Feb. 10, 2010, 10:39 a.m.
Wednesday, 20 January 2010 Anna Salleh
German researchers have worked out how the malaria parasite is able to burrow through the skin and into our body.
The study of sporozoites - the highly mobile stages of the malaria parasite - is published in the current issue [until Jan 21] of the journal Cell Host & Microbe.
"We show that sporozoite motility is characterised by a continuous sequence of stick-and-slip phases," write Dr Sylvia Münter of the Heidelberg University and colleagues.
When a malaria-carrying mosquito bites a human, single-celled 'worm-like' sporozoites burrow through the skin and into the body.
They breed in liver cells and are then released into the bloodstream, where they infect red blood cells and cause the symptoms of malaria.
But how exactly do these parasites move around mosquitoes and humans?
Sporozoites don't have a tail to propel them along, like sperm, and they don't move like amoeba, says Australian malaria expert, Dr Paul Gilson of the Burnet Institute in Melbourne, who writes an accompanying commentary on the new research.
Gilson says scientists had suggested that sporozoites moved using sticky patches on their outside connected to an internal 'motor' of actin and myosin, which are the proteins that enable human muscle to flex.
The idea was that the motor pulled the sticky proteins from one end of the cell to the other, enabling it to glide slowly forward.
But Münter and colleagues took a closer look and found sporozoites moved in a more complicated way.
They found that the cells showed bursts of fast and slow movement.
"The sporozoites had two gears - like a slow gear and a fast gear," says Gilson.
Using a special microscopy technique Münter and colleagues identified exactly when the cells were attaching to a surface and when they weren't.
"It would be like sitting underneath a glass coffee table watching a slug crawl across the top of it and looking at the waves of movement that are happening," says Gilson.
"They were able to do that but at a microscopic level."
Münter and colleagues found the cell has a very complex pattern of attachment points underneath.
First, small sticky patches form on the front and rear and the actin-myosin motor pushes the cell forward very slowly, says Gilson.
Then a large sticky patch forms in the middle of the cell, wanting to push it forward quickly.
But the slow-moving sticky patches at either end hold the cell back and tension builds up until the back and front patches break.
"The cell then shoots forward using the middle sticky patch," says Gilson.
The front and rear patches reform, causing the middle sticky patch to break and the whole process repeats itself.
"There's a degree of complexity to this movement that people haven't realised before," says Gilson.
He says better understanding the basic biology of how the parasites move helps with the development of anti-malarial drugs.
He says vaccines against sticky proteins are currently being developed and drugs could be designed to disrupt the actin-myosin motor.