AKAP6

AKAP6 (mAKAP) is a perinuclear scaffold protein in striated muscle and neurons that integrates cAMP, calcium, and MAPK signaling at the nuclear envelope to control cardiac hypertrophy, myogenic differentiation, hypoxic gene expression, and microtubule organization. Anchored to the nuclear envelope via direct binding of its spectrin repeats to nesprin-1α, AKAP6 assembles a signalosome containing PKA, PDE4D3, PP2A (B56δ), Epac1, ERK5, calcineurin, ryanodine receptors, and MEF2 transcription factors, enabling coordinated feedback loops in which PKA activates PDE4D3 to dampen local cAMP while ERK5 opposes this effect, and calcineurin activates NFATc and MEF2 to drive hypertrophic and myogenic transcription (PMID:11296225, PMID:16177794, PMID:23261540). AKAP6 also serves as the key organizer of the nuclear envelope microtubule-organizing center (NE-MTOC) by recruiting centrosomal proteins Pcnt and AKAP9 through its spectrin repeats, a process driven by myogenin-dependent transcriptional induction of AKAP6 and nesprin-1α (PMID:33295871, PMID:34605406). In neurons, AKAP6-anchored calcineurin activates NFATc4 downstream of BDNF to mediate neuroprotection, and in cardiomyocytes Wnt/β-catenin signaling upregulates AKAP6 to enhance PKA-dependent RyR2 phosphorylation and SR calcium release (PMID:39578909, PMID:40097291).

1. 1995 High

   Establishing that AKAP6 is an A-kinase anchoring protein resolved how type II PKA is targeted to specific subcellular compartments in cardiac and skeletal muscle.

   Evidence Interaction cloning with RIIα, overlay binding assay, and cAMP-agarose affinity chromatography in cardiac tissue

   PMID:7721854

   Open questions at the time

   • Exact subcellular targeting mechanism unknown
   • No identified substrates of the anchored PKA pool
   • Functional consequence of PKA anchoring not tested
2. 1999 High

   Mapping nuclear envelope targeting to two spectrin-like repeat regions explained how AKAP6 achieves perinuclear localization in differentiated myocytes, distinguishing it from other AKAPs.

   Evidence GFP-fusion construct imaging and dominant-negative displacement of endogenous mAKAP from the nuclear membrane

   PMID:10413680 PMID:9679148

   Open questions at the time

   • Identity of the nuclear envelope receptor for AKAP6 unknown
   • Whether spectrin repeats mediate protein-protein interactions beyond targeting unclear
3. 2001 High

   Discovery that AKAP6 co-assembles PKA, PDE4D3, ryanodine receptors, and PP2A into one complex established the concept of an integrated cAMP/Ca²⁺ signaling module with built-in negative feedback at the nuclear envelope.

   Evidence Co-immunoprecipitation from cardiac tissue, kinase and phosphodiesterase activity assays, dominant-negative PKA-anchoring disruption

   PMID:11296225 PMID:11590243

   Open questions at the time

   • Stoichiometry and dynamics of the complex not determined
   • Whether ryanodine receptor regulation by this complex operates in vivo unclear
4. 2005 High

   Identification of dual opposing feedback loops (PKA→PDE4D3 activation vs. ERK5→PDE4D3 inhibition, with Epac1 attenuating ERK5) and calcineurin/NFATc recruitment revealed how AKAP6 integrates cAMP and MAPK signaling to control cardiomyocyte hypertrophy.

   Evidence Co-IP, pharmacological and dominant-negative disruption, ERK5 kinase assay, NFATc reporter, cardiomyocyte hypertrophy cell-size measurement

   PMID:16177794 PMID:16306226

   Open questions at the time

   • In vivo cardiac phenotype of AKAP6 disruption not yet shown
   • Whether ERK5 module operates independently of PKA feedback in pathological hypertrophy unclear
5. 2005 High

   Identification of nesprin-1α as the nuclear envelope receptor that directly binds the third spectrin repeat of AKAP6 solved how AKAP6 is anchored at the nuclear envelope.

   Evidence Direct binding assay between nesprin-1α and mAKAP spectrin repeat domains, overexpression displacement, co-immunoprecipitation

   PMID:15652351

   Open questions at the time

   • Structural basis of spectrin repeat–nesprin interaction not resolved
   • Whether other nuclear envelope proteins contribute to anchoring unknown
6. 2008 High

   Showing that AKAP6 organizes E3 ubiquitin ligases controlling HIF-1α stability expanded the scaffold's role beyond kinase/phosphatase signaling to include regulation of hypoxic gene expression.

   Evidence Co-IP of E3 ligases with mAKAP, siRNA knockdown, HIF-1α stability immunoblot under normoxia/hypoxia, transcriptional reporter

   PMID:19109240

   Open questions at the time

   • Identity and specificity of the E3 ligases in the complex not fully defined
   • Whether HIF-1α regulation is relevant outside cardiomyocytes unknown
7. 2010 High

   Characterization of PP2A (B56δ subunit) as a PKA-activated phosphatase within the AKAP6 complex that dephosphorylates PDE4D3 uncovered a cAMP-induced positive feedback loop layered onto the existing negative feedback circuit.

