Serum samples
The patients who were admitted to the First Affiliated Hospital of Fujian Medical University from June 2019 to January 2020 were included in the study. According to the medical history and results of 75 g oral glucose tolerance test (OGTT), these patients were divided into normal pregnant women group (n=8) and GDM pregnant women group (n=12). After written informed consents were obtained, blood samples were collected. 5 mL blood samples were taken and placed in a glass tube without any additives. The blood was clotted for 30 min at room temperature and centrifuged at 3000g for 10 min at 4 ℃, and the separated serum was stored at -80 ℃. The study was approved by the hospital ethical committee.
Protein extraction
An appropriate amount of SDT lysis buffer (4% sodium dodecyl sulfate, 0.1 M dithiothreitol, 100 mM Tris-HCl) was added to the serum, mixed well, and then bathed in boiling water for 10 min. After centrifugation at 14000 g for 15 min, the supernatant was taken and then filtered using a 0.22 µm centrifuge tube to collect the filtrate. The BCA method was used for protein quantification. The protein samples were divided and stored at -80 ℃. The protein extraction was performed on serum samples of 12 pregnant women with GDM and 8 normal pregnant women.
SDS-PAGE electrophoresis
The quality of protein samples was evaluated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). For each sample, 20 μg of protein was dissolved in SDS-PAGE loading buffer (6X) and boiled for 5 min. The protein was loaded on a 12% SDS-PAGE gel, stained with Coomassie Brilliant Blue, and then electrophoresed (constant voltage 250 V, 40 min).
FASP enzymatic hydrolysis
200 ug of protein solution was taken from each sample and mixed with dithiothreitol at a final concentration of 100 mM. The mixture was incubated in boiling water bath for 5 min and then allowed to cool to room temperature. 100 mM iodoacetamide buffer was added to the mixture, oscillated and reacted at room temperature in dark for 30 min prior to centrifugation. Subsequently, 4 g Trypsin was added to the filtrate, shaken for 1min, and centrifuged at 37 ℃ for 16-18 h. After being collected, the peptides were desalted using C18 Cartridge, lyophilized, reconstituted with 40 μL of 0.1% formic acid solution, and subjected to a quantification.
HPLC Fractionation
The tryptic peptides were fractionated by high pH reverse-phase HPLC using XBridge Peptide BEH C18 Column (Waters, 130Å, 5 µm, 4.6 mm × 100 mm). Briefly, peptides were first separated into 48 fractions with a gradient of 5% to 45% acetonitrile (pH 10.0) over 40 min, and then combined into 12 fractions and dried by vacuum centrifuging.
LC-MS/MS Analysis
The tryptic peptides were dissolved in 0.1% formic acid (solvent A) and then directly loaded onto the Acclaim PepMap RSLC C18 column (Scientific Thermo Fisher, 50um × 15cm). The gradient was comprised of holding at 1% for the first 5 min, an increase from 1% to 28% solvent B (0.1% formic acid in 80% acetonitrile) over 90 min, 28% to 38% for 15 min, and climbing to 100% in 5 min, at a constant flow rate of 300 nL/min on an Easy nLC system (Scientific Thermo Fisher).
The peptides were analyzed by the electrospray ionization tandem mass spectrometry (ESI-MS/MS) in Q-Exactive HF-X (Scientific Thermo Fisher) coupled online to the Easy nLC. The electrospray voltage of 2.0 kV was applied. The m/z scan range of primary MS and scanning resolution were set as 350 to 1500 and 60,000, respectively. Peptides were then selected for MS analysis using NCE setting of 28, and the fragments were detected at a resolution of 15,000. Automatic gain control (AGC) was set at 2E5, while the maximum injection time (IT) and dynamic exclusion duration were set to 45 ms and 30.0 s, respectively.
The original mass spectrometry data were merged and analyzed by Spectronaut Pulsar X (version 12, Biognosys AG) to establish a spectral database, Uniprot_HomoSapiens_20386_20180905 (http://www.uniprot.org). For the database search, trypsin digestion was set, with two missed cutting sites allowed. Carbamidomethylation on cysteine (C) was defined as a fixed modification of library search parameters, while variable modifications included oxidation on methionine (M) and acetylation on the protein N-terminus. Both precursor and peptide FDR were controlled at 1%. For identification of differentially expressed proteins, the fold changes and p-value (Student's t test) should be >1.2 or <0.83 and <0.05, respectively.
PRM assays
The parallel reaction monitoring (PRM) method was used to verify the differentially expressed proteins. Peptide separation was performed on the Acclaim PepMap RSLC C18 analytical column using a multistep gradient of B: 2% to 8% for 1 min, 8% to 28% for 10 min, 28% to 40% for 10 min and 40% to 90% for 1 min. Then B was held at 90% for 3 min. A flow rate of 300 nL/min was used and the analysis duration was 60 min. ESI source settings were the same as above. The PRM analysis included a full MS1 event followed by up to 10 targeted MS2 events. The m/z scan range of MS1 and scanning resolution were set as 350 to 1500 and 60,000, respectively. AGC target value was set to 3e6 and maximum IT was 45 ms. For MS2, scan settings were 15,000 resolving power, AGC target 2e5, maximum IT 45 ms, normalized collision energy 27 eV and isolation window was set to 2.0 m/z.
Bioinformatics analysis
NCBI BLAST+ (ncbi-blast-2.3.0+) was utilized to compare the gene ontology (GO) annotations of differentially expressed proteins with the appropriate protein sequence database, and the top 10 aligned sequences that meet the appropriate E-value<=1e-3 were retained for subsequent analysis. Functional annotation was carried out using the online Kyoto Encyclopedia of Genes and Genomes (KEGG) database (http://geneontology.org/). Both GO and KEGG pathway enrichment analyses were performed based on Fisher's exact test. In clustering analysis, quantitative data of the target protein set were normalized first. Then, the software matplotlib was applied to classify samples and protein expression in two dimensions at the same time (Euclidean distance, average linkage clustering). The hierarchical cluster membership was defined by plotting a heat map.