The Intertwined Life Cycles of Enterovirus and Autophagy

Coxsackievirus B3 Activates Macrophages Independently of CAR-Mediated Viral Entry

Enteroviruses are a genus of small RNA viruses that are responsible for approximately one billion global infections annually. These infections range in severity from the common cold and flu-like symptoms to more severe diseases, such as viral myocarditis, pancreatitis, and neurological disorders, that continue to pose a global health challenge with limited therapeutic strategies currently available. In the current study, we sought to understand the interaction between coxsackievirus B3 (CVB3), which is a model enterovirus, and macrophage cells, as there is limited understanding of how this virus interacts with macrophage innate immune cells. Our study demonstrated that CVB3 can robustly activate macrophages without apparent viral replication in these cells. We also showed that myeloid cells lacked the viral entry receptor coxsackievirus and adenovirus receptor (CAR). However, the expression of exogenous CAR in RAW264.7 macrophages was unable to overcome the viral replication deficit. Interestingly, the CAR expression was associated with altered inflammatory responses during prolonged infection. Additionally, we identified the autophagy protein LC3 as a novel stimulus for macrophage activation. These findings provide new insights into the mechanisms of CVB3-induced macrophage activation and its implications for viral pathogenesis.

A review of the influence of environmental pollutants (microplastics, pesticides, antibiotics, air pollutants, viruses, bacteria) on animal viruses

Microorganisms, especially viruses, cause disease in both humans and animals. Environmental chemical pollutants including microplastics, pesticides, antibiotics sand air pollutants arisen from human activities affect both animal and human health. This review assesses the impact of chemical and biological contaminants (virus and bacteria) on viruses including its life cycle, survival, mutations, loads and titers, shedding, transmission, infection, re-assortment, interference, abundance, viral transfer between cells, and the susceptibility of the host to viruses. It summarizes the sources of environmental contaminants, interactions between contaminants and viruses, and methods used to mitigate such interactions. Overall, this review provides a perspective of environmentally co-occurring contaminants on animal viruses that would be useful for future research on virus-animal-human-ecosystem harmony studies to safeguard human and animal health.

Activation of cGAS-STING suppresses coxsackievirus replication via interferon-dependent signaling

Coxsackievirus B3 (CVB3) is a non-enveloped, single-stranded, positive RNA virus known for its role in provoking inflammatory diseases that affect the heart, pancreas, and brain, leading to conditions such as myocarditis, pancreatitis, and meningitis. Currently, there are no FDA-approved drugs treating CVB3 infection; therefore, identifying potential molecular targets for antiviral drug development is imperative. In this study, we examined the possibility of activating the cyclic GMP-AMP (cGAMP) synthase (cGAS)-stimulator of interferon genes (STING) pathway, a cytosolic DNA-sensing pathway that triggers a type-I interferon (IFN) response, in inhibiting CVB3 infection. We found that activation of the cGAS-STING pathway through the application of cGAS (poly dA:dT and herring testes DNA) or STING agonists (2'3'-cGAMP and diamidobenzimidazole), or the overexpression of STING, significantly suppresses CVB3 replication. Conversely, gene-silencing of STING enhances viral replication. Mechanistically, we demonstrated that cGAS-STING activation combats CVB3 infection by inducing IFN response. Notably, we discovered that knockdown of IFN-α/β receptor, a key membrane receptor in type-I IFN signaling, or inhibition of the downstream JAK1/2 signaling with ruxolitinib, mitigates the effects of STING activation, resulting in increased viral protein production. Furthermore, we investigated the interplay between CVB3 and the cGAS-STING pathway. We showed that CVB3 does not trigger cGAS-STING activation; instead, it antagonizes STING and the downstream TBK1 activation induced by cGAMP. In summary, our results provide insights into the interaction of an RNA virus and the DNA-sensing pathway, highlighting the potential for agonist activation of the cGAS-STING pathway in the development of anti-CVB3 drugs.

