This Week in Science

Science  05 Dec 2008:
Vol. 322, Issue 5907, pp. 1431
  1. A Closer Look


    During development, the early embryo undergoes massive restructuring due to the organized migration of large numbers of cells. However, imaging complex cellular movements inside living organisms is a huge technical challenge. In species like Drosophila, researchers have been limited to making observations from fixed specimens. Now, as technologies have advanced, it is possible to visualize complex cellular movements as they occur in real-time. McMahon et al. (p. 1546) have used advances in imaging and quantitative analysis to produce a detailed description of cell behavior during Drosophila gastrulation. Two-photon excitation fluorescence imaging techniques were used to visualize cells inside the developing Drosophila embryo, while still retaining viability. The position, movement, and division of over 1500 cells from two separate germ layers could be followed, three-dimensional (3D) over the course of 2 hours, revealing the role of a fibroblast growth factor in coordinating cell migration during gastrulation.

  2. See-Through Perovskite

    Much of Earth's dynamics and plate tectonics reflect the loss of heat through its interior. Thus, understanding the nature of that heat flow and the extent to which it occurs by advection, conduction, or radiation is critical. Usually, advection and conduction have been thought to dominate, but the role of radiation at high pressures and temperatures in the mantle has also been considered. Keppler et al. (p. 1529) have now measured the absorption spectra of silicate perovskite, a main mineral in the lower mantle, and show that, under high pressures, it is surprisingly transparent. This would imply that the overall thermal conductivity in the deep mantle could be higher than generally assumed.

  3. Solid State in a Cloud of Atoms

    The interactions between electrons in solid-state systems can manifest a number of different electronic phases. These systems tend, however, to be fixed in terms of tunability, and complex, with the underlying physics being masked by defects and other electronic bands. Cold atoms trapped in optical lattices have offered the potential of a clean, defect free, highly tunable system in which to explore the complex interactions between interacting particles. Schneider et al. (p. 1520; see the Perspective by Fallani and Inguscio) demonstrate this potential, working with potassium atoms confined to a 3D optical lattice. Tuning the depth of the trap and the interaction strength between the atoms and the physical confinement of the gas cloud allowed the system to be converted from a metallic state through a Mott-insulating state to a band-insulator. The results illustrate the possibility of using cold atoms to mimic and model complex solid state systems.

  4. A Tunnel Clock

    Among the more counterintuitive phenomena fostered by quantum mechanics at the atomic scale is the ability of particles to access spatial regions by tunneling through a barrier while lacking the energy to climb over it. Eckle et al. (p. 1525) have devised a technique for measuring the extremely rapid rate of this process in the context of electrons escaping from helium atoms in the presence of a strong laser field. Once free, the electrons are accelerated in whatever direction the field vector points at that instant. Applying an elliptical polarization to the laser pulse established a fixed reference point in the laboratory frame. The momentum of emergent electrons can then be resolved, and a tunneling time delay calculated, based on the known evolution of the field polarization. Intensities on the order of 1014 watts/square centimeter generated an upper bound of 34 attoseconds for the delay.

  5. Bio-Inspired Materials Engineering


    Nature uses many tricks to form strong composites from otherwise weak starting materials. Using an ice templating procedure, Munch et al. (p. 1516) combined aluminum oxide and poly(methyl methacrylate) to make a composite with a high toughness and properties comparable to some aluminum alloys. This freeze-casting technique produces the layered materials with remarkable toughness against crack growth, similar to the toughening mechanisms found in some biological composites like nacre.

  6. Martian Rhythmic Rocks

    Mars has been known to have some layered sedimentary rocks. With the camera on the Mars Reconnaissance Orbiter, Lewis et al. (p. 1532) have constructed stereo images of layered rocks exposed on the floors of several large craters and are now able to measure the thicknesses of individual beds at a resolution of about 1 meter. The layering is rhythmic, containing several cycles over 10 meters, organized into larger units, which implies that deposition was cyclic. It is possible that variations in Mars's orbit yielded climatic variations and thereby cyclical deposition, as seen in some sedimentary rocks on Earth.

  7. Blue-Light Response

    Plants respond to light with a variety of developmental and physiological changes. The receptor for the blue-light wavelengths is cryptochrome. How blue light causes cryptochrome to alter cellular function has been a puzzle. Now, using a yeast two-hybrid screen, Liu et al. (p. 1535, published online 6 November) have identified a protein from Arabidopsis, CIB1, which, in the presence of blue light, interacts with the cryptochrome. CIB1 and cryptochrome colocalize in the plant cell nucleus, where CIB1 functions as a transcription factor. Together, these proteins bring the input of blue light into the signaling pathways that regulate flowering.

  8. A Matter of Life and Death


    Pooling data from a population of cells can hide physiologically important differences that can only be seen at the level of individual cells. Cohen et al. (p. 1511, published online 20 November) describe a system to observe the dynamics in space and time of about 1000 different endogenous proteins in individual living human cells at large-scale, with high temporal resolution and accuracy. A retroviral strategy was used to construct a library of 1200 cell lines, each of which had incorporated a fluorescent protein into an intron in a different gene. The cells were previously tagged for red fluorescence in the nucleus and the cytosol, making automated imaging much easier. The approach was used to identify proteins that show widely different behaviors in individual cells in response to treatment with camptothecin, a commonly used chemotherapeutic, in a way that corresponds to outcome—cell death or survival.

