Caveola-dependent endocytic entry of amphotropic murine leukemia virus

Attachment, Entry, and Intracellular Trafficking of Classical Swine Fever Virus

Classical swine fever virus (CSFV), which is a positive-sense, single-stranded RNA virus with an envelope, is a member of the Pestivirus genus in the Flaviviridae family. CSFV causes a severe and highly contagious disease in pigs and is prevalent worldwide, threatening the pig farming industry. The detailed mechanisms of the CSFV life cycle have been reported, but are still limited. Some receptors and attachment factors of CSFV, including heparan sulfate (HS), laminin receptor (LamR), complement regulatory protein (CD46), MER tyrosine kinase (MERTK), disintegrin, and metalloproteinase domain-containing protein 17 (ADAM17), were identified. After attachment, CSFV internalizes via clathrin-mediated endocytosis (CME) and/or caveolae/raft-dependent endocytosis (CavME). After internalization, CSFV moves to early and late endosomes before uncoating. During this period, intracellular trafficking of CSFV relies on components of the endosomal sorting complex required for transport (ESCRT) and Rab proteins in the endosome dynamics, with a dependence on the cytoskeleton network. This review summarizes the data on the mechanisms of CSFV attachment, internalization pathways, and intracellular trafficking, and provides a general view of the early events in the CSFV life cycle.

Neurological manifestations of SARS-CoV-2: complexity, mechanism and associated disorders

BACKGROUND

Coronaviruses such as Severe Acute Respiratory Syndrome coronavirus (SARS), Middle Eastern Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) are associated with critical illnesses, including severe respiratory disorders. SARS-CoV-2 is the causative agent of the deadly COVID-19 illness, which has spread globally as a pandemic. SARS-CoV-2 may enter the human body through olfactory lobes and interact with the angiotensin-converting enzyme2 (ACE2) receptor, further facilitating cell binding and entry into the cells. Reports have shown that the virus can pass through the blood-brain barrier (BBB) and enter the central nervous system (CNS), resulting in various disorders. Cell entry by SARS-CoV-2 largely relies on TMPRSS2 and cathepsin L, which activate S protein. TMPRSS2 is found on the cell surface of respiratory, gastrointestinal and urogenital epithelium, while cathepsin-L is a part of endosomes.

AIM

The current review aims to provide information on how SARS-CoV-2 infection affects brain function.. Furthermore, CNS disorders associated with SARS-CoV-2 infection, including ischemic stroke, cerebral venous thrombosis, Guillain-Barré syndrome, multiple sclerosis, meningitis, and encephalitis, are discussed. The many probable mechanisms and paths involved in developing cerebrovascular problems in COVID patients are thoroughly detailed.

MAIN BODY

There have been reports that the SARS-CoV-2 virus can cross the blood-brain barrier (BBB) and enter the central nervous system (CNS), where it could cause a various illnesses. Patients suffering from COVID-19 experience a range of neurological complications, including sleep disorders, viral encephalitis, headaches, dysgeusia, and cognitive impairment. The presence of SARS-CoV-2 in the cerebrospinal fluid (CSF) of COVID-19 patients has been reported. Health experts also reported its presence in cortical neurons and human brain organoids. The possible mechanism of virus infiltration into the brain can be neurotropic, direct infiltration and cytokine storm-based pathways. The olfactory lobes could also be the primary pathway for the entrance of SARS-CoV-2 into the brain.

CONCLUSIONS

SARS-CoV-2 can lead to neurological complications, such as cerebrovascular manifestations, motor movement complications, and cognitive decline. COVID-19 infection can result in cerebrovascular symptoms and diseases, such as strokes and thrombosis. The virus can affect the neural system, disrupt cognitive function and cause neurological disorders. To combat the epidemic, it is crucial to repurpose drugs currently in use quickly and develop novel therapeutics.

Entry of bovine parainfluenza virus type 3 into MDBK cells occurs via clathrin-mediated endocytosis and macropinocytosis in a acid-dependent manner

Bovine parainfluenza virus 3 (BPIV3) is an important respiratory pathogen of both young and adult cattle. No specific therapies are available for BPIV3. Understanding the viral internalization pathway of BPIV3 will provide new strategies for the development of antiviral treatments. Here, the entry mechanism of BPIV3 into MDBK cells was analyzed using chemical inhibitors and RNA silencing. Our data demonstrated that treatment with an inhibitor targeting the clathrin-mediated pathway or clathrin heavy chain (CHC) knockdown suppressed the entry of BPIV3 into MDBK cells. In contrast, sequestration of cellular cholesterol by nystatin or silencing of caveolin-1 had no effect on viral entry. Moreover, inhibition of critical modulators of macropinocytosis significantly reduced BPIV3 uptake. In addition, fluid-phase uptake was significantly increased in cells infected with BPIV3, which is indicative of virus-induced facilitation of macropinocytosis. These results suggest that BPIV3 enters MDBK cells via macropinocytosis and clathrin- but not caveolar-dependent endocytosis. Furthermore, inhibition of endosomal acidification and activation of cathepsin blocked BPIV3 entry, demonstrating that BPIV3 entered MDBK cells in a acid-dependent manner and required cathepsin L. Finally, we demonstrated that macropinocytosis but not clathrin-mediated endocytosis is dependent on actin dynamics during BPIV3 infection.

Bovine Parainfluenza Virus Type 3 (BPIV3) Enters HeLa Cells via Clathrin-Mediated Endocytosis in a Cholesterol- and Dynamin-Dependent Manner

Bovine parainfluenza virus 3 (BPIV3) is a crucial causative agent of respiratory disease in young and adult cattle. No specific therapies are available for BPIV3 infection. Understanding the internalization pathway of the virus will provide a new strategy for the development of antiviral therapy. Here, the mechanism of BPIV3 entry into HeLa cells was analyzed using RNA silencing and pharmacological inhibitors. Treatment of HeLa cells with hypertonic medium prevented BPIV3 internalization. These results indicated that BPIV3 entered HeLa cells via receptor-mediated endocytosis. Moreover, removing cell membrane cholesterol through MβCD treatment hampered viral penetration but not viral replication. In addition, BPIV3 infection was inhibited by pretreatment with dynasore or chlorpromazine (CPZ) or knockdown of dynamin II or clathrin heavy chain. However, virus entry was unaffected by nystatin, EIPA, wortmannin, or cytochalasin D treatment or caveolin-1 knockdown. These data demonstrated that the entry of BPIV3 into HeLa cells was dependent on clathrin-mediated endocytosis but not on caveolae-mediated endocytosis or the macropinocytosis pathway. Many viruses are transported to endosomes, which provide an acidic environment and release their genome upon separation from primary endocytic vesicles. However, we found that BPIV3 infection required endosomal cathepsins, but not a low pH. In summary, we show, for the first time, that BPIV3 enters HeLa cells through the clathrin-mediated endocytosis pathway, presenting novel insights into the invasion mechanism of Paramyxoviridae.

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

Caveolae as Potential Hijackable Gates in Cell Communication

Caveolae are membrane microdomains described in many cell types involved in endocytocis, transcytosis, cell signaling, mechanotransduction, and aging. They are found at the interface with the extracellular environment and are structured by caveolin and cavin proteins. Caveolae and caveolins mediate transduction of chemical messages via signaling pathways, as well as non-chemical messages, such as stretching or shear stress. Various pathogens or signals can hijack these gates, leading to infectious, oncogenic and even caveolin-related diseases named caveolinopathies. By contrast, preclinical and clinical research have fallen behind in their attempts to hijack caveolae and caveolins for therapeutic purposes. Caveolae involvement in human disease is not yet fully explored or understood and, of all their scaffold proteins, only caveolin-1 is being considered in clinical trials as a possible biomarker of disease. This review briefly summarizes current knowledge about caveolae cell signaling and raises the hypothesis whether these microdomains could serve as hijackable “gatekeepers” or “gateways” in cell communication. Furthermore, because cell signaling is one of the most dynamic domains in translating data from basic to clinical research, we pay special attention to translation of caveolae, caveolin, and cavin research into clinical practice.

