Fusion-defective gibbon ape leukemia virus vectors can be rescued by homologous but not heterologous soluble envelope proteins

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.

Pseudotype-based neutralization assays for influenza: a systematic analysis

The use of vaccination against the influenza virus remains the most effective method of mitigating the significant morbidity and mortality caused by this virus. Antibodies elicited by currently licensed influenza vaccines are predominantly hemagglutination-inhibition (HI)-competent antibodies that target the globular head of hemagglutinin (HA) thus inhibiting influenza virus entry into target cells. These antibodies predominantly confer homosubtypic/strain specific protection and only rarely confer heterosubtypic protection. However, recent academia or pharma-led R&D toward the production of a “universal vaccine” has centered on the elicitation of antibodies directed against the stalk of the influenza HA that has been shown to confer broad protection across a range of different subtypes (H1–H16). The accurate and sensitive measurement of antibody responses elicited by these “next-generation” influenza vaccines is, however, hampered by the lack of sensitivity of the traditional influenza serological assays HI, single radial hemolysis, and microneutralization. Assays utilizing pseudotypes, chimeric viruses bearing influenza glycoproteins, have been shown to be highly efficient for the measurement of homosubtypic and heterosubtypic broadly neutralizing antibodies, making them ideal serological tools for the study of cross-protective responses against multiple influenza subtypes with pandemic potential. In this review, we will analyze and compare literature involving the production of influenza pseudotypes with particular emphasis on their use in serum antibody neutralization assays. This will enable us to establish the parameters required for optimization and propose a consensus protocol to be employed for the further deployment of these assays in influenza vaccine immunogenicity studies.

A soluble envelope protein of endogenous retrovirus (FeLIX) present in serum of domestic cats mediates infection of a pathogenic variant of feline leukemia virus

T-lymphotropic feline leukemia virus (FeLV-T), a highly pathogenic variant of FeLV, induces severe immunosuppression in cats. FeLV-T is fusion defective because in its PHQ motif, a gammaretroviral consensus motif in the N terminus of an envelope protein, histidine is replaced with aspartate. Infection by FeLV-T requires FeLIX, a truncated envelope protein encoded by an endogenous FeLV, for transactivation of infectivity and Pit1 for binding FeLIX. Although Pit1 is present in most tissues in cats, the expression of FeLIX is limited to certain cells in lymphoid organs. Therefore, the host cell range of FeLV-T was thought to be restricted to cells expressing FeLIX. However, because FeLIX is a soluble factor and is expressed constitutively in lymphoid organs, we presumed it to be present in blood and evaluated its activities in sera of various mammalian species using a pseudotype assay. We demonstrated that cat serum has FeLIX activity at a functional level, suggesting that FeLIX is present in the blood and that FeLV-T may be able to infect cells expressing Pit1 regardless of the expression of FeLIX in vivo. In addition, FeLIX activities in sera were detected only in domestic cats and not in other feline species tested. To our knowledge, this is the first report to prove that a large amount of truncated envelope protein of endogenous retrovirus is circulating in the blood to facilitate the infection of a pathogenic exogenous retrovirus.

A new site and mechanism of action for the widely used adenylate cyclase inhibitor SQ22,536

