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Posted: September 12, 2006
Quantum dots used to track protein traffic
(Nanowerk News) The natural flow of proteins inside living cells can be tracked using quantum dots, researchers have shown.
Quantum dots – brightly reflective nano-crystals – could allow biologists to watch biological activities that cannot normally be resolved under a microscope, using conventional dyes and florescent labels. When excited by laser light, the tiny crystals emit photons and shine more brightly and for longer than any comparable label.
Researchers have inserted quantum dots into cells before, but this is the first time they have been used to trace the movement of internal cellular activity.
Tania Vu and Sujata Rajan at Oregon Health and Science University in the US tracked a protein within rat cells that regulates the growth of nerve tissue. They used quantum dots, each 10 nanometres in diameter.
The dots were chemically bound to a protein that sits on the outside of each nerve cell. Natural growth factor proteins bind to this external protein and the pair move into the cell together. They travel around the cell, altering processes related to the maintenance, growth and regeneration of nerve fibres.
In Vu and Rajan's experiments, the quantum dot went along for the ride, allowing them to watch through a microscope as the two proteins went about their business. For example, they saw the partner proteins move into areas within the cell that subsequently grew outwards towards other nerve cells.
As quantum dots are thousands of times brighter than the fluorescent proteins often used to track cells, they can be introduced and observed individually. They can also be watched for longer periods of time – the researchers followed dots for four days,and believe they should work for longer – while fluorescent protein labels go dim in less than 24 hours.
Watching cellular activity in such detail would be impossible using conventional labelling methods, Vu says. "They are much dimmer so generally you're looking at the average of many of those individual labels," she told New Scientist.
Vu adds that quantum dots could one day be used to watch new drug compounds as they travel inside cells. They could even be used in living animals or humans: "You could use quantum dots in any system where you can access visually – under the skin, for example."
Giovanni Cappello at the Curie Institute, Paris, France, says the work could prove significant. Earlier in 2006, Cappello and colleagues attached quantum dots to molecular motors – molecules that perform mechanical work – and injected them into cancer cells.
"The major criticism we received was that we injected proteins from outside, instead of targeting the ones inside," Cappello explains. "Rajan and Vu found a trick to overcome this problem by targeting a protein which naturally [moves to within the cell]."
Many other proteins also "internalise" in this way, but Cappello says quantum-dot labelling is not yet universally useful. "Our dream is to be able to tag any protein in a cell and follow it from its creation to the death of the cell," he says. "No one is quite sure how to do that yet."