Wnt16 Elicits a Protective Effect Against Fractures and Supports Bone Repair in Zebrafish

Bone homeostasis is a dynamic, multicellular process which is required throughout life to maintain bone integrity, prevent fracture and respond to skeletal damage. WNT16 has been linked to bone fragility and osteoporosis in humans, as well as functional haematopoiesis of leukocytes in vivo , but the mechanisms by which it promotes bone health and repair are not fully understood. We used CRISPR-Cas9 to generate mutant zebrafish lacking Wnt16 ( wnt16 -/- ) to study its effect on bone dynamically. wnt16 mutants displayed variable tissue mineral density and were susceptible to spontaneous fractures and the accumulation of bone calluses at an early age. Fractures were induced in the lepidotrichia of the caudal fins of wnt16 -/- and wild type (WT) zebrafish; this model was used to probe the mechanisms by which Wnt16 regulates skeletal and immune cell-dynamics in vivo. wnt16 mutants repaired fractures more slowly compared to WT zebrafish. Osteoblast cell number was reduced at the fracture site 4 days post-injury in wnt16 mutants, coinciding with prolonged activation of the canonical Wnt signalling pathway. Surprisingly, we found no evidence that the recruitment of innate immune cells to fractures was altered in wnt16 mutants. This study highlights zebrafish as an emerging model for functionally validating osteoporosis-associated genes and investigating fracture repair dynamically in vivo . Using this model, we demonstrate that Wnt16 protects against fracture and is likely to support bone repair by attenuating the activation of the canonical Wnt signalling pathway to facilitate osteoblast recruitment and bone matrix deposition. ### Competing Interest Statement The authors have declared no competing interest.