Recombinant Human Olfactory receptor 4K13 (OR4K13)

What is OR4K13 and how is it classified within the human olfactory receptor family?

OR4K13 (Olfactory receptor 4K13, also known as Olfactory receptor OR14-27) is one of the 388 human olfactory receptors that belong to the G-protein-coupled receptor (GPCR) superfamily . Like other olfactory receptors, OR4K13 is involved in the molecular recognition of odor molecules, triggering signal transduction pathways that ultimately lead to odor perception.

Human olfactory receptors are classified into families and subfamilies based on sequence homology. Each OR gene is named according to a systematic nomenclature that reflects its phylogenetic relationship with other OR genes. The "4" in OR4K13 denotes the family, "K" represents the subfamily, and "13" is the individual member identifier within that subfamily.

What is the molecular mechanism by which OR4K13 detects odor molecules?

Similar to other human olfactory receptors, OR4K13 likely functions by binding specific odor molecules within a binding pocket in its transmembrane domain. Based on structural studies of other olfactory receptors such as OR51E2, when an odor molecule binds to the receptor, it likely causes conformational changes that activate intracellular signaling pathways .

The binding pocket of olfactory receptors is typically surrounded by transmembrane domains, with a flexible loop that can act as a "lid" to trap the odor molecule. This interaction triggers the associated G protein (typically Golf), leading to increased cAMP production through adenylyl cyclase, opening of cyclic nucleotide-gated channels, and calcium influx that generates action potentials in olfactory sensory neurons .

What expression patterns does OR4K13 show in human tissues?

While the provided information doesn't specifically detail OR4K13's expression pattern, research on human olfactory receptors has shown that many ORs are expressed not only in the olfactory epithelium but also in non-olfactory tissues. For example, OR51E2 has been found in the gut, kidney, prostate, and other organs .

To determine OR4K13's expression pattern, researchers should consider techniques such as:

• RT-PCR analysis of tissue samples

• RNA-seq data mining from tissue expression databases

• Immunohistochemistry using specific antibodies against OR4K13

• Single-cell RNA sequencing of olfactory epithelium to identify OR4K13-expressing neurons

What are the primary challenges in heterologous expression of OR4K13?

Expressing functional olfactory receptors, including OR4K13, in heterologous systems presents several challenges:

• Protein misfolding and aggregation: Olfactory receptors often aggregate and accumulate in the endoplasmic reticulum when expressed in heterologous cells .

• Poor trafficking to the cell surface: Even when expressed, many ORs fail to reach the plasma membrane efficiently .

• Low stability: Olfactory receptors tend to be unstable when removed from their native environment.

• Limited functional coupling: In heterologous systems, ORs may not efficiently couple with the signaling machinery required for response measurement.

These challenges necessitate specialized approaches for successful expression of functional OR4K13 protein.

What expression systems and strategies are most effective for producing functional OR4K13?

Based on advances in olfactory receptor expression technology, the following strategies are recommended for OR4K13:

Expression Systems:

• Yeast expression systems have been used successfully for producing OR4K13

• HEK293 cells (particularly Hana3A cells) with enhanced OR expression capabilities

Expression Enhancement Strategies:

• Use of chaperone proteins:

  • Receptor-transporting proteins (RTP1, particularly RTP1S, and RTP2)

  • Receptor expression-enhancing protein 1 (REEP1)

• Addition of N-terminal tags:

  • Rho-tag (rhodopsin-derived signal peptide)

  • Lucy-tag

  • IL-6-Halo-tag

• Co-expression with other GPCRs:

  • β2-adrenergic receptor

  • M3 muscarinic acetylcholine receptor (which suppresses β-arrestin 2-mediated OR internalization)

• Enhanced signaling components:

  • Co-expression of olfactory-specific G protein α (GNAL/Gαolf)

  • Addition of Ric-8B (a chaperone of Gα protein)

  • Use of GloSensor™ for sensitive cAMP detection

What are the optimal storage and handling conditions for recombinant OR4K13?

