Mechanistic Target of Rapamycin Signaling Activation Antagonizes Autophagy To Facilitate Zika Virus Replication

Abstract

Zika virus (ZIKV), a mosquito-transmitted flavivirus, is linked to microcephaly and other neurological defects in neonates and Guillain–Barré syndrome in adults. The molecular mechanisms regulating ZIKV infection and pathogenic outcomes are incompletely understood. Signaling by the mechanistic (mammalian) target of rapamycin (mTOR) kinase is important for cell survival and proliferation, and viruses are known to hijack this pathway for their replication. Here, we show that in human neuronal precursors and glial cells in culture, ZIKV infection activates both mTOR complex 1 (mTORC1) and 2 (mTORC2). Inhibition of mTOR kinase by torin1 or rapamycin results in reduction in ZIKV protein expression and progeny production. Depletion of Raptor, the defining subunit of mTORC1, by small interfering RNA (siRNA), negatively affects ZIKV protein expression and viral replication. Although depletion of Rictor, the unique subunit of mTORC2 or the mTOR kinase itself also inhibits the viral processes, the extent of inhibition is less pronounced. Autophagy is transiently induced early by ZIKV infection and impairment of autophagosome elongation by the Class III PI3K inhibitor 3-methyladenine (3-MA) enhances viral protein accumulation and progeny production. mTOR phosphorylates and inactivates ULK1 (S757) at later stages of ZIKV infection, suggesting a link between autophagy inhibition and mTOR activation by ZIKV. Accordingly, inhibition of ULK1 (by MRT68921) or autophagy (by 3-MA) reversed the effects of mTOR-inhibition, leading to increased levels of ZIKV protein expression and progeny production. Our results demonstrate that ZIKV replication requires the activation of both mTORC1 and mTORC2, which negatively regulates autophagy to facilitate ZIKV replication. IMPORTANCE The re-emergence of Zika virus (ZIKV) and its association with neurological complications necessitates studies on the molecular mechanisms that regulate ZIKV pathogenesis. The mTOR signaling cascade is tightly regulated and central to normal neuronal development and survival. Disruption of mTOR signaling can result in neurological abnormalities. In the studies reported here, we demonstrate for the first time that ZIKV infection results in activation of both mTORC1 and mTORC2 to promote the virus replication. Although autophagy is activated early in infection to counter virus replication, it is subsequently suppressed by the mTOR. These results reveal critical roles of mTOR signaling and autophagy in ZIKV infection and point to a possible mechanism underlying ZIKV-induced pathogenesis. Elucidating the role of mTOR signaling in ZIKV infection will provide insights into our understanding of the mechanisms of ZIKV-induced neurological complications and potential targets for therapeutic approaches.

LINC00680 enhances hepatocellular carcinoma stemness behavior and chemoresistance by sponging miR-568 to upregulate AKT3

“…The mammalian target of rapamycin (mTOR) is aberrantly phosphorylated and activated by PI3K-AKT signaling in many human malignancies, including HCC, and plays a critical role in HCC oncogenesis [19,33,34]. mTOR is a serine/threonine protein kinase and exist as two distinct forms: mTORC1 and mTORC2 [35,36]. Rapamycin-sensitive mTORC1 promote HCC growth and progression via directly targeting phosphorylation and activation of ribosomal protein S6 kinase (p70S6K) and eukaryotic translation initiation factor 4E-binding protein 1 (eIF4EBP1) [37].…”

Anti-flavivirus Properties of Lipid-Lowering Drugs

“…It should be noted that this mechanism also plays an essential role in the degradation of labeled intracellular pathogens and in the induction of the antiviral response (Lee and Iwasaki, 2008;Deretic et al, 2013). Even though autophagy restricts WNV replication (Shoji-Kawata et al, 2013;Kobayashi et al, 2014), viruses such as DENV and ZIKV have successfully subverted this process to enhance their replication Chu et al, 2014;Liang et al, 2016;Sahoo et al, 2020).…”

“…Interestingly, ZIKV-induced autophagy activation has been associated with the early stages of infection. Sahoo et al (2020) reported that ZIKV induces autophagy early and transiently, and subsequently, the virus can reverse this activation to allow viral protein accumulation and virus replication in neuronal and glial cells; therefore, suppression of autophagy at late times of ZIKV infection is suggested necessary for its replication (Sahoo et al, 2020). Similarly, Metz et al (2015) reported a biphasic autophagy response to DENV infection, in which DENV infection initially activates it and then, later on, inhibits autophagy (Metz et al, 2015, p. 62).…”

“…Although the mechanism is unclear, the evidence suggests that flaviviruses can dynamically modulate autophagy throughout infection. In this sense, it has been demonstrated that ZIKV can induce changes in the activity of the mTORC1 protein (mammalian target of rapamycin complex 1), the master regulator of the autophagic pathway (Sahoo et al, 2020); in turn, mTORC1 can regulate lipogenesis mediated by SREBPs (sterol responsive element binding protein), the transcription factors that regulate cholesterol and fatty acid synthesis (Porstmann et al, 2008;Düvel et al, 2010, p. 1;Li et al, 2010;Peterson et al, 2011, p. 1). Therefore, it cannot be ruled out that ZIKV-induced changes in mTORC1 activity contribute to the switch between lipid catabolism and lipid biogenesis.…”

Expanding TOR Complex 2 Signaling: Emerging Regulators and New Connections

“…In neuronal cells, Zika virus (ZIKV) infection activates mTORC2 through unknown mechanisms. The activation of mTORC2 facilitates ZIKV replication by negatively regulating autophagy (Sahoo et al, 2020). Upon influenza virus infection, the viral protein NS1 promotes mTORC2-mediated Akt-S473 phosphorylation, which inhibits cell apoptosis (Kuss-Duerkop et al, 2017).…”