All ETDs from UAB

Advisory Committee Chair

Amjad Javed

Advisory Committee Members

Haiyan Chen

Noel Childers

Dobrawa Napierala

Somsak Sittitavornwong

Document Type

Thesis

Date of Award

2014

Degree Name by School

Master of Science (MS) School of Dentistry

Abstract

Runx2 and its downstream target Specificity Protein 7 (Sp7) are essential for skeletogenesis. In humans, mutations in Sp7 gene are associated with osteogenesis imperfecta. Unlike Runx2, the role of Sp7 in palatogenesis, tooth development or differentiation of ameloblasts and odontoblasts remains unknown. The main goal of this research was to identify the functional requirement of Sp7 transcription factor during tooth development and palatogenesis. Molecular, biochemical, and histological approaches were employed to answer this question. We established Sp7-null mice that are completely void of mineralized tissue, exhibit craniofacial dysmorphogenesis and die shortly after birth. Surprisingly, Sp7 homozygous mutants exhibited normal tooth morphogenesis that progressed until the late bell stage. Thus formation of alveolar bone is not a prerequisite for tooth morphogenesis. The uni-cusp incisors and multi-cusped first and second molars in the Sp7-null mutants were comparable to wild-type littermates. However tooth organs in the Sp7-null mice were significantly smaller in size and poorly vascularized. Furthermore, only few progenitor cells at the labial cervical loop of Sp7 deficient tooth organs showed Ki-67 and BrdU-labeling. These data strongly suggests that the small size of tooth organs in Sp7 mutants is due to impaired proliferative capacity of progenitor cells and poor delivery of nutrients. Interestingly, despite normal progression of tooth morphogenesis, mineralization was completely absent in dental tissues of Sp7-null mice due to maturational failure of both ameloblasts and odontoblasts. Epithelial and mesenchyme portions of Sp7 mutant teeth contained only cuboidal and disorganized cells. Impaired differentiation and matrix synthesis was evident by reduced production of collagen type I and loss of expression of stage and cell-specific markers such as dmp1, dspp, oc, amelogenin, klk4, amelotin and enamelin in the Sp7-null incisors. Failed maturation of both ameloblasts and odontoblast prompted us to examine if this involves a cell-specific or cell-autonomous effect of Sp7. We demonstrate that during embryonic development, expression of the Sp7 gene is restricted to cells of the dental mesenchyme, such as odobtoblasts and pulp. In postnatal life, Sp7 is expressed in cementoblasts, periodontal ligament cells, dental follicular cells, and root odontoblasts. To our surprise, the Sp7 gene is turned on in secretory ameloblasts after birth. Thus Sp7 is expressed in all mineralizing cell types involved in formation of the tooth organ. We found promoter of odontoblast (dspp, oc) and ameloblast (amelx, enam) specific genes contain multiple binding motifs for zinc finger transcription factors such as Sp7. Luciferase-reporter studies revealed that Sp7 directly induces promoter activity of dspp (~45 fold), enam (~7 fold) and amelx (~4 fold). During embryonic development, Sp7 is not expressed in ameloblasts, yet they fail to mature in Sp7-null mice. We find FGF mediated signaling required for a cross-talk between dental epithelium and mesenchyme is blocked in Sp7 mutant mice. Both FGF8 and FGF3 ligands are not expressed in Sp7-null teeth. Moreover, Sp7 directly induces expression of the FGF3 promoter. Another craniofacial structure that involves coordinated interaction between epithelial and mesenchymal cell types is the palate. Sp7 is expressed in mesenchyme of both the primary and secondary palate. Surprisingly, Sp7 gene deletion does not result in clefting of primary or secondary palatal structures, however mineralization is absent in both tissues. These results indicate that like tooth development, Sp7 does not play a role in the original morphogenesis but serves to drive the maturation of the palate. In conclusion, the Sp7 transcription factor is obligatory for the maturation of both ameloblasts and odontoblasts but not for the initial tooth or palate morphogenesis.

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Dentistry Commons

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