Real-Time Ligand Binding of Fluorescent VEGF-A Isoforms that Discriminate between VEGFR2 and NRP1 in Living Cells

Neuropilin-1 is a co-receptor for NGF and TrkA-evoked pain

Nerve growth factor (NGF) monoclonal antibodies (mAb) are one of the few patient-validated non-opioid treatments for chronic pain, despite failing to gain FDA approval due to worsened joint damage in some osteoarthritis patients. Herein, we demonstrate that neuropilin-1 (NRP1) is a nociceptor-enriched co-receptor for NGF that is necessary for tropomyosin-related kinase A (TrkA) signaling of pain. NGF binds NRP1 with nanomolar affinity. NRP1 and G Alpha Interacting Protein C-terminus 1 (GIPC1), a NRP1/TrkA adaptor, are coexpressed with TrkA in human and mouse nociceptors. NRP1 small molecule inhibitors and blocking mAb prevent NGF-stimulated action potential firing and activation of Na+ and Ca2+ channels in human and mouse nociceptors and abrogate NGF-evoked and inflammatory nociception in mice. NRP1 knockdown blunts NGF-stimulated TrkA phosphorylation, kinase signaling and transcription, whereas NRP1 overexpression enhances NGF and TrkA signaling. As well as interacting with NGF, NRP1 forms a heteromeric complex with TrkA. NRP1 thereby chaperones TrkA from the biosynthetic pathway to the plasma membrane and then to signaling endosomes, which enhances NGF-induced TrkA dimerization, endocytosis and signaling. Knockdown of GIPC1, a PDZ-binding protein that scaffolds NRP1 and TrkA to myosin VI, abrogates NGF-evoked excitation of nociceptors and pain-like behavior in mice. We identify NRP1 as a previously unrecognized co-receptor necessary for NGF/TrkA pain signaling by direct NGF binding and by chaperoning TrkA to the plasma membrane and signaling endosomes via the adaptor protein GIPC1. Antagonism of NRP1 and GIPC1 in nociceptors offers a long-awaited alternative to systemic sequestration of NGF with mAbs for the treatment of pain.

CXCL17 is an allosteric inhibitor of CXCR4 through a mechanism of action involving glycosaminoglycans

CXCL17 is a chemokine principally expressed by mucosal tissues, where it facilitates chemotaxis of monocytes, dendritic cells, and macrophages and has antimicrobial properties. CXCL17 is also implicated in the pathology of inflammatory disorders and progression of several cancers, and its expression is increased during viral infections of the lung. However, the exact role of CXCL17 in health and disease requires further investigation, and there is a need for confirmed molecular targets mediating CXCL17 functional responses. Using a range of bioluminescence resonance energy transfer (BRET)-based assays, here we demonstrated that CXCL17 inhibited CXCR4-mediated signaling and ligand binding. Moreover, CXCL17 interacted with neuropillin-1, a VEGFR2 coreceptor. In addition, we found that CXCL17 only inhibited CXCR4 ligand binding in intact cells and demonstrated that this effect was mimicked by known glycosaminoglycan binders, surfen and protamine sulfate. Disruption of putative GAG binding domains in CXCL17 prevented CXCR4 binding. This indicated that CXCL17 inhibited CXCR4 by a mechanism of action that potentially required the presence of a glycosaminoglycan-containing accessory protein. Together, our results revealed that CXCL17 is an endogenous inhibitor of CXCR4 and represents the next step in our understanding of the function of CXCL17 and regulation of CXCR4 signaling.

An efficient site-selective, dual bioconjugation approach exploiting N-terminal cysteines as minimalistic handles to engineer tailored anti-HER2 affibody conjugates

Site-selective, dual-conjugation approaches for the incorporation of distinct payloads are key for the development of molecularly targeted biomolecules, such as antibody conjugates, endowed with better properties. Combinations of cytotoxic drugs, imaging probes, or pharmacokinetics modulators enabled for improved outcomes in both molecular imaging, and therapeutic settings. We have developed an efficacious dual-bioconjugation strategy to target the N-terminal cysteine of a chemically-synthesized, third-generation anti-HER2 affibody. Such two-step, one-purification approach can be carried out under mild conditions (without chaotropic agents, neutral pH) by means of a slight excess of commercially available N-hydroxysuccinimidyl esters and maleimido-functionalized payloads, to generate dual conjugates displaying drugs (DM1/MMAE) or probes (sulfo-Cy5/biotin) in high yields and purity. Remarkably, the double drug conjugate exhibited an exacerbated cytoxicity against HER2-expressing cell lines as compared to a combination of two monoconjugates, demonstrating a potent synergistic effect. Consistently, affibody-drug conjugates did not decrease the viability of HER2-negative cells, confirming their specificity for the target.

