HnRNP A1 Alters the Structure of a Conserved Enterovirus IRES Domain to Stimulate Viral Translation

Host RNA-binding proteins and specialized viral RNA translation mechanisms: Potential antiviral targets

RNA-binding proteins (RBPs) are the key regulators of the metabolism of RNA, from its genesis to its degradation. Qualitative and quantitative alterations of RBPs, including their post-translational modifications, impact cellular physiology and are associated with disease processes. Many cellular RBPs also play essential roles in the replication of viruses, especially RNA viruses, which, as obligatory parasites, rely on the host cell's biosynthetic and structural machinery. Viral protein synthesis is a key step in viral lifecycles and critically depends on host RBPs. In many cases, the translation of viral mRNAs employs specialized mechanisms that give viral mRNAs advantages over cellular RNAs. Host RBPs regulate these specialized mechanisms. In this work, we review the role of RBPs in specialized viral RNA translation, focusing on these RBPs as potential antiviral drug targets.

Identification of Small-Molecule Inhibitors for Enterovirus A71 IRES by Structure-Based Virtual Screening

Structured RNAs play a crucial role in regulating gene expression, which includes both protein synthesis and RNA processing. Dysregulation of these processes is associated with various conditions, including viral and bacterial infections, as well as cancer. The unique tertiary structures of structured RNAs provide an opportunity for small molecules to directly modulate such processes, making them promising targets for drug discovery. Although small-molecule inhibitors targeting RNA have shown early success, in silico strategies like structure-based virtual screening remain underutilized for RNA-targeted drug discovery. In this study, we developed a virtual screening scheme targeting the structural ensemble of EV-A71 IRES SL II, a noncoding viral RNA element essential for viral replication. We subsequently optimized the experimentally validated hit compound IRE-03 from virtual screening through an "analog-by-catalog" search. This led to the identification of a more potent IRES inhibitor, IRE-03-3, validated through biochemical and functional assays with an EC50 value of 11.96 μM against viral proliferation. Our findings demonstrate that structure-based virtual screening can be effectively applied to RNA targets, providing exciting new opportunities for future antiviral drug discovery.

Targeting WDPF domain of Hsp27 achieves a broad spectrum of antiviral

Enterovirus A71 (EV-A71) is a positive-sense single-stranded RNA virus, which hijacks host proteins to benefit viral internal ribosome entry site (IRES)-dependent protein translation and further propagation. We demonstrated that serine 78 (S78) phosphorylation of Hsp27 is critical for Hsp27/hnRNP A1 relocalization upon EV-A71 infection. Here, we report that the deletion of WDPF and ACD domains disturbs subcellular localization of Hsp27, resulting in partial nuclear translocation. The domain deletion-induced Hsp27 nuclear translocation fails to direct hnRNP A1 translocation. The 2Apro-induced IRES activity and viral replication are suppressed by the deletion of WDPF or ACD domain. Surprisingly, a peptide (WDPF) dramatically inhibits S78 phosphorylation. Therefore, hnRNP A1 translocation, viral IRES activity, and viral protein translation and propagation are all strongly suppressed by the WDPF peptide, but not by peptide without WDPFR sequence (ΔWDPF). Moreover, the WDPF peptide also has potent antiviral activity on other RNA virus (e.g., coronavirus HCoV-OC43) and DNA virus (e.g., HSV-1 and HBV). Peptide treatment with kinase inhibitor Sorafenib brings an additional inhibitory effect on HCoV-OC43 and HSV-1. Taken together, we uncover a crucial role of WDPF domain in S78 phosphorylation for EV-A71-induced hnRNP A1 nuclear translocation, IRES-dependent viral protein translation, and EV-A71 propagation. Our results explore a new path for target-based pan-antiviral strategy.

Targeting Ser78 phosphorylation of Hsp27 achieves potent antiviral effects against enterovirus A71 infection

A positive-sense (+) single-stranded RNA (ssRNA) virus (e.g. enterovirus A71, EV-A71) depends on viral polypeptide translation for initiation of virus replication after entry. We reported that EV-A71 hijacks Hsp27 to induce hnRNP A1 cytosol redistribution to initiate viral protein translation, but the underlying mechanism is still elusive. Here, we show that phosphorylation-deficient Hsp27-3A (Hsp27S15/78/82A) and Hsp27S78A fail to translocate into the nucleus and induce hnRNP A1 cytosol redistribution, while Hsp27S15A and Hsp27S82A display similar effects to the wild type Hsp27. Furthermore, we demonstrate that the viral 2A protease (2Apro) activity is a key factor in regulating Hsp27/hnRNP A1 relocalization. Hsp27S78A dramatically decreases the IRES activity and viral replication, which are partially reduced by Hsp27S82A. However, Hsp27S15A displays the same activity as the wild-type Hsp27. Peptide S78 potently suppresses EV-A71 protein translation and reproduction through blockage of EV-A71-induced Hsp27 phosphorylation and Hsp27/hnRNP A1 relocalization. A point mutation (S78A) on S78 impairs its inhibitory functions on Hsp27/hnRNP A1 relocalization and viral replication. Taken together, we demonstrate the importance of Ser78 phosphorylation of Hsp27 regulated by virus infection in nuclear translocation, hnRNP A1 cytosol relocation, and viral replication, suggesting a new path (such as peptide S78) for target-based antiviral strategy.

Heterogeneous nuclear ribonucleoprotein A3 binds to the internal ribosomal entry site of enterovirus A71 and affects virus replication in neural cells

Enterovirus A71 (EV-A71) belongs to the genus Enterovirus of the Picornaviridae family and often causes outbreaks in Asia. EV-A71 infection usually causes hand, foot, and mouth disease and can even affect the central nervous system, causing neurological complications or death. The 5'-untranslated region (5'-UTR) of EV-A71 contains an internal ribosome entry site (IRES) that is responsible for the translation of viral proteins. IRES-transacting factors can interact with the EV-A71 5'-UTR to regulate IRES activity. Heterogeneous nuclear ribonucleoprotein (hnRNP) A3 is a member of the hnRNP A/B protein family of RNA-binding proteins and is involved in RNA transport and modification. We found that hnRNP A3 knockdown promoted the replication of EV-A71 in neural calls. Conversely, increasing the expression of hnRNP A3 within cells inhibits the growth of EV-A71. HnRNP A3 can bind to the EV-A71 5'-UTR, and knockdown of hnRNP A3 enhances the luciferase activity of the EV-A71 5'-UTR IRES. The localization of hnRNP A3 shifts from the nucleus to the cytoplasm of infected cells during viral infection. Additionally, EV-A71 infection can increase the protein expression of hnRNP A3, and the protein level is correlated with efficient viral growth. Based on these findings, we concluded that hnRNP A3 plays a negative regulatory role in EV-A71 replication within neural cells.

RNA-Small-Molecule Interaction: Challenging the "Undruggable" Tag

RNA targeting, specifically with small molecules, is a relatively new and rapidly emerging avenue with the promise to expand the target space in the drug discovery field. From being "disregarded" as an "undruggable" messenger molecule to FDA approval of an RNA-targeting small-molecule drug Risdiplam, a radical change in perspective toward RNA has been observed in the past decade. RNAs serve important regulatory functions beyond canonical protein synthesis, and their dysregulation has been reported in many diseases. A deeper understanding of RNA biology reveals that RNA molecules can adopt a variety of structures, carrying defined binding pockets that can accommodate small-molecule drugs. Due to its functional diversity and structural complexity, RNA can be perceived as a prospective target for therapeutic intervention. This perspective highlights the proof of concept of RNA-small-molecule interactions, exemplified by targeting of various transcripts with functional modulators. The advent of RNA-oriented knowledge would help expedite drug discovery.

