Cathepsin D activity in isolated odontoblasts

Does inhibition of proteolytic activity improve adhesive luting?

Endogenous enzymes may be involved in the biodegradation of adhesive restoration-tooth interfaces. Inhibitors of matrix metalloproteinases (MMPs) have been suggested to retard the bond-degradation process. Limited data are available on whether composite cements may also benefit from MMP inhibitors. Therefore, the aim of this study was to determine the effect of two MMP inhibitors--chlorhexidine digluconate (CHX) and galardin--on the microtensile bond strength (μTBS) of two self-adhesive composite cements to dentin. Ceramic specimens were cemented to bur-cut dentin surfaces using the self-adhesive composite cements RelyX Unicem 2 (3M ESPE) or Clearfil SA (Kuraray), or the etch-and-rinse composite cement Nexus 3 (Kerr) that served as the control. The surfaces were left untreated or were pretreated with MMP inhibitors (2% CHX or 0.2 mM galardin). The μTBS was determined 'immediately' and upon ageing (water storage for 6 months). Statistical analysis revealed a significant effect of the factors 'composite cement' and 'storage', as well as all interactions, but no effect of the MMP inhibitors. After 6 months of ageing, the μTBS decreased for all cements, except for the multistep etch-and-rinse luting composite when it was applied without MMP inhibitors. The MMP inhibitors could not prevent the decrease in μTBS upon ageing and therefore do not improve the luting durability of the composite cements tested.

Matrix metalloproteinase-8 (MMP-8) is the major collagenase in human dentin

OBJECTIVE

Previously an unidentified collagenolytic metalloprotease together with gelatinase (matrix metalloproteinase-2, MMP-2), and enamelysin (MMP-20) have been detected in human dentin. The aim of the study was to characterize dentinal collagenolytic enzymes. Furthermore, we hypothesized that the dentinal MMPs are protected by the mineral phase, and studied the stability of dentinal MMPs.

DESIGN

To characterize dentinal collagenolytic enzymes, we used Western blotting with specific antibodies against MMP collagenases (MMP-1, -8, and -13) and cathepsin K. MMP-8 immunofluorometric assay (IFMA) was also used for MMP-8 detection, and functional collagenase activity was examined with type I collagen degradation assay. The stability of dentinal MMPs was examined by autoclaving dentin blocks before protein extraction and subsequent examination of protein levels and the activities of dentin collagenase and gelatinases.

RESULTS

MMP-8 (collagenase-2) was detected in dentin both with Western blot and IFMA, and dentinal samples also cleaved the intact type I collagen into characteristic 3/4(alphaA)-cleavage products in vitro. No other collagenases or cathepsin K were detected. In autoclaved samples no MMP-8 was found, but gelatinase activity was observed in protein fractions of mineralized dentin.

CONCLUSIONS

MMP-8 represents the major collagenase in human dentin. Unlike MMP-8, dentinal gelatinases can be detected after autoclave treatment of dentin, indicating their high resistance to external sample treatment procedures.

Structure and organization of odontoblasts

Differentiation of odontoblasts involves cell-to-cell recognition, contact stabilization involving the formation of attachment specializations, cytoplasmic polarization, development of the protein synthetic and secretory apparatus, and the active transport of mineral ions. The secretory odontoblast is characterized by an extensive rough-surfaced endoplasmic reticulum, a highly developed Golgi complex, and the presence of specific secretion granules. Type I collagen, a major constituent of dentin matrix, appears to be secreted by the odontoblast into predentin at the proximal portion of the odontoblast process, the major cytoplasmic process extending from the odontoblast cell body into the dentin. The odontoblast process contains a rich network of microtubules and microfilaments. The proximal portion of the process is also a site of fluid-phase endocytosis. Adjacent odontoblasts are held together by numerous macula adherens junctions and a well-developed distal junctional complex adjacent to be predentin. Junctional strands of the occludens type have been observed to be a component of this junctional complex. Tracer studies employing horseradish peroxidase indicate that this junctional complex does not form a tight barrier to the diffusion of tissue fluid from the interodontoblast spaces into the predentin. Many well-developed gap junctions are formed between adjacent odontoblasts and between odontoblasts and the fibroblasts that make up the subodontoblastic layer. Ca-ATPase activity is demonstrated in the Golgi complex and mitochondrial cristae and along the distal plasma membranes of odontoblasts. ALPase activity is also intense along the entire odontoblast cell surface. The osmium tetroxide-pyroantimonate technique for calcium localization demonstrates prominent reaction precipitates in mitochondria of odontoblasts. Energy-dispersive x-ray microanalysis of anhydrously fixed and processed odontoblasts detected Ca and P peaks throughout the cytoplasm. A sulfur peak is noted in the distal cytoplasm of odontoblasts and in matrix vesicles. Together, these results demonstrate the complexity and variety of cell functions involved in dentinogenesis.

