Recombinant Human Olfactory receptor 13C4 (OR13C4)

What is OR13C4 and what gene encodes it?

OR13C4 is an olfactory receptor protein encoded by the OR13C4 gene in humans. It belongs to the large family of G-protein-coupled receptors (GPCRs) that interact with odorant molecules in the nasal epithelium to initiate a neuronal response that triggers smell perception. The protein is also known by several aliases including HSHTPCRX17, HTPCRX17, OR2K1, OR37F, and OR9-7 . As an olfactory receptor, it plays a crucial role in the olfactory system by converting chemical signals from odorant molecules into electrical signals that can be processed by the brain.

How does OR13C4 function in the olfactory system?

OR13C4 functions through a complex signal transduction pathway. When an odorant molecule binds to OR13C4, it triggers a conformational change in the receptor structure that activates the associated G-protein. This activation initiates a cascade of intracellular signaling events that ultimately leads to the opening of ion channels and generation of action potentials in olfactory sensory neurons. The receptor shares a characteristic 7-transmembrane domain structure with many neurotransmitter and hormone receptors, which is essential for its ability to transduce signals across the cell membrane . The specific odorants that activate OR13C4 have not been fully characterized, but the receptor contributes to the remarkable discriminatory power of the human olfactory system.

What is the evolutionary significance of OR13C4?

OR13C4 belongs to Class O2 tetrapod-specific odorant receptors , indicating its evolutionary development occurred after the emergence of land vertebrates. The olfactory receptor gene family is the largest in the human genome, reflecting the critical importance of smell for survival throughout evolutionary history. The nomenclature assigned to the olfactory receptor genes and proteins is independent of other organisms, highlighting the significant species-specific adaptations in olfactory perception . Comparative genomic analyses of OR13C4 orthologs across species can provide insights into how environmental pressures and dietary adaptations have shaped the evolution of olfactory perception in humans compared to other mammals.

What is the complete amino acid sequence and structural organization of OR13C4?

The complete amino acid sequence of human OR13C4 consists of approximately 318 amino acids organized into the characteristic seven-transmembrane domain structure of G-protein-coupled receptors. The sequence begins with an N-terminal domain exposed to the extracellular environment, followed by seven transmembrane helices (TM1-TM7) connected by alternating intracellular and extracellular loops, and ending with a C-terminal domain inside the cell . The amino acid sequence includes specific motifs that are highly conserved among olfactory receptors and play crucial roles in receptor activation and signal transduction.

The detailed amino acid sequence structure includes:

• N-terminal domain: MDKINQTFVREFILLGLSGY

• Transmembrane domain 1 (TM1): PKLEIIFFALILVMYVVILIENG

• TM2: VLIIASILDSRLHMPMYFFLGNLSFLD

• TM3: ICYTTSSIPSTLVSLISKKRN

• TM4: ISFSGCAVQMFFGFAMGSTECFLLGMMAFDRYVAICNPLRYPIIMN

• TM5: KVVYVLLTSVSW

• TM6: LSGGINSTV

• TM7: QTSLAMRWPFCGNNIINH

• C-terminal domain: FLCEILAVLKLACSDISVNIVTLAVSNIAFLVLPLLVIFFSYMFILYTILRTNSATGRHKAFS

How does the three-dimensional structure of OR13C4 relate to its function?

The three-dimensional structure of OR13C4, like other olfactory receptors, features a central binding pocket formed by the arrangement of its seven transmembrane helices. This binding pocket contains amino acid residues that directly interact with odorant molecules. The specific arrangement of these residues determines the receptor's selectivity for particular odorants.

The transmembrane domains form a barrel-like structure that creates a hydrophobic environment for binding lipophilic odorant molecules. Key regions involved in G-protein coupling include the intracellular loops, particularly the third intracellular loop, and portions of the C-terminal domain. These regions undergo conformational changes upon odorant binding that enable interaction with G-proteins to initiate signal transduction .

Specific amino acid residues within the transmembrane regions are highly conserved across olfactory receptors and are critical for maintaining the structural integrity required for proper folding, trafficking to the cell membrane, and signaling function.

What are the key post-translational modifications that affect OR13C4 function?

