Mouse transferrin receptor 1 is the cell entry receptor for mouse mammary tumor virus Susan R. Ross*†, Jason J. Schofield*, Christine J. Farr‡, and Maja Bucan§ *Department of Microbiology and Cancer Center, University of Pennsylvania, Philadelphia, PA 19104; ‡Department of Genetics, University of Cambridge, Cambridge CB2 3EH, United Kingdom; and §Center for Neurobiology and Behavior, Department of Psychiatry, University of Pennsylvania, Philadelphia, PA 19104 Edited by John M. Coffin, Tufts University School of Medicine, Boston, MA, and approved July 23, 2002 (received for review June 14, 2002) Enveloped viruses enter cells by binding to their entry receptors mapped a receptor to mouse Chr 16 (4). We also identified a and fusing with the membrane at the cell surface or after traffick- potential MMTV receptor that mapped to Chr 19, termed ing through acidic endosomal compartments. Species-specific virus MTVR2, using virus binding as an assay for expression cloning tropism is usually determined by these entry receptors. Because (5). To determine whether the bona fide MMTV receptor mouse mammary tumor virus (MMTV) is unable to infect Chinese mapped to Chr 16, Chr 19, or elsewhere in the mouse genome, hamster cells, we used phenotypic screening of the T31 mouse͞ we screened the T31 mouse͞Chinese hamster RH cell panel (6) hamster radiation hybrid panel to map the MMTV cell entry with Moloney murine leukemia virus (MoMLV) pseudotypes receptor gene and subsequently found that it is transferrin recep- bearing the MMTV Env protein on their surface. As described tor 1. MMTV-resistant human cells that expressed mouse trans- here, we found that the receptor gene mapped to Chr 16. This ferrin receptor 1 became susceptible to MMTV infection, and placement allowed us to test bacterial artificial chromosomes treatment of mouse cells with a monoclonal antibody that down- (BACs) from the candidate region for receptor activity and then regulated cell surface expression of the receptor blocked infection. to identify mouse transferrin receptor 1 (mTfR1) as the MMTV MMTV, like vesicular stomatitis virus, depended on acid pH for cell entry receptor. infection. MMTV may use transferrin receptor 1, a membrane TfR1 is a type II, single membrane-spanning glycoprotein with protein that is endocytosed via clathrin-coated pits and traffics a short cytoplasmic domain and is the major means by which through the acidic endosomes, to rapidly get to a compartment most cells take up iron (7). The receptor has a high affinity for where acid pH triggers the conformational changes in envelope iron-loaded transferrin at neutral pH, and upon binding ligand protein required for membrane fusion. traffics to the early acidic endosome, where it releases iron, recycles back to the cell surface, and releases transferrin. The n recent years, a large number of viral receptors have been transferrin binding site has been determined by mutagenesis and Iidentified using a variety of approaches. These include the binding studies, and maps to an RGD-containing sequence in the biochemical purification of virus-binding molecules, the gener- extracellular domain that is highly conserved between species (8, ation of antibodies to cell surface proteins that block infection, 9). In addition to transferrin, peptides that bind at other sites on and the introduction of genomic or cDNA libraries from sus- TfR1 cause it to be endocytosed and traffic to the acidic ceptible into resistant cells, followed by infection with the virus endosome (10). of interest, usually engineered to encode an identifiable marker The identification of a protein that traffics to the acidic such as a drug resistance gene. Although such techniques are endosome upon ligand binding is interesting in light of a previous extremely powerful and have been successful in many cases, the observation that treatment of MMTV-infected mammary tumor identification of some virus receptors has been refractive to cells with acid pH induced cell–cell fusion (11). We show here these methods. Recently, Rai and colleagues (1) showed that phenotypic that MMTV pseudovirus infection is also dependent on acidified screening of radiation hybrid panels could be used for receptor endosomes. Taken together, these results suggest that MMTV binds mTfR1 at the cell surface and then traffics to an acidic identification. These investigators tested the Stanford G3 Chi- ͞ nese hamster͞human radiation hybrid (RH) panel for infection endosomal compartment where virus cell membrane fusion with Jaagsietke sheep retrovirus (JSRV) pseudoviruses and occurs. mapped the receptor gene. Historically, somatic cell hybrid Materials and Methods panels between Chinese hamster cells, which are refractive to infection by many viruses including mouse mammary tumor Infection of the RH Panel. MMTV-Env pseudotyped Mo-MuLV virus (MMTV), and cells from different species have been used vectors were prepared as described (5). A23 hamster cells and to map receptor