This Week in Science

Science  13 Mar 2009:
Vol. 323, Issue 5920, pp. 1401
  1. Immunological Balancing Act


    Secretory immunoglobulin A (IgA) plays a crucial role in the gut by promoting immune tolerance to food and the microflora that colonize the gut. IgA is produced by B cells with the help of CD4+ T follicular helper cells. How these helper CD4+ T cells are generated in the gut is unclear. Tsuji et al. (p. 1488) found that suppressor Foxp3+CD4+ T cells can differentiate selectively into follicular B cell helper T cells (TFH) in Peyer's patches, specialized lymphoid tissue in the gut, but not in other lymphoid tissue. Differentiation of suppressor T cells into TFH cells requires down-regulation of the suppressor T cell-specific transcription factor Foxp3 and interaction with B cells. Thus, flexibility of T cell populations and the specific environment of the gut help to maintain the critical balance between immune tolerance and activation.

  2. Out of the Deep

    The Southern Ocean may have provided a reservoir for atmospheric CO2 during cold glacial periods, which could help to explain why the concentration of atmospheric CO2 rose by roughly 50% during each of the last five deglaciations. Anderson et al. (p. 1443; see the Perspective by Toggweiler) find that burial rates of biogenic opal increased in the Southern Ocean during the last deglaciation, indicating more vigorous upwelling. More intense upwelling would have brought more CO2, dissolved in deeper water, to the surface and vented to the atmosphere, potentially leading to the observed glacial-to-deglacial CO2 rise.

  3. Fusion Confusion

    Vesicle fusion and neurotransmitter release are fundamental aspects of brain function. Efficient use of synaptic vesicles and the dynamics of neurotransmission depend critically on the mode of fusion. So-called kiss-and-run fusion and subsequent reuse of the same vesicle could in principle greatly expand information transfer at synapses. Zhang et al. (p. 1448, published online 12 February) used a quantum dot-based optical approach to investigate synaptic vesicle behavior that allowed kiss-and-run and classical full collapse fusion to be distinguished at the kind of synapses involved in learning and memory. The prevalence of kiss-and-run, relative to classical fusion, changed progressively over the course of a series of stimuli.

  4. Crystallization Visualized

    Crystal nucleation and growth from a melt or supersaturated solution is often kinetically driven by the presence of secondary materials or surfaces that alter the free energy of the system. Pouget et al. (p. 1455; see the cover) provide a description of calcium carbonate precipitation under Langmuir monolayers in which the monolayer controls the orientation of the precipitating crystals. Cryogenic transmission electron microscopy allowed the sequence of events to be traced starting from amorphous mineral particles, which attach to the ordered monolayer, until crystalline particles form on a structured template. Throughout the process amorphous calcium carbonate particles were observed, confirming their role in the crystallization process.

  5. Self-Healing Polyurethanes

    Self-healing materials may allow the fabrication of objects that can be repaired in situ. Ghosh and Urban (p. 1458) devised a self-repairing polyurethane in which the process that damages the material also initiates the first step in the healing process. Oxetane-substituted chitosan was incorporated into a polyurethane and, upon mechanical damage, the oxetane rings open up to form reactive end groups. With exposure to ultraviolet light, the chitosan chains are cut, and link to the oxetane reactive sites, thus leading to rapid healing and repair in a scratched polyurethane coating.

  6. Squeezing in Fluorocarbons

    Chemical separations using porous solids mostly rely on size selection: Compounds too large to squeeze through the pores are excluded. Metrangolo et al. (p. 1461) describe a class of ionic solids that can selectively capture certain fluorocarbons dynamically. The crystals comprise dicationic hydrocarbon chains capped at each end by positive ammonium groups, with negative iodide ions to balance the charge. Although structurally nonporous, the solids spontaneously stretch to accommodate iodine-capped fluorocarbon chains, which form robust intermolecular halogen bonds with iodides at each end. The encapsulation is highly selective for the fluorocarbon with a chain length scaled to the lattice dication. Moreover, the process is reversible, with the guests liberated on heating, offering potential for use in industrial fluorocarbon separations.

