Chimeras of receptors for gibbon ape leukemia virus/feline leukemia virus B and amphotropic murine leukemia virus reveal different modes of receptor recognition by retrovirus

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.

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.

Primary Brain Calcification Causal PiT2 Transport-Knockout Variants can Exert Dominant Negative Effects on Wild-Type PiT2 Transport Function in Mammalian Cells

Primary brain calcification (PBC) is a neurodegenerative disorder characterized by calcium-phosphate deposits in the basal ganglia and often also other areas of the brain. The prevalent clinical manifestations are cognitive impairment, neuropsychiatric symptoms, and movement disorders. In recent years, monoallelic variants in SLC20A2, which encodes the type III sodium-dependent inorganic phosphate (P_i) transporter 2 (PiT2), have been linked to the familial form of PBC in 40–50% of the families reported worldwide as well as to sporadic cases of PBC. Further insight into the disease mechanism is, however, needed. Based on co-expression studies of wild-type and variant PiT2 in Xenopus laevis oocytes, the molecular disease mechanism associated with SLC20A2 missense variants has formerly been suggested to be haploinsufficiency. We have here used mammalian cells isolated from a Slc20a2^−/− mouse and co-expression of human wild-type and variant PiT2. Two of the variants studied have both been reported twice in unrelated PBC cases: PiT2D28N in two sporadic cases and PiT2E575K in a familial and a sporadic case. We find that in mammalian cells, the analyzed SLC20A2 missense variants can exert their effect in a dominant negative manner resulting in decreased wild-type PiT2 P_i transport. Thus, compared to monoallelic lack of functional PiT2 protein expression, which reasonably points towards haploinsufficiency, certain SLC20A2 missense variants may be more detrimental for cellular P_i uptake and potentially contribute to an earlier disease onset and/or a more severe phenotype as observed for Slc20a2^−/− mice compared to Slc20a2^+/− mice.

Endogenous Gibbon Ape Leukemia Virus Identified in a Rodent (Melomys burtoni subsp.) from Wallacea (Indonesia)

Gibbon ape leukemia virus (GALV) and koala retrovirus (KoRV) most likely originated from a cross-species transmission of an ancestral retrovirus into koalas and gibbons via one or more intermediate as-yet-unknown hosts. A virus highly similar to GALV has been identified in an Australian native rodent (Melomys burtoni) after extensive screening of Australian wildlife. GALV-like viruses have also been discovered in several Southeast Asian species, although screening has not been extensive and viruses discovered to date are only distantly related to GALV. We therefore screened 26 Southeast Asian rodent species for KoRV- and GALV-like sequences, using hybridization capture and high-throughput sequencing, in the attempt to identify potential GALV and KoRV hosts. Only the individuals belonging to a newly discovered subspecies of Melomys burtoni from Indonesia were positive, yielding an endogenous provirus very closely related to a strain of GALV. The sequence of the critical receptor domain for GALV infection in the Indonesian M. burtoni subsp. was consistent with the susceptibility of the species to GALV infection. The second record of a GALV in M. burtoni provides further evidence that M. burtoni, and potentially other lineages within the widespread subfamily Murinae, may play a role in the spread of GALV-like viruses. The discovery of a GALV in the most western part of the Australo-Papuan distribution of M. burtoni, specifically in a transitional zone between Asia and Australia (Wallacea), may be relevant to the cross-species transmission to gibbons in Southeast Asia and broadens the known distribution of GALVs in wild rodents.

IMPORTANCE

Gibbon ape leukemia virus (GALV) and the koala retrovirus (KoRV) are very closely related, yet their hosts neither are closely related nor overlap geographically. Direct cross-species infection between koalas and gibbons is unlikely. Therefore, GALV and KoRV may have arisen via a cross-species transfer from an intermediate host whose range overlaps those of both gibbons and koalas. Using hybridization capture and high-throughput sequencing, we have screened a wide range of rodent candidate hosts from Southeast Asia for KoRV- and GALV-like sequences. Only a Melomys burtoni subspecies from Wallacea (Indonesia) was positive for GALV. We report the genome sequence of this newly identified GALV, the critical domain for infection of its potential cellular receptor, and its phylogenetic relationships with the other previously characterized GALVs. We hypothesize that Melomys burtoni, and potentially related lineages with an Australo-Papuan distribution, may have played a key role in cross-species transmission to other taxa.

Vitamin-D receptor agonist calcitriol reduces calcification in vitro through selective upregulation of SLC20A2 but not SLC20A1 or XPR1

Vitamin D deficiency (hypovitaminosis D) causes osteomalacia and poor long bone mineralization. In apparent contrast, hypovitaminosis D has been reported in patients with primary brain calcifications (“Fahr’s disease”). We evaluated the expression of two phosphate transporters which we have found to be associated with primary brain calcification (SLC20A2, whose promoter has a predicted vitamin D receptor binding site, and XPR1), and one unassociated (SLC20A1), in an in vitro model of calcification. Expression of all three genes was significantly decreased in calcifying human bone osteosarcoma (SaOs-2) cells. Further, we confirmed that vitamin D (calcitriol) reduced calcification as measured by Alizarin Red staining. Cells incubated with calcitriol under calcifying conditions specifically maintained expression of the phosphate transporter SLC20A2 at higher levels relative to controls, by RT-qPCR. Neither SLC20A1 nor XPR1 were affected by calcitriol treatment and remained suppressed. Critically, knockdown of SLC20A2 gene and protein with CRISPR technology in SaOs2 cells significantly ablated vitamin D mediated inhibition of calcification. This study elucidates the mechanistic importance of SLC20A2 in suppressing the calcification process. It also suggests that vitamin D might be used to regulate SLC20A2 gene expression, as well as reduce brain calcification which occurs in Fahr’s disease and normal aging.

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.

Regulation of cell proliferation and cell density by the inorganic phosphate transporter PiT1

ABSTACT:

BACKGROUND

The inorganic phosphate (Pi) transporter, PiT1 (SLC20A1), is ubiquitously expressed in mammalian cells. It has previously been shown that down-regulation of PiT1 severely impaired the proliferation of two transformed human cells lines, HepG2 and HeLa, and the tumorigenicity of HeLa cells in nude mice. Moreover, PiT1 knock-out mice do not survive past E12.5 and from E10.5, the embryos were found to be growth-retarded and showed reduced proliferation of liver cells. Isolated mouse embryonic fibroblasts with knocked out as well as reduced PiT1 expression levels also exhibited impaired proliferation. Together these results suggest that a certain level of PiT1 is important for proliferation. We have here investigated the role of PiT1 in regulation of cell proliferation using two strictly density-inhibited cells lines, the murine MC3T3-E1 and NIH3T3 cells.

RESULTS

We found that knock-down of PiT1 in MC3T3-E1 cells led to impaired proliferation supporting that at least a certain level of PiT1 is important for wildtype level of proliferation. We, however, also observed that MC3T3-E1 and NIH3T3 cells themselves regulate their endogenous PiT1 mRNA levels with lower levels in general correlating with decreased proliferation/increased cell density. Moreover, over-expression of human PiT1 led to increased proliferation of both MC3T3-E1 and NIH3T3 cultures and resulted in higher cell densities in cultures of these two strictly density-inhibited cell lines. In addition, when we transformed NIH3T3 cells by cultivation in fetal bovine serum, cells over-expressing human PiT1 formed more colonies in soft agar than control cells.

