Endocrine tissues

In the pig transcriptomic analysis, adrenal, thyroid, pituitary and pineal gland are all endocrine tissues included for quantitative RNA measurements. The gene classification strategy highlights genes with an elevated level of expression in one or a group of tissues compared to all other tissues.

The endocrine system is the body’s “slow” chemical communication system, which includes a series of glands, such as pituitary, adrenal gland, pineal gland and thyroid gland that secret hormone into the bloodstream. These hormones are then transported to the target organs and cells, where they can bind to the receptors and trigger series of reactions.

291 genes are classified as adrenal gland elevated out of which 26 genes are highlighted as adrenal gland enriched. Based on the expression in thyroid gland 265 genes are classified as elevated compared to other tissue types, out of which 13 genes are highlighted as tissue enriched in thyroid gland. RNA expression levels in pituitary gland resulted in 477 genes classified as elevated in pituitary gland compared to other tissue types, out of which 26 genes are highlighted as tissue enriched in pituitary gland. 629 genes are classified as pineal gland elevated out of which 24 genes are highlighted as pineal gland enriched.

The function of the endocrine tissues, the basic architecture and histology is in general similar between pig and human. Histological image of the pig tissues used in the analysis can be found in the pig tissue dictionary.

Adrenal gland

In pigs, the adrenal glands are elongated and located near the aorta, cephalic end of the kidneys. They are responsible for producing a variety of hormones including adrenaline and the steroids aldosterone and cortisol. Each gland has an outer cortex and an inner medulla, the former can be further divided into three zones: the zona glomerulosa, the zona fasciculata and the zona reticularis. The adrenal cortex secretes three main kinds of steroid hormones: mineralocorticoids, glucocorticoids and androgens. The mineralocorticoids (such as aldosterone) produced in the zona glomerulus involves in regulating the blood pressure and homeostasis of extracellular electrolytes. The glucocorticoids cortisol, secreted by the zona fasciculata, plays an essential role in normal metabolism. The androgens are synthesized in the zona reticularis, which engages in regulating development and maintenance of male characteristics. The adrenal medulla under neuron control produces catecholamine (adrenaline and norepinephrine) that is involved in stress response.

Gene expression in adrenal gland is categorized based on two gene classification strategies, tissue detection and tissue specificity. Figure 1 summarizes the number of genes in respective category. In total, 15146 genes are detected above cut of (1NX) in pig adrenal gland. The tissue distribution category highlights 2 genes only detected in adrenal gland while 291 genes are classified as adrenal gland elevated compared to other tissues. Table 1 shows the overlap for the adrenal gland elevated genes and tissue distribution category.

Figure 1. (A) The distribution of all genes across the five categories based on transcript specificity in adrenal 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 adrenal gland as well as in all other tissues. The combination of the two categories is shown in table 1.

Table 1. Number of genes in the subdivided categories of elevated expression and tissue distribution in adrenal gland

Adrenal gland enriched expression

The adrenal cortex produces steroid hormones that are vital to life. To meet this need, there are several genes with adrenal gland enriched expression. One typical example is CYP21A2, which plays a main role in synthesis of steroid hormones including cortisol and aldosterone. Another example is NR5A1, which is essential for formation of the primary steroidogenic tissues. Additionally, TH is not among the top 10 enriched genes, but plays a key role in the physiology of adrenergic neurons involved in the conversion of tyrosine to dopamine.

Table 2. The 10 genes with the highest level of enriched expression in adrenal gland. "mRNA (tissue)" shows the transcript level in retina as NX values. "Tissue specificity score (TS)" corresponds to the fold-change between the expression level in adrenal gland and the tissue with second highest expression level.

The cromaffin cells of the adrenal medulla have a neuronal developmental origin and function, releasing hormones instead of neurotransmitters. In the adrenal medulla proteins such as tyrosine hydroxylase (TH), vital in the synthesis of catecholamines as well as dopamine beta-hydroxylase (DBH) that catalyzes the conversion of dopamine to norepinephrine are highly expressed.

Immunohistochemical detection of medulla specific proteins in pig adrenal gland, TH detected using HPA061003 and DBH detected using HPA002130.

Thyroid gland

In pigs, the pair of thyroid glands are joined ventrally. They can produce the hormones thyroxine (T4) and triiodothyronine (T3), which are involved in regulating the metabolic rate, protein synthesis and body’s sensitivity to other hormones. They also secrete calcitonin, which regulates the balance of calcium in the body.

Gene expression in thyroid gland is categorized based on two gene classification strategies, tissue detection and tissue specificity. Figure 2 summarizes the number of genes in respective category. In total, 15100 genes are detected above cut of (1NX) in pig thyroid gland. The tissue distribution category highlights 3 genes only detected in thyroid gland while 265 genes are classified as thyroid gland elevated compared to other tissues. Table 3 shows the overlap for the thyroid gland elevated genes and tissue distribution category.

Figure 2. (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 (NX≥1) in thyroid gland as well as in all other tissues. The combination of the two categories is shown in table 3.

