Involvement of Hsp90 in assembly and nuclear import of influenza virus RNA polymerase subunits

Novel Hsp90α inhibitor inhibits HSV-1 infection by suppressing the Akt/β-catenin pathway

OBJECTIVE

The prevalence of herpes simplex virus type 1 (HSV-1) infection and the emergence of drug-resistant HSV-1 strains posts a significant global health challenge, necessitating the urgent development of effective anti-HSV-1 drugs. As one of the most prevalent molecular chaperones, heat shock protein 90 α (Hsp90α) has been extensively demonstrated to regulate a range of viral infections, thus representing a promising antiviral target. In this study, we identified JD-13 as a novel Hsp90α inhibitor and explored its capability in inhibiting HSV-1 infection.

METHODS

The inhibitory effect of JD-13 on Hsp90α activity was confirmed by molecular docking, molecular dynamic stimulations, fluorescence quench titration and cellular thermal shift assay. The antiviral activity of JD-13 was examined by viral plaque assay, RT-qPCR, Western blot, flow cytometry, fluorescence microscopy and time-of-addition assay. The in vivo antiviral efficacy of JD-13 was evaluated in the HSV-1 skin infection guinea pig model by analyzing skin lesions and herpes formation.

RESULTS

JD-13 significantly inhibited the infection of both normal and acyclovir-resistant HSV-1 strains. In addition, JD-13 alleviated skin damage in guinea pigs caused by cutaneous HSV-1 infection. Further studies revealed that JD-13 impaired HSV-1 early infection and suppressed the Akt/β-catenin signalling pathway by promoting Akt degradation. Consequently, the inhibition of the Akt/β-catenin signalling pathway restricted HSV-1 infection.

CONCLUSIONS

These results suggest JD-13 as a novel HSP90α inhibitor with the potential to be developed as an antiviral agent for the treatment of HSV-1-related diseases.

Glucose and glutamine drive hepatitis E virus replication

Viruses have undergone evolutionary adaptations to tune their utilization of carbon sources, enabling them to extract specific cellular substrates necessary for their replication. The lack of a reliable cell culture system and a small-animal model has hampered our understanding of the molecular mechanism of replication of hepatitis E virus (HEV) genotype 1. Our recent identification of a replicative ensemble of mutant HEV RNA libraries has allowed us to study the metabolic prerequisites for HEV replication. Initial assessments revealed increased glucose and glutamine utilization during HEV replication. Inhibition of glycolysis and glycolysis + glutaminolysis reduced the levels of HEV replication to similar levels. An integrated analysis of protein-metabolite pathways suggests that HEV replication markedly alters glycolysis, the TCA cycle, and glutamine-associated metabolic pathways. Cells supporting HEV replication showed a requirement for fructose-6-phosphate and glutamine utilization through the hexosamine biosynthetic pathway (HBP), stimulating HSP70 expression to facilitate virus replication. Observations of mannose utilization and glutamine dependence suggest a crucial role of the HBP in supporting HEV replication. Inhibition of glycolysis and HSP70 activity or knockdown of glutamine fructose-6-phosphate amidotransferase expression led to a substantial reduction in HEV RNA and ORF2 expression accompanied by a significant decrease in HSP70 levels. This study demonstrates that glucose and glutamine play critical roles in facilitating HEV replication.

Litopenaeus vannamei heat shock protein 90 (LvHSP90) interacts with white spot syndrome virus protein, WSSV322, to modulate hemocyte apoptosis during viral infection

As cellular chaperones, heat shock protein can facilitate viral infection in different steps of infection process. Previously, we have shown that the suppression of Litopenaeus vannamei (Lv)HSP90 not only results in a decline of white spot syndrome virus (WSSV) infection but also induces apoptosis in shrimp hemocyte cells. However, the mechanism underlying how LvHSP90 involved in WSSV infection remains largely unknown. In this study, a yeast two-hybrid assay and co-immunoprecipitation revealed that LvHSP90 interacts with the viral protein WSSV322 which function as an anti-apoptosis protein. Recombinant protein (r) LvHSP90 and rWSSV322 inhibited cycloheximide-induced hemocyte cell apoptosis in vitro. Co-silencing of LvHSP90 and WSSV322 in WSSV-infected shrimp led to a decrease in expression level of viral replication marker genes (VP28, ie-1) and WSSV copy number, while caspase 3/7 activity was noticeably induced. The number of apoptotic cells, confirmed by Hoechst 33342 staining assay and annexin V/PI staining, was significantly higher in LvHSP90 and WSSV322 co-silenced-shrimp than the control groups. Moreover, the co-silencing of LvHSP90 and WSSV322 triggered apoptosis by the mitochondrial pathway, resulting in the upregulation of pro-apoptotic protein expression (bax) and the downregulation of anti-apoptotic protein expression (bcl, Akt). This process also involved the release of cytochrome c (CytC) from the mitochondria and a decrease in mitochondrial membrane potential (MMP). These findings suggest that LvHSP90 interacts with WSSV322 to facilitate viral replication by inhibiting host apoptosis during WSSV infection.

Evaluation of Oxidative Stress and Endothelial Dysfunction in COVID-19 Patients

Background and Objectives: Heat shock proteins (HSPs) are stress proteins. The endogenous nitric oxide (NO) synthase inhibitor asymmetric dimethyl arginine (ADMA) is a mediator of endothelial dysfunction. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus causes endothelial dysfunction and coagulopathy through severe inflammation and oxidative stress. Using these markers, we analyzed the prognostic value of serum ADMA and HSP-90 levels for early prediction of severe coronavirus disease (COVID-19) patients. Materials and Methods: A total of 76 COVID-19 patients and 35 healthy control subjects were included in this case-control study. COVID-19 patients were divided into two groups: mild and severe. Results: Serum ADMA and HSP-90 levels were significantly higher in the COVID-19 patients compared to the control subjects (p < 0.001). Additionally, serum ADMA and HSP-90 levels were determined to be higher in a statistically significant way in severe COVID-19 compared to mild COVID-19 (p < 0.001). Univariable logistic regression analysis revealed that ADMA and HSP-90, respectively, were independent predictors of severe disease in COVID-19 patients (ADMA (OR = 1.099, 95% CI = 1.048-1.152, p < 0.001) and HSP-90 (OR = 5.296, 95% CI = 1.719-16.316, p = 0.004)). When the cut-off value for ADMA was determined as 208.94 for the prediction of the severity of COVID-19 patients, the sensitivity was 72.9% and the specificity was 100% (AUC = 0.938, 95%CI = 0.858-0.981, p < 0.001). When the cut-off value for HSP-90 was determined as 12.68 for the prediction of the severity of COVID-19 patients, the sensitivity was 88.1% and the specificity was 100% (AUC = 0.975, 95% CI= 0.910-0.997, p < 0.001). Conclusions: Increased levels of Heat shock proteins-90 (HSP-90) and ADMA were positively correlated with increased endothelial damage in COVID-19 patients, suggesting that treatments focused on preventing and improving endothelial dysfunction could significantly improve the outcomes and reduce the mortality rate of COVID-19. ADMA and HSP-90 might be simple, useful, and prognostic biomarkers that can be utilized to predict patients who are at high risk of severe disease due to COVID-19.

