Study question: In addition to sperm motility, which major biological characteristics of sperm fertility potential are associated with mitochondrial functionality?
Summary answer: Sperm fertilization capacities, including acrosin activity, acrosome reaction (AR) capability and chromatin integrity, are related to the mitochondria functionality as evaluated by the mitochondrial membrane potential (MMP).
What is known already: Correlative studies suggest a potential role of sperm MMP in predicting sperm fertilization ability and ensuring sperm motility. However, researches characterizing other determinants of sperm fertility potential according to MMP are lacking.
Study design, size, duration: The sperm MMP was examined in 627 young college students in the Male Reproductive Health in Chongqing College Students (MARHCS) cohort study in 2014. Among these participants, acrosin activity and chromatin integrity were measured in 378 and 604 subjects, respectively. These two determinants of sperm fertility potential were first compared among high-, moderate- and low-MMP groups in the college population. The effects of MMP collapse caused by carbonyl cyanide 3-chlorophenylhydrazone (CCCP) on acrosin activity, AR, DNA fragmentation, reactive oxygen species (ROS) production, and ATP content in human spermatozoa were evaluated in vitro.
Participants/materials, setting, methods: The sperm MMP was evaluated by using JC-1 staining, acrosin activity was measured using a N-α-benzoyl-dl-arginine-para-nitroanilide HCl (BAPNA) substrate method, the integrity of chromatin represented by DNA fragmentation index (DFI) was measured by sperm chromatin structure assay (SCSA), AR was evaluated with chlortetracycline staining, and intracellular ROS production was evaluated with dihydroethidium. ATP concentration was determined with luciferase. Measurements were performed by spectrophotometry or flow cytometry.
Main results and the role of chance: Nonparametric analysis revealed significantly higher acrosin activity and a lower DFI in subjects with moderate or high MMP compared to those with low MMP. After adjustment for potential confounders, increases of 7.9 and 44.4% in sperm acrosin activity and deceases of 12.0 and 25.2% in the sperm DFI were found in the moderate- and high-MMP groups, respectively. The MMP dissipation induced by CCCP caused significant declines in acrosin activity and AR capacity and increased DFI in human spermatozoa. Moreover, sperm MMP dissipation induced ROS overproduction and decreased ATP content.
Limitations, reasons for caution: We cannot exclude a contribution of leukocytes to ROS production and no size gating was used to exclude these cells from the FACS measurements. No simultaneous live-dead staining was done and a contribution of dead sperm to the MMP and acrosome assays cannot be excluded.
Wider implications of the findings: Mitochondrial functionality might be necessary to maintain sperm acrosin activity, AR and chromatin integrity. Tests of mitochondrial functionality should be developed and used independently of or in addition to conventional semen parameters in infertility diagnosis or risk-assessment processes.
Study funding/competing interest(s): This study was supported by the Key Program of the National Natural Science Foundation of China (No. 81630087) and the National Natural Science Foundation of China (No. 81703254). None of the authors have any competing interests to declare.