CAR1, a TNFR-related protein, is a cellular receptor for cytopathic avian leukosis-sarcoma viruses and mediates apoptosis

Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

After the onset of the AIDS pandemic, HIV-1 (genus Lentivirus) became the predominant model for studying retrovirus Env glycoproteins and their role in entry. However, HIV Env is an inadequate model for understanding entry of viruses in the Alpharetrovirus, Gammaretrovirus and Deltaretrovirus genera. For example, oncogenic model system viruses such as Rous sarcoma virus (RSV, Alpharetrovirus), murine leukemia virus (MLV, Gammaretrovirus) and human T-cell leukemia viruses (HTLV-I and HTLV-II, Deltaretrovirus) encode Envs that are structurally and functionally distinct from HIV Env. We refer to these as Gamma-type Envs. Gamma-type Envs are probably the most widespread retroviral Envs in nature. They are found in exogenous and endogenous retroviruses representing a broad spectrum of vertebrate hosts including amphibians, birds, reptiles, mammals and fish. In endogenous form, gamma-type Envs have been evolutionarily coopted numerous times, most notably as placental syncytins (e.g., human SYNC1 and SYNC2). Remarkably, gamma-type Envs are also found outside of the Retroviridae. Gp2 proteins of filoviruses (e.g., Ebolavirus) and snake arenaviruses in the genus Reptarenavirus are gamma-type Env homologs, products of ancient recombination events involving viruses of different Baltimore classes. Distinctive hallmarks of gamma-type Envs include a labile disulfide bond linking the surface and transmembrane subunits, a multi-stage attachment and fusion mechanism, a highly conserved (but poorly understood) "immunosuppressive domain", and activation by the viral protease during virion maturation. Here, we synthesize work from diverse retrovirus model systems to illustrate these distinctive properties and to highlight avenues for further exploration of gamma-type Env structure and function.

Advances on genetic and genomic studies of ALV resistance

Avian leukosis (AL) is a general term for a variety of neoplastic diseases in avian caused by avian leukosis virus (ALV). No vaccine or drug is currently available for the disease. Therefore, the disease can result in severe economic losses in poultry flocks. Increasing the resistance of poultry to ALV may be one effective strategy. In this review, we provide an overview of the roles of genes associated with ALV infection in the poultry genome, including endogenous retroviruses, virus receptors, interferon-stimulated genes, and other immune-related genes. Furthermore, some methods and techniques that can improve ALV resistance in poultry are discussed. The objectives are willing to provide some valuable references for disease resistance breeding in poultry.

Residues L55 and W69 of Tva Mediate Entry of Subgroup A Avian Leukosis Virus

The receptor of the subgroup A avian leukosis virus (ALV-A) in chicken is Tva, which is the homologous protein of human CD320 (huCD320), contains a low-density lipoprotein (LDL-A) module and is involved in the uptake of transcobalamin bound vitamin B₁₂/cobalamin (Cbl). To map the functional determinants of Tva responsible for ALV-A receptor activity, a series of chimeric receptors were created by swapping the LDL-A module fragments between huCD320 and Tva. These chimeric receptors were then used for virus entry and binding assays to map the minimal ALV-A functional domain of Tva. The results showed that Tva residues 49 to 71 constituted the minimal functional domain that directly interacted with the ALV-A gp85 protein to mediate ALV-A entry. Single-residue substitution analysis revealed that L55 and W69, which were spatially adjacent on the surface of the Tva structure, were key residues that mediate ALV-A entry. Structural alignment results indicated that L55 and W69 substitutions did not affect the Tva protein structure but abolished the interaction force between Tva and gp85. Furthermore, substituting the corresponding residues of huCD320 with L55 and W69 of Tva converted huCD320 into a functional receptor of ALV-A. Importantly, soluble huCD320 harboring Tva L55 and W69 blocked ALV-A entry. Finally, we constructed a Tva gene-edited cell line with L55R and W69L substitutions that could fully resist ALV-A entry, while Cbl uptake was not affected. Collectively, our findings suggested that amino acids L55 and W69 of Tva were key for mediating virus entry. IMPORTANCE Retroviruses bind to cellular receptors through their envelope proteins, which is a crucial step in infection. While most retroviruses require two receptors for entry, ALV-A requires only one. Various Tva alleles conferring resistance to ALV-A, including Tva^r1 (C40W substitution), Tva^r2 (frame-shifting four-nucleotide insertion), Tva^r3, Tva^r4, Tva^r5, and Tva^r6 (deletion in the first intron), are known. However, the detailed entry mechanism of ALV-A in chickens remains to be explored. We demonstrated that Tva residues L55 and W69 were key for ALV-A entry and were important for correct interaction with ALV-A gp85. Soluble Tva and huCD320 harboring the Tva residues L55 and W69 effectively blocked ALV-A infection. Additionally, we constructed gene-edited cell lines targeting these two amino acids, which completely restricted ALV-A entry without affecting Cbl uptake. These findings contribute to a better understanding of the infection mechanism of ALV-A and provided novel insights into the prevention and control of ALV-A.

Nanomaterials for virus sensing and tracking

The effect of the on-going COVID-19 pandemic on global healthcare systems has underlined the importance of timely and cost-effective point-of-care diagnosis of viruses. The need for ultrasensitive easy-to-use platforms has culminated in an increased interest for rapid response equipment-free alternatives to conventional diagnostic methods such as polymerase chain reaction, western-blot assay, etc. Furthermore, the poor stability and the bleaching behavior of several contemporary fluorescent reporters is a major obstacle in understanding the mechanism of viral infection thus retarding drug screening and development. Owing to their extraordinary surface-to-volume ratio as well as their quantum confinement and charge transfer properties, nanomaterials are desirable additives to sensing and imaging systems to amplify their signal response as well as temporal resolution. Their large surface area promotes biomolecular integration as well as efficacious signal transduction. Due to their hole mobility, photostability, resistance to photobleaching, and intense brightness, nanomaterials have a considerable edge over organic dyes for single virus tracking. This paper reviews the state-of-the-art of combining carbon-allotrope, inorganic and organic-based nanomaterials with virus sensing and tracking methods, starting with the impact of human pathogenic viruses on the society. We address how different nanomaterials can be used in various virus sensing platforms (e.g. lab-on-a-chip, paper, and smartphone-based point-of-care systems) as well as in virus tracking applications. We discuss the enormous potential for the use of nanomaterials as simple, versatile, and affordable tools for detecting and tracing viruses infectious to humans, animals, plants as well as bacteria. We present latest examples in this direction by emphasizing major advantages and limitations.

PANoptosis in Viral Infection: The Missing Puzzle Piece in the Cell Death Field

In the past decade, emerging viral outbreaks like SARS-CoV-2, Zika and Ebola have presented major challenges to the global health system. Viruses are unique pathogens in that they fully rely on the host cell to complete their lifecycle and potentiate disease. Therefore, programmed cell death (PCD), a key component of the host innate immune response, is an effective strategy for the host cell to curb viral spread. The most well-established PCD pathways, pyroptosis, apoptosis and necroptosis, can be activated in response to viruses. Recently, extensive crosstalk between PCD pathways has been identified, and there is evidence that molecules from all three PCD pathways can be activated during virus infection. These findings have led to the emergence of the concept of PANoptosis, defined as an inflammatory PCD pathway regulated by the PANoptosome complex with key features of pyroptosis, apoptosis, and/or necroptosis that cannot be accounted for by any of these three PCD pathways alone. While PCD is important to eliminate infected cells, many viruses are equipped to hijack host PCD pathways to benefit their own propagation and subvert host defense, and PCD can also lead to the production of inflammatory cytokines and inflammation. Therefore, PANoptosis induced by viral infection contributes to either host defense or viral pathogenesis in context-specific ways. In this review, we will discuss the multi-faceted roles of PCD pathways in controlling viral infections.