Enterovirus evolution reveals the mechanism of an RNA-targeted antiviral and determinants of viral replication

Selective pressures on viruses provide opportunities to establish target site specificity and mechanisms of antivirals. Enterovirus (EV)-A71 with resistant mutations in the stem loop (SL) II internal ribosome entry site (IRES) (SLIIresist) were selected at low doses of the antiviral dimethylamiloride (DMA)-135. The EV-A71 mutants were resistant to DMA-135 at concentrations that inhibit replication of wild-type virus. EV-A71 IRES structures harboring resistant mutations induced efficient expression of Luciferase messenger RNA in the presence of noncytotoxic doses of DMA-135. Nuclear magnetic resonance indicates that the mutations change the structure of SLII at the binding site of DMA-135 and at the surface recognized by the host protein AU-rich element/poly(U)-binding/degradation factor 1 (AUF1). Biophysical studies of complexes formed between AUF1, DMA-135, and either SLII or SLIIresist show that DMA-135 stabilizes a ternary complex with AUF1-SLII but not AUF1-SLIIresist. This work demonstrates how viral evolution elucidates the (DMA-135)-RNA binding site specificity in cells and provides insights into the viral pathways inhibited by the antiviral.

Stability-Based Proteomics for Investigation of Structured RNA-Protein Interactions

RNA-protein interactions are essential to RNA function throughout biology. Identifying the protein interactions associated with a specific RNA, however, is currently hindered by the need for RNA labeling or costly tiling-based approaches. Conventional strategies, which commonly rely on affinity pull-down approaches, are also skewed to the detection of high affinity interactions and frequently miss weaker interactions that may be biologically important. Reported here is the first adaptation of stability-based mass spectrometry methods for the global analysis of RNA-protein interactions. The stability of proteins from rates of oxidation (SPROX) and thermal protein profiling (TPP) methods are used to identify the protein targets of three RNA ligands, the MALAT1 triple helix (TH), a viral stem loop (SL), and an unstructured RNA (PolyU), in LNCaP nuclear lysate. The 315 protein hits with RNA-induced conformational and stability changes detected by TPP and/or SPROX were enriched in previously annotated RNA-binding proteins and included new proteins for hypothesis generation. Also demonstrated are the orthogonality of the SPROX and TPP approaches and the utility of the domain-specific information available with SPROX. This work establishes a novel platform for the global discovery and interrogation of RNA-protein interactions that is generalizable to numerous biological contexts and RNA targets.

Advances in anti-EV-A71 drug development research

BACKGROUND

Enterovirus A71 (EV-A71) is capable of causing hand, foot and mouth disease (HFMD), which may lead to neurological sequelae and even death. As EV-A71 is resistant to environmental changes and mutates easily, there is still a lack of effective treatments or globally available vaccines.

AIM OF REVIEW

For more than 50 years since the HFMD epidemic, related drug research has been conducted. Progress in this area can promote the further application of existing potential drugs and develop more efficient and safe antiviral drugs, and provide useful reference for protecting the younger generation and maintaining public health security.

KEY SCIENTIFIC CONCEPTS OF REVIEW

At present, researchers have identified hundreds of EV-A71 inhibitors based on screening repurposed drugs, targeted structural design, and rational modification of previously effective drugs as the main development strategies. This review systematically introduces the current potential drugs to inhibit EV-A71 infection, including viral inhibitors targeting key sites such as the viral capsid, RNA-dependent RNA polymerase (RdRp), 2C protein, internal ribosome entry site (IRES), 3C proteinase (3Cpro), and 2A proteinase (2Apro), starting from each stage of the viral life cycle. Meanwhile, the progress of host-targeting antiviral drugs and their development are summarized in terms of regulating host immunity, inhibiting autophagy or apoptosis, and regulating the cellular redox environment. In addition, the current clinical methods for the prevention and treatment of HFMD are summarized and discussed with the aim of providing support and recommendations for the treatment of enterovirus infections including EV-A71.

The Repurposing of Cellular Proteins during Enterovirus A71 Infection

Viruses pose a great threat to people's lives. Enterovirus A71 (EV-A71) infects children and infants all over the world with no FDA-approved treatment to date. Understanding the basic mechanisms of viral processes aids in selecting more efficient drug targets and designing more effective antivirals to thwart this virus. The 5'-untranslated region (5'-UTR) of the viral RNA genome is composed of a cloverleaf structure and an internal ribosome entry site (IRES). Cellular proteins that bind to the cloverleaf structure regulate viral RNA synthesis, while those that bind to the IRES also known as IRES trans-acting factors (ITAFs) regulate viral translation. In this review, we survey the cellular proteins currently known to bind the 5'-UTR and influence viral gene expression with emphasis on comparing proteins' functions and localizations pre- and post-(EV-A71) infection. A comprehensive understanding of how the host cell's machinery is hijacked and reprogrammed by the virus to facilitate its replication is crucial for developing effective antivirals.

Advances and Breakthroughs in IRES-Directed Translation and Replication of Picornaviruses

Viruses lack the properties to replicate independently due to the limited resources encoded in their genome; therefore, they hijack the host cell machinery to replicate and survive. Picornaviruses get the prerequisite for effective protein synthesis through specific sequences known as internal ribosome entry sites (IRESs). In the past 2 decades, significant progress has been made in identifying different types of IRESs in picornaviruses. This review will discuss the past and current findings related to the five different types of IRESs and various internal ribosome entry site trans-acting factors (ITAFs) that either promote or suppress picornavirus translation and replication. Some IRESs are inefficient and thus require ITAFs. To achieve their full efficiency, they recruit various ITAFs, which enable them to translate more effectively and efficiently, except type IV IRES, which does not require any ITAFs. Although there are two kinds of ITAFs, one promotes viral IRES-dependent translation, and the second type restricts. Picornaviruses IRESs are classified into five types based on their use of sequence, ITAFs, and initiation factors. Some ITAFs regulate IRES activity by localizing to the viral replication factories in the cytoplasm. Also, some drugs, chemicals, and herbal extracts also regulate viral IRES-dependent translation and replication. Altogether, this review will elaborate on our understanding of the past and recent advancements in the IRES-dependent translation and replication of picornaviruses. IMPORTANCE The family Picornaviridae is divided into 68 genera and 158 species. The viruses belonging to this family range from public health importance, such as poliovirus, enterovirus A71, and hepatitis A virus, to animal viruses of great economic importance, such as foot-and-mouth disease virus. The genomes of picornaviruses contain 5' untranslated regions (5' UTRs), which possess crucial and highly structured stem-loops known as IRESs. IRES assemble the ribosomes and facilitate the cap-independent translation. Virus-host interaction is a hot spot for researchers, which warrants deep insight into understanding viral pathogenesis better and discovering new tools and ways for viral restriction to improve human and animal health. The cap-independent translation in the majority of picornaviruses is modulated by ITAFs, which bind to various IRES regions to initiate the translation. The discoveries of ITAFs substantially contributed to understanding viral replication behavior and enhanced our knowledge about virus-host interaction more effectively than ever before. This review discussed the various types of IRESs found in Picornaviridae, past and present discoveries regarding ITAFs, and their mechanism of action. The herbal extracts, drugs, and chemicals, which indicated their importance in controlling viruses, were also summarized. In addition, we discussed the movement of ITAFs from the nucleus to viral replication factories. We believe this review will stimulate researchers to search for more novel ITAFs, drugs, herbal extracts, and chemicals, enhancing the understanding of virus-host interaction.

