Animals and Ethics Statement
Three BJY chickens were obtained from the Institute of Animal Sciences, CAAS (Beijing, China), which were raised under the same recommended environmental and nutritional conditions. Animal experiments were approved by the Science Research Department (in charge of animal welfare issues) at the Institute of Animal Sciences, Chinese Academy of Agricultural Sciences (CAAS) (Beijing, China). Three birds were individually euthanized by carbon dioxide anesthesia and exsanguination by severing the carotid artery at 10 days of age, the pectoralis major and abdominal fat tissues were excised to use the cell isolation.
Preadipocyte acquisition
The mature adipocytes from the pectoralis major and abdominal fat tissue were respectively isolated as previous method, and then the preadipocytes were obtained with the dedifferentiation treatment as previous method [16]. The main protocol as follows:
The abdominal fat tissue and pectoralis major of three chickens were collected, and then washed with phosphate-buffered saline (PBS) containing 1% penicillin- streptomycin (Gibco, Thermo Fisher Scientific Inc., Shang hai, China). The abdominal fat tissue and pectoralis major from the same chicken were recorded for the one-to-one correspondence of cell samples in subsequent experiments. After removal of the blood vessels and connective tissue, the samples were finely minced to 1 mm3 with scissors, and then digested in DMEM/F12(1:1) medium (Gibco, Thermo Fisher Scientific Inc., Shang hai, China) contained 0.1% Type I collagenase (Sigma-Aldrich, Shanghai, China) in a water bath with continuous shaking at 37°C for 60 min. After terminated digestion and filtered, the cell suspension was centrifuged with 600 g for 15 min. The top layer containing mature adipocytes was collected and placed to a 25 cm2 cell culture flask, which was inverted and completely filled with DMEM/F12(1:1) medium containing 10% FBS. The floating mature adipocytes would adhere to the bottom of the flask incubate in a 37°C incubator with 5% CO2. After 3 days, the mature adipocytes would gradually converse to preadipocytes by releasing the lipid by exocytosis, and the medium was replaced and the flask was re-inverted so that the preadipocytes were on the bottom to proliferate massively. Up to 15 days, the confluence of preadipocytes would reach 80%-90%, cells were sub-cultured.
Preadipocytes culture and treatment
The medium was changed every 3 days. After reaching 80 % confluence, cells were passaged, and the second passage (P2) preadipocytes by the dedifferentiation of mature adipocytes from the pectoralis major (DIMFP) and abdominal fat tissue (DAFP) were used for further experiments. Both of DIMFP and DAFP were respectively plated in 24-well or 100-mm dishes, and cells were collected after 2 days with 100 % confluence for further RNA extraction and Measurement of biochemical indices (in 100-mm dishes), or Oil-red-O staining assay (in 24-well) .
Oil-red-O staining assay
The cellular lipid content of DIMFP and DAFP in 24-well were determined by oil-red-O staining. Staining procedure was conducted as follows: cell medium was discarded, washed 3 times with PBS, and then fixed with 4% formalin for 30 min. After fixed, the cells were washed 3 times with PBS and stained with oil-red-O (WuHan AmyJet Scientific Inc., Wuhan, China) for 60 min. Subsequently, the oil-red-O was discarded and washed 3 times. After water evaporated, isopropanol was added to extract the oil-red-O for 10 min, and then the absorbance values of the solutions were measured by the Varioskan™ LUX Multimode Microplate Reader (Thermo Fisher Scientific Inc., Shang hai, China) at 510 nm.
Measurement of biochemical indices
The TG, TCHO and PLIP contents in cell samples were measured using TG (Nanjing Jiancheng Bioengineering Institute, Nanjing, China), TCHO (Nanjing Jiancheng Bioengineering Institute, Nanjing, China) and PLIP (Beijing Lidemann Biochemical Co. , Ltd. , Beijing, China) assay kits. Cell samples from each group were homogenized with absolute ethanol at room temperature and centrifuged (1000×g, 20 min). After centrifugation, the supernatant was used for TG, TCHO and PLIP measurement, respectively. The assay was performed according to the manufacturer’s instructions.
