Important note: Kelley's Textbook of Rheumatology and Hochberg's Rheumatology are not available in this textbook library. The following is compiled from closely related authoritative sources (Robbins Cotran & Kumar Pathologic Basis of Disease, Harrison's Principles of Internal Medicine 22E, Rockwood & Green's Fractures in Adults, Miller's Review of Orthopaedics, and Katzung's Basic and Clinical Pharmacology), supplemented by a 2025 peer-reviewed review article specifically on Wnt and RA bone pathology.

Wnt Signalling Pathway in Rheumatology

1. Overview of the Canonical Wnt/β-Catenin Pathway

• Wnt OFF state: In the absence of Wnt ligand, a "destruction complex" - consisting of Axin, APC (adenomatous polyposis coli), CK1, and GSK-3β - phosphorylates β-catenin, targeting it for ubiquitin-mediated proteasomal degradation. β-catenin levels in the cytoplasm remain low, and Wnt target genes are transcriptionally repressed.
• Wnt ON state: Wnt ligands bind to a co-receptor complex of Frizzled (FZD) and LRP5/6 (low-density lipoprotein receptor-related protein 5/6) on the cell surface. This activates Dishevelled (Dvl), which inhibits the destruction complex → GSK-3β is inactivated → β-catenin accumulates in the cytoplasm → translocates to the nucleus → forms a complex with TCF/LEF transcription factors → drives expression of target genes (cyclin D1, c-Myc, RUNX2, FOXO family, OPG, etc.).

• Wnt/PCP (planar cell polarity) pathway - activates Rho-GTPases and JNK
• Wnt/Ca²⁺ pathway - activates PKC and CaMKII These are less well-defined in rheumatology but implicated in synovial fibroblast invasion and cartilage degradation.

2. Wnt Signalling in Normal Bone Physiology

Osteoblastogenesis

• Wnt proteins produced by osteoprogenitor cells bind LRP5/LRP6 co-receptors on osteoblast precursors
• Activates β-catenin signalling → promotes osteoblast differentiation and maturation
• β-catenin is the major transcription factor driving cells toward the osteoblast lineage (rather than adipocyte lineage)
• Wnt signalling also stimulates osteoblasts to produce osteoprotegerin (OPG) - the decoy receptor for RANKL - thereby indirectly suppressing osteoclastogenesis

Fig. Wnt binding to osteoblasts/stromal cells drives OPG secretion, which acts as a decoy receptor blocking RANKL-RANK interaction and thus preventing osteoclast differentiation. (Robbins Cotran & Kumar, Fig. 26.4)

Sclerostin and DKK-1: Key Endogenous Inhibitors

Fig. Wnt promotes osteoprogenitor proliferation and maturation via LRP5/6 and β-catenin. Sclerostin (produced by osteocytes) blocks Wnt activation, suppressing bone formation. (Robbins Cotran & Kumar, Fig. 26.5)

• Sclerostin and DKK-1 inhibit binding of Wnt to LRP5/6
• Inhibition of sclerostin or DKK-1 leads to increased bone mass (Miller's Review of Orthopaedics)

3. Wnt Pathway in Rheumatoid Arthritis (RA)

3a. DKK-1 and the "Repair Switch"

• RA → high DKK-1 → Wnt pathway suppressed → bone erosions form BUT periosteal new bone formation / osteophyte formation is absent
• Spondyloarthritis (SpA) → lower DKK-1 → Wnt pathway relatively active → syndesmophytes and new bone formation occur alongside erosions

3b. Sclerostin in RA

• Systemic inflammation in RA disrupts osteoblastogenesis through elevated sclerostin and DKK-1, which enhance inhibition of the Wnt signalling pathway
• Bone biopsies from RA patients undergoing joint replacement show excessive osteocyte apoptosis, high sclerostin expression, and empty osteocyte lacunae
• Both immature osteoblasts and osteocytes produce sclerostin under inflammatory conditions mediated by TNF-α family cytokines
• Fibroblast-like synoviocytes (FLS) in RA also express sclerostin, acting in a paracrine manner to suppress osteoblast activity in juxta-articular bone

