Retinoic acid receptor-related orphan receptor a (RORa) is a transcription factor belonging to the superfamily of nuclear receptors. Disruption of the Rora gene in the mouse results in a defect in the development of Purkinje cells leading to a cerebellar atrophy, which suggests a neuroprotective role for RORa. To test this hypothesis, the survival rate of lentiviralmediated human RORa1-overexpressing neurones has been evaluated in response to different stressors disturbing the redox homeostasis, such as b-amyloid peptide, c 2 -ceramide and H 2 O 2 . We show that overexpression of human RORa1 provides neuroprotection by increasing the expression of the antioxidant proteins glutathione peroxidase 1 and peroxiredoxin 6, leading to a reduction in the accumulation of stressinduced reactive oxygen species. We further demonstrate that the neuroprotective effect of RORa is predominantly mediated by glutathione peroxidase 1 and peroxiredoxin 6. These results suggest a new role for RORa in the control of the neuronal oxidative stress and thus represents a new transcription factor of interest in the regulation of reactive oxygen species-induced neurodegenerative processes during ageing. Keywords: apoptosis, lentivirus, neuroprotection, oxidative stress, retinoic acid receptor-related orphan receptor a. J. Neurochem. 96, 1778-1789.There is accumulating evidence that a proper balance between oxidants and antioxidants is of systemic importance for health and longevity. Homeostasis of redox status is normally maintained by intracellular regulatory molecules and enzymes such as glutathione, thioredoxin, catalase, superoxide dismutase and peroxidases. Exposure to oxidants can lead to oxidative stress wherein the antioxidant status of the cell is thrown out of balance and may lead to a variety of symptoms including apoptosis. How the expression of these enzymes, implicated in homeostasis of redox status, is regulated and which factors are involved is still poorly understood.Retinoic acid receptor-related orphan receptor a (RORa) is a member of the superfamily of the nuclear receptors (Giguere 1999). Initially described as an orphan receptor, RORa had long been considered a constitutive activator of transcription in the absence of exogenous ligand (Atkins et al. 1999;Harris et al. 2002). Recently, however, cholesterol and 7-dehydrocholesterol were shown to be natural ligands of the transcription factor RORa (Kallen et al. 2002;Bitsch et al. 2003).Mutant mice lacking functional RORa protein (Hamilton et al. 1996;Steinmayr et al. 1998) develop a severe cerebellar ataxia due to a massive cerebellar neurodegeneration (Sidman et al. 1962 Abbreviations used: Ab, b-amyloid peptide; GFP, green fluorescent protein; Gpx, glutathione peroxidase; hRORa, human retinoic acid receptor-related orphan receptor a; Prx, peroxiredoxin; RORa, retinoic acid receptor-related orphan receptor a; ROS, reactive oxygen species; si-RNA, small interfering RNA.

Ror alpha is an orphan nuclear receptor. In homozygous staggerer mutant mice (Ror alpha(sg/sg)), a deletion within the Ror alpha gene leads to an overexpression of inflammatory cytokines. Because inflammation and hypoxia are 2 key stimuli of ischemia-induced angiogenesis, we studied the role of Ror alpha in this setting. Ischemia was induced by ligation of the right femoral artery in C57BL/6 Ror alpha(+/+) and Ror alpha(sg/sg) mice. After 3 and 28 days, angiogenesis was evaluated by microangiography, measurement of capillary density using immunohistochemistry (anti-CD31), and measurement of blood flow by laser Doppler imaging. At day 3, angiographic score and blood flow were similar in Ror alpha(sg/sg) mice and in Ror alpha(+/+) littermates. Conversely, at day 28, Ror alpha(sg/sg) mice showed a significant 2-fold increase in angiographic score and a 3-fold increase in capillary density within the ischemic hindlimb compared with control. Functionally, this coincided with a significant rise in leg perfusion in Ror alpha(sg/sg) mice (0.83+/-0.05 for ischemic/nonischemic leg perfusion ratio) compared with Ror(+/+) mice (0.66+/-0.04, P<0.05). In addition, more extensive angiogenesis in Ror alpha(sg/sg) mice correlated with an increased expression of eNOS protein by 83+/-12% and 71+/-24% at 3 and 28 days, respectively (P<0.05), whereas the level of the antiangiogenic cytokine IL-12 was significantly reduced by 38+/-10% at day 28 (P<0.05). Conversely, no changes in VEGF expression were observed. Our study identifies for the first time a new role for Ror alpha as a potent negative regulator of ischemia-induced angiogenesis.