Effects of reduced mitochondrial DNA content on secondary mitochondrial toxicant exposure in Caenorhabditis elegans

Abstract

The mitochondrial genome (mtDNA) is intimately linked to cellular and organismal health, as demonstrated by the fact that mutations in and depletion of mtDNA result in severe mitochondrial disease in humans. However, cells contain hundreds to thousands of copies of mtDNA, which provides genetic redundancy, and creates a threshold effect in which a large percentage of mtDNA must be lost prior to clinical pathogenesis. As certain pharmaceuticals and genetic mutations can result in depletion of mtDNA, and as many environmental toxicants target mitochondria, it is important to understand whether reduced mtDNA will sensitize an individual to toxicant exposure. Here, using ethidium bromide (EtBr), which preferentially inhibits mtDNA replication, we reduced mtDNA 35-55% in the in vivo model organism Caenorhabditis elegans. Chronic, lifelong, low-dose EtBr exposure did not disrupt nematode development or lifespan, and induced only mild alterations in mitochondrial respiration, while having no effect on steady-state ATP levels. Next, we exposed nematodes with reduced mtDNA to the known and suspected mitochondrial toxicants aflatoxin B1, arsenite, paraquat, rotenone or ultraviolet C radiation (UVC). EtBr pre-exposure resulted in mild sensitization of nematodes to UVC and arsenite, had no effect on AfB1 and paraquat, and provided some protection from rotenone toxicity. These mixed results provide a first line of evidence suggesting that reduced mtDNA content may sensitize an individual to certain environmental exposures.

Cell Biology of the Mitochondrion

“…Interestingly, they also showed that the effects were NAD + dependent and could be alleviated by pretreatment of the nematodes with exogenous NAD + . In closely related work, Luz and colleagues showed that arsenite caused mitochondrial dysfunction, and that arsenite toxicity was increased in the presence of some, but not all, genetic defects in mitochondrial function (Luz et al 2016a,c;Luz and Meyer 2016). They concluded that the interaction between arsenite toxicity and mitochondrial function was a novel class of interactions first identified in the nematode that needed study in humans.…”

Effects of methyl and inorganic mercury exposure on genome homeostasis and mitochondrial function in Caenorhabditis elegans

“…However, the degree to which mitochondrial copy number reductions impact cellular health is not well understood, because many cells have high mtDNA redundancy, copy number varies between tissues, and there is a wide range of apparently normal mtDNA copy number in humans (Shokolenko and Alexeyev 2015). We recently found that small decreases in mtDNA copy number have only mild impacts on ATP levels in C. elegans (Luz and Meyer 2016). Nonetheless, mtDNA depletions around 65% can cause disease in humans (Suomalainen and Isohanni 2010), and a certain level of mtDNA is required to pass some developmental milestones in C. elegans (Tsang and Lemire 2002).…”

Beyond the polymerase-γ theory: Production of ROS as a mode of NRTI-induced mitochondrial toxicity

“…This effect, however, is probably small for the other NRTIs as a 55% reduction in mtDNA copy number in C . elegans has recently been shown to have limited influence on life history traits [ 72 ].…”