Internalization of rabies virus glycoprotein differs between pathogenic and attenuated virus strains

The zoonotic rabies virus (RABV) is a non-segmented negative-sense RNA virus classified within the family Rhabdoviridae, and is the most common aetiological agent responsible for fatal rabies disease. The RABV glycoprotein (G) forms trimeric spikes that protrude from RABV virions and mediate virus attachment, entry and spread, and is a major determinant of RABV pathogenesis. A range of RABV strains exist that are highly pathogenic in part due to their ability to evade host immune detection. However, some strains are disease-attenuated and can be cleared by host defences. A detailed molecular understanding of how strain variation relates to pathogenesis is currently lacking. Here, we reveal key differences in the trafficking profiles of RABV-G proteins from the challenge virus standard strain (CVS-11) and a highly attenuated vaccine strain SAD-B19 (SAD). We show that CVS-G traffics to the cell surface and undergoes rapid internalization through both clathrin- and cholesterol-dependent endocytic pathways. In contrast, SAD-G remains resident at the plasma membrane and internalizes at a significantly slower rate. Through engineering hybrids of CVS-G and SAD-G, we show that the cytoplasmic tail of CVS-G is the key determinant of these different internalization profiles. Alanine scanning further revealed that mutation of Y497 in CVS-G (H497 in SAD-G) could reduce the rate of internalization to SAD-G levels. Together, these data reveal new phenotypic differences between CVS-G and SAD-G proteins that may contribute to altered in vivo pathogenicity.

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.

Exploitation of ATP-sensitive potassium ion (KATP) channels by HPV promotes cervical cancer cell proliferation by contributing to MAPK/AP-1 signalling

Persistent infection with high-risk human papillomaviruses (HPVs) is the causal factor in multiple human malignancies, including >99% of cervical cancers and a growing proportion of oropharyngeal cancers. Prolonged expression of the viral oncoproteins E6 and E7 is necessary for transformation to occur. Although some of the mechanisms by which these oncoproteins contribute to carcinogenesis are well-characterised, a comprehensive understanding of the signalling pathways manipulated by HPV is lacking. Here, we present the first evidence to our knowledge that the targeting of a host ion channel by HPV can contribute to cervical carcinogenesis. Through the use of pharmacological activators and inhibitors of ATP-sensitive potassium ion (KATP) channels, we demonstrate that these channels are active in HPV-positive cells and that this activity is required for HPV oncoprotein expression. Further, expression of SUR1, which forms the regulatory subunit of the multimeric channel complex, was found to be upregulated in both HPV+ cervical cancer cells and in samples from patients with cervical disease, in a manner dependent on the E7 oncoprotein. Importantly, knockdown of SUR1 expression or KATP channel inhibition significantly impeded cell proliferation via induction of a G1 cell cycle phase arrest. This was confirmed both in vitro and in in vivo tumourigenicity assays. Mechanistically, we propose that the pro-proliferative effect of KATP channels is mediated via the activation of a MAPK/AP-1 signalling axis. A complete characterisation of the role of KATP channels in HPV-associated cancer is now warranted in order to determine whether the licensed and clinically available inhibitors of these channels could constitute a potential novel therapy in the treatment of HPV-driven cervical cancer.

Induction of Pro-Fibrotic CLIC4 in Dermal Fibroblasts by TGF-β/Wnt3a Is Mediated by GLI2 Upregulation

Chloride intracellular channel 4 (CLIC4) is a recently discovered driver of fibroblast activation in Scleroderma (SSc) and cancer-associated fibroblasts (CAF). CLIC4 expression and activity are regulated by TGF-β signalling through the SMAD3 transcription factor. In view of the aberrant activation of canonical Wnt-3a and Hedgehog (Hh) signalling in fibrosis, we investigated their role in CLIC4 upregulation. Here, we show that TGF-β/SMAD3 co-operates with Wnt3a/β-catenin and Smoothened/GLI signalling to drive CLIC4 expression in normal dermal fibroblasts, and that the inhibition of β-catenin and GLI expression or activity abolishes TGF-β/SMAD3-dependent CLIC4 induction. We further show that the expression of the pro-fibrotic marker α-smooth muscle actin strongly correlates with CLIC4 expression in dermal fibroblasts. Further investigations revealed that the inhibition of CLIC4 reverses morphogen-dependent fibroblast activation. Our data highlights that CLIC4 is a common downstream target of TGF-β, Hh, and Wnt-3a through signalling crosstalk and we propose a potential therapeutic avenue using CLIC4 inhibitors

The intracellular chloride channel 4 (CLIC4) activates systemic sclerosis fibroblasts

OBJECTIVES

Tissue fibrosis in SSc is driven by active fibroblasts (myofibroblasts). Previous studies have shown the intracellular chloride channel 4 (CLIC4) mediates the activation of cancer-associated fibroblasts. In this study we investigated the role of CLIC4 in SSc fibroblast activation.

