12.1.4 - Primary Cilia Drive Murine Articular Cartilage Organization by Directing Responses to Hedgehog Signaling and Local Ambulatory Load
- D. Rux (Philadelphia, US)
- D. Rux (Philadelphia, US)
- K. Helbig (Philadelphia, US)
- B. Han (Philadelphia, US)
- C. Cortese (Philadelphia, US)
- E. Koyama (Philadelphia, US)
- L. Han (Philadelphia, US)
- M. Pacifici (Philadelphia, US)
Abstract
Purpose
Synovial joints are essential for body movement. Unfortunately, articular cartilage (AC) is highly susceptible to disease and exhibits poor repair capacity that current clinical strategies fall short of rectifying. To improve these strategies, more information is needed on AC development and, specifically, how a functional multizone organization is acquired. Mature AC consists of: flat, lubricant-producing surface zone cells; round, column-aligned, and load-resistant deep zone cells; and a mineralized zone of cells demarcated by the tidemark. How this organization is generated during postnatal growth remains poorly understood. To explore mechanisms, we asked whether and how primary cilia regulate AC morphogenesis. Primary cilia are mechanical- and morphogen-transducing cell surface organelles that are crucial in morphogenesis of many mammalian tissues. Their roles in growth plate chondrocytes are well appreciated and functions have been implicated in AC but remain unclear.
Methods and Materials
We used a conditional loss-of-function approach (Ift88-flox) targeting joint-lineage progenitor cells (Gdf5Cre) and monitored structural/functional consequences on postnatal knee AC development.
Results
We found that embryonic joint development and growth up to 3 weeks of age were largely unaffected in mutants. However, mature (8 weeks) tissue exhibited: highly disorganized extracellular matrix (ECM) (i.e. aggrecan and collagen2); disrupted tidemark patterning and zonal organization; and markedly reduced mechanical properties (AFM-based testing). Further analyses revealed zonal disorganization was likely driven by disorganized chondrocyte differentiation (not degradation) and that hedgehog signaling was markedly disrupted in mutant joints. Notably, the changes in hedgehog response gene patterns related to, and likely caused, changes in tidemark topography and regional ambulatory load responses, increasing dramatically in loaded regions of the mutant tibial plateau. Interestingly, Prg4 expression was also increased in those loaded sites.
Conclusion
Overall, our data provide clear evidence that primary cilia orchestrate -and are essential for- postnatal AC morphogenesis, dictating tidemark topography, zonal chondrocyte composition and responses to local ambulatory loads.