Remote controlled sperm
How sperm gear themselves up to fertilise an egg is very difficult to study. For a start, standardising the conditions and then being able to reproducibly study each sperm cell without unintentionally altering its biochemistry is extremely tricky. And this is what inspired a German team to come up with a way to make remote controlled sperm that you can turn on with a flash of light. Chris Smith spoke to Dagmar Wachten and Vera Jansen from The Center of Advanced European Studies and Research.
Dagmar - We wanted to control sperm function by light. And the reason was because light has no side-effects, at least in the way we are using it. So pharmacology, for instance, when you use pharmacological substances they have loads of side-effects and we wanted to get away from that by using optogenetics.
Chris - You can shine a light on sperm and adjust its level of activity?
Vera - Yes, that's true. So, basically what we have done is we have genetically engineered mice. Introducing an enzyme that is sensitive to light and what this enzyme does is by stimulating it using light, it produces cyclic AMP. This is an intracellular messenger that is really important for sperm function. And what cyclic AMP does in sperm is that it controls sperm motility. And now, when we activate this enzyme in sperm by shining light on them, they produce cyclic AMP and thereby we can control sperm motility.
Chris - And when you shine the light one the sperm, Vera, what's the response? They make this second messenger but how do they respond? Is it just a change in motility or do other effects kick in as well?
Vera - So, various functions in sperm are controlled by cyclic AMP. One immediate effect of the increase in cyclic AMP is the change in motility which is an acceleration of the flagella beat frequency. But there are also long-term effects of elevated cyclic AMP and this the so called sperm capacitation which is a maturation process which is needed for sperm to fertilize an egg successfully. By illumination, we could actually trigger this long-term maturation process which is called capacitation.
Chris - Does this mean, Vera, that you could potentially follow the signals through the sperm cells. So, if you were to illuminate one small part of the sperm could you then unpick how the signal is propagated or cascades through the sperm turning on these different effector functions which are all involved in activating a sperm so it can fertilize an egg?
Vera - We really hope to solve this issue in the future. So, we want to expand actually the optogenetic tool box in sperm by not only introducing a photoactivated adenylyl cyclase but also photoactivated phosphodiesterase. And we want to combine these tools together with tools to actually follow the dynamics of cyclic AMP on the single cell level. And with this expanded tool box, we really hope to decipher the processes in the sperm which actually make sperm fertile.
Chris - How reliable do you think your method is from a physiological standpoint? Because if one takes a sample of sperm and shines light on them, the amount of response or the amount of light hitting an individual sperm is not going to be the same from one sperm to the next. And therefore, the degree of activation to make a certain amount of cyclic AMP may differ from one sperm to the next. So, how sure are you that this is a faithful and reproducible way to study how these sperm work?
Vera - We're really trying to use the very same light dose that we shine on individual sperm. And thereby, we can more or less get the same response in individual sperm. When we do that in sperm populations, the response is also similar in the whole population. So, we're quite sure that independent of the light dose that we're using, we get a very similar response in the whole population of sperm.
Chris - And there's no danger, Vera, that by making this particular construct, adjusting or altering the sperm in this way to make them light-sensitive, that you have in some way changed them in a way you don't know and therefore some of the responses you're seeing to light might be ones that you didn't expect or can't account for?
Vera - The side-effects, of course, we don't really know if we changed something by introducing these enzymes but the processes that we know which are regulated by cyclic AMP, these processes we checked by control experiments to show that actually the normal cyclic AMP dependent processes which take place in the sperm are not altered in the transgenic sperm.