This Week in Science

Science  08 Jan 2016:
Vol. 351, Issue 6269, pp. 134
  1. Ancient Microbiome

    Stomach ache for a European mummy

    1. Caroline Ash

    The 5000-year-old remains of the “Iceman” hold clues to deciphering the mummy's microbiome

    PHOTO: ©EURAC/MARION LAFOGLER

    Five thousand years ago in the European Alps, a man was shot by an arrow, then clubbed to death. His body was subsequently mummified by ice until glacier retreat exhumed him in 1991. Subsequently, this ancient corpse has provided a trove of intriguing information about copper-age Europeans. Now, Maixner et al. have identified the human pathogen Helicobacter pylori within the mummy's stomach contents. The strain the “Iceman” hosted appears to most closely resemble pathogenic Asian strains found today in Central and Southern Asia.

    Science, this issue p. 162

  2. Solar Cells

    Perovskites for tandem solar cells

    1. Phil Szuromi

    Improving the performance of conventional single-crystalline silicon solar cells will help increase their adoption. The absorption of bluer light by an inexpensive overlying solar cell in a tandem arrangement would provide a step in the right direction by improving overall efficiency. Inorganic-organic perovskite cells can be tuned to have an appropriate band gap, but these compositions are prone to decomposition. McMeekin et al. show that using cesium ions along with formamidinium cations in lead bromide–iodide cells improved thermal and photostability. These improvements lead to high efficiency in single and tandem cells.

    Science, this issue p. 151

  3. Stem Cell Niche

    How HSCs populate the fetal liver

    1. Beverly A. Purnell

    Hematopoietic stem cells (HSCs) undergo dramatic expansion in the fetal liver before migrating to their definitive site in the bone marrow. Khan et al. identify portal vessel–associated Nestin+NG2+ pericytes as critical HSC niche components (see the Perspective by Cabezas-Wallscheid and Trumpp). The portal vessel niche and HSCs expand according to fractal geometries, suggesting that niche cells—rather than factors expressed by the niche—drive HSC proliferation. After birth, arterial portal vessels transform into portal veins, and lose Nestin+NG2+ pericytes. When this happens, the niche is lost and HSCs migrate away from the neonatal liver.

    Science, this issue p. 176; see also p. 126

  4. Paleoclimate

    The difference is all in the water

    1. H. Jesse Smith

    Glacial cycles are in part controlled by the pattern of incident solar energy determined by the geometry of Earth's orbit around the Sun. The classic record of the penultimate deglaciation from Devils Hole, Nevada, did not reconcile the presumption of so-called orbital forcing, however, suggesting that deglaciation began ~10,000 years too early. Moseley et al. present analyses of a new set of data from Devils Hole that show that the deglaciation indeed occurred at the time expected on the basis of orbital forcing. The age offset displayed by the older samples apparently was caused by interaction with groundwater, which preferentially affected the deeper original samples but not the new shallower samples.

    Science, this issue p. 165

  5. HIV Transmission

    The ART of HIV prevention

    1. Angela Colmone

    Despite the relative success of antiretroviral therapy (ART) for individuals infected with human immunodeficiency virus (HIV), the rate of new diagnoses has remained fairly constant. Now, Ratmann et al. examined probable sources of transmission in the Netherlands for men who have sex with men. They found that neither ineffective ART nor inadequate retention in care contributed to new infections. Rather, many of these cases could have been averted with available ART had there been more comprehensive testing coverage among men at risk of transmission. These findings support broader, more frequent testing followed by immediate ART as a strategy to decrease transmission rates.

    Sci. Transl. Med. 8, 320ra2 (2016)

  6. Protein Aggregates

    Location, location, location

    1. Stella M. Hurtley

    Aggregates of certain disease-associated proteins are involved in neurodegeneration. Woerner et al. now show that the exact location of these aggregates in the cell may be the key to their pathology (see the Perspective by Da Cruz and Cleveland). An artificial aggregate-prone protein caused problems when expressed in the cytoplasm but not when expressed in the nucleus. Cytoplasmic aggregates interfered with nucleocytoplasmic import and export. Perhaps if we can shunt pathological aggregates to the nucleus in the future, we will be able to ameliorate some forms of degenerative disease.