Granzymes in health and diseases: the good, the bad and the ugly

Granzymes are a family of serine proteases, composed of five human members: GA, B, H, M and K. They were first discovered in the 1980s within cytotoxic granules released during NK cell- and T cell-mediated killing. Through their various proteolytic activities, granzymes can trigger different pathways within cells, all of which ultimately lead to the same result, cell death. Over the years, the initial consideration of granzymes as mere cytotoxic mediators has changed due to surprising findings demonstrating their expression in cells other than immune effectors as well as new intracellular and extracellular activities. Additional roles have been identified in the extracellular milieu, following granzyme escape from the immunological synapse or their release by specific cell types. Outside the cell, granzyme activities mediate extracellular matrix alteration via the degradation of matrix proteins or surface receptors. In certain contexts, these processes are essential for tissue homeostasis; in others, excessive matrix degradation and extensive cell death contribute to the onset of chronic diseases, inflammation, and autoimmunity. Here, we provide an overview of both the physiological and pathological roles of granzymes, highlighting their utility while also recognizing how their unregulated presence can trigger the development and/or worsening of diseases.

Hsp90 β is critical for the infection of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome (SFTS) caused by the SFTS virus (SFTSV) is an emerging disease in East Asia with a fatality rate of up to 30%. However, the viral-host interaction of SFTSV remains largely unknown. The heat-shock protein 90 (Hsp90) family consists of highly conserved chaperones that fold and remodel proteins and has a broad impact on the infection of many viruses. Here, we showed that Hsp90 is an important host factor involved in SFTSV infection. Hsp90 inhibitors significantly reduced SFTSV replication, viral protein expression, and the formation of inclusion bodies consisting of nonstructural proteins (NSs). Among viral proteins, NSs appeared to be the most reduced when Hsp90 inhibitors were used, and further analysis showed that their translation was affected. Co-immunoprecipitation of NSs with four isomers of Hsp90 showed that Hsp90 β specifically interacted with them. Knockdown of Hsp90 β expression also inhibited replication of SFTSV. These results suggest that Hsp90 β plays a critical role during SFTSV infection and could be a potential target for the development of drugs against SFTS.

K64 acetylation of heat shock protein 90 suppresses nucleopolyhedrovirus replication in Bombyx mori

HSP90 is a highly conserved chaperone that facilitates the proliferation of many viruses, including silkworm (bombyx mori) nucleopolyhedrovirus (BmNPV), but the underlying regulatory mechanism was unclear. We found that suppression of HSP90 by 17-AAG, a HSP90-specific inhibitor, significantly reduced the expression of BmNPV capsid protein gp64 and viral genome replication, whereas overexpression of B. mori HSP90(BmHSP90) promoted BmNPV replication. Furthermore, in a recent study of the lysine acetylome of B. mori infected with BmNPV, we focused on the reduced viral proliferation due to changes of BmHSP90 lysine acetylation. Site-directed introduction of acetylated (K/Q) or deacetylated (K/R) mimic mutations into BmHSP90 revealed that lysine 64 (K64) acetylation activated the JAK/STAT pathway and reduced BmHSP90 ATPase activity, leading to diminished chaperone activity and ultimately inhibiting BmNPV proliferation. In this study, a single lysine 64 acetylation change of BmHSP90 was elucidated as a model of posttranslational modifications occurring in the wake of host-virus interactions, providing novel insights into potential antiviral strategies.

ASK1 inhibitors are potential pan-antiviral drugs, which dampen replication of diverse viruses including SARS-CoV2

Apoptosis signal-regulating kinase 1 (ASK1)/MAP3K5 is a stress response kinase that is activated by various stimuli. It is known as an upstream activator of p38- Mitogen-activated protein kinase (p38MAPK) and c-Jun N-terminal kinase (JNK) that are reactive oxygen species (ROS)-induced kinases. Accumulating evidence show that ROS accumulate in virus-infected cells. Here, we investigated the relationship between viruses and ASK1/p38MAPK or ASK1/JNK pathways. Our findings suggest that virus infection activates ASK1 related pathways. In parallel, ASK1 inhibition led to a remarkable reduction in the replication of a broad range of viruses including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), vaccinia virus (VV), vesicular stomatitis virus (VSV), Herpes Simplex Virus (HSV), and Human Immunodeficiency virus (HIV) in different human cell lines. Our work demonstrates the potential therapeutic use of Selonsertib, an ASK1 inhibitor, as a pan-antiviral drug in humans. Surprisingly, we observed differential effects of Selonsertib in in vitro and in vivo hamster models, suggesting caution in using rodent models to predict clinical and therapeutic outcomes in humans.

LGP2 directly interacts with flavivirus NS5 RNA-dependent RNA polymerase and downregulates its pre-elongation activities

LGP2 is a RIG-I-like receptor (RLR) known to bind and recognize the intermediate double-stranded RNA (dsRNA) during virus infection and to induce type-I interferon (IFN)-related antiviral innate immune responses. Here, we find that LGP2 inhibits Zika virus (ZIKV) and tick-borne encephalitis virus (TBEV) replication independent of IFN induction. Co-immunoprecipitation (Co-IP) and confocal immunofluorescence data suggest that LGP2 likely colocalizes with the replication complex (RC) of ZIKV by interacting with viral RNA-dependent RNA polymerase (RdRP) NS5. We further verify that the regulatory domain (RD) of LGP2 directly interacts with RdRP of NS5 by biolayer interferometry assay. Data from in vitro RdRP assays indicate that LGP2 may inhibit polymerase activities of NS5 at pre-elongation but not elongation stages, while an RNA-binding-defective LGP2 mutant can still inhibit RdRP activities and virus replication. Taken together, our work suggests that LGP2 can inhibit flavivirus replication through direct interaction with NS5 protein and downregulates its polymerase pre-elongation activities, demonstrating a distinct role of LGP2 beyond its function in innate immune responses.

ASFV transcription reporter screening system identifies ailanthone as a broad antiviral compound

African swine fever (ASF) is an acute, highly contagious and deadly viral disease in swine that jeopardizes the worldwide pig industry. Unfortunately, there are no authoritative vaccine and antiviral drug available for ASF control. African swine fever virus (ASFV) is the etiological agent of ASF. Among the ASFV proteins, p72 is the most abundant component in the virions and thus a potential target for anti-ASFV drug design. Here, we constructed a luciferase reporter system driven by the promoter of p72, which is transcribed by the co-transfected ASFV RNA polymerase complex. Using this system, we screened over 3200 natural product compounds and obtained three potent candidates against ASFV. We further evaluated the anti-ASFV effects and proved that among the three candidates, ailanthone (AIL) inhibits the replication of ASFV at the nanomolar concentration (IC50 ​= ​15 ​nmol/L). Our in vitro experiments indicated that the antiviral effect of AIL is associated with its inhibition of the HSP90-p23 cochaperone. Finally, we showed the antiviral activity of AIL on Zika virus and hepatitis B virus (HBV), which supports that AIL is a potential broad-spectrum antiviral agent.

