Editors' Choice

Science  02 Jan 2009:
Vol. 323, Issue 5910, pp. 16

    Moving Toward Asymmetry

    Asymmetric division intrudes at various stages of development in order to generate daughter cells with distinctly different fates. An extreme instance occurs during oocyte maturation when the oocyte expels half of its chromosomal capital by packaging them into a small polar body, leaving behind the vastly larger haploid egg. To pull off this feat the oocyte needs to move its centrally located mitotic spindle to the cell periphery, in a process that relies on the actin cytoskeleton.

    Schuh and Ellenberg describe the role played by actin by watching spindle relocation in real time in live mouse oocytes. The actin cytoskeleton undergoes constant remodeling that relies on the activity of an actin-nucleating protein, formin-2. In particular, actin (shown at right, green) in the vicinity of the spindle (red) showed net flux toward the spindle poles, whereas actin elsewhere in the cell did not perform concerted directional movements. This poleward flux became especially apparent as the spindle approached the cell cortex. Along with this recruitment of actin, the activated form of the actin motor myosin-2 was also observed to accumulate near the spindle pole-associated actin filaments. Inhibition of myosin-2 motor activity interfered with the movement of actin toward the spindle poles and with spindle movement toward the periphery. Thus, spindle relocation depends on the interaction between the mitotic spindle poles and the actin cytoskeleton, which promotes the relocation of the spindle to the cell periphery and allows for the asymmetric division that produces egg and polar body. — SMH

    Curr. Biol. 18, 10.1016/j.cub.2008.11.022 (2008).


    A Star at Death's Door

    Some stars die violent deaths. If they are born with masses above about eight times that of the Sun, they become unstable as they approach the end of their nuclear fusion lives and collapse under the force of gravity. As a result they explode as supernovae, producing a sudden radiation burst that can outshine an entire galaxy for a short period of time. Such events don't usually go unnoticed; astronomers have been observing them over the centuries. However, very few stars have been observed right before their death, given the need to presciently acquire high-resolution images of the galaxy in question at just the right time.

    Mattila et al. used the European Southern Observatory Science Archive to find high-quality images of the pre-explosion site of supernova 2008bk, which was discovered in March 2008 in a galaxy 13 million light-years from Earth. By comparing the archive observations with a new image of the supernova they took using the adaptive optics system on the Very Large Telescope, they were able to unambiguously identify the star that exploded—an unfortunate red supergiant that started its life with a mass 8.5 times that of the Sun. The observation is consistent with theoretical models of supernovae, and provides further constraints on their progenitors. — MJC

    Astrophys. J. 688, L91 (2008).


    Finding Negatives Wherever

    Many autoimmune diseases are associated with specific alleles of the major histocompatibility complex (MHC), but how a particular MHC allele contributes to a particular disease is often not known. The MHC class II allele HLA-DQ8 has been linked to celiac disease, an autoimmune disease driven by T cell responses against dietary gluten, as occurs in wheat, for example. In contrast to other MHC alleles, HLA-DQ8 lacks an aspartic acid residue in its peptide-binding pocket and thus preferentially binds to peptides and T cell receptors (TCRs) that carry a negative charge. In celiac disease, negatively charged gluten-derived peptides are generated by tissue transglutaminase (which converts glutamine to glutamic acid); however, how HLA-DQ8 contributes to disease is not clear because transglutaminase requires inflammation for its activation.

    Hovhannisyan et al. demonstrate that antigluten T cell responses can be mediated by HLADQ8 bound to native (nondeamidated) gluten peptides. Recruited T cells contained negatively charged TCRs, which helped to stabilize HLA-DQ8 binding of nondeamidated peptide. Inflammation resulting from this response led to the activation of transglutaminase and to peptide deamidation. The T cells were highly cross-reactive and mounted stronger responses to deamidated gluten peptides due to the enhanced stability of TCR-peptide-HLA-DQ8 interaction. These studies suggest how HLA-DQ8 may drive an amplified T cell response to gluten in celiac disease. — KM

    Nature 456, 534 (2008).


