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.