The mRNA for zona pellucida proteins B1, C and D in two genetic lines of turkey hens that differ in fertility

Proteo-transcriptomic profiles reveal key regulatory pathways and functions of LDHA in the ovulation of domestic chickens (Gallus gallus)

BACKGROUND

In poultry, the smooth transition of follicles from the preovulatory-to-postovulatory phase impacts egg production in hens and can benefit the poultry industry. However, the regulatory mechanism underlying follicular ovulation in avians is a complex biological process that remains unclear.

RESULTS

Critical biochemical events involved in ovulation in domestic chickens (Gallus gallus) were evaluated by transcriptomics, proteomics, and in vitro assays. Comparative transcriptome analyses of the largest preovulatory follicle (F1) and postovulatory follicle (POF1) in continuous laying (CL) and intermittent laying (IL) chickens indicated the greatest difference between CL_F1 and IL_F1, with 950 differentially expressed genes (DEGs), and the smallest difference between CL_POF1 and IL_POF1, with 14 DEGs. Additionally, data-independent acquisition proteomics revealed 252 differentially abundant proteins between CL_F1 and IL_F1. Perivitelline membrane synthesis, steroid biosynthesis, lysosomes, and oxidative phosphorylation were identified as pivotal pathways contributing to ovulation regulation. In particular, the regulation of zona pellucida sperm-binding protein 3, plasminogen activator, cathepsin A, and lactate dehydrogenase A (LDHA) was shown to be essential for ovulation. Furthermore, the inhibition of LDHA decreased cell viability and promoted apoptosis of ovarian follicles in vitro.

CONCLUSIONS

This study reveals several important biochemical events involved in the process of ovulation, as well as crucial role of LDHA. These findings improve our understanding of ovulation and its regulatory mechanisms in avian species.

Quantitative Protein Analysis of ZPB2, ZPB1 and ZPC in the Germinal Disc and a Non-Germinal Disc Region of the Inner Perivitelline Layer in Two Genetic Lines of Turkey Hens That Differ in Fertility

The avian inner perivitelline layer (IPVL), containing the zona pellucida (ZP) family of proteins, surrounds the ovulated ovum. In mammalian species, ZP proteins serve as key component(s) in binding sperm and initiating the acrosome reaction. Sperm binding at the germinal disc (GD) region of the IPVL initiates fertilization in avian species, and the amount of sperm binding at the GD reflects female fertility. The current research determined whether reported differences in mRNA expression in two genetic lines of turkey hens (E, high fertility and F, low fertility) translated to the protein level. ZPB2 in the IPVL is greater in the GD region compared with the nongerminal disc (NGD) region, as indicated by both mRNA and protein expression. However, protein expressions of ZPB1 and ZPC in the IPVL of E- and F-line turkey hens was in contrast to previously reported mRNA expression. The results indicate that the mRNA expression of ZP proteins at their site of synthesis in E- and F-line hens often does not directly correlate with the IPVL abundance of these proteins. The greater protein concentration of ZPB2 in the GD region compared with the NGD regions suggests that this protein may be critical for sperm binding at the GD region.

Transcriptome and Proteome Analysis Revealed Key Pathways Regulating Final Stage of Oocyte Maturation of the Turkey (Meleagris gallopavo)

In birds, the zona pellucida (ZP) matrix that surrounds the ovulated oocyte—called the inner perivitelline layer—is involved in sperm–zona interaction and successful fertilization. To identify the important genes and proteins connected with the final step of egg development, next-generation sequencing and two-dimensional electrophoresis, combined with mass spectrometry, were used for the analysis of mature oocytes at the F1 developmental stage. A total of 8161 genes and 228 proteins were annotated. Six subfamilies of genes, with codes ZP, ZP1–4, ZPD, and ZPAX, were identified, with the dominant expression of ZPD. The main expression site for ZP1 was the liver; however, granulosa cells may also participate in local ZP1 secretion. A ubiquitination system was identified in mature oocytes, where ZP1 was found to be the main ubiquitinated protein. Analysis of transcripts classified in estrogen receptor (ESR) signaling indicated the presence of ESR1 and ESR2, as well as a set of estrogen-dependent genes involved in both genomic and nongenomic mechanisms for the regulation of gene expression by estrogen. Oxidative phosphorylation was found to be a possible source of adenosine triphosphate, and the nuclear factor erythroid 2-related factor 2 signaling pathway could be involved in the response against oxidative stress. Oocyte–granulosa cell communication by tight, adherens, and gap junctions seems to be essential for the final step of oocyte maturation.

