This Week in Science

Science  18 Feb 2011:
Vol. 331, Issue 6019, pp. 817
  1. Don't Wake the Bears


      In hibernating mammals, temperature and metabolic rate decrease, allowing survival over long winter periods of starvation and cold. In small hibernators, such as hamsters, decreases in metabolic rate and temperature are linked; temperature decreases follow a physiological reduction in metabolic rate. In bears, body temperature remains relatively high during hibernation, which has been assumed to be owing to their large size. Tøien et al. (p. 906; see the Perspective by Heldmaier) undertook the challenging task of maintaining hibernating bears in artificial conditions and found, surprisingly, that metabolic rate and body temperature were completely unlinked. Instead, the bears cycled through elevated temperatures (between 30° to 36°C) over multiday cycles throughout hibernation, while undergoing a fourfold reduction in heart rate and maintaining a metabolic rate at 25% of normal. Despite a return to active body temperatures, metabolic rate remained depressed for several weeks post-hibernation.

    1. Mixing Sand and Water

        The flow properties of a suspension of sand in water will be controlled by how much sand is added and will change over time as the sand sediments out. Koos and Willenbacher (p. 897; see the Perspective by Butt) studied the agglomeration of particles suspended in a liquid under the influence of a small amount of a secondary, immiscible liquid. For the case where the second liquid preferentially wets the solid particles, one would expect the flow properties to change, but significant changes were also observed even when a small amount of a non-wetting fluid was added to the mix.

      1. Membrane-Docking Complex

          Correct targeting of membrane and secretory proteins involves the binding of the signal recognition particle (SRP), a ribonucleoprotein complex, to the ribosome nascent chain (RNC) complex. This joint complex then binds to a receptor (SR) in the target membrane and delivers the protein cargo to the protein translocation machinery, the translocon, in a guanosine triphosphate (GTP)–dependent process. Ataide et al. (p. 881) now describe the structure of the eubacterial SRP:SR complex bound to a nonhydrolysable GTP analog. The structure shows the arrangement of the SRP proteins relative to the RNA in a conformation that is likely to represent the cargo release state, and gives insight into how GTP hydrolysis may be coupled to conformational changes that facilitate transfer of the signal sequence to the translocon.

        1. Lasing in Reverse

            Recent theoretical work has shown that the time-reversal symmetry of electromagnetism allows a lasing process to be run backwards, so that photons of the correct amplitude and phase incident on a cavity medium can be coherently absorbed. Wan et al. (p. 889) experimentally demonstrate that such carefully tuned beams incident from either side on a silicon wafer multiply scatter within the wafer so that the total transmitted and reflected beams destructively interfere, leading to the predicted enhanced absorption within the silicon cavity. The effect provides a route to control absorption through coherent illumination, with numerous potential applications in optical circuitry.

          1. Turned Away at a Distance

              When a gas-phase molecule scatters off a solid surface, the interaction potential attracts at a distance because of favorable alignment of dipoles in the molecule and the surface. At a certain point, the molecule gets close enough to the surface so that electron repulsions dominate and the molecule is repelled. In many ways, the scattering of molecules can largely be described as classical scattering off these potential surfaces. Zhao et al. (p. 892) now show that the helium dimer, an extremely weak molecule, can scatter off a grating intact, but does so several nanometers above the surface, when it is still in the attractive part of the interaction potential. The scattering is quantum mechanical in nature, with the wave functions of the repulsive potential and the dimer extending evanescently and interacting at a distance.

            1. Spin Filtering with DNA Monolayers

                Spin filters can create an excess of one electron spin population over another with applications in spin-based electronics. Typically, such devices use magnetic materials. Göhler et al. (p. 894; see the Perspective by Rikken) measured the spin distribution of photoelectrons emitted from layers of double-stranded DNA (up to 80 base pairs in length) adsorbed on gold. Excitation with unpolarized ultraviolet light at room temperature led to a 60% increase in spin polarization.

              1. Prelude to an Earthquake?

                  CREDIT: BOUCHON ET AL.

                  To improve warning systems and minimize damage, it is important to understand the moments leading up to large earthquakes. Bouchon et al. (p. 877) observed a persistent and repeating low-frequency seismic signal in the hour before the 1999 magnitude 7.6 Izmit earthquake in Turkey that may have been the beginning stages of the major slip along the fault. The signal originated from the base of the brittle crust near the hypocenter, suggesting that slip accumulated leading up to the earthquake. The increase of seismic noise over this time frame indicates possible movement along the fault before the main quake. It is not yet clear whether similar patterns are likely to have occurred before other large ruptures, or whether such patterns occur in the absence of any subsequent rupture.

                1. Frozen to Death

                    Global biodiversity throughout geologic time has been punctuated by mass extinction events, the causes of which are often controversial. Major swings in climate are often accompanied by variations in temperature, sea level, and glacial coverage. Finnegan et al. (p. 903, published online 27 January) used an isotope-based paleothermometry technique to tease out ocean temperatures during one of the most devastating losses of marine life around 440 to 450 million years ago. Ocean temperatures cooled by up to 5°C in the tropics at the same time that the mass extinction was in full swing, which also coincides with a disruption of the marine carbon cycle.

