Lrp4, a Novel Receptor for Dickkopf 1 and Sclerostin, Is Expressed by Osteoblasts and Regulates Bone Growth and Turnover In Vivo

Hong Y Choi,Joachim Herz,Andreas Niemeier,Marco Dieckmann,Ellen A A Nollen

doi:10.1371/journal.pone.0007930

Abstract

Lrp4 is a multifunctional member of the low density lipoprotein-receptor gene family and a modulator of extracellular cell signaling pathways in development. For example, Lrp4 binds Wise, a secreted Wnt modulator and BMP antagonist. Lrp4 shares structural elements within the extracellular ligand binding domain with Lrp5 and Lrp6, two established Wnt co-receptors with important roles in osteogenesis. Sclerostin is a potent osteocyte secreted inhibitor of bone formation that directly binds Lrp5 and Lrp6 and modulates both BMP and Wnt signaling. The anti-osteogenic effect of sclerostin is thought to be mediated mainly by inhibition of Wnt signaling through Lrp5/6 within osteoblasts. Dickkopf1 (Dkk1) is another potent soluble Wnt inhibitor that binds to Lrp5 and Lrp6, can displace Lrp5-bound sclerostin and is itself regulated by BMPs. In a recent genome-wide association study of bone mineral density a significant modifier locus was detected near the SOST gene at 17q21, which encodes sclerostin. In addition, nonsynonymous SNPs in the LRP4 gene were suggestively associated with bone mineral density. Here we show that Lrp4 is expressed in bone and cultured osteoblasts and binds Dkk1 and sclerostin in vitro. MicroCT analysis of Lrp4 deficient mutant mice revealed shortened total femur length, reduced cortical femoral perimeter, and reduced total femur bone mineral content (BMC) and bone mineral density (BMD). Lumbar spine trabecular bone volume per total volume (BV/TV) was significantly reduced in the mutants and the serum and urinary bone turnover markers alkaline phosphatase, osteocalcin and desoxypyridinoline were increased. We conclude that Lrp4 is a novel osteoblast expressed Dkk1 and sclerostin receptor with a physiological role in the regulation of bone growth and turnover, which is likely mediated through its function as an integrator of Wnt and BMP signaling pathways.

Highlights

Osteoporosis and the inherently increased susceptibility to sustain fractures associated with this disease represent a major challenge in our aging western societies
Many of the current approaches to identify potential therapeutic targets are focused on the Wnt/b-catenin signaling pathway, which is of fundamental importance for osteogenesis [4,5,6]
The current study was prompted by the association of both the SOST and the LRP4 gene with bone mineral density (BMD) [7,8,47,48] the established function of both Sost and Lrp4 in the modulation of bone morphogenetic protein (BMP) and Wnt signaling [13,49,50], the partially overlapping developmental phenotypes in genetically manipulated mice of the Sost, the Dkk1 and the Lrp4 genes, and our previous findings that Lrp4 binds Wise

Summary

Introduction

Osteoporosis and the inherently increased susceptibility to sustain fractures associated with this disease represent a major challenge in our aging western societies. The current study was prompted by the association of both the SOST and the LRP4 gene with BMD [7,8,47,48] the established function of both Sost and Lrp in the modulation of BMP and Wnt signaling [13,49,50], the partially overlapping developmental phenotypes in genetically manipulated mice of the Sost, the Dkk and the Lrp genes, and our previous findings that Lrp binds Wise (a.k.a. Sostdc1) through its extracellular domain which is homologous to that of Lrp5/6 which interacts with sclerostin and Dkk. We have used in vitro and in vivo analysis of wild type mice and of two Lrp mutant mouse strains, one fully deficient (null mutant, Lrp4-/-; Dietrich et al, in preparation) and a functional hypomorph (Lrp4-ECD; [18]) to show that Lrp is an osteoblast expressed receptor for Dkk and sclerostin and regulates bone growth and bone turnover in vivo

Materials and Methods

Findings

Discussion