Nucleus pulposus cells synthesize a functional extracellular matrix and respond to inflammatory cytokine challenge following long-term agarose culture

Abstract

Intervertebral disc degeneration is characterized by a cascade of cellular, biochemical and structural changes that may lead to functional impairment and low back pain. Interleukin-1 beta (IL-1β) is strongly implicated in the etiology of disc degeneration, however there is currently no direct evidence linking IL-1β upregulation to downstream biomechanical changes. The objective of this study was to evaluate long-term agarose culture of nucleus pulposus (NP) cells as a potential in vitro model system to investigate this. Bovine NP cells were cultured in agarose for 49 days in a defined medium containing transforming growth factor-beta 3, after which both mechanical properties and composition were evaluated and compared to native NP. The mRNA levels of NP cell markers were compared to those of freshly isolated NP cells. Glycosaminoglycan (GAG) content, aggregate modulus and hydraulic permeability of mature constructs were similar to native NP, and aggrecan and SOX9 mRNA levels were not significantly different from freshly isolated cells. To investigate direct links between IL-1β and biomechanical changes, mature agarose constructs were treated with IL-1β, and effects on biomechanical properties, extracellular matrix composition and mRNA levels were quantified. IL-1β treatment resulted in upregulation of a disintegrin and metalloproteinase with thrombospondin motifs 4, matrix metalloproteinase-13 and inducible nitric oxide sythase, decreased GAG and modulus, and increased permeability. To evaluate the model as a test platform for therapeutic intervention, co-treatment with IL-1β and IL-1 receptor antagonist (IL-1ra) was evaluated. IL-1ra significantly attenuated degradative changes induced by IL-1β. These results suggest that this in vitro model represents a reliable and cost-effective platform for evaluating new therapies for disc degeneration.