Differential Mechanisms for the Involvement of Polyamines and Hypusinated eIF5A in Ebola Virus Gene Expression

Molecular insights into the Ebola virus life cycle

Ebola virus and other orthoebolaviruses cause severe haemorrhagic fevers in humans, with very high case fatality rates. Their non-segmented single-stranded RNA genome encodes only seven structural proteins and a small number of non-structural proteins to facilitate the virus life cycle. The basics of this life cycle are well established, but recent advances have substantially increased our understanding of its molecular details, including the viral and host factors involved. Here we provide a comprehensive overview of our current knowledge of the molecular details of the orthoebolavirus life cycle, with a special focus on proviral host factors. We discuss the multistep entry process, viral RNA synthesis in specialized phase-separated intracellular compartments called inclusion bodies, the expression of viral proteins and ultimately the assembly of new virus particles and their release at the cell surface. In doing so, we integrate recent studies into the increasingly detailed model that has developed for these fundamental aspects of orthoebolavirus biology.

The Role of Spermidine and Its Key Metabolites in Important, Pathogenic Human Viruses and in Parasitic Infections Caused by Plasmodium falciparum and Trypanosoma brucei

The triamine spermidine is a key metabolite of the polyamine pathway. It plays a crucial role in many infectious diseases caused by viral or parasitic infections. Spermidine and its metabolizing enzymes, i.e., spermidine/spermine-N¹-acetyltransferase, spermine oxidase, acetyl polyamine oxidase, and deoxyhypusine synthase, fulfill common functions during infection in parasitic protozoa and viruses which are obligate, intracellular parasites. The competition for this important polyamine between the infected host cell and the pathogen determines the severity of infection in disabling human parasites and pathogenic viruses. Here, we review the impact of spermidine and its metabolites in disease development of the most important, pathogenic human viruses such as SARS-CoV-2, HIV, Ebola, and in the human parasites Plasmodium and Trypanosomes. Moreover, state-of-the-art translational approaches to manipulate spermidine metabolism in the host and the pathogen are discussed to accelerate drug development against these threatful, infectious human diseases.

New K50R mutant mouse models reveal impaired hypusination of eif5a2 with alterations in cell metabolite landscape

The eukaryotic translation initiation factor 5A1 (eIF5A1) and 5A2 (eIF5A2) are important proteins in a variety of physiological and pathophysiological processes and their function has been linked to neurodevelopmental disorders, cancer, and viral infections. Here, we report two new genome-edited mouse models, generated using a CRISPR-Cas9 approach, in which the amino acid residue lysine 50 is replaced with arginine 50 (K50R) in eIF5A1 or in the closely related eIF5A2 protein. This mutation prevents the spermidine-dependent post-translational formation of hypusine, a unique lysine derivative that is necessary for activation of eIF5A1 and eIF5A2. Mouse brain lysates from homozygous eif5a2-K50R mutant mice (eif5a2K50R/K50R) confirmed the absence of hypusine formation of eIF5A2, and metabolomic analysis of primary mouse dermal fibroblasts revealed significant alterations in the metabolite landscape compared to controls including increased levels of tryptophan, kyrunenine, pyridoxine, nicotinamide adenine dinucleotide, riboflavin, flavin adenine dinucleotide, pantothenate, and coenzyme A. Further supported by new publicly available bioinformatics data, these new mouse models represent excellent in vivo models to study hypusine-dependent biological processes, hypusination-related disorders caused by eIF5A1 and eIF5A2 gene aberrations or mRNA expression dysregulation, as well as several major human cancer types and potential therapies.

Good cop, bad cop: Polyamines play both sides in host immunity and viral replication

Viruses rely on host cells for energy and synthesis machinery required for genome replication and particle assembly. Due to the dependence of viruses on host cells, viruses have evolved multiple mechanisms by which they can induce metabolic changes in the host cell to suit their specific requirements. The host immune response also involves metabolic changes to be able to react to viral insult. Polyamines are small ubiquitously expressed polycations, and their metabolism is critical for viral replication and an adequate host immune response. This is due to the variety of functions that polyamines have, ranging from condensing DNA to enhancing the translation of polyproline-containing proteins through the hypusination of eIF5A. Here, we review the diverse mechanisms by which viruses exploit polyamines, as well as the mechanisms by which immune cells utilize polyamines for their functions. Furthermore, we highlight potential avenues for further study of the host-virus interface.

