Scientists from the USA recently published the first Cryo-EM image of New World Hantaviruses. The team, led by Colleen B. Jonsson, obtained the Cryo-EM images of three New World Hantaviruses – Andes Virus (ANDV), Sin Nombre Virus (SNV), and Black Creek Canal Virus (BCCV). The study was published in the journal Viruses.
Left: Cryo-EM image of Sin Nombre Virus (SNV). Right: Members of the research team. L to R: Mariah Taylor, Amar Parvate, Colleen B. Jonsson
The team of seven researchers employed cryo-electron microscopy (cryo-EM) to understand the structural features a group of viruses called Hantaviruses. The technique helped them unravel the features of two types of Hantaviruses – Old World Hantavirus and New World Hantaviruses. While the former is prevalent in Eurasia, the latter is found in the Americas. Old World Hantaviruses include the viruses that cause haemorrhagic fever with renal syndrome (HFRS), whereas New World hantaviruses include those viruses that cause hantavirus pulmonary syndrome (HPS).
“I had not intended to work on these structures from the onset,” Amar Parvate, the first author of the paper says. “The idea was to investigate the spread of hantaviruses through cells using Transmission Electron Microscopy (TEM). But I realized that these viruses were not safe. We had to work on them in a highly contained environment (BSL3). There was no way to safely load them on a cryo-EM.” The team then worked on a method to inactive these viruses, without compromising their quality, and load them safely to see how they look through the cryo-EM. It took Amar and his collaborators three years to optimize the method on a prototype virus. They then applied it to several BSL3 hantaviruses the results of which are published in the paper.
The cryo-EM images of Andes Virus (ANDV), Sin Nombre Virus (SNV), and Black Creek Canal Virus (BCCV) published by this team are the first cryo-EM images of New World Hantaviruses. These images have revealed diverse features and sizes of New World viruses. They are round, tubular or irregular. While BCCV were mostly tubular, SNV were mostly irregular.
“Most of the Hantavirus community was looking at Old World Hantavirus morphology and assuming that the virions are all round. One reason was that they could not put these viruses on a cryo-EM was the containment restriction I mentioned earlier. Even after I had the initial results, other researchers were sceptical if the viruses were inactivated or whether the method itself distorted the morphology. My images proved that there are more diverse features to these viruses rather than just being round” Amar explained. “The most striking finding was the long tube-like morphology of one of the New World Hantaviruses.”
Amar’s method is now available to other researchers working on these kinds of viruses and paves the way to further discoveries in viral studies. “I am hoping other groups use my method to finally tease out structural details of other dangerous (BSL3/4) viruses that had been recalcitrant cryo-EM and structural studies,” Amar said.
Of course, the team had to do some things very differently to achieve this. Electron microscopists have traditionally fixed their room temperature biological samples on the grids using glutaraldehyde. To get an image of higher resolution using cryo-EM, they fixed the samples with a very mild fixation technique using glutaraldehyde. Amar and colleagues combined the two techniques into one. This was the thought that brought the breakthrough.
Amar is excited about the possibilities that his work has opened to researchers studying viruses. “Currently, there are very few cryo-EM facilities in the world that can handle BSL3 samples. Although there are advances being made in this direction, most highly contained (BSL3) labs do not have access to cryo-EM. My method proposes a way to use cryo-EM outside the containment for any BSL3 viruses” Amar said. “Once the virus is inactivated, it can be safely taken out and even shipped to a completely different institute for cryo-EM analysis. This type of extension of my method may eventually help us analyse morphologies of multiple viruses for which the most we have now are one or two TEM images collected in the 1980s.”
The implications of this research, however, do not end there. All these findings will ultimately lead to the greater goal of developing drugs and vaccines to fight these viruses. But those will come only later – when we have a better understanding of the structural features of the viruses. For all we know now, the images published by the group and the method that they have put has given the scientific community a great stride in studying dangerous viruses.
Joel P Joseph 