A new WNT on the bone: WNT16, cortical bone thickness, porosity and fractures

Abstract

The last decade has provided abundant data implicating the WNT pathway in bone development and in the regulation of skeletal homeostasis. Rare human mutations together with gain- and loss-of-function approaches in mice have clearly demonstrated that disrupted regulation of this pathway leads to altered bone mass. In addition to these rare human and mice mutations, large population-based genome-wide association studies (GWASs) have identified single-nucleotide polymorphisms in ∼60 loci strongly associated with variations in bone mineral density (BMD) at different skeletal sites. Among the loci/genes identified by BMD GWAS, components of the WNT signaling pathway are numerous and have been shown to contribute to skeletal development and homeostasis. Within the components of WNT signaling, the gene coding for WNT16, one of the 19 WNT ligands of the human genome, has been found strongly associated with specific bone traits such as cortical bone thickness, cortical porosity and fracture risk. Recently, the first functional characterization of Wnt16 has confirmed the critical role of Wnt16 in the regulation of cortical bone mass and bone strength in mice. These reports have extended our understanding of Wnt16 function in bone homeostasis and have not only confirmed the unique association of Wnt16 with cortical bone and fracture susceptibility, as suggested by GWAS in human populations, but have also provided novel insights into the biology of this WNT ligand and the mechanism(s) by which it regulates cortical but not trabecular bone homeostasis. Most interestingly, Wnt16 appears to be a strong anti-resorptive soluble factor acting on both osteoblasts and osteoclast precursors.

Figure 1

WNT signaling. (a) Canonical WNT signal on. Binding of Wnt ligands to the frizzled (Fzd) family of receptors activates the cytoplasmic signaling protein Dishevelled (Dvl), which in turn recruits the axin-glycogen synthase kinase 3 (GSK3) complex, leading to LRP5/6 phosphorylation. LRP5/6 phosphorylation prevents phosphorylation of β-catenin and thereby its degradation. R-spondin (Rspo) proteins are secreted agonists that enhance activation of canonical WNT signaling. Subsequently, β-catenin accumulates in the cytoplasm and enters the nucleus to initiate gene transcription. (b) Canonical WNT signal off. In the absence of WNTs, or when secreted WNT inhibitors such as Dickkopf1 (Dkk1), sclerostin (Sost) and secreted frizzled-related proteins (Sfrps) antagonize WNT signaling by either binding directly to the receptors or by functioning as decoy receptors for WNT proteins, the key protein β-catenin is phosphorylated by the destruction complex and degraded by ubiquitin-mediated proteolysis in the cytosol. Tcf/Lef assembles a transcriptional repressor complex to silence WNT target genes. (c) Non-canonical WNT signaling triggers its effects through alternative pathways including WNT/Rho-Rac and WNT/G-protein coupled receptors. In these pathways, WNT ligands signal through the Fzd receptors, or directly through membrane receptors such as Ror2 and Ryk, and dependently or independently of Dvl lead to the activation of multiple distinct downstream effectors, which eventually affect expression of genes involved in osteoblast differentiation.

Figure 2

Deletion of WNT16 in mice decreases cortical bone thickness, representative μCT and histological images of 16-week-old WT and Wnt16^-/- female mice. WT, wild type.

Figure 3

Model for WNT16 function in regulation of cortical bone. Wnt16 is expressed in osteoblasts lining the cortical bone and maintains cortical bone homeostasis by regulating endocortical resorption. Osteoblast-expressed Wnt16 signals via the canonical WNT pathway to regulate opg expression, which in turn functions as a decoy receptor for rankl expressed by osteoblasts and osteoclasts and regulates osteoclastogenesis. Osteoblast-expressed Wnt16 also functions directly on osteoclast precursors to regulate osteoclastogenesis via a non-canonical JNK cascade. OB, osteoblasts; Oc, osteocytes; OCL, osteoclasts.