Identification of the effect and mechanism of Yiyi Fuzi Baijiang powder against colorectal cancer using network pharmacology and experimental validation

Background: Yiyi Fuzi Baijiang powder (YFBP) is a traditional Chinese medicine used to treat colorectal cancer, although its bioactivity and mechanisms of action have not been studied in depth yet. The study intended to identify the potential targets and signaling pathways affected by YFBP during the treatment of colorectal cancer through pharmacological network analysis and to further analyze its chemical compositions and molecular mechanisms of action. Methods: The Traditional Chinese Medicine Systems Pharmacology (TCMSP), Traditional Chinese Medicine Integrated Database (TCMID), HitPredict (HIT), and Search Tool for Interactions of Chemicals (STITCH) databases were used to screen the bioactive components and promising targets of YFBP. Targets related to colorectal cancer were retrieved from the GeneCards and Gene Ontology databases. Cytoscape software was used to construct the "herb-active ingredient-target" network. The STRING database was used to construct and analyze protein-protein interactions (PPIs). Afterward, the R packages clusterProfiler and Cytoscape Hub plug-in were used to perform Gene Ontology (GO) functional and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of target genes. The results of the network pharmacological analysis were also experimentally validated. Results: In total, 33 active components and 128 target genes were screened. Among them, 46 target genes were considered potential therapeutic targets that crossed the CRC target genes. The network pharmacology analysis showed that the active components of YFBP were correlated positively with CRC inflammatory target genes such as TLR4, TNF, and IL-6. The inflammation-related signaling pathways affected by the active components included the TNF-α, interleukin-17, and toll-like receptor signaling pathways. The active ingredients of YFBP, such as luteolin, β-sitosterol, myristic acid, and vanillin, may exert anti-tumor effects by downregulating SMOX expression via anti-inflammatory signaling and regulation of the TLR4/NF-κB signaling pathway. Conclusion: In the present study, the potential active components, potential targets, and key biological pathways involved in the YFBP treatment of CRC were determined, providing a theoretical foundation for further anti-tumor research.

Herpesvirus-induced spermidine synthesis and eIF5A hypusination for viral episomal maintenance

Spermidine is essential for cellular growth and acts as a prerequisite of hypusination, a post-translational modification of eukaryotic initiation factor 5A (eIF5A), allowing the translation of polyproline-containing proteins. Here, we show that oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV) increases spermidine synthesis and eIF5A hypusination to enhance expression of polyproline-containing latency-associated nuclear antigen (LANA) for viral episomal maintenance. KSHV upregulates intracellular spermidine levels by dysregulating polyamine metabolic pathways in three-dimensional (3D) culture and 2D de novo infection conditions. Increased intracellular spermidine leads to increased eIF5A hypusination, ultimately enhancing LANA expression. In contrast, inhibition of spermidine synthesis or eIF5A hypusination alleviates LANA expression, decreasing viral episomal maintenance and KSHV-infected cell proliferation in vitro and in vivo, which is reversed by spermidine supplement. This demonstrates that KSHV hijacks spermidine synthesis and eIF5A hypusination pathways to enhance LANA expression for viral episomal maintenance, suggesting polyamine metabolism and eIF5A hypusination as therapeutic targets for KSHV-induced tumorigenesis.

eIF5A is activated by virus infection or dsRNA and facilitates virus replication through modulation of interferon production

Active hypusine-modified initiation elongation factor 5A is critical for cell proliferation and differentiation, embryonic development, and innate immune response of macrophages to bacterial infection. Here, we demonstrate that both virus infection and double-stranded RNA viral mimic stimulation induce the hypusination of eIF5A. Furthermore, we show that activation of eIF5A is essential for the replication of several RNA viruses including influenza A virus, vesicular stomatitis virus, chikungunya virus, mayaro virus, una virus, zika virus, and punta toro virus. Finally, our data reveal that inhibition of eIF5A hypusination using the spermidine analog GC7 or siRNA-mediated downmodulation of eIF5A1 induce upregulation of endoplasmic reticulum stress marker proteins and trigger the transcriptional induction of interferon and interferon-stimulated genes, mechanisms that may explain the broad-spectrum antiviral activity of eIF5A inhibition.

Transcriptional and Translational Dynamics of Zika and Dengue Virus Infection

Zika virus (ZIKV) and dengue virus (DENV) are members of the Flaviviridae family of RNA viruses and cause severe disease in humans. ZIKV and DENV share over 90% of their genome sequences, however, the clinical features of Zika and dengue infections are very different reflecting tropism and cellular effects. Here, we used simultaneous RNA sequencing and ribosome footprinting to define the transcriptional and translational dynamics of ZIKV and DENV infection in human neuronal progenitor cells (hNPCs). The gene expression data showed induction of aminoacyl tRNA synthetases (ARS) and the translation activating PIM1 kinase, indicating an increase in RNA translation capacity. The data also reveal activation of different cell stress responses, with ZIKV triggering a BACH1/2 redox program, and DENV activating the ATF/CHOP endoplasmic reticulum (ER) stress program. The RNA translation data highlight activation of polyamine metabolism through changes in key enzymes and their regulators. This pathway is needed for eIF5A hypusination and has been implicated in viral translation and replication. Concerning the viral RNA genomes, ribosome occupancy readily identified highly translated open reading frames and a novel upstream ORF (uORF) in the DENV genome. Together, our data highlight both the cellular stress response and the activation of RNA translation and polyamine metabolism during DENV and ZIKV infection.