Licochalcone A regulates viral IRES activity to inhibit enterovirus replication

Enterovirus D68 (EV-D68), belonging to the genus Enterovirus of the Picornavirus family, is an emerging pathogen that can cause neurological and respiratory diseases in children. However, there is little understanding of the pathogenesis of EV-D68, and no effective vaccine or drug for the prevention or treatment of the diseases caused by this virus is available. Autophagy is a cellular process that targets cytoplasmic proteins or organelles to the lysosomes for degradation. Enteroviruses strategically harness the host autophagy pathway to facilitate the completion of their life cycle. Therefore, we selected an autophagy compound library to screen for autophagy-related compounds that may affect viral growth. By using the neutralization screening assay, we identified a compound, 'licochalcone A' that significantly inhibited EV-D68 replication. To investigate the mechanism by which licochalcone A inhibits EV-D68 replication and to identify the viral life cycle stage it inhibits, the time-of-addition, viral attachment, viral entry, and dual-luciferase reporter assays were performed. The results of the time-of-addition assay showed that licochalcone A, a characteristic chalcone found in liquorice roots and widely used in traditional Chinese medicine, inhibits EV-D68 replication during the early stages of the viral life cycle, while those of the dual-luciferase reporter assay showed that licochalcone A does not regulate viral attachment and entry, but inhibits EV-D68 IRES-dependent translation. Licochalcone A also inhibited enterovirus A71 and coxsackievirus B3 but did not significantly inhibit dengue virus 2 or human coronavirus 229E replication. Licochalcone A regulates IRES translation to inhibit EV-D68 viral replication.

Mitochondria Dysfunction at the Heart of Viral Myocarditis: Mechanistic Insights and Therapeutic Implications

The myocardium/heart is the most mitochondria-rich tissue in the human body with mitochondria comprising approximately 30% of total cardiomyocyte volume. As the resident "powerhouse" of cells, mitochondria help to fuel the high energy demands of a continuously beating myocardium. It is no surprise that mitochondrial dysfunction underscores the pathogenesis of many cardiovascular ailments, including those of viral origin such as virus-induced myocarditis. Enteroviruses have been especially linked to injuries of the myocardium and its sequelae dilated cardiomyopathy for which no effective therapies currently exist. Intriguingly, recent mechanistic insights have demonstrated viral infections to directly damage mitochondria, impair the mitochondrial quality control processes of the cell, such as disrupting mitochondrial antiviral innate immune signaling, and promoting mitochondrial-dependent pathological inflammation of the infected myocardium. In this review, we briefly highlight recent insights on the virus-mitochondria crosstalk and discuss the therapeutic implications of targeting mitochondria to preserve heart function and ultimately combat viral myocarditis.

Enterovirus infection and its relationship with neurodegenerative diseases

Neurodegenerative diseases (NDs) are increasingly common, especially in populations with higher life expectancies. They are associated mainly with protein metabolism and structure changes, leading to neuronal cell death. Viral infections affect these cellular processes and may be involved in the etiology of several neurological illnesses, particularly NDs. Enteroviruses (EVs) frequently infect the central nervous system (CNS), causing neurological disease. Inflammation, disruption of the host autophagy machinery, and deregulation and accumulation/misfolding of proteins are the main alterations observed after infection by an EV. In this perspective, we discuss the most recent findings on the subject, examining the possible role of EVs in the development of NDs, and shedding light on the putative role played by these viruses in developing NDs.

Coxsackievirus Protease 2A Targets Host Protease ATG4A to Impair Autophagy

Enteroviruses (EVs) are medically important RNA viruses that cause a broad spectrum of human illnesses for which limited therapy exists. Although EVs have been shown to usurp the cellular recycling process of autophagy for pro-viral functions, the precise manner by which this is accomplished remains to be elucidated. In the current manuscript, we sought to address the mechanism by which EVs subvert the autophagy pathway using Coxsackievirus B3 (CVB3) as a model. We showed that CVB3 infection selectively degrades the autophagy cysteine protease ATG4A but not other isoforms. Exogenous expression of an N-terminally Flag-labeled ATG4A demonstrated the emergence of a 43-kDa cleavage fragment following CVB3 infection. Furthermore, bioinformatics analysis coupled with site-directed mutagenesis and in vitro cleavage assays revealed that CVB3 protease 2A cleaves ATG4A before glycine 374. Using a combination of genetic silencing and overexpression studies, we demonstrated a novel pro-viral function for the autophagy protease ATG4A. Additionally, cleavage of ATG4A was associated with a loss of autophagy function of the truncated cleavage fragment. Collectively, our study identified ATG4A as a novel substrate of CVB3 protease, leading to disrupted host cellular function and sheds further light on viral mechanisms of autophagy dysregulation.

