The invasion of tobacco mosaic virus RNA induces endoplasmic reticulum stress-related autophagy in HeLa cells

Li, Li; Wang, Li; Xiao, Ruijing; Zhu, Guoguo; Li, Yan; Liu, Changxuan; Yang, Ru; Tang, Zhiqing; Li, Jie; Huang, Wei; Chen, Lang; Zheng, Xiaoling; He, Yuling; Tan, Jinquan

As shown, the ER membrane resembling swollen fingers might have just produced a vacuole ( Figure 6B-c, arrowheads) ; the distal ER membrane end expanded to resemble forks that extend towards the anterior vacuole ( Figure 6B-b, pane) ; and another distal ER membrane end expanded towards the anterior vacuole and appeared to encapsulate it ( Figure 6B-d) . Moreover, the other distal end of the expanded ER membrane was sequestered in a vacuole ( Figure 6B-d, arrowheads) . The ER membrane expanded so greatly that it might have produced more vacuoles ( Figure 6B -e, arrowheads), with nascent vacuoles emerging from the end points of the expanded ER membranes ( Figure 6B -f, arrowheads). Moreover, fusion events might have occurred between lysosomes and autophagic vacuoles ( Figure 6B-f, pane) . Finally, a huge autolysosome formed in the cytoplasm, some partial ER membranes expanded to resemble forks extending towards the vacuole ( Figure 6B -g, two small panes) and one vacuole had just been produced from other partial ER membranes ( Figure 6B-g, arrowheads) . Based on these data, it appeared that the process of autophagy, including the maturation and formation of autophagosomes, was required for the expansion of the ER membrane. Hence all of these observations confirmed that the membranes of TMV-RNA-related autophagic vacuoles might be contiguous with or originate from ER membranes. This result explains why more and more vacuoles appeared and accumulated around the expanded ER membranes when the transfection time was extended. We presume that, under ERS, the distal end of the ER membrane expands to maximize the transformation into more vacuoles, which when encapsulated by a monolayer membrane, are called phagophores (which might form from partially expanded ER membranes; see Figures 6B-c, 6B-d and B-f) . When expanded phagophore cisterns wrap around a portion of the cytoplasm and/or organelles, autophagy is triggered. During the autophagosomal maturation process, the segregated cytoplasm that is engulfed by a phagophore is delivered to the endo/lysosomal lumen by fusion events between autophagosomes and endosomal and/or lysosomal vesicles. Both the cytoplasm and the surrounding membrane are then degraded by lysosomal hydrolases, and the degradation products are transported back to the cytoplasm where they can be re-used for metabolism. Together with our data, the expansion of the ER induced by ERS results in the formation of autophagosomes in TMV-RNA-transfected HeLa cells.