  9. A Divisive Tale

    To ensure that each daughter cell receives a single genomic complement at the end of cell division, the central spindle—a set of microtubule bundles that forms between the separating chromosomes during anaphase—controls assembly and constriction of a cortical contractile actomyosin ring that bisects the separated chromosome masses. A key regulator of signaling by the central spindle is the protein complex centralspindlin. The current dominant model for signaling during cytokinesis proposes a simple positive signaling cascade in which centralspindlin tethers a Rho guanine nucleotide exchange factor (RhoGEF) to the central spindle, thereby activating the small GTPase RhoA locally at the cell equator. RhoA and its effectors, in turn, direct assembly and constriction of the contractile ring that divides the cell. Now Canman et al. (p. 1543) challenge this view by demonstrating that the critical function of centralspindlin in promoting contractile ring constriction is the negative regulation of Rac, a different Rho family GTPase. Thus centralspindlin may inactivate Rac and its effectors at the cell equator, and it is this negative regulation that promotes the RhoA-dependent constriction of the contractile ring.

  10. How to Feed Your Neurons

    The role of neuroglial interactions in brain functions has recently gained attention. Indeed, astrocytes are now viewed as integral active elements of the brain circuitry that play a crucial role in modulating synaptic activity. Astrocytes deliver metabolic substrates to neurons to face energy needs upon increased neuronal activity. However, the fine details of these processes are still unclear. Rouach et al. (p. 1551) demonstrate an essential role for astroglial gap junction proteins, connexin 30 and 43, enriched in perivascular astrocyte endfeet, in the activity-dependent delivery of energetic metabolites to neurons. Glucose metabolites were delivered specifically to astrocytes via a patch pipette in wild-type or double-knockout mice for these connexins, and local neuronal activity was recorded simultaneously. The perivascular connexins allowed intercellular trafficking of glucose metabolites through astroglial networks, which were used by neurons to sustain their excitatory synaptic transmission.

  11. Tolerating Maternal Influences

    The perception that the developing fetal immune system is sensitive to tolerance induction to foreign antigen has existed for more than 50 years. Although it is clear that tolerance can be achieved following experimental administration of foreign antigens in animal models, there is some debate about whether similar findings would be made in human beings. Mold et al. (p. 1562; see the news story by Leslie) now provide evidence that the human fetal immune system can be exposed to maternal cells during development, prompting the development of regulatory T cells that can then suppress fetal antimaternal immunity. These findings provide an explanation for the long-standing observation that some individuals are “tolerant” of certain noninherited maternal alloantigens.

  12. Toward Normalizing Metabolic Regulation

    Mice fed a diet high in fat develop resistance to the action of the hormone insulin, much as do humans suffering from the disease known as metabolic syndrome. Sabio et al. (p. 1539; see the Perspective by Ogawa and Kasuga) present evidence that the mitogen-activated protein kinase JNK1 is a key player in this process. The authors created conditional knockout mice in which JNK was no longer expressed in adipose tissue (fat). When fed a diet high in fat, these animals still showed increased concentrations of glucose and insulin in the blood, but measures of insulin responsiveness in the liver showed that the knockout animals retained a stronger response than did wild-type mice. Adipose tissue appeared to communicate with the liver through release of the cytokine interleukin 6 (IL-6). The knockout animals had less IL-6 in the blood after fasting than did control animals, and restoration of IL-6 in the knockouts reduced liver responsiveness to insulin. Thus, JNK1 represents a potential target for therapies aimed at improving metabolic control in diseases in which normal regulation is disrupted.

  13. What Is Behind the Secondary Inward Current?

    Hemichannels can form membrane channels that generate large ionic currents and allow the passage of small molecules across plasma membranes. Activation of N-methyl-D-aspartate (NMDA) receptors leads to a prolonged but unidentified secondary inward current. Thompson et al. (p. 1555) now show that this current is likely to be due to Pannexin-1 hemichannels. It seems excessive activation of NMDA receptors causes the opening of Pannexin-1 channels, which then leads to an increase in bursting activity. This type of hemichannel might thus represent a target for the treatment of epilepsy and stroke.

  14. Selfish Flowers

    Meiotic drive systems are those that change the inheritance of specific chromosomes from the expected ratio of 50:50 among the progeny, and thus can impact reproductive fitness and evolution. Fishman and Saunders (p. 1559; see the Perspective by Charlesworth) describe a system of female meiotic drive in a population of the wild monkeyflower plant that appears to be balanced by fitness costs in the pollen parent. A driving allele, D, is preferentially transmitted over alternative alleles through the female parents. The D allele is tightly linked to a candidate centromeric satellite found on all chromosomes and a particular haplotype of markers within the population. Thus, the drive element is centromeric and homozygotes for the D allele have significantly reduced pollen viability, explaining the polymorphism of D in the population.