A modified lysosomal organelle mediates nonlytic egress of reovirus

Abstract

Mammalian orthoreoviruses (reoviruses) are nonenveloped viruses that replicate in cytoplasmic membranous organelles called viral inclusions (VIs) where progeny virions are assembled. To better understand cellular routes of nonlytic reovirus exit, we imaged sites of virus egress in infected, nonpolarized human brain microvascular endothelial cells (HBMECs) and observed one or two distinct egress zones per cell at the basal surface. Transmission electron microscopy and 3D electron tomography (ET) of the egress zones revealed clusters of virions within membrane-bound structures, which we term membranous carriers (MCs), approaching and fusing with the plasma membrane. These virion-containing MCs emerged from larger, LAMP-1–positive membranous organelles that are morphologically compatible with lysosomes. We call these structures sorting organelles (SOs). Reovirus infection induces an increase in the number and size of lysosomes and modifies the pH of these organelles from ∼4.5–5 to ∼6.1 after recruitment to VIs and before incorporation of virions. ET of VI–SO–MC interfaces demonstrated that these compartments are connected by membrane-fusion points, through which mature virions are transported. Collectively, our results show that reovirus uses a previously undescribed, membrane-engaged, nonlytic egress mechanism and highlights a potential new target for therapeutic intervention.

Ins and Outs of Reovirus: Vesicular Trafficking in Viral Entry and Egress

“…Instead, these alterations misdirect reovirus virions to lysosome -like organelles during entry, resulting in diminished infectivity. By contrast, lysosome-like organelles function productively in the reovirus life cycle during nonlytic egress following replication ( Figure 1 ) [ 36 ]. Whether reovirus directly engages β1 integrin, and how src signaling is triggered to influence virus sorting after uptake, is not understood.…”

“…Viruses must exit infected cells to spread to new cells and hosts. The mechanisms by which reoviruses exit the bloodstream to initiate infection in organs of secondary replication remain to be identified, but the release process may be mediated by nonlytic egress from infected endothelial cells [ 36 ]. Likewise, much remains to be discovered about mechanisms of reovirus neuronal egress and spread through neural networks.…”

“…We discovered that the reovirus egress machinery is composed of two different membranous elements called sorting organelles ( SOs ) and membranous carriers ( MCs ) ( Figure 1 ) [ 36 ]. SOs are recruited to the periphery of viral factories ( VFs ) during late phases of infection and appear to be modified lysosomes.…”

“…In fact, reovirus infection promotes the modification of lysosome dynamics, morphology, and function. The number and size of lysosomes are increased in reovirus-infected cells, and these organelles frequently form aggregates [ 36 ]. It is possible that these modifications alter lysosome function and activity for subsequent use in reovirus egress.…”

Inhibition of HIF-1α accumulation in prostate cancer cells is initiated during early stages of mammalian orthoreovirus infection

“…In either case, the virus core particle escapes and resides in the cytoplasm where transcription, translation, assortment, assembly, and replication occur at or near virus formed inclusions termed viral factories (Broering et al, 2004;Dales, 1965;Desmet et al, 2014;Miller et al, 2010;Silverstein and Dales, 1968). Intact virions are then released by cell lysis or fusion of modified lysosomal compartments containing virus particles with the plasma membrane (Fernández de Castro et al, 2020).…”