[Frontiers in Bioscience 1, d146-160, August 1, 1996]
Reprints
PubMed
CAVEAT LECTOR



INTERACTION OF HUMAN SPERMATOZOA WITH THE ZONA PELLUCIDA OF OOCYTE: DEVELOPMENT OF THE ACROSOME REACTION

Patricio Morales1 and Miguel Llanos2

1 P. Catholic University of Chile, Faculty of Biological Sciences. P. O. Box 114-D. Santiago, Chile.

2INTA, University of Chile, P. O. Box 138-11. Santiago, Chile.

4. SPERM BINDING PROTEINS/RECEPTORS FOR THE ZONA PELLUCIDA

As indicated, the glycoprotein composition of the ZP from several mammalian species has been well elucidated (18, 19). However, only limited information exists regarding the molecular identities and biochemical characteristics of the sperm surface components involved in zona binding. Several glycoprotein candidates have been proposed over the past years, among them are:

4.1. Galactosyltransferase

Galactosyltransferase is an enzyme located within the plasma membrane of spermatozoa that has a defined role in gamete recognition in the mouse (20). This enzyme mediates sperm-egg recognition by binding to N-acetylglucosamine residues on ZP glycoconjugates. Recently, the mouse zona protein ZP3 was identified as the complementary binding site for interaction with sperm ß-1,4-galactosyltransferase. This interaction mediates tight spermatozoa-ZP binding (21). A potential role of multivalent binding between ZP3 and several galactosyltransferase moieties in the induction of the AR has been proposed (21). This enzyme, however, may not be present in human spermatozoa (22).

4.2. D-mannosidase

Human spermatozoa possess a D-mannosidase activity (22) which may be involved in binding to ZP3 and induction of the AR (23). The sugar moieties involved in these processes are mannose-rich oligosaccharides on zona glycoconjugates (22). A positive relationship has been demonstrated between the appearance of mannose-binding sites on the surface of human spermatozoa and the outcome of in vitro fertilization (24). Previous studies, however, have failed to demonstrate inhibition of spermatozoa-ZP binding by treating the spermatozoa with mannose (25, 26) (see table 1).

Table 1. Effect of carbohydrates on human spermatozoa binding and penetration through the human zona pellucida.

Control*D-mannose
Oocyte NoNo Sperm BoundNo Sperm PenetratingNo Sperm BoundNo Sperm Penetrating

117240
2161500
316890
48320
516890
6138110
73320
841
X±SEM11.6±2.06.0±1.65.3±1.60±0

* Sperm aliquots treated for 30 min with 50 mmoles/l of D-mannose were use to inseminate single oocytes for 6 hr. Control spermatozoa were incubated with medium alone. Modified from (25 and 26).

4.3. Protein tyrosine kinase (ZRK)

One class of molecule that could account for both ZP3 recognition and signal transduction is a receptor tyrosine kinase (27). Exposure of both mouse and human spermatozoa to ZP3 results in the rapid autophosphorylation of tyrosine residues on the putative, 95 kDa, zona receptor kinase (ZRK). Using a monoclonal antibody directed against ZRK to screen a human testicular complementary DNA library, Burks et al. isolated a full-length clone predicting a 600 amino acid receptor that contains a unique cysteine-rich extracellular domain (28). Peptides from the extracellular domain of this molecule suppress spermatozoa-zona binding. Recently, Saling and colleagues reported that recombinant human ZP3 (hZP3rec), expressed in COS cells, stimulates tyrosine phosphorylation of human ZRK, in vivo and in vitro (29). Tyrphostin, a tyrosine phosphorylation inhibitor, inhibited the AR induced by hZP3rec (29). These results suggest that human ZRK is a receptor for human ZP3 and that ligand-stimulated tyrosine kinase activity in human spermatozoa is essential for the signaling cascade regulating exocytosis of the sperm acrosome. However, the 95 kDa phosphotyrosine-containing protein found in the mouse spermatozoa has been shown to be a unique form of phosphorylated hexokinase (30).

Naz and Ahmad reported on several sperm proteins that have the capacity to bind ZP proteins with molecular weights of 95, 63, 51 and 14-18 kDa (15). These proteins showed autophosphorylating activity. Potentially, the 95 kDa protein corresponds to the ZRK described by Saling and colleagues (28).

4.4. Lectin

An unusual galactosyl receptor, C-type (Ca2+-dependent) lectin on human spermatozoa may be the zona receptor (31, 32). This lectin is a transmembrane protein with carbohydrate-recognition domains on the C-terminal extracellular segment and an N-terminal cytoplasmic anchor. This protein was characterized as a single protein component of 50 kDa confined to the plasma membrane of the head of the spermatozoa overlying the acrosome (32).

[Table of Contents ] [Next Section] [Previous Section]