Deletion of a Single Allele of the Dkk1 Gene Leads to an Increase in Bone Formation and Bone Mass

Abstract

Wnt/␤-catenin signaling has been proven to play a central role in bone biology. Unexpectedly, the Wnt antagonist Dkk2 is required for terminal osteoblast differentiation and mineralized matrix formation. We show that Dkk1, unlike Dkk2, negatively regulates osteoblast differentiation and bone formation. Introduction:The Wnt co-receptor LRP5 is a critical regulator of bone mass. Dickkopf (Dkk) proteins act as natural Wnt antagonists by bridging LRP5/6 and Kremen, inducing the internalization of the complex. Wnt antagonists are thus expected to negatively regulation bone formation. However, Dkk2 deficiency results in increased bone, questioning the precise role of Dkks in bone metabolism. Materials and Methods:In this study, we investigated specifically the role of Dkk1 in bone in vitro and in vivo. Using rat primary calvaria cells, we studied the effect of retroviral expression of Dkk1 on osteoblast differentiation. In addition, the effect of Dkk1 osteoblast was studied in MC3T3-E1 cells by means of recombinant protein. Finally, to address the role of Dkk1 in vivo, we analyzed the bone phenotype of Dkk1 +/− animals. Results: Retroviral expression of Dkk1 in rat primary calvaria cells resulted in a complete inhibition of osteoblast differentiation and formation of mineralized nodules, with a marked decrease in the expression of alkaline phosphatase. Dkk1 expression also increased adipocyte differentiation in these cell cultures. Recombinant murine Dkk1 (rmDkk1) inhibited spontaneous and induced osteoblast differentiation of MC3T3-E1 cells. To determine the role of Dkk1 in vivo and overcome the embryonic lethality of homozygous deletion, we studied the bone phenotype in heterozygous Dkk1-deficient mice. Structural, dynamic, and cellular analysis of bone remodeling in Dkk1 +/− mice showed an increase in all bone formation parameters, with no change in bone resorption, leading to a marked increase in bone mass. Importantly, the number of osteoblasts, mineral apposition, and bone formation rate were all increased several fold. Conclusions: We conclude that Dkk1 protein is a potent negative regulator of osteoblasts in vitro and in vivo. Given that a heterozygous decrease in Dkk1 expression is sufficient to induce a significant increase in bone mass, antagonizing Dkk1 should result in a potent anabolic effect.

Functional characterization of Wilms tumor-suppressor WTX and tumor-associated mutants

“…WTX may be in a stochiometric relationship with other members of the b-catenin destruction complex and loss of its expression may hinder the complex, whereas overexpression does not lead to further assembly of such complexes. Nevertheless, as increased Wnt signaling is associated with increased skeletal ossification as seen in congenital WTX mutation, the available evidence does suggest that WTX is a negative regulator of canonical Wnt signaling (Bennett et al, 2005;Morvan et al, 2006;Jenkins et al, 2009). However, our results indicate that the ability of overexpressed WTX to slow cell growth and induce cell death is not associated with changes in b-catenin levels.…”

Wnt signaling antagonists are potential prognostic biomarkers for the progression of radiographic hip osteoarthritis in elderly Caucasian women

“…However, Dkk-1 has been shown to retard new bone formation and is a potent negative regulator of osteoblast differentiation in vitro and in vivo (19,31). Thus, the reduced risk of RHOA progression with elevated serum levels of Dkk-1 may reflect the ability of Dkk-1 to inhibit bone remodeling around the diseased joint.…”

Function and biological roles of the Dickkopf family of Wnt modulators

“…Conversely, mice that overexpress the G171V LRP5 mutant, have a high bone mass phenotype (Babij et al, 2003). Unlike homozygous mutants, heterozygous Dkk1 þ /À mice are viable and they show an increase in bone mineral density (Morvan et al, 2006). Conversely, transgenic mice overexpressing Dkk1 in bone develop osteopenia (Li et al, 2006).…”