Recombinant Human Popeye domain-containing protein 2 (POPDC2)

What is POPDC2 and where is it predominantly expressed?

POPDC2 belongs to the Popeye domain-containing family of proteins, which are plasma membrane-localized proteins abundantly expressed in heart and skeletal muscle tissues. Protein modeling and binding experiments have established that POPDC proteins constitute a novel class of cyclic nucleotide binding proteins . While POPDC2 is strongly expressed in cardiac myocytes generally, the highest expression is observed in cardiac conduction tissue .

Immunolocalization studies using monoclonal antibodies specific for POPDC2 have demonstrated that this protein localizes to the sarcolemma of skeletal muscle and is found at intercalated discs, the cardiac sarcolemma, and in transverse tubules (T-tubules) in heart muscle . Single-cell RNA sequencing from human hearts has shown that POPDC2 is abundantly expressed in sinoatrial node cells, and co-expression of POPDC1 and POPDC2 is particularly prevalent in atrioventricular (AV) node, AV node pacemaker, and AV bundle cells .

What are the functional interactions between POPDC2 and ion channels in cardiac tissue?

POPDC2 plays a crucial role in cardiac pacemaking and conduction, partially through its cAMP-dependent binding and regulation of TREK-1 potassium channels . In electrophysiological studies, co-expression of wild-type POPDC2 with TREK-1 increases TREK-1 current density, while POPDC2 variants identified in patients with cardiac conduction disorders fail to enhance TREK-1 current density .

This functional interaction is critical for normal cardiac function, as evidenced by the development of stress-induced bradycardia and sinus node dysfunction in Popdc2-null mice. These mice exhibit chronotropic incompetence when challenged with swimming exercise, mental stress, or after beta-adrenergic stimulation, characterized by severe sinoatrial node dysfunction with long pauses interrupting periods of normal sinus rhythm .

How do researchers distinguish between the roles of POPDC1 and POPDC2?

While POPDC1 and POPDC2 share many functions and often form complexes, their roles can be distinguished through several experimental approaches:

• Genetic models: Knockout mice studies show that loss of Popdc2 results in sinus pauses and bradycardia, while Popdc1 knockout may present different phenotypes .

• Expression analysis: Single-cell RNA sequencing demonstrates differential expression patterns. For example, sinoatrial node cells abundantly express POPDC2, while POPDC1 expression in these cells is sparse . This helps explain why sinus node disease is present in POPDC2-related disease in humans but not in POPDC1-related disease.

• Specific antibodies: Monoclonal antibodies prepared against recombinant protein preparations can specifically recognize either POPDC1 or POPDC2. The specificity of these antibodies can be demonstrated through competition experiments, where pre-incubation with recombinant POPDC1 inhibits localization of POPDC1-specific antibodies but not POPDC2-specific antibodies, and vice versa .

What experimental models are commonly used to study POPDC2 function?

Several experimental models have proven valuable for investigating POPDC2 function:

• Mouse knockout models: Mice with null mutations in the Popdc2 gene have been engineered to study the protein's role in vivo. These models show that while Popdc2-null mice survive into adulthood, they develop age-dependent chronotropic incompetence with stress or exercise .

• Zebrafish models: Morpholino knockdown of popdc2 in zebrafish embryos results in cardiac conduction defects, specifically atrioventricular (AV) block, suggesting an evolutionarily conserved function of POPDC2 in regulating cardiac conduction .

• Cell culture systems: Human skeletal myotubes and transfected cell lines (like COS7 cells) have been used to study protein-protein interactions and cellular localization of POPDC2 .

• In vitro electrophysiological studies: These have been employed to demonstrate the functional effects of POPDC2 on ion channels such as TREK-1, showing that wild-type POPDC2 increases TREK-1 current density while pathogenic variants fail to do so .

What methodological approaches are used to identify POPDC2 interaction partners?

Researchers have employed several complementary techniques to identify and validate POPDC2 interaction partners:

• Protein pull-down assays with mass spectrometry: GST-fusion "bait" proteins (mouse POPDC1 or POPDC2) attached to glutathione beads have been used to pull down interacting proteins from tissue extracts (such as RIPA extracts of human skeletal myotubes). The pulled-down proteins are then identified by mass spectrometry .

• Co-immunoprecipitation: This technique confirms direct protein-protein interactions in cellular contexts. For example, the interaction between POPDC1/2 and XIRP1 has been validated using this approach .

• Immunofluorescence co-localization: Antibodies specific for POPDC2 and potential binding partners are used to visualize their localization in tissue sections. Co-localization at specific cellular structures (such as intercalated discs or T-tubules) provides evidence for potential functional interactions .

