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Loss of HIF-1α in the notochord results in cell death and complete disappearance of the nucleus pulposus.

Merceron C, Mangiavini L, Robling A, Wilson TL, Giaccia AJ, Shapiro IM, Schipani E, Risbud MV - PLoS ONE (2014)

Bottom Line: 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.Loss of the NP in mutant mice significantly reduced the IVD biomechanical properties by decreasing its ability to absorb mechanical stress.

View Article: PubMed Central - PubMed

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.

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.

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Metabolic changes in the mutant NP.In situ hybridization for PGK mRNA in the NP of control (HIF-1αf/f) (a,c) and mutant (Foxa2iCre;HIF-1αf/f) (b,d) mice at E15.5. Darkfield (a,b) and brightfield (c,d) pictures are shown. Bar = 50 µm.
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pone-0110768-g011: Metabolic changes in the mutant NP.In situ hybridization for PGK mRNA in the NP of control (HIF-1αf/f) (a,c) and mutant (Foxa2iCre;HIF-1αf/f) (b,d) mice at E15.5. Darkfield (a,b) and brightfield (c,d) pictures are shown. Bar = 50 µm.

Mentions: Last, we asked why mutant NP cells died. In situ hybridizations showed that phosphoglycerate kinase-1 (PGK) mRNA is highly expressed in the normal NP, consistent with the notion that, likewise the avascular fetal growth plate, the avascular NP lives on anaerobic glycolysis rather than on oxidative phosphorylation. PGK is a known downstream target of HIF-1α, and it catalyzes the reversible transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP producing 3-phosphoglycerate and ATP during glycolysis. Notably, expression of PGK mRNA was not anymore detectable in the NP of mice deficient for HIF-1α, though, as expected was still present in their mutant VBs (Figure 11). Taken together, these findings confirmed that HIF-1α had been efficiently deleted in notochordal cells at E15.5 and suggest that changes in glycolytic metabolism may contribute to the cell death observed in the mutant NP.


Loss of HIF-1α in the notochord results in cell death and complete disappearance of the nucleus pulposus.

Merceron C, Mangiavini L, Robling A, Wilson TL, Giaccia AJ, Shapiro IM, Schipani E, Risbud MV - PLoS ONE (2014)

Metabolic changes in the mutant NP.In situ hybridization for PGK mRNA in the NP of control (HIF-1αf/f) (a,c) and mutant (Foxa2iCre;HIF-1αf/f) (b,d) mice at E15.5. Darkfield (a,b) and brightfield (c,d) pictures are shown. Bar = 50 µm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4206488&req=5

pone-0110768-g011: Metabolic changes in the mutant NP.In situ hybridization for PGK mRNA in the NP of control (HIF-1αf/f) (a,c) and mutant (Foxa2iCre;HIF-1αf/f) (b,d) mice at E15.5. Darkfield (a,b) and brightfield (c,d) pictures are shown. Bar = 50 µm.
Mentions: Last, we asked why mutant NP cells died. In situ hybridizations showed that phosphoglycerate kinase-1 (PGK) mRNA is highly expressed in the normal NP, consistent with the notion that, likewise the avascular fetal growth plate, the avascular NP lives on anaerobic glycolysis rather than on oxidative phosphorylation. PGK is a known downstream target of HIF-1α, and it catalyzes the reversible transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP producing 3-phosphoglycerate and ATP during glycolysis. Notably, expression of PGK mRNA was not anymore detectable in the NP of mice deficient for HIF-1α, though, as expected was still present in their mutant VBs (Figure 11). Taken together, these findings confirmed that HIF-1α had been efficiently deleted in notochordal cells at E15.5 and suggest that changes in glycolytic metabolism may contribute to the cell death observed in the mutant NP.

Bottom Line: 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.Loss of the NP in mutant mice significantly reduced the IVD biomechanical properties by decreasing its ability to absorb mechanical stress.

View Article: PubMed Central - PubMed

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.

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.

Show MeSH
Related in: MedlinePlus