Profiling epidermal growth factor receptor and heregulin receptor 3 heteromerization using receptor tyrosine kinase heteromer investigation technology

A peptide toxin in ant venom mimics vertebrate EGF-like hormones to cause long-lasting hypersensitivity in mammals

The targeting of mammalian ErbB receptor signaling by a venom toxin to cause hypersensitivity is a mode of action that has not previously been described. Natural selection of a defensive toxin to target ErbB signaling provides compelling independent evidence for a fundamental role of this receptor and its ligands in mammalian pain. The evolution of a toxin in ant venom to mimic a vertebrate nociceptive hormone serves as an example of both convergent evolution and molecular mimicry, illustrating how natural selection can shape the gene product of one organism to resemble that of another.

Venoms are excellent model systems for studying evolutionary processes associated with predator–prey interactions. Here, we present the discovery of a peptide toxin, MIITX₂-Mg1a, which is a major component of the venom of the Australian giant red bull ant Myrmecia gulosa and has evolved to mimic, both structurally and functionally, vertebrate epidermal growth factor (EGF) peptide hormones. We show that Mg1a is a potent agonist of the mammalian EGF receptor ErbB1, and that intraplantar injection in mice causes long-lasting hypersensitivity of the injected paw. These data reveal a previously undescribed venom mode of action, highlight a role for ErbB receptors in mammalian pain signaling, and provide an example of molecular mimicry driven by defensive selection pressure.

Profiling novel pharmacology of receptor complexes using Receptor-HIT

Many receptors are able to undergo heteromerisation, leading to the formation of receptor complexes that may have pharmacological profiles distinct from those of the individual receptors. As a consequence of this, receptor heteromers can be classed as new drug targets, with the potential for achieving greater specificity and selectivity over targeting their constituent receptors. We have developed the Receptor-Heteromer Investigation Technology (Receptor-HIT), which enables the detection of receptor heteromers using a proximity-based reporter system such as bioluminescence resonance energy transfer (BRET). Receptor-HIT detects heteromers in live cells and in real time, by utilising ligand-induced signals that arise from altered interactions with specific biomolecules, such as ligands or proteins. Furthermore, monitoring the interaction between the receptors and the specific biomolecules generates functional information about the heteromer that can be pharmacologically quantified. This review will discuss various applications of Receptor-HIT, including its use with different classes of receptors (e.g. G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and others), its use to monitor receptor interactions both intracellularly and extracellularly, and also its use with genome-edited endogenous proteins.

Complex interactions between the angiotensin II type 1 receptor, the epidermal growth factor receptor and TRIO-dependent signaling partners

Transactivation of the epidermal growth factor receptor (EGFR) by the angiotensin II (AngII) type 1 (AT₁) receptor is involved in AT₁ receptor-dependent growth effects and cardiovascular pathologies, however the mechanisms underpinning this transactivation are yet to be fully elucidated. Recently, a potential intermediate of this process was identified following the discovery that a kinase called TRIO was involved in AngII/AT₁ receptor-mediated transactivation of EGFR. To investigate the mechanisms by which TRIO acts as an intermediate in AngII/AT₁ receptor-mediated EGFR transactivation we used bioluminescence resonance energy transfer (BRET) assays to investigate proximity between the AT₁ receptor, EGFR, TRIO and other proteins of interest. We found that AngII/AT₁ receptor activation caused a Gα_q-dependent increase in proximity of TRIO with Gγ₂ and the AT₁-EGFR heteromer, as well as trafficking of TRIO towards the Kras plasma membrane marker and into early, late and recycling endosomes. In contrast, we found that AngII/AT₁ receptor activation caused a Gα_q-independent increase in proximity of TRIO with Grb2, GRK2 and PKCζ, as well as trafficking of TRIO up to the plasma membrane from the Golgi. Furthermore, we confirmed the proximity between the AT₁ receptor and the EGFR using the Receptor-Heteromer Investigation Technology, which showed AngII-induced recruitment of Grb2, GRK2, PKCζ, Gγ₂ and TRIO to the EGFR upon AT₁ coexpression. In summary, our results provide further evidence for the existence of the AT₁-EGFR heteromer and reveal potential mechanisms by which TRIO contributes to the transactivation process.

