Loading-Induced Stress Response in the Intervertebral Disc

Abstract

Introduction Previous research has been done to study the effect of mechanical loading on intervertebral disc (IVD) cells. However, few studies have investigated in whether the IVD cells perceive mechanical loading as stress and respond by expression of stress response proteins such as heat shock proteins (HSP). Studies have shown that stress response can be seen in cell line chondrocytes under hydrostatic pressure. On the other hand, studies have also shown that expression of heat shock protein-72 (HSP72) and HSP27 was associated with disc degeneration and IVD cells can secrete HSP70 in response to oxidative stress. This study aims to study the stress response in the IVD in response to compressive loading and whether the disc cells are able to adapt to the loading. The outcome of the study will help to understand how the disc cells adapt or cope with mechanical stress. Materials and Methods Fresh adult bovine caudal discs were harvested and cultured with dynamic compressive loading applied at physiological range magnitude, 0.1 to 0.6 MPa. The culture condition was such that the discs underwent 2 hours of dynamic loading, followed by 22 hours of resting for 2 days. Samples were retrieved at different time points: right after loading (Dyna) and right after resting (DyNa+rest). Positive control discs were put under static loading (0.35 MPa, static) and heat shock (43°C, HS) exposed for 2 h/d during 2 days and gene expression was quantified right after the treatments. Both nucleus pulposus (NP) and annulus fibrosus (AF) were retrieved for gene expression study of the cellular stress response genes. HSP72 and heat shock factor-1 (HSF1). HSP72 is the general stress response protein which is upregulated in the cell in response to stress while HSF1 is the transcriptional factor of HSP72. The expression was normalized to free swelling control. Results In the NP of the bovine disc, both positive controls (HS and static) expressed high level of HSP72, confirming their expression in the NP tissues and their response to stress. For the experimental groups, the expression of HSP72 was upregulated after loading, decreased after resting but was again increased after second round of loading at day 2. On the other hand, HSF1 expression increased after resting in the day 1 loading and peaked at day 2 after loading. For the AF tissues, the expression of HSF1 was low in most of the groups including the positive control, even the HSP72 expression was high in these two groups. The expression of HSP72 in AF tissues was decreasing with both resting and an additional round of loading. The pattern of HSF1 expression of AF tissues was similar to the NP tissues where the expression was the highest 2 days after loading. Conclusion This study showed that the IVD cells do upregulate the stress response proteins expression in response to loading induced stress. The cells express HSP72 in response to the stress while HSF1 may have a slower and transient expression. The increase in HSP72 and HSF1 expression after two rounds of loading may indicate more cycles are needed to see whether there is adaptation in stress response induced by mechanical loading. Disclosure of Interest None declared