This Week in Science

Science  18 Mar 2011:
Vol. 331, Issue 6023, pp. 1363
  1. The Brain's Alarm Clock


      Circadian rhythms are linked to the external light-dark cycle through inputs from photoreceptors that signal into networks that regulate neural and physiological function. One of the key photoreceptors is CRYPTOCHROME, which is sensitive to blue-light wavelengths. Fogle et al. (p. 1409, published online 3 March; see the Perspective by Im and Taghert) now find that CRYPTOCHROME has an unexpectedly direct effect on circadian physiology in fruit flies. A small group of neurons that are part of the circadian circuit and that are usually more active in the morning express CRYPTOCHROME. These neurons normally receive plenty of input from the circadian circuit that perceives cycles and drives responses. However, when those inputs are blocked, it seems that these neurons are able to respond directly to blue light.

    1. Moral Determinism?

        Experimental philosophers have recently begun to ask fundamental, but seemingly straightforward, questions about the human condition. Nichols (p. 1401) reviews the problem that free will, which is accepted as a given by most people, poses for determinism: Does free will absolve people of moral responsibility for their actions? Although a definitive resolution of this issue is not in sight, the experimental findings suggest explanations for why people behave as they do, even if the underlying motivations for their reasoning are still debated.

      1. Gassing Up

          Large igneous provinces (LIPs) are extremely large geological features formed by episodes of rapid and extensive volcanism, which, in the Late Triassic/Early Jurassic (about 200 million years ago), were often associated with massive extinction events. It is believed that LIP volcanism can have dramatic impacts on the composition of the atmosphere, including its CO2 burden. Using evidence from soil carbonates, Schaller et al. (p. 1404, published online 17 February) reconstructed variations in the concentration of atmospheric CO2 at the time of the emplacement of the Central Atlantic Magmatic Province of the Late Triassic/Early Jurassic. The data confirm that geological events can affect climate and thus drive large global extinctions.

        1. Chroming Earth's Core

            Earth originally had a bulk composition not unlike meteorites, and comparing the composition of meteorites to Earth's differentiated layers—the metallic core and silicate-rich mantle—reveals clues about Earth's formation. For example, why are elements like chromium, which are moderately volatile, depleted in the mantle relative to meteorites and the core? Moynier et al. (p. 1417, published online 24 February; see the Perspective by McDonough) found that the isotopic signature of chromium in a number of meteorites deviates from the bulk silicate portion of Earth. This effect suggests that chromium depletion from the mantle and enrichment in the core occurred because chromium followed iron and other metals down to the core during the differentiation of Earth's interior. Further modeling suggests that Earth's chromium isotope signature was imparted from Moon-sized masses that differentiated before colliding together to form Earth.

          1. Titan's April Showers


              The equatorial regions of Saturn's largest moon, Titan, are predominantly arid with vast expanses of dunes. Dry fluvial channels have been observed in these regions, which have been proposed to represent remnants of a past wetter equatorial climate. Using the Cassini Imaging Science Subsystem, Turtle et al. (p. 1414; see the Perspective by Tokano) observed sudden surface changes near Titan's equator after a cloud outburst. The best explanation for these changes is rainfall from a low-latitude storm in Titan's early northern spring, suggesting that the equatorial climate is not static year-round and that the fluvial channels may not necessarily be remnants of an early wetter climate.

            1. Persistent Vibrations

                In the gas phase, bimolecular reactions proceed in a tightly regimented fashion. Quantum mechanics dictates that only specific vibrations and rotations can accompany the electronic rearrangements associated with the scission and formation of bonds. In principle, these rules still apply in solution, but frequent collisions with surrounding solvent molecules tend to blur the energy distribution. By applying sensitive ultrafast spectroscopy, Greaves et al. (p. 1423, published online 3 February; see the Perspective by Bradforth) have now found that when CN snatches a hydrogen atom from cyclohexane in solution, the HCN product exhibits specific H-C stretching and bending motions that are only partially suppressed by solvent during the tens of picoseconds following its formation.

              1. A Boost for Hydrogen Power

                  Hydrogen is in many respects an excellent automotive fuel: it is lightweight, releases a great deal of energy in reacting with oxygen, and forms a completely innocuous product—water—in the process. Unfortunately, the gaseous element is not so easy to store at high density. Hydrogen-rich solid compounds such as ammonia borane can be coaxed into releasing H2 under fairly mild conditions, but the polymeric material left behind is difficult to replenish. Sutton et al. (p. 1426) now show that treatment of such material with hydrazine in liquid ammonia regenerates ammonia borane in high yield over the course of a day. Though only demonstrated at a small scale, the method bodes well for further development of ammonia borane as a hydrogen storage medium.

