Uncovering the secret weapon of rabies viruses

By Dr. Liji Thomas, MDNov 13 2019

In a new study published in the journal Cell Reports, scientists have discovered the exact site at which the immune system is disabled by the rabies virus, opening the way for a new and highly effective means of rabies prevention. This is a long-awaited discovery and could transform the field of rabies immunization.

Rabies is a deadly viral disease of the nervous system, 3d illustration Credit: nobeastsofierce / Shutterstock

Rabies is a viral illness which is invariably fatal. It is transmitted through the body fluids of rabid animals. It is still rampant in developing countries, where it takes 60,000 lives a year, mostly because of dog bites. The causative organism comes from the family of lyssaviruses, and includes the classic rabies virus as well as a bat virus from Australia, among others.

All over the world, scientists are looking at more effective ways to stop rabies. In many countries stray dogs are culled, but to no effect. Mass vaccination is an effective way to stop transmission in animals but it is difficult to capture and immunize stray animals.

The researchers started from the fundamental assumption that the virus binds to the human protein called STAT1 as well as closely similar proteins, to trigger a shutdown of the immune response. STAT1 is one of the transcription factors that mediates the decoding of required bits of DNA to RNA in preparation for making the necessary proteins for any bodily process. STAT1 is an essential component of the immune response to viral infection. Inactivation of this protein is therefore crucial to establishing the disease in the host, and many viruses produce proteins that interfere with IFN binding to STAT proteins.

The P protein is a viral protein, with multiple functions. One of these involves antagonizing type 1 interferon (IFN), a key set of inflammatory cell signaling molecules that mediate the earliest antiviral immune response. The cascade proceeds as follows: once a cell is infected by a virus, the cell secretes IFNs which bind to type 1 IFN receptors. This binding switches on a protein that transduces the signal to activate transcription – called STAT (Signal Transducers and Activators of Transcription). As a result, IFN-stimulated genes (ISGs) start working to produce first an RNA copy (transcription) which is then converted to proteins which have antiviral and immunostimulating properties. The immune system has begun its work of clearing out the virus.

The P protein is the main IFN antagonist produced by the rabies virus. However, nobody had discovered how exactly it binds to STAT1.  

The study

To elucidate the nature of P-STAT1 binding, the current study attempted a structural analysis of the complex formed by the STAT1-viral protein binding. Researcher Greg Moseley says, “The challenge was to produce the key proteins on the viral and host sides in a test tube and keep them stable so we could interrogate the interaction directly; this hadn't been done before, at least for the full-size human protein.”

After bringing the two types of protein components together in the test tube, the researchers used nuclear magnetic resonance (NMR) spectroscopy to show the physical structure of the complex. In particular they focused on the exact place at which the P protein was attached to the STAT1 protein. This adherence prevents the complex from being translocated into other cell regions where it is necessary to switch on the immune response.

The findings

Incredibly, the scientists found a very complex attachment site, constituted by three different regions on both sides of the interface, that is, involving different sites on both viral P protein and host STAT1 protein.

This discovery also means they found new targets to attack the virus. Introducing a mutation at certain of these sites could inactivate viral binding to STAT1 and thus prevent it from disabling normal immune reactions to virus infection. They found also that mutations in two regions were more effective at preventing binding than in only one region.

The researchers say this is the first time a full-sized STAT1 protein bound to a viral protein has been subjected to structural analysis. A variety of common pathogenic viruses, including Hendra virus and measles virus, attack this protein target.

Another team of researchers then introduced one of the nonlethal but inhibitory mutations they had studied into a wild strain of rabies vaccine. This enabled them to demonstrate that even powerful disease-causing viruses can be stopped from causing infection by simply preventing them from binding to STAT1. The researchers have applied for an international patent on their findings, which took five long years to obtain.

Implications

Moseley says it would be a big step forward if they could help develop a new, safe and effective rabies vaccine which can be given by mouth or hidden in baits, as this would obviate the challenge of catching and injecting strays and other animals. Like him, fellow researcher Paul Gooley says he is excited to have been part of a project that could end in a safer oral rabies vaccine that could even end this plague, especially in developing countries. Describing himself as a “discovery scientist, driven by curiosity,” the specialist in NMR spectroscopy says, “The state-of-the-art technological tools and methods used in the study could also be applied more broadly to counter other viruses that target STAT proteins.”