Bunyavirus requirement for endosomal K+ reveals new roles of cellular ion channels during infection

The plant rhabdovirus viroporin P9 facilitates insect-mediated virus transmission in barley

Potassium (K+) plays crucial roles in both plant development and immunity. However, the function of K+ in plant-virus interactions remains largely unknown. Here, we utilized Barley yellow striate mosaic virus (BYSMV), an insect-transmitted plant cytorhabdovirus, to investigate the interplay between viral infection and plant K+ homeostasis. The BYSMV accessory P9 protein exhibits viroporin activity by enhancing membrane permeability in Escherichia coli. Additionally, P9 increases K+ uptake in yeast (Saccharomyces cerevisiae) cells, which is disrupted by a point mutation of glycine 14 to threonine (P9G14T). Furthermore, BYSMV P9 forms oligomers and targets to both the viral envelope and the plant membrane. Based on the recombinant BYSMV-GFP (BYGFP) virus, a P9-deleted mutant (BYGFPΔP9) was rescued and demonstrated infectivity within individual plant cells of Nicotiana benthamiana and insect vectors. However, BYGFPΔP9 failed to infect barley plants after transmission by insect vectors. Furthermore, infection of barley plants was severely impaired for BYGFP-P9G14T lacking P9 K+ channel activity. In vitro assays demonstrate that K+ facilitates virion disassembly and the release of genome RNA for viral mRNA transcription. Altogether, our results show that the K+ channel activity of viroporins is conserved in plant cytorhabdoviruses and plays crucial roles in insect-mediated virus transmission.

KCNE4 is a crucial host factor for Orf virus infection by mediating viral entry

The orf virus (ORFV) poses a serious threat to the health of domestic small ruminants (i.e., sheep and goats) and humans on a global scale, causing around $150 million in annual losses to livestock industry. However, the host factors involved in ORFV infection and replication are still elusive. In this study, we compared the RNA-seq profiles of ORFV-infected or non-infected sheep testicular interstitial cells (STICs) and identified a novel host gene, potassium voltage-gated channel subfamily E member 4 (KCNE4), as a key host factor involved in the ORFV infection. Both RNA-seq data and RT-qPCR assay revealed a significant increase in the expression of KCNE4 in the infected STICs from 9 to 48 h post infection (hpi). On the other hand, the RT-qPCR assay detected a decrease in ORFV copy number in both the STICs transfected by KCNE4 siRNA and the KCNE4 knockout (KO) HeLa cells after the ORFV infection, together with a reduced fluorescence ratio of ORFV-GFP in the KO HeLa cells at 24 hpi, indicating KCNE4 to be critical for the ORFV infection. Furthermore, the attachment and internalization assays showed decreased ORFV attachment, internalization, replication, and release by the KO HeLa cells, demonstrating a potential inhibition of ORFV entry into the cells by KCNE4. Pretreatment with the KCNE4 inhibitors such as quinidine and fluoxetine significantly repressed the ORFV infection. All our findings reveal KCNE4 as a novel host regulator of the ORFV entry and replication, shedding new insight into the interactive mechanism of ORFV infection. The study also highlights the K+ channels as possible druggable targets to impede viral infection and disease.

Cellular endosomal potassium ion flux regulates arenavirus uncoating during virus entry

Lymphocytic choriomeningitis virus (LCMV) is an enveloped and segmented negative-sense RNA virus classified within the Arenaviridae family of the Bunyavirales order. LCMV is associated with fatal disease in immunocompromised populations and, as the prototypical arenavirus member, acts as a model for the many highly pathogenic members of the Arenaviridae family, such as Junín, Lassa, and Lujo viruses, all of which are associated with devastating hemorrhagic fevers. To enter cells, the LCMV envelope fuses with late endosomal membranes, for which two established requirements are low pH and interaction between the LCMV glycoprotein (GP) spike and secondary receptor CD164. LCMV subsequently uncoats, where the RNA genome-associated nucleoprotein (NP) separates from the Z protein matrix layer, releasing the viral genome into the cytosol. To further examine LCMV endosome escape, we performed an siRNA screen which identified host cell potassium ion (K+) channels as important for LCMV infection, with pharmacological inhibition confirming K+ channel involvement during the LCMV entry phase completely abrogating productive infection. To better understand the K+-mediated block in infection, we tracked incoming virions along their entry pathway under physiological conditions, where uncoating was signified by separation of NP and Z proteins. In contrast, K+ channel blockade prevented uncoating, trapping virions within Rab7 and CD164-positive endosomes, identifying K+ as a third LCMV entry requirement. K+ did not increase GP-CD164 binding or alter GP-CD164-dependent fusion. Thus, we propose that K+ mediates uncoating by modulating NP-Z interactions within the virion interior. These results suggest K+ channels represent a potential anti-arenaviral target.IMPORTANCEArenaviruses can cause fatal human disease for which approved preventative or therapeutic options are not available. Here, using the prototypical LCMV, we identified K+ channels as critical for arenavirus infection, playing a vital role during the entry phase of the infection cycle. We showed that blocking K+ channel function resulted in entrapment of LCMV particles within late endosomal compartments, thus preventing productive replication. Our data suggest K+ is required for LCMV uncoating and genome release by modulating interactions between the viral nucleoprotein and the matrix protein layer inside the virus particle.

Illuminating bunyavirus entry into host cells with fluorescence

Bunyavirales constitute the largest order of enveloped RNA viruses, many members of which cause severe diseases in humans and domestic animals. In recent decades, innovative fluorescence-based methods have paved the way to visualize and track single fluorescent bunyaviral particles in fixed and live cells. This technological breakthrough has enabled imaging of the early stages of infection and the quantification of every step in the bunyavirus cell entry process. Here, we describe the latest procedures for rendering bunyaviral particles fluorescent and discuss the advantages and disadvantages of each approach in light of the most recent advances in fluorescence detection and monitoring of bunyavirus entry. In this mini-review, we also illustrate how fluorescent viral particles are a powerful tool for deciphering the cellular entry process of bunyaviruses, the vast majority of which have not yet been analyzed.

