Selenium has been known for its beneficial function. In recent years, accumulated evidence has indicated that much of the beneficial effect of selenium is attributed to its antioxidant nature and being a functional component of selenoproteins including selenoenzymes. Selenium supplementation has been reported to have positive effect in various diseases and stress conditions including Keshan disease, thyroid hormone metabolism, excitotoxicity, neurodegenerative diseases and cancer . Lifelong low selenium level is associated with compromised cognitive function . In the present study, we investigated the effect of selenium pretreatment on glutamate toxicity, hypoxic and ischemic brain injury. Our data show that selenium treatment decreased cell death and improved cell viability from glutamate toxicity and hypoxia. The positive effect of selenium is mediated through lowering ROS production/accumulation, and preserving mitochondrial membrane potential and mitochondrial functional performance. These in vitro effects of selenium were positively translated to in vivo stroke model. Therefore, selenium pretreatment decreased infarct volume, reduced oxidative DNA damage and showed neuroprotection. Additionally, we detected the increased protein levels of mitochondrial biogenesis regulators NRF1 and PGC-1α whereas autophagy modulators Beclin 1 and LC3 significantly decreased following selenium pretreatment.
Cerebral ischemia leads to severe structural and functional loss of neurons in the affected region of the brain. Our study with NeuN and Fluoro-Jade B staining revealed that under ischemic conditions selenium pretreatment reduced neurodegeneration and neuronal loss, thereby preserving neuronal integrity. Moreover, selenium pretreatment markedly reduced DNA oxidation following cerebral ischemia. Available evidence suggests that ischemia/reperfusion induces mitochondrial dysfunction by enhancing ROS generation, leading to the damage of intracellular proteins, lipids, and DNA [2, 31, 32]. Presently, we observed that selenium pretreatment significantly reduced ROS production in our in vitro model of glutamate toxicity  and hypoxia , which may be associated with the selenium-induced increase in activities of antioxidant enzymes [12, 34]. Likewise, our in vitro study has shown that selenium pretreatment protects mitochondrial functional performance by preserving mitochondrial membrane potential and the activities of mitochondrial complexes. These results are in direct correlation with the available evidence that indicates the important role of selenium in regulating ATP production and activities of mitochondrial respiratory chain complexes [16, 35, 36]. Thus, it seems most likely that selenium protects mitochondrial function and inhibits mitochondria-initiated cell death pathway, which thereby improves neuro-survival . Additionally, reduction in DNA oxidation observed presently may be attributed to the anti-oxidative nature of selenium, which under these conditions significantly reduced neuronal loss as compared to normal animals.
Reported evidence suggests that selenium accumulates mainly in mitochondria and nuclei in rat in vivo pretreatment trials  and also present as the crucial component in selenoproteins. Deficiency of selenium or mutation in selenoenzymes such as glutathione peroxidase (GPx) decreases the expression or activity of these enzymes  and may exacerbate neuronal loss, whereas selenium pretreatment-dependent increase in activity or overexpression of selenoenzymes ameliorates outcome during endogenous or exogenous stimuli, trauma and other neurodegenerative conditions including cerebral stroke [10–14].
Selenium has been shown to protect mitochondrial function by upregulating mitochondrial biogenesis [12, 37, 38]. Cerebral ischemia on the other hand is known to damages mitochondria, increases ROS production and impairs ATP generation. Neurons in these conditions initiate adaptive response through activation of mitochondrial biogenesis . Therefore, in the present study we analyzed the protein markers of mitochondrial biogenesis (PGC-1α and NRF1). We found the increased protein levels of PGC-1α and NRF1 at 5- and/or 24 h of recirculation. These results are in accordance with the reports that showed marked increase in mitochondrial DNA content, mitochondrial proteins and numbers, and mRNA levels of NRF1 and Tfam after hypoxia and ischemia [40, 41]. Interestingly, selenium pretreatment increased the protein levels of PGC-1α and NRF1 at basal level and increased further after cerebral ischemia and recirculation as compared to respective control. Previous reports have also shown that selenite supplementation increased the level of NRF1, which clearly support our result that mitochondrial biogenesis could be modulated by selenite application . In a parallel study (Li and collegues, unpublished data), we have observed that selenium increases mitochondrial biogenesis markers and mitochondrial proteins cytochrome c and COX IV under normal culture condition, inhibits mitochondrial fission induced by glutamate exposure, and induces phosphorylations of Akt, PKA and CREB, transcription factors that are known to activate mitochondrial biogenesis. Thus, selenium induced mitochondrial biogenesis could be an important strategy to improve mitochondrial function in various stress conditions including neurodegenerative diseases. In fact, our in vitro hypoxic study demonstrated that selenium increased mitochondrial oxidative phosphorylation and ameliorated the hypoxia-induced suppression to respiratory complex activity.
Autophagy is a major catabolic contributor to degrade and recycle macromolecules and organelles. We and others have found that autophagy markers Beclin 1 and LC3-II were increased following cerebral ischemia [25, 43]. Autophagy activation following cerebral ischemia is a process to recycle injured cells or a process responsible for cell demise [43–45]. Interestingly, selenium pretreatment reduced the protein level of Beclin 1 and LC3-II cleavage. It has been reported that ROS is a major factor involved in activation of autophagy [46, 47] and selenium lowers ROS production and prevents mitochondrial dysfunction [34, 48]. Therefore, it is possible that selenium preserve mitochondrial function, lowers ROS production, reduces autophagy and thereby provides neuroprotection.