From endocytosis to membrane fusion: emerging roles of dynamin in virus entry

Receptor-binding proteins from animal viruses are broadly compatible with human cell entry factors

Cross-species transmission of animal viruses poses a threat to human health. However, systematic experimental assessments of these risks remain scarce. A critical step in viral infection is cellular internalization mediated by viral receptor-binding proteins (RBPs). Here we constructed viral pseudotypes bearing the RBPs of 102 enveloped RNA viruses and assayed their infectivity across 5,202 RBP-cell combinations. This showed that most of the tested viruses have the potential to enter human cells. Pseudotype infectivity varied widely among the 14 viral families examined and was influenced by RBP characteristics, host of origin and target cell type. Cellular gene expression data revealed that the availability of specific cell-surface receptors is not necessarily the main factor limiting viral entry and that additional host factors must be considered. Altogether, these results suggest weak interspecies barriers in the early stages of infection and advance our understanding of the molecular interactions driving viral zoonosis.

Recent advances in solid-state nuclear magnetic resonance studies on membrane fusion proteins

Membrane fusion is an essential biological process that merges two separate lipid bilayers into a whole one. Membrane fusion proteins facilitate this process by bringing lipid bilayers in close proximity to reduce the repulsive energy between membranes. Along with their interactions with membranes, the structures and dynamics of membrane fusion proteins are key to elucidating the mechanisms of membrane fusion. Solid-state NMR (SSNMR) spectroscopy has unique advantages in determining the structures and dynamics of membrane fusion proteins in their membrane-bound states. It has been extensively applied to reveal conformational changes in intermediate states of viral membrane fusion proteins and to characterize the critical lipid-membrane interactions that drive the fusion process. In this review, we summarize recent advancements in SSNMR techniques for studying membrane fusion proteins and their applications in elucidating the mechanisms of membrane fusion.

Lumpy skin disease virus enters into host cells via dynamin-mediated endocytosis and macropinocytosis

Lumpy skin disease virus (LSDV), a ruminant poxvirus of the Capripoxvirus genus, is the etiologic agent of an economically important cattle disease categorized as a notifiable disease by the World Organization for Animal Health. However, the endocytic pathway and their regulatory molecules have not been characterized for LSDV. In the present study, specific pharmacological inhibitors were used to analyze the mechanism of LSDV entry into Mardin-Darby Bovine Kidney cell (MDBK) and bovine mammary epithelial cell (BMEC). The results showed that LSDV entered MDBK and BMEC cells depends on low-pH conditions and dynamin. However, the inhibitor of caveolae- and clathrin-mediated endocytosis cann't inhibit LSDV entry into MDBK and BMEC cells. Furthermore, treatment with specific inhibitors demonstrated that LSDV entry into MDBK and BMEC cells via macropinocytosis depended on the Na1/H1 exchanger (NHE) but not phosphatidylinositol 3-kinase (PI3K). In addition, results demonstrated that these inhibitors inhibited LSDV entry but did not have effect on LSDV binding. Taken together, our study demonstrated that LSDV enters MDBK and BMEC cells through macropinocytosis pathway in a low-PH- and dynamin-dependent manner while independent on PI3K. Results presented in this study potentially provides insight into the entry mechanisms of LSDV, and it may facilitate the development of therapeutic interventions.

Schisandra chinensis inhibits the entry of BoHV-1 by blocking PI3K-Akt pathway and enhances the m6A methylation of gD to inhibit the entry of progeny virus

Schisandra chinensis, a traditional Chinese medicine known for its antitussive and sedative effects, has shown promise in preventing various viral infections. Bovine herpesvirus-1 (BoHV-1) is an enveloped DNA virus that causes respiratory disease in cattle, leading to significant economic losses in the industry. Because the lack of previous reports on Schisandra chinensis resisting BoHV-1 infection, this study aimed to investigate the specific mechanisms involved. Results from TCID50, qPCR, IFA, and western blot analyses demonstrated that Schisandra chinensis could inhibit BoHV-1 entry into MDBK cells, primarily through its extract Methylgomisin O (Meth O). The specific mechanism involved Meth O blocking BoHV-1 entry into cells via clathrin- and caveolin-mediated endocytosis by suppressing the activation of PI3K-Akt signaling pathway. Additionally, findings from TCID50, qPCR, co-immunoprecipitation and western blot assays revealed that Schisandra chinensis blocked BoHV-1 gD transcription through enhancing m6A methylation of gD after virus entry, thereby hindering gD protein expression and preventing progeny virus entry into cells and ultimately inhibiting BoHV-1 replication. Overall, these results suggest that Schisandra chinensis can resist BoHV-1 infection by targeting the PI3K-Akt signaling pathway and inhibiting gD transcription.

