31 Oct 2012

NewsFLASH: Globular star clusters seen in spectacular detail

Stunning new images released this week by the European Southern Observatory (ESO) and the Hubble Space Telescope show globular star clusters in spectacular detail.

Globular clusters are spherical groups of over hundreds of thousands of ancient stars pulled toward each other by gravity. They are amongst the oldest objects in the universe, and for many years astronomers have used them to study the structure and evolution of stars.

Globular clusters are very common; our galaxy alone has about 150 million visible clusters orbiting its periphery, and many remained undiscovered.

This wide view of the globular star cluster NGC 6362 was captured by the 67-million pixels Wide Field Imager (WFI) attached to the MPG/ESO 2.2-metre telescope at La Silla Observatory in Chile. This is one of many new color images created from data obtained during the ESO Imaging Survey concluded in 2002.

Wide view of NGC 6362 captured by the Wide Field Imager in MPG/ESO 2.2-metre telescope  (credit: European Southern Observatory)

This survey collected images of several regions of the sky in preparation for observations with the new huge Very Large Telescope (VLT). ‘The full set of data obtained by the ESO Imaging Survey would be better compared to a set of maps’ says Richard Hook, ESO’s public information officer.

Another dazzling image of NGC 6362 was created combining ultraviolet, visible-light and infrared images taken by the Hubble Space Telescope- a space-based observatory run by the European Space Agency and NASA.

Detailed view of the NGC 6362 core captured by the Hubble Space Telecope (credit: European Space Agency/NASA)

This view takes a closer look at the compact core of the globular cluster. Hook says ‘[Research] groups are combining WFI and Hubble data on globular clusters very successfully in other projects.’ The wide-angle and detailed views from these two telescopes ‘complement each other perfectly’.

Most stars in a globular cluster are over 10 billion years old, nearly as old as the universe itself, and they look typically yellowish or red. In recent years, however, younger-looking massive blue stars- blue stragglers- have been found in the core regions of star clusters, including NGC 6362.

Since all stars in a globular cluster were presumably born at the same time and therefore have similar ages, astronomers came up with two theories to explain blue stragglers: they must be the result of a star collision or a transfer of material between two neighboring stars.

The Paranal platform of the Very Large Telescope (VLT) with the four main units and four auxiliary telescopes (credit: ESO/H.H.Heyer) 

The new images from ESO’s WFI/2.2-meter and Hubble telescopes will help astronomers from over 25 countries solve this and many other mysteries. 

It will be exciting to see new answers and questions arise with the new generation of powerful super-telescopes like the VLT.

30 Oct 2012

Bees know their way around

The 'travelling salesman problem' has puzzled mathematicians for over eight decades, but bees might just have the answer. In a study published in September in the journal PLoS Biology, scientists show that bumblebees quickly work out the shortest route to feed from several flowers and return to their nest. 

Worker bee with transponder (credit: Stephan Wolf from Rothamsted Research)

Pollinator insects like bumblebees, and other foraging animals such as hummingbirds, bats and even primates, establish stable routes between regular feeding sites and their homes. To save energy, foragers have to come up with a way of visiting multiple locations while traveling the shortest possible distance. This is a task identical to the travelling salesman mathematical problem, which tries to calculate the shortest route to visit several cities once and return to the starting point. Now Lars Chittka's team at Queen Mary University of London found how bees manage to solve this problem without computers or even a map. Andy Reynolds from Rothamsted Research and a co-author in the study says 'We showed how this complex routing problem can be solved by small-brained animals without requiring 'map-like' memory'.

The scientists trained bees to collect sugar from artificial flowers, and then followed their flying routes from the nest to five artificial flowers arranged as a pentagon in a field. To do this, they attached a tiny wire antenna to the back of each bee and tracked their movements with a radar, much like a GPS navigation system. They found that the bees first visited the nearest flowers to the nest, and that, in only eight round-trips, they had discovered all the flowers. After this initial 'random' explorative phase, the bees gradually began visiting the flowers in a specific sequence, as though in each trip they were learning and progressively optimizing the 5-flower circuit. 

Amazingly, in just about a couple dozen trips, each bee chose the shortest possible route to visit all five flowers and return to the nest- amongst the 120 other possible ways- and stuck to it 'Stable routes (...) that linked together all the flowers in an optimal sequence were typically established after a bee made 26 foraging bouts, during which time only about 20 of the 120 possible routes were tried.' explains Reynolds. 

