Skeletal muscle development is mainly regulated by genes and influenced by endogenous metabolites. The identification of key genes and metabolites of skeletal muscle development in embryonic stage is of great useful in explaining the molecular mechanism of muscle development in meat ducks. If only through single-omics analysis, it can not fully explain the relevant issues, so multi-omics analysis has been widely used [16, 17]. In this study, the transcriptome of Jiaji duck breast muscle during embryonic period was analyzed. The key genes and related metabolites of Jiaji duck breast muscle development during embryonic period were explained in this study. The results of this paper combined with our previous results of metabolomics analysis could provide a basis for the further study in exploring the mechanism of breast muscle development.
Through previous experiments, we found that the embryonic breast muscle cells proliferated rapidly and the weight of the breast muscle increased from E18 to E27. After E27, the cell proliferation almost stopped, and the weight of breast muscle did not change. The E34 was basically developed and ready for shelling. Therefore, we chose E18, E27, E34 to explore the key genes and metabolites that may be the regulators for skeletal muscle development of Jiaji ducks.
In this study, found many DEGs in E18 VS E27, E27 VS E34, E18 VS E34 and conducted GO and KEGG pathway analysis for these DEGs. We obtained two pathways shared by E18 VS E27, E27 VS E34, E18 VS E34, ECM-receptor interaction and focal adhesion. ECM consists of a complex mixture of structural and functional macromolecules, mainly including collagen, fibronectin, and laminin [18]. Studies have found that cell differentiation is affected by the interaction between cells and ECM [21]. For focal adhesion, studies have shown the site where integrin and proteoglycan mediated adhesion connects with the actin cytoskeleton is called focal adhesion (FA), which is dynamic multi-protein complexes that connect ECM with the intracellular cytoskeleton [22, 23]. The formation and maturation of FA is a key procedure during myoblast differentiation. Thus, the GO and KEGG pathway analysis of DEGs of this study indicate that DEGs tightly correlated with the proliferation of breast muscle cell and the weight of breast muscle for Jiajia ducks.
In this study, DEGs in transcriptome trend 1 was contrast to the trend of breast development and myoblast cell proliferation. Thus, DEGs in transcriptome trend 1 may inhibit the development of embryonic breast muscle of Jiaji duck. KEGG analysis showed that TGF-beta signaling pathway was the extremely significant pathway, which was related to cell proliferation and differentiation [24]. DEGs in this pathway included NOG, NBL1, CHRD, ID4, GREM1, GDF5, BMP2, BMP6, BMP7, SMAD6, LTBP1, ENSAPLG00000018058, ID2. Among them, ID4, ID2, and GDF5 were tightly related to skeletal muscle development. Study shows that ID4 gene can inhibit cell differentiation [25]. In stem cells, ID4 prevents other transcription factors from binding to DNA, thereby inhibiting the initiation of cell differentiation. This allows stem cells to continue to self-renew and proliferate, thereby maintaining their undifferentiated state [25]. Melnikova et al found that ID2 over-expression inhibited the differentiation of Sol8 myoblasts [26]. Sullivan et al also showed that ID2 inhibits cell differentiation but promotes the proliferation of different types of cells [27]. GDF5 belongs to the TGF-β family and follows the same Smad-dependent and Smad-independent cell signaling pathways [28–31]. Moore et al found that the addition of GDF5 to the medium can lead to higher proliferation and ECM deposition of various cell types in a dose-dependent manner [29].