Bracovirus Sneaks Into Apoptotic Bodies Transmitting Immunosuppressive Signaling Driven by Integration-Mediated eIF5A Hypusination

A typical characteristics of polydnavirus (PDV) infection is a persistent immunosuppression, governed by the viral integration and expression of virulence genes. Recently, activation of caspase-3 by Microplitis bicoloratus bracovirus (MbBV) to cleave Innexins, gap junction proteins, has been highlighted, further promoting apoptotic cell disassembly and apoptotic body (AB) formation. However, whether ABs play a role in immune suppression remains to be determined. Herein, we show that ABs transmitted immunosuppressive signaling, causing recipient cells to undergo apoptosis and dismigration. Furthermore, the insertion of viral–host integrated motif sites damaged the host genome, stimulating eIF5A nucleocytoplasmic transport and activating the eIF5A-hypusination translation pathway. This pathway specifically translates apoptosis-related host proteins, such as P53, CypA, CypD, and CypJ, to drive cellular apoptosis owing to broken dsDNA. Furthermore, translated viral proteins, such Vank86, 92, and 101, known to complex with transcription factor Dip3, positively regulated DHYS and DOHH transcription maintaining the activation of the eIF5A-hypusination. Mechanistically, MbBV-mediated extracellular vesicles contained inserted viral fragments that re-integrated into recipients, potentially via the homologous recombinant repair system. Meanwhile, this stimulation regulated activated caspase-3 levels via PI3K/AKT 308 and 473 dephosphorylation to promote apoptosis of granulocyte-like recipients Sf9 cell; maintaining PI3K/AKT 473 phosphorylation and 308 dephosphorylation inhibited caspase-3 activation leading to dismigration of plasmatocyte-like recipient High Five cells. Together, our results suggest that integration-mediated eIF5A hypusination drives extracellular vesicles for continuous immunosuppression.

Polyamine biosynthesis and eIF5A hypusination are modulated by the DNA tumor virus KSHV and promote KSHV viral infection

Polyamines are critical metabolites involved in various cellular processes and often dysregulated in cancers. Kaposi’s sarcoma-associated Herpesvirus (KSHV), a defined human oncogenic virus, leads to profound alterations of host metabolic landscape to favor development of KSHV-associated malignancies. In our studies, we identified that polyamine biosynthesis and eIF5A hypusination are dynamically regulated by KSHV infection through modulation of key enzymes (ODC1 and DHPS) of these pathways. During KSHV latency, ODC1 and DHPS are upregulated along with increase of hypusinated eIF5A (hyp-eIF5A), while hyp-eIF5A is further induced along with reduction of ODC1 and intracellular polyamines during KSHV lytic reactivation. In return these metabolic pathways are required for both KSHV lytic reactivation and de novo infection. Further analysis unraveled that synthesis of critical KSHV latent and lytic proteins (LANA, RTA) depends on hypusinated-eIF5A. We also demonstrated that KSHV infection can be efficiently and specifically suppressed by inhibitors targeting these pathways. Collectively, our results illustrated that the dynamic and profound interaction of a DNA tumor virus (KSHV) with host polyamine biosynthesis and eIF5A hypusination pathways promote viral propagation, thus defining new therapeutic targets to treat KSHV-associated malignancies.

Understanding virus-host interactions is crucial to develop and improve therapies. Kaposi’s sarcoma associated Herpesvirus (KSHV) is a human gamma-herpesvirus which deeply modulates the host metabolism and is associated with various cancers of endothelial and lymphoid origin. Polyamines are critical metabolites often dysregulated in cancers. In this study we demonstrated KSHV dynamically modulates polyamine metabolism to favor eIF5A hypusination and translation of critical KSHV latent and lytic proteins (LANA, RTA). Consequently, we found KSHV lytic switch from latency and de novo infection were dependent on polyamines and hypusination and pharmacological inhibition efficiently and specifically restricted KSHV infection. Our study provides new insights into KSHV alteration of the host metabolism and describe new therapeutic targets to treat KSHV-associated malignancies.

PEDV infection affects the expression of polyamine-related genes inhibiting viral proliferation

Porcine epidemic diarrhea virus (PEDV) is an alpha-coronavirus that causes epidemic diarrhea in swines. The mortality of PEDV infection in one-week-old piglets is extremely high, which causes a huge significant economic loss to the global pig husbandry and blocks its healthy development. There was a lack of adequate studies to elucidate pathogenic mechanism associated with PEDV infection. In the present study, we detected the expression profiles of polyamine metabolism associated genes in Vero cells infected with PEDV by RT-qPCR. It is shown that PAOX(acetylpolyamine oxidase), SMOX(spermine oxidase), SAT1(spermidine-spermine acetyltransferase 1), ODC1(ornithine decarboxylase 1), DHPS(deoxyhypusine synthase) and EIF5A( eukaryotic initiation factor 5A) were significantly upregulated. Through intervening SAT1 level in PEDV-infected Vero cells, it is identified that overexpression of SAT1 inhibited PEDV replication by reducing polyamine levels. Furthermore, polyamine depletion and upregulation were found to regulate the proliferation of PEDV. PEDV infection in Vero cells did not result in a significant change in the protein level of eIF5A, and in addition, the activated eIF5A did not affect the proliferation of PEDV. Our results provided new insights into the influence of polyamine metabolism on the proliferation of PEDV.