Multifaceted Functions of Host Cell Caveolae/Caveolin-1 in Virus Infections

Virus infection has drawn extensive attention since it causes serious or even deadly diseases, consequently inducing a series of social and public health problems. Caveolin-1 is the most important structural protein of caveolae, a membrane invagination widely known for its role in endocytosis and subsequent cytoplasmic transportation. Caveolae/caveolin-1 is tightly associated with a wide range of biological processes, including cholesterol homeostasis, cell mechano-sensing, tumorigenesis, and signal transduction. Intriguingly, the versatile roles of caveolae/caveolin-1 in virus infections have increasingly been appreciated. Over the past few decades, more and more viruses have been identified to invade host cells via caveolae-mediated endocytosis, although other known pathways have been explored. The subsequent post-entry events, including trafficking, replication, assembly, and egress of a large number of viruses, are caveolae/caveolin-1-dependent. Deprivation of caveolae/caveolin-1 by drug application or gene editing leads to abnormalities in viral uptake, viral protein expression, or virion release, whereas the underlying mechanisms remain elusive and must be explored holistically to provide potential novel antiviral targets and strategies. This review recapitulates our current knowledge on how caveolae/caveolin-1 functions in every step of the viral infection cycle and various relevant signaling pathways, hoping to provide a new perspective for future viral cell biology research.

Disparate Entry of Adenoviruses Dictates Differential Innate Immune Responses on the Ocular Surface

Human adenovirus infection of the ocular surface is associated with severe keratoconjunctivitis and the formation of subepithelial corneal infiltrates, which may persist and impair vision for months to years following infection. Long term pathology persists well beyond the resolution of viral replication, indicating that the prolonged immune response is not virus-mediated. However, it is not clear how these responses are sustained or even initiated following infection. This review discusses recent work from our laboratory and others which demonstrates different entry pathways specific to both adenovirus and cell type. These findings suggest that adenoviruses may stimulate specific pattern recognition receptors in an entry/trafficking-dependent manner, leading to distinct immune responses dependent on the virus/cell type combination. Additional work is needed to understand the specific connections between adenoviral entry and the stimulation of innate immune responses by the various cell types present on the ocular surface.

IFITM3 Clusters on Virus Containing Endosomes and Lysosomes Early in the Influenza A Infection of Human Airway Epithelial Cells

Interferon-induced transmembrane proteins (IFITMs) have been shown to strongly affect influenza A virus (IAV) infectivity in tissue culture. Moreover, polymorphisms in IFITM3 have been associated with the severity of the disease in humans. IFITM3 appears to act early in the infection, but its mechanism of action and potential interactions with incoming IAV structures are not yet defined. Here, we visualized endogenous IFITM3 interactions with IAV in the human lung epithelial cell line A549 and in primary human airway epithelial cells employing stimulated emission depletion super-resolution microscopy. By applying an iterative approach for the cluster definition and computational cluster analysis, we found that IFITM3 reorganizes into clusters as IAV infection progresses. IFITM3 cluster formation started at 2-3 h post infection and increased over time to finally coat IAV-containing endosomal vesicles. This IAV-induced phenotype was due to the endosomal recruitment of IFITM3 rather than to an overall increase in the IFITM3 abundance. While the IAV-induced IFITM3 clustering and localization to endosomal vesicles was comparable in primary human airway epithelial cells and the human lung epithelial cell line A549, the endogenous IFITM3 signal was higher in primary cells. Moreover, we observed IFITM3 signals adjacent to IAV-containing recycling endosomes.

Infectious bronchitis virus entry mainly depends on clathrin mediated endocytosis and requires classical endosomal/lysosomal system

Although several reports suggest that the entry of infectious bronchitis virus (IBV) depends on lipid rafts and low pH, the endocytic route and intracellular trafficking are unclear. In this study, we aimed to shed greater light on early steps in IBV infection. By using chemical inhibitors, RNA interference, and dominant negative mutants, we observed that lipid rafts and low pH was indeed required for virus entry; IBV mainly utilized the clathrin mediated endocytosis (CME) for entry; GTPase dynamin 1 was involved in virus containing vesicle scission; and the penetration of IBV into cells led to active cytoskeleton rearrangement. By using R18 labeled virus, we found that virus particles moved along with the classical endosome/lysosome track. Functional inactivation of Rab5 and Rab7 significantly inhibited IBV infection. Finally, by using dual R18/DiOC labeled IBV, we observed that membrane fusion was induced after 1 h.p.i. in late endosome/lysosome.

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Intact lipid rafts is involved in IBV entry.

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Low pH in intracyplasmic vesicles is required for IBV entry.

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IBV penetrates cells via clathrin mediated endocytosis.

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IBV moves along with the classical endosome/lysosome track, finally fuses with late endosome/lysosome.

A CRISPR screen for factors regulating SAMHD1 degradation identifies IFITMs as potent inhibitors of lentiviral particle delivery

The InterFeron Induced TransMembrane (IFITM) proteins are interferon stimulated genes that restrict many viruses, including HIV-1. SAMHD1 is another restriction factor blocking replication of HIV-1 and other viruses. Some lentiviruses evolved Vpx/Vpr proteins to degrade SAMHD1. However, this viral antagonism can be perturbed by host mechanisms: a recent study showed that in interferon (IFN) treated THP1 cells, Vpx is unable to degrade SAMHD1. In the present work, we designed an Interferon Stimulated Genes (ISGs)-targeted CRISPR knockout screen in order to identify ISGs regulating this phenotype. We found that IFITM proteins contribute to the IFNα-mediated protection of SAMHD1 by blocking VSV-G-mediated entry of the lentiviral particles delivering Vpx. Consistent with this, IFNα treatment and IFITM expression had no effect when the A-MLV envelope was used for pseudotyping. Using an assay measuring viral entry, we show that IFNα and IFITMs directly block the delivery of Vpx into cells by inhibiting VSV-G viral fusion. Strikingly, the VSV-G envelope was significantly more sensitive to this IFNα entry block and to IFITMs than HIV-1's natural envelope. This highlights important differences between VSV-G pseudotyped and wild-type HIV-1, in particular relative to the pathways they use for viral entry, suggesting that HIV-1 may have evolved to escape restriction factors blocking entry.

Characterization of two distinct early post-entry blocks to HIV-1 in common marmoset lymphocytes

In nature, primate lentiviruses infect humans and several Old World monkeys and apes. However, to date, lentiviruses infecting New World monkeys have not been described. We studied the susceptibility of common marmoset cells to HIV-1 infection and observed the presence of post-entry blocks to the early phase of HIV-1 infection in peripheral blood lymphocytes (PBLs) and a B lymphocytic cell line (B-LCL). The blocks present in these cells are dominant and phenotypically different from each other. In PBLs, the block occurs at the level of reverse transcription, reducing the accumulation of early and late transcripts, similar to the block imposed by TRIM5α. However, we have found that marmoset TRIM5α does not block HIV-1. In contrast, the restriction factor present in B-LCLs blocks HIV-1 replication at a later step, after nuclear entry, and inhibits integration. Additionally, we have identified an HIV-1 capsid mutant, N74D, that is able to escape the restriction in the marmoset B-LCLs. Our results suggest that the factors responsible for the blocks present in marmoset PBLs and B-LCLs are different. We propose the existence of at least two new restriction factors able to block HIV-1 infection in marmoset lymphocytes.

Dynein Regulators Are Important for Ecotropic Murine Leukemia Virus Infection

During the early steps of infection, retroviruses must direct the movement of the viral genome into the nucleus to complete their replication cycle. This process is mediated by cellular proteins that interact first with the reverse transcription complex and later with the preintegration complex (PIC), allowing it to reach and enter the nucleus. For simple retroviruses, such as murine leukemia virus (MLV), the identities of the cellular proteins involved in trafficking of the PIC in infection are unknown. To identify cellular proteins that interact with the MLV PIC, we developed a replication-competent MLV in which the integrase protein was tagged with a FLAG epitope. Using a combination of immunoprecipitation and mass spectrometry, we established that the microtubule motor dynein regulator DCTN2/p50/dynamitin interacts with the MLV preintegration complex early in infection, suggesting a direct interaction between the incoming viral particles and the dynein complex regulators. Further experiments showed that RNA interference (RNAi)-mediated silencing of either DCTN2/p50/dynamitin or another dynein regulator, NudEL, profoundly reduced the efficiency of infection by ecotropic, but not amphotropic, MLV reporters. We propose that the cytoplasmic dynein regulators are a critical component of the host machinery needed for infection by the retroviruses entering the cell via the ecotropic envelope pathway.

IMPORTANCE

Retroviruses must access the chromatin of host cells to integrate the viral DNA, but before this crucial event, they must reach the nucleus. The movement through the cytoplasm-a crowded environment where diffusion is slow-is thought to utilize retrograde transport along the microtubule network by the dynein complex. Different viruses use different components of this multisubunit complex. We found that the preintegration complex of murine leukemia virus (MLV) interacts with the dynein complex and that regulators of this complex are essential for infection. Our study provides the first insight into the requirements for retrograde transport of the MLV preintegration complex.

