Relationships between mitochondrial DNA content, mitochondrial activity, and boar sperm motility

Energy produced by mitochondria via oxidative phosphorylation (OXPHOS) is essential for mammalian sperm motility. Mammalian mitochondrial DNA (mtDNA)-encoded proteins are subunits of the OXPHOS system. Paradoxically, there are less mitochondrial and mtDNA contents in motile sperm than less motile sperm. Here, mature boar sperm was used as a model to investigate the relationships between mtDNA content, mitochondrial activity, and sperm motility. Motile and less motile sperm were separated by centrifugation on a discontinuous percoll density gradient. The contents and expression of mtDNA as well as mitochondrial activity and biosynthesis were determined to reveal possible mechanisms. Motile sperm showed less mitochondrial (P < 0.01) and mtDNA (P < 0.05) contents as compared to less motile sperm. Higher mitochondrial activity in motile sperm indicated by mitochondrial ultrastructure, higher mitochondrial transmembrane potential (P < 0.01), as well as higher mitochondrial respiratory chain complex I activity (P < 0.05). Moreover, more mitochondrial reactive oxygen species (P < 0.01) suggested higher mitochondrial biosynthesis in motile sperm. Although less mtDNA contents in motile sperm, accompanied by the higher expression of transcription factors, the level of mtDNA-encoded protein (cytochrome c oxidase subunit 1) which play pivotal role in OXPHOS was higher in motile sperm. The results are helpful to interpret why mtDNA-less sperm have higher mitochondrial activity and better motility.