Structure of a murine leukemia virus receptor-binding glycoprotein at 2.0 angstrom resolution

Cryo-EM structures elucidate the multiligand receptor nature of megalin

Megalin (low-density lipoprotein receptor-related protein 2) is a giant glycoprotein of about 600 kDa, mediating the endocytosis of more than 60 ligands, including those of proteins, peptides, and drug compounds [S. Goto, M. Hosojima, H. Kabasawa, A. Saito, Int. J. Biochem. Cell Biol. 157, 106393 (2023)]. It is expressed predominantly in renal proximal tubule epithelial cells, as well as in the brain, lungs, eyes, inner ear, thyroid gland, and placenta. Megalin is also known to mediate the endocytosis of toxic compounds, particularly those that cause renal and hearing disorders [Y. Hori et al., J. Am. Soc. Nephrol. 28, 1783-1791 (2017)]. Genetic megalin deficiency causes Donnai-Barrow syndrome/facio-oculo-acoustico-renal syndrome in humans. However, it is not known how megalin interacts with such a wide variety of ligands and plays pathological roles in various organs. In this study, we elucidated the dimeric architecture of megalin, purified from rat kidneys, using cryoelectron microscopy. The maps revealed the densities of endogenous ligands bound to various regions throughout the dimer, elucidating the multiligand receptor nature of megalin. We also determined the structure of megalin in complex with receptor-associated protein, a molecular chaperone for megalin. The results will facilitate further studies on the pathophysiology of megalin-dependent multiligand endocytic pathways in multiple organs and will also be useful for the development of megalin-targeted drugs for renal and hearing disorders, Alzheimer's disease [B. V. Zlokovic et al., Proc. Natl. Acad. Sci. U.S.A. 93, 4229-4234 (1996)], and other illnesses.

Receptor-recognition and antiviral mechanisms of retrovirus-derived human proteins

Human syncytin-1 and suppressyn are cellular proteins of retroviral origin involved in cell-cell fusion events to establish the maternal-fetal interface in the placenta. In cell culture, they restrict infections from members of the largest interference group of vertebrate retroviruses, and are regarded as host immunity factors expressed during development. At the core of the syncytin-1 and suppressyn functions are poorly understood mechanisms to recognize a common cellular receptor, the membrane transporter ASCT2. Here, we present cryo-electron microscopy structures of human ASCT2 in complexes with the receptor-binding domains of syncytin-1 and suppressyn. Despite their evolutionary divergence, the two placental proteins occupy similar positions in ASCT2, and are stabilized by the formation of a hybrid β-sheet or 'clamp' with the receptor. Structural predictions of the receptor-binding domains of extant retroviruses indicate overlapping binding interfaces and clamping sites with ASCT2, revealing a competition mechanism between the placental proteins and the retroviruses. Our work uncovers a common ASCT2 recognition mechanism by a large group of endogenous and disease-causing retroviruses, and provides high-resolution views on how placental human proteins exert morphological and immunological functions.

The crystal structure of a simian Foamy Virus receptor binding domain provides clues about entry into host cells

The surface envelope glycoprotein (Env) of all retroviruses mediates virus binding to cells and fusion of the viral and cellular membranes. A structure-function relationship for the HIV Env that belongs to the Orthoretrovirus subfamily has been well established. Structural information is however largely missing for the Env of Foamy viruses (FVs), the second retroviral subfamily. In this work we present the X-ray structure of the receptor binding domain (RBD) of a simian FV Env at 2.57 Å resolution, revealing two subdomains and an unprecedented fold. We have generated a model for the organization of the RBDs within the trimeric Env, which indicates that the upper subdomains form a cage-like structure at the apex of the Env, and identified residues K342, R343, R359 and R369 in the lower subdomain as key players for the interaction of the RBD and viral particles with heparan sulfate.

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.

Identification of Copper Transporter 1 as a Receptor for Feline Endogenous Retrovirus ERV-DC14

Vertebrates harbor hundreds of endogenous retroviral (ERV) sequences in their genomes, which are considered signs of past infections that occurred during evolution. On rare occasions, ERV genes like env are maintained and coopted by hosts for physiological functions, but they also participate in recombination events with exogenous retroviruses to generate rearranged viruses with novel tropisms. In domestic cats, feline leukemia virus type D (FeLV-D) has been described as a recombinant virus between the infectious FeLV-A and likely the ERV-DC14 env gene that resulted in an extended tropism due to the usage of a new uncharacterized retroviral receptor. Here, we report the identification of SLC31A1 encoding the copper transporter 1 (CTR1) as a susceptibility gene for ERV-DC14 infection. Expression of human CTR1 into nonpermissive cells was sufficient to confer sensitivity to ERV-DC14 pseudotype infection and to increase the binding of an ERV-DC14 Env ligand. Moreover, inactivation of CTR1 by genome editing or cell surface downmodulation of CTR1 by a high dose of copper dramatically decreased ERV-DC14 infection and binding, while magnesium treatment had no effect. We also investigated the role of CTR1 in the nonpermissivity of feline and hamster cells. While feline CTR1 was fully functional for ERV-DC14, we found that binding was strongly reduced upon treatment with conditioned medium of feline cells, suggesting that the observed resistance to infection was a consequence of CTR1 saturation. In contrast, hamster CTR1 was inactive due to the presence of a N-linked glycosylation site at position 27, which is absent in the human ortholog. These results provide evidence that CTR1 is a receptor for ERV-DC14. Along with chimpanzee endogenous retrovirus type 2, ERV-DC14 is the second family of endogenous retrovirus known to have used CTR1 during past infections of vertebrates. IMPORTANCE Receptor usage is an important determinant of diseases induced by pathogenic retroviruses. In the case of feline leukemia viruses, three subgroups (A, B, and C) based on their ability to recognize different cell host receptors, respectively, the thiamine transporter THTR1, the phosphate transporter PiT1, and the heme exporter FLVCR1, are associated with distinct feline diseases. FeLV-A is horizontally transmitted and found in all naturally infected cats, while FeLV-B and FeLV-C have emerged from FeLV-A, respectively, by recombination with endogenous retroviral env sequences or by mutations in the FeLV-A env gene, both leading to a switch in receptor usage and in subsequent in vivo tropism. Here, we set up a genetic screen to identify the retroviral receptor of ERV-DC14, a feline endogenous provirus whose env gene has been captured by infectious FeLV-A to give rise to FeLV-D in a process similar to FeLV-B. Our results reveal that the copper transporter CTR1 was such a receptor and provide new insights into the acquisition of an expanded tropism by FeLV-D.