We evaluated the efficacy, potency, and selectivity of the three most commonly used adenylate cyclase (AC) inhibitors in a battery of cell lines constructed to study signaling via three discrete cAMP sensors identified in neuroendocrine cells. SQ22,536 [9-(tetrahydrofuryl)-adenine] and 2',5'-dideoxyadenosine (ddAd) are effective and potent AC inhibitors in HEK293 cells expressing a cAMP response element (CRE) reporter gene, and MDL-12,330A [cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine hydrochloride] is not. Neuroscreen-1 (NS-1) cells were used to assess the specificity of the most potent AC inhibitor, SQ22,536, to block downstream cAMP signaling to phosphorylate CREB (via PKA); to activate Rap1 (via Epac); and to activate ERK signaling leading to neuritogenesis (via the newly described neuritogenic cAMP sensor NCS). SQ22,536 failed to inhibit the effects of cAMP analogs 8-Br-cAMP and 8-CPT-2'-O-Me-cAMP on PKA-mediated CREB activation/phosphorylation and Epac-mediated Rap1 activation, indicating that it does not inhibit these cAMP pathways beyond the level of AC. On the other hand, SQ22,536, but not ddAd, inhibited the effects of cAMP analogs 8-Br-cAMP and 8-CPT-cAMP on ERK phosphorylation and neuritogenesis, indicating that it acts not only as an AC blocker, but also as an inhibitor of the NCS. The observed off-target actions of SQ22,536 are specific to cAMP signaling: SQ22,536 does not block the actions of compounds not related to cAMP signaling, including ERK induction by PMA, and ERK activation and neuritogenesis induced by NGF. These data led us to indicate a second target for SQ22,536 that should be considered when interpreting its effects in whole cell and in vivo experiments.

A mutant retroviral receptor restricts virus superinfection interference and productive infection

Background

Both cell-free and cell-associated infection routes are important for retroviral dissemination. Regardless of the mechanism, the driving force of retroviral entry is the interaction between the viral envelope and its receptor. To date it remains unclear how decreased affinity of viruses for their receptors affects viral cell-free infection, cell-cell transmission, and spreading kinetics. We have previously characterized a mutant form of the amphotropic murine retrovirus receptor human phosphate transporter 2 (PiT2) wherein the single substitution of a glutamic acid for the lysine residue at position 522 of this receptor is sufficient to render it to function as a gibbon ape leukemia virus (GALV) receptor.

Results

In this study we analyzed the binding affinity of the mutant receptor PiT2K522E and determined that it has a 1000 fold decreased GALV envelope binding affinity compared to the GALV wild type receptor. The decreased affinity does not restrict the initiation of cell-free GALV infection. The diminished binding affinity does, however, correlate with a decrease in the ability of GALV to spread in cells expressing this mutant receptor.

Conclusions

The reduced ability of GALV to subsequently spread among cells expressing PiT2K522E is likely resulted from reduced cell-cell transmission, the decreased ability of PiT2K522E-expressing cells to establish superinfection interference, and attendant cytopathic affects.

The receptors for gibbon ape leukemia virus and amphotropic murine leukemia virus are not downregulated in productively infected cells

Background

Over the last several decades it has been noted, using a variety of different methods, that cells infected by a specific gammaretrovirus are resistant to infection by other retroviruses that employ the same receptor; a phenomenon termed receptor interference. Receptor masking is thought to provide an earlier means of blocking superinfection, whereas receptor down regulation is generally considered to occur in chronically infected cells.

Results

We used replication-competent GFP-expressing viruses containing either an amphotropic murine leukemia virus (A-MLV) or the gibbon ape leukemia virus (GALV) envelope. We also constructed similar viruses containing fluorescence-labeled Gag proteins for the detection of viral particles. Using this repertoire of reagents together with a wide range of antibodies, we were able to determine the presence and availability of viral receptors, and detect viral envelope proteins and particles presence on the cell surface of chronically infected cells.

Conclusions

A-MLV or GALV receptors remain on the surface of chronically infected cells and are detectable by respective antibodies, indicating that these receptors are not downregulated in these infected cells as previously proposed. We were also able to detect viral envelope proteins on the infected cell surface and infected cells are unable to bind soluble A-MLV or GALV envelopes indicating that receptor binding sites are masked by endogenously expressed A-MLV or GALV viral envelope. However, receptor masking does not completely prevent A-MLV or GALV superinfection.

