Animals and semen collection
The animal study protocol was approved by the Animal Ethics Committee of the Anhui Agricultural University (approval no. AHAU20210826). The experimental animals were selected from the Wannan Black Pig National Breeder Farm of Guangde Sanxi Ecological Farming Co., Ltd.. Twenty Wannan black pigs, each aged 1 (n=10) and 7 (n=10) years, with similar body weights and no physiological diseases were chosen.
Fresh ejaculations were continuously collected through an artificial vagina stimulated by a sow. Discard the anterior and caudal parts of the ejaculated semen and take the middle part into the semen collection cup. Semen was collected for each boar twice a week. Sperm sample which is free from any foreign matter or odor, exhibiting a milk-white appearance and with a survival rate exceeding 90% is selected for subsequent testing. The sperm were purified by centrifugation after mixing with semen in a percoll solution. Animal experiments in this study were performed in full accordance with the ARRIVE guideline reporting guidelines. All methods were carried out in accordance with relevant guidelines and regulations.
Sample preparation
Samples were suspended on ice in 200 μL lysis buffer (4% SDS, 100 mM DTT, 150 mM Tris-HCl pH 8.0). The cells and tissues were disrupted by agitation using a homogenizer and boiled for 5 min. The samples were then ultrasonicated and boiled for 5 min. Undissolved cellular debris were removed using centrifugation at 16000 rpm for 15 min. The supernatant was collected and quantified using a BCA protein assay kit (Bio-Rad, USA).
Protein digestion
Digestion of protein (200 μg for each sample) was performed according to the FASP procedure described by Wisniewski, Zougman et al. Briefly, the detergent, DTT and other low-molecular-weight components were removed using 200 μL UA buffer (8 M Urea, 150 mM Tris-HCl pH 8.0) via repeated ultrafiltration (Microcon units, 30 kD) facilitated using centrifugation. Then, 100 μL 0.05 M iodoacetamide in UA buffer was added to block reduced cysteine residues and the samples were incubated for 20 min in the dark. The filter was washed with 100 μL UA buffer three times and then 100 μL 25 mM NH4HCO3 twice. Finally, the protein suspension was digested with 4 μg trypsin (Promega) in 40 μL 25 mM NH4HCO3 overnight at 37 °C, and the resulting peptides were collected as a filtrate. The peptide concentration was determined at OD280 using a Nanodrop device.
TMT Labeling of peptides
The peptides were labeled with TMT reagent according to the manufacturer’s instructions (Thermo Fisher Scientific). Each aliquot (100 μg of peptide equivalent) was reacted with one tube of TMT reagent, respectively. After the sample was dissolved in 100 μL of 0.05M TEAB solution, pH 8.5, the TMT reagent was dissolved in 41 μL of anhydrous acetonitrile. The mixture was then incubated at room temperature for 1 h. Then, 8 μL of 5% hydroxylamine was added to the sample and incubated for 15 min to quench the reaction. The multiplex-labeled samples were pooled and lyophilized.
High pH Reverse Phase Fractionation (HPRP)
TMT-labeled peptides mixture was fractionated using a Waters XBridge BEH130 column (C18, 3.5 μm, 2.1 × 150 mm) on a Agilent 1290 HPLC operating at 0.3 mL/min. Buffer A consisted of 10 mM ammonium formate and buffer B consisted of 10 mM ammonium formate with 90% acetonitrile; both buffers were adjusted to pH 10 using ammonium hydroxide. A total of 30 fractions were collected from each peptide mixture and concatenated to 15 (pooling equal-interval RPLC fractions). The fractions were dried for nano-LC-MS/MS analysis.
LC-MS Analysis (TMT10plex)
The LC-MS analysis was performed using a Q Exactive mass spectrometer coupled to an Easy nLC (Thermo Fisher Scientific). Peptide from each fraction was loaded onto a the C18-reversed phase column (12 cm long, 75 μm ID, 3μm) in buffer A (2% acetonitrile and 0.1% Formic acid) and separated with a linear gradient of buffer B (90% acetonitrile and 0.1% Formic acid) at a flow rate of 300 nL/min over 90 min. The linear gradient was set as follows: 0–2 min, linear gradient from 2% to 5% buffer B; 2–62 min, linear gradient from 5% to 20% buffer B; 62–80 min, linear gradient from 20% to 35% buffer B; 80–83 min, linear gradient from 35% to 90% buffer B; and 83–90 min, buffer B maintained at 90%. MS data were acquired using a data-dependent top15 method dynamically choosing the most abundant precursor ions from the survey scan (300–1800 m/z) for HCD fragmentation. The target value was determined based on predictive Automatic Gain Control (pAGC). The AGC target values of 1e6 and a maximum injection time of 50 ms were used for full MS, and the target AGC value of 1e5 and a maximum injection time of 100 ms were used for MS2. The dynamic exclusion duration was 30 s. Survey scans were acquired at a resolution of 70,000 at m/z 200, and the resolution of the HCD spectra was set to 35,000 at m/z 200. The normalized collision energy was 30. The instrument was run in the peptide recognition mode.