• Functional assays: For ion channel interactions, co-expression of POPDC2 with channel proteins (like TREK-1) in heterologous expression systems allows measurement of functional effects on channel properties .

How are recombinant POPDC2 proteins produced for research applications?

While the search results don't provide extensive details on the production of recombinant POPDC2, we can infer some methodological approaches based on the described experiments:

• Bacterial expression systems: GST-fusion proteins of mouse POPDC1 or POPDC2 have been produced for pull-down experiments . These likely involve cloning the POPDC2 coding sequence into expression vectors with GST tags, expressing in bacterial systems like E. coli, and purifying using affinity chromatography.

• Mammalian expression systems: For functional studies, POPDC2 has been expressed in mammalian cell lines such as COS7 cells . This approach is particularly important for studies requiring proper post-translational modifications.

• Quality control: Recombinant protein preparations are verified using techniques such as:

  • Western blotting with specific antibodies

  • Testing functionality through binding assays

  • Confirming proper folding and activity through functional assays

• Application in antibody production: Recombinant POPDC2 has been used as an immunogen for producing monoclonal antibodies. These antibodies are then selected for their specific recognition of human POPDC2 proteins .

What molecular mechanisms underlie POPDC2-associated cardiac arrhythmias?

The pathophysiology of POPDC2-associated cardiac arrhythmias involves several molecular mechanisms:

• Disrupted cAMP binding: Homology modeling shows that pathogenic POPDC2 variants diminish the protein's ability to bind cAMP . Sequence analysis reveals that conserved amino acids cluster around the putative cyclic nucleotide-binding domain, and mutagenesis of these conserved amino acids impairs cyclic nucleotide binding .

• Altered ion channel regulation: Wild-type POPDC2 increases TREK-1 potassium channel current density, but pathogenic variants fail to do so . This disruption in ion channel modulation likely contributes to the observed arrhythmias.

• Structural degeneration of cardiac conduction tissue: In Popdc2-null mice, the bradycardic phenotype is accompanied by degeneration of the sinoatrial node (SAN) structure, with specific loss of tissue in the inferior part . Since the inferior SAN becomes the predominant pacemaker center after sympathetic stimulation, the loss of SAN myocytes in this region may explain the observed stress-induced sinus node dysfunction.

• Impaired POPDC1-POPDC2 complex formation: Pathogenic variants in either POPDC1 or POPDC2 significantly reduce the expression of both proteins, suggesting that the stability and/or membrane trafficking of the POPDC1-POPDC2 complex is impaired .

How do biallelic variants in POPDC2 affect protein function at the molecular level?

Biallelic variants in POPDC2 impact protein function through several mechanisms:

• Reduced cAMP binding: Homology modeling demonstrates that pathogenic POPDC2 variants diminish the protein's ability to bind cAMP , disrupting its role in cAMP-dependent signaling.

• Failed regulation of TREK-1 channels: While co-expression of wild-type POPDC2 with TREK-1 increases TREK-1 current density, POPDC2 variants found in patients fail to increase TREK-1 current density .

• Disrupted protein-protein interactions: Patient muscle biopsies show significant reduction in the expression of both POPDC1 and POPDC2, suggesting that the stability and/or membrane trafficking of the POPDC1-POPDC2 complex is impaired by pathogenic variants in either protein .

• Phenotypic spectrum: The functional effects of these variants manifest as a clinical spectrum consisting of sinus node dysfunction, atrioventricular conduction defects, and hypertrophic cardiomyopathy .

Importantly, population-level genetic data analysis of more than 1 million individuals has shown that familial variants are not associated with clinical outcomes in heterozygous state, suggesting that heterozygous family members are unlikely to develop clinical manifestations .

What is the relationship between POPDC2 and the XIRP1 protein in cardiac conduction?

The interaction between POPDC2 and XIRP1 (Xin actin binding repeat-containing protein 1) represents a significant molecular partnership in cardiac conduction:

• Strong physical interaction: Proteomic analysis of proteins pulled down by POPDC1 and POPDC2 from human skeletal myotubes identified XIRP1 as the highest-scoring binding partner for both proteins, along with cardiac actin .

• Co-localization in cardiac structures: Both POPDC2 and XIRP1 are present at intercalated discs and near the Z-line of myofibrils in rat heart . In human heart, co-localization of XIRP1 with Connexin-43 at intercalated discs has been demonstrated .

