- Stella M. Hurtley
The muscular dystrophies represent a group of heritable disorders causing progressive physical decline due to skeletal muscle wasting. Mutations in the gene that encodes dysferlin cause two distinct muscular dystrophies—Miyoshi myopathy and limb-girdle muscular dystrophy type 2B. Basal et al. examined the cellular defects that are responsible for the decline seen in these patients. The muscle cells from mice engineered to lack dysferlin possessed normal levels of the dystrophin glycoprotein complex, whose disruption is responsible for the most common form of muscular dystrophy. However, dysferlin was shown to be key to the repair of muscle cell membranes damaged during the normal wear and tear caused by movement. Small tears in the muscle cell membranes are a common occurrence during activity, and it appears that dysferlin-rich vesicles normally repair these tears by undergoing calcium-dependent fusion with the cell membrane. In mutant dysferlin-deficient muscle fibers, vesicles accumulate at the site of membrane disruption but fail to fuse and seal the damaged membranes.—SMH
Nature 423, 168 (2003).
Lipids on the Brain
- Lisa D. Chong
The hippocampus of the vertebrate brain has tremendous structural plasticity. Robust axon outgrowth is observed during development and upon hippocampal lesion. Bräuer et al. show that a membrane-associated lipid phosphatase called plasticity-related gene 1 (PRG-1) is expressed in hippocampal neurons and controls axon outgrowth by modulating the extracellular phospholipid environment. PRG-1 is expressed in the rat hippocampus during active axon outgrowth and it is up-regulated in the denervated hippocampus. PRG-1 is a lipid phosphate phosphatase that acts on extracellular phospholipids such as lysophosphatidic acid (LPA). Hippocampal explants expressing PRG-1 showed resistance to neurite retraction induced by exposure to LPA, and overexpression of PRG-1 in a neuronal cell line had the same effect. LPA receptor expression in the hippocampus remains constant during embryogenesis and after lesion. Thus, it seems that expression of PRG-1 allows hippocampal neurons to overcome the effects of bioactive phospholipids such as LPA by reducing their local concentrations around axons and growth cones.—LDC
Nature Neurosci. 10.1038/nn1052 (2003).
- Andrew M. Sugden
Migrant birds fly to the same overwintering and summer breeding regions each year. Furthermore, in many species, individuals also return to the same precise location, a feat that would be expected to require a good spatial memory. Mettke-Hoffmann and Gwinner compared the memory for particular feeding grounds in two species of warbler: one migrant and one nonmigrant. In their experimental design, captive birds were allowed to explore a number of “rooms” in their enclosures, some containing food and others empty. The birds were then exposed to the same rooms at intervals of up to 1 year after the initial exposure. The migratory garden warblers displayed a significant preference for the rooms that had contained food a year earlier, whereas the nonmigratory Sardinian warblers were unable to remember the same rooms after only 2 weeks. These results indicate a link between migratory behavior and long-term memory.—AMS
Proc. Natl. Acad. Sci. U.S.A. 100, 5863 (2003).
Eocene Arctic Climate
- H. Jesse Smith
The climate of the Eocene (57.8 to 36.6 million years ago) was so warm that deciduous conifers lived in the Arctic. Despite this warmth, it is unclear what other environmental conditions prevailed to allow coniferous forests to survive through 4 months of continuous daylight during the summer and 4 months of uninterrupted darkness during the winter. Jahren and Sternberg measured the oxygen and hydrogen isotopic compositions of the cellulose from a suite of astoundingly well-preserved fossilized Metasequoia trees that grew 45 million years ago at latitudes of 80°N, near what is now Nunavit, Canada. This allowed them to determine local Eocene atmospheric moisture levels. They also estimated mean annual surface air temperatures by determining the oxygen isotopic composition of secondary carbonates in the fossilized wood. Eocene atmospheric moisture levels were 20 to 120% higher than modern Arctic values, and the mean annual temperature was more than 13°C. These warm temperatures could have increased moisture delivery to high latitudes, and high water vapor levels would have helped to maintain the warmth, particularly through the dark polar winters. These paleotemperatures and relative humidity values are like those of the spring climate of the seasonal rainforests of the North American Pacific Northwest, where relatives of these ancient Metasequoia now grow.—HJS
Geology 31, 463 (2003).
