The adrenal gland-specific proteome

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Figure 1. The distribution of all genes across the five categories based on transcript abundance in adrenal gland as well as in all other tissues.

252 genes show some level of elevated expression in the adrenal gland compared to other tissues. The three categories of genes with elevated expression in the adrenal gland compared to other organs are shown in Table 1. In Table 2, the 12 genes with the highest level of expression among 33 enriched genes are defined.

Table 2. The 12 genes with the highest level of enriched expression in adrenal gland. "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 TPM values, TS-score (Tissue Specificity score) corresponds to the score calculated as the fold change to the second highest tissue.

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 surface of the cell membrane by a GPI anchor.

The adrenal gland 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 87% of the mRNA molecules in the adrenal gland correspond to housekeeping genes and only 5% of the mRNA pool corresponds to genes categorized to be either adrenal gland enriched, group enriched, or enhanced. Thus, most of the transcriptional activity in the adrenal gland relates to proteins with presumed housekeeping functions as they are found in all tissues and cells analyzed.

Protein expression of genes elevated in adrenal gland

In-depth analysis of the elevated genes in the adrenal gland using antibody-based protein profiling allowed us to visualize where these proteins are expressed with regards to localization in the adrenal cortex and adrenal medulla.

Proteins specifically expressed in the adrenal cortex

The hormones released from the adrenal cortex are vital to life. Glucocorticoids regulate the body's metabolic processes and response to stress. Mineralocorticoids are involved in control of electrolyte balance and blood pressure. Small amounts of sex hormones are also released from the adrenal gland but the greater part of the body's sex hormones are released from other sex hormone producing organs (testis and ovaries).

Several members of the cytochrome P450 family are specific to the adrenal gland. These are well-known proteins that are involved in steroid hormone biosynthesis. CYP11A1 converts cholesterol to pregnenolone, this protein is not exclusively expressed in the adrenal gland, but also in other steroid hormone producing organs. Nor are HSD3B2 or STAR exclusively expressed in the adrenal gland since these proteins are essential for all steroid hormone synthesis. However, transcripts are present at a much higher level in the adrenal gland than in any other tissue, signifying the adrenal cortex's very specialized role in steroid hormone synthesis. FDX1 is also enriched in the adrenal gland; the gene encodes an iron-sulfur protein involved in electron transfer.

Figure 2. Full section of an adrenal gland with immunohistochemical staining of the cortex using an antibody towards CYP11A1.

Proteins specifically expressed in the adrenal medulla

The adrenal medulla is stimulated by preganglionic sympathetic neurons to release adrenalin and noradrenalin. It has a rapid response to external and internal stress. The release of adrenalin and noradrenalin leads to increased heart rate increased blood pressure and increased blood flow to the muscles.

Proteins specific to the adrenal medulla are predominantly related to noradrenalin and adrenalin synthesis. Transcripts of DBH and PNMT are detected at high mRNA levels in the analysis.

Figure 3. Full section of an adrenal gland with immunohistochemical staining of the medulla using an antibody towards DBH.

Genes shared between adrenal gland and other tissues

There are 75 group enriched genes expressed in the adrenal gland. 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 adrenal gland, compared to all other tissues.

In order to illustrate the relation of adrenal gland tissue to other tissue types, a network plot was generated, displaying the number of genes shared between different tissue types. Most group enriched genes were shared with the brain.

Figure 4. An interactive network plot of the adrenal gland enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of adrenal gland 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 3 tissues, but the resulting lists show the complete set of group enriched genes in the particular tissue.

Adrenal gland function

The paired endocrine adrenal glands are located on top of the kidneys. Each gland is divided into two distinct parts, a centrally located adrenal medulla and an outer adrenal cortex. These two different parts are derived from different embryological layers, the medulla from the ectoderm and the cortex from the mesoderm.

The adrenal cortex secretes three groups of corticosteroid hormones: mineralocorticoids, glucocorticoids, and sex hormones. The principal mineralocorticoid is aldosterone, which is involved in controlling normal electrolyte balance and blood pressure. The principal glucocorticoid is cortisol, a hormone that is essential for normal metabolic function. The adrenal sex hormones have primarily androgenic effect; among them are dehydroepiandrosterone (DHEA) and androstenedione.

The adrenal medulla secretes the hormones noradrenalin and adrenalin in response to sympathetic nerve stimulation. When noradrenalin and adrenalin are released to the bloodstream they cause almost the same effect in the body as direct sympathetic stimulation.

Adrenal gland histology

The adrenal glands are small endocrine glands with triangular to semi lunar shape. They are located right on top of the kidneys and are enveloped by a fibrous capsule surrounded by adipose tissue. Each gland has two parts, the adrenal cortex and the adrenal medulla. The adrenal gland has rich blood supply and nervous innervation, for rapid release of hormones into the bloodstream.

The outermost adrenal cortex secretes corticosteroids and sex hormones and consists of three layers, namely zona glomerulosa, zona fasciculata and zona reticularis. Microscopically the three layers can be easily distinguished from each other looking at the cell's shapes and orientation. The outermost zona glomerulosa produces mineralocorticoids, the middle zona fasciculata produces glucocorticoids, and the inner zona reticularis produce sex hormones and some glucocorticoids.

The cells of the adrenal medulla are innervated by presynaptic sympathetic neurons and release the catecholamines noradrenalin and adrenalin in direct response to nerve impulses. These chromaffin cells of the medulla are lighter stained than the cells of the adrenal cortex and are arranged in close proximity to capillaries.

The histology of human adrenal gland including detailed images and information can be viewed in the Protein Atlas Histology Dictionary.

Background

Here, the protein-coding genes expressed in the adrenal gland 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 adrenal gland.

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 172 individual samples corresponding to 37 different human normal tissue types. RNA sequencing results of 3 fresh frozen tissues representing normal adrenal gland was compared to 169 other tissue samples corresponding to 36 tissue types, in order to determine genes with elevated expression in adrenal gland. A tissue-specific score, defined as the ratio between mRNA levels in adrenal gland 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 adrenal gland, genes expressed in all tissues, genes with a mixed expression pattern, genes not expressed in adrenal gland, and genes not expressed in any tissue. Genes with elevated expression in adrenal gland were further sub-categorized as i) genes with enriched expression in adrenal gland, ii) genes with group enriched expression including adrenal gland and iii) genes with enhanced expression in adrenal gland.

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

Uhlén M et al, 2015. Tissue-based map of the human proteome. Science
PubMed: 25613900 DOI: 10.1126/science.1260419

Yu NY et al, 2015. Complementing tissue characterization by integrating transcriptome profiling from the Human Protein Atlas and from the FANTOM5 consortium. Nucleic Acids Res.
PubMed: 26117540 DOI: 10.1093/nar/gkv608

Fagerberg L et al, 2014. Analysis of the human tissue-specific expression by genome-wide integration of transcriptomics and antibody-based proteomics. Mol Cell Proteomics.
PubMed: 24309898 DOI: 10.1074/mcp.M113.035600

Bergman J et al, 2016. The human adrenal gland proteome defined by transcriptomics and antibody-based profiling. Endocrinology.
PubMed: 27901589 DOI: 10.1210/en.2016-1758

Histology dictionary - the adrenal gland

Symptoms and causes of endocrine disorders and how they can be treated.