Advances in deciphering the interactions between viral proteins of influenza A virus and host cellular proteins

Influenza A virus (IAV) poses a severe threat to the health of animals and humans. The genome of IAV consists of eight single-stranded negative-sense RNA segments, encoding ten essential proteins as well as certain accessory proteins. In the process of virus replication, amino acid substitutions continuously accumulate, and genetic reassortment between virus strains readily occurs. Due to this high genetic variability, new viruses that threaten animal and human health can emerge at any time. Therefore, the study on IAV has always been a focus of veterinary medicine and public health. The replication, pathogenesis, and transmission of IAV involve intricate interplay between the virus and host. On one hand, the entire replication cycle of IAV relies on numerous proviral host proteins that effectively allow the virus to adapt to its host and support its replication. On the other hand, some host proteins play restricting roles at different stages of the viral replication cycle. The mechanisms of interaction between viral proteins and host cellular proteins are currently receiving particular interest in IAV research. In this review, we briefly summarize the current advances in our understanding of the mechanisms by which host proteins affect virus replication, pathogenesis, or transmission by interacting with viral proteins. Such information about the interplay between IAV and host proteins could provide insights into how IAV causes disease and spreads, and might help support the development of antiviral drugs or therapeutic approaches.

Mode of Action of Heat Shock Protein (HSP) Inhibitors against Viruses through Host HSP and Virus Interactions

Misfolded proteins after stress-induced denaturation can regain their functions through correct re-folding with the aid of molecular chaperones. As a molecular chaperone, heat shock proteins (HSPs) can help client proteins fold correctly. During viral infection, HSPs are involved with replication, movement, assembly, disassembly, subcellular localization, and transport of the virus via the formation of macromolecular protein complexes, such as the viral replicase complex. Recent studies have indicated that HSP inhibitors can inhibit viral replication by interfering with the interaction of the virus with the HSP. In this review, we describe the function and classification of HSPs, the transcriptional mechanism of HSPs promoted by heat shock factors (HSFs), discuss the interaction between HSPs and viruses, and the mode of action of HSP inhibitors at two aspects of inhibiting the expression of HSPs and targeting the HSPs, and elaborate their potential use as antiviral agents.

Avian Influenza Virus Tropism in Humans

An influenza pandemic happens when a novel influenza A virus is able to infect and transmit efficiently to a new, distinct host species. Although the exact timing of pandemics is uncertain, it is known that both viral and host factors play a role in their emergence. Species-specific interactions between the virus and the host cell determine the virus tropism, including binding and entering cells, replicating the viral RNA genome within the host cell nucleus, assembling, maturing and releasing the virus to neighboring cells, tissues or organs before transmitting it between individuals. The influenza A virus has a vast and antigenically varied reservoir. In wild aquatic birds, the infection is typically asymptomatic. Avian influenza virus (AIV) can cross into new species, and occasionally it can acquire the ability to transmit from human to human. A pandemic might occur if a new influenza virus acquires enough adaptive mutations to maintain transmission between people. This review highlights the key determinants AIV must achieve to initiate a human pandemic and describes how AIV mutates to establish tropism and stable human adaptation. Understanding the tropism of AIV may be crucial in preventing virus transmission in humans and may help the design of vaccines, antivirals and therapeutic agents against the virus.

Heat Shock Protein 60 Is Involved in Viral Replication Complex Formation and Facilitates Foot and Mouth Virus Replication by Stabilizing Viral Nonstructural Proteins 3A and 2C

The maintenance of viral protein homeostasis depends on the machinery of the infected host cells, giving us an insight into the interplay between host and virus. Accumulating evidence suggests that heat shock protein 60 (HSP60), as one molecular chaperone, is involved in regulating virus infection. However, the role of HSP60 during foot-and-mouth disease virus (FMDV) replication and its specific mechanisms have not been reported. We demonstrate that HSP60 modulates the FMDV life cycle. HSP60 plays a role at the postentry stage of the viral life cycle, including RNA replication and mRNA translation; however, HSP60 does not affect viral replication of Seneca Valley virus (SVA) or encephalomyocarditis virus (EMCV). We found that HSP60 is involved in FMDV replication complex (RC) formation. Furthermore, our results indicate that HSP60 interacts with FMDV nonstructural proteins 3A and 2C, key elements of the viral replication complex. We also show that HSP60 regulates the stability of 3A and 2C via caspase-dependent and autophagy-lysosome-dependent degradation, thereby promoting FMDV RNA synthesis and mRNA translation mediated by the RC. Additionally, we determined that the apical domain of HSP60 is responsible for interacting with 3A and 2C. The N terminus of 3A and ATPase domain of 2C are involved in binding to HSP60. Importantly, HSP60 depletion potently reduced FMDV pathogenicity in infected mice. Altogether, this study demonstrates a specific role of HSP60 in promoting FMDV replication. Furthermore, targeting host HSP60 will help us design the FMDV-specific antiviral drugs.

Identification and validation of the necroptosis-related gene signature related to prognosis and tumor immune in hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is the sixth most common cancer, which is characterized by complicated etiology, excessive heterogeneity, and poor prognosis. Necroptosis is a new kind of programmed cell death, which is intently associated with the occurrence and development of tumors. Although researchers have had a deep understanding of necroptosis in recent years, the expression level of necroptosis-related genes in HCC and its relationship with the survival time of HCC patients are not clear.

METHODS

According to the expression of necroptosis-related genes and the survival of HCC patients, HCC patients in the TCGA database were divided into 2 groups that were relatively independent of each other. The genes related to the survival time of HCC patients were screened from the 2 groups of differentially expressed genes. By using the Least Absolute Shrinkage and Selection Operator Cox regression analysis, the optimal λ value was obtained, and the 10-gene signature model was established.

RESULTS

According to the median risk score of the TCGA cohort, HCC patients were averagely divided into high- and low-risk groups. Compared with the low-risk group, the death toll of the high-risk group was relatively higher and the survival time was relatively shorter. Principal component analysis and t-distributed stochastic neighbor embedding analysis showed that there was a significant separation between high- and low-risk groups. Through Kaplan-Meier analysis, it was found that the survival time of HCC patients in the high-risk group was significantly shorter than that in the low-risk group. Through receiver operating characteristic analysis, it was found that the sensitivity and specificity of the model were good. We also make a comprehensive analysis of the international cancer genome consortium database as a verification queue and prove the reliability of the 10-gene signature model. Gene Ontolog, Kyoto Encyclopedia of Genes and Genomes, and single-sample gene set enrichment analysis showed that many biological processes and pathways related to immunity had been enriched, and the antitumor immune function was weakened in the high-risk population.

CONCLUSION

The risk score can be considered as an independent prognostic factor to predict the prognosis of patients with HCC, and necroptosis-related genes are also closely related to tumor immune function.

Heat shock protein 90 of Pacific white shrimp (Litopenaeus vannamei) is possibly involved in promoting white spot syndrome virus infection

Viruses cause up to 60% of disease-associated losses in shrimp aquaculture, and the white spot syndrome virus (WSSV) is a major viral pathogen in shrimp. Heat shock proteins (HSPs) are host chaperones that help promote many viral infections. We investigated the involvement of Litopenaeus vannamei (Lv) HSP90 in WSSV infections. Expression of LvHSP90 at the transcript and protein levels were upregulated after WSSV infection. Silencing LvHSP90 resulted in the increased cumulative mortality rate and the reduction of circulating hemocytes. The inhibition of LvHSP90 also induced the expression of apoptosis-related genes which indicated the induction of apoptotic pathway and might lead to shrimp death. However, lower the number of WSSV-infected cells and viral copy numbers were detected in the LvHSP90-silenced shrimp compared with those of the controls, corresponding with significantly decreased expressions of viral genes, including the immediate-early genes WSV083 and WSV249 and viral DNA polymerase. Conversely, injecting shrimp with WSSV that had been co-incubated with a recombinant LvHSP90 (rLvHSP90) promoted WSSV infection as evidenced by an increased cumulative mortality rate and viral copy numbers at 40-48 h post infection (hpi). Subcellular localization of LvHSP90 in WSSV-infected hemocytes at 3, 6 and 12 hpi demonstrated increased expression and translocation of LvHSP90 into the nucleus where WSSV DNA can replicate. Thus, LvHSP90 might be involved in the WSSV pathogenesis by promoting WSSV replication.

