Exploiting a Bacterial System to Combat Viruses

by Stephen Braigen

In the February issue of PLoS One, a group of researchers reported a technique to selectively clear populations of cells infected with the Hepatitis C virus (HCV), leaving uninfected cells unharmed. What is particularly nifty about the system that they developed is that it uses a protein that bacteria use to keep their growth in check in a low nutrient environment.

These bacterial components are called Toxin-Antitoxin Systems (or TA systems, TA loci, TA genes, etc…) This nomenclature does not quite do justice to these genetic systems, and also confuses people because when they hear “bacterial toxin” they think of something like botulinum toxin or anthrax toxin or any number of nasty compounds which bacteria use to gain a foothold in their war for food, either against other microbes or against host organisms. Anyway, TA systems are not themselves toxins in that sense. They are toxic to the bacteria that carry them. While there is a wealth of TA systems amongst many bacteria and archaea, they all revolve around the same basic mechanism. The toxin portion of the system poisons some cellular function (these targets include DNA synthesis, mRNA stabilization, and protein translation) and the conjugate antitoxin portion inhibits that toxic function by directly interacting with the toxin. Seem bizarre? Good, because it is. The next piece of the puzzle is that the antitoxin is not stable, while the toxin is. So the bacterium is regularly producing antitoxin while toxin production is fairly limited. When the bacterium enters into a nutrient-deprived environment, antitoxin is not replenished as it is degraded, and the counterpart toxin molecules are freed up and put the kibosh on cellular processes, according to the current model. Now the system begins to make sense; you don’t want to be spendthrift when times are tough.

The mazEF heterotrimer complex; two red mazF toxin proteins and one yellow mazE antitoxin protein. Image generated by SZB from the PDB file for 1UB4.

What Shapira et al. has done is exploited the activation mechanism of the Toxin-Antitoxin system in order to tackle a viral infection in a eukaryotic organism. Drawing from the established armory of gene therapy, they generated an adenoviral vector carrying a version of a well-studied TA system called mazEF that had been mutated into a single protein, inserting in between the toxin and antitoxin the site that HCV protease recognizes and cuts (HCV generates large polyproteins which are then processed by cleavage into smaller proteins). This modified mazEF complex follows the general functionality of the Toxin-Antitoxin systems that I laid out earlier – the mazF toxin leads to cell death by cleaving messenger RNA and thereby preventing protein synthesis, once it is freed from interactions with its conjugate antitoxin, mazE. In this case, instead of keeping the toxin in check by continually generating the antitoxin, the toxin is only activated if the fusion is cleaved, which can only occur in Hepatitis C virus-infected cells.

Figure 7 from Shapira et al. demonstrating the selective killing power of this system upon cells expressing the HCV protease.

Upon infection, the researchers found that their modified TA system was able to drastically reduce the number of HCV-infected cells, with no similar effect in non-infected cells. This is quite promising, and dovetails with similar research using adenovirus-based gene therapy to kill HCV-infected cells in mice. Shapira et al. point out that by using this bacterial system circumvents the issues of off-target effects that may occur by targeting or overexpressing components of mammalian systems to achieve the same goal.

Since this work was done with the aim of advancing the fight on a major disease like Hepatitis C, I should make it clear to any readers not familiar with translational research or have much experience in the lab that this work is far away from any sort of clinical implementation. This work was only done in cell cultures, and while it may very well work in the mouse model I mentioned before, there is a lot of work to be done.

At the very least, this sort of system could wind up being used to eke out some information about mechanisms of replication in HCV (or any other virus which makes use of proteases, of which there are many).

References:
Assaf Shapira, Shiran Shapira, Meital Gal-Tanamy, Romy Zemel, Ran Tur-Kaspa, Itai Benhar1 (2012). Removal of Hepatitis C Virus-Infected Cells by a Zymogenized Bacterial Toxin. PLoS ONE 7(2): e32320. doi:10.1371/journal.pone.0032320
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