Background

MicroRNAs (miRNAs) can modulate almost all biological processes at the post-transcriptional level in viruses, plants and animals. miRNAs are a class of small endogenous non-coding RNAs that are approximately 19 to 24 nucleotides in length [1]. miRNAs negatively regulate extensive gene expression through sequence-specific interactions with the 3′untranslated regions (UTRs) of target mRNAs and thereby cause mRNA destabilization or translational repression [2, 3]. Currently, 35,828 mature miRNAs from 223 species have been discovered and deposited in the publicly available miRNA database miRBase (Release 21.0, June 2014) [4]. Genome-wide miRNA expression studies demonstrate that miRNAs have numerous significant biological functions, especially for signaling pathways implicated in development, cellular differentiation, hematopoiesis, proliferation, apoptosis and oncogenesis [1, 5–8].

Chickens (Gallus gallus domesticus) are an important model organism that bridges the evolutionary gap between mammals and other vertebrates [9]. Chickens were domesticated in Asia at least by 5400 BC, perhaps as early as 8000 BC [10]. Previous studies have reported abundant miRNA identification in various chicken tissues, such as embryo [8, 11–14], ovary and testis [13, 15], lung and trachea [16], somite [17], adipose tissue [18], skeletal muscle [19] and immune organs [20]. Further, some studies have assessed diseases in chicken [21–23]. However, a role for miRNAs in chicken pituitary and hypothalamic development related to high and low egg-laying performance has not been reported clearly. The characteristics of pituitary and hypothalamic tissues are highly related with growth development and reproductive traits of chickens, and they are ideal tissues to identify molecular markers associated with egg production [24].

In the present study, we constructed small-RNA cDNA libraries from pituitary and hypothalamic tissues of Luhua chicken using individuals at the age of 300 days with relatively high and low rates of egg production. In poultry breeding programs, the egg number at 300 days of age is generally used as the most valuable indicator of total egg production potential [25]. Through high-throughput sequencing of small RNA libraries and subsequent bioinformatic analysis, comprehensive miRNA profiles of pituitary and hypothalamic tissues from low and high rate of egg production chickens were generated, and comparative analysis of miRNA data was performed. The discovery of miRNA resources from this study will contribute to a better understanding in miRNAs regulating chicken egg production biological processes. Moreover, we will be able to screen suitable miRNAs for use as molecular markers in the application of genetic selection in the chicken breeding programs.

Methods

Experimental animals, tissue collection, rna extraction and sequencing

Eight hundred Luhua chickens from the Experimental Chicken Farm of Sichuan Agricultural University were used in this study. Then, low rate of egg production (LP) chicken and high rate of egg production (HP) chickens were categorized according to their egg number at 250 days of age. The egg production distribution of 800 birds at 300 days of age was shown in Additional file 1: Figure S1. According to the similar reproductive traits and regular egg production cycle, three LP and three HP chickens (Additional file 2: Table S1) were selected for tissue collection at 300 days of age. More detailed animal descriptions, sample grouping and egg production cycle recording methods can be found in our previous study [26].

The pituitary and hypothalamic tissues of six chickens were dissected as described [27], then snap frozen in liquid nitrogen and stored at −80 °C until RNA extraction. The total RNA of pituitary and hypothalamus was isolated by using RNeasy Mini Kit (Qiagen, Hilden, Germany) followed the manufacturer’s instructions. Twelve complementary DNA (cDNA) libraries for small RNA from chicken pituitary and hypothalamic tissues were constructed according to published miRNA cloning protocols [28, 29]. All samples were sequenced by 50-bp paired end reads (PE-50) on an Illumina HiSeq 2500 platform. The Institutional Animal Care and Use Committee of Sichuan Agricultural University approved animal experimentation in this study under permit number DKY- S20143204.

