This Week in Science

Science  08 Jul 2011:
Vol. 333, Issue 6039, pp. 133
  1. Operating at the Limits

    Control theory provides a mathematical basis for engineering the dynamic behavior of a system by using feedback. Design is constrained by trade-offs between making the system efficient and robust while minimizing output error. Glycolysis is a central metabolic pathway that consumes glucose to generate adenosine triphosphate (ATP) with two key enzymes that are feedback-inhibited by ATP. Under certain conditions, glycolytic intermediates in yeast oscillate, but the basis for these oscillations has been unclear. Chandra et al. (p. 187) have now applied control theory to analysis of a minimal model of glycolysis and show that the oscillations are a consequence of operating at the hard limits of maximizing robustness, while minimizing metabolic overhead and enzyme complexity.

  2. Volatile Moon Rocks

    CREDIT: HAURI ET AL.

    The presence of water in the Moon is a highly debated question. Hauri et al. (p. 213, published online 26 May) present measurements of dissolved volatiles, including water, in melt inclusions from a sample of ancient lunar magma brought back by Apollo 17. Melt inclusions are small pieces of molten rock that get trapped in crystals that grow in magma. Melt inclusions thus preserve their volatile content, which would otherwise evaporate during volcanic eruption. The results suggest that parts of the lunar interior are more volatile-rich than prior studies have indicated, and may be more akin to Earth's modern upper mantle. This is surprising because the Moon was thought to have lost most of its volatile content immediately after its formation, which was caused by a Mars-sized object hitting Earth.

  3. Turbulent Times

    In 1883, Osborne Reynolds described the key factors that influence the transition of a flowing fluid from a smooth, laminar flow to a choppy, chaotic, turbulent flow. Known as the Reynolds number (Re), the ratio of inertial forces to viscous forces is used to predict the change in flow behavior at a critical value for a specific flow geometry. In simple pipe flow, the onset of turbulence has been estimated to occur at Re values between 1900 and 2100, but it has not been possible to pin down a critical transition point. Avila et al. (p. 192; see the Perspective by Eckhardt) examined the fate of water jet puffs injected into a stream of flowing water. At low Re, the puff eventually decayed, while at high Re, a puff split into two by absorbing energy from the flowing liquid. Finding the point where the lifetime of a single puff reached a maximum allowed the minimum Re required to sustain turbulent flow to be determined.

  4. Swift Encounter

    CREDIT: MARK GARLICK/UNIVERSITY OF WARWICK

    On 28 March 2011, the NASA Swift satellite detected a high-energy outburst that behaved completely differently from the classical gamma-ray bursts that the telescope was designed to study and detect. Levan et al. (p. 199, published online 16 June) present comprehensive observations of this unusual event, concluding that it occurred in the nucleus of a galaxy with a redshift of 0.35. Bloom et al. (p. 203, published online 16 June) present a physical model to explain the origin of the burst: The tidal disruption of a star as it passed close to the galaxy's central black hole produced a relativistic jet pointed toward Earth.

  5. Forbidden Territory

    In the parlance of quantum mechanics, certain transitions between specific energy states are termed “forbidden.” These transitions, however, do occur, but they are extremely improbable. Using precise laser spectroscopy in tandem with laser cooling, van Rooij et al. (p. 196; see the Perspective by Eyler) measured the frequency of one such transition in helium (from the lowest triplet state to the second-lowest singlet state), relying on extended interaction times between the atoms and light field, as well as highly sensitive detection, to characterize the rare excitation events. This frequency, in turn, afforded a stringent test of the theoretical framework describing atomic structure and light-matter interactions.

  6. Caught in the Act

    Under conditions where a structural phase transition can occur, multiple phases of a material coexist as it transforms from one equilibrium phase to the other. Zheng et al. (p. 206) devised a method to study the transformation between two solid phases in copper sulfide nanoparticles. Extremely high-quality transmission electron microscopy and image analysis were able to distinguish between the two phases within the nanoparticle as the transition temperature was approached. Regions were observed to flip back and forth between the two phases, which could be described in terms of simplified thermodynamic fluctuation arguments.

  7. A Different Route to Phenols

    Phenol derivatives are essential intermediates in the preparation of many commercial organic compounds, such as drugs, pesticides, pigments, and plastics. Traditionally, these intermediates are prepared through modification of intact aromatic rings. Izawa et al. (p. 209, published online 9 June) present an alternative strategy: palladium-catalyzed oxidation of cyclohexanone derivatives (essentially hydrogenated precursors of the desired phenols), with oxygen accepting the liberated hydrogen to form water. The advantage of this approach is that the cyclohexanone precursors are available with substitution patterns that are difficult to achieve selectively when using standard protocols for direct aromatic ring elaboration.

