Development of skeleton in mammals is an exceedingly complex process and involves both intramembranous and endochondral ossification. Completion of either class of ossification implies a highly intricate but well coordinated process of patterning, cell fate commitment, differentiation, growth and remodeling. These events are specified by a coordinated temporal and spatial pattern of gene expression. At first, secreted morphogens such as bone morphogenetic proteins, hedgehog, wingless proteins and others, signal to key transcription factors to specify gene expression. Runx2 is an essential transcription factor for chondrocyte and osteoblast differentiation. Runx2 gene deletion results in complete failure of skeleton formation and embryonic lethality. In humans, mutation of Runx2 gene causes cleidocranial dysplasia, a dominantly inherited skeletal disorder. Other master regulator of skeletogenesis is the Specificity protein-7 (Sp7). Sp7 belongs to the Sp subgroup of the Kruppel-like family of transcription factors characterized by three zinc-finger DNA-binding domains. Targeted disruption of Sp7/Osterix gene, results in absence of endochondral and intramembranous bone formation. The Sp7 deficient mesenchymal cells do not deposit bone matrix and cannot differentiate into osteoblasts. Very little is known about the underlying molecular mechanism for the surprisingly similar phenotype from the two seemingly unrelated proteins. Runx2 is required for the expression of Sp7 and possibly for its function as mice with targeted disruption of Runx2 do not show expression of Sp7. Interestingly, Runx2 expression is normal in the mesenchymal cells of membranous and the endochondral skeleton of Sp7 null animals. The functional incompetency of Runx2 in Sp7 deficient cells, suggest that Sp7 presence is obligatory for completion of Runx2 osteogenic activity. It is important to note that the observation of Runx2 presence in Sp7 null mice is limited to only RNA, determined by in situ hybridization of tissue section from Sp7 null embryos. Our recent data demonstrate that in skeletal cells, Sp7 acts as a molecular rheostat and is necessary for functional stability and turnover of Runx2 protein. Given that a complex post-transcription regulatory network is operative in skeletal cells, a strong possibility exist that Runx2 protein is never made or rapidly degraded in Sp7 null cells. We will experimentally address this by assessing endogenous levels of Runx2 protein in Sp7 null cells and by a regulated and selective gene reconstitution/ ablation in osteoprogenitor cells. The goal of this application is to identify and define a) spatial and temporal organization and assembly of Runx2 and Sp7 regulatory complexes for formation/maintenance of osteoblasts and b) mechanisms supporting stable complex formation and retention of competency for skeletal gene expression. Knowledge obtained from this study will provide molecular insights into components of bone regulatory complex that can be targeted for innovative therapy to improve cartilage and bone formation and repair.
Crucial understanding of molecular mechanism involved in the regulation of bone cell maturation has significant potential for developing interventional therapies in growth anomalies and metabolic bone disorders. Findings from this study will help us in understanding the pathophysiology of skeletal tissues and cartilage and bone disorders.
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