Cellular behavior and extracellular matrix turnover in bovine annulus fibrosus cells under hydrostatic pressure and deviatoric strain

Abstract

Intervertebral disc herniation is a common spinal disorder that is often treated with discectomy when conservative measures fail. To devise therapeutic strategies for tears in the annulus fibrosus (AF), the regenerative capability of AF cells under spinal loading needs to be addressed. We hypothesized that the compressive loading associated with deformation in AF cells reduces synthetic and degradative activities in extracellular matrix and cell proliferation. We evaluated expression of key matrix molecules and cell proliferation by RT-PCR and immunohistochemistry by inner and outer bovine AF cells incubated under hydrostatic pressure (HP), arc-bending strain (Strain), and combined HP and Strain (HP/Strain) mimicking spinal loading. Inner AF cells showed significantly increased levels of aggrecan core protein, chondroitin sulfate N-acetylgalactosaminyltransferase-1, and tissue inhibitor of metalloproteinases-2 by 6 days under HP (p < 0.05), with a tendency toward increased matrix metalloproteinase-13. Outer AF cells demonstrated a significant decline in collagen type-2 under Strain and HP/Strain (p < 0.05) and a tendency toward suppression of collagen type-1 and elastin expression compared to HP and unloaded control. On the other hand, proliferating cell nucleus antigen increased significantly under Strain and HP/Strain in inner AF and declined under unloaded and HP in outer AF (p < 0.05). Immunohistology findings supported reductions in gene expressions of matrix molecules. Thus, changes in HP/Strain in AF appear to diminish synthetic and degradative activities while increasing cell proliferation. To promote regeneration, continuous overloading should be avoided, as it converts the synthetic activity to a state in which tissue repair is limited.

Keywords: deviatoric strain; hydrostatic pressure; inner and outer annulus fibrosus; matrix homeostasis.