Virus-induced inhibition of cardiac pacemaker channel HCN4 triggers bradycardia in human-induced stem cell system

The enterovirus Coxsackievirus B3 (CVB3) is known to be a major source for the development of cardiac dysfunctions like viral myocarditis (VMC) and dilatative cardiomyopathy (DCM), but also results in bradycardia and fatal cardiac arrest. Besides clinical reports on bradycardia and sudden cardiac death, very little is known about the influence of CVB3 on the activity of human cardiac pacemaker cells. Here, we address this issue using the first human induced pluripotent stem cell (hiPSC)-derived pacemaker-like cells, in which the expression of a transgenic non-infectious variant of CVB3 can be controlled dose- and time-dependently. We found that CVB3 drastically changed hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) distribution and function in hiPSC-derived pacemaker-like tissue. In addition, using HCN4 cell expression systems, we found that HCN4 currents were decreased with altered voltage dependency of activation when CVB3 was expressed. Increased autophagosome formation and autophagosomal HCN4 insertion was observed in hiPSC-derived pacemaker-like cells under CVB3 expression as well. Individual effects of single, non-structural CVB3 proteins were analyzed and demonstrated that CVB3 proteins 2C and 3A had the most robust effect on HCN4 activity. Treatment of cells with the Rab7 inhibitor CID 106770 or the CVB3-3A inhibitor GW5074 led to the recovery of the cytoplasmatic HCN4 accumulation into a healthy appearing phenotype, indicating that malfunctioning Rab7-directed autophagosome transport is involved in the disturbed, cytoplasmatic HCN4 accumulation in CVB3-expressing human pacemaker-like cells. Summarizing, the enterovirus CVB3 inhibits human cardiac pacemaker function by reducing the pacemaker channel plasma membrane density, an effect that can be corrected by pharmacological intervention of endocytic vesicle trafficking.

Density Analysis of Enterovirus D68 Shows Viral Particles Can Associate with Exosomes

Enterovirus D68 (EV-D68) is an emerging pathogen which causes respiratory disease and is associated with an acute flaccid myelitis that predominately affects children. EV-D68 can infect motor neurons, causing cell death and a loss of motor control leading to flaccid paralysis. However, it remains unknown how viral particles gain entry into the central nervous system (CNS). Here, we show that three distinct densities of EV-D68 particle can be isolated from infected muscle and neural cell lines (RD and SH-SY5Y) using high-speed density centrifugation to separate cell supernatant. The lowest-density peak is composed of viral particles, which have adhered to the exterior surface of a small extracellular vesicle called an exosome. Analysis of prototypic (historic) and contemporary EV-D68 strains suggests that binding to exosomes is a ubiquitous characteristic of EV-D68. We further show that interaction with exosomes increases viral infectivity in a neural cell line. Analysis of the two higher-density peaks, which are not associated with exosomes, revealed that a significant amount of viral titer in the modern (2014) EV-D68 strains is found at 1.20 g/cm3, whereas this density has a very low viral titer in the prototypic Fermon strain. IMPORTANCE Despite the strong causal link between enterovirus D68 (EV-D68) and acute flaccid myelitis (AFM), it remains unclear how EV-D68 gains entry into the central nervous system and what receptors enable it to infect motor neurons. We show that EV-D68 particles can adhere to exosomes, placing EV-D68 among a handful of other picornaviruses which are known to interact with extracellular vesicles. Uptake and infection of permissive cells by virally contaminated exosomes would have major implications in the search for the EV-D68 receptor, as well as providing a possible route for viral entry into motor neurons. This work identifies a novel cellular entry route for EV-D68 and may facilitate the identification of genetic risk factors for development of AFM.

