BORCS8

BORCS8 is a subunit of the BLOC-one-related complex (BORC) that associates with the cytosolic face of lysosomes and drives their anterograde transport toward the cell periphery in non-neuronal cells and toward the distal axon in neurons, acting by sequentially recruiting the small GTPase ARL8 and kinesin-1 and -3 microtubule motors (PMID:38128568). Within this pathway BORCS8/BORC function is required for lysosomal dynamics and autophagy in both non-neuronal cells and iPSC-derived neurons (PMID:41887224). BORCS8 additionally mediates acidification of early endosomes during viral entry, facilitating degradation of virus particles and thereby modulating SARS-CoV-2 infection (PMID:42103727). Disease-associated missense variants (p.Ser29Pro, p.Ser42Pro, p.Thr66Pro) are expressed normally but assemble poorly with other BORC subunits and fail to drive peripheral lysosome distribution, while a frameshift variant (p.Asn26Trpfs*51) is expressed at reduced levels and cannot assemble with BORC or promote lysosome distribution, causing an infantile-onset neurodegenerative disorder (PMID:38128568).

1. 2024 High

   Established that BORCS8 is a functional BORC subunit whose disease variants impair complex assembly and lysosome positioning, linking molecular defects to an inherited neurodegenerative disorder.

   Evidence Heterologous transfection rescue and co-assembly assays in cultured cells with lysosome distribution imaging; zebrafish borcs8 knockout for in vivo validation

   PMID:38128568

   Open questions at the time

   • Structural basis for how the Ser/Thr-to-Pro substitutions disrupt BORC assembly is not resolved
   • Direct biochemical demonstration of ARL8/kinesin recruitment by BORCS8 specifically is not isolated from the complex
   • Mechanism connecting impaired lysosome distribution to neuronal degeneration is not defined
2. 2026 Medium

   Extended BORCS8 function beyond lysosome positioning to early endosome acidification during viral entry, implicating it in antiviral degradation of internalized particles.

   Evidence Arrayed siRNA/shRNA loss-of-function screen across eight viruses with mechanistic follow-up on early endosome acidification

   PMID:42103727

   Open questions at the time

   • Single study with limited orthogonal validation of the acidification mechanism
   • How a BORC/lysosome-positioning subunit controls early endosome pH is not mechanistically explained
   • Whether the antiviral role depends on the same BORC assembly required for lysosome transport is unknown
3. 2026 Medium

   Confirmed within a BLOC1S1-focused study that the BORCS8/BORC pathway is required for anterograde lysosome transport and autophagy in iPSC-derived neurons, reinforcing its neuronal relevance.

   Evidence BLOC1S1-KO functional studies with lysosome transport and autophagy assays in iPSC-derived neurons, referencing BORCS8 disease variants as comparators

   PMID:41887224

   Open questions at the time

   • BORCS8 is referenced indirectly as a comparator rather than directly perturbed in this study
   • The contribution of BORCS8 versus other BORC subunits to the autophagy defect is not separated

• How the same BORCS8/BORC machinery coordinates lysosome positioning, autophagy, and early endosome acidification, and how its disruption produces neurodegeneration, remains unresolved.
• No structural model of BORCS8 within the assembled BORC complex
• Mechanistic link between lysosome mispositioning and neuronal cell death undefined
• Reconciliation of the endosomal acidification role with the established lysosome-transport role