Regulatory effects of potassium channel blockers on potassium channel genes upon nervous necrosis virus infection in sevenband grouper Hyporthodus septumfasciatus

Ion channels in fishes regulate the flow of important ions that play an active role in the excitation and transmission of impulses through neuronal cells. Specific housekeeping genes translates into proteins and selectively permeabilize and facilitate ion crossover transmissions. Potassium (K+) channels play a crucial role in a wide range of functions such as cell volume regulation, hormone secretion, synaptic transmission and muscle contraction. The dysfunction of ion channels result in channelopathies, which hinder critical cellular activities. Recent studies have indicated that viral pathogens tend to regulate cellular ion channels for entry into host cells. Hence, the present study aimed to elucidate the role of K+ channels during nervous necrosis virus (NNV) infections in the sevenband grouper (Hyporthodus septumfasciatus). Real-time PCR with the standardized potassium genes revealed the downregulation of potassium two pore domain channel subfamily member - KCNK10, KCNK9, KCNK2, and KCNK1 genes post infection at both 17 °C and 25 °C whereas an upregulation was noted in the case of gill tissues. SMART analysis revealed a transmembrane region in all genes. Multiple sequence alignment using MultAlin and phylogenetic analysis revealed true homology of potassium genes with other higher vertebrates. In vitro and in vivo challenge study of NNV using Tetra ethyl ammonium (TEA) as potential drug showed inverse relation to that of viral replication and a corresponding downregulation of K+ channel gene expression was observed which was further confirmed by an immunofluorescence assay. These findings indicate that K+ channels play a crucial role during viral infection. Moreover, the observed downregulation can be related to rapid endocytosis resulting from recycling endosomes during a viral infection. Hence, further studies are warranted to better understand the role of K+ channel genes during NNV infection.

Organisation of the orthobunyavirus tripodal spike and the structural changes induced by low pH and K+ during entry

Following endocytosis, enveloped viruses employ the changing environment of maturing endosomes as cues to promote endosomal escape, a process often mediated by viral glycoproteins. We previously showed that both high [K+] and low pH promote entry of Bunyamwera virus (BUNV), the prototypical bunyavirus. Here, we use sub-tomogram averaging and AlphaFold, to generate a pseudo-atomic model of the whole BUNV glycoprotein envelope. We unambiguously locate the Gc fusion domain and its chaperone Gn within the floor domain of the spike. Furthermore, viral incubation at low pH and high [K+], reminiscent of endocytic conditions, results in a dramatic rearrangement of the BUNV envelope. Structural and biochemical assays indicate that pH 6.3/K+ in the absence of a target membrane elicits a fusion-capable triggered intermediate state of BUNV GPs; but the same conditions induce fusion when target membranes are present. Taken together, we provide mechanistic understanding of the requirements for bunyavirus entry.

IFN-stimulated metabolite transporter ENT3 facilitates viral genome release

An increasing amount of evidence emphasizes the role of metabolic reprogramming in immune cells to fight infections. However, little is known about the regulation of metabolite transporters that facilitate and support metabolic demands. In this study, we found that the expression of equilibrative nucleoside transporter 3 (ENT3, encoded by solute carrier family 29 member 3, Slc29a3) is part of the innate immune response, which is rapidly upregulated upon pathogen invasion. The transcription of Slc29a3 is directly regulated by type I interferon-induced signaling, demonstrating that this metabolite transporter is an interferon-stimulated gene (ISG). Suprisingly, we unveil that several viruses, including SARS-CoV-2, require ENT3 to facilitate their entry into the cytoplasm. The removal or suppression of Slc29a3 expression is sufficient to significantly decrease viral replication in vitro and in vivo. Our study reveals that ENT3 is a pro-viral ISG co-opted by some viruses to gain a survival advantage.

High Levels of Extracellular Potassium Can Delay Myxoma Virus Replication by Preventing Release of Virions from the Endosomes

Potassium (K+) is one of the most abundant cations in the human body. Under normal conditions, the vast majority of K+ is found within cells, and the extracellular [K+] is tightly regulated to within 3.0 to 5.0 mM. However, it has recently been shown that high levels of localized necrosis can increase the extracellular concentration of K+ to above 50 mM. This raises the possibility that elevated extracellular K+ might influence a variety of biological processes that occur within regions of necrotic tissue. For example, K+ has been shown to play a central role in the replication cycles of numerous viral families, and in cases of lytic infection, localized regions containing large numbers of necrotic cells can be formed. Here, we show that the replication of the model poxvirus myxoma virus (MYXV) is delayed by elevated levels of extracellular K+. These increased K+ concentrations alter the cellular endocytic pathway, leading to increased phagocytosis but a loss of endosomal/lysosomal segregation. This slows the release of myxoma virus particles from the endosomes, resulting in delays in genome synthesis and infectious particle formation as well as reduced viral spread. Additionally, mathematical modeling predicts that the extracellular K+ concentrations required to impact myxoma virus replication can be reached in viral lesions under a variety of conditions. Taken together, these data suggest that the extracellular [K+] plays a role in determining the outcomes of myxoma infection and that this effect could be physiologically relevant during pathogenic infection. IMPORTANCE Intracellular K+ homeostasis has been shown to play a major role in the replication of numerous viral families. However, the potential impact of altered extracellular K+ concentrations is less well understood. Our work demonstrates that increased concentrations of extracellular K+ can delay the replication cycle of the model poxvirus MYXV by inhibiting virion release from the endosomes. Additionally, mathematical modeling predicts that the levels of extracellular K+ required to impact MYXV replication can likely be reached during pathogenic infection. These results suggest that localized viral infection can alter K+ homeostasis and that these alterations might directly affect viral pathogenesis.