Physiological factors influencing female fertility in birds

Fertility is fundamental to reproductive success, but not all copulation attempts result in a fertilized embryo. Fertilization failure is especially costly for females, but we still lack a clear understanding of the causes of variation in female fertility across taxa. Birds make a useful model system for fertility research, partly because their large eggs are easily studied outside of the female's body, but also because of the wealth of data available on the reproductive productivity of commercial birds. Here, we review the factors contributing to female infertility in birds, providing evidence that female fertility traits are understudied relative to male fertility traits, and that avian fertility research has been dominated by studies focused on Galliformes and captive (relative to wild) populations. We then discuss the key stages of the female reproductive cycle where fertility may be compromised, and make recommendations for future research. We particularly emphasize that studies must differentiate between infertility and embryo mortality as causes of hatching failure, and that non-breeding individuals should be monitored more routinely where possible. This review lays the groundwork for developing a clearer understanding of the causes of female infertility, with important consequences for multiple fields including reproductive science, conservation and commercial breeding.

Zona pellucida protein B2 messenger ribonucleic acid expression varies with follicular development and granulosa cell location

The freshly ovulated ovum in avian species is surrounded by a protein layer called the inner perivitelline layer (IPVL). The IPVL contains zona pellucida proteins and 6 distinct zona pellucida genes have been identified (ZPA, ZPB1, ZPB2, ZPC, ZPD and ZPX1) in the chicken. In the present research, the expression of the mRNA for ZPA, ZPB2, and ZPX1 was investigated in 2 lines of turkey hens selected for either increased egg production (E line) or increased body weight (F line). Theca and granulosa cell expression of the mRNA for ZPA and ZPB2 was also investigated in hierarchical and prehierarchical follicles from broiler breeder hens. Granulosa tissue was collected from F1 through F4 and F1 through F10 follicles in E line and F line hens, respectively. A one cm2 section of the granulosa layer around the germinal disc (GD) and an equivalent sized nongerminal disc (NGD) area was also collected from the F1 and F2 follicles from other hens from each genetic line. Granulosa and theca tissue was collected from hierarchical and prehierarchical follicles of broiler breeder hens. Total RNA was extracted from the samples. Minor groove-binding probes and primers for detecting ZPA, ZPB2, and ZPX1, were made for real-time PCR analyses. Expression of ZPA, ZPB2, and ZPX1 was detected in all follicle sizes from both genetic lines of hens. No significant differences in ZPA and ZPX1 mRNA expression were detected between the GD and NGD granulosa cells. However, the expression of the mRNA for ZPB2 was significantly greater in the GD granulosa cells when compared to the NGD granulosa cells in F1 and F2 follicles from E line and F line hens. In broiler breeder hens, the mRNA expression of ZPA and ZPB2 was greatest in the smallest prehierarchical follicles. The results suggest that higher expression of ZPB2 in the germinal disc area may be important for the preferential binding of sperm to this region of the IPVL.

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.

Egg-Coat and Zona Pellucida Proteins of Chicken as a Typical Species of Aves

Birds are oviparous vertebrates in terrestrial animals. Birds' eggs accumulate mass of egg yolk during the egg development and are accordingly much larger than the eggs of viviparous vertebrates. Despite such difference in size and contents, the birds' eggs are surrounded with the egg-coat morphologically and compositionally resembling the mammalian egg-coat, zona pellucida. On the other hand, there are some differences in part between the two egg-coats, though relationships of such structural differences to any biological roles specific for the extracellular matrix of birds' eggs are not fully understood. In birds, unlike mammals, ZP proteins constituting the egg-coat are highly conserved and therefore those of chicken are described as a representative of birds. The egg-coat ZP proteins, ZP1, ZP3, and ZPD as the majors, accumulate and form the matrix by self-assembly around the egg rapidly growing in the ovarian follicle, in which ZP1 is from liver and both ZP3 and ZPD are from follicular granulosa cells. Although details of the egg-coat-sperm interaction on fertilization remain to be investigated, the lytic degradation process of egg-coat matrix for the sperm penetration has become to be clarified gradually. ZP1 is the primary target of sperm acrosin, and the limited cleavage in the specific region leading to the loss of intermolecular cross-linkages is crucial for the lysis of egg-coat matrix. Possible roles of the ZP1 with the additional sequence characteristic to birds are discussed from a viewpoint of giving both robustness and elastomeric nature to the egg-coat matrix for the birds' eggs.