Primate gammaretroviruses require an ancillary factor not required for murine gammaretroviruses to infect BHK cells

BHK cells remain resistant to xenotropic murine retrovirus-related virus (XMRV) or gibbon ape leukemia virus (GALV) infection, even when their respective receptors, Xpr1 or PiT1, are expressed. We set out to determine the stage at which viral infection is blocked and whether this block is mediated by a dominant-negative factor or the absence of a requisite ancillary factor. BHK cells bind neither XMRV nor GALV envelope proteins. BHK cells expressing the appropriate receptors bind XMRV or GALV envelope proteins. BHK cells can be infected by NZB-XMV(New Zealand Black mouse xenotropic murine virus)-enveloped vectors, expressing an envelope derived from a xenotropic retrovirus that, like XMRV, employs Xpr1 as a receptor, and also by vectors bearing the envelope of 10A1 murine leukemia virus (MLV), a murine retrovirus that can use PiT1 as a receptor. The retroviral vectors used in these analyses differ solely in their viral envelope proteins, suggesting that the block to XMRV and GALV infection is mediated at the level of envelope-receptor interactions. N-linked glycosylation of the receptors was not found to mediate resistance of receptor-expressing BHK cells to GALV or XMRV, as shown by tunicamycin treatment and mutation of the specific glycosylation site of the PiT1 receptor. Hybrid cells produced by fusing BHKXpr1 or BHKPiT1 to XMRV- or GALV-resistant cells, respectively, can mediate efficient XMRV or GALV infection. These findings indicate that BHK cells lack a factor that is required for infection by primate xenotropic viruses. This factor is not required for viruses that use the same receptors but were directly isolated from mice.

Changes in viral protein function that accompany retroviral endogenization

Endogenous retroviruses (ERVs) are the remnants of ancient retroviral infections of germ cells and have been maintained in whole or part as heritable genomic elements. The last known endogenization events occurred several million years ago, and therefore stepwise analysis of retroviral endogenization has not been possible. A unique opportunity to study this process became available when a full-length ERV isolated from koalas (KoRV) was shown to have integrated into their germ line within the past 100 years. Even though KoRV shares 78% nucleotide identity with the exogenous and highly infectious gibbon ape leukemia virus (GALV), the infectivity of KoRV, like that of other ERVs, is substantially lower than that of GALV. Differences in the protein coding regions of KoRV that distinguish it from GALV were introduced into the GALV genome, and their functional consequences were assessed. We identified a KoRV gagpol L domain mutation as well as five residues present in the KoRV envelope (env) that, when substituted for the corresponding residues of GALV, resulted in vectors exhibiting substantially reduced titers similar to those observed with KoRV vectors. In addition, KoRV env protein lacks an intact CETTG motif that we have identified as invariant among highly infectious gammaretroviruses. Disruption of this motif in GALV results in vectors with reduced syncytia forming capabilities. Functional assessment of specific sequences that contribute to KoRV's attenuation from a highly infectious GALV-like progenitor virus has allowed the identification of specific modifications in the KoRV genome that correlate with its endogenization.

Selection of feline leukemia virus envelope proteins from a library by functional association with a murine leukemia virus envelope

Libraries of feline leukemia virus subgroup A (FeLV-A)-derived envelope (Env) proteins with random peptides incorporated into the cell-targeting region were screened for productive gene delivery to the PC-3 human prostate cell line. In order to increase the efficiency of recovering and testing functional clones, the screen was performed in the presence of a replication-competent 4070A Env-expressing virus under conditions of viral interference. The Env proteins resulting from this library screen were able to mediate gene delivery to 4070A-infected human PC-3, DU145 prostate and TE671 rhabdomyosarcoma cells in the presence, but not absence, of 4070A helper virus. FeLV-A, FeLV-B and Moloney murine leukemia virus (Mo-MuLV) Env proteins were unable to substitute for 4070A Env. Flow cytometry and Western blot analyses indicated increased cell-surface expression and virion incorporation of library-derived Env proteins in the presence of 4070A Env. Interference assays on cells infected with both 4070A and FeLV-B are consistent with the combination of library-derived and 4070A Env proteins utilizing the Pit1 receptor.

