Paradoxical roles of serine racemase and d-serine in the G93A mSOD1 mouse model of amyotrophic lateral sclerosis

d-Serine is an endogenous neurotransmitter that binds to the NMDA receptor, thereby increasing the affinity for glutamate, and the potential for excitotoxicity. The primary source of d-serine in vivo is enzymatic racemization by serine racemase (SR). Regulation of d-serine in vivo is poorly understood, but is thought to involve a combination of controlled production, synaptic reuptake by transporters, and intracellular degradation by d-amino acid oxidase (DAO). However, SR itself possesses a well-characterized eliminase activity, which effectively degrades d-serine as well. d-Serine is increased two-fold in spinal cords of G93A Cu,Zn-superoxide dismutase (SOD1) mice the standard model of amyotrophic lateral sclerosis (ALS). ALS mice with SR disruption show earlier symptom onset, but survive longer (progression phase is slowed), in an SR-dependent manner. Paradoxically, administration of d-serine to ALS mice dramatically lowers cord levels of d-serine, leading to changes in the onset and survival very similar to SR deletion. d-Serine treatment also increases cord levels of the alanineserinecysteine transporter 1 (Asc-1). 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 pre-symptomatic and progression phases of disease, and offer a direct link between mutant SOD1 and a glial-derived toxic mediator.