Investigation of Receptor Heteromers Using NanoBRET Ligand Binding

Receptor heteromerization is the formation of a complex involving at least two different receptors with pharmacology that is distinct from that exhibited by its constituent receptor units. Detection of these complexes and monitoring their pharmacology is crucial for understanding how receptors function. The Receptor-Heteromer Investigation Technology (Receptor-HIT) utilizes ligand-dependent modulation of interactions between receptors and specific biomolecules for the detection and profiling of heteromer complexes. Previously, the interacting biomolecules used in Receptor-HIT assays have been intracellular proteins, however in this study we have for the first time used bioluminescence resonance energy transfer (BRET) with fluorescently-labeled ligands to investigate heteromerization of receptors on the cell surface. Using the Receptor-HIT ligand binding assay with NanoBRET, we have successfully investigated heteromers between the angiotensin II type 1 (AT1) receptor and the β2 adrenergic receptor (AT1-β2AR heteromer), as well as between the AT1 and angiotensin II type 2 receptor (AT1-AT2 heteromer).

Interplay Between Angiotensin II Type 1 Receptor and Thrombin Receptor Revealed by Bioluminescence Resonance Energy Transfer Assay

The key hormone of the renin-angiotensin system (RAS), angiotensin II (AngII), and thrombin are known to play major roles in the vascular system and its related disorders. Previous studies reported connections between AngII and thrombin in both physiological and pathophysiological models. However, the molecular mechanisms controlling such interplay at the level of their receptors belonging to the family of G protein-coupled receptors (GPCRs) are not fully understood. In this study, we investigated the functional interaction between the AngII type 1 receptor (AT1R) and the thrombin receptor [or protease-activated receptor 1 (PAR1)] in human embryonic kidney 293 (HEK293) cells. For this, we used various bioluminescence resonance energy transfer (BRET) proximity-based assays to profile the coupling to the heterotrimeric Gαq protein, β-arrestin recruitment, and receptor internalization and trafficking in intact cells. The overall dose-response and real-time kinetic BRET data demonstrated the specific molecular proximity between AT1R and PAR1 resulting in their functional interaction. This was characterized by thrombin inducing BRET increase within AT1R/Gαq and AT1R/β-arrestin pairs and synergistic effects observed upon the concomitant activation of both receptors suggesting a positive allosteric interaction. The BRET data corroborated with the data on the downstream Gαq/inositol phosphate pathway. Moreover, the selective pharmacological blockade of the receptors revealed the implication of both AT1R and PAR1 protomers in such a synergistic interaction and the possible transactivation of AT1R by PAR1. Interestingly, the positive action of PAR1 on AT1R activation was contrasted with its apparent inhibition of AT1R internalization and its endosomal trafficking. Finally, BRET saturation and co-immunoprecipitation assays supported the physical AT1-PAR1 interaction in HEK293 cells. Our study reveals for the first time the functional interaction between AT1R and PAR1 in vitro characterized by a transactivation and positive allosteric modulation of AT1R and inhibition of its desensitization and internalization. This finding may constitute the molecular basis of the well-known interplay between RAS and thrombin. Thus, our data should lead to revising some findings on the implication of RAS and thrombin in vascular physiology and pathophysiology revealing the importance to consider the functional and pharmacological interaction between AT1R and thrombin receptors.

The orphan receptor GPR88 blunts the signaling of opioid receptors and multiple striatal GPCRs

GPR88 is an orphan G protein-coupled receptor (GPCR) considered as a promising therapeutic target for neuropsychiatric disorders; its pharmacology, however, remains scarcely understood. Based on our previous report of increased delta opioid receptor activity in Gpr88 null mice, we investigated the impact of GPR88 co-expression on the signaling of opioid receptors in vitro and revealed that GPR88 inhibits the activation of both their G protein- and β-arrestin-dependent signaling pathways. In Gpr88 knockout mice, morphine-induced locomotor sensitization, withdrawal and supra-spinal analgesia were facilitated, consistent with a tonic inhibitory action of GPR88 on µOR signaling. We then explored GPR88 interactions with more striatal versus non-neuronal GPCRs, and revealed that GPR88 can decrease the G protein-dependent signaling of most receptors in close proximity, but impedes β-arrestin recruitment by all receptors tested. Our study unravels an unsuspected buffering role of GPR88 expression on GPCR signaling, with intriguing consequences for opioid and striatal functions.

