Deficiencies in mitochondrial dynamics sensitize Caenorhabditis elegans to arsenite and other mitochondrial toxicants by reducing mitochondrial adaptability

Abstract

Mitochondrial fission, fusion, and mitophagy are interlinked processes that regulate mitochondrial shape, number, and size, as well as metabolic activity and stress response. The fundamental importance of these processes is evident in the fact that mutations in fission (DRP1), fusion (MFN2, OPA1), and mitophagy (PINK1, PARK2) genes can cause human disease (collectively >1/10,000). Interestingly, however, the age of onset and severity of clinical manifestations varies greatly between patients with these diseases (even those harboring identical mutations), suggesting a role for environmental factors in the development and progression of certain mitochondrial diseases. Using the model organism Caenorhabditis elegans, we screened ten mitochondrial toxicants (2, 4-dinitrophenol, acetaldehyde, acrolein, aflatoxin B1, arsenite, cadmium, cisplatin, doxycycline, paraquat, rotenone) for increased or decreased toxicity in fusion (fzo-1, eat-3)-, fission (drp-1)-, and mitophagy (pdr-1, pink-1)-deficient nematodes using a larval growth assay. In general, fusion-deficient nematodes were the most sensitive to toxicants, including aflatoxin B1, arsenite, cisplatin, paraquat, and rotenone. Because arsenite was particularly potent in fission- and fusion-deficient nematodes, and hundreds of millions of people are chronically exposed to arsenic, we investigated the effects of these genetic deficiencies on arsenic toxicity in more depth. We found that deficiencies in fission and fusion sensitized nematodes to arsenite-induced lethality throughout aging. Furthermore, low-dose arsenite, which acted in a “mitohormetic” fashion by increasing mitochondrial function (in particular, basal and maximal oxygen consumption) in wild-type nematodes by a wide range of measures, exacerbated mitochondrial dysfunction in fusion-deficient nematodes. Analysis of multiple mechanistic changes suggested that disruption of pyruvate metabolism and Krebs cycle activity underlie the observed arsenite-induced mitochondrial deficits, and these disruptions are exacerbated in the absence of mitochondrial fusion. This research demonstrates the importance of mitochondrial dynamics in limiting arsenite toxicity by permitting mitochondrial adaptability. It also suggests that individuals suffering from deficiencies in mitodynamic processes may be more susceptible to the mitochondrial toxicity of arsenic and other toxicants.

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).…”

“…2), arsenite, and paraquat (Luz et al 2017; Yang et al 2011), and a recent report supports a role for mitochondrial fission in the adaptive response to mitochondrial dysfunction (Benard et al 2013), loss or inhibition of fission is currently more frequently associated with protection in the literature. For example, DRP1 siRNA reduced cell death in human SH-SY5Y cells exposed to 6-hydroxydopamine (Gomez-Lazaro et al 2008), reduced cell death in rat astrocytoma C6 cells exposed to manganese (Alaimo et al 2014), reduced cell death in mouse HT22 neurons exposed to glutamate (Grohm et al 2012), reduced cell death in lung epithelial cells exposed to cigarette smoke extract (Mizumura et al 2014), reduced mitochondrial dysfunction in human L02 hepatocytes exposed to cadmium (Xu et al 2013a), and protected drp-1 -deficient nematodes from acrolein-induced larval growth delay (Luz et al 2017). These results suggest hyperfusion caused by mutations in drp-1 may be protective under certain conditions.…”

“…Additionally, a recent study has demonstrated that the fatty acid metabolite, stearic acid (C18:0), reversed mitochondrial fragmentation by limiting ubiquitination and degradation of mitofusins by HECT, UBA, and WWE domain containing E3 ubiquitin protein ligase (Senyilmaz et al 2015). However, many of these studies have been carried out in vitro , and the extent to which SIMH occurs in vivo , and what other factors may regulate its occurrence, are unclear; for example, we recently failed to observed SIMH with chronic, non-lethal arsenic exposure in Caenorhabditis elegans (Luz et al 2017). Mitochondrial fusion may be triggered upon viral infection, and appears to support the innate immune response (Khan et al 2015).…”

“…2) and larval growth delay ( fzo-1 & eat-3 only: (Luz et al 2017)); rotenone reduces the viability of cortical neurons isolated from heterozygous OPA1 +/− rats (Millet et al 2016); and transfection with OPA1 G300E , an OPA1 variant carrying a missense mutation in the GTPase domain, sensitizes primary mice neurons to rotenone-induced axonal degeneration (Lassus et al 2016). Furthermore, mutations in drp-1 have been reported to mildly sensitize nematodes to paraquat-induced lethality and larval growth delay (Luz et al 2017; Yang et al 2011) while fusion-deficient C. elegans ( eat-3, fzo-1 ) are sensitive to paraquat-induced lethality and larval growth delay, and D. melanogaster ( OPA1 -deficient) display reduced survival following paraquat exposure (Kanazawa et al 2008; Luz et al 2017; Tang et al 2009). Interestingly, paraquat and rotenone exposure have been associated with the development of PD in farmworkers (Tanner et al 2011), and are also used in animal models of PD [reviewed in (Cicchetti et al 2009)], and MFN2 deficiency can cause dopaminergic neurodegeneration (Pham et al 2012).…”

“…Finally, we have recently demonstrated that fusion ( fzo-1, eat-3 ) -deficient nematodes are hypersensitive to arsenite-induced toxicity (Luz et al 2017). In fusion-deficient nematodes, exposure to arsenite resulted in impaired mitochondrial respiration, lower ATP levels, and reduced pyruvate and isocitrate dehydrogenase activity, suggesting that disruption of pyruvate metabolism and Krebs cycle activity may underlie the observed mitochondrial dysfunction.…”