Nrf2 target genes are induced under marginal selenium-deficiency

A suboptimal selenium supply appears to prevail in Europe. The current study, therefore, was focused on the changes in gene expression under a suboptimal selenium intake. Previous microarray analyses in the colon of mice fed either a selenium-adequate or a moderately deficient diet revealed a change in genes of several pathways. Severe selenium-deficiency has been found previously to influence Nrf2-regulated genes of the adaptive response. Since the previous pathway analyses were done with a program not searching for Nrf2 target genes, respective genes were manually selected and confirmed by qPCR. qPCR revealed an induction of phase II (Nqo1, Gsts, Sult1b1 and Ugt1a6) and antioxidant enzymes (Hmox1, Mt2, Prdx1, Srxn1, Sod1 and Gclc) under the selenium-poor diet, which is considered to compensate for the loss of selenoproteins. The strongest effects were observed in the duodenum where preferentially genes for antioxidant enzymes were up-regulated. These also include the mRNA of the selenoproteins TrxR1 and GPx2 that would enable their immediate translation upon selenium refeeding. The down-regulation of Gsk3β in moderate selenium-deficiency observed in the previous paper provides a possible explanation for the activation of the Nrf2 pathway, because inhibition of GSK3β results in the nuclear accumulation of Nrf2.     

Impaired spare respiratory capacity in cortical synaptosomes from Sod2 null mice

Presynaptic nerve terminals require high levels of ATP for the maintenance of synaptic function. Failure of synaptic mitochondria to generate adequate ATP has been implicated as a causative event preceding the loss of synaptic networks in neurodegenerative disease. Endogenous oxidative stress has often been postulated as an etiological basis for this pathology, but has been difficult to test in vivo. Inactivation of the superoxide dismutase gene (Sod2) encoding the chief defense enzyme against mitochondrial superoxide radicals results in neonatal lethality. However, intervention with an SOD mimetic extends the life span of this model and uncovers a neurodegenerative phenotype providing a unique model for the examination of in vivo oxidative stress. We present here studies on synaptic termini isolated from the frontal cortex of Sod2 null mice demonstrating impaired bioenergetic function as a result of mitochondrial oxidative stress. Cortical synaptosomes from Sod2 null mice demonstrate a severe decline in mitochondrial spare respiratory capacity in response to physiological demand induced by mitochondrial respiratory chain uncoupling with FCCP or by plasma membrane depolarization induced by 4-aminopyridine treatment. However, Sod2 null animals compensate for impaired oxidative metabolism in part by the Pasteur effect allowing for normal neurotransmitter release at the synapse, setting up a potentially detrimental energetic paradigm. The results of this study demonstrate that high-throughput respirometry is a facile method for analyzing specific regions of the brain in transgenic models and can uncover bioenergetic deficits in subcellular regions due to endogenous oxidative stress.