GPC4

GPC4 is a GPI-anchored heparan sulfate proteoglycan that functions as a cell-surface co-receptor modulating multiple signaling pathways, including non-canonical Wnt/planar cell polarity, β-catenin, TLR4/NF-κB, and Sonic Hedgehog signaling. In zebrafish, GPC4 is required for non-canonical Wnt signaling during convergent-extension cell movements and chondrocyte maturation in palate morphogenesis and endochondral ossification (PMID:27287801, PMID:27908786). CD36 promotes proteasome-dependent ubiquitination and degradation of GPC4, thereby suppressing β-catenin/c-myc-driven glycolysis in colorectal cancer (PMID:31484922), while in glial cells GPC4 activates TLR4/NF-κB inflammatory signaling (PMID:37492748, PMID:41724289), and Aβ-induced shedding of GPC4 from microglia facilitates tau aggregate seeding in neurons (PMID:40883746). Loss-of-function mutations in GPC4 that eliminate the GPI anchor and key glycosylation sites cause Keipert syndrome, a craniofacial-digital disorder recapitulated by Gpc4 knockout mice (PMID:30982611).

1. 1995 High

   Establishing GPC4's molecular identity as a GPI-anchored heparan sulfate proteoglycan resolved what class of molecule GPC4 encodes and how it is tethered to the cell surface.

   Evidence cDNA transfection in MDCK cells with biochemical characterization of GPI anchor and HS attachment sites

   PMID:7657705

   Open questions at the time

   • No signaling function yet assigned
   • No in vivo requirement demonstrated
   • Heparan sulfate chain composition and length uncharacterized
2. 2000 Medium

   Demonstration that a conserved tryptophan residue is required for glypican processing and surface HS presentation revealed a structural requirement shared across the glypican family including GPC4.

   Evidence Recombinant GPC3 W296R mutant expression and surface HS functional assay, with conservation-based inference to GPC4

   PMID:10814714

   Open questions at the time

   • Direct mutagenesis of GPC4 at this residue not performed
   • Processing mechanism (protease identity, ER/Golgi steps) undefined
3. 2016 High

   Genetic epistasis in zebrafish revealed that GPC4 functions specifically in non-canonical Wnt signaling to drive convergent-extension cell intercalation in chondrocytes and to regulate the timing of endochondral ossification, establishing GPC4 as a co-receptor in PCP signaling.

   Evidence Zebrafish gpc4 mutant analysis with epistasis to wnt5b, wnt9a, wls, and frzb mutants across two independent studies

   PMID:27287801 PMID:27908786

   Open questions at the time

   • Direct biochemical interaction between GPC4 and Wnt ligands not shown
   • Whether heparan sulfate chains are essential for PCP signal transduction not tested
   • Mammalian in vivo PCP role not directly demonstrated
4. 2019 High

   Identification of CD36 as a GPC4 interactor that triggers its proteasomal degradation revealed a mechanism by which GPC4 protein levels are controlled and linked GPC4 abundance to β-catenin/c-myc-driven glycolytic reprogramming in colorectal cancer.

   Evidence Reciprocal Co-IP, proteasome inhibitor rescue, knockdown, and in vivo tumor xenograft models

   PMID:31484922

   Open questions at the time

   • E3 ubiquitin ligase responsible for GPC4 ubiquitination not identified
   • Whether GPC4 directly binds β-catenin or acts via an intermediate not resolved
   • How CD36-GPC4 interaction triggers ubiquitination mechanistically unclear
5. 2019 High

   Human genetics linked GPC4 loss-of-function truncating mutations to Keipert syndrome and Gpc4 knockout mice recapitulated the craniofacial-digital phenotype, establishing GPC4 as the causal gene for this Mendelian disorder.

   Evidence Whole-exome sequencing of affected families, recombinant protein stability assays, Gpc4 KO mouse phenotyping

   PMID:30982611

   Open questions at the time

   • Which signaling pathway (Wnt, Shh, FGF) is primarily disrupted in Keipert syndrome not determined
   • Whether partial loss-of-function alleles produce milder phenotypes unknown
6. 2022 Medium

   Showing that GPC4 activates TLR4/NF-κB inflammatory signaling in microglia, with its own mRNA stability regulated by FTO-mediated m6A demethylation via YTHDF3, connected GPC4 to innate immune activation and revealed an epitranscriptomic layer of GPC4 regulation.

   Evidence RNA stability assays, siRNA knockdown, FTO inhibitor treatment, and rescue experiments in microglia during autoimmune uveitis

   PMID:37492748

   Open questions at the time

   • Whether GPC4 directly binds TLR4 or acts through HS-mediated ligand presentation not distinguished
   • Single-lab finding without independent replication
   • In vivo validation of m6A-GPC4 axis limited
7. 2025 Medium

   Multiple studies converged to show GPC4 functions in neuroinflammation and neurodegeneration: Aβ-induced GPC4 upregulation and shedding from microglia facilitates tau seeding in neurons, and GPC4 knockdown in astrocytes suppresses TLR4/NF-κB signaling in ischemic stroke.

   Evidence Microglia surfaceome profiling, tau uptake/seeding assays, Drosophila in vivo amyloidosis model, astrocyte siRNA knockdown with signaling readouts

   PMID:40883746 PMID:41724289

   Open questions at the time

   • Molecular mechanism of GPC4 shedding (sheddase identity) not identified
   • Whether GPC4-mediated tau seeding is HS-dependent not tested
   • In vivo mammalian validation of GPC4 in tauopathy lacking

• Key unresolved questions include the structural basis of GPC4 co-receptor function, the identity of the E3 ligase mediating its ubiquitination, whether HS chains are required for each of its signaling roles, and how GPC4 shedding is regulated in neurodegeneration.
• No crystal or cryo-EM structure of GPC4 or GPC4–ligand complexes
• HS chain requirements not dissected for individual signaling pathways
• Sheddase(s) responsible for GPC4 release not identified