The eukaryotic initiation factor 5A (eIF5A1), the molecule, mechanisms and recent insights into the pathophysiological roles

Since the demonstration of its involvement in cell proliferation, the eukaryotic initiation factor 5A (eIF5A) has been studied principally in relation to the development and progression of cancers in which the isoform A2 is mainly expressed. However, an increasing number of studies report that the isoform A1, which is ubiquitously expressed in normal cells, exhibits novel molecular features that reveal its new relationships between cellular functions and organ homeostasis. At a first glance, eIF5A can be regarded, among other things, as a factor implicated in the initiation of translation. Nevertheless, at least three specificities: (1) its extreme conservation between species, including plants, throughout evolution, (2) its very special and unique post-translational modification through the activating-hypusination process, and finally (3) its close relationship with the polyamine pathway, suggest that the role of eIF5A in living beings remains to be uncovered. In fact, and beyond its involvement in facilitating the translation of proteins containing polyproline residues, eIF5A is implicated in various physiological processes including ischemic tolerance, metabolic adaptation, aging, development, and immune cell differentiation. These newly discovered physiological properties open up huge opportunities in the clinic for pathologies such as, for example, the ones in which the oxygen supply is disrupted. In this latter case, organ transplantation, myocardial infarction or stroke are concerned, and the current literature defines eIF5A as a new drug target with a high level of potential benefit for patients with these diseases or injuries. Moreover, the recent use of genomic and transcriptomic association along with metadata studies also revealed the implication of eIF5A in genetic diseases. Thus, this review provides an overview of eIF5A from its molecular mechanism of action to its physiological roles and the clinical possibilities that have been recently reported in the literature.

Identification and characterization of short leader and trailer RNAs synthesized by the Ebola virus RNA polymerase

Transcription of non-segmented negative sense (NNS) RNA viruses follows a stop-start mechanism and is thought to be initiated at the genome’s very 3’-end. The synthesis of short abortive leader transcripts (leaderRNAs) has been linked to transcription initiation for some NNS viruses. Here, we identified the synthesis of abortive leaderRNAs (as well as trailer RNAs) that are specifically initiated opposite to (anti)genome nt 2; leaderRNAs are predominantly terminated in the region of nt ~ 60–80. LeaderRNA synthesis requires hexamer phasing in the 3’-leader promoter. We determined a steady-state NP mRNA:leaderRNA ratio of ~10 to 30-fold at 48 h after Ebola virus (EBOV) infection, and this ratio was higher (70 to 190-fold) for minigenome-transfected cells. LeaderRNA initiation at nt 2 and the range of termination sites were not affected by structure and length variation between promoter elements 1 and 2, nor the presence or absence of VP30. Synthesis of leaderRNA is suppressed in the presence of VP30 and termination of leaderRNA is not mediated by cryptic gene end (GE) signals in the 3’-leader promoter. We further found different genomic 3’-end nucleotide requirements for transcription versus replication, suggesting that promoter recognition is different in the replication and transcription mode of the EBOV polymerase. We further provide evidence arguing against a potential role of EBOV leaderRNAs as effector molecules in innate immunity. Taken together, our findings are consistent with a model according to which leaderRNAs are abortive replicative RNAs whose synthesis is not linked to transcription initiation. Rather, replication and transcription complexes are proposed to independently initiate RNA synthesis at separate sites in the 3’-leader promoter, i.e., at the second nucleotide of the genome 3’-end and at the more internally positioned transcription start site preceding the first gene, respectively, as reported for Vesicular stomatitis virus.

The RNA polymerase (RdRp) of Ebola virus (EBOV) initiates RNA synthesis at the 3’-leader promoter of its encapsidated, non-segmented negative sense (NNS) RNA genome, either at the penultimate 3’-end position of the genome in the replicative mode or more internally (position 56) at the transcription start site (TSS) in its transcription mode. Here we identified the synthesis of abortive replicative RNAs that are specifically initiated opposite to genome nt 2 (termed leaderRNAs) and predominantly terminated in the region of nt ~ 60–80 near the TSS. The functional role of abortive leaderRNA synthesis is still enigmatic; a role in interferon induction could be excluded. Our findings indirectly link leaderRNA termination to nucleoprotein (NP) availability for encapsidation of nascent replicative RNA or to NP removal from the template RNA. Our findings further argue against the model that leaderRNA synthesis is a prerequisite for each transcription initiation event at the TSS. Rather, our findings are in line with the existence of distinct replicase and transcriptase complexes of RdRp that interact differently with the 3’-leader promoter and intiate RNA synthesis independently at different sites (position 2 or 56 of the genome), mechanistically similar to another NNS virus, Vesicular stomatitis virus.

