Recombinant Chicken Popeye domain-containing protein 3 (POPDC3)

Basic Research Questions

• What is the structure and function of POPDC3 in chickens?

POPDC3 belongs to the Popeye domain containing gene family, which encodes a novel class of membrane-bound cyclic AMP (cAMP) effector proteins. The protein contains three transmembrane domains with a short extracellular N-terminus (20-40 residues) and a cytosolic portion featuring the highly conserved Popeye domain, followed by a variable carboxyterminal domain (CTD) .

Structurally, POPDC3 is characterized as a medium-sized protein (290-360 residues) with regions of high sequence conservation, particularly in the Popeye domain which functions as a high-affinity cAMP binding site . The Popeye domain shows structural similarities with the cyclic nucleotide binding domain (CNBD) of PKA, although the primary sequence of the phosphate binding cassette (PBC) is divergent .

In chickens, POPDC3 is primarily expressed in heart and skeletal muscle tissue, as well as smooth muscle cells . Of particular interest in avian species, POPDC3 may play a crucial role in uterine function, as the gene is expressed in the uterus, which is composed of smooth muscle and contains the shell gland involved in depositing eggshell . Research methodologies exploring its function should include tissue-specific expression analysis using qPCR and immunohistochemistry focusing on these tissues.

• How is gene copy number variation of POPDC3 studied in chickens?

Copy number variation (CNV) in chicken POPDC3 has been studied using multiple complementary approaches:

• Whole genome sequencing: High-throughput sequencing followed by bioinformatic analysis using programs like CNVnator can identify regions with altered copy numbers .

• Array comparative genomic hybridization (aCGH): This technique provides validation of sequencing-based CNV predictions, with studies showing Pearson's correlation values between sequencing and aCGH results ranging from 0.395 to 0.740 .

• Quantitative PCR (qPCR): This approach offers high reliability for CNV validation, with reported positive validation rates of 91.71% and false negative rates of 22.43% .

• Heatmap visualization: This technique helps visualize gene duplication and clustering features, as demonstrated in White Leghorn (WL) chickens, which showed approximately twice as many copies of POPDC3 compared to other chicken breeds .

To effectively study CNVs in chicken POPDC3, researchers should employ at least two of these methods for cross-validation, with qPCR being particularly valuable for accurately quantifying copy number differences between breeds or experimental groups.

• How does chicken POPDC3 compare with mammalian POPDC3 in terms of structure and function?

Comparative analysis between chicken and mammalian POPDC3 reveals several important similarities and differences:

For researchers working across species, it's important to note that while the core Popeye domain and cAMP binding function appear conserved, the regulatory mechanisms and physiological roles may differ significantly. Western blotting and protein interaction studies should use species-specific antibodies to account for potential epitope differences.

• What signaling pathways involve POPDC3 in chicken tissues?

POPDC3, like other POPDC proteins, functions as a cAMP effector protein, suggesting its involvement in β-adrenergic signaling pathways that are crucial in muscle tissues. The most well-characterized pathway involves:

• cAMP binding and protein interactions: POPDC proteins bind cAMP with high affinity (EC₅₀ approximately 330 nM for POPDC1) . This binding modulates interactions with partner proteins, most notably the 2-pore domain potassium channel TREK-1 .

• Membrane trafficking: In heterologous expression systems, POPDC proteins recruit TREK-1 to the plasma membrane, and this interaction is negatively modulated by cAMP binding .

• Electrophysiological effects: The regulation of TREK-1 by POPDC proteins suggests a role in setting resting membrane potential and modulating action potentials, particularly in excitable tissues like heart and skeletal muscle .

While most detailed studies have been performed with POPDC1 and POPDC2, the conserved Popeye domain suggests similar mechanisms for POPDC3. In chickens, researchers should investigate these pathways using techniques such as FRET or BRET to monitor protein-protein interactions in real-time, coupled with electrophysiological measurements to assess functional outcomes in native tissues.

• What techniques are most effective for detecting POPDC3 expression in chicken tissues?

For comprehensive analysis of POPDC3 expression in chicken tissues, multiple complementary techniques should be employed:

• Quantitative RT-PCR: Provides accurate quantification of mRNA expression levels. When studying POPDC3, researchers should normalize to stable housekeeping genes like GAPDH and RPLP0 . This approach has been effectively used to compare expression across different chicken breeds .

