When disease-causing viruses break into their hosts’ cells, it is invariably an “inside job.” Viral pathogens can only invade cells and replicate with the assistance of the cells’ own molecular machinery.
SARS-CoV-2 is no exception. Before the new coronavirus can enter a human cell, enzymes called proteases on the cell’s surface must split open the protein spikes that give the virus its characteristic crown-like appearance.
This splitting changes the shape of the spikes, exposing the binding sites that allow the virus to gain entry to the cell.
The spikes of coronaviruses contain three “cleavage sites,” where particular proteases can split the proteins. A coronavirus can, therefore, only invade cells that bear the appropriate proteases.
The cleavage sites and their respective proteases help determine how pathogenic the virus is, which tissues it can infect, and whether it can jump from species to species.
Scientists at the University of California, Riverside’s School of Medicine and the Sanford Burnham Prebys Medical Discovery Institute, in La Jolla, wanted to find out whether a compound that inhibits two particular proteases would protect cells from invasion by SARS-CoV-2.
Their findings have been published in the journal Molecules.
A previous study had suggested that one of the proteases, called furin, is used by some of the most pathogenic coronaviruses. It may be one factor that helps SARS-CoV-2 spread so easily.
Rather than working directly with SARS-CoV-2, the researchers used anthrax toxin as a model.
This is because furin not only helps viruses infect cells, it also activates anthrax toxin, allowing it to enter and kill cells.
Crucially, furin cleaves the same sequence of peptides — the units that form protein — in both the SARS-CoV-2 spike protein and the anthrax toxin. This makes the toxin an ideal model.
First, the researchers checked whether their agent, called compound 1, could protect human cells in a dish from the toxin.
Once they confirmed this, they went on to investigate whether compound 1 would protect mice from the toxin.
They discovered that even a single dose of the compound significantly improved the animals’ survival.
Compound 1 inhibits both furin and another protease, called TMPRSS2.
In their paper, the scientists argue that further research is needed to develop compounds like theirs that inhibit both proteases, rather than just one. Alternatively, a cocktail of different protease inhibitors could also work, they argue.
The study authors cite two lines of evidence for their argument.
First, when scientists in the past have genetically engineered host cells so that they were unable to make furin, this has failed to stop the virus from infecting the cells.
Second, when the authors of the present study looked at the peptide sequence of the SARS-CoV-2 spike protein, they found evidence that newly acquired mutations allow the virus to exploit both furin and TMPRSS2 cleavage sites.
These mutations have given the virus the ability to infect a wider variety of tissues in the body.
“In other words, SARS-CoV-2, unlike other, less pathogenic strains, can more efficiently use both proteases, TMPRSS2 and furin, to start the invasion of host cells,” says Maurizio Pellecchia, a professor of biomedical sciences at the University of California, Riverside, who led the research team.
“While TMPRSS2 is more abundant in the lungs, furin is expressed in other organs, perhaps explaining why SARS-CoV-2 is capable of invading and damaging multiple organs.”
A clinical trial of the TMPRSS2 inhibitor camostat in people with COVID-19 recently began.
However, research from the team in California suggests that camostat is a poor furin inhibitor. “Our current study, therefore, calls for the development of additional protease inhibitors or inhibitor cocktails that can simultaneously target both TMPRSS2 and furin and suppress SARS-CoV-2 from entering the host cell,” says Prof. Pellecchia.
He and colleagues are seeking funding to design and develop protease inhibitors that target both TMPRSS2 and furin.
In addition to treating SARS-CoV-2 infections, such agents could combat other highly pathogenic coronaviruses that may jump from other species into humans.
“The funding would allow us to explore new possibly effective therapeutics against COVID-19 and support studies that could have far-reaching applications — to ward off possible future pandemics,” says Prof Pellecchia.
The new research was a lab-based, preclinical study. Clinical trials would, therefore, be needed to test whether an agent such as compound 1 is safe and effective in people.
One shortcoming of protease inhibitors is that they work by disabling enzymes that the body needs for everyday functioning.
While protease inhibitors have proved highly effective in treatments for HIV, for example, they can cause severe side effects in some people.
Cells use furin, in particular, to activate a wide variety of important proteins.
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