METHODS

Fibroblasts were obtained from full thickness skin biopsies from SSc patients (early-diffuse). RNA and protein were collected from the fibroblasts and CLIC4 transcript and protein levels were assessed by qPCR and western blot. SSc patient fibroblasts were treated with the chloride channel inhibitors nitro-2-(3-phenylpropylamino)benzoic acid and indyanyloxyacetic acid 94.

RESULTS

CLIC4 was expressed at significantly higher levels in SSc patients' fibroblasts compared with healthy controls, at both the transcript (3.7-fold) and protein (1.7-fold) levels. Inhibition of the TGF-β receptor and its downstream transcription factor SMAD3 led to a reduction in CLIC4 expression, confirming this pathway as the main driver of CLIC4 expression. Importantly, treatment of SSc fibroblasts with known pharmacological inhibitors of CLIC4 led to reduced expression of the myofibroblast markers collagen type 1 and α-smooth muscle actin, inferring a direct role for CLIC4 in disease pathogenesis.

CONCLUSIONS

We have identified a novel role for CLIC4 in SSc myofibroblast activation, which strengthens the similarities of SSc fibroblasts with cancer-associated fibroblasts and highlights this channel as a novel target for therapeutic intervention.

P149 The intracellular chloride channel 4 (CLIC4) plays an important role in systemic sclerosis fibroblast activation

Background/Aims 

The intracellular chloride ion channel CLIC4 mediates the activation of cancer associated fibroblasts. Interestingly, systemic sclerosis (SSc) fibroblasts display a number of similar properties to cancer associated fibroblasts. Tissue fibrosis in SSc is driven by active fibroblasts (myofibroblasts). Therefore in this study we investigated the role of CLIC4 in SSc fibroblast activation.

Methods 

Dermal fibroblasts were obtained from full thickness skin biopsies from SSc patients (early-diffuse). RNA and protein were collected from the fibroblasts and CLIC4 transcript and protein levels were assessed by qPCR and western blot. SSc patient fibroblasts were treated with the chloride ion channel inhibitors NPPB and IAA-94.

Results 

CLIC4 was found to be expressed at significantly higher levels in SSc patients fibroblasts compared to healthy controls, at both the transcript (3.7 fold) and protein (1.7 fold) levels. Inhibition of the TGF-β signalling pathway led to reduced CLIC4 expression in SSc fibroblasts, confirming this pathway as the main driver of CLIC4 expression. Finally, treatment of SSc fibroblasts with small molecule inhibitors that target the channel led to reduced expression of the myofibroblast markers collagen type 1 and alpha-smooth muscle actin, suggesting a direct role for CLIC4 in SSc associated skin fibrosis.

Conclusion 

We have identified a novel role for CLIC4 in SSc myofibroblast activation, which further strengthen the similarities between SSc fibroblasts and cancer associated fibroblasts. Furthermore this study highlights this channel as a novel target for therapeutic intervention.

Disclosure 

C. Wasson: None. R. Ross: None. R. Morton: None. J. Mankouri: None. F. Del Galdo: None.

Rationally derived inhibitors of hepatitis C virus (HCV) p7 channel activity reveal prospect for bimodal antiviral therapy

Since the 1960s, a single class of agent has been licensed targeting virus-encoded ion channels, or 'viroporins', contrasting the success of channel blocking drugs in other areas of medicine. Although resistance arose to these prototypic adamantane inhibitors of the influenza A virus (IAV) M2 proton channel, a growing number of clinically and economically important viruses are now recognised to encode essential viroporins providing potential targets for modern drug discovery. We describe the first rationally designed viroporin inhibitor with a comprehensive structure-activity relationship (SAR). This step-change in understanding not only revealed a second biological function for the p7 viroporin from hepatitis C virus (HCV) during virus entry, but also enabled the synthesis of a labelled tool compound that retained biological activity. Hence, p7 inhibitors (p7i) represent a unique class of HCV antiviral targeting both the spread and establishment of infection, as well as a precedent for future viroporin-targeted drug discovery.

TMEM16A/ANO1 calcium-activated chloride channel as a novel target for the treatment of human respiratory syncytial virus infection

Introduction

Human respiratory syncytial virus (HRSV) is a common cause of respiratory tract infections (RTIs) globally and is one of the most fatal infectious diseases for infants in developing countries. Of those infected, 25%–40% aged ≤1 year develop severe lower RTIs leading to pneumonia and bronchiolitis, with ~10% requiring hospitalisation. Evidence also suggests that HRSV infection early in life is a major cause of adult asthma. There is no HRSV vaccine, and the only clinically approved treatment is immunoprophylaxis that is expensive and only moderately effective. New anti-HRSV therapeutic strategies are therefore urgently required.

