Nucleus Pulposus Cell Response to Confined and Unconfined Compression Implicates Mechanoregulation by Fluid Shear Stress

Abstract

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.

Dynamic Compression of Chondrocyte-Agarose Constructs Reveals New Candidate Mechanosensitive Genes

“…Biglycan (Bgn) , an extracellular matrix protein [49], Mmp9 , a matrix metalloprotease [50], Cyr61 , a regulator of chondrogenesis from the CCN family [43], Cited2 , a transcription co-regulator playing a key role in shear-induced regulation of MMPs in chondrocytes [51], or in other tissues, e.g. Thrombomodulin (Thbd) , a gene coding for an anticoagulant factor [52], Lmo4 , a fluid flow-responsive transcription factor [53], Ptgs1/Cox1 , a cyclooxygenase involved in the production of prostaglandin E2 [54], or Ahnak , a protein involved in Ca 2+ signalling pathways and regulated exocytosis [55], [56].…”

Reconstruction of an In Vitro Niche for the Transition from Intervertebral Disc Development to Nucleus Pulposus Regeneration

“…Other than hydrostatic pressure, the NP also experiences high interstitial fluid and osmotic pressure during loading that arises from a high fluid content and PGassociated negative charges [91,92]. NPCs are also sensitive to fluid shear stress [93]. The magnitude-and durationdependent changes in gene expression resulting from compression were identified in the NPC.…”

Ethanol‐mediated compaction and cross‐linking enhance mechanical properties and degradation resistance while maintaining cytocompatibility of a nucleus pulposus scaffold

“…Furthermore, introducing compaction into our cross‐linking treatment likely induced changes in the biphasic properties of our constructs, which we quantified via dynamic mechanical analyses. Confined compression could provide a more direct evaluation of these changes and has been well described in the literature for NP tissue . Another limitation is our implementation of the enzymes used for degradation, which was based on the concentrations thought to occur in a degenerative environment rather than their activity levels .…”