Identification of an extracellular domain within the human PiT2 receptor that is required for amphotropic murine leukemia virus binding

Sodium-dependent phosphate transporter PiT1/SLC20A1 as the receptor for the endogenous retroviral envelope syncytin-B involved in mouse placenta formation

Syncytins are envelope genes of retroviral origin that play a critical role in the formation of a syncytial structure at the fetomaternal interface via their fusogenic activity. The mouse placenta is unique among placental mammals since the fetomaternal interface comprises two syncytiotrophoblast layers (ST-I and ST-II) instead of one observed in all other hemochorial placentae. Each layer specifically expresses a distinct mouse syncytin, namely syncytin-A (SynA) for ST-I and syncytin-B (SynB) for ST-II, which have been shown to be essential to placentogenesis and embryonic development. The cellular receptor for SynA has been identified as the membrane protein LY6E and is not the receptor for SynB. Here, by combining a cell-cell fusion assay with the screening of a human ORFeome-derived expression library, we identified the transmembrane multipass sodium-dependent phosphate transporter 1 PiT1/SLC20A1 as the receptor for SynB. Transfection of cells with the cloned receptor, but not the closely related PiT2/SLC20A2, leads to their fusion with cells expressing SynB, with no cross-reactive fusion activity with SynA. The interaction between the two partners was further demonstrated by immunoprecipitation. PiT1/PiT2 chimera and truncation experiments identified the PiT1 N-terminus as the major determinant for SynB-mediated fusion. RT-qPCR analysis of PiT1 expression on a panel of mouse adult and fetal tissues revealed a concomitant increase of PiT1 and SynB specifically in the developing placenta. Finally, electron microscopy analysis of the placenta of PiT1 null embryo before they die (E11.5) disclosed default of ST-II formation with lack of syncytialization, as previously observed in cognate SynB null placenta, and consistent with the present identification of PiT1 as the SynB partner.IMPORTANCESyncytins are envelope genes of endogenous retroviruses, coopted for a physiological function in placentation. They are fusogenic proteins that mediate cell-cell fusion by interacting with receptors present on the partner cells. Here, by devising an in vitro fusion assay that enables the screening of an ORFeome-derived expression library, we identified the long-sought receptor for syncytin-B (SynB), a mouse syncytin responsible for syncytiotrophoblast formation at the fetomaternal interface of the mouse placenta. This protein - PiT1/SLC20A1 - is a multipass transmembrane protein, also known as the receptor for a series of infectious retroviruses. Its profile of expression is consistent with a role in both ancestral endogenization of a SynB founder retrovirus and present-day mouse placenta formation, with evidence-in PiT1 knockout mice-of unfused cells at the level of the cognate placental syncytiotrophoblast layer.

Mechanisms of PiT2-loop7 Missense Mutations Induced Pi Dyshomeostasis

PiT2 is an inorganic phosphate (Pi) transporter whose mutations are linked to primary familial brain calcification (PFBC). PiT2 mainly consists of two ProDom (PD) domains and a large intracellular loop region (loop7). The PD domains are crucial for the Pi transport, but the role of PiT2-loop7 remains unclear. In PFBC patients, mutations in PiT2-loop7 are mainly nonsense or frameshift mutations that probably cause PFBC due to C-PD1131 deletion. To date, six missense mutations have been identified in PiT2-loop7; however, the mechanisms by which these mutations cause PFBC are poorly understood. Here, we found that the p.T390A and p.S434W mutations in PiT2-loop7 decreased the Pi transport activity and cell surface levels of PiT2. Furthermore, we showed that these two mutations attenuated its membrane localization by affecting adenosine monophosphate-activated protein kinase (AMPK)- or protein kinase B (AKT)-mediated PiT2 phosphorylation. In contrast, the p.S121C and p.S601W mutations in the PD domains did not affect PiT2 phosphorylation but rather impaired its substrate-binding abilities. These results suggested that missense mutations in PiT2-loop7 can cause Pi dyshomeostasis by affecting the phosphorylation-regulated cell-surface localization of PiT2. This study helps understand the pathogenesis of PFBC caused by PiT2-loop7 missense mutations and indicates that increasing the phosphorylation levels of PiT2-loop7 could be a promising strategy for developing PFBC therapies.

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.

