A moving tail – studying sperm motility

Human sperm sample showing an exceptional sperm count.

Infertility affects around 80 million couples worldwide, with problems in sperm motility playing a major role. Chrissie Giles spoke to some of the leading researchers exploring how sperm move to understand more about this fascinating and complex process.

For sperm it’s all about being in the right place at the right time. On average, a human ejaculation contains over 100 hundred million sperm, yet fertilisation will not happen unless one of these moves towards, binds to and fuses with an egg.

Some one in 15 men are thought to have defects in their sperm, making this the single most common cause of infertility. Defects include abnormal shape, low number and low motility. Some 80 per cent of cases of low motility – essentially, where a sperm cannot swim towards the egg – involve defects in the sperm’s flagellum, or tail, which drives its movement.

The flagellum drives the sperm towards its target and, through furious waggling, helps it burrow through the outer layers of the egg. Both these processes, it has emerged, are controlled by calcium within the sperm. Researchers are examining sperm from organisms as diverse as sea urchins and humans to understand how calcium is involved – work that has implications not only for the treatment of infertility but also for the development of a ‘male Pill’.

Micrograph of a single human sperm

Professor Alberto Darszon from the Universidad Nacional Autonoma de Mexico has been studying sperm motility for over two decades. Much of his work has involved sea urchin sperm, which are particularly useful for imaging as they swim in circles when placed on a flat surface, such as the bottom of a Petri dish.

But to be able to relate how tiny changes in calcium concentration in different parts of sperm affects the flagellum, he needed to be able to measure calcium levels at the same time as watching them move. And so, through several Trust-supported collaborations, Professor Darszon joined forces with imaging specialist Professor Michael Whitaker from the University of Newcastle to develop new ways of studying sea urchin sperm

As Professor Whitaker explains: “Previously, people measured calcium in bulk solution, without imaging. When you do this you’re effectively looking at calcium in the sperm’s head as the volume of the tail is tiny in comparison. The calcium levels in the head don’t oscillate very much, but in the tail they do, so when you image sperm you can see the changes in the tail.”

Unlike humans, the sea urchin is an external fertiliser – which means that the egg and sperm are spawned into sea water at the same time. Sperm must locate the egg, a process that is made easier by chemical attractants released by the egg, such as the peptide speract. Using the new techniques, in 2003 Professors Darszon and Whitaker working with Dr Christopher Wood were the first to show that speract caused transient calcium oscillations in the tails of sea urchin sperm.

Movie showing movement of sea urchin sperm before and after exposure to the peptide speract. The sperm cells contain a fluorescent dye for quantifying cellular calcium concentrations. The ultraviolet flash 8 seconds in activates the caged speract. Credit: Professor Alberto Darszon.

Building on this work, Professor Darszon and colleagues from the National Institute of Advanced Industrial Science and Technology in Japan then produced a synthetic version of speract that could be activated very quickly, and developed a specialised fluorescence microscopy imaging system. This has allowed the researchers to record calcium changes in a single swimming sperm, the form of its tail and its trajectory at the same time. He is now applying the techniques developed with sea urchin sperm to track mouse and human sperm.

Making tracks

Constrained within the female tract, human sperm have perhaps an easier task finding an egg than sea urchin sperm, floating in the sea, do. But when human sperm reach the egg, they still have to break into it, a process aided by switching from forward (progressive) swimming to a ‘hyperactivated’ style.

In progressive swimming, the sperm’s tail is relatively symmetrical and conservative, taking it on a zig-zagged path to the egg. But in hyperactivated swimming, the front end of the tail bends and its beating becomes asymmetrical. This causes the sperm to swim in figures of eight and circles, and their heads see-saw frantically from side to side, helping them burrow through the layers of the egg.

“Calcium seems to control two things that sperm have to do in the right place at the right time: progressive swimming, to move towards the egg, and hyperactivated swimming, to get into it,” says Dr Stephen Publicover from the University of Birmingham. He is working with Professor Chris Barratt from the University of Dundee to unpick the biological basis of low sperm motility.