    Science, this issue p. 173; see also p. 125

  7. Forest Ecology

    Size distributions of tropical trees

    1. Andrew M. Sugden

    The distribution of tree size in tropical forests follows a power-law regardless of location. This pattern has largely eluded mechanistic explanation. Using 30 years of tree demography and growth data from a forest plot in Panama, Farrior et al. show that the power-law size structure emerges after natural local disturbances such as the gaps formed by falling trees. A model of forest dynamics identifies the structural parameter governing the power-law distribution. A mechanistic understanding of tropical forest structural dynamics will benefit forest carbon cycling studies.

    Tropical rainforest in Panama with a fresh treefall gap

    PHOTO: ©DANITA DELIMONT/ALAMY STOCK PHOTO

    Science, this issue p. 155

  8. Quorum Sensing

    Plants send out a bacterial mimic

    1. Nancy R. Gough

    Bacteria use the quorum-sensing pathway to regulate community-level interactions, such as the formation of biofilms. Corral-Lugo et al. determined that a common plant compound mimics a quorum-sensing signal. Rosmarinic acid stimulated the activity of a transcriptional regulator in the quorum-sensing pathway of the plant and human pathogen Pseudomonas aeruginosa, increasing biofilm formation. Because rosmarinic acid could stimulate a premature quorum-sensing response, this compound may strategically disrupt bacterial communication.

    Sci. Signal. 9, ra1 (2016).

  9. Thermoelectrics

    Heat conversion gets a power boost

    1. Brent Grocholski

    Thermoelectric materials convert waste heat into electricity, but often achieve high conversion efficiencies only at high temperatures. Zhao et al. tackle this problem by introducing small amounts of sodium to the thermoelectric SnSe (see the Perspective by Behnia). This boosts the power factor, allowing the material to generate more energy while maintaining good conversion efficiency. The effect holds across a wide temperature range, which is attractive for developing new applications.

    Science, this issue p. 141; see also p. 124

  10. Earth History

    Evidence of an Anthropocene epoch

    1. Nicholas S. Wigginton

    Humans are undoubtedly altering many geological processes on Earth—and have been for some time. But what is the stratigraphic evidence for officially distinguishing this new human-dominated time period, termed the “Anthropocene,” from the preceding Holocene epoch? Waters et al. review climatic, biological, and geochemical signatures of human activity in sediments and ice cores. Combined with deposits of new materials and radionuclides, as well as human-caused modification of sedimentary processes, the Anthropocene stands alone stratigraphically as a new epoch beginning sometime in the mid–20th century.

    Science, this issue p. 10.1126/science.aad2622

  11. Ecology

    Reforest with care

    1. Julia Fahrenkamp-Uppenbrink

    Grasslands throughout the tropics are targeted for forest planting. In his Perspective, Bond argues that such efforts put ancient, highly biodiverse ecosystems at risk. Although some grasslands are the result of deforestation, wide areas have supported mosaics of grassland and forest for millions of years. Examples include grasslands in southern Brazil, South Africa, and Madagascar. Many of these grasslands require frequent fires to maintain them. Plants have evolved to survive fires, for example, by developing extensive root systems. It is crucial that efforts are made to protect these ancient tropical grassy ecosystems.

    Science, this issue p. 120

  12. Brain Circuits

    Fine-tuned information flow in the brain

    1. Peter Stern

    In addition to providing well-characterized excitatory inputs, the entorhinal cortex also sends long-range inhibitory projections to the hippocampus. Basu et al. described this input in detail and characterized its role for learning and memory. Multimodal sensory stimuli activate long-range inhibitory input in vivo. This input enables precisely timed information transfer within the cortico-hippocampal circuit. In this way, long-range inhibitory projections play an important role in providing specificity of fear conditioning, and thus help prevent overgeneralization.

    Science, this issue p. 10.1126/science.aaa5694

  13. Hematopoiesis

    Adjusting hematopoietic hierarchy

    1. Beverly A. Purnell

    In adults, more than 300 billion blood cells are replenished daily. This output arises from a cellular hierarchy where stem cells differentiate into a series of multilineage progenitors, culminating in unilineage progenitors that generate over 10 different mature blood cell types. Notta et al. mapped the lineage potential of nearly 3000 single cells from 33 different cell populations of stem and progenitor cells from fetal liver, cord blood, and adult bone marrow (see the Perspective by Cabezas-Wallscheid and Trumpp). Prenatally, stem cell and progenitor populations were multilineage with few unilineage progenitors. In adults, multilineage cell potential was only seen in stem cell populations.

