The small intestine-specific proteome
The duodenum is succeeded by jejunum and ileum, the remaining two sections of the small intestine. Here, the jejunum/ileum region is referred to as small intestine. The major function of this part of the digestive system is the absorption of nutrients derived from the digestion of food in more distal regions, water and electrolytes. The transcriptome analysis shows that 69% of all human proteins (n=20344) are expressed in the small intestine and 328 of these genes show an elevated expression in small intestine compared to other tissue types.
An analysis of the genes with elevated expression in the small intestine with regards to subcellular localization reveals that the corresponding proteins are most often located in the brush boarder, secreted mucus, cellular membrane, or cytoplasm.
- 4 small intestine enriched genes
- 328 genes defined as elevated in the small intestine
- Large number of genes show shared expression in small intestine and duodenum
Figure 1. The distribution of all genes across the five categories based on transcript abundance in small intestine as well as in all other tissues.
328 genes show some level of elevated expression in the small intestine compared to other tissues. The four categories of genes with elevated expression in small intestine compared to other organs are shown in Table 1.
Table 1. The genes with elevated expression in small intestine
Number of genes
||At least five-fold higher mRNA levels in a particular tissue as compared to all other tissues
||At least five-fold higher mRNA levels in a group of 2-7 tissues
||At least five-fold higher mRNA levels in a particular tissue as compared to average levels in all tissues
||Total number of elevated genes in small intestine
Table 2. The 4 genes with the highest level of enriched expression in small intestine. "Predicted localization" shows the classification of each gene into three main classes: Secreted, Membrane, and Intracellular, where the latter consists of genes without any predicted membrane and secreted features. "mRNA (tissue)" shows the transcript level as FPKM values, TS-score (Tissue Specificity score) corresponds to the score calculated as the fold change to the second highest tissue.
||fatty acid binding protein 6, ileal
||potassium inwardly-rectifying channel, subfamily J, member 13
||solute carrier family 17 (vesicular glutamate transporter), member 8
Some of the proteins predicted to be membrane-spanning are intracellular, e.g., in the Golgi or mitochondrial membranes, and some of the proteins predicted to be secreted can potentially be retained in a compartment belonging to the secretory pathway, such as the ER, or remain attached to the outer face of the cell membrane by a GPI anchor.
The small intestine transcriptome
An analysis of the expression levels of each gene makes it possible to calculate the relative mRNA pool for each of the categories. The analysis shows that 78% of the mRNA molecules derived from small intestine correspond to housekeeping genes and that 12% of the mRNA pool corresponds to genes categorized as small intestine enriched, group enriched or, small intestine enhanced. Thus, most of the transcriptional activity in the small intestine relates to proteins with presumed housekeeping functions as they are found in all tissues and cells analyzed.
Protein expression of genes elevated in small intestine
In-depth analysis of the elevated genes in small intestine using antibody-based protein profiling allowed us to visualize the protein expression patterns in the small intestine with respect to cellular compartments. The expression profile of small intestine is very similar to duodenum, FABP6 is one of the genes that are present in small intestine but not in duodenum and is involved in ileal fatty acid binding. ALPI, DEFA6, and FABP2 are examples of genes shared by duodenum and small intestine and with less expression in the other parts of gastrointestinal tract.
Genes shared between small intestine and other tissues
There are 185 group-enriched genes expressed in the small intestine. Group enriched genes are defined as genes showing a 5-fold higher average level of mRNA expression in a group of 2-7 tissues, including small intestine, compared to all other tissues.
In order to illustrate the relation of small intestine tissue to other tissue types, a network plot was generated, displaying the number of commonly expressed genes between different tissue types.
Figure 2. An interactive network plot of the small intestine enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of small intestine enriched genes and orange nodes represent the number of genes that are group enriched. The sizes of the red and orange nodes are related to the number of genes displayed within the node. Each node is clickable and results in a list of all enriched genes connected to the highlighted edges. The network is limited to group enriched genes in combinations of up to 4 tissues, but the resulting lists show the complete set of group enriched genes in the particular tissue.
Small intestine histology
The small intestine (jejunum and ileum) measures about 6 meters and absorbs nutrition, water and electrolytes. It is similar to the duodenum in histology and composition. The permanent transverse submucosal fold extending into the lumen of the intestine are termed plica circularis. The plica circularis consist of mucosa as well as submucosa. The core is the submucosa composed of loose connective tissue, blood vessels, nerves and dispersed lymphoid tissue. A distinctive feature of the jejunum and ileum is the lack of glands in the submucosa.
The mucosa is characterized by numerous finger-like villi that protrude into the lumen of the intestine. Enterocytes, which are columnar epithelial cells with basally located oval nuclei and an apical brush border line the villi. The enterocytes located to the villi have mainly absorptive function. Interspersed between the enterocytes are goblet cells, which are recognized by their content of a large mucous globule, resembling a small "empty bubble" within the epithelial lining. The goblet cells are connected to the basement membrane by a thin, cytoplasmic strand that is difficult to distinguish in haematoxylin and eosin staining. Underlying the intestinal villi are the intestinal glands. They are straight tubular glands that are slightly dilated at their bottom. Intestinal stem cells line the proximal part of the glands. The stem cells give rise to all the cells in the epithelium, which are the paneth cells, the enterocytes, the goblet cells and the enteroendocrine cells. The paneth cells secrete antibacterial enzymes and are located in the lower portion of the intestinal glands. Paneth cells are recognized by eosinophilic granules in the cytoplasm.
The histology of human small intestine including detailed images and information about the different cell types can be viewed in the Protein Atlas Histology Dictionary.
Here, the protein-coding genes expressed in the small intestine are described and characterized, together with examples of immunohistochemically stained tissue sections that visualize protein expression patterns of proteins that correspond to genes with elevated expression in the small intestine.
Transcript profiling and RNA-data analyses based on normal human tissues have been described previously (Fagerberg et al., 2013). Analyses of mRNA expression including over 99% of all human protein-coding genes was performed using deep RNA sequencing of 122 individual samples corresponding to 32 different human normal tissue types. RNA sequencing results of 4 fresh frozen tissues representing normal small intestine was compared to 118 other tissue samples corresponding to 31 tissue types, in order to determine genes with elevated expression in small intestine. A tissue-specific score, defined as the ratio between mRNA levels in small intestine compared to the mRNA levels in all other tissues, was used to divide the genes into different categories of expression.
These categories include: genes with elevated expression in small intestine, genes expressed in all tissues, genes with a mixed expression pattern, genes not expressed in small intestine, and genes not expressed in any tissue. Genes with elevated expression in small intestine were further sub-categorized as i) genes with enriched expression in small intestine, ii) genes with group enriched expression including small intestine and iii) genes with enhanced expression in small intestine.
Human tissue samples used for protein and mRNA expression analyses were collected and handled in accordance with Swedish laws and regulation and obtained from the Department of Pathology, Uppsala University Hospital, Uppsala, Sweden as part of the sample collection governed by the Uppsala Biobank. All human tissue samples used in the present study were anonymized in accordance with approval and advisory report from the Uppsala Ethical Review Board.
Relevant links and publications
Uhlen et al (2015). Tissue-based map of the human proteome. Science.
Analysis of the Human Tissue-specific Expression by Genome-wide Integration of Transcriptomics and Antibody-based Proteomics
Histology dictionary - the small intestine