This Week in Science

Science  16 May 2008:
Vol. 320, Issue 5878, pp. 845
  1. The Logic of Neurite Outgrowth


    Cannabinoid receptor 1 (CB1R) regulates neurite outgrowth, has important functions in central nervous system development, and is a drug target for several diseases. Bromberg et al. (p. 903) combined transcriptional profiling on DNA arrays with graph theory analysis of known signaling networks to explore the effects of signaling by the CB1R. Unexpectedly, the analysis predicted that the product of the breast cancer susceptibility gene BRCA1 was likely to regulate transcription factors activated during CB1R-stimulated neurite outgrowth. Furthermore, depletion of BRCA1 did indeed inhibit CB1R-stimulated neurite outgrowth. The transcription factor PAX6 was also regulated in response to cannabinoid signaling. This type of network analysis is useful to define the logic of complex signaling decision processes.

  2. Manmade Sources of Nitrogen

    Manmade sources of biologically available nitrogen may enhance the capacity of the ocean to assimilate carbon dioxide. However, this assimilation capacity is likely to be offset by the production of nitrous oxide, itself a potent greenhouse gas. Duce et al. (p. 893) review the current status of atmospheric emission and deposition of nitrogen species and its impact on the biological nitrogen cycle. As anthropogenic mobilization of nitrogen increases in many areas of the world, negative environmental impacts are becoming apparent. The distressing paradox is that parts of the world still do not receive enough nitrogen to sustain food production. The N-related issues facing society are numerous, complex, and interrelated. Galloway et al. (p. 889) review some of the most critical factors and propose a strategy for how society might manage nitrogen.

  3. From Folds to Wrinkles

    Thin films on fluid or elastic substrates occur in many situations on many length scales and will deform from their flat geometries when compressed. Both wrinkled and folded states can occur, but the transition between them is not well understood. Pocivavsek et al. (p. 912) examine the compression of a set of supported membranes that span a range of length scales and stiffness and find a universal transition from wrinkling to localized folds when the compression exceeds one-third of the length of the membrane.

  4. Cosmic Shock Waves

    Intergalactic space is filled with magnetic fields, cosmic rays, and wisps of turbulent plasma. How these magnetic fields arose during the evolution of the universe is not well understood. Ryu et al. (p. 909) have conducted computer simulations showing that during the formation of the large-scale structures in the cosmos, shock waves created swirling regions that led to turbulent mixing. Very weak magnetic fields in the early universe could have been amplified by this turbulence, leading to the fields and structures we see today. These predictions should be testable using the new generation of radio telescopes such as the Square Kilometer Array.

  5. Melting and Mixing the Mantle

    The geochemistry of many types of basalt rich in sodium and potassium and relatively poor in calcium has been thought to imply derivation from the Earth's mantle containing some recycled oceanic crust. Pilet et al. (p. 916; see the Perspective by Niu) show experimentally, however, that many of the same signatures—both the compositions of the basalts and trends with time—can be produced by melting mantle that has previously interacted with a hydrous melt or fluid, forming veins of hydrous minerals. These hydrous phases dominate the composition of early melts and also buffer mantle melting temperatures.

  6. Two Places at Once

    Molecules heavier than H2 have an inner layer, or core, of electrons that are held more tightly to individual nuclei than the constantly rearranging outer-valence electrons. What happens to the vacancy created when one such core electron is expelled by a high-energy photon? Does the hole remain localized beside one nucleus until a valence electron drops down to fill it, or does it spread out along the molecular axis? Schöffler et al. (p. 920; see the Perspective by Ueda) use ion imaging to probe this question in N2, deriving the symmetry of the hole state based on the trajectory of an Auger electron emitted after relaxation. Depending on the angle of the Auger electron detected, the state could be described as either localized or delocalized, a consequence of quantum entanglement.

  7. Microwaves in a Hurry


    Rotational spectroscopy is widely used to characterize molecular structures in the gas phase. However, bandwidth limitations have generally restricted the technique to characterization of stable ground state geometries. Dian et al. (p. 924; see the Perspective by Melnik and Miller) have devised a Fourier Transform Microwave Spectrometer that uses an amplified chirped pulse to acquire data over an 11-GHz spectral range in a single burst. As a result, they can acquire spectra rapidly enough to probe the rotational dynamics of vibrationally excited molecules. Specifically, they examine the rotational isomerization of cyclopropane carboxaldehyde about a carbon-carbon single bond after exciting the aldehyde C-H stretch and using lineshape analysis, extract-mode-specific rates less than a tenth as rapid as statistical theory predicts.

  8. Similar But Not the Same

    The level of species diversity in the tropics—especially among so-called cryptic species, which are genetically distinct but resemble other closely related species—is unclear. By sampling all morphologically similar larvae found on plants in the cucumber family across the New World tropics with molecular markers, Condon et al. (p. 928; cover) demonstrate a much higher than expected insect diversity on these plants: The insects tend to be specific not only to a single plant species but within that species to a single part of the plant.

