Interaction Mechanism of NiV Receptor Ephrin-B2/B3 and Applications in Drug Screening

Introduction

Nipah virus (NiV) is a highly pathogenic zoonotic paramyxovirus belonging to the genus Henipavirus, capable of causing severe respiratory disease and fatal encephalitis with case fatality rates ranging from 40% to 75%. NiV invades host cells through specific recognition of Ephrin-B2 and Ephrin-B3 receptors on the cell surface by its attachment glycoprotein (G protein), mediating viral entry and cell-to-cell spread. Understanding the interaction mechanism between NiV G protein and Ephrin receptors is crucial for developing therapeutic strategies targeting viral entry.

Tissue Distribution of Ephrin-B2/B3 Receptors and Association with NiV Pathogenesis

Receptor Expression Profile and Viral Tropism

Ephrin-B2 (EFNB2) and Ephrin-B3 (EFNB3) belong to the B-class Ephrin ligand family and participate in physiological processes such as neuronal axon guidance, angiogenesis, and cell boundary formation under normal conditions. The tissue distribution of these two receptors directly determines NiV host cell tropism and pathological characteristics:

Table 1. Tissue Distribution of Ephrin-B2 and Ephrin-B3 and Association with NiV Pathology

Receptor Type	Primary Expression Tissue	Cell Type	NiV-Associated Pathology	Relative Expression Level
Ephrin-B2	Central Nervous System	Neurons, microvascular endothelial cells	Multifocal encephalitis, vasculitis	++++
	Respiratory System	Alveolar epithelial cells, bronchial epithelium	Interstitial pneumonia, ARDS	+++
	Cardiovascular System	Arterial endothelium, smooth muscle cells	Vasculitis, thrombosis	++++
	Lymphatic System	Lymph node sinus endothelium, spleen	Lymphoid tissue necrosis	++
	Placental Tissue	Trophoblast cells	Vertical transmission (speculated)	++
Ephrin-B3	Central Nervous System	Brainstem neurons, spinal cord neurons	Brainstem encephalitis, neurological dysfunction	+++
	Cardiovascular System	Cardiomyocytes (primary expression)	Myocarditis (secondary manifestation)	++
	Lymphatic System	Lymphocytes (specific subsets)	Acute lymphoid tissue necrosis	+

Note: Expression levels are assessed based on immunohistochemistry and RNA-seq data; + indicates low expression, ++++ indicates high expression.

The high expression of Ephrin-B2 in arterial endothelial cells and neurons is closely associated with vasculitis and encephalitis following NiV infection. The virus disrupts the blood-brain barrier by infecting cerebrovascular endothelial cells, subsequently invading neurons and causing severe neuropathological changes. In contrast, Ephrin-B3 is primarily expressed in the brainstem and heart, explaining the pathological basis for brainstem symptoms and occasional myocarditis in NiV infection.

Regulation of Infection Efficiency by Receptor Expression Levels

Research indicates that Ephrin-B2 expression levels do not show a simple positive correlation with NiV infection efficiency. Diederich et al. found that overexpression of Ephrin-B2 actually reduces cell-to-cell fusion efficiency and viral entry rates. This phenomenon is attributed to the downregulation of G protein caused by imbalanced ratios between viral envelope glycoproteins and surface receptors. Importantly, even when using Ephrin-B2 mutants with truncated cytoplasmic tails (lacking signaling functions, ΔcEB2), their function as NiV entry receptors remains intact, and overexpression similarly interferes with viral replication. This demonstrates that Ephrin-B2-mediated viral entry is independent of downstream signal transduction and relies purely on physical receptor-ligand interactions.

Binding Kinetics of NiV G Protein with Ephrin Receptors

Structural Basis and Binding Interface

NiV G protein is a type II membrane glycoprotein composed of 602 amino acids, containing an N-terminal cytoplasmic tail, transmembrane domain, stalk domain, and C-terminal globular head domain. The G protein forms characteristic tetrameric structures through α-helices in the stalk region, while the globular head is responsible for binding host receptors.

Fig 1. Schematic Diagram of NiV G Protein-Ephrin-B2 Complex Structure

Crystal structure analysis reveals that the G protein head forms a six-bladed β-propeller structure with a conserved hydrophobic binding cavity at its center. Ephrin-B2 inserts into this cavity through its G-H loop (residues 119-125), forming high-affinity protein-protein interactions. The binding interface involves approximately 2,700 Ų of surface area, including two major regions:

1. Polar peripheral region: Stabilized by 24 hydrogen bonds, 4 salt bridges, and multiple hydrophobic interactions;
2. Central hydrophobic cavity: Residues Phe120, Pro122, Leu124, and Trp125 of the Ephrin-B2 G-H loop interact with the G protein binding pocket through van der Waals forces.

