Optogenetics · Lobeworks/17

Optogenetics makes chosen neurons respond to light by giving them a light-sensitive ion channel from algae or microbes, and it is the main tool for testing what a given set of neurons does: switch them on or off with millisecond precision and watch what changes.

Optogenetics. Optogenetics makes chosen neurons respond to light by giving them a light-sensitive ion channel from algae or microbes, and it is the main tool for testing what a given set of neurons does: switch them on or off with millisecond precision and watch what changes.

The method has two parts. A gene for an opsin, most famously channelrhodopsin-2, is delivered into the brain by a harmless virus, with genetic controls that restrict it to the chosen cells (one kind of interneuron, say, or only the cells that project to one area). Light of the right colour is then delivered through an optical fibre or a window: channelrhodopsin opens and lets cations in, firing the cell within milliseconds; other opsins pump chloride or protons and silence it. Its first use for millisecond control of mammalian neurons was published in 2005.

It writes where most methods read. Recording shows that activity goes with a behaviour; optogenetics tests whether that activity causes it, which is why it reshaped systems neuroscience.

It can write patterns as well as switches. With holographic light aimed at single cells, mice have been made to behave as if they saw a stimulus by reactivating a small ensemble of visual cortex neurons that the real stimulus used to activate.

In humans it has begun in the eye. A clinical study reported in 2021 partial restoration of light perception in retinitis pigmentosa by expressing an opsin in retinal ganglion cells; use in the human brain would need gene delivery and light access deep in tissue, both unsolved for routine use.

It disturbs what it controls. Light heats tissue, and expressing a foreign protein changes cells, so careful studies include controls for both.

Questions: Can the light used in optogenetics change the brain it is meant to control? Yes, through heat. Part of the light delivered through a fibre is absorbed by tissue, and modelling and measurements show that light powers within the range used in experiments can warm the tissue near the fibre tip by around a degree or more, which in some conditions is enough to change firing on its own, in neurons that carry no opsin at all. Careful experiments therefore keep light power and duty cycle low and run the same light in animals without the opsin, so that any effect of light alone can be subtracted from the effect of switching the cells. Could light-sensitive proteins replace a retinal implant? Optogenetics tries exactly that: a gene therapy makes the surviving ganglion cells of the retina respond to light, so no chip is needed. In 2021 a man blind from retinitis pigmentosa, treated this way and wearing goggles that projected intensified light onto his retina, could perceive, locate and count some objects on a table. Both approaches still depend on glasses that turn the scene into a strong signal, and for now the implant gives the sharper result: PRIMA patients gained on average about five lines on a reading chart. How do researchers find the cells that hold one memory? They mark the neurons that are active while an animal learns and then switch those same neurons back on. In the mice used by Tonegawa's laboratory, a neuron that turns on the activity gene c-fos also makes a light-gated channel, but only during a window opened by withdrawing a drug from the diet, so only the cells active during that one experience are tagged. Shining light through an implanted fibre onto the tagged cells of the dentate gyrus made the mice freeze as if they were back in the box where they had learned to fear a shock. Cells tagged in a harmless box did not, which is what shows the tag captured that particular memory. What would a molecular brain interface add to ultrasound? Ultrasound sees blood flow, a slow and blurred echo of what neurons do. Optogenetics found a way around a similar limit for light: give chosen neurons a gene for a protein that responds to it, and light can switch them on or off with cell-type precision. The ultrasound version would give neurons proteins that echo sound or respond to it; Mikhail Shapiro, a co-founder of Merge Labs, showed in 2014 that tiny gas-filled protein shells made by microbes return an ultrasound signal. Merge Labs has said it combines such molecular approaches with ultrasound, without publishing which molecules or how they would reach a human brain. Why does a tool that can switch chosen neurons on raise questions of mental integrity? Optogenetics has shown in mice that writing activity into a small number of chosen neurons can stand in for a real experience: reactivating a small ensemble of visual cortex cells with holographic light made trained mice behave as if they had seen the stimulus that ensemble encoded. The same group that ran that work, led by Rafael Yuste, co-authored the call for neurorights, arguing that devices able to write perceptions or shape decisions threaten agency and identity. No such writing is possible in people today, and the proposed right to mental integrity is meant to be in place before it is. How do optogenetics and calcium imaging divide the work of writing to neurons and reading them? Both rely on genes delivered into chosen neurons and on light: calcium imaging makes active cells glow so a microscope can read which ones fired, and optogenetics makes cells fire or fall silent when light reaches them. Combined in one experiment, with an indicator and an opsin excited by different colours, they close the loop, so a researcher can watch an ensemble form during a task and later switch the same cells on to test what they do. Both are limited to the depth that light can reach and to animals into which genes can be introduced.