Sequence analysis

The Illumina 5’ and 3’ sequencing adapters were trimmed with Cutadapt software [30], and the small RNA libraries were further filtered to a minimum length of 17 nt and a maximum length of 35 nt. The clean reads were mapped to the chicken genome (galGal4) using MegaBLAST, and rRNA, tRNA, miscRNA, snRNA and snoRNA were discarded from the small RNA sequences. The remaining sequences were again searched against the miRBase 21.0 database [31–33] of Gallus gallus known miRNA sequences with zero or one mismatch. The sequences matching the Gallus gallus miRBase database were considered as known miRNA sequences. Next, after filtering known miRNA sequences, the remaining sequences were BLAST searched against the Gallus gallus genome. The sequences matching the chicken genome were used to predict the novel miRNA by the mirDeep2 [34–36] using default parameters. These sequences were considered as potential novel miRNAs, and expression of all miRNAs was assayed. Differential expression for known and novel miRNAs were analyzed using edgeR [37]. Reads per million miRNAs mapped (RPM) values were used to represent miRNA expression levels. P-values were calculated using right-tailed Fisher’s exact test. P < 0.05 and |Log2Fold Change| ≥1(LogFC) were used to screen differentially expressed miRNAs. The miRNA target prediction software miRDB [38] was used to predict the binding sites of the differentially expressed miRNA. The TargetScan principle (http://www.targetscan.org/) was also applied in the prediction procedures. The main functions of the predicted target genes regulated by differentially expressed miRNAs were determined using GO and KEGG functional classifications by Blast2GO program [39]. In GO terms, P-value ≤ 0.001 was used to identify the significantly enriched GO terms, and the P-value cut-off was 0.05 for KEGG terms.

Validation of miRNA expression by RT-qPCR

To validate the reliability of Illumina analysis, we tested the expression of eight miRNAs expressed in both tissues, including six significantly differentially expressed miRNAs and two non-significant expressed miRNAs, using reverse transcription (RT) Real-time PCR. The RT-qPCR primers were designed using Primer 5.0 (http://downloads.fyxm.net/Primer-Premier-101178.html), and listed in Additional file 2: Table S2 (miRNA-specific primers were synthesized by the Shanghai Biological Technology Co., and universal primers were provided by the miRcute miRNA qPCR Detection kit, Aidlab, Beijing, China). Real-time PCR was performed in a 96-wells plate using a Bio-Rad iQ5 Real-time PCR Detection System (Bio-Rad, California, USA) according to the protocol. In addition, 5.8S rRNA, which has relatively stable expression in most tissues, was used as an endogenous control [40], and the expression level of 5.8S rRNA was used to normalize the RT-qPCR results for each miRNA. All reactions were run in three technical replicates and included negative controls without template. Fold-changes of miRNA expression were calculated using the 2-ΔΔCt method (versus 5.8S rRNA) [41]. All data are expressed as the mean ± standard deviation, and statistical analysis using Student’s t-tests was performed with SPSS 16.0 software (SPSS Inc.).

Validation of candidate miRNA and miRNA-target pairs in the Jiuyuan black fowl

The Jiuyuan black fowl (Gallus Domesticus) is similar with Luhua bird, which has been recognized as a commercial dual-purpose egg-meat type chicken, but Luhua bird has a more superior reproductive performance such as high rates of egg production. Four hundred Jiuyuan black fowls from the Experimental Chicken Farm of Sichuan Agricultural University were used in this study. The processes of samples selection, tissues collection, RNA extraction and RT-qPCR validation of miRNAs and genes expression were according to previous described methods. The reproductive performance information of selected chicken samples was shown in Additional file 2: Table S1 and Additional file 1: Figure S2. The RT-qPCR primers of miRNAs and genes were listed in Additional file 2: Table S2. In addition, the expression levels of 5.8S rRNA and GAPDH were used to normalize the RT-qPCR results for each miRNA and gene, respectively.

Detection of SNPs in miRNAs

To explore the effects of SNPs on six known miRNAs differentially expressed in both tissues, SNPs in 120 low and 120 high rate of egg production chickens were scanned. Blood samples (0.5 to 1 ml) were collected in a 1-ml syringe primed with EDTA anticoagulation agent. Genomic DNA was phenol-extracted following standard procedures [42]. Six pairs of primer (Additional file 2: Table S2) were used for PCR, and products were sequenced by an ABI 3730xl automatic sequencer. The obtained sequences were aligned with the miRNA precursors using the program Seqman 5.01 of DNAstar Software (DNAstar Inc. Madison, WI, USA) [43]. To study the effect of SNPs on miRNA biogenesis, we calculated the second structure energy of different SNP-type precursors using RNAfold (http://nhjy.hzau.edu.cn/kech/swxxx/jakj/dianzi/Bioinf4/miRNA/miRNA1.htm) [44] and compared the energy changes between SNP-type pre-miRNAs and wild type pre-miRNAs.