3c. The RA Bone-Synovium-Muscle Crosstalk Axis

• Inflammatory cytokines (TNF-α, IL-6, IL-17) → ↑ sclerostin and DKK-1 expression → suppressed Wnt signalling → ↓ osteoblast activity
• ACPAs directly hyperactivate osteoclasts → juxta-articular bone degradation
• Excess RANKL production → osteoclast-mediated systemic bone loss
• Sclerostin secreted into circulation may represent an endocrine mechanism connecting synovial inflammation, systemic osteoporosis, and muscle wasting (sarcopenia)
• In early RA: biological therapies (anti-TNF, anti-IL-6) can restore bone homeostasis by lowering sclerostin/DKK-1; in long-standing RA, persistent dysregulation suggests irreversible alterations in bone biology beyond inflammation alone

4. Wnt Pathway in Spondyloarthritis (SpA) / Ankylosing Spondylitis (AS)

• In AS/SpA, DKK-1 levels are lower (partly due to different cytokine milieu - IL-17 dominant rather than TNF-α dominant)
• Wnt signalling is therefore less suppressed → osteoblast/chondrocyte activity is permissive → new bone formation, syndesmophytes, and ankylosis result
• BMP (bone morphogenic protein) and Wnt pathways cooperate to drive entheseal new bone formation in SpA
• Anti-IL-17 therapy (secukinumab) may partly act by modulating this Wnt/DKK-1 balance

5. Wnt Pathway in Osteoarthritis (OA)

• Wnt/β-catenin signalling in chondrocytes is finely balanced - both excess activation and excess inhibition cause cartilage pathology
• Gain-of-function Wnt/β-catenin in chondrocytes → promotes chondrocyte hypertrophy, matrix degradation (MMP upregulation), and subchondral bone remodelling → OA phenotype
• Loss-of-function mutations in Wnt pathway components → impaired chondrocyte maturation
• Elevated canonical Wnt activity in synovial fibroblasts in OA contributes to synovitis
• FRZB (secreted Frizzled-related protein 3, an endogenous Wnt antagonist): polymorphisms in FRZB have been associated with increased hip OA susceptibility in women

6. Wnt Pathway in Osteoporosis and Therapeutic Implications

Romosozumab (anti-sclerostin antibody)

• Mechanism: inhibits sclerostin → relieves suppression of Wnt/β-catenin → increased osteoblast proliferation and activity → anabolic bone formation + reduces bone resorption
• FDA-approved for osteoporosis; dual mechanism (promotes formation AND inhibits resorption, though the anti-resorptive mechanism is not fully understood)
• Katzung: "Sclerostin blocks wnt activation, suppressing bone formation. Antibodies to sclerostin have been developed, of which only romosozumab has been FDA approved."
• Limitation: restricted to 1 year of use; after which transition to antiresorptive is recommended
• Black box warning: increased risk of MI, stroke, and cardiovascular death in patients with prior MI/stroke within past year

Anti-DKK-1 antibodies

• In development for RA-associated bone erosion and osteolytic lesions (e.g., multiple myeloma)
• Theoretically: blocking DKK-1 → restores Wnt signalling → promotes bone repair at erosion sites

7. Key Modulators Summary Table

8. Therapeutic Targets in Rheumatology

• Robbins, Cotran & Kumar - Pathologic Basis of Disease, Bone Homeostasis and Remodeling (Figs. 26.4, 26.5)
• Harrison's Principles of Internal Medicine, 22E (2025) - RA Pathogenesis
• Rockwood & Green's Fractures in Adults, 10th ed. (2025) - Wnt Signaling, Bone Formation
• Miller's Review of Orthopaedics, 9th ed. - Section I: Bone
• Katzung's Basic and Clinical Pharmacology, 16th ed. - Sclerostin Antibodies
• Padmanaban D, Krishnan M. "Targeting NAV2-mediated Wnt/β-catenin signaling in rheumatoid arthritis." Mol Biol Rep (2026) [PMID: 42047909]
• MJR 2025 In Press - "Impaired Bone Physiology in Rheumatoid Arthritis: Can the Wnt Pathway Be the Connecting Link?"