Cell entry of bovine respiratory syncytial virus through clathrin-mediated endocytosis is regulated by PI3K-Akt and Src-JNK pathways

Bovine respiratory syncytial virus (BRSV) is an RNA virus with envelope that causes acute, febrile, and highly infectious respiratory diseases in cattle. However, the manner and mechanism of BRSV entry into cells remain unclear. In this study, we aimed to explore the entry manner of BRSV into MDBK cells and its regulatory mechanism. Our findings, based on virus titer, virus copies, western blot and IFA analysis, indicate that BRSV enters MDBK cells through endocytosis, relying on dynamin, specifically via clathrin-mediated endocytosis rather than caveolin-mediated endocytosis and micropinocytosis. We observed that the entered BRSV initially localizes in early endosomes and subsequently localizes in late endosomes. Additionally, our results of western blot, virus titer and virus copies demonstrate that BRSV entry through clathrin-mediated endocytosis is regulated by PI3K-Akt and Src-JNK signaling pathways. Overall, our study suggests that BRSV enters MDBK cells through clathrin-mediated endocytosis, entered BRSV is trafficked to late endosome via early endosome, BRSV entry through clathrin-mediated endocytosis is regulated by PI3K-Akt and Src-JNK signaling pathways.

Cell entry of Bovine herpesvirus-1 through clathrin- and caveolin-mediated endocytosis requires activation of PI3K-Akt-NF-κB and Ras-p38 MAPK pathways as well as the interaction of BoHV-1 gD with cellular receptor nectin-1

Bovine herpesvirus-1 (BoHV-1) can infect all breeds of cattle and cause severe respiratory organs and genital tract diseases. However, the mechanism of BoHV-1 entering the cells remains unclear. In this study, we explored the mechanism of BoHV-1 entering MDBK cells. We found that the entry of BoHV-1 was blocked by NH₄Cl and bafilomycin A1, indicating that BoHV-1 entry is dependent on the acidic environment of endosome. Specific inhibitor dynasore and small interfering RNA (siRNA) knockdown of dynamin-2 inhibited BoHV-1 entry, showing that dynamin is required in BoHV-1 entry. The results of specific inhibitor, siRNA knockdown and co-localization indicating clathrin- and caveolin- mediated endocytosis play a role in BoHV-1 entry. BoHV-1 infection was not affected by EIPA which is a specific inhibitor of macropinocytosis. In addition, we found that BoHV-1 triggered PI3K-Akt-NF-κB and Ras-p38 MAPK signaling pathways to induce clathrin-mediated and caveolin-mediated endocytosis at the early stage of BoHV-1 infection. BoHV-1 binding was sufficient to activate the endocytic signaling pathways and promote viral entry. These two signaling pathways were activated by transfection of viral gD protein, and were inhibited by deletion of viral gD protein and the siRNA knockdown of cellular receptor nectin-1. The results of co-localization indicating the entered BoHV-1 is traced to late endosomes via early endosomes. Our results suggested the interaction of viral gD protein and cellular receptor nectin-1 triggered the PI3K-Akt-NF-κB and Ras-p38 MAPK signaling pathways and induced clathrin-mediated and caveolin-mediated endocytosis to promote BoHV-1 entry into MDBK cells at the early stage of BoHV-1 infection.

SARS-CoV-2 pseudovirus enters the host cells through spike protein-CD147 in an Arf6-dependent manner

The spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants is threatening public health around the world. Endocytosis functions as an important way for viral infection, and SARS-CoV-2 bears no exception. However, the specific endocytic mechanism of SARS-CoV-2 remains unknown. In this study, we used endocytic inhibitors to evaluate the role of different endocytic routes in SARS-CoV-2 pseudovirus infection and found that the viral infection was associated with caveolar/lipid raft- and cytoskeleton-mediated endocytosis, but independent of the clathrin-mediated endocytosis and macropinocytosis. Meanwhile, the knockdown of CD147 and Rab5a in Vero E6 and Huh-7 cells inhibited SARS-CoV-2 pseudovirus infection, and the co-localization of spike protein, CD147, and Rab5a was observed in pseudovirus-infected Vero E6 cells, which was weakened by CD147 silencing, illustrating that SARS-CoV-2 pseudovirus entered the host cells via CD147-mediated endocytosis. Additionally, Arf6 silencing markedly inhibited pseudovirus infection in Vero E6 and Huh-7 cells, while little change was observed in CD147 knockout-Vero E6 cells. This finding indicated Arf6-mediated CD147 trafficking plays a vital role in SARS-CoV-2 entry. Taken together, our findings provide new insights into the CD147-Arf6 axis in mediating SARS-CoV-2 pseudovirus entry into the host cells, and further suggest that blockade of this pathway seems to be a feasible approach to prevent the SARS-CoV-2 infection clinically.