Lifting the Lid on Undercooled Metals
- Ian S. Osborne
As the temperature of molten metals is reduced below the melt temperature, Tm, they tend to solidify and crystallize. In many instances, however (for example, if the number of nucleation sites can be suppressed), it is possible to maintain the metal as a liquid at temperatures well below Tm. Up to now, the mechanism giving rise to this undercooling has been unclear. Some 50 years ago, it was proposed that the local structure of the liquid may contain a significant amount of icosahedral ordering that would present a barrier to long-range ordering and prevent the metal from solidifying. However, direct verification has so far been lacking. Kelton et al. now remove the ambiguities that may arise with the presence of interfaces by using an electrostatic levitation technique to isolate the molten metal and then perform x-ray scattering to provide direct confirmation of the formation of the icosahedral phase within the liquid metal.—ISO
Phys. Rev. Lett. 90, 195504 (2003).
- PLANETARY SCIENCE
The Trail of a Wild Comet
- Linda Rowan
Comets lose mass as they orbit close to the Sun, producing their distinctive coma and tail of gas and very fine particles. Comets also leave a longer trail of larger particles that are harder to see, except when Earth passes through a trail, causing a meteor shower. The Infrared Astronomical Satellite (IRAS) detected trails from eight short-period comets because the trail particles were very hot. Ishiguro et al. have detected a 2.3°(0.14 AU) long trail behind comet 81P/Wild2 at optical wavelengths—the first optical image of a trail without an infrared counterpart.
Wild2 is of particular interest because the Stardust mission will sample it in January 2004. The spacecraft will fly through the trail at a relative velocity of 6.1 km/s and sample the inferred 1-mm-sized particles. These particles may penetrate to the base of the aerogel sample collectors, but the Whipple bumper shields should protect the spacecraft from catastrophic disruption unless the particles are much larger. If all goes well, Stardust will return these samples to Earth in 2006 and we'll really get to see a comet up close.—LR
Astrophys. J. 589, L101 (2003).
Zn+ Now in Sites
- Phil D. Szuromi
Zinc is normally oxidized all the way to Zn2+, and previous attempts to create the Zn+ species have produced diamagnetic dimers of monovalent cation. Now Tian et al. have used a molecular sieve, a silicoaluminophosphate that adopts the chabazite structure (SAPO-CHA), to create isolated Zn+ cations. The phosphate groups within the lattice lead to the presence of one to two acidic protons that can react with metallic Zn to liberate hydrogen. These sites are sufficiently isolated, however, to prevent any one Zn atom from liberating a second hydrogen atom and oxidizing further to Zn2+. Electron spin resonance spectra revealed the presence of the paramagnetic Zn+ cations.—PDS
J. Am. Chem. Soc. 10.1021/ja0343757 I (2003).
- CELL BIOLOGY
A Code for Cell Death
- Guy Riddihough
There is increasing evidence that the regulation of gene transcription is controlled in part by a “histone code.” This code would consist of a series of distinct and specific covalent modifications to the N-terminal tails of the histones within the nucleosome: the core structural component of chromatin. The condensation of chromatin during cell division is similarly known to involve histone modification, specifically the phosphorylation of serine 10 in the N-terminal tail of histone H3.
Hacker et al. now show that programmed cell death, or apoptosis, which involves chromatin condensation and the subsequent destruction of genomic DNA, may also be regulated by a histone code. Phosphorylation of Ser14 in the tail of histone H2B is correlated with the onset of apoptosis, suggesting that it may be involved in triggering chromatin condensation, which is then followed by DNA fragmentation and cell death. The kinase responsible for phosphorylating Ser14 is Mst1 (Mammalian Sterile Twenty), and both the covalent mark and the activity of Mst1 are dependent on caspase-3, a key regulatory enzyme in apoptosis.—GR
Cell 113, 507 (2003).