Upregulation of a novel eukaryotic translation initiation factor 5A (eIF5A) in dengue 2 virus-infected mosquito cells

A comprehensive review of Wolbachia-mediated mechanisms to control dengue virus transmission in Aedes aegypti through innate immune pathways

The Dengue virus (DENV), primarily spread by Aedes aegypti and also by Aedes albopictus in some regions, poses significant global health risks. Alternative techniques are urgently needed because the current control mechanisms are insufficient to reduce the transmission of DENV. Introducing Wolbachia pipientis into Ae. aegypti inhibits DENV transmission, however, the underlying mechanisms are still poorly understood. Innate immune effector upregulation, the regulation of autophagy, and intracellular competition between Wolbachia and DENV for lipids are among the theories for the mechanism of inhibition. Furthermore, mainly three immune pathways Toll, IMD, and JAK/STAT are involved in the host for the suppression of the virus. These pathways are activated by Wolbachia and DENV in the host and are responsible for the upregulation and downregulation of many genes in mosquitoes, which ultimately reduces the titer of the DENV in the host. The functioning of these immune pathways depends upon the Wolbachia, host, and virus interaction. Here, we summarize the current understanding of DENV recognition by the Ae. aegypti's immune system, aiming to create a comprehensive picture of our knowledge. Additionally, we investigated how Wolbachia regulates the activation of multiple genes associated with immune priming for the reduction of DENV.

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.

Antiviral strategies targeting host factors and mechanisms obliging +ssRNA viral pathogens

The ongoing COVID-19 pandemic, periodic recurrence of viral infections, and the emergence of challenging variants has created an urgent need of alternative therapeutic approaches to combat the spread of viral infections, failing to which may pose a greater risk to mankind in future. Resilience against antiviral drugs or fast evolutionary rate of viruses is stressing the scientific community to identify new therapeutic approaches for timely control of disease. Host metabolic pathways are exquisite reservoir of energy to viruses and contribute a diverse array of functions for successful replication and pathogenesis of virus. Targeting the host factors rather than viral enzymes to cease viral infection, has emerged as an alternative antiviral strategy. This approach offers advantage in terms of increased threshold to viral resistance and can provide broad-spectrum antiviral action against different viruses. The article here provides substantial review of literature illuminating the host factors and molecular mechanisms involved in innate/adaptive responses to viral infection, hijacking of signalling pathways by viruses and the intracellular metabolic pathways required for viral replication. Host-targeted drugs acting on the pathways usurped by viruses are also addressed in this study. Host-directed antiviral therapeutics might prove to be a rewarding approach in controlling the unprecedented spread of viral infection, however the probability of cellular side effects or cytotoxicity on host cell should not be ignored at the time of clinical investigations.

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.

Life as a Vector of Dengue Virus: The Antioxidant Strategy of Mosquito Cells to Survive Viral Infection

Dengue fever is a mosquito-borne viral disease of increasing global importance. The disease has caused heavy burdens due to frequent outbreaks in tropical and subtropical areas of the world. The dengue virus (DENV) is generally transmitted between human hosts via the bite of a mosquito vector, primarily Aedes aegypti and Ae. albopictus as a minor species. It is known that the virus needs to alternately infect mosquito and human cells. DENV-induced cell death is relevant to the pathogenesis in humans as infected cells undergo apoptosis. In contrast, mosquito cells mostly survive the infection; this allows infected mosquitoes to remain healthy enough to serve as an efficient vector in nature. Overexpression of antioxidant genes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), glutaredoxin (Grx), thioredoxin (Trx), and protein disulfide isomerase (PDI) have been detected in DENV2-infected mosquito cells. Additional antioxidants, including GST, eukaryotic translation initiation factor 5A (eIF5a), and p53 isoform 2 (p53-2), and perhaps some others, are also involved in creating an intracellular environment suitable for cell replication and viral infection. Antiapoptotic effects involving inhibitor of apoptosis (IAP) upregulation and subsequent elevation of caspase-9 and caspase-3 activities also play crucial roles in the ability of mosquito cells to survive DENV infection. This article focused on the effects of intracellular responses in mosquito cells to infection primarily by DENVs. It may provide more information to better understand virus/cell interactions that can possibly elucidate the evolutionary pathway that led to the mosquito becoming a vector.

