Raptor and Drp1 Function Synthetically to Control Hypoxic Death and the Mitochondrial Network in <i>Caenorhabditis elegans</i>

Hypoxia induces mitochondrial fragmentation. Whether this fragmentation promotes or prevents cell death and whether the mitochondrial dynamics machinery plays a role are unresolved. To address these questions, we measured the effect of hypoxia on mitochondrial morphology in a Caenorhabditis elegans Raptor mutant resistant to hypoxic death and in mutants with disrupted mitochondrial fission and fusion. The Raptor loss-of-function mutant reduced hypoxia-induced mitochondrial fragmentation and death. However, forcing mitochondrial fragmentation prior to hypoxia by combining the Raptor mutation with a loss-of-function mutation in mitofusin did not increase hypoxic death. A loss-of-function mutation in drp-1, which is required for mitochondrial fission, did not block hypoxia-induced mitochondrial fragmentation nor enhance Raptor hypoxia resistance; rather, drp-1(lf) was surprisingly mildly hypoxia resistant and partially suppressed the high-level hypoxia resistance of the Raptor mutant. Likewise, loss of DRP-1 function interacted synthetically with the Raptor(lf) mutant to produce tangled mitochondria, demonstrating a role of Raptor in maintenance of the mitochondrial network. Vitamin B12 supplementation and feeding with a bacterial strain replete in vitamin B12 mitigated hypoxia-induced mitochondrial fragmentation. Our results demonstrate that fragmented mitochondria do not necessarily promote hypoxic cell death, and hypoxia-induced mitochondrial fragmentation is mechanistically distinct from physiological mitochondrial fission.