TrkA serves as a virulence modulator in Porphyromonas gingivalis by maintaining heme acquisition and pathogenesis

Periodontitis is an inflammatory disease of the supporting tissues of the teeth, with polymicrobial infection serving as the major pathogenic factor. As a periodontitis-related keystone pathogen, Porphyromonas gingivalis can orchestrate polymicrobial biofilm skewing into dysbiosis. Some metatranscriptomic studies have suggested that modulation of potassium ion uptake might serve as a signal enhancing microbiota nososymbiocity and periodontitis progression. Although the relationship between potassium transport and virulence has been elucidated in some bacteria, less is mentioned about the periodontitis-related pathogen. Herein, we centered on the virulence modulation potential of TrkA, the potassium uptake regulatory protein of P. gingivalis, and uncovered TrkA as the modulator in the heme acquisition process and in maintaining optimal pathogenicity in an experimental murine model of periodontitis. Hemagglutination and hemolytic activities were attenuated in the case of trkA gene loss, and the entire transcriptomic profiling revealed that the trkA gene can control the expression of genes in relation to electron transport chain activity and translation, as well as some transcriptional factors, including cdhR, the regulator of the heme uptake system hmuYR. Collectively, these results link the heme acquisition process to the potassium transporter, providing new insights into the role of potassium ion in P. gingivalis pathogenesis.

The Orthobunyavirus Germiston Enters Host Cells from Late Endosomes

With more than 80 members worldwide, the Orthobunyavirus genus in the Peribunyaviridae family is a large genus of enveloped RNA viruses, many of which are emerging pathogens in humans and livestock. How orthobunyaviruses (OBVs) penetrate and infect mammalian host cells remains poorly characterized. Here, we investigated the entry mechanisms of the OBV Germiston (GERV). Viral particles were visualized by cryo-electron microscopy and appeared roughly spherical with an average diameter of 98 nm. Labeling of the virus with fluorescent dyes did not adversely affect its infectivity and allowed the monitoring of single particles in fixed and live cells. Using this approach, we found that endocytic internalization of bound viruses was asynchronous and occurred within 30 to 40 min. The virus entered Rab5a-positive (Rab5a+) early endosomes and, subsequently, late endosomal vacuoles containing Rab7a but not LAMP-1. Infectious entry did not require proteolytic cleavage, and endosomal acidification was sufficient and necessary for viral fusion. Acid-activated penetration began 15 to 25 min after initiation of virus internalization and relied on maturation of early endosomes to late endosomes. The optimal pH for viral membrane fusion was slightly below 6.0, and penetration was hampered when the potassium influx was abolished. Overall, our study provides real-time visualization of GERV entry into host cells and demonstrates the importance of late endosomal maturation in facilitating OBV penetration. IMPORTANCE Orthobunyaviruses (OBVs), which include La Crosse, Oropouche, and Schmallenberg viruses, represent a growing threat to humans and domestic animals worldwide. Ideally, preventing OBV spread requires approaches that target early stages of infection, i.e., virus entry. However, little is known about the molecular and cellular mechanisms by which OBVs enter and infect host cells. Here, we developed accurate, sensitive tools and assays to investigate the penetration process of GERV. Our data emphasize the central role of late endosomal maturation in GERV entry, providing a comprehensive overview of the early stages of an OBV infection. Our study also brings a complete toolbox of innovative methods to study each step of the OBV entry program in fixed and living cells, from virus binding and endocytosis to fusion and penetration. The information gained herein lays the foundation for the development of antiviral strategies aiming to block OBV entry.

Structural Modifications and Biological Evaluations of Rift Valley Fever Virus Inhibitors Identified from Chemical Library Screening

The Rift Valley fever virus (RVFV) is an emerging high-priority pathogen endemic in Africa with pandemic potential. There is no specific treatment or approved antiviral drugs for the RVFV. We previously developed a cell-based high-throughput assay to screen small molecules targeting the RVFV and identified a potential effective antiviral compound (1-N-(2-(biphenyl-4-yloxy)ethyl)propane-1,3-diamine) as a lead compound. Here, we investigated how structural modifications of the lead compound affected the biological properties and the antiviral effect against the RVFV. We found that the length of the 2-(3-aminopropylamino)ethyl chain of the compound was important for the compound to retain its antiviral activity. The antiviral activity was similar when the 2-(3-aminopropylamino)ethyl chain was replaced with a butyl piperazine chain. However, we could improve the cytotoxicity profile of the lead compound by changing the phenyl piperazine linker from the para-position (compound 9a) to the meta-position (compound 13a). Results from time-of-addition studies suggested that compound 13a might be active during virus post-entry and/or the replication phase of the virus life cycle and seemed to affect the K⁺ channel. The modifications improved the properties of our lead compound, and our data suggest that 13a is a promising candidate to evaluate further as a therapeutic agent for RVFV infection.

Chemical Evolution of Rhinovirus Identifies Capsid-Destabilizing Mutations Driving Low-pH-Independent Genome Uncoating

Rhinoviruses (RVs) cause recurrent infections of the nasal and pulmonary tracts, life-threatening conditions in chronic respiratory illness patients, predisposition of children to asthmatic exacerbation, and large economic cost. RVs are difficult to treat. They rapidly evolve resistance and are genetically diverse. Here, we provide insight into RV drug resistance mechanisms against chemical compounds neutralizing low pH in endolysosomes. Serial passaging of RV-A16 in the presence of the vacuolar proton ATPase inhibitor bafilomycin A1 (BafA1) or the endolysosomotropic agent ammonium chloride (NH4Cl) promoted the emergence of resistant virus populations. We found two reproducible point mutations in viral proteins 1 and 3 (VP1 and VP3), A2526G (serine 66 to asparagine [S66N]), and G2274U (cysteine 220 to phenylalanine [C220F]), respectively. Both mutations conferred cross-resistance to BafA1, NH4Cl, and the protonophore niclosamide, as identified by massive parallel sequencing and reverse genetics, but not the double mutation, which we could not rescue. Both VP1-S66 and VP3-C220 locate at the interprotomeric face, and their mutations increase the sensitivity of virions to low pH, elevated temperature, and soluble intercellular adhesion molecule 1 receptor. These results indicate that the ability of RV to uncoat at low endosomal pH confers virion resistance to extracellular stress. The data endorse endosomal acidification inhibitors as a viable strategy against RVs, especially if inhibitors are directly applied to the airways. IMPORTANCE Rhinoviruses (RVs) are the predominant agents causing the common cold. Anti-RV drugs and vaccines are not available, largely due to rapid evolutionary adaptation of RVs giving rise to resistant mutants and an immense diversity of antigens in more than 160 different RV types. In this study, we obtained insight into the cell biology of RVs by harnessing the ability of RVs to evolve resistance against host-targeting small chemical compounds neutralizing endosomal pH, an important cue for uncoating of normal RVs. We show that RVs grown in cells treated with inhibitors of endolysosomal acidification evolved capsid mutations yielding reduced virion stability against elevated temperature, low pH, and incubation with recombinant soluble receptor fragments. This fitness cost makes it unlikely that RV mutants adapted to neutral pH become prevalent in nature. The data support the concept of host-directed drug development against respiratory viruses in general, notably at low risk of gain-of-function mutations.

