Ciliary IFT88 Protects Coordinated Adolescent Growth Plate Ossification From Disruptive Physiological Mechanical Forces

Abstract

Compared with our understanding of endochondral ossification, much less is known about the coordinated arrest of growth defined by the narrowing and fusion of the cartilaginous growth plate. Throughout the musculoskeletal system, appropriate cell and tissue responses to mechanical force delineate morphogenesis and ensure lifelong health. It remains unclear how mechanical cues are integrated into many biological programs, including those coordinating the ossification of the adolescent growth plate at the cessation of growth. Primary cilia are microtubule-based organelles tuning a range of cell activities, including signaling cascades activated or modulated by extracellular biophysical cues. Cilia have been proposed to directly facilitate cell mechanotransduction. To explore the influence of primary cilia in the mouse adolescent limb, we conditionally targeted the ciliary gene Intraflagellar transport protein 88 (Ift88 fl/fl ) in the juvenile and adolescent skeleton using a cartilage-specific, inducible Cre (AggrecanCreER T2 Ift88 fl/fl ). Deletion of IFT88 in cartilage, which reduced ciliation in the growth plate, disrupted chondrocyte differentiation, cartilage resorption, and mineralization. These effects were largely restricted to peripheral tibial regions beneath the load-bearing compartments of the knee. These regions were typified by an enlarged population of hypertrophic chondrocytes. Although normal patterns of hedgehog signaling were maintained, targeting IFT88 inhibited hypertrophic chondrocyte VEGF expression and downstream vascular recruitment, osteoclastic activity, and the replacement of cartilage with bone. In control mice, increases to physiological loading also impair ossification in the peripheral growth plate, mimicking the effects of IFT88 deletion. Limb immobilization inhibited changes to VEGF expression and epiphyseal morphology in Ift88cKO mice, indicating the effects of depletion of IFT88 in the adolescent growth plate are mechano-dependent. We propose that during this pivotal phase in adolescent skeletal maturation, ciliary IFT88 protects uniform, coordinated ossification of the growth plate from an otherwise disruptive heterogeneity of physiological mechanical forces.

Roles of Local Soluble Factors in Maintaining the Growth Plate: An Update

“…Mechanical loading also stimulates the formation of a secondary ossification center, protecting the hypertrophic chondrocytes in the growth plate from potential injury due to high mechanical stress [ 8 ]. In addition to the protection from bone tissue, the growth-plate chondrocytes resist mechanical injuries through several mechanisms, such as inducing autophagy [ 15 ] and extension of the primary cilium [ 16 , 17 ]. For example, ciliary gene intraflagellar transport protein 88 (Ift88) knock-out (KO) mice showed narrow growth plate and endochondral ossification inhibition [ 16 ], demonstrating that Ift88 helped promote chondrocyte ciliation, cartilage resorption, mineralization, and, finally, coordinated ossification of growth plate from the disruptive mechanical force.…”

“…In addition to the protection from bone tissue, the growth-plate chondrocytes resist mechanical injuries through several mechanisms, such as inducing autophagy [ 15 ] and extension of the primary cilium [ 16 , 17 ]. For example, ciliary gene intraflagellar transport protein 88 (Ift88) knock-out (KO) mice showed narrow growth plate and endochondral ossification inhibition [ 16 ], demonstrating that Ift88 helped promote chondrocyte ciliation, cartilage resorption, mineralization, and, finally, coordinated ossification of growth plate from the disruptive mechanical force. Additionally, the Sprouty family, a modulator of the receptor tyrosine kinase signaling pathway, maintains the morphology of the primary cilium of chondrocytes in the growth plate [ 17 ].…”

Primary cilium‐mediated signaling cascade suppresses age‐related biliary fibrosis

“…Several ciliopathies are associated with liver diseases, including biliary fibrosis (Mansini et al, 2018). Intraflagellar transport 88 (IFT88), a component of the IFT system, is vital for cilium formation and maintenance (Coveney, Samvelyan, et al, 2022; W. Li et al, 2022; Y. Yang et al, 2021). Mutations or disruptions in IFT88 have been implicated in cilia dysfunction, leading to the development of ciliopathies, including polycystic liver and kidney disease (Coveney, Zhu, et al, 2022; R. Fujii et al, 2021; Shao et al, 2020; Y. Wu et al, 2023; S. Yang et al, 2023).…”

New and Unexpected Roles for Primary Cilia in Coordinating Response to Mechanical Load in Articular and Growth Plate Cartilages

“…The study by Coveney and colleagues ( 9 ) in this issue of JBMR goes on to describe an unexpected function for primary cilia in cartilage. Based on the known role of primary cilia in other cell types, it was presumed that primary cilia would be required to transmit mechanical signals in chondrocytes; however, evidence for a positive role for primary cilia in propagating mechanical signals in cartilage in vivo had been elusive.…”

“…In this issue of JBMR, two studies address broad questions about cartilage patterning and the role of primary cilia in coordinating chemical and mechanical signals. (8,9) Rux and colleagues (8) generated mice with a conditional deletion of Ift88 in joint progenitor cells via Gdf5-expressed Cre. They observed that subsequent reduction in primary cilia resulted in disruptions to the organization and localization of the tidemark in articular cartilage in a way that was dependent on the type of load experienced by the cartilage.…”