UNC93B1 physically associates with human TLR8 and regulates TLR8-mediated signaling

Toll-like receptors (TLRs) 3, 7, 8, and 9 are localized to intracellular compartments where they encounter foreign or self nucleic acids and activate innate and adaptive immune responses. The endoplasmic reticulum (ER)-resident membrane protein, UNC93B1, is essential for intracellular trafficking and endolysosomal targeting of TLR7 and TLR9. TLR8 is phylogenetically and structurally related to TLR7 and TLR9, but little is known about its localization or function. In this study, we demonstrate that TLR8 localized to the early endosome and the ER but not to the late endosome or lysosome in human monocytes and HeLa transfectants. UNC93B1 physically associated with human TLR8, similar to TLRs 3, 7, and 9, and played a critical role in TLR8-mediated signaling. Localization analyses of TLR8 tail-truncated mutants revealed that the transmembrane domain and the Toll/interleukin-1 receptor domain were required for proper targeting of TLR8 to the early endosome. Hence, although UNC93B1 participates in intracellular trafficking and signaling for all nucleotide-sensing TLRs, the mode of regulation of TLR localization differs for each TLR.     

Hepatitis A Virus 3C Protease Cleaves NEMO To Impair Induction of Beta Interferon

NEMO (NF-κB essential modulator) is a bridging adaptor indispensable for viral activation of interferon (IFN) antiviral response. Herein, we show that hepatitis A virus (HAV) 3C protease (3C^pro) cleaves NEMO at the Q304 residue, negating its signaling adaptor function and abrogating viral induction of IFN-β synthesis via the retinoic acid-inducible gene I/melanoma differentiation-associated protein 5 (RIG-I/MDA5) and Toll-like receptor 3 (TLR3) pathways. NEMO cleavage and IFN antagonism, however, were lost upon ablation of the catalytic activity of 3C^pro. These data describe a novel immune evasion mechanism of HAV.     

The interaction between the ER membrane protein UNC93B and TLR3, 7, and 9 is crucial for TLR signaling

Toll-like receptors (TLRs) sense the presence of microbial and viral pathogens by signal transduction mechanisms that remain to be fully elucidated. A single point mutation (H412R) in the polytopic endoplasmic reticulum (ER)-resident membrane protein UNC93B abolishes signaling via TLR3, 7, and 9. We show that UNC93B specifically interacts with TLR3, 7, 9, and 13, whereas introduction of the point mutation H412R in UNC93B abolishes their interactions. We establish the physical interaction of the intracellular TLRs with UNC93B in splenocytes and bone marrow-derived dendritic cells. Further, by expressing chimeric TLRs, we show that TLR3 and 9 bind to UNC93B via their transmembrane domains. We propose that a physical association between UNC93B and TLRs in the ER is essential for proper TLR signaling.     

The Coxsackievirus B 3Cpro Protease Cleaves MAVS and TRIF to Attenuate Host Type I Interferon and Apoptotic Signaling

The host innate immune response to viral infections often involves the activation of parallel pattern recognition receptor (PRR) pathways that converge on the induction of type I interferons (IFNs). Several viruses have evolved sophisticated mechanisms to attenuate antiviral host signaling by directly interfering with the activation and/or downstream signaling events associated with PRR signal propagation. Here we show that the 3C^pro cysteine protease of coxsackievirus B3 (CVB3) cleaves the innate immune adaptor molecules mitochondrial antiviral signaling protein (MAVS) and Toll/IL-1 receptor domain-containing adaptor inducing interferon-beta (TRIF) as a mechanism to escape host immunity. We found that MAVS and TRIF were cleaved in CVB3-infected cells in culture. CVB3-induced cleavage of MAVS and TRIF required the cysteine protease activity of 3C^pro, occurred at specific sites and within specialized domains of each molecule, and inhibited both the type I IFN and apoptotic signaling downstream of these adaptors. 3C^pro-mediated MAVS cleavage occurred within its proline-rich region, led to its relocalization from the mitochondrial membrane, and ablated its downstream signaling. We further show that 3C^pro cleaves both the N- and C-terminal domains of TRIF and localizes with TRIF to signalosome complexes within the cytoplasm. Taken together, these data show that CVB3 has evolved a mechanism to suppress host antiviral signal propagation by directly cleaving two key adaptor molecules associated with innate immune recognition.     

