is an accepted marker to measure the extent of neurodegeneration in multiple sclerosis (MS) patients. NAA is catabolized by the enzyme aspartoacylase (ASPA) which is predominantly expressed in oligodendrocytes. Since the formation of MS lesions is paralleled by oligodendrocyte loss, NAA might accumulate in the brain, and therefore, the extent of neurodegeneration might be underestimated. In the present study, we used the well-characterized cuprizone model. There, the loss of oligodendrocytes is paralleled by a reduction in ASPA expression and activity as demonstrated by genome-wide gene expression analysis and enzymatic activity assays. Notably, brain levels of NAA were not increased as determined by gas chromatography-mass spectrometry and 1H-MRS. These important findings underpin the reliability of NAA quantification as a valid marker for the paraclinical determination of the extent of neurodegeneration, even under conditions of oligodendrocyte loss in which impaired metabolization of NAA is expected. Future studies have to reveal whether other enzymes are able to metabolize NAA or whether an excess of NAA is cleared by other mechanisms rather than enzymatic metabolism.

Localisation of N-acetylaspartate in oligodendrocytes/myelin

The role of N-acetylaspartate in the brain is unclear. Here we used specific antibodies against N-acetylaspartate and immunocytochemistry of carbodiimide-fixed adult rodent brain to show that, besides staining of neuronal cell bodies in the grey matter, N-acetylaspartate labelling was present in oligodendrocytes/myelin in white matter tracts. Immunoelectron microscopy of the rat hippocampus showed that N-acetylaspartate was concentrated in the myelin. Also neuronal cell bodies and axons contained significant amounts of N-acetylaspartate, while synaptic elements and astrocytes were low in N-acetylaspartate. Mitochondria in axons and neuronal cell bodies contained higher levels of N-acetylaspartate compared to the cytosol, compatible with synthesis of N-acetylaspartate in mitochondria. In aspartoacylase knockout mice, in which catabolism of N-acetylaspartate is blocked, the levels of N-acetylaspartate were largely increased in oligodendrocytes/myelin. In these mice, the highest myelin concentration of N-acetylaspartate was found in the cerebellum, a region showing overt dysmyelination. In organotypic cortical slice cultures there was no evidence for N-acetylaspartate-induced myelin toxicity, supporting the notion that myelin damage is induced by the lack of N-acetylaspartate for lipid production. Our findings also implicate that N-acetylaspartate signals on magnetic resonance spectroscopy reflect not only vital neurons but also vital oligodendrocytes/myelin.

Extensive aspartoacylase expression in the rat central nervous system

Aspartoacylase (ASPA) catalyzes deacetylation of N-acetylaspartate (NAA) to generate acetate and aspartate. Mutations in the gene for ASPA lead to reduced acetate availability in the CNS during development resulting in the fatal leukodystrophy Canavan disease. Highly specific polyclonal antibodies to ASPA were used to examine CNS expression in adult rats. In white matter, ASPA expression was associated with oligodendrocyte cell bodies, nuclei, and some processes, but showed a dissimilar distribution pattern to myelin basic protein and oligodendrocyte specific protein. Microglia expressed ASPA in all CNS regions examined, as did epiplexus cells of the choroid plexus. Pial and ependymal cells and some endothelial cells were ASPA positive, as were unidentified cellular nuclei throughout the CNS. Astrocytes did not express ASPA in their cytoplasm. In some fiber pathways and nerves, particularly in the brainstem and spinal cord, the axoplasm of many neuronal fibers expressed ASPA, as did some neurons. Acetyl coenzyme A synthase immunoreactivity was also observed in the axoplasm of many of the same fiber pathways and nerves. All ASPA-immunoreactive elements were unstained in brain sections from tremor rats, an ASPA-null mutant. The strong expression of ASPA in oligodendrocyte cell bodies is consistent with a lipogenic role in myelination. Strong ASPA expression in cell nuclei is consistent with a role for NAA-derived acetate in nuclear acetylation reactions, including histone acetylation. Expression of ASPA in microglia may indicate a role in lipid synthesis in these cells, whereas expression in axons suggests that some neurons can both synthesize and catabolize NAA.

Diagnostic methods for extra-temporal neocortical focal epilepsies: present and future

The progress of epilepsies diagnosis has been great, but, amongst the diagnostic detailing that demand research, one of the most important is the essential lateralization and localization of epileptogenic zone, considered as the cerebral cortex region, that removed, will result in a free state of seizures. The present study aims to analyze the possible uses of proton spectroscopy for clinical and pre-surgical evaluation of focal extratemporal epilepsies, since this group presents the highest difficulty degree for lateralizing and locating epileptogenic zones. In almost all cases, a non invasive diagnosis can be performed using routine electroencephalography, video-electroencephalography - considered as gold standard, and magnetic resonance imaging. However, when the results of these exams are contradictory, some patients need invasive techniques, as the intra-cranial video-EEG, using deep electrodes, sub-dural strip and grid, that are associated with increased diagnostic cost and risk of complications, as cerebral hemorrhages and intra-cranial infections. Proton spectroscopy appears as a possibility, given its capacity to evaluate cerebral metabolism, by N-acetyl-aspartate (NAA), creatine (Cre) and choline (Cho) concentrations, amongst other metabolites. This non invasive method may provide time reduction of this evaluation and reliable level improvement for this topographical diagnosis.

Upregulation of N-acetylaspartic acid resulting nitric oxide toxicity induces aspartoacylase mutations and protein interaction to cause pathophysiology seen in Canavan disease

Aspartoacylase (ASPA) converts N-acetylaspartic acid into aspartate and acetate. In Canavan disease (CD), N-acetylaspartic acid (NAA) is found to be increased and over 65 mutations including IVS4+1 G → T, deletion of introns and exons have been reported in the ASPA gene. These changes lead to severe form or mild form of CD. The present study was aimed to understand mechanism in the cause of mutations in ASPA and pathophysiology seen in patients with CD. We have reported that elevated levels of NAA induce inducible nitric oxide (iNOS) to produce nitric oxide toxicity in CD. Nitric oxide toxicity has been shown to induce several mutations including base change G → T and deletion and enhances protein interaction in several genes. Therefore we hypothesize that upregulation of NAA stimulates NOS and the resulting nitric oxide toxicity induces ASPA mutations and protein interaction to result pathophysiological abnormalities seen in patients with CD.

Aspartoacylase deficiency affects early postnatal development of oligodendrocytes and myelination

Canavan disease (CD) is a neurodegenerative disease, caused by a deficiency in the enzyme aspartoacylase (ASPA). This enzyme has been localized to oligodendrocytes; however, it is still undefined how ASPA deficiency affects oligodendrocyte development. In normal mice the pattern of ASPA expression coincides with oligodendrocyte maturation. Therefore, postnatal oligodendrocyte maturation was analyzed in ASPA-deficient mice (CD mice). Early in development, CD mice brains showed decreased expression of neural cell markers that was later compensated. In addition, the levels of myelin proteins were decreased along with abnormal myelination in CD mice compared to wild-type (WT). These defects were associated with increased global levels of acetylated histone H3, decreased chromatin compaction and increased GFAP protein, a marker for astrogliosis. Together, these findings strongly suggest that, early in postnatal development, ASPA deficiency affects oligodendrocyte maturation and myelination.

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.