Several post-translational modifications are critical for OR13C4 function:

• N-linked glycosylation: Occurs at specific asparagine residues in the N-terminal domain and extracellular loops, affecting receptor trafficking and stability

• Palmitoylation: Cysteine residues in the C-terminal domain can be modified with palmitate groups, anchoring this region to the membrane and affecting receptor signaling properties

• Phosphorylation: Serine and threonine residues in the intracellular loops and C-terminal domain can be phosphorylated, typically leading to receptor desensitization

• Disulfide bond formation: Conserved cysteine residues form disulfide bonds that stabilize the tertiary structure

These modifications are essential for proper receptor folding, cell surface expression, odorant binding, and signal transduction. Alterations in these modifications can lead to receptor dysfunction and altered olfactory perception.

What expression systems are optimal for producing functional recombinant OR13C4?

Producing functional recombinant OR13C4 presents significant challenges due to the hydrophobic nature of the protein and its complex folding requirements. Several expression systems have been developed with varying degrees of success:

For functional studies, HEK293 cells have proven most effective as they provide the cellular machinery needed for proper folding and trafficking of olfactory receptors. For structural studies requiring larger quantities of protein, insect cell or yeast expression systems modified with specific chaperones and folding enhancers have shown promise .

How can researchers verify the proper folding and functionality of recombinant OR13C4?

Verifying proper folding and functionality of recombinant OR13C4 requires multiple complementary approaches:

• Cell surface expression assays:

  • Immunofluorescence microscopy with antibodies targeting epitope tags or the receptor itself

  • Flow cytometry analysis of non-permeabilized cells

  • Surface biotinylation followed by Western blot analysis

• Ligand binding assays:

  • Radioligand binding assays using tritiated odorants

  • Fluorescence-based binding assays with fluorescent ligand analogs

  • Surface plasmon resonance to measure binding kinetics

• Functional signaling assays:

  • Calcium imaging to detect intracellular calcium release upon receptor activation

  • cAMP accumulation assays to measure G-protein activation

  • BRET/FRET-based assays to monitor protein-protein interactions

  • GTPγS binding assays to directly measure G-protein activation

• Biophysical characterization:

  • Circular dichroism to assess secondary structure content

  • Tryptophan fluorescence to monitor tertiary structure

  • Thermal stability assays to assess protein stability

  • Size-exclusion chromatography to verify monodispersity

A properly folded and functional OR13C4 should demonstrate appropriate cell surface localization, specific ligand binding with expected affinity constants, and the ability to activate downstream signaling pathways upon exposure to its cognate odorants.

What are the most effective methods for studying OR13C4-ligand interactions?

Studying OR13C4-ligand interactions requires specialized approaches due to the typically low affinity and transient nature of odorant binding:

• High-throughput screening approaches:

  • Cell-based reporter assays using luciferase or fluorescent proteins

  • Automated calcium imaging platforms

  • Odorant library screening using functional readouts

• Direct binding measurement techniques:

  • Isothermal titration calorimetry (ITC) for thermodynamic binding parameters

  • Microscale thermophoresis (MST) for binding in solution

  • Surface plasmon resonance (SPR) for real-time binding kinetics

• Structural biology approaches:

  • X-ray crystallography (challenging but provides highest resolution)

  • Cryo-electron microscopy (increasingly viable for GPCRs)

  • NMR spectroscopy (for studying dynamic interactions)

• Computational methods:

  • Homology modeling based on other GPCR structures

  • Molecular docking simulations

  • Molecular dynamics simulations to study binding site flexibility

• Mutagenesis approaches:

  • Alanine scanning of binding pocket residues

  • Chimeric receptor construction with other olfactory receptors

  • Site-directed mutagenesis of predicted key residues

A comprehensive understanding of OR13C4-ligand interactions typically requires integrating data from multiple of these approaches, as each provides complementary information about different aspects of the binding process.

What is the association between OR13C4 and cancer?

OR13C4 has been implicated in cancer biology through several research findings. Expression of OR13C4 was shown to be significantly associated with risk of pancreatic cancer through pathway analysis in genome-wide studies . This suggests that altered OR13C4 expression or function may contribute to pancreatic cancer pathogenesis or serve as a biomarker for disease risk assessment.

