Editors' Choice

Science  10 Feb 2012:
Vol. 335, Issue 6069, pp. 636
  1. Immunology

    Energy to Burn

    1. Kristen L. Mueller
    CREDIT: IMMUNITY 36, 68 (2012)

    The life of a T cell can be very demanding—at least as far as metabolism is concerned. An encounter with a pathogen calls for rapid proliferation and, for the subset of cells that go on to become memory cells, long-term survival. How do cells manage such metabolic demands? Upon activation, T cells switch to glycolysis. Largely quiescent memory cells, however, use oxidative phosphorylation but do have the extra challenge of being able to respond under conditions of increased work or stress. van der Windt et al. now show that mouse CD8+ memory T cells have substantial mitochondrial spare respiratory capacity compared to naïve or effector T cells, which helps them keep up with their energetic demands. The cytokine interleukin-15 (IL-15), which is critical for memory T cell differentiation, was required for this elevated spare respiratory capacity. IL-15 promoted mitochondrial biogenesis and expression of a metabolic enzyme necessary for fatty acid oxidation. Thus, signals encountered along the way to becoming a memory T cell also endow these cells with the ability to respond to future energetic demands.

    Immunity 36, 68 (2012).

  2. Applied Physics

    Magnetized at the Interface

    1. Jelena Stajic

    Magnetism arises when electron spins in a solid align parallel (ferromagnetism) or antiparallel (antiferromagnetism) to each other. Especially useful in today's computer industry is the so-called exchange bias, a somewhat counterintuitive phenomenon whereby an antiferromagnet (AFM) interfaced with a ferromagnet (FM) biases the preferred direction of the magnetization of the FM despite having zero net magnetization of its own. Gibert et al. now demonstrate the exchange bias effect in an artificial lattice consisting of alternating thin layers of the normally nonmagnetic LaNiO3 and the magnetic LaMnO3. When sliced thinly, LaMnO3 is a FM, and the authors show that its magnetization curve is shifted by the presence of LaNiO3, as is typical in exchange bias systems; this suggests that LaNiO3 develops magnetic ordering as well. Previous work has indicated the presence of AFM order in thin films of LaNiO3; the authors' numerical modeling suggests that a modulated spin distribution resembling a spin wave is created in the nickelate.

    Nat. Mater. 11, 10.1038/nmat3224 (2012).

  3. Genetics

    Mom's in Charge

    1. Laura M. Zahn
    CREDIT: J. YUAN

    Imprinting is a genetic mechanism that directs an offspring to use only one copy of a parental gene. Typically, this occurs through the directed expression of an allele from only one parent. In the mammalian placenta, imprinting ensures proper nutrient allocation to the developing offspring. Costa et al. investigated whether this was also the case in the endosperm, the nutritive accessory tissue found in the seeds of flowering plants. In maize plants, the imprinted gene Maternally expressed gene1 (Meg1), which encodes a small peptide known to control cell differentiation in plants, regulated nutrient allocation and affected embryo growth. Meg1 promoted the development and differentiation of the endosperm nutrient transfer tissue, which is responsible for the uptake and partitioning of sugars within the seed. In this system, imprinting was necessary for fine-balancing nutrient allocation from the maternal tissue to both the embryo and endosperm. These findings suggest that, similar to their roles in the mammalian placenta, imprinted genes function in the plant endosperm to control the nourishment of the embryo.

    Curr. Biol. 22, 160 (2012).

  4. Materials Science

    A Model for Aging

    1. Marc S. Lavine

    When cooled below the glass transition temperature, amorphous polymers show a sudden and rapid increase in viscosity, which macroscopically manifests as a hardening or solidifying of the material. Nonetheless, even at these temperatures, local segmental motion is possible, and the polymers thereby age with time as the chains relax into a lower-energy state. Recent experiments have shown that when deformed at a constant load, glassy polymers initially flow rapidly but then suddenly stop, with a striking decrease in the segmental relaxation time, but the basis for this behavior has not been well understood. Fielding et al. tackle this problem by developing a simple model of elastic polymer dumbbells immersed in a glassy solvent, in effect. Under a constant load, rejuvenation of the glassy segments occurs as initial fluidization leads to an accelerated rate of motion. However, the associated increase in the stress borne by the polymer segments then causes an arrest in the strain, allowing the glassy segments to resolidify. The authors note that the unloading behavior of the glassy chains is not well captured by the simple model, as the collapsing polymer chains carry enough stress to cause the glassy solvent to flow. By including a simple modification of the effective modulus of the polymer to reflect more realistic conditions, they are able to capture much of the behavior seen experimentally.