Methods

It is now established that viruses require cellular ion channel functionality to infect cells. Here, we infected human lung epithelial cell lines and ex vivo human lung slices with HRSV in the presence of a defined panel of chloride (Cl⁻) channel modulators to investigate their role during the HRSV life-cycle.

Results

We demonstrate the requirement for TMEM16A, a calcium-activated Cl⁻ channel, for HRSV infection. Time-of-addition assays revealed that the TMEM16A blockers inhibit HRSV at a postentry stage of the virus life-cycle, showing activity as a postexposure prophylaxis. Another important negative-sense RNA respiratory pathogen influenza virus was also inhibited by the TMEM16A-specific inhibitor T16Ainh-A01.

Discussion

These findings reveal TMEM16A as an exciting target for future host-directed antiviral therapeutics.

Unravelling the requirement for host cell chloride channels during HRSV infection

Ion channels are a diverse class of transmembrane proteins that selectively allow ions across membranes, influencing a multitude of cellular processes. Modulation of these channels by viruses is emerging as an important host-pathogen interaction that regulates critical stages of the virus multiplication cycle including entry, replication and egress.

Human respiratory syncytial virus (HRSV) causes severe respiratory tract infections globally and is one of the most lethal respiratory pathogens for infants in developing countries, with frequent development of bronchiolitis. Furthermore, it is the most significant cause of hospitalisation of infants in the UK. Evidence also indicates that severe childhood HRSV infection contributes towards the increased incidence of adult asthma. No HRSV vaccine is available, and currently the only treatment is immunoprophylaxis which is prohibitively expensive and only moderately effective; thus new treatment options are required.

Utilising GFP-expressing HRSV in combination with an extensive panel of channel specific pharmacological inhibitors, we have identified an important role of cellular chloride (Cl-) channels during HRSV infection. Interestingly, pharmacological inhibition of specific Cl- channel families has ruled out involvement of the CFTR and instead highlighted a critical requirement for calcium-activated Cl- channels (CaCCs). Time of addition studies using CaCC blockers have indicated that these channels play a post-entry role during HRSV infection. Using genetic knockdown techniques we have isolated a single channel of interest and are now further investigating its role in facilitating HRSV multiplication, as well as assessing the importance of Cl- channels in replication cycles of other negative sense RNA viruses.

The RNA Replication Site of Tula Orthohantavirus Resides within a Remodelled Golgi Network

The family Hantaviridae within the Bunyavirales order comprises tri-segmented negative sense RNA viruses, many of which are rodent-borne emerging pathogens associated with fatal human disease. In contrast, hantavirus infection of corresponding rodent hosts results in inapparent or latent infections, which can be recapitulated in cultured cells that become persistently infected. In this study, we used Tula virus (TULV) to investigate the location of hantavirus replication during early, peak and persistent phases of infection, over a 30-day time course. Using immunofluorescent (IF) microscopy, we showed that the TULV nucleocapsid protein (NP) is distributed within both punctate and filamentous structures, with the latter increasing in size as the infection progresses. Transmission electron microscopy of TULV-infected cell sections revealed these filamentous structures comprised aligned clusters of filament bundles. The filamentous NP-associated structures increasingly co-localized with the Golgi and with the stress granule marker TIA-1 over the infection time course, suggesting a redistribution of these cellular organelles. The analysis of the intracellular distribution of TULV RNAs using fluorescent in-situ hybridization revealed that both genomic and mRNAs co-localized with Golgi-associated filamentous compartments that were positive for TIA. These results show that TULV induces a dramatic reorganization of the intracellular environment, including the establishment of TULV RNA synthesis factories in re-modelled Golgi compartments.

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.

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.

Tula orthohantavirus nucleocapsid protein is cleaved in infected cells and may sequester activated caspase-3 during persistent infection to suppress apoptosis

The family Hantaviridae mostly comprises rodent-borne segmented negative-sense RNA viruses, many of which are capable of causing devastating disease in humans. In contrast, hantavirus infection of rodent hosts results in a persistent and inapparent infection through their ability to evade immune detection and inhibit apoptosis. In this study, we used Tula hantavirus (TULV) to investigate the interplay between viral and host apoptotic responses during early, peak and persistent phases of virus infection in cell culture. Examination of early-phase TULV infection revealed that infected cells were refractory to apoptosis, as evidenced by the complete lack of cleaved caspase-3 (casp-3C) staining, whereas in non-infected bystander cells casp-3C was highly abundant. Interestingly, at later time points, casp-3C was abundant in infected cells, but the cells remained viable and able to continue shedding infectious virus, and together these observations were suggestive of a TULV-associated apoptotic block. To investigate this block, we viewed TULV-infected cells using laser scanning confocal and wide-field deconvolution microscopy, which revealed that TULV nucleocapsid protein (NP) colocalized with, and sequestered, casp-3C within cytoplasmic ultrastructures. Consistent with casp-3C colocalization, we showed for the first time that TULV NP was cleaved in cells and that TULV NP and casp-3C could be co-immunoprecipitated, suggesting that this interaction was stable and thus unlikely to be solely confined to NP binding as a substrate to the casp-3C active site. To account for these findings, we propose a novel mechanism by which TULV NP inhibits apoptosis by spatially sequestering casp-3C from its downstream apoptotic targets within the cytosol.

