Centrosomes and cilia in human disease

Abstract

Centrioles are microtubule-derived structures that are essential for the formation of centrosomes, cilia and flagella. The centrosome is the major microtubule organiser in animal cells, participating in a variety of processes, from cell polarisation to cell division, whereas cilia and flagella contribute to several mechanisms in eukaryotic cells, from motility to sensing. Although it was suggested more than a century ago that these microtubule-derived structures are involved in human disease, the molecular bases of this association have only recently been discovered. Surprisingly, there is very little overlap between the genes affected in the different diseases, suggesting that there are tissue-specific requirements for these microtubule-derived structures. Knowledge of these requirements and disease mechanisms has opened new avenues for therapeutical strategies. Here, we give an overview of recent developments in this field, focusing on cancer, diseases of brain development and ciliopathies.

Figure 1. Centrosome and cilia structure

The centriole is a structural constituent of centrosomes, cilia and flagella. (A) The canonical centriole has nine MT triplets and is ~0.5 µm long and 0.2 µm in diameter. Each centrosome is composed by a mother and daugther centriole present in an orthogonal configuration and surrounded by a matrix of proteins called the pericentriolar material (PCM). The older centriole (mother) shows subdistal appendages, where MTs are docked, and distal appendages, which are important for docking to the plasma membrane. Satellites are granular structures surrounding the centrosome that are implicated in trafficking of material involved in centriole assembly. (A’) Electron micrograph of a centrosome; scale bar: 0.2 µm (reproduced with permission from ). (B) In many cells the centriole, then called basal body, migrates and tethers to the plasma membrane via its appendages and seeds the growth of cilia and flagella. The skeleton of cilia and flagella, called the axoneme, results from a continuation of the basal body structure and might be composed of nine doublets with no dynein arms nor central pair, as it is in the case of most immotile cilia; or nine MT doublets with dynein arms and a central MT pair, as it is for most motile cilia. The distal part of the basal body is called transition zone, where the outer tubule stops growing. During centriole to basal body differentiation the acquisition of specialized structures such as striated rootlets, basal feet and transitional fibres, will provide mechanical support to cilia, anchor the basal body to the apical cytoskeleton and serve as platforms for the docking of ciliary components, respectively .

Figure 2. Centrosome and cilia biogenesis and human disease

A. Centriole biogenesis. PLK4 triggers centriole biogenesis. It is recruited to the centrosome by CEP152. CEP152 also binds other molecules essential to this process, such as CPAP (also called SAS4 in Caenohabditis elegans). Procentriole formation begins in S phase upon recruitment of SAS6, CEP135 and STIL (also called SAS5), which are needed to form the cartwheel, a structure that helps in defining the centriole nine-fold symmetry. CPAP also plays a role in centriole elongation (for a review see ^, ). CDK2 activity may be necessary for speeding up procentriole formation and elongation, hence coordinating this event with DNA replication. In G2, the daughter centriole reaches full elongation and maturation with the recruitment of several molecules that are needed for microtubule nucleation, stability and focusing to the pericentriolar material (PCM), including pericentrin (PCTN), CEP192 (also called SPD2 in C. elegans), CDK5RAP2 (also called CNN in Drosophila) and ASPM. CDK1 activity increases in G2, regulating a variety of molecules and processes needed for entry into mitosis, such as changes in microtubule dynamics. Through the concerted action of molecules such as the kinase Nek2, the two centrosomes separate. When a cell exits mitosis, the centrioles within the centrosome disengage through the action of PLK1 and separase. That process may allow recruitment or activation of molecules necessary for duplication and ensures that daughter centrioles can only form after this point, preventing reduplication. Molecules involved in preventing DNA rereplication, such as ORC1 have a similar role in preventing centriole reduplication (for a review see ^, ). B. Cilia assembly/function and human disease. Mother centriole appendices dock to a vesicle, after which axoneme growth starts, followed by fusion of the vesicle to the plasma membrane. Migration of the centriole is dependent on the actin cytoskeleton and molecules such as MKS1, which is mutated in Meckel-Grueber syndrome. Several signalling pathways operate in ciliated cells, some of which are dependent on the presence of the cilium. Calcium signalling operates through membrane receptors and calcium channels on the ciliary membrane such as polycystin 1 and 2 respectively (PC1 and PC2). Hedgehog (Hh) signalling operates through the cilium in vertebrates: upon binding of Hh to its receptor Patched (Ptch1) a cascade of events starts within cilia and the body of the cell leading to the expression of target genes (see text; for a review see ). Wnt signalling is modulated through several components that localise at the centrosome and cilia. Several players in human disease, such as inversin (also called NPHP2) and the BBS proteins, which are part of the BBsome and are involved in protein trafficking, have been shown to play an important role in the switch between canonical and non-canonical Wnt pathways, through the regulation of β-catenin degradation, hence potentially regulating polarity, spindle positioning and proliferation (for a review see ^, ). The centriolar satellites may play an important role in human disease as several ciliopathy proteins, such as BBS4, CEP290 (also called NPHP6) and OFD1 have been shown to localise both to centrioles and satellites. Several proteins mutated in human disease, such as the NPHP1,3,4,5 and 10 localise to the transition zone. IFT components, which are involved in transporting molecules in and out of the cilia, including molecules involved in cilia assembly are also mutated in ciliopathies (e.g. IFT80, Dync2h1).

Figure 3. Centrosome and cancer

Possible mechanisms of how extra centrosomes could affect tumorigenesis. (A) Extra centrosomes can directly induce aneuploidy by forcing cells to undergo a multipolar intermediate during spindle assembly that lead to an increase of merotelic chromosome attachments and lagging chromosomes during mitosis. (B) Centrosome amplification can affect signalling, for example by modulating cilia number. (C) The presence of extra centrosomes can cause defects in asymmetric cell division in Drosophila neuroblasts, leading to an overproliferation of the stem cell population.