EXPANDED CUMULI INDUCE ACROSOME REACTION IN BOAR SPERM

The authors investigated acrosomal changes occurring in boar sperm tha t interact with the expanded cumulus matrix surrounding ovulated pig o ocytes. Samples of washed boar sperm obtained from six donors were inc ubated for 4 hr under capacitating conditions and exposed either to so lubilized zonae pellucidae (ZP) or solubilized expanded pig cumuli (SE C) obtained from IVM oocytes. Alternatively, hyaluronic acid, laminin, or fibronectin, components of the extracellular matrix (ECM) were add ed to capacitated sperm. Acrosomal integrity was evaluated 1hr later b y using FITC-PSA staining. Solubilized cumuli induced acrosome reactio n (AR) in a dose-dependent manner with a saturating effect exerted at 2.5 SEC/50 mu l. Both 500 nM fibronectin and 500 nM laminin stimulated acrosomal exocytosis, the latter being more effective and inducing sa turating levels of AR. By contrast, hyaluronic acid did not affect acr osomal status. Preincubation with anti-laminin antibodies completely p revented the inducing activity of SEC without affecting the activity o f solubilized ZP. Consistent with these data, the integrin VLA-6, a re ceptor with high affinity for laminin, was detected by immunoblotting on the plasma membrane of capacitated boar spermatozoa. In addition, i ts immunoneutralization, obtained with the preincubation of capacitate d sperm with the antibody raised against the or chain of VLA-6 integri n, prevented AR upon exposure to laminin or SEC (10.7 +/- 3.2 and 10.2 +/- 1.0% respectively), while the samples retained their responsivene ss to ZP (29.6 +/- 1.2%). The results demonstrate that the interaction between laminin, entrapped in the expanded cumuli, and specific integ rins present on the sperm membrane can initiate AR, thus taking part i n the process of sperm-egg recognition. Mol. Reprod. Dev. 51.445-453, 1998. (C) 1998 Wiley-Liss, Inc.