Medicine Nobel: how an algal protein became a switch for nerve cells

A flash of blue light can make a suitably prepared nerve cell fire. That gives scientists a way to investigate something deeply personal: how the activity of brain cells contributes to memories, feelings and behavior. The surprising ingredient is a protein borrowed from a single-celled alga that swims toward light.

On October 5, the Nobel Assembly awarded the 2026 Nobel Prize in Physiology or Medicine jointly to Karl Deisseroth, Peter Hegemann and Georg Nagel for discoveries concerning light-gated ion channels and optogenetics—the use of genetic tools and light to control cell activity.

The value of that control becomes clear when considering an ambiguous observation. If certain nerve cells become active during a movement, their activity might help produce it, respond to it or accompany something else happening nearby. Changing their activity and measuring the consequences gives researchers a way to distinguish those possibilities.

The route to that experiment began with Hegemann’s interest in how the green alga Chlamydomonas responds to light. One of its proteins, channelrhodopsin-2, or ChR2, turned out to combine two jobs: detecting light and opening a passage through the cell’s outer membrane. Electrically charged particles called ions could then cross that membrane.

In their 2003 study, Nagel, Hegemann and seven coauthors demonstrated light-triggered electrical currents in frog eggs and mammalian cells containing ChR2. Those collaborators were Tanjef Szellas, Wolfram Huhn, Suneel Kateriya, Nona Adeishvili, Peter Berthold, Doris Ollig and Ernst Bamberg. The finding showed that the protein’s useful property could work beyond its original algal setting.

But an electrical response to light left an important question: was ChR2 itself the gate, or was it telling another part of the cell to open one?

The researchers tested detached patches of cell membrane, replacing the liquid facing the membrane’s inner surface. Light-triggered currents persisted. Together with the rapid response, that supported their conclusion that ChR2 directly opened an ion channel, without requiring a messenger floating through the cell’s interior. The paper also reports that eggs without the introduced protein lacked these light-triggered currents. These comparisons helped separate the protein’s action from illumination alone. The original experiments established a mechanism, rather than merely an intriguing response.

The next step was to put that mechanism to work in nerve cells. According to the Nobel Assembly’s account, Deisseroth introduced the gene for channelrhodopsin into rat nerve cells and triggered nerve signals with blue light, publishing the breakthrough in 2005. The announcement dates his subsequent demonstration in the brains of living mice to 2007.

The chain is straightforward. Introduced genetic instructions allow a cell to make the light-sensitive protein. The protein sits in the membrane; blue light opens its channel; moving ions change the cell’s electrical state. In a nerve cell, that change can trigger firing—the electrical signal used to communicate with other cells.

The distinction between the two stages matters. The 2003 experiments established that an algal protein could provide a directly light-controlled channel in other cells. The neuronal work showed that this property could be used to trigger nerve signals. A working channel and a working tool for studying the brain are related achievements, but they answer different questions.

Consider a clearly hypothetical experiment: researchers notice that a group of nerve cells fires when a mouse turns left. They arrange for those cells to make ChR2, activate them with light and measure whether turning changes. Comparisons would include otherwise matched animals receiving the same illumination without ChR2, and animals with ChR2 receiving no illumination. These help test whether a change depends on the combination of protein and light.

If activation increased left turns, that would support the conclusion that those cells can influence turning under the tested conditions. It would leave open whether they are required for an ordinary left turn, whether other cells can produce the same movement and whether the result applies beyond that experiment. ChR2 supplies a route to activation; it is not a universal switch that both starts and stops any nerve cell.

The October 5 announcement also notes attempts to use optogenetics to restore sight in people with visual impairment. It gives no patient outcomes. The frog-cell and nerve-cell experiments establish how light can alter electrical activity; a clinical treatment must also establish what that change achieves for a person.