Loss-of-function mutations in multiple morphological abnormalities of the sperm flagella (MMAF)-associated genes lead to decreased sperm motility and impaired male fertility. As an MMAF gene, the function of fibrous sheath-interacting protein 2 (FSIP2) remains largely unknown. In this work, we identified a homozygous truncating mutation of FSIP2 in an infertile patient. Accordingly, we constructed a knock-in (KI) mouse model with this mutation. In parallel, we established an Fsip2 overexpression (OE) mouse model. Remarkably, KI mice presented with the typical MMAF phenotype, whereas OE mice showed no gross anomaly except for sperm tails with increased length. Single-cell RNA sequencing of the testes uncovered altered expression of genes related to sperm flagellum, acrosomal vesicle and spermatid development. We confirmed the expression of Fsip2 at the acrosome and the physical interaction of this gene with Acrv1, an acrosomal marker. Proteomic analysis of the testes revealed changes in proteins sited at the fibrous sheath, mitochondrial sheath and acrosomal vesicle. We also pinpointed the crucial motifs of Fsip2 that are evolutionarily conserved in species with internal fertilization. Thus, this work reveals the dosage-dependent roles of Fsip2 in sperm tail and acrosome formation.

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