Intramuscular fat (IMF) is one of the important factors influencing meat quality, however, for chickens, the molecular regulatory mechanisms underlying this trait have not yet been clear. In this study, a systematic identification of differentially expressed genes (DEGs) and molecular regulatory mechanism related to IMF metabolism between Beijing-you chicken breast and thigh at 42 and 90 days of age was performed. IMF contents, Gene Ontology (GO) terms, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were analyzed, The results showed that both IMF contents in breast at 42 and 90 d were significantly lower (P < 0.05 or P < 0.01) than those in thigh. By microarray, 515 common known DEGs and 36 DEGs related to IMF metabolism were identified between the breast and thigh at 42 and 90 d. Compared to thigh, the expression levels of PPARG had significantly down-regulated (P < 0.01) in breast, but the expression levels of RXRA and CEBPB had significantly up-regulated (P < 0.01). However, the expression levels of LPL, FABP4, THRSP, RBP7, LDLR, FABP3, CPT2 and PPARGC1A had significantly down-regulated in breast (P < 0.01), supporting that PPARG and its down-stream genes had the important regulatory function to IMF deposition. In addition, based on of DEGs, KEGG analysis revealed that PPAR signaling pathway and cell junction-related pathways (focal adhesion and ECM-receptor interaction, which play a prominent role in maintaining the integrity of tissues), might contribute to the IMF metabolism in chicken. Our data had screened the potential candidate genes associated with chicken IMF metabolism, and imply that IMF metabolism in chicken is regulated and mediated not only by related functional genes and PPAR pathway, but also by others involved in cell junctions. These findings establish the groundwork and provide new clues for deciphering the molecular mechanisms underlying IMF deposition in poultry. Further studies at the translational and posttranslational level are now required to validate the genes and pathways identified here.

Skeletal muscle development is closely linked to meat production and its quality. This study is the first to quantify the proteomes and metabolomes of breast muscle in two distinct chicken breeds at embryonic day 12 (ED 12), ED 17, post-hatch D 1 and D 14 using mass spectrometry-based approaches. Results found that intramuscular fat (IMF) accumulation increased from ED 17 to D 1 and that was exactly the opposite of when most obvious growth of muscle occurred (ED 12 - ED 17 and D 1 - D 14). For slow-growing Beijing-You chickens, Ingenuity Pathway Analysis of 77–99 differential abundance (DA) proteins and 63–72 metabolites, indicated significant enrichment of molecules and pathways related to protein processing and PPAR signaling. For fast-growing Cobb chickens, analysis of 68–95 DA proteins and 56–59 metabolites demonstrated that molecules and pathways related to ATP production were significantly enriched after ED12. For IMF, several rate-limiting enzymes for beta-oxidation of fatty acid (ACADL, ACAD9, HADHA and HADHB) were identified as candidate biomarkers for IMF deposition in both breeds. This study found that ED 17 - D 1 was the earliest period for IMF accumulation. Pathways related to protein processing and PPAR signaling were enriched to support high capacity of embryonic IMF accumulation in Beijing-You. Pathways related to ATP production were enriched to support the fast muscle growth in Cobb. The beta-oxidation of fatty acid is identified as the key pathway regulating chicken IMF deposition at early stages.

The lipid from egg yolk is largely consumed in supplying the energy for embryonic growth until hatching. The remaining lipid in the yolk sac is transported into the hatchling’s tissues. The gene expression profiles of fast- and slow-growing chickens, Arbor Acres (AA) and Beijing-You (BJY), were determined to identify global differentially expressed genes and enriched pathways related to lipid metabolism in the pectoralis major at hatching. Between these two breeds, the absolute and weight-specific amounts of total yolk energy (TYE) and intramuscular fat (IMF) content in pectoralis major of fast-growing chickens were significantly higher (P < 0.01, P < 0.01, P < 0.05, respectively) than those of the slow-growing breed. IMF content and u-TYE were significantly related (r = 0.9047, P < 0.01). Microarray analysis revealed that gene transcripts related to lipogenesis, including PPARG, RBP7, LPL, FABP4, THRSP, ACACA, ACSS1, DGAT2, and GK, were significantly more abundant in breast muscle of fast-growing chickens than in slow-growing chickens. Conversely, the abundance of transcripts of genes involved in fatty acid degradation and glycometabolism, including ACAT1, ACOX2, ACOX3, CPT1A, CPT2, DAK, APOO, FUT9, GCNT1, and B4GALT3, was significantly lower in fast-growing chickens. The results further indicated that the PPAR signaling pathway was directly involved in fat deposition in pectoralis major, and other upstream pathways (Hedgehog, TGF-beta, and cytokine–cytokine receptor interaction signaling pathways) play roles in its regulation of the expression of related genes. Additional energy from the yolk sac is transported and deposited as IMF in the pectoralis major of chickens at hatching. Genes and pathways related to lipid metabolism (such as PPAR, Hedgehog, TGF-beta, and cytokine–cytokine receptor interaction signaling pathways) promote the deposition of IMF in the pectoralis major of fast-growing chickens compared with those that grow more slowly. These findings provide new insights into the molecular mechanisms underlying lipid metabolism and deposition in hatchling chickens.