Deep-Time Structural Evolution of Retroviral and Filoviral Surface Envelope Proteins

The retroviral surface envelope protein subunit (SU) mediates receptor binding and triggers membrane fusion by the transmembrane (TM) subunit. SU evolves rapidly under strong selective conditions, resulting in seemingly unrelated SU structures in highly divergent retroviruses. Structural modeling of the SUs of several retroviruses and related endogenous retroviral elements with AlphaFold 2 identifies a TM-proximal SU β-sandwich structure that has been conserved in the orthoretroviruses for at least 110 million years. The SU of orthoretroviruses diversified by the differential expansion of the β-sandwich core to form domains involved in virus-host interactions. The β-sandwich domain is also conserved in the SU equivalent GP₁ of Ebola virus although with a significantly different orientation in the trimeric envelope protein structure relative to the β-sandwich of human immunodeficiency virus type 1 gp120, with significant evidence for divergent rather than convergent evolution. The unified structural view of orthoretroviral SU and filoviral GP₁ identifies an ancient, structurally conserved, and evolvable domain underlying the structural diversity of orthoretroviral SU and filoviral GP₁. IMPORTANCE The structural relationships of SUs of retroviral groups are obscured by the high rate of sequence change of SU and the deep-time divergence of retroviral lineages. Previous data showed no structural or functional relationships between the SUs of type C gammaretroviruses and lentiviruses. A deeper understanding of structural relationships between the SUs of different retroviral lineages would allow the generalization of critical processes mediated by these proteins in host cell infection. Modeling of SUs with AlphaFold 2 reveals a conserved core domain underlying the structural diversity of orthoretroviral SUs. Definition of the conserved SU structural core allowed the identification of a homologue structure in the SU equivalent GP₁ of filoviruses that most likely shares an origin, unifying the SU of orthoretroviruses and GP₁ of filoviruses into a single protein family. These findings will allow an understanding of the structural basis for receptor-mediated membrane fusion mechanisms in a broad range of biomedically important retroviruses.

Domain Organization of Lentiviral and Betaretroviral Surface Envelope Glycoproteins Modeled with AlphaFold

The surface envelope glycoproteins of non-primate lentiviruses and betaretroviruses share sequence similarity with the inner proximal domain β-sandwich of the human immunodeficiency virus type 1 (HIV-1) gp120 glycoprotein that faces the transmembrane glycoprotein as well as patterns of cysteine and glycosylation site distribution that points to a similar two-domain organization in at least some lentiviruses. Here, high reliability models of the surface glycoproteins obtained with the AlphaFold algorithm are presented for the gp135 glycoprotein of the small ruminant caprine arthritis-encephalitis (CAEV) and visna lentiviruses and the betaretroviruses jaagsiekte sheep retrovirus (JSRV), mouse mammary tumor virus (MMTV) and consensus human endogenous retrovirus type K (HERV-K). The models confirm and extend the inner domain structural conservation in these viruses and identify two outer domains with a putative receptor binding site in the CAEV and visna virus gp135. The location of that site is consistent with patterns of sequence conservation and glycosylation site distribution in gp135. In contrast, a single domain is modeled for the JSRV, MMTV and HERV-K betaretrovirus envelope proteins that is highly conserved structurally in the proximal region and structurally diverse in apical regions likely to interact with cell receptors. The models presented here identify sites in small ruminant lentivirus and betaretrovirus envelope glycoproteins likely to be critical for virus entry and virus neutralization by antibodies and will facilitate their functional and structural characterization. Importance Structural information on the surface envelope proteins of lentiviruses and related betaretroviruses is critical to understand mechanisms of virus-host interactions. However, experimental determination of these structures has been challenging and only the structure of the human immunodeficiency virus type 1 gp120 has been determined. The advent of the AlphaFold artificial intelligence method for structure prediction allows high-quality modeling of the structures of small ruminant lentiviral and betaretroviral surface envelope proteins. The models are consistent with much of previously described experimental data, show regions likely to interact with receptors and identify domains that may be involved in mechanisms of antibody neutralization resistance in the small ruminant lentiviruses. The models will allow more precise design of mutants to further determine mechanisms of viral entry and immune evasion in this group of viruses and constructs for structure of these surface envelope proteins.

Structure of the Receptor Binding Domain of EnvP(b)1, an Endogenous Retroviral Envelope Protein Expressed in Human Tissues

Organisms can access genetic and functional novelty by capturing viral elements within their genomes, where they can evolve to drive new cellular or organismal processes. We demonstrate that a retroviral envelope gene, EnvP(b)1, has been maintained and its fusion activity preserved for 40 to 71 million years. It is expressed as a protein in multiple healthy human tissues. We determined the structure of its inferred receptor binding domain and compared it with the same domain in modern viruses. We found a common conserved architecture that underlies the varied receptor binding activity of divergent Env genes. The modularity and versatility of this domain may underpin the evolutionary success of this clade of fusogens.

EnvP(b)1 is an endogenous retroviral envelope gene found in human and other primate genomes. We report EnvP(b)1 sequences in primate genomes consistent with an integration event between 40 and 71 million years ago. Using a highly specific polyclonal antiserum raised against the putative receptor binding domain (RBD) of human EnvP(b)1, we detected expression in human placenta, ovaries, and thymus. We found that EnvP(b)1 is proteolytically processed, and using cell-cell fusion assays in multiple primate cell lines, we demonstrated that extant EnvP(b)1 proteins from a variety of primate genomes are fusogenic. This work supports the idea that EnvP(b)1 is under purifying selection and its fusogenic activity has been maintained for over 40 million years. We determined the structure of the RBD of human EnvP(b)1, which defines structural similarities with extant leukemia viruses, despite little sequence conservation. This structure highlights a common scaffold from which novel receptor binding specificities likely evolved. The evolutionary plasticity of this domain may underlie the diversity of related Envs in circulating viruses.

Design, molecular docking analysis of an anti-inflammatory drug, computational analysis and intermolecular interactions energy studies of 1-benzothiophene-2-carboxylic acid

In this paper, theoretical study on molecular geometry, vibrational, pharmaceutical and electronic properties of the monomeric and dimeric structures of 1-benzothiophene-2-carboxylic acid (2BT) were carried out using B3LYP hybrid functional with 6-311++G(d,p) as basis set. The structural study show that the stability of 2BT crystalline structure arising from O-H…O, C-H…O as well as S-H…O hydrogen bonding interactions. Vibrational analysis, for monomer and dimer species, show a good compatibility between experimental and theoretical frequencies. Then, the 1H and 13C NMR chemical shifts were calculated using Gauge Independent Atomic Orbital (GIAO) technical. In addition, the UV-Vis spectrum was simulated in gas phase and in water throughout TD-DFT calculation. The electronic transitions were identified based on HOM-LUMO energies. However, donor-acceptor interactions and charge delocalization has been studied via natural bond orbital (NBO). The nucleophilic and electrophilic site localization is identified by molecular electrostatic potential. Hirshfeld surface analysis has been discussed based on color code demonstrating the various non covalent interactions. Besides, molecular docking analysis was reported to evince the pharmaceutical properties of the studied molecule.