COVID-19: the CaMKII-like system of S protein drives membrane fusion and induces syncytial multinucleated giant cells

The SARS-CoV-2 S protein on the membrane of infected cells can promote receptor-dependent syncytia formation, relating to extensive tissue damage and lymphocyte elimination. In this case, it is challenging to obtain neutralizing antibodies and prevent them through antibodies effectively. Considering that, in the current study, structural domain search methods are adopted to analyze the SARS-CoV-2 S protein to find the fusion mechanism. The results show that after the EF-hand domain of S protein bound to calcium ions, S2 protein had CaMKII protein activities. Besides, the CaMKII_AD domain of S2 changed S2 conformation, facilitating the formation of HR1-HR2 six-helix bundles. Apart from that, the Ca2+-ATPase of S2 pumped calcium ions from the virus cytoplasm to help membrane fusion, while motor structures of S drove the CaATP_NAI and CaMKII_AD domains to extend to the outside and combined the viral membrane and the cell membrane, thus forming a calcium bridge. Furthermore, the phospholipid-flipping-ATPase released water, triggering lipid mixing and fusion and generating fusion pores. Then, motor structures promoted fusion pore extension, followed by the cytoplasmic contents of the virus being discharged into the cell cytoplasm. After that, the membrane of the virus slid onto the cell membrane along the flowing membrane on the gap of the three CaATP_NAI. At last, the HR1-HR2 hexamer would fall into the cytoplasm or stay on the cell membrane. Therefore, the CaMKII_like system of S protein facilitated membrane fusion for further inducing syncytial multinucleated giant cells.

Neural Infection by Oropouche Virus in Adult Human Brain Slices Induces an Inflammatory and Toxic Response

Oropouche virus (OROV) is an emerging arbovirus in South and Central Americas with high spreading potential. OROV infection has been associated with neurological complications and OROV genomic RNA has been detected in cerebrospinal fluid from patients, suggesting its neuroinvasive potential. Motivated by these findings, neurotropism and neuropathogenesis of OROV have been investigated in vivo in murine models, which do not fully recapitulate the complexity of the human brain. Here we have used slice cultures from adult human brains to investigate whether OROV is capable of infecting mature human neural cells in a context of preserved neural connections and brain cytoarchitecture. Our results demonstrate that human neural cells can be infected ex vivo by OROV and support the production of infectious viral particles. Moreover, OROV infection led to the release of the pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-α) and diminished cell viability 48 h post-infection, indicating that OROV triggers an inflammatory response and tissue damage. Although OROV-positive neurons were observed, microglia were the most abundant central nervous system (CNS) cell type infected by OROV, suggesting that they play an important role in the response to CNS infection by OROV in the adult human brain. Importantly, we found no OROV-infected astrocytes. To the best of our knowledge, this is the first direct demonstration of OROV infection in human brain cells. Combined with previous data from murine models and case reports of OROV genome detection in cerebrospinal fluid from patients, our data shed light on OROV neuropathogenesis and help raising awareness about acute and possibly chronic consequences of OROV infection in the human brain.

Orthobunyaviruses: From Virus Binding to Penetration into Mammalian Host Cells

With over 80 members worldwide, Orthobunyavirus is the largest genus in the Peribunyaviridae family. Orthobunyaviruses (OBVs) are arthropod-borne viruses that are structurally simple, with a trisegmented, negative-sense RNA genome and only four structural proteins. OBVs are potential agents of emerging and re-emerging diseases and overall represent a global threat to both public and veterinary health. The focus of this review is on the very first steps of OBV infection in mammalian hosts, from virus binding to penetration and release of the viral genome into the cytosol. Here, we address the most current knowledge and advances regarding OBV receptors, endocytosis, and fusion.

Common Dysregulation of Innate Immunity Pathways in Human Primary Astrocytes Infected With Chikungunya, Mayaro, Oropouche, and Zika Viruses

Arboviruses pose a major threat throughout the world and represent a great burden in tropical countries of South America. Although generally associated with moderate febrile illness, in more severe cases they can lead to neurological outcomes, such as encephalitis, Guillain-Barré syndrome, and Congenital Syndromes. In this context astrocytes play a central role in production of inflammatory cytokines, regulation of extracellular matrix, and control of glutamate driven neurotoxicity in the central nervous system. Here, we presented a comprehensive genome-wide transcriptome analysis of human primary astrocytes infected with Chikungunya, Mayaro, Oropouche, or Zika viruses. Analyses of differentially expressed genes (DEGs), pathway enrichment, and interactomes have shown that Alphaviruses up-regulated genes related to elastic fiber formation and N-glycosylation of glycoproteins, with down-regulation of cell cycle and DNA stability and chromosome maintenance genes. In contrast, Oropouche virus up-regulated cell cycle and DNA maintenance and condensation pathways while down-regulated extracellular matrix, collagen metabolism, glutamate and ion transporters pathways. Zika virus infection only up-regulated eukaryotic translation machinery while down-regulated interferon pathways. Reactome and integration analysis revealed a common signature in down-regulation of innate immune response, antiviral response, and inflammatory cytokines associated to interferon pathway for all arboviruses tested. Validation of interferon stimulated genes by reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) corroborated our transcriptome findings. Altogether, our results showed a co-evolution in the mechanisms involved in the escape of arboviruses to antiviral immune response mediated by the interferon (IFN) pathway.

