Human disease: Unfair sisterly exchange
Hum. Mol. Genet. 13, 417–428 (2004)
DiGeorge syndrome, a rare condition characterized by symptoms including facial and heart abnormalities, arises through the spontaneous loss of a segment of chromosome 22 when it is passed from parent to child. Sulagna C. Saitta et al. show that this segment is lost in an unexpected way.
They compared copies of chromosome 22 from children with DiGeorge syndrome with those of their parents and grandparents. In 19 out of 20 families, the missing segment was lost when eggs or sperm were formed during meiotic cell division, because the resulting two ‘sister’ chromosomes had misaligned and incorrectly exchanged genetic information.
This is surprising — only a handful of the 20 families would be predicted to undergo such chromosomal crossovers in this particular region. It also differs from other deletion syndromes, such as Williams syndrome, in which a segment is often deleted from a single chromosome without the involvement of its homologue.
For DiGeorge syndrome, the blame may lie with large duplicated segments of DNA that are present in the deletion region, the team suspects. They hope to find out whether critical sequences might predict whether a person's chromosome 22 is susceptible to this disease-causing deletion.
Data storage: Hot bits are smaller
Appl. Phys. Lett. 84, 810–812 (2004)
The size of individual information-storing bits in magnetic recording can now be made smaller than the accepted limit for the traditional technology. Hendrik F. Hamann et al. report bit sizes of less than 40 nm across in a magnetic film. This permits storage densities of 400 billion bits (400 Gbit) per square inch, and further increases of more than twofold seem possible. In contrast, the conventional technology was expected to plateau at around 150 Gbit per square inch.
The problem is that smaller bits require a finer grain size for the magnetic recording medium, but if the grains are too small they can no longer maintain their magnetic order in the face of thermal fluctuations. One answer is to use materials in which the magnetic spins are harder to flip, which are said to have higher coercivity. This makes them more resistant to thermal scrambling, but they are then also harder to write in the first place with conventional recording heads. To record magnetic data, Hamann et al. use a high-coercivity material whose coercivity can be temporarily lowered by heating. By using the hot tip of an atomic force microscope, they can keep the heating highly localized.
Animal behaviour: Bees clean up
Ethology 110, 1–10 (2004)
Honeybees need to keep themselves clean — not least because their wings can become clogged by dirt, which hampers flying. But as anyone who has had a troublesome itch knows, some areas are more difficult to reach than others. Benjamin B. Land and Thomas D. Seeley have carried out a detailed investigation of the ‘grooming invitation dance’, by which one bee solicits a helping hand from another. This behaviour is distinct from the ‘waggle dance’ used to point fellow workers towards food.
Land and Seeley's first approach was to film bees in the hive. In the grooming dance, they found, a worker vibrates her body from side to side about four times a second, sometimes stopping to groom herself with her legs. In two-thirds of cases, another worker sprang to the grubby bee's aid and began grooming her. Such helpful behaviour was always preceded by the dance, and workers always touched the dancer with their antennae before beginning to groom, suggesting that the signal is transmitted by touch.
Bees can't reach the bases of their own wings. Is this the reason for the dance? To find out, Land and Seeley puffed chalk dust onto the wing bases of workers. Sure enough, dusty bees produced more dances than control workers puffed with air.
Biochemistry: Flipping lipids
Org. Biomol. Chem. 2, 214–219 (2004)
Cells are wrapped in a double layer of phospholipids. The inside of this membrane is rich in aminophospholipids such as phosphatidylserine (PS); the outer surface has more phosphatidylcholine (PC). This asymmetry is a key feature of cell plasma membranes, and a family of translocase enzymes, including those known as ‘flippases’, is dedicated to maintaining the correct distribution of lipids. Moving PS to the outside of this membrane acts as a flag to attract phagocytes, the assassin cells of the body. Consequently, synthetic flippases are being investigated as tools to elucidate membrane processes and as potential therapeutic agents.
Yoshihiro Sasaki and colleagues have previously reported synthetic flippases that move PC to the inner layer of the cell membrane. Now they have made a flippase that can swap both PC and PS lipids in a ‘flip-flop’ reaction. From their studies on red blood cells, the authors believe that the flippase acts as a ferry, shuttling between the inner and outer membrane layers, carrying a lipid passenger on each journey. The authors also found that there was virtually no leakage from the cells, indicating that the flip-flop had not opened up the membrane to unregulated traffic.
Neuroscience: Astrocytes in action
Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0306731101 (2004)
Glial cells were long thought of as bystanders to the main movers of brain activity, the neurons. Increasingly, however, they are taking the research spotlight. The latest developments are reported by Qing-song Liu et al., who have investigated the influence of astrocytes, a type of glial cell, in the rat hippocampus.
Working with brain slices, the authors followed up evidence that, in response to intracellular rises in the levels of calcium, astrocytes release the signalling molecule glutamate. They found that increased levels of astrocyte calcium resulted in increased activity in another cell type, interneurons. Using various approaches, they came up with a chain of events in which calcium indeed prompts the production of glutamate by astrocytes, and this activates a glutamate-sensitive receptor — the kainate receptor — on interneurons.
What can be inferred about astrocyte function from these results? Interneurons have an inhibitory effect on neurons. So Liu et al. conclude that astrocytes may be involved in regulating the excitability level of the hippocampus, with the interneurons acting as intermediaries.