Cell-autonomous and non-cell-autonomous neuroprotective functions of RORα in neurons and astrocytes during hypoxia

There is increasing evidence to suggest that the neuronal response to hypoxia is regulated through their interactions with astrocytes. However, the hypoxia-induced molecular mechanisms within astrocytes which influence neuronal death have yet to be characterized. In this study, we investigated the roles of the nuclear receptor RORα (retinoid-related orphan receptor-α) respectively in neurons and astrocytes during hypoxia using cultures and cocultures of neurons and astrocytes obtained from RORα-deficient mice. We found that loss of RORα function in neuronal cultures increases neuronal death after hypoxia, suggesting a cell-autonomous neuroprotective effect of RORα. Moreover, wild-type neurons cocultured with RORα-deficient astrocytes are characterized by a higher death rate after hypoxia than neurons cocultured with wild-type astrocytes, suggesting that RORα also has a non-cell-autonomous action. By using cocultures of neurons and astrocytes of different genotypes, we showed that this neuroprotective effect of RORα in astrocytes is additive to its effect in neurons, and is mediated in part by cell-to-cell interactions between neurons and astrocytes. We also found that RORα is upregulated by hypoxia in both neurons and astrocytes. Furthermore, our data showed that RORα does not alter oxidative mechanisms during hypoxia but regulates hypoxic inducible factor 1α (HIF-1α) expression, a major regulator of hypoxia sensing, in a cell-specific manner. Indeed, the neuroprotective function of RORα in astrocytes correlates with a downregulation of HIF-1α selectively in these cells. Altogether, our results show that RORα is a key molecular player in hypoxia, protecting neurons through its dual action in neurons and astrocytes.