Knock-Out of Retrovirus Receptor Gene Tva in the Chicken Confers Resistance to Avian Leukosis Virus Subgroups A and K and Affects Cobalamin (Vitamin B12)-Dependent Level of Methylmalonic Acid

The chicken Tva cell surface protein, a member of the low-density lipoprotein receptor family, has been identified as an entry receptor for avian leukosis virus of classic subgroup A and newly emerging subgroup K. Because both viruses represent an important concern for the poultry industry, we introduced a frame-shifting deletion into the chicken tva locus with the aim of knocking-out Tva expression and creating a virus-resistant chicken line. The tva knock-out was prepared by CRISPR/Cas9 gene editing in chicken primordial germ cells and orthotopic transplantation of edited cells into the testes of sterilized recipient roosters. The resulting tva −/− chickens tested fully resistant to avian leukosis virus subgroups A and K, both in in vitro and in vivo assays, in contrast to their susceptible tva +/+ and tva +/− siblings. We also found a specific disorder of the cobalamin/vitamin B₁₂ metabolism in the tva knock-out chickens, which is in accordance with the recently recognized physiological function of Tva as a receptor for cobalamin in complex with transcobalamin transporter. Last but not least, we bring a new example of the de novo resistance created by CRISPR/Cas9 editing of pathogen dependence genes in farm animals and, furthermore, a new example of gene editing in chicken.

Heterologous avian system for quantitative analysis of Syncytin-1 interaction with ASCT2 receptor

Background

Human Syncytin-1 is a placentally-expressed cell surface glycoprotein of retroviral origin. After interaction with ASCT2, its cellular receptor, Syncytin-1 triggers cell–cell fusion and formation of a multinuclear syncytiotrophoblast layer of the placenta. The ASCT2 receptor is a multi-spanning membrane protein containing a protruding extracellular part called region C, which has been suggested to be a retrovirus docking site. Precise identification of the interaction site between ASCT2 and Syncytin-1 is challenging due to the complex structure of ASCT2 protein and the background of endogenous ASCT2 gene in the mammalian genome. Chicken cells lack the endogenous background and, therefore, can be used to set up a system with surrogate expression of the ASCT2 receptor.

Results

We have established a retroviral heterologous chicken system for rapid and reliable assessment of ectopic human ASCT2 protein expression. Our dual-fluorescence system proved successful for large-scale screening of mutant ASCT2 proteins. Using this system, we demonstrated that progressive deletion of region C substantially decreased the amount of ASCT2 protein. In addition, we implemented quantitative assays to determine the interaction of ASCT2 with Syncytin-1 at multiple levels, which included binding of the soluble form of Syncytin-1 to ASCT2 on the cell surface and a luciferase-based assay to evaluate cell–cell fusions that were triggered by Syncytin-1. Finally, we restored the envelope function of Syncytin-1 in a replication-competent retrovirus and assessed the infection of chicken cells expressing human ASCT2 by chimeric Syncytin-1-enveloped virus. The results of the quantitative assays showed that deletion of the protruding region C did not abolish the interaction of ASCT2 with Syncytin-1.

Conclusions

We present here a heterologous chicken system for effective assessment of the expression of transmembrane ASCT2 protein and its interaction with Syncytin-1. The system profits from the absence of endogenous ASCT2 background and implements the quantitative assays to determine the ASCT2-Syncytin-1 interaction at several levels. Using this system, we demonstrated that the protruding region C was essential for ASCT2 protein expression, but surprisingly, not for the interaction with Syncytin-1 glycoprotein.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12977-021-00558-0.

The avian retroviral receptor Tva mediates the uptake of transcobalamin bound vitamin B12 (cobalamin)

The Avian sarcoma and leukosis viruses (ASLVs) are important chicken pathogens. Some of the virus subgroups, including ASLV-A and K, utilize the Tva receptor for cell entrance. Though Tva was identified three decades ago, its physiological function remains unknown. Previously, we have noted an intriguing resemblance and orthology between the chicken gene coding for Tva and the human gene coding for CD320, a receptor involved in cellular uptake of transcobalamin (TC) in complex with vitamin B12/cobalamin (Cbl).Here we show that both the transmembrane and the glycosylphosphatidylinositol (GPI)-anchored form of Tva in the chicken cell line DF-1 promotes the uptake of Cbl with help of expressed and purified chicken TC. The uptake of TC-Cbl complex was monitored using an isotope- or fluorophore-labeled Cbl. We show that (i) TC-Cbl is internalized in chicken cells; and (ii) the uptake is lower in the Tva-knockout cells and higher in Tva-overexpressing cells when compared with wild type chicken cells. The relation between physiological function of Tva and its role in infection was elaborated by showing that infection with ASLV subgroups (targeting Tva) impairs the uptake of TC-Cbl, while this is not the case for cells infected with ASLV-B (not recognized by Tva). In addition, exposure of the cells to a high concentration of TC-Cbl alleviates the infection with Tva-dependent ASLV.IMPORTANCE: We demonstrate that the ASLV receptor Tva participates in the physiological uptake of TC-Cbl, because the viral infection suppresses the uptake of Cbl and vice versa. Our results pave the road for future studies addressing the issues: (i) whether a virus infection can be inhibited by TC-Cbl complexes in vivo; and (ii) whether any human virus employs the human TC-Cbl receptor CD320. In broader terms, our study sheds light on the intricate interplay between physiological roles of cellular receptors and their involvement in virus infection.

Multiplex Neural Circuit Tracing With G-Deleted Rabies Viral Vectors

Neural circuits interconnect to organize large-scale networks that generate perception, cognition, memory, and behavior. Information in the nervous system is processed both through parallel, independent circuits and through intermixing circuits. Analyzing the interaction between circuits is particularly indispensable for elucidating how the brain functions. Monosynaptic circuit tracing with glycoprotein (G) gene-deleted rabies viral vectors (RVΔG) comprises a powerful approach for studying the structure and function of neural circuits. Pseudotyping of RVΔG with the foreign envelope EnvA permits expression of transgenes such as fluorescent proteins, genetically-encoded sensors, or optogenetic tools in cells expressing TVA, a cognate receptor for EnvA. Trans-complementation with rabies virus glycoproteins (RV-G) enables trans-synaptic labeling of input neurons directly connected to the starter neurons expressing both TVA and RV-G. However, it remains challenging to simultaneously map neuronal connections from multiple cell populations and their interactions between intermixing circuits solely with the EnvA/TVA-mediated RV tracing system in a single animal. To overcome this limitation, here, we multiplexed RVΔG circuit tracing by optimizing distinct viral envelopes (oEnvX) and their corresponding receptors (oTVX). Based on the EnvB/TVB and EnvE/DR46-TVB systems derived from the avian sarcoma leukosis virus (ASLV), we developed optimized TVB receptors with lower or higher affinity (oTVB-L or oTVB-H) and the chimeric envelope oEnvB, as well as an optimized TVE receptor with higher affinity (oTVE-H) and its chimeric envelope oEnvE. We demonstrated independence of RVΔG infection between the oEnvA/oTVA, oEnvB/oTVB, and oEnvE/oTVE systems and in vivo proof-of-concept for multiplex circuit tracing from two distinct classes of layer 5 neurons targeting either other cortical or subcortical areas. We also successfully labeled common input of the lateral geniculate nucleus to both cortico-cortical layer 5 neurons and inhibitory neurons of the mouse V1 with multiplex RVΔG tracing. These oEnvA/oTVA, oEnvB/oTVB, and oEnvE/oTVE systems allow for differential labeling of distinct circuits to uncover the mechanisms underlying parallel processing through independent circuits and integrated processing through interaction between circuits in the brain.