Background: The development of skeletal muscle is closely related to the efficiency of meat production and meat quality. Chicken skeletal muscle development depends on myogenesis and adipogenesis and occurs in two phases—hyperplasia and hypertrophy. However, cell profiles corresponding to the two-phase muscle development have yet to be determined. Single-cell RNA-sequencing (scRNA-seq) can elucidate the cell subpopulations in tissue and capture the gene expression of individual cells, which can provide new insights into the myogenesis and intramuscular adipogenesis. Results: Ten cell clusters at the post-hatching developmental stage at Day 5 and seven cell clusters at the late developmental stage at Day 100 were identified in chicken breast muscles by scRNA-seq. Five myocyte-related clusters and two adipocyte clusters were identified at Day 5, and one myocyte cluster and one adipocyte cluster were identified at Day 100. The pattern of cell clustering varied between the two stages. The cell clusters showed clear boundaries at the terminal differentiation stage at Day 100; by contrast, cell differentiation was not complete at Day 5. APOA1 and COL1A1 were selected from up-regulated genes in the adipocyte cluster and found to be co-expressed with the ADIPOQ adipocyte marker gene in breast muscles by RNA in situ hybridization. Conclusions: This study is the first to describe the heterogeneity of chicken skeletal muscle at two developmental stages. The genes APOA1 and COL1A1 were identified as biomarkers for chicken intramuscular fat cells.

The present study aimed to find a blood marker for the prediction of wooden breast (WB) in live broiler to assist the genetic selection of fast-growing chickens. The experiments were carried out with two chicken flocks: 250 male broilers in flock 1 and 100 male and female broilers in flock 2. Both flocks were slaughtered and measured. The breast filets were assessed by combining subjective scoring and compression force at 28 (flock 1 only) and 42 days of age. The enzyme activity in serum and breast tissue (flock 1 only) of normal and affected groups was tested. The results showed that the compression force was significantly different between the normal and affected groups at 28 and 42 days of age (P < 0.001), and it increased significantly with rising WB and WS scores. The serum creatine kinase (CK) value increased significantly with rising compression force at 42 days of age (P < 0.001). The serum CK positively correlated with compression force (r = 0.608; P < 0.001) and the linear regression equation (serum CK = 0.9960^∗compression force + 1.884) was established for the line studied. The association between serum CK and compression force is consistent between flocks 1 and 2. In conclusion, our study confirmed that compression force could be the quantitative indicator to differentiate breast filets and found that serum CK could be a candidate biomarker to predict WB in live broilers and assist genetic selection in broiler breeding.

Caponization results in reduced androgen levels, which leads to abdominal fat accumulation in capons. In this study, we sought to understand the molecular mechanisms behind this fat accumulation. Abdominal fat (AF) content increased significantly (P < 0.05) and serum and AF testosterone levels decreased significantly (P < 0.05 or P < 0.01) after caponization. In AF tissue, 90 differentially expressed genes related to lipid metabolism were screened by gene expression profiling in caponized and sham-treated chickens. Among these, six representative genes were significantly up-regulated (APOA1, SCD, FABP7, RXRG, and FADS2) or down-regulated (FABP3) (P < 0.05 or P < 0.01) and were strongly associated with the PPAR pathway. In addition, cell junction pathways were also enriched. In vitro, Fat content was significantly lower in cells treated with testosterone compared with control cells (P < 0.01), and mRNA levels of RXRG, FABP7, and FABP3 changed accordingly, confirming the effect of testosterone on fat deposition. The results of this study indicate that testosterone reduction likely regulates gene expression through PPAR and cell junction pathways resulting in increased fat accumulation. These results provide increase our understanding of the biological mechanisms by which caponization induces greater fat accumulation.