RNA Structure in the 5' Untranslated Region of Enterovirus D68 Strains with Differing Neurovirulence Phenotypes

Enterovirus-D68 (EV-D68) is a positive-sense single-stranded RNA virus within the family Picornaviridae. EV-D68 was initially considered a respiratory virus that primarily affected children. However, in 2014, EV-D68 outbreaks occurred causing the expected increase in respiratory illness cases, but also an increase in acute flaccid myelitis cases (AFM). Sequencing of 2014 outbreak isolates revealed variations in the 5' UTR of the genome compared to the historical Fermon strain. The structure of the 5' UTR RNA contributes to enterovirus virulence, including neurovirulence in poliovirus, and could contribute to neurovirulence in contemporary EV-D68 strains. In this study, the secondary and tertiary structures of 5' UTR RNA from the Fermon strain and 2014 isolate KT347251.1 are analyzed and compared. Secondary structures were determined using SHAPE-MaP and TurboFold II and tertiary structures were predicted using 3dRNAv2.0. Comparison of RNA structures between the EV-D68 strains shows significant remodeling at the secondary and tertiary levels. Notable secondary structure changes occurred in domains II, IV and V. Shifts in the secondary structure changed the tertiary structure of the individual domains and the orientation of the domains. Our comparative structural models for EV-D68 5' UTR RNA highlight regions of the molecule that could be targeted for treatment of neurotropic enteroviruses.

Reviewing the Potential Links between Viral Infections and TDP-43 Proteinopathies

Transactive response DNA binding protein 43 kDa (TDP-43) was discovered in 2001 as a cellular factor capable to inhibit HIV-1 gene expression. Successively, it was brought to new life as the most prevalent RNA-binding protein involved in several neurological disorders, such as amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Despite the fact that these two research areas could be considered very distant from each other, in recent years an increasing number of publications pointed out the existence of a potentially important connection. Indeed, the ability of TDP-43 to act as an important regulator of all aspects of RNA metabolism makes this protein also a critical factor during expression of viral RNAs. Here, we summarize all recent observations regarding the involvement of TDP-43 in viral entry, replication and latency in several viruses that include enteroviruses (EVs), Theiler's murine encephalomyelitis virus (TMEV), human immunodeficiency virus (HIV), human endogenous retroviruses (HERVs), hepatitis B virus (HBV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), West Nile virus (WNV), and herpes simplex virus-2 (HSV). In particular, in this work, we aimed to highlight the presence of similarities with the most commonly studied TDP-43 related neuronal dysfunctions.

Biophysical Analysis of Small Molecule Binding to Viral RNA Structures

RNA molecules are essential for carrying genetic information and regulating gene expression in most organisms including human pathogenic RNA and relate retro viruses. Targeting viral RNA (vRNA) structures provide broad opportunities to develop chemical tools to probe molecular virology and to discover novel targets for therapeutic intervention. An increasing number of RNA binding small molecules are being identified, stimulating increased interests in small molecule drug discovery for RNA targets. In this chapter, we describe protocols to characterize and robustly validate vRNA-small molecule (vRNA-sm) interactions starting from vRNA sample preparation, followed by small molecule screening against vRNA targets and finally to validating the vRNA-sm interactions via NMR spectroscopy and calorimetric titrations.

Ribosomal control in RNA virus-infected cells

Viruses are strictly intracellular parasites requiring host cellular functions to complete their reproduction cycle involving virus infection of host cell, viral genome replication, viral protein translation, and virion release. Ribosomes are protein synthesis factories in cells, and viruses need to manipulate ribosomes to complete their protein synthesis. Viruses use translation initiation factors through their own RNA structures or cap structures, thereby inducing ribosomes to synthesize viral proteins. Viruses also affect ribosome production and the assembly of mature ribosomes, and regulate the recognition of mRNA by ribosomes, thereby promoting viral protein synthesis and inhibiting the synthesis of host antiviral immune proteins. Here, we review the remarkable mechanisms used by RNA viruses to regulate ribosomes, in particular, the mechanisms by which RNA viruses induce the formation of specific heterogeneous ribosomes required for viral protein translation. This review provides valuable insights into the control of viral infection and diseases from the perspective of viral protein synthesis.

Phase Separation of Heterogeneous Nuclear Ribonucleoprotein A1 upon Specific RNA-Binding Observed by Magnetic Resonance

Interaction of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) with specific single-stranded RNA and its relation to liquid-liquid phase separation (LLPS) were studied in vitro by magnetic resonance based on site-directed spin labelling. An ensemble model of dispersed hnRNP A1 in the absence of RNA was derived from distance distributions between spin labelled sites and small angle X-ray scattering. This model revealed a compact state of the low-complexity domain and its interaction with the RNA recognition motifs. Paramagnetic relaxation enhancement NMR spectroscopy confirmed this interaction. Addition of RNA to dispersed hnRNP A1 induced liquid-droplet formation. Such LLPS depended on RNA concentration and sequence, with continuous wave EPR spectroscopy showing an influence of RNA point mutations on local protein dynamics. We propose that an interplay of sequence-specific RNA binding and LLPS contributes to regulation of specific RNA segregation during stress response.

Synergistic combination of ritonavir and cisplatin as an efficacious therapy in human cervical cancer cells: a computational drug discovery and in vitro insight

HIV-protease inhibitor Ritonavir (RTV) is a clinical-stage drug. We exhibit here the synergistic effect of RTV coupled with cisplatin as potential combination therapy for treatment of cervical cancer. Knowledge about the interaction of RTV with the high-expression signatures in cancer is limited. Therefore, we utilized computational techniques to understand and assess the drug-binding affinity and drug-target interaction of RTV with these altered protein signatures. Computational studies revealed the potential interaction ability of RTV along with few other HIV protease inhibitors against these altered cancer targets. All targets exhibited good affinity towards RTV and the highest affinity was exhibited by CYP450 3A4, PDGFR and ALK. RTV established stable interaction with PDGFR and molecular dynamics simulation confirms their frequent interaction for 300 ns. Control docking of PDGFR with standard PDGFR inhibitor exhibited lower binding affinity when compared with RTV-PDGFR complex. In search of drugs as a part of combination therapy to reduce side effects of Cisplatin, this paper further evaluated the effect of combination of RTV and Cisplatin in cervical cancer cells. We propose several combination models that combines anti-viral drug RTV and standard chemotherapeutic agent, Cisplatin to be synergistic with CI value ranging from of 0.01 to 1.14. These observations suggest that anti-viral compound (RTV) could act synergistically with Cisplatin for cervical cancer therapy. However, further studies are warranted to investigate the combinatorial mode of action of RTV and Cisplatin on different molecular pathways to have a translational outcome in cervical cancer.Communicated by Ramaswamy H. Sarma.

RNA-Binding Proteins as Regulators of Internal Initiation of Viral mRNA Translation

Viruses are obligate intracellular parasites that depend on the host’s protein synthesis machinery for translating their mRNAs. The viral mRNA (vRNA) competes with the host mRNA to recruit the translational machinery, including ribosomes, tRNAs, and the limited eukaryotic translation initiation factor (eIFs) pool. Many viruses utilize non-canonical strategies such as targeting host eIFs and RNA elements known as internal ribosome entry sites (IRESs) to reprogram cellular gene expression, ensuring preferential translation of vRNAs. In this review, we discuss vRNA IRES-mediated translation initiation, highlighting the role of RNA-binding proteins (RBPs), other than the canonical translation initiation factors, in regulating their activity.

