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

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.

Osteoblasts in osteoporosis: past, emerging, and future anabolic targets

“…Sclerostin, the product of the SOST gene expressed by mature osteocytes, is a physiological negative modulator of bone formation (49,50). Sclerostin binds only weakly to BMPs and acts by antagonizing Wnt binding to LRP4-6 co-receptors (51,52). The pre-clinical observation that loss-of-function mutation of sclerostin results in increased bone formation and bone mass (53) led to the interesting concept that targeting sclerostin may increase bone formation in vivo (54).…”

The effects of Dickkopf-1 antibody on metaphyseal bone and implant fixation under different loading conditions

“…It is a potent Wnt antagonist [2], vital for head and limb development [2][3]. Dkk1 binds to low-density lipoprotein related proteins 4, 5 and 6 (LRP4/5/6) [4][5][6][7]. The exact mode of action is unclear, but it appears that Dkk1 directly competes with Wnt-ligand in binding to LRP6 [8] thus antagonizing Wnt signalling [9] and increasing β-catenin degradation.…”

Sclerostin: A gem from the genome leads to bone-building antibodies

“…Analysis of hypomorphic (partial loss-offunction) LRP4 alleles in mice showed that LRP4 appears to play a role in achieving and/or maintaining normal bone mass. (49) This, coupled with recent exciting data demonstrating that certain missense mutations in LRP4 cause increased bone mineral density in humans, (50) strongly suggests that sclerostin interacts with LRP4 in vivo.…”

“…Recently, it has been reported that sclerostin binds to LRP4, a singletransmembrane protein related to LRP5 and LRP6, and, additionally, that LRP4 is expressed in bone by osteoblasts and osteocytes. (48,49) Knockdown of LRP4 expression in cell culture was found to block sclerostin's inhibitory activity on Wnt signaling and on in vitro mineralization. (48) Thus in these in vitro systems, sclerostin's activity was shown to depend on the presence of LRP4.…”