Effect of methyl-beta-cyclodextrin on the acrosomal responsiveness of human sperm

Human sperm incubated in vitro gradually become capable of acrosome-reacting in response to the agonist, progesterone (P4). Loss of unesterified cholesterol is an obligatory step in the development of acrosomal responsiveness. These experiments tested the ability of methyl-beta-cyclodextrin (MbetaCD) to accelerate sperm cholesterol loss and the development of acrosomal responsiveness. Incubating sperm 30 min in MbetaCD (2.5-10 mM) decreased sperm cholesterol by as much as 89% in a dose-dependent fashion. MbetaCD caused some sperm (maximum of 16% following treatment with 5 mM MbetaCD) to become responsive to P4, and it caused a dose-dependent increase in spontaneous acrosome reactions. The number of responsive sperm increased in the first 3 hr following their removal from MbetaCD. Continuing incubation to 24 hr increased the numbers of spontaneously reacted sperm and dead sperm, but not P4-responsive sperm. It appears, therefore, that up to 3 hr are required for the full expression of P4-responsiveness in cholesterol-depleted sperm. The observed effects of MbetaCD are due to its cholesterol-depleting properties, because including sufficient cholesterol with MbetaCD to reduce the loss of sperm cholesterol inhibited the effects of MbetaCD on cell viability, spontaneous acrosome reactions, and responsiveness to P4. MbetaCD accelerates the appearance of the functional stages that sperm normally pass through during incubation in vitro, reinforcing the view that cholesterol loss is an important determinant of the rate at which sperm become acrosomally responsive.