The Novel Avian Leukosis Virus Subgroup K Shares Its Cellular Receptor with Subgroup A

Avian leukosis virus subgroup K (ALV-K) is composed of newly emerging isolates, which, in sequence analyses, cluster separately from the well-characterized subgroups A, B, C, D, E, and J. However, it remains unclear whether ALV-K represents an independent ALV subgroup with regard to receptor usage, host range, and superinfection interference. In the present study, we examined the host range of the Chinese infectious isolate JS11C1, an ALV-K prototype, and we found substantial overlap of species that were either resistant or susceptible to ALV-A and JS11C1. Ectopic expression of the chicken tva gene in mammalian cells conferred susceptibility to JS11C1, while genetic ablation of the tva gene rendered chicken DF-1 cells resistant to infection by JS11C1. Thus, tva expression is both sufficient and necessary for JS11C1 entry. Receptor sharing was also manifested in superinfection interference, with preinfection of cells with ALV-A, but not ALV-B or ALV-J, blocking subsequent JS11C1 infection. Finally, direct binding of JS11C1 and Tva was demonstrated by preincubation of the virus with soluble Tva, which substantially decreased viral infectivity in susceptible chicken cells. Collectively, these findings indicate that JS11C1 represents a new and bona fide ALV subgroup that utilizes Tva for cell entry and binds to a site other than that for ALV-A.IMPORTANCE ALV consists of several subgroups that are particularly characterized by their receptor usage, which subsequently dictates the host range and tropism of the virus. A few newly emerging and highly pathogenic Chinese ALV strains have recently been suggested to be an independent subgroup, ALV-K, based solely on their genomic sequences. Here, we performed a series of experiments with the ALV-K strain JS11C1, which showed its dependence on the Tva cell surface receptor. Due to the sharing of this receptor with ALV-A, both subgroups were able to interfere with superinfection. Because ALV-K could become an important pathogen and a significant threat to the poultry industry in Asia, the identification of a specific receptor could help in the breeding of resistant chicken lines with receptor variants with decreased susceptibility to the virus.

TRIM67 promotes NF‑κB pathway and cell apoptosis in GA‑13315‑treated lung cancer cells

13‑Chlorine‑3,15‑dioxy‑gibberellic acid methyl ester (GA‑13315), a gibberellin derivative, possesses strong anti‑tumor activity in vitro and in vivo. The present study aimed to investigate the underlying mechanisms of GA‑13315‑induced apoptosis in human non‑small cell lung cancer cell lines. Lung cancer cells were treated with different doses of GA‑13315 (4, 8, 16 and 32 ng/µl) for 48 h, and a CCK8 assay was performed to measure cell viability. Alteration in gene expression was identified using RNA‑sequencing (RNA‑Seq). Quantitative polymerase chain reaction (qPCR) was used to confirm the differentially expressed genes (DEGs) identified in RNA‑Seq. Gene expression plasmids or small interfering RNA were used to overexpress or silence targeted genes, in order to investigate downstream signals. Chromatin immunoprecipitation was conducted to evaluate the binding of transcription factors to the target genes. A Student's t‑test or one‑way analysis of variance followed by Tukey's honestly significant difference post‑hoc test were performed to evaluate the significance between groups. P<0.05 was considered to indicate a statistically significant difference. GA‑13315 significantly decreased the number of viable cells and induced apoptosis among lung cancer cells (median lethal dose =12‑16 ng/µl). RNA‑Seq identified 250 significant DEGs, including 94 upregulated and 156 downregulated genes in A549 cells (P<0.05; fold change ≥1.5). Upregulation of TRIM67, NF‑κB subunit 2 (NF‑κB2) and FAS was additionally confirmed using qPCR and western blot analysis in A549 and H460 cells. Apoptosis of A549 cells was significantly decreased following knockdown of TRIM67. GA‑13315 promoted TRIM67 expression to increase FAS expression and cell apoptosis. TRIM67 promoted the processing of NF‑κB2 into its active form, p52, which then enhanced the NF‑κB pathway and GA‑13315‑induced apoptosis.

Avian Sarcoma and Leukosis Virus Envelope Glycoproteins Evolve to Broaden Receptor Usage Under Pressure from Entry Competitors †

The subgroup A through E avian sarcoma and leukosis viruses (ASLV(A) through ASLV(E)) are a group of highly related alpharetroviruses that have evolved their envelope glycoproteins to use different receptors to enable efficient virus entry due to host resistance and/or to expand host range. Previously, we demonstrated that ASLV(A) in the presence of a competitor to the subgroup A Tva receptor, SUA-rIgG immunoadhesin, evolved to use other receptor options. The selected mutant virus, RCASBP(A)Δ155–160, modestly expanded its use of the Tvb and Tvc receptors and possibly other cell surface proteins while maintaining the binding affinity to Tva. In this study, we further evolved the Δ155–160 virus with the genetic selection pressure of a soluble form of the Tva receptor that should force the loss of Tva binding affinity in the presence of the Δ155–160 mutation. Viable ASLVs were selected that acquired additional mutations in the Δ155–160 Env hypervariable regions that significantly broadened receptor usage to include Tvb and Tvc as well as retaining the use of Tva as a receptor determined by receptor interference assays. A similar deletion in the hr1 hypervariable region of the subgroup C ASLV glycoproteins evolved to broaden receptor usage when selected on Tvc-negative cells.

Mutations in Both the Surface and Transmembrane Envelope Glycoproteins of the RAV-2 Subgroup B Avian Sarcoma and Leukosis Virus Are Required to Escape the Antiviral Effect of a Secreted Form of the TvbS3 Receptor †

The subgroup A through E avian sarcoma and leukosis viruses ASLV(A) through ASLV(E) are a group of highly related alpharetroviruses that have evolved to use very different host protein families as receptors. We have exploited genetic selection strategies to force the replication-competent ASLVs to naturally evolve and acquire mutations to escape the pressure on virus entry and yield a functional replicating virus. In this study, evolutionary pressure was exerted on ASLV(B) virus entry and replication using a secreted for of its Tvb receptor. As expected, mutations in the ASLV(B) surface glycoprotein hypervariable regions were selected that knocked out the ability for the mutant glycoprotein to bind the sTvb^S3-IgG inhibitor. However, the subgroup B Rous associated virus 2 (RAV-2) also required additional mutations in the C-terminal end of the SU glycoprotein and multiple regions of TM highlighting the importance of the entire viral envelope glycoprotein trimer structure to mediate the entry process efficiently. These mutations altered the normal two-step ASLV membrane fusion process to enable infection.

Reverse Engineering Provides Insights on the Evolution of Subgroups A to E Avian Sarcoma and Leukosis Virus Receptor Specificity

The initial step of retrovirus entry—the interaction between the virus envelope glycoprotein trimer and a cellular receptor—is complex, involving multiple, noncontiguous determinants in both proteins that specify receptor choice, binding affinity and the ability to trigger conformational changes in the viral glycoproteins. Despite the complexity of this interaction, retroviruses have the ability to evolve the structure of their envelope glycoproteins to use a different cellular protein as receptors. The highly homologous subgroup A to E Avian Sarcoma and Leukosis Virus (ASLV) glycoproteins belong to the group of class 1 viral fusion proteins with a two-step triggering mechanism that allows experimental access to intermediate structures during the fusion process. We and others have taken advantage of replication-competent ASLVs and exploited genetic selection strategies to force the ASLVs to naturally evolve and acquire envelope glycoprotein mutations to escape the pressure on virus entry and still yield a functional replicating virus. This approach allows for the simultaneous selection of multiple mutations in multiple functional domains of the envelope glycoprotein that may be required to yield a functional virus. Here, we review the ASLV family and experimental system and the reverse engineering approaches used to understand the evolution of ASLV receptor usage.

Sequential disruption of ALV host receptor genes reveals no sharing of receptors between ALV subgroups A, B, and J

Background

Previously, we showed that targeted disruption of viral receptor genes in avian leukosis virus (ALV) subgroups using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9))-based genome editing confers resistance to ALV subgroups B and J. Here, we used the same strategy to target the receptor expressed by ALV subgroup A (TVA) and generate chicken cells resistant to infection by this virus.

Results

CRISPR/Cas9-based disruption of exon 2 within the tva gene of DF-1 fibroblasts conferred resistance to infection by ALV subgroup A regardless of whether frameshift mutations were introduced during editing. Conversely, overexpression of the wild-type TVA receptor (wtTVA) by tva-modified DF-1 clones restored susceptibility to ALV subgroup A. The results confirm that exon 2, which contains the low-density lipoprotein receptor class A domain of TVA, is critical for virus entry. Furthermore, we sequentially modified DF-1 cells by editing the tva, tvb, and Na⁺/H⁺ exchange 1 (chNHE1) genes, which are the specific receptors for ALV subgroups A, B, and J, respectively.

Conclusions

Simultaneous editing of multiple receptors to block infection by different subgroups of ALV confirmed that ALV subgroups A, B, and J do not share host receptors. This strategy could be used to generate cells resistant to multiple viral pathogens that use distinct receptors for cell entry.

