What is optogenetics?

Optogenetics was developed in the early 2000s; it is the combined use of genetic and optical (light) methods to control genes and neurons. It is among the most rapidly advanced technologies in neuroscience and has the potential to revolutionize the way scientists study the brain. 

What is optogenetics

Optogenetics is an experimental method in biological research that involves the combination of optics and genetics in technologies that are designed to control (through excitation or inhibition) well-defined events in the cells of living animal tissue. Unlike experimental methods previously developed for light control, optogenetics allows researchers to use light to turn cells on or off with remarkable precision and resolution (down to individual cells or even regions of cells) in living animals and in movement. As a result, it can be used not only to control specific behaviors in animals, such as triggering or blocking fear or pain but also to thereby deduce the contributions of individual cells to those behaviors.

Since the 1970s, many laboratories have identified various trans-membrane proteins called opsins, which react to light by opening and allowing the selective passage of charged ions through them. In the late 90s, the first research was carried out in some laboratories to exploit these proteins in the control of neurons, in a very elegant way to provide those who want to study. Despite the fact that it was conceptually simple, it is only in 2005 that the team of K. Deisseroth, MIT. They managed to publish a revolutionary article, integrating one of these opsins, called Channelrhododopsina-2 (ChR2), to those normally present in a culture of murine brain cells. The researchers were able to activate the neurons with a temporal precision of the order of milliseconds by turning on a laser. Now optogenetics has become a relatively widespread and applied technique on numerous species, from worms to non-human primates.

Its use can serve to confirm hypotheses or knowledge previously acquired in another way, to clarify the functioning of some phenomena, to distinguish the influence of the various neurons on the behavior of the guinea pig live, or to manipulate the mind.

It is a technique that "will allow many brain diseases to be treated effectively in the near future,” so researchers promise.

With precise time pulses of light aimed at targeted tissue regions or cells, optogenetics allows the researchers to activate or block events in specific cells of living animals. A mouse with a paw made hypersensitive to touch. For example, the pain response can be eliminated by making the yellow light shine on the affected paw, the cells in which they were aimed at expressing a type of light-sensitive microbial protein known as opsin.

Optogenetics on humans

The first human trial involving optogenetics began in 2016 and was designed to explore the potential use of technology for the treatment of patients with hereditary retinitis pigmentosa. The progressive degeneration of the retina, the hallmark of the disease, causes serious vision problems. At least 15 patients who were blind or mostly blind were expected to participate in the study, and each of them would receive an injection of virus that carried genes encoding the opsin targeted specifically to retinal ganglion cells (RGC). 

One of the main objectives of the test was to establish sensitivity to light in RGCs, which are usually not affected by retinitis pigmentosa and normally transmit visual information from the photoreceptors in the eye to the brain. In the presence of blue light, the RGCs expressing the opsin would start sending visual signals to the brain.

Optogenetics seems to open immense horizons to researchers, who can already modulate nerve activity through the stimulation with optic fibers of photosensitive proteins called opsins produced ad hoc inside neurons; but with light.

It is also possible to control the switching on or off of the genes, in a refined form of epigenetics. This way, we could define it as artificial that will give us the opportunity to activate the beneficial genes and deactivate the dangerous genes at our convenience.

Although the extent to which optogenetic treatment improved vision was uncertain, the results of the study were highly anticipated. Other optogenetic therapies are being developed for a wide range of diseases, including chronic pain and Parkinson's disease.

Optogenetics: Controlling the brain with light

Our brain is undoubtedly the most complex machine. It must be said that in humans, there are not far from 100 billion neurons linked together by nearly a million connections. No wonder it is hard to understand how this damn brain works.

And yet over the past ten years, a new technique has appeared which is perhaps poised to revolutionize neuroscience i.e., optogenetics.

The brain, a delicate organ to study

A brain is a large bundle of neurons linked together, and which spend their time activating and deactivating. When a neuron is activated, it sends an electrical signal to the other neurons to which it is connected; it is said that the neuron discharges.

By simplifying, we can see the brain as a huge machine with billions of switches that would spend their time alternating between "on" and "off." 

And what do you normally do to try to understand how a machine full of buttons works? Perhaps we press the buttons one after the other and watch what is happening. The problem is that we can't do that with the brain. It is indeed almost impossible to stimulate only one given neuron while leaving the others unchanged. We are generally reduced to observing the working brain and trying to deduce something about the role of neurons or given brain areas.

How to stimulate neurons?

Of course, in certain cases, it is possible to activate a region of the brain that interests us by inserting an electrode delivering electrical impulses (as opposite on a mouse). However, this method has the defect of generally exciting an entire area, without making it possible to target a given type of neuron.

