The pituitary gland-specific proteome
The pituitary gland (also called hypophysis) is a hormone-producing gland of dual embryonic origin, the anterior endocrine gland and the posterior neuronal gland. The anterior lobe is divided into three regions: pars distalis, pars tuberalis and pars intermedia. These structures consist mainly of hormone-producing epithelial cells (somatotropes, corticotropes, thyrotropes, gonadotropes and lactotropes) that store hormones and other molecules in secretory granules, to later be released into the bloodstream and facilitates further downstream effects in peripheral tissues. The posterior lobe consists mainly of unmyelinated axons of hypothalamic secretory neuronal cells, from the supraoptic and paraventricular hypothalamic nuclei, and pituicytes, a type of glial cell that supports the storage and release of hormones. Neurohypophysial hormones, containing antidiuretic hormones and oxytocin, travels along the axons from the hypothalamic neurons to the nerve endings in the posterior pituitary, specifically to the pars nervosa - the main structure of the posterior gland. Transcriptome analysis shows that 75% (n=14719) of all human proteins (n=19670) are expressed in the pituitary gland and 353 of these genes show an elevated expression in the pituitary gland compared to other tissue types.
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:
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:
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).
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.
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.
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.
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.
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.
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.
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.
Relevant links and publications
Uhlén M et al., Tissue-based map of the human proteome. Science (2015)