CONCLUSIONS

We conclude that not only is a certain level of PiT1 necessary for normal cell division as suggested by previously published studies, rather the cellular PiT1 level is involved in regulating cell proliferation and cell density and an increased PiT1 expression can indeed make NIH3T3 cells more sensitive to transformation. We have thus provided the first evidence for that expression of the type III Pi transporter, PiT1, above the endogenous level can drive cell proliferation and overrule cell density constraints, and the results bridge previous observations showing that a certain PiT1 level is important for regulating normal embryonic growth/development and for tumorigenicity of HeLa cells.

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.

Mapping of the minimal inorganic phosphate transporting unit of human PiT2 suggests a structure universal to PiT-related proteins from all kingdoms of life

BACKGROUND

The inorganic (Pi) phosphate transporter (PiT) family comprises known and putative Na(+)- or H(+)-dependent Pi-transporting proteins with representatives from all kingdoms. The mammalian members are placed in the outer cell membranes and suggested to supply cells with Pi to maintain house-keeping functions. Alignment of protein sequences representing PiT family members from all kingdoms reveals the presence of conserved amino acids and that bacterial phosphate permeases and putative phosphate permeases from archaea lack substantial parts of the protein sequence when compared to the mammalian PiT family members. Besides being Na(+)-dependent P(i) (NaP(i)) transporters, the mammalian PiT paralogs, PiT1 and PiT2, also are receptors for gamma-retroviruses. We have here exploited the dual-function of PiT1 and PiT2 to study the structure-function relationship of PiT proteins.

RESULTS

We show that the human PiT2 histidine, H(502), and the human PiT1 glutamate, E(70),--both conserved in eukaryotic PiT family members--are critical for P(i) transport function. Noticeably, human PiT2 H(502) is located in the C-terminal PiT family signature sequence, and human PiT1 E(70) is located in ProDom domains characteristic for all PiT family members.A human PiT2 truncation mutant, which consists of the predicted 10 transmembrane (TM) domain backbone without a large intracellular domain (human PiT2ΔR(254)-V(483)), was found to be a fully functional P(i) transporter. Further truncation of the human PiT2 protein by additional removal of two predicted TM domains together with the large intracellular domain created a mutant that resembles a bacterial phosphate permease and an archaeal putative phosphate permease. This human PiT2 truncation mutant (human PiT2ΔL(183)-V(483)) did also support P(i) transport albeit at very low levels.

CONCLUSIONS

The results suggest that the overall structure of the P(i)-transporting unit of the PiT family proteins has remained unchanged during evolution. Moreover, in combination, our studies of the gene structure of the human PiT1 and PiT2 genes (SLC20A1 and SLC20A2, respectively) and alignment of protein sequences of PiT family members from all kingdoms, along with the studies of the dual functions of the human PiT paralogs show that these proteins are excellent as models for studying the evolution of a protein's structure-function relationship.

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.

Matrix fibronectin binds gammaretrovirus and assists in entry: new light on viral infections

A major entry route for the gammaretrovirus amphotropic murine leukemia virus (A-MLV) into NIH 3T3 fibroblasts is via caveola-dependent endocytosis. However, during the infection time, few viral particles can be observed intracellularly. Analyzing the dynamics of the A-MLV infection process by using total internal reflection fluorescence microscopy, we show that the majority of viruses are extracellular and bound to the fibronectin matrix. Moreover, the amounts of bound virus and of fibronectin correlated. Using confocal microscopy, nanoparticles targeted to fibronectin by a III1C-fibronectin fragment or anti-fibronectin antibody were detected intracellularly in NIH 3T3 cells; unconjugated nanoparticles neither bound to cells nor were detectable intracellularly. Furthermore, A-MLV colocalized intracellularly with the fibronectin-targeted nanoparticles, suggesting that they were taken up by the same cellular pathway. Both A-MLV entry and fibronectin turnover depend on caveolar endocytosis, and we found that inhibiting viral binding to the extracellular NIH 3T3 fibronectin-matrix dramatically reduced A-MLV infection, indeed, showing an active role of fibronectin in infection. We suggest that binding to the cellular fibronectin matrix provides a new mechanism by which viruses can enter cells.

Marking of peripheral T-lymphocytes by retroviral transduction and transplantation of CD34+ cells in a canine X-linked severe combined immunodeficiency model

A retrovirus vector containing an enhanced green fluorescent protein complimentary DNA (EGFP cDNA) was used to mark and dynamically follow vector-expressing cells in the peripheral blood of bone marrow transplanted X-linked severe combined immunodeficient dogs. CD34(+) cells isolated from young normal dogs were transduced, using a 2 day protocol, with an amphotropic retroviral vector that expressed enhanced green fluorescent protein (EGFP) and the canine common gamma chain (gammac) cDNAs. Following transplantation of the transduced cells, normal donor peripheral blood lymphocytes (PBL) appeared by 1 month post-bone marrow transplant (BMT) and rescued three of five treated dogs from their lethal immunodeficiency. PCR and flow cytometric analysis of post-BMT PBL documented the peripheral EGFP expressing cells as CD3(+) T cells, which varied from 0% to 28%. Sorting of EGFP(+) and EGFP(-) peripheral blood T cells from two dogs, followed by vector PCR analysis, showed no evidence of vector shutdown. EGFP expression in B cells or monocytes was not detected. These marking experiments demonstrate that the transduction protocol did not abolish the lymphoid engraftment capability of ex vivo transduced canine CD34(+) cells and supports the potential utility of the MSCV retroviral vector for gene transfer to XSCID affected canine hematopoietic progenitor cells (HPC).

Optimized Transduction of Canine Paediatric CD34+ Cells Using an MSCV-based Bicistronic Vector

We have used a murine MSCV-based bicistronic retroviral vector, containing the common gamma chain (gammac) and enhanced green fluorescent protein (EGFP) cDNAs, to optimize retroviral transduction of canine cells, including an adherent canine thymus fibroblast cell line, Cf2Th, as well as normal canine CD34(+) bone marrow (BM) cells. Both canine cell types were shown to express Ram-1 (the amphotropic retroviral receptor) mRNA. Supernatants containing infectious viruses were produced using both stable (PA317) and transient (Phoenix cells) amphotropic virus producer cell lines. Centrifugation (spinfection) combined with the addition of polybrene produced the highest transduction efficiencies, infecting approximately 75% of Cf2Th cells. An average of 11% of highly enriched canine CD34(+) cells could be transduced in a protocol that utilized spinfection and plates coated with the fibronectin fragment CH-296 (Retronectin). Indirect assays showed the vector-encoded canine gammac cDNA produced a gammac protein that was expressed on the cell surface of transduced cells. This strategy may result in the transduction of sufficient numbers of CD34(+) BM cells to make the treatment of canine X-linked severe combined immunodeficiency and other canine genetic diseases feasible.

