Mitochondrial function and oxidative stress in response to induced reactive oxygen species and reproduction

Abstract

Mitochondria are pivotal in the survival of complex organisms, generating metabolic energy in all eukaryotic cells by breaking down carbohydrates, fatty acids, and protein from diet and converting them to ATP (adenosine triphosphate) through oxidative phosphorylation. As a byproduct of this process, reactive oxygen species (ROS) can form, a form of free radical that have the capacity to damage macromolecules. When there is an imbalance between relative levels of ROS and antioxidants, damage can accumulate within the cell; this condition is referred to as “oxidative stress” and results in macromolecular damage such as lipid peroxidation, protein oxidation, and DNA mutations that are associated with various diseases and aging. Paradoxically, however, the notion that oxidative stress is always harmful is controversial. Besides generating damage, ROS have also been shown to act as signaling molecules stimulating processes that promote increased mitochondrial biogenesis, antioxidant production and repair of damaged macromolecules. Thus, mitochondria are hypothesized to display a biphasic response to ROS exposure referred to as mitochondrial hormesis. For my thesis, I investigated the mitochondrial function and oxidative stress under two scenarios in mice: induced ROS production and natural life-history event of reproduction. The majority of studies evaluating the impact of oxidative stress on animals have largely focused on quantifying damage to proteins and lipids. Therefore, in chapter one, I examined the temporal response to induced ROS via radiation on DNA in wild-derived mice, extrapolating upon a previous study from our lab (Zhang et al., 2017) looking at the effect of radiation-induced ROS on mitochondrial function and physiological parameters. I measured a biomarker of oxidative DNA damage 8-oxo-7,8-dihydroguanine (8-OHdG), and its primary repair protein 8-oxoguanine glycosylate (OGG1), using the frozen tissues from the past study. I found there to be a mitohormetic response at the DNA level, consistent with past study’s findings of the same pattern on other macromolecules. In chapter two, I asked if mitohormesis can be seen in a natural life-history setting. Reproduction is an energetically demanding activity, with individuals that allocate more to reproduction typically having reduced longevity. But despite numerous studies and reviews, the mechanisms behind the tradeoff between reproduction, bioenergetics and longevity are still poorly understood. I asked if oxidative stress could be the mediator of this tradeoff, if and when the cost of reproduction is seen with varying parity in laboratory mice. I hypothesized that females that have undergone one bout of reproduction will display improved mitochondrial function relative to non-reproductive mice, while multiparous females display negative effects of continuous reproduction. The results revealed no negative effects of reproductive effort on the bioenergetic capacity of female lab mice. Instead, females with the highest reproductive performance had a heavier liver and heart, which would equate to more liver and heart mitochondria, and their skeletal muscle mitochondria displayed higher respiratory performance when oxidizing lipid. Together, my results suggest that there is evidence of a mitohormetic response at the DNA level under induced ROS, and that the assumed negatively linear tradeoff between reproduction and longevity does not hold in the context of oxidative stress.