SDHA is a component of the succinate dehydrogenase (SDH) complex and plays a critical role in mitochondria in both the citric acid cycle and mitochondrial respiration. In the citric acid cycle SDHA converts succinate to fumarate. Additionally, this reaction contributes electrons to the electron transport chain, responsible for driving ATP synthesis. C. elegans have two orthologues of SDHA, SDHA-1 and SDHA-2.
Here, we show that mutation in sdha-2 results in dramatically reduced male fertility due to defective sperm activation. We found that C. elegans harbouring an sdha-2 SNP produce a significantly diminished brood size, one fifth of that of wild-type animals. In vitro sperm activation assays reveal that most mutant sperm do not activate from spermatids to spermatozoa, failing to grow the pseudopod required for motility. We show that, as a result, mutant sperm fail to localise to the spermatheca in hermaphrodites, the site of oocyte fertilisation. We repaired the sdha-2 SNP in the endogenous locus to wild-type sequence using CRISPR-Cas9, and demonstrate complete rescue of brood size and sperm activation.
We have identified similar sperm motility defects in sdha-1 mutant animals, further implicating succinate dehydrogenase function, and in icl-1 mutant animals. ICL-1 catalyzes the cleavage of isocitrate to succinate and glyoxylate in the glyoxylate cycle. This suggests that an imbalance in succinate may underlie the dramatic brood size defect. Additionally, we performed an EMS suppressor screen in sdha-2 mutant animals and have identified several candidate strains in which the brood size is rescued. Characterising the suppressor mutations in these strains should provide clues about the pathway by which SDHA-2-dependent sperm activation failure occurs.
Our results demonstrate a role for SDHA-2 in sperm motility and male reproductive health. In humans, mutations in SDHA are associated with Leigh Syndrome, and this strain may provide a new animal model of Leigh Syndrome.