Receptor expression is essential for forward motility in the course of sperm cell maturation

Forward motility stimulating factor (FMSF) is a glycoprotein previously purified from buffalo blood serum that promotes progressive motility of caprine caudal spermatozoa. We have prepared a functionally active covalent conjugate of this factor with horseradish-peroxidase (HRP) to obtain an idea of its binding efficacy on maturing spermatozoa. Receptor-assay was performed using FMSF-HRP conjugate in saturating conditions to bind with spermatozoa isolated from different epididymal segments. Activity and binding profile of the motility stimulating factor coincided, suggesting both these parameters come into play only partially when spermatozoa reach the maturation state in the distal-corpus region and largely in caudal part (around 24% and 80% binding and 10% and 79% forward motility, respectively). Spermatozoa from caput up to mid-corpus regions neither displayed any substantial binding with FMSF nor exhibited significant induction in forward motility. Study of cell surface-bound FMSF on maturing spermatozoa in physiological milieu demonstrated their presence on anterior spermhead and suggests a nearly similar pattern of occurrence. Flow-cytometric analysis also implies analogous presence of this receptor. The factor was also immunodetected in uterine fluids of cattle species. This study displays a maturation-dependent expression of FMSF-receptor and consequential stimulation of forward motility that may be crucial for its journey to meet the ovum.