Role and Regulation of Methionine Sulfoxide Reductase (Msr) in a model of oxidative stress tolerance: Trachemys scripta

The detrimental effects of oxidative stress caused by the accumulation of Reactive
Oxygen Species (ROS) have been acknowledged as major factors in aging, senescence and
several neurodegenerative diseases and conditions such as Parkinson’s disease and stroke
(ischemia/reperfusion). Mammalian models are extremely susceptible to these stresses that
follow the restoration of oxygen after anoxia; however, some organisms including the
freshwater turtle Trachemys scripta can withstand several bouts of anoxia and repeated
reoxygenation without any apparent pathology. T. scripta thus provides us with an
alternate vertebrate model in which we can investigate physiological mechanisms of
neuroprotection without the damaging effects that come with oxidative stress. The major
objective of this study was to investigate the protective mechanisms in the turtle brain
under conditions of anoxia and oxidative stress. Specifically, the focus is on the Methionine Sulfoxide Reductase system (Msr), an antioxidant and cellular repair system,
and how it is regulated to protect the brain against such stressors.
Previous studies in my lab have demonstrated that Msr mRNA and protein levels
are differentially upregulated during anoxia and reoxygenation. To investigate the
regulation of Msr, FOXO3a was directly induced by transfecting a human FOXO3a
plasmid into turtle brain cell cultures, as FOXO3a has been shown to regulate MsrA levels
in other animal models. Pharmacological manipulation of FOXO3a was also performed
using the green tea extract Epigallocatechin gallate (EGCG) as it has been shown to
increase expression of FOXO3a during oxidative stress conditions in other models. I found
that an induction of human FOXO3a increased FOXO3a levels and showed protection
against cell death during oxidative stress. Furthermore, treatment of cells with EGCG
increased expression of FOXO3a only when the cells were exposed to oxidative stress and
decreased cell death. Induction of FOXO3a and EGCG treatment did not increase MsrA
levels, however MsrB3 levels were upregulated under both treatments but only in the
presence of oxidative stress. These results suggest that MsrA and MsrB3 protect the cells
from oxidative stress damage through different molecular pathways and that EGCG may
be a therapeutic target to treat diseases related to damage by oxidative stress.