• Western blotting: For protein-level detection, commercial antibodies against POPDC3 are available (see search result #9), though validation in chicken tissues is essential before use.

• Immunohistochemistry: For spatial localization within tissues. Based on approaches used for POPDC1 in chick embryos , whole-mount immunohistochemistry can effectively visualize expression patterns during development.

• In situ hybridization: Provides precise spatial information about mRNA expression. Whole-mount in situ hybridization using full-length probes has successfully detected novel expression domains for POPDC family members in chick embryos .

• RNA-seq: For genome-wide expression analysis, which can place POPDC3 expression in context with other genes and identify potential co-regulated networks.

For developmental studies, researchers should consider the temporal dynamics of expression, as POPDC proteins show stage-specific patterns during embryogenesis .

Advanced Research Questions

• How do copy number variations in chicken POPDC3 affect phenotypic traits?

Copy number variations (CNVs) in chicken POPDC3 have been associated with important phenotypic traits, particularly in commercially valuable breeds:

White Leghorn (WL) chickens demonstrate approximately twice as many copies of POPDC3 compared to other chicken breeds . This increased copy number appears to be functionally significant given POPDC3's expression pattern and putative biological roles:

• Uterine function: POPDC3 belongs to the Popeye family of proteins expressed in smooth muscle cells, including those in the uterus . Given that "the expression of two Popeye family members was upregulated in uterus of pregnant mice," researchers hypothesize that "duplication in POPDC3 gene may facilitate myometrium maturation and labor as well as uterine fluid secretion during the egg laying period" .

• Economic implications: The duplication of POPDC3 in high egg-producing breeds suggests potential "great economic importance in poultry breeding" .

To investigate these associations further, researchers should:

• Perform broad CNV screening across diverse chicken breeds with varying egg production traits

• Correlate POPDC3 copy number with physiological measurements of uterine function

• Consider functional studies using CRISPR-Cas9 to modify copy number in chicken cell lines or embryos

• Analyze the regulatory elements around POPDC3 to understand how increased copy number affects expression

• What experimental approaches can determine the functional impact of POPDC3 in avian muscle physiology?

To comprehensively investigate POPDC3's role in avian muscle physiology, researchers should consider these methodological approaches:

• Genetic modification: While no POPDC3 knockout studies in chickens are described in the search results, models in other species have provided valuable insights. Zebrafish popdc3 knockdown using splice-site blocking morpholinos resulted in larvae with tail curling and dystrophic muscle features . Similar approaches could be adapted for chicken embryos.

• Electrophysiology: Given that POPDC proteins regulate the potassium channel TREK-1 , patch-clamp electrophysiology of chicken cardiac and skeletal muscle cells with modulated POPDC3 expression would reveal functional impacts on membrane potential and excitability.

• Muscle functional testing: Assessing muscle contractility, fatigue resistance, and recovery in tissue explants with altered POPDC3 expression or activity would provide physiological context. This is particularly relevant given that human POPDC3 mutations cause limb girdle muscular dystrophy with exercise intolerance .

• Protein-protein interaction studies: Using proximity ligation assays, FRET, or co-immunoprecipitation to identify POPDC3 binding partners in chicken muscle tissues .

• Stress response evaluation: Since Popdc1 and Popdc2 null mutant mice develop stress-induced cardiac bradycardia , stress testing of avian models with altered POPDC3 function might reveal similar phenotypes.

• Histopathological analysis: Examining muscle tissue structure and integrity, similar to the approaches used in human patients with POPDC3 mutations who show dystrophic changes in muscle biopsies .

• How can recombinant chicken POPDC3 be used to study protein-protein interactions?

