HIV in a new light
Marc Johnson’s images could help solve an infectious puzzle
Mizzou virologist Marc Johnson uses special imaging techniques to observe virus assembly, the process by which viruses such as HIV reproduce themselves.
If the HIV virus were a weapons factory, the Gag protein would be the shell for one of the most explosive missiles imaginable. The Env protein would be the missile-guidance system. Other proteins would be other missile components.
Once you put the pieces of a missile together, destruction follows.
Mizzou virologist Marc Johnson uses this missile metaphor to describe the process of “virus assembly.” With nearly 40 million HIV/AIDS cases worldwide as of 2005, Johnson and his colleagues at the Bond Life Sciences Center know the stakes of understanding the assembly process are high.
Johnson uses a new microscopic technique to study assembly in HIV and other retroviruses. Retroviruses enter a body’s cell and copy their toxic genetic makeup into that cell’s chromosome. This causes irreversible damage.
During this process, Gag actually goes beyond being a shell and becomes more like a bossy factory foreman. It “recruits” Env and other viral proteins to take over healthy cells and turn them into little factories that churn out more of the virus.
Current HIV treatments focus on keeping these viral cells in check — something like deactivating the missiles once they have been launched. This approach has come a long way in terms of increasing patients’ length and quality of life.
Johnson thinks researchers can do better, though. With the proper understanding of virus assembly, they could create a treatment that renders the virus incapable of multiplying in the first place — effectively shutting those factories down.
New ways of seeing
Researchers have known about virus assembly since the 1950s, but they still don’t know the how or the why. It’s a puzzle. Johnson devotes his career to solving it.
“I love puzzles,” he says.
To solve a puzzle, one needs to see all of the pieces. In the past few years, Johnson and his colleagues have advanced an imaging technique they use to do just that.
In a 2005 paper published in the Proceedings of the National Academy of Sciences, Johnson first discussed this technique, called correlative imaging. Correlative imaging combines two types of microscopic images: fluorescence microscopy and scanning electron microscopy.
In fluorescence microscopy, Johnson “tags” Gag proteins with fluorescent proteins from green jellyfish. When he hits them with the right kind of light, these proteins illuminate, and he can view them in real time. The problem? The images are low resolution, so exactly what and where they show are hazy.
Electron microscopy shows images at about 100 times higher resolution. Correlative imaging uses both forms to show both the big picture and the minute details.
Johnson has another handy metaphor to describe it: “Fluorescence microscopy is kind of like satellite imagery. You can see everything, but you can’t tell what everything is. Electron microscopy is like having someone on the ground taking a picture for you. It tells you what you’re looking at.
“I’m able to visualize things we could never visualize before,” Johnson says. “In particular, I can see steps of assembly. I can actually take a picture and see the virus recruiting certain parts of its components to the right site. Now I’m trying to figure out how it’s doing that.”
The next steps
Fluorescence microscopy images (top) allow Johnson and fellow researchers to "tag" special viral proteins and observe them in real time. Such images are low-resolution, though, so Johnson correlates them with high-res electron microscopy images (bottom) to see crucial details of virus assembly.
Johnson’s next publication likely will deal with what correlative imaging allows him to see.
For example, he can see the patterns that emerge when both Gag and Env interact. In an electron microscope image of Env without Gag, the protein appears scattered in random areas. Add Gag into those images, though, and suddenly Env exists almost exclusively at viral “budding sites” — the little factories where viruses grow and reproduce. To Johnson, that means that Gag specifically seeks and recruits Env; their interaction isn’t random.
On top of that, Gag recruits indiscriminately. Gag from one virus will recruit Env from another virus. For example, Gag from HIV will recruit Env from a rabies-like virus, even though the two viruses differ like apples and oranges.
What does this tell researchers? “It means there’s some mechanism in there that all viruses seem to be using,” Johnson says. The long-term goal, then, would be to understand that mechanism.
That understanding could lead to treatments that deal with the virus early in its life cycle. Future drugs or gene therapy could render Gag incapable of recruiting Env, thereby destroying the little virus factories that destroy the body. The treatment would be proactive rather than defensive, and it could work for anything from HIV to the flu.
“The story does actually have an end, but this is probably going to take five or 10 years to flesh out,” Johnson says. “But it has been a riddle for 50 years. I think 10 years is reasonable if I can find an answer.”