The alteration of d-serine levels is associated with the pathogenesis of sporadic ALS and mutant SOD1 (G93A) animal model of ALS. However, the exact mechanism of d-serine transport is not known in ALS. To better understand the distribution of d-serine in ALS, we determined the activity and the expression of serine transporter in a motor neuronal cell line model of ALS (NSC-34/hSOD1G93A cells). The uptake of [3H]d-serine was significantly lower in NSC-34/hSOD1G93A cells than in control NSC-34 and NSC-34/hSOD1wt cells. In contrast, the uptake of [3H]l-serine, precursor of d-serine, was markedly increased in NSC-34/hSOD1G93A cells compared to control NSC-34 and NSC-34/hSOD1wt cells. Both [3H]d-serine and [3H]l-serine uptake were saturable in these cells. The estimated Michaelis-Menten constant, Km, for d-serine uptakes was higher in NSC-34/hSOD1G93A cells than in NSC-34/hSOD1wt cells while the Km for l-serine uptake was 2 fold lower in NSC-34/hSOD1G93A cells than in control cells. [3H]d-serine and [3H]l-serine uptakes took place in a Na+-dependent manner, and both uptakes were significantly inhibited by system ASC (alanine-serine-cysteine) substrates. As a result of small interfering RNA experiments, we found that ASCT2 (SLC1A5) and ASCT1 (SLC1A4) are involved in [3H]d-serine and [3H]l-serine uptake in NSC-34/hSOD1G93A cells, respectively. The level of SLC1A4 mRNA was significantly increased in NSC-34/hSOD1G93A compared to NSC-34 and NSC-34/hSOD1wt cells. In contrast, the level of SLC7A10 mRNA was relatively lower in NSC-34/hSOD1G93A cells than the control cells. Together, these data suggest that the pathological alteration of d- and l-serine uptakes in ALS is driven by the affinity change of d-and l-serine uptake system.
Keywords: Amyotrophic lateral sclerosis (ALS); Motor neuron disease; d-Serine; l-Serine.
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