From algae to the brain: the 2026 Nobel Prize in Medicine honours optogenetics

The 2026 Nobel Prize in Physiology or Medicine has been awarded to Karl Deisseroth, Peter Hegemann and Georg Nagel for their discoveries concerning light-activated ion channels and for the development of optogenetics. Behind this rather technical wording lies a story that begins with a fundamental problem in biology: understanding how a single-celled organism manages to perceive light and alter its behaviour almost immediately.

Peter Hegemann was studying Chlamydomonas, a small green alga with two flagella, capable of orienting its movement in response to light. The speed of the response suggested that light perception was directly linked to an electrical change in the cell membrane. The work carried out with Georg Nagel led to the identification of rhodopsins, proteins that possess precisely this property. When they absorb light, they change their conformation and open a channel across the membrane; ions – that is, particles with an electric charge – pass through that channel, and the movement of the ions alters the cell’s electrical potential.

To understand why this discovery has had such far-reaching consequences, we need to recall how a neuron works. Nerve activity depends on changes in the membrane’s electrical potential. When depolarisation exceeds a certain threshold, the neuron generates an action potential – the electrical impulse through which it transmits information to other cells. A protein capable of opening an ion channel in response to light could therefore serve as a means of externally controlling a neuron’s electrical activity.

In 2005, Edward Boyden, Feng Zhang, Ernst Bamberg, Georg Nagel and Karl Deisseroth demonstrated that the rhodopsin-2 gene could be introduced into mammalian neurons and that brief pulses of blue light were sufficient to generate action potentials with a temporal precision of the order of milliseconds. This breakthrough gave rise to optogenetics. The genetic aspect of the method allows the light-sensitive protein to be expressed only in specific cell types; light then enables its activity to be controlled at a time chosen by the experimenter. The development of different proteins has subsequently also made it possible to reduce neuronal activity and to use different wavelengths.

The result has transformed the kind of experiments that can be carried out on the brain. For a long time, neuroscience has studied the association between the activity of specific brain regions and behaviour, or has used electrical stimulation. Both strategies have contributed significantly to our understanding, but they leave the question of causality unresolved. If a group of neurons becomes active whilst an animal experiences fear, that correlation alone does not prove that it is precisely those cells that produce the response. Electrical stimulation, in turn, involves different cell populations and fibres that traverse the same region, making it difficult to determine which element of the circuit is responsible for the observed effect.

Using optogenetics, it is possible to genetically select a specific population of neurons, activate them during behaviour and measure what happens. The same group of cells can be silenced to see whether a function ceases. The experimental question thus takes on a much more rigorous form: is the activity of those cells sufficient to produce a certain behaviour? Is their activity necessary for that behaviour to occur?

One of the best-known examples concerns memory. In 2012, Susumu Tonegawa’s team succeeded in labelling certain neurons in the hippocampus—which had been activated during the formation of a fear-associated memory—using a light-sensitive protein. These same neurons were then reactivated by light whilst the mouse was in a different environment. The animal exhibited the behavioural response associated with the memory. The problem of the engram – that is, the physical trace left in the brain by a memory – was addressed through the direct manipulation of the cells involved: a portion of the neuronal population active during memory formation could be artificially reactivated, and this reactivation produced a component of the learned response.

Experiments of this kind have made it possible to analyse circuits involved in reward, fear, sleep, movement, sensory responses and numerous other functions. The importance of optogenetics stems from the ability to link the activity of identified cells to observable outcomes using time scales compatible with those of neuronal communication. A variation lasting just a few milliseconds can have different effects from prolonged stimulation of a brain region, and temporal control therefore becomes an essential part of the experiment.

Therapeutic applications have also emerged. In 2021, the partial restoration of visual function was reported in a patient with retinitis pigmentosa, achieved by inducing the expression of a light-sensitive protein in the retinal ganglion cells and then delivering appropriate light stimuli via special glasses. This is a significant achievement, though still a long way from the widespread clinical use of optogenetics in neurological disorders. The 2026 prize recognises, above all, what this technology has already achieved as a method of gaining knowledge.

Precisely for this reason, this year’s Nobel Prize prompts a question that also applies to other recent awards. Looking back at the history of medicine and biology, one sometimes gets the impression that major conceptual revolutions have become rarer and that recognition is increasingly being given to technologies capable of producing extraordinary results. The comparison springs to mind when thinking of discoveries that have transformed our understanding of biological phenomena, from the structure of DNA to the genetic code, from gene regulation to prions. These were discoveries that forced us to revise a significant part of the theory used to describe life.

The history of the Nobel Prize, however, shows that the distinction between fundamental knowledge and its application has always been less clear-cut than it appears. Computed tomography, magnetic resonance imaging and in vitro fertilisation were awarded the prize for introducing new technical capabilities. During the same period, and indeed over the last twenty-five years, the Nobel Prize has continued to recognise discoveries concerning cell function, development, immunology, genetics and the nervous system. In 2012, the prize for cell reprogramming recognised the discovery that an adult cell can be restored to a pluripotent state; in 2019, the mechanisms by which cells sense the availability of oxygen were honoured; in 2024, the discovery of microRNAs and their role in regulating gene expression was recognised. Even when a discovery rapidly leads to practical applications, its scientific significance may precede and surpass them.

Optogenetics brings another phenomenon into particularly sharp focus. A scientific revolution can stem from a new theory, and it can also stem from the emergence of a tool that makes an experiment possible which was previously unfeasible. The microscope changed biology because it made the cellular world visible; DNA sequencing transformed genetics because it allowed the information contained in genomes to be read directly; optogenetics allows us to manipulate the activity of selected cells whilst a neural circuit is functioning and whilst an animal is behaving.

When the type of experiment we can carry out changes, so too does the quality of the results we can obtain. The distinction between conceptual discovery and technical innovation then becomes insufficient. Hegemann and Nagel elucidated the functioning of light-sensitive proteins found in microorganisms; Deisseroth and other researchers transformed those proteins into tools for probing the nervous system. From that point onwards, it became possible to establish causal relationships that previously could only be inferred through indirect observations.

If there is a recognisable shift in recent Nobel Prizes, it concerns the way in which scientific revolutions come about. A growing part of contemporary biology is advancing by developing tools that extend our ability to observe and manipulate living systems, whilst new knowledge emerges from the use of those tools. In such cases, technical invention becomes an integral part of the structure of scientific discovery.

The 2026 Nobel Prize falls into this category. It stems from research into the light response of a single-celled alga; its outcome is the ability to control specific groups of neurons with the temporal precision required for the brain to function. The medical applications could prove significant, and some have already begun to emerge. The achievement that justifies the prize today relates above all to what, thanks to those proteins, we have become able to ask ourselves, in order to obtain experimental answers as to how our nervous system works.