Seneca Valley virus 3Cpro degrades heterogeneous nuclear ribonucleoprotein A1 to facilitate viral replication

Seneca Valley virus (SVV) is a recently-identified important pathogen that is closely related to idiopathic vesicular disease in swine. Infection of SVV has been shown to induce a variety of cellular factors and their activations are essential for viral replication, but whether heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) involved in SVV replication is unknown. The cytoplasmic redistribution of hnRNP A1 is considered to play an important role in the virus life cycle. Here, we demonstrated that SVV infection can promote redistribution of the nucleocytoplasmic shuttling RNA-binding protein hnRNP A1 to the cytoplasm from the nucleus, whereas hnRNP A1 remained mainly in the nucleus of mock-infected cells. siRNA-mediated knockdown of the gene encoding hnRNP A1 attenuated viral replication as evidenced by decreased viral protein expression and virus production, whereas its overexpression enhanced replication. Moreover, infection with SVV induced the degradation of hnRNP A1, and viral 3 C protease (3 C^pro) was found to be responsible for its degradation and translocation. Further studies demonstrated that 3 C^pro induced hnRNP A1 degradation through its protease activity, via the proteasome pathway. This degradation could be attenuated by a proteasome inhibitor (MG132) and inactivation of the conserved catalytic box in 3 C^pro. Taken together, these results presented here reveal that SVV 3 C protease targets cellular hnRNP A1 for its degradation and translocation, which is utilized by SVV to aid viral replication, thereby highlighting the control potential of strategies for infection of SVV.

An integrative NMR-SAXS approach for structural determination of large RNAs defines the substrate-free state of a trans-cleaving Neurospora Varkud Satellite ribozyme

The divide-and-conquer strategy is commonly used for protein structure determination, but its applications to high-resolution structure determination of RNAs have been limited. Here, we introduce an integrative approach based on the divide-and-conquer strategy that was undertaken to determine the solution structure of an RNA model system, the Neurospora VS ribozyme. NMR and SAXS studies were conducted on a minimal trans VS ribozyme as well as several isolated subdomains. A multi-step procedure was used for structure determination that first involved pairing refined NMR structures with SAXS data to obtain structural subensembles of the various subdomains. These subdomain structures were then assembled to build a large set of structural models of the ribozyme, which was subsequently filtered using SAXS data. The resulting NMR-SAXS structural ensemble shares several similarities with the reported crystal structures of the VS ribozyme. However, a local structural difference is observed that affects the global fold by shifting the relative orientation of the two three-way junctions. Thus, this finding highlights a global conformational change associated with substrate binding in the VS ribozyme that is likely critical for its enzymatic activity. Structural studies of other large RNAs should benefit from similar integrative approaches that allow conformational sampling of assembled fragments.

Host restriction factor A3G inhibits the replication of Enterovirus D68 through competitively binding 5' UTR with PCBP1

The host restriction factor APOBEC3G (A3G) presents extensively inhibition on a variety of viruses, including retroviruses, DNA and RNA viruses. Our recent study showed that A3G inhibits enterovirus 71 (EV71) and coxsackievirus A16 (CA16) via competitively binding 5'UTR with the host protein poly(C)-binding protein 1 (PCBP1) that is required for multiple EVs replication. However, in addition to EV71 and CA16, whether A3G inhibits other EVs has not been investigated. Here, we demonstrate that A3G could inhibit EVD68 replication, which needs PCBP1 for its replication, but not CA6 that PCBP1 is dispensable for CA6 replication. Further investigation revealed that nucleic acid binding activity of A3G is required for EVD68 restriction, which is similar to the mechanism presented in EV71 restriction. Mechanistically, A3G competitively binds to the cloverleaf (1-123) and the stem-loop IV (234-446) domains of EVD68 5'UTR with PCBP1, thereby inhibiting the 5'UTR activity of EVD68, whereas A3G doesn't interact with CA6 5'UTR results in no effect on CA6 replication. Moreover, non-structural protein 2C encoded by EVD68 overcomes A3G suppression through inducing A3G degradation via the autophagy-lysosome pathway. Our finding revealed that A3G might have broad spectrum antiviral activity against multiple EVs through the general mechanism, which might provide important information for the development of anti-EVs strategy. Importance As the two major pathogens causing hand, food, and mouth disease (HFMD), EV71 and CA16 attract more attention for the discovery of pathogenesis, the involvement of cellular proteins and so on. However, other EVs such as CA6 or EVD68 constantly occurred sporadic or might spread widely in recent years worldwide. Therefore, more information related to these EVs needs to be further investigated so as to develop broad-spectrum anti-EVs inhibitor. In this study, we first reveal that PCBP1 involved in PV and EV71 virus replication, also is required for the replication of EVD68 but not CA6. Then we found that the host restriction factor A3G specifically inhibits the replication of EVD68 but not CA6 via competitively binding to the 5'UTR of EVD68 with PCBP1. Our findings broaden the knowledge related to EVs replication and the interplay between EVs and host factors.

Esculetin inhibits endometrial cancer proliferation and promotes apoptosis via hnRNPA1 to downregulate BCLXL and XIAP

Endometrial cancer represents one of the most common gynecological tumors in the world. Advanced and relapsed patients rely on drug therapy. Therefore, it is extremely important to seek more effective targeted drugs. This study found that esculetin has an anti-tumor effect on endometrial cancer through cellular proliferation and apoptosis. At the same time, its anti-tumor effect has also been verified in human endometrial cancer xenograft models in nude mice. Western blot results showed that BCLXL, XIAP, and pAKT protein expression level were down-regulated. A pulldown experiment and LC-MS/MS analysis technology revealed that esculetin targets the hnRNPA1 protein. Cellular proliferation experiments following si-hnRNPA1 transfection verified the tumor-promoting effect of hnRNPA1 in endometrial cancer cells. Nuclear and cytoplasmic separation experiment demonstrated esculetin affecting the export of the hnRNPA1/mRNA complex from the nucleus into the cytoplasm. Thus, esculetin targets hnRNPA1, thereby downregulates BCLXL and XIAP mRNA transcription and translation, resulting in apoptosis and an arrest in proliferation.

RNA-Binding Proteins at the Host-Pathogen Interface Targeting Viral Regulatory Elements

Viral RNAs contain the information needed to synthesize their own proteins, to replicate, and to spread to susceptible cells. However, due to their reduced coding capacity RNA viruses rely on host cells to complete their multiplication cycle. This is largely achieved by the concerted action of regulatory structural elements on viral RNAs and a subset of host proteins, whose dedicated function across all stages of the infection steps is critical to complete the viral cycle. Importantly, not only the RNA sequence but also the RNA architecture imposed by the presence of specific structural domains mediates the interaction with host RNA-binding proteins (RBPs), ultimately affecting virus multiplication and spreading. In marked difference with other biological systems, the genome of positive strand RNA viruses is also the mRNA. Here we focus on distinct types of positive strand RNA viruses that differ in the regulatory elements used to promote translation of the viral RNA, as well as in the mechanisms used to evade the series of events connected to antiviral response, including translation shutoff induced in infected cells, assembly of stress granules, and trafficking stress.

Optimized photochemistry enables efficient analysis of dynamic RNA structuromes and interactomes in genetic and infectious diseases

Direct determination of RNA structures and interactions in living cells is critical for understanding their functions in normal physiology and disease states. Here, we present PARIS2, a dramatically improved method for RNA duplex determination in vivo with >4000-fold higher efficiency than previous methods. PARIS2 captures ribosome binding sites on mRNAs, reporting translation status on a transcriptome scale. Applying PARIS2 to the U8 snoRNA mutated in the neurological disorder LCC, we discover a network of dynamic RNA structures and interactions which are destabilized by patient mutations. We report the first whole genome structure of enterovirus D68, an RNA virus that causes polio-like symptoms, revealing highly dynamic conformations altered by antiviral drugs and different pathogenic strains. We also discover a replication-associated asymmetry on the (+) and (-) strands of the viral genome. This study establishes a powerful technology for efficient interrogation of the RNA structurome and interactome in human diseases.