    Winter Carbonate Collapse

    Anthropogenic fossil-fuel burning is increasing the concentration of CO2 in the atmosphere, which in turn is causing more CO2 to dissolve in the ocean, thereby lowering the water's pH. Such ocean acidification in turn decreases the concentration of carbonate ion (CO3)2−, which makes it more difficult for calcifying organisms such as foraminifera, pteropods, and corals to build their skeletons. So far, most of the attention paid to this process has focused on the time-averaged chemistry of the ocean, but organisms actually experience seasonal carbonate and pH variations. McNeil and Matear examine these variations and show that anthropogenic CO2 uptake is likely to induce winter aragonite undersaturation in some regions of the ocean when atmospheric CO2 levels reach 450 parts per million. These findings underscore the importance of understanding the seasonal dynamics of marine carbonate chemistry, as natural variability could hasten the deleterious impacts of future ocean acidification. — HJS

    Proc. Natl. Acad. Sci. U.S.A. 105, 18860 (2008).


    Diverse Scaffolds

    Chemical libraries are widely used in bioactivity studies and drug discovery, yet many such libraries cover only a limited range of chemical shapes, or molecular scaffolds. Morton et al. have now achieved a library of over 80 different scaffolds by exploiting the power of ring-closing metathesis, a process involving scission and redistribution of carbon-carbon double bonds that leads to the formation of ring systems. Using two types of building blocks and fluorous-tagged linkers, the authors prepared cyclic products in four or five steps using combinations of just six reaction types. The compounds prepared contained numerous structural features such as isolated, fused, and spirocyclic ring systems; intramolecular hydrogen bonding; unsaturation; and dense substitution that were reminiscent of natural products. Many of the scaffolds had not been previously synthesized. — JFU

    Angew. Chem. Int. Ed. 10.1002/anie.200804486 (2008).


    Follow the Leader

    Cell migration is fundamental, being an integral part of human development and a driving force behind tissue repair. Many cells possess the ability to migrate directionally in response to environmental signals, a process known as chemotaxis. When starving, individual Dictyostelium discoideum cells aggregate in a head-to-tail fashion to form a migrating stream. This coordination is achieved by transmitting an amplified cAMP signal to neighboring cells. Adenylyl cyclase, the enzyme that makes cAMP, is concentrated at the rear of the cell and is required for this collective migration. Kriebel et al. discovered that cyclase is present in intracellular vesicles and that vesicular trafficking via actin and microtubule networks is required to generate its asymmetric distribution. Migrating cells were also found to shed cyclase-containing vesicles from the back of the cell. This directional release of vesicles could form extracellular tracks along which other cells follow, in what may prove a general mechanism for coordinated migration processes. — HP*

    J. Cell Biol. 183, 949 (2008).

    • *Helen Pickersgill is a locum editor in Science's editorial department.


    Controlling Salt Intake

    It's hard to know which we might run out of sooner, fresh water or fresh land. Most agricultural systems depend on a plentiful supply of both. The problem is that irrigation, especially in brackish environments, and intense land usage can increase soil salinity, degrading the productivity of the land. Decreasing agricultural productivity in the face of increasing community needs moves us in the wrong direction. Meanwhile, demand rises for feed crops used to support milk and meat production. Khan et al. have devised a feed crop growth system that can be managed in the midst of brackish waters. Surveys of Pakistani herdsmen and herbalists led them to investigate the grass Panicum turgidum, which can grow in brackish waters and salty soils, but avoids accumulating salt itself, which would make it unappetizing and harmful as animal fodder. Cocultivation within a grid of Saueda fruticosa, a salt-accumulating plant that is used locally to make soap, kept the soil salinity stable. Farmers in southwestern Pakistan may be willing to replace their standard maize feed with this system. — PJH

    Agric. Ecosyst. Environ. 10.1016/j.agee.2008.10.014 (2008).