Novel African Rhinolophus bat ACE2 sequences reveal the determinants of Afro-Eurasian sarbecovirus entry

This study characterizes novel ACE2 sequences from African *Rhinolophus* bats in Zambia, identifying specific amino acid residues that determine the susceptibility of these hosts to Afro-Eurasian sarbecoviruses and revealing distinct receptor usage patterns among different bat species.

The Gist

The Big Picture: The Bat-Virus Lock and Key

Imagine that viruses like SARS-CoV-2 are burglars trying to break into a house (a human or animal cell). To get in, they need a specific key (the virus's Spike protein) to fit into a specific lock on the front door (the ACE2 receptor on the cell).

If the key doesn't fit the lock, the burglar can't get in, and the infection stops.

For a long time, scientists have been studying the locks found in Asian bats because that's where the original SARS-CoV-2 came from. But this paper asks a crucial question: What about the locks in African bats? Africa is a huge continent with many different bat species, and we know very little about how their "locks" work compared to the viruses circulating there.

The Mission: Exploring the Zambian Bat Locks

The researchers went to Zambia in Africa and caught five horseshoe bats (Rhinolophus). They identified two different species:

1. Four bats were Rhinolophus simulator (let's call them the "Simulators").
2. One bat was Rhinolophus blasii (let's call him "Blasius").

The team took a sample of the "locks" (ACE2 genes) from these bats' lungs and built a model of them in the lab. Then, they tested these locks against a panel of eight different virus keys (representing various sarbecoviruses from Asia, Europe, and Africa) to see which ones could open the door.

The Findings: Who Fits Which Key?

Here is what they discovered, broken down simply:

1. The "Simulator" Bats (The Generalists)
The four R. simulator bats had slightly different locks (genetic variations), but surprisingly, all four locks worked exactly the same way.

• They could be opened by the "Asian" virus keys (like SARS-CoV and SARS-CoV-2).
• They could be opened by some "European/African" virus keys.
• The Twist: They could not be opened by two specific African/European viruses (RhGB01 and BM48-31). It was as if those two keys were shaped completely wrong for these locks.

2. The "Blasius" Bat (The Specialist)
The single R. blasii bat had a lock that was very similar to the Simulators for most viruses. However, it had a special superpower:

• It was the only one of the five bats that could be opened by the two tricky viruses (RhGB01 and BM48-31) that the Simulators rejected.
• This suggests that even though these two bat species live in the same country, their "locks" have evolved differently to handle specific local viruses.

The Secret Sauce: Two Tiny Screws

The researchers wanted to know: What makes the Blasius lock special?

They zoomed in on the lock and found two tiny screws (amino acids) at positions 31 and 41.

• In the Simulator bats, these screws were shaped one way.
• In the Blasius bat, they were shaped differently.

To prove this, the scientists played "Lego" with the locks. They took the Blasius lock and swapped its screws to match the Simulator bat.

• Result: The Blasius lock immediately lost its ability to open the door for those two tricky viruses.
• Conclusion: Those two tiny screws are the master keys that determine whether a specific African virus can infect a bat.

Why Does This Matter?

Think of this like a security system upgrade.

• Pandemic Preparedness: By understanding exactly which "locks" exist in African bats and which "keys" fit them, scientists can predict which viruses might jump from bats to humans in the future.
• The "One Health" Connection: The study shows that even small differences in a bat's biology (like those two tiny screws) can completely change which viruses can infect them. This helps us understand why some viruses stay in bats while others might spill over to humans.

The Limitations (The "But...")

The authors are honest about what they didn't do:

• They only tested five bats. It's like testing the security of a whole city by checking just five houses. There might be other bat locks out there we haven't found yet.
• They used fake viruses (pseudoviruses) in the lab to test the locks. While safe and effective for testing the "key fit," real viruses have other tricks up their sleeves that might change the outcome.

The Takeaway

This paper is a map of the "locks" in African bats. It tells us that location and tiny genetic details matter. Just because two bat species live in the same village doesn't mean they are vulnerable to the same viruses. By identifying the specific "screws" (residues 31 and 41) that control this interaction, scientists are one step closer to predicting the next potential pandemic threat before it happens.

Technical Summary

1. Problem Statement

Sarbecoviruses, including SARS-CoV and SARS-CoV-2, utilize the host Angiotensin-Converting Enzyme 2 (ACE2) receptor for cellular entry. While the interactions between Asian Rhinolophus bats and sarbecoviruses are well-characterized, African Rhinolophus species remain underrepresented in genomic and functional studies. This gap limits the understanding of the zoonotic potential of Afro-Eurasian sarbecoviruses (specifically Clade 3 viruses found in Africa and Europe) and their ability to cross species barriers. The study aims to fill this gap by characterizing novel ACE2 sequences from African bats and determining their susceptibility to a panel of sarbecoviruses.