Affinity and Kinetic Parameters

Biolayer interferometry (BLI) and surface plasmon resonance (SPR) experiments provide precise data for quantifying the interaction between NiV G protein and Ephrin receptors:

Table 2. Binding Kinetic Parameters of NiV G Protein with Ephrin-B2/B3

Interaction Pair	Binding Format	Affinity Constant (KD)	Association Rate (Kon)	Dissociation Rate (Koff)	Experimental Method
NiV-G / Ephrin-B2 (wild-type)	Soluble protein	3.36 × 10⁻¹⁰ M	6.72 × 10⁴ M⁻¹s⁻¹	2.26 × 10⁻⁵ s⁻¹	BLI
NiV-G / Ephrin-B2 (L124A mutant)	Soluble protein	2.66 × 10⁻¹¹ M	8.89 × 10⁴ M⁻¹s⁻¹	2.37 × 10⁻⁶ s⁻¹	BLI
NiV-G / Ephrin-B3	Cell surface	Lower than Ephrin-B2	Not reported	Faster dissociation	Flow cytometry
HeV-G / Ephrin-B2	Soluble protein	~10⁻¹⁰ M	Not reported	Not reported	SPR

Data sources: Xu et al., 2008; Negrete et al., 2006

Notably, the Ephrin-B2 L124A mutant shows more than 10-fold higher binding affinity for NiV-G compared to wild-type (KD decreased from 336 pM to 26.6 pM), primarily due to significantly reduced dissociation rates (Koff decreased approximately 10-fold). This mutant substantially enhances pseudovirus and live virus infection efficiency, providing a new platform for establishing highly sensitive viral isolation and drug screening cell lines.

Receptor Selectivity Mechanism

Although both Ephrin-B2 and Ephrin-B3 can serve as NiV receptors, Ephrin-B2 exhibits higher binding affinity. Molecular dynamics simulations show that the NiV-G/Ephrin-B2 complex forms more hydrogen bonds and has higher conformational stability. Ephrin-B3, as an alternative receptor, is primarily expressed in specific brain regions (such as the corpus callosum and spinal cord), potentially explaining viral tropism in these areas.

Blocking Strategies Based on Receptor-Ligand Interactions

Soluble Receptor Decoys

Using soluble Ephrin-B2 as a competitive inhibitor is an effective strategy for blocking NiV entry. Soluble Ephrin-B2 ligands can effectively block henipavirus infection mediated by both Ephrin-B2 and Ephrin-B3 in vitro. However, since Ephrin-B2 is also a physiological ligand for EphB receptor tyrosine kinases involved in angiogenesis and neural development, direct use of soluble Ephrin-B2 may interfere with normal physiological functions.

Recent research has employed deep mutational scanning technology to engineer Ephrin-B2, developing decoy molecules that selectively bind NiV-G without binding Eph receptors. These engineered receptors retain high-affinity viral neutralization capabilities while avoiding interference with host signaling pathways, showing promising therapeutic potential.

Monoclonal Antibodies and Nanobodies

Monoclonal antibodies targeting the G protein receptor binding site exert neutralizing effects through competitive inhibition of Ephrin binding:

Table 3. Therapeutic Antibodies Targeting NiV G Protein/Ephrin Interactions

Antibody Name	Target Epitope	Mechanism of Action	Neutralizing Activity	Development Stage
m102.4	G protein head, receptor binding cavity	Competitive inhibition of Ephrin-B2/B3 binding	Broad neutralization of HeV/NiV	Preclinical/Emergency use
HENV-26	G protein receptor binding site	Blocks Ephrin binding	High-efficiency neutralization	Research stage
n425 (single-domain antibody)	Conserved cryptic epitope	Conformational locking, prevents receptor-induced G protein conformational changes	Broad neutralization of multiple henipaviruses	Research stage
HENV-32	G protein non-receptor binding region	No direct competition, may interfere with oligomerization	Moderate neutralizing activity	Research stage

Data sources: Zhu et al., 2024; Broder et al., 2024

The m102.4 antibody mimics the structure of the Ephrin-B2 G-H loop through its CDR-H3 loop, occupying the G protein binding cavity through "molecular mimicry" to block viral interaction with natural receptors. This antibody has completed validation in non-human primate models, demonstrating good post-exposure prophylactic efficacy.