The Role of Mitophagy in Viral Infection

Mitophagy, which is able to selectively clear excess or damaged mitochondria, plays a vital role in the quality control of mitochondria and the maintenance of normal mitochondrial functions in eukaryotic cells. Mitophagy is involved in many physiological and pathological processes, including apoptosis, innate immunity, inflammation, cell differentiation, signal transduction, and metabolism. Viral infections cause physical dysfunction and thus pose a significant threat to public health. An accumulating body of evidence reveals that some viruses hijack mitophagy to enable immune escape and self-replication. In this review, we systematically summarize the pathway of mitophagy initiation and discuss the functions and mechanisms of mitophagy in infection with classical swine fever virus and other specific viruses, with the aim of providing a theoretical basis for the prevention and control of related diseases.

Mechanistic insights into COVID-19 by global analysis of the SARS-CoV-2 3CLpro substrate degradome

The main viral protease (3CLpro) is indispensable for SARS-CoV-2 replication. We delineate the human protein substrate landscape of 3CLpro by TAILS substrate-targeted N-terminomics. We identify more than 100 substrates in human lung and kidney cells supported by analyses of SARS-CoV-2-infected cells. Enzyme kinetics and molecular docking simulations of 3CLpro engaging substrates reveal how noncanonical cleavage sites, which diverge from SARS-CoV, guide substrate specificity. Cleaving the interactors of essential effector proteins, effectively stranding them from their binding partners, amplifies the consequences of proteolysis. We show that 3CLpro targets the Hippo pathway, including inactivation of MAP4K5, and key effectors of transcription, mRNA processing, and translation. We demonstrate that Spike glycoprotein directly binds galectin-8, with galectin-8 cleavage disengaging CALCOCO2/NDP52 to decouple antiviral-autophagy. Indeed, in post-mortem COVID-19 lung samples, NDP52 rarely colocalizes with galectin-8, unlike in healthy lungs. The 3CLpro substrate degradome establishes a foundational substrate atlas to accelerate exploration of SARS-CoV-2 pathology and drug design.

SNAP47 Interacts with ATG14 to Promote VP1 Conjugation and CVB3 Propagation

Coxsackievirus B3 (CVB3), an enterovirus (EV) in the family of Picornaviridae, is a global human pathogen for which effective antiviral treatments and vaccines are lacking. Previous research demonstrated that EV-D68 downregulated the membrane fusion protein SNAP47 (synaptosome associated protein 47) and SNAP47 promoted EV-D68 replication via regulating autophagy. In the current study, we investigated the interplay between CVB3 and cellular SNAP47 using HEK293T/HeLa cell models. We showed that, upon CVB3 infection, protein levels of SNAP47 decreased independent of the activity of virus-encoded proteinase 3C. We further demonstrated that the depletion of SNAP47 inhibited CVB3 infection, indicating a pro-viral function of SNAP47. Moreover, we found that SNAP47 co-localizes with the autophagy-related protein ATG14 on the cellular membrane fractions together with viral capsid protein VP1, and expression of SNAP47 or ATG14 enhanced VP1 conjugation. Finally, we revealed that disulfide interactions had an important role in strengthening VP1 conjugation. Collectively, our study elucidated a mechanism by which SNAP47 and ATG14 promoted CVB3 propagation through facilitating viral capsid assembly.

Escaping the Lion's Den: redirecting autophagy for unconventional release and spread of viruses

Autophagy is an evolutionarily conserved process, designed to maintain cellular homeostasis during a range of internal and external stimuli. Conventionally, autophagy is known for coordinated degradation and recycling of intracellular components and removal of cytosolic pathogens. More recently, several lines of evidence have indicated an unconventional, nondegradative role of autophagy for secretion of cargo that lacks a signal peptide. This process referred to as secretory autophagy has also been implicated in the infection cycle of several virus species. This review focuses on the current evidence available on the nondegradative features of autophagy, emphasizing its potential role and unresolved questions in the release and spread of (-) and (+) RNA viruses.