  7. Mistier Blue Sky


    Atmospheric aerosols have a large effect on how, and how much, the atmosphere is warmed by solar radiation, and uncertainties about their abundance has impeded a more nuanced understanding of the mechanisms of global warming. Recently, “global dimming” and “global brightening” have been observed when the amount of solar radiation incident on Earth's surface apparently decreased and increased. Wang et al. (p. 1468) analyzed records of aerosol optical depth gathered worldwide since 1973 and constructed a global map of trends in clear sky visibility. Skies have become less transparent over most, but not all, of the world, and the changes in aerosol optical depth correspond well to reports of dimming or brightening over the past 20 to 30 years.

  8. The CS2 Point of View

    Molecules in the liquid and gas phases tumble constantly, and techniques that probe reaction dynamics in these states often yield blurry information averaged over multiple orientations. Bisgaard et al. (p. 1464) used a short laser burst to push a sample of gaseous CS2 molecules into temporary alignment and applied ultrafast spectroscopy to study photodissociation in the molecular reference frame, free of blurring. A complex progression of electronic motion was observed coupled to C-S bending and stretching vibrations preceding scission of a C-S bond.

  9. Too Little of a Bad Thing

    Mutations in the amyloid precursor protein (APP) are known to cause familial Alzheimer's disease. Di Fede et al. (p. 1473) now describe an APP mutation that is associated with disease only in the homozygous state. The A673V mutation affects APP processing, which leads to enhanced β-amyloid (Aβ) production, and also increases the Aβ's tendency to aggregate and form amyloid fibrils. However, co-incubation of mutated and wild-type Aβ peptides confers instability on Aβ aggregates and inhibits amyloidogenesis, consistent with the observation that heterozygous carriers do not develop the disease even at a very old age.

  10. Check Out the Checkpoint


    “Job one” for any living organism is to make cells that can reproduce and faithfully distribute chromosomes. Not only do eukaryotic cells manage to line up and segregate their chromosomes during cell division, but they have a fail-safe mechanism that prevents cell division from proceeding if all of the chromosomes are not lined up appropriately. Herzog et al. (p. 1477) provide a glimpse of the molecular machinery that makes this fail-safe mechanism work. At the heart of the mechanism is a ubiquitin ligase complex, the anaphase-promoting complex/cyclosome (APC/C), whose activity is modulated by an activator, Cdc20, or by an inhibitory complex called the mitotic checkpoint complex (MCC), which includes of Cdc20 together with other proteins. Proteins were purified from cells in which the mitotic checkpoint was active or inactive and the structure of APC/C complexes with or without its various partners bound was observed by single-particle electron microscopy. MCC inhibited APC/C by physically obstructing binding of APC/C's substrates for ubiquitinylation and by inducing changes in APC/C that lock it in a more compact conformation.

  11. Selective Removal of Memories

    How are memories encoded and stored in the brain, and which neurons are part of a specific memory? Han et al. (p. 1492) tried to establish a causal link between a defined population of neurons and the disruption of a specific fear memory. Neurons in the lateral amygdala overexpressing the transcription factor CREB are preferentially activated by fear memory testing, which suggests that this population of neurons is selectively recruited to the fear memory test. Selective killing of just these neurons after training impaired subsequent memory expression, whereas random destruction of a similar number of lateral amygdala neurons did not impact subsequent memory expression. This memory loss was robust, persistent, and not due to a disruption in reconsolidation or overall lateral amygdala function.

  12. Windfall Wonder

    Research on primates suggests that midbrain dopaminergic neurons encode unexpected reward signals that drive reinforcement learning. The reward-related properties of these neurons have not been measured directly in humans. Using microelectrode recordings during deep brain stimulation, Zaghloul et al. (p. 1496) investigated reinforcement learning mechanisms by analyzing neural activity in the substantia nigra of patients with Parkinson's disease while they engaged in a probability learning task. Neuronal responses were classified into four distinct categories based on an improved model for expectation: expected wins and losses, and unexpected wins and losses. Significant differences in spike activity were observed in response to unexpected wins and losses, while the difference in response to expected wins and losses was not significant. The human substantia nigra thus preferentially encodes unexpected reward and differentiates between unexpected positive and negative feedback.

