Toll-like receptor 3 associates with c-Src tyrosine kinase on endosomes to initiate antiviral signaling

Dual Roles of Host Zinc Finger Proteins in Viral RNA Regulation: Decay or Stabilization

Host defense mechanisms against viral infections have been extensively studied over the past few decades and continue to be a crucial area of research in understanding human diseases caused by acute and chronic viral infections. Among various host mechanisms, recent findings have revealed that several host RNA-binding proteins play pivotal roles in regulating viral RNA to suppress viral replication and eliminate infection. We have focused on identifying host proteins that function as regulators of viral RNA, specifically targeting viral components without adversely affecting host cells. Interestingly, these proteins exhibit dual roles in either restricting viral infections or promoting viral persistence by interacting with cofactors to either degrade viral genomes or stabilize them. In this review, we discuss RNA-binding zinc finger proteins as viral RNA regulators, classified into two major types: ZCCCH-type and ZCCHC-type. By highlighting the functional diversity of these zinc finger proteins, this review provides insights into their potential as therapeutic targets for the development of novel antiviral therapies.

Identification and Validation of Glomeruli Cellular Senescence-Related Genes in Diabetic Nephropathy by Multiomics

Accumulating evidence indicates that cellular premature senescence of the glomerulus, including endothelial cells, mesangial cells, and podocytes leads to diabetic nephropathy (DN), and DN is regarded as a clinical model of premature senescence. However, the role of cellular senescence-associated genes in the glomerulus in DN progression remains unclear. Therefore, this work aims to identify and validate potential cellular aging-related genes in the glomerulus in DN to provide novel clues for DN treatment based on anti-aging. The microarray GSE96804 dataset, including 41 diabetic glomeruli and 20 control glomeruli, is retrieved from the Gene Expression Omnibus (GEO) database and cellular senescence-related genes (CSRGs) are obtained from the GeneCards database and literature reports. Subsequently, PPI, GO, and KEGG enrichment are analyzed by screening the intersection between differentially expressed genes (DEGs) and CSRGs. scRNA-seq dataset GSE127235 is used to verify core genes expression in glomerulocytes of mice. Finally, db/db mice are utilized to validate the hub gene expression in the glomeruli, and high glucose-induced mesangial cells are used to confirm key gene expression. This study reveals that FOS and ZFP36 may play an anti-aging role in DN to ameliorate cell intracellular premature aging in mesangial cells of glomeruli.

Epoxyeicosatrienoic Acids Prevent Cardiac Dysfunction in Viral Myocarditis via Interferon Type I Signaling

Myocarditis is a challenging inflammatory disease of the heart, and better understanding of its pathogenesis is needed to develop specific drug therapies. Epoxyeicosatrienoic acids (EETs), active molecules synthesized by CYP (cytochrome P450) enzymes from arachidonic acids and hydrolyzed to less active dihydroxyeicosatrienoic acids by sEH (soluble epoxide hydrolase), have been attributed anti-inflammatory activity. Here, we investigated whether EETs have immunomodulatory activity and exert protective effects on coxsackie B3 virus-induced myocarditis. Viral infection altered eicosanoid epoxide and diol levels in both patients with myocarditis and in the murine heart and correlated with the increased expression and activity of sEH after coxsackie B3 virus infection. Administration of a sEH inhibitor prevented coxsackie B3 virus-induced cardiac dysfunction and inflammatory infiltration. Importantly, EET/sEH inhibitor treatment attenuated viral infection or improved viral resistance by activating type I IFN (interferon) signaling. At the molecular level, EETs enhanced the interaction between GSK3β (glycogen synthase kinase-3 beta) and TBK1 (TANK-binding kinase 1) to promote IFN-β production. Our findings revealed that EETs and sEH inhibitors prevent the progress of coxsackie B3 virus-induced myocarditis, particularly by promoting viral resistance by increasing IFN production.

Siglec cis-ligands and their roles in the immune system

Sialic acid-binding immunoglobulin-like lectins are a family of membrane molecules primarily expressed in immune cells. Most of them are inhibitory receptors containing immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic tail. On the cell surface, sialic acid-binding immunoglobulin-like lectins are mostly bound by sialylated glycans on membrane molecules expressed in the same cell (cis-ligands). Although ligands of sialic acid-binding immunoglobulin-like lectins are not efficiently identified by conventional methods such as immunoprecipitation, in situ labeling including proximity labeling is useful in identifying both cis-ligands and the sialylated ligands expressed by other cells (trans-ligands) of sialic acid-binding immunoglobulin-like lectins. Interaction of the inhibitory sialic acid-binding immunoglobulin-like lectins with cis-ligands including both those with and without signaling function modulates the inhibitory activity of sialic acid-binding immunoglobulin-like lectins by multiple different ways. This interaction also modulates signaling function of the cis-ligands. So far, little is known about the role of the interaction between sialic acid-binding immunoglobulin-like lectins and the cis-ligands. Nonetheless, recent studies showed that the inhibitory activity of CD22 (also known as Siglec-2) is regulated by endogenous ligands, most likely cis-ligands, differentially in resting B cells and those in which B-cell antigen receptor is ligated. This differential regulation plays a role in quality control of signaling-competent B cells and also partial restoration of B-cell antigen receptor signaling in immunodeficient B cells.

The Src-ZNRF1 axis controls TLR3 trafficking and interferon responses to limit lung barrier damage

Type I interferons are important antiviral cytokines, but prolonged interferon production is detrimental to the host. The TLR3-driven immune response is crucial for mammalian antiviral immunity, and its intracellular localization determines induction of type I interferons; however, the mechanism terminating TLR3 signaling remains obscure. Here, we show that the E3 ubiquitin ligase ZNRF1 controls TLR3 sorting into multivesicular bodies/lysosomes to terminate signaling and type I interferon production. Mechanistically, c-Src kinase activated by TLR3 engagement phosphorylates ZNRF1 at tyrosine 103, which mediates K63-linked ubiquitination of TLR3 at lysine 813 and promotes TLR3 lysosomal trafficking and degradation. ZNRF1-deficient mice and cells are resistant to infection by encephalomyocarditis virus and SARS-CoV-2 because of enhanced type I interferon production. However, Znrf1-/- mice have exacerbated lung barrier damage triggered by antiviral immunity, leading to enhanced susceptibility to respiratory bacterial superinfections. Our study highlights the c-Src-ZNRF1 axis as a negative feedback mechanism controlling TLR3 trafficking and the termination of TLR3 signaling.

Targeting endosomal receptors, a new direction for polymers in nanomedicine

In this perspective, we outline a new opportunity for exploiting nanoparticle delivery of antagonists to target G-protein coupled receptors localized in intracellular compartments. We discuss the specific example of antagonizing endosomal receptors involved in pain to develop long-lasting analgesics but also outline the broader application potential of this delivery approach. We discuss the materials used to target endosomal receptors and indicate the design requirements for future successful applications.

Toll-Like Receptor 3 (TLR3) Is Engaged in the Intracellular Survival of the Protozoan Parasite Leishmania (Leishmania) amazonensis

The protozoan parasite Leishmania (L.) amazonensis infects and replicates inside host macrophages due to subversion of the innate host cell response. In the present study, we demonstrate that TLR3 is required for the intracellular growth of L. (L.) amazonensis. We observed restricted intracellular infection of TLR3-/- mouse macrophages, reduced levels of IFN1β and IL-10, and increased levels of IL-12 upon L. (L.) amazonensis infection, compared with their wild-type counterparts. Accordingly, in vivo infection of TLR3-/- mice with L. (L.) amazonensis displayed a significant reduction in lesion size. Leishmania (L.) amazonensis infection induced TLR3 proteolytic cleavage, which is a process required for TLR3 signaling. The chemical inhibition of TLR3 cleavage or infection by CPB-deficient mutant L. (L.) mexicana resulted in reduced parasite load and restricted the expression of IFN1β and IL-10. Furthermore, we show that the dsRNA sensor molecule PKR (dsRNA-activated protein kinase) cooperates with TLR3 signaling to potentiate the expression of IL-10 and IFN1β and parasite survival. Altogether, our results show that TLR3 signaling is engaged during L. (L.) amazonensis infection and this component of innate immunity modulates the host cell response.

