This Week in Science

Science  24 Aug 2012:
Vol. 337, Issue 6097, pp. 886
  1. Stellar Explosions


    Stars that are born with masses greater than eight times that of the Sun end their lives in luminous explosions known as supernovae. Over the past decade, access to improved sky surveys has revealed rare types of supernovae that are much more luminous than any of those that were known before. Gal-Yam (p. 927) reviews these superluminous events and groups them into three classes that share common observational and physical characteristics. Gamma-ray bursts are another type of extreme explosive events related to the death of massive stars, which occur once per day somewhere in the universe and produce short-lived bursts of gamma-ray light. Gehrels and Mészáros (p. 932) review what has been learned about these events since the launch of NASA's Swift (2004) and Fermi (2008) satellites. The current interpretation is that gamma-ray bursts are related to the formation of black holes. Type Ia supernovae are used as cosmological distance indicators. They are thought to be the result of the thermonuclear explosion of white dwarf stars in binary systems, but the nature of the stellar companion to the white dwarf is still debated. Dilday et al. (p. 942) report high-resolution spectroscopy of the supernova PTF 11kx, which was detected on 26 January 2011 by the Palomar Transient Factory survey. The data suggest a red giant star companion whose material got transferred to the white dwarf.

  2. Sticky Mucus?

    Mucus—experienced, for example, in the form of a runny nose or productive cough—is one of the tools the body uses to expel or prevent the uptake of foreign matter. In a number of diseases, a failure of the normal mucus-control system leads to obstructions of the airways and respiratory problems. Button et al. (p. 937; see the Perspective by Dickey) examine the existing gel-on-liquid model, where the mucus is thought to sit on a watery periciliary layer around the beating lung cilia that has been used to explain the flow of mucus. A gel-on-brush model is proposed where the mucus sits on a brushlike periciliary layer. The key elements of this layer are membrane-tethered macromolecules that cause normal flow and clearance of mucus. When dehydrated, the interface is disrupted, preventing normal mucus motion.

  3. Interrogating Growing Nanoparticles


    Several methods can be used to follow the mechanistic steps of a chemical reaction, but the growth of a nanoparticle from a small seed crystal is more difficult to follow. Langille et al. (p. 954) used plasmonic gold nanocrystals of different shapes (cubes and octahedral) as seeds for the growth of larger silver nanoparticles. Electron microscopy was used to track the formation of several different particle shapes and internal structures during the growth process.

  4. Finger on the Pulsar

    Pulsars, the most commonly seen neutron stars, have been known for over four decades. They are rotating, dense stars with very strong magnetic fields, whose spectra have been understood as a superposition of continuum components of thermal and nonthermal origin. Kargaltsev et al. (p. 946) now report the detection of absorption features in the x-ray spectrum of an ordinary radio pulsar. These features may be more common than was previously thought and may provide the opportunity to probe the environment and radiation physics of neutron stars.

  5. Oscillating Black Hole

    The massive black holes that reside in the centers of galaxies can occasionally capture and tidally disrupt stars that wander too close. One such tidal disruption event was detected last year by the Swift satellite. Follow-up x-ray observations analyzed by Reis et al. (p. 949, published online 2 August; see the Perspective by McKinney) show quasi-periodic oscillations that suggest that an accretion disk formed around the black hole shortly after the tidal disruption event. This type of oscillation is commonly seen in the x-ray light from the much lighter black holes that result from the gravitational collapse of stars, but has been seen only once in a massive black hole residing in the center of a galaxy.

  6. Metal Manipulation

    Reducing the grain size below 100 micrometers can vastly improve the properties of a metal. However, these nanocrystalline metals are not thermally stable; at elevated temperatures the grains will grow and merge. Alloying with a second metal to slow grain growth can slow down this process, which has shown some success on a trial-and-error basis. Chookajorn et al. (p. 951; see the Perspective by Weertman) now provide a theoretical framework to create stability maps to identify potential alloys with the greatest thermal stability. For tungsten, counterintuitively, the theory suggests that atoms with the largest size differential or lowest solubility are not the best alloying choice. Indeed, an alloy of tungsten and titanium was processed more easily than pure nanocrystalline tungsten and also showed better stability at high temperatures.

  7. Keep on Neurexin

    Mutations altering neurexin and neuroligin have been linked to several psychiatric disorders, including autism and schizophrenia. However, it remains uncertain how these mutations alter neural circuit development and function. Working in the nematode Caenorhabditis elegans, Hu et al. (p. 980, published online 2 August) found that neurexin and neuroligin function downstream of a muscle microRNA (miR-1), mediating a retrograde synaptic signal that inhibits neurotransmitter release. This retrograde signal inhibited transmission by adjusting the rate and duration of synaptic vesicle release.

