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Far-Red Stain Safer for Live-Cell Imaging

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Emitting at the far-red end of the visible light spectrum, a DNA stain could enable imaging of live cells over the course of a day or more.

The stain has three key advantages over existing fluorescent tags used in live-cell imaging, according to its developers at the Swiss Federal Institute of Technology in Lausanne (EPFL). 

Live-cell imaging can help scientists track key biological processes such as cell division. But the fluorescent stains that light up DNA within cells are toxic, or require blue or UV light that can damage cells.

In addition, many DNA stains are not compatible with superresolution microscopy techniques, which capture higher-resolution images of cells than regular optical microscopes.

The new stain, called SiR-Hoechst, is superresolution-compatible, displays minimal toxicity and emits red light with very little noise once bound to a part of the DNA helix known as the minor groove.

Mitosis of a live HeLa cell stained with SiR-Hoechst, whose chemical structure is superimposed.
Mitosis of a live HeLa cell stained with SiR-Hoechst, whose chemical structure is superimposed. Courtesy of Kai Johnsson/EPFL.


"The introduction of SiR-Hoechst brings bioimaging closer to one of its main goals: observing the wonders of nature without disturbing them," said EPFL professor Kai Johnsson.


SiR-Hoechst incorporates two molecules. The first is a fluorescent molecule (silicon rhodamine, or SiR) that works in the far-red spectrum and was developed previously in Johnsson's lab. The second is the well-known DNA stain Hoechst (bisbenzimide).

Because SiR-Hoechst works with far-red light, there is little risk of photodamage to cells. In addition, the light it emits can be easily distinguished from any background fluorescence of living cells. The stain lasts for more than 24 hours, allowing biologists to identify individual cells in tissue or culture, or to track delicate processes such as cell division in real time.

And because all cells possess DNA, the probe can be used across numerous species and types of cells and tissues.

The researchers are now preparing to commercialize SiR-Hoechst through the EPFL spinoff company Spirochrome SA. The company already provides fluorescent probes for imaging the cytoskeletons of living cells.

The research was published in Nature Communications (doi: 10.1038/ncomms9497).

Published: October 2015
Glossary
superresolution
Superresolution refers to the enhancement or improvement of the spatial resolution beyond the conventional limits imposed by the diffraction of light. In the context of imaging, it is a set of techniques and algorithms that aim to achieve higher resolution images than what is traditionally possible using standard imaging systems. In conventional optical microscopy, the resolution is limited by the diffraction of light, a phenomenon described by Ernst Abbe's diffraction limit. This limit sets a...
fluorescence
Fluorescence is a type of luminescence, which is the emission of light by a substance that has absorbed light or other electromagnetic radiation. Specifically, fluorescence involves the absorption of light at one wavelength and the subsequent re-emission of light at a longer wavelength. The emitted light occurs almost instantaneously and ceases when the excitation light source is removed. Key characteristics of fluorescence include: Excitation and emission wavelengths: Fluorescent materials...
Research & TechnologysuperresolutionEuropeSwitzerlandEPFLKai JohnssonMicroscopyfluorescenceImagingBiophotonicsSiR-HoechstTech Pulse

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