Proteolytic Processing Regulates Toll-like Receptor 3 Stability and Endosomal Localization

Toll-like receptors (TLRs) 3, 7, and 9 are innate immune receptors that recognize nucleic acids from pathogens in endosomes and initiate signaling transductions that lead to cytokine production. Activation of TLR9 for signaling requires proteolytic processing within the ectodomain by endosome-associated proteases. Whether TLR3 requires similar proteolytic processing to become competent for signaling remains unclear. Herein we report that human TLR3 is proteolytically processed to form two fragments in endosomes. Unc93b1 is required for processing by transporting TLR3 through the Golgi complex and to the endosomes. Proteolytic cleavage requires the eight-amino acid Loop1 within leucine-rich repeat 12 of the TLR3 ectodomain. Proteolytic cleavage is not required for TLR3 signaling in response to poly(I:C), although processing could modulate the degree of response toward viral double-stranded RNAs, especially in mouse cells. Both the full-length and cleaved fragments of TLR3 can bind poly(I:C) and are present in endosomes. However, although the full-length TLR3 has a half-life in HEK293T cells of 3 h, the cleaved fragments have half-lives in excess of 7 h. Inhibition of TLR3 cleavage by either treatment with cathepsin inhibitor or by a mutation in Loop1 decreased the abundance of TLR3 in endosomes targeted for lysosomal degradation.     

Secretion of the Human Toll-like Receptor 3 Ectodomain Is Affected by Single Nucleotide Polymorphisms and Regulated by Unc93b1

The innate immune receptor Toll-like receptor 3 (TLR3) can be present on the surface of the plasma membranes of cells and in endolysosomes. The Unc93b1 protein has been reported to facilitate localization of TLR7 and 9 and is required for TLR3, -7, and -9 signaling. We demonstrate that siRNA knockdown of Unc93b1 reduced the abundance of TLR3 on the cell surface without altering total TLR3 accumulation. In addition, siRNA to Unc93b1 reduced the secretion of the TLR3 ectodomain (T3ECD) into the cell medium. Furthermore, two human single nucleotide polymorphisms that affected herpesvirus and influenza virus encephalopathy as well as a natural isoform generated by alternative splicing were found to be impaired for T3ECD secretion and decreased the abundance of TLR3 on the cell surface. The locations of the SNP P554S and the deletion in the isoform led to the identification of a loop in the TLR3 ectodomain that is required for secretion and a second whose presence decreased secretion. Finally, a truncated protein containing the N-terminal 10 leucine-rich repeats of T3ECD was sufficient for secretion in an Unc93b1-dependent manner.     

Structural basis for antiviral inhibition of the main protease, 3C, from human enterovirus 93

Members of the Enterovirus genus of the Picornaviridae family are abundant, with common human pathogens that belong to the rhinovirus (HRV) and enterovirus (EV) species, including diverse echo-, coxsackie- and polioviruses. They cause a wide spectrum of clinical manifestations ranging from asymptomatic to severe diseases with neurological and/or cardiac manifestations. Pandemic outbreaks of EVs may be accompanied by meningitis and/or paralysis and can be fatal. However, no effective prophylaxis or antiviral treatment against most EVs is available. The EV RNA genome directs the synthesis of a single polyprotein that is autocatalytically processed into mature proteins at Gln↓Gly cleavage sites by the 3C protease (3C^pro), which has narrow, conserved substrate specificity. These cleavages are essential for virus replication, making 3C^pro an excellent target for antivirus drug development. In this study, we report the first determination of the crystal structure of 3C^pro from an enterovirus B, EV-93, a recently identified pathogen, alone and in complex with the anti-HRV molecules compound 1 (AG7404) and rupintrivir (AG7088) at resolutions of 1.9, 1.3, and 1.5 Å, respectively. The EV-93 3C^pro adopts a chymotrypsin-like fold with a canonically configured oxyanion hole and a substrate binding pocket similar to that of rhino-, coxsackie- and poliovirus 3C proteases. We show that compound 1 and rupintrivir are both active against EV-93 in infected cells and inhibit the proteolytic activity of EV-93 3C^pro in vitro. These results provide a framework for further structure-guided optimization of the tested compounds to produce antiviral drugs against a broad range of EV species.     

