Oxidative stress in semen: bad for reproduction?

In the last few years, there have been a strong focus on the physiology of sperm, and andrology in general is going through a sort of golden age in Assisted Reproduction. One aspect that has received significant attention is the effect of Oxidative Stress (OS) on sperm health, and its possible relationship with reproductive outcomes. In order to understand how OS is created and how it affects the cell, we need a short reminder in basic cell biology. Reactive oxygen species (ROS) are normal by-products of metabolism and small amounts of them are required for some cellular processes (Wright et al 2014); ROS include oxidative species such as hydroxyl radicals (OH) and non radical species such as superoxide anion (O-2) or hydrogen peroxide (H2O2). In spermatozoa, O-2 is the main ROS produced, and generates H2O2 spontaneously (Aitken and Clarkson 1987; Alvarez et al 1987; Wright 2014); O-2 and H2O2 are needed for functions such as sperm capacitation (i.e. the process by which the glycoprotein coat and the seminal proteins are removed from the surface of the sperm permitting binding of sperm to the oocyte). A delicate balance of ROS and antioxidants is required for efficient fertilization. H2O2 stimulates capacitation while the antioxidant enzyme Catalase prevents it (Aitken et al 1995; Wright 2014). Moreover, it has been shown that while low concentrations of H2O2 are required for capacitation, elevated concentrations reduce hyperactivation, zona pellucida binding and acrosome reaction (Oehninger et al 1995).

What factors affect ROS generation in semen? ROS can be generated by spermatozoa. Immature and teratozoospermic sperm produces more ROS, mainly due to excess cytoplasmic residues in the middle-piece (Tremellen et al 2008); however, there are other sources of ROS and some factors that could modify the production. Leukocytes are another important source of ROS. An increased concentration of ROS and decreased antioxidant capacity has been found in men with varicocele (Wright et al 2014). Xenobiotics, toxic metals, heat, mobile phone radiations, chronic disease, smoking, age, and obesity could also increase the production of ROS by different mechanisms (Wright et al 2014, Tremellen et al 2008).We encounter Oxidative stress (OS) when the concentration of ROS is too high and/or antioxidant protection ability is exceeded. Seminal plasma contains antioxidants that prevent sperm oxidative attack after ejaculation to a certain extent; however, during spermatogenesis and epididymal storage, the sperm are not in contact with seminal plasma and must rely on the antioxidant capacity of the epididymal and testicular environment (Tremellen et al 2008).

What damage can ROS make in semen? ROS can damage DNA directly by producing oxidized DNA adducts leading to DNA sites without bases, which in turn could cause single-strand breaks (Badouard et al 2008, Wright et al 2014). ROS also damage the sperm membrane and, consequently, the spermatozoon’s motility, as well as its ability to bind to the oocyte membrane, is affected (Tremellen 2008). Mitochondrial DNA is also damaged by ROS, limiting ATP production (Shamsi et al 2008). OS is considered to be the principal cause of DNA damage in spermatozoa (Aitken et al 2010). Single-strand DNA breaks (ssDNA) across the genome is related to OS in sperm (Ribas-Maynou et al 2012; Ribas et al, 2013). DNA repair in spermatozoa occurs during the chromatin remodeling steps in elongating spermatids and ends during the nuclear condensation in the epididymis (Leduc et al 2008, Marcon et al 2004). However, spermatozoa can be exposed to oxidative damage not only in the epididymis but also in vas deferens (Ribas-Maynou 2012); from this stage, the next chance of DNA repair is by the oocyte, which may be capable of repairing DNA adducts but not ssDNA breaks, leading to impaired fertilization and poor pregnancy outcomes (Gonzalez-Marin et al 2012, Menezo et al 2007).

Post testicular oxidative DNA damage targets specific chromatin domains of lower compaction associated with histones and attached to the sperm nuclear matrix (Noblanc et al 2013). These specific domains of the paternal genome are enriched in genes involved in the control of post-fertilization DNA replication events and in the developmental program of the embryo (Hammoud 2009).

Does OS in semen affect reproductive outcomes? Although there are clear evidences of a negative effect of OS on sperm physiology, the relationship between sperm OS and ART outcomes is unclear. On one hand, a relationship with severe seminal alterations has been shown (Tremellen 2008), which should lead in theory to worse reproductive outcomes at least in natural conception. However a sound relationship with pregnancy rates outcomes in ART has not been established and rigorous studies are needed to overcome this knowledge gap.

Rita Vassena
Scientific Director, Clinica EUGIN