Lysosomal ion channels involved in cellular entry and uncoating of enveloped viruses: Implications for therapeutic strategies against SARS-CoV-2

Ion channels are necessary for correct lysosomal function including degradation of cargoes originating from endocytosis. Almost all enveloped viruses, including coronaviruses (CoVs), enter host cells via endocytosis, and do not escape endosomal compartments into the cytoplasm (via fusion with the endolysosomal membrane) unless the virus-encoded envelope proteins are cleaved by lysosomal proteases. With the ongoing outbreak of severe acute respiratory syndrome (SARS)-CoV-2, endolysosomal two-pore channels represent an exciting and emerging target for antiviral therapies. This review focuses on the latest knowledge of the effects of lysosomal ion channels on the cellular entry and uncoating of enveloped viruses, which may aid in development of novel therapies against emerging infectious diseases such as SARS-CoV-2.

A DNA-based voltmeter for organelles

The role of membrane potential in most intracellular organelles remains unexplored because of the lack of suitable tools. Here, we describe Voltair, a fluorescent DNA nanodevice that reports the absolute membrane potential and can be targeted to organelles in live cells. Voltair consists of a voltage-sensitive fluorophore and a reference fluorophore for ratiometry, and acts as an endocytic tracer. Using Voltair, we could measure the membrane potential of different organelles in situ in live cells. Voltair can potentially guide the rational design of biocompatible electronics and enhance our understanding of how membrane potential regulates organelle biology.

Valinomycin as a potential antiviral agent against coronaviruses: A review

Human coronaviruses (HCoVs), including severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), have been resulting in global epidemics with heavy morbidity and mortality. Unfortunately, there are currently no specific medicines that can better treat these coronaviruses. Drug repurposing is an effective and economical strategy for drug discovery from existing drugs, natural products, and synthetic compounds. In this review, the broad-spectrum antiviral activity of valinomycin (VAL), especially its activity against coronaviruses such as SARS-CoV, MERS-CoV, human coronavirus OC43 (HCoV-OC43), were summarized, it highlights that VAL has tremendous potential for use as a novel antiviral agent against SARS-CoV-2.

Ion Channels as Therapeutic Targets for Viral Infections: Further Discoveries and Future Perspectives

Ion channels play key roles in almost all facets of cellular physiology and have emerged as key host cell factors for a multitude of viral infections. A catalogue of ion channel-blocking drugs have been shown to possess antiviral activity, some of which are in widespread human usage for ion channel-related diseases, highlighting new potential for drug repurposing. The emergence of ion channel–virus interactions has also revealed the intriguing possibility that channelopathies may explain some commonly observed virus induced pathologies. This field is rapidly evolving and an up-to-date summary of new discoveries can inform future perspectives. We herein discuss the role of ion channels during viral lifecycles, describe the recently identified ion channel drugs that can inhibit viral infections, and highlight the potential contribution of ion channels to virus-mediated disease.

Novel Ionophores Active against La Crosse Virus Identified through Rapid Antiviral Screening

Bunyaviruses are significant human pathogens, causing diseases ranging from hemorrhagic fevers to encephalitis. Among these viruses, La Crosse virus (LACV), a member of the California serogroup, circulates in the eastern and midwestern United States. While LACV infection is often asymptomatic, dozens of cases of encephalitis are reported yearly. Unfortunately, no antivirals have been approved to treat LACV infection. Here, we developed a method to rapidly test potential antivirals against LACV infection. From this screen, we identified several potential antiviral molecules, including known antivirals. Additionally, we identified many novel antivirals that exhibited antiviral activity without affecting cellular viability. Valinomycin, a potassium ionophore, was among our top targets. We found that valinomycin exhibited potent anti-LACV activity in multiple cell types in a dose-dependent manner. Valinomycin did not affect particle stability or infectivity, suggesting that it may preclude virus replication by altering cellular potassium ions, a known determinant of LACV entry. We extended these results to other ionophores and found that the antiviral activity of valinomycin extended to other viral families, including bunyaviruses (Rift Valley fever virus, Keystone virus), enteroviruses (coxsackievirus, rhinovirus), flavirivuses (Zika virus), and coronaviruses (human coronavirus 229E [HCoV-229E] and Middle East respiratory syndrome CoV [MERS-CoV]). In all viral infections, we observed significant reductions in virus titer in valinomycin-treated cells. In sum, we demonstrate the importance of potassium ions to virus infection, suggesting a potential therapeutic target to disrupt virus replication.

Identification of potassium and calcium channel inhibitors as modulators of polyomavirus endosomal trafficking

During virus entry, members of the Polyomaviridae transit the endolysosomal network en route to the endoplasmic reticulum (ER), from which degraded capsids escape into the cytoplasm and enter the nucleus. Emerging evidence suggests that viruses require both endosomal acidification and the correct ionic balance of K⁺ and Ca²⁺ ions in endosomes for correct virus trafficking and genome release. Here, using two polyomaviruses with different capsid architectures, namely Simian virus 40 (SV40) and Merkel cell polyomavirus (MCPyV), we describe methods to rapidly quantify virus infection using IncuCyte ZOOM imaging analysis, and use this system to investigate the role of both K⁺ and Ca²⁺ channels during the early stages of virus entry. Using broad spectrum blockers of both K⁺ and Ca²⁺ channels to specifically target host cell ion channel functionality, we show that MCPyV, but not SV40 can be inhibited by K⁺ channel modulators, whilst both viruses are restricted by the broad spectrum Ca²⁺ channel inhibitor verapamil. Using a panel of more specific Ca²⁺ blockers, we show that both MCPyV and SV40 are dependent on the activity of two-pore Ca²⁺ channels (TPCs), as the TPC-specific blocker tetrandrine prevented capsid disassembly and nuclear transport required for virus entry. We therefore reveal a novel target to restrict the entry of polyomaviruses, which given the known role of TPCs during endolysosomal-ER fusion, is likely to be applicable to other viruses that transit this pathway.

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We describe novel high-throughput assays to study SV40 and MCPyV infection.

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MCPyV, but not SV40, is sensitive to K⁺ channel inhibition.

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Verapamil inhibits MCPyV and SV40 infection.

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Tetrandrine is a potent inhibitor of MCPyV and SV40 infection.

