This Week in Science

Science  03 Dec 1999:
Vol. 286, Issue 5446, pp. 1813
  1. Discontinuous Plumes

    Several volcanic regions, such as Hawaii and Iceland, are associated with hot spot plumes, but their depth of origin and thus the heat sources that lead to the formation of these plumes have been debated. Two reports show how seismic tomography, which generates an image of the variations in velocity in the mantle, can be used to try to resolve plume structures. Goes et al. (p. 1928) have resolved a lower mantle upwelling of hot material beneath central Europe that may be broken up into smaller upper mantle plumes. Ritsema et al. (p. 1925) have resolved an upper mantle upwelling beneath Iceland that does not extend into the lower mantle. They have also found evidence for a tilted upwelling in the lower mantle beneath East Africa that does not connect with the upper mantle plume-like structures that have been related to the East African Rift. In a Perspective, Ritter discusses how these discontinuous plumes may be related to upper mantle structure and crustal volcanism and rifting.

  2. Skating on Thinning Ice

    Sea ice is a sensitive indicator of climate change—if global climate were warming, then the amount of permanent sea ice in the Arctic should decrease. Satellite measurements and surface observations have shown that at least the areal extent of Northern Hemisphere sea ice has been decreasing for almost 50 years. Two reports present new measurements and analysis of changes in Arctic sea ice (see the news story by Kerr). Johannessen et al. (p. 1937) measured the areal extent of the perennial Arctic sea ice pack and found that the area of multiyear ice has decreased by more than twice as much as that of total sea ice. This change in ice cover has been accompanied by a roughly coincident decrease in ice thickness, which suggests that the heat and fresh water balances of the Arctic may be changing in significant ways. Vinnikov et al. (p. 1934) have examined the decrease in areal extent of sea ice. They combined five separate data sets and two independent climate models to test whether these sea ice reductions can be attributed to natural climate variations. Their model results suggest that the probability of this change being caused by natural variability is less than 2% for the last 20 years and that the decreases seen should continue during the next century. They conclude that the observed decrease in Northern Hemisphere sea ice is probably a result of anthropogenically driven global warming.

  3. B Cells and Mucosal Immunity

    The immune system that develops along the length of the small intestine consists of organized lymphoid tissue called Peyer's patches. Within these patches are M cells, which allow microbes to pass into the lymphoid tissue where they initiate an immune response or gain a foothold for infection. Golovkina et al. (p. 1965) report that mice lacking B lymphocytes were deficient in M cells, and that M cells were restored by exposure to B cells. The resistance of mice with no B cells to infection by murine mammary tumor virus was found to be due to the deficiency of M cells rather than the direct lack of B cells. Thus, B cells not only participate in immune responses but are involved in the generation of the organs for mucosal immunity.

  4. Quantum Dots Grow Up

    One way to self-assemble quantum dots or nanocrystals on a surface is through strain. When there is sufficient crystallographic mismatch between a thin film several monolayers thick and its underlying substrate, the resulting strain can be relieved by rearrangements that create quantum dots. For germanium-silicon dots, low-volume pyramids and high-volume domes form, but the origin of these two size distributions has been unclear. Ross et al. (p. 1931) used a low-energy electron microscopic technique to look at the evolution of the dots in real time. The dots begin as square-based pyramids, and proceed through a series of metastable transition states before maturing into multifaceted dome-shaped dots.

  5. Did You Get That Flu Shot?

    The value of the annual flu vaccine is directly related to how well it can be determined which flu strain will actually predominate in the seasonal onslaught. Bush et al. (p. 1921; see the Perspective by Hillis) have analyzed the molecular evolution of flu strains through recent years and arrived at a means to predict more accurately which strain will emerge in the next flu season. Their process, which involves identification of codons under positive selection to change, successfully predicted in retrospective tests the flu strains for the last 8 years.

  6. A Class Difference in Where You Dock

    The adaptive immune response is stimulated when peptides bound to major histocompatibility complex (MHC) molecules bind to T cells. Structures of T cell receptors (TCRs) in complex with peptide-MHC class I ligands have been determined. Now Reinherz et al. (p. 1913) present the structure of a TCR binding to a complex of a 16-residue peptide antigen and a murine MHC class II molecule (pMHCII). The authors propose that the angle of TCR-pMHCII docking will be restricted to a more “orthogonal” interaction than that seen in the class I “diagonal” mode, and that this difference in docking specifies the preferential binding of pMHCI and pMHCII to CD8 or CD4 co-receptors, respectively. In a Perspective, Wilson analyzes the similarities and differences between the pMHCII and pMHCI structures, and cautions that while this work is a valuable contribution, more TCR-pMHC structures are needed to understand fully the structural basis of TCR-MHC recognition.

  7. Going with the Group

    Spontaneous activity is seen during recording from a single neuron in the brain. Is this seemingly random firing simply “noise” that lacks any informational content or could it also represent some kind of meaningful signal? Tsodyks et al. (p. 1943) combined single-neuron recording with real-time optical imaging of a large surrounding cortical region and found that the firing of a cell depends on the overall activity pattern in the surrounding area. It appears that nerve cells are not really firing at random, but rather that they are participating in larger cortical activity states that seem to correspond to the functional architecture maps.

