A Direct Role for DKK1 in Calvarial Osteogenesis

Objective: The Wnt/β-catenin pathway plays a significant role in many developmental processes, including osteogenesis. DKK1, a secreted protein, is a potent inhibitor of Wnt/β-catenin signaling via LRP5/6 and Kremen1/2. Deletion of Dkk1 leads to embryonic lethality due to the lack of anterior head structures, whereas over-expression of Dkk1 under the 2.3 kb Col1a1 promoter (Dkk1-Tg) results in an osteopenic phenotype and severe calvarial bone abnormalities. The objectives of this study are to delineate the direct role of DKK1 in calvarial osteogenesis.

Method: We analyzed the phenotype of a Dkk1 over-expression transgenic (Tg) mouse model, driven by the 2.3 kb Col1a1 promoter, active in osteoblasts. Age groups included: E16.5, P0, 1-wk, 2-wks, 4-wks, 12-wks, and 24-wks. Phenotypic analyses included: radiographs, micro-CT, SEM, acid etch SEM, calcein double-labeling, immuno- and histo-chemical analyses. 4-day-old primary calvarial osteoblast isolation was performed on WT control and Dkk1-Tg mice to study the differential gene expressions of SOST, DKK1, RANKL, OPG, and OPN.

Result: Dkk1-Tg mice possessed a markedly reduced calvarial bone thickness and multiple areas lacking bone formation in the frontal, parietal, and interparietal bones. Micro-CT confirmed >60% reduction of bone volume/total volume within the parietal bone (N = 4; p<0.001). The cranial sutures appear to position correctly during development and undergo normal posterior frontal suture fusion. However, excessive gaps present between the bone fronts indicate a bone formation deficiency. Acid etching reveals morphologically abnormal calvarial osteocytes and a decreased number of dendritic processes. In addition, there was an increase in immature osteoclast number on the endocranial surface of the calvarium. In contrast to the striking differences in the calvaria, Dkk1-Tg mice possess a mild long bone phenotype.

Conclusion: These data indicate that DKK1 controls calvarial osteogenesis in a unique way through its direct role on osteoblasts and its indirect role on osteoclasts.