Transmissible Gastroenteritis Virus Infection Up-Regulates FcRn Expression via Nucleocapsid Protein and Secretion of TGF-ß in Porcine Intestinal Epithelial Cells

Qian, Shaoju; Gao, Zitong; Cao, Rui; Yang, Kang; Cui, Yijie; Li, Shaowen; Meng, Xianrong; He, Qigai; Li, Zili

The ability of TGEV proteins to stimulate the FcRn promoter was identified by constructing a series of recombinant expression plasmids. However, efficient expression was obtained only from plasmids encoding the N, E, 3a, nsp1, nsp2, nsp5, nsp7, nsp8, nsp9, nsp10, nsp13, and nsp14. The expression of viral protein was not high or related to cell type and state. Among these proteins, N was the most potent NF-κB activator, and it induced FcRn expression. The proteins 3a, nsp1, and nsp5 also enhance but have little influence on NF-κB promoter activity. While nsp2 and nsp14 are also potential NF-κB activators, consistent with that reported in previous studies (Zhou et al., 2017; Wang et al., 2018) . Since the essential and multifunctional N protein is fundamental for immune regulation and pathogenesis, we examined it more closely. Porcine epidemic diarrhea virus (PEDV) N protein mediates NF-κB activation through TLR pathways and has functions in cell growth, cell cycle and ER stress (Xu et al., 2013) . In fact, the interaction of TGEV N protein with several host cell proteins has been shown (Zhang et al., 2014 (Zhang et al., , 2015 . In addition, overexpression of TGEV N protein induces cell cycle arrest (Ding et al., 2014a) . Using domainspecific mutations, we determined that the domain at aa 128-252 of the N protein (immunodominant central region) is critical for FcRn activation; it contains a ZnF structure and a SR-rich region. Both the SR-rich truncated proteins of the N protein of SARS coronavirus and PEDV can effectively promote the activation of the NF-κB signaling pathway (Zhang et al., 2007; Cao et al., 2015a) .