Alu RNA induces NLRP3 expression through TLR7 activation in α-1-antitrypsin-deficient macrophages

α-1 antitrypsin (AAT) is a serine protease inhibitor that plays a pivotal role in maintaining lung homeostasis. The most common AAT allele associated with AAT deficiency (AATD) is PiZ. Z-AAT accumulates in cells due to misfolding, causing severe AATD. The major function of AAT is to neutralize neutrophil elastase in the lung. It is generally accepted that loss of antiprotease function is a major cause of COPD in individuals with AATD. However, it is now being recognized that the toxic gain-of-function effect of Z-AAT in macrophage likely contributes to lung disease. In the present study, we determined that TLR7 signaling is activated in Z-MDMs, and the expression level of NLRP3, one of the targets of TLR7 signaling, is significantly higher in Z- compared with M-MDMs. We also determined that the level of endosomal Alu RNA is significantly higher in Z-compared with M-MDMs. Alu RNA is a known endogenous ligand that activates TLR7 signaling. Z-AAT likely induces the expression of Alu elements in MDMs and accelerates monocyte death, leading to the higher level of endosomal Alu RNA in Z-MDMs. Taken together,this study identifies a mechanism responsible for the toxic gain of function of Z-AAT macrophages.

Identification and functional characterization of an immune adapter molecular TRIF in Northeast Chinese lamprey (Lethenteron morii)

The TIR domain-containing adaptor inducing IFN-β (TRIF) is an adaptor molecule that plays a critical role in the Toll-like receptors (TLRs)-mediated innate immune signaling pathway. Lamprey, as the most primitive jawless vertebrate, rely mainly on innate immunity to defend against various pathogens infection. The function of TRIF in lamprey remains unknown. In this study, a homologous adaptor molecule TRIF, named LmTRIF, was identified in Northeast Chinese lamprey (Lethenteron morii). The LmTRIF coding sequence (cds) is 1242 bp in length and encodes 413 amino acids (aa). Domain analysis showed that LmTRIF is characterized with the classical TIR domain and a lack of TRAF6 binding motif. The results of evolutionary tree indicated that the relationship between LmTRIF and other homologous proteins was consistent with the position of lamprey in the species evolutionary history. The relative expression of LmTRIF was highest in the liver of larvae and in the gill of adults, respectively. Cellular immunofluorescence assays showed that LmTRIF was expressed in the cytoplasma in both mammalian cell line HEK 293T and the fish cell line EPC. The double luciferase reporter gene assay showed that the overexpression of LmTRIF promoted the activity of NF-κB, an immune transcription factor downstream of the classical TLR signaling pathway. In this study, we identified the TLR adaptor molecule TRIF from L. morii, a vertebrate more primitive than fish. Our results suggested an important role of LmTRIF in the innate immune signal transduction process of L. morii and is the basis for the origin and evolution of the TLR signaling pathway in the innate immune system in vertebrates.

Scavenger Receptors in the Pathogenesis of Viral Infections

Scavenger receptors (SR) are not only pattern recognition receptors involved in the immune response against pathogens but are also important receptors exploited by different virus to enter host cells, and thus represent targets for antiviral therapy. The high mutation rates of viruses, as well as their small genomes are partly responsible for the high rates of virus resistance and effective treatments remain a challenge. Most currently approved formulations target viral-encoded factors. Nevertheless, host proteins may function as additional targets. Thus, there is a need to explore and develop new strategies aiming at cellular factors involved in virus replication and host cell entry. SR-virus interactions have implications in the pathogenesis of several viral diseases and in adenovirus-based vaccination and gene transfer technologies, and may function as markers of severe progression. Inhibition of SR could reduce adenoviral uptake and improve gene therapy and vaccination, as well as reduce pathogenesis. In this review, we will examine the crucial role of SR play in cell entry of different types of human virus, which will allow us to further understand their role in protection and pathogenesis and its potential as antiviral molecules. The recent discovery of SR-B1 as co-factor of SARS-Cov-2 (severe acute respiratory syndrome coronavirus 2) entry is also discussed. Further fundamental research is essential to understand molecular interactions in the dynamic virus-host cell interplay through SR for rational design of therapeutic strategies.

Endosomal Cholesterol in Viral Infections - A Common Denominator?

Cholesterol has gained tremendous attention as an essential lipid in the life cycle of virtually all viruses. These seem to have developed manifold strategies to modulate the cholesterol metabolism to the side of lipid uptake and de novo synthesis. In turn, affecting the cholesterol homeostasis has emerged as novel broad-spectrum antiviral strategy. On the other hand, the innate immune system is similarly regulated by the lipid and stimulated by its derivatives. This certainly requires attention in the design of antiviral strategies aiming to decrease cellular cholesterol, as evidence accumulates that withdrawal of cholesterol hampers innate immunity. Secondly, there are exceptions to the rule of the abovementioned virus-induced metabolic shift toward cholesterol anabolism. It therefore is of interest to dissect underlying regulatory mechanisms, which we aimed for in this minireview. We further collected evidence for intracellular cholesterol concentrations being less important in viral life cycles as compared to the spatial distribution of the lipid. Various routes of cholesterol trafficking were found to be hijacked in viral infections with respect to organelle-endosome contact sites mediating cholesterol shuttling. Thus, re-distribution of cellular cholesterol in the context of viral infections requires more attention in ongoing research. As a final aim, a pan-antiviral treatment could be found just within the transport and re-adjustment of local cholesterol concentrations. Thus, we aimed to emphasize the importance of the regulatory roles the endosomal system fulfils herein and hope to stimulate research in this field.

A New Look at Vaccine Strategies Against PPRV Focused on Adenoviral Candidates

Peste des petits ruminants virus (PPRV) is a virus that mainly infects goats and sheep causing significant economic loss in Africa and Asia, but also posing a serious threat to Europe, as recent outbreaks in Georgia (2016) and Bulgaria (2018) have been reported. In order to carry out the eradication of PPRV, an objective set for 2030 by the Office International des Epizooties (OIE) and the Food and Agriculture Organization of the United Nations (FAO), close collaboration between governments, pharmaceutical companies, farmers and researchers, among others, is needed. Today, more than ever, as seen in the response to the SARS-CoV2 pandemic that we are currently experiencing, these goals are feasible. We summarize in this review the current vaccination approaches against PPRV in the field, discussing their advantages and shortfalls, as well as the development and generation of new vaccination strategies, focusing on the potential use of adenovirus as vaccine platform against PPRV and more broadly against other ruminant pathogens.

RNA-induced inflammation and migration of precursor neurons initiates neuronal circuit regeneration in zebrafish

Tissue regeneration and functional restoration after injury are considered as stem- and progenitor-cell-driven processes. In the central nervous system, stem cell-driven repair is slow and problematic because function needs to be restored rapidly for vital tasks. In highly regenerative vertebrates, such as zebrafish, functional recovery is rapid, suggesting a capability for fast cell production and functional integration. Surprisingly, we found that migration of dormant "precursor neurons" to the injury site pioneers functional circuit regeneration after spinal cord injury and controls the subsequent stem-cell-driven repair response. Thus, the precursor neurons make do before the stem cells make new. Furthermore, RNA released from the dying or damaged cells at the site of injury acts as a signal to attract precursor neurons for repair. Taken together, our data demonstrate an unanticipated role of neuronal migration and RNA as drivers of neural repair.

