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Published in Volume
119, Issue 7 (July 1, 2009)
J Clin Invest.
2009;119(7):1849–1851.
doi:10.1172/JCI39688.
Copyright © 2009, American Society for Clinical
Investigation
Commentary
Gpx5 protects the family jewels
R. John Aitken
ARC Centre of Excellence in Biotechnology and Development, Discipline of
Biological Sciences, University of Newcastle, Newcastle, New South Wales, Australia.
Address correspondence to: R. John Aitken, ARC Centre of Excellence in
Biotechnology and Development, Discipline of Biological Sciences, Room LS4.32,
University of Newcastle, Callaghan, New South Wales, Australia. Phone:
61-2-4921-2082; Fax: 61-2-4921-6308; E-mail:
john.aitken@newcastle.edu.au. First published June 22, 2009
Immature spermatozoa are vulnerable to oxidative stress after their release from the
testes, due in part to an innate deficiency in antioxidant enzymes. The male
reproductive tract compensates for this deficiency by secreting antioxidant enzymes
such as glutathione peroxidase 5 (Gpx5) into the epididymal lumen. In this issue of
the JCI, Chabory et al. examined the phenotype of
Gpx5–/– mice and found that
while deletion of this gene did not seem to affect fertility per se, it did influence
the incidence of miscarriage and embryonic defects in mated wild-type female mice
(see the related article beginning on page 2074). Importantly, the appearance of
these problems was age dependent and associated with signs of oxidative stress in the
spermatozoa. These results demonstrate the key importance of Gpx5 as an extracellular
antioxidant designed to protect maturing mammalian spermatozoa from oxidative stress.
See the related article beginning on page 2074. Imagine you are a spermatozoon. As an immature germ cell, you lived your life within the
seminiferous tubules of the testes, in cytoplasmic communion with your brothers and
sisters, nourished and nurtured by Sertoli cells (Figure 1). Then, just when you had completed the complexities of meiosis and your gene
transcription machinery had closed down, you were forced to undergo one of the most
dramatic morphological transformations in biology — the metamorphosis of a
small, spherical haploid cell, the round spermatid, into the epitome of specialized cell
types, the spermatozoon. Once created, you were then cast free from the germinal epithelium
in a carefully regulated process known as spermiation. At this point in
your life history you have, apart from a little too much cytoplasm around your waist, the
outward appearance of a mature, functional spermatozoon. However, in reality, you are
neither of these things. You may look like a spermatozoon, but you have none of the
functional attributes of this cell; you cannot swim, cannot recognize the egg, and cannot
engage in the complex array of cell-cell interactions that lead to fertilization. You have
virtually no cytoplasm, no capacity for DNA repair, and little facility for apoptosis. And
from now on, you are on your own.
Spermatozoon functionality is acquired during two major phases of post-testicular
maturation. The first phase involves passage through a complex, convoluted, regionally
differentiated organ known as the epididymis (Figure 1). Within this organ, spermatozoa acquire the potential for progressive movement
and fertilization. However, as long as they remain within the epididymal lumen, this
potential is never realized. Instead, these cells are stored in a potentially functional
but quiescent state, exhibiting little, if any, movement, until the moment of ejaculation.
At this point spermatozoa suddenly explode into a state of vigorous progressive motility
that allows these cells to begin their ascent of the female reproductive tract to the site
of fertilization, high in the fallopian tubes. During this journey spermatozoa undergo the
second phase of their maturation known as capacitation, which allows these
cells to express the range of potential functions acquired during epididymal maturation.
These functional attributes are illustrated in Figure 1 and include: (a) a specialized form of movement known as
hyperactivation that facilitates the final stages of sperm transport and
penetration of the zona pellucida, a thick shell surrounding the oocyte; (b) an exquisitely
specific cell-cell recognition event that allows the spermatozoon to recognize a single
cell, the oocyte, while ignoring the hundreds of thousands of other cells encountered on
their journey through the female tract; (c) a secretory event, known as the
acrosome reaction that releases enzymes to further facilitate zona
penetration; (d) penetration of the zona matrix; and (e) remodeling of a specific region of
the plasma membrane in the equatorial segment of the sperm head so that it will recognize,
and fuse with, the surface of the oocyte (Figure 1).
Remarkably, the highly complex series of changes depicted in Figure 1 occur in a cell that is transcriptionally and translationally silent
and driven entirely by posttranslational protein modifications. Furthermore, this
maturation process involves a week or more of isolated existence for the spermatozoa as
they transit the epididymis, followed by a further period of isolation in the female
reproductive tract as the cells capacitate (a process by which the spermatozoon becomes
capable of fertilizing an egg) and wait for an egg to arrive.
