Convergent Recognition of the IgE Binding Site on the High-Affinity IgE Receptor

IgE-FcεRI protein-protein interaction as a therapeutic target against allergic asthma: An updated review

In the last decade, research has clarified the binding interactions between immunoglobulin E (IgE) and its high-affinity receptor, the FcεRI alpha chain (FcεRI). The formation of the IgE-FcεRI complex is crucial in the context of atopic allergies, linking allergen recognition to cellular activation and disease manifestation. Consequently, pharmacological strategies that disrupt these interactions are vital for managing atopic conditions. Historically, the complexity of the IgE-FcεRI binding process and challenges in producing functional recombinant derivatives has complicated data interpretation. However, advancements in structural biology, protein engineering, and immunological studies have enhanced our understanding of these protein-protein interactions (PPI), facilitating the development of more effective therapies. The introduction of anti-IgE therapies underscores the significance of the IgE-FcεRI PPI in allergic asthma. IgE, that is present locally and systemically, serves as a sensory mechanism in the adaptive immune response, particularly in mast cells (MCs) and basophils. When bound to FcεRI, IgE enables rapid memory responses to allergens, but dysregulation can lead to severe allergic asthma. Thus, the reactivity of IgE sensors can be finely modulated using various IgE-associated molecules. This review explores the mechanisms underlying IgE-dependent MC activation and its regulation by these molecules, including the latest therapeutic candidates under investigation.

Molecular modelling studies in explaining the higher GPVI antagonistic activity of the racemic 2-(4-methoxyphenylsulfonyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxamide than its enantiomers

The GPVI receptor on the platelets plays a major role in inhibiting arterial thrombosis with limited risk of bleeding and is considered a potential anti-thrombotic target for arterial thrombosis. In the reported anti-thrombotics, tetrahydropyridoindoles, the title compound was the best inhibitor of the collagen mediated platelet aggregation by antagonizing the platelet receptor GPVI. Interestingly, the racemic title compound showed better antagonism (IC₅₀ racemate = 6.7 μM) than either of its enantiomers (IC₅₀ S enantiomer = 25.3 μM; IC₅₀ R enantiomer = 126.3 μM). In order to explain this, the molecular modelling approaches viz. site map analysis, protein-protein docking and molecular dynamics simulation were carried out, which led to the identification of a second binding site located near the primary antagonist binding site known to bind losartan. The induced fit docking studies for both the enantiomers at the primary and secondary binding sites showed that the S-enantiomer has better interactions at the primary binding site than the R-enantiomer, while the R-enantiomer has better interactions at the secondary site than the S-enantiomer. Hence, the overall interactions of the racemic compound containing equimolar mixture may be higher than any one of the enantiomers and may explain the higher activity than its enantiomers of the racemic compound.

TRB3 links insulin/IGF to tumour promotion by interacting with p62 and impeding autophagic/proteasomal degradations

High insulin/IGF is a biologic link between diabetes and cancers, but the underlying molecular mechanism remains unclear. Here we report a previously unrecognized tumour-promoting mechanism for stress protein TRB3, which mediates a reciprocal antagonism between autophagic and proteasomal degradation systems and connects insulin/IGF to malignant promotion. We find that several human cancers express higher TRB3 and phosphorylated insulin receptor substrate 1, which correlates negatively with patient's prognosis. TRB3 depletion protects against tumour-promoting actions of insulin/IGF and attenuates tumour initiation, growth and metastasis in mice. TRB3 interacts with autophagic receptor p62 and hinders p62 binding to LC3 and ubiquitinated substrates, which causes p62 deposition and suppresses autophagic/proteasomal degradation. Several tumour-promoting factors accumulate in cancer cells to support tumour metabolism, proliferation, invasion and metastasis. Interrupting TRB3/p62 interaction produces potent antitumour efficacies against tumour growth and metastasis. Our study opens possibility of targeting this interaction as a potential novel strategy against cancers with diabetes.