Toll-like receptor 3 (TLR3) regulation mechanisms and roles in antiviral innate immune responses

Toll-like receptor 3 (TLR3) is a member of the TLR family, mediating the transcriptional induction of type I interferons (IFNs), proinflammatory cytokines, and chemokines, thereby collectively establishing an antiviral host response. Studies have shown that unlike other TLR family members, TLR3 is the only RNA sensor that is utterly dependent on the Toll-interleukin-1 receptor (TIR)‍-domain-containing adaptor-inducing IFN-‍β (TRIF). However, the details of how the TLR3-TRIF signaling pathway works in an antiviral response and how it is regulated are unclear. In this review, we focus on recent advances in understanding the antiviral mechanism of the TRIF pathway and describe the essential characteristics of TLR3 and its antiviral effects. Advancing our understanding of TLR3 may contribute to disease diagnosis and could foster the development of novel treatments for viral diseases.

ZCCHC3 modulates TLR3-mediated signaling by promoting recruitment of TRIF to TLR3

Toll-like receptor 3 (TLR3)-mediated signaling is important for host defense against RNA virus. Upon viral RNA stimulation, toll and interleukin-1 receptor domain-containing adaptor inducing IFN-β (TRIF) is recruited to TLR3 and then undergoes oligomerization, which is required for the recruitment of downstream molecules to transmit signals. Here, we identified zinc finger CCHC-type containing 3 (ZCCHC3) as a positive regulator of TLR3-mediated signaling. Overexpression of ZCCHC3 promoted transcription of downstream antiviral genes stimulated by the synthetic TLR3 ligand poly(I:C). ZCCHC3-deficiency markedly inhibited TLR3- but not TLR4-mediated induction of type I interferons (IFNs) and proinflammatory cytokines. Zcchc3^−/− mice were more resistant to poly(I:C)- but not lipopolysaccharide-induced inflammatory death. Mechanistically, ZCCHC3 promoted recruitment of TRIF to TLR3 after poly(I:C) stimulation. Our findings reveal that ZCCHC3 plays an important role in TLR3-mediated innate immune response by promoting the recruitment of TRIF to TLR3 after ligand stimulation.

TLR3 agonists: RGC100, ARNAX, and poly-IC: a comparative review

Toll-like receptors 3 (TLR3) have been broadly studied among all TLRs over the last few decades together with its agonists due to their contribution to cancer regression. These agonists undeniably have some shared characteristics such as mimicking dsRNA but pathways through which they exhibit antitumor properties are relatively diverse. In this review, three widely studied agonists RGC100, ARNAX, and poly-IC are discussed along with their structural and physiochemical differences including the signaling cascades through which they exert their actions. Comparison has been made to identify the finest agonist with maximum effectivity and the least side effect profile.

Cell Surface Expression of Endosomal Toll-Like Receptors-A Necessity or a Superfluous Duplication?

Timely and precise delivery of the endosomal Toll-like receptors (TLRs) to the ligand recognition site is a critical event in mounting an effective antimicrobial immune response, however, the same TLRs should maintain the delicate balance of avoiding recognition of self-nucleic acids. Such sensing is widely known to start from endosomal compartments, but recently enough evidence has accumulated supporting the idea that TLR-mediated signaling pathways originating in the cell membrane may be engaged in various cells due to differential expression and distribution of the endosomal TLRs. Therefore, the presence of endosomal TLRs on the cell surface could benefit the host responses in certain cell types and/or organs. Although not fully understood why, TLR3, TLR7, and TLR9 may occur both in the cell membrane and intracellularly, and it seems that activation of the immune response can be initiated concurrently from these two sites in the cell. Furthermore, various forms of endosomal TLRs may be transported to the cell membrane, indicating that this may be a normal process orchestrated by cysteine proteases-cathepsins. Among the endosomal TLRs, TLR3 belongs to the evolutionary distinct group and engages a different protein adapter in the signaling cascade. The differently glycosylated forms of TLR3 are transported by UNC93B1 to the cell membrane, unlike TLR7, TLR8, and TLR9. The aim of this review is to reconcile various views on the cell surface positioning of endosomal TLRs and add perspective to the implication of such receptor localization on their function, with special attention to TLR3. Cell membrane-localized TLR3, TLR7, and TLR9 may contribute to endosomal TLR-mediated inflammatory signaling pathways. Dissecting this signaling axis may serve to better understand mechanisms influencing endosomal TLR-mediated inflammation, thus determine whether it is a necessity for immune response or simply a circumstantial superfluous duplication, with other consequences on immune response.

The tyrosine kinase SRC of grass carp (Ctenopharyngodon idellus) up-regulates the expression of IFN I by activating TANK binding kinase 1

In response to viral infections, various pattern recognition receptors (PRRs) are activated for the production of type I interferon (IFN I). As a center of these receptor responses, TANK binding kinase-1 (TBK1) activates interferon regulatory factor 3 (IRF3). SRC is a member of Src family kinases (SFK) which participates in TBK1-mediated IFN I signaling pathway. In mammals, the immunological function of SRC is depended on its interaction with TBK1. To date, SRC has not been studied in fish. In this paper, we cloned the ORF of grass carp (Ctenopharyngodon idellus) SRC (CiSRC). CiSRC has a closer relationship with Sinocyclocheilus rhinocerous SRC (SrSRC). The expression level of CiSRC was significantly up-regulated following poly (I:C) stimulation in grass carp tissues and cells. Subcellular localization results showed that CiSRC is located both in the cytoplasm and nucleus, while CiTBK1 is only located in the cytoplasm of CIK cells. When GFP-CiSRC and FLAG-CiTBK1 were co-transfected into CIK cells, we found that they were co-localized in the cytoplasm. GST-pulldown and Co-immunoprecipitation analysis revealed that CiSRC and CiSRC tyrosine kinase domain deletion mutant (SRC-ΔTyrkc) can interact with CiTBK1, respectively. CiSRC promotes the phosphorylation of CiTBK1. Furthermore, the phosphorylation of TBK1 is more strongly under poly (I:C) stimulation. We also demonstrated that SRC can up-regulate IFN I expression. These results above unraveled that CiSRC initiates innate immune response by binding to and then up-regulating the phosphorylation of TBK1.

Toll-like receptor-mediated innate immunity against herpesviridae infection: a current perspective on viral infection signaling pathways

Background

In the past decades, researchers have demonstrated the critical role of Toll-like receptors (TLRs) in the innate immune system. They recognize viral components and trigger immune signal cascades to subsequently promote the activation of the immune system.

Main body

Herpesviridae family members trigger TLRs to elicit cytokines in the process of infection to activate antiviral innate immune responses in host cells. This review aims to clarify the role of TLRs in the innate immunity defense against herpesviridae, and systematically describes the processes of TLR actions and herpesviridae recognition as well as the signal transduction pathways involved.

Conclusions

Future studies of the interactions between TLRs and herpesviridae infections, especially the subsequent signaling pathways, will not only contribute to the planning of effective antiviral therapies but also provide new molecular targets for the development of antiviral drugs.

Participation of Endosomes in Toll-Like Receptor 3 Transportation Pathway in Murine Astrocytes

TLR3 provides immediate type I IFN response following entry of stimulatory PAMPs into the CNS, as it is in HSV infection. The receptor plays a vital role in astrocytes, contributing to rapid infection sensing and suppression of viral replication, precluding the spread of virus beyond neurons. The route of TLR3 mobilization culminating in the receptor activation remains unexplained. In this research, we investigated the involvement of various types of endosomes in the regulation of the TLR3 mobility in C8-D1A murine astrocyte cell line. TLR3 was transported rapidly to early EEA1-positive endosomes as well as LAMP1-lysosomes following stimulation with the poly(I:C). Later, TLR3 largely associated with late Rab7-positive endosomes. Twenty-four hours after stimulation, TLR3 co-localized with LAMP1 abundantly in lysosomes of astrocytes. TLR3 interacted with poly(I:C) intracellularly from 1 min to 8 h following cell stimulation. We detected TLR3 on the surface of astrocytes indicating constitutive expression, which increased after poly(I:C) stimulation. Our findings contribute to the understanding of cellular modulation of TLR3 trafficking. Detailed analysis of the TLR3 transportation pathway is an important component in disclosing the fate of the receptor in HSV-infected CNS and may help in the search for rationale therapeutics to control the replication of neuropathic viruses.

