Controllers of apoptosis in herniated and diseased intervertebral disc
Authors: Menage J. , Wojcik A. , Krajewski M. , Reed J.C. , Eisenstein S.M. , Johnson W.E.B. , Roberts S. 
Published: September 2009

Introduction  Cell death in intervertebral disc is common, and this may account for the degeneration of discs occurring relatively early in life in comparison to other connective tissues. Apoptosis, or programmed cell death, provides an efficient mechanism for degrading and disposing of dying cells, minimizing the chance of an inflammatory response; necrosis, in contrast, is uncontrolled and more damaging. The type of cell death in intervertebral disc is not well studied. Disruption of the control system for apoptosis could well play a role in disc diseases, resulting in premature cell death and malfunctioning of the tissue. We have studied the presence of three proteins, members of the Bcl‐2 family, which are either pro‐ or anti‐apoptotic, in herniated and diseased intervertebral disc.

Materials and methods  Thirty‐seven intervertebral discs from 36 patients aged 16–71 years (with spondylolisthesis, low back pain or disc herniations) were immunostained for the presence of members of the Bcl‐2 family of proteins: BAK, which is pro‐apoptotic and Bcl‐X and Bcl‐2, which are anti‐apoptotic. Polyclonal antibodies were raised against synthetic peptides corresponding to the human proteins. Indirect immunohistochemistry was carried out labelling with DAB. Controls were performed using antibody preabsorbed with the antigen in place of the primary antibody. The prevalence of positively stained cells was scored on a scale of 0–3.

Results  There was staining of some cells for all the markers studied in all but three samples. Most samples had approximately 30% of cells staining positively. Staining was generally greater for cells in clusters (more commonly found in degenerate discs) than single cells, particularly for BAK. All antigens were present in the same cluster of cells, sometimes possibly produced by the same cells. There was a positive correlation between staining for all three members of the Bcl‐2 family, both apoptosis‐promoting and ‐inhibiting.

Discussion  This preliminary study suggests that members of the Bcl‐2 family play a role in regulating cell death via apoptosis, at least in diseased human intervertebral discs. Apoptosis of prolapsed disc tissue may be the least deleterious way for the tissue to ‘self destruct’. Both pro‐ and anti‐apoptotic markers are present in the same region, which at first glance may appear surprising. However, Bcl‐2 and Bcl‐x promote apoptosis on homodimerization. One manner in which BAK inhibits apoptosis is by binding to Bcl‐2 and Bcl‐x and preventing this occurring. In addition, the presence of pro‐ and anti‐apoptotic members of the Bcl family may simply indicate that regulatory pathways of cell survival or death have been initiated within the cells. The fate of these cells may well depend on relative levels of expression or activity of these pro‐ or anti‐apoptotic factors within a given cell. Hence, the production and interaction of various members of these multigene families within the apoptotic pathway in intervertebral disc remains to be clarified.