Transcriptomic Profiling of the study population
Human colon biopsies (n = 20) including 12 samples from constipation and 8 samples from non-constipation (Table 1). All selected patients had similar gender and ages. Besides, there were no significant differences in disease duration or concomitant ileus between two groups.
Table 1
Characteristics of Human Colon Specimens
Characters | constipation group (n = 12) | non-constipation group (n = 8) | P value |
Gender (M/F) | 3/9 | 3/5 | 0.550 |
Age (years) | 45.14 ± 12.21 | 52.67 ± 17.62 | 0.451 |
Disease duration (years) | 15.29 ± 12.87 | 0.83 ± 0.29 | 0.097 |
Concomitant ileus | 58.3% | 37.5% | 0.361 |
Identification Of Biopsy Proteins
In this study, a total of 5,208 proteins were identified, of which 4,522 had quantitative information. All the differentially expressed proteins which were displayed fold change greater than 1.3 were considered dysregulated. As a result, 42 proteins were up-regulated and 23 proteins were down-regulated in constipation samples.
The box-plot was plotted by the deviation standard coefficient calculated by repeated experimental values of each group of samples, indicating the dispersion and fluctuation degree of repeated data. The quantitative results of biological or technical duplicates were statistically consistent.
Functional Classification And Enrichment Analysis Of Differentially Expressed Proteins
In order to thoroughly understand the identified and quantified proteins in the data, functions and characteristics of those proteins were annotated in detail from the aspects of subcellular localization, GO, protein domain and KEGG pathway. For the annotation of all identified proteins and the screening of differentially expressed proteins, enrichment analysis of GO, KEGG, domain and other functional annotation types were conducted to detect whether the differentially expressed proteins have a significant enrichment trend in some functional types.
Wolfpsort software was used to predict and classify the subcellular structures of differentially expressed proteins (Fig. 1). 33% of up-regulated proteins were annotated as belonging to cytoplasm, while more than 30% of down-regulated proteins were extracellular and nucleus.
The distribution of the quantified proteins in the GO secondary annotation was statistically analyzed. Biological process, cellular component and molecular function explained the biological role of proteins from different perspectives. Using GO, glucose 1-phosphate metabolic process and response to toxic substance in biological process, lyase activity and carbonate dehydratase activity in molecular function, organellar and mitochondrial small ribosomal subunit in cellular component were significantly enriched in up-regulated proteins (Fig. 2A), while negative regulation and regulation of cell-substrate adhesion in biological process, extracellular matrix structural constituent and platelet-derived growth factor binding in molecular function, proteinaceous extracellular matrix and extracellular matrix in cellular component were significantly enriched in down-regulated proteins (Fig. 2B).
KEGG pathways include metabolism, genetic information processing, environmental information processing, cell processes, human diseases, drug development, etc. KEGG generated networks that demonstrated significant hubs of dysregulation in constipation patients in the work (Fig. 3). The regulation of KEGG pathway obtained by enrichment analysis. The up-regulated proteins mainly participated in drug metabolism-cytochrome P450, nitrogen metabolism, galactose metabolism, chemical carcinogenesis, metabolism of xenobiotics by cytochrome P450, amino sugar and nucleotide sugar metabolism, glutathione metabolism, platinum drug resistance and drug metabolism-other enzymes, while the down-regulated proteins took part in protein digestion and absorption, AGE-RAGE signaling pathway in diabetic complications, amoebiasis, relaxin signaling pathway and platelet activation. Of these categories, drug metabolism-cytochrome P450, nitrogen metabolism and protein digestion and absorption were the highest scoring KEGG network (Fig. 4). Constipation might have unique molecular mechanisms and molecular characteristics.
Protein domain refers to some components that occur repeatedly in different protein molecules and have similar sequences, structures and functions. It is the unit of protein evolution. The domain length is usually between 25 amino acids and 500 amino acids. Alpha carbonic anhydrase and S-adenosyl-L-methionine-dependent methyltransferase were in up-regulated proteins, and "VWFC domain, fibrillar collagen, C-terminal, "Zinc finger, LIM-type, Butyrophylin-like, SPRY domain, B30.2/SPRY domain, Immunoglobulin subtype, Immunoglobulin-like fold, leucine-rich repeat domain, L domain-like, Immunoglobulin-like domain, C2 domain and concanavalin A-like lectin/glucanase domain were in down-regulated proteins. Alpha carbonic anhydrase, "VWFC domain and fibrillar collagen, C-terminal were statistically significant. Unique protein domains were appeared in constipation.
Clustering Analysis Of Dysregulated Proteins
Differentially expressed proteins were divided into four parts according to their differentially expressed multiples, called Q1 to Q4: Q1 (0 < Ratio ≤ 1/1.3), Q2 (1/1.3 < Ratio ≤ 1/1.2), Q3 (1.2 < Ratio ≤ 1.3) and Q4 (Ratio > 1.3). GO classification, KEGG and protein domain enrichment were conducted for each Q group, and cluster analysis was conducted to find the correlation of protein function with different differentially expressed multiples. In results, Q1 had 23 proteins, Q2 had 14 proteins, Q3 had 22 proteins and Q4 had 42 proteins.
According to Fisher's exact test, p value which was obtained from enrichment analysis used clustering method to gather the relevant functions in different groups together and draw a heatmap. The heatmap represents the enrichment test results of different groups horizontally, and the longitudinal description of GO, KEGG pathway, protein domain. The color blocks corresponding to the differentially expressed proteins and functional descriptions in different groups indicated the degree of enrichment. Red indicated strong enrichment and blue represented weak enrichment. For example, response to transforming growth factor beta in Q1 and phosphorylation in Q4 were strong enrichment in biological process of GO terms, and AGE-RAGE signaling pathway in diabetic complications in Q1 and metabolism of xenobiotics by cytochrome P450 in Q4 were strong enrichment in KEGG pathway (Fig. 5,6).