Mammalian oocytes exhibit specific recognition of the RGD (ARG‐Gly‐Asp) tripeptide and express oolemmal integrins

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Abstract
Integrins are a family of cell adhesion receptors involved in many cell–cell and cell–matrix interactions. Some of these heterodimeric receptors, such as α5b̃1 and αvb̃1, specifically recognize the amino acid sequence Arg-Gly-Asp (RGD) within their ligands. The RGD sequence is found in fibronectin, vitronectin, and other extracellular matrix proteins. Our results demonstrate that the oolemmas of eggs from human and several other mammalian species contain receptors capable of binding to RGD ligands, and that integrin subunits are expressed by oocytes. Four distinct techniques were utilized to identify the presence of functional integrins on mammalian eggs. RGD-binding receptors were detected on the surfaces of zona-free eggs from all species tested. Covaspheres coated with PepTite-2000, which contains RGD, bound to the eggs and formed rosettes. Rosetting was competitively inhibited by PepTite-2000 and by GRGDTP, a soluble RGD peptide, but not by RGES. An ELISA using polyclonal antibodies directed against the cytoplasmic tails of the integrin subunits identified the integrin subunits α5, b̃1, and αv, but not b̃3, in detergent extracts of Syrian hamster eggs. A dot blot confirmed the presence of αv in hamster egg lysates. Finally, the integrin subunits α2, α5, α6, but not α4, were detected on the surfaces of zona-free eggs from human and Syrian hamster. Immunobeads coated with monoclonal antibodies specific for α2, α5, and α6 bound to the eggs and formed rosettes. A similar rosetting was obtained from beads coated with polyclonal antibody against the vitronectin receptor (αvb̃3/b̃5). These observations suggest that the integrins α2b̃1, α5b̃1, α6b̃1, and one or more integrins containing αv are expressed by oocytes. We propose that these integrins may play important roles in fertilization and implantation.