The nutritional requirements for the propagation of poliomyelitis virus by the HeLa cell

ISG15 blocks cardiac glycolysis and ensures sufficient mitochondrial energy production during Coxsackievirus B3 infection

AIMS

Virus infection triggers inflammation and, may impose nutrient shortage to the heart. Supported by type I interferon (IFN) signalling, cardiomyocytes counteract infection by various effector processes, with the IFN-stimulated gene of 15 kDa (ISG15) system being intensively regulated and protein modification with ISG15 protecting mice Coxsackievirus B3 (CVB3) infection. The underlying molecular aspects how the ISG15 system affects the functional properties of respective protein substrates in the heart are unknown.

METHODS AND RESULTS

Based on the protective properties due to protein ISGylation, we set out a study investigating CVB3-infected mice in depth and found cardiac atrophy with lower cardiac output in ISG15-/- mice. By mass spectrometry, we identified the protein targets of the ISG15 conjugation machinery in heart tissue and explored how ISGylation affects their function. The cardiac ISGylome showed a strong enrichment of ISGylation substrates within glycolytic metabolic processes. Two control enzymes of the glycolytic pathway, hexokinase 2 (HK2) and phosphofructokinase muscle form (PFK1), were identified as bona fide ISGylation targets during infection. In an integrative approach complemented with enzymatic functional testing and structural modelling, we demonstrate that protein ISGylation obstructs the activity of HK2 and PFK1. Seahorse-based investigation of glycolysis in cardiomyocytes revealed that, by conjugating proteins, the ISG15 system prevents the infection-/IFN-induced up-regulation of glycolysis. We complemented our analysis with proteomics-based advanced computational modelling of cardiac energy metabolism. Our calculations revealed an ISG15-dependent preservation of the metabolic capacity in cardiac tissue during CVB3 infection. Functional profiling of mitochondrial respiration in cardiomyocytes and mouse heart tissue by Seahorse technology showed an enhanced oxidative activity in cells with a competent ISG15 system.

CONCLUSION

Our study demonstrates that ISG15 controls critical nodes in cardiac metabolism. ISG15 reduces the glucose demand, supports higher ATP production capacity in the heart, despite nutrient shortage in infection, and counteracts cardiac atrophy and dysfunction.

Virus-induced host cell metabolic alteration

Viruses change the host cell metabolism to produce infectious particles and create optimal conditions for replication and reproduction. Numerous host cell pathways have been modified to ensure available biomolecules and sufficient energy. Metabolomics studies conducted over the past decade have revealed that eukaryotic viruses alter the metabolism of their host cells on a large scale. Modifying pathways like glycolysis, fatty acid synthesis and glutaminolysis could provide potential energy for virus multiplication. Thus, almost every virus has a unique metabolic signature and a different relationship between the viral life cycle and the individual metabolic processes. There are enormous research in virus induced metabolic reprogramming of host cells that is being conducted through numerous approaches using different vaccine candidates and antiviral drug substances. This review provides an overview of viral interference to different metabolic pathways and improved monitoring in this area will open up new ways for more effective antiviral therapies and combating virus induced oncogenesis.

Niclosamide inhibits Newcastle disease virus replication in chickens by perturbing the cellular glycolysis

Newcastle disease virus (NDV), a member of Paramyxoviridae family, is one of the most important pathogens in poultry. To ensure optimal environments for their replication and spread, viruses rely largely on host cellular metabolism. In the present study, we evaluated the small drug molecule niclosamide for its anti-NDV activity. Our study has shown that a sublethal dose of 1 μM niclosamide could drastically reduce NDV replication. The results showed that niclosamide has antiviral activity against NDV infection during in vitro, in ovo and in vivo assays. Pharmacologically inhibiting the glycolytic pathway remarkably reduced NDV RNA synthesis and infectious virion production. Our results suggest that the effect of niclosamide on cellular glycolysis could be the possible reason for the specific anti-NDV effect. This study could help us understand antiviral strategies against similar pathogens and may lead to novel therapeutic approaches through targeted inhibition of specific cellular metabolic pathways.

Plasma-Like Culture Medium for the Study of Viruses

Viral infections attract more and more attention, especially after the emergence of novel zoonotic coronaviruses and the monkeypox virus over the last 2 decades. Research on viruses is based to a great extent on mammalian cell lines that are permissive to the respective viruses. These cell lines are usually cultivated according to the protocols established in the 1950s to 1970s, although it is clear that classical media have a significant imprint on cell growth, phenotype, and especially metabolism. So, recently in the field of biochemistry and metabolomics novel culture media have been developed that resemble human blood plasma. As perturbations in metabolic and redox pathways during infection are considered significant factors of viral pathogenesis, these novel medium formulations should be adapted by the virology field. So far, there are only scarce data available on viral propagation efficiencies in cells cultivated in plasma-like media. But several groups have presented convincing data on the use of such media for cultivation of uninfected cells. The aim of the present review is to summarize the current state of research in the field of plasma-resembling culture media and to point out the influence of media on various cellular processes in uninfected cells that may play important roles in viral replication and pathogenesis in order to sensitize virology research to the use of such media.

Lactate is useful for the efficient replication of porcine epidemic diarrhea virus in cell culture

Porcine epidemic diarrhea virus (PEDV) is a deadly pathogen infecting pig herds, and has caused significant economic losses around the world. Vaccination remains the most effective way of keeping the PEDV epidemic under control. Previous studies have shown that the host metabolism has a significant impact on viral replication. In this study, we have demonstrated that two substrates of metabolic pathway, glucose and glutamine, play a key role in PEDV replication. Interestingly, the boosting effect of these compounds toward viral replication appeared to be dose-independent. Furthermore, we found that lactate, which is a downstream metabolite, promotes PEDV replication, even when added in excess to the cell culture medium. Moreover, the role of lactate in promoting PEDV was independent of the genotype of PEDV and the multiplicity of infection (MOI). Our findings suggest that lactate is a promising candidate for use as a cell culture additive for promoting PEDV replication. It could improve the efficiency of vaccine production and provide the basis for designing novel antiviral strategies.

