HPCAL1

HPCAL1 (also known as VILIP-3) is a neuronal calcium sensor protein that employs a calcium-myristoyl switch for reversible, calcium-dependent translocation to specific membrane compartments including the Golgi, enabling cell-type-specific signaling (PMID:14664824, PMID:12445467). HPCAL1 functions as a selective autophagy receptor that, upon PRKCQ-mediated phosphorylation at Thr149, engages LC3 via a non-classical LIR motif to target CDH2 for lysosomal degradation, reducing membrane tension and promoting lipid peroxidation during ferroptosis (PMID:35403545). HPCAL1 also binds the mitophagy receptor BNIP3 in a calcium-dependent manner, stabilizing BNIP3 and amplifying mitophagy-driven ROS production to further potentiate ferroptosis (PMID:41482082). Beyond autophagy, HPCAL1 interacts with PHOX2B to regulate its nuclear translocation and promote sympathetic neuronal differentiation (PMID:23873030), inhibits TGF-β/Smad2 signaling by controlling Smad2 ubiquitination via its EF-hand 4 domain (PMID:39910671), and activates Wnt/β-catenin–ERK signaling to drive glioblastoma proliferation (PMID:30843345).

1. 2002 Medium

   Establishing that HPCAL1 has calcium-dependent signaling properties distinct from its paralog VILIP-1 answered whether NCS family members are functionally redundant, showing they activate different cGMP pathways and bind different partners.

   Evidence Subcellular fractionation with calcium titration, cGMP signaling assays, and pull-down interaction screens in cell lines

   PMID:12445467

   Open questions at the time

   • Specific HPCAL1 interaction partners not identified
   • Downstream effectors of HPCAL1-specific cGMP signaling unknown
   • In vivo relevance not tested
2. 2003 Medium

   Demonstrating that HPCAL1 undergoes a fast, reversible calcium-myristoyl switch with translocation to Golgi and other compartments resolved how HPCAL1 achieves membrane targeting specificity distinct from VILIP-1.

   Evidence GFP-tagged HPCAL1 live-cell imaging in cell lines and primary hippocampal neurons with calcium-dependent translocation assays

   PMID:14664824

   Open questions at the time

   • Membrane receptor or tethering factor at the Golgi not identified
   • Whether myristoylation is required for all downstream signaling functions untested
   • Endogenous dynamics in non-neuronal cells uncharacterized
3. 2013 High

   Identifying HPCAL1 as a physical interactor of PHOX2B that undergoes calcium-independent nuclear translocation upon PHOX2B binding, and showing that HPCAL1 knockdown impairs neurite outgrowth, established a direct role for HPCAL1 in sympathetic neuronal differentiation.

   Evidence Yeast two-hybrid screen, reciprocal co-immunoprecipitation, subcellular localization imaging, shRNA knockdown with neurite outgrowth readouts in neuroblastoma cells

   PMID:23873030

   Open questions at the time

   • Transcriptional targets jointly regulated by HPCAL1-PHOX2B unknown
   • Whether HPCAL1 modulates PHOX2B DNA-binding activity untested
   • In vivo neuronal differentiation role not confirmed in animal models
4. 2019 Medium

   Showing that HPCAL1 activates Wnt/β-catenin signaling through GSK3β modulation and requires downstream ERK activity to drive CCND1 and c-Myc transcription revealed a proliferative signaling axis in glioblastoma.

   Evidence Overexpression and shRNA knockdown in GBM cell lines and xenograft models with Western blotting for β-catenin, GSK3β, and ERK phosphorylation

   PMID:30843345

   Open questions at the time

   • Direct molecular target linking HPCAL1 to GSK3β or β-catenin not identified
   • Whether calcium-myristoyl switch is required for Wnt pathway activation unknown
   • Single-lab finding not independently replicated
5. 2022 High

   Identifying HPCAL1 as a selective autophagy receptor that delivers CDH2 for lysosomal degradation via PRKCQ phosphorylation and a non-classical LIR motif resolved the molecular mechanism coupling autophagy to membrane tension reduction during ferroptosis.

   Evidence Quantitative proteomics, site-directed mutagenesis of Thr149 and LIR motif, in vitro kinase assays, co-immunoprecipitation, genetic knockdown/knockout in cell lines and mouse pancreatitis/tumor models, drug screen of 4208 compounds

   PMID:35403545

   Open questions at the time

   • Structural basis of non-classical LIR–LC3 interaction unresolved
   • How HPCAL1 selectively recognizes CDH2 cargo remains unknown
   • Whether HPCAL1 mediates autophagy of additional cargo beyond CDH2 untested
6. 2025 Medium

   Demonstrating that HPCAL1 inhibits TGF-β signaling by binding Smad2 through its EF-hand 4 domain and controlling Smad2 ubiquitination established HPCAL1 as a negative regulator of hepatic stellate cell activation and liver fibrosis.

   Evidence Co-immunoprecipitation with domain mapping, luciferase reporter assays, immunofluorescence, in vivo and in vitro fibrosis models, miR-342-3p target validation

   PMID:39910671

   Open questions at the time

   • E3 ubiquitin ligase recruited by HPCAL1 to ubiquitinate Smad2 not identified
   • Whether calcium binding by EF-hand 4 is required for Smad2 interaction untested
   • Single-lab observation awaiting independent replication
7. 2025 Medium

   Showing that HPCAL1 binds and stabilizes the mitophagy receptor BNIP3 in a calcium-dependent manner, enhancing LC3-II interaction and driving excessive mitophagy-ROS-ferroptosis, established a second autophagy-related mechanism by which HPCAL1 promotes ferroptotic cell death.

   Evidence Co-immunoprecipitation, ROS and lipid peroxidation fluorescent probes, mitochondrial membrane potential and autophagic flux assays, genetic disruption in mouse I/R and rat IEC-6 H/R models

   PMID:41482082

   Open questions at the time

   • Whether the HPCAL1-BNIP3 interaction requires myristoylation or specific EF-hand domains not determined
   • Relationship between CDH2-targeted autophagy and BNIP3-mediated mitophagy in the same cell not clarified
   • Single-lab finding in ischemia-reperfusion context, generalizability unknown

• A unified structural and regulatory model explaining how HPCAL1 selectively engages diverse cargo (CDH2, BNIP3, Smad2, PHOX2B) and switches between autophagy receptor, signaling modulator, and nuclear functions remains unresolved.
• No crystal or cryo-EM structure of HPCAL1 in complex with any partner
• How calcium-myristoyl switching versus calcium-independent nuclear translocation are regulated in the same cell is unknown
• Whether HPCAL1 autophagy receptor and signaling functions are coordinated or context-exclusive is unresolved