[Availability of D-cysteine as a protectant for cerebellar neurons]

Hydrogen sulfide (H₂S) has been focused as a biological mediator, which modulates signal transduction and protects cells and tissues from oxidative stress. H₂S is also expected as a neuroprotectant because it has a neuroprotective activity. Endogenous H₂S is mainly generated from L-cysteine. However, it is difficult to use L-cysteine as a neuroprotectant because of its neurotoxicity. In 2013, a novel biogenesis pathway of H₂S from D-cysteine has been identified. In this pathway, D-amino acid oxidase (DAO) converts D-cysteine to 3-mercaptopyruvate (3MP), followed by the generation of H₂S from 3MP by 3-mercaptopyrvate sulfurtransferase. DAO is especially abundant in cerebellum among various brain regions and mediates efficient generation of H₂S from D-cysteine in the cerebellar tissues. In addition, D-cysteine has more potent neuroprotective activity in cerebellar primary neurons than L-cysteine. Cerebella Purkinje cells (PCs) are characterized by the highly-branched dendrites and are important for cerebellar functions. The dendritic shrinkage and degeneration of PCs are frequently observed in patients and model mice of cerebellar ataxias. We revealed that D-cysteine enhanced dendritic development of primary cultured PCs, but L-cysteine impaired the dendritic development. This effect of D-cysteine was inhibited by DAO inhibitors and reproduced by 3MP and a H₂S donor, suggesting that this enhancement of dendritic development is caused by the production of H₂S from D-cysteine. Taken together, D-cysteine would be available as a neuroprotectant against cerebellar ataxias, which are accompanied with dendritic shrinkage of cerebellar PCs.