Table 3. Number of genes in the subdivided categories of elevated expression and tissue distribution in thyroid gland

Thyroid gland enriched expression

Proteins related to thyroid hormone synthesis are highly enriched in thyroid gland. One typical example is the thyroid specific protein thyroglobulin (TG), which acts as a substrate for thyroxine (T4) production. Triiodothyronine (T3) can be formed from T4 under the catalyzation of iodine tyrosine deiodinase (IYD). And finally, FOXE1 functions as a thyroid transcription factor that plays an essential role in thyroid morphogenesis.

Table 4. The 10 genes with the highest level of enriched expression in thyroid gland. "mRNA (tissue)" shows the transcript level in retina as NX values. "Tissue specificity score (TS)" corresponds to the fold-change between the expression level in thyroid gland and the tissue with second highest expression level.

Pituitary gland

Gene expression in pituitary gland is categorized based on two gene classification strategies, tissue detection and tissue specificity. Figure 4 summarizes the number of genes in respective category. In total, 15625 genes are detected above cut of (1NX) in pig pituitary gland. The tissue distribution category highlights 8 genes only detected in pituitary gland while 477 genes are classified as pituitary gland elevated compared to other tissues. Table 5 shows the overlap for the pituitary gland elevated genes and tissue distribution category.

Figure 4. (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. The combination of the two categories is shown in table 5.

Table 5. Number of genes in the subdivided categories of elevated expression and tissue distribution in pituitary gland

Pituitary gland enriched expression

The genes with enriched expression in pituitary gland fit very well with the function and unique cell types of the pituitary gland. TSHB, POMC, CGA and FSHB are either precursors or parts of the different hormones secreted by the anterior pituitary. GHRHR codes for the growth hormone releasing hormone receptor, crucial part of the growth hormone releasing axis receiving signals from the GHRH from the hypothalamus and signaling release of the growth hormone (GH1).

POU1F1 and TBX19 are both cell lineage specific transcription factors. NNAT and IGSF1 are genes with high pituitary gland enrichment but rather uncharacterized so far.

Table 6. The 10 genes with the highest level of enriched expression in pituitary gland. "mRNA (tissue)" shows the transcript level in retina 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.

Pineal gland

The main function of the pineal gland is to produce melatonin. Melatonin has multiple functions in the central nervous system, the most important of which is to help regulate sleep patterns. Pineal gland could only be collected from one of the pigs in this analysis, due to the small size and difficult location inside the brain. Although being represented by a single sample only, we chose to keep pineal gland in the analysis due to its unique expression profile and relevance in comparison to other tissue types, for example the close relation with retina and circadian rhythm.

Gene expression in pineal gland is categorized based on two gene classification strategies, tissue detection and tissue specificity. Figure 5 summarizes the number of genes in respective category. In total, 15648 genes are detected above cut of (1NX) in pig pineal gland. The tissue distribution category highlights 2 genes only detected in pineal gland while 629 genes are classified as pineal gland elevated compared to other tissues. Table 7 shows the overlap for the pineal gland elevated genes and tissue distribution category.

Figure 5. (A) The distribution of all genes across the five categories based on transcript specificity in pineal 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 pineal gland as well as in all other tissues. The combination of the two categories is shown in table 7.

Table 7. Number of genes in the subdivided categories of elevated expression and tissue distribution in pineal gland

Pineal gland enriched expression

CRABP1 encodes a binding protein for members of the vitamin A family and may play an important role in retinoic acid-mediated differentiation and proliferation. CNGB3 encodes the beta subunit of a cyclic nucleotide-gated ion channel, which plays a role in modulation of channel function in cone photoreceptors. The transcription factor BSX is essential for normal postnatal growth and nursing.

Table 8. The 10 genes with the highest level of enriched expression in adrenal gland. "mRNA (tissue)" shows the transcript level in retina as NX values. "Tissue specificity score (TS)" corresponds to the fold-change between the expression level in pineal gland and the tissue with second highest expression level.

Gene expression in endocrine tissues compared to other tissues

Endocrine tissues share several basic cellular functions, but also includes hormone specific functions related to the different tissue types. Therefore, rather few group-enriched genes are shared among the endocrine tissues. Adrenal and pituitary gland share some unique expression profiles, such as CHGB and KIAA1024L. Both pineal and pituitary gland share brain related proteins due to the developmental origin and location. Thyroid gland, on the other hand shares most group-enriched genes with testis.

The adrenal gland share expression of many genes with testis and ovary, related to steroid hormones, for example STAR, SCARB1and CYP11A1.

Immunohistochemical detection of CYP11A1 in pig adrenal gland, ovary and testis, using antibody HPA016436.

In order to illustrate the relation of endocrine tissue to other tissue types, a network plot was generated, displaying the number of genes shared between different tissue types. Group enriched genes are defined as genes showing a 4-fold higher average level of mRNA expression in a group of 2-5 tissues, compared to all other tissues.

Figure 6. An interactive network plot of the endocrine tissue enriched and group enriched genes connected to their respective enriched tissues (grey circles). Black circles shows tissues representing the endocrine system. Red nodes represent the number of tissue 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.