SLC1A3 facilitates Newcastle disease virus replication by regulating glutamine catabolism

As obligate intracellular parasites, viruses rely completely on host metabolic machinery and hijack host nutrients for viral replication. Newcastle disease virus (NDV) causes acute, highly contagious avian disease and functions as an oncolytic agent. NDV efficiently replicates in both chicken and tumour cells. However, how NDV reprograms host cellular metabolism for its efficient replication is still ill-defined. We previously identified a significantly upregulated glutamate transporter gene, solute carrier family 1 member 3 (SLC1A3), during NDV infection via transcriptome analysis. To investigate the potential role of SLC1A3 during NDV infection, we first confirmed the marked upregulation of SLC1A3 in NDV-infected DF-1 or A549 cells through p53 and NF-κB pathways. Knockdown of SLC1A3 inhibited NDV infection. Western blot analysis further confirmed that glutamine, but not glutamate, asparagine, or aspartate, was required for NDV replication. Metabolic flux data showed that NDV promotes the decomposition of glutamine into the tricarboxylic acid cycle. Importantly, the level of glutamate and glutaminolysis were reduced by SLC1A3 knockdown, indicating that SLC1A3 propelled glutaminolysis for glutamate utilization and NDV replication in host cells. Taken together, our data identify that SLC1A3 serves as an important regulator for glutamine metabolism and is hijacked by NDV for its efficient replication during NDV infection. These results improve our understanding of the interaction between NDV and host cellular metabolism and lay the foundation for further investigation of efficient vaccines.

Physiological Media in Studies of Cell Metabolism

Changes in cell metabolism accompany the development of a wide spectrum of pathologies including cancer, autoimmune, and inflammatory diseases. Therefore, usage of inhibitors of metabolic enzymes are considered a promising strategy for the development of therapeutic agents. However, the investigation of cellular metabolism is hampered by the significant impact of culture media, which interfere with many cellular processes, thus making cellular models irrelevant. There are numerous reports that show that the results from in vitro systems are not reproduced in in vivo models and patients. Over the last decade a novel approach has emerged, which consists of adaptation of the culture medium composition to that closer to the composition of blood plasma. In 2017‒2019, two plasma-like media were proposed, Plasmax and HPLM. In the review, we have summarized the drawbacks of common media and have analyzed changes in the metabolism of cells cultivated in common and plasma-like media in normal and pathological conditions.

'Warburg effect' controls tumor growth, bacterial, viral infections and immunity - Genetic deconstruction and therapeutic perspectives

The evolutionary pressure for life transitioning from extended periods of hypoxia to an increasingly oxygenated atmosphere initiated drastic selections for a variety of biochemical pathways supporting the robust life currently present on the planet. First, we discuss how fermentative glycolysis, a primitive metabolic pathway present at the emergence of life, is instrumental for the rapid growth of cancer, regenerating tissues, immune cells but also bacteria and viruses during infections. The 'Warburg effect', activated via Myc and HIF-1 in response to growth factors and hypoxia, is an essential metabolic and energetic pathway which satisfies nutritional and energetic demands required for rapid genome replication. Second, we present the key role of lactic acid, the end-product of fermentative glycolysis able to move across cell membranes in both directions via monocarboxylate transporting proteins (i.e. MCT1/4) contributing to cell-pH homeostasis but also to the complex immune response via acidosis of the tumour microenvironment. Importantly lactate is recycled in multiple organs as a major metabolic precursor of gluconeogenesis and energy source protecting cells and animals from harsh nutritional or oxygen restrictions. Third, we revisit the Warburg effect via CRISPR-Cas9 disruption of glucose-6-phosphate isomerase (GPI-KO) or lactate dehydrogenases (LDHA/B-DKO) in two aggressive tumours (melanoma B16-F10, human adenocarcinoma LS174T). Full suppression of lactic acid production reduces but does not suppress tumour growth due to reactivation of OXPHOS. In contrast, disruption of the lactic acid transporters MCT1/4 suppressed glycolysis, mTORC1, and tumour growth as a result of intracellular acidosis. Finally, we briefly discuss the current clinical developments of an MCT1 specific drug AZ3965, and the recent progress for a specific in vivo MCT4 inhibitor, two drugs of very high potential for future cancer clinical applications.

Pathogenic Mechanism of a Highly Virulent Infectious Hematopoietic Necrosis Virus in Head Kidney of Rainbow Trout (Oncorhynchus mykiss) Analyzed by RNA-Seq Transcriptome Profiling