PiT2 regulates neuronal outgrowth through interaction with microtubule-associated protein 1B

PiT2 is a member of the inorganic phosphate transporter family, and is extensively expressed in the nervous system. It was found that loop7 domain of PiT2 is not required for retroviral recognition and transport function. The exact functions of loop7 remain poorly understood. Here we show that loop7 of PiT2 is necessary for the transport of PiT2 protein to the cell surface. Further, loop7 is also related to the outgrowth of neurite in Neuro2A cells interacts with the light chain 1 of microtubule-associated protein 1B (MAP1B). PiT2 with mutated MAP1B binding sites affect neurite outgrowth whereas Pi transport function deficient mutants of PiT2 do not. We also show that Drosophila dPiT interacts with microtubule-associated protein Futsch, and dPiT is crucial for the normal development of neuromuscular junctions (NMJs). These results indicate that PiT2 might participate in the regulation of neuronal outgrowth by interacting with MAP1B and independently of its Pi transport function in the nervous system.

Characterization of retroviral infectivity and superinfection resistance during retrovirus-mediated transduction of mammalian cells

Retroviral vectors including lentiviral vectors are commonly used tools to stably express transgenes or RNA molecules in mammalian cells. Their utilities are roughly divided into two categories, stable overexpression of transgenes and RNA molecules, which requires maximal transduction efficiency, or functional selection with retrovirus (RV)-based libraries, which takes advantage of retroviral superinfection resistance. However, the dynamic features of RV-mediated transduction are not well characterized. Here, we engineered two murine stem cell virus-based retroviral vectors expressing dual fluorescence proteins and antibiotic markers, and analyzed virion production efficiency and virion stability, dynamic infectivity and superinfection resistance in different cell types, and strategies to improve transduction efficiency. We found that the highest virion production occurred between 60 and 72 h after transfection. The stability of the collected virion supernatant decreased by >60% after 3 days in storage. We found that RV infectivity varied drastically in the tested human cancer lines, while low transduction efficiency was partially overcome with increased virus titer, prolonged infection duration and/or repeated infections. Furthermore, we demonstrated that RV receptors PIT1 and PIT2 were lowly expressed in the analyzed cells, and that PIT1 and/or PIT2 overexpression significantly improved transduction efficiency in certain cell lines. Thus, our findings provide resourceful information for the optimal conditions of retroviral-mediated gene delivery.

Macropinocytosis is the entry mechanism of amphotropic murine leukemia virus

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

IMPORTANCE

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

High levels of the type III inorganic phosphate transporter PiT1 (SLC20A1) can confer faster cell adhesion

The inorganic phosphate transporter PiT1 (SLC20A1) is ubiquitously expressed in mammalian cells. We recently showed that overexpression of human PiT1 was sufficient to increase proliferation of two strict density-inhibited cell lines, murine fibroblastic NIH3T3 and pre-osteoblastic MC3T3-E1 cells, and allowed the cultures to grow to higher cell densities. In addition, upon transformation NIH3T3 cells showed increased ability to form colonies in soft agar. The cellular regulation of PiT1 expression supports that cells utilize the PiT1 levels to control proliferation, with non-proliferating cells showing the lowest PiT1 mRNA levels. The mechanism behind the role of PiT1 in increased cell proliferation is not known. We, however, found that compared to control cells, cultures of NIH3T3 cells overexpressing PiT1 upon seeding showed increased cell number after 24h and had shifted more cells from G0/G1 to S+G2/M within 12h, suggesting that an early event may play a role. We here show that expression of human PiT1 in NIH3T3 cells led to faster cell adhesion; this effect was not cell type specific in that it was also observed when expressing human PiT1 in MC3T3-E1 cells. We also show for NIH3T3 that PiT1 overexpression led to faster cell spreading. The final total numbers of attached cells did, however, not differ between cultures of PiT1 overexpressing cells and control cells of neither cell type. We suggest that the PiT1-mediated fast adhesion potentials allow the cells to go faster out of G0/G1 and thereby contribute to their proliferative advantage within the first 24h after seeding.

Phosphate transporters of the SLC20 and SLC34 families

Transport of inorganic phosphate (Pi) across the plasma membrane is essential for normal cellular function. Members of two families of SLC proteins (SLC20 and SLC34) act as Na(+)-dependent, secondary-active cotransporters to transport Pi across cell membranes. The SLC34 proteins are expressed in specific organs important for Pi homeostasis: NaPi-IIa (SLC34A1) and NaPi-IIc (SLC34A3) fulfill essential roles in Pi reabsorption in the kidney proximal tubule and NaPi-IIb (SLC34A2) mediates Pi absorption in the gut. The SLC20 proteins, PiT-1 (SLC20A1), PiT-2 (SLC20A2) are expressed ubiquitously in all tissues and although generally considered as "housekeeping" transport proteins, the discovery of tissue-specific activity, regulatory pathways and gene-related pathophysiologies, is redefining their importance. This review summarizes our current knowledge of SLC20 and SLC34 proteins in terms of their basic molecular characteristics, physiological roles, known pathophysiology and pharmacology.

