Human glucose-regulated protein 78 modulates intracellular production and secretion of nonstructural protein 1 of dengue virus

The interaction of GRP78 and Zika virus E and NS1 proteins occurs in a chaperone-client manner

Glucose regulated protein 78 (GRP78) is a chaperone protein that is a central mediator of the unfolded protein response, a key cellular stress response pathway. GRP78 has been shown to be critically required for infection and replication of a number of flaviviruses, and to interact with both non-structural (NS) and structural flavivirus proteins. However, the nature of the specific interaction between GRP78 and viral proteins remains largely unknown. This study aimed to characterize the binding domain and critical amino acid residues that mediate the interaction of GRP78 to ZIKV E and NS1 proteins. Recombinant EGFP fused GRP78 and individual subdomains (the nucleotide binding domain (NBD) and the substrate binding domain (SBD)) were used as a bait protein and co-expressed with full length or truncated ZIKV E and NS1 proteins in HEK293T/17 cells. Protein-protein interactions were determined by a co-immunoprecipitation assay. From the results, both the NBD and the SBD of GRP78 were crucial for an effective interaction. Single amino acid substitutions in the SBD showed that R492E and T518A mutants significantly reduced the binding affinity of GRP78 to ZIKV E and NS1 proteins. Notably, the interaction of GRP78 with ZIKV E was stably maintained against various single amino acid substitutions on ZIKV E domain III and with all truncated ZIKV E and NS1 proteins. Collectively, the results suggest that the principal binding between GRP78 and viral proteins is mainly a classic canonical chaperone protein-client interaction. The blocking of GRP78 chaperone function effectively inhibited ZIKV infection and replication in neuronal progenitor cells. Our findings reveal that GRP78 is a potential host target for anti-ZIKV therapeutics.

Dengue virus NS1 secretion is regulated via importin-subunit β1 controlling expression of the chaperone GRp78 and targeted by the clinical drug ivermectin

IMPORTANCE

Dengue virus (DENV) is a major human pathogen that can cause hemorrhagic fever and shock syndrome. One important factor of DENV pathogenicity is non-structural protein 1 (NS1), a glycoprotein that is secreted from infected cells. Here we study the mode of action of the widely used drug ivermectin, used to treat parasitic infections and recently shown to lower NS1 blood levels in DENV-infected patients. We found that ivermectin blocks the nuclear transport of transcription factors required for the expression of chaperones that support the folding and secretion of glycoproteins, including NS1. Impairing nuclear transport of these transcription factors by ivermectin or depleting them from infected cells dampens NS1 folding and thus its secretion. These results reveal a novel mode of action of ivermectin that might apply to other flaviviruses as well.

Role of endoplasmic reticulum stress-related unfolded protein response and its implications in dengue virus infection for biomarker development

Dengue virus (DENV) causes debilitating disease in humans, which varies at different rates in host cells, such as monocytes, macrophages, dendritic cells, Langerhans cells, and other cell types. Such heterogeneity in DENV infection in cells could be attributed to a range of factors, including host cell immune response, anti-viral cellular proteins, and virus mediated cellular autophagy. This review delineates an important feature of every cell, the unfolded protein response (UPR) that is attributed to the accumulation of several viral and unfolded/misfolded proteins, such as in DENV infection. UPR is a normal process to counteract endoplasmic reticulum (ER) stress that leads to cell autophagy; though the phenomenon is markedly upregulated during DENV infection. This could be attributed to the uncontrolled activation of the key UPR signaling pathways: inositol-requiring transmembrane kinase/endoribonuclease 1 (IRE1), protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK), and activating transcription factor-6 (ATF6), which promote cell autophagy under normal and diseased conditions through the downstream regulation of apoptosis promoting factors such as X-box binding protein (XBP1), GADD34, and ATF-6. Because DENV can modulate these signaling cascades, by promoting dysregulated cell autophagy, the ER stress mediated UPR pathways and the inherent agents could play an important role in delineating the severity of dengue infection with a potential for developing DENV targeted therapeutics.

The chaperone BiP promotes dengue virus replication and mosquito vitellogenesis in Aedes aegypti

Binding immunoglobulin protein (BiP, also known as GRP78), a chaperone and master regulator of the unfolded protein response (UPR) pathway, plays an essential role in several flavivirus infections, but its functional role in regulating dengue virus replication in the mosquito remains largely unknown. We here demonstrated the interaction between a dengue virus serotype 2 (DENV2) and BiP in Aedes aegypti and report the discovery of a novel functional role of BiP in mosquito vitellogenesis. Silencing Ae. aegypti BiP (AaBiP) expression resulted in the significant inhibition of DENV2 viral genome replication, viral protein production, and infectious viral particle biogenesis. Co-immunoprecipitation assays showed that the DENV2 non-structural protein 1 (NS1) interacts with the AaBiP protein, and silencing AaBiP expression led to enhanced DENV2 NS1 aggregation, indicating that AaBiP plays a role in viral protein stability. A kinetic study focusing on pulse treatment of MG132, a proteasome inhibitor, in AaBiP-silenced mosquitoes showed that DENV2 NS1 was drastically elevated, which further suggests that AaBiP-mediated viral protein degradation is mediated by proteasomal machinery. Silencing of AaBiP also resulted in a reduction in mosquito fertility and fecundity. Depletion of AaBiP inhibited mosquito vitellogenesis due to the reduction of vitellogenin mRNA and elevated aggregation of vitellogenin protein post blood meal, further suppressing ovary development and fecundity. Overall, our results suggest that AaBiP is a dual-function protein with roles in both the regulation of dengue virus replication and mosquito reproduction. Our findings will be useful in the establishment of more efficient strategies for vector-borne disease control.