   Evidence Domain mapping, cardiac tissue co-IP, phosphatase activity assay, B56δ PKA-site mutagenesis

   PMID:20106966

   Open questions at the time

   • Net outcome of simultaneous positive and negative feedback on local cAMP dynamics not modeled quantitatively
   • Whether other PP2A substrates in the complex exist unknown
8. 2012 High

   Demonstrating that AKAP6 directly binds MEF2 and bridges calcineurin to MEF2 established the scaffold as essential for myogenic transcription and differentiation beyond its kinase-anchoring role.

   Evidence Co-IP, direct domain binding, dominant-negative peptide interference, MEF2 reporter, myotube formation assay

   PMID:22484155 PMID:23261540

   Open questions at the time

   • Which MEF2 isoform is preferentially bound in vivo unknown
   • Structural basis of MEF2–AKAP6 interface not determined
9. 2015 High

   Discovery of a positive feedback loop in which AKAP6 promotes myogenin expression via MEF2A, and myogenin in turn transactivates the AKAP6 promoter, explained how AKAP6 levels are amplified during myogenesis and linked AKAP6 to in vivo muscle regeneration.

   Evidence siRNA/shRNA knockdown, ChIP showing myogenin at AKAP6 E-box, luciferase promoter assay, in vivo cardiotoxin injury model

   PMID:26563778

   Open questions at the time

   • Whether the feedback loop is sufficient for full differentiation or requires additional cofactors unknown
   • Temporal dynamics of the feedback loop during regeneration not resolved
10. 2020 High

    Establishing AKAP6 as the key organizer of the nuclear envelope MTOC — recruiting Pcnt and AKAP9 via its spectrin repeats and nesprin-1α — redefined AKAP6 function beyond signaling scaffold to include cytoskeletal architecture essential for cardiomyocyte hypertrophy and osteoclast bone resorption.

    Evidence Gain-of-function (ectopic AKAP6 in epithelial cells recruits centrosomal proteins), loss-of-function (KD/KO), MTOC activity assays, hypertrophy and resorption assays in two cell types

    PMID:33295871

    Open questions at the time

    • How AKAP6-mediated MTOC function is coordinated with its signaling scaffold role is unclear
    • Whether AKAP6 recruits additional MTOC components beyond Pcnt and AKAP9 unknown
11. 2021 High

    Showing that myogenin is both necessary and sufficient to induce the muscle-specific AKAP6β and nesprin-1α isoforms that together reconstitute NE-MTOC formation placed AKAP6 MTOC assembly under direct transcriptional control of the myogenic program.

    Evidence siRNA and overexpression of myogenin, promoter/transcriptional reporter, immunofluorescence for MTOC protein recruitment in myoblasts

    PMID:34605406

    Open questions at the time

    • Whether other transcription factors cooperate with myogenin for AKAP6 induction unknown
    • Mechanism by which NE-MTOC formation feeds back on differentiation not explored
12. 2024 Medium

    Extension of the AKAP6–calcineurin–NFATc axis to neurons demonstrated that AKAP6-anchored calcineurin activates NFATc4 downstream of BDNF for neuroprotection, broadening AKAP6 function beyond striated muscle.

    Evidence Co-IP of CaN/NFATc4 with AKAP6, dominant-negative disruption, NFATc4 reporter, NFATc4−/− mouse neuroprotection assay

    PMID:39578909

    Open questions at the time

    • Single-lab finding; independent replication in neuronal systems needed
    • Identity of neuroprotective NFATc4 target genes downstream of AKAP6 unknown
13. 2025 Medium

    Identification of AKAP6 as a Wnt/β-catenin target gene whose upregulation enhances PKA-mediated RyR2 phosphorylation and SR calcium leak linked an upstream developmental pathway to AKAP6-dependent calcium dysregulation in cardiac dysfunction.

    Evidence Transcriptomics, Wnt pathway activation/inhibition, RyR2 phosphorylation assay, AKAP6 overexpression/knockdown, Ca²⁺ imaging in cardiomyocytes

    PMID:40097291

    Open questions at the time

    • Whether Wnt-driven AKAP6 upregulation is relevant in human heart failure not established
    • Mechanism of β-catenin regulation of AKAP6 promoter not mapped

• A full structural model of the AKAP6 signalosome — including how simultaneous binding of PKA, PDE4D3, PP2A, ERK5, calcineurin, MEF2, and MTOC components is coordinated on one scaffold — and in vivo phenotyping of AKAP6 knockout in adult heart remain unresolved.
• No high-resolution structure of any AKAP6 multi-protein complex
• No conditional cardiac-specific AKAP6 knockout phenotype published
• Quantitative modeling of the integrated feedback circuits has not been performed