The seminal vesicle-specific proteome

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Figure 1. The distribution of all genes across the five categories based on transcript abundance in seminal vesicle as well as in all other tissues.

137 genes show some level of elevated expression in the seminal vesicle compared to other tissues. The three categories of genes with elevated expression in seminal vesicle compared to other organs are shown in Table 1. The function and cellular localization of known genes with tissue enriched expression in seminal vesicle (n=8), are well in-line with the function of the seminal vesicle. In Table 2, the 8 enriched genes are defined.

Table 2. The 8 genes with the highest level of enriched expression in seminal vesicle. "Predicted localization" shows the classification of each gene into three main classes: Secreted, Membrane, and Intracellular, where the latter consists of genes without any predicted membrane and secreted features. "mRNA (tissue)" shows the transcript level as TPM values, TS-score (Tissue Specificity score) corresponds to the score calculated as the fold change to the second highest tissue.

Some of the proteins predicted to be membrane-spanning are intracellular, e.g. in the Golgi or mitochondrial membranes, and some of the proteins predicted to be secreted can potentially be retained in a compartment belonging to the secretory pathway, such as the ER, or remain attached to the outer surface of the cell membrane by a GPI anchor.

The seminal vesicle transcriptome

An analysis of the expression levels of each gene made it possible to calculate the relative mRNA pool for each of the categories. The analysis shows that 79% of the mRNA molecules derived from seminal vesicle correspond to housekeeping genes and only 10% of the mRNA pool corresponds to genes categorized to be either seminal vesicle enriched, group enriched or, seminal vesicle enhanced. Thus, most of the transcriptional activity in the seminal vesicle relates to proteins with presumed housekeeping functions as they are found in all tissues and cells analyzed.

Gene Ontology-based analysis of all the 137 genes elevated in the seminal vesicle indicates a clear overrepresentation of proteins associated with ion-channel function, contractility, protection and secretion of sperm.

Protein expression of genes elevated in seminal vesicle

In-depth analysis of the elevated genes in seminal vesicle using antibody-based protein profiling allowed us to visualize the expression patterns of these proteins in different functional compartments including proteins related to insemination and sperm motility as well as protection.

Proteins specifically expressed in seminal vesicle

The proteins that are encoded by the genes SEMG1 and SEMG2 are the predominant proteins in semen. These secreted proteins are involved in the formation of a gel matrix that encases ejaculated spermatozoa. The preproproteins of and are proteolytically processed by the prostate-specific antigen (PSA) protease to generate multiple peptide products that exhibit distinct functions. One of these peptides is an antimicrobial peptide with antibacterial activity. This proteolysis process also breaks down the gel matrix and allows the spermatozoa to move more freely.

Tight junctions hold cells together and function as a protective barrier preventing unwanted molecules and ions to pass the plasma membrane and the adjacent cells. The protein expressed by CLDN2 belongs to the claudin protein family whose members have been identified as major integral membrane proteins localized exclusively at tight junctions. Claudins are expressed in an organ-specific manner and regulate tissue-specific physiologic properties of tight junctions. The ZNF185 gene encodes a zinc-finger proteins that binds to nucleic acids and play important roles in various cellular functions, including cell proliferation, differentiation, and apoptosis. This gene encodes a LIM-domain zinc finger protein. The LIM domain is composed of two contiguous zinc finger domains, separated by a two-amino acid residue hydrophobic linker. The LIM domain mediates protein-protein interactions.

Genes shared between the seminal vesicle and other tissues

There are 33 group enriched genes expressed in the seminal vesicle. Group enriched genes are defined as genes showing a 5-fold higher average level of mRNA expression in a group of 2-7 tissues, including seminal vesicle, compared to all other tissues.

In order to illustrate the relation of seminal vesicle to other tissue types, a network plot was generated, displaying the number of genes shared between different tissue types.

Figure 2. An interactive network plot of the seminal vesicle enriched and group enriched genes connected to their respective enriched tissues (grey circles). Red nodes represent the number of seminal vesicle 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.

The seminal vesicle share most genes with cerebral cortex and epididymis. An example of a group enriched gene in the seminal vesicle and the cerebral cortex is CPNE6. This gene encodes a member of the copine family. Members of this family are calcium-dependent, phospholipid-binding proteins with C2 domains, two calcium- and phospholipid-binding domains. Through their domain structure and lipid binding capabilities, these proteins may play a role in membrane trafficking. CPNE6 is expressed in the smooth muscle layer surrounding the secretory glands in the seminal vesicles and in all cell types and structures in cerebral cortex.