Spermatozoa are vulnerable to oxidative stress
Throughout the whole of this period, spermatozoa are vulnerable to attack from ROS. This
vulnerability is partly due to the fact that these cells are loaded with substrates for
free radical attack, including polyunsaturated fatty acids and DNA (1, 2). These
cells are also professional generators of ROS. Indeed, they were the first cell type
ever shown to generate ROS, as described in a Nature paper published in
1946 (3), predating the discovery of this activity
in leukocytes by more than a decade. In addition, spermatozoa adopt the unusual strategy
of divesting themselves of most of their cytoplasm during spermiogenesis, thereby
shedding a key cellular constituent that, in all other cell types, offers protection
against oxidative stress through the presence of specialized antioxidant enzymes such as
catalase, superoxide dismutase, and glutathione peroxidase (Gpx).
While the highly compacted nature of mammalian spermatozoa confers upon these cells some
resistance against oxidative damage (4), they are
exquisitely dependent on the antioxidant properties of the surrounding fluids to protect
them from ROS attack. It is for this reason that both epididymal and seminal plasma are
highly enriched with antioxidants, including small-molecular-mass free radical
scavengers (vitamin C, uric acid, taurine, thioredoxin) and highly specialized
extracellular antioxidant enzymes, including unique isoforms of superoxide dismutase and
Gpx, particularly Gpx5 (5).
Gpx5 is an unusual Gpx, in that it is solely expressed in the caput epididymis under
androgenic control. It is also unusual in that it lacks a selenocysteine residue, while
still retaining its antioxidant properties (6).
This protein associates with the sperm surface during epididymal transit and protects
the spermatozoa from peroxide-mediated attack while they are undergoing maturation
(5, 6).
The functional significance of this enzyme has been beautifully illustrated by a
landmark paper in this issue of the JCI by Chabory et al. describing
the phenotype of the Gpx5-knockout mouse (7). Although deletion of the Gpx5 gene did not
appear to have any effect on male fertility, it did result in a high rate of miscarriage
in mated wild-type female mice, as well as developmental defects in the offspring.
Furthermore, these changes were only observed in embryos sired by
Gpx5-deficient mice more than one year old and were associated with
increased oxidative damage in the spermatozoa. The onset of this pathology was delayed
because of a concomitant upregulation of alternative antioxidant genes in the
epididymis, including other isoforms of Gpx and catalase
(Cat), in response to the increased oxidative stress.
Clinical relevance
These findings have substantial clinical relevance. The levels of sperm DNA damage are
often markedly elevated in cases of male infertility (2, 8). This damage is also associated with
a wide range of adverse clinical outcomes, including impaired fertilization, disrupted
preimplantation embryonic development, and, as was observed in the current study of male
Gpx5–/– mice (7), increased rates of miscarriage and offspring
morbidity (1, 2, 8, 9). Although these adverse clinical effects are not always observed with the
same degree of penetration in independent clinical studies, the overall weight of
evidence suggests that oxidative DNA damage in the male germ line is potentially
damaging for both the progress of pregnancy and the health and wellbeing of the
offspring. Moreover, like the
Gpx5–/– mice, the DNA damage we see
in human spermatozoa is commonly oxidative in nature (10) and is positively associated with male age (11, 12). Furthermore, recent studies
in an animal model for aging studies, the brown Norway rat, have confirmed that
increased paternal age is associated with increased levels of DNA damage and oxidative
stress in epididymal spermatozoa (13).
The clinical consequences of this DNA damage to sperm can be seen in the wide variety of
pathologies that have been observed in children and young adults in association with
advanced paternal age, including dominant genetic diseases and complex polygenic
neurological conditions such as bipolar disease, epilepsy, spontaneous schizophrenia,
and autism (2, 8, 14–16). In keeping with these data, the experimental injection of
DNA-damaged spermatozoa into mouse oocytes has not only been shown to affect embryo
development but also to significantly affect the behavior, postnatal growth, and
longevity of the offspring as well as their susceptibility to tumors (17). These data also echo previous reports
associating paternal smoking, oxidative damage to sperm DNA, and the incidence of cancer
in children (18, 19).
Under physiological circumstances, spermatozoa possessing oxidatively damaged DNA would
be prevented from fertilizing the oocyte because collateral peroxidative damage to the
sperm plasma membrane would disrupt the process of sperm-oocyte fusion (20). However, when intracytoplasmic sperm injection
(ICSI) is used as the in vitro fertilization technique, such biological barriers to
fertilization are completely bypassed (21). Since
the use of such assisted reproductive technologies is rising exponentially, it is highly
likely that conceptions involving oxidatively damaged spermatozoa are a relatively
common occurrence in current clinical practice. The data generated by Chabory et al.
(7) serve to remind us that the health and
well-being of children generated in assisted conception clinics is at risk and should be
carefully monitored, or we will have to deal with the consequences in the years to come.
Footnotes
Conflict of interest: The author has declared that no conflict of
interest exists. Nonstandard abbreviations used: Gpx5, glutathione peroxidase 5. Citation for this article:J. Clin. Invest.119:1849–1851 (2009). doi:10.1172/JCI39688 See the related article beginning on page 2074.
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