For recombinant OR4K13 protein stability and functionality:

Storage Recommendations:

• Lyophilized form: Stable for 12 months at -20°C/-80°C

• Liquid form: Stable for 6 months at -20°C/-80°C

• Avoid repeated freezing and thawing

• Working aliquots can be stored at 4°C for up to one week

Reconstitution Protocol:

• Briefly centrifuge the vial before opening

• Reconstitute in deionized sterile water to 0.1-1.0 mg/mL

• Add glycerol to a final concentration of 5-50% (50% is recommended)

• Aliquot for long-term storage at -20°C/-80°C

What methodologies are recommended for identifying ligands of OR4K13?

To identify molecules that activate OR4K13, researchers should consider these methodological approaches:

High-Throughput Screening:

• Cell-based assays using OR4K13-expressing cells with:

  • Calcium imaging using fluorescent calcium indicators or GCaMP

  • cAMP measurements using FRET-based sensors or GloSensor technology

  • Membrane potential assays

• Screening library design:

  • Chemical libraries based on structural similarity to known olfactory receptor ligands

  • Odor molecule collections organized by molecular features

  • Natural product extracts and essential oils

• Validation of hits:

  • Dose-response curves to determine EC50 values

  • Structure-activity relationship studies

  • Competitive binding assays with known ligands (if available)

Real-time Measurement Considerations:
Implement real-time measurement systems rather than endpoint assessments, as continuous exposure to odors can lead to:

• Denaturation of odor molecules (oxidation, hydrolysis)

• Receptor desensitization

• Cytotoxic effects at higher concentrations

How can OR4K13 be integrated into an olfactory receptor array for complex odor analysis?

Building on the human OR sensor technology described in the literature:

• Create stable cell lines expressing OR4K13 along with:

  • CNGs (cyclic nucleotide-gated channels)

  • Calcium-dependent fluorescent proteins (GCaMP)

  • Appropriate G proteins and chaperones

• Integration into microwell arrays:

  • Place 400-500 OR4K13-expressing cells in 0.5 mm square microwells

  • Incorporate OR4K13-expressing cells alongside cells expressing other ORs for comparative analysis

  • Use hydrophobic ink printing to create microwell boundaries

• Measurement protocol:

  • Set up reflux apparatus under a fluorescence microscope with video capability

  • Use Ringer's solution for odor delivery (concentration: 0.01-0.1 mM for simple odors; 0.05-5.0 mM for complex odors)

  • Capture fluorescent images at 0.67 frames per second for at least 7 minutes

  • Define significant responses as fluorescence intensity changes of 5% or more

• Data analysis:

  • Use image analysis software (e.g., ImageJ) to quantify fluorescence changes at single-cell resolution

  • Compare OR4K13 responses with responses from other ORs to build an odor response profile

  • Create multidimensional odor matrices to characterize complex odors

What controls should be included when assessing OR4K13 functionality?

Essential Controls for OR4K13 Functional Assays:

• Negative controls:

  • Untransfected cells

  • Cells expressing a non-responsive olfactory receptor

  • Vehicle-only stimulation

• Positive controls:

  • Cells expressing a well-characterized OR with known ligands

  • Direct activation of signaling pathway components (e.g., forskolin to activate adenylyl cyclase)

• Expression controls:

  • Fluorescent tags or epitope tags to confirm surface expression

  • Western blotting to verify protein expression

  • Immunofluorescence to assess subcellular localization

• Specificity controls:

  • Dose-response relationships

  • Structurally related odor molecules to assess selectivity

  • Competitive binding with known ligands (if available)

What computational methods can predict OR4K13 binding sites and potential ligands?

Based on advances in olfactory receptor structural biology:

Homology Modeling:

• Use the recently solved structure of OR51E2 as a template

• Identify conserved regions in the transmembrane domains

• Model the binding pocket and the flexible loop "lid" region

• Validate the model through energy minimization and molecular dynamics simulations

Virtual Screening:

• Structure-based virtual screening:

  • Dock libraries of odor molecules into the predicted binding pocket

  • Score interactions based on binding energy and pose stability

  • Filter hits based on chemical properties relevant to olfaction

• Ligand-based virtual screening:

  • Use pharmacophore models based on known ligands of related ORs

  • Apply machine learning approaches trained on OR-ligand interaction data

  • Implement fingerprint-based similarity searches

Molecular Dynamics Simulations:

• Simulate OR4K13 behavior in a lipid bilayer environment

• Analyze conformational changes upon ligand binding

• Identify key residues involved in ligand recognition and receptor activation

• Investigate the dynamics of the binding pocket and the flexible loop

What experimental approaches can elucidate the structure-function relationship of OR4K13?

