Severe Fever with Thrombocytopenia Syndrome Virus NSs Interacts with TRIM21 To Activate the p62-Keap1-Nrf2 Pathway

Abstract

Nuclear factor erythroid 2-related factor 2 (Nrf2) dissociates from its inhibitor, Keap1, upon stress signals and subsequently induces an antioxidant response that critically controls the viral life cycle and pathogenesis. Besides intracellular Fc receptor function, tripartite motif 21 (TRIM21) E3 ligase plays an essential role in the p62-Keap1-Nrf2 axis pathway for redox homeostasis. Specifically, TRIM21-mediated p62 ubiquitination abrogates p62 oligomerization and sequestration activity and negatively regulates the Keap1-Nrf2-mediated antioxidant response. A number of viruses target the Nrf2-mediated antioxidant response to generate an optimal environment for their life cycle. Here we report that a nonstructural protein (NSs) of severe fever with thrombocytopenia syndrome virus (SFTSV) interacts with and inhibits TRIM21 to activate the Nrf2 antioxidant signal pathway. Mass spectrometry identified TRIM21 to be a binding protein for NSs. NSs bound to the carboxyl-terminal SPRY subdomain of TRIM21, enhancing p62 stability and oligomerization. This facilitated p62-mediated Keap1 sequestration and ultimately increased Nrf2-mediated transcriptional activation of antioxidant genes, including those for heme oxygenase 1, NAD(P)H quinone oxidoreductase 1, and CD36. Mutational analysis found that the NSs-A46 mutant, which no longer interacted with TRIM21, was unable to increase Nrf2-mediated transcriptional activation. Functionally, the NS wild type (WT), but not the NSs-A46 mutant, increased the surface expression of the CD36 scavenger receptor, resulting in an increase in phagocytosis and lipid uptake. A combination of reverse genetics and assays with Ifnar−/− mouse models revealed that while the SFTSV-A46 mutant replicated similarly to wild-type SFTSV (SFTSV-WT), it showed weaker pathogenic activity than SFTSV-WT. These data suggest that the activation of the p62-Keap1-Nrf2 antioxidant response induced by the NSs-TRIM21 interaction contributes to the development of an optimal environment for the SFTSV life cycle and efficient pathogenesis. IMPORTANCE Tick-borne diseases have become a growing threat to public health. SFTSV, listed by the World Health Organization as a prioritized pathogen, is an emerging phlebovirus, and fatality rates among those infected with this virus are high. Infected Haemaphysalis longicornis ticks are the major source of human SFTSV infection. In particular, the recent spread of this tick to over 12 states in the United States has increased the potential for outbreaks of this disease beyond Far East Asia. Due to the lack of therapies and vaccines against SFTSV infection, there is a pressing need to understand SFTSV pathogenesis. As the Nrf2-mediated antioxidant response affects viral life cycles, a number of viruses deregulate Nrf2 pathways. Here we demonstrate that the SFTSV NSs inhibits the TRIM21 function to upregulate the p62-Keap1-Nrf2 antioxidant pathway for efficient viral pathogenesis. This study not only demonstrates the critical role of SFTSV NSs in viral pathogenesis but also suggests potential future therapeutic approaches to treat SFTSV-infected patients.

Comparison of Habitat Suitability Models for Haemaphysalis longicornis Neumann in North America to Determine Its Potential Geographic Range

“…Globally, this tick species is associated with at least 59 pathogens, of which 30 are potentially pathogenic to humans [45]. The most noted human pathogen transmitted by the H. longicornis in its native range in East Asia is the SFTSV [4]. Since its discovery in 2009, cases of SFTSV have been increasingly reported in East Asia [46].…”

“…It has also become established in Australia, New Zealand, and the western Pacific islands (New Caledonia, Fiji, Western Samoa, Tonga, Vanuatu) [1][2][3]. In East Asia, H. longicornis is the main vector for transmission of severe fever with thrombocytopenia syndrome virus (SFTSV) in humans [4]. This emerging zoonotic disease caused by a novel bunyavirus was first identified in China in 2009 [5] and then in South Korea and Japan in 2013 [6,7].…”

A Look into Bunyavirales Genomes: Functions of Non-Structural (NS) Proteins

“…Lastly, SFTSV NSs functions to increase virulence independent of the IFN systems. This includes upregulating the p62-Keap1-Nrf2 antioxidant pathway [ 144 ], inducing interleukin 10 (IL-10) expression involved in host immune response [ 145 ], suppressing NF-κB promoter activities to avoid innate immunity signaling [ 146 ], interacting with TRIM25 to mediate antiviral signaling [ 147 ], and promoting cell cycle arrest [ 148 ]. Further, although SFTSV NSs is dispensable for viral replication [ 128 ], it can form viroplasm-like structures (VLSs) in infected cells and these serve as sites of viral dsRNA localization, indicating a potential novel role of NSs in enhancing SFTSV replication [ 149 ].…”

TRIM Proteins in Host Defense and Viral Pathogenesis

“…In context of SFTSV infection, the viral proteins bind to the C-terminal SPRY subdomain of TRIM21, enhancing p62 stability and oligomerization. This allows p62mediated Keap1 sequestration and activates the Nrf2mediated antioxidant response, promoting viral replication and pathogenesis [103].…”