Infectious hematopoietic necrosis virus (IHNV) is a pathogen that causes high rates of mortality in salmonid fishes. Therefore, an RNA-seq-based transcriptome analysis was performed in the head kidney of rainbow trout infected with a highly virulent IHNV strain to understand the pathogenesis of and defense strategies for IHNV infection in rainbow trout. The results showed that the numbers of DEGs were 618, 2626, and 774 (control vs. IHNV) on days 1, 3, and 5, respectively. Furthermore, the enrichment analysis of gene ontology (GO) annotations to classify DEGs showed that GO terms considerably associated with DEGs were gluconeogenesis, inflammatory response, and cell adhesion in the Biological Process (BP) category, apical plasma membrane, extracellular matrix (ECM) in the Cellular Component category, and transporter activity, integrin binding, and protein homodimerization activity in the Molecular Function category, on days 1, 3, and 5, respectively. Notably, GO terms in the BP category, including the negative regulation of type I interferon production and positive regulation of interleukin-1β secretion, were commonly identified at all time points. In the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, complement and coagulation cascades were commonly identified at all time points. Importantly, the widely recognized GO terms and KEGG pathways extensively linked to DEGs were related to energy metabolism on day 1, the immune response on day 3, and cell proliferation on day 5. Furthermore, protein–protein interaction networks and centrality analysis showed that the metabolism and signaling transduction pathways were majorly upregulated. Conclusively, the virulent IHNV infection drives pathogenesis by activating the metabolic energy pathway for energy use for viral replication, facilitating necrosis through autophagy, and causing a shutoff response of the host immune system through the downregulation of type I IFN at the initial stage of infection.

Characterization and function of mandarin fish c-Myc during viral infection process

c-Myc is a transcription factor and master regulator of cellular metabolism, and plays a critical role in virus replication by regulating glutamine metabolism. In this study, the open-reading frame (ORF) of c-Myc, designated as Sc-c-Myc, was cloned and sequenced. Multiple alignment of the amino acid sequence showed that the conserved domain of Sc-c-Myc, including the helix-loop-helix-zipper (bHLHzip) domain and Myc N-terminal region, shared high identities with other homologues from different species. Sc-c-Myc mRNA was widely expressed in the examined tissues of mandarin fish, and the higher mRNA levels was expressed in hind kidney. Moreover, mRNA and protein level of Sc-c-Myc was significantly increased in the Chinese perch brain (CPB) cells and spleen of mandarin fish post infection with infectious spleen and kidney necrosis virus (ISKNV) and Siniperca chuatsi rhabdovirus (SCRV). Sc-c-Myc overexpression promoted ISKNV and SCRV replication, on the contrary, knocking down Sc-c-Myc restrained ISKNV and SCRV replication. These results indicated that Sc-c-Myc involved in ISKNV and SCRV replication and proliferation, providing a potential target for the development of new therapic strategy against ISKNV and SCRV.

Fueling influenza and the immune response: Implications for metabolic reprogramming during influenza infection and immunometabolism

Recent studies support the notion that glycolysis and oxidative phosphorylation are rheostats in immune cells whose bioenergetics have functional outputs in terms of their biology. Specific intrinsic and extrinsic molecular factors function as molecular potentiometers to adjust and control glycolytic to respiratory power output. In many cases, these potentiometers are used by influenza viruses and immune cells to support pathogenesis and the host immune response, respectively. Influenza virus infects the respiratory tract, providing a specific environmental niche, while immune cells encounter variable nutrient concentrations as they migrate in response to infection. Immune cell subsets have distinct metabolic programs that adjust to meet energetic and biosynthetic requirements to support effector functions, differentiation, and longevity in their ever-changing microenvironments. This review details how influenza coopts the host cell for metabolic reprogramming and describes the overlap of these regulatory controls in immune cells whose function and fate are dictated by metabolism. These details are contextualized with emerging evidence of the consequences of influenza-induced changes in metabolic homeostasis on disease progression.

Glutamine addiction in virus-infected mammalian cells: A target of the innate immune system?

Control of core cell metabolism is a key aspect of the evolutionary conflict between viruses and the host's defence mechanisms. From their side, the invading viruses press the accelerator on their host cell's glycolysis, fatty acid, and glutaminolytic metabolic processes among others. It is also well established that activation of innate immune system responses modulates facets of metabolism such as that of polyamine, cholesterol, tryptophan and many more. But what about glutamine, a proteogenic amino acid that is a crucial nutrient for multiple cellular biosynthetic processes? Although mammalian cells can normally synthesize glutamine de novo, it has been noted that infections with genetically and phylogenetically diverse viruses are followed by the acquisition of a dependency on supplies of exogenous glutamine i.e. "glutamine addiction". Here we present our novel hypothesis that glutamine metabolism is also a target of the innate immune system, possibly through the action of interferons, as part of the evolutionary conserved antiviral metabolic reprogramming.

Host cell glutamine metabolism as a potential antiviral target

A virus minimally contains a nucleic acid genome packaged by a protein coat. The genome and capsid together are known as the nucleocapsid, which has an envelope containing a lipid bilayer (mainly phospholipids) originating from host cell membranes. The viral envelope has transmembrane proteins that are usually glycoproteins. The proteins in the envelope bind to host cell receptors, promoting membrane fusion and viral entry into the cell. Virus-infected host cells exhibit marked increases in glutamine utilization and metabolism. Glutamine metabolism generates ATP and precursors for the synthesis of macromolecules to assemble progeny viruses. Some compounds derived from glutamine are used in the synthesis of purines and pyrimidines. These latter compounds are precursors for the synthesis of nucleotides. Inhibitors of glutamine transport and metabolism are potential candidate antiviral drugs. Glutamine is also an essential nutrient for the functions of leukocytes (lymphocyte, macrophage, and neutrophil), including those in virus-infected patients. The increased glutamine requirement for immune cell functions occurs concomitantly with the high glutamine utilization by host cells in virus-infected patients. The development of antiviral drugs that target glutamine metabolism must then be specifically directed at virus-infected host cells to avoid negative effects on immune functions. Therefore, the aim of this review was to describe the landscape of cellular glutamine metabolism to search for potential candidates to inhibit glutamine transport or glutamine metabolism.

