SEPTIN8

SEPTIN8 is a GTP-binding cytoskeletal protein that operates at the interface of vesicle trafficking, regulated exocytosis, and the actin cytoskeleton, partnering with other septins and SNARE machinery to control membrane dynamics in neurons and platelets (PMID:19196426, PMID:33202444, PMID:38051778). At presynaptic terminals it binds VAMP2 directly and suppresses the VAMP2–synaptophysin interaction, thereby gating SNARE complex availability, an interaction modulated by SNAP-25 and accompanied by complex formation with syntaxin1A (PMID:19196426). SEPTIN8 assembles with other septins, including SEPT4 and SEPT5, the former in platelets where both surround α-granules and translocate to the surface upon activation (PMID:15116257, PMID:12023038). In platelets, SEPTIN8 is selectively required for integrin αIIbβ3 activation and α-granule exocytosis but not δ-granule or lysosome release (PMID:33202444). A brain-specific variant, Sept8-204, is palmitoylated at Cys469/Cys470/Cys472 by ZDHHC7 and ZDHHC17 and depalmitoylated by PPT1; this lipid modification licenses F-actin binding, filopodia and neurite outgrowth, and Sept5-dependent formation of synaptophysin-positive vesicle-like structures, while non-palmitoylatable mutants fail in these functions, and in vivo loss of Sept8 or Zdhhc7 produces learning, memory, and anxiety phenotypes (PMID:38051778, PMID:38308620). SEPTIN8 is a direct substrate of MK5, which phosphorylates it at Ser242 and Ser271 (PMID:22649572), and it limits amyloid-β generation by reducing BACE1 levels post-translationally, with a disease-associated transcript variant extending BACE1 half-life and shifting it to recycling endosomes to elevate Aβ (PMID:27084579).

1. 2002 Medium

   Establishing that SEPT8 physically associates with other septins defined it as a member of an oligomeric septin network rather than an isolated protein.

   Evidence Yeast two-hybrid, GST pull-down, and co-IP showing SEPT8–SEPT5 interaction in K-562 cells, plus an interaction with the kinase hPFTAIRE1

   PMID:12023038 PMID:12098780

   Open questions at the time

   • Stoichiometry and architecture of the SEPT8-containing complex not resolved
   • Functional consequence of the hPFTAIRE1 interaction not established
2. 2004 Medium

   Linking SEPT8 to SEPT4 around platelet α-granules connected the protein to regulated secretion in a non-neuronal cell type.

   Evidence Yeast two-hybrid, reciprocal co-IP, and TEM localization in human platelets

   PMID:15116257

   Open questions at the time

   • Causal role in secretion not tested at this stage
   • Mechanism of granule association unknown
3. 2009 Medium

   Identifying VAMP2 as a direct binding partner positioned SEPT8 as a regulator of SNARE complex assembly at presynapses.

   Evidence Yeast two-hybrid screen plus co-IP and interaction assays showing SEPT8 suppresses VAMP2–synaptophysin binding and forms a complex with syntaxin1A

   PMID:19196426

   Open questions at the time

   • Effect on actual neurotransmitter release not measured
   • Whether SNAP-25 competition is physiologically regulated unknown
4. 2010 Low

   Biochemical characterization tested the oligomeric state of the SEPT8 GTP-binding module in isolation.

   Evidence Recombinant expression with gel filtration, DLS, CD, and homology modelling indicating a dimer via the GTP-binding interface

   PMID:20544379

   Open questions at the time

   • No experimental structure beyond a homology model
   • No mutagenesis validating the proposed dimer interface
   • Relevance to native heteromeric filaments unclear
5. 2012 High

   Defining SEPT8 as an MK5 substrate introduced a post-translational regulatory input on the protein.

   Evidence Y2H, GST pull-down, co-IP, FRET, and in vitro kinase assay with peptide-array site mapping to Ser242/Ser271

   PMID:22649572

   Open questions at the time

   • Functional consequence of Ser242/Ser271 phosphorylation in cells not determined
   • Phosphorylation not demonstrated in vivo
6. 2016 Medium

   Connecting SEPT8 to BACE1 turnover and endosomal sorting linked it to amyloid-β generation.

   Evidence Knockdown/overexpression in neuronal cells with BACE1 half-life, recycling-endosome localization, and Aβ/sAPPβ readouts

   PMID:27084579

   Open questions at the time

   • Direct molecular interaction with BACE1 not shown
   • In vivo relevance to Alzheimer pathology untested
7. 2020 High

   A knockout mouse established a selective, causal requirement for Sept8 in platelet integrin activation and α-granule exocytosis.

   Evidence Sept8 knockout platelets analyzed by flow cytometry, aggregation, fibrinogen/lactadherin binding, and thrombin generation assays

   PMID:33202444

   Open questions at the time

   • Molecular mechanism coupling Sept8 to αIIbβ3 inside-out signaling unresolved
   • Why δ-granule and lysosome exocytosis are spared unexplained
8. 2023 High

   Identifying palmitoylation of the brain-specific Sept8-204 variant as a switch for F-actin binding tied a defined lipid modification to neuronal morphogenesis and behavior.

   Evidence Palmitoylation-site mutagenesis (3CA), acyl-RAC, F-actin co-sedimentation, live imaging, and Sept8/Zdhhc7 knockout behavioral assays

   PMID:38051778

   Open questions at the time

   • How palmitoylation structurally enables F-actin binding unknown
   • Behavioral deficits not mechanistically traced to a single circuit
9. 2024 Medium

   Extending the palmitoylation switch to vesicle-like structure formation showed it dynamically controls Sept8-204/Sept5 assembly and synaptophysin co-localization.

   Evidence 3CA mutant, 2-BP inhibition, ZDHHC17/PPT1 knockout cells with synaptophysin co-localization microscopy

   PMID:38308620

   Open questions at the time

   • Identity and cargo of the vesicle-like structures undefined
   • Functional output for synaptic transmission not measured

• How SEPT8's septin-filament assembly, palmitoylation-driven actin coupling, SNARE regulation, and BACE1 sorting are integrated into a single mechanistic logic across neurons and platelets remains unresolved.
• No structure of native SEPT8-containing heterofilaments
• Unknown whether MK5 phosphorylation, palmitoylation, and partner binding are coordinately regulated
• Cell-type-specific determinants of SEPT8 function not defined