Nipah Virus C Protein Recruits Tsg101 to Promote the Efficient Release of Virus in an ESCRT-Dependent Pathway

Discovery of plant-based phytochemical as effective antivirals that target the non-structural protein C of the Nipah virus through computational methods

Nipah Virus (NiV) belongs to the Paramyxoviridae family and was first identified during an outbreak in Malaysia. Some initial symptoms include mild fever, headache and sore throat, which could escalate to respiratory illness and brain inflammation. The mortality rate of NiV infection can range from 40% to 75%, which is quite high. This is mainly due to the lack of efficient drugs and vaccines. In most instances, NiV is transmitted from animals to humans. Non-Structural Proteins (C, V and W) of the Nipah virus impede the host immune response by obstructive the JAK/STAT pathway. However, Non-Structural Proteins - C (NSP-C) plays a vital role in NiV pathogenesis, which includes IFN antagonist activity and viral RNA production. In the present study, the full-length structure of NiV-NSP-C was predicted using computational modelling, and the stability of the structure was analysed using 200 ns molecular dynamic (MD) simulation. Further, the structure-based virtual screening identified five potent phytochemicals (PubChem CID: 9896047, 5885, 117678, 14887603 and 5461026) with better binding affinity against NiV-NSP-C. DFT studies clearly showed that the phytochemicals had higher chemical reactivity, and the complex MD simulation depicted that the identified inhibitors exhibited stable binding with NiV-NSP-C. Furthermore, experimental validation of these identified phytochemicals would likely control the infection of NiV.Communicated by Ramaswamy H. Sarma.

ESCRT machinery and virus infection

The endosomal sorting complex required for transport (ESCRT) machinery plays a significant role in the spread of human viruses. However, our understanding of how the host ESCRT machinery responds to viral infection remains limited. Emerging evidence suggests that the ESCRT machinery can be hijacked by viruses of different families to enhance their replication. Throughout their life cycle, these viruses can interfere with or exploit ESCRT-mediated physiological processes to increase their chances of infecting the host. In contrast, to counteract virus infection, the interferon-stimulated gene 15 (ISG15) or the E3 ISG15-protein ligase (HERC5) system within the infected cells is activated to degrade the ESCRT proteins. Many retroviral and RNA viral proteins have evolved "late (L) domain" motifs, which enable them to recruit host ESCRT subunit proteins to facilitate virus transport, replication, budding, mature, and even endocytosis, Therefore, the L domain motifs and ESCRT subunit proteins could serve as promising drug targets for antiviral therapy. This review investigated the composition and essential functions of the ESCRT, shedding light on the impact of ESCRT subunits and viral L domain motifs on the replication of viruses. Furthermore, the antiviral effects facilitated by the ESCRT machinery have been investigated, aiming to provide valuable insights to guide the development and utilization of antiviral drugs.

Insights into the function of ESCRT and its role in enveloped virus infection

The endosomal sorting complex required for transport (ESCRT) is an essential molecular machinery in eukaryotic cells that facilitates the invagination of endosomal membranes, leading to the formation of multivesicular bodies (MVBs). It participates in various cellular processes, including lipid bilayer remodeling, cytoplasmic separation, autophagy, membrane fission and re-modeling, plasma membrane repair, as well as the invasion, budding, and release of certain enveloped viruses. The ESCRT complex consists of five complexes, ESCRT-0 to ESCRT-III and VPS4, along with several accessory proteins. ESCRT-0 to ESCRT-II form soluble complexes that shuttle between the cytoplasm and membranes, mainly responsible for recruiting and transporting membrane proteins and viral particles, as well as recruiting ESCRT-III for membrane neck scission. ESCRT-III, a soluble monomer, directly participates in vesicle scission and release, while VPS4 hydrolyzes ATP to provide energy for ESCRT-III complex disassembly, enabling recycling. Studies have confirmed the hijacking of ESCRT complexes by enveloped viruses to facilitate their entry, replication, and budding. Recent research has focused on the interaction between various components of the ESCRT complex and different viruses. In this review, we discuss how different viruses hijack specific ESCRT regulatory proteins to impact the viral life cycle, aiming to explore commonalities in the interaction between viruses and the ESCRT system.