Avian Reovirus σB Interacts with Caveolin-1 in Lipid Rafts during Dynamin-Dependent Caveolae-Mediated Endocytosis

Caveolin-1 (Cav-1) is the basic component of caveolae, a specialized form of lipid raft that plays an essential role in endocytic viral entry. However, the evidence of direct involvement of caveolae and Cav-1 in avian reovirus (ARV) entry remains insufficient. In this study, the membrane lipid rafts were isolated as detergent-resistant microdomains (DRMs) by sucrose gradient centrifugation, and the capsid protein σB of ARV was found to associate with Cav-1 in DRMs fractions. Additionally, the interaction between ARV σB protein and Cav-1 was demonstrated by immunofluorescence co-localization and co-immunoprecipitation assays. Furthermore, we found that the internalization of ARV is sensitive to caveolae and dynamin inhibitors, while it is insensitive to clathrin inhibitors. In conclusion, these results indicate that the ARV σB protein interacts with Cav-1 during dynamin-dependent caveolae-mediated endocytosis for the entry of ARV.

A multi-targeting drug design strategy for identifying potent anti-SARS-CoV-2 inhibitors

The COVID-19, caused by SARS-CoV-2, is threatening public health, and there is no effective treatment. In this study, we have implemented a multi-targeted anti-viral drug design strategy to discover highly potent SARS-CoV-2 inhibitors, which simultaneously act on the host ribosome, viral RNA as well as RNA-dependent RNA polymerases, and nucleocapsid protein of the virus, to impair viral translation, frameshifting, replication, and assembly. Driven by this strategy, three alkaloids, including lycorine, emetine, and cephaeline, were discovered to inhibit SARS-CoV-2 with EC50 values of low nanomolar levels potently. The findings in this work demonstrate the feasibility of this multi-targeting drug design strategy and provide a rationale for designing more potent anti-virus drugs.

Synthesis of Phthaladyn-29 and Naphthalimide-10, GTP Site Directed Dynamin GTPase Inhibitors

Herein we describe the detailed synthesis of the dynamin inhibitors Phthaladyn-29 and Napthaladyn-10, and their chemical scaffold matched partner inactive compounds. Combined with the assay data provided, this allows the interrogation of dynamin in vitro and potentially in vivo.

CryoEM structure of the super-constricted two-start dynamin 1 filament

Dynamin belongs to the large GTPase superfamily, and mediates the fission of vesicles during endocytosis. Dynamin molecules are recruited to the neck of budding vesicles to assemble into a helical collar and to constrict the underlying membrane. Two helical forms were observed: the one-start helix in the constricted state and the two-start helix in the super-constricted state. Here we report the cryoEM structure of a super-constricted two-start dynamin 1 filament at 3.74 Å resolution. The two strands are joined by the conserved GTPase dimeric interface. In comparison with the one-start structure, a rotation around Hinge 1 is observed, essential for communicating the chemical power of the GTPase domain and the mechanical force of the Stalk and PH domain onto the underlying membrane. The Stalk interfaces are well conserved and serve as fulcrums for adapting to changing curvatures. Relative to one-start, small rotations per interface accumulate to bring a drastic change in the helical pitch. Elasticity theory rationalizes the diversity of dynamin helical symmetries and suggests corresponding functional significance.

