This Week in Science

Science  03 Dec 2010:
Vol. 330, Issue 6009, pp. 1283
  1. The Yin and Yang of Plant Caspases


      The function of plant metacaspases, identified by limited sequence homology to the animal caspases that control cell death, has remained elusive. Coll et al. (p. 1393) have now elucidated the actions of two metacaspases in the small plant Arabidopsis. One metacaspase, AtMC1, promoted cell death, and the other, AtMC2, acted antagonistically to stall cell death. The results help to elucidate the mechanisms by which plants control cell survival during development and defend against pathogen attack.

    1. Sunny and Cool

        Changes in solar output cause changes in the amount of radiation that Earth receives from the Sun, which in turn can cause climate variations. The effects of solar variations are not uniform over the globe—owing to the complexity of the climate system, larger solar fluxes may produce warming in one area but cooling in another. Marchitto et al. (p. 1378) present a record of Holocene sea surface temperature in the eastern equatorial Pacific Ocean that shows cooling as solar output increased and warming as the Sun dimmed. These temperature changes resulted from dynamical control of El Niño and La Niña episodes by solar radiative forcing of Earth's climate.

      1. Reading, Writing, and Face Recognition

          Reading, not to mention writing and texting, is a relatively recent invention, and hence it is believed that a preliterate brain must adapt on the fly, so to speak, in learning how to process written words, rather than being able to rely upon evolutionarily ancient modifications of the visual system pathways. Dehaene et al. (p. 1359, published online 11 November) examined the neural response to a range of visual stimuli in three groups: illiterate adults, adults who learned to read as children, and adults who learned to read as adults. Reading induced a greater facility in processing horizontally oriented stimuli at early stages in the visual pathway and was also associated with the appearance of an area specialized for words. This gain of function appeared to occur at a cost—the area in the temporal cortex devoted to face processing shrank.

        1. Shake It to Wake It

            Viscoelastic materials combine the recoverable stretchiness found in elastic materials with the slow-flowing behavior of a thick fluid, like honey. When subjected to an oscillatory motion, the response will depend on the frequency. At low frequencies, the viscous behavior will dominate and lead to a dissipation of the applied energy as heat, while at fast frequencies the elastic behavior dominates. Xu et al. (p. 1364; see the Perspective by Gogotsi) developed a viscoelastic material with an exceptionally broad operating temperature range, based on a network of carbon nanotubes. The responsiveness of the material was probably caused by the “zipping” and “unzipping” of the nanotubes at points of contact.

          1. Skin-Deep Raman Spectroscopy

              CREDIT: SAAR ET AL.

              Raman spectroscopy allows for molecular identification via vibrational spectra at optical wavelengths. However, if the optical signal is scattered, as occurs when trying to image tissue, the signal becomes very weak, and it becomes difficult to image a sample with high time resolution. Saar et al. (p. 1368) now show that by improving the optics and electronics of the acquisition of the backscattered signal, stimulated Raman scattering spectroscopy can be performed at video rates on human skin, which should enable label-free studies of tissues and, for example, the tracking of the delivery of a drug.

            1. An Upside of Asymmetry

                Advances in synthetic techniques have enabled the preparation of nanometer-scale semiconductors in a wide range of precise shapes and sizes, including core-shell morphologies that layer several different materials in the same particle. Such two-in-one motifs are promising for light-harvesting applications because they allow optically induced charge separation across the internal interface. Borys et al. (p. 1371) studied a series of rod-shaped cadmium sulfide–cadmium selenide hybrid particles using single-particle–resolved optical spectroscopy and found that smooth versus bulbous geometries produced distinct emission spectra. Further analysis of more complex, tetrapodal particles (with four arms aligned tetrahedrally) suggested that nonuniform geometries facilitate interfacial charge transfer by reducing the likelihood of electronic band misalignment.

              1. Cryptic Sulfur Cycling

                  Aerobic bacteria and ocean circulation patterns control the formation and distribution of oxygen-minimum zones at moderate depth in the oceans. These habitats host microorganisms that thrive on other metabolic substrates in the absence of oxygen—most commonly, metabolizing thermodynamically favorable nitrogen compounds like nitrate. Off the coast of Chile, however, Canfield et al. (p. 1375, published online 11 November; see the Perspective by Teske) suggest that bacteria may often reduce sulfate as well. Metagenomic sequencing revealed the presence of both sulfate-reducing and sulfide-oxidizing bacteria. With the coincidence of sulfate and nitrate reduction, the sulfur and nitrogen cycles may be intimately linked; for example, sulfate reduction could provide nitrogen-rich ammonium for bacteria that ultimately transform it into nitrogen gas.

                1. Deadly Trio

                    The proteins BAX and BAK act as a key decision point, regulating apoptosis by controlling the permeability of the mitochondrial outer membrane. Evidence has been presented for two mechanisms of activation of BAX and BAK: an indirect mechanism where proapoptotic proteins neutralize the antiapoptotic effects of the protein BCL-2 and its relatives; or direct activation of BAX and BAK by BIM, BID, or PUMA. Analysis of the situation in vivo is complicated by the overlapping function of BIM, BID, and PUMA. Ren et al. (p. 1390; see the Perspective by Martin) thus analyzed triple-knockout mice lacking BIM, BID, and PUMA. Apoptosis during mouse development required a direct effect of one of these proteins to activate BAX or BAK, thereby promoting cell death.

