The rotavirus progeny is released from the apical pole of...

The rotavirus progeny is released from the apical pole of human enterocytes before cell lysis. NSP4 acts as a viroporin, mediating the release of Ca2+ from intracellular stores, which leads to the activation of an autophagy-related kinase cascade (Crawford et al. 2012; reviewed in Suzuki 2019). Using a reassortant rotavirus A (RV-A) between the human strain D and simian strain RRV, it was shown that the exit of viral particles from the ER lumen is dependent on VP4, as when VP4 expression is silenced by siRNA, immature nonenveloped viral particles accumulate and aggregate inside the distended ER (Cuadras et al. 2006). As determined by co-staining for VP4, VP7, and LMAN1 (also known as ERGIC-53), a component of ER-Golgi intermediate compartment, mature TLPs are found in the transport vesicles belonging to this compartment, but not in cis-Golgi (Cuadras et al. 2006). TLPs migrate to the lipid rafts (Cuadras et al. 2006) and previously reported targeting of VP4 to the lipid rafts of the plasma membrane (Sapin et al. 2002; Delmas te al. 2004) is likely mostly mediated by the formation of TLPs in the ER (Cuadras et al. 2006). Migration to the lipid rafts is dependent on both VP4 and NSP4 (Cuadras et al. 2006). VP4 has an intrinsic propensity to interact with cholesterol- and sphingolipid-enriched model lipid membranes (Sapin et al. 2002) and has been reported to localize to lipid raft microdomains of the plasma membrane independently of the rest of the virion (Delmas et al. 2004). An alternative model of rotavirus assembly and release has been that VP4 becomes incorporated into virions at the plasma membrane lipid rafts (reviewed in Chwetzoff and Trugnan 2006), but the existing experimental evidence does not favor this model, although a portion of VP4 may associate with mature TLPs at the plasma membrane during release (Trejo-Cerro et al. 2018: RRV-infected rhesus monkey embryonic kidney cell line MA104 was used). VP4 binds to and remodels actin bundles at the apical brush border of the intestinal epithelial cell line Caco-2, derived from colon carcinoma, and this VP4-mediated actin remodeling is involved in the release of rotavirus virions from the apical surface of infected host enterocytes (Gardet et al. 2006, Gardet et al. 2007: bovine RV-A strain RF was used). VP4-mediated remodeling of actin filaments is also involved in rotavirus exit from the monkey kidney cell line COS-7 (Condemine et al. 2019). Functional actin cytoskeleton is also required for targeting of VP4 to lipid rafts and the release of RV-A particles from nonpolarized cells in culture (Trejo-Cerro et al. 2018).

Cytoskeleton alterations caused by the infection of Caco-2 cells with RRV block the transport of sucrase-isomaltase to the brush border of enterocytes, resulting in decreased apical surface expression of sucrase-isomaltase (Jourdan et al. 1998).