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Biophysics

Biophysics

Photoswitches selectively activate individual neurons

Activating worm neurons
Worm neurons are individually activated by two-photon stimulation thanks to azobenzene photoswitches. (Courtesy: Montserrat Porta; Aida Garrido)
Activating worm neurons

Researchers in Spain have developed azobenzene “photoswitches” that are able to efficiently and selectively activate neurons in brain tissue and in living nematodes, an animal model for the study of neuronal circuits (Nature Communications 10.1038/s41467-019-08796-9).

Azobenzenes are aromatic molecules that change their shape (configuration) under light excitation. Recently, researchers have designed azobenzenes conjugated with ligands that attach to neuronal channel receptors, and demonstrated control of the cell channels with infrared light. This approach means that cells located under the light beam in a tissue can be selectively and remotely activated.

Azobenzene conformations

Activating cells at a particular depth, rather than along the whole path of the light beam, requires two-photon (2P) excitation. To this end, near-infrared light pulses are employed, since they easily penetrate tissue, offer good spatial resolution and cause low photodamage in cells. Unfortunately, azobenzenes do not absorb much energy from 2P infrared pulses, resulting in a very low cell activation efficiency.

Rational development of precise photoswitches

To address this shortfall, Pau Gorostiza from the Institute for Bioengineering of Catalonia (IBEC), Ramon Alibés from Univeristat Autonoma de Barcelona (UAB) and colleagues  have used computational models to design azobenzenes with improved 2P absorption, while maintaining a suitable thermal stability.

Once the researchers identified suitable candidates, they synthesized the photoreactive compounds and tested their photochemical properties. They subsequently selected two of these compounds for cell activation assays in vitro.

When tested on genetically modified cells, the chosen photoswitches precisely activated cell receptors using determined wavelengths of near infrared light. In addition, one of the azobenzene photoswitches presented a remarkable 2P stimulation efficiency, which encouraged its trial in brain tissue slices. In these tests, the photoswitch not only maintained this high efficiency in the cells of the tissue slices, but also allowed selective activation of different cells at selected depths. Furthermore, the team also demonstrated photoswitching in living nematodes, manipulating the activity of individual neurons.

A new tool for studying neural networking

These findings represent new possibilities in the study of single-cell behaviour and photoactivated drugs. As Gorostiza points out: “It’s a development that opens the door to a large number of applications. From drugs that only act at the point of our body that is illuminated and are therefore free from unwanted side-effects in other regions, to the spatial and temporal control of any protein whose function we want to study in the context of an organism.”

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