A Survey of Henipavirus Tropism-Our Current Understanding from a Species/Organ and Cellular Level

Henipaviruses are single-stranded RNA viruses that have been shown to be virulent in several species, including humans, pigs, horses, and rodents. Isolated nearly 30 years ago, these viruses have been shown to be of particular concern to public health, as at least two members (Nipah and Hendra viruses) are highly virulent, as well as zoonotic, and are thus classified as BSL4 pathogens. Although only 5 members of this genus have been isolated and characterized, metagenomics analysis using animal fluids and tissues has demonstrated the existence of other novel henipaviruses, suggesting a far greater degree of phylogenetic diversity than is currently known. Using a variety of molecular biology techniques, it has been shown that these viruses exhibit varying degrees of tropism on a species, organ/tissue, and cellular level. This review will attempt to provide a general overview of our current understanding of henipaviruses, with a particular emphasis on viral tropism.

Orthoparamyxovirinae C Proteins Have a Common Origin and a Common Structural Organization

The protein C is a small viral protein encoded in an overlapping frame of the P gene in the subfamily Orthoparamyxovirinae. This protein, expressed by alternative translation initiation, is a virulence factor that regulates viral transcription, replication, and production of defective interfering RNA, interferes with the host-cell innate immunity systems and supports the assembly of viral particles and budding. We expressed and purified full-length and an N-terminally truncated C protein from Tupaia paramyxovirus (TupV) C protein (genus Narmovirus). We solved the crystal structure of the C-terminal part of TupV C protein at a resolution of 2.4 Å and found that it is structurally similar to Sendai virus C protein, suggesting that despite undetectable sequence conservation, these proteins are homologous. We characterized both truncated and full-length proteins by SEC-MALLS and SEC-SAXS and described their solution structures by ensemble models. We established a mini-replicon assay for the related Nipah virus (NiV) and showed that TupV C inhibited the expression of NiV minigenome in a concentration-dependent manner as efficiently as the NiV C protein. A previous study found that the Orthoparamyxovirinae C proteins form two clusters without detectable sequence similarity, raising the question of whether they were homologous or instead had originated independently. Since TupV C and SeV C are representatives of these two clusters, our discovery that they have a similar structure indicates that all Orthoparamyxovirine C proteins are homologous. Our results also imply that, strikingly, a STAT1-binding site is encoded by exactly the same RNA region of the P/C gene across Paramyxovirinae, but in different reading frames (P or C), depending on which cluster they belong to.

The nanoscale organization of Nipah virus matrix protein revealed by super-resolution microscopy

The matrix proteins (M) of many enveloped RNA viruses mediate virus assembly and budding. However, it remains poorly understood how M is involved in virus budding and how they interact with envelope proteins. Here, we show that the expression level of Nipah (NiV) M in particles produced by the host cells deviates from a Gamma distribution and does not reflect that of the host cells, indicating assembly of the NiV-M in the process. Our data reveal that NiV-M affects the circularity of the particles while the NiV envelope proteins do not. The organization of NiV envelope proteins on the membrane of the particles is similar to those that do not express NiV-M, suggesting that NiV-M does not directly interact with the envelope proteins during assembly and budding.

Tumor Susceptibility Gene 101 (TSG101) Contributes to Virion Formation of Porcine Reproductive and Respiratory Syndrome Virus via Interaction with the Nucleocapsid (N) Protein along with the Early Secretory Pathway

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused huge economic losses to global swine industry. As an intracellular obligate pathogen, PRRSV exploits host cellular machinery to establish infection. The endocytic sorting complex required for transport (ESCRT) system has been shown to participate in different life cycle stages of multiple viruses. In the current study, a systematic small interference RNA (siRNA) screening assay identified that certain ESCRT components contributed to PRRSV infection. Among them, tumor susceptibility gene 101 (TSG101) was demonstrated to be important for PRRSV infection by knockdown and overexpression assays. TSG101 was further revealed to be involved in virion formation rather than viral attachment, internalization, RNA replication and nucleocapsid (N) protein translation within the first round of PRRSV life cycle. In detail, TSG101 was determined to specially interact with PRRSV N protein and take effect on its subcellular localization along with the early secretory pathway. Taken together, these results provide evidence that TSG101 is a pro-viral cellular factor for PRRSV assembly, which will be a promising target to interfere with the viral infection. IMPORTANCE PRRSV infection results in a serious swine disease affecting pig farming in the world. However, efficient prevention and control of PRRSV is hindered by its complicated infection process. Up to now, our understanding of PRRSV assembly during infection is especially limited. Here, we identified that TSG101, an ESCRT-I subunit, facilitated virion formation of PRRSV via interaction with the viral N protein along with the early secretory pathway. Our work actually expands the knowledge of PRRSV infection and provides a novel therapeutic target for prevention and control of the virus.

