For sea urchin sperm, finding an egg to fertilize in a vast ocean might seem like looking for a needle in a haystack. However, these prickly creatures have devised a highly effective strategy to overcome this hurdle: eggs release chemical factors that guide the sperm towards them, a process called chemotaxis. Now, scientists from the Center of Advanced European Studies and Research in Germany have discovered how sea urchin sperm navigate up a gradient of attractant.
|Tracking of calcium signals (green) from a sperm cell swimming in a chemoattractant gradient (blue)|
(credits: Luis Alvarez and René Pascal from Stiftung Caesar)
Sperm chemotaxis is commonly found in nature and is important for fertilization. Most animal species with external fertilization- such as marine invertebrates like sea urchins- and even some plants, use chemical attractants to guide sperm towards the egg. However, the molecular details of sperm chemotaxis, particularly in mammals, such as humans, are still not well understood.
Research on mammalian sperm chemotaxis presents many challenges: direct measurements can only be carried out in vitro and only about 10% of sperm respond to attractants. In contrast, fertilization in sea urchins can be mimicked in the laboratory, and 'sperm are mostly homogeneous in their responses' the researchers say.
When sea urchin sperm detect an attractant, they adjust their swimming trajectory by changing the beating of the tail (flagellum). The attractant of Arbacia punctulata, the sea urchin species used in this study, is a small molecule called 'resact'. Resact released by the egg binds to receptor proteins on a sperm’s flagellum, and this causes calcium ions to enter the cell. The calcium rise controls the flagellar beat and tunes the swimming path of sperm, but exactly how this happens remains unclear.
In the study published in September in The Journal of Cell Biology, Benjamin Kaupp’s group shows how sea urchin sperm sample and integrate the attractant cues to adjust their course as they swim towards the egg.
|Sperm oozing out of the sea urchin gonopores (credit: René Pascal from Stiftung Caesar)|
The scientists placed sea urchin sperm in tiny chambers and then added caged resact, a modified version of the molecule that is activated by a flash of UV light. Using caged resact, the scientists were able to stimulate the sperm with the attractant at precise time intervals. They found that sperm count resact molecules for about 0.2 to 0.6 seconds before producing a calcium response- they called this 'sampling time'. 'A defined or optimal sampling time is essential,' says Nichiket Kashikar, leading author in the study 'either too short or too long sampling times will leave the sperm astray'.
Sperm are also able to correct themselves, for instance, by stopping a calcium response and initiating a new one, or 'resetting'. But how does resetting affect swimming? To answer this question, the scientists recorded videos of single sperm cells stimulated with resact during a calcium surge. 'During the reset, sperm show an extended period of straight swimming, thereby spending more time swimming up the gradient of attractant.' explains Kashikar 'Simple rule: if the conditions are improving, continue in the same direction'.
The authors propose that this newly found sperm 'navigation system' might be used by other species. 'Although there are likely to be species-specific differences, there might be some commonalities across species' Kashikar says. It remains to be discovered whether similar mechanisms exist in human sperm.
'Chemotaxis is clearly important for sea urchins' notes David Clapham, an expert on calcium sensors from the Howard Hughes Medical Institute Boston Children’s Hospital in the United States 'However, [in mammals] investigators will have to demonstrate that a progesterone gradient exists in the path of swimming sperm in females and that sperm respond to this gradient, not the factor alone'. Kaupp’s team trusts that this might be possible in a near future. 'The experimental tools developed to study chemotaxis in model systems (such as sea urchin) and the chemotactic principles identified might help to design experiments to study chemotaxis of sperm in human and other mammalian species'.
A shorter version of this article was published in ScienceNow on the 19th of September 2012. You can read it here.