HnRNP A1/A2 Proteins Assemble onto 7SK snRNA via Context Dependent Interactions

7SK small nuclear RNA (snRNA) is an abundant and ubiquitously expressed noncoding RNA that functions to modulate the activity of RNA Polymerase II (RNAPII) in part by stabilizing distinct pools of 7SK-protein complexes. Prevailing models suggest that the secondary structure of 7SK is dynamically remodeled within its alternative RNA-protein pools such that its architecture differentially regulates the exchange of cognate binding partners. The nuclear hnRNP A1/A2 proteins influence the biology of 7SK snRNA via processes that require an intact stem loop (SL) 3 domain; however, the molecular details by which hnRNPs assemble onto 7SK snRNA are yet to be described. Here, we have taken an integrated approach to present a detailed description of the 7SK-hnRNP A1 complex. We show that unbound 7SK snRNA adopts at least two major conformations in solution, with significant structural differences localizing to the SL2-3 linker and the base of SL3. Phylogenetic analysis indicates that this same region is the least genetically conserved feature of 7SK snRNA. By performing DMS modifications with the presence of excess protein, we reveal that hnRNP A1 binds with selectivity to SL3 through mechanisms that increase the flexibility of the RNA adjacent to putative binding sites. Calorimetric titrations further validate that hnRNP A1-SL3 assembly is complex with the affinity of discrete binding events modulated by the surrounding RNA structure. To interpret this context-dependent binding phenomenon, we determined a 3D model of SL3 to show that it folds to position minimal hnRNP A1/A2 binding sites (5'-Y/RAG-3') within different local environments. SL3-protein complexes resolved by SEC-MALS-SAXS confirm that up to four hnRNP A1 proteins bind along the entire surface of SL3 via interactions that preserve the overall structural integrity of this domain. In sum, the collective results presented here reveal a specific role for a folded SL3 domain to scaffold hnRNP A1/A2-7SK assembly via mechanisms modulated by the surrounding RNA structure.

TAR DNA-Binding Protein 43 is Cleaved by the Protease 3C of Enterovirus A71

Enterovirus A71 (EV-A71) is one of the etiological pathogens leading to hand, foot, and mouth disease (HFMD), which can cause severe neurological complications. The neuropathogenesis of EV-A71 infection is not well understood. The mislocalization and aggregation of TAR DNA-binding protein 43 (TDP-43) is the pathological hallmark of amyotrophic lateral sclerosis (ALS). However, whether TDP-43 was impacted by EV-A71 infection is unknown. This study demonstrated that TDP-43 was cleaved during EV-A71 infection. The cleavage of TDP-43 requires EV-A71 replication rather than the activated caspases due to viral infection. TDP-43 is cleaved by viral protease 3C between the residues 331Q and 332S, while mutated TDP-43 (Q331A) was not cleaved. In addition, mutated 3C which lacks the protease activity failed to induce TDP-43 cleavage. We also found that TDP-43 was translocated from the nucleus to the cytoplasm, and the mislocalization of TDP-43 was induced by viral protease 2A rather than 3C. Taken together, we demonstrated that TDP-43 was cleaved by viral protease and translocated to the cytoplasm during EV-A71 infection, implicating the possible involvement of TDP-43 in the pathogenesis of EV-A71infection.

Coxsackievirus B3 Infection of Human iPSC Lines and Derived Primary Germ-Layer Cells Regarding Receptor Expression

The association of members of the enterovirus family with pregnancy complications up to miscarriages is under discussion. Here, infection of two different human induced pluripotent stem cell (iPSC) lines and iPSC-derived primary germ-layer cells with coxsackievirus B3 (CVB3) was characterized as an in vitro cell culture model for very early human development. Transcriptomic analysis of iPSC lines infected with recombinant CVB3 expressing enhanced green fluorescent protein (EGFP) revealed a reduction in the expression of pluripotency genes besides an enhancement of genes involved in RNA metabolism. The initial distribution of CVB3-EGFP-positive cells within iPSC colonies correlated with the distribution of its receptor coxsackie- and adenovirus receptor (CAR). Application of anti-CAR blocking antibodies supported the requirement of CAR, but not of the co-receptor decay-accelerating factor (DAF) for infection of iPSC lines. Among iPSC-derived germ-layer cells, mesodermal cells were especially vulnerable to CVB3-EGFP infection. Our data implicate further consideration of members of the enterovirus family in the screening program of human pregnancies. Furthermore, iPSCs with their differentiation capacity into cell populations of relevant viral target organs could offer a reliable screening approach for therapeutic intervention and for assessment of organ-specific enterovirus virulence.

Regulation of the proteostasis network during enterovirus infection: A feedforward mechanism for EV-A71 and EV-D68

The proteostasis network guarantees successful protein synthesis, folding, transportation, and degradation. Mounting evidence has revealed that this network maintains proteome integrity and is linked to cellular physiology, pathology, and virus infection. Human enterovirus A71 (EV-A71) and EV-D68 are suspected causative agents of acute flaccid myelitis, a severe poliomyelitis-like neurologic syndrome with no known cure. In this context, further clarification of the molecular mechanisms underlying EV-A71 and EV-D68 infection is paramount. Here, we summarize the components of the proteostasis network that are intercepted by EV-A71 and EV-D68, as well as antivirals that target this network and may help develop improved antiviral drugs.

Small molecule-RNA targeting: starting with the fundamentals

The structural and regulatory elements in therapeutically relevant RNAs offer many opportunities for targeting by small molecules, yet fundamental understanding of what drives selectivity in small molecule:RNA recognition has been a recurrent challenge. In particular, RNAs tend to be more dynamic and offer less chemical functionality than proteins, and biologically active ligands must compete with the highly abundant and highly structured RNA of the ribosome. Indeed, the only small molecule drug targeting RNA other than the ribosome was just approved in August 2020, and our recent survey of the literature revealed fewer than 150 reported chemical probes that target non-ribosomal RNA in biological systems. This Feature outlines our efforts to improve small molecule targeting strategies and gain fundamental insights into small molecule:RNA recognition by analyzing patterns in both RNA-biased small molecule chemical space and RNA topological space privileged for differentiation. First, we synthesized libraries based on RNA binding scaffolds that allowed us to reveal general principles in small molecule:recognition and to ask precise chemical questions about drivers of affinity and selectivity. Elaboration of these scaffolds has led to recognition of medicinally relevant RNA targets, including viral and long noncoding RNA structures. More globally, we identified physicochemical, structural, and spatial properties of biologically active RNA ligands that are distinct from those of protein-targeted ligands, and we have provided the dataset and associated analytical tools as part of a publicly available online platform to facilitate RNA ligand discovery. At the same time, we used pattern recognition protocols to identify RNA topologies that can be differentially recognized by small molecules and have elaborated this technique to visualize conformational changes in RNA secondary structure. These fundamental insights into the drivers of RNA recognition in vitro have led to functional targeting of RNA structures in biological systems. We hope that these initial guiding principles, as well as the approaches and assays developed in their pursuit, will enable rapid progress toward the development of RNA-targeted chemical probes and ultimately new therapeutic approaches to a wide range of deadly human diseases.

