Science Roundup

Progress in Stem Cell Research
Introduction to Special Issue

The capacity of human stem cells to self-renew and to differentiate into multiple cell types (pluripotency) makes them valuable for modeling human disease and attractive candidates for targeted therapies. A special section of the 26 Jun 2009 Science highlighted recent progress in stem cell research made possible by interdisciplinary efforts and improved communication among researchers, clinicians, and bioengineers. Review articles examined the role of stromal stem cells in wound repair, research on cancer stem cells and their relevance for cancer therapy, and new technologies being used to elucidate the biophysical properties of stem cells and their microenvironments. Two Perspectives looked at the limits of acceptable medical innovation when it comes to stem cell research and the role of the U.S. Food and Drug Administration in evaluating the safety of stem cell–based products intended for clinical application. Elsewhere in the magazine, a Policy Forum outlined draft guidelines for stem cell research established by the National Institutes of Health and an Editorial highlighted the need for revised federal funding policies.

Amazon Booms and Busts

The Brazilian Amazon is renowned for its biodiversity and for its influence on climate regulation and geochemical cycles. It is also one of the country's poorest regions. For decades, economic development has been pursued through the clearing of forests for cattle ranching and agriculture. In a Report in the 12 Jun 2009 Science, Rodrigues et al. investigated whether such patterns of land use are indeed associated with a sustained improvement in people's well-being across the region's deforestation frontier. The team analyzed data on the economic development of 286 municipalities in different stages of deforestation and found a boom-and-bust pattern in levels of human development across the frontier. That is, relative standards of living, literacy, and life expectancy tend to increase as deforestation begins but then decline as deforestation progresses. As a result, people in areas that have cleared their forests do not end up better off than those in areas where forests remain intact. In a related podcast interview, lead author Ana Rodrigues discussed the challenges of ensuring sustained improvements in human well-being while avoiding the depletion of natural resources. This will likely require a combination of approaches, including better use of land that has already been cleared, promoting reforestation in degraded landscapes, direct incentives to encourage forest-based livelihoods based on the sustainable harvesting of timber and other forest products, and payments for carbon sequestration.

Seeds in Flight

Each spring, winged maple seeds twirl like helicopters as they descend to the ground. On a windy day, these rotating seeds can travel up to a kilometer, generating unexpectedly high lift forces despite their small size and slow velocity. Now, in a Report in the 12 Jun 2009 Science, Lentik et al. explain that winged seeds employ the same aerodynamic mechanism that helps moths, bats, and probably birds stay aloft (see the ScienceNOW story by J. Grom). Using a robot model seed and a three-dimensional flow measurement technique, the team found that falling maple seeds create a spiraling flow of air known as a leading-edge vortex that is similar to the flow structures that are responsible for the high lift generated by the wings of hovering insects and bats. This vortex decreases the pressure above the seed, essentially sucking it upward to slow its descent. The use of leading-edge vortices thus represents a convergent aerodynamic solution in the evolution of high-performance flight in both animals and plants.

Controlling Chronic Viral Infections

T cell–mediated immune responses are critical for controlling viral infections such as HIV infection and hepatitis B and C. During the initial phases of infection, cytotoxic CD8+ T cells whittle away at the viral load by killing off infected cells, but progressively become "exhausted" and lose function during a chronic infection. This exhaustion becomes more severe in the absence of helper CD4+ T cells, but exactly how CD4+ T cells help CD8+ T cells has been unclear. Three Reports published in the 19 Jun 2009 Science -- by Elsaesser et al. (published online 7 May), Yi et al. (published online 14 May), and Fröhlich et al. (published online 28 May) -- now show that the cytokine interleukin-21 (IL-21) is an essential factor produced by CD4+ T cells, known to be important for the differentiation of certain subsets of CD4+ T cells, that helps CD8+ T cells to control chronic lymphocytic choriomeningitis virus (LCMV) infection in mice. When CD8+ T cells were unable to signal through IL-21 or when IL-21 was not available, they were reduced in number, exhibited a more exhausted phenotype, and were not able to control the virus. In contrast, the absence of IL-21–dependent signaling did not affect primary CD8+ T cell responses to acute infection or responses to a viral rechallenge, suggesting that differentiation of memory CD8+ T cells is independent of IL-21. Whether IL-21 also acts on other immune cell types to help resolve chronic LCMV infection remains to be determined. An accompanying Perspective by L. D. S. Johnson and S. C. Jameson highlighted the findings.

