The adrenal gland-specific proteome
The main function of the adrenal gland is to supply the body with two different sets of hormones, steroid hormones from the adrenal cortex and catecholamines from the adrenal medulla. The cortical steroid hormones are involved in metabolic function, electrolyte balance and have androgenic effects. Catecholamines are released in response to stress and their effect is mainly described as the flight-and-fight response. Transcriptome analysis shows that 68% (n=13755) of all human proteins (n=20162) are expressed in the adrenal gland and 226 of these genes show an elevated expression in the adrenal gland compared to other tissue types.
The adrenal gland transcriptome
Transcriptome analysis of the adrenal gland can be visualized with regard to the specificity and distribution of transcribed mRNA molecules (Figure 1). Specificity illustrates the number of genes with elevated or non-elevated expression in the adrenal gland compared to other tissues. Elevated expression includes three subcategory types of elevated expression:
Distribution, on the other hand, visualizes how many genes have, or do not have, detectable levels (nTPM≥1) of transcribed mRNA molecules in the adrenal gland compared to other tissues. As evident in Table 1, all genes elevated in adrenal gland are categorized as:
Figure 1. (A) The distribution of all genes across the five categories based on transcript specificity in adrenal gland as well as in all other tissues. (B) The distribution of all genes across the six categories, based on transcript detection (nTPM≥1) in adrenal gland as well as in all other tissues.
Table 1. The number of genes in the subdivided categories of elevated expression in adrenal gland.
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 stress response. Mineralocorticoids are involved in the control of electrolyte balance and blood pressure. Small amounts of sex hormones are also released from the adrenal gland but mainly produced in other organs (testis and ovaries).
One example is CYP11A1 which converts cholesterol to pregnenolone. CYP11A1 is also expressed in other steroid hormone-producing organs. HSD3B2 and STAR are also essential for all steroid hormone synthesis and therefore expressed in other steroid hormone-producing organs as well. However, these genes are expressed at much higher levels in the adrenal gland than in any other tissue, signifying in the adrenal cortex which is highly specialized in steroid hormone synthesis. Another example of a protein expressed in the adrenal gland is FDX1 which 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 and 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 noradrenaline and adrenaline synthesis. Examples of these proteins are DBH and PNMT, both expressed in the medulla of the adrenal gland.
Figure 3. A full section of an adrenal gland with immunohistochemical staining of the medulla using an antibody towards DBH.
Gene expression shared between adrenal gland and other tissues
There are 49 group enriched genes expressed in adrenal gland. Group enriched genes are defined as genes showing a 4-fold higher average level of mRNA expression in a group of 2-5 tissues, including adrenal gland, compared to all other tissues.
To illustrate the relation of adrenal gland tissue to other tissue types, a network plot was generated, displaying the number of genes with a shared expression between different tissue types.
Figure 2. 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.
The adrenal gland mainly shares group enriched gene expression with brain (n=9). Two examples of such genes are proenkephalin (PENK) and secretogranin II (SCG2). PENK is a hormone that plays a role in a number of functions including pain perception and responses to stress. SCG2 is involved in the biogenesis of secretory vesicle by the suggested mechanism of packaging or sorting peptide hormones and neuropeptides into secretory vesicles.
Proteins analyzed in extended samples of adrenal gland
The standard setup in the Tissue Atlas is based on Tissue Microarray technique (TMA), thus saving valuable tissue material as well as reagents and providing a wide tissue representation for protein profiling. However, due to the complex nature of the adrenal gland, with different layers, a larger tissue sample is required to fully understand protein localization. The full list of genes used for protein profiling on extended samples of the adrenal gland is defined in Table 3.
Table 3. Following 59 genes have been analyzed in extended samples of adrenal gland.
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 an androgenic effect, two examples are dehydroepiandrosterone (DHEA) and androstenedione.
Adrenal gland histology
The adrenal glands are small endocrine glands with triangular to semilunar shapes. 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 a rich blood supply and nervous innervation, for the 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.
Here, the protein-coding genes expressed in adrenal gland are described and characterized, together with examples of immunohistochemically stained tissue sections that visualize corresponding protein expression patterns of genes with elevated expression in adrenal gland.
Transcript profiling was based on a combination of two transcriptomics datasets (HPA and GTEx), corresponding to a total of 14590 samples from 50 different human normal tissue types. The final consensus normalized expression (nTPM) value for each tissue type was used for the classification of all genes according to the tissue-specific expression into two different categories, based on specificity or distribution.
Relevant links and publications
Uhlén M et al., Tissue-based map of the human proteome. Science (2015)