Targeting Polyamines Inhibits Coronavirus Infection by Reducing Cellular Attachment and Entry

Coronaviruses first garnered widespread attention in 2002 when the severe acute respiratory syndrome coronavirus (SARS-CoV) emerged from bats in China and rapidly spread in human populations. Since then, Middle East respiratory syndrome coronavirus (MERS-CoV) emerged and still actively infects humans. The recent SARS-CoV-2 outbreak and the resulting disease (coronavirus disease 2019, COVID19) have rapidly and catastrophically spread and highlighted significant limitations to our ability to control and treat infection. Thus, a basic understanding of entry and replication mechanisms of coronaviruses is necessary to rationally evaluate potential antivirals. Here, we show that polyamines, small metabolites synthesized in human cells, facilitate coronavirus replication and the depletion of polyamines with FDA-approved molecules significantly reduces coronavirus replication. We find that diverse coronaviruses, including endemic and epidemic coronaviruses, exhibit reduced attachment and entry into polyamine-depleted cells. We further demonstrate that several molecules targeting the polyamine biosynthetic pathway are antiviral in vitro. In sum, our data suggest that polyamines are critical to coronavirus replication and represent a highly promising drug target in the current and any future coronavirus outbreaks.

Cucurbit[7]uril as a Broad-Spectrum Antiviral Agent against Diverse RNA Viruses

The emergence and re-emergence of RNA virus outbreaks highlight the urgent need for the development of broad-spectrum antivirals. Polyamines are positively-charged small molecules required for the infectivity of a wide range of RNA viruses, therefore may become good antiviral targets. Cucurbit[7]uril (CB[7]), a synthetic macrocyclic molecule, which can bind with amine-based organic compounds with high affinity, has been shown to regulate bioactive molecules through competitive binding. In this study, we tested the antiviral activity of CB[7] against diverse RNA viruses, including a panel of enteroviruses (i.e. human enterovirus A71, coxsackievirus A16, coxsackievirus B3, and echovirus 11), some flaviviruses (i.e. dengue virus and Zika virus), and an alphavirus representative Semliki forest virus. CB[7] can inhibit virus replications in a variety of cell lines, and its mechanism of action is through the competitive binding with polyamines. Our findings not only for the first time provide evidence that CB[7] can be a promising broad-spectrum antiviral agent, but more importantly, offer a novel therapeutic strategy to fight against RNA viruses by supramolecular sequestration of polyamines.

Alternative human eIF5A protein isoform plays a critical role in mitochondria

The eukaryotic translation initiation factor 5A (eIF5A) is the only known protein containing the amino acid residue hypusine, essential for its activity. Hypusine residue is produced by a posttranslational modification involving deoxyhypusine synthetase and deoxyhypusine hydroxylase. Herein, we aimed to describe the role of the alternative human isoform A on mitochondrial processes. Isoform A depletion modulates oxidative metabolism in association with the downregulation of mitochondrial biogenesis-related genes. Through positive feedback, it increases cell respiration leading to highly reactive oxygen species production, which impacts mitochondrial bioenergetics. These metabolic changes compromise mitochondrial morphology, increasing its electron density and fission, observed by transmission electron microscopy. This set of changes leads the cells to apoptosis, evidenced by increased DNA fragmentation and proapoptotic BAK protein content increase. Thus, we show that the alternative eIF5A isoform A is crucial for energy metabolism controlled by mitochondria and cellular survival.

Targeting Polyamine Metabolism for Control of Human Viral Diseases

A virus is an infectious particle which generally contains nucleic acid genome (DNA or RNA inside a protein shell), except for human immunodeficiency virus (HIV). Viruses have to reproduce by infecting their host cells. Polyamines are ubiquitous compounds in mammalian cells and play key roles in various cellular processes. The metabolic pathways of polyamines have been well studied. Targeting these metabolic pathways can reduce infections caused by viruses. In the study, we systematically reviewed the association of polyamine metabolic pathways and viruses including coxsackievirus B3 (CVB3), enterovirus 71 (EV71), poliovirus (PV), Zika virus (ZKV), hepatitis C virus (HCV), hepatitis B virus (HBV), dengue virus (DENV), Japanese encephalitis virus (JEV), yellow fever virus (YFV), Ebola virus (EBOV), marburgvirus (MARV), chikungunya virus (CHIKV), sindbis virus (SINV), Semliki Forest virus (SFV), Epstein-Barr virus (EBV), herpes simplex virus 1 (HSV), human cytomegalovirus (HCMV), vesicular stomatitis virus (VSV), Rabies virus (RABV), Rift Valley fever virus (RVFV), La Crosse virus (LACV), human immunodeficiency virus (HIV), Middle East respiratory syndrome virus (MERS-CoV), and coronavirus disease 2019 (SARS-CoV-2). This review revealed that targeting polyamine metabolic pathways may be a potential approach to control human viral infection.

