title: Menisci of the rabbit knee require mechanical loading to maintain homeostasis: cyclic hydrostatic compression in vitro prevents derepression of catabolic genes.
abstract: BACKGROUND
  The purpose of this study was to examine the influence of removing menisci from their in vivo loading environment on gene expression patterns and to determine whether in vitro loading can maintain the tissues in their in vivo phenotype.
  
  
  METHODS
  Lateral and medial rabbit meniscal explants from one leg were cultured in vitro and subjected to intermittent cyclic hydrostatic pressure (CHP) of 1 MPa at 0.5 Hz for 1 min and a rest period of 14 min (4 h of culture). The contralateral menisci were incubated at atmospheric pressure for 4 h. Menisci from both legs of another set of rabbits were frozen immediately to yield time zero values reflective of in vivo mRNA levels. Total RNA was isolated from all groups and processed for reverse transcription-polymerase chain reaction analysis for a subset of relevant genes (matrix molecules, cytokines, proteinases and inhibitors, enzymes).
  
  
  RESULTS
  It was found that mRNA levels for MMP-1, MMP-3, TIMPs, iNOS, COX-2, interleukin-1beta in both menisci, and interleukin-6 in medial menisci were significantly elevated in tissues cultured under nonloading conditions compared to the time zero controls. Subjecting menisci to CHP significantly prevented these increases in mRNA levels for nearly all of the indicated molecules. In contrast, there were no significant differences in mRNA levels for collagens, biglycan, MMP-13, or TIMP-4 between the time zero values and those cultured under either nonloading or loading conditions.
  
  
  CONCLUSIONS
  These studies demonstrate that removing rabbit menisci from their normal in vivo mechanical environment leads to an apparent up-regulation of a subset of potent effector molecules that could mediate catabolic activities, and that in vitro CHP can largely prevent this apparent up-regulation.
authors.affiliation: Department of Orthopaedic Surgery, Osaka Rosai Hospital, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Natsu-Ume
authors.forename: Tokifumi
authors.initials: T
authors.lastname: Majima
authors.forename: Carol
authors.initials: C
authors.lastname: Reno
authors.forename: Nigel G
authors.initials: NG
authors.lastname: Shrive
authors.forename: Cyril B
authors.initials: CB
authors.lastname: Frank
authors.forename: David A
authors.initials: DA
authors.lastname: Hart
mesh_terms: D000818:Animals; D017364:Collagenases / Q000378:metabolism; D005260:Female; D006706:Homeostasis; D020781:Matrix Metalloproteinase 1 / Q000378:metabolism; D053509:Matrix Metalloproteinase 13; D019278:Matrix Metalloproteinase 3 / Q000378:metabolism; D008592:Menisci, Tibial / Q000502:physiology*; D012333:RNA, Messenger / Q000032:analysis; D011817:Rabbits; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013314:Stress, Mechanical; D015854:Up-Regulation
chemical_list: D012333:RNA, Messenger; D017364:Collagenases; D053509:Matrix Metalloproteinase 13; D019278:Matrix Metalloproteinase 3; D020781:Matrix Metalloproteinase 1