Introduction

Lamb is one of the healthiest meat options since it has high contents of iron, zinc, selenium, vitamins B1, B2, and B61,2. However, fat deposition directly affects the flavor and quality of meat, and it can reduce sheep productivity. For example, intramuscular fat (IMF) is a key contributor to meat flavor and palatability3,4,5. In this context, understanding the growth and differentiation of adipose tissue in sheep may aid in controlling the amount and distribution of fat deposits, with this potentially boosting consumer demand for sheep meat, thus allowing for improved market growth and development.

Lipogenesis is a complicated process that has multiple stages and complex regulatory control6. The differentiation of adipocytes from mesenchymal precursors (termed adipogenesis), is an important process for develo** and maintaining functional adipose tissues7. The factors that affect adipocyte differentiation also affect fat deposition, and include genetic factors, dietary factors, and physiological conditions6. However, these factors likely all act through regulation of the expression of specific genes, and the modulation of various signaling pathways.

In recent years, research into lipogenesis and its regulatory mechanisms has become a focus for breeding and animal production. Researchers have identified several genes, associated proteins and transcription factors that affect preadipocyte differentiation, including the genes for the CCAAT enhancer binding proteins (C/EBPs)8,9, the peroxisome proliferator-activated receptor (PPAR)10, the early B-cell factor (EBF)11, the fatty acid binding proteins (FABPs)12, and the β-linked protein β-catenin13.

More recently, analysis of the genome and transcriptome has revealed a group of RNA transcripts that have been called long non-coding RNAs (lncRNAs). They appear to play an important role as epigenetic regulators14, and some of them have been shown to have regulatory functions in adipogenesis or lipid metabolism15. For example, a human study by Sun et al.16 analyzed the transcriptomes of primary adipocytes and identified 175 lncRNAs that were up- or down-regulated (greater than two-fold) during differentiation of both brown and white adipocytes. Some of the lncRNAs were induced during adipogenesis and bound at their promoters by key transcription factors such as PPARG and the and CCAAT/enhancer-binding protein α (C/EBPα)16.

Xu et al.17 revealed that overexpression of lncRNA steroid receptor RNA activator (SRA) in mesenchymal preadipocytes promoted their differentiation to adipocytes, and that contrastingly, knockdown of this lncRNA inhibited 3T3-L1 preadipocyte differentiation. In addition, ** to the reference genome.

Figure 1
figure 1

An overview of the lncRNAs that were detected during the differentiation of the ovine preadipocytes, and their expression characteristics. (A) The distribution of the lncRNAs in intergenic, intronic and exonic regions based on their location in the ovine reference genome v4.0. (B) Venn diagram summarizing the number of lncRNAs identified using the CNCI v2 and CPC v0.9-r2 software. (C) Summary of the type of lncRNAs identified in this study. (D) Venn diagram summarizing the number of lncRNAs expressed at D0, D2 and D8 in the ovine preadipocytes. (E, F) Comparison of expression levels of lncRNAs and mRNAs identified in the D0, D2 and D8 preadipocytes.

A total of 8819 transcripts were produced. Upon identification, there was a total of 2517 novel lncRNAs and 3943 known lncRNAs (Fig. 1B). The combined total of 6460 lncRNAs were classified into five categories based on their position in the ovine genome relative to protein-coding genes, and they included intergenic lncRNAs (59.33%), bidirectional lncRNAs (10.99%), intronic lncRNAs (2.32%), antisense lncRNAs (10.54%) and nonsense lncRNAs (7.80%) (Fig. 1C). The intergenic lncRNAs was the most common, with the number of known lncRNAs and novel lncRNAs being 2445 and 1388, respectively (Fig. 1C). Among all the lncRNAs, there were 3455 (53.48%) lncRNAs that were expressed in the preadipocytes at all three stages of differentiation (Fig. 1D), while 214, 226 and 228 lncRNAs were specifically expressed in preadipocytes at D0, D2 and D8, respectively. Among the lncRNAs that were expressed at all three stages, the novel lncRNA MSTRG.65945.1 was expressed at the highest level. It was revealed that the majority of lncRNAs detected were present at a low to medium abundance when compared to mRNA levels (Fig. 1E), with them predominantly in the fragments per kilobase of transcript per million mapped reads (FPKM) range of 0–10 (Fig. 1E,F).

Based on the results of the differential analyses, 405 (244 up-regulated, 161 down-regulated), 272 (167 up-regulated, 105 down-regulated) and 359 (166 up-regulated, 193 down-regulated) differentially expressed lncRNAs were identified in D0-vs-D2, D0-vs-D8 and D2-vs-D8 comparisons, respectively (Fig. 2A). Among them, 19 lncRNAs were differentially expression in all three stages (Fig. 2B). The hierarchical clustering results of differentially expressed lncRNAs revealed a significant difference in the gene expression profiles among the three groups (Fig. 2C–E).

Figure 2
figure 2

Summary of the differential expression analysis of the annotated lncRNAs. (A) The number of differentially expression lncRNAs. (B) The shared and unique lncRNAs when comparing three groups of differentially expressed lncRNAs. (C-E) Heat maps of differentially expressed lncRNAs were analyzed using the z-score method according to the FPKM values21. Blue denotes the genes with lower levels of expression, while red denotes the genes with higher levels of expression.

