# Tight junction protein claudin-1 is a novel internalization factor for swine enteric coronaviruses infection

## TL;DR

This study identifies claudin-1 as a new factor that helps swine enteric coronaviruses infect cells, including SARS-CoV-2, by promoting viral internalization.

## Contribution

Claudin-1 is newly identified as an internalization factor for multiple swine coronaviruses and SARS-CoV-2.

## Key findings

- Claudin-1 overexpression increases susceptibility to TGEV, PEDV, and PDCoV infections.
- Claudin-1 interacts with the S1 protein of SeCoVs and SARS-CoV-2, promoting viral internalization.
- Upregulation of claudin-1 in infected piglets correlates with increased viral infection in the intestine.

## Abstract

Swine enteric coronaviruses (SeCoVs), including transmissible gastroenteritis virus (TGEV), porcine epidemic diarrhea virus (PEDV), and porcine deltacoronavirus (PDCoV), have been reported to use aminopeptidase N (APN) as a cellular receptor. However, APN alone cannot effectively explain the infection of both APN-positive and APN-negative enterocytes by PEDV and TGEV, nor the wide host range of PDCoV, suggesting the involvement of other host factors. In this study, we demonstrate that TGEV infection in piglets upregulates claudin-1 expression not only in infected cells but also in uninfected cells. Claudin-1 levels correlated strongly with TGEV N protein levels in the jejunum of infected piglets. Functional studies revealed that claudin-1 overexpression enhanced cellular susceptibility to TGEV, PEDV, and PDCoV, whereas its knockout significantly attenuated infection. Mechanistically, claudin-1 specifically interacts with the S1 or receptor-binding domain (RBD) of SeCoVs and promotes viral internalization. Furthermore, induction of claudin-1 in piglets promotes PDCoV infection in the intestine. Notably, claudin-1 also binds to the S1 protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Collectively, our results identify claudin-1 as a novel internalization factor for porcine enteric coronaviruses, playing a critical role in facilitating infection within the digestive tract, and highlight its potential as a target for future clinical interventions.

We observed a downregulation in the expression of the majority of tight junction proteins in intestinal tissues infected with transmissible gastroenteritis virus (TGEV). However, unexpectedly, claudin-1 exhibited a significant upregulation in intestinal epithelial cells. This intriguing finding prompted us to delve deeper into the potential role of claudin-1 in facilitating virus invasion of epithelial cells. Utilizing overexpression and knockout cell lines, we demonstrate that claudin-1 is an internalization factor for swine enteric coronaviruses (SeCoVs), including TGEV, porcine epidemic diarrhea virus (PEDV), and porcine deltacoronavirus (PDCoV). Notably, claudin-1 interacts with the S1 protein of TGEV, PEDV, PDCoV, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), spanning across alpha, beta, and delta coronaviruses. Our findings provide deeper insights into the infection mechanisms and pathogenesis of SeCoVs and SARS-CoV-2.