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

Significance

d -amino acids are increasingly recognized as important signaling molecules in the mammalian central nervous system. Cysteine is the amino acid with the fastest in vitro spontaneous racemization rate, but its d -stereoisomer has not been examined. Here, we establish the presence of endogenous d -cysteine in the mammalian brain. Using sensitive and specific assays, we delineate its actions as a negative regulator of growth factor signaling during cortical development and identify a putative binding partner mediating these effects. By describing the newest member of the d -amino acid family, we open an avenue of research into the functions of these multifaceted signaling molecules.

Abstract

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.