C Proteins: Controllers of Orderly Paramyxovirus Replication and of the Innate Immune Response

Particles of many paramyxoviruses include small amounts of proteins with a molecular weight of about 20 kDa. These proteins, termed “C”, are basic, have low amino acid homology and some secondary structure conservation. C proteins are encoded in alternative reading frames of the phosphoprotein gene. Some viruses express nested sets of C proteins that exert their functions in different locations: In the nucleus, they interfere with cellular transcription factors that elicit innate immune responses; in the cytoplasm, they associate with viral ribonucleocapsids and control polymerase processivity and orderly replication, thereby minimizing the activation of innate immunity. In addition, certain C proteins can directly bind to, and interfere with the function of, several cytoplasmic proteins required for interferon induction, interferon signaling and inflammation. Some C proteins are also required for efficient virus particle assembly and budding. C-deficient viruses can be grown in certain transformed cell lines but are not pathogenic in natural hosts. C proteins affect the same host functions as other phosphoprotein gene-encoded proteins named V but use different strategies for this purpose. Multiple independent systems to counteract host defenses may ensure efficient immune evasion and facilitate virus adaptation to new hosts and tissue environments.

Structural Insight into the Interaction of Sendai Virus C Protein with Alix To Stimulate Viral Budding

The Sendai virus (SeV), belonging to the Respirovirus genus of the family Paramyxoviridae, harbors an accessory protein, named C protein, which facilitates the viral pathogenicity in mice. In addition, the C protein is known to stimulate the budding of virus-like particles through the binding to the host ALG-2 interacting protein X (Alix), a component of the endosomal sorting complexes required for transport (ESCRT) machinery. However, siRNA-mediated gene knockdown studies suggested that neither Alix nor C protein are related to the SeV budding. In the present study, we determined the crystal structure of a complex comprising of the C-terminal half of the C protein (Y3) and the Bro1 domain of Alix at a resolution of 2.2 Å, to investigate the role of the association in the SeV budding. The structure revealed that a novel consensus sequence, LxxW, which is conserved among the Respirovirus C proteins, is important for the Alix-binding. SeV possessing a mutated C protein with a reduced Alix-binding affinity showed impaired virus production, which correlated with the binding affinity. Infectivity analysis showed a 160-fold reduction at 12 h post-infection compared with non-mutated virus, while C protein competes with CHMP4, one subunit of the ESCRT-III complex, on the binding to Alix. Altogether, these results highlight the critical role of C protein in the SeV budding. IMPORTANCE Human parainfluenza virus type I (hPIV1) is a respiratory pathogen affecting in young children, immunocompromised patients, and the elderly, with no available vaccines or antiviral drugs. Sendai virus (SeV), a murine counterpart of hPIV1, has been extensively studied to determine the molecular and biological properties of hPIV1. These viruses possess a multifunctional accessory protein, C protein, which is essential for stimulating the viral reproduction, however, its role in budding remains controversial. In the present study, the crystal structure of the C-terminal half of the SeV C protein associated with the Bro1 domain of Alix, a component of a cell membrane modulating machinery ESCRT, was elucidated. Based on the structure, we designed mutated C proteins with different binding affinity to Alix, and showed that the interaction between C and Alix is vital for the viral budding. These findings provide new insights into the development of a new antiviral drugs against hPIV1.

