Mitochondrial Functionality and Mammalian Sperm Quality: Implications for Fertility

Mammalian sperm quality is often clinically investigated by assessing parameters such as sperm motility and morphology as indicators of sperm health and male fertility. Though commonly assessed, the physiology and pathophysiology underlying sperm quality parameters remains to be fully elucidated. This knowledge gap limits the clinical and scientific knowledge of male fertility across mammalian species. Study of mammalian sperm physiology is complicated by interspecies variation, which has left differing knowledge gaps between species. The interspecies variation also challenges efforts to advance the field of human male fertility, for which more appropriate models than the commonly used rodent models are needed. The overall objective of this dissertation research was to investigate the physiology and pathophysiology of mammalian sperm quality parameters frequently assessed in the clinical setting, specifically morphology and motility. This dissertation includes assessment of bull, stallion and canine sperm to address specific knowledge gaps in sperm physiology and pathophysiology of said species. I aimed to further explore the physiologic significance of sperm quality parameters by investigating their relationship with mitochondrial functionality and oxidative metabolism in multiple mammalian species. This was accomplished in four chapters. Chapter 1, Mammalian sperm quality and oxidative metabolism- a review, provides a review of the literature pertaining to mammalian sperm quality and oxidative metabolism, emphasizing knowledge gaps, disagreements in existing literature, and implications for male fertility. In Chapter 2, Effects from disruption of mitochondrial electron transport chain function on bull sperm motility, I investigate the relationship between mitochondrial function and motility in bull sperm by simultaneously measuring motility parameters and mitochondrial oxygen consumption in the presence of several mitochondrial effector drug treatments. The bull was chosen as the model for this chapter due to conflicting results in the literature regarding the role of mitochondria in fueling bull sperm motility. Motility parameters were only observed to differ significantly from the vehicle with antimycin (inhibitor of complex III of the electron transport chain) treatment, for which significant decreases in numerous parameters, including total motility (p=0.007), progressive motility (p=0.01), and velocity parameters, such as VAP and VSL (p<0.05), were identified. These results indicate that bovine sperm motility is impacted by mitochondrial functionality. In Chapter 3, the novel use of imaging flow cytometry was employed to investigate the relationship between sperm morphology and reactive oxygen species generation in fresh and cool-stored stallion sperm. The stallion was used as the model in this chapter in part due to literature associating the percentage of morphologically abnormal sperm with elevated ROS and decreased fertility, but limited information regarding ROS generation of specific morphologic abnormalities. Compared to morphologically normal sperm, ROS production was significantly higher in cells with abnormal heads, proximal droplets and abnormal midpieces in both fresh and cooled semen. These results indicate that excessive oxidative stress contributes to the pathophysiology of morphologic abnormalities, which may contribute to findings previously reported in the literature linking certain morphologic abnormalities to decreased fertility in stallions. In Chapter 4, we evaluated the repeatability and accuracy of the tablet-based Canine iSperm® instrument compared to computer-assisted sperm analysis (CASA) for assessment of motility (total and progressive) in fresh and frozen-thawed canine sperm. This tablet-based system is available for both stallion and canine sperm, but prior to our study, published validation was only available for stallion sperm. Correlational analysis revealed significant positive correlations between CASA and iSperm® assessment for both fresh and frozen-thawed samples. These results indicate the iSperm® system offers an accurate alternative to CASA for measurement of canine sperm motility. This tablet-based system for motility assessment is substantially more affordable to veterinary practitioners than conventional CASA systems and could allow for more clinical data collection of canine sperm motility, which would be invaluable to future research efforts to expand knowledge of canine sperm physiology. The multispecies work in this dissertation reports findings in sperm quality, physiology and bioenergetics. The results of this work directly contribute to current clinical knowledge by improving the diagnostic power of commonly employed sperm quality measures and by elucidating some of the underlying pathophysiology of sperm morphologic abnormalities and sperm motility. Further, the findings from this dissertation research informs future research efforts, with potential applications in advancement and improvement of mammalian male fertility, sperm preservation techniques, species conservation efforts and mitochondrial physiology