Results

Overview of miRNA sequencing

In total, 70.23 and 74.02 million raw reads were obtained from LP and HP chickens, respectively. After filtering the low-quality sequences, a total of 64.39 (clean ratio: 91.64%) and 67.20 (clean ratio: 90.85%) million clean reads in LP and HP chickens, respectively, were used for further analysis (Additional file 2: Table S3). Among the clean reads, 50.18 M reads from LP and 51.05 M reads from HP were successfully mapped and annotated, amounting to 77.92% and 75.96% of the total reads, respectively. In addition, 48.13 M reads in LP and 47.88 M reads in HP were found to be similar to miRNAs. HP chickens have more small RNAs and unique small RNAs than LP chicken in both tissues. The remaining of the mapped reads were other types of RNA, including rRNA, tRNA, miscRNA (repeat and polII-transcribed), snRNA and snoRNA (Additional file 2: Table S4). In total small RNAs, most small RNA were miRNA (approximately 70%) followed by unannotated RNAs (approximately 20%). However, in the unique small RNAs, unannotated RNAs account for the biggest proportion (approximately 40%), and miRNA (approximately 30%) ranked second, except in the hypothalamus of HP chickens (Additional file 1: Figure S3). Proportions of the remaining categories of small RNAs, including tRNA, rRNA, miscRNA, snRNA or snoRNA, were relatively lower (less than 2%). The size distribution of small RNAs was similar in all tissues, and the majority of them changed from 20 to 24 nt, which was consistent with the typical size range of small RNAs (Additional file 1: Figure S4). The most abundant size class was 22 nt, which accounted for approximately 10% in the unique reads and 40% in the total reads in the four libraries.

Expression patterns of known and novel miRNAs in chicken pituitary and hypothalamus

In the present study, a total of 651 (LP:562, HP:594) and 645 known miRNAs (LP:585, HP:555) was identified in pituitary and hypothalamic tissues, respectively (Additional file 2: Table S5). In the pituitary, 505 (77.6%) unique miRNAs were co-expressed in LP and HP chickens; 57 (8.7%) and 89 (13.7%) were specifically expressed in LP and HP chickens, respectively. In the hypothalamus, 495 (76.7%) unique miRNAs were co-expressed in LP and HP chickens; 90 (14%) and 60 (9.3%) were specifically expressed in LP and HP chickens, respectively (Fig. 1). The minimal differences in the specifically expressed miRNAs of LP and HP chickens reflect the organizational complementarity of the two tissues.

Fig. 1
figure 1

The distribution of known and novel miRNAs in pituitary and hypothalamus tissues

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A total of 649 novel miRNAs (LP:538, HP:574) and 635 novel miRNAs (LP:541, HP:556) were predicted in pituitary and hypothalamic tissues, respectively (Additional file 2: Table S6). In pituitary, 463 (71.3%) unique miRNAs were co-expressed in LP and HP chickens; 75 (11.6%) and 111 (17.1%) were specifically expressed in LP and HP chickens, respectively. In the hypothalamus, 462 (72.8%) unique miRNAs were co-expressed in LP and HP chickens; 79 (12.4%) and 94 (14.8%) were specifically expressed in LP and HP chickens, respectively (Fig. 1).

