Proteolysis of the zona pellucida by acrosin: the nature of the hydrolysis products

New Insights on Sperm Function in Male Infertility of Unknown Origin: A Multimodal Approach

The global trend of rising (male) infertility is concerning, and the unidentifiable causes in half of the cases, the so-called unknown origin male infertility (UOMI), demands a better understanding and assessment of both external/internal factors and mechanisms potentially involved. In this work, it was our aim to obtain new insight on UOMI, specifically on idiopathic (ID) and Unexplained male infertility (UMI), relying on a detailed evaluation of the male gamete, including functional, metabolic and proteomic aspects. For this purpose, 1114 semen samples, from males in couples seeking infertility treatment, were collected at the Reproductive Medicine Unit from the Centro Hospitalar e Universitário de Coimbra (CHUC), from July 2018-July 2022. Based on the couples' clinical data, seminal/hormonal analysis, and strict eligibility criteria, samples were categorized in 3 groups, control (CTRL), ID and UMI. Lifestyle factors and anxiety/depression symptoms were assessed via survey. Sperm samples were evaluated functionally, mitochondrially and using proteomics. The results of Assisted Reproduction Techniques were assessed whenever available. According to our results, ID patients presented the worst sperm functional profile, while UMI patients were similar to controls. The proteomic analysis revealed 145 differentially expressed proteins, 8 of which were specifically altered in ID and UMI samples. Acrosin (ACRO) and sperm acrosome membrane-associated protein 4 (SACA4) were downregulated in ID patients while laminin subunit beta-2 (LAMB2), mannose 6-phosphate isomerase (MPI), ATP-dependent 6-phosphofructokinase liver type (PFKAL), STAR domain-containing protein 10 (STA10), serotransferrin (TRFE) and exportin-2 (XPO2) were downregulated in UMI patients. Using random forest analysis, SACA4 and LAMB2 were identified as the sperm proteins with a higher chance of distinguishing ID and UMI patients, and their function and expression variation were in accordance with the functional results. No alterations were observed in terms of lifestyle and psychological factors among the 3 groups. These findings obtained in an experimental setting based on 3 well-defined groups of subjects, might help to validate new biomarkers for unknown origin male infertility (ID and UMI) that, in the future, can be used to improve diagnostics and treatments.

Mechanisms of Sperm–Egg Interactions: What Ascidian Fertilization Research Has Taught Us

Fertilization is an essential process in terrestrial organisms for creating a new organism with genetic diversity. Before gamete fusion, several steps are required to achieve successful fertilization. Animal spermatozoa are first activated and attracted to the eggs by egg-derived chemoattractants. During the sperm passage of the egg’s extracellular matrix or upon the sperm binding to the proteinaceous egg coat, the sperm undergoes an acrosome reaction, an exocytosis of acrosome. In hermaphrodites such as ascidians, the self/nonself recognition process occurs when the sperm binds to the egg coat. The activated or acrosome-reacted spermatozoa penetrate through the proteinaceous egg coat. The extracellular ubiquitin–proteasome system, the astacin-like metalloproteases, and the trypsin-like proteases play key roles in this process in ascidians. In the present review, we summarize our current understanding and perspectives on gamete recognition and egg coat lysins in ascidians and consider the general mechanisms of fertilization in animals and plants.

