Editors' Choice

Science  19 Aug 2011:
Vol. 333, Issue 6045, pp. 920
  1. Astrophysics

    How Stars Explode

    1. Maria Cruz

    White dwarfs in binary systems with other stars can sometimes explode thermonuclearly as supernovae. How these very dense stars, the remnants of stars like our Sun, explode is still not fully understood. One aspect that needs explanation is the deflagration-to-detonation transition; i.e., how a subsonic flame can form a supersonic detonation. The physics of this transition has been explained in confined systems, where there are walls, internal obstacles, or preexisting shocks, but not in unconfined media, such as the interior of a white dwarf. Current models of supernovae explosions do not predict the deflagration-to-detonation transition in the interior of a white dwarf, but the same process is thought to occur in chemical explosions, such as those responsible for major industrial disasters. Poludnenko et al. model chemical combustion using direct numerical simulations and show that unconfined turbulent flames can spontaneously transition to detonation if they become fast enough. The mechanism is different from what had been expected, and it may help model supernovae explosions in a more realistic way.

    Phys. Rev. Lett. 107, 54501 (2011).

  2. Development

    Spare the Brain

    1. Lisa D. Chong
    CREDIT: CHENG ET AL., CELL 146, 435 (2011)

    During development, periods of nutrient deprivation can shut down body tissue and organ growth as a survival strategy, but somehow the brain is protected and continues to mature. This phenomenon occurs in many animals, from flies to humans, but little has been known about how the central nervous system gains this growth advantage. Cheng et al. report that in the model organism Drosophila melanogaster, nutrient withdrawal blocks late larval-stage body growth, but brain neural stem cells (neuroblasts) continue to grow and divide at a normal rate. This neural sparing requires a tyrosine kinase receptor called anaplastic lymphoma kinase (ALK). ALK activity during nutrient deprivation specifically suppressed the reliance of neuroblasts on the insulin receptor (which controls the cellular growth response to insulin-like peptides) and TOR (a component of a signaling pathway that senses amino acids) to survive. Disabling these pathways did not affect growth of the brain, yet growth was blunted in other larval tissues. ALK promotes neural growth by somehow activating downstream targets of the insulin receptor and TOR. The authors also determined that secretion of Jelly belly—the activating ligand for ALK—by glia does not depend on nutrient availability either. Besides providing insight into neural growth during nutrient deprivation, the fact that ALK regulates two growth-controlling signaling pathways may explain why ALK mutations are associated with human cancers, including neuroblastoma.

    Cell 146, 435 (2011).

  3. Cell Signaling

    Tweaked Receptors

    1. L. Bryan Ray

    β2-adrenergic receptors (β2ARs) are important drug targets in humans, mediating, for example, effects of drugs that help restore lung function in patients with asthma. There are common mutant forms of the β2AR gene in humans, though, and some of these can prevent such beneficial therapeutic actions. Using cultured human cells, Ahles et al. examined the signaling by common variants of β2AR, including one variant in which Arg16 is replaced by Gly. The response time of these receptors was similar to that of wild-type receptors after a single exposure to an agonist, but differed in response when exposed to repeated stimulation. The receptors with Gly16 were activated more rapidly after a second exposure to epinephrine, whereas the wild-type receptors responded more slowly to the second stimulus. The variant receptors with Gly16 also caused formation of greater amounts of the second messenger adenosine 3′-5′-monosphosphate, which would lead to enhanced downstream signaling. These results may explain why asthma patients who have the Gly variant respond better to β-agonist bronchodilator therapy.

    Sci. Signal. 4, ra53 (2011).

  4. Applied Physics

    Video Holography

    1. Ian S. Osborne

    A hologram captures and stores the interference patterns created when coherent light is scattered from an object. As such, holograms store sufficient information to enable reconstruction of a true three-dimensional (3D) representation of the object that can be rotated and viewed from any direction. In contrast, the 3D displays currently becoming more popular in consumer electronics and cinemas differ in that they provide a sense of depth but give the same perspective; that is, a viewer watching a 3D film from one side of the room sees the same 3D image as someone watching from the other side. Holographic displays would provide both a sense of depth and a different perspective. However, the capture, processing, and storage of holograms and their reconstruction are computationally intensive. Tsang et al. have developed a method that allows holograms to be captured and processed at up to 40 frames per second, good enough for video-rate holographic imaging. The prospect of a holographic video display for true-to-life 3D video conferencing, or, more practically, real-time medical imaging, may not be too far off.

    Opt. Express 19, 15205 (2011).