The acrosome is an organelle of the spermatozoon that covers the apical part of the sperm head. This organelle comprises an outer acrosomal membrane, an inner acrosomal membrane, and plasma membranes. The acrosome plays an important role during the early stages of fertilization, in that the acrosome and its associated molecules mediate recognition of the egg to be fertilized. Although acrosome morphology varies with the species, the function of the acrosome and its contents as regulating elements during the early stages of fertilization are fairly similar among species. The acrosome contains proteolytic enzymes (see Proteinases) as well as proteins for binding to the zona pellucida and plasma membrane of the Egg (1). Therefore, the associated molecules of the acrosome have been proposed as targets for nonhormonal antifertilization agents.
1. Primary Ligands
The primary ligands are proteins that are located on or close to the acrosome; their function is to ensure that recognition between the gametes is species-specific. This is especially necessary in animals with extracorporal fertilization (eg, frog and sea urchin). On the surface of the acrosomal cap, there is a plethora of adhesion molecules (2), but their origin is not in all cases clear. Sperm adhesins become associated only upon ejaculation (3). Many of these primary ligands, such as galactosyltransferase (4), are lectins and bind to O-linked oligosaccharides and glycans of the ZP3 receptor of the oocyte’s extracellular matrix, the zona pellucida (5, 6). Other proteins were discovered that induce metabolic events; a 95-kDa protein was detected that induces phosphorylation events (7).
One of the most fascinating events during the early stages of fertilization is the acrosome reaction . After the initial contact of the sperm with the zona pellucida of the oocyte, and under the influence of progesterone (8), a unique event called the acrosome reaction takes place, which is calcium-dependent (9). Although in mammals the acrosome reaction can be induced without zona pellucida glycoproteins, they act in a catalytic manner (10). The acrosome reaction involves exocytosis of the outer membrane, causes the formation of hybrid vesicles, and leads to exposure of the inner contents of the acrosome. In many invertebrates, such as starfish, sea cucumbers, or sea urchin, a polymerization event takes place that results in the formation of actin filaments. In mollusks, the actin filaments are already present in the unreacted spermatozoon and exposed on activation.
Recent investigations report the presence of the inositol phosphate system in mammalian sperm. It involves the activation of a sperm receptor that stimulates a G-protein. This activates phospholipase C, which cleaves subsequently phosphatidyl inositol diphosphate (PIP2) into diacylglycerol and inositol triphosphate (IP3). IP3 triggers the release of calcium from intercellular stores, and diacylglycerol activates protein kinasekinase C, which is calcium-dependent. This causes proteins to be phosphorylated and leads subsequently to the acrosome reaction (11). Upon their exposure, the molecules within the acrosome become important for the fertilization process.
2. Secondary Ligands: Acrosin/Proacrosin
A major component of the inner acrosome is the protein proacrosin . This is the best-characterized molecule of those involved in the early stages of fertilization. It is important for local lysis of the zona pellucida and therefore plays a central role at the stage of sperm penetration. Proacrosin is stored as its inactive form, a zymogen. On raising the pH, or on contact with zona pellucida glycoproteins, the zymogen converts into its active proteolytic form, acrosin . This is accomplished by several intramolecular reorganizations (12). Proacrosin is a two-polypeptide chain molecule and, typical of serine proteinases, the heavy chain starts at residue Val24. Major alterations of the molecule involve proteolytic cleavage of the proline-rich C-terminal tail, cleavage of the light chain, and linkage to the heavy chain via disulfide bonds.
Proacrosin is a multifunctional protein; it does not act solely as a proteinase, but is also a fucose-binding protein (13). The molecule shows high binding affinity for fucoidan, heparin, and zona pellucida glycoprotein preparations (14). The primary mechanism of proacrosin binding to the zona pellucida is not only an interaction with carbohydrates, but involves positively charged amino acid residues of the proacrosin and negatively charged sulfate groups of the zona pellucida glycoproteins. Homology modeling of proacrosin revealed that the groove for binding to the zona pellucida contains loops in which the positively charged amino acids are located. The active site for proteolytic activity is located in the center, surrounded by the positively charged amino acids. The binding groove and proteolytic center appear to form a single reactive unit, even though they are located on different regions on the polypeptide chain. The proteolytic activity could be blocked without affecting proacrosin’s binding characteristics.
The arrangement of the binding site and proteolytic center suggests two major tasks of the enzyme during fertilization. The molecule recognizes the zona pellucida specifically, binds to it, and digests it locally to promote sperm entry through the extracellular matrix. However, there must be further binding and proteolytic proteins involved in fertilization, as mouse knockout strains lacking the proacrosin gene did not produce infertile males (15); other molecules must be able to take over the role of proacrosin. On the other hand, male mice with a functionally active proacrosin gene demonstrated a significantly greater fitness in terms of reproductive success in a competition-mating experiment with the proacrosin knockout mice (16). Although proacrosin is the major compound of the acrosomal vesicle, there are further adhesion proteins. Fertilin , formerly designated PH-30, is a molecule that is involved in interactions between the sperm and egg (17, 18).
