Structure and developmental expression of hatching enzyme genes of the Japanese eel Anguilla japonica : an aspect of the evolution of fish hatching enzyme gene

Molecular evolution of hatching enzymes and their paralogous genes in vertebrates

BACKGROUND

Hatching is identified as one of the most important events in the reproduction of oviparous vertebrates. The genes for hatching enzymes, which are vital in the hatching process, are conserved among vertebrates. However, especially in teleost, it is difficult to trace their molecular evolution in detail due to the presence of other C6astacins, which are the subfamily to which the genes for hatching enzymes belong and are highly diverged. In particular, the hatching enzyme genes are diversified with frequent genome translocations due to retrocopy.

RESULTS

In this study, we took advantage of the rapid expansion of whole-genome data in recent years to examine the molecular evolutionary process of these genes in vertebrates. The phylogenetic analysis and the genomic synteny analysis revealed C6astacin genes other than the hatching enzyme genes, which was previously considered to be retained only in teleosts, was also retained in the genomes of basal ray-finned fishes, coelacanths, and cartilaginous fishes. These results suggest that the common ancestor of these genes can be traced back to at least the common ancestor of the Gnathostomata. Moreover, we also found that many of the C6astacin genes underwent multiple gene duplications during vertebrate evolution, and the results of gene expression analysis in frogs implied that genes derived from hatching enzyme genes underwent neo-functionalization.

CONCLUSIONS

In this study, we describe in detail the molecular evolution of the C6astacin gene in vertebrates, which has not been summarized previously. The results revealed the presence of the previously unknown C6astacin gene in the basal-lineage of jawed vertebrates and large-scale gene duplication of hatching enzyme genes in amphibians. The comprehensive investigation reported in this study will be an important basis for studying the molecular evolution of the vertebrate C6astacin genes, hatching enzyme, and its paralogous genes and for identifying these genes without the need for gene expression and functional analysis.

Effects of hatching enzymes on egg envelope digestion in the male-brooding seahorse

In the present study, we aimed to evaluate the effects of hatching enzymes on the egg envelope digestion during the hatching period in the male brooding seahorse. The complementary DNAs encoding two hatching-enzyme genes, high choriolytic enzyme (HCE) and low choriolytic enzyme (LCE), were cloned and functionally characterized from the lined seahorse (Hippocampus erectus). The genomic-synteny analysis confirmed that teleosts shared LCE gene synteny. In contrast, the genomic location of HCE was found to be conserved with pipefish, but not other teleosts, suggesting that translocation into a novel genomic location occurred. Whole-mount in situ hybridization showed that HCE and LCE mRNAs were expressed in hatching gland cells. To determine the digestion mechanisms of HCE and LCE in hatching, recombinant HCE and LCE were generated and their enzyme activities were examined using fertilized egg envelopes and synthetic peptides. Seahorse HCE and LCE independently digested and softened the egg envelopes of the lined seahorse. Although the egg envelope was digested more following HCE and LCE co-treatment, envelope solubilization was not observed. Indeed, both HCE and LCE showed similar substrate specificities toward four different synthetic peptides designed from the cleavage sites of egg envelope proteins. HCE and LCE proteins from other euteleostean fishes showed different specificities, and the egg envelope was solubilized by the cooperative action of HCE and LCE. These results suggest that the function of LCE was degenerated in the lined seahorse. Our results imply a digestion mechanism for evolutionary adaptation in ovoviviparous fish with male pregnancy.

Release of eDNA by different life history stages and during spawning activities of laboratory-reared Japanese eels for interpretation of oceanic survey data

To assist in detection of offshore spawning activities of the Japanese eel Anguilla japonica and facilitate interpretation of results of environmental DNA (eDNA) analysis in their spawning area, we examined the eDNA concentration released by each life history stage of artificially reared Japanese eels in the laboratory using quantitative real-time PCR (qPCR). We also compared eDNA concentrations between before and after artificially induced spawning activities. eDNA was not detected from three 30 L seawater tanks containing each single fertilized egg, but eDNA was found from other tanks each containing single individuals of larval stages (preleptocephalus and leptocephalus), juvenile stages (glass eel, elver and yellow eel) or adult stage (silver eel). The eDNA concentrations increased in the life history stages, showed a significant difference among all stages, and were positively correlated with the total length and wet weight. Moreover, the eDNA concentration after spawning was 10-200 times higher than that before spawning, which indicated that the spawning events in the ocean would produce relatively high eDNA concentration. These results in the laboratory suggested that eDNA analysis appears to be an effective method for assisting oceanic surveys to estimate the presence and spawning events of the Japanese eel in the spawning area.

