Host factors involved in retroviral budding and release

Infection and intracellular transport of white spot syndrome virus require the ESCRT machinery in shrimp

The cellular endosomal sorting complex required for transport (ESCRT) system comprises five distinct components and is involved in many different physiological processes. Recent studies have shown that different viruses rely upon the host ESCRT system for viral infection. However, whether this system is involved in white spot syndrome virus (WSSV) infection remains unclear. Here, we identified 24 homologs of ESCRT subunits in kuruma shrimp, Marsupenaeus japonicus, and found that some key components were strongly upregulated in shrimp after WSSV infection. Knockdown of key components of the ESCRT system using RNA interference inhibited virus replication, suggesting that the ESCRT system is beneficial for WSSV infection. We further focused on TSG101, a crucial member of the ESCRT-I family that plays a central role in recognizing cargo and activating the ESCRT-II and ESCRT-III complexes. TSG101 colocalized with WSSV in hemocytes. The addition of N16 (a TSG101 inhibitor) markedly decreased WSSV replication. TSG101 and ALIX of the ESCRT system interact with WSSV envelope proteins. The host proteins TSG101, RAB5, and RAB7, the viral protein VP28, and DNA were detected in endosomes isolated from hemocytes of WSSV-infected shrimp. Knockdown of Rab5 and Rab7 expression reduced viral replication. Taken together, these results suggest that the ESCRT system is hijacked by WSSV for transport through the early to late endosome pathway. Our work identified a novel requirement for the intracellular trafficking and infection of WSSV, and provided novel therapeutic targets for the prevention and control of WSSV in shrimp aquaculture.

IMPORTANCE

Viruses utilize the ESCRT machinery in a variety of strategies for their replication and infection. This study revealed that the interaction of ESCRT complexes with WSSV envelope proteins plays a crucial role in WSSV infection in shrimp. The ESCRT system is conserved in the shrimp Marsupenaeus japonicus, and 24 homologs of the ESCRT system were identified in the shrimp. WSSV exploits the ESCRT system for transport and propagation via the interaction of envelope proteins with host TSG101 and ALIX in an endosome pathway-dependent manner. Understanding the underlying mechanisms of WSSV infection is important for disease control and breeding in shrimp aquaculture.

Structural Relationships to Efficacy for Prazole-Derived Antivirals

Here, an in vitro characterization of a family of prazole derivatives that covalently bind to the C73 site on Tsg101 and assay their ability to inhibit viral particle production is presented. Structurally, increased steric bulk on the 4-pyridyl of the prazole expands the prazole site on the UEV domain toward the β-hairpin in the Ub-binding site and is coupled to increased inhibition of virus-like particle production in HIV-1. Increased bulk also increased toxicity, which is alleviated by increasing flexibility. Further, the formation of a novel secondary Tsg101 adduct for several of the tested compounds and the commercial drug lansoprazole. The secondary adduct involved the loss of the 4-pyridyl substituent to form an irreversible species, with implications for increasing the half-life of the active species or its specificity toward Tsg101 UEV. It is also determined that sulfide derivatives display effective viral inhibition, presumably through cellular sulfoxidation, allowing for delayed conversion within the cellular environment, and identify SARS-COV-2 as a target of prazole inhibition. These results open multiple avenues for the design of prazole derivatives for antiviral applications.

Two-step evolution of HIV-1 budding system leading to pandemic in the human population

The pandemic HIV-1, HIV-1 group M, emerged from a single spillover event of its ancestral lentivirus from a chimpanzee. During human-to-human spread worldwide, HIV-1 diversified into multiple subtypes. Here, our interdisciplinary investigation mainly sheds light on the evolutionary scenario of the viral budding system of HIV-1 subtype C (HIV-1C), a most successfully spread subtype. Of the two amino acid motifs for HIV-1 budding, the P(T/S)AP and YPxL motifs, HIV-1C loses the YPxL motif. Our data imply that HIV-1C might lose this motif to evade immune pressure. Additionally, the P(T/S)AP motif is duplicated dependently of the level of HIV-1 spread in the human population, and >20% of HIV-1C harbored the duplicated P(T/S)AP motif. We further show that the duplication of the P(T/S)AP motif is caused by the expansion of the CTG triplet repeat. Altogether, our results suggest that HIV-1 has experienced a two-step evolution of the viral budding process during human-to-human spread worldwide.

Evidence for Involvement of ADP-Ribosylation Factor 6 in Intracellular Trafficking and Release of Murine Leukemia Virus Gag

Murine leukemia viruses (MuLVs) are simple retroviruses that cause several diseases in mice. Retroviruses encode three basic genes: gag, pol, and env. Gag is translated as a polyprotein and moves to assembly sites where viral particles are shaped by cleavage of poly-Gag. Viral release depends on the intracellular trafficking of viral proteins, which is determined by both viral and cellular factors. ADP-ribosylation factor 6 (Arf6) is a small GTPase that regulates vesicular trafficking and recycling of different types of cargo in cells. Arf6 also activates phospholipase D (PLD) and phosphatidylinositol-4-phosphate 5-kinase (PIP5K) and produces phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2). We investigated how Arf6 affected MuLV release with a constitutively active form of Arf6, Arf6Q67L. Expression of Arf6Q67L impaired Gag release by accumulating Gag at PI(4,5)P2-enriched compartments in the cytoplasm. Treatment of the inhibitors for PLD and PIP5K impaired or recovered MuLV Gag release in the cells expressing GFP (control) and Arf6Q67L, implying that regulation of PI(4,5)P2 through PLD and PIP5K affected MuLV release. Interference with the phosphoinositide 3-kinases, mammalian target of rapamycin (mTOR) pathway, and vacuolar-type ATPase activities showed further impairment of Gag release from the cells expressing Arf6Q67L. In contrast, mTOR inhibition increased Gag release in the control cells. The proteasome inhibitors reduced viral release in the cells regardless of Arf6Q67L expression. These data outline the differences in MuLV release under the controlled and overactivated Arf6 conditions and provide new insight into pathways for MuLV release.

E3 Ubiquitin Ligases in Gammaherpesviruses and HIV: A Review of Virus Adaptation and Exploitation

For productive infection and replication to occur, viruses must control cellular machinery and counteract restriction factors and antiviral proteins. Viruses can accomplish this, in part, via the regulation of cellular gene expression and post-transcriptional and post-translational control. Many viruses co-opt and counteract cellular processes via modulation of the host post-translational modification machinery and encoding or hijacking kinases, SUMO ligases, deubiquitinases, and ubiquitin ligases, in addition to other modifiers. In this review, we focus on three oncoviruses, Epstein-Barr virus (EBV), Kaposi's sarcoma herpesvirus (KSHV), and human immunodeficiency virus (HIV) and their interactions with the ubiquitin-proteasome system via viral-encoded or cellular E3 ubiquitin ligase activity.

A Review Study of the Participation of Late Domains in Sorting and Transport of Viral Factors to Exosomes

Cellular communication depends heavily on the participation of vesicular systems generated by most cells of an organism. Exosomes play central roles in this process. Today, these vesicles have been characterized, and it has been determined that the cargo they transport is not within a random system. In fact, it depends on various molecular signals and the recruitment of proteins that participate in the biogenesis of exosomes. It has also been shown that multiple viruses can recruit these vesicles to transport viral factors such as genomes or proteins. It has been shown that the late domains present in viral proteins are critical for the exosomal selection and biogenesis systems to recognize these viral proteins and introduce them into the exosomes. In this review, the researchers discuss the evidence related to the characterization of these late domains and their role in exosome recruitment during viral infection.