An IgE receptor mimetic peptide (PepE) protects mice from IgE mediated anaphylaxis

Crosslinking of receptor-bound Immunoglobulin E (IgE) triggers immediate hypersensitivity reactions including anaphylaxis. Blocking the interaction of IgE with its high-affinity receptor, FcεRI, on mast cells and basophils is an attractive strategy for the treatment of allergies. This approach has seen clinical success using the anti-IgE monoclonal antibody, omalizumab. We recently designed and characterized a novel FcεRI-mimetic peptide (PepE) which contains the two key FcεRI α-chain receptor loops known to interact with the ε-heavy chain of IgE, C'-E and B-C, with an optimized linker for joining them. PepE has high specificity and affinity for IgE, blocks IgE binding to FcεRI and prevents IgE-induced mediator release from RBL2H3 cells. We have now investigated the biological effects of this peptide in vivo using a line of mice (BALB/c Il4raF709) very sensitive to IgE-mediated systemic anaphylaxis. IgE-deficient (IgE-/-) Il4raF709 mice were passively sensitized with the anti-DNP IgE monoclonal antibody (SPE-7) and subsequently challenged i.v. with DNP-BSA. Mice receiving a single dose of PepE prior to sensitization with SPE-7 IgE were fully protected from anaphylaxis while vehicle control-treated mice displayed strong reactions with significant core body temperature drops and elevated levels of mouse mast cell protease-1 (mMCP-1) in the serum. However, PepE had no effect on IgE-mediated anaphylaxis if given after IgE administration in IgE-/- mice, suggesting that PepE can block binding of free IgE to FcεRI but cannot compete with the receptor for already bound IgE in vivo. A single dose of PepE treatment did not protect IgE sufficient mice from IgE mediated anaphylaxis. However, a 3 week long course of PepE treatment protected IgE sufficient Il4raF709 mice from body temperature drops and elevation of serum mMCP-1. Our findings establish the potential of this type of structure for blocking IgE binding to mast cells in vivo and suggest that related peptides might have the potential to attenuate clinical allergic reactions.

Accelerated dissociation of IgE-FcεRI complexes by disruptive inhibitors actively desensitizes allergic effector cells

BACKGROUND

The remarkably stable interaction of IgE with its high-affinity receptor FcεRI on basophils and mast cells is critical for the induction of allergic hypersensitivity reactions. Because of the exceptionally slow dissociation rate of IgE-FcεRI complexes, such allergic effector cells permanently display allergen-specific IgE on their surface and immediately respond to allergen challenge by releasing inflammatory mediators. We have recently described a novel macromolecular inhibitor that actively promotes the dissociation of IgE from FcεRI through a molecular mechanism termed facilitated dissociation.

OBJECTIVE

Here we assessed the therapeutic potential of this non-immunoglobulin-based IgE inhibitor E2_79, a designed ankyrin repeat protein (DARPin), as well as a novel engineered biparatopic DARPin bi53_79, and directly compared them with the established anti-IgE antibody omalizumab.

METHODS

IgE-FcεRI complex dissociation was analyzed in vitro by using recombinant proteins in ELISA and surface plasmon resonance, ex vivo by using human primary basophils with flow cytometry, and in vivo by using human FcεRI α-chain transgenic mice in a functional passive cutaneous anaphylaxis test.

RESULTS

We show that E2_79-mediated removal of IgE from primary human basophils fully abrogates IgE-dependent cell activation and release of proinflammatory mediators ex vivo. Furthermore, we report that omalizumab also accelerates the dissociation of IgE from FcεRI, although much less efficiently than E2_79. Using the biparatopic IgE targeting approach, we further improved the disruptive potency of E2_79 by approximately 100-fold and show that disruptive IgE inhibitors efficiently prevent passive cutaneous anaphylaxis in mice expressing the human FcεRI α-chain.

CONCLUSION

Our findings highlight the potential of such novel IgE inhibitors as important diagnostic and therapeutic tools for management of allergic diseases.