Cell Walls and the Convergent Evolution of the Viral Envelope

Why some viruses are enveloped while others lack an outer lipid bilayer is a major question in viral evolution but one that has received relatively little attention. The viral envelope serves several functions, including protecting the RNA or DNA molecule(s), evading recognition by the immune system, and facilitating virus entry. Despite these commonalities, viral envelopes come in a wide variety of shapes and configurations. The evolution of the viral envelope is made more puzzling by the fact that nonenveloped viruses are able to infect a diverse range of hosts across the tree of life. We reviewed the entry, transmission, and exit pathways of all (101) viral families on the 2013 International Committee on Taxonomy of Viruses (ICTV) list. By doing this, we revealed a strong association between the lack of a viral envelope and the presence of a cell wall in the hosts these viruses infect. We were able to propose a new hypothesis for the existence of enveloped and nonenveloped viruses, in which the latter represent an adaptation to cells surrounded by a cell wall, while the former are an adaptation to animal cells where cell walls are absent. In particular, cell walls inhibit viral entry and exit, as well as viral transport within an organism, all of which are critical waypoints for successful infection and spread. Finally, we discuss how this new model for the origin of the viral envelope impacts our overall understanding of virus evolution.

Disruption of clathrin-dependent trafficking results in the failure of grass carp reovirus cellular entry

Background

Grass carp reovirus (GCRV) is responsible for viral hemorrhagic disease in cultured grass carp (Ctenopharyngon idellus). GCRV is a non-enveloped, double-stranded RNA virus in the genus Aquareovirus, of the family Reoviridae, which encodes seven structural proteins (VP1-VP7) and five nonstructural proteins (NS80, NS38, NS31, NS26, and NS16). To date, the mechanism of GCRV entry into CIK Ctenopharyngon idellus kidney (CIK) cells remains poorly understood.

Results

Here, we present a study of the GCRV internalization mechanism in CIK cells. Our results indicated that GCRV infection was inhibited by chlorpromazine, the specific inhibitor for clathrin-mediated endocytosis. Colocalization of GCRV virions with endogenous clathrin was observed during early infection by confocal microscopy. Moreover, GCRV infection of CIK cells depended on acidification of the endosome. This was indicated by significant inhibition of viral infection following prophylactic treatment with the lysosomotropic drugs chloroquine or ammonium chloride. In addition, the disturbance of dynamin activity blocked GCRV entry, which confirmed the dynamin-dependent nature of clathrin-mediated endocytosis.

Conclusion

Our findings suggest that GCRV might enter CIK cells via clathrin-mediated endocytosis in a pH-dependent manner. Additionally, dynamin is critical for efficient viral entry.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0485-7) contains supplementary material, which is available to authorized users.

Herpes Simplex Virus 1 Suppresses the Function of Lung Dendritic Cells via Caveolin-1

Caveolin-1 (Cav-1), the principal structural protein of caveolae, has been implicated as a regulator of virus-host interactions. Several viruses exploit caveolae to facilitate viral infections. However, the roles of Cav-1 in herpes simplex virus 1 (HSV-1) infection have not fully been elucidated. Here, we report that Cav-1 downregulates the expression of inducible nitric oxide synthase (iNOS) and the production of nitric oxide (NO) in dendritic cells (DCs) during HSV-1 infection. As a result, Cav-1 deficiency led to an accelerated elimination of virus and less lung pathological change following HSV-1 infection. This protection was dependent on iNOS and NO production in DCs. Adoptive transfer of DCs with Cav-1 knockdown was sufficient to confer the protection to wild-type (WT) mice. In addition, Cav-1 knockout (KO) (Cav-1(-/-)) mice treated with an iNOS inhibitor exhibited significantly reduced survival compared to that of the nontreated controls. We found that Cav-1 colocalized with iNOS and HSV-1 in caveolae in HSV-1-infected DCs, suggesting their interaction. Taken together, our results identified Cav-1 as a novel regulator utilized by HSV-1 to evade the host antiviral response mediated by NO production. Therefore, Cav-1 might be a valuable target for therapeutic approaches against herpesvirus infections.

Intracellular trafficking of hybrid gene delivery vectors

Viral and non-viral gene delivery vectors are in development for human gene therapy, but both exhibit disadvantages such as inadequate efficiency, lack of cell-specific targeting or safety concerns. We have recently reported the design of hybrid delivery vectors combining retrovirus-like particles with synthetic polymers or lipids that are efficient, provide sustained gene expression and are more stable compared to native retroviruses. To guide further development of this promising class of gene delivery vectors, we have investigated their mechanisms of intracellular trafficking. Moloney murine leukemia virus-like particles (M-VLPs) were complexed with chitosan (Chi) or liposomes (Lip) comprising DOTAP, DOPE and cholesterol to form the hybrid vectors (Chi/M-VLPs and Lip/M-VLPs, respectively). Transfection efficiency and cellular internalization of the vectors were quantified in the presence of a panel of inhibitors of various endocytic pathways. Intracellular transport and trafficking kinetics of the hybrid vectors were dependent on the synthetic component and used a combination of clathrin- and caveolar-dependent endocytosis and macropinocytosis. Chi/M-VLPs were slower to transfect compared to Lip/M-VLPs due to the delayed detachment of the synthetic component. The synthetic component of hybrid gene delivery vectors plays a significant role in their cellular interactions and processing and is a key parameter for the design of more efficient gene delivery vehicles.

A sorting signal suppresses IFITM1 restriction of viral entry

The interferon-induced transmembrane proteins (IFITMs) broadly inhibit virus infections, particularly at the viral entry level. However, despite this shared ability to inhibit fusion, IFITMs differ in the potency and breadth of viruses restricted, an anomaly that is not fully understood. Here, we show that differences in the range of viruses restricted by IFITM1 are regulated by a C-terminal non-canonical dibasic sorting signal KRXX that suppresses restriction of some viruses by governing its intracellular distribution. Replacing the two basic residues with alanine (KR/AA) increased restriction of jaagsiekte sheep retrovirus and 10A1 amphotropic murine leukemia virus. Deconvolution microscopy revealed an altered subcellular distribution for KR/AA, with fewer molecules in LAMP1-positive lysosomes balanced by increased levels in CD63-positive multivesicular bodies, where jaagsiekte sheep retrovirus pseudovirions are colocalized. IFITM1 binds to cellular adaptor protein complex 3 (AP-3), an association that is lost when the dibasic motif is altered. Although knockdown of AP-3 itself decreases some virus entry, expression of parental IFITM1, but not its KR/AA mutant, potentiates inhibition of viral infections in AP-3 knockdown cells. By using the substituted cysteine accessibility method, we provide evidence that IFITM1 adopts more than one membrane topology co-existing in cellular membranes. Because the C-terminal dibasic sorting signal is unique to human IFITM1, our results provide novel insight into understanding the species- and virus-specific antiviral effect of IFITMs.

Macropinocytosis is the entry mechanism of amphotropic murine leukemia virus

The entry mechanism of murine amphotropic retrovirus (A-MLV) has not been unambiguously determined. We show here that A-MLV is internalized not by caveolae or other pinocytic mechanisms but by macropinocytosis. Thus, A-MLV infection of mouse embryonic fibroblasts deficient for caveolin or dynamin, and NIH 3T3 cells knocked down for caveolin expression, was unaffected. Conversely, A-MLV infection of NIH 3T3 and HeLa cells was sensitive to amiloride analogues and actin-depolymerizing drugs that interfere with macropinocytosis. Further manipulation of the actin cytoskeleton through conditional expression of dominant positive or negative mutants of Rac1, PAK1, and RhoG, to increase or decrease macropinocytosis, similarly correlated with an augmented or inhibited infection with A-MLV, respectively. The same experimental perturbations affected the infection of viruses that use clathrin-coated-pit endocytosis or other pathways for entry only mildly or not at all. These data agree with immunofluorescence studies and cryo-immunogold labeling for electron microscopy, which demonstrate the presence of A-MLV in protrusion-rich areas of the cell surface and in cortical fluid phase (dextran)-filled macropinosomes, which also account for up to a half of the cellular uptake of the cell surface-binding lectin concanavalin A. We conclude that A-MLV use macropinocytosis as the predominant entry portal into cells.

IMPORTANCE

Binding and entry of virus particles into mammalian cells are the first steps of infection. Understanding how pathogens and toxins exploit or divert endocytosis pathways has advanced our understanding of membrane trafficking pathways, which benefits development of new therapeutic schemes and methods of drug delivery. We show here that amphotropic murine leukemia virus (A-MLV) pseudotyped with the amphotropic envelope protein (which expands the host range to many mammalian cells) gains entry into host cells by macropinocytosis. Macropinosomes form as large, fluid-filled vacuoles (up to 10 μm) following the collapse of cell surface protrusions and membrane scission. We used drugs or the introduction of mutant proteins that affect the actin cytoskeleton and cell surface dynamics to show that macropinocytosis and A-MLV infection are correlated, and we provide both light- and electron-microscopic evidence to show the localization of A-MLV in macropinosomes. Finally, we specifically exclude some other potential entry portals, including caveolae, previously suggested to internalize A-MLV.