Polyamine regulation of porcine reproductive and respiratory syndrome virus infection depends on spermidine-spermine acetyltransferase 1

•

Inhibition of polyamine synthesis suppresses PRRSV proliferation.

•

PRRSV infection relies on spermidine and spermine.

•

PRRSV decreases the levels of intracellular polyamines.

•

PRRSV infection increases the mRNA level of SAT1.

•

SAT1 inhibits PRRSV propagation.

Like obligate intracellular parasites, viruses co-opt host cell resources to establish productive infections. Polyamines are key aliphatic molecules that perform important roles in cellular growth and proliferation. They are also needed for the successful multiplication of various viruses. Little is known about the effects of polyamines on Arteriviridae infections. Here, porcine reproductive and respiratory syndrome virus (PRRSV), an economically prominent porcine virus, was used to investigate virus–polyamine interactions. We found that PRRSV infection significantly downregulated the levels of cellular polyamines. Using an inhibitor or specific short interfering RNAs (siRNAs) of ornithine decarboxylase 1, a key anabolic enzyme involved in the classical de novo biosynthesis of polyamines, we found that polyamine depletion abrogated PRRSV proliferation, and this effect was recoverable by adding exogenous spermidine and spermine, but not putrescine to the cells, suggesting that the host inhibits polyamine biosynthesis to restrict PRRSV proliferation. Further analysis revealed that the expression level of spermidine-spermine acetyltransferase 1 (SAT1), a catabolic enzyme that reduces spermidine and spermine levels, was upregulated during PRRSV infection, but conversely, SAT1 had an inhibitory effect on PRRSV reproduction. Our data show that polyamines are important molecules during PRRSV-host interactions, and polyamines and their biosynthetic pathways are potential therapeutic targets against PRRSV infection.

Ribavirin Induces Polyamine Depletion via Nucleotide Depletion to Limit Virus Replication

Common antivirals include nucleoside or nucleotide analogs with base prodrugs. The antiviral ribavirin, a US Food and Drug Administration (FDA)-approved nucleoside antimetabolite, halts guanine production, mutagenizes viral genomes, and activates interferon signaling. Here, we find that ribavirin induces spermidine-spermine N1-acetyltransferase (SAT1), a polyamine catabolic enzyme. Polyamines are small, positively charged molecules involved in cellular functions such as transcription and translation. Previous work showed that SAT1 activation and polyamine depletion interfere with RNA virus replication. We show ribavirin depletes polyamines via SAT1, in conjunction with its known mechanisms. SAT1 transcripts, protein, and activity are induced in a dose-dependent manner, which depletes polyamine levels and reduces viral titers. Inhibition of SAT1 activity, pharmacologically or genetically, reduces ribavirin's effectiveness against three virus infection models. Additionally, ribavirin-mediated polyamine depletion results from nucleotide pool depletion. These data demonstrate another mechanism of ribavirin that inform its clinical effectiveness, which may provide insight for improved therapies.

Virion-Associated Polyamines Transmit with Bunyaviruses to Maintain Infectivity and Promote Entry

Viruses require host cell metabolites to productively infect, and the mechanisms by which viruses usurp these molecules are diverse. One group of cellular metabolites important in virus infection is the polyamines, small positively charged molecules involved in cell cycle, translation, and nucleic acid metabolism, among other cellular functions. Polyamines support replication of diverse viruses, and they are important for processes such as transcription, translation, and viral protein enzymatic activity. Rift Valley fever virus (RVFV) is a negative and ambisense RNA virus that requires polyamines to produce infectious particles. In polyamine depleted conditions, noninfectious particles are produced that interfere with virus replication and stimulate immune signaling. Here, we find that RVFV relies on virion-associated polyamines to maintain infectivity and enhance viral entry. We show that RVFV replication is facilitated by a limited set of polyamines and that spermidine and closely related molecules associate with purified virions and transmit from cell to cell during infection. Virion-associated spermidine maintains virion infectivity, as virions devoid of polyamines rapidly lose infectivity and are temperature sensitive. Further, virions without polyamines bind to cells but exhibit a defect in entry, requiring more acidic conditions than virions containing spermidine. These data highlight a unique role for polyamines, and spermidine particularly, to maintain virus infectivity. Further, these studies are the first to identify polyamines associated with RVFV virions. Targeting polyamines represents a promising antiviral strategy, and this work highlights a new mechanism by which we can inhibit virus replication through FDA-approved polyamine depleting pharmaceuticals.

