Climate and human evolution
It has recently been suggested that a change to a drier climate in Africa about 2.5 million years ago spurred early human evolution and led to a turnover in mammalian fauna. Behrensmeyer et al. (p. 1589) analyzed a large collection of mammalian fossils dating from 3 to 1.8 million years ago from the Turkana Basin in Kenya and Ethiopia. Although the data show that perhaps up to 78 percent of the mammalian fauna turned over during this time, the major change in fauna was not at 2.5 million years ago, but occurred continually afterward.
Polymers that form through hydrogen bonding could have interesting properties, such as reversibility of the polymerization through heating or concentration change. However, insufficient strength and directionality of the bonding between the monomer units have hampered progress. Sijbesma et al. (p. 1601; see the Perspective by McLeish, p. 1577) found that highly directional polymerization resulted for monomer units bearing a self-complimentary array of four hydrogen bonds. Viscosity, chain length, and composition can be tuned by changing conditions.
Ocean currents are now dominated by thermohaline circulation, the sinking of dense, cool, salty water at high latitudes. Today, approximately equal amounts of deep water are formed in the Arctic and Antarctic. Changes in this pattern are thought to have affected climate greatly in the recent past. Broecker (p. 1582) reviews our present understanding of this circulation pattern and its evolution through time. A critical question is whether increases in greenhouse gases in the atmosphere might disturb this balance and flow pattern.
Melting in 2D
The physics of phase transitions, such as melting, in two-dimensional (2D) systems can differ from that in bulk; theory suggests that a hexagonal crystal, a typical packing, will show an intermediate hexatic phase before melting, but this phase has been difficult to observe. One problem is that defects will distort the packing to orthorhombic, in which case theory predicts that a smectic phase will precede the hexatic phase. Sikes and Schwartz (p. 1604), using high-temperature atomic force microscopy, present evidence for such a smectic phase in supported Langmuir-Blodgett films of fatty acids.
Several times a year, large stable weather patterns form and persist for weeks to up to a month or so. Weeks et al. (p. 1598) performed an experiment with a rotating annulus to show that this pattern, and the transition to more normal eastward atmospheric flow, reflects interactions of the rotating atmosphere with topography.
Types of attention
Two opposing views of neural implementations of attention are a top-down (or roving spotlight) mechanism or a bottom-up (or competitive inputs) mechanism. Both views are supported by studies of neurological patients, primate experiments, and functional imaging of normal humans. Rees et al. (p. 1616) describe a combined functional imaging and psychophysical study of how the degree of visual perception is influenced by the difficulty of simultaneously performing a word-based task. Irrelevant visual stimuli were not perceived only when the task commands large processing resources.
Experiments in space
The spatial aspect of population ecology can be more readily modeled than it can be tested and studied in the field. Two reports help redress the balance. Maron and Harrison (p. 1619) demonstrate experimentally how a host-parasitoid interaction can result in the patchy distribution of a population within a continuous habitat, a result predicted theoretically. Ranta et al. (p. 1621) used a 68-year record of the population size of Canadian lynx from eight provinces of Canada to investigate the dynamics of populations in time and space. Notable, but unstable, synchronicities occur, particularly among neighboring populations. These dynamics can be captured by a straightforward model of population dynamics, which suggests that there may be a simple explanation for the phenomenon.
Clocks all over
Most animals are thought to contain central circadian clocks in the brain—mammals in the superchiasmatic nucleus, and birds and fish in the pineal. Plautz et al. (p. 1632; see the cover and the news story by Pennisi, p. 1560) have visualized the expression of the key clock component PER by engineering the protein luciferase together with PER to form a fusion protein. This construct revealed that the fly Drosophila contains multiple independent oscillators in peripheral tissues, notably in the chemosensory cells of the wing margin. Each independent clock can be reset directly by light, likely through as yet undefined photoreceptors.
NKT cells and tumors
An unusual subset of T cells, natural killer T (NKT) cells, all have the same T cell receptor and all bear some NK cell markers; two reports show that they play a role in fighting tumors. Cui et al. (p. 1623) now show that these cells seem to be indispensable for the control of syngenic tumors mediated by interleukin-12. The mechanism involves the production of interferon γ by these cells. Kawano et al. (p. 1626) found that NKT cells recognize a galactosylceramide lipid found in marine sponges in the context of CD1d; an analog of this lipid may be the natural ligand for antigen recognition. Addition of galactosylceramide in a model of tumor metastasis activated these cells and inhibited development of tumor metastases. Activation of the NKT cells is similar to T cells, but their effector function is more like NK cells.