An alternative is to inject molecules capable of binding to certain categories of neurons to stimulate or inhibit them. This can allow better targeting, but in this case, we lose all reactivity because the effects are manifested on time scales of several hours. Not ideal when you know that in the brain, signals act in a few milliseconds. The dream would be to have a technique allowing both to target certain neurons selectively while allowing stimulations over very short periods of time. Well, that's what allows optogenetics to do. But to understand how this technique works, we must first review the way communications work in our brain.

The brain, a giant electrical circuit

In the most classic configuration, one neuron in the brain can be connected to another by means of an extension called an axon, which allows the propagation of electrical signals. It is quite tempting to see the axon as conducting wire carrying electricity, but we will see that this analogy is a little incorrect.

Almost everywhere in our body, there are ions, that is to say, charged atoms, some positively like Na + or K +, others negatively like Cl-. However, the distribution of these ions is not the same on each side of the membrane, which delimits the neurons; the charge is thus slightly lower inside. 

This results in a small potential difference of around -70mV on either side of the membrane of our neurons. It is said to be polarized. However, neurons are able to modify this polarization, and the ingredient that allows them to do so is a protein inserted into their membrane, the sodium channel.

The sodium channel, in fact, behaves like a door capable of allowing Na + ions to pass or not from the outside to the inside. When this happens, the charge inside increases and the potential can go from -70 to + 100mV, a threshold beyond which the sodium channel closes, and the polarization drops to -70mV. This momentary change in polarization takes only a few milliseconds and is called an action potential. Initially, it will generally be created in the main part of the neuron. We must now understand why this potential can propagate along the axon.

What allows this propagation is a peculiarity of these channels letting the sodium ions pass, they tend to open precisely if their vicinity becomes depolarized. So if a channel opens, a depolarization takes place, stimulating the opening of the neighboring channel, and so on by domino effect all along the axon. And it is through this kind of chain reaction that a signal can propagate along the axon of a neuron to the neurons to which it is connected. This is how the neuron discharges.

Channelrhodopsin: the key ingredient in optogenetics

Now that we have seen how electrical communications propagate in cells, we can present the star of the show, the key ingredient in optogenetics, channelrhodopsin 2 (ChR2). ChR2 is a protein discovered in 2002 in a unicellular alga with the sweet name of Chlamydomonas reinhardtii. It is very similar to the sodium channel since it inserts into the cell membrane and can pass ions. But its big peculiarity is that its opening is controlled by light. In fact, when you throw blue light at it, the protein ChR2 changes shape and gives rise to a small hole of around 6 Angstroms, enough to let the ions pass inside the membrane.

It was while reading a publication on ChR2 that the biologist Karl Deisseroth and his team came up with the idea that underlies optogenetics. If ChR2 behaves like a channel responding to light, it can be used to unload neurons on demand.

The founding experiment then took place in 2005 at Stanford University in the United States. Deisseroth and his team cultivated neurons in which they implanted the protein ChR2 in a Petri dish. And they then observed that the latter started to discharge when they were lit with blue light. Here is the technique that everyone was waiting for! It is a way to activate neurons in a given region by simply sending them lightly. It remained to implement the technique with a real brain.

Control the brain with light

When the blue light is on, the mouse starts to spin frantically counterclockwise. And it stops when the light signal is turned off. It's a little scary and even cruel. But you can imagine that the purpose of the technique is not to have fun and laugh on the back of our rodent friends. Optogenetics makes it possible to stimulate in a rapid and targeted manner specific areas and neurons of the brain and to understand the impact of their activation on the behavior of the animal. So it's a great way to unravel the web of neural connection to our favorite organ and to better understand how it is wired.

At this point, you may be wondering what the advantage of the method is over conventional electrical stimulation, where electrodes are inserted into the brain to balance electrical impulses. 

Well, the main difference is that with optogenetics, we can target specific types of neurons because all neurons are not identical. Take, for example, the dopaminergic neurons, those that use this hormone called dopamine. These neurons are comparatively very few (on the order of only 400,000 in the human brain), and yet they play an essential role to the point that their dysfunction is considered to be one of the causes of Parkinson's disease. One of the treatments for Parkison's disease consists precisely in performing stimulations using electrodes implanted in the deep areas of the brain. 

Control the expression of ChR2

For the moment, we have passed over in silence the way in which it is done so that the protein ChR2 is found inserted in the membrane of neurons. Naturally, this protein is produced in a very specific unicellular alga, but not at all in the brains of animals. In order for this to work, we must, therefore, ensure that the neurons that we target begin to produce this protein. As you may know, in the living world, proteins are produced from DNA. Part of the genes serves as a blueprint for making proteins. So for a cell to start producing the ChR2 protein, we must provide it with the corresponding DNA that we have previously extracted from our unicellular alga.

Author: Vicki Lezama