Mechanobiology of the relocation of proteins in advecting cells: in vitro experiments, multi-physics modeling, and simulations

Cell motility-a cellular behavior of paramount relevance in embryonic development, immunological response, metastasis, or angiogenesis-demands a mechanical deformation of the cell membrane and influences the surface motion of molecules and their biochemical interactions. In this work, we develop a fully coupled multi-physics model able to capture and predict the protein flow on endothelial advecting plasma membranes. The model has been validated against co-designed in vitro experiments. The complete picture of the receptor dynamics has been understood, and limiting factors have been identified together with the laws that regulate receptor polarization. This computational approach might be insightful in the prediction of endothelial cell behavior in different tumoral environments, circumventing the time-consuming and expensive empirical characterization of each tumor.

Aerocyte specification and lung adaptation to breathing is dependent on alternative splicing changes

Adaptation to breathing is a critical step in lung function and it is crucial for organismal survival. Alveoli are the lung gas exchange units and their development, from late embryonic to early postnatal stages, requires feedbacks between multiple cell types. However, how the crosstalk between the alveolar cell types is modulated to anticipate lung adaptation to breathing is still unclear. Here, we uncovered a synchronous alternative splicing switch in multiple genes in the developing mouse lungs at the transition to birth, and we identified hnRNP A1, Cpeb4, and Elavl2/HuB as putative splicing regulators of this transition. Notably, we found that Vegfa switches from the Vegfa 164 isoform to the longer Vegfa 188 isoform exclusively in lung alveolar epithelial AT1 cells. Functional analysis revealed that VEGFA 188 (and not VEGFA 164) drives the specification of Car4-positive aerocytes, a subtype of alveolar endothelial cells specialized in gas exchanges. Our results reveal that the cell type-specific regulation of Vegfa alternative splicing just before birth modulates the epithelial-endothelial crosstalk in the developing alveoli to promote lung adaptation to breathing.

Optimization of Peptide Linker-Based Fluorescent Ligands for the Histamine H1 Receptor

The histamine H₁ receptor (H₁R) has recently been implicated in mediating cell proliferation and cancer progression; therefore, high-affinity H₁R-selective fluorescent ligands are desirable tools for further investigation of this behavior in vitro and in vivo. We previously reported a H₁R fluorescent ligand, bearing a peptide-linker, based on antagonist VUF13816 and sought to further explore structure-activity relationships (SARs) around the linker, orthostere, and fluorescent moieties. Here, we report a series of high-affinity H₁R fluorescent ligands varying in peptide linker composition, orthosteric targeting moiety, and fluorophore. Incorporation of a boron-dipyrromethene (BODIPY) 630/650-based fluorophore conferred high binding affinity to our H₁R fluorescent ligands, remarkably overriding the linker SAR observed in corresponding unlabeled congeners. Compound 31a, both potent and subtype-selective, enabled H₁R visualization using confocal microscopy at a concentration of 10 nM. Molecular docking of 31a with the human H₁R predicts that the optimized peptide linker makes interactions with key residues in the receptor.

Serine-arginine-rich protein kinase-1 inhibition for the treatment of diabetic retinopathy

