Fanni Gergely

Aurora-A: the maker and breaker of spindle poles.

The gene encoding the Aurora-A protein kinase is located in the 20q13 breast cancer amplicon and is also overexpressed in colorectal, pancreatic and gastric tumours. Although Aurora-A may not be a bona fide oncoprotein in humans, it is a promising drug target in cancer therapy. Thus, it is surprising that so little is known of its role in normal cells. The primary function of Aurora-A is to promote bipolar spindle assembly, but the molecular details of this process remained obscure until recently. The discovery of several novel Aurora-A-binding proteins and substrates has implicated Aurora-A in centrosome maturation and separation, acentrosomal and centrosomal spindle assembly, kinetochore function, cytokinesis and in cell fate determination. Here we discuss recent advances in determining the early mitotic role of Aurora-A, with a strong emphasis on its function at the mitotic spindle poles.

Msps/XMAP215 interacts with the centrosomal protein D-TACC to regulate microtubule behaviour

The XMAP215/ch-TOG/Msps family of microtubule-associated proteins (MAPs) promote microtubule growth in vitro and are concentrated at centrosomes in vivo. We show here that Msps (mini-spindles protein) interacts with the centrosomal protein D-TACC, and that this interaction strongly influences microtubule behaviour in Drosophila embryos. If D-TACC levels are reduced, Msps does not concentrate at the centrosomes efficiently and the centrosomal microtubules appear to be destabilized. If D-TACC levels are increased, both D-TACC and Msps accumulate around the centrosomes/spindle poles, and the centrosomal microtubules appear to be stabilized. We show that the interaction between D-TACC and Msps is evolutionarily conserved. We propose that D-TACC and Msps normally cooperate to stabilize centrosomal microtubules by binding to their minus ends and binding to their plus ends as they grow out from the centrosome.

INPP5E mutations cause primary cilium signaling defects, ciliary instability and ciliopathies in human and mouse.

The primary cilium is an antenna-like structure that protrudes from the cell surface of quiescent/differentiated cells and participates in extracellular signal processing. Here, we report that mice deficient for the lipid 5-phosphatase Inpp5e develop a multiorgan disorder associated with structural defects of the primary cilium. In ciliated mouse embryonic fibroblasts, Inpp5e is concentrated in the axoneme of the primary cilium. Inpp5e inactivation did not impair ciliary assembly but altered the stability of pre-established cilia after serum addition. Blocking phosphoinositide 3-kinase (PI3K) activity or ciliary platelet-derived growth factor receptor α (PDGFRα) restored ciliary stability. In human INPP5E, we identified a mutation affecting INPP5E ciliary localization and cilium stability in a family with MORM syndrome, a condition related to Bardet-Biedl syndrome. Together, our results show that INPP5E plays an essential role in the primary cilium by controlling ciliary growth factor and PI3K signaling and stability, and highlight the consequences of INPP5E dysfunction.

The mago nashi gene is required for the polarisation of the oocyte and the formation of perpendicular axes in Drosophila

BACKGROUND Drosophila axis formation requires a series of inductive interactions between the oocyte and the somatic follicle cells. Early in oogenesis, Gurken protein, a member of the transforming growth factor alpha family, is produced by the oocyte to induce the adiacent follicle cells to adopt a posterior cell fate. These cells subsequently send an unidentified signal back to the oocyte to induce the formation of a polarised microtubule array that defines the anterior-posterior axis. The polarised microtubules also direct the movement of the nucleus and gurken mRNA from the posterior to the anterior of the oocyte, where Gurken signals a second time to induce the dorsal follicle cells, thereby polarising the dorsal-ventral axis. RESULTS In addition to its previously described role in the localisation of oskar mRNA, the mago nashi gene is required in the germ line for the transduction of the polarising signal from the posterior follicle cells. Using a new in vivo marker for microtubules, we show that mago nashi mutant oocytes develop a symmetric microtubule cytoskeleton that leads to the transient localisation of bicoid mRNA to both poles. Furthermore, the oocyte nucleus often fails to migrate to the anterior, causing the second Gurken signal to be sent in the same direction as the first. This results in a novel phenotype in which the anterior of the egg is ventralised and the posterior dorsalised, demonstrating that the migration of the oocyte nucleus determines the relative orientation of the two principal axes of Drosophila. The mago nashi gene is highly conserved from plants to animals, and encodes a protein that is predominantly localised to nuclei. CONCLUSIONS The mago nashi gene plays two essential roles in Drosophila axis formation: it is required downstream of the signal from the posterior follicle cells for the polarisation of the oocyte microtubule cytoskeleton, and has a second, independent role in the localisation of oskar mRNA to the posterior of the oocyte.

