Scientists studying pond scum discovered its peculiar ability to sense light, even without eyes. This discovery would eventually lead to a technique called optogenetics, one of the most powerful techniques for mapping the human brain. Find out how in this original video from BrainFacts.org:
For millennia, the filmy green coating floating on stagnant summer ponds held a secret. Far from a massive goo, pond scum is made up of trillions upon trillions of microscopic organisms known as green algae. Like most life on Earth, these organisms rely on sunlight for energy. What’s more, they can actually detect and move towards sunlight, even though they do not possess eyes or brains. Unraveling how they do that has given a new tool to study the most complicated structure in the universe, the brain.
Scientists figured out the first piece of the puzzle in 2002 when they discovered that green algae detect light using proteins called channelrhodopsins. Curiously, these proteins function like those found in the light-sensitive cells of the retina, lining the back of the mammalian eye. As scientists investigated these unusual proteins, they discovered a larger role for them. In addition to sensing light, channelrhodopsins might also function as ion channels. Embedded in cell membranes, ion channels are tiny pores that allow charged atoms to move in and out of cells. In the nervous system, this flux of charged atoms generates the electrical signals neurons use to communicate with each other. But the real question was, could light trigger channelrhodopsins to open and allow the flow of charged ions, and the answer to this question proved a pivotal advance in the way we study the brain. Experiments in laboratory dishes demonstrated that light can indeed open these channels and let ions flow in sufficient quantities to excite neurons and turn them on. This discovery piqued the interest of scientists studying the brain.
If these light activated channels could be introduced into neurons, then perhaps scientists could turn neurons on by simply shining a light. That kind of control could help scientists determine which neurons were talking to each other and map the brain’s neural circuits. Using modern genetic techniques, this is now possible. Scientists can insert light activated channels into neurons in a living brain and turn them on with light. The technology is called optogenetics, and thanks to some salt-loving bacteria found in desert lakes, we can also add an off switch. A light-activated pump found in the bacteria shuttles negatively charged atoms into neurons, shutting down activity. With these tools, we can turn neurons off with the flip of a switch. As a result, we can map neural circuitry with unprecedented precision.
What’s more, optogenetics has the potential to advance our understanding of the brain in both health and disease. A study in blind mice, for example, found that adding channelrhodopsins to retinal neurons restored the animals’ responses to light. And the technique could also revolutionize our understanding of a number of neurological disorders. In Neurodegenerative disease where dopaminergic neurons of the ..., for example, the loss of dopamine-producing cells results in the over-stimulation of some neurons and the under-simulation of others. We could imagine one day using optogenetics to treat neurological disorders, like Parkinson’s disease. For example, when medicines no longer help, some Parkinson’s patients undergo deep brain stimulation where electrodes implanted in specific regions of the brain activate the regions to improve symptoms like tremor. But sometimes surrounding brain areas also become activated, which can produce serious side-effects. A therapy using light, however, may be less invasive and more precise, targeting only neurons possessing genetically-added light-activated on or off switch.
When scientists revealed channelrhodopsins as the secret behind pond scum, they found an innovative way to investigate the brain’s many mysteries. Further research and exploration could also one day help develop treatments for people suffering from Parkinson’s disease and other neurological diseases.
Illustration by Sean Noah.