Sirtuin Inhibitors Are Broadly Antiviral against Arboviruses

Tissue specific innate immune responses impact viral infection in Drosophila

All organisms sense and respond to pathogenic challenge. Tissue-specific responses are required to combat pathogens infecting distinct cell types. Cyclic dinucleotides (CDNs) are produced endogenously downstream of pathogen recognition or by pathogens themselves which bind to STING to activate NF-kB-dependent antimicrobial gene expression programs. It remains unknown whether there are distinct immune responses to CDNs in Drosophila tissues. Here, we investigated tissue specific CDN-STING responses and uncovered differences in gene-induction patterns across tissues that play important roles in viral infections. Using tissue-and cell-specific genetic studies we found that dSTING in the fat body controls CDN-induced expression of dSTING-regulated gene 1 (Srg1) but not dSTING-regulated gene 2 (Srg2) or 3 (Srg3). In contrast, the gastrointestinal tract largely controls expression of Srg2 and Srg3. We found that Srg3 is antiviral against the natural fly pathogen Drosophila C virus and the human arthropod-borne Rift Valley Fever virus (RVFV), but not other arthropod-borne viruses including Sindbis virus and dengue virus. Furthermore, we found that Srg3 has an important role in controlling RVFV infection of the ovary which has important implications in understanding vertical transmission of viruses and RVFV in mosquitoes. Overall, our study underscores the importance of tissue-specific responses in antiviral immunity and highlights the complex tissue regulation of the CDN-STING pathway.

Drugs Targeting Sirtuin 2 Exhibit Broad-Spectrum Anti-Infective Activity

Direct-acting anti-infective drugs target pathogen-coded gene products and are a highly successful therapeutic paradigm. However, they generally target a single pathogen or family of pathogens, and the targeted organisms can readily evolve resistance. Host-targeted agents can overcome these limitations. One family of host-targeted, anti-infective agents modulate human sirtuin 2 (SIRT2) enzyme activity. SIRT2 is one of seven human sirtuins, a family of NAD+-dependent protein deacylases. It is the only sirtuin that is found predominantly in the cytoplasm. Multiple, structurally distinct SIRT2-targeted, small molecules have been shown to inhibit the replication of both RNA and DNA viruses, as well as intracellular bacterial pathogens, in cell culture and in animal models of disease. Biochemical and X-ray structural studies indicate that most, and probably all, of these compounds act as allosteric modulators. These compounds appear to impact the replication cycles of intracellular pathogens at multiple levels to antagonize their replication and spread. Here, we review SIRT2 modulators reported to exhibit anti-infective activity, exploring their pharmacological action as anti-infectives and identifying questions in need of additional study as this family of anti-infective agents advances to the clinic.

An allosteric inhibitor of sirtuin 2 blocks hepatitis B virus covalently closed circular DNA establishment and its transcriptional activity

296 million people worldwide are predisposed to developing severe end-stage liver diseases due to chronic hepatitis B virus (HBV) infection. HBV forms covalently closed circular DNA (cccDNA) molecules that persist as episomal DNA in the nucleus of infected hepatocytes and drive viral replication. Occasionally, the HBV genome becomes integrated into host chromosomal DNA, a process that is believed to significantly contribute to circulating HBsAg levels and HCC development. Neither cccDNA accumulation nor expression from integrated HBV DNA are directly targeted by current antiviral treatments. In this study, we investigated the antiviral properties of a newly described allosteric modulator, FLS-359, that targets sirtuin 2 (SIRT2), an NAD+-dependent deacylase. Our results demonstrate that SIRT2 modulation by FLS-359 and by other tool compounds inhibits cccDNA synthesis following de novo infection of primary human hepatocytes and HepG2 (C3A)-NTCP cells, and FLS-359 substantially reduces cccDNA recycling in HepAD38 cells. While pre-existing cccDNA is not eradicated by short-term treatment with FLS-359, its transcriptional activity is substantially impaired, likely through inhibition of viral promoter activities. Consistent with the inhibition of viral transcription, HBsAg production by HepG2.2.15 cells, which contain integrated HBV genomes, is also suppressed by FLS-359. Our study provides further insights on SIRT2 regulation of HBV infection and supports the development of potent SIRT2 inhibitors as HBV antivirals.