Functional characterization of β2-adrenergic and insulin receptor heteromers

This study aimed to functionally characterize β₂-adrenergic (β₂AR) and insulin receptor (IR) heteromers in regard to β-arrestin 2 (βarr2) recruitment and cAMP signaling and to examine the involvement of the cytoplasmic portion of the IR β chain in heteromerization with β₂AR. Evidence for β₂AR:IR:βarr2 complex formation and the specificity of the IR:βarr2 interaction was first provided by bioinfomatics analysis. Receptor-heteromer investigation technology (HIT) then provided functional evidence of β₂AR:IR heterodimerization by showing isoproterenol-induced but not insulin-induced GFP²-βarr2 recruitment to the β₂AR:IR complex; the IR:βarr2 interaction was found to only be constitutive. The constitutive IR:βarr2 BRET signal (BRET_const) was significantly smaller in cells coexpressing IR-RLuc8 and a GFP²-βarr2 1-185 mutant lacking the proposed IR binding domain. β₂AR:IR heteromerization also influenced the pharmacological phenotype of β₂AR, i.e., its efficacy in recruiting βarr2 and activating cAMP signaling. Evidence suggesting involvement of the cytoplasmic portion of the IR β chain in the interaction with β₂AR was provided by BRET² saturation and HIT assays using an IR 1-1271 stop mutant lacking the IR C-terminal tail region. For the complex consisting of IR 1-1271-RLuc8:β₂AR-GFP², saturation was not reached, most likely reflecting random collisions between IR 1-1271 and β₂AR. Furthermore, in the HIT assay, no substantial agonist-induced increase in the BRET² signal was detected that would be indicative of βarr2 recruitment to the IR 1-1271:β₂AR heteromer. Complementary 3D visualization of β₂AR:IR provided supporting evidence for stability of the heterotetramer complex and identified amino acid residues involved in β₂AR:IR heteromerization. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

BRET-based assay to monitor EGFR transactivation by the AT1R reveals Gq/11 protein-independent activation and AT1R-EGFR complexes

The type 1 angiotensin II (AngII) receptor (AT₁R) transactivates the epidermal growth factor receptor (EGFR), which leads to pathological remodeling of heart, blood vessels and kidney. End-point assays are used as surrogates of EGFR activation, however these downstream readouts are not applicable to live cells, in real-time. Herein, we report the use of a bioluminescence resonance energy transfer (BRET)-based assay to assess recruitment of the EGFR adaptor protein, growth factor receptor-bound protein 2 (Grb2), to the EGFR. In a variety of cell lines, both epidermal growth factor (EGF) and AngII stimulated Grb2 recruitment to EGFR. The BRET assay was used to screen a panel of 9 G protein-coupled receptors (GPCRs) and further developed for other EGFR family members (HER2 and HER3); the AT₁R was able to transactivate HER2, but not HER3. Mechanistically, AT₁R-mediated ERK1/2 activation was dependent on G_q/11 and EGFR tyrosine kinase activity, whereas the recruitment of Grb2 to the EGFR was independent of G_q/11 and only partially dependent on EGFR tyrosine kinase activity. This G_q/11 independence of EGFR transactivation was confirmed using AT₁R mutants and in CRISPR cell lines lacking G_q/11. EGFR transactivation was also apparently independent of β-arrestins. Finally, we used additional BRET-based assays and confocal microscopy to provide evidence that both AngII- and EGF-stimulation promoted AT₁R-EGFR heteromerization. In summary, we report an alternative approach to monitoring AT₁R-EGFR transactivation in live cells, which provides a more direct and proximal view of this process, including the potential for complexes between the AT₁R and EGFR.