In vitro characterization of a koala retrovirus

Recently, a new endogenous koala gammaretrovirus, designated KoRV, was isolated from koalas. The KoRV genome shares 78% nucleotide identity with another gammaretrovirus, gibbon ape leukemia virus (GALV). KoRV is endogenous in koalas, while GALV is exogenous, suggesting that KoRV predates GALV and that gibbons and koalas acquired the virus at different times from a common source. We have determined that subtle adaptive differences between the KoRV and GALV envelope genes account for differences in their receptor utilization properties. KoRV represents a unique example of a gammaretrovirus whose envelope has evolved to allow for its expanded host range and zoonotic potential.

The infectivity and host range of the ecotropic porcine endogenous retrovirus, PERV-C, is modulated by residues in the C-terminal region of its surface envelope protein

Endogenous retroviral genetic material serves as a reservoir for the generation of retroviral pathogens by recombination between activated endogenous or exogenous infectious agents. Some porcine tissues actively express infectious porcine endogenous retroviruses (PERVs). Of the three classes of PERV characterized to date, two, PERV-A and B, are capable of infecting human cells in vitro, whereas PERV-C cannot. Here, we demonstrate that the PERV-C envelope surface protein (SU) when disassociated from its C-terminus binds human cells. Further, we show that PERV-C binding to human cells is not inhibited in 293 cells productively infected with PERV-A, confirming that the molecule PERV-C interacts with on human cells is distinct from that used by PERV-A. Moreover, we demonstrate that the envelope region encompassing the proline-rich region is required for binding to cells in addition to the putative variable region A (VRA) and B (VRB). The region in the C-terminus of the SU that alters the binding and infectivity properties of PERV-C differs by only nine residues from the analogous region of PERV-A. Caution may be warranted even when a xenotransplantation product is from source pigs that do not express human-tropic viruses, as minimal mutations within PERV-C combined with selection in a human recipient could render PERV-C infectious in humans.

Dissection of gammaretroviral receptor function by using type III phosphate transporters as models

Gammaretroviruses that enter cells via binding to a surface receptor use one of two fundamental mechanisms. In the first, binding of the virus particle to its cognate receptor is followed by fusion and internalization. The second, less common mechanism requires the addition of an accessory protein in order to achieve fusion and entry into the target cells; this protein is usually the soluble form of the envelope protein containing the receptor-binding domain (RBD). For some viruses, such as amphotropic murine leukemia virus (A-MLV), particles with fusion-defective envelope proteins can enter cells in the presence of their own RBD or that of another viral envelope, regardless of its cognate receptor, suggesting that these viruses share a common entry mechanism. A notable exception is gibbon ape leukemia virus (GALV). Fusion-impaired GALV envelope mutants can be trans-activated for infectivity only by GALV RBDs. Using dually functional GALV/A-MLV receptors, we examined the role of receptor with respect to which RBD could overcome fusion impaired virus entry.

Cell signaling through the protein kinases cAMP-dependent protein kinase, protein kinase Cepsilon, and RAF-1 regulates amphotropic murine leukemia virus envelope protein-induced syncytium formation

