Metabolism and function of N-acetylaspartate, N-acetylaspartylglutamate and related metabolites
Final Report Abstract
N-acetylaspartate (NAA) and the peptide N-acetylaspartylglutamate (NAAG) are present in high concentrations in the brain of vertebrates. The physiological roles of these compounds are, however, not fully understood. One widely excepted hypothesis on the role of NAA is that it serves as a carrier to supply oligodendrocytes with acetyl-groups. According to this model, oligodendrocytes, which express high levels of the NAA degrading enzyme aspartoacylase (ASPA), use the liberated acetate for lipid synthesis during myelin formation. Deficiency in ASPA causes a severe leukodystrophy, Canavan disease. This disease is characterized by a progressive loss of myelin accompanied by a severe spongy degeneration of the brain's white matter. Loss of myelin is thought to be a result of insufficient acetate supply. The results of our project, however, disproved this hypothesis. When the NAA synthesizing enzyme (NAA synthase) is inactivated in ASPA-deficient mice, an established animal model of Canavan disease, spongy degeneration, loss of myelin and changes in specific myelin lipids is fully prevented. These observations prove that the myelin phenotype of ASPA-deficient mice is caused by the accumulation of NAA and not by the absence of any NAA-degradation products due to the deficiency of the ASPA enzyme. It strongly suggests that NAA accumulation causes spongy degeneration of white matter tracts and that this primary event is responsible for myelin loss. An important implication of this result is that inhibition of the NAA synthase is a promissing therapeutic option for Canavan disease. The dipeptide NAAG is synthesized by NAAG synthases in many neurons in the brain and spinal cord of vertebrates. It is believed to be a neuropeptide that may be released at synapses as a co-transmitter with neurotransmitters like glutamate or GABA. This of cause requires the uptake of NAAG into synaptic vesicles. Our studies identified the membrane protein sialin as a vesicular NAAG transporter. In most cells sialin is localized to lysosomes and responsible for the transport of sialic acids from the lysosome into the cytosol. In neurons, however, sialin also localize to synaptic vesicles. Uptake of NAAG could be measured in isolated synaptic vesicles isolated from mouse brains. In contrast, when synaptic vesicles were isolated from sialin-deficient mice, NAAG uptake was no longer detectable. Furthermore, the sialin-dependent NAAG transport could be confirmed using purified sialin incorporated into artificial vesicles (liposomes).
Publications
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2013. Vesicular uptake of N-acetylaspartylglutamate is catalysed by sialin (SLC17A5). Biochem. J. 454:31-38
Lodder-Gadaczek J, Gieselmann V, Eckhardt M
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2015. A commercial human protamine-2 antibody used in several studies to detect mouse protamine-2 recognizes mouse transition protein-2 but not protamine-2. Mol. Hum. Reprod. 21:825-831
Eckhardt M, Wang-Eckhardt L
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2015. N-Acetylaspartate Synthase Deficiency Corrects the Myelin Phenotype in a Canavan Disease Mouse Model But Does Not Affect Survival Time. J. Neurosci. 35:14501-14516
Maier H, Wang-Eckhardt L, Hartmann D, Gieselmann V, Eckhardt M
