Isolation and characterization of ovochymase, a chymotrypsin-like protease released during Xenopus laevis egg activation

Mechanisms of harmful effects of Microcystis aeruginosa on a brackish water organism Moina mongolica based on physiological and transcriptomic responses

To investigate the detrimental impacts of cyanobacterial bloom, specifically Microcystis aeruginosa, on brackish water ecosystems, the study used Moina mongolica, a cladoceran species, as the test organism. In a chronic toxicology experiment, the survival and reproductive rates of M. mongolica were assessed under M. aeruginosa stress. It was observed that the survival rate of M. mongolica fed with M. aeruginosa significantly decreased with time and their reproduction rate dropped to zero, while the control group remained maintained stable and normal reproduction. To further explore the underlying molecular mechanisms of the effects of M. aeruginosa on M. mongolica, we conducted a transcriptomic analysis on newly hatched M. mongolica cultured under different food conditions for 24 h. The results revealed significant expression differences in 572 genes, with 233 genes significantly up-regulated and 339 genes significantly down-regulated. Functional analysis of these differentially expressed genes identified six categories of physiological functional changes, including nutrition and metabolism, oxidative phosphorylation, neuroimmunology, cuticle and molting, reproduction, and programmed cell death. Based on these findings, we outlined the basic mechanisms of microcystin toxicity. The discovery provides critical insights into the mechanisms of Microcystis toxicity on organisms and explores the response mechanisms of cladocerans under the stress of Microcystis.

Delegating Sex: Differential Gene Expression in Stolonizing Syllids Uncovers the Hormonal Control of Reproduction

Stolonization in syllid annelids is a unique mode of reproduction among animals. During the breeding season, a structure resembling the adult but containing only gametes, called stolon, is formed generally at the posterior end of the animal. When stolons mature, they detach from the adult and gametes are released into the water column. The process is synchronized within each species, and it has been reported to be under environmental and endogenous control, probably via endocrine regulation. To further understand reproduction in syllids and to elucidate the molecular toolkit underlying stolonization, we generated Illumina RNA-seq data from different tissues of reproductive and nonreproductive individuals of Syllis magdalena and characterized gene expression during the stolonization process. Several genes involved in gametogenesis (ovochymase, vitellogenin, testis-specific serine/threonine-kinase), immune response (complement receptor 2), neuronal development (tyrosine-protein kinase Src42A), cell proliferation (alpha-1D adrenergic receptor), and steroid metabolism (hydroxysteroid dehydrogenase 2) were found differentially expressed in the different tissues and conditions analyzed. In addition, our findings suggest that several neurohormones, such as methyl farnesoate, dopamine, and serotonin, might trigger stolon formation, the correct maturation of gametes and the detachment of stolons when gametogenesis ends. The process seems to be under circadian control, as indicated by the expression patterns of r-opsins. Overall, our results shed light into the genes that orchestrate the onset of gamete formation and improve our understanding of how some hormones, previously reported to be involved in reproduction and metamorphosis processes in other invertebrates, seem to also regulate reproduction via stolonization.

Proteomics of phosphorylation and protein dynamics during fertilization and meiotic exit in the Xenopus egg

Fertilization releases the meiotic arrest and initiates the events that prepare the egg for the ensuing developmental program. Protein degradation and phosphorylation are known to regulate protein activity during this process. However, the full extent of protein loss and phosphoregulation is still unknown. We examined absolute protein and phosphosite dynamics of the fertilization response by mass spectrometry-based proteomics in electroactivated eggs. To do this, we developed an approach for calculating the stoichiometry of phosphosites from multiplexed proteomics that is compatible with dynamic, stable, and multisite phosphorylation. Overall, the data suggest that degradation is limited to a few low-abundance proteins. However, this degradation promotes extensive dephosphorylation that occurs over a wide range of abundances during meiotic exit. We also show that eggs release a large amount of protein into the medium just after fertilization, most likely related to the blocks to polyspermy. Concomitantly, there is a substantial increase in phosphorylation likely tied to calcium-activated kinases. We identify putative degradation targets and components of the slow block to polyspermy. The analytical approaches demonstrated here are broadly applicable to studies of dynamic biological systems.