ROLE OF TOLL-LIKE RECEPTORS IN CORONAVIRUS INFECTION AND IMMUNE RESPONSE

The emergence of a novel coronavirus referred to as SARS-CoV-2 has become a global health apprehension due to rapid transmission tendency, severity, and wide geographical spread. This emergence was started from Wuhan, China in 2019 from the zoonotic source and spread worldwide, infecting almost half of the community on this earth. Many of the receptors are involved in proceeding with this infection in the organism's body. Toll-like receptors (TLRs) play essential and protective functions from a wide range of microbial pathogens. Small setup of TLR adaptor proteins leads to activate nuclear factor kappa B (NF-kB) and interferon-regulatory factor (IRF). Consequently, various advanced inflammatory cytokines, chemokines, and interferon reaction properties can be up-regulated. Similarly, TLR flagging works on autophagy in macrophages. Autophagy is a cell response to starvation that helps to eliminate damaged cytosol organelles and persistent proteins. It is also able to prevent the replication of intracellular pathogens. Several microbes subvert the autophagy pathways to sustain their viability. This review investigates how TLRs can modulate a macrophagic system and analyze the role of natural resistance autophagy.

Tumoural activation of TLR3-SLIT2 axis in endothelium drives metastasis

Blood vessels support tumours by providing nutrients and oxygen, while also acting as conduits for the dissemination of cancer1. Here we use mouse models of breast and lung cancer to investigate whether endothelial cells also have active 'instructive' roles in the dissemination of cancer. We purified genetically tagged endothelial ribosomes and their associated transcripts from highly and poorly metastatic tumours. Deep sequencing revealed that metastatic tumours induced expression of the axon-guidance gene Slit2 in endothelium, establishing differential expression between the endothelial (high Slit2 expression) and tumoural (low Slit2 expression) compartments. Endothelial-derived SLIT2 protein and its receptor ROBO1 promoted the migration of cancer cells towards endothelial cells and intravasation. Deleting endothelial Slit2 suppressed metastatic dissemination in mouse models of breast and lung cancer. Conversely, deletion of tumoural Slit2 enhanced metastatic progression. We identified double-stranded RNA derived from tumour cells as an upstream signal that induces expression of endothelial SLIT2 by acting on the RNA-sensing receptor TLR3. Accordingly, a set of endogenous retroviral element RNAs were upregulated in metastatic cells and detected extracellularly. Thus, cancer cells co-opt innate RNA sensing to induce a chemotactic signalling pathway in endothelium that drives intravasation and metastasis. These findings reveal that endothelial cells have a direct instructive role in driving metastatic dissemination, and demonstrate that a single gene (Slit2) can promote or suppress cancer progression depending on its cellular source.

Covid-19: Perspectives on Innate Immune Evasion

The ongoing outbreak of Coronavirus disease 2019 infection achieved pandemic status on March 11, 2020. As of September 8, 2020 it has caused over 890,000 mortalities world-wide. Coronaviral infections are enabled by potent immunoevasory mechanisms that target multiple aspects of innate immunity, with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) able to induce a cytokine storm, impair interferon responses, and suppress antigen presentation on both MHC class I and class II. Understanding the immune responses to SARS-CoV-2 and its immunoevasion approaches will improve our understanding of pathogenesis, virus clearance, and contribute toward vaccine and immunotherepeutic design and evaluation. This review discusses the known host innate immune response and immune evasion mechanisms driving SARS-CoV-2 infection and pathophysiology.

Myeloid Differentiation Primary Response 88-Cyclin D1 Signaling in Breast Cancer Cells Regulates Toll-Like Receptor 3-Mediated Cell Proliferation

Toll-like receptor 3 (TLR3)-mediated apoptotic changes in cancer cells are well-documented, and hence, several synthetic ligands of TLR3 are being used for adjuvant therapy, but there are reports showing a contradictory effect of TLR3 signaling, which include our previous report that had shown cell proliferation following surface localization of TLR 3. However, the underlying mechanism of cell surface localization of TLR3 and subsequent cell proliferation lacks clarity. This study addresses the TLR3 ligand-mediated signaling cascade that regulates a proliferative effect in breast cancer cells (MDA-MB-231 and T47D) challenged with TLR3 ligand in the presence of myeloid differentiation primary response 88 (MyD88) inhibitor. Evidences were obtained using immunoblotting, coimmunoprecipitation, confocal microscopy, immunocytochemistry, ELISA, and flow cytometry. Results had revealed that TLR3 ligand treatment significantly enhanced breast cancer cell proliferation marked by an upregulated expression of cyclinD1, but the same was suppressed by the addition of MyD88 inhibitor. Also, expression of interleukin 1 receptor-associated kinase 1 (IRAK1)-TNF receptor-associated factor 6 (TRAF6)-transforming growth factor beta-activated kinase 1 (TAK1) was altered in the given TLR3-signaling pathway. Inhibition of MyD88 disrupted the downstream adaptor complex and mediated signaling through the TLR3-MyD88-NF-κB (p65)-IL-6-cyclin D1 pathway. TLR3-mediated alternative signaling of the TLR3-MyD88-IRAK1-TRAF6-TAK1-TAB1-NF-κB axis leads to upregulation of IL6 and cyclin D1. This response is hypothesized to be via the MyD88 gateway that culminates in the proliferation of breast cancer cells. Overall, this study provides first comprehensive evidence on the involvement of canonical signaling of TLR3 using MyD88-cyclin D1-mediated breast cancer cell proliferation. The findings elucidated herein will provide valuable insights into understanding the TLR3-mediated adjuvant therapy in cancer.

Ubiquitination of TLR3 by TRIM3 signals its ESCRT-mediated trafficking to the endolysosomes for innate antiviral response

Trafficking of toll-like receptor 3 (TLR3) from the endoplasmic reticulum (ER) to endolysosomes and its subsequent proteolytic cleavage are required for it to sense viral double-stranded RNA (dsRNA) and trigger antiviral response, yet the underlying mechanisms remain enigmatic. We show that the E3 ubiquitin ligase TRIM3 is mainly located in the Golgi apparatus and transported to the early endosomes upon stimulation with the dsRNA analog poly(I:C). TRIM3 mediates K63-linked polyubiquitination of TLR3 at K831, which is enhanced following poly(I:C) stimulation. The polyubiquitinated TLR3 is recognized and sorted by the ESCRT (endosomal sorting complex required for transport) complexes to endolysosomes. Deficiency of TRIM3 impairs TLR3 trafficking from the Golgi apparatus to endosomes and its subsequent activation. Trim3 ^-/- cells and mice express lower levels of antiviral genes and show lower levels of inflammatory response following poly(I:C) but not lipopolysaccharide (LPS) stimulation. These findings suggest that TRIM3-mediated polyubiquitination of TLR3 represents a feedback-positive regulatory mechanism for TLR3-mediated innate immune and inflammatory responses.