• Functional overlap in cardiac conduction: Mutations in both POPDC2 and XIRP1 cause cardiac arrhythmias:

  • Loss of Popdc2 in mice results in sinus pauses and bradycardia

  • XIRP1 mutations are also associated with cardiac conduction defects

  This functional overlap suggests a potential cooperative role for both proteins in maintaining normal cardiac conduction .

• Evolutionary conservation: The functional relationship between these proteins appears to be evolutionarily conserved, as evidenced by cardiac conduction defects observed in both mouse models and zebrafish morphants with reduced Popdc2 expression .

How can researchers differentiate between primary effects of POPDC2 dysfunction and secondary consequences?

Differentiating between primary effects of POPDC2 dysfunction and secondary consequences requires sophisticated experimental approaches:

• Temporal analysis in model systems: Studying the progression of phenotypes in Popdc2-null mice reveals that under baseline conditions, a normal heart rate is observed initially, with age-dependent development of chronotropic incompetence emerging later when mice are challenged with exercise, stress, or β-adrenergic stimulation . This temporal progression helps distinguish primary defects from secondary adaptations.

• Structure-function correlation: The observation that stress-induced cardiac pauses in Popdc2-null mice are associated with degeneration of the sinoatrial node structure, particularly in the inferior part (which becomes the predominant pacemaker center after sympathetic stimulation), provides a direct link between structural changes and functional deficits .

• Molecular pathway analysis: In vitro electrophysiological studies demonstrate that wild-type POPDC2 increases TREK-1 current density, while pathogenic variants fail to do so . This direct functional effect can be considered a primary consequence of POPDC2 dysfunction.

• Comparative analysis across species: The observation of cardiac conduction defects in both Popdc2-null mice and zebrafish popdc2 morphants suggests evolutionarily conserved primary functions of POPDC2 .

• Human genetics correlation: The identification of biallelic variants in POPDC2 in families with a phenotypic spectrum of sinus node dysfunction, AV conduction defects, and hypertrophic cardiomyopathy provides strong evidence for primary effects of POPDC2 dysfunction .

What antibodies and detection methods are most effective for studying POPDC2?

Based on the search results, several effective antibodies and detection methods have been developed for POPDC2 research:

• Monoclonal antibodies: Specifically developed monoclonal antibodies like POPDC2 12G12 have been shown to effectively and specifically recognize human POPDC2 proteins . The specificity of these antibodies has been validated through:

  • Staining of COS7 cells transfected with recombinant POPDC2

  • Confirmation that POPDC2-specific antibodies do not recognize POPDC1

  • Competition experiments where pre-incubation with recombinant POPDC2 inhibits localization of the antibody to the sarcolemma of human skeletal muscle

• Immunofluorescence microscopy: This technique has been used to visualize POPDC2 localization in:

  • Cardiac tissue, where POPDC2 was found at intercalated discs, the cardiac sarcolemma, and T-tubules

  • Skeletal muscle, where POPDC2 localizes to the sarcolemma and T-tubules

  • Identifying co-localization with other proteins such as XIRP1, annexin A5, and markers for specific cellular structures (e.g., connexin 43 for intercalated discs)

• Western blotting: While not explicitly detailed in the search results, Western blotting with specific antibodies is typically used to confirm the presence and quantity of POPDC2 in tissue samples and recombinant preparations.

• Mass spectrometry: This technique has been successfully employed to identify POPDC2 interaction partners in pull-down experiments .

What are the critical considerations when designing experiments to study POPDC2 function in cardiac tissue?

When designing experiments to study POPDC2 function in cardiac tissue, researchers should consider:

• Tissue-specific expression patterns: POPDC2 shows differential expression across cardiac tissues, with highest expression in cardiac conduction tissue . Single-cell RNA sequencing has shown that POPDC2 is abundantly expressed in sinoatrial node cells, and co-expression of POPDC1 and POPDC2 is particularly prevalent in AV node, AV node pacemaker, and AV bundle cells . Experimental designs should account for this heterogeneity.

• Stress-dependent phenotypes: In Popdc2-null mice, cardiac dysfunction is particularly evident under stress conditions (exercise, mental stress, β-adrenergic stimulation) rather than at baseline . Experiments should incorporate appropriate stress protocols to unmask POPDC2-dependent phenotypes.

• Age-dependent effects: Popdc2-null mice develop an age-dependent chronotropic incompetence , suggesting that age should be a controlled variable in experimental designs.

• Interaction with binding partners: POPDC2 interacts with multiple proteins, including XIRP1, cardiac actin, and annexin A5 , as well as ion channels like TREK-1 . Comprehensive functional studies should consider these interactions.

• cAMP dependency: As POPDC2 is a cyclic nucleotide binding protein, experiments should consider the cAMP signaling context and how manipulations of this pathway might affect POPDC2 function .