Development of small molecules to target the IgE:FcεRI protein-protein interaction in allergies

The protein-protein interaction (PPI) between IgE and its high-affinity receptor (FcεRI) is a key component of the allergic response. Inhibiting the IgE:FcεRI PPI is an attractive strategy for therapeutic intervention and the development of allergy treatments. This PPI has been validated as a viable target by the monoclonal anti-IgE antibody omalizumab (Xolair(®)), which has demonstrated clinical efficacy when prescribed to treat moderate-to-severe asthma and hay fever, but small molecules would be a more convenient form of treatment. Cyclic peptides, small proteins and a natural product have all been developed to target the IgE:FcεRI PPI, and these will be discussed in this review. Targeting the IgE:FcεRI complex with small molecules presents various challenges, some of which are inherent in all PPI targets but some of which are unique to this system, which presents great opportunities for the development of new therapeutics for the treatment of allergies.

IgE and mast cells in allergic disease

Immunoglobulin E (IgE) antibodies and mast cells have been so convincingly linked to the pathophysiology of anaphylaxis and other acute allergic reactions that it can be difficult to think of them in other contexts. However, a large body of evidence now suggests that both IgE and mast cells are also key drivers of the long-term pathophysiological changes and tissue remodeling associated with chronic allergic inflammation in asthma and other settings. Such potential roles include IgE-dependent regulation of mast-cell functions, actions of IgE that are largely independent of mast cells and roles of mast cells that do not directly involve IgE. In this review, we discuss findings supporting the conclusion that IgE and mast cells can have both interdependent and independent roles in the complex immune responses that manifest clinically as asthma and other allergic disorders.

Synthesis and incorporation into cyclic peptides of tolan amino acids and their hydrogenated congeners: construction of an array of A-B-loop mimetics of the Cε3 domain of human IgE

The disruption of the human immunolobulin E-high affinity receptor I (IgE-FcεRI) protein-protein interaction (PPI) is a validated strategy for the development of anti asthma therapeutics. Here, we describe the synthesis of an array of conformationally constrained cyclic peptides based on an epitope of the A-B loop within the Cε3 domain of IgE. The peptides contain various tolan (i.e., 1,2-biarylethyne) amino acids and their fully and partially hydrogenated congeners as conformational constraints. Modest antagonist activity (IC(50) ∼660 μM) is displayed by the peptide containing a 2,2'-tolan, which is the one predicted by molecular modeling to best mimic the conformation of the native A-B loop epitope in IgE.

Association of polymorphisms in the promoter region of FCER1A gene with atopic dermatitis, chronic uticaria, asthma, and serum immunoglobulin E levels in a Han Chinese population

The high-affinity receptor for immunoglobulin E (IgE) plays a central role in allergy diseases. Previous studies have reported the association of variants in the proximal promoter of FCER1A with IgE levels as well as allergy disorders. Another promoter gene polymorphism that is located upstream of exon 1 has not been investigated. We investigated the association of variants in the promoter located upstream of FCER1A exon 1 with serum IgE levels and allergy diseases in a Han Chinese population. A total of 97 patients with atopic dermatitis (AD), 123 patients with chronic urticaria (CU), 286 children with asthma, and control groups were screened for polymorphisms in the promoter region located upstream of FCER1A exon 1 by the polymerase chain reaction-ligation detection reaction method. Total serum IgE levels were tested in groups. The rare allele A of the rs2427837 A/G polymorphism was significantly different in the AD group compared with the controls. No association with the polymorphism was observed in the CU group. In asthmatic patients, IgE levels were higher in the mutation genotypes GA of rs2427837 and TC of rs2251746 compared with normal genotype individuals. The minor allele of rs2427837 and rs2251746 in FCER1A is a genetic risk factor of high IgE levels.