In the known miRNA expression profile of the pituitary and hypothalamus, the read numbers of the top 10 miRNAs accounted for 73.65% and 79.26% of the total reads, respectively. In the novel miRNA expression profile, the read numbers of the top 10 miRNAs accounted for 97.35% and 97.63% of the total reads for the pituitary and hypothalamus, respectively. The expression profile revealed that most miRNAs were expressed by a small portion of miRNA genes. Interestingly, although there were some changes in the rank of miRNA expression, the known and novel miRNAs with the highest expression levels were consistent in the two tissues. In known miRNAs, gga-miR-26a-5p displayed greater than 12 M reads and exhibited the highest expression level followed by gga-miR-99a-5p, which displayed greater than 5 M reads. Some miRNAs (such as gga-miR-6516-5p and gga-miR-130a-5p) displayed less than approximately 1,000 reads, indicating that expression significantly varied among different miRNA families, which is consistent with previous studies [45]. In novel miRNAs, we found that the most abundant novel miRNA, gga-novel-1-mature, displayed greater than approximately 0.96 M reads in pituitary, it only displayed approximately 0.39 M reads in hypothalamus. The significant difference of miRNA expression may reflect the miRNA different functions in the two tissues.

Moreover, compared with known miRNAs, the sequencing frequencies of novel miRNAs were considerably reduced. The same expression pattern has also been reported in other species [46, 47], which suggests that novel miRNAs are typically weakly expressed, whereas known miRNA genes are highly expressed.

miRNA:miRNA* pairs in chicken pituitary and hypothalamus

The miRNA:miRNA* pairs are the mature miRNAs and miRNA*s (miR-#-5p and miR-#-3p) align to the 5′ and 3′ end regions of the precursors, respectively. In the study, a total of 181 duplex like miRNA:miRNA* pairs were obtained from known miRNA sequences of the two tissues (Additional file 2: Table S7). Commonly, the miR-#-5 ps were detected at the same or relatively high expression levels than miR-#-3 ps (Table 1), suggesting that the expression level of miR-#-3p mainly relied on the degradation degree and rate because both strands of miRNA duplex were produced in equal amounts by transcription. However, some miR-#-3p exhibited relatively increased expression levels compared with miR-#-5p (such as gga-miR-92-3p, gga-miR-7471-3p, gga-miR-130c-3p). Recently, some miRNA* sequences (miR-#-3p) with abundant expression were reported as mature functional miRNAs [48]. The relatively high number of reads of these miRNA*s indicates that they may play a functional role in regulating gene expression, such as reproduction regulation, in present study. Such a phenomenon has also been described in several previous studies [46, 47].

Table 1 The detail distribution of miRNA:miRNA* pairs in pituitary and hypothalamus tissues
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Hierarchical cluster analysis of differentially expressed miRNAs

Correlation analysis revealed high reproducibility in the same group, with Pearson correlation coefficients >0.75 (Additional file 1: Figure S5). By comparing the abundance of known and novel miRNAs between HP and LP chickens (Fig. 2), a total of 46 known miRNAs and 27 novel miRNAs that are differentially expressed (P < 0.05) in the two tissues were identified (Additional file 2: Table S8). Specifically, 21 up-regulated and 21 down-regulated miRNAs were identified in pituitary tissues, and 24 up-regulated and 15 down-regulated miRNAs were identified in hypothalamus tissues. In addition, for the absolute values of logFC, the majority of differentially expressed miRNAs exhibit a 1- to 4-fold difference, and 18 miRNAs showed differences greater than 4-fold between the LP and HP in the two tissues. Among the up-regulated miRNAs, gga-novel-148-mature had the highest logFC at 5.01-fold. Among the down-regulated miRNAs, gga-novel-16-mature and gga-novel-220-mature had the highest |logFC| with 9.70-fold, followed by gga-novel-306-mature, gga-miR-1682, gga-miR-1683 and gga-miR-6549-3p, |logFC| with more than 5-fold.

Fig. 2
figure 2

Scatter plot of the high-throughput sequencing data. The high-throughput sequencing data (differentially expressed miRNAs) are graphed on the scatter plot to visualize variations in miRNA expression between HP and LP chickens. Diagrams reflect fold change value (HP/LP) distribution in the differentially expressed miRNA numbers. In MA and volcano plots, red dots represent the differentially expressed miRNAs, whereas black represent miRNAs with similar expression

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To validate the Illumina small RNA deep sequencing data, RT-qPCR detection assays were used to confirm the expression of eight miRNAs expressed in both tissues, including six differentially expressed known miRNAs and two non-significant expressed miRNAs. As shown in Table 2 and Fig. 3, the general expression patterns of eight miRNAs from the Illumina sequencing are consistent with the RT-qPCR results, which further support the reliability of the Illumina sequencing data. The discrepancies with respect to ratio may be attributed to the essentially different algorithms and sensitivities between the two techniques.

