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J. Biol. Chem., Vol. 281, Issue 20, 13964-13971, May 19, 2006
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1
1
2
3


4
From the
Biochemistry Department, University of Lausanne, CH-1066 Epalinges, Switzerland and
BiogenIdec, Cambridge, Massachusetts 02142
Received for publication, February 17, 2006 , and in revised form, March 15, 2006.
| ABSTRACT |
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, lymphotoxin 
, OX40L, RANKL, TL1A, TNF, TWEAK, and TRAIL) bind members of the TNF receptor superfamily (TNFRSF). A comprehensive survey of ligand-receptor interactions was performed using a flow cytometry-based assay. All ligands engaged between one and five receptors, whereas most receptors only bound one to three ligands. The receptors DR6, RELT, TROY, NGFR, and mouse TNFRH3 did not interact with any of the known TNFSF ligands, suggesting that they either bind other types of ligands, function in a ligand-independent manner, or bind ligands that remain to be identified. The study revealed that ligand-receptor pairs are either cross-reactive between human and mouse (e.g. Tweak/Fn14, RANK/RANKL), strictly species-specific (GITR/GITRL), or partially species-specific (e.g. OX40/OX40L, CD40/CD40L). Interestingly, the receptor binding patterns of lymphotoxin
and
are redundant in the human but not in the mouse system. Ligand oligomerization allowed detection of weak interactions, such as that of human TNF with mouse TNFR2. In addition, mouse APRIL exists as two different splice variants differing by a single amino acid. Although human APRIL does not interact with BAFF-R, the shorter variant of mouse APRIL exhibits weak but detectable binding to mouse BAFF-R. | INTRODUCTION |
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TNF family ligands are type II transmembrane proteins with an extracellular, homotrimeric C-terminal TNF homology domain that is frequently released as a soluble cytokine upon proteolytic processing (11). Lymphotoxin
is remarkable in that it cannot form homotrimers but instead heterotrimerizes with lymphotoxin
(12). Receptors are usually type I and sometimes type III (BCMA, TACI, BAFF-R, and XEDAR) membrane proteins, with the exception of OPG and DcR3, which are secreted. Receptors are characterized by the presence of one to four cysteine-rich domains (CRD) in their extracellular portion. Those receptors with several CRDs adopt an elongated structure and bind at the interface between two ligand monomers within a trimer, whereas single CRD receptors are more compact and contact a single ligand monomer in a trimeric ligand (1316). Generally, one trimeric ligand engages three monomeric receptors, a key event for the activation of intracellular signaling pathways.
The TNF ligands have been shown to bind to one, two, or more different receptors and vice versa. For example, RANKL promotes bone resorption by stimulating the maturation of osteoclast precursors through the membrane-bound receptor RANK and is regulated through binding to the soluble decoy receptor OPG (10, 17). Experimental data regarding TNFSF-TNFRSF interactions in human and mouse and more specifically those addressing interspecies cross-reactivities are scattered in the literature (if available at all) and rely on different experimental settings or on specific reagents. In this study, we have conducted a systematic survey of TNFSF-TNFRSF interactions and report reactivities and cross-reactivities of human and mouse proteins.
| MATERIALS AND METHODS |
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Expression ConstructsAn expression vector for Fc:ligands has been described previously (18) and was used for the expression of the following ligands (amino acid numbers are given in parentheses; = means amino acid sequence is 100% identical to): h41BBL-(85254), m41BBL-(140310), hAPRIL-H98-(98233), mAPRIL-H98(
A112)-(98232), mAPRIL-H98(A112)-(98233), hBAFF-(134285), mBAFF-(127309), hCD27L-(39193), mCD27L-(52195), hCD30L-(63235), mCD30L-(71239), hCD40L-(11661), mCD40L-(115260), mEDA1-(=hEDA1)-(245391), mEDA2(=hEDA2)-(245389), hFasL-(139281), mFasL-(106279), hGITRL-(47177), mGITRL-(38173), hLIGHT-(89240), mLIGHT-(87239), hLT
-(43205), mLT
-(52202), hLT
-(63244), mLT
-(63306), hOX40L-(50183), mOX40L-(140310), hRANKL(152317), mRANKL-(157316), hTL1A-(93251), mTL1A-(94252), hTNF-(85233), mTNF-(76235), hTRAIL-(95281), mTRAIL-(120291), hTWEAK-(141284), mTWEAK-(94249).