The RNA helicase DDX39A binds a conserved structure in chikungunya virus RNA to control infection

Alphaviruses are a large group of re-emerging arthropod-borne RNA viruses. The compact viral RNA genomes harbor diverse structures that facilitate replication. These structures can be recognized by antiviral cellular RNA-binding proteins, including DExD-box (DDX) helicases, that bind viral RNAs to control infection. The full spectrum of antiviral DDXs and the structures that are recognized remain unclear. Genetic screening identified DDX39A as antiviral against the alphavirus chikungunya virus (CHIKV) and other medically relevant alphaviruses. Upon infection, the predominantly nuclear DDX39A accumulates in the cytoplasm inhibiting alphavirus replication, independent of the canonical interferon pathway. Biochemically, DDX39A binds to CHIKV genomic RNA, interacting with the 5' conserved sequence element (5'CSE), which is essential for the antiviral activity of DDX39A. Altogether, DDX39A relocalization and binding to a conserved structural element in the alphavirus genomic RNA attenuates infection, revealing a previously unknown layer to the cellular control of infection.

Actomyosin-dependent cell contractility orchestrates Zika virus infection

Emerging pathogen infections, such as Zika virus (ZIKV), pose an increasing threat to human health, but the role of mechanobiological attributes of host cells during ZIKV infection is largely unknown. Here, we reveal that ZIKV infection leads to increased contractility of host cells. Importantly, we investigated whether host cell contractility contributes to ZIKV infection efficacy, from both the intracellular and extracellular perspective. By performing drug perturbation and gene editing experiments, we confirmed that disruption of contractile actomyosin compromises ZIKV infection efficiency, viral genome replication and viral particle production. By culturing on compliant matrix, we further demonstrate that a softer substrate, leading to less contractility of host cells, compromises ZIKV infection, which resembles the effects of disrupting intracellular actomyosin organization. Together, our work provides evidence to support a positive correlation between host cell contractility and ZIKV infection efficacy, thus unveiling an unprecedented layer of interplay between ZIKV and the host cell.

Sirtuins: Key pieces in the host response to pathogens' puzzle

Global warming is changing the distribution of different pathogens around the globe, and humans are more susceptible to new or re-emerging infections. The human response to microbes is complex and involves different mechanisms of the immune system. Regulation of gene expression of immunity genes and of metabolism of immune cells are essential in this process. Both mechanisms could be regulated by protein lysine acetylation that will control chromatin structure affecting gene expression or key enzyme activity involved in cellular processes. Protein acetylation is crucial for the immunity and involves two families of enzymes: lysine acetyltransferases (KATs), which will promote protein acetylation, and lysine deacetylases (KDACs) that will reduce this modification. Lysine deacetylases are divided into Zinc-dependent or HDACs and NAD⁺ -dependent, or Sirtuins. These enzymes are in the nucleus, cytosol, and mitochondria of mammalian cells affecting different cellular pathways, such as metabolism, gene expression, DNA repair, cell proliferation, and apoptosis, opening the opportunity to explore these proteins as drug targets in different diseases, including cancer and neurodegenerative illness. Although widely explored in chronic diseases, very little is known about the role of Sirtuins during host response against microbes' infection. In this review we aim to explore the most recent literature evidencing a role for these enzymes during host responses to viruses, bacterial and protozoan infections, pointing out how these proteins can be manipulated by these pathogens to progress in the infection. Moreover, we will uncover the potential of host KDACs as therapeutic targets to prevent infections by activating effector immune functions.