Investigating protein-protein interactions involving recombinant chicken POPDC3 requires a strategic experimental approach:

• Expression system selection: While the search results don't specify optimal systems for chicken POPDC3, successful expression of other POPDC proteins has been achieved in:

  • HEK293 cells for fluorescently tagged constructs

  • Xenopus oocytes for functional studies with ion channels

• Protein interaction methods:

  • Fluorescence/Bioluminescence Resonance Energy Transfer (FRET/BRET): These techniques have been successfully employed to study POPDC protein interactions and their modulation by cAMP. For example, co-expression of cyan fluorescent protein-tagged POPDC1 (POPDC1-CFP) and yellow fluorescent protein-tagged TREK-1 (YFP-TREK-1) in cells produced a robust FRET signal that changed upon addition of isoproterenol or forskolin to stimulate cAMP production . NanoBRET analysis using POPDC constructs with C-terminal NanoLuc luciferase or HaloTag domains has also proven effective .

  • Co-immunoprecipitation: This approach has revealed differential interactions between POPDC family members, showing that "POPDC1 was found to specifically co-precipitate with POPDC2, but this was not the case for POPDC3" .

  • Affinity precipitation: This method has demonstrated binding of native POPDC proteins to cAMP, with elution by free cAMP showing specificity of binding .

  • Proximity ligation: This technique has been used to demonstrate heteromeric complex formation between POPDC proteins .

• Functional validation: For ion channel interactions, electrophysiological recordings in Xenopus oocytes can assess functional consequences of POPDC3-channel interactions .

When designing these experiments for chicken POPDC3, researchers should consider species-specific differences and validate the functionality of the recombinant protein through cAMP binding assays, as binding to cAMP is a hallmark of POPDC family proteins.

• What approaches can determine if chicken POPDC3 functions similarly to human POPDC3 in disease contexts?

To investigate functional conservation between chicken and human POPDC3, particularly in disease contexts, researchers should employ these approaches:

• Mutant protein expression: Express human POPDC3 disease-causing variants (such as p.Leu155His, p.Leu217Phe, and p.Arg261Gln identified in limb-girdle muscular dystrophy ) in chicken cell systems to assess cellular localization, binding partner interactions, and downstream signaling effects.

• Heterologous expression systems: Studies have shown that expressing mutant human POPDC3 in Xenopus oocytes disrupts the function of the mechano-gated potassium channel TREK-1 . Similar approaches with chicken POPDC3 could determine if the functional interaction is conserved.

• Zebrafish models: Knockdown studies using morpholinos targeting popdc3 in zebrafish resulted in "larvae with tail curling and dystrophic muscle features" , providing a valuable model organism for comparative studies.

• CRISPR/Cas9 engineering: Introduce specific human disease mutations into chicken cell lines or embryos to assess phenotypic consequences.

• Tissue-specific expression analysis: Compare expression patterns between species, particularly in heart and skeletal muscle tissues affected in human disease.

• Comparative protein interaction studies: Use techniques like proximity ligation and BRET to determine if chicken POPDC3 interacts with the same partner proteins as human POPDC3, and if these interactions are similarly affected by disease-causing mutations .

Data from these studies should be integrated to create a comprehensive understanding of conserved and divergent functions between species.

• How does cAMP binding to POPDC3 affect its molecular interactions and cellular function?

The impact of cAMP binding on POPDC3 function can be investigated through strategic experimental approaches:

• Binding affinity determination: While specific data for POPDC3 is not provided in the search results, POPDC1 demonstrated high-affinity cAMP binding with an EC₅₀ of approximately 330 nM . Similar radioligand binding assays should be employed for chicken POPDC3, using competitive displacement with unlabeled cAMP.

• Structural changes upon binding: cAMP binding to POPDC proteins likely induces conformational changes that alter interactions with binding partners. To study this:

  • Use FRET sensors to monitor real-time changes in protein conformation and interaction upon cAMP binding

  • Apply mutagenesis to identified cAMP-binding residues to create binding-deficient mutants, as demonstrated with the POPDC1 D200A mutant that failed to respond to isoproterenol in FRET experiments

• Specificity of nucleotide binding: The search results indicate that "physiological concentrations of cGMP did not affect the FRET signal" for POPDC1-TREK-1 interaction , suggesting specificity for cAMP. Similar specificity should be investigated for POPDC3.

