Critical Role for Pleiotrophin during Tooth Development

Pleiotrophin (PTN) is an 18kDa heparin-binding protein expressed in mesodermal and ectodermal cells.  It plays an important role in osteoblast differentiation and recruitment. Notably, when PTN is upregulated there is an increase in bone thickness.  PTN binds to receptor protein tyrosine phosphatase beta/zeta, dimerizing and leading to increased tyrosine phosphorylation of beta-catenin, in addition to other proteins. Despite known effects of PTN on osteoblasts, there is limited information available about PTN involvement during tooth formation. Objectives: 1) to determine the expression of PTN in teeth; 2) determine the spatiotemporal expression pattern of PTN by immunohistochemistry; and 3) determine if PTN knock-out (KO) mice have a dental phenotype. Methods: Immortalized mouse dental pulp (MD10D3, MD10A11), odontoblast (MO6-G3) and ameloblast (3M-EOE) cell lines were grown and samples prepared for RT-PCR analysis, immunohistochemistry and Western blot analysis.  PTN KO mice were analyzed for gross tooth morphological and mineralization defects by high resolution radiography, micro-computed tomography and histology as compared to age- and sex- matched wildtype animals. Results: PTN, at the mRNA and protein level, was expressed in both the epithelial and mesenchymal dental cell lines tested although at different levels. Immunohistochemistry reveal initial expression of PTN in the inner enamel epithelium (E-16) with prolonged expression in the ameloblasts throughout their stages of maturation (days 1-10 postnatal).  PTN was also expressed at lower levels in the dental pulp mesenchyme (E-16) with increased staining associated with odontoblast cytodifferentation. Staining was also seen within sub-populations of the stellate reticulum, the dental follicle and the periodontal ligament. PTN KO mice showed a dramatic dentin phenotype with enlargement of the pulp chambers and decreased dentin mineralization.   Conclusions: These studies demonstrate PTN is expressed during tooth development in both the ameloblasts and odontoblasts and plays a critical role in dentinogenesis. Support: NIDCR T35-HL007473 and UAB-SOD Faculty Development Grants (HE).