SARS-CoV-2 and Glutamine: SARS-CoV-2 Triggered Pathogenesis via Metabolic Reprograming of Glutamine in Host Cells

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, as coronavirus disease 2019 (COVID-19) pandemic, has killed more than a million people worldwide, and researchers are constantly working to develop therapeutics in the treatment and prevention of this new viral infection. To infect and induced pathogenesis as observed in other viral infections, we postulated that SARS-CoV-2 may also require an escalation in the anabolic metabolism, such as glucose and glutamine, to support its energy and biosynthetic requirements during the infection cycle. Recently, the requirement of altered glucose metabolism in SARS-CoV-2 pathogenesis was demonstrated, but the role of dysregulated glutamine metabolism is not yet mentioned for its infection. In this perspective, we have attempted to provide a summary of possible biochemical events on putative metabolic reprograming of glutamine in host cells upon SARS-CoV-2 infection by comparison to other viral infections/cancer metabolism and available clinical data or research on SARS-CoV-2 pathogenesis. This systematic hypothesis concluded the vital role of glutaminase-1 (GLS1), phosphoserine aminotransferase (PSAT1), hypoxia-inducible factor-1 alpha (HIF-1α), mammalian target of rapamycin complex 1 (mTORC1), glutamine-fructose amidotransferase 1/2 (GFAT1/2), and transcription factor Myc as key cellular factors to mediate and promote the glutamine metabolic reprogramming in SARS-CoV-2 infected cells. In absence of concrete data available for SARS-CoV-2 induced metabolic reprogramming of glutamine, this study efforts to connect the gaps with available clinical shreds of evidence in SARS-CoV-2 infection with altered glutamine metabolism and hopefully could be beneficial in the designing of strategic methods for therapeutic development with elucidation using in vitro or in vivo approaches.

The role of cancer metabolism in defining the success of oncolytic viro-immunotherapy

Oncolytic viruses infect, replicate in, and kill cancer cells selectively without harming normal cells. The rapidly expanding clinical development of oncolytic virotherapy is an exciting interdisciplinary field that provides insights into virology, oncology, and immunotherapy. Recent years have seen greater focus on rational design of cancer-selective viruses together with strategies to exploit their immunostimulatory capabilities, ultimately to develop powerful oncolytic cancer vaccines. However, despite great interest in the field, many important experiments are still conducted under optimum conditions in vitro, with many nutrients present in excess and with cellular stress kept to a minimum. Whilst this provides a convenient platform for cell culture, it bears little relation to the typical conditions found within a tumour in vivo, where cells are often subject to a range of metabolic and environmental stresses. Viral infection and cancer will both lead to production of metabolites that are also not present in media in vitro. Understanding how oncolytic viruses interact with cells exposed to more representative metabolic conditions in vitro represents an under-explored area of study that could provide valuable insight into the intelligent design of superior oncolytic viruses and help bridge the gap between bench and bedside. This review summarises the major metabolic pathways altered in cancer cells, during viral infection and highlights possible targets for future studies.

Host Mitochondrial Requirements of Cytomegalovirus Replication

Purpose of Review

Metabolic rewiring of the host cell is required for optimal viral replication. Human cytomegalovirus (HCMV) has been observed to manipulate numerous mitochondrial functions. In this review, we describe the strategies and targets HCMV uses to control different aspects of mitochondrial function.

Recent Findings

The mitochondria are instrumental in meeting the biosynthetic and bioenergetic needs of HCMV replication. This is achieved through altered metabolism and signaling pathways. Morphological changes mediated through biogenesis and fission/fusion dynamics contribute to strategies to avoid cell death, overcome oxidative stress, and maximize the biosynthetic and bioenergetic outputs of mitochondria.

Summary

Emerging data suggests that cytomegalovirus relies on intact, functional host mitochondria for optimal replication. HCMV large size and slow replication kinetics create a dependency on mitochondria during replication. Targeting the host mitochondria is an attractive antiviral target.

Complexity and ultrastructure of infectious extracellular vesicles from cells infected by non-enveloped virus

Enteroviruses support cell-to-cell viral transmission prior to their canonical lytic spread of virus. Poliovirus (PV), a prototype for human pathogenic positive-sense RNA enteroviruses, and picornaviruses in general, transport multiple virions en bloc via infectious extracellular vesicles, 100~1000 nm in diameter, secreted from host cells. Using biochemical and biophysical methods we identify multiple components in secreted microvesicles, including mature PV virions; positive-sense genomic and negative-sense replicative, template viral RNA; essential viral replication proteins; and cellular proteins. Using cryo-electron tomography, we visualize the near-native three-dimensional architecture of secreted infectious microvesicles containing both virions and a unique morphological component that we describe as a mat-like structure. While the composition of these mat-like structures is not yet known, based on our biochemical data they are expected to be comprised of unencapsidated RNA and proteins. In addition to infectious microvesicles, CD9-positive exosomes released from PV-infected cells are also infectious and transport virions. Thus, our data show that, prior to cell lysis, non-enveloped viruses are secreted within infectious vesicles that also transport viral unencapsidated RNAs, viral and host proteins. Understanding the structure and function of these infectious particles helps elucidate the mechanism by which extracellular vesicles contribute to the spread of non-enveloped virus infection.