Entry of a novel marine DNA virus, Singapore grouper iridovirus, into host cells occurs via clathrin-mediated endocytosis and macropinocytosis in a pH-dependent manner

Iridoviruses are nucleocytoplasmic DNA viruses which cause great economic losses in the aquaculture industry but also show significant threat to global biodiversity. However, a lack of host cells has resulted in poor progress in clarifying iridovirus behavior. We investigated the crucial events during virus entry using a combination of single-virus tracking and biochemical assays, based on the established virus-cell infection model for Singapore grouper iridovirus (SGIV). SGIV infection in host cells was strongly inhibited when cells were pretreated with drugs blocking clathrin-mediated endocytosis, including sucrose and chlorpromazine. Inhibition of key regulators of macropinocytosis, including Na(+)/H(+) exchanger, Rac1 GTPase, p21-activated kinase 1 (PAK1), protein kinase C (PKC), and myosin II, significantly reduced SGIV uptake. Cy5-labeled SGIV particles were observed to colocalize with clathrin and macropinosomes. In contrast, disruption of cellular cholesterol by methyl-β-cyclodextrin and nystatin had no effect on virus infection, suggesting that SGIV entered grouper cells via the clathrin-mediated endocytic pathway and macropinocytosis but not via caveola-dependent endocytosis. Furthermore, inhibitors of endosome acidification such as chloroquine and bafilomycin A1 blocked virus infection, indicating that SGIV entered cells in a pH-dependent manner. In addition, SGIV particles were observed to be transported along both microtubules and actin filaments, and intracellular SGIV motility was remarkably impaired by depolymerization of microtubules or actin filaments. The results of this study for the first time demonstrate that not only the clathrin-dependent pathway but also macropinocytosis are involved in fish DNA enveloped virus entry, thus providing a convenient tactic for exploring the life cycle of DNA viruses.

IMPORTANCE

Virus entry into host cells is critically important for initiating infections and is usually recognized as an ideal target for the design of antiviral strategies. Iridoviruses are large DNA viruses which cause serious threats to ecological diversity and the aquaculture industry worldwide. However, the current understanding of iridovirus entry is limited and controversial. Singapore grouper iridovirus (SGIV) is a novel marine fish DNA virus which belongs to genus Ranavirus, family Iridoviridae. Here, using single-virus tracking technology in combination with biochemical assays, we investigated the crucial events during SGIV entry and demonstrated that SGIV entered grouper cells via the clathrin-mediated endocytic pathway in a pH-dependent manner but not via caveola-dependent endocytosis. Furthermore, we propose for the first time that macropinocytosis is involved in iridovirus entry. Together, this work not only contributes greatly to understating iridovirus pathogenesis but also provides an ideal model for exploring the behavior of DNA viruses in living cells.

Tyrosine-based signal mediates LRP6 receptor endocytosis and desensitization of Wnt/β-catenin pathway signaling

Wnt/β-catenin signaling orchestrates a number of critical events including cell growth, differentiation, and cell survival during development. Misregulation of this pathway leads to various human diseases, specifically cancers. Endocytosis and phosphorylation of the LDL receptor-related protein 6 (LRP6), an essential co-receptor for Wnt/β-catenin signaling, play a vital role in mediating Wnt/β-catenin signal transduction. However, its regulatory mechanism is not fully understood. In this study, we define the mechanisms by which LRP6 endocytic trafficking regulates Wnt/β-catenin signaling activation. We show that LRP6 mutant with defective tyrosine-based signal in its cytoplasmic tail has an increased cell surface distribution and decreased endocytosis rate. These changes in LRP6 endocytosis coincide with an increased distribution to caveolae, increased phosphorylation, and enhanced Wnt/β-catenin signaling. We further demonstrate that treatment of Wnt3a ligands or blocking the clathrin-mediated endocytosis of LRP6 leads to a redistribution of wild-type receptor to lipid rafts. The LRP6 tyrosine mutant also exhibited an increase in signaling activation in response to Wnt3a stimulation when compared with wild-type LRP6, and this activation is suppressed when caveolae-mediated endocytosis is blocked. Our results reveal molecular mechanisms by which LRP6 endocytosis routes regulate its phosphorylation and the strength of Wnt/β-catenin signaling, and have implications on how this pathway can be modulated in human diseases.

Efficient in vitro gene delivery by hybrid biopolymer/virus nanobiovectors

Recombinant retroviruses provide highly efficient gene delivery and the potential for sustained gene expression, but suffer from significant disadvantages including low titer, expensive production, poor stability and limited flexibility for modification of tropism. In contrast, polymer-based vectors are more robust and allow cell- and tissue-specific deliveries via conjugation of ligands, but are comparatively inefficient. The design of hybrid gene delivery agents comprising both virally derived and synthetic materials (nanobiovectors) represents a promising approach to development of safe and efficient gene therapy vectors. Non-infectious murine leukemia virus-like particles (M-VLPs) were electrostatically complexed with chitosan (χ) to replace the function of the viral envelope protein. At optimal fabrication conditions and compositions, ranging from 6 to 9μg chitosan/10(9) M-VLPs at 10×10(9)M-VLPs/ml to 40μg chitosan/10(9) M-VLPs at 2.5×10(9)M-VLPs/ml, χ/M-VLPs were ~300-350nm in diameter and exhibited efficient transfection similar to amphotropic MLV vectors. In addition, these nanobiovectors were non-cytotoxic and provided sustained transgene expression for at least three weeks in vitro. This combination of biocompatible synthetic agents with inactive viral particles to form a highly efficient hybrid vector is a significant extension in the development of novel gene delivery platforms.

Syncytin-1 in differentiating human myoblasts: relationship to caveolin-3 and myogenin

Myoblasts fuse to form myotubes, which mature into skeletal muscle fibres. Recent studies indicate that an endogenous retroviral fusion gene, syncytin-1, is important for myoblast fusions in man. We have now expanded these data by examining the immunolocalization of syncytin in human myoblasts induced to fuse. Additionally, we have compared the localization of syncytin with the localization of caveolin-3 and of myogenin, which are also involved in myoblast fusion and maturation. Syncytin was localized to areas of the cell membrane and to filopodial structures connecting myoblasts to each other and to myotubes. Weaker staining was present over intracellular vesicles and tubules. Caveolin-3 was detected in the sarcolemma and in vesicles and tubules in a subset of myoblasts and myotubes. The strongest staining occurred in multinucleated myotubes. Wide-field fluorescence microscopy indicated a partial colocalization of syncytin and caveolin-3 in a subset of myoblasts. Super-resolution microscopy showed such colocalization to occur in the sarcolemma. Myogenin was restricted to nuclei of myoblasts and myotubes and the strongest staining occurred in multinucleated myotubes. Syncytin staining was observed in both myogenin-positive and myogenin-negative cells. Antisense treatment downmodulated syncytin-1 expression and inhibited myoblast cell fusions. Importantly, syncytin-1 antisense significantly decreased the frequency of multinucleated myotubes demonstrating that the treatment inhibited secondary myoblast fusions. Thus, syncytin is involved in human myoblast fusions and is localized in areas of contact between fusing cells. Moreover, syncytin and caveolin-3 might interact at the level of the sarcolemma.

Bioengineered baculoviruses as new class of therapeutics using micro and nanotechnologies: principles, prospects and challenges

Designing a safe and efficient gene delivery system is required for success of gene therapy trials. Although a wide variety of viral, non-viral and polymeric nanoparticle based careers have been widely studied, the current gene delivery vehicles are limited by their suboptimal, non-specific therapeutic efficacy and acute immunological reactions, leading to unwanted side effects. Recently, there has been a growing interest in insect-cell-originated baculoviruses as gene delivery vehicles for diverse biomedical applications. Specifically, the emergence of diverse types of surface functionalized and bioengineered baculoviruses is posed to edge over currently available gene delivery vehicles. This is primarily because baculoviruses are comparatively non-pathogenic and non-toxic as they cannot replicate in mammalian cells and do not invoke any cytopathic effect. Moreover, emerging advanced studies in this direction have demonstrated that hybridizing the baculovirus surface with different kinds of bioactive therapeutic molecules, cell-specific targeting moieties, protective polymeric grafts and nanomaterials can significantly improve the preclinical efficacy of baculoviruses. This review presents a comprehensive overview of the recent advancements in the field of bioengineering and biotherapeutics to engineer baculovirus hybrids for tailored gene therapy, and articulates in detail the potential and challenges of these strategies for clinical realization. In addition, the article illustrates the rapid evolvement of microfluidic devices as a high throughput platform for optimizing baculovirus production and treatment conditions.