Characterization of transport mechanisms and determinants critical for Na+-dependent Pi symport of the PiT family paralogs human PiT1 and PiT2

The general phosphate need in mammalian cells is accommodated by members of the P(i) transport (PiT) family (SLC20), which use either Na(+) or H(+) to mediate inorganic phosphate (P(i)) symport. The mammalian PiT paralogs PiT1 and PiT2 are Na(+)-dependent P(i) (NaP(i)) transporters and are exploited by a group of retroviruses for cell entry. Human PiT1 and PiT2 were characterized by expression in Xenopus laevis oocytes with (32)P(i) as a traceable P(i) source. For PiT1, the Michaelis-Menten constant for P(i) was determined as 322.5 +/- 124.5 microM. PiT2 was analyzed for the first time and showed positive cooperativity in P(i) uptake with a half-maximal activity constant for P(i) of 163.5 +/- 39.8 microM. PiT1- and PiT2-mediated Na(+)-dependent P(i) uptake functions were not significantly affected by acidic and alkaline pH and displayed similar Na(+) dependency patterns. However, only PiT2 was capable of Na(+)-independent P(i) transport at acidic pH. Study of the impact of divalent cations Ca(2+) and Mg(2+) revealed that Ca(2+) was important, but not critical, for NaP(i) transport function of PiT proteins. To gain insight into the NaP(i) cotransport function, we analyzed PiT2 and a PiT2 P(i) transport knockout mutant using (22)Na(+) as a traceable Na(+) source. Na(+) was transported by PiT2 even without P(i) in the uptake medium and also when P(i) transport function was knocked out. This is the first time decoupling of P(i) from Na(+) transport has been demonstrated for a PiT family member. Moreover, the results imply that putative transmembrane amino acids E(55) and E(575) are responsible for linking P(i) import to Na(+) transport in PiT2.

Use of different but overlapping determinants in a retrovirus receptor accounts for non-reciprocal interference between xenotropic and polytropic murine leukemia viruses

Background

Retrovirus infection depends on binding of the retroviral envelope (Env) protein to specific cell-surface protein receptors. Interference, or superinfection resistance, is a frequent consequence of retroviral infection, and occurs when newly-synthesized Env binds to receptor proteins resulting in a block to entry by retroviruses that use the same receptors. Three groups of viruses demonstrate a non-reciprocal pattern of interference (NRI), which requires the existence of both a common receptor utilized by all viruses within the group, and a specific receptor that is used by a subset of viruses. In the case of amphotropic and 10A1 murine leukemia viruses (MLV), the common and specific receptors are the products of two related genes. In the case of avian sarcoma and leukosis virus types B, D, and E, the two receptors are distinct protein products of a single gene. NRI also occurs between xenotropic and polytropic MLV. The common receptor, Xpr1, has been identified, but a specific receptor has yet to be described.

Results

Using chimeric receptor proteins and interference studies, we have identified a region of Xpr1 that is uniquely utilized by xenotropic MLV and show that this receptor domain is required for non-reciprocal interference.

Conclusion

We propose a novel pattern of receptor usage by xenotropic and polytropic MLV to explain the NRI observed between these viruses. We propose that the specific and common receptor determinants for xenotropic and polytropic viruses are simultaneously present in discreet domains of a single Xpr1 protein.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Caveola-dependent endocytic entry of amphotropic murine leukemia virus

Early results suggested that the amphotropic murine leukemia virus (A-MLV) does not enter cells via endocytosis through clathrin-coated pits and this gammaretrovirus has therefore been anticipated to fuse directly with the plasma membrane. However, here we present data implicating a caveola-mediated endocytic entry route for A-MLV via its receptor Pit2. Caveolae belong to the cholesterol-rich microdomains characterized by resistance to nonionic detergents such as Triton X-100. Extraction of murine fibroblastic NIH 3T3 cells in cold Triton X-100 showed the presence of the A-MLV receptor Pit2 in detergent-insoluble microdomains. Using coimmunoprecipitation of cell extracts, we were able to demonstrate direct association of Pit2 with caveolin-1, the structural protein of caveolae. Other investigations revealed that A-MLV infection in contrast to vesicular stomatitis virus infection is a slow process (t(1/2) approximately 5 h), which is dependent on plasma membrane cholesterol but independent of NH4Cl treatment of cells; NH4Cl impairs entry via clathrin-coated pits. Furthermore, expression of dominant-negative caveolin-1 decreased the susceptibility to infection via Pit2 by approximately 70%. These results show that A-MLV can enter cells via a caveola-dependent entry route. Moreover, increase in A-MLV infection by treatment with okadaic acid as well as entry of fusion-defective fluorescent A-MLV virions in NIH 3T3 cells further confirmed our findings and show that A-MLV can enter mouse fibroblasts via an endocytic entry route involving caveolae. Finally, we also found colocalization of fusion-defective fluorescent A-MLV virions with caveolin-1 in NIH 3T3 cells. This is the first time substantial evidence has been presented implicating the existence of a caveola-dependent endocytic entry pathway for a retrovirus.

Evolutionary and experimental analyses of inorganic phosphate transporter PiT family reveals two related signature sequences harboring highly conserved aspartic acids critical for sodium-dependent phosphate transport function of human PiT2

The mammalian members of the inorganic phosphate (P(i)) transporter (PiT) family, the type III sodium-dependent phosphate (NaP(i)) transporters PiT1 and PiT2, have been assigned housekeeping P(i) transport functions and are suggested to be involved in chondroblastic and osteoblastic mineralization and ectopic calcification. The PiT family members are conserved throughout all kingdoms and use either sodium (Na+) or proton (H+) gradients to transport P(i). Sequence logo analyses revealed that independent of their cation dependency these proteins harbor conserved signature sequences in their N- and C-terminal ends with the common core consensus sequence GANDVANA. With the exception of 10 proteins from extremophiles all 109 proteins analyzed carry an aspartic acid in one or both of the signature sequences. We changed either of the highly conserved aspartates, Asp28 and Asp506, in the N- and C-terminal signature sequences, respectively, of human PiT2 to asparagine and analyzed P(i) uptake function in Xenopus laevis oocytes. Both mutant proteins were expressed at the cell surface of the oocytes but exhibited knocked out NaP(i) transport function. Human PiT2 is also a retroviral receptor and we have previously shown that this function can be exploited as a control for proper processing and folding of mutant proteins. Both mutant transporters displayed wild-type receptor functions implying that their overall architecture is undisturbed. Thus the presence of an aspartic acid in either of the PiT family signature sequences is critical for the Na+-dependent P(i) transport function of human PiT2. The conservation of the aspartates among proteins using either Na+- or H+-gradients for P(i) transport suggests that they are involved in H+-dependent P(i) transport as well. Current results favor a membrane topology model in which the N- and C-terminal PiT family signature sequences are positioned in intra- and extracellular loops, respectively, suggesting that they are involved in related functions on either side of the membrane. The present data are in agreement with a possible role of the signature sequences in translocation of cations.

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.