                  1. Two Ways to Plant Symbiosis

                      CREDIT: OP DEN CAMP ET AL.

                      Many plants form symbiotic relationships with mycorrhizal fungi. A much smaller group of plants, mostly the legumes, form symbiotic relationships with rhizobial bacteria. Op den Camp et al. (p. 909, published online 23 December; see the Perspective by Kereszt and Kondorosi) analyzed the molecular pathways underlying the early stages of mycorrhizal and rhizobial symbioses. The nonleguminous small tree Parasponia can form symbiotic relationships with rhizobia. Comparisons of the nodulation pathways in legumes with that in Parasponia suggest that the rhizobial pathway is derived from the mycorrhizal pathway.

                    1. Progesterone-Estrogen Antagonism

                        In reproductive biology, the steroid hormone progesterone counteracts estrogen-induced uterine growth. The antiproliferative action of progesterone in the uterus relates to female infertility as well as to estrogen-driven hyperplasia and endometrial cancer. Using targeted genetic mutation in mice, Li et al. (p. 912; see the Perspective by Hewitt and Korach) demonstrate that the transcription factor Hand2 is a target of progesterone regulation in the uterine stroma and that Hand2 controls estrogen-induced epithelial proliferation via a paracrine mechanism involving fibroblast growth factors.

                      1. Minding Your Xs and Ys

                          The double (or “reduction”) division of meiosis generates haploid gametes from diploid cells. Meiotic recombination is required for pairing of homologous chromosomes, which ensures correct segregation of the chromosomes. The X and Y chromosomes of mammals have only a small segment of homology, the pseudoautosomal region, where recombination must take place to hold the chromosomes together. Although sex chromosomes missegregate more frequently than autosomes, X-Y nondisjunction is rare, which suggests that mechanisms exist to ensure X-Y recombination. Kauppi et al. (p. 916; see the Perspective by Hawley) show that, in mice, a distinct chromosome structure renders the PAR more conducive to recombination-initiating double-strand break formation, which occur at a 10- to-20-fold higher frequency than the genome average.

                        1. Mastering Memory Maintenance

                            The mechanisms involved in the formation of enduring memories are still largely unknown. Lesburguères et al. (p. 924; see the Perspective by Sweatt) investigated the hippocampal-cortical dialogue during memory consolidation. Neurons in the rat orbitofrontal cortex appeared to be “tagged” at the time of initial encoding, even though the initial learning was hippocampal. Inactivating the orbitofrontal cortex at the time of encoding affected late memory while not affecting memory during the period when it was still supported by the hippocampus.

                          1. Taxol and Spinal Cord Repair

                              Because neuron-intrinsic factors and the lesion site itself impose obstacles to axon regeneration, a combination of therapeutic approaches is required to promote axonal regeneration after spinal injury. Hellal et al. (p. 928, published online 27 January) now provide evidence from spinal cord injury experiments on rats that the licensed anticancer drug Taxol can induce axon regeneration by altering microtubule dynamics. Moderate microtubule stabilization reduced scarring and decreased the deposition of factors that prevent axonal regeneration after spinal cord injury so that axons could regenerate through the Taxol-treated lesion site.

                            1. Changes at the Boundary

                                The two-dimensional electron gas created at the interface of insulating complex oxides such as SrTiO3 and LaAlO3 may exhibit properties not present in either of the bulk materials, like superconductivity and integer and fractional quantum Hall effects. The transport behavior of the boundary layer depends on the chemistry of the surrounding structure. Jang et al. (p. 886) used pulsed-laser deposition to engineer heterostructures consisting of a single atomic rare-earth oxide layer inserted into a SrTiO3 matrix. When the type of the rare-earth ion was varied systematically, the interface changed character from metallic for La, Pr, and Nd ions to insulating for Sm and Y ions.

                              1. Staying Aligned

                                  Hydrogen abstractions seem fairly simple as chemical reactions go. Consider, for instance, the reaction of methane (CH4) with a single oxygen, fluorine, or chlorine atom: One H atom is stripped (abstracted) to form OH, HF, or HCl and a CH3 fragment is left behind. However, recent studies have uncovered remarkable complexity in this single atom exchange. Working with CHD3 (D is deuterium), vibrational excitation of the C-H bond promotes its reactivity toward O and Cl but hinders its reactivity toward F. Efforts to explain these findings have focused on steering effects, which minimize the importance of the initial relative alignment of the two reagents. Wang et al. (p. 900) now find that CHD3 alignment plays a major role in the reaction outcome with Cl, even after C-H stretch excitation, partially contradicting previous explanations.

                                1. So, How Did Evolution Happen?

                                    Studies of the process of evolution have often focused on naming the specific genetic changes that have resulted in modern-day humans, but relatively few human-specific genes or elements have been identified. Hernandez et al. (p. 920) argue that the tools we have been using identify genes that have undergone selective sweeps—strong selection on newly arisen alleles. Data from the pilot for the 1000 Genomes Project suggest that classic selective sweeps were not the primary mode of evolution of the human genome. Instead, it seems that the majority of human genetic diversity is best explained by purifying selection against deleterious mutations.