Centrosome aberrations: cause or consequence of cancer progression?

Regulated expression of the centrosomal protein DdCP224 affects microtubule dynamics and reveals mechanisms for the control of supernumerary centrosome number

The Dictyostelium XMAP215 family member DdCP224 is involved in centrosome duplication and cytokinesis and is concentrated at the centrosome and microtubule tips. Herein, we have created a DdCP224 promoter replacement mutant that allows both over- and underexpression. Overexpression led to supernumerary microtubule-organizing centers and, independently, an increase of the number of multinuclear cells. Electron microscopy demonstrated that supernumerary microtubule-organizing centers represented bona fide centrosomes. Live cell imaging of DdCP224-green fluorescent protein mutants also expressing green fluorescent protein-histone2B as a DNA label revealed that supernumerary centrosomes were also competent of cell cycle-dependent duplication. In contrast, underexpression of DdCP224 inhibited cell growth, reduced the number and length of astral microtubules, and caused nocodazole hypersensitivity. Moreover, microtubule regrowth after nocodazole removal was dependent on DdCP224. Underexpression also resulted in a striking disappearance of supernumerary centrosomes and multinuclear cells caused by previous overexpression. We show for the first time by live cell observation that the number of supernumerary centrosomes can be reduced either by centrosome fusion (coalescence) or by the formation of cytoplasts containing supernumerary centrosomes during cytokinesis.

Does the intrinsic instability of aneuploid genomes have a causal role in cancer?

The role of aneuploidy in carcinogenesis has long been debated. We argue here that aneuploid genomes are naturally more susceptible to the types of chromosome rearrangement and epigenetic aberration that are found typically in tumor cells. In some cases, the formation of an aneuploid genome might be the initiating step in neoplastic conversion.

Nuclear localization of the cell cycle regulator CDH1 and its regulation by phosphorylation

The anaphase-promoting complex activated by CDC20 and CDH1 is a major ubiquitination system that controls the destruction of cell cycle regulators. Exactly how ubiquitination is regulated in time and space is incompletely understood. Here we report on the cell cycle-dependent localization of CDH1 and its regulation by phosphorylation. CDH1 localizes dynamically to the nucleus during interphase and to the centrosome during metaphase and anaphase. The nuclear accumulation of CDH1 correlates with a reduction in the steady-state amount of cyclin A, but not of cyclin E. A nuclear localization signal conserved in various species was identified in CDH1, and it sufficiently targets green fluorescent protein to the nucleus. Interestingly, a CDH1-4D mutant mimicking the hyperphosphorylated form was constitutively found in the cytoplasm. In further support of the notion that phosphorylation inhibits nuclear import, the nuclear localization signal of CDH1 with two phospho-accepting serine/threonine residues changed into aspartates was unable to drive heterologous protein into the nucleus. On the other hand, abolition of the cyclin-binding ability of CDH1 has no influence on its nuclear localization. Taken together, our findings document the phosphorylation-dependent localization of CDH1 in vertebrate cells.

Multistep tumorigenesis of multiple myeloma: its molecular delineation

Multiple myeloma (MM) is an incurable malignant neoplasm affecting terminally differentiated B-cells. It derives from post-germinal center B-cells and develops as a result of multistep tumorigenic events, because approximately one third of all MM cases have a history of preceding monoclonal gammopathy of undetermined significance (MGUS) or smoldering myeloma. MM terminates in the formation of extramedullary invasion or in secondary plasma cell leukemia. To account for this clinical experience, investigators have found that intrinsic chromosomal instability followed by complex chromosomal translocations/deletions plays a crucial role in the development from MGUS to MM. Representative aberrations include chromosomal rearrangements involving 14q32 loci and deletion at the long arm of chromosome 13. Contributing to the progression of MM itself are genomic instability and altered methylation of the specific gene promoters. The former results in activation of specific oncogenes such as RAS and FGFR3 or in inactivation of p53, and the latter results in inactivation of tumor suppressor genes, including p16. An accurate understanding of each of these molecular events should help clarify the development of specific molecular targeting therapies based on the differences in dysfunctional signaling pathways found in the cells of all MM patients.

SAS-4 is essential for centrosome duplication in C elegans and is recruited to daughter centrioles once per cell cycle

The mechanisms governing centrosome duplication remain poorly understood. We identified a gene called sas-4 that is essential for this process in C. elegans. SAS-4 encodes a predicted coiled-coil protein that localizes to a tiny dot in the center of centrosomes throughout the cell cycle. FRAP experiments with GFP-SAS-4 transgenic embryos reveal that SAS-4 is recruited to the centrosome once per cell cycle, at the time of organelle duplication. Additional evidence indicates that SAS-4 is recruited to the daughter centriole or a closely associated structure. These findings identify SAS-4 recruitment as a key step in the centrosome duplication cycle.

Aurora-A in cell fate control

Key cells divide asymmetrically during the development of multicellular organisms to give rise to offspring with different fates. In the Drosophila external sensory organ, asymmetrical division depends on polarization of the precursor cells during interphase and the consequent unequal distribution during mitosis of the protein Numb, which determines cell fate. Gonzalez discusses recent research implicating the mitotic kinase Aurora-A in the asymmetric localization of Numb in sensory organ pI precursor cells, a new function that appears to be independent of Aurora-A's known roles in regulating centrosomal maturation and the organization of mitotic spindle microtubules.