Postnatal Skeletal Deletion of Dickkopf-1 Increases Bone Formation and Bone Volume in Male and Female Mice, Despite Increased Sclerostin Expression

Abstract

The Wnt antagonist Dickkopf-1 (Dkk1) is a negative regulator of osteoblast function and bone mass. However, because of the lack of appropriate models, many aspects of its role in the regulation of postnatal bone turnover and its cellular source have remained unknown. In this study, we deleted Dkk1 postnatally and in different cell types using various Cre-drivers (Rosa26-ERT2-Cre, Osx-cre, Dmp1-Cre) and assessed to which extent cells of the osteoblastic lineage contribute to the effects of Dkk1 on bone turnover and homeostasis. Female and male mice were examined at 12 weeks of age. Mice with a global or cell type-specific deletion of Dkk1 showed a two- to threefold higher bone volume compared with their Cre-negative littermates. The mineral apposition rate and the bone formation rate were increased two- to fourfold in all three mouse lines, despite a significant increase in systemic and skeletal levels of sclerostin. Dkk1 deletion further reduced the number of osteoclasts about twofold, which was accompanied by a strong decrease in the receptor activator of nuclear factor-κB ligand/osteoprotegerin mRNA ratio in femoral bone. Despite similar increases in bone mass, the deletion of Dkk1 in osterix-expressing cells reduced circulating Dkk1 significantly (males, -79%; females, -77%), whereas they were not changed in Dkk1 ;Dmp1-Cre mice. However, both lines showed significantly reduced Dkk1 mRNA levels in bone. In summary, we show that lack of Dkk1 in cells of the osteoblastic lineage leads to high bone mass with increased bone formation, despite increased levels of sclerostin. Moreover, the majority of systemic Dkk1 appears to originate from osteoprogenitors but not from mature osteoblasts or osteocytes. Nevertheless, the amount of Dkk1 produced locally by more mature osteogenic cells is sufficient to modulate bone mass. Thus, this study highlights the importance of local Wnt signaling on postnatal bone homeostasis. © 2018 American Society for Bone and Mineral Research.

Osteogenic Dkk1 Mediates Glucocorticoid-Induced but Not Arthritis-Induced Bone Loss

“…This is in contrast to a previous study that showed that deficiency of Sost protected mice from GC‐mediated bone loss via inhibition of bone resorption and not as expected by increasing bone formation . Potentially, this discrepancy may be explained by the already increased serum levels of sclerostin in conditional Dkk1‐deficient mice …”

“…D , E ). Consistent with earlier studies, we found that an osteoprogenitor‐specific deletion of Dkk1 led to a significant reduction of Dkk1 serum levels, confirming that these cells are the major contributors to the systemic amount of Dkk1 …”

“…Consistent with earlier studies, osteoprogenitor‐ as well as osteocyte‐specific Dkk1 deletion was accompanied by an increased sclerostin serum level. This compensatory mechanism has been confirmed in many studies, and in fact, increased sclerostin has been proposed to mask some of the anabolic effects of Dkk1 . As sclerostin neutralization also prevents systemic bone loss in arthritis mice, blockade of both sclerostin and Dkk1 using bispecific antibodies as proposed by Florio and colleagues may be an effective strategy to prevent inflammation‐induced bone loss …”

“…We previously showed that besides osterix‐expressing cells, Dmp1‐expressing cells (ie, late osteoblasts and osteocytes) also modulate bone mass via their production of Dkk‐1 . Therefore, we next investigated whether osteocytic deletion of Dkk1 also diminishes inflammation in arthritis.…”

“…Dkk1 is expressed in various organs (bone, brain, reproductive system, kidney/urogenital tract) and by several cell types within bone, including osteoprogenitors, osteoblasts, osteocytes, and adipocytes . We recently found that osterix‐expressing osteoprogenitors are the main source of circulating Dkk1 levels . However, also late osteoblasts/osteocytes regulate bone homeostasis via their production of Dkk1, even though contributing only to local Dkk1 levels …”