IFITM3 restricts influenza A virus entry by blocking the formation of fusion pores following virus-endosome hemifusion

Interferon-induced transmembrane proteins (IFITMs) inhibit infection of diverse enveloped viruses, including the influenza A virus (IAV) which is thought to enter from late endosomes. Recent evidence suggests that IFITMs block virus hemifusion (lipid mixing in the absence of viral content release) by altering the properties of cell membranes. Consistent with this mechanism, excess cholesterol in late endosomes of IFITM-expressing cells has been reported to inhibit IAV entry. Here, we examined IAV restriction by IFITM3 protein using direct virus-cell fusion assay and single virus imaging in live cells. IFITM3 over-expression did not inhibit lipid mixing, but abrogated the release of viral content into the cytoplasm. Although late endosomes of IFITM3-expressing cells accumulated cholesterol, other interventions leading to aberrantly high levels of this lipid did not inhibit virus fusion. These results imply that excess cholesterol in late endosomes is not the mechanism by which IFITM3 inhibits the transition from hemifusion to full fusion. The IFITM3's ability to block fusion pore formation at a post-hemifusion stage shows that this protein stabilizes the cytoplasmic leaflet of endosomal membranes without adversely affecting the lumenal leaflet. We propose that IFITM3 interferes with pore formation either directly, through partitioning into the cytoplasmic leaflet of a hemifusion intermediate, or indirectly, by modulating the lipid/protein composition of this leaflet. Alternatively, IFITM3 may redirect IAV fusion to a non-productive pathway, perhaps by promoting fusion with intralumenal vesicles within multivesicular bodies/late endosomes.

Interferon-induced transmembrane proteins (IFITMs) block infection of many enveloped viruses, including the influenza A virus (IAV) that enters from late endosomes. IFITMs are thought to prevent virus hemifusion (merger of contacting leaflets without formation of a fusion pore) by altering the properties of cell membranes. Here we performed single IAV imaging and found that IFITM3 did not interfere with hemifusion, but prevented complete fusion. Also, contrary to a current view that excess cholesterol in late endosomes of IFITM3-expressing cells inhibits IAV entry, we show that cholesterol-laden endosomes are permissive for virus fusion. The ability of IFITM3 to block the formation of fusion pores implies that this protein stabilizes the cytoplasmic leaflet of endosomal membranes, either directly or indirectly, through altering its physical properties. IFITM3 may also redirect IAV to a non-productive pathway by promoting fusion with intralumenal vesicles of late endosomes instead of their limiting membrane.

Caveolin-1 associated adenovirus entry into human corneal cells

The cellular entry of viruses represents a critical area of study, not only for viral tropism, but also because viral entry dictates the nature of the immune response elicited upon infection. Epidemic keratoconjunctivitis (EKC), caused by viruses within human adenovirus species D (HAdV-D), is a severe, ocular surface infection associated with corneal inflammation. Clathrin-mediated endocytosis has previously been shown to play a critical role in entry of other HAdV species into many host cell types. However, HAdV-D endocytosis into corneal cells has not been extensively studied. Herein, we show an essential role for cholesterol rich, lipid raft microdomains and caveolin-1, in the entry of HAdV-D37 into primary human corneal fibroblasts. Cholesterol depletion using methyl-β-cyclodextrin (MβCD) profoundly reduced viral infection. When replenished with soluble cholesterol, the effect of MβCD was reversed, allowing productive viral infection. HAdV-D37 DNA was identified in caveolin-1 rich endosomal fractions after infection. Src kinase activity was also increased in caveolin-1 rich endosomal fractions after infection, and Src phosphorylation and CXCL1 induction were both decreased in caveolin-1-/- mice corneas compared to wild type mice. siRNA knock down of caveolin-1 in corneal cells reduced chemokine induction upon viral infection, and caveolin-1-/- mouse corneas showed reduced cellular entry of HAdV-D37. As a control, HAdV-C2, a non-corneal pathogen, appeared to utilize the caveolar pathway for entry into A549 cells, but failed to infect corneal cells entirely, indicating virus and cell specific tropism. Immuno-electron microscopy confirmed the presence of caveolin-1 in HAdV-D37-containing vesicles during the earliest stages of viral entry. Collectively, these experiments indicate for the first time that HAdV-D37 uses a lipid raft mediated caveolin-1 associated pathway for entry into corneal cells, and connects the processes of viral entry with downstream proinflammatory cell signaling.

MicroRNA-124 reduces caveolar density by targeting caveolin-1 in porcine kidney epithelial PK15 cells

Caveolin-1 is the principal component of caveolae, and it is implicated in endocytosis, cholesterol homeostasis, signal transduction and tumorigenesis. MicroRNAs play key regulatory roles in many cellular processes. However, the molecular mechanism by which porcine caveolin-1 is regulated by microRNAs remains unclear. In the present study, we found that miR-124 could directly target caveolin-1 in porcine kidney epithelial cells (PK15). A luciferase reporter assay revealed that miR-124 directly bound to Cav1 mRNA. Ectopic expression of miR-124 decreased porcine Cav1 expression at both the mRNA and protein levels. Furthermore, we used transmission electron microscopy to count caveolae in the cytosolic space next to the membrane and we found that the overexpression of miR-124 in PK15 cells reduced the density of the caveolae. Our results suggested that miR-124 reduced caveolar density by targeting caveolin-1 in PK15 cells; therefore, miR-124 could play an important role in the caveolae-mediated endocytosis of pathogens and signal transduction.

Glycoprotein H and α4β1 integrins determine the entry pathway of alphaherpesviruses

Herpesviruses enter cells either by direct fusion at the plasma membrane or from within endosomes, depending on the cell type and receptor(s). We investigated two closely related herpesviruses of horses, equine herpesvirus type 1 (EHV-1) and EHV-4, for which the cellular and viral determinants routing virus entry are unknown. We show that EHV-1 enters equine epithelial cells via direct fusion at the plasma membrane, while EHV-4 does so via an endocytic pathway, which is dependent on dynamin II, cholesterol, caveolin 1, and tyrosine kinase activity. Exchange of glycoprotein H (gH) between EHV-1 and EHV-4 resulted in rerouting of EHV-1 to the endocytic pathway, as did blocking of α4β1 integrins on the cell surface. Furthermore, a point mutation in the SDI integrin-binding motif of EHV-1 gH also directed EHV-1 to the endocytic pathway. Cumulatively, we show that viral gH and cellular α4β1 integrins are important determinants in the choice of alphaherpesvirus cellular entry pathways.

Retrovirus entry by endocytosis and cathepsin proteases

Retroviruses include infectious agents inducing severe diseases in humans and animals. In addition, retroviruses are widely used as tools to transfer genes of interest to target cells. Understanding the entry mechanism of retroviruses contributes to developments of novel therapeutic approaches against retrovirus-induced diseases and efficient exploitation of retroviral vectors. Entry of enveloped viruses into host cell cytoplasm is achieved by fusion between the viral envelope and host cell membranes at either the cell surface or intracellular vesicles. Many animal retroviruses enter host cells through endosomes and require endosome acidification. Ecotropic murine leukemia virus entry requires cathepsin proteases activated by the endosome acidification. CD4-dependent human immunodeficiency virus (HIV) infection is thought to occur via endosomes, but endosome acidification is not necessary for the entry whereas entry of CD4-independent HIVs, which are thought to be prototypes of CD4-dependent viruses, is low pH dependent. There are several controversial results on the retroviral entry pathways. Because endocytosis and endosome acidification are complicatedly controlled by cellular mechanisms, the retrovirus entry pathways may be different in different cell lines.

Cholesterol depletion inactivates XMRV and leads to viral envelope protein release from virions: evidence for role of cholesterol in XMRV infection

Membrane cholesterol plays an important role in replication of HIV-1 and other retroviruses. Here, we report that the gammaretrovirus XMRV requires cholesterol and lipid rafts for infection and replication. We demonstrate that treatment of XMRV with a low concentration (10 mM) of 2-hydroxypropyl-β-cyclodextrin (2OHpβCD) partially depleted virion-associated cholesterol resulting in complete inactivation of the virus. This effect could not be reversed by adding cholesterol back to treated virions. Further analysis revealed that following cholesterol depletion, virus-associated Env protein was significantly reduced while the virions remained intact and retained core proteins. Increasing concentrations of 2OHpβCD (≥20 mM) resulted in loss of the majority of virion-associated cholesterol, causing disruption of membrane integrity and loss of internal Gag proteins and viral RNA. Depletion of cholesterol from XMRV-infected cells significantly reduced virus release, suggesting that cholesterol and intact lipid rafts are required for the budding process of XMRV. These results suggest that unlike glycoproteins of other retroviruses, the association of XMRV glycoprotein with virions is highly dependent on cholesterol and lipid rafts.