Ovochymase in amphioxus Branchiostoma belcheri is an ovary-specific trypsin-like serine protease with an antibacterial activity

Ovochymases have been shown to be present in vertebrates; little information is available at present regarding ovochymase in invertebrates. Here we isolated a cDNA encoding an ovochymase homolog from amphioxus Branchiostoma belcheri, named BbOvc. The cDNA contained a 1248bp open reading frame corresponding to a deduced protein of 415 amino acids with a predicted molecular mass of approximately 44.4kDa. Phylogenetic analysis showed that BbOvc was located at the base of its vertebrate counterparts, suggesting that it represents the archetype of vertebrate ovochymases. BbOvc is found to display a tissue- and stage-specific expression pattern, with a predominant expression in the ovary of sexually matured females and in the early stage embryos (1-16-cell embryos). The recombinant ovochymase expressed in vitro shows a trypsin-like activity capable of hydrolysing the trypsin prototypic substrate N(a)-benzoyl-l-arginine ethyl ester (60UBAEE/mg), which can be inhibited by the trypsin-specific inhibitor soybean trypsin inhibitor. It also exhibits an antibacterial activity capable of inhibiting the growth of bacteria like E. coli and V. parahaemolyticus. Taken together, these data indicate that BbOvc is a novel ovochymase with an antibacterial activity and offer first clues to its role as an immune-relevant molecule which may protect the early embryos from pathogenic attacks.

The distribution of transglutaminase in the rat oocytes and embryos

Transglutaminases (TGs) are calcium-dependent enzymes that catalyze the transamidation of glutamine residues of a protein substrate to form intermolecular isopeptide bonds. The zona pellucida (ZP) is an extracellular, glycoprotein matrix that surrounds the oocytes of all Eutherian mammals. We aimed to identify the immunoreactivity of tissue transglutaminase (tTG) and ultrastructural changes occuring in rat oocytes before and after fertilization. Female rats were stimulated to superovulate, then mated with males. Oocytes and embryos were collected and examined by immunohistochemistry and electron microscopy. Before fertilization, tTG was present only in the oolemma and the cortical cytoplasm. After fertilization, tTG reactivity increased in the ZP of the early zygote and the preimplantation embryos, but decreased in the cytoplasm and perivitelline space (PVS). After fertilization, the PVS ultrastructure became asymmetrical and large around the polar bodies with many cortical granule contents. In conclusion, tTG immunoreactivity was found to be spatially and temporarily heterogeneous in the rat oocytes and embryos, especially in the ZP.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Effects of proteinase inhibitors on fertilization in sea lamprey (Petromyzon marinus)

A search for alternative sterilants in parasitic fish encouraged us to explore the usefulness of proteinase inhibitors for this purpose. Fertilization in sea lamprey species (Petromyzon marinus L.) was inhibited by chymotrypsin and trypsin inhibitors 4'-acetamidophenyl 4-guanidinobenzoate (AGB), chymostatin, tosyl-L-lysine chloromethyl ketone (TLCK), and N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) when these substances were added into a fertilization medium at the time of fertilization. Preincubation of eggs before fertilization with 100 microM TPCK, but not TLCK, resulted in inhibition of fertilization. Conversely, preincubation of spermatozoa with TLCK, but not TPCK, produced inhibition of fertilization. These data suggest the involvement of the chymotrypsin-like activity of eggs and trypsin-like activity of spermatozoa in fertilization. However, enzymes present in sperm suspensions were able to hydrolyze a chymotrypsin substrate N-glutaryl-L-phenylalanine-p-nitroanilide (GPNA) but not trypsin substrate N-alpha-benzoyl-DL-arginine-p-nitroanilide (BAPNA). The nature of this activity can be characterized as serine protease and our results indicate the involvement of serine proteinases in the fertilization of sea lamprey.

