Background Loss-of-function mutations in FSIP2 result in multiple morphological abnormalities of the flagella in humans and mice. Intriguingly, a recent study found that FSIP2 might regulate the expression of acrosomal proteins, indicating that Fsip2 might be involved in acrosome development in mice. However, whether FSIP2 also function in acrosome biogenesis in humans is largely unknown, and the underlying mechanism of which is unexplored.

Objective Our objective was to reveal potential function of FSIP2 in regulating sperm acrosome formation.

Methods We performed whole exome sequencing on four asthenoteratozoospermic patients. Western blot analysis and immunofluorescence staining were conducted to assess the protein expression of FSIP2. Proteomics approach, liquid chromatography-tandem mass spectrometry and co-immunoprecipitation were implemented to clarify the molecules in acrosome biogenesis regulated by FSIP2.

Results Biallelic FSIP2 variants were identified in four asthenoteratozoospermic individuals. The protein expression of MUT-FSIP2 was sharply decreased or absent in vitro or in vivo. Interestingly, aside from the sperm flagellar defects, the acrosomal hypoplasia was detected in numerous sperm from the four patients. FSIP2 co-localised with peanut agglutinin in the acrosome during spermatogenesis. Moreover, FSIP2 interacted with proteins (DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2 and CLTC) involved in acrosome biogenesis. In addition, spermatozoa from patients carrying FSIP2 mutations showed downregulated expression of DPY19L2, ZPBP, SPACA1, CCDC62, CCIN, SPINK2 and CSNK2A2.

Conclusion Our findings unveil that FSIP2 might involve in sperm acrosome development, and consequently, its mutations might contribute to globozoospermia or acrosomal aplasia. We meanwhile first uncover the potential molecular mechanism of FSIP2 regulating acrosome biogenesis.