Thiosulfate mediates cytoprotective effects of hydrogen sulfide against neuronal ischemia

Background-Hydrogen sulfide (H₂S) exhibits protective effects in various disease models including cerebral ischemia-reperfusion (I/R) injury. Nonetheless, mechanisms and identity of molecules responsible for neuroprotective effects of H₂S remain incompletely defined. In the current study, we observed that thiosulfate, an oxidation product of H₂S, mediates protective effects of an H₂S donor compound sodium sulfide (Na₂S) against neuronal I/R injury. Methods and Results-We observed that thiosulfate in cell culture medium is not only required but also sufficient to mediate cytoprotective effects of Na₂S against oxygen glucose deprivation and reoxygenation of human neuroblastoma cell line (SH-SY5Y) and murine primary cortical neurons. Systemic administration of sodium thiosulfate (STS) improved survival and neurological function of mice subjected to global cerebral I/R injury. Beneficial effects of STS, as well as Na₂S, were associated with marked increase of thiosulfate, but not H₂S, in plasma and brain tissues. These results suggest that thiosulfate is a circulating "carrier" molecule of beneficial effects of H₂S. Protective effects of thiosulfate were associated with inhibition of caspase-3 activity by persulfidation at Cys163 in caspase-3. We discovered that an SLC13 family protein, sodium sulfate cotransporter 2 (SLC13A4, NaS-2), facilitates transport of thiosulfate, but not sulfide, across the cell membrane, regulating intracellular concentrations and thus mediating cytoprotective effects of Na₂S and STS. Conclusions-The protective effects of H₂S are mediated by thiosulfate that is transported across cell membrane by NaS-2 and exerts antiapoptotic effects via persulfidation of caspase-3. Given the established safety track record, thiosulfate may be therapeutic against ischemic brain injury.