FNDC8

FNDC8 is a testis-enriched protein that localizes to the perinuclear theca (PT) of spermatids, where it physically interacts with CCIN and ACTL7A to maintain PT structural integrity during spermiogenesis (PMID:41169243). Genetic disruption of Fndc8 in mice causes male infertility characterized by acrosome detachment from the nuclear membrane and sperm head surface collapse, with concomitant destabilization of its binding partners CCIN and ACTL7A (PMID:41169243).

1. 2025 Medium

   The first functional characterization of FNDC8 established it as a PT-localized scaffolding protein required for sperm head morphogenesis, resolving that its loss leads to acrosome detachment, sperm head collapse, and male infertility through destabilization of CCIN and ACTL7A.

   Evidence Fndc8 knockout mouse with fertility, morphology, immunofluorescence localization, and co-immunoprecipitation assays

   PMID:41169243

   Open questions at the time

   • Single-lab, single-study finding awaiting independent replication
   • Molecular basis of FNDC8–CCIN and FNDC8–ACTL7A interactions (e.g., domain mapping, structural data) is unknown
   • Whether FNDC8 has additional roles outside spermiogenesis or interacts with other PT components has not been explored

• It remains unknown how FNDC8 is recruited to the perinuclear theca, whether its fibronectin type III domain directly mediates its protein–protein interactions, and whether FNDC8 dysfunction contributes to human male infertility.
• No structural or domain-function analysis of the FNIII domain in FNDC8
• No human genetic or clinical data linking FNDC8 variants to infertility
• Temporal hierarchy of PT assembly relative to FNDC8, CCIN, and ACTL7A is unresolved