Characterization of progressive changes in ZPC of the vitelline membrane of quail oocyte following oviductal transport

Avian Egg and Egg Coat

An ovulated egg of vertebrates is surrounded by unique extracellular matrix, the egg coat or zona pellucida, playing important roles in fertilization and early development. The vertebrate egg coat is composed of two to six zona pellucida (ZP) glycoproteins that are characterized by the evolutionarily conserved ZP-domain module and classified into six subfamilies based on phylogenetic analyses. Interestingly, investigations of biochemical and functional features of the ZP glycoproteins show that the roles of each ZP-glycoprotein family member in the egg-coat formation and the egg-sperm interactions seemingly vary across vertebrates. This might be one reason why comprehensive understandings of the molecular basis of either architecture or physiological functions of egg coat still remain elusive despite more than 3 decades of intensive investigations. In this chapter, an overview of avian egg focusing on the oogenesis are provided in the first section, and unique features of avian egg coat, i.e., perivitelline layer, including the morphology, biogenesis pathway, and physiological functions are discussed mainly on chicken and quail in terms of the characteristics of ZP glycoproteins in the following sections. In addition, these features of avian egg coat are compared to mammalian zona pellucida, from the viewpoint that the structural and functional varieties of ZP glycoproteins might be associated with the evolutionary adaptation to their reproductive strategies. By comparing the egg coat of birds and mammals whose reproductive strategies are largely different, new insights into the molecular mechanisms of vertebrate egg-sperm interactions might be provided.

Transcriptomic profiling of proteases and antiproteases in the liver of sexually mature hens in relation to vitellogenesis

Background

Most egg yolk precursors are synthesized by the liver, secreted into the blood and transferred into oocytes, to provide nutrients and bioactive molecules for the avian embryo. Three hundred and sixteen distinct proteins have been identified in egg yolk. These include 37 proteases and antiproteases, which are likely to play a role in the formation of the yolk (vitellogenesis), as regulators of protein metabolism. We used a transcriptomic approach to define the protease and antiprotease genes specifically expressed in the hen liver in relation to vitellogenesis by comparing sexually mature and pre-laying chickens showing different steroid milieu.

Results

Using a 20 K chicken oligoarray, a total of 582 genes were shown to be over-expressed in the liver of sexually mature hens (1.2 to 67 fold-differences). Eight of the top ten over-expressed genes are known components of the egg yolk or perivitelline membrane. This list of 582 genes contains 12 proteases and 3 antiproteases. We found that “uncharacterized protein LOC419301/similar to porin” (GeneID:419301), an antiprotease and “cathepsin E-A-like/similar to nothepsin” (GeneID:417848), a protease, were the only over-expressed candidates (21-fold and 35-fold difference, respectively) that are present in the egg yolk. Additionally, we showed the 4-fold over-expression of “ovochymase-2/similar to oviductin” (GeneID:769290), a vitelline membrane-specific protease.

Conclusions

Our approach revealed that three proteases and antiproteases are likely to participate in the formation of the yolk. The role of the other 12 proteases and antiproteases which are over-expressed in our model remains unclear. At least 1/3 of proteases and antiproteases identified in egg yolk and vitelline membrane proteomes are expressed similarly in the liver regardless of the maturity of hens, and have been initially identified as regulators of haemostasis and inflammatory events. The lack of effect of sex steroids on these genes expressed in the liver but the products of which are found in the yolk suggests that these may be passively incorporated into the yolk rather than actively produced for that purpose. These results raise the question of the biological significance of egg yolk proteases and antiproteases, and more generally of all minor proteins that have been identified in egg yolk.

Absence of donor-derived zona pellucida protein C homolog in the inner perivitelline layer of Peking duck (Anas platyrhynchos)-Japanese quail (Coturnix coturnix japonica) chimeras (Duails)

Avian blastodermal cells at stage X are used to produce interspecies chimeras for heterogenous poultry reproduction. However, recipient-derived inner perivitelline layer (IPVL)-enclosed donor-derived ova may affect the efficiency of germline transmission via chimera. Among the proteins in the IPVL, zona pellucida protein C (ZPC) plays an important role in sperm-egg binding and inducing the acrosome reaction. In the present study, Peking duck blastodermal cells at stage X were transferred into subgerminal cavities of Japanese quail embryos at the same stage. Fourteen female duck-quail chimeras (duails) were hatched and raised to sexual maturity. After being screened by PCR, 3 duails were selected for examination of donor-derived ZPC. A total of 152 IPVL protein samples from the individual eggs laid by the 3 duails then underwent a preliminary examination for the presence of donor-derived ZPC by means of SDS-PAGE, periodic acid-Schiff staining, and Western blotting. A novel 35-kDa ZPC, not observed in quail but in duck, was found in the IPVL of the duails. Further analysis of peptide mass fingerprinting of Peking duck ZPC, Japanese quail ZPC, and the 35-kDa duail ZPC by matrix-assisted laser desorption-ionization reflectron time-of-flight mass spectrometry revealed that the novel ZPC was an isoform of quail ZPC. Moreover, comparison of N-terminal amino acid sequences of these 3 ZPC confirmed that the 35-kDa quail ZPC had more amino acids at the N terminus than did native quail ZPC, and none of the donor-derived ZPC was found in the duails. These findings suggest that it would be difficult to obtain donor-derived offspring by natural mating of interspecies chimeras.