Translocation of promoter-conserved hatching enzyme genes with intron-loss provides a new insight in the role of retrocopy during teleostean evolution

The hatcing enzyme gene (HE) encodes a protease that is indispensable for the hatching process and is conserved during vertebrate evolution. During teleostean evolution, it is known that HE experienced a drastic transfiguration of gene structure, namely, losing all of its introns. However, these facts are contradiction with each other, since intron-less genes typically lose their original promoter because of duplication via mature mRNA, called retrocopy. Here, using a comparative genomic assay, we showed that HEs have changed their genomic location several times, with the evolutionary timings of these translocations being identical to those of intron-loss. We further showed that HEs maintain the promoter sequence upstream of them after translocation. Therefore, teleostean HEs are unique genes which have changed intra- (exon-intron) and extra-genomic structure (genomic loci) several times, although their indispensability for the reproductive process of hatching implies that HE genes are translocated by retrocopy with their promoter sequence.

The light cycle controls the hatching rhythm in Bombyx mori via negative feedback loop of the circadian oscillator

Hatching behavior is a key target in silkworm (Bombyx mori) rearing, especially for the control of Lepidoptera pests. According to previous research, hatching rhythms appear to be controlled by a clock mechanism that restricts or "gates" hatching to a particular time. However, the underlying mechanism remains elusive. Under 12-h light:12-h dark photoperiod (LD) conditions, the transcriptional levels of the chitinase5 (Cht5) and hatching enzyme-like (Hel) genes, as well as the enzymatic activities of their gene products, oscillated in time with ambient light cycles, as did the transcriptional levels of the cryptochrome 1, cryptochrome 2, period (per), and timeless genes, which are key components of the negative feedback loop of the circadian rhythm. These changes were related to the expression profile of the ecdysteroid receptor gene and the hatching behavior of B. mori eggs. However, under continuous light or dark conditions, the hatching behavior, the expression levels of Cht5 and Hel, as well as the enzymatic activities of their gene products, were not synchronized unlike under LD conditions. In addition, immunohistochemistry experiments showed that light promoted the translocation of PER from the cytoplasm to the nucleus. In conclusion, LD cycles regulate the hatching rhythm of B. mori via negative feedback loop of the circadian oscillator.

Evaluation of Tanshinone IIA Developmental Toxicity in Zebrafish Embryos

Tanshinone IIA (Tan-IIA) is derived from the dried roots of Salvia miltiorrhiza Bunge, a traditional Chinese medicine. Although Salvia miltiorrhiza has been applied for many years, the toxicity of the mono-constituent of Salvia miltiorrhiza, tanshinone IIA, is still understudied. This study evaluated the cardiotoxicity and developmental malformations of Tan-IIA by using zebrafish normal embryos and dechorionated embryos. After treatment with Tan-IIA in different concentrations for four-day periods, obvious pericardial edema, spinal curvature, and even missing tails were observed in zebrafish embryos. The LC₅₀ values in the dechorionated embryo group at 72 h post-fertilization (hpf) and 96 hpf were 18.5 μM and 12.8 μM, respectively, and the teratogenicity was manifested at a concentration of about 1 µM. The main endpoints of teratogenicity were scoliosis, malformation of tail, and pericardium edema. Our findings displayed the potential cardiotoxicity and severe impact on the abnormal development of Tan-IIA in zebrafish embryo at high concentrations, which may help avoid the risk of its clinical application.

Evolutionary Changes in the Developmental Origin of Hatching Gland Cells in Basal Ray-Finned Fishes

Hatching gland cells (HGCs) originate from different germ layers between frogs and teleosts, although the hatching enzyme genes are orthologous. Teleostei HGCs differentiate in the mesoendodermal cells at the anterior end of the involved hypoblast layer (known as the polster) in late gastrula embryos. Conversely, frog HGCs differentiate in the epidermal cells at the neural plate border in early neurula embryos. To infer the transition in the developmental origin of HGCs, we studied two basal ray-finned fishes, bichir (Polypterus) and sturgeon. We observed expression patterns of their hatching enzyme (HE) and that of three transcription factors that are critical for HGC differentiation: KLF17 is common to both teleosts and frogs; whereas FoxA3 and Pax3 are specific to teleosts and frogs, respectively. We then inferred the transition in the developmental origin of HGCs. In sturgeon, the KLF17, FoxA3, and HE genes were expressed during the tailbud stage in the cell mass at the anterior region of the body axis, a region corresponding to the polster in teleost embryos. In contrast, the bichir was suggested to possess both teleost- and amphibian-type HGCs, i.e. the KLF17 and FoxA3 genes were expressed in the anterior cell mass corresponding to the polster, and the KLF17, Pax3 and HE genes were expressed in dorsal epidermal layer of the head. The change in developmental origin is thought to have occurred during the evolution of basal ray-finned fish, because bichir has two HGCs, while sturgeon only has the teleost-type.

