Aurora-A Kinase Is Essential for Bipolar Spindle Formation and Early Development

Aurora-A is a conserved kinase implicated in mitotic regulation and carcinogenesis. Aurora-A was previously implicated in mitotic entry and spindle assembly, although contradictory results prevented a clear understanding of the roles of Aurora-A in mammals. We developed a conditional null mutation in the mouse Aurora-A gene to investigate Aurora-A functions in primary cells ex vivo and in vivo. We show here that conditional Aurora-A ablation in cultured embryonic fibroblasts causes impaired mitotic entry and mitotic arrest with a profound defect in bipolar spindle formation. Germ line Aurora-A deficiency causes embryonic death at the blastocyst stage with pronounced cell proliferation failure, mitotic arrest, and monopolar spindle formation. Aurora-A deletion in mid-gestation embryos causes an increase in mitotic and apoptotic cells. These results indicate that murine Aurora-A facilitates, but is not absolutely required for, mitotic entry in murine embryonic fibroblasts and is essential for centrosome separation and bipolar spindle formation in vitro and in vivo. Aurora-A deletion increases apoptosis, suggesting that molecular therapies targeting Aurora-A may be effective in inducing tumor cell apoptosis. Aurora-A conditional mutant mice provide a valuable system for further defining Aurora-A functions and for predicting effects of Aurora-A therapeutic intervention.The equal partitioning of chromosomes at mitosis is critical for avoiding aneuploidy, a condition associated with spontaneous miscarriage, developmental disorders, and cancer (50). Mitosis requires coordinated completion of multiple events including nuclear envelope breakdown, chromosome condensation and congression to the metaphase plate, centrosome separation, spindle formation, chromosome-spindle attachment and error correction, sister chromatid separation, and cytokinesis. Multiple regulators, many of which are kinases, are required to ensure that each event is completed in a timely fashion and in the proper order (reviewed in reference 46). Although a number of mitotic kinases have been identified, their targets and the intricacies of mitotic signal transduction pathways are just beginning to be understood.The Aurora kinases are key mitotic regulators in eukaryotes (reviewed in reference 45). The Aurora family includes a single member in yeasts (Saccharomyces cerevisiae Ipl1p, Schizosaccharomyces pombe Ark1), two members each in Caenorhabditis elegans and Drosophila, and two or three members in vertebrates. Although originally given a variety of names, Aurora kinases in multicellular eukaryotes have subsequently been classified into A, B, and C groups based on patterns of mitotic subcellular localization and homology, which also appear to reflect functional distinctions (8, 46). Aurora-A kinases are observed at centrosomes and adjacent spindle fibers, and current evidence supports key roles in regulating protein localization and function at centrosomes, as well as regulation of the assembly, stability, and function of the mitotic spindle (reviewed in reference 43). Aurora-B kinases display “chromosomal passenger” localization, residing on mitotic chromosomes and subsequently moving to the spindle midzone after separation of sister chromatids. Aurora-B family members have been implicated in the regulation of kinetochore-spindle attachment, the spindle checkpoint, and cytokinesis (reviewed in references 1 and 8). Aurora-C kinases, which have only been identified in mammals, have a limited expression pattern and appear to have functions that overlap those of Aurora-B (7, 53).The human Aurora-A kinase (hAurA) was first identified because of its overexpression in cancer cell lines (5, 58). The hAurA gene (stk15) resides on chromosome 20q13, a region frequently amplified in human cancers (5, 58). hAurA has been dubbed an oncogene because of the fact that its overexpression transforms immortalized rodent fibroblasts (5, 70). Polymorphisms in hAurA are associated with an increased risk of colon cancer, while murine AurA (mAurA) polymorphisms confer increased susceptibility to experimentally induced skin tumors (14). The mAurA gene is frequently amplified in radiation-induced lymphomas from p53 heterozygous mice, while loss of one mAurA allele has been observed in lymphomas from p53-null mice (41). Thus, aberrant AurA expression is associated with tumorigenesis, suggesting that insight into AurA functions will lead to a better understanding of tumorigenesis mechanisms.A number of experimental observations suggest that AurA kinases are required for normal centrosome maturation and bipolar spindle assembly. The AurA ortholog in Drosophila melanogaster (Aurora) was identified in a screen for mutations that impact the centrosome cycle (21). Syncytial embryos from hypomorphic Aurora mutant females display a variety of mitotic abnormalities resulting from a failure to separate centrosomes. Aurora-null flies die at the larval stage with characteristic monopolar spindles and circular chromosome arrays in larval neuroblasts. Such monopolar spindles arise from failed centrosome separation (21). Subsequent studies of Drosophila Aurora mutant alleles revealed additional defects in centrosome maturation (including a failure to localize transforming acidic coiled-coil protein, centrosomin, and γ-tubulin at centrosomes) and in asymmetric localization of Numb protein in sensory organ precursor cells (3, 17). Similar to the case in Drosophila, disruption of the C. elegans AurA ortholog AIR-1 by RNA interference (RNAi) or mutation causes defects in centrosome maturation and monopolar spindle formation. Centrosomes undergo normal separation but collapse, leading to monopolar spindle formation (16, 24, 56). Studies of the Xenopus AurA homolog pEg2 revealed similar phenotypes after overexpression of kinase-dead mutants, antibody-mediated inhibition, or immunodepletion (18, 19, 38, 52). Furthermore, Xenopus AurA has been shown to interact with and phosphorylate Eg5, a mitotic kinesin required for bipolar spindle formation, suggesting a possible mechanism by which AurA could influence bipolar spindle formation and/or stabilization (19). Thus, existing reports from these systems are quite consistent in implicating AurA in centrosome separation and function.In contrast to the systems described above, published reports of RNAi-mediated reduction of AurA expression in mammalian cell lines have contained conflicting results about the role of AurA in mitotic entry, bipolar spindle formation, and mitotic progression. AurA RNAi in HeLa cells was reported to block or delay mitotic entry, prompting the conclusion that AurA is essential for mitotic commitment in mammalian cells (27, 36). In contrast, other AurA RNAi studies showed accumulation of mitotic cells with monopolar spindles (12, 20, 67). These discrepancies call into question the functional conservation of AurA in mammals and highlight a need for additional studies to definitively address the roles of AurA. This is particularly critical for understanding the roles of AurA in cancer and for projecting possible effects of AurA inhibitors currently in development as anticancer agents. We used gene targeting in mouse embryonic stem (ES) cells to produce a conditional null allele at the AurA locus. Here we describe cellular phenotypes of AurA deletion in primary cells in vitro and developmental phenotypes of AurA mutant mice. We show that AurA deletion in primary embryonic fibroblasts causes delayed mitotic entry with accumulation of cells in early prophase, consistent with a role for AurA in mitotic entry. Nevertheless, AurA-deficient cells that enter prometaphase arrest with monopolar spindles and eventually exit mitosis without segregating their chromosomes. Prolonged culture of AurA-deficient cells leads to polyploidy with abnormal nuclear structure. Germ line AurA deficiency causes embryonic death at the blastocyst stage with mitotic arrest and monopolar spindle formation, while AurA deletion in mid-gestation embryos causes an increased mitotic index and increased apoptosis. Together, our findings indicate that AurA is required for timely mitotic entry and bipolar spindle formation in vitro and in vivo.     