RNA Extraction and Identification
Total RNA was extracted from DIMFP and DAFP in 100-mm dishes, using the Trizol reagent (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. The quality of RNA was detected by 1.5% gel electrophoresis, and RNA concentration was determined by NANODROP2000 spectrophotometer (Thermo Fisher Scientific Inc., Shang hai, China). The OD260/280 values of all samples were limited to the range of 1.8 to 2.0. The RNA samples were subsequently used for gene expression profiling.
Gene Expression Profiling
Based on ultra-high-throughput sequencing (HiSeq2500; Illumina, San Diego, CA, USA), gene expression profiling was undertaken by Berry Genomics (Beijing, China). Raw data were converted to FASTQ files using bcl2fastq (Illumina). Clean reads were generated by removing reads with adapter and low-quality sequences and mapped to the reference chicken genome and genes (Gallus gallus, Galgal6; available at https://www.ncbi.nlm.nih.gov/assembly/GCF_000002315.6) using TopHat 1.3.2 (https://ccb.jhu.edu/software/tophat). Gene expression levels were calculated using the RPKM method, as described by Mortazavi et al [42]. Differentially expressed genes (DEGs) between the DIMFP and DAFP were analyzed using the edgeR R package. DEGs were screened by the following criteria: |log2 FC| ≥ 1.0, with FDR < 0.05. Based on the DEGs, clustering analysis was performed in each sample by the pheatmap package of R software. Hierarchical clustering was performed on both rows and columns, and the resulting dendrogram was saved as an image file.
Gene Ontology and Kyoto Encyclopedia of Genes and Genomes Analysis
Gene Ontology (GO) enrichment analysis was performed to identify the gene function classes and categories corresponding to the DEGs using the ClueGO plug-in and CluePedia plugin of Cytoscape (https://cytoscape.org/). The significance level of GO terms enrichment was set at P < 0.05 as indicated. According to the results of GO enrichment analysis, DEGs related to abdominal adipose tissue metabolism were screened. The significantly enriched signaling pathways of DEGs were analyzed by the Kyoto Encyclopedia of Genes and Genomes (KEGG) [43]. P < 0.05 was considered to be indicative of statistical significance.
Real-time quantitative polymerase chain reaction
Using the same RNA samples, real-time quantitative polymerase chain reaction (qRT-PCR) was performed to confirm the results of gene expression profiling. RNA samples were reverse transcribed using TIANGEN® FastQuant RT Kit (Tiangen, Beijing, China), and specific primers were designed placing at or just outside of exon/exon junctions using Primer 5.0 software dependent on GeneBank sequences (Additional file 6:Table S6).
Samples were amplified using the real-time PCR Detection System ABI 7500 (Applied Biosystems, Carlsbad, California, USA). The PCR mixture contained 10 μL of 2 × iQ™ SYBR Green Supermix, 0.5 μL (10 mmol) of each primer, and 1 μL of cDNA, along with ddH2O for a total volume of 20 μL. After initial denaturation for 30 s at 95 °C, amplification was performed for 40 cycles (95 °C for 5 s and 60 °C for 32 s). The actin beta (β-actin) expressions were used as the normalization control by the 2−△△Ct method to determine fold-changes in gene expression [44]. A melting curve was constructed to verify the single amplified PCR product. Samples were assayed in triplicate, with standard deviations of cycle threshold values did not exceed 0.5 on a within-run basis. Correlations between relative abundance from qRT-PCR and gene expression profiling data were also calculated.
Statistical analysis
Three comparison replicates (DIMFP vs DAFP) of the cell experiment were set according to the one-to-one correspondence of cell samples from the abdominal fat tissue and pectoralis major of the same chicken. All data are presented as the means ± SEM, for three replicates of the experiment. Statistically significant differences between the two culture conditions were tested by independent-samples t-tests using SAS 9.2 software. P < 0.05 (*) or P < 0.01 (**) was considered to be significant. All figures were constructed using GraphPad Prism version 5.02 (GraphPad Software Inc, La Jolla, CA).