IgE-binding properties and selectivity of peptide mimics of the FcvarepsilonRI binding site

FcvarepsilonRIalpha found on the surface of mast cells and basophiles mediates allergic diseases, anaphylaxis and asthma through binding of IgE. Disrupting this interaction with anti-IgE mAbs has proven an efficient approach to control these diseases. The crystallographic structure of the complex formed between the IgE-Fc and FcvarepsilonRIalpha extracellular domain has shown that recognition is mediated by residues in the second Ig-like domain of the receptor (D2) and in the loop connecting the D1 and D2 domains. In an attempt to obtain specific IgE antagonists, we have designed and prepared a polypeptide named IgE-Trap that partially reproduces the IgE receptor-binding sites and binds with micromolar affinity to soluble IgE. The polypeptide contains loops C'-E [residues 129-134] and F-G [residues 151-161] from the D2 domain joined by a linker, and loop B-C [residues 110-113]. Peptide binding to IgE has been assessed by SPR analyses and the data fit with a biphasic model of interaction, in agreement with the two-site mechanism reported for the native receptor. The polypeptide binds to immobilized IgE in a dose-dependent manner with a K(D) estimated to be around 6muM, while it does not recognize IgG nor IgA. Polypeptide sub-domains involved in IgE binding have also been defined, showing that loop C'-E connected to loop B-C, but also the isolated loop B-C alone suffice to bind immunoglobulins E with high selectively though with reduced affinity compared to IgE-Trap. ELISA and cytometric assays on RBL2H3 cells demonstrate that the interacting peptides are able to displace the binding of IgE to receptor, confirming affinity and specificity of these ligands and suggesting a potential application as modulators of disorders associated with inappropriate IgE production.

Identification of cyclic peptides able to mimic the functional epitope of IgG1-Fc for human Fc gammaRI

Identification of short, structured peptides able to mimic potently protein-protein interfaces remains a challenge in drug discovery. We report here the use of a naive cyclic peptide phage display library to identify peptide ligands able to recognize and mimic IgG1-Fc functions with FcγRI. Selection by competing off binders to FcγRI with IgG1 allowed the isolation of a family of peptides sharing the common consensus sequence TX₂CXXθPXLLGCΦXE (θ represents a hydrophobic residue, Φ is usually an acidic residue, and X is any residue) and able to inhibit IgG1 binding to FcγRI. In soluble form, these peptides antagonize superoxide generation mediated by IgG1. In complexed form, they trigger phagocytosis and a superoxide burst. Unlike IgG, these peptides are strictly FcγRI-specific among the FcγRs. Molecular modeling studies suggest that these peptides can adopt 2 distinct and complementary conformers, each able to mimic the discontinuous interface contacts constituted by the Cγ2-A and -B chains of Fc for FcγRI. In addition, by covalent homodimerization, we engineered a synthetic bivalent 37-mer peptide that retains the ability to trigger effector functions. We demonstrate here that it is feasible to maintain IgG-Fc function within a small structured peptide. These peptides represent a new format for modulation of effector functions.—Bonetto, S., Spadola, L., Buchanan, A. G., Jermutus, L. Lund, J. Identification of cyclic peptides able to mimic the functional epitope of IgG1-Fc for human FcγRI.

IgE in allergy and asthma today

The spreading epidemic of allergies and asthma has heightened interest in IgE, the central player in the allergic response. The activity of IgE is associated with a network of proteins; prominent among these are its two principal receptors, FcepsilonRI (high-affinity Fc receptor for IgE) and CD23, as well as galectin-3 and several co-receptors for CD23, notably CD21 and various integrins. Here, we review recent progress in uncovering the structures of these proteins and their complexes, and in our understanding of how IgE exerts its effects and how its expression is regulated. The information that has emerged suggests new therapeutic directions for combating allergic disease.

