Cementomimetics——constructing a cementum-like biomineralized microlayer via amelogenin-derived peptides

Cementum is the outer-, mineralized-tissue covering the tooth root and an essential part of the system of periodontal tissue that anchors the tooth to the bone. Periodontal disease results from the destructive behavior of the host elicited by an infectious biofilm adhering to the tooth root and left untreated, may lead to tooth loss. We describe a novel protocol for identifying peptide sequences from native proteins with the potential to repair damaged dental tissues by controlling hydroxyapatite biomineralization. Using amelogenin as a case study and a bioinformatics scoring matrix, we identified regions within amelogenin that are shared with a set of hydroxyapatite-binding peptides （HABPs） previously selected by phage display. One 22-amino acid long peptide regions referred to as amelogenin-derived peptide 5 （ADP5） was shown to facilitate cell-free formation of a cementum-like hydroxyapatite mineral layer on demineralized human root dentin that, in turn, supported attachment of periodontal ligament cells in vitro. Our findings have several implications in peptide-assisted mineral formation that mimic biomineralization. By further elaborating the mechanism for protein control over the biomineral formed, we afford new insights into the evolution of protein-mineral interactions. By exploiting small peptide domains of native proteins, our understanding of structure-function relationships of biomineralizing proteins can be extended and these peptides can be utilized to engineer mineral formation. Finally, the cementomimetic layer formed by ADP5 has the potential clinical application to repair diseased root surfaces so as to promote the regeneration of periodontal tissues and thereby reduce the morbiditv associated with tooth loss.

Counter-regulatory phosphatases TNAP and NPP1 temporally regulate tooth root cementogenesis

Cementum is critical for anchoring the insertion of periodontal ligament fibers to the tooth root. Several aspects of cementogenesis remain unclear, including differences between acellular cementum and cellular cementum, and between cementum and bone. Biomineralization is regulated by the ratio of inorganic phosphate （Pi） to mineral inhibitor pyrophosphate （PPi）, where local Pi and PPi concentrations are controlled by phosphatases including tissue-nonspecific alkaline phosphatase （TNAP） and ectonucleotide pyrophosphatase/phosphodiesterase 1 （NPP1）. The focus of this study was to define the roles of these phosphatases in cementogenesis. TNAP was associated with earliest cementoblasts near forming acellular and cellular cementum. With loss of TNAP in the Alpl null mouse, acellular cementum was inhibited, while cellular cementum production increased, albeit as hypomineralized cementoid. In contrast, NPP1 was detected in cementoblasts after acellular cementum formation, and at low levels around cellular cementum. Loss of NPP1 in the Enppl null mouse increased acellular cementum, with little effect on cellular cementum. Developmental patterns were recapitulated in a mouse model for acellular cementum regeneration, with early TNAP expression and later NPP1 expression. In vitro, cementoblasts expressed Alpl gene/protein early, whereas Enppl gene/protein expression was significantly induced only under mineralization conditions. These patterns were confirmed in human teeth, including widespread TNAP, and NPP1 restricted to cementoblasts lining acellular cementum. These studies suggest that early TNAP expression creates a low PPi environment promoting acellular cementum initiation, while later NPP1 expression increases PPi, restricting acellular cementum apposition. Alterations in PPi have little effect on cellular cementum formation, though matrix mineralization is affected.

Laser capture microdissection enables cellular and molecular studies of tooth root development

Epithelial-mesenchymal interactions（EMIs） are critical for tooth development.Molecular mechanisms mediating these interactions in root formation is not well understood.Laser capture microdissection（LCM） and subsequent microarray analyses enable large scale in situ molecular and cellular studies of root formation but to date have been hindered by technical challenges of gaining intact histological sections of non-decalcified mineralized teeth or jaws with well-preserved RNA.Here,we describe a new method to overcome this obstacle that permits LCM of dental epithelia,adjacent mesenchyme,odontoblasts and cementoblasts from mouse incisors and molars during root development.Using this method,we obtained RNA samples of high quality and successfully performed microarray analyses.Robust differences in gene expression,as well as genes not previously associated with root formation,were identified.Comparison of gene expression data from microarray with real-time reverse transcriptase polymerase chain reaction（RT-PCR） supported our findings.These genes include known markers of dental epithelia,mesenchyme,cementoblasts and odontoblasts,as well as novel genes such as those in the fibulin family.In conclusion,our new approach in tissue preparation enables LCM collection of intact cells with well-preserved RNA allowing subsequent gene expression analyses using microarray and RT-PCR to define key regulators of tooth root development.