• Downstream signaling effects: The interaction between POPDC proteins and TREK-1 is "negatively modulated by cAMP" , suggesting that cAMP binding reduces this interaction. For POPDC3, researchers should:

  • Investigate if the same mechanism applies using co-immunoprecipitation under varying cAMP concentrations

  • Assess membrane trafficking of interaction partners like TREK-1 in response to cAMP level modulation

  • Determine if the PKA inhibitor H89 affects POPDC3-partner interactions, as it did not alter the cAMP-mediated modulation of POPDC1-TREK-1 interaction , suggesting a direct allosteric effect

• Physiological context: Investigate how β-adrenergic stimulation, which increases cAMP levels, affects POPDC3 function in chicken cardiac and skeletal muscle tissues.

• What role might POPDC3 play in chicken reproductive physiology?

The potential role of POPDC3 in chicken reproductive physiology, particularly egg production, can be investigated through several research approaches:

• Copy number and expression correlation: White Leghorn (WL) chickens, known for high egg production, have approximately twice as many copies of POPDC3 compared to other chicken breeds . To explore this correlation:

  • Compare POPDC3 expression levels in the uterus (shell gland) across chicken breeds with varying egg production capacity

  • Analyze temporal expression changes during the egg-laying cycle

• Uterine function hypothesis: Since "the expression of two Popeye family members was upregulated in uterus of pregnant mice," and the chicken uterus is "composed of smooth muscle and containing the shell gland in favor of depositing eggshell," POPDC3 may "facilitate myometrium maturation and labor as well as uterine fluid secretion during the egg laying period" . To test this hypothesis:

  • Perform immunohistochemistry to localize POPDC3 in the chicken oviduct, focusing on the uterine smooth muscle and shell gland

  • Use RNA interference or CRISPR-Cas9 to modulate POPDC3 expression in primary uterine cell cultures

  • Measure contractility and secretory function in ex vivo uterine tissue preparations with altered POPDC3 expression

• Signaling pathway integration: Given POPDC3's role as a cAMP effector:

  • Investigate crosstalk with hormonal signaling pathways important in egg formation

  • Determine if cAMP-dependent modulation of POPDC3 function varies during different phases of the egg-laying cycle

  • Explore interactions with calcium signaling pathways crucial for uterine contraction

• Comparative studies: Analyze POPDC3 expression and function in birds with different reproductive strategies (continuous vs. seasonal layers, varying clutch sizes).

This research could provide valuable insights into molecular mechanisms underlying egg production efficiency, with potential applications in poultry breeding.

• What experimental systems are most suitable for functional characterization of chicken POPDC3?

For comprehensive functional characterization of chicken POPDC3, researchers should consider these experimental systems, each offering distinct advantages:

• Cell culture systems:

  • Primary chicken cell cultures: Derived from relevant tissues (cardiac, skeletal muscle, uterine smooth muscle) to study POPDC3 in its native cellular context

  • DF-1 chicken fibroblast cell line: Useful for basic expression studies and initial characterization in an avian cellular background

  • HEK293 cells: Successfully used for heterologous expression of POPDC proteins for interaction studies

  • Xenopus oocytes: Particularly valuable for electrophysiological studies of POPDC3 interactions with ion channels like TREK-1

• Ex vivo tissue preparations:

  • Muscle strip preparations: To study contractility and physiological function

  • Perfused heart preparations: For cardiac electrophysiology studies

• In vivo models:

  • Chicken embryos: Accessible for developmental studies and amenable to techniques like in ovo electroporation for genetic manipulation

  • Zebrafish: Proven useful for POPDC functional studies, with popdc3 knockdown showing "tail curling and dystrophic muscle features"

  • Mouse models: While not directly relevant to chicken POPDC3, provide comparative insights from mammalian systems

• Functional readout systems:

  • FRET/BRET sensors: For real-time monitoring of protein-protein interactions and their modulation by cAMP

  • Patch-clamp electrophysiology: To assess effects on ion channel function

  • Calcium imaging: To monitor signaling in muscle cells

  • Contractility measurements: Particularly relevant for smooth muscle function in the uterus

Selection should be guided by the specific research questions, with multiple complementary systems providing the most comprehensive characterization.

Methodological Questions

• What expression systems yield optimal functional recombinant chicken POPDC3?

Optimal expression of functional recombinant chicken POPDC3 requires careful consideration of expression systems based on downstream applications:

Key considerations for functional expression include:

• Construct design: Include appropriate purification tags (His, FLAG) positioned to avoid interference with protein function. Consider fluorescent protein fusions (CFP, YFP) for interaction studies .