Metabolomic Analysis of Cricket paralysis virus Infection in Drosophila S2 Cells Reveals Divergent Effects on Central Carbon Metabolism as Compared with Silkworm Bm5 Cells

High-throughput approaches have opened new opportunities for understanding biological processes such as persistent virus infections, which are widespread. However, the potential of persistent infections to develop towards pathogenesis remains to be investigated, particularly with respect to the role of host metabolism. To explore the interactions between cellular metabolism and persistent/pathogenic virus infection, we performed untargeted and targeted metabolomic analysis to examine the effects of Cricket paralysis virus (CrPV, Dicistroviridae) in persistently infected silkworm Bm5 cells and acutely infected Drosophila S2 cells. Our previous study (Viruses 2019, 11, 861) established that both glucose and glutamine levels significantly increased during the persistent period of CrPV infection of Bm5 cells, while they decreased steeply during the pathogenic stages. Strikingly, in this study, an almost opposite pattern in change of metabolites was observed during different stages of acute infection of S2 cells. More specifically, a significant decrease in amino acids and carbohydrates was observed prior to pathogenesis, while their abundance significantly increased again during pathogenesis. Our study illustrates the occurrence of diametrically opposite changes in central carbon mechanisms during CrPV infection of S2 and Bm5 cells that is possibly related to the type of infection (acute or persistent) that is triggered by the virus.

Metabolic host response and therapeutic approaches to influenza infection

Based on available metabolomic studies, influenza infection affects a variety of cellular metabolic pathways to ensure an optimal environment for its replication and production of viral particles. Following infection, glucose uptake and aerobic glycolysis increase in infected cells continually, which results in higher glucose consumption. The pentose phosphate shunt, as another glucose-consuming pathway, is enhanced by influenza infection to help produce more nucleotides, especially ATP. Regarding lipid species, following infection, levels of triglycerides, phospholipids, and several lipid derivatives undergo perturbations, some of which are associated with inflammatory responses. Also, mitochondrial fatty acid β-oxidation decreases significantly simultaneously with an increase in biosynthesis of fatty acids and membrane lipids. Moreover, essential amino acids are demonstrated to decline in infected tissues due to the production of large amounts of viral and cellular proteins. Immune responses against influenza infection, on the other hand, could significantly affect metabolic pathways. Mainly, interferon (IFN) production following viral infection affects cell function via alteration in amino acid synthesis, membrane composition, and lipid metabolism. Understanding metabolic alterations required for influenza virus replication has revealed novel therapeutic methods based on targeted inhibition of these cellular metabolic pathways.

Accelerated Metabolite Levels of Aerobic Glycolysis and the Pentose Phosphate Pathway Are Required for Efficient Replication of Infectious Spleen and Kidney Necrosis Virus in Chinese Perch Brain Cells

Glucose is a main carbon and energy source for virus proliferation and is usually involved in the glycolysis, pentose phosphate pathway (PPP), and tricarboxylic acid cycle (TCA cycle) pathways. In this study, we investigated the roles of glucose-related metabolic pathways during the replication of infectious spleen and kidney necrosis virus (ISKNV), which has caused serious economic losses in the cultured Chinese perch (Siniperca chuatsi) industry. We found that ISKNV infection enhanced the metabolic pathways of the PPP and the TCA cycle at the early stage of the ISKNV infection cycle and enhanced the glycolysis pathway at the late stage of the ISKNV infection cycle though the comprehensive analysis of transcriptomics, proteomics, and metabolomics. The advanced results proved that ISKNV replication induced upregulation of aerobic glycolysis at the late stage of ISKNV infection cycle and aerobic glycolysis were required for ISKNV multiplication. In addition, the PPP, providing nucleotide biosynthesis, was also required for ISKNV multiplication. However, the TCA cycle involving glucose was not important and necessary for ISKNV multiplication. The results reported here provide new insights into viral pathogenesis mechanism of metabolic shift, as well as antiviral treatment strategies.

Hijacking the Supplies: Metabolism as a Novel Facet of Virus-Host Interaction

Viral replication is a process that involves an extremely high turnover of cellular molecules. Since viruses depend on the host cell to obtain the macromolecules needed for their proper replication, they have evolved numerous strategies to shape cellular metabolism and the biosynthesis machinery of the host according to their specific needs. Technologies for the rigorous analysis of metabolic alterations in cells have recently become widely available and have greatly expanded our knowledge of these crucial host–pathogen interactions. We have learned that most viruses enhance specific anabolic pathways and are highly dependent on these alterations. Since uninfected cells are far more plastic in their metabolism, targeting of the virus-induced metabolic alterations is a promising strategy for specific antiviral therapy and has gained great interest recently. In this review, we summarize the current advances in our understanding of metabolic adaptations during viral infections, with a particular focus on the utilization of this information for therapeutic application.

Investigation of cell culture conditions for optimal foot-and-mouth disease virus production

Background

Foot-and-mouth disease is a highly contagious and economically devastating disease with endemic occurrence in many parts of the world. Vaccination is the method of choice to eradicate the disease and to limit the viral spread. The vaccine production process is based on mammalian cell culture, in which the viral yield varies in dependence of the composition of the culture media. For foot-and-mouth disease virus (FMDV), very little is known about the culture media components that are necessary to grow the virus to high titers in cell culture.

Results

This study examined the influence of increasing concentrations of glucose, glutamine, ammonium chloride and different cell densities on the yield of FMDV. While an excess of glucose or glutamine does not affect the viral yield, increasing cell density reduces the viral titer by a log₁₀ step at a cell density of 3 × 10⁶ cells/mL. This can be mitigated by performing a 100% media exchange before infection of the cells.

Conclusions

The reasons for the diminished viral growth, if no complete media exchange has been performed prior to infection, remain unclear and further studies are necessary to investigate the causes more deeply. For now, the results argue for a vaccine production process with 100% media exchange to reliably obtain high viral titers.

Electronic supplementary material

The online version of this article (10.1186/s12896-019-0527-5) contains supplementary material, which is available to authorized users.

