Regulatory analyses of the beneficial effects by trace amount of selenite in Caenorhabditis elegans

Selenium (Se), an essential trace element, is involved in many important cell processes and is vital for health. Although several beneficial effects of Se have been described, the associated regulatory mechanisms by Se remain further eluciared. The goal of this dissertion is to investigate the beneficial effects of Se and its associated regulatory mechanisms in Caenorhabditis elegans. There are 3 specific aims in this study: (1) the antioxidant activities of selenite (Se(IV)) and its related mechanisms; (2) the neuroprotective activities of (Se(IV)) against the neurotoxin (i.e., lead); and (3) the effects of (Se(IV)) on immune systems against human pathogen Pseudomonas aeruginosa PA14 and its related mechanisms.
For specific aim 1, trace amount of Se(IV)-treated (0.01 μM) C. elegans showed an increased survival under oxidative stress and thermal stress compared to untreated controls. Further studies demonstrated that the significant stress resistance of Se(IV) on C. elegans could be attributable to its in vivo free radical-scavenging ability. The oxidative and thermal stress resistance phenotypes by Se(IV) were also found to be absent from the forkhead transcription factor daf-16 mutant worms. Moreover, Se(IV) influenced the subcellular distribution of DAF-16 in C. elegans. Furthermore, Se(IV) increased mRNA levels of stress-resistance-related proteins, including superoxide dismutase-3 and heat shock protein-16.2. Additionally, Se(IV) (0.01 μM) upregulated expressions of transgenic C. elegans carrying sod-3::green fluorescent protein (GFP) and hsp-16.2::GFP, whereas this effect was abolished by feeding daf-16 RNA interference in C. elegans. Finally, unlike the wild-type N2 worms, the oxidative stress resistance phenotypes by Se(IV) were both absent from the C. elegans selenoprotein trxr-1 mutant worms and trxr-1 mutants feeding with daf-16 RNA interference. These findings suggest that the antioxidant effects of Se(IV) in C. elegans are mediated via DAF-16 and TRXR-1.
For specific aim 2, the study investigated the protective potential of selenite (IV) against lead (Pb(II))-induced neurotoxicity in C. elegans. The results showed that Se(IV) (0.01 μM) pretreatment ameliorated the decline of locomotion behaviors (frequencies of body bends, head thrashes, and reversal ) of C. elegans that are damaged by Pb(II) (100 μM) exposure. The intracellular ROS level of C. elegans induced by Pb(II) exposure was significantly lowered by Se(IV) supplementation prior to Pb exposure. Finally, Se(IV) protects AFD sensory neurons from Pb(II)-induced toxicity. Our study suggests that Se(IV) has protective activities against Pb(II)-induced neurotoxicity through its antioxidant property.
For specific aim 3, the immune effects of Se(IV) were investigated by examining the responses of C. elegans to P. aerugonisa PA14 strain. Se(IV)-treated C. elegans showed increased survival under PA14 infection compared with untreated controls. The significant pathogen resistance of Se(IV) on C. elegans might not be attributable to the effects of Se(IV) on PA14 as Se(IV) showed no effect on bacterial quorum-sensing and virulence factors of PA14. This study showed that Se(IV) enhanced the expression of a gene pivotal for the innate immunity in C. elegans. The study found that the pathogen-resistant phenotypes contributed by Se(IV) was absent from the skn-1 mutant worms. Moreover, Se(IV) influenced the subcellular distribution of SKN-1/Nrf in C. elegans upon PA14 infection. Furthermore, Se(IV) increased mRNA levels of SKN-1 target genes (gst-4 and gcs-1). This study found evidence of Se(IV) protecting C. elegans against P. aeruginosa PA14 infection by exerting effects on the innate immunity of C. elegans that is likely to be mediated via regulation of a SKN-1-dependent signaling pathway.
In conclusion, results in this dissertion showed that trace amount of Se(IV) can: (1) protect C. elegans from oxidative stress via DAF-16 and TRXR-1; (2) provide protection against lead-induced neurotoxicity in C. elegans; and (3) enhance immune responses against P. aeruginosa PA14 infection via SKN-1 in C. elegans. This dissertion demonstrates that (Se(IV)) modulates activation of stress-related or immune-related genes to offer beneficial effects on C. elegans in antioxidant defense systems, nerve systems, and immune systems.