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

Human PiT2 (PiT2) is a multiple-membrane-spanning protein that functions as a type III sodium phosphate cotransporter and as the receptor for amphotropic murine leukemia virus (A-MuLV). Human PiT1 (PiT1), another type III sodium phosphate cotransporter, is a highly related protein that functions as a receptor for gibbon ape leukemia virus but not for A-MuLV. The ability of PiT1 and PiT2 to function as discrete viral receptors with unique properties presumably is reflected in critical residue differences between these two proteins. Early efforts to map the region(s) within PiT2 that is important for virus binding and/or entry relied on infection results obtained with PiT1-PiT2 chimeric cDNAs expressed in Chinese hamster ovary (CHOK1) cells. These attempts to localize the PiT2 virus-binding site were hampered because they were based on infectivity, not binding, assays, and therefore, receptors that bound but failed to facilitate virus entry could not be distinguished from receptors that did not bind virus. Using a more accurate topological model for PiT2 as well as an A-MuLV receptor-binding assay, we have identified extracellular domain one (ECD1) of the human PiT2 receptor as being important for A-MuLV binding and infection.

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.

Two highly conserved glutamate residues critical for type III sodium-dependent phosphate transport revealed by uncoupling transport function from retroviral receptor function

Type III sodium-dependent phosphate (NaP(i)) cotransporters, Pit1 and Pit2, have been assigned housekeeping P(i) transport functions and suggested involved in chondroblastic and osteoblastic mineralization and ectopic calcification. Both proteins exhibit dual function, thus, besides being transporters, they also serve as receptors for several gammaretroviruses. We here show that it is possible to uncouple transport and receptor functions of a type III NaP(i) cotransporter and thus exploit the retroviral receptor function as a control for proper processing and folding of mutant proteins. Thus exchanging two putative transmembranic glutamate residues in human Pit2, Glu(55) and Glu(575), with glutamine or with lysine severely impaired or knocked out, respectively, P(i) transport function, but left viral receptor function undisturbed. Both glutamates are conserved in type III NaP(i) cotransporters, in fungal NaP(i) cotransporters PHO-4 and Pho89, and in other known or putative phosphate permeases from a number of species and are the first residues shown to be critical for type III NaP(i) cotransport. Their putative transmembranic positions together with the presented data are consistent with Glu(55) and Glu(575) being parts of a cation liganding site or playing roles in conformational changes associated with substrate transport. Finally, the results also show that Pit2 retroviral receptor function per se is not dependent on Pit2 P(i) transport 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.

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

Murine leukemia virus (MLV)-derived envelope proteins containing alterations in or adjacent to the highly conserved PHQ motif present at the N terminus of the envelope surface subunit (SU) are incorporated into vector particles but are not infectious due to a postbinding block to viral entry. These mutants can be rendered infectious by the addition of soluble receptor-binding domain (RBD) proteins in the culture medium. The RBD proteins that rescue the infectivity of these defective MLV vectors can be derived from the same MLV or from other MLVs that use distinct receptors to mediate entry. We have now constructed functional immunologically reactive gibbon ape leukemia virus (GALV) envelope proteins, tagged with a feline leukemia virus (FeLV)-derived epitope tag, which are efficiently incorporated into infectious particles. Tagged GALV envelope proteins bind specifically to cells expressing the phosphate transporter protein Pit1, demonstrating for the first time that Pit1 is the binding receptor for GALV and not a coreceptor or another type of GALV entry factor. We have also determined that GALV particles bearing SU proteins with an insertion C-terminal to the PHQ motif (GALV I(10)) bind Pit1 but fail to infect cells. Incubation with soluble GALV RBD renders GALV I(10) particles infectious, whereas incubation with soluble RBDs from MLV or FeLV-B does not. This finding is consistent with the results obtained by Lauring et al. using FeLV-T, a virus that employs Pit1 as a receptor but requires soluble FeLV RBD for entry. MLV and GALV RBDs are not able to render FeLV-T infectious (A. S. Lauring, M. M. Anderson, and J. Overbaugh, J. Virol. 75:8888-8898, 2001). Together, these results suggest that fusion-defective FeLV-T and GALV are restricted to homologous RBD rescue of infectivity.

Feline Pit2 functions as a receptor for subgroup B feline leukemia viruses

Different subgroups of feline leukemia virus (FeLV) use different host cell receptors for entry. Subgroup A FeLV (FeLV-A) is the virus that is transmitted from cat to cat, suggesting that cells expressing the FeLV-A receptor are important targets at the earliest stages of infection. FeLV-B evolves from FeLV-A in the infected cat through acquisition of cellular sequences that are related to the FeLV envelope gene. FeLV-Bs have been shown to infect cells using the Pit1 receptor, and some variants can infect cells at a lower efficiency using Pit2. Because these observations were made using receptor proteins of human or rodent origin, the role that Pit1 and Pit2 may play in FeLV-B replication in the cat is unclear. In this study, the feline Pit receptors were cloned and tested for their ability to act as receptors for different FeLV-Bs. Some FeLV-Bs infected cells expressing feline Pit2 and feline Pit1 with equal high efficiency. Variable region A (VRA) in the putative receptor-binding domain (RBD) was a critical determinant for both feline Pit1 and feline Pit2 binding, although other domains in the RBD appear to influence how efficiently the FeLV-B surface unit can bind to feline Pit2 and promote entry via this receptor. An arginine residue at position 73 in VRA was found to be important for envelope binding to feline Pit2 but not feline Pit1. Interestingly, this arginine is not found in endogenous FeLV sequences or in recombinant viruses recovered from feline cells infected with FeLV-A. Thus, while FeLV-Bs that are able to use feline Pit2 can evolve by recombination with endogenous sequences, a subsequent point mutation during reverse transcription may be needed to generate a virus that can efficiently enter the cells using the feline Pit2 as its receptor. These studies suggest that cells expressing the feline Pit2 protein are likely to be targets for FeLV-B infection in the cat.

Receptors and entry cofactors for retroviruses include single and multiple transmembrane-spanning proteins as well as newly described glycophosphatidylinositol-anchored and secreted proteins

In the past few years, many retrovirus receptors, coreceptors, and cofactors have been identified. These molecules are important for some aspects of viral entry, although in some cases it remains to be determined whether they are required for binding or postbinding stages in entry, such as fusion. There are certain common features to the molecules that many retroviruses use to gain entry into the cell. For example, the receptors for most mammalian oncoretroviruses are multiple membrane-spanning transport proteins. However, avian retroviruses use single-pass membrane proteins, and a sheep retrovirus uses a glycosylphosphatidylinositol-anchored molecule as its receptor. For some retroviruses, particularly the lentiviruses, two cell surface molecules are required for efficient entry. More recently, a soluble protein that is required for viral entry has been identified for a feline oncoretrovirus. In this review, we will focus on the various strategies used by mammalian retroviruses to gain entry into the cell. The choice of receptors will also be discussed in light of pressures that drive viral evolution and persistence.