We also found the expression profiles of DEGs in transcriptome trend 6 was consistent with the trend of breast muscle development and breast muscle cell proliferation of Jiaji ducks. Therefore, we supposed that DEGs in transcriptome trend 6 may promote the development of breast muscle and myoblast proliferation. In this study, we found DEGs were extremely significantly enriched in the MAPK signaling pathway, a pathway related to cell proliferation [32]. DEGs in this pathway included LRIT2 and DOK5. DOK5 protein can bind to a variety of growth factor receptors, such as EGFR and DGFR [33], and affect the transmission of growth factors, thereby affecting cell proliferation and differentiation. Xu et al showed that over-expression of DOK5 promoted cell proliferation and osteogenesis, and activated the canonical Wnt/β-catenin signaling pathway [34]. LRIT2 with its expression level consistent with skeletal muscle development and myoblast proliferation of Jiaji duck may be a promotor for skeletal muscle development and myoblast proliferation. However, there is no report about the role of LRIT2 up to date. Thus, the further study should be performed in the future. In addition, focal adhesion pathway, a pathway with extremely significant enrichment by DEGs in transcriptome trend 6, is closely related to cell proliferation and differentiation [35]. EGF is enriched in this pathway. Study shows EGF encodes epidermal growth factor [36], which is a signaling molecule that participates in the regulation of biological processes such as proliferation, differentiation by binding and activating EGF receptors on the cell surface [37].
Through the joint analysis, we found 16 overlapped pathways between transcriptome trend 1 and metabolome trend 1. Among them, the protein digestion and absorption pathway has been reported to be closely related to cell proliferation and differentiation [38, 39]. Further analysis found arginine and COL8A2 were in the pathway. Arginine can be used as a synthetic substance and involve in the synthesis of protein and other cellular active substances [40]. Some studies have found that arginine regulates signal transduction of cell proliferation and differentiation by participating in mechanisms such as nitric oxide synthesis and release [41]. In addition, arginine is also involved in the synthesis of DNA and protein [42], which plays an important role in cell proliferation and differentiation. COL8A2 involves in the construction and maintenance of extracellular matrix, and has a regulatory effect on cell proliferation and migration [13]. COL8A2 is co-expressed with activating enhancer binding protein 2 (TFAP2) during corneal development, which has been reported to be related to the proliferation of CEC [43]. The co-expression of COL8A2 and TFAP2 plays an important role in the migration and proliferation of vascular smooth muscle [44]. Hwang et al found that the cell survival rate, cell proliferation rate, cyclin D1 expression and the number of cells in S phase decreased in cells transfected with siCOL8A2 [45], indicating COL8A2 has a inhibitory role for cell proliferation. In this study, the expression trend of COL8A2 (Fig. 6) was contrast to the development trend of breast muscle of Jiaji duck, indicating that it had an inhibitory effect on the proliferation of muscle cells in the embryonic stage. In addition, a correlation analysis showed that arginine was significantly positively correlated with COL8A2, suggesting that the expression level of COL8A2 has a tight linkage with the content of arginine. At the same time, this also shows that the decrease of COL8A2 gene expression will lead to the decrease of arginine content, thus inhibiting the development of chest muscle of embryonic Jiaji ducks.
Through the joint analysis transcriptome trend 6 and metabolome trend 6, the arginine and proline metabolism pathway stood out because it was closely related to cell proliferation and differentiation. This pathway contains proline, which plays an important role in cell proliferation and differentiation [46] and can increase the rate of protein synthesis in muscle [47]. Proline can synthesize molecules related to protein synthesis, signal transduction and cell growth through the catalysis of a series of enzymes. Studies have shown that the regulation of the enzyme activity in the proline metabolic pathway can affect cell proliferation and differentiation [48]. DEGs enriched in this pathway are GAMT, SAT1, and AGMAT. The enzyme encoded by GAMT gene plays an important role in muscle cells [49], which is involved in the synthesis of creatine in muscle. Creatine directly affect the myogenic process (formation of muscle tissue), by altering secretions of myokines [50]. Arginine is the raw material for the synthesis of creatine [51]. There is a conversion relationship between proline and arginine[52–54], so proline has a certain effect on the synthesis of creatine. According to the correlation analysis, there is a significant positive correlation between GAMT and proline, so we believe that GAMT and proline jointly promote the proliferation of muscle cells.