SPHKAP

SPHKAP (SKIP) is a type I-specific A-kinase anchoring protein that scaffolds cAMP/PKA signaling at distinct subcellular compartments to couple it with calcium handling, hormone secretion, and mitochondrial biology (PMID:20394097, PMID:37633939). It is the first mammalian AKAP shown to preferentially bind the type I regulatory subunit PKA-RIα through a characteristic amphipathic helix, and it distributes to the cytoplasm consistent with PKA-RIα (PMID:20394097). SPHKAP localizes type I PKA to ER-associated membrane contact sites by additionally binding ER-resident VAP proteins: in neurons it concentrates type I PKA at Kv2.1-dependent ER-PM junctions to enable reciprocal PKA-Ca2+ regulation and excitation-transcription coupling (PMID:37633939), and in β-cells the active GLP-1 receptor recruits SPHKAP with VAPB at ER-mitochondria contact sites to nucleate a PKA-RIα condensate that drives MICOS phosphorylation and mitochondrial remodelling (PMID:41372122). SPHKAP, AKAP11, and VAPA/B also form a complex that mediates selective-autophagy degradation of the PKA-RI complex at the ER (PMID:41315293, PMID:39803523, PMID:40162211). Beyond scaffolding, SPHKAP regulates endocrine physiology — it is expressed in pancreatic β-cells and intestinal K/L-cells and acts as a negative regulator of glucose-stimulated insulin and incretin secretion, placing it upstream of GIP/GLP-1 signaling (PMID:28396589, PMID:30789757) — and modulates sphingolipid metabolism and apoptotic signaling in a cell-context-dependent manner (PMID:32161116). Reorganization of the PKA-SPHKAP interaction is observed in failing human hearts (PMID:21712045).

1. 2010 High

   Established SPHKAP as the founding mammalian type I-specific AKAP, answering whether an AKAP exists that preferentially anchors PKA-RIα rather than RII.

   Evidence Recombinant protein binding assays, chemical proteomics with cAMP resins in heart/spleen, amphipathic helix modeling, and immunolocalization

   PMID:20394097

   Open questions at the time

   • Subcellular anchoring sites and physiological consequences not yet defined
   • Structural basis of RI selectivity not resolved at atomic level
2. 2011 Medium

   Showed the PKA-SPHKAP scaffold is dynamically remodeled in disease, linking type I AKAP anchoring to human heart failure.

   Evidence Chemical proteomics on human failing vs control heart tissue

   PMID:21712045

   Open questions at the time

   • Whether upregulated anchoring is causal or compensatory unknown
   • No mechanistic readout of altered PKA signaling in cardiomyocytes
3. 2017 Medium

   Defined a physiological role for SPHKAP in endocrine pancreas, showing it negatively regulates glucose-stimulated insulin secretion through a pathway distinct from cAMP/PDE/sphingosine kinase.

   Evidence SKIP-/- mice, glucose tolerance tests, islet ATP/cAMP/insulin measurements, PDE inhibition

   PMID:28396589

   Open questions at the time

   • Molecular mechanism of secretion suppression unidentified
   • β-cell-specific contribution vs systemic effects not separated
4. 2019 Medium

   Extended SPHKAP's secretory role to enteroendocrine cells, placing it genetically upstream of incretin (GIP/GLP-1) signaling and metabolic phenotypes.

   Evidence SKIP-/- mice with GIP genetic depletion cross and GLP-1R antagonism, plasma hormone/lipid measurement

   PMID:30789757

   Open questions at the time

   • Direct molecular target of SPHKAP in K/L-cells unknown
   • Connection to PKA anchoring not established here
5. 2020 Medium

   Showed SPHKAP modulates sphingolipid metabolism and apoptosis in a cell-type-dependent manner, increasing SK activity/ceramide and inactivating ERK in leukemia cells, contrasting earlier fibroblast data.

   Evidence SPHKAP transfection in leukemia lines, UPLC-MS/MS sphingolipidomics, SK activity assays

   PMID:32161116

   Open questions at the time

   • Mechanism reconciling opposing fibroblast vs leukemia effects unresolved
   • Whether sphingolipid effects depend on PKA anchoring untested
6. 2023 High

   Resolved the subcellular mechanism by which SPHKAP anchors type I PKA, showing it tethers PKA-RIα to ER-PM junctions via VAP binding to couple PKA with Ca2+ signaling and excitation-transcription coupling in neurons.

   Evidence Super-resolution/live-cell imaging, Co-IP of SPHKAP with PKA-RI and VAP, neuronal fractionation, electrophysiology and Ca2+ assays

   PMID:37633939

   Open questions at the time

   • Downstream transcriptional targets of the signalosome not enumerated
   • Generalizability beyond Kv2.1-dependent junctions unclear
7. 2025 Medium

   Identified SPHKAP as part of a degradation machinery, showing an SPHKAP-AKAP11-VAPA/B complex routes the PKA-RI complex for selective autophagy at the ER.

   Evidence Multi-omics, Co-IP in mouse models and human induced neurons, cell biology and electrophysiology

   PMID:39803523 PMID:40162211 PMID:41315293

   Open questions at the time

   • Trigger/regulation of degradation not defined
   • Single-lab finding; reciprocal validation across systems limited
8. 2025 Medium

   Linked SPHKAP scaffolding to GLP-1R signaling and mitochondria, showing endosomal GLP-1R/VAPB/SPHKAP at ER-mitochondria contacts nucleates a PKA-RIα condensate driving MICOS phosphorylation and β-cell mitochondrial remodelling.

   Evidence Co-IP, proximity ligation, imaging in β-cell lines and primary islets, condensate detection, MICOS phosphorylation assays after GLP-1RA

   PMID:41372122

   Open questions at the time

   • Biophysical basis of PKA-RIα condensate formation unresolved
   • Relationship to SPHKAP's negative regulation of secretion not reconciled

• How SPHKAP's distinct roles — secretion suppression, PKA anchoring, PKA-RI degradation, and sphingolipid/apoptosis control — are integrated into a single coherent signaling logic remains unresolved.
• No unified model linking secretory phenotype to PKA anchoring
• Tissue-specific determinants of SPHKAP function uncharacterized
• Structure of full SPHKAP complexes undetermined