Identification of envelope determinants of feline leukemia virus subgroup B that permit infection and gene transfer to cells expressing human Pit1 or Pit2

The retroviral vector systems that are in common use for gene therapy are designed to infect cells expressing either of two widely expressed phosphate transporter proteins, Pit1 or Pit2. Subgroup B feline leukemia viruses (FeLV-Bs) use the gibbon ape leukemia virus receptor, Pit1, as a receptor for entry. Our previous studies showed that some chimeric envelope proteins encoding portions of FeLV-B could also enter cells by using a related receptor protein, Pit2, which serves as the amphotropic murine leukemia virus receptor (S. Boomer, M. Eiden, C. C. Burns, and J. Overbaugh, J. Virol. 71:8116--8123, 1997). Here we show that an arginine at position 73 within variable region A (VRA) of the FeLV-B envelope surface unit (SU) is necessary for viral entry into cells via the human Pit2 receptor. However, C-terminal SU sequences have a dominant effect in determining human Pit2 entry, even though this portion of the protein is outside known receptor binding domains. This suggests that a combination of specific VRA sequences and C-terminal sequences may influence interactions between FeLV-B SU and the human Pit2 receptor. Binding studies suggest that the C-terminal sequences may affect a postbinding step in viral entry via the Pit2 receptor, although in all cases, binding of FeLV-B SU to human Pit2 was weak. In contrast, neither the arginine 73 nor specific C-terminal sequences are required for efficient binding or infection with Pit1. Taken together, these data suggest that different residues in SU may interact with these two receptors. The specific FeLV-Bs described here, which can enter cells using either human Pit receptor, may be useful as envelope pseudotypes for viruses used in gene therapy.

Transmembrane topology of PiT-2, a phosphate transporter-retrovirus receptor

PiT-1 and PiT-2 are related multiple transmembrane proteins which function as sodium-dependent phosphate transporters and as the cell receptors of several oncoretroviruses. Two copies of a homology domain that is found in distantly related species assign these proteins to a large family of phosphate transporters. A current membrane topology model of PiT-1 and PiT-2 predicts 10 transmembrane domains. However, the validity of this model has not been addressed experimentally. We addressed this issue by a comprehensive study of human PiT-2. Evidence was obtained for glycosylation of asparagine 81. Epitope tagging showed that the N- and C-terminal extremities are extracellular. The orientation of C-terminal-truncation mutants expressed in cell-free translation assays and incorporated into microsomal membranes was examined by immunoprecipitation. Data were interpreted with respect to previous knowledge about retrovirus binding sites, to the existence of repeated homology domains, and to predictions made in family members. A model in which PiT-2 has 12 transmembrane domains and extracellular N- and C-terminal extremities is proposed. This model, which differs significantly from previous predictions about PiT-2 topology, may be useful for further investigations of PiT-2 interactions with other proteins and for the understanding of PiT-2 transporter and virus receptor functions.

Expression of type III sodium-dependent phosphate transporters/retroviral receptors mRNAs during osteoblast differentiation

Inorganic phosphate (Pi) is essential for the formation of bone. Pi transport in osteoblastic cells is mainly handled by sodium-dependent Pi (NaPi) transporters, different from the renal type I and II transporters; their molecular identities are, however, still subject to investigation. Recently, two type III NaPi transporters, Pit1 and Pit2, were identified, both of which exhibit some of the biochemical and regulatory characteristics of osteoblastic cell-associated NaPi transporters. Here, we have investigated the Pit1 and Pit2 steady-state mRNA levels during the osteoblast differentiation in cultures of the nontransformed MC3T3-E1 cell line. While Pit2 mRNAs were invariably expressed at low levels, Pit1 mRNA levels were found to increase during osteoblast differentiation concomitantly with osteocalcin mRNA. Moreover, the increase in Pit1 mRNA levels also correlated with the time in culture at which mineralization could be observed. The increase in Pit1 mRNA levels over time in culture was only observed in cultures grown under conditions allowing for osteoblast differentiation. This is the first time that osteoblast differentiation-dependent regulation of expression of a NaPi transporter has been demonstrated. Moreover, we show here for the first time the presence of Pit1 and Pit2 mRNAs in undifferentiated and differentiated nontransformed osteoblastic cells. Our data suggest that both Pit1 and Pit2 NaPi transporters are involved in Pi transport in preosteoblastic and osteoblastic cells, and they represent the first evidence consistent with a potential role for Pit1, but not for Pit2, in differentiation-dependent Pi transport. The observed upregulation of Pit1 mRNA levels during osteoblast differentiation suggests that Pit1 might be used as a marker for osteoblast maturation.

Efficient retrovirus-mediated gene transfer to transplantable human bone marrow cells in the absence of fibronectin

The low frequency of transplantable hematopoietic stem cells in adult human bone marrow (BM) and other differences from cord blood stem cells have impeded studies to optimize the retroviral transduction of stem cells from adult sources. To address this problem, first a cytokine combination was defined that would both maximize the kinetics of adult BM CD34+CD38− cell mitogenesis and minimize the period of prestimulation required for the transduction of these cells by a MSCV-GFP/neor virus in tissue culture dishes in the absence of fibronectin. Three days of stimulation with flt3-ligand, Steel factor, interleukin (IL)-3, and hyper-IL-6 proved both necessary and sufficient to obtain 83% ± 2% GFP+ CD34+CD38− cells, 75% ± 10% G418-resistant clonogenic progenitors, and 50% ± 20% transduced long-term culture-initiating cells as recovered 48 hours after a single exposure to virus. Moreover, this was accompanied by a several-fold increase in viral receptor (pit-1) messenger RNA transcripts in the target cells. Using this prestimulation protocol, repeated daily exposure to new virus (3×) did not alter the proportion of transduced cells over that obtained with a single exposure. Adult human BM cells able to engraft immunodeficient (NOD/SCID-β2M−/−) mice were also efficiently transduced (10%-20% GFP+ human lymphoid and myeloid cells present 6-8 weeks after transplant) using a 6-day prestimulation and infection protocol. A clinically useful efficiency of retrovirus-mediated gene transfer to transplantable adult human BM stem cells can thus be obtained with a protocol that allows their semisynchronous activation into cycle and concomitant increased expression of virus receptor transcripts before virus exposure.

Efficient retrovirus-mediated gene transfer to transplantable human bone marrow cells in the absence of fibronectin

The low frequency of transplantable hematopoietic stem cells in adult human bone marrow (BM) and other differences from cord blood stem cells have impeded studies to optimize the retroviral transduction of stem cells from adult sources. To address this problem, first a cytokine combination was defined that would both maximize the kinetics of adult BM CD34+CD38− cell mitogenesis and minimize the period of prestimulation required for the transduction of these cells by a MSCV-GFP/neor virus in tissue culture dishes in the absence of fibronectin. Three days of stimulation with flt3-ligand, Steel factor, interleukin (IL)-3, and hyper-IL-6 proved both necessary and sufficient to obtain 83% ± 2% GFP+ CD34+CD38− cells, 75% ± 10% G418-resistant clonogenic progenitors, and 50% ± 20% transduced long-term culture-initiating cells as recovered 48 hours after a single exposure to virus. Moreover, this was accompanied by a several-fold increase in viral receptor (pit-1) messenger RNA transcripts in the target cells. Using this prestimulation protocol, repeated daily exposure to new virus (3×) did not alter the proportion of transduced cells over that obtained with a single exposure. Adult human BM cells able to engraft immunodeficient (NOD/SCID-β2M−/−) mice were also efficiently transduced (10%-20% GFP+ human lymphoid and myeloid cells present 6-8 weeks after transplant) using a 6-day prestimulation and infection protocol. A clinically useful efficiency of retrovirus-mediated gene transfer to transplantable adult human BM stem cells can thus be obtained with a protocol that allows their semisynchronous activation into cycle and concomitant increased expression of virus receptor transcripts before virus exposure.

