Podoplanin is a protein that in humans is encoded by the PDPN gene. It is a small glycoprotein located on the surface membranes of various cell types. While termed PDPN in humans, it is often named: a) OTS-8, gp38, aggrus, antigen PA2.26, or RANDAM-2 (i.e., retinoic acid-induced neuronal differentiated-associated molecule-2) in mice; b) T1α protein or E11 antigen in rats; c) aggrus or gp40 in canines; and d) aggrus in hamsters and cows. The PDPN gene is located on the "p", i.e., short, arm of chromosome 1, region 3, band 1 (location notated as 1p36.21; see Gene nomenclature). This gene directs the formation of PDPN messenger RNA (i.e., mRNA) which in turn directs formation of the PDPN glycoprotein. Here, the term PDPN is used for the non-human as well as human glycoprotein, PDPN is used for the human gene, and Pdpn is used for the animal gene.
Studies to date have suggested that PDPN acts to promote or inhibit a wide range of physiological and pathological reactions in rodents and, in a few studies, humans. However, almost all of these studies are preliminary and require far larger follow-up studies to determine if regulating PDPN levels could be used in humans to treat the various PDPN-regulated functional responses and PDPD-induced disorders. Indeed, studies have not yet determined if the promotion or inhibition of PDPN actions can be used safely in humans.
Tissues distribution A study on the levels of PDPN in 20 human tissues reported that it was: most highly expressed on the cells in the lung, placenta, heart, trachea, uterus, cerebellum, fetal brain, stomach, thymus, and prostate; less strongly expressed in skeletal muscle, adult brain, thyroid gland, adrenal gland, kidney, salivary gland, and small intestine; and minimally expressed or not detected in cells of the fetal liver, non-fetal liver, and spleen. Other studies have reported that PDPN is expressed by human and/or rodent: a) type I alveolar cells of the lung and kidney podocytes (podoplanin was given its name based on its expression in podocytes); b) endothelial cells lining the lymphatic system but not endothelial cells lining blood vessels; c) reticular cells (i.e., a type of fibroblast that makes extracellular reticular fibers in, e.g., the liver, bone marrow, and lymphatic system) and epithelial cells (also termed ependymocytes or ependymal cells) in the choroid plexuses of the four brain ventricles; d) the glial and microglia cells located in the central nervous system; cells in nasal polyps, mesothelial cells, e) stromal cells, macrophages, activated T helper 17 cells; cells in the basal layer of sweat glands, and external layer of hair follicles; f) a wide range of cancer cells including 80% of squamous cell carcinomas of the lung, larynx, cervix, skin, and esophagus, 25% of oral squamous cell carcinoma cells, 98% of seminoma cells; 69% of embryonal carcinoma cells, 29% of the cells in teratomas, 25% of the cells in endodermal sinus tumors (also termed yoke sac tumors), 98% of the cells in brain germinomas, 71% of the cells in immature brain teratomas, 47% of the cells in brain glioblastomas, 25% of brain anaplastic astrocytomas, 100% of lymphangiomas, 90-100% of Kaposi's sarcomas, 93% of mesotheliomas; and g) the cancer-associated fibroblasts in the tissues of various cancers.
Structure Human PDPN is a mucin-containing, O-linked glycosyl, type I transmembrane glycoprotein. Type 1 glycoproteins pass through a cell's surface membrane once and have their N-terminal and C-terminal ends located respectively on the extracellular and intracellular sides of their cell's surface membrane. PDPN consists of 162 amino acids with about 128 residing on the outside the cell, about 25 spanning the cell's surface membrane, and 9 residing inside the cell. Human PDPN's structure is similar to that of animal PDPNs in its transmembrane and cytoplasmic portions but has a somewhat different structure in its extracellular portions than that of animals (which vary between different animal species). Human PDPN has a molecular mass of 36 to 43 kilodaltons, depending on the amount of O-linked glycosyl residues it contains. Its extracellular portion consist of four amino acid tandem repeat areas termed platelet aggregation-stimulating domains 1-4, i.e., PLAG 1-4. In addition, PDPN can be released from its parent cells as a PDPN-expressing secreted vesicle or as a free intact protein, circulate in the blood, and be measured in the plasma of humans and animals.