Another example of a group enriched gene that is shared with the seminal vesicle is the MUC6 gene that encodes a member of the mucin protein family. Mucins are high molecular weight glycoproteins produced by many epithelial tissues, including stomach and duodenum. MUC6 protein expression has been found in the gastrointestinal system as well as in the seminal vesicles. The protein encoded by this gene is secreted and forms an insoluble mucous barrier that protects the lumen in several tissues, including the seminal vesicles.

The KIAA1210 gene is group enriched in seminal vesicle, epididymis and testis. The protein expression in seminal vesicle and epididymis, excretory male glands, is restricted to the fibromuscular tissues surrounding the glandular epithelia, while the expression in testis is mainly detected in Sertoli cells.

Seminal vesicle function

Testosterone is the main regulator of morphology and secretory function in the seminal vesicles and the thick layer of smooth muscle surrounding the glands force the sperm out through the urethra during ejaculation. Spermatozoa requires fructose as their largest nutrient which is provided by the secreting cells in the seminal vesicles. The secretory cells provide 70-80% of the total ejaculate content. Proteins, amino acids, citric acid and ascorbic acids among other important nutrients are released to facilitate sperm survival and transportation. Large amounts of prostaglandins are produced in the glands of the seminal vesicle. Prostaglandins stimulates inflammatory reactions among several other processes in our body.

Seminal vesicle histology

The seminal vesicles are paired, highly coiled, tubular structures located above the prostate and behind the urinary bladder. The seminal vesicles merge with vas deferens before debouching into the intra-prostatic urethra. The pseudostratified columnar epithelium in seminal vesicles is surrounded by thick fibromuscular tissue. The vesicle epithelium is composed of columnar and basal epithelial cells. The columnar epithelium typically contains large amounts of lipofuscin pigment. An unusual feature of normal epithelial cells is the feature of cellular atypia. However, atypical cells are often encountered in the seminal vesicle epithelium, where epithelial cells with large atypical nuclei often are present.

The histology of human seminal vesicle 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 the seminal vesicle are described and characterized, together with examples of immunohistochemically stained tissue sections that visualize protein expression patterns of proteins that correspond to genes with elevated expression in the seminal vesicle.

Transcript profiling and RNA-data analyses based on normal human tissues have been described previously (Fagerberg et al., 2013). Analyses of mRNA expression including over 99% of all human protein-coding genes was performed using deep RNA sequencing of 172 individual samples corresponding to 37 different human normal tissue types. RNA sequencing results of 3 fresh frozen tissues representing normal seminal vesicle was compared to 169 other tissue samples corresponding to 36 tissue types, in order to determine genes with elevated expression in seminal vesicle. A tissue-specific score, defined as the ratio between mRNA levels in seminal vesicle compared to the mRNA levels in all other tissues, was used to divide the genes into different categories of expression. These categories include: genes with elevated expression in seminal vesicle, genes expressed in all tissues, genes with a mixed expression pattern, genes not expressed in seminal vesicle, and genes not expressed in any tissue. Genes with elevated expression in seminal vesicle were further sub-categorized as i) genes with enriched expression in seminal vesicle, ii) genes with group enriched expression including seminal vesicle and iii) genes with enhanced expression in seminal vesicle.

Human tissue samples used for protein and mRNA expression analyses were collected and handled in accordance with Swedish laws and regulation and obtained from the Department of Pathology, Uppsala University Hospital, Uppsala, Sweden as part of the sample collection governed by the Uppsala Biobank. All human tissue samples used in the present study were anonymized in accordance with approval and advisory report from the Uppsala Ethical Review Board.

Relevant links and publications

Uhlén M et al, 2015. Tissue-based map of the human proteome. Science
PubMed: 25613900 DOI: 10.1126/science.1260419

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

Fagerberg L et al, 2014. Analysis of the human tissue-specific expression by genome-wide integration of transcriptomics and antibody-based proteomics. Mol Cell Proteomics.
PubMed: 24309898 DOI: 10.1074/mcp.M113.035600

Histology dictionary - the seminal vesicle