title: The relationship of autophagy defects to cartilage damage during joint aging in a mouse model.
abstract: OBJECTIVE
  Aging is a main risk factor for osteo arthritis (OA), the most prevalent musculoskeletal disorder. Defects in autophagy, an essential cellular homeostasis mechanism, have recently been observed in OA articular cartilage. The objectives of this study were to establish the constitutive level of autophagy activation in normal cartilage and to monitor the temporal relationship between changes in autophagy and aging-related degradation of cartilage in a mouse model.
  
  
  METHODS
  In GFP-LC3-transgenic mice, green fluorescent protein (GFP)-light chain 3 (LC3) is ubiquitously expressed, and the accumulation of GFP puncta, representing autophagosomes, was quantified by confocal microscopy as a measure of autophagy activation. Expression of the autophagy proteins autophagy-related protein 5 (ATG-5) and microtubule-associated protein 1 light chain 3 (LC3) was analyzed by immunohistochemistry. Cartilage cellularity, apoptotic cell death, and cartilage structural damage and changes in synovium and bone were examined by histology and immunohistochemistry.
  
  
  RESULTS
  Basal autophagy activation was detected in liver and knee articular cartilage from young (6-month-old) mice, with higher levels in cartilage than in liver in the same animals. In 28-month-old mice, there was a statistically significant reduction in the total number of autophagic vesicles per cell (P < 0.01) and in the total area of vesicles per cell (P < 0.01) in the articular cartilage as compared to that from young 6-month-old mice. With increasing age, the expression of ATG-5 and LC3 decreased, and this was followed by a reduction in cartilage cellularity and an increase in the apoptosis marker poly(ADP-ribose) polymerase p85. Cartilage structural damage progressed in an age-dependent manner subsequent to the autophagy changes.
  
  
  CONCLUSIONS
  Autophagy is constitutively activated in normal cartilage. This is compromised with aging and precedes cartilage cell death and structural damage.
authors.affiliation: The Scripps Research Institute La Jolla California and Instituto de Investigación Biomédica de A Coruña Complexo Hospitalario Universitario de A Coruña SERGAS and Universidade da Coruña, A Coruña, Spain.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Caramés
authors.affiliation: The Scripps Research Institute, La Jolla, California.
authors.forename: Merissa
authors.initials: M
authors.lastname: Olmer
authors.affiliation: The Scripps Research Institute, La Jolla, California.
authors.forename: William B
authors.initials: WB
authors.lastname: Kiosses
authors.affiliation: The Scripps Research Institute, La Jolla, California.
authors.forename: Martin K
authors.initials: MK
authors.lastname: Lotz
mesh_terms: D000375:Aging / Q000502:physiology*; D000818:Animals; D001343:Autophagy / Q000502:physiology*; D000071187:Autophagy-Related Protein 5; D002358:Cartilage, Articular / Q000378:metabolism* / Q000503:physiopathology; D019902:Chondrocytes / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000235:genetics; D007150:Immunohistochemistry; D051379:Mice; D008822:Mice, Transgenic; D018613:Microscopy, Confocal; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism*; D023421:Models, Animal; D010003:Osteoarthritis / Q000378:metabolism* / Q000503:physiopathology; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D013264:Stifle
chemical_list: C495773:Atg5 protein, mouse; D000071187:Autophagy-Related Protein 5; C480860:Map1lc3b protein, mouse; D008869:Microtubule-Associated Proteins; D049452:Green Fluorescent Proteins; D011065:Poly(ADP-ribose) Polymerases