Overview of circRNAs data of Ribo-Zero RNA-Seq in chicken preadipocytes and adipocytes
In the present study, a total of 960 million raw paired-end (PE) reads generated by Ribo-Zero RNA-Seq data were analyzed. After fliting with low-quality reads, we finally got 910 million clean reads. The 88%-94% unique mapping clean reads were mapped to the chicken genome 5 (galGal5) (Table 1). CircRNAs distributed in the majority of chromosomes of chicken genome. To better understand the cirRNAs in chicken adipocytes, we identified and analyzed the expression levels of circRNAs in different adipose tissues-derived adipocytes. As shown in Fig. 1A, intramuscular preadipocytes (IM_Pre) and adipocytes (IM_Ad), abdominal preadipocytes (Ab_Pre) and adipocytes (Ab_Ad) were derived into two clusters, suggesting that the differences between intramuscular and abdominal fat might higher than that between preadipocytes and adipocytes. Moreover, the results of Pearson correlation coefficient (r = 0.86 ~ 0.93) suggested that cell samples within groups have good consistency. The bar plot showed that no obvious difference of global expression levels of circRNAs across different groups (Fig. 1B). To classify the length distribution of circRNAs, we compared the resource of all circRNAs in different gene regions, including exon, intergenic and intron regions. In the present study, we finally identified 787, 710, 980, 1083, 1406, 1426, 1447 and 1004 circRNAs in eight groups (Fig. 1C). Our results showed that the majority of circRNAs (65%) generated from exon regions, followed with intergenic regions (27%) (Fig. 1D) and intron regions (8%). Moreover, our results suggested that most of the intersect circRNAs shared with different software were 200–1000 nt in length (Fig. 1E). The circRNAs distributed across 31 chromosomes (chromosomes 1–28, 33, Z and W) of chicken (Fig. 2). Most circRNAs distributed in chicken chromosome 1, 2, 3 and 21 (Fig. 1F).
Identification of differentially expressed (DE) circRNAs during adipogenic differentiation across various groups
To explore functional roles of circRNAs in tissues-specific adipogenic differentiation, we analyzed and compared the expression levels of circRNAs between different groups. We finally identified 17, 14, 20 and 9 differentiated expressed (DE) circRNAs across different groups (IMPre_vs_IMAd, AbPre_vs_AbAd, AbPre_vs_IMPre and AbAd_vs_IMAb) (Fig. 3, Table 2). We noticed that most of DE circRNAs were specific found in preadipocytes or adipocytes, several DE circRNAs showed different expression patterns during adipogenic differentiation between AbF and IMF groups (Table S1). Among them, we noticed that 7:22323550|22327655 was significantly downregulated in mature adipocytes when comparison with preadipocytes in both IMF and AbF groups. Z:78636651|78640537 (circCDKN2A) was considerably downregulated in AbAd group, while upregulated in IMAd group. In addition, 7:22754779|22756470 (circTNS1) was only found in intramuscular fat preadipocytes and adipocytes, 21:6563897|6564555 (circHSPG2), 3:66699169|66707019 (circSLC22A16), 17:10086299|10108615 (circGAPVD1), 16:242878|309387 (circKIFC1L) were specifically found in intramuscular adipocytes, 3:79378778|79415580 (circBCKDHB), 28:1095851|1096498 and 1:83410785|83437136 were specifically found in abdominal adipocytes.
Functional characterization of DE circRNAs
To investigate the potential functions of DE circRNAs, the host genes of DE circRNAs were performed GO terms analysis. We found that the molecular function (MF) of gene ontology (GO) analysis during adipogenesis were mainly enriched in lipid binding, phospholipase activity and lipase activity, cell component (CC) were mainly enriched in regulation of autophagy, glycerophospholipid biosynthetic process, phosphatidylinositol-mediated signaling and inositol lipid-mediated signaling (Fig. 4).
Validation of circRNAs
To confirm the circRNAs in chicken adipocytes, five DE circRNAs (circFNDC3AL, circHSPG2, circCLEC19A, circARMH1 and circLCLAT1) were randomly selected for validation by PCR and sanger sequencing (Fig. 5A). Divergent and convergent primers were designed for the amplification of circRNAs back-spliced junction (BSJ) site and linear mRNAs. As showed in Fig. 5B, BSJ sites of circRNAs were amplified and validated by sanger sequencing. The PCR products amplified by convergent primers of circRNAs were distinct in both cDNA and gDNA samples, while the divergent primers could only be amplified in the cDNA samples. In addition, PCR products amplified by the divergent primers were identified by Sanger sequencing (Fig. 5B). The sequencing results were consistent with Ribo-zero RNA-Seq data, which suggested that the circRNAs data is reliable.
Expression characteristics of miRNAs in chicken intramuscular and abdominal adipogenesis
To comprehensively analyze the characteristics of miRNAs during tissue-specific adipogenesis, miRNAs expression levels were compared across various groups. We finally identified 80 DE miRNAs (58 upregulated and 22 downregulated) between AbPre and AbAd groups, 225 DE miRNAs (85 downregulated and 140 DE miRNAs upregulated) between AbPre and IMPre groups, 206 DE miRNAs (98 downregulated and 108 upregulated) between AbAd and IMAd groups and 111 DE miRNAs (89 upregulated and 22 upregulated) between IMPre and IMAd groups, respectively (Fig. 6A) (Table S2). Among them, we noticed that 81 and 25 DE miRNAs were specifically identified during Ab adipogenic differentiation and IM adipogenic differentiation, respectively (Fig. 6B). We noticed that most of DE miRNAs were significantly downregulated in mature adipocytes, including miR-130b-5p, miR-148a-5p, miR-15c-5p, miR-16c-5p, miR-30a-3p (Fold change > 2, Qvalue < 0.001). However, miR-146b-5p was significantly upregulated in mature adipocytes in comparison with preadipocytes (Fold change > 4, Qvalue < 0.001). In the present study, heatmaps of DE miRNAs across various groups were displayed in Fig. 6C.
