Centrosome

Summary

In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progression. The centrosome provides structure for the cell. The centrosome is thought to have evolved only in the metazoan lineage of eukaryotic cells. Fungi and plants lack centrosomes and therefore use other structures to organize their microtubules. Although the centrosome has a key role in efficient mitosis in animal cells, it is not essential in certain fly and flatworm species. Centrosomes are composed of two centrioles arranged at right angles to each other, and surrounded by a dense, highly structured mass of protein termed the pericentriolar material (PCM). The PCM contains proteins responsible for microtubule nucleation and anchoring — including γ-tubulin, pericentrin and ninein. In ge

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Functional characterization of the SAS-4-related protein CPAP in centrosome biology of human cells

Gregor Kohlmaier

The centrosome is an organelle that resides at the center of most animal cells and comprises two microtubule-based centriole cylinders surrounded by pericentriolar material (PCM). The centrosome plays a fundamental role for nucleating and organizing a radial array of cytoplasmic microtubules during interphase and promoting the assembly of the mitotic spindle during mitosis. In proliferating cells, the centrosome duplicates once per cell cycle, a process that involves notably the formation of one procentriole at the base of each centriole. The procentriole then grows until it reaches the same size of its associated centriole. The formation of supernumerary centrosomes causes severe problems, since this can lead to multipolar spindle formation, chromosome missegregation and genomic instability, which are hallmarks of cancer cells. The mechanisms regulating centrosome duplication are still incompletely understood. We have studied the role of the centrosomal P4.1-associated protein (CPAP) in human cells. We found that this protein is required for efficient growth of cytoplasmic microtubules from centrosomes as well as for centrosome duplication. There, CPAP is required at an early step during procentriole assembly, after the incorporation of HsSAS-6 but before that of Centrin. Importantly, we found also that the overexpression of CPAP leads to abnormal elongation of procentrioles and centrioles, indicating that the levels of CPAP determine centriole length. Excess CPAP levels furthermore lead to the formation of multiple procentrioles along overly elongated centrioles, multipolar spindle formation and cell division errors. Therefore, proper regulation of proteins such as CPAP that set centriole length contributes to ensure genome integrity. Overall, we gained important insights into the functions of CPAP at the centrosome and identified a novel control mechanism of centriole length. This will be relevant for a better understanding of how centrosomes function in the progression of cancer and other diseases.

EPFL2009

Asymmetric cell division is essential for the generation of diversity during development and the function of stem cell lineages. The Caenorhabditis elegans zygote is an attractive model to investigate the mechanisms of spindle positioning during asymmetric cell division. In this polarized cell, the asymmetric distribution of cortical force generators along the antero-posterior axis and pulling on astral microtubules leads to the unequal cleavage of the one-cell embryo. The mechanisms underlying such cortical force generation are thought to act strictly at the protein level. In this thesis work we report that the mRNA encoding the cortical force generator component LIN-5 is enriched around centrosomes in early embryos, in a manner that depends on microtubules and dynein. We established that the lin-5 coding sequence is necessary and sufficient for mRNA enrichment around centrosomes in C. elegans. In addition, we found that lin-5 mRNA is mislocalized in lin-5(ev571) mutant embryos, which harbor a 9 nucleotide insertion in the coding sequence. Moreover, an intragenic revertant of lin-5(ev571), lin-5(ev571he63), also exhibits mislocalized lin-5 mRNA distribution. We demonstrated that this is accompanied by diminished pulling forces on the posterior spindle pole, suggesting that centrosomal localization of lin-5 mRNA is important for robust pulling forces. We found also that lin-5 mRNA centrosomal enrichment is slightly asymmetric during anaphase, with more transcripts present on the anterior side. We developed a novel FRAP-based assay, which revealed that lin-5 is translated/folded preferentially in the cytoplasm compared to centrosomes. Furthermore, we found that morpholino-mediated inhibition of lin-5 translation diminishes pulling forces on the posterior side during anaphase. Together, these findings lead us to propose that preferential translation/folding of lin-5 in the posterior cytoplasm following release of the mRNA from the posterior centrosome contributes to asymmetric cortical distribution of force generators, and thus to proper spindle positioning. Moreover, we found that the mRNA of an uncharacterized gene, era-1 is enriched on the anterior side of the zygote and is inherited by the anterior blastomeres. Similar to era-1 mRNA, a YFP fusion of ERA-1 protein is also asymmetrically distributed. Moreover, asymmetric distribution of both era-1 mRNA and YFP-ERA-1 protein requires the era-1 3'UTR. Furthermore, the RNA-binding protein MEX-5 is needed for both asymmetric era-1 mRNA localization and for its translational activation. Furthermore, we report that the clathrin heavy chain CHC-1 negatively regulates pulling forces acting on centrosomes during interphase and on spindle poles during mitosis in one-cell C. elegans embryos. We establish a similar role for the cytokinesis/apoptosis/RNA-binding protein CAR-1 and uncover that CAR-1 is needed to maintain normal levels of CHC-1. We demonstrate that CHC-1 is necessary for proper organization of the cortical acto-myosin network and for full cortical tension. Furthermore, we establish that the centrosome positioning phenotype of embryos depleted of CHC-1 is alleviated by stabilizing the acto-myosin network. Conversely, we demonstrate that slight perturbations of the acto-myosin network results in excess centrosome movements. Overall, our findings lead us to propose that clathrin plays a critical role in centrosome positioning by promoting acto-myosin cortical tension.

EPFL2014

The centrosome is the principal microtubule organizing center (MTOC) of animal cells. Accurate centrosome duplication is fundamental for genome integrity and entails the formation of one procentriole next to each existing centriole, once per cell cycle. The procentriole then elongates to eventually reach the same size as the centriole. The mechanisms that govern elongation of the centriolar cylinder and their potential relevance for cell division are not known. Here, we show that the SAS-4-related protein CPAP is required for centrosome duplication in cycling human cells. Furthermore, we demonstrate that CPAP overexpression results in the formation of abnormally long centrioles. This also promotes formation of more than one procentriole in the vicinity of such overly long centrioles, eventually resulting in the presence of supernumerary MTOCs. This in turn leads to multipolar spindle assembly and cytokinesis defects. Overall, our findings suggest that centriole length must be carefully regulated to restrict procentriole number and thus ensure accurate cell division

2009