Editors' Choice

Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1871

    Not So Spineless

    Sea urchins are sophisticated invertebrates whose biology holds many clues to the evolution of the vertebrates. These organisms very effectively remove any invading bacterial pathogens and other foreign material from within their coeloms by means of a range of macrophage-like cells. It seems that sea urchins have a simplified version of the complement system that can mediate opsonization of pathogens. Nair et al. have been investigating the immune responses of sea urchins by analysis of expressed sequence tags generated from coelomocytes and discovered that a wide range of genes are up-regulated in response to bacterial lipopolysaccharide. What was particularly interesting was a previously unknown gene family that represented 60% of the ESTs and could undergo alternative splicing to yield around 15 translatable elements. The evidence suggested that these were immune response proteins under positive selection for diversification, and revealed a greater level of complexity of putative responses than anticipated for an invertebrate group. — CA

    Physiol. Genomics 22, 33 (2005)


    A Well-Fitted Coating

    Most heterogenous metal catalysts consist of metal nanoparticles on a ceramic oxide support, but for systems that exhibit strong metal-support interactions (such as noble metals with cerium oxide), the maximum interaction might involve completely coating a metal nanoparticle with oxide. Yeung et al. have used a modified microencapsulation method, previously demonstrated for silica, for encapsulating platinum nanoparticles with ceria. Increasing the Pt loading from 1 to 5% created particles with larger Pt cores and thinner coatings of ceria. The thinnest coating (1.7 nm) increased the band transition for ceria from 3.18 to 3.33 electron volts, and increased the water-gas shift (WGS) activity (H2O + CO → H2 + CO2) from negligible CO conversion for pure ceria to 63%. Unlike most other noble metal-ceria catalysts, these nanoparticles, which expose few noble metal sites, exhibit no activity for the competing reactions of methanation and higher hydrocarbon formation. — PDS

    J. Am. Chem. Soc. 10.1021/ja056102c (2005).


    Electrostatic Stitching

    Supported lipid bilayers are often used as model systems for studying surface phenomena because of their well-defined planar geometry. One interesting phenomenon relates to the “fluid-to-gel” phase transition that occurs when the mobile liquid-crystalline ordering crystallizes as the temperature is lowered. However, it is difficult to study this phase transition in simple single-component bilayers, such as those composed of zwitterionic phospholipids, because of a high density of defects that form in the gel phase. The defects are most likely caused by shrinkage of the area occupied by a lipid molecule as the tilt angle of the head-group changes on cooling. Zhang et al. tested this hypothesis by studying the effects of adding cationic or anionic lipids to a bilayer composed of a zwitterionic phosphatidylcholine. With the addition of the cationic lipid, defects no longer formed on gelation. Measurements of the head-group orientation showed that the cationic lipid increased the tilt angle in the fluid phase, but that it no longer changed on cooling. The cationic lipid is expected to be well dispersed because of electrostatic repulsions, and they may act to stitch together the bilayer, thus giving it stability through the phase transition. The addition of anionic lipids, which is expected to be unfavorable, led to defects in the gel phase, supporting the stitching hypothesis. — MSL

    J. Phys. Chem. B, 10.1021/jp055995s (2005).


    Parkinson's and Potassium Channels

    Parkinson's disease (PD) results from the selective loss of dopaminergic neurons in the substantia nigra of the brain. However, dopaminergic neurons in nearby parts of the brain are not affected, even though the genes implicated in familial inherited PD, as well as toxins that can induce symptoms of PD, are not restricted in their effects. Why then is this small region targeted for destruction in PD? There are hints that substantia nigra neurons show disruptions in mitochondrial respiratory function. Diminished cellular metabolism, as well as oxidative stress, can in turn cause the potassium (K)-ATP channels of dopaminergic neurons to open. Liss et al. investigated the interaction between these channels, the signals that control their function, and the degeneration of neurons. The K-ATP channel mediates dopaminergic neuron degeneration in response to mitochondrial complex 1 inhibition, in response to PD-inducing treatment of susceptible mice, and also in the mutant weaver mouse, in which dopaminergic neuron degeneration is due to constitutive activation of another potassium channel. The inappropriate function of K-ATP channels is characteristic of substantia nigra neurons, but not of dopaminergic neurons in other nearby brain areas, in which the channels seem to be connected to cellular metabolism through different signaling networks. — PJH

    Nat. Neuro 8, 1742 (2005).


    Kuiper Belt Curiosity

    Pluto is only one of several large bodies in the outer solar system's Kuiper belt. Brown et al. describe the discovery of the largest object in orbit beyond Neptune, 2003 UB313, whose brightness suggests that it exceeds the size of Pluto. By tracing the object's motion in archival images, they show that it follows a highly eccentric orbit inclined 44° from the ecliptic plane, which contains most of the objects that are orbiting the Sun. Such an extreme orbit may have arisen if the body formed closer to the Sun and was scattered outward by gravitational interactions. Frozen methane is detected on the surface in infrared spectra, with characteristics very similar to Pluto. However, 2003 UB313 is not as red as Pluto, suggesting that the distribution of methane and other hydrocarbons on its surface is different and may even change with temperature as it swings closer to the Sun. — JB

    Astrophys. J. 635, L97 (2005).


    Organization Without an Organizer

    Within cells, the tracks provided by microtubules are important for a whole variety of cellular processes, not least when microtubules form into a spindle in order to promote the separation of chromosomes during mitosis. Such microtubule arrays are arranged around organizing centers known as the centrosomes. However, within the cell there also exist well-organized arrays of microtubules that form without the aid of centrosomes. Reilein et al. describe the organizing principles involved in producing acentrosomal microtubule networks found in the basal cortex of epithelial cells. Microtubules are formed from tubulin monomers, and microtubule networks in a steady state contain growing and shrinking microtubules. Typically, in order to grow, microtubules need to be anchored somehow. By imaging microtubule dynamics in cytoplasts derived from the base of epithelial cells, the authors showed that networks of microtubules form based on microtubule-microtubule interactions and microtubule-cortex interactions. Each type of interaction increased microtubule stability. By modeling the parameters involved, in particular by including stabilizing interactions, the authors could replicate in silico the type of stable arrays observed within cells. — SMH

    J. Cell Biol. 171, 845 (2005).

  7. STKE

    β-Arrestin Regulates Notch Abundance

    β-Arrestin, well known for its role in G protein-coupled receptor regulation, is also being recognized for its roles in regulating other types of receptors. Mukherjee et al. report that Drosophila β-arrestin, Kurtz (Krz), is involved in controlling the abundance of the receptor Notch. Notch is a single transmembrane receptor that is cleaved in response to ligand binding, releasing a fragment that translocates to the nucleus to regulate transcription. Krz was found in two different screens for proteins that interacted with the Notch regulator and with putative E3 ubiquitin ligase Deltex (Dx). In flies, loss of Krz function led to increased Notch abundance. Overexpression of both Krz and Dx produced Notch loss-of-function phenotypes and reduced Notch protein abundance. In transfected Drosophila S2 cells, Krz and Dx together promoted ubiquitination of Notch. Notch signaling is highly sensitive to gene dosage effects, and β-arrestin appears to be one component that contributes to this sensitivity. — NG

    Nat. Cell Biol. 7, 1191 (2005).