The ESCRT-I Subunit Tsg101 Plays Novel Dual Roles in Entry and Replication of Classical Swine Fever Virus

Classical swine fever (CSF), caused by classical swine fever virus (CSFV), is a highly contagious disease of swine with high morbidity and mortality that negatively affects the pig industry worldwide, in particular in China. Soon after the endocytosis of CSFV, the virus makes full use of the components of host cells to complete its life cycle. The endocytosis sorting complex required for transport (ESCRT) system is a central molecular machine for membrane protein sorting and scission in eukaryotic cells that plays an essential role in many physiological metabolic processes, including invasion and egress of envelope viruses. However, the molecular mechanism that ESCRT uses to regulate the replication of CSFV is unknown. In this study, we demonstrated that the ESCRT-I complex Tsg101 protein participates in clathrin-mediated endocytosis of CSFV and is also involved in CSFV trafficking. Tsg101 assists the virus in entering the host cell through the late endosome (Rab7 and Rab9) and finally reaching the lysosome (Lamp-1). Interestingly, Tsg101 is also involved in the viral replication process by interacting with nonstructural proteins 4B and 5B of CSFV. Finally, confocal microscopy showed that the replication complex of Tsg101 and double-stranded RNA (dsRNA) or NS4B and NS5B protein was close to the endoplasmic reticulum (ER), not the Golgi, in the cytoplasm. Collectively, our finding highlights that Tsg101 regulates the process of CSFV entry and replication, indicating that the ESCRT plays an important role in the life cycle of CSFV. Thus, ESCRT molecules could serve as therapeutic targets to combat CSFV infection.IMPORTANCE CSF, caused by CSFV, is a World Organization for Animal Health (OIE) notifiable disease and causes significant financial losses to the pig industry globally. The ESCRT machinery plays an important regulatory role in several members of the genera Flavivirus and Hepacivirus within the family Flaviviridae, such as hepatitis C virus, Japanese encephalitis virus, and dengue virus. Previous reports have shown that assembling and budding of these viruses require ESCRT. However, the role of ESCRT in Pestivirus infection remains to be elucidated. We determined the molecular mechanisms of the regulation of CSFV infection by the major subunit Tsg101 of ESCRT-I. Interestingly, Tsg101 plays an essential regulatory role in both clathrin-mediated endocytosis and genome replication of CSFV. Overall, the results of this study provide further insights into the molecular function of ESCRT-I complex protein Tsg101 during CSFV infection, which may serve as a molecular target for pestivirus inhibitors.

Henipavirus infection of the central nervous system

Nipah virus (NiV) and Hendra virus are highly pathogenic zoonotic viruses of the genus Henipavirus, family Paramyxoviridae. These viruses were first identified as the causative agents of severe respiratory and encephalitic disease in the 1990s across Australia and Southern Asia with mortality rates reaching up to 75%. While outbreaks of Nipah and Hendra virus infections remain rare and sporadic, there is concern that NiV has pandemic potential. Despite increased attention, little is understood about the neuropathogenesis of henipavirus infection. Neuropathogenesis appears to arise from dual mechanisms of vascular disease and direct parenchymal brain infection, but the relative contributions remain unknown while respiratory disease arises from vasculitis and respiratory epithelial cell infection. This review will address NiV basic clinical disease, pathology and pathogenesis with a particular focus on central nervous system (CNS) infection and address the necessity of a model of relapsed CNS infection. Additionally, the innate immune responses to NiV infection in vitro and in the CNS are reviewed as it is likely linked to any persistent CNS infection.

A key region of molecular specificity orchestrates unique ephrin-B1 utilization by Cedar virus

The emergent zoonotic henipaviruses, Hendra, and Nipah are responsible for frequent and fatal disease outbreaks in domestic animals and humans. Specificity of henipavirus attachment glycoproteins (G) for highly species-conserved ephrin ligands underpins their broad host range and is associated with systemic and neurological disease pathologies. Here, we demonstrate that Cedar virus (CedV)-a related henipavirus that is ostensibly nonpathogenic-possesses an idiosyncratic entry receptor repertoire that includes the common henipaviral receptor, ephrin-B2, but, distinct from pathogenic henipaviruses, does not include ephrin-B3. Uniquely among known henipaviruses, CedV can use ephrin-B1 for cellular entry. Structural analyses of CedV-G reveal a key region of molecular specificity that directs ephrin-B1 utilization, while preserving a universal mode of ephrin-B2 recognition. The structural and functional insights presented uncover diversity within the known henipavirus receptor repertoire and suggest that only modest structural changes may be required to modulate receptor specificities within this group of lethal human pathogens.

