ABSTRACT
Background@#We investigated RNA sequencing-based transcriptome profiling and the transformation of mature osteoblasts into bone lining cells (BLCs) through a lineage tracing study to better understand the effect of mechanical unloading on bone loss. @*Methods@#Dmp1-CreERt2(+):Rosa26R mice were injected with 1 mg of 4-hydroxy-tamoxifen three times a week starting at postnatal week 7, and subjected to a combination of botulinum toxin injection with left hindlimb tenotomy starting at postnatal week 8 to 10. The animals were euthanized at postnatal weeks 8, 9, 10, and 12. We quantified the number and thickness of X-gal(+) cells on the periosteum of the right and left femoral bones at each time point. @*Results@#Two weeks after unloading, a significant decrease in the number and a subtle change in the thickness of X-gal(+) cells were observed in the left hindlimbs compared with the right hindlimbs. At 4 weeks after unloading, the decrease in the thickness was accelerated in the left hindlimbs, although the number of labeled cells was comparable. RNA sequencing analysis showed downregulation of 315 genes in the left hindlimbs at 2 and 4 weeks after unloading. Of these, Xirp2, AMPD1, Mettl11b, NEXN, CYP2E1, Bche, Ppp1r3c, Tceal7, and Gadl1 were upregulated during osteoblastogenic/osteocytic and myogenic differentiation in vitro. @*Conclusion@#These findings demonstrate that mechanical unloading can accelerate the transformation of mature osteoblasts into BLCs in the early stages of bone loss in vivo. Furthermore, some of the genes involved in this process may have a pleiotropic effect on both bone and muscle.
ABSTRACT
OBJECTIVES: The inhibitory effect of Dickkopf (Dkk)-1 on osteoblastic differentiation through blocking Wnt signaling has been well studied. However, the role of other members of the subfamily of Dkks remains unclear. We have examined the role of different Dkks on osteoblastic differentiation of mesenchymal progenitor cells and apoptosis of osteoblasts. METHODS: Osteoblastic differentiation was induced by treatment of Wnt-3a with Dkks or vehicle in C3H10T1/2 cells and alkaline phosphatase (ALP) activity was measured. Serum deprivation induced apoptosis was performed with pre-treatment of Dkks or vehicle in MC3T3-E1 cells and methyl thiazolyl tetrazolium (MTT) assay was done. RESULTS: Dkk-2 at low concentrations (5 and 20 nM) and Dkk-3, -4 at any concentrations (5 to 100 nM) significantly increased Wnt-3a-induced ALP activity, whereas Dkk-2 at high concentration (100 nM) significantly reduced. Treatment of Dkk-2, -3 and -4 at high concentration (100 nM) showed significant decreases of Wnt/beta-catenin transcriptional activity, whereas no effects were seen at low concentration (20 nM). In parallel experiments, treatment of Dkk-1 showed robust dose dependent inhibition not only in ALP activity but also in Wnt/beta-catenin transcriptional activity. Dkk-2, -3 and -4 increased serum deprivation-induced apoptosis in MC3T3-E1 mouse osteoblasts, while Dkk-1 had no effect. CONCLUSIONS: We found that unlike Dkk-1, Dkk-3 and -4 stimulated early osteoblastic differentiation at various concentrations regardless of their inhibitory effects on Wnt/beta-catenin transcriptional activity at high concentration. Dkk-2 had a biphasic effect where the lower doses significantly increased ALP activity while the high dose was inhibitory. Dkk-2, -3 and -4 stimulated osteoblast apoptosis whereas Dkk-1 had no effect.