HPCAL1

HPCAL1 (VILIP-3) is a neuronal calcium sensor that uses an EF-hand calcium-myristoyl switch to undergo fast, reversible calcium-dependent translocation between subcellular compartments, distinguishing its behavior and interaction partners from the related VILIP-1 (PMID:14664824, PMID:12445467). Beyond calcium sensing, HPCAL1 functions as a selective autophagy receptor: during ferroptosis it is phosphorylated on Thr149 by PRKCQ and, through a non-classical LIR motif (residues 46-51), targets CDH2 for autophagic degradation, lowering membrane tension and promoting lipid peroxidation (PMID:35403545). It also binds the mitophagy receptor BNIP3 in a calcium-dependent manner, stabilizing BNIP3 and its LC3-II interaction to drive excessive mitophagy and a ROS burst that potentiates ferroptosis (PMID:41482082). HPCAL1 promotes Wnt/β-catenin signaling by augmenting β-catenin accumulation and nuclear localization and by forming β-catenin complexes with TCF7 or p65 to transactivate Wnt ligands, supporting proliferation in glioblastoma and colorectal cancer (PMID:30843345, PMID:42091587). Conversely, in hepatic stellate cells it suppresses TGF-β signaling by promoting Smad2 ubiquitination through its EF-hand 4 domain, acting as a fibrogenesis suppressor (PMID:39910671). In the nervous system, HPCAL1 binds PHOX2B and is required for neurite outgrowth and sympathetic neuronal differentiation (PMID:23873030).

1. 1999 Medium

   Establishing where HPCAL1 protein resides was the first step in defining its biological context; immunohistochemistry showed it is a neuron-restricted protein expressed in specific brain populations.

   Evidence Immunohistochemistry with specific polyclonal antisera on human brain tissue

   PMID:10851344

   Open questions at the time

   • Does not address molecular function
   • No mechanism for the cell-type-restricted expression pattern
2. 2002 Medium

   To determine whether HPCAL1 is a functional calcium sensor distinct from related VILIPs, biochemical work showed it has unique calcium-dependent membrane association, distinct interaction partners, and activates a different cGMP pathway than VILIP-1.

   Evidence Subcellular fractionation, calcium-dependent membrane association and cGMP signaling assays comparing two proteins

   PMID:12445467

   Open questions at the time

   • Interaction partners not individually identified
   • cGMP pathway components downstream unresolved
3. 2003 Medium

   The mechanism of calcium-triggered relocalization was resolved by demonstrating a fast reversible calcium-myristoyl switch driving translocation to distinct compartments, differing from VILIP-1.

   Evidence Live-cell imaging of GFP-tagged proteins in cell lines and hippocampal neurons with calcium stimulation

   PMID:14664824

   Open questions at the time

   • Functional consequence of translocation not defined
   • Target compartments and effectors at each site unknown
4. 2013 High

   Linking HPCAL1 to a developmental program, a yeast two-hybrid screen identified PHOX2B as a partner whose disease mutants disrupt HPCAL1 localization, and knockdown impaired sympathetic neuronal differentiation.

   Evidence Yeast two-hybrid screen, subcellular localization imaging, shRNA knockdown with transcriptional profiling and neurite outgrowth assay

   PMID:23873030

   Open questions at the time

   • Direct biochemical mechanism by which HPCAL1 drives differentiation not defined
   • Whether PHOX2B-induced nuclear translocation of HPCAL1 alters transcription directly is unresolved
5. 2019 Medium

   The first oncogenic mechanism placed HPCAL1 upstream of Wnt/β-catenin and ERK signaling, showing it stabilizes β-catenin and drives proliferation in glioblastoma.

   Evidence Ectopic expression and shRNA knockdown in GBM cells in vitro and in xenografts, signaling Western blots, ERK inhibition and β-catenin rescue

   PMID:30843345

   Open questions at the time

   • Direct molecular target of HPCAL1 in the pathway not defined
   • Relationship between calcium-sensing activity and signaling role unaddressed
6. 2022 High

   HPCAL1 was redefined as a selective autophagy receptor, establishing a phosphorylation-dependent (PRKCQ/Thr149) and LIR-dependent mechanism that degrades CDH2 to sensitize cells to ferroptosis.

   Evidence Quantitative proteomics, site-directed mutagenesis of Thr149 and the LIR motif, drug screening, and genetic/pharmacological models in mice

   PMID:35403545

   Open questions at the time

   • How calcium sensing relates to receptor function not defined
   • Generality of HPCAL1 cargo selection beyond CDH2 unknown
7. 2025 Medium

   Two studies extended the mechanism: HPCAL1 suppresses TGF-β fibrogenesis via EF-hand 4-dependent Smad2 ubiquitination, and drives ferroptosis by calcium-dependent stabilization of the mitophagy receptor BNIP3.

   Evidence Co-IP, ubiquitination and luciferase assays in hepatic stellate cells (Smad2); Co-IP, ROS/lipid peroxidation, mitochondrial and autophagic flux assays in I/R and H/R models (BNIP3)

   PMID:39910671 PMID:41482082

   Open questions at the time

   • Whether HPCAL1 directly recruits a ubiquitin ligase to Smad2 is unresolved
   • Structural basis of calcium-dependent BNIP3 binding undefined
8. 2026 Medium

   The Wnt mechanism was refined by showing HPCAL1 forms distinct β-catenin complexes with TCF7 or p65 to differentially transactivate specific Wnt ligands in colorectal cancer.

   Evidence Reciprocal co-IP, RNA sequencing, knockdown/overexpression in CRC cells and xenografts, pharmacological inhibition with desloratadine

   PMID:42091587

   Open questions at the time

   • Whether HPCAL1 binds β-catenin directly or via an adaptor is unresolved
   • How complex choice (TCF7 vs p65) is regulated unknown

• How HPCAL1's core calcium-myristoyl sensor activity mechanistically governs its diverse roles as an autophagy receptor, ubiquitination regulator, and transcriptional cofactor remains unresolved.
• No unifying model linking calcium sensing to receptor and signaling functions
• No structural data on the relevant complexes
• Tissue-specific selection between opposing pro-ferroptotic and anti-fibrotic roles undefined