A novel human dendritic cell-derived C1r-like serine protease analog inhibits complement-mediated cytotoxicity

Trypsin-like serine proteases are involved in diverse biological processes such as complement activation, tissue remodeling, cellular migration, tumor invasion, and metastasis. Here we report a novel human C1r-like serine protease analog, CLSPa, derived from dendritic cells (DC). The 487-residue CLSPa protein contains a CUB domain and a serine protease domain, possessing characteristic catalytic triad but lacking typical activation/cleavage sequence. It shares great homology with complement C1r/C1s and mannose-associated serine proteases. CLSPa mRNA is widely expressed, especially abundant in placenta, liver, kidney, pancreas, and myeloid cells, which are a major resources of serine proteases. Upon stimulation by agonistic anti-CD40 Ab, TNF-alpha, or LPS, CLSPa mRNA expression was significantly up-regulated in monocytic cells and monocyte-derived immature DC. When overexpressed in 293T cells, CLSPa protein was synthesized into the culture supernatants as a secretory protein, which had an inhibitory effect on complement-mediated cytotoxicity to antibody-sensitized erythrocytes. However, CLSPa itself possesses little protease activity, but it plays an inhibitory role in other active protease catalytic processes. The identification of human CLSPa as a novel Clr-like protein might facilitate future investigation of the regulatory mechanism of CLSPa in complement pathways during inflammation.

The envelopes of amphibian oocytes: physiological modifications in Bufo arenarum

A characterization of the Amphibian Bufo arenarum oocyte envelope is presented. It was made in different functional conditions of the oocyte: 1) when it has been released into the coelomic cavity during ovulation (surrounded by the coelomic envelope, (CE), 2) after it has passed through the oviduct and is deposed (surrounded by the viteline envelope, (VE), and 3) after oocyte activation (surrounded by the fertilization envelope, (FE). The characterization was made by SDS-PAGE followed by staining for protein and glycoproteins. Labeled lectins were used to identify glycosidic residues both in separated components on nitrocellulose membranes or in intact oocytes and embryos. Proteolytic properties of the content of the cortical granules were also analyzed. After SDS-PAGE of CE and VE, a different protein pattern was observed. This is probably due to the activity of a protease present in the pars recta of the oviduct. Comparison of the SDS-PAGE pattern of VE and FE showed a different mobility for one of the glycoproteins, gp75. VE and FE proved to have different sugar residues in their oligosaccharide chains. Mannose residues are only present in gp120 of the three envelopes. N-acetyl-galactosamine residues are present in all of the components, except for gp69 in the FE. Galactose residues are present mainly in gp120 of FE. Lectin-binding assays indicate the presence of glucosamine, galactose and N-acetyl galactosamine residues and the absence (or non-availability) of N-acetyl-glucosamine or fucose residues on the envelopes surface. The cortical granule product (CGP) shows proteolytic activity on gp75 of the VE.

Fibroin-like substance is a major component of the outer layer of fertilization envelope via which carp egg adheres to the substratum

Seven cDNA encoding silkworm fibroin homologues were cloned from a carp ovarian cDNA library. The encoded proteins are denoted as carp ovarian fibroin-like substances (FLS). FLS contain a repetitive domain consisting of tandem repeats of dipeptide of Gly-X, where X may be any amino acid. Each FLS has its own unique repeating sequence, such as GQGAGQGS, GQGMGQGM, GRGQGEGHGS, and GFGFGQGS, indicating a family of FLS genes exists in carp. FLS is exclusively expressed in oocytes and is stored in cortical granules. During cortical reaction, FLS is exocytosed to perivitelline space and then gradually added to the outer layer of the fertilization envelope (FEo). The FLS of fertilization envelope is conjugated with cystatin and cathepsin-like substance (CLS) and appears in multiple bands of molecular weights ranging from 40 to 205 kDa. After fertilization or artificial activation, carp eggs adhere firmly to the substratum via FEo. FLS is a major component of FEo. The presence of transglutaminase inhibitor, cadaverine or ethylene diaminetetraacetic acid, in the cortical reaction medium can impair or block the recruitment of FLS and other substances to FEo. As a consequence, FEo is not formed or is greatly reduced, resulting in a great reduction of egg adhesion.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Purification, characterization, and molecular cloning of an outer layer protein of carp fertilization envelope