Nucleoporin 85 interacts with influenza A virus PB1 and PB2 to promote its replication by facilitating nuclear import of ribonucleoprotein

Transcription and replication of the influenza A virus (IAV) genome take place in the nucleus of infected cells, which rely on host factors to aid viral ribonucleoprotein (vRNP) to cross the nuclear pore complex (NPC) and complete the bidirectional nucleocytoplasmic trafficking. Here, we showed that nucleoporin 85 (NUP85), a component of NPC, interacted with RNP subunits polymerase basic 1 (PB1) and polymerase basic 2 (PB2) in an RNA-dependent manner during IAV infection. Knockdown of NUP85 delayed the nuclear import of vRNP, PB1 and PB2, inhibiting polymerase activity and ultimately suppressing viral replication. Further analysis revealed that NUP85 assisted the binding of PB1 to nuclear transport factor Ran-binding protein 5 (RanBP5) and the binding of PB2 to nuclear transport factor importin α1 and importin α7. We also found that NUP85 expression was downregulated upon IAV infection. Together, our study demonstrated that NUP85 positively regulated IAV infection by interacting with viral PB1 and PB2, which may provide new insight into the process of vRNP nuclear import and a novel target for effective antivirals.

Heat shock proteins and viral infection

Heat shock proteins (HSPs) are a kind of proteins which mostly found in bacterial, plant and animal cells, in which they are involved in the monitoring and regulation of cellular life activities. HSPs protect other proteins under environmental and cellular stress by regulating protein folding and supporting the correctly folded structure of proteins as chaperones. During viral infection, some HSPs can have an antiviral effect by inhibiting viral proliferation through interaction and activating immune pathways to protect the host cell. However, although the biological function of HSPs is to maintain the homeostasis of cells, some HSPs will also be hijacked by viruses to help their invasion, replication, and maturation, thereby increasing the chances of viral survival in unfavorable conditions inside the host cell. In this review, we summarize the roles of the heat shock protein family in various stages of viral infection and the potential uses of these proteins in antiviral therapy.

p17-Modulated Hsp90/Cdc37 Complex Governs Oncolytic Avian Reovirus Replication by Chaperoning p17, Which Promotes Viral Protein Synthesis and Accumulation of Viral Proteins σC and σA in Viral Factories

In this work we have determined that heat shock protein 90 (Hsp90) is essential for avian reovirus (ARV) replication by chaperoning the ARV p17 protein. p17 modulates the formation of the Hsp90/Cdc37 complex by phosphorylation of Cdc37, and this chaperone machinery protects p17 from ubiquitin-proteasome degradation. Inhibition of the Hsp90/Cdc37 complex by inhibitors (17-N-allylamino-17-demethoxygeldanamycin 17-AGG, and celastrol) or short hairpin RNAs (shRNAs) significantly reduced expression levels of viral proteins and virus yield, suggesting that the Hsp90/Cdc37 chaperone complex functions in virus replication. The expression levels of p17 were decreased at the examined time points (2 to 7 h and 7 to 16 h) in 17-AAG-treated cells in a dose-dependent manner while the expression levels of viral proteins σA, σC, and σNS were decreased at the examined time point (7 to 16 h). Interestingly, the expression levels of σC, σA, and σNS proteins increased along with coexpression of p17 protein. p17 together with the Hsp90/Cdc37 complex does not increase viral genome replication but enhances viral protein stability, maturation, and virus production. Virus factories of ARV are composed of nonstructural proteins σNS and μNS. We found that the Hsp90/Cdc37 chaperone complex plays an important role in accumulation of the outer-capsid protein σC, inner core protein σA, and nonstructural protein σNS of ARV in viral factories. Depletion of Hsp90 inhibited σA, σC, and p17 proteins colocalized with σNS in viral factories. This study provides novel insights into p17-modulated formation of the Hsp90/Cdc37 chaperone complex governing virus replication via stabilization and maturation of viral proteins and accumulation of viral proteins in viral factories for virus assembly. IMPORTANCE Molecular mechanisms that control stabilization of ARV proteins and the intermolecular interactions among inclusion components remain largely unknown. Here, we show that the ARV p17 is an Hsp90 client protein. The Hsp90/Cdc37 chaperone complex is essential for ARV replication by protecting p17 chaperone from ubiquitin-proteasome degradation. p17 modulates the formation of Hsp90/Cdc37 complex by phosphorylation of Cdc37, and this chaperone machinery protects p17 from ubiquitin-proteasome degradation, suggesting a feedback loop between p17 and the Hsp90/Cdc37 chaperone complex. p17 together with the Hsp90/Cdc37 complex does not increase viral genome replication but enhances viral protein stability and virus production. Depletion of Hsp90 prevented viral proteins σA, σC, and p17 from colocalizing with σNS in viral factories. Our findings elucidate that the Hsp90/Cdc37 complex chaperones p17, which, in turn, promotes the synthesis of viral proteins σA, σC, and σNS and facilitates accumulation of the outer-capsid protein σC and inner core protein σA in viral factories for virus assembly.

Differential gene expression reveals host factors for viral shedding variation in mallards (Anas platyrhynchos) infected with low-pathogenic avian influenza virus

Intraspecific variation in pathogen shedding impacts disease transmission dynamics; therefore, understanding the host factors associated with individual variation in pathogen shedding is key to controlling and preventing outbreaks. In this study, ileum and bursa of Fabricius tissues of wild-bred mallards (Anas platyrhynchos) infected with low-pathogenic avian influenza (LPAIV) were evaluated at various post-infection time points to determine genetic host factors associated with intraspecific variation in viral shedding. By analysing transcriptome sequencing data (RNA-seq), we found that LPAIV-infected wild-bred mallards do not exhibit differential gene expression compared to uninfected birds, but that gene expression was associated with cloacal viral shedding quantity early in the infection. In both tissues, immune gene expression was higher in high/moderate shedding birds compared to low shedding birds, and significant positive relationships with viral shedding were observed. In the ileum, expression for host genes involved in viral cell entry was lower in low shedders compared to moderate shedders at 1 day post-infection (DPI), and expression for host genes promoting viral replication was higher in high shedders compared to low shedders at 2 DPI. Our findings indicate that viral shedding is a key factor for gene expression differences in LPAIV-infected wild-bred mallards, and the genes identified in this study could be important for understanding the molecular mechanisms driving intraspecific variation in pathogen shedding.

Protein lysine acetylation and its role in different human pathologies: a proteomic approach

INTRODUCTION

Lysine acetylation is a reversible post-translational modification (PTM) regulated through the action of specific types of enzymes: lysine acetyltransferases (KATs) and lysine deacetylases (HDACs), in addition to bromodomains, which are a group of conserved domains which identify acetylated lysine residues, several of the players in the process of protein acetylation, including enzymes and bromodomain-containing proteins, have been related to the progression of several diseases. The combination of high-resolution mass spectrometry-based proteomics, and immunoprecipitation to enrich acetylated peptides has contributed in recent years to expand the knowledge about this PTM described initially in histones and nuclear proteins, and is currently reported in more than 5000 human proteins, that are regulated by this PTM.