Induction of sperm acrosome reaction by perivitelline membrane glycoprotein ZP1 in Japanese quail (Coturnix japonica)

The extracellular matrix surrounding avian oocytes, called the perivitelline membrane (PL), consists of at least two major glycoproteins, ZP3 and ZP1. Our previous study using Japanese quail had demonstrated that the PL obtained from the preovulatory follicles was incubatedin vitrowith spermatozoa, and perforations were observed. This result indicated that the PL might contain a constituent that possesses activity to initiate the acrosome reaction (AR) in quail. In order to elaborate upon our previous findings, we evaluated the effects of ZP3 and ZP1 on the induction of sperm AR in Japanese quail. Ejaculated sperm were incubated with or without the purified PL glycoprotein, and their acrosome status was determined based on the presence or absence of the acrosome. Treatment of spermatozoa with increasing doses of the purified monomeric ZP1 led to a concentration-dependent stimulation of AR. The purified dimeric ZP1 had similar effect. Moreover, we found that the ZP1-induced AR was significantly blocked by the digestion of the PL protein with PNGaseF. In contrast, the addition of purified ZP3 failed to induce AR at any doses tested. These results indicate thatN-linked glycans on ZP1 play an important role in triggering the AR in Japanese quail.

Involvement of interaction of ZP1 and ZPC in the formation of quail perivitelline membrane

The extracellular matrix surrounding the oocyte before ovulation is called the perivitelline membrane (PL) in avian species. We have previously reported that one of its components, ZPC, is produced in ovarian granulosa cells by the stimulation of follicle-stimulating hormone and testosterone. Another component, ZP1, is synthesized in the liver and might be transported to the surface of the oocyte of the follicles. These glycoproteins are assembled to form a three-dimensional network of coarse fibers between the granulosa cells and the oocyte. In the present study, we have evaluated the involvement of the interaction of ZPC and ZP1 in the formation of the PL of Japanese quail. By measuring the incorporation of tritium-labeled proteins into the PL, we have found that tritium-labeled ZPC is specifically incorporated into the PL. Whole-mount autoradiographic analysis of the PL has also revealed the incorporation of the secreted ZPC into the isolated PL. To study which component in the PL is responsible for the specific incorporation of ZPC, PL lysates were incubated with the conditioned medium of the granulosa cells and were immunoprecipitated with anti-ZPC antiserum. Western blot analysis of the immunoprecipitated materials indicated that the 175-kDa and 97-kDa ZP1 forms were co-immunoprecipitated with anti-ZPC antiserum. These results demonstrate that ZPC secreted from the granulosa cells specifically binds with ZP1, and that the phenomenon might be involved in insoluble PL fiber formation in quail ovary.

Variant of perivitelline membrane glycoprotein ZPC of Japanese quail (Coturnix japonica) lacking its cytoplasmic tail exhibits the retention in the endoplasmic reticulum of Chinese hamster ovary (CHO-K1) cells

Avian perivitelline membrane, an investment homologous to the mammalian zona pellucida, is composed of at least two glycoproteins. Our previous studies demonstrated that one of its components, ZPC, which is synthesized in the ovarian granulosa cells, is secreted after carboxy-terminal proteolytic processing, and this event is a prerequisite event for ZPC secretion in quail. In the present study, we examined the role of the cytoplasmic tail, which is successfully removed after proteolytic processing, in membrane transport, proteolytic processing, and the secretion of quail ZPC. In pursuit of this, we produced a truncated ZPC mutant lacking the cytoplasmic tail located in its C-terminus and examined its expression in the mammalian cell line. Western blot analyses demonstrated that the cytoplasmic tail-deficient ZPC was neither secreted nor underwent proteolytic processing in the cells. Immunofluorescence analysis and the acquisition of resistance to endoglycosidase H digestion of the cytoplasmic tail-deficient ZPC demonstrated that the deletion of the cytoplasmic tail interferes with the intracellular trafficking of the protein from the endoplasmic reticulum to the Golgi apparatus. These results indicate that the cytoplasmic tail of quail ZPC might possess the determinant responsible for the efficient transport of the newly synthesized ZPC from the endoplasmic reticulum to the Golgi apparatus.

