The pituitary gland-specific proteome

Figure 1. Schematic drawing of the pituitary gland and its position underneath the hypothalamus.

The pituitary gland transcriptome

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

• Tissue enriched: At least four-fold higher mRNA level in pituitary 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 pituitary gland compared to the average level in all other tissues.

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 pituitary gland compared to other tissues. As evident in Table 1, all genes elevated in pituitary 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 pituitary 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 pituitary gland as well as in all other tissues.

As shown in Figure 1, 353 genes show some level of elevated expression in the pituitary gland compared to other tissues. The three categories of genes with elevated expression in pituitary gland compared to other organs are shown in Table 1. In Table 2, the 12 genes with the highest enrichment in pituitary gland are defined.

Table 1. Number of genes in the subdivided categories of elevated expression in pituitary gland.

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

Protein expression of genes elevated in pituitary gland

In-depth analysis of the elevated genes in pituitary gland using antibody-based protein profiling allowed us to create a map of where the corresponding proteins are expressed within the anterior (adeno) pituitary gland.

Adenopituitary proteins and hormones

The anterior pituitary hormone secretion is under the influence of hypothalamic hormone releasing hormones, and stimulates the release of several different hormones.

The hypothalamic thyrotropin-releasing hormone binds the thyrotropin-releasing hormone receptor (TRHR) which in turn stimulates the thyrotropic cells to release thyroid-stimulating hormone (TSHB). In a similar fashion, the secretion of proopiomelanocortin (POMC) from the corticotropes is stimulated by corticotropin-releasing hormone (CRH) binding to the receptor (CRHR1). POMC is a precursor to ACTH, opioid peptides and alpha-MSH. Furthermore, the release of follicle-stimulating hormone (FSHB) and luteinizing hormone (LHB) is dependant on the stimulation of gonadotropin-releasing hormone receptor (GNRHR) by hypothalamic gonadotropin-releasing hormone (GNRH1).

TRHR
TSHB

CRHR1
POMC

GNRHR
FSHB
LHB

Prolactin (PRL) is a secreted anterior pituitary hormone and functions as a growth regulator for many tissues, and it is essential for stimulating lactation. The gene (AVPR1B) encodes a G-coupled receptor for arginine vasopressin. The AVPR1B receptor is expressed in the anterior pituitary and is implicated in ACTH release. Another protein, Galanin (GAL) is a physiologically active neuropeptide that is implicated in nociception, feeding and energy homeostasis, osmotic regulation and water balance. Galanin immunoreactivity can be observed in the pituitary but also in the hypothalamus.

PRL
AVPR1B
GAL

Proteins involved in adenopituitary cell type-specific differentiation

Cellular differentiation in the anterior pituitary is dependent on specific transcription factors. PROP1 is believed to mainly be involved in the differentiation of gonadotropes, as well as somatotropes and lactotropes. T-Pit (T-box family member TBX19) regulates the POMC lineage and corticotrophs. While the Pit-1 transcription factor, encoded by the POU1F1 gene, specifies lineage of the lactotropes, somatotropes and thyrotropes. Lineage-markers against different transcription factors are useful in classifying pituitary neuroendocrine tumors.

PROP1
TBX19
POU1F1

Gene expression shared between the pituitary gland and other tissues

There are 111 group enriched genes expressed in pituitary 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 pituitary gland, compared to all other tissues.

In order to illustrate the relation of pituitary 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 pituitary gland enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of pituitary 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.

There are 111 group enriched genes expressed in the pituitary gland and most of that expression is shared with the brain. The neurosecretory protein (VGF), which shows sequence similarities with the secretogranin/chromogranin family, is group enriched in the pituitary gland and brain tissue. The exact function of VGF protein is not yet fully understood, however, there is evidence that the protein is involved in body fluid homeostasis by regulating vasopressin release.