An allosteric inhibitor of sirtuin 2 deacetylase activity exhibits broad-spectrum antiviral activity

Most drugs used to treat viral disease target a virus-coded product. They inhibit a single virus or virus family, and the pathogen can readily evolve resistance. Host-targeted antivirals can overcome these limitations. The broad-spectrum activity achieved by host targeting can be especially useful in combating emerging viruses and for treatment of diseases caused by multiple viral pathogens, such as opportunistic agents in immunosuppressed patients. We have developed a family of compounds that modulate sirtuin 2, an NAD+-dependent deacylase, and now report the properties of a member of that family, FLS-359. Biochemical and x-ray structural studies show that the drug binds to sirtuin 2 and allosterically inhibits its deacetylase activity. FLS-359 inhibits the growth of RNA and DNA viruses, including members of the coronavirus, orthomyxovirus, flavivirus, hepadnavirus, and herpesvirus families. FLS-359 acts at multiple levels to antagonize cytomegalovirus replication in fibroblasts, causing modest reductions in viral RNAs and DNA, together with a much greater reduction in infectious progeny, and it exhibits antiviral activity in humanized mouse models of infection. Our results highlight the potential of sirtuin 2 inhibitors as broad-spectrum antivirals and set the stage for further understanding of how host epigenetic mechanisms impact the growth and spread of viral pathogens.

Identification of a novel acylthiourea-based potent broad-spectrum inhibitor for enterovirus 3D polymerase in vitro and in vivo

Enterovirus infections have become a serious public health threat to young children, leading to hand-foot-and-mouth disease and more severe nervous system diseases. Due to the lack of licensed anti enterovirus drugs, we reported herein that a Tenovin-1 analog, acylthiourea-based 4-(tert-butyl)-N-((4-(4-(tert-butyl)benzamido)phenyl)carbamothioyl) benzamide (AcTU), displayed low nanomolar anti-EV-A71 activity with an EC₅₀ of 1.0 nM in RD cells. Moreover, AcTU exhibited nanomolar to picomolar inhibitory activity against a series of enteroviruses including EV-D68, CV-A21, CV-A16 and CV-B1 (EC₅₀ = 0.75-17.15 nM). Mechanistic studies indicated that AcTU inhibited enterovirus proliferation by targeting 3D polymerase. In addition, AcTU displayed moderate pharmacokinetic properties in rats (F = 7.4%, T_1/2 = 3.26 h), and in vivo protection studies demonstrated that AcTU orally administered at 0.6 mg/kg/d was highly protective against lethal EV-A71 challenge in mice, potentially reducing mortality from 100% to 20% as well as alleviating symptoms. These results suggested that AcTU could be a potent clinical candidate for the treatment of enterovirus infections.

Redox Regulation and Metabolic Dependency of Zika Virus Replication: Inhibition by Nrf2-Antioxidant Response and NAD(H) Antimetabolites

Viral infections alter host cell metabolism and homeostasis; however, the mechanisms that regulate these processes have only begun to be elucidated. We report here that Zika virus (ZIKV) infection activates the antioxidant nuclear factor erythroid 2-related factor 2 (Nrf2), which precedes oxidative stress. Downregulation of Nrf2 or inhibition of glutathione (GSH) synthesis resulted in significantly increased viral replication. Interestingly, 6-amino-nicotinamide (6-AN), a nicotinamide analog commonly used as an inhibitor of the pentose phosphate pathway (PPP), decreased viral replication by over 1,000-fold. This inhibition was neither recapitulated by the knockdown of PPP enzymes, glucose 6-phosphate dehydrogenase (G6PD), or 6-phosphogluconate dehydrogenase (6PGD), nor prevented by supplementation with ribose 5-phosphate. Instead, our metabolomics and metabolic phenotype studies support a mechanism in which 6-AN depletes cells of NAD(H) and impairs NAD(H)-dependent glycolytic steps resulting in inhibition of viral replication. The inhibitory effect of 6-AN was rescued with precursors of the salvage pathway but not with those of other NAD+ biosynthesis pathways. Inhibition of glycolysis reduced viral protein levels, which were recovered transiently. This transient recovery in viral protein synthesis was prevented when oxidative metabolism was inhibited by blockage of the mitochondrial pyruvate carrier, fatty acid oxidation, or glutaminolysis, demonstrating a compensatory role of mitochondrial metabolism in ZIKV replication. These results establish an antagonistic role for the host cell Nrf2/GSH/NADPH-dependent antioxidant response against ZIKV and demonstrate the dependency of ZIKV replication on NAD(H). Importantly, our work suggests the potential use of NAD(H) antimetabolite therapy against the viral infection. IMPORTANCE Zika virus (ZIKV) is a major public health concern of international proportions. While the incidence of ZIKV infections has declined substantially in recent years, the potential for the reemergence or reintroduction remains high. Although viral infection alters host cell metabolism and homeostasis to promote its replication, deciphering the mechanism(s) involved in these processes is important for identifying therapeutic targets. The present work reveals the complexities of host cell redox regulation and metabolic dependency of ZIKV replication. An antagonistic effect of the Nrf2/GSH/NADP(H)-dependent antioxidant response against ZIKV infection and an essential role of NAD(H) metabolism and glycolysis for viral replication are established for the first time. These findings highlight the potential use of NAD(H) antimetabolites to counter ZIKV infection and pathogenesis.

