MacDonald Bryan T, Joiner Danese M, Oyserman Sivan M, Sharma Parul, Goldstein Steven A, He Xi, Hauschka Peter V
Division of Neuroscience, Department of Orthopedic Surgery, Children's Hospital, Harvard Medical School, Boston, MA 02115, USA.
Bone. 2007 Sep;41(3):331-9. doi: 10.1016/j.bone.2007.05.009. Epub 2007 Jun 5.
The Wnt/beta-catenin signaling pathway has emerged as a key regulator in bone development and bone homeostasis. Loss-of-function mutations in the Wnt co-receptor LRP5 result in osteoporosis and "activating" mutations in LRP5 result in high bone mass. Dickkopf-1 (DKK1) is a secreted Wnt inhibitor that binds LRP5 and LRP6 during embryonic development, therefore it is expected that a decrease in DKK1 will result in an increase in Wnt activity and a high bone mass phenotype. Dkk1-/- knockout mice are embryonic lethal, but mice with hypomorphic Dkk1d (doubleridge) alleles that express low amounts of Dkk1 are viable. In this study we generated an allelic series by crossing Dkk1+/- and Dkk1+/d mice resulting in the following genotypes with decreasing Dkk1 expression levels: +/+, +/d, +/- and d/-. Using muCT imaging we scanned dissected left femora and calvariae from 8-week-old mice (n=60). We analyzed the distal femur to represent trabecular bone and the femur diaphysis for cortical endochondral bone. A region of the parietal bones was used to analyze intramembranous bone of the calvaria. We found that trabecular bone volume is increased in Dkk1 mutant mice in a manner that is inversely proportional to the level of Dkk1 expression. Trabeculae number and thickness were significantly higher in the low Dkk1 expressing genotypes from both female and male mice. Similar results were found in cortical bone with an increase in cortical thickness and cross sectional area of the femur diaphysis that correlated with lower Dkk1 expression. No consistent differences were found in the calvaria measurements. Our results indicate that the progressive Dkk1 reduction increases trabecular and cortical bone mass and that even a 25% reduction in Dkk1 expression could produce significant increases in trabecular bone volume fraction. Thus DKK1 is a negative regulator of normal bone homeostasis in vivo. Our study suggests that manipulation of DKK1 function or expression may have therapeutic significance for the treatment of low bone mass disorders.
Wnt/β-连环蛋白信号通路已成为骨骼发育和骨稳态的关键调节因子。Wnt共受体LRP5的功能丧失突变会导致骨质疏松，而LRP5的“激活”突变会导致骨量增加。Dickkopf-1（DKK1）是一种分泌型Wnt抑制剂，在胚胎发育过程中与LRP5和LRP6结合，因此预计DKK1的减少将导致Wnt活性增加和高骨量表型。Dkk1-/-基因敲除小鼠胚胎致死，但具有低表达Dkk1的低表达Dkk1d（双脊）等位基因的小鼠是存活的。在本研究中，我们通过将Dkk1+/-和Dkk1+/d小鼠杂交产生了一个等位基因系列，得到了以下Dkk1表达水平逐渐降低的基因型：+/+、+/d、+/-和d/-。使用微型计算机断层扫描（muCT）成像，我们扫描了8周龄小鼠（n=60）解剖后的左股骨和颅骨。我们分析了股骨远端以代表小梁骨，分析了股骨干以代表皮质软骨内骨。使用顶骨区域分析颅骨的膜内骨。我们发现，Dkk1突变小鼠的小梁骨体积增加，其方式与Dkk1表达水平成反比。在低表达Dkk1的基因型中，雌性和雄性小鼠的小梁数量和厚度均显著更高。在皮质骨中也发现了类似的结果，股骨干的皮质厚度和横截面积增加，这与较低的Dkk1表达相关。在颅骨测量中未发现一致的差异。我们的结果表明，Dkk1的逐渐减少会增加小梁骨和皮质骨的骨量，即使Dkk1表达降低25%也会导致小梁骨体积分数显著增加。因此，DKK1是体内正常骨稳态的负调节因子。我们的研究表明，操纵DKK1的功能或表达可能对治疗低骨量疾病具有治疗意义。