  8. Off With Her Head!

    The Red Queen hypothesis suggests that coevolution results from the evolutionary race between interacting species resulting in a seemingly stationary situation. Morran et al. (p. 216; see the Perspective by Brockhurst) explore the role of the Red Queen in the case of a host-pathogen system with the presence and absence of outcrossing and coevolution. Caenorhabditis elegans nematodes were infected with the bacteria Serratia marcescens. The populations of C. elegans were either wild-type (can cross with other individuals or self-fertilize), obligate outcrossers, or obligately self-fertilizing lines. Outcrossers had lower mortality rates when infected than obligate selfers, even if infected by coevolving bacterial strains. Wild-type populations initially showed higher levels of outcrossing in the presence of the virulent bacteria, but reduced levels of outcrossing in later generations. Thus, selection imposed by coevolving pathogens may account for the widespread prevalence of outcrossing in nature.

  9. Cord Blood Delivers

    Continuous blood-cell production during an organism's life depends on rare, self-renewing hematopoietic stem cells (HSCs). The ability to characterize human HSCs has been limited because isolation methods cannot distinguish HSCs from progenitors that only participate in transient hematopoietic regeneration. Notta et al. (p. 218) now report isolation of a near-homogeneous population of HSCs, from human umbilical cord blood and successful transplantation of a single HSC into immunodeficient mice. These findings provide a method to assist in systematic studies of human HSC biology and the development of approaches to harness the regenerative potential of HSCs for clinical applications, such as transplantation.

  10. Bacterial Elongation Mechanism

    CREDIT: DOMÍNGUEZ-ESCOBAR ET AL.
    CREDIT: DOMÍNGUEZ-ESCOBAR ET AL.

    One of the defining concepts of recent research on bacterial cytoskeleton organization and morphogenesis is the helical organization of actin-like MreB proteins and the role of these helices in the spatial organization of cell wall biosynthesis. Two papers now challenge this picture. Domínguez-Escobar et al. (p. 225, published online 2 June) and Garner et al. (p. 222, published online 2 June) studied the dynamic interactions between bacterial actins and the cell wall elongation machinery in Bacillus subtilis cells and found that MreB proteins in actively growing cells did not form helical filamentous structures. Instead, the proteins formed discrete patches that moved processively along peripheral tracks perpendicular to the cell axis.

  11. Optineurin in Autophagic Bacterial Clearance

    Autophagy receptors bind both ubiquitin and autophagy markers, including microtubule-associated protein light chain 3 (LC3), and promote the specific clearance of protein aggregates, defective organelles, and intracellular pathogens. Wild et al. (p. 228, published online 26 May) describe optineurin (OPTN) as an autophagy receptor whose function is regulated by phosphorylation of its LC3-interacting motif. Phosphorylation by the protein kinase Tank binding kinase 1 (TBK1) increased the affinity of OPTN for autophagy modifiers by 13-fold. OPTN is also a ubiquitin-binding protein and was recruited to cytosolic Salmonella to promote bacterial clearance via the autophagy pathway. Thus, TBK1 and OPTN represent critical components of the cell defense system for restricting the growth of bacteria in the cell.

  12. Waste Not

    Cancer-associated cachexia is a wasting syndrome characterized by uncontrolled loss of fat and muscle mass, which kills about 15% of cancer patients. Studying two models of tumor-bearing mice, Das et al. (p. 233, published online 16 June; see the Perspective by Arner) show that adipose triglyceride lipase (ATGL), an enzyme that breaks down stored fat, is essential to the pathogenesis of cancer-associated cachexia. Mutant mice that were genetically deficient in ATGL were protected from cancer–associated cachexia; they retained a normal fat mass and showed reduced loss of skeletal muscle. Pharmacological inhibition of ATGL may thus merit investigation as a potential treatment for cachexia.

  13. Pericytes in Neuronal Scarring

    Scarring can serve the valuable purpose of reestablishing tissue integrity after damage. This immediate response to damage can, however, interfere with slower but more effective tissue repair processes. In the central nervous system, the scars left after damage to neuronal tracts have been thought to be derived from astrocytes, a type of glial cell. Göritz et al. (p. 238) now identify pericytes as important contributors to scars in neural tissue. Pericytes, usually found wrapping small blood vessels, are already known for their contributions to scars in dermal and kidney tissues. A subgroup of pericytes formed the core of scars after spinal cord damage in the mouse, and when the contribution of pericytes was reduced, the lesion was more likely to remain unclosed.

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