Editors' Choice

Science  03 Aug 2007:
Vol. 317, Issue 5838, pp. 572

    Choose Calcifiers with Care

    In contrast to near-term estimates, prediction of atmospheric CO2 content several centuries from now is severely hampered by the multitude of poorly understood feedback mechanisms. The most important of these is probably the interaction between atmospheric CO2 and marine calcification. In a nutshell, the amount of CO2 absorbed by the ocean depends on the quantity depleted through calcium carbonate incorporation into the skeletons of calcifying organisms such as foraminifera and coccolithophorids. This bioactivity is a function of the alkalinity and the pH of the ocean, which in turn depend largely on the partial pressure of atmospheric CO2, as well as the type of calcifying organism (E. huxleyi and O. universa are shown). Although the carbonate chemistry of the ocean is well known, the response of different species to changes in pH and alkalinity is incompletely understood, and large differences exist between the species that have been studied. Ridgwell et al. have performed model calculations for a range of calcifying behaviors. They find that the strength of CO2 calcification feedback is dominated by the assumption of which species of calcifier contributes most to carbonate production, and that ocean CO2 sequestration could reduce the atmospheric fossil fuel CO2 burden by 4 to 13% in the year 3000. This long-term view is needed to help understand the full impact of current energy use. — HJS

    Biogeosciences 4, 481 (2007).


    Degrees of Tolerance

    The affinity of a T cell receptor for its ligand [a peptide bound by a molecule of the major histocompatibility complex (pMHC)] dictates whether a T lymphocyte will become active. However, activation also depends critically on a series of parallel signals, and when these are lacking, the effect of any pMHC complex is a state of anergy (or permanent inactivation) of the T cell. This is an important means by which immune tolerance to self-constituents of the body is maintained.

    Using intravital imaging, Skokos et al. observed that as T cells engaged pMHC complexes of differing affinities for the T cell receptor in lymph nodes in the absence of coactivating signals, their behavior varied considerably. Thus, although under these conditions pMHCs of all affinities induced anergy and led to the retention of circulating T cells in the lymph nodes, only those with a high affinity triggered the flux of Ca2+ that signals the T cells to slow down. Medium-affinity complexes, on the other hand, failed to stimulate Ca2+ flux, but did induce a low level of division and cytokine production. Finally, the low-affinity ligands did not evoke any biochemical event or change in cellular motility but instead rapidly induced an inactive state. Thus, a hierarchy of anergic states may exist for T cells, depending on differences in the binding strength of antigens they meet under steady-state conditions. — SJS

    Nat. Immunol. 8, 835 (2007).


    Getting a Leg Up

    The reversible cis/trans isomerization of azobenzene and its derivatives, induced by alternating irradiation with ultraviolet and visible light, can be used for on-demand control of properties ranging from large-area hydrophobicity (by variation of surface-exposed groups) to the opening and closing of ion channels on the nanoscale. However, adsorption on metal surfaces tends to impede the isomerization process, in all likelihood through electronic effects or induced changes in the molecule's optical absorption spectrum. Comstock et al. show that if the phenyl rings in azobenzene are derivatized with four bulky tert-butyl groups, the molecule in the planar trans configuration is lifted far enough off a gold surface for irradiation with ultraviolet light to induce photoisomerization. The addition of only two tert-butyl “legs” proved insufficient. The reversible isomerization was monitored with scanning tunneling microscopy. — PDS

    Phys. Rev. Lett. 99, 038301 (2007).


