Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and disease

Abstract

Wnts are secreted, lipid-modified proteins that bind to different receptors on the cell surface to activate canonical or non-canonical Wnt signaling pathways, which control various biological processes throughout embryonic development and adult life. Aberrant Wnt signaling pathway underlies a wide range of human disease pathogeneses. In this review, we provide an update of Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and diseases. The Wnt proteins, receptors, activators, inhibitors, and the crosstalk of Wnt signaling pathways with other signaling pathways are summarized and discussed. We mainly review Wnt signaling functions in bone formation, homeostasis, and related diseases, and summarize mouse models carrying genetic modifications of Wnt signaling components. Moreover, the therapeutic strategies for treating bone diseases by targeting Wnt signaling, including the extracellular molecules, cytosol components, and nuclear components of Wnt signaling are reviewed. In summary, this paper reviews our current understanding of the mechanisms by which Wnt signaling regulates bone formation, homeostasis, and the efforts targeting Wnt signaling for treating bone diseases. Finally, the paper evaluates the important questions in Wnt signaling to be further explored based on the progress of new biological analytical technologies.

Fig. 1

Schematic representation of canonical and non-canonical Wnt signaling pathway. a The canonical Wnt signaling pathway in inactive and active status. Without Wnt binding, β-catenin is sequestered by a destruction complex composed of GSK3β, Axin, APC, and CK1, which leads to the phosphorylation of β-catenin at serine/threonine residues. Phosphorylated β-catenin is then undergoing the proteosomal degradation mediated by polyubiquitination. The Wnt signaling is in inactive status. When Wnt binds to its receptor complex, including the seven-transmembrane receptor Fzd and the co-receptor LRP5 or LRP6, Wnt/β-catenin is initiated. This binding mobilizes GSK3β and CK1 to the cell membrane, where they phosphorylate serines on Lrp5/6, promoting the formation of a signalosome, and the recruitment of Dvl and Axin. Then, β-catenin is released from the destruction complex, accumulates in the cytoplasm and translocates into the nucleus to activate target gene expression by binding to TCF/LEF. Thus, The Wnt signaling is in active status. b Non-canonical Wnt signaling pathway includes Wnt/PCP and Wnt/Ca²⁺ signaling pathway. In the Wnt/PCP signaling pathway, non canonical Wnts bind to Fzd and the coreceptor (e.g., ROR2) to initiate the signaling. The Dvl is recruited to Fzds, which further activates the small GTPases Rac1 and RhoA. The activated GTPases induces changes in the actin cytoskeleton, and activates JNK and ROCK to regulate downstream signals. In the Wnt/Ca²⁺ signaling pathway, Wnts bind to Fzd to mediate the activation of a G protein, which in turn activate the PLC. The activated PLC leads to the generation of IP3 and DAG, which increase intracellular Ca²⁺ concentration. Alternatively, Wnt/Fzd activates cGMP-specific PDE6, which results in decrease of cGMP and the inactivation of PKG, thus increases intracellular Ca²⁺ concentration. The Ca²⁺ activates CaMKII, calcineurin, or PKC, which further activates various transcription factors

Fig. 2

Activators/agonists and inhibitors/antagonists of Wnt signaling. a Activators/agonists of Wnt signaling. RSpo maintains the Wnt signal by binding to LGR and RNF43/ZNRF3 to prevent the polyubiquitination and endocytosis of Fzd induced by RNF43/ZNRF3. Norrin, acting as a mimic of Wnts, specifically binds to Fzd4 with high affinity and activates the Wnt/β-catenin signaling pathway in a LRP5/6-dependent manner. MACF1 promotes Wnt/β-catenin signaling by translocating the Axin complex (Axin, β-catenin, and GSK3β) from cytoplasm to cell membrane, where GSK3β is inactivated by phosphorylation and β-catenin is released and enters the nucleus to activate target genes. FOXB2 interacts with multiprotein transcriptional complex to induce multiple Wnt ligands, including Wnt1 and Wnt7b to increase TCF/LEF-dependent transcription. b Inhibitors/antagonists of Wnt signaling. Sclerostin, DKK and Wise bind to LRP5/6 to interfere with the binding between LRP5/6 and Fzd to inhibit Wnt signaling. Krm1 and Krm2 (Krm1/2) cooperate with Dkks to form a complex with LRP6 and inhibit the Wnt signaling. Wise also binds to LRP4 to inhibit Wnt/β-catenin signaling. sFRPs, WIF-1 and Cerberus inhibit Wnt signaling by interacting with Wnts. IGFBP-4 binds to both Fzd and LRP6 to antagonize Wnt signaling. Bighead, Tiki, Waif1/5T4, and APCDD1 prevent ligand–receptor interaction to antagonize Wnt signaling. Shisa impairs Fzd maturation to inhibit Wnt signaling