Neofunctionalization of a duplicate hatching enzyme gene during the evolution of teleost fishes

BACKGROUND

Duplication and subsequent neofunctionalization of the teleostean hatching enzyme gene occurred in the common ancestor of Euteleostei and Otocephala, producing two genes belonging to different phylogenetic clades (clade I and II). In euteleosts, the clade I enzyme inherited the activity of the ancestral enzyme of swelling the egg envelope by cleavage of the N-terminal region of egg envelope proteins. The clade II enzyme gained two specific cleavage sites, N-ZPd and mid-ZPd but lost the ancestral activity. Thus, euteleostean clade II enzymes assumed a new function; solubilization of the egg envelope by the cooperative action with clade I enzyme. However, in Otocephala, the clade II gene was lost during evolution. Consequently, in a late group of Otocephala, only the clade I enzyme is present to swell the egg envelope. We evaluated the egg envelope digestion properties of clade I and II enzymes in Gonorynchiformes, an early diverging group of Otocephala, using milkfish, and compared their digestion with those of other fishes. Finally, we propose a hypothesis of the neofunctionalization process.

RESULTS

The milkfish clade II enzyme cleaved N-ZPd but not mid-ZPd, and did not cause solubilization of the egg envelope. We conclude that neofunctionalization is incomplete in the otocephalan clade II enzymes. Comparison of clade I and clade II enzyme characteristics implies that the specificity of the clade II enzymes gradually changed during evolution after the duplication event, and that a change in substrate was required for the addition of the mid-ZPd site and loss of activity at the N-terminal region.

CONCLUSIONS

We infer the process of neofunctionalization of the clade II enzyme after duplication of the gene. The ancestral clade II gene gained N-ZPd cleavage activity in the common ancestral lineage of the Euteleostei and Otocephala. Subsequently, acquisition of cleavage activity at the mid-ZPd site and loss of cleavage activity in the N-terminal region occurred during the evolution of Euteleostei, but not of Otocephala. The clade II enzyme provides an example of the development of a neofunctional gene for which the substrate, the egg envelope protein, has adapted to a gradual change in the specificity of the corresponding enzyme.

Primitive duplicate Hox clusters in the European eel's genome

The enigmatic life cycle and elongated body of the European eel (Anguilla anguilla L., 1758) have long motivated scientific enquiry. Recently, eel research has gained in urgency, as the population has dwindled to the point of critical endangerment. We have assembled a draft genome in order to facilitate advances in all provinces of eel biology. Here, we use the genome to investigate the eel's complement of the Hox developmental transcription factors. We show that unlike any other teleost fish, the eel retains fully populated, duplicate Hox clusters, which originated at the teleost-specific genome duplication. Using mRNA-sequencing and in situ hybridizations, we demonstrate that all copies are expressed in early embryos. Theories of vertebrate evolution predict that the retention of functional, duplicate Hox genes can give rise to additional developmental complexity, which is not immediately apparent in the adult. However, the key morphological innovation elsewhere in the eel's life history coincides with the evolutionary origin of its Hox repertoire.

Molecular cloning and characterization of hatching enzyme-like geneII (BmHELII) in the silkworm, Bombyx mori

Hatching enzyme (HE) is an enzyme that digests an egg envelop at the time of embryo hatching. Previously, we have reported a kind of Bombyx mori hatching enzyme-like gene (BmHEL). In this paper, the full length of another BmHEL cDNA sequence (BmHELII, GenBank ID: JN627443) was cloned from bluish-silkworm-eggs. The cDNA was 977 bp in length with an open reading frame of 885 bp which encodes a polypeptide of 294 amino acids including a putative signal peptide of 16 amino acid residues and a mature protein of 278 amino acids. The deduced BmHELII had a predicted molecular mass of 33.62 kDa, isoelectric point of 5.44 and two conserved signature sequences of astacin family. Bioinformatic analysis results showed that the deduced protease domain amino acid sequence of BmHELII had 29.5-87.0% identities to that of HE identified in the other species. The BmHELII gene structure was 6-exon-5-intron, and the promoter region harbored some basal promoter elements and some embryo development related transcription factor binding sites. Semi-quantitative RT-PCR analysis revealed that the relative level of BmHELII transcripts at different stages during egg incubation increased with the development of embryos and reached to a maximum just before hatching, hence declined gradually after hatching. The spatio-temporal expression pattern of BmHELII basically resembled that of hatching enzyme gene. Moreover, the BmHELII transcript was detected in testis of the silkworm, and semi-quantitative RT-PCR analysis showed that it kept at the high level in testis of silkworm from larvae to moth, which suggested that BmHELII might take part in the development of sperm. These results will be helpful to provide a molecular basis for understanding the mechanism underlying silkworm hatching as well as spermatogenesis.