The DDX23 Negatively Regulates Translation and Replication of Foot-and-Mouth Disease Virus and Is Degraded by 3C Proteinase

DEAD-box helicase 23 (DDX23) is a host nuclear helicase, which is a part of the spliceosomal complex and involved in pre-mRNA splicing. To investigate whether DDX23, an internal ribosomal entry sites transacting factor (ITAF) affects foot-and-mouth disease virus (FMDV) replication and translation through internal ribosome entry site (IRES)-dependent manner. For this, we utilized a pull-down assay, Western blotting, quantitative real-time PCR, confocal microscopy, overexpression and small interfering RNA knockdown, as well as the median tissue culture infective dose. Our findings showed that FMDV infection inhibited DDX23 expression and the overexpression of DDX23 reduced viral replication, however, CRISPR Cas9 knockout/small interfering RNA knockdown increased FMDV replication. FMDV IRES domain III and IV interacted with DDX23, whereas DDX23 interacted with FMDV 3C proteinase and significantly degraded. The enzymatic activity of FMDV 3C proteinase degraded DDX23, whereas FMDV degraded DDX23 via the lysosomal pathway. Additionally, IRES-driven translation was suppressed in DDX23-overexpressing cells, and was enhanced in DDX23 knocked down. Collectively, our results demonstrated that DDX23 negatively affects FMDV IRES-dependent translation, which could be a useful target for the design of antiviral drugs.

Integrated approaches to reveal mechanisms by which RNA viruses reprogram the cellular environment

RNA viruses are major threats to global society and mass outbreaks can cause long-lasting damage to international economies. RNA and related retro viruses represent a large and diverse family that contribute to the onset of human diseases such as AIDS; certain cancers like T cell lymphoma; severe acute respiratory illnesses as seen with COVID-19; and others. The hallmark of this viral family is the storage of genetic material in the form of RNA, and upon infecting host cells, their RNA genomes reprogram the cellular environment to favor productive viral replication. RNA is a multifunctional biomolecule that not only stores and transmits heritable information, but it also has the capacity to catalyze complex biochemical reactions. It is therefore no surprise that RNA viruses use this functional diversity to their advantage to sustain chronic or lifelong infections. Efforts to subvert RNA viruses therefore requires a deep understanding of the mechanisms by which these pathogens usurp cellular machinery. Here, we briefly summarize several experimental techniques that individually inform on key physicochemical features of viral RNA genomes and their interactions with proteins. Each of these techniques provide important vantage points to understand the complexities of virus-host interactions, but we attempt to make the case that by integrating these and similar methods, more vivid descriptions of how viruses reprogram the cellular environment emerges. These vivid descriptions should expedite the identification of novel therapeutic targets.

IRES-targeting small molecule inhibits enterovirus 71 replication via allosteric stabilization of a ternary complex

Enterovirus 71 (EV71) poses serious threats to human health, particularly in Southeast Asia, and no drugs or vaccines are available. Previous work identified the stem loop II structure of the EV71 internal ribosomal entry site as vital to viral translation and a potential target. After screening an RNA-biased library using a peptide-displacement assay, we identify DMA-135 as a dose-dependent inhibitor of viral translation and replication with no significant toxicity in cell-based studies. Structural, biophysical, and biochemical characterization support an allosteric mechanism in which DMA-135 induces a conformational change in the RNA structure that stabilizes a ternary complex with the AUF1 protein, thus repressing translation. This mechanism is supported by pull-down experiments in cell culture. These detailed studies establish enterovirus RNA structures as promising drug targets while revealing an approach and mechanism of action that should be broadly applicable to functional RNA targeting.

Human enterovirus 71 (EV71) contains an internal ribosome entry site (IRES) that promotes translation of viral RNA. Here the authors show that an antiviral small molecule DMA-135 binds to the EV71 IRES RNA, inducing conformational change and stabilizing a ternary complex to repress translation.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

Molecular Pathogenicity of Enteroviruses Causing Neurological Disease

Enteroviruses are single-stranded positive-sense RNA viruses that primarily cause self-limiting gastrointestinal or respiratory illness. In some cases, these viruses can invade the central nervous system, causing life-threatening neurological diseases including encephalitis, meningitis and acute flaccid paralysis (AFP). As we near the global eradication of poliovirus, formerly the major cause of AFP, the number of AFP cases have not diminished implying a non-poliovirus etiology. As the number of enteroviruses linked with neurological disease is expanding, of which many had previously little clinical significance, these viruses are becoming increasingly important to public health. Our current understanding of these non-polio enteroviruses is limited, especially with regards to their neurovirulence. Elucidating the molecular pathogenesis of these viruses is paramount for the development of effective therapeutic strategies. This review summarizes the clinical diseases associated with neurotropic enteroviruses and discusses recent advances in the understanding of viral invasion of the central nervous system, cell tropism and molecular pathogenesis as it correlates with host responses.

Viral RNA structure analysis using DMS-MaPseq

RNA structure is critically important to RNA viruses in every part of the replication cycle. RNA structure is also utilized by DNA viruses in order to regulate gene expression and interact with host factors. Advances in next-generation sequencing have greatly enhanced the utility of chemical probing in order to analyze RNA structure. This review will cover some recent viral RNA structural studies using chemical probing and next-generation sequencing as well as the advantages of dimethyl sulfate (DMS)-mutational profiling and sequencing (MaPseq). DMS-MaPseq is a robust assay that can easily modify RNA in vitro, in cell and in virion. A detailed protocol for whole-genome DMS-MaPseq from cells transfected with HIV-1 and the structure of TAR as determined by DMS-MaPseq is presented. DMS-MaPseq has the ability to answer a variety of integral questions about viral RNA, including how they change in different environments and when interacting with different host factors.

Functional analyses of mammalian virus 5'UTR-derived, small RNAs that regulate virus translation

Enterovirus A71 (EV-A71¹) RNA contains an internal ribosomal entry site (IRES) to direct cap-independent translation. IRES-dependent translation requires the host's translation initiation factors and IRES-associated trans-acting factors (ITAFs). We previously showed that hnRNP A1, the mRNA stability factor HuR, and the RISC subunit Argonaute 2 (Ago2) are ITAFs that associate with stem loop II (SL-II) of the IRES and promote IRES-dependent translation. By contrast, the mRNA decay factor AUF1 is a negative-acting ITAF that also binds SL-II. Moreover, the small RNA-processing enzyme Dicer produces at least four virus-derived, small RNAs (vsRNAs 1-4) from the EV-A71 5'UTR in infected cells. One of these, vsRNA1, derived from SL-II, inhibits IRES activity via an unknown mechanism. In vitro RNA-binding assays revealed that vsRNA1 can alter association of Ago2, HuR, and AUF1 with SL-II. This presents a possible mechanism by which vsRNA1 could control association of ITAFs with the IRES and modulate viral translation. Here, we describe methods for functional analyses of vsRNA1-mediated regulation of IRES activity. These methods should be applicable to other virus-derived, small RNAs as well.

Translation control of Enterovirus A71 gene expression

Upon EV-A71 infection of a host cell, EV-A71 RNA is translated into a viral polyprotein. Although EV-A71 can use the cellular translation machinery to produce viral proteins, unlike cellular translation, which is cap-dependent, the viral RNA genome of EV-A71 does not contain a 5′ cap and the translation of EV-A71 protein is cap-independent, which is mediated by the internal ribosomal entry site (IRES) located in the 5′ UTR of EV-A71 mRNA. Like many other eukaryotic viruses, EV-A71 manipulates the host cell translation devices, using an elegant RNA-centric strategy in infected cells. During viral translation, viral RNA plays an important role in controlling the stage of protein synthesis. In addition, due to the cellular defense mechanism, viral replication is limited by down-regulating translation. EV-A71 also utilizes protein factors in the host to overcome antiviral responses or even use them to promote viral translation rather than host cell translation. In this review, we provide an introduction to the known strategies for EV-A71 to exploit cellular translation mechanisms.