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.

Hazara nairovirus elicits differential induction of apoptosis and nucleocapsid protein cleavage in mammalian and tick cells

The Nairoviridae family within the Bunyavirales order comprise tick-borne segmented negative-sense RNA viruses that cause serious disease in a broad range of mammals, yet cause a latent and lifelong infection in tick hosts. An important member of this family is Crimean-Congo haemorrhagic fever virus (CCHFV), which is responsible for serious human disease that results in case fatality rates of up to 30 %, and which exhibits the most geographically broad distribution of any tick-borne virus. Here, we explored differences in the cellular response of both mammalian and tick cells to nairovirus infection using Hazara virus (HAZV), which is a close relative of CCHFV within the CCHFV serogroup. We show that HAZV infection of human-derived SW13 cells led to induction of apoptosis, evidenced by activation of cellular caspases 3, 7 and 9. This was followed by cleavage of the classical apoptosis marker poly ADP-ribose polymerase, as well as cellular genome fragmentation. In addition, we show that the HAZV nucleocapsid (N) protein was abundantly cleaved by caspase 3 in these mammalian cells at a conserved DQVD motif exposed at the tip of its arm domain, and that cleaved HAZV-N was subsequently packaged into nascent virions. However, in stark contrast, we show for the first time that nairovirus infection of cells of the tick vector failed to induce apoptosis, as evidenced by undetectable levels of cleaved caspases and lack of cleaved HAZV-N. Our findings reveal that nairoviruses elicit diametrically opposed cellular responses in mammalian and tick cells, which may influence the infection outcome in the respective hosts.

The Structure of the Human Respiratory Syncytial Virus M2-1 Protein Bound to the Interaction Domain of the Phosphoprotein P Defines the Orientation of the Complex

Human respiratory syncytial virus (HRSV) is a negative-stranded RNA virus that causes a globally prevalent respiratory infection, which can cause life-threatening illness, particularly in the young, elderly, and immunocompromised. HRSV multiplication depends on replication and transcription of the HRSV genes by the virus-encoded RNA-dependent RNA polymerase (RdRp). For replication, this complex comprises the phosphoprotein (P) and the large protein (L), whereas for transcription, the M2-1 protein is also required. M2-1 is recruited to the RdRp by interaction with P and also interacts with RNA at overlapping binding sites on the M2-1 surface, such that binding of these partners is mutually exclusive. The molecular basis for the transcriptional requirement of M2-1 is unclear, as is the consequence of competition between P and RNA for M2-1 binding, which is likely a critical step in the transcription mechanism. Here, we report the crystal structure at 2.4 Å of M2-1 bound to the P interaction domain, which comprises P residues 90 to 110. The P90-110 peptide is alpha helical, and its position on the surface of M2-1 defines the orientation of the three transcriptase components within the complex. The M2-1/P interface includes ionic, hydrophobic, and hydrogen bond interactions, and the critical contribution of these contacts to complex formation was assessed using a minigenome assay. The affinity of M2-1 for RNA and P ligands was quantified using fluorescence anisotropy, which showed high-affinity RNAs could outcompete P. This has important implications for the mechanism of transcription, particularly the events surrounding transcription termination and synthesis of poly(A) sequences.IMPORTANCE Human respiratory syncytial virus (HRSV) is a leading cause of respiratory illness, particularly in the young, elderly, and immunocompromised, and has also been linked to the development of asthma. HRSV replication depends on P and L, whereas transcription also requires M2-1. M2-1 interacts with P and RNA at overlapping binding sites; while these interactions are necessary for transcriptional activity, the mechanism of M2-1 action is unclear. To better understand HRSV transcription, we solved the crystal structure of M2-1 in complex with the minimal P interaction domain, revealing molecular details of the M2-1/P interface and defining the orientation of M2-1 within the tripartite complex. The M2-1/P interaction is relatively weak, suggesting high-affinity RNAs may displace M2-1 from the complex, providing the basis for a new model describing the role of M2-1 in transcription. Recently, the small molecules quercetin and cyclopamine have been used to validate M2-1 as a drug target.

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.