  8. A Family of Languages

    English is part of the large Indo-European language family, which includes Celtic, Germanic, Italic, Balto-Slavic, and Indo-Iranian languages. The origin of this family is hotly debated: one hypothesis places the origin north of the Caspian Sea in the Pontic steppes, from where it was disseminated by Kurgan semi-nomadic pastoralists; a second suggests that Anatolia, in modern-day Turkey, is the source, and the language radiated with the spread of agriculture. Bouckaert et al. (p. 957) used phylogenetic methods and modeling to assess the geographical spread of the Indo-European language group. The findings support the suggestion that the origin of the language family was indeed Anatolia 7 to 10 thousand years ago—contemporaneous with the spread of agriculture.

  9. Making Arabidopyrones


    Plants possess specialized metabolic systems that rapidly explore new chemical space as a means to adapt to their environments. Weng et al. (p. 960) describe the evolutionary trajectory underlying the emergence of a class of α-pyrone–bearing metabolites—arabidopyrones—in the model plant Arabidopsis. The first step of the arabidopyrone synthesis pathway is catalyzed by a cytochrome P450 enzyme neofunctionalized following a recent gene duplication event. This development led to the synthesis of a catechol-substituted substrate, which became a new substrate for a highly conserved extradiol ring-cleavage dioxygenase. Thus, plants can rapidly assemble a specialized metabolic pathway by substituting a new biological activity into an existing catalytic framework.

  10. BMP Signaling and Transcription

    Bone morphogenetic protein signaling (BMP) is widely used for dorsoventral axis determination in animal embryos, which involves specific interactions between BMPs and their antagonists. In the genomes of many animals, the gene Pinhead is encoded as a neighbor of one of the BMP ligand genes, Admp (antidorsalizing morphogenetic protein). Working in ascidian tailbud-stage embryos, Imai et al. (p. 964) now show that Pinhead is an antagonist of ADMP and is important for the establishment of the sharp boundary of the ventral epidermis. Transcription of Pinhead disturbs transcription of Admp by a cis-acting mechanism, which may explain the mutually exclusive expression of these two functionally opposed genes.

  11. Movement in the Cancer Genome

    Transposable elements are genetic sequences that can replicate and move within the genome. The factors that make an element mobile are unknown but are generally considered rare in mammals. Lee et al. (p. 967, published online 28 June) analyzed five cancer types occurring among several individuals and found that three types of epithelial tumors exhibited high rates of element movement relative to brain and blood cancers. Furthermore, these somatically acquired, tumor-specific elements targeted genes in colorectal cancer that, when disrupted, impact gene expression and thus may be a factor in the progression of the cancers.

  12. Coding Complex Movements

    The dorsal premotor cortex is a critical area for planning reaching movements. Pearce and Moran (p. 984, published online 19 July) trained two macaques to reach around randomly placed objects in a virtual reality task while simultaneously recording neuronal activity in the premotor cortex. Neurons in the premotor cortex coded for the direction of the hand movement and at the same time also coded other variables, including the position of the hand and the final location of the target. Population vector analyses were able to decode multiple kinematic variables from the neural activity. In one monkey, the population vector was first directed to the target but when the obstacle appeared the population vector rotated. However, the other monkey waited until the obstacle appeared. The differences in the neural planning between these two monkeys suggest different optimization strategies.

  13. Maintaining Repression

    The Polycomb Repressive Complex 2 (PRC2) plays a critical role in gene silencing in metazoans, methylating histone H3 on lysine 27 (H3K27) to generate a repressive chromatin mark. The catalytic subunit E(z)/Ezh2 requires the presence of two other subunits—ESC/EED and Su(z)12—for enzyme activity. Yuan et al. (p. 971; see the Perspective by Pirrotta) show that both a fragment of the histone H3 N-terminal tail, and histone H1 stimulated PRC2 enzyme activity on poor, low-density chromatin substrates, indicating that that PRC2 is regulated by the density and compaction states of chromatin. The histone H3 fragment binds to the Su(z)12 subunit of PRC2 to stimulate E(z)/Ezh2. Local chromatin compaction preceded establishment of histone H3K27 methylation indicating how PRC2 might maintain the repressed state.

  14. Metabolic Sensor

    The enzyme O-GlcNAc transferase (OGT) catalyzes the transfer of N-acetylglucosamine from uridine diphospho-N-acetylglucosamine (UDP-GlcNAc) to serine or threonine residues of intracellular proteins and responds to the metabolic status of the cell. Yi et al. (p. 975; see the Perspective by Mattaini and Vander Heiden) show that O-GlcNAcylation of phosphofructokinase 1 (PFK1) reduces its activity, thus influencing rates of glycolysis within cells. O-GlcNAcylation of PFK1 was increased in cells exposed to hypoxia, and was increased in several cell lines derived from human tumors. Thus, metabolic changes mediated by O-GlcNAcylation may benefit anabolism and growth of cancer cells. However, glycosylation of PFK1 was not detected in rapidly proliferating normal cells.