A 13-amino-acid Pit1-specific loop 4 sequence confers feline leukemia virus subgroup B receptor function upon Pit2

Feline leukemia virus subgroup B (FeLV-B) and gibbon ape leukemia virus (GALV) utilize the human protein Pit1 but not the related protein, Pit2, as receptor. A stretch of 9 amino acids, named region A, was identified in the putative fourth extracellular loop of Pit1 (residues 550 through 558) as critical for FeLV-B and GALV receptor function. However, the presence of Pit1 region A did not confer receptor function for FeLV-B upon Pit2, while it did so for GALV. We have here shown that the presence of two Pit1-specific loop 4 residues (tyrosine 546 and valine 548) in addition to Pit1 region A is sufficient to make Pit2 an efficient FeLV-B receptor; that is, a stretch of 13 amino acids encompassing all loop 4 amino acid differences between Pit1 and Pit2 comprises a C-terminal determinant for FeLV-B receptor function. Thus, the same limited receptor region is sufficient to confer receptor function for both viruses upon Pit2.

A comprehensive approach to mapping the interacting surfaces of murine amphotropic and feline subgroup B leukemia viruses with their cell surface receptors

Because mutations in envelope glycoproteins of retroviruses or in their cell surface receptors can eliminate function by multiple mechanisms, it has been difficult to unambiguously identify sites for their interactions by site-directed mutagenesis. Recently, we developed a gain-of-function approach to overcome this problem. Our strategy relies on the fact that feline leukemia virus subgroup B (FeLV-B) and amphotropic murine leukemia virus (A-MLV) have closely related gp70 surface envelope glycoproteins and use related Na(+)-dependent phosphate symporters, Pit1 and Pit2, respectively, as their receptors. We previously observed that FeLV-B/A-MLV envelope glycoprotein chimeras spliced between the variable regions VRA and VRB were unable to use Pit1 or Pit2 as a receptor but could efficiently use specific Pit1/Pit2 chimeras. The latter study suggested that the VRA of A-MLV and FeLV-B functionally interact with the presumptive extracellular loops 4 and 5 (ECL4 and -5) of their respective receptors, whereas VRB interacts with ECL2. We also found that FeLV-B gp70 residues F60 and P61 and A-MLV residues Y60 and V61 in the first disulfide-bonded loop of VRA were important for functional interaction with the receptor's ECL4 or -5. We have now extended this approach to identify additional VRA and VRB residues that are involved in receptor recognition. Our studies imply that FeLV-B VRA residues F60 and P61 interact with the Pit1 ECL5 region, whereas VRA residues 66 to 78 interact with Pit1 ECL4. Correspondingly, A-MLV VRA residues Y60 and V61 interact with the Pit2 ECL5 region, whereas residues 66 to 78 interact with Pit2 ECL4. Similar studies that focused on the gp70 VRB implicated residues 129 to 139 as contributing to specific interactions with the receptor ECL2. These results identify three regions of gp70 that interact in a specific manner with distinct portions of their receptors, thereby providing a map of the functionally interacting surfaces.

Gibbon ape leukemia virus receptor functions of type III phosphate transporters from CHOK1 cells are disrupted by two distinct mechanisms

The Chinese hamster cell lines E36 and CHOK1 dramatically differ in susceptibility to amphotropic murine leukemia virus (A-MuLV) and gibbon ape leukemia virus (GALV); E36 cells are highly susceptible to both viruses, CHOK1 cells are not. We have previously shown that GALV can infect E36 cells by using both its own receptor, HaPit1, and the A-MuLV receptor, HaPit2. Given that the two cell lines are from the same species, the loss of function of both of these receptors in CHOK1 cells is surprising. Other studies have shown that CHOK1 cells secrete proteins that block A-MuLV entry into CHOK1 as well as E36, suggesting the two A-MuLV receptors are functionally identical. However, CHOK1 conditioned medium does not block GALV entry into E36, indicating the secreted inhibitors do not block HaPit1. HaPit1 and ChoPit1 therefore differ as receptors for GALV; ChoPit1 is either inactivated by secreted factors or intrinsically nonfunctional. To determine why GALV cannot infect CHOK1, we cloned and sequenced ChoPit1 and ChoPit2. ChoPit2 is almost identical to HaPit2, which explains why CHOK1 conditioned medium blocks A-MuLV entry via both receptors. Although ChoPit1 and HaPit1 are 91% identical, a notable difference is at position 550 in the fourth extracellular region, shown by several studies to be crucial for GALV infection. Pit1 and HaPit1 have aspartate at 550, whereas ChoPit1 has threonine at this position. We assessed the significance of this difference for GALV infection by replacing the aspartate 550 in Pit1 with threonine. This single substitution rendered Pit1 nonfunctional for GALV and suggests that threonine at 550 inactivates ChoPit1 as a GALV receptor. Whether native ChoPit1 functions for GALV was determined by interference assays using Lec8, a glycosylation-deficient derivative of CHOK1 that is susceptible to both viruses and that has the same receptors as CHOK1. Unlike with E36, GALV and A-MuLV exhibited reciprocal interference when infecting Lec8, suggesting that they use the same receptor. We conclude both viruses can use ChoPit2 in the absence of the inhibitors secreted by CHOK1 and ChoPit1 is nonfunctional.

Amphotropic murine leukemia virus entry is determined by specific combinations of residues from receptor loops 2 and 4

Pit2 is the human receptor for amphotropic murine leukemia virus (A-MuLV); the related human protein Pit1 does not support A-MuLV entry. Interestingly, chimeric proteins in which either the N-terminal or the C-terminal part of Pit2 was replaced by the Pit1 sequence all retained A-MuLV receptor function. A possible interpretation of these observations is that Pit1 harbors sequences which can specify A-MuLV receptor function when presented in a protein context other than Pit1, e.g., in Pit1-Pit2 hybrids. We reasoned that such Pit1 sequences might be identified if presented in the Neurospora crassa protein Pho-4. This protein is distantly related to Pit1 and Pit2, predicted to have a similar membrane topology with five extracellular loops, and does not support A-MuLV entry. We show here that introduction of the Pit1-specific loop 2 sequence conferred A-MuLV receptor function upon Pho-4. Therefore, we conclude that (i) a functional A-MuLV receptor can be constructed by combining sequences from two proteins each lacking A-MuLV receptor function and that (ii) a Pit1 sequence can specify A-MuLV receptor function when presented in another protein context than that provided by Pit1 itself. Previous results indicated a role of loop 4 residues in A-MuLV entry, and the presence of a Pit2-specific loop 4 sequence was found here to confer A-MuLV receptor function upon Pho-4. Moreover, the introduction of a Pit1-specific loop 4 sequence, but not of a Pit2-specific loop 4 sequence, abolished the A-MuLV receptor function of a Pho-4 chimera harboring the Pit1-specific loop 2 sequence. Together, these data suggest that residues in both loop 2 and loop 4 play a role in A-MuLV receptor function. A-MuLV is, however, not dependent on the specific Pit2 loop 2 and Pit2 loop 4 sequences for entry; rather, the role played by loops 2 and 4 in A-MuLV entry can be fulfilled by several different combinations of loop 2 and loop 4 sequences. We predict that the residues in loops 2 and 4, identified in this study as specifying A-MuLV receptor function, are to be found among those not conserved among Pho-4, Pit1, and Pit2.