Expression levels of the PiT-2 receptor explain, in part, the gestational age-dependent alterations in transduction efficiency after in utero retroviral-mediated gene transfer

BACKGROUND

A fundamental obstacle to using retroviral-mediated gene transfer (GT) to treat human diseases is the relatively low transduction levels that have been achieved in clinically relevant human cells. We previously showed that performing GT in utero overcomes this obstacle and results in significant levels of transduction within multiple fetal organs, with different tissues exhibiting optimal transduction at different developmental stages. We undertook the present study aiming to elucidate the mechanism for this age-dependent transduction, testing the two factors that we hypothesized could be responsible: (i) the proliferative status of the tissue at the time of GT and (ii) the expression level of the amphotropic PiT-2 receptor.

METHODS

Immunofluorescence was performed on tissues from sheep of varying developmental stages to assess the proliferative status of the predominant cells within each organ as a function of age. After developing an enzyme-linked immunosorbent assay (ELISA) and a quantitative reverse transcription chain reaction (qRT-PCR) assay, we then quantified PiT-2 expression at the protein and mRNA levels, respectively.

RESULTS

The results obtained indicate that the proliferative status of organs at the time of fetal GT is not the major determinant governing transduction efficiency. By contrast, our ELISA and qRT-PCR analyses demonstrated that PiT-2 mRNA and protein levels vary with gestational age, correlating with the observed differences in transduction efficiency.

CONCLUSIONS

The findings of the present study explain the age-related differences that we previously observed in transduction efficiency after in utero GT. They also suggest it may be possible to achieve relatively selective GT to specific tissues by performing in utero GT when levels of PiT-2 are maximal in the desired target organ.

Phosphate transport kinetics and structure-function relationships of SLC34 and SLC20 proteins

Transport of inorganic phosphate (P(i)) is mediated by proteins belonging to two solute carrier families (SLC20 and SLC34). Members of both families transport P(i) using the electrochemical gradient for Na(+). The role of the SLC34 members as essential players in mammalian P(i) homeostasis is well established, whereas that of SLC20 proteins is less well defined. The SLC34 family comprises the following three isoforms that preferentially cotransport divalent P(i) and are expressed in epithelial tissue: the renal NaPi-IIa and NaPi-IIc are responsible for reabsorbing P(i) in the proximal tubule, whereas NaPi-IIb is more ubiquitously expressed, including the small intestine, where it mediates dietary P(i) absorption. The SLC20 family comprises two members (PiT-1, PiT-2) that preferentially cotransport monovalent P(i) and are expressed in epithelial as well as nonepithelial tissue. The transport kinetics of members of both families have been characterized in detail using heterologous expression in Xenopus oocytes. For the electrogenic NaPi-IIa/b, and PiT-1,-2, conventional electrophysiological techniques together with radiotracer methods have been applied, as well as time-resolved fluorometric measurements that allow new insights into local conformational changes of the protein during the cotransport cycle. For the electroneutral NaPi-IIc, conventional tracer uptake and fluorometry have been used to elucidate its transport properties. The 3-D structures of these proteins remain unresolved and structure-function studies have so far concentrated on defining the topology and identifying sites of functional importance.

Phosphate transporters in renal, gastrointestinal, and other tissues

Inorganic phosphate (Pi) is essential for all living organisms. Bound to organic molecules, Pi fulfills structural, metabolic, and signaling tasks. Therefore, cell growth and maintenance depends on efficient transport of Pi across cellular membranes into the intracellular space. Uptake of Pi requires energy because the substrate is transported against its electrochemical gradient. Till recently, 2 major families of physiologically relevant Pi-specific transporters have been identified: the solute carrier families Slc34 and Slc20. Interestingly, phylogenetic links can be detected between prokaryotic and eukaryotic transporters in both families. Because less complex model organisms are often instrumental in establishing paradigms for protein function in human beings, a brief assessment of Slc34 and Slc20 phylogeny is of interest.

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.