Green Diatom Genes

Diatoms are the dominant component of marine phytoplankton and account for 20% of global carbon fixation. Together with other chromalveolates (e.g., dinoflagellates and coccolithophorids), they represent many thousands of eukaryotic taxa in the world's oceans. In a Report in the 26 Jun 2009 Science, Moustafa et al.  presented surprising new insights into the evolution of these organisms, made possible by comparative genome analyses. Diatoms are known to have acquired their photosynthetic organelles, or plastids, from red algae via a process known as secondary endosymbiosis. One would therefore expect diatom genomes to contain many nuclear genes with a red algal origin. But the new study reveals that diatom genomes are more green than red. The team compared the thousands of genes in two completely sequenced diatom genomes to hundreds of other sequenced genomes and found that although a number of diatom genes appear to derive from red algae, more of them -- more than 1000 genes, constituting ~16% of the genome -- have a green algal origin. The researchers suggest that ancient diatoms once possessed a green algal endosymbiont that donated green genes to the nucleus and that this green plastid was later replaced by an endosymbiotic event that gave rise to the contemporary red plastid. An accompanying Perspective by T. Dagan and W. Martin highlighted the work noting that, like much other recent genomic data, the new findings "do not fit comfortably into current theories of algal evolution."

Human Social Evolution

The origins of modern human behavior are marked by increased symbolic and technological complexity in the archaeological record, including evidence of sophisticated tools, musical instruments, and art. Two studies in the 5 Jun 2009 Science examined the roles of human interactions and population dynamics in shaping how human social behavior evolved. Bowles constructed a model of the impact of competition between groups of humans and incorporated ethnographic and archaeological evidence of adult mortality due to warfare in prehistoric and hunter-gather populations. His analysis suggests that intergroup warfare was sufficiently prevalent that altruistic behaviors could have evolved because they improved a group's chances to win lethal conflicts. Turning to the evolution of technological and cultural complexity, Powell et al.  presented a population model that shows that the development of modern behaviors may rely on the attainment of critical population densities and migratory patterns required for stable cultural transmission. The model is consistent with genetically based estimates of population dynamics in Africa and Europe and suggests that the appearance of modern behavior may not solely reflect increased cognitive capacity. R. Mace discussed the findings in a related Perspective and podcast interview.

From X-ray Binary to Pulsar

Millisecond pulsars are rapidly spinning neutron stars whose lighthouse-like beams of radio waves sweep Earth, producing highly regular pulses with periods shorter than 10 ms. These extreme objects are thought to be formed in low-mass x-ray binary systems -- which consist of a neutron star and a solar-like companion star -- through a long-lasting mass transfer process whereby accreting matter from the companion falls onto the neutron star, reducing its magnetic field while increasing its spin rate until a point at which it begins emitting radio waves. Although researchers have previously identified several rapidly spinning neutron stars in low-mass x-ray binaries, no radio pulsations from these systems have been detected -- until now. In a paper in the 12 Jun 2009 Science (published online 21 May), Archibald et al. reported the detection of a neutron star in a low-mass x-ray binary that is in the process of turning into a radio millisecond pulsar (listen to the related podcast interview with lead author Anne Archibald). Optical observations indicate that the system, which consists of a solar-like star and a 1.69-millisecond radio pulsar, has gone through an accretion phase in the last 10 years, but that no accretion disk exists today, thus confirming the evolutionary connection between millisecond radio pulsars and low-mass x-ray binaries. An accompanying Perspective by M. Kramer highlighted the Report.

Aerosols and Climate

Some aerosols, such as sulfates, have a cooling effect on climate because they reflect solar radiation, while other aerosols, such as black carbon, have a warming effect because they absorb radiation. The preponderance of reflective aerosols in the atmosphere has caused a net cooling effect on climate, offsetting the warming caused by carbon dioxide by almost a third according to an estimate by the Intergovernmental Panel on Climate Change (IPCC). However, the amount of cooling is highly uncertain because estimates based on global aerosol models and observation-based estimates differ widely. In a Report published online in ScienceExpress on 18 Jun, G. Myhre used a combination of observational data and modeling to reconcile the two approaches. The study finds that cooling from the direct effect of aerosols is about 40% less than the IPCC estimate (0.3 Watts per square meter rather than 0.5 Watts per square meter). The reason for the discrepancy is that the relative increase in heat-absorbing black carbon aerosols has been much larger than the overall increase in the abundance of aerosols caused by human activities. Dr. Myhre discussed his work in a related podcast interview.