Vero Cell Proteomic Changes Induced by Zika Virus Infection

The re-emergence and the recent spread of the Zika virus (ZIKV) has raised significant global concerns due to lack of information in patient diagnosis and management. Thus, in addition to gaining more basic information about ZIKV biology, appropriate interventions and management strategies are being sought to control ZIKV-associated diseases and its spread. This study's objective is to identify host cell proteins that are significantly dysregulated during ZIKV infection. SOMAScan, a novel aptamer-based assay, is used to simultaneously screen >1300 host proteins to detect ZIKV-induced host protein dysregulation at multiple time points during infection. A total of 125 Vero cell host proteins, including cytokines such as CXCL11 and CCL5, interferon stimulated gene 15, and translation initiation factors EIF5A and EIF4G2, are significantly dysregulated after ZIKV infection. Bioinformatic analyses of 77 host proteins, that are significantly dysregulated ≥1.25-fold, identify several activated biological processes, including the JAK/STAT, Tec kinase, and complement cascade pathways.

XBP1-Mediated BiP/GRP78 Upregulation Copes with Oxidative Stress in Mosquito Cells during Dengue 2 Virus Infection

Dengue viruses (DENVs) cause dengue fever which is an important mosquito-borne disease in tropical areas. Generally, DENV does not cause cellular damage in mosquito cells. However, alterations in cytosolic calcium ions ([Ca²⁺]cyt) and the mitochondrial membrane potential (MMP), as well as accumulated reactive oxygen species (ROS), including superoxide anions (O₂^∙-) and hydrogen peroxide (H₂O₂), can be detected in C6/36 cells with DENV2 infection. Evident upregulation of BiP/GRP78 also appeared at 24 h postinfection in DENV2-infected C6/36 cells. As expression of BiP/GRP78 mRNA was reduced when the transcription factor X-box-binding protein-1 (XBP1) was knocked down in C6/36 cells, it demonstrated that BiP/GRP78 is the target gene regulated by the XBP1 signal pathway. We further demonstrated that the expression and splicing activity of XBP1 were upregulated in parallel with DENV2 infection in C6/36 cells. In C6/36 cells with BiP/GRP78 overexpression, oxidative stress indicators including [Ca²⁺]cyt, MMP, O₂^∙-, and H₂O₂ were all pushed back to normal. Taken together, DENV2 activates XBP1 at earlier stage of infection, followed by upregulating BiP/GRP78 in mosquito cells. This regulatory pathway contributes a cascade in relation to oxidative stress alleviation. The finding provides insights into elucidating how mosquitoes can healthily serve as a vector of arboviruses in nature.

Systems Biology Reveals NS4B-Cyclophilin A Interaction: A New Target to Inhibit YFV Replication

Yellow fever virus (YFV) replication is highly dependent on host cell factors. YFV NS4B is reported to be involved in viral replication and immune evasion. Here interactions between NS4B and human proteins were determined using a GST pull-down assay and analyzed using 1-DE and LC-MS/MS. We present a total of 207 proteins confirmed using Scaffold 3 Software. Cyclophilin A (CypA), a protein that has been shown to be necessary for the positive regulation of flavivirus replication, was identified as a possible NS4B partner. 59 proteins were found to be significantly increased when compared with a negative control, and CypA exhibited the greatest difference, with a 22-fold change. Fisher's exact test was significant for 58 proteins, and the p value of CypA was the most significant (0.000000019). The Ingenuity Systems software identified 16 pathways, and this analysis indicated sirolimus, an mTOR pathway inhibitor, as a potential inhibitor of CypA. Immunofluorescence and viral plaque assays showed a significant reduction in YFV replication using sirolimus and cyclosporine A (CsA) as inhibitors. Furthermore, YFV replication was strongly inhibited in cells treated with both inhibitors using reporter BHK-21-rep-YFV17D-LucNeoIres cells. Taken together, these data suggest that CypA-NS4B interaction regulates YFV replication. Finally, we present the first evidence that YFV inhibition may depend on NS4B-CypA interaction.