Infectious KoRV-related retroviruses circulating in Australian bats

Bats are reservoirs of emerging viruses that are highly pathogenic to other mammals, including humans. Despite the diversity and abundance of bat viruses, to date they have not been shown to harbor exogenous retroviruses. Here we report the discovery and characterization of a group of koala retrovirus-related (KoRV-related) gammaretroviruses in Australian and Asian bats. These include the Hervey pteropid gammaretrovirus (HPG), identified in the scat of the Australian black flying fox (Pteropus alecto), which is the first reproduction-competent retrovirus found in bats. HPG is a close relative of KoRV and the gibbon ape leukemia virus (GALV), with virion morphology and Mn²⁺-dependent virion-associated reverse transcriptase activity typical of a gammaretrovirus. In vitro, HPG is capable of infecting bat and human cells, but not mouse cells, and displays a similar pattern of cell tropism as KoRV-A and GALV. Population studies reveal the presence of HPG and KoRV-related sequences in several locations across northeast Australia, as well as serologic evidence for HPG in multiple pteropid bat species, while phylogenetic analysis places these bat viruses as the basal group within the KoRV-related retroviruses. Taken together, these results reveal bats to be important reservoirs of exogenous KoRV-related gammaretroviruses.

Computational analysis of envelope glycoproteins from diverse geographical isolates of bovine leukemia virus identifies highly conserved peptide motifs

BACKGROUND

Bovine leukemia virus (BLV) is a deltaretrovirus infecting bovine B cells and causing enzootic bovine leucosis. The SU or surface subunit, gp51, of its envelope glycoprotein is involved in receptor recognition and virion attachment. It contains the major neutralizing and CD4+ and CD8+ T cell epitopes found in naturally infected animals. In this study, we aimed to determine global variation and conservation within gp51 in the context of developing an effective global BLV vaccine.

RESULTS

A total of 256 sequences extracted from the NCBI database and collected in different parts of the world, were studied to identify conserved segments along the env gene sequences that encode the gp51 protein. Using the MEME server and the conserved DNA Region module for analysis within DnaSP, we identified six conserved segments, referred to as A-F, and five semi-conserved segments, referred to as G-K. The amino acid conservation ranged from 98.8 to 99.8% in conserved segments A to F, while segments G to K had 89.6-95.2% conserved amino acid sequence. Selection analysis of individual segments revealed that residues of conserved segments had undergone purifying selection, whereas, particular residues in the semi-conserved segments are currently undergoing positive selection, specifically at amino acid positions 48 in segment K, 74 in segment G, 82 in segment I, 133 and 142 in segment J, and residue 291 in segment H. Each of the codons for these six residues contain the most highly variable nucleotides within their respective semi-conserved segments.

CONCLUSIONS

The data described here show that the consensus amino acid sequence constitutes a strong candidate from which a global vaccine can be derived for use in countries where eradication by culling is not economically feasible. The most conserved segments overlap with amino acids in known immunodeterminants, specifically in epitopes D-D', E-E', CD8+ T-cell epitopes, neutralizing domain 1 and CD4+ T-cell epitopes. Two of the segments reported here represent unique segments that do not overlap with previously identified antigenic determinants. We propose that evidence of positive selection in some residues of the semi-conserved segments suggests that their variation is involved in viral strategy to escape immune surveillance of the host.

Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

Permissive XPR1 gammaretrovirus receptors in four mammalian species are functionally distinct in interference tests

Xenotropic/polytropic mouse leukemia viruses (X/P-MLVs) use the XPR1 gammaretrovirus receptor for entry. X/P-MLV host range is defined by usage of naturally occurring restrictive XPR1 receptors, and is governed by polymorphisms in the virus envelope glycoprotein and in XPR1. Here, we examined receptors of four mammalian species permissive to all X/P-MLVs (Mus dunni, human, rabbit, mink). Interference assays showed the four to be functionally distinct. Preinfection with X-MLVs consistently blocked all nine XPR1-dependent viruses, while preinfection with P-MLVs and wild mouse X/P-MLVs produced distinctive interference patterns in the four cells. These patterns indicate shared usage of independent, but not always fully functional, receptor sites. XPR1 sequence comparisons identified candidate sites in receptor-determining regions that correlate with some interference patterns. The evolutionary record suggests that the X/P-MLV tropism variants evolved to adapt to host receptor polymorphisms, to circumvent blocks by competing viruses or to avoid host-encoded envelope glycoproteins acquired for defense.

Native structure of a retroviral envelope protein and its conformational change upon interaction with the target cell

Enveloped viruses enter their host cells by membrane fusion. The process of attachment and fusion in retroviruses is mediated by a single viral envelope glycoprotein (Env). Conformational changes of Env in the course of fusion are a focus of intense studies. Here we provide further insight into the changes occurring in retroviral Env during its initial interaction with the cell, employing murine leukemia virus (MLV) as model system. We first determined the structure of both natively membrane anchored MLV Env and MLV Env tagged with YFP in the proline rich region (PRR) by electron cryo tomography (cET) and sub-volume averaging. At a resolution of ∼20 Å, native MLV Env presents as a hollow trimer (height ∼85 Å, diameter ∼120 Å) composed of step-shaped protomers. The major difference to the YFP-tagged protein was in regions outside of the central trimer. Next, we focused on elucidating the changes in MLV Env upon interaction with a host cell. Virus interaction with the plasma membrane occurred over a large surface and Env clustering on the binding site was observed. Sub-volume averaging did yield a low-resolution structure of Env interacting with the cell, which had lost its threefold symmetry and was elongated by ∼35 Å in comparison to the unbound protein. This indicates a major rearrangement of Env upon host cell binding. At the site of virus interaction, the otherwise clearly defined bilayer structure of the host cell plasma membrane was much less evident, indicative of integral membrane protein accumulation and/or a change in membrane lipid composition.

Role of Cysteines in Stabilizing the Randomized Receptor Binding Domains within Feline Leukemia Virus Envelope Proteins

Retargeting of gammaretroviral envelope proteins has shown promising results in the isolation of novel isolates with therapeutic potential. However, the optimal conditions required to obtain high-affinity retargeted envelope proteins with narrow tropism are not understood. This study highlights the advantage of constrained peptides within receptor binding domains and validates the random library screening technique of obtaining novel retargeted Env proteins. Using a modified vector backbone to screen the envelope libraries on 143B osteosarcoma cells, three novel and unique retargeted envelopes were isolated. The use of complex disulfide bonds within variable regions required for receptor binding is found within natural gammaretroviral envelope isolates. Interestingly, two of the isolates, named AII and BV2, have a pair of cysteines located within the randomized region of 11 amino acids similar to that identified within the CP Env, an isolate identified in a previous Env library screen on the human renal carcinoma Caki-1 cell line. The amino acids within the randomized region of AII and BV2 envelopes that are essential for viral infection have been identified in this study and include these cysteine residues. Through mutagenesis studies, the putative disulfide bond pairs including and beyond the randomized region were examined. In parallel, the disulfide bonds of CP Env were identified using mass spectrometry. The results indicate that this pair of cysteines creates the structural context to position key hydrophobic (F and W) and basic (K and H) residues critical for viral titer and suggest that AII, BV2, and CP internal cysteines bond together in distinct ways.

IMPORTANCE

Retargeted gammaretroviral particles have broad applications for therapeutic use. Although great advances have been achieved in identifying new Env-host cell receptor pairs, the rules for designing optimal Env libraries are still unclear. We have found that isolates with an additional pair of cysteines within the randomized region have the highest transduction efficiencies. This emphasizes the importance of considering cysteine pairs in the design of new libraries. Furthermore, our data clearly indicate that these cysteines are essential for viral infectivity by presenting essential residues to the host cell receptor. These studies facilitate the screening of Env libraries for functional entry into target cells, allowing the identification of novel gammaretroviral Envs targeting alternative host cell receptors for gene and protein delivery.