Small Molecule Inhibitors and Peptide Inhibitors

Based on structural information of the G protein-Ephrin interaction interface, researchers have designed various small molecule and peptide inhibitors:

• G-H loop peptide analogs: Short peptides containing key Ephrin-B2 residues (Phe120, Pro122, Leu124, Trp125) as competitive inhibitors;
• Small molecules targeting the binding cavity: Compounds identified through virtual screening that can embed into the central hydrophobic cavity of G protein, blocking Ephrin insertion;
• Glycoprotein processing inhibitors: Blocking post-translational modifications of G protein (such as N-glycosylation), affecting proper folding and receptor binding capacity.

Applications of In Vitro Binding Assays in Drug Screening

Recombinant Protein Preparation and Quality Control

High-quality recombinant proteins are fundamental for conducting in vitro interaction studies. Currently, various commercial recombinant NiV G proteins and Ephrin receptors are available for SPR and BLI experiments:

Table 4. Characteristics of Commercial NiV G Protein and Ephrin Receptor Products

Product Name	Expression System	Tag	Purity	Bioactivity Validation	Application Platform
NiV G protein (aa 71-602)	HEK293	His-tag (C-terminal)	≥95% (SDS-PAGE)	Binding to Ephrin-B2, EC50: 16-48 ng/mL	SPR, BLI, ELISA
NiV G protein (aa 71-602)	HEK293	Human Fc-tag (C-terminal)	≥90% (SEC-HPLC)	Receptor binding validated	Pseudovirus packaging, neutralization assays
Ephrin-B2 (full-length extracellular domain)	HEK293	His-tag	≥95%	Binding to NiV-G, KD ~10⁻¹⁰ M	SPR, BLI
Ephrin-B3 (full-length extracellular domain)	HEK293	Fc-tag	≥90%	Supports NiV pseudovirus entry	Cell assays, BLI

SPR and BLI Experimental Protocol Examples

Case Study: BLI Binding Kinetics Analysis Using Creative BioMart NiV G Protein

Experimental Design:

• Ligand immobilization: Biotinylated NiV G protein (Cat. No.: DRA265, Creative BioMart) immobilized via streptavidin sensors;
• Analyte: Serially diluted soluble Ephrin-B2 (concentration range: 1.25-100 nM);
• Buffer: PBS pH 7.4 containing 0.1% BSA and 0.02% Tween-20;
• Experimental procedure: Baseline (60 s) → Association (300 s) → Dissociation (600 s) → Regeneration (10 mM Glycine-HCl pH 2.0).

Data Analysis: Association rate constant (Kon), dissociation rate constant (Koff), and equilibrium dissociation constant (KD) are obtained through 1:1 binding model fitting. Typical results show high-affinity binding between NiV G protein and Ephrin-B2 (KD ~10⁻¹⁰ M), consistent with literature reports.

Fig 2. Example BLI Sensorgram

High-Throughput Drug Screening Applications

SPR/BLI-based high-throughput screening platforms can be used for:

1. Small molecule compound library screening: Detecting inhibition rates of compounds on G protein-Ephrin binding;
2. Antibody epitope binning: Identifying competitive/non-competitive antibodies to guide combination therapy design;
3. Affinity maturation: Optimizing affinity of lead antibodies/proteins to reduce therapeutic doses.

Screening Case Study: Using the Octet RED96 system with immobilized NiV G protein as bait, 1,200 natural product extracts were screened. By monitoring inhibition signals of Ephrin-B2 binding, three candidate compounds with significant inhibitory activity were identified, with IC50 values in the 1-10 μM range, providing starting points for subsequent structure optimization.

Conclusions and Perspectives

Ephrin-B2/B3, as key receptors for NiV entry into host cells, research on their molecular interaction mechanisms has provided multiple druggable targets for antiviral drug development. The high-affinity, high-specificity binding interface between the G protein head domain and the Ephrin G-H loop is an ideal target for structure-based drug design (SBDD). Structure-based drug design has successfully guided the development of high-affinity decoy receptors and broadly neutralizing antibodies.

Future research directions include:

• Resolving the structure of G protein tetramers in complex with Ephrin to understand the impact of oligomerization on binding affinity;
• Developing peptide mimetics or small molecule inhibitors with high oral bioavailability;
• Establishing animal models based on humanized Ephrin-B2 to evaluate in vivo efficacy of soluble receptors and antibodies;
• Utilizing artificial intelligence algorithms to predict the impact of viral G protein mutations on receptor binding and provide early warning of escape mutations.

References

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