This Week in Science

Science  29 Jan 2010:
Vol. 327, Issue 5965, pp. 501
  1. X-rays to Order


    Self-assembly of molecules is often irreversible. Cui et al. (p. 555, published online 17 December; see the Perspective by Safinya and Li) examined the ordering of a short peptide sequence (Ala6Glu3) terminated with an alkyl chain. Aqueous solutions of this molecule could form hexagonally ordered filaments, but more dilute solutions were disordered. However, prolonged x-ray exposure caused these arrays to become ordered. These arrays were stable for several hours but eventually returned to a disordered state; the addition of salts slowed the ordering processes. It is possible that during the ordering process, X-ray–induced charging affected the repulsive forces that balance tension within the filament.

  2. In the (Stem Cell) Zone

    Efforts to isolate and characterize various types of stem cells have revealed much about the molecular mechanisms of stem cell self-renewal and differentiation, as well as the function of the microenvironment, or stem cell niche. Li and Clevers (p. 542) review what is known about stem cells in the hair follicle, bone marrow, and intestinal epithelium and suggest that two stem cell populations, one that is quiescent and one that is actively in the cell cycle, may coexist in these tissues in a so-called “zoned” stem cell model.

  3. Phase Transitions on Carbon Nanotubes

    The nature of phase transitions changes with system dimensionality. Many aspects of two-dimensional systems have been explored by adsorbing rare gases on graphite surfaces. Wang et al. (p. 552) reduce the dimensionality further by examining phase transitions of argon and krypton on single-walled carbon nanotubes, following the extent of surface coverage and mapping out phase transitions by using the nanotube as a resonator. Changing the conductance and thus the density of surface electrons also allowed exploration of the effect of adsorbate-surface interactions.

  4. Packing Puzzle

    The packing of a large number of spheres is a well-studied problem with maximal packing based on the arrangement of nearest neighbors. With much smaller numbers of particles, it is the free energy that governs which packing arrangements dominate. Meng et al. (p. 560; see the Perspective by Crocker) looked at the assembly of colloidal clusters where the number of particles was limited from 2 to 10. For five particles or fewer, only one packing arrangement was found. For six or more particles, while a number of similar energy structures could form, the probability of formation was biased toward those structures with the greater number of nearest-neighbor connections.

  5. Big Finger

    Alvarezsauroidea are an enigmatic group of theropods presumed to be closely related to birds, though most specimens are younger than Archaeopteryx. Choiniere et al. (p. 571; see news story by Stone) now describe a more complete early specimen, dating to about 160 million years ago, which supports the conclusion that Alvarezsauroidea are a basal group of the clade containing both birds and their close theropod relatives. The fossil also helps to reveal the evolution of this group's peculiar forelimb, which includes one enlarged functional finger.

  6. Sex Avoidance Strategy

    In metazoans, sex, as a means of reproduction, is almost universal, with popular theories suggesting that the high rate of genome mixing that sex promotes helps protect us from deleterious mutations and/or rapidly evolving parasites. Asexual organisms are both rare and often evolutionarily short-lived. If this rarity is due to failure to deal with parasites, then bdelloid rotifers, which have been abstaining from sex for millions of years, must be able to dodge the parasite bullet in some arrange-other way. Wilson and Sherman (p. 574; see the cover) suggest that bdelloid rotifers do so by being both so resistant to desiccation and so amenable to dispersal on the air that their parasites can neither survive such punishing conditions nor spread as far nor as fast, allowing the asexual bdelloids to start afresh in (parasite-free) pastures anew.

  7. Aromatic Silicon


    Benzene has long intrigued chemists on account of the energy stabilization, termed aromaticity, which arises from π-electron delocalization around its ring framework. A persistent question has been how such stabilization would be impacted were the carbons to be replaced by heavier atoms such as silicon. Abersfelder et al. (p. 564) have prepared a benzene analog with Si atoms in place of all six-ring carbons, but a slightly altered bonding framework in which substituents outside the ring are no longer evenly distributed. Instead, the substituents pair up at two Si sites, leaving two other ring sites with no external appendages. The resulting compound no longer has a continuous network of π-electrons, but retains a degree of aromatic stabilization involving sigma and nonbonding electrons.

  8. The Best Things in Life Are Free?

    Does money buy happiness? Answers to this question differ, depending, in part, on whether one asks an economist or a psychologist. The former would point to correlations between higher incomes and greater self-reported well-being, whereas the latter would argue that happiness shows little correlation with absolute material goods and is instead dictated largely by an individual's so-called set-point. Another strand of research invokes a hedonic treadmill, whereby income matters until subsistence requirements are met, at which point comparisons with one's neighbors are what influence one's sense of life satisfaction. Oswald and Wu (p. 576, published online 17 December; see the Perspective by Layard) establish that the subjective responses from 1 million adults, collected within health surveys conducted by the U.S. Centers for Disease Control and Prevention, do indeed correlate with objective measures of quality of life.