The cellular chloride channels CLIC1 and CLIC4 contribute to virus-mediated cell motility

Ion channels regulate many aspects of cell physiology, including cell proliferation, motility, and migration, and aberrant expression and activity of ion channels is associated with various stages of tumor development, with K⁺ and Cl⁻ channels now being considered the most active during tumorigenesis. Accordingly, emerging in vitro and preclinical studies have revealed that pharmacological manipulation of ion channel activity offers protection against several cancers. Merkel cell polyomavirus (MCPyV) is a major cause of Merkel cell carcinoma (MCC), primarily because of the expression of two early regulatory proteins termed small and large tumor antigens (ST and LT, respectively). Several molecular mechanisms have been attributed to MCPyV-mediated cancer formation but, thus far, no studies have investigated any potential link to cellular ion channels. Here we demonstrate that Cl⁻ channel modulation can reduce MCPyV ST-induced cell motility and invasiveness. Proteomic analysis revealed that MCPyV ST up-regulates two Cl⁻ channels, CLIC1 and CLIC4, which when silenced, inhibit MCPyV ST-induced motility and invasiveness, implicating their function as critical to MCPyV-induced metastatic processes. Consistent with these data, we confirmed that CLIC1 and CLIC4 are up-regulated in primary MCPyV-positive MCC patient samples. We therefore, for the first time, implicate cellular ion channels as a key host cell factor contributing to virus-mediated cellular transformation. Given the intense interest in ion channel modulating drugs for human disease. This highlights CLIC1 and CLIC4 activity as potential targets for MCPyV-induced MCC.

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

In order to multiply and cause disease a virus must transport its genome from outside the cell into the cytosol, most commonly achieved through the endocytic network. Endosomes transport virus particles to specific cellular destinations and viruses exploit the changing environment of maturing endocytic vesicles as triggers to mediate genome release. Previously we demonstrated that several bunyaviruses, which comprise the largest family of negative sense RNA viruses, require the activity of cellular potassium (K+) channels to cause productive infection. Specifically, we demonstrated a surprising role for K+ channels during virus endosomal trafficking. In this study, we have used the prototype bunyavirus, Bunyamwera virus (BUNV), as a tool to understand why K+ channels are required for progression of these viruses through the endocytic network. We report three major findings: First, the production of a dual fluorescently labelled bunyavirus to visualize virus trafficking in live cells. Second, we show that BUNV traffics through endosomes containing high [K+] and that these K+ ions influence the infectivity of virions. Third, we show that K+ channel inhibition can alter the distribution of K+ across the endosomal system and arrest virus trafficking in endosomes. These data suggest high endosomal [K+] is a critical cue that is required for virus infection, and is controlled by cellular K+ channels resident within the endosome network. This highlights cellular K+ channels as druggable targets to impede virus entry, infection and disease.

Viral dependence on cellular ion channels - an emerging anti-viral target?

The broad range of cellular functions governed by ion channels represents an attractive target for viral manipulation. Indeed, modulation of host cell ion channel activity by viral proteins is being increasingly identified as an important virus-host interaction. Recent examples have demonstrated that virion entry, virus egress and the maintenance of a cellular environment conducive to virus persistence are, in part, dependent on virus manipulation of ion channel activity. Most excitingly, evidence has emerged that targeting ion channels pharmacologically can impede virus life cycles. Here, we discuss current examples of virus-ion channel interactions and the potential of targeting ion channel function as a new, pharmacologically safe and broad-ranging anti-viral therapeutic strategy.

Early events in the generation of autophagosomes are required for the formation of membrane structures involved in hepatitis C virus genome replication

Hepatitis C virus (HCV) infection has been shown to induce autophagy but the mechanisms underpinning this process remain to be elucidated. Induction of autophagy requires the class III phosphatidylinositol 3-kinase, Vps34, which produces phosphatidylinositol 3-phosphate (PI3P) within the endoplasmic reticulum (ER) membrane. This recruits proteins with PI3P binding domains such as the double-FYVE-containing protein 1 (DFCP1). DFCP1 generates cup-shaped protrusions from the ER membrane, termed omegasomes, which provide a platform for the production of autophagosomes. Here we present data demonstrating that both Vps34 and DFCP1 are required for HCV genome replication, in the context of both a subgenomic replicon and virus infection, but did not affect virus entry or initial translation. Using live cell fluorescence microscopy we demonstrated that early during HCV infection the nascent viral genome replication complexes (identified by using non-structural protein NS5A as a marker) transiently colocalize with DFCP1-positive punctae (omegasomes), before the two structures move apart from each other. This observation is reminiscent of the transient association of LC3 and DFCP1 during omegasome formation, and therefore we propose that omegasomes are utilized by HCV to generate the double-membrane vesicles which are the hallmark of HCV replication complexes.