Angiogenic VEGF isoforms are upregulated in diabetic retinopathy (DR), driving pathological growth and fluid leakage. Serine-arginine-rich protein kinase-1 (SRPK1) regulates VEGF splicing, and its inhibition blocks angiogenesis. We tested the hypothesis that SRPK1 is activated in diabetes, and an SRPK1 inhibitor (SPHINX31) switches VEGF splicing in DR and prevents increased vascular permeability into the retina. SRPK1 was activated by high glucose (HG), in a PKC-dependent manner, and was blocked by SPHINX31. HG induced release of SRSF1 from the nuclear speckles, which was also SRPK1 dependent, and increased retinal pigment epithelial (RPE) monolayer admittance, which was reversed by SRPK1 inhibition (P < 0.05). Diabetes increased retinal permeability and thickness after 14 days which was blocked by treatment with SPHINX31 eye drops (P < 0.0001). These results show that SRPK1 inhibition, administered as an eye drop, protected the retinal barrier from hyperglycemia-associated loss of integrity in RPE cells in vitro and in diabetic rats in vivo. A clinical trial of another SRPK1 inhibitor has now been initiated in patients with diabetic macular edema.NEW & NOTEWORTHY VEGF-A₁₆₅b splicing is induced by hyperglycemia through PKC-mediated activation of SRPK1 in RPE cells, increasing their permeability and angiogenic capability. SRPK1 inhibitors can be given as eye drops to reduce retinal permeability and edema in diabetic retinopathy.

Measuring the rapid kinetics of receptor-ligand interactions in live cells using NanoBRET

The importance of receptor-ligand binding kinetics has often been overlooked during drug development, however, over the past decade it has become increasingly clear that a better understanding of the kinetic parameters is crucial for fully evaluating pharmacological effects of a drug. One technique enabling us to measure the real-time kinetics of receptor-ligand interactions in live cells is NanoBRET, which is a bioluminescence resonance energy transfer (BRET)-based assay that uses Nano luciferase. The assay described here allows the measurement of kinetic parameters of a fluorescent ligand and an unlabeled ligand binding to the same place at the receptor, as well as monitoring the effects of another compound like an allosteric modulator on the ligand binding.

Use of NanoBiT and NanoBRET to monitor fluorescent VEGF-A binding kinetics to VEGFR2/NRP1 heteromeric complexes in living cells

BACKGROUND AND PURPOSE

VEGF-A is a key mediator of angiogenesis, primarily signalling via VEGF receptor 2 (VEGFR2). Endothelial cells also express the co-receptor neuropilin-1 (NRP1) that potentiates VEGF-A/VEGFR2 signalling. VEGFR2 and NRP1 had distinct real-time ligand binding kinetics when monitored using BRET. We previously characterised fluorescent VEGF-A isoforms tagged at a single site with tetramethylrhodamine (TMR). Here, we explored differences between VEGF-A isoforms in living cells that co-expressed both receptors.

EXPERIMENTAL APPROACH

Receptor localisation was monitored in HEK293T cells expressing both VEGFR2 and NRP1 using membrane-impermeant HaloTag and SnapTag technologies. To isolate ligand binding pharmacology at a defined VEGFR2/NRP1 complex, we developed an assay using NanoBiT complementation technology whereby heteromerisation is required for luminescence emissions. Binding affinities and kinetics of VEGFR2-selective VEGF165 b-TMR and non-selective VEGF165 a-TMR were monitored using BRET from this defined complex.

KEY RESULTS

Cell surface VEGFR2 and NRP1 were co-localised and formed a constitutive heteromeric complex. Despite being selective for VEGFR2, VEGF165 b-TMR had a distinct kinetic ligand binding profile at the complex that largely remained elevated in cells over 90 min. VEGF165 a-TMR bound to the VEGFR2/NRP1 complex with kinetics comparable to those of VEGFR2 alone. Using a binding-dead mutant of NRP1 did not affect the binding kinetics or affinity of VEGF165 a-TMR.

CONCLUSION AND IMPLICATIONS

This NanoBiT approach enabled real-time ligand binding to be quantified in living cells at 37°C from a specified complex between a receptor TK and its co-receptor for the first time.

Interactions between Ligand-Bound EGFR and VEGFR2

In this work, we put forward the provocative hypothesis that the active, ligand-bound RTK dimers from unrelated subfamilies can associate into heterooligomers with novel signaling properties. This hypothesis is based on a quantitative FRET study that monitors the interactions between EGFR and VEGFR2 in the plasma membrane of live cells in the absence of ligand, in the presence of either EGF or VEGF, and in the presence of both ligands. We show that direct interactions occur between EGFR and VEGFR2 in the absence of ligand and in the presence of the two cognate ligands. However, there are not significant heterointeractions between EGFR and VEGFR2 when only one of the ligands is present. Since RTK dimers and RTK oligomers are believed to signal differently, this finding suggests a novel mechanism for signal diversification.