Similar to findings with the related olfactory receptor OR7C1, which has been identified as a novel marker for colorectal cancer-initiating cells (CICs) , OR13C4 may participate in cancer-related cellular processes beyond its canonical role in olfaction. The mechanisms underlying this connection may involve:

• Aberrant activation of proliferative signaling pathways

• Influence on cell migration and invasive properties

• Modulation of tumor microenvironment

• Potential role in cancer stem cell maintenance

Understanding the precise role of OR13C4 in cancer biology requires further investigation, but these initial associations suggest that olfactory receptors may have broader physiological functions beyond smell perception, potentially including roles in cellular homeostasis that, when dysregulated, contribute to cancer development.

How might OR13C4 be utilized as a potential therapeutic target?

Based on current understanding of OR13C4 and research on related olfactory receptors, several therapeutic strategies could be developed:

• Targeted immunotherapy approaches: Similar to the OR7C1-derived antigenic peptide-specific CTLs that showed specific cytotoxicity for cancer-initiating cells , OR13C4 could potentially serve as a target for cancer immunotherapy if its expression is confirmed to be enriched in cancer cells.

• Small molecule modulators: Designing agonists or antagonists that specifically target OR13C4 could help modulate its activity in disease states.

• Gene therapy approaches: Modifying OR13C4 expression levels through RNA interference or CRISPR-based technologies could be explored if aberrant expression contributes to disease.

• Diagnostic applications: OR13C4 expression patterns or genetic variations could serve as biomarkers for disease risk assessment, particularly for pancreatic cancer.

The development of these therapeutic approaches would require:

• Validation of OR13C4 as a causative factor in disease pathogenesis rather than merely a correlative marker

• Development of highly selective compounds that target OR13C4 without affecting other olfactory receptors

• Understanding of the signaling pathways and cellular processes regulated by OR13C4 in disease contexts

What are the challenges in targeting OR13C4 for therapeutic applications?

Developing therapeutic strategies targeting OR13C4 faces several significant challenges:

• Receptor specificity: The high sequence similarity among olfactory receptors makes it difficult to develop compounds that selectively target OR13C4 without cross-reactivity with other family members.

• Expression heterogeneity: Variability in OR13C4 expression levels between individuals and across tissues complicates the development of standardized therapeutic approaches.

• Functional redundancy: Other olfactory receptors may compensate for altered OR13C4 function, potentially limiting the efficacy of targeted therapies.

• Limited structural information: Despite advances in GPCR structural biology, high-resolution structures of olfactory receptors remain elusive, hampering structure-based drug design efforts.

• Delivery challenges: For cancer applications, delivering therapeutics specifically to OR13C4-expressing cells within tumors presents substantial technical obstacles.

Overcoming these challenges will require multidisciplinary approaches combining expertise in structural biology, medicinal chemistry, immunology, and innovative drug delivery methods.

How can single-cell analysis advance our understanding of OR13C4 function?

Single-cell analysis technologies offer powerful approaches to unravel the complex biology of OR13C4:

• Single-cell RNA sequencing: This approach can reveal the heterogeneity of OR13C4 expression across individual cells within tissues, potentially identifying previously unrecognized cell populations that express this receptor. It can also uncover co-expression patterns with other genes that may provide insights into the functional networks involving OR13C4.

• Single-cell proteomics: Emerging mass spectrometry-based methods can identify the protein interactome of OR13C4 at the single-cell level, revealing cell-specific signaling complexes.

• Spatial transcriptomics: Technologies like MERFISH or Slide-seq can map OR13C4 expression within the tissue architecture, providing crucial information about its spatial distribution relative to other cell types and anatomical structures.

• CyTOF and spectral flow cytometry: These approaches can simultaneously measure multiple protein markers alongside OR13C4, enabling detailed characterization of cells expressing this receptor.

• Single-cell ATAC-seq: This method can reveal the chromatin accessibility landscape in OR13C4-expressing cells, providing insights into the regulatory mechanisms controlling its expression.

These technologies are particularly valuable for studying OR13C4 in heterogeneous tissues like tumors, where they can identify specific cellular contexts in which the receptor may have pathological roles.

What computational approaches can predict OR13C4 ligand interactions?