Exosomal MicroRNA and Protein Profiles of Hepatitis B Virus-Related Hepatocellular Carcinoma Cells

Infection with hepatitis B virus (HBV) is a main risk factor for hepatocellular carcinoma (HCC). Extracellular vesicles, such as exosomes, play an important role in tumor development and metastasis, including regulation of HBV-related HCC. In this study, we have characterized exosome microRNA and proteins released in vitro from hepatitis B virus (HBV)-related HCC cell lines SNU-423 and SNU-182 and immortalized normal hepatocyte cell lines (THLE2 and THLE3) using microRNA sequencing and mass spectrometry. Bioinformatics, including functional enrichment and network analysis, combined with survival analysis using data related to HCC in The Cancer Genome Atlas (TCGA) database, were applied to examine the prognostic significance of the results. More than 40 microRNAs and 200 proteins were significantly dysregulated (p < 0.05) in the exosomes released from HCC cells in comparison with the normal liver cells. The functional analysis of the differentially expressed exosomal miRNAs (i.e., mir-483, mir-133a, mir-34a, mir-155, mir-183, mir-182), their predicted targets, and exosomal differentially expressed proteins (i.e., POSTN, STAM, EXOC8, SNX9, COL1A2, IDH1, FN1) showed correlation with pathways associated with HBV, virus activity and invasion, exosome formation and adhesion, and exogenous protein binding. The results from this study may help in our understanding of the role of HBV infection in the development of HCC and in the development of new targets for treatment or non-invasive predictive biomarkers of HCC.

Getting on the right track: Interactions between viruses and the cytoskeletal motor proteins

The cytoskeleton is an essential component of the cell and it is involved in multiple physiological functions, including intracellular organization and transport. It is composed of three main families of proteinaceous filaments; microtubules, actin filaments and intermediate filaments and their accessory proteins. Motor proteins, which comprise the dynein, kinesin and myosin superfamilies, are a remarkable group of accessory proteins that mainly mediate the intracellular transport of cargoes along with the cytoskeleton. Like other cellular structures and pathways, viruses can exploit the cytoskeleton to promote different steps of their life cycle through associations with motor proteins. The complexity of the cytoskeleton and the differences among viruses, however, has led to a wide diversity of interactions, which in most cases remain poorly understood. Unveiling the details of these interactions is necessary not only for a better comprehension of specific infections, but may also reveal new potential drug targets to fight dreadful diseases such as rabies disease and acquired immunodeficiency syndrome (AIDS). In this review, we describe a few examples of the mechanisms that some human viruses, that is, rabies virus, adenovirus, herpes simplex virus, human immunodeficiency virus, influenza A virus and papillomavirus, have developed to hijack dyneins, kinesins and myosins.

The unconventional way out-Egress of HCMV through multiviral bodies

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus and the leading cause of congenital disabilities as well as a significant cause of disease in immunocompromised patients. The envelopment and egress of HCMV particles is an essential step of the viral life cycle as it determines viral spread and potentially tropism. Here we review the current literature on HCMV envelopment and egress with a particular focus on the role of virus-containing multivesicular body-like vesicles for virus egress and spread. We discuss the difficulties of determining the cellular provenance of these structures in light of viral redistribution of cellular marker proteins and provide potential paths to illuminate their genesis. Finally, we discuss how divergent egress pathways could result in virions of different tropisms.

PPRV-Induced Autophagy Facilitates Infectious Virus Transmission by the Exosomal Pathway

Peste des petits ruminants virus (PPRV) is an important pathogen that seriously influences the productivity of small ruminants worldwide. We showed previously that PPRV induced sustained autophagy for their replication in host cells. Many studies have shown that exosomes released from virus-infected cells contain a variety of viral and host cellular factors that are able to modulate the recipient's cellular response and result in productive infection of the recipient host. Here, we show that PPRV infection results in packaging of the viral genomic RNA and partial viral proteins into exosomes of Vero cells and upregulates exosome secretion. We provide evidence showing that the exosomal viral cargo can be transferred to and establish productive infection in a new target cell. Importantly, our study reveals that PPRV-induced autophagy enhances exosome secretion and exosome-mediated virus transmission. Additionally, our data show that TSG101 may be involved in the sorting of the infectious PPRV RNA into exosomes to facilitate the release of PPRV through the exosomal pathway. Taken together, our results suggest a novel mechanism involving autophagy and exosome-mediated PPRV intercellular transmission. IMPORTANCE Autophagy plays an important role in PPRV pathogenesis. The role of exosomes in viral infections is beginning to be appreciated. The present study examined the role of autophagy in secretion of infectious PPRV from Vero cells. Our data provided the first direct evidence that ATG7-mediated autophagy enhances exosome secretion and exosome-mediated PPRV transmission. TSG101 may be involved in the sorting of the infectious PPRV RNA genomes into exosomes to facilitate the release of PPRV through the exosomal pathway. Inhibition of PPRV-induced autophagy or TSG101 expression could be used as a strategy to block exosome-mediated virus transmission.

Characterization of Bovine Foamy Virus Gag Late Assembly Domain Motifs and Their Role in Recruiting ESCRT for Budding

A large number of retroviruses, such as human immunodeficiency virus (HIV) and prototype foamy virus (PFV), recruit the endosomal sorting complex required for transport (ESCRT) through the late domain (L domain) on the Gag structural protein for virus budding. However, little is known about the molecular mechanism of bovine foamy virus (BFV) budding. In the present study, we report that BFV recruits ESCRT for budding through the L domain of Gag. Specifically, knockdown of VPS4 (encoding vacuolar protein sorting 4), ALIX (encoding ALG-2-interacting protein X), and TSG101 (encoding tumor susceptibility 101) indicated that BFV uses ESCRT for budding. Mutational analysis of BFV Gag (BGag) showed that, in contrast to the classical L domain motifs, BGag contains two motifs, P₅₆LPI and Y₁₀₃GPL, with L domain functions. In addition, the two L domains are necessary for the cytoplasmic localization of BGag, which is important for effective budding. Furthermore, we demonstrated that the functional site of Alix is V498 in the V domain and the functional site of Tsg101 is N69 in the UBC-like domain for BFV budding. Taken together, these results demonstrate that BFV recruits ESCRT for budding through the PLPI and YGPL L domain motifs in BGag.

Roles of ESCRT proteins (ALIX and CHIMP4A) and their interplay with ISG15 during tick-borne flavivirus infection

Flaviviruses are usually transmitted to humans via mosquito or tick bites. During infection, virus replication and assembly, whose cellular sites are relatively close, are controlled by virus proteins and a diverse range of host proteins. By siRNA-mediated gene silencing, we showed that ALIX and CHMP4A, two members of the host endosomal sorting complex required for transport (ESCRT) protein machinery, are required during flavivirus infection. Using cell lines expressing subgenomic replicons and replicon virus-like particles, we demonstrated specific roles for ALIX and CHMP4A in viral replication and assembly, respectively. Employing biochemical and imaging methodology, we showed that the ESCRT proteins are recruited by a putative specific late (L) domain motif LYXLA within the NS3 protein of tick-borne flaviviruses. Furthermore, to counteract the recruitment of ESCRT proteins, the host cells may elicit defense mechanisms. We found that ectopic expression of the interferon-stimulated gene 15 (ISG15) or the E3 ISG15-protein ligase (HERC5) reduced virus replication by suppressing the positive effects of ALIX and CHMP4A. Collectively, these results have provided new insights into flavivirus-host cell interactions that function as checkpoints, including the NS3 and the ESCRT proteins, the ISG15 and the ESCRT proteins, at essential stages of the virus life cycle.

IMPORTANCE Flaviviruses are important zoonotic viruses with high fatality rates worldwide. Here, we report that during infection, the virus employs members of ESCRT proteins for virus replication and assembly. Among the ESCRT proteins, ALIX acts during virus replication, while CHMP4A is required during virus assembly. Another important ESCRT protein, TSG101, is not required for virus production. The ESCRT, complex, ALIX-CHMP4A, is recruited to NS3 through their interactions with the putative L domain motif of NS3, while CHMP4A is recruited to E. In addition, we demonstrate the antiviral mechanism of ISG15 and HERC5, which degrades ALIX and CHIMP4A, indirectly targets virus infection. In summary, we reveal host-dependency factors supporting flavivirus infection, but these factors may also be targeted by antiviral host effector mechanisms.