Neuropsychiatric Symptoms of COVID-19 Explained by SARS-CoV-2 Proteins' Mimicry of Human Protein Interactions

The first clinical symptoms focused on the presentation of coronavirus disease 2019 (COVID-19) have been respiratory failure, however, accumulating evidence also points to its presentation with neuropsychiatric symptoms, the exact mechanisms of which are not well known. By using a computational methodology, we aimed to explain the molecular paths of COVID-19 associated neuropsychiatric symptoms, based on the mimicry of the human protein interactions with SARS-CoV-2 proteins. Methods: Available 11 of the 29 SARS-CoV-2 proteins' structures have been extracted from Protein Data Bank. HMI-PRED (Host-Microbe Interaction PREDiction), a recently developed web server for structural PREDiction of protein-protein interactions (PPIs) between host and any microbial species, was used to find the "interface mimicry" through which the microbial proteins hijack host binding surfaces. Classification of the found interactions was conducted using the PANTHER Classification System. Results: Predicted Human-SARS-CoV-2 protein interactions have been extensively compared with the literature. Based on the analysis of the molecular functions, cellular localizations and pathways related to human proteins, SARS-CoV-2 proteins are found to possibly interact with human proteins linked to synaptic vesicle trafficking, endocytosis, axonal transport, neurotransmission, growth factors, mitochondrial and blood-brain barrier elements, in addition to its peripheral interactions with proteins linked to thrombosis, inflammation and metabolic control. Conclusion: SARS-CoV-2-human protein interactions may lead to the development of delirium, psychosis, seizures, encephalitis, stroke, sensory impairments, peripheral nerve diseases, and autoimmune disorders. Our findings are also supported by the previous in vivo and in vitro studies from other viruses. Further in vivo and in vitro studies using the proteins that are pointed here, could pave new targets both for avoiding and reversing neuropsychiatric presentations.

Inhibitors of endosomal acidification suppress SARS-CoV-2 replication and relieve viral pneumonia in hACE2 transgenic mice

Background

Coronavirus disease 2019 (COVID-19) is caused by SARS-CoV-2 and broke out as a global pandemic in late 2019. The acidic pH environment of endosomes is believed to be essential for SARS-CoV-2 to be able to enter cells and begin replication. However, the clinical use of endosomal acidification inhibitors, typically chloroquine, has been controversial with this respect.

Methods

In this study, RT-qPCR method was used to detect the SARS-CoV-2N gene to evaluate viral replication. The CCK-8 assay was also used to evaluate the cytotoxic effect of SARS-CoV-2. In situ hybridization was used to examine the distribution of the SARS-CoV-2 gene in lung tissues. Hematoxylin and eosin staining was also used to evaluate virus-associated pathological changes in lung tissues.

Results

In this study, analysis showed that endosomal acidification inhibitors, including chloroquine, bafilomycin A1 and NH₄CL, significantly reduced the viral yields of SARS-CoV-2 in Vero E6, Huh-7 and 293T-ACE2 cells. Chloroquine and bafilomycin A1 also improved the viability and proliferation of Vero E6 cells after SARS-CoV-2 infection. Moreover, in the hACE2 transgenic mice model of SARS-CoV-2 infection, chloroquine and bafilomycin A1 reduced viral replication in lung tissues and alleviated viral pneumonia with reduced inflammatory exudation and infiltration in peribronchiolar and perivascular tissues, as well as improved structures of alveolar septum and pulmonary alveoli.

Conclusions

Our research investigated the antiviral effects of endosomal acidification inhibitors against SARS-CoV-2 in several infection models and provides an experimental basis for further mechanistic studies and drug development.

A pharmacological perspective of chloroquine in SARS-CoV-2 infection: An old drug for the fight against a new coronavirus?

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is having serious consequences on health and the economy worldwide. All evidence-based treatment strategies need to be considered to combat this new virus. Drugs need to be considered on scientific grounds of efficacy, safety and cost. Chloroquine (CQ) and hydroxychloroquine (HCQ) are old drugs used in the treatment of malaria. Moreover, their antiviral properties have been previously studied, including against coronaviruses, where evidence of efficacy has been found. In the current race against time triggered by the COVID-19 pandemic, the search for new antivirals is very important. However, consideration should be given to old drugs with known anti-coronavirus activity, such as CQ and HCQ. These could be integrated into current treatment strategies while novel treatments are awaited, also in light of the fact that they display an anticoagulant effect that facilitates the activity of low-molecular-weight heparin, aimed at preventing acute respiratory distress syndrome (ARDS)-associated thrombotic events. The safety of CQ and HCQ has been studied for over 50 years, however recently published data raise concerns for cardiac toxicity of CQ/HCQ in patients with COVID-19. This review also re-examines the real information provided by some of the published alarming reports, although concluding that cardiac toxicity should in any case be stringently monitored in patients receiving CQ/HCQ.