Construction of circRNAs-miRNAs-mRNAs Network
We noticed that IM and Ad groups shared with 31 DE miRNAs during adipogenesis, including miR-15c-5p, miR-206, miR-148a-5p, miR-128-1-5p, miR-30a-3p, miR-27b-5p, miR-92-5p (Fig. 7A). To further investigate the functional roles of miRNAs in chicken adipogenesis, the target genes of DE miRNAs were predicted by the bioinformatics analyses. KEGG pathway analysis suggested that their target genes were mainly enriched in PPAR signaling pathway, Metabolic pathways, Fatty acid metabolism pathway (Fig. 7B). Through integrating analysis of DE cirRNAs, miRNAs and mRNAs data, we constructed a ceRNAs network during chicken adipogenesis (Fig. 7C). It was worth noting that circFNDC3AL, circHSPG2, circLCLAT1 and circCLEC19A were the hub cirRNAs of the ceRNAs network (Fig. 7C). Furthermore, KEGG pathway analysis showed that the ceRNAs network regulate target genes mainly via Fatty acid metabolism, Focal adhesion and ECM-receptor interaction pathways (Fig. 7D).
Candidate circRNAs related to adipogenic differentiation
To further investigate the crucial roles of circRNAs in chicken adiogenesis, correlation analysis between circRNAs and their target genes which based on the ceRNAs network was performed. As shown in Fig. 8A, the expression levels of circFNDC3AL and circHSPG2 were significantly positively related with those of genes related to PPAR signaling pathway in intramuscular adipogenesis (r > 0.90, p < 0.01), such as peroxisome proliferator-activated receptor γ (PPARγ), fatty acid binding protein 4 (FABP4), perilipin 2 (PLIN2), CD36 molecule (CD36). In contrast, the expression level of circLCLAT1 was significantly negatively related with those of genes, while positively related with lysocardiolipin acyltransferase 1 (LCLAT1) and fatty acid desaturase 2 (FADS2) (r > 0.95, p < 0.01) (Fig. 8A). Moreover, the expression level of circARMH1 was significantly negatively related with those of genes in abdominal adipocytes (r > 0.92, p < 0.01), while circCLEC19A was significantly positively related with those of genes (r > 0.90, p < 0.01) (Fig. 8B). To further investigate the potential roles of circRNAs in fat tissue-specific deposition, the expression of cirRNAs were analyzed in different tissues. Our results showed that these four candidate circRNAs were highly expressed in the abdominal fat and breast muscle tissue of chicken (Fig. 8C). In addition, the dynamistic expression patterns of candidate circRNAs during tissue-specific adipogenetic differentiation were detected. Our results showed that circFNDC3AL and circCLEC19A expression levels were gradually increased during intramuscular and abdominal adipogenic differentiation, (Fig. 8D). In contrast, the expression levels of circLCLAT1 and circARMH1 were gradually decreased during intramuscular and abdominal adipogenic differentiation, respectively (Fig. 8D). Interestingly, we noticed that chicken circLCLAT1 was highly conservative among human, mouse and pig (Fig.S1), which suggested circLCLAT1 might play significant roles in animal adipogenesis or lipid metabolism.
Validation of circRNA-miRNA-mRNAs ceRNA networks
To verify the circRNA-miRNA-mRNAs regulation networks during intramuscular adipogenic differentiation, we analyzed their expression levels in intramuscular preadipocytes and mature adipocytes. Bioinformatics analysis suggested that cirFNDC3AL has many MREs (miRNAs response element), such as miR-130a-5p, miR-15c-5p, miR-18a/b-3p (Fig. 9A). RNA-Seq and RT-qPCR results showed that cirFNDC3AL and its’ downstream genes were significantly upregulated in differentiated-mature intramuscular adipocytes (Fig. 9C, 9D and 9F), while several adipogenic differentiation-related miRNAs were significantly downregulated, including miR-222, miR-456-3p, miR-15C-5p, miR-130b-5p, miR-130a-5p (Fig. 9B and 9E). It was worth noting that the target genes of circFNDC3AL were mainly associated with lipid metabolism and adipogenesis, such as PPARG, PLIN2, CPT1A, INSIG1. In addition, our results found that miR-146b-5p and miR-147 might regulate RUNXT1 and FADS2, while miR-34a-5p and miR-30e-5p might regulate PDGFRA, IGF2BP1, FNDC3B, MYH9. circLCLAT1 might affect chicken adipogenesis by sponging these miRNAs, therefore regulating target genes. Our results suggested that circLCLAT1-miRNAs (miR-146b-5p, miR-147, miR-30e-5p and miR-34a-5p)-mRNAs pathway showed the opposite expression patterns during intramuscular adipogenic differentiation (Fig. 10). CircLCLAT1 and its’ downstream genes were significantly downregulated in differentiated-mature intramuscular adipocytes (Fig. 10C, 10D and 10F), while miR-146b-5p and miR-147 were significantly upregulated in mature adipocytes in comparison with preadipocytes (Fig. 10B and 10E).