Murine BST2/tetherin promotes measles virus infection of neurons

BST2/tetherin is a transmembrane protein with antiviral activity; it is synthesized following exposure to interferons, and restricts the release of budding virus particles by tethering them to the host cell membrane. We previously showed that BST2 is induced in primary neurons following measles virus (MV) infection or type I interferon; however, BST2 was dispensable for protection against challenge with neuron-restricted MV. Here, we define the contribution of BST-2 in neuronal MV infection. Surprisingly, and in contrast to its antiviral role in non-neuronal cells, murine BST2 promotes MV infection in brains of permissive mice and in primary neuron cultures. Moreover, BST2 expression was predominantly observed in the non-synaptic fraction of purified neurons. These studies highlight a cell-type dependent role of a well-characterized antiviral protein in enhancing neuronal infection.

Cell type-specific biogenesis of novel vesicles containing viral products in human cytomegalovirus infection

Human cytomegalovirus (HCMV), while highly restricted for the human species, infects an diverse array of cell types in the host. Patterns of infection are dictated by the cell type infected, but cell type-specific factors and how they impact tropism for specific cell types is poorly understood. Previous studies in primary endothelial cells showed that HCMV infection induces large multivesicular-like bodies (MVBs) that incorporate viral products, including dense bodies (DBs) and virions. Here we define the nature of these large vesicles using a recombinant virus where UL32, encoding the pp150 tegument protein, is fused in frame with green fluorescent protein (GFP, TB40/E-UL32-GFP). In fibroblasts, UL32-GFP-positive vesicles were marked with classical markers of MVBs, including CD63 and lysobisphosphatidic acid (LBPA), both classical MVB markers, as well as the clathrin and LAMP1. Unexpectedly, UL32-GFP-positive vesicles in primary human microvascular endothelial cells (HMVECs) were not labeled by CD63, and LBPA was completely lost from infected cells. We defined these UL32-positive vesicles in endothelial cells using markers for the cis-Golgi (GM130), lysosome (LAMP1), and autophagy (LC3B). These findings suggest that UL32-GFP containing MVBs in fibroblasts are derived from the canonical endocytic pathway and takeover classical exosomal release pathway. However, UL32-GFP containing MVBs in HMVECs are derived from the early biosynthetic pathway and exploit a less characterized early Golgi-LAMP1-associated non- canonical secretory autophagy pathway. These results reveal striking cell-type specific membrane trafficking differences in host pathways that are exploited by HCMV, which may reflect distinct pathways for virus egress.ImportanceHuman cytomegalovirus (HCMV) is a herpesvirus that, like all herpesvirus, that establishes a life-long infection. HCMV remains a significant cause of morbidity and mortality in the immunocompromised and HCMV seropositivity is associated with age-related pathology. HCMV infects many cells in the human host and the biology underlying the different patterns of infection in different cell types is poorly understood. Endothelial cells are important target of infection that contribute to hematogenous spread of the virus to tissues. Here we define striking differences in the biogenesis of large vesicles that incorporate virions in fibroblasts and endothelial cells. In fibroblasts, HCMV is incorporated into canonical MVBs derived from an endocytic pathway, whereas HCMV matures through vesicles derived from the biosynthetic pathway in endothelial cells. This work defines basic biological differences between these cell types that may impact how progeny virus is trafficked out of infected cells.

From Entry to Egress: Strategic Exploitation of the Cellular Processes by HIV-1

HIV-1 employs a rich arsenal of viral factors throughout its life cycle and co-opts intracellular trafficking pathways. This exquisitely coordinated process requires precise manipulation of the host microenvironment, most often within defined subcellular compartments. The virus capitalizes on the host by modulating cell-surface proteins and cleverly exploiting nuclear import pathways for post entry events, among other key processes. Successful virus–cell interactions are indeed crucial in determining the extent of infection. By evolving defenses against host restriction factors, while simultaneously exploiting host dependency factors, the life cycle of HIV-1 presents a fascinating montage of an ongoing host–virus arms race. Herein, we provide an overview of how HIV-1 exploits native functions of the host cell and discuss recent findings that fundamentally change our understanding of the post-entry replication events.

Functions of Wnt and Hedgehog-containing extracellular vesicles in development and disease

Secreted morphogens play a major role in the intercellular communication necessary for animal development. It was initially thought that, in order to organize tissue morphogenesis and control cell fate and proliferation, morphogens diffused freely in the extracellular space. This view has since changed following the discovery that morphogens of the Wnt and Hedgehog (Hh) families are modified by various lipid adducts during their biosynthesis, providing them with high affinity for the membrane bilayer. Recent work performed in model organisms suggests that Wnt and Hh proteins are carried on extracellular vesicles. In this Review, we provide our perspectives on the mechanisms of formation of Wnt- and Hh-containing extracellular vesicles, and discuss their functions during animal development, as well as in various human physiopathologies.

Intracellular Trafficking of HBV Particles

The human hepatitis B virus (HBV), that is causative for more than 240 million cases of chronic liver inflammation (hepatitis), is an enveloped virus with a partially double-stranded DNA genome. After virion uptake by receptor-mediated endocytosis, the viral nucleocapsid is transported towards the nuclear pore complex. In the nuclear basket, the nucleocapsid disassembles. The viral genome that is covalently linked to the viral polymerase, which harbors a bipartite NLS, is imported into the nucleus. Here, the partially double-stranded DNA genome is converted in a minichromosome-like structure, the covalently closed circular DNA (cccDNA). The DNA virus HBV replicates via a pregenomic RNA (pgRNA)-intermediate that is reverse transcribed into DNA. HBV-infected cells release apart from the infectious viral parrticle two forms of non-infectious subviral particles (spheres and filaments), which are assembled by the surface proteins but lack any capsid and nucleic acid. In addition, naked capsids are released by HBV replicating cells. Infectious viral particles and filaments are released via multivesicular bodies; spheres are secreted by the classic constitutive secretory pathway. The release of naked capsids is still not fully understood, autophagosomal processes are discussed. This review describes intracellular trafficking pathways involved in virus entry, morphogenesis and release of (sub)viral particles.

Plasmacytoid dendritic cells in the eye

Plasmacytoid dendritic cells (pDCs) are a unique subpopulation of immune cells, distinct from classical dendritic cells. pDCs are generated in the bone marrow and following development, they typically home to secondary lymphoid tissues. While peripheral tissues are generally devoid of pDCs during steady state, few tissues, including the lung, kidney, vagina, and in particular ocular tissues harbor resident pDCs. pDCs were originally appreciated for their potential to produce large quantities of type I interferons in viral immunity. Subsequent studies have now unraveled their pivotal role in mediating immune responses, in particular in the induction of tolerance. In this review, we summarize our current knowledge on pDCs in ocular tissues in both mice and humans, in particular in the cornea, limbus, conjunctiva, choroid, retina, and lacrimal gland. Further, we will review our current understanding on the significance of pDCs in ameliorating inflammatory responses during herpes simplex virus keratitis, sterile inflammation, and corneal transplantation. Moreover, we describe their novel and pivotal neuroprotective role, their key function in preserving corneal angiogenic privilege, as well as their potential application as a cell-based therapy for ocular diseases.

Distinct Molecular Mechanisms of Host Immune Response Modulation by Arenavirus NP and Z Proteins

Endemic to West Africa and South America, mammalian arenaviruses can cross the species barrier from their natural rodent hosts to humans, resulting in illnesses ranging from mild flu-like syndromes to severe and fatal haemorrhagic zoonoses. The increased frequency of outbreaks and associated high fatality rates of the most prevalent arenavirus, Lassa, in West African countries, highlights the significant risk to public health and to the socio-economic development of affected countries. The devastating impact of these viruses is further exacerbated by the lack of approved vaccines and effective treatments. Differential immune responses to arenavirus infections that can lead to either clearance or rapid, widespread and uncontrolled viral dissemination are modulated by the arenavirus multifunctional proteins, NP and Z. These two proteins control the antiviral response to infection by targeting multiple cellular pathways; and thus, represent attractive targets for antiviral development to counteract infection. The interplay between the host immune responses and viral replication is a key determinant of virus pathogenicity and disease outcome. In this review, we examine the current understanding of host immune defenses against arenavirus infections and summarise the host protein interactions of NP and Z and the mechanisms that govern immune evasion strategies.