  9. Platelet Microparticles Drive Inflammatory Arthritis


    Platelets are best known for their critical role in blood clot formation during wound repair, but an appreciation for their role in inflammatory processes is growing. Platelet-derived cellular microparticles (MPs) are small membrane vesicles released from platelets in response to cell activation that can transport biomolecules throughout the body that have also been implicated in inflammatory processes. Boilard et al. (p. 580; see the Perspective by Zimmerman and Weyrich) have now found that platelet-derived MPs probably contribute to the inflammatory processes underlying rheumatoid arthritis, an autoimmune disease. The majority of MPs in synovial fluid from patients with various types of inflammatory arthritis were platelet-derived and, importantly, platelet-derived MPs were lacking in synovial fluid from osteoarthritis patients. Furthermore, platelet depletion abrogated disease development in a mouse model of inflammatory arthritis.

  10. Columns, Connections, and Correlations

    What is the nature of interactions between neurons in neural circuits? The prevalent hypothesis suggests that dense local connectivity causes nearby cortical neurons to receive substantial amounts of common input, which in turn leads to strong correlations between them. Now two studies challenge this view, which impacts our fundamental understanding of coding in the cortex. Ecker et al. (p. 584) investigated the statistics of correlated firing in pairs of neurons from area V1 of awake macaque monkeys. In contrast to previous studies, correlations turned out to be very low, irrespective of the stimulus being shown to the animals, the distances of the recording sites, and the similarity of the neuron's receptive fields or response properties. In an accompanying modeling and recording paper, Renart et al. (p. 587) demonstrate how it is possible to have zero noise correlation, even among cells with common input.

  11. When Polymerases Collide

    Genomes must be replicated by DNA polymerase and copied by RNA polymerase. The two processes occur concurrently in bacteria and, although the majority of bacterial genes are co-oriented with replication, head-on collisions still occur and can result in double-strand breaks in the genome. Pomerantz and O'Donnell (p. 590) show, in vitro, that both Escherichia coli DNA and RNA polymerases grind to a halt when they collide bumper to bumper but that the replication fork remains intact, with the RNA polymerase being shunted off the DNA and out of the way. Mfd, a transcription-repair coupling factor, promotes DNA replication restart probably by rewinding separated DNA stands ahead of the halted transcription complex, promoting its displacement from the DNA.

  12. Gut Check

    The gastrointestinal (GI) tract is particularly sensitive to damage by ionizing radiation. Despite decades of study, fundamental questions such as which cells and which molecular mechanisms mediate this GI damage remain a source of great controversy. Studying a series of genetically manipulated mice, Kirsch et al. (p. 593, published online 17 December) conclude that GI epithelial cells, rather than endothelial cells, are the critical cellular targets of radiation damage and that apoptosis (a well-studied mechanism of cell death) is not a major contributor to the damage. Rather, an alternative cell-death pathway whose activity is inhibited by the tumor suppressor protein p53 appears to mediate GI damage. Further insights into this pathway may assist the development of medical counter-measures for preventing and treating radiation-induced tissue damage.

  13. Promoting Axon Formation

    How do neurons initiate one axon and lots of dendrites? Using an in vitro assay involving stripes of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), Shelly et al. (p. 547) show that an increase in cAMP initiates axon formation while an increase in cGMP initiates dendrites. Moreover, cAMP and cGMP reciprocally inhibit each other via the activation of specific phosphodiesterases, as well as protein kinase A and protein kinase G. Finally, long-range self-inhibition of cAMP can explain why only one axon, yet multiple dendrites, is initiated in single hippocampal neurons in culture. Locally increasing cAMP in one neurite causes long-range cAMP reduction in the rest of the neurites, accompanied by corresponding reciprocal changes in cGMP.

  14. Secondary Selectivity

    Organic molecules consist principally of rings and chains of methylene (secondary) CH2 groups, intermittently adorned with oxygen or nitrogen centers and more heavily substituted carbons at the junctions. Synthetic transformations would be most efficient if the C–H bonds in any particular methylene group along the framework could be targeted for selective modification. However, for the most part, these carbon centers prove remarkably hard to differentiate for reactive purposes. Chen and White (p. 566) now show that an iron catalyst can direct peroxide to oxidize specific secondary C–H bonds preferentially, and with reasonable efficiency, in a range of complex molecules. The observed selectivities follow predictable trends correlated with the electronic and steric environment of the target site.