Amphotropic murine leukemia virus (A-MuLV) utilizes the PiT2 sodium-dependent phosphate transporter as its cell surface receptor to infect mammalian cells. The process of A-MuLV infection requires cleavage of the R peptide from the envelope protein. This occurs within virions thereby rendering them competent to fuse with target cells. Envelope proteins lacking the inhibitory R peptide (e.g. envelope (R-) proteins) induce viral envelope-mediated cell-cell fusion (syncytium). Here we have performed studies to determine if cell signaling through protein kinases is involved in the regulation of PiT2-mediated A-MuLV envelope (R-)-induced syncytium formation. Truncated A-MuLV retroviral envelope protein lacking the inhibitory R peptide (R-) was used to induce viral envelope-mediated cell-cell fusion. Signaling through cyclic AMP to activate PKA was found to inhibit envelope-induced cell-cell fusion, whereas treatment of cells with PKA inhibitors H89, KT5720, and PKA Catalpha siRNA all enhanced this cell fusion process. It was noted that activation of PKC, as well as overexpression of PKCepsilon, up-regulated A-MuLV envelope protein-induced cell-cell fusion, whereas exposure to PKC inhibitors and expression of a kinase-inactive dominant-negative mutant of PKCepsilon (K437R) inhibited syncytium formation. v-ras transformed NIH3T3 cells were highly susceptible to A-MuLV envelope-induced cell-cell fusion, whereas expression of a dominant-negative mutant of Ras (N17Ras) inhibited this cell fusion process. Importantly, activation of Raf-1 protein kinase also is required for A-MuLV envelope-induced syncytium formation. Expression of constitutively active BXB Raf supported, whereas expression of a dominant-negative mutant of Raf-1 (Raf301) blocked, A-MuLV-induced cell-cell fusion. These results indicate that specific cell signaling components are involved in regulating PiT2-mediated A-MuLV-induced cell-cell fusion. Selective pharmacological modulation of these signaling components may be an effective means of altering cell susceptibility to viral-mediated cytopathic effects.

Subtle mutational changes in the SU protein of a natural feline leukemia virus subgroup A isolate alter disease spectrum

FeLV-945 is a representative isolate of the natural feline leukemia virus (FeLV) variant predominant in non-T-cell malignant, proliferative, and degenerative diseases in a geographic cohort. The FeLV-945 surface glycoprotein (SU) is closely related to natural horizontally transmissible FeLV subgroup A (FeLV-A) but was found to differ from a prototype to a larger extent than the members of FeLV-A differ among themselves. The sequence differences included point mutations restricted largely to the functional domains of SU, i.e., VRA, VRB, and PRR. Despite the sequence differences in these critical domains, measurements of receptor utilization, including host range and superinfection interference, confirmed the assignment of FeLV-945 to subgroup A. Other proviruses isolated from the cohort contained similar sequence hallmarks and were assigned to FeLV subgroup A. A provirus from cat 1046 contained a histidine-to-proline change at SU residue 6 within an SPHQ motif that was previously identified as a critical mediator of fusion events during virus entry. The 1046 pseudotype virus entered cells only in the presence of the soluble cofactor FeLIX provided in trans, but it retained an ecotropic host range even in the presence of FeLIX. The mutational changes in FeLV-945 were shown to confer significant functional differences compared to prototype FeLV-A viruses. The substitution of FeLV-945 envelope gene sequences for FeLV-A/61E sequences conferred a small but statistically significant replicative advantage in some feline cells. Moreover, substitution of the unique FeLV-945 long terminal repeat and envelope gene for those of FeLV-A/61E altered the disease spectrum entirely, from a thymic lymphoma of a T-cell origin to an as yet uncharacterized multicentric lymphoma that did not contain T cells.

HTLV-1 and -2 envelope SU subdomains and critical determinants in receptor binding

Background

Human T-cell leukemia virus (HTLV) -1 and -2 are deltaretroviruses that infect a wide range of cells. Glut1, the major vertebrate glucose transporter, has been shown to be the HTLV Env receptor. While it is well established that the extracellular surface component (SU) of the HTLV envelope glycoprotein (Env) harbors all of the determinants of interaction with the receptor, identification of SU subdomains that are necessary and sufficient for interaction with the receptor, as well as critical amino acids therein, remain to be precisely defined. Although highly divergent in the rest of their genomes, HTLV and murine leukemia virus (MLV) Env appear to be related and based on homologous motifs between the HTLV and MLV SU, we derived chimeric HTLV/MLV Env and soluble HTLV-1 and -2 truncated amino terminal SU subdomains.