Centrosomal protein 55: A new paradigm in tumorigenesis

Centrosomal Protein 55 (Cep55), also known as c10orf3 and FLJ10540, was initially discovered as a major player in abscission, the final stage of cytokinesis. Subsequent studies have described its role in regulating the PI3K/AKT pathway, increasing cancer cell stemness, and promoting tumor formation. Clinically, Cep55 has been found to be overexpressed in many cancer types. Cep55 overexpression has been notably associated with tumor stage, tumor aggressiveness, poor prognosis, and metastasis. The present review discusses the role of Cep55 as a crucial biomarker and model in tumorigenesis.

Amino Acid Deletions in p6Gag Domain of HIV-1 CRF07_BC Ameliorate Galectin-3 Mediated Enhancement in Viral Budding

HIV-1 CRF07_BC is a recombinant virus with amino acid (a.a.) deletions in p6^Gag, which are overlapped with the Alix-binding domain. Galectin-3 (Gal3), a β-galactose binding lectin, has been reported to interact with Alix and regulate HIV-1 subtype B budding. This study aims to evaluate the role of Gal3 in HIV-1 CRF07_BC infection and the potential effect of a.a. deletions on Gal3-mediated regulation. A total of 38 HIV-1+ injecting drug users (IDUs) were enrolled in the study. Viral characterization and correlation of Gal3 were validated. CRF07_BC containing 7 a.a. deletions and wild-type in the p6^Gag (CRF07_BC-7d and -wt) were isolated and infectious clones were generated. Viral growth kinetic and budding assays using Jurkat-CCR5/Jurkat-CCR5-Gal3 cells infected with CRF07_BC were performed. Results indicate that 69.4% (25/38) of the recruited patients were identified as CRF07_BC, and CRF07_BC-7d was predominant. Slow disease progression and significantly higher plasma Gal3 were noted in CRF07_BC patients (p < 0.01). Results revealed that CRF07_BC infection resulted in Gal3 expression, which was induced by Tat. Growth dynamic and budding assays indicated that Gal3 expression in Jurkat-CCR5 cells significantly enhanced CRF07_BC-wt replication and budding (p < 0.05), while the promoting effect was ameliorated in CRF07_BC-7d. Co-immunoprecipitation found that deletions in the p6^Gag reduced Gal-3-mediated enhancement of the Alix–Gag interaction.

The Late Domain of Prototype Foamy Virus Gag Facilitates Autophagic Clearance of Stress Granules by Promoting Amphisome Formation

Prototype foamy virus (PFV), a complex retrovirus belonging to Spumaretrovirinae, maintains lifelong latent infection. The maintenance of lifelong latent infection by viruses relies on the repression of the type I interferon (IFN) response. However, the mechanism involving PFV latency, especially regarding the suppression of the IFN response, is poorly understood. Our previous study showed that PFV promotes autophagic flux. However, the underlying mechanism and the role of PFV-induced autophagy in latent infection have not been clarified. Here, we report that the PFV viral structural protein Gag induced amphisome formation and triggered autophagic clearance of stress granules (SGs) to attenuate type I IFN production. Moreover, the late domain (L-domain) of Gag played a central role in Alix recruitment, which promoted endosomal sorting complex required for transport I (ESCRT-I) formation and amphisome accumulation by facilitating late endosome formation. Our data suggest that PFV Gag represses the host IFN response through autophagic clearance of SGs by activating the endosome-autophagy pathway. More importantly, we found a novel mechanism by which a retrovirus inhibits the SG response to repress the type I IFN response.IMPORTANCE Maintenance of lifelong latent infection for viruses relies on repression of the type I IFN response. Autophagy plays a double-edged sword in antiviral immunity. However, the role of autophagy in the regulation of the type I IFN response and the mechanism involving virus-promoted autophagy have not been fully elucidated. SGs are an immune complex associated with the antiviral immune response and are critical for type I IFN production. Autophagic clearance of SGs is one means of degradation of SGs and is associated with regulation of immunity, but the detailed mechanism remains unclear. In this article, we demonstrate that PFV Gag recruits ESCRT-I to facilitate amphisome formation. Our data also suggest that amphisome formation is a critical event for autophagic clearance of SGs and repression of the type I IFN response. More importantly, we found a novel mechanism by which a retrovirus inhibits the SG response to repress the type I IFN response.

Development of a Cyclic Peptide Inhibitor of the p6/UEV Protein-Protein Interaction

The budding of HIV from infected cells is driven by the protein-protein interaction between the p6 domain of the HIV Gag protein and the UEV domain of the human TSG101 protein. We report the development of a cyclic peptide inhibitor of the p6/UEV interaction, from a non cell-permeable parent that was identified in a SICLOPPS screen. Amino acids critical for the activity of the parent cyclic peptide were uncovered using alanine-scanning, and a series of non-natural analogues synthesized and assessed. The most potent molecule disrupts the p6/UEV interaction with an IC₅₀ of 6.17 ± 0.24 μM by binding to UEV with a K_d of 11.9 ± 2.8 μM. This compound is cell permeable and active in a cellular virus-like particle budding assay with an IC₅₀ of ∼2 μM. This work further demonstrates the relative simplicity with which the potency and activity of cyclic peptides identified from SICLOPPS libraries can be optimized.

A PLPPV sequence in the p8 region of Gag provides late domain function for mouse mammary tumor virus

The late (L) domain sequence used by mouse mammary tumor virus (MMTV) remains undefined. Similar to other L domain-containing proteins, MMTV p8 and p14NC proteins are monoubiquitinated, suggesting L domain function. Site-directed mutagenesis of p8, PLPPV, and p14NC, PLPPL, sequences in MMTV Gag revealed a requirement only for the PLPPV sequence in virion release in a position-dependent manner. Electron microscopy of a defective Gag mutant confirmed an L domain budding defect morphology. The equine infectious anemia virus (EIAV) YPDL core L domain sequence and PLPPV provided L domain function in reciprocal MMTV and EIAV Gag exchange mutants, respectively. Alanine scanning of the PLPPV sequence revealed a strict requirement for the valine residue but only minor requirements for any one of the other residues. Thus, PLPPV provides MMTV L domain function, representing a fourth type of retroviral L domain that enables MMTV Gag proteins to co-opt cellular budding pathways for release.

HIV-1 Subtype C with PYxE Insertion Has Enhanced Binding of Gag-p6 to Host Cell Protein ALIX and Increased Replication Fitness

Human immunodeficiency virus type 1 subtype C (HIV-1C) has a natural deletion of a YPxL motif in its Gag-p6 late domain. This domain mediates the binding of Gag to host cell protein ALIX and subsequently facilitates viral budding. In a subset of HIV-1C-infected individuals, the tetrapeptide insertion PYxE has been identified at the deleted YPxL motif site. Here, we report the consequences of PYxE insertion on the interaction with ALIX and the relevance regarding replication fitness and drug sensitivity. In our three HIV-1C cohorts, PYKE and PYQE were most prevalent among PYxE variants. Through in silico predictions and in vitro experiments, we showed that HIV-1C Gag has an increased binding to ALIX when the PYxE motif is present. To go more into the clinical relevance of the PYxE insertion, we obtained patient-derived gag-pol sequences from HIV-1C_PYxEi viruses and inserted them in a reference HIV-1 sequence. Viral growth was increased, and the sensitivity to the protease inhibitor (PI) lopinavir (LPV) and nucleoside reverse transcriptase inhibitor tenofovir alafenamide (TAF) was decreased for some of the HIV-1C PYxE variants compared to that of wild-type variants. Our data suggest that PYxE insertion in Gag restores the ability of Gag to bind ALIX and correlates with enhanced viral fitness in the absence or presence of LPV and TAF. The high prevalence and increased replication fitness of the HIV-1C virus with PYxE insertion indicates the clinical importance of these viral variants.IMPORTANCE Genomic differences within HIV-1 subtypes is associated with various degrees of viral spread, disease progression, and clinical outcome. Viral budding is essential in the HIV-1 life cycle and mainly mediated through the interaction of Gag with host proteins. Two motifs within Gag-p6 mediate binding of host cell proteins and facilitate budding. HIV-1C has a natural deletion of one of these two motifs, resulting in an inability to bind to host cell protein ALIX. Previously, we have identified a tetrapeptide (PYxE) insertion at this deleted motif site in a subset of HIV-1C patients. Here, we report the incidence of PYxE insertions in three different HIV-1C cohorts, and the insertion restores the binding of Gag to ALIX. It also increases viral growth even in the presence of the antiretroviral drugs lopinavir and tenofovir alafenamide. Hence, PYxE insertion in HIV-1C might be biologically relevant for viruses and clinically significant among patients.

