Tight junction proteins
Tight junction proteins widely present between the epithelial cells and endothelial cells, are responsible for closing the cell gap and preventing the free entry and exit of substances inside and outside the epithelial layer. Meanwhile, paracellular transport of bacteria, toxins and other substances in the intestinal cavity is also required to maintain the integrity of epithelial barrier function, which is achieved by regulating the role of tight junction proteins in accordance with physiological state (JA & BB, 2009). Tight junction proteins composed of transmembrane proteins and cytoplasmic proteins, are complex structures formed by the interaction of various proteins. They are linked to microfilaments by cytoplasmic binding proteins, which can be divided into transmembrane proteins (e.g. occludin, claudin) and cytoplasmic proteins (e.g. ZO-1, ZO-2, ZO-3) (G.-M. L, R, & D, 2008). The extracellular components of the transmembrane proteins between adjacent cells interact to form intercellular spaces, while the intracellular components are bound to cytoplasmic scaffold proteins ZO-1 and ZO-3, and then directly linked to the prejunctional ring structure composed of actin filaments and myosins (T. S, M, K, & MS, 2010). Importantly, several tightly linked proteins including occludin, claudin, CAR and JAM, have been found to act as receptors for viruses (JM & CF, 2015; M. M, A, PL, & R, 2015). Moreover, it has been regarded that viruses invade epithelium by binding and destroying these tight junction proteins. For instance, the invasion of HCV into hepatic epithelial cells is mediated by tight junction proteins claudin-1 and occludin. Similarly, claudins-6/9 has also been identified to be an invasive co-receptor in endothelial cells (P. A et al., 2009; MJ et al., 2007). Notably, PEDV has been confirmed to cause structural alterations in the barrier integrity both in vitro and in vivo through modulating related proteins of the tight junction and adhesion junction in the early stage of infection (Z. S, J, L, & Q, 2014). Furthermore, this effect of PEDV on cell junction is achieved by affecting the MAPK pathway, since inhibition of MAPK pathway could regulate the changes in tight junction of cells (Z. S et al., 2014). In particular, the essential role of the tight junction protein occludin in PEDV infection during late entry events has been suggested and characterized (L. X et al., 2017). The tight junctional distribution of occludin is pronouncedly affected by PEDV infection. Furthermore, overexpression or downregulation of occludin promotes or alleviates the susceptibility of target cells to PEDV infection, respectively (L. X et al., 2017). Interestingly, although of PEDV and occludin are mutually influenced by each other, their direct interaction is absent (L. X et al., 2017). Additionally, it remains undetermined whether other tight junction proteins play similar or various roles in the infection of PEDV. Future works are warranted to elucidate these issues.
Genome Replication and Transcription
When PEDV enters the cytoplasmic exfoliation, the RNA genome in the virion is released. The infected cells generally contain 7 to 9 virus-specific mRNAs, which carry the same 3’ mRNA, whereas the longest one is viral genomic RNA. The PEDV replicase synthesizes full-length negative stranded RNAs using genomic RNA as a template, and then these newly synthesized RNAs function as a template for the synthesis of new genomic RNA (Masters, 2006). Although genome replication/transcription is regarded to be mediated primarily by viral replicating enzymes, multiple host factors are also involved in these processes. Moreover, the N protein of PEDV, as an RNA companion, plays a key role in the interaction with host factors.