Giardia under the microscope. [Suha Al Naimi and Kevin Tyler]
Researchers at the University of East Anglia (UEA) say they have discovered how the Giardia parasite is able to break apart cells in the gut and feed off them. The team reports (“Giardia Secretome Highlights Secreted Tenascins as a Key Component of Pathogenesis”) in GigaScience that the organism produces two main types of protein that enable it to cut through layers of protective mucus in the gut, breaking the links that knit cells together in order to access the nutrients within them.
This protozoan parasite, which people typically pick up through drinking infected water or contaminated food, causes giardiasis, with symptoms including diarrhea and stomach pains. Currently more than 200,000 people are ill with giardiasis and there are 500,000 new cases a year.
The team looked at laboratory cell cultures infected with Giardia to see what the parasite was producing that could be interacting with cells in the gut. Of the two families of protein identified, the researchers discovered that one mimics tenascins, which regulate cell adhesion during wound healing and tissue remodeling. They help cells to break apart when necessary and balance the proteins that glue the cells together. The Giardia parasite has evolved by making proteins that are similar to humans and able to trigger this process.
However, the scientists found that the Giardia tenascins are used instead to upset the body's balance by preventing healing of the junctions between cells that hold them together.
“The genome of Giardia is believed to contain ORFs [open reading frames] which could encode as many as 6,000 proteins. By successfully applying quantitative proteomic analyses to the whole parasite and to the supernatants derived from parasite culture of assemblages A and B, we confirm expression of ~1,600 proteins from each assemblage, the vast majority of which being common to both lineages. To look for signature enrichment of secreted proteins, we considered the ratio of proteins in the supernatant compared with the pellet which defined a small group of enriched proteins, putatively secreted at a steady state by cultured growing trophozoites of both assemblages. This secretome is enriched with proteins annotated to have N-terminal signal peptidewrite the investigators.
“The most abundant secreted proteins include known virulence factors such as cathepsin B cysteine proteases and members of a Giardia superfamily of cysteine rich proteins which comprises VSPs [variant-specific surface proteins], HCMPs [high cysteine membrane proteins] and a new class of virulence factors, the Giardia tenascins. We demonstrate that physiological function of human enteric epithelial cells is disrupted by such soluble factors even in the absence of the trophozoites. We are able to propose a straightforward model of Giardia pathogenesis incorporating key roles for the major Giardia derived soluble mediators.”
“We've discovered an entirely new model for how this disease develops in the gut, which can also explain why in some people the symptoms can be more severe. Because the Giardia have broken down the cell barriers and made all these nutrients available, other, opportunistic bacteria can move in to take advantage of these 'ready meals' which can make giardiasis even more severe for some,” said Kevin Tyler, Ph.D., from UEA's Norwich Medical School. “Giardia was one of the very first disease-causing microbes to be visualized; scientists have known of its existence since 1681. But this is the first time we have been able properly to understand why this parasite is so successful.”
The next step for the team is to look at whether neutralizing these proteins can provide therapy for the illness and to ask whether differences in these molecules, between parasites that cause more severe disease and those that do not, can be used to identify the more dangerous strains, which is not currently possible.