FLAG-tagged hLT
-(43205) and mLT
-(52202) were expressed with the pFLAG vector (19) and co-transfected with the corresponding Fc:LT
to produce mouse and human LT
. Although we did not determine the ratio of LT
1
2 and LT
2
1 in these preparations, they probably contained a mixture of both. hTNF-(85233), mTNF-(76235), mBAFF-(127309), mAPRIL(
A112)-(98232), and mAPRIL(A112)-(98233) were also cloned into the pFLAG vector.
An expression vector for the extracellular domains of receptors fused to a portion of hTRAILR3 (157259), including the GPI addition signal, has been described previously (20) and was used to clone the indicated sequence of various receptors that either contained their own signal peptide or, when indicated, were cloned after the hemagglutinin or immunoglobulin G1 heavy chain signal peptides (HA signal and Ig signal, respectively): h41BB-(1186), m41BB-(1181), hBAFF-R-(271) (HA signal), mBAFF-R-(270) (HA signal), hBCMA-(254) (Ig signal), mBCMA-(146) (Ig signal), hCD27-(1191), mCD27-(1182), hCD30-(1380), mCD30-(1285), hCD40-(1193), mCD40-(1193), hDcR3-(1300), hDR3-(25199) (Ig signal), mDR3-(1194), hDR6-(1351), mDR6-(1350), mDcTRAILR1-(1158), mDcTRAILR2-(40171) (HA signal), hEDAR-(1183), mEDAR-(1183), hFas-(1170), mFas-(1169), hFn14-(175), mFn14-(175), hGITR-(26161) (HA signal), mGITR-(1153), hHVEM-(1200), mHVEM-(1206), hLT
R-(1220), mLT
R-(1217), hNGFR-(1250), mNGFR-(1243), hOPG-(1202), mOPG-(1214), hOX40-(1214), mOX40-(1209), hRANK-(29213) (HA signal), mRANK-(1200), hRELT-(1125), mRELT-(1165), hTACI-(2160) (HA signal), mTACI-(278) (HA signal), hTNFR1-(1211), mTNFR1-(1210), hTNFR2-(1257), mTNFR2 Thr-102/Ile-108-(1257), mTNFR2 Ser-102/Thr-108-(1257), mTNF-RH3-(1162), hTRAIL-R1-(1239), hTRAIL-R2-(1212), mTRAIL-R-2-(1166), hTRAIL-R3-(1259) (full-length), hTRAIL-R4-(1211), hT-ROY-(1168), mTROY-(1168), hXEDAR-(1134) (Ig signal), and mXEDAR-(2133) (HA signal). An expression vector for Receptor:Fc has been described previously (21) and was used for the expression of hCD27-(1191), mCD27-(1182), mDcTRAILR1-(1158), mDcTRAI-LR2-(1171), hFas-(1170), mFas-(1169), hHVEM-(1200), mHVE-M-(1206), hLT
R-(1220), mLT
R-(1217), hOPG-(1202), mOPG-(1214), mTNFRH3-(1162), hTRAIL-R1-(1239), hTRAIL-R2-(1212), hTRAIL-R3-(1240), and hTRAIL-R4-(1211). Full-length hFasL-(1281), mFasL-(1279), hTRAIL-(1281) (FLAG), mTRAIL-(1291) (FLAG), hCD27L-(1193) (FLAG), mCD27L-(1195), hLIGHT-(1240), and mLIGHT-(1239) were cloned in the PCR3 mammalian expression vector (Invitrogen).
TransfectionFor secreted proteins, 293T cells transiently transfected using the calcium phosphate procedure were grown in serum-free Opti-MEM I medium for 47 days. Supernatants were collected and frozen until use. In one instance (see Fig. 2D), supernatants were concentrated 60x before use. When concentration is indicated, protein concentration was estimated by immunoblot using anti-FLAG or anti-Fc antibodies with purified proteins of known concentration as standards.
