Expression of podoplanin in the mouse salivary glands
Introduction
Podoplanin, a 43-kDa transmembrane glycoprotein, is one of the most highly expressed lymphatic-specific genes and is regulated by homeobox gene Prox1.1, 2, 3, 4 The podoplanin expression in lymphatic endothelium was first reported as E11 antigen and was further identified as podoplanin because of the expression in kidney glomerular epithelial cells (podocytes).5, 6Podoplanin is homologous to T1α encodes an antigen expressed at alveolar type I cells in rat lung.7, 8Podoplanin−/− mice die at birth because of respiratory defect and congenital lymphedema due to the failure in lymphatic pattern formation. In the level of cultured cell it has been indicated that the lymphatic tube formation is based on the cell adhesion with podoplanin at the cell–cell contact.9
On the other hand it is well established that the mouse parotid, submandibular, and sublingual glands are composed of serous, seromucous, and mucous-dominant mixed acinar cells, respectively.10, 11, 12 The determinant for the viscosity of saliva is the glycoprotein mucins which protect mucosal cell membranes against proteases. The submandibular gland secretes 30% of the salivary mucins while sublingual and a large number of minor glands of palate, cheeks and lips secrete 70%. Concentration of mucin secreted by the sublingual gland is higher than that secreted by the submandibular gland, while the secretion of parotid gland is almost devoid of mucins.13, 14, 15 It has been reported that podoplanin is resistant to proteases because of the negatively charged mucin-type protein.6, 14 It is thought that the expression of transmembrane protein podoplanin may contribute to protecting salivary gland cells from protease by covering the cell surface with mucin. The expression of podoplanin in the several epithelial cells, such as epidermis and alveolar epithelia, has been noticed but little is known on the expression in salivary glands.16, 17
Platelet-endothelial cell adhesion molecule-1 (PECAM-1) is a well-established endothelial cell marker generally expressed on leukocytes and endothelial cells. The PECAM-1 is a 130-kDa type I transmembrane glycoprotein belonging to the immunoglobulin superfamily and acts as an adhesion molecule.18, 19, 20, 21 In this study the expression of PECAM-1 was immunohistochemically tested to discriminate lymphatic vessels from salivary gland cells since the lymphatic endothelium expresses both podoplanin and PECAM-1. The study here was designed to investigate the distribution of cells expressing podoplanin in mouse major salivary glands.
Section snippets
Materials and methods
Eight-week-old wild-type male mice (C57BL6/J, n = 5) purchased from the Jackson Laboratory (Bar Harbor, ME, USA) were used. The collection of the tissue was conducted after euthanasia by intraperitoneal injection with sodium pentobarbital (10 ml/kg, Nembutal, Abbott Laboratories, North Chicago, IL). Mice were perfused through the heart with 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4). The protocol for animal use was reviewed and approved by the animal experiment committee of Fukuoka
Immunohistochemistry of the mouse kidney and tongue
The mouse kidney and tongue tissue was used to test the specificity of anti-podoplanin (Fig. 1). The immunostaining with anti-PECAM-1 was simultaneously performed to identify lymphatic vessels because anti-podoplanin also reacts to the lymphatic endothelium. Reaction products with anti-PECAM-1 were only detected on blood vessels including glomerular arteriole and on lymphatic vessels. Reaction products with anti-podoplanin were only detected on podocytes and lymphatic vessels.
Immunohistochemistry of the mouse major salivary glands
In the mouse
Discussion
Podoplanin is a transmembrane protein expressed in podocytes, alveolar epithelial type I cells, mesothelial cells and lymphatic endothelial cells.1, 6 In this study the strong reaction with anti-podoplanin was detected on podocytes and lymphatic vessels but not on blood vessels, uriniferous tubules, and muscle and connective tissue, suggesting that the specificity of the antibody for podoplanin is reliable (Fig. 1). The expression of podoplanin was rarely found in serous acini of the parotid
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