NEDD4 family ubiquitin ligases associate with LCMV Z's PPXY domain and are required for virus budding, but not via direct ubiquitination of Z

Viral late domains are used by many viruses to recruit the cellular endosomal sorting complex required for transport (ESCRT) to mediate membrane scission during viral budding. Unlike the P(S/T)AP and YPX_(1–3)L late domains, which interact directly with the ESCRT proteins Tsg101 and ALIX, the molecular linkage connecting the PPXY late domain to ESCRT proteins is unclear. The mammarenavirus lymphocytic choriomeningitis virus (LCMV) matrix protein, Z, contains only one late domain, PPXY. We previously found that this domain in LCMV Z, as well as the ESCRT pathway, are required for the release of defective interfering (DI) particles but not infectious virus. To better understand the molecular mechanism of ESCRT recruitment by the PPXY late domain, affinity purification-mass spectrometry was used to identify host proteins that interact with the Z proteins of the Old World mammarenaviruses LCMV and Lassa virus. Several Nedd4 family E3 ubiquitin ligases interact with these matrix proteins and in the case of LCMV Z, the interaction was PPXY-dependent. We demonstrated that these ligases directly ubiquitinate LCMV Z and mapped the specific lysine residues modified. A recombinant LCMV containing a Z that cannot be ubiquitinated maintained its ability to produce both infectious virus and DI particles, suggesting that direct ubiquitination of LCMV Z alone is insufficient for recruiting ESCRT proteins to mediate virus release. However, Nedd4 ligases appear to be important for DI particle release suggesting that ubiquitination of targets other than the Z protein itself is required for efficient viral ESCRT recruitment.

Enveloped viruses derive their lipid bilayer from either the cellular plasma membrane or an intracellular organelle during the process of viral budding in which a virus particle is formed at a membrane. Many enveloped viruses recruit the cellular endosomal sorting complex required for transport (ESCRT) in order to efficiently cut the membrane that connects a newly budded, but not released, virus particle from its parent membrane. Late domains, which are short protein motifs found in numerous enveloped viruses, specifically recruit ESCRT for this process. Two types of late domains accomplish this by binding directly to ESCRT proteins. A third late domain, PPXY, recruits ESCRT proteins through an unknown, indirect linkage. In this study, we sought to identify proteins that may bridge the PPXY late domain and ESCRT proteins. We found that Nedd4 family ubiquitin ligases interact with the PPXY domain in the mammarenavirus Z protein resulting in ubiquitination of Z at two lysine residues. However, Z ubiquitination was largely dispensable for the virus. Conversely, Nedd4 ubiquitin ligases were critical during infection suggesting that the most important contribution made to virus release by Nedd4 ligases is not direct ubiquitination of the viral matrix protein, but possibly the ubiquitination of cellular proteins or other viral proteins.

Nipah Virus-Like Particle Egress Is Modulated by Cytoskeletal and Vesicular Trafficking Pathways: a Validated Particle Proteomics Analysis

Classified as a biosafety level 4 (BSL4) select agent, Nipah virus (NiV) is a deadly henipavirus in the Paramyxoviridae family, with a nearly 75% mortality rate in humans, underscoring its global and animal health importance. Elucidating the process of viral particle production in host cells is imperative both for targeted drug design and viral particle-based vaccine development. However, little is understood concerning the functions of cellular machinery in paramyxoviral and henipaviral assembly and budding. Recent studies showed evidence for the involvement of multiple NiV proteins in viral particle formation, in contrast to the mechanisms understood for several paramyxoviruses as being reliant on the matrix (M) protein alone. Further, the levels and purposes of cellular factor incorporation into viral particles are largely unexplored for the paramyxoviruses. To better understand the involvement of cellular machinery and the major structural viral fusion (F), attachment (G), and matrix (M) proteins, we performed proteomics analyses on virus-like particles (VLPs) produced from several combinations of these NiV proteins. Our findings indicate that NiV VLPs incorporate vesicular trafficking and actin cytoskeletal factors. The involvement of these biological processes was validated by experiments indicating that the perturbation of key factors in these cellular processes substantially modulated viral particle formation. These effects were most impacted for NiV-F-modulated viral particle formation either autonomously or in combination with other NiV proteins, indicating that NiV-F budding relies heavily on these cellular processes. These findings indicate a significant involvement of the NiV fusion protein, vesicular trafficking, and actin cytoskeletal processes in efficient viral particle formation.IMPORTANCE Nipah virus is a zoonotic biosafety level 4 agent with high mortality rates in humans. The genus to which Nipah virus belongs, Henipavirus, includes five officially recognized pathogens; however, over 20 species have been identified in multiple continents within the last several years. As there are still no vaccines or treatments for NiV infection, elucidating its process of viral particle production is imperative both for targeted drug design as well as for particle-based vaccine development. Developments in high-throughput technologies make proteomic analysis of isolated viral particles a highly insightful approach to understanding the life cycle of pathogens such as Nipah virus.