TICAM2-related pathway mediates neutrophil exhaustion

Pathogenic inflammation and immune suppression are the cardinal features that underlie the pathogenesis of severe systemic inflammatory syndrome and sepsis. Neutrophil exhaustion may play a key role during the establishment of pathogenic inflammation and immune suppression through elevated expression of inflammatory adhesion molecules such as ICAM1 and CD11b as well as immune-suppressors such as PD-L1. However, the mechanism of neutrophil exhaustion is not well understood. We demonstrated that murine primary neutrophils cultured in vitro with the prolonged lipopolysaccharides (LPS) stimulation can effectively develop an exhaustive phenotype resembling human septic neutrophils with elevated expression of ICAM1, CD11b, PD-L1 as well as enhanced swarming and aggregation. Mechanistically, we observed that TICAM2 is involved in the generation of neutrophil exhaustion, as TICAM2 deficient neutrophils have the decreased expression of ICAM1, CD11b, PD-L1, and the reduced aggregation following the prolonged LPS challenge as compared to wild type (WT) neutrophils. LPS drives neutrophil exhaustion through TICAM2 mediated activation of Src family kinases (SFK) and STAT1, as the application of SFK inhibitor Dasatinib blocks neutrophil exhaustion triggered by the prolonged LPS challenge. Functionally, TICAM2 deficient mice were protected from developing severe systemic inflammation and multi-organ injury following the chemical-induced mucosal damage. Together, our data defined a key role of TICAM2 in facilitating neutrophil exhaustion and that targeting TICAM2 may be a potential approach to treating the severe systemic inflammation.

A genome-wide CRISPR screen identifies regulation factors of the TLR3 signalling pathway

A subset of TLRs is specialised in the detection of incoming pathogens by sampling endosomes for nucleic acid contents. Among them, TLR3 senses the abnormal presence of double-stranded RNA in the endosomes and initiates a potent innate immune response via activation of NF-κB and IRF3. Nevertheless, mechanisms governing TLR3 regulation remain poorly defined. To identify new molecular players involved in the TLR3 pathway, we performed a genome-wide screen using CRISPR/Cas9 technology. We generated TLR3⁺ reporter cells carrying a NF-κB-responsive promoter that controls GFP expression. Cells were next transduced with a single-guide RNA (sgRNA) library, subjected to sequential rounds of stimulation with poly(I:C) and sorting of the GFP-negative cells. Enrichments in sgRNA estimated by deep sequencing identified genes required for TLR3-induced activation of NF-κB. Among the hits, five genes known to be critically involved in the TLR3 pathway, including TLR3 itself and the chaperone UNC93B1, were identified by the screen, thus validating our strategy. We further studied the top 40 hits and focused on the transcription factor aryl hydrocarbon receptor (AhR). Depletion of AhR had a dual effect on the TLR3 response, abrogating IL-8 production and enhancing IP-10 release. Moreover, in primary human macrophages exposed to poly(I:C), AhR activation enhanced IL-8 and diminished IP-10 release. Overall, these results reveal AhR plays a role in the TLR3 cellular innate immune response.

The Role of Dendritic Cells During Infections Caused by Highly Prevalent Viruses

Dendritic cells (DCs) are a type of innate immune cells with major relevance in the establishment of an adaptive response, as they are responsible for the activation of lymphocytes. Since their discovery, several reports of their role during infectious diseases have been performed, highlighting their functions and their mechanisms of action. DCs can be categorized into different subsets, and each of these subsets expresses a wide arrange of receptors and molecules that aid them in the clearance of invading pathogens. Interferon (IFN) is a cytokine -a molecule of protein origin- strongly associated with antiviral immune responses. This cytokine is secreted by different cell types and is fundamental in the modulation of both innate and adaptive immune responses against viral infections. Particularly, DCs are one of the most important immune cells that produce IFN, with type I IFNs (α and β) highlighting as the most important, as they are associated with viral clearance. Type I IFN secretion can be induced via different pathways, activated by various components of the virus, such as surface proteins or genetic material. These molecules can trigger the activation of the IFN pathway trough surface receptors, including IFNAR, TLR4, or some intracellular receptors, such as TLR7, TLR9, and TLR3. Here, we discuss various types of dendritic cells found in humans and mice; their contribution to the activation of the antiviral response triggered by the secretion of IFN, through different routes of the induction for this important antiviral cytokine; and as to how DCs are involved in human infections that are considered highly frequent nowadays.

Human PSC-Derived Hepatocytes Express Low Levels of Viral Pathogen Recognition Receptors, but Are Capable of Mounting an Effective Innate Immune Response

Hepatocytes are key players in the innate immune response to liver pathogens but are challenging to study because of inaccessibility and a short half-life. Recent advances in in vitro differentiation of hepatocyte-like cells (HLCs) facilitated studies of hepatocyte-pathogen interactions. Here, we aimed to define the anti-viral innate immune potential of human HLCs with a focus on toll-like receptor (TLR)-expression and the presence of a metabolic switch. We analysed cytoplasmic pattern recognition receptor (PRR)- and endosomal TLR-expression and activity and adaptation of HLCs to an inflammatory environment. We found that transcript levels of retinoic acid inducible gene I (RIG-I), melanoma differentiation antigen 5 (MDA5), and TLR3 became downregulated during differentiation, indicating the acquisition of a more tolerogenic phenotype, as expected in healthy hepatocytes. HLCs responded to activation of RIG-I by producing interferons (IFNs) and IFN-stimulated genes. Despite low-level expression of TLR3, receptor expression was upregulated in an inflammatory environment. TLR3 signalling induced expression of proinflammatory cytokines at the gene level, indicating that several PRRs need to interact for successful innate immune activation. The inflammatory responsiveness of HLCs was accompanied by the downregulation of cytochrome P450 3A and 1A2 activity and decreased serum protein production, showing that the metabolic switch seen in primary hepatocytes during anti-viral responses is also present in HLCs.

Roles of GSK-3 and β-Catenin in Antiviral Innate Immune Sensing of Nucleic Acids

The rapid activation of the type I interferon (IFN) antiviral innate immune response relies on ubiquitously expressed RNA and DNA sensors. Once engaged, these nucleotide-sensing receptors use distinct signaling modules for the rapid and robust activation of mitogen-activated protein kinases (MAPKs), the IκB kinase (IKK) complex, and the IKK-related kinases IKKε and TANK-binding kinase 1 (TBK1), leading to the subsequent activation of the activator protein 1 (AP1), nuclear factor-kappa B (NF-κB), and IFN regulatory factor 3 (IRF3) transcription factors, respectively. They, in turn, induce immunomodulatory genes, allowing for a rapid antiviral cellular response. Unlike the MAPKs, the IKK complex and the IKK-related kinases, ubiquitously expressed glycogen synthase kinase 3 (GSK-3) α and β isoforms are active in unstimulated resting cells and are involved in the constitutive turnover of β-catenin, a transcriptional coactivator involved in cell proliferation, differentiation, and lineage commitment. Interestingly, studies have demonstrated the regulatory roles of both GSK-3 and β-catenin in type I IFN antiviral innate immune response, particularly affecting the activation of IRF3. In this review, we summarize current knowledge on the mechanisms by which GSK-3 and β-catenin control the antiviral innate immune response to RNA and DNA virus infections.

Glycogen Synthase Kinase 3β Regulates Antiviral Responses of TLR3 via TRAF2-Src Axis

The protein tyrosine kinase Src regulates the synthesis of TLR3-mediated IFN-β via the TBK1-IFN regulatory factor 3 axis. However, the molecular mechanisms regulating Src activity in TLR3 signaling remain unclear. In this study, we report that GSK3β regulates Src phosphorylation via TNFR-associated factor 2 (TRAF2)-mediated Src ubiquitination. GSK3β deficiency in mouse embryonic fibroblasts significantly reduces polyinosinic:polycytidylic acid-induced IFN-β and IFN-stimulated gene expression, which is caused by diminished phosphorylation of Src at tyrosine 416. Src undergoes polyinosinic:polycytidylic acid-dependent lysine 63 chain ubiquitination, and TRAF2 is a direct E3 ligase for Src. Our study reveals novel mechanisms underlying TLR3-mediated antiviral responses mediated via the GSK3β-TRAF2-Src axis.

