Mutations in Cu, Zn superoxide dismutase (SOD1) are a confirmed cause of amyotrophic lateral sclerosis (ALS), selective death of motor neurons, which endows the enzyme with a toxic gain of function. In the presence of mSOD1, motor neurons are subjected to glutamate excitoxicity via the NMDA receptor. D-serine is an endogenous neurotransmitter that binds to the NMDA receptor, thereby increasing the affinity for glutamate. Production of D-Serine in vivo is thought to result from pyridoxal-5'phosphate dependent serine racemase (SR) acting on L-serine. D-Serine is increased two-fold in spinal cords of G93A SOD1 mice—the standard model of ALS. Chapter two shows that disrupting SR in ALS mice decreases D-Serine to non-transgenic levels. These mice show symptom onset earlier, but survive longer in an SR-dependent manner. Paradoxically, feeding D-Serine to ALS mice dramatically lowers cord levels of D-Serine, leading to changes in onset and survival very similar to SR deletion.
Extensive time course studies in chapter three reveal that this drop in spinal cord D-serine levels occurs almost immediately after D-serine administration begins, and SR expression increases over time which could be explained by the eliminase activity of SR. Glycine and GABA were found to be reduced in treated mice at the time symptoms manifested, which may play a role in the premature symptom onset. The exploratory time course study of Srr -/-:G93A mice in chapter four, showed that levels of D-serine may be higher at earlier stages of disease, and glycine and GABA are lower, possibly resulting in the earlier presentation of onset. The D-serine in these knockout animals is likely from some other enzymatic origin, and in chapter five, we employed 2D DIGE proteomics to identify uncharacterized proteins which are reactive to serine racemase antibodies, and to identify proteins which are altered in expression by D-serine treatment. The identity of these proteins could be important keys to determining the protective effects we observed and may also prove to be therapeutic targets.
Although the mechanism by which SOD1 mutations increases D-Serine is not known, these results strongly suggest that SR and D-Serine are fundamentally involved in both the presymptomatic and progression phases of disease, and offer a direct link between mutant SOD1 and a glial-derived toxic mediator.
|Advisor:||Crow, John P.|
|Commitee:||Drew, Paul, Gottschall, Paul, Macmillan-Crow, Lee Ann, Rusch, Nancy|
|School:||University of Arkansas for Medical Sciences|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 72/08, Dissertation Abstracts International|
|Subjects:||Neurosciences, Toxicology, Surgery, Pharmacology|
|Keywords:||Amyotrophic lateral sclerosis, D-serine, G93A mSOD1, Glutamate excitoxicity, Motor neurons, Superoxide dismutase|
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