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

Abstract

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.