Why Is Longevity Still a Scientific Mystery? Sirtuins-Past, Present and Future

The sirtuin system consists of seven highly conserved regulatory enzymes responsible for metabolism, antioxidant protection, and cell cycle regulation. The great interest in sirtuins is associated with the potential impact on life extension. This article summarizes the latest research on the activity of sirtuins and their role in the aging process. The effects of compounds that modulate the activity of sirtuins were discussed, and in numerous studies, their effectiveness was demonstrated. Attention was paid to the role of a caloric restriction and the risks associated with the influence of careless sirtuin modulation on the organism. It has been shown that low modulators' bioavailability/retention time is a crucial problem for optimal regulation of the studied pathways. Therefore, a detailed understanding of the modulator structure and potential reactivity with sirtuins in silico studies should precede in vitro and in vivo experiments. The latest achievements in nanobiotechnology make it possible to create promising molecules, but many of them remain in the sphere of plans and concepts. It seems that solving the mystery of longevity will have to wait for new scientific discoveries.

Host cytoskeletal vimentin serves as a structural organizer and an RNA-binding protein regulator to facilitate Zika viral replication

We discovered a dual role of vimentin underlying Zika virus (ZIKV) replication. The vimentin network reorganizes to surround the replication complex. Depletion of vimentin resulted in drastic segregation of viral proteins and subsequent defective infection, indicating its function as an “organizer” that ensures the concentration of all necessary factors for high replication efficacy. With omics analysis, we prove that vimentin also functions as a “regulator” that dominates RNA-binding proteins during infection. These two roles complement one another to make an integrated view of vimentin in regulating ZIKV infection. Collectively, our study fills the long-term gap in our knowledge of the cellular function of intermediate filaments in addition to structural support and provides a potential target for ZIKV therapy.

Emerging microbe infections, such as Zika virus (ZIKV), pose an increasing threat to human health. Investigations on ZIKV replication have revealed the construction of replication complexes (RCs), but the role of cytoskeleton in this process is largely unknown. Here, we investigated the function of cytoskeletal intermediate filament protein vimentin in the life cycle of ZIKV infection. Using advanced imaging techniques, we uncovered that vimentin filaments undergo drastic reorganization upon viral protein synthesis to form a perinuclear cage-like structure that embraces and concentrates RCs. Genetic removal of vimentin markedly disrupted the integrity of RCs and resulted in fragmented subcellular dispersion of viral proteins. This led to reduced viral genome replication, viral protein production, and release of infectious virions, without interrupting viral binding and entry. Furthermore, mass spectrometry and RNA-sequencing screens identified interactions and interplay between vimentin and hundreds of endoplasmic reticulum (ER)-resident RNA-binding proteins. Among them, the cytoplasmic-region of ribosome receptor binding protein 1, an ER transmembrane protein that directly binds viral RNA, interacted with and was regulated by vimentin, resulting in modulation of ZIKV replication. Together, the data in our work reveal a dual role for vimentin as a structural element for RC integrity and as an RNA-binding-regulating hub during ZIKV infection, thus unveiling a layer of interplay between Zika virus and host cell.

