Pig acute diarrhea coronavirus promotes replication by inducing autophagy

A recently iscience Study shows that porcine acute diarrhea syndrome (SADS-CoV) coronavirus promotes autophagy to sustain its replication in host cells. More specifically, the virus downregulates the AKT/mammalian target of the rapamycin (mTOR) pathway to induce autophagy.

To learn: Swine acute diarrhea syndrome Coronavirus induces autophagy to promote its replication via the Akt/mTOR pathway. Credit: Kateryna Kon / Shutterstock.com

background

SADS-CoV is an enveloped, positive-sense, single-stranded ribonucleic acid (RNA) virus belonging to the Coronaviridae family. Other highly pathogenic members of the same virus family include severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and more recently SARS-CoV-2.

SADS-CoV is a bat-derived zoonotic coronavirus that was recently discovered in 2017. The virus has potential species transmissibility and is able to infect a range of cells derived from pigs, rats, monkeys and humans. This underscores the need to understand host-pathogen interactions to identify potential antiviral therapeutics.

Autophagy is a vital host defense mechanism against invading viruses. This process helps destroy and eliminate viral components via the lysosomal pathway. However, some viruses, such as Zika virus, human papillomavirus (HPV), and herpes simplex virus type 2, have been found to block host autophagy to promote replication and survival.

In the current study, scientists are investigating the connection between SADS-CoV infection and the regulation of autophagy.

Impact of SADS-CoV infection on autophagy

Cells derived from monkeys and pigs were infected with SADS-CoV-2 and subjected to autophagy analysis at different time points. The modulation of autophagy was assessed by estimating the expression of a vital autophagosome marker LC3-II.

SADS-CoV infection was found to induce expression of LC3-II at all post-infection time points tested. The highest expression was observed after 24 and 36 hours, depending on the cell type. Furthermore, microscopic analysis confirmed the accumulation of autophagosomes in response to SADS-CoV infection.

To determine whether a replication-incompetent virus can induce autophagy, SADS-CoV was first inactivated by ultraviolet (UV) radiation and then used to infect cells. This experiment demonstrated that SADS-CoV must maintain its replication in host cells in order to stimulate the autophagy process.

The effects of autophagy on SADS-CoV replication were assessed using rapamycin and 3-methylhlademine, which are a well-established inducer and inhibitor of autophagy, respectively. While rapamycin induces both autophagy and viral replication in host cells, an opposite effect was observed in cells treated with 3-methylademine.

These results suggest that SADS-CoV induces autophagy to facilitate its replication in host cells during infection.

Mechanism of SADS-CoV-induced autophagy

Autophagy is characterized by the formation of autophagosomes and the subsequent fusion of autophagosomes with lysosomes to degrade viral components. While some viruses induce fusion of autophagosomes with endosomes for survival, others prevent fusion between autophagosomes and lysosomes and subsequently inhibit autophagic flux.

A series of experiments performed to determine the mechanistic details of SADS-CoV-induced autophagy showed that the virus induces full autophagic flux to promote its replication. Inhibition of autophagosome-lysosome fusion has been found to disrupt viral replication.

Further analysis revealed that SADS-CoV induces autophagy via the ATG5-dependent signaling pathway. ATG5 is a protein required for autophagosome formation. At the molecular level, SADS-CoV inhibited AKT/mTOR signaling to promote autophagy and maintain replication.

The mTOR signaling pathway plays a crucial role in initiating autophagy. AKT is a serine/threonine kinase that functions as an upstream signaling component of mTOR signaling to regulate autophagy.

Influence of autophagy inhibition on SADS-CoV replication

Proteomics analysis of SADS-CoV infected cells was performed to identify potential antiviral targets. This led to the identification of eight differentially expressed proteins associated with the PI3K/AKT pathway. Of these proteins, only integrin α3 (ITGA3) showed antiviral effects against SADS-CoV replication.

ITGA3 is a cell membrane adhesion protein closely related to autophagy. Overexpression of ITGA3 in SADS-CoV-infected cells caused down-regulation of autophagy and viral replication, and up-regulation of AKT and mTOR activities. In contrast, opposite events were observed after suppression of ITGA3 in the virus-infected cell.

These results suggest that ITGA3 prevents SADS-CoV replication by inhibiting autophagy via the AKT/mTOR pathway.

study meaning

The current study describes a novel mechanism of SADS-CV-induced autophagy, which is required for viral replication and survival in host cells. In addition, the study identifies ITGA3 as a potential antiviral molecule that can prevent viral replication by inhibiting autophagy.