Title

Roles of ADAM family proteins and egg integrins in mouse sperm-egg plasma membrane adhesion and fusion

Date of Completion

January 1998

Keywords

Biology, Cell

Degree

Ph.D.

Abstract

Peptide mimetics from the predicted binding sites in the disintegrin domains of the five testis-expressed ADAMs(A Disintegrin And Metalloprotease), fertilin $\alpha$, fertilin $\beta$, cyritestin, ADAM 4 and ADAM 5, were tested in a sperm-egg plasma membrane adhesion and fusion assay. The active site peptide from cyritestin strongly inhibited (80-90%) sperm-egg adhesion and fusion and was a more potent inhibitor than the fertilin $\beta$ active site peptide, which showed 55-60% of inhibition for sperm-egg fusion. Antibodies generated against the active site region of either cyritestin or fertilin $\beta$ also strongly inhibited (80-90%) both sperm-egg adhesion and fusion. The active site peptide from fertilin $\alpha$ only showed limited inhibition ($\sim$30%). The active site peptides from ADAM 4 and ADAM 5 did not show any inhibition.^ Fertilin $\alpha$, fertilin $\beta$, cyritestin and ADAM 4 were expressed as large precursors and proteolytically processed into mature forms during sperm maturation. ADAM 5 was only detected as the processed form and only in testicular cells. Cyritestin and fertilin $\beta$ was localized by indirect immunofluorescence on the inner acrosomal membrane (IAM) and the equatorial region of acrosome reacted sperm, respectively.^ The minimal binding sequence of ADAMs is not as restricted as the RGD sequence in PII SVMP disintegrins. For fertilin $\beta$, at least a five amino acid sequence QDECD is required for the maximal function. The additional cysteine residue in the predicted binding loop of ADAMs seems to be a free thiol. Inhibition of sperm-egg adhesion and fusion with antibodies against integrins indicates $\alpha$6 and $\beta$1 containing integrins on egg plasma membrane might be the egg receptors for sperm.^ In summary, three ADAMs, fertilin $\alpha$, fertilin $\beta$ and cyritestin, are involved in sperm-egg plasma membrane adhesion and fusion. Sperm-egg plasma membrane interactions might be mediated by the disintegrin-like domains of ADAMs on sperm and integrins on egg surface. ^