For flow cytometry stainings, 293T cells were co-transfected overnight with expression constructs for the receptor of interest and EGFP, washed and cultured for an additional 24 h in complete medium. For staining and analysis, cells were detached by pipetting.
Flow Cytometry StainingStainings (
2 x 105 transfected 293T cells) were performed in round-bottomed 96-well plates for 20 min on ice with either (a) Fc-tagged ligands followed by PE-coupled goat anti-human IgG, (b) Fc-tagged receptor followed by PE-coupled goat anti-human IgG, (c) 0.5 µg of LEIA antibody followed by PE-coupled goat anti-rat IgG, or (d) FLAG-tagged ligands followed by biotinylated anti-FLAG M2 antibody and PE-coupled streptavidin. Stainings were performed with 515 µl (
1050 ng) of recombinant proteins in Opti-MEM supernatants in a final volume of 25 µl of phosphate-buffered saline and 5% fetal calf serum. Heparin (0.1 µl) was added in all stainings to prevent unspecific binding of recombinant proteins to glycosaminoglycans (20). The cells were analyzed on a FACScan (BD Biosciences) using the CellQuest program.
| RESULTS AND DISCUSSION |
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Ligands are expressed as soluble fusion proteins comprising the Fc portion of human immunoglobulin G1 and the TNF homology domain of TNFSF members. These fusion proteins are predicted to form hexamers (i.e. to contain two trimeric ligands) and to display higher avidity for their receptors than a regular trimer (18). The Fc is detected with a secondary PE-labeled antibody directed against human IgG1.
Cells co-transfected with EGFP, and various receptor-GPI constructs were screened for Fc:ligands binding. Although background staining of the various ligands varied, more than a hundred specific interactions were readily identified based on the curved appearance of the scattergrams (Fig. 1). Under these experimental conditions, there were several exceptions. Fc:mLIGHT (but not Fc:hLIGHT) failed to bind its cognate receptors, despite the fact that the portions of hLIGHT and mLIGHT contained in these constructs were exactly equivalent. The same was true for Fc:mTRAIL. In addition, Fc:hTRAIL and Fc:mFasL did not recognize at least one of their described binding partners (Fig. 1 and data not shown). This could result from Cys-230 oxidation in TRAIL (25, 26), from putative detrimental effects of the Fc portion on some ligands in the Fc:ligand fusion proteins, or could be due to the portion of the ligand expressed. Fc:mFasL was expressed as two constructs, one containing the TNF homology domain only (data not shown) and the other containing the entire extracellular domain (Fig. 1). Only the long form demonstrated binding to hDcR3, a described receptor for hFasL (27), but neither form interacted with mFas. We therefore expressed mFasL, mLIGHT, mTRAIL, and hTRAIL as untagged, full-length proteins in 293T cells and detected interactions with recombinant receptor:Fc fusion proteins. In this format, these four ligands interacted with their cognate receptors (Fig. 2). mLIGHT interacted readily with mLT
R, but only weak binding to mHVEM was observed (Fig. 2D).
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An interaction between TWEAK(Apo3L) and DR3 has been reported in an earlier study (28). This interaction has not been reproduced here or in a number of other settings (Fig. 1) (2931) and should be considered an artifact. Instead, TWEAK interacts with Fn14, and DR3 binds to TL1A (Fig. 1) (32, 33).
The RANKL, TRAIL NetworkExpression of soluble TRAIL in 293 cells results in the oxidation of Cys-230, which is required for the chelation of a zinc atom at the center of the trimer, and may explain the total or partial lack of activity of Fc:hTRAIL and Fc:mTRAIL (25, 26). However, the membrane-bound forms of both human and mouse TRAIL were suitable for this study of receptor binding and allowed us to reproduce the previously published interactions (Fig. 2A) (34).