    Science, this issue p. 10.1126/science.aab2116; see also p. 126

  14. Geomorphology

    Nepal's quake-driven landslide hazards

    1. Brent Grocholski

    Large earthquakes can trigger dangerous landslides across a wide geographic region. The 2015 Mw 7.8 Gorhka earthquake near Kathmandu, Nepal, was no exception. Kargal et al. used remote observations to compile a massive catalog of triggered debris flows. The satellite-based observations came from a rapid response team assisting the disaster relief effort. Schwanghart et al. show that Kathmandu escaped the historically catastrophic landslides associated with earthquakes in 1100, 1255, and 1344 C.E. near Nepal's second largest city, Pokhara. These two studies underscore the importance of determining slope stability in mountainous, earthquake-prone regions.

    Science, this issue p. 10.1126/science.aac8353; see also p. 147

  15. Microbiome

    Decomposition spawns a microbial zoo

    1. Caroline Ash

    The death of a large animal represents a food bonanza for microorganisms. Metcalf et al. monitored microbial activity during the decomposition of mouse and human cadavers. Regardless of soil type, season, or species, the microbial succession during decomposition was a predictable measure of time since death. An overlying corpse leaches nutrients that allow soil- and insect-associated fungi and bacteria to grow. These microorganisms are metabolic specialists that convert proteins and lipids into foul-smelling compounds such as cadaverine, putrescine, and ammonia, whose signature may persist in the soil long after a corpse has been removed.

    Science, this issue p. 158

  16. Cell Biology

    Mitochondria migration during mitosis

    1. L. Bryan Ray

    Every time a cell divides, it is faced with the problem of ensuring adequate distribution of all components to the daughter cells. This includes components that are present in relatively small numbers, like the mitochondria and their DNA-containing nucleoids. Jajoo et al. used quantitative time-lapse fluorescence microscopy of single yeast cells to explore how this works. Unlike chromosomes or macromolecules, mitochondria and nucleoids are distributed in proportion to the volume of cytoplasm received by each daughter cell. Partitioning errors are kept low by even distribution of the mitochondrial volume and roughly equal spacing of the nucleoids before they are distributed.

    Science, this issue p. 169

  17. Geophysics

    Mantle minerals won't share the strain

    1. Brent Grocholski

    The deformation of a mixed block of material depends on the strength of the components of which it is made. Weak materials will deform more than the strong ones in a mixture that is squished or stretched. Girard et al. find a large difference in strength between the two primary minerals making up Earth's lower mantle (see the Perspective by Chen). Deformation in the convecting mantle may occur only near boundary layers as a result, leaving large regions potentially unaffected. This could explain long-lived chemical reservoirs in Earth's interior and the lack of seismic anisotropy in the lower mantle.

    Science, this issue p. 144; see also p. 122

  18. Chemotaxis

    A chemokine's sugary release

    1. Kristen L. Mueller

    As immune cells survey the body for pathogens, they circulate through the blood and migrate through the lymphatic system. The latter route allows for tissues and lymph nodes—the central hubs of the immune system—to communicate. Kiermaier et al. reveal the importance of the monosaccharide sialic acid in keeping immune cells in motion. Multiple sialic acids decorate the surface CCR7 on immune cells. CCR7 recognizes proteins called chemokines, which direct where cells move in the body. Sialic acids on CCR7 release one such chemokine present on lymph node endothelial cells from an inhibited state, allowing immune cells to enter lymph nodes.

    Science, this issue p. 186

  19. Protein Structure

    Going in with a BAM

    1. Valda Vinson

    Integral membrane proteins in bacterial outer membranes play roles in nutrient import and infectivity. These proteins are folded into a barrel shape composed of β-strands and inserted into the outer membrane by the β-barrel assembly machinery (BAM) complex. Bakelar et al. determined the crystal structure of a four-component BAM subcomplex. The structure of a central β barrel in BAM changes in the presence of the accessory components to create a lateral opening that may be involved in how BAM inserts proteins into the outer membrane.

    Science, this issue p. 180

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