    Phys. Rev. Lett. 108, 48301 (2012).

  5. Evolution

    Getting from One to Two

    1. Guy Riddihough

    Life on Earth was initially unicellular, but at some point these morphologically simple organisms started forming cooperative clusters of cells that allowed them to overcome the limited size and complexity of their single-celled brethren and thus colonize new environments.

    To explore how this transition might have occurred, Ratcliff et al. subjected the unicellular yeast Saccharomyces cerevisiae to a gravity selection protocol. Heavy (and therefore more rapidly sinking) “snowflake” clusters of cells arose through dividing cells remaining stuck together. The close genetic relatedness of the cells in the clusters reduced the potential evolutionary conflict. Further selection resulted in larger snowflake clusters, which nonetheless grew to a specific size limit and reproduced through the production of smaller, but still multicellular, daughter snowflakes. The asymmetric divisions needed to generate the daughter snowflakes were driven by the formation of specialized apoptotic (self-killing) cells that allowed fragmentation of the interconnected cluster of parent snowflake cells. Higher rates of apoptosis correlated with smaller, more numerous daughter snowflakes, which, given their relatively fast growth rates, probably increased the chance of cluster survival through the next selection step. These results suggest that multicellularity may have arisen early in the evolution of life, and indeed there is evidence of filamentous and mat-like organisms dating back to over 3 billion years ago.

    Proc. Natl. Acad. Sci. U.S.A. 109, 10910.1073/pnas.1115323109 (2012).

  6. Biomedicine

    All Eyes on Epigenetics

    1. Paula A. Kiberstis

    Cancer geneticists who are cataloging the genes mutated in human tumors have encountered a recurring theme: Many tumor types carry mutations in genes implicated in epigenetic mechanisms that regulate gene expression. New work on a rare childhood eye cancer called retinoblastoma reinforces the notion that the development and behavior of tumors involve both genetic and epigenetic mechanisms. Retinoblastoma arises when both copies of a tumor suppressor gene called RB1 are inactivated. It had been postulated that RB1 loss destabilizes the genome, leading to the accumulation of additional mutations that increase tumor aggressiveness. Through whole-genome sequencing of four retinoblastoma samples, Zhang et al. instead discovered that these tumors have a low mutation rate and a stable genome. However, several known oncogenes and tumor suppressor genes were found to have epigenetic changes that correlated with changes in their expression pattern in tumor versus normal tissue. Thus, RB1 mutational status appears to affect the epigenetic mechanisms that turn genes on and off. One of the genes overexpressed in retinoblastoma encodes the protein tyrosine kinase SYK, and experiments with preclinical models suggest that SYK inhibitors may be useful drugs for targeting this type of tumor.

    Nature 10.1038/nature10733 (2012).

  7. Chemistry

    Polymer, Heal Thyself

    1. Phil Szuromi
    CREDIT: ZHENG AND MCCARTHY, J. AM. CHEM. SOC. 134, 10.1021/JA2113257 (2012)

    Damage to a polymer, such as a cut or crack, can be repaired if it's possible to recover the reactive groups that initiate and sustain polymerization. Zheng and McCarthy show that one of the oldest polymerization schemes—the formation of silicones from cyclic siloxanes—can be used in this fashion. An 80:1 mixture of octamethylcyclotetrasiloxane (D4) and its ethylene-linked dimer (bis-D4) was polymerized with an anion initiator by heating at 90°C for 4 hours. The resulting polymers contained active silanolate end groups under ambient conditions. The rubbery material could be cut with a knife and then repaired by holding the pieces in place next to one another and reheating at 90°C for 4 hours. When the repaired material was stressed by bending, it split at a location different from the knife cut. The authors note that this propensity was probably evident ∼60 years ago when these polymerization methods were originally explored.

    J. Am. Chem. Soc. 134, 10.1021/ja2113257 (2012).

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