Exposure to Mitochondrial Genotoxins and Dopaminergic Neurodegeneration in Caenorhabditis elegans

Abstract

Neurodegeneration has been correlated with mitochondrial DNA (mtDNA) damage and exposure to environmental toxins, but causation is unclear. We investigated the ability of several known environmental genotoxins and neurotoxins to cause mtDNA damage, mtDNA depletion, and neurodegeneration in Caenorhabditis elegans. We found that paraquat, cadmium chloride and aflatoxin B1 caused more mitochondrial than nuclear DNA damage, and paraquat and aflatoxin B1 also caused dopaminergic neurodegeneration. 6-hydroxydopamine (6-OHDA) caused similar levels of mitochondrial and nuclear DNA damage. To further test whether the neurodegeneration could be attributed to the observed mtDNA damage, C. elegans were exposed to repeated low-dose ultraviolet C radiation (UVC) that resulted in persistent mtDNA damage; this exposure also resulted in dopaminergic neurodegeneration. Damage to GABAergic neurons and pharyngeal muscle cells was not detected. We also found that fasting at the first larval stage was protective in dopaminergic neurons against 6-OHDA-induced neurodegeneration. Finally, we found that dopaminergic neurons in C. elegans are capable of regeneration after laser surgery. Our findings are consistent with a causal role for mitochondrial DNA damage in neurodegeneration, but also support non mtDNA-mediated mechanisms.

Mitochondrial fusion, fission, and mitochondrial toxicity

“…However, UVC-induced mtDNA damage only caused larval growth arrest in fusion ( fzo-1, eat-3 )-deficient nematodes, suggesting that individuals with fusion deficiencies are more susceptible to toxicity induced by irreparable mtDNA damage. In agreement with this, we have also observed sensitivity to the mycotoxin aflatoxin B 1 and the chemotherapeutic cisplatin, both of which can cause irreparable mtDNA damage (González-Hunt et al 2014; Niranjan et al 1982; Podratz et al 2011), in fzo-1 - and eat-3 -deficient but not drp-1 C. elegans (Luz et al 2017). Although the precise reason for sensitivity to irreparable mtDNA damage in fusion-deficient C. elegans remains unknown, one possibility is the existence of a threshold effect in which >65% of mtDNA must typically be damaged or lost prior to pathogenesis (Taylor and Turnbull 2005).…”

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

“…AfB 1 , a foodborne mycotoxin and important carcinogen in the developing world, causes bulky DNA lesions when its epoxide metabolite reacts with DNA. We and others have found that AfB 1 preferentially, and irreparably (due to a lack of NER) damages mtDNA [15–17]. Arsenite, a global drinking water contaminant associated with the development of skin, lung, and bladder cancer, is a well-known inhibitor of several Krebs cycle dehydrogenases [48], ETC complexes [49], and is also capable of causing metabolic shifts from OXPHOS to aerobic glycolysis, otherwise known as the Warburg effect [36, 50].…”

6-hydroxydopamine (6-OHDA) Oxidative Stress Assay for Observing Dopaminergic Neuron Loss in Caenorhabditis elegans

“…We use 30 h of incubation time for all our experiments and encourage standardizing this time interval as starvation renders the animals more resistant to 6-OHDA treatment (González-Hunt et al , 2014; Offenburger et al , 2018b). After 30 h, several hundred L1s will accumulate in the wells, of which only around 200 will be used in the assay and only 50-100 randomly selected animals will be scored after treatment.…”