The placenta-specific proteome

The placenta transcriptome

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

• Tissue enriched: At least four-fold higher mRNA level in placenta 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 placenta 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 placenta compared to other tissues. As evident in Table 1, all genes elevated in placenta 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 placenta as well as in all other tissues. (B) The distribution of all genes across the six categories, based on transcript detection (nTPM≥1) in placenta as well as in all other tissues.

As shown in Figure 1, 293 genes show some level of elevated expression in the placenta compared to other tissues. The three categories of genes with elevated expression in placenta compared to other organs are shown in Table 1. In Table 2, the 12 genes with the highest enrichment in placenta are defined.

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

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

Protein expression of genes elevated in placenta

In-depth analysis of the elevated genes in placenta using antibody-based protein profiling allowed us to create a map of where these proteins are expressed with regards to the different cellular compartments that constitute the placenta, i.e. cytotrophoblast cells, syncytiotrophoblast cells, extravillous trophoblast cells, decidual cells and blood vessels.

Proteins expressed in cytotrophoblasts

The placenta begins to develop upon implantation of the blastocyst (fertilized egg) as it attaches to the endometrium. The outer layer of the blastocyst consists of trophoblast cells, which also form the outer layers of the placenta. The trophoblast layer is further divided into two layers: the cytotrophoblast and the overlying syncytiotrophoblast layer. Examples of proteins expressed in cytotrophoblasts include the highly conserved PEG10, involved in cell proliferation, differentiation and apoptosis, and PAGE4, a member of the GAGE gene-family.

PEG10
PAGE4

Proteins expressed in syncytiotrophoblasts

Syncytiotrophoblasts form when undifferentiated and highly proliferative cytotrophoblast fuse together. Syncytiotrophoblasts are specialized epithelial cells that cover the floating placental villi and are involved in maintaining pregnancy through the production of growth factors and hormones. Since the syncytiotrophoblasts are in direct contact with the maternal blood these cells are closely involved in the exchange of gas, nutrients and waste between the mother and the fetus. Examples of proteins expressed in syncytiotrophoblasts include CSH1, a member of the somatotropin/prolactin family of hormones, KISS1, a stimulator of gonadotropin secretion, and transcription factor GCM1, glial cells missing homolog 1.

CSH1
KISS1
GCM1

Proteins expressed in extravillous trophoblasts

Extravillous trophoblasts are formed when cytotrophoblasts proliferate to form anchoring villi that attach to the uterine wall. From the anchoring villi, extravillous trophoblasts can form by detaching from the placental villi and migrate into the decidua. Examples of proteins expressed in extravillous trophoblast cells include PAPPA2, a metalloproteinase thought to be a local regulator of insulin-like growth factor (IGF), PRG2, a predominant constituent in the eosinophil granule and present in placenta where it exists as a complex with several other proteins including PAPPA, and HLA-G, a major histocompatibility complex (MHC) class I molecule, which plays a central role in the establishment of immune tolerance during pregnancy.

PAPPA2
PRG2
HLA-G

Proteins expressed in placental blood vessels and stroma

The placenta is highly vascularized with capillaries and blood vessels composed of endothelial cells, pericytes and vascular smooth muscle cells. Examples of proteins expressed in endothelial cells in the placenta include FCGR2B, a low affinity receptor for the Fc region of immunoglobulin gamma complexes involved in the phagocytosis of immune complexes and LIN28B, a suppressor of microRNA biogenesis. FBN2, a component of connective tissue microfibrils potentially involved in elastic fiber assembly, shows expression in the stroma of placental villi.

FCGR2B
LIN28B
FBN2

Gene expression shared between placenta and other tissues

There are 47 group enriched genes expressed in placenta. 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 placenta, compared to all other tissues.

To illustrate the relation of placenta 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 placenta enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of placenta 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.

The placenta shares group enriched gene expression of a few genes with several other tissue types, but mostly with brain and testis. One example of a group enriched gene expressed in the placenta and other tissues is MAGEA10, which is also expressed in testis. As of yet, no known biological function has been found for this protein. Another example is DLK1, a transmembrane protein involved in the differentiation of several cell types including adipocytes and neuroendocrine cells, also expressed in the adrenal gland.

MAGEA10 - placenta
MAGEA10 - testis

DLK1 - placenta
DLK1 - adrenal gland

Placenta function

The placenta is a temporary organ formed during pregnancy. It connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. The placenta consists of a fetal component, chorionic villi, which develops from the blastocyst that forms the fetus, and a maternal component, decidua, which develops from the maternal uterine tissue.

Besides supplying nutrients and oxygen to the fetus the placenta also metabolizes substances, protects against pathogens, is involved in maternal-fetal tolerance, and has endocrine functions where it for example produces several pregnancy-associated hormones and growth factors including human placental lactogen (hPL), human chorionic gonadotropin (hCG), placental growth hormone (PGH), corticotrophin-releasing hormone (CRH), gonadotrophin-releasing hormone (GnRH), thyrotrophin-releasing hormone (TRH), estrogen and progesterone.

Placenta histology

The normal term placenta measures 15-20 cm in diameter, 1.5-3 cm in thickness and weighs approximately 450-600 grams. The main components are the chorionic villi protruding from the chorionic plate, decidual tissue, umbilical cord (54-61 cm in length, containing two umbilical arteries and one umbilical vein) and placental membranes. The placental membranes include the amnion (the innermost lining of the amniotic cavity composed of a single layer of flat epithelial cells) and the chorion (connective tissue that carries the fetal vasculature).

The main structure of the placenta is the villous "tree". The placenta villi can float freely in the intervillous space, or be attached to the decidua (anchoring villi) which provides structural stability for the placenta.

The cell types of the placenta villi include:

• Cytotrophoblasts and syncytiotrophoblasts which cover the surface of the villous tree and are surrounded by maternal blood.
• Mesenchymal cells, Hofbauer cells and fibroblasts which are located within the villous core stroma between the trophoblasts.
• Fetal blood vessels, including vascular smooth muscle cells, pericytes and endothelial cells.

The other part of the placenta, the decidua, includes maternal decidual cells and vascular cells. Moreover there are extravillous trophoblast cells, of fetal origin, that migrate into the decidua and remodel uterine arteries.

The histology of human placenta including detailed images and information about the different cell types can be viewed in the Protein Atlas Histology Dictionary.

Background

Here, the protein-coding genes expressed in placenta are described and characterized, together with examples of immunohistochemically stained tissue sections that visualize corresponding protein expression patterns of genes with elevated expression in placenta.

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 placenta