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.

d-氨基酸越来越被认为是哺乳动物中枢神经系统中的重要信号分子。然而，在体外具有最快自发消旋率的氨基酸半胱氨酸的d-立体异构体尚未在哺乳动物中进行过检测。我们使用手性高效液相色谱法和立体特异性荧光素酶测定法，鉴定哺乳动物大脑中的内源性d-半胱氨酸。我们鉴定了丝氨酸消旋酶 (SR)，它产生N-甲基-d-天冬氨酸 (NMDA) 谷氨酸受体共激动剂d-丝氨酸，作为d-半胱氨酸的候选生物合成酶。d与成年大脑相比，胚胎小鼠大脑中的半胱氨酸含量高出 20 倍以上。d-半胱氨酸可将培养的小鼠胚胎神经祖细胞 (NPC) 的增殖减少 ∼50%，这种作用与d-丝氨酸或l-半胱氨酸不同。d-半胱氨酸的抗增殖作用由转录因子 FoxO1 和 FoxO3a 介导。d-半胱氨酸对 NPC 增殖的选择性影响反映在新生 SR 基因敲除小鼠大脑皮层的过度生长和异常分层中。最后，我们通过用d免疫沉淀对 NPC 中的d-半胱氨酸结合蛋白进行无偏筛选。-半胱氨酸特异性抗体，然后进行质谱分析。这种方法将肉豆蔻酰化富含丙氨酸的 C 激酶底物 (MARCKS) 鉴定为推定的d-半胱氨酸结合蛋白。总之，这些结果建立了内源性哺乳动物d-半胱氨酸，并暗示它是发育中大脑中 NPC 稳态的生理调节剂。