Fighting infection, the true confessions of a little worm barely... a millimeter long
Caenorhabditis elegans is a roundworm widely used by geneticists to study the development and function of living organisms. In 1998, it was the first multicellular animal whose genome sequence was published in full. The majority of human genes are known to have functional equivalents in this worm. Importantly, it is technically very easy to turn off or turn on a gene of C. elegans and to analyze the biological consequences of this manipulation.
Jonathan Ewbank and Nathalie Pujol are using this "simplified model" to better understand the mechanisms of defense against infections by fungi and bacteria.
In the competition for the title of best model for research in the life sciences, C. elegans offers several advantages over other models, such as drosophila or the mouse. Its genome and its anatomy are very simple, it is fast growing, and above all it is perfectly transparent. To study the role of a gene, in addition to the usual manipulations (deletions, mutations ...), we can, thanks to a clever construction based on a gene borrowed from a fluorescent* jellyfish, observe visually where and when this gene literally ... lights up!
C. elegans lives in soil among decaying plants and feeds on bacteria, which exposes it to the danger of infection. It protects itself by three main strategies:
- Escape: the worm has olfactory neurons that permit it to detect hostile microorganisms and move away from them (in the same way it detects edible microorganisms before ingesting them).
- Tools, such as the cuticle, made of chitin and collagen which surrounds and protects the organism, or a small grinder in its throat that crushes ingested bacteria so they do not pass into the intestine intact.
- Finally, an inducible defense system, which resembles the immediate innate immunity of mammals. This mechanism allows the recognition of pathogens and harmful factors they produce, but also the prevention of damage caused by these attacks by inducing the release of antimicrobial compounds and molecules responsible for tissue repair.
A living organism that may be manipulated genetically
on a very large scale
The team of Jonathan Ewbank has studied in particular the response of C.elegans to infection by a fungus called Drechmeria coniospora.
Worms crawling through their bacterial food on a Petri dish, seen under the microscope. Courtesy of Parafilms
This organism attacks from the outside by secreting enzymes that break through the cuticle and then infects through the skin. In response, the genes responsible for production of antifungal molecules are activated in the epidermis, the site of the attack by the fungus.
"If we establish a list of genes turned on in response to infection by different types of agents, these may be divided into two subsets," according to J. Ewbank, "genes specifically induced by a single agent, according to its nature, and common genes induced by all microorganisms. If you cause a sterile wound in the cuticle of a worm, it induces some of the defense genes observed during an attack of D. coniospora. This strategy allows C. elegans to prevent an increased risk of infection due simply to the rupture of a physical barrier."
"Each gene may be inactivated, which allows us to measure its impact on the ability of the worms to protect themselves from infection"
Some pathogens such as Pseudomonas aeruginosa infect both nematodes and man, which makes the model more relevant to study the mechanisms of innate immunity against infection. "We have developed a technology platform that allows us to evaluate the involvement of each of the 19,000 genes of C. elegans in antimicrobial defense and host-pathogen interactions," says Jonathan Ewbank.
"Each gene may be inactivated individually, which allows us to measure its impact on the ability of the worms to protect themselves from infection. Using reporter genes (a gene that produces a fluorescent protein upon contact between the worm and microbe), we may directly identify all genes whose activity is required for the response to infection. This systematic approach is automated and computational tools integrate and analyze all the data collected on a large-scale."
The simple genetic model makes this type of exhaustive investigation possible and permits applications in the exploration of other complex biological processes. Hence today this "universal" C. elegans genetic platform is available for immunologists and neurobiologists as well as developmental biologists who want to benefit from the opportunities offered by this small worm, barely a millimeter long.
* GFP (Green Fluorescent Protein) is a naturally fluorescent protein from a small jellyfish called Aequorea victoria. Its gene may be fused in vitro with the gene of a protein to be studied. This fusion gene, called recombinant, is then introduced into a worm that will synthesize the corresponding protein of interest, which is now fluorescent. It can be followed over time and space using a simple fluorescence microscope. In 2008, Martin Chalfie, Osamu Shimomura and Roger Tsien were awarded the Nobel Prize in Chemistry for this discovery.