Table 2 Evaluation of the expression profile variation between RNA-Seq and RT-qPCR for the selected miRNAs
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Fig. 3
figure 3

Validation of the miRNA expression profile by qRT-PCR. The relative expression levels of eight selected miRNAs were calculated according to the 2-ΔΔCt method using 5.8S rRNA as an internal reference RNA. Error bars represent the standard deviation. The x-axis indicates different miRNAs in the two tissues. *P < 0.05, **P <0.01, ***P < 0.001

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Target prediction and Gene Functional Annotation

In total, we predicted 2541 target genes in pituitary and 2108 target genes in hypothalamic tissues (Additional file 2: Table S9). Some predicted targets were likely to be targeted by multiple miRNAs at multiple targeting sites. Typically, tyrosine-protein kinase receptor (CTK-1) can be targeted by three miRNAs, including gga-miR-15c-5p, gga-miR-15a and gga-miR-16-5p.

To probe the biological roles of differentially expressed miRNAs, all of the predicted targets of them were mapped to terms in the GO and KEGG databases. In total, 369 GO terms and 28 KEGG pathways in pituitary tissues and 201 GO terms and 19 KEGG pathways in hypothalamic tissues were significantly enriched (Additional file 2: Table S10). GO enrichment analysis revealed that target genes were main functionally enriched in cellular process, cell, cell part and protein binding in the two tissues (Additional file 1: Figure S6). KEGG analysis results showed that most targets in pituitary tissues were mainly involved in focal adhesion, endocytosis, insulin signaling pathway, hepatitis B and FoxO signaling pathway. In hypothalamic tissues, most targets were mainly involved in protein processing in endoplasmic reticulum, microRNAs in cancer, wnt signaling pathway, dopaminergic synapse and ubiquitin mediated proteolysis (Fig. 4). Notably, a specific enrichment of genes was observed in some reproduction regulation pathways, such as ovarian steroidogenesis, oocyte meiosis, GnRH signaling pathways, progesterone-mediated oocyte maturation, calcium signaling pathways, endocrine and other factor-regulated calcium reabsorption, dopaminergic synapse, oxytocin signaling pathway and MAPK signaling pathway (Additional file 2: Table S11).

Fig. 4
figure 4

Selected significant pathway annotation in pituitary and hypothalamus tissues

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The identification of reciprocally expressed miRNA-target pairs

In order to further explore the relationship between differentially expressed miRNAs and their predicted targets, we added a transcriptome analysis in pituitary and hypothalamic tissues of the six samples using RNA-seq. A total of 662 and 336 differentially expressed genes (P < 0.05) were identified in pituitary and hypothalamic tissues, respectively (Fig. 5a and Additional file 2: Table S12). By comparing miRNAs predicted targets and these differentially expressed genes, 124 and 30 miRNA-target pairs demonstrated a reciprocal expression pattern in pituitary and hypothalamic tissues, respectively (Additional file 2: Table S13). KEGG analysis were also conducted on these miRNA-target pairs, and most mapped pathways were demonstrated to play important roles in regulating metabolism, development, reproduction, tumorigenesis and many other processes. These predicted reciprocally expressed miRNA-target pairs will provide invaluable insights into candidate miRNAs and genes for reproductive traits and selective breeding of chicken.

Fig. 5
figure 5

a The distribution of differentially expressed genes in pituitary and hypothalamic tissues between low- and high-rate egg production chickens. b Validation of eight miRNA expression profile by qRT-PCR in pituitary and hypothalamic tissues of Jiuyuan black fowl. c, d The expression of randomly selected four miRNA-target pairs in the pituitary and hypothalamic tissues of Jiuyuan black fowl. The same color indicated the miRNA and its corresponding reciprocally expressed target genes. The expression of each miRNA was normalized to 5.8S rRNA and then transformed to a log 2 scale. The expression of each target gene was relative to GAPDH and also transformed to a log 2 scale. All four miRNA-target pairs showed significantly reciprocal expression patterns

