# Beyond TFRC: The Pivotal Role of mGluR2 in Feline Calicivirus Entry and Replication

## TL;DR

This study reveals that mGluR2 is a new host protein involved in feline calicivirus entry, independent of TFRC, offering insights into viral infection mechanisms.

## Contribution

The study identifies mGluR2 as a novel, independent host factor for FCV entry, distinct from TFRC.

## Key findings

- mGluR2 interacts directly with FCV and enhances viral entry into host cells.
- Reduced mGluR2 levels significantly decrease FCV infectivity, even when TFRC is present.
- mGluR2 influences endocytic vesicle formation and F-actin polymerization during FCV infection.

## Abstract

Feline calicivirus (FCV) is a prevalent virus that induces respiratory and oral diseases in felines and serves as a valuable model for investigating other challenging caliciviruses, such as human norovirus. In previous research, we identified a cellular protein known as transferrin receptor (TFRC) that facilitates the entry of FCV into feline cells; however, we sought to determine whether additional proteins are implicated in this process. To explore this, we examined the role of another protein, metabotropic glutamate receptor 2 (mGluR2), in the context of FCV infection through laboratory techniques designed to assess protein interactions and cellular infection. Our results demonstrated that mGluR2 directly interacts with FCV and enhances the virus’s entry into host cells. Notably, when mGluR2 levels were diminished, FCV exhibited significantly reduced infectivity, even in the presence of TFRC, indicating that mGluR2 operates independently of TFRC. These findings contribute to a deeper understanding of the mechanisms underlying FCV infection in cats and position mGluR2 as a novel target for the prevention of FCV-related diseases. Furthermore, they offer valuable insights for the study of human norovirus, which poses significant research challenges.

Feline calicivirus (FCV) is among the few members of the Caliciviridae family that can replicate efficiently in vitro. Our recent studies have found the Transferrin Receptor Protein (TFRC) is an entry receptor that facilitates the internalization of FCV. To explore the potential involvement of additional host factors in conjunction with TFRC during the viral entry process, we identified metabotropic glutamate receptor 2 (mGluR2) as a specific interacting partner for both TFRC and the FCV VP1 protein by Co-IP analysis. Our findings indicate that the downregulation of mGluR2, along with its downstream signaling molecule, Calcium-activated potassium channel subunit alpha-1 (KCa1.1), significantly inhibits FCV replication by impairing viral internalization. Importantly, the knockout of TFRC did not diminish the effects of mGluR2 and KCa1.1 on FCV infection. Furthermore, mGluR2 was found to interact directly with FCV VP1, rather than with TFRC, and the rate of F-actin polymerization induced by FCV infection was reduced solely by the downregulation of mGluR2 protein expression, not by TFRC knockout. These results suggest that mGluR2 may independently mediate FCV internalization, operating independently of TFRC, and plays a critical role in the formation of endocytic vesicles. Overall, the results indicate that multiple host factors, including TFRC and mGluR2, are involved in the internalization of FCV into host cells. Further research is necessary to explore the propagation of other caliciviruses, such as norovirus, in vitro.