VGF - pituitary gland
VGF - cerebral cortex

The pituitary gland also shares many proteins with the adrenal gland and other endocrine tissues. The neuroendocrine protein secretogranin 2, encoded by the (SCG2) gene, is group enriched in the adrenal gland, brain and pituitary gland. Secretogranin 2 has an important function in the packaging and sorting of hormones and neuropeptides into secretory vesicles.

SCG2 - pituitary gland
SCG2 - adrenal gland

SCG2 - cerebral cortex
SCG2 - hypothalamus

The gene NPTX2 encodes a neuronal petraxin protein, a synaptic protein of the neuronal petraxin family. (NPTX2) is group enriched in the pituitary gland, adrenal gland, brain, testis and pancreas. The protein is believed to be involved in excitatory synapse formation and studies have shown that the up-regulation of this protein may be associated with Parkinson's disease pathology.

NPTX2 - pituitary gland
NPTX2 - hippocampus

NPTX2 - pancreas
NPTX2 - testis
NPTX2 - adrenal gland

The pituitary glycoprotein hormones: luteinizing hormone (LHB), follicle stimulating hormone (FSHB), thyroid-stimulating hormone (TSHB), and the placental chorionic gonadotropin consists of an alpha and a beta subunit. The alpha subunit for these proteins is encoded by the CGA gene, and is group enriched in the pituitary gland and placenta.

CGA - pituitary gland
CGA - placenta

Proteins analyzed in pituitary gland tissue

The standard setup in the Tissue Atlas is based on Tissue Micro array technique (TMA), thus saving valuable tissue material as well as reagents and provides a wide tissue representation for protein profiling. In addition to the standard setup, extended tissue profiling in the pituitary gland is performed for selected proteins, to give a more complete overview of where the proteins are expressed. The full list of genes used for protein profiling on pituitary gland samples is defined in Table 3.

Table 3. The following 58 genes have been analyzed in pituitary gland.

Pituitary gland histology and function

The pituitary gland, or hypophysis (Gr. hypo, under, + physis, growth) is a small hormone-producing gland of dual embryonic origin that is about 1 cm in diameter and weighs approximately 0.5g. The pituitary gland lies in the cavity of the sphenoid bone (sella turcica), beneath the third ventricle of the brain. The dual origin of the pituitary gland derives from the gland being partly oral ectoderm and partly nervous tissue. During early gestation, a "loop" of ectodermal tissue arises from the roof of the mouth (called Rathke’s pouch) and combines together with a "loop" from the diencephalon of the developing brain. The function of the two parts stays separated as the anterior and posterior lobes, also called adenohypophysis and neurohypophysis, respectively.

The pituitary gland plays a crucial role in human physiology, and together with the hypothalamus, the gland forms a link between the nervous and endocrine system, to control the functions of the thyroid, adrenal glands, and the gonads. The pituitary also regulates growth, lactation, and water preservation.

The anterior glands consists of epithelial cells that produce different hormones:

The posterior pituitary gland consists mainly of unmyelinated axons from hypothalamic secretory neuronal cells located in the supraoptic and periventricular hypothalamic nuclei. The axons form the hypothalamo-hypophyseal tract and their terminals end in the posterior lobe. The neurons secrete a neurosecretory compound that mostly contains oxytocin or vasopressin. Surrounding these axons are pituicytes, which are glial cells that support the storage and release of the hormones.

Background

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

Transcript profiling was based on a combination of two transcriptomics datasets (GTEx and FANTOM5), corresponding to a total of 8849 samples from 109 different human normal tissue types. The final consensus normalized expression (NX) value for each tissue type was used for 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

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

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

Sjöstedt E et al., A specific antibody to detect transcription factor T-Pit: a reliable marker of corticotroph cell differentiation and a tool to improve the classification of pituitary neuroendocrine tumours. Acta Neuropathol. (2017)
PubMed: 28823042 DOI: 10.1007/s00401-017-1768-9

Sjöstedt E et al., Integration of Transcriptomics and Antibody-Based Proteomics for Exploration of Proteins Expressed in Specialized Tissues. J Proteome Res. (2018)
PubMed: 30272454 DOI: 10.1021/acs.jproteome.8b00406