Anterograde trafficking of Toll-like receptors requires the cargo sorting adaptors TMED-2 and 7

Toll-Like Receptors (TLRs) play a pivotal role in immunity by recognising conserved structural features of pathogens and initiating the innate immune response. TLR signalling is subject to complex regulation that remains poorly understood. Here we show that two small type I transmembrane receptors, TMED2 and 7, that function as cargo sorting adaptors in the early secretory pathway are required for transport of TLRs from the ER to Golgi. Protein interaction studies reveal that TMED7 interacts with TLR2, TLR4 and TLR5 but not with TLR3 and TLR9. On the other hand, TMED2 interacts with TLR2, TLR4 and TLR3. Dominant negative forms of TMED7 suppress the export of cell surface TLRs from the ER to the Golgi. By contrast TMED2 is required for the ER-export of both plasma membrane and endosomal TLRs. Together, these findings suggest that association of TMED2 and TMED7 with TLRs facilitates anterograde transport from the ER to the Golgi.     

Toll-like receptor 3 (TLR3) signaling requires TLR4 Interactor with leucine-rich REPeats (TRIL)

Toll-like receptors (TLRs) are a family of proteins that act as the primary sensors of microbial products. Many TLRs require accessory molecules in order to recognize these microbial products and initiate signal transduction cascades. We have identified TRIL (TLR4 interactor with leucine-rich repeats) as a novel modulator of TLR4 signaling showing high expression in the brain. We now show that TRIL also plays a role in TLR3 signaling. TRIL is expressed intracellularly in the astrocytoma cell line U373 and in the monocytic cell line THP1. TRIL co-localizes with the endosomal compartment. These data are consistent with a role for TRIL in TLR3 signaling and endosomal TLR4 signaling. TRIL was induced by the TLR3 ligand poly(I:C). Overexpression of TRIL enhanced cytokine production and interferon-stimulated response element (ISRE) luciferase activity following poly(I:C) stimulation in U373. TRIL interacted with TLR3, and this interaction was enhanced following poly(I:C) stimulation. Transient knockdown of TRIL with siRNA or stable knockdown using shRNA in U373 cells inhibited TLR3 signaling, reducing ISRE luciferase, RANTES, and type I interferon production. Knockdown of TRIL did not affect TLR2 signaling. Most accessory molecules identified to date, such as CD14, gp96, PRAT4a, and Unc93B, all play roles in multiple TLR signaling pathways, and we now show that this is also the case for TRIL.     

Foot-and-Mouth Disease Virus 3C Protease Induces Fragmentation of the Golgi Compartment and Blocks Intra-Golgi Transport

Picornavirus infection can cause Golgi fragmentation and impose a block in the secretory pathway which reduces expression of major histocompatibility antigens at the plasma membrane and slows secretion of proinflammatory cytokines. In this study, we show that Golgi fragmentation and a block in secretion are induced by expression of foot-and-mouth disease virus (FMDV) 3C^pro and that this requires the protease activity of 3C^pro. 3C^pro caused fragmentation of early, medial, and late Golgi compartments, but the most marked effect was on early Golgi compartments, indicated by redistribution of ERGIC53 and membrin. Golgi fragments were dispersed in the cytoplasm and were able to receive a model membrane protein exported from the endoplasmic reticulum (ER). Golgi fragments were, however, unable to transfer the protein to the plasma membrane, indicating a block in intra-Golgi transport. Golgi fragmentation was coincident with a loss of microtubule organization resulting from an inhibition of microtubule regrowth from the centrosome. Inhibition of microtubule regrowth also required 3C^pro protease activity. The loss of microtubule organization induced by 3C^pro caused Golgi fragmentation, but loss of microtubule organization does not block intra-Golgi transport. It is likely that the block of intra-Golgi transport is imposed by separate actions of 3C^pro, possibly through degradation of proteins required for intra-Golgi transport.     