Fig. 3

The interaction of Wnt signaling pathway with other signaling pathways. Wnt signaling interacts with Notch signaling pathway. β-catenin, the key component of Wnt signaling, activates Notch signaling by targeting Jagged 1 to activate Notch signaling. GSK3β also activates Notch signaling by phosphorating NICD. However, Dvl inhibits Notch signaling by inducing NICD degradation. In contrast, Notch negatively regulates β-catenin stability by inducing its lysosomal degradation. Wnt signaling inhibits Hedgehog signaling by regulating the expression of Gli3, the main repressor of Hedgehog signaling. TGFβ/BMP signaling and Wnt signaling determine the expression of ligand and components (e.g., Wnts, LRP5, Sost, Axin, BMP2, and TGFβ) of each other and the interaction between Smad7 and Axin links these two signaling pathways. Moreover, Dvl is targeted for degradation by Smurf2, a regulator of TGF-β/BMP signaling pathway. Conversely, Dvl activates Smurf2. PTH signal stabilizes the β-catenin to activate Wnt/β-catenin signaling. In addition, PTH inhibits sclerostin expression by promoting nuclear accumulation of HDACs to repress MEF2C-dependent Sost enhancer. Estrogen signaling interacts with Wnt signaling. The estrogen 17β-Estradiol (E2) activates estrogen signaling by binding to ERα to suppress the expression of WNT5B, but to increase the expression and activation levels of β-catenin. Besides, ERβ mediates the E2 suppression on the expression of Sost, an antagonist of Wnt signaling

Fig. 4

Schematic representation of Wnt signaling modulates bone homeostasis. Wnt signaling regulates bone homeostasis by modulating the biological function of bone cells, including BM-MSCs, osteoblasts, osteoclasts, and osteocytes. The canonical Wnt signaling promotes bone formation, inhibits bone resorption and adipocyte differentiation during maintaining bone homeostasis. When canonical Wnt signaling is activated by Wnts binding to the receptors or by the activators of Wnt signaling (e.g., RSpo, Norrin, and MACF1), β-catenin accumulates in the cytoplasm and translocates into the nucleus to regulate the target gene expression in bone cells to control bone cell capacity. Runx1 activates Wnt signaling by increasing Wnts expression to promote osteoblast differentiation. ZBP1 facilitates β-catenin nuclear translocation to promote Wnt signaling, while β-catenin in turn induces ZBP1 expression. HMGA1 transcriptionally regulates LRP5 expression to activate Wnt signaling. The non-canonical Wnt signaling promotes bone formation and bone resorption and inhibits adipocyte differentiation. Non-canonical Wnt5a signals through ROR2 to activate RhoA that is necessary and sufficient for osteogenic differentiation. Wnts also promote osteoblast differentiation and bone formation via PLC/PKCδ signaling. Wnt5a-ROR2 signals increase the expression of RANK by activating c-Jun to enhance RANKL-induced osteoclastogenesis and also promote actin ring formation via Rho to increase bone resorption. Besides, sFRP4, a Wnt inhibitor, dramatically suppresses the osteoclast differentiation by inhibiting non-canonical Wnt/ROR2/JNK signaling

Fig. 5

Wnt signaling involved in bone disease. The Wnt signaling is involved in bone disease. including osteoporosis, sclerosteosis, osteoarthritis (OA), and rheumatoid arthritis (RA), as shown in Table 3 for detail