Quantifying the Interaction between EGFR Dimers and Grb2 in Live Cells

Adaptor proteins are a class of cytoplasmic proteins that bind to phosphorylated residues in receptor tyrosine kinases and trigger signaling cascades that control critically important cellular processes, such as cell survival, growth, differentiation, and motility. Here, we seek to characterize the interaction between epidermal growth factor receptor (EGFR) and the cytoplasmic adaptor protein growth factor receptor-bound protein 2 (Grb2) in a cellular context. To do so, we explore the utility of a highly biologically relevant model system, mammalian cells under reversible osmotic stress, and a recently introduced Förster resonance energy transfer microscopy method, fully quantified spectral imaging. We present a method that allows us to quantify the stoichiometry and the association constant of the EGFR-Grb2 binding interaction in the plasma membrane, in the presence and absence of activating ligand. The method that we introduce can have broad utility in membrane protein research, as it can be applied to different membrane protein-cytoplasmic protein pairs.

HER-3 targeting alters the dimerization pattern of ErbB protein family members in breast carcinomas

Breast carcinogenesis is a multi-step process in which membrane receptor tyrosine kinases are crucial participants. Lots of research has been done on epidermal growth factor receptor (EGFR) and HER-2 with important clinical results. However, breast cancer patients present intrinsic or acquired resistance to available HER-2-directed therapies, mainly due to HER-3. Using new techniques, such as proximity ligation assay, herein we evaluate the dimerization pattern of HER-3 and the importance of context-dependent dimer formation between HER-3 and other HER protein family members. Additionally, we show that the efficacy of novel HER-3 targeting agents can be better predicted in certain breast cancer patient sub-groups based on the dimerization pattern of HER protein family members. Moreover, this model was also evaluated and reproduced in human paraffin-embedded breast cancer tissues.

Differential Effects of Camel Milk on Insulin Receptor Signaling - Toward Understanding the Insulin-Like Properties of Camel Milk

Previous studies on the Arabian camel (Camelus dromedarius) showed beneficial effects of its milk reported in diverse models of human diseases, including a substantial hypoglycemic activity. However, the cellular and molecular mechanisms involved in such effects remain completely unknown. In this study, we hypothesized that camel milk may act at the level of human insulin receptor (hIR) and its related intracellular signaling pathways. Therefore, we examined the effect of camel milk on the activation of hIR transiently expressed in human embryonic kidney 293 (HEK293) cells using bioluminescence resonance energy transfer (BRET) technology. BRET was used to assess, in live cells and real-time, the physical interaction between hIR and insulin receptor signaling proteins (IRS1) and the growth factor receptor-bound protein 2 (Grb2). Our data showed that camel milk did not promote any increase in the BRET signal between hIR and IRS1 or Grb2 in the absence of insulin stimulation. However, it significantly potentiated the maximal insulin-promoted BRET signal between hIR and Grb2 but not IRS1. Interestingly, camel milk appears to differentially impact the downstream signaling since it significantly activated ERK1/2 and potentiated the insulin-induced ERK1/2 but not Akt activation. These observations are to some extent consistent with the BRET data since ERK1/2 and Akt activation are known to reflect the engagement of Grb2 and IRS1 pathways, respectively. The preliminary fractionation of camel milk suggests the peptide/protein nature of the active component in camel milk. Together, our study demonstrates for the first time an allosteric effect of camel milk on insulin receptor conformation and activation with differential effects on its intracellular signaling. These findings should help to shed more light on the hypoglycemic activity of camel milk with potential therapeutic applications.

Assessing Gonadotropin Receptor Function by Resonance Energy Transfer-Based Assays

