Microtubule dynamics in Xenopus egg extracts

Screening for small molecule inhibitors of embryonic pathways: sometimes you gotta crack a few eggs

Extract prepared from Xenopus eggs represents a cell-free system that has been shown to recapitulate a multitude of cellular processes, including cell cycle regulation, DNA replication/repair, and cytoskeletal dynamics. In addition, this system has been used to successfully reconstitute the Wnt pathway. Xenopus egg extract, which can be biochemically manipulated, offers an ideal medium in which small molecule screening can be performed in near native milieu. Thus, the use of Xenopus egg extract for small molecule screening represents an ideal bridge between targeted and phenotypic screening approaches. This review focuses on the use of this system for small molecules modulators of major signal transduction pathways (Notch, Hedgehog, and Wnt) that are critical for the development of the early Xenopus embryo. We describe the properties of Xenopus egg extract and our own high throughput screen for small molecules that modulate the Wnt pathway using this cell-free system. We propose that Xenopus egg extract could similarly be adapted for screening for modulators of the Notch and Hedgehog pathways.

Xenomics

Xenopus genomics, or Xenomics for short, is coming of age. Indeed, biological insight into processes such as growth factor signaling and patterning of the early embryo is now being gained by combining the value of Xenopus as a model organism for cell and developmental biology with genomic approaches. In this review I address these recent advances and explore future possibilities gained from combining this powerful experimental system with genomic approaches, as well as how our quest to understand basic biological principles will be greatly facilitated though the marriage of Xenopus and genomics.

Mechanical properties of Xenopus egg cytoplasmic extracts

Cytoplasmic extracts prepared from Xenopus laevis eggs are used for the reconstitution of a wide range of processes in cell biology, and offer a unique environment in which to investigate the role of cytoplasmic mechanics without the complication of preorganized cellular structures. As a step toward understanding the mechanical properties of this system, we have characterized the rheology of crude interphase extracts. At macroscopic length scales, the extract forms a soft viscoelastic solid. Using a conventional mechanical rheometer, we measure the elastic modulus to be in the range of 2-10 Pa, and loss modulus in the range of 0.5-5 Pa. Using pharmacological and immunological disruption methods, we establish that actin filaments and microtubules cooperate to give mechanical strength, whereas the intermediate filament cytokeratin does not contribute to viscoelasticity. At microscopic length scales smaller than the average network mesh size, the response is predominantly viscous. We use multiple particle tracking methods to measure the thermal fluctuations of 1 microm embedded tracer particles, and measure the viscosity to be approximately 20 mPa-s. We explore the impact of rheology on actin-dependent cytoplasmic contraction, and find that although microtubules modulate contractile forces in vitro, their interactions are not purely mechanical.

A novel centrosome-associated protein with affinity for microtubules

We have identified a novel mammalian protein, MIR1, with microtubule-binding activity. MIR1 is a relative of MID1/midin, the protein implicated in Opitz G/BBB syndrome. In tissue culture cells, MIR1 is enriched at the centrosome. MIR1 dissociates from centrosomes at the G2/M transition and is recruited back to spindle poles during anaphase. When overexpressed during interphase, MIR1 binds along microtubule filaments, which become stabilized, bundled and detached from the centrosome. In mitosis,overexpressed MIR1 dissociates from microtubules but still affects the normally focused localization of γ-tubulin in spindle poles. Tight binding to microtubules in interphase appears to require an oligomeric state of MIR1, and phosphorylation in mitosis at predicted cyclin-dependent kinase(cdk) sites weakens the interaction.

Drosophila Aurora A kinase is required to localize D-TACC to centrosomes and to regulate astral microtubules

Disruption of the function of the A-type Aurora kinase of Drosophila by mutation or RNAi leads to a reduction in the length of astral microtubules in syncytial embryos, larval neuroblasts, and cultured S2 cells. In neuroblasts, it can also lead to loss of an organized centrosome and its associated aster from one of the spindle poles, whereas the centrosome at the other pole has multiple centrioles. When centrosomes are present at the poles of aurA mutants or aurA RNAi spindles, they retain many antigens but are missing the Drosophila counterpart of mammalian transforming acidic coiled coil (TACC) proteins, D-TACC. We show that a subpopulation of the total Aurora A is present in a complex with D-TACC, which is a substrate for the kinase. We propose that one of the functions of Aurora A kinase is to direct centrosomal organization such that D-TACC complexed to the MSPS/XMAP215 microtubule-associated protein may be recruited, and thus modulate the behavior of astral microtubules.