Animals and experimental design
The experimental procedures used in this study were approved by the Animal Care Committee of Zhejiang University (Hangzhou, China) and conducted in accordance with the university’s guidelines for animal research. The design of the experiment is described in our previous study [19]. In brief, thirty multiparous Chinese Holstein dairy cows at wk 4 before parturition with similar BW (657 kg, SD = 58) were assigned to 15 blocks according to parity and the 305-day milk yield (8692 kg, SD = 607) of the previous lactation. The first week (wk 4 before parturition) was used as the adaptive week. The dairy cows were then randomly allocated into two groups and fed basal diet alone (CON) or supplemented with 20 g/d NCG (NCG) (Beijing Animore Sci. & Tech. Co., Ltd., Beijing, China). The chemical compositions of the diets are listed in Supplementary Table S1. Throughout the trial period, cows were housed in a barn with individual tie stalls and had free access to water. NCG was added once per day at 1400 h by scattering it on the total mixed ration for individual cows. Calves were weighed soon after parturition and before colostrum consumption.
Bloodsample collection and analyses
Fourteen and fifteen blood samples from each newborn calf was collected from the jugular vein before colostrum consumption in the CON and NCG group respectively. Then these blood samples centrifuged at 3,000 × g for 15 min at 4°C for collection of plasma and then frozen at -80°C until subsequent analysis. A subsample of each plasma sample was used to analyze biochemical variables, including glucose, total protein, blood urea nitrogen, non-esterified fatty acids, β-hydroxybutyrate, cholesterol, triglyceride, total bilirubin, albumin, and globulin using an AutoAnalyzer 7020 instrument (Hitachi High-technologies Corporation, Tokyo, Japan) with colorimetric commercial kits (Ningbo Medical System Biotechnology Co., Ltd.). The plasma AA compositions were determined as described elsewhere [19]. Six plasma subsamples in each group were collected based on the following criteria: parities of the dairy cows, number of days from the data, the cows began receiving NCG to the calving day, and calf sex (female) to analyze the metabolome profiles with GC-MS.
Placentome collection and analyses
The placentome samples were collected referring to the methods reported by Batistel et al. [4]. Briefly, after natural delivery (within 2 h), the placenta was rinsed with physiologic saline, and then, 4-6 placentomes from the central area of the placenta were dissected, rinsed with physiologic saline until clear and then stored at -80°C for later analysis. The criteria of sample selection were based on the following: parities and body conditions of the dairy cows, number of days from the data the cows began receiving NCG to the calving day, time when the placenta was released after parturition, and calf sex. According to these criteria, six placentas were selected from each group for further analysis.
Total RNA from the placentome was extracted with TRIzol reagent according to the manufacturer’s instructions (Aidlab Inc.; Code: RN03). The concentration and purity of the total RNA were measured using a NanoDrop® ND-1000 Spectrophotometer (NanoDrop Technologies Inc., Wilmington, DE, USA). The total RNA of the placentome was reverse transcribed to cDNA using the PrimeScript 1st Strand cDNA Synthesis Kit (TOYOBO, Osaka, Japan; Code: FSQ-101). Quantitative real-time PCR (qRT-PCR) was performed with the 2 × SYBR® Premix Ex Taq kit (Aidlab Inc., Beijing, China; Code No. PC5902) and the Applied Biosystems 7500 system (Foster City, CA). The PCR conditions were set as follows: 1 cycle at 95°C for 2 min, 40 cycles of 95°C for 15 s and 60°C for 34 s, followed by a melting curve program (from 60 to 95°C).
Twenty-three genes of interest were selected based on their key roles in placental functions: (1) genes encoding Arg transporters (SLC6A14, SLC7A1, SLC7A6, SLC7A7, SLC7A9); (2) genes encoding glucose transporters (SLC2A1, SLC2A3, SLC2A4); (3) genes encoding fatty acid transporters (SLC27A1, SLC27A2, SLC27A3); (4) genes associated with angiogenesis (VEGFA, NOS3, GUCY1B3, HIF1A), and (5) genes involved in the mTOR pathway (AKT1, mTOR, RPS6KB1, EIF4BP1, EIF4EBP2, EEF1A1, ELF2, IRS1). In addition, two references genes (GAPDH and ACTB) were included based on published recommendations [4]. Some of the primers targeting these genes were selected based on previous studies, and others were designed using National Center for Biotechnology Information Nucleotide database (http://www.ncbi.nlm.nih.gov/nuccore/), with Bos taurus as the species (Supplementary Table S2). The specificity of the primers was validated by primer-BLAST (https://www.ncbi.nlm.nih.gov/tools/primer-blast/). The relative changes at the mRNA level for each individual gene were analyzed using the 2-ΔΔCT method, where ΔCT = CT target mRNA – CT housekeeping mRNA (where CT = cycle threshold).
Metabolomics analysis
Fifty microliters of sample was transferred into a 2 mL tube, and 200 μL prechilled extraction mixture (methanol) and 5 μL internal standard (L-2-chlorophenylalanine, 1 mg/mL stock) were added. The mixture was then vortex mixed for 30 s. The samples were then ultrasonicated for 10 min in ice water and centrifuged at 4°C for 15 min at 16,065 × g. To prepare the quality control (QC) sample, 20 μL of each sample was collected and pooled together. After evaporation in a vacuum concentrator, 30 μL of methoxyamination hydrochloride (20 mg/mL in pyridine) was added. The mixture was then incubated at 80°C for 30 min and then derivatized by 40 μL of BSTFA regent (1% TMCS, v/v) at 70°C for 1.5 h. After gradually cooling the samples to room temperature, 5 μL of FAMEs (in chloroform) was added to the QC sample. All samples were then analyzed by a gas chromatograph coupled with a time-of-flight mass spectrometer (GC-TOF-MS).
Raw data processing, including peak extraction, baseline adjustment, deconvolution, alignment and integration [20], was performed with Chroma TOF (V 4.3x, LECO) software, and the LECO-Fiehn Rtx5 database was used for metabolite identification by matching the mass spectrum and retention index. Finally, the peaks detected in fewer than half the QC samples or RSD>30% in the QC samples was removed [21].
The pattern recognition multivariate analyses, including principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA), were performed with SIMCA 14.1 software package (V14.1, Sartorius Stedim Data Analytics AB, Umea, Sweden) with log transformation and the unit variance scaling conversion mode. The significantly different metabolites (SDMs) were defined based on the variable importance for the projection (VIP) > 1.0 and P value < 0.05 [22]. The SDMs were further identified and validated by searching the online Kyoto Encyclopedia of Genes and Genomes (KEGG) and Bovine Metabolome Database (BMDB). Metaboanalyst 3.0 (http://www.metaboanalyst.ca/) was employed to identify relevant pathways. The Bos taurus (cow) pathway library was applied in this procedure.
Statistical analysis
Blood parameters and mRNA expression levels of the placentome were analyzed using SAS software (version 9.0) with covariance type AR(1) and the MIXED model procedure, with treatment included as a fixed effect and block and cow as random effects. The means were separated using the PDIFF option of the LSMEANS procedure. The experimental results were reported as least squares means. Significance was declared at P ≤ 0.05, and 0.05 < P ≤ 0.10 was considered indicative of a trend.