WDR62 is associated with the spindle pole and is mutated in human microcephaly

Autosomal recessive primary microcephaly (MCPH) is a disorder of neurodevelopment resulting in a small brain. We identified WDR62 as the second most common cause of MCPH after finding homozygous missense and frame-shifting mutations in seven MCPH families. In human cell lines, we found that WDR62 is a spindle pole protein, as are ASPM and STIL, the MCPH7 and MCHP7 proteins. Mutant WDR62 proteins failed to localize to the mitotic spindle pole. In human and mouse embryonic brain, we found that WDR62 expression was restricted to neural precursors undergoing mitosis. These data lend support to the hypothesis that the exquisite control of the cleavage furrow orientation in mammalian neural precursor cell mitosis, controlled in great part by the centrosomes and spindle poles, is critical both in causing MCPH when perturbed and, when modulated, generating the evolutionarily enlarged human brain.

D‐TACC: a novel centrosomal protein required for normal spindle function in the early Drosophila embryo

We identify Drosophila TACC (D‐TACC) as a novel protein that is concentrated at centrosomes and interacts with microtubules. We show that D‐TACC is essential for normal spindle function in the early embryo; if D‐TACC function is perturbed by mutation or antibody injection, the microtubules emanating from centrosomes in embryos are short and chromosomes often fail to segregate properly. The C‐terminal region of D‐TACC interacts, possibly indirectly, with microtubules, and can target a heterologous fusion protein to centrosomes and microtubules in embryos. This C‐terminal region is related to the mammalian transforming, acidic, coiled‐coil‐containing (TACC) family of proteins. The function of the TACC proteins is unknown, but the genes encoding the known TACC proteins are all associated with genomic regions that are rearranged in certain cancers. We show that at least one of the mammalian TACC proteins appears to be associated with centrosomes and microtubules in human cells. We propose that this conserved C‐terminal ‘TACC domain’ defines a new family of microtubule‐interacting proteins.

A primary microcephaly protein complex forms a ring around parental centrioles

Autosomal recessive primary microcephaly (MCPH) is characterized by a substantial reduction in prenatal human brain growth without alteration of the cerebral architecture and is caused by biallelic mutations in genes coding for a subset of centrosomal proteins. Although at least three of these proteins have been implicated in centrosome duplication, the nature of the centrosome dysfunction that underlies the neurodevelopmental defect in MCPH is unclear. Here we report a homozygous MCPH-causing mutation in human CEP63. CEP63 forms a complex with another MCPH protein, CEP152, a conserved centrosome duplication factor. Together, these two proteins are essential for maintaining normal centrosome numbers in cells. Using super-resolution microscopy, we found that CEP63 and CEP152 co-localize in a discrete ring around the proximal end of the parental centriole, a pattern specifically disrupted in CEP63-deficient cells derived from patients with MCPH. This work suggests that the CEP152-CEP63 ring-like structure ensures normal neurodevelopment and that its impairment particularly affects human cerebral cortex growth.

BRCA1-Independent Ubiquitination of FANCD2.

Monoubiquitination of the FANCD2 protein is a key step in the Fanconi anemia (FA) tumor suppressor pathway, coinciding with this molecules accumulation at sites of genome damage. Strong circumstantial evidence points to a requirement for the BRCA1 gene product in this step. Here, we show that the purified BRCA1/BARD1 complex, together with E1 and UbcH5a, is sufficient to reconstitute the monoubiquitination of FANCD2 in vitro. Although siRNA-mediated knockdown of BRCA1 in human cells results in defective targeting of FANCD2 to sites of DNA damage, it does not lead to a defect in FANCD2 ubiquitination. Furthermore, ablation of the RING finger domains of either BRCA1 or BARD1 in the chicken B cell line DT40 also leaves FANCD2 modification intact. Consequently, while BRCA1 affects the accumulation of FANCD2 at sites of DNA damage, BRCA1/BARD1 E3 ligase activity is not essential for the monoubiquitination of FANCD2.

Centrosome function in cancer: guilty or innocent?

The regulation of centrosome number and function underlies bipolar mitotic spindle formation and genetic integrity. Cancer cells both in culture and in situ exhibit a wide range of centrosome abnormalities. Here, we briefly review advances in our understanding of the pathways that govern normal centrosome function and outline the potential causes and consequences of their deregulation in disease. There is ample observational but little experimental evidence to support the conventional model that centrosome dysfunction causes genomic instability and, as a result, cancer. This model has been challenged by recent studies that have uncovered evidence of a direct link between centrosome function in asymmetric cell division and tumourigenesis. Thus, it is timely to discuss the provocative idea that, in certain tissues, abnormal centrosomes drive malignant transformation not by generating genomic instability but by deregulating asymmetric cell division.

CDK5RAP2 functions in centrosome to spindle pole attachment and DNA damage response

Two domains of centrosomal protein CDK5RAP2, CNN1 and CNN2, link centrosomes to mitotic spindle poles. CNN1 lacking centrosomes are unable to recruit pericentriolar matrix components that mediate attachment to spindle poles.