Mass spectrometry proteomics data were deposited in the ProteomeXchange Consortium (https://proteomecentral.proteomexchange.org) via the iProX partner repository [53,54] with the dataset identifier PXD050879.
Database Searching and Analysis
The resulting LC-MS/MS raw files were imported into the Proteome Discoverer 2.4 software (version 1.6.0.16) for data interpretation and protein identification against the Uniprot-Sus scrofa (Pig) [9823]-122175-220104.fasta) database. The initial search was performed using a precursor mass window of 10 ppm. The search followed the enzymatic cleavage rule of trypsin/phosphate and allowed two maximal missed cleavage sites and a mass tolerance of 20 ppm for fragment ions. The modification set was as follows: fixed modification, carbamidomethyl (C), TMT10plex(K), and TMT10plex(N-term); and variable modification, oxidation (M) and acetyl (Protein N-term). The minimum 6 amino acids for peptide, ≥1 unique peptides were required per protein. For peptide and protein identification, the false discovery rate (FDR) was set to 1%. The TMT reporter ion intensity was used for quantification.
Bioinformatic analysis
Analyses of the bioinformatics data were performed using Perseus software, Microsoft Excel, and R statistical computing software. Differentially expressed proteins were screened with a cutoff of a fold-change ratio of >1.20 or <0.83 and nominal p-value of <0.05. Expression data were grouped together via hierarchical clustering according to the protein level. To annotate the sequences, information was extracted from the UniProtKB/Swiss-Prot, Kyoto Encyclopedia of Genes and Genomes (KEGG), and Gene Ontology (GO). GO and KEGG enrichment analyses were performed using Fisher’s exact test, and FDR correction for multiple testing was performed. GO terms were grouped into three categories: biological processes (BP), molecular functions (MF), and cellular components (CC). The enriched GO and KEGG pathways were nominally statistically significant at nominal p<0.05. Protein–protein interaction (PPI) networks were constructed using the STRING database and Cytoscape software.
Parallel reaction monitoring (PRM) analysis
To verify the protein expression levels obtained by TMT analysis, the expression levels of selected proteins were quantified using LC-PRM/MS [1]. Briefly, peptides were prepared according to the TMT protocol. Tryptic peptides were loaded onto C18 stage tips for desalting before reverse-phase chromatography using an Easy nLC-1200 system (Thermo Scientific). One-hour liquid chromatography gradients with acetonitrile ranging from 5 to 35% over 45 min were used. The PRM analysis was performed using a Q Exactive Plus mass spectrometer (Thermo Scientific). Methods optimized for the collision energy, charge state, and retention times of the most significantly regulated peptides were generated experimentally using unique peptides with high intensity and confidence for each target protein. The mass spectrometer was operated in positive ion mode with the following parameters: the full MS1 scan was acquired with a resolution of 70000 (at 200 m/z), AGC target values of 3.0×106, and a 250 ms maximum ion injection time. Full MS scans were followed by 20 PRM scans at 35000 resolution (at m/z 200) with AGC 3.0×106 and a maximum injection time of 200 ms. The targeted peptides were isolated using a 2Th window and fragmented at a normalized collision energy of 27 in a higher-energy dissociation (HCD) collision cell. Raw data were analyzed using Skyline (MacCoss Lab, University of Washington) [2] to get the signal intensities of the individual peptide sequences.
For the PRM MS data, the average base peak intensity of each sample was extracted from the full scan acquisition using RawMeat (version 2.1, VAST Scientific, www.vastscientific.com). The normalization factor for sample N was calculated as fN = the average base peak intensity of sample N/median of the average base peak intensities of all samples. The area under curve (AUC) for each transition from sample N was multiplied by this factor. After normalization, the AUC of each transition were summed to obtain the AUCs at the peptide level. The relative protein abundance was defined as the intensity of a specific peptide.