Polyamines and Hypusination Are Required for Ebolavirus Gene Expression and Replication

Ebolavirus (EBOV) is an RNA virus that is known to cause severe hemorrhagic fever in humans and other primates : EBOV successfully enters and replicates in many cell types. This replication is dependent on the virus successfully coopting a number of cellular factors. Many of these factors are currently unidentified but represent potential targets for antiviral therapeutics. Here we show that cellular polyamines are critical for EBOV replication. We found that small-molecule inhibitors of polyamine synthesis block gene expression driven by the viral RNA-dependent RNA polymerase. Short hairpin RNA (shRNA) knockdown of the polyamine pathway enzyme spermidine synthase also resulted in reduced EBOV replication. These findings led us to further investigate spermidine, a polyamine that is essential for the hypusination of eukaryotic initiation factor 5A (eIF5A). Blocking the hypusination of eIF5A (and thereby inhibiting its function) inhibited both EBOV gene expression and viral replication. The mechanism appears to be due to the importance of hypusinated eIF5A for the accumulation of VP30, an essential component of the viral polymerase. The same reduction in hypusinated eIF5A did not alter the accumulation of other viral polymerase components. This action makes eIF5A function an important gate for proper EBOV polymerase assembly and function through the control of a single virus protein.

IMPORTANCE

Ebolavirus (EBOV) is one of the most lethal human pathogens known. EBOV requires host factors for replication due to its small RNA genome. Here we show that the host protein eIF5A in its activated form is necessary for EBOV replication. We further show that the mechanism is through the accumulation of a single EBOV protein, VP30. To date, no other host proteins have been shown to interfere with the translation or stability of an EBOV protein. Activated eIF5A is the only protein in the cell known to contain the specific modification of hypusine; therefore, this pathway is a target for drug development. Further investigation into the mechanism of eIF5A interaction with VP30 could provide insight into therapeutics to combat EBOV.

Viral Interference and Persistence in Mosquito-Borne Flaviviruses

Mosquito-borne flaviviruses are important pathogens for humans, and the detection of two or more flaviviruses cocirculating in the same geographic area has often been reported. However, the epidemiological impact remains to be determined. Mosquito-borne flaviviruses are primarily transmitted through Aedes and Culex mosquitoes; these viruses establish a life-long or persistent infection without apparent pathological effects. This establishment requires a balance between virus replication and the antiviral host response. Viral interference is a phenomenon whereby one virus inhibits the replication of other viruses, and this condition is frequently associated with persistent infections. Viral interference and persistent infection are determined by several factors, such as defective interfering particles, competition for cellular factors required for translation/replication, and the host antiviral response. The interaction between two flaviviruses typically results in viral interference, indicating that these viruses share common features during the replicative cycle in the vector. The potential mechanisms involved in these processes are reviewed here.

Proteomic changes occurring in the malaria mosquitoes Anopheles gambiae and Anopheles stephensi during aging

The age of mosquitoes is a crucial determinant of their ability to transmit pathogens and their resistance to insecticides. We investigated changes to the abundance of proteins found in heads and thoraces of the malaria mosquitoes Anopheles gambiae and Anopheles stephensi as they aged. Protein expression changes were assessed using two-dimensional difference gel electrophoresis and the identity of differentially expressed proteins was determined by using either matrix-assisted laser desorption ionization tandem time-of-flight mass spectrometry or capillary high-pressure liquid chromatography coupled with a linear ion-trap (LTQ)-Orbitrap XL hybrid mass spectrometer. Protein biomarkers were validated by semi quantitative Western blot analysis. Nineteen and nine age dependent protein spots were identified for A. stephensi and A. gambiae, respectively. Among the proteins down-regulated with age were homologs of ADF/Cofilin, cytochome c1, heat shock protein-70 and eukaryotic translation initiation factor 5A (eIF5a). Proteins up-regulated with age included probable methylmalonate-semialdehyde dehydrogenase, voltage-dependent anion-selective channel and fructose bisphosphate aldolase. Semi quantitative Western blot analysis confirmed expression patterns observed by 2-D DIGE for eIF5a and ADF/Cofilin. Further work is recommended to determine whether these biomarkers are robust to infection, blood feeding and insecticide resistance. Robust biomarkers could then be incorporated into rapid diagnostic assays for ecological and epidemiological studies.