Genetic Resistance to Avian Leukosis Viruses Induced by CRISPR/Cas9 Editing of Specific Receptor Genes in Chicken Cells

Avian leukosis viruses (ALVs), which are pathogens of concern in domestic poultry, utilize specific receptor proteins for cell entry that are both necessary and sufficient for host susceptibility to a given ALV subgroup. This unequivocal relationship offers receptors as suitable targets of selection and biotechnological manipulation with the aim of obtaining virus-resistant poultry. This approach is further supported by the existence of natural knock-outs of receptor genes that segregate in inbred lines of chickens. We used CRISPR/Cas9 genome editing tools to introduce frame-shifting indel mutations into tva, tvc, and tvj loci encoding receptors for the A, C, and J ALV subgroups, respectively. For all three loci, the homozygous frame-shifting indels generating premature stop codons induced phenotypes which were fully resistant to the virus of respective subgroup. In the tvj locus, we also obtained in-frame deletions corroborating the importance of W38 and the four amino-acids preceding it. We demonstrate that CRISPR/Cas9-mediated knock-out or the fine editing of ALV receptor genes might be the first step in the development of virus-resistant chickens.

Naturally Occurring Frameshift Mutations in the tvb Receptor Gene Are Responsible for Decreased Susceptibility of Chicken to Infection with Avian Leukosis Virus Subgroups B, D, and E

The group of highly related avian leukosis viruses (ALVs) in chickens are thought to have evolved from a common retroviral ancestor into six subgroups, A to E and J. These ALV subgroups use diverse cellular proteins encoded by four genetic loci in chickens as receptors to gain entry into host cells. Hosts exposed to ALVs might be under selective pressure to develop resistance to ALV infection. Indeed, resistance alleles have previously been identified in all four receptor loci in chickens. The tvb gene encodes a receptor, which determines the susceptibility of host cells to ALV subgroup B (ALV-B), ALV-D, and ALV-E. Here we describe the identification of two novel alleles of the tvb receptor gene, which possess independent insertions each within exon 4. The insertions resulted in frameshift mutations that reveal a premature stop codon that causes nonsense-mediated decay of the mutant mRNA and the production of truncated Tvb protein. As a result, we observed that the frameshift mutations in the tvb gene significantly lower the binding affinity of the truncated Tvb receptors for the ALV-B, ALV-D, and ALV-E envelope glycoproteins and significantly reduce susceptibility to infection by ALV-B, ALV-D and ALV-E in vitro and in vivo Taken together, these findings suggest that frameshift mutation can be a molecular mechanism of reducing susceptibility to ALV and enhance our understanding of virus-host coevolution.IMPORTANCE Avian leukosis virus (ALV) once caused devastating economic loss to the U.S. poultry industry prior the current eradication schemes in place, and it continues to cause severe calamity to the poultry industry in China and Southeast Asia, where deployment of a complete eradication scheme remains a challenge. The tvb gene encodes the cellular receptor necessary for subgroup B, D, and E ALV infection. Two tvb allelic variants that resulted from frameshift mutations have been identified in this study, which have been shown to have significantly reduced functionality in mediating subgroup B, D, and E ALV infection. Unlike the control of herpesvirus-induced diseases by vaccination, the control of avian leukosis in chickens has relied totally on virus eradication measures and host genetic resistance. This finding enriches the allelic pool of the tvb gene and expands the potential for genetic improvement of ALV resistance in varied chicken populations by selection.

Precise gene editing of chicken Na+/H+ exchange type 1 (chNHE1) confers resistance to avian leukosis virus subgroup J (ALV-J)

Avian leukosis virus subgroup J (ALV-J), first isolated in the late 1980s, has caused economic losses to the poultry industry in many countries. As all chicken lines studied to date are susceptible to ALV infection, there is enormous interest in developing resistant chicken lines. The ALV-J receptor, chicken Na⁺/H⁺ exchange 1 (chNHE1) and the critical amino acid sequences involved in viral attachment and entry have already been characterized. However, there are no reported attempts to induce resistance to the virus by targeted genome modification of the receptor sequences. In an attempt to induce resistance to ALV-J infection, we used clustered regularly interspaced short palindromic repeats (CRISPR)-associated (CRISPR/Cas9)-based genome editing approaches to modify critical residues of the chNHE1 receptor in chicken cells. The susceptibility of the modified cell lines to ALV-J infection was examined using enhanced green fluorescent protein (EGFP)-expressing marker viruses. We showed that modifying the chNHE1 receptor by artificially generating a premature stop codon induced absolute resistance to viral infection, with mutations of the tryptophan residue at position 38 (Trp38) being very critical. Single-stranded oligodeoxynucleotide (ssODN)-mediated targeted recombination of the Trp38 region revealed that deletions involving the Trp38 residue were most effective in conferring resistance to ALV-J. Moreover, protein structure analysis of the chNHE1 receptor sequence suggested that its intrinsically disordered region undergoes local conformational changes through genetic alteration. Collectively, these results demonstrate that targeted mutations on chNHE1 alter the susceptibility to ALV-J and the technique is expected to contribute to develop disease-resistant chicken lines.

A premature stop codon within the tvb receptor gene results in decreased susceptibility to infection by avian leukosis virus subgroups B, D, and E

Avian leukosis virus (ALV) is an oncogenic virus causing a variety of neoplasms in chickens. The group of avian leukosis virus in chickens contains six closely related subgroups, A to E and J. The prevalence of ALVs in hosts may have imposed strong selective pressure toward resistance to ALVs infection. The tvb gene encodes Tvb receptor and determines susceptibility or resistance to the subgroups B, D, and E ALV. In this study, we characterized a novel resistant allele of the tvb receptor gene, tvbr3, which carries a single-nucleotide substitution (c.298C>T) that constitutes a premature termination codon within the fourth exon and leads to the production of a truncated TvbR3 receptor protein. As a result, we observed decreased susceptibility to infection by ALV-B, ALV-D and ALV-E both in vitro and in vivo, and decreased the binding affinity of the TvbR3 receptor for the subgroups B, D, and E ALV envelope glycoproteins. Additionally, we found that the tvbr3 allele was prevalent in Chinese broiler lines. This study demonstrated that premature termination codon in the tvb receptor gene can confer genetic resistance to subgroups B, D, and E ALV in the host, and indicates that tvbr3 could potentially serve as a resistant target against ALV-B, ALV-D and ALV-E infection.

Genome Modification Technologies and Their Applications in Avian Species

The rapid development of genome modification technology has provided many great benefits in diverse areas of research and industry. Genome modification technologies have also been actively used in a variety of research areas and fields of industry in avian species. Transgenic technologies such as lentiviral systems and piggyBac transposition have been used to produce transgenic birds for diverse purposes. In recent years, newly developed programmable genome editing tools such as transcription activator-like effector nuclease (TALEN) and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9) have also been successfully adopted in avian systems with primordial germ cell (PGC)-mediated genome modification. These genome modification technologies are expected to be applied to practical uses beyond system development itself. The technologies could be used to enhance economic traits in poultry such as acquiring a disease resistance or producing functional proteins in eggs. Furthermore, novel avian models of human diseases or embryonic development could also be established for research purposes. In this review, we discuss diverse genome modification technologies used in avian species, and future applications of avian biotechnology.

Sequencing the Biology of Entry: The Retroviral env Gene

The surface envelope protein of any virus is major determinant of the host cell that is infected and as a result a major determinant of viral pathogenesis. Retroviruses have a single surface protein named Env. It is a trimer of heterodimers and is responsible for binding to the host cell receptor and mediating fusion between the viral and host membranes. In this review we will discuss the history of the discovery of the avian leukosis virus (ALV) and human immunodeficiency virus type 1 (HIV-1) Env proteins and their receptor specificity, comparing the many differences but having some similarities. Much of the progress in these fields has relied on viral genetics and genetic polymorphisms in the host population. A special feature of HIV-1 is that its persistent infection in its human host, to the point of depleting its favorite target cells, allows the virus to evolve new entry phenotypes to expand its host range into several new cell types. This variety of entry phenotypes has led to confusion in the field leading to the major form of entry phenotype of HIV-1 being overlooked until recently. Thus an important part of this story is the description and naming of the most abundant entry form of the virus: R5 T cell-tropic HIV-1.

