Apoptosis in the connective tissues of the periodontal ligament and gingivae of rat incisor and molar teeth at various stages of development

Apoptotic signaling in mouse odontogenesis

Apoptosis is an important morphogenetic event in embryogenesis as well as during postnatal life. In the last 2 decades, apoptosis in tooth development (odontogenesis) has been investigated with gradually increasing focus on the mechanisms and signaling pathways involved. The molecular machinery responsible for apoptosis exhibits a high degree of conservation but also organ and tissue specific patterns. This review aims to discuss recent knowledge about apoptotic signaling networks during odontogenesis, concentrating on the mouse, which is often used as a model organism for human dentistry. Apoptosis accompanies the entire development of the tooth and corresponding remodeling of the surrounding bony tissue. It is most evident in its role in the elimination of signaling centers within developing teeth, removal of vestigal tooth germs, and in odontoblast and ameloblast organization during tooth mineralization. Dental apoptosis is caspase dependent and proceeds via mitochondrial mediated cell death with possible amplification by Fas-FasL signaling modulated by Bcl-2 family members.

Age-dependent differential expression of apoptosis markers in the gingival tissue

OBJECTIVE

The current study was performed to test the hypothesis that periodontal disease produces age-dependent activation of apoptotic markers in the gingival tissues.

METHODS

To address the hypothesis a prospective experimental study was designed and twenty-two patients were enrolled. Out of the twenty-two patients, gingival tissue biopsies samples were obtained from active sites of ten and twelve periodontal-healthy (HS) and periodontal disease (PD, probing depths >5mm patients, respectively. The groups were further divided into 25-50 and <5 years age subgroups.

RESULTS

A significant decrease in the expression of TRADD (Tumour Necrosis Factor Receptor-Associated Death Domain) was observed in 25-50 years of PD group compared to the HS group. Bax (BCL(2)-associated X protein) expression in the PD groups was significantly decreased in PD 25-50 years age group but increased in the >50 years age group compared to respective HS age groups. PD patients of both 25-50 years and >50 years age exhibited a significant increase in the expression of Cytochrome C and Caspase-3 compared to the respective HS groups. The PD patients exhibited a stronger correlation with age in the expression of TRADD and Bax compared to the HS groups. Further analyses revealed that the expression of Caspase-3 correlated with an increase in the age of the healthy patients.

CONCLUSIONS

The data suggested that modulation of apoptotic cascade may contribute to the damage of gingival tissues particularly in PD patients >50 years age.

Inhibition of SFRP1 reduces severity of periodontitis

Porphyromonas gingivalis (P. gingivalis) is implicated as a major pathogen in periodontitis, a common infectious disease characterized by the inflammation and destruction of periodontal tissues. Secreted frizzled-related protein 1 (SFRP1) modulates apoptosis in different cell types. To characterize the roles of SFRP1 in periodontitis, we used a P. gingivalis-induced murine periodontitis model. Inflammatory responses were measured by morphometric and histomorphometric analysis, apoptosis assay, and immunohistochemistry. We found that P. gingivalis-infected mouse periodontal tissues expressed significantly more SFRP1 compared with those of control mice. Also, in P. gingivalis-infected animals, more apoptosis of inflammatory cells, fibroblasts, and bone-lining cells was observed compared with controls. Antibody experiments aimed at inhibiting SFRP1 expression in periodontitis resulted in a reduction of periodontal breakdown, inflammatory cell infiltrate, osteoclastogenesis, and apoptosis of inflammatory cells and fibroblasts. The results of our studies suggest that SFRP1 may be involved in the development of periodontitis, since inhibiting SFRP1 resulted in reduced periodontal breakdown.

Fluorosis: a new model and new insights

Fluoride is an effective agent for the prevention of dental caries. However, the mechanism of how excessive fluoride exposure causes fluorosis remains uncertain. Zebrafish (Danio rerio) exhibit periodic tooth replacement throughout their lives, thereby providing continuous access to teeth at developmental stages susceptible to fluoride exposure. Zebrafish teeth do not contain true enamel, but consist of a hard enameloid surface. Therefore, we asked whether zebrafish could be used as a model organism for the study of dental fluorosis. Scanning electron microscopy of fluoride-treated teeth demonstrated that the enameloid was pitted and rough, and FTIR analysis demonstrated that the teeth also contained a significantly higher organic content when compared with untreated controls. Furthermore, we demonstrate for the first time that decreased expression of an important signaling molecule (Alk8) in tooth development may contribute to the observed fluorotic phenotype, and that increased cell apoptosis may also play a role in the mechanism of fluorosis.

The effects of aging on the eruptive behavior of the mandibular incisor in male rats

Several studies have highlighted the lack of age changes in the extracellular matrix of the periodontal ligament, but more needs to be known about cellular and functional changes (including the effects upon eruption). For this study, impeded and unimpeded eruption rates were measured over a 2-week period for the mandibular incisors of a group of 24-month-old rats. The technique used for measuring eruption was similar to that described by Bryer [15]. Both impeded and unimpeded eruption rates were found to be significantly increased (p < .01) compared with those obtained from a group of rats aged 8 weeks. These changes might be related to changes in the mechanism(s) responsible for the generation of the eruptive force(s) or to changes in the resistance of the tissues to such forces.