Restoring lakes by using artificial plant beds: habitat selection of zooplankton in a clear and a turbid shallow lake

1. Return of large‐bodied zooplankton populations is of key importance for creating a shift from a turbid to a clear‐water state in shallow lakes after a nutrient loading reduction. In temperate lakes, recovery is promoted by submerged macrophytes which function as a daytime refuge for large zooplankton. However, recovery of macrophytes is often delayed and use of artificial plant beds (APB) has been suggested as a tool to enhance zooplankton refuges, thereby reinforcing the shift to a clear‐water state and, eventually, colonisation of natural plants. 2. To further evaluate the potential of APB in lake restoration, we followed the day–night habitat choices of zooplankton throughout summer in a clear and a turbid lake. Observations were made in the pelagic and littoral zones and in APB in the littoral representing three different plant densities (coverage 0%, 40% and 80%). 3. In the clear lake, the zooplankton (primarily Daphnia) were mainly found in the pelagic area in spring, but from mid‐May they were particularly abundant in the APB and almost exclusively so in mid‐June and July, where they appeared in extremely high densities during day (up to 2600 ind. L⁻¹). During night Daphnia densities were overall more equally distributed between the five habitats. Ceriodaphnia was proportionally more abundant in the APB during most of the season. Cyclopoids were more abundant in the high APB during day but were equally distributed between the five habitats during night. 4. In the turbid lake, however, no clear aggregation was observed in the APB for either of the pelagic genera (Daphnia and Bosmina). This may reflect a higher refuge effect in the open water due to the higher turbidity, reduced ability to orient to plant beds and a significantly higher fish density (mainly of roach, Rutilus rutilus, and perch, Perca fluviatilis) in the plant beds than in the clear lake. Chydorus was found in much higher proportions among the plants, while cyclopoids, particularly the pelagic Cyclops vicinus, dominated in the pelagic during day and in the pelagic and high density plants during night. 5. Our results suggest that water clarity is decisive for the habitat choice of large‐bodied zooplankton and that introduction of APB as a restoration measure to enhance zooplankton survival is only a useful tool when water clarity increases following loading reduction. Our results indicate that dense APB will be the most efficient.     