Retroviral host range extension is coupled with Env-activating mutations resulting in receptor-independent entry

The extent of virus transmission among individuals and species is generally determined by the presence of specific membrane-embedded virus receptors required for virus entry. Interaction of the viral envelope glycoprotein (Env) with a specific cellular receptor is the first and crucial step in determining host specificity. Using a well-established retroviral model-avian Rous sarcoma virus (RSV)-we analyzed changes in an RSV variant that had repeatedly been able to infect rodents. By envelope gene (env) sequencing, we identified eight mutations that do not match the already described mutations influencing the host range. Two of these mutations-one at the beginning (D32G) of the surface Env subunit (SU) and the other at the end of the fusion peptide region (L378S)-were found to be of critical importance, ensuring transmission to rodent, human, and chicken cells lacking the appropriate receptor. Furthermore, we carried out assays to examine the virus entry mechanism and concluded that these two mutations cause conformational changes in the Env variant and that these changes lead to an activated, or primed, state of Env (normally induced after Env interaction with the receptor). In summary, our results indicate that retroviral host range extension is caused by spontaneous Env activation, which circumvents the need for original cell receptor. This activation is, in turn, caused by mutations in various env regions.

Viruses and the Diversity of Cell Death

Cell death is a common outcome of virus infection. In some cases, cell death curbs virus replication. In others, cell death enhances virus dissemination and contributes to tissue injury, exacerbating viral disease. Three forms of cell death are observed following virus infection-apoptosis, necroptosis, and pyroptosis. In this review, I describe the core machinery needed for each of these forms of cell death. Using representative viruses, I highlight how distinct stages of virus replication initiate signaling pathways that elicit these forms of cell death. I also discuss viral strategies to overcome the deleterious effects of cell death on virus propagation and the consequences of cell death for host physiology.

Genetic Diversity of NHE1, Receptor for Subgroup J Avian Leukosis Virus, in Domestic Chicken and Wild Anseriform Species

J subgroup avian leukosis virus (ALV-J) infects domestic chicken, jungle fowl, and turkey and enters the host cell through a receptor encoded by tvj locus and identified as Na+/H+ exchanger 1 (NHE1). The resistance to ALV-J in a great majority of examined galliform species was explained by deletions or substitutions of the critical tryptophan 38 in the first extracellular loop of NHE1, and genetic polymorphisms around this site predict the susceptibility or resistance of a given species or individual. In this study, we examined the NHE1 polymorphism in domestic chicken breeds and documented quantitative differences in their susceptibility to ALV-J in vitro. In a panel of chicken breeds assembled with the aim to cover the maximum variability encountered in domestic chickens, we found a completely uniform sequence of NHE1 extracellular loop 1 (ECL1) without any source of genetic variation for the selection of ALV-J-resistant poultry. In parallel, we studied the natural polymorphisms of NHE1 in wild ducks and geese because of recent reports on ALV-J positivity in feral Asian species. In anseriform species, we demonstrate a specific and highly conserved critical ECL1 sequence without any homologue of tryptophan 38 in accordance with the resistance of duck cells to prototype ALV-J. Last, we demonstrated that the new Asian strains of ALV-J have not evolved their envelope glycoprotein to the entry the duck cells. Our results contribute substantially to the current discussion of possible heterotransmission of ALV-J and its spill-over into the wild ducks and geese.

Avian sarcoma leukosis virus receptor-envelope system for simultaneous dissection of multiple neural circuits in mammalian brain

Pathway-specific gene delivery is requisite for understanding complex neuronal systems in which neurons that project to different target regions are locally intermingled. However, conventional genetic tools cannot achieve simultaneous, independent gene delivery into multiple target cells with high efficiency and low cross-reactivity. In this study, we systematically screened all receptor-envelope pairs resulting from the combination of four avian sarcoma leukosis virus (ASLV) envelopes (EnvA, EnvB, EnvC, and EnvE) and five engineered avian-derived receptors (TVA950, TVB(S3), TVC, TVB(T), and DR-46TVB) in vitro. Four of the 20 pairs exhibited both high infection rates (TVA-EnvA, 99.6%; TVB(S3)-EnvB, 97.7%; TVC-EnvC, 98.2%; and DR-46TVB-EnvE, 98.8%) and low cross-reactivity (<2.5%). Next, we tested these four receptor-envelope pairs in vivo in a pathway-specific gene-transfer method. Neurons projecting into a limited somatosensory area were labeled with each receptor by retrograde gene transfer. Three of the four pairs exhibited selective transduction into thalamocortical neurons expressing the paired receptor (>98%), with no observed cross-reaction. Finally, by expressing three receptor types in a single animal, we achieved pathway-specific, differential fluorescent labeling of three thalamic neuronal populations, each projecting into different somatosensory areas. Thus, we identified three orthogonal pairs from the list of ASLV subgroups and established a new vector system that provides a simultaneous, independent, and highly specific genetic tool for transferring genes into multiple target cells in vivo. Our approach is broadly applicable to pathway-specific labeling and functional analysis of diverse neuronal systems.

Intronic deletions of tva receptor gene decrease the susceptibility to infection by avian sarcoma and leukosis virus subgroup A

The group of avian sarcoma and leukosis virus (ASLV) in chickens contains six highly related subgroups, A to E and J. Four genetic loci, tva, tvb, tvc and tvj, encode for corresponding receptors that determine the susceptibility to the ASLV subgroups. The prevalence of ASLV in hosts may have imposed strong selection pressure toward resistance to ASLV infection, and the resistant alleles in all four receptor genes have been identified. In this study, two new alleles of the tva receptor gene, tva^r5 and tva^r6, with similar intronic deletions were identified in Chinese commercial broilers. These natural mutations delete the deduced branch point signal within the first intron, disrupting mRNA splicing of the tva receptor gene and leading to the retention of intron 1 and introduction of premature TGA stop codons in both the longer and shorter tva isoforms. As a result, decreased susceptibility to subgroup A ASLV in vitro and in vivo was observed in the subsequent analysis. In addition, we identified two groups of heterozygous allele pairs which exhibited quantitative differences in host susceptibility to ASLV-A. This study demonstrated that defective splicing of the tva receptor gene can confer genetic resistance to ASLV subgroup A in the host.

How do viruses control mitochondria-mediated apoptosis?

There is no doubt that viruses require cells to successfully reproduce and effectively infect the next host. The question is what is the fate of the infected cells? All eukaryotic cells can "sense" viral infections and exhibit defence strategies to oppose viral replication and spread. This often leads to the elimination of the infected cells by programmed cell death or apoptosis. This "sacrifice" of infected cells represents the most primordial response of multicellular organisms to viruses. Subverting host cell apoptosis, at least for some time, is therefore a crucial strategy of viruses to ensure their replication, the production of essential viral proteins, virus assembly and the spreading to new hosts. For that reason many viruses harbor apoptosis inhibitory genes, which once inside infected cells are expressed to circumvent apoptosis induction during the virus reproduction phase. On the other hand, viruses can take advantage of stimulating apoptosis to (i) facilitate shedding and hence dissemination, (ii) to prevent infected cells from presenting viral antigens to the immune system or (iii) to kill non-infected bystander and immune cells which would limit viral propagation. Hence the decision whether an infected host cell undergoes apoptosis or not depends on virus type and pathogenicity, its capacity to oppose antiviral responses of the infected cells and/or to evade any attack from immune cells. Viral genomes have therefore been adapted throughout evolution to satisfy the need of a particular virus to induce or inhibit apoptosis during its life cycle. Here we review the different strategies used by viruses to interfere with the two major apoptosis as well as with the innate immune signaling pathways in mammalian cells. We will focus on the intrinsic mitochondrial pathway and discuss new ideas about how particular viruses could activately engage mitochondria to induce apoptosis of their host.