Mutations in the HIV-1 envelope glycoprotein can broadly rescue blocks at multiple steps in the virus replication cycle

The p6 domain of HIV-1 Gag contains highly conserved peptide motifs that recruit host machinery to sites of virus assembly, thereby promoting particle release from the infected cell. We previously reported that mutations in the YPX_nL motif of p6, which binds the host protein Alix, severely impair HIV-1 replication. Propagation of the p6-Alix binding site mutants in the Jurkat T cell line led to the emergence of viral revertants containing compensatory mutations not in Gag but in Vpu and the envelope (Env) glycoprotein subunits gp120 and gp41. The Env compensatory mutants replicate in Jurkat T cells and primary human peripheral blood mononuclear cells, despite exhibiting severe defects in cell-free particle infectivity and Env-mediated fusogenicity. Remarkably, the Env compensatory mutants can also rescue a replication-delayed integrase (IN) mutant, and exhibit reduced sensitivity to the IN inhibitor Dolutegravir (DTG), demonstrating that they confer a global replication advantage. In addition, confirming the ability of Env mutants to confer escape from DTG, we performed de novo selection for DTG resistance and observed resistance mutations in Env. These results identify amino acid substitutions in Env that confer broad escape from defects in virus replication imposed by either mutations in the HIV-1 genome or by an antiretroviral inhibitor. We attribute this phenotype to the ability of the Env mutants to mediate highly efficient cell-to-cell transmission, resulting in an increase in the multiplicity of infection. These findings have broad implications for our understanding of Env function and the evolution of HIV-1 drug resistance.

HIV-1 gag recruits PACSIN2 to promote virus spreading

The p2b domain of Rous sarcoma virus (RSV) Gag and the p6 domain of HIV-1 Gag contain late assembly (L) domains that engage the ESCRT membrane fission machinery and are essential for virus release. We now show that the PPXY-type RSV L domain specifically recruits the BAR domain protein PACSIN2 into virus-like particles (VLP), in addition to the NEDD4-like ubiquitin ligase ITCH and ESCRT pathway components such as TSG101. PACSIN2, which has been implicated in the remodeling of cellular membranes and the actin cytoskeleton, is also recruited by HIV-1 p6 independent of its ability to engage the ESCRT factors TSG101 or ALIX. Moreover, PACSIN2 is robustly recruited by NEDD4-2s, a NEDD4-like ubiquitin ligase capable of rescuing HIV-1 budding defects. The NEDD4-2s-induced incorporation of PACSIN2 into VLP correlated with the formation of Gag-ubiquitin conjugates, indicating that PACSIN2 binds ubiquitin. Although PACSIN2 was not required for a single cycle of HIV-1 replication after infection with cell-free virus, HIV-1 spreading was nevertheless severely impaired in T cell lines and primary human peripheral blood mononuclear cells depleted of PACSIN2. HIV-1 spreading could be restored by reintroduction of wild-type PACSIN2, but not of a SH3 domain mutant unable to interact with the actin polymerization regulators WASP and N-WASP. Overall, our observations indicate that PACSIN2 promotes the cell-to-cell spreading of HIV-1 by connecting Gag to the actin cytoskeleton.

Porcine Endogenous Retrovirus (PERV) - Molecular Structure and Replication Strategy in the Context of Retroviral Infection Risk of Human Cells

The xenotransplantation of porcine tissues may help overcome the shortage of human organs for transplantation. However, there are some concerns about recipient safety because the risk of porcine endogenous retrovirus (PERV) transmission to human cells remains unknown. Although, to date, no PERV infections have been noted in vivo, the possibility of such infections has been confirmed in vitro. Better understanding of the structure and replication cycle of PERVs is a prerequisite for determining the risk of infection and planning PERV-detection strategies. This review presents the current state of knowledge about the structure and replication cycle of PERVs in the context of retroviral infection risk.

Bone marrow stem cells to destroy circulating HIV: a hypothetical therapeutic strategy

Human immunodeficiency virus (HIV) still poses enigmatic threats to human life. This virus has mastered in bypassing anti retroviral therapy leading to patients’ death. Circulating viruses are phenomenal for the disease outcome. This hypothesis proposes a therapeutic strategy utilizing receptor-integrated hematopoietic, erythroid and red blood cells. Here, HIV specific receptors trap circulating viruses that enter erythrocyte cytoplasm and form inactive integration complex. This model depicts easy, effective removal of circulating HIV without any adverse effect.

Inhibiting the Ins and Outs of HIV Replication: Cell-Intrinsic Antiretroviral Restrictions at the Plasma Membrane

Like all viruses, human immunodeficiency viruses (HIVs) and their primate lentivirus relatives must enter cells in order to replicate and, once produced, new virions need to exit to spread to new targets. These processes require the virus to cross the plasma membrane of the cell twice: once via fusion mediated by the envelope glycoprotein to deliver the viral core into the cytosol; and secondly by ESCRT-mediated scission of budding virions during release. This physical barrier thus presents a perfect location for host antiviral restrictions that target enveloped viruses in general. In this review we will examine the current understanding of innate host antiviral defences that inhibit these essential replicative steps of primate lentiviruses associated with the plasma membrane, the mechanism by which these viruses have adapted to evade such defences, and the role that this virus/host battleground plays in the transmission and pathogenesis of HIV/AIDS.

Aedes aegypti microRNA miR-2b regulates ubiquitin-related modifier to control chikungunya virus replication

Arboviruses that replicate in mosquitoes activate innate immune response within mosquitoes. Regulatory non-coding microRNAs (miRNA) are known to be modulated in mosquitoes during chikungunya infection. However, information about targets of these miRNAs is scant. The present study was aimed to identify and analyze targets of miRNAs that are regulated during chikungunya virus (CHIKV) replication in Aedes aegypti cells and in the mosquito. Employing next-generation sequencing technologies, we identified a total of 126 miRNAs from the Ae. aegypti cell line Aag2. Of these, 13 miRNAs were found to be regulated during CHIKV infection. Putative targets of three of the most significantly regulated miRNAs- miR-100, miR-2b and miR-989 were also analyzed using quantitative PCRs, in cell lines and in mosquitoes, to validate whether they were the targets of the miRNAs. Our study expanded the list of miRNAs known in Ae. aegypti and predicted targets for the significantly regulated miRNAs. Further analysis of some of these targets revealed that ubiquitin-related modifier is a target of miRNA miR-2b and plays a significant role in chikungunya replication.