Nano-based approach to combat emerging viral (NIPAH virus) infection

Emergence of new virus and their heterogeneity are growing at an alarming rate. Sudden outburst of Nipah virus (NiV) has raised serious question about their instant management using conventional medication and diagnostic measures. A coherent strategy with versatility and comprehensive perspective to confront the rising distress could perhaps be effectuated by implementation of nanotechnology. But in concurrent to resourceful and precise execution of nano-based medication, there is an ultimate need of concrete understanding of the NIV pathogenesis. Moreover, to amplify the effectiveness of nano-based approach in a conquest against NiV, a list of developed nanosystem with antiviral activity is also a prerequisite. Therefore the present review provides a meticulous cognizance of cellular and molecular pathogenesis of NiV. Conventional as well several nano-based diagnosis experimentations against viruses have been discussed. Lastly, potential efficacy of different forms of nano-based systems as convenient means to shield mankind against NiV has also been introduced.

Extracellular Vesicles and Ebola Virus: A New Mechanism of Immune Evasion

Ebola virus (EBOV) disease can result in a range of symptoms anywhere from virtually asymptomatic to severe hemorrhagic fever during acute infection. Additionally, spans of asymptomatic persistence in recovering survivors is possible, during which transmission of the virus may occur. In acute infection, substantial cytokine storm and bystander lymphocyte apoptosis take place, resulting in uncontrolled, systemic inflammation in affected individuals. Recently, studies have demonstrated the presence of EBOV proteins VP40, glycoprotein (GP), and nucleoprotein (NP) packaged into extracellular vesicles (EVs) during infection. EVs containing EBOV proteins have been shown to induce apoptosis in recipient immune cells, as well as contain pro-inflammatory cytokines. In this manuscript, we review the current field of knowledge on EBOV EVs including the mechanisms of their biogenesis, their cargo and their effects in recipient cells. Furthermore, we discuss some of the effects that may be induced by EBOV EVs that have not yet been characterized and highlight the remaining questions and future directions.

Angiomotin-Like 1 Links Paramyxovirus M Proteins to NEDD4 Family Ubiquitin Ligases

To define the links between paramyxovirus budding and cellular ESCRT machinery, we previously identified angiomotin-like 1 (AMOTL1) in a screen for host factors that bind to the matrix (M) protein of parainfluenza virus 5 (PIV5). This protein harbors three L/PPXY sequences, allowing it to interact with WW domain containing proteins including NEDD4 family members. We hypothesize that paramyxoviruses use AMOTL1 as a linker to indirectly recruit the same NEDD4 ubiquitin ligases for budding that other enveloped viruses recruit directly through their PPXY late domains. In support of this hypothesis, we found that AMOTL1 could link together M proteins and NEDD4 family proteins in three-way co-IP experiments. Both PIV5 and mumps virus M proteins could be linked to the NEDD4 family proteins NEDD4-1, NEDD4L, and NEDL1, provided that AMOTL1 was co-expressed as a bridging protein. AMOT and AMOTL2 could not substitute for AMOTL1, as they lacked the ability to bind with paramyxovirus M proteins. Attachment of a PPXY late domain sequence to PIV5 M protein obviated the need for AMOTL1 as a linker between M and NEDD4 proteins. Together, these results suggest a novel host factor recruitment strategy for paramyxoviruses to achieve particle release.

In vitro methods for testing antiviral drugs

Despite successful vaccination programs and effective treatments for some viral infections, humans are still losing the battle with viruses. Persisting human pandemics, emerging and re-emerging viruses, and evolution of drug-resistant strains impose continuous search for new antiviral drugs. A combination of detailed information about the molecular organization of viruses and progress in molecular biology and computer technologies has enabled rational antivirals design. Initial step in establishing efficacy of new antivirals is based on simple methods assessing inhibition of the intended target. We provide here an overview of biochemical and cell-based assays evaluating the activity of inhibitors of clinically important viruses.