RNA levels of human retrovirus receptors Pit1 and Pit2 do not correlate with infectibility by three retroviral vector pseudotypes

The gibbon ape leukemia virus (GaLV) and the amphotropic murine leukemia virus (A-MuLV) infect human cells via specific receptors, Pit1 and Pit2, respectively. mRNA levels of these receptors were determined by Northern analysis and for Pit2 in addition by quantitative RT-PCR. Pit1 and Pit2 were expressed in different amounts in human tissues and cell lines; Pit1-specific mRNA was generally more abundant than Pit2 mRNA. No correlation was found between Pit1 and Pit2 RNA levels and infectibility by GaLV and A-MuLV pseudotyped vectors, respectively. GaLV and A-MuLV revealed a partial reciprocal interference. MuLV-10A1 can utilize both Pit1 and Pit2 for entry into cells but could not infect any of the 14 human cell lines more efficiently than A-MuLV or GaLV. Interference assays suggested that MuLV-10A1 has a higher affinity for and infected most cells predominantly by Pit2. However, at least in one cell line it used Pit1 more efficiently for entry. We conclude that (1) Pit1 and Pit2 mRNA levels in human cells are not indicative of the infectibility by GaLV and A-MuLV pseudotypes, respectively; (2) A-MuLV can infect target cells as efficiently as can GaLV, although Pit2 RNA is less abundant than Pit1 RNA; (3) factor(s) in addition to the presence of Pit1 and Pit2 are involved in retroviral infection; and (4) MuLV-10A1 pseudotype does not infect human cells more efficiently than do A-MuLV and GaLV pseudotypes.

Entry of amphotropic murine leukemia virus is influenced by residues in the putative second extracellular domain of its receptor, Pit2

Human cells express distinct but related receptors for the gibbon ape leukemia virus (GALV) and the amphotropic murine leukemia virus (A-MuLV), termed Pit1 and Pit2, respectively. Pit1 is not able to function as a receptor for A-MuLV infection, while Pit2 does not confer susceptibility to GALV. Previous studies of chimeric receptors constructed by interchanging regions of Pit1 and Pit2 failed to clarify the determinants unique to Pit2 which correlate with A-MuLV receptor function. In order to identify which regions of Pit2 are involved in A-MuLV receptor function, we exchanged the putative second and third extracellular domains of Pit1, either individually or together, with the corresponding regions of Pit2. Our functional characterization of these receptors indicates a role for the putative second extracellular domain (domain II) in A-MuLV infection. We further investigated the influence of domain II with respect to A-MuLV receptor function by performing site-specific mutagenesis within this region of Pit2. Many of the mutations had little or no effect on receptor function. However, the substitution of serine for methionine at position 138 (S138M) in a Pit1 chimera containing domain II of Pit2 resulted in a 1,000-fold reduction in A-MuLV receptor function. Additional mutations made within domain II of the nonfunctional S138M mutant restored receptor function to nearly wild-type efficiency. The high degree of tolerance for mutations as well as the compensatory effect of particular substitutions observed within domain II suggests that an element of secondary structure within this region plays a critical role in the interaction of the receptor with A-MuLV.

Single amino acid insertion in loop 4 confers amphotropic murine leukemia virus receptor function upon murine Pit1

Pit1 is the human receptor for gibbon ape leukemia virus (GALV) and feline leukemia virus subgroup B (FeLV-B), while the related human protein Pit2 is a receptor for amphotropic murine leukemia virus (A-MuLV). The A-MuLV-related isolate 10A1 can utilize both Pit1 and Pit2 as receptors. A stretch of amino acids named region A was identified in Pit1 (residues 550 to 558 in loop 4) as critical for GALV and FeLV-B receptor function. We have here investigated the role of region A in A-MuLV and 10A1 entry. Insertion of a single amino acid in region A of mouse Pit1 resulted in a functional A-MuLV receptor, showing that region A plays a role in A-MuLV infection. Moreover, the downregulation of 10A1 receptor function by changes in region A of human Pit1 indicates that this region is also involved in 10A1 entry. Therefore, region A seems to play a role in infection by all viruses utilizing Pit1 and/or Pit2 as receptors.

Variable regions A and B in the envelope glycoproteins of feline leukemia virus subgroup B and amphotropic murine leukemia virus interact with discrete receptor domains

The surface (SU) envelope glycoproteins of feline leukemia virus subgroup B (FeLV-B) and amphotropic murine leukemia virus (A-MLV) are highly related, even in the variable regions VRA and VRB that have been shown to be required for receptor recognition. However, FeLV-B and A-MLV use different sodium-dependent phosphate symporters, Pit1 and Pit2, respectively, as receptors for infection. Pit1 and Pit2 are predicted to have 10 membrane-spanning domains and five extracellular loops. The close relationship of the retroviral envelopes enabled us to generate pseudotype virions carrying chimeric FeLV-B/A-MLV envelope glycoproteins. We found that some of the pseudotype viruses could not use Pit1 or Pit2 proteins but could efficiently utilize specific chimeric Pit1/Pit2 proteins as receptors. By studying Mus dunni tail fibroblasts expressing chimeric Pit1/Pit2 proteins and pseudotype virions carrying chimeric FeLV-B/A-MLV envelopes, we show that FeLV-B and A-MLV VRA and VRB interact in a modular manner with specific receptor domains. Our results suggest that FeLV-B VRA interacts with Pit1 extracellular loops 4 and 5 and that residues Phe-60 and Pro-61 of FeLV-B VRA are essential for receptor choice. However, this interaction is insufficient for infection, and an additional interaction between FeLV-B VRB and Pit1 loop 2 is essential. Similarly, A-MLV infection requires interaction of A-MLV VRA with Pit2 loops 4 and 5 and VRB with Pit2 loop 2, with residues Tyr-60 and Val-61 of A-MLV VRA being critical for receptor recognition. Together, our results suggest that FeLV-B and A-MLV infections require two major discrete interactions between the viral SU envelope glycoproteins and their respective receptors. We propose a common two-step mechanism for interaction between retroviral envelope glycoproteins and cell surface receptors.