Diverse Functions of Polyamines in Virus Infection

As obligate intracellular parasites, viruses rely on host cells for the building blocks of progeny viruses. Metabolites such as amino acids, nucleotides, and lipids are central to viral proteins, genomes, and envelopes, and the availability of these molecules can restrict or promote infection. Polyamines, comprised of putrescine, spermidine, and spermine in mammalian cells, are also critical for virus infection. Polyamines are small, positively charged molecules that function in transcription, translation, and cell cycling. Initial work on the function of polyamines in bacteriophage infection illuminated these molecules as critical to virus infection. In the decades since early virus-polyamine descriptions, work on diverse viruses continues to highlight a role for polyamines in viral processes, including genome packaging and viral enzymatic activity. On the host side, polyamines function in the response to virus infection. Thus, viruses and hosts compete for polyamines, which are a critical resource for both. Pharmacologically targeting polyamines, tipping the balance to favor the host and restrict virus replication, holds significant promise as a broad-spectrum antiviral strategy.

Difluoromethylornithine, a Decarboxylase 1 Inhibitor, Suppresses Hepatitis B Virus Replication by Reducing HBc Protein Levels

Current treatments of hepatitis B virus (HBV) are limited to Interferon-alpha or the nucleos(t)ide analogs antiviral therapies, and it is crucial to develop and define new antiviral drugs to cure HBV. In this study, we explored the anti-HBV effect of difluoromethylornithine (DFMO), an irreversibly inhibitor of decarboxylase 1(ODC1) on HBV replication. Firstly, we found that polyamines contributed to HBV DNA replication via increasing levels of the HBV core protein (HBc) and capsids. In contrast, depletion of polyamines either by silencing the expression of ODC1 or DFMO treatment, resulted in decreasing viral DNA replication and levels of HBc protein and capsids. Furthermore, we found that DFMO decreased the stability of the HBc protein without affecting mRNA transcription and protein translation. Taken together, our findings demonstrate that DFMO inhibits HBV replication by reducing HBc stability and this may provide a new approach for HBV therapeutics.

Polyamine Transport Protein PotD Protects Mice against Haemophilus parasuis and Elevates the Secretion of Pro-Inflammatory Cytokines of Macrophage via JNK-MAPK and NF-κB Signal Pathways through TLR4

The potD gene, belonging to the well-conserved ABC (ATP-binding cassette) transport system potABCD, encodes the bacterial substrate-binding subunit of the polyamine transport system. In this study, we found PotD in Haemophilus (Glaesserella) parasuis could actively stimulate both humoral immune and cellular immune responses and elevate lymphocyte proliferation, thus eliciting a Th1-type immune response in a murine immunity and infection model. Stimulation of Raw 264.7 macrophages with PotD validated that Toll-like receptor 4, rather than 2, participated in the positive transcription and expression of pro-inflammatory cytokines IL–1β, IL–6, and TNF–α using qPCR and ELISA. Blocking signal-regulated JNK–MAPK and RelA(p65) pathways significantly decreased PotD-induced pro-inflammatory cytokine production. Overall, we conclude that vaccination of PotD could induce both humoral and cellular immune responses and provide immunoprotection against H. parasuis challenge. The data also suggest that Glaesserella PotD is a novel pro-inflammatory mediator and induces TLR4-dependent pro-inflammatory activity in Raw 264.7 macrophages through JNK–MAPK and RelA(p65) pathways.

A Chimeric Lloviu Virus Minigenome System Reveals that the Bat-Derived Filovirus Replicates More Similarly to Ebolaviruses than Marburgviruses

Recently, traces of zoonotic viruses have been discovered in bats and other species around the world, but despite repeated attempts, full viral genomes have not been rescued. The absence of critical genetic sequences from these viruses and the difficulties to isolate infectious virus from specimens prevent research on their pathogenic potential for humans. One example of these zoonotic pathogens is Lloviu virus (LLOV), a filovirus that is closely related to Ebola virus. Here, we established LLOV minigenome systems based on sequence complementation from other filoviruses. Our results show that the LLOV replication and transcription mechanisms are, in general, more similar to ebolaviruses than to marburgviruses. We also show that a single nucleotide at the 3' genome end determines species specificity of the LLOV polymerase. The data obtained here will be instrumental for the rescue of infectious LLOV clones for pathogenesis studies.