FUS/TLS Suppresses Enterovirus Replication and Promotes Antiviral Innate Immune Responses

During viral infection, the dynamic virus-host relationship is constantly in play. Many cellular proteins, such as RNA-binding proteins (RBPs), have been shown to mediate antiviral responses during viral infection. Here, we report that the RBP FUS/TLS (fused in sarcoma/translocated in liposarcoma) acts as a host-restricting factor against infection with coxsackievirus B3 (CVB3). Mechanistically, we found that deletion of FUS leads to increased viral RNA transcription and enhanced internal ribosome entry site (IRES)-driven translation, with no apparent impact on viral RNA stability. We further demonstrated that FUS physically interacts with the viral genome, which may contribute to direct inhibition of viral RNA transcription/translation. Moreover, we identified a novel function for FUS in regulating host innate immune response. We show that in the absence of FUS, gene expression of type I interferons and proinflammatory cytokines elicited by viral or bacterial infection is significantly impaired. Emerging evidence suggests a role for stress granules (SGs) in antiviral innate immunity. We further reveal that knockout of FUS abolishes the ability to form SGs upon CVB3 infection or poly(I·C) treatment. Finally, we show that, to avoid FUS-mediated antiviral response and innate immunity, CVB3 infection results in cytoplasmic mislocalization and cleavage of FUS through the enzymatic activity of viral proteases. Together, our findings in this study identify FUS as a novel host antiviral factor which restricts CVB3 replication through direct inhibition of viral RNA transcription and protein translation and through regulation of host antiviral innate immunity.IMPORTANCE Enteroviruses are common human pathogens, including those that cause myocarditis (coxsackievirus B3 [CVB3]), poliomyelitis (poliovirus), and hand, foot, and mouth disease (enterovirus 71). Understanding the virus-host interaction is crucial for developing means of treating and preventing diseases caused by these pathogens. In this study, we explored the interplay between the host RNA-binding protein FUS/TLS and CVB3 and found that FUS/TLS restricts CVB3 replication through direct inhibition of viral RNA transcription/translation and through regulation of cellular antiviral innate immunity. To impede the antiviral role of FUS, CVB3 targets FUS for mislocalization and cleavage. Findings from this study provide novel insights into interactions between CVB3 and FUS, which may lead to novel therapeutic interventions against enterovirus-induced diseases.

Coxsackievirus B3 targets TFEB to disrupt lysosomal function

Coxsackievirus B3 (CVB3) is a prevalent etiological agent for viral myocarditis and neurological disorders, particularly in infants and young children. Virus-encoded proteinases have emerged as cytopathic factors that contribute to disease pathogenesis in part through targeting the cellular recycling machinery of autophagy. Although it is appreciated that CVB3 can usurp cellular macroautophagy/autophagy for pro-viral functions, the precise mechanisms by which viral proteinases disrupt autophagy remain incompletely understood. Here we identified TFEB (transcription factor EB), a master regulator of autophagy and lysosome biogenesis, as a novel target of CVB3 proteinase 3 C. Time-course infections uncovered a significant loss of full-length TFEB and the emergence of a lower-molecular mass (~63 kDa) fragment. Cellular and in vitro cleavage assays revealed the involvement of viral proteinase 3 C in the proteolytic processing of TFEB, while site-directed mutagenesis identified the site of cleavage after glutamine 60. Assessment of TFEB transcriptional activity using a reporter construct discovered a loss of function of the cleavage fragment despite nuclear localization and retaining of the ability of DNA and protein binding. Furthermore, we showed that CVB3 infection was also able to trigger cleavage-independent nuclear translocation of TFEB that relied on the serine-threonine phosphatase PPP3/calcineurin. Finally, we demonstrated that both TFEB and TFEB [Δ60] serve roles in viral egress albeit through differing mechanisms. Collectively, this study reveals that CVB3 targets TFEB for proteolytic processing to disrupt host lysosomal function and enhance viral infection.