New structural arrangement of the extracellular regions of the phosphate transporter SLC20A1, the receptor for gibbon ape leukemia virus

Infection of a host cell by a retrovirus requires an initial interaction with a cellular receptor. For numerous gammaretroviruses, such as the gibbon ape leukemia virus, woolly monkey virus, feline leukemia virus subgroup B, feline leukemia virus subgroup T, and 10A1 murine leukemia virus, this receptor is the human type III sodium-dependent inorganic phosphate transporter, SLC20A1, formerly known as PiT1. Understanding the critical receptor functionalities and interactions with the virus that lead to successful infection requires that we first know the surface structure of the cellular receptor. Previous molecular modeling from the protein sequence, and limited empirical data, predicted a protein with 10 transmembrane helices. Here we undertake the biochemical approach of substituted cysteine accessibility mutagenesis to resolve the topology of this receptor in live cells. We discover that there are segments of the protein that are unexpectedly exposed to the outside milieu. By using information determined by substituted cysteine accessibility mutagenesis to set constraints in HMMTOP, a hidden Markov model-based transmembrane topology prediction method, we now propose a comprehensive topological model for SLC20A1, a transmembrane protein with 12 transmembrane helices and 7 extracellular regions, that varies from previous models and should permit approaches that define both virus interaction and transport function.

Identification of two distinct structural regions in a human porcine endogenous retrovirus receptor, HuPAR2, contributing to function for viral entry

BACKGROUND

Of the three subclasses of Porcine Endogenous Retrovirus (PERV), PERV-A is able to infect human cells via one of two receptors, HuPAR1 or HuPAR2. Characterizing the structure-function relationships of the two HuPAR receptors in PERV-A binding and entry is important in understanding receptor-mediated gammaretroviral entry and contributes to evaluating the risk of zoonosis in xenotransplantation.

RESULTS

Chimeras of the non-permissive murine PAR and the permissive HuPAR2, which scanned the entire molecule, revealed that the first 135 amino acids of HuPAR2 are critical for PERV-A entry. Within this critical region, eighteen single residue differences exist. Site-directed mutagenesis used to map single residues confirmed the previously identified L109 as a binding and infectivity determinant. In addition, we identified seven residues contributing to the efficiency of PERV-A entry without affecting envelope binding, located in multiple predicted structural motifs (intracellular, extracellular and transmembrane). We also show that expression of HuPAR2 in a non-permissive cell line results in an average 11-fold higher infectivity titer for PERV-A compared to equal expression of HuPAR1, although PERV-A envelope binding is similar. Chimeras between HuPAR-1 and -2 revealed that the region spanning amino acids 152-285 is responsible for the increase of HuPAR2. Fine mapping of this region revealed that the increased receptor function required the full sequence rather than one or more specific residues.

CONCLUSION

HuPAR2 has two distinct structural regions. In one region, a single residue determines binding; however, in both regions, multiple residues influence receptor function for PERV-A entry.

Human T Cell Leukemia Virus Envelope Binding and Virus Entry Are Mediated by Distinct Domains of the Glucose Transporter GLUT1

The glucose transporter GLUT1, a member of the multimembrane-spanning facilitative nutrient transporter family, serves as a receptor for human T cell leukemia virus (HTLV) infection. Here, we show that the 7 amino acids of the extracellular loop 6 of GLUT1 (ECL6) placed in the context of the related GLUT3 transporter were sufficient for HTLV envelope binding. Glutamate residue 426 in ECL6 was identified as critical for binding. However, binding to ECL6 was not sufficient for HTLV envelope-driven infection. Infection required two additional determinants located in ECL1 and ECL5, which otherwise did not influence HTLV envelope binding. Moreover the single N-glycosylation chain located in ECL1 was not required for HTLV infection. Therefore, binding involves a discrete determinant in the carboxyl terminal ECL6, whereas post-binding events engage extracellular sequences in the amino and carboxyl terminus of GLUT1.

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.

The central half of Pit2 is not required for its function as a retroviral receptor

The type III sodium-dependent phosphate (NaPi) cotransporter, Pit2, is a receptor for amphotropic murine leukemia virus (A-MuLV) and 10A1 MuLV. In order to determine what is sufficient for Pit2 receptor function, a deletion mutant lacking about the middle half of the protein was made. The mutant supported entry for both viruses, unequivocally narrowing down the identification of the sequence that is sufficient to specify the receptor functions of Pit2 to its N-terminal 182 amino acids and C-terminal 170 amino acids.