The parathyroid gland-specific proteome

The parathyroid gland transcriptome

Transcriptome analysis of the parathyroid 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 parathyroid gland compared to other tissues. Elevated expression includes three subcategory types of elevated expression:

• Tissue enriched: At least four-fold higher mRNA level in parathyroid gland compared to any other tissues.
• Group enriched: At least four-fold higher average mRNA level in a group of 2-5 tissues compared to any other tissue.
• Tissue enhanced: At least four-fold higher mRNA level in parathyroid gland compared to the average level in all other tissues.

Distribution, on the other hand, visualizes how many genes have, or do not have, detectable levels (nTPM≥1) of transcribed mRNA molecules in the parathyroid gland compared to other tissues. As evident in Table 1, all genes elevated in parathyroid gland are categorized as:

• Detected in single: Detected in a single tissue
• Detected in some: Detected in more than one but less than one-third of tissues
• Detected in many: Detected in at least a third but not all tissues
• Detected in all: Detected in all tissues

Figure 1. (A) The distribution of all genes across the five categories based on transcript specificity in parathyroid 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 parathyroid gland as well as in all other tissues.

As shown in Figure 1, 201 genes show some level of elevated expression in the parathyroid gland compared to other tissues. The three categories of genes with elevated expression in parathyroid gland compared to other organs are shown in Table 1. In Table 2, the 12 genes with the highest enrichment in parathyroid gland are defined.

Table 1. The number of genes in the subdivided categories of elevated expression in parathyroid gland.

Table 2. The 12 genes with the highest level of enriched expression in parathyroid gland. "Tissue distribution" describes the transcript detection (nTPM≥1) in parathyroid gland as well as in all other tissues. "mRNA (tissue)" shows the transcript level in parathyroid gland as nTPM values. "Tissue specificity score (TS)" corresponds to the fold-change between the expression level in parathyroid gland and the tissue with the second-highest expression level.

Protein expression of genes elevated in parathyroid gland

In-depth analysis of the elevated genes in the parathyroid gland using antibody-based protein profiling allowed us to visualize the expression patterns of these proteins in different functional compartments including proteins involved in extracellular calcium homeostasis and proteins that bind intracellular calcium.

Proteins involved in extracellular calcium homeostasis in parathyroid gland

The parathyroid gland regulates calcium homeostasis by producing and releasing parathyroid hormone (PTH). Extracellular calcium level is continuously monitored by CASR, a receptor located at the cell membrane of chief cells. In response to decreased calcium level, PTH synthesis is triggered in chief cells whereupon it is released into the blood. It restores calcium levels in the blood by binding parathyroid hormone receptors in bone and kidney which results in increased bone resorption and calcium reabsorption. Calcium reabsorption in the kidney is also regulated by CASR, located at the cell membrane of epithelial cells in the thick ascending limb of the loop of Henle.

CASR
PTH

Proteins that bind intracellular calcium in parathyroid gland

Several genes with an elevated expression in the parathyroid gland encode proteins that bind intracellular calcium ions. CHGA is abundant in secretory granules and has a high affinity for calcium ions but does not bind them strongly. Thus, CHGA facilitates the storage of calcium ions in secretory granules and a buffering mechanism between these vesicles and the cytosol. PVALB binds calcium ions in the cytosol and is believed to be involved in the buffering of intracellular calcium. STXBP5 is a less characterized intracellular protein that may regulate calcium-dependent exocytosis by inhibiting membrane fusion between intracellular vesicles and cell membrane.

CHGA
PVALB
STXBP5

Gene expression shared between parathyroid gland and other tissues

There are 36 group enriched genes expressed in parathyroid 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 parathyroid gland, compared to all other tissues.

To illustrate the relation of parathyroid 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 parathyroid gland enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of parathyroid 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 4 tissues, but the resulting lists show the complete set of group enriched genes in the particular tissue.

Parathyroid gland mainly shares group enriched gene expression with brain (n= 12) and testis (n= 7). One example of a protein expressed in both parathyroid gland and the testis is HTR7, one of several receptors for the hormone serotonin, which is involved in cognitive and behavoural functions. Its possible roles in parathyroid gland and testis are yet to be characterized.

HTR7 - parathyroid gland
HTR7 - testis

Parathyroid gland function

The parathyroid gland regulates calcium homeostasis by producing and releasing parathyroid hormone (PTH). PTH restores blood calcium levels by 1) activating osteoclasts to release calcium from bone into blood, 2) increasing calcium reabsorption in the kidney and, 3) triggering activation of vitamin D in the kidney. The active form of vitamin D, calcitriol, stimulates the uptake of calcium from food through the small intestines and further release of calcium from the bone.

PTH concentration in the blood is dependent on the release of preformed PTH stored in secretory granules and by the synthesis of new PTH. The activity of calcium sensing receptor (CASR) regulates the secretion of preformed PTH. This receptor is located at the cell membrane of chief cells and detects changes in extracellular calcium levels by binding calcium ions. High extracellular calcium concentration activates CASR and consequently triggers an intracellular signal pathway that inhibits the secretion of preformed PTH. Low calcium level has an opposite effect: PTH secretion is increased in the absence of CASR activation. Thus, the level of PTH in blood changes within minutes and the delicate balance in calcium homeostasis is maintained. Calcitriol alters the transcription of the PTH gene, thus affecting the replenishment of PTH stored in secretory granules. It also may have an indirect effect on PTH release by increasing the expression of CASR.

Parathyroid gland histology

Parathyroid glands are highly vascularized endocrine organs located behind the thyroid gland. There are typically four parathyroid glands, each about 5 mm in size and weighing 130 mg, however, the exact number and size of the glands may vary depending on the individual. Chief cells are the predominant cell type characterized by a round nucleus surrounded by scarce cytoplasm. They produce and secrete PTH in response to low extracellular calcium levels detected by receptors in the cell membrane. Larger oxyphil cells with eosinophilic cytoplasm and a slightly smaller nucleus form clusters scattered between chief cells. The function of oxyphil cells is still unknown and whether they are derived from or they are a deactivated form of chief cells is debated. A third cell type, transitional oxyphil cells are similar in size to chief cells but have more eosinophilic staining, which may be evidence of a transition from chief cells to oxyphil cells.

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

Background

Here, the protein-coding genes expressed in parathyroid 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 parathyroid 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)
PubMed: 25613900 DOI: 10.1126/science.1260419

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

Histology dictionary - the parathyroid gland