426. The role of sperm mitochondria in the process of epididymal maturation

Abstract

On leaving the testis, spermatozoa can neither swim nor achieve fertilisation of the oocyte. These functional properties are acquired as spermatozoa engage a process of post-testicular maturation in the epididymis. Research into the biochemical basis of sperm maturation has revealed that this process is associated with activation of sperm mitochondria. Immature spermatozoa from the caput epididymis displayed a low mitochondrial membrane potential (MMP) whereas mature spermatozoa from the caudal epididymis actively maintained a high MMP. Moreover mitochondrial generation of reactive oxygen species could be triggered by antimycin in mature caudal epididymal spermatozoa but not in immature cells recovered from the caput epididymis. The molecular mechanisms responsible for regulating mitochondrial function were reversible since washing the cells free of epididymal fluid allowed immature spermatozoa to acquire a high MMP while incubating mature caudal cells in caput epididymal fluid, suppressed MMP. These results strongly suggested that fluid from the caput epididymis contains a mitochondrial inhibitor and that activation of mitochondrial activity is due to the removal or inactivation of this inhibitor during epididymal transit. The causative factor is not species specific because incubation of ejaculated human spermatozoa in murine epididymal fluid effectively suppressed their MMP. Caput epididymal fluid was fractionated using FPLC and each of the fractions was tested for their bioactivity. A major protein band with a molecular mass ~150 kDa was present in the active fraction. Further characterisation of this reversible mitochondrial inhibitor may reveal the mechanisms by which epididymal spermatozoa control mitochondrial function during maturation. It may also contribute to our understanding of human male infertility and potentially serve as a novel target for male fertility regulation.