The alteration of serine transporter activity in a cell line model of amyotrophic lateral sclerosis (ALS).

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/hSOD1 cells). The uptake of [H]d-serine was significantly lower in NSC-34/hSOD1 cells than in control NSC-34 and NSC-34/hSOD1 cells. In contrast, the uptake of [H]l-serine, precursor of d-serine, was markedly increased in NSC-34/hSOD1 cells compared to control NSC-34 and NSC-34/hSOD1 cells. Both [H]d-serine and [H]l-serine uptake were saturable in these cells. The estimated Michaelis-Menten constant, K, for d-serine uptakes was higher in NSC-34/hSOD1 cells than in NSC-34/hSOD1 cells while the K for l-serine uptake was 2 fold lower in NSC-34/hSOD1 cells than in control cells. [H]d-serine and [H]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 [H]d-serine and [H]l-serine uptake in NSC-34/hSOD1 cells, respectively. The level of SLC1A4 mRNA was significantly increased in NSC-34/hSOD1 compared to NSC-34 and NSC-34/hSOD1 cells. In contrast, the level of SLC7A10 mRNA was relatively lower in NSC-34/hSOD1 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.