Identification of gangliosides GD1b and GT1b as receptors for BK virus

Gangliosides have been shown to be plasma membrane receptors for both murine polyomavirus and SV40, while JC virus uses serotonin receptors. In contrast, little is known of the membrane receptor and entry pathway for BK virus (BKV), which can cause severe disease in immunosuppressed bone marrow and renal transplant patients. Using sucrose flotation assays, we investigated BKV binding to and interaction with human erythrocyte membranes and determined that this interaction was dependent on a neuraminidase-sensitive, proteinase K-resistant molecule. BKV was found to interact with the gangliosides GT1b and GD1b. The terminal alpha2-8-linked disialic acid motif, present in both of these gangliosides, is likely to be important for this interaction. We also determined that the addition of GD1b and GT1b to LNCaP cells, which are normally resistant to BKV infection, made them susceptible to the virus. In addition, BKV interacted with membranes extracted from the endoplasmic reticulum (ER) and infection was blocked by the addition of brefeldin A, which interferes with transport from the ER to the Golgi apparatus. These data demonstrate that BKV uses the gangliosides GT1b and GD1b as receptors and passes through the ER on the way to the nucleus.     

RIP-Cre revisited, evidence for impairments of pancreatic beta-cell function

The Cre/loxP recombinase system for performing conditional gene targeting experiments has been very useful in exploring genetic pathways that control both the development and function of pancreatic beta-cells. One particular line of transgenic mice (B6.Cg-Tg(Ins2-cre)25Mgn/J), commonly called RIP-Cre, in which expression of Cre recombinase is controlled by a short fragment of the rat insulin II gene promoter has been used in at least 21 studies on at least 17 genes. In most of these studies inactivation of the gene of interest was associated with either glucose intolerance or frank diabetes. Experimental evidence has been gradually emerging to suggest that RIP-Cre mice alone display glucose intolerance. In this study experiments from three laboratories demonstrate that RIP-Cre mice, in the absence of genes targeted by loxP sites, are glucose intolerant, possibly due to impaired insulin secretion. In addition, we review the use of RIP-Cre mice and discuss possible molecular underpinnings and ramifications of our findings.     

Simultaneous fMRI and Electrophysiology in the Rodent Brain

To examine the neural basis of the blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI) signal, we have developed a rodent model in which functional MRI data and in vivo intracortical recording can be performed simultaneously. The combination of MRI and electrical recording is technically challenging because the electrodes used for recording distort the MRI images and the MRI acquisition induces noise in the electrical recording. To minimize the mutual interference of the two modalities, glass microelectrodes were used rather than metal and a noise removal algorithm was implemented for the electrophysiology data. In our studies, two microelectrodes were separately implanted in bilateral primary somatosensory cortices (SI) of the rat and fixed in place. One coronal slice covering the electrode tips was selected for functional MRI. Electrode shafts and fixation positions were not included in the image slice to avoid imaging artifacts. The removed scalp was replaced with toothpaste to reduce susceptibility mismatch and prevent Gibbs ringing artifacts in the images. The artifact structure induced in the electrical recordings by the rapidly-switching magnetic fields during image acquisition was characterized by averaging all cycles of scans for each run. The noise structure during imaging was then subtracted from original recordings. The denoised time courses were then used for further analysis in combination with the fMRI data. As an example, the simultaneous acquisition was used to determine the relationship between spontaneous fMRI BOLD signals and band-limited intracortical electrical activity. Simultaneous fMRI and electrophysiological recording in the rodent will provide a platform for many exciting applications in neuroscience in addition to elucidating the relationship between the fMRI BOLD signal and neuronal activity.Download video file.^{(95M, mp4)}     

Progesterone analogs influence germination of Clostridium sordellii and Clostridium difficile spores in vitro

Clostridium sordellii and Clostridium difficile are closely related anaerobic Gram-positive, spore-forming human pathogens. C. sordellii and C. difficile form spores that are believed to be the infectious form of these bacteria. These spores return to toxin-producing vegetative cells upon binding to small molecule germinants. The endogenous compounds that regulate clostridial spore germination are not fully understood. While C. sordellii spores require three structurally distinct amino acids to germinate, the occurrence of postpregnancy C. sordellii infections suggests that steroidal sex hormones might regulate its capacity to germinate. On the other hand, C. difficile spores require taurocholate (a bile salt) and glycine (an amino acid) to germinate. Bile salts and steroid hormones are biosynthesized from cholesterol, suggesting that the common sterane structure can affect the germination of both C. sordellii and C. difficile spores. Therefore, we tested the effect of sterane compounds on C. sordellii and C. difficile spore germination. Our results show that both steroid hormones and bile salts are able to increase C. sordellii spore germination rates. In contrast, a subset of steroid hormones acted as competitive inhibitors of C. difficile spore germination. Thus, even though C. sordellii and C. difficile are phylogenetically related, the two species' spores respond differently to steroidal compounds.     

Harmonised Australian Reference Intervals for Serum PINP and CTX in Adults

Bone turnover markers (BTMs) are classified as either formation or resorption markers. Their concentrations in blood or urine of adults are considered to reflect the rate of bone remodelling and may be of use in the management of patients with bone disease. Major inter-method differences exist for BTMs, and harmonisation of methods is currently being pursued at an international level. Based on published data, this article describes age- and sex-specific Australian consensus reference intervals for adults for serum procollagen type I amino-terminal propeptide (s-PINP) and serum β-isomerised carboxy-terminal cross-linking telopeptide of type I collagen (s-CTX).