AREAS COVERED

This review presents an overview of the main participant elements, the scenario in the development of protein lysine acetylation, and its role in different human pathologies.

EXPERT OPINION

Acetylation targets are practically all cellular processes in eukaryotes and prokaryotes organisms. Consequently, this modification has been linked to many pathologies like cancer, viral infection, obesity, diabetes, cardiovascular, and nervous system-associated diseases, to mention a few relevant examples. Accordingly, some intermediate mediators in the acetylation process have been projected as therapeutic targets.

Host Cell Factors That Interact with Influenza Virus Ribonucleoproteins

Influenza viruses hijack host cell factors at each stage of the viral life cycle. After host cell entry and endosomal escape, the influenza viral ribonucleoproteins (vRNPs) are released into the cytoplasm where the classical cellular nuclear import pathway is usurped for nuclear translocation of the vRNPs. Transcription takes place inside the nucleus at active host transcription sites, and cellular mRNA export pathways are subverted for export of viral mRNAs. Newly synthesized RNP components cycle back into the nucleus using various cellular nuclear import pathways and host-encoded chaperones. Replication of the negative-sense viral RNA (vRNA) into complementary RNA (cRNA) and back into vRNA requires complex interplay between viral and host factors. Progeny vRNPs assemble at the host chromatin and subsequently exit from the nucleus-processes orchestrated by sets of host and viral proteins. Finally, several host pathways appear to play a role in vRNP trafficking from the nuclear envelope to the plasma membrane for egress.

Diversity in heat shock protein families: functional implications in virus infection with a comprehensive insight of their role in the HIV-1 life cycle

Heat shock proteins (HSPs) are a group of cellular proteins that are induced during stress conditions such as heat stress, cold shock, UV irradiation and even pathogenic insult. They are classified into families based on molecular size like HSP27, 40, 70 and 90 etc, and many of them act as cellular chaperones that regulate protein folding and determine the fate of mis-folded or unfolded proteins. Studies have also shown multiple other functions of these proteins such as in cell signalling, transcription and immune response. Deregulation of these proteins leads to devastating consequences, such as cancer, Alzheimer's disease and other life threatening diseases suggesting their potential importance in life processes. HSPs exist in multiple isoforms, and their biochemical and functional characterization still remains a subject of active investigation. In case of viral infections, several HSP isoforms have been documented to play important roles with few showing pro-viral activity whereas others seem to have an anti-viral role. Earlier studies have demonstrated that HSP40 plays a pro-viral role whereas HSP70 inhibits HIV-1 replication; however, clear isoform-specific functional roles remain to be established. A detailed functional characterization of all the HSP isoforms will uncover their role in cellular homeostasis and also may highlight some of them as potential targets for therapeutic strategies against various viral infections. In this review, we have tried to comprehend the details about cellular HSPs and their isoforms, their role in cellular physiology and their isoform-specific functions in case of virus infection with a specific focus on HIV-1 biology.

Role of Heat Shock Proteins (HSP70 and HSP90) in Viral Infection

Heat shock proteins (HSPs) are a large group of chaperones found in most eukaryotes and bacteria. They are responsible for the correct protein folding, protection of the cell against stressors, presenting immune and inflammatory cytokines; furthermore, they are important factors in regulating cell differentiation, survival and death. Although the biological function of HSPs is to maintain cell homeostasis, some of them can be used by viruses both to fold their proteins and increase the chances of survival in unfavorable host conditions. Folding viral proteins as well as replicating many different viruses are carried out by, among others, proteins from the HSP70 and HSP90 families. In some cases, the HSP70 family proteins directly interact with viral polymerase to enhance viral replication or they can facilitate the formation of a viral replication complex and/or maintain the stability of complex proteins. It is known that HSP90 is important for the expression of viral genes at both the transcriptional and the translational levels. Both of these HSPs can form a complex with HSP90 and, consequently, facilitate the entry of the virus into the cell. Current studies have shown the biological significance of HSPs in the course of infection SARS-CoV-2. A comprehensive understanding of chaperone use during viral infection will provide new insight into viral replication mechanisms and therapeutic potential. The aim of this study is to describe the molecular basis of HSP70 and HSP90 participation in some viral infections and the potential use of these proteins in antiviral therapy.

Escaping the Lion's Den: redirecting autophagy for unconventional release and spread of viruses

Autophagy is an evolutionarily conserved process, designed to maintain cellular homeostasis during a range of internal and external stimuli. Conventionally, autophagy is known for coordinated degradation and recycling of intracellular components and removal of cytosolic pathogens. More recently, several lines of evidence have indicated an unconventional, nondegradative role of autophagy for secretion of cargo that lacks a signal peptide. This process referred to as secretory autophagy has also been implicated in the infection cycle of several virus species. This review focuses on the current evidence available on the nondegradative features of autophagy, emphasizing its potential role and unresolved questions in the release and spread of (-) and (+) RNA viruses.

Radicicol Inhibits Chikungunya Virus Replication by Targeting Nonstructural Protein 2

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus that causes a debilitating febrile illness characterized by persistent muscle and joint pain. The widespread distribution of transmission-competent vectors, Aedes species mosquitoes, indicates the potential risk of large-scale epidemics with high attack rates that can severely impact public health globally. Despite this, currently, there are no antivirals available for the treatment of CHIKV infections. Thus, we aimed to identify potential drug candidates by screening a chemical library using a cytopathic effect-based high-throughput screening assay. As a result, we identified radicicol, a heat shock protein 90 (Hsp90) inhibitor that effectively suppressed CHIKV replication by blocking the synthesis of both positive- and negative-strand viral RNA as well as expression of viral proteins. Interestingly, selection for viral drug-resistant variants and mutational studies revealed nonstructural protein 2 (nsP2) as a putative molecular target of radicicol. Moreover, coimmunoprecipitation and in silico modeling analyses determined that G641D mutation in the methyltransferase (MT)-like domain of nsP2 is essential for its interaction with cytoplasmic Hsp90β chaperone. Our findings collectively support the potential application of radicicol as an anti-CHIKV agent. The detailed study of the underlying mechanism of action further contributes to our understanding of virus-host interactions for novel therapeutics against CHIKV infection.

Inhibition of Influenza Virus Polymerase by Interfering with Its Protein-Protein Interactions

Influenza (flu) virus is a serious threat to global health with the potential to generate devastating pandemics. The availability of broad spectrum antiviral drugs is an unequaled weapon during pandemic events, especially when a vaccine is still not available. One of the most promising targets for the development of new antiflu therapeutics is the viral RNA-dependent RNA polymerase (RdRP). The assembly of the flu RdRP heterotrimeric complex from the individual polymerase acidic protein (PA), polymerase basic protein 1 (PB1), and polymerase basic protein 2 (PB2) subunits is a prerequisite for RdRP functions, such as mRNA synthesis and genome replication. In this Review, we report the known protein-protein interactions (PPIs) occurring by RdRP that could be disrupted by small molecules and analyze their benefits and drawbacks as drug targets. An overview of small molecules able to interfere with flu RdRP functions exploiting the PPI inhibition approach is described. In particular, an update on the most recent inhibitors targeting the well-consolidated RdRP PA-PB1 subunit heterodimerization is mainly reported, together with pioneer inhibitors targeting other virus-virus or virus-host interactions involving RdRP subunits. As demonstrated by the PA-PB1 interaction inhibitors discussed herein, the inhibition of flu RdRP functions by PPI disrupters clearly represents a valid means to identify compounds endowed with a broad spectrum of action and a reduced propensity to develop drug resistance, which are the main issues of antiviral drugs.

