Rabbits and Rodents Arose Relatively Recently
Rodents and their close relatives, including rabbits, make up much of the diversity of mammals, but their fossil record is sparse, and the time of divergence from other placental mammals has been controversial, with estimates spanning from near the Cretaceous-Tertiary (K-T) boundary to much earlier times. Asher et al. (p. 1091) now describe several specimens of a fossil rabbit dating to about 50 million years ago that collectively provide a more complete view of early glires (rabbits and rodents). Its primitive features imply that this group had diverged from placental mammals near the K-T boundary.
Origins of Olmec Pottery
Olmec ceramics are associated with the origination of culture and language in Mesoamerica. Their widespread distribution has raised the questions of whether cultural development was concentrated along the Gulf Coast or arose from the interactions of several societies. Blomster et al. (p. 1068; see the Perspective by Diehl) tested these ideas by using trace-element chemistry of numerous Olmec ceramics to determine their provenance. The data show that all Olmec ceramics originated from the San Lorenzo region of the Gulf Coast, and other wares were not imported into this region.
Slicing Through Ammonia
During the last 40 years, late transition-metal catalysts have been developed to insert small molecules into H2 and into Si-H, B-H, and C-H bonds. However, a homogeneous catalyst to break the N-H bond in ammonia remains elusive. Zhao et al. (p. 1080) have prepared an iridium compound with an electron-rich alkyl ligand that reacts with ammonia in room-temperature solution. Kinetic and isotopic labeling studies to show that the N-H insertion process occurs from a 14-electron Ir(I) intermediate. The studies could point the way toward a catalyst for ammonia transfer to olefins and other organic substrates.
Walking the Walk
Conventional walking robots require large amounts of energy and complex control mechanisms. In the 1990s, researchers developed gravitationally propelled bipedal passive-dynamic walking machines that mimic human walking without active control. Collins et al. (p. 1082) have extended these passive-dynamic designs by including simple powered actuators and controllers. The bipedal robot walkers exhibit improved energy efficiency and offer insights into the mechanics of human walking.
Oceanic Iron and Atmospheric Oxygen
The oxygenation of Earth's atmosphere began 2.3 billion years ago, and some evidence, such as the presence of banded iron formations (BIFs), suggests the oceans remained largely anoxic until 1.8 billion years ago. Rouxel et al. (p. 1088; see the Perspective by Kump) present evidence from sedimentary sulfides which shows that the rise of atmospheric O2 had a direct affect on iron cycling in the ocean and the ocean's redox state. Based on changes they see in the Fe isotopic composition of these rocks, they conclude that most of the Paleoproterozoic ocean became strongly stratified after 2.3 billion years ago, and that BIFs continued to form until 1.8 billion years ago by upwelling of ferrous Fe-rich plumes and rapid oxidation in the oxygenated upper layer of the ocean.
In 1994, researchers operating NASA's Compton Gamma-Ray Observatory detected gamma rays emitted toward space from Earth's atmosphere. These unusual emissions appeared to be correlated with lightning and other electrical discharges such as sprites and blue jets. Smith et al. (p. 1085; see the Perspective by Inan) have observed a series of gamma-ray events at energies up to 20 million electron volts in their data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite launched in 2002 to study solar flares. The data lend support to the explanation that a powerful electron-accelerating mechanism in the atmosphere propels particles to relativistic velocities.
A Rainbow of Human Variation
Individual differences in DNA sequence are the genetic basis of human variability. Hinds et al. (p. 1072; see the cover, the Policy Forum by Duster, and the Perspective by Altshuler and Clark) describe a large, publicly available collection of human genetic variation data consisting of 1.58 million single-nucleotide polymorphisms genotyped in each of 71 individuals. They present an initial characterization of the structure of variation within and among three human populations, and explore the application of these data for uncovering the genetic basis of complex traits. These results represent the first draft of what will eventually be a detailed haplotype map describing human variation.
Modeling Conflict, Error, and Decision-Making
We constantly have to make decisions based on integrating many types of information (see the Perspective by Ridderinkhof and van den Wildenberg). The anterior cingulate cortex (ACC) and neighboring areas play a role in monitoring and controlling goal-directed behavior, but how it knows that an error has occurred, or that a given set of response processes are in conflict with each other, is unclear. Brown and Braver (p. 1118) developed a computational model that shows how the ACC might represent a prediction of error-likelihood, such that its response to a given task condition is proportional to the perceived likelihood of an error in that condition. Machens et al. (p. 1121) studied a two-stimulus interval decision task in which subjects first perceived an initial stimulus, then held it in working memory, and finally made a decision by comparing it with a second stimulus. A simple mechanistic model for how primates may solve two-interval discrimination tasks suggests a testable mechanistic architecture that bridges the gaps from neural mechanism to neural phenomenology to behavior.