Modulation of Potassium Channels Inhibits Bunyavirus Infection

Bunyaviruses are considered to be emerging pathogens facilitated by the segmented nature of their genome that allows reassortment between different species to generate novel viruses with altered pathogenicity. Bunyaviruses are transmitted via a diverse range of arthropod vectors, as well as rodents, and have established a global disease range with massive importance in healthcare, animal welfare, and economics. There are no vaccines or anti-viral therapies available to treat human bunyavirus infections and so development of new anti-viral strategies is urgently required. Bunyamwera virus (BUNV; genus Orthobunyavirus) is the model bunyavirus, sharing aspects of its molecular and cellular biology with all Bunyaviridae family members. Here, we show for the first time that BUNV activates and requires cellular potassium (K(+)) channels to infect cells. Time of addition assays using K(+) channel modulating agents demonstrated that K(+) channel function is critical to events shortly after virus entry but prior to viral RNA synthesis/replication. A similar K(+) channel dependence was identified for other bunyaviruses namely Schmallenberg virus (Orthobunyavirus) as well as the more distantly related Hazara virus (Nairovirus). Using a rational pharmacological screening regimen, two-pore domain K(+) channels (K2P) were identified as the K(+) channel family mediating BUNV K(+) channel dependence. As several K2P channel modulators are currently in clinical use, our work suggests they may represent a new and safe drug class for the treatment of potentially lethal bunyavirus disease.

Hepatitis C virus NS5A protein blocks epidermal growth factor receptor degradation via a proline motif- dependent interaction

Hepatitis C virus (HCV) establishes a persistent infection that in many cases leads to cirrhosis and hepatocellular carcinoma. The non-structural 5A protein (NS5A) has been implicated in this process as it contains a C-terminal polyproline motif (termed P2) that binds to Src homology 3 (SH3) domains to regulate cellular signalling and trafficking pathways. We have shown previously that NS5A impaired epidermal growth factor (EGF) receptor (EGFR) endocytosis, thereby inhibiting EGF-stimulated EGFR degradation by a mechanism that remained unclear. As EGFR has been implicated in HCV cell entry and trafficking of the receptor involves several SH3-domain containing proteins, we investigated in more detail the mechanisms by which NS5A perturbs EGFR trafficking. We demonstrated that the P2 motif was required for the NS5A-mediated disruption to EGFR trafficking. We further demonstrated that the P2 motif was required for an interaction between NS5A and CMS, a homologue of CIN85 that has previously been implicated in EGFR endocytosis. We provided evidence that CMS was involved in the NS5A-mediated perturbation of EGFR trafficking. We also showed that NS5A effected a loss of EGFR ubiquitination in a P2-motif-dependent fashion. These data provide clues to the mechanism by which NS5A regulates the trafficking of a key cellular receptor and demonstrate for the first time the ability of NS5A to regulate host cell ubiquitination pathways.

Hepatitis C virus NS5A inhibits mixed lineage kinase 3 to block apoptosis

Hepatitis C virus (HCV) infection results in the activation of numerous stress responses including oxidative stress, with the potential to induce an apoptotic state. Previously we have shown that HCV attenuates the stress-induced, p38MAPK-mediated up-regulation of the K(+) channel Kv2.1, to maintain the survival of infected cells in the face of cellular stress. We demonstrated that this effect was mediated by HCV non-structural 5A (NS5A) protein, which impaired p38MAPK activity through a polyproline motif-dependent interaction, resulting in reduction of phosphorylation activation of Kv2.1. In this study, we investigated the host cell proteins targeted by NS5A to mediate Kv2.1 inhibition. We screened a phage-display library expressing the entire complement of human SH3 domains for novel NS5A-host cell interactions. This analysis identified mixed lineage kinase 3 (MLK3) as a putative NS5A interacting partner. MLK3 is a serine/threonine protein kinase that is a member of the MAPK kinase kinase (MAP3K) family and activates p38MAPK. An NS5A-MLK3 interaction was confirmed by co-immunoprecipitation and Western blot analysis. We further demonstrate a novel role of MLK3 in the modulation of Kv2.1 activity, whereby MLK3 overexpression leads to the up-regulation of channel activity. Accordingly, coexpression of NS5A suppressed this stimulation. Additionally we demonstrate that overexpression of MLK3 induced apoptosis, which was also counteracted by NS5A. We conclude that NS5A targets MLK3 with multiple downstream consequences for both apoptosis and K(+) homeostasis.

Carbon monoxide protects against oxidant-induced apoptosis via inhibition of Kv2.1

Oxidative stress induces neuronal apoptosis and is implicated in cerebral ischemia, head trauma, and age-related neurodegenerative diseases. An early step in this process is the loss of intracellular K(+) via K(+) channels, and evidence indicates that K(v)2.1 is of particular importance in this regard, being rapidly inserted into the plasma membrane in response to apoptotic stimuli. An additional feature of neuronal oxidative stress is the up-regulation of the inducible enzyme heme oxygenase-1 (HO-1), which catabolizes heme to generate biliverdin, Fe(2+), and carbon monoxide (CO). CO provides neuronal protection against stresses such as stroke and excitotoxicity, although the underlying mechanisms are not yet elucidated. Here, we demonstrate that CO reversibly inhibits K(v)2.1. Channel inhibition by CO involves reactive oxygen species and protein kinase G activity. Overexpression of K(v)2.1 in HEK293 cells increases their vulnerability to oxidant-induced apoptosis, and this is reversed by CO. In hippocampal neurons, CO selectively inhibits K(v)2.1, reverses the dramatic oxidant-induced increase in K(+) current density, and provides marked protection against oxidant-induced apoptosis. Our results provide a novel mechanism to account for the neuroprotective effects of CO against oxidative apoptosis, which has potential for therapeutic exploitation to provide neuronal protection in situations of oxidative stress.

