title: Cartilage to bone transformation during fracture healing is coordinated by the invading vasculature and induction of the core pluripotency genes.
abstract: Fractures heal predominantly through the process of endochondral ossification. The classic model of endochondral ossification holds that chondrocytes mature to hypertrophy, undergo apoptosis and new bone forms by invading osteoprogenitors. However, recent data demonstrate that chondrocytes transdifferentiate to osteoblasts in the growth plate and during regeneration, yet the mechanism(s) regulating this process remain unknown. Here, we show a spatially-dependent phenotypic overlap between hypertrophic chondrocytes and osteoblasts at the chondro-osseous border in the fracture callus, in a region we define as the transition zone (TZ). Hypertrophic chondrocytes in the TZ activate expression of the pluripotency factors [Sox2, Oct4 (Pou5f1), Nanog], and conditional knock-out of Sox2 during fracture healing results in reduction of the fracture callus and a delay in conversion of cartilage to bone. The signal(s) triggering expression of the pluripotency genes are unknown, but we demonstrate that endothelial cell conditioned medium upregulates these genes in ex vivo fracture cultures, supporting histological evidence that transdifferentiation occurs adjacent to the vasculature. Elucidating the cellular and molecular mechanisms underlying fracture repair is important for understanding why some fractures fail to heal and for developing novel therapeutic interventions.
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco General Hospital (SFGH), Department of Orthopaedic Surgery, Orthopaedic Trauma Institute, 2550 23rd Street, Building 9, 3rd Floor, San Francisco, CA 94110, USA.
authors.forename: Diane P
authors.initials: DP
authors.lastname: Hu
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco General Hospital (SFGH), Department of Orthopaedic Surgery, Orthopaedic Trauma Institute, 2550 23rd Street, Building 9, 3rd Floor, San Francisco, CA 94110, USA.
authors.forename: Federico
authors.initials: F
authors.lastname: Ferro
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco General Hospital (SFGH), Department of Orthopaedic Surgery, Orthopaedic Trauma Institute, 2550 23rd Street, Building 9, 3rd Floor, San Francisco, CA 94110, USA.
authors.forename: Frank
authors.initials: F
authors.lastname: Yang
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco General Hospital (SFGH), Department of Orthopaedic Surgery, Orthopaedic Trauma Institute, 2550 23rd Street, Building 9, 3rd Floor, San Francisco, CA 94110, USA.
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Taylor
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco Veterans Affairs Medical Center (VAMC), Department of Medicine, 1700 Owens Street, 4th Floor, San Francisco, CA 94158, USA.
authors.forename: Wenhan
authors.initials: W
authors.lastname: Chang
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco General Hospital (SFGH), Department of Orthopaedic Surgery, Orthopaedic Trauma Institute, 2550 23rd Street, Building 9, 3rd Floor, San Francisco, CA 94110, USA.
authors.forename: Theodore
authors.initials: T
authors.lastname: Miclau
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco General Hospital (SFGH), Department of Orthopaedic Surgery, Orthopaedic Trauma Institute, 2550 23rd Street, Building 9, 3rd Floor, San Francisco, CA 94110, USA Chelsea.Bahney@UCSF.edu Ralph.Marcucio@UCSF.edu.
authors.forename: Ralph S
authors.initials: RS
authors.lastname: Marcucio
authors.affiliation: University of California, San Francisco (UCSF) & San Francisco General Hospital (SFGH), Department of Orthopaedic Surgery, Orthopaedic Trauma Institute, 2550 23rd Street, Building 9, 3rd Floor, San Francisco, CA 94110, USA Chelsea.Bahney@UCSF.edu Ralph.Marcucio@UCSF.edu.
authors.forename: Chelsea S
authors.identifier: 0000-0001-9808-8888
authors.initials: CS
authors.lastname: Bahney
mesh_terms: D000818:Animals; D001842:Bone and Bones / Q000166:cytology / Q000502:physiology; D002146:Bony Callus / Q000254:growth & development / Q000378:metabolism; D002356:Cartilage / Q000166:cytology / Q000502:physiology; D054338:Cell Transdifferentiation / Q000235:genetics*; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology / Q000502:physiology*; D020219:Chondrogenesis / Q000502:physiology; D017102:Fracture Healing / Q000235:genetics / Q000502:physiology; D061307:Human Umbilical Vein Endothelial Cells; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018919:Neovascularization, Physiologic / Q000235:genetics / Q000502:physiology*; D010006:Osteoblasts / Q000166:cytology / Q000502:physiology*; D010012:Osteogenesis / Q000502:physiology*; D039904:Pluripotent Stem Cells / Q000502:physiology*; D015854:Up-Regulation / Q000235:genetics
keywords: Chondrocyte transformation; Endochondral ossification; Fracture repair; Pluripotency programs