24S-hydroxycholesterol and 25-hydroxycholesterol differentially impact hippocampal neuronal survival following oxygen-glucose deprivation

N-methyl-D-Aspartate receptors (NMDARs), a major subtype of glutamate receptor mediating excitatory transmission throughout the CNS, participate in ischemia-induced neuronal death. Unfortunately, undesired side effects have limited the strategy of inhibiting/blocking NMDARs as therapy. Targeting endogenous positive allosteric modulators of NMDAR function may offer a strategy with fewer downsides. Here, we explored whether 24S-hydroxycholesterol (24S-HC), an endogenous positive NMDAR modulator characterized recently by our group, participates in NMDAR-mediated excitotoxicity following oxygen-glucose deprivation (OGD) in primary neuron cultures. 24S-HC is the major brain cholesterol metabolite produced exclusively in neurons near sites of glutamate transmission. By selectively potentiating NMDAR current, 24S-HC may participate in NMDAR-mediated excitotoxicity following energy failure, thus impacting recovery after stroke. In support of this hypothesis, our findings indicate that exogenous application of 24S-HC exacerbates NMDAR-dependent excitotoxicity in primary neuron culture following OGD, an ischemic-like challenge. Similarly, enhancement of endogenous 24S-HC synthesis reduced survival rate. On the other hand, reducing endogenous 24S-HC synthesis alleviated OGD-induced cell death. We found that 25-HC, another oxysterol that antagonizes 24S-HC potentiation, partially rescued OGDmediated cell death in the presence or absence of exogenous 24S-HC application, and 25-HC exhibited NMDAR-dependent/24S-HC-dependent neuroprotection, as well as NMDARindependent neuroprotection in rat tissue but not mouse tissue. Our findings suggest that both endogenous and exogenous 24S-HC exacerbate OGD-induced damage via NMDAR activation, while 25-HC exhibits species dependent neuroprotection through both NMDARdependent and independent mechanisms.