Friday, 30 November 2012

Restricted access post 3 of 3

I left the last blog touching on the fact that human schalfen 11 may potentially be able to act on influenza virus in addition to HIV. One protein that is already established as an influenza restriction factor is IFITM3, the topic of this blog post. The reason research in the field of IFITM3 is of much interest is that our current stock of drugs to tackle the virus are struggling. We currently have four licensed drugs to tackle influenza, however only two of these remain truly effective due to the emergence of resistant strains. Both Tamiflu and Relenza remain effective, but there are a growing number of cases of Tamiflu resistant influenza. If Tamiflu resistance continues to spread we will be left with only Relenza as an effective drug. Having a single drug is never a great situation as it makes it very likely that resistance will emerge, which would leave us with nothing! We therefore need novel approaches to tackle influenza; enter IFITM3…

IFITM3 was first defined in 2009. The protein was identified in a large-scale screen of proteins that are ‘turned on’ when flu infects cells. This switching on is controlled by a set of immune proteins known as interferons (IFN) that are produced by cells when they are infected (IFITM stands for InterFeron Induced TransMembrane protein). The IFN response turns on around 2000 protective proteins that produce an ‘antiviral state’ to help protect cells from the invading virus. The team wanted to find which of these 2000 or so proteins are most important for tackling influenza infection. In order to do this they systematically switched off individual proteins to see which ones provided the most protection (defined by exacerbated infection in the absence of the protein). By this approach they found 120 proteins that play an inhibitory role on influenza infection. Through further probing they found the best “hit” was the IFITM3 protein. IFITM3 is closely related to two other proteins, IFITM1 and IFITM2 (as the names would suggest), and both of these proteins were also found to inhibit influenza infection, albeit to a lesser extent.

The IFITM proteins play a major role in protecting cells from influenza based on the fact that their removal from cells allows a higher level of infection, as seen in the study published in 2009, and many other since. Interestingly it isn’t only influenza that these proteins protect us against. The team that originally identified the IFITM proteins found that they could also protect cells from West Nile and Dengue viruses (WNV and DENV). These two additional viruses are closely related to each other, but not very closely related to influenza, making the finding somewhat mysterious.  Since the original discovery, IFITM proteins have been found to act on yet more viruses including Marburg, SARS, the highly feared Ebola and HIV (though there is some debate about the link to HIV).

As with most things in biology, viruses are grouped together based on certain characteristics and their evolutionary origins, just like animals. As I have mentioned WNV and DENV are very closely related (let’s say similarly to chimps and humans) and are in the same viral evolutionary family, the Flaviviridae. The SARS virus is similar to the flaviviruses but is within a different family, known as the Coronaviridae (more distantly related, such as humans/chimps compared to orang-utans).  Influenza falls into a different viral family (Orthomyxoviridae) and is more distantly related to the flaviruses and coronaviruses (let’s say like a domestic cat compared to the apes). However influenza is reasonably closely related to the Filoviridae family that contains Ebola and Marburg viruses (like a house cat to a lion). HIV is not particularly related to any of the others mentioned here and falls into the family of Retroviridae (like a kangaroo to all the other mentioned animals). I direct your attention to the image to see how these families all interconnect. The viruses that the IFITM proteins target are all very diverse, which begs the question of how these proteins are able to target such a wide array of invaders. (I’d just like to point out that the animal examples I have used are just for demonstration).
Relationship of different viral families
 Even though the IFITM sensitive viruses are very diverse, they all, with the exception on HIV (hence the dispute), enter cells in a very similar manner. Initially the virus binds to receptors on the surface of a cell, somewhat akin to grabbing a door handle. Once bound the virus is able to enter into the cell (open the door) through a process known as endocytosis.  At this point the virus is essentially encased within a small bubble inside the cell that is known as a vesicle (think of a bubble within a glass of drink, sorry to mix metaphors). This vesicle joins up to a larger structure known as an endosome, which over time becomes acidic. This acidification eventually causes a change to the virus that allows it to fuse with the bubble and escape to the rest of the cell where it will then move on to complete its life cycle. Don’t be distracted by the names (or the metaphors), just have a look at the picture if my description has been a bit hard to follow. The main point to take from this paragraph is that all of the viruses that the IFITM proteins inhibit, except HIV, enter cells through this endocytosis and acidification process.

Entry of influenza through the endocytic pathway

Since all the viruses that IFITM has been found to have an inhibitory affect on enter cells in the same way it seems fair to assume that the IFITMs must be targeting this pathway. Evidence is stacking up that this is indeed the case as numerous studies have found that IFITM3 (less work has been done on the other two) is localised to vesicles and the endosome. However, it is still not clear exactly how the IFITM proteins actually interfere with the processes involved with the viral life cycle. The current hypothesis is that the protein may interfere with acidification, which would block exit of the virus from the endosome. This eventually leads to destruction of the virus since the acidification continues to a point that kills the virus. This idea is supported by the fact that less virus is seen inside the cell cytoplasm when IFITM3 is present (the cytoplasm is simply the contents of the cell, using the analogy of the vesicle being like a bubble in a drink from before; the cytoplasm is the liquid).

A comparison of the different entry mechanisms used by Influenza and HIV
We need to fully resolve how the IFITM proteins are functioning. Once that has been achieved it will be possible to look towards finding drugs that act in a similar fashion. If we are able achieve this, we may well be able to develop drugs that are highly active against a very broad set of viruses. In addition, these potential drugs would be targeting a human process meaning that resistance is less likely to evolve since humans mutate so very much slower than viruses. That’s a long way off yet but shows why we need basic research into these proteins.

As an extra little add-on, IFITM3 has been linked to severe flu in patients as well as simple studies in a lab. A study was conducted to look at patients hospitalised by pandemic H1N1 (Swine Flu) and season flu and found that these patients were enriched for a certain mutation to the IFITM3 gene (the mutation was more common in people who were hospitalised than the general public). This hints towards the fact that IFITM3 truly does impact on influenza and helps most people to control it (flu is rarely fatal). It also hints towards the fact that this mutation could potentially be used to find those people who are at the highest risk of influenza infection, which would help with deciding who receives the limited stocks of influenza vaccine produced each year.

IFITM3 is a very interesting protein that has the potential to make a large impact in our efforts to tackle influenza (and potentially other viruses). I will be keeping a close eye as the story develops and will, in all likelihood, write future blogs on the topic as the story evolves.

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