The parathyroid gland-specific proteome
The parathyroid gland is an endocrine organ that plays a pivotal role in the maintenance of blood calcium levels. In response to decreased calcium levels, parathyroid hormone is released into the blood and restores calcium level mainly by regulating bone and kidney functions. Transcriptome analysis shows that 65% (n=12798) of all human proteins (n=19670) are expressed in the parathyroid gland and 224 of these genes show an elevated expression in the parathyroid gland compared to other tissue types.
The parathyroid gland transcriptome
Transcriptome analysis of the parathyroid gland can be visualized with regard to 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:
Distribution, on the other hand, visualizes how many genes that have, or do not have, detectable levels (NX≥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:
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 (NX?1) in parathyroid gland as well as in all other tissues.
As shown in Figure 1, 224 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. 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 (NX?1) in parathyroid gland as well as in all other tissues. "mRNA (tissue)" shows the transcript level in parathyroid gland as NX values. "Tissue specificity score (TS)" corresponds to the fold-change between the expression level in parathyroid gland and the tissue with 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.
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.
Gene expression shared between parathyroid gland and other tissues
There are 34 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.
In order to illustrate the relation of parathyroid gland tissue to other tissue types, a network plot was generated, displaying the number of genes with 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 3 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= 23). One example of a protein expressed in both parathyroid gland and the cerebral cortex of the brain is PEX5L, an intracellular protein that may regulate hyperpolarization-activated cyclic nucleotide-gated channels.
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 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 level 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 to size to chief cells but have a more eosinophilic staining, which may be evidence of a transition from chief cells to oxyphil cells.
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.
Relevant links and publications
Uhlén M et al., Tissue-based map of the human proteome. Science (2015)