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Two-pore channel 1/2 activity is essential for polyomavirus entry.

Glibenclamide inhibits BK polyomavirus infection in kidney cells through CFTR blockade

BK polyomavirus (BKPyV) is a ubiquitous pathogen in the human population that is asymptomatic in healthy individuals, but can be life-threatening in those undergoing kidney transplant. To-date, no vaccines or anti-viral therapies are available to treat human BKPyV infections. New therapeutic strategies are urgently required. In this study, using a rational pharmacological screening regimen of known ion channel modulating compounds, we show that BKPyV requires cystic fibrosis transmembrane conductance regulator (CFTR) activity to infect primary renal proximal tubular epithelial cells. Disrupting CFTR function through treatment with the clinically available drug glibenclamide, the CFTR inhibitor CFTR₁₇₂, or CFTR-silencing, all reduced BKPyV infection. Specifically, time of addition assays and the assessment of the exposure of VP2/VP3 minor capsid proteins indicated a role for CFTR during BKPyV transport to the endoplasmic reticulum, an essential step during the early stages of BKPyV infection. We thus establish CFTR as an important host-factor in the BKPyV life cycle and reveal CFTR modulators as potential anti-BKPyV therapies.

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BK polyomavirus (BKPyV) is life-threatening in those undergoing kidney transplant.

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BKPyV requires CFTR to infect primary kidney cells.

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Disrupting CFTR function pharmacologically reduces BKPyV infection.

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CFTR is required during BKPyV transport to the endoplasmic reticulum.

Evasion of a Human Cytomegalovirus Entry Inhibitor with Potent Cysteine Reactivity Is Concomitant with the Utilization of a Heparan Sulfate Proteoglycan-Independent Route of Entry

The dependence of viruses on the host cell to complete their replicative cycle renders cellular functions potential targets for novel antivirals. We screened a panel of broadly acting cellular ion channel inhibitors for activity against human cytomegalovirus (HCMV) and identified the voltage-gated chloride ion channel inhibitor 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) as a potent inhibitor of HCMV replication. Time-of-addition studies demonstrated that DIDS inhibited entry via direct interaction with the virion that impeded binding to the plasma membrane. Synthesis and analysis of pharmacological variants of DIDS suggested that intrinsic cysteine, and not lysine, reactivity was important for activity against HCMV. Although sequencing of DIDS-resistant HCMV revealed enrichment of a mutation within UL100 (encoding glycoprotein M) and a specific truncation of glycoprotein RL13, these did not explain the DIDS resistance phenotype. Specifically, only the introduction of the RL13 mutant partially phenocopied the DIDS resistance phenotype. Serendipitously, the entry of DIDS-resistant HCMV also became independent of heparan sulfate proteoglycans (HSPGs), suggesting that evasion of DIDS lowered dependence on an initial interaction with HSPGs. Intriguingly, the DIDS-resistant virus demonstrated increased sensitivity to antibody neutralization, which mapped, in part, to the presence of the gM mutation. Taken together the data characterize the antiviral activity of a novel HCMV inhibitor that drives HCMV infection to occur independently of HSPGs and the generation of increased sensitivity to humoral immunity. The data also demonstrate that compounds with cysteine reactivity have the potential to act as antiviral compounds against HCMV via direct engagement of virions.IMPORTANCE Human cytomegalovirus (HCMV) is major pathogen of nonimmunocompetent individuals that remains in need of new therapeutic options. Here, we identify a potent antiviral compound (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid [DIDS]), its mechanism of action, and the chemical properties required for its activity. In doing so, the data argue that cysteine-reactive compounds could have the capacity to be developed for anti-HCMV activity. Importantly, the data show that entry of DIDS-resistant virus became independent of heparan sulfate proteoglycans (HSPGs) but, concomitantly, became more sensitive to neutralizing antibody responses. This serendipitous observation suggests that retention of an interaction with HSPGs during the entry process in vivo may be evolutionarily advantageous through better evasion of humoral responses directed against HCMV virions.

Selectivity filter instability dominates the low intrinsic activity of the TWIK-1 K2P K+ channel

Two-pore domain K⁺ (K2P) channels have many important physiological functions. However, the functional properties of the TWIK-1 (K2P1.1/KCNK1) K2P channel remain poorly characterized because heterologous expression of this ion channel yields only very low levels of functional activity. Several underlying reasons have been proposed, including TWIK-1 retention in intracellular organelles, inhibition by posttranslational sumoylation, a hydrophobic barrier within the pore, and a low open probability of the selectivity filter (SF) gate. By evaluating these potential mechanisms, we found that the latter dominates the low intrinsic functional activity of TWIK-1. Investigating this further, we observed that the low activity of the SF gate appears to arise from the inefficiency of K⁺ in stabilizing an active (i.e. conductive) SF conformation. In contrast, other permeant ion species, such as Rb⁺, NH₄ ⁺, and Cs⁺, strongly promoted a pH-dependent activated conformation. Furthermore, many K2P channels are activated by membrane depolarization via an SF-mediated gating mechanism, but we found here that only very strong nonphysiological depolarization produces voltage-dependent activation of heterologously expressed TWIK-1. Remarkably, we also observed that TWIK-1 Rb⁺ currents are potently inhibited by intracellular K⁺ (IC₅₀ = 2.8 mm). We conclude that TWIK-1 displays unique SF gating properties among the family of K2P channels. In particular, the apparent instability of the conductive conformation of the TWIK-1 SF in the presence of K⁺ appears to dominate the low levels of intrinsic functional activity observed when the channel is expressed at the cell surface.

