title: Enthesis fibrocartilage cells originate from a population of Hedgehog-responsive cells modulated by the loading environment.
abstract: Tendon attaches to bone across a specialized tissue called the enthesis. This tissue modulates the transfer of muscle forces between two materials, i.e. tendon and bone, with vastly different mechanical properties. The enthesis for many tendons consists of a mineralized graded fibrocartilage that develops postnatally, concurrent with epiphyseal mineralization. Although it is well described that the mineralization and development of functional maturity requires muscle loading, the biological factors that modulate enthesis development are poorly understood. By genetically demarcating cells expressing Gli1 in response to Hedgehog (Hh) signaling, we discovered a unique population of Hh-responsive cells in the developing murine enthesis that were distinct from tendon fibroblasts and epiphyseal chondrocytes. Lineage-tracing experiments revealed that the Gli1 lineage cells that originate in utero eventually populate the entire mature enthesis. Muscle paralysis increased the number of Hh-responsive cells in the enthesis, demonstrating that responsiveness to Hh is modulated in part by muscle loading. Ablation of the Hh-responsive cells during the first week of postnatal development resulted in a loss of mineralized fibrocartilage, with very little tissue remodeling 5 weeks after cell ablation. Conditional deletion of smoothened, a molecule necessary for responsiveness to Ihh, from the developing tendon and enthesis altered the differentiation of enthesis progenitor cells, resulting in significantly reduced fibrocartilage mineralization and decreased biomechanical function. Taken together, these results demonstrate that Hh signaling within developing enthesis fibrocartilage cells is required for enthesis formation.
authors.affiliation: Department of Orthopaedic Surgery, Washington University in St Louis, St Louis, MO 63110, USA.
authors.forename: Andrea G
authors.initials: AG
authors.lastname: Schwartz
authors.affiliation: Department of Orthopaedic Surgery, Washington University in St Louis, St Louis, MO 63110, USA Department of Medicine, Washington University in St Louis, St Louis, MO 63110, USA Department of Developmental Biology, Washington University in St Louis, St Louis, MO 63110, USA.
authors.forename: Fanxin
authors.initials: F
authors.lastname: Long
authors.affiliation: Department of Orthopaedic Surgery, Washington University in St Louis, St Louis, MO 63110, USA Department of Biomedical Engineering, Washington University in St Louis, St Louis, MO 63110, USA Department of Mechanical Engineering & Materials Science, Washington University in St Louis, St Louis, MO 63110, USA ThomopoulosS@wudosis.wustl.edu.
authors.forename: Stavros
authors.initials: S
authors.lastname: Thomopoulos
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001696:Biomechanical Phenomena; D001905:Botulinum Toxins / Q000633:toxicity; D002113:Calcification, Physiologic; D051445:Fibrocartilage / Q000166:cytology*; D053823:Hedgehog Proteins / Q000378:metabolism*; D019426:Integrases / Q000378:metabolism; D008822:Mice, Transgenic; D008954:Models, Biological; D009132:Muscles / Q000502:physiology*; D010243:Paralysis / Q000139:chemically induced / Q000473:pathology; D015398:Signal Transduction; D016474:Weight-Bearing; D055114:X-Ray Microtomography
chemical_list: D053823:Hedgehog Proteins; C045073:Cre recombinase; D019426:Integrases; D001905:Botulinum Toxins
keywords: Biomineralization; Fibrocartilage; Indian hedgehog; Lineage tracing; Mouse; Postnatal development; Tendon enthesis