Viruses and the fuel sensor: the emerging link between AMPK and virus replication

Adenosine 5' monophosphate-activated protein kinase (AMPK) is conserved in all eukaryotic cells and functions as the key regulator of cellular metabolism by responding to the energy status of the cell. It is activated by an increase in the AMP : ATP ratio and then attempts to redress the balance by upregulating catabolic processes, whilst concomitantly inhibiting anabolic processes. Despite its critical importance in the functioning of eukaryotic cells, there has been a paucity of studies investigating the potential for dysregulation of AMPK by viruses. Recently, however, there have been a number of reports that have begun to address this gap in our knowledge. In this article, we will review this emerging field, outlining how a variety of viruses have been shown to either stimulate or inhibit AMPK activity. We will also document the effects of these perturbations on the biology of virus infection, in particular with regard to the ability of viruses to persist or cause cytopathogenesis.

Hepatitis C virus NS5A protein interacts with beta-catenin and stimulates its transcriptional activity in a phosphoinositide-3 kinase-dependent fashion

Hepatitis C virus (HCV) infection is increasingly associated with the development of hepatocellular carcinoma (HCC). HCV is not thought to be directly oncogenic but, by modulating a range of cellular functions, may predispose patients to the development of liver tumours. However, the molecular mechanisms by which HCV infection might contribute to HCC remain to be characterized. In this regard, we showed previously that the HCV NS5A protein bound to the p85 regulatory subunit of phosphoinositide-3 kinase (PI3K), thereby stimulating the activity of the p110 catalytic subunit of the enzyme. One of the downstream consequences of this was the stabilization of the proto-oncogene, beta-catenin, with a concomitant stimulation of its transcriptional activity. Here, we further analyse the mechanism by which NS5A mediates activation of beta-catenin. Although our previous data were consistent with a role for the PI3K downstream effector kinases, Akt and glycogen synthase kinase-3beta, in NS5A-mediated activation of beta-catenin, we demonstrate here that it is in fact independent of both of these kinases. Truncation analysis revealed that both the N and C termini of NS5A are required for full activation of beta-catenin. Furthermore, we demonstrate that NS5A, either alone or in complex with p85, is able to bind directly to beta-catenin; again both N and C termini contribute to this interaction. We propose that NS5A activates beta-catenin via a novel mechanism that involves a direct interaction between the two proteins and is augmented by PI3K activity. This may contribute to the association between chronic HCV infection and the development of HCC.

Expression of the NS3 protease of cytopathogenic bovine viral diarrhea virus results in the induction of apoptosis but does not block activation of the beta interferon promoter

Bovine viral diarrhea virus (BVDV; genus Pestivirus) can exist as two biotypes, cytopathogenic (CP) and non-cytopathogenic (NCP). The CP form differs from NCP by the continual expression of free non-structural protein 3 (NS3). CP BVDV infection of cultured cells induces apoptosis, whereas NCP BVDV infection has been reported to block the induction of beta interferon (IFN-beta). To investigate the viral mechanisms underlying these effects, NS3 or NS2-3 proteins of NCP and CP BVDV biotypes, together with the cognate NS3 co-factor NS4A, were expressed in cells, and their effect on apoptosis and induction of IFN-beta was investigated. Expression of NS3/4A resulted in increased activity of caspase-9 and caspase-3, indicating induction of the intrinsic apoptosis pathway. Mutational analysis revealed that a protease-inactive NS3/4A was unable to induce apoptosis, suggesting that NS3 protease activity is required for initiation of apoptosis during CP BVDV infection. The ability of NS2-3 to modulate activation of the IFN-beta promoter was also investigated. These studies confirmed that, unlike the related hepatitis C virus and GB virus-B, BVDV proteases are unable to inhibit TLR3- and RIG-I-dependent activation of the IFN-beta promoter. These data suggest that BVDV NS3/4A is responsible for regulating the levels of cellular apoptosis and provide new insights regarding the viral elements associated with CP biotype pathogenesis.