Oocyte specific oolemmal SAS1B involved in sperm binding through intra-acrosomal SLLP1 during fertilization

Molecular mechanisms by which fertilization competent acrosome-reacted sperm bind to the oolemma remain uncharacterized. To identify oolemmal binding partner(s) for sperm acrosomal ligands, affinity panning was performed with mouse oocyte lysates using sperm acrosomal protein, SLLP1 as a target. An oocyte specific membrane metalloproteinase, SAS1B (Sperm Acrosomal SLLP1 Binding), was identified as a SLLP1 binding partner. cDNA cloning revealed six SAS1B splice variants, each containing a zinc binding active site and a putative transmembrane domain, with signal peptides in three variants. SAS1B transcripts were ovary specific. SAS1B protein was first detected in early secondary follicles in day 3 ovaries. Immunofluorescence localized SAS1B to the microvillar oolemma of M2 oocytes. After fertilization, SAS1B decreased on the oolemma and became virtually undetectable in blastocysts. In transfected CHO-K1 cells SAS1B localized to the surface of unpermeabilized cells. Recombinant and native SLLP1 co-localized with SAS1B to the microvillar domain of ovulated M2 oocytes. Molecular interactions between mouse SLLP1 and SAS1B were demonstrated by surface plasmon resonance, far-western, yeast two-hybrid, recombinant- and native-co-IP analyses. SAS1B bound to SLLP1 with high affinity. SAS1B had protease activity, and SAS1B protein or antibody significantly inhibited fertilization. SAS1B knockout female mice showed a 34% reduction in fertility. The study identified SAS1B-SLLP1 as a pair of novel sperm-egg binding partners involving the oolemma and intra-acrosomal compartment during fertilization.

Taking the plunge: California Grunion embryos emerge rapidly with environmentally cued hatching

The process of hatching has been well studied in some model species of teleosts: the medaka Oryzias latipes, the mummichog Fundulus heteroclitus, and the zebrafish Danio rerio. These models are compared to the California Grunion, Leuresthes tenuis that has some unique features of reproduction related to tidal synchrony of spawning and environmentally cued hatching (ECH). During oviposition at spring tides, this marine teleost spawns out of water to bury its clutches on sandy beaches in the high intertidal zone. After embryos of L. tenuis reach hatching competence, hatching can be triggered at any time. Incubation above the water line inhibits hatching until ECH is triggered by rising tides during the following lunar phase, and hatching occurs within a few seconds. We review the embryo's response to environmental cues at hatching and the effects of the surrounding medium on the chorionase and chorion for this form of ECH. Leuresthes tenuis shares some similarities as well as some important differences with the model species. Comparison of hatching across teleostean taxa indicates great variability in stage at hatching and in duration of incubation that suggest hatching plasticity in response to environmental cues may be more widespread than currently appreciated.

Regulation of zebrafish hatching by tetraspanin cd63

Tetraspanins cause the clustering of membrane proteins into a level of organisation essential for cellular function. Given the importance and complicated nature of this mechanism, we attempted a novel approach to identify the function of a single component in a biologically relevant context. A morpholino knockdown strategy was used to investigate the role of cd63, a membrane protein associated with intracellular transport and a melanoma marker, in embryonic zebrafish. By using three separate morpholinos targeting cd63, we were able to identify a specific phenotype. Strikingly, morphant fish failed to hatch due to the lack of secreted proteolytic enzymes required for chorion-softening. The morphology of the hatching gland at both the cellular and intracellular levels was disorganised, suggesting a role for cd63 in the functioning of this organ. This work identifies a specific role for cd63 in the zebrafish embryo and provides evidence for the suitability of zebrafish as a model system for the investigation of tetraspanin enriched microdomains.

Intron-loss evolution of hatching enzyme genes in Teleostei

Background

Hatching enzyme, belonging to the astacin metallo-protease family, digests egg envelope at embryo hatching. Orthologous genes of the enzyme are found in all vertebrate genomes. Recently, we found that exon-intron structures of the genes were conserved among tetrapods, while the genes of teleosts frequently lost their introns. Occurrence of such intron losses in teleostean hatching enzyme genes is an uncommon evolutionary event, as most eukaryotic genes are generally known to be interrupted by introns and the intron insertion sites are conserved from species to species. Here, we report on extensive studies of the exon-intron structures of teleostean hatching enzyme genes for insight into how and why introns were lost during evolution.

Results

We investigated the evolutionary pathway of intron-losses in hatching enzyme genes of 27 species of Teleostei. Hatching enzyme genes of basal teleosts are of only one type, which conserves the 9-exon-8-intron structure of an assumed ancestor. On the other hand, otocephalans and euteleosts possess two types of hatching enzyme genes, suggesting a gene duplication event in the common ancestor of otocephalans and euteleosts. The duplicated genes were classified into two clades, clades I and II, based on phylogenetic analysis. In otocephalans and euteleosts, clade I genes developed a phylogeny-specific structure, such as an 8-exon-7-intron, 5-exon-4-intron, 4-exon-3-intron or intron-less structure. In contrast to the clade I genes, the structures of clade II genes were relatively stable in their configuration, and were similar to that of the ancestral genes. Expression analyses revealed that hatching enzyme genes were high-expression genes, when compared to that of housekeeping genes. When expression levels were compared between clade I and II genes, clade I genes tends to be expressed more highly than clade II genes.