The Immediate TNF NetworkTNF, LT
, LT
, LIGHT, TL1A, and FasL share at least one receptor with another ligand of the immediate TNF family. We have confirmed the published interactions within this subfamily (Figs. 1, 2, and 5). Curiously, in our hands, hDcR3:Fc failed to bind efficiently to its three cognate ligands LIGHT, FasL, and TL1A (data not shown), but the same portion of DcR3 fused to the GPI anchor of TRAIL-R3 did show an interaction (Fig. 1). It has been shown that DcR3 specifically loses its ability to interact with FasL (but not LIGHT) upon proteolytic cleavage at residue Arg-247, which separates the CRDs from the C-terminal extension (35). In contrast to DcR3-Ig, GPI-anchored DcR3 may escape proteolytic processing or maintain a conformation that allows ligand binding. We have not been able to test the hDcR3-mLIGHT interaction.
There were marked differences in the patterns of interaction of the lymphotoxins with their receptors in mouse and human. In the human, both LT
and LT
interacted with three receptors each, whereas in the mouse, LT
bound specifically to TNFR1 and LT
to LT
R. This raises the question as to whether the interactions of human LT
with HVEM or human LT
with TNFR1 are physiologically relevant or whether they represent weak interactions with no real signaling potential in vivo.
Fc:ligands allow the detection of weak interactions, because they are predicted to contain more than one trimeric ligand (18) and to display higher avidity for the receptors than regular trimeric soluble ligands. This is illustrated by the unexpected observation that human TNF did bind to mouse TNFR2 in our experimental system (Fig. 1), although it is known that, under physiological conditions, hTNF does not bind and cannot activate mTNFR2 (36, 37). We tested whether this result was because of the increased avidity of Fc:hTNF or to polymorphisms in mTNFR2. Indeed, non-obese diabetic and 129/SvJ mouse strains contain the T102S and I108T polymorphisms (38, 39) (Fig. 3A) that could potentially alter the specificity for hTNF as they correspond to amino acids that are present in the ligand binding domain of human TNFR2 (Fig. 3A) (16). However, both polymorphic forms of mTNFR2 bound to Fc:hTNF nearly as efficiently as the positive controls hTNFR2, hTNFR1, and mTNFR1 but only weakly bound trimeric human TNF (Fig. 3, B and C). We conclude that the use of Fc:ligands, which may mimic membrane-bound ligands, contributes to the sensitivity of the screen by allowing detection of weaker interactions. However, additional experiments are required to determine whether a given interaction is biologically relevant.
The BAFF and APRIL NetworkThe ligands BAFF and APRIL share two receptors, TACI and BCMA. However, only BAFF can bind to BAFF-R. We indeed confirmed these findings with hBAFF and hAPRIL (Fig. 1). Unexpectedly, mAPRIL interacted with mBAFF-R, although the binding was weaker than that observed with mBAFF (Fig. 1). To our knowledge, the binding of mAPRIL to mBAFF-R has not been reported previously in the literature. We noticed that APRIL exists as two splice variants differing by a single amino acid residue, Ala-112, as a result of differential usage of alternative splice acceptor sites that are separated by only three nucleotides (Fig. 4A). A similar case has been described for EDA isoforms that differ by two amino acid residues (40). Both APRIL variants were found in mouse expressed sequence tags at approximately equal frequencies and in both "regular" APRIL and in the membrane-bound TWE-PRIL. TWE-PRIL is the result of an intergenic splicing event and contains the entire TNF homology domain of APRIL (41). The APRIL splice variant lacking the Ala-112 (or Asp-112 in human) form cannot be produced in human and was not detected in dog, pig, or cow expressed sequence tags. The available rat expressed sequence tags encode only the short form of APRIL. When expressed as an Fc:ligand, both APRIL splice variants displayed equivalent binding to mTACI and mBCMA, weaker but significant binding to mBAFF-R, and no detectable binding to hBAFF-R (Fig. 4, B and C). However, only the shorter splice variant bound to mBAFF-R when expressed as FLAG-tagged (trimeric) mAPRIL (Fig. 4, B and C). This points to a specific interaction between APRIL and BAFF-R in the mouse, which depends in part on the splicing of APRIL. Although this interaction is weak, and despite the fact that the phenotypes of BAFF-deficient, BAFF-R-deficient, and APRIL-deficient mice all indicate that APRIL does not compensate for the loss of BAFF with regard to BAFF-R activation (9, 42), it remains possible that, in the mouse, APRIL or TWE-PRIL may exert some effects through mBAFF-R. It is also possible that administration of high doses of murine BAFF-R-Ig in the mouse might affect APRIL function in addition to that of BAFF.