Litopenaeus vannamei Src64B restricts white spot syndrome virus replication by modulating apoptosis

The Src family kinases (SFK) are involved in signaling transductions that regulate numerous biological activities including host-virus interaction. These features of SFK have been well explored in vertebrates, however, in shrimp, the invertebrate SFK family member Src64B, has not been characterized and therefore its role in shrimp-virus interaction remains unknown. In this study, two Litopenaeus vannamei Src64B isoforms (designated LvSrc64B1 and LvSrc64B2) were first cloned and their role in white spot syndrome virus (WSSV) infection was explored. Bioinformatics analysis revealed that LvSrc64B1 and LvSrc64B2 were similar to other Src64B family members, with high homology in primary and tertiary structures, and contained the conserved SFK functional domains, as well as the putative myristylation and phosphorylation sites. Tissue distribution analysis showed that both LvSrc64B isoforms were ubiquitously expressed, albeit distinctively in the tested tissues. In addition, transcript levels of LvSrc64B1 and LvSrc64B2 were significantly induced following WSSV challenge and had similar expression patterns. Furthermore, siRNA-mediated knockdown of LvSrc64B1 and LvSrc64B2 followed by WSSV infection resulted in increased expression of viral genes, enhanced viral DNA replication, and elevation of hemocytes apoptosis. Depletion of LvSrc64B1 and LvSrc64B2 also reduced shrimp survival upon WSSV infection. In conclusion, the current data strongly suggest that Src64B is a host factor that inhibits WSSV replication by modulating apoptosis in shrimp.

The zinc-finger protein ZFYVE1 modulates TLR3-mediated signaling by facilitating TLR3 ligand binding

Recognition of viral dsRNA by Toll-like receptor 3 (TLR3) leads to the induction of downstream antiviral effectors and the innate antiviral immune response. Here, we identified the zinc-finger FYVE domain-containing protein ZFYVE1, a guanylate-binding protein (GBP), as a positive regulator of TLR3-mediated signaling. Overexpression of ZFYVE1 promoted the transcription of downstream antiviral genes upon stimulation with the synthetic TLR3 ligand poly(I:C). Conversely, ZFYVE1 deficiency had the opposite effect. Zfyve1^-/- mice were less susceptible than wild-type mice to inflammatory death induced by poly(I:C) but not LPS. ZFYVE1 was associated with TLR3, and the FYVE domain of ZFYVE1 and the ectodomain of TLR3 were shown to be responsible for their interaction. ZFYVE1 was bound to poly(I:C) and increased the binding affinity of TLR3 to poly(I:C). These findings suggest that ZFYVE1 plays an important role in the TLR3-mediated innate immune and inflammatory responses by promoting the ligand binding of TLR3.

Transcriptome Analysis Reveals Differential Expression of Genes Regulating Hepatic Triglyceride Metabolism in Pekin Ducks During Dietary Threonine Deficiency

Dietary threonine (Thr) deficiency increases hepatic triglyceride accumulation in Pekin ducks, which results in fatty liver disease and impairs hepatic function. However, the underlying molecular mechanisms altered by dietary Thr deficiency are still unknown. To identify the underlying molecular changes, 180 one-day-old ducklings were divided into three groups, including Thr deficiency group (Thr-D), Thr sufficiency group (Thr-S), and pair-fed group (Pair-F) that was fed with a Thr-sufficient diet but with reduced daily feed intake. The results showed that feed intake was similar between Thr-D and Pair-F groups, but weight gain rate and final body weight in the Thr-D group were lower than those in the Pair-F group. Feed intake, weight gain, and body weight in Thr-D and Pair-F groups were lower than those in the Thr-S group. The Thr-D diet reduced abdominal fat percentage but increased hepatic triglyceride content when compared with that of the Thr-S and Pair-F groups. The Pair-F reduced hepatic levels of C15:0, C17:0, C18:0, C20:0, C20:4n6, and C22:0 and also reduced total fatty acid, saturated fatty acid, and unsaturated fatty acid content when compared with those of the Thr-D and Thr-S groups. The Thr-D diet increased hepatic content of C6:0, C17:1, C18:3n6, C20:0, C20:1n9, and C22:2, as well as reduced the content of C18:2n6t and C23:0 when compared with those of the Thr-S group. Transcriptome analysis in the liver indicated that the Thr-D diet upregulated genes related to fatty acid and triglyceride synthesis and downregulated genes related to fatty acid oxidation and triglyceride transport. Gene ontology analysis showed that more genes related to lipid metabolism processes and molecular function were differentially expressed in the Thr-D group relative to Thr-S and Pair-F groups than in the Pair-F group relative to the Thr-S group. KEGG pathway analysis showed that differentially expressed genes were enriched in signal transduction, immune, hormone, lipid, and amino acid metabolism pathways. Our findings indicated that the Thr-D diet increased hepatic triglyceride and fatty acid accumulation via increasing fatty acid and triglyceride synthesis and reducing fatty acid oxidation and triglyceride transport. These findings provide novel insights into our understanding of the molecular mechanisms underlying fat accumulation in the liver caused by dietary threonine deficiency.

CCR5 is a required signaling receptor for macrophage expression of inflammatory genes in response to viral double-stranded RNA

Double-stranded (ds) RNA, both synthetic and produced during virus replication, rapidly stimulates MAPK and NF-κB signaling that results in expression of the inflammatory genes inducible nitric oxide synthase, cyclooxygenase 2, and IL-1β by macrophages. Using biochemical and genetic approaches, we have identified the chemokine ligand-binding C-C chemokine receptor type 5 (CCR5) as a cell surface signaling receptor required for macrophage expression of inflammatory genes in response to dsRNA. Activation of macrophages by synthetic dsRNA does not require known dsRNA receptors, as poly(inosinic:cytidylic) acid [poly(I:C)] activates signaling pathways leading to expression of inflammatory genes to similar levels in wild-type and Toll-like receptor 3- or melanoma differentiation antigen 5-deficient macrophages. In contrast, macrophage activation in response to poly(I:C) is attenuated in macrophages isolated from mice lacking CCR5. These findings support a role for CCR5 as a cell surface signaling receptor that participates in activation of inflammatory genes in macrophages in response to the viral dsRNA mimetic poly(inosinic:cytidylic) acid by pathways that are distinct from classical dsRNA receptor-mediated responses.

Cell entry of BmCPV can be promoted by tyrosine-protein kinase Src64B-like protein

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a non-enveloped dsRNA virus, which specifically infect the midgut epithelium of B. mori. BmCPV enters permissive cells via clathrin-dependent endocytosis employing β1 integrin mediated internalization. Until now, the cell entry mechanism of BmCPV has not been known clearly. Here, we investigated whether tyrosine-protein kinase Src64B-like is involved in the cell entry of BmCPV. The Src64B-like gene was cloned and expressed in Escherichia coli (E. coli), and the recombinant protein Src64B-like was used to immunize mouse for preparation of anti-Src64B-like polyclonal antibody (pAb). After Src64B-like gene was silenced by RNAi, the infection of BmCPV was reduced by 59.48% ± 2.18% and 92.22% ± 1.12% in vitro and in vivo autonomously. Contrary to it, BmCPV infection could be enhanced by increasing the expression of Src64B-like. In addition, immunofluorescence assay showed that Src64B-like protein did not co-localize with BmCPV in the cultured BmN cells during viral infection. These results indicate that Src64B-like protein participates and plays an important role in the cell entry of BmCPV, but not contacting directly with BmCPV.

