Monday, April 26, 2010

Preventing Parasitic Infection

Nearly one third of the people that will die this year will die from an Infectious disease worldwide. So what exactly is an Infectious disease? An infectious disease is an illness derived from a pathogenic microbe. A pathogenic microbe can range from bacteria, a virus, a fungus, or a parasite. Current research is focused on preventative and medicinal treatments that can attack the microbe before it can invade the host body. One way a drug can disrupt the microbe from invading the host cell is to use small-molecules to prevent the pathogen from invading the host cell.
Invading pathogens have proteins on their outer shell that can be used to identify the pathogen or the proteins can be used to attach to a host cell. These proteins can also be used to locate and disable a foreign microbe from invading your body. This study focuses on identifying small-molecules that can disrupt the ability of a certain pathogen, Toxoplasma gondii. T. gondii is the causative agent in toxoplasmosis, and is related to Plasmodium which causes malaria.  1Toxoplasmosis is a parasite that can infect humans, but is transmitted to humans by the common housecat.  2People and animals can become infected by being exposed to contaminated meat, fecal matter of an infected cat, or from a mother to her fetus. Roughly one third of the world is estimated to be carrying the Toxoplasma parasite. Symptoms of infection are mild flu like symptoms. However, if you have a weakened Immune System or are pregnant, the infection may cause more serious symptoms such as swelling of the brain and neurological disease, or it can be fatal especially to the fetus.
T. gondii has two distinct phases in its lifecycle. The first phase is the sexual stage. The sexual stage takes place in humans and in cats, the pathogen invades a cell and produce bradyzoites (form of the pathogen).  Bradyzoites most commonly found in muscle or in the brain, are continually being produced until the host cell bursts from the infection. The burst cell releases the replicated bradyzoites which are now called tachyzoites.  Tachyzoites are the mobile form of the pathogen that can infect new cells or pass into the small intestine. The tachyzoites can be killed off by the host immune system once the host cell has burst. However, if the tachyzoites reach the small intestine, the tachyzoites produce oocytes which get excreted in fecal matter. The production of the oocytes is the sexual phase of the T. gondii life cycle. The shed oocytes can then be passed onto humans by consuming unwashed vegetables or eating infected meat.
The molecular mechanism by which T. gondii invades cell is still unknown, but is crucial to survival of the pathogen. Although the 3mechanism by which cells are invaded isn’t known, it is known that small-molecules can inhibit the invasion of host cells by the parasite T. gondii. The current experiment tested 12,160 small molecules for their ability to prevent the pathogen from invading cells. The experiment was carried out by placing equal amounts of the differing small-molecules into wells with possible host cells and one invading parasite (T. gondii) and one non-invasive parasite. The invasive T. gondii pathogens were labeled with a yellow fluorescent protein that allows the parasite to be visualized using a microscope. The effectiveness of the small-molecules on preventing the Toxoplasma pathogen from invading host cells was determined visually by looking to see if any Toxoplasma pathogens made it into the cell. If yellow specks were seen in the cell, the cell was invaded by the pathogen and the small-molecule did not prevent the pathogen from entering the cell.
Of the 12,160 small-molecules tested, only twenty-four molecules non-cytotoxic prevented invasion by the Toxoplasma parasite. After identifying the twenty-four inhibitory small-molecules, nineteen of the small-molecules effects could be reversed. That leaves five small-molecules that cause irreversible effects to the Toxoplasma pathogen.  
The twenty-four small molecules were then examined to determine how they exerted their effects on the parasite.  There are five ways that the parasite can be inhibited, but only three were examined. The first mechanism studied was the motility of the Toxoplasma pathogen. Of the 24 inhibitory molecules, 21 prevented the parasite from becoming mobile by inhibiting slime trail formation which helps the parasite glide across a surface. A second mechanism that was studied was the formation of a conoid extension. A conoid extension extends and retracts repeatedly as the parasite moves across a cell. None of the inhibitory small-molecules caused a conoid extension, while three inhibited extension but did not affect motility of the parasite. The final mechanism studied was the secretion of microneme. Micronemes are secretory organelles that help the parasite attach to the host cell.  18 of the 24 small-molecules inhibited the secretion of a certain microneme protein. However, the effect of inhibiting microneme protein secretion on parasite-host relationships was not studied.
The study found 24 out of 12,160 small molecules inhibited the invasion of T. gondii into a host cell.  The 24 molecules that inhibited invasion of a pathogen into a host cell can be used to study how the parasite infects the host cell. Further characterization of the inhibitory molecules can be used to help determine how each of the molecules prevents the invasion into a host cell. By studying Toxoplasma gondii, the molecular mechanism by which the parasite infects cells can be studied. By identifying the mechanism of invasion, further infections of the Toxoplasma pathogen and other pathogens related to it can be prevented.
3Carey, K et al. “A Small-Molecule approach to studying invasive mechanisms of Toxoplasma gondii.” Proceedings of the National Academy of Sciences in the United States of America. Doi. 10.1073

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