    Let's Get Sorted

    Epithelial cells provide a barrier function in tissues by establishing and maintaining a distinctive polarity with an apical surface that faces the lumen of the tissue and a basolateral surface that faces the blood. The maintenance of the two distinct membrane domains has been studied in great detail for many years, but how the polarity originates is much less clear. Nejsum and Nelson examined this process as epithelial cells in tissue culture generated a polarized cell layer and determined whether cell-cell adhesion and the generation of distinct membrane domains were linked. By looking at fluorescently tagged versions of two similar proteins, aquaporins 3 and 5, one of which (AQP3) is basolateral, the other (AQP5) apical, while simultaneously monitoring a component of the cell adhesion machinery, E-cadherin, they observed a precise correlation between the basolateral membrane protein and newly formed cell adhesions. It seems that during the establishment of polarity, newly synthesized basolateral membrane proteins leave the Golgi complex in vesicles that are specifically targeted to and fuse with the growing sites of cell adhesion, which are enriched for the docking and fusion machinery involved in basolateral membrane protein targeting. — SMH

    J. Cell Biol. 178, 323 (2007).


    Softening into Shape

    Polymer particles with nanometer and micrometer dimensions have been applied to fields ranging from medical imaging to fluid rheology, but controlling particle shape during synthesis has been challenging. Champion et al. have devised a simple method for manipulating spherical polystyrene particles into a diverse array of shapes and sizes (including the bicones shown left). The particles were embedded in a sheet of polyvinyl alcohol and then either softened (by solvent or heat) and stretched, or else softened after first stretching the film (which created voids filled by the softened particles). Dissolution of the film then afforded the shaped free particles. Stretching was possible in either one or two directions, and it was possible to combine or repeat the two techniques. — MSL

    Proc. Nat. Acad. Sci. U.S.A. 104, 11901 (2007).


    Appalachian Twins

    The striking change in the trend of the Appalachian mountain range—from south-southwesterly to easterly through central Pennsylvania—reflects a bend in the direction of large folds in the upper crust, which elevate and expose old resistant rocks that support many of the mountains there. Similar bends are common in other mountain ranges, and their origin and timing during continent-continent collision have been enigmatic. Ong et al. looked at twinning in calcite crystals in limestone across Pennsylvania. The orientation of twins reflects stress acting during the initial deformation of the rocks before most of the folding began (strain hardening then locks the twins in). The data suggest that, initially, compression was to the northeast, orthogonal to the southwesterly direction of the southern Appalachians; the early-formed twins in the north were then rotated clockwise. These data, and other studies of later strain, imply that the folds and thrust faults formed after this initial deformation on top of the bend and were not later rotated themselves. A rigid salient in the North American crust that became increasingly important as a barrier during collision with what is now western Africa ~300 million years ago may have caused these dynamics. — BH

    Geol. Soc. Am. Bull. 119, 796 (2007).

  7. STKE

    Hearing New Things About Calcium

    Otoconia, particles in the inner ear composed of a proteinaceous core coated with CaCO3 crystals, underlie our sense of balance. Otoconial development depends on otopetrin 1 (Otop1), a member of a protein family defined by a highly conserved transmembrane domain of unknown function. Its predicted structure, together with its extracellular location, suggested to Hughes et al. that Otop1 was likely to associate with globular substance vesicles that have previously been shown to respond to ATP with an increase in intravesicular Ca2+. When overexpressed in COS7 cells, Otop1 abolished an early peak in intracellular Ca2+ concentration that was elicited in wild-type cells by extracellular ATP or UTP. Both nucleotides activate P2Y purinergic receptors, which signal through the Gαq family of heterotrimeric guanine nucleotide-binding proteins. Signaling through other Gαq-coupled receptors was maintained in the presence of Otop1, suggesting that the Otop1-mediated decrease in endoplasmic reticulum Ca2+ stores was not sufficient to abolish all rapid Gαq-dependent Ca2+ signals and that P2Y inhibition occurred upstream of Gαq. ATP also elicited a slower sustained phase of increased intracellular Ca2+ (a Ca2+ plateau), yet cells overexpressing Otop1 exhibited a Ca2+ plateau even after treatment with thapsigargin. The effects of Otop1 on the Ca2+ response to ATP were reversibly inhibited by suramin (which inhibits the ATP-dependent Ca2+ response of globular substance vesicles), and the authors conclude that Otop1 has marked effects on the Ca2+ response to ATP that could be critical to its role in otoconia formation. — EMA

    Proc. Natl. Acad. Sci. U.S.A. 104, 12023 (2007).

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