How can researchers effectively model POPDC2-related cardiac disorders in vitro?

Based on the search results, several approaches have been utilized to model POPDC2-related cardiac disorders in vitro:

• Heterologous expression systems: Co-expression of wild-type or mutant POPDC2 with binding partners (like TREK-1) in cell culture systems allows assessment of functional interactions. For example, electrophysiological studies have demonstrated that while wild-type POPDC2 increases TREK-1 current density, pathogenic POPDC2 variants fail to do so .

• Structural modeling: Homology modeling has been used to predict how pathogenic POPDC2 variants might affect the protein's ability to bind cAMP . This approach can guide experimental designs to test specific hypotheses about structure-function relationships.

• Primary cell cultures: Though not explicitly mentioned in the search results, primary cultures of cardiomyocytes or conduction system cells would provide a more physiologically relevant system for studying POPDC2 function than heterologous expression systems.

• Induced pluripotent stem cell (iPSC) models: While not specifically mentioned in the search results, patient-derived iPSCs differentiated into cardiomyocytes or specialized cardiac conduction cells would provide a powerful system for modeling POPDC2-related cardiac disorders, particularly given the availability of patients with identified pathogenic POPDC2 variants .

• Organoid or engineered heart tissue models: These advanced 3D culture systems could recapitulate the tissue architecture in which POPDC2 functions, potentially providing insights not obtainable from 2D culture systems.

What aspects of POPDC2 function remain poorly understood?

Despite significant advances in understanding POPDC2 biology, several aspects remain poorly characterized:

• Complete interactome: While some interaction partners have been identified (XIRP1, cardiac actin, annexin A5, TREK-1) , the complete set of POPDC2 interacting proteins in different cardiac cell types remains to be fully elucidated.

• Regulation of POPDC2 expression: The search results provide information about where POPDC2 is expressed, but little about how its expression is regulated during development or in response to physiological or pathological stimuli.

• Post-translational modifications: Beyond cAMP binding, other potential post-translational modifications that might regulate POPDC2 function are not well characterized in the available search results.

• Subcellular trafficking: While POPDC2 localization has been described at the sarcolemma, intercalated discs, and T-tubules , the mechanisms controlling its subcellular trafficking and how mutations might affect this process remain to be fully elucidated.

• Therapeutic targeting: Approaches to therapeutically modulate POPDC2 function in the context of cardiac arrhythmias are not addressed in the search results and represent an important area for future research.

How might POPDC2 research inform development of therapeutics for cardiac arrhythmias?

While the search results don't directly address therapeutic development, several aspects of POPDC2 biology suggest potential therapeutic approaches:

• cAMP pathway modulation: Since POPDC2 functions as a cAMP binding protein , therapies that modulate cAMP levels or mimic cAMP binding to POPDC2 might be effective in treating POPDC2-related cardiac disorders.

• Ion channel targeting: Given POPDC2's regulation of ion channels like TREK-1 , compounds that modulate these channels might compensate for loss of POPDC2 function.

• Gene therapy approaches: For patients with loss-of-function POPDC2 variants, gene replacement or gene editing therapies could potentially restore normal POPDC2 function.

• Protein-protein interaction modulation: Therapies that stabilize interactions between POPDC2 and its binding partners might preserve function in the context of certain pathogenic variants.

• Precision medicine: The identification of specific POPDC2 variants in patients with cardiac arrhythmias suggests the potential for variant-specific therapeutic approaches.

What emerging technologies might advance POPDC2 research in the near future?

Several emerging technologies could significantly advance POPDC2 research:

• Single-cell technologies: Further application of single-cell RNA sequencing, as already demonstrated , and emerging single-cell proteomics could provide unprecedented resolution of POPDC2 expression and function across different cardiac cell populations.

• CRISPR-based approaches: CRISPR/Cas9 technology could enable:

  • Generation of isogenic cell lines with specific POPDC2 variants

  • High-throughput screening for POPDC2 modifiers

  • In vivo modeling of POPDC2 variants in model organisms

• Advanced imaging techniques: Super-resolution microscopy and live-cell imaging could provide new insights into POPDC2 dynamics and interactions at specialized cardiac structures like intercalated discs and T-tubules.

• Biomaterials and tissue engineering: Engineered cardiac tissues incorporating cells with normal or mutant POPDC2 could provide physiologically relevant platforms for studying POPDC2 function and testing potential therapeutics.

• Computational modeling: Integration of structural, functional, and clinical data could enable predictive modeling of the effects of POPDC2 variants and guide therapeutic development.