Editors' Choice

Science  10 Jan 2003:
Vol. 299, Issue 5604, pp. 167

    Reduced-Fat Milk

    1. Beverly A. Purnell

    Sometimes the most natural of events presents the most unnatural problems. Breastfeeding might be considered such an event, and about 5% of women experience lactation problems.

    Vorbach et al. describe their work on mice that have only one copy of the gene for xanthine oxidoreductase (XOR). These mice appear normal and fertile but are incapable of nursing successfully, and hence their pups die of starvation within about 2 weeks of birth. Haploinsufficiency of XOR in mice results in a defect in the enveloping of milk fat droplets, preventing their secretion into the lumen of the mammary alveoli. The involvement of XOR appears to be unrelated to its enzymatic function in purine catabolism; instead, it appears to contribute to the structural packaging of milk fat within the epithelial cells. — BAP

    Genes Dev. 16, 3223 (2002).


    Bacterial Iron Deposits

    1. Linda Rowan

    Banded iron formations (BIFs) are enigmatic, yet stunning, sedimentary rocks deposited mainly in the Late Archean to Early Proterozoic (2.7 to 1.9 billion years ago). They consist of micrometer- to centimeter-wide alternating layers of iron-rich and silica-rich material. The laminae were probably deposited in a quiet, marine, near-shore environment at a time when the ocean was nearly saturated in amorphous silica. The source of iron for the iron-rich layers may have been hydrothermal activity of the mid-ocean ridge or plume volcanism. The puzzling aspect is how ferrous iron from hydrothermal sources was oxidized into ferric iron in an oxygen-poor, early terrestrial setting. Konhauser et al. show quantitatively that iron-oxidizing bacteria, primarily chemolithoautotrophs, at cell densities much lower than what is found in modern environments could account for all of the ferric iron found in Precambrian BIFs. — LR

    Geology30, 1079 (2002).


    Synthesizing Siliceous Spiny Shells

    1. Julia F. Uppenbrink

    Unicellular organisms such as diatoms and radiolaria make intricately patterned shells or tests, consisting mainly of amorphous silica. It has been suggested that these structures form through repeated phase separation events at decreasing length scales. To study this process, Volkmer et al. have developed a simple biomimetic model system.

    A surfactant-stabilized oil droplet is microinjected into an aqueous solution. Organic additives in the oil droplet induce spontaneous emulsification: the surface of the droplet develops protruding spines, which grow and then separate from the original droplet to form smaller droplets until the parent droplet disappears. When the oil droplet also contains a metal (silicon or titanium) oxide precursor that can hydrolyze at the oil-water interface, a mineralized shell results. The concentration of the metal oxide precursor can be varied so that the spines are preserved. The star-shaped morphology bears some resemblance to that of the radiolaria, which have silica shells with fine radial spines. — JFU

    Angew. Chem. Int. Ed. 42, 58 (2003).


    Freshwater Triggers

    1. H. Jesse Smith

    Climate records of the late Pleistocene from the North Atlantic region indicate a strong link between large-scale ocean circulation and iceberg discharges from ice sheets. Climate models and observational data point to the impact of meltwater on thermohaline forcing, but the mechanism responsible for the submillennial scale instability of marine ice margins remains unclear.

    Knutz et al. present a paleoclimatic record of the last deglaciation, from the northeast Atlantic margin, which captures a detailed picture of the interaction between ocean circulation and ice sheets in northwest Europe. A moderate elevation of sea surface temperatures triggered a series of multidecadal ice-rafting events, culminating in a major meltwater discharge 17,500 years ago. A similar sequence is also apparent during the B∅lling-Aller∅d to Younger Dryas transition 12,700 years ago. The authors suggest that the initial cooling observed in many North Atlantic records before the massive iceberg discharge event (Heinrich-1) that occurred near the beginning of the Younger Dryas is likely to be the effect of meltwater discharge from European ice sheets. These results provide important details in support of the idea that a sensitive and rapid response of ice sheets in northwest Europe occurred because of transient increases in thermohaline heat transport. — HJS

    Geochem. Geophys. Geosys. 3, 10.1029/2002GC000351 (2002).


    Bridging the Gap

    1. Marc S. Lavine

    In molecular electronic devices, a critical step is making the connection between the active molecules and a pair of electrical terminals. For vertical devices, a connection can be made by depositing a layer of metal on top of the molecular layer, but the quality of the interface is unpredictable, and short circuits often occur. An alternative is to build a lateral device, but here the problem is that the smallest achievable electrode separation is about 6 nm, which is much larger than the length of the typical molecules of interest.

    Lin and Kagan solve the lateral problem by using a layer-by-layer deposition process to grow a metal-metal paddlewheel complex based on ruthenium. This sort of complex has been shown to undergo multiple redox processes, with extensive coupling between the metal centers, and can be tuned by changing the ligand chemistry. The authors demonstrate that the complexes can be layered onto two gold electrodes until the intervening gap of 60 to 80 nm closes. Measurement of the current-voltage characteristics showed a voltage-controlled negative differential resistance (a rise and then a fall of current as voltage was increased). However, this was observed only during the first cycling of the device, possibly due to irreversible oxidation of the ruthenium units in the solid state or to the use of symmetric electrodes. — MSL

    J. Am. Chem. Soc. 10.1021/ja028653y (2002).


    Sugarcoating Male Contraception

    1. Paula A. Kiberstis

    Over a billion people will be entering reproductive age by the year 2020, yet little progress has been made toward the development of new birth control methods (News, 21 June, p. 2172). Research on male contraceptives has focused primarily on hormonal manipulations that disrupt spermatogenesis, but these strategies can have undesired side effects.

    van der Spoel et al. report that oral delivery of a sugar molecule called NB-DNJ (for N-butyldeoxynojirimycin) causes sterility in male mice that is fully reversible after withdrawal of the drug. Although NB-DNJ did not affect sperm counts, the epididymal spermatozoa in the treated mice showed morphological abnormalities and severely impaired motility. These effects may relate to the drug's ability to inhibit synthesis of certain glucosphingolipids that are required for spermatogenesis. NB-DNJ is a particularly promising lead in the search for a male pill, because it already has been evaluated in clinical trials for other indications and is known to be well tolerated by humans. –PAK

    Proc. Natl. Acad. Sci. U.S.A. 99, 17173 (2002).

  7. STKE

    Keeping Cells in Their Places

    1. Elizabeth Adler

    Vascular proliferative diseases, such as atherosclerosis and coronary restenosis, are characterized by dedifferentiation, abnormal proliferation, and migration of vascular smooth muscle cells, a pathophysiological reaction to injury that has been interpreted as a response to cytokines released by inflammatory cells. By comparing vascular smooth muscle cells from mice lacking elastin to cells from wild-type mice, Karnik et al. show that the extracellular matrix protein elastin, which is secreted by vascular smooth cells, inhibited cell proliferation and promoted the development of actin stress fibers, a marker for a mature contractile phenotype. Vascular smooth muscle cells migrated through a filter in response to an elastin concentration gradient, but elastin inhibited cellular migration in response to a platelet-derived growth factor gradient. In an in vivo porcine model of coronary artery stenosis, insertion of an elastin sheath into arteries after vascular injury reduced the pathophysiological response, suggesting the possibility that elastin's effects on vascular smooth muscle could be exploited in therapy to treat vascular proliferative diseases. — EA

    Development130, 411 (2002).