Results

Using these SU constructs, we found that the 183 and 178 amino terminal residues of the HTLV-1 and -2 Env, respectively, were sufficient to efficiently bind target cells of different species. Binding resulted from bona fide interaction with the HTLV receptor as isolated SU subdomains specifically interfered with HTLV Env-mediated binding, cell fusion, and cell-free as well as cell-to-cell infection. Therefore, the HTLV receptor-binding domain (RBD) lies in the amino terminus of the SU, immediately upstream of a central immunodominant proline rich region (Env residues 180 to 205), that we show to be dispensible for receptor-binding and interference. Moreover, we identified a highly conserved tyrosine residue at position 114 of HTLV-1 Env, Tyr₁₁₄, as critical for receptor-binding and subsequent interference to cell-to-cell fusion and infection. Finally, we observed that residues in the vicinity of Tyr₁₁₄ have lesser impact on receptor binding and had various efficiency in interference to post-binding events.

Conclusions

The first 160 residues of the HTLV-1 and -2 mature cleaved SU fold as autonomous domains that contain all the determinants required for binding the HTLV receptor.

Porcine endogenous retroviruses infect cells lacking cognate receptors by an alternative pathway: implications for retrovirus evolution and xenotransplantation

A PHQ motif near the amino termini of gammaretroviral envelope glycoprotein surface (SU) subunits is important for infectivity but not for incorporation into virions or binding to cognate receptors. The H residue of this motif is most critical, with all substitutions we tested being inactive. Interestingly, porcine endogenous retroviruses (PERVs) of all three host-range groups, A, B, and C, lack full PHQ motifs, but most members have an H residue at position 10. H10A PERV mutants are noninfectious but were efficiently transactivated by adding to the assays a PHQ-containing SU or receptor-binding subdomain (RBD) derived from a gibbon ape leukemia virus (GALV). A requirement of this transactivation was a functional GALV receptor on the cells. In contrast to this heterologous transactivation, PERV RBDs and SUs were inactive in all tested cells, including porcine ST-IOWA cells. Surprisingly, transactivation by GALV RBD enabled wild-type or H10A mutant PERVs of all three host-range groups to efficiently infect cells from humans and rodents that lack functional PERV receptors and it substantially enhanced infectivities of wild-type PERVs, even for cells with PERV receptors. Thus, PERVs can suboptimally infect cells that contain cognate receptors or they can employ a transactivation pathway to more efficiently infect all cells. This ability to infect cells lacking cognate receptors was previously demonstrated only for nontransmissible variant gammaretroviruses with recombinant and mutant envelope glycoproteins. We conclude that some endogenously inherited mammalian retroviruses also have a receptor-independent means for overcoming host-range and interference barriers, implying a need for caution in xenotransplantation, especially of porcine tissues.

A point mutation in the binding subunit of a retroviral envelope protein arrests virus entry at hemifusion

The transmembrane subunits of viral envelope proteins are thought to perform all of the functions required for membrane fusion during entry of enveloped viruses. However, changes in a conserved SPHQ motif near the N terminus of the receptor binding subunit of a murine leukemia virus (MLV) envelope protein block infection and induction of cell-cell fusion but not receptor binding. Here we report evidence that a histidine-to-arginine change at position 8 (H8R) in the SPHQ motif of Moloney MLV blocks infection by arresting virus-cell fusion at the hemifusion state. In cell-cell fusion assays, H8R envelope protein induced mixing of membrane outer leaflet lipids but did not lead to content mixing, a finding indicative of fusion pore formation. Kinetic studies of virus-cell fusion showed that lipid mixing of H8R virus membranes begins much later than for wild-type virus. The length of the delay in lipid mixing decreased upon addition of two second-site changes that increase H8R virus infection to 100-fold less than the wild-type virus. Finally, chlorpromazine, dibucaine, and trifluoperazine, agents that induce pores in an arrested hemifusion state, rescued infection by H8R virus to within 2.5-fold of the level of wild-type virus infection and cell-cell fusion to half that mediated by wild-type envelope protein. We interpret these results to indicate that fusion progressed to the hemifusion intermediate but fusion pore formation was inhibited. These results establish that membrane fusion of Moloney MLV occurs via a hemifusion intermediate. We also interpret these findings as evidence that histidine 8 is a key switch-point residue between the receptor-induced conformation changes that expose fusion peptide and those that lead to six-helix bundle formation.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Na+-dependent phosphate transporters in the murine osteoclast: cellular distribution and protein interactions