An FcgammaRIIa-binding peptide that mimics the interaction between FcgammaRIIa and IgG

A disulphide-constrained peptide that binds to the low affinity Fc receptor, FcgammaRIIa (CD32) has been identified and its structure solved by NMR. Linear (7-mer and 12-mer) and disulphide-constrained (7-mer) phage display peptide libraries were panned on recombinant soluble FcgammaRIIa genetically fused to HSA (HSA-FcgammaRIIa). Peptides were isolated only from the constrained peptide library and these contained the consensus sequence, CWPGWxxC. Phage clones displaying variants of the peptide consensus sequence bound to FcgammaRIIa and the strongest binding clone C7C1 (CWPGWDLNC) competed with IgG for binding to FcgammaRIIa and was inhibited from binding to FcgammaRIIa by the FcgammaRIIa-blocking antibody, IV.3, suggesting that C7C1 and IgG share related binding sites on FcgammaRIIa. A synthetic disulphide-constrained peptide, pep-C7C1 bound to FcgammaRIIa by biosensor analysis, albeit with low affinity (KD approximately 100microM). It was significant that the FcgammaRIIa consensus peptide sequence contained a Proline (Pro3), which when substituted with alanine abrogated FcgammaRIIa binding, consistent with Pro3 contributing to receptor binding. Upon binding of IgG and IgE to their respective Fc receptors (FcgammaRs and FcepsilonRI) Pro329 in the Fc makes a critical interaction with two highly conserved Trp residues (Trp90 and Trp113) of the FcRs. The NMR structure of pep-C7C1 revealed a stabilizing type II beta-turn between Trp2 and Trp5, with Pro3 solvent exposed. Modelling of the pep-C7C1 structure in complex with FcgammaRIIa suggests that Pro3 of C7C1 binds to FcgammaRIIa by inserting between Trp90 and Trp113 of FcgammaRIIa thereby mimicking the molecular interaction made between FcgammaRIIa and IgG.

Disulfide linkage controls the affinity and stoichiometry of IgE Fcepsilon3-4 binding to FcepsilonRI

IgE antibodies cause long-term sensitization of tissue mast cells and blood basophils toward allergen-induced cross-linking and triggering of allergic inflammation. This persistence of IgE binding is due to its uniquely high affinity for the receptor FcepsilonRI and in particular its slow rate of dissociation once bound. The binding interface consists of two subsites, one contributed by each Cepsilon3 domain of IgE Fc in a 1:1 complex. We have investigated the contributions of Cepsilon3 disulfide linkage and glycosylation to the kinetics and affinity of binding of an Fc subfragment (Fcepsilon3-4) to a soluble receptor fragment (sFcepsilonRIalpha). In contrast to IgG Fc where deglycosylation abrogates receptor binding activity, the removal of the N-linked carbohydrate at Asn-394 in Fcepsilon3-4 only reduces binding affinity by a factor of 4, principally because of a faster off-rate. Removal of the inter-heavy chain disulfide bond unexpectedly resulted in a fragment with a much faster off-rate and the potential to form a complex with a 2:1 stoichiometry (sFcepsilonRIalpha:Fcepsilon3-4). This permitted the determination of the affinity of a single, natively folded Cepsilon3 domain for the first time. The low affinity Ka approximately 10(5)-10(6) m-1, similar to that determined previously for an isolated and partially folded Cepsilon3 domain, demonstrates that substantial reduction in affinity can be achieved by preventing the engagement of one of the two Cepsilon3 domains. Recent structural data indicate that conformational change in IgE is required to allow both Cepsilon3 domains to bind, and thus an allosteric inhibitor that prevents access to the second Cepsilon3 has the potential to reduce the ability of IgE to sensitize allergic effector cells.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.

Data-driven docking for the study of biomolecular complexes

With the amount of genetic information available, a lot of attention has focused on systems biology, in particular biomolecular interactions. Considering the huge number of such interactions, and their often weak and transient nature, conventional experimental methods such as X-ray crystallography and NMR spectroscopy are not sufficient to gain structural insight into these. A wealth of biochemical and/or biophysical data can, however, readily be obtained for biomolecular complexes. Combining these data with docking (the process of modeling the 3D structure of a complex from its known constituents) should provide valuable structural information and complement the classical structural methods. In this review we discuss and illustrate the various sources of data that can be used to map interactions and their combination with docking methods to generate structural models of the complexes. Finally a perspective on the future of this kind of approach is given.