Ancestral Mutations Acquired in Refrex-1, a Restriction Factor against Feline Retroviruses, during its Cooption and Domestication

Endogenous retroviruses (ERVs) are remnants of ancestral retroviral infections of germ cells. Retroviral endogenization is an adaptation process for the host genome, and ERVs are gradually attenuated or inactivated by mutation. However, some ERVs that have been "domesticated" by their hosts eventually gain physiological functions, such as placentation or viral resistance. We previously reported the discovery of Refrex-1, a soluble antiretroviral factor in domestic cats that specifically inhibits infection by feline leukemia virus subgroup D (FeLV-D), a chimeric virus of FeLV, and a feline ERV, ERV-DC. Refrex-1 is a truncated envelope protein (Env) encoded by both ERV-DC7 and ERV-DC16 proviral loci. Here, we reconstituted ancestral and functional Env from ERV-DC7 and ERV-DC16 envelope genes (env) by inducing reverse mutations. Unexpectedly, ERV-DC7 and ERV-DC16 full-length Env (ERV-DC7 fl and ERV-DC16 fl), reconstructed by removing stop codons, did not produce infectious viral particles. ERV-DC7 fl and ERV-DC16 fl were highly expressed in cells but were not cleaved into surface subunits (SU) and transmembrane subunits, nor were they incorporated into virions. G407R/N427I-A429T and Y431D substitutions within the SU C-terminal domain of ERV-DC7 fl and ERV-DC16 fl, respectively, caused these dysfunctions. The residues glycine 407 and tyrosine 431 are relatively conserved among infectious gammaretroviruses, and their substitution causes the same dysfunctions as the tested retroviruses. Our results reveal that specific mutations within the SU C-terminal domain suppressed Env cleavage and incorporation into virions and indicate that these mutations contributed to the domestication of Refrex-1 through multistep events that occurred in the postintegration period.

IMPORTANCE

Domestic cats are colonized with various exogenous retroviruses (exRVs), such as feline leukemia virus (FeLV), and their genomes contain numerous ERVs, some of which are replication-competent proviruses. The feline hosts, exRVs, and ERVs have complicated genetic interactions and provide an interesting field model for triangular relationships: recombination between FeLV and ERV-DC, which is a feline ERV, generated FeLV-D, a chimeric virus, and FeLV-D is restricted by Refrex-1, an antiretroviral factor corresponding to truncated Env of ERV-DC7 and ERV-DC16. Here, we reconstructed ancestral, functional Env from ERV-DC7 and ERV-DC16 env by inducing reverse mutations to elucidate how Refrex-1 was generated from its ancestor. Our results reveal that they were repeatedly inactivated by mutations preventing Env maturation. Our results provide insights into how ERVs were "domesticated" by their hosts and identify the mutations that mediated these evolutions. Notably, experiments that restore inactivated ERVs might uncover previously unrecognized features or properties of retroviruses.

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.

Postinhibitory rebound neurons and networks are disrupted in retrovirus-induced spongiform neurodegeneration

Certain retroviruses induce progressive spongiform motor neuron disease with features resembling prion diseases and amyotrophic lateral sclerosis. With the neurovirulent murine leukemia virus (MLV) FrCasE, Env protein expression within glia leads to postsynaptic vacuolation, cellular effacement, and neuronal loss in the absence of neuroinflammation. To understand the physiological changes associated with MLV-induced spongiosis, and its neuronal specificity, we employed patch-clamp recordings and voltage-sensitive dye imaging in brain slices of the mouse inferior colliculus (IC), a midbrain nucleus that undergoes extensive spongiosis. IC neurons characterized by postinhibitory rebound firing (PIR) were selectively affected in FrCasE-infected mice. Coincident with Env expression in microglia and in glia characterized by NG2 proteoglycan expression (NG2 cells), rebound neurons (RNs) lost PIR, became hyperexcitable, and were reduced in number. PIR loss and hyperexcitability were reversed by raising internal calcium buffer concentrations in RNs. PIR-initiated rhythmic circuits were disrupted, and spontaneous synchronized bursting and prolonged depolarizations were widespread. Other IC neuron cell types and circuits within the same degenerative environment were unaffected. Antagonists of NMDA and/or AMPA receptors reduced burst firing in the IC but did not affect prolonged depolarizations. Antagonists of L-type calcium channels abolished both bursts and slow depolarizations. IC infection by the nonneurovirulent isogenic virus Friend 57E (Fr57E), whose Env protein is structurally similar to FrCasE, showed no RN hyperactivity or cell loss; however, PIR latency increased. These findings suggest that spongiform neurodegeneration arises from the unique excitability of RNs, their local regulation by glia, and the disruption of this relationship by glial expression of abnormal protein.

Furin cleavage of the Moloney murine leukemia virus Env precursor reorganizes the spike structure

The trimeric Moloney murine leukemia virus Env protein matures by two proteolytic cleavages. First, furin cleaves the Env precursor into the surface (SU) and transmembrane (TM) subunits in the cell and then the viral protease cleaves the R-peptide from TM in new virus. Here we analyzed the structure of the furin precursor, by cryoelectron microscopy. We transfected 293T cells with a furin cleavage site provirus mutant, R466G/K468G, and produced the virus in the presence of amprenavir to also inhibit the R-peptide cleavage. Although Env incorporation into particles was inhibited, enough precursor could be isolated and analyzed by cryoelectron microscopy to yield a 3D structure at 22 Å resolution. This showed an open cage-like structure like that of the R-peptide precursor and the mature Env described before. However, the middle protrusion of the protomeric unit, so prominently pointing out from the side of the more mature forms of the Env, was absent. Instead, there was extra density in the top protrusion. This suggested that the C-terminal SU domain was associated alongside the receptor binding N-terminal SU domain in the furin precursor. This was supported by mapping with a SU C-terminal domain-specific antigen binding fragment. We concluded that furin cleavage not only separates the subunits and liberates the fusion peptide at the end of TM but also allows the C-terminal domain to relocate into a peripheral position. This conformational change might explain how the C-terminal domain of SU gains the potential to undergo disulfide isomerization, an event that facilitates membrane fusion.

Novel bifunctional single-chain variable antibody fragments to enhance virolysis by complement: generation and proof-of-concept

When bound to the envelope of viruses, factor H (FH), a soluble regulator of complement activation, contributes to the protection against a potent immune defense mechanism, the complement-mediated lysis (CML). Thus, removing FH from the surface renders viruses, such as HIV, susceptible to CML. For a proof of concept, we developed a construct consisting of recombinant bifunctional single-chain variable fragment (scFv) based on a monoclonal antibody against Friend murine leukemia virus (F-MuLV) envelope protein gp70, which was coupled to specific binding domains (short consensus repeats 19-20; SCR1920) of FH. We used Pichia pastoris as expression system in common shake flasks and optimized expression in high density bench top fermentation. Specific binding of recombinant scFv was proven by flow cytometry. The recombinant scFv-SCR significantly enhanced CML of F-MuLV in vitro implying that FH binding to the viral surface was impaired by the scFv-SCR. This novel concept to enhance virolysis may provide a new approach for antiviral treatment.

