Intra-acrosomally located acrosin-inhibitors: Evolution and developmental patterns in mammals

Characterization of mouse epididymal acrosin: comparative studies with acrosin from boar and human ejaculated spermatozoa

Acrosin is an acrosomal protease believed to play a major role in fertilization. It is synthesized as an inactive precursor, proacrosin, which is processed via (auto)proteolysis into active form(s). In this paper, comparative studies on the characteristics of acrosin from mouse, boar and human are reported. The mouse proacrosin/acrosin was especially investigated to clarify whether or not the enzyme undergoes modifications during epididymal maturation. Acrosomal extracts from mature and immature mouse spermatozoa, as well as from ejaculated boar and human spermatozoa, were analysed by means of SDS-electrophoresis, Western blot and activity measurements. The studies showed that epididymal maturation produced a shift in the molecular weight of proacrosin. It was also observed that the activation kinetics differ strongly between the three species studied. Human proacrosin showed a constant substrate turnover, acrosin from boar showed sigmoidal activation kinetics and mouse acrosin, either from the caput or the cauda epididymides, showed a rapid decay in activity, suggesting the presence of an endogenous specific inhibitor.

Changes in sperm-bound amidase activity suggest subtle damage to ram sperm acrosomes by freezing/thawing, not detected by light microscopy

We have measured sperm-bound amidase activity in fresh, cooled and frozen/thawed ram spermatozoa, in order to study if freezing and thawing led to some degree of acrosome damage of motile/viable spermatozoa not detected by optical methods. This assay was based on the fact that membrane damage would result in an increased access of the enzyme substrate to the sperm acrosome. Semen was collected from adult Australian Merino rams, and spermatozoa were washed by centrifugation through a Ficoll solution. Sperm-bound amidase activity was measured in whole spermatozoa using the protease substrate benzoyl-arginyl-p-nitroanilide (BAPNA). Acrosomal status was also assessed by light microscopy after Giemsa staining. Most amidase activity was shown to be sperm-bound, as only a minor fraction of the enzyme activity was release into the medium after induced damage. Simultaneous assessment of sperm-bound amidase activity and the percentage of spermatozoa with microscopically evident acrosomal damage, after mild sonication for different times, showed a high correlation between both parameters (r = 0.97, p < 0.001). In separate experiments, fresh, cooled and frozen/thawed semen samples were filtered through Sephadex G-10 to obtain a subpopulation of motile, mostly acrosome-intact spermatozoa. As controls, spermatozoa from the same samples to which extensive acrosome damage was induced were evaluated. Slow cooling to 4 degrees C had no effect on amidase activity or percent acrosomal damage with respect to fresh samples. Freezing and thawing resulted in a sperm population that, after filtration through Sephadex, had a low percentage of acrosome damage (9.4%, vs. 2.1% for fresh filtered controls), which was 11% of that obtained after extensive acrosome damage (83%). However, amidase activity in these samples was markedly increased, showing values of activity that were 56% of those obtained in extensively damaged spermatozoa. This effect was not due to an alteration in the enzyme kinetics. We conclude that sperm-bound amidase activity is useful to detect subtle changes, provoked by a standard freezing/thawing procedure, in the permeability of acrosomes from ram spermatozoa which are not detected by direct observation of the acrosomes after Giemsa staining.

Boar proacrosin expressed in spermatids of transgenic mice does not reach the acrosome and disrupts spermatogenesis

Transgenic mice that express boar proacrosin were produced to examine mechanisms for targeting hydrolytic enzymes to the acrosome. A 2.3 kb transgene was constructed by ligating the cDNA for boar preproacrosin with the mouse protamine 2 promoter region. Six founder mice that incorporated the transgene were identified by polymerase chain reaction and Southern blot analysis. Northern blots indicated that the two male founders (Ac.2 and Ac.5) and male progeny from three female founders (Ac.3, Ac.4, Ac.6) expressed the transgene mRNA in testis, but not in somatic tissues. In these transgenic animals boar proacrosin was detected by immunohistochemistry in condensing spermatids, but was not localized in the acrosome. This acrosomal targeting defect of the transgene product may result from its delayed expression during the later steps of haploid differentiation. Furthermore, both male founders and all Ac.4 and Ac.6 males were infertile, as determined by multiple matings for at least 2 months. Ac.3 males were either infertile or rarely transmitted the transgene to their offspring. The infertile males mated, produced copulatory plugs, and had seminal vesicle weights and testosterone levels within the normal range. However, they produced significantly fewer spermatozoa and had lower testis weights than controls. Although the mitotic and meiotic phases of spermatogenesis appeared normal by histological criteria, condensing spermatids were missing from most tubules, and multinucleated cells were present in the lumen of seminiferous tubules and in the epididymis. We hypothesize that boar proacrosin which fails to reach the acrosome is activated in these transgenic mice, and that its proteolytic activity disrupts spermatogenesis during spermatid formation.

Evaluation of the acrosomal membrane in bovine spermatozoa: Effects of proteinase inhibitors

Thawed bovine spermatozoa are characterized by a lack of homogeneity in the acrosomal membrane. Therefore, it is difficult to visualize the acrosome to assess morphology. Synthetic proteinase inhibitors were tested on thawed bovine semen for their effect on the integrity of acrosomal membranes. The proteinase inhibitors 4-nitrophenyl-4-guanidinobenzoate (NPGB) and N-L-p-tosyl-L-lysine-chloromethylketone (TLCK) were added to a medium containing spermatozoa separated on a percoll gradient. After incubation for 30 min at 38 degrees C in 5% CO(2), 95% air (final concentration 1 mM), the action of these inhibitors was controlled by measuring the activity of acrosome proteinases. The acrosomal membrane was evaluated by means of a dual stain procedure (trypan blue, Giemsa). In contrast to spermatozoa that had been incubated with proteinase inhibitor-free solution, samples that had been incubated with TLCK showed homogeneity in 90% of the acrosomal membranes and excellent visualization of the acrosome itself; in the NPGB-treated samples, homogeneous staining was observed in 83% of spermatozoa (P < 0.0005). It is concluded that alteration of the acrosomal membrane in thawed semen is not directly caused by freezing-thawing, but may be due to activation of acrosomal proteinases, which is increased during staining procedures. The addition of proteinase inhibitors before staining offers a new possibility for improved assessment of the acrosome in bovine spermatozoa.

