Nucleus pulposus cell response to confined and unconfined compression implicates mechanoregulation by fluid shear stress.

Mechanical loading of the intervertebral disc (IVD) plays an important role in governing the function of nucleus pulposus (NP) cells. In situ, the loading environment of the NP is neither fully confined nor unconfined. To investigate the potential influences of these idealized loading modes, we utilized an alginate cell culture system to compare the effects of 1 h confined and unconfined compression on glycoprotein gene expression in mixed population NP cells. For dynamic compression, we applied 10 kPa peak stress using a sinusoidal waveform at 0.5 Hz. For static compression, displacements were applied at a constant rate and held at 20% strain. Aggrecan, laminin, fibronectin, glypican, biglycan, and fibromodulin were generally stimulated, and trends were similar between confined and unconfined compression. On the other hand, lumican expression was differentially regulated between confined and unconfined groups. More interestingly, trends under static loading were opposite that under dynamic loading; lumican was upregulated in static confined compression and in dynamic unconfined compression. We theorize that this pattern is indicative of regulation by fluid flow within loaded alginate disks. Calculations using the linear biphasic model suggest that spatial pressure gradients exist in static confined specimens for the majority of the loading duration. Using a parallel plate flow chamber, we report for the first time that NP cells are sensitive to fluid shear stress and confirm that lumican expression can be regulated by short durations of exposure to fluid shear.