A multiscale computational model of angiogenesis after traumatic brain injury, investigating the role location plays in volumetric recovery

Vascular endothelial growth factor (VEGF) is a key protein involved in the process of angiogenesis. VEGF is of particular interest after a traumatic brain injury (TBI), as it re-establishes the cerebral vascular network in effort to allow for proper cerebral blood flow and thereby oxygenation of damaged brain tissue. For this reason, angiogenesis is critical in the progression and recovery of TBI patients in the days and weeks post injury. Although well established experimental work has led to advances in our understanding of TBI and the progression of angiogenisis, many constraints still exist with existing methods, especially when considering patient progression in the days following injury. To better understand the healing process on the proposed time scales, we develop a computational model that quickly simulates vessel growth and recovery by coupling VEGF and its interactions with its associated receptors to a physiologically inspired fractal model of the microvascular re-growth. We use this model to clarify the role that diffusivity, receptor kinetics and location of the TBI play in overall blood volume restoration in the weeks post injury and show that proper therapeutic angiogenesis, or vasculogenic therapies, could speed recovery of the patient as a function of the location of injury.

Evidence that GPVI is Expressed as a Mixture of Monomers and Dimers, and that the D2 Domain is not Essential for GPVI Activation

Collagen has been proposed to bind to a unique epitope in dimeric glycoprotein VI (GPVI) and the number of GPVI dimers has been reported to increase upon platelet activation. However, in contrast, the crystal structure of GPVI in complex with collagen-related peptide (CRP) showed binding distinct from the site of dimerization. Further fibrinogen has been reported to bind to monomeric but not dimeric GPVI. In the present study, we have used the advanced fluorescence microscopy techniques of single-molecule microscopy, fluorescence correlation spectroscopy (FCS) and bioluminescence resonance energy transfer (BRET), and mutagenesis studies in a transfected cell line model to show that GPVI is expressed as a mixture of monomers and dimers and that dimerization through the D2 domain is not critical for activation. As many of these techniques cannot be applied to platelets to resolve this issue, due to the high density of GPVI and its anucleate nature, we used Förster resonance energy transfer (FRET) to show that endogenous GPVI is at least partially expressed as a dimer on resting and activated platelet membranes. We propose that GPVI may be expressed as a monomer on the cell surface and it forms dimers in the membrane through diffusion, giving rise to a mixture of monomers and dimers. We speculate that the formation of dimers facilitates ligand binding through avidity.

The Production of Pro-angiogenic VEGF-A Isoforms by Hypoxic Human NK Cells Is Independent of Their TGF-β-Mediated Conversion to an ILC1-Like Phenotype

Circulating natural killer (NK) cells have been shown to adopt a type 1 innate lymphoid cell (ILC1)-like phenotype in response to TGF-β and secrete VEGF-A when exposed to hypoxia. Although these changes are often considered to be linked attributes of tissue residency, it has yet to be determined if TGF-β and hypoxia work in concert to coordinate NK cellular phenotype and angiogenic potential. Examination of human circulating NK cells treated with TGF-β demonstrated heterogeneity in their potential to adopt an ILC1-like phenotype, as indicated by the upregulation of CD9 and CD103 on only a subset of cells in culture. Culturing NK cells in chronic hypoxia did not induce a similar ILC1-like conversion and did not enhance the degree of conversion for cells exposed to TGF-β. Similarly, hypoxic culture of circulating NK cells induced VEGF-A secretion, but this production was not enhanced by TGF-β. Fluorescent in-situ hybridization flow cytometry demonstrated that hypoxia-induced VEGF-A production was uniform across all NK cells in culture and was not a selective feature of the cellular subset that adopted an ILC1-like phenotype in response to TGF-β. Examination of VEGF-A isoforms demonstrated that hypoxia induces the production of pro-angiogenic VEGF-A isoforms, including VEGF-A₁₆₅ and VEGF-A₁₂₁, and does not stimulate any meaningful production of anti-angiogenic isoforms, such as VEGF-A_b transcriptional variants or VEGF-Ax. In summary, TGF-β-mediated ILC1-like conversion and hypoxia-induced VEGF-A production are discrete processes in NK cells and are not part of a linked cellular program associated with tissue residency.

