Validation of extracellular ligand–receptor interactions by Flow-TriCEPS
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The advent of ligand-based receptor capture methodologies, allows the identification of unknown receptor candidates for orphan extracellular ligands. However, further target validation can be tedious, laborious and time-consuming. Here, we present a methodology that provides a fast and cost-efficient alternative for candidate target verification on living cells.
In the described methodology a ligand of interest (e.g. transferrin, epidermal growth factor or insulin) was conjugated to a linker (TriCEPS) that carries a biotin. To confirm ligand/receptor interactions, the ligand–TriCEPS conjugates were first added onto living cells and cells were subsequently labeled with a streptavidin-fluorophore and analyzed by flow cytometry (thus referred as Flow-TriCEPS). Flow-TriCEPS was also used to validate identified receptor candidates when combined with a siRNA knock down approach (i.e. reduction of expression levels). This approach is versatile as it can be applied for different classes of ligands (proteins, peptides, antibodies) and different cell lines. Moreover, the method is time-efficient since it takes advantage of the large variety of commercially available (and certified) siRNAs.
KeywordsFlow-TriCEPS HATRIC LRC Knock down siRNA Candidate verification
ligand-based receptor capture
epidermal growth factor receptor
bovine serum albumin
insulin-like growth factor receptor
Identifying the protein binding receptors of key ligands provides valuable mechanistic information regarding signal transduction, drug action or off-target effects. Recognizing the unmet need to decipher transient extracellular interactions, the ligand-based receptor capture (LRC) technology was developed [1, 2]. The advantages of the LRC-TriCEPS™ (and its latest development LRC-HATRIC ) methodology include versatility, no need for genetic manipulation, target identification on living cells and the ability to capture both stable and transient interactions.
The LRC methodologies have been applied to study numerous ligands ranging from small molecules to intact viruses and have revealed known and novel interactors [1, 3, 4, 5, 6, 7, 8]. As with every discovery-based methodology, the resulting candidate targets should be further validated and confirmed, which can be a tedious, laborious and time-consuming exercise.
In the present technical note, we present a method for the fast validation and confirmation of receptor candidates, which was demonstrated using different ligands and their well-established targets.
20 μg of transferrin (TRFE), epidermal growth factor receptor (EGFR) antibody, insulin (INS) and glycine (Gly) were coupled to 10 μg of TriCEPS v.2.0 (biotin moiety) or TriCEPS-TAMRA (fluorophore moiety) in a total volume of 50 μl 25 mM HEPES pH 8.2 (90 min at 22 °C). Upon coupling, non-reacted N-hydroxysuccinimide (NHS) was quenched with 20 μg Gly.
Cell incubation with TriCEPS-coupled ligand
MDA-MB-231 and HEK293 cells were collected using 5 mM ethylene-diamine-tetra-acetic acid, washed once with phosphate-buffered saline (PBS) pH 6.5 and incubated with 1.2 μg of the corresponding TriCEPS-ligands (500,000 cells in 200 μl PBS pH 6.5 for 60 min at 4 °C). For competition experiments, cells were incubated with different amounts of unlabeled ligand TRFE or bovine serum albumin (BSA) (20 min at 22 °C) in PBS pH 7.4 prior to addition of the TriCEPS-coupled ligand.
Upon incubation of cells with TriCEPS–coupled ligands, cells were washed twice with 400 μl of ice cold PBS buffer pH 6.5, labelled with streptavidin-R-phycoerythrin (PE) (Jackson) (30 min at 4 °C in the dark), washed twice and analyzed by flow cytometry.