Chikungunya virus requires cellular chloride channels for efficient genome replication

Chikungunya virus (CHIKV) is a re-emerging, pathogenic alphavirus that is transmitted to humans by Aedes spp. mosquitoes-causing fever and debilitating joint pain, with frequent long-term health implications and high morbidity. The CHIKV lifecycle is poorly understood and specific antiviral therapeutics or vaccines are lacking. In this study, we investigated the role of host-cell chloride (Cl-) channels on CHIKV replication.We demonstrate that specific pharmacological Cl- channel inhibitors significantly inhibit CHIKV replication in a dose-dependent manner, suggesting that Cl-channels are pro-viral factors in human cells. Further analysis of the effect of the inhibitors on CHIKV attachment, entry, viral protein expression and replicon replication demonstrated that Cl- channels are specifically required for efficient CHIKV genome replication. This was conserved in mosquito cells, where CHIKV replication and genome copy number was significantly reduced following Cl- channel inhibition. siRNA silencing identified chloride intracellular channels 1 and 4 (CLIC1 and CLIC4, respectively) as required for efficient CHIKV replication and protein affinity chromatography showed low levels of CLIC1 in complex with CHIKV nsP3, an essential component of the viral replication machinery. In summary, for the first time we demonstrate that efficient replication of the CHIKV genome depends on cellular Cl- channels, in both human and mosquito cells and identifies CLIC1 and CLIC4 as agonists of CHIKV replication in human cells. We observe a modest interaction, either direct or indirect, between CLIC1 and nsP3 and hypothesize that CLIC1 may play a role in the formation/maintenance of CHIKV replication complexes. These findings advance our molecular understanding of CHIKV replication and identify potential druggable targets for the treatment and prevention of CHIKV mediated disease.

Selectivity filter instability dominates the low intrinsic activity of the TWIK-1 K2P K+ Channel.. [DOI: 10.1101/735704] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Two-pore domain (K2P) K+ channels have many important physiological functions. However, the functional properties of the TWIK-1 (K2P1.1/KCNK1) K2P channel remain poorly characterized because heterologous expression of this ion channel yields only very low levels of functional activity. Several underlying reasons have been proposed, including TWIK-1 retention in intracellular organelles, inhibition by post-translational sumoylation, a hydrophobic barrier within the pore, and a low open probability of the selectivity filter (SF) gate. By evaluating these various potential mechanisms, we found that the latter dominates the low intrinsic functional activity of TWIK-1. Investigating the underlying mechanism, we observed that the low activity of the SF gate appears to arise from the inefficiency of K+ in stabilizing an active (i.e. conductive) SF conformation. In contrast, other permeant ion species, such as Rb+, NH4+, and Cs+, strongly promoted a pH-dependent activated conformation. Furthermore, many K2P channels are activated by membrane depolarization via a SF-mediated gating mechanism, but we found here that only very strong, non-physiological depolarization produces voltage-dependent activation of heterologously expressed TWIK-1. Remarkably, we also observed that TWIK-1 Rb+ currents are potently inhibited by intracellular K+ (IC50 = 2.8 mM). We conclude that TWIK-1 displays unique SF gating properties among the family of K2P channels. In particular, the apparent instability of the conductive conformation of the TWIK-1 SF in the presence of K+ appears to dominate the low levels of intrinsic functional activity observed when the channel is expressed at the cell surface.

An exploration of conditions proposed to trigger the Ebola virus glycoprotein for fusion

Ebolaviruses continue to inflict horrific disease and instill fear. The 2013-2016 outbreak in Western Africa caused unfathomable morbidity and mortality (over 11,000 deaths), and the second largest outbreak is on-going in the Democratic Republic of the Congo. The first stage of an Ebolavirus infection is entry, culminating in delivery of the viral genome into the cytoplasm to initiate replication. Among enveloped viruses, Ebolaviruses use a complex entry pathway: they bind to attachment factors on cell surfaces, are engulfed by macropinocytosis, and traffic through the endosomal system. En route, the receptor binding subunit of the glycoprotein (GP) is reduced from ~130 to ~19 kDa by cathepsins. This event allows cleaved GP (GPcl) to bind to Niemann-Pick C1 (NPC1), its endosomal receptor. The virus then fuses with a late endosomal membrane, but how this occurs remains a subject of debate. An early, but standing, observation is that entry of particles bearing GPcl is inhibited by agents that raise endosomal pH or inhibit cysteine proteases, suggesting the need for an additional factor(s). Yet, some have concluded that NPC1 is sufficient to trigger the fusion activity of GPcl. Here, we re-examined this question using sensitive cell-cell and pseudovirus-cell fusion assays. We did not observe detectable GPcl-mediated fusion with NPC1 or its GPcl binding domain at any pH tested, while robust fusion was consistently observed with GP from lymphocytic choriomeningitis virus at low pH. Addition of proposed fusion-enhancing factors-cations (Ca++ and K+), a reducing agent, the anionic lipid Bis(Monoacylglycero)Phosphate, and a mixture of cathepsins B and L-did not induce detectable fusion. Our findings are in line with the earlier proposal that an additional factor is required to trigger the full fusion activity of GPcl after binding to NPC1. We discuss caveats to our study and what the missing factor(s) might be.

Cellular cholesterol abundance regulates potassium accumulation within endosomes and is an important determinant in bunyavirus entry

The Bunyavirales order of segmented negative-sense RNA viruses includes more than 500 isolates that infect insects, animals, and plants and are often associated with severe and fatal disease in humans. To multiply and cause disease, bunyaviruses must translocate their genomes from outside the cell into the cytosol, achieved by transit through the endocytic network. We have previously shown that the model bunyaviruses Bunyamwera virus (BUNV) and Hazara virus (HAZV) exploit the changing potassium concentration ([K⁺]) of maturing endosomes to release their genomes at the appropriate endosomal location. K⁺ was identified as a biochemical cue to activate the viral fusion machinery, promoting fusion between viral and cellular membranes, consequently permitting genome release. In this study, we further define the biochemical prerequisites for BUNV and HAZV entry and their K⁺ dependence. Using drug-mediated cholesterol extraction along with viral entry and K⁺ uptake assays, we report three major findings: BUNV and HAZV require cellular cholesterol during endosomal escape; cholesterol depletion from host cells impairs K⁺ accumulation in maturing endosomes, revealing new insights into endosomal K⁺ homeostasis; and "priming" BUNV and HAZV virions with K⁺ before infection alleviates their cholesterol requirement. Taken together, our findings suggest a model in which cholesterol abundance influences endosomal K⁺ levels and, consequently, the efficiency of bunyavirus infection. The ability to inhibit bunyaviruses with existing cholesterol-lowering drugs may offer new options for future antiviral interventions for pathogenic bunyaviruses.