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Influence of SNPs in miRNAs on the energy of the miRNA secondary structure

Mutations in miRNAs or in their target sites have been demonstrated to potentially enhance or interrupt miRNA biogenesis or target alteration [49–52], resulting in phenotypic changes associated with diseases or traits [51, 53, 54]. To date, the effects of SNPs on miRNA biogenesis and regulation in chickens have not been reported. In the present study, using conventional Sanger sequencing, we PCR-amplified and sequenced six precursors of known miRNAs, gga-miR-1684a-3p, gga-miR-1744-3p, gga-miR-34b-3p, gga-miR-34c-3p, gga-miR-122-5p and gga-miR-1434, which are differentially expressed in both tissues. All the precursor regions were successfully covered. When the Sanger reads were compared with the reference precursors, we found that two miRNA precursors, gga-mir-1684a and gga-mir-1434, were identified as containing SNPs. In total, 83 haplotypes, 81 SNP sites and 112 SNPs were successfully detected (Table 3). By comparison of statistical significance, we found that the distribution of haplotypes showed no significant differences between HP and LP chickens. However, regarding the distribution of SNP sites and SNP numbers, HP chickens had significantly greater SNP sites and SNP numbers compared with LP chickens. These data indicate that HP chickens with a high egg-laying trait have more mutations in the miRNA biogenesis, which may have special physiological significance in the egg-laying process.

Table 3 Overview of PCR data processing of all sequencing samples
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Next, we investigated the effect of the SNPs on the energy change (ΔΔG) of the secondary structures of the two precursors. Given the large sample sizes for SNP identification and to avoid the calculation of too many SNPs and effect of sequence error, haplotypes in which the sample size was less than 3 were not considered. In total, we statistically analyzed 10 haplotypes, 8 SNP sites and 10 SNPs in the two miRNA precursors (Additional file 2: Table S14). We found that an SNP in the mature regions did not change the energy of the structure, whereas the remaining 9 SNPs changed the energy of the predicted secondary structures (Table 4). For seven of the SNPs, the absolute energy change values were ≥ 0.3 kcal/mol, which is the minimum energy change reported to be required to change the production of mature miRNAs [49]. Gong et al. summarized the rule that if an SNP increases the hairpin structure energy, the production of the mature miRNA will increase. If the SNP decreases the energy, the production of the mature miRNA will be reduced [55]. The identified seven SNPs therefore enhance or reduce the production of the mature miRNAs and may play special physiological roles in the egg-laying process.

Table 4 Effect of SNPs on miRNA precursor energy changes
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Validation of candidate miRNAs and miRNA-target pairs expression in the Jiuyuan black fowl

In the present study, in order to explore the applicability of these identified candidate miRNAs on other native birds, we detected the expression of previously selected eight candidate miRNAs in pituitary and hypothalamic tissues of Jiuyuan black fowl. Although the ratio between LP and HP chickens for each miRNA may be different, the expression pattern of most selected miRNAs in Jiuyuan black fowl is consistent with the high-throughput sequencing results of Luhua chicken except gga-miR-34c-3p shown no difference in pituitary and gga-miR-34b-3p shown up-regulated pattern in hypothalamus (Fig. 5b).

In addition, we also validated the expression of four egg production-related miRNA-target pairs which were randomly selected from identified miRNA-target pairs of Luhua chickens using RT-qPCR in pituitary and hypothalamic tissues of Jiuyuan black fowl. All four pairs showed significantly reciprocal expression patterns (Fig. 5c, d), consistent with the observation that miRNAs predominantly function to decrease target gene levels [56–58].

Discussion

The miRNA sequences we identified were generally close to 22 nucleotides in size, which is consistent with previous reports [59, 60], and the annotation of RNA distribution revealed that the clean reads were highly enriched and included a myriad of miRNA sequences. Correlation analysis revealed that the twelve samples have high repeatability in the same group and were obviously different in different group. In addition, abundant known and novel miRNAs are identified and predicted in the pituitary and hypothalamic tissues. By comparing HP and LP chickens, 46 known miRNAs and 27 novel miRNAs that are differentially expressed (P < 0.05) in the two tissues were found. Moreover, the RT-qPCR results provide evidence that Illumina small RNA deep sequencing is a sensitive and reliable approach to identifying differentially expressed miRNAs in chicken pituitary and hypothalamic tissues. In summary, all of the above results suggested that the deep sequence data are representative and reliable for the subsequent analyses.