siRNA knock down of receptor candidates
Ten nanomolar of scramble siRNA (AllStars Negative Control, Qiagen) and siRNA against human transferrin receptor (TFR1) (Hs_TFRC_7, Hs_TFRC_5 and Hs_TFRC_11, Qiagen), human EGFR (Hs_EGFR_10, Qiagen), human insulin receptor (INSR) (Hs_INSR_3 and Hs_INSR_4, Qiagen) and human insulin-like growth factor receptor (IGF1R) (Hs_IGF1R_8 and Hs_IGF1R_6, Qiagen) were used for reverse transfection of MDA-MB-231 and HEK293 cells on a 6 well plate format using Lipofectamine RNAiMax (Invitrogen). The knock down efficiency of TFR1 on mRNA level in MDA-MB-231 cells was monitored by qRT-PCR. Knock down efficiency was tested by flow cytometric analysis using PE anti human CD71 (TFR1) (BioLegend), PE anti human EGFR (BioLegend), PE anti human CD220 (INSR) (BD Bioscience) and PE anti human CD221 (IGF1R) (BD Bioscience) antibodies using recommended conditions.
Microscopy using TriCEPS-TAMRA
HEK293 cells were plated on cover slips previously coated with Poly l-Lys (0.01%) (EMD Millipore) on 12 well plates and the ligand–TriCEPS–TAMRA conjugates (8 μg) were added in 500 μl RPMI media containing 1% fetal bovine serum (45 min at 4 °C, in the dark). Upon incubation, cells were fixed with 4% paraformaldehyde (15 min at 22 °C in the dark), and the cover slips were transferred to a microscope slide containing 30 μl 4,6-diamidino-2-phenylindole (DAPI) (Invitrogen). Imaging was performed using the Leica SP8 inverse confocal microscope with support of the Center for Microscopy and Image Analysis, University of Zurich.
Development of Flow-TriCEPS as a method to determine ligand binding on living cells
In the first step of a LRC experiment, the tri-functional molecule (TriCEPS or HATRIC) is conjugated to the primary amines of the ligand of interest via a NHS-ester (Additional file 1: Figure S1). During the incubation of tri-functional molecule–ligand conjugates with mildly oxidized cells the second moiety (hydrazone) reacts with aldehydes of cell surface glycoproteins and decorates the cell surface. Finally, the third moiety (i.e. biotin) is used in order to pull down and thus enrich the tagged proteins [1, 3]. When conjugates are added on non-oxidized cells, the hydrazone should not react (no aldehydes) and conjugates should bind only to the target of interest (if expressed). To confirm the binding of a ligand to its receptor, the third moiety (i.e. biotin) can be replaced or interact with a fluorophore and the binding visualized by microscopy or flow cytometry.
To explore this possibility, we used TRFE as ligand, because its target TFR1 (also known as CD71) is expressed in the vast majority of cell types including the breast cancer cell line (MDA-MB-231) . TRFE coupled to TriCEPS (TRFE-TriCEPS) was added on oxidized and non-oxidized MDA-MB-231 cells. The TriCEPS molecule quenched with glycine to hydrolyze the NHS group was used as a negative control. Samples were labeled with streptavidin-PE (Str-PE) that reacts with biotin of TriCEPS and analyzed by flow cytometry. In parallel, the same number of cells was incubated with Str-PE to assess the background signal. As shown by the shift of median fluorescence values, both conjugates bound to the surface of oxidized cells to a comparable extent likely due to the covalent labeling of glycoproteins (Additional file 1: Figure S1, left panel). Non-oxidized cells incubated with quenched TriCEPS (Gly-TriCEPS) showed a strong reduction of the median fluorescence values compared to cells incubated with TRFE-TriCEPS (Additional file 1: Figure S1, right panel). Given that quenched TriCEPS should not react with non-oxidized cells, the observed shift in the median fluorescence intensity in the cells treated with TRFE-TriCEPS was very likely due to the binding of TRFE to its corresponding target on the surface of the cells.