Conclusions

Hatching enzyme genes evolved to lose their introns, and the intron-loss events occurred at the specific points of teleostean phylogeny. We propose that the high-expression hatching enzyme genes frequently lost their introns during the evolution of teleosts, while the low-expression genes maintained the exon-intron structure of the ancestral gene.

Crystal structure of zebrafish hatching enzyme 1 from the zebrafish Danio rerio

Fish hatching enzymes are zinc metalloproteases that digest the egg envelope (chorion) at the time of hatching. The crystal structure of zebrafish hatching enzyme 1 (ZHE1) has been solved at 1.10 Å resolution. ZHE1 is monomeric, is mitten shaped, and has a cleft at the center of the molecule. ZHE1 consists of three 3(10)-helices, three α-helices, and two β-sheets. The central cleft represents the active site of the enzyme that is crucial for substrate recognition and catalysis. Alanine-scanning mutagenesis of the two substrate peptides has shown that AspP1' contributes the most and that the residues at P4-P2' also contribute to the recognition of the major substrate peptide by ZHE1, whereas GluP3' and the hydrophobic residues at P4-P2, P2', and P5' contribute significantly to the recognition of the minor substrate peptide by ZHE1. Molecular models of these two substrate peptides bound to ZHE1 have been built based on the crystal structure of a transition-state analog inhibitor bound to astacin. In substrate-recognition models, the AspP1' in the major substrate peptide forms a salt bridge with Arg182 of ZHE1, while the GluP3' in the minor substrate peptide instead forms a salt bridge with Arg182. Thus, these two substrate peptides would be differently recognized by ZHE1. The shapes and electrostatic potentials of the substrate-binding clefts of ZHE1 and the structurally similar proteins astacin and bone morphogenetic protein 1 are significantly dissimilar due to different side chains, which would confer their distinctive substrate preferences.

Purification and characterization of hatching enzyme from brine shrimp Artemia salina

By using Artemia chorion as a specific substrate, the hatching enzyme from Artemia salina (AHE) was purified by gel-filtration and ion-exchange chromatography, and characterized biochemically and enzymatically in this study. It was found that the AHE had a molecular weight of 82.2 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and often contained 73.3 kDa molecules in preparation. The AHE had obvious choriolytic activity, which was optimal at pH 7.0 and a temperature of 408C. The Km value of the AHE for dimethyl casein was 8.20 mg/ml. The AHE activity was almost completely inhibited by soybean trypsin inhibitor and p-amidinophenyl methane sulfonyl fluoride hydrochloride, greatly inhibited by N-tosyl-L-lysyl chloromethyl ketone, phenylmethanesulfonyl fluoride, and lima bean trypsin inhibitor, slightly inhibited by pepstatin, N-tosyl-L-phenylalanyl chloromethyl ketone, leupeptin, N-ethylmaleimide, and iodoacetamide, and not inhibited by chymostatin and bestatin. All these results imply that AHE is most probably a trypsin-type serine protease. Besides of these, AHE was also sensitive to EDTA and Zn21. Combined with the results that the EDTA-pre-treated HE activity could be perfectly recovered by Zn21, it is indicated that AHE might be also a kind of Zn-metalloprotease.

Different hatching strategies in embryos of two species, pacific herring Clupea pallasii and Japanese anchovy Engraulis japonicus, that belong to the same order Clupeiformes, and their environmental adaptation

Pacific herring Clupea pallasii and Japanese anchovy Engraulis japonicus, which belong to the same order Clupeiformes, spawn different types of eggs: demersal adherent eggs and pelagic eggs, respectively. We cloned three cDNAs for Pacific herring hatching enzyme and five for Japanese anchovy. Each of them was divided into two groups (group A and B) by phylogenetic analysis. They were expressed specifically in hatching gland cells (HGCs), which differentiated from the pillow and migrated to the edge of the head in both species. HGCs of Japanese anchovy stopped migration at that place, whereas those of Pacific herring continued to migrate dorsally and distributed widely all over the head region. During evolution, the program for the HGC migration would be varied to adapt to different hatching timing. Analysis of the gene expression revealed that Pacific herring embryos synthesized a large amount of hatching enzyme when compared with Japanese anchovy. Chorion of Pacific herring embryo was about 7.5 times thicker than that of Japanese anchovy embryo. Thus, the difference in their gene expression levels between two species is correlated with the difference in the thickness of chorion. These results suggest that the hatching system of each fish adapted to its respective hatching environment. Finally, hatching enzyme genes were cloned from each genomic DNA. The exon-intron structure of group B genes basically conserved that of the ancestral gene, whereas group A genes lost one intron. Several gene-specific changes of the exon-intron structure owing to nucleotide insertion and/or duplication were found in Japanese anchovy genes.

