Tick-borne encephalitis virus inhibits rRNA synthesis and host protein production in human cells of neural origin

The immune response to RNA suppresses nucleic acid synthesis by limiting ribose 5-phosphate

During infection viruses hijack host cell metabolism to promote their replication. Here, analysis of metabolite alterations in macrophages exposed to poly I:C recognises that the antiviral effector Protein Kinase RNA-activated (PKR) suppresses glucose breakdown within the pentose phosphate pathway (PPP). This pathway runs parallel to central glycolysis and is critical to producing NADPH and pentose precursors for nucleotides. Changes in metabolite levels between wild-type and PKR-ablated macrophages show that PKR controls the generation of ribose 5-phosphate, in a manner distinct from its established function in gene expression but dependent on its kinase activity. PKR phosphorylates and inhibits the Ribose 5-Phosphate Isomerase A (RPIA), thereby preventing interconversion of ribulose- to ribose 5-phosphate. This activity preserves redox control but decreases production of ribose 5-phosphate for nucleotide biosynthesis. Accordingly, the PKR-mediated immune response to RNA suppresses nucleic acid production. In line, pharmacological targeting of the PPP during infection decreases the replication of the Herpes simplex virus. These results identify an immune response-mediated control of host cell metabolism and suggest targeting the RPIA as a potential innovative antiviral treatment.

Cellular stress is triggered by tick-borne encephalitis virus and limits the virus replication in PMJ2-R mouse macrophage cell line

Viral infection may represent a stress condition to the host cell. Cells react to it by triggering the defence programme to restore homeostasis and these events may in turn impact the viral replication. The knowledge about tick-borne encephalitis virus (TBEV) infection-associated stress is limited. Here we investigated the interplay between TBEV infection and stress pathways in PMJ2-R mouse macrophage cell line, as macrophages are the target cells in early phases of TBEV infection. First, to determine how stress influences TBEV replication, the effect of stress inducers H2O2 and tunicamycin (TM) was tested. Viral multiplication was decreased in the presence of both stress inducers suggesting that the stress and cellular stress responses restrict the virus replication. Second, we investigated the induction of oxidative stress and endoplasmic reticulum (ER) stress upon TBEV infection. The level of oxidative stress was interrogated by measuring the reactive oxygen species (ROS). ROS were intermittently increased in infected cells at 12 hpi and at 72 hpi. As mitochondrial dysfunction may result in increased ROS level, we evaluated the mitochondrial homeostasis by measuring the mitochondrial membrane potential (MMP) and found that TBEV infection induced the hyperpolarization of MMP. Moreover, a transient increase of gene expression of stress-induced antioxidative enzymes, like p62, Gclm and Hmox1, was detected. Next, we evaluated the ER stress upon TBEV infection by analysing unfolded protein responses (UPR). We found that infection induced gene expression of two general sensors BiP and CHOP and activated the IRE1 pathway of UPR. Finally, since the natural transmission route of TBEV from its tick vector to the host is mediated via tick saliva, the impact of tick saliva from Ixodes ricinus on stress pathways in TBEV-infected cells was tested. We observed only marginal potentiation of UPR pathway. In conclusion, we found that TBEV infection of PMJ2-R cells elicits the changes in redox balance and triggers cellular stress defences, including antioxidant responses and the IRE1 pathway of UPR. Importantly, our results revealed the negative effect of stress-evoked events on TBEV replication and only marginal impact of tick saliva on stress cellular pathways.

Rescue and in vitro characterization of a divergent TBEV-Eu strain from the Netherlands

Tick-borne encephalitis virus (TBEV) may cause tick-borne encephalitis (TBE), a potential life-threatening infection of the central nervous system in humans. Phylogenetically, TBEVs can be subdivided into three main subtypes, which differ in endemic region and pathogenic potential. In 2016, TBEV was first detected in the Netherlands. One of two detected strains, referred to as Salland, belonged to the TBEV-Eu subtype, yet diverged ≥ 2% on amino acid level from other members of this subtype. Here, we report the successful rescue of this strain using infectious subgenomic amplicons and its subsequent in vitro characterization by comparison to two well-characterized TBEV-Eu strains; Neudoerfl and Hypr. In the human alveolar epithelial cell line A549, growth kinetics of Salland were comparable to the high pathogenicity TBEV-Eu strain Hypr, and both strains grew considerably faster than the mildly pathogenic strain Neudoerfl. In the human neuroblastoma cell line SK-N-SH, Salland replicated faster and to higher infectious titers than both reference strains. All three TBEV strains infected primary human monocyte-derived dendritic cells to a similar extent and interacted with the type I interferon system in a similar manner. The current study serves as the first in vitro characterization of the novel, divergent TBEV-Eu strain Salland.

