title: Jagged1 expression by osteoblast-lineage cells regulates trabecular bone mass and periosteal expansion in mice.
abstract: Loss-of-function mutations in the Notch ligand, Jagged1 (Jag1), result in multi-system developmental pathologies associated with Alagille syndrome (ALGS). ALGS patients present with skeletal manifestations including hemi-vertebrae, reduced bone mass, increased fracture incidence and poor bone healing. However, it is not known whether the increased fracture risk is due to altered bone homeostasis (primary) or nutritional malabsorption due to chronic liver disease (secondary). To determine the significance of Jag1 loss in bone, we characterized the skeletal phenotype of two Jag1-floxed conditional knockout mouse models: Prx1-Cre;Jag1(f/f) to target osteoprogenitor cells and their progeny, and Col2.3-Cre;Jag1(f/f) to target mid-stage osteoblasts and their progeny. Knockout phenotypes were compared to wild-type (WT) controls using quantitative micro-computed tomography, gene expression profiling and mechanical testing. Expression of Jag1 and the Notch target genes Hes1 and Hey1 was downregulated in all Jag1 knockout mice. Osteoblast differentiation genes were downregulated in whole bone of both groups, but unchanged in Prx1-Cre;Jag1(f/f) cortical bone. Both knockout lines exhibited changes in femoral trabecular morphology including decreased bone volume fraction and increased trabecular spacing, with males presenting a more severe trabecular osteopenic phenotype. Prx1-Cre;Jag1(f/f) mice showed an increase in marrow mesenchymal progenitor cell number and, counterintuitively, developed increased cortical thickness resulting from periosteal expansion, translating to greater mechanical stiffness and strength. Similar alterations in femoral morphology were observed in mice with canonical Notch signaling disrupted using Prx1-Cre-regulatable dominant-negative mastermind like-protein (dnMAML). Taken together, we report that 1) Jag1 negatively regulates the marrow osteochondral progenitor pool, 2) Jag1 is required for normal trabecular bone formation and 3) Notch signaling through homotypic Jag1 signaling in osteochondral progenitors, but not mature osteoblasts, inhibits periosteal expansion. Therefore, Jag1 signaling within the osteoblast lineage regulates bone metabolism in a compartment-dependent manner. Moreover, loss of Jag1 function in osteoblast lineage cells may contribute to the skeletal phenotype associated with ALGS.
authors.affiliation: Department of Small Animal Clinical Sciences, Michigan State University, East Lansing, MI, United States; Department of Physiology, Michigan State University, East Lansing, MI, United States.
authors.forename: D W
authors.initials: DW
authors.lastname: Youngstrom
authors.affiliation: Department of Bioengineering, University of Pennsylvania, Philadelphia, PA, United States.
authors.forename: M I
authors.initials: MI
authors.lastname: Dishowitz
authors.affiliation: Division of Gastroenterology, Hepatology and Nutrition, The Children's Hospital of Philadelphia, Philadelphia, PA, United States; Department of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, United States.
authors.forename: C B
authors.initials: CB
authors.lastname: Bales
authors.affiliation: Department of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, United States.
authors.forename: E
authors.initials: E
authors.lastname: Carr
authors.affiliation: Department of Bioengineering, University of Pennsylvania, Philadelphia, PA, United States.
authors.forename: P L
authors.initials: PL
authors.lastname: Mutyaba
authors.affiliation: Department of Orthopaedic Surgery, University of Michigan School of Medicine, Ann Arbor, MI, United States.
authors.forename: K M
authors.initials: KM
authors.lastname: Kozloff
authors.affiliation: Medical Scientist Training Program, University of Michigan School of Medicine, Ann Arbor, MI, United States.
authors.forename: H
authors.initials: H
authors.lastname: Shitaye
authors.affiliation: Department of Small Animal Clinical Sciences, Michigan State University, East Lansing, MI, United States; Department of Physiology, Michigan State University, East Lansing, MI, United States.
authors.forename: K D
authors.initials: KD
authors.lastname: Hankenson
authors.affiliation: Division of Gastroenterology, Hepatology and Nutrition, The Children's Hospital of Philadelphia, Philadelphia, PA, United States; Department of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, United States. Electronic address: loomes@email.chop.edu.
authors.forename: K M
authors.initials: KM
authors.lastname: Loomes
mesh_terms: D000818:Animals; D001846:Bone Development; D000071556:Cancellous Bone / Q000166:cytology* / Q000000981:diagnostic imaging / Q000196:embryology / Q000378:metabolism; D019070:Cell Lineage; D000071538:Cortical Bone / Q000166:cytology / Q000000981:diagnostic imaging; D017353:Gene Deletion; D005786:Gene Expression Regulation; D000072100:Jagged-1 Protein / Q000378:metabolism*; D059630:Mesenchymal Stem Cells / Q000166:cytology / Q000378:metabolism; D008810:Mice, Inbred C57BL; D009929:Organ Size; D010006:Osteoblasts / Q000166:cytology* / Q000378:metabolism*; D010521:Periosteum / Q000166:cytology*; D051880:Receptors, Notch / Q000378:metabolism; D015398:Signal Transduction; D055114:X-Ray Microtomography
chemical_list: D000072100:Jagged-1 Protein; D051880:Receptors, Notch
keywords: Bone QCT/μCT; Genetic animal models; Osteoblasts; Osteoporosis; Stromal/stem cells