Tooth development depends on reciprocal interactions between oral epithelium and mesenchyme. The mesenchymal-derived odontoblasts secrete several collagenous and non-collagenous proteins to form a unique extracellular matrix . Among the non-collagenous proteins, dentin sialoprotein (Dsp) and dentin phosphoproteins (Dpp) are highly tooth-specific and are believed to play a crucial role in converting predentin to form mineralized dentin. These two proteins are derived from the cleavage of a 940 amino acid polypeptide, dentin sialophosphoprotein (Dspp). The Dspp gene consists of 5 exons spanning 16 kb and is transcribed as 4.4 kb mRNA . Messenger RNA for Dspp is expressed predominantly by odontoblasts and transiently by pre-ameloblasts. However, recently low levels of Dspp were observed in the ear and in bone. The Dsp, the amino terminal part of Dspp, is sialic acid rich and glycosylated protein that shares similarities with the other sialoproteins including bone sialoprotein, dentin matrix protein-1, and osteopontin. The Dpp, a highly phosphorylated protein with repeats of aspartic acid and phosphoserine, is thought to play a key role in the nucleation of hydroxyapatite formation during dentin calcification. Due to its restricted expression, and its physical localization on the human chromosome 4q within the dentinogenesis imperfecta-II (DGI-II) locus, Dspp was implicated as a potential candidate gene for this disorder. The other important genes involved in biomineralization mapped to this region of chromosome 4q include dentin matrix protein-1, bone sialoprotein, DSPP and osteopontin. Dentinogenesis imperfecta (DGI), an autosomal dominant disorder of the tooth that primarily affects dentin biomineralization, is classified into three subtypes based on the clinical features in an order of degree severity with type-I being the least severe and type-III the most severe . DGI-I is associated with osteogenesis imperfecta while the more severe forms (DGI-II and DGI-III) are restricted to the dentin. Opalescent dentin with an obliterated pulp chambers are the characteristic features in DGI-II. The teeth of patients with DGI-III are referred to as shell teeth in which the dentin mineralization does not occur after mantle dentin is formed. Radiographically, the pulp cavities in these teeth appear as enlarged pulp chambers along with high incidence of pulp exposures .Recently, several mutations in the Dspp gene have been identified in families with DGI-II disorder. These mutations include a C-T transition at the end of 3rd exon (Gln45stop) resulting in a premature termination of Dspp protein, G-A transition mutation in intron 3 (splice donor site) causing exon skipping, and Pro17Thr and Val18Phe transitions. Dentin dysplasia-II (DD-II), another human disorder of dentin mineralization, that shares similarities with DGI-II, and is attributed to a Tyr6Asp protein transition mutation in the hydrophobic core sequence of dspp gene. This mutation disabled the entry of Dspp into the endoplasmic reticulum . All these mutations indicate a potential function for Dspp in tooth mineralization. In order to characterize the molecular events that control dentin mineralization during normal tooth development and disease, we have deleted the entire Dspp coding region in embryonic stem (ES) cells and generated Dspp -/- mice. These null mice displayed an enlarged pulp cavity, widened predentin zone, decreased dentin width and high incidence of pulp exposures similar to DGI-III. Additionally, these mice showed an increased accumulation of biglycan and decorin within the widened predentin and scalloped (void spaces) regions in the dentin correlating well with defective mineralization. Amelogenins form a major protein component of developing enamel and are predominantly involved in the formation of enamel. Additionally, amelogenins have been implicated in the formation and maintenance of cementum. However, their precise expression profile and molecular role in the cementogenesis are not well understood. We have identified for the first time expression of the splice variants of amelogenin mRNA in the periodental region of tooth roots of the wild-type mice but not in the amelogenin-null mice. Progressive cementum defects in the amelogenin-null mice are associated with the increased expression of RANKL, an essential molecule for osteoclastogenesis, in cementoblast/periodontal ligament cells. These findings indicate a novel role for the amelogenin proteins, derived from the splice variants, in regulating RANKL pathway affecting the maintenance of cementum through osteoclastogenisis.
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