FSIP2

FSIP2 is a fibrous sheath protein required for sperm flagellar assembly and acrosome biogenesis during spermiogenesis, and its loss causes the multiple morphological abnormalities of the sperm flagella (MMAF) phenotype (PMID:30137358, PMID:34125190). Loss-of-function variants produce complete disorganization of the fibrous sheath together with axonemal defects, including abnormal central-pair microtubules and inner/outer dynein arms, and uniquely abolish AKAP4 from the flagellum, distinguishing FSIP2-related MMAF from other MMAF genes (PMID:30137358). Mouse knock-in and overexpression models establish a dosage-dependent role in both tail and acrosome formation, with overexpression elongating tails and truncation reproducing MMAF, while FSIP2 physically interacts with the acrosomal protein Acrv1 (PMID:34125190). FSIP2 co-localizes at the acrosome and interacts with acrosome-biogenesis proteins including DPY19L2, SPACA1, HSP90B1, HSPA2, and CLTC; its loss downregulates DPY19L2, ZPBP, and SPACA1 and produces acrosomal hypoplasia alongside flagellar defects (PMID:35654582). FSIP2 deficiency also drives pathological 'super-length' mitochondrial sheaths with elevated TOMM20 and reduced annulus protein SEPT4, implicating it in terminating mitochondrial sheath extension (PMID:36632462).

1. 2018 Medium

   Established FSIP2 as a causal MMAF gene by showing its loss disorganizes the fibrous sheath and disrupts axonemal architecture, defining its place in flagellar biogenesis.

   Evidence Whole-exome sequencing with IF and TEM in patient sperm carrying loss-of-function mutations

   PMID:30137358

   Open questions at the time

   • No protein-absence confirmation due to lack of reliable antibody
   • Mechanism by which FSIP2 loss abolishes AKAP4 not defined
   • Patient-only observation without functional rescue
2. 2021 High

   Demonstrated that FSIP2 acts in a dosage-dependent manner on both sperm tail and acrosome formation and physically engages an acrosomal protein, extending its role beyond the fibrous sheath.

   Evidence Knock-in and overexpression mouse models, scRNA-seq, reciprocal co-IP with Acrv1, and proteomics

   PMID:34125190

   Open questions at the time

   • Direct binding interface with Acrv1 not mapped
   • How overexpression lengthens tails mechanistically unresolved
   • Causal hierarchy among proteomic changes not established
3. 2022 Medium

   Connected FSIP2 directly to the acrosome-biogenesis machinery by identifying multiple interacting partners and linking its loss to acrosomal hypoplasia.

   Evidence Co-IP and LC-MS/MS proteomics with western blot and IF in patient samples

   PMID:35654582

   Open questions at the time

   • Co-IP interactions not validated reciprocally for all partners
   • Whether interactions are direct or complex-mediated unknown
   • No in vivo rescue of acrosomal defects
4. 2023 Medium

   Implicated FSIP2 in terminating mitochondrial sheath extension, revealing a structural role in midpiece organization beyond the fibrous sheath.

   Evidence Exome sequencing, TEM, IF, mitochondrial ATP consumption assay, and western blot for TOMM20 and SEPT4 in patient sperm

   PMID:36632462

   Open questions at the time

   • Molecular link between FSIP2 and SEPT4/annulus not defined
   • Cause of decreased mitochondrial ATP consumption unclear
   • No functional reconstitution
5. 2025 Low

   Showed FSIP2 absence coincides with loss of intraflagellar transport (IFT20) and acrosomal (ACTL7A) proteins, broadening the set of proteins dependent on FSIP2 for localization or stability.

   Evidence IF, RT-PCR, and TEM in patient spermatozoa

   PMID:40968718

   Open questions at the time

   • Single method per protein target with no rescue
   • Cannot distinguish loss of stability from loss of localization
   • No direct interaction data for IFT20 or ACTL7A
6. 2026 Low

   Reinforced that FSIP2 variants dysregulate axonemal assembly factors and produce combined midpiece and principal-piece ultrastructural defects.

   Evidence Whole-exome and third-generation sequencing with IF, WB, SEM, and TEM in patient sperm

   PMID:42169611

   Open questions at the time

   • No functional reconstitution or epistasis
   • Specific dysregulated factors and their direct dependence on FSIP2 not resolved
   • Patient-based correlative evidence only

• The biochemical mechanism by which FSIP2 organizes the fibrous sheath and stabilizes its diverse partners remains unresolved.
• No structural model of FSIP2 or its complexes
• Direct versus indirect basis of partner dependence not separated
• No reconstitution of FSIP2-dependent assembly steps