The thyroid gland-specific proteome
The main function of the thyroid gland is the regulation of metabolic rate. It produces the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which increase heart rate, respiration and gastrointestinal motility and stimulate carbohydrate and fat metabolism. The thyroid gland also produces calcitonin, a hormone that regulates blood calcium levels. Transcriptome analysis shows that 69% (n=13914) of all human proteins (n=20090) are expressed in the thyroid gland and 171 of these genes show an elevated expression in the thyroid gland compared to other tissue types.
The thyroid gland transcriptome
Transcriptome analysis of the thyroid 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 thyroid 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 thyroid gland compared to other tissues. As evident in Table 1, all genes elevated in thyroid gland are categorized as:
Figure 1. (A) The distribution of all genes across the five categories based on transcript specificity in thyroid 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 thyroid gland as well as in all other tissues.
As shown in Figure 1, 171 genes show some level of elevated expression in the thyroid gland compared to other tissues. The three categories of genes with elevated expression in thyroid gland compared to other organs are shown in Table 1. In Table 2, the 12 genes with the highest enrichment in thyroid gland are defined.
Table 1. The number of genes in the subdivided categories of elevated expression in thyroid gland.
Protein expression of genes elevated in thyroid gland
In-depth analysis of the elevated genes in the thyroid gland using antibody-based protein profiling allowed us to visualize the expression of the corresponding genes.
Proteins specifically involved in thyroid hormone synthesis
An essential step of thyroid hormone production is the oxidation of iodide to iodine, which is carried out by the enzyme thyroid peroxidase (TPO). In autoimmune thyroiditis antibodies target this enzyme leading to underproduction of thyroid hormone. After oxidation of iodide to iodine, iodine is bound to tyrosine amino acids within the thyroid-specific protein thyroglobulin (TG) and thyroxine (T4) is formed. The thyroxine remains bound to the thyroglobulin molecule and is stored within thyroid follicles. Thyroxine (T4) can be considered a prohormone to the more potent hormone triiodothyronine (T3). T3 is formed from T4 by the action of enzyme iodotyrosine deiodinase (IYD).
Gene expression shared between thyroid gland and other tissues
There are 30 group enriched genes expressed in thyroid 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 thyroid gland, compared to all other tissues.
To illustrate the relation of thyroid 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 thyroid gland enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of thyroid 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 5 tissues, but the resulting lists show the complete set of group enriched genes in the particular tissue.
Examples of group enriched gene expression include NKX2-1, a transcription factor, expressed in the thyroid gland and lung. NKX2-1 regulates the expression of thyroid-specific genes but also genes involved in morphogenesis. Another example is PAX8, expressed in the thyroid gland and the kidney, and that encodes a transcription factor crucial for the development of these organs. Dysregulation of PAX8 has been observed in thyroid cancers.
Thyroid gland function
The thyroid gland produces the hormones thyroxine (T4) and triiodothyronine (T3). Follicular cells of the thyroid produce the protein thyroglobulin that is secreted into the colloid of the thyroid gland. T4 and T3 are formed by iodination of thyroglobulin tyrosine residues. The thyroid gland is stimulated to release T4 and T3 by thyroid stimulating hormone (TSH), produced in the pituitary. Increased blood TSH concentration leads to increased release of T4 and T3. These thyroid hormones control the metabolic rate of the body. An elevated level of these hormones will increase heart rate, respiration, gastrointestinal motility and stimulate carbohydrate and fat metabolism. And a decreased production will in turn have the opposite effect. For infants and younger children, normal thyroid function is essential for normal growth and brain development.
The thyroid gland also produces calcitonin, a peptide hormone that decreases the plasma concentration of calcium. Calcitonin is secreted from a completely different set of cells than the T4 and T3 secreting cells, called parafollicular cells, or c-cells.
Thyroid gland histology
The thyroid gland is a bi-lobar endocrine gland, located to the lower part of the neck, immediately under the thyroid cartilage. The thyroid gland has a lobular organization. Thyroglobulin is stored as a homogeneous colloid material within densely packed thyroid follicles. The follicles are lined with a single layer of follicular cells. A vascular network surrounds each follicle. Interspersed between the thyroid follicles another group of cells are present, the calcitonin producing parafollicular cells.
Here, the protein-coding genes expressed in thyroid 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 thyroid gland.
Transcript profiling was based on a combination of two transcriptomics datasets (HPA and GTEx), corresponding to a total of 14590 samples from 54 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)