A genetic tweak can make light work of some nervous disorders. Using flashes of light to stimulate modified neurons can restore movement to paralysed muscles. A study demonstrating this, carried out in mice, lays the path for using such "optogenetic" approaches to treat nerve disorders ranging from spinal cord injury to epilepsy and motor neuron disease.

Optogenetics has been hailed as one of the most significant recent developments in neuroscience. It involves genetically modifying neurons so they produce a light-sensitive protein, which makes them "fire", sending an electrical signal, when exposed to light.

So far optogenetics has mainly been used to explore how the brain works, but some groups are exploring using it as therapy. One stumbling block has been fears about irreversibly genetically manipulating the brain.

In the latest study, a team led by Linda Greensmith of University College London altered mouse stem cells in the lab before transplanting them into nerves in the leg this means they would be easier to remove if something went wrong.

"It's a very exciting approach that has a lot of potential," says Ziv Williams of Harvard Medical School in Boston.

Greensmith's team inserted an algal gene that codes for a light-responsive protein into mouse embryonic stem cells. They then added signalling molecules to make the stem cells develop into motor neurons, the cells that carry signals to and from the spinal cord to the rest of the body. They implanted these into the sciatic nerve which runs from the spinal cord to the lower limbs of mice whose original nerves had been cut.

After waiting five weeks for the implanted neurons to integrate with the muscle, Greensmith's team anaesthetised the mice, cut open their skin and shone pulses of blue light on the nerve. The leg muscles contracted in response. "We were surprised at how well this worked," says Greensmith.

Most current approaches being investigated to help people who are paralysed involve electrically stimulating their nerves or muscles. But this can be painful because they may still have working pain neurons. Plus, the electricity makes the muscles contract too forcefully, making them tire quickly.

Using the optogenetic approach, however, allows the muscle fibres to be stimulated more gently, because the light level can be increased with each pulse. "It gives a very smooth contraction," says Greensmith.

To make the technique practical for use in people, the researchers are developing a light-emitting diode in the form of a cuff that would go around the nerve, which could be connected to a miniature battery pack under the skin.

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Muscle paralysis eased by light-sensitive stem cells