Antioxidant and anti-apoptotic defenses in the anoxia-tolerant turtle, Trachemys scripta elegans.
The freshwater turtle, Trachemys scripta elegans, utilizes various biochemical adaptations to survive anoxia-reoxygenation cycles without apparent tissue damage. This thesis focused on changes in the expression and regulation of selected enzymes and proteins involved in antioxidant and anti-apoptotic defense in turtle tissues in response to anoxic submergence and reoxygenation recovery. Western blotting showed that levels of the antioxidant enzyme, manganese superoxide dismutase (Mn SOD), were significantly higher (P<0.05) in heart and skeletal muscle during anoxia. Among four isozymes of glutathione S-transferase (GST), GST K1 expression level was enhanced in kidney, liver and muscle during anoxia, but remained stable in heart. GSTT1, GSTP1 and GSTM3 were elevated in a tissue-specific manner. Anoxia-induced upregulation of the transcription factor, Nrf2, coupled with translocation of Nrf2 into the nucleus in anoxia, indicated that Nrf2 is probably involved in activating downstream antioxidant genes such as GST. Analysis of antiapoptotic proteins (Bcl-XL, Bcl-2 and Mcl-1) also showed enhanced expression in selected tissues during anoxia exposure whereas the pro-apoptotic protein, Bad, was suppressed via phosphorylation during anoxia in muscle. Levels of bcl-xl mRNA were also quantified to assess the relationship between bcl-xl gene and Bcl-XL protein expression under anoxia. The results indicate that enhancement of antioxidant and anti-apoptotic defenses is an important adaptive mechanism for effectively dealing with low oxygen and oxidative stresses over cycles of anoxia/reoxygenation in turtles.