Wnt10b increases postnatal bone formation by enhancing osteoblast differentiation.
Overexpression of Wnt10b from the osteocalcin promoter in transgenic mice increases postnatal bone mass. Increases in osteoblast perimeter, mineralizing surface, and bone formation rate without detectable changes in pre-osteoblast proliferation, osteoblast apoptosis, or osteoclast number and activity suggest that, in this animal model, Wnt10b primarily increases bone mass by stimulating osteoblastogenesis.


BACKGROUND
Wnt signaling regulates many aspects of development including postnatal accrual of bone. Potential mechanisms for how Wnt signaling increases bone mass include regulation of osteoblast and/or osteoclast number and activity. To help differentiate between these possibilities, we studied mice in which Wnt10b is expressed specifically in osteoblast lineage cells or in mice devoid of Wnt10b.


METHODS
Transgenic mice, in which mouse Wnt10b is expressed from the human osteocalcin promoter (Oc-Wnt10b), were generated in C57BL/6 mice. Transgene expression was evaluated by RNase protection assay. Quantitative assessment of bone variables was done by radiography, muCT, and static and dynamic histomorphometry. Mechanisms of bone homeostasis were evaluated with assays for BrdU, TUNEL, and TRACP5b activity, as well as serum levels of C-terminal telopeptide of type I collagen (CTX). The endogenous role of Wnt10b in bone was assessed by dynamic histomorphometry in Wnt10b(-/-) mice.


RESULTS
Oc-Wnt10b mice have increased mandibular bone and impaired eruption of incisors during postnatal development. Analyses of femoral distal metaphyses show significantly higher BMD, bone volume fraction, and trabecular number. Increased bone formation is caused by increases in number of osteoblasts per bone surface, rate of mineral apposition, and percent mineralizing surface. Although number of osteoclasts per bone surface is not altered, Oc-Wnt10b mice have increased total osteoclast activity because of higher bone mass. In Wnt10b(-/-) mice, changes in mineralizing variables and osteoblast perimeter in femoral distal metaphyses were not observed; however, bone formation rate is reduced because of decreased total bone volume and trabecular number.


CONCLUSIONS
High bone mass in Oc-Wnt10b mice is primarily caused by increased osteoblastogenesis, with a minor contribution from elevated mineralizing activity of osteoblasts.
Bennett
Christina N
CN
Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor, Michigan, USA.
Ouyang
Hongjiao
H
Ma
Yanfei L
YL
Zeng
Qingqiang
Q
Gerin
Isabelle
I
Sousa
Kyle M
KM
Lane
Timothy F
TF
Krishnan
Venkatesh
V
Hankenson
Kurt D
KD
MacDougald
Ormond A
OA
D000135:Acid Phosphatase / Q000096:biosynthesis / Q000235:genetics; D000818:Animals; D000831:Animals, Newborn; D017209:Apoptosis / Q000235:genetics; D015519:Bone Density / Q000235:genetics; D002454:Cell Differentiation / Q000235:genetics; D049109:Cell Proliferation; D006706:Homeostasis / Q000235:genetics; D006801:Humans; D007180:Incisor / Q000254:growth & development / Q000378:metabolism / Q000473:pathology; D007527:Isoenzymes / Q000096:biosynthesis / Q000235:genetics; D008334:Mandible / Q000254:growth & development / Q000378:metabolism / Q000473:pathology; D051379:Mice; D018345:Mice, Knockout; D008954:Models, Biological; D009929:Organ Size / Q000235:genetics; D010006:Osteoblasts / Q000378:metabolism* / Q000473:pathology; D015675:Osteocalcin; D010010:Osteoclasts / Q000378:metabolism / Q000473:pathology; D010012:Osteogenesis; D013234:Stem Cells / Q000378:metabolism* / Q000473:pathology; D000071681:Tartrate-Resistant Acid Phosphatase; D019076:Transgenes; D051153:Wnt Proteins / Q000096:biosynthesis* / Q000235:genetics
D007527:Isoenzymes; D051153:Wnt Proteins; C102068:Wnt10b protein, mouse; D015675:Osteocalcin; D000135:Acid Phosphatase; D000071681:Tartrate-Resistant Acid Phosphatase