• Transmembrane domain handling: As POPDC3 contains three transmembrane domains , expression systems must support proper membrane insertion and folding.

• Post-translational modifications: Inclusion of N-glycosylation sites is important, as glycosylation of POPDC proteins appears to be tissue-specific .

• Validation of functionality: Verify cAMP binding capability and appropriate subcellular localization before proceeding with detailed studies.

For most research applications, mammalian expression in HEK293 cells with appropriate epitope or fluorescent tags represents the most versatile starting point.

• How can protein-protein interactions of chicken POPDC3 be effectively analyzed?

For comprehensive analysis of chicken POPDC3 protein interactions, researchers should employ multiple complementary techniques:

• Fluorescence Resonance Energy Transfer (FRET):

  • Implementation: Express chicken POPDC3 fused to CFP and potential interacting proteins fused to YFP in appropriate cells

  • Advantages: Real-time monitoring of interactions in living cells; detection of cAMP-induced changes in interaction

  • Example application: POPDC1-CFP and TREK-1-YFP co-expression showed robust FRET that decreased upon isoproterenol or forskolin treatment to stimulate cAMP production

  • Controls: Include binding-deficient mutants and non-interacting protein pairs

• Bioluminescence Resonance Energy Transfer (BRET):

  • Implementation: Express POPDC3 with NanoLuc luciferase tag and partner proteins with HaloTag in HEK293 cells

  • Example application: NanoBRET analysis has been successfully used for POPDC1 and POPDC2 interaction studies

  • Quantification: Measure BRET signal 5 minutes after adding furimazine NanoLuc substrate

• Co-immunoprecipitation:

  • Implementation: Express tagged versions of POPDC3 and potential partners, immunoprecipitate with tag-specific antibodies

  • Analysis approach: Western blotting to detect co-precipitated proteins

  • Precedent: "POPDC1 was found to specifically co-precipitate with POPDC2, but this was not the case for POPDC3"

• Proximity Ligation Assay (PLA):

  • Implementation: Use specific antibodies against POPDC3 and potential partners in fixed cells/tissues

  • Advantages: High sensitivity; detects endogenous proteins; spatial information

  • Application: Has been used to demonstrate heteromeric complex formation between POPDC proteins

• Membrane trafficking analysis:

  • Implementation: Co-express POPDC3 and partner proteins tagged with different fluorophores

  • Visualization: Use membrane stains like CellBrite Red containing DiD to mark plasma membrane

  • Analysis: Confocal microscopy with plan-apochromat 63X/1.40 oil objective

For physiological relevance, initial screening with heterologous expression systems should be followed by validation in chicken-derived cells or tissues.

• What are the optimal methods for purifying recombinant chicken POPDC3 while preserving its functional properties?

Purification of functional recombinant chicken POPDC3 presents challenges due to its transmembrane nature. Based on approaches used for similar proteins, a comprehensive purification strategy should include:

• Expression system selection:

  • For structural studies: Insect cells (Sf9, Hi5) using baculovirus expression

  • For functional studies: Mammalian cells (HEK293) for proper post-translational modifications

• Construct design:

  • N-terminal tag placement (avoiding disruption of signal peptides)

  • Consideration of glycosylation sites important for POPDC protein function

  • Inclusion of cleavable purification tags (His, FLAG, Strep-II)

• Membrane extraction:

  • Gentle detergent solubilization is critical for maintaining structure and function

  • Recommended detergents for initial screening: n-dodecyl-β-D-maltoside (DDM), digitonin, and LMNG

  • Buffer optimization to include stabilizing agents (glycerol, cholesterol hemisuccinate)

• Multi-step purification protocol:

  • Affinity chromatography using the engineered tag (e.g., IMAC for His-tagged protein)

  • Size exclusion chromatography to remove aggregates and ensure homogeneity

  • Optional ion exchange chromatography for removing contaminants

• Functional validation during purification:

  • cAMP binding assay using techniques described in the literature for POPDC proteins

  • Thermal stability assessment using differential scanning fluorimetry

  • Verification of oligomeric state (native POPDC1 exists as dimers )

• Storage optimization:

  • Test stability in different buffer compositions

  • Assess functionality after freeze-thaw cycles

  • Consider detergent exchange to more stable options for long-term storage

Throughout purification, researchers should monitor not only protein purity but also functional integrity by testing cAMP binding capability, as this is central to POPDC3 function.