Distinct Roles of Cellular ESCRT-I and ESCRT-III Proteins in Efficient Entry and Egress of Budded Virions of Autographa californica Multiple Nucleopolyhedrovirus

The endosomal sorting complex required for transport (ESCRT) machinery is necessary for budding of many enveloped viruses. Recently, it was demonstrated that Vps4, the key regulator for recycling of the ESCRT-III complex, is required for efficient infection by the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, ESCRT assembly, regulation, and function are complex, and little is known regarding the details of participation of specific ESCRT complexes in AcMNPV infection. In this study, the core components of ESCRT-I (Tsg101 and Vps28) and ESCRT-III (Vps2B, Vps20, Vps24, Snf7, Vps46, and Vps60) were cloned from Spodoptera frugiperda Using a viral complementation system and RNA interference (RNAi) assays, we found that ESCRT-I and ESCRT-III complexes are required for efficient entry of AcMNPV into insect cells. In cells knocking down or overexpressing dominant negative (DN) forms of the components of ESCRT-I and ESCRT-III complexes, entering virions were partially trapped within the cytosol. To examine only egress, cells were transfected with the double-stranded RNA (dsRNA) targeting an individual ESCRT-I or ESCRT-III gene and viral bacmid DNA or viral bacmid DNA that expressed DN forms of ESCRT-I and ESCRT-III components. We found that ESCRT-III components (but not ESCRT-I components) are required for efficient nuclear egress of progeny nucleocapsids. In addition, we found that several baculovirus core or conserved proteins (Ac11, Ac76, Ac78, GP41, Ac93, Ac103, Ac142, and Ac146) interact with Vps4 and components of ESCRT-III. We propose that these viral proteins may form an "egress complex" that is involved in recruiting ESCRT-III components to a virus egress domain on the nuclear membrane.IMPORTANCE The ESCRT system is hijacked by many enveloped viruses to mediate budding and release. Recently, it was found that Vps4, the key regulator of the cellular ESCRT machinery, is necessary for efficient entry and egress of Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, little is known about the roles of specific ESCRT complexes in AcMNPV infection. In this study, we demonstrated that ESCRT-I and ESCRT-III complexes are required for efficient entry of AcMNPV into insect cells. The components of ESCRT-III (but not ESCRT-I) are also necessary for efficient nuclear egress of progeny nucleocapsids. Several baculovirus core or conserved proteins were found to interact with Vps4 and components of ESCRT-III, and these interactions may suggest the formation of an "egress complex" involved in the nuclear release or transport of viral nucleocapsids.

ESCRT-dependent control of membrane remodelling during cell division

The Endosomal Sorting Complex Required for Transport (ESCRT) proteins form an evolutionarily conserved membrane remodelling machinery. Identified originally for their role in cargo sorting and remodelling of endosomal membranes during yeast vacuolar sorting, an extensive body of work now implicates a sub-complex of this machinery (ESCRT-III), as a transplantable membrane fission machinery that is dispatched to various cellular locations to achieve a topologically unique membrane separation. Surprisingly, several ESCRT-III-regulated processes occur during cell division, when cells undergo a dramatic and co-ordinated remodelling of their membranes to allow the physical processes of division to occur. The ESCRT machinery functions in regeneration of the nuclear envelope during open mitosis and in the abscission phase of cytokinesis, where daughter cells are separated from each other in the last act of division. Roles for the ESCRT machinery in cell division are conserved as far back as Archaea, suggesting that the ancestral role of these proteins was as a membrane remodelling machinery that facilitated division and that was co-opted throughout evolution to perform a variety of other cell biological functions. Here, we will explore the function and regulation of the ESCRT machinery in cell division.

Inhibition of endocytic pathways impacts cytomegalovirus maturation

Endocytic processes are critical for cellular entry of several viruses; however, the role of endocytosis in cellular trafficking of viruses beyond virus entry is only partially understood. Here, we utilized two laboratory strains (AD169 and Towne) of human cytomegalovirus (HCMV), which are known to use cell membrane fusion rather than endocytosis to enter fibroblasts, in order to study a post-entry role of endocytosis in HCMV life cycle. Upon pharmacological inhibition of dynamin-2 or clathrin terminal domain (TD) ligand association, these strains entered the cells successfully based on the expression of immediate early viral protein. However, both the inhibitors significantly reduced the growth rates and final virus yields of viruses without inhibiting the expression of early to late viral proteins. Clathrin accumulated in the cytoplasmic virus assembly compartment (vAC) of infected cells co-localizing with virus tegument protein pp150 and the formation of vAC was compromised upon endocytic inhibition. Transmission electron micrographs (TEM) of infected cells treated with endocytosis inhibitors showed intact nuclear stages of nucleocapsid assembly but the cytoplasmic virus maturation was greatly compromised. Thus, the data presented here implicate endocytic pathways in HCMV maturation and egress.

A pilot study on interaction between donkey tetherin and EIAV stains with different virulent and replication characteristics

Tetherin (BST-2) is an important host restriction factor that can inhibit the release of a diverse array of enveloped viruses from infected cells. Conversely, to facilitate their release and spread, many viruses have evolved various strategies to overcome the antiviral effect of tetherin in a species-specific manner. During the development of an attenuated equine infectious anemia virus (EIAV) vaccine in our laboratory, we found that serial passage of a field-isolated virulent EIAV strains in horse and donkey as well as the cultivated donkey cells, produces several typical EIAV strains, including EIAV_DV, EIAV_DLV, and EIAV_FDDV, which exhibit distinct virulence and replication features in vivo and in vitro. However, the role of host restriction factors in EIAV evolution during the serial passage is not well understood. This study aimed to evaluate whether these newly generated strains adapt differently to donkey tetherin (do-tetherin) based on their virulence. We found that do-tetherin exerts an inhibition on the release of the viral particles produced by all three strains, albeit with varying intensity: EIAV_DV < EIAV_DLV < EIAV_FDDV. Additionally, all three EIAV strains could counteract the restriction mediated by do-tetherin via their envelope proteins (Env) with varying strength: EIAV_DV > EIAV_DLV > EIAV_FDDV. These results indicate that donkey tetherin is involved in shaping of EIAV evolution during serial passage.

An Essential Role of INI1/hSNF5 Chromatin Remodeling Protein in HIV-1 Posttranscriptional Events and Gag/Gag-Pol Stability

INI1/hSNF5/SMARCB1/BAF47 is an HIV-specific integrase (IN)-binding protein that influences HIV-1 transcription and particle production. INI1 binds to SAP18 (Sin3a-associated protein, 18 kDa), and both INI1 and SAP18 are incorporated into HIV-1 virions. To determine the significance of INI1 and the INI1-SAP18 interaction during HIV-1 replication, we isolated a panel of SAP18-interaction-defective (SID)-INI1 mutants using a yeast reverse two-hybrid screen. The SID-INI1 mutants, which retained the ability to bind to IN, cMYC, and INI1 but were impaired for binding to SAP18, were tested for their effects on HIV-1 particle production. SID-INI1 dramatically reduced the intracellular Gag/Gag-Pol protein levels and, in addition, decreased viral particle production. The SID-INI1-mediated effects were less dramatic in trans complementation assays using IN deletion mutant viruses with Vpr-reverse transcriptase (RT)-IN. SID-INI1 did not inhibit long-terminal-repeat (LTR)-mediated transcription, but it marginally decreased the steady-state gag RNA levels, suggesting a posttranscriptional effect. Pulse-chase analysis indicated that in SID-INI1-expressing cells, the pr55Gag levels decreased rapidly. RNA interference analysis indicated that small hairpin RNA (shRNA)-mediated knockdown of INI1 reduced the intracellular Gag/Gag-Pol levels and further inhibited HIV-1 particle production. These results suggest that SID-INI1 mutants inhibit multiple stages of posttranscriptional events of HIV-1 replication, including intracellular Gag/Gag-Pol RNA and protein levels, which in turn inhibits assembly and particle production. Interfering INI1 leads to a decrease in particle production and Gag/Gag-Pol protein levels. Understanding the role of INI1 and SAP18 in HIV-1 replication is likely to provide novel insight into the stability of Gag/Gag-Pol, which may lead to the development of novel therapeutic strategies to inhibit HIV-1 late events.

