Overexpression of Gremlin Results in Defects in Enamel and Dentin

Bone morphogenetic proteins (BMPs) and BMP antagonists play crucial roles in tooth development. Gremlin, a BMP extracellular antagonist, is a highly conserved 20.7-kDa glycoprotein. Transgenic mice overexpressing gremlin under the control of the osteocalcin promoter exhibit a skeletal phenotype and tooth fragility; however, the tooth phenotype details have not been explored. Objectives: To define the tooth phenotype of gremlin transgenic (GT) mice, and to determine the effect of gremlin on pulp cell differentiation and mineralization in vitro. Methods: Teeth and surrounding tissues obtained from 4-month-old GT mice were examined by gross anatomy, radiography, histology and scanning electronic microscopy (SEM). For in vitro studies, murine pulp cells were treated with gremlin (50 nM) with or without BMP-4 (0.3 nM, positive control) in mineralization media (DMEM with 2% FBS, 10 mM β-glycerophosphate and 50 μg/ml ascorbic acid). At day 14, cells were processed for measuring expression of Dspp by quantitative RT-PCR, and degree of mineralization by Alizarin red staining. Results: GT mice exhibited enlarged pulp chambers with ectopic calcification, thin dentin and irregular enamel crystal formation compared to wild-type control. In vitro, at day 14, control (vehicle treated) cells expressed Dspp and were able to promote mineral formation. A 3 fold increase in Dspp expression and a 9 fold increase in mineral formation were observed in BMP-4 treated cells compared to control cells. Gremlin inhibited BMP-4 induction of Dspp and mineralization, but gremlin alone had no effect on cell behavior beyond that of control. Conclusion: These in vivo and in vitro data provide evidence that balanced interactions between BMP agonists/antagonists are required for proper development of teeth and suggest that BMPs may be useful for regeneration of dentin.

Acknowledgements: This work was supported by NIH/NIDCR Grant DE09532, and NIH/NIAMS Grant AR021707.