A comparative cell biological analysis reveals only limited functional homology between the NS5A proteins of hepatitis C virus and GB virus B

GB virus B (GBV-B) is the closest relative to hepatitis C virus (HCV) with which it shares a common genome organization, however, unlike HCV in humans, it generally causes an acute resolving hepatitis in New World monkeys. It is important to understand the factors regulating the different disease profiles of the two viruses and in this regard, as well as playing a key role in viral RNA replication, the HCV NS5A non-structural protein modulates a variety of host-cell signalling pathways. We have shown previously that HCV NS5A, expressed either alone, or in the context of the complete polyprotein, inhibits the Ras-extracellular-signal-regulated kinase (Erk) pathway and activates the phosphoinositide 3-kinase (PI3K) pathway. In this report, we investigate whether these functions are shared by GBV-B NS5A. Immunofluorescence analysis revealed that a C-terminally FLAG-tagged GBV-B NS5A exhibited a punctate cytoplasmic distribution. However, unlike HCV NS5A, the GBV-B protein did not partially co-localize with early endosomes. Utilizing a transient luciferase reporter system, we observed that GBV-B NS5A failed to inhibit Ras-Erk signalling, however GBV-B NS5A expression did result in the elevation of beta-catenin-dependent transcription via activation of the PI3K pathway. These effects of GBV-B and HCV NS5A on the PI3K and Ras-Erk pathways were confirmed in cells harbouring subgenomic replicons derived from the two viruses. Based on these data we speculate that the differential effects of the two NS5A proteins on cellular signalling pathways may contribute to the differences in the natural history of the two viruses.

The hepatitis C virus non-structural protein NS5A alters the trafficking profile of the epidermal growth factor receptor

Hepatitis C virus (HCV) frequently establishes a persistent infection, leading to chronic liver disease. The NS5A protein has been implicated in this process as it modulates a variety of intracellular signalling pathways that control cell survival and proliferation. In particular, NS5A associates with several proteins involved in the endocytosis of the epidermal growth factor receptor (EGFR) and has been previously shown to inhibit epidermal growth factor (EGF)-stimulated activation of the Ras-Erk pathway by a mechanism that remains unclear. As EGFR signalling involves trafficking to late endosomes, we investigated whether NS5A perturbs EGFR signalling by altering receptor endocytosis. We demonstrate that NS5A partially localizes to early endosomes and, although it has no effect on EGF internalization, it colocalizes with the EGFR and alters its distribution. This redistribution correlates with a decrease in the amount of active EGF-EGFR ligand-receptor complexes present in the late endosomal signalling compartment and also results in a concomitant increase in the total levels of EGFR. These observations suggest that NS5A controls EGFR signalling by diverting the receptor away from late endosomes. This represents a novel mechanism by which a viral protein attenuates cell signalling and suggests that NS5A may perturb trafficking pathways to maintain an optimal environment for HCV persistence.

Molecular cell biology of KATPchannels: implications for neonatal diabetes

ATP-sensitive potassium (KATP) channels play a key role in the regulation of insulin secretion by coupling glucose metabolism to the electrical activity of pancreatic β-cells. To generate an electric signal of suitable magnitude, the plasma membrane of the β-cell must contain an appropriate number of channels. An inadequate number of channels can lead to congenital hyperinsulinism, whereas an excess of channels can result in the opposite condition, neonatal diabetes. KATPchannels are made up of four subunits each of Kir6.2 and the sulphonylurea receptor (SUR1), encoded by the genesKCNJ11andABCC8, respectively. Following synthesis, the subunits must assemble into an octameric complex to be able to exit the endoplasmic reticulum and reach the plasma membrane. While this biosynthetic pathway ensures supply of channels to the cell surface, an opposite pathway, involving clathrin-mediated endocytosis, removes channels back into the cell. The balance between these two processes, perhaps in conjunction with endocytic recycling, would dictate the channel density at the cell membrane. In this review, we discuss the molecular signals that contribute to this balance, and how an imbalance could lead to a disease state such as neonatal diabetes.

Kir6.2 mutations causing neonatal diabetes prevent endocytosis of ATP-sensitive potassium channels

ATP-sensitive potassium (KATP) channels couple the metabolic status of a cell to its membrane potential-a property that endows pancreatic beta-cells with the ability to regulate insulin secretion in accordance with changes in blood glucose. The channel comprises four subunits each of Kir6.2 and the sulphonylurea receptor (SUR1). Here, we report that KATP channels undergo rapid internalisation from the plasma membrane by clathrin-mediated endocytosis. We present several lines of evidence to demonstrate that endocytosis is mediated by a tyrosine based signal (330YSKF333) located in the carboxy-terminus of Kir6.2 and that SUR1 has no direct role. We show that genetic mutations, Y330C and F333I, which cause permanent neonatal diabetes mellitus, disrupt this motif and abrogate endocytosis of reconstituted mutant channels. The resultant increase in the surface density of KATP channels would predispose beta-cells to hyperpolarise and may account for reduced insulin secretion in these patients. The data imply that endocytosis of KATP channels plays a crucial role in the (patho)-physiology of insulin secretion.