Towards the male contraceptive

The viability of male sperm is preserved in many species by the location of the testes in the body, where the temperature is maintained at a lower value than the core temperature of the body. This really is a vital factor in the reproductive system, because many studies have shown that mildly increased temperatures precipitate increased sperm cell death.

This apoptotic process is reversible, with cell viability returning to normal several weeks following the cessation of heating. While proving problematical for the reproductive process, this effect has been examined as a potential route to human male contraception.

In a Chinese-US collaboration, a team of scientists published a report in 2007 which confirmed that testicular warming suppressed spermatogenesis. Following a short heat treatment over 6 consecutive days, the sperm concentration began to fall after 3 weeks, reaching a minimum of 21 x 10⁶/mL after 6 weeks. Thereafter, the concentration began to rise until it reached the pre-treatment value of about 87 x 10⁶/mL 12 weeks after treatment.

Although the effect has been proven, and the associated physiological and cellular responses are well documented, the molecular mechanisms behind the sperm depletion and recovery processes remain somewhat of a mystery. So, the same scientific team have undertaken a proteomic study to try and identify the proteins and pathways involved in heat-induced spermatogenesis suppression.

Senior reporter Jiahao Sha and co-researchers from Nanjing Medical University, the Jiangsu Family Planning Research Institute, Nanjing, and Harbor-UCLA Medical Center & Los Angeles Biomedical Research Institute, Torrance, CA, published their findings in Proteomics.

They obtained testicular biopsies from men before treatment began, then subjected them to 6 consecutive days of testicular warming at 43°C for 30 min. Further biopsies were taken 2 weeks following treatment, when increased cell death had set in, and 9 weeks following treatment, when recovery was under way.

Proteins were extracted from all of the tissue samples and separated by 2D gel electrophoresis. They were quantified by image analysis following silver staining to reveal that 45 and 36 proteins were expressed in different quantities for the 2 and 9 week samples, respectively.

The proteins were identified following in-gel digestion with trypsin and MALDI mass spectrometric analysis on a time-of-flight instrument. The peptide masses were searched against the IPI human database and protein identification was confirmed by sequence information obtained by MS/MS.

A total of 31 and 26 known proteins were identified from the 2 and 9 week samples, respectively and they were subjected to network analysis using a commercial software program.

After 2 weeks of heat treatment, 25 of the 26 identified proteins were found to fit in the functional network, with most of them being involved in proliferation, differentiation, apoptosis and cell survival. The levels of most proteins involved in reducing apoptosis or aiding proliferation were reduced, while those promoting apoptosis and preventing proliferation were increased. The result was sperm depletion.

The situation was reversed 9 weeks after treatment, with apoptosis being suppressed and cell proliferation and differentiation on the increase.

These trends in the functional protein abundances indicate that they are the key targets of heat treatment during spermatogenesis.

The researchers performed more detailed studies on one of the heat-regulated proteins, heterogeneous nuclear ribonucleoprotein H1 (HNRNPH1), using the mouse testis model from birth to mature animals. HNRNPH1 is an anti-apoptosis protein with the ability to regulate the expression of heat-related proteins. When HNRNPH1 expression was reduced, the levels of 10 other proteins fell markedly.

So, the molecular targets implicated in the reversible suppression of spermatogenesis following heat treatment have been identified. The team suggested that further studies will be able to provide more specific information on the mechanisms involved and could lead to the identification of specific molecules as targets for male contraceptive development.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.