Primary Cilia Direct Murine Articular Cartilage Tidemark Patterning Through Hedgehog Signaling and Ambulatory Load

Articular cartilage (AC) is essential for body movement but is highly susceptible to degenerative diseases and has poor self-repair capacity. To improve current subpar regenerative treatments, developmental mechanisms of AC should be clarified and, specifically, how its postnatal multizone organization is acquired. Primary cilia are cell surface organelles crucial for mammalian tissue morphogenesis. Although their importance for chondrocyte function is appreciated, their specific roles in postnatal AC morphogenesis remain unclear. To explore these mechanisms, we used a murine conditional loss-of-function approach (Ift88-flox) targeting joint-lineage progenitors (Gdf5Cre) and monitored postnatal knee AC development. Joint formation and growth up to juvenile stages were largely unaffected. However, mature AC (aged 2 months) exhibited disorganized extracellular matrix, decreased aggrecan and collagen II due to reduced gene expression (not increased catabolism), and marked reduction of AC modulus by 30%-50%. In addition, and unexpectedly, we discovered that tidemark patterning was severely disrupted, as was hedgehog signaling, and exhibited specificity based on regional load-bearing functions of AC. Interestingly, Prg4 expression was markedly increased in highly loaded sites in mutants. Together, our data provide evidence that primary cilia orchestrate postnatal AC morphogenesis including tidemark topography, zonal matrix composition, and ambulation load responses. © 2022 American Society for Bone and Mineral Research (ASBMR).