Saracatinib Inhibits Middle East Respiratory Syndrome-Coronavirus Replication In Vitro

The Middle East respiratory syndrome-coronavirus (MERS-CoV), first identified in Saudi Arabia, is an emerging zoonotic pathogen that causes severe acute respiratory illness in humans with a high fatality rate. Since its emergence, MERS-CoV continues to spread to countries outside of the Arabian Peninsula and gives rise to sporadic human infections following the entry of infected individuals to other countries, which can precipitate outbreaks similar to the one that occurred in South Korea in 2015. Current therapeutics against MERS-CoV infection have primarily been adapted from previous drugs used for the treatment of severe acute respiratory syndrome. In search of new potential drug candidates, we screened a library composed of 2334 clinically approved drugs and pharmacologically active compounds. The drug saracatinib, a potent inhibitor of Src-family of tyrosine kinases (SFK), was identified as an inhibitor of MERS-CoV replication in vitro. Our results suggest that saracatinib potently inhibits MERS-CoV at the early stages of the viral life cycle in Huh-7 cells, possibly through the suppression of SFK signaling pathways. Furthermore, saracatinib exhibited a synergistic effect with gemcitabine, an anticancer drug with antiviral activity against several RNA viruses. These data indicate that saracatinib alone or in combination with gemcitabine can provide a new therapeutic option for the treatment of MERS-CoV infection.

Activation of TLR3 and its adaptor TICAM-1 increases miR-21 levels in extracellular vesicles released from human cells

Pattern-recognition receptors (PRRs) recognizes viral RNAs and trigger the innate immune responses. Toll-like receptor 3 (TLR3), a PRR, recognizes viral double-stranded RNA (dsRNA) in endolysosomes, whereas cytoplasmic dsRNA is sensed by another PRR, MDA5. TLR3 and MDA5 utilize TICAM-1 and MAVS, respectively, to trigger the signal for inducing innate immune responses. Extracellular vesicles (EVs) include the exosomes and microvesicles; an accumulating body of evidence has shown that EVs delivers functional RNA, such as microRNAs (miRNAs), to other cells and thus mediate intercellular communications. Therefore, EVs carrying miRNAs affect innate immune responses in macrophages and dendritic cells. However, the mechanism underlying the regulation of miRNA levels in EVs remains unclear. To elucidate the mechanism, we sought to reveal the pathway that control miRNA expression levels in EVs. Here, we found that TLR3 stimulation increased miR-21 levels in EVs released from various types of human cells. Ectopic expression of the TLR3 adaptor, TICAM-1, increased miR-21 levels in EVs but not intracellular miR-21 levels, suggesting that TICAM-1 augmented sorting of miR-21 to EVs. In contrast, the MDA5 adaptor, MAVS, did not increase miR-21 levels in EVs. The siRNA for TICAM-1 reduced EV miR-21 levels after stimulation of TLR3. Collectively, our data indicate a novel role of the TLR3-TICAM-1 pathway in controlling miR-21 levels in EVs.

Characterization of a novel non-receptor tyrosine kinase Src from Litopenaeus vannamei and its response to white spot syndrome virus infection

Src family kinases (SFKs), a class of non-receptor tyrosine kinases, mediate a wide aspect of cellular signaling pathways that regulate cell proliferation, differentiation, motility and survival. In this study, we identified and characterized for the first time a novel SFK homologue from Litopenaeus vannamei (designated as LvSrc). Sequence analysis showed that LvSrc had a high homology with the identified SFKs, especially those from invertebrates. LvSrc contained the conserved SH3, SH2 and tyrosine kinase domains, as well as the potential phosphorylation and lipid modification sites. Immunofluorescence analysis demonstrated that LvSrc was mostly localized at the plasma membrane and partly resided in the perinuclear vesicle and nucleus or whole cell. Infection with white spot syndrome virus (WSSV) could up-regulate the transcription and expression levels of LvSrc and further induced its phosphorylation, suggesting that LvSrc was implicated in WSSV infection. Furthermore, our co-immunoprecipitation result confirmed the interaction between Src and focal adhesion kinase (FAK) in shrimp, while the phosphorylation of FAK was markedly enhanced by co-expression with LvSrc. In sum, our studies suggested that LvSrc might act in the FAK-regulated signaling pathway during WSSV infection, which would give us a better insight in understanding the role of SKFs in host-virus interactions in crustaceans.

Surface Toll-like receptor 3 expression in metastatic intestinal epithelial cells induces inflammatory cytokine production and promotes invasiveness

Toll-like receptors (TLRs) are innate immune receptors for sensing microbial molecules and damage-associated molecular patterns released from host cells. Double-stranded RNA and the synthetic analog polyinosinic:polycytidylic acid (poly(I:C)) bind and activate TLR3. This stimulation leads to recruitment of the adaptor molecule TRIF (Toll/IL-1 resistance (TIR) domain-containing adapter-inducing interferon β) and activation of the transcription factors nuclear factor κB (NF-κB) and interferon regulatory factor 3 (IRF-3), classically inducing IFNβ production. Here we report that, unlike non-metastatic intestinal epithelial cells (IECs), metastatic IECs express TLR3 and that TLR3 promotes invasiveness of these cells. In response to poly(I:C) addition, the metastatic IECs also induced the chemokine CXCL10 in a TLR3-, TRIF-, and IRF3-dependent manner but failed to produce IFNβ. This was in contrast to healthy and non-metastatic IECs, which did not respond to poly(I:C) stimulation. Endolysosomal acidification and the endosomal transporter protein UNC93B1 was required for poly(I:C)-induced CXCL10 production. However, TLR3-induced CXCL10 was triggered by immobilized poly(I:C), was only modestly affected by inhibition of endocytosis, and could be blocked with an anti-TLR3 antibody, indicating that TLR3 can still signal from the cell surface of these cells. Furthermore, plasma membrane fractions from metastatic IECs contained both full-length and cleaved TLR3, demonstrating surface expression of both forms of TLR3. Our results imply that metastatic IECs express surface TLR3, allowing it to sense extracellular stimuli that trigger chemokine responses and promote invasiveness in these cells. We conclude that altered TLR3 expression and localization may have implications for cancer progression.

Activation of cell membrane-localized Toll-like receptor 3 by siRNA

Small interfering RNA molecules (siRNA) are short dsRNAs that are used for different therapeutic applications. On the other hand, dsRNAs can bind to and activate cell RNA sensors and consequently trigger inflammatory response. Here we show that siRNA activates primary human endothelial cells and human lymphatic endothelial cells and that this response is inhibited by antibodies against TLR3. In contrast, the activation of human lymphatic endothelial cells by poly(I:C) was inhibited by bafilomycin but not by anti-TLR3 antibodies. Bafilomycin also inhibited poly(I:C) but not siRNA cell stimulation in TLR3-transfected HEK293. The response to siRNA required the expression of UNC93B1, which directs TLR3 to the surface of HEK293 cells. We propose that the engaged signaling pathway of TLR3 depends on the receptor localization and on the length of the dsRNA, where the activation of cell membrane TLR3 by short dsRNA leads to a predominantly proinflammatory response, whereas TLR3 activation in endosomal compartments by long dsRNA is characterized by the production of type I IFN. A molecular model suggests that the siRNA can bind to the binding sites of the TLR3 ectodomain and trigger receptor dimerization. These results contribute to understanding of the mechanism of side effects seen in the therapeutic application of naked, unmodified siRNA as a result of the activation of TLR3 localized at the plasma membrane.

Cordycepin negatively modulates lipopolysaccharide-induced cytokine production by up-regulation of heme oxygenase-1

AIMS

The present study is to investigate the effect of cordycepin on the expression of heme oxygenase-1 (HO-1) in lipopolysaccharide (LPS)-activated microphages, as well as its mechanism of action.

METHODS

Mouse RAW264.7 cells were treated with different concentrations of cordycepin for 0-16h. Western blotting was used to determine the expression of HO-1 and the phosphorylation of c-Src and the p47^phox subunit of NADPH oxidase. Intracellular reactive oxygen species (ROS) level was determined using H₂DCFDA as fluorescent probe. Laser-scanning confocal microscopy was used to visualize the nuclear translocation of NF-E2-related factor 2 (Nrf2). Enzyme-linked immunosorbent assay was performed to measure the inhibitory effect of cordycepin on LPS-induced secretion of tumor necrosis factor-α and interleukin-6.