How Nanocrystals Grow

Nanocrystals -- aggregates of hundreds to tens of thousands of atoms that combine to form crystalline clusters -- are attractive materials because of their chemical and electrical properties and are being used in applications from fuel cell catalysts to light-emitting cellular probes. Understanding the mechanisms by which these particles grow can help researchers synthesize nanocrystals with more complex shapes and tailor-made physical properties. In a Report in the 5 Jun 2009 Science, Zheng et al. described their use of transmission electron microscopy to study the growth dynamics of platinum nanocrystals in unprecedented detail. Using a specialized liquid cell that allows observation of crystal growth in situ, the team discovered that nanocrystals can either grow steadily through the addition of metal-containing monomers from solution or by merging with other nanocrystals in random coalescence events. This latter process had not been considered in the classical models of nanocrystal growth. The researchers suggest that nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies. An accompanying Perspective by C. B. Murray highlighted the Report.

Minimal Acidic Breakup

A current driving force in modern science is an interest in studying chemical and physical phenomena in ever-smaller size regimes: the fields of molecular electronics, single-molecule spectroscopy, and atomic-scale microscopy are apt examples. In a Report in the 19 Jun 2009 Science, Gutberlet et al. considered acid dissociation at its smallest scale, probing the dissociation of a single hydrochloric acid (HCl) molecule into its ions under conditions where just a handful of water molecules are present. The researchers used theoretical simulations together with laser-based infrared spectroscopy in ultracold helium droplets that effectively isolated small aqueous HCl clusters. They found that HCl remained intact upon solvation by one, two, or three water molecules. Dissociation into an ion pair, as occurs in bulk water, required the approach of a fourth water molecule and was facilitated by the geometry of the existing cluster of three water molecules. An accompanying Perspective by T. S. Zwier discussed some of questions left to answer in future studies of acid-base reactions in this small size limit.

Charge Detection

In molecular electronic devices, such as single-electron transistors, interactions between molecules and the underlying substrate must be minimized to ensure that their electronic properties are not disturbed. In a Report the 12 Jun 2009 Science Gross et al. described a technique that will afford researchers a new level of control over these nanoscale systems. Scientists have recently shown that a technique called noncontact atomic force microscopy (AFM) can be used to image immobilized molecules with submolecular resolution. The new study now shows that an extension of noncontact AFM can be used to detect the charge state of individual atoms on thin insulating films. Using a tuning-fork atomic force microscope operating at cryogenic temperatures, the team was able to resolve the charge state of gold and silver atoms absorbed on a sodium chloride film. A related Perspective by E. Meyer and T. Glatzel noted that the approach can be extended to molecules or molecular networks, where charges can be added or removed at specific sites of the molecule. It will therefore be of interest not only for molecular electronics, but also for catalysis, material synthesis, and photovoltaics. Lead author Leo Gross discussed the work in a related podcast interview.

In Science Signaling

A Mammalian Cos2 Homolog

Hedgehog (Hh) signaling is an evolutionarily conserved pathway required for tissue patterning and cell fate determination during development. In the absence of Hh, the Drosophila kinesin-like protein Costal2 (Cos2) mediates the proteolytic processing of the transcription factor Cubitus interruptus (Ci) into a repressor of gene transcription. However, in the presence of Hh, Cos2 interacts with the transmembrane protein Smoothened (Smo) and the repressor form of Ci is not produced. In a Research Article in the 23 June 2009 issue, Cheung et al. provide several lines of evidence suggesting that the kinesin family member Kif7 functions similarly to Cos2 in mammalian Hh signaling. Thus, Hh signaling in Drosophila and vertebrates appears to be more mechanistically similar than previously suspected.

Also in Science Signaling this month:
-- Liao and Xu discussed recent research on insulin signaling in sporadic Alzheimer’s disease (9 June 2009)
-- Mizwicki and Norman reviewed the contributions of ligand and receptor conformation to vitamin D signaling (16 June 2009)
-- Tuomi et al. reported that phosphorylated tight junction protein ZO-1 moves from cell junctions to a cell's leading edge to regulate migration in cancer cells (30 June 2009)

Image credits (in order of appearance): Science cover, 26 June 2009; David Lentink; T. Dagan, W. Martin, Science 324, 1651 (2009); Anne M. Archibald; T. S. Zwier, Science 324, 1522 (2009).