Mapping the RNA structural landscape of viral genomes

Functional RNA structures are prevalent in viral genomes, and have been shown to play roles in almost every aspect of their biology. However, the majority of viral RNA remains structurally uncharacterized. This is likely to remain true as the cost of sequencing decreases much faster than the cost of structural characterizations. Because of this, there is a need for rapid, inexpensive methods to highlight regions of viral RNA which are ideal candidates for structure-function analyses. The ScanFold method was developed as a single sequence alternative to traditional RNA structural motif discovery pipelines, which rely heavily on well curated sequence alignments to identify conserved RNA structures. ScanFold focuses on identifying (based on their more stable than expected folding energies) the most likely functional structures encoded within a single large RNA sequence, while allowing predicted motifs to be tested for evidence of structural conservation later. Decoupling these processes can be a benefit to researchers studying viruses lacking the ideal phylogenetic depth to yield evidence of structural conservation. Here, we demonstrate how the most significant ScanFold predicted structures correspond to higher base pairing probabilities, SHAPE reactivities, and predict known functional structures within the ZIKV and HIV-1 genomes with accuracy. Best practices and examples are also shown to aid users in utilizing ScanFold for their own systems of interest. ScanFold is available as a Webserver (https://mosslabtools.bb.iastate.edu/scanfold) or can be downloaded (https://github.com/moss-lab/ScanFold) and run locally.

Picornaviruses and RNA Metabolism: Local and Global Effects of Infection

Due to the limiting coding capacity for members of the Picornaviridae family of positive-strand RNA viruses, their successful replication cycles require complex interactions with host cell functions. These interactions span from the down-modulation of many aspects of cellular metabolism to the hijacking of specific host functions used during viral translation, RNA replication, and other steps of infection by picornaviruses, such as human rhinovirus, coxsackievirus, poliovirus, foot-and-mouth disease virus, enterovirus D-68, and a wide range of other human and nonhuman viruses. Although picornaviruses replicate exclusively in the cytoplasm of infected cells, they have extensive interactions with host cell nuclei and the proteins and RNAs that normally reside in this compartment of the cell. This review will highlight some of the more recent studies that have revealed how picornavirus infections impact the RNA metabolism of the host cell posttranscriptionally and how they usurp and modify host RNA binding proteins as well as microRNAs to potentiate viral replication.

Heads or tails? Differential translational regulation in cercarial heads and tails of schistosome worms

Schistosomes are obligate helminths responsible for over 218 million cases of human schistosomiasis in 78 countries around the world. Infection occurs when free-swimming cercariae penetrate human skin and initiate developmental progression into parasitic obligate worms that consume red blood cells. Transcriptomic studies of infectious cercariae reveal abundant mRNAs associated with energy metabolism and host invasion. However, the cercaria is mostly transcriptionally quiescent, suggesting that most mRNAs are primed prior to cercarial escape from the snail host. The use of transcriptomics to understand protein expression presumes that transcription and translation are functionally coupled and the cercarial stage has categorically been treated as a single unit for -omic analysis. Per contra, the relationship between transcription and translation in infectious cercariae has not been described. To understand the correlation between transcription and translation in cercariae, we separately measured nascent translation levels in cercarial heads, cercarial tails and in the developing schistosomula, the next stage of its life cycle. The loss of the cercarial tail is essential for the transformation from a cercaria to a schistosomulum. We observed that translation was initially limited and the translation rate accelerated during the first 72-hours after tail loss. When we tested nascent translation in cercarial heads, cercarial tails, whole cercariae, and 4-hour schistosomula, we found that translation is significantly upregulated in the cercarial tail when compared to the cercarial head and that translation was undetectable in heads using immunofluorescent image quantification (p = .0005). These data represent a major shift in how we understand the cercarial stage. The cercarial head is mostly transcriptionally and translationally quiescent while being sufficient for progression into a schistosomulum. In addition, transcription and translation are not linked in Schistosoma mansoni cercaria. Thus, our current conceptual approach of treating the cercaria as a single functional unit for -omic studies may be insufficient to understand cercarial development.

EV71 3C protease induces apoptosis by cleavage of hnRNP A1 to promote apaf-1 translation

Enterovirus 71 (EV71) induces apoptosis to promote viral particle release. Earlier work showed that EV71 utilizes its 3C protease to induce apoptosis in a caspase-3-dependent pathway, though the mechanism is unknown. However, work from Vagner, Holcik and colleagues showed that host protein heterogeneous ribonucleoprotein A1 (hnRNP A1) binds the IRES of cellular apoptotic peptidase activating factor 1 (apaf-1) mRNA to repress its translation. In this work, we show that apaf-1 expression is essential for EV71-induced apoptosis. EV71 infection or ectopic expression of 3C protease cleaves hnRNP A1, which abolishes its binding to the apaf-1 IRES. This allows IRES-dependent synthesis of apaf-1, activation of caspase-3, and apoptosis. Thus, we reveal a novel mechanism that EV71 utilizes for virus release via a 3C protease-hnRNP A1-apaf-1-caspase-3-apoptosis axis.

RGG-box in hnRNPA1 specifically recognizes the telomere G-quadruplex DNA and enhances the G-quadruplex unfolding ability of UP1 domain

hnRNPA1 is a member of heteronuclear ribonucleoproteins that has been shown to promote telomere elongation apart from its roles in RNA transport and alternative splicing. It is a modular protein with an N-terminal domain called UP1 that consists of two RNA Recognition Motifs (RRM1 and RRM2 domains) and a C-terminal region that harbors functional motifs such as RGG-box, a prion-like domain, and a nuclear shuttling sequence. UP1 has been reported to bind and destabilize telomeric DNA G-quadruplexes and thereby participate in DNA telomere remodeling. An RGG-box motif that consists of four RGG repeats (containing arginine and glycine residues) is located C-terminal to the UP1 domain and constitutes an additional nucleic acid and protein-binding domain. However, the precise role of the RGG-box of hnRNPA1 in telomere DNA recognition and G-quadruplex DNA unfolding remains unexplored. Here, we show that the isolated RGG-box interacts specifically with the structured telomere G-quadruplex DNA but not with the single-stranded DNA. Further the interaction of the RGG-box with the G-quadruplex DNA is dependent on the loop nucleotides of the G-quadruplex. Finally, we show that the RGG-box enhances the G-quadruplex unfolding activity of the adjacent UP1 domain. We propose that UP1 and RGG-box act synergistically to achieve complete telomere G-quadruplex DNA unfolding.

Recent advances on the role of host factors during non-poliovirus enteroviral infections

Non-polio enteroviruses are emerging viruses known to cause outbreaks of polio-like infections in different parts of the world with several cases already reported in Asia Pacific, Europe and in United States of America. These outbreaks normally result in overstretching of health facilities as well as death in children under the age of five. Most of these infections are usually self-limiting except for the neurological complications associated with human enterovirus A 71 (EV-A71). The infection dynamics of these viruses have not been fully understood, with most inferences made from previous studies conducted with poliovirus.Non-poliovirus enteroviral infections are responsible for major outbreaks of hand, foot and mouth disease (HFMD) often associated with neurological complications and severe respiratory diseases. The myriad of disease presentations observed so far in children calls for an urgent need to fully elucidate the replication processes of these viruses. There are concerted efforts from different research groups to fully map out the role of human host factors in the replication cycle of these viral infections. Understanding the interaction between viral proteins and human host factors will unravel important insights on the lifecycle of this groups of viruses.This review provides the latest update on the interplay between human host factors/processes and non-polio enteroviruses (NPEV). We focus on the interactions involved in viral attachment, entry, internalization, uncoating, replication, virion assembly and eventual egress of the NPEV from the infected cells. We emphasize on the virus- human host interplay and highlight existing knowledge gaps that needs further studies. Understanding the NPEV-human host factors interactions will be key in the design and development of vaccines as well as antivirals against enteroviral infections. Dissecting the role of human host factors during NPEV infection cycle will provide a clear picture of how NPEVs usurp the human cellular processes to establish an efficient infection. This will be a boost to the drug and vaccine development against enteroviruses which will be key in control and eventual elimination of the viral infections.