Viral Manipulation of the Host Metabolic Network

Viruses are intracellular parasites that rely on host machinery to replicate and achieve a successful infection. Viruses have evolved to retain a broad range of strategies to manipulate host cell metabolism and metabolic resources, channeling them toward the production of virion components leading to viral production. Although several viruses share similar strategies for manipulating host cell metabolism, these processes depend on several factors, namely, the viral life cycle and the metabolic and energetic status of the infected cell. Based on this knowledge, the development of new therapeutic approaches that circumvent viral spread through the target of altered metabolic pathways is an opportunity to tackle the infection. However, finding effective broad-spectrum strategies that aim at restoring to homeostasis the metabolic alterations induced upon virus infection is still a Holy Grail quest for antiviral therapies. Here, we review the strategies by which viruses manipulate host metabolism for their own benefit, with a particular emphasis on carbohydrate, glutamine, and lipid metabolism.

Antagonism of Glycolysis and Reductive Carboxylation of Glutamine Potentiates Activity of Oncolytic Adenoviruses in Cancer Cells

Tumor cells exhibiting the Warburg effect rely on aerobic glycolysis for ATP production and have a notable addiction to anaplerotic use of glutamine for macromolecular synthesis. This strategy maximizes cellular biosynthetic potential while avoiding excessive depletion of NAD+ and provides an attractive anabolic environment for viral infection. Here, we evaluate infection of highly permissive and poorly permissive cancer cells with wild-type adenoviruses and the oncolytic chimeric adenovirus enadenotucirev (EnAd). All adenoviruses caused an increase in glucose and glutamine uptake along with increased lactic acid secretion. Counterintuitively, restricting glycolysis using 2-deoxyglucose or by limiting glucose supply strongly improved virus activity in both cell types. Antagonism of glycolysis also boosted EnAd replication and transgene expression within human tumor biopsies and in xenografted tumors in vivo. In contrast, the virus life cycle was critically dependent on exogenous glutamine. Virus activity in glutamine-free cells was rescued with exogenous membrane-permeable α-ketoglutarate, but not pyruvate or oxaloacetate, suggesting an important role for reductive carboxylation in glutamine usage, perhaps for production of biosynthetic intermediates. This overlap between the metabolic phenotypes of adenovirus infection and transformed tumor cells may provide insight into how oncolytic adenoviruses exploit metabolic transformation to augment their selectivity for cancer cells. SIGNIFICANCE: This study describes changes in glucose and glutamine metabolism induced by oncolytic and wild-type adenoviruses in cancer cells, which will be important to consider in the preclinical evaluation of oncolytic viruses.

Interferons: Reprogramming the Metabolic Network against Viral Infection

Viruses exploit the host and induce drastic metabolic changes to ensure an optimal environment for replication and the production of viral progenies. In response, the host has developed diverse countermeasures to sense and limit these alterations to combat viral infection. One such host mechanism is through interferon signaling. Interferons are cytokines that enhances the transcription of hundreds of interferon-stimulated genes (ISGs) whose products are key players in the innate immune response to viral infection. In addition to their direct targeting of viral components, interferons and ISGs exert profound effects on cellular metabolism. Recent studies have started to illuminate on the specific role of interferon in rewiring cellular metabolism to activate immune cells and limit viral infection. This review reflects on our current understanding of the complex networking that occurs between the virus and host at the interface of cellular metabolism, with a focus on the ISGs in particular, cholesterol-25-hydroxylase (CH25H), spermidine/spermine acetyltransferase 1 (SAT1), indoleamine-2,3-dioxygenase (IDO1) and sterile alpha motif and histidine/aspartic acid domain-containing protein 1 (SAMHD1), which were recently discovered to modulate specific metabolic events and consequently deter viral infection.

Latent KSHV Infected Endothelial Cells Are Glutamine Addicted and Require Glutaminolysis for Survival

Kaposi’s Sarcoma-associated Herpesvirus (KSHV) is the etiologic agent of Kaposi’s Sarcoma (KS). KSHV establishes a predominantly latent infection in the main KS tumor cell type, the spindle cell, which is of endothelial cell origin. KSHV requires the induction of multiple metabolic pathways, including glycolysis and fatty acid synthesis, for the survival of latently infected endothelial cells. Here we demonstrate that latent KSHV infection leads to increased levels of intracellular glutamine and enhanced glutamine uptake. Depletion of glutamine from the culture media leads to a significant increase in apoptotic cell death in latently infected endothelial cells, but not in their mock-infected counterparts. In cancer cells, glutamine is often required for glutaminolysis to provide intermediates for the tri-carboxylic acid (TCA) cycle and support for the production of biosynthetic and bioenergetic precursors. In the absence of glutamine, the TCA cycle intermediates alpha-ketoglutarate (αKG) and pyruvate prevent the death of latently infected cells. Targeted drug inhibition of glutaminolysis also induces increased cell death in latently infected cells. KSHV infection of endothelial cells induces protein expression of the glutamine transporter, SLC1A5. Chemical inhibition of SLC1A5, or knockdown by siRNA, leads to similar cell death rates as glutamine deprivation and, similarly, can be rescued by αKG. KSHV also induces expression of the heterodimeric transcription factors c-Myc-Max and related heterodimer MondoA-Mlx. Knockdown of MondoA inhibits expression of both Mlx and SLC1A5 and induces a significant increase in cell death of only cells latently infected with KSHV, again, fully rescued by the supplementation of αKG. Therefore, during latent infection of endothelial cells, KSHV activates and requires the Myc/MondoA-network to upregulate the glutamine transporter, SLC1A5, leading to increased glutamine uptake for glutaminolysis. These findings expand our understanding of the required metabolic pathways that are activated during latent KSHV infection of endothelial cells, and demonstrate a novel role for the extended Myc-regulatory network, specifically MondoA, during latent KSHV infection.

