7 Sep 2015

Filming living cells in close-up

Physicists have upgraded super-resolution microscopes to see cells move in all their inner beauty. Viviane Richter reports.

Inside every cell, billions of molecules continually writhe in an endless, carefully choreographed dance. Each step and shimmy can now be captured in super-resolution video, thanks to researchers led by Eric Betzig at the Howard Hughes Medical Institute in Maryland, US, who published their work in Science in August.

Peter Lock, a cell microscopist at Melbourne’s La Trobe University, says the work is “awe-inspiring”.

The performance may not go viral on YouTube, but Betzig’s movie making method has a huge range of possible applications – not least, finding new ways to stop cancer in its tracks.

This mouse embryo cell moves around thanks to two proteins - actin (purple) and myosin (green) - dancing in sync. Credit: Li et al. (Science 2015)

We’ve come a long way since the humble 17th century light microscope. Today’s electron microscopes show us individual atoms. But capturing images of the fine detail of living cells remains a challenge. “There’s beauty in watching the cell and then there’s incredibly complex science,” says Betzig, who was awarded a Nobel prize last year for his super-resolution microscopy work.

Inside each cell, 10,000 types of protein – and 10 billion molecules in total – wriggle and squirm in what Betzig describes as a “crazy dance”. Figuring out their steps is important: they are the ones carrying out the chemistry of life.

To watch individual proteins through the microscope, scientists label them with tags that glow in the light. To snap them at high resolution, cells are photographed through a filter that illuminates the cell in narrow ‘slices’. The filter position is adjusted fractionally for each photograph, until the whole cell has been captured. Combining this sequence of shots using a computer, the researchers get a single high resolution video frame.

Actin proteins (red) help this monkey cell create surface pockets (green) by which they can engulf nutrients from outside. Credit: Li et al. (Science 2015)

Until now, using the technique meant waiting for the glowing proteins in one shot to fade before snapping the next shot, lest they appear as ghostly fuzzy smudges blurring the following image – like the fading afterglow you see on your retina after a light flash. The researchers wait for a moment to let the proteins fade, but can only wait so long or they end up with a hopelessly jerky, disjointed film.

Also, light can also trigger the synthesis of toxic free radicals lethal to the cell. You can’t make a movie if your lighting rig makes the star sick.

So Betzig’s team used a new kind of fluorescent tag that glows as soon as the light is switched on - reducing light exposure time - and stops glowing immediately the light is switched off, reducing blur.

By combining these tags with a new commercially available lens, Betzig’s team was able to snap images up to 100 times faster than before – making smooth movies of intracellular dance routines. They were also able to see structures down to 62 nanometres in size (the previous resolution limit had been 100 nanometres). Whereas before, the entire flu virus was represented by a single pixel, the new resolution allows scientists to make out the shape of its outer coat.

Using their new movie camera and tagging technique, the team has already discovered that actin – a protein ‘railway’ that shuttles cargo around the cell – also helps the cell’s membrane fold around and engulf molecules from outside. Viruses, for example, hijack this transport machinery to travel to the nucleus of a host cell.

Lock, for one, is intrigued by the possibilities offered by super-resolution video. He studies how rouge cancer cells spread to other tissues using tentacle-like protrusions termed “invadopodia”. Invadopodia are packed with equipment to clear the path for the moving cancer cell. The tentacles are held together by actin but how they’re built is not understood. “At the moment we can’t tease apart the dynamics of what’s happening,” Lock says.

He thinks Betzig’s method could uncover the steps cancer cells need to assemble invadopodia. The “dream” would be to target and disrupt the proteins involved to stop cancer in its tracks. “I wish [the facility] was around the corner; I would be sending people there in the blink of an eye!”

Actin proteins (purple) let this monkey cell form "ruffles" that help the cell crawl along a surface. Credit: Li et al. (Science 2015)

Viviane Richter is a staff writer at COSMOS.