Orthobunyavirus spike architecture and recognition by neutralizing antibodies

Orthobunyaviruses (OBVs) form a distinct genus of arthropod-borne bunyaviruses that can cause severe disease upon zoonotic transmission to humans. Antigenic drift or genome segment re-assortment have in the past resulted in new pathogenic OBVs, making them potential candidates for causing emerging zoonoses in the future. Low-resolution electron cryo-tomography studies have shown that OBV particles feature prominent trimeric spikes, but their molecular organization remained unknown. Here we report X-ray crystallography studies of four different OBVs showing that the spikes are formed by an N-terminal extension of the fusion glycoprotein Gc. Using Schmallenberg virus, a recently emerged OBV, we also show that the projecting spike is the major target of the neutralizing antibody response, and provide X-ray structures in complex with two protecting antibodies. We further show that immunization of mice with the spike domains elicits virtually sterilizing immunity, providing fundamental knowledge essential in the preparation for potential newly emerging OBV zoonoses.

Orthobunyaviruses (OBVs) cause severe disease in humans and farm animals, but the molecular basis for infection is not fully understood. Here, the authors present crystal structures of free and antibody-bound OBV envelope glycoproteins and show that their domains enable efficient immunization in a mouse model.

Structural and functional similarities in bunyaviruses: Perspectives for pan-bunya antivirals

The order of Bunyavirales includes numerous (re)emerging viruses that collectively have a major impact on human and animal health worldwide. There are no vaccines for human use or antiviral drugs available to prevent or treat infections with any of these viruses. The development of efficacious and safe drugs and vaccines is a pressing matter. Ideally, such antivirals possess pan‐bunyavirus antiviral activity, allowing the containment of every bunya‐related threat. The fact that many bunyaviruses need to be handled in laboratories with biosafety level 3 or 4, the great variety of species and the frequent emergence of novel species complicate such efforts. We here examined the potential druggable targets of bunyaviruses, together with the level of conservation of their biological functions, structure, and genetic similarity by means of heatmap analysis. In the light of this, we revised the available models and tools currently available, pointing out directions for antiviral drug discovery.

Calibration and characterization of intracellular Asante Potassium Green probes, APG-2 and APG-4

The response of fluorescent ion probes to ions is affected by intracellular environment. To properly calibrate them, intracellular and extracellular concentrations of the measured ion must be made equal. In the first, computational, part of this work, we show, using the example of potassium, that the two requirements for ion equilibration are complete dissipation of membrane potential and high membrane permeability for both potassium and sodium. In the second part, we tested the ability of various ionophores to achieve potassium equilibration in Jurkat and U937 cells and found a combination of valinomycin, nigericin, gramicidin and ouabain to be the most effective. In the third part, we applied this protocol to two potassium probes, APG-4 and APG-2. APG-4 shows good sensitivity to potassium but its fluorescence is sensitive to cell volume. Because ionophores cause cell swelling, calibration buffers had to be supplemented with 50 mM sucrose to keep cell volume constant. With these precautions taken, the average potassium concentrations in U937 and Jurkat cells were measured at 132 mM and 118 mM, respectively. The other tested probe, APG-2, is nonselective for cations; this is, however, a potentially useful property because the sum [K⁺] + [Na⁺] determines the amount of intracellular water.

A scorpion venom peptide Ev37 restricts viral late entry by alkalizing acidic organelles

Viral infections still threaten human health all over the world, and many people die from viral diseases every year. However, there are no effective vaccines or drugs for preventing or managing most viral diseases. Thus, the discovery and development of broad-spectrum antiviral agents remain urgent. Here, we expressed and purified a venom peptide, Ev37, from the scorpion Euscorpiops validus in a prokaryotic system. We found that rEv37 can inhibit dengue virus type 2 (DENV-2), hepatitis C virus (HCV), Zika virus (ZIKV), and herpes simplex virus type 1 (HSV-1) infections in a dose-dependent manner at noncytotoxic concentrations, but that it has no effect on Sendai virus (SeV) and adenovirus (AdV) infections in vitro Furthermore, rEv37 alkalized acidic organelles to prevent low pH-dependent fusion of the viral membrane-endosomal membrane, which mainly blocks the release of the viral genome from the endosome to the cytoplasm and then restricts viral late entry. Taken together, our results indicate that the scorpion venom peptide Ev37 is a broad-spectrum antiviral agent with a specific molecular mechanism against viruses undergoing low pH-dependent fusion activation during entry into host cells. We conclude that Ev37 is a potential candidate for development as an antiviral drug.

Potassium is a trigger for conformational change in the fusion spike of an enveloped RNA virus

Many enveloped viruses enter cells through the endocytic network, from which they must subsequently escape through fusion of viral and endosomal membranes. This membrane fusion is mediated by virus-encoded spikes that respond to the dynamic endosomal environment, which triggers conformational changes in the spikes that initiate the fusion process. Several fusion triggers have been identified and include pH, membrane composition, and endosome-resident proteins, and these cues dictate when and where viral fusion occurs. We recently reported that infection with an enveloped bunyavirus requires elevated potassium ion concentrations [K⁺], controlled by cellular K⁺ channels, that are encountered during viral transit through maturing endosomes. Here we reveal the molecular basis for the K⁺ requirement of bunyaviruses through the first direct visualization of a member of the Nairoviridae family, namely Hazara virus (HAZV), using cryo-EM. Using cryo-electron tomography, we observed HAZV spike glycoproteins within infectious HAZV particles exposed to both high and low [K⁺], which showed that exposure to K⁺ alone results in dramatic changes to the ultrastructural architecture of the virion surface. In low [K⁺], the spikes adopted a compact conformation arranged in locally ordered arrays, whereas, following exposure to high [K⁺], the spikes became extended, and spike–membrane interactions were observed. Viruses exposed to high [K⁺] also displayed enhanced infectivity, thus identifying K⁺ as a newly defined trigger that helps promote viral infection. Finally, we confirmed that K⁺ channel blockers are inhibitory to HAZV infection, highlighting the potential of K⁺ channels as anti-bunyavirus targets.