  9. Sipping with Tweezers

    The surface of water in a pipette is higher at the edges than in the center, due to the relatively stronger attraction of a water molecule for glass in comparison to other molecules of water. Prakash et al. (p. 931; see the Perspective by Denny) demonstrate how surface tension and cycles of opening and closing its beak allow the shorebird Phalaropes to transport droplets of water uphill into its mouth. A droplet of water, suspended between the upper and lower mandibles of a tweezers-like model of the Phalaropes beak, moves toward the hinged end as the mandibles are brought closer together; it slips back slightly as the tweezers are opened, but the net motion is still forward. Closing and opening its beak several times thus enables the bird to ingest the droplet of water, along with the small invertebrates contained therein.

  10. Keeping Foreign DNA Silent

    Bacterial genomes are densely packed, so it is critical that transcription of operons is precisely terminated to prevent transcription of downstream genes. Regulation of many Escherichia coli genes uses three factors—Rho, NusA, and NusG—that work together to promote accurate transcription termination. Cardinale et al. (p. 935) now show that this termination is required to suppress expression of toxic genes from cryptic prophages. The E. coli derivative strain MDS42 lacking these prophages and other phylogenetically unique genes is highly resistant to a Rho inhibitor and can sustain deletions of the essential nusA and nusG genes. Thus Rho acts globally to prevent read-through of downstream operons, to match transcriptional yield to translational needs, and to suppress expression of foreign DNA.

  11. Improving Imperfect Predictions

    Although the genome encodes the proteins, there is variety in the regulatory choices available in translating the genome into the proteome. Baerenfaller et al. (p. 938, published online 24 April) analyzed the proteome of Arabidopsis and compared it to the known genome. As expected, proteins were identified from many of the genes predicted from genome. However, some proteins highlighted the presence of genes not yet predicted, for example, from sequences thought to be introns or pseudogenes. Further analysis of different organs and developmental stages confirms that, while the genome remains constant, the proteome shifts with development.

  12. Plant Responses to Salt Stress

    Detrimental levels of salt can result when agriculture is extended to marginal lands or relies on irrigation. Using the Arabidopsis root tip, Dinneny et al. (p. 942, published online 24 April; see the Perspective by Voesenek and Pierik) examined how different cells within a tissue respond to the physiological stresses due to salinity. Different layers of cells, whether at the surface of the root or more internal, responded differently to the environmental stress of too much salt. Furthermore, stressed cells could influence their neighbors, and gene expression patterns changed over the duration of the stress.

  13. Cadherins and Guidepost Neurons

    The Celsr3 gene, which encodes a cell-surface cadherin molecule, is widely expressed in neurons of the developing brain after they have migrated when they are refining their connections. Zhou et al. (p. 946) prevented Celsr3 expression in a variety of specific regions of the developing mouse brain. Celsr3 expression was critical to the function of guidepost neurons—cells that developing axons use as flags to find their way. In particular, the axon tracts that connect thalamus and cortex depend upon Celsr3 interactions as they develop.

  14. Charge Learns Its Fractions

    Electronic current is usually expected to be quantized in units of the electron charge, e*. However, in the Fractional Quantum Hall Effect, which arises when a two-dimensional electron gas (2DEG) is subjected to a large magnetic field and current is carried in conduction channels around the edge of the 2DEG, the current carriers can have fractional units of electronic charge. This fractionation of charge arises from electron-electron interactions. Under specific conditions, the so-called Graphic filling state, the charge is expected to be e/4. This particular Graphic state is of interest because recent theory predicts that it is a robust quantum state suitable for quantum computation, as well as other exotic properties. Radu et al. (p. 899, published online 17 April 2007) perform tunneling measurements between the counterpropagating conduction channel of this state and present compelling evidence confirming the fractions of charge in units of e/4.

  15. Signaling Clock Timing

    Transcriptional feedback loops are at the heart of circadian clocks of mammals and other organisms, but evidence is emerging that suggests that other signaling mechanisms also have important roles in basic functions of the clock. O'Neill et al. (p. 949; see the Perspective by Harrisingh and Nitabach) show that cytosolic signaling through adenosine 3′,5′-monophosphate (cAMP) integrates with the transcriptional timing mechanism, sustains it, and determines the fundamental properties (amplitude, phase, and period) of the transcriptional feedback loops of the mammalian circadian pacemaker. This expands the paradigm of cellular circadian oscillation control by transcriptional feedback loops to include small-molecule cytoplasmic signals that differ between species—cAMP in mammals, Ca2+ in mammals and insects, and cyclic ADP ribose in plants.