title: Prolonged spinal loading induces matrix metalloproteinase-2 activation in intervertebral discs.
abstract: METHODS
  An established in vivo mouse model of compression-induced disc degeneration was used to investigate the effects of load on matrix catabolism.
  
  
  OBJECTIVE
  To determine whether matrix metalloproteinase-2 expression in discs is modulated by mechanical load and to characterize the regulation of matrix metalloproteinase-2 activity.
  
  
  BACKGROUND
  We have previously shown that static compression of discs elicits changes in tissue architecture consistent with those seen with degeneration. Evidence in the literature demonstrates the existence of matrix metalloproteinases in both healthy and pathologic discs and suggests that mechanical load may influence matrix metalloproteinase expression and activity.
  
  
  METHODS
  Static compression was applied to mouse coccygeal discs in vivo for 1, 4, or 7 days, with adjacent discs serving as sham control. An activity assay was used to measure concentrations of active and total matrix metalloproteinase-2, and changes in matrix metalloproteinase-2 gene expression relative to beta-actin were assessed by reverse transcriptase-polymerase chain reaction.
  
  
  RESULTS
  Although no change was seen relative to sham after 1 day of load, the proportion of total matrix metalloproteinase-2 that was active increased after 4 days. This elevation was sustained through 7 days of compression, with no significant differences in total matrix metalloproteinase-2 concentrations among discs throughout the range of time points examined. Semiquantitative reverse transcriptase-polymerase chain reaction demonstrated no significant changes in matrix metalloproteinase-2 gene expression at 1 day or 4 days.
  
  
  CONCLUSIONS
  In this model, regulation of matrix metalloproteinase-2 activity occurs primarily through enhanced molecular activation of the proenzyme rather than through elevated gene expression or translation. Our results suggest that matrix metalloproteinase-2 may have a role in load-induced changes in disc architecture.
authors.affiliation: Orthopaedic Bioengineering Laboratory, Department of Orthopaedic Surgery, University of California, San Francisco 94143-0514, USA.
authors.forename: Adam H
authors.initials: AH
authors.lastname: Hsieh
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Lotz
mesh_terms: D000818:Animals; D004789:Enzyme Activation; D015971:Gene Expression Regulation, Enzymologic; D007403:Intervertebral Disc / Q000378:metabolism*; D008297:Male; D020778:Matrix Metalloproteinase 2 / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D023421:Models, Animal; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013131:Spine / Q000201:enzymology / Q000502:physiology*; D013314:Stress, Mechanical; D013997:Time Factors; D016474:Weight-Bearing
chemical_list: D012333:RNA, Messenger; D020778:Matrix Metalloproteinase 2