Human LRRC15 protein is a receptor inhibitor of SARS-CoV-2 peak-mediated entry

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) was first identified in Wuhan, China at the end of December 2019, and subsequently led to the 2019 Coronavirus Disease Pandemic (COVID- 19).

Infection with SARS-CoV-2 is facilitated by the input receptor for angiotensin-converting enzyme 2 (ACE2). Attachment factors and co-receptors facilitating entry have been extensively studied. However, the cellular entry factors inhibiting viral entry are largely unknown.

In a recent study, published on the bioRxiv* preprint server, researchers identified human LRRC15 as an entry inhibitor receptor for SARS-CoV-2, which binds directly to the receptor binding domain (RBD) of spike protein with moderate affinity and inhibits spike-mediated entry.

To study: LRRC15 is an inhibitory receptor blocking spike-mediated entry of SARS-CoV-2 into trans. Image Credit: Dotted Yeti / Shutterstock

Context and motivation

Similar to SARS-CoV-1, SARS-CoV-2 uses ACE2 as the primary input receptor. The viral structural protein peak (S) binds to ACE2 and mediates cellular entry of the virus. The S1 subunit consists of the N-terminal domain (NTD) and the receptor binding domain (RBD).

The interaction between RBD and ACE2 is important in determining several critical characteristics of SARS-CoV-2 infection. The Spike protein (especially RBD) is the main target antigen for COVID-19 vaccines, which interfere with the binding between RBD and ACE2. This highlights the importance of RBD and its binding to the cellular receptor in controlling SARS-CoV-2.

Scientists have identified several cellular factors that facilitate cellular entry of SARS-CoV-2, but it is not clear whether cell receptors inhibit viral entry. In the current study, the researchers generated a targeted CRISPRa library called the surfaceome. The library covers approximately 6000 of all known / predicted surface proteins on the cell plasma membrane.

The screening method used revealed that human LRRC15 (repeat rich leucine containing 15) is a novel receptor inhibitor of SARS-CoV-2.

Main conclusions

Scientists attempted to identify cellular receptors for SARS-CoV-2 by staining cells with a recombinant spike protein, which produced two separate results, ACE2 and LRRC15. ACE2 is, of course, the bona fide entry receptor, while LRRC15 is the new inhibitory receptor. In addition, LRRC15 has been observed to interact directly with the S RBD with moderate affinity.

It is known from previous research that ACE2 also interacts with S (through RBD), but the LRRC15-RBD interaction has not been found to compete with or compete with the ACE2-RBD interaction. stabilizes. However, more high resolution studies will be needed to determine the LRRC15-RBD interface.

A notable observation was that in addition to the same cells, LRRC15 also inhibited viral entry into neighboring cells in trans.

By interacting with the spike protein itself, inhibition of viral entry by LRRC15 demonstrated a direct effector. In many cases, the LRR domain has been linked to the detection of pathogen-associated molecular models (PAMPs).

A notable feature of LRR domain proteins is that they are highly conserved (even in plants). This provides superior protection against pathogens.

It is possible that humans develop a pattern recognition protein for certain types of coronavirus, given the role of LRR domains in pattern recognition.

The inhibitory effect of LRRC15 against coronaviruses signifies an arms race between humans and coronaviruses. Additionally, at the time of preparation of the current research manuscript, LRRC15 was identified as a SARS-CoV-2 peak binding factor in two other studies, proving the robust and detectable nature of the LRRC15-peak interaction.

Many interferon-stimulated genes (ISGs), such as LY6E, CH25H, and IFITM, have been shown to hinder coronavirus entry by interfering with spike protein-mediated membrane fusion.

Alternatively, ISGs could also interfere with endosome-mediated processes. However, the mechanism of action of ISGs and LRRC15 is quite different.

LRRC15 is not induced by interferons and the inhibitory action is directly mediated by the interaction with the peak, which resembles PAMP receptors.

Scientists said that LRRC15 could act as an antiviral factor via different modes as it redistributes adenovirus receptors, thereby affecting adenovirus delivery to cells.

Future research will need to determine whether LRRC15 requires intracellular signaling (via the cytosolic domain) or other cellular proteins for inhibition.

The researchers said the trans-the inhibition of the virus was notable. They co-cultivated ACE2 + LRRC15- and ACE2-LRRC15 + cells and observed significant suppression of viral infection. This indicated that the antiviral effect of LRRC15 may be broad in a physiological setting. In the human lung, during infection with SARS-CoV-2, LRRC15 could function as an entry inhibitor and act as a viral control.

Conclusion

The attachment factors and co-receptors facilitating entry of SARS-CoV-2 have been extensively studied, but the cellular entry factors that inhibit viral entry are largely unknown.

The current study showed that LRRC15 (by directly interacting with the spike protein) was a receptor for the SARS-CoV-2 spike and represented pattern recognition-type inhibition of viral entry.

Therefore, the study provided new information for therapeutic development and a better understanding of COVID-19.

*Important Notice

bioRxiv publishes preliminary scientific reports that are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.