Sec62 Suppresses Foot-and-Mouth Disease Virus Proliferation by Promotion of IRE1α-RIG-I Antiviral Signaling

Foot-and-mouth disease virus (FMDV) is highly infectious and causes a major plague in animal farming. Unfolded protein response is one of the major cellular responses to pathogenic infections, which performs a crucial role in cell survival, apoptosis, and antiviral innate immune response. In this study, we showed that FMDV infection activated two unfolded protein response branches (PERK-eIF2α and ATF6 signaling) in both baby hamster kidney cells (BHK-21) and porcine kidney (PK-15) cells, whereas it suppressed the IRE1α-XBP1 signaling by decreasing IRE1α level. Further study revealed IRE1α signaling as an important antiviral innate immune mechanism against FMDV. Sec62, the transport protein, was greatly decreased at the late stages of FMDV infection. By overexpression and knockdown study, we also found that the expression of Sec62 was positively involved in the levels of IRE1α and RIG-I and subsequent activation of downstream antiviral signaling pathways in FMDV-infected PK-15 cells. Taken together, our study demonstrates that Sec62 is an important antiviral factor that upregulates IRE1α-RIG-I-dependent antiviral innate immune responses, and FMDV evades antiviral host defense mechanism by downregulating Sec62-IRE1α/RIG-I.

Entry of sapelovirus into IPEC-J2 cells is dependent on caveolae-mediated endocytosis

BACKGROUND

Porcine sapelovirus (PSV), a species of the genus Sapelovirus within the family Picornaviridae, are a significant cause of enteritis, pneumonia, polioencephalomyelitis and reproductive disorders in pigs. However, the life cycle of PSV on the molecular level is largely unknown.

METHODS

Here, we used chemical inhibitors, RNA interference, and overexpression of dominant negative (DN) mutant plasmids to verify the roles of distinct endocytic pathways involved in PSV entry into porcine small intestinal epithelial cell line (IPEC-J2).

RESULTS

Our experiments indicated that PSV infection was inhibited when cells were pre-treated with NH₄Cl or chloroquine. Inhibitors nystatin, methyl-β-cyclodextrin, dynasore and wortmannin dramatically reduced PSV entry efficiency, whereas the inhibitors chlorpromazine and EIPA had no effect. Furthermore, overexpression caveolin DN mutant and siRNA against caveolin also decreased virus titers and VP1 protein synthesis, whereas overexpression EPS15 DN mutant and siRNA against EPS15 did not reduce virus infection.

CONCLUSIONS

Our findings suggest that PSV entry into IPEC-J2 cells depends on caveolae/lipid raft mediated-endocytosis, that is pH-dependent and requires dynamin and PI3K but is independent of clathrin and macropinocytosis.

Dynamin Is Required for Efficient Cytomegalovirus Maturation and Envelopment

Cytomegalovirus secondary envelopment occurs in a virus-induced cytoplasmic assembly compartment (vAC) generated via a drastic reorganization of the membranes of the secretory and endocytic systems. Dynamin is a eukaryotic GTPase that is implicated in membrane remodeling and endocytic membrane fission events; however, the role of dynamin in cellular trafficking of viruses beyond virus entry is only partially understood. Mouse embryonic fibroblasts (MEF) engineered to excise all three isoforms of dynamin were infected with mouse cytomegalovirus (MCMV-K181). Immediate-early (IE1; m123) viral protein was detected in these triple dynamin knockout (TKO) cells, as well as in mock-induced parental MEF, at early times postinfection, although levels were reduced in TKO cells, indicating that virus entry was affected but not eliminated. Levels of IE1 protein and another viral early protein (m04) were normalized by 48 h postinfection; however, late protein (m55; gB) expression was reduced in infected TKO cells compared to parental MEF. Ultrastructural analysis revealed intact stages of nuclear virus maturation in both cases with equivalent numbers of nucleocapsids containing packaged viral DNA (C-capsids), indicating successful viral DNA replication, capsid assembly, and genome packaging. Most importantly, severe defects in virus envelopment were visualized in TKO cells but not in parental cells. Dynamin inhibitor (dynasore)-treated MEF showed a phenotype similar to TKO cells upon mouse cytomegalovirus infection, confirming the role of dynamin in late maturation processes. In summary, dynamin-mediated endocytic pathways are critical for the completion of cytoplasmic stages of cytomegalovirus maturation.IMPORTANCE Viruses are known to exploit specific cellular functions at different stages of their life cycle in order to replicate, avoid immune recognition by the host and to establish a successful infection. Cytomegalovirus (CMV)-infected cells are characterized by a prominent cytoplasmic inclusion (virus assembly compartment [vAC]) that is the site of virus maturation and envelopment. While endocytic membranes are known to be the functional components of vAC, knowledge of specific endocytic pathways implicated in CMV maturation and envelopment is lacking. We show here that dynamin, which is an integral part of host endocytic machinery, is largely dispensable for early stages of CMV infection but is required at a late stage of CMV maturation. Studies on dynamin function in CMV infection will help us understand the host-virus interaction pathways amenable to targeting by conventional small molecules, as well as by newer generation nucleotide-based therapeutics (e.g., small interfering RNA, CRISPR/CAS gRNA, etc.).