                  1. Better Brain Maps

                      A high-resolution atlas of the complete neuronal connectivity in a whole brain should fundamentally advance our understanding of the organization and function of animal nervous systems. Now, A. Li. et al. (p. 1404, published online 4 November) describe an automated system, micro-optical sectioning tomography, that allowed three-dimensional mapping of the morphology and spatial location of neurons and traces of neurites in a whole, intact mouse brain.

                    1. Wired for Life


                        Syntrophic bacteria live on the metabolic by-products of a partner species. The exchange of the by-products accompanies a flow of electrons in the opposite direction that helps some species grow in conditions that would otherwise be unfavorable. In mixed anaerobic cultures of two related Geobacter species, Summers et al. (p. 1413) observed that one species evolved to promote the transfer of electrons directly to the other, in large aggregated cell clusters, without coupling to common anaerobic by-products such as hydrogen or formate. Selection pressures in nine parallel populations all resulted in a point mutation that truncated a protein involved in the production of small hairlike projections involved in intercellular communication—pili—and indirectly increased the expression of a c-type multiheme cytochrome responsible for extracellular electron transfer. The evolved aggregates were conductive, suggesting that the direct exchange of electrons between partner species is a possible alternative route to anaerobic syntrophy rather than interspecies hydrogen transfer; indeed, deleting a gene that encodes a hydrogenase involved in hydrogen transfer conferred a growth advantage in the cocultures.

                      1. Novel or Familiar?

                          Amnesia is characterized by a number of memory deficits, including the apparent inability to distinguish between novel and familiar stimuli. McTighe et al. (p. 1408; see the Perspective by Eichenbaum) observed that the recognition memory of brain-damaged rats in a standard model of amnesia was impaired not because previously experienced objects seemed to be novel, but because objects not previously experienced seemed to be familiar. Furthermore, simply placing the animal in a visually deprived environment during the delay, reducing visual interference, completely rescued the impairment. This counterintuitive finding contradicts the predominant “multiple memory systems” model in which amnesia is usually considered and forces a reconsideration of fundamental assumptions underlying our understanding of amnesia.

                        1. An Eye on Metastasis

                            Despite the considerable progress being made in elucidating the cell biology of metastasis, little is known about the genetic alterations that promote metastasis of human tumors, the cause of most cancer deaths. A potentially important clue now emerges from the work of Harbour et al. (p. 1410, published online 4 November), who used an exome-sequencing approach to search for genetic mutations in uveal melanomas, an eye cancer associated with a high rate of fatal metastasis. Remarkably, over 80% of tumor samples with a high metastatic risk had inactivating somatic mutations in the gene encoding BAP1 (BRCA1-associated protein 1), a nuclear protein involved in controlling protein degradation. Thus, in this tumor type, mutational inactivation of BAP1 may be a key event in the acquisition of metastatic competence.

                          1. Ocean Dweller Sequenced

                              The Tunicates, which include the solitary free-swimming larvaceans that are a major pelagic component of our oceans, are a basal lineage of the chordates. In order to investigate the major evolutionary transition represented by these organisms, Denoeud et al. (p. 1381, published online 18 November) sequenced the genome of Oikopleura dioica, a chordate placed by phylogeny between vertebrates and amphioxus. Surprisingly, the genome showed little conservation in genome architecture when compared to the genomes of other animals. Furthermore, this highly compacted genome contained intron gains and losses, as well as species-specific gene duplications and losses that may be associated with development. Thus, contrary to popular belief, global similarities of genome architecture from sponges to humans are not essential for the preservation of ancestral morphologies.

                            1. DNA Damage Pathways Revealed

                                Despite the dynamic nature of cellular responses, the genetic networks that govern these responses have been mapped primarily as static snapshots. Bandyopadhyay et al. (p. 1385; see the Perspective by Friedman and Schuldiner) report a comparison of large genetic interactomes measured among all yeast kinases, phosphatases, and transcription factors, as the cell responded to DNA damage. The interactomes revealed were highly dynamic structures that changed dramatically with changing conditions. These dynamic interactions reveal genetic relationships that can be more effective than classical “static” interactions (for example, synthetic lethals and epistasis maps) in identifying pathways of interest.

                              1. Just Beet It

                                  Flowering time regulation is important for plants to maximize their reproductive output. By investigating copies of genes that are strong and central activators of flowering in many different species (homologs of the FT gene in Arabidopsis), Pin et al. (p. 1397) found that during evolution, the regulation of flowering time in sugar beet (Beta vulgaris) has come under the control of two FT-like genes. Functional differences in these genes owing to small mutations in a critical domain have caused a duplicated copy of the flowering promoter FT to turn into a flowering repressor in sugar beet. These changes may explain why cultivated beets are unable to flower until their second year after passing through the winter, a behavior important for increasing crop yield.

                                1. Pain in the Brain

                                    One of the major challenges in pain research is finding ways to reverse chronic pain. Synaptic long-term potentiation (LTP) at spinal or cortical levels is a cellular model of chronic pain. X.-Y. Li. et al. (p. 1400) studied the role of the enzyme protein kinase M zeta (PKMζ) in neurons of the anterior cingulate cortex (ACC) in the maintenance of LTP and for enhanced pain sensitivity after peripheral nerve injury in mice. Nerve injury appeared to lead to the up-regulation and phosphorylation of PKMζ. This triggered LTP at some synapses in the ACC by increasing the number of AMPA receptors. LTP was restricted to ACC neurons that were activated by nerve injury. Blocking PKMζ in the ACC days after nerve injury normalized pain behavior. Thus, PKMζ may represent a promising target for the treatment of chronic pain.