Debunking the Myth of the Endogenous Antiangiogenic Vegfaxxxb Transcripts

In this opinion article we critically assess evidence for the existence of a family of antiangiogenic vascular endothelial growth factor (Vegfaxxxb) transcripts, arising from the use of a phylogenetically conserved alternative distal splice site within exon 8 of the VEGFA gene. We explain that prior evidence for Vegfaxxxb transcripts in tissues rests heavily upon flawed RT-PCR methodologies, with the extensive use of 5'-tailing in primer design being the main issue. Furthermore, our analysis of large RNA-seq data sets (human and ovine) fails to identify a single Vegfaxxxb transcript. Therefore, we challenge the very existence of Vegfaxxxb transcripts, which further questions the physiological relevance of studies based on the use of 'anti-VEGFAxxxb' antibodies. Our analysis has implications for the proposed therapeutic use of isoform-specific anti-VEGFA strategies for treating cancer and retinopathies.

Quantifying Target Occupancy of Small Molecules Within Living Cells

The binding affinity and kinetics of target engagement are fundamental to establishing structure-activity relationships (SARs) for prospective therapeutic agents. Enhancing these binding parameters for operative targets, while minimizing binding to off-target sites, can translate to improved drug efficacy and a widened therapeutic window. Compound activity is typically assessed through modulation of an observed phenotype in cultured cells. Quantifying the corresponding binding properties under common cellular conditions can provide more meaningful interpretation of the cellular SAR analysis. Consequently, methods for assessing drug binding in living cells have advanced and are now integral to medicinal chemistry workflows. In this review, we survey key technological advancements that support quantitative assessments of target occupancy in cultured cells, emphasizing generalizable methodologies able to deliver analytical precision that heretofore required reductionist biochemical approaches.

Live Cell Membranome cDNA Screen: A Novel Homogenous Live Cell Binding Assay to Study Membrane Protein-Ligand Interaction

Interactions between transmembrane receptors and their ligands play important roles in normal biological processes and pathological conditions. However, the binding partners for many transmembrane-like proteins remain elusive. To identify potential ligands of these orphan receptors, we developed a screening platform using a homogenous nonwash binding assay in live cells. A collection of ~1900 cDNA clones, encoding full-length membrane proteins, was assembled. As a proof of concept, cDNA clones were individually transfected into CHO-K1 cells in a high-throughput format, and soluble PD-L1-Fc fusion protein was used as bait. The interaction between the putative receptor and PD-L1-Fc was then detected by Alexa Fluor 647 conjugated anti-human immunoglobulin G Fc antibody and visualized using the Mirrorball fluorescence plate cytometer. As expected, PDCD1, the gene encoding programmed cell death protein 1 (PD-1), was revealed as the predominant hit. In addition, three genes that encode Fc receptors (FCGR1A, FCGR1B, and FCGR2A) were also identified as screen hits as the result of the Fc-tag fused to PD-L1, which has provided a reliable internal control for the screen. Furthermore, the potential of using a biotinylated ligand was explored and established to expand the versatility of the cDNA platform. This novel screening platform not only provides a powerful tool for the identification of ligands for orphan receptors but also has the potential for small-molecule target deconvolution.

Comparison of the ligand-binding properties of fluorescent VEGF-A isoforms to VEGF receptor 2 in living cells and membrane preparations using NanoBRET

Background and Purpose

Vascular endothelial growth factor A (VEGF‐A) is a key mediator of angiogenesis. A striking feature of the binding of a fluorescent analogue of VEGF₁₆₅a to nanoluciferase‐tagged VEGF receptor 2 (VEGFR2) in living cells is that the BRET signal is not sustained and declines over time. This may be secondary to receptor internalisation. Here, we have compared the binding of three fluorescent VEGF‐A isoforms to VEGFR2 in cells and isolated membrane preparations.

Experimental Approach

Ligand‐binding kinetics were monitored in both intact HEK293T cells and membranes (expressing nanoluciferase‐tagged VEGFR2) using BRET between tagged receptor and fluorescent analogues of VEGF₁₆₅a, VEGF₁₆₅b, and VEGF₁₂₁a. VEGFR2 endocytosis in intact cells expressing VEGFR2 was monitored by following the appearance of fluorescent ligand‐associated receptors in intracellular endosomes using automated quantitative imaging.