In pituitary and hypothalamic libraries, gga-miR-26a-5p (>12 M reads) and gga-miR-99a-5p (>5 M reads) were the most two frequently sequenced miRNAs. Despite the different read numbers in the two miRNAs, this result is consisted with our previous study [26]. miR-26a regulates tissue and cell growth and differentiation [61, 62] and has anti-apoptotic effects on many cancers [63–65]. mir-99a and mir-99b inhibit proliferation of c-Src-transformed cells and prostate cancer cells by targeting mTOR [66, 67] and were identified as novel targets in some important biological process, such as the TGF-β-induced epithelial to mesenchymal transition [68]. Among other miRNAs, we found that the let-7 miRNA family was another abundant cluster with let-7f-5p being the most abundantly expressed miRNA. The let-7 miRNA family is abundantly expressed in bovines [69–71] and in murine ovaries and testis [72]. Furthermore, gga-miR-7, gga-miR-148a-3p, gga-miR-146c-5p, gga-miR-125b-5p, gga-miR-30d, gga-miR-153-3p and gga-miR-126-3p were abundant in our sequencing libraries and have been shown to occur in other animals [15, 69, 73, 74]. The significant biological functions of these miRNAs imply that they have important roles in the female reproductive physiology of chicken.

In significantly differentially expressed miRNAs, some miRNAs play important roles in various aspects. For example, miR-138 [75–77], miR-29a [78–81], miR-490-3p [82, 83], miR-9-3p [84–86], and miR-135 [87–90] have pivotal roles in tumorigenesis and tumor progression by acting as tumor suppressors. miR-138 modulates the DNA damage response by repressing histone H2AX expression [91]. miR-490-3p [92] and miR-135 [93] as potential regulators of myogenesis also modulate the proliferation of muscle cells and are involved in skeletal muscle development. The important roles of these miRNAs and their significant expression pattern between LP and HP suggested that they might have important physiological roles in the process of chicken reproduction.

Furthermore, we found eight significantly differentially expressed miRNAs in both tissues, including six known miRNAs (gga-miR-1684a-3p, gga-miR-1744-3p, gga-miR-34b-3p, gga-miR-34c-3p, gga-miR-122-5p and gga-miR-1434) and two novel miRNAs (gga-novel-367-mature and gga-novel-465-mature). Intriguingly, among the eight miRNAs, gga-miR-1684a-3p and gga-miR-1744-3p have the same expression pattern, and the remaining six miRNAs exhibited a contrasting expression pattern between the two tissues. We speculated the contrary expression pattern of these candidate miRNAs may be beneficial to the improvement of egg-laying in hens. In addition, miRNA target identification is important to explore the functions of the miRNAs. In the present study, in order to further explore the relationship between differentially expressed miRNAs and their predicted targets, we added a transcriptome analysis of pituitary and hypothalamic tissues to identify reciprocally expressed miRNA-target pairs. These successfully identified miRNA-target pairs suggested they have highly possibility to be related in the egg-laying process. This will provide a great convenience for future studies to functionally validate these miRNA targets. These miRNA-target pairs will also provide invaluable insights into candidate miRNAs and genes for reproductive traits and selective breeding of chicken.

The miR-34 family was significantly differentially expressed between LP and HP chickens in the two tissues. The results were also presented in our previous study [26]. Compared with LP chickens, gga-miR-34b-3p and gga-miR-34c-3p exhibited a significant increase in HP pituitary tissues. In HP hypothalamus, gga-miR-34b (include 3p and 5p) and gga-miR-34c (include 3p and 5p) exhibited significant reductions. Previous studies reported that the mir-34 family was identified as a p53 target and a potential tumor suppressor to regulate processes, such as proliferation, cell cycle, apoptosis and metastasis [94–100]. Some studies also observed that miR-34 can regulate age-associated events and long-term brain integrity to modulate aging and neurodegeneration in Drosophila [101]. In the present study, gga-miR-34 were significantly differentially expressed between LP and HP chickens, suggesting that gga-miR-34 has an important impact on reproductive process of hens. However, the contrary expression pattern between pituitary and hypothalamus tissues indicates that the different members of miR-34 family play different roles in the two tissues in the egg-laying process.

