How do viruses hijack our brains to make us vomit - and can we stop it?

Human rotavirus particles - http://faculty.riohondo.edu

We've all experienced it; that is, the awful unwell sickness that overwhelms you when you've picked up a nasty viral infection. Remember the last time you had a cold or the 'flu -  when coughs, headaches, sore muscles, vomiting, diarrhea and general fatigue forced you to remain in bed. Yet, just exactly how does the virus manage to do this to you? Especially when in some cases your sickness allowed the virus to spread from you to an uninfected family member; sometimes there is an evolutionary advantage to making you unwell. A recent paper published in PLoS Pathogens this week (which can be read here) investigates the molecular mechanisms behind why the oft' fatal rotavirus causes those infected to vomit and through doing so sheds light on possible therapeutic strategies. 

A gut villus . http://rezidentiat.3x.ro

This research isn't just aimed at answering some obscure academic problem - although it does do it - rotavirus induced gastroenteritis is serious business worldwide, with over half a million deaths each year (and many, many more hospitalised) mostly due to severe dehydration caused by the excessive vomiting and diarrhea. In many countries, the rotavirus vaccines just haven't reached the population at large and the reasons why this virus causes vomiting and diarrhea are poorly understood - hence this paper.

Now, vomiting - like many physiological processes - is controlled by our brain. Usually vomiting is induced by eating something nasty and is hence a kind of defense mechanism to rid ourselves of any harmful things within the gut. In order to sense if there is anything bad in your intestines, the brain is wired up to it's surface through what is known as the enteric nervous system or ENS. This mesh of neurons is connected to particular endocrine sensor cells (see left), called enterochromaffin cells (EC cells) that line our intestines and respond to the harmful stuff through the release of chemical messengers - like serotonin (5-HT), which in turn activate the ENS and end up inducing vomiting and diarrhea via our brain. But, specifically how does rotavirus fit in here?

 A straightforward hypothesis here would be that following ingestion of rotavirus, it would infect the EC cells lining the gut and somehow alter them to release serotonin and stimulate vomiting/diarrhea. And so, the used a primary EC tumour cell line to show that rotavirus does indeed infect the cells. So, now that the virus infects the cell, is this how it causes induction of the ENS? A rapid induction of calcium release was observed soon after infection as well as a quick secretion of serotonin - which itself is dependent on calcium signalling yet the fact that this release occured so soon after addition of  virus suggested that replication was not responsible for it - maybe something in the virus particle or something that was in the solution with the virus was to blame? Following seperation of virus particles from the culture media used to grow it, the group showed that the virus particles themselves were not behind it. The group tracked this effect down to NSP4, a protein secreted from rotavirus infected cells following infection and would therefore have been carried over with the virus in these experiments. In the end it was this protein itself that was responsible for the induction of calcium signalling and downstream serotonin release. 

Gut villi from mice infected with rotavirus. Arrows indicate rotavirus infected cells (those with large vacuoles) and stars indicated EC cells stained brown. Below, red = rotavirus and green = serotonin and yellow = colocalisation or a rotavirus infected EC cell. Yellow cells are rare - non-EC infected cells (enterocytes) are not.

All done and dusted now? No - up to this point, all work was done under tissue culture conditions in vitro - all be it, mostly in primary cells. The group then moved on to a small-animal mouse model of rotavirus infection to determine if this was the case in vivo - does rotavirus infect EC cells and induce calcium/serotonin signalling that stimulates gut neurons to induce vomiting/diarrhea. This could not have been addressed in cell culture. Following mouse infection, cross-sections of the gut wall were studied to determine the location of rotavirus infected cells and EC cells (see above). Many of the enterocyte cells lining the mouse gut were infected while only rarely were the EC cells targeted - despite what was observed in vitro. Yet for this model to work, rotavirus did not require infection of EC cells as NSP4, the protein behind this effect is secreted from cells and could hence stimulate EC from a distance. In the mouse, the group show that serotonin administration results in diarrhea (oddly, mice do not - and cannot - vomit) and also rotavirus infection leads to the activation of regions in the brain associated with sickness in humans (done through fos staining of brain sections).

A model for how rotavirus makes us vomit/diarrhea

This work has built up a kind of nice and simple picture of a mechanism of rotavirus causes vomiting and diarrhea. They have shown that the virus is able to enter and replicate inside epithelial cells and endcocrine cells lining the gut; they showed that this led to the release of serotonin - the stimulator of vomiting/diarrhea - and that this was down to NSP4 synthesis and release and finally that mice infected with rotavirus activated brain regions associated with vomiting and diarrhea.

The model: (see above) - Rotavirus infects gut cells - expresses and releases NSP4 - NSP4 stimulates calcium signalling and serotonin release from nearby EC cells - serotonin stimulates close by neurons that activate vomiting/diarrhea areas of the brain, which causes serious fluid loss, dehydration and subsequent death. Also this results in rapid spread of virus particles and transmission of infection.

The good thing is now, with this mechanism at hand, we may be able to partially inhibit this response and save the lives of many children and prevent the spread of rotavirus in these populations. Luckily for us, a number of generic - and hence cheap - already in use anti-sickness tablets (serotonin receptor antagonists) exist and could be applied in this setting. Although previous research had established the efficacy of using this drug to attenuate diarrhea in rotavirus infections, this paper established a working mechanism for its activity.

Granisetron - a serotonin receptor antogonist used as an anti-sickness medication and could be developed to aid rotavirus-induced vomiting/diarrhea. http://upload.wikimedia.org/wikipedia/

Hagbom, M., Istrate, C., Engblom, D., Karlsson, T., Rodriguez-Diaz, J., Buesa, J., Taylor, J., Loitto, V., Magnusson, K., Ahlman, H., Lundgren, O., & Svensson, L. (2011). Rotavirus Stimulates Release of Serotonin (5-HT) from Human Enterochromaffin Cells and Activates Brain Structures Involved in Nausea and Vomiting PLoS Pathogens, 7 (7) DOI: 10.1371/journal.ppat.1002115