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. 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.