Viral Sneakiness

Viruses are fascinating little buggers. They sneak into cells and take over their entire machinery and pathways for their own use, sometimes with as few as three genes. The amazing thing is that every virus has evolved a different way in which to do this. This week's papers are from the same issue of Cell Host and Microbe, and describe the MOs of two different classes of virus. (I love it when I find pairs of complementary papers in the same journal issue; it makes my job so much easier.)

The first paper, by George Katsafanas and Bernard Moss at the National Institute of Allergy and Infectious Diseases in Maryland, concerns poxvirus replication. It has been known for almost sixty years that poxviruses, unlike many other DNA-based viruses, do not replicate in the cell nucleus where the host's DNA replication machinery resides. Instead, the infectious particles that enter the cell contain several viral enzymes; these proteins allow the poxvirus genome to replicate in the cell's cytoplasm. This process occurs in distinct cytoplasmic regions known as DNA factories.

The authors began with high-resolution visualisation of viral DNA factories. Dual colour staining revealed cavities and tunnels within each factory that appeared to contain RNA, the cell's intermediate between DNA and protein. This suggested that transcription (DNA→RNA) and translation (RNA→protein) may also occur within the factories.

A direct test of this hypothesis would have been technically very tricky, as RNA is notoriously unstable and difficult to work with. An alternative approach is to determine whether molecules that are associated with transcription and translation co-localise to the viral DNA factories. Indeed, viral and host proteins that are known to be involved in viral transcription and translation were found in the RNA cavities within the DNA factories.

As the authors wrote in the Discussion section of their paper: “Although the presence of translation initiation factors... provided circumstantial evidence that protein synthesis occurs in the factory, more compelling data were needed”. But how do you prove that viral proteins are made in the factory, rather than being produced elsewhere and then imported? One solution is to insert a foreign gene that encodes a visible protein into the viral genome, and to track the accumulation of the protein over time. In fact, the blue marker protein was synthesised within the viral DNA factory, as predicted by the presence of translation machinery components. Further experiments with other coloured proteins revealed that a single virus genome is often transcribed and translated within a single DNA factory.

Packaging everything needed for viral transcription, translation and DNA replication into a distinct cell region makes sense in terms of increasing the efficiency of viral replication. However, a secondary effect is to sequester vital molecules away from the cell's own DNA and RNA, decreasing the efficiency of host transcription and translation. So increasing your own replication efficiency also helps to suppress the host's immune responses. I told you these things were sneaky.

The second paper concerns a modern media darling, the West Nile Virus (WNV). Although this small RNA-based virus is very different to the large DNA-based poxvirus, WNV also replicates in distinct structures outside the cell nucleus – in this case, in specialised cell membrane subdomains¹ – and knocks out aspects of the host's immune system. Jason Mackenzie and colleagues from the University of Queensland, Australia, spotted similarities between the WNV-induced membrane structures and those that arise when the cell's usual cholesterol levels are disrupted, and investigated whether disruption of cholesterol synthesis is part of the viral arsenal. And just like that, a brand new avenue of investigation was opened...

The first finding was that cholesterol is distributed differently in WNV-infected cells. There was less overall cholesterol present in infected cell membranes, and the remaining cholesterol molecules clustered in the membrane subdomains associated with viral replication. By observing the effects of various cholesterol-modulating chemicals, the researchers concluded that WNV replication depends on a key enzyme that is usually involved in cholesterol synthesis, and that is upregulated when the host cell detects the virus-induced changes in cholesterol distribution.

Cholesterol is also involved in communication between some cell membrane proteins. The redistribution of membrane cholesterol by WNV severely hampered that communication in one specific molecular pathway – yes, you've guessed it, a pathway that usually participates in anti-viral immune responses. While supplementing the cell with additional cholesterol did help to restore the usual immune response, don't resort to eating deep fried Mars Bars² if you get bitten by a mosquito - just trust that this potential treatment for WNV will be followed up.

The tactics of WNV and the poxvirus family are, then, strikingly similar: infect a cell, claim a specific region of the cell as your own, sequester cellular molecules away into those regions in order to replicate efficiently, and in doing so, deprive your host of the means to mount an effective response against you. Two very different viruses converging on a very similar solution (using different pathways) to the same problem. A double double whammy of viral sneakiness. So what are you waiting for? Go out there and evolve some resistance, why don't you?

1. Imagine the membrane as an oily bag that surrounds the cell. Molecules such as cholesterol and various proteins are embedded into this layer; the molecules are mobile and can move through the oil to contact one another. Some embedded proteins connect to the outside of the cell, and some to the inside, allowing communication between cells.

2. Imagine the deep fried Mars Bar as an oily bag that surrounds the artery... seriously, they exist, I've eaten one, it was tasty. And gross. Don't try this at home.