Redox Regulation of Human Sperm Function: from the Physiological Control of Sperm Capacitation to the Etiology of Infertility and DNA Damage in the Germ Line

Defective sperm function is the largest single defined cause of human infertility and one of the major reasons we are witnessing an exponential increase in the uptake of assisted conception therapy in the developed world. A major characteristic of defective human spermatozoa is the presence of large amounts of DNA damage, which is, in turn, associated with reduced fertility, increased rates of miscarriage, and an enhanced risk of disease in the offspring. This DNA damage is largely oxidative and is closely associated with defects in spermiogenesis. To explain the origins of this DNA damage, we postulate that spermiogenesis is disrupted by oxidative stress, leading to the creation of defective gametes with poorly remodeled chromatin that are particularly susceptible to free radical attack. To compound the problem, these defective cells have a tendency to undergo an unusual truncated form of apoptosis associated with high amounts of superoxide generation by the sperm mitochondria. This leads to significant oxidative DNA damage that eventually culminates in the DNA fragmentation we see in infertile patients. In light of the significance of oxidative stress in the etiology of defective sperm function, a variety of antioxidant therapies are now being assessed for their therapeutic potential.