Distinct Genome Replication and Transcription Strategies within the Growing Filovirus Family

Research on filoviruses has historically focused on the highly pathogenic ebola- and marburgviruses. Indeed, until recently, these were the only two genera in the filovirus family. Recent advances in sequencing technologies have facilitated the discovery of not only a new ebolavirus, but also three new filovirus genera and a sixth proposed genus. While two of these new genera are similar to the ebola- and marburgviruses, the other two, discovered in saltwater fishes, are considerably more diverse. Nonetheless, these viruses retain a number of key features of the other filoviruses. Here, we review the key characteristics of filovirus replication and transcription, highlighting similarities and differences between the viruses. In particular, we focus on key regulatory elements in the genomes, replication and transcription strategies, and the conservation of protein domains and functions among the viruses. In addition, using computational analyses, we were able to identify potential homology and functions for some of the genes of the novel filoviruses with previously unknown functions. Although none of the newly discovered filoviruses have yet been isolated, initial studies of some of these viruses using minigenome systems have yielded insights into their mechanisms of replication and transcription. In general, the Cuevavirus and proposed Dianlovirus genera appear to follow the transcription and replication strategies employed by the ebola- and marburgviruses, respectively. While our knowledge of the fish filoviruses is currently limited to sequence analysis, the lack of certain conserved motifs and even entire genes necessitates that they have evolved distinct mechanisms of replication and transcription.

Polyamine Depletion Inhibits Bunyavirus Infection via Generation of Noninfectious Interfering Virions

Several host and viral processes contribute to forming infectious virions. Polyamines are small host molecules that play diverse roles in viral replication. We previously demonstrated that polyamines are crucial for RNA viruses; however, the mechanisms by which polyamines function remain unknown. Here, we investigated the role of polyamines in the replication of the bunyaviruses Rift Valley fever virus (vaccine strain MP-12) and La Crosse virus (LACV). We found that polyamine depletion did not impact viral RNA or protein accumulation, despite significant decreases in titer. Viral particles demonstrated no change in morphology, size, or density. Thus, polyamine depletion promotes the formation of noninfectious particles. These particles interfere with virus replication and stimulate innate immune responses. We extended this phenotype to Zika virus; however, coxsackievirus did not similarly produce noninfectious particles. In sum, polyamine depletion results in the accumulation of noninfectious particles that interfere with replication and stimulate immune signaling, with important implications for targeting polyamines therapeutically, as well as for vaccine strategies.IMPORTANCE Bunyaviruses are emerging viral pathogens that cause encephalitis, hemorrhagic fevers, and meningitis. We have uncovered that diverse bunyaviruses require polyamines for productive infection. Polyamines are small, positively charged host-derived molecules that play diverse roles in human cells and in infection. In polyamine-depleted cells, bunyaviruses produce an overabundance of noninfectious particles that are indistinguishable from infectious particles. However, these particles interfere with productive infection and stimulate antiviral signaling pathways. We further find that additional enveloped viruses are similarly sensitive to polyamine depletion but that a nonenveloped enterovirus is not. We posit that polyamines are required to maintain bunyavirus infectivity and that polyamine depletion results in the accumulation of interfering noninfectious particles that limit infectivity. These results highlight a novel means by which bunyaviruses use polyamines for replication and suggest promising means to target host polyamines to reduce virus replication.

Ebolavirus polymerase uses an unconventional genome replication mechanism

Most nonsegmented negative strand (NNS) RNA virus genomes have complementary 3' and 5' terminal nucleotides because the promoters at the 3' ends of the genomes and antigenomes are almost identical to each other. However, according to published sequences, both ends of ebolavirus genomes show a high degree of variability, and the 3' and 5' terminal nucleotides are not complementary. If correct, this would distinguish the ebolaviruses from other NNS RNA viruses. Therefore, we investigated the terminal genomic and antigenomic nucleotides of three different ebolavirus species, Ebola (EBOV), Sudan, and Reston viruses. Whereas the 5' ends of ebolavirus RNAs are highly conserved with the sequence ACAGG-5', the 3' termini are variable and are typically 3'-GCCUGU, ACCUGU, or CCUGU. A small fraction of analyzed RNAs had extended 3' ends. The majority of 3' terminal sequences are consistent with a mechanism of nucleotide addition by hairpin formation and back-priming. Using single-round replicating EBOV minigenomes, we investigated the effect of the 3' terminal nucleotide on viral replication and found that the EBOV polymerase initiates replication opposite the 3'-CCUGU motif regardless of the identity of the 3' terminal nucleotide(s) and of the position of this motif relative to the 3' end. Deletion or mutation of the first residue of the 3'-CCUGU motif completely abolished replication initiation, suggesting a crucial role of this nucleotide in directing initiation. Together, our data show that ebolaviruses have evolved a unique replication strategy among NNS RNA viruses resulting in 3' overhangs. This could be a mechanism to avoid antiviral recognition.

Current status of small molecule drug development for Ebola virus and other filoviruses

The filovirus family includes some of the deadliest viruses known, including Ebola virus and Marburg virus. These viruses cause periodic outbreaks of severe disease that can be spread from person to person, making the filoviruses important public health threats. There remains a need for approved drugs that target all or most members of this virus family. Small molecule inhibitors that target conserved functions hold promise as pan-filovirus therapeutics. To date, compounds that effectively target virus entry, genome replication, gene expression, and virus egress have been described. The most advanced inhibitors are nucleoside analogs that target viral RNA synthesis reactions.