Molecular cloning and characterization of hatching enzyme-like gene in the silkworm, Bombyx mori

Hatching is the important process for the life of the metazoan, in which hatching enzyme (HE) plays a key role. In this paper, we cloned the full-length sequence of hatching enzyme-like cDNA from bluish-silkworm-eggs of Bombyx mori (BmHEL) by the method of in silico cloning, SMART cDNA synthesis and RACE-PCR technique. The BmHEL is 974 bp in length, and contains an ORF of 885 bp, encoding 294 amino acids residues. The deduced amino acid sequence of BmHEL has 30.3-47.1% identities to that of HE identified in the other species. Two similar signature sequences of HE gene family harbor in the BmHEL. The BmHEL gene structure is 6-exon-5-intron, and a promoter region with high scores has been predicted, which harbors some basal elements and some embryo-development related transcription factor binding sites. In the silkworm eggs at different developmental stages during incubation, the BmHEL transcripts can be detected and keep at a low level during the early stages, increase dramatically since 7th day of incubation, and reach to the maximum on 9th day. Change of BmHEL transcripts is in accordance with the process of embryo development and hatching, indicated that it plays an important role in these processes. Moreover, BmHEL transcript can be detected in the midgut and testis at larval stage, suggested that BmHEL may have other biological functions. To the best of our knowledge, this is the first report on HE gene in the Lepidoptera insects and will be helpful to provide a molecular basis for understanding the complicated mechanism underlying silkworm hatching.

Purification and characterization of zebrafish hatching enzyme - an evolutionary aspect of the mechanism of egg envelope digestion

There are two hatching enzyme homologues in the zebrafish genome: zebrafish hatching enzyme ZHE1 and ZHE2. Northern blot and RT-PCR analysis revealed that ZHE1 was mainly expressed in pre-hatching embryos, whereas ZHE2 was rarely expressed. This was consistent with the results obtained in an experiment conducted at the protein level, which demonstrated that one kind of hatching enzyme, ZHE1, was able to be purified from the hatching liquid. Therefore, the hatching of zebrafish embryo is performed by a single enzyme, different from the finding that the medaka hatching enzyme is an enzyme system composed of two enzymes, medaka high choriolytic enzyme (MHCE) and medaka low choriolytic enzyme (MLCE), which cooperatively digest the egg envelope. The six ZHE1-cleaving sites were located in the N-terminal regions of egg envelope subunit proteins, ZP2 and ZP3, but not in the internal regions, such as the ZP domains. The digestion manner of ZHE1 appears to be highly analogous to that of MHCE, which partially digests the egg envelope and swells the envelope. The cross-species digestion using enzymes and substrates of zebrafish and medaka revealed that both ZHE1 and MHCE cleaved the same sites of the egg envelope proteins of two species, suggesting that the substrate specificity of ZHE1 is quite similar to that of MHCE. However, MLCE did not show such similarity. Because HCE and LCE are the result of gene duplication in the evolutionary pathway of Teleostei, the present study suggests that ZHE1 and MHCE maintain the character of an ancestral hatching enzyme, and that MLCE acquires a new function, such as promoting the complete digestion of the egg envelope swollen by MHCE.

A global role for zebrafish klf4 in embryonic erythropoiesis

There are two waves of erythropoiesis, known as primitive and definitive waves in mammals and lower vertebrates including zebrafish. The founding member of the Kruppel-like factor (KLF) family of CACCC-box binding proteins, EKLF/Klf1, is essential for definitive erythropoiesis in mammals but only plays a minor role in primitive erythropoiesis. Morpholino knockdown experiments have shown a role for zebrafish klf4 in primitive erythropoiesis and hatching gland formation. In order to generate a global understanding of how klf4 might influence gene expression and differentiation, we have performed expression profiling of klf4 morphants, and then performed validation of many putative target genes by qRT-PCR and whole mount in situ hybridization. We found a critical role for klf4 in embryonic globin, heme synthesis and hatching gland gene expression. In contrast, there was an increase in expression of definitive hematopoietic specific genes such as larval globin genes, runx1 and c-myb from 24 hpf, suggesting a selective role for klf4 in primitive rather than definitive erythropoiesis. In addition, we show klf4 preferentially binds CACCC box elements in the primitive zebrafish beta-like globin gene promoters. These results have global implications for primitive erythroid gene regulation by KLF-CACCC box interactions.