Prions at Sites of Inflammation
So-called prion diseases, like bovine spongiform encephalopathy (“mad cow” disease), are thought to be caused by infectious proteins (prions) that accumulate in the brain. Neuronal and lymphoid organs have thus been excluded from the food chain with the aim of protecting public health. Under inflammatory conditions, however, immune cells are not confined to lymphoid organs, which suggests that inflammation could shift the tissue tropism of prions. Heikenwalder et al. (p. 1107, published online 20 January 2005) report that in mouse models of prion diseases, conditions that lead to inflammation of the liver, pancreas, or kidney can indeed lead to the accumulation of high levels of prion infectivity within the affected organs through the infiltration of prion-infected immune cells. The findings have far-reaching implications for prion biosafety, for example, if prion-infected farm animals have ongoing inflammation.
The Attractive Cockroach
The cockroach is despised for good reason, as it is a vector for pathogens and a major cause of allergic disease. Nojima et al. (p. 1104; see the news story by Pennisi) have characterized a sex pheromone from the German cockroach Blatella germanica that may provide a new tool in pest control. The pheromone (blattelaquinone) was purified from adult female cockroaches and characterized as gentisyl quinone isovalerate. In field tests on a cockroach-infested pig farm, adult males, but not nymphs or adult females, were attracted to traps baited with synthetic pheromone.
The Roots of Patterning
Root hairs develop on the emerging roots of Arabidopsis plants in a regular pattern of tidy files of neatly spaced hairs. A suite of transcription factors manages the fates of root cells in response to lateral inhibition. Kwak et al. (p. 1111, published online 23 December 2004) have now identified a gene termed SCRAMBLED, which encodes a putative receptor-like kinase protein that seems to function as a regulator of the overall transcriptional response. Scrambled enables the developing epidermal cells to interpret their position and establish the appropriate cell type pattern.
Keeping Hair Cells Cycling
In the mammalian ear, the hair cells critical for hearing and for maintaining balance cease proliferating and differentiate early in life. Thus, hearing loss caused by damaged hair cells is irreversible. Sage et al. (p. 1114, published online 13 January 2005; see the Perspective by Taylor and Forge) have now analyzed the relation between proliferation and differentiation in the mouse by manipulating the expression of one of the retinoblastoma protein family members, which can regulate cell cycle exit. In the absence of the relevant retinoblastoma protein, hair cells of the inner ear can differentiate and yet continue to proliferate. Further research is required to determine whether this effect can be extended to later in life.
The action of cytidine deaminase enzymes on nucleic acids and subsequent repair of the resulting lesion can lead to base-pair modification, or “editing” of coding sequences. This process can have beneficial results, as in the case of class switching and somatic mutation of immunoglobulin loci. For a retroviral genome, however, related intracellular editing enzymes can be detrimental to viral replication, and such viruses have evolved mechanisms to counteract the activity of these host proteins. Turelli and Trono (p. 1061) review the evolution and relation of the diverse activity of cytidine deaminases in these different contexts of host defense.
Golgin with the Flow
There has been much discussion concerning the mechanism of transport through the Golgi apparatus, a stack of flattened membrane cisternae involved in the modification and packaging of secretory and membrane proteins. Malsam et al. (p. 1095) developed a biochemical approach that allows subpopulations of Golgi vesicles bearing proteins known as golgins to be isolated and characterized. Golgin tethers were used as affinity ligands to isolate Golgi vesicles and further analysis revealed the existence of two subpopulations that appear to modulate different flow patterns in the Golgi. The flow patterns through the Golgi will determine the nature and extent of posttranslational modifications within the Golgi complex and so determine the chemical fingerprint of the cell surface.
Getting in on the Akt
Akt/protein kinase B (PKB) is deregulated in many human cancers. Sarbassov et al.(p. 1098) have identified an enzyme in Drosophila and human cells that initiates its activation. Upon association with the cytoplasmic protein rictor, the cytoplasmic kinase mammalian target of rapamycin (mTOR) phosphorylates a critical hydrophobic motif in Akt/PKB. This modification is required for full activation of Akt/PKB and its subsequent regulation of cell survival. In contrast, association of mTOR with another cytoplasmic protein, raptor, does not target Akt/PKB and is sensitive to the drug rapamycin. Reduced expression of mTOR or rictor blocked Akt/PKB activity, which suggests that the complex may serve as an effective target in cancer cells for drug development.
Long-Lived Bone Marrow Precursors
Hematopoietic stem cells (HSCs) give rise to all of the cells of the blood, including those of the immune system. The abundance of these cells is determined not only by proliferation, but by whether the developing cells undergo apoptosis, or programmed cell death. Opferman et al. (p. 1101) show that MCL-1 is a critical regulator of this cell death decision in HSCs. MCL-1 is an antiapoptotic factor that prevents activation of the apoptosis-inducing proteins BAX and BAK. Conditional deletion of the MCL-1 gene in mice caused loss of HSCs. When cells lacking MCL-1 were isolated from conditional knockout animals and cultured in vitro, MCL-1 was required to allow stem cell factor to promote survival of bone marrow cell progenitors.