Viral journeys on the intracellular highways

Viruses are obligate intracellular pathogens that are dependent on cellular machineries for their replication. Recent technological breakthroughs have facilitated reliable identification of host factors required for viral infections and better characterization of the virus-host interplay. While these studies have revealed cellular machineries that are uniquely required by individual viruses, accumulating data also indicate the presence of broadly required mechanisms. Among these overlapping cellular functions are components of intracellular membrane trafficking pathways. Here, we review recent discoveries focused on how viruses exploit intracellular membrane trafficking pathways to promote various stages of their life cycle, with an emphasis on cellular factors that are usurped by a broad range of viruses. We describe broadly required components of the endocytic and secretory pathways, the Endosomal Sorting Complexes Required for Transport pathway, and the autophagy pathway. Identification of such overlapping host functions offers new opportunities to develop broad-spectrum host-targeted antiviral strategies.

Cryo-EM of the dynamin polymer assembled on lipid membrane

Membrane fission is a fundamental process in the regulation and remodeling of cell membranes. Dynamin, a large GTPase, mediates membrane fission by assembling around, constricting and cleaving the necks of budding vesicles¹. Here, we report a 3.75 Å resolution cryo-EM structure of the membrane-associated helical polymer of human dynamin-1 in the GMPPCP bound state. The structure defines the helical symmetry of the dynamin polymer and the positions of the oligomeric interfaces, which were validated by cell-based endocytosis assays. Compared to the lipid-free tetramer form², membrane-associated dynamin binds to the lipid bilayer with its pleckstrin homology domain (PHD) and self-assembles across the helical rungs via the GTPase domain³. Notably, interaction with the membrane and helical assembly is accommodated by a severely bent bundle signaling element (BSE), which connects the GTPase domain with the rest of the protein. The BSE conformation is asymmetric across the inter-rung GTPase interface, and is unique compared to all known nucleotide-bound states of dynamin. The structure suggests that the BSE bends from forces generated from the GTPase dimer interaction that are transferred across the stalk to the PHD and lipid membrane. Mutations disrupting the BSE kink impaired endocytosis. We also report a 10.1 Å resolution cryo-EM map of a super-constricted dynamin polymer showing localized conformational changes at the BSE and GTPase domains induced by GTP hydrolysis that drive membrane constriction. Altogether, the results provide a structural basis for dynamin’s mechanism of action on lipid membrane.

Bunyavirales ribonucleoproteins: the viral replication and transcription machinery

The Bunyavirales order is one of the largest groups of segmented negative-sense single-stranded RNA viruses, which includes many pathogenic strains that cause severe human diseases. The RNA segments of the bunyavirus genome are separately encapsidated by multiple copies of nucleoprotein (N), and both termini of each N-encapsidated genomic RNA segment bind to one copy of the viral L polymerase protein. The viral genomic RNA, N and L protein together form the ribonucleoprotein (RNP) complex that constitutes the molecular machinery for viral genome replication and transcription. Recently, breakthroughs have been achieved in understanding the architecture of bunyavirus RNPs with the determination of the atomic structures of the N and L proteins from various members of this order. In this review, we discuss the structures and functions of these bunyavirus RNP components, as well as viral genome replication and transcription mechanisms.

