Oxidized human neuroglobin acts as a heterotrimeric Galpha protein guanine nucleotide dissociation inhibitor

Neuroglobin (Ngb) is a newly discovered vertebrate heme protein that is expressed in the brain and can reversibly bind oxygen. It has been reported that Ngb expression levels increase in response to oxygen deprivation and that it protects neurons from hypoxia in vitro and in vivo. However, the mechanism of this neuroprotection remains unclear. In the present study, we tried to clarify the neuroprotective role of Ngb under oxidative stress in vitro. By surface plasmon resonance, we found that ferric Ngb, which is generated spontaneously as a result of the rapid autoxidation, binds exclusively to the GDP-bound form of the alpha subunit of heterotrimeric G protein (Galphai). In GDP dissociation assays or guanosine 5'-O-(3-thio)triphosphate binding assays, ferric Ngb behaved as a guanine nucleotide dissociation inhibitor (GDI), inhibiting the rate of exchange of GDP for GTP. The interaction of GDP-bound Galphai with ferric Ngb will liberate Gbetagamma, leading to protection against neuronal death. In contrast, ferrous ligand-bound Ngb under normoxia did not have GDI activities. Taken together, we propose that human Ngb may be a novel oxidative stress-responsive sensor for signal transduction in the brain.