Gonadotropin receptors belong to the super family of G protein-coupled receptors and mediate the physiological effects of follicle-stimulating hormone (FSHR) and luteinizing hormone (LHR). Their central role in the control of reproductive function has made them the focus of intensive studies. Upon binding to their cognate hormone, they trigger complex signaling and trafficking mechanisms that are tightly regulated in concentration, time, and space. Classical cellular assays often fail to capture all these dynamics. Here, we describe the use of various bioluminescence and fluorescence resonance energy transfer (BRET and FRET) assays to investigate the activation and regulation of FSHR and LHR in real-time, in living cells (i.e., transiently expressed in human embryonic kidney 293 cells). Indeed, the dynamics of hormone-mediated heterotrimeric G protein activation, cyclic adenosine-monophosphate (cAMP) production, calcium release, β-arrestin 2 recruitment, and receptor internalization/recycling was assessed. Kinetics and dose-response analyses confirmed the expected pharmacological and signaling properties of hFSHR and hLHR but revealed interesting characteristics when considering the two major pathways (cAMP and β-arrestin 2) of the two receptors assessed by BRET. Indeed, the EC50 values were in picomolar range for cAMP production while nanomolar range was observed for β-arrestin 2 recruitment as well as receptor internalization. Interestingly, the predicted receptor occupancy indicates that the maximal G protein activation and cAMP response occur at <10% of receptor occupancy whereas >90% of activated receptors is required to achieve full β-arrestin 2 recruitment and subsequent receptor internalization. The rapid receptor internalization was also followed by a recycling phase. Collectively, our data reveal that β-arrestin-mediated desensitization, internalization, and the subsequent fast recycling of receptors at the plasma membrane may provide a mechanistic ground to the "spare receptor" paradigm. More generally, the novel tools described here will undoubtedly provide the scientific community investigating gonadotropin receptors with powerful means to decipher their pharmacology and signaling with the prospect of pathophysiological and drug discovery applications.

Signalling profiles of H3 relaxin, H2 relaxin and R3(BΔ23-27)R/I5 acting at the relaxin family peptide receptor 3 (RXFP3)

BACKGROUND AND PURPOSE

Relaxin family peptide receptor 3 (RXFP3) is expressed in brain areas important for processing sensory information and feeding, suggesting that it may be a target for anti-anxiety and anti-obesity drugs. We examined the effects of H3 relaxin, the biased agonist H2 relaxin and the antagonist, R3(BΔ23-27)R/I5, on RXFP3 signalling to establish their suitability as tools to assess the physiological roles of RXFP3.

EXPERIMENTAL APPROACH

The signalling profile of the RXFP3 ligands was determined using reporter gene assays, multiplexed signalling assays and direct examination of receptor-G protein and receptor-β-arrestin interactions using BRET.

KEY RESULTS

H2 relaxin activated p38MAPK and ERK1/2 with lower efficacy than H3 relaxin, but had similar efficacy for JNK1/2 phosphorylation. H2 or H3 relaxin activation of p38MAPK, JNK1/2 or ERK1/2 involved Pertussis toxin-sensitive G-proteins. R3(BΔ23-27)R/I5 blocked H3 relaxin AP-1 reporter gene activation, but not H2 relaxin AP-1 activation or H3 relaxin NF-κB activation. R3(BΔ23-27)R/I5 activated the SRE reporter, but did not inhibit either H2 or H3 relaxin SRE activation. R3(BΔ23-27)R/I5 blocked H3 relaxin-stimulated p38MAPK and ERK1/2 phosphorylation, but was a weak partial agonist for p38MAPK and ERK1/2 signalling. p38MAPK activation by R3(BΔ23-27)R/I5 was G protein-independent. H3 relaxin-activated RXFP3 interacts with Gαi2 , Gαi3 , Gαo A and Gαo B whereas H2 relaxin or R3(BΔ23-27)R/I5 induce interactions only with Gαi2 or Gαo B . Only H3 relaxin promoted RXFP3/β-arrestin interactions that were blocked by R3(BΔ23-27)R/I5.

CONCLUSION AND IMPLICATIONS

Understanding signalling profile of drugs acting at RXFP3 is essential for development of therapies targeting this receptor.

Biophysical Detection of Diversity and Bias in GPCR Function

Guanine nucleotide binding protein (G protein)-coupled receptors (GPCRs) function in complexes with a range of molecules and proteins including ligands, G proteins, arrestins, ubiquitin, and other receptors. Elements of these complexes may interact constitutively or dynamically, dependent upon factors such as ligand binding, phosphorylation, and dephosphorylation. They may also be allosterically modulated by other proteins in a manner that changes temporally and spatially within the cell. Elucidating how these complexes function has been greatly enhanced by biophysical technologies that are able to monitor proximity and/or binding, often in real time and in live cells. These include resonance energy transfer approaches such as bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET). Furthermore, the use of fluorescent ligands has enabled novel insights into allosteric interactions between GPCRs. Consequently, biophysical approaches are helping to unlock the amazing diversity and bias in G protein-coupled receptor signaling.