The pulp contains a relatively large concentration of glycosaminoglycans and proteoglycans.180,181 Versican, a chondroitin-6-sulfate–rich proteoglycan, has been detected in high concentrations in peripheral pulp.182 Fibroblasts are distributed evenly throughout the middle regions of the pulp and concentrated beneath the odontoblastic layer in the coronal pulp of erupted teeth to form a cell-rich zone. The cell-rich zone also contains numerous major histocompatibility complex–positive dendritic cells that have an increased capacity for capturing and processing antigens.172 These cells have numerous cell processes that make contact with odontoblasts and nerves.183 Dendritic cells are part of the surveillance arm of the immune system (see chapter 13). Because the cell-rich zone makes its appearance after the tooth has erupted into the oral cavity and is limited in its extent to the coronal pulp (excluding the floor of the coronal pulp), it is believed to form as a defensive response to external stimulation.
Dental pulp cells respond to a variety of growth factors.184 Deoxyribonucleic acid synthesis in human dental pulp cells is stimulated by basic fibroblast growth factor and platelet-derived growth factor and is inhibited by interleukin 1 β. Transforming growth factor β stimulates the synthesis of collagen and fibronectin in cultures of pulp cells.184 Vitamin D stimulates pulp fibroblasts to express osteopontin, a phosphoprotein typically found in bone.185
Basic Science Correlation: The Secretory Pathway
During the 1960s and 1970s, the ultrastructure of the RER-Golgi system was characterized, and the morphologic aspects of a secretory pathway were established. It is now known that proteins destined to be exported from the cell, or to lysosomes and endosomes, are synthesized in the rough endoplasmic reticulum and transported to the Golgi complex.186 In the Golgi complex, proteins are posttranslationally modified, sorted, and packaged for further transport to their ultimate destination, whether it be a secretory granule, a primary lysosome, or the cell membrane.
No specific signal recognition event appears to be required for the transport of proteins from the RER to the Golgi apparatus. The only prerequisite is that the proteins undergo correct three-dimensional folding within the RER. Transport vesicles destined for the Golgi apparatus develop from smooth-membrane segments of the RER, called transitional elements.
The Golgi complex is subdivided into a cis-Golgi network, Golgi stacks, and a trans-Golgi network (Fig 2-12).187 The cis-Golgi network acts as a quality control gate, preventing the transport of defective proteins through the Golgi complex to the cell surface and/or secretion into the extracellular space. The small percentage of RER-resident proteins that escape during the formation of transport vesicles are recognized in the cis-Golgi network by their lysine–aspartic acid–glutamic acid–leucine amino acid sequence and are returned to the RER in a retrograde vesicular pathway (see Fig 2-12).188 Retrograde traffic also returns membrane lipids to the RER compartment. Transport from the RER to the Golgi apparatus requires microtubules. However, the retrograde pathway from the Golgi apparatus back to the RER does not depend on an intact microtubular network.
Fig 2-12 Secretory pathway from the rough endoplasmic reticulum (RER) to the cis-Golgi network (CGN), across the Golgi stacks, and into the trans-Golgi network (TGN), where proteins are directed to appropriate destinations. (arrows) Unidirectional anterograde vesicular transport. (dashed lines) Retrograde pathways used to retrieve membrane and proteins that have escaped from the RER. (Adapted from Rothman and Orci187 with permission from MacMillan Publishers.)
Secretory and cell membrane proteins undergo successive compartment-specific reactions during their transit through the Golgi stacks. Glycosyltransferase and glycosidases contained in the Golgi cisternae sequentially decorate the peptide backbone of the protein by the addition of carbohydrate side chains. These posttranslational modifications involve the addition of oligosaccharides by nitrogen-linkage to asparagine, and/or oxygen-linkage at serine and threonine residues. Formation of oxygen-linked glycans involves a two-step process consisting of the addition of N-acetyl-galactosamine, followed by the addition of galactose and sialic acid (N-acetyl-neuraminic acid). Studies have shown that the addition of N-acetyl-galactosamine occurs in transitional elements of the RER, while the addition of galactose and sialic acid occurs in the most mature cisternae of the Golgi apparatus.
The two-way traffic of vesicles from the RER to the Golgi complex, and from the Golgi complex to the cell membrane, requires numerous regulatory mechanisms.189 The complex machinery for sorting proteins and controlling vesicular traffic inside the cell began to be deciphered in the 1980s and was accelerated by the advent of newly discovered molecular biology techniques.190 Cell biologists view the Golgi complex as a dynamic system of membrane-bound compartments whose function requires constant intercompartmental vesicular exchange. Movement of substances from the RER to the cis-Golgi network, between Golgi stacks, and from the trans-Golgi network to the final target membrane is carried out in small transport vesicles that bud from surfaces of the donor compartment.191,192 A great deal of research is being focused on identifying the molecular nature of the sorting, docking, and fusion events needed for this operation.
The budding process requires the recruitment and attachment of specific coat proteins (coatomers) on the parent cisternal membrane to form a mechanochemical “patch” capable of deforming the membrane into a separate vesicle.193–195 As the vesicle forms, it concentrates a microscopic sample of specific cargo proteins from the cisternal fluid. Coatomer recruitment requires ATP, Ca2+, guanosine triphosphate (GTP), and several cytosolic proteins.196
In the first step of the process (Fig 2-13), a transmembrane protein in the donor membrane, guanine nucleotide–releasing protein (GNRP), interacts with a cytosolic GTP-binding protein called adenosine diphosphate ribosylation factor (ARF). In the cytosol, ARF is in its guanosine diphosphate (GDP)-bound state (ARF-GDP). When ARF-GDP interacts with GNRP, GDP is released and GTP is bound in its place. Subsequently, ARF-GTP undergoes conformational change, exposing a fatty acid chain that anchors ARF-GTP to the donor membrane.
Fig 2-13 Formation of a coatomer-coated membrane. The first step involves the insertion of guanine nucleotide–releasing protein (GNRP) into the membrane of the donor compartment. In the second step, GNRP reacts with adenosine diphosphate ribosylation factor (ARF), converting ARF–guanosine diphosphate (GDP) to ARF–guanosine triphosphate (GTP). Attached to the donor membrane, ARF-GTP is then able to bind coatomer proteins. As more coatomer proteins are bound to the site, a vesicle will start to form from the donor compartment by a budding process. Soluble N-ethylmaleimide–sensitive fusion attachment protein receptors (SNAREs) project from the surface of the transport vesicle (v-SNARE).
In step two, segments of the membrane covered by ARF-GTP favor the recruitment and attachment of coatomer proteins (see Fig 2-13). In mechanisms yet to be clarified, the coatomer-coated membrane is deformed and pinched off to form a coatomer-coated transport vesicle (Fig 2-14). Transport vesicles retain their coatomer coats until they begin docking to the appropriate target membrane.
Sorting products to their appropriate destinations requires specific signals to control the docking of transport vesicles with the correct target compartment. This is accomplished by transmembrane