Mitochondrial respiratory chain proteins can be organised into supercomplexes, such as ATP synthase dimers and respirasomes (comprised of complexes I, III2 and IV). Dissociation of both ATP synthase dimers and respirasomes into monomeric complexes is associated with ageing, and the latter with age-related disorders such as Parkinson’s disease. Conversely, increased respirasome stability has been observed in certain types of cancer. Unfortunately, it has thus far been impossible to exploit this known relationship for drug development due to lack of knowledge regarding the biological role of respiratory supercomplexes.
We have used electron cryo-tomography supported by biochemistry to study the effect of reducing respirasome stability on mitochondrial morphology and respiratory chain organisation in the model organism C. elegans. RNAi technology was used to reduce respirasome stability by knocking down an accessory subunit of CI, called NDUFA11, required for interaction with CIII. Mitochondria were then isolated from NDUFA11 knockdown and empty vector control animals for analysis. Mitochondrial morphology was studied through segmentation and quantification of tomograms of whole mitochondria, and respiratory chain organisation by sub-tomogram averaging of densities corresponding to the ATP synthase and complex I.
We found that reduced respirasome stability was associated with aberrant crista membrane morphology, impaired respiration and severely inhibited reproduction, and are currently exploring the effect on supercomplex abundance and stoichiometry. This study also reveals a novel ATP synthase architecture unique to C. elegans. Further investigation revealed a relationship between ATP synthase architecture and crista shape, when we compared sub-tomogram averages and segmentations with those from other organisms. We speculate that a range of dimer angles may have evolved to alter crista diameter and thus suit bespoke energetic needs.