IGSF8

IGSF8 (EWI-2) is a cell-surface immunoglobulin superfamily transmembrane protein that organizes tetraspanin-enriched microdomains and modulates integrin, growth factor receptor, and immune receptor signaling. It forms highly stoichiometric, direct complexes with tetraspanins CD9 and CD81 — requiring a transmembrane glycine zipper motif and palmitoylation of cytoplasmic cysteines — and links these microdomains to the actin cytoskeleton through direct binding of ERM proteins and α-actinin, regulated by phosphoinositide lipids (PMID:11504738, PMID:21343309, PMID:16690612, PMID:22689882). IGSF8 negatively regulates cell migration, EGFR clustering/endocytosis, and TGF-β signaling by sequestering CD9/CD81 away from TGF-β receptors and by promoting endolysosomal degradation of growth factor receptors and integrins via TFEB-dependent lysosomogenesis, thereby suppressing EMT and metastasis (PMID:25656846, PMID:35773608, PMID:35339615). IGSF8 also functions as an innate immune checkpoint that suppresses NK cell cytotoxicity through direct interaction with inhibitory receptors KIR3DL2 and Klra9, and serves as a presynaptic regulator in hippocampal mossy fiber circuits controlling bouton filopodia density and excitation/inhibition balance (PMID:38657602, PMID:33057002).

1. 2001 High

   Identification of IGSF8 as a direct, highly stoichiometric partner of tetraspanins CD9 and CD81 established it as a core component of tetraspanin-enriched microdomains, resolving which IgSF proteins are bona fide tetraspanin partners versus fortuitous co-precipitants.

   Evidence Co-purification, mass spectrometry, chemical cross-linking, and immunodepletion in multiple cell lines; domain-deletion mapping of the Ig3–Ig4 domains required for CD81 binding

   PMID:11504738 PMID:11673522

   Open questions at the time

   • Structural basis of the IGSF8–tetraspanin interface not resolved at atomic level
   • Whether IGSF8 can engage CD9 and CD81 simultaneously or competitively was not determined
2. 2003 High

   IGSF8 was shown to modulate integrin-dependent adhesion and migration by reorganizing tetraspanin–integrin complexes, revealing a mechanism by which IgSF8 indirectly controls integrin function through tetraspanin scaffolding rather than direct integrin binding.

   Evidence Overexpression and cytoplasmic-tail mutant analysis in cell reaggregation, migration, and shear-flow adhesion assays; co-IP with α3β1 and α4β1 integrins; chimeric CD9/CD82 domain mapping of the EWI-2 binding regions on CD9

   PMID:12708969 PMID:12750295 PMID:14662754 PMID:15070678

   Open questions at the time

   • Whether IGSF8 controls integrin signaling through conformational or purely spatial mechanisms was not distinguished
   • Endogenous stoichiometry of IGSF8–tetraspanin–integrin complexes in primary cells not established
3. 2006 High

   Discovery that IGSF8 directly binds ERM proteins via a basic cytoplasmic stretch and that its silencing augments ERM phosphorylation and cell migration established IGSF8 as a negative regulator linking tetraspanin microdomains to the cortical actin cytoskeleton.

   Evidence GST pulldown (direct binding), confocal co-localization at microspikes/uropods, dominant-negative moesin, siRNA knockdown

   PMID:16690612

   Open questions at the time

   • Whether ERM binding and tetraspanin binding are mutually exclusive or cooperative was not resolved
   • Upstream signals controlling IGSF8–ERM dissociation remain undefined
4. 2011 High

   Mutagenesis of the transmembrane glycine zipper and cytoplasmic palmitoylation sites, together with phosphoinositide-binding mapping, revealed the post-translational logic governing IGSF8 assembly into tetraspanin webs and its regulation of migration and adhesion.

   Evidence Site-directed mutagenesis of TM glycine zipper and palmitoylation cysteines, in vitro lipid-binding assays, palmitoylation assays, functional migration/lamellipodia assays

   PMID:21343309 PMID:21609323

   Open questions at the time

   • Identity of the palmitoyl acyltransferase(s) modifying IGSF8 unknown
   • How PIP binding and palmitoylation are dynamically coordinated in live cells not shown
5. 2012 High

   Identification of α-actinin as a PIP2-regulated direct cytoplasmic partner of IGSF8, and the finding that IGSF8 localizes to the immune synapse cSMAC where its loss elevates IL-2 secretion and HIV infectivity, established IGSF8 as a functional brake on T cell activation and viral entry.