An outer layer protein of carp fertilization envelope (FE), FEO-1, was purified from carp oocytes. The cDNAs encoding FEO-1 were cloned. The mature protein of FEO-1 is 21 kDa in molecular weight and contains 177 amino acid residues whose sequence has 58% identity to the outer layer protein of chick vitelline membrane. In situ hybridization and immunocytochemistry show that FEO-1 is expressed in oocytes and liver. In oocytes, FEO-1 is stored in the cortical granules. During cortical reaction, it is exocytosed to the perivitelline space and then gradually added to the outer layer of FE (FE(o)). FEO-1 first appears as discrete deposits along FE(o), then merges to form a continuous layer. The thickness of FE(o) increases as cortical reaction proceeds. In addition to FEO-1, FE(o) contains cystatin, fibroin-like substance (FLS), and cathepsin-like substance (CLS) as well. They are stored in the cortical granules and are exocytosed to FE(o) simultaneously with FEO-1 during cortical reaction. In FE(o), FEO-1 is present in monomer form and can be completely extracted by sodium dodecyl sulfate (SDS)-mercaptoethanol (MSH). On the other hand, the cystatin, FLS, and CLS present in FE(o)are cross-linked together. They are partially extracted by SDS-MSH but can be completely extracted by guanidium thiocyanate-lauroylsarcosine.

Ovochymase, a Xenopus laevis egg extracellular protease, is translated as part of an unusual polyprotease

Ovochymase, an extracellular Xenopus laevis egg serine active-site protease with chymotrypsin-like (Phe-X) substrate specificity, is released during egg activation. Molecular cloning results revealed that ovochymase is translated as part of an unusual polyprotein proenzyme. In addition to the ovochymase protease domain at the C terminus of the deduced amino acid sequence, two unrelated serine protease domains were present, each with apparent trypsin-like (Arg/Lys-X) substrate specificity, and thus, they were designated ovotryptase1 (at the N terminus) and ovotryptase2 (a mid domain). Also, a total of five CUB domains were interspersed between the protease domains. The presence of a hydrophobic signal sequence indicated that the polyprotein was secreted. Immunolocalization and Western blot studies of all three proteases showed that they are all present in the perivitelline space of unactivated eggs, apparently as proenzymes processed away from the original polyprotein. Western blot analysis also showed that the vast majority of the proteases in ovary, eggs, and embryos were present as the proenzyme forms, suggesting that the functions of these proteases depend on very limited levels of activation.

The cortical granule serine protease CGSP1 of the sea urchin, Strongylocentrotus purpuratus, is autocatalytic and contains a low-density lipoprotein receptor-like domain

Trypsin-like activity is secreted from eggs of many species at fertilization, and this activity is believed to be critical for the block to polyspermy. Here we show that a cortical granule serine protease of sea urchins is the major and perhaps only protease family member important for fertilization. Zymography assays suggest that the cortical granules contain a single serine protease that can undergo autocatalysis and is secreted upon egg activation. We used this finding to identify a cDNA clone from a Strongylocentrotus purpuratus ovary cDNA library that encodes a 581-amino-acid-residue protein that we refer to as cortical granule serine protease 1 (CGSP1). The catalytic domain of the protein contains the essential residues of the catalytic triad characteristic of a member of the trypsin-like family of serine proteases and the N-terminus of CGSP1 resembles the ligand-binding domain of the low-density lipoprotein (LDL) receptor. Antibodies raised separately to both the protease and LDL receptor-like domains each localize to the cortical granules of unfertilized eggs. Furthermore, the full-length form of CGSP1, as well as intermediate and active forms of the protease, is detected in cortical granules by immunoblot analysis. Our evidence suggests that CGSP1 is activated at fertilization and is responsible for the protease-mediated reactions that follow cortical granule exocytosis and contribute to the block to polyspermy.

