Identification and characterization of stem cell-like populations in primate intervertebral disc

Abstract

Upon aging, the intervertebral disc (IVD) inevitably undergoes degeneration characterized by biochemical and morphologic changes. IVD degeneration can lead to multiple clinical disorders such as back and neck pain, and myelopathy. Low back pain can disable up to 85% of the adult population and results in a significant restriction of social activities and inability to work. Such disorder incurs billions of dollars in medical expenditures each year. Despite advances in the detection and treatment of the degeneration, the regeneration of the IVD remains low because current therapies are limited by exogenous curing approaches. New strategies for the reversal of IVD degeneration, including gene, cytokine, and stem cell therapies that can influence the anabolic and catabolic pathways in disc cells have been under investigation. These therapies aim to rejuvenate or replace diminished nucleus pulposus cells in the degenerative IVD. Recent reports have put forth a proposal of “endogenous disc stem cells”, suggesting that cells derived from the degenerative IVD tissue possess stem cell properties. These putative stem cells are believed to regulate the development and homeostasis of the IVD tissue. In this study, we identified and characterized a stem cell population from the IVD of healthy Rhesus monkey, termed disc stem/progenitor cells (DSCs). We show that the DSCs possess clonogenicity, multipotency and self-renewal capacity. The DSCs are phenotypically similar to bone marrow mesenchymal stem cells (BMSCs) but they are not identical. The DSCs show a faster growth rate under hypoxia than normoxia. DSCs derived from nucleus pulposus (DSCNP) show a stable expression level of hypoxia inducible factor-1 alpha (Hif-1ａ) in response to hypoxia. DSCs derived from annulus fibrosus (DSCAF) are more resistant to apoptosis under hypoxia than DSCNP. More importantly, small leucine-rich proteoglycans (SLRPs) are identified as important DSC niche components. We show that biglycan (bgn) and decorin (dcn) reduce the susceptibility of DSCs to hypoxia-induced apoptosis by promoting the expression of hypoxia inducible factors (HIFs). Our findings suggest that DSCs rely on the unique niche components for survival.

In summary, our findings propose the existence of endogenous stem cells in IVD. Further study of the DSCs may provide new insights into the biology of IVD and facilitate the design of new strategies to treat disc degeneration in future.