The binding of zinc ions to Emericella nidulans endo-β-1,4-galactanase is essential for crystal formation

A novel Emericella nidulans endo-β-1,4-galactanase (EnGAL) demonstrates a strong capacity to generate high levels of very potent prebiotic oligosaccharides from potato pulp, a by-product of the agricultural potato-starch industry. EnGAL belongs to glycoside hydrolase family 53 and shows high (72.5%) sequence identity to an endo-β-1,4-galactanase from Aspergillus aculeatus. Diffraction data extending to 2.0 Å resolution were collected from a crystal of EnGAL grown from conditions containing 0.2 M zinc acetate. The crystal structure showed a high similarity between EnGAL and other endo-β-1,4-galactanases belonging to GH53. It also revealed 15 zinc ions bound to the protein, one of which is located in the active site, where it is coordinated by residues Glu136 and Glu246 which comprise the catalytic machinery. The majority of the zinc ions are located on the surface of the enzyme, in some cases with side chains from two different molecules as ligands, thus explaining why the presence of zinc ions was essential for crystallization.

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.

Second site mutation in the virus envelope expands the host range of a cytopathic variant of Moloney murine leukemia virus

Spl574 MLV (murine leukemia virus) is a variant of Moloney ecotropic MLV (MoMLV) that is cytopathic in Mus dunni cells and restricted by other mouse cells. Its host range and cytopathicity are due to a mutation, S82F, at a site critical for binding to the CAT-1 receptor. To identify residues that affect affinity for receptor variants, virus with S82F was passed in restrictive cells. The env genes of the adapted viruses contained 18 novel mutations, including one, E114G, present in 6 of 30 sequenced envs. MoMLV-E114G efficiently infected all mouse cells as well as ecotropic MLV resistant Chinese hamster cells. Virus with E114G and S82F induced large multinucleated syncytia in NIH 3T3 and SC-1 cells as well as M. dunni cells. Inoculation of Mo-S82F,E114G into mice produced lymphomas typical of MoMLV. Residues at env position 114 are thus important determinants of host range, and E114G suppresses host range restriction due to S82F, but does not affect S82F-governed cytopathicity.

One hundred twenty years of koala retrovirus evolution determined from museum skins

Although endogenous retroviruses are common across vertebrate genomes, the koala retrovirus (KoRV) is the only retrovirus known to be currently invading the germ line of its host. KoRV is believed to have first infected koalas in northern Australia less than two centuries ago. We examined KoRV in 28 koala museum skins collected in the late 19th and 20th centuries and deep sequenced the complete proviral envelope region from five northern Australian specimens. Strikingly, KoRV env sequences were conserved among koalas collected over the span of a century, and two functional motifs that affect viral infectivity were fixed across the museum koala specimens. We detected only 20 env polymorphisms among the koalas, likely representing derived mutations subject to purifying selection. Among northern Australian koalas, KoRV was already ubiquitous by the late 19th century, suggesting that KoRV evolved and spread among koala populations more slowly than previously believed. Given that museum and modern koalas share nearly identical KoRV sequences, it is likely that koala populations, for more than a century, have experienced increased susceptibility to diseases caused by viral pathogenesis.

Construction of a gammaretrovirus with a novel tropism and wild-type replication kinetics capable of using human APJ as entry receptor

We have constructed a replication-competent gammaretrovirus (SL3-AP) capable of using the human G-protein-coupled receptor hAPJ as its entry receptor. The envelope protein of the virus was made by insertion of the 13-amino-acid peptide ligand for hAPJ, flanked by linker sequences, into one of the variable loops of the receptor binding domain of SL3-2, a murine leukemia virus (MLV) that uses the xenotropic-polytropic virus receptor Xpr1 and which has a host range limited to murine cells. This envelope protein can utilize hAPJ as well as murine Xpr1 for entry into host cells with equal efficiencies. In addition, the SL3-AP virus replicates in cells expressing either of its receptors, hAPJ and murine Xpr1, and causes resistance to superinfection and downregulation of hAPJ in infected cells. Thus, SL3-AP is the first example of a retargeted replication-competent retrovirus, with replication characteristics and receptor interference properties similar to those of natural isolates.

Phylogenetic and biological analysis of a laboratory-generated gammaretrovirus xenotropic murine leukemia virus-related virus (XMRV)

A xenotropic murine leukemia virus-related virus (XMRV) has been reported to be an emerging pathogen associated with prostate cancer (PC) and chronic fatigue syndrome (CFS). However, recent studies have demonstrated that XMRV is a laboratory-derived virus resulting from genetic recombination between two mouse viral genomes during serial xenograft tissue transplantation. This study describes a phylogenetic analysis that compared XMRV with the ecotropic murine leukemia viruses (E-MLV), xenotropic MLV (X-MLV), and other retroviruses, including HTLV-1 and HIV-1. We found that sequences corresponding to three XMRV structural proteins (Env, Gag, and Pol) exhibited high degrees of homology with X-MLV (>91 %) and E-MLV (67-96 %), but not HTLV-1 (13-16 %) or HIV-1 (10-15 %), indicating that XMRV was derived from X-MLV and/or E-MLV. We then compared the infectivity of XMRV and E-MLV for human and murine lymphocytes, respectively. Results showed that human PBMCs were not susceptible to XMRV infection, suggesting that XMRV exhibits host cell tropism similar to E-MLV that only infects murine PBMCs. These data suggest that it is unlikely that this laboratory-generated retrovirus could cause disease in humans.

Maturation cleavage of the murine leukemia virus Env precursor separates the transmembrane subunits to prime it for receptor triggering

The Env protein of murine leukemia virus matures by two cleavage events. First, cellular furin separates the receptor binding surface (SU) subunit from the fusion-active transmembrane (TM) subunit and then, in the newly assembled particle, the viral protease removes a 16-residue peptide, the R-peptide from the endodomain of the TM. Both cleavage events are required to prime the Env for receptor-triggered activation. Cryoelectron microscopy (cryo-EM) analyses have shown that the mature Env forms an open cage-like structure composed of three SU-TM complexes, where the TM subunits formed separated Env legs. Here we have studied the structure of the R-peptide precursor Env by cryo-EM. TM cleavage in Moloney murine leukemia virus was inhibited by amprenavir, and the Envs were solubilized in Triton X-100 and isolated by sedimentation in a sucrose gradient. We found that the legs of the R-peptide Env were held together by trimeric interactions at the very bottom of the Env. This suggested that the R-peptide ties the TM legs together and that this prevents the activation of the TM for fusion. The model was supported by further cryo-EM studies using an R-peptide Env mutant that was fusion-competent despite an uncleaved R-peptide. The Env legs of this mutant were found to be separated, like in the mature Env. This shows that it is the TM leg separation, normally caused by R-peptide cleavage, that primes the Env for receptor triggering.