  13. Discontinuities in Random Network

    Phase transitions might first bring to mind water boiling or freezing, but slight changes in the number of connections within a network that form at random (for example, connections in social networks) can lead to a huge (explosive) increase in the number of vertices in the longest path available in the network. These percolation transitions are often studied with the Erdös-Rényi (ER) model, in which edges connecting pairs of vertices in a network are added randomly. It has been thought that these transitions are continuous in nature. Now Achlioptas et al. (p. 1453; see the Perspective by Bohman) have studied a variation of the ER model in which one of two candidate edges are chosen based on a product rule (which applies a mathematical operation to the lengths of the paths being connected). Simulations based on this model reveal that these percolation transitions can be discontinuous.

  14. Antarctic Meltdown

    Paleo-records for the western Antarctic Peninsula show it is particularly prone to rapid climate change. With global warming continuing apace, Montes-Hugo et al. (p. 1470) show that major transitions in primary productivity in the western Antarctic accompany the spectacular loss of ice-cover. Less ice in the northern zone causes more cloud cover, reducing the amount of light reaching the plankton. A loss of light, together with less ice-melt freshwater and stronger winds (both inhibiting water column stratification) means fewer large phytoplankton blooms. By contrast, in the south, the skies are staying cloudless for longer and the Antarctic current has increased its flow rate, pulling up more micronutrients. Both factors contribute to greater primary productivity. These physical changes explain the striking shifts recently observed in krill and the vertebrate communities of the western Antarctic.

  15. Closing the Loop

    Circadian clocks keep us, and our plants, reasonably attuned to the cycle of days and nights. Analyses of the molecules in the plant Arabidopsis that achieve this steady coordination have led to interlocking networks of logic and feedback loops of gene regulation. Pruneda-Paz et al. (p. 1481; see the Perspective by McClung) have now used a targeted genomics approach with yeast to add another molecule to the game: the transcription factor CHE. CHE binds to already known clock components of Arabidopsis and forms a linkage that was predicted but not yet closed, namely, bringing together a known clock component TOC1 with its expected target gene CCA1.

  16. Paternal Influences on Zygote Development

    The signals that get embryonic development going in plants reveal a finely timed interaction between paternal and maternal gametes. Bayer et al. (p. 1485; see the Perspective by Grossniklaus) have discovered the messenger RNA (mRNA) encoding a kinase SSP in the paternal pollen grains. After fertilization and fusion of male and female gametes, the mRNA is translated into the SSP protein. SSP then triggers activity of YODA, a kinase supplied from the maternal gamete. YODA in turn regulates early phases of zygotic development. In this way, embryonic development begins only after fusion of male and female gametes.

  17. Limiting Potential

    Synapse formation is tightly controlled to achieve a specific size, shape, and number of synapses. While recent advances have been made in discovering key molecules that are important for the initiation and maintenance of synaptic recruitment, negative regulators that restrict this aggregation to control the size and number of synapses are poorly understood. Patel and Shen (p. 1500) have identified the evolutionarily conserved protein RSY-1 as a local inhibitor of synaptogenesis, controlling the number of presynaptic terminals and the amount of material recruited to presynaptic sites. RSY-1 inhibited synaptogenesis by antagonizing master assembly molecules essential for presynaptic development. This type of negative regulation is likely to be as important for synaptic regulation as the intensively studied mechanisms of positive regulation.

  18. Fingertip Search

    Why do humans have epidermal ridges and why are they arranged in whorls? Previous work has established two cutaneous mechanoreceptor channels by which the perception of coarse and fine spatial features is transmitted via the finger surfaces to tactile processing centers; the slowly adapting mechanoreceptor class is primarily used for the detection of edges and skin stretches, whereas the Pacinian corpuscles pick up vibrations at a few hundred hertz. Scheibert et al. (p. 1503, published online 29 January) do not answer the second question, but are able to offer a response to the first by using an artificial spherical cap (like a miniature swimming cap) whose surface has been patterned with ridges of similar size and frequency to what are found on human fingerpads. Rubbing this cap across patterned surfaces reveals that the presence of the ridges serves to amplify the pressure deflection by 100-fold and that this occurs optimally within the range of the Pacinian fibers.