The peri-albumen layer: a novel structure in the envelopes of an avian egg

The present paper describes a novel structure, termed the peri-albumen layer, in the egg-envelopes of the quail Coturnix japonica. It reacts with Alcian blue and exists between the egg white and the shell membrane. Ultrastructurally, it is of fine granular structure and forms a fenestrate sheet, the width of which is 190 nm or less. Isolated materials of the peri-albumen layer include an Alcian-blue-positive polysaccharide of 260 kDa, and three glycoproteins of 160, 108 and 52 kDa. The layer is supplied to an egg when it passes through the magnum-isthmus junction, the normalized length of which is 0.62-0.63 of the oviduct. The mucosa of the junction consists exclusively of a luminal epithelium. It is apparently distinct from the mucosa of the magnum and the isthmus, which consist of a luminal epithelium and tubular glands. The luminal epithelium of the magnum-isthmus junction stains prominently with Alcian blue and consists of alternately distributed ciliated cells and granular cells. Immunohistochemistry with an antiserum raised against the materials of the peri-albumen layer revealed the staining of the peri-albumen layer of the egg, and secretory cells of the luminal epithelium at the magnum-isthmus junction. It was concluded that the materials of the peri-albumen layer are produced by secretory cells at the magnum-isthmus junction of the oviduct.

Asparagine-linked oligosaccharide-independent secretion of egg envelope glycoprotein ZPC of the Japanese quail (Coturnix japonica)

In avian species, a glycoprotein homologous to mammalian ZPC is synthesized in the granulosa cells of developing follicles. We have previously reported that the newly synthesized ZPC (proZPC) in the granulosa cells is cleaved at the consensus furin cleavage site to generate mature ZPC prior to secretion. In the present study, we examined the role of asparagine (N)-linked oligosaccharides in the proteolytic processing of proZPC and the subsequent secretion of ZPC by using site-directed mutagenesis of the consensus sequence for N-glycosylation, and tunicamycin, an inhibitor for N-glycosylation of glycoprotein. Western blot analysis demonstrated that tunicamycin did not block either proteolytic cleavage of proZPC or the subsequent ZPC secretion. Moreover, a site-directed mutant that possesses a mutated sequence for N-glycosylation was efficiently secreted from the cells. These results indicate that proteolytic cleavage of proZPC, and the subsequent ZPC secretion occur in the absence of N-linked oligosaccharides. Therefore, the addition of N-glycans to ZPC polypeptide is not required for quail ZPC secretion.

Expression of perivitelline membrane glycoprotein ZP1 in the liver of Japanese quail (Coturnix japonica) after in vivo treatment with diethylstilbestrol

Avian perivitelline membrane, an oocyte extracellular matrix homologous to the zona pellucida in mammals or chorion in fish, is composed of at least two glycoproteins. Previous studies have indicated that one of the components, a glycoprotein homologous to mammalian ZPC, is produced in the granulosa cells of the developing follicles of quail ovary on stimulation with testosterone. However, little is known about the molecular biology of the other component of the avian perivitelline membrane, ZP1, and information about gene expression is particularly lacking. We have cloned the ZP1 in Japanese quail and examined its gene expression. A cDNA encoding quail ZP1 was isolated from the livers of mature females using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. It encoded a 934-amino acid protein that showed greatest homology (87.8% identity) with the chicken ZP1. RT-PCR amplification indicated that the ZP1 mRNA in the liver was restricted to mature laying females. The expression of ZP1 mRNA was stimulated by in vivo treatment with diethylstilbestrol in immature females as well as males. These results suggested that androgens and estrogens coordinately regulate the formation of quail perivitelline membrane proteins. In addition, the use of ZP1 transcriptional induction in males or immature females as a biological marker of environmental estrogens is discussed.

Effects of testosterone on production of perivitelline membrane glycoprotein ZPC by granulosa cells of Japanese quail (Coturnix japonica)

Avian perivitelline membrane, an investment homologous to the zona pellucida of mammalian oocytes, is composed of at least two glycoproteins. Previous studies have indicated that one of the components, a glycoprotein homologous to mammalian ZPC, is produced and secreted by the granulosa cells of developing follicles of the chicken ovary. In the present study, we evaluated the expression and regulation of ZPC in Japanese quail (Coturnix japonica) granulosa cells both in vivo and in vitro. Western blot analysis of the SDS-solubilized granulosa layer using anti-quail ZPC antiserum showed that the amount of ZPC increased in parallel with follicular development. Northern blot analysis of total RNA using cDNA of quail ZPC showed that the increase in mRNA expression was also correlated with follicular development. To investigate the regulation of ZPC production, the granulosa cells were cultured in a medium containing steroid hormones such as progesterone, estradiol-17ss, or testosterone. By measuring ZPC protein and mRNA with Western and Northern blot analyses, respectively, we found that addition of testosterone maintained ZPC contents in the culture of the granulosa cells, and that ZPC mRNA expression was high in the culture with testosterone compared to the control. These results suggest that testosterone stimulates ZPC protein production at the gene transcription level.