To further validate that the signal was due to a specific interaction of TRFE with its receptor, the expression of TFR1 was downregulated in MDA-MB-231 cells (Additional file 2: Figure S2a). Cells were transfected with anti-TFR1 siRNA for 24, 48 or 72 h and the expression levels of TFR1 on the cell surface were quantified by incubating cells with an anti-CD71-PE antibody followed by flow cytometric analysis. Cells transfected with scramble siRNA served as negative control (Additional file 2: Figure S2b, left panel). Based on the observed shift of median fluorescence with the transfected cells (Fig. 1c, Additional file 2: Figure S2b), the TFR1 levels were over time gradually reduced, indicating that the used siRNA was indeed specific for TFR1. When the transfected cells were subsequently incubated with the TRFE–TriCEPS conjugate and labeled with Str-PE, the acquired fluorescence signal closely mimicked the signal intensity staining of the anti-CD71 antibody, suggesting that the observed signal was due to the interaction of TRFE with TFR1 (Fig. 1d).
To ensure that the ligand–TriCEPS conjugate did not enter the cells, cells treated in the same manner as described above were analyzed by microscopy (Fig. 1e). The TRFE–TriCEPS–TAMRA conjugate was only located on the surface of cells. Together, these experiments provide strong evidence that coupling of TriCEPS to TRFE followed by flow cytometry is suitable to monitor the binding of ligands on the surface of living cells.
Verification of ligand binding to LRC-HATRIC identified targets by siRNA knockdown of putative candidates and subsequently analysis with Flow-TriCEPS
We next investigated if the developed Flow-TriCEPS pipeline was also applicable to different classes of ligands. Two ligands were selected (i) an antibody against EGFR (anti-EGFR) and (ii) a peptide (insulin) that is known to bind to the INSR and IGF1R . The targets of the above-mentioned ligands were confirmed by independent LRC-HATRIC experiments (Additional file 3: Figure S3).
Finally, we investigated the binding of INS in transfected cells using Flow-TriCEPS. When compared to cells transfected with scrambled siRNA, the generated signal by INS-TriCEPS was not reduced in cells expressing neither INSR nor IGF1R (Fig. 3b, left and right top panels respectively). This could be due to functional compensation between INSR and IGF1R . However, fluorescence signal is drastically reduced in cells lacking both, INSR and IGF1R (Fig. 3b, low panel), suggesting that combinatorial knock-down of multiple targets may be necessary to demonstrate reduced ligand binding.
Validating previously identified ligand–receptor interactions can for different reasons be challenging and time consuming. Here, we provide evidence in three different conditions that the developed Flow-TriCEPS methodology can monitor binding of extracellular ligands to their receptors on living cells. Additionally, we showed that candidate targets identified by the LRC methodologies can be quickly validated by combining Flow-TriCEPS with a siRNA mediated knock down approach for a particular receptor protein. This approach is versatile as it can be applied for different classes of ligands (proteins, peptides, antibodies) and different cell lines. The method is also time- and cost-efficient and takes advantage of the large variety of commercially available (and certified) siRNAs. Additionally, should a ligand bind to several interaction candidates, the combinatorial knock down of these targets can be easily performed as demonstrated in the case of INS.
Not all cell types can be readily transfected, which might require additional optimization.
In case that a ligand has more than one candidate targets, the combinatorial knock down of these should be performed, as single knock downs may lead to increase of the other targets compensating ligand binding.
LALG participated in the design of the study, performed experiments, performed data analysis and results interpretation, prepared manuscript figures. LD and SM participated in the design of the study and performed experiments. AKH helped with the siRNA treatment and immunofluorescence analysis. MOH was involved in drafting the manuscript and provided critical revisions. PH participated in the design of the study and provided editorial assistance for the writing of the manuscript. MPP participated in the design of the study, performed experiments and wrote the manuscript. All authors read and approved the final manuscript.
L.A.L.G, L.D, S.R.M, P.M.H and M.P.P are employees of Dualsystems Biotech AG. Dualsystems Biotech A.G. has the worldwide exclusive license to use the LRC-TriCEPS and the LRC-HATRIC technologies. M.O.H. is member of the board of directors of Dualsystems Biotech AG.
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