3.1 Establishment of a Model of Inflammation and Coagulation Dysfunction Caused by Sepsis in Mouse
The mice were treated as described. After 24 hours, we used ELISA to detect the levels of inflammatory factors such as Il-1β, Il-6 and Tnf-α in the plasma, and found that inflammatory factor levels were higher in CLP group than in the sham group (Fig. 1G). The lung is usually the first organ to become damaged, contributing to pulmonary dysfunction in septic shock(29, 30). To evaluate CLP-induced lung injury, Hematoxylin and Eosin (HE) staining was used for histological analysis of the lung tissue. CLP significantly destroys the normal structure of lung tissue and causes severe pulmonary interstitial edema (Fig. 1A). Septic acute kidney injury (AKI) is also the most common AKI syndrome in clinical and animal models(31).Fig.1B shows mesenchymal congestion and edema in kidney. We also found that the levels of iNos, Ccl2, Il-1β, Il-6 Tnf-α and Dpp4 were upregulated in the CLP group at both in the mRNA (Fig. 1C and 1E) and protein (Fig. 1D and 1F) level. The results described above indicated an inflammatory response in septic mice.
In terms of coagulation dysfunction, we tested the levels of coagulation-related substances in the serum of mice. The levels of Tf, Pai-1, Tat and D-dimer in the CLP group were higher than those in the sham group (Fig. 1H). Simultaneously, qRT-PCR analysis of Tf, Tfpi, t-Pa and Pai-1 found that these substances were elevated in the kidney tissue of the CLP group (Fig 1I), so was the protein level of Tf (Fig. 1H). These results suggested that the mice in the CLP model had coagulation dysfunction.
3.2 Inflammatory and Coagulatory Responses after LPS Stimulation for 6 hours in Macrophages
Macrophages are closely associated with inflammatory responses, among which M1 macrophages are mainly involved in proinflammatory responses(32). We detected the level of inflammation in RAW264.7 cells after LPS stimulation for 6 hours, and found that the levels of iNos and Ccl2 increased in the LPS group (Fig. 2A and 2B). At the same time, the levels of inflammatory mediators such as Il-1β, Il-6 Tnf-α and Dpp4 were also elevated (Fig. 2C and 2D). In the cell supernatant culture medium, we found upregulation of Il-1β, Il-6 and Tnf-α levels (Fig 2E). The levels of the coagulation-related substances Tf and Pai-1 increased in the cells and supernatant, respectively (Fig. 2F,2G and 2H).
3.3 Differentially expresses gene (DEGs) in macrophages between the PBS group and LPS group
Quality control of filtered reads was performed with respect to reference genome comparison (Table S1), reference gene analysis (Table S2), random distribution of reads on the transcript (Fig.S1) and sequencing saturation curve (Fig. S2). The data quality control results indicated that our sequencing data were suitable for further analysis. First, the limma package was used to screen 11,921 potential DEGs. Then, different thresholds were used to further screen the different genes. For example, we identified 2715 differentially expressed genes between the PBS group and LPS group, with 716 upregulated and 1999 downregulated by LPS according to the threshold applied. The results using other thresholds are shown in Table 4. This suggested that LPS can cause great changes in macrophage gene levels.
3.3 GO Enrichment Analysis of DEGs
To evaluate the function of the DEGs in the sepsis model, we first annotated the genes at level 2(Table 4) to identify GO analysis. The GO classification analysis showed that there was a strong correlation among genes involved in DNA repair (Fig. 3A). Then, org.Mm.eg.db in R was used to perform enrichment analysis of the cell component(CC), biological process(BP), and molecular function(MF) categories.
According to GO enrichment analysis, DEGs between the PBS group and LPS group were significantly enriched in DNA replication, DNA repair, cell cycle phase transition, mitotic cell cycle phase transition, DNA-dependent DNA replication, negative regulation of cell cycle, chromosomal region, regulation of mitotic cell cycle phase transition, regulation of cell cycle phase transition and chromosome segregation (Table 5).
3.4 KEGG Pathway Analysis of DEGs
The KEGG classification analysis of DEGs is shown in Fig. 3B. The cell cycle pathway was significantly enriched relative to other pathways. The top 10 most highly enriched pathways were as follows: cell cycle, DNA replication, apoptosis, TNF signaling pathway, Epstein-Barr virus infection, hepatitis B, mismatch repair, NF-kappa B signaling pathway, osteoclast differentiation and human T-cell leukemia virus 1 infection (Table 6).
3.5 PPI Network of the DEGs and Seed Genes.
To explore whether there are a close and significant relationships among the proteins encoded by the DEGs, a PPI network was constructed from 465 (Level 3, Table 4) candidate DEGs obtained by screening that were significantly enriched. The PPI included 388 nodes and 4498 edges (Fig. 4). PPI enrichment means that there are more interactions between these proteins than would be expected from similarly sized random proteins selected from the mouse genome. Subsequently, to further discover the cluster structure of these proteins, MCODE was used to screen protein clusters and seed genes. Finally, 9 protein clusters with more than 4 genes and the seed genes from each cluster (Bub1b, C5ar1, Ptprj, Zw10, Pfas, Ikbke, Dok2, Ezh2 and Pygb) were defined (Fig. 4, Table S2).
To detect the expression of seed genes, firstly, we counted the expression of seed genes in the sequencing data (Fig. 5A) and calculated the logFC value. C5ar1, Ptprj, Ikbke and Dok2 were overexpressed in LPS group, well Bub1b, Zw10, Pfas, Ezh2 and Pygb had low expression (Fig. 5B). Then, the qRT-PCR results verified the expression of seed genes, and the trend was consistent with the sequencing results (Fig. 5C).
3.6 KEGG mapping analysis of the seed genes
KEGG Mapper was used to reanalyze the seed genes and found that these 9 genes were involved in a total of 29 pathways (Table 7). Pygb, Ezh2 and Pfas were involved in metabolic pathways; C5ar1 and Ikbke were involved in the coronavirus disease-COVID-19 pathway (Fig. 6); and Ikbke was also involved in the Toll-like receptor signaling pathway which was related to inflammation (Fig.7). The gene participating in the most pathways was Ikbke. Other proteins are listed in Table S4.