Development of a Genetically Stable Live Attenuated Influenza Vaccine Strain Using an Engineered High-Fidelity Viral Polymerase

RNA viruses demonstrate a vast range of variants, called quasispecies, due to error-prone replication by viral RNA-dependent RNA polymerase. Although live attenuated vaccines are effective in preventing RNA virus infection, there is a risk of reversal to virulence after their administration. To test the hypothesis that high-fidelity viral polymerase reduces the diversity of influenza virus quasispecies, resulting in inhibition of reversal of the attenuated phenotype, we first screened for a high-fidelity viral polymerase using serial virus passages under selection with a guanosine analog ribavirin. Consequently, we identified a Leu66-to-Val single amino acid mutation in polymerase basic protein 1 (PB1). The high-fidelity phenotype of PB1-L66V was confirmed using next-generation sequencing analysis and biochemical assays with the purified influenza viral polymerase. As expected, PB1-L66V showed at least two-times-lower mutation rates and decreased misincorporation rates, compared to the wild type (WT). Therefore, we next generated an attenuated PB1-L66V virus with a temperature-sensitive (ts) phenotype based on FluMist, a live attenuated influenza vaccine (LAIV) that can restrict virus propagation by ts mutations, and examined the genetic stability of the attenuated PB1-L66V virus using serial virus passages. The PB1-L66V mutation prevented reversion of the ts phenotype to the WT phenotype, suggesting that the high-fidelity viral polymerase could contribute to generating an LAIV with high genetic stability, which would not revert to the pathogenic virus.IMPORTANCE The LAIV currently in use is prescribed for actively immunizing individuals aged 2 to 49 years. However, it is not approved for infants and elderly individuals, who actually need it the most, because it might prolong virus propagation and cause an apparent infection in these individuals, due to their weak immune systems. Recently, reversion of the ts phenotype of the LAIV strain currently in use to a pathogenic virus was demonstrated in cultured cells. Thus, the generation of mutations associated with enhanced virulence in LAIV should be considered. In this study, we isolated a novel influenza virus strain with a Leu66-to-Val single amino acid mutation in PB1 that displayed a significantly higher fidelity than the WT. We generated a novel LAIV candidate strain harboring this mutation. This strain showed higher genetic stability and no ts phenotype reversion. Thus, our high-fidelity strain might be useful for the development of a safer LAIV.

Epigenetic reprogramming mechanisms of immunity during influenza A virus infection

This paper reviews epigenetic mechanisms by which influenza viruses affect cellular gene activity to control their life cycles, aiming to provide new insights into the complexity of functional interactions between viral and cellular factors, as well as to introduce novel targets for therapeutic intervention and vaccine development against influenza infections.

Insights into RNA-dependent RNA Polymerase Inhibitors as Antiinfluenza Virus Agents

BACKGROUND

Influenza is a seasonal disease that affects millions of people every year and has a significant economic impact. Vaccines are the best strategy to fight this viral pathology, but they are not always available or administrable, prompting the search for antiviral drugs. RNA-dependent RNA polymerase (RdRp) recently emerged as a promising target because of its key role in viral replication and its high conservation among viral strains.

DISCUSSION

This review presents an overview of the most interesting RdRp inhibitors that have been discussed in the literature since 2000. Compounds already approved or in clinical trials and a selection of inhibitors endowed with different scaffolds are described, along with the main features responsible for their activity.

RESULTS

RdRp inhibitors are emerging as a new strategy to fight viral infections and the importance of this class of drugs has been confirmed by the FDA approval of baloxavir marboxil in 2018. Despite the complexity of the RdRp machine makes the identification of new compounds a challenging research topic, it is likely that in the coming years, this field will attract the interest of a number of academic and industrial scientists because of the potential strength of this therapeutic approach.

The importance of virion-incorporated cellular RNA-Binding Proteins in viral particle assembly and infectivity

RNA is a central molecule in RNA virus biology due to its dual function as messenger and genome. However, the small number of proteins encoded by viral genomes is insufficient to enable virus infection. Hence, viruses hijack cellular RNA-binding proteins (RBPs) to aid replication and spread. In this review we discuss the 'knowns' and 'unknowns' regarding the contribution of host RBPs to the formation of viral particles and the initial steps of infection in the newly infected cell. Through comparison of the virion proteomes of ten different human RNA viruses, we confirm that a pool of cellular RBPs are typically incorporated into viral particles. We describe here illustrative examples supporting the important functions of these RBPs in viral particle formation and infectivity and we propose that the role of host RBPs in these steps can be broader than previously anticipated. Understanding how cellular RBPs regulate virus infection can lead to the discovery of novel therapeutic targets against viruses.

[Functional analysis of host factors involved in mumps virus propagation]

Mumps virus (MuV) is the causative agent of mumps, a common childhood illness characterized by fever and swelling of the salivary glands. Like other viral infections, a number of host proteins are thought to involve in MuV infection. We have shown the function of several host factors in MuV infection. The chaperone proteins, heat shock protein 70 (Hsp70) and Hsp90, interact with the P and L proteins that form the polymerase complex and function in the protein quality control of these viral proteins, and thus they are essential host factors in MuV RNA synthesis. The R2TP complex is a host factor that contributes to effective viral propagation by precise regulation of viral RNA synthesis and evasion of host immune responses, and Rab11 is a host factor involved in viral RNP trafficking to the plasma membrane. This article summarizes the functions of host factors involved in MuV infection based on our researches.

Host factors involved in influenza virus infection

Influenza virus causes an acute febrile respiratory disease in humans that is commonly known as 'flu'. Influenza virus has been around for centuries and is one of the most successful, and consequently most studied human viruses. This has generated tremendous amount of data and information, thus it is pertinent to summarise these for, particularly interdisciplinary readers. Viruses are acellular organisms and exist at the interface of living and non-living. Due to this unique characteristic, viruses require another organism, i.e. host to survive. Viruses multiply inside the host cell and are obligate intracellular pathogens, because their relationship with the host is almost always harmful to host. In mammalian cells, the life cycle of a virus, including influenza is divided into five main steps: attachment, entry, synthesis, assembly and release. To complete these steps, some viruses, e.g. influenza utilise all three parts - plasma membrane, cytoplasm and nucleus, of the cell; whereas others, e.g. SARS-CoV-2 utilise only plasma membrane and cytoplasm. Hence, viruses interact with numerous host factors to complete their life cycle, and these interactions are either exploitative or antagonistic in nature. The host factors involved in the life cycle of a virus could be divided in two broad categories - proviral and antiviral. This perspective has endeavoured to assimilate the information about the host factors which promote and suppress influenza virus infection. Furthermore, an insight into host factors that play a dual role during infection or contribute to influenza virus-host adaptation and disease severity has also been provided.