We previously demonstrated that inhibition of Na-dependent phosphate (P(i)) transport in osteoclasts led to reduced ATP levels and diminished bone resorption. These findings suggested that Na/P(i) cotransporters in the osteoclast plasma membrane provide P(i) for ATP synthesis and that the osteoclast may utilize part of the P(i) released from bone resorption for this purpose. The present study was undertaken to define the cellular localization of Na/P(i) cotransporters in the mouse osteoclast and to identify the proteins with which they interact. Using glutathione S-transferase (GST) fusion constructs, we demonstrate that the type IIa Na/P(i) cotransporter (Npt2a) in osteoclast lysates interacts with the Na/H exchanger regulatory factor, NHERF-1, a PDZ protein that is essential for the regulation of various membrane transporters. In addition, NHERF-1 in osteoclast lysates interacts with Npt2a in spite of deletion of a putative PDZ-binding domain within the carboxy terminus of Npt2a. In contrast, deletion of the carboxy-terminal TRL amino acid motif of Npt2a significantly reduced its interaction with NHERF-1 in kidney lysates. Studies in osteoclasts transfected with green fluorescent protein-Npt2a constructs indicated that Npt2a colocalizes with NHERF-1 and actin at or near the plasma membrane of the osteoclast and associates with ezrin, a linker protein associated with the actin cytoskeleton, likely via NHERF-1. Furthermore, we demonstrate by RT/PCR of osteoclast RNA and in situ hybridization that the type III Na/P(i) cotransporter, PiT-1, is also expressed in mouse osteoclasts. To examine the cellular distribution of PiT-1, we infected mouse osteoclasts with a retroviral vector encoding PiT-1 fused to an epitope tag. PiT-1 colocalizes with actin and is present on the basolateral membrane of the polarized osteoclast, similar to that previously reported for Npt2a. Taken together, our data suggest that association of Npt2a with NHERF-1, ezrin, and actin, and of PiT-1 with actin, may be responsible for membrane sorting and regulation of these Na/P(i) cotransporters in the osteoclast.

Identification of receptors for pig endogenous retrovirus

Xenotransplantation of porcine tissues has the potential to treat a wide variety of major health problems including organ failure and diabetes. Balanced against the potential benefits of xenotransplantation, however, is the risk of human infection with a porcine microorganism. In particular, the transmission of porcine endogenous retrovirus (PERV) is a major concern [Chapman, L. E. & Bloom, E. T. (2001) J. Am. Med. Assoc. 285, 2304-2306]. Here we report the identification of two, sequence-related, human proteins that act as receptors for PERV-A, encoded by genes located on chromosomes 8 and 17. We also describe homologs from baboon and porcine cells that also are active as receptors. Conversely, activity could not be demonstrated with a syntenic murine receptor homolog. Sequence analysis indicates that PERV-A receptors [human PERV-A receptor (HuPAR)-1, HuPAR-2, baboon PERV-A receptor 2, and porcine PERV-A receptor] are multiple membrane-spanning proteins similar to receptors for other gammaretroviruses. Expression is widespread in human tissues including peripheral blood mononuclear cells, but their biological functions are unknown. The identification of the PERV-A receptors opens avenues of research necessary for a more complete assessment of the retroviral risks of pig to human xenotransplantation.