Alpha-mangostin inhibits viral replication and suppresses nuclear factor kappa B (NF-κB)-mediated inflammation in dengue virus infection

Severe dengue virus (DENV) infection results from viral replication and dysregulated host immune response, which trigger massive cytokine production/cytokine storm. The result is severe vascular leakage, hemorrhagic diathesis, and organ dysfunction. Subsequent to previously proposing that an ideal drug for treatment of DENV infection should efficiently inhibit both virus production and cytokine storm, we discovered that α-mangostin (α-MG) from the pericarp of the mangosteen fruit could inhibit both DENV infection and cytokine/chemokine production. In this study, we investigated the molecular mechanisms underlying the antiviral and anti-inflammatory effects of α-MG. Time-of-drug-addition and time-of-drug-elimination studies suggested that α-MG inhibits the replication step of the DENV life cycle. α-MG inhibited polymerization activity of RNA-dependent RNA polymerase (RdRp) with IC50 values of 16.50 μM and significantly reduced viral RNA and protein syntheses, and virion production. Antiviral and cytokine/chemokine gene expression profiles of α-MG-treated DENV-2-infected cells were investigated by polymerase chain reaction array. α-MG suppressed the expression of 37 antiviral and cytokine/chemokine genes that relate to the NF-κB signaling pathway. Immunofluorescence and immunoblot analyses revealed that α-MG inhibits NF-κB nuclear translocation in DENV-2-infected cells in association with reduced RANTES, IP-10, TNF-α, and IL-6 production. These results suggest α-MG as a potential treatment for DENV infection.

The Dengue Virus Nonstructural Protein 1 (NS1) Interacts with the Putative Epigenetic Regulator DIDO1 to Promote Flavivirus Replication in Mosquito Cells

Dengue virus (DENV) NS1 is a multifunctional protein essential for viral replication. To gain insights into NS1 functions in mosquito cells, the protein interactome of DENV NS1 in C6/36 cells was investigated using a proximity biotinylation system and mass spectrometry. A total of 817 mosquito targets were identified as protein-protein interacting with DENV NS1. Approximately 14% of them coincide with interactomes previously obtained in vertebrate cells, including the oligosaccharide transferase complex, the chaperonin containing TCP-1, vesicle localization, and ribosomal proteins. Notably, other protein pathways not previously reported in vertebrate cells, such as epigenetic regulation and RNA silencing, were also found in the NS1 interactome in mosquito cells. Due to the novel and strong interactions observed for NS1 and the epigenetic regulator DIDO1 (Death-Inducer Obliterator 1), the role of DIDO1 in viral replication was further explored. Interactions between NS1 and DIDO1 were corroborated in infected mosquito cells, by colocalization and proximity ligation assays. Silencing DIDO1 expression results in a significant reduction in DENV and ZIKV replication and progeny production. Comparison of transcription analysis of mock or DENV infected cells silenced for DIDO1 revealed variations in multiple gene expression pathways, including pathways associated with DENV infection such as RNA surveillance, IMD, and Toll. These results suggest that DIDO1 is a host factor involved in the negative modulation of the antiviral response necessary for flavivirus replication in mosquito cells. Our findings uncover novel mechanisms of NS1 to promote DENV and ZIKV replication, and add to the understanding of NS1 as a multifunctional protein. IMPORTANCE Dengue is the most important mosquito-borne viral disease to humans. Dengue virus NS1 is a multifunctional protein essential for replication and modulation of innate immunity. To gain insights into NS1 functions, the protein interactome of dengue virus NS1 in Aedes albopictus cells was investigated using a proximity biotinylation system and mass spectrometry. Several protein pathways, not previously observed in vertebrate cells, such as transcription and epigenetic regulation, were found as part of the NS1 interactome in mosquito cells. Among those, DIDO1 was found to be a necessary host factor for dengue and Zika virus replication in mosquito cells. Transcription analysis of infected mosquito cells silenced for DIDO1 revealed alterations of the IMD and Toll pathways, part of the antiviral response in mosquitoes. The results suggest that DIDO1 is a host factor involved in modulation of the antiviral response and necessary for flavivirus replication.

