Telling sperm to sort themselves out.

Gatica et al. (2013) report the development of a new device, the sperm selection assay (SSA), for enriching the content of high-quality, functionally competent cells in a human sperm sample. The device is technically simple, but exploits a sophisticated characteristic of many motile cells and organisms—their ability to direct their movement in response to the concentration gradient of a chemical cue. If the cell or organism responds to a chemical by moving up the concentration gradient (positive chemotaxis) it is directed towards the source of the chemical. This behaviour is clearly valuable for a sperm as it seeks the oocyte and is known to occur in external fertilizers, where the oocytes of a number of species have been shown to release chemoattractants (Burnett et al., 2008; Kaupp et al., 2008). Whether the guidance of sperm to the cumulus–oocyte complex occurs within the female tract of mammals has been the subject of research and controversy for some years (Eisenbach and Giojalas, 2006; Kaupp et al., 2008). One of the best candidate molecules for chemotactic guidance of human sperm is progesterone, which is synthesized by the cumulus and the oocyte. Analysis of the behaviour of human spermobserved under laboratory conditions shows that they bias the direction of their swimming in response to gradients formed by low concentrations of this steroid (Teves et al., 2006; Guidobaldi et al., 2008). These authors now report that they can use this response to select sperm of high functional quality. A simple assay to test the ability of an agent to induce positive chemotaxis in vitro is to provide the chemical at a single point and observe whether the resulting concentration gradient causes cells to accumulate at the source.Progesterone is effective in such anassaybut interpretation is complicated by the fact that progesterone at high concentrations can induce hyperactivated motility in a minority of cells (Calogero et al., 1996). Since hyperactivated sperm are non-progressive (or often negligibly so) sperm arriving randomly at the progesterone source may become ‘trapped’ at the progesterone source, giving the impression of accumulation due to chemotaxis (Jaiswal et al., 1999). Teves et al. (2006) tackled this problem elegantly by using video microscopy to monitor the vectors of human sperm swimming in a progesterone gradient—amore direct assessment of chemotactic behaviour. This revealed that nanomolar to micromolar concentrations of progesterone increased hyperactivation but did not affect the direction in which the cells were moving, but at very low (10 pM) concentrations the distribution of motility vectors showed a statistically significant bias towards the progesterone source. This effect was subtle, suggesting that only a small minority of cells (≈10%) were directing their movement up the progesterone gradient. This is believed to reflect the limitation of chemotactic responsiveness in mammalian sperm to cells that are competent to fertilize—capacitatedcells (Cohen-Dayag et al., 1995). Though this complicates attempts to characterize chemotaxis in human sperm (see below) it is also potentially very useful. The quality of cells in a single human ejaculate is highly variable. If the ability to capacitate (and thus respond chemotactically) is limited to a subpopulation of ‘high-quality’ cells it may be possible to exploit chemotactic activity to select those cells (Cohen-Dayag et al., 1995). This is the basis of the SSA. Gatica et al. present evidence that cells can indeed be selected in this way, not only showing increased levels of markers for capacitation (tyrosine phosphorylation, ionophore-induced acrosome reaction) but also having lower levels of oxidative stress and less DNA damage (Comet assay). Furthermore, since the run time of the SSA (20 min) is relatively short, it was possible to run a ‘selected’ sample for a second time and further enhance the proportion of cells that were classified as capacitated. The SSA was particularly effective when the level of sperm classified as capacitated was low in the initial sample and worked even with samples obtained from subfertile men. A surprising aspect of the findings is that migration through a chemotactic gradient apparently increased the absolute number of cells that were classified as capacitated, whereas exposing cells to similar concentrations of progesterone, but not as a gradient, did not have this effect. This is an observation that demands further investigation. In sperm of marine invertebrates positive chemotaxis in response to cues released by the oocyte brings about the contact between the gametes that is required for fertilization (Kaupp et al., 2008). These sperm tend to swim in circles or spirals but when they encounter the chemoattractant molecule released by the egg (typically a peptide) they perform a stereotypical behaviour that has been called ‘turn and run’, composed of a sharp turn and brief near-linear segment, before returning to their circling. A series of these behaviours moves the cell incrementally towards the chemoattractant source. Though there is undoubtedly more to be studied and understood, the signalling events that are induced by binding of the chemoattractant and the way in which they modify activity of the flagellum to cause this behaviour have been characterized (Darszon et al., 2008; Kaupp et al., 2008). The occurrence of sperm chemotaxis in externally fertilizing vertebrates iswell established. For instance, a chemoattractantmolecule (allurin) and behavioural responses to that molecule have been described for the sperm of Xenopus laevis (Burnett et al., 2008). In contrast, our understanding of chemotactic behaviour in mammalian sperm is poor. Undoubtedly, the fact that only a small minority of sperm will (or can)