TSPAN9

TSPAN9 is a tetraspanin membrane protein that organizes specialized membrane microdomains and tunes signaling and membrane-trafficking events across diverse cell types (PMID:18795891, PMID:28032533). In platelets, TSPAN9 is a component of tetraspanin-enriched microdomains containing the collagen receptor GPVI and integrin α6β1, and it associates directly with GPVI; genetic deletion in mice impairs GPVI-induced platelet aggregation, secretion, and tyrosine phosphorylation, acting not by altering GPVI clustering but by promoting GPVI lateral diffusion within the membrane (PMID:18795891, PMID:28032533). TSPAN9 also localizes to the plasma membrane and endosomes, where it is required for the early-endosomal membrane fusion of certain enveloped viruses without affecting endosomal pH, protease activity, or cargo delivery, and its depletion shifts late-endosomal protein composition (PMID:26865714). In disease contexts TSPAN9 modulates intracellular signaling and autophagy: in cardiomyocytes it binds p62/SQSTM1, impairs its cargo-receptor function, and suppresses autophagic flux, thereby exacerbating pressure-overload cardiac hypertrophy and failure in a manner dependent on autophagy and p62 (PMID:40406987). In cancer cells TSPAN9 engages partners including β1 integrin and PI3K, and its effects on migration, invasion, and drug resistance are context-dependent—suppressing ERK1/2 signaling and EMILIN1-cooperative invasion in gastric cancer while activating FAK/Ras/ERK1/2 signaling to drive osteosarcoma metastasis (PMID:27177197, PMID:31242895, PMID:31911756, PMID:35280793).

1. 2009 Medium

   Established that TSPAN9 is a bona fide platelet surface tetraspanin and defined its selective microdomain partners, addressing whether it physically associates with specific platelet receptors.

   Evidence Co-immunoprecipitation, antibody/flow cytometry, and SAGE/microarray in megakaryocytes and platelets

   PMID:18795891

   Open questions at the time

   • Functional consequence of the GPVI/α6β1 association not tested
   • Mechanism of selective partner inclusion/exclusion unresolved
2. 2016 High

   Resolved how TSPAN9 supports GPVI function by showing it promotes receptor lateral diffusion rather than clustering, settling the mechanistic basis of its platelet role.

   Evidence Tspan9 gene-trap knockout mice, reciprocal Co-IP, super-resolution imaging, single-particle tracking by TIRF, aggregation/secretion and phosphorylation assays

   PMID:28032533

   Open questions at the time

   • Structural basis of TSPAN9–GPVI interaction unknown
   • Whether TSPAN9 affects other platelet receptors in vivo untested
3. 2016 Medium

   Identified a trafficking role for TSPAN9 distinct from its platelet function, showing it is required for early-endosomal viral membrane fusion and influences endosomal protein composition.

   Evidence siRNA depletion in U-2 OS cells, confocal colocalization, membrane fusion and multi-virus infection assays, endosomal pH/protease measurements, endosomal marker westerns

   PMID:26865714

   Open questions at the time

   • Molecular mechanism by which TSPAN9 confers fusion competence unknown
   • Direct endosomal interaction partners not identified
4. 2016 Medium

   Provided first evidence that TSPAN9 acts as a migration/invasion suppressor in gastric cancer via ERK1/2 pathway downregulation.

   Evidence TSPAN9 overexpression in SGC7901 cells with proliferation, wound-healing, Transwell assays, pERK1/2 western, MMP-9/uPA ELISA

   PMID:27177197

   Open questions at the time

   • No pathway epistasis rescue
   • Mechanism linking TSPAN9 to ERK1/2 not defined
5. 2019 Medium

   Linked TSPAN9 to EMILIN1 as a cooperating partner that amplifies its tumor-suppressive effect in gastric cancer.

   Evidence Immunofluorescence co-localization, Co-IP, western blot, wound-healing and invasion assays with TSPAN9 and EMILIN1 manipulation

   PMID:31242895

   Open questions at the time

   • Direct vs indirect nature of TSPAN9–EMILIN1 interaction unresolved
   • Mechanism of reciprocal expression upregulation unknown
6. 2020 Medium

   Connected TSPAN9 to PI3K/AKT/mTOR signaling and autophagy-mediated chemoresistance, proposing direct inhibition of PI3K catalytic activity.

   Evidence Co-IP, PI3K/AKT/mTOR pathway westerns, CCK-8, gain/loss-of-function in parental and 5-FU-resistant gastric cancer cells

   PMID:31911756

   Open questions at the time

   • PI3K catalytic inhibition not reconstituted in vitro
   • Apparent contradiction with tumor-suppressive role across gastric cancer studies not reconciled
7. 2022 Medium

   Demonstrated a pro-metastatic role for TSPAN9 in osteosarcoma via β1 integrin engagement and FAK/Ras/ERK1/2 activation, contrasting its gastric cancer phenotype.

   Evidence Mass spectrometry, Co-IP, pathway westerns, CCK-8/Transwell/wound-healing assays, in vivo tail-vein metastasis in nude mice with knockdown/overexpression

   PMID:35280793

   Open questions at the time

   • Basis for opposite signaling outcomes across cancer types unexplained
   • Direct β1 integrin binding interface not mapped
8. 2025 High

   Established a cardiac role through which TSPAN9 binds p62/SQSTM1, impairs cargo-receptor function, and suppresses autophagy to drive hypertrophy, with genetic epistasis confirming the pathway.

   Evidence Co-IP, AAV9 cardiac overexpression/knockdown in mouse TAC model, neonatal rat cardiomyocyte hypertrophy model, autophagy-inhibition and p62-knockdown rescue epistasis

   PMID:40406987

   Open questions at the time

   • Molecular mechanism by which TSPAN9 impairs p62 cargo function unknown
   • Relationship to TSPAN9's PI3K-autophagy axis in cancer not addressed
9. 2026 Low

   Implicated TSPAN9 in mitochondrial quality control by promoting mitocytosis in senescent chondrocytes alongside KIF5B and TSPAN4.

   Evidence Lentiviral TSPAN9 overexpression in IL-1β-induced rat chondrocytes, SA-β-gal, JC-1, TEM, qRT-PCR

   PMID:41981440

   Open questions at the time

   • Interaction with KIF5B/TSPAN4 not demonstrated by Co-IP or biochemistry
   • Abstract-level detail only; mechanism of mitocytosis induction unknown

• How a single tetraspanin produces opposite signaling outcomes (suppressive vs activating ERK; autophagy-suppressing vs autophagy-promoting) across cell types remains unresolved.
• No unifying biochemical model reconciling context-dependent partner engagement
• No structural data on TSPAN9 microdomain organization
• Direct vs adaptor-mediated nature of most reported interactions undefined