Mutagenesis Studies:

• Alanine scanning mutagenesis of predicted binding pocket residues

• Site-directed mutagenesis based on computational predictions

• Creation of chimeric receptors with other ORs to identify functionally important domains

Structural Biology Approaches:

• Cryo-electron microscopy:

  • Stabilize OR4K13 with nanobodies or other protein engineering approaches

  • Capture the receptor in complex with ligands

• NMR spectroscopy for dynamic studies:

  • Analyze conformational changes

  • Study the flexible loop dynamics

• Cross-linking studies:

  • Identify proximity relationships between receptor domains

  • Map ligand interaction sites

• Hydrogen-deuterium exchange mass spectrometry:

  • Monitor conformational changes upon ligand binding

  • Identify regions with altered solvent accessibility

How can OR4K13 be utilized in developing biomimetic olfactory sensors?

Integration Strategies for Sensor Development:

• Cell-based biosensors:

  • Immobilize OR4K13-expressing cells in biocompatible matrices

  • Couple with optical or electrical readout systems

  • Optimize for stability and longevity

• Receptor-functionalized surfaces:

  • Extract and purify OR4K13 for direct application on sensor surfaces

  • Use supported lipid bilayers to maintain receptor functionality

  • Couple with surface plasmon resonance or impedance measurement systems

• Hybrid systems:

  • Combine OR4K13 with artificial intelligence for pattern recognition

  • Integrate with other ORs to create broad-spectrum sensors

  • Develop multiplexed detection systems for complex odor analysis

Sensitivity Enhancement Approaches:

• Inhibit mechanisms that dampen receptor responses:

  • PDE (phosphodiesterase) inhibitors

  • CaMKII (Ca²⁺-calmodulin-dependent protein kinase II) inhibitors

  • CaM (calmodulin) modulators

  • NCX (Na⁺/Ca²⁺ exchanger) and PMCA (plasma membrane calcium pump) inhibitors

• Incorporate auxiliary proteins:

  • Odorant binding proteins (OBPs) to enhance odor molecule presentation

  • Olfactory marker proteins (OMPs) to increase cAMP levels

  • GRK2 (G-protein-coupled receptor kinase 2) inhibitors to suppress β-arrestin-mediated receptor internalization

What is known about OR4K13 polymorphisms and their impact on olfactory perception?

While specific information about OR4K13 polymorphisms is not provided in the search results, research on olfactory receptor polymorphisms suggests:

• Single nucleotide polymorphisms (SNPs) in olfactory receptor genes can alter:

  • Ligand binding affinity

  • Receptor signaling efficiency

  • Expression levels

• Methodological approaches to study OR4K13 polymorphisms:

  • Genomic sequencing to identify variants

  • Functional characterization of variants in heterologous expression systems

  • Population studies correlating genotypes with odor perception phenotypes

  • Computational modeling to predict the impact of amino acid substitutions

• Research implications:

  • Individual differences in odor perception

  • Population-specific olfactory capabilities

  • Potential links to disorders involving chemosensation

What are the non-olfactory functions of OR4K13 in human physiology?

Based on the emerging understanding of ectopic expression of olfactory receptors:

• Potential non-olfactory roles:

  • Some olfactory receptors, like OR51E2, are expressed in multiple tissues including gut, kidney, and prostate

  • These ectopically expressed ORs may function in:

    • Cell-cell communication

    • Metabolic regulation

    • Development and differentiation

    • Detection of endogenous ligands as opposed to environmental odors

• Investigative approaches:

  • Tissue-specific expression analysis of OR4K13

  • Functional studies in non-olfactory cell types

  • Knockout/knockdown experiments to identify phenotypic consequences

  • Identification of endogenous ligands in non-olfactory tissues

• Potential biomedical applications:

  • Diagnostic biomarkers if expression is altered in disease states

  • Therapeutic targets if involved in pathological processes

  • Tools for tissue engineering if involved in cellular differentiation