Cytoplasmic Motifs in the Nipah Virus Fusion Protein Modulate Virus Particle Assembly and Egress

Nipah virus (NiV), a paramyxovirus in the genus Henipavirus, has a mortality rate in humans of approximately 75%. While several studies have begun our understanding of NiV particle formation, the mechanism of this process remains to be fully elucidated. For many paramyxoviruses, M proteins drive viral assembly and egress; however, some paramyxoviral glycoproteins have been reported as important or essential in budding. For NiV the matrix protein (M), the fusion glycoprotein (F) and, to a much lesser extent, the attachment glycoprotein (G) autonomously induce the formation of virus-like particles (VLPs). However, functional interactions between these proteins during assembly and egress remain to be fully understood. Moreover, if the F-driven formation of VLPs occurs through interactions with host cell machinery, the cytoplasmic tail (CT) of F is a likely interactive domain. Therefore, we analyzed NiV F CT deletion and alanine mutants and report that several but not all regions of the F CT are necessary for efficient VLP formation. Two of these regions contain YXXØ or dityrosine motifs previously shown to interact with cellular machinery involved in F endocytosis and transport. Importantly, our results showed that F-driven, M-driven, and M/F-driven viral particle formation enhanced the recruitment of G into VLPs. By identifying key motifs, specific residues, and functional viral protein interactions important for VLP formation, we improve our understanding of the viral assembly/egress process and point to potential interactions with host cell machinery.IMPORTANCE Henipaviruses can cause deadly infections of medical, veterinary, and agricultural importance. With recent discoveries of new henipa-like viruses, understanding the mechanisms by which these viruses reproduce is paramount. We have focused this study on identifying the functional interactions of three Nipah virus proteins during viral assembly and particularly on the role of one of these proteins, the fusion glycoprotein, in the incorporation of other viral proteins into viral particles. By identifying several regions in the fusion glycoprotein that drive viral assembly, we further our understanding of how these viruses assemble and egress from infected cells. The results presented will likely be useful toward designing treatments targeting this aspect of the viral life cycle and for the production of new viral particle-based vaccines.

Nuclear localization and secretion competence are conserved among henipavirus matrix proteins

Viruses of the genus Henipavirus of the family Paramyxoviridae are zoonotic pathogens, which have emerged in Southeast Asia, Australia and Africa. Nipah virus (NiV) and Hendra virus are highly virulent pathogens transmitted from bats to animals and humans, while the henipavirus Cedar virus seems to be non-pathogenic in infection studies. The full replication cycle of the Paramyxoviridae occurs in the host cell's cytoplasm, where viral assembly is orchestrated by the matrix (M) protein. Unexpectedly, the NiV-M protein traffics through the nucleus as an essential step to engage the plasma membrane in preparation for viral budding/release. Comparative studies were performed to assess whether M protein nuclear localization is a common feature of the henipaviruses, including the recently sequenced (although not yet isolated) Ghanaian bat henipavirus (Kumasi virus, GH-M74a virus) and Mojiang virus. Live-cell confocal microscopy revealed that nuclear translocation of GFP-fused M protein is conserved between henipaviruses in both human- and bat-derived cell lines. However, the efficiency of M protein nuclear localization and virus-like particle budding competency varied. Additionally, Cedar virus-, Kumasi virus- and Mojiang virus-M proteins were mutated in a bipartite nuclear localization signal, indicating that a key lysine residue is essential for nuclear import, export and induction of budding events, as previously reported for NiV-M. The results of this study suggest that the M proteins of henipaviruses may utilize a similar nucleocytoplasmic trafficking pathway as an essential step during viral replication in both humans and bats.

Understanding the interaction between henipaviruses and their natural host, fruit bats: Paving the way toward control of highly lethal infection in humans