Key Results

Quantitative analysis of the effect of fluorescent VEGF‐A isoforms on VEGFR2 endocytosis in cells demonstrated that they produce a rapid and potent translocation of ligand‐bound VEGFR2 into intracellular endosomes. NanoBRET can be used to monitor the kinetics of the binding of fluorescent VEGF‐A isoforms to VEGFR2. In isolated membrane preparations, ligand‐binding association curves were maintained for the duration of the 90‐min experiment. Measurement of the k_off at pH 6.0 in membrane preparations indicated shorter ligand residence times than those obtained at pH 7.4.

Conclusions and Implications

These studies suggest that rapid VEGF‐A isoform‐induced receptor endocytosis shortens agonist residence times on the receptor (1/k_off) as VEGFR2 moves from the plasma membrane to the intracellular endosomes.

Utilizing a Simple Method for Stoichiometric Protein Labeling to Quantify Antibody Blockade

Ligand binding assays routinely employ fluorescently-labeled protein ligands to quantify the extent of binding. These ligands are commonly generated through chemical modification of accessible lysine residues, which often results in heterogeneous populations exhibiting variable binding properties. This could be remedied by quantitative, site-specific labeling. Recently, we reported on a single-step method integrating recombinant protein purification with 2-cyanobenzothiazole (CBT) condensation for labeling a proteolytically exposed N-terminal cysteine. Here, using three growth factors, we show that unlike random lysine labeling, this site-specific approach yielded homogeneous populations of growth factors that were quantitatively labeled at their N-termini and retained their binding characteristics. We demonstrate the utility of this labeling method through the development of a novel assay that quantifies the capacity of antibodies to block receptor-ligand interactions (i.e. antibody blockade). The assay uses bioluminescence resonance energy transfer (BRET) to detect binding of CBT-labeled growth factors to their cognate receptors genetically fused to NanoLuc luciferase. The ability of antibodies to block these interactions is quantified through decrease in BRET. Using several antibodies, we show that the assay provides reliable quantification of antibody blockade in a cellular context. As demonstrated here, this simple method for generating uniformly-labeled proteins has potential to promote more accurate and robust ligand binding assays.

Binding kinetics of ligands acting at GPCRs

The influence of drug-receptor binding kinetics has often been overlooked during the development of new therapeutics that target G protein-coupled receptors (GPCRs). Over the last decade there has been a growing understanding that an in-depth knowledge of binding kinetics at GPCRs is required to successfully target this class of proteins. Ligand binding to a GPCR is often not a simple single step process with ligand freely diffusing in solution. This review will discuss the experiments and equations that are commonly used to measure binding kinetics and how factors such as allosteric regulation, rebinding and ligand interaction with the plasma membrane may influence these measurements. We will then consider the molecular characteristics of a ligand and if these can be linked to association and dissociation rates.

Complex Formation between VEGFR2 and the β2-Adrenoceptor

Vascular endothelial growth factor (VEGF) is an important mediator of endothelial cell proliferation and angiogenesis via its receptor VEGFR2. A common tumor associated with elevated VEGFR2 signaling is infantile hemangioma that is caused by a rapid proliferation of vascular endothelial cells. The current first-line treatment for infantile hemangioma is the β-adrenoceptor antagonist, propranolol, although its mechanism of action is not understood. Here we have used bioluminescence resonance energy transfer and VEGFR2 genetically tagged with NanoLuc luciferase to demonstrate that oligomeric complexes involving VEGFR2 and the β₂-adrenoceptor can be generated in both cell membranes and intracellular endosomes. These complexes are induced by agonist treatment and retain their ability to couple to intracellular signaling proteins. Furthermore, coupling of β₂-adrenoceptor to β-arrestin2 is prolonged by VEGFR2 activation. These data suggest that protein-protein interactions between VEGFR2, the β₂-adrenoceptor, and β-arrestin2 may provide insight into their roles in health and disease.