Advanced computational methods are increasingly important for understanding OR13C4-ligand interactions:

• AlphaFold2 and RoseTTAFold-based structure prediction: These AI approaches can now predict protein structures with remarkable accuracy, potentially overcoming the challenges of obtaining experimental structures for OR13C4.

• Molecular dynamics simulations: All-atom and coarse-grained simulations can model the dynamic behavior of OR13C4 in lipid bilayers, including conformational changes associated with activation.

• Fragment-based virtual screening: This approach can identify small molecular fragments that bind to specific pockets within OR13C4, which can then be elaborated into full ligands.

• Machine learning methods for ligand prediction:

  • Deep neural networks trained on known GPCR-ligand pairs

  • Graph convolutional networks that leverage the chemical structure of potential ligands

  • Transfer learning approaches that apply knowledge from well-characterized GPCRs to OR13C4

• Quantum mechanical calculations: For detailed understanding of binding energetics and electronic interactions between OR13C4 and putative ligands.

• Metadynamics and other enhanced sampling methods: These approaches can explore rare events in protein dynamics that may be crucial for understanding ligand binding and receptor activation.

Integration of these computational methods with experimental validation creates a powerful feedback loop for accelerating OR13C4 research.

What are the emerging technologies for studying OR13C4 trafficking and signaling dynamics?

Cutting-edge technologies that can advance our understanding of OR13C4 dynamics include:

• Advanced imaging techniques:

  • Super-resolution microscopy (STED, PALM, STORM) to visualize OR13C4 localization beyond the diffraction limit

  • Single-particle tracking to follow individual receptor molecules in live cells

  • Lattice light-sheet microscopy for long-term imaging with minimal phototoxicity

• Biosensor technologies:

  • FRET/BRET-based sensors to monitor receptor conformational changes in real-time

  • Genetically encoded calcium indicators (GECIs) to measure signaling dynamics

  • Nanobody-based biosensors that recognize specific receptor states

• Optogenetic and chemogenetic tools:

  • Light-activated OR13C4 variants for precise temporal control of receptor activation

  • Chemically induced dimerization systems to manipulate receptor clustering

  • DREADD technology adapted for olfactory receptors

• Cryo-electron tomography:

  • Visualization of OR13C4 in its native cellular environment at near-atomic resolution

  • Capturing different conformational states during trafficking and signaling

• Proximity labeling methods:

  • BioID, APEX, or TurboID fused to OR13C4 to identify proximal proteins in different cellular compartments

  • Time-resolved proximity labeling to capture dynamic interaction networks

These technologies provide unprecedented capability to study the spatiotemporal dynamics of OR13C4 from its synthesis in the endoplasmic reticulum to its functional activity at the cell membrane and subsequent internalization and recycling.

What are the most significant knowledge gaps in OR13C4 research?

Despite advances in our understanding of OR13C4, several critical knowledge gaps remain:

• The complete spectrum of odorants that activate OR13C4 has not been fully characterized, limiting our understanding of its physiological role in olfaction.

• The precise signaling pathways downstream of OR13C4 activation in different cell types, particularly in non-olfactory tissues, remain poorly understood.

• The three-dimensional structure of OR13C4 has not been experimentally determined, hampering structure-based drug design efforts.

• The mechanisms underlying the association between OR13C4 and cancer risk, particularly for pancreatic cancer, require further investigation to determine whether this relationship is causal or merely correlative.

• The potential roles of OR13C4
  in normal cellular physiology beyond olfaction have not been systematically explored.

Addressing these knowledge gaps will require integrated approaches combining structural biology, functional genomics, advanced imaging, and computational modeling.

How might OR13C4 research contribute to broader scientific advances?

Research on OR13C4 has the potential to catalyze advances in multiple scientific domains:

• Enhanced understanding of GPCR signaling mechanisms, particularly how subtle structural differences among highly similar receptors can confer ligand specificity.

• Development of novel therapeutic approaches targeting olfactory receptors in cancer and other diseases, potentially opening a new class of drug targets.

• Improved computational methods for predicting protein-ligand interactions that could be applied across the GPCR superfamily.

• New insights into the evolution of sensory perception and how genetic variation in olfactory receptors contributes to individual differences in sensory experience.

• Better understanding of the unexpected roles of sensory receptors in non-sensory tissues, challenging our traditional views of receptor function.