Insights into Cleavage Specificity from the Crystal Structure of Foot-and-Mouth Disease Virus 3C Protease Complexed with a Peptide Substrate

Picornavirus replication is critically dependent on the correct processing of a polyprotein precursor by 3C protease(s) (3C^pro) at multiple specific sites with related but non-identical sequences. To investigate the structural basis of its cleavage specificity, we performed the first crystallographic structural analysis of non-covalent complexes of a picornavirus 3C^pro with peptide substrates. The X-ray crystal structure of the foot-and-mouth disease virus 3C^pro, mutated to replace the catalytic Cys by Ala and bound to a peptide (APAKQ|LLNFD) corresponding to the P5-P5′ region of the VP1-2A cleavage junction in the viral polyprotein, was determined up to 2.5 Å resolution. Comparison with free enzyme reveals significant conformational changes in 3C^pro on substrate binding that lead to the formation of an extended interface of contact primarily involving the P4-P2′ positions of the peptide. Strikingly, the deep S1′ specificity pocket needed to accommodate P1′-Leu only forms when the peptide binds. Substrate specificity was investigated using peptide cleavage assays to show the impact of amino acid substitutions within the P5-P4′ region of synthetic substrates. The structure of the enzyme-peptide complex explains the marked substrate preferences for particular P4, P2 and P1 residue types, as well as the relative promiscuity at P3 and on the P′ side of the scissile bond. Furthermore, crystallographic analysis of the complex with a modified VP1-2A peptide (APAKE|LLNFD) containing a Gln-to-Glu substitution reveals an identical mode of peptide binding and explains the ability of foot-and-mouth disease virus 3C^pro to cleave sequences containing either P1-Gln or P1-Glu. Structure-based mutagenesis was used to probe interactions within the S1′ specificity pocket and to provide direct evidence of the important contribution made by Asp84 of the Cys-His-Asp catalytic triad to proteolytic activity. Our results provide a new level of detail in our understanding of the structural basis of polyprotein cleavage by 3C^pro.     

Crystal Structure of the Middle East Respiratory Syndrome Coronavirus (MERS-CoV) Papain-like Protease Bound to Ubiquitin Facilitates Targeted Disruption of Deubiquitinating Activity to Demonstrate Its Role in Innate Immune Suppression

Middle East respiratory syndrome coronavirus (MERS-CoV) is a newly emerging human pathogen that was first isolated in 2012. MERS-CoV replication depends in part on a virus-encoded papain-like protease (PL^pro) that cleaves the viral replicase polyproteins at three sites releasing non-structural protein 1 (nsp1), nsp2, and nsp3. In addition to this replicative function, MERS-CoV PL^pro was recently shown to be a deubiquitinating enzyme (DUB) and to possess deISGylating activity, as previously reported for other coronaviral PL^pro domains, including that of severe acute respiratory syndrome coronavirus. These activities have been suggested to suppress host antiviral responses during infection. To understand the molecular basis for ubiquitin (Ub) recognition and deconjugation by MERS-CoV PL^pro, we determined its crystal structure in complex with Ub. Guided by this structure, mutations were introduced into PL^pro to specifically disrupt Ub binding without affecting viral polyprotein cleavage, as determined using an in trans nsp3↓4 cleavage assay. Having developed a strategy to selectively disable PL^pro DUB activity, we were able to specifically examine the effects of this activity on the innate immune response. Whereas the wild-type PL^pro domain was found to suppress IFN-β promoter activation, PL^pro variants specifically lacking DUB activity were no longer able to do so. These findings directly implicate the DUB function of PL^pro, and not its proteolytic activity per se, in the inhibition of IFN-β promoter activity. The ability to decouple the DUB activity of PL^pro from its role in viral polyprotein processing now provides an approach to further dissect the role(s) of PL^pro as a viral DUB during MERS-CoV infection.     

