Primary cilia disassembly down-regulates mechanosensitive hedgehog signalling: a feedback mechanism controlling ADAMTS-5 expression in chondrocytes.
OBJECTIVE
Hedgehog signalling is mediated by the primary cilium and promotes cartilage degeneration in osteoarthritis. Primary cilia are influenced by pathological stimuli and cilia length and prevalence are increased in osteoarthritic cartilage. This study aims to investigate the relationship between mechanical loading, hedgehog signalling and cilia disassembly in articular chondrocytes.


METHODS
Primary bovine articular chondrocytes were subjected to cyclic tensile strain (CTS; 0.33 Hz, 10% or 20% strain). Hedgehog pathway activation (Ptch1, Gli1) and A Disintegrin And Metalloproteinase with Thrombospondin Motifs 5 (ADAMTS-5) expression were assessed by real-time PCR. A chondrocyte cell line generated from the Tg737(ORPK) mouse was used to investigate the role of the cilium in this response. Cilia length and prevalence were quantified by immunocytochemistry and confocal microscopy.


RESULTS
Mechanical strain upregulates Indian hedgehog expression and activates hedgehog signalling. Ptch1, Gli1 and ADAMTS-5 expression were increased following 10% CTS, but not 20% CTS. Pathway activation requires a functioning primary cilium and is not observed in Tg737(ORPK) cells lacking cilia. Mechanical loading significantly reduced cilium length such that cilia became progressively shorter with increasing strain magnitude. Inhibition of histone deacetylase 6 (HDAC6), a tubulin deacetylase, prevented cilia disassembly and restored mechanosensitive hedgehog signalling and ADAMTS-5 expression at 20% CTS.


CONCLUSIONS
This study demonstrates for the first time that mechanical loading activates primary cilia-mediated hedgehog signalling and ADAMTS-5 expression in adult articular chondrocytes, but that this response is lost at high strains due to HDAC6-mediated cilia disassembly. The study provides new mechanistic insight into the role of primary cilia and mechanical loading in articular cartilage.
Institute of Bioengineering, School of Engineering and Material Sciences, Queen Mary University of London, London, UK; Centre for Endocrinology, William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK. Electronic address: clare.l.thompson@qmul.ac.uk.
C L
CL
Thompson
Centre for Endocrinology, William Harvey Research Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK. Electronic address: j.p.chapple@qmul.ac.uk.
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JP
Chapple
Institute of Bioengineering, School of Engineering and Material Sciences, Queen Mary University of London, London, UK. Electronic address: m.m.knight@qmul.ac.uk.
M M
MM
Knight
D051722:ADAM Proteins / Q000378:metabolism*; D000818:Animals; D002358:Cartilage, Articular / Q000378:metabolism*; D002417:Cattle; D019902:Chondrocytes / Q000378:metabolism*; D002923:Cilia / Q000378:metabolism; D053823:Hedgehog Proteins / Q000378:metabolism*; D006655:Histone Deacetylases / Q000378:metabolism; D051379:Mice; D018613:Microscopy, Confocal; D060888:Real-Time Polymerase Chain Reaction; D013314:Stress, Mechanical
D053823:Hedgehog Proteins; D051722:ADAM Proteins; D006655:Histone Deacetylases
ADAMTS-5; Chondrocyte; Cilia length; Hedgehog; Primary cilium