Members of the SNARE hypothesis are associated with cortical granule exocytosis in the sea urchin egg

Cortical granule exocytosis is important for the block to polyspermy at fertilization in the eggs of most vertebrates and many invertebrates. Cortical granules are poised at the cell surface and exocytose in response to sperm stimulation. Following exocytosis, the cortical granule contents modify the extracellular environment of the egg, the major result of which is to block additional sperm binding. Here we show that proteins homologous to members of the SNARE hypothesis-a molecular model designed to explain the trafficking, docking, and exocytosis of vesicles in the secretory compartment-are present in eggs at the right time and place to be involved in the regulation of cortical granule exocytosis. Using polymerase chain reaction (PCR) screens we have found homologues of synaptobrevin/VAMP, syntaxin, synaptotagmin, and rab3. Antibodies generated to fusion proteins or to synthetic peptides encoded by the cloned cDNAs were used in an immunofluorescence assay to show that each of the cognate proteins are present in the cortex of the egg. A synaptobrevin/VAMP homologue appears to be specifically associated with the membrane of cortical granules before fertilization and, following cortical granule exocytosis, is incorporated into the plasma membrane of the zygote. A rab3 homologue is also associated with cortical granules specifically but, following fertilization, the protein reassociates with different, yet undefined, vesicles throughout the cytoplasm of the zygote. Homologues of synaptotagmin and syntaxin are also present at the egg cortex but, in contrast to rab3 and VAMP, appear to be associated with the plasma membrane. Following fertilization, syntaxin and tagmin remain associated with the plasma membrane and are more readily immunolabeled, presumably due to an increased accessibility of the antibodies to the target protein domains. We also show by immunoblotting experiments that the cognate proteins are of the sizes predicted for these homologues. These results suggest that at least some steps in the biology of cortical granules may be mediated by SNARE homologues, and this finding, along with the unique biology of cortical granules, should facilitate examination of specific events of the fertilization reaction and the mechanism of stimulus-dependent exocytosis.

A major glycoprotein of Xenopus egg vitelline envelope, gp41, is a frog homolog of mammalian ZP3

A predominant glycoprotein in the vitelline envelope (VE) of the anuran Xenopus laevis is gp41, known to be proteolytically converted from gp43 of the coelomic egg envelope concomitant with the acquisition of egg fertilizability. To characterize the protein core of gp41, purified gp41 from VE was digested with lysyl endopeptidase, and peptides isolated from the digests were sequenced for amino acids to design degenerate primers for polymerase chain reaction. By reverse transcription-polymerase chain reaction with a poly(A)+ RNA from the ovary of an ovulated female Xenopus, a specifically amplified band was obtained and sequenced. The upstream and downstream sequences of the sequenced region were completed by 5'- and 3'-rapid amplification of cDNA ends, respectively. The cDNA, referred to as gp43 cDNA, comprises 1423 base pairs and contains one open reading frame with a sequence for 460 amino acids. The predicted amino acid sequence of gp43 cDNA has a close similarity with that of mammalian ZP3. Northern blot and in situ hybridization studies indicated that gp43 mRNA is expressed in oocytes, particularly in the previtellogenic oocytes. A comparison of the N-terminal sequences of gp41 and gp43 strongly suggested that gp41 is generated at least by processing of the N-terminal portion of gp43 with oviductin.