Three distinct envelope domains, variably present in subgroup B feline leukemia virus recombinants, mediate Pit1 and Pit2 receptor recognition

Subgroup B feline leukemia viruses (FeLV-Bs) evolve from subgroup A FeLV (FeLV-A) by recombining with portions of endogenous FeLV envelope sequences in the cat genome. The replication properties of FeLV-B are distinct from those of FeLV-A; FeLV-B infects many nonfeline cell lines and recognizes the human Pit1 (HuPit1) receptor, whereas FeLV-A infects primarily feline cells, using a distinct but as yet undefined receptor. Here, we demonstrate that some FeLV-Bs can also use human Pit2 (HuPit2) and hamster Pit2 (HaPit2) for entry. By making viruses that contain chimeric surface (SU) envelope proteins from FeLV-A and FeLV-B, and testing their infectivity, we have defined genetic determinants that confer host range and specific receptor recognition. HuPit1 receptor recognition determinants localize to the N-terminal region of the FeLV-B SU, amino acids 83 to 116, encompassing the N-terminal portion of variable region A (VRA). While this 34-amino-acid domain of the FeLV-B VRA is sufficient for infection of some cells (feline, canine, and human), amino acids 146 to 249 of FeLV-B, which include variable region B (VRB), were required for efficient infection in other cell types (hamster, bovine, and rat). Chimeras encoding FeLV-B VRA and VRB also infected cells expressing HaPit2 and HuPit2 receptors more efficiently than chimeras encoding only the VRA of FeLV-B, suggesting that VRB provides a secondary determinant that is both cell and receptor specific. However, viruses containing additional FeLV-B sequences in the C terminus of SU could not recognize HuPit2, implying that there is a determinant beyond VRB that negatively affects HuPit2 interactions. Thus, Pit2 recognition may drive selection for the generation of specific FeLV-B recombinants, offering an explanation for the two major classes of FeLV-B that have been observed in vivo. Furthermore, the finding that some FeLV-Bs can use both Pit1 and Pit2 may explain previous observations that FeLV-B and GALV, which primarily uses Pit1, display nonreciprocal interference on many cell types.

Fungal phosphate transporter serves as a receptor backbone for gibbon ape leukemia virus

Pit1, the receptor for gibbon ape leukemia virus (GALV), is proposed to be an integral membrane protein with five extracellular loops. Chimeras made between Pit1 homologs differing in permissivity for infection and between Pit1 and the related protein Pit2 have shown that the fourth extracellular loop plays a critical role in infection. However, further elucidation of the roles of the extracellular loops in infection is hampered by the high level of sequence similarity among these proteins. The sodium-dependent phosphate transporter, Pho-4, from the filamentous fungus Neurospora crassa is distantly related to Pit1 and -2, showing an amino acid identity of only 35% to Pit1 in the putative extracellular loops. We show here that Pho-4 itself does not function as a receptor for GALV. Introduction of 12 Pit1-specific amino acid residues in the putative fourth extracellular loop of Pho-4 resulted in a functional GALV receptor. Therefore, the presence of a Pit1 loop 4-specific sequence is sufficient to confer receptor function for the mammalian retrovirus GALV on the fungal phosphate transporter Pho-4.

Mutational analysis of the proposed gibbon ape leukemia virus binding site in Pit1 suggests that other regions are important for infection

Region A of Pit1 (residues 550 to 558 in domain IV) and related receptors has remained the only sequence implicated in gibbon ape leukemia virus (GALV) infection, and an acidic residue at the first position appeared indispensable. The region has also been proposed to be the GALV binding site, but this lacks empirical support. Whether an acidic residue at the first position in this sequence is a definitive requirement for GALV infection has also remained unclear; certain receptors retain function even in the absence of this acidic residue. We report here that in Pit1 an acidic residue is dispensable not only at position 550 but also at 553 alone and at both positions. Further, the virus requires no specific residue at either position. Mutations generated a collection of region A sequences, often with fundamentally different physicochemical properties (overall hydrophobicity or hydrophilicity and net charge of -1, or 0, or +1), and yet Pit1 remained an efficient GALV receptor. A comparison of these sequences and a few previously published ones from highly efficient GALV receptors revealed that every position in region A can vary without affecting GALV entry. Even Pit2 is nonfunctional for GALV only because it has lysine at the first position in its region A, which is otherwise highly diverse from region A of Pit1. We propose that region A itself is not the GALV binding motif and that other sequences are required for virus entry. Indeed, certain Pit1/Pit2 chimeras revealed that sequences outside domain IV are specifically important for GALV infection.

The amphotropic murine leukemia virus receptor gene encodes a 71-kilodalton protein that is induced by phosphate depletion

The amphotropic murine leukemia virus (MuLV) can infect cells from a number of mammals, including humans, via its specific receptor. Basic knowledge of amphotropic MuLV receptor expression is likely to be useful in the development and improvement of gene therapy protocols based on amphotropic-pseudotyped vectors. To investigate the expression of the human receptor for the amphotropic MuLV (GLVR-2, newly termed Pit2), we determined its mRNA levels in several cell lines and found them to vary significantly. Induction of increased levels of mRNA after removal of phosphate from the media was observed in two osteosarcoma cell lines. The increase in GLVR-2 mRNA resulted in a concomitant rise in the levels of a 71-kDa protein specifically recognized by affinity-purified antibodies against GLVR-2. Using these antibodies, we were able to confirm the intracellular topology of the large hydrophilic domain between the proposed sixth and seventh transmembrane domains of the GLVR-2 protein. This assignment is in agreement with the fourth extracellular loop being outside the cell, consistent with the proposal that the fourth extracellular loop of GLVR-2 contains the envelope binding site.

Residue Trp-48 of Tva is critical for viral entry but not for high-affinity binding to the SU glycoprotein of subgroup A avian leukosis and sarcoma viruses

Previously, mutant Tva receptors were classified as either partially or completely defective in mediating subgroup A avian leukosis and sarcoma virus (ALSV-A) entry (C. Bélanger, K. Zingler, and J. A. T. Young, J. Virol. 69:1019-1024, 1995; K. Zingler, C. Bélanger, R. Peters, D. Agard, and J. A. T. Young, J. Virol. 69:4261-4266, 1995). To specifically test the abilities of these mutant Tva proteins to bind ALSV-A surface (SU) protein, binding studies were performed with a subgroup A SU-immunoadhesin. This fusion protein is composed of the subgroup A Schmidt-Ruppin SU protein fused in frame to a rabbit immunoglobulin constant region. This reagent was conjugated to fluorescein isothiocyanate and used for flow cytometric analysis with transfected human 293 cells expressing different forms of Tva. The SU-immunoadhesin bound the wild-type Tva protein with a KD of approximately 1.5 nM. Amino acid substitutions that reduced viral entry at Asp-46 and at Cys-35 and Cys-50, which are predicted to form an intrachain disulfide bond in Tva, drastically reduced the binding affinity for the SU-immunoadhesin. Thus, the effects on viral entry of some mutations could be explained solely by changes in the binding affinity for ALSV-A SU. However, this was not true for other mutations tested, especially those with amino acid substitutions that replaced Trp-48. Compared with the wild-type receptor, these latter mutations led to approximately 43- to 200-fold reductions in viral infectivity but only to approximately 2.5- to 3.4-fold reductions in the binding affinity for the SU-immunoadhesin. These results support a role for Trp-48 of Tva in mediating steps of viral entry subsequent to binding ALSV-A SU.