Regulation of reactionary dentin formation by odontoblasts in response to polymicrobial invasion of dentin matrix

Abstract

Odontoblast synthesis of dentin proceeds through discrete but overlapping phases characterized by formation of a patterned organic matrix followed by remodelling and active mineralization. Microbial invasion of dentin in caries triggers an adaptive response by odontoblasts, culminating in formation of a structurally altered reactionary dentin, marked by biochemical and architectonic modifications including diminished tubularity. Scanning electron microscopy of the collagen framework in reactionary dentin revealed a radically modified yet highly organized meshwork as indicated by fractal and lacunarity analyses. Immuno-gold labelling demonstrated increased density and regular spatial distribution of dentin sialoprotein (DSP) in reactionary dentin. DSP contributes putative hydroxyapatite nucleation sites on the collagen scaffold. To further dissect the formation of this altered dentin matrix, the associated enzymatic machinery was investigated. Analysis of extracted dentin matrix indicated increased activity of matrix metalloproteinase-2 (MMP-2) in the reactionary zone referenced to physiologic dentin. Likewise, gene expression analysis of micro-dissected odontoblast layer revealed up-regulation of MMP-2. Parallel up-regulation of tissue inhibitor of metalloproteinase-2 (TIMP-2) and membrane type 1- matrix metalloproteinase (MT1-MMP) was observed in response to caries. Next, modulation of odontoblastic dentinogenic enzyme repertoire was addressed. In the odontoblast layer expression of Toll-like receptors was markedly altered in response to bacterial invasion. In carious teeth TLR-2 and the gene encoding the corresponding adaptor protein MyD88 were down-regulated whereas genes encoding TLR-4 and adaptor proteins TRAM and Mal/TIRAP were up-regulated. TLR-4 signalling mediated by binding of bacterial products has been linked to up-regulation of MMP-2. Further, increased expression of genes encoding components of the TGF-β signalling pathway, namely SMAD-2 and SMAD-4, may explain the increased synthesis of collagen by odontoblasts in caries. These findings indicate a radical adaptive response of odontoblasts to microbial invasion of dentin with resultant synthesis of modified mineralized matrix.

Role of Dentin MMPs in Caries Progression and Bond Stability

“…Even in healthy teeth, dentinal tubules have high gelatinolytic activity (Mazzoni et al 2012), and MMP-2 has been demonstrated in dentinal fluid (Zehnder et al 2011). Caries stimulates MMP-2 expression in human odontoblasts in vivo (Charadram et al 2012). MMP-2 seems to be actively secreted by odontoblasts in response to carious insult (Boushell et al 2011), resulting in the differential expression of this protease in sound, caries-affected, and caries-infected dentin (Toledano et al 2010;Charadram et al 2012).…”

“…Caries stimulates MMP-2 expression in human odontoblasts in vivo (Charadram et al 2012). MMP-2 seems to be actively secreted by odontoblasts in response to carious insult (Boushell et al 2011), resulting in the differential expression of this protease in sound, caries-affected, and caries-infected dentin (Toledano et al 2010;Charadram et al 2012). Increased levels of MMP-9 (Zehnder et al 2011) and MMP-20 (Sulkala et al 2002) in dentinal tubules under caries lesions have also been demonstrated.…”

Bone Sialoprotein, Matrix Metalloproteinases and Type I Collagen Expression after Sealing Infected Caries Dentin in Primary Teeth

“…In demineralized dentin, the literature has suggested that remnant intrafibrillar mineral crystallites that are present within the collagen might act as sites for apatite nucleation and re-growth [Bertassoni et al, 2011]. BSP provides the connection between collagen and the inorganic phase of dentin due to its high affinity for calcium and hydroxyapatite [Charadram et al, 2012] and has been identified around dentin tubules from caries dentin [Boushell et al, 2011]. In fact, the higher expression of BSP after cavity sealing, particularly around dentin tubules, may be considered as evidence of the remineralization process to which the carious dentin is susceptible after sealing.…”

“…The repair and regeneration that occurs in the dentin-pulp complex is similar to the natural wound healing responses seen in many of the body's systems [Smith et al, 2012], where MMPs play a fundamental role [Charadram et al, 2012]. So far, few authors have attempted to investigate the expression of MMPs and other non-collagenous proteins, such as bone sialoprotein (BSP), in the healing process of dentin after incomplete caries removal and cavity sealing.…”

Biochemical composition of carious dentin and different layers of sound dentin

“…The process of dental caries results in demineralization of hard dental tissues, leading to a considerable reduction in their mineral content (1)(2)(3)(4)(5)(6). Calcium, inorganic phosphate and fluoride are the main constituents of the mineral phase of dentin (2,(7)(8)(9).…”