Proteoglycans (PGs), which consist of a central core protein to which one or more glycosaminoglycan (GAG) side chains are covalently bound, are distributed throughout the extracellular matrix (ECM), at the cell surface, and in intracellular storage granules.
The cell-surface PGs are diverse in structure and function. Via their extracellular GAG chains they bind a multitude of ECM molecules. Via their cytoplasmic domain they interact with the cytoskeleton and potential downstream signal transducers. The majority of currently known cell-surface PGs is associated with one of two classes of core proteins. The integral transmembrane cell-surface PGs are the syndecan family, betaglycan (transforming growth factor β type III receptor) and CD44 (lymphocyte homing receptor or epican). The other class of cell-surface PGs is the glypican family which is anchored to cell membrane via a glycosylphosphatidylinositol link.
Many of the above cell-surface PGs have been studied in a variety of cells and tissues. However, to date, there has been no comprehensive study of the expression of these important molecules by cells of the periodontium. The three major resident cells (periodontal ligament fibroblasts (PDLF), osteoblasts (OB), and gingival connective tissue fibroblasts (GF)) which each contribute to the unique functions of the various tissues within the periodontium have been selected to study the expression of periodontal cell-surface PGs. The hypothesis was that a number of cell-surface PGs are expressed by periodontal cells and this may be related to the source and function of the cell.
An initial in vitro study was carried out (chapter 3) utilizing Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR) methods to investigate the expression of cell-surface PGs (syndecan-1, -2, -4, glypican and betaglycan) by human periodontal cells (PDLF, OB, and GF). The results demonstrated the expression of the protein cores for syndecan-1 (43 kDa), syndecan-2 (48 kDa), syndecan-4 (35 kDa), glypican (64 kDa), and betaglycan (100-110 kDa). RT-PCR results confirmed that all of these cells produced mRNA for the cell-surface PGs under study, of which syndecan-2 showed a significant difference in expression between the PDLF and OB with GF. This finding indicated that human periodontal cells produce, at least five different cell-surface PGs. The differential expression in syndecan-2 amongst these cells may be related to the source and function of the cell.
The expression and distribution of cell-surface PGs (syndecan-1, -2, -4, glypican and betaglycan) in the normal Lewis rat periodontium was investigated using an immunohistochemical technique (chapter 4). The results demonstrated the expression and distribution of all studied cell-surface PGs in the suprabasal gingival epithelium, as well as the soft and hard connective tissues of the periodontium. Marked expression of syndecan-2 was noted in the acellular cementum and also at the mineralization fronts of alveolar bone. This observation of differential expression and distribution of syndecan-2 in hard connective tissues, correlated well with the earlier in vitro studies and further reinforces the hypothesis that syndecan-2 expression is tissue and function specific.
Using RT-PCR, the regulation by growth factors and cytokines on the expression of cell-surface PG gene by cultured human PDLF was studied (chapter 5). Cells were treated with various concentrations of serum, fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF-BB), transforming growth factor (TGF-βi), interleukin (IL-1β), and interferon (IFN-γ) and the effects of these agents on the mRNA expression of five cell-surface PGs (syndecan-1, -2, -4, glypican and betaglycan) were examined. The results demonstrated that the gene expression of studied cell-surface PGs were differentially regulated by growth factors and cytokines in PDLF. The data lend support to the notion of distinct functions of these cell-surface PGs.
In chapter 6, syndecan-1 and -2 gene expression was investigated in human periodontal cells (PDLF, OB, and GF) in response to PDGF-BB and TGF-βi, separately or in combination, in the prolonged presence of IL-iβ by Northern blot analyses. The results demonstrated that the three cell lines regulated the expression of syndecan-1 and -2 in response to the stimulants in different manners. The data lend support to the distinct functions of the studied cell-surface PGs which may be correlated to the source and function of the cell.
The results of this series of investigations suggest that periodontal cells (PDLF, OB, and GF) are able to synthesize, at least five cell-surface PGs (syndecan-1, -2, -4, glypican, and betaglycan). The differences of expression and distribution of studied cell-surface PGs by specific cells of the periodontium indicate the potential for a relationship to the source of the cell, as well as lending support to the notion of distinct functions for these cell-surface PGs. It is clear that among the studied cell-surface PGs, syndecan-2 is uniquely expressed and distributed in the hard tissue of the periodontium which may be related to cellular function during hard tissue formation and mineralization.
The findings of this study contribute to a broader understanding of possible functions related to the source of the cells which may, in part, impact on the way of viewing the structure and cellular function in health and disease of the periodontium.