Distribution of filipin-sterol complexes in Japanese abalone sperm during the acrosome reaction.

Abstract

ABSTRACT. The distribution of membrane sterols in the acrosome reaction of Japanese abalone sperm was studied by freeze-fracture techniques using a polyene antibiotic, filipin, as a probe for membrane sterols. In the unreacted sperm, the plasma membrane covering the acrosome showed a random distribution of filipin-sterol complexes, while in the outer acrosomal membrane, filipinsterol complexes were rarely observed. In the initial stage of the acrosome reaction, filipin-sterol complexes were found densely distributed in the plasma membrane throughout the acrosomal region. On the subjacent outer acrosomal membrane, filipin-sterol complexes were densely packed in the apical area, corresponding to the truncated cone region. This apical area consistently showed such regionalization of filipin-sterol complexes during the acrosome reaction. As the acrosomal process elongated, the intramembrane particle (IMP)-free areas which appeared in the acrosomal process membrane were found to be displaced by densely distributed filipin-sterol complexes. These results indicate a possible regionalization of membrane sterols related to the acrosome reaction.In marine invertebrates the acrosome reaction consists fundamentally of two requisites; exocytosis of the acrosomal vesicle to release lytic substances for dissolving the egg coats, and exposure of the newly formed acrosomal process to generate a site for fusion with the egg. Of particular interest is that membrane transformations occurring during the acrosome reaction involve the fusion between the plasma membrane and the outer acrosomal membrane at the apex of the acrosome, resulting in exocytosis, and rapid extension of the inner acrosomal membrane to form the acrosomal process membrane which may eventually fuse with the egg plasma membrane.A few freeze-fracture studies of abalone (29) and mammalian sperm (9,10,15,40) have demonstrated the regionalization of intramembrane particles (IMPs) associated with the acrosome reaction, while little information is available concerning the distribution of sterols in the sperm membrane during the acrosome reaction (12). Sterol is an important component of the cell membrane and plays a fundamental role in modulating the permeability and fluidity of membranes (6,24,33). Thus, the study of the distribution of sterol in the membranes is important in order to further understand the structural properties and functions of the membrane in relation to the acrosome reaction.In the present study, attempts have been made to investigate the distribution of sterols (usually cholesterol) in the membranes of abalone sperm during the 73