Bi-allelic mutation in Fsip1 impairs acrosome vesicle formation and attenuates flagellogenesis in mice.

Fibrous sheath interacting protein 1 (Fsip1) is a cytoskeletal structural protein of the sperm flagellar proteome. A few studies have reported that it plays a vital role in the tumorigenesis and cancer progression. However, little is known about the role of Fsip1 in spermatogenesis and mammalian sperm flagellogenesis. Fsip1 protein showed the highest expression in round spermatids, and was translocated from nucleus to the anterior region of the elongating spermatid head. To investigate its role we constructed homozygous Fsip1 null (Fsip1) mice. We found that the homozygous Fsip1 mutant mice were infertile, with a low sperm count and impaired motility. Interestingly, a subtle phenotype characterized by abnormal head shape, and flagella deformities was observed in the sperm of Fsip1 mutant mice similar to the partial globozoospermia phenotype. Electron microscopy analysis of Fsip1 sperm revealed abnormal accumulation of mitochondria, disrupted axoneme and retained cytoplasm. Testicular sections showed increased cytoplasmic vacuoles in the elongated spermatid of Fsip1mice, which indicated an intraflagellar transport (IFT) defect. Using proteomic approaches, we characterized the cellular components and the mechanism underlying this subtle phenotype. Our result indicated that Fsip1downregulates the formation of acrosomal membrane and vesicles proteins, intraflagellar transport particles B, and sperm flagellum components. Our results suggest that Fsip1 is essential for normal spermiogenesis, and plays an essential role in the acrosome biogenesis and flagellogenesis by attenuating intraflagellar transport proteins.