NanoBRET: The Bright Future of Proximity-Based Assays

Bioluminescence resonance energy transfer (BRET) is a biophysical technique used to monitor proximity within live cells. BRET exploits the naturally occurring phenomenon of dipole-dipole energy transfer from a donor enzyme (luciferase) to an acceptor fluorophore following enzyme-mediated oxidation of a substrate. This results in production of a quantifiable signal that denotes proximity between proteins and/or molecules tagged with complementary luciferase and fluorophore partners. BRET assays have been used to observe an array of biological functions including ligand binding, intracellular signaling, receptor-receptor proximity, and receptor trafficking, however, BRET assays can theoretically be used to monitor the proximity of any protein or molecule for which appropriate fusion constructs and/or fluorophore conjugates can be produced. Over the years, new luciferases and approaches have been developed that have increased the potential applications for BRET assays. In particular, the development of the small, bright and stable Nanoluciferase (NanoLuc; Nluc) and its use in NanoBRET has vastly broadened the potential applications of BRET assays. These advances have exciting potential to produce new experimental methods to monitor protein-protein interactions (PPIs), protein-ligand interactions, and/or molecular proximity. In addition to NanoBRET, Nluc has also been exploited to produce NanoBiT technology, which further broadens the scope of BRET to monitor biological function when NanoBiT is combined with an acceptor. BRET has proved to be a powerful tool for monitoring proximity and interaction, and these recent advances further strengthen its utility for a range of applications.

NanoBRET ligand binding at a GPCR under endogenous promotion facilitated by CRISPR/Cas9 genome editing

Bioluminescence resonance energy transfer (BRET) is a versatile tool used to investigate membrane receptor signalling and function. We have recently developed a homogenous NanoBRET ligand binding assay to monitor interactions between G protein-coupled receptors and fluorescent ligands. However, this assay requires the exogenous expression of a receptor fused to the nanoluciferase (Nluc) and is thus not applicable to natively-expressed receptors. To overcome this limitation in HEK293 cells, we have utilised CRISPR/Cas9 genome engineering to insert Nluc in-frame with the endogenous ADORA2B locus this resulted in HEK293 cells expressing adenosine A_2B receptors under endogenous promotion tagged on their N-terminus with Nluc. As expected, we found relatively low levels of endogenous (gene-edited) Nluc/A_2B receptor expression compared to cells transiently transfected with expression vectors coding for Nluc/A_2B. However, in cells expressing gene-edited Nluc/A_2B receptors we observed clear saturable ligand binding of a non-specific fluorescent adenosine receptor antagonist XAC-X-BY630 (K_d = 21.4 nM). Additionally, at gene-edited Nluc/A_2B receptors we derived pharmacological parameters of ligand binding; K_d as well as K_on and K_off for binding of XAC-X-BY630 by NanoBRET association kinetic binding assays. Lastly, cells expressing gene-edited Nluc/A_2B were used to determine the pK_i of unlabelled adenosine receptor ligands in competition ligand binding assays. Utilising CRISPR/Cas9 genome engineering here we show that NanoBRET ligand binding assays can be performed at gene-edited receptors under endogenous promotion in live cells, therefore overcoming a fundamental limitation of NanoBRET ligand assays.

Gain of affinity for VEGF165 binding within the VEGFR2/NRP1 cellular complex detected by an HTRF-based binding assay

Neuroplin 1 (NRP1), a transmembrane protein interacting with Vascular Endothelial Growth Factor VEGF-A₁₆₅ (called here VEGF165) and the tyrosine kinase Receptor 2 (VEGFR2) promote angiogenesis and vascular homeostasis. In a pathophysiological context, several studies suggested that VEGFR2 and NRP1 mediate tumor development and progression. Given the involvement of the VEGF165 network in promoting tumor angiogenesis, NRP1, VEGFR2 and VEGF165 have been identified as targets for anti-angiogenic therapy. No binding assay exists to monitor specifically the binding of VEGF165 to the VEGFR2/NRP1 complex in intact cells. We established a binding assay based on the homogenous time-resolved fluorescence (HTRF®) technology. This unique binding assay enables to assess the interaction of VEGF165 with VEGFR2 or NRP1 within the VEGFR2/NRP1 complex. Ligand binding saturation experiments revealed that VEGF165 binds the VEGFR2/NRP1 complex at the cell surface with a ten to twenty-fold higher affinity compared to SNAP-VEGFR2 or SNAP-NRP1 receptors alone not engaged in the heteromeric complex. The assay allows characterizing the impact of NRP1 ligands on VEGF165 to the complex. It shows high specificity, reproducibility and robustness, making it compatible with high throughput screening (HTS) applications for identifying new VEGF165 antagonists selective for NRP1 or the VEGFR2/NRP1 complex.