Page 221 - Avian Virology: Current Research and Future Trends

214 | Zheng

endosomal membrane by forming pores (Galloux et al., 2007). 2015). Macropinocytosis, one form of endocytosis, accompa-
The release of pep46 from viral capsid to cellular endosome nies the membrane ruffling that is induced in many cell types
was dependent on the low calcium concentration environment, upon stimulation, and involves Rho-family GTPases, which
suggesting that the entry of IBDV, via pep46-formed pores in trigger the actin-driven formation of membrane protrusions
the endosomal membrane, needs endocytosis to release pep46 that collapse onto and fuse with the plasma membrane to gen-
first. Interestingly, this hypothesis was partially proven by the erate a large endosome, called macropinosomes (Conner and
successive study, which showed that IBDV was endocytosed Schmid, 2003). Thus, macropinocytosis of IBDV is supposed
and transported to the V-ATPase positive vesicles for uncoating to be the major route of the viral entry and internalization
(Yip et al., 2012). Furthermore, IBDV-mediated endocytosis in a receptor-mediated manner. Uncoating of the virus occurs
was demonstrated to be clathrin-independent, and inhibitions within the endosome in response to a low-pH environment
or depletions of lipid raft, c-Src tyrosine kinase, dynamin and (Galloux et al., 2007), followed by the release of Pep46 from
actin polymerization markedly reduced the entry of caIBDV in the viral capsid to the endosomes, which induces endosome
DF-1 cells (Yip et al., 2012). Annexin II (Anx2), a calcium- and permeabilization and facilitates the escape of viral genome with
phospholipid-binding protein that has shown to function in VP1 into the cytosol of host cell (Fig. 7.2). The machinery for
membrane traffic within endocytosis, exocytosis and cell adhe- viral replication will soon be started after the viral genome and
sion (Futter and White, 2007; Tebar et al., 2014; Rentero et al., VP1 (a RNA-dependent RNA polymerase) enter the cytosol. It
2018), also acts as a cell surface receptor that binds to IBDV was proposed that viral replication and maturation are mainly
VP2 (Ren et al., 2015). completed in the low-pH environment of amphisomes and/
Virus uptake is a complex process involving an initial or autolysosomes and the virions are packaged within the cel-
step conducted to deliver the genome of RNA virus into lular membrane, which facilitates fusion with the membranes of
the cytosol or the genome of DNA virus into the nucleus. adjacent cells so as to deliver a large number of virions directly
Experimental evidence shows that IBDV uptake involves and rapidly into host cells via cell-to-cell transmission (Wang et
macropinocytosis as a primary entry mechanism and traffick- al., 2017). Currently, the exact mechanism for IBDV replication
ing to early endosomes in a Rab5-dependent manner and this is still not clear. It is necessary to uncover the detailed processes
process is clathrin- and dynamin-independent (Gimenez et al., of IBDV replication in host cells.

Figure 7.2 The proposed model for IBDV entry and internalization. (A) Attachment of IBDV to host cells. The first step of IBDV infection
is the attachment of virus to the cellular membrane specific receptors (sIgM, chHSP90, α4β1 integrin, and/or Annexin II). (B–D) Formation
of macropinocytosis. Upon stimulation by virus attachment, the cell membrane ruffles, involving Rho-family GTPases, which triggers the
actin-driven formation of membrane protrusions that collapse onto and fuse with the plasma membrane to generate a large endosome,
called macropinosome. (E and F). Release of viral genome from endosomes. The endosome, via fusing with the lysosome, harbours acids,
free oxygen radicals, acid hydrolases, protease and so on (The early endosome becomes acidic by the action of V-ATPase accumulating
on its membrane. This V-ATPase translocate protons (H+) into the lumen of the endosome using cytosolic ATP as an energy source). Inside
the endolysosome, IBDV particles were detached by enzymatic cleavages in low pH environment, yielding Pep46 that forms pores enabling
IBDV genome with VP1 to escape into the cytosol.