POPDC2

POPDC2 is a plasma membrane-associated cAMP effector that controls cardiac and skeletal muscle excitability by regulating the surface expression of ion channel partners through its high-affinity cAMP-binding Popeye domain (PMID:22354168). It interacts with the two-pore potassium channel TREK-1 to increase TREK-1 surface expression and current density, an effect reversed when cAMP is elevated (PMID:22354168, PMID:24646234), and conversely downregulates the cardiac sodium channel Nav1.5 (SCN5A) by reducing its surface expression in a cAMP-dependent manner (PMID:38689644). POPDC2 functions within a heteromeric POPDC complex, assembling with the other Popeye-domain proteins through a helix-helix interface at the C-terminus of the Popeye domain whose integrity is required for correct membrane trafficking of the POPDC1-POPDC2 complex (PMID:36624536). At the cardiomyocyte membrane, POPDC2 localizes to the intercalated disc and T-tubules, where it associates with the Xin actin-binding protein XIRP1 and actin (PMID:33261556, PMID:27148881) and with the sodium-calcium exchanger NCX1 (PMID:28755400). Loss of POPDC2 function in mice and zebrafish produces stress-induced sinus node dysfunction, conduction block, and arrhythmia (PMID:22354168, PMID:22290329), and bi-allelic loss-of-function variants in POPDC2 that abolish TREK-1 current augmentation cause an autosomal recessive cardiac syndrome in humans (PMID:32535041, PMID:40409267). Expression of Popdc2 during cardiogenesis is controlled by mutually exclusive binding of NKX2-5 and MEIS1 to an overlapping enhancer motif (PMID:26411676).

1. 2012 High

   Established the founding molecular function: that the POPDC2 Popeye domain is a cAMP sensor and that POPDC2 couples cAMP signaling to TREK-1 channel surface expression, defining it as a membrane cAMP effector in the cardiac conduction system.

   Evidence cAMP binding assays, co-IP with TREK-1, Xenopus oocyte electrophysiology, and ECG of Popdc2 null mice

   PMID:22354168

   Open questions at the time

   • Did not resolve the structural basis of cAMP-regulated trafficking
   • Mechanism by which cAMP suppresses TREK-1 surface delivery not defined
2. 2012 High

   Demonstrated in vivo that POPDC2 is required for both skeletal muscle morphogenesis and cardiac conduction, independently corroborating the conduction phenotype seen in mouse knockouts.

   Evidence Morpholino knockdown in zebrafish with optical contractility and calcium-transient imaging

   PMID:22290329

   Open questions at the time

   • Morpholino specificity not orthogonally controlled by mutant
   • Molecular effector linking knockdown to conduction defect not identified in this study
3. 2014 Medium

   Extended the channel-trafficking model by showing all three Popdc family members enhance TREK-1 membrane transport in a cAMP-blockable manner and linking the family to caveolar organization.

   Evidence Xenopus oocyte electrophysiology, interaction studies, caveolae analysis in Popdc1 null mice

   PMID:24646234

   Open questions at the time

   • Review summarizing prior data without new independent replication
   • POPDC2-specific contribution to caveolar effects not separated from POPDC1
4. 2015 High

   Connected POPDC dysfunction to human disease by showing a POPDC1 cAMP-affinity-reducing mutation impairs membrane trafficking of both POPDC1 and POPDC2, establishing complex interdependence.

   Evidence Exome sequencing, cAMP affinity assays, patient muscle biopsy immunostaining, zebrafish homologous mutation

   PMID:26642364

   Open questions at the time

   • Did not test whether POPDC2 mutations reciprocally affect POPDC1 trafficking
   • Functional channel consequence in patient tissue not directly measured
5. 2015 Medium

   Identified the transcriptional control of Popdc2 expression, showing NKX2-5 and MEIS1 compete at an overlapping enhancer motif to time expression during cardiac progenitor differentiation.

   Evidence Transcription factor binding assays, reporter assays, and chromatin studies during cardiac differentiation

   PMID:26411676

   Open questions at the time

   • In vivo requirement of this enhancer for endogenous Popdc2 expression not established
   • Single lab, Popdc2 used as model locus
6. 2016 Medium

   Pinpointed the subcellular site of POPDC2 action by localizing it to the cardiac intercalated disc.

   Evidence Quantitative membrane proteomics with immunofluorescence co-localization with n-cadherin across species

   PMID:27148881

   Open questions at the time

   • No direct functional consequence of intercalated-disc localization tested
   • Localization-trafficking link not mechanistically resolved
7. 2017 Medium

   Expanded the POPDC2 interactome with the sodium-calcium exchanger NCX1, hinting at broader regulation of cardiac ion handling beyond potassium and sodium channels.

   Evidence Affinity purification-mass spectrometry from rat left ventricle with co-precipitation validation

   PMID:28755400

   Open questions at the time

   • Functional consequence of POPDC2-NCX1 interaction not tested
   • Single lab, no cAMP dependence examined
8. 2020 High

   Provided the first direct human genetic evidence for POPDC2 as a disease gene, linking a loss-of-function nonsense variant to impaired TREK-1 modulation and a sinus node phenotype reproduced in knock-in mice.

   Evidence Exome sequencing, Xenopus oocyte co-expression electrophysiology, knock-in mouse ECG, qPCR of TREK-1 in AV node

   PMID:32535041

   Open questions at the time

   • Mechanism of stress-dependence of the bradycardia not resolved
   • Whether heterozygous loss is sufficient in humans not fully established
9. 2023 High

   Defined the structural basis of POPDC complex assembly, mapping a C-terminal helix-helix interface required for membrane trafficking and separating trafficking-disrupting from interaction-preserving pathogenic mutations.

   Evidence Co-IP, PLA, BRET, BiFC, site-directed mutagenesis, and patient muscle biopsy immunostaining

   PMID:36624536

   Open questions at the time

   • The 'other essential function' of interaction-preserving mutants not identified
   • No high-resolution structure of the interface
10. 2024 High

    Broadened the effector model by establishing that POPDC proteins also suppress Nav1.5 surface expression in a cAMP-dependent manner, showing POPDC2 reciprocally tunes both repolarizing and depolarizing currents.

    Evidence Functional co-expression screen in oocytes, co-IP from native cardiac tissue, zebrafish knockdown with action potential recordings

    PMID:38689644

    Open questions at the time

    • Trafficking machinery mediating Nav1.5 internalization not identified
    • Relative in vivo contribution of Nav1.5 vs TREK-1 modulation to arrhythmia unresolved
11. 2025 High

    Confirmed POPDC2 as a recessive disease gene, showing bi-allelic loss-of-function variants fail to augment TREK-1 current and reduce abundance of the POPDC1-POPDC2 complex in patient muscle.

    Evidence In vitro TREK-1 co-expression electrophysiology, patient muscle protein quantification, homology modeling, single-cell RNA-seq of human heart

    PMID:40409267

    Open questions at the time

    • Whether reduced complex abundance reflects degradation or trafficking failure not distinguished
    • Skeletal muscle phenotype mechanism in patients not fully defined

• How cAMP binding to the Popeye domain is mechanistically transduced into altered channel trafficking, and what the trafficking machinery and 'other essential function' of interaction-competent POPDC2 mutants are, remain unresolved.
• No structure of the cAMP-bound Popeye domain or POPDC complex
• Identity of the membrane trafficking adaptors engaged by POPDC2 unknown
• Functional consequence of NCX1 and XIRP1 interactions for channel regulation untested