Long-term infection and vertical transmission of a gammaretrovirus in a foreign host species

Increasing evidence has indicated natural transspecies transmission of gammaretroviruses; however, viral-host interactions after initial xeno-exposure remain poorly understood. Potential association of xenotropic murine leukemia virus-related virus (XMRV) in patients with prostate cancer and chronic fatigue syndrome has attracted broad interests in this topic. Although recent studies have indicated that XMRV is unlikely a human pathogen, further understanding of XMRV xenoinfection would allow in vivo modeling of the initial steps of gammaretroviral interspecies transmission, evolution and dissemination in a new host population. In this study, we monitored the long-term consequences of XMRV infection and its possible vertical transmission in a permissive foreign host, wild-derived Mus pahari mice. One year post-infection, XMRV-infected mice showed no notable pathological changes, while proviral DNA was detected in three out of eight mice. XMRV-infected mice remained seropositive throughout the study although the levels of gp70 Env- and p30 capsid-specific antibodies gradually decreased. When vertical XMRV transmission was assessed, no viremia, humoral immune responses nor endogenization were observed in nine offspring from infected mothers, yet one offspring was found PCR-positive for XMRV-specific sequences. Amplified viral sequences from the offspring showed several mutations, including one amino acid deletion in the receptor binding domain of Env SU. Our results therefore demonstrate long-term asymptomatic infection, low incidence of vertical transmission and limited evolution of XMRV upon transspecies infection of a permissive new host, Mus pahari.

O-sulfate groups of heparin are critical for inhibition of ecotropic murine leukemia virus infection by heparin

There is increasing evidence that soluble glycosaminoglycans such as heparin can interfere with the infectivity of various viruses, including ecotropic murine leukemia viruses (MLVs). The ecotropic MLV, Friend MLV (F-MLV) and the neuropathogenic variants A8 MLV and PVC-211 MLV, were susceptible to heparin-mediated inhibition of infection of NIH 3T3 cells. To investigate the interaction between the envelope glycoprotein (Env) of MLV and heparin, we prepared vesicular stomatitis virus-based pseudotyped viruses carrying the Env of F-, A8, or PVC-211 MLVs. Surface plasmon resonance analyses indicated that the Env of A8 and PVC-211 MLVs had a higher binding activity to heparin than that of F-MLV. We examined the influence of N- or O-sulfation of heparin on binding activity to Env and on the inhibition of the infectivity of MLV and pseudotyped viruses carrying Env. This analysis indicated that the O-sulfate groups of heparin play a major role in determining Env-dependent inhibitory effects.

Narrowing down the critical region within env gene for determining neuropathogenicity of murine leukemia virus A8

Friend murine leukemia virus clone A8 causes spongiform neurodegeneration in the rat brain, and the env gene of A8 is a primary determinant of neuropathogenicity. In order to narrow down the critical region within the env gene that determines neuropathogenicity, we constructed chimeric viruses having chimeric env between A8 and non-neuropathogenic 57 on the background of A8 virus. After replacement of the BamHI (at nucleotide 5715)-AgeI (at nucleotide 6322) fragment of A8 virus with the corresponding fragment of 57, neuropathogenicity was lost. In contrast, the chimeric viruses that have the BamHI (5715)-AgeI (6322) fragment of A8 induced spongiosis in 100% of infected rats at the same or slightly lower intensity than A8 virus. These results indicate that the BamHI (5715)-AgeI (6322) fragment of A8, which contains the signal sequence and the N-terminal half of RBD, is crucial for the induction of spongiform neurodegeneration. In the BamHI (5715)-AgeI (6322) fragment, seven amino acids differed between A8 and 57, one in the signal sequence and six in RBD, which suggests that these amino acids significantly contribute to the neuropathogenicity of A8.

GRB2 interaction with the ecotropic murine leukemia virus receptor, mCAT-1, controls virus entry and is stimulated by virus binding

For retroviruses such as HIV-1 and murine leukemia virus (MLV), active receptor recruitment and trafficking occur during viral entry. However, the underlying mechanisms and cellular factors involved in the process are largely uncharacterized. The viral receptor for ecotropic MLV (eMLV), a classical model for retrovirus infection mechanisms and pathogenesis, is mouse cationic amino acid transporter 1 (mCAT-1). Growth factor receptor-bound protein 2 (GRB2) is an adaptor protein that has been shown to couple cell surface receptors, such as epidermal growth factor receptor (EGFR) and hepatocyte growth factor receptor, to intracellular signaling events. Here we examined if GRB2 could also play a role in controlling infection by retroviruses by affecting receptor function. The GRB2 RNA interference (RNAi)-mediated suppression of endogenous GRB2 resulted in a consistent and significant reduction of virus binding and membrane fusion. The binding between eMLV and cells promoted increased GRB2-mCAT-1 interactions, as detected by immunoprecipitation. Consistently, the increased colocalization of GRB2 and mCAT-1 signals was detected by confocal microscopy. This association was time dependent and paralleled the kinetics of cell-virus membrane fusion. Interestingly, unlike the canonical binding pattern seen for GRB2 and growth factor receptors, GRB2-mCAT-1 binding does not depend on the GRB2-SH2 domain-mediated recognition of tyrosine phosphorylation on the receptor. The inhibition of endogenous GRB2 led to a reduction in surface levels of mCAT-1, which was detected by immunoprecipitation and by a direct binding assay using a recombinant MLV envelope protein receptor binding domain (RBD). Consistent with this observation, the expression of a dominant negative GRB2 mutant (R86K) resulted in the sequestration of mCAT-1 from the cell surface into intracellular vesicles. Taken together, these findings suggest a novel role for GRB2 in ecotropic MLV entry and infection by facilitating mCAT-1 trafficking.

Murine leukemia viruses: objects and organisms

Murine leukemia viruses (MLVs) are among the simplest retroviruses. Prototypical gammaretroviruses encode only the three polyproteins that will be used in the assembly of progeny virus particles. These are the Gag polyprotein, which is the structural protein of a retrovirus particle, the Pol protein, comprising the three retroviral enzymes—protease, which catalyzes the maturation of the particle, reverse transcriptase, which copies the viral RNA into DNA upon infection of a new host cell, and integrase, which inserts the DNA into the chromosomal DNA of the host cell, and the Env polyprotein, which induces the fusion of the viral membrane with that of the new host cell, initiating infection. In general, a productive MLV infection has no obvious effect upon host cells. Although gammaretroviral structure and replication follow the same broad outlines as those of other retroviruses, we point out a number of significant differences between different retroviral genera.

Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity

Gammaretroviruses of several different host range subgroups have been isolated from laboratory mice. The ecotropic viruses infect mouse cells and rely on the host CAT-1 receptor. The xenotropic/polytropic viruses, and the related human-derived XMRV, can infect cells of other mammalian species and use the XPR1 receptor for entry. The coevolution of these viruses and their receptors in infected mouse populations provides a good example of how genetic conflicts can drive diversifying selection. Genetic and epigenetic variations in the virus envelope glycoproteins can result in altered host range and pathogenicity, and changes in the virus binding sites of the receptors are responsible for host restrictions that reduce virus entry or block it altogether. These battleground regions are marked by mutational changes that have produced 2 functionally distinct variants of the CAT-1 receptor and 5 variants of the XPR1 receptor in mice, as well as a diverse set of infectious viruses, and several endogenous retroviruses coopted by the host to interfere with entry.

