title: The Wnt serpentine receptor Frizzled-9 regulates new bone formation in fracture healing.
abstract: Wnt signaling is a key regulator of bone metabolism and fracture healing. The canonical Wnt/β-catenin pathway is regarded as the dominant mechanism, and targeting this pathway has emerged as a promising strategy for the treatment of osteoporosis and poorly healing fractures. In contrast, little is known about the role of non-canonical Wnt signaling in bone. Recently, it was demonstrated that the serpentine receptor Fzd9, a Wnt receptor of the Frizzled family, is essential for osteoblast function and positively regulates bone remodeling via the non-canonical Wnt pathway without involving β-catenin-dependent signaling. Here we investigated whether the Fzd9 receptor is essential for fracture healing using a femur osteotomy model in Fzd9(-/-) mice. After 10, 24 and 32 days the fracture calli were analyzed using biomechanical testing, histomorphometry, immunohistochemistry, and micro-computed tomography. Our results demonstrated significantly reduced amounts of newly formed bone at all investigated healing time points in the absence of Fzd9 and, accordingly, a decreased mechanical competence of the callus tissue in the late phase of fracture healing. In contrast, cartilage formation and numbers of osteoclasts degrading mineralized matrix were unaltered. β-Catenin immunolocalization showed that canonical Wnt-signaling was not affected in the absence of Fzd9 in osteoblasts as well as in proliferating and mature chondrocytes within the fracture callus. The expression of established differentiation markers was not altered in the absence of Fzd9, whereas chemokines Ccl2 and Cxcl5 seemed to be reduced. Collectively, our results suggest that non-canonical signaling via the Fzd9 receptor positively regulates intramembranous and endochondral bone formation during fracture healing, whereas it does not participate in the formation of cartilage or in the osteoclastic degradation of mineralized matrix. The finding that Fzd9, in addition to its role in physiological bone remodeling, regulates bone repair may have implications for the development of treatments for poorly or non-healing fractures.
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Aline
authors.initials: A
authors.lastname: Heilmann
authors.affiliation: Department of Osteology and Biomechanics, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Schinke
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Ronny
authors.initials: R
authors.lastname: Bindl
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Tim
authors.initials: T
authors.lastname: Wehner
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Anna
authors.initials: A
authors.lastname: Rapp
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Melanie
authors.initials: M
authors.lastname: Haffner-Luntzer
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Claudia
authors.initials: C
authors.lastname: Nemitz
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Astrid
authors.initials: A
authors.lastname: Liedert
authors.affiliation: Department of Osteology and Biomechanics, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Amling
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research, University of Ulm, Ulm, Germany.
authors.forename: Anita
authors.initials: A
authors.lastname: Ignatius
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002146:Bony Callus / Q000502:physiology; D017102:Fracture Healing / Q000502:physiology*; D051157:Frizzled Receptors; D006652:Histological Techniques; D051379:Mice; D010012:Osteogenesis / Q000502:physiology*; D017981:Receptors, Neurotransmitter / Q000378:metabolism*; D018709:Statistics, Nonparametric; D060449:Wnt Signaling Pathway / Q000502:physiology*; D055114:X-Ray Microtomography
chemical_list: D051157:Frizzled Receptors; C403212:Fzd9 protein, mouse; D017981:Receptors, Neurotransmitter