Mitochondrial Adaptions Involved in Brain Tolerance to Hypoxia and Reoxygenation in New Zealand Triplefin Fish (Tripterygiidae)

Abstract

Oxygen is fundamental for the survival of eukaryotic life due to its central role in mitochondrial oxidative phosphorylation, which produces ~90% of cellular ATP. Fluctuations in oxygen represent a major stressor to mitochondrial function, with exposure to low oxygen or hypoxia leading to a sequalae of detrimental physiological changes that alter cell operations and ultimately induce death. Nonetheless, some species are remarkably tolerant of low oxygen environments, and sustain function even during extended periods of hypoxic exposure. A diverse array of coordinated physiological, biochemical and morphological traits have been described in species which allow them to survive large fluctuations in oxygen availability. However, with the exception of studies on freshwater turtles (T. scripta and C. picta) and the Crucian carp (C. carassius), the role of the mitochondria in hypoxia tolerance has remained largely undefined. Tripterygiidae (New Zealand triplefin fish) present an excellent model for study on adaptive hypoxia tolerance, as species exhibit considerable diversity in habitat occupation and level of oxygen tolerance despite their close genetic background. This thesis aimed to determine whether hypoxia tolerant species have evolved adaptive mitochondrial mechanisms to avoid oxidative and reductive stress and(or) prevent damage induced by intermittent hypoxic episodes that frequently occur in their natural environment. Given the vulnerability of brain tissue to hypoxia, mitochondrial function was tested in brain homogenates of three closely related species which displayed varying degrees of hypoxia tolerance (F. lapillum, B. medius, and F. varium). High resolution respirometry coupled with fluorometric measurements of membrane potential and spectrometric analysis of cytochrome redox state allowed comprehensive assessment of differences in mitochondrial function and integrity in response to intermittent hypoxia. Traditional steady state measures of mitochondrial respiration flux and membrane potential showed no differences among species. However, in the transition into hypoxia, B. medius and F. lapillum were able to maintain membrane potential at oxygen pressures 7 and 4.4 fold lower than sensitive F. varium, respectively, and exhibited faster repolarisation upon reoxygenation. Cytochrome redox states also revealed differences in hypoxic responses between tolerant species that may reflect differences in phylogeny and habitat. Notably, these species appeared to reestablish redox equilibrium following reoxygenation, while F. varium remained compromised. Overall, this reveals some elements of mitochondrial function may underlie the hypoxia tolerance exhibited by intertidal triplefin fish.