imprimer la page

-A +A

Biology of centrosomes and Cilia

Group leader : Renata Basto

basto cloud

 

The Basto lab is interested in studying the basic mechanisms that regulate centriole and cilia biogenesis and to understand how dysfunction of these organelles can lead to the establishment of developmental defects and disease. The centrosome is the major microtubule-organising center of animal cells. It contains two centrioles (microtubule-based cylindrical organelles) that recruit and organise a large number of proteins to form the pericentriolar material. In a normal cell, the two centrioles of the centrosome replicate only once during the cell cycle to ensure the formation of a bipolar spindle that accurately segregates the chromosomes into two daughter cells. Recently, we have shown that centrosome amplification, the presence of supernumerary centrosomes in stem cells can lead to tumour formation. We are therefore focusing on better defining the nature of the link between centrosomes and cancer. We use two different model organisms, the fruit fly Drosophila melanogaster and mouse as a vertebrate model system. Through a combination of complementary approaches, our target objectives are therefore to identify and dissect the molecular pathways that allow transformation of stem cells when extra centrosomes are present.

figure 1

Figure 1 : A- The two centrosomes of the cell (shown in yellow) nucleate and organize the MT network (red) to assemble a bipolar mitotic spindle. The chromosomes are shown in blue. B- Electron microscopy (EM) picture of a cross section of Drosophila sperm tail flagellum.

In addition to their role at the centrosome, centrioles can also be found close to the plasma membrane where they behave as basal bodies. Basal bodies nucleate cilia or flagella, microtubule microtubule-based structures with crucial roles in motility, sensory perception and signaling. Recently, it has been recognized that the majority of the cells in the human body contain primary cilia and that mutations that affect basal body and /or cilia structure can lead to the establishment of severe developmental defects and pathologies such as cancer. We are also investigating the basic mechanisms that regulate basal body duplication and ciliogenesis and how defects in these structures can lead to tumourigenesis.

figure 2

Figure 2: A. Extra centrosomes in the mouse neural stem cells cause multipolar divisions, which are normally unviable. B. Inhibition of neural stem cell death results in the progressive degeneration of the mammalian brain.

Key publications

  • Year of publication : 2014

  • Cilia and flagella are organelles essential for motility and sensing of environmental stimuli. Depending on the cell type, cilia acquire a defined set of functions and, accordingly, are built with an appropriate length and molecular composition. Several ciliary proteins display a high degree of conservation throughout evolution and mutations in ciliary genes are associated with various diseases such as ciliopathies and infertility. Here, we describe the role of the highly conserved ciliary protein, Bug22, in Drosophila. Previous studies in unicellular organisms have shown that Bug22 is required for proper cilia function, but its exact role in ciliogenesis has not been investigated yet. Null Bug22 mutant flies display cilia-associated phenotypes and nervous system defects. Furthermore, sperm differentiation is blocked at the individualization stage, due to impaired migration of the individualization machinery. Tubulin post-translational modifications (PTMs) such as polyglycylation, polyglutamylation or acetylation, are determinants of microtubule (MT) functions and stability in centrioles, cilia and neurons. We found defects in the timely incorporation of polyglycylation in sperm axonemal MTs of Bug22 mutants. In addition, we found that depletion of human Bug22 in RPE1 cells resulted in the appearance of longer cilia and reduced axonemal polyglutamylation. Our work identifies Bug22 as a protein that plays a conserved role in the regulation of PTMs of the ciliary axoneme.

  • Year of publication : 2013

  • Mutations in ASPM are the most frequent cause of microcephaly, a disorder characterized by reduced brain size at birth. ASPM is recognized as a major regulator of brain size, yet its role during neural development remains poorly understood. Moreover, the role of ASPM proteins in invertebrate brain morphogenesis has never been investigated. Here, we characterized the function of the Drosophila ASPM orthologue, Asp, and found that asp mutants present severe defects in brain size and neuroepithelium morphogenesis. We show that size reduction depends on the mitotic function of Asp, whereas regulation of tissue shape depends on an uncharacterized function. Asp interacts with myosin II regulating its polarized distribution along the apico-basal axis. In the absence of Asp, mislocalization of myosin II results in interkinetic nuclear migration and tissue architecture defects. We propose that Asp regulates neuroepithelium morphogenesis through myosin-II-mediated structural and mechanical processes to maintain force balance and tissue cohesiveness.

  • Centrosome amplification is a hallmark of human tumours. In flies, extra centrosomes cause spindle position defects that result in the expansion of the neural stem cell (NSC) pool and consequently in tumour formation. Here we investigated the consequences of centrosome amplification during mouse brain development and homeostasis. We show that centrosome amplification causes microcephaly due to inefficient clustering mechanisms, where NSCs divide in a multipolar fashion producing aneuploid cells that enter apoptosis. Importantly, we show that apoptosis inhibition causes the accumulation of highly aneuploid cells that lose their proliferative capacity and differentiate, thus depleting the pool of progenitors. Even if these conditions are not sufficient to halt brain development, they cause premature death due to tissue degeneration. Our results support an alternative concept to explain the etiology of microcephaly and show that centrosome amplification and aneuploidy can result in tissue degeneration rather than overproliferation and cancer.

  • Year of publication : 2011

  • The LIM-domain protein Ajuba localizes at sites of epithelial cell-cell adhesion and has also been implicated in the activation of Aurora-A (Aur-A). Despite the expected importance of Ajuba, Ajuba-deficient mice are viable, which has been attributed to functional redundancy with the related LIM-domain protein LIMD1. To gain insights into the function of Ajuba, we investigated its role in Drosophila, where a single gene (jub) encodes a protein closely related to Ajuba and LIMD1. We identified a key function in neural stem cells, where Jub localizes to the centrosome. In these cells, mutation in jub leads to centrosome separation defects and aberrant mitotic spindles, which is a phenotype similar to that of aur-A mutants. We show that in jub mutants Aur-A activity is not perturbed, but that Aur-A recruitment and maintenance at the centrosome is affected. As a consequence the active kinase is displaced from the centrosome. On the basis of our studies in Drosophila neuroblasts, we propose that a key function of Ajuba, in these cells, is to maintain active Aur-A at the centrosome during mitosis.

  • Year of publication : 2008

  • Centrosome amplification is a common feature of many cancer cells, and it has been previously proposed that centrosome amplification can drive genetic instability and so tumorigenesis. To test this hypothesis, we generated Drosophila lines that have extra centrosomes in approximately 60% of their somatic cells. Many cells with extra centrosomes initially form multipolar spindles, but these spindles ultimately become bipolar. This requires a delay in mitosis that is mediated by the spindle assembly checkpoint (SAC). As a result of this delay, there is no dramatic increase in genetic instability in flies with extra centrosomes, and these flies maintain a stable diploid genome over many generations. The asymmetric division of the larval neural stem cells, however, is compromised in the presence of extra centrosomes, and larval brain cells with extra centrosomes can generate metastatic tumors when transplanted into the abdomens of wild-type hosts. Thus, centrosome amplification can initiate tumorigenesis in flies.