genes. These mapping studies resulted in the A23-derived RH clones (gifts from P. N. Goodfellow, Glaxo- chromosomal placement of the virus receptor genes, but not SmithKline Pharmaceuticals, Stevenage, U.K., and L. C. Mc- their identification. RH panels, in which the cell line from the Carthy, Glaxo Wellcome Medicines Research, Stevenage, U.K.) species of interest are X-irradiated to generate small chromo- were grown in hypoxanthine͞aminopterin͞thymidine (HAT)- somal fragments before fusion with the hamster cells, have been DMEM, supplemented with 10% fetal bovine sera. The RH cells one of the major tools used in generating high-resolution maps were plated at 5 ϫ 104 cells per well in a 6-well plate and infected of different organisms and have resulted in maps that integrate with the MMTV-Env pseudotypes the following day. After 2 genetic, physical, and expressed sequence tag (EST) data (2). days, the cells were stained for ␤-galactosidase activity. Wells The ability to precisely map the JSRV receptor gene on this RH panel in conjunction with the availability of cosmid͞P1 contig maps and genomic sequence from the region led to cloning of the This paper was submitted directly (Track II) to the PNAS office. entry receptor, HYAL2 (3). Abbreviations: MMTV, mouse mammary tumor virus; NMuMG, normal murine mammary gland; Chr, chromosome; BAC, bacterial artificial chromosome; Env, envelope; RH, radia- We undertook this approach to identify the receptor for tion hybrid; mTfR, mouse transferrin receptor; LFU, lacZ-forming units. MMTV, a betaretrovirus that causes breast carcinomas in mice. †To whom reprint requests should be addressed at: University of Pennsylvania, 313 BRBII͞ Previous work using vesicular stomatitis virus (VSV)- 6142, 421 Curie Boulevard, Philadelphia, PA 19104-6142. E-mail: [email protected]. pseudotyped with the MMTV envelope (Env) protein had upenn.edu. 12386–12390 ͉ PNAS ͉ September 17, 2002 ͉ vol. 99 ͉ no. 19 www.pnas.org͞cgi͞doi͞10.1073͞pnas.192360099 Downloaded by guest on September 30, 2021 containing greater than 25 ␤-galactosidase colonies were Table 1. BAC transfection to test for MMTV receptor activity counted as positive. Cell line BAC͞plasmid LFU͞ml LFU͞ml ϩ anti-MMTV Recombinant DNAs. BAC clones were obtained from BACPAC 293T 320J17 0 0 Resources (Oakland, CA) or Research Genetics (Birmingham, 219I11 0 0 AL) and were purified according to standard techniques. An 342H15 0 0 insert containing the mouse TFR1 cDNA (a gift from N. 248H7 1 0 Andrews, Howard Hughes Medical Institute, The Children’s 27L21 0 ND Hospital, Boston) was transferred from the pSPORT to the 121N24 0 0 pcDNA3.1 mammalian expression vector (mTfR1). Human 308K23 3 0 TFR1 in pCDM8 was a gift from M. Marks (University of 73P12 77 1 Pennsylvania) and mouse TFR2 (IMAGE clone 4196597; 322N13 84 0 pCMV-SPORT6 vector) was obtained from Research Genetics. 324J8 1 0 342H15 4 0 Transfection of 293T Cells and Pseudovirus Infection. Two micro- mTfR1 7,300 0 grams of BAC or plasmid DNA was transiently transfected into hTfR1 0 ND 293T cells plated in 6-well dishes, using calcium phosphate — 00 precipitation. Twenty-four hours after transfection, the cells A23 — 0ND were infected with MMTV pseudotypes containing the lacZ NMuMG — 3,461 0 gene under the control of the MoMLV long terminal repeat (pHIT60) (12) and the cells were stained for LacZ activity 48 hr 293T cells were transfected by the CaPO4 method in duplicate with 2 ug of later as described (5). Normal murine mammary gland each DNA in 6-well dishes. Twenty-four hours after transfection, the cells were (NMuMG) cells were infected as positive controls. A20 cells infected with MMTV Env-pseudotyped Mo-MLV vectors in the absence or ␮ presence of goat polyclonal anti-MMTV antisera (ϩ anti-MMTV) (Quality were infected with 100 l of pseudovirus stock in 96-well dishes Biotech, Camden, NJ). The normal murine mammary gland cell line, NMuMG, ϫ 5 at 1 10 cells per well for 2 hr; five wells for each treatment served as a positive control for MMTV infection (13). Shown are the lacZ- ϫ 6 (1 10 cells in total) were pooled and diluted to 5 ml final forming units (LFU) per milliliter of virus supernatant. ND, not determined. volume with media. Two days after infection, cell extracts were made and ␤-galactosidase assays were performed using a com- mercial kit, according to the manufacturer’s instructions (Pro- (ES) cells (6). We first established that A23 cells were resistant mega). Data are presented as milliunits͞␮g protein. In some to infection by MMTV pseudotypes (Table 1); we did not have cases, duplicate wells were treated with 1 ␮l͞ml goat anti- the 129 ES cell line available, but we have found that all tested MMTV antisera (13) at the time of infection or 1 ␮g͞ml rat murine tissue culture lines were susceptible (13). We then anti-mouse CD71 monoclonal antibody C2 (MOPC-315; RDI, screened 98͞100 T31 RH panel cell lines for infection by these Flanders, NJ) for 10 min at room temperature before the pseudotypes.