The effects of cyclic tensile strain on the organisation and expression of cytoskeletal elements in bovine intervertebral disc cells: an in vitro study.

It is still relatively unclear how intervertebral disc (IVD) cells sense a mechanical stimulus and convert this signal into a biochemical response. Previous studies demonstrated that the cytoskeletal elements are mechano-responsive in many cell types and may contribute to mechano-signalling pathways. The objective of this study was to determine the response of cells from the outer annulus fibrosus (OAF) to physiological levels of cyclic tensile strain; further, cells from the nucleus pulposus (NP) were also subjected to an identical loading regime to compare biological responses across the IVD populations. We determined whether the organisation and expression of the major cytoskeletal elements and their associated accessory proteins are responsive to mechanical stimulation in these cells, and whether these changes correlated with either a catabolic or anabolic phenotype. OAF and NP cells from immature bovine IVD were seeded onto Flexcell® type I collagen coated plates. Cells were subjected to cyclic tensile strain (10 %, 1 Hz) for 60 minutes. Post-loading, cells were processed for immunofluorescence microscopy, RNA extracted for quantitative PCR and protein extracted for Western blotting analysis. F-actin reorganisation was evident in OAF and NP cells subjected to tensile strain; strain induced β-actin at the transcriptional and translational level in OAF cells. β-tubulin mRNA and protein synthesis increased in strained OAF cells, but vimentin expression was significantly inhibited. Cytoskeletal element organisation and expression were less responsive to strain in NP cells. Tensile strain increased type I collagen and differentially regulated extracellular matrix (ECM)-degrading enzymes' mRNA levels in OAF cells. Strain induced type II collagen transcription in NP cells, but had no effect on the transcription of any other genes analysed. Tensile strain induces different mechano-responses in the organisation and/or expression of cytoskeletal elements and on markers of IVD metabolism. Differential mechano-regulation of anabolic and catabolic ECM components in the OAF and NP populations reflects their respective mechanical environments in situ.