Wednesday, April 3, 2013

Coming Attractions: Sperm Wars—The Bourgeonal Strikes Back


Earlier this year I declared that the Bourgeonal Sperm Wars were over. Now it seems that announcement may have been a bit . . . premature.

The Prequel
Bourgeonal, which smells like lily of the valley, activates the OR1D2 olfactory receptor found in the human nose and on human sperm cells. Bourgeonal is also the only odorant to which men are more sensitive than are women, and sperm respond to it as well. This led some researchers to suggest that bourgeonal is a chemosignal that guides sperm to the egg.

However, Timo Strünker and colleagues demonstrated that the CatSper Ca2+ ion channel, the molecular mechanism which triggers the relevant behavioral changes in sperm, is activated not only by bourgeonal but by steroids, prostaglandins, menthol, and helional (everything but the kitchen sink). As a finishing touch, Strünker et al. found that the OR1D2 receptor itself is not necessary for bourgeonal to activate the CatSper channel. So much for the notion of sperm tracking the egg by lily of the valley scent.

The New Releases
Now come two papers linking olfactory perception of bourgeonal to human infertility. The first is by olfaction researcher Thomas Hummel and colleagues at the University of Dresden Medical School. They gave smell tests to men with idiopathic infertility, i.e., guys from couples trying unsuccessfully for two years to get pregnant in the absence of a medical cause. The infertile guys were compared to proven fathers of similar age.

The groups did not differ in overall smell ability, or in sensitivity (detection threshold) to bourgeonal and a couple of other floral scents: helional and phenylethyl alcohol. However, infertile guys perceived the odor of bourgeonal as significantly less intense at above-threshold concentrations. The perceived the other two scents the same as the fertile guys. According to Hummel et al., decreased olfactory sensitivity to bourgeonal may relate “to a decreased functionality of OR1D2, which in turn may be linked to idiopathic infertility.”

I know what you’re thinking: this is a slender thread on which to hang the bourgeonal story. But the same week, a paper by an Italian research group at the University of Padua came up with a finding that took the story even further.

Led by Giancarlo Ottaviano, the group compared a group of infertile men to a set of healthy controls. The infertile guys were significantly less sensitive to the odor of bourgeonal (detection threshold), and their sperm were significantly less likely to migrate in the direction of bourgeonal. Genetic analysis of the OR1D2 receptor showed that single nucleotide polymorphisms (tiny genetic mutations) were more common among the infertile men. The Padua team believes they have confirmed the importance of olfactory receptor-based bourgeonal detection in human male infertility.

The Review
Based on the Strünker papers I thought the sperm-guided-by-bourgeonal idea was probably an artifact. However, these new findings are not easy to dismiss. It looks like there will be more episodes of Sperm Wars in the future.

The studies discussed here are “Decreased perception of bourgeonal may be linked to male idiopathic infertility,” by Charlott Sinding, Eva Kemper, Petra Spornraft-Ragaller, and Thomas Hummel, published online in Chemical Senses, March 27, 2013, and “Human olfactory sensitivity for bourgeonal and male infertility: a preliminary investigation,” by Giancarlo Ottaviano, D. Zuccarello, M. Menegazzo, L. Perilli, G. Marioni, A.C. Frigo, A. Staffieri, and C. Foresta, published online in European Archives of Otorhinolaryngology, March, 2013.

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