Synaptogyrin-2 Promotes Replication of a Novel Tick-borne Bunyavirus through Interacting with Viral Nonstructural Protein NSs

Abstract

Synaptogyrin-2 is a non-neuronal member of the synaptogyrin family involved in synaptic vesicle biogenesis and trafficking. Little is known about the function of synaptogyrin-2. Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease characterized by high fever, thrombocytopenia, and leukocytopenia with high mortality, caused by a novel tick-borne phlebovirus in the family Bunyaviridae. Our previous studies have shown that the viral nonstructural protein NSs forms inclusion bodies (IBs) that are involved in viral immune evasion, as well as viral RNA replication. In this study, we sought to elucidate the mechanism by which NSs formed the IBs, a lipid droplet-based structure confirmed by NSs co-localization with perilipin A and adipose differentiation-related protein (ADRP). Through a high throughput screening, we identified synaptogyrin-2 to be highly up-regulated in response to SFTS bunyavirus (SFTSV) infection and to be a promoter of viral replication. We demonstrated that synaptogyrin-2 interacted with NSs and was translocated into the IBs, which were reconstructed from lipid droplets into large structures in infection. Viral RNA replication decreased, and infectious virus titers were lowered significantly when synaptogyrin-2 was silenced in specific shRNAexpressing cells, which correlated with the reduced number of the large IBs restructured from regular lipid droplets. We hypothesize that synaptogyrin-2 is essential to promoting the formation of the IBs to become virus factories for viral RNA replication through its interaction with NSs. These findings unveil the function of synaptogyrin-2 as an enhancer in viral infection. Synaptogyrin-2 (SYNGR2)2 is a tetraspan vesicle membrane protein (1-3) and a member of the synaptogyrin family. Synaptogyrin-1 and -3, abundantly present in synaptic vesicles, are believed to be involved in various aspects in the synaptic vesicle cycle, including vesicle biogenesis, exocytosis and endocytotic recycling, and neurotransmission (2, 4). They are encoded by multigene classes in mammals and evolutionarily conserved throughout the animal kingdom (5). The synaptogyrin-2 gene transcript of 1.6 kb is expressed at high levels in all tissues, except brain, encoding a protein of 224 amino acids in length (5). However, studies on synaptogyrin-2 have been limited, and the function of this protein is largely unknown.Emerging tick-borne bunyaviruses have been recognized to be a serious threat to public health and include severe fever with thrombocytopenia syndrome virus (SFTSV) (6). Since first identified in 2010 in China (7), SFTSV has caused over 3000 reported cases with mortalities ranging from 5 to 15% and has quickly spread to more than 13 provinces (8, 9). Although the symptoms of SFTS are mainly nonspecific, drastic loss of white blood cells and platelets are reported in most cases, and multiorgan failure can occur in severe cases, which usually are fatal (10, 11). SFTSV has also been isolated from patients in Korea and Japan since 2013 (12-15). The clo...

Internalization of the Active Subunit of the Aggregatibacter actinomycetemcomitans Cytolethal Distending Toxin Is Dependent upon Cellugyrin (Synaptogyrin 2), a Host Cell Non-Neuronal Paralog of the Synaptic Vesicle Protein, Synaptogyrin 1

“…Further support that cellugyrin may exist in cytoplasmic vesicles comes from Chapel et al ( 2013 ) who have suggested that this cellular protein is a lysosomal transporter protein. Of particular importance to our study is a recent report by Sun et al ( 2016 ) who were studying Bunyavirus infection in mammalian cells and demonstrated that synaptogyrin-2 (cellugyrin) interacts with viral nonstructural proteins and together were transported into inclusion bodies “reconstructed from lipid droplets” during infection. They further reported that this translocation was critical to viral replication as silencing of cellugyrin expression reduced inclusion body formation and decreased virus titers.…”

Lipid droplet functions beyond energy storage

“…There are indications that a range of other viruses may also exploit LDs for virus maturation. LD-association has been reported for non-structural proteins of orbiviruses [147], bovine viral diarrhea virus [148], and bunyavirus [149, 150], but the functional significance of this LD localization remains to be investigated.…”

Suppression of the IFN-α and -β Induction through Sequestering IRF7 into Viral Inclusion Bodies by Nonstructural Protein NSs in Severe Fever with Thrombocytopenia Syndrome Bunyavirus Infection

“…We conclude that the induction of the IRF3 and IRF7 transcripts varied greatly among the tested cell types, with the HepG2 cells to be the most potent for induction. HepG2, originated from human liver epithelial cells, could be unique in response to SFTSV infection as we know that the liver is one of the severely affected organs during SFTSV infection (30).…”