BIOLOGICAL SIGNIFICANCE

In this study, we have identified several proteins with characteristic changes in abundance in both A. gambiae and A. stephensi during their aging process. These changes may highlight underlying mechanisms beneath the relationship between mosquito age and factors affecting Plasmodium transmission and mosquito control. The similarity of changes in protein abundance between these species and the primary dengue vector Aedes aegypti, has revealed conserved patterns of aging-specific protein regulation.

Heat shock cognate protein 70 isoform D is required for clathrin-dependent endocytosis of Japanese encephalitis virus in C6/36 cells

Japanese encephalitis virus (JEV), one of encephalitic flaviviruses, is naturally transmitted by mosquitoes. During infection, JEV generally enters host cells via receptor-mediated clathrin-dependent endocytosis that requires the 70 kDa heat-shock protein (Hsp70). Heat-shock cognate protein 70 (Hsc70) is one member of the Hsp70 family and is constitutively expressed; thus, it may be expressed under physiological conditions. In C6/36 cells, Hsc70 is upregulated in response to JEV infection. Since Hsc70 shows no relationship with viruses attaching to the cell surface, it probably does not serve as the receptor according to our results in the present study. In contrast, Hsc70 is evidently associated with virus penetration into the cell and resultant acidification of intracellular vesicles. It suggests that Hsc70 is highly involved in clathrin-mediated endocytosis, particularly at the late stage of viral entry into host cells. Furthermore, we found that Hsc70 is composed of at least three isoforms, including B, C and D; of these, isoform D helps JEV to penetrate C6/36 cells via clathrin-mediated endocytosis. This study provides relevant evidence that sheds light on the regulatory mechanisms of JEV infection in host cells, especially on the process of clathrin-mediated endocytosis.

Proteomic biomarkers for ageing the mosquito Aedes aegypti to determine risk of pathogen transmission

Biomarkers of the age of mosquitoes are required to determine the risk of transmission of various pathogens as each pathogen undergoes a period of extrinsic incubation in the mosquito host. Using the 2-D Difference Gel Electrophoresis (2-D DIGE) procedure, we investigated the abundance of up to 898 proteins from the Yellow Fever and dengue virus vector, Aedes aegypti, during ageing. By applying a mixed-effects model of protein expression, we identified five common patterns of abundance change during ageing and demonstrated an age-related decrease in variance for four of these. This supported a search for specific proteins with abundance changes that remain tightly associated with ageing for use as ageing biomarkers. Using MALDI-TOF/TOF mass spectrometry we identified ten candidate proteins that satisfied strict biomarker discovery criteria (identified in two out of three multivariate analysis procedures and in two cohorts of mosquitoes). We validated the abundances of the four most suitable candidates (Actin depolymerising factor; ADF, Eukaryotic initiation factor 5A; eIF5A, insect cuticle protein Q17LN8, and Anterior fat body protein; AFP) using semi-quantitative Western analysis of individual mosquitoes of six ages. The redox-response protein Manganese superoxide dismutase (SOD2) and electron shuttling protein Electron transfer oxidoreductase (ETO) were subject to post-translational modifications affecting their charge states with potential effects on function. For the four candidates we show remarkably consistent decreases in abundance during ageing, validating initial selections. In particular, the abundance of AFP is an ideal biomarker candidate for whether a female mosquito has lived long enough to be capable of dengue virus transmission. We have demonstrated proteins to be a suitable class of ageing biomarkers in mosquitoes and have identified candidates for epidemiological studies of dengue and the evaluation of new disease reduction projects targeting mosquito longevity.