Series of images of human sperm showing calcium being mobilised at the neck, and the flagellum (tail) bending. When the cell removes the calcium, the flagellum straightens out. Hot colours represent high calcium levels; cold colours represent low calcium.

“We’re interested in how the sperm get activated,” says Professor Barratt. ”The data we have suggest that if you can’t ‘flip’ [hyperactivate] then your chances of fertilising an egg are almost zero.” He adds that calcium does have an important role so we will be able to determine more about the mechanisms and how to change them. If there are specific defects in some of the sperm we study then we would have a good starting point for drug discovery.”

CatSpers are sperm-specific proteins that are thought to form a calcium channel in the tails of sperm. They are known to be important in hyperactivation of mammalian sperm, but this may not be the whole story.

In their research, Dr Publicover and Professor Barratt are particularly interested in the role of calcium stored in the neck region of the sperm, where the head and tail join. As the researchers work with humans, they cannot use sperm that have been genetically modified to investigate the function of specific proteins. Instead, they are using drugs to manipulate processes that occur inside the sperm.

One of these drugs is a compound called 4-aminopyridine. A very potent inducer of hyperactivation in human sperm, it’s thought that this compound may activate CatSpers, as well as potentially triggering the emptying of the calcium store in the sperm neck.

Using semen from men with motility-related low fertility, the researchers are therefore examining hyperactivation when sperm are exposed to 4-aminopyridine, the female hormone progesterone or increased pH - the latter two being conditions that sperm naturally meet in the female tract.

Ultimately, Professor Barratt hopes that this work will produce a more effective way of diagnosing defective sperm motility in men. This would enable doctors to identify which couples could, and which could not, benefit from expensive and potentially stressful procedures such as in vitro fertilisation and intrauterine insemination, which require motile sperm.

As well as its use in helping patients conceive, this research could be applied to preventing pregnancy, for example, in the development of a ‘male Pill’. “Putting it simply, if you have lots of infertile men who produce sperm but cannot fertilise an egg, then all you have to do is repeat what is wrong with them,” says Professor Barratt. “It’s no good shutting down spermatogenesis entirely, nor should you interfere with calcium on a systemic level. You’d need very specific drugs, and that’s where I think the tremendous excitement is.”

Further reading

• Darszon A et al. Sperm-activating peptides in the regulation of ion fluxes, signal transduction and motility. Int J Dev Biol 2008;52:595-606.
• Kaupp UB et al. Mechanisms of sperm chemotaxis. Annu Rev Physiol 2008;70:93-117.
• Publicover S et al. [Ca2+]i signalling in sperm – making the most of what you’ve got. Nat Cell Biol 2007;9:235-42.
• Wood CD et al. Speract induces calcium oscillations in the sperm tail. J Cell Biol 2003;161:89-101.
• Wood CD et al. Real-time analysis of the role of Ca(2+) in flagellar movement and motility in single sea urchin sperm. J Cell Biol 2005;169:725-31.
• Wood CD et al. Altering the speract-induced ion permeability changes that generate flagellar Ca2+ spikes regulates their kinetics and sea urchin sperm motility. Dev Biol 2007;306:525-37.

Glossary

• 4-amino pyridine: A chemical that is commonly used to block potassium channels but also releases stored Ca2+ from and induces hyperactivation in human sperm.
• Calcium: A chemical element that, as Ca2+, is involved in many processes in living cells and organisms, including signal transduction and muscle contraction.
• CatSpers: Short for cation channels of sperm, these proteins form channels for the movement of calcium ions and are thought to be specific to mammalian sperm.
• Hyperactivation: A type of sperm motility in which the tail beating becomes asymmetrical, thought to help the sperm burrow through the layers of the egg before fertilisation.
• Motility: The ability to move spontaneously.
• Speract: A peptide hormone produced by the eggs of some sea urchins that attracts sperm towards the egg.