Hendra virus and Nipah virus (NiV) are highly pathogenic zoonotic paramyxoviruses, from henipavirus genus, that have emerged in late 1990s in Australia and South-East Asia, respectively. Since their initial identification, numerous outbreaks have been reported, affecting both domestic animals and humans, and multiple rounds of person-to-person NiV transmission were observed. Widely distributed fruit bats from Pteropodidae family were found to be henipavirus natural reservoir. Numerous studies have reported henipavirus seropositivity in pteropid bats, including bats in Africa, thus expanding notably the geographic distribution of these viruses. Interestingly, henipavirus infection in bats seems to be asymptomatic, in contrast to severe disease induced in numerous other mammals. Unique among the mammals by their ability to fly, these intriguing animals are natural reservoir for many other emerging and remerging viruses highly pathogenic for humans. This feature, combined with absence of clinical symptoms, has attracted the interest of scientific community to virus-bat interactions. Therefore, several bat genomes were sequenced and particularities of the bat immune system have been intensively analyzed during the last decade to understand their coexistence with viruses in the absence of disease. The peculiarities in inflammasome activation, a constitutive expression of interferon alpha, and some differences in adaptive immunity have been recently reported in fruit bats. Studies on virus-bat interactions have thus emerged as an exciting novel area of research that should shed new light on the mechanisms that regulate viral infection and may allow development of novel therapeutic approaches to control this highly lethal emerging infectious disease in humans.

ESCRT-I Protein Tsg101 Plays a Role in the Post-macropinocytic Trafficking and Infection of Endothelial Cells by Kaposi's Sarcoma-Associated Herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) binding to the endothelial cell surface heparan sulfate is followed by sequential interactions with α3β1, αVβ3 and αVβ5 integrins and Ephrin A2 receptor tyrosine kinase (EphA2R). These interactions activate host cell pre-existing FAK, Src, PI3-K and RhoGTPase signaling cascades, c-Cbl mediated ubiquitination of receptors, recruitment of CIB1, p130Cas and Crk adaptor molecules, and membrane bleb formation leading to lipid raft dependent macropinocytosis of KSHV into human microvascular dermal endothelial (HMVEC-d) cells. The Endosomal Sorting Complexes Required for Transport (ESCRT) proteins, ESCRT-0, -I, -II, and-III, play a central role in clathrin-mediated internalized ubiquitinated receptor endosomal trafficking and sorting. ESCRT proteins have also been shown to play roles in viral egress. We have recently shown that ESCRT-0 component Hrs protein associates with the plasma membrane during macropinocytosis and mediates KSHV entry via ROCK1 mediated phosphorylation of NHE1 and local membrane pH change. Here, we demonstrate that the ESCRT-I complex Tsg101 protein also participates in the macropinocytosis of KSHV and plays a role in KSHV trafficking. Knockdown of Tsg101 did not affect virus entry in HMVEC-d and human umbilical vein endothelial (HUVEC) cells but significantly inhibited the KSHV genome entry into the nucleus and consequently viral gene expression in these cells. Double and triple immunofluorescence, proximity ligation immunofluorescence and co-immuoprecipitation studies revealed the association of Tsg101 with the KSHV containing macropinosomes, and increased levels of Tsg101 association/interactions with EphA2R, c-Cbl, p130Cas and Crk signal molecules, as well as with upstream and downstream ESCRT components such as Hrs (ESCRT-0), EAP45 (ESCRT-II), CHMP6 (ESCRT-III) and CHMP5 (ESCRT-III) in the KSHV infected cells. Tsg101 was also associated with early (Rab5) and late endosomal (Rab7) stages of KSHV intracellular trafficking, and CHMP5 (ESCRT-III) was also associated with Rab 5 and Rab 7. Knockdown of Tsg101 significantly inhibited the transition of virus from early to late endosomes. Collectively, our studies reveal that Tsg101 plays a role in the trafficking of macropinocytosed KSHV in the endothelial cells which is essential for the successful viral genome delivery into the nucleus, viral gene expression and infection. Thus, ESCRT molecules could serve as therapeutic targets to combat KSHV infection.

Nipah virus matrix protein: expert hacker of cellular machines

Nipah virus (NiV, Henipavirus) is a highly lethal emergent zoonotic paramyxovirus responsible for repeated human outbreaks of encephalitis in South East Asia. There are no approved vaccines or treatments, thus improved understanding of NiV biology is imperative. NiV matrix protein recruits a plethora of cellular machinery to scaffold and coordinate virion budding. Intriguingly, matrix also hijacks cellular trafficking and ubiquitination pathways to facilitate transient nuclear localization. While the biological significance of matrix nuclear localization for an otherwise cytoplasmic virus remains enigmatic, the molecular details have begun to be characterized, and are conserved among matrix proteins from divergent paramyxoviruses. Matrix protein appropriation of cellular machinery will be discussed in terms of its early nuclear targeting and later role in virion assembly.