EV71 3C protease cleaves host anti-viral factor OAS3 and enhances virus replication

The global spread of enteroviruses (EVs) has become more frequent, severe and life-threatening. Intereron (IFN) I has been proved to control EVs by regulating IFN-stimulated genes (ISG) expression. 20-50-oligoadenylate synthetases 3 (OAS3) is an important ISG in the OAS/RNase L antiviral system. The relationship between OAS3 and EVs is still unclear. Here, we reveal that OAS3, superior to OAS1 and OAS2, significantly inhibited EV71 replication in vitro. However, EV71 utilized autologous 3C protease (3C^pro) to cleave intracellular OAS3 and enhance viral replication. Rupintrivir, a human rhinovirus 3C protease inhibitor, completely abolished the cleavage of EV71 3C^pro on OAS3. And the proteolytically deficient mutants H40G, E71A, and C147G of EV71 3C^pro also lost the ability of OAS3 cleavage. Mechanistically, the Q982-G983 motif in C-terminal of OAS3 was identified as a crucial 3C^pro cutting site. Further investigation indicated that OAS3 inhibited not only EV71 but also Coxsackievirus B3 (CVB3), Coxsackievirus A16 (CA16), Enterovirus D68 (EVD68), and Coxsackievirus A6 (CA6) subtypes. Notably, unlike other four subtypes, CA16 3C^pro could not cleave OAS3. Two key amino acids variation Ile36 and Val86 in CA16 3C^pro might result in weak and delayed virus replication of CA16 because of failure of OAS and 3AB cleavage. Our works elucidate the broad anti-EVs function of OAS3, and illuminate a novel mechanism by which EV71 use 3C^pro to escape the antiviral effect of OAS3. These findings can be an important entry point for developing novel therapeutic strategies for multiple EVs infection.     

Enterovirus 71 3C Protease Cleaves a Novel Target CstF-64 and Inhibits Cellular Polyadenylation

Identification of novel cellular proteins as substrates to viral proteases would provide a new insight into the mechanism of cell–virus interplay. Eight nuclear proteins as potential targets for enterovirus 71 (EV71) 3C protease (3C^pro) cleavages were identified by 2D electrophoresis and MALDI-TOF analysis. Of these proteins, CstF-64, which is a critical factor for 3′ pre-mRNA processing in a cell nucleus, was selected for further study. A time-course study to monitor the expression levels of CstF-64 in EV71-infected cells also revealed that the reduction of CstF-64 during virus infection was correlated with the production of viral 3C^pro. CstF-64 was cleaved in vitro by 3C^pro but neither by mutant 3C^pro (in which the catalytic site was inactivated) nor by another EV71 protease 2A^pro. Serial mutagenesis was performed in CstF-64, revealing that the 3C^pro cleavage sites are located at position 251 in the N-terminal P/G-rich domain and at multiple positions close to the C-terminus of CstF-64 (around position 500). An accumulation of unprocessed pre-mRNA and the depression of mature mRNA were observed in EV71-infected cells. An in vitro assay revealed the inhibition of the 3′-end pre-mRNA processing and polyadenylation in 3C^pro-treated nuclear extract, and this impairment was rescued by adding purified recombinant CstF-64 protein. In summing up the above results, we suggest that 3C^pro cleavage inactivates CstF-64 and impairs the host cell polyadenylation in vitro, as well as in virus-infected cells. This finding is, to our knowledge, the first to demonstrate that a picornavirus protein affects the polyadenylation of host mRNA.     

Seneca Valley Virus 3Cpro Substrate Optimization Yields Efficient Substrates for Use in Peptide-Prodrug Therapy

The oncolytic picornavirus Seneca Valley Virus (SVV-001) demonstrates anti-tumor activity in models of small cell lung cancer (SCLC), but may ultimately need to be combined with cytotoxic therapies to improve responses observed in patients. Combining SVV-001 virotherapy with a peptide prodrug activated by the viral protease 3C^pro is a novel strategy that may increase the therapeutic potential of SVV-001. Using recombinant SVV-001 3C^pro, we measured cleavage kinetics of predicted SVV-001 3C^pro substrates. An efficient substrate, L/VP4 (k_cat/K_M = 1932 ± 183 M^-1s^-1), was further optimized by a P2’ N→P substitution yielding L/VP4.1 (k_cat/K_M = 17446 ± 2203 M^-1s^-1). We also determined essential substrate amino acids by sequential N-terminal deletion and substitution of amino acids found in other picornavirus genera. A peptide corresponding to the L/VP4.1 substrate was selectively cleaved by SVV-001 3C^pro in vitro and was stable in human plasma. These data define an optimized peptide substrate for SVV-001 3C^pro, with direct implications for anti-cancer therapeutic development.     