IMPORTANCE Significant gaps exist in our current understanding of the mechanisms and host factors that influence HIV-1 posttranscriptional events, including gag RNA levels, Gag/Gag-Pol protein levels, assembly, and particle production. Our previous studies suggested that the IN-binding host factor INI1 plays a role in HIV-1 assembly. An ectopically expressed minimal IN-binding domain of INI1, S6, potently and selectively inhibited HIV-1 Gag/Gag-Pol trafficking and particle production. However, whether or not endogenous INI1 and its interacting partners, such as SAP18, are required for late events was unknown. Here, we report that endogenous INI1 and its interaction with SAP18 are necessary to maintain intracellular levels of Gag/Gag-Pol and for particle production. Interfering INI1 or the INI1-SAP18 interaction leads to the impairment of these processes, suggesting a novel strategy for inhibiting posttranscriptional events of HIV-1 replication.

Reverse-topology membrane scission by the ESCRT proteins

The narrow membrane necks formed during viral, exosomal and intra-endosomal budding from membranes, as well as during cytokinesis and related processes, have interiors that are contiguous with the cytosol. Severing these necks involves action from the opposite face of the membrane as occurs during the well-characterized formation of coated vesicles. This 'reverse' (or 'inverse')-topology membrane scission is carried out by the endosomal sorting complex required for transport (ESCRT) proteins, which form filaments, flat spirals, tubes and conical funnels that are thought to direct membrane remodelling and scission. Their assembly, and their disassembly by the ATPase vacuolar protein sorting-associated 4 (VPS4) have been intensively studied, but the mechanism of scission has been elusive. New insights from cryo-electron microscopy and various types of spectroscopy may finally be close to rectifying this situation.

Functional Equivalence of Retroviral MA Domains in Facilitating Psi RNA Binding Specificity by Gag

Retroviruses specifically package full-length, dimeric genomic RNA (gRNA) even in the presence of a vast excess of cellular RNA. The “psi” (Ψ) element within the 5′-untranslated region (5′UTR) of gRNA is critical for packaging through interaction with the nucleocapsid (NC) domain of Gag. However, in vitro Gag binding affinity for Ψ versus non-Ψ RNAs is not significantly different. Previous salt-titration binding assays revealed that human immunodeficiency virus type 1 (HIV-1) Gag bound to Ψ RNA with high specificity and relatively few charge interactions, whereas binding to non-Ψ RNA was less specific and involved more electrostatic interactions. The NC domain was critical for specific Ψ binding, but surprisingly, a Gag mutant lacking the matrix (MA) domain was less effective at discriminating Ψ from non-Ψ RNA. We now find that Rous sarcoma virus (RSV) Gag also effectively discriminates RSV Ψ from non-Ψ RNA in a MA-dependent manner. Interestingly, Gag chimeras, wherein the HIV-1 and RSV MA domains were swapped, maintained high binding specificity to cognate Ψ RNAs. Using Ψ RNA mutant constructs, determinants responsible for promoting high Gag binding specificity were identified in both systems. Taken together, these studies reveal the functional equivalence of HIV-1 and RSV MA domains in facilitating Ψ RNA selectivity by Gag, as well as Ψ elements that promote this selectivity.

The HIV-1 late domain-2 S40A polymorphism in antiretroviral (or ART)-exposed individuals influences protease inhibitor susceptibility

Background

The p6 region of the HIV-1 structural precursor polyprotein, Gag, contains two motifs, P₇TAP₁₁ and L₃₅YPLXSL₄₁, designated as late (L) domain-1 and -2, respectively. These motifs bind the ESCRT-I factor Tsg101 and the ESCRT adaptor Alix, respectively, and are critical for efficient budding of virus particles from the plasma membrane. L domain-2 is thought to be functionally redundant to PTAP. To identify possible other functions of L domain-2, we examined this motif in dominant viruses that emerged in a group of 14 women who had detectable levels of HIV-1 in both plasma and genital tract despite a history of current or previous antiretroviral therapy.

Results

Remarkably, variants possessing mutations or rare polymorphisms in the highly conserved L domain-2 were identified in seven of these women. A mutation in a conserved residue (S40A) that does not reduce Gag interaction with Alix and therefore did not reduce budding efficiency was further investigated. This mutation causes a simultaneous change in the Pol reading frame but exhibits little deficiency in Gag processing and virion maturation. Whether introduced into the HIV-1 NL4-3 strain genome or a model protease (PR) precursor, S40A reduced production of mature PR. This same mutation also led to high level detection of two extended forms of PR that were fairly stable compared to the WT in the presence of IDV at various concentrations; one of the extended forms was effective in trans processing even at micromolar IDV.

Conclusions

Our results indicate that L domain-2, considered redundant in vitro, can undergo mutations in vivo that significantly alter PR function. These may contribute fitness benefits in both the absence and presence of PR inhibitor.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0298-1) contains supplementary material, which is available to authorized users.

Interactions of Prototype Foamy Virus Capsids with Host Cell Polo-Like Kinases Are Important for Efficient Viral DNA Integration

Unlike for other retroviruses, only a few host cell factors that aid the replication of foamy viruses (FVs) via interaction with viral structural components are known. Using a yeast-two-hybrid (Y2H) screen with prototype FV (PFV) Gag protein as bait we identified human polo-like kinase 2 (hPLK2), a member of cell cycle regulatory kinases, as a new interactor of PFV capsids. Further Y2H studies confirmed interaction of PFV Gag with several PLKs of both human and rat origin. A consensus Ser-Thr/Ser-Pro (S-T/S-P) motif in Gag, which is conserved among primate FVs and phosphorylated in PFV virions, was essential for recognition by PLKs. In the case of rat PLK2, functional kinase and polo-box domains were required for interaction with PFV Gag. Fluorescently-tagged PFV Gag, through its chromatin tethering function, selectively relocalized ectopically expressed eGFP-tagged PLK proteins to mitotic chromosomes in a Gag STP motif-dependent manner, confirming a specific and dominant nature of the Gag-PLK interaction in mammalian cells. The functional relevance of the Gag-PLK interaction was examined in the context of replication-competent FVs and single-round PFV vectors. Although STP motif mutated viruses displayed wild type (wt) particle release, RNA packaging and intra-particle reverse transcription, their replication capacity was decreased 3-fold in single-cycle infections, and up to 20-fold in spreading infections over an extended time period. Strikingly similar defects were observed when cells infected with single-round wt Gag PFV vectors were treated with a pan PLK inhibitor. Analysis of entry kinetics of the mutant viruses indicated a post-fusion defect resulting in delayed and reduced integration, which was accompanied with an enhanced preference to integrate into heterochromatin. We conclude that interaction between PFV Gag and cellular PLK proteins is important for early replication steps of PFV within host cells.

Viruses are masters at exploiting host cell machineries for their replication. For human immunodeficiency virus type 1 (HIV-1), the best-studied representative of the Orthoretrovirinae subfamily from the genus lentiviruses, numerous important virus-host interactions have been described. In contrast, only a few cellular proteins are known to influence the replication of foamy viruses (FVs, also known as spumaviruses), an intriguing type of complex retrovirus of the Spumaretrovirinae subfamily that combines features of both retroviruses and hepadnaviruses in its replication strategy. Given the increasing interest in FVs as gene transfer tools and their unique status within the retrovirus family, this discrepancy urged the identification of novel host cell interaction partners of FV structural components. This study focused on prototype FV (PFV), the best-characterized member of FVs, and its capsid protein, Gag, as the central player of viral replication. Members of the mitosis-regulatory, polo-like kinase (PLK) family were identified as novel Gag binding partners. The Gag interaction with PLK1 (and possibly also PLK2) facilitated efficient PFV genome integration into host chromatin, ensuring successful replication and viral spread in infected target cell cultures. Collectively, our results elucidate the first link between cell cycle regulatory networks and the mitosis-dependent PFV integration process.