2. Methodology

The study employed a multi-disciplinary approach combining field sampling, genomic sequencing, phylogenetic analysis, and virological assays:

• Sample Collection & Species Identification:
  • Five Rhinolophus bat lung samples were collected in Zambia (four from Shimabala, one from Suesueman).
  • Species identification was performed via RNA sequencing of the mitochondrial Cytochrome b (CYTB) gene, followed by Maximum Likelihood (ML) phylogenetic analysis.
• ACE2 Sequencing & Phylogeny:
  • ACE2 genes were cloned and sequenced from the cDNA of the lung samples.
  • ML trees were constructed for both CYTB and ACE2 coding sequences to assess evolutionary relationships and polymorphism.
• Cell Line Engineering:
  • Human osteosarcoma (HOS) cells stably expressing human TMPRSS2 were transduced with lentiviral vectors to express the newly identified bat ACE2 orthologs (Rhinolophus simulator variants Z-Rsi-01 to 04 and Rhinolophus blasii Z-Rbla).
  • Protein expression was confirmed via Western blotting.
• Pseudovirus Infection Assays:
  • HIV-1-based pseudoviruses bearing the Spike (S) proteins of eight sarbecoviruses were generated:
    • Clade 1a: SARS-CoV.
    • Clade 1b: SARS-CoV-2 (Wuhan-Hu-1) and BANAL-20-236.
    • Clade 3 (Afro-Eurasian): BtKY72, PRD-0038, PDF-2370, RhGB01, and BM48-31.
  • Infectivity was quantified using a luciferase reporter system in cells expressing the various bat ACE2s.
• Site-Directed Mutagenesis:
  • To identify critical residues, specific amino acid substitutions (N31D, H41Y, and the double mutant N31D/H41Y) were introduced into the R. blasii ACE2 sequence to test their impact on viral entry.

3. Key Contributions

• First Characterization of African ACE2s: The study reports the first ACE2 sequences for two African bat species: Rhinolophus simulator and Rhinolophus blasii.
• Discovery of Intraspecies Polymorphism: The study identified three distinct ACE2 genotypes within the R. simulator population sampled in Zambia, highlighting genetic diversity within a single bat species.
• Functional Mapping of Receptor Determinants: The research pinpointed specific amino acid residues (31 and 41) in the ACE2 receptor that dictate the host range of Clade 3 sarbecoviruses, specifically distinguishing between R. simulator and R. blasii susceptibility.

4. Key Results

• Species Identification: Phylogenetic analysis confirmed four bats as R. simulator and one as R. blasii.
• ACE2 Polymorphism: Three unique ACE2 sequences were found among the four R. simulator individuals. However, these intraspecies polymorphisms did not significantly alter susceptibility to any of the tested sarbecoviruses; all R. simulator variants supported infection by Clade 1a, 1b, and three Clade 3 viruses (BtKY72, PRD-0038, PDF-2370) similarly to human ACE2.
• Species-Specific Susceptibility:
  • Clade 3 Viruses (RhGB01 & BM48-31): These viruses failed to infect cells expressing R. simulator ACE2 but successfully infected cells expressing R. blasii ACE2.
  • Clade 1 & 2 Viruses: Both species supported infection by SARS-CoV, SARS-CoV-2, and BANAL-20-236.
• Residue 31 and 41 are Critical Determinants:
  • Residue 31: The R. blasii ACE2 possesses Asparagine (N) at position 31, whereas R. simulator has Aspartic acid (D). The N31D mutation in R. blasii ACE2 completely abolished infectivity by the BM48-31 virus.
  • Residue 41: R. blasii has Histidine (H) at position 41, while R. simulator has Tyrosine (Y).
  • Cooperative Interaction: For the RhGB01 virus, neither single mutation (N31D or H41Y) alone abolished infectivity. However, the double mutation (N31D/H41Y) significantly reduced entry, suggesting that residues 31 and 41 act cooperatively to facilitate RhGB01 binding.
• Structural Insight: The study hypothesizes that the N31 residue in R. blasii forms favorable interactions with the Spike protein of BM48-31 (which has an Alanine at the corresponding Spike contact point), a mechanism not possible with the D31 residue found in R. simulator.

5. Significance

• Pandemic Preparedness: By identifying specific ACE2 residues that govern viral entry, the study provides a molecular basis for predicting which bat species may serve as reservoirs for emerging sarbecoviruses.
• Host Range Determinants: The findings demonstrate that interspecies differences in ACE2 (e.g., between R. blasii and R. simulator) are more critical for viral entry than intraspecies polymorphisms for certain Clade 3 viruses.
• Surveillance Implications: The identification of N31 as a "hotspot" for Clade 3 virus interaction suggests that surveillance should prioritize bat species possessing this specific residue profile.
• Evolutionary Context: The discrepancy between the CYTB (mitochondrial) and ACE2 phylogenies suggests that ACE2 evolution in these bats may be driven by positive selection (host-virus coevolution) rather than neutral drift.

Limitations Noted: The study utilized a pseudovirus system rather than live virus assays (due to biosafety constraints with Clade 3 live viruses) and had a small sample size for R. blasii (n=1), indicating a need for broader sampling to confirm the universality of these findings across African populations.