Fertilization 1: Sperm-Egg Interaction

In birds in the reproductive season, an egg is ovulated without cumulus cells from the largest follicle with the highest hierarchy in the ovary. The outermost part of the ovulated eggs is the perivitelline layer, a glycoprotein matrix consisting of a few ZP-glycoproteins. The fertilization starts from sperm penetration of the perivitelline layer predominantly in the germinal disc region, followed by uptake of the sperm into the egg, and goes through by the fusion of sperm male pronucleus with the female pronucleus in the egg. A series of these fertilization steps occurs in the infundibulum of the oviduct within a short period after ovulation. Some pioneering microstructural studies using electron microscopy and supporting biochemical data from later studies indicate that, in avian fertilization, sperm interacts with the perivitelline layer covering the germinal disc, locally degrade and dissolve the matrix of the perivitelline layer, and penetrate it through the hole made proteolytically at the sperm-binding site on the perivitelline layer. Several molecules and structures presumably involved in the sperm-perivitelline interaction have been characterized, especially sperm proteases and their targets in the egg perivitelline layer. On the other hand, no molecules involved in the sperm-egg membrane fusion for the male pronucleus uptake into the egg have yet been identified or characterized and, moreover, no orthologue but one have been annotated so far in the chicken genome for the mouse genes involved in the sperm-egg membrane fusion.

Sperm proteases and extracellular ubiquitin-proteasome system involved in fertilization of ascidians and sea urchins

Ascidians (primitive chordates) are hermaphroditic animals that release spermatozoa and eggs almost simultaneously, but some species, including Halocynthia roretzi, show strict self-sterility. In H. roretzi, a 70-kDa vitelline coat (VC) protein consisting of 12 EGF-like repeats, named HrVC70, appears to be a promising candidate for the self/nonself-recognition (or allorecognition) system during gamete interaction. After spermatozoon recognizes the VC as nonself, sperm 700-kDa extracellular ubiquitin-conjugating enzyme complex appears to ubiquitinate Lys234 of HrVC70, and the ubiquitinated HrVC70 is degraded by the sperm 26S proteasome that is located on the sperm head surface. This novel ubiquitin-proteasome system enables spermatozoa to penetrate through the VC. Sperm trypsin-like proteases, acrosin and spermosin, also participate in fertilization, probably as sperm-side 'movable' binding proteins to the VC.

Inhibition of sperm-zona pellucida binding by a 55 kDa pig sperm protein in vitro

This study was conducted to evaluate the role of a 55 kDa pig sperm protein on the oocytesperm binding process, and its location in situ. For this purpose, in vitro matured oocytes were incubated with isolated and purified protein, and incubated with capacitated spermatozoa. In addition, capacitated sperm were incubated with anti-55 kDa antiserum and later with mature oocytes. Immunolocalization assays were performed using non-capacitated, capacitated and acrosome reacted sperm, which were incubated independently with anti-55 kDa protein antibodies and analyzed under fluorescence light microscopy. The 55 kDa protein concentrations correlated negatively with the amounts of sperm bound to the zona pellucida (ZP); the presence of the anti-55 kDa protein totally inhibited this binding. The immunolocalization assays revealed that fluorescence was located preferentially at the apical edge of the head in capacitated sperm, but not in acrosome reacted sperm. It would appear that the 55 kDa protein binds specifically to the oocyte ZP, and that it may be responsible for primary gamete binding during fertilization.

Evaluation of the proacrosin/acrosin system and its mechanism of activation in human sperm extracts

Acrosin is an acrosomal protease synthesized as a proenzyme and activated into beta-acrosin during the acrosome reaction. In the present study, a set of sensitive assays was developed to identify the proacrosin/acrosin system and to evaluate its activation pattern in human sperm extracts. Immunocytochemical analysis with monoclonal antibody (Mab) AcrC5F10 showed specific staining on the acrosome of permeabilized ejaculated and capacitated spermatozoa. Acrosome reaction was associated with a decrease in staining. AcrC5F10 specifically recognized a 55-kDa band (proacrosin) in Western immunoblots. Activation studies showed enzymatically active intermediates of 39 and 35 kDa after zymography. Immunoreactive bands of 52, 43, 34, 21-26 and 16 kDa were identified in the activation patterns developed with AcrC5F10. Activation was completely inhibited in the presence of 9 mM CaCl(2) or 100 mM benzamidine. A multiple sequence alignment revealed partial conservation of putative cleavage sites in the proacrosin sequence. The tests described allow the detection of human proacrosin in spermatozoa and sperm protein extracts, as well as the evaluation of the proenzyme activation pattern. They can be used to study the effect of inhibitors upon proenzyme activation. In addition, alterations in proacrosin activation in semen samples with abnormal acrosin enzymatic activity can be analyzed using these assays.

