Viruses are the ultimate parasites. They are incapable of performing any biochemical functions associated with "life" outside of a host cell, making them completely dependent on another organism for their survival. At the same time they are marvels of evolutionary efficiency. With just a few genes and gene products, viruses are able to hijack Host cells. This process leads to all sorts of different disease manifestations. How these seemingly simple entities exert their effects is fascinating.
Our group studies the pathogenic mechanisms of the type-2 turkey astrovirus (TAstV2) as a model for understanding how astroviruses (and other small round RNA viruses) causes acute diarrhea in poultry and other animals.
TAstV2 induces Na+ malabsorption
Most medical textbooks describe five major causes of diarrhea. Secretory: something happens that causes the body to pump fluid into the intestine. Osmotic: there are high levels of solutes in the intestine that create osmotic forces drawing water into the gut. Exudative: there is destruction of the cells lining the intestine which can lead to bloody and/or pussy stool. Malabsorption: something affects how well the body can digest and absorb nutrients leading to osmotic imbalance and voluminous stool. Deranged motility: something happens to change the contractions of the muscles in the gut and therefore the speed and volume of contents being excreted.
Most intestinal pathogens are associated with exudative diarrhea. Part of how they infect, replicate, or interact with the intestine leads to the killing of gut cells and inducing inflammation. This is part of why the term gastroenteritis (gut inflammation) is almost a synonym for diarrhea. Astroviruses; however, cause diarrhea without inducing inflammation, or killing the cells of the intestine. While there are other gut pathogens known to causing diarrhea without inflammation or other tissue damage, they are more the exception than the rule, and leads to the question how is astrovirus causing diarrhea? Work in our lab and in partnership with Dr. Anthony Blikslager we have found that during astrovirus replication inside the gut epithelia cells, the function of various host surface proteins involved in digesting and absorbing ions are suppressed. Specifically, a protein called sodium-hydrogen exchanger-3 (NHE3) that is responsible for moving sodium ions from the intestine into the cell, in exchange for hydrogen ions so it can maintain the cell's electrical charge. During astrovirus infection the total amount of expression of this protein seems to be the same, but where the protein is located changes. Instead of being inserted in the cell membrane so it can interact with the sodium ions in the gut, most of the protein is found inside the cell leading to a build-up of sodium ions in the gut lumen and the production of a malabsorption diarrhea.
It is currently not clear if this is the only way astrovirus infection leads to diarrhea. In reality, the five major causes listed above never really exist in isolation. The start of diarrhea by one mechanism will lead to changes in other systems making the diarrhea worse. We are currently trying to understand how well this sodium malabsorption explains the amount of diarrhea produced, and how exactly does the astrovirus infection lead to changes in where NHE3, or other host proteins, are located in the cell.
Gut epithelial cells induce iNOS in response to TAstV2 infection
A lot of the diarrhea produced by infections, is the direct result of the immune system responding to the invader and trying to kill the pathogen. Curiously, following astrovirus infection there is very little response by the immune system. There is no strong antibody response, no protection to subsequent infections, and no influx of immune cells to the site of infection. In spite of the lack of response by "professional" immune cells, the virus is cleared. We are interested in understanding why doesn't the immune system respond like one would expect. Like it does to so many other intestinal infections. Additionally, if the cells of the immune system aren't primarily responsible for getting rid of the astrovirus infection, what is?
In previous studies we had seen that immune cells, specifically macrophages, were exposed to the astrovirus they would produce a compound called nitric oxide (NO). This is a highly reactive nitrogen-oxygen radical and is made by immune cells to kill bacteria, viruses, and other disease agents. So we then asked the question, could the cells of the gut epithelium be responding to astrovirus infection with their own NO and using it to protect themselves since no one else was coming to their rescue. While its still not clear how big a role NO plays in clearing astrovirus infections, it does appear that these cells can, and do express the enzyme (inducible nitric oxide synthase, iNOS) that makes NO and expression of iNOS is increased following astrovirus infection and in areas of the gut where astrovirus in replicating. We are currently working to understand what other defense factors are also made by gut cells, and what exactly is preventing the "professional" immune system from helping in the fight against this pathogen.
Meliopoulos VA, Marvin SA, Freiden P, Moser LA, Nighot P, Ali R, Blikslager A, Reddivari M, Heath RJ, Koci MD, Schultz-Cherry S. Oral Administration of Astrovirus Capsid Protein Is Sufficient To Induce Acute Diarrhea In Vivo. MBio. 2016 Nov 1;7(6). pii: e01494-16. doi: 10.1128/mBio.01494-16. PubMed PMID: 27803180; PubMed Central PMCID: PMC5090040.
R. R. Meyerhoff, P. K. Nighot, R. A. Ali, A. T. Blikslager, and M. D. Koci. Characterization of turkey inducible nitric oxide synthase and identification of its expression in the intestinal epithelium following astrovirus infection. Comparative Immunology, Microbiology, and Infectious Diseases. 35:63-9, 2012.
P. K. Nighot, A. Moeser, R. A. Ali, A. T. Blikslager, and M. D. Koci. Astrovirus Infection Induces Sodium Malabsorption and Redistributes Sodium Hydrogen Exchanged Expression. Virology. 401:146-54, 2010.
E. Strain, L. A. Kelley, S. Schultz-Cherry, S. V. Muse, and M. D. Koci. Genomic Analysis of Closely Related Astroviruses. Journal of Virology. 82: 5099-5103, 2008.
M. D. Koci, L. A. Kelley, D. L. Larsen, and S. Schultz-Cherry. Astrovirus-Induced Synthesis of Nitric Oxide Contributes to Virus Control During Infection. Journal of Virology. 78:1564-1574, 2004.
M. D. Koci, L. A. Moser, L. A. Kelley, D. L. Larsen, C. C. Brown, and S. Schultz-Cherry. Astrovirus Induces Diarrhea in the Absence of Inflammation and Cell Death. Journal of Virology. 77:11798-11808, 2003.
E. Behling-Kelly, S. Schultz-Cherry, M. D. Koci, L. A. Kelley, D. L. Larsen, and C. Brown. Localization of Astrovirus in Experimentally Infected Turkeys, as Determined by In Situ Hybridization. Veterinary Pathology. 39:595-598, 2002.