Analysis of the exon–intron structures of fish, amphibian, bird and mammalian hatching enzyme genes, with special reference to the intron loss evolution of hatching enzyme genes in Teleostei

Using gene cloning and in silico cloning, we analyzed the structures of hatching enzyme gene orthologs of vertebrates. Comparison led to a hypothesis that hatching enzyme genes of Japanese eel conserve an ancestral structure of the genes of fishes, amphibians, birds and mammals. However, the exon-intron structure of the genes was different from species to species in Teleostei: Japanese eel hatching enzyme genes were 9-exon-8-intron genes, and zebrafish genes were 5-exon-4-intron genes. In the present study, we further analyzed the gene structures of fishes belonging to Acanthopterygii. In the species of Teleostei we examined, diversification of hatching enzyme gene into two paralogous genes for HCE (high choriolytic enzyme) and LCE (low choriolytic enzyme) was found only in the acanthopterygian fishes such as medaka Oryzias latipes, Fundulus heteroclitus, Takifugu rubripes and Tetraodon nigroviridis. In addition, the HCE gene had no intron, while the LCE gene consisted of 8 exons and 7 introns. Phylogenetic analysis revealed that HCE and LCE genes were paralogous to each other, and diverged during the evolutionary lineage to Acanthopterygii. Analysis of gene synteny and cluster structure showed that the syntenic genes around the HCE and LCE genes were highly conserved between medaka and Teraodon, but such synteny was not found around the zebrafish hatching enzyme genes. We hypothesize that the zebrafish hatching enzyme genes were translocated from chromosome to chromosome, and lost some of their introns during evolution.

Evolution of teleostean hatching enzyme genes and their paralogous genes

We isolated genes for hatching enzymes and their paralogs having two cysteine residues at their N-terminal regions in addition to four cysteines conserved in all the astacin family proteases. Genes for such six-cysteine-containing astacin proteases (C6AST) were searched out in the medaka genome database. Five genes for MC6AST1 to 5 were found in addition to embryo-specific hatching enzyme genes. RT-PCR and whole-mount in situ hybridization evidenced that MC6AST1 was expressed in embryos and epidermis of almost all adult tissues examined, while MC6AST2 and 3 were in mesenterium, intestine, and testis. MC6AST4 and 5 were specifically expressed in jaw. In addition, we cloned C6AST cDNA homologs from zebrafish, ayu, and fugu. The MC6AST1 to 5 genes were classified into three groups in the phylogenetic positions, and the expression patterns and hatching enzymes were clearly discriminated from other C6ASTs. Analysis of the exon-intron structures clarified that genes for hatching enzymes MHCE and MAHCE were intron-less, while other MC6AST genes were basically the same as the gene for another hatching enzyme MLCE. In the basal Teleost, the C6AST genes having the ancestral exon-intron structure (nine exon/eight intron structure) first appeared by duplication and chromosomal translocation. Thereafter, maintaining such ancestral exon-intron structure, the LCE gene was newly diversified in Euteleostei, and the MC6AST1 to 5 gene orthologs were duplicated and diversified independently in respective fish lineages. The HCE gene lost all introns in Euteleostei, whereas in the lineage to zebrafish, it was translocated from chromosome to chromosome and lost some of its introns.

Mechanical analysis of chorion softening in prehatching stages of zebrafish embryos

During early development, the chorion envelope of the zebrafish embryo undergoes a thinning process called "chorion softening," which has so far only been characterized chemically. In this study, a micromechanical force sensing system was used to characterize and quantitate mechanical modifications of the zebrafish embryo chorion during early development. Quantitative relationships between applied forces and chorion structural deformations were established at various embryonic stages. The measured penetration force into the chorion at the blastula stage was 1.3-fold greater than those at the prehatching stage. Furthermore, chorion elastic modulus values were determined by using a biomembrane elastic model. The elastic modulus of the chorion at the blastula stage was 1.66-fold greater than that at the prehatching stage, thus indicating that the chorion envelope become mechanically "softened" at the prehatching stage. The experimental results quantitatively describe "chorion softening," which is most likely due to proteolytic activities at the prehatching stage. Gradual chorion softening during embryonic development was also artificially achieved by treating blastula chorion with pronase, a proteolytic enzyme. The forces required to penetrate the pronase-treated chorion were similar to those at the prehatching stage. This similarity suggests that "chorion softening" may be induced by the release of protease from the embryos, and the chemical nature of the process involves proteolytic fragmentation of the ZP2 protein.