In significantly enriched KEGG pathways, the most overrepresented pathways belonged to the metabolic pathways, such as glycerophospholipid metabolism, glycerolipid metabolism and butanoate metabolism. Some pathways associated with endocytosis, cancer, oocyte meiosis, focal adhesion, protein processing in endoplasmic reticulum, cutin, snare interactions in vesicular transport endocrine, other factor-regulated calcium reabsorption, dopaminergic synapse, GABAergic synapse and some signal transduction pathways, such as Renin-angiotensin system and GnRH signaling pathway, were all significantly enriched, indicating the role of the differentially expressed miRNAs in the regulation of cell motility, cell proliferation, the cytoskeleton, cell nutrition, nervous system development and function, communication between cells and the extracellular matrix. Moreover, the insulin signaling pathway was also enriched in our results. Insulin is the most potent anabolic hormone, mediating a wide spectrum of biological responses, including the synthesis and storage of carbohydrates, proteins and lipids and inhibiting their degradation and release back into circulation [102]. In addition, a small number of pathways involved in shigellosis, bacterial invasion of epithelial cells and hepatitis B suggested that the hens were involved in a stage of immune regulation. In general, the results indicated that these differentially expressed miRNAs were mainly involved in cell proliferation and development, signal transduction, metabolic and immune processes and nervous system development and function.

In these KEGG pathways, a specific enrichment of predicted targeted genes was involved in some reproduction related pathways, such as ovarian steroidogenesis, oocyte meiosis and GnRH signaling pathways. Some of these genes play important roles in ovary development and the reproductive management of hens, such as FSHβ. FSHβ produces the pituitary glycoprotein hormone FSH, which plays a key role in the reproductive system of chickens, including steroidogenesis, folliculogenesis and follicular maturation [103]. In addition, the significance level of some reproduction regulation-related pathways in the hypothalamus were increased compared with pituitary tissues, such as GnRH signaling pathway and dopaminergic synapse, suggesting that the hypothalamic tissues play a greater role in reproduction regulation-related activities compared with pituitary tissues in hens. Moreover, we found that most identified miRNA-target pairs participated in reproduction regulation-related pathways, suggesting that these miRNA-target pairs were closely associated with high egg-laying performance in chickens. Also, these candidate miRNA and miRNA-target pairs were validated in Jiuyuan black fowl suggested the highly applicability of these egg production-related miRNA-target pairs in other chicken breeds. These identified miRNA-target pairs will provide an opportunity for early high egg-laying performance determination and to select individuals with rapid growth and disease resistance for breeding purposes. Thus, it is important for future studies to functionally validate these miRNA-target pairs. In addition, most of these miRNAs have not been reported their biological function. This information provides a guideline to explore their unknown roles in the reproductive management of hens.

Conclusions

The current study demonstrated that a diverse and dynamic set of miRNA is expressed in the pituitary and hypothalamic tissues during the egg-laying process in hens. By comparing miRNA expression between low and high rate of egg production chicken in each tissue, miRNAs with significant differences were screened as key factors to participate in the pituitary and hypothalamic reproduction regulatory mechanisms. A comprehensive analysis of the pituitary and hypothalamic microRNA transcriptomes was performed. Plenty of reciprocally expressed miRNA-target pairs were identified and randomly selected candidate miRNA and miRNA-target pairs were validated by RT-qPCR in Jiuyuan black fowl. This information will aid greatly in understanding the complexity of miRNA regulation at egg-laying stages of hens. Furthermore, previous reports demonstrated that chicken miRNA share a high degree of homology with other vertebrate species; therefore, knowledge of miRNA in chicken tissues will also assist in vertebrate development studies and enhance the understanding of the functions of miRNA and regulatory mechanisms of miRNA expression during vertebrate development.