The understanding of the receptors that viruses use to enter cells is a key component to understanding the emergence of zoonotic diseases in humans. It is well understood that the original SARS virus from 2003, and SARS-CoV-2, uses the angiotensin-converting enzyme 2 (ACE2) receptor to enter human cells. A related coronavirus, Middle East respiratory syndrome coronavirus (MERS), and several bat coronaviruses use dipeptidyl peptidase-4 (DPP4) as an entry receptor. The receptor for NeoCoV—the closest known MERS-CoV relative found in bats—has remained unclear.
Now, new research shows that NeoCoV and its close relative, PDF-2180, can efficiently bind to (and use) specific bat ACE2 orthologs and human ACE2 as entry receptors through the receptor-binding domains (RBDs) on the spike (S) proteins. However, at present these coronaviruses only weakly bind to human ACE2 cell receptors as an entry point into cells. They also are not known to cause disease outbreaks in people.
“To date,” the authors noted, “there is no evidence that NeoCoV and PDF-2180 can infect any mammals other than bats.”
This is published in Nature in the paper, “Close relatives of MERS-CoV in bats use ACE2 as their functional receptors.”
Many sarbecoviruses, an alphacoronavirus (setracovirus), and a group of merbecoviruses share the ability to bind to ACE2 on the cells of certain animals. The convergent evolution that led widely different coronaviruses to use ACE2 as entry receptors remains unclear.
Cryo-electron microscopy analysis revealed that NeoCoV has a distinct way of binding with the bat ACE2 receptor, compared to the other coronaviruses known to use that receptor.
More specifically, microscopy revealed “an RBD–ACE2 binding interface involving protein–glycan interactions, distinct from those of other known ACE2-using coronaviruses.” The authors wrote that they identified residues 337–342 of human ACE2 as a molecular determinant restricting NeoCoV entry, whereas a NeoCoV S pseudotyped virus containing a T510F RBD mutation efficiently entered cells expressing human ACE2.
Discovering that ACE2 is the host cell receptor for NeoCoV and PDF-2180, which are closely related to the human pathogen MERS-CoV, was unexpected and opens new avenues to prepare for the possible future zoonotic transmission of these viruses. This information is critical in setting up a watch-list of families of animal viruses that present a risk of emerging as zoonotic diseases in people.
The lack of knowledge about which cell receptors bat coronaviruses use, the researchers explained, limits scientific understanding of their cell entry strategies and of the virus’ propensity for interspecies transmission. The researchers on this current study also sought to gather data that might provide clues to the origin of MERS-CoV. Their results support earlier hypotheses that MERS-CoV may have appeared after a recombination event between a virus that was like NeoCoV and a virus that preferred the DPP4 cell receptor.
The two viruses, NeoCoV and PDF-2180, prefer the ACE2 cell receptors in vesper bats, part of the species Vespertilionidae. Most merbecoviruses are found in that species, which is a remarkably diverse, widely dispersed bat species.
While NeoCoV does not have a strong affinity for the ACE2 receptor on human cells, the scientists pointed out NeoCoV could undergo adaptive changes that would allow it to enter human cells more readily. The extensive mutations in the key cell binding regions of SARS-CoV-2, particularly in the Omicron variant, indicate that other coronaviruses may hold the still unrealized ability to adapt through mutations.
On the other hand, spillover of viruses from one animal species to another is a complicated transition that involves not just recognizing a cell receptor in the jumped-to species, but also subverting host immune responses, contact opportunity, conditions suitable for the virus to replicate, and many other factors.
The researchers also mentioned that MERS-CoV antibodies and antibodies generated by current COVID-19 vaccinations do not adequately neutralize NeoCoV and PDF 2180. However, their results showed that broadly neutralizing antibodies that target evolutionarily conserved regions of NeoCoV and PDF-2180 do inhibit the entry of these viruses into cells. They suggest that these broader-acting antibodies should be considered for preparedness against spillover to humans by these viruses.