A STING-dependent innate-sensing pathway mediates resistance to corneal HSV-1 infection via upregulation of the antiviral effector tetherin

Type 1 interferons (IFNs; IFNα/β) mediate immunological host resistance to numerous viral infections, including herpes simplex virus type 1 (HSV-1). The pathways responsible for IFNα/β signaling during the innate immune response to acute HSV-1 infection in the cornea are incompletely understood. Using a murine ocular infection model, we hypothesized that the stimulator of IFN genes (STING) mediates resistance to HSV-1 infection at the ocular surface and preserves the structural integrity of this mucosal site. Viral pathogenesis, tissue pathology, and host immune responses during ocular HSV-1 infection were characterized by plaque assay, esthesiometry, pachymetry, immunohistochemistry, flow cytometry, and small interfering RNA transfection in wild-type C57BL/6 (WT), STING-deficient (STING(-/-)), and IFNα/β receptor-deficient (CD118(-/-)) mice at days 3-5 postinfection. The presence of STING was critical for sustained control of HSV-1 replication in the corneal epithelium and resistance to viral neuroinvasion, but loss of STING had a negligible impact with respect to gross tissue pathology. Auxiliary STING-independent IFNα/β signaling pathways were responsible for maintenance of corneal integrity. Lymphatic vessels, mast cells, and sensory innervation were compromised in CD118(-/-) mice concurrent with increased tissue edema. STING-dependent signaling led to the upregulation of tetherin, a viral restriction factor we identify is important in containing the spread of HSV-1 in vivo.

The Race against Protease Activation Defines the Role of ESCRTs in HIV Budding

HIV virions assemble on the plasma membrane and bud out of infected cells using interactions with endosomal sorting complexes required for transport (ESCRTs). HIV protease activation is essential for maturation and infectivity of progeny virions, however, the precise timing of protease activation and its relationship to budding has not been well defined. We show that compromised interactions with ESCRTs result in delayed budding of virions from host cells. Specifically, we show that Gag mutants with compromised interactions with ALIX and Tsg101, two early ESCRT factors, have an average budding delay of ~75 minutes and ~10 hours, respectively. Virions with inactive proteases incorporated the full Gag-Pol and had ~60 minutes delay in budding. We demonstrate that during budding delay, activated proteases release critical HIV enzymes back to host cytosol leading to production of non-infectious progeny virions. To explain the molecular mechanism of the observed budding delay, we modulated the Pol size artificially and show that virion release delays are size-dependent and also show size-dependency in requirements for Tsg101 and ALIX. We highlight the sensitivity of HIV to budding “on-time” and suggest that budding delay is a potent mechanism for inhibition of infectious retroviral release.

ESCRTs are implicated in cellular processes which require fission of budding membranes. Likely the most studied of these processes is the HIV-ESCRT interactions. The canonical view is that interference with ESCRT recruitment results in a late budding arrest of virions at the plasma membrane and this mechanistic view of ESCRTs has shaped our understanding of their function in almost all cell biology. In this manuscript, we present a full kinetic analysis of HIV virion release under all known mutations in Gag that affect HIV-ESCRT interactions. Our data show that contrary to the canonical view, a defect in ESCRT recruitment does not inhibit virion budding, however it creates a delay. We further show that during budding delay, activated proteases release critical HIV enzymes back to host cytosol, leading to budding of non-infectious progeny virions. We suggest that budding delay is a potent mechanism for inhibition of infectious retroviral release and can be the basis for developing antiviral treatments which slow the budding process and therefore disproportionally affect infectious retroviral release. We also suggest that such budding delay may be one of the mechanisms underlying cellular innate immune responses which inhibit the spread of retroviral infection.

Budding of Tiger Frog Virus (an Iridovirus) from HepG2 Cells via Three Ways Recruits the ESCRT Pathway

The cellular endosomal sorting complex required for transport (ESCRT) pathway is a multifunctional pathway involved in cell physiological activities. While the majority of RNA viruses bearing L-domains are known to hijack the ESCRT pathway to complete the budding process, the budding of large and complex enveloped DNA viruses, especially iridoviruses, has been rarely investigated. In the present study, we use the tiger frog virus (TFV) as a model to investigate whether iridoviruses are released from host cells through the ESCRT pathway. Inhibition of class E proteins and auxiliary proteins (VPS4A, VPS4B, Tsg101, Alix, and Nedd4.1) reduces extracellular virion production, which preliminarily indicates that the ESCRT pathway is involved in TFV release. The respective interactions of TFV VP031L, VP065L, VP093L with Alix, Tsg101, Nedd4 suggest the underlying molecular mechanism by which TFV gets access to the ESCRT pathway. Co-depletion of Alix, Tsg101, and Nedd4.1 induces a significant reduction in extracellular virion production, which implies the functional redundancy of host factors in TFV budding. Those results are first observation that iridovirus gains access to ESCRT pathway through three ways of interactions between viral proteins and host proteins. Our study provides a better understanding of the budding mechanism of enveloped DNA viruses.

Late Maturation Steps Preceding Selective Nuclear Export and Egress of Progeny Parvovirus

Although the mechanism is not well understood, growing evidence indicates that the nonenveloped parvovirus minute virus of mice (MVM) may actively egress before passive release through cell lysis. We have dissected the late maturation steps of the intranuclear progeny with the aims of confirming the existence of active prelytic egress and identifying critical capsid rearrangements required to initiate the process. By performing anion-exchange chromatography (AEX), we separated intranuclear progeny particles by their net surface charges. Apart from empty capsids (EC), two distinct populations of full capsids (FC) arose in the nuclei of infected cells. The earliest population of FC to appear was infectious but, like EC, could not be actively exported from the nucleus. Further maturation of this early population, involving the phosphorylation of surface residues, gave rise to a second, late population with nuclear export potential. While capsid surface phosphorylation was strictly associated with nuclear export capacity, mutational analysis revealed that the phosphoserine-rich N terminus of VP2 (N-VP2) was dispensable, although it contributed to passive release. The reverse situation was observed for the incoming particles, which were dephosphorylated in the endosomes. Our results confirm the existence of active prelytic egress and reveal a late phosphorylation event occurring in the nucleus as a selective factor for initiating the process.

IMPORTANCE

In general, the process of egress of enveloped viruses is active and involves host cell membranes. However, the release of nonenveloped viruses seems to rely more on cell lysis. At least for some nonenveloped viruses, an active process before passive release by cell lysis has been reported, although the underlying mechanism remains poorly understood. By using the nonenveloped model parvovirus minute virus of mice, we could confirm the existence of an active process of nuclear export and further characterize the associated capsid maturation steps. Following DNA packaging in the nucleus, capsids required further modifications, involving the phosphorylation of surface residues, to acquire nuclear export potential. Inversely, those surface residues were dephosphorylated on entering capsids. These spatially controlled phosphorylation-dephosphorylation events concurred with the nuclear export-import potential required to complete the infectious cycle.

ESCRT Requirements for Murine Leukemia Virus Release

The Murine Leukemia Virus (MLV) is a gammaretrovirus that hijack host components of the endosomal sorting complex required for transport (ESCRT) for budding. To determine the minimal requirements for ESCRT factors in MLV viral and viral-like particles (VLP) release, an siRNA knockdown screen of ESCRT(-associated) proteins was performed in MLV-producing human cells. We found that MLV VLPs and virions primarily engage the ESCRT-I factor Tsg101 and marginally the ESCRT-associated adaptors Nedd4-1 and Alix to enter the ESCRT pathway. Conversely, the inactivation of ESCRT-II had no impact on VLP and virion egress. By analyzing the effects of individual ESCRT-III knockdowns, VLP and virion release was profoundly inhibited in CHMP2A- and CHMP4B-knockdown cells. In contrast, neither the CHMP2B and CHMP4A isoforms nor CHMP3, CHMP5, and CHMP6 were found to be essential. In case of CHMP1, we unexpectedly observed that the CHMP1A isoform was specifically required for virus budding, but dispensable for VLP release. Hence, MLV utilizes only a subset of ESCRT factors, and viral and viral-like particles differ in ESCRT-III factor requirements.