Tick-borne encephalitis virus capsid protein induces translational shut-off as revealed by its structural-biological analysis

Tick-borne encephalitis virus (TBEV) is the most medically relevant tick-transmitted Flavivirus in Eurasia, targeting the host central nervous system and frequently causing severe encephalitis. The primary function of its capsid protein (TBEVC) is to recruit the viral RNA and form a nucleocapsid. Additional functionality of Flavivirus capsid proteins has been documented, but further investigation is needed for TBEVC. Here, we show the first capsid protein 3D structure of a member of the tick-borne flaviviruses group. The structure of monomeric Δ16-TBEVC was determined using high-resolution multidimensional NMR spectroscopy. Based on natural in vitro TBEVC homodimerization, the dimeric interfaces were identified by hydrogen deuterium exchange mass spectrometry (MS). Although the assembly of flaviviruses occurs in endoplasmic reticulum-derived vesicles, we observed that TBEVC protein also accumulated in the nuclei and nucleoli of infected cells. In addition, the predicted bipartite nuclear localization sequence in the TBEVC C-terminal part was confirmed experimentally, and we described the interface between TBEVC bipartite nuclear localization sequence and import adapter protein importin-alpha using X-ray crystallography. Furthermore, our coimmunoprecipitation coupled with MS identification revealed 214 interaction partners of TBEVC, including viral envelope and nonstructural NS5 proteins and a wide variety of host proteins involved mainly in rRNA processing and translation initiation. Metabolic labeling experiments further confirmed that TBEVC and other flaviviral capsid proteins are able to induce translational shutoff and decrease of 18S rRNA. These findings may substantially help to design a targeted therapy against TBEV.

Integrative RNA profiling of TBEV-infected neurons and astrocytes reveals potential pathogenic effectors

Tick-borne encephalitis virus (TBEV), the most medically relevant tick-transmitted flavivirus in Eurasia, targets the host central nervous system and frequently causes severe encephalitis. The severity of TBEV-induced neuropathogenesis is highly cell-type specific and the exact mechanism responsible for such differences has not been fully described yet. Thus, we performed a comprehensive analysis of alterations in host poly-(A)/miRNA/lncRNA expression upon TBEV infection in vitro in human primary neurons (high cytopathic effect) and astrocytes (low cytopathic effect). Infection with severe but not mild TBEV strain resulted in a high neuronal death rate. In comparison, infection with either of TBEV strains in human astrocytes did not. Differential expression and splicing analyses with an in silico prediction of miRNA/mRNA/lncRNA/vd-sRNA networks found significant changes in inflammatory and immune response pathways, nervous system development and regulation of mitosis in TBEV Hypr-infected neurons. Candidate mechanisms responsible for the aforementioned phenomena include specific regulation of host mRNA levels via differentially expressed miRNAs/lncRNAs or vd-sRNAs mimicking endogenous miRNAs and virus-driven modulation of host pre-mRNA splicing. We suggest that these factors are responsible for the observed differences in the virulence manifestation of both TBEV strains in different cell lines. This work brings the first complex overview of alterations in the transcriptome of human astrocytes and neurons during the infection by two TBEV strains of different virulence. The resulting data could serve as a starting point for further studies dealing with the mechanism of TBEV-host interactions and the related processes of TBEV pathogenesis.

History of Arbovirus Research in the Czech Republic

The aim of this review is to follow the history of studies on endemiv arboviruses and the diseases they cause which were detected in the Czech lands (Bohemia, Moravia and Silesia (i.e., the Czech Republic)). The viruses involve tick-borne encephalitis, West Nile and Usutu flaviviruses; the Sindbis alphavirus; Ťahyňa, Batai, Lednice and Sedlec bunyaviruses; the Uukuniemi phlebovirus; and the Tribeč orbivirus. Arboviruses temporarily imported from abroad to the Czech Republic have been omitted. This brief historical review includes a bibliography of all relevant papers.

NS5-independent Ablation of STAT2 by Zika virus to antagonize interferon signalling

Flavivirus genus includes numerous arthropod-borne human pathogens that are clinically important. Flaviviruses are notorious for their ability to antagonize host interferon (IFN) induced anti-viral signalling. It has been documented that NS5s of flaviviruses mediate proteasome degradation of STAT2 to evade IFN signalling. Deciphering the molecular mechanism of the IFN antagonism by the viruses and reversing this antagonism may dictate anti-viral responses and provide novel antiviral approaches. In this report, by using Zika virus (ZIKV) as a model, we first demonstrated that ZIKV antagonized interferon signalling in an infectious dose-dependent manner; in other words, the virus antagonized interferon signalling at a high multiple of infection (MOI) and was sensitive to interferon signalling at a low MOI. Mechanistically, we found that ZIKV infection triggered degradation of ubiquitinated STAT2 and host short-lived proteins while didn't affect the proteasome activity per se. ZIKV infection resulted in suppression of host de novo protein synthesis. Overexpression of NS5 alone only marginally reduced STAT2 and had no effect on the host de novo protein synthesis. Ectopically expressed murine STAT2 that was resistant to NS5- and ZIKV-induced ablation exaggerated the IFN-induced anti-viral signalling. These data favour a new model of the innate immune evasion of ZIKV in which the viral infection triggers suppression of host de novo protein synthesis to accelerate the degradation of short-lived, ubiquitinated STAT2. As flaviviruses share a very conserved replication strategy, the mechanisms of IFN antagonism elucidated here might also be employed by other flaviviruses.