Structural insight into equine lentivirus receptor 1

Equine lentivirus receptor 1 (ELR1) has been identified as a functional cellular receptor for equine infectious anemia virus (EIAV). Herein, recombinant ELR1 and EIAV surface glycoprotein gp90 were respectively expressed in Drosophila melanogaster S2 cells, and purified to homogeneity by Ni-NTA affinity chromatography and gel filtration chromatography. Gel filtration chromatography and analytical ultracentrifugation (AUC) analyses indicated that both ELR1 and gp90 existed as individual monomers in solution and formed a complex with a stoichiometry of 1:1 when mixed. The structure of ELR1 was first determined with the molecular replacement method, which belongs to the space group P42 21 2 with one molecule in an asymmetric unit. It contains eight antiparallel β-sheets, of which four are in cysteine rich domain 1 (CRD1) and two are in CRD2 and CRD3, respectively. Alignment of ELR1 with HVEM and CD134 indicated that Tyr61, Leu70, and Gly72 in CRD1 of ELR1 are important residues for binding to gp90. Isothermal titration calorimetry (ITC) experiments further confirmed that Leu70 and Gly72 are the critical residues.

Identification of novel viral receptors with cell line expressing viral receptor-binding protein

The viral cell receptors and infection can be blocked by the expression of the viral receptor-binding protein. Thus, the viral cell receptor is an attractive target for anti-viral strategies, and the identification of viral cell receptor is critical for better understanding and controlling viral disease. As a model system for viral entry and anti-retroviral approaches, avian sarcoma/leukosis virus (ASLV, including the A-J ten subgroups) has been studied intensively and many milestone discoveries have been achieved based on work with ASLV. Here, we used a DF1 cell line expressed viral receptor-binding protein to efficiently identify chicken Annexin A2 (chANXA2) as a novel receptor for retrovirus ALV-J (avian leukosis virus subgroup J). Our data demonstrate that antibodies or siRNA to chANXA2 significantly inhibited ALV-J infection and replication, and over-expression of chANXA2 permitted the entry of ALV-J into its non-permissible cells. Our findings have not only identified chANXA2 as a novel biomarker for anti-ALV-J, but also demonstrated that cell lines with the expression of viral receptor-binding protein could be as efficient tools for isolating functional receptors to identify novel anti-viral targets.

Model of the TVA receptor determinants required for efficient infection by subgroup A avian sarcoma and leukosis viruses

The study of the interactions of subgroup A avian sarcoma and leucosis viruses [ASLV(A)] with the TVA receptor required to infect cells offers a powerful experimental model of retroviral entry. Several regions and specific residues in the TVA receptor have previously been identified to be critical determinants of the binding affinity with ASLV(A) envelope glycoproteins and to mediate efficient infection. Two homologs of the TVA receptor have been cloned: the original quail TVA receptor, which has been the basis for most of the initial characterization of the ASLV(A) TVA, and the chicken TVA receptor, which is 65% identical to the quail receptor overall but identical in the region thought to be critical for infection. Our previous work characterized three mutant ASLV(A) isolates that could efficiently bind and infect cells using the chicken TVA receptor homolog but not using the quail TVA receptor homolog, with the infectivity of one mutant virus being >500-fold less with the quail TVA receptor. The mutant viruses contained mutations in the hr1 region of the surface glycoprotein. Using chimeras of the quail and chicken TVA receptors, we have identified new residues of TVA critical for the binding affinity and entry of ASLV(A) using the mutant glycoproteins and viruses to probe the function of those residues. The quail TVA receptor required changes at residues 10, 14, and 31 of the corresponding chicken TVA residues to bind wild-type and mutant ASLV(A) glycoproteins with a high affinity and recover the ability to mediate efficient infection of cells. A model of the TVA determinants critical for interacting with ASLV(A) glycoproteins is proposed.

IMPORTANCE

A detailed understanding of how retroviruses enter cells, evolve to use new receptors, and maintain efficient entry is crucial for identifying new targets for combating retrovirus infection and pathogenesis, as well as for developing new approaches for targeted gene delivery. Since all retroviruses share an envelope glycoprotein organization, they likely share a mechanism of receptor triggering to begin the entry process. Multiple, noncontiguous interaction determinants located in the receptor and the surface (SU) glycoprotein hypervariable domains are required for binding affinity and to restrict or broaden receptor usage. In this study, further mechanistic details of the entry process were elucidated by characterizing the ASLV(A) glycoprotein interactions with the TVA receptor required for entry. The ASLV(A) envelope glycoproteins are organized into functional domains that allow changes in receptor choice to occur by mutation and/or recombination while maintaining a critical level of receptor binding affinity and an ability to trigger glycoprotein conformational changes.

Neuroinvasion by Chandipura virus

Chandipura virus (CHPV) is an arthropod borne rhabdovirus associated with acute encephalitis in children below the age of 15 years in the tropical states of India. Although the entry of the virus into the nervous system is among the crucial events in the pathogenesis of CHPV, the exact mechanism allowing CHPV to invade the central nervous system (CNS) is currently poorly understood. In the present review, based on the knowledge of host interactors previously predicted for CHPV, along with the support from experimental data available for other encephalitic viruses, the authors have speculated the various plausible modes by which CHPV could surpass the blood-brain barrier and invade the CNS to cause encephalitis whilst evading the host immune surveillance. Collectively, this review provides a conservative set of potential interactions that can be employed for future experimental validation with a view to better understand the neuropathogenesis of CHPV.

Avian retroviral replication

Avian retroviruses were originally identified as cancer-inducting filterable agents in chicken neoplasms at the beginning of the 20th century. Since their discovery, the study of these simple retroviruses has contributed greatly to our understanding of viral replication and cancer. Avian retroviruses continue to evolve and have great economic importance in the poultry industry worldwide. The aim of this review is to provide a broad overview of the genome, pathology, and replication of avian retroviruses. Notable gaps in our current knowledge are highlighted, and areas where avian retroviruses differ from other retroviruses are emphasized.

Nonconserved tryptophan 38 of the cell surface receptor for subgroup J avian leukosis virus discriminates sensitive from resistant avian species

Subgroup J avian leukosis virus (ALV-J) is unique among the avian sarcoma and leukosis viruses in using the multimembrane-spanning cell surface protein Na(+)/H(+) exchanger type 1 (NHE1) as a receptor. The precise localization of amino acids critical for NHE1 receptor activity is key in understanding the virus-receptor interaction and potential interference with virus entry. Because no resistant chicken lines have been described until now, we compared the NHE1 amino acid sequences from permissive and resistant galliform species. In all resistant species, the deletion or substitution of W38 within the first extracellular loop was observed either alone or in the presence of other incidental amino acid changes. Using the ectopic expression of wild-type or mutated chicken NHE1 in resistant cells and infection with a reporter recombinant retrovirus of subgroup J specificity, we studied the effect of individual mutations on the NHE1 receptor capacity. We suggest that the absence of W38 abrogates binding of the subgroup J envelope glycoprotein to ALV-J-resistant cells. Altogether, we describe the functional importance of W38 for virus entry and conclude that natural polymorphisms in NHE1 can be a source of host resistance to ALV-J.

Induction of apoptosis by the Amsacta moorei entomopoxvirus

CF-70-B2 cells derived from the spruce budworm (Choristoneura fumiferana) undergo apoptosis when infected with Amsacta moorei entomopoxvirus (AMEV), as characterized by membrane blebbing, formation of apoptotic bodies, TdT-mediated dUTP nick-end labelling (TUNEL) staining, condensed chromatin and induction of caspase-3/7 activity. The apoptotic response was reduced when cells were infected with UV-inactivated AMEV, but not when infected in the presence of the DNA synthesis inhibitor, cytosine β-d-arabinofuranoside. Hence, only pre-DNA replication events were involved in inducing the antiviral response in CF-70-B2 cells. The virus eventually overcame the host’s antiviral response and replicated to high progeny virus titres accompanied by high levels of caspase-3/7 activity. The CF-70-B2 cells were less productive of progeny virus in comparison to LD-652, a Lymantria dispar cell line routinely used for propagation of AMEV. At late stages of infection, LD-652 cells also showed characteristics of apoptosis such as oligosomal DNA fragmentation, TUNEL staining, condensed chromatin and increased caspase-3/7 activity. Induction of apoptosis in LD-652 cells was dependent on viral DNA replication and/or late gene expression. A significantly reduced rate of infection was observed in the presence of general caspase inhibitors Q-VD-OPH and Z-VAD-FMK, indicating caspases may be involved in productive virus infection.