Evidence for sequential deployment of secretory enzymes during the normal acrosome reaction of guinea pig sperm in vitro

Experiments were conducted to determine if acrosomal enzymes are released simultaneously or in sequence during the normal acrosome reaction. Epididymal guinea pig sperm were incubated in a chemically defined, calcium-containing medium which supports normal acrosome reactions within 4-5 hours at 37 degrees C. The sperm suspensions were monitored for motility, normal acrosome reactions, and false acrosome reactions during in vitro incubation. At specified time intervals, the sperm were separated from the incubation medium by centrifugation, and the distribution of dipeptidyl peptidase (DPP II) and acrosin activity was determined by biochemically assaying the hydrolysis of trialanine and N-benzoyl-L-arginine ethyl ester (BAEE), respectively. When calcium was present, there was a significant increase in DPP II activity in the supernatants by 1 hour of incubation and a slight decline at later time points. This release was not correlated with false or normal acrosome reactions (loss of the acrosomal cap) monitored by phase-contrast microscopy but probably represents a very early stage in the normal acrosome reaction. This early stage is difficult to detect at the light microscope level because sperm are still in rouleaux and because membrane fusion is not directly observable. In contrast, acrosin activity, which was assayed in the same supernatants, increased at later times when sperm were observed to have completed normal acrosome reactions. The ultrastructural distribution of DPP II was determined in sperm pellets collected during in vitro incubation by using the DPP II substrate lysyl-alanyl-4-methoxy-2-naphthyamide. In freshly isolated cauda epidiymal sperm, reaction product is confined to the light-staining area in the dorsal bulge of the acrosome. However, by 1 hour of incubation, the light-staining area of many sperm was partially or completely dispersed, while other regions of the acrosome were unchanged. Our data are consistent with the conclusions that DPP II is a highly soluble component of the guinea pig sperm acrosome and that its release occurs during the initial phase of the acrosome reaction while sperm are still in rouleaux. Structural changes in the acrosome associated with DPP II release were detectable by electron microscopy but not by light microscopy. Acrosin, which is less soluble than DPP II, is released at a later time during the acrosome reaction. Both DPP II and acrosin appear to be partially inhibited following their release from sperm. A complete understanding of the sequential release and extracellular activities of the acrosomal enzymes will be necessary to fully define their functions in fertilization.

Isolation of a stable apical segment of the guinea pig sperm acrosome

The large apical segments of guinea pig sperm acrosomes were mechanically separated from the spermatozoa and subsequently isolated by density gradient centrifugation. The isolated acrosomal caps were very stable and maintained their crescent morphology when suspended in sucrose-based medium buffered at pH 5.6, with or without the acrosin inhibitor p-aminobenzamidine (pAB). Examination under the electron microscope showed that the acrosomal caps were free of plasma membrane and were bound by an outer acrosomal membrane which was discontinuous. Enzymatic analysis after lysis of the caps indicated that acrosin and hyaluronidase were present with high specific activity, while only a trace amount of acid phosphatase activity and no arylsulphatase, phospholipase A2, or phospholipase C activities were present. Significant particulate acrosin activity, but only trace amounts of soluble acrosin activity, could be detected in the isolated acrosomal caps if assayed immediately after isolation in the absence of pAB. However, soluble acrosin activity of high specific activity was obtained after the acrosomal caps were extracted by 10% glycerol buffered at low pH (pH 3.0). The new procedures provide a means to isolate and purify guinea pig sperm apical acrosomal segments rapidly.

Evolution and development of the outer acrosomal membrane (OAM) and evidence that acrosin-inhibitors are proteins of the OAM

An antiserum to the purified porcine outer acrosomal membrane (OAM) was raised in rabbits and the IgG fraction isolated by ammonium sulphate precipitation and ion exchange chromatography. The antibodies reacted exclusively with the acrosomal cap of the sperm head as revealed by indirect immunofluorescence. In addition they cross-reacted not only with the acrosomal part of the spermatozoa of all mammalian species tested (bull, horse, rabbit, rat, mouse, hamster, mole, antelope, monkey, man) but also with the spermatozoa of the cock (Class: birds) and the rainbow trout (Class: fish). All the species exhibited similar development of the acrosomal cap during spermatogenesis, with the appearance of the immunofluorescent stain in early round spermatids. In the mole the localization of the acrosome in elongated testicular spermatids differed from that in all other species: Instead of prominent fluorescence over the apical part of the sperm an equatorial belt was formed. The cross-reactivity of the anti-boar OAM antibody with the acrosomes of different vertebrate species at the morphological level was supported by the results of Western blotting experiments with purified boar OAM proteins and the SDS-extractable proteins of bull and human spermatozoa, respectively. Using anti-OAM antibodies and antibodies against the acrosin inhibitors I and II described recently by Tschesche et al. (1982), in absorption and Western blotting experiments, it was demonstrated that the acrosin inhibitor proteins are integrated in the outer acrosomal membrane.