Xenopus laevis sperm-egg adhesion is regulated by modifications in the sperm receptor and the egg vitelline envelope

The biochemical and ultrastructural changes in the envelope of the Xenopus laevis egg that occur during oviposition and fertilization have been thoroughly studied (Hedrick, J. L., and Nishihara, D. M., Methods Cell Biol. 36, 231-247, 1991; Larabell, C. A., and Chandler, D. E., J. Electron Microsc. Tech. 17, 294-318, 1991). However, the biological significance of these changes with respect to gamete interaction has been unclear. In the current study, it was found that changes in the envelope are directly responsible for regulating sperm-egg adhesion, an initial step of fertilization. As a result of these transformations, sperm bind only to unfertilized oviposited eggs, not to oocytes or coelomic eggs. In addition, they do not bind to fertilized eggs. The molecular and cellular basis of the regulation of the sperm binding process was investigated in the context of our recent findings that two structurally related envelope glycoproteins, gp69/64, serve as sperm receptors during fertilization (Tian, J.-D., Gong, H., Thomsen, G. H., and Lennarz, W. J., J. Cell Biol. 136, 1099-1108, 1997). Although the purified gp69/64 glycoproteins isolated from the oocyte or coelomic egg envelopes exhibited sperm binding activity, when these proteins are part of the intact oocyte or coelomic egg envelopes, they are not accessible to either anti-gp69/64 antibodies or to sperm. During the conversion from the coelomic to the vitelline envelope, the gp69/64 sperm receptors become exposed on the surface, an event that correlates with proteolytic cleavage of gp43 and accompanying ultrastructural alterations in the envelope. Conversely, after fertilization, when the vitelline envelope of the egg is converted to the fertilization envelope of the zygote, limited proteolytic cleavage of the sperm receptor results in loss of sperm binding activity. In addition, formation of a fertilization layer on top of the structurally altered VE adds another physical block to sperm binding. These results provide new insights into structure-function relationships between envelope components of the anuran egg, and provide further evidence supporting the key role of gp69/64 as sperm receptors during X. laevis fertilization.

Characterization of pNiXa, a serpin of Xenopus laevis oocytes and embryos, and its histidine-rich, Ni(II)-binding domain

A Ni(II)-binding serpin, pNiXa, is abundant in Xenopus oocytes and embryos. Kinetic assays show that purified pNiXa strongly inhibits bovine alpha-chymotrypsin (Ki = 3 mM), weakly inhibits porcine elastase (K1 = 0.5 microM), and does not inhibit bovine trypsin. The reversible, slow-binding inhibition of alpha-chymotrypsin by pNiXa is unaffected by Ni(II). Ovochymase in egg exudates is inhibited by pNiXa, but to a limited extent, even at high pNiXa concentrations. An octadecapeptide that models the His-rich domain (-HRHRHEQQGHHDSAKHGH-) of pNiXa forms six-coordinate, octahedral Ni(II)-complexes when the N-terminus is acetylated, and a square-planar Ni(II)-complex when the N-terminus is unblocked. Spectroscopy reveals two distinct types of octahedral Ni(II)-coordination to the N-acetylated octadecapeptide, involving, respectively, 3-4 and 5-6 imidazole nitrogens; the octadecapeptide undergoes partial, reversible precipitation in pH- and Ni(II)-dependent fashion, suggesting an insoluble, Ni(II)-coupled (Hx)n-dimer. Such (Hx)n-peptide interaction is confirmed by an enzyme-linked biotinavidin assay with N-biotin-KHRHRHE-amide and N-acetyl-KHRHRHE-resin beads, which become coupled after adding Ni(II) or Zn(II). H2O2 oxidation of 2'-deoxyguanosine to mutagenic 8-hydroxy-2'-deoxyguanosine is enhanced by the octahedral Ni(II)-octadecapeptide complex, although the effect is more intense with the square-planar Ni(II)-octadecapeptide complex. Immunoperoxidase staining of whole mounts with pNiXa antibody shows that pNiXa is distributed throughout gastrula-stage embryos and is localized during organogenesis in the brain, eye, spinal cord, myotomes, craniofacial tissues, and other sites of Ni(II)-induced anomalies. Patterns of pNiXa staining are similar in controls and Ni(II)-exposed embryos. Binding of Ni(II) to pNiXa may cause embryotoxicity by enhancing oxidative reactions that produce tissue injury and genotoxicity. Although the natural target proteinases for pNiXa inhibition have not been established, pNiXa may be an important regulator of proteolysis during embryonic development.