RESULTS

Cordycepin induced the phosphorylation of c-Src and p47^phox subunit of NADPH oxidase in RAW264.7 cells. Cordycepin increased the secretion of ROS by activating NADPH oxidase. In addition, cordycepin enhanced the expression of HO-1 in RAW264.7 cells in both dose- and time-dependent manners. Of note, elevated HO-1 expression induced by cordycepin treatment was regulated by c-Src/NADPH oxidase/ROS pathway. HO-1 expression induced by cordycepin was dependent on the activation of Nrf2, which was regulated by c-Src/NADPH oxidase/ROS. Cordycepin reduced LPS-induced secretion of proinflammatory cytokines through up-regulation of HO-1.

CONCLUSION

The present study demonstrates that cordycepin induces the expression of HO-1 in RAW264.7 cells via c-Src/NADPH oxidase/ROS/Nrf2 pathway, and plays an anti-inflammatory role by inhibiting the secretion of cytokines from macrophages.

Extracellular Vesicles Deliver Host and Virus RNA and Regulate Innate Immune Response

The innate immune system plays a crucial role in controlling viral infection. Pattern recognition receptors (PRRs), such as Toll-like receptors and RIG-I-like receptors, sense viral components called pathogen-associated molecular patterns (PAMPs) and trigger signals to induce innate immune responses. Extracellular vesicles (EVs), including exosomes and microvesicles, deliver functional RNA and mediate intercellular communications. Recent studies have revealed that EVs released from virus-infected cells deliver viral RNA to dendritic cells and macrophages, thereby activating PRRs in recipient cells, which results in the expression of type I interferon and pro-inflammatory cytokines. On the other hand, EVs transfer not only viral RNA but also host microRNAs to recipient cells. Recently, infection of hepatocytes with hepatitis B virus (HBV) was shown to affect microRNA levels in EVs released from virus-infected cells, leading to attenuation of host innate immune response. This suggests that the virus utilizes the EVs and host microRNAs to counteract the antiviral innate immune responses. In this review, we summarize recent findings related to the role of EVs in antiviral innate immune responses.

The tyrosine kinase Src promotes phosphorylation of the kinase TBK1 to facilitate type I interferon production after viral infection

Various pattern recognition receptors (PRRs) are activated in response to viral infection to stimulate the production of type I interferons (IFNs). However, central to the responses of all of these receptors is their activation of the kinase TBK1, which stimulates transcription by IFN regulatory factor 3 (IRF3). We investigated the mechanism by which the kinase activity of TBK1 is stimulated in response to viral infection. We found that the tyrosine kinase Src promoted the phosphorylation of TBK1 on Tyr¹⁷⁹ upon viral infection of RAW264.7 macrophages. Mutation of Tyr¹⁷⁹ to alanine resulted in impaired autophosphorylation of TBK1 at Ser¹⁷², which is required for TBK1 activation. The TBK1 Y179A mutant failed to rescue type I IFN production by virally infected RAW264.7 macrophages deficient in TBK1. Pharmacological inhibition of Src with AZD0530 and clustered regularly interspaced short palindromic repeats/Cas9-mediated knockout of Src demonstrated that Src was critical for activating the TBK1-IRF3 pathway and stimulating type I IFN production. However, Src did not directly bind to recombinant TBK1 in vitro but instead bound to the proline-X-X-proline motifs within key PRR adaptor proteins, such as TRIF, MAVS, and STING, which formed complexes with TBK1 after PRR engagement. Together, our data suggest that Src is the major tyrosine kinase that primes TBK1 for autophosphorylation and activation, thus providing mechanistic insights into the regulation of TBK1 activity by various PRRs as part of the innate antiviral response.

Nucleic acid-sensing TLRs and autoimmunity: novel insights from structural and cell biology

Invasion of pathogenic microorganisms or tissue damage activates innate immune signaling receptors that sample subcellular locations for foreign molecular structures, altered host molecules, or signs of compartment breaches. Upon engagement of innate immune receptors an acute but transient inflammatory response is initiated, aimed at the clearance of pathogens and cellular debris. Among the molecules that are sensed are nucleic acids, which activate several members of the transmembrane Toll-like receptor (TLR) family. Inappropriate recognition of nucleic acids by TLRs can cause inflammatory pathologies and autoimmunity. Here, we review the mechanisms involved in triggering nucleic acid-sensing TLRs and indicate checkpoints that restrict their activation to endolysosomal compartments. These mechanisms are crucial to sample the content of endosomes for nucleic acids in the context of infection or tissue damage, yet prevent accidental activation by host nucleic acids under physiological conditions. Decoding the molecular mechanisms that regulate nucleic acid recognition by TLRs is central to understand pathologies linked to unrestricted nucleic acid sensing and to develop novel therapeutic strategies.

The ORF3 Protein of Genotype 1 Hepatitis E Virus Suppresses TLR3-induced NF-κB Signaling via TRADD and RIP1

Hepatitis E virus (HEV) genotype 1 infection is common and can emerge as outbreaks in developing areas, thus posing a threat to public health. However, due to the absence of feasible animal models, the mechanism of HE pathogenesis remains obscure. The HEV pathogenic mechanism has been suggested to be mediated by the immune system and not by direct viral duplication. We firstly discovered that the open reading frame 3 (ORF3) protein of genotype 1 HEV downregulates TLR3-mediated NF-κB signaling in Human A549 Lung Epithelial Cells (A549 cells) which were exposed to different TLR agonists associated with viral nucleic acids. Additionally, we identified the P2 domain of ORF3 as being responsible for this inhibition. Intriguingly, tumor necrosis factor receptor 1-associated death domain protein (TRADD) expression and receptor-interacting protein kinase 1 (RIP1) K63-ubiquitination were reduced in the presence of both ORF3 and Poly(I:C). Furthermore, we found that Lys377 of RIP1 acts as the functional ubiquitination site for ORF3-associated inhibition. Overall, we found that ORF3 protein downregulates TLR3-mediated NF-κB signaling via TRADD and RIP1. Our findings provide a new perspective on the cellular response in HEV infection and expand our understanding of the molecular mechanisms of HEV pathogenesis in innate immunity.

Interferon β (IFN-β) Production during the Double-stranded RNA (dsRNA) Response in Hepatocytes Involves Coordinated and Feedforward Signaling through Toll-like Receptor 3 (TLR3), RNA-dependent Protein Kinase (PKR), Inducible Nitric Oxide Synthase (iNOS), and Src Protein

The sensing of double-stranded RNA (dsRNA) in the liver is important for antiviral defenses but can also contribute to sterile inflammation during liver injury. Hepatocytes are often the target of viral infection and are easily injured by inflammatory insults. Here we sought to establish the pathways involved in the production of type I interferons (IFN-I) in response to extracellular poly(I:C), a dsRNA mimetic, in hepatocytes. This was of interest because hepatocytes are long-lived and, unlike most immune cells that readily die after activation with dsRNA, are not viewed as cells with robust antimicrobial capacity. We found that poly(I:C) leads to rapid up-regulation of inducible nitric oxide synthase (iNOS), double-stranded RNA-dependent protein kinase (PKR), and Src. The production of IFN-β was dependent on iNOS, PKR, and Src and partially dependent on TLR3/Trif. iNOS and Src up-regulation was partially dependent on TLR3/Trif but entirely dependent on PKR. The phosphorylation of TLR3 on tyrosine 759 was shown to increase in parallel to IFN-β production in an iNOS- and Src-dependent manner, and Src was found to directly interact with TLR3 in the endosomal compartment of poly(I:C)-treated cells. Furthermore, we identified a robust NO/cGMP/PKG-dependent feedforward pathway for the amplification of iNOS expression. These data identify iNOS/NO as an integral component of IFN-β production in response to dsRNA in hepatocytes in a pathway that involves the coordinated activities of TLR3/Trif and PKR.