Eukaryotic Translation Elongation Factor 1 Delta Inhibits the Nuclear Import of the Nucleoprotein and PA-PB1 Heterodimer of Influenza A Virus

The viral ribonucleoprotein (vRNP) of the influenza A virus (IAV) is responsible for the viral RNA transcription and replication in the nucleus, and its functions rely on host factors. Previous studies have indicated that eukaryotic translation elongation factor 1 delta (eEF1D) may associate with RNP subunits, but its roles in IAV replication are unclear. Herein, we showed that eEF1D was an inhibitor of IAV replication because knockout of eEF1D resulted in a significant increase in virus yield. eEF1D interacted with RNP subunits polymerase acidic protein (PA), polymerase basic 1 (PB1), polymerase basic 2 (PB2), and also with nucleoprotein (NP) in an RNA-dependent manner. Further studies revealed that eEF1D impeded the nuclear import of NP and PA-PB1 heterodimer of IAV, thereby suppressing the vRNP assembly, viral polymerase activity, and viral RNA synthesis. Together, our studies demonstrate eEF1D negatively regulating the IAV replication by inhibition of the nuclear import of RNP subunits, which not only uncovers a novel role of eEF1D in IAV replication but also provides new insights into the mechanisms of nuclear import of vRNP proteins.IMPORTANCE Influenza A virus is the major cause of influenza, a respiratory disease in humans and animals. Different from most other RNA viruses, the transcription and replication of IAV occur in the cell nucleus. Therefore, the vRNPs must be imported into the nucleus for viral transcription and replication, which requires participation of host proteins. However, the mechanisms of the IAV-host interactions involved in nuclear import remain poorly understood. Here, we identified eEF1D as a novel inhibitor for the influenza virus life cycle. Importantly, eEF1D impaired the interaction between NP and importin α5 and the interaction between PB1 and RanBP5, which impeded the nuclear import of vRNP. Our studies not only reveal the molecular mechanisms of the nuclear import of IAV vRNP but also provide potential anti-influenza targets for antiviral development.

Hsp90 is involved in pseudorabies virus virion assembly via stabilizing major capsid protein VP5

Many viruses utilize molecular chaperone heat shock protein 90 (Hsp90) for protein folding and stabilization, however, the role of Hsp90 in herpesvirus lifecycle is obscure. Here, we provide evidence that Hsp90 participates in pseudorabies virus (PRV) replication. Viral growth kinetics assays show that Hsp90 inhibitor geldanamycin (GA) abrogates PRV replication at the post-penetration step. Transmission electron microscopy demonstrates that dysfunction of Hsp90 diminishes the quantity of PRV nucleocapsids. Overexpression and knockdown of Hsp90 suggest that de novo Hsp90 is involved in PRV replication. Mechanismly, dysfunction of Hsp90 inhibits PRV major capsid protein VP5 expression. Co-immunoprecipitation and indirect immunofluorescence assays indicate that Hsp90 interacts with VP5. Interestingly, Hsp70, a collaborator of Hsp90, also interacts with VP5, but doesn't affect PRV growth. Finally, inhibition of Hsp90 results in PRV VP5 degradation in a proteasome-dependent manner. Collectively, our data suggest that Hsp90 contributes to PRV virion assembly and replication via stabilization of VP5.

The Role of Molecular Chaperones in Virus Infection and Implications for Understanding and Treating COVID-19

The COVID-19 pandemic made imperative the search for means to end it, which requires a knowledge of the mechanisms underpinning the multiplication and spread of its cause, the coronavirus SARS-CoV-2. Many viruses use members of the hosts’ chaperoning system to infect the target cells, replicate, and spread, and here we present illustrative examples. Unfortunately, the role of chaperones in the SARS-CoV-2 cycle is still poorly understood. In this review, we examine the interactions of various coronaviruses during their infectious cycle with chaperones in search of information useful for future research on SARS-CoV-2. We also call attention to the possible role of molecular mimicry in the development of autoimmunity and its widespread pathogenic impact in COVID-19 patients. Viral proteins share highly antigenic epitopes with human chaperones, eliciting anti-viral antibodies that crossreact with the chaperones. Both, the critical functions of chaperones in the infectious cycle of viruses and the possible role of these molecules in COVID-19 autoimmune phenomena, make clear that molecular chaperones are promising candidates for the development of antiviral strategies. These could consist of inhibiting-blocking those chaperones that are necessary for the infectious viral cycle, or those that act as autoantigens in the autoimmune reactions causing generalized destructive effects on human tissues.

Influenza A Virus Induces Autophagy by Its Hemagglutinin Binding to Cell Surface Heat Shock Protein 90AA1

Autophagy can be utilized by the influenza A virus (IAV) to facilitate its replication. However, whether autophagy is induced at the stage of IAV entry is still unclear. Here, we report that IAV induces autophagy by hemagglutinin (HA) binding to heat shock protein 90AA1 (HSP90AA1) distributed on the cell surface. Virus overlay protein binding assay and pull-down assay indicated that IAV HA bound directly to cell surface HSP90AA1. Knockdown of HSP90AA1 weakened H1N1 infection. Incubation of IAV viral particles with recombinant HSP90AA1 or prior blockade of A549 cells with an anti-HSP90AA1 antibody could inhibit attachment of IAV. Moreover, we found that recombinant HA1 protein binding to cell surface HSP90AA1 was sufficient to induce autophagy through the AKT-MTOR pathway. Our study reveals that the HSP90AA1 on cell surface participates in IAV entry by directing binding to the HA1 subunit of IAV and subsequently induces autophagy.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

Proteostasis in Viral Infection: Unfolding the Complex Virus-Chaperone Interplay

Viruses are obligate intracellular parasites that rely on their hosts for protein synthesis, genome replication, and viral particle production. As such, they have evolved mechanisms to divert host resources, including molecular chaperones, facilitate folding and assembly of viral proteins, stabilize complex structures under constant mutational pressure, and modulate signaling pathways to dampen antiviral responses and prevent premature host death. Biogenesis of viral proteins often presents unique challenges to the proteostasis network, as it requires the rapid and orchestrated production of high levels of a limited number of multifunctional, multidomain, and aggregation-prone proteins. To overcome such challenges, viruses interact with the folding machinery not only as clients but also as regulators of chaperone expression, function, and subcellular localization. In this review, we summarize the main types of interactions between viral proteins and chaperones during infection, examine evolutionary aspects of this relationship, and discuss the potential of using chaperone inhibitors as broad-spectrum antivirals.

Chemical Biology Framework to Illuminate Proteostasis

Protein folding in the cell is mediated by an extensive network of >1,000 chaperones, quality control factors, and trafficking mechanisms collectively termed the proteostasis network. While the components and organization of this network are generally well established, our understanding of how protein-folding problems are identified, how the network components integrate to successfully address challenges, and what types of biophysical issues each proteostasis network component is capable of addressing remains immature. We describe a chemical biology-informed framework for studying cellular proteostasis that relies on selection of interesting protein-folding problems and precise researcher control of proteostasis network composition and activities. By combining these methods with multifaceted strategies to monitor protein folding, degradation, trafficking, and aggregation in cells, researchers continue to rapidly generate new insights into cellular proteostasis.