GO and KEGG enrichment analyses of the target genes of the differentially expressed lncRNAs

To investigate the functions of the lncRNAs associated with ovine preadipocyte differentiation, analyses were undertaken to predict the target genes of the differentially expressed lncRNAs using antisense, cis-action and trans-action analysis. In the antisense analysis, 703 lncRNA-mRNA pairs were identified, including 14 significantly different antisense-mRNA pairs. The GO functional enrichment analysis results revealed that the target genes were mainly enriched in 544 biological processes (BP), 52 cellular components (CC), and 69 molecular functions (MF) (Fig. 3A). Most target genes were significantly enriched in the regulation of immune processes and enzyme activities, including immune responses (GO:0002440), positive regulation of lymphocyte activation (GO:0051251), phospholipase activity (GO:0004620), tRNA methyltransferase activity (GO:0008175), phosphodiester hydrolase activity (GO:0008081), cellular glucose homeostasis (GO:0001678), and response to fatty acids (GO. 0070542) (Supplementary Table 1).

Figure 3
figure 3

Functional categorization of the target genes of differentially expressed lncRNAs during the selected stages of differentiation of the ovine preadipocytes. (A) Antisense target genes. (B) Cis target genes. (C) Trans target genes.

In the context of a cis mode of action, we identified 3517 cis-regulatory (cis-mRNA) relationship pairs, of which 39 were significantly different cis-mRNA relationship pairs (P < 0.05). The GO functional enrichment analysis of the target genes revealed that they were mainly enriched in 902 GO-BP terms. Most of the target genes were enriched in GO terms related to transporter protein activity and catabolic processes, including protein transmembrane transporter protein activity (GO:0008320), protein transporter protein activity (GO:0008565), aromatic amino acid family catabolic processes (GO:0009074), long-chain fatty acid transport (GO:0015909), and cellular amino acid catabolic processes (GO:0009063) (Fig. 3B, Supplementary Table 1).

A total of 31,502 trans-mRNA pairs were identified (P < 0.05). The GO functional enrichment analysis results revealed that the target genes were enriched in 3772 GO-BP, 333 GO-CC and 665 GO-MF terms. The majority of target genes were mainly enriched in the regulation of cell proliferation and differentiation and protein binding, including cell proliferation (GO:0008283), regulation of TGF-β receptor signaling pathway (GO:0017015), cell differentiation (GO:0030154), epithelial cell proliferation (GO:0050673), actin binding (GO:0003779), epithelial cell differentiation (GO:0030855) and adipocyte differentiation (GO:0045444), among others (Fig. 3C, Supplementary Table 1).

To further analyze the possible pathways involved in ovine preadipocyte differentiation for the target genes, a KEGG enrichment analysis was undertaken. This revealed that most of antisense target genes were significantly enriched in the fatty acid degradation pathway, the glycerolipid metabolism, ether lipid metabolism, amino acid metabolism pathways, and glycolysis/gluconeogenesis signaling pathways (P < 0.05, Fig. 4A). Most cis target genes were significantly enriched in pathways including metabolic pathways, chemokine signaling pathways, amino acid biosynthesis pathways, cytokine-cytokine receptor interactions, and taurine and hypotaurine metabolism pathways (P < 0.05, Fig. 4B). In contrast, most trans target genes were significantly enriched in pathways, including the MAPK signaling pathway, the PI3K-Akt signaling pathway and the TGF-β signaling pathway (P < 0.05, Fig. 4C). These results suggest that lncRNAs may act on protein-coding genes through antisense, cis and trans modes of action, thereby regulating preadipocyte differentiation in sheep.

Figure 4
figure 4

KEGG enrichment analysis of the target genes of differentially expressed lncRNAs during the differentiation stage of ovine preadipocytes. (A) Antisense target genes (B) Cis target genes. (C) Trans target genes.

The structure-interaction network of ceRNAs

This analysis revealed a total of 12,528 lncRNA-miRNA-mRNA binding pairs. That is, 427 differentially expressed lncRNAs would bind 421 miRNAs to increase the expression of 1571 target mRNAs (Supplementary Table 2). The 18 most significant ceRNA relationship pairs from all ceRNAs were chose to construct a network diagram (Fig. 5), and together the above findings suggest that differentially expressed lncRNAs may act as ‘molecular sponges’ for miRNAs during ovine preadipocyte differentiation.

Figure 5
figure 5

Sankey plot of the lncRNA-miRNA-mRNA network related to ovine preadipocyte differentiation.

RT-qPCR verification of the RNA-Seq data

To verify the RNA-Seq results, nine differentially expressed lncRNAs were selected and analysed using RT-qPCR. The RT-qPCR expression patterns of the selected differentially expressed lncRNAs were found to be consistent with the expression trends from the RNA-Seq results during preadipocyte differentiation (Fig. 6), with this confirming the reliability of the RNA-Seq method used in this study.

Figure 6
figure 6

Comparison of RT-qPCR and RNA-Seq results for nine selected lncRNAs.