NMR characterization of RNA small molecule interactions

Exciting discoveries of naturally occurring ligand-sensing and disease-linked noncoding RNAs have promoted significant interests in understanding RNA-small molecule interactions. NMR spectroscopy is a powerful tool for characterizing intermolecular interactions. In this review, we describe protocols and approaches for applying NMR spectroscopy to investigate interactions between RNA and small molecules. We review protocols for RNA sample preparation, methods for identifying RNA-binding small molecules, approaches for mapping RNA-small molecule interactions, determining complex structures, and characterizing binding kinetics. We hope this review will provide a guideline to streamline NMR applications in studying RNA-small molecule interactions, facilitating both basic mechanistic understandings of RNA functions and translational efforts in developing RNA-targeted therapeutics.

Hsp27 Responds to and Facilitates Enterovirus A71 Replication by Enhancing Viral Internal Ribosome Entry Site-Mediated Translation

Outbreaks of infections with EV-A71, which causes hand, foot, and mouth disease, severe neurological disorders, and even death, have been repeatedly reported worldwide in recent decades and are a great public health problem for which no approved treatments are available. We show that Hsp27, a heat shock protein, supports EV-A71 infection in two distinct ways to promote viral IRES-dependent translation. A small-molecule Hsp27 inhibitor isolated from a traditional Chinese medicinal herb effectively reduces virus yields. Together, our findings demonstrate that Hsp27 plays an important role in EV-A71 infection and may serve as an antiviral target.

Enterovirus 71 (EV-A71) is a human pathogen that causes hand, foot, and mouth disease (HFMD) and fatal neurological diseases, and no effective treatment is available. Characterization of key host factors is important for understanding its pathogenesis and developing antiviral drugs. Here we report that Hsp27 is one of the most upregulated proteins in response to EV-A71 infection, as revealed by two-dimensional gel electrophoresis-based proteomics studies. Depletion of Hsp27 by small interfering RNA or CRISPR/Cas9-mediated knockout significantly inhibited viral replication, protein expression, and reproduction, while restoration of Hsp27 restored such virus activities. Furthermore, we show that Hsp27 plays a crucial role in regulating viral internal ribosome entry site (IRES) activities by two different mechanisms. Hsp27 markedly promoted 2A^pro-mediated eukaryotic initiation factor 4G cleavage, an important process for selecting and initiating IRES-mediated translation. hnRNP A1 is a key IRES trans-acting factor (ITAF) for enhancing IRES-mediated translation. Surprisingly, knockout of Hsp27 differentially blocked hnRNP A1 but not FBP1 translocation from the nucleus to the cytoplasm and therefore abolished the hnRNP A1 interaction with IRES. Most importantly, the Hsp27 inhibitor 1,3,5-trihydroxy-13,13-dimethyl-2H-pyran [7,6-b] xanthone (TDP), a compound isolated from a traditional Chinese herb, significantly protected against cytopathic effects and inhibited EV-A71 infection. Collectively, our results demonstrate new functions of Hsp27 in facilitating virus infection and provide novel options for combating EV-A71 infection by targeting Hsp27.

IMPORTANCE Outbreaks of infections with EV-A71, which causes hand, foot, and mouth disease, severe neurological disorders, and even death, have been repeatedly reported worldwide in recent decades and are a great public health problem for which no approved treatments are available. We show that Hsp27, a heat shock protein, supports EV-A71 infection in two distinct ways to promote viral IRES-dependent translation. A small-molecule Hsp27 inhibitor isolated from a traditional Chinese medicinal herb effectively reduces virus yields. Together, our findings demonstrate that Hsp27 plays an important role in EV-A71 infection and may serve as an antiviral target.

Molecular Dynamics Simulations of 2-Aminopurine-Labeled Dinucleoside Monophosphates Reveal Multiscale Stacking Kinetics

Molecular dynamics (MD) simulations of 2-aminopurine (2Ap)-labeled DNA dinucleoside monophosphates (DNMPs) were performed to investigate the hypothesis that base stacking dynamics occur on timescales sufficiently rapid to influence the emission signals measured in time-resolved fluorescence experiments. Analysis of multiple microsecond-length trajectories shows that the DNMPs sample all four coplanar stacking motifs. In addition, three metastable unstacked conformations are detected. A hidden Markov-state model (HMSM) was applied to the simulations to estimate transition rates between the stacked and unstacked states. Transitions between different stacked states generally occur at higher rates when the number of nucleobase faces requiring desolvation is minimized. Time constants for structural relaxation range between 1.6 and 25 ns, suggesting that emission from photoexcited 2Ap, which has an excited-state lifetime of 10 ns, is sensitive to base stacking kinetics. A master equation model for the excited-state population of 2Ap predicts multiexponential emission decays that reproduce the sub-10 ns emission decay lifetimes and amplitudes seen in experiments. Combining MD simulations with HMSM analysis is a powerful way to understand the dynamics that influence 2Ap excited-state relaxation and represents an important step toward using observed emission signals to validate MD simulations.

Polypyrimidine Tract-Binding Protein Regulates Enterovirus 71 Translation Through Interaction with the Internal Ribosomal Entry Site

Enterovirus 71 (EV71), a major causative agent of hand, foot, and mouth disease, has caused periodic infection outbreaks in children in the Asia–Pacific region. In order to describe the largely unknown life cycle of EV71, the molecular basis of its virus-host interactions must first be determined. The 5′ untranslated region of EV71 contains a cloverleaf-like structure and internal ribosomal entry site (IRES), which play an important role in transcription and translation of viral protein. We found that polypyrimidine tract-binding protein 1 (PTB) bound to the IRES of EV71. RNA recognition motifs 1 and 2 of PTB were responsible for its binding to the EV71 IRES. Moreover, PTB protein was shuttled from nucleus to cytoplasm after EV71 infection. Additionally, IRES activity and viral protein production were inhibited by PTB knockdown. These results suggest that PTB interacts with the EV71 IRES, and positively regulates viral protein translation.

Idiosyncrasies of hnRNP A1-RNA recognition: Can binding mode influence function

The heterogeneous nuclear ribonucleoproteins (hnRNPs) are a diverse family of RNA binding proteins that function in most stages of RNA metabolism. The prototypical member, hnRNP A1, is composed of three major domains; tandem N-terminal RNA Recognition Motifs (RRMs) and a C-terminal mostly intrinsically disordered region. HnRNP A1 is broadly implicated in basic cellular RNA processing events such as splicing, stability, nuclear export and translation. Due to its ubiquity and abundance, hnRNP A1 is also frequently usurped to control viral gene expression. Deregulation of the RNA metabolism functions of hnRNP A1 in neuronal cells contributes to several neurodegenerative disorders. Because of these roles in human pathologies, the study of hnRNP A1 provides opportunities for the development of novel therapeutics, with disruption of its RNA binding capabilities being the most promising target. The functional diversity of hnRNP A1 is reflected in the complex nature by which it interacts with various RNA targets. Indeed, hnRNP A1 binds both structured and unstructured RNAs with binding affinities that span several magnitudes. Available structures of hnRNP A1-RNA complexes also suggest a degree of plasticity in molecular recognition. Given the reinvigoration in hnRNP A1, the goal of this review is to use the available structural biochemical developments as a framework to interpret its wide-range of RNA functions.