A phosphoserine phosphatase variant present in the brain of Alzheimer's disease patients favors nuclear mistargeting.

Phosphoserine phosphatase (PSP) catalyzes the dephosphorylation of 3-phosphoserine, which is the final step in the de novo biosynthesis of l-serine (l-Ser) via the phosphorylated pathway in human astrocytes. Individuals who are homozygous or compound heterozygous for functionally defective PSP variants exhibit reduced cerebrospinal fluid l-Ser levels and severe neurological symptoms. In the present study, single nucleotide polymorphisms in PSP were identified in hippocampal samples from Alzheimer's disease (AD) patients. Two single nucleotide polymorphisms, likely forming a haplotype (chr7:56088825 T>A and chr7:56088811 T>C, encoding R27S and D32G PSP variants, respectively), were detected exclusively in AD patients (three females and one male). Biochemical characterization of the recombinant R27S/D32G PSP enzyme revealed a slight decrease in thermostability, a 38-fold reduction in catalytic efficiency and a two-fold increase in IC₅₀ for l-Ser, with the D32G substitution being the primary contributor to these effects. Despite its reduced enzyme activity, the R27S/D32G variant did not impair l-Ser biosynthesis either in an in vitro reconstructed pathway or in U251 human glioblastoma cells ectopically expressing the variant under heterozygous conditions. In these cells, PSP colocalized extensively with the other two phosphorylated pathway enzymes, namely phosphoglycerate dehydrogenase and phosphoserine aminotransferase, suggesting that they assemble into a functional complex, known as the serinosome. Notably, the R27S/D32G PSP variant exhibited increased nuclear localization compared to the wild-type enzyme. This mislocalization raises the intriguing possibility that PSP's moonlighting functions, including its putative role as a protein phosphatase, may be affected, potentially implicating it in pathways beyond l-Ser biosynthesis.