KSHV is the etiologic agent of KS, the most common tumor of AIDS patients worldwide. Currently, there are no therapeutics available to directly treat latent KSHV infection. This study reveals that latent KSHV infection induces endothelial cells to become glutamine addicted, similarly to cancer cells. Extracellular glutamine is required to feed the TCA cycle through glutaminolysis, a process called anaplerosis. KSHV induces protein expression of the glutamine transporter SLC1A5 and SLC1A5 expression is required for the survival of latently infected cells. KSHV also induces the expression of the proto-oncogene Myc and its binding partner Max, as well as, the nutrient-sensing transcription factor, MondoA and its binding partner Mlx. MondoA regulates SLC1A5 and glutaminolysis during latent KSHV infection, and its expression is required for the survival of latently infected endothelial cells. These studies show that glutaminolysis and a single glutamine transporter, under the regulation of MondoA, are required for the survival of latently infected cells, providing novel druggable targets for latently infected endothelial cells. This work supports that a cancer-like metabolic signature is established by latent KSHV infection, opening the door to further therapeutic targeting specifically of KSHV latently infected cells.

Viral activation of cellular metabolism

To ensure optimal environments for their replication and spread, viruses have evolved to alter many host cell pathways. In the last decade, metabolomic studies have shown that eukaryotic viruses induce large-scale alterations in host cellular metabolism. Most viruses examined to date induce aerobic glycolysis also known as the Warburg effect. Many viruses tested also induce fatty acid synthesis as well as glutaminolysis. These modifications of carbon source utilization by infected cells can increase available energy for virus replication and virion production, provide specific cellular substrates for virus particles and create viral replication niches while increasing infected cell survival. Each virus species also likely requires unique metabolic changes for successful spread and recent research has identified additional virus-specific metabolic changes induced by many virus species. A better understanding of the metabolic alterations required for the replication of each virus may lead to novel therapeutic approaches through targeted inhibition of specific cellular metabolic pathways.

Vaccinia virus requires glutamine but not glucose for efficient replication

Viruses require host cell metabolism to provide the necessary energy and biosynthetic precursors for successful viral replication. Vaccinia virus (VACV) is a member of the Poxviridae family, and its use as a vaccine enabled the eradication of variola virus, the etiologic agent of smallpox. A global metabolic screen of VACV-infected primary human foreskin fibroblasts suggested that glutamine metabolism is altered during infection. Glutamine and glucose represent the two main carbon sources for mammalian cells. Depriving VACV-infected cells of exogenous glutamine led to a substantial decrease in infectious virus production, whereas starving infected cells of exogenous glucose had no significant impact on replication. Viral yield in glutamine-deprived cells or in cells treated with an inhibitor of glutaminolysis, the pathway of glutamine catabolism, could be rescued by the addition of multiple tricarboxylic acid (TCA) cycle intermediates. Thus, VACV infection induces a metabolic alteration to fully rely on glutamine to anaplerotically maintain the TCA cycle. VACV protein synthesis, but not viral transcription, was decreased in glutamine-deprived cells, which corresponded with a dramatic reduction in all VACV morphogenetic intermediates. This study reveals the unique carbon utilization program implemented during poxvirus infection and provides a potential metabolic pathway to target viral replication.

IMPORTANCE

Viruses are dependent on the metabolic machinery of the host cell to supply the energy and molecular building blocks needed for critical processes including genome replication, viral protein synthesis, and membrane production. This study investigates how vaccinia virus (VACV) infection alters global cellular metabolism, providing the first metabolomic analysis for a member of the poxvirus family. Unlike most viruses examined to date, VACV does not activate glycolysis, and exogenous glucose is not required for maximal virus production. Instead, VACV requires exogenous glutamine for efficient replication, and inhibition of glutamine metabolism effectively blocks VACV protein synthesis. This study defines a major metabolic perturbation essential for the replication of a poxvirus and may lead to the discovery of novel antiviral therapies based on metabolic inhibitors.

Joys and surprises of a career studying eukaryotic gene expression

In this Reflections, I review a few early and very lucky events that gave me a running start for the rest of a long and wonderfully enjoyable career. For the main part, a discussion is provided of what I recall as the main illuminating results that my many dozens of students and postdoctoral fellows (approximately 140 in all) provided to our biochemical/molecular biological world.

Latent viral infection of cells in tissue culture. VI. Role of amino acids, glutamine, and glucose in psittacosis virus propagation in L cells

Mouse fibroblasts (L cells) fail to support the growth of psittacosis virus (6BC strain) if they are maintained on a medium containing only inorganic salts and glucose for 2 days prior to infection. Virus propagation can be stimulated by the addition of a synthetic medium containing amino acids, water-soluble vitamins, glutamine, glucose, and inorganic salts. By omitting single amino acids from the complete synthetic medium, tyrosine, threonine, methionine, isoleucine, phenylalanine, tryptophan, leucine, valine, and cysteine or cystine were found to be essential for stimulation, while lysine, arginine, histidine, hydroxyproline, proline, glutamic acid, aspartic acid, serine, alanine, and glycine were not essential. The cells on deficient media showed varying degrees of degenerative changes, but there was little correlation between ability to support psittacosis virus growth and morphologic condition of the cells. Glucose is also an essential component of the medium for viral growth, but the absence of glutamine had no effect on stimulation of virus propagation. L cell cultures maintained on media deficient in phenylalanine or tryptophan for 2 days before infection were also found to be incapable of supporting virus growth. The implications of this study in latent viral infections are discussed.