Targeting endosomal acidification by chloroquine analogs as a promising strategy for the treatment of emerging viral diseases

Emerging viruses such as HIV, dengue, influenza A, SARS coronavirus, Ebola, and other viruses pose a significant threat to human health. Majority of these viruses are responsible for the outbreaks of pathogenic lethal infections. To date, there are no effective therapeutic strategies available for the prophylaxis and treatment of these infections. Chloroquine analogs have been used for decades as the primary and most successful drugs against malaria. Concomitant with the emergence of chloroquine‐resistant Plasmodium strains and a subsequent decrease in the use as antimalarial drugs, other applications of the analogs have been investigated. Since the analogs have interesting biochemical properties, these drugs are found to be effective against a wide variety of viral infections. As antiviral action, the analogs have been shown to inhibit acidification of endosome during the events of replication and infection. Moreover, immunomodulatory effects of analogs have been beneficial to patients with severe inflammatory complications of several viral diseases. Interestingly, one of the successful targeting strategies is the inhibition of HIV replication by the analogs in vitro which are being tested in several clinical trials. This review focuses on the potentialities of chloroquine analogs for the treatment of endosomal low pH dependent emerging viral diseases.

Viral entry pathways: the example of common cold viruses

For infection, viruses deliver their genomes into the host cell. These nucleic acids are usually tightly packed within the viral capsid, which, in turn, is often further enveloped within a lipid membrane. Both protect them against the hostile environment. Proteins and/or lipids on the viral particle promote attachment to the cell surface and internalization. They are likewise often involved in release of the genome inside the cell for its use as a blueprint for production of new viruses. In the following, I shall cursorily discuss the early more general steps of viral infection that include receptor recognition, uptake into the cell, and uncoating of the viral genome. The later sections will concentrate on human rhinoviruses, the main cause of the common cold, with respect to the above processes. Much of what is known on the underlying mechanisms has been worked out by Renate Fuchs at the Medical University of Vienna.

Two host microRNAs influence WSSV replication via STAT gene regulation

MicroRNAs (miRNAs) have important roles in post-transcriptional regulation of gene expression. During viral infection, viruses utilize hosts to enhance their replication by altering cellular miRNAs. The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway plays crucial roles in the antiviral responses. In this study, two miRNAs (miR-9041 and miR-9850) from Macrobrachium rosenbergii were found to promote white spot syndrome virus (WSSV) replication. The up-regulation of miR-9041 or miR-9850 suppresses STAT expression in the gills of M. rosenbergii, which subsequently down-regulates the expression of its downstream dynamin (Dnm) genes: Dnm1, Dnm2, and Dnm3. Knockdown of miR-9041 and miR-9850 restricts WSSV replication by up-regulating STAT and Dnm gene expression. The silencing of STAT, Dnm1, Dnm2, or Dnm3 led to an increase of the number of WSSV copies in shrimp. The injection of recombinant Dnm1, Dnm2, or Dnm3 proteins could inhibit WSSV replication in vivo. Overall, our research indicates the roles of host miRNAs in the enhancement of WSSV replication by regulating the host JAK/STAT pathway.

1,8-Naphthalimide derivatives: new leads against dynamin I GTPase activity

Fragment-based in silico screening against dynamin I (dynI) GTPase activity identified the 1,8-naphthalimide framework as a potential scaffold for the design of new inhibitors targeting the GTP binding pocket of dynI. Structure-based design, synthesis and subsequent optimization resulted in the development of a library of 1,8-naphthalimide derivatives, called the Naphthaladyn™ series, with compounds 23 and 29 being the most active (IC50 of 19.1 ± 0.3 and 18.5 ± 1.7 μM respectively). Compound 29 showed effective inhibition of clathrin-mediated endocytosis (IC50(CME) 66 μM). The results introduce 29 as an optimised GTP-competitive lead Naphthaladyn™ compound for the further development of naphthalimide-based dynI GTPase inhibitors.

Direct cytosolic delivery of siRNA using nanoparticle-stabilized nanocapsules

The use of nanoparticle-stabilized nanocapsules (NPSCs) for the direct cytosolic delivery of siRNA is reported. In this approach, siRNA is complexed with cationic arginine-functionalized gold nanoparticles by electrostatic interactions, with the resulting ensemble self-assembled onto the surface of fatty acid nanodroplets to form a NPSC/siRNA nanocomplex. The complex rapidly delivers siRNA into the cytosol through membrane fusion, a mechanism supported by cellular uptake studies. Using destabilized green fluorescent protein (deGFP) as a target, 90% knockdown was observed in HEK293 cells. Moreover, the delivery of siRNA targeting polo-like kinase 1 (siPLK1) efficiently silenced PLK1 expression in cancer cells with concomitant cytotoxicity.

