Characterization, expression dynamics, and potential function of OPA1 for regulation of mitochondrial morphology during spermiogenesis in Phascolosoma esculenta

Mitochondria undergo morphological changes during spermatogenesis in some animals. The mechanism and role of mitochondrial morphology regulation, however, remain somewhat unclear. In this study, we analyzed the molecular characteristics, expression dynamics and subcellular localization of optic atrophy protein 1 (OPA1), a mitochondrial fusion and cristae maintenance-related protein, to reveal the possible regulatory mechanisms underlying mitochondrial morphology in Phascolosoma esculenta spermiogenesis. The full-length cDNA of the P. esculenta opa1 gene (Pe-opa1) is 3 743 bp in length and encodes 975 amino acids. The Pe-OPA1 protein is highly conservative and includes a transmembrane domain, a GTPase domain, two helical bundle domains, and a lipid-interacting stalk. Gene and protein expression was higher in the coelomic fluid (a site of spermatid development) of male P. esculenta and increased first and then decreased from March to December. Moreover, their expression during the breeding stage was significantly higher than during the non-breeding stage, suggesting that Pe-OPA1 is involved in P. esculenta reproduction. The Pe-OPA1 protein was more abundant in components consisting of many spermatids than in components without, indicating that Pe-OPA1 mainly plays a role in the spermatid in coelomic fluid. Moreover, Pe-OPA1 was mainly detected in the spermatid mitochondria. Immunofluorescence experiments showed that the Pe-OPA1 are constitutively expressed and co-localized with mitochondria during spermiogenesis, suggesting its involvement in P. esculenta spermiogenesis. These results provide evidence for Pe-OPA1's involvement in the regulation of mitochondrial morphology during spermiogenesis.