Activation of CLEC-2 The PLAGs of PDPN expressed on cells, vesicles, or the free protein interact with proteins on the surface of other cells, particularly the C-type lectin-like receptor 2, i.e., CLEC-2 (also termed CLEC1B, 1810061I13Rik, CLEC2, CLEC2B, PRO1384, QDED721, and C-type lectin domain family 1 member B). CLEC-2 is a member of the C-type lectin receptors in the superfamily of pattern recognition receptors (see C-type lectin section on Classification). It is expressed on: the surface membranes of megakaryocytes, platelets, dendritic cells, follicular dendritic cells, mesothelial cells (i.e., simple squamous epithelial cells of mesodermal origin in the mesothelium), epithelial cells in lymphatic vessels, and cancer-associated fibroblasts (i.e., fibroblasts located in cancer tissues). The binding of PLAG-3/PLAG-4 to the extracellular portion of CLEC-2 causes it to be phosphorylated on the tyrosine and lysine amino acids in its intracellular single cytoplasmic tyrosine-XX-lysine sequence (known as a hemi-Immunoreceptor Tyrosine-based Activation Motif or hemITAM, a key structural motif in a protein that is critical for the protein's actions (hemITAM is related to the Immunoreceptor tyrosine-based activation motif, i.e., ITAM). The phosphorylated CLEC-2 then activates tyrosine-protein kinase SYK, i.e., Syk, which in turn activates various pathways that trigger these cells to make certain types of responses (see Signal transduction). CLEC-2 is also activated by the human immunodeficiency virus (i.e., HIV), rhodocytin (a platelet-activating snake venom also termed aggretin), hemin, galectin-9, dextran sulfate, sulfated polysaccharides, fucoidan, ketacine (i.e., an extract of the Polygonaceae family of flowering plants), S100A13 (i.e., S100 calcium-binding protein A13), CLEC7A (also termed C-type lectin domain family 7 member A or dectin-1), and the soot, carbon, and other particles in the exhaust gas of diesel engines.
Function and clinical significance The PLAG-3/PLAG-4 part of PDPN interacts with the CLEC-2 expressed on the surfaces of platelets and megakaryocytes to promote blood clots, inflammation, lymphangiogenesis (i.e., formation of lymphatic vessels), angiogenesis (i.e., formation of blood vessels), immune surveillance (e.g., killing cancer cells), remodeling the extracellular matrix of normal and cancerous tissues, and epithelial–mesenchymal transitions by which epithelial cells gain migratory and invasive properties to become mesenchymal stem cells that differentiate into cell types that promote cancer metastases. PDPN's interaction with CLEC-2 is generally considered to involve the binding of PDPN on the surface of cells to CLEC-2 on the surface of adjacent cells. However, studies have found that cultured mice and human PDPN-expressing glioblastoma cell lines release PDPN-positive vesicles which when injected into mice activate their platelets. The injection of PDPN-negative vesicles into mice did not activate the platelets suggesting that the PDPN-expressing vesicles activated platelets by binding to their CLEC-2 receptors. Similar platelet-activating effects were seen in cultures of mouse ovarian cancer cells and a mouse model of ovarian cancer. These studies suggest that the release of PDPN-expressing vesicles into the circulation activate blood platelets to cause intravascular blood clots. Finally: a) the plasma levels of soluble PDPN were significantly elevated in patients with various forms of squamous cell carcinoma, adenocarcinomas, rectal cancer, lung cancer, and gastric cancer compared to those of individuals who did not have cancer; b) the levels of soluble plasma PDPN in patients who had metastatic cancer were significantly higher than those of patients with non‐metastatic cancer; and c) the levels of soluble plasma PDPN significantly decreased in patients who were treated for these cancers. While further studies that include larger numbers of patients are needed, this study suggest that measurements of soluble plasma PDPN may be useful for detecting the presence, metastasis, and responses to treatment of these and perhaps other cancers.
Platelet activation PDPN activates platelets by binding to their CLEC-2 receptors and thereby causing them to phosphorylate the immunoreceptor tyrosine-based activation motifs on their intercellular portions and activate cell signaling pathways that cause the platelets to bind fibrinogen, aggregate with other platelets, and release various agents such as fibrinogen, adenosine diphosphate, serotonin, von Willebrand factor, platelet-derived growth factor, and transforming growth factor-β, all of which act to further increase platelet activation.
Platelet formation Studies in mice have shown that the PDPN expressed on reticular cells in the bone marrow stimulates megakaryocytes (i.e., cells that produce platelets) to proliferate, form platelets, and thereby increase the levels of platelets that circulate in the blood. This effect is due to PDPN binding to the CLEC-2 receptors on megakaryocytes.
Hair follicle growth The hair follicle is an organ that resides in the dermal layer of the skin in mammals. The follicle continuously cycles through an anagen phase of growth, catagen phase of apoptosis-driven regression, and telogen phase of relative quiescence. Stem cells in the hair follicle's bulge promote repetitive regeneration of its follicle along with the follicle's hair. Studies in mice showed that PDPN was expressed in the stem cells and keratinocyte-rich regions of the hair follicle during the late anagen but not telogen phase of the hair growth cycle. The application of wax to the skin of female C57BL/6 mice caused hair removal and hair regeneration at days 1 (early-anagen phase) and 5 (mid-anagen phase) after hair removal. PDPN was expressed in lymphatic vessels but not the hair follicles' keratinocytes. At days 8 and 12 (late-anagen phase), however, PDPN was expressed in hair follicle keratinocytes and stem cells. At day 18 (catagen phase), PDPN expression was still present in the keratinocytes; and at day 22 (telogen phase) PDPN was detected in the lymphatic vessel but not hair follicles. Mice that were made to lack PDPN in the keratinocytes of their hair follicles showed increased hair follicle growth during the anagen phase of hair growth compared to mice that had normal levels of PDPN in their keratinocytes. These findings suggest that PDPN deletion promotes hair follicle cycling and growth and that inhibiting PDPN may prove useful for treating hair loss in animals and humans. The involvement of CLEC-2 in the DPDN's inhibition of hair growth was not determined in these studies.
Development of blood vessels, lymphatic vessels, and the heart