Primary cilia disassembly down-regulates mechanosensitive hedgehog signalling: a feedback mechanism controlling ADAMTS-5 expression in chondrocytes

Abstract

SummaryObjectiveHedgehog 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.MethodsPrimary 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 Tg737ORPK mouse was used to investigate the role of the cilium in this response. Cilia length and prevalence were quantified by immunocytochemistry and confocal microscopy.ResultsMechanical 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 Tg737ORPK 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.ConclusionsThis 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.

The primary cilium as a signaling nexus for growth plate function and subsequent skeletal development

“…Mechanically induced activation of Ihh signaling in primary chondrocytes is also lost when primary cilia are absent. In articular cartilage, cilia are important for chondrocytes to respond to compression‐induced osmotic changes . Similarly, chondrocyte cilia also play a role in the growth plate's ability to sense compressive loading.…”

Reduced primary cilia length and altered Arl13b expression are associated with deregulated chondrocyte Hedgehog signaling in alkaptonuria

“…Several studies have now shown that very minor changes in cilia structure and trafficking can have dramatic effects upon functionality (Cruz et al, ; Thompson et al, ; Thompson et al, ; Tran et al, ). Thus, while the cilia disruption in +HGA cells does not influence cilia structure to the same extent as in AKU cells, it is sufficient to considerably disrupt function.…”

The Mechanobiology of Articular Cartilage: Bearing the Burden of Osteoarthritis

“…Though less is known about the transduction of mechanical load in pathologic settings, these processes appear to involve the enhancement of catabolic effectors, such as MMPs and aggrecanases [65, 66], and recent studies suggest the involvement of ciliary signaling [67] and histone modification [68]. Further mechanistic studies in the setting of injurious loading, or joint impact followed by physiologic loading, will reveal clinically relevant information for the treatment of patients following destabilizing and traumatic joint injuries.…”