The proteins actively secreted to human blood
A particularly interesting subgroup of the human secretome is the 729 proteins predicted to translocate to peripheral blood, thus having systemic effects in the human body. We here define blood proteins as the proteins actively secreted to or released into the blood, as opposed to leakage products from cell death and proteins ending up in the blood due to disease causes. The secretome blood proteins include both known secreted proteins but also secreted isoforms or processed variants of e.g membrane-bound receptors. Blood proteins are attractive to study, since they are involved in many important body functions and also of great interest as possible molecular targets for clinical diagnostics and precision medicine.
Functions of blood proteins
Blood proteins are part of many important processes, such as inflammation, coagulation, defense, signaling and blood homeostasis. They are involved in functions including transport of lipids, hormones, vitamins and minerals and functioning of the immune system, and some of them act as enzymes, complement components, hormones, protease inhibitors or kinin precursors. The transport protein serum albumin is the most abundant blood protein and accounts for 55% of blood proteins, while globulins that transport ions, hormones, and lipids and are assisting in immune function make up 38%. Another 7% consists of fibrinogen which is essential for blood clotting, and the remainder of the plasma proteins are mainly regulatory proteins, such as enzymes, proenzymes, and hormones. The dynamic range of the protein concentrations is more than 10 orders in magnitude, from albumin at 45 mg per mL blood to low abundant proteins, such as the cytokine IL1, at less than 5 pg per mL. In Figure 1 the function of the 729 blood proteins were annotated, based mainly on Uniprot molecular function and biological process keywords, to a single function term according to the following hierarchy: Blood coagulation, Complement pathway, Acute phase, Cytokine, Hormone, Neuropeptide, Growth factor, Receptor, Lectin, Transport, Developmental protein, Defence, Enzyme, Enzyme inhibitor, Transcription, Immunity, Cell adhesion. Other Uniprot terms to which only a few proteins were annotated were grouped together under the term Other. According to these criteria, 638 genes encode proteins with a known function, while the remaining 91 are still not mapped to Uniprot keywords relating to function. The largest groups are the cytokines (n= 154), enzymes (n=83), hormones (n=75) and proteins related to blood coagulation and the complement system (n=81).
Tissue specificity classification of the blood proteins
The genes encoding proteins secreted into blood were further analyzed based on mRNA expression and categorized according to tissue specificity and tissue distribution. The majority of the genes (82%) showed a tissue restricted expression including 217 being tissue enriched and 89 group enriched, i.e. having either at least five-fold higher mRNA level in one tissue or in a group of two to five tissues compared to all other tissues (Fig. 2). Less than a fifth of the genes showed low tissue specificity, and 4 genes, mainly interferons and interleukins, were not detected in any of the analyzed samples. Regarding the distribution of expression over the different analyzed tissues, about half of the genes were detected either in a single tissue (including the classic plasma proteins expressed in the liver, insulin expressed in the pancreas and the pituitary hormones TSHB and FSHB) or in less than 30% of the tissues (e. g. the apolipoprotein APOB exclusively expressed in the intestine and the liver) (Fig. 3).Figure 2. Tissue specificity for the genes classified as secreted to blood. Categories include: tissue enriched, defined as mRNA level in one tissue at least five-fold higher than in all other tissues; group enriched, defined as five-fold higher average mRNA level in a group of two to five tissues all other tissues; tissue enhanced, defines as five-fold higher average mRNA level in one or more tissues compared to the mean mRNA level of all tissues; expressed in all, defined as ≥ 1 NX in all tissues; and not detected, defined as < 1 NX in all tissues. Figure 3. The genes classified as secreted to blood, categorized according to tissue distribution. Categories include: detected in all, defined as n=100%; detected in many, defined as 31%=< n <100%; detected in some, defined as 1< n <31%; detected in single defined as single n=1; and not detected, n=0.
The origin of the blood proteins
In analyzing the tissue of origin for the 217 tissue enriched blood proteins based on RNA seq the liver was, as expected, found to be the tissue of origin for more than half of the proteins, including albumin (ALB), many of the apolipoproteins and complement related proteins. The proteins secreted to blood from other tissues include for example the apolipoprotein APOA4 produced in the intestine, the antimicrobial protein granulysin (GNLY) enriched in NK-cells and T-cells, the pituitary hormone prolactin (PRL), the anti-Müllerian hormone (AMH) expressed by testis and the hormone oxytocin (OXT) produced in the hypothalamus and secreted into the bloodstream from the neurohypophysis.