A role of biglycan C-terminal peptide in regulating intervertebral disc homeostasis

Abstract

Intervertebral discs (IVDs) are essential to spinal motion. Each IVD comprises a proteoglycan-enriched gelatinous core, the nucleus pulposus (NP), which confers mechanical strength to the disc to resist axial loads. NP cells are derived from the notochord, and their normal biological regulation exerts a role in maintaining IVD homeostasis during IVD development. IVD degeneration (IDD) is associated with aging and chronic inflammation, involving excessive degradation and remodeling of the extracellular matrix (ECM). However, the mechanism of IDD pathogenesis is not yet fully understood, and effective treatments for IDD remain to be investigated. Biglycan (BGN) is a member in the first class of small leucine-rich proteoglycans (SLRPs) that have been suggested to have diverse biological functions predominantly in the ECM. Sequestered BGN has been reported to interact with collagen and thus deposit in the ECM to promote fibrosis or scar formation, while soluble BGN is often released from the ECM to trigger cell-mediated inflammation. The depletion of BGN can accelerate mouse IDD, and fragmentation of BGN is coupled with the severity of human IDD. These findings suggest the role of BGN in IVD homeostasis. Nevertheless, the role played by changes in BGN activity during IDD so far remains elusive.

An earlier study reported that the secreted BGN neoepitope 344YWEVQPATFR (Bgm1) is associated with disease progression related to dysregulated matrix remodeling. However, no work to date has exclusively studied the expression, function and mechanism of Bgm1. Here, I hypothesized that Bgm1 may serve as a regulator of NP homeostasis in IVDs. To explore the involvement of Bgm1 in the progression of IDD, a custom rabbit polyclonal antibody that recognizes Bgm1 (Bgm1 Y-R) was generated. By immunostaining, Bgm1 signal was found to be stronger in human non- degenerative NP samples than in degenerative samples. Moreover, Bgm1 showed specific expression surrounding vacuolar structure cells in mouse NP, but the expression was lost in an induced mouse IDD model. After comparing the protein expression of intracellular and extracellular Bgm1 by immunostaining localization and western blotting, Bgm1 was shown to be mainly expressed intracellularly and differently from BGN. In addition, proteomic analysis revealed a candidate protein eukaryotic translation initiation factor 6 (eIF6) that can interact with Bgm1 and form a complex in the cell. Combined with the transcriptome bulk RNA-sequencing results, this implied that IVD homeostasis may require a dynamic equilibrium of the Bgm1- eIF6 interaction. Disrupting the equilibrium might be associated with the onset of IDD.

In conclusion, this study is the first to imply a role of Bgm1 in IDD progression and implicates an active role of proteoglycan in maintaining IVD homeostasis.