Ectopic Expression of Dentin Sialoprotein during Amelogenesis Hardens Bulk Enamel

Abstract

Dentin sialophosphpoprotein (Dspp) is transiently expressed in the early stage of secretory ameloblasts. The secretion of ameloblast-derived Dspp is short-lived, correlates to the establishment of the dentinoenamel junction (DEJ), and is consistent with Dspp having a role in producing the specialized firstformed harder enamel adjacent to the DEJ. Crack diffusion by branching and dissipation within this specialized first-formed enamel close to the DEJ prevents catastrophic interfacial damage and tooth failure. Once Dspp is secreted, it is subjected to proteolytic cleavage that results in two distinct proteins referred to as dentin sialoprotein (Dsp) and dentin phosphoprotein (Dpp). The purpose of this study was to investigate the biological and mechanical contribution of Dsp and Dpp to enamel formation. Transgenic mice were engineered to overexpress either Dsp or Dpp in their enamel organs. The mechanical properties (hardness and toughness) of the mature enamel of transgenic mice were compared with genetically matched and age-matched nontransgenic animals. Dsp and Dpp contributions to enamel formation greatly differed. The inclusion of Dsp in bulk enamel significantly and uniformly increased enamel hardness (20%), whereas the inclusion of Dpp weakened the bulk enamel. Thus, Dsp appears to make a unique contribution to the physical properties of the DEJ. Dsp transgenic animals have been engineered with superior enamel mechanical properties.Until recently, it was believed that ectoderm-derived ameloblasts and ectomesenchyme-derived odontoblasts only expressed matrix proteins that were exclusive to themselves. But it is now known that there is also a transient expression of dentin sialophosphpoprotein (Dspp) 2 in early ameloblasts, adjacent to the dentinoenamel junction (DEJ) (1-6). We propose that Dspp has a role in producing the specialized first-formed harder enamel adjacent to the DEJ. Such a localized spatio-temporal pattern of Dspp protein expression may contribute to remarkable crack-resistant properties of the DEJ.Specialized highly mineralized first-formed enamel, and specialized tubule-rich mantle dentin have long been known to be present close to the DEJ interface (7-9). Although there is an abrupt transition between dentin and enamel tissues at the DEJ interface (10 -13), it is now known that the DEJ area functions as a broad mechanical zone (14 -17). Crack diffusion by branching and dissipation within the specialized first-formed enamel close to the DEJ prevents catastrophic interfacial damage and tooth failure (18,19).We tested the prediction that overexpression of Dspp proteins throughout enamel formation would produce altered enamel material properties. In this study, we have examined the mechanical properties of enamel of transgenic mice that overexpress either Dsp or Dpp in ameloblasts within their enamel organs. EXPERIMENTAL PROCEDURESOur strategy was to ascertain the functions of selected known proteins acting during enamel biomineralization by overexpressing them in quantity and time so that a l...

Temporal and Spatial Localization of the Dentin Matrix Proteins During Dentin Biomineralization

“…Furthermore, strong DSP expression was observed in the enamel matrix, particularly in the interprismatic region. White et al (2007) have shown that overexpression of DSP in mouse ameloblasts significantly increased enamel hardness. Thus, expression of DSP during early enamel formation suggests that DSP might contribute to the mechanical properties of the dentin-enamel junction (DEJ), confirming published data (White et al 2007).…”

“…White et al (2007) have shown that overexpression of DSP in mouse ameloblasts significantly increased enamel hardness. Thus, expression of DSP during early enamel formation suggests that DSP might contribute to the mechanical properties of the dentin-enamel junction (DEJ), confirming published data (White et al 2007). However, Baba et al (2004), in decalcified rat molars, showed the presence of DSP only in the dentinal tubules of all stages of rat molar development.…”

Partial Rescue of the Amelogenin Null Dental Enamel Phenotype

“…The absence of amelogenin does not appear to alter histological appearance, hardness, elastic modulus, or volume of dentin in the KO mice, but a slight increase in enamel density and a significant increase (p Ͻ 0.001) in dentin density were seen in TgM180 mice. Overexpression of TgM180 on this wild-type background could lead to this increase as it has been shown that overexpression in enamel of a dentin sialophosphoprotein protein involved in dentin formation positively impacted the physical properties of enamel in the transgenic mice (52).…”

Ultrastructure of dentine carious lesions

“…DPP covalently cross-linked to type I collagen and induces the mineralization of the collagen in calcium phosphate solutions, therefore plays crucial role in calcification of dentine 106,107 , which was directly confirmed when DSPP-null mice developed dentine defects in the absence of other symptoms 108 . The role of DSP on dentine mineralization, which accounts for 5 to 8% of the dentine matrix, is not clear at the present time 109 but using transgenic mice overexpressing DSP the increased hardness of enamel was demonstrated and it was concluded that DSP may contribute to structural properties of the innermost layer of enamel close to DEJ that is formed during the initial stages of amelogenesis when DSP is naturally expressed 110 . DGP was recently characterized 111 but its role has not yet been elucidated.…”