Tenovin-1 inhibited dengue virus replication through SIRT2

Dengue fever is a common arbovirus disease, which has been spread to the entire tropical world. At present, effective drugs for the treatment of dengue fever have not yet appeared, and the dengue vaccines studied in various countries have also experienced severe adverse reactions. Thus it is urgent to find new chemicals against dengue virus. Now we found Sirtuins (SIRTs) were increased during dengue virus infection and tenovin-1, a SIRT1/2 inhibitor, showed an impressive antiviral ability in vitro. In BHK-21 cells, tenovin-1 inhibited the replication of DENV2 with an EC50 at 3.41 ± 1.10 μM, also inhibited other three types of dengue viruses with EC50 at 0.97 ± 1.11 μM, 1.81 ± 1.08 μM, 3.81 ± 1.34 μM respectively. Moreover, the cytopathic effect-induced DENV2 was largely improved by tenovin-1 treatment and the release of progeny viruses was inhibited by tenovin-1 treatment. At the same time, the viral protein level and mRNA level were decreased with tenovin-1 treatment after dengue virus infection. From the drug-addition assay, the tenovin-1 played its antiviral after viral infection, which indicated tenovin-1 was not a microbicide. Apart from its antiviral effect, tenovin-1 inhibited the inflammatory response caused by DENV2, reducing the release of inflammatory factors during viral infection. The antiviral effect of tenovin-1 was abrogated with SIRT agonist or SIRT2 knockdown treatment, which indicated the effect of tenovin-1 was on-target. In conclusion, tenovin-1 was proved to be a promising compound against flavivirus infection through SIRT2, which should be pay more attention for further study.

Antivirals against the Chikungunya Virus

Chikungunya virus (CHIKV) is a mosquito-transmitted alphavirus that has re-emerged in recent decades, causing large-scale epidemics in many parts of the world. CHIKV infection leads to a febrile disease known as chikungunya fever (CHIKF), which is characterised by severe joint pain and myalgia. As many patients develop a painful chronic stage and neither antiviral drugs nor vaccines are available, the development of a potent CHIKV inhibiting drug is crucial for CHIKF treatment. A comprehensive summary of current antiviral research and development of small-molecule inhibitor against CHIKV is presented in this review. We highlight different approaches used for the identification of such compounds and further discuss the identification and application of promising viral and host targets.

Melatonin Inhibits Dengue Virus Infection via the Sirtuin 1-Mediated Interferon Pathway

Dengue virus (DENV) is the causative pathogen in the life-threatening dengue hemorrhagic fever and dengue shock syndrome. DENV is transmitted to humans via the bite of an infected Aedes mosquito. Approximately 100 million people are infected annually worldwide, and most of those live in tropical and subtropical areas. There is still no effective drug or vaccine for treatment of DENV infection. In this study, we set forth to investigate the effect of melatonin, which is a natural hormone with multiple pharmacological functions, against DENV infection. Treatment with subtoxic doses of melatonin dose-dependently inhibited DENV production. Cross-protection across serotypes and various cell types was also observed. Time-of-addition assay suggested that melatonin exerts its influence during the post-entry step of viral infection. The antiviral activity of melatonin partly originates from activation of the sirtuin pathway since co-treatment with melatonin and the sirtuin 1 (SIRT1) inhibitor reversed the effect of melatonin treatment alone. Moreover, melatonin could modulate the transcription of antiviral genes that aid in suppression of DENV production. This antiviral mechanism of melatonin suggests a possible new strategy for treating DENV infection.

Modelling of pathogen-host systems using deeper ORF annotations and transcriptomics to inform proteomics analyses

The Zika virus is a flavivirus that can cause fulminant outbreaks and lead to Guillain-Barré syndrome, microcephaly and fetal demise. Like other flaviviruses, the Zika virus is transmitted by mosquitoes and provokes neurological disorders. Despite its risk to public health, no antiviral nor vaccine are currently available. In the recent years, several studies have set to identify human host proteins interacting with Zika viral proteins to better understand its pathogenicity. Yet these studies used standard human protein sequence databases. Such databases rely on genome annotations, which enforce a minimal open reading frame (ORF) length criterion. An ever-increasing number of studies have demonstrated the shortcomings of such annotation, which overlooks thousands of functional ORFs. Here we show that the use of a customized database including currently non-annotated proteins led to the identification of 4 alternative proteins as interactors of the viral capsid and NS4A proteins. Furthermore, 12 alternative proteins were identified in the proteome profiling of Zika infected monocytes, one of which was significantly up-regulated. This study presents a computational framework for the re-analysis of proteomics datasets to better investigate the viral-host protein interplays upon infection with the Zika virus.