Male Infertility and Oxidative Stress: A Focus on the Underlying Mechanisms

Abstract

Reactive oxygen species (ROS) play a critical role in defining the functional competence of human spermatozoa. When generated in moderate amounts, ROS promote sperm capacitation by facilitating cholesterol efflux from the plasma membrane, enhancing cAMP generation, inducing cytoplasmic alkalinization, increasing intracellular calcium levels, and stimulating the protein phosphorylation events that drive the attainment of a capacitated state. However, when ROS generation is excessive and/or the antioxidant defences of the reproductive system are compromised, a state of oxidative stress may be induced that disrupts the fertilizing capacity of the spermatozoa and the structural integrity of their DNA. This article focusses on the sources of ROS within this system and examines the circumstances under which the adequacy of antioxidant protection might become a limiting factor. Seminal leukocyte contamination can contribute to oxidative stress in the ejaculate while, in the germ line, the dysregulation of electron transport in the sperm mitochondria, elevated NADPH oxidase activity, or the excessive stimulation of amino acid oxidase action are all potential contributors to oxidative stress. A knowledge of the mechanisms responsible for creating such stress within the human ejaculate is essential in order to develop better antioxidant strategies that avoid the unintentional creation of its reductive counterpart.

Keywords: NADPH oxidase; amino acid oxidase; antioxidants; oxidative stress; reactive oxygen species; sperm mitochondria; spermatozoa.

Figure 1

Examples of some of the estrogenic compounds that have been found to induce mitochondrial ROS generation in purified populations of human spermatozoa. ROS generation generally occurs via the redox cycling on these compounds within the mitochondria.

Figure 2

Postulated role for NOX 5 in driving sperm capacitation in human spermatozoa. This is a calcium sensitive NADPH oxidase that generates both the superoxide anion and protons. A possible pathway for human sperm capacitation begins with a reduction in ambient zinc concentrations as they move from the high zinc environment provided by seminal plasma to the relatively low zinc environment offered by the female reproductive tract. Since zinc inhibits the Hv1 proton channel, this change favours cytoplasmic alkalinization. As the spermatozoa ascend the female tract and prepare for fertilization, they are stimulated by progesterone which, in combination with the independent increase in cytoplasmic pH induced by Hv1 activation, triggers an influx of calcium via the low-voltage-dependent calcium channel, CatSper. CatSper-dependent calcium influx activates NOX5, generating O₂^•− which, along with bicarbonate and calcium, stimulates adenylyl cyclase and the production of cAMP. NOX5 also produces the protons required for optimal Hv1 activity. Furthermore, the intracellular conversion of O₂^•− into H₂O₂ by superoxide dismutase consumes protons, thereby facilitating the alkalinization of the cells’ interior. The H₂O₂ generated by this dismutation reaction promotes cholesterol oxidation and efflux from the plasma membrane [56,120] and simultaneously suppresses tyrosine phosphatase activity, thereby facilitating the increase in tyrosine phosphorylation that accompanies capacitation. Thus, in a synergistic cooperation, Catsper, Hv1, and NOX5 control the primary drivers of capacitation including cholesterol efflux, cAMP generation, cytoplasmic alkalinization, and elevated intracellular free calcium. After Seredenina et al. [121].

Figure 3

A summary of some of the factors contributing to oxidative stress in human spermatozoa. The major sources of ROS include disturbed electron flow in the sperm mitochondria, enhanced amino acid oxidase activity, or an increase in ROS generation by NADPH oxidases such as NOX5. Some of the drivers for enhanced ROS generation via these pathways are highlighted. Oxidative stress can also arise because of deficiencies in antioxidant protection due to impaired intake in the diet, or high levels of antioxidant turnover/consumption due to chronic ROS generation, associated with conditions such as obesity, varicocele, and the detoxification of environmental toxicants.