Purification and characterization of hatching enzyme from shrimp Penaeus chinensis

By using Penaeus chorion as a specific substrate, the hatching enzyme (HE) from Penaeus chinensis was purified by gel-filtration and ion-exchange chromatography, and characterized in terms of its molecular weight and enzymatic properties in this study. It was found that the molecular weight of Penaeus HE is about 43.0 kDa in SDS-PAGE. The Penaeus HE had obvious choriolytic activity, which was optimal at pH 6.0 and temperature of 40 degrees C, respectively. The Km value of the HE for casein was 7.47 mg ml(-1). The HE activity was almost completely inhibited by SBTI, p-APMSF, bestatin, and NEM, greatly inhibited by ovomucoid, TLCK, IAM, chymostatin, and PMSF, and slightly inhibited by pepstatin A, TPCK, LBTI, and leupeptin. These results indicate that the HE is most probably a trypsin-type serine protease. Besides of these, the HE was extremely sensitive to EDTA, Zn2+, Ca2+, Mg2+, and Cu2+. Combined with the results that the EDTA-pretreated HE activity could be perfectly recovered by Zn2+, it is indicated that shrimp HE is most probably a kind of Zn-metalloprotease.

Purification and characterization of hatching enzyme from flounder Paralichthys olivaceus

Using chorion of Paralichthys as a specific substrate, hatching enzyme (HE) from Paralichthys olivaceus (PHE) was purified by gel-filtration and ion-exchange chromatography, and characterized in terms of its molecular weight and enzymatic properties in this study. It was found that the molecular size of PHE is about 34.8 kDa in SDS-PAGE. The PHE had obvious choriolytic activity, which was optimal at pH 7.0 and temperature of 35 degrees C, respectively. The Km value of the PHE for casein was 4.28 mg ml(-). The PHE was very sensitive to trypsin-specific inhibitors, especially serine protease-specific inhibitors, such as LBTI, SBTI, bestatin and p-APMSF, leupeptin, ovomucoid, PMSF, pepstatin A and TLCK, indicates that it is a trypsin-type serine protease. The PHE was also extremely sensitive to Cu(2+) and Ca(2+), combined with the results that it was inhibited by EDTA in a dose-dependent manner, indicates this PHE is also a kind of metalloprotease.

Expression and tissue distribution of astacin-like squid metalloprotease (ALSM)

Astacin metalloprotease family members function in a wide variety of biologic events, including cell differentiation and morphogenesis during embryonic development and adult tissue differentiation. We previously isolated and characterized an astacin-like squid metalloprotease (ALSM). To elucidate the embryonic expression of ALSM, we performed immunohistochemical analysis with specific antibodies and examined the expression profiles of ALSM isoforms by in situ hybridization analysis. Tissue distribution and expression were also examined in adult spear squid. mRNA expression of ALSM isoforms I and III was first detected in newly hatched squid and was restricted to the liver. No mRNA signals were detected in other tissues even in adult squids. At the protein level, both isoforms were prominent in the liver of embryos and later in digestive organs of adult squid. Both isoforms were also detected in muscle tissues, including mantle and tentacle muscle. Staining for ALSM III was also identified in the iris and in tissues near the eye in squid embryos. However, no reactive bands were detected by immunoblotting of adult squid eyes. Thus, ALSM is initially expressed at the late stage of embryogenesis in spear squid, and expression is restricted to the liver. Thereafter, ALSM isoforms function in various tissues in an isoform-dependent manner.

Purification and gene cloning of Fundulus heteroclitus hatching enzyme. A hatching enzyme system composed of high choriolytic enzyme and low choriolytic enzyme is conserved between two different teleosts, Fundulus heteroclitus and medaka Oryzias latipes

Two cDNA homologues of medaka hatching enzyme -- high choriolytic enzyme (HCE) and low choriolytic enzyme (LCE) -- were cloned from Fundulus heteroclitus embryos. Amino acid sequences of the mature forms of Fundulus HCE (FHCE) and LCE (FLCE) were 77.9% and 63.3% identical to those of medaka HCE and LCE, respectively. In addition, phylogenetic analysis clearly showed that FHCE and FLCE belonged to the clades of HCE and LCE, respectively. Exon-intron structures of FHCE and FLCE genes were similar to those of medaka HCE (intronless) and LCE (8-exon-7-intron) genes, respectively. Northern blotting and whole-mount in situ hybridization showed that both genes were concurrently expressed in hatching gland cells. Their spatio-temporal expression pattern was basically similar to that of medaka hatching enzyme genes. We separately purified two isoforms of FHCE, FHCE1 and FHCE2, from hatching liquid through gel filtration and cation exchange column chromatography in the HPLC system. The two isoforms, slightly different in molecular weight and in MCA-peptide-cleaving activity, swelled the inner layer of chorion by their limited proteolysis, like the medaka HCE isoforms. In addition, we identified FLCE by TOF-MS. Similar to the medaka LCE, FLCE hardly digested intact chorion. FHCE and FLCE together, when incubated with chorion, rapidly and completely digested the chorion, suggesting their synergistic effect in chorion digestion. Such a cooperative digestion was confirmed by electron microscopic observation. The results suggest that a hatching enzyme system composed of HCE and LCE is conserved between two different teleosts Fundulus and medaka.