Nutritional requirements for the production of herpes simplex virus. I. Influence of glucose and glutamine of herpes simplex virus production by HeLa cells

Lewis, Vester J., Jr. (University of Oklahoma School of Medicine, Oklahoma City) and L. Vernon Scott. Nutritional requirements for the production of herpes simplex virus. I. Influence of glucose and glutamine on herpes simplex virus production by HeLa cells. J. Bacteriol. 83:475-482. 1962.-The importance of glucose and glutamine in herpes simplex virus production by HeLa cells was evaluated by variation of the composition of Eagle's basal medium. Simultaneous omission of glucose and glutamine from the serum-containing medium resulted in marked curtailment of viral synthesis. The effect was attributable neither to decreased survival time of infectious particles or of cells, nor to decreased rate of viral adsorption or penetration in the presence of the deficient medium. Therefore, the effect was probably on intracellular viral replication.When the requirements for glucose and glutamine were determined separately, it was found that viral production was reduced in glucose-deficient medium. In contrast, a medium which was complete except for restriction of the glutamine content to that occurring naturally in the serum component supported significantly greater viral multiplication than did the complete medium. Limited viral replication occurred with medium which lacked both glutamine and serum. Viral multiplication in the presence of serum-free medium was stimulated by the addition of glutamine.

AMINO ACID REQUIREMENTS OF HERPES SIMPLEX VIRUS IN HUMAN CELLS

Tankersley, Robert W., Jr. (Medical College of Virginia, Richmond). Amino acid requirements of herpes simplex virus in human cells. J. Bacteriol. 87:609-613. 1964.-Progressive infection of human cells minimally infected with herpes simplex virus requires 11 of the amino acids of Eagle's medium, and glutamine. Lysine is not required, but rather exerts a partially inhibitory effect upon virus multiplication. Infected cells deprived of arginine support neither cytopathogenic effects nor virus replication; when arginine is replaced, a prompt and extensive infection follows. The effect of nutritional deficiencies on virus infection is discussed.

METABOLIC CONTROLS IN CULTURED MAMMALIAN CELLS

A number of apparently unrelated factors are known to have a profound effect on the metabolism of cultured mammalian cells; and some of these may be operative as metabolic controls in the whole animal as well. The more complete exploration of (i) homotypic and heterotypic cellular interactions, (ii) the spontaneous transformations sometimes observed in cultured cells, (iii) the mode of action of cytotoxic agents, (iv) the multiple metabolic effects of viral infection, and (v) the conditions necessary for the maintenance of specialized function in cultured cells, can be expected to throw light on the basic mechanisms underlying such complex processes as differentiation, senescence, and cancer.

The major oligosaccharides in the large subunit of the hemagglutinin from fowl plague virus, strain Dutch. Structure elucidation by one-dimensional and two-dimensional 1H nuclear magnetic resonance and by methylation analysis

The N-glycosidically linked glycans in the large subunit (HA1) of the hemagglutinin from fowl plague virus, strain Dutch (containing about 15%, w/w, of carbohydrates), were liberated by alkaline hydrolysis, and were filtrated through Bio-Gel as the re-N-acetylated oligosaccharide alditols. One major fraction (90%, mol/mol) was obtained. It was subfractionated by concanavalin A affinity chromatography and was analyzed by methylation/capillary gas chromatography/mass fragmentography and especially by one-dimensional and two-dimensional 1H nuclear magnetic resonance. The major HA1 glycans, which are not sialylated, were thus found to comprise about 40%, 30% and 20% (mol/mol), respectively, of biantennary intersected, biantennary, and triantennary N-acetyllactosaminic ('complex') oligosaccharides. About two thirds of the internal GlcNAc residues in these glycans are substituted by Fuc(alpha 1----6), all the triantennary species carry the third Gal(beta 1----4)GlcNAc(beta 1----unit at the Man(alpha 1----6)-branch, and roughly one fourth of the N-acetyllactosamine units in the non-intersected biantennary oligosaccharides are incomplete.

Inhibition of glycosylation of the influenza virus hemagglutinin

d-Glucosamine and 2-deoxy-d-glucose interfere with the biosynthesis of the hemagglutinin glycoproteins. With increasing inhibitor concentrations a progressive decrease in size of the precursor HA and the cleavage products, HA(1) and HA(2) can be observed. The shift in molecular weight is paralleled by a decrease of the carbohydrate content. This was shown by labeling studies with radioactive sugars which revealed that the inhibitors block the incorporation into glycoproteins, whereas they have no or only slight effects on the uptake and activation of sugars. Under conditions of maximal inhibition, the hemagglutinin proteins lack all or most of their carbohydrates. These findings indicate that the inhibitory effect of d-glucosamine and 2-deoxy-d-glucose is due to an impairment of glycosylation. When glycosylation is inhibited, the precursor polypeptide is synthesized at normal rates. Its cleavage products, however, are very heterogeneous. This suggests that carbohydrate protects the hemagglutinin from proteolytic degradation.

Formation of influenza virus proteins

Eight virus-specific proteins have been found in chicken embryo fibroblasts infected with fowl plague virus. Among them are two glycoproteins which are the constituents of the hemagglutinin on the virus particle. They are derived from a large precursor glycoprotein by cleavage of a covalent linkage. The reaction can be blocked by the protease inhibitor diisopropylfluorophosphate and the amino acid analogue fluorophenylalanine. This indicates that a peptide bond is cleaved. If infected cells are kept at 25 C, a temperature at which virus maturation is inhibited, the precursor glycoprotein is cleaved at a significantly slower rate than at 37 C. It appears, however, that a reduced synthesis of the carbohydrate-free envelope protein is responsible for the block of virus maturation at 25 C rather than the lower cleavage rate of the precursor.