The Role of Calpain-Myosin 9-Rab7b Pathway in Mediating the Expression of Toll-Like Receptor 4 in Platelets: A Novel Mechanism Involved in α-Granules Trafficking

Toll-like receptors (TLRs) plays a critical role in innate immunity. In 2004, Aslam R. and Shiraki R. first determined that murine and human platelets express functional TLRs. Additionally, Andonegui G. demonstrated that platelets express TLR4, which contributes to thrombocytopenia. However, the underlying mechanisms of TLR4 expression by platelets have been rarely explored until now. The aim of this study was to identify the mechanism of TLR4 expression underlying thrombin treatment. The human washed platelets were used in this study. According to flowcytometry and western blot analysis, the surface levels of TLR4 were significantly enhanced in thrombin-activated human platelets and decreased by TMB-8, calpeptin, and {"type":"entrez-nucleotide","attrs":{"text":"U73122","term_id":"4098075"}}U73122, but not Y27632 (a Rho-associated protein kinase ROCK inhibitor) indicating that thrombin-mediated TLR4 expression was modulated by PAR/PLC pathway, calcium and calpain. Co-immunoprecipitation (co-IP) assay demonstrated that the interaction between TLR4 and myosin-9 (a substrate of calpain) was regulated by calpain; cleavage of myosin-9 enhanced TLR4 expression in thrombin treated platelets. Transmission electron microscope data indicated that human platelets used α-granules to control TLR4 expression; the co-IP experiment suggested that myosin-9 did not coordinate with Rab7b to negatively regulate TLR4 trafficking in thrombin treated platelets. In summary, phospholipase Cγ-calpain-myosin 9-Rab7b axis was responsible for the mechanism underlying the regulation of TLR4 containing α-granules trafficking in thrombin-stimulated platelets, which was involved in coagulation.     

Silencing of Nuclear Mitotic Apparatus protein (NuMA) accelerates the apoptotic disintegration of the nucleus

One main feature of apoptosis is the sequential degradation of the nuclear structure, including the fragmentation of chromatin and caspase-mediated cleavage of various nuclear proteins. Among these proteins is the Nuclear Mitotic Apparatus protein (NuMA) which plays a specific role in the organization of the mitotic spindle. The exact function of NuMA in the interphase nucleus is unknown, but a number of reports have suggested that it may play a role in chromatin organization and/or gene expression. Here we show that upon cleavage in apoptotic cells, the N-terminal cleavage fragment of NuMA is solubilized while the C-terminal fragment remains associated with the condensed chromatin. Using pancaspase inhibitor z-VAD-fmk and caspase-3 deficient MCF-7 cells, we further show that the solubilization is dependent on caspase-mediated cleavage of NuMA. Finally, the silencing of NuMA by RNAi accelerated nuclear breakdown in apoptotic MCF-7 cells. These results suggest that NuMA may provide structural support in the interphase nucleus by contributing to the organization of chromatin.     

Differential Regulation of TLR Signaling on the Induction of Antiviral Interferons in Human Intestinal Epithelial Cells Infected with Enterovirus 71

Enterovirus 71 (EV71) causes hand-foot-and-mouth disease, which can lead to fatal neurological complications in young children and infants. Few gastrointestinal symptoms are observed clinically, suggesting the presence of a unique immunity to EV71 in the gut. We reported a robust induction of interferons (IFNs) in human intestinal epithelial cells (HT-29), which was suppressed in other types such as RD and HeLa cells. The underlying mechanism for the apparent difference remains obscure. In this study we report that in EV71-infected HT-29 cells, TLR/TRIF signaling was essential to IFN induction; viral replication increased and the induction of IFN-α, -β, -ω, -κ, and -ε decreased markedly in TRIF-silenced HT-29 cells. Importantly, TRIF was degraded by viral 3C^pro in RD cells, but resisted cleavage, and IRF3 was activated and translocated into the nucleus in HT-29 cells. Taken together, our data suggest that IFNs were induced differentially in human HT-29 cells through an intact TLR/TRIF signaling, which differs from other cell types and may be implicated in viral pathogenesis in EV71 infection.