Rift Valley fever virus strain MP-12 enters mammalian host cells via caveola-mediated endocytosis

Rift Valley fever virus (RVFV) is a zoonotic pathogen capable of causing serious morbidity and mortality in both humans and livestock. The lack of efficient countermeasure strategies, the potential for dispersion into new regions, and the pathogenesis in humans and livestock make RVFV a serious public health concern. The receptors, cellular factors, and entry pathways used by RVFV and other members of the family Bunyaviridae remain largely uncharacterized. Here we provide evidence that RVFV strain MP-12 uses dynamin-dependent caveola-mediated endocytosis for cell entry. Caveolae are lipid raft domains composed of caveolin (the main structural component), cholesterol, and sphingolipids. Caveola-mediated endocytosis is responsible for the uptake of a wide variety of host ligands, as well as bacteria, bacterial toxins, and a number of viruses. To determine the cellular entry mechanism of RVFV, we used small-molecule inhibitors, RNA interference (RNAi), and dominant negative (DN) protein expression to inhibit the major mammalian cell endocytic pathways. Inhibitors and RNAi specific for macropinocytosis and clathrin-mediated endocytosis had no effect on RVFV infection. In contrast, inhibitors of caveola-mediated endocytosis, and RNAi targeted to caveolin-1 and dynamin, drastically reduced RVFV infection in multiple cell lines. Expression of DN caveolin-1 also reduced RVFV infection significantly, while expression of DN EPS15, a protein required for the assembly of clathrin-coated pits, and DN PAK-1, an obligate mediator of macropinocytosis, had no significant impact on RVFV infection. These results together suggest that the primary mechanism of RVFV MP-12 uptake is dynamin-dependent, caveolin-1-mediated endocytosis.

Infectious spleen and kidney necrosis virus (a fish iridovirus) enters Mandarin fish fry cells via caveola-dependent endocytosis

Infectious spleen and kidney necrosis virus (ISKNV) is the type species of the genus Megalocytivirus from the family Iridoviridae. Megalocytiviruses have been implicated in more than 50 fish species infections and currently threaten the aquaculture industry, causing great economic losses in China, Japan, and Southeast Asia. However, the cellular entry mechanisms of megalocytiviruses remain largely uncharacterized. In this study, the main internalization mechanism of ISKNV was investigated by using mandarin fish fry (MFF-1) cells. The progression of ISKNV infection is slow, and infection is not inhibited when the cells are treated with ammonium chloride (NH(4)Cl), chloroquine, sucrose, and chlorpromazine, which are inhibitors of clathrin-dependent endocytosis. The depletion of cellular cholesterol by methyl-β-cyclodextrin results in the significant inhibition of ISKNV infection; however, the infection is resumed with cholesterol replenishment. Inhibitors of caveolin-1-involved signaling events, including phorbol 12-myristate 13-acetate (PMA), genistein, and wortmannin, impair ISKNV entry into MFF-1 cells. Moreover, ISKNV entry is dependent on dynamin and the microtubule cytoskeleton. Cofraction analysis of ISKNV and caveolin-1 showed that ISKNV colocates with caveolin-1 during virus infection. These results indicate that ISKNV entry into MFF-1 cells proceeds via classical caveola-mediated endocytosis and is dependent on the microtubules that serve as tracks along which motile cavicles may move via a caveola-caveosome-endoplasmic reticulum (ER) pathway. As a fish iridovirus, ISKNV entry into MFF-1 cells is different from the clathrin-mediated endocytosis of frog virus 3 entry into mammalian cells (BHK-21) at 28°C, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection.

Cholesterol dependence of Newcastle Disease Virus entry

Lipid rafts are membrane microdomains enriched in cholesterol, sphingolipids, and glycolipids that have been implicated in many biological processes. Since cholesterol is known to play a key role in the entry of some other viruses, we investigated the role of cholesterol and lipid rafts in the host cell plasma membrane in Newcastle Disease Virus (NDV) entry. We used methyl-β-cyclodextrin (MβCD) to deplete cellular cholesterol and disrupt lipid rafts. Our results show that the removal of cellular cholesterol partially reduces viral binding, fusion and infectivity. MβCD had no effect on the expression of sialic acid containing molecule expression, the NDV receptors in the target cell. All the above-described effects were reversed by restoring cholesterol levels in the target cell membrane. The HN viral attachment protein partially localized to detergent-resistant membrane microdomains (DRMs) at 4°C and then shifted to detergent-soluble fractions at 37°C. These results indicate that cellular cholesterol may be required for optimal cell entry in NDV infection cycle.

Inhibition of HIV-1 endocytosis allows lipid mixing at the plasma membrane, but not complete fusion

Background

We recently provided evidence that HIV-1 enters HeLa-derived TZM-bl and lymphoid CEMss cells by fusing with endosomes, whereas its fusion with the plasma membrane does not proceed beyond the lipid mixing step. The mechanism of restriction of HIV-1 fusion at the cell surface and/or the factors that aid the virus entry from endosomes remain unclear.

Results

We examined HIV-1 fusion with a panel of target cells lines and with primary CD4⁺ T cells. Kinetic measurements of fusion combined with time-resolved imaging of single viruses further reinforced the notion that HIV-1 enters the cells via endocytosis and fusion with endosomes. Furthermore, we attempted to deliberately redirect virus fusion to the plasma membrane, using two experimental strategies. First, the fusion reaction was synchronized by pre-incubating the viruses with cells at reduced temperature to allow CD4 and coreceptors engagement, but not the virus uptake or fusion. Subsequent shift to a physiological temperature triggered accelerated virus uptake followed by entry from endosomes, but did not permit fusion at the cell surface. Second, blocking HIV-1 endocytosis by a small-molecule dynamin inhibitor, dynasore, resulted in transfer of viral lipids to the plasma membrane without any detectable release of the viral content into the cytosol. We also found that a higher concentration of dynasore is required to block the HIV-endosome fusion compared to virus internalization.

Conclusions

Our results further support the notion that HIV-1 enters disparate cell types through fusion with endosomes. The block of HIV-1 fusion with the plasma membrane at a post-lipid mixing stage shows that this membrane is not conducive to fusion pore formation and/or enlargement. The ability of dynasore to interfere with the virus-endosome fusion suggests that dynamin could be involved in two distinct steps of HIV-1 entry - endocytosis and fusion within intracellular compartments.

Lipid raft endocytosis and exosomal transport facilitate extracellular trafficking of annexin A2

Annexin A2 (AnxA2), a Ca(2+)-dependent phospholipid-binding protein, is known to associate with the plasma membrane and the endosomal system. Within the plasma membrane, AnxA2 associates in a Ca(2+) dependent manner with cholesterol-rich lipid raft microdomains. Here, we show that the association of AnxA2 with the lipid rafts is influenced not only by intracellular levels of Ca(2+) but also by N-terminal phosphorylation at tyrosine 23. Binding of AnxA2 to the lipid rafts is followed by the transport along the endocytic pathway to be associated with the intralumenal vesicles of the multivesicular endosomes. AnxA2-containing multivesicular endosomes fuse directly with the plasma membrane resulting in the release of the intralumenal vesicles into the extracellular environment, which facilitates the exogenous transfer of AnxA2 from one cell to another. Treatment with Ca(2+) ionophore triggers the association of AnxA2 with the specialized microdomains in the exosomal membrane that possess raft-like characteristics. Phosphorylation at Tyr-23 is also important for the localization of AnxA2 to the exosomal membranes. These results suggest that AnxA2 is trafficked from the plasma membrane rafts and is selectively incorporated into the lumenal membranes of the endosomes to escape the endosomal degradation pathway. The Ca(2+)-dependent exosomal transport constitutes a novel pathway of extracellular transport of AnxA2.

