Hypoxia-inducible factor (HIF)-1alpha knockout accelerates intervertebral disc degeneration in mice

The abnormality of nucleus pulposus (NP) plays a critical role in intervertebral disc (IVD) degeneration, in which NP cells show apoptosis and fibrosis, leading to the ability of the disc to transfer and distribute loads between the vertebrae is decreased. Considering that hypoxia inducible factor-1α (HIF-1α) is abundantly expressed in NP and that it mediates cell proliferation, migration and apoptosis in various cell types, we hypothesized that NP-HIF-1α plays an important role in NP and evaluate whether NP-HIF-1α is involved in IVD degeneration.

Sonic Hedgehog-Cre^+/- mice were crossed with HIF-1α^flox/flox mice to generate NP specific HIF-1α-deficient (HIF-1α^-/-) mice. Magnetic resonance imaging (MRI) study was used to evaluate NP dehydration and X-ray study was used to acquire the changes of disc height. Histological changese, content of glycoproteins and the in situ expression of aggrecan were evaluated by hematoxylin & eosin (H&E) staining, safranin-O/fast green staining and immunohistochemistry assay, respectively. Western bloting was used to detect the change of extracellular matrix in IVD.

Firstly, the results of in situ hybridization confirmed that HIF-1α in NP was successfully knocked out in HIF-1α^-/- mice. Next, for HIF-1α deficiency mice, imaging study shows IVD was narrowed in X-ray and signal intensity of NP was decreased in MR T2-weight imaging. Accordingly, the size and cell number of NP and proteoglycan content was decreased in NP-HIF-1α^-/- mice. Finally, Western bloting shows that protein level of collagen II and aggrecan, two main matrix in disc, were both decreased in NP-HIF-1α^-/- mice.

The present study demonstrates that HIF-1α is essential for NP development and homeostasis and the deficiency of NP-HIF-1α leads to IVD degeneration in mice.