EPN2

EPN2 (epsin-2) is a clathrin-mediated endocytic adaptor that, together with its paralog EPN1, functions as a specialized sorting factor linking ubiquitinated cargo to the clathrin coat machinery during receptor-mediated endocytosis. EPN2 contains an N-terminal ENTH domain and interacts with AP-2 via DPW motifs, with clathrin directly, and with Eps15 through C-terminal NPF motifs; it localizes to clathrin-coated vesicle fractions and the peri-Golgi region (PMID:10567358). Combined loss of EPN1 and EPN2 in mice causes embryonic lethality due to impaired Notch signaling activation without disrupting housekeeping endocytosis (PMID:19666558), while endothelial-specific deletion elevates VEGFR2 levels and drives aberrant angiogenesis that is rescued by genetic reduction of VEGFR2 (PMID:24311377). In podocytes, epsins including EPN2 sustain cell adhesion through a Cdc42–SRF–β1 integrin signaling axis, and their loss leads to foot process effacement and albuminuria (PMID:33051360).

1. 1999 High

   Identification and molecular characterization of EPN2 established it as a brain-enriched epsin-1 paralog that bridges cargo to the clathrin coat through its ENTH domain, AP-2-binding DPW motifs, clathrin-binding capacity, and Eps15-binding NPF motifs, answering whether mammals possess more than one epsin isoform and defining its interaction repertoire.

   Evidence cDNA cloning, co-immunoprecipitation, pulldown assays, subcellular fractionation, GFP-fusion imaging, and endocytosis inhibition assay in mammalian cells

   PMID:10567358

   Open questions at the time

   • No assessment of whether EPN2 has non-redundant functions compared with EPN1
   • No structural information on the ENTH domain or cargo-recognition interface
   • Physiological cargo specificity unresolved
2. 2009 High

   Genetic ablation of both Epn1 and Epn2 in mice revealed that epsins are dispensable for housekeeping endocytosis but essential for ligand-dependent Notch receptor activation, resolving the long-standing question of whether mammalian epsins have a cargo-selective signaling role analogous to Drosophila liquid facets.

   Evidence Epn1/Epn2 double-knockout mouse embryos with Notch target gene expression analysis and endocytosis assays in knockout-derived cells

   PMID:19666558

   Open questions at the time

   • Whether EPN2 alone is sufficient for Notch activation remains untested
   • Mechanism by which epsins promote Notch ligand endocytosis versus receptor processing is not defined
   • Tissue-specific requirements beyond embryonic development not addressed
3. 2013 High

   Endothelial-specific deletion of EPN1/EPN2 demonstrated that epsins specifically mediate activated VEGFR2 internalization and degradation, and that aberrant VEGFR2 accumulation is the causal driver of the resulting pathological angiogenesis — establishing a second major cargo-selective endocytic role for epsins.

   Evidence Conditional endothelial Epn1/Epn2 double-knockout mice with VEGFR2 haploinsufficiency rescue, in vitro EC assays, and in vivo wound healing and tumor angiogenesis models

   PMID:24311377

   Open questions at the time

   • Whether EPN2 binds VEGFR2 directly or through an intermediary adaptor is not resolved
   • Relative contribution of EPN2 versus EPN1 in endothelial cells unknown
   • Fate of internalized VEGFR2 (lysosomal vs. proteasomal degradation) not mechanistically dissected
4. 2017 Medium

   EPN2 was identified as a direct target of miR-1224, linking post-transcriptional regulation of EPN2 levels to the balance between VEGF and Notch signaling outputs and providing a mechanism by which EPN2 expression tunes angiogenic responses.

   Evidence Luciferase reporter assay for direct miR-1224 targeting, siRNA knockdown, EPN2 overexpression rescue, and tube-formation assays in HUVECs

   PMID:28717225

   Open questions at the time

   • Single-lab finding; independent confirmation of miR-1224–EPN2 axis in vivo is lacking
   • Quantitative relationship between EPN2 protein levels and angiogenic output not defined
   • Whether miR-1224 also targets EPN1 was not assessed
5. 2020 High

   Podocyte-specific loss of all three epsins (including EPN2) revealed an unexpected role for epsins in maintaining cell adhesion through a Cdc42–SRF–β1 integrin signaling axis, extending epsin function beyond classical endocytic cargo sorting to adhesion-dependent signaling in specialized epithelial cells.

   Evidence Triple-knockout mice with albuminuria, electron microscopy, primary podocyte adhesion/spreading assays, Cdc42 activation measurement, and phenocopy by podocyte-specific Srf deletion

   PMID:33051360

   Open questions at the time

   • Individual contributions of EPN1, EPN2, and EPN3 in podocytes not dissected
   • Mechanism linking epsin endocytic activity to Cdc42 activation remains undefined
   • Whether the Cdc42–SRF axis operates in non-podocyte cell types downstream of epsins is unknown

• Open question: how EPN2 distinguishes cargo-selective endocytic events (Notch ligand, VEGFR2) from housekeeping endocytosis at the molecular level, and what non-redundant functions EPN2 has versus EPN1 and EPN3, remain unresolved.
• No structural basis for cargo selectivity of EPN2
• Isoform-specific knockout phenotypes for EPN2 alone are lacking
• Role of EGF-stimulated phosphorylation of EPN2 is functionally uncharacterized