Objective: To define specific function of Runx2 exclusive to committed osteoblasts and postnatal-skeletogenesis.
Method: Molecular, biochemical, histomorphometric and genetic approaches were employed to evaluate postnatal-skeletogenesis in wild-type and osteoblast-specific Runx2-null model (Runx2CKO).
Result: Collagen type-1 is a marker for committed osteoblasts. We utilized 2.3kbCol1a1-Cre transgenic mice to achieve osteoblast-specific deletion of Runx2. Specificity of Runx2 deletion in committed-osteoblasts was confirmed by two independent reporter lines. To our surprise, Runx2CKO mice were born alive and were similar to wild-type littermates in size and body-weight. However, by 4-weeks of age, Runx2CKO mice were noticeably smaller (30%) than wild-type littermates. Micro-CT analyses revealed dramatic decrease in cortical BV/TV (30%), trabecular number (35%), and increase in trabecular space (62%) in tibias of Runx2CKO mice. We next elucidated the underlying cellular defect responsible for poor bone acquisition. Runx2CKO exhibit proteoglycan-enriched hyaline cartilage, and a marked decrease of Collagen type-1, reflecting abnormal extracellular-matrix synthesis. In vivo BrdU incorporation revealed no change in proliferative capacity of osteoprogenitors in Runx2CKO mice. Moreover, calcein double-labeling revealed a decrease in rate of bone synthesis, confirming impaired function of osteoblasts. Interestingly, Runx2CKO also exhibit delayed resorption of calcified cartilage and poor deposition of mineralized matrix, indicating disrupted recruitment of both osteoblasts and osteoclasts via invading blood vessels. Together, these data suggest impaired bone synthesis is not due to decrease in osteoblast number, rather loss of osteoblast function.
Conclusion: Runx2 is essential for both initial commitment and for sustained function of mature osteoblast.