Editors' Choice

Science  08 Apr 2011:
Vol. 332, Issue 6026, pp. 150
  1. Biochemistry

    Enzymes Aren't Perfect

    1. L. Bryan Ray

    Cells have efficient quality-control systems to detect and repair errors in polymerization of DNA or RNA. Tagliabracci et al. reveal the biological effects of errors by another polymerase, glycogen synthase, which creates glycogen—branched chains of glucose that serve as an important energy store in cells. About 1 in 1000 of the glucose residues in normal glycogen contain covalently linked phosphate. The authors found that the major enzymatic activity from mouse muscle catalyzing such incorporation of phosphate was in fact glycogen synthase. Although the incorporated phosphate may serve a useful function, current evidence indicates it is probably a mistake. A phosphatase that removes phosphate from glycogen is known as laforin because mutations in the enzyme are associated with Lafora disease, a deadly human disease that causes neurodegeneration and epilepsy. The disease appears to result from insolubility of excessively phosphorylated glycogen. Thus, laforin is likely required to prevent deleterious effects of catalytic errors made by glycogen synthase.

    Cell Metab. 13, 274 (2011).

  2. Materials Science

    Flat Pack

    1. Marc S. Lavine
    CREDIT: DONG ET AL., NANO LETT. 11, 10.1021/NL200468P (2011)

    Superlattices combining colloidal particles or nanocrystals with two distinct sizes can exhibit unusual magnetic and electronic properties. It has proven easier to make three-dimensional rather than two-dimensional structures of this sort, particularly for samples that extend over large areas. Dong et al. used a liquid-air fabrication method with a low concentration of nanocrystals in the feedstock to form single and binary superlattices that extend over cm2 areas. The nanocrystals were deposited from hexane onto the surface of diethylene glycol and allowed to dry. Cocrystallization of Fe3O4 with Au, Fe3O4 with FePt, and NaYF4:Yb/Er with Fe3O4 nanocrystals showcased the generality of the method. The superlattice structure could be controlled by changing the ratio of the diameter of the particles, with both AB- and AB2-type lattices obtained. In hexagonally packed AB2 lattices, the smaller particles occupied the interstices of three larger neighboring particles. Shifting to A2B3 bilayer structures (above), the authors found that the smaller particles sat on only half of the larger particles, and this anisotropy drove the formation of a beltlike morphology rather than a membrane. The method was extended to ternary ABC2 structures using a mixture of three types of particles, although in this case the structure was more prone to defects.

    Nano Lett. 11, 10.1021/nl200468p (2011).

  3. Cell Biology

    Have miRNA, Will Travel

    1. Guy Riddihough

    MicroRNAs (miRNAs), small noncoding RNAs encoded in the genomes of many eukaryotes, have a pervasive role in regulating gene expression. miRNAs normally act cell-autonomously, but increasing evidence suggests that they might also act far from their site of synthesis.

    High-density lipoprotein (HDL) particles facilitate the transport of lipids, and other biomolecules, in the bloodstream. Given that lipid-based transport of RNAs can be used as a systemic delivery system, Vickers et al. investigated the nucleic acid component of HDL and found that they are able to carry miRNAs. The profile of these miRNAs differed between healthy individuals and individuals suffering from familial hypercholesterolemia (which can lead to atherosclerosis), an effect also seen in a mouse model for atherosclerosis. HDL loaded with exogenous miRNAs was able to modulate the expression of specific miRNA target genes when added to tissue-culture cells. Indeed, the changes in gene expression in cells exposed to HDL from individuals with familial hypercholesterolemia were enriched for genes involved in lipid metabolism, inflammation, and atherosclerosis, which suggests that some of the HDL miRNAs may play a direct role in disease progression.

    Nat. Cell Biol. 13, 10.1038/ncb2210 (2011).