                1. Selectivity Switch

                    Chiral molecular catalysts can usually be synthesized in either of their two mirror image forms (enantiomers) enabling preparation of either product enantiomer in the reaction they accelerate. However, to synthesize the independent catalysts often involves complex, multistep procedures. Wang and Feringa (p. 1429, published online 10 February; see the Perspective by Ooi) designed a single catalyst that, through photoisomerization, could be modulated between conformations that select for opposite product enantiomers in an organic reaction. The catalyst cycled through isomers with distinct helicities and a varied relative disposition of two functional groups (an amine and a thiourea) that needed to cooperate to activate the reagents.

                  1. Tortoise or Hare Evolution

                      The “mutator” phenomenon allows clones of asexually reproducing bacteria to evolve at different rates, for example, enabling bacteria to escape selection pressure by antibiotics. Woods et al. (p. 1433) compared Escherichia coli that had been collected and frozen at different points in a long-term evolution experiment. Surprisingly, after several hundred generations, clones that had initially showed low competitive fitness outcompeted initially more successful clones. The apparent losers turned into winners, not by increasing mutation rate and running off with the innovations, but because their competitors acquired mutations that, while conferring superior fitness in the short term, in the longer term hindered their “evolvability.”

                    1. Help for Heart Failure?

                        CREDIT: MALIK ET AL.

                        Heart failure is often caused by decreased cardiac contraction. Current therapies increase contractility indirectly by acting on signaling pathways, which can have adverse side effects. A small molecule, omecamtiv mecarbil (OM), has been shown to activate cardiac myosin directly. Malik et al. (p. 1439; see the Perspective by Leinwand and Moss) now show that OM acts allosterically to increase the transition rate of myosin into the strongly actin-bound force-producing state. OM increased cardiac function in animal models, which provides a proof-of-principle that direct cardiac myosin activation is a potential strategy for treating systolic heart failure.

                      1. Storage and Retrieval

                          In primates, visual long-term memory is stored in the inferotemporal cortex. Neuronal activity related to presented and retrieved images has been demonstrated at the single-neuron level. However, the underlying neuronal network dynamic remains largely unknown. Takeuchi et al. (p. 1443) investigated the functional interactions between different layers of the inferotemporal cortex while monkeys performed a paired associate memory task. While the cue stimulus was acquired, the direction of interactions followed the classical interlaminar pathway from granular via supragranular to infragranular layers as suggested by the anatomy of the canonical microcircuitry. During the subsequent delay period, when networks had to hold or retrieve the stimulus, the functional interactions indicated a reversal with infragranular neurons spiking before supragranular neurons.

                        1. You're Not Alone

                            One of the key transitions for young people, especially in the developed world, occurs when they begin tertiary education and, in many cases, move away from home and out of their accustomed social milieu. Their capacity to adjust and to develop a new social network can influence their level of academic achievement, and these adjustments may be more challenging for some than others. Walton and Cohen (p. 1447) examined the effectiveness of supplying African Americans a 1-hour exercise designed to bolster their psychological well-being against events that might make them feel that they do not fit in or belong in higher-education environments. The intervention was associated with a 50% reduction of the gap in grades between African Americans and European Americans when assessed at the end of college, and appeared also to enhance physiological measures of well-being.

                          1. Closing Out Dipole Depolarization

                              Nanometer-sized domains of ferroelectric materials have been considered for use in future nonvolatile memory devices. However, at these sizes the materials can spontaneously depolarize. Indirect evidence has suggested that a flux-closure domain structure could inhibit the spontaneous depolarization. Using aberration-corrected transmission electron microscopy, Jia et al. (p. 1420) were able to observe the continuous and gradual rotation of the dipoles to form a closed-domain structure directly.

                            1. When More CO2 Is Good

                                So-called C4 photosynthesis is associated with increased CO2 fixation, compared with the ancestral C3 form, and its repeated evolution suggests that it may underlie the success of several flowering plant families. Brown et al. (p. 1436) demonstrate that key photosynthetic enzymes contain a specific 240-nucleotide sequence. When this sequence was expressed in a model C3 plant, gene expression was localized to regions similar to those found in specialized C4 cells. This specific expression may have been due to a trans-factor important for the development of C4 photosynthesis that recognizes the element from both C3 and C4 plants. Thus, the evolutionary transition from C3 to C4 photosynthesis is relatively simple, not requiring multiple independent steps, which may explain the convergent origins of C4 plants.

                              1. Monitoring Mediator

                                  Gene expression must be temporally and spatially controlled. In eukaryotes, the Mediator complex is required for most regulated transcription. Mutations in human Mediator are implicated in developmental diseases and cancer. However, the mechanism of Mediator action remains poorly understood. Soutourina et al. (p. 1451) used a highly specific in vivo protein cross-linking method, functional genomics, and yeast genetics to show that the Rpb3 subunit of RNA polymerase II (Pol II) and the Med17 subunit of Mediator are in direct contact to position Pol II on promoters and activate transcription. The results suggest that an element in the Mediator–Pol II interface is functionally important for global transcription.