A study published in the journal Cell revealed β-coronaviruses exploit lysosomal organelles for egress. This finding provides information on the cellular and immunological abnormities seen in patients and suggests new avenues for therapies, said investigators.
The kinetics of replication and egress were first investigated in HeLa-ATCC cells expressing murine CEACAM1 that were infected with the MHV-A59 strain of mouse hepatitis virus (MHV). The replication rate was highest up to 8 hours postinfection (pi) and egress was highest from 8 to 12 hours pi, levelling off at 14 hours pi. The virus released 8 to 14 hours pi was infectious, could be titrated, and occurred without cell lysis and with no changes to membrane permeability.
Experiments to block the biosynthetic pathway, which is commonly used by other enveloped viruses, failed to stop viral egress, suggesting another pathway for β-coronaviruses. Labelling and imaging techniques found that MHV is instead associated with lysosomes and/or atypical late endosomes, said investigators. Importantly, this association was determined not to be caused by endocytic reuptake of the virus.
The association of the β-coronaviruses with lysosomes was further investigated using transmission electron microscopy (TEM) and immune-electron (Immuno-EM) microscopy in both MHV- and SARS-CoV-2-infected cells. TEM imaging of MHV- and SARS-CoV-2-infected cells revealed what were likely to be lysosomes filled with intact virus. Further evidence for the use of lysosomal organelles was provided by the presence of host chaperone proteins co-localized and secreted with MHV and lysosomes.
Importantly, investigators also found that viral egress was occurring via a route that bypassed the BFA-sensitive biosynthetic secretory pathway. Instead, egress is being mediated by the Arf-like small GTPase Arl8b.
The Rab7 GTPase competitive inhibitor CID1067700 was able to block egress while having no effect on cell viability, viral infection, or replication, said investigators. The functional consequences of this egress pathway were lysosome deacidification, inactivation of lysosomal degradation enzymes, and disruption of antigen presentation pathways.
According to investigators, work is underway to find the route β-coronaviruses use to reach the lysosomes, and two nonbiosynthetic, secretory trafficking routes are possible. The discovery of this lysosome-based egressed pathway also points to potential new treatment options targeting regulators of lysosome trafficking and biogenesis as well as reversing lysosome deacidification and improving immune responses against lysosomal defects.
Ghosh S, Dellibovi-Ragheb TA, Kerviel A, et al. β-coronaviruses use lysosomes for egress instead of the biosynthetic secretory pathway. Cell. 2020;183(6):1520-1535. doi: 10.1016/j.cell.2020.10.039