Recovery of a Far-Eastern Strain of Tick-Borne Encephalitis Virus with a Full-Length Infectious cDNA Clone

Tick-borne encephalitis virus (TBEV) is a pathogenic virus known to cause central nervous system (CNS) diseases in humans, and has become an increasing public health threat nowadays. The rates of TBEV infection in the endemic countries are increasing. However, there is no effective antiviral against the disease. This underscores the urgent need for tools to study the emergence and pathogenesis of TBEV and to accelerate the development of vaccines and antivirals. In this study, we reported an infectious cDNA clone of TBEV that was isolated in China (the WH2012 strain). A beta-globin intron was inserted in the coding region of nonstructural protein 1 (NS1) gene to improve the stability of viral genome in bacteria. In mammalian cells, the inserted intron was excised and spliced precisely, which did not lead to the generation of inserted mutants. High titers of infectious progeny viruses were generated after the transfection of the infectious clone. The cDNA-derived TBEV replicated efficiently, and caused typical cytopathic effect (CPE) and plaques in BHK-21 cells. In addition, the CPE and growth curve of cDNA-derived virus were similar to that of its parental isolate in cells. Together, we have constructed the first infectious TBEV cDNA clone in China, and the clone can be used to investigate the genetic determinants of TBEV virulence and disease pathogenesis, and to develop countermeasures against the virus.

Inflammasome dysregulation in human gingival fibroblasts in response to periodontal pathogens

OBJECTIVE

Uncontrolled production of Interleukin-1β (IL-1β), a major proinflammatory cytokine, is associated with tissue destruction in periodontal disease. IL-1β production is controlled by inflammasomes which are multiprotein regulatory complexes. The current study aimed to elucidate potential regulatory pathways by monitoring the effects of periodontal pathogens Fusobacterium nucleatum (Fn) and Porphyromonas gingivalis (Pg) on inflammasomes and their regulators in human gingival fibroblasts (HGFs) in vitro.

METHODS

HGFs were exposed to Fn and Pg alone or in combination for 24 hr at a multiplicity of infection of 100, ±30 min exposure with 5 mM adenosine triphosphate (ATP) incubation. Gene expression of NLRP3 and AIM2, inflammasome regulatory proteins POP1, CARD16 and TRIM16, and inflammasome components ASC and CASPASE 1, and IL-1β, were evaluated by RT-PCR. Pro- and mature IL-1β levels were monitored intracellularly by immunocytochemistry and extracellularly by ELISA.

RESULTS

Fn + ATP significantly upregulated NLRP3, AIM2, IL-1β, ASC, and CASPASE 1; however, it downregulated POP1 and TRIM16. Pg + ATP downregulated NLRP3, ASC, POP1, but upregulated IL-1β and CARD16. Pg + Fn+ATP significantly upregulated AIM2, IL-1β and CARD16, and downregulated POP1, TRIM16, and CASPASE 1. Pg + ATP exposure significantly increased pro- and mature IL-1β production.

CONCLUSION

Bacterial exposure with ATP may deregulate IL-1β by dysregulating inflammasomes and their regulators in HGFs.

Effects of Porphyromonas gingivalis and Fusobacterium nucleatum on inflammasomes and their regulators in H400 cells

INTRODUCTION

Inflammasomes are multiprotein complexes that regulate immune processes in response to infections and tissue damage. They modulate Interleukin-1beta (IL-1β) expression, a major proinflammatory cytokine. The inflammasome/IL-1β pathway is involved in head and neck squamous cell carcinoma (HNSCC) progression and the periodontal pathogens Fusobacterium nucleatum (Fn) and Porphyromonas gingivalis (Pg) have been reported to cause chronic inflammation in HNSCC. The aim of this study was to characterise the role of these pathogens in regulating inflammasome activity and the IL-1β response in HNSCC in vitro.

METHODS

An HNSCC cell line (H400) was exposed to Fn and Pg individually or in combination for 24h, ± incubation for 30 min with 5 mM adenosine triphosphate (ATP). Transcript levels of inflammasomes, NLRP3 and AIM2; inflammasome-regulatory proteins, POP1, CARD16 and TRIM16; and inflammasome-component, ASC and caspase 1 and IL-1β, were assayed by RT-PCR. Expression of IL-1β was by immunocytochemistry and ELISA.

RESULTS

NLRP3 expression was significantly upregulated in response to Pg, Fn + Pg, Pg + ATP and Fn + Pg + ATP. AIM2 was significantly upregulated by Fn, Pg and Fn + Pg + ATP exposure. All conditions significantly upregulated IL-1β gene expression. POP1 expression was significantly downregulated by Pg or Fn exposure but not by Fn + Pg. Intracellular pro- and mature IL-1β were significantly higher following Fn and Pg + ATP exposure.

CONCLUSION

Pg alone increased IL-1β by upregulating AIM2, NLRP3 and downregulating POP1. Fn promoted IL-1β by increasing AIM2 and downregulating POP1. Pg + ATP with or without Fn upregulated NLRP3, IL-1β by downregulating POP1. Periodontal pathogens may contribute to HNSCC pathogenesis by increasing the IL-1β response due to inflammasome dysregulation.