SFTS bunyavirus NSs protein sequestrates mTOR into inclusion bodies and deregulates mTOR‐ULK1 signaling, provoking pro‐viral autophagy

Abstract

Autophagy is emerging as a critical player in host defense against diverse infections, in addition to its conserved function to maintain cellular homeostasis. Strikingly, some pathogens have evolved strategies to evade, subvert or exploit different steps of the autophagy pathway for their lifecycles. Here, we present a new viral mechanism of manipulating autophagy for its own benefit with severe fever with thrombocytopenia syndrome bunyavirus (SFTSV, an emerging high‐pathogenic virus) as a model. SFTSV infection triggers autophagy, leading to complete autophagic flux. Mechanistically, we show that the nonstructural protein of SFTSV (NSs) interacts with mTOR, the pivotal regulator of autophagy, by targeting its kinase domain and captures mTOR into viral inclusion bodies (IBs) induced by NSs itself. Furthermore, NSsimpairs mTOR‐mediated phosphorylation of unc‐51‐like kinase 1 (ULK1) at Ser757, disrupting the inhibitory effect of mTOR on ULK1 activity and thus contributing to autophagy induction. Pharmacologic treatment and Beclin‐1 knockout experimental results establish that, in turn, autophagy enhances SFTSV infection and propagation. Moreover, the minigenome reporter system reveals that SFTSV ribonucleoprotein (the transcription and replication machinery) activity can be bolstered by autophagy. Additionally, we found that the NSs proteins of SFTSV‐related bunyaviruses have a conserved function of targeting mTOR. Taken together, we unravel a viral strategy of inducing pro‐viral autophagy by interacting with mTOR, sequestering mTOR into IBs and hence provoking the downstream ULK1 pathway, which presents a new paradigm for viral manipulation of autophagy and may help inform future development of specific antiviral therapies against SFTSV and related pathogens.

RNA transcriptome analysis of platelets revealed altered platelet responses and the mechanism of thrombocytopenia in SFTS

“…Previous studies reported that SFTSV infection induces autophagic processes in HeLa cells [53] and inhibits autophagic degradation in Vero cells [54]. LC3 accumulation, which is required for autophagosome formation, was observed in both studies [53,54]. In this study, increased levels of LC3B were observed, which suggested the occurrence of autophagy in the platelets of SFTS patients.…”

“…In SFTS patients, although the increase in platelet MLKL levels was not as great as the levels of pyroptosis and apoptosis biomarkers, their increase still indicated a role of necroptosis in thrombocytopenia. Previous studies reported that SFTSV infection induces autophagic processes in HeLa cells [53] and inhibits autophagic degradation in Vero cells [54]. LC3 accumulation, which is required for autophagosome formation, was observed in both studies [53,54].…”

The Effect of Tryptophan-to-Tyrosine Mutation at Position 61 of the Nonstructural Protein of Severe Fever with Thrombocytopenia Syndrome Virus on Viral Replication through Autophagosome Modulation

“…Furthermore, viral replication levels were affected by the NS mutants, which was verified with quantitative real-time polymerase chain reaction (qRT-PCR) and the 50% fluorescent antibody infectious dose (FAID 50 ) [ 16 ]. It has been reported that NSs and NP are involved in autophagosome formation and replication [ 12 , 17 , 18 ]. Therefore, we further explored whether these viral proteins alter LC3 and p62 expression.…”

“…Thus, mTOR-dependent signaling is crucial in regulating autophagy. Previous studies revealed that SFTSV infection decreased the phosphorylation of mTOR and the downstream effector unc-51-like kinase 1 (ULK1) [ 17 , 18 ]. Therefore, we also examined the changes in regulatory factors according to the presence or absence of viral proteins and their binding.…”

Genetic Characteristics of Wuxiang Virus in Shanxi Province, China

“…The function of the NSs protein in WUXV remains elusive. Nonetheless, the NSs protein is widely recognized as a pivotal virulence determinant among phleboviruses, exerting significant influence on the evasion strategies employed by the virus to counteract the host’s innate immune response [ 22 , 44 ]. Among the mutation sites mentioned above, only two (M-312 and M-340) in the M segment exhibited both amino acid mutations and positive selection pressure.…”