Function and regulation of primary cilia and intraflagellar transport proteins in the skeleton

Abstract

Primary cilia are microtubule-based organelles that project from the cell surface to enable transduction of various developmental signaling pathways. The process of intraflagellar transport (IFT) is crucial for the building and maintenance of primary cilia. Ciliary dysfunction has been found in a range of disorders called ciliopathies, some of which display severe skeletal dysplasias. In recent years, interest has grown in uncovering the function of primary cilia/IFT proteins in bone development, mechanotransduction, and cellular regulation. We summarize recent advances in understanding the function of cilia and IFT proteins in the regulation of cell differentiation in osteoblasts, osteocytes, chondrocytes, and mesenchymal stem cells (MSCs). We also discuss the mechanosensory function of cilia and IFT proteins in bone cells, cilia orientation, and other functions of cilia in chondrocytes.

Keywords: IFT proteins; MSCs; bone; chondrocytes; cilium; osteoblasts; osteocytes; primary cilia.

Fig. 1

Schema of cilia structure, IFT complexes, and BBSome. Basal bodies are modified centrioles, which provide a template to build the axoneme, the backbone of cilia. IFT occurs in two directions. Anterograde transport is from the base to the tip of the cilium and retrograde transport is from the tip back to the cell body. Anterograde transport is operated by a kinesin-II motor and the IFT-B complex. Kinesin-II complexes are composed of either KIF3A/KIF3B/KAP3 or KIF3A/KIF3C/KAP3 in vertebrates. IFT-B complex is composed of 14 IFT proteins (IFT20, IFT22, IFT25, IFT27, IFT46, IFT52, IFT54, IFT57, IFT70, IFT74/IFT72, IFT80, IFT81, IFT88 and IFT172). IFT81 and IFT74/72 form a tetrameric complex and interact with IFT88, IFT52, IFT46, and IFT27 to form the core of complex B. Retrograde transport is regulated by Dynein-2 and IFT-A complex. Dynein-2 is composed of the heavy chains D2IC, D2LIC, and LC. IFT-A contains six proteins (IFT144, IFT140, IFT139, IFT122, IFT121, and IFT43). The BBSome, composed of seven highly conserved BBS proteins (BBS1, BBS2, BBS4, BBS5, BBS7, BBS8 and BBS9), assembles IFT-A and IFT-B together to facilitate IFT complex and cargo transport. Polycystin-1 (PC-1), a G protein couple receptor (GPCR), and PC-2, a receptor-activated calcium channel, form a heterodimer that is located in cilia. EVC-1 and EVC-2 are located at the basal body and at the base of the axoneme.

Fig. 2

The primary cilium is a HH signal transduction machine. When HH is absent, the HH receptor PTCH1 is located at the base of the cilium and inhibits SMO function. SUFU processes GLI in the cilia into its repressor form (GLIR), which inhibits related gene expression. However, when HH is present, it binds to PTCH1 and triggers PTCH1 to move out of cilia, which releases the inhibition of SMO and allows SMO to translocate into the cilium. Active SMO goes into cilia and forms a complex with EVC2 at the EVC zone and activates GLI (GLIA) by relieving the inhibition by SUFU. GLIA then turns on related gene expression. Adapted from Refs. and .

Fig. 3

Cilia in chondrocytes with ciliary pocket and matrix–cilium–golgi continuum. Ciliary pocket is a membrane domain, in which the cilium is rooted. It is also an endocytic membrane domain with actin-based cytoskeleton and clathrin-coated pits. CD44, the receptor for hyaluronan, was found in the calthrincoated pits with the ciliary pocket. The collagen fibers are closely associated with the axoneme. The contact points on the membrane of the axoneme are receptors for cartilage matrix components such as integrin and NG2. The matrix–cilium–Golgi continuum exists in chondrocytes to potentially regulate the cellular secretion of ECM components. Adapted from Ref. .

Fig. 4

Chondrocytes rotation and cilia orientation in the articular and growth plate cartilage. The cilia in cartilage have a clear orientation. In articular cartilage, cilia in the superficial articular cartilage project away from the articulating surface. In the growth plate, chondrocytes in the proliferative and hypertrophic zones are formed as stacked columns that are parallel to the longitudinal axis of the bone. Cilia in these areas are found parallel to this axis as well. However, the proliferation of chondrocytes is perpendicular to this axis and their daughter cells are displayed laterally to each other. To maintain columnar organization, the daughter cells rotate after division to merge into the stacked columnar. The cilia are believed to have an important role in the regulation of chondrocyte rotation.^{, ,} Adapted from Refs. and .