Cells use integrins to adhere to a variety of extracellular matrix molecules and to communicate chemically in a bidirectional way with their environment. Information from the extracellular matrix is gathered when ligands bind to the extracellular portion of the integrins, producing conformational changes in the cytoplasmic portion of the molecules and thereby altering their interaction with adjacent cytoplasmic molecules. Ligand binding to integrins can also exert an intracellular effect through the activation of tyrosine kinases.
Conversely, the binding of certain cytoplasmic proteins to the cytoplasmic domain can induce conformational changes in the external part of the integrin molecules, affecting their affinity for extracellular ligands. Through this process, the cell can interact with its environment, creating adhesive contacts and/or activating specific differentiation cascades.
The expression of integrin receptors for laminin has been shown to oscillate between IEE and dental papilla ectomesenchyme during tooth formation.95 Whether integrin-laminin signaling pathways have a significant role in ameloblast differentiation remains to be determined. Additional discussions of the role of integrins in cell activation and muscle differentiation are contained in chapters 11, 13, and 14.
Syndecan Syndecans are integral membrane proteoglycans. Four types have been identified based on differences in the core protein. Each syndecan molecule consists of a short cytoplasmic domain, a helical hydrophobic domain inserted into the plasma membrane, and a large extracellular domain containing several glycosaminoglycan side chains.
Syndecan 1 is typically located in epithelia and in embryonic mesenchymal tissues, especially in areas of epithelial-mesenchymal interaction, such as in developing teeth.96 Because of its binding interaction with tenascin, it may play a role in condensation of ectomesenchymal cells to form the dental papilla.37 In addition to binding tenascin, syndecan 1 also binds fibronectin, and collagen types I, III, and V.
Syndecan 4 is the smallest and most widely distributed type of syndecan. It colocalizes with integrins in focal adhesions to extracellular fibronectin. Syndecans are not only matrix receptors but also coreceptors for growth factors and cytokines, capable of potentiating signal transduction events.
Fibronectin Fibronectin is a large extracellular glycoprotein with multiple binding sites capable of forming attachments to cells, collagen, heparin, fibrin, tenascin, bacteria, and other proteoglycans.97,98 Fibronectin has a dimeric structure composed of two equal polypeptide chains joined near their C-terminal by disulfide bonds. Binding sites on each chain have been identified for cell membrane integrins and a variety of extracellular matrix molecules (Fig 1-20). Fibronectin is a significant component of basement membranes in developing organ systems, where it stabilizes cells and thereby permits them to establish polarity and to undergo further differentiation. A good example of this type of interaction occurs during the differentiation of the preodontoblasts.
Fig 1-20 The elongated fibronectin molecule is made up of two similar subunits. Each consists of globular domains joined by flexible polypeptide sections. Specific binding sites have been mapped on the molecule for various cells and molecules as shown.
The interaction of cells with fibronectin is important not only during embryonic development but also in the migration and stabilization of cells in the adult organism. Fibronectin plays an important role in wound healing by interacting with fibrin to create a scaffold for the migration of fibroblasts. Fibronectin stimulates fibroblast invasion of collagen gels. The gelatin-binding domain of the fibronectin molecule is essential to this migratory action. The gelatin-binding domain segment interacts with a fibroblast surface integrin protein to induce a transition to the migratory phenotype.
The recognition site of the cell-binding domain of fibronectin has been identified to consist of the tripeptide, arginine-glycine–aspartic acid (the RGD sequence). This sequence binds to the cell membrane integrins (fibronectin receptors). The α5β1 integrin is the main fibronectin receptor. The association of integrin fibronectin receptors to extracellular fibronectin triggers the recruitment of cytoskeletal and signaling molecules to the plasma membrane site of attachment to form focal adhesions. Fibronectin is concentrated at the IEE basal lamina and along the cytoplasmic surface of preodontoblasts.4,99–101 The role of fibronectin and its receptor in odontoblast differentiation is discussed in chapter 2.
Laminin Laminin is a major constituent of the basal lamina complex. It is a large glycoprotein with a molecular weight of about 800,000 d. The laminin molecule is a heterotrimer of β1, β2, and α subunits. The three chains assemble to form a cross-shaped molecule (Fig 1-21).102 Laminin binds to type IV collagen, to heparan sulfate proteoglycans (perlecan) of the basal lamina, and to receptors in the cell membrane of various cells, especially epithelial cells. Laminin 5 subunits are expressed in the enamel organ, and the protein is localized in the basal lamina beneath the IEE.99,103
Fig 1-21 Structure of the laminin molecule.
Immunocytochemical studies reveal temporospatial changes in laminin subunit expression during odontoblast and ameloblast differentiation.103 The results of tissue recombination experiments suggest that the dental ectomesenchyme controls the expression of laminin in the dental epithelium.104 Laminin is discussed further in chapter 4.
Tenascin Tenascin, a large extracellular matrix molecule, also known as cytotactin and hexabrachion, is made up of six polypeptide chains assembled to form a sixarm structure capable of interacting with a variety of cells and extracellular matrix molecules. Because the six polypeptide chains appear to represent separate gene products, it has been suggested that tenascin molecules may have tissue specificity.
Tenascin binds to cell surface proteoglycan (syndecan). Expression of tenascin in dental ectomesenchyme coincides with the concentration of the dental papilla.100,105 It has been demonstrated that tenascin prevents the migration of certain neural crest cells, causing them to assume a round shape characteristic of stationary cells.
Nidogen Nidogen (also called entactin) is a rod-shaped protein consisting of a single polypeptide chain, approximately 30 nm long, with globular domains at each end and one centrally located domain.106,107 Because nidogen binds with high affinity to collagen IV and laminin, it has an organizing role in assembly of the basal lamina. Nidogen also binds perlecan, the large heparan sulfate proteoglycan of the basal lamina.
The coexpression of laminin 1 and nidogen results in a relatively stable basal lamina. In general, laminin is produced by epithelial cells and nidogen by mesenchymal cells. Temporospatial differences in the expression of laminin and nidogen are thought to have significance in epithelial-mesenchymal tissue remodeling because of resulting changes in the stability of the basement membranes.108
Basal lamina The basal lamina is a supramolecular aggregate of type IV collagen, laminin, fibronectin, nidogen, and perlecan. They form a macromolecular network with the dual function of supporting epithelial cells and providing for a permeability barrier or filter. Meyer et al109 have reviewed the role of the basal lamina in tooth development and odontoblast differentiation. The basal lamina is discussed in detail in chapter 4.