Primary cilia and coordination of receptor tyrosine kinase (RTK) signalling

Abstract

Primary cilia are microtubule-based sensory organelles that coordinate signalling pathways in cell-cycle control, migration, differentiation and other cellular processes critical during development and for tissue homeostasis. Accordingly, defects in assembly or function of primary cilia lead to a plethora of developmental disorders and pathological conditions now known as ciliopathies. In this review, we summarize the current status of the role of primary cilia in coordinating receptor tyrosine kinase (RTK) signalling pathways. Further, we present potential mechanisms of signalling crosstalk and networking in the primary cilium and discuss how defects in ciliary RTK signalling are linked to human diseases and disorders.

Figure 1

Overview on ciliary structures, signalling systems, diseases and disorders. (A) The primary cilium emerges from the ciliary pocket from the centrosomal mother centriole (basal body) that lies in close proximity to the Golgi apparatus at the plasma membrane in growth-arrested cells. Golgi-derived vesicles (GDVs) deliver ciliary axonemal and membrane proteins from the Golgi complex to the cilium, and clathrin-coated vesicles (CCVs) are formed from the ciliary pocket. Inserts to the right show primary cilia (arrows) visualized with differential interference contrast (DIC) microscopy, Immunofluorescence microscopy (IFM) and scanning electron microscopy (SEM; courtesy of Johan Kolstrup). In IFM, the cilium is localized with anti-acetylated α-tubulin (blue), the centrioles are marked with anti-centrin (green) and the nucleus is stained with DAPI (magenta). (B) The primary cilium is assembled by intraflagellar transport (IFT) from the basal body (see text for details). The upper inserts to the right show transmission electron microscopy (TEM) images of a longitudinal section of a mouse embryonic fibroblast primary cilium emerging from the ciliary basal body (BB) at the surface of the cell in close proximity to the Golgi and the nucleus (courtesy of Johan Kolstrup), and a cross-section of the cilium in a human embryonic stem cell that shows the axonemal arrangement of nine outer doublets of microtubules (9 + 0; from [139]), with permission from the Journal of Cell Biology). The lower insert shows a TEM image of a longitudinal section of a human foreskin fibroblast primary cilium emerging from the basal body (BB) in the ciliary pocket (CP; reproduced from [140] with permission from the Journal of Cell Science); AX: axoneme. (C) Schematic view on types of signal transduction systems that are coordinated by primary cilia and relayed through the cilium/centrosome axis to control cellular responses. (D) Examples on ciliopathy syndromes and primary cilia diseases and disorders associated with defects in ciliary assembly and function.

Figure 2

Schematic view on the formation and resorption of the primary cilium during the cell cycle. After mitosis and cytokinesis the two daughter cells enter the G₁/G₀ phase and begin their assembly of a primary cilium by IFT from the centrosomal mother centriole at the plasma membrane and into the extracellular environment for registration and relaying of extracellular cues. Upon stimulation with growth factors that triggers cell cycle entry at the G₁/S phase transition the cilium is resorbed and the centrosome is duplicated to form the two spindle poles for mitotic spindle formation and mitosis (M). At the end of mitosis the midbody is created from the spindle microtubules to coordinate cytokinesis followed by cell cycle arrest and formation of a new primary cilium in each daughter cell. Images show immunofluorescence microscopy analysis of the different phases of the cell cycle, using anti-acetylated α-tubulin (blue) to mark primary cilia (bold arrow) and cytoplasmic microtubules. Centrosomes are marked with anti-Pericentrin (red) and DNA with DAPI (green). Open arrows indicate time points for formation and resorption of the primary cilium.

Figure 3

Overview on ciliary RTKs. (A) list of RTKs localizing to primary and motile cilia. Motile cilia are indicated by ‘9 + 2’. (B-E) immunofluorescence microscopy (IFM) and immunogold electron microscopy (IEM) analysis of RTK localization to primary+cilia (bold arrows) and to the ciliary base (open arrows). Cilia were localized with either anti-acetylated γ-tubulin (ac.tb) of detyrosinated α-tubulin (glu.tb). (B) Localization of PDGFRα to the primary cilium (upper images) and PDGF-AA-induced phosphorylaton of Mek1/2 (pMek1/2) and Akt (pAkt) in the cilium and at the ciliary base NIH3T3 cells (middle images) (reproduced from [48] with permission from Elsevier, and from [49] with permission from S. Karger AG Basel). Localization of PDGFRα to the primary ciliary membrane (lower images) in rat astrocytes (reproduced from [51] with permission from John Wiley and Sons). (C) insulin-induced phosphorylation of IGF1R in preadipocytes (upper images) and ciliary localization of IRS-1 and insulin-induced phosphorylation of Akt (pAkt) at the ciliary base (middle images). Lower images show insulin-induced phoshorylation of IRS-1 (pIRS-1) in the primary cilium and at the ciliary base (reproduced from [53] with permission from Journal of Cell Science). (D) Co-localization of EGFR and polycystin 2, PKD2, to primary cilia in kidney epithelial (LLC-PK₁) cells (upper images; reproduced from [50] with permission from the American Society for Microbiology), and localization of EGFR to the primary ciliary membrane in rat astrocytes (lower image; reproduced from [51] with permission from John Wiley and Sons). (E) Localization of Tie-2 to epithelial cells in mouse extra-ovarian rete ducts (upper images with differential interference contrast, DIC) and to human ovary surface epithelium (lower image; reproduced from [54] with permission from Elsevier).

Figure 4

Summary of ciliary RTK signalling pathways and examples on potential crosstalk between signalling pathways at the cilium-centrosome axis to regulate cell cycle control, cell migration, cell differentiation, cell growth and metabolism.