Male bee with transponder
(credit: Stephan Wolf from Rothamsted Research)

But what happens if the flower spatial arrangement changes? In the wild, bees have to modify their foraging routes in response to changes in the environment. To understand how bees do this, the scientists removed an artificial flower from the pentagonal experimental set-up and tracked the bees with the radar. They found that the bees continued to visit all four flowers and the empty feeding location using the optimal route, as if they could remember where the missing flower had been. 

Previous work on honeybees suggested that bees have a 'map-like memory', but the authors in this study believe bees can develop optimal routes simply by using 'a highly effective and versatile trial and error method' Reynolds says. The scientists used their experimental data to develop a mathematical 'trial and error' model based on heuristic, or experience-based, algorithms. Similar heuristic models describing how ants find the shortest routes between feeding locations and their nest are widely used by mathematicians and computer scientists. These models work only for a low number of locations, however, and in nature bees can feed from hundreds or even thousands of flowers. So what happens then? Reynolds explains 'The trial-and-error model becomes impractical for 20 or more locations but is effective for up to about 10 locations, which in practice could facilitate the linking up of flower patches'.
Bees may move randomly between flowers within a flower patch, but have a fixed order of flying between patches. 'This could be quite effective because there could be much to gain by minimizing the distances flown between patches but little to gain by minimizing the distances flown within patches' he says.

The team had recently made similar observations in the laboratory, but this is the first study examining the bee's routing behavior over long distances and in a natural setting. Thomas Collett, a neurobiologist at the University of Sussex who specializes in insect navigation says 'Such a study of the ontogeny of routes over the kinds of distances that bumblebees normally fly has had to wait for the right technology'. 
In the future, Chittka's team would like to use their radar tracking method to answer questions such as whether bees can solve the travelling salesmen problem when more feeding sites are available. Collett says 'Testing [this] and other models will be exciting and may give new insights into navigation and sequence learning'.

This article was published in The Munich Eye on the 27th of September 2012. You can read it here.

Lihoreau et al PLoS Biology (2012) DOI: 10.1371/journal.pbio.1001392

27 Oct 2012

Sperm use navigation system to find eggs

For sea urchin sperm, finding an egg to fertilize in a vast ocean might seem like looking for a needle in a haystack. However, these prickly creatures have devised a highly effective strategy to overcome this hurdle: eggs release chemical factors that guide the sperm towards them, a process called chemotaxis. Now, scientists from the Center of Advanced European Studies and Research in Germany have discovered how sea urchin sperm navigate up a gradient of attractant.

Tracking of calcium signals (green) from a sperm cell swimming in a chemoattractant gradient (blue)
Luis Alvarez and René Pascal from Stiftung Caesar)

Sperm chemotaxis is commonly found in nature and is important for fertilization. Most animal species with external fertilization- such as marine invertebrates like sea urchins- and even some plants, use chemical attractants to guide sperm towards the egg. However, the molecular details of sperm chemotaxis, particularly in mammals, such as humans, are still not well understood.

Research on mammalian sperm chemotaxis presents many challenges: direct measurements can only be carried out in vitro and only about 10% of sperm respond to attractants. In contrast, fertilization in sea urchins can be mimicked in the laboratory, and 'sperm are mostly homogeneous in their responses' the researchers say.

When sea urchin sperm detect an attractant, they adjust their swimming trajectory by changing the beating of the tail (flagellum). The attractant of Arbacia punctulata, the sea urchin species used in this study, is a small molecule called 'resact'. Resact released by the egg binds to receptor proteins on a sperm’s flagellum, and this causes calcium ions to enter the cell. The calcium rise controls the flagellar beat and tunes the swimming path of sperm, but exactly how this happens remains unclear.

In the study published in September in The Journal of Cell Biology, Benjamin Kaupp’s group shows how sea urchin sperm sample and integrate the attractant cues to adjust their course as they swim towards the egg.

Sperm oozing out of the sea urchin gonopores (credit: René Pascal from Stiftung Caesar)      

The scientists placed sea urchin sperm in tiny chambers and then added caged resact, a modified version of the molecule that is activated by a flash of UV light. Using caged resact, the scientists were able to stimulate the sperm with the attractant at precise time intervals. They found that sperm count resact molecules for about 0.2 to 0.6 seconds before producing a calcium response- they called this 'sampling time'. 'A defined or optimal sampling time is essential,' says Nichiket Kashikar, leading author in the study 'either too short or too long sampling times will leave the sperm astray'.

