title: Loss of HIF-1α in the notochord results in cell death and complete disappearance of the nucleus pulposus.
abstract: The intervertebral disc (IVD) is one of the largest avascular organs in vertebrates. The nucleus pulposus (NP), a highly hydrated and proteoglycan-enriched tissue, forms the inner portion of the IVD. The NP is surrounded by a multi-lamellar fibrocartilaginous structure, the annulus fibrosus (AF). This structure is covered superior and inferior side by cartilaginous endplates (CEP). The NP is a unique tissue within the IVD as it results from the differentiation of notochordal cells, whereas, AF and CEP derive from the sclerotome. The hypoxia inducible factor-1α (HIF-1α) is expressed in NP cells but its function in NP development and homeostasis is largely unknown. We thus conditionally deleted HIF-1α in notochordal cells and investigated how loss of this transcription factor impacts NP formation and homeostasis at E15.5, birth, 1 and 4 months of age, respectively. Histological analysis, cell lineage studies, and TUNEL assay were performed. Morphologic changes of the mutant NP cells were identified as early as E15.5, followed, postnatally, by the progressive disappearance and replacement of the NP with a novel tissue that resembles fibrocartilage. Notably, lineage studies and TUNEL assay unequivocally proved that NP cells did not transdifferentiate into chondrocyte-like cells but they rather underwent massive cell death, and were completely replaced by a cell population belonging to a lineage distinct from the notochordal one. Finally, to evaluate the functional consequences of HIF-1α deletion in the NP, biomechanical testing of mutant IVD was performed. Loss of the NP in mutant mice significantly reduced the IVD biomechanical properties by decreasing its ability to absorb mechanical stress. These findings are similar to the changes usually observed during human IVD degeneration. Our study thus demonstrates that HIF-1α is essential for NP development and homeostasis, and it raises the intriguing possibility that this transcription factor could be involved in IVD degeneration in humans.
authors.affiliation: Department of Orthopaedic Surgery, Medical School, University of Michigan, Ann Arbor, Michigan, United States of America; Inserm, UMRS 791-LIOAD, Centre for Osteoarticular and Dental Tissue Engineering, Group STEP 'Skeletal Tissue Engineering and Physiopathology', Nantes, France; LUNAM, Nantes University, Faculty of Dental Surgery, Nantes, France.
authors.forename: Christophe
authors.initials: C
authors.lastname: Merceron
authors.affiliation: Department of Orthopaedic Surgery, Medical School, University of Michigan, Ann Arbor, Michigan, United States of America; Department of Orthopaedic and Traumatology, Milano-Bicocca University, Monza (MB), Italy.
authors.forename: Laura
authors.initials: L
authors.lastname: Mangiavini
authors.affiliation: School of Medicine, Department of Anatomy and Cell Biology, Indiana University, Indianapolis, Indiana, United States of America.
authors.forename: Alexander
authors.initials: A
authors.lastname: Robling
authors.affiliation: Department of Orthopaedic Surgery, Medical School, University of Michigan, Ann Arbor, Michigan, United States of America.
authors.forename: Tremika LeShan
authors.initials: TL
authors.lastname: Wilson
authors.affiliation: School of Medicine, Department of Radiation Oncology, Division of Radiation and Cancer Biology, Stanford University, Stanford, California, United States of America.
authors.forename: Amato J
authors.initials: AJ
authors.lastname: Giaccia
authors.affiliation: Department of Orthopaedic Surgery and Graduate Program in Cell and Developmental Biology, Thomas Jefferson University, Philadelphia, Pennsylvania, United States of America.
authors.forename: Irving M
authors.initials: IM
authors.lastname: Shapiro
authors.affiliation: Department of Orthopaedic Surgery, Medical School, University of Michigan, Ann Arbor, Michigan, United States of America; Department of Medicine, Division of Endocrinology, Medical School, University of Michigan, Ann Arbor, Michigan, United States of America.
authors.forename: Ernestina
authors.initials: E
authors.lastname: Schipani
authors.affiliation: Department of Orthopaedic Surgery and Graduate Program in Cell and Developmental Biology, Thomas Jefferson University, Philadelphia, Pennsylvania, United States of America.
authors.forename: Makarand V
authors.initials: MV
authors.lastname: Risbud
mesh_terms: D000818:Animals; D017209:Apoptosis; D001696:Biomechanical Phenomena; D002454:Cell Differentiation; D049109:Cell Proliferation; D002470:Cell Survival; D005260:Female; D006706:Homeostasis; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000172:deficiency / Q000235:genetics*; D007403:Intervertebral Disc / Q000002:abnormalities / Q000166:cytology / Q000378:metabolism; D008297:Male; D008822:Mice, Transgenic; D009672:Notochord / Q000166:cytology* / Q000378:metabolism
chemical_list: C497443:Hif1a protein, mouse; D051795:Hypoxia-Inducible Factor 1, alpha Subunit