P53-mediated rapid induction of apoptosis conveys resistance to viral infection in Drosophila melanogaster

Arthropod-borne pathogens account for millions of deaths each year. Understanding the genetic mechanisms controlling vector susceptibility to pathogens has profound implications for developing novel strategies for controlling insect-transmitted infectious diseases. The fact that many viruses carry genes that have anti-apoptotic activity has long led to the hypothesis that induction of apoptosis could be a fundamental innate immune response. However, the cellular mechanisms mediating the induction of apoptosis following viral infection remained enigmatic, which has prevented experimental verification of the functional significance of apoptosis in limiting viral infection in insects. In addition, studies with cultured insect cells have shown that there is sometimes a lack of apoptosis, or the pro-apoptotic response happens relatively late, thus casting doubt on the functional significance of apoptosis as an innate immunity. Using in vivo mosquito models and the native route of infection, we found that there is a rapid induction of reaper-like pro-apoptotic genes within a few hours following exposure to DNA or RNA viruses. Recapitulating a similar response in Drosophila, we found that this rapid induction of apoptosis requires the function of P53 and is mediated by a stress–responsive regulatory region upstream of reaper. More importantly, we showed that the rapid induction of apoptosis is responsible for preventing the expression of viral genes and blocking the infection. Genetic changes influencing this rapid induction of reaper-like pro-apoptotic genes led to significant differences in susceptibility to viral infection.

Arthropod-borne pathogens account for millions of deaths each year. Understanding the genetic mechanisms controlling arthropod susceptibility to pathogens has profound implications for developing novel strategies for controlling insect-transmitted infectious diseases. Although it was postulated that apoptosis (a genetically controlled form of cellular suicide) may play a very important role in insect innate immunity against viral infection, direct evidence has been lacking due to the lack of knowledge on the regulatory pathways responsible for the induction of apoptosis following viral infection. In this study, we found that there is a rapid induction of pro-apoptotic genes within 1–3 hours of exposure to virus. This rapid pro-apoptotic response was only observed in live animals but not in cultured cells. Genetic analysis indicated that animals lacking this rapid pro-apoptotic response were hypersensitive to viral infection. Thus our work provides unequivocal evidence indicating that rapid induction of apoptosis plays a very important role in mediating insect resistance to viral infection.

Additive protection by antioxidant and apoptosis-inhibiting effects on mosquito cells with dengue 2 virus infection

Cytopathic effects (CPEs) in mosquito cells are generally trivial compared to those that occur in mammalian cells, which usually end up undergoing apoptosis during dengue virus (DENV) infection. However, oxidative stress was detected in both types of infected cells. Despite this, the survival of mosquito cells benefits from the upregulation of genes related to antioxidant defense, such as glutathione S transferase (GST). A second defense system, i.e., consisting of antiapoptotic effects, was also shown to play a role in protecting mosquito cells against DENV infection. This system is regulated by an inhibitor of apoptosis (IAP) that is an upstream regulator of caspases-9 and -3. DENV-infected C6/36 cells with double knockdown of GST and the IAP showed a synergistic effect on activation of these two caspases, causing a higher rate of apoptosis (>20%) than those with knockdown of each single gene (∼10%). It seems that the IAP acts as a second line of defense with an additional effect on the survival of mosquito cells with DENV infection. Compared to mammalian cells, residual hydrogen peroxide in DENV-infected C6/36 cells may signal for upregulation of the IAP. This novel finding sheds light on virus/cell interactions and their coevolution that may elucidate how mosquitoes can be a vector of DENV and probably most other arboviruses in nature.

This study demonstrated an idea that mosquito cells can survive dengue virus (or other arboviruses) infection through antioxidant defense and an additional effect by induction of IAP expression for protection of infection. It makes mosquito eligible to support virus replication efficiently, leading to a goal which is important to explain how mosquitoes can be a vector even when they have been seriously infected by the virus. Our findings opened an avenue for studies on virus/vector co-evolution that benefits for both virus replication and its transmission to humans or susceptible hosts.