The Response of Human Anulus Fibrosus Cells to Cyclic Tensile Strain Is Frequency-Dependent and Altered with Disc Degeneration

Objective. Mechanical loads are important for homeostasis of the intervertebral disc (IVD) cell matrix, with physiologic and nonphysiologic loads leading to matrix anabolism and catabolism, respectively. Previous investigations into the effects of load on disc cells have predominantly used animal models, with the limited number of human studies focusing primarily on nucleus pulposus cells. The aim of this study was to examine the effect of cyclic tensile strain (CTS) on human anulus fibrosus (AF) cells to ascertain whether the response was frequency-dependent and to compare AF cells derived from nondegenerated and degenerated tissue samples. Methods. AF cells were isolated from nondegenerated and degenerated human IVDs, expanded in monolayer, and cyclically strained for 20 minutes, applying 10% strain at a frequency of 1.0 Hz or 0.33 Hz with the use of a Flexcell strain device. Total RNA was extracted from the cells at baseline (control) and at 1, 3, and 24 hours following application of CTS. Real-time quantitative polymerase chain reaction was used to analyze gene expression of matrix proteins (aggrecan, type I collagen, and type II collagen) and enzymes (matrix metalloproteinases [MMPs] 3, 9, 13, and ADAMTS-4). Results. The expression of catabolic genes (MMP-3 and ADAMTS-4) in AF cells derived from nondegenerated tissue decreased in response to 1.0 Hz of CTS, whereas changing the frequency to 0.33 Hz resulted in a shift toward matrix catabolism. Application of 1.0 Hz of CTS reduced anabolic gene expression (aggrecan and type 1 collagen) in AF cells derived from degenerated tissue, with 0.33 Hz of CTS resulting in increased catabolic gene expression. Conclusion. The response of human AF cells to CTS is frequency-dependent and is altered by degeneration.