Medicinal chemistry strategies toward host targeting antiviral agents

Direct‐acting antiviral agents (DAAs) represent a class of drugs targeting viral proteins and have been demonstrated to be very successful in combating viral infections in clinic. However, DAAs suffer from several inherent limitations, including narrow‐spectrum antiviral profiles and liability to drug resistance, and hence there are still unmet needs in the treatment of viral infections. In comparison, host targeting antivirals (HTAs) target host factors for antiviral treatment. Since host proteins are probably broadly required for various viral infections, HTAs are not only perceived, but also demonstrated to exhibit broad‐spectrum antiviral activities. In addition, host proteins are not under the genetic control of viral genome, and hence HTAs possess much higher genetic barrier to drug resistance as compared with DAAs. In recent years, much progress has been made to the development of HTAs with the approval of chemokine receptor type 5 antagonist maraviroc for human immunodeficiency virus treatment and more in the pipeline for other viral infections. In this review, we summarize various host proteins as antiviral targets from a medicinal chemistry prospective. Challenges and issues associated with HTAs are also discussed.

Discovery of Influenza Polymerase PA-PB1 Interaction Inhibitors Using an In Vitro Split-Luciferase Complementation-Based Assay

The limited therapeutic options and increasing drug-resistance call for next-generation influenza antivirals. Due to the essential function in viral replication and high sequence conservation among influenza viruses, influenza polymerase PA-PB1 protein-protein interaction becomes an attractive drug target. Here, we developed an in vitro split luciferase complementation-based assay to speed up screening of PA-PB1 interaction inhibitors. By screening 10,000 compounds, we identified two PA-PB1 interaction inhibitors, R160792 and R151785, with potent and broad-spectrum antiviral activity against a panel of influenza A and B viruses, including amantadine-, oseltamivir-, or dual resistant strains. Further mechanistic study reveals that R151785 inhibits PA nuclear localization, reduces the levels of viral RNAs and proteins, and inhibits viral replication at the intermediate stage, all of which are in line with its antiviral mechanism of action. Overall, we developed a robust high throughput-screening assay for screening broad-spectrum influenza antivirals targeting PA-PB1 interaction and identified R151785 as a promising antiviral drug candidate.

The Host Heat Shock Protein MRJ/DNAJB6 Modulates Virus Infection

A variety of pathogens take advantage of cellular heat shock proteins (HSPs) to complete their life cycle and exert pathogenic effects. MRJ (DNAJB6), a member of the heat shock protein 40 family, acts as a molecular chaperone for a wide range of cellular processes. MRJ mutations are linked to human diseases, such as muscular dystrophy and neurodegenerative diseases. There are two MRJ isoforms generated by alternative use of terminal exons, which differ in their C-terminus. This mini-review summarizes how these two MRJ isoforms participate differentially in viral production and virulence, and the possibility for MRJ as a therapeutic target.

Influenza Virus Mounts a Two-Pronged Attack on Host RNA Polymerase II Transcription

Influenza virus intimately associates with host RNA polymerase II (Pol II) and mRNA processing machinery. Here, we use mammalian native elongating transcript sequencing (mNET-seq) to examine Pol II behavior during viral infection. We show that influenza virus executes a two-pronged attack on host transcription. First, viral infection causes decreased Pol II gene occupancy downstream of transcription start sites. Second, virus-induced cellular stress leads to a catastrophic failure of Pol II termination at poly(A) sites, with transcription often continuing for tens of kilobases. Defective Pol II termination occurs independently of the ability of the viral NS1 protein to interfere with host mRNA processing. Instead, this termination defect is a common effect of diverse cellular stresses and underlies the production of previously reported downstream-of-gene transcripts (DoGs). Our work has implications for understanding not only host-virus interactions but also fundamental aspects of mammalian transcription.

•

Influenza virus infection dysregulates host transcription

•

Viral infection depletes Pol II from gene bodies downstream of the TSS

•

Virus-induced stress leads to a catastrophic failure of Pol II termination

•

Defective termination does not require viral NS1: host CPSF30 interaction

Tyr82 Amino Acid Mutation in PB1 Polymerase Induces an Influenza Virus Mutator Phenotype

In various positive-sense single-stranded RNA viruses, a low-fidelity viral RNA-dependent RNA polymerase (RdRp) confers attenuated phenotypes by increasing the mutation frequency. We report a negative-sense single-stranded RNA virus RdRp mutant strain with a mutator phenotype. Based on structural data of RdRp, rational targeting of key residues, and screening of fidelity variants, we isolated a novel low-fidelity mutator strain of influenza virus that harbors a Tyr82-to-Cys (Y82C) single-amino-acid substitution in the PB1 polymerase subunit. The purified PB1-Y82C polymerase indeed showed an increased frequency of misincorporation compared with the wild-type PB1 in an in vitro biochemical assay. To further investigate the effects of position 82 on PB1 polymerase fidelity, we substituted various amino acids at this position. As a result, we isolated various novel mutators other than PB1-Y82C with higher mutation frequencies. The structural model of influenza virus polymerase complex suggested that the Tyr82 residue, which is located at the nucleoside triphosphate entrance tunnel, may influence a fidelity checkpoint. Interestingly, although the PB1-Y82C variant replicated with wild-type PB1-like kinetics in tissue culture, the 50% lethal dose of the PB1-Y82C mutant was 10 times lower than that of wild-type PB1 in embryonated chicken eggs. In conclusion, our data indicate that the Tyr82 residue of PB1 has a crucial role in regulating polymerase fidelity of influenza virus and is closely related to attenuated pathogenic phenotypes in vivo IMPORTANCE Influenza A virus rapidly acquires antigenic changes and antiviral drug resistance, which limit the effectiveness of vaccines and drug treatments, primarily owing to its high rate of evolution. Virus populations formed by quasispecies can contain resistance mutations even before a selective pressure is applied. To study the effects of the viral mutation spectrum and quasispecies, high- and low-fidelity variants have been isolated for several RNA viruses. Here, we report the discovery of a low-fidelity RdRp variant of influenza A virus that contains a substitution at Tyr82 in PB1. Viruses containing the PB1-Y82C substitution showed growth kinetics and viral RNA synthesis levels similar to those of the wild-type virus in cell culture; however, they had significantly attenuated phenotypes in a chicken egg infection experiment. These data demonstrated that decreased RdRp fidelity attenuates influenza A virus in vivo, which is a desirable feature for the development of safer live attenuated vaccine candidates.

Heat shock proteins in infection

Heat shock proteins (HSPs) are constitutively expressed under physiological conditions in most organisms but their expression can significantly enhance in response to four types of stimuli including physical (e.g., radiation or heat shock), chemical and microbial (e.g., pathogenic bacteria, viruses, parasites and fungi) stimuli, and also dietary. These proteins were identified for their role in protecting cells from high temperature and other forms of stress. HSPs control physiological activities or virulence through interaction with various regulators of cellular signaling pathways. Several roles were determined for HSPs in the immune system including intracellular roles (e.g., antigen presentation and expression of innate receptors) as well as extracellular roles (e.g., tumor immunosurveillance and autoimmunity). It was observed that exogenously administered HSPs induced various immune responses in immunotherapy of cancer, infectious diseases, and autoimmunity. Moreover, virus interaction with HSPs as molecular chaperones showed important roles in regulating viral infections including cell entry and nuclear import, viral replication and gene expression, folding/assembly of viral protein, apoptosis regulation, and host immunity. Viruses could regulate host HSPs at different levels such as transcription, translation, post-translational modification and cellular localization. In this review, we attempt to overview the roles of HSPs in a variety of infectious diseases.