• What experimental designs best evaluate the functional impact of POPDC3 gene variants?

To comprehensively evaluate the functional impact of POPDC3 gene variants, researchers should implement a multi-faceted experimental approach:

• In silico analysis and prediction:

  • Structural modeling to predict effects on the Popeye domain and cAMP binding pocket

  • Conservation analysis across species to identify functionally critical residues

  • Consider the location of variants relative to key functional motifs, such as the DSPE and FQVT motifs involved in cyclic nucleotide binding

• Cell-based functional assays:

  • Expression and localization studies: Compare subcellular localization of wild-type and variant POPDC3 using fluorescent tags and confocal microscopy

  • cAMP binding assays: Assess impact on cAMP binding affinity using competition binding assays

  • Protein-protein interaction analysis: Use FRET/BRET assays to measure how variants affect interactions with binding partners such as TREK-1

  • Heterologous expression in Xenopus oocytes: Measure effects on ion channel function, as demonstrated for human POPDC3 variants that "caused an aberrant modulation of the mechano-gated potassium channel, TREK-1"

• Animal models:

  • Zebrafish model: popdc3 knockdown resulted in "larvae with tail curling and dystrophic muscle features" , providing a system to test variant rescue capability

  • Chicken embryo model: Introduce variants using techniques like in ovo electroporation

• Tissue-specific considerations:

  • Muscle physiology assays: Given POPDC3's role in muscle function, assess contractility and fatigue resistance

  • Uterine function tests: In chicken models, evaluate impact on uterine smooth muscle function and egg production

• Controls and comparisons:

  • Include known pathogenic variants as positive controls

  • Test multiple variants affecting different protein domains

  • Compare effects to other POPDC family members

This approach has been successfully applied to human POPDC3 variants, revealing that missense mutations affecting conserved residues in the Popeye and carboxy-terminal domains lead to limb girdle muscular dystrophy .

• What strategies can optimize the production of antibodies against chicken POPDC3 for research applications?

Developing specific antibodies against chicken POPDC3 requires careful planning and strategic approaches:

• Antigen design considerations:

  • Epitope selection: The Popeye domain is highly conserved across species and might generate cross-reactive antibodies. Consider using:

    • Unique regions in the carboxyterminal domain (CTD) which is variable and isoform-specific

    • Chicken-specific sequences that differ from mammalian POPDC3

    • Synthetic peptides corresponding to extracellular or cytoplasmic loops

  • Recombinant protein fragments: Express soluble domains (e.g., the cytoplasmic Popeye domain) as fusion proteins with tags like GST or MBP to enhance immunogenicity

  • Avoid transmembrane segments: These are typically hydrophobic and poorly immunogenic

• Immunization strategies:

  • Host selection: Rabbits for polyclonal antibodies; mice or rats for monoclonal development

  • Adjuvant selection: Complete Freund's adjuvant for primary immunization, incomplete for boosters

  • Prime-boost protocols: Multiple immunizations to enhance antibody affinity and titer

• Screening and validation approaches:

  • Initial screening: ELISA against the immunizing antigen

  • Cross-reactivity testing: Western blotting against recombinant human, mouse, and chicken POPDC3

  • Specificity validation:

    • Western blotting of chicken tissue lysates (heart, skeletal muscle, uterus)

    • Immunohistochemistry on chicken tissue sections

    • Testing in POPDC3-transfected cells versus controls

    • Pre-absorption controls with immunizing peptide

  • Functional validation: Immunoprecipitation to confirm ability to recognize native protein

• Application-specific considerations:

  • For immunohistochemistry: Test antibodies with different fixation methods

  • For immunoprecipitation: Verify compatibility with detergents used for membrane protein extraction

  • For FRET/proximity studies: Ensure antibody binding doesn't interfere with protein function or interactions

Researchers should note that commercial antibodies against POPDC3 exist (see search result #9), but these should be carefully validated for cross-reactivity with chicken POPDC3 before use in research applications.