Mitochondria : Hubs of Hypoxia-Tolerance in Naked Mole-Rats

Abstract

Most adult mammals are highly sensitive to reduced oxygen availability; however, some species have evolved to live in hypoxic environments. Naked mole-rats (NMRs, Heterocephalus glaber) are among the most hypoxia-tolerant mammals and rapidly reduce whole animal oxygen consumption (>80% in 3% O₂) during hypoxia, presumably by downregulating numerous cellular functions across multiple tissues. Mitochondria are cellular oxygen sensors, major consumers of oxygen, and regulators of numerous cellular signalling pathways, and thus likely play key roles in cellular responses to hypoxia. However, little is known about the specific mechanisms and pathways that mitochondria regulate in NMRs, nor about how these roles vary between tissues with divergent metabolic demands in hypoxia. Of particular interest are mechanisms that regulate mitochondrial oxidative phosphorylation (OXPHOS) function, and reactive oxygen species (ROS) and Ca²⁺ homeostasis, since ATP deficits, and ROS and Ca²⁺ dysregulation are central to hypoxic/ischemic cell death in hypoxia-intolerant mammalian cells. Therefore, the goal of my thesis is to uncover mechanisms via which mitochondria regulate OXPHOS, ROS, and Ca²⁺, and the impact of these mechanisms on hypoxia/ischemia tolerance in NMR interscapular brown adipose tissue (iBAT), brain, and skeletal muscle. These tissues were chosen because 1) thermoregulation is a major energy drain in small rodents and decreased thermoregulation is a common strategy to save energy during hypometabolism in such species, 2) maintenance of brain function is both energetically expensive but also obligatory in all animals, and 3) NMRs remain physically active in hypoxia, and therefore require the maintenance of skeletal muscle function. Based on the divergent energy demands and sensitivity to hypoxia of these tissues, we expected to find highly divergent tissue-specific regulation at the mitochondrial level. Using multiple approaches, including high-resolution respirometry, optical fluorescence measurements, western blot, qPCR, and transmission electron microscopy, our integrated findings revealed numerous novel responses in each of these tissues that help to explain the remarkable tolerance to hypoxic/ischemic stress in NMRs. Specifically: (1) In hypoxia, iBAT mitochondria significantly suppress respiration (by 45-70%) and rate of Ca²⁺ uptake. These functional changes were accompanied by rapid reductions in the expression of OXPHOS and UCP1 proteins, which was likely mediated by mitochondrial membrane remodeling, including the activation of mitochondrial fission and inhibition of apoptosis. (2) NMR brain mitochondria have a very high capacity to buffer Ca²⁺. Elevated mitochondrial Ca²⁺ suppresses the O₂ consumption rate without compromising membrane integrity in NMRs but not in mice. The mechanism underlying this enhanced capacity likely involves the occurrence of larger and more interconnected mitochondrial networks in NMR brain. As a result, NMR brain is better able to regulate redox state, minimize excitotoxity (i.e., glutamate, Ca²⁺), and retain OXPHOS function under in vitro ischemia than mouse brain. (3) Skeletal muscle mitochondria exhibit a mild decrease in OXPHOS function but reduce mitochondrial superoxide (O₂ᣟ⁻) emission in acute and chronic hypoxia, which may support continuous exercise in intermittent hypoxic burrow systems in nature. Overall, these results suggested that NMR mitochondria play key roles in maintaining essential functions (e.g., brain function, physical activity), and also suppressing non-essential functions (e.g., thermogenesis) in a tissue-specific fashion to minimize the O₂ consumption and hypoxia-induced cell damage.