Human cells derived from degenerate intervertebral discs respond differently to those derived from non-degenerate intervertebral discs following application of dynamic hydrostatic pressure

The intervertebral disc (IVD) is one of the body's most important load-bearing structures with the major mechanical force experienced in the nucleus pulposus (NP) being hydrostatic pressure (HP). Physiological levels of HP have an anabolic effect on IVD matrix metabolism in cells derived from non-degenerate animal and herniated IVD while excessive HP has a catabolic effect. However, no studies have investigated the response of non-degenerate and degenerate human disc cells derived from non-herniated discs to HP. Here we investigate the effect of physiological HP on such cells using a novel loading rig. Human IVD cells (both NP and AF) cultured in alginate were subjected to dynamic HP (0.8-1.7 MPa 0.5 Hz) for 2 h. Cell viability was assessed, RNA extracted and qRT-PCR for 18 s, c-fos, Sox-9, collagen type II, aggrecan and MMP-3 performed. Cell viability was unaffected by the loading regime. In non-degenerate NP cells, HP increased c-fos, aggrecan, Sox-9 and collagen type II (significantly so in the case of c-fos and aggrecan), but not MMP-3 gene expression. In contrast, application of HP to AF or degenerate NP cells had no effect on target gene expression. Our data shows that cells obtained from the healthy NP respond to dynamic HP by up-regulating genes indicative of healthy matrix homeostasis. However, responses differed in degenerate NP cells suggesting that an altered mechanotransduction pathway may be operational.