Gadalla, M. M. | Search bioRxiv

bioRxiv2022

Serine Racemase is a Cysteine Racemase and Physiologic Down Regulator of Insulin Promoter Methylation

D-amino acids are being recognized in mammals as important molecules with function. This is a first identification of endogenous D-cysteine in mammalian pancreas. D-cysteine is synthesized by serine racemase (SR) and SR-/- mice produce 6-10 fold higher levels of insulin in the pancreas and plasma including higher glycogen and ketone bodies in the liver. The excess insulin is stored as amyloid in secretory vesicles and exosomes. In glucose stimulated insulin secretion studies in mouse and human islets, equimolar amount of D-cysteine showed higher inhibition of insulin secretion compared to D-serine, another closely related stereoisomer synthesized by SR. In mouse models of diabetes (STZ and NOD) and human pancreas, the diabetic state showed increased expression of D-cysteine compared to D-serine followed by increased expression of SR. SR-/- mice show decreased cAMP in the pancreas followed by reduced phosphorylation of CREB (S133), lower DNA methyltransferase enzymatic and promoter activities resulting in decreased methylation of the Ins1 promoter. D-cysteine is efficiently metabolized by D-amino acid oxidase and transported by ASCT2 and Asc1. Dietary supplementation with methyl donors restored the high insulin levels and low DNMT enzymatic activity in SR-/- mice. Our data show that endogenous D-cysteine in the mammalian pancreas is a regulator of insulin secretion. HighlightsO_LISerine Racemase also functions as a cysteine racemase. C_LIO_LILack of Serine Racemase results in significantly high levels of insulin in the pancreas, plasma and larger islets. C_LIO_LID-cysteine shows greater inhibition of insulin secretion compared to D-serine. C_LIO_LIEndogenous D-cysteine signals via cyclic AMP that mediates downstream CREB-DNMT1 interaction. C_LIO_LICREB-DNMT1 interaction results in hypomethylation of Ins1 promoter that can be rescued by high methyl donor dietary supplementation rescuing high insulin levels. C_LI

biochemistry↗

bioRxiv2021

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 in vitro spontaneous racemization rate, cysteine, has not been examined in mammals. Using chiral high-performance liquid chromatography and an stereospecific luciferase assay, we identify endogenous D-cysteine in the mammalian brain. We identify serine racemase (SR), which generates the NMDA glutamate receptor co-agonist D-serine, as a candidate biosynthetic enzyme for D-cysteine. Levels of D-cysteine are enriched over twentyfold in the embryonic mouse brain compared to the adult. D-cysteine reduces the proliferation of cultured mouse embryonic neural progenitor cells (NPCs) by approximately 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.

neuroscience↗