D-cysteine is an endogenous regulator of neural progenitor cell dynamics in the mammalian brain

D-amino acids are increasingly recognized as important signaling molecules in the mammalian central nervous system. However, the D-stereoisomer of the amino acid with the fastest spontaneous racemization ratein vitro in vitro, cysteine, has not been examined in mammals. Using chiral high-performance liquid chromatography and a stereospecific luciferase assay, we identify endogenous D-cysteine in the mammalian brain. We identify serine racemase (SR), which generates the N-methyl-D-aspartate (NMDA) glutamate receptor coagonist D-serine, as a candidate biosynthetic enzyme for D-cysteine. D-cysteine is enriched more than 20-fold in the embryonic mouse brain compared with the adult brain. D-cysteine reduces the proliferation of cultured mouse embryonic neural progenitor cells (NPCs) by ∼50%, effects not shared with D-serine or L-cysteine. The antiproliferative effect of D-cysteine is mediated by the transcription factors FoxO1 and FoxO3a. The selective influence of D-cysteine on NPC proliferation is reflected in overgrowth and aberrant lamination of the cerebral cortex in neonatal SR knockout mice. Finally, we perform an unbiased screen for D-cysteine–binding proteins in NPCs by immunoprecipitation with a D-cysteine–specific antibody followed by mass spectrometry. This approach identifies myristoylated alanine-rich C-kinase substrate (MARCKS) as a putative D-cysteine–binding protein. Together, these results establish endogenous mammalian D-cysteine and implicate it as a physiologic regulator of NPC homeostasis in the developing brain.