With new data in hand, our first preprint on SARS-cov-2 receptor binding domain (RBD) interacting with heparin now has a sibling, which demonstrates that heparin inhibits the infection of Vero cells by SARS-cov-2
Some of the key points of the team’s new work are:
- Inhibition of viral infectivity in a Vero cell model by heparin, which is a better inhibitor for SARS-cov-2 than SARS-cov.
- Analysis of the interactions of a more extended library of model heparins with the SARS-cov-2 receptor binding domain. As with many other heparin-binding proteins, these data show that while sulfation is critical for RBD binding, the amount of sulfate is not, but instead it is the spatial arrangement of sulfate groups that is most important.
Together the data point to heparin being a potentially useful therapeutic to reduce infectivity.
From a bioinformatics standpoint, we would expect heparin to inhibit virus infection, since the ACE2 binding site on the protein overlaps the heparin binding site. Moreover, heparin was established to inhibit SARS-cov infectivity and the two viruses have good sequence homology in their RBDs, including the ACE2 binding site and the putative overlapping heparin binding site.
However, to determine if this is actually the case takes time. It is nearly a month since the the team’s initial preprint (now updated with low molecular weight heparins), which was the first demonstration of an interaction between the SARS-cov-2 receptor binding domain and heparin. In the ensuing month, the usual lab gremlins have been working overtime to thwart us. In particular we should cite the Scale up Protein Production gremlin, who is a really awkward so and so. Then there is the Devil’s very own side kick, the Surface Gremlin, who has the uncanny ability to mess up the most careful of preparations of sensing surfaces. Anyway, we know them well and wrestled them into submission, though it did take a couple of all nighters to finally send them packing to some other lab (rest assured, they will not stay with you forever, they know where we work and have 24/7 access to our building, they will be back here and leave you alone for a while).
We have a large library of sugars and sugar-related structures to investigate, courtesy of friends and colleagues in the glycoworld. Some relate to potential therapeutics, others to mechanism. So our work is far from finished on this front.
It has been a lot of fun, with the team’s work varying in intensity from the equivalent of lazing in the sun in front of the trench, to long stints in the lab, including a couple of 22 h ones.
It has also been profoundly rewarding. There are some strange features of the interaction of the SARS-cov-2 receptor binding domain with heparin, not the least of which is the need for SDS to regenerate heparin surfaces with bound SARS-cov-2 receptor binding domain, rather than the more usual 2 M NaCl. This is not unprecedented, as many moons ago one of John Gallagher’s team saw something similar with the interaction of thrombospondin with heparin – I think in that case we had to use 1 M urea to regenerate surfaces.
Cellular heparan sulfate is integral to the infectivity of many pathogens and exogenous heparins have been found to be effective inhibitors. In the case of respiratory viruses this extends beyond the coronaviruses to include influenza viruses. HIV and herpes have long been known to be inhibited by heparins, while more recently this has been shown for Zika and other flaviviruses. On the parasite front, Plasmodium falciparun rosetting is inhibited by heparin and a literature search using your favourite pathogen and ‘heparin OR heparan sulfate’ will pull out many more examples.
Somewhat surprisingly, particularly since heparins lacking anticoagulant activity show good inhibition of pathogens, none of this has hit the clinic yet. There are a number of reasons behind this. Heparin and its derivatives are not single chemical entities, which makes companies nervous in relation to regulatory approval. Moreover, this is not blockbuster territory, partly because once one heparin drug is in use, it will not be so difficult to make another that is not covered by the patent. This relates in part to the fact that heparins are not single chemical entities and the myriad routes to making derivatives. There are, for example, a number of different proprietary low molecular weight heparins, though these are not clinically equivalent. Then there is the market. Many of those in need of such therapeutics are poor and a ‘blockbuster’ drug with a hefty price tag is not going to have much of a market.
On the plus side, it is unlikely that a heparan sulfate-dependent pathogen will be able to evolve resistance to a heparin therapeutic – heparan sulfate is too fundamental to metazoan biology for it to be bypassed. So perhaps covid19 will mark the point at which heparins start to be tested clinically as therapeutics in infectious diseases.
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