Inhibition of acrosin by protein C inhibitor and localization of protein C inhibitor to spermatozoa

Protein C inhibitor (PCI) is synthesized by cells throughout the male reproductive tract and is present in high concentrations (220 micrograms/ml) in seminal plasma. Seminal plasma as well as the acrosome of spermatozoa are rich in possible target proteases for PCI. We analyzed the interaction of PCI with acrosin, a serine protease stored in its zymogen form in the acrosome of spermatozoa. Purified human PCI inhibited the amidolytic activity of purified boar acrosin with an apparent second-order rate constant of 3.7 x 10(4) M-1.s-1. Inhibition was paralleled by the degradation of PCI from its 57- to its 54-kDa form. Human PCI also inhibited the amidolytic activity of activated human sperm extracts and formed complexes with acrosin as determined by an enzyme-linked immunosorbent assay. Immunocytochemistry revealed that morphologically abnormal spermatozoa stained for PCI antigen, whereas morphologically normal spermatozoa were negative. In immunoelectron microscopy, PCI was exclusively localized in the immediate vicinity of disrupted acrosomal membranes of sperm heads. These data suggest that PCI might function as a scavenger of prematurely activated acrosin, thereby protecting intact surrounding cells and seminal plasma proteins from possible proteolytic damage.

Immunolocalization of proacrosin/acrosin in rabbit sperm during acrosome reaction and in spermatozoa recovered from the perivitelline space

The participation of acrosin in mammalian sperm penetration through the zona pellucida has been amply debated. In this paper we report the immunolocalization--by silver enhanced immunogold technique using ACRO-8C10 monoclonal antibody to human acrosin--of proacrosin/acrosin on ejaculated rabbit spermatozoa incubated in vitro in a capacitating medium and on spermatozoa recovered from the perivitelline space. After incubation in a capacitating medium, four different patterns were observed: (1) no labeling on acrosome intact spermatozoa; (2) labeling on the rim of the head; (3) labeling on the whole acrosome area; and (4) no labeling on acrosome reacted spermatozoa. At the start of incubation, spermatozoa with pattern 1 were the most abundant, whereas at the end of the 32 h incubation period, patterns 2 and 3 were the most frequent. On the other hand, 625 perivitelline spermatozoa were recovered from 17 fertilized rabbit eggs, of which 26% were labeled with the antiacrosin monoclonal antibody ACRO-8C10 in two different areas: (1) only on the equatorial region; and (2) only on the postacrosomal area. These results are consistent with the idea that proacrosin/acrosin remains associated to the acrosome reacted spermatozoa for long periods of time, and that proacrosin/acrosin associated to perivitelline spermatozoa could be responsible for the second penetration of fresh rabbit eggs by perivitelline spermatozoa.

Activation and subsequent degradation of proacrosin is mediated by zona pellucida glycoproteins, negatively charged polysaccharides, and DNA

Boar proacrosin (E.C. 3.4.21.10, Mw 53 kD) was isolated by a modified method and subjected to autoactivation. Previously described molecular intermediates of 49 and 43 kD and a stable form (beta-acrosin, 35 kD) were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Autoactivation was expedited in the presence of either zona pellucida glycoproteins, fucoidan, or DNA. The end point of this accelerated conversion was the complete degradation of otherwise stable beta-acrosin via the formation of a characteristic active intermediate protein of 30 kD. All intermediate molecular forms observed during proacrosin activation/conversion exhibited the N-terminal sequence of the boar acrosin heavy chain, indicating a C-terminal processing mechanism. Hence zona pellucida glycoproteins stimulate proacrosin activation as well as acrosin degradation. Such a mechanism of proenzyme activation and degradation is to our knowledge described here for the first time and points to a previously unrecognized role of zona pellucida during gamete interaction.