  8. Boning Up on Statins

    Nearly 100 million people worldwide are at risk for osteoporosis, a debilitating disease of bone loss, and this number is expected to rise as the population ages. Mundy et al. (p. 1946; see the news story by Vogel) show that the statins, a group of drugs commonly prescribed to lower serum cholesterol levels, also stimulate bone formation in vitro and in rodent models. This effect was associated with increased expression of bone morphogenetic protein-2, a growth factor produced by bone cells. These results raise the possibility that with optimization of dose and tissue distribution properties, the statins may have beneficial effects in the prevention and treatment of osteoporosis.

  9. Too Many Doing Too Little

    Familial hemophagocytic lymphohistiocytosis (FHL) is a genetic disorder that dysregulates the immune system and leads to excesses of lymphocytes, macrophages, and inflammatory cytokines. Two genetic loci have been identified and linked to the disease. Stepp et al. (p. 1957) identified the defective gene in the 10q21-22-linked patients as the gene coding for perforin, a protein released from cytotoxic lymphocytes to kill the target cell. This finding provides insight into the biological role of perforin as a mechanism for turning off immune responses, perhaps through the elimination of antigen presenting cells or the cytotoxic T cells themselves.

  10. Blocking Development of B Cells

    Not all of the causes of genetic immunodeficiencies have been worked out. In cases in which B cells are lacking in the periphery, the Btk protein kinase signaling pathway is suspect, as it has turned out to be critical for B cell activation and development. Minegishi et al. (p. 1954) have characterized the defect in a patient that had normal Btk. The mutation responsible in this case was the BLNK adaptor protein, which coordinates signaling from the B cell antigen receptor by binding to various kinases, lipases, and adaptors. Pappu et al. (p. 1949) generated mice deficient in BLNK and report that these mice have the same defect—a block in the development of immature B cells from the pro-B to the pre-B stage that effectively prevents maturation of B cells and antibody production.

  11. Pox Entry Pathways

    The ability of chemokine receptors to act as viral receptors is best known for the case of the human immunodeficiency virus (HIV). Lalani et al. (p. 1968) have now identified a receptor for a very different pathogenic virus, myxoma, a rabbit pox virus. They show that 3T3 cells can be made permissive for this virus if the cells are transfected with one of a number of chemokine receptors, including CCR1, CCR5, and CXCR4. Infection could be blocked by treatment with a chemokine or specific antibody. The authors speculate that the existence of poxviruses may have led to the selection in the human population for mutant alleles of CCR5 that would later provide resistance to HIV.

  12. Reconciling Holocene CO2 Records

    Wagner et al. (Reports, 18 June, p. 1971) used measurements of stomata of fossil leaves from a peat bog in the Netherlands to infer atmospheric CO2 levels during deglaciation and the beginning of the Holocene. They concluded that changes in CO2 levels accompanied small climatic oscillations and suggested that CO2 levels were high [above 300 parts per million by volume (ppmv)] in the beginning of the Holocene. Two comments critique their second interpretation in particular. Birks et al. comment that other fossil leaf data and ice core data “are consistent with atmospheric CO2 levels…at ca. 260 to 280 ppmv” and question the calibration set used by Wagner et al. Indermühl et al. provide further discussion of the discrepancy with the ice core measurements of atmospheric CO2 levels from Taylor Dome, Antarctica. Wagner et al. respond that the other data from leaf stomata have low temporal resolution and may be biased by comparison of samples across latitudes, and “agree that [their] results cannot be reconciled with the ice-core records from Antarctica.” The full text of these comments can be seen at

  13. Gelsolin Activation

    Changes in cell shape and cell movement require remodeling of the actin cytoskeleton. At elevated calcium concentrations, the protein gelsolin severs and caps actin filaments and thus is involved in regulation of filament length. Gelsolin is comprised of six domains with G1 to G3 similar to G4 to G6. Robinson et al. 1939 have determined the structure at 3.4 angstroms of the carboxyl-terminal half of gelsolin (G4 to G6) bound to actin and calcium. This structure shows that when gelsolin binds calcium, bonds between β-strands in G4 and G6 are broken, and G6 is rotated and translated about 40 angstroms relative to G4 and G5. This process activates gelsolin by exposing the actin-binding site on G4. The interactions with actin and the residues involved in calcium binding are similar between G4 and G1, and thus a similar calcium-activated switch mechanism is likely to operate in the amino-terminal half of gelsolin (G1 to G3).

  14. Delivering Flowers

    The timing with which plants switch from producing only vegetative tissue to producing flowers as well is regulated by a complex interplay of time, day length, and light. Kardailsky et al. 1962 and Kobayashi et al. 1960 have characterized the Arabidopsis gene FT (FLOWERING LOCUS T) and show that, seemingly in parallel with LFY (LEAFY), it regulates floral transition. Although FT is similar in sequence to the TFL (TERMINAL FLOWER) gene, their functions are divergent. Thus, the complex genetic network that regulates the onset of flowering is brought into view.