Abbreviations:ACTB: actin beta; CLEAR: coordinated lysosomal enhancement and regulation; CVB3: coxsackievirus B3; DAPI: 4′,6-diamidino-2-phenylindole; GFP: green fluorescent protein; LAMP1: lysosomal associated membrane protein 1; LTR: LysoTracker Red; PPP3/calcineurin: protein phosphatase 3; PPP3CA: protein phosphatase 3 catalytic subunit A; p-TFEB: phospho-Ser211 TFEB; si-CON: scramble control siRNA; TFEB: transcription factor EB; TFEB [Δ60]: TFEB cleavage fragment that lacks the first 60 amino acids; VP1: viral capsid protein 1

The papain-like protease of coronaviruses cleaves ULK1 to disrupt host autophagy

The ongoing pandemic of COVID-19 alongside the outbreaks of SARS in 2003 and MERS in 2012 underscore the significance to understand betacoronaviruses as a global health challenge. SARS-CoV-2, the etiological agent for COVID-19, has infected over 50 million individuals' worldwide with more than ∼1 million fatalities. Autophagy modulators have emerged as potential therapeutic candidates against SARS-CoV-2 but recent clinical setbacks urge for better understanding of viral subversion of autophagy. Using MHV-A59 as a model betacoronavirus, time-course infections revealed significant loss in the protein level of ULK1, a canonical autophagy-regulating kinase, and the concomitant appearance of a possible cleavage fragment. To investigate whether virus-encoded proteases target ULK1, we conducted in-vitro and cellular cleavage assays and identified ULK1 as a novel bona fide substrate of SARS-CoV-2 papain-like protease (PL^pro). Mutagenesis studies discovered that ULK1 is cleaved at a conserved PL^pro recognition sequence (LGGG) after G499, separating its N-terminal kinase domain from a C-terminal substrate recognition region. Over-expression of SARS-CoV-2 PL^pro is sufficient to impair starvation-induced autophagy and disrupt formation of ULK1-ATG13 complex. Finally, we demonstrated a dual role for ULK1 in MHV-A59 replication, serving a pro-viral functions during early replication that is inactivated at late stages of infection. In conclusion, our study identified a new mechanism by which PL^pro of betacoronaviruses induces viral pathogenesis by targeting cellular autophagy.

Innate immune evasion mediated by picornaviral 3C protease: Possible lessons for coronaviral 3C-like protease?

Severe acute respiratory syndrome coronavirus-2 is the etiological agent of the ongoing pandemic of coronavirus disease-2019, a multi-organ disease that has triggered an unprecedented global health and economic crisis. The virally encoded 3C-like protease (3CL^pro ), which is named after picornaviral 3C protease (3C^pro ) due to their similarities in substrate recognition and enzymatic activity, is essential for viral replication and has been considered as the primary drug target. However, information regarding the cellular substrates of 3CL^pro and its interaction with the host remains scarce, though recent work has begun to shape our understanding more clearly. Here we summarized and compared the mechanisms by which picornaviruses and coronaviruses have evolved to evade innate immune surveillance, with a focus on the established role of 3C^pro in this process. Through this comparison, we hope to highlight the potential action and mechanisms that are conserved and shared between 3C^pro and 3CL^pro . In this review, we also briefly discussed current advances in the development of broad-spectrum antivirals targeting both 3C^pro and 3CL^pro .

Brequinar inhibits enterovirus replication by targeting biosynthesis pathway of pyrimidines

Infection of human enteroviruses could cause diverse diseases ranging from mild respiratory symptoms to neurological complications, and even death. Currently, no-FDA approved antiviral drug is available for clinical treatment of human enteroviruses infection. Brequinar is an immunosuppressive drug currently being used for the prevention of organ graft rejection. The drug repurposing studies show that Brequinar exhibits potent antiviral activity against diverse viruses, including flaviviruses, alphavirus, rhabdovirus, and influenza viruses. The antiviral effect of Brequinar on human enterovirus infection has not been investigated yet. Here, the in vitro study shows that Brequinar potently inhibited EV71, EV70, and CVB3 replication at 50% inhibitory concentration (IC50) of 82.40 nM, 29.26 nM, and 35.14 nM, respectively. The antiviral activity of Brequinar was reversed by supplement exogenous pyrimidines, indicating that the antiviral effect of Brequinar against enterovirus relies on the inhibition of dihydroorotate dehydrogenase (DHODH) activity, which is responsible for the de novo biosynthesis of pyrimidines. These data extend the antiviral spectrum of Brequinar and indicate that Brequinar could serve as a promising antiviral drug to treat EV71 and other enterovirus infections.

