title: Increased caveolin-1 in intervertebral disc degeneration facilitates repair.
abstract: BACKGROUND
  Preceding intervertebral disc (IVD) degeneration, the cell phenotype in the nucleus pulposus (NP) shifts from notochordal cells (NCs) to chondrocyte-like cells (CLCs). Microarray analysis showed a correlation between caveolin-1 expression and the phenotypic transition of NCs to CLCs. With a clinical directive in mind, the aim of this study was to determine the role of caveolin-1 in IVD degeneration. As a scaffolding protein, caveolin-1 influences several signaling pathways, and transforming growth factor (TGF)-β receptors have been demonstrated to colocalize with caveolin-1. Therefore, the hypothesis of this study was that caveolin-1 facilitates repair by enhancing TGF-β signaling in the IVD.
  
  
  METHODS
  Protein expression (caveolin-1, apoptosis, progenitor cell markers, extracellular matrix, and phosphorylated Smad2 [pSmad2]) was determined in IVDs of wild-type (WT) and caveolin-1-null mice and canine IVDs of different degeneration grades (immunofluorescence, immunohistochemistry, and TUNEL assay). Canine/human CLC microaggregates were treated with chondrogenic medium alone or in combination with caveolin-1 scaffolding domain (CSD) peptide and/or caveolin-1 silencing RNA. After 28 days, gene and protein expression profiles were determined.
  
  
  RESULTS
  The NP of WT mice was rich in viable NCs, whereas the NP of caveolin-1-null mice contained more collagen-rich extracellular matrix and fewer cells, together with increased progenitor cell marker expression, pSmad2 TGF-β signaling, and high apoptotic activity. During canine IVD degeneration, caveolin-1 expression and apoptotic activity increased. In vitro caveolin-1 silencing decreased the CLC microaggregate glycosaminoglycan (GAG) content, which could be rescued by CSD treatment. Furthermore, CSD increased TGF-β/pSmad2 signaling at gene and protein expression levels and enhanced the anabolic effects of TGF-β1, reflected in increased extracellular matrix deposition by the CLCs.
  
  
  CONCLUSIONS
  Caveolin-1 plays a role in preservation of the NC phenotype. Additionally, it may be related to CLC apoptosis, given its increased expression in degenerated IVDs. Nevertheless, CSD enhanced CLC GAG deposition in vitro, and hence the increased caveolin-1 expression during IVD degeneration may also facilitate an ultimate attempt at repair. Further studies are needed to investigate how caveolin-1 modifies other signaling pathways and facilitates IVD repair.
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. f.c.bach@uu.nl.
authors.forename: Frances C
authors.initials: FC
authors.lastname: Bach
authors.affiliation: Department of Biochemistry, The University of Hong Kong, Hong Kong, China. joycezhang1205@gmail.com.
authors.forename: Ying
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. a.mirandabedate@uu.nl.
authors.forename: Alberto
authors.initials: A
authors.lastname: Miranda-Bedate
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. l.c.verdonschot@uu.nl.
authors.forename: Lucy C
authors.initials: LC
authors.lastname: Verdonschot
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. n.bergknut@uu.nl.
authors.forename: Niklas
authors.initials: N
authors.lastname: Bergknut
authors.affiliation: Department of Orthopaedics, University Medical Center Utrecht, Utrecht, The Netherlands. l.b.creemers@umcutrecht.nl.
authors.forename: Laura B
authors.initials: LB
authors.lastname: Creemers
authors.affiliation: Department of Orthopaedics, University Medical Center Utrecht, Utrecht, The Netherlands. k.ito@tue.nl.|Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands. k.ito@tue.nl.
authors.forename: Keita
authors.initials: K
authors.lastname: Ito
authors.affiliation: Department of Orthopaedic Surgery, Tokai University School of Medicine, Isehara, Japan. daisakai@is.icc.u-tokai.ac.jp.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Sakai
authors.affiliation: Department of Biochemistry, The University of Hong Kong, Hong Kong, China. chand@hku.hk.
authors.forename: Danny
authors.initials: D
authors.lastname: Chan
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. b.p.meij@uu.nl.
authors.forename: Björn P
authors.initials: BP
authors.lastname: Meij
authors.affiliation: Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands. m.a.tryfonidou@uu.nl.
authors.forename: Marianna A
authors.initials: MA
authors.lastname: Tryfonidou
mesh_terms: D000818:Animals; D051242:Caveolin 1 / Q000378:metabolism*; D002478:Cells, Cultured; D004285:Dogs; D005260:Female; D005455:Fluorescent Antibody Technique; D006801:Humans; D007150:Immunohistochemistry; D020287:In Situ Nick-End Labeling; D007403:Intervertebral Disc / Q000378:metabolism; D055959:Intervertebral Disc Degeneration / Q000378:metabolism*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008875:Middle Aged; D010641:Phenotype; D014945:Wound Healing / Q000502:physiology
chemical_list: D051242:Caveolin 1