PDPN
PDPN, i.e., podoplanin, 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 T1α protein or E11 antigen in rats; c) aggrus or gp40 in canines; and d)' aggrus in hamsters and cows. Human PDPN is encoded by the PDPN gene located on the "p", i.e., short, arm of chromosome 1, region 3, band 1. This gene directs the formation of PDPN messenger RNA 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 endothelial cells lining the lymphatic system but not endothelial cells lining blood vessels; c) reticular cells and epithelial cells 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 ''' 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. 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. CLEC-2 is a member of the C-type lectin receptors in the superfamily of pattern recognition receptors. It is expressed on: the surface membranes of megakaryocytes, platelets, dendritic cells, follicular dendritic cells, mesothelial cells, epithelial cells in lymphatic vessels, and cancer-associated fibroblasts. 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. 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. CLEC-2 is also activated by the human immunodeficiency virus, rhodocytin, hemin, galectin-9, dextran sulfate, sulfated polysaccharides, fucoidan, ketacine, S100A13, CLEC7A, and the soot, carbon, and other particles in the exhaust gas of diesel engines.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, angiogenesis, immune surveillance 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 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 and 5 after hair removal. PDPN was expressed in lymphatic vessels but not the hair follicles' keratinocytes. At days 8 and 12, however, PDPN was expressed in hair follicle keratinocytes and stem cells. At day 18, PDPN expression was still present in the keratinocytes; and at day 22 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 been determined in these studies.Development of blood vessels, lymphatic vessels, and the heart
Mouse embryos made deficient in PDPN, CLEC-2, the tyrosine-based activation residues in CLEC-2's cytoplasmic domain, or the cell signaling molecules activated by PDPN's binding to CLEC-2, i.e., Syk, SLP-76, or PLCG2 the lymphatic endothelial cells did not express PDPN, b) the blood platelets did not express CLEC-2, or c)''' PDPN-bound CLEC-2 lacked the tyrosine residues that activate platelets or one of the cited platelet-activating pathways. Other studies suggested that the activation of CLEC-2 by the PDPN on lymphatic endothelial cells causes the release of one or more TGF-β proteins that inhibit the migration and proliferation of the lymphatic endothelial cells which otherwise would facilitate blood–lymphatic vessel separation. Studies on these vascular deficiencies in brain tissues reported that early in their embryonic development mice embryos deficient in PDPN or CLEC-2 developed spontaneous hemorrhages throughout their forebrains, midbrains, and hindbrains. This appeared due to a defect in the recruitment of pericytes. Pericytes lie adjacent to vascular endothelial cells and act to protect these cells, alter the blood flow in the developing vessels, regulate the tightness of the blood–brain barrier, and influence new blood vessel formation. It has also been noted that PDPN-deficient or CLEC-2-deficient mice developed brain aneurysms and brain hemorrhages during their embryonic gestation. Treating the mothers carrying PDPN-deficient embryos with a combination of two inhibitors of platelet activation, aspirin and ticagrelor, almost completely blocked the development of these brain hemorrhages. Finally, mouse embryos made to lack PDPN also had a small proepicardial organ, reduced sizes of the cardiac muscle, and defects in their developing hearts' atrium dorsal wall and septum.