Osteoblast-specific Krm2 overexpression and Lrp5 deficiency have different effects on fracture healing in mice.

Astrid Liedert,Ludger Klein-Hitpass,Anita Ignatius,Viktoria Röntgen,Peggy Benisch,Franz Jakob,Jochen K Lennerz,Michael Amling,Thorsten Schinke,Regina Ebert

doi:10.1371/journal.pone.0103250

Abstract

The canonical Wnt/β-catenin pathway plays a key role in the regulation of bone remodeling in mice and humans. Two transmembrane proteins that are involved in decreasing the activity of this pathway by binding to extracellular antagonists, such as Dickkopf 1 (Dkk1), are the low-density lipoprotein receptor related protein 5 (Lrp5) and Kremen 2 (Krm2). Lrp 5 deficiency (Lrp5−/−) as well as osteoblast-specific overexpression of Krm2 in mice (Col1a1-Krm2) result in severe osteoporosis occurring at young age. In this study, we analyzed the influence of Lrp5 deficiency and osteoblast-specific overexpression of Krm2 on fracture healing in mice using flexible and semi-rigid fracture fixation. We demonstrated that fracture healing was highly impaired in both mouse genotypes, but that impairment was more severe in Col1a1-Krm2 than in Lrp5−/− mice and particularly evident in mice in which the more flexible fixation was used. Bone formation was more reduced in Col1a1-Krm2 than in Lrp5−/− mice, whereas osteoclast number was similarly increased in both genotypes in comparison with wild-type mice. Using microarray analysis we identified reduced expression of genes mainly involved in osteogenesis that seemed to be responsible for the observed stronger impairment of healing in Col1a1-Krm2 mice. In line with these findings, we detected decreased expression of sphingomyelin phosphodiesterase 3 (Smpd3) and less active β-catenin in the calli of Col1a1-Krm2 mice. Since Krm2 seems to play a significant role in regulating bone formation during fracture healing, antagonizing KRM2 might be a therapeutic option to improve fracture healing under compromised conditions, such as osteoporosis.

Highlights

The canonical Wnt/b-catenin pathway has been intensively investigated over the past decade because of its key role in the regulation of skeletal development and bone mass maintenance [1]
In the present study we examined the influence of lipoprotein receptor related protein 5 (Lrp5) deficiency and osteoblast-specific Kremen 2 (Krm2) overexpression on fracture healing in mice with flexible and semi-rigid fixation, respectively
We investigated the influence of Lrp5 deficiency and osteoblast-specific overexpression of Krm2 in fracture healing using Lrp52/2 and Col1a1-Krm2 mice

Summary

Introduction

The canonical Wnt/b-catenin pathway has been intensively investigated over the past decade because of its key role in the regulation of skeletal development and bone mass maintenance [1]. The remarkable finding that loss-of-function mutations in the Wnt coreceptor low-density lipoprotein receptor related protein 5 (LRP5) gene cause the osteoporosis-pseudoglioma syndrome (OPPG), a rare autosomal recessive disorder of severe juvenile osteoporosis and congenital blindness, and that gain-of-function mutations in this gene result in a high bone mass phenotype, provided first evidence for the considerable influence of Lrp signaling on bone remodeling [2,3]. Studies using transgenic mouse models are currently reflecting the high impact of Wnt signaling on bone mass regulation [4,5,6]. Most of the transgenic animal models that affect bone mass target canonical Wnt signaling, there is increasing evidence that noncanonical Wnt signaling pathways, the Wnt-planar cell polarity (Wnt-PCP) and the Wnt-calcium (Ca2+) pathway, play a significant role in bone mass homeostasis. It has been demonstrated that there may be a crosstalk between these pathways and that some Wnts are able to activate more than one of these pathways in a receptor-dependent manner. [1,5]

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Conclusion