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Orphan Receptors and Atypical InteractionsIn this screen, all TNFSF members bound to one or more receptors, but five receptors, namely NGFR, DR6, TROY, RELT, and mTNFRH3, remained orphan. As interactions were tested in both human and mouse, it was unlikely that an interaction with one of the known TNF ligands would have been missed. One possibility is that these receptors may bind heterotrimeric ligands, similar to the LT
R-LT
interaction. In addition, we have limited our screen to ligands of the TNF family, although NGFR is known to interact with several dimeric ligands of the neurotrophin family, such as nerve growth factor, brain-derived neurotrophic factor, and other neurotrophins (NT-3, NT-4/5) (47, 48). NGFR and TROY have been reported to interact with membrane-bound partners, such as LINGO-1 and NogoR. In these cases, myelin-derived ligands bind the GPI-anchored NogoR that transmits intracellular signals through the transmembranous protein complexes composed of NGFR, LINGO-1, and NogoR or TROY, LINGO-1, and NogoR (4951). There are also reports that receptors can function in the absence of ligand (52, 53), and some of the orphan receptors might function uniquely in this manner. Finally, a number of "atypical" interactions have been reported, for example APRIL and TACI with proteoglycans (20, 54), HVEM with the herpes simplex virus glycoprotein D (55), CD40 with Hsp70-peptide complexes (56), TNF with Tanaxapox virus 2L protein, (57), LIGHT with B and T cell attenuator (58, 59), and TNF with specific oligosaccharides (24). None of these interactions would have been uncovered in the present screen.
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| FOOTNOTES |
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1 These authors contributed equally to this work. ![]()
2 Present address: Merck & Co, Inc., West Point, PA 19486. ![]()
3 Present address: University Medical Center, CH-1211 Geneva, Switzerland. ![]()
4 To whom correspondence should be addressed: Dept. of Biochemistry, University of Lausanne, Boveresses 155, CH-1066 Epalinges, Switzerland. Tel.: 41-21-692-5709; Fax: 41-21-692-5705; E-mail: pascal.schneider{at}unil.ch.
5 The abbreviations used are: TNF, tumor necrosis factor; TNFSF, TNF superfamily; TNFRSF, TNF receptor superfamily; BAFF, B cell-activating factor of the TNF family; BCMA, B cell maturation antigen; TACI, transmembrane activator and calcium modulator and cyclophilin ligand interactor; APRIL, a proliferation-inducing ligand; EDA, ectodysplasin A; EDAR, EDA receptor; XEDAR, X-linked EDAR; GITR, glucocorticoid-induced TNF receptor; GITRL, GITR ligand; HVEM, herpes virus entry mediator; LIGHT, lymphotoxin homologue inducible competing with glycoprotein D for HVEM (a receptor expressed on T cells); LINGO-1, leucine-rich repeat and Ig domain-containing Nogo receptor-interacting protein; NogoR, Nogo receptor; LT
, lymphotoxin 
; LT
R, lymphotoxin
receptor; RANK, receptor activator of NF
B; RANKL, RANK ligand; TWEAK, TNF family member with weak apoptosis-inducing activity; DcR3, decoy receptor 3; DR, death receptor; Fn14, product of fibroblast growth factor-inducible gene 14; NGF, nerve growth factor; NGFR, nerve growth factor receptor; OPG, osteoprotegerin; RELT, receptor expressed in lymphoid tissues; GPI, glycosyl-phosphatidylinositol; EGFP, enhanced green fluorescent protein; HA, hemagglutinin; PE, phosphatidylethanolamine; TRAIL, TNF-related apoptosis-inducing ligand; CD, cluster of differentiation; TL, TNF family ligand; TNFRH3, TNF receptor homologue 3; CRD, cysteine-rich domain; h, human; m, mouse. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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