   Evidence Mass spectrometry pulldown, co-IP, PIP2 regulation, confocal at immune synapse, siRNA/cytoplasmic-truncation functional assays, cell–cell fusion assay

   PMID:22689882

   Open questions at the time

   • Whether α-actinin and ERM binding are competitive or spatially segregated was not tested
   • Mechanism by which IGSF8 restrains IL-2 signaling beyond clustering is undefined
6. 2015 High

   Epistasis experiments demonstrated that IGSF8 negatively regulates TGF-β signaling by sequestering CD9/CD81 away from TGF-β receptors, so that IGSF8 loss frees tetraspanins to promote TβR2–TβR1 association, driving EMT and metastasis — a new signaling-level mechanism for IGSF8's tumor-suppressive activity.

   Evidence RNAi/overexpression epistasis (CD9/CD81 depletion rescues EWI-2 knockdown phenotype), co-IP of TGF-β receptor complexes, in vivo lung metastasis model

   PMID:25656846

   Open questions at the time

   • Whether IGSF8 physically contacts TGF-β receptors or acts exclusively through tetraspanin redistribution not clarified
   • Applicability beyond melanoma not tested
7. 2020 High

   Conditional presynaptic knockout of Igsf8 in hippocampal mossy fibers revealed a non-immune, neurodevelopmental role: IGSF8 is required for bouton filopodia formation and excitation/inhibition balance in CA3 circuits, establishing it as a synaptic organizer.

   Evidence Presynaptic conditional knockout, electron microscopy, electrophysiology, cell-surface interactome screening

   PMID:33057002

   Open questions at the time

   • Trans-synaptic binding partners mediating filopodia formation not fully identified
   • Whether IGSF8 synaptic function depends on tetraspanin association was not tested
8. 2022 High

   Super-resolution imaging and gene ablation showed that IGSF8 prevents EGFR clustering and endocytosis, and that IGSF8 promotes endolysosomal degradation of growth factor receptors and integrins via TFEB-dependent lysosomogenesis, providing two complementary mechanisms for receptor downregulation and tumor suppression.

   Evidence Single-molecule localization microscopy (EGFR clustering), endocytosis assays, TFEB nuclear localization assay, EGFR/ERK inhibitor rescue, in vivo xenograft and lung cancer models

   PMID:35339615 PMID:35773608

   Open questions at the time

   • How IGSF8 promotes TFEB nuclear retention mechanistically is unclear
   • Whether EGFR clustering prevention and lysosome-mediated degradation operate in the same or distinct cell contexts is unresolved
9. 2024 High

   CRISPR screening and direct binding studies revealed IGSF8 as an innate immune checkpoint: tumor-expressed IGSF8 engages inhibitory NK receptors KIR3DL2/Klra9 to suppress NK cell killing, and antibody blockade of this axis enhances antitumor immunity alone or with anti-PD1.

   Evidence CRISPR screen, co-immunoprecipitation/interaction assay, antibody blockade, in vitro NK killing assay, syngeneic tumor models

   PMID:38657602

   Open questions at the time

   • Structural basis of IGSF8–KIR3DL2 interaction not determined
   • Whether tetraspanin association modulates the immune checkpoint function is unknown
   • Patient-level correlation between IGSF8 expression and NK cell infiltration/response to immunotherapy not yet established

• Key unresolved questions include the structural basis of IGSF8's multivalent interactions (tetraspanins, KIR3DL2, ERM, α-actinin), whether its immune checkpoint and tetraspanin-scaffolding functions are mechanistically coupled, and the identity of its trans-synaptic partners in mossy fiber circuits.
• No atomic-resolution structure of IGSF8 or its complexes available
• Functional interdependence of immune checkpoint and tetraspanin-organizing roles untested
• Trans-synaptic ligands at mossy fiber synapses remain unidentified