Dysregulated autophagy contributes to the pathogenesis of enterovirus A71 infection

Enterovirus A71 (EVA71) infection continues to remain a vital threat to global public health, especially in the Asia–Pacific region. It is one of the most predominant pathogens that cause hand, foot, and mouth disease (HFMD), which occurs mainly in children below 5 years old. Although EVA71 prevalence has decreased sharply in China with the use of vaccines, epidemiological studies still indicate that EVA71 infection involves severe and even fatal HFMD cases. As a result, it remains more fundamental research into the pathogenesis of EVA71 as well as to develop specific anti-viral therapy. Autophagy is a conserved, self-degradation system that is critical for maintaining cellular homeostasis. It involves a variety of biological functions, such as development, cellular differentiation, nutritional starvation, and defense against pathogens. However, accumulating evidence has indicated that EVA71 induces autophagy and hijacks the process of autophagy for their optimal infection during the different stages of life cycle. This review provides a perspective on the emerging evidence that the “positive feedback” between autophagy induction and EVA71 infection, as well as its potential mechanisms. Furthermore, autophagy may be involved in EVA71-induced nervous system impairment through mediating intracranial viral spread and dysregulating host regulator involved self-damage. Autophagy is a promising therapeutic target in EVA71 infection.

Coxsackievirus infection induces a non-canonical autophagy independent of the ULK and PI3K complexes

Coxsackievirus B3 (CVB3) is a single-stranded positive RNA virus that usurps cellular machinery, including the evolutionarily anti-viral autophagy pathway, for productive infections. Despite the emergence of double-membraned autophagosome-like vesicles during CVB3 infection, very little is known about the mechanism of autophagy initiation. In this study, we investigated the role of established autophagy factors in the initiation of CVB3-induced autophagy. Using siRNA-mediated gene-silencing and CRISPR-Cas9-based gene-editing in culture cells, we discovered that CVB3 bypasses the ULK1/2 and PI3K complexes to trigger autophagy. Moreover, we found that CVB3-induced LC3 lipidation occurred independent of WIPI2 and the transmembrane protein ATG9 but required components of the late-stage ubiquitin-like ATG conjugation system including ATG5 and ATG16L1. Remarkably, we showed the canonical autophagy factor ULK1 was cleaved through the catalytic activity of the viral proteinase 3C. Mutagenesis experiments identified the cleavage site of ULK1 after Q524, which separates its N-terminal kinase domain from C-terminal substrate binding domain. Finally, we uncovered PI4KIIIβ (a PI4P kinase), but not PI3P or PI5P kinases as requisites for CVB3-induced LC3 lipidation. Taken together, our studies reveal that CVB3 initiates a non-canonical form of autophagy that bypasses ULK1/2 and PI3K signaling pathways to ultimately converge on PI4KIIIβ- and ATG5–ATG12–ATG16L1 machinery.

Autophagy and metabolism

Metabolism consists of diverse life-sustaining chemical reactions in living organisms. Autophagy is a highly conservative process that responds to various internal and external stresses. Both processes utilize surrounding resources to provide energy and nutrients for the cell. Autophagy progression may proceed to the degradative or secretory pathway determined by Rab family proteins. The former is a degradative and lysosome-dependent catabolic process that produces energy and provides nutrients for the synthesis of essential proteins. The degradative pathway also balances the energy source of the cell and regulates tissue homeostasis. The latter is a newly discovered pathway in which the autophagosome is fused with the plasma membrane. Secretory autophagy participates in diverse functions and diseases ranging from the spread of viral particles to cancer and neurodegenerative diseases. Aberrant metabolism in the body causes various metabolic syndromes. This review explores the relationships among autophagy, metabolism, and related diseases.