Simian hemorrhagic fever virus cell entry is dependent on CD163 and uses a clathrin-mediated endocytosis-like pathway

Simian hemorrhagic fever virus (SHFV) causes a severe and almost uniformly fatal viral hemorrhagic fever in Asian macaques but is thought to be nonpathogenic for humans. To date, the SHFV life cycle is almost completely uncharacterized on the molecular level. Here, we describe the first steps of the SHFV life cycle. Our experiments indicate that SHFV enters target cells by low-pH-dependent endocytosis. Dynamin inhibitors, chlorpromazine, methyl-β-cyclodextrin, chloroquine, and concanamycin A dramatically reduced SHFV entry efficiency, whereas the macropinocytosis inhibitors EIPA, blebbistatin, and wortmannin and the caveolin-mediated endocytosis inhibitors nystatin and filipin III had no effect. Furthermore, overexpression and knockout study and electron microscopy results indicate that SHFV entry occurs by a dynamin-dependent clathrin-mediated endocytosis-like pathway. Experiments utilizing latrunculin B, cytochalasin B, and cytochalasin D indicate that SHFV does not hijack the actin polymerization pathway. Treatment of target cells with proteases (proteinase K, papain, α-chymotrypsin, and trypsin) abrogated entry, indicating that the SHFV cell surface receptor is a protein. Phospholipases A2 and D had no effect on SHFV entry. Finally, treatment of cells with antibodies targeting CD163, a cell surface molecule identified as an entry factor for the SHFV-related porcine reproductive and respiratory syndrome virus, diminished SHFV replication, identifying CD163 as an important SHFV entry component.

IMPORTANCE

Simian hemorrhagic fever virus (SHFV) causes highly lethal disease in Asian macaques resembling human illness caused by Ebola or Lassa virus. However, little is known about SHFV's ecology and molecular biology and the mechanism by which it causes disease. The results of this study shed light on how SHFV enters its target cells. Using electron microscopy and inhibitors for various cellular pathways, we demonstrate that SHFV invades cells by low-pH-dependent, actin-independent endocytosis, likely with the help of a cellular surface protein.

Molecular Signaling and Cellular Pathways for Virus Entry

The cell signaling plays a pivotal role in regulating cellular processes and is often manipulated by viruses as they rely on the functions offered by cells for their propagation. The first stage of their host life is to pass the genetic materials into the cell. Although some viruses can directly penetrate into cytosol, in fact, most virus entry into their host cells is through endocytosis. This machinery initiates with cell type specific cellular signaling pathways, and the signaling compounds can be proteins, lipids, and carbohydrates. The activation can be triggered in a very short time after virus binds on target cells, such as receptors. The signaling pathways involved in regulation of viral entry are wide diversity that often cross-talk between different endocytosis results. Furthermore, some viruses have the ability to use the multiple internalization pathways which leads to the regulation being even more complex. In this paper, we discuss some recent advances in our understanding of cellular pathways for virus entry, molecular signaling during virus entry, formation of endocytic vesicles, and the traffic.

Productive entry of HIV-1 during cell-to-cell transmission via dynamin-dependent endocytosis

HIV-1 can be transmitted as cell-free virus or via cell-to-cell contacts. Cell-to-cell transmission between CD4(+) T cells is the more efficient mode of transmission and is predominant in lymphoid tissue, where the majority of virus resides. Yet the cellular mechanisms underlying productive cell-to-cell transmission in uninfected target cells are unclear. Although it has been demonstrated that target cells can take up virus via endocytosis, definitive links between this process and productive infection remain undefined, and this route of transmission has been proposed to be nonproductive. Here, we report that productive cell-to-cell transmission can occur via endocytosis in a dynamin-dependent manner and is sensitive to clathrin-associated antagonists. These data were obtained in a number of CD4(+) T-cell lines and in primary CD4(+) T cells, using both CXCR4- and CCR5-tropic virus. However, we also found that HIV-1 demonstrated flexibility in its use of such endocytic pathways as certain allogeneic transmissions were seen to occur in a dynamin-dependent manner but were insensitive to clathrin-associated antagonists. Also, depleting cells of the clathrin accessory protein AP180 led to a viral uptake defect associated with enhanced infection. Collectively, these data demonstrate that endosomal uptake of HIV-1 during cell-to-cell transmission leads to productive infection, but they are also indicative of a flexible model of viral entry during cell-to-cell transmission, in which the virus can alter its entry route according to the pressures that it encounters.