Fusion of Sperm with Prostasomes: Effects on Membrane Fluidity

Prostasomes are membranous vesicles (150–200 nm diameter) present in human semen. They are secreted by the prostate gland and contain large amounts of cholesterol, sphingomyelin, and Ca2+. In addition, some of their proteins are enzymes. Prostasomes enhance the motility of ejaculated sperm and are involved in a number of biological functions. In this work, we study the fusion of prostasomes to sperm by determining the relief of octadecylrhodamine self-quenching and the fluidity of membranes by measuring the fluorescence anisotropy of diphenylhexatriene. We present the following findings: (a) the contact of sperm cells with prostasomes at slightly acidic pH causes the fusion of the membranes; (b) the amount of transferred lipid depends on the prostasome/sperm ratio; (c) the fluidity of sperm is much higher than that of prostasomes; (d) the fusion changes some properties of sperm cells, such as fluidity, which decreases greatly; and (e) the extent of fluidity variations depends on the prostasome to sperm ratio. We propose that the H+-dependent fusion of prostasomes to sperm may have physiological consequences. In fact, this process can modify the lipid and protein pattern of sperm plasma membranes.