Small leucine-rich proteoglycans (SLRPs): characteristics and function in the intervertebral disc.

The intervertebral disc (IVD) is responsible for normal spinal motion and load distribution. However, degeneration may occur due to age- and non-age-related processes and is primarily characterized by a reduction in the number of chondrocyte-like cells and abnormal extracellular matrix (ECM) structure in the nucleus pulposus. Although IVD progenitor cells have been identified, the local microenvironment components regulating the behaviour of these progenitor cell populations remain unknown. Small leucine-rich proteoglycans (SLRPs) are bioactive components of the ECM associated with fibrillogenesis, cellular growth and apoptosis and tissue remodelling. SLRPs support the survival of IVD progenitor cells under hypoxic conditions via the activation of specific hypoxia-inducible factors. Additionally, SLRPs deficiency (biglycan) in knockout mice is sufficient to accelerate the IVD degenerative process. These data suggest that SLRPs play an important role in the homeostasis of IVD. Given their specific properties and physiological functions, we propose a role of SLRPs in IVD degeneration and potential application in its regeneration.