Library screening and receptor-directed targeting of gammaretroviral vectors

Gene- and cell-based therapies hold great potential for the advancement of the personalized medicine movement. Gene therapy vectors have made dramatic leaps forward since their inception. Retroviral-based vectors were the first to gain clinical attention and still offer the best hope for the long-term correction of many disorders. The fear of nonspecific transduction makes targeting a necessary feature for most clinical applications. However, this remains a difficult feature to optimize, with specificity often coming at the expense of efficiency. The aim of this article is to discuss the various methods employed to retarget retroviral entry. Our focus will lie on the modification of gammaretroviral envelope proteins with an in-depth discussion of the creation and screening of envelope libraries.

Reovirus activates a caspase-independent cell death pathway

Virus-induced apoptosis is thought to be the primary mechanism of cell death following reovirus infection. Induction of cell death following reovirus infection is initiated by the incoming viral capsid proteins during cell entry and occurs via NF-κB-dependent activation of classical apoptotic pathways. Prototype reovirus strain T3D displays a higher cell-killing potential than strain T1L. To investigate how signaling pathways initiated by T3D and T1L differ, we methodically analyzed cell death pathways activated by these two viruses in L929 cells. We found that T3D activates NF-κB, initiator caspases, and effector caspases to a significantly greater extent than T1L. Surprisingly, blockade of NF-κB or caspases did not affect T3D-induced cell death. Cell death following T3D infection resulted in a reduction in cellular ATP levels and was sensitive to inhibition of the kinase activity of receptor interacting protein 1 (RIP1). Furthermore, membranes of T3D-infected cells were compromised. Based on the dispensability of caspases, a requirement for RIP1 kinase function, and the physiological status of infected cells, we conclude that reovirus can also induce an alternate, necrotic form of cell death described as necroptosis. We also found that induction of necroptosis requires synthesis of viral RNA or proteins, a step distinct from that necessary for the induction of apoptosis. Thus, our studies reveal that two different events in the reovirus replication cycle can injure host cells by distinct mechanisms.

Virus-induced cell death is a determinant of pathogenesis. Mammalian reovirus is a versatile experimental model for identifying viral and host intermediaries that contribute to cell death and for examining how these factors influence viral disease. In this study, we identified that in addition to apoptosis, a regulated form of cell death, reovirus is capable of inducing an alternate form of controlled cell death known as necroptosis. Death by this pathway perturbs the integrity of host membranes and likely triggers inflammation. We also found that apoptosis and necroptosis following viral infection are activated by distinct mechanisms. Our results suggest that host cells can detect different stages of viral infection and attempt to limit viral replication through different forms of cellular suicide. While these death responses may aid in curbing viral spread, they can also exacerbate tissue injury and disease following infection.

Cellular entry of retroviruses

The retrovirus family contains several important human and animal pathogens, including the human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome (AIDS). Studies with retroviruses were instrumental to our present understanding of the cellular entry of enveloped viruses in general. For instance, studies with alpharetroviruses defined receptor engagement, as opposed to low pH, as a trigger for the envelope protein-driven membrane fusion. The insights into the retroviral entry process allowed the generation of a new class of antivirals, entry inhibitors, and these therapeutics are at present used for treatment of HIV/AIDS. In this chapter, we will summarize key concepts established for entry of avian sarcoma and leukosis virus (ASLV), a widely used model system for retroviral entry. We will then review how foamy virus and HIV, primate- and human retroviruses, enter target cells, and how the interaction of the viral and cellular factors involved in the cellular entry of these viruses impacts viral tropism, pathogenesis and approaches to therapy and vaccine development.

African swine fever virus-cell interactions: from virus entry to cell survival

Viruses have adapted to evolve complex and dynamic interactions with their host cell. The viral entry mechanism determines viral tropism and pathogenesis. The entry of African swine fever virus (ASFV) is dynamin-dependent and clathrin-mediated, but other pathways have been described such as macropinocytosis. During endocytosis, ASFV viral particles undergo disassembly in various compartments that the virus passes through en route to the site of replication. This disassembly relies on the acid pH of late endosomes and on microtubule cytoskeleton transport. ASFV interacts with several regulatory pathways to establish an optimal environment for replication. Examples of these pathways include small GTPases, actin-related signaling, and lipid signaling. Cellular cholesterol, the entire cholesterol biosynthesis pathway, and phosphoinositides are central molecular networks required for successful infection. Here we report new data on the conformation of the viral replication site or viral factory and the remodeling of the subcellular structures. We review the virus-induced regulation of ER stress, apoptosis and autophagy as key mechanisms of cell survival and determinants of infection outcome. Finally, future challenges for the development of new preventive strategies against this virus are proposed on the basis of current knowledge about ASFV-host interactions.

Dengue virus infection perturbs lipid homeostasis in infected mosquito cells

Dengue virus causes ∼50-100 million infections per year and thus is considered one of the most aggressive arthropod-borne human pathogen worldwide. During its replication, dengue virus induces dramatic alterations in the intracellular membranes of infected cells. This phenomenon is observed both in human and vector-derived cells. Using high-resolution mass spectrometry of mosquito cells, we show that this membrane remodeling is directly linked to a unique lipid repertoire induced by dengue virus infection. Specifically, 15% of the metabolites detected were significantly different between DENV infected and uninfected cells while 85% of the metabolites detected were significantly different in isolated replication complex membranes. Furthermore, we demonstrate that intracellular lipid redistribution induced by the inhibition of fatty acid synthase, the rate-limiting enzyme in lipid biosynthesis, is sufficient for cell survival but is inhibitory to dengue virus replication. Lipids that have the capacity to destabilize and change the curvature of membranes as well as lipids that change the permeability of membranes are enriched in dengue virus infected cells. Several sphingolipids and other bioactive signaling molecules that are involved in controlling membrane fusion, fission, and trafficking as well as molecules that influence cytoskeletal reorganization are also up regulated during dengue infection. These observations shed light on the emerging role of lipids in shaping the membrane and protein environments during viral infections and suggest membrane-organizing principles that may influence virus-induced intracellular membrane architecture.

Intronic deletions that disrupt mRNA splicing of the tva receptor gene result in decreased susceptibility to infection by avian sarcoma and leukosis virus subgroup A

The group of closely related avian sarcoma and leukosis viruses (ASLVs) evolved from a common ancestor into multiple subgroups, A to J, with differential host range among galliform species and chicken lines. These subgroups differ in variable parts of their envelope glycoproteins, the major determinants of virus interaction with specific receptor molecules. Three genetic loci, tva, tvb, and tvc, code for single membrane-spanning receptors from diverse protein families that confer susceptibility to the ASLV subgroups. The host range expansion of the ancestral virus might have been driven by gradual evolution of resistance in host cells, and the resistance alleles in all three receptor loci have been identified. Here, we characterized two alleles of the tva receptor gene with similar intronic deletions comprising the deduced branch-point signal within the first intron and leading to inefficient splicing of tva mRNA. As a result, we observed decreased susceptibility to subgroup A ASLV in vitro and in vivo. These alleles were independently found in a close-bred line of domestic chicken and Indian red jungle fowl (Gallus gallus murghi), suggesting that their prevalence might be much wider in outbred chicken breeds. We identified defective splicing to be a mechanism of resistance to ASLV and conclude that such a type of mutation could play an important role in virus-host coevolution.

Simple, automated, high resolution mass spectrometry method to determine the disulfide bond and glycosylation patterns of a complex protein: subgroup A avian sarcoma and leukosis virus envelope glycoprotein

Enveloped viruses must fuse the viral and cellular membranes to enter the cell. Understanding how viral fusion proteins mediate entry will provide valuable information for antiviral intervention to combat associated disease. The avian sarcoma and leukosis virus envelope glycoproteins, trimers composed of surface (SU) and transmembrane heterodimers, break the fusion process into several steps. First, interactions between SU and a cell surface receptor at neutral pH trigger an initial conformational change in the viral glycoprotein trimer followed by exposure to low pH enabling additional conformational changes to complete the fusion of the viral and cellular membranes. Here, we describe the structural characterization of the extracellular region of the subgroup A avian sarcoma and leukosis viruses envelope glycoproteins, SUATM129 produced in chicken DF-1 cells. We developed a simple, automated method for acquiring high resolution mass spectrometry data using electron capture dissociation conditions that preferentially cleave the disulfide bond more readily than the peptide backbone amide bonds that enabled the identification of disulfide-linked peptides. Seven of nine disulfide bonds were definitively assigned; the remaining two bonds were assigned to an adjacent pair of cysteine residues. The first cysteine of surface and the last cysteine of the transmembrane form a disulfide bond linking the heterodimer. The surface glycoprotein contains a free cysteine at residue 38 previously reported to be critical for virus entry. Eleven of 13 possible SUATM129 N-linked glycosylation sites were modified with carbohydrate. This study demonstrates the utility of this simple yet powerful method for assigning disulfide bonds in a complex glycoprotein.

