Medicine Gene therapy for prion diseases?

J. Cell Sci. 117, 5591–5597 (2004)

Transmissible spongiform encephalopathies — such as scrapie in sheep, mad cow disease and human Creutzfeldt–Jakob disease — are thought to be triggered by an aberrant, infectious form of prion protein. This converts other, healthy prion proteins into the infectious form, which accumulate and eventually kill neurons.

Carole Crozet et al. have found that a type of gene therapy may stop the aberrant prion protein from proliferating in infected cells. They based their technique on two naturally occurring, disease-resistant forms of prion protein. These proteins do not switch into the infectious form and are thought to protect some sheep and humans from brain-wasting disease.

The authors engineered mouse prion genes to be similarly disease-resistant, placed them in a lentiviral ‘carrier’, and allowed the virus to inject the genes into cultured mouse cells already infected with disease-causing prions. The cells started to manufacture the disease-resistant prion, and this stopped the infectious prion from multiplying.

Crozet et al. went on to show that the lentivirus, unlike many other viruses used in gene therapy, can infect brain cells that have ceased dividing. Gene therapy for prion diseases — for which there is currently no treatment — may one day prove a reality.

Helen Pearson

Molecular physics Liquid mix

Science 306, 848–851, 845–848 (2004)

There are many ways to stack atoms and molecules in a solid — ice has at least 12 crystalline forms. But a liquid is a liquid, a mere disordered jumble of particles. Or so one might think. Yet over the past few years there have been signs that some substances may have more than one liquid state. This has been claimed both for water and for a melt of Al2O3–Y2O3 when they are in supercooled (and thus metastable) states.

Yoshinori Katayama et al. have previously reported signs of a first-order phase transition (like the jump from liquid to gas) between two stable liquid phases of phosphorus. Now they clinch the case by directly observing the coexistence of high- and low-density phases of phosphorus at 1,000 °C and high pressure. X-ray diffraction measurements show that the liquids have different structures.

Meanwhile, also in the latest issue of Science, Rei Kurita and Hajime Tanaka add to the story. They report a transition between two liquid phases of the compound triphenyl phosphite.

Philip Ball

Development Eye for stem cells

Proc. Natl Acad. Sci. USA 101, 15772–15777 (2004)

The human retina maintains a small population of stem cells, say Brenda L. K. Coles and her colleagues, and does so throughout life.

Coles et al. studied retinas taken from human cadavers ranging from newborns to subjects in their seventies. They isolated and cultured cells from various parts of the retina, and found that those from a region called the retinal ciliary margin had the hallmarks of stem cells: the ability to proliferate and self-renew, and (crucially) to develop into a range of different types. Each eye contained around 10,000 such cells.

When the researchers injected the stem cells into developing mouse or chick eyes, the cells' progeny migrated to the retina and differentiated — most showing the characteristic markers of photoreceptor cells, the retina's light-sensing apparatus. The authors argue that this shows that there is no qualitative difference between the retinal stem cells of different animal species. What's more, the lifelong presence of these cells might mean they could be used in tackling the human retinal degeneration that is associated with ageing.

Michael Hopkin

Food science Down the hatch

J. Agric. Food Chem. 52, 6564–6571 (2004)

The difficulty of simulating our acutely tuned olfactory system isn't the only reason that taste-testing of food and wine is hard to automate. Our sensory experience of foods depends not just on the complex blend of aromas that the food gives off, but on exactly how these are released in the mouth and throat. The heady effects of a fine whisky or a rich Cambozola seem to derive mostly from a thin film of liquid or semi-liquid material that is deposited in the throat during swallowing and which discharges its aroma molecules into the breath.

Koen G. C. Weel et al. have devised an artificial throat that can mimic this release process. The ‘throat’ is a simple piece of rubber tubing that bridges glass tubes above and below it. The upper tube can be filled with liquids, including saliva, while the rubber tube is clamped shut. To ‘swallow’, the clamp is opened and the liquids drain through, leaving a film on the rubber tube. Air is then pumped upwards to carry the aroma molecules to a mass spectrometer, which acts as the nose. The release patterns are generally similar to those measured in human tasters — except that this device could be used for unpalatable or even toxic substances that no taster could be expected to stomach.

Philip Ball

Glaciology Under the ice

Geophys. Res. Lett. 31, L20401 (2004)

Credit: LANDSAT

Ice slips off the edge of Antarctica primarily in focused, relatively fast-moving flows called ice streams. Understanding the behaviour of these streams is critical for predicting how the Antarctic ice sheets will respond to climate change and how they might alter sea level. It is generally thought that ice streams are lubricated by a mixture of meltwater and fine-grained sediment (known as till) at their base. But only now have Edward C. King and colleagues taken a good look at how the meltwater is distributed.

Their seismic soundings of the Rutford Ice Stream (pictured) in West Antarctica indicate that beneath the ice there are channels or ‘canals’ of water about 0.4–0.7 m deep, carved into the till. The seismic anomaly that pinpoints the water is about 200 m wide and 1 km long, but the researchers can't tell yet if this is a single channel or an unresolved series of narrower canals 3–5 m wide. Either way, the liquid surely helps speed the ice flow towards the ocean.

Bill Happil