Cell entry of avian reovirus follows a caveolin-1-mediated and dynamin-2-dependent endocytic pathway that requires activation of p38 mitogen-activated protein kinase (MAPK) and Src signaling pathways as well as microtubules and small GTPase Rab5 protein

Very little is known about the mechanism of cell entry of avian reovirus (ARV). The aim of this study was to explore the mechanism of ARV entry and subsequent infection. Cholesterol mainly affected the early steps of the ARV life cycle, because the presence of cholesterol before and during viral adsorption greatly blocked ARV infectivity. Although we have demonstrated that ARV facilitating p38 MAPK is beneficial for virus replication, its mechanism remains unknown. Here, we show that ARV-induced phosphorylation of caveolin-1 (Tyr(14)), dynamin-2 expression, and Rac1 activation through activation of p38 MAPK and Src in the early stage of the virus life cycle is beneficial for virus entry and productive infection. The strong inhibition by dynasore, a specific inhibitor of dynamin-2, and depletion of endogenous caveolin-1 or dynamin-2 by siRNAs as well as the caveolin-1 colocalization study implicate caveolin-1-mediated and dynamin-2-dependent endocytosis as a significant avenue of ARV entry. By means of pharmacological inhibitors, dominant negative mutants, and siRNA of various cellular proteins and signaling molecules, phosphorylation of caveolin-1, dynamin-2 expression, and Rac1 activation were suppressed, suggesting that by orchestrating p38 MAPK, Src, and Rac1 signaling cascade in the target cells, ARV creates an appropriate intracellular environment facilitating virus entry and productive infection. Furthermore, disruption of microtubules, Rab5, or endosome acidification all inhibited ARV infection, suggesting that microtubules and small GTPase Rab5, which regulate transport to early endosome, are crucial for survival of ARV and that exposure of the virus to acidic pH is required for productive infection.

Involvement of caveolin-1 in the Jak-Stat signaling pathway and infectious spleen and kidney necrosis virus infection in mandarin fish (Siniperca chuatsi)

Caveolae, the major source of caveolin-1 protein, are specialized invaginated microdomains of the plasma membrane that act as organizing centers for signaling molecules in the immune system. In the present study, we report the cloning and characterization of caveolin-1 (mCav-1) from mandarin fish (Siniperca chuatsi) and study on the roles of mCav-1 in the fish Jak–Stat signaling pathway and in virus infection. The cDNA sequence of mCav-1 was 707 bp in size, encoding a protein of 181 amino acids, which was different from the mammalian protein (178 amino acids). The deduced amino acid sequence of mCav-1 shared similar architecture with vertebrate caveolin-1 proteins, but mCav-1 lacked a phosphorylation site (y14). The major subcellular location of mCav-1 was in the caveolae, where the protein appeared to have major functions. Real-time PCR revealed that the expression of the mandarin fish Mx, IRF-1, SOCS1, and SOCS3 genes involved in the poly(I:C)-induced Jak–Stat signaling pathway was impaired by the mCav-1 scaffolding domain peptide (mSDP). In mandarin fish fry (MFF-1) cells, the protein levels of mCav-1 were markedly up-regulated at 12 and 24 h post-infection with ISKNV (infectious spleen and kidney necrosis virus). In addition, ISKNV entry into MFF-1 cells was significantly inhibited by mSDP, and the inhibition was dose-dependent. Thus, ISKNV infection was apparently associated with mCav-1 protein and may utilize the caveolae-related endocytosis pathway. The findings reported here further our understanding of the function of caveolin-1 in the complex signal transduction network in fish immune systems and in the cellular entry mechanism of iridoviruses.

Entry of tiger frog virus (an Iridovirus) into HepG2 cells via a pH-dependent, atypical, caveola-mediated endocytosis pathway

Tiger frog virus (TFV), in the genus Ranavirus of the family Iridoviridae, causes high mortality of cultured tiger frog tadpoles in China. To explore the cellular entry mechanism of TFV, HepG2 cells were treated with drugs that inhibit the main endocytic pathways. We observed that TFV entry was inhibited by NH(4)Cl, chloroquine, and bafilomycin, which can all elevate the pH of acidic organelles. In contrast, TFV entry was not influenced by chlorpromazine or overexpression of a dominant-negative form of Esp15, which inhibit the assembly of clathrin-coated pits. These results suggested that TFV entry was not associated with clathrin-mediated endocytosis, but was related to the pH of acidic organelles. Subsequently, we found that endocytosis of TFV was dependent on membrane cholesterol and was inhibited by the caveolin-1 scaffolding domain peptide. Dynamin and actin were also required for TFV entry. In addition, TFV virions colocalized with the cholera toxin subunit B, indicating that TFV enters as caveola-internalized cargo into the Golgi complex. Taken together, our results demonstrated that TFV entry occurs by caveola-mediated endocytosis with a pH-dependent step. This atypical caveola-mediated endocytosis is different from the clathrin-mediated endocytosis of frog virus 3 (FV3) by BHK cells, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection in lower vertebrates.

Visualization of retrovirus uptake and delivery into acidic endosomes

Diverse enveloped viruses enter cells by endocytosis and fusion with intracellular compartments. Recent evidence suggests that HIV also infects permissive cell lines by fusing with endosomes in a pH-independent manner. This finding highlights the importance of time-resolved monitoring of viral uptake. In the present study, we designed an imaging-based assay to measure endocytosis in real-time through probing the virus' accessibility to external solutions. Exposure of viruses bearing a pH-sensitive GFP (green fluorescent protein) variant on their surface to solutions of different acidity altered the fluorescence of surface-accessible particles, but not internalized viruses. By sequentially applying acidic and alkaline buffers with or without ammonium chloride, we were able to quantify the fractions of internalized and non-internalized virions, as well as the fraction of detached particles, over time. The exact time of single-virus internalization was assessed from the point when a particle ceased to respond to a perfusion with alternating acidic and alkaline buffers. We found that, surprisingly, HIV pseudoparticles entered acidic compartments shortly after internalization. These results suggest that the virus might be sorted to a quickly maturing pool of endocytic vesicles and thus be trafficked to fusion-permissive sites near the cell nucleus.

Interplay between HIV entry and transportin-SR2 dependency

BACKGROUND

Transportin-SR2 (TRN-SR2, TNPO3, transportin 3) was previously identified as an interaction partner of human immunodeficiency virus type 1 (HIV-1) integrase and functions as a nuclear import factor of HIV-1. A possible role of capsid in transportin-SR2-mediated nuclear import was recently suggested by the findings that a chimeric HIV virus, carrying the murine leukemia virus (MLV) capsid and matrix proteins, displayed a transportin-SR2 independent phenotype, and that the HIV-1 N74D capsid mutant proved insensitive to transportin-SR2 knockdown.

RESULTS

Our present analysis of viral specificity reveals that TRN-SR2 is not used to the same extent by all lentiviruses. The DNA flap does not determine the TRN-SR2 requirement of HIV-1. We corroborate the TRN-SR2 independent phenotype of the chimeric HIV virus carrying the MLV capsid and matrix proteins. We reanalyzed the HIV-1 N74D capsid mutant in cells transiently or stably depleted of transportin-SR2 and confirm that the N74D capsid mutant is independent of TRN-SR2 when pseudotyped with the vesicular stomatitis virus glycoprotein (VSV-G). Remarkably, although somewhat less dependent on TRN-SR2 than wild type virus, the N74D capsid mutant carrying the wild type HIV-1 envelope required TRN-SR2 for efficient replication. By pseudotyping with envelopes that mediate pH-independent viral uptake including HIV-1, measles virus and amphotropic MLV envelopes, we demonstrate that HIV-1 N74D capsid mutant viruses retain partial dependency on TRN-SR2. However, this dependency on TRN-SR2 is lost when the HIV N74D capsid mutant is pseudotyped with envelopes mediating pH-dependent endocytosis, such as the VSV-G and Ebola virus envelopes.

CONCLUSION

Here we discover a link between the viral entry of HIV and its interaction with TRN-SR2. Our data confirm the importance of TRN-SR2 in HIV-1 replication and argue for careful interpretation of experiments performed with VSV-G pseudotyped viruses in studies on early steps of HIV replication including the role of capsid therein.

Imaging single retrovirus entry through alternative receptor isoforms and intermediates of virus-endosome fusion

A large group of viruses rely on low pH to activate their fusion proteins that merge the viral envelope with an endosomal membrane, releasing the viral nucleocapsid. A critical barrier to understanding these events has been the lack of approaches to study virus-cell membrane fusion within acidic endosomes, the natural sites of virus nucleocapsid capsid entry into the cytosol. Here we have investigated these events using the highly tractable subgroup A avian sarcoma and leukosis virus envelope glycoprotein (EnvA)-TVA receptor system. Through labeling EnvA pseudotyped viruses with a pH-sensitive fluorescent marker, we imaged their entry into mildly acidic compartments. We found that cells expressing the transmembrane receptor (TVA950) internalized the virus much faster than those expressing the GPI-anchored receptor isoform (TVA800). Surprisingly, TVA800 did not accelerate virus uptake compared to cells lacking the receptor. Subsequent steps of virus entry were visualized by incorporating a small viral content marker that was released into the cytosol as a result of fusion. EnvA-dependent fusion with TVA800-expressing cells occurred shortly after endocytosis and delivery into acidic endosomes, whereas fusion of viruses internalized through TVA950 was delayed. In the latter case, a relatively stable hemifusion-like intermediate preceded the fusion pore opening. The apparent size and stability of nascent fusion pores depended on the TVA isoforms and their expression levels, with TVA950 supporting more robust pores and a higher efficiency of infection compared to TVA800. These results demonstrate that surface receptor density and the intracellular trafficking pathway used are important determinants of efficient EnvA-mediated membrane fusion, and suggest that early fusion intermediates play a critical role in establishing low pH-dependent virus entry from within acidic endosomes.