Enterovirus Replication Organelles and Inhibitors of Their Formation

Enteroviral replication reorganizes the cellular membrane. Upon infection, viral proteins and hijacked host factors generate unique structures called replication organelles (ROs) to replicate their viral genomes. ROs promote efficient viral genome replication, coordinate the steps of the viral replication cycle, and protect viral RNA from host immune responses. More recent researches have focused on the ultrastructure structures, formation mechanism, and functions in the virus life cycle of ROs. Dynamic model of enterovirus ROs structure is proposed, and the secretory pathway, the autophagy pathway, and lipid metabolism are found to be associated in the formation of ROs. With deeper understanding of ROs, some compounds have been found to show inhibitory effects on viral replication by targeting key proteins in the process of ROs formation. Here, we review the recent findings concerning the role, morphology, biogenesis, formation mechanism, and inhibitors of enterovirus ROs.

Antiviral activity of berberine

Plants are a rich source of new antiviral, pharmacologically active agents. The naturally occurring plant alkaloid berberine (BBR) is one of the phytochemicals with a broad range of biological activity, including anticancer, anti-inflammatory and antiviral activity. BBR targets different steps in the viral life cycle and is thus a good candidate for use in novel antiviral drugs and therapies. It has been shown that BBR reduces virus replication and targets specific interactions between the virus and its host. BBR intercalates into DNA and inhibits DNA synthesis and reverse transcriptase activity. It inhibits replication of herpes simplex virus (HSV), human cytomegalovirus (HCMV), human papillomavirus (HPV), and human immunodeficiency virus (HIV). This isoquinoline alkaloid has the ability to regulate the MEK-ERK, AMPK/mTOR, and NF-κB signaling pathways, which are necessary for viral replication. Furthermore, it has been reported that BBR supports the host immune response, thus leading to viral clearance. In this short review, we focus on the most recent studies on the antiviral properties of berberine and its derivatives, which might be promising agents to be considered in future studies in the fight against the current pandemic SARS-CoV-2, the virus that causes COVID-19.

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.

Dual NDP52 Function in Persistent CSFV Infection

Viruses have evolved many mechanisms to escape host antiviral responses. Previously, we found that classical swine fever virus (CSFV) infection induces autophagy using the autophagosome as a self-replication site, thereby evading the host immune response and promoting long-term infection. However, the underlying mechanisms used by CSFV to enter autophagosomes and the mechanism by which autophagy promotes viral replication remain unclear. We found that CSFV infection inhibited autophagy receptor nuclear dot protein 52 kDa (NDP52) expression, ubiquitination, and SUMO2-4 modification. Further analyses revealed that CSFV mediated ubiquitination and SUMOylation of NDP52 via Pten-induced kinase 1 (PINK1)-Parkin. Moreover, NDP52 inhibition also inhibited CSFV replication and the induction of mitophagy marker proteins expression. Inhibition of NDP52 reduced CD63 expression and binding to CSFV E2 protein, which has an essential role in persistent CSFV infection. As NDP52 has a close relationship with the NF-κB innate immunity pathway and plays an important role in the antiviral response, we investigated whether NDP52 inhibited CSFV replication through the release of immune factors and antivirus signals. Our results showed that inhibiting NDP52 boosted interferon and TNF release and promoted NF-κB pathway activation. In summary, we found that NDP52 inhibition not only reduces CSFV binding and entry into autophagic vesicles, but also inhibits CSFV replication by active NF-κB antiviral immune pathways. Our data reveal a novel mechanism by which NDP52, an autophagy receptor, mediates CSFV infection, and provide new avenues for the development of antiviral strategies.

Multifunctionality of structural proteins in the enterovirus life cycle

Members of the genus Enterovirus have a significant effect on human health, especially in infants and children. Since the viral genome has limited coding capacity, Enteroviruses subvert a range of cellular processes for viral infection via the interaction of viral proteins and numerous cellular factors. Intriguingly, the capsid-receptor interaction plays a crucial role in viral entry and has significant implications in viral pathogenesis. Moreover, interactions between structural proteins and host factors occur directly or indirectly in multiple steps of viral replication. In this review, we focus on the current understanding of the multifunctionality of structural proteins in the viral life cycle, which may constitute valuable targets for antiviral and therapeutic interventions.