| Matrix metalloproteinases (MMPs) and their specific tissue inhibitors (TIMPs) in mature human odontoblasts and pulp tissue | ||
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The dentin organic matrix primarily consists of fibrous collagens and other proteins such as proteoglycans, phosphoproteins and phospholipids, etc. The matrix provides a framework for mineralization. Collagens comprise 90% of the dentin matrix, and are principally type I (Gage et al. 1984, Lukinmaa & Waltimo 1992). Type I collagen is composed of two identical α1(I) chains and one α2(I) chain, and a glycine in every third amino acid position in an individual chain is needed for the formation of a triple helix structure (Beier & Engel 1966, Kielty et al. 1993). Proα2(I) mRNA has been shown to be expressed by mature human odontoblasts (Lukinmaa et al. 1992), whereas the expression of other collagen chain coding mRNA of proα1(I) has not been studied in fully developed human odontoblasts.
Type I collagen is synthesised as a larger procollagen, which contains extensions at both the N- and C-terminal ends, called the aminoterminal and carboxyterminal propeptides, which prevent premature collagen aggregation into fibrils. After procollagen secretion from cells, extracellular modification takes place, and propeptides are removed by specific proteinases and mature collagen molecules aggregate into a fibrous matrix (Kielty et al. 1993), which then serves as a support for mineral deposition.
Type I trimer, a more unusual type I collagen consisting of three α1(I) chains, is synthesised by odontoblasts actively forming mineralized dentin in rodent and bovine teeth. However, whether this occurs in the human tooth is unknown (Munksgaard 1979, Sodek & Mandell 1982).
Type III collagen, a homopolymer of three α1(III) chains, is a conspicuous constituent of soft connective tissues, such as pulp tissue, where it comprises approximately half of the collagen matrix (Shuttleworth et al. 1978, van Amerongen et al. 1983). In addition, there is strong evidence that calcified tissues are also able to express type III collagen, as Karjalainen and colleagues (1986) have shown that mature and intact human odontoblasts produce type III collagen after tooth development. Type III procollagen has been observed to be transiently located in human predentin during matrix formation, but not in mineralized dentin (Becker et al. 1986). The role of type III collagen in normal physiological dentin mineralization is unknown and awaits clarification. Type III collagen may be a more relevant constituent of the abnormal dental matrix, since it has been detected in dentinogenesis imperfecta patients (Waltimo et al. 1994). Type III collagen has also been detected in reparative dentin of carious human teeth (Karjalainen et al. 1986, Magloire et al. 1988b).
Additionally, other collagens may exist in the organic matrix of human dentin, since some expression of type V has been observed in the predentin of mature human teeth but not in dentin (Lukinmaa & Waltimo 1992). Instead, type VI was detected both in predentin and dentin of intact teeth (Becker et al. 1986), and it has also been found in the teeth of dentinogenesis imperfecta patients (Waltimo et al. 1994).
Proteins other than collagens comprise the remaining 10% of the dentin organic matrix. In addition, a minor part of the dentin matrix is composed of lipids, which possibly participate in mineral formation (Linde & Goldberg 1993). Of the noncollagenous proteins, dentin phosphoprotein (DPP; phosphophoryn) and dentin sialoprotein (DSP) represent the most abundant dentin-specific acid proteins in the dental matrix (Gu et al. 2000). A single gene encodes both, and after protein synthesis they are cleaved into the separate proteins of DPP and DSP. DPP is a highly phosphorylated protein, hydrophilic in character. It is capable of binding a large amount of calcium, facilitating the initial mineralization of the organic collagen frame. DPP is secreted by odontoblasts just ahead of the mineralization front (Butler et al. 1979, Takagi et al. 1986). The function of DSP is not yet known, but possibly it also has a role in the matrix mineralization reaction (Gu et al. 2000). Proteoglycans, such as decorin, bigclycan, fibromodulin and lumican, which carry glycosaminoglycan (GAG) carbohydrate side chains within their structures, comprise another sizeable portion of the noncollagenous proteins (reviewed in Embery et al. 2001). Since proteoglycans are also able to bind calcium (Embery et al. 1998), they may play a part in mineralization of the organic matrix of dentin, together with acid phosphoproteins.