To Wnt or not to Wnt: the bone and joint health dilemma

Abstract

The Wnt signalling cascades have essential roles in development, growth and homeostasis of joints and the skeleton. Progress in basic research, particularly relating to our understanding of intracellular signalling cascades and fine regulation of receptor activation in the extracellular space, has provided novel insights into the roles of Wnt signalling in chronic arthritis. Cartilage and bone homeostasis require finely tuned Wnt signalling; both activation and suppression of the Wnt-β-catenin cascade can lead to osteoarthritis in rodent models. Genetic associations with the Wnt antagonist encoded by FRZB and the transcriptional regulator encoded by Dot1l with osteoarthritis further corroborate the essential part played by Wnts in the joint. In rheumatoid arthritis, inhibition of Wnt signalling has a role in the persistence of bone erosions, whereas Wnts have been associated with the ankylosing phenotype in spondyloarthritis. Together, these observations identify the Wnt pathway as an attractive target for therapeutic intervention; however, the complexity of the Wnt signalling cascades and the potential secondary effects of drug interventions targeting them highlight the need for further research and suggest that our understanding of this exciting pathway is still in its infancy.

Figure 1

The complex synthesis and signalling properties of Wnt molecules. Wnts are glycosylated and palmitoylated in the ER. The latter process results in covalent attachment of a fatty acid chain to the Wnt protein, catalyzed by the enzyme porcupine. Wntless is a Golgi apparatus molecule essential for further trafficking of Wnts and their secretion. Membrane bound heparan sulphate proteoglycans on the surface of the Wnt-producing source cell and the target cells can interact with Wnt, thereby contributing to establishment of concentration gradient of Wnt expression. Wnt antagonists such as FRZB (also known as sFRP3) and other sFRPs can act as direct antagonist, sequestering secreted Wnts, but in doing so can also increase the signalling range of Wnts by acting as molecular shuttles that carry the ligand away from the source cells. Abbreviations: ER, endoplasmic reticulum; sFRPs, secreted frizzled-related proteins.

Figure 2

Wnt signalling cascades. a | The canonical Wnt signalling cascade is dependent on intracellular signalling molecule β-catenin. In the absence of Wnt binding to Fz receptors, β-catenin is sequestered into a destruction complex composed of Axin, CK1α, APC and GSK3β, phosphorylated, ubiquitinylated and subsequently degraded by the proteasome. Upon binding of Wnt to Fz receptors and LRP5/6 co-receptors, DSH interacts with the receptor complex and recruits the destruction complex to the cell membrane allowing newly synthesized β-catenin to accumulate within the cytoplasm and to translocate to the nucleus. Nuclear β-catenin can displace the transcriptional co-repressor groucho from TCF transcription factors and promote activation of a gene transcription program. Both Wnt-binding antagonists (sFRPs/WIF) and Wnt receptor antagonists (Dkk/SOST) inhibit the canonical cascade. b | In the noncanonical Wnt signalling cascade, different phosphorylation cascades are activated by specific ligand–receptor interactions, seemingly without engagement of the LRP co-receptors. Many of these cascades are triggered by an increase in intracellular Ca²⁺ concentrations secondary to PLC- and DAG production. Subsequently, PKC and CaMKII can activate transcription factors like NFκB and CREB, mediated IP₃ and calmodulin is involved in the activation of NFAT. Only the Wnt-binding antagonists inhibit the noncanonical cascade. Abbreviations: APC, adenomatous polyposis coli; CaMKII, calcium/calmodulin-dependent protein kinase type II; CK1α, caseine kinase 1-α; CREB, cyclic AMP-responsive element-binding protein; DAG, diacylglycerol; Dkk, Dickkopf; DSH, disheveled; GSK3β, glycogen synthase kinase-3 β; IP₃, inositol 1,4,5-triphosphate; LRP, low-density lipoprotein receptor-related protein; NFAT, nuclear factor of activated T cells; NFκB, nuclear factor κB; PIP₂, phosphatidylinositol 4,5-bisphosphate; PKC, protein kinase C; PLC, phospholipase C; sFRPs, secreted frizzled-related proteins; SOST, sclerostin; WIF, Wnt inhibitory factor.

Figure 3

The varied roles of Wnts in skeletal development. a | At the early phase of skeletogenesis, expression of Wnts, such as Wnt3 (known as Wnt3a in some organisms) and Wnt5a, at the AER keeps the progenitor cells (red cells) in a proliferative state and prevents differentiation into chondrocytes (yellow cells). Absence of sufficient Wnt signalling in cells further along the concentration gradient allows them to express the chondrogenic transcription factor SOX9 (green cells). Condensations of these chondrocytes occur distant from the Wnt source and form the cartilage template for the prospective skeletal elements. Canonical, β-catenin-dependent Wnt signalling by Wnt3a, with co-activation by R-spondin-2, has been linked to inhibition of chondrogenesis, whereas noncanonical signalling, induced by Wnt5a and involving NFAT, has been associated with proximal–distal growth. b | During endochondral bone formation, Wnt signalling is critical in the final differentiation steps towards chondrocytes hypertrophy when cells start expressing ColX, MMP-13 and VEGF (orange cells); canonical Wnt signalling stimulates this process. Chondrocyte differentiation might be triggered by Wnt7b and Wnt16 that are expressed in developing chondrocytes, which are positive for Col2 (large green cells). By contrast, noncanonical cascades triggered by Wnt5a or Wnt5b might inhibit chondrocytes hypertrophy. c | During direct bone formation, Wnts stimulate the differentiation from osteoblast precursors towards mature osteoblasts, which produce BSAP and osteocalcin. Wnt7b has been specifically associated with this process. Abbreviations: AER, apical ectodermal ridge; BSAP; bone-specific alkaline phosphatase; Col2, collagen type II; ColX, collagen type X; MMP-13, matrix metalloproteinase-13 (also known as collagenase 3); NFAT, nuclear factor of activated T cells; VEGF, vascular endothelial growth factor.

Figure 4

The complex roles of Wnts in cartilage homeostasis and disease. a | Both overexpression and loss of β-catenin in the articular cartilage lead to joint damage. Overexpression β-catenin results in chondrocytes hypertrophy and loss of matrix quality, whereas loss of β-catenin function results in tissue damage through chondrocyte death. b | Canonical and noncanonical Wnt signalling pathways keep each other in check through reciprocal inhibition. The canonical pathway appears to stimulate proliferation. The noncanonical pathway stimulates dedifferentiation.