Morphological alterations in dental and periodontal tissues in murine mucopolysaccharidosis type VII

Mucopolysaccharidoses (MPSs) in humans are frequently associated with tooth and periodontal aberrations. Although the cause is known, namely, enzyme deficiency, the pathophysiology of these alterations is not well defined. A murine MPS VII (beta-glucuronidase deficiency) model has earlier been identified with morphological, genetic, and biochemical characteristics that closely mimic those of human MPS VII. The present investigation describes the histopathological alterations in dental and periodontal tissues from such mutant mice. Homozygous animals were identified by external phenotypical features and as being beta-glucuronidase deficient by a fluorometric assay of liver samples. In the incisor and the periodontium, abnormalities were evident in both cells and the extracellular matrices. Mesenchyme-derived cells were more aberrant than epithelial cells. Moreover, undifferentiated cells appeared unaffected, whereas actively synthesizing and resorbing cells were distended by virtually empty or granular material-containing vacuoles, the content presumably being glycosaminoglycans. The cells most affected were those in which macromolecular turnover is normally the highest, namely, odontoblasts, postsecretory ameloblasts, and periodontal ligament fibroblasts. Extracellularly, predentin displayed abnormal collagen fibrils, whereas mineralization defects occurred in both dentin and enamel. This murine model of MPS VII provides a good tool for understanding the pathophysiology of this disease in bone, periodontium, and teeth.

Dentinogenesis

The formation of dentin, dentinogenesis, comprises a sophisticated interplay between several factors in the tissue, cellular as well as extracellular. Dentin may be regarded as a calcified connective tissue. In this respect, as well as in its mode of formation, it is closely related to bone. Using dentinogenesis as an experimental model to study biomineralization provides several practical advantages, and the results may be extrapolated to understand similar processes in other tissues, primarily bone. After describing dentin structure and composition, this review discusses items such as the morphology of dentinogenesis; the dentinogenically active odontoblast, transport, and concentrations of mineral ions; the constituents of the dentin organic matrix; and the presumed mechanisms involved in mineral formation.

Suramin-induced mucopolysaccharidosis in rat incisor

Two weeks after a single injection of suramin, the secretory and post-secretory ameloblasts of the rat incisor were filled with large lysosome-like vacuoles. At the light-microscope level, these vacuoles were positively stained with Alcian blue when MgCl2 was used at a critical electrolyte concentration varying between 0.1 and 0.3 M, whereas no staining appeared when MgCl2 varied between 0.7 and 0.9 M. Hyaluronidase digestion markedly reduced but did not totally abolish the staining, indicating that glycosaminoglycans were accumulated inside these vacuoles. Examination of these cells with the electron microscope revealed a polymorphic population of large vesicles, filled to various degrees with cetylpyridinium chloride (CPC)-positive and malachite green aldehyde (MGA)-positive material. The same pattern was observed in secretory odontoblasts but to a lesser extent. In the extracellular matrix, suramin-induced alterations appeared as large defects occurring during enamel formation. In predentin and dentin, the number and/or size of electron-dense aggregates resulting from CPC and MGA fixation, were enhanced in the suramin-injected rats. These aggregates were largely reduced or suppressed respectively by hyaluronidase digestion and chloroform/methanol treatment of the sections. The accumulation of glycosaminoglycans and phospholipids reported here inside ameloblasts and odontoblasts and in predentin and dentin supports the occurrence of suramin-induced mucopolysaccharidosis and lipidosis in this experimental animal model.

Immunolocalization of cathepsin D in dental tissues

Cathepsin D antigenicity was localized at the light and electron microscopic levels within dental cells, but not in extracellular matrix. Different intracellular sites for cathepsin D were found depending on the cell type: the enzyme was detected in secretory vesicles of the odontoblasts and in the lysosome-like structures of the ameloblasts. Otherwise, these results suggest that the secretory vesicles of the odontoblasts may contain both cathepsin D and type I collagen. These data might implicate cathepsin D in the enamel and the dentin formations.