Functional interaction between the N- and C-terminal domains of murine leukemia virus surface envelope protein

A series of murine leukemia viruses (MuLVs) with chimeric envelope proteins (Env) was generated to map functional interactions between the N- and the C-terminal domains of surface proteins (SU). All these chimeras have the 4070A amphotropic receptor-binding region flanked by various lengths of Moloney ecotropic N- and C-terminal Env. A charged residue, E49 (E16 on the mature protein), was identified at the N-terminals of Moloney MuLV SU that is important for the interaction with the C-terminal domain of the SU. The region that interacts with E49 was localized between junction 4 (R265 of M-MuLV Env) and junction 6 (L374 of M-MuLV Env) of SU. Sequencing the viable chimeric Env virus populations identified residues within the SU protein that improved the replication kinetics of the input chimeric Env viruses. Mutations in the C-domain of SU (G387E/R, L435I, L442P) were found to improve chimera IV4, which displayed a delayed onset of replication. The replication of AE6, containing a chimeric junction in the SU C-terminus, was improved by mutations in the N-domain (N40H, E80K), the proline-rich region (Q252R), or the transmembrane protein (L538N). Altogether, these observations provide insights into the structural elements required for Env function.

Reassessing the role of region A in Pit1-mediated viral entry

The mammalian gammaretroviruses gibbon ape leukemia virus (GALV) and feline leukemia virus subgroup B (FeLV-B) can use the same receptor, Pit1, to infect human cells. A highly polymorphic nine-residue sequence within Pit1, designated region A, has been proposed as the virus binding site, because mutations in this region abolish Pit1-mediated cellular infection by GALV and FeLV-B. However, a direct correlation between region A mutations deleterious for infection and loss of virus binding has not been established. We report that cells expressing a Pit1 protein harboring mutations in region A that abolish receptor function retain the ability to bind virus, indicating that Pit1 region A is not the virus binding site. Furthermore, we have now identified a second region in Pit1, comprising residues 232 to 260 (region B), that is required for both viral entry and virus binding. Epitope-tagged Pit1 proteins were used to demonstrate that mutations in region B result in improper orientation of Pit1 in the cell membrane. Compensatory mutations in region A can restore proper orientation and full receptor function to these region B mutants. Based on these results, we propose that region A of Pit1 confers competence for viral entry by influencing the topology of the authentic binding site in the membrane and hence its accessibility to a viral envelope protein. Based on glycosylation studies and results obtained by using N- and C-terminal epitope-tagged Pit1, region A and region B mutants, and the transmembrane helices predicted with the PHD PredictProtein algorithm, we propose a new Pit1 topology model.

Genetic and biochemical analyses of receptor and cofactor determinants for T-cell-tropic feline leukemia virus infection

Entry by retroviruses is mediated through interactions between the viral envelope glycoprotein and the host cell receptor(s). We recently identified two host cell proteins, FeLIX and Pit1, that are necessary for infection by cytopathic, T-cell-tropic feline leukemia viruses (FeLV-T). Pit1 is a classic multiple transmembrane protein used as a receptor by several other simple retroviruses, including subgroup B FeLV (FeLV-B), and FeLIX is a secreted cellular protein expressed from endogenous FeLV-related sequences (enFeLV). FeLIX is nearly identical to FeLV-B envelope sequences that encode the N-terminal half of the viral surface unit (SU), because these FeLV-B sequences are acquired by recombination with enFeLV. FeLV-B SUs can functionally substitute for FeLIX in mediating FeLV-T infection. Both of these enFeLV-derived cofactors can efficiently facilitate FeLV-T infection only of cells expressing Pit1, not of cells expressing the related transport protein Pit2. We therefore have used chimeric Pit1/Pit2 receptors to map the determinants for cofactor binding and FeLV-T infection. Three distinct determinants appear to be required for cofactor-dependent infection by FeLV-T. We also found that Pit1 sequences within these same domains were required for binding by FeLIX to the Pit receptor. In contrast, these determinants were not all required for receptor binding by the FeLV-B SU cofactors used in this study. These data indicate that cofactor binding is not sufficient for FeLV-T infection and suggest that there may be a direct interaction between FeLV-T and the Pit1 receptor.