Targeted entry via somatostatin receptors using a novel modified retrovirus glycoprotein that delivers genes at levels comparable to those of wild-type viral glycoproteins

Here we report a novel viral glycoprotein created by replacing a natural receptor-binding sequence of the ecotropic Moloney murine leukemia virus envelope glycoprotein with the peptide ligand somatostatin. This new chimeric glycoprotein, which has been named the Sst receptor binding site (Sst-RBS), gives targeted transduction based on three criteria: (i) a gain of the use of a new entry receptor not used by any known virus; (ii) targeted entry at levels comparable to gene delivery by wild-type ecotropic Moloney murine leukemia virus and vesicular stomatitis virus (VSV) G glycoproteins; and (iii) a loss of the use of the natural ecotropic virus receptor. Retroviral vectors coated with Sst-RBS gained the ability to bind and transduce human 293 cells expressing somatostatin receptors. Their infection was specific to target somatostatin receptors, since a synthetic somatostatin peptide inhibited infection in a dose-dependent manner and the ability to transduce mouse cells bearing the natural ecotropic receptor was effectively lost. Importantly, vectors coated with the Sst-RBS glycoprotein gave targeted entry of up to 1 × 10(6) transducing U/ml, a level comparable to that seen with infection of vectors coated with the parental wild-type ecotropic Moloney murine leukemia virus glycoprotein through the ecotropic receptor and approaching that of infection of VSV G-coated vectors through the VSV receptor. To our knowledge, this is the first example of a glycoprotein that gives targeted entry of retroviral vectors at levels comparable to the natural capacity of viral envelope glycoproteins.

Molecular aspects of HTLV-1 entry: functional domains of the HTLV-1 surface subunit (SU) and their relationships to the entry receptors

The initial step in retroviral infection involves specific interactions between viral envelope proteins (Env) and specific receptors on the surface of target cells. For many years, little was known about the entry receptors for HTLV-1. During this time, however, functional domains of the HTLV-1 Env were identified by analyzing the effects of neutralizing antibodies and specific mutations in Env on HTLV-1 infectivity. More recent studies have revealed that HTLV-1 infectivity involves interactions with three different molecules: heparan sulfate proteoglycans (HSPG), the VEGF-165 receptor Neuropilin 1 (NRP-1) and glucose transporter type 1 (GLUT1). Here, we revisit previously published data on the functional domains of Env in regard to the recent knowledge acquired about this multi-receptor complex. We also discuss the similarities and differences between HTLV-1 and other deltaretroviruses in regards to receptor usage.

Misfolding of CasBrE SU is reversed by interactions with 4070A Env: implications for gammaretroviral neuropathogenesis

Background

CasBrE is a neurovirulent murine leukemia virus (MLV) capable of inducing paralytic disease with associated spongiform neurodegeneration. The neurovirulence of this virus has been genetically mapped to the surface expressed subunit (SU) of the env gene. However, CasBrE SU synthesized in the absence of the transmembrane subunit (TM) does not retain ecotropic receptor binding activity, indicating that folding of the receptor binding domain (RBD) requires this domain. Using a neural stem cell (NSC) based viral trans complementation approach to examine whether misfolded CasBrE SU retained neurovirulence, we observed CasBrE SU interaction with the "non-neurovirulent" amphotropic helper virus, 4070A which restored functional activity of CasBrE SU.

Results

Herein, we show that infection of NSCs expressing CasBrE SU with 4070A (CasES+4070A-NSCs) resulted in the redistribution of CasBrE SU from a strictly secreted product to include retention on the plasma membrane. Cell surface cross-linking analysis suggested that CasBrE SU membrane localization was due to interactions with 4070A Env. Viral particles produced from CasES+4070A-NSCS contained both CasBrE and 4070A gp70 Env proteins. These particles displayed ecotropic receptor-mediated infection, but were still 100-fold less efficient than CasE+4070A-NSC virus. Infectious center analysis showed CasBrE SU ecotropic transduction efficiencies approaching those of NSCs expressing full length CasBrE Env (CasE; SU+TM). In addition, CasBrE SU-4070A Env interactions resulted in robust ecotropic superinfection interference indicating near native intracellular SU interaction with its receptor, mCAT-1.

Conclusions

In this report we provided evidence that 4070A Env and CasBrE SU physically interact within NSCs leading to CasBrE SU retention on the plasma membrane, incorporation into viral particles, restoration of mCAT-1 binding, and capacity for initiation of TM-mediated fusion events. Thus, heterotropic Env-SU interactions facilitates CasBrE SU folding events that restore Env activity. These findings are consistent with the idea that one protein conformation acts as a folding scaffold or nucleus for a second protein of similar primary structure, a process reminiscent of prion formation. The implication is that template-based protein folding may represent an inherent feature of neuropathogenic proteins that extends to retroviral Envs.

Coupling of receptor interference and a host-dependent post-binding entry deficiency in a gammaretroviral envelope protein

Background

SL3-2 is a unique polytropic murine gammaretroviral isolate that is only able to infect murine cells. We have previously shown that two mutations R212G and T213I located on the surface of the receptor binding domain in a region designated the VR3 loop can alter the species tropism of this envelope protein. This location suggests that the VR3 loop composition has an influence on receptor interaction and thereby affects binding as well as superinfection resistance. In order to investigate this further, we have studied the binding and interference patterns of the SL3-2 envelope and its mutants.

Results

We find unexpectedly that wild type SL3-2 envelope binds equally well to both permissive and non-permissive cells, indicating a post binding defect when interacting with the human Xpr1. Using replication competent viruses containing envelopes from SL3-2 or its mutants we find that the same amino acid mutations can dramatically alter the interference profile of this polytropic ENV, suggesting that the same amino acid changes that cause the post binding defect also influence interaction with the receptor.

Conclusions

The envelope protein of SL3-2 MLV shows an entry defect on non-murine cells. This is coupled to a dramatically reduced ability to interfere with entry of other polytropic viruses. Two point mutations in the VR3 loop of the receptor binding domain of this envelope result both in a much increased interference ability and in removing the post-binding defect on non-murine cells, suggesting that both of these phenotypes are a consequence of insufficient interaction between the envelope and the receptor

Host range and cellular tropism of the human exogenous gammaretrovirus XMRV

Recently, the first human infection with an exogenous gammaretrovirus (XMRV) was reported. In its initial description, XMRV was confined to prostate stromal fibroblasts, although subsequent reports demonstrated XMRV protein expression in prostate epithelial cells. Most recently, XMRV has been detected in blood cells of patients with chronic fatigue syndrome. The aim of this study was to elucidate the transmission routes and tissue tropism of XMRV by comparing its host range, receptor usage and LTR functionality with other MLV isolates. We demonstrate using pseudotype experiments that XMRV Env mediates efficient infection of cells from different species. We show that replication competent XMRV infects various human cell types, including hematopoietic cell lines and prostate stromal fibroblasts. XMRV-LTR activity is significantly higher in the prostate cancer cell line LNCaP and in prostate stromal fibroblasts, compared to other cell types tested and could be one factor contributing to efficient viral spread in prostate tissue.