Further evidence of an intravesicular Ca2+- pump in odontoblasts from rat incisors

Intracellular vesicles containing alkaline phosphatases were isolated from isolated odontoblasts using several centrifugation techniques, gradient media and filtering procedures. With a combined centrifugation technique using 0.32 M sucrose layered on 1.23 M sucrose, a fraction containing alkaline phosphatases with a 36-40-fold increased specific activity was obtained. This fraction also revealed a high Ca2+-accumulating ability. The vesicle fraction was totally free from mitochondria but to some extent contaminated by lysosomes. Characteristics of Ca2+-uptake were obtained. The Ca2+-uptake was maximal at 37-40 degrees C whereas no Ca2+-accumulated at 4 degrees C. Temperatures above 40 degrees C strongly inhibited Ca2+-uptake. ATP was the most potent stimulator of Ca2+-uptake whereas ITP, GTP, CTP, ADP, PPi and AMP also promoted Ca2+-uptake. Cysteine, EDTA and Triton X-100 were inhibitory to Ca2+-uptake. A correlation between alkaline phosphatases in intracellular vesicles as well as their relation to extracellular matrix vesicles and to the mineralization process is suggested.

Localization of fibronectin during dentinogenesis in rat incisor

Fibronectin (FN) was localized in the dental pulp and predentine at different stages of dentinogenesis by indirect immunofluorescence. Fibronectin was present in the basement membrane between the inner enamel epithelium and the underlying dental mesenchyme and also in mantle predentine. Fibronectin was absent from predentine during further (circumpulpal) dentine formation, indicating that the FN molecule is not directly involved in mineralization. The findings emphasize the difference between the mantle and circumpulpal dentine formation. Fibronectin was localized in the odontoblast layer at the level where the cell processes leave the cell bodies and where the odontoblasts adhere to each other by junctional complexes.

Identification of hyaluronidase activity in rabbit dental pulp

A hyaluronidase activity was demonstrated in rabbit dental pulp. The optimum pH of the enzyme was 3.8. The enzyme activity was enhanced by protamine and poly-L-lysine and was inhibited by iodoacetamide, ferric ion, and ferrous ion in decreasing order. The product of the enzyme reaction was identified as tetrasaccharide. From these results it was concluded that the enzyme exists in pulp tissue and is functioning for degradation of proteoglycans in situ.

Cathepsin D: ultra-immunohistochemical localization in dentinogenesis

Cathepsin D was purified from rat liver using a new affinity chromatographic method, based on the coupling to the specific inhibitor pepstatin. This preparation was used for the production of specific antibodies from rabbit. The purified IgG fraction was conjugated to horseradish peroxidase in a two-step coupling procedure and used for electron microscopic immunohistochemistry of the odontoblast-predentine region of the rat incisor. Precipitates, indicating the presence of cathepsin D, were seen in the odontoblast, odontoblast process, and in the extracellular unmineralized matrix, the predentine. The observations are discussed in relation to proteoglycan degradation at the mineralization front simultaneous with crystal formation, and in relation to the function of lysosomal enzymes in the turnover of connective tissue.

Odontoblast metabolism in rats deficient in vitamin D and calcium. III. Protein synthesis in vitro

Dentinogenically active rat incisor odontoblasts were dissected out from animals fed a low calcium, vitamin D free diet (R 25). Protein synthesis by these cells was studied by means of short-time incubation in an in vitro system in the presence of the radioactive labeled precursors L-leucine, L-fucose and L-proline. A significantly increased leucine and proline incorporation into the protein synthesized was noted in the cells from rats fed the deficient R 25 diet compared with odontoblasts from rats fed an adequate control diet (R 47). No difference between the two groups was found when fucose was utilized as a precursor. The well known increase in predentin width when feeding a rachitogenic diet may thus be explained by an increase in organic matrix synthesis in addition to the possible negative direct effects of lowered serum Ca content. Prior to this study, the behavior of proline as a precursor in the in vitro system was studied. The possibility of separating leucine-labeled proteins synthesized in the in vitro system by means of SDS-polyacrylamide gel electrophoresis was also shown.

Purification fo cathepsin D by AH-sepharose affinity chromatography

A rapid and reliable method for coupling the protease inhibitor pepstatin to AH-Sepharose 48 was developed. The matrix prepared was used to purify cathepsin D from rat liver. The enzyme was eluted in one fraction and proved to be pure by gel electrophoresis, two types of ion exchange chromatography, molecular sieve chromatography, and immunologically homogenous by immunoelectrophoresis. This method is more rapid and gives a higher yield than previous techniques. The possibility to use this technique for the purification of other enzymes inhibitable by pepstatin should be considered.