Ice role for STATs
STATs (signal transducers and activators of transcription) are important mediators of regulated transcription induced by exposure of cells to various cytokines and growth factors. Kumar et al. (p. 1630; see the Perspective by Hoey, p. 1578) describe a different role for these transcription factors—constitutive activation of transcription of Ice-family proteases. These proteases function in apoptosis (programmed cell death), and cells that lack STAT1 failed to undergo apoptosis in response to tumor necrosis factor-α. The constitutive transcription of the protease genes is distinct from the regulated transcriptional activation mediated by STAT1 in that dimerization is apparently not required for the former.
Upper-crust magma tracers
The platinum-group elements (PGEs) have been used as tracers of magma dynamics and mantle differentiation in Earth's interior because their depletions and enrichments relative to each other indicate how much melting, mixing, and water-rock interaction a magma may have experienced during its solidification. However, because the PGEs represent parts-per-billion concentrations in most rocks, an accurate determination of their abundance has been difficult. Rehkämper et al. measured the abundances of platinum, palladium, iridium, and ruthenium in a set of Cameroon and northern Tanzania xenoliths. From this limited but precise data, they infer that the magmas that formed the northern Tanzanian crust were extracted from a fluid-rich, deep subduction zone, whereas those magmas that produced the Cameroon crust melted and were extracted under anhydrous conditions; these results imply different mantle environments rather than varying degrees of partial melting to produce these different rock suites.
Magnetoresistive CrO2 films
Magnetoresistive materials change their resistance in response to an applied magnetic field and thus have applications in sensing and magnetic readout. Half-metallic ferromagnets are potential candidate materials, and Hwang and Cheong investigated CrO2, a material already used as a magnetic recording. Polycrystalline CrO2 films show enhanced low-field magnetoresistance after annealing, which modified the grain barriers by depositing insulating CrO3.
Structures within turbulence
Turbulence can be thought of as a case where inertial forces cause large structures to decompose or cascade into progressively smaller ones. One key to analyzing turbulence is being able to image this process. Wright et al. show that diffusing light photography can be used to image ripple turbulence on a fluid. The analysis shows that the turbulent state consists of a broad distribution of large-scale structures that appear intermittently even during the cascade to smaller structures.
Inside IL-1 signaling
One of the central cytokines of the inflammatory response is interleukin-1, and yet our knowledge of how interleukin-1 receptors (IL-1Rs) transmit a signal to activate the transcription factor NF-κB is quite incomplete. Muzio et al. have elucidated some of the proximal components of the pathway. Taking clues from the “Toll” pathway, a homologous signaling pathway in Drosophila, they have cloned a second IL-1R-associated kinase, IRAK-2, and found that both IRAK-2, a protein similar to Drosophila's Pelle protein, and MyD88, similar to Toll itself, are in the IL-1R complex. MyD88 acts as an adapter that functions upstream of IRAK-2 activity, perhaps to regulate or enable kinase action to be regulated by receptor-cytokine interactions.
The pIV protein of filamentous phage pIV is part of a family of proteins that include flagellar assembly systems and systems in pathogenic bacteria that deliver toxins into mammalian cells. Linderoth et al. used scanning transmission electron microscopy to image purified pIV protein and show that it is a multimer that forms a channel-like structure.
JNK and T cells
The c-Jun amino-terminal kinase (JNK) phosphorylates and activates c-Jun and other transcription factors when it is activated in response to cytokines or environmental stress. In searching for substrates of JNK, Chow et al. found it could bind and phosphorylate NFAT4, a member of the nuclear factor of activated T cells (NFAT) family. NFAT proteins mediate responses of T cells in the immune system, but are also expressed in other cell types. When NFAT4 is activated by calcineurin dephosphorylation, it is transported to the nucleus. Phosphorylation of NFAT4 by JNK caused it to be excluded from the nucleus and inhibited its trancriptional activity. The results reveal specific regulation of the NFAT4 isoform through the JNK signaling pathway and show that the NFAT4 protein itself receives input from multiple cellular signaling pathways.
Obesity and leptin desensitization
The hormone leptin induces dramatic weight loss in leptin-deficient lepob/lepob mice but is much less effective in normal mice and in other mouse models of obesity. In the case of the lethal yellow (AY/a) obesity model, leptin resistance has been postulated to occur because the genetically programmed disruption of signaling by arcuate nucleus proopiomelanocortin neurons in these mice presents a direct block to leptin action. Boston et al. disprove this hypothesis by showing that mice carrying both the AY/a and lepob/lepob mutations are fully sensitive to exogenous leptin. Thus, leptin resistance in the AYa model is more likely caused by classic desensitization.