Enter the kill zone: initiation of death signaling during virus entry

Infection of host cells by a variety of viruses results in programmed cell death or apoptosis. In many cases, early events in virus replication that occur prior to synthesis of viral proteins and replication of viral genomes directly or indirectly activate signaling pathways that culminate in cell death. Using examples of viruses for which prodeath signaling is better defined, this review will describe how cell entry steps including virus attachment to receptors, virus uncoating in endosomes, and events that occur following membrane penetration lead to apoptosis. The relevance and physiologic consequences of early induction of prodeath signaling to viral pathogenesis also will be discussed.

Selective viral vector transduction of ErbB4 expressing cortical interneurons in vivo with a viral receptor-ligand bridge protein

Both treatment of disease and basic studies of complex tissues can benefit from directing viral vector infection to specific cell types. We have used a unique cell targeting method to direct viral vector transduction to cerebral cortical neurons expressing the neuregulin (NRG) receptor ErbB4; both NRG and ErbB4 have been implicated in schizophrenia, and ErbB4 expression in cerebral cortex is known to be restricted to inhibitory neurons. We find that a bridge protein composed of the avian viral receptor TVB fused to NRG, along with EnvB-pseudotyped virus, is able to direct infection selectively to ErbB4-expressing inhibitory cortical neurons in vivo. Interestingly, although ErbB4 is expressed in a broad range of cortical inhibitory cell types, NRG-dependent infection is restricted to a more selective subset of inhibitory cell types. These results demonstrate a tool that can be used for further studies of NRG and ErbB receptors in brain circuits and demonstrate the feasibility for further development of related bridge proteins to target gene expression to other specific cell types in complex tissues.

[Receptors for animal retroviruses]

Diseases caused by animal retroviruses have been recognized since 19th century in veterinary field. Most livestock and companion animals have own retroviruses. To disclose the receptors for these retroviruses will be useful for understanding retroviral pathogenesis, developments of anti-retroviral drugs and vectors for human and animal gene therapies. Of retroviruses in veterinary field, receptors for the following viruses have been identified; equine infectious anemia virus, feline immunodeficiency virus, feline leukemia virus subgroups A, B, C, and T, Jaagsiekte sheep retrovirus, enzootic nasal tumor virus, avian leukosis virus subgroups A, B, C, D, E, and J, reticuloendotheliosis virus, RD-114 virus (a feline endogenous retrovirus), and porcine endogenous retrovirus subgroup A. Primate lentiviruses require two molecules (CD4 and chemokine receptors such as CXCR4) as receptors. Likewise, feline immunodeficiency virus also requires two molecules, i.e., CD134 (an activation marker of CD4 T cells) and CXCR4 in infection. Gammaretroviruses utilize multi-spanning transmembrane proteins, most of which are transporters of amino acids, vitamins and inorganic ions. Betaretroviruses and alpharetroviruses utilize transmembrane and/or GPI-anchored proteins as receptors. In this review, I overviewed receptors for animal retroviruses in veterinary field.

Cell-specific targeting of lentiviral vectors mediated by fusion proteins derived from Sindbis virus, vesicular stomatitis virus, or avian sarcoma/leukosis virus

Background

The ability to efficiently and selectively target gene delivery vectors to specific cell types in vitro and in vivo remains one of the formidable challenges in gene therapy. We pursued two different strategies to target lentiviral vector delivery to specific cell types. In one of the strategies, vector particles bearing a membrane-bound stem cell factor sequence plus a separate fusion protein based either on Sindbis virus strain TR339 glycoproteins or the vesicular stomatitis virus G glycoprotein were used to selectively transduce cells expressing the corresponding stem cell factor receptor (c-kit). An alternative approach involved soluble avian sarcoma/leukosis virus receptors fused to cell-specific ligands including stem cell factor and erythropoietin for targeting lentiviral vectors pseudotyped with avian sarcoma/leukosis virus envelope proteins to cells that express the corresponding receptors.

Results

The titers of unconcentrated vector particles bearing Sindbis virus strain TR339 or vesicular stomatitis virus G fusion proteins plus stem cell factor in the context of c-kit expressing cells were up to 3.2 × 10⁵ transducing units per ml while vector particles lacking the stem cell factor ligand displayed titers that were approximately 80 fold lower. On cells that lacked the c-kit receptor, the titers of stem cell factor-containing vectors were approximately 40 times lower compared to c-kit-expressing cells.

Lentiviral vectors pseudotyped with avian sarcoma/leukosis virus subgroup A or B envelope proteins and bearing bi-functional bridge proteins encoding erythropoietin or stem cell factor fused to the soluble extracellular domains of the avian sarcoma/leukosis virus subgroup A or B receptors resulted in efficient transduction of erythropoietin receptor or c-kit-expressing cells. Transduction of erythropoietin receptor-expressing cells mediated by bi-functional bridge proteins was found to be dependent on the dose, the correct subgroup-specific virus receptor and the correct envelope protein. Furthermore, transduction was completely abolished in the presence of anti-erythropoietin antibody.

Conclusions

Our results indicate that the avian sarcoma/leukosis virus bridge strategy provides a reliable approach for cell-specific lentiviral vector targeting. The background levels were lower compared to alternative strategies involving Sindbis virus strain TR339 or vesicular stomatitis virus fusion proteins.

Bovine ephemeral fever virus-induced apoptosis requires virus gene expression and activation of Fas and mitochondrial signaling pathway

Although induction of apoptosis by bovine ephemeral fever virus (BEFV) in several cell lines has been previously demonstrated by our laboratory, less information is available on the process of BEFV-induced apoptosis in terms of cellular pathways and specific proteins involved. In order to determine the step in viral life cycle at which apoptosis of infected cells is triggered, chemical and physical agents were used to block viral infection. Treatment of BHK-21 infected cells with ammonium chloride (NH4Cl) or cells infected with UV-inactivated BEFV was seen to abrogate virus apoptosis induction, suggesting that virus uncoating and gene expression are required for the induction of apoptosis. Using soluble death receptors Fc:Fas chimera to block Fas signaling, BEFV-induced apoptosis was inhibited in cells. BEFV infection of BHK-21 cells results in the Fas-dependent activation of caspase 8 and cleavage of Bid. This initiated the dissipation of the membrane potential and the release of cytochrome c but not AIF or Smac/DIABLO from mitochondrial into cytoplasm leading to activation of caspase 9. Combined activation of the death receptor and mitochondrial pathways results in activation of the downstream effecter caspase 3 leading to cleavage of PARP. Fas-mediated BEFV-induced apoptosis could be suppressed by the overexpression of Bcl-2 or by treatment with caspase inhibitors and soluble death receptors Fc:Fas chimera. Taken together, this study provided first evidence demonstrating that BEFV-induced apoptosis requires viral gene expression and occurs through the activation of Fas and mitochondrion-mediated caspase-dependent pathways.

Apoptosis in human reproductive tissues: emerging concepts

In summary, apoptosis is an important concept in understanding many facets of human reproduction. Recent advances in the understanding of molecular mechanisms of apoptosis will allow us to understand this physiologically important process. How can the modulation of this process be applied to human reproduction? Studies to further understand the abnormalities of apoptosis, either too much or too little, may lead to a better understanding of the clinical problems in human reproduction.

We summarize future directions towards further understanding the roles of apoptotic processes in human reproduction in Table 3. The diseases listed in Table 3 are problems which could be approached from the apoptosis point of view. With further study using this concept as the lens, new diagnostic tools or therapies may be developed for these problems.