Oxidative stress and male reproductive health

Abstract

One of the major causes of defective sperm function is oxidative stress, which not only disrupts the integrity of sperm DNA but also limits the fertilizing potential of these cells as a result of collateral damage to proteins and lipids in the sperm plasma membrane. The origins of such oxidative stress appear to involve the sperm mitochondria, which have a tendency to generate high levels of superoxide anion as a prelude to entering the intrinsic apoptotic cascade. Unfortunately, these cells have very little capacity to respond to such an attack because they only possess the first enzyme in the base excision repair (BER) pathway, 8-oxoguanine glycosylase 1 (OGG1). The latter successfully creates an abasic site, but the spermatozoa cannot process the oxidative lesion further because they lack the downstream proteins (APE1, XRCC1) needed to complete the repair process. It is the responsibility of the oocyte to continue the BER pathway prior to initiation of S-phase of the first mitotic division. If a mistake is made by the oocyte at this stage of development, a mutation will be created that will be represented in every cell in the body. Such mechanisms may explain the increase in childhood cancers and other diseases observed in the offspring of males who have suffered oxidative stress in their germ line as a consequence of age, environmental or lifestyle factors. The high prevalence of oxidative DNA damage in the spermatozoa of male infertility patients may have implications for the health of children conceived in vitro and serves as a driver for current research into the origins of free radical generation in the germ line.

Figure 1

Proposed cycle of cause and effect by which oxidative stress in the male germ line impacts upon the health and well-being of future generations. (1) A variety of primary factors can initiate oxidative stress in the male germ line including infection, age, obesity and exposure to a variety of adverse environmental influences. (2) This initial oxidative stress induces lipid peroxidation culminating in the production of lipid aldehydes such as 4HNE, which bind to proteins in the mitochondrial electron transport chain, stimulating the generation of reactive oxygen species (ROS). The latter stimulate yet more lipid peroxidation in a self-propagating cycle that culminates in apoptosis. (3) One of the most sensitive targets of oxidative stress is the DNA in the sperm nucleus, generating 8-hydroxy, 2’deoxyguanosine (8OHdG) base adducts. (4) The first enzyme in the base excision repair pathway, 8-oxoguanine glycosylase 1 (OGG1), is present in spermatozoa and its activity creates abasic sites. The remainder of the DNA repair pathway is present in the oocyte. The oocyte has to repair the DNA damage brought into the zygote by the fertilizing spermatozoon before the initiation of S-phase for the first mitotic division. (5) If the oocyte makes a mistake at this stage of DNA repair, it has the potential to create a mutation that will be represented in every cell in the body and could account for the range of pathologies seen in the offspring of fathers exhibiting high levels of oxidative DNA damage in their spermatozoa. Abbreviations: IVF, in vitro fertilization; ROS, reactive oxygen species.

Figure 2

The unique architecture of spermatozoa influences the impact of apoptosis on DNA integrity. (a) Conventional somatic cells feature a centrally placed nucleus surrounded by mitochondria embedded in the cytoplasm. Under these circumstances, endonucleases activated in the cytoplasm or released from the mitochondria during apoptosis are able to move into the nucleus (arrows) and cleave the DNA. (b) Spermatozoa are completely different from such somatic cells because their mitochondria (stained black) and most of their cytoplasm are located in the midpiece of the cell, physically separated from the nucleus. (c) As a consequence of this compartmentalization key effectors of apoptosis such as apoptosis inducing factor (AIF) or Endonuclease G (Endo G) remain resolutely locked in the sperm midpiece even when apoptosis is induced by the powerful PI3 kinase inhibitor, wortmannin and cannot move into the sperm nucleus. Because of this physical limitation, most DNA damage in mature spermatozoa is induced by membrane permeant reactive oxygen species emanating from the mitochondria, rather than nucleases.