Osmond J. D'Cruz, Ph.D

Section of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynecology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73190

3. SPERM-OOCYTE FUSION

Membrane fusion is a key event in a variety of cellular phenomena such as transport between organelles, endo-and exocytosis, myogenesis, viral infection, and fertilization (5). The specificity of membrane fusion suggests the involvement of specific modulators regulating recognition of fusion partners. A number of specific proteins have been identified as candidates for the control of membrane fusion in several model systems (6). It is thought that similar principles may apply to membrane fusion events leading to sperm plasma membrane-oolemma adhesion and fusion. The responses of the oocyte following sperm-oocyte binding and fusion are strikingly similar to the responses that occur within lymphocytes following their activation by an antigen presenting cell (7). Based on this premise, numerous investigators have begun to dissect the relative contributions of various cell adhesion molecules in sperm-oocyte fusion as well as to implicate these molecules in specific defects, such as failure in IVF systems. Similar to fusion in somatic cells, sperm-oocyte adhesion and fusion appear to involve receptor/counter-receptor binding and to be under the influence of the mechanical forces exerted by sperm on the oocyte. Furthermore, metabolic changes that lead to increased affinity (receptor clustering, phosphorylation induced by cell signaling) for sperm-oocyte fusion may also be operative. Reproductive biologists have begun to search for the adhesion molecules on the sperm plasma membrane and oolemma, and to cell signaling events in gametes during the onset of fertilization in an attempt to find an analogy with the well-characterized membrane fusion model systems such as cell-cell differentiation and immune-microbe interactions. The best characterized membrane fusion proteins are found in viruses (6). Virus binding and fusion are mediated by either separate proteins or these separate attributes remain on a single protein but at topographically distinct sites. A conformational change in the fusion protein leads to exposure of a "fusion peptide", which then recognize counter-receptors on the target cell surface.

The hypothesis that sperm plasma membrane and oocyte membranes carry complementary molecules involved in a multistep fusion process has been validated by studies of adhesion molecules in sperm-oocyte interaction in a number of animal models (8-9). The species-specificity of sperm-oocyte recognition and binding primarily resides at the level of ZP (2). Human sperm bind and penetrate hamster oocytes denuded of ZP (10). Sperm binding and fusion with oocyte occurs at discrete regions on both the sperm membrane and oolemma. The equatorial segment of the sperm head, which constitutes the post-acrosomal region toward the posterior of the sperm head, appears to be the region of the sperm membrane that is involved in binding and fusion with the microvillar region of the oolemma (11).

In somatic cells, cell adhesion receptors have been grouped into distinct families based upon their structure and binding. The major families of membrane-anchored adhesion molecules include integrins, the immunoglobulin supergene family, selectins, cadherins, syndecans, and ADAMs (a disintegrin and metalloprotease domain) (12). Members of the ADAM family possess a potential adhesion domain as well as a potential protease domain (13). As a group, adhesion molecules are characterized by an extracellular domain anchored to the membrane of the cell by a short, hydrophobic transmembrane domain that is followed by a cytoplasmic tail of variable length. They act as "molecular bridges" connecting molecules on the outside of the cell with cytoskeletal and signal-transducing machinery within the cell (14). The primary mediators of cell-cell interaction appear to be adhesion molecules of the integrin family. This family is composed of non-covalently associated alpha and ß subunits and classified into subfamilies based on their ß subunits (called ß1, ß2, ß3, etc.) (15). At present, at least 15 alpha subunits and 8 ß subunits have been characterized. The alpha and ß subunits in various combinations are known to form at least 19 integrins. In general, integrins are subclassified into: (a) ß1 integrins, sharing the ß1 chain and functioning in both cell-cell and cell-substratum adhesion, (b) ß2 integrins, sharing the ß2 chain and mainly participating in cell-cell interactions; and (c) ß3 integrins, which share the ß3 chain and have variable adhesive functions. The integrin expression profile of individual cells varies greatly among different cell types and determines the ability of cells to recognize different adhesive substrates (16). In addition, different ß integrins may recognize different peptide sequences of the same protein or the same structural element presented by a particular ligand (17). Integrins are involved in a variety of biological processes including platelet aggregation, leukocyte recognition, and adhesion during immune response and cell migration during embryonic development (18-20). The ability of ligands to be recognized by integrins is often associated with the presence of the tripeptide sequence Arg-Gly-Asp (RGD) in the substrate (14, 19). Not all substrates for integrins contain this sequence, however, and other sequences are clearly important for recognition by integrins.

Integrin-ligand interactions also trigger specific organizational and physiologic events (21). In general, ligand binding leads to receptor clustering within the surface membrane, the organization of cytoskeletal elements around the occupied receptors, and the generation of an intracellular signal (22). Integrin engagement also stimulates phosphoprotein kinases known to be required for information transduction and gene activation (23). Various lines of evidence suggests that integrins, or integrin-like molecules, may be present on the surface of mammalian gametes, and might be involved in the interactions between oolemma and sperm membrane at fertilization (8-9, 24-25). The molecular mechanisms by which this occurs are currently the topic of intense investigation by reproductive biologists.