Removal of either N-glycan site from the envelope receptor binding domain of Moloney and Friend but not AKV mouse ecotropic gammaretroviruses alters receptor usage

Three N-linked glycosylation sites were removed from the envelope glycoproteins of Friend, Moloney, and AKV mouse ecotropic gammaretroviruses: gs1 and gs2, in the receptor binding domain; and gs8, in a region implicated in post-binding cell fusion. Mutants were tested for their ability to infect rodent cells expressing 4 CAT-1 receptor variants. Three mutants (Mo-gs1, Mo-gs2, and Fr-gs1) infect NIH 3T3 and rat XC cells, but are severely restricted in Mus dunni cells and Lec8, a Chinese hamster cell line susceptible to ecotropic virus. This restriction is reproduced in ferret cells expressing M. dunni dCAT-1, but not in cells expressing NIH 3T3 mCAT-1. Virus binding assays, pseudotype assays, and the use of glycosylation inhibitors further suggest that restriction is primarily due to receptor polymorphism and, in M. dunni cells, to glycosylation of cellular proteins. Virus envelope glycan size or type does not affect infectivity. Thus, host range variation due to N-glycan deletion is receptor variant-specific, cell-specific, virus type-specific, and glycan site-specific.

The degree of folding instability of the envelope protein of a neurovirulent murine retrovirus correlates with the severity of the neurological disease

A small group of ecotropic murine retroviruses cause a spongiform neurodegenerative disease manifested by tremor, paralysis, and wasting. The neurovirulence of these viruses has long been known to be determined by the sequence of the viral envelope protein, although the nature of the neurotoxicity remains to be clarified. Studies on the neurovirulent viruses FrCas(NC) and Moloney murine leukemia virus ts1 indicate that the nascent envelope protein misfolds, is retained in the endoplasmic reticulum (ER), and induces an unfolded protein response. In the present study we constructed a series of viruses with chimeric envelope genes containing segments from virulent and avirulent retroviruses. Each of the viruses studied was highly neuroinvasive but differed in the severity of the neurological disease they induced. Only viruses that contained the receptor-binding domain (RBD) of the neurovirulent virus induced neurological disease. Likewise, only viruses containing the RBD of the neurovirulent virus exhibited increased binding of the ER chaperone BiP to the envelope precursor protein and induced the unfolded protein response. Thus, the RBD determined both neurovirulence and folding instability. Among viruses carrying the neurovirulent RBD, the severity of the disease was increased when envelope sequences from the neurovirulent virus outside the RBD were also present. Interestingly, these sequences appeared to further increase the degree of folding instability (BiP binding) of the viral envelope protein. These results provide strong support for the hypothesis that this spongiform neurodegenerative disease represents a virus-induced protein folding disorder.

Turning of the receptor-binding domains opens up the murine leukaemia virus Env for membrane fusion

The activity of the membrane fusion protein Env of Moloney mouse leukaemia virus is controlled by isomerization of the disulphide that couples its transmembrane (TM) and surface (SU) subunits. We have arrested Env activation at a stage prior to isomerization by alkylating the active thiol in SU and compared the structure of isomerization-arrested Env with that of native Env. Env trimers of respective form were isolated from solubilized particles by sedimentation and their structures were reconstructed from electron microscopic images of both vitrified and negatively stained samples. We found that the protomeric unit of both trimers formed three protrusions, a top, middle and a lower one. The atomic structure of the receptor-binding domain of SU fitted into the upper protrusion. This was formed similar to a bent finger. Significantly, in native Env the tips of the fingers were directed against each other enclosing a cavity below, whereas they had turned outward in isomerization-arrested Env transforming the cavity into an open well. This might subsequently guide the fusion peptides in extended TM subunits into the target membrane.

Binding of equine infectious anemia virus to the equine lentivirus receptor-1 is mediated by complex discontinuous sequences in the viral envelope gp90 protein

The identification and characterization of a functional cellular receptor for equine infectious anemia virus (EIAV), designated equine lentivirus receptor-1 (ELR1), a member of the tumour necrosis factor receptor protein family, has been reported previously [Zhang, B. et al. (2005). Proc Natl Acad Sci U S A, 102 , 9918-9923]. The finding of a single receptor for EIAV is distinct from feline, simian and human immunodeficiency viruses, which typically utilize two co-receptors for infection, but is similar to avian and murine oncoviruses, which use single receptors. This study sought to determine ELR1-binding domains of EIAV gp90. Towards this goal, a GFP-tagged gp90 fusion protein (gp90GFP) expression vector was constructed and a specific cell-cell binding assay was developed to measure EIAV gp90 binding to ELR1. Using these assays, the receptor-binding properties of 41 gp90GFP mutants were evaluated, each with a sequential replacement 11 aa linear epitope peptide from the vesicular stomatitis virus glycoprotein (VSV-G tag), as well as eight mutants containing individual gp90 variable-domain deletions. The results of these studies demonstrated that, in general, gp90 constructs containing substitutions or deletions in the N-terminal third of gp90 retained their receptor-binding activity. In contrast, segment substitutions or deletions in the C-terminal two-thirds of gp90 eliminated receptor-binding activity. Thus, these results reveal for the first time that the ELR1-binding domains of EIAV gp90 are located in the C-terminal two-thirds of EIAV gp90, apparently as a complex of discontinuous determinants.

Neurovirulence of polytropic murine retrovirus is influenced by two separate regions on opposite sides of the envelope protein receptor binding domain

Changes in the envelope proteins of retroviruses can alter the ability of these viruses to infect the central nervous system (CNS) and induce neurological disease. In the present study, nine envelope residues were found to influence neurovirulence of the Friend murine polytropic retrovirus Fr98. When projected on a three-dimensional model, these residues were clustered in two spatially separated groups, one in variable region B of the receptor binding site and the other on the opposite side of the envelope. Further studies indicated a role for these residues in virus replication in the CNS, although the residues did not affect viral entry.

Receptor-induced thiolate couples Env activation to retrovirus fusion and infection

According to current models of retrovirus infection, receptor binding to the surface subunit (SU) of the envelope glycoprotein (Env) triggers a conformational change in the transmembrane subunit (TM) that mediates virus fusion to cell membranes. To understand how this occurs, we investigated the role of the receptor Tva in avian leukosis virus-A (ALV-A) infection. We find that Tva binding induced the formation of a reactive thiolate on Cys38 (Cys38-S- in SU. Both chemical and genetic inactivation of Cys38-S- completely abrogated ALV fusion and infection. Remarkably, Cys38-S- does not mediate isomerization of the SU-TM disulfide bond and is not required for Tva-induced activation of TM, including pre-hairpin association with membranes and low pH assembly of helical bundles. These findings indicate that, contrary to current models, receptor activation of TM is not sufficient for ALV fusion and infection and that formation of a reactive thiolate is an additional receptor-dependent step.