Gene silencer and quantum dots reduce protein production to a whisper

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Gene silencer and quantum dots reduce protein production to a whisper

http://www.eurekalert.org/pub_releases/2008-06/uow-gsa062308.php

Each of these jars contains the same substance. The difference is the

size of the particles. Quantum dots, suspended in liquid, absorb

white light and then reemit it in a...

More than 15 years ago scientists discovered a way to stop a

particular gene in its tracks. The Nobel Prize-winning finding holds

tantalizing promise for medical science, but so far it has been

difficult to apply the technique, known as RNA interference, in

living cells.

Now scientists at the University of Washington in Seattle and Emory

University in Atlanta have succeeded in using nanotechnology known as

quantum dots to address this problem. Their technique is 10 to 20

times more effective than existing methods for injecting the gene-

silencing tools, known as siRNA, into cells.

A fluorescent image of the cell taken 15 minutes after introducing

the quantum dot-siRNA complex. At this early stage the particles are

in the cell membrane.

" We believe this is going to make a very important impact to the

field of siRNA delivery, " said Xiaohu Gao, a UW assistant professor

of bioengineering and co-author of a study published online this week

in the Journal of the American Chemical Society.

" This work helps to overcome the longstanding barrier in the siRNA

field: How to achieve high silencing efficiency with low toxicity, "

said co-author Shuming Nie, a professor in the Wallace H. Coulter

Department of Biomedical Engineering, jointly affiliated with the

Georgia Institute of Technology and Emory University.

Other co-authors are Maksym Yezhelyev and Ruth O'Regan at Emory and

Lifeng Qi at the UW.

Short pieces of RNA, the working copy of DNA, can disable production

of a protein by silencing, or deactivating, a stretch of genetic

code. Research laboratories regularly use the technique to figure out

what a particular gene does. In the body, RNA interference could be

used to treat conditions ranging from breast cancer to deteriorating

eyesight.

The recent experiments used quantum dots, fluorescent balls of

semiconductor material just six nanometers across (lining up 9,000

dots end to end would equal the width of a human hair). Quantum dots'

unique optical properties cause them to emit light of different

colors depending on their size. The dots are being developed for

cellular imaging, solar cells and light-emitting diodes.

This paper describes one of the first applications of quantum dots to

drug delivery.

Each quantum dot was surrounded by a proton sponge that carried a

positive charge. Without any quantum dots attached, the siRNA's

negative charge would prevent it from penetrating a cell's wall. With

the quantum-dot chaperone, the more weakly charged siRNA complex

crosses the cellular wall, escapes from the endosome (a fatty bubble

that surrounds incoming material) and accumulates in the cellular

fluid, where it can do its work disrupting protein manufacture.

Key to the newly published approach is that researchers can adjust

the chemical makeup of the quantum dot's proton-sponge coating,

allowing the scientists to precisely control how tightly the dots

attach to the siRNA.

Quantum dots were dramatically better than existing techniques at

stopping gene activity. In experiments, a cell's production of a test

protein dropped to 2 percent when siRNA was delivered with quantum

dots. By contrast, the test protein was produced at 13 percent to 51

percent of normal levels when the siRNA was delivered with one of

three commercial reagents, or reaction-causing substances, now

commonly used in laboratories.

Central to the finding is that fluorescent quantum dots allow

scientists to watch the siRNA's movements. Previous siRNA trackers

gave off light for less than a minute, while quantum dots, developed

for imaging, emit light for hours at a time. In the experiments the

authors were able to watch the process for many hours to track the

gene-silencer's path.

The new approach is also five to 10 times less toxic to the cell than

existing chemicals, meaning the quantum dot chaperones are less

likely to harm cells. The ideal delivery vehicle would have no

effect; the only biological change would be siRNA blocking cells'

production of an unwanted protein.

The exact reason that the quantum dots were more effective than

previous techniques is, however, still a mystery.

" We believe the improvement is caused by the endosome escape, and the

ability of the quantum dots to separate from the siRNA, " Gao said.

Quantum dots are not yet approved for use in humans. The authors are

now transferring their techniques to particles of iron oxide, several

types of which have been approved by the Food and Drug Administration

for use in humans. They are also working to target cancer cells by

attaching to specific markers on the cells' surface. " Looking

forward, this work will have important implications in in-vivo siRNA

therapeutics, which will require the use of nontoxic iron oxide and

biodegradable polymeric carriers rather than quantum dots, " Nie said.