PCV2 and PRV Coinfection Induces Endoplasmic Reticulum Stress via PERK-eIF2α-ATF4-CHOP and IRE1-XBP1-EDEM Pathways

Porcine circovirus 2 (PCV2) and pseudorabies virus (PRV) are two important pathogens in the pig industry. PCV2 or PRV infection can induce endoplasmic reticulum stress (ERS) and unfolded protein response (UPR). However, the effect of PCV2 and PRV coinfection on the ERS and UPR pathways remains unclear. In this study, we found that PRV inhibited the proliferation of PCV2 mainly at 36 to 72 hpi, while PCV2 enhanced the proliferation of PRV in the middle stage of the infection. Notably, PRV is the main factor during coinfection. The results of the transcriptomic analysis showed that coinfection with PCV2 and PRV activated cellular ERS, and upregulated expressions of the ERS pathway-related proteins, including GRP78, eIF2α, and ATF4. Further research indicated that PRV played a dominant role in the sequential infection and coinfection of PCV2 and PRV. PCV2 and PRV coinfection induced the ERS activation via the PERK-eIF2α-ATF4-CHOP axis and IRE1-XBP1-EDEM pathway, and thus may enhance cell apoptosis and exacerbate the diseases.

Potential Phosphorylation of Viral Nonstructural Protein 1 in Dengue Virus Infection

Dengue virus (DENV) infection causes a spectrum of dengue diseases that have unclear underlying mechanisms. Nonstructural protein 1 (NS1) is a multifunctional protein of DENV that is involved in DENV infection and dengue pathogenesis. This study investigated the potential post-translational modification of DENV NS1 by phosphorylation following DENV infection. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), 24 potential phosphorylation sites were identified in both cell-associated and extracellular NS1 proteins from three different cell lines infected with DENV. Cell-free kinase assays also demonstrated kinase activity in purified preparations of DENV NS1 proteins. Further studies were conducted to determine the roles of specific phosphorylation sites on NS1 proteins by site-directed mutagenesis with alanine substitution. The T27A and Y32A mutations had a deleterious effect on DENV infectivity. The T29A, T230A, and S233A mutations significantly decreased the production of infectious DENV but did not affect relative levels of intracellular DENV NS1 expression or NS1 secretion. Only the T230A mutation led to a significant reduction of detectable DENV NS1 dimers in virus-infected cells; however, none of the mutations interfered with DENV NS1 oligomeric formation. These findings highlight the importance of DENV NS1 phosphorylation that may pave the way for future target-specific antiviral drug design.

Peptides targeting dengue viral nonstructural protein 1 inhibit dengue virus production

Viruses manipulate the life cycle in host cells via the use of viral properties and host machineries. Development of antiviral peptides against dengue virus (DENV) infection has previously been concentrated on blocking the actions of viral structural proteins and enzymes in virus entry and viral RNA processing in host cells. In this study, we proposed DENV NS1, which is a multifunctional non-structural protein indispensable for virus production, as a new target for inhibition of DENV infection by specific peptides. We performed biopanning assays using a phage-displayed peptide library and identified 11 different sequences of 12-mer peptides binding to DENV NS1. In silico analyses of peptide-protein interactions revealed 4 peptides most likely to bind to DENV NS1 at specific positions and their association was analysed by surface plasmon resonance. Treatment of Huh7 cells with these 4 peptides conjugated with N-terminal fluorescent tag and C-terminal cell penetrating tag at varying time-of-addition post-DENV infection could inhibit the production of DENV-2 in a time- and dose-dependent manner. The inhibitory effects of the peptides were also observed in other virus serotypes (DENV-1 and DENV-4), but not in DENV-3. These findings indicate the potential application of peptides targeting DENV NS1 as antiviral agents against DENV infection.

NLRP6 suppresses gastric cancer growth via GRP78 ubiquitination

Nod-like receptor pyrin domain-containing protein 6 (NLRP6) plays a key role in innate immunity, host defense and tumorigenesis. Our previous study has demonstrated the tumor suppressor role of NLRP6 in gastric cancer. In the present study, we explored the interaction protein of NLRP6 by Flag-tagged immunoprecipitation assay and liquid chromatography/mass spectrometry-based proteomics analysis. The 78 kDa glucose-regulated protein (GRP78), a heat shock protein, was identified as an interaction protein of NLRP6. The binding of NLRP6 to GRP78 was through the Pyrin domain, and the substrate binding domain (SBD) domain of GRP78 was responsible for the interaction with NLRP6. NLRP6 overexpression enhanced the polyubiquitination of GRP78 in gastric cancer cells. Overexpression of GRP78 abolished the effects of NLRP6 overexpression in gastric cancer cell proliferation, cell cycle progression, cell apoptosis, migration and Cyclin D1 expression. GRP78 knockdown reversed the effects of NLRP6 knockdown on cell proliferation and cell cycle progression. NLRP6 expression was negatively correlated with GRP78 expression in human gastric tissues. Tumorigenicity assay indicated that GRP78 mediated the functions of NLRP6 on gastric cancer cell growth in vivo. ON-013100, which could inhibit Cyclin D1 expression, was less effective in treating xenografts of gastric cancer cells with higher level of NLRP6 than in those with lower level of NLRP6. In conclusion, our study suggested that NLRP6 exerted inhibitory effects on gastric cancer cell growth by promoting the ubiquitination of GRP78.