  4. Evolution

    Shape-Shifting Cyanobacteria

    1. Caroline Ash
    CREDIT: BETTINA SCHIRRMEISTER

    Two-and-a-half billion years ago, cyanobacteria may have caused the first mass extinction on Earth by inventing oxygenic photosynthesis. During their slow evolution, these prokaryotes donated their light-harvesting organelles to plants, developed a nitrogen-harvesting mechanism, and, not least, adopted many multicellular morphotypes. Few cyanobacteria have been sequenced, however, and as a result, their evolutionary history and phylogeny are largely unknown. Schirrmeister et al. used multiple phylogenetic analyses on the limited number of 16S rDNA cyanobacterial sequences available to compare the resulting trees with established morphologically derived clades and to the scant fossil record. They obtained a monophyletic tree with Gloeobacter being the nearest outgroup and Chlamydia as the closest eubacterium. Rates of evolution among the cyanobacteria may be slow, but they organized into multicellular forms that displayed some functional specialization several times—and well before eukaryotes undertook this major transition. Since then, cyanobacteria appear merely to have evolved into three major clades, which have shifted between multicellular and unicellular morphotypes. Most modern lineages appear to have multicellular common ancestors, including the important marine genera Synechococcus and Prochlorococcus.

    BMC Evol. Biol. 11, 45 (2011).

  5. Policy

    Framing the Climate Debate

    1. Barbara R. Jasny

    How concerned are Americans on the whole about global warming? Yeager et al. have found that the answer may depend on what exactly the question is. Pollsters regularly pose a “most important problem” (MIP) question originally devised by George Gallup in the 1930s: “What do you think is the most important problem facing the country today?” Only 1 to 2% of respondents to this question in three surveys conducted by the authors offered global warming or the environment as an answer. However, if instead the question posed was “What do you think will be the most serious problem facing the world in the future if nothing is done to stop it?” global warming/environment emerged as the most frequent response, its percentage more than 10-fold higher than before. Shuldt et al. examined the partisan subtleties of wording choice. They found that the Web sites of conservative think tanks use the phrase “global warming” more frequently than “climate change,” whereas the reverse was true of liberal think tank sites. They then surveyed a sample of Americans to probe the impact of these distinct phrases and found that self-identified Republicans were more likely (by a ∼3:2 margin) to consider climate change a real phenomenon than global warming. Democrats were not affected by the wording, nor did educational attainment appear to favor one response over the other.

    Public Opin. Q. 75, 125;115 (2011).

  6. Materials Science

    The (S)lowdown on Crystallization

    1. Phil Szuromi

    One way to resolve the many steps that occur during crystallization is to use small droplets of solution, in part to avoid heterogeneous nucleation by impurities. Stephens et al. created picoliter droplets by using self-assembled monolayers (SAMs) that contained hydrophilic (carboxylic acid terminated) islands inside hydrophobic (fluorous terminated) layers. These islands supported hemispherical droplets, with radii varying from 4 to 10 µm (0.04 to 2 pl in volume), that were supersaturated with calcium carbonate. Bulk precipitation created many rhombohedral crystals, but in 90% of the droplets, a single smaller tetrahedral crystal formed. Redissolution of these tetrahedral crystals in undersaturated solution, or further growth from bulk solution, initiated recrystallization into rhombohedral crystals. The SAMs appear to slow down crystallization and capture the tetrahedron as an early intermediate crystal form.

    J. Am. Chem. Soc. 133, 10.1021/ja200309m (2011).

  7. Biomedicine

    Axon Damage Illuminated

    1. Kristen L. Mueller

    Normal-appearing white matter

    CREDIT: NIKIĆ ET AL., NAT. MED. 17, 10.1038/NM.232 (2011)

    Lesion

    CREDIT: NIKIĆ ET AL., NAT. MED. 17, 10.1038/NM.232 (2011)

    Pathogenesis of the autoimmune disease multiple sclerosis (MS) is associated with progressive deterioration of the myelin sheath surrounding neuronal axons; however, axon damage may also contribute to MS-associated neurodegeneration. Nikić et al. used in vivo imaging and electron microscopy to examine axon damage in a mouse model of MS (EAE, experimental autoimmune encephalitis). In EAE mice, swelling in discrete sites on axons was observed, which was then followed by axon fragmentation. In many cases, damaged axons retained myelin; in some cases, axon damage was reversible. Axon damage was preceded by mitochondrial pathology, which was associated with the presence of microglia and the production of reactive oxygen and nitrogen species. Induction of oxidative or nitrosative stress was sufficient to induce mitochondrial pathology and axon damage in normal mice, and their blockade in EAE mice alleviated axon damage. Lesion biopsies from MS patients also showed similar axon (above, right) and mitochondrial damage, which suggests that reversing axon damage may be an important therapeutic strategy in the treatment of MS.

    Nat. Med. 17, 10.1038/nm.2324 (2011).

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