Cellular entry of ebola virus involves uptake by a macropinocytosis-like mechanism and subsequent trafficking through early and late endosomes

Zaire ebolavirus (ZEBOV), a highly pathogenic zoonotic virus, poses serious public health, ecological and potential bioterrorism threats. Currently no specific therapy or vaccine is available. Virus entry is an attractive target for therapeutic intervention. However, current knowledge of the ZEBOV entry mechanism is limited. While it is known that ZEBOV enters cells through endocytosis, which of the cellular endocytic mechanisms used remains unclear. Previous studies have produced differing outcomes, indicating potential involvement of multiple routes but many of these studies were performed using noninfectious surrogate systems such as pseudotyped retroviral particles, which may not accurately recapitulate the entry characteristics of the morphologically distinct wild type virus. Here we used replication-competent infectious ZEBOV as well as morphologically similar virus-like particles in specific infection and entry assays to demonstrate that in HEK293T and Vero cells internalization of ZEBOV is independent of clathrin, caveolae, and dynamin. Instead the uptake mechanism has features of macropinocytosis. The binding of virus to cells appears to directly stimulate fluid phase uptake as well as localized actin polymerization. Inhibition of key regulators of macropinocytosis including Pak1 and CtBP/BARS as well as treatment with the drug EIPA, which affects macropinosome formation, resulted in significant reduction in ZEBOV entry and infection. It is also shown that following internalization, the virus enters the endolysosomal pathway and is trafficked through early and late endosomes, but the exact site of membrane fusion and nucleocapsid penetration in the cytoplasm remains unclear. This study identifies the route for ZEBOV entry and identifies the key cellular factors required for the uptake of this filamentous virus. The findings greatly expand our understanding of the ZEBOV entry mechanism that can be applied to development of new therapeutics as well as provide potential insight into the trafficking and entry mechanism of other filoviruses.

Filoviruses, including Zaire ebolavirus (ZEBOV), are among the most pathogenic viruses known. Our understanding of how these viruses enter into host cells is very limited. A deeper understanding of this process would enable the design of better targeted antiviral therapies. This study defines in detail, key steps of ZEBOV cellular uptake and trafficking into cells using wild type virus as well as the host factors that are responsible for permitting virus entry into cells. Our data indicated that the primary mechanism of ZEBOV uptake is a macropinocytosis-like process that delivers the virus to early endosomes and subsequently to late endosomes. These findings aid in our understanding of how filoviruses infect cells and suggest that disruption of macropinocytosis may be useful in treatment of infection.

A clathrin independent macropinocytosis-like entry mechanism used by bluetongue virus-1 during infection of BHK cells

Acid dependent infection of Hela and Vero cells by BTV-10 occurs from within early-endosomes following virus uptake by clathrin-mediated endocytosis (Forzan et al., 2007: J Virol 81: 4819–4827). Here we report that BTV-1 infection of BHK cells is also dependent on a low endosomal pH; however, virus entry and infection were not inhibited by dominant-negative mutants of Eps15, AP180 or the ‘aa’ splice variant of dynamin-2, which were shown to inhibit clathrin-mediated endocytosis. In addition, infection was not inhibited by depletion of cellular cholesterol, which suggests that virus entry is not mediated by a lipid-raft dependent process such as caveolae-mediated endocytosis. Although virus entry and infection were not inhibited by the dominant-negative dynamin-2 mutant, entry was inhibited by the general dynamin inhibitor, dynasore, indicating that virus entry is dynamin dependent. During entry, BTV-1 co-localised with LAMP-1 but not with transferrin, suggesting that virus is delivered to late-endosomal compartments without first passing through early-endosomes. BTV-1 entry and infection were inhibited by EIPA and cytochalasin-D, known macropinocytosis inhibitors, and during entry virus co-localised with dextran, a known marker for macropinocytosis/fluid-phase uptake. Our results extend earlier observations with BTV-10, and show that BTV-1 can infect BHK cells via an entry mechanism that is clathrin and cholesterol-independent, but requires dynamin, and shares certain characteristics in common with macropinocytosis.

Role of Abl kinase and the Wave2 signaling complex in HIV-1 entry at a post-hemifusion step

Entry of human immunodeficiency virus type 1 (HIV-1) commences with binding of the envelope glycoprotein (Env) to the receptor CD4, and one of two coreceptors, CXCR4 or CCR5. Env-mediated signaling through coreceptor results in Galphaq-mediated Rac activation and actin cytoskeleton rearrangements necessary for fusion. Guanine nucleotide exchange factors (GEFs) activate Rac and regulate its downstream protein effectors. In this study we show that Env-induced Rac activation is mediated by the Rac GEF Tiam-1, which associates with the adaptor protein IRSp53 to link Rac to the Wave2 complex. Rac and the tyrosine kinase Abl then activate the Wave2 complex and promote Arp2/3-dependent actin polymerization. Env-mediated cell-cell fusion, virus-cell fusion and HIV-1 infection are dependent on Tiam-1, Abl, IRSp53, Wave2, and Arp3 as shown by attenuation of fusion and infection in cells expressing siRNA targeted to these signaling components. HIV-1 Env-dependent cell-cell fusion, virus-cell fusion and infection were also inhibited by Abl kinase inhibitors, imatinib, nilotinib, and dasatinib. Treatment of cells with Abl kinase inhibitors did not affect cell viability or surface expression of CD4 and CCR5. Similar results with inhibitors and siRNAs were obtained when Env-dependent cell-cell fusion, virus-cell fusion or infection was measured, and when cell lines or primary cells were the target. Using membrane curving agents and fluorescence microscopy, we showed that inhibition of Abl kinase activity arrests fusion at the hemifusion (lipid mixing) step, suggesting a role for Abl-mediated actin remodeling in pore formation and expansion. These results suggest a potential utility of Abl kinase inhibitors to treat HIV-1 infected patients.

Human host factors required for influenza virus replication

Influenza A virus is an RNA virus that encodes up to 11 proteins and this small coding capacity demands that the virus use the host cellular machinery for many aspects of its life cycle. Knowledge of these host cell requirements not only informs us of the molecular pathways exploited by the virus but also provides further targets that could be pursued for antiviral drug development. Here we use an integrative systems approach, based on genome-wide RNA interference screening, to identify 295 cellular cofactors required for early-stage influenza virus replication. Within this group, those involved in kinase-regulated signalling, ubiquitination and phosphatase activity are the most highly enriched, and 181 factors assemble into a highly significant host-pathogen interaction network. Moreover, 219 of the 295 factors were confirmed to be required for efficient wild-type influenza virus growth, and further analysis of a subset of genes showed 23 factors necessary for viral entry, including members of the vacuolar ATPase (vATPase) and COPI-protein families, fibroblast growth factor receptor (FGFR) proteins, and glycogen synthase kinase 3 (GSK3)-beta. Furthermore, 10 proteins were confirmed to be involved in post-entry steps of influenza virus replication. These include nuclear import components, proteases, and the calcium/calmodulin-dependent protein kinase (CaM kinase) IIbeta (CAMK2B). Notably, growth of swine-origin H1N1 influenza virus is also dependent on the identified host factors, and we show that small molecule inhibitors of several factors, including vATPase and CAMK2B, antagonize influenza virus replication.

HIV enters cells via endocytosis and dynamin-dependent fusion with endosomes

Enveloped viruses that rely on a low pH-dependent step for entry initiate infection by fusing with acidic endosomes, whereas the entry sites for pH-independent viruses, such as HIV-1, have not been defined. These viruses have long been assumed to fuse directly with the plasma membrane. Here we used population-based measurements of the viral content delivery into the cytosol and time-resolved imaging of single viruses to demonstrate that complete HIV-1 fusion occurred in endosomes. In contrast, viral fusion with the plasma membrane did not progress beyond the lipid mixing step. HIV-1 underwent receptor-mediated internalization long before endosomal fusion, thus minimizing the surface exposure of conserved viral epitopes during fusion and reducing the efficacy of inhibitors targeting these epitopes. We also show that, strikingly, endosomal fusion is sensitive to a dynamin inhibitor, dynasore. These findings imply that HIV-1 infects cells via endocytosis and envelope glycoprotein- and dynamin-dependent fusion with intracellular compartments.