Divergently expressed gene identification and interaction prediction of long noncoding RNA and mRNA involved in duck reproduction

Long noncoding RNAs (lncRNAs) and divergently expressed genes exist widely in different tissues of mammals and birds, in which they are involved in various biological processes. However, there is limited information on their role in the regulation of normal biological processes during differentiation, development, and reproduction in birds. In this study, whole transcriptome strand-specific RNA sequencing of the ovary from young ducks (60days), first-laying ducks (160days), and old ducks, i.e., ducks that stopped laying eggs (490days) was performed. The lncRNAs and mRNAs from these ducks were systematically analyzed and identified by duck genome sequencing in the three study groups. The transcriptome from the duck ovary comprised 15,011 protein-coding genes and 2905 lncRNAs; all the lncRNAs were identified as novel long noncoding transcripts. The comparison of transcriptome data from different study groups identified 2240 divergent transcription genes and 135 divergently expressed lncRNAs, which differed among the groups; most of them were significantly downregulated with age. Among the divergent genes, 38 genes were related to the reproductive process and 6 genes were upregulated. Further prediction analysis revealed that 52 lncRNAs were closely correlated with divergent reproductive mRNAs. More importantly, 6 remarkable lncRNAs were correlated significantly with the conversion of the ovary in different phases. Our results aid in the understanding of the divergent transcriptome of duck ovary in different phases and the underlying mechanisms that drive the specificity of protein-coding genes and lncRNAs in duck ovary.

Early expression of zona pellucida proteins under octylphenol exposure in Cichlasoma dimerus (Perciformes, Cichlidae)

An increasing number of widely used industrial and agricultural chemicals are being found to cause endocrine disruption. In fishes, xenoestrogens can induce female proteins, and in some cases, the development of testis-ova, demonstrating feminization of males. In this study we analyzed the effect of an acute exposure of adult male Cichlasoma dimerus fish to estradiol (E(2)) and octylphenol (OP). E(2) and OP were injected at 10 and 50 μg/g body weight doses, respectively. After a single OP dose, liver was processed for RNA extraction at 1, 3, 12, 24, and 72 h. PCR was performed using cDNA and primers for egg coat or zona pellucida proteins (ZP). Genes encoding ZPB and ZPC isoforms were sequenced. E(2)-induced fish were sacrificed at 72 h. Using multiple OP or E(2) injections, blood and surface mucus were sampled on days 0, 3, 6, 9, and 13. On day 13 fish were sacrificed for liver and testis dissection. Histological examination of E(2) and OP-treated fish livers showed cellular disarray and intense cytoplasmatic basophilia within hepatocytes, probably due to increased mRNA synthesis, as well as hypertrophied euchromatic nuclei, and conspicuous nucleoli, indicative of augmented cell activity. An abnormal amount of sperm and immature germ cells within the testis lumen were seen in treated fish, suggesting reproductive impairment. Both plasma and mucus revealed the presence of ZP (and vitellogenin) at day 3 and thereafter with E(2) treatment, using Western and Dot blot techniques; OP effects were delayed in time. These results validate the analysis of mucus by Dot blot as an easy and rapid technique to address endocrine disruption caused by OP. Quantitative gene expression showed induction of liver ZPB and ZPC upon OP injection; muscle, brain, and intestine did not express any ZP. Both ZPs were induced at 1h post injection, but only ZPB expression was statistically significant. At 12h, both ZPs increased significantly, reaching the same levels of E(2)-challenged males after 72 h. Therefore, OP mimicked the action of E(2) with a prompt and strong xenoestrogenic effect, evidenced by the early response through mRNA and protein expression of ZP and the concomitant histological liver and testis alterations.