Proteases are not involved in the membrane fusion events of the lysolecithin-mediated guinea pig sperm acrosome reaction

Transitional states of acrosomal exocytosis and proteolytic processing of the acrosomal matrix in guinea pig sperm

In this study, we adapted a FluoSphere bead-binding assay to study the exposure and release of guinea pig sperm acrosomal components during the course of capacitation and acrosomal exocytosis. Prior to capacitation or the initiation of exocytosis, acrosomal proteins were not accessible to FluoSpheres coated with antibodies against two acrosomal matrix (AM) proteins, AM67 and AM50; during the course of capacitation and ionophore-induced acrosomal exocytosis, however, we detected the transient exposure of the solid-phase AM proteins on the surface of guinea pig sperm using the antibody-coated fluorescent beads. Several different transitional stages leading to complete acrosomal exocytosis were classified, and we propose these represent true, functional intermediates since some of the AM proteins are orthologues of mouse proteins that bind the zona pellucida (ZP) of unfertilized eggs. In addition, we present evidence that implicates acrosin in the proteolytic processing of AM50 during AM disassembly. Thus, we propose that the transitional states of acrosomal exocytosis involve early binding of AM proteins to the ZP (by what visually appear to be "acrosome-intact" sperm), maintenance of ZP binding that coincides with the progressive exposure of AM proteins, and gradual proteolytic disassembly of the AM to allow sperm movement through the ZP. We feel this "transitional states" model provides a more refined view of acrosomal function that supports a move away from the widely held, overly simplistic, and binary "acrosome-reaction" model, and embraces a more dynamic view of acrosomal exocytosis that involves intermediate stages of the secretory process in ZP binding and penetration.

In guinea pig spermatozoa, the procaine-promoted synchronous acrosome reaction results in highly fertile cells exhibiting normal F-actin distribution

In guinea pig spermatozoa, procaine induces Ca(2+) independent hyperactivated motility suggestive of sperm capacitation. Nonetheless, in the presence of high extracellular Ca(2+), procaine increases cytoplasmic Ca(2+). We analyze the procaine effect on the acrosome reaction (AR) processes in guinea pig spermatozoa. Results indicated that: (i) in spermatozoa pre-incubated 5-30 min in MCM-PLG medium, procaine produced synchronous AR, (ii) the acrosome-reacted sperm number increased with the capacitation period before procaine treatment and with procaine concentration, (iii) acrosome reaction was blocked when Ca(2+) was omitted, (iv) plasma membrane-outer acrosomal membrane fusion started within 2 min after procaine treatment, (v) in acrosome-reacted spermatozoa, actin polymerization occurred and F-actin was located in the equatorial and post-acrosomal regions and (vi) procaine treatment resulted in highly fertile acrosome-reacted spermatozoa. This is the first report indicating that procaine promotes synchronic AR in mammalian spermatozoa. If procaine promotes premature AR of spermatozoa in vivo, it might be a factor for infertility in patients exposed to this local anesthetic.

Dexamethasone reduces acrosin activity of ram spermatozoa

The aim of this study was to investigate the effect of dexamethasone on acrosin activity of spermatozoa in Chios rams during autumn (breeding season for sheep in Greece), in correlation with possible changes in blood testosterone. Dexamethasone was administered in four equal consecutive intramuscular injections, one every four hours (total dose: 3 mg kg(-1)). Total acrosin activity was determined in semen samples collected 48 h before and on the 4th and 7th day and thereafter once every week until the 77th day after dexamethasone administration. Blood samples for testosterone radioimmunoassays were collected 24 h before, during dexamethasone administration and on the 4th, 7th, 14th and 21st day after administration. Total acrosin activity in spermatozoa was reduced between days 7-28 after dexamethasone administration. Dexamethasone also induced a reduction in mean value and basal level of blood testosterone and inhibited its episodic secretion between 1 and 4 days after administration. As the reduction of acrosin activity appeared relatively soon after dexamethasone administration (7th day), it is likely that the increased amount of dexamethasone did not influence the synthesis of proacrosin in the late spermatids. As glucocorticoid receptors exist in the epididymis and accessory glands in various species, dexamethasone may have a direct influence on the synthesis and/or release of acrosin inhibitors in epididymal fluid or seminal plasma. These changes in acrosin activity in ovine spermatozoa mediated by dexamethasone may be of importance regarding the role of stress in the reduction of sperm fertilizing ability.

Dynamics of the mammalian sperm plasma membrane in the process of fertilization

Sexual reproduction requires the fusion of sperm cell and oocyte during fertilization to produce the diploid zygote. In mammals complex changes in the plasma membrane of the sperm cell are involved in this process. Sperm cells have unusual membranes compared to those of somatic cells. After leaving the testes, sperm cells cease plasma membrane lipid and protein synthesis, and vesicle mediated transport. Biophysical studies reveal that lipids and proteins are organized into lateral regions of the sperm head surface. A delicate reorientation and modification of plasma membrane molecules take place in the female tract when sperm cells are activated by so-called capacitation factors. These surface changes enable the sperm cell to bind to the extra cellular matrix of the egg (zona pellucida, ZP). The ZP primes the sperm cell to initiate the acrosome reaction, which is an exocytotic process that makes available the enzymatic machinery required for sperm penetration through the ZP. After complete penetration the sperm cell meets the plasma membrane of the egg cell (oolemma). A specific set of molecules is involved in a disintegrin-integrin type of anchoring of the two gametes which is completed by fusion of the two gamete plasma membranes. The fertilized egg is activated and zygote formation preludes the development of a new living organism. In this review we focus on the involvement of processes that occur at the sperm plasma membrane in the sequence of events that lead to successful fertilization. For this purpose, dynamics in adhesive and fusion properties, molecular composition and architecture of the sperm plasma membrane, as well as membrane derived signalling are reviewed.