Sperm are also able to correct themselves, for instance, by stopping a calcium response and initiating a new one, or 'resetting'. But how does resetting affect swimming? To answer this question, the scientists recorded videos of single sperm cells stimulated with resact during a calcium surge. 'During the reset, sperm show an extended period of straight swimming, thereby spending more time swimming up the gradient of attractant.' explains Kashikar 'Simple rule: if the conditions are improving, continue in the same direction'.

The authors propose that this newly found sperm 'navigation system' might be used by other species. 'Although there are likely to be species-specific differences, there might be some commonalities across species' Kashikar says. It remains to be discovered whether similar mechanisms exist in human sperm.

'Chemotaxis is clearly important for sea urchins' notes David Clapham, an expert on calcium sensors from the Howard Hughes Medical Institute Boston Children’s Hospital in the United States 'However, [in mammals] investigators will have to demonstrate that a progesterone gradient exists in the path of swimming sperm in females and that sperm respond to this gradient, not the factor alone'. Kaupp’s team trusts that this might be possible in a near future. 'The experimental tools developed to study chemotaxis in model systems (such as sea urchin) and the chemotactic principles identified might help to design experiments to study chemotaxis of sperm in human and other mammalian species'.

A shorter version of this article was published in ScienceNow on the 19th of September 2012. You can read it here.


26 Oct 2012

Cancer stem cell discovery could lead to new therapies

Scientists have discovered that cancers are fueled by small populations of cancer stem cells. These cells are resistant to current therapies and are thought to drive cancer relapse and metastasis, which are the main cause of death in cancer patients. The exciting findings published in August in the journals Nature and Science could lead to revolutionary new strategies for cancer treatment. 

Cancer is the second cause of death in the US and Europe, and despite an increase in cancer survival in some cancers due to prevention and early diagnosis, the survival rate for patients with cancers in advanced stages has not changed significantly in the past decades.

After a tumor is removed surgically or by chemo and radiotherapy, it often grows back (relapse) and spreads to other parts of the body (metastasis). Scientists have believed for many years that a small population of cancerous stem cells is resistant to therapy and responsible for tumor growth, including during relapse and metastasis- this is called 'cancer stem cell hypothesis'. 

During the past 15 years, several research groups have described cancer stem cells in many types of cancer, and transplantation experiments, in which cells from biopsies of cancer patients are injected into mice, have shown that such cells could generate new tumors. However, these studies did not provide direct evidence for the existence of cancer stem cells. "This manipulation of tumors could potentially bring pitfalls and stronger evidence from unperturbed tumors were needed," said Gregory Driessens, a molecular biologist from the Université Libre de Bruxelles in Belgium.

Now researchers from three independent groups were able to 'see' cancer stem cells labeled with fluorescent markers promoting tumor growth in the brain, skin and digestive system of mice. "Our finding confirms that cancer stem cells really exist as it was suggested but not formally proven so far by grafting experiments," said Driessens, who led the study that identified cancer stem cells in skin.

Cancer stem cells consist of only about 1-3% of all cells in a tumor. So why is their discovery so important? Cancer stem cells could be the source of the most aggressive cancers with a poor prognostic. "This the first time researchers have traced the cell of origin within different tumors. Because cancers are proving to be so complex, we don't yet know how relevant this research in mice is to humans, but it gives us new insights into how cancers might develop and why they can sometimes grow back after therapy." explains Michaela Frye, a Cancer Research UK scientist based at the University of Cambridge (UK). 

"Anticancer treatments should not only be evaluated on their efficacy on the bulk tumor but also specifically for the effect on cancer stem cells, since these cells could be more resistant to chemo and radiotherapies," adds Driessens. These discoveries therefore open the way for the development of new therapies targeting cancer stem cells, which could revolutionize the treatment of cancer.

This article was published in The Munich Eye on the 14th of September 2012. You